Evaluation of Recent Treatment Techniques for Removal of Heavy ...

-RESZr a7w EVALUATION OF RECENT TREATMENT TECHNIQUES

FOR REMOVAL OF HEAVY METALS FROM INDUSTRIAL WASTEWATERS

Robert W. Peters and Young Ku H Environmental Engineering, School of Civil Engineering

Dibakar Bhattacharyya Department of Chemical Engineering, University of Kentucky Purdue University, West Lafayette, IN 47907

Lexington, KY 40506

Removal of heavy metals from industrial wastewaters can be accomplished through various treatment options, including such unit operations as chemical precipitation, coagulation, complexation, activated carbon adsorption, ion exchange, solvent extraction, foam flotation, electrodeposition, cementation, and membrane operations. This paper describes these various treatment strategies and methodologies employed for heavy metal removal. Comparison of their applications in industry are addressed. Advantages and disadvantages are addressed for each treatment scheme.

INTRODUCTION

Growing concern f o r t h e presence and con- tam ina t ion o f heavy meta ls i n our water sup- p l i e s has s t e a d i l y increased over t h e l a s t few years. Elements such as mercury and cadmium e x h i b i t human t o x i c i t y a t ext remely low con- c e n t r a t i o n s . The elements s i l v e r , chromium, lead, copper, and z i n c a l s o e x h i b i t t o x i c pro- p e r t i e s t o humans a l though the concen t ra t i ons a r e orders o f magnitude h ighe r than t h a t r e - q u i r e d f o r Cd o r Hg t o x i c i t y . The most pub- l i c i z e d case o f i n d u s t r i a l heavy metal p o l l u - t i o n i n v o l v e d t h e discharge o f t h e c a t a l y s t methy lated mercury c h l o r i d e i n t o Minamato Bay, Japan, f rom a p l a s t i c manufactur ing f a c t o r y . Microorganisms converted t h e sedimented compound t o monomethyl-mercury, which l e d t o an enr ichment o f t h i s t o x i c metal i n f i s h con- sumed by l o c a l people, causing severe ch ron ic mercury po i son ing diseases . (31. Removal s o r reduc t i ons o f t o t a l heavy metal concen t ra t i ons below 10 mg/l a re u s u a l l y desirable p r i o r t o any wastewater t reatment o p e r a t i o n s ince many heavy metals, adverse ly a f f e c t b i o l o g i c a l o x i - d a t i o n processes, (Z,27,113), such as trickling f i l t e r s , a c t i v a t e d sludge, and anaerobic d i - ges t i on.

t h e p l a t i n g i n d u s t r i e s a re obvious sources of these heavy metals. The na tu re o f t h e waste d i r e c t l y r e f l e c t s t h e p a r t i c u l a r combinat ion o f metals and manufactur ing processes used by a g i v e n p l a n t . For example, p l a t i n g i s commonly done i n e i t h e r an ammonia o r a cyanide

Metal processing, metal f i n i s h i n g , and

bath. t a i n h i g h concen t ra t i ons o f ammonia o r cyanide which may i n t e r f e r e w i t h t h e removal o f the heavy meta ls by hydroxide p r e c i p i t a t i o n . Genera l ly , t h e waste problems are r e l a t e d t o the t r a n s f e r o f d i sso l ved metal from p l a t i n g baths o r m e t a l sur face c lean ing baths by drag ou t .

The spent process waters u s u a l l y con-

A number o f s p e c i a l i z e d processes have been developed f o r t h e removal o f metals from waste discharges. These u n i t operat ions i n - c l ude: chemical precipitation (18,67,85,87,111,124), coagulation/flocculation (38,431,ion exchange/ solvent extraction (36.44,76,93), cement- a t i o n (41,681, complexation (66,99,1511, e l e c t r o - chemical operat ions (391, b i o l o g i c a l operat ions (27,301, adso rp t i on (11,60,77,110), evaporat ion (1131, filtration (401, and membrane processes (23,291. A number o f authors have presented summaries on var ious techniques a v a i l a b l e f o r heavy metal removal f rom s o l u t i o n .

The purpose o f t h i s paper i s t o review the processes c u r r e n t l y a v a i l ab le f o r wastewaters laden w i t h heavy meta ls . The removal e f f i c i e n c i e s and r e s i d u a l heavy metal con- c e n t r a t i o n s repo r ted i n t h e l i t e r a t u r e are summarized i n t h i s paper and w e l l as r e l a t e d c o s t i n f o r m a t i o n . This paper cou ld n o t pos- s i b l y i n c l u d e a l l t he research and app l i ca - t i o n s o f heavy metal removal f rom wastewaters; r a t h e r t h i s paper seeks t o h i g h l i g h t many o f t he s i g n i f i c a n t c o n t r i b u t i o n s t o the f i e l d d u r i n g t h e pas t twenty years.

165

166 Separation of Heavy Metals AlChE SYMPOSIUM SERIES

PRECIPITATION PROCESSES

I n i ndus t r y , by f a r t h e most w i d e l y used process f o r removal o f heavy metals f rom s o l - u t i o n i s t h a t o f chemical p r e c i p i t a t i o n ; ap- p rox ima te l y 75% o f t he e l e c t r o p l a t i n g f a c i l i - t i e s employ p r e c i p i t a t i o n t reatment ( i i 3 ) ,us ing e i t h e r hydroxide, carbonate, o r s u l f i d e t r e a t - ment, o r some combinat ion o f these treatments t o t r e a t t h e i r wastewaters. The most commonly used p r e c i p i t a t i o n technique i s hydrox ide t reatment due t o i t s r e l a t i v e s i m p l i c i t y , low c o s t o f p r e c i p i t a n t ( l i m e ) , and ease o f auto- ma t i c pH c o n t r o l . The s o l u b i l i t i e s o f t h e va r ious metal hydroxides a re minimized f o r pH i n t h e range o f 8.0 t o 11.0. Peters and Ku (124)have reviewed the c u r r e n t s t a t e - o f - t h e - a r t

Dean e t a1 .MI), p o i n t o u t t h a t iron manganese, copper, z inc, n i c k e l , and c o b a l t r e s u l t i n almost complete removal by hydroxide p r e c i p i t a t i o n w i t h a l - most no spec ia l m o d i f i c a t i o n requ i red . However, p r e c i p i t a t i o n o f mercury, cadmium, and l e a d may be slow and imcomplete. chromium i s present, r e d u c t i o n o f t he s o l u t i o n w i t h sodium m e t a b i s u l f i t e , f e r r o u s s u l f a t e , o r m e t a l l i c i r o n p r i o r t o l i m e t reatment i s necessary. t h e t r i v a l e n t form, chrome bear ing streams are general l yseg rega ted and t r e a t e d separa te l y . C h l o r i n a t i o n i s sometimes r e q u i r e d t o break down t h e con:,)l';xed organic metal 1 i c compounds p r i o r t o chemical p r e c i p i t a t i o n . Employing hydrox ide p r e c i p i t a t i o n a t e leva ted pH p rov ides c o n d i t i o n s where t h e metal hydroxides have 1 ow so l ub i 1 i t i e s and p r e c i p i t a t e o u t upon s e t t l i n g , t y p i c a l l y over t ime pe r iods o f 2 t o 4 hours. L;uxxletk [ a ) , p o i n t s o u t t h a t when two o r more heavy meta ls are found i n t h e same waste stream, the optimum pH f o r p rec - ip i -

The ques t i on then becomes whether i t i s p o s s i b l e and p r a c t i c a l t o p r e c i p i t a t e one o r more o f t h e meta ls separa te l y a t t h e source a t one pH and t r e a t t h e remaining s t ream(s) a t another pH. d i t i o n e d can p rov ide s a t i s f a c t o r y , a1 though n o t optimum, removal o f the metals p resen t i n t h e wastewater.

' * for heavy metal p r e c i p i t a t i o n .

When

To reduce hexavalent chromium t o

, t a t i o n o f each c a t i o n may be d i f f e r e n t .

I t must a l s o be determined i f one pH con-

Chemical p r e c i p i t a t i o n o f heavy metals may be accomplished by e i t h e r ba tch o r con- t i nuous systems. For smal l f l o w r a t e s ( l e s s than 50,000 g a l l o n s pe r day), s imp le r and l e s s expensive ba tch systems are more f e a s i b l e . Another a p p l i c a t i o n o f t h e ba tch system i s where t h e waste c h a r a c t e r i s t i c s may be v a r i a b l e and r e q u i r e m o d i f i c a t i o n o f t reatment f rom t ime t o t ime. A c o n t i n u o u s t reatment system i s a p p l i c a b l e when wastewater c h a r a c t e r i s t i c s a re u n i f o r m o r when f l o w r a t e s are l a r g e .

Hydroxide P r e c i p i t a t i o n

Arumugam ( 5 ) s t u d i e d hydrox ide p r e c i p i t- a t i o n f o r recovery o f chromium f rom spent tan l i q u o r . cheapest f o r t h e removal and recovery o f chromium. The optimum pH fo r maximum removal w i t h 1 ime i s 6.6; removal o f chromium exceeded 98% a t t h a t pH. The p r e c i p i t a t e d chromium hydrox ide i s separated by s e t t l i n g , f i l t e r e d , and r e d i s s o l v e d i n s u l f u r i c a c i d t o form chromium s u l f a t e which can be recyc led f o r f u r t h e r tanning. The use o f l i m e was more economical than t h e use o f o t h e r a l k a l i e s (NaOH,Na2C03, and NH40H).

h i s t o r y o f wastewater t reatment f o r a small chrome p l a t i n g shop. Caust ic soda was used t o a d j u s t t h e wastewater pH t o 9.5 - 10.0 t o p r e c i p i t a t e t h e meta ls as metal hydroxides. For t h e copper wastewaters, t h e p r e c i p i t a t i o n was c a r r i e d o u t a t pH 10.5 u s i n g NaOH.

This p r e c i p i t a t i o n process was t h e

Rabosky and A1 tared131 1 presented a case

S h e f f i e l d (138) i n v e s t i g a t e d 1 ime p r e c i p i - t a t i o n f o r removal o f copper, i r on , n i c k e l , chromium, and lead. These meta ls f rom these e l e c t r o p l a t i n g shops cou ld be success fu l l y removed by p r e c i p i t a t i n g w i t h a hydrox ide (such as l i m e ) or soda ash w i t h a d d i t i o n o f s u l f a t e o r s u l f i d e f o r t he enhancement i n removing t h e copper / i r on complexes.

t a t i o n o f z inc, cadmium, and n i c k e l by both hydrox ide and s u l f i d e p r e c i p i t a t i o n f o r va r ious pH cond i t i ons , r e a c t i o n times, and type and concen t ra t i on o f complexing agents. The metal hydroxide p r e c i p i t a t e s tend t o be c o l l o i d a l a n d amorphous i n nature, causing t h e r e s u l t i n g sludqe t o be voluminous. The presence o f complexing agents severe ly i n - h i b i t e d metal hydroxide p r e c i p i t a t i o n . Gen- e r a l l y h ighe r pH c o n d i t i o n s enhance t h e n u c l e a t i o n r a t e and enhances the r e s u l t i n g p a r t i c l e s i z e d i s t r i b u t i o n . I n the absence o f c h e l a t i n q agents, ext remely low res idua l z i n c and cadmium concentrat ions (Zn < 0 . 5 mg/l, Cd i 0.3 mq/ l ) cou ld be obtained.

Peters and Ku (124) s t u d i e d batch precipi-

Hydroxide prec'ipi t a t i o n o f heavy metats i s w e l l s u i t e d f o r automat ic pH c o n t r o l and has been shown t o be an e f f e c t i v e t reatment technique i n i n d u s t r y . As an example o f the e f f e c t i v e na tu re o f hydroxide p r e c i p i t a t i o n reTyyal e f f i c f p c i e s exceeded 98% f o r Cd++, C r , and Pb us ing sp iked w e l l waters and r i v e r waters ( 4 3 , 1 4 3 , 1 4 4 ) . L i m i t a t i o n s associated w i t h t h e use o f hydroxide t r e a t - ment(i2i) i nc lude :

No. 243, Vol. 81 1 67

Hydroxide p r e c i p i t a t e s tend t o reso l ub i 1 i z e i f the s o l u t i o n pH i s changed. Chromium ( V I ) i s n o t removed by hydrox ide p r e c i p i t a t i o n . Removal o f metals hydrox ide p r e c i p i t a t i o n o f mixed metal wastes may n o t be e f f e c t i v e because the minimum s o l u b i l i t i e s f o r d i f - f e r e n t metals occur a t d i f f e r e n t pH con- d i t i o n s . The presence o f complexing agents may have an adverse e f f e c t on metal removal. Cyanide i n t e r f e r e s w i t h heavy metal r e - moval by hydroxide p r e c i p i t a t i o n . Hydroxide sludge q u a n t i t i e s can be sub- s t a n t i a l and are g e n e r a l l y d i f f i c u l t t o dewater due t o the amorphous p a r t i c l e s t ruc tu re .

Carbonate P r e c i p i t a t i o n

Pat terson e t a1 . ( i l l ) s tud ied the feasibility o f carbonate p r e c i p i t a t i o n f o r heavy meta ls removal. Carbonate p r e c i p i t a t i o n has severa l advantages over t h a t o f convent ional hydrox ide p r e c i p i t a t i o n :

Optimum carbonate p r e c i p i t a t i o n t reatment occurs a t lower pH c o n d i t i o n s than those f o r optimum hydrox ide t reatment . Metal carbonate p r e c i p i t a t e s are r e p o r t e d t o be denser than the hydrox ide p r e c i p i t a t e causing inproved s o l i d s separat ion. Carbonate sludges have b e t t e r f i l t r a t i o n c h a r a c t e r i s t i c s than hydrox ide sludges.

For the case o f n i c k e l and z inc , no b e n e f i t i s d e r i v e d by u t i l i z i n g carbonate p r e c i p i t a t i o n as opposed t o hydrox ide p r e c i p i t a t i o n ; the optimum pH f o r metal removal corresponds t o the pH values p r e d i c t e d by the t h e o r e t i c a l metal hydrox ide s o l u b i l i t y diagram. No advantage i n terms o f denser sludges o r b e t t e r f i l t r a t i o n c h a r a c t e r i s t i c s were observed f o r the z inc carbonate and n i c k e l carbonate systems over t h a t f o r t he corresponding hydrox ide systems. B e n e f i c i a l r e s u l t s were observed us ing carbonate p r e c i p i t a t i o n f o r cadmium and lead removal. Comparable res idua l cadmium concent ra t ions were observed approx imate ly 2 pH u n i t s lower f o r carbonate t reatment versus hydrox ide t reatment . The cadmium carbonate system p r e c i p i t a t e had r e l a t i v e f i l t r a t i o n r a t e s approx imate ly tw ice t h a t o f t he cadmium hydrox ide system. Treatment e q u i v a l e n t t o t h a t f o r l ead hydrox ide a t pH 10.5 was obta ined w i t h the lead, carbonate system a t pH {.? and a t o t a l carbonate concent ra t ion o f 10- *

moles/ l , o r a t pH 10 a d a t o t a l carbonate concent ra t ion o f 10-2.9 moles / l . The lead carbonate system y i e l d e d a denser p r e c i p i t a t e than the l ead hydrox ide system w i t h improved

f i l t e r a b i l i t y c h a r a c t e r i s t i c s .

p r e c i p i t a t e the heavy metals ou t o f s o l u t i o n (8). Such t reatment has the dual advantage o f p r e c i p i t a t i n g the metal carbonate ho ld ing pH w i t h i n a narrow range a t nea r l y optimum l e v e l s . as e f f i c i e n t i n removing metal f rom s o l u t i o n as o t h e r bases, i t has the advantage o f n e u t r a l i z i n g excess a c i d i t y and t h i s he lps meet wastewater discharge standards. The sodium b icarbonate ac ts as a b u f f e r t o main- t a i n a l k a l i n i t y near the optimum pH l e v e l . Some metals, such as z inc, do n o t r e a d i l y p r e c i p i t a t e regard less o f the amount o f car - bonate added. However, by mix ing soda ash, sodium bicarbonate, and l ime, i t i s p o s s i b l e t o p r e c i p i t a t e z inc as hydroxide w h i l e us ing the carbonates t o s t a b i l i z e pH. Sodium b i - carbonate t reatment has the a d d i t i o n a l advantage o f easy handling, simple app l i ca t i on , a b i l i t y t o f u n c t i o n i n cont inuous f l o w operat ion, and moderate cos t .

Sodium b icarbonate can a l so be used t o

Al though sodium b icarbonate i s n o t

S u l f i d e P r e c i p i t a t i o n

S u l f i d e p r e c i p i t a t i o n has been demon- s t r a t e d t o be an e f f e c t i v e a l t e r n a t i v e t o

hydracide precipitaticn ( 17,18,45,56,73,81,119,121,124,125 1 f o r removal o f heavy metals from i n d u s t r i a l wastewaters. A t t r a c t i v e fea tures o f t h i s process inc lude:

Attainment o f a h igh degree o f metal r e - moval even a t low pH (pH - 2-3). Low d e t e n t i o n t ime requirements i n the reac to r . because o f the h igh r e a c t i v i t i e s o f s u l f i d e s . F e a s i b i l i t y o f s e l e c t i v e metal removal and recovery. Metal s u l f i d e sludge i s th ree times l e s s sub jec t t o leach ing a t pH 5 as compared t o metal hydrox i de sl udge(l5g)making f i n a l d isposal sa fe r and eas ie r . Metal s u l f i d e sludges e x h i b i t b e t t e r t h i c k e n i n g and dewater ing c h a r a c t e r i s t i c s than the corresponding metal hydroxide sludges.

L i m i t a t i o n s o f the process i nvo l ve the po- t e n t i a l o f H S gas e v o l u t i o n and the concern f o r s u l f i d e ? o x i c i t y . E l im ina t i ng s u l f i d e reagent overdose prevents format ion o f the odor causing H S. so lub le s u l f i d g systems which do n o t match demand, the process tanks must be enclosed and vacuum evacuated t o minimize s u l f i d e odor problems.

I n c u r r e n t l y operated

Two main processes e x i s t f o r s u l f i d e


precipitation of heavy metals (45) : sulfide precipitation (SSP) and insoluble su l - f ide precipitation ( I S P ) , the difference being on how the sulfide ion i s introduced i n t o the wastewater. In the SSP process, sulfide i s added in the form of a water soluble sulfide reagent such as sodium sulfide (Na S ) or sodium hydrosulfide. I n the ISP pfocess, a sl ightly soluble ferrous sulfide (FeS) slurry i s added t o the wastewater t o supply the needed sulfide ions needed t o precipitate the heavy metals. Since most of the heavy metals have sulfides less soluble than ferrous sul- f ide, they will precipitate as metal sulfides. Advantages of the ISP process include the absence of any6detect$gle H S gas and reduction of Cr t o Cr . D?sadvantages of the process include considerably larger t h a n stoichiometric reagent consumption and 1 arge quantities of sludge being generated due t o the ferrous hydroxide formation, In the SSP process, the high sulfide concentration often causes rapid precipitation of metal sulfides (high nucleation rates) resulting in small particulate fines and hydrated colloidal particles. This causes poor set t l ing and f i l t e r ing of the flocs. chelating agents, hydroxide precipitation of heavy metals i s n o t possible even a t high pH. With sulfide precipitation, heavy metal removal i s possible even with chelants present, a l - t h o u g h the heavy metal precipitation i s in- fluenced by the presence of chelating agents. In the absence of chelating agents, l i t t l e metal hydroxide precipitation occurs fo r pH < 6. An excellent description and review of the sulfide precipitation processes has been pro- vi ded by EPA (45).

Bhattacharyya e t a l . (17) found sulfide precipitation using Na2S to be highly effective for removal of Cd, Zn , C u , Pb, As, and Se from complex wastewaters. The separation character- i s t i c s were evaluated using a dilute synthetic wastewater and an actual copper smelting p l a n t wastewater. Overall separation and precipi- t a te set t l ing rates were optimum for under- stoichiometric addition of sulfide (-0.60 x theoretical stoichiometric sul fide requi rement) and pH > 8.0. For the copper smelting plant wastewater, removal of Cd, Zn, and Cu exceeded 99%, w i t h the removals of As and Se being 98% and > 92%, respectively. The residual concentrations achieved for Cd, Cu, and Zn were consistently in the range of 0.05-0.10 mg/l. Using only hydroxide treatment, the set t l ing rates and metal separations were considerably lower t h a n t h a t obtained by sulfide precipi- t a t i o n . The Zn, Cd, and Se removals were much poorer, even a t pH 10.5, using hydroxide t reatment. A t pH 8.5, the residual metal concentra-

soluble

In the presence of

tions of Se, Cd, and Zn were 9,2, and 5 mg/l, respectively. A t pH 10.5, the residual Cd and Zn concentrations decreased t o 0.6 and 1.1 mg/l, respectively. The set t l ing rate of the sludges was a function of pH and the suifide dose. sulted from hydroxide precipitation and excess sulfide precipitation. The set t l ing rate with 0 . 6 ~ was twice t h a t obtained by hydroxide precipitation. Bhattacharyya e t a l . (21) observed essentially complete removal of zinc using sulfide precipitation ( 1 . 0 ~ ) for pH > 4. Measurements taken on H2S loss (as gas) showed H S loss was negligible due t o the preference f$r the metal sulfide reaction over the H -S- reaction. The fact t h a t nickel precipitation with sulfide was found to be a s t r o n g function of reaction time for pH < 10 i n open systems was attributed to nickel dissolution causing the formation of Ni(SOH)2 and NiSO i n the presence of oxygen. Using sulfide ffrecipi tation, residual concentrations less than 0.1 mg/l for Cu, Cd, Pb, and Zn can be achieved(l8) for the pH range of 4-12. The combined hydroxide-sulfide treatment a t pH 8-9 was shown effective for removal of As, Z n , Cd, Pb, Cu, Hg, and Fe in both synthetic wastewaters and for full-scale operation of wastewaters a t the Boliden Metal1 Corporation, Skelleftehamn, Sweden. Even with sulfide overdoses a t low pH, no H2S gas loss was observed.

Poor set t l ing velocities re-

Fender e t a l . (49) studied sulfide precipi tation for zinc-1 aden foundry wastewaters. The wastewater pH was adjusted with 1 ime t o pH's ranging from 8.5 t o 11.0. fide (750 mg/l) was added t o each sample. The residual lead and iron concentrations were consistently less t h a n 0.1 mg/l for in i t ia l wastewater concentrations of 0.1 and 21.1 mg/l, respectively. The zinc concentration decreased from 775 mg/l t o 1.7-3.7 mg/l; however this removal was s t i l l inadequate. Two stage hydroxi de/sul f i de prec i pi t a t i on was investigated. for lime treatment a t pH 9.6 t o which 20 mg/l FeS was added t o the supernatant, resulting i n a final f i l t e red effluent concentration of 0.05 mg/l Zn.

A full scale SSP treatment plant(l33)was constructed by the U.S. Army a t the Belvoir Research and Development Center in Fort Belvoir. Virainia. becomina oDerationa1 in

Ferrous sul-

The best treatment occurred

February 1985. Safety f e a h included: neutralization of pH prior t o sulfide a d d i t i o n t ro l of the sulfide feed via probe, ferrous sulfate addi t the excess sulfide, hydrogen

es of the plant the wastewater automatic con-

a specific ion on t o remove peroxide oxidatioi

No. 243, Vol. 81 169

o f t h e res idua l e f f l u e n t s u l f i d e , and cover ing and v e n t i 1 a t i o n o f the process tanks. Removal s exceeding 90% were observed f o r Cd, C r y Cu, N i , and Zn, w i t h removal o f Pb exceeding 80%. The sludge generated averaged 0.3 g a l l o n s o f sludge/1000 ga l l ons o f wastewater t rea ted ; t he s o l i d s conten t o f the f i l t e r cake averaged 23.4% w i t h o u t t he use o f any sludge condi - t i o n e r s . The sludge samples were found t o be nonhazardous w i t h respec t t o t o x i c i t y us ing the standard EPA t o x i c e x t r a c t i o n procedure f o r hazardous waste determinat ion. Chemical cos ts averaged $0.30/1000 gal 1 ons - S1 udge d isposal cos ts averaged $0.41/1000 ga l l ons o f wastewater t rea ted , causing the t o t a l opera t iona l c o s t t o be $0.71/1000 ga l l ons (exc lud ing manpower and energy cos ts ) .

p a r t i c l e s i z e d i s t r i b u t i o n s o f ZnS, CdS, and N i S p r e c i p i t a t i o n , because the removal e f - f i c i e n c y and ease o f removing the sludge are coupled. p a r t i c l e s i z e d i s t r i b u t i o n s (PSD) were r e p o r t - ed. Using s u l f i d e p r e c i p i t a t i o n v i r t u a l l y a l l t he z i n c was removed(l2l)at pH 8-10 ( r e s i - dual Zn < 0.30 mg/l; removal e f f i c i e n c y >99.7%) Few p a r t i c l e s l a r g e r than 20 pm i n s i z e were observed; the dominant p a r t i c l e s i z e was on ly 5-7 pm f o r the p r e c i p i t a t e s , causing a c loudy appearance f o r the r e a c t o r suspension and making sedimentat ion and f i l t r a t i o n operat ions d i f f i c u l t . Th is suggests a f l o c c u l a n t o r coagulant a i d would be advantageous f o r t h i s system. A1 though very h igh supersa tura t ions were achieved, t he k i n e t i c o rder was low (i < 1.02) f o r cont inuous ZnS p r e c i p i t a t i o n . L i t t l e e f f e c t i s observed on the r e s u l t i n g PSD by v a r y i n g the r e a c t o r d e t e n t i o n time, T. t i o n increases bo th the p a r t i c l e growth r a t e and the p r e c i p i t a t e dominant s i z e y i e l d i n g a more f a v o r a b l e PSD. The presence o f ca lc ium improved the s e t t l i n g c h a r a c t e r i s t i c s o f both t h e Cd-Ca-Na S and the Cd-Cas s l u r r y systems. The metal sug f ide reac t i ons are ext remely rap id ; chemical e q u i l i b r i a i s achieved w i t h i n 5 minutes r e a c t i o n t ime. a t i o n f rom an i n i t i a l amorphous k i n e t i c a l l y favored p r e c i p i t a t e t o a more c r y s t a l l i n e thermodynamically favored p r e c i p i t a t e was i n - d i c a t e d f o r t he ZnS system as the p r e c i p i t a t e ages. Minear(l l3)have a l so noted such phase t rans - format ions. P a r t i c l e s i z e r a t h e r than com- p le teness o f t he s o l i d phase fo rmat ion may o f t e n t imes c o n t r o l t he apparent removal e f - f e c t i veness .

