PAOs Amb.pdf

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W A S T E M A N A G E M E N T Vol. 13, pp.361-377, 1993 0956-053X/93 6.00 + .00

Printed in the U.S.A. All rights reserved. Copyright 1993 Pergamon Press Ltd.

O R I G I N L C O N T R I B U T I O N

A D V A N C E D C H E M I C A L O X I D A T I O N I T S P R E S E N T

R O L E A N D P O T E N T I A L F U T U R E I N H A Z A R D O U S

W A S T E T R E A T M E N T

C. P. Huang Chengdi Dong, and Zhonghung Tang

Department of Civil Engineering University of Delaware Newark DE 19716

ABSTRACT.

Che mica l oxidation reactions involv ing hydrox yl radicals ha ve been extremely effective in the destruction of organic

pol lu tants. These advanced chem ical oxidat ion processes AO P) genera l ly use a combinat ion of oxidat ion agents such

as

H202 o r

O3), irradiation such as uv or ultrasound), and catalysts such as metal ions or photo catalysts) as a means to generate hydro xyl radi-

cal. Th e hyd roxy l radical is one o f the stronge st inorganic oxidan ts next to elemental f luorine. Th e hyd roxy l radical is stable ov er a

wide pH range , up to pH 10. The h ydroxy l radica l reac ts wi th orga nic by three major mechanisms: hy droxy addi tion, hydrogen ab-

straction, and elec tron transfer. Several AO P system s are reviewed first. Th e merits as well as l imitations o f these system s are dis-

cussed. The potential of AO P for future hazard ous wastes treatment is then dem onstrated by fou r A O P systems, H2 02/Fe 2+,

TiO2/uv/O2, H2Oz/uv/O2, and TiO2/uv/H202, exemplified by chlorophenols. A reaction sch em e can be generalized for the oxida-

tion of haloge nated phen ols by adva nced chem ical oxidation, specifically, o nes involvin g hyd roxy l radicals. Upo n the attack of a

haloge nated phenol, ArX nOH , by a hydrox yl radical, OH., a free radical, Ar OH)2Xn. , is forme d. This free radical can und ergo two

reaction paths: l)hy dro xyl atio n witho ut dechlorination Typ e A) and 2) hyd roxy lation with deehlorination Typ e B). I t has been

observed tha t mono -halogenated phenols n= l ) only fo l low Type A path; d ichlorophenols n = 2 ) and tr ichlorophenols n = 3 ) can

have both Type A and Ty pe B reac t ion pathways; te t rahalogenated n = 4) and pentahalogenated n = 5) phenols only fo l low Type B

reaction pathway.

I N T R O D U T I O N

O x i d a t i v e d e s t r u c t i o n o f c h e m i c a l s p r o v i d e s u l t i m a t e s o -

l u t io n s f o r t h e tr e a t m e n t o f h a z a r d o u s w a s t e s . O x i d a t i v e

d e s t ru c t io n o f o r g a n i c p o l l u ta n t s c a n b e a c c o m p l i s h e d b y

v a r i o u s m e a n s : b i o l o g i c a l , c h e m i c a l a n d p h y s i c a l t h e r -

m a l ) . W h i l e t h e re a r e d i f f e r e n c e s i n t h e m o d e o f o p e r a -

t i o n , th e b a s i c p r i n c i p l e o f e n e r g y d i s s i p a t i o n p r o c e s s ,

i n t ri n s ic t o o x i d a t i v e d e s t r u c t i o n o f o r g a n i c m a t t e r s , i s

l a r ge l y th e s a m e a m o n g t h e s e s y s t e m s .

T r a d i t i o n a l l y , a t t e n ti o n r e c e i v e d b y b i o l o g i c a l o x i d a -

t i o n p r o c e s s e s f a r e x c e e d s t h a t o f t h e c h e m i c a l a n d p h y s i -

c a l o x i d a t i o n s y s t e m s , d u e i n p a r t t o i ts c o s t e f f e c t i v e n e s s

a n d v e r s a t i l i ty i n h a n d l i n g a w i d e v a r i e t y o f o r g a n i c p o l -

l u t a n t s . S i nc e i ts i n t r odu c t i on i n t he 1900s , s e ve r a l b i o -

l o g ic a l tr e a t m e n t s y s t e m s h a v e e v o l v e d a n d b e e n a d o p t e d

*To wh om corresponden ce may be addressed.

Acknowledgements-This

work i s suppor ted by a grant No.

R815081) f rom the Explora tory Research Program, US

Env ironm ental Protection Ag enc y. Con tents of this publication

do not necessarily reflect the view s or policy of the fundin g

a ge nc y . Any c onc l u s i ons o r me n t i on o f c he mi c a l s a nd

processes are made by the authors only and should not be

impl ied as the i r endorsem ent by the funding agen cy.

361

f o r t h e t re a t m e n t o f c o m p l e x w a s t e s w i t h a g o o d d e g r e e

o f s u c c e s s . H o w e v e r , t h e p r e s e n c e o f t o x i c c h e m i c a l s i n

t h e w a s t e s t r e a m s c a n c a u s e u n w a n t e d o p e r a t i o n a l d i f f i -

c u l ti e s . M u c h h a s b e e n r e p o r t e d o n t h e d i f f ic u l t i e s i n t h e

o p e r a t i o n o f b i o l o g i c a l o x i d a t i o n s y s t e m s i n t r e a t i n g m u -

n i c i p a l w a s t e w a t e r l et a l o n e h a z a r d o u s w a s t e s . T h e s e

d i f f i c u l t i e s a r e i n t r i n s i c t o t he p r oc e s s i n t ha t i t i s s e ns i -

t i v e t o a m b i e n t c o n d i t i o n s a s w e l l a s w a s t e c h a r a c -

t e r is t ic s . M o r e o v e r , t h e g e n e r a l l y l o n g re t e n t i o n t im e a n d

s t a r t- u p t i m e o f t h e b i o l o g i c a l o x i d a t i o n p r o c e s s m a k e i t

u n a t t r a c t iv e f o r th e t r e a t m e n t o f t o x i c a n d r e f r a c t o r y o r -

g a n i c p o l l u t an t s .