Peters e t a l . (119,121,124) have s t u d i e d the

Both heavy metals removals and

Increas ing the suspended s o l i d s concentra-

A phase t ransform-

Pat terson e t a1 . (1l l)and Pat terson and

Peters and Ku (124) s t u d i e d batch p r e c i p i t- a t i o n o f z inc, cadmium, and n i c k e l us ing both

hydroxide and s u l f i d e p r e c i p i t a t i o n a t var ious pH c o n d i t i o n s w i t h and w i t h o u t t he presence and complexing agents. EDTA i n h i b i t e d z inc removal by bo th hydroxide and s u l f i d e t r e a t - ment due t o the fo rmat ion o f s tab le metal chelates; the e f f e c t was more pronounced f o r the Zn(0H) system. Extremely low res idua l metal c o n c h t r a t i o n s can be achieved u s i n g s u l f i d e p r e c i p i t a t i o n i n the absence o f che la t - i n g agents. Enhanced removals are achieved us ing s u l f i d e p r e c i p i t a t i o n even i n the presence o f complexing agents, as compared t o s i m i 1 a r hydrox ide p r e c i p i t a t i o n cond i t i ons . The presence o f phosphate enhances the PSD due t o a flocculation/agglomeration mechanism. The presence o f ammonia has a minimal e f f e c t on metal s u l f i d e removal and the p r e c i p i t a t i o n k i n e t i c s . e t h e r severe ly i n h i b i t s CdS p r e c i p i t a t i o n . E q u i l i b r i u m c o n d i t i o n s are q u i c k l y reached, w i t h i n 5.0 minutes r e a c t i o n time, f o r the ZnS and CdS systems, w h i l e e q u i l i b r i u m i s achieved a f t e r 40 minutes r e a c t i o n t ime f o r t he N i S system, due t o o x i d a t i o n o f s u l f i d e i n the open system.

1

p r e c i p i t a t i o n s was addressed by Peters e t a1 . (120)EDTA forms s t rong metal che la tes which i n t e r f e r e w i t h ZnS p r e c i p i t a t i o n . Weak che l - a t i n g agents ( c i t r a t e , g lucon ic acid, t a r t r a t e , e t c . ) form weak metal chelates; fo rmat ion o f the metal s u l f i d e p r e c i p i t a t e predominates. Removal o f copper i s nea r l y complete even i n the presence o f EDTA.

Hohman(5)select ively p r e c i p i t a t e d Cd f r o m a Cd-Fe wastewater a t pH 2 by employing a s t o i c h i o m e t r i c + $ y l f i d e dose on ly f o r the cadmium ions. Fe was s e l e c t i v e l y removed from Cd a t pH 6 us ing hydrox ide p r e c i p i t a t i o n . The CdS p r e c i p i t a t i o n s tep goes t o complet ion w i t h i n 1.0 minute r e a c t i o n time, w h i l e FeS p r e c i p i t a t i o n i s much slower. l eve l s , FeS r e s o l u b i l i z e s on format ion o f so lub le species o r c o l l o i d s .

p r e c i p i t a t i o n o f z inc, n i c k e l , copper, and chromium by both hydroxide and s u l f i d e t r e a t - ment us ing syn the t i c and ac tua l i n d u s t r i a l p l a t i n g wastewaters. For a z inc-n icke l wastewater, s l i g h t l y lower res idua ls and enhanced f l o c s i z e were observed f o r s u l f i d e t reatment versus hydrox i de t reatment . Low pH t reatment pH - 7.2-7.4) r e s u l t s i n incomplete removal o f n i c k e l ; z inc p r e c i p i t a t i o n i s p r e f e r e n t i a l t o t h a t o f n i c k e l . A t h igher pH l e v e l s (pH-lo), removal o f both n i c k e l and z inc exceeded 98% us ing e i t h e r hydroxide o r s u l f i d e treatment. Larger s u l f i d e doses and h igher pH cond i t ions

The presence o f EDTA and crown

The e f f e c t on che lan ts on metal s u l f i d e

A t h igh s u l f i d e

Peters and Ku (125) s tud ied the cont inuous

’ 170 Separation of Heavy Metals AlChE SYMPOSIUM SERIES

r e s u l t e d i n both g r e a t e r metal removals and l a r g e r s e t t l i n g v e l o c i t i e s . Chromium was n o t e f f e c t i v e l y removed from the Zn-Cu-Ni-Cr wastewater us ing hydrox ide treatment. I n - c reas ing the s u l f i d e dose causes lower res idua l heavy metal concent ra t ions f o r Zn, Cu, and C r , w h i l e the n i c k e l concent ra t ion increased due t o the fo rmat ion o f f i n e f l o c s . A t pH 10.0 opera t ion us ing s u l f i d e treatment, r e - movals exceeded 97%, 97%, 55%, and 83%, r e - s p e c t i v e l y f o r Zn, Cu, C r , and N i . The under- s t o i c h i o m e t r i c a d d i t i o n o f s u l f i d e appears ext remely promis ing as a t reatment technique t o achieve ext remely low metal r e s i d u a l s w h i l e min imiz ing the H2S e v o l u t i o n p o t e n t i a l and s u l f i d e t o x i c i t y problems.

Kim(75) found t h a t ca lc ium s u l f i d e s l u r r i e s , prepared s u l f i d e o r sodium hydrosu l f ide , a re e f f e c t i v e f o r metal s u l f i d e p r e c i p i t a t i o n . The a d d i t i o n of Cas can be c o n t r o l l e d i n most cases by measuring the pH. The use o f Cas enabled wastewaters c o n t a i n i n g o i l emulsions and d i s - so lved copper t o be e f f e c t i v e l y t r e a t e d (73) The a d d i t i o n o f Cas produces e a s i l y s e t t e a b l e p r e c i p i t a t e s . s i t e s f o r the produc t ion o f the metal s u l f i d e p r e c i p i t a t e s . The d isso lved ca lc ium a l so f u n c t i o n s as a coagulant. Since the ca lc ium added i n the form o f CaS i s d isso lved i n the wastewater a f t e r reac t ion , the increase i n s ludge volume i s minimal.

by r e a c t i n g l ime w i t h e i t h e r hydrogen

The Cas p a r t i c l e s a c t as n u c l e i

Coprec ip i tation/Combined Chemical Treatment

McAnnally e t a l . (91) s tud ied the use o f so lub le s u l f i d e and carbonate f o r t h e i r e f - fec t i veness i n reducing n i c k e l i n a s y n t h e t i c n i c k e l p l a t i n g wastewater. Employing j a r t es ts , an optimum pH range f o r n i c k e l removal f rom the s y n t h e t i c wastewater was found t o be 10.0- 11.0. Optimum removal occurred a t pH 11 where a res idua l t o t a l n i c k e l concent ra t ion o f 0.1 mg/l was obta ined w i t h a s u l f i d e : weight r a t i o o f 2.0 and a carbonate: n i c k e l weight r a t i o o f 20.0. A t pH 10, a s i m i l a r degree o f removal (0.2 mg/l r e s i d u f l t o t a l N i ) was obta ined us’?? a CO -: N i r a t i o o f 10.0 and a S - : N i r a t i a o f 0.5. The authors o f t h i s c u r r e n t paper speculate the e x c e l l e n t N i removals observed are due t o a cop rec ip i t a t i o n phenomena. s y n t h e t i c n i c k e l wastewater, the pH was ad- j u s t e d by dropwise a d d i t i o n o f 1N NaOH and an e q u i v a l e n t amount o f 1N CaCl s imu la te l i m e add i t i on . The ca6bonate was added i n the form o f NaHCO . Such c o n d i t i o n s l i k e l y l e d t o the p r e c i p i t a t i o n o f ca lc ium carbonate, which has been shown t o be an excellent adsorbant for Cd,Ca,pb, and Zn(31,48,122,123).

n i c k e l

TO t r e a t t he

s o l u t i o n t o

Coprec ip i t a t i o n and adsorp t ion o f N i (OH)2, and N i S onto the CaC03 sur faces may have caused such excel l e n t N i removals. McAnnal l y e t a1 .(91) however, d i d n o t r e p o r t the res idua l ca lc ium concent ra t ions t o a s c e r t a i n whether t h i s was the case o r no t .

I n a s tudy s i m i l a r t o t h a t o f McAnally e t a l . (91) , McFadden e t a1 (74) i n v e s t i g a t e d the e f f e c t o f i r o n as a c o p r e c i p i t a t o r o f n i c k e l as w e l l as carbonate add i t i on , pH adjustment, and polymer add i t i on . For pH adjustment a long w i t h carbonate add i t ion , the optimum n i c k e l removal occurred f o r a t o t a l carbonate concent ra t ion ( C ) o f 50 mg/l a t pH 11. so lub le and t o t a l n i c k e l concent ra t ions being CO.10 and 0.10 mg/l, respec t i ve l y . A l l th ree C concent ra t ions employed (50, 100, and 2’60 mg/ l ) a t pH 10 and 11 achieved a t l e a s t 96% and 99% removal o f the t o t a l and so lub le n i c k e l , r e s p e c t i v e l y . McFadden e t a l . (94) suggest the ca lc ium may prov ide a nucleus f o r CaCO format ion thereby i n c r e a s i n g s e t t l e a b j l i ty, a1 though they do n o t r e p o r t t he f i n a l r e s i d u a l ca lc ium concent ra t ions ++ o f the t r e a t e d wastewater. The i n i t i a l Ca concent ra t ions were of t he same manner as t h a t o f McAnally e t a1 .(91). C o p r e c i p i t a t i o n o f N i onto the CaCO sur faces may indeed be an exp lanat ion f o r Zhe h igh n i c k e l removals.

These c o n d i t i o n s resuTted i n the

N icke l removal us ing hydrox ide p r e c i p i - t a t i o n was most e f f i c i e n t f o r t he syn the t i c wastewater a t @I 10-11, depending on the Fe: N i r a t i o and C (94). Both the so lub le and t o t a l n i c k e l a i pH 10, Fe:Ni = 2, and C = 0, were reduced t o CO.10 mg/l. I d e n t i c a l $e- s u l t s occurred a t pH ll, Fe:Ni = 2, and C - 100 mg/l as CaCO . A t pH 9 the bes t o v e r a i l removal was obta?ned f o r a Fe:Ni r a t i o of 1.0 and CT = 50 mg/l as CaCO , where the t o t a l and so lub le res idua l nqckel concentra- t i o n s were 0.20 and 0.10 mg/l, respec t i ve l y . For the ac tua l wastewater, the most e f f i c i e n t s o l u b l e n i c k e l removal occurred a t pH 10 w i t h a Fe:Ni r a t i o o f 0.7 and C o f 0 mg/l as CaC03. A t pH 9, the most z f f i c i e n t n i c k e l removal f o r the ac tua l wastewater occured a t an Fe:Ni r a t i o o f 2.0 and C = 0 r e s u l t i n g i n a t o t a l and so lub le n i c k e l l o n c e n t r a t i o n o f 0.30 and 0.25 mg/ l , r espec t i ve l y . The use o f an ion ic and c a t i o n i c polymers d i d n o t enhance the removal o f n i c k e l apprec iab ly . On the ac tua l wastewater, the lowest c o s t t o t r e a t t he wastewater was $0.5349/1000 ga l l ons f o r c o n d i t i o n s o f pH 10, Fe:Ni r a t i o o f 0.7, and CT = 0.

heavy metals (Cu, Cd, N i , C r , and Zn) were Using fe r rous s u l f i d e as a c o p r e c i p i t a t o r

No. 243, Vol. 81 171

shown t o be s i g n i f i c a n t l y reduced i n concen- t r a t i o n f rom the i n f l u e n t wastewater (136).FeS t reatment was found t o be super io r t o conven t iona l hydroxide p r e c i p i t a t i o n employing l i m e as the p r e c i p i t a n t .

P i l o t p l a n t s tud ies (%)employing t h e con- d i t i o n s o f l i m e a d d i t i o n (260 mg/l g i v i n g r i s e t o a pH o f 10.0) p l u s 20 mg/l f e r r o u s s u l f a t e were used t o t r e a t a n i c k e l wastewater i n i t i a l - l y c o n t a i n i n g 5 mg/l N i . The r e s i d u a l n i c k e l concen t ra t i on was reduced t o 0.35 mg/l a f t e r sedimentat ion and f i l t r a t i o n . Hydroxide p r e c i p i t a t i o n alone (pH 11.5, Ca(0H) dosage = 600 mg/ l ) r e s u l t e d i n a r e s i d u a l n i c a e l con- c e n t r a t i o n o f 0.15 mg/l a f t e r sedimentat ion and f i l t r a t i o n . Leckie e t a1 484) . l i k e w i s e observed t h a t t r a c e elements o f cadmium, z inc, lead, arsenic , selenium, s i l v e r , chromium, copper, and vanadium cou ld be removed by ad- s o r p t i o n / c o p r e c i p i t a t i o n w i t h amorphous i r o n oxyhydroxide. c a t i o n adsorpt ion, i r o n dose and s o l u t i o n pH were the two adsorpt ion c o n t r o l 1 i n g parameters.

Chang and Peters (31) observed t h a t cadmium cou ld be very e f f e c t i v e l y removed us ing conven- t i o n a l l i m e s o f t e n i n g operat ions; t h e maximum contaminant l e v e l o f 0.01 mg/l f o r Cd cou ld be met f o r t he pH range o f 7.3-11.0. C a l c i t e was t h e o n l y morphological form observed f o r the cont inuous CaCO p r e c i p i t a t i o n . The r e - s idua l ca lc ium concsn t ra t i on increased 1-30-40 mg/l i n t h e presence o f cadmium i n d i c a t i n g an i n h i b i t o r y e f f e c t o f cadmium on CaCO pre- c i p i t a t i o n . Removal o f cadmium was a t r j b u t e d p r i m a r i l y t o phys i ca l adso rp t i on onto t h e CaC03 s 1 udges .

than s t o i c h i o m e t r i c a d d i t i o n o f s u l f i d e g i v i n g a combined hyd rox ide -su l f i de t reatment . A t pH 8.0, a s o l u t i o n i n i t i a l l y c o n t a i n i n g 15.0 mg/l Cd was lowered t o <O 05 mg/l us ing t h e Ta lbo t process, w h i l e hydroxide t reatment p ro - v ided a r e s i d u a l concen t ra t i on o f 4.8 mg/ l . For a water c o n t a i n i n g 2.9 mg/l Hg, convent ion- a l hydrox ide t reatment r e s u l t e d i n no mercury removal , w h i l e t h e Tal b o t process lowered the mercury l e v e l t o <0.001 mg/l a t pH 8.0. The opera t i ng c o s t o f t he Ta lbo t process i s com- pa rab le t o t h a t o f convent ional hydrox i de p re - c i p i t a t i o n . i s generated by the Ta lbo t process (as compared t o convent ional hydrox ide p r e c i p i t a t i o n ) the re - by l ower ing t h e sludge d isposal cos ts . Peters e t a1 .(118)likewise observed t h a t unders to i ch - i o m e t r i c a d d i t i o n o f s u l f i d e , even as low as 0.5 x s t o i c h i o m e t r i c requirement, l i k e w i s e p rov ided e x c e l l e n t removal o f z i n c and cadmium, and decreased the r e s u l t i n g sludge volume.

For the case o f both an ion and

Tal b o t (153)descri bed a process us ing l e s s

A sma l le r quani t i ty o f s ludge

They proposed the idea t h a t metals c o u l d be s e l e c t i v e l y p r e c i p i t a t e d , removed, and r e - covered from a mixed-metal wastewater, by proper c o n t r o l o f the pH, s u l f i d e dose, che lan t dose, type o f chelant , and temperature, i n a cascading s e r i e s o f r e a c t o r s . For example, z i n c can be s e l e c t i v e l y p r e c i p i t a t e d from n i c k e l a t pH 6.0, s t o i c h i o m e t r i c s u l f i d e dose, and a d d i t i o n o f EDTA (118)Pugsley e t a1 .(129) were t h e f i r s t i n v e s t i g a t o r s t o note t h a t s e l e c t i v e removal o f a p a r t i c u l a r heavy metal could be achieved i n a cascading r e a c t o r system through proper c o n t r o l o f t h e dosage r a t e s . P re l im ina ry c o s t est imates show the s u l f i d e t reatment process t o be -$9.27/1000 ga l l ons o f p l a t i n g wastewater, which f a v o r a b l y compares w i t h t h e -$9.45/1000 ga l l ons o f p l a t i n g wastewater by convent ional hydrox ide t reatment (118)Considerable c o s t savings can a l s o be r e a l i z e d through reuse, recyc le, and recovery o f t h e waste metals f rom t h e p l a t i n g process.

Higg ins and Slater (26) observed t h a t t reatment o f a m i x t u r e o f metals i s somewhat more e f f e c t i v e than t reatment o f meta ls i n - d i v i d u a l l y S u l f i d e t reatment lowers the s o l u b i l i t y o f n i c k e l and cadmium. F e r r i c hydroxide p r e c i p i t a t e s reduce the metal so l - ub i 1 i t i e s by i n c o r p o r a t i o n o f o t h e r metals i n an amorphous p r e c i p i t a t e and prov ides sur face s i t e s f o r adsorpt ion. A d d i t i o n o f f e r r o u s s u l f a t e t o an a l k a l i n e environment (7spH(10) causes the i r o n t o p r e c i p i t a t e as i r o n hydroxide and a ids i n t h e f l o c c u l a t i o n o f s o l i d s i n t h e process. Such t reatment was very e f f e c t i v e i n reduc t i ng hexavalent chromium t o the t r i v a l e n t form. A combinat ion o f f e r r o u s s u l f a t e and sodium s u l f i d e produces a sludge t h a t i s e a s i l y removed, y e t minimizes sludge product ion.

Brantner and Cichon (26)studied and compared the hydroxide, carbonate, and s u l f i d e t reatments f o r removal o f heavy metals (Zn, C r , Cd, Cu, and Pb). E f f e c t i v e z i n c removal was obta ined by a l l t h ree chemical p r e c j p i - t a t i o n processes. l e v e l s i n the c l a r i f i e r over f low, i t appeared z i n c removal was n o t l i m i t e d by s o l u b i l i t y b u t r a t h e r by the e f fec t i veness o f t he s o l i d s - l i q u i d separat ion. The i n a b i l i t y o f carbonate p r e c i p i t a t i o n t o c o n s i s t e n t l y achieve low r e s i d u a l z inc concentrat ions was a t t r i b u t e d t o the k i n e t i c s o f z inc carbonate format ion causing the s o l u b i l i t y o f z inc hydroxide t o govern t h e removal o f z i nc . E f f e c t i v e r e - moval o f chromium was o n l y achieved by s u l - f i d e p r e c i p i t a t i o n . P r e c i p i t a t i o n o f cadmium and copper was very e f f e c t i v e l y achieved by a l l t h ree processes. Removal o f l ead was

For low so lub le z inc

172 Separation of Heavy Metals

e f f e c t i v e l y achieved by carbonate and s u l f i d e p r e c i p i t a t i o n causing a r e s i d u a l f i l t e r e d l e a d concen t ra t i on o f <0.1 mg/l; hydroxide p r e c i p i - t a t i o n r e s u l t e d i n a mean e f f l u e n t l e v e l o f 0.2 mg/ l . cesses, carbonate p r e c i p i t a t i o n produced t h e sma l les t sludge volume. sludges o r t he s u l f i d e sludges th ickened w e l l i n t he c l a r i f i e r . When subjected t o a c i d i f i - ca t i on , t he hydroxide sludge was the l e a s t s t a b l e w h i l e the s u l f i d e s ludge was the most s tab le .

COAGULATION/FLOCCULATION

O f t h e th ree p r e c i p i t a t i o n pro-

Ne i the r the hydroxide

Coagul a t i o n / f l o c c u l a t i o n has been known capable o f removing heavy meta ls f rom s o l u t i o n . Coagulat ion r e f e r s t o the charge n e u t r a l i z a t i o n o f t he p a r t i c l e s . F l o c c u l a t i o n i nvo l ves slow m ix ing t o promote the agglomerat ion o f t h e d e s t a b i l i z e d p a r t i c l e s . EPA (43) i n v e s t i g a t e d the use o f l i m e s o f t e n i n g and coagu la t i on (us ing f e r r i c s u l f a t e o r a&m) f y i remyyal o f such heavy meta ls as Pb , Cd , C r , C r , e t c . Whi le l i m e s o f t e n i n g achieved removals o f g rea te r than 98% i n the pH range o f 8.5-11.3 f o r cadmium, cadmium removals by f e r r i c s u l f a t e and alum coagu la t i on were lower than t h a t of l i m e s o f t e n i n g and were shown t o depend on pH. Cadmium removals increased w i t h i n c r e a s i n g pH. F e r r i c s u l f a t e coagu la t i on o f a r i v e r water c o n t a i n i n g 0.3 mg/l Cd showed removal t o increase from 20% a t pH 7.2 t o above 90% a t pH 2 8. creased w i t h i n c r e a s i n g pH; however above pH 8, removals may depend on the raw water t u r b i d i t y . Both f e r r i c s u l f a t e and alum coagu la t i on achieved g r e a t e r than 97% removal o f l e a d from a r i v e r water c o n t a i n i n g 0.15 mg/l Pb i n t h e pH range o f 6-10. Experiments on w e l l waters under s i m i l a r c o n d i t i o n s had removal r a t e s by f e r r i c s u l f a t e and a1 um coagu la t i on o f >97% and 80-90%, r e s p e c t i v e l y . When the l e a d con- c e n t r a t i o n was increased t o 10 mg/l, f e r r i c s u l f a t e coagu la t i on again achieved removals i n excess o f 95% whereas alum coagulaJAon achieved o n l y -80% removal. Using C r waters, f e r r i c s u l f a t e achieved the bes t r e s u l t s , r e - moving -35% a t pQ65.5 on a r i v e r water c o n t a i n - i n g 0.15 mg/l C r . Alum coagu la t i on c o u l d do no b e t t e r than 10% removal throughout t h e e n t i r e pH range.+3pH has o n l y a s l i g h t e f f e c t qn removal o f C r by alum and i r o n coagula- t i o n . F e r r i $ 3 s u l f a t e coagd la t i on achieved P y c e l l e n t C r removals, i n excess of 98%, Tnroughout the pH range o f 6.5-9.3.

rz i iova ls exceeding 90% f o r t he pH range 6.7-

3t pH 9 . 2 the remyyal had decreased t o 78%. F o r an i n i t i a l C r concen t ra t i on o f 10 mg/l,

+6

Alum coagu la t i on r e s u l t s a l s o i n -

Alum oagulat ion, a l though l e s s e f f e c t i v e , ob ta ined

5 Above pH 8.5, removals began t o decrease;

AIChE SYMPOSIUM SERIES

f e r r i c s u l f a t e and alum coagu la t i on bo th achieved removals i n excess o f 98% i n the optimum pH range.

Daniel s(38) prov ided an e x c e l l e n t rev iew on t h e removal o f heavy meta ls by i r o n s a l t s and p o l y e l e c t r o l y t e f l o c c u l a n t s . Removal o f c e r t a i n 'heavy meta ls f r o m wastewaters can occur s imul taneously w i t h removal o f suspended s o l i d s , BOD, and t o t a l phosphorus when chemical t rea tmen t i s employed. The i n s o l u b l e f r a c t i o n s o f Fe, C r y Zn, Cu, and N i are captured by sequent ia l coagu la t i on and f l o c c u l a t i o n .