T h e r m a l d e s t r u c t i o n o f s p e c i f i c c h e m i c a l s a t e x t r e m e

t e mp e r a t u r e , i .e . i nc i ne r a t i on , ha s be e n s uc c e s s f u l l y a p -

p l i e d t o t h e t re a t m e n t o f s o li d w a s t e , s l u d g e , a n d g a s e s .

W h i l e s i m p l e i n p r i n c i p l e , t h e r m a l s y s t e m s h a v e s u f f e r e d

f r o m m a n y d i f f i c u l ti e s i n o p e r a t i o n s . A d d i t i o n a l l y , t h e r -

m a l p r o c e s s e s a r e r a t h e r l i m i t e d i n t r e a t i n g a q u e o u s

w a s t e s a s c o n s i d e r a b l e e n e r g y i s r e q u i r e d t o h e a t a n d v a -

p o r i z e t h e w a t e r m a s s b e f o r e s p e c i f i c o r g a n i c p o l l u t a n t s

a r e d e s t r o y e d a t r e l a t i v e l y e l e v a t e d t e m p e r a t u r e s u s u a l l y

g r e a t e r t h a n 8 5 0 C ) .

T o o v e r c o m e t h i s d i f f i c u l t y , a t h e r m a l c a t a l y t i c o x i d a -

t i o n p r o c e s s , w e t - a i r o x i d a t i o n , h a s b e e n d e v e l o p e d . I n


2/17

36 C . P . H U A N G , C . D O N G , A N D Z . T A N G

w e t - a i r o x i d a t i o n , o r g a n i c c o m p o u n d s a r e d e c o m p o s e d

w i th a tmosphe re a i r o r pu re oxyge n a t 150 to 370C a nd

10 to 220 ba r p re ssu re . O rga n ic subs t r a te s a re f ir s t c a r -

b o n i z e d ( l ) . T h e d i s s o l v e d o x y g e n r e a c ts c a t a ly t i c al l y o n

t h e s u r f a c e o f t h e c a r b o n c e n t e r s o a s t o y i e l d h y d r o g e n

p e r o x i d e w h i c h t h e n d e c o m p o s e s t o f o r m o x y g e n a n d h y -

d roxy l r a d i c a l s . The s e r a d i c al s t he n re a c t w i th t he c a rbon

to y i e ld c a rbon d iox ide ( a u toc a t a ly t i c ox ida t ion ) . Sub -

s t r a t e s suc h a s a c e t i c a c id , w h ic h doe s no t c a rbon iz e be -

low 300C , w i l l be de g ra de d by w e t -a i r ox ida t ion .

A b ove 300C , a c e t i c a c id i s r e po r t e d to be ox id i z e d ve ry

s low ly . Subs t ra t e s suc h a s suga rs , w h ic h a re e a sy to c a r -

bon iz e , w i l l be c omple t e ly de g ra de d .

Se ve ra l be nc h sc a l e s tud ie s ha ve be e n c onduc te d to

de t e rmin e the su sc e p t ib i l it y o f spe c i f ic c ompo unds to

w e t - a i r o x i d a t io n . T h e f o l l o w i n g c o m p o u n d s c a n b e d e-

s t roye d by w e t -a i r ox ida t ion (2 ,3 ) : a l i pha t i c c ompounds

inc lud ing those w i th mu l t ip l e ha loge n a toms c a n be ox i -

d i z ed ; r e s id u a l o x y g e n a t e d c o m p o u n d s o f l o w m o l e c u l e

w e igh t suc h a s a l c oho l s , a lde hyde s , ke tone s , a nd c a rbox -

y l i c a ci d s m i g h t b e f o r m e d ; a r o m a t i c h y d r o c a r b o n s s u c h

a s t o lue ne a nd py re ne a re e a s i ly ox id i z e d ; ha loge na te d

a roma t i c s w i th a t l e a s t one nonha loge n func t iona l g roups

o f e l e c t ron dona t ing na tu re p re s e n t on the r i ng c a n be de -

s t roye d . The se subs t i tu t e s i nc lude hyd ro xy l , a mino , o r

me thy l g roup . H a lo ge na te d a roma t i c s suc h a s 1 ,2 -d ibe n -

z e ne a nd PC B s a re r e s i s t a n t t o de g ra da t ion un le ss c a t a -

ly s t s are e mp loy e d .

A sp e c i f i c c a t a ly s t sy s t e m ba se d on a c id i c so lu t ions o f

b r o m i d e , n i t ra t e , a n d m a n g a n e s e i o n s h a s b e e n d e v e l o p e d

(4 ) . K a to x p roc e ss i s a c a t a ly t i c ox ida t ion t e c hno log y

e m ploy ing spe c i a l c on ta c t me d ia . The o rga n ic s a re

b a t h e d i n o x y g e n - c o n t a i n i n g w a t e r a n d o x i d i z e d a t th e

su r fa c e ( i n pa r t i c u l a r i n t he i n t e rna l su r fa c e ) o f t he c a t a -

ly s t. C op pe r i ons ha ve a s ign i f i c a n t c a t a ly t i c e f fe c t .

M n ( V ) c a t a l y s t s a n d a c t i v a t e d c h r o m i u m c o m p o u n d s

ha ve a l so be e n inve s t iga t e d (5 ) . The ma jo r p rob le m s a s -

soc i a t e d w i th t he se p roc e sse s a re t he se pa ra t ion o f t he

ca ta lysts .

T h e w e t - a i r o x i d a t i o n p r o c e s s h a s g r e a t l y i m p r o v e d

the e f f i c i e n c y o f t he i nc ine ra t ion p roc e ss a s w e l l a s t he

e a s e o f o p e r a ti o n . H o w e v e r , d u e t o t h e s e v e r i ty o f t h e r e -

a c t ion , c ons t ruc t ion ma te r i a l s fo r t he sy s t e m b e c om e the

ma jo r c os t f a c to r . D ue m a in ly to it s h igh c os t , w e t -a i r

ox ida t ion p roc e ss r e m a ins a n ope ra t ion o f t he p re l imi -

na ry t r e a tme n t na tu re w i thou t t o t a l de s t ruc t ion o f t he o r -

ga n ic po l lu t a n t s .

More ove r , t he w e t -a i r ox ida t ion p roc e ss i s l im i t e d by

the so lub i l i t y o f oxyge n in w a te r . Th i s ga s (02 ) t o l iqu id

(H 20) ma ss l im i t a t i on c a n be e l imina t e d a t supe rc r i t i c a l

w a te r t e mpe ra tu re s . In t he supe rc r i t i c a l r e g ion , w a te r e x -

h ib i t s a de ns i ty o f 0 .1 t o 0 5 g /c m 3 a nd a d i e l e c t r i c c on -

s t a n t o f 3 t o 10. M os t impor t a n t ly , i t ha s a lmos t 100

so lub i l i t y o f o rga n ic s , be c a use the rma l e n e rgy c on ta ine d

i n t h e w a t e r m o l e c u l e r e d u c es h y d r o g e n b o n d i n g . S u p e r -

c r it i ca l w a t e r b e c o m e s a n e x c e l l e n t s o l v e n t f o r n o n p o l a r

o rga n ic s . The su r fa c e t e ns ion o f t he w a te r i s z e ro , w h ic h

a l low s oxy ge n to pe ne t ra t e e ve n the sma l l e s t po re s a nd

ox id i z e a ny o rga n ic ma te r i a l s p re se n t . W he n t e mpe ra -

tu re s r e a c h 500C , ne a r t o t a l i n so lub i l i t y o f i no rga n ic s

t a ke p la c e . The ma jo r a dva n ta ge s o f supe rc r i ti c a l w a te r

o x i d a t io n ( S C W O ) a r e: ( 1 ) e n h a n c e d s o l u b i l it y o f o x y -

ge n a nd a i r in w a te r a nd e l imina t ion o f t he ma ss t r a ns fe r

be tw e e n tw o pha se s ; (2 ) r a p id ox ida t ion o f o rga n ic s i n

sho r t r e s ide nc e t ime ; (3 ) c omple t e ox ida t ion o f o rga n ic s ;

(4 ) po te n t i a l r e mova l o f i no rga n ic c ons t i t ue n t s a s so l id s

o r b r i n e ; ( 5 ) r e c o v e r y o f t h e h e a t o f c o m b u s t i o n a s h i g h

te mpe ra tu re p roc e ss he a t o r pow e r . Thus , th i s p roc e ss

h a s b e e n s h o w n a s a v i a b l e t e c h n o l o g y f o r t h e t h e r m a l

ox ida t ion o f ha z a rdous w a s t e s (6 ). I t ha s be e n de m on-

s t r a t e d tha t SC W O proc e ss i s p romis ing in ox id i z ing

c onc e n t ra t e d ha z a rdous w a s t e s . H ow e ve r , l i ke the w e t -a i r

p roc e ss , t he se ve r i t y o f the sy s t e m ma k e s i t una t t r a c t ive

fo r the t r e a tme n t o f d i lu t e w a s t e s . In h i s r e c e n t s tudy ,

T h o r n t o n r e p o r t e d t h a t h i g h m o l e c u l a r w e i g h t p r o d u c t s

s u c h a s d i b e n z o f u r a n a n d d i b e n z o - p - d i o x i n m a y b e p r o -

d u c e d i n t h e S C W O t r e a tm e n t o f p h e n o l a n d t h a t c h l o r in -

a t e d d i b e n z o f u r a n a n d d i b e n z o - p - d i o x i n m a y b e

p rodu c e d (7 ) .

In l i gh t o f t he i nc re a s ing c onc e rn ove r t he c on ta mina -

t i on o f t he e nv i ronm e n t by ha z a rdous c he m ic a l s , t he re i s

g re a t ne e d to de ve lop innova t ive t e c hno log ie s fo r t he

sa fe de s t ruc t ion o f t ox i c po l lu t a n t s . The p roc e sse s m us t

be c os t e f f e c t ive , e a sy to ope ra t e , a nd c a pa b le o f a c h ie v -

ing a t o t a l o r ne a r - to t a l m ine ra l i z a t ion . Th i s ha s

p rompte d re se a rc he r s t o i nve s t iga t e i nnova t ive c he mic a l

ox ida t ion te c hno log ie s . Th i s r e po r t g ive s a n ove rv i e w o f

re c e n t de ve lopme n t s i n a dva nc e d ox ida t ion p roc e sse s ,

w i t h a n e m p h a si s o n t h e c h e m i c al s y st e m s . F o u r A O P

sys t e ms a re i l lu s t r a t e d u s ing c h lo rop he no l s .