.For t reatment o f base metal mine drainage, (64),use o f an a n i o n i c po lymer ic f l o c c u l a n t p rov ided g r e a t e r c l a r i f i e r ope ra t i ona l r e - 1 i ab i 1 i t y and reduced mean metal concentrations i n t h e ove r f l ow . l y improved t h e sludge s e t t l i n g characteristics Aulenbach e t a1 .(6) observed t h a t a d d i t i o n o f alum o r sodium aluminate a t dosages t h a t e f f e c t i v e l y remove phosphorus i s b e n e f i c i a l i n t h e removal o f Cu, C r , and Pb f rom wastewaters. Chromium removal i s enhanced by sodium aluminate add i t i on , b u t i s una f fec ted by alum a d d i t i o n . crease t h e removal o f 1 ead. N i 1 ssy$lOtj) itj- veS.igaty$ the+remov$i o f &++, Cu C r , Hg , Cd , Zn , N i ' , Co , and As'5 by aluminum s u l f a t e and ca lc ium hydrox ide t r e a t - ment. Orthophosphate had no i n f l u e n c e upon the p r e c i p i t a t i o n . The concentrat ions of Pb, Cu, Cr, Hg, Cd, and As were reduced t o low l e v e l s by bo th p r e c i p i t a n t s , w h i l e Zn, N i , and Co were p r e c i p i t a t e d o n l y f o r pH 9.5. The p r e c i p i t a t i o n o f Cu and Pb were severe ly i n h i b i t e d by t h e presence o f NTA a t pH 1 9 . 0 .

Polymer a d d i t i o n s u b s t a n t i a l -

Both aluminum s a l t s i n -

FLOTATION

Foam f l o t a t i o n depends on the use o f a s u r f a c t a n t t h a t causes a nonsurface a c t i v e m a t e r i a l t o become sur face a c t i v e forming a product t h a t i s removed by bubb l i ng a gas through the bu lk s o l u t i o n t o form a foam. The use o f foam f l o t a t i o n techniques f o r r e - moval o f heavy meta ls has been w e l l s tud ied. (32,52,142,155). With dilute wastewaters containing heavy metals i n t h e p a r t s per b i l l i o n o r pa r t s per m i l l i o n ranges, foam f l o t a t i o n o f f e r s several d i s t i n c t advantages:

s i m p l i c i t y f l e x i b i l i t y and e f f e c t i v e n e s s o f ope ra t i on . l i m i t e d space requirements due t o r a p i d reac t i ons . p roduc t i on o f small, concentrated volumes of sludge. moderate cos ts comparable t o t h a t o f l i m e

No. 243, Vol. 81 1 73

p r e c i p i t a t i o n .

energy, and chemical s . and l a r g e scales.

c e n t r a t i o n s w e l l below the standards e s t a b l i s h e d by r e g u l a t o r y agencies

9 low cos ts i n terms o f l abo r , equipment,

capable o f a p p l i c a t i o n on small i n te rmed ia te

capabi 1 i ty o f reducing t h e contaminant con-

To s imu la te a lead-bear ing waste, f e r r i c c h l o r i d e was added t o form a p r e c i p i t a t e o f f e r r i c hydroxide; t h e l ead r a p i d l y adsorbed on to t h e sur face o f t he Fe(0H) f l o c (155) . Fo l low ing pH adjustment, a s u r f a c t a n t sodium l a u r y l s u r f a t e (NLS) was added. P o s i t i v e l y charged Fe(0H) t o the n e g a t i v h y charged bubbles formed by bubb l i ng a i r through the s o l u t i o n . The r i s i n g bubbles o f NLS along w i t h t h e Fe(0H) and adsorbed l e a d were removed as a foam and then co l l apsed forming a foamate o f about 2-3% o f t h e i n f l u e n t volume.

f l o c p a r t i c l e s were a t t r a c t e d

f l o c

Two phys i ca l models are used t o descr ibe t h e attachment o f f l o c p a r t i c f e s t o bubbles i n the presence o f a s u r f a c t a n t (154,155). I n t h e coulombic model, i o n i c s u r f a c t a n t i s adsorbed on t h e a i r - w a t e r i n t e r f a c e o f t h e bubble, r e s u l t i n g i n a sur face charge d e n s i t y on the bubbles. The charge d e n s i t y i s u s u a l l y nega t i ve due t o t h e use o f an ion i c s u r f a c t a n t s . The f l o c i s u s u a l l y g iven a sur face charge d e n s i t y opposi te t o t h a t on the bubbles through pH adjustment o r concen t ra t i on o f o t h e r p o t e n t i a l determin ing i ons . The e l e c t r i c a l a t t r a c t i o n between the f l o c p a r t i c l e and t h e bubble i s reduced by the d i f f u s e e l e c t r i c a l double l a y e r i n t h e v i c i n i t y o f t h e su r face . Wi lson (160) descr ibed methods t o c a l c u l a t e the e l e c t r i c a l p o t e n t i a l i n s o l u t i o n , t he p o t e n t i a l energy o f i n t e r a c t i o n between t h e two i n t e r - faces, and t h e adso rp t i on isotherms.

I n the con tac t angle model, s u r f a c t a n t i o n s adsorb onto t h e pr imary l a y e r o f t h e f l o c p a r t i c l e . They present t h e i r i o n i c o r p o l a r ends t o the s o l i d and t h e i r hydrophobic hydrocarbon t a i l s t o t h e s o l u t i o n . For s u f f i c i e n t l y h i g h concen t ra t i ons o f s u r f a c t a n t , t he s o l i d sur face i s o f a hydrocarbon cha rac te r and i s hydrophobi c. t h a t t h e c o n t a c t angle o f t h e a i r - w a t e r i n t e r - face on the s o l i d i s nonzero, thereby pe rm i t - t i n g attachment o f the p a r t i c l e t o the bubble. Using s t a t i s t i c a l mechanics, t he b i n d i n g energ ies and adso rp t i on isotherms can be c a l - cu la ted . Conclusions commoli t o both models a re summarized below (154): 1. I nc reas ing i o n i c s t r e n g t h decreases the

The s u r f ace tens ions are such

f l o t a t i o n e f f i c i e n c y .

2.

3 .

4.

I nc reas ing the l e n g t h o f the s u r f a c t a n t hydrocarbon t a i l decreases t h e b u l k l i q u i d concen t ra t i on o f su r fac tan t r e - q u i r e d t o produce f l o t a t i o n . I nc reas ing p a r t i c l e s i z e increases f l o t a - t i o n e f f i c i e n c y . I nc reas ing temperature increases t h e r e - q u i r e d s u r f a c t a n t concentrat ion.

P l o t s o f F ( t h e r a t i o o f t he f l u x o f metal o u t the bottom o f a column t o t h e f l u x o f metal a t t h e top o f t he column) as a f u n c t i o n of t h e column, parameters shm (154,155):

F decreases ( b e t t e r performance) as speci -

F decreases as sur face v e l o c i t y increases. F i s rough ly p r o p o r t i o n a l t o l i q u i d v e l o c i t y . F increases as the l i q u i d and foam tu rbu - l e n t d i f f u s i o n c o e f f i c i e n t s increase . F decreases as the mass t r a n s f e r c o e f f i c i e n t increases approaching a f i n i t e e q u i l i b r i u m - c o n t r o l l e d 1 i m i t i ng value.

r i s e s .

c e n t r a t i o n s increase.

f i c foam area increases.

F decreases as the adsorpt ion isotherm

F increases s low ly as the l o w feed con-

P re l im ina ry batch s tud ies o f Thackston e t a ] . (154,155)showed the optimum pH f o r removal o f l e a d was approximately 6.5 and t h a t i nc reas - i n g i o n i c s t r e n g t h was de t r imen ta l t o t h e process; above 0.05M NaNO , the separat ion e f f i c i e n c y dec l i ned markedly. F e r r i c hydro- x ide was much more e f f e c t i v e f o r adsorbing l e a d than was aluminum hydroxide Sodium carbonate was a more e f f e c t i v e n e u t r a l i z i n g agent than was soda ash. High s u l f a t e con- c e n t r a t i o n s reduced the optimum pH l e v e l somewhat .

I n cont inuous f l o w studies4+l 'ackstonet a1 . (154,155) found the optimum Fe t r a t i o n was -150 mg/l and a s t a b l e foam cou ld be produced us ing -35 mg/l NLS. The i n f l u e n t f l o w r a t e and h y d r a u l i c l oad ing r a t e d i d n o t a l t e r t he removal e f f i c i e n c y u n t i l i t became so g r e a t as t o produce ove r tu rn ing o f she fFam ( o c c u r r i n g a t a l oad ing o f -120 m /day- m ) . E f f l u e n t l ead concentrat ions l ess than 0.15 mg/l cou ld be maintained over a wide pH range ( 5 . 0 t o 6 . 5 ) and i n f l u e n t f l ow r a t e range. the r e s i d u a l l ead averaged 0 .5-1 .5 mg/ l . I nc reas ing the i o n i c s t reng th t o 85011 a n d 17,000 mg/l Na N O 3 increased the res iaua l lead t o 0.4 and 2 . 0 mg/l, respect ive:v

concen-

I f channel ing and ove r tu rn ing occurred,

Experiments were performed by Mu.ai r t a 1 . (98) i n v o l v i n g cop rec ip i t a t i o n of 9 f 11

1 74 Separation of Heavy Metals AlChE SYMPOSIUM SERIES

i o n (Cd, Cu, Zn, o r Hg) w i t h f e r r i c hydroxide f o l l o w e d by f l o t a t i o n us ing sodium o l e a t e as a c o l l e c t o r The t e s t s o l u t i o n s a l l con- t i n u e d 1.0 mg/l o f the he$YY metal i nvo l ved and va ry ing amounts o f Fe . For the optimum pH range f o r Cd removal (8<pH<10), t he r e s i d u a l cadmium concen t ra t i on was l e s s than 0.05 mg/ l . Favorable Cu removal was mainta ined over a wide pH range o f 6-10, w h i l e the optimum pH c o n d i t i o n f o r Zn removal was pH 9-10. The Hg reyyva l was -98% a t pH 8-9 w i t h t h e r e s i d u a l Hg con- c e n t r a t i o n i n the t a i l i n g s o l u t i o n being 0.03 mg/ l . Because Hg i s hard t o c o p r e c i p i t a t e , adopt ion o f a mu1 t i s t a g e + f l o t a t i o n o r a d d i t i o n o f Na2S together w i t h Fe i n the c o p r e c i p i t - a t i o n stag$ i s r e q u i r e d t o minimize t h e r e - s idua l Hg concen t ra t i on . F l o t a t i o n s o f c u p r i t e and ch rysoco l l a were performed w i t h LIX65N reagent as a f u n c t i o n o f LIX concentra- t i o n , pH, c o n d i t i o n i n g time, i o n i c s t reng th , and s o l u t i o n copper concentrat ion. F l o t a t i o n was a maximum around pH 5.5 and pH 10 i n general agreement w i t h t h a t p r e d i c t e d on the bas i s o f t he so l ub i1 i t y dependence w i t h pH. F l o t a t i o n was low f o r the pH range o f 6.5-9.0. Bei te lshees e t a1 .(9) s tud ied p r e c i p i t a t e f l o - t a t i o n as a method t o remove copper f rom aqueous s o l u t i o n s con ta in ing 1-1000 mg/l d i s - so lved copper. s l i g h t excess t o p r e c i p i t a t e CuS, producing a r e l a t i v e l y s t a b l e suspension o f submicron (-0.1-0.5 Vm) c o l l o i d a l CuS p a r t i c l e s . Varying amounts o f Hyamine 2389 and Amide 23 were em- p loyed i n t h e study. Hyamine was used t o serve as the c o l l e c t o r s u r f a c t a n t . The pr imary use o f Amide 23 was t h a t o f a foam s t a b i l i z e r . The minimum a d d i t i o n s o f chemicals necessary f o r good removal and steady column opera t i on were 25 mg/7 Hyamine 2389 and 5 mg/l Amide 23. I n 1976, the chemical cos ts were est imated t o be 37.6$/1000 ga l l ons t r e a t e d o r 45.3Q/pound o f copper removed. The CuS p a r t i c l e s were r e a d i l y removed by f l o t a t i o n i n a bubble column. Copper removals exceeding 90% f o r s o l u t i o n s c o n t a i n i n g 10-1000 mg/l d i sso l ved copper were achieved. A t 1 and 5 mg/l o f c a t i o n i c su r - f a c t a n t , t h e removal e f f i c i e n c y was 50% and 80%, r e s p e c t i v e l y . The p r e c i p i t a t e was con- c e n t r a t e d i n a foamate f r a c t i o n , t he volume o f which was g e n e r a l l y l e s s than 5% o f t he i n i t i a l s o l u t i o n volume.

S u l f i d e i o n was added i n a

The comparative c o s t o f foam f l o t a t i o n i s repo r ted (95,140) t o be compe t i t i ve w i t h t h a t o f l i m e p r e c i p i t a t i o n ; meigl waS$ewater c y y i a i n i n g 20 mg/l each of Cu , Zn , and C r , the t reatment cos ts were r e p o r t e d t o be $0.51/1000 l i t e r s f o r foam f l o t a t i o n versus $0.60/1000 l i t e r s f o r l i m e p r e c i p i t a t i o n .

i n t r e a t i n g a mixed

I O N EXCHANGE

I o n exchange i s an e f f e c t i v e means o f removing heavy metals f rom wastewaters. i s a r e v e r s i b l e chemical reac t i on , where the removal o f heavy metals i s accomplished by the exchange o f i o n s on the r e s i n f o r those i n wastewater. they must be regenerated w i t h an a c i d o r a l k a l i n e medium t o remove the metal i o n s f rom the r e s i n bed. sma l le r i n volume and h ighe r i n concentrat ion than the o r i g i n a l wastewater, b u t these metals must then be adequately t r e a t e d o r recovered.

There are a v a r i e t y o f r e s i n s f o r s p e c i f i c a p p l i c a t i o n s w i t h var ious metals. Syn the t i c organic r e s i n s are most ly used because o f t h e i r c a p a b i l i t i e s o f be ing manu- f a c t u r e d f o r s p e c i f i c a p p l i c a t i o n s (44) The use o f several i no rgan ic ge l s f o r the removal o f heavy metals have a l so been studied(l47)due t o t h e i r s t a b i l i t y under more d r a s t i c phys i ca l cond i t i ons . L i q u i d - i o n exchange i s a e f f e c t i v e u n i t ope ra t i on used t o p u r i f y and concentrate m ine ra l s from va r ious s o l u t i o n s i n the hydrometal lurgy i n d u s t r y . There have been l i m i t e d app l i ca - t i o n s i n i n d u s t r i a1 wastewater treatment, mos t l y i n Europe. Removal o f heavy metals, such as Zn, C r , and Cd are e f f e c t v e l y handl- ed by l i q u i d i o n exchange. Technica l ly , a lmost every metal i o n can be removed o r recovered by some i o n exchange process b u t economical concern p lays a very d e c i s i v e r o l e i n t h e commerical a p p l i c a t i o n o f i o n exchange. Ion exchange i s an i d e a l method f o r t he removal o f t r a c e amounts o f i m - p u r i t i e s f rom d i l u t e wastewaters, w i t h the h i g h q u a l i t y t r e a t e d water ready f o r reuse.

It

When t h e r e s i n s are saturated,

The regenerant b r i n e i s

Due t o the f a c t t h a t i o n exchange i s e f f i c i e n t i n removal o f d i sso l ved s o l i d s f rom normal ly d i l u t e spent r i n s e waters, i t i s w e l l s u i t e d f o r use i n water p u r i f i c a t i o n and r e c y c l e . Many of t h e p l a t i n g chemicals, acids, and bases used i n metal f i n i s h i n g are i o n i z e d i n s o l u t i o n and can be removed by i o n exchange. Factors making i o n exchange e f f e c t i v e f o r such a p p l i c a t i o n s (44) inc lude :

I o n exchange can economical ly separate d i l u t e concentrat ions o f i o n i c species f rom s o l u t i o n s . The process can c o n s i s t e n t l y p rov ide h igh p u r i t y water over a broad range o f con- d i t i o n s . The r e s i n s used f o r separat ion are durable under severe chemical environments.

I on exchange i s a l s o app l i ed f o r “end-of

No. 243, Vol. 81 1 75

p ipe " p o l l u t i o n c o n t r o l where t o x i c heavy meta ls and metal cyanide complexes are s e l e c t - i v e l y removed from combined waste streams be- f o r e discharge. I on exchange has been em- p loyed as a p o l i s h i n g s tep f o l l o w i n g covent ion- a1 hydroxide t reatment t o lower t h e metal concen t ra t i on i n the discharge; i t has a l so been a p p l i e d as a means o f d i r e c t l y t r e a t i n g wastewaters t o remove heavy metals and metal cyanide complexes. I n comparison w i t h conven t iona l p r e c i p i t a t i o n t reatment, i o n exchange o f f e r s the f o l l o w i n g advantages(53):

P r e c i p i t a t i o n and c l a r i f i c a t i o n equipment r e q u i r e a l o t o f space w h i l e i o n exchange equipment i s very compact. Metal hydroxide sludges must be t ranspor ted t o a l a n d f i l l l i c e n s e d t o handle them; i o n exchange avoids the generat ion o f sludges. No economical method i s c u r r e n t l y a v a i l a b l e t o recover the metal values f rom metal f i n i s h i n g sludges so the metals cannot be recyc led. I o n exchange a l l ows convenient recovery o f t h e metal s . I on exchange i s a v e r s a t i l e process which accommodates metal i o n concen t ra t i on v a r i a t i o n s and reasonable changes i n f l o w r a t e w i t h o u t d e t e r i o r a t i o n i n performance.

I o n exchange has been used t o a l i m i t e d e x t e n t t o remove t o x i c p o l l u t a n t s s e l e c t i v e l y f rom an un t rea ted wastewater w h i l e a l l o w i n g most o f t he non tox i c i o n s t o pass through. To f a c i l i t a t e t h i s a p p l i c a t i o n , approaches (44) i nc lude :

Weak a c i d c a t i o n exchange r e s i n i n an ap- p l i c a t i o n o f t h e wastewater s o f t e n i n g type t o remove heavy meta ls and o t h e r d i v a l e n t ca t i ons f rom a wastewater laden w i t h sodium ions. Heavy-metal-select ive weak a c i d o r c h e l a t i n g c a t i o n exchange r e s i n f o r removal o n l y o f the heavy metal i ons w h i l e a l l o w i n g sodium, calcium, and magnesium ions t o pass through. A s t r a t i f i e d r e s i n bed c o n t a i n i n g s t r o n g and weak a c i d c a t i o n and s t rong base anion r e s i n s t o remove heavy meta ls and metal cyanide complexes from s o l u t i o n w h i l e a l l o w i n g t h e m a j o r i t y o f t h e wastewater i o n i c c o n s t i t u e n t s t o pass through.

Each of these approaches i n v o l v e s pH a d j u s t - ment ( t o ensure t h e pH i s w i t h i n the o p e r a t i n g range o f t he r e s i n ) and f i l t r a t i o n ( t o remove suspended s o l i d s t h a t would otherwise f o u l t h e r e s i n bed). The p o l l u t a n t s f rom t h e wastewater are concentrated i n t h e i o n exchange regenerant s o l u t i o n s . This regenerant b r i n e i s much sma l le r i n volume and h ighe r i n con- c e n t r a t i o n than t h a t o f t h e o r i g i n a l wastewater. Such process ing serves as an e x c e l l e n t

pret reatment s tep f o r subsequent p h y s i c a l - chemical t reatment o r as a p o l i s h i n g s tep. Such a case h i s t o r y was o u t l i n e d where i o n exchange t reatment f o l l o w i n g hydroxide pre- c i p i t a t i o n enabled HurdLock and Manufactur ing Company t o meet the s t a t e l i m i t s f o r Cd, Cr, Cu, Fe, Pb, N i , and Zn. As a r u l e , i o n exchange systems are s u i t a b l e f o r chemical r e - covery a p p l i c a t i o n s where the r i n s e water feed has a low concen t ra t i on o f p l a t i n g chemicals and where a r e l a t i v e l y low degree o f concen t ra t i on i s r e q u i r e d f o r r e c y c l e o f t he concentrate. I o n exchange has been demonstrated commercial ly f o r recovery o f p l a t i n g chemicals f rom acid-copper, ac id-z inc, n i c k e l , t i n , coba l t , and chromium p l a t i n g baths (44). spent a c i d s o l u t i o n s and t o p u r i f y p l a t i n g s o l u t i o n s f o r l onger se rv i ce l i f e . Typica l process condi ti ons, removal costs , and opera t i ng cos ts are l i s t e d i n Reference(44).

It has a l s o been used t o recover

Several i o n exchange systems have been developed. i s a convent ional w i d e l y used design which r e q u i r e s m u l t i p l e columns and a l o t o f operator a t t e n t i o n . Two approaches have been used t o s i m p l i f y t he opera t i on o f i o n exchange systems: the cont inuous i o n exchanger and the r e c i p r o c a t i n g f l o w i o n exchanger. The cont inuous i o n exchange system prov ides simultaneous i o n exchange, regenerat ion, back wash, and r i n s e cyc les i n separate sect ions. However, t h e system's c a p i t a l c o s t i s h ighe r than t h a t o f f ixed-bed systems. The r e c i p r o - c a t i n g f l o w i o n exchanger operates on the p r i n c i p l e t h a t f o r t h e s h o r t p e r i o d time, t h e system goes o f f stream f o r regenerat ion. Cap i ta l c o s t and l a b o r are minimized i n t h i s system.

As an example o f recovery o f va luable metals f rom indus t r iOQ waste streams, hex- ava len t chromium (Cr ) can be success fu l l y recovered us ing i o n exchange t reatment (158). The waste stream i s f i r s t passed through a c a t i o n exchanger t o remove extraqT9us metals, p r i m a r i l y Fe, Cu, Zn, N i , and C r . The - hexavalent chromium passes through as Cr04- and i s removed i n an anion exchanger; t h e e f f l u e n t i s a deminera l ized wa+%r s u i t a b l e f o r reuse. For recovery o f C r , t he regenerated water w i t h NaOH and Na Cr04 i s released. The Na C r O s o l u t i o n i s th6n passed through anotheg c a t i o n exchanger which exchanges sodium f o r hydrogen r e l e a s i n g chranic a c i d (H2Cr04) i n the e f f l u e n t f o r recovery.

on the removal o f Pb, Ag, and Cd by c l i n o p t i - l o l i t e i n the presence o f competing concen-

The f ixed-bed i o n exchanger system

Semmens and M a r t i n (137)performed s tud ies

I 76 Separation of Heavy Metals

t r a t i o n s o f calcium, magnesium, and sodiyy. Th$ obseryed s e l e c t i v i t y sequence was Pb Ag > Cd . The competing ca t i ons s t r o n g l y i n f l uenced the metal exchange w i t h the z e o l i t e . Metal removal was g rea tes t i n exchange y i t h magnT-ium and decreased i n the o rde r Mg > Na+

Mg > " > Ca++ > Na f o r s i l v e r . The presence o f any ca lc ium i n s o l u t i o n had a profound e f f e c t on the e x t e n t o f Cd removal. Both batch and cont inuous f l o w s tud ies were performed i n the i n v e s t i g a t i o n . very e f f e c t i v e l y removed; l a r g e through pu ts cou ld be obta ined be fo re breakthrough occurred A1 k a l i n e pret reatment o f t he z e o l i t e improved the metal removal e f f i c i e n c y .

drawback i n the use o f i o n exchange i s the f a c t t h a t t h e r e s i n must be regenerated a f t e r exhaust ion, thereby comp l i ca t i ng opera t i on o f t h e system. Chemical cos ts are h i g h pe r u n i t o f meta ls removed; i t a l s o r e s u l t s i n volumes o f regenerant s o l u t i o n which must be t r e a t e d p r i o r t o discharge. Etze l and Tseng(46) i n v e s t i g a t e d a new approach t o regenerat ion o f a heavy metal exhausted c a t i o n exchange r e s i n us ing a recoverable c h e l a t i n g agent, whereby the regenerant s o l u t i o n cou ld be reused and the metal removed by t h e exchangeF r e - covered. The i r s tud ies showed EDTA, NTA, and c i t r a t e coul'd s u c c e s s f u l l y regenerate a s t rong a c i d c a t i o n exchange r e s i n exhausted by Cu, Zn, N i , o r a metal mix ture, t o i t s o r i g i n a l sodium form. The optimum pH range f o r a l l t he c h e l a t i n g agent s o l u t i o n s was between 8and 9 f o r regenera t i on o f t h e r e s i n . Regeneration e f f e c t i v e n e s s o f t he c h e l a t i n g agents was i n t h e o rde r Cu>Zn>Ni; regenerat ion e f f e c t i v e n e s s o f EDTA, NTA, and c i t r a t e was i n the o rde r EDTA = C i t > NTA.

>

f o r l e a d $nd cadmium and i n the o rde r

Lead was found t o be

E tze l and Tseng (&)recognized t h a t a

Problems t y p i c a l l y i n v o l v e d w i t h i o n exchange t reatment i n c l u d e :

M e t a l l i c f o u l i n g ( f r o m Fe, Mn, Cu, e t c . ) on the i o n exchange media.