A D V A N C ED C H E M I C A L O X I D A T IO N S Y S T E M S

The ox ida t ion po te n t i a l o f a n ox ida n t i s r e l a t e d to i t s ox i -

da t ion - re duc t ion po te n t i a l , E . A n ox ida n t w i th a h igh E

va lue i s a s t rong ox ida t ion a ge n t . Ta b le 1 li s ts t he E

va lue fo r a hos t o f ox ida t ion a ge n t s a ga in s t t ha t o f mo-

l e c u la r 02 (8 ) . A l l c he mic a l spe c i e s show n in Ta b le 1 a re

s t ronge r ox ida t ion a ge n t s t ha n 02 , a l t hough i t mus t be

me n t ione d tha t k ine t i c f a c to r s o f t e n ou tw e igh the rmody-

na mic p rope r t i e s i n c on t ro l l i ng the ox ida t ion r e a c t ions .

N e ve r the l e ss , a s t ronge r ox ida n t w i l l ge ne ra l ly e xh ib i t a

fa s t e r ox ida t ion r e a c tion . B a se d on th i s s imp le p re mise ,

F2 is the s t rong est amo ng the l i st . Ho we ver , F2, a ha lo-

g e n , m a y p r o d u c e h a l o g e n a t e d c o m p o u n d s d u r i n g t h e

ox ida t ion p roc e ss . A m ong the se ox ida n t s , 03 ha s re -

c e ive d the g re a te s t a t t e n tion . H ow e ve r , t he me c ha n i sm

of oz ona t ion w a s no t fu l ly unde r s too d un t i l the l a t e se v -

e n t i e s , w he n H o igne a nd c ow orke r s (9 ,10 ) f i r s t r e po r t e d

t h e m e c h a n i s m o f o z o n a t i o n , a l t h o u g h o v e r o n e h u n d r e d

pub l i c w a te r t r e a tme n t f a c i l i ti e s w e re bu i l t i n Eu ro pe du r -


3/17

A D V A N CE D C H E M I C A L O X I D A T I O N 3 63

TABLE 1

Oxidation Reduction Potentials of Oxidation Agents

(8)

Redox Reaction Eo (NHE)

volt, 25C

F2 + 2e = 2F- 2.87

OH + H + + e = H2 0 2.33

03 + 2H + 2e = 02 + H2 0 2.07

H202 + 2H + 2e = H30 2+ 1.76

MnO4- + 4H + + 3e = MnO2 + 2H2 0 1.68

HCIO2 + 3H + 4e = Cl + 2H 20 1.57

MnO4- + 8H+ + 5e = Mn 2+ + 4H 20 1.49

HOCI + H + 2e = CI + H20 1.49

C12 + 2e = 2CI- 1.36

HBr O + H + 2e = Br + H2 0 1.33

03 + H20 + 2e = 02 + 2O H 1.24

CIO2( g) + e = CIO2- 1.15

Br2 + 2e = 2B r 1.07

HIO + H + 2e = I + H2 0 0.99

CIO2( aq) + e = CIO2- 0.95

CI O + 2H2 0 + 2e = CI + 2OH- 0.90

H202 + 2H + + 2e = 2H 20 0.87

CIO 2 + 2H 20 + 4e = CI + 4OH- 0.78

BrO + H2 0 + 2e = Br + 4O H 0.70

12 + 2e = 21 0.54

13 + 3e = 31 0.53

IO + H20 + 2e = I- + 2OH 0.49

i ng the e a r ly se ve n t i e s (11 ) . I t w a s thou gh t t ha t mo le c u -

l a r oz on e w a s the ma jo r ox ida t ion spe c i e s . H o ig ne e t a l

(9 ,10 ) p ropose d tha t 03 c a n ha ve tw o re a c t ion mode s : d i -

r e c t oz ona t ion r e a c t ions a nd f r e e r a d i c a l ( suc h a s hy -

d roxy l r a d i c a l ) de c ompos i t i on r e a c t ions (9 ,10 ) . The

d i re c t 03 r e a c t ion invo lve s mo le c u la r 03 w h ic h i s h igh ly

se l e c t ive a nd p roduc e s a r e l a ti ve ly s low re a c t ion . The

typica l ra te constants a re on the ord er of 1-103 (M-1 sec-1) .

A s a r e su l t , t he re i s no c omple t e mine ra l i z a t ion a nd a

v a r i e t y o f i n t e r m e d i a t e s c o u l d b e f o r m e d ( 1 2 ) . T h e h y -

d r o x y l r a d i c a l f o r m e d d u r i n g t h e o z o n e d e c o m p o s i t i o n

r e a c t s m u c h m o r e r a p i d l y w i t h o r g a n i c c o m p o u n d s t h a n

TABLE 2

List of Typical AO P Systems

Homogeneous System

with irradiation

O3/ultraviolet (uv)

HzOz/uv

electron beam

ultrasound (us)

H202/us

uv/us

without irradiation

O3/[t202

O 3 / O H

HEO2/Fe2 (Fenton's)

Heterogeneous Systems

with irradiation

TiO2/O2/ultraviolet

TiO2/HEOE/ultraviolet

without irradiation

electro-Fenton

mo le c u la r 03 . The ra t e c ons t a n ts a re u sua l ly on the o rde r

o f 10s-10 l (M lse c z ) (13 ) . C he m ic a l ly , hyd ro xy l r a d i -

c a l s a re muc h l e ss se l e c t ive t ha n mo le c u la r 03 .

T h e c o n c e p t o f " a d v a n c e d o x i d a t io n p r o c es s e s " (A O P )

w a s e s t a b l i she d by G la z e e t a l (14 ,15 ) . A O P w a s de -

f ine d a s t he ox ida t ion p roc e sse s w h ic h ge ne ra t e hyd roxy l

ra d i c a l s i n su f f i c i e n t qua n t i t y t o a f fe c t w a te r t r e a tme n t

(14 ,15 ) . M a ny sys t e ms a re qua l i f i e d unde r th i s b roa d

d e f i n it i o n o f A O P . M o s t o f t h e s e s y s te m s u s e a co m b i -

na t ion o f s t rong ox ida n t s , e .g . 03 a nd H 202 , c a t a ly s ts , e .g .

t r a ns i t i on me ta l i ons o r pho toc a t a ly s t , a nd i r r a d i a t ion ,

e .g . u l t r a v io l e t (uv ) , u l t r a sound (u s ) , o r e l e c t ron be a m

(e b ) . Ta b le 2 li s ts t yp i c a A O P sys t e ms c u r re n t ly be e n

reported in the l i te ra ture .

H y d r o x y l R a d i c a l

The h yd ro xy l r a d i c a l is one o f t he mo s t r e a c t ive f r e e ra d i -

c a l s a nd one o f t he s t ronge s t ox ida n t s (16 ) :

OH + H + + e" = H2 0; E = 2 .33 v [1]

Thus , i n a c id ic c ond i t i ons , t he hyd rox y l ra d i c a l w i l l ha ve

high er oxid a t ion potent ia l . In basic so lu t ion , i .e . pH 11,

the hyd rox y l r a d i c a l a nd H 202 re a c t t o g ive a n ox ide a nd

a pe rox ide ion (17 ) . I r r a d ia t ion o f w a te r ge ne ra t e s O H

re la t ive ly i nde pe nde n t o f pH un t i l pH 10. The re a c t ion

o f O H w i t h o r g a n i c c o m p o u n d s c a n b e c l a s s i fi e d i n to

t h r e e m e c h a n i s m s : h y d r o x y a d d i ti o n , h y d r o g e n a b s tr a c -

t i on , a nd e l e c t ron t r a ns fe r . O rga n ic c omp ound s c on ta in -

ing a roma t i c sy s t e ms o r c a rbon -c a rbon mu l t ip l e bonds

unde rgo a dd i t i ona l r e a c t ion w i th O H due to t he r i c h n -

e l e c t ron c loud on the a roma t i c r i ng . Fo r e xa mp le :

OH . + C6H6 ~ .C6H6O H [2]

H ydroge n a bs t r a c t ion i s t he u sua l r e a c t ion w i th unsa tu -

ra t e d o rga n ic c ompounds :

O H . + C H 3 C O H 3 - + C H 2 C O C H 3 + H 2 0 [ 3]

E le c t ron t r a ns fe r i s u sua l ly found in r e a c t ions be tw e e n

hyd roxy l r a d i c a l a nd ino rga n ic i ons. Fo r e xa mp le :

F e 2+ + OH . --~ OH - + Fe 3+ [4]

Th i s i s impor t a n t t o t he Fe n ton ' s r e a ge n t r e a c t ion . S inc e


4/17

3 64 C . P . H U A N G C . D O N G A N D Z . T A N G

O H ha s h igh a nd ind i sc r imina t e r e a c t iv i ty , i t c a n r e a c t

w i th a lm os t a l l type s o f o rga n ic s , suc h a s e thy le n ic , l ip id ,

a roma t i c , a nd a l i pha t i c , a nd ino rga n ic s , suc h a s a n ions

a n d c a t io n s . T h e a d d i t i o n o f O H t o b e n z e n e i s f o u n d t o

b e v e r y f as t . T h e p r o c e s s p r o d u c e s t h e c y c l o h e x a d i e n y l

ra d i c a l w i th a n unpa i re d e l e c t ron . The d i e ny l r a d i c a l d is -

p r o p o r t io n a t e s t o a c o m p l e x m i x t u r e o f p r o d u c t s i n c lu d -

i n g p h e n o l , c y c l o h e x a d i e n e s , h y d r o c y c l o h e x a d i e n e s ,

b iphe ny l , a nd h yd ro xy la t e d b iphe no l s r t he r i ng i s

ope ne d , ox ida t ion by O H c a n re su l t i n t he f i na l p roduc t s

suc h a s C O 2 , H 20 th rough a se r i a l r e a c t ion :

d r o g e n p e ro x i d e . T h e d e p r o t o n a t e d f o r m o f h y d r o g e n

pe rox id e (H O 2- ) c a n re a c t w i th oz on e to p rodu c e oz on ide

(O 3-) , t he n hyd rox y l r a d i c a l (O H ) (27 ) . The se spe c i e s

in i ti a t e t he c ha in re a c t ion . A s a r e su l t, c omp ound s no r -

m a l l y r e f r a c t o r y t o o z o n a t i o n a l o n e a r e r a p i d l y c o n v e r t e d

to C O 2 a nd w a te r . A dd i t i ona l ly , po ly -c h lo r ina t e d

b iphe ny l (PC B s) , w h ic h a re ve ry s t a b l e t o oz one , a re de -

s t roye d ra p id ly t o l e ss t ha n 0 .1 ppm by the c ombina t ion

03 a nd uv me thod . Th i s p roc e ss w a s in i t i a l ly de ve lop e d

by H ous ton R e se a rc h Inc . a nd ha s be e n spe c i f i e d a s B e s t

P r a c t ic a b l e C o n t r o l T e c h n o l o g y C u r r e n t l y A v a i l a b le f o r

t h e t re a t m e n t o f P C B s b y t h e U . S . E P A .

H O O C - C 4 H ~ C O O H + O H .

+ H 2 O C O 2 + H 2 0

H O O C - C O O H

[Sl

H 2 0 2 / u v s y st e m U V p h o t o l y s is o f H 2 0 2 g e n e r a te s O H :

T h e a b o v e r e a c t i o n i n v o l v e s b o t h h y d r o x y a d d it i on a n d

hydro ge n a bs t r a c t ion fo r t he mine ra l i z a t ion o f be nz e ne .