* Fou l i ng due t o o i l , grease, s i l t , c l a y , c o l l o i d a l s i l i c a , organic ma te r ia l s , and microbes. The choice o f a proper c lean ing program can restoremuch o f t he l o s t e f f i c i ency (115,116). The presence o f f r e e a c i d reduces the e f f i c i e n c y o f ope ra t i on . F a i r l y h igh opera t i ona l cos ts .

LIQUID I O N EXCHANGE/LIQUID-LIQUID EXTRACTION

Recent ly research has been performed on the use o f l i q u i d i o n exchange f o r removal of heavy metals f rom p l a t i n g wastes(76,126).The

AlChE SYMPOSIUM SERIES

process b a s i c a l l y i n v o l v e s a two phase system (composed o f an organic l i q u i d c o n t a i n i n g a d isso lved, wa te r - i nso lub le a c t i v e compound and the heavy meta ls i n the aqueous phase) i n v o l v i n g l i q u i d - l i q u i d e x t r a c t i o n . The l i q u i d e x t r a c t a n t i s t y p i c a l l y p resen t a t a 10-40% a c t i v e l e v e l i n a so l ven t , such as kerosene. The e x t r a c t a n t s o l u t i o n i s run c o u n t e r c u r r e n t t o an aqueous feed con ta in ing one o r more types o f metal ion, u s u a l l y a t a temperature s l i g h t l y above ambient t o i m - prove t h e exchange k i n e t i c s and t o f a c i l i t a t e phase separat ion. A c i d - t r e a t i n g t h e organic f r a c t i o n re leases the metal i n a concentrated wa te r -so lub le form.

Several advantages have been noted w i t h a p p l i c a t i o n o f l i q u i d i o n exchange (L IE ) technology (76,126):

L IE can s e l e c t i v e l y e x t r a c t a des i red m e t a l l i c c a t i o n f rom a feed s o l u t i o n con- t a i n i n g a s i g n i f i c a n t amount o f m e t a l l i c

. i m p u r i t i e s .

i n f i n a l aqueous s o l u t i o n so t h a t i t can be t r e a t e d by methods i n a p p r o p r i a t e f o r t h e i n i t i a l d i l u t e feed. L IE i s w e l l s u i t e d f o r process automation and e f f i c i e n t metal recovery f rom a mixed- metal hydrox ide s l u r r y .

LIE can concentrate t h e d e s i r e d c a t i o n

One drawback w i t h l i q u i d - l i q u i d e x t r a c t i o n i s i t s 1 i m i t e d a b i l i t y t o concentrate t h e meta ls d u r i n g t h e e x t r a c t i o n process; i n most cases, e x t r a c t i o n produces no more than a t e n - f o l d increase i n metal concen t ra t i on .

This process has been s u c c e s s f u l l y adopt- ed by the uranium and copper i n d u s t r i e s (39). and o f f e r s promise i n the p l a t i n g and metal f i n i s h i n g i n d u s t r i e s . Petersen e t a l . (126) i n v e s t i g a t e d t h e use o f l i q u i d i o n exchange i n removing cadmium and n i c k e l f rom both segregated and composite wastestreams. Langmuir and F reund l i ch isotherm equat ions adequately descr ibed t h e removal o f heavy metals i n t h e i r system. They est imated the c o s t o f a t y p i c a l moderate s i z e d p l a t i n g f a c i i i t y employing reduc t i on and l i m e pre- c i p i t a t i o n was $4.71/1000 ga l l ons . As a comparison, us ing anion exchange t o remove chromate and l i q u i d i o n exchange f o r removal o f n i c k e l and cadmium, the t reatment cos t w a s est imated t o be $2.45/1000 ga l l ons .

Knocke e t a l . (76) i n v e s t i g a t e d the use o f

l i q u i d i o n exchange f o r t reatment o f wastewaters from T inke r A i r Force Base conta in ing N i , C r , Cu, Cd, and Ag. P re l im ina ry ext rac- t i o n s tud ies were performed u t i l i z i n g a mixed

No. 243, Vol. 81 1 77

metal s o l u t i o n c o n t a i n i n g 100 mg/l each o f C r y Cd, Cu, N i , and Zn. S i g n i f i c a n t r e s u l t s o f t h e i r s tudy are summarized below:

Thenoyl t r i f l u o r o a c e t o n e possesses h i g h

8-Hydroxyquinol ine was ab le t o e x t r a c t s e l e c t i v i t y f o r copper a t low pH cond i t i ons .

over 99% o f t h e n i c k e l i n t o t h e organic l aye r ; a t pH 2 over 95% o f t h e e x t r a c t e d n i c k e l c o u l d be s t r i p p e d o f f i n t o the aqueous phase us ing 2.4M HC1. Using 0 .5M 8-Hydroxyquinol ineY e x t r a c t i o n a t pH 5.4 r e s u l t e d i n 99% o f t h e cadmium being e x t r a c t e d i n t o the ch lo ro fo rm laye r ; back e x t r a c t i o n r e s u l t e d i n a 79% recdvery o f t he cadmium i n t h e aqueous phase a t pH 2.0. A l l seven c h e l a t i n g compounds i n v e s t i g a t e d performed p o o r l y f o r s e l e c t i v e chromium e x t r a c t i o n .

CEMENTATION

Cementation i s a metal-replacement p ro - cess i n which a s o l u t i o n c o n t a i n i n g t h e d i s - so lved m e t a l l i c i o n ( s ) comes i n c o n t a c t w i t h a more a c t i v e metal such as i r o n . Cementa- t i o n i s thus t h e recovery o f an i o n i z e d metal f rom s o l u t i o n by spontaneous e lect rochemical r e d u c t i o n . t o t h e elemental m e t a l l i c s t a t e w i t h subsequent o x i d a t i o n o f a s a c r i f i c i a l metal (such as i r o n ) . The r e a c t i o n f o r copper and i r o n ' i s :

Cu++ + Feo + Cuo + Fe++

The r e s u l t i s t o rep lace copper w i t h i r o n i n s o l u t i o n ; t h e copper p l a t i n g on to the s o l i d i r o n sur face. I f hexavalent chromium i s p re - sen t i n a wastewater, i t can r e a c t w i t h e i t h e r elemental o r f e r r o u s i r o n t o y i e l d t r i v a l e n t chromium:

2 C r + 6 + 3 Feo + 2 C r + 3 + 3 Fe+2

C r + 3 + 3 Fet3

The cementat ion process can be p r e d i c t e d i n terms o f e l e c t r o d e p o t e n t i a l s . Advantages o f t h e process i nc lude :

Simple c o n t r o l requirements. Low energy u t i l i z a t i o n . Recovery o f va luab le h i g h p u r i t y metals,

Pat terson and Jancuk( l l2)s tud ied cementat ion t reatment o f copper i n wastewaters. They found copper cementat ion i s a f i r s t - o r d e r r e a c t i o n w i t h respec t t o removal o f copper f rom t h e wastewater. The r a t e o f cementat ion was independent o f t h e presence o f oxygen. Copper cementat ion was independent o f pH; howeve, c.':t3ve ptl 3, f e r r i c hydrox ide p re -

such as copper.

c i p i t a t i o n masked and i n t e r f e r e d w i t h copper recovery. The copper f rom t h e cont inuous r e a c t o r s had a mo is tu re con ten t o f -38%; the d r i e d cement con ta ined -95.5% pure copper on a d r y weight bas i s .

i n v o l v e d t h e suspension o f scrap i r o n i n a p e r f o r a t e d r o t a t i n g drum through which the wastewater f l o w s (69). on to t h e i r o n and scraped o f f as p a r t i c u l a t e copper as i t tumbles w i t h i n t h e drum.

COMPLEXATION/SEQUESTRATION

A recen t a p p l i c a t i o n o f t h i s technology

Copper i s cemented

Complexation i n v o l v e s the fo rma t ion o f a complex compound through a complexing o r c h e l a t i n g agent. Sequestrat ion i n v o l v e s the removal o f a metal i o n f rom s o l u t i o n by fo rma t ion o f a complex i o n t h a t does n o t have t h e chemical r e a c t i o n s o f t he i o n t h a t i s removed, i n o t h e r words t h a t metal i o n i s t i e d up o r complexed. a1 t e r s t h e chemical c h a r a c t e r i s t i c s o f t h e metal i o n s and a f f e c t s t h e removal mechanisms i n v o l v e d (59). For example, t he fo rma t ion o f metal complexes increases the s o l u b i l i t y o f metal p r e c i p i t a t e s as hydroxides, carbonates, and s u l f i d e s . i s a f f e c t e d by s o l u t i o n pH and t h e concentra- t i o n o f p a r t i c i p a t i n g species. Lagvankar (%Found t h a t d i sso l ved chelated n i c k e l c o u l d be e f f e c t i v e l y removed by con- t a c t i n g t h e wastewater w i t h a bed o f i r o n f i l i n g s ; t h e type o f c h e l a t i n g agent present a f f e c t e d t h e r e a c t i o n r a t e . inyQst igated+&he removal o f C y , Zn , Fe , C r , and C r by u l t r a f i l t r a t i o n by t y i n g up t h e metal i o n s i n l a r g e molecularcomplexes us ing EDTA. Al though from s e l e c t i v i t y con- s i d e r a t i o n s , i t appeared separat ion o f copper and z i n c i n t h e pH range o f 5-6 was p o s s i b l e , such was n o t t he case. For t h a t case, separa t i on o f metal c a t i o n s by c h e l a t i o n and u l t r a f i l t r a t i o n d i d n o t appear promising. Using s o l e l y camplexat ion f o r recovery o f meta ls does n o t h o l d much promise i n the p l a t i n g i n d u s t r y .

ELECTROCHEMICAL OPERATIONS

Complex fo rma t ion

The e x t e n t o f complex fo rma t ion

Mayenkar and

O;#)eilltpt a l i $ a )

E l e c t r o l y t i c metal recovery i s one o f a number o f technologies capable o f removing meta ls f rom process wastewaters. no logy has been used f o r many years i n the min ing i n d u s t r y f o r e lec t ro -w inn ing and e l e c t r o r e f i n i n g o f ores, and has been used t o recover copper f rom p i c k l e l i q u o r s . Dur inq t h e l a s t 25-30 years, e l e c t r o l y t i c metal recovery has been i n v e s t i g a t e d f o r recovery

The tech-

1 78

o f meta

Separation of Heavy Metals AlChE SYMPOSIUM SERIES

; i n dragout from p l a t i n g tanks. e l e c t r o l y t i c recovery, a d i r e c t c u r r e n t i s passed through an aqueous s o l u t i o n c o n t a i n i n g metal i o n s between cathode p l a t e s and i n - s o l u b l e anodes. The p o s i t i v e charged meta l - l i c ionsadhere t o the n e g a t i v e l y charged cathodes l e a v i n g a metal depos i t t h a t can be s t r i p p e d o f f and recovered. and techniques o f e lect rochemical techniques have been descr ibed i n t h e l i t e r a t u r e (2,117) addressing such t o p i c s as e lec t rode p o t e n t i a l s ; e q u i l i b r i u m , ox ida t i on - reduc t i on , and mixed p o t e n t i a l s ; vo l tammetry, e l e c t r o c a p i l l a r i ty capaci ty , s h o r t - c i r c u i t c u r r e n t measurements, r e d u c t i o n by s u l f u r d i o x i d e and carbon mono- xide, e t c . Pemsler and Rappas(ll7)point o u t t h a t e lec t ro -w inn ing i s a h i g h l y energy- dependent and l a b o r i n t e n s i v e process. c a p i t a l c o s t o f an e lec t row inn ing process i s ext remely h i g h and represents a s i g n i f i c a n t p o r t i o n o f t he t o t a l cos t .

I n

The p r i n c i p l e s

The

A graph ica l rep resen ta t i on o f e l e c t r o - chemical e q u i l i b r i a ( i n c l u d i n g chemical r e - a c t i o n s such as h y d r o l y s i s and complex form- a t i o n i s t he Pourbaix diagram (128). These diagrams p l o t the r e d u c t i o n p o t e n t i a l on the hydrogen scale (E-HH) versus pH, showing t h e domains o f predominance f o r t h e var ious species i n t h e system. represent t h e locus o f equal a c t i v i t y o f two species i n e q u i l i b r i u m ; h o r i z o n t a l l i n e s are f o r o x i d a t i o n o r r e d u c t i o n w i t h o u t h y d r o l y s i s o r complex format ion, w h i l e v e r t i c a l l i n e s are f o r h y d r o l y s i s o r complex format ion. Areas between l i n e s represent f i e l d s o f s t a - b i l i t y f o r a g i ven a c t i v i t y l e v e l f o r t h e chemical species considered.

blem because cathode p o l a r i z a t i o n becomes s i g n i f i c a n t a t low concen t ra t i ons . As p l a t i n g proceeds, t he l a y e r o f s o l u t i o n adjacent t o t h e cathode becomes depleted i n metal ions, forming a p o l a r i z e d l a y e r . f u s i o n i n t o and across t h e p o l a r i z e d l a y e r i s lower; the l a y e r becomes t h i c k e r and more depleted. Cathode p o l a r i z a t i o n leads t o a number o f ope ra t i ona l problems i n c l u d i n g a low r a t e o f metal depos i t i on . A number o f means e x i s t t o reduce cathode p o l a r i z a t i o n i n c l u d i n g opera t i on a t lower c u r r e n t dens i t y , a d j u s t i n g the chemist ry and s o l u t i o n tempera- tu re , a g i t a t i n g t h e s o l u t i o n , employing h i g h cathode sur face areas, and reducing the d i f - f u s i o n l a y e r th ickness.

The theo ry o f ope ra t i on b a s i c a l l y i n - vo lves an ox ida t i on - reduc t i on r e a c t i o n whereby e l e c t r o n s are supp l i ed by an e x t e r n a l e l e c t r i c a l source reducing the metal i o n s i n

The l i n e s i n the diagrams

D i l u t e r i n s e waters pose a spec ia l pro-

The r a t e o f d i f -

t he e l e c t r o l y 2 t o form elemental metal a t t h e cathode sur face. As an example, t he ca thod ic and anodic r e a c t i o n s f o r copper are as f o l l o w s :

Cathode: Cu++ + 2 e- + Cuo

Anode: H20 + 2H+ + 30, + 2e-

Benni on and Newman (13) devel oped an e l e c t r o l y t i c c e l l u s i n g porous, f i xed , f low- through e lec t rodes as a means o f removing metal i ons f rom d i l u t e s o l u t i o n s . Using t h i s c e l l , t he copper concen t ra t i ons were reduced from 670 mg/l t o l e s s than 1 mg/ l . The con- t r o l l i n g f a c t o r f o r use o f t h e c e l l was i t s c a p i t a l c o s t . E l e c t r o l y t i c recovery o f z i n c f rom a p l a t i n g ba th r i n s e stream (con ta in ing z i n c cyanide s o l u t i o n ) was accomplished us ing a s imple ba tch e lec t rochemica l r e a c t o r w i t h s t a i n l e s s s t e e l e l e c t r o d e s (157).Test s o l u t i o n s rangPd i n concen t ra t i on f rom 100-980 mg/ l . The h ighes t z i n c removal r a t e s were achieved us ing h igh a p p l i e d cu r ren ts . The s i n g l e most impor tan t f a c t o r i n ach iev ing h i g h depos i t i on r a t e s was a g i t a t i o n ; mechanical m ix ing and n i t r o g e n gas a e r a t i o n were bo th e f f e c t i v e f o r a g i t a t i o n . Zinc d e p o s i t i o n was h ighe r f o r h ighe r n i t r o g e n gas feed ra tes , due t o the increased a g i t a t i o n reducing the concentrat ion p o l a r i z a t i o n . A g i t a t i o n promotes the re - plenishment o f z i n c i ons a t t h e boundary l aye r as z i n c deposi ts on t h e e lec t rodes . Higher c u r r e n t d e n s i t i e s p rov ide moderately h ighe r depos i t i on r a t e s than low c u r r e n t d e n s i t i e s . The NaCl dose had l i t t l e e f f e c t on the de- p o s i t i o n r a t e s as l ong as s u f f i c i e n t NaCl was added t o achieve a c r i t i c a l minimum con- d u c t i v i t y l e v e l . t he e f f e c t o f cathode sur face area on t h e recovery o f copper f rom d i l u t e , chelated copper r i n s e waters, by employing an e l e c t r o - l y t i c c e l l c o n s i s t i n g o f a s e r i e s o f f low- through cathodes and anodes i n an e l e c t r o d e c e l l box. The copper removal e f f i c i e n c y ranged from 80-851. I n f l u e n t pH had l i t t l e e f f e c t on the che la ted copper removal over the pH range o f 3 t o 11. concen t ra t i on and f l o w r a t e had t h e g rea tes t i n f l u e n c e on the performance o f t h e e l e c t r o - l y t i c u n i t w h i l e pH and r e c i r c u l a t i o n r a t e had l i t t l e o r no e f f e c t on the o v e r a l l t r e a t - ment e f f i c i e n c y . per cou ld be removed t o l e v e l s o f l e s s than 1 mg/l f rom an i n i t i a l l e v e l o f 100 mg/l f r o m s o l u t i o n s con ta in ing EDTA, Quadrol , t a r t r a t e , pyrophosphate, ammonium persul f a t e etch, and perox ide s u l f u r i c copper etch.

Bishop and Breton(24) s tud ied

I n f l u e n t copper

F i e l d data (146)indicated cop-

Electrochemical processes p rov ide a means o f i n t r o d u c i n g o x i d a n t w i t h o u t adding ex- traneous chemicals o r i on~( l27 )~Ayres and

No. 243, Vol. 81

Fedkiw(7) s t u d i e d t h e depos i t i on o f l e a d and copper on a r e t i c u l a t e d v i t r e o u s carbon sur face . . as+$ ca t$ l y t i c+$gen t fo r+$ lec t rodepos i t i o n o f Cu , N i . Pb , and Zn f rom d i l u t e so lu - t i o n s (10 mg/l ) us ing a f low- through porous e lec t rode reac tor+ depos i t i on o f Zn a l though i t d i d i n h i b i t t h e H2 s i d e r e a c t i o n 2 ? t t h e h i g h e s t l o a d i n g l e v e l used (50 Vg/cm ) . c a t a l y t i c a c t i v i t y f o r Zn and N i e l y t r o - depos i t i on a t load ings f rom 2Oi30 pg/cm . The presence o f 10-100 mg/l Cu acted as a c a t a l y s t f o r CN- e lec t roox ida t i on . A c u r r e n t e f f i c i e n c y of+$!7% was measured as the concen- t r a t i o n o f Cu was lowered f rom 95.6 mg/l t o 0.05 mg/l. Ayres and Fedkiw (7)estimated the e l e c t r i c a l ope ra t i ng c o s t t o be 11B/lb o f copper recovered (which compares w i t h 65.3C1 l b o f copper removed by hydrox ide p r e c i p i t a - t i o n ) . e ra ted i s cons iderab ly smal le r by the e l e c t r o - l y t i c recovery r o u t e than by t h e p r e c i p i t a t i o n r o u t e (0.0279 g a l l o n s per l b copper e l e c t r o - chemica l l y generated versus 4.45 g a l l o n s [ o f 4% s o l i d s ] per l b o f copper removed by hydro- x i d e p r e c i p i t a t i o n ) .

Farkas and M i t c h e l l (47) developed a process f o r e lect rochemical heavy metal recovery f rom wastewaters. The equipment cons is t s o f a r e a c t o r module con ta in ing the anode and cathode assemblies, two c o n t r o l l a b l e power suppl ies, p l u s pumps, e l e c t r o l y t e tanks, e t c . The cathode c o n s i s t s o f a bundle o f t h i n carbon f i b e r s connected t o a feeder sheet. The sur face area t o volume r a t i o i s ext remely h igh, -12,000~ h igher than t h e apparent sur face area t o volume r a t i o . The l a r g e sur face area enhances t h e mass t r a n s f e r r a t e severa l o rders o f magnitude. Features o f t h e process i n c l u d e no genera t ion o f sludge, low opera t i ng costs , no consumable reagents requ i red f o r opera t ion , and meta ls are generated i n a sa leab le form.

Lead d i d n o t ca ta l yze t h e

&pper shyred a s t rong

The sludge volume of t h e waste gen-

B I O LOG1 CAL TREATMENT

Reid e t a l . (132)studied the e f f e c t s o f m e t a l l i c i o n s on b i o l o g i c a l waste t rea tment processes. Act ivatec i s ludge t reatment p ro- cesses can+&olerate up t o 10 mg/l o f heavy meta ls (Cr , Cu, N i , and/or Zn) e i t h e r s i n g l y o r i n combination w i t h o n l y a 5% reduc t i on i n e f f i c i e n c y . These same f o u r heavy meta ls may be present, e i t h e r s i n g l y o r i n conibina- t i o n , up t o concent ra t ions o f 10 mg/l w i t h o u t any adverse e f f e c t on anerobic sludge d i g e s t i o n . B i o l o g i c a l t rea tment o f domestic sewage w i l l n o t bf6adversely e f f e c t e d i f t h e concen t ra t i on o f C r does n o t exceed 2 mg/ l . Biomass con- c e n t r a t i o n s t r o n g l y a f f e c t e d t h e amount o f copper t h a t c o u l d be removed f rom s o l u t i o n .

1 79

Removal of copper was more e f f i c i e n t w i t h inc reas ing pH f o r adsorp t ion by Zoogloea ramigera. However, a t h igher pH, o n l y p a r t o f t h e copper removed i s due t o adsorp t ion t o b a c t e r i a l biomass s ince copper hydrox ide p r e c i p i t a t i o n becomes apprec iab le above pH 5.0. The uptake o f copper i s r a p i d and e f f i c i e n t ; about 0.17 grams o f copper are adsorbed per gram o f biomass w i t h i n 10 minutes.

Several s t u d i e s have addressed t h e e f f e c t o f meta ls on t h e a c t i v a t e d sludge process. Moul t o n and Shumate(97)observed t h a t an ac- c l ima ted a c t i v a t e d sludge system cou ld g i v e a r e d u c t i o n o f 80-85% f o r t h e copper concen- t r a t i o n when t h e i n f l u e n t copper l e v e l was 50 mg/l. The a c t i v a t e d sludge process was ab le t o remove 50-79% o f t h e copper f o r an i n f l u e n t concen t ra t i on o f 0.4 t o 25.0 mg/l (104). When t h e i n f l u e n t metal concent ra t ion was v a r i e d f rom 2.1 t o 25.5 mg/l, a 7 4 4 6 % uptake o f cadmium and a 8 1 4 2 % uptake o f copper r e s u l t e d (33). Adsorpt ion of cadmium (101) increased f rom 15-20% a t pH 4 t o adsorp t ions exceeding 90% a t pH 10; maximum adsorp t ion o f copper occurred i n t h e pH range 7<pH<8 Kodukula and Pat te rson (78)observed the s o l u b l e cadmium and n i c k e l concent ra t ion remained cons tan t a t low suspended s o l i d s concent ra t ions and was c o n t r o l l e d by p r e c i p i t a t e s o l u b i l i t y . These concent ra t ions decreased as the s o l i d s l e v e l increased; s o l u b l e metal i s adsorbed t o the s o l i d s . The percent metal adsorp t ion increased by more than 70% over 1-2 pH u n i t s change. For cadmium, s i g n i f i c a n t changes i n the adsorp t ion c a p a b i l i t y occur i n t h e range 5.0<pH<7.0 whereas f o r n i c k e l t h i s change i s observed f o r 6.0<pH<8.0. Removals o f both n i c k e l and cadmium were about 90%, a t pH 8, whereas a t pH 7, removal o f n i c k e l and cadmium were approx imate ly 60 and 90% r e s p e c t i v e l y . Thus, i f t h e pH o f the a c t i v a t - ed sludge mixed l i q u o r i s below 8.0, poor n i c k e l removals are a n t i c i p a t e d . Kodukula and Patterson(78)demonstrated t h e meta ls were removed p r i m a r i l y by s o r p t i o n processes i n a c t i v a t e d sludge t reatment . F r i s t o e and Nelson(51) developed a chemical spec ia t ion- d i s t r i b u t i o n model f o r heavy meta ls and a p p l i e d i t t o cadmium i n a c t i v a t e d sludge from a f u l l - s c a l e wastewater t reatment p l a n t . I n t h e model, cadmium adsorp t ion t o b a c t e r i a l s o l i d s was q u a n t i f i e d by t h e de terminat ion o f c o n d i t i o n a l adsorp t ion constants; t h e cadmium complexation by d isso lved organic compounds was q u a n t i f i e d by the de terminat ion o f condi t i o n a l formation constants . Both t h e adsorp t ion by b a c t e r i a l s o l i d s and the cadmium complexation are s t r o n g l y pH dependent. I n general, below pH 4, adsorp t ion i s l e s s than 20%; above pH 7, adsorp t ion exceeds 80%.