T h e h y d r o x y l r a d i ca l c a n b e e f f i c i e n t l y g e n e r a t e d

f rom H 202 a nd c a t a ly s t s suc h a s t r a ns i t i on me ta l i ons

w i thou t uv i r r a d i at ion . O ne suc h sys t e m i s Fe n ton s r e -

a ge n t , i n w h ic h H 202 i s de c ompose d in to a hyd roxy l

ra d i c a l a nd hyd ro x ide ion in the p re se nc e o f Fe2+ ions.

F e n t o n s r e a g e n t w a s d i s c o v e r e d b y F e n t o n i n 1 8 94

(18 ,19 ) . I t w a s no t a c c e p te d w ide ly un t i l 50 ye a r s l a t e r in

o rga n ic c he mis t ry , w he re se l e c t iv i ty i s e mpha s i z e d , be -

c a u s e o f t h e u n s e l e c ti v e o x i d a t io n p r o p e r t y o f h y d r o x y l

r a d ic a ls . T h e u s e o f F e n t o n s r e a g e n t t o o x i d i z e t o x i c o r -

ga n ic s on ly be ga n a t t he l a t e s ix t ie s . The m a jo r a dva n -

t a g es o f F e n t o n s r e a g e n t a s a h a z a r d o u s w a s t e t r e a tm e n t

t e c hn o log y a re : (1 ) t he re a re no c h lo r ina t e d o rga n ic

c o m p o u n d s f o r m e d d u r i n g t h e o x i d a t i o n p r o c e s s es a s i n

c h lo r ina t ion o r oz ona t ion ; (2 ) bo th i ron a nd H 202 a re

c he a p a nd non tox ic ; (3 ) t he re i s no ma ss t r a ns fe r l im i t a -

t i on due to i t s homoge ne ous c a t a ly t i c na tu re ; (4 ) t he re i s

no l i gh t i nvo lve d a s c a t a ly s t so t ha t t he r e a c to r de s ign i s

mu c h e a s i e r t ha n those uv l i gh t sy s t ems .

D ue to t he se a d va n ta ge s , Fe n ton s r e a ge n t ha s be e n

w ide ly a pp l i e d in t he t r e a tme n t o f ha z a rdous o rga n ic s .

The d i r e c t ox ida t ion o f phe no l i c w a s t e s ha s be e n s tud ie d

by E i se nha ue r (20 ) . B i shop e t a l (21 ) ha ve inve s t iga t e d

t h e f e a s ib i l it y o f F e n t o n s o x i d a t io n o f r e f r a c t o r y o r-

ga n ic s i n mun ic ipa l w a s t e w a te r . R e c e n t ly , Fe n ton s ox i -

da t ion ha s a l so be e n u se d a s t he p re t r e a tme n t o f

nonb iode g ra da b le i ndus t r i a l w a s t e s t o r e nde r t he o rga n ic s

more b iode g ra da b le fo r bo th a e rob ic p roc e sse s (22 ) a nd

a na e rob ic p roc e sse s (23 ). F ina l ly , Fe n ton s r e a ge n t ha s

be e n a t t e mp te d to ox id i z e pe n ta c h lo rophe no l i n a s imp l i -

f i e d so i l sy s t e m (24 ) a nd po lyc h lo r ina t e d b iphe ny l s

( P C B s ) a n d c h l o r o b e n z e n e ( 2 5 , 2 6 ) .

H om oge ne ou s Sy s t e m s w i th I r r ad ia t ion

O J u v S y s t e m P h o t o l y s i s o f a q u e o u s o z o n e p r o d u c e s h y -

H 20 2 + hv = 2 O H [6]

S u n d s t r o m e t a l (28 ,29 ,30 ) ha ve s tud ie d the de s t ruc -

t i on o f i nd iv idua l a l i pha ti c a nd a roma t i c c om poun ds by

u v c a t a l y z e d o x i d a t io n w i t h h y d r o g e n p e r o x i d e . T h e r e -

su l ts de m ons t ra t e d tha t t he sy s t e m c a n de s t roy a w ide va -

r i e t y o f h a za r d o u s c o m p o u n d s p r e s e n t i n w a t e r a t l o w

c onc e n t ra t i on le ve l s . Fo r c h lo r ina t e d a l i pha t ic c om -

pounds , t he o rga n ic c h lo r ine w a s c on ve r t e d to c h lo r ide

ion , i nd i c a t ing tha t t he c h lo r ina t e d s t ruc tu re s w e re e f fe c -

t i ve ly de s t roye d . In t he c a se o f a roma t i c c ompoun ds ,

m a n y i n t e r m e d i a t e s w e r e f o r m e d w h i c h c o u l d b e e l i m i -

na t e d by e x t e nd ing the t r e a tme n t t ime . I t w a s fou nd tha t

be nz e n e a nd t r i c h lo roe thy le ne ha ve s imi l a r r e a c t ion r a t e s,

w i th t r i c h lo roe thy le ne (TC E) h a v ing a s l i gh t ly f a s t e r r a t e

tha n be nz e ne . The ox ida t ion r a t e i nc re a se s w i th i nc re a s -

ing hyd roge n pe rox ide a nd de c re a se s w i th i nc re a s ing o r -

ga n ic c onc e n t ra t i on . The rat e c ons t a n t fo r hyd roxy l

ra d i c a l a dd i t i on to be nz e ne r ing i s

7 8109

M-lsec ~ . The

a dd i t i on o f hyd ro xy l r a d i c a l t o TC E i s 4.O 109 M lse c ~

(31 ) . Thus , hyd r oxy l r a d i c a l a dd i t i on to be nz e n e is a bou t

tw ic e a s f a s t a s t ha t t o TC E. The T C E ox ida t ion r a t e

s h o u l d b e e x p e c t e d t o b e r e d u c e d b y h a l f a t th e c o n s t a n t

o r g a n i c c a r b o n c o n c e n tr a t io n . T h e d e c o m p o s i t i o n o f

H 2 02 b y u v h a s m u c h l o w e r e f f i c ie n t t h a n t h a t o f o z o n e

due to t he muc h low e r e x t inc t ion c oe f f i c i e n t o f H 202 by

uv pho to ly s i s .

Processes such as HzO2/uv, O3/uv, and

H202/O3/uv

h a v e b e e n s h o w n t o b e e f f e c t i v e f o r g r o u n d w a t e r d e c o n -

t a mina t ion a nd so i l r e me d ia t ion . B e c a use the a c t iva t ion

e ne rgy be tw e e n a c t ive r a d i c a ls , suc h a s hy d rox y l r a d ic a l s

a nd o rga n ic s i s c lo se t o z e ro , ox ida t ion oc c u rs r a p id ly .

H ow e ve r , t he se sy s t e ms a re no t w i thou t d i sa dva n ta ge s .

F i r s t , w he n the p roc e sse s i nvo lve oz one , ma ss t r a ns fe r

b e t w e e n g a s e o u s o z o n e a n d a q u e o u s s o l u t i o n b e c o m e s

the ma jo r l im i t ing f a c to r (31 ) . Se c ond , a l though H 202

c a n e l imina t e t he ma ss t r a ns fe r l im i t a t i on , t he de c ompo-

s i t i on o f H 202 by uv ha s be e n p rove d ine f f i c i e n t . Th i s i s


5/17

A D V A N CE D C H E M I C A L O X I D A T I O N

b e c a u s e t h e e x t i n c t i o n c o e f f i c i e n t o f H2 2 a t 254 nm i s

o n l y 1 9. 6 ( M ~ c m ~ ) (3 3 ) c o m p a r e d w i t h a b o u t 3 , 0 0 0 ( M

~sec~ ) f o r ozone ( 34) . T h i r d , t he uv pene t r a t i on dep t h i s

s t il l d i ff i cu l t to ma t ch t he r eac t o r s i ze. I f a p r ec i se ma t ch

cou l d no t be s a t i s f i ed , t he r eac t i on r a t e cou l d be r educed

due t o t he a r ea wher e uv i s no t i n t ens i ve enough t o acce l -

e r a t e t h e d e c o m p o s i t io n o f o x id a n t s. A t t h e s a m e t im e , a

s i g n i fi c a n t a m o u n t o f u v c o u l d b e a b s o r b e d b y w a t e r

mol ecu l es wher e t he l i gh t i n t ens i t y i s h i ghe r t han neces -

s a r y a n d t h e e n e r g y c o u l d b e w a s t e d i n t h e f o r m o f h ea t .

S i nce t he r eac t i on r a t e i s d i r ec t l y p r opor t i ona l t o l i gh t i n -

t ens i t y , t u r b i d i t y and co l o r w i l l a l so s i gn i f i can t l y r educe

t h e e f f i c i e n c y o f t h e s y s t e m s .

Electron-bea m eb) system.

I r r ad i a ti o n o f w a t e r w i t h

h i gh - ene r g y e l ec t r ons r e su l t s i n t he f o r m a t i on o f t he reac -

t i ve f ree rad i ca l s e - ( aq . ), H ' , and OH. T hese r ad i ca l s a r e

f o r m e d t h r o u g h t h e e x c i t e d s t a t e m o l e c u l e s s u c h a s

H 2 0 * , H 2 0 + , a n d e ' . W h e n t h e e x c it e d m o l e c u l e s a n d

e l ec t r ons i n t e r ac t and t r ans f e r t he i r ene r gy t o o t he r mol e -

cu l e s , s eve r a l s econdar y r eac t i ve spec i e s a r e f o r med .

T h e r e l a t i v e e f fi c i e n c ie s ( G ) a r e d e f i n e d a s t h e n u m b e r o f

r ad i ca l s , exc i t ed s t a t e s , o r o t he r p r oduc t s f o r med o r l os t

in a s y s te m a b s o r b i n g 1 00 e V o f e n er g y . A c c o r d i n g t o

t he i r p r oduc t i on abundance , t he f o l l owi ng o r de r ex i s t s :

'OH ( 2 .7 ) = H 30 + ( 2 .7 ) > e ( 2 .6 ) > H202 ( 0 .71)

> H' (0.55) I7]

T he e and H a r e t he r educ i ng spec i e s . O f t hese rad i ca l s ,

e a n d O H m a k e u p 9 0 % o f t h e r e a c ti v e sp e c i e s a n d a r e

t he ma j or r ad i ca l s t o r ende r o r gan i c des t r uc t ion . H202 is

a l so an i mpor t an t spec i e s i n ox i d i z i ng t ox i c o r gan i cs , a l -

t hough i t i s no t a s abun dan t a s t he o t he r t wo spec i e s . T he

i n t e r ac t i on be t ween aqueous e l ec t r ons and spec i f i c o r -

gan i cs and i nor gan i cs has been ex t ens i ve l y s t ud i ed ( 35) .