A t pH 10 adsorp t i on increases t o 90-99%. Soluble cadmium s p e c i a t i o n i s dominated by f r e e cadmium i o n below pH 6, by cadmium-organic l i g a n d complexation i n the pH range o f 61pH17, and by i no rgan ic species a t pH 8 and 9. cadmium adsorbed by b a c t e r i a l s o l i d s i n c r e a s e d f rom -30% a t pH 4 t o near 90% a t pH 9. l i g a n d complexat ion o f cadmiumin a c t i v a t e d sludge i s s i g n i f i c a n t i n the pH range o f 5 t o 8. Wu and Hilgev(163)observed t h a t mercury contaminants e n t e r i n g an a c t i v a t e d sludge wastewater t reatment p l a n t may undergo several a1 t e r n a t e b io t rans fo rma t ions du r ing the t r e a t - ment process. Adsorpt ion o f mercury by sludge and v o l a t i l i z a t i o n o f m e t a l l i c mercury by b a c t e r i a l reduc t i on o f mercur ic mercury were observed t o be complementary processes d u r i n g the t r a n s i e n t p e r i o d f o l l o w i n g the i n t r o d u c t i o n o f mercury. Adsorpt ion was the major mechanism f o r mercury removal a t steady s ta te ; me thy la t i on was i n s i g n i f i c a n t a t t he dosages s tud ied.

The

Organic

To date, most o f the research on heavy meta ls removal i n b i o l o g i c a l systems has been d i r e c t e d towards t h e suspended growth a c t i v a t - ed sludge process. Chang e t a1 .(30) s tud ied the removal o f heavy metals i n a f i x e d - f i l m b i o l o g i c a l system f i n d i n g i t p r i m a r i l y a t t r i b u - t a b l e t o t h e s o r p t i o n o f so lub le and f i n e metal p a r t i c u l a t e s by b i o l o g i c a l f l o c s ; t he r a t e o f uptake was g r e a t l y a f f e c t e d by pH. The average removal o f s o l u b l e Cd was 95 and 85% a t i n f l u e n t concen t ra t i ons o f 5 and 20 mg/l, r e s p e c t i v e l y . Copper removals were g e n e r a l l y lower than t h a t o f cadmium. The s o l u b l e Cu removal increased from 40% a t 1 mg/l sp i ke o f Cu t o 90% when the sp i ke was increased t o 5 mg/ l . When the copper feed was increased t o 10, 25, and 50 mg/l. the average so lub le Cu removal decreased t o 75, 40, and 30%, r e - s p e c t i v e l y . Approximately 80-90% o f the heavy metal r e d u c t i o n occurred i n t h e f i r s t stage o f t he three-stage r o t a t i n g b i o l o g i c a l con- t a c t o r (RBC) system. L i m i t e d p a r a l l e l s tud ies on t h e Cd and Cu removals between1 the RBC and the a c t i v a t e d sludge systems i n d i c a t e d t h a t a c t i v a t e d sludge was more e f f i c i e n t i n removing metals a t low metal s p i k i n g concen t ra t i ons were used.

Pugsley e t a1 .(12g)showed t h a t w i t h su i t a b l e n u t r i e n t s and a pH i n the range o f 5.5-9.0, c u l t u r e s o f D e s u l f o v i b r i g desu l fu r i cans w i l l grow and w i l l reduce SO4- t o S- under anaerobic c o n d i t i o n s . Given s u f f i c i e n t concen t ra t i ons o f SO4, n u t r i e n t s , D. desul fur icans, and a l ong growth p e r i o & suf f lc ien$+S- i s gerirated t o p r e c i p i t a t e Cu , Fe , Zn , and Mn f rom raw mine drainage wastewater.

EVAPORATION/DISTILLATION

The p r imary use o f evapora t i on and d i s t i l - l a t i o n t reatment has been f o r product recovery, w i t h some l i m i t e d use t o t r e a t f i n a l concen- t r a t e d wastewater res idues t o dryness. These techniques a re b a s i c a l l y end -o f - the - l i ne processes. Genera l l y evapora t i ve processes are economical o n l y f o r concentrated r i n s e s and mu1 t i s t a g e coun te rcu r ren t r i n s i n g ( 134) .This technique r e q u i r e s segregat ion o f wastes by compat ib le types and use o f va r ious means f o r e x c l u s i o n and/or removal o f i m p u r i t i e s . Evaporat ive recovery concentrates t h e chemical dragout i n t h e r i n s e water t o ba th s t r e n g t h r e t u r n i n g t h e concentrated s o l u t i o n t o t h e process tanks. densed and r e t u r n e d t o t h e r i n s i n g system, thereby m in im iz ing water consumption.

Two types o f evaporat ive recovery systems are commonly used: t he atmospheric evaporator( l l3) . The atmospheric evaporator operates a t atmospheric pressure and the normal b o i l i n g temperature o f t he s o l u t i o n be ing t rea ted . operates a t subatmospheric pressures enabl ing evaporat ion t o occur a t temperatures i n t h e range o f 130-190°F. can be operated i n e i t h e r open o r c losed loop process ing cyc les . t i o n , t he system i s designed t o recover 100% o f t h e processing bath chemicals l o s t i n the dragout f o r reuse i n t h e metal f i n i s h i n g cyc le . No e x t e r n a l r i n s e water i s added f o r makeup except t h a t r e q u i r e d t o rep lace the l o s s due t o atmospheric evaporat ion. r e q u i r e d are those requ i red t o rep lace what has been deposi ted on p a r t s o r due t o any s p i l l a g e and acc iden ta l losses. The open loop system i s o f t e n employed i n i n s t a l l a t i o n s where the re are an i n s u f f i c i e n t number o f r i n s e tanks It i s designed f o r p a r t i a l recovery o f t he process ing bath.

Four d i s t i l l a t i o n processes are commonly used t o t r e a t spent p i c k l e l iquors(l48).These d i s t i l l a t i o n processes have i n common t h e use o f vacuum evaporat ion and a d d i t i o n o f s u l f u r i c a c i d t o the spent p i c k l e l i q u o r a t some stage i n the recovery scheme. i n g under vacuum inc lude:

The evaporated water i s con-

t h e vacuum evaporator and

A vacuum evaporator

Both types o f evaporators

I n the c losed loop opera-

The o n l y chemicals

Advantages o f work-

The c o r r o s i v e a c t i o n o f t he mixed a c i d i s reduced due t o t h e lower d i s t i l l a t i o n temperatures i nvo l ved . Lower c o s t m a t e r i a l s o f c o n s t r u c t i o n can be used.

The i n t e r e c t e d reader i s r e f e r r e d t o reference (148) f o r d e s c r i p t i o n s and d e t a i l s o f t he

No. 243, Vol. 81 181

d i s t i l 1 a t i o n processes.

Pat te rson and Minear ( l l3 )po in t o u t t h a t evapora t ion i s a we l l -es tab l i shed technology f o r recover ing p l a t i n g chemicals and water f rom p l a t i n g waste e f f l u e n t s . Commericial u n i t s have been b u i l t f o r hand l ing z inc, copper, n i c k e l , chromium, and o t h e r metal p l a t i n g baths. Disadvantages o f e v a p o r a t i o n / d i s t i l l a t i o n processes inc lude:

R e l a t i v e l y h i g h c a p i t a l c o s t s . R e l a t i v e l y h i g h opera t iona l cos ts ( p a r t i c u - l a r l y f o r vacuum systems.) D i s t i l l a t i o n processes are energy i n t e n s i v e . The economics o f d i s t i l l a t i o n imposes a c o n s t r a i n t on t h e s i z e range o f these systems. These systems are complex, r e q u i r i n g t r a i n e d personnel t o operate and ma in ta in them.

ADSORPTION

Due t o t h e h i s t o r i c a l development and use o f a c t i v a t e d carbon i n water and wastewater treatment, most o f t h e a p p l i c a t i o n s and research e f f o r t on a c t i v a t e d carbon have been o r i e n t e d towards organ ics removal (63). Research e f f o r t s on i no rgan ics removal by a c t i v a t e d carbon, s p e c i f i c a l l y m e t a l l i c ions, have been markedly 1 i m i ted . Huang(63) p rov ides an excel l e n t rev iew o f i no rgan ics removal by a c t i v a t e d carbon by cons ide r ing such f a c t o r s as sur face p r o p e r t i e s (and t h e i r measurement) and adsorp t ion charac- t e r i s t i c s o f c a t i o n i c and a n i o n i c species on to a c t i v a t e d carbon sur faces. Impor tan t phys i ca l - chemical p r o p e r t i e s a f f e c t i n g i no rgan ic e l e c t r o - l y t e adsorp t ion i n c l u d e : s p e c i f i c sur face area, pore s t r u c t u r e , e l e c t r o p h o r e t i c p r o p e r t i e s , and sur face a c i d i t y .

Very l i t t l e has been repo r ted on t h e use o f a c t i v a t e d carbon f o r removal o f inorgan ics f rom wastewaters. Ac t i va ted carbon was r e - po r ted t o be a p o t e n t i a l adsorbent f o r heavy metal removal (4,35). Salvaged automobi le t i r e s a re capable o f removing t r a c e meta ls (A1 , Hg, Ni, Cd, C r , Co, Cu, Fe, Pb, Mn, Ag, and Zn) f rom s o l u t i o n due t o var ious m a t e r i a l s p resent i n t h e t i r e s , such as s u l f u r , carbon b lack, f i l l e r s , s y n t h e t i c rubber, an t i ox idan ts , e t c . (103-05).Saito(134)2sed a c t i v a t e d carbon and su l - fona ted coal f o r removal o f Cu, Cd, and Fe f rm wastewaters. Removals exceeding 98% were achieved f o r cuper ic ions t r e a t e d w i t h s u l f o - nated coal a f t e r a one-stage e x t r a c t i o n . For an i n i t i a l copper concen t ra t i on o f 312 mg/l, t h e r e s i d u a l copper concen t ra t i on was reduced t o 0.08 mg/l a f t e r a two-stage e x t r a c t i o n . S i g n i f i c a n t removals were achieved l i k e w i s e w i t h a c t i v a t e d carbon. The e x t e n t of heavy metal adsorp t ion on a s p e c i f i c type o f carbon

i s expected t o be a f u n c t i o n o f : c h a p f i + $ i s t r i b y t h o r ( f o r example ML , MH L ' MOHL - - ) ; f r e e metal and f r e e l i g a n d charges; t h e pH o f t h e carbon sur face and t h e p o l a r i t y o f l i g % % molecules. The pH w i l l a f f e c t t h e charge d i s t r i b u t i o n o f t h e var ious species as w e l l as t h e hydroxo group d i s t r i b u t i o n a t t h e a c t i v a t e d carbon sur face. J e v t i t c h and Bhattacharyya (69) es tab l i shed t h e adsorp t ion capac i t i es o f com- p lexed heavy metal i o n s by a c t i v a t e d carbons under e q u i l i b r i u m c o n d i t i o n s . An ex tens ive experimental i n v e s t i g a t i o n was conducted w i t h a c t i v a t e d carbons t o establ ish2$he a$$or;;$p" capaci5 jes o f heavy meta ls (Cd , N i , Y

and Zn ) i n t h e presence o f complexing agents (EDTA, t r i e thy lene te t ram ine (TRIEN), c i t r a t e , e t c ) . Adsorpt ion e q u i l i b r i a a re exp la ined by species charges and carbon sur face charge c h a r a c t e r i s t i c s . I n t h e pH range 7.5-8.0, t h e a c t i v e s i t e s a re p o s i t i y f l y charged. adsorp t ion capac i t i es o f Cd meta ls ) , f r e e l igands, and cadmium-ligands were a l s o a f u n c t i o n o f feed metal concentra- t i o n , typesof l igands , molar r a t i o o f l i g a n d / metal, and pH. Complete me ta l - l i gand species d i s t r i b u t i o n s were c a l c u l a t e d by computer s o l u t i o n s o f m u l t i p l e r e a c t i o n e q u i l i b r i a . For an e q u i l i b r i u m cadmium concent ra t ion o f 0.1 mM (pH 7.5-8.0), t h e sequence i n adsorp-

m?td l - l igand

The (and o t h e r heavy

t i o n capac i ty f o r cadmium che la tes i s Qc,-,,,, > QCD-c i t ra te > QCd-TRIEN > QCd- ta r t ra te .

Huang e t a1 . evaluated t h e a c t i v a t e d carbon adsorp t ion process f o r removal o f Co( I1) f rom s o l u t i o n w i t h spec ia l emphasis p laced on the e f f e c t of var ious organic sub- stances on Co( I1) removal. T h e i r r e s u l t s showed Co( 11) was removed by adsorp t ion r a t h e r than be p r e c i p i t a t i o n alone. Fourteen types o f a c t i v a t e d carbon were evaluated f o r t h e i r Co( I1) removal c a p a b i l i t i e s . Two a c t i v a t e d carbons, Nuclar SA and Nuchar SN, gave s i m i l a r h i g h metal removal c a p a c i t i e s (removals approached loo%, depending on the s o l u t i o n pH). The Co( I1) removal percentage decreased w i t h i nc reas ing sur face load ings . The authors showed t h a t t h e presence o f o rgan ic a c i d r e - s u l t s i n f i v e d i f f e r e n t cases o f e f f e c t s i n Co(I1) adsorp t ion : 1. anion induced c a t i o n adsorpt ion, 2. complexes n o t adsorbable ( o r compe t i t i ve anion adsorp t ion) , 3 . complexes adsorbable ( o r non-compet i t ive anion adsorp t ion) , 4. anion-induced c a t i o n adsorp t ion and com- p lexes n o t adsorbable, and 5 . anion-induced c a t i o n adsorp t ion and cumplexes adsorbable. Depending on the extenL o f complex format ion and t h e a d s o r b a b i l i t y o f t h e complexed Co( I1) and o f t h e l igand, i t w a s poss ib le t o d i f - f e r e n t i a t e f o u r d i f f e r e n t e f f e c t groups:

1 82 Separation of Heavy Metals AlChE SYMPOSIUM SERIES

a. enhancement over the mid-pH range, b. i n h i b i t i o n as the pH increases, c. no e f f e c t , and d. enhancement fo l l owed by i n - h i b i t i o n as the pH increases. t i o n o f the exhausted a c t i v a t e d carbon by s t rong a c i d was determined f e a s i b l e . The adso rp t i ve capac i t y o f the recyc led a c t i v a t e d carbon was mainta ined a t t he 92% l e v e l a f t e r t h e 5 t h cyc le .

Ku and Peters (80) i n v e s t i g a t e d the use o f a c t i v a t e d carbon as a p o l i s h i n g s tep f o r t reatment o f i n d u s t r i a1 p l a t i ng wastewaters. When hydroxide p r e c i p i t a t i o n i s employed f o r removal o f heavy metals, t he use o f a c t i v a t e d carbon a l l ows s i g n i f i c a n t l y lower e f f l u e n t metal concentrat ions t o be achieved. The r e s i d u a l z i n c and cadmium concen t ra t i ons were reduced g r e a t e r than 70% through use of a c t i v a t e d carbon f o l l o w i n g hydroxide p r $ $ i p i - t a t i o n . The adsorpt ion capac i t y f o r Cd was c a l c u l a t e d t o be between 5 and 20 Vmole/gm o f powdered a c t i v a t e d carbon f o r 9 i p H i l l which i s muchlower than the 40-50 vmoles/gm o f powdered a c t i v a t e d carbon repo r ted by Huang and Wi r th (60) . This i s l i k e l y due t o the i n t e r f e r e n c e o f ammonia and cyanide on t h e a c t i v a t e d carbon adso rp t i on process(80) . Because o f t h e completeness and r a p i d i t y f o r removal o f heavy metal s by s u l f i d e p r e c i p i t a - t i on , l i t t l e enhancement i n heavy metal removal i s achieved us ing a c t i v a t e d carbon t reatment as a p o l i s h i n g s tep. The s i g n i f i c a n t r e s u l t o f such a p o l i s h i n g s tep f o l l o w i n g s u l f i d e p r e c i p i t a t i o n l i e s i n i t s a b i l i t y t o sub- s t a n t i a l l y reduce the s u l f i d e concentrat ion, thereby lessening the p o t e n t i a l f o r H2S gas e v o l u t i o n .

The regenera-

Algae has a l so been used f o r removal of heavy meta ls f rom wastewaters and f o r con- c e n t r a t i o n o f va luable metals f rom d i l u t e s o l u t i o n s (50,54). When algae grown i n a sewage lagoon were mixed w i t h heavy metal s o l u t i o n s and subsequent ly were dewatered by an i n t e r m i t t e n t sand f i l t e r , 98% o f t he Cu and 100% o f the Cd were removed from s o l u t i o n (50 ) . Rapid uptake o f cadmium by a lgae was observed i n the f i r s t stage o f a t e r t i a r y t reatment system (72 ) . Sloan e t a l . (141) s t u d i e d the removal o f f o u r d i f f e r e n t meta ls (Cd, Cu, Pb, and Zn) a t d i f f e r e n t concentra- t i o n s us ing th ree d i f f e r e n t a l g a l species. Cadmium, copper, and l e a d cou ld be removed i n a two stage process w i t h the f i r s t stage be ing e i t h e r i o n exchange o r pass ive adsorp- t i o n and t h e second stage being removal f rom s o l u t i o n by passive d i f f u s i o n through t h e c e l l membrane.

Swallow e t a l . (150) i n v e s t i g a t e d the

s o r p t i v e p r o p e r t i e s o f hydrous f e r r i c ox ide us ing copper and l e a d as sorbates. f r e s h hydrous f e r r i c ox ide sorbed more l e a d than aged and l e a d sorbed on t h e f r e s h ox ide was l e s s e f f i c i e n t l y recovered suggest ing occ lus ion occurs i n the c o p r e c i p i t a t i o n process. Va r ia t i ons i n i o n i c s t r e n g t h and com- p o s i t i o n o f t h e background e l f l t r o l y t e s o l - u t i o n had no e f f e l i on t h e Cu s o r p t i o n isotherm. The Pb s o r p t i o n isotherm was s i m i l a r l y una f fec ted by changes i n i o n i c s t reng th ; t he presence o f C1 g r a m a t i c a l l y decreased the percentage o f Pb sorbed a t any g iven pH. the e f f e c t s o f complexat ion by C1, SO , and S 0 on adso rp t i on o f Cd on to f o u r d i f f e r e n t oaide sur faces. I n t e r a c t i o n s between metal i ons and complexing l i gands i n the presence o f an adsorbent sur face can be d i v i d e d i n t o th ree groups based on the o r i g i n and s t reng th o f t h e i n t e r a c t i o n :

They noted

Benjamin and Leck ie (12) s tud ied

a.

b.

me ta l - l i gand complexes may form i n s o l u t i o n and adsorb o n l y weakly o r n o t a t a l l . t he species may i n t e r a c t i n d i r e c t l y a t t h e sur face a1 t e r i n g t h e sur face e l e c t r i - c a l p r o p e r t i e s . t h e m e t a l - l i g a n d complex can adsorb strong- l y enhancing t h e removal o f metal , l i gand , o r both f rom s o l u t i o n compared t o the case when e i t h e r one i s present alone.

c .

The c h l o r i d e and s u l f a t e complexes adsorb less s t r o n g l y than the uncomplexed cadmium ions. Adsorpt ion o f l i k e t h a t o f f r e e l i g a n d .

t h i o s u l f a t e complexes i s more

The e f f e c t o f anions on the adsorpt ion o f heavy metals was addressed by Huang e t a l . ( 5 7 ) . The r o l e o f anions i n the adsorpt ion o f heavy metals i s de l i nea ted by cons ide ra t i on o f t h e f o l l o w i n g r e a c t i o n steps:

1. Anions accumulate i n the double l a y e r o f a p o s i t i v e l y charged p a r t i c l e .

2 . Approach o f metal i o n s t o the soil surface being f a c i l i t a t e d .

3. Complexes form between t h e heavy metal i ons and the a l ready adsorbed aqueous an ion i c l i gands .

For the s o i l - w a t e r environment, copper and z i n c are more removable than l e a d and cadmium.

Various research s tud ies have been performed t o i n v e s t i g a t e the removal o f a s ing le heavy metal f rom s o l u t i o n s . are summarized i n the f o l l o w i n g subsect ions.

These s tud ies

Cadmium

Cedar wood f 1 our cooked w i t h a1 k a l i (17.5% NaOH) f o r 30 minutes t o conver t p a r t of the

No. 243, Vol. 81 183

way$ t o a l k a l i c e l l u l o s e was ab le t o reduce Cd from 8.7 t o 0.4 mg/l f rom a so l y$ ion (164) . The adso rp t i on c h a r a c t e r i s t i c s o f Cd onto t h r e e a c t i v a t e d carbons ( F i l t r a s o r b 400 Nuchar 722, and Nuchar C-190-N) were s tud ied by Huang (63) i n v e s t i g a t i n g such f a c t o r s as pH, i n i t i a l cadmium concentrat ion, dose and type o f carbon, and t h e presence o f c h e l a t i n g agents on cadmium adsorpt ion. Removal e f f i c i e n c y increased w i t h i nc reas ing pH f o r a l l t h ree carbons. Since t h e F i l t r a s o r b 400 and Nuchar 722 are both H-type carbons, p r e c i p i t a t i o n o f CdCO occurs a t h i g h pH and i s a s i g n i f i c a n t removh megtanism. A t t he pH , a f i n i t e amount of Cd adsorbed on the keEhar 722 and s i g n i f i - c a n t l y adsorbed on the Nuchar C-190-N. Nuchar C-190-N, a L-type carbon which adsorbs o n l y a c i d i c ma te r ia l s , i s an e x c e l l e n t adsorbent f o r c a t i o n metal removal; t he Nuchar C-190-r+had the g r e a t e s t adso rp t i ve c a p a c i t y f o r Cd adso rp t i on o f t he carbons tested. I nc reas ing the carbon tocadmium r a t i o lOOx, a t h r e e - f o l d increase i n removal e f f i c i e n c y resu l ted ; however the t ime t o equi 1 i br ium adsorp t i on remains una f fec ted . Strong chel a t - i n g agents were added i n an at tempt t o improve t h e cadmium removal c a p a c i t y o f t h e F i 1 t r a s o r b 400 ( F i 1 t r a s o r b 400 adsorbs p r i m a r i l y an ion i c species) . An improvement i n terms o f t h i s percentage metal removal, f rom 20% w i t h o u t NTA t o 40% w i t h NTA a t 0 . 0 1 ~ Cd. and 50% w i t h NTA a t 0 . 1 ~ Cdi r e s u l t e d a t n e u t r a l pH, w i t h comparable r e s u l t s observed f o r Cd removal performed i n t h e presence o f EDTA. Huang (63) a t t r i b u t e d t h i s improvement t o t h e fo rma t ion o f CdEDTA- o r CdNTA- complexes which cou ld be e l e c t r o s t a t i c a l l y a t t r a c t e d by F i l t r a s o r b 400 (whose sur face charge i s p o s i t i v e f o r pH < 7) f o l l o w e d by f u r t h e r assoc ia t i on o f Cdt+ i ons w i t h t h e adsorbed anions.

The

Copper

Using a s y n t h e t i c seawater of h i g h i o n i c s t r e n g t h (1.6!jy NaCl), Moore (96) s t u d i e d the removal o f Cu by s i x d i f f e r e n t a c t i v a t e d carbons, w i t h Barneby-Cheney PC-859$+being t h e most e f f e c t i v e adsorbent f o r Cu removal. The removal e f f i c i e n c y was ext remely poor however (6%) . were a l l o f t he H-type; a t pH 4-5, t he carbon sur faces a re p o s i t i v e l y charqTd thereby r e p e i l i n g the predominant Cu species (Cu", CuCl , and CuOH ) f rom t h e c g b o n sur face. Al though pH and the t o t a l Cu concen t ra t i on 3r$+themost impor tan t f a c t o r s i n con1.olling Cu adsorpt ion, t he magnitude o f Cu removal by a c t i v a t e d carbon was i n s i q q i f i c a n t even under t h e optimum pH and Cu concen t ra t i on c o n d i t i o n s . Huang (63) i n v e s t i g a t e d us ing e i g h t d i f f e r e n t c h e l a t i n g agents t o improve

The a c t i v a t e d carbons t e s t e d

1

t he Cu++ adsorp t i ve capac i t y . Using 8-hydroxyquinol ine, t he most e f f e c t i v e c h e l a t i p g agent used, a s l i g h t improvement i n Cu ad- s o r p t i o n f rom $+to 7% was adsorbed. The ad- s o r p t i o n o f Cu onto a c t i v a t e d carbon (Aqua Nuchar A) was found t o depend on t h e i o n i c s t reng th (102). For t h e concen t ra t i on range o f 0.001 t o 0.100 M NaCl adsorbabi 1 i t y o f CU+' was h i g h w i t h a d i s t r i b u t i o n c o e f f i c i e n t D o f -700. For h ighe r NaCl concentrat ions, D i n - creases w i t h i n c r e a s i n g NaCl concentrat ions; D - 5+OtoO near 6M NaC1. i n Cu adsorpt ion w i t h i nc reas ing NaCl con- c e n t r a t i o n s was a t t r i b u t e d t o t h e adso rp t i on o f n e g a t i v e l y charged CuC12 complexes.

Mercury

The r a p i d increase

The amount o f mercury removed i s known t o increase w i t h decreasing pH. mercury concen t ra t i on f rom a p r e c i p i t a t i o n / f i l t r a t i o n / a d s o r p t i o n process was reduced f rom 30-50 ppb t o 3-5 ppb by l ower ing t h e pH from 7 t o 2-4 (109). The mercury adsorpt ion capac i t y increased s t e a d i l y by decreasing t h e pH from 9 t o 2, as noted by Huang (@). e t a1 . (56) noted t h e removal o f Hg by a c t i v a t e d carbon was s e n s i t i v e t o pH; ap- p rox ima te l y tw ice as much mercury was removed a t pH 7 was compared t o pH 9. Humenick e t a l . (63) showed t h a t good mercury removal was a t t a i n e d a t low pH. For a carbon column i n which the i n f l u e n t was a c i d i f i e d t o pH 4 and employing a con tac t t ime o f 11 minutes, t h e e f f l u e n t concen t ra t i on averaged 1 ppb over a 5 day per iod. The column mercury removal capac i t y increased from 0.3 t o 4.1 mg Hg removed pe r gram o f carbon ( F i l t r a s o r b 400) by a d j u s t i n g t h e pH from 10 t o 4. i n g c h e l a t i n g agent (ammonium 1 -py r ro l i d i n e d i jQio-carbamate) t h e removal capac i t y o f Hg by a c t i v a t e d carbon was s i g n i f i c a n t l y increased. Treatment o f t h e a c t i v a t e d carbon sur face by+$S was a l s o e f f e c t i v e i n increas- i n g t h e Hg i d s o r p t i o n capaci ty . Dur ing a s tudy o f t h e c a t a l y t i c o x i d a t i o n o f H S over carbon, Sinha and Walker (139) observgd t h a t a l a r g e amount o f mercury f rom a mercury d i f f u s i o n pump and mercury manometer was taken up by the s u l f u r i z e d carbon c a t a l y s t a t 140°C. I n subsequent experiments, s i g n i f i c a n t mercury adsorpt ion was observed us ing the su l fona ted a c t i v a t e d carbon.

The e f f l u e n t

Thiem

Research by us ing column experiments

By adding a p r e c i p i t a t -

Chromium

Chromates can be e f f e c t i v e l y removed from wastewaters by passing the chromate 1 aden wastewater through a column packed w i t h p la t i num b lack cata lyst - impregnated a c t i v a t e d carbon


A f t e r t reatment, a wastewater i n i t i a l l y con- t a i n i n g 100 mg/l chromate had a r e s i d u a l concen t ra t i on o f l e s s than 0.1 mg/ l . 200 m l K C r 0 s o l u t i o n w i t h 5.0 gms o f powdered2c060t?ut she1 1 charcoal and hea t ing i n an autoc lave a t 200°C f o r 30 minutes, t h e concen t ra t i on o f Cr( V I ) was reduced below 0.01 mg/l (152). Huang and Wu (61) found t h a t t he removal o f Cr(V1) by c a l c i n a t e d charcoal was most s i g n i f i c a n t a t low pH and f o r low i n i t i a l Cr(V1) concentrat ions. They specu- l a t e d t h a t the H C r O - i o n s were t h e major species being removgd. Landigan and Hal 1 owe1 1 (82) demonstrated t h a t a c t i v a t e d carbon c o u l d be used by small p l a t i n g f a c i l i t i e s f o r r e - moval o f chromium.

By m ix ing

I n a study on the adso rp t i on o f C r ( I I 1 ) and Cr(V1) as a f u n c t i o n o f pH and t h e amount o f t o t a l C r and Cr(V1) e l u t e d from a c t i v a t e d carbon a t pH 4-6.5, Yoshida e t a l . (166) found t h a t Cr(V1) was r e a d i l y adsorbed on a c t i v a t e d carbo! as anioni$+species such as HCr04- and C r O - w h i l e C r adsorbed by ac?ivated carbon. s o l u t i o n , Cr(V1) i s r e a d i l y reduced t o C r ( I I 1 ) i n t h e presence o f a c t i v a t e d carbon. and Wu (62) l i k e w i s e s t u d i e d t h e e f f e c t o f pH on C r ( I I 1 ) and Cr(V1) adsorpt ion by F i l t r a s o r b 400 a c t i v a t e d carbon. a t l e a s t tw ice as adsorbable as C r ( I I 1 ) . The optimum pH f o r adso rp t i ve removal was 5.5-6.0 and 5.0 f o r Cr(V1) and C r ( I I I ) , r e s p e c t i v e l y . The removal o f Cr(V1) i n v o l v e d r e d u c t i o n and adsorpt ion steps o c c u r r i n g consecu t i ve l y (63) :

-d[ Cr( V I ) ] d t

where t h e f i r s t term on the r i g h t hand s i d e o f t he equat ion denotes the reduc t i on r a t e o f Cr(V1) w h i l e the second term denotes the r a t e o f Cr(V1) adsorpt ion. The concentrat ions are a l l expressed i n moles/ l w i t h t h e excep- t i o n o f C y repo r ted i n gm/l . By p r e t r e a t i n g the Cr(V1) wastewater w i t h C1, and hea t ing the a c t i v a t e d carbon i n 1M HNO s o l u t i o n f o r 30 minutes, t he r e s i d u a l C r ( I I ? ) was s i g n i f i - c a n t l y reduced i n batch-mode opera t i on .

I n summary, a c t i v a t e d carbon w i l l adsorb hexavalent chromium, mercury, and many meta ls complexed i n organic form. The adso rp t i ve capac i t y depends on t h e carbon pore s i ze , t h e s i z e o f t h e molecule, s o l u t i o n pH, and the i n i t i a l and f i n a l concen t ra t i on o f t h e m e t a l ( s ) . Adsorpt ive capac i t y increases as the pH decreases; adso rp t i on e f f i c i e n c y increases as the concen t ra t i on increases. Most e f f e c t i v e t reatment r e s u l t s w i t h d i 1 Ute wastes. A c t i - vated carbon t reatment shows considerable promise f o r removing the l a s t t r a c e o f metal

was m i n i m a l l y I n a c i d i c

Huang

Cr(V1) was

t 1 /6 4. 4 x m 3 [ Cr( V I ) ]3/4[ C][ H ] + 2 . 3 ~ 1 0 - ~ [ C r ( V I ) I[ C I 3 [ H’] a -

( i n t h e range o f 1-2 ppm) f o l l ow ing o t h e r t reatments (such as hydrox ide p r e c i p i t a t i o n , e lec t rodepos i t i on , cementation, e t c . ) . A c t i - vated carbon t reatment has been success fu l l y used f o r e x t r a c t i n g g o l d f rom cyanide so lu t i ons .

To determine t h e e f f e c t i v e n e s s o f a c t i - vated carbon f o r a p a r t i c u l a r metal bear ing wastewater, adso rp t i on isotherms a re developed i n d i c a t i n g t h e amount o f m a t e r i a l adsorbed a t a p a r t i c u l a r concen t ra t i on . The t e s t s are run on severa l d i f f e r e n t carbons t o determine which one p rov ides t h e most e f f e c t i v e t r e a t - ment. Granular a c t i v a t e d carbon (GAC) i s u s u a l l y p r e f e r r e d s ince i t can be chemical ly regenerated and reused. Powdered a c t i v a t e d carbon (PAC) i s l e s s expensive, b u t i t can o n l y be used on a once-through bas i s . PAC i s d i f f i c u l t to handle due t o a tendency t o dust and i t must be removed from t h e waste- stream by coagu la t i on and/or f i l t r a t i o n . A c t i v a t e d carbon t reatment has n o t been widely employed f o r removal o f heavy meta ls : (29 ) .

MEMBRANE OPERATIONS

The use o f msrbrane processes for water reuse, wastewater v o l m &tion, and byproducts (such as, valuable m t a l s recovery) i s gaining considerable atten- tion i n many industries. W r a n e pmesses can be divi- ded i n thwe cakqories: high pressure reverse o m s i s (500-1500 psi), lm pressure reverse o m s i s (200-500 psi), and u l t ra f i l t ra t ion (20-100 psi). The pnxess consists i n let t ing the solution flw under pressure through an appropriate poms “ne (cellulose acetate. polyamide, polysulfone, etc.) and withdrawing the m brane permeate product water a t atmospheric pressure. For reverse o m s i s (Ro), the applied pressure nust be cm- siderably greater than the o m t i c pressure of the re- jected solutes i n order to obtain adequate water flux. Ul t raf i l t rat ion with neutral “ n e s i s normally used to m v e large mlecular w igh t (W > 1ooO) solutes.

Recent ly Bel f o r t (10) prov ided an ex- tens i ve overview o f s y n t h e t i c membrane processes and va r ious wastewater t reatment a p p l i - ca i tons . m a t e r i a l s p repara t i on and c h a r a c t e r i z a t i o n was repo r ted by L loyd ( 8 6 ) . Reverse osmosis membrane f l u x and s o l u t e t r a n s p o r t have been commonly descr ibed by two models: d i f f u s i o n model (88) and the p r e f e r e n t i a l s o r p t i on-capi 11 a r y f 1 ow model : ( 145).

An e x c e l l e n t rev iew o f membrane

the s o l u t i o

The development o f s y n t h e t i c t h i n - f i l m composi t e membranes has resu l t e d i n so l u t e removals a t lower pressures over a broad pH range (pH 2-12). The new generat ion membranes

No. 243, Vol. 81 185

a1 low simultaneous separat ion o f metal s a l t s and o rgan ics f rom aqueous systems. pressure processes w i t h composite membranes have d e f i n i t e advantages i n terms o f energy savings and c a p i t a l cost . T h i n - f i l m membranes r e s y l t i n a h ighe r water f l u x (25-35 g a l l o n s / ( f t ) (day ) ) and 95-99% removal o f var ious chem- i c a l s (14,16).

Much o f t he development work and commer- c i a l u t i l i z a t i o n o f t h e RO process has occurred w i t h i n t h e l a s t 15 years, w i t h p a r t i c u l a r emphasis devoted t o d e s a l i n a t i o n and water t r e a t - ment and recovery. amount t o 4 t o f hp/1000 g a l l o n s wastewater t rea ted . T h i s technique has shown promise f o r removal and recovery o f metal i o n s f rom waste s o l u t i o n s . Reverse osmosis has been used t o dewater mixed p l a t i n g r i n s e streams p r i o r t o chemical p r e c i p i t a t i o n / c l a r i f i c a t i o n t o reduce t h e h y d r a u l i c l oad ing t o the c l a r i f i e r ( 28). Typ ica l o p e r a t i n g c o n d i t i o n s are summarized below:

These low

The power requirements

feed stream t o t a l d i sso l ved s o l i d s 1000-10000

t o x i c meta ls concen t ra t i on 1-100 ppm feed r a t e s 5-100 gpm

C a r t w r i g h t (28) est imated ghe e x i s t i n g market p o t e n t i a l t o be $5-10 x 10 p r o j e c t e d growth r a t e o f 10-15%/year. osmosis has a1 so been used f o r e l e c t r o p l a t i n g s o l u t e recovery ( p l a t i n g s a l t s ) f rom r i n s e - waters. The concentrate i s r e t u r n e d t o t h e p l a t i n g pa th and the permeate i s r e t u r n e d t o t h e l a s t r i n s e . Over 170 baths a re c u r r e n t l y t r e a t e d w i t h RO; t y p i c a l feed r a t e s are 2-10 gal /min. The e x i s t i n g magket p o t e n t i a l was es t ima ted t o be $5-10 x 10 /year w i t h a p r o j e c t e d growth r a t e o f lO%/year ( 2 8 ) . Continuous s ide - stream u l t r a f i l t r a t i o n has a l s o been employed f o r e l e c t r o d e p o s i t i o n p a i n t baths f o r removal o f water and contaminants. The concentrate i s r e t u r n e d t o t h e baths; permeate i s used as t h e f i r s t rinse o r i s discharged.

I n another membrane process, e l e c t r o - d i a l y s i s i n v o l v e s t h e t r a n s p o r t o f i o n i c species through membranes by a p p l i c a t i o n o f a d.c. p o t e n t i a l . an ion exchange res ins , s e l e c t i v e f o r o n l y c a t i o n s o r anions i n the waste s o l u t i o n . The c a t i o n exchange membrane pe rm i t s the passage o f c a t i o n s w h i l e r e j e c t i n g anions w h i l e the anion exchange membrane a l l ows anions t o pass and r e j e c t s c a t i o n s .

PPm

per year a long w i t h a Reverse

The membrane con ta ins c a t i o n o r

These membrane separa t i on processes have become standard procedures f o r separa t i on o f mo lecu la r s o l u t i o n s . I n UF, the d i f f e r e n t chemical components a re separated e x c l u s i v e l y

by molecular s i z e . I n RO, p a r t i c l e s i z e and t h e chemical na tu re bo th are impor tant f o r t h e separat ion o f m a t e r i a l s . F o r mult icom- ponent m ix tu res where h i g h l y t o x i c o r v a l u a b l e m a t e r i a l s (heavy me ta l s ) have t o be removed from i n d u s t r i a l wastewaters c o n t a i n i n g these i o n s i n low concen t ra t i ons i n a d d i t i o n t o a m ix tu re o f o t h e r s a l t s , Strathman and Kock (149) p o i n t o u t t h a t RO i s n o t w e l l s u i t e d f o r t h i s s o r t o f separat ion problem. I n t h e i r study, po l ye thy len im ine showed a good s e l e c t i v i t y for+Cu ag$ was somzwhat l e s s s e l e c t i v e f o r Zn , N i , and Ag ions . Quatern ized p o l y - e thy len imine had a good s e l e c t i v i t y f o r Pd, P t , Hg, and Au. P o l y t h i o u r i e a was w e l l s u i t e d t o b i n d Hg.

pressure u l t r a f i l t r a t i o n mebranes (800 mM charge c a p a c i t y ) have been s tud ied f o r several a l k a l i n e - e a r t h , heavy metal ions, and oxyanions: (23) . For many closed- loop process i n v o l v i n g water reuse, adequate r e j e c t i o n s o f up t o 97% w i t h charged UF membranes prevent t h e b u i l d u p o f low molecular weight i o n i c s o l u t e s i n t h e recyc led water : (15,19,20) . Low-flux, t i g h t uncharged membranes (such as c e l l u l o s e acetate) are commonly used f o r r e - moval o f i n o r g a n i c e l e c t r o l y t e s . The s a l t r e j e c t i o n s o f t e n exceed 98%; however t h e h i g h e f f e c t i v e osmotic pressures and the membrane t i g h t n e s s o f t e n necess i ta te h i g h pressure operat ion, which causes membrane compaction problems and a r e d u c t i o n i n t h e water f l u x (23 ) . Bhattacharyya and Grieves (23) a l s o p o i n t o u t t h a t s e l e c t i v e separat ion o f s p e c i f i c i o n i c s o l u t e s f rom mixed metal mix tures i s n o t g e n e r a l l y poss ib le . Membrane s w e l l i n g i s un- d e s i r a b l e s ince i t would produce a loss i n r e j e c t i o n o f monovalentanions g rea te r than t h a t o f d i v a l e n t anions. Bhattacharyya and Grieves (23) prov ided t h e f o l 1 owing resu]i$s foy+the ch loy jde and s u l f a t e s a l t s of Cu , N i , and Zn r e l a t i n g the u l t r a f i l t r a t e stream concen t ra t i on t o the i n l e t stream concen t r a t i on :

The r e j e c t i o n c h a r a c t e r i s t i c s o f low-

1.36 C f = 0.093 Ci

* 0.36 R = 1-0.093 C . 1

Th$+reje$$ion depeggence on concentrat ion f o r Cu , N i , and Zn s u l f a t e s a l t s was l i k e - wise descr ibed bv:

1.21 Cf = 0.082 Ci 0.21 i R* = 1-0.082 C

P o t e n t i a l a p p l i c a t i o n s o f t h i s technology i n c l u d e u l t r a f i l t r a t i o n o f p l a t i n g r i n s e waters, u l t r a f i l t r a t i o n o f t o x i c metal con -


s t i t uen ts f rom metal manufactur ing wastewaters, d i sso l ved s o l i d s reduct ion, and u l t r a f i l t r a t i o n o f photographic processing water c o n s t i t u e n t s . As an example, a s e r i e s o f UF exper iments(23) were performed w i t h ac tua l r insewaters by d i 1 u t i n g a Watts- type b r i g h t n i c k e l - p l a t i n g s o l u t i o n . The unadjusted pH ranged from 5.2 t o 6.8; t he i n l e t n i c k e l concen t ra t i on ranged from 35 mg/l 80 949 mg/l. The r e j e c t i o n s ( a t hp = 2.8 x 10 N/m ) of t o t a l organic carbon, c o n d u c t i v i t y , and o p t i c a l d e n s i t y ( a t 400 nm) were 0.67, 0.73, and 0.90+, r e s p e c t i v e l y . Bhattacharyya and Grieves (23) a1 so observed t h a t cadmium s u l f a t e r e j e c t i o n (88%) was somewhat greater+than t h a t of++the o t h e r s u l f a t e s a l t s o f Ca , N i , and Zn i n a metal manu- f a c t u r i n g wastewater. Using a n e g a t i y T l y charged UF mgybrane, r e j e c t i o n s o f Cu , Ni", Zn , and Cd were 0.88, 0.88, 0.88, and 0.90, r e s p e c t i v e l y .

Pusch and Walch (130) p o i n t o u t t h a t membrane separat ion processes r e q u i r e o n l y about 1/10 o f t he energy cos ts o f a correspond- i n g d i s t i l l a t i o n process i f smal l t o medium wastewater ( p l a n t s i zes o f 100-1000 m /day). R e s t r i c t i o n s associated w i t h t h e use o f membrane processing i n v o l v e membrane l i f e t i m e and s e l e c t i v i t y s ince s t rong a c i d i c , a l k a l i n e , and/or o x i d i z i n g s o l u t i o n s o f t e n have t o be concentrated r a t h e r than n e u t r a l o r pH-con t ro l l ed r i n s e water . Conventional membranes r a p i d l y d e t e r i o r a t e i n such chemical ly a c t i v e b r ines . Recyc l ing i s l e s s e f f e c t i v e i f va luab le m a t e r i a l s must be i s o l a t e d i n t h e presence o f l a r g e amounts o f low value by-products. I t thus becomes necessary f o r development o f processes f o r separat ion o f d i f f e r e n t s a l t s i n a d d i t i o n t o t h e p u r i f i c a t i o n o f t h e r i n s e water. goals can be approached through several d i f l f e r e n t means (130):

1. Use o f complexing agents t o complex s p e c i f i c i ons . The f i l t r a t e s o l u t i o n con ta in ing o t h e r contaminants cou ld be f u r t h e r processed by h y p e r f i l t r a t i o n t o y i e l d a good q u a l i t y r i n s e water f o r reuse. Use o f "Ac t i ve t r a n s p o r t membranes" con- s i s t i n g of a water- immisc ib le l i q u i d organic complexing agent immobi l ized w i t h - i n the pores o f a microporous membrane.

3. A d d i t i o n o f chemicals (chelate, po l ye lec - t r o l y t e s , e t c . ) f o r eventual separat ion o f t h e va luable i o n f rom the complexing

t reatment p l angs are considered

These

2 .

agent. separat ion of two so lu tes was more e f f e c t i v e a t 40 atm than a t 100 atm due t o the l e s s r a p i d increase i n t h e contaminant r e j e c t i o n

Pusch and Walch (130) found the


w i t h i n c r e a s i n g pressure. By adapt ing the p r o p e r t i e s o f t h e membrane polymer t o t h e p r o p e r t i e s o f t h e so lu tes and by v a r i a t i o n o f t h e process parameters ( a p p l i e d pressure, s o l u t e concentrat ion, pH, e t c . ) , separat ion numbers g r e a t e r than 1000 f o r low molecular weight s o l u t e s can be achieved by h y p e r f i l t r a - t i o n . o f t h e species y i e l d s a r e j e c t i o n g r e a t e r than 99% a t a h i g h water recovery r a t e .

membranes p rov ide the advantages o f good water f l y x a t !ow t r a smem rane pressures (5.0 x 10 t o 7.0 x 10 N/m ) and t h e s e l e c t i v e separat ion o f s imple and complexed ions f rom aqueous s o l u t i o n s . The separa t i on o f heavy meta ls i n the presence o f complexing agents i s p a r t i c u l a r l y d i f f i c u l t w i t h convent ional processes. An ex tens i ve exper imental i n v e s t i - g a t i o n (22) was conducted w i t h nega t i ve l y - charged, n o n - c e l l u l o s i c u l t r a f i l t r a t i o n membranes t o e s t a b l i s h t h e r e l a t i v e r e j e c t i o n behaviors o f complexed heavy meta ls under i n s i g n i f i c a n t concen t ra t i on p o l a r i z a t i o n con- d i t i o n . Three types of complexing agents were u t i l i z e d : cyanide (CN), ethylenediamine t e t r a - a c e t i c a c i d (EDTA), and oxa la te ( O X ) . The w g a t i v e l y - c h a r g e d ( s u l f o n i c a c i d groups) mem- brages used had a t y p i c a l water f lux 10-

Separat ion i s most e f f e c t i v e i f one

Charged, n o n c e l l u l o s i c u l t r a f i l t r a t i o n

! 3 P

o[ 1.32x cm/sec a t a pressure o f 5.6 x 10 N/m . The r 9 J e c t i g g deptrrdence ofl$.he heavy

meta ls (Zn , Cd , Cu , and Cu ) and f r e e complexing agents (CN-, EDTA, and oxa la te ) was found t o be a f u n c t i o n o f feed r a t i o (L/M) o f complexing agent to metal P H , ~ i o n i c s t rength, and pressure (below 5.0 x 10 n/M ) . For EDTA and o x a l a t e systems, t h e r e j e c t i o n s o f metal were independent o f i n i t i a l metal concentrat ion whereas f o r t he cyanide system t h e r e j e c t i o n s o f both metal and cyanide decreased w i t h feed concen t ra t i on . The concen t ra t i on e f f e c t was f u r t h e r v e r i f i e d by the h i g h water recovery exper imegts. A t a transmembrane pressure o f 5.6 x 10 n/M2, metal r e j e c t i o n s ranged between 77% and 96%. The r e j e c t i o n behavior i s exp la ined i n terms o f metal complex species d i s t r i b u t i o o and Donnan Exc lus ion model. The r e j e c t i o n s o f a l l complexed metal i o n s are s t rong f u n c t i o n s o f average species charge. Because negat ive ly-charged membranes r e j e c t d i v a l e n t anions b e t t e r than monovalent anions, t h e r e j e c t i o n o f ML- > ML would be expected. The complexed metal i o n charge i s a f u n c t i o n o f the L/M r a t i o and pH.

The membranes used i n the study had a t y e l c a l CuC12 r e j e c g i o n $ R ) o f 34% a t 1.6 x 10 M and 5.6 x 10 N/m pressure. Typica l r e j e c t i o n s obta ined w i t h complexed copper

No. 243, Vol. 81 species were 94%, 96%, and 80% f o r Cu)CN)- Cu(EDTA)-, and Cu(0X)-, r e s p e c t i v e l y . The r e - j e c t i o n s o f f r e e CN-, EDTA-, and OX- were 30%, 85%, and 93%, r e s p e c t i v e l y . For a l l cases, t h e meta l r e j e c t i o n s show t h e f o l l o w i n g t rend :

were ob ta ined a t L/M = 1.0, pH 4-10 w i t h EDTA; a t L/M = 4-6, pH 9-10 w i t h cyanide; and L/M = 10-12, pH 6-7 w i t h oxa la te . of va r ious spec ies - fo l l ow the order : Cu(CN1- > Zn(CN)i > Cd(CN)- ; ZN(EDTA)- > Cd(EDTA)- Removals a t h i g h wa?er recove r ies were a l s o computed from t h e concen t ra t i on dependence c o r r e l a t i o n and f u r t h e r v e r i f i e d by e x p e r i - ments. S e l e c t i v e separat ion o f meta ls c o u l d be ob ta ined by a d j u s t i n g the pH and complex- i n g agent t o metal feed r a t i o .

Car twr igh t (29) descr ibes the present a p p l i c a t i o n s and p o t e n t i a l uses o f membrane processes i n t h e p l a t i n g shop. 100 RO systems a re used t o t r e a t n i c k e l p l a t i n g r insewaters. Typ ica l ope ra t i ng cos ts ( i n c l u d i n g l a b o r and maintenance, e l e c t r i c a l expendi tures f o r t h e h i g h pressure pump and t r a n s f e r pump and rep1 acement o f t he c a r t r i d g e f i l t e r and RO membrane element) range f rom $0.75 t o $2.00/1000 g a l l o n s o f feedwater processed. The economic va lue o f recovered n i c k e l - p l a t i n g s a l t s u s u a l l y r e s u l t s i n a c a p i t a l payback p e r i o d of l e s s than 18 months. C a r t w r i g h t (29) p o i n t s art that few RO i n s t a l - l a t i o n s a r e used t o t r e a t wastewaters such as a c i d copper o r t i n - l e a d f l u o r o b o r a t e due to low evaporat ion r a t e s and low va lue o f t h e p l a t i n g s a l t s .

High metal r e j e c t i o n s %-EDTA > R ~ - ~ ~ > R ~ - o ~ .

The r e j e c t i o n s

More than

187

For membrane e x t r a c t i o n o f heavy metals, the netals a re removed through membranes which separates these two phases (74 ) . No m i x i n g i s i n v o l v e d and no w i n g p a r t s are used, thereby e l i m i n a t i n g many problems assoc ia ted w i t h d i m t mixing necessary i n convent ional s o l v e n t e x t r a c t i o n processes. This new technology o f t h e membrane e x t r a c t i o n process employes two ho l l ow f i b e r modules, one f o r e x t r a c t i o n and theother f o r s t r i p p i n g . The o rgan ic so l ven ts c i r c u l a t e between t h e two modules p i c k i n g up metal i o n s i n the e x t r a c - t i o n module. The meta ls are removed f rom t h e o rgan ic s o l v e n t i n t h e s t r i p p i n g module. The regenerated s o l v e n t i s r e c y c l e d back t o t h e e x t r a c t i o n module. Water d r o p l e t s i n t h e o rgan ic phase s i g n i f i c a n t l y d e t e r i o r a t e t h e performance o f t h e system. The water d r o p l e t s i n the o rgan ic phase w i l l c a r r y s t r i p p i n g s o l u t i o n t o t h e aqueous stream, thereby s h i f t i n g t h e e q u i l i b r i u m r e l a t i o n s h i p and causing a r e d u c t i o n i n t h e removal e f - f i c i e n c y . However, s i n c e o rgan ic l o s s i s l e s s than t h a t w i t h convent ional s o l v e n t e x t r a c t i o n , i t i s p o s s i b l e t o use more ex-

pensive so l ven t m a t e r i a l s . t i o n can be used f o r separat ion and concen- t r a t i o n o f copper, n i c k e l , z inc, chromium, coba l t , uranium, and o t h e r heavy meta ls . Major advantages o f t h i s technology ove r o t h e r compe t i t i ve processes are h i g h s e l e c t i v i t i y f o r a s p e c i f i c metal , s i m p l i c i t y o f operat ion, and h i g h chemical and phys i ca l s t a b i l i t i e s .

I n summary, membrane processes a r e be- coming more and more accepted w i t h i n the i n - dus t r y . To da te however, t h i s system i s e s s e n t i a l l y a concen t ra t i on technique. Major l i m i t a t i o n s associated w i t h t h e use o f membrane processes i n c l u d e membrane f o u l i n g , l i m i t e d l i f e o f t he membranes, d i s s o l u t i o n o f t h e membrane by s t rong o x i d i z i n g agents, so lvents , and o t h e r organic compounds. New generat ion composite membranes o f f e r broad pH (pH 2-12) and temperature (up t o 50°C) o p e r a t i n g 1 i m i t s .

Membrane e x t r a c -

EMERGING TECHNOLOGIES

Several i n n o v a t i v e technologies have emerged over t h e l a s t few years. and Lancy ( 7 1 ) s t u d i e d t h e bas ic f a c t o r s causing erratic r e a c t i o n r a t e s i n s u l f i d e p re - c i p i t a t i o n o f heavy meta ls i n a secondary t rea tmen t s tep a f t e r n e u t r a l i z a t i o n and se t - t l i n g . Since EDTA forms t h e t i g h t e s t complex encountered among t h e usual organic a d d i t i o n s i n metal f i n i s h i n g process so lu t i ons , syn- t h e t i c s o l u t i o n s were prepared by adding a s l i g h t excess o f metal s a l t s above t h e s t o i c h i o m e t r i c amount t o an EDTA s o l u t i o n a t pH 4. s t i r r e d f o r several hours , and f i 1 t e r e d through a 0.45 filter, The r e a c t i o n r a t e s were s lugg ish and e r r a t i c . The i n i t i a l r e a c t i o n r a t e was acce le ra ted through a d d i t i o n o f unchelated metal s a l t s t o p rov ide seeds t o hasten t h e r e a c t i o n . Residual suspended s o l i d s from a p r e v i o u s l y c l a r i f i e d e f f l u e n t a l s o served as a seed f o r t h e r e a c t i o n . I n - c reas ing t h e pH reduced t h e t i gh tness o f t h e EDTA complex a l l o w i n g a more complete r e a c t i o n t o occur . a f i l t e r f o r which t h e r e s u l t a n t media i s e f f e c t i v e i n adsorbing s o l u b l e heavy metals and r e t a i n s a c t i v e s u l f i d e s i n i t s body a l l o w i n g f u r t h e r r e d u c t i o n i n the so lub le heavy metal res idues. That f e a t u r e n e a r l y e l i m i n a t e s t h e s l i ppage o f excess s u l f i d e p reven t ing t h e contaminat ion o f t he e f f l e u n t by s u l f i d e .

Kamperman

The s o l u t i o n pH was r a i s e d t o 9.0,

Kamperman and Lancy ( 7 1 ) developed

I t o e t a1 . (65) developed a cont inuous t reatment system f o r t he magnetic separat ion o f heavy metal i o n s us ing e i t h e r f e r r i t e o r magnet i te . Advantages o f t he process inc lude:


Various heavy metal ions can be t r e a t e d

The sludge formed i s s tab le t o pH and

The f e r r i t e sludge can be separated by

The sludge formed i s a p p l i c a b l e t o the

together . temperature changes.

app ly ing a magnetic f i e l d .

ox ide magnetic m a t e r i a l .

Water i n s o l u b l e s ta rch xanthate (ISX) has been shown t o be an e f f e c t i v e a l t e r n a t i v e t o heavy metal m v a l and recovery (161,162). Using th i s l w c o s t product, I S X i n e f f e c t i v e i n removing metal ions a t d i f f e r e n t concen- t r a t i o n l e v e l s . Wing and Rayford (161) p o i n t ou t t h a t i f the i n i t i a l metal concentrat ions exceed 100 mg/l, i t i s n o t economical t o use I S X treatment, and removal by chemical pre- c i p i t a t i o n or sane other process should be used. I S X i s a h i g h l y cos t compet i t i ve method compared t o o t h e r t reatment processes f o r f i n a l p o l i s h o f process o r e f f l e u n t discharge water c o n t a i n i n g l e s s than 10 mg/l o f heavy metal contaminants i n s i n g u l a r o r complexed f o r m (162) . The I S X process was originally Qveloped a t the U.S. Department o f Ag r i cu l tu re . I S X i s a cerea l grain-based product chemica l l y c r o s s - l i n k e d t o make i t i n s o l u b l e i n water, and then is xanthated t o form an an ion ic polymer. When added t o wastewater c o n t a i n i n g heavy metals, I S X exchanges sodium and magnesium ions f o r the heavy metal ca t i ons presented severa l case h i s t o r i e s demonstrat- i n g t h a t very low res idua l metal concent ra t ions can be achieved i n the t r e a t e d e f l u e n t s .

Wing (162)

Advantages and c h a r a c t e r i s t I S X process inc lude:

cs o f the

1.

2.

The average c a p a c i t y i s i n t h e range o f 1.1-1.5 meq o f metal i ons /gm I S X . The I S X process i s e f f e c t i v e over the pH range o f 3-11 w i t h maximum e f f e c t i v e n e s s shown f o r pH > 7. S a l t concent ra t ions up t o 10% have 1 i t t l e e f f e c t on the removal o f heavy meta ls by the I S X process.

4. I S X r a p i d l y removes heavy metal ions f rom s o l u t i o n .

5. Treatment i s a p p l i c a b l e t o batch o r c o n t i n - uous f l o w systems.

6. Meta ls can be recovered from the ISX-metal s ludge by n i t r i c a c i d t reatment o r i n - c i n e r a t i o n .

7. ISX-metal s ludge s e t t l e s r a p i d l y and de- waters t o 30-90% s o l i d s conten t a f t e r f i l t e r a t i o n o r c e n t r i f u g a t i o n . P r e l i m i n a r y est imates t o manufacture I S X amount t o $0.30/1 b .

3.

8.

Johannesmeyer and Ghosh (70) i n v e s t i g a t e d


two techniques f o r f i x a t i o n o f heavy meta ls i n e l e c t r o p l a t i n g wastes. Waste f i x a t i o n i n - vo lves s t a b i l i z a t i o n / s o l i d i f i c a t i o n t h a t immobi l izes, isolates,. o r con ta ins i n d u s t r i a l waste products . The Silicate and cement based and l i m e based techniques were evaluated i n t h e i r s tudy. The i n i ti a1 concent ra t ion o f chromium and cadmium were 0.490 and 0.102 mgll, r e s p e c t i v e l y . It i s p o s s i b l e f o r t r a c e amounts of mta ls t o escape f i x a t i o n due t o improper f i x a t i o n i n g r e d i e n t t o s ludge r a t i o , i n s u f - f i c i e n t mix ing of f i x a t i v e and sludge, improper mo is tu re conten t o f t he sludge, e t c . Both f i x a t i o n techniques were capable o f reducing the leach ing o f chromium and cadmium from the t r e a t e d product . Sodium s i l i c a t e and cement was more e f f e c t i v e than f l y ash and l i m e i n i m m o b i l i z i n g chromium and cadmium dur ing the EP t o x i c i t y and column leach t e s t s . methods showed a l o s s o f chromium and cadmium over the s tudy per iod; t h e s i l i c a t e method re1 eased cons iderab ly lower amounts than the l i m e based method. content , the more e f f e c t i v e the method was a t immobi l i z ing cadmium and chromium.

Both

The l a r g e r the s i l i c a t e

SUMMARY

The var ious t reatment techniques a v a i l - ab le f o r removal o f heavy meta ls f rom wastewaters have been addressed. By f a r , t he most commonly used technique i s t h a t o f chemical p r e c i p i t a t i o n . assoc iated w i t h each process have been d i s- cussed. Table 1 summarized the t reatment cond i t ions , i n f l u e n t cond i t ions , res idua l metal concentrat ions, and removal e f f i c i e n c i e s r e p o r t e d i n the 1 i t e r a t u r e .

ACKNOWLEDGEMENTS

The advantages and 1 i m i t a t i o n s

The authors wish t o acknowledge the suppor t o f t he School o f C i v i l Engineer ing a t Purdue U n i v e r s i t y and the Department of Chemical Engineer ing a t the U n i v e r s i t y of Kentucky i n per forming the review and conduct- i n the heavy metal removal research.

No. 243, Vol. 81 1 89

Table 1.

Technology Metal(s) pH I n i t i a l Metal, Residual Metal Removal Conrilents Ref e rence Em1 oyed Con cent r a t i o n Concentration E f f i c i ency ,%

S m m r y o f heavy metal removals by various treatment techniwies.

Hydroxide 0a 10-11 P r e d p i t a - 9.2 t ion 10.5

11.6 10.5 10.3

Cd 8.5-11.3 11.2-11.3 6.0-10.0

10.0

8.0 8.6 9.4

10.4 11.9

C r 6.6 9.5-10.0

9.5-1 0.5

Cr+3 10.6-11.3

Cr+6 9.5-11.6 9.21

cu 10.5 9.5-10.5

Pb 8.5-11.3 6.0 7.4 8.8

10.5 11.9 12.3

tlg Ag

20

N1

Mixed Metal cu N i Pb

Hq 10.7-11.4 ( inorganic) 9.4

9.3-11.3 9.0

11.5 10.0

6.2 7.5 8.3 9.5

11.0 11.8 6.8 7.4 8.3 9.4

11.0 12.3

: >8.5

Mixed Eletals: 8.5-9.1 Fe Pb Sn

cu Pb

Mixed k t a l s : 8-10

S i Mixed Metals: 9.7-10.2

Cd C r cu Pb Zn

7.0-8.5 10.0-12.0 10.0-12.0 10.0-12 .o

7.5 17.4 0.3 10 100 100 100 100 -- _- _ _ --

Crt3.i;25

Cr+6:2 .23 ~ r + ~ :4.0 ~ r + ~ : 4 . 5

c~+~I I .40

0.15 0.15 0.15 0.45 5.7 0.15 -- -- -_

-- -- _ _ 9 . 3 u9/1 9.3 P9/1 9.3 u9/1

0.15 0.15 100 100 -- -_ _- -- --

-- -_ _- -- _ _ -- -- 60.0

1.9 1.2

20-60 0.1 1 .o

20-150 0.5-1.0 0.1-0.5

1.66 1.11 0.29 1.7 31

-- -- -- -- -- _ _ _ _ - -

e0.3 <0.3 2.0 5.0

2350 126 5

0.2 0.3

Crt3.26.0 Crt3:0.3 Cr+6 :<0.01 Cr;: ; O .03 C r .<0.01 -- _ _ _ _

0.08 0.89 --

1700 25.6 6.0 0.6 280

1050 -- -- _ _ _ - _ _ 0.3

32.0 1900 27.5 0.55 0.25 0.68 0.95

1450 930 15 0.5 0.3 0.5

0.9 0.3 0.4

0.3-2.4 0.5 0.5

0.5-2.0 <0.05 0.1

0.04 0.97 0.03 0.2 0.28

>go 84 93 82 88 95

>98 > 98

>99.7 .99.7

98 95 _ _ _ _ _- -- _ _

>98 78.6

939.5 99.3

>99.7 >98 >70 < l o 02.2 84.4 >98 _ _ _ _ _ _ _ _ _ _ _ _

60-00 30 e 5 70 90

99.7 68 -_ _ _ _ _ - - _ _ -- _ - -_ _ _ _ - _ _ _ _

98.5 84.2 66.7

>96

>50

>97.5 90-95

<EO

97.6 12.6 89.7 88.2 99.1

- -

_ _ P i l o t p lan t tes ts

F u l l scale t e s t s

_ _ _ --- Synthet ic p l a t i n g wastwater Synthet ic p l a t i n g wastewater

10 mg/l CO= IO mg/l co3 IO nig/l CO: 55 mgl l CO - 100 mg/l d; _ _ _ Nickellctyonte r i n s e

F u l l scale p l a n t

--- _-- - - -

Copper r i n s e F u l l scale p l a n t

15mg/l CO= 15mgil C03 15mg/1 C03 40nig11 co3 12311gll ca 275mg/1 cni

_ _ _

--- _ _ _ _ _ _ , _ _ _ _ _ -

25 my11 COJ 19 mg/l co3 30 mgl l C03 38 mg/l co3 38 mg/l co3 90 mg/l co3 225 mg/l Cc! Electroplat?r ig p lan t da

E lec t rop la t ing p l a n t data

Pr in ted c i r c u i t board manufacturer

1

43 43

43

43

124 124

111

5

5

43

43 131

43 111

43

43 43

124

111

111

138

138

138

26

Carbonate Cd 7.2 _- 440 _ _ 5 IIKJ/l co= 111 P r e d p l t a - 8.4 -- 7.5 -- 5 niqll co3 t i o n 9.5 -- 0.6 -- 10 mg/l ca

10.7 -- 0.35 -- 50 m q / l C03 11.9 _ _ 0.5 -- 225 mg/l ca

Cd 8.1 _ _ 5 -- 1200 mg/l cd 111 8.4 -- 1.2 -- 2800 mg/l co3 8.7 -- 1.7 _ _ 3350 mg/l C03

10.0 -- 0.25 _ _ 4200 mg/l C03 10.8 -- 0.25 _ _ 4400 mg/l C03 11.7 -- 0.35 _ _ 4300 mg/l C03

8.2 -- 60 _- 3000 nig/l C03 9.0 -_ 3.8 _ _ 3500 mg/l co3

10.5 -- 2.7 _ _ 5500 n q l l C03 11.5 -- I .4 -- 5500 mg/l C03 12.5 -- 1.4 _ _ 5500 nq/1 CO;

N i 7.2 -_ 8M) _ _ 1500 mg/l C03 111


Table 1. (cont. 1

Technology Metal(s) pH I n i t i a l Metal, Residual Metal Reinoval Comnents Reference Etiipl Dyed Concentration Concent r a t ion E f f i c i e n c y ,%

Preci p i t a - Carbonate Pb 5.6 -- 4.6 _ _ 10 mg/l co= 111 15 m g / ~ co3

t i o n 6.8 -- 17.4 -- 20 w / ~ co3 10.1 -- 0.6 -- 55 ~1 co3 11.6 -- 10.0 _ _ 55 mg/l co3 12.3 -- 1260 -- 55 ~1 co3 8.4 -- 2.0 -- 4000 w / ~ co3

6.1 -- 43.6 --

Pb 7.5 -- 1 .o -- 1200 mg/l 20 111

9.2 -- 3.6 _ - 4650 mg/l C03 10.5 -- 8.0 _ _ 5500 mg/l C03 11.4 -- 6.0 -- 5500 mg/l C03 12.4 -- 150 -- 5500 mg/l C03

Zn 6.6 -- 260 _ _ Sample n:t adalyzed f o r CO; 111 8.3 -- 0.95 _- 9.1 -- 0.75 --

10.0 -- 0.60 -- 5500 mg/l CO= 10.8 -- 0.85 _- 6500 ng/l C03 11.9 -- 1.60 _ _ 5500 s g / l C03 12.5 -- 49.6 -- 3500 m g i i ro:

Mixed Metals: 7.8-8.5 -- Cd 1.37 0.04 97.1 C r 0.67 0.60 10.4 cu 0.18 <O .03 .83.3 Pb 1.4 <0.1 >92.9 Zn 26 1.18 95.4

26

Su l f ide cu Prec ip i t a - t i o n

cu

cu

cu

cu

cu Cd

Zn

Zn

Zn

4.0 6.0 8.0

10.0 4.0 4.0 4.0 4.0 3.0 4.0 6.0 8.0

10.0 4.0

8.0

8.0 4.0-10.0

4.0 9.0

3.0 4.0 8.0

10.0 4.0 6.0 8.0

10.0 4.0 8.0

Mixed Metals: 8.0 Cd C r cu N i Zn

Mixed Metals: Zn 10.8 Fe Pb Zn 11.1 Fe Pb Zn 10.6 Fe Pb Zn 10.4 Fe Pb Zn 10.85 Fe Pb Zn 10.7

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 500 100 100

100 100 100 100 100 100 100 100 100 100

2.06 2.61 1 .a2 3.50 5.8

81 3 < O . l 23.5 81 3

<o. 1 23.5 81 3

< O . l 23.5 813

go.1 23.5 81 3

co.1 23.5 81 3

0.08 0.08 0.05 0.05 1.3 0.6 0.3 0.2 0.9 0.8 0.6 0.6 1 .o 0.08 0.85 0.65 0.25 0. I 5 0.05 0.7 0.4 0.1

0.5-1.0 0.01 1.2 0.16

12.0 0.3 0.2 0.15

16.5 15.0 17.8 12.0 8.0

12.0

0.10 0.32 0.04 0.07 0.41

6.66

0.09 6.74

<o. 1 g0.03

3.90 4 . 1 0.03 4.59

0.10 4.40

0.03 4.64

<o. 1

<0.1

co.1

99.92 99.92 99.95 99.95 98.7 99.4 99.7 99.8 99.1 99.2 99.4 99.4 99.0 99.92 99.15 99.35 99.75 99.85 99.95 99.3 99.6 99.9

>99.0 .99.99

98.8 99.84

88.0 99.1 99.8 99.85 83.5 85.0 87.2 88.0 92.0 88.0

95.1 87.7 97.8 98.0 92.9

99.2

99.6 95.2

>9Y.8 99.5

99.8 99.4

99.6 99.45

99.8 99.4

._

._

_ _

_ _

_ _

No chelants present, S==l.O5x

100 mg/l EDTA, S==l.Ox 100 mg/l Gluconic Afid. S==l.Ox 100 t g / l C i t ra te , 5 ~ 1 . 0 ~ 100 mg/l Tartrate. S-=I .Ox 100 mg/l EDTA. 5 =I.O5x

No chelants present, S==l.O5x 100 mg/l EDTA, S'= 1 . 0 5 ~ 100 W g / l C i t ra te , S'=1.05x 100 iwg/l t i luconic Acid. S - = 1 . 0 5 ~ 100 mg/l Tar t ra te , S =-l.O5x No chelants present. S==1.05x 100 mg/l €OTA, S-=1,05x 100 mg/l C i t ra te , S ~ 1 . 0 5 ~ 100 mq/l Tartrate, S-=l.O5x 100 m q / l EDTA. S =1.05x s==1 .bx 100 mal l EDTA. S==I.OX No cheiauts present, CaS p r e c i p i t a t i o n No chelants present. S==l.O5x

100 nig/l ,,EDTA. S==l.O5x

100 mq/l EDTA, Sz71.Ox 100 mg/l EUTA, S - 1 . 0 5 ~ Wastewater froin e lec t rop l a t ing and metal f i n i s h i n g operat ion a t For t Be lvo i r . VA

Pure hydroxide treatment

50 mg/ l 7s

100 mg/l,,FeS

500 mg/l ,,FeS

1000 mg/;, FeS

2000 m/l FeS

119

21

119

81

81

120 119 21 75

119

119

21 120 133

49

49

49

49

49

49 Fe Ph

<0.1 < O . l _ _ 23.5 0.04 99.8

No. 243, Vol. 81 191

Table 1. (cant.)

Techology Metal(s) pH I n i t i a l Metal. Residual Metal Removal Conmnts Reference Fmployed Concentration Concentration Eff ic iency ,% Su l f ide Mixed Metals: 8.0 S-=0.6x, f u l l scale p lan t 18 Prec ip l ta - Zn 30-60 -- 51-75

20-40 -- 97-99 -- 99 tion Pb

3-16 cu 3-5 -- 90 Cd

2 -4 -- 99 5-20 _- 2 -26

H9 Fe

Mixed Metals: Cd 8.5 10.5 0.6 94.3 wastewater; Hydroxide treatment cu 297 0.5 99.8 only Zn 85.5 3.1 96.4 Pb 39 0.7 98.2 Fe 149 ~ 0 . 5 >99.7 se 3.0 <1 .o >66.7 As 100 6.8 93.2

Cd 8.0-8.5 10.5 <o .os ~ 1 9 . 5 s== 5 mM cu 2 97 0.2 99.9 Zn 85.5 0.9 98.9 Pb 39 0.4 99.0 Fe 149 <0.5 >99.7 se 3.0 <1 .o >66.7 As 100 2.0 98.0

Copper smelting p l a n t scrubber 17

17

Cd 8.0-8.5 cu Zn Pb Fe se As

Cd 8.0-8.5 cu Zn Pb Fe se AS

Mixed Metals: cu 8.0 Zn cu 8.0 zn cu 8.0 zn cu 8.0 zn

10.5 297 85.5 39 149 3.0 100 10.5 297 85.5 39 149 3.0 100

100 100 100 100

100 100 100 100

Mixed Metals: Cd cu C r Pb zn

Mixed Metals: Zn cu C r N i zn cu C r Ni

zn cu C r Ni Zn cu C r N i

Zn cu C r Ni Zn cu C r N i

9.0 7.95

18.6 1.34 3.5

47.0

8.0 24.5 62.4 28.0

8.0 24.5 62.4 28.0

8.0 24.5 62.4 28.0

8.0 24.5 62.4 28.0

10.0 24.5 62.4 28.0

10.0 24.5 62.4 28.0

1.04

1.04

1.04

1.04

1.04

1.04

<O .05 0.1 0.05 0.4 <0.5

1.2 11.0

17

<0.05 <0.05 ~ 0 . 0 5 0.3

<0.5 1 .0

21 .o 0.4 0.3 0.3

15.0

0.4 27.0

0.5 74.0

<0.05 <0.05 <O .05 <0.5 ~ 0 . 0 5

2.4 1.45

13.6 0.36 1.70 1.325 8.8 0.30 0.40 1.40 9.6 0.30 0.39 1.475 8.0 0.20 0.5

14.4 2.35 0.15 0.53

12.2 2.05 0.175

>99.5 S== 8 nt4 >99.9

99.4 99.0

>9Y. 7 60.0 89.0

P99.9 >99.5 99.2

>99.7 66.7 79.0

>99.5 s== 12 mM

99.6 No EDTA. S== 1 . 0 5 ~ 120 99.7 99.7 100 mg/l EDTA, S'=l.O5x 120 85.0

73.0

26.0

99.6 200 mg/l EDTA, S==l.OSx 120

99.5 200 mg/l EDTA. S==0.8x 120

17

>99.4 >99.7 >96.2 ~ 8 5 . 7 >98.9

90.2 97.7 51.4 65.4

93.0 97.9 68.6 71.1

98.4 97.75 65.7 71.1

98.4 97.6 71.4 80.8 97.9 76.9 91.6 85.5 97.8 80.4 92.7 83.1

CaS p r e c i p i t a t i o n process 75

I n d u s t r i a l p l a t i n g wastewater Hydroxide treatment only 125

S== 0 . 8 3 ~ 125

s== 1.ox 125

s== l . l x 125

Hydroxide treatment only 125

S== 0 . 8 3 ~ 125

zn 10.0 24.5 0.605 97.5 s== 1.ox cu 62.4 12.4 80.1 C r 28.0 1.75 93.8 N i 1.04 0.175 83.1

125

zn 10.0 24.5 1.2 95.1 s== l . l x 125 cu 62.4 10.8 82.7 C r 28.0 1 .so 94.6 Ni 1.04 0.20 80.8

Separation of Heavy Metals AlChE SYMPOSIUM SERIES Table 1. (cont.)

Technology Metal ( 5 ) pH I n i t i a l Metal, Residual Metal Rewval Comnen t s Reference Employed Loncent r a t i on Concent r a t i o n Eff ic iency ,%

Su l f ide Mixed Metals: 8.0-8.4 Prec ip i ta - Cd 3.3 0.06 98.2 t i o n C r 0.52 <O .05 >90.4

cu 0.35 e0.03 >91.4 Pb 4.5 eo. 1 >97.7 Zn 93 0.68 99.3

FeS dose = 1 . 5 ~ 26

Hydmxide treatnlent only 91 Combined N i 6 Prec ip i ta t ion 7 Treatment 8

q 10 11

N i 6 7 8 9

10 11

6 7 8 9

10 11

6 7 8 9

10 11 6 7 8 9

10 11 6 7 8 9

10 11 6 7 8 9

10 11 6 7 8 9

10 11 6 7 8 9

10 11 6 7 8 9

10 11 6 7 8 9

10 11

N i 6 7 8 9

10 11

N i 6 7 8 9

10 11

N i

N i

N i

N i

N i

N i

N i

N i

N i

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

10 10 10 10 10 10

8.2 9.5 6.8 4.0 0.2 0.1 7.3 7.8 8.3 1.6 0.05 0.1 9.8 8.8 8.9 2.3 0.12 0.05 9.7 9.6 8.7 6.4 0.1 0.05 7.0 9.4 7.8 1.5 0.1 0.05 8.7 8.4 8.8 3.2 0.3 0.05

10.0 9.5 9.2 4.8 0.1 0.05 9.2 9.4 9.1 4.8 0.15 0.05 9.1 7.6 7.7 4.6 0.1 0.05

8.9 7.8 8.1 4.5 0.1 0.05 9.1 9.2 8.6 6.8 0.9 0.05

9.2 8.8 8.5 6.6 0.1 0.05

9.7 8.2 1 .o 3.1 0.1 0.05

18 5

32 60 98 99 27 22 17 84 99.5 99 2

12 11 77 98.8 99.5

3 4

13 36 99 99.5 30 6

22 85 99 99.5

13 16 12 68 97 99.5 0 5 8

52 99 99.5 8 6 9 52 98.5 99.5

9 24 23 54 99 99.5

11 22 19 55 99

'99.5 9 8

14 37 91 99.5

8 12 15 34 99 99.5

3 18 90 69 99 99.5

Hydroxide treatment. CT= 5Onq/l 91

Hydroxide treatment. CT= 100 mg/l 91

Hydroxide treatment, CT= 200 mg/l 91

5 = 5 mg/l. cT= 0 T

ST= 5 mg/l. CT= 50 ing/l

ST= 5 mg/l, CT= 100 mg/l

ST= 5 mg/l. CT= 200 mg/l

ST= 10 mg/1. CT= 0

ST= 10 w/l. CT= 50 nq/1

sT= 10 mg/l. cT= 100 mg/l 91

ST= 10 mg/l, CT= 200 mg/l 91

sT= 20 mg/l, cT= 0 91

91

91

91

91

91

91

7 ,

Table 1. (cant.) 1 93

Conbined Ni 6 10 9.3 7 ST= 20 ng / l . CT= 50 mg/l 91

Technology Metal(s) pH I n i t i a l Metal,, Residual Metal Reinoval Cmnents Reference Employed Concentration Concentrat i on E f f i c i e n c y ,%

P r e c i p i t a t i o n 7 10 6.1 39 Treatment 8 10 5.2 48

9 10 2.8 72 10 10 0.15 98.5 11 10 0.1 99

N i 6 10 9.5 5 ST= 20 mg/l. CT= 100 mg/l 91 7 10 9.2 8 8 10 9.1 9 9 10 3.4 66

10 10 0.15 98.5 11 10 0.05 99.5

Ni 6 7 8 9

10 11

Ni 6

10 10 10 10 10 10

8.5 9.0 8.8 5.4 0.3 0.05 8.2 8.2 4.9 6.7 7.6 9.2 9.5 8.8 9.9 9.6 9.3 6.9 9.5 9.0 8.8 8.8 7.7 0.5 0.35 0.35 2.0 1.7 0.2 0.3

2.8 1.9 0.3 0.22 6.8 2.0 0.12 0.20 0.20 0.10

co.10 co.10 co.10 co.10 <0.10 co.10 0.14 0.10

co.10 co.10 co.10 co.10 co.10 co.10 0.35

Cd=0.08:Ca=147.5 Cd4.01 ;Ca=129.5 CdcO.01 ;Ca=ll9.4 Cd<O .01 ;Ca=75 .O Cd:ND; Ca r39.0 Cd:NO; Ca = 4.6 Cd:ND; Ca = 3.0 Cd:NO; Ca = 3.1 CdcO.01 ;Ca=152.3 Cdc0.01 ;Ca=125.9 Cdc0.01 ;Ca=92.5 Cd:NO; Ca =51.7 Cd:NO; Ca =11.5 Cd:ND; Ca = 4.2 Cd:ND; Ca = 3.7 Cd:ND; Ca = 3.7 Cdq.01 :Ca -191.6 Cd:NO: Ca=156.7

15 sT= 20 mg/l . cT= 200 mg/l 91 10 12 46 97 99.5 18 18 51 33 24

8 5

12 1 4 7

11 5

10 12 12 23 95 96.5 96.5 80 83 98 97

72 81 97 97.8 32 80 98.8 98 98 99 >99 >99 >99 >99 >99 >99 98.6 99 >99 >99 >99 >99 >99 >99 93 92 299 ;99 >99 >99 >99 ? 99 >99 >99 > 99 >99 > 99 >99 >99 >99 >99 >99 >99

10 10 10 10 10 10 10 10 10 10 10

C = 0, Fe/Ni = 0 CT= 0. Fe/Ni = 0.5 CT= 0. Fe/Ni = 1.0 CT= 0, Fe/Ni = 2.0 CT= 50 mg/l. Fe/Ni = 0 CT= 50 mg/l, Fe/Ni = 0.5 CT= 50 mg/l, Fe/Ni = 1 .O CT= 50 mgll. Fe/Ni = 2.0 CT= 100 mg/l. Fe/Ni = 0 CT= 100 w/l, Fe/Ni = 0.5 CT= 100 mg/l, Fe/Ni = 1 .O CT= 100 n g / l . Fe/Ni = 2.0 CT= 200 mg/ l . Fe/Ni = 0 CT= 200 mg/l, Fe/Ni = 0.5 CT= 200 mg/l, Fe/ni = 1.0 CT= 200 mg/l, Fe/Ni = 2.0 CT= 0, Fe/Ni = 0 CT= 0, Fe/Ni = 0.5 CT= 0, Fe/Ni = 1.0 CT= 0, Fe/Ni = 2.0 C!= 50 m/l. Fe/Ni = 0

94

94

94

94

94

94

6

6

10 10 10 10 10 10 10

6

Ni 9

10 10 10 9 10 10 10

C'= 50 m g / l - Fe/Ni = 0.5 CT= 50 mg/l: Fe/Ni = 1 .O C+ 50 mg/l, Fe/Ni = 2.0

Ni 9 10 10 10

C = 100 mg/l. Fe/Ni = 0 <- 100 nig/l. Fe/Ni = 0.5 C 100 mg/l, Fe/Ni = 1.0 CT= 100 mg/l. Fe/Ni = 2.0 <= 200 mg/l. Fe/Ni = 0 C = 200 mg/l. Fe/Ni = 0.5 CT= 200 ng/l. Fe/Ni = 1 . O CT= 200 mg/l, Fe/Ni = 2.0 CT- 0. Fe/Ni - 0 CT= 0, Fe/Ni = 0.5 CT= 0. Fe/Ni = 1.0 CT= 0. Fe/Ni = 2.0 CT= 50 mg/l. Fe/Ni = 0 CT= 50 mgl l . Fe/Ni = 0.5 CT= 50 mg/l, Fe/Ni = 1 .O CT= 50 w/l, Fe/Ni = 2.0 CT= 100 mg/l. Fe/Ni = 0 C:= 100 mg/l. Fe/Ni = 0.5 CTz 100 mg/l. Fe/Ni = 1.0 5- 100 mg/l. Fe/Ni = 2.0 Cf 200 mg/l, Fe/Ni = 0 C = 200 mg/l. Fe/Ni = 0.5 CT= 200 mg/l , Fe/Ni = 1.0 CT= 200 mg/l. Fe/Ni = 2.0 FTSO = 20 mg/l Cai 9 1511 mg/l as CaCO3

Cai = 2 5 i mg/l as CaC03

Cai = 35: mg/l as CaC03

94

10 10 10 10 10 10 10 10

9 94

Ni 10 94

10 10 10 10 10 10 10 10 10 10 10 10

10 94

10 94

10 94

10 5.0 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1.0 1 .o 1 .o 1 .o 1 .o

Ni 10.0 Cd 6.90

7.32 7.60 7.76 8.04 9.44

10.38 10.84

Cd 7.42 7.45 7.66 7.96 8.66 9.74

10.62 11.01

cd 7.36 7.41

89 31

31

31

7.57 1.0 Cd:NOi Ca-117.0 >99 7.62 1.0 Cd:NO; Ca= 92.1 >99 7.64 1.0 Cd:ND; Ca-106.5 >99 8.03 1.0 Cd:NO; Ca- 37.1 >99 9.58 1.0 Cd:ND; Cam 10.3 '99

10.81 1.0 Cd:ND; Ca= 2.7 >99


Table 1. (cont.)

Technology Hetal(s) pH I n i t i a l Metal, Residual Etetal Removal Coiinlents Reference Einploved Concentration Concentration E f f i c i e n c y .%

Combined Cd 7.79 Prec ip i ta t ion 8.05 Treatment 9.42

10.96 11.30

Cd 7.82 8.21 8.82

10.47 11.07 11.39

Cd 7.67 7.97 8.38

10.09 10.66 10.95 11.30

Mixed Metals: Zn 8.0 N i Pb Cd cu Hg

Zn 9.5 E l i Pb Cd cu Hq

5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

50 15 15 15 15 2.9

50 15 15 15 15 2.9

Cd=O.O2, Ca=148.9 Cd-0.01. Ca=llO.O Cd<O.Ol, Ca138.7 Cd:ND, Ca=52.2 Cd<0.01, Ca-32.2 Cd=0.02, Ca=l15.8

Cd<0.01, Ca=40.5

Cd:ND. Ca=14.4

Cd=0.02. Ca=75.0

Cd:NO, Ca.11.2

Cd-0.02, Ca=10.6 Cd=O .03. Ca.208.1 Cd<0.01, Ca=119.4 Cd:NO, Ca.67.2 Cd<O.Ol, Ca=22.8 Cd-0.01, Ca.14.8 Cd<O.Ol, Ca=15.3 Cd:NO. Ca110.8

0.57 1.8 ~0.05 ~0.05 <O .03 <0.001

0.04 e0.05 <0.05 <O .05 <0.03 <0.001

98 Cai= 150 mg/l as CaC03 31 99

>99 >99 >99 98 Cai= 250 mg/l as CaC03 31 98

>99 .99 $99

98 97 Cai= 350 mg/l as CaC03 31

>99 > 99 299

99 >99 > 99

98.9 88.0

>99.7 >99.7 >99.8 >99.9

99.9 .99.7 >99.7 >99.7 >99.8 >99.9

153

153

Coagulation/ Cd 7.2 Floccu 1 a t ion 8.0

Pb 6-10 6-10 6-10

6-10 6-10

6-10

CrC6 5.5 6-10

Cr+3 6.5-9.3 6.7-8.5

9.2 optimum optimum

Mixed Metals: -- C r cu Fe Ni Zn

Mixed Metals: -- C r cu Fe N i 2n

Pb Mixed Metals: 6.5-7.0

cu Cd cr+3 Cr+6 Zn

Pb cu Mo ~ r + ~ Zn Ni

Mixed Metals5.5-7.0

co Mixed Metals: ---

Cd C r cu Pb Hg N i Aa

0.3 0.3 0.15 0.15 0.15 0.15 10 10

0.15 0.15 0.15 0.15 0.15

10 10

1.2 0.8 2.2 1.3 2.8

1.2 0.4 3.6 0.8 3.7

3.4 3.0 3.1 3.2 3.0 2.6

17 15 11 15 17 16 12

--- _-- --- --- --- --- --- --- --- --- --- __- __- _-- --- 0.3 0.4 1.9 0.9 0.9

0.2 0.2 1.1 0.4 1.3

0.5 0.2

1.2-1.7 0.2-0.4 0.9-2.3

1.7

0.7 1.8

0.3 2.6

11 .o 12.5

12

20 90

>97 >97 >97

80-90 > 95 80

-35 <10 '98 >90

78 >98 >98

75 50 14 31 68

83 50 69 50 65

85.3 93.3

.45.1 s37.5 ~ 2 3 . 3 34.6

95.9 88.0 0

98.0 84.7 26.3 0

FeS04 coagulat ion o f r i v e r water 43

FeSO coagulat ion o f r i v e r water 43 A l ~ m ~ c o a g u l a t i o n o f r i v e r water FeSO coagulat ion o f we l l water 43 A l ~ m ~ c o a g u l a t i o n o f we l l water FeSO coagulat ion o f w e l l water 43 A l ~ m ~ c o a g u l a t i o n o f we l l water

FeSO coagulat ion of r i v e r water 43 Alum4coagulation of r i v e r water

A1 uotcoagul a t i on Alum coagulation

A1 um4coagulat i o n

Primary treatment w i t h 18.3 mg/l f e r r i c i r o n + 0.32 mg/l P u r i f l o r A23 f loccu lan t

FeSO coagulat ion 43

FeSO coaqulat ion 43

38

Secondary treatment w i t h 150 mg/l 38 f e r r i c i r o n + 0.20 mg/l P u r i f l o c A23 f l o c c u l a n t

Alum Sulfate treatment 106

Alum Su l fa te treatment 106

A1 um coagulation 6 5.86 0.57 90.3

158.8 17.3 89.1 528:9 767.3 0.29

42.3 92 .O 78.2 89.8 0.20 > 31.0

1000 2.0 > 93.1 _ _ -- 28.79 Z i 550 300 M5.5

No. 243. Vol. 81 195 ' 0

Table 1 (cont.)

Technology Metal(s) pH I n i t i a l Metal Residual Metal Reilloval Comnennts Reference Employed Concent r a t i o n * Concentration* E f f i c iency ,%

Coagula- Mixed Metals: -- Sodium Aluminate Coagulation 6 t i o n l Cd 17.29 4.10 76.3 F 1 occu 1 - C r 48.1 3.8 92.1 a t i o n cu 536.1 57.9 89.2

Pb 72.3 5.9 91 .E 0.23 64.1 H9

Ni 1000 20.86 2.0 90.4

Zn 990 300 '69.7 A9

-_ - 0.64 ---

F lo ta t ion Pb 6.0 50 1.4 97.2 100 mg/l Feftt, 30 mg/l NLS, 155 6.5 50 0.0 100 A i r f l o r r a t e = 1.4 ml.sec, 7.0 50 - 0.1 99.8 0 M NaN03 7.5 50 0.0 100 8.0 50

Pb 6.0 50 6.5 50 7.0 50 7.5 50 0.0 50

Pb 6.0 50 6.5 50 7 .O 50 7.5 50 R.0 50

Pb 6.0 50 6.5 50 7.0 50 7.5 50 8.0 50

Pb 6.0 50 6.5 50 7.0 50 7.5 50

0.1 1.4 0.0 0.14 0.20 1 .I 1.5 0.10 0.42 0.56 7.8 1.3 1 .0 0.7 2.2 6.3 2.1 1.1 5.8 2.6

99.8 97.2 0.025 M NaN03

99.7 100

99.6 97.8 97.0 0.050 M NaN03 99.8 99.2 98.9 94.4 97.4 0.075 M NaN03 98.0 98.6 95.6

155

155

155

155

8.0 50 19.0 62.0

6.4 20 1.5 92.5 I n f l u e n t f low r a t e = 2 g a l h i i n , 6.6 20 0.9 95.5 A i r f low ra te = 84 m3/hr 6.7 20 0.5 97.5 6 .9 20 0.6 97.0 7.0 20 0.8 96.0 7.2 20 0.9 95.5

Pb 5.6 20 7.0 70.0 100 mg/l Fettt, 40-50 nig/l NLS, 155

Cd 8-10 1 .o ~ 0 . 0 5 295 Fe(0H) copreci p i t a t i o n 98 Hg 8-10 1 .o <0.03 .97 followsd bv f l o t a t i o n w i t h sodium

Ion Mixed Metals: 8.4 Exchange N i

cu

Cd Mixed Metals: - -

Ca C r cu Ni Fe Zn CN

Cd C r cu Fe Pb N i 2n

Cd C r cu Fe Pb N i Zn

Cd C r CU

Mixed Metals: 11.0

Mixed Metals: 11.0

Mixed Metals: 6.9

8.9 0.3

0.4 28.8 0.52 0.28 2.92 3.3

23.0 31.4

0.55 688

33.6 64.0

1.6 64.2

1050

<0.02

3.2 5.65 0.20 0.45

450

245

<0.02 0.33 2.1

0.16 98.2 0.02 93.3

0.0001 >99.9 1.637 94.3 0.356 31.5 0.41 0 0.425 85.4 0.195 94.1 2.62 88.6 3.0 90.4

q0.005 >99.1 <0.02 >9Y. 99 <0.05 >99.85 ~ 0 . 0 5 ~ 9 9 . 9 2 (0.05 >96.87 <0.05 >99.92 co.02 >99.99

<0.005 >75.0 <0.02 >99.99 <0.05 >98.43 < O . 05 .99.11 ~ 0 . 0 5 >75.0 <0,G? >88.88 c C . G ; .9Y.99

Ion exchange po l i sh ing system 44

Metal cyanide wastewater 44

Chrome f l o o r sump

Chrome r i v e r sump

Nickel r inse sump

165

165

165

_ _ Fe 0.8 .0.05 >93.75 Pb 2.0 ~ 0 . 0 5 >97.50 N i 133.75 <0.05 >99.96 Zn 30.0 <0.02 >99.93

Separation of Heavy Metals

Table 1 (cont.)


Technology Metal(s) pH I n i t i a l Metal Residual Metal Removal Comnents Reference Employed Concentration* Concentration* Eff ic iency,%

I o n Mixed Metals: 11.6 Zinc sump 165 Exchange Cd <0.01 <0.005 >50.0

C r 0.10 <0.02 >80.0 cu 3.24 <0.05 >98.46 Fe 0.08 <0.05 >37.5 Pb ~ 0 . 0 5 <0.05 --- Ni 1.08 <0.05 >95.37 Zn 19.0 <0.02 >99.89

L iqu id Ion N i 2;0 100 <1 > 99 Mixed nletal wastewater 76 Exchange cu 3.1 100 C1 > 99 containing 100 mg/l each of

Cenlentation Cu 2.1 100 89.3 11.7 7.25 mg/l Fe.27.9 min. reac t ion t ime 112 Cd 5.4 100 < I > 99 Cd, C r . Ni, and Zn

100 79.6 20.4 13.55 mg/l Fe.25.7 min. reac t ion tinie 100 56.3 43.7 36.75 mg/l Fe.25.8 min. reac t ion t ime 100 63.0 37.0 7.25 mg/l Fe.52.0 min. reac t ion time 100 12.9 87.1 36.75 mq/1 Fe.46.0 niin. react ion tinie

Electrochemical C r 4.0 195 0.1 99.94 30 min., cur ren t density-0.0085 Alcm2 127 Operations 6.8 180 0.04 99.97 50 min., cur ren t density=0.007 A/"

7.7 150 0.0 -100.0 30 min., cur ren t density=0.011 A/" 8.8 185 0.06 99.96 40 min., current density=0.012 A / c d 7.6 175 0.1 99.94 60 min., current density=0.0085 A / c d 8.9 188 0.18 99.90 50 niin., cur ren t density=0.011 A/"

cu --- 100 < l . O >99.0 F i e l d data f o r solut ions containing 146 EDTA. Quadrol . t a r t r a t e pyrophosphate, amnoniuni persu l fa te etch, and peroxide s u l f u r i c copper etch

cu --- 670 c1 .o >99.85 --- 13 cu _ _ _ 95.6 0.05 99.95 87 % cur ren t e f f i c i e n c y 7 cu -9 -230 -0 -100 -3.2 hours 47 Cu 3.0.9.0 >50 --- >80 Pr in ted c i r c u i t board wastewater 24 Zn _-- 180-300 --- >60 Zinc p l a t i n g l i n e 157

Cir.Pb -9 -150 -0 -1 00 -2.6 hours treatment 47 Mixed Metals:

~~~

B io log ica l Cu Operations Cu

Cd Cd

N i

Cd

cu

cu

--- ---

7 .O-8.0 e 4 5-6

' 7 10 7.0 8.0

7.2-7.4

7.2-7.4

7.2-7.4

50 0.4-25.0

5.16-10.4 5.16-10.4 5.16-10.4 5.16-10.4

__ -

_ _ - ---

1.9-2.3 2 .O-2.4

0.18 0.45 1 .u 1.1 1.2

0.18-0.45 1.0-1.2

- - - <0.1

0.2-0.5 0.10-0.14

0.05

80-85 50-79

- 90 < 20

50 > 80

90-99 -60 -90 -95 -85 - 40 -90

0.25 0.63 0.84

75 43 30

Adsorption Co

Zn

Cd

Cd

>9 >9 >8 8.5 9.0 9.5

10.0 10.5 11 .o 9.0 9.5

10.0 10.5 9.5

Cd

rr(

10.0 10.5

_-_ --- _ _ - 2.1 0.7 0.6 0.4 0.45 0.55 1.55 0.5 0.25 0.2 1.3 0.9 0.8 0.35

--- >80 --- >8U

0.25,,0.9 88.1,,57.1, 0.15,,0.25, 78.61~64.3, 0.10,,0.15, 83.3,,75.02 0.101,0.057 75.0, ,87.57 0.10, ,0.052 77.71.88.8, 0.12 ,0.05 78.2 .90.9

0.50 67.7 0.05 90.0

r - - 2 ?80 7

0.05 80.0 0.05 75.0 0.45 65.4 0.10 88.9 0.10 87.5 0.05 85.7

8.7

-_-

0.4 95.4

--- -100

Acclimated ac t iva ted sludge 97 Act ivated sludge 104 Suspended so l ids = 2.6 gm/l 78 Activate! sludge 51

Suspende! so l ids = 4.4 gm/l 70

5 mg/l Cd spike i n R8C operat ion 30 20 mg/l Cd spike i n RBC operation 1 mg/l Cu spike i n RBC operat ion 30 5 mg/l Cu spike i n RBC operat ion 10 mg/l Cu spike i n R8C operation 25 mg/l Cu spike i n RBC operation 50 mg/l Cu spike i n RBC operat ion Reniovals were 60-90% i n the 1s t 30 stage, 15% i n the 2nd stage, and 10% i n the 3rd staqe. 1 gui/l F i l t r a s o r b ac t iva ted carbon 58 1 gm/l Nuchar ac t iva ted carbon 1 gm/l Oarco ac t iva ted carbon Carbon adsorption po l i sh ing EO f o l 1 owing hydroxide prec i p i t a t i on treatnlent using 1.0 qm/l o f : 1.Darco 5-51 and 2.Calgon F300

Carbon adsorption po l i sh ing 80 fo l low ing hydroxide p r e c i p i t a t i o n treatment using 1.0 sm/l Oarco S-51 ac t iva ted carbon Carbon adsorption po l i sh ing 80 f o l 1 owing hydroxide prec i p i t a t i on treatment using 1.0 gm/l Oarco S-51 ac t iva ted carbon i n the presence o f 100 mg/l CN- Cedar wood f l o u r cooked w i t h a l k a l i fo r 30 min. t o convert the wood t o a l k a l i ce l lu lose Alaae used t o adsorb heavy 50

164

--- 98 i i s i a l s --- >80 Adsorption onto 0.5 gm/l y-FeOOH 12

-_ - >80 Adsorption onto Fe 0 (0 .Wl k ?e) --- >80 Adsomtion onto hv&rdus f e r r i c

>80 Adsorption onto 1.0 gm/l y-A1 0 ---

-- hydroxi de

Membrane Ni - - - _-- --- 90-97 Cel lu lose acetate membranes 29

cu --- -0.6 0.17-0.03 >71 --- 74 used i n reverse osmosis operation Operations

* A l l concentrations are reported i n mg/l unless otherwise noted.

No. 243, Vol. 81

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