T he e ( aq . ) i s a power f u l r educ i ng agen t w i t h a po t en t i a l

E o f 2 .77 v . a s :

e + H + = 1/2H2 [8]

H + 0 2 = H O 2 ; k = 2 .1 1 0 1 0 ( M l s e c 1 )

365

[ ]

Ultrasound us) system. Ul t r a sound ( us ) can a f f ec t o r -

gan i c ox i da t i on t h r ough t h r ee mechan i sms : nuc l ea t i on ,

g r owt h , and cav i t a t i on . Cav i t a t i on whi ch i s c r ea t ed by

t h e c o l l a p s e o f a g a s - o r v a p o r - f i l le d b u b b l e i n a b o d y o f

l i qu id i s t he mo s t imp or t an t phen om eno n . T he i ns t an ta -

neous p r es su r e and t emp er a t u r e a t the cen t e r o f t he co l -

l aps i ng bubb l e has been e s t i ma t ed t o be abou t 75 ,000 ps i

and 13 ,000 F r e spec t i ve l y (36) . Due t o t h i s h i gh l oca l

p r es sur e and t emper a t u r e , i t has been r ecogn i zed t ha t

cav i t a t i on can enhan ce t he r a t e o f chem i ca l r eac t i ons s i g -

n i f i can t l y . Nex t , so l ven t w i l l be compr es sed and r e -

f r a c te d b y t h e r a p i d m o v e m e n t o f f l u id s c a u s e d b y a

va r i a t i on o f son i c p r es sur e . T hen , a l a r ge amo unt o f en -

e r gy wi l l be pu t i n t o a sma l l vo l ume wi t h l i t t l e hea t i ng .

T h e p h e n o m e n o n i s c a l le d m i c r o s t r e a m i n g .

U S r a d i a t i o n i s k n o w n t o d e c o m p o s e w a t e r v a p o r

mol ecu l es i n t he bubb l es i n t o f r ee r ad i ca l s , such a s hy-

d r o x y l r a d i c a l s , h y d r o g e n r a d i c a l s , a n d h y d r o p e r o x y l

r ad i ca l s ( 37 ,38) . P r asad et al . s t ud i ed t he l i be r a t ion o f a

ch l o r i de i on i n a s a t u r a t ed aqueo us so l u t i on o f ch l o r ob en-

z e n e a n d m - d i c h l o r o b e n z e n e b y u l t r a s o u n d ( U S ) ( 4 0 ) .

M o r e t h a n 9 0 % d e c h l o r in a t i o n o f c h l o ro b e n z e n e a n d m -

d i ch l o r obenzene occur s i n 60 and 100 mi nu t es , r e spec -

t i ve l y (40) . I t i s be l i eved t ha t t he hydr ox y l rad i ca l

f o r med by t he r eac t i on be t ween H and O2 i s r e spons i b l e

f o r th e o x i d a t i o n o f c h l o r o b e n z e n e a n d m - d i c h l o r o b e n -

z e n e i n w a t e r. A l t h o u g h h y d r o g e n p e r o x i d e c a n b e p ro -

d u c e d b y a p p l i c a t io n o f U S a l o n e t o a d i l u te d a q u e o u s

so l u t i on , t he amount may be t oo sma l l t o be s i gn i f i can t .

H y d r o g e n p e r o x i d e c a n b e a d d e d t o t h i s p r o c e ss a s a n i n -

i t i a t o r t o i nc r ease f r ee r ad i ca l concen t r a t i on i n t he so l u -

t ion.

H202 /us sys t em. C h e n et al . ( 41) i nves t i ga t ed t he ox i da -

t io n o f c h l o r o f o r m b y u s / H 2 0 2 p r o c e s s . T h e o p t i m a l m o -

l a r r a ti o s o f H 2 0 2 t o C H C I 3 a r e b e t w e e n 3 0 a n d 5 0.

W h en t he r a t i o o f H2 02 t o CH CI 3 i s 50 : 1 , t he bes t pe r -

c e n t a g e r e m o v a l ( 9 4 % ) w a s a c h i e v e d . T h e o v e r a ll re a c -

t i on can be p r esen t ed a s f o l l ows :

C H C I 3 + 2 ' O H

~ C O 2

+ 3Cl + 3H +

[1 ]

T h e a q u e o u s e l e c t r o n s c a n r e s u lt i n t h e d e h a i o g e n a t i o n o f

o r g a n o h a l o g e n c o m p o u n d s . F u r t h e r r e a c t io n o f t h e o r -

gan i c r ad i ca l s l eads t o t he comp l e t e des t r uc t i on o f t he

c o m p o u n d s . H y d r o g e n r a d i c al s c a n u n d e r g o h y d r o g e n

add i t i on o r hyd r ogen abs t r ac t i on . T hes e r eac t i ons a r e

c o n s i d e r e d a s t h e m i n o r o n e s d u e t o t h e f o l l o w i n g e x -

t r e m e l y f a s t r e a c ti o n b e t w e e n t h e h y d r o g e n r a d ic a l a n d

o x y g e n :

T he r eac t i on i s fi r s t o r de r w i t h a r a t e cons t an t o f 0 .0177

( l / mi n ) . S i nce t he concen t r a t i on o f hyd r oxy l r ad i ca l s

g e n e r a t e d i s m u c h h i g h e r t h a n t h a t o f c h l o r o f o r m , t h e o r -

de r o f r eac t ion w i t h r e spec t t o t he f r ee r ad i ca l i s ze ro .

Ho wev er , the op t i ma l r a t i o o f 50 :1 f o r H202 t o ch l o r o-

f o r m i s qu i t e h i gh f o r p r ac t ica l app l i ca t i on .

I t a p p e a r s t h a t a n e x c e s s a m o u n t o f H 2 0 2 w o u l d


6/17

66

s l ig h t ly r e t ar d t h e d e c o m p o s i t i o n o f c h lo r o f o r m . T h a t

e xc e s s H 202 c a n re t a rd the ox ida t ion re a c t ion ha s be e n

r e p o r t e d b y H o

e t a l

i n t h e s t u d y o f p h o t o o x i d a t i o n o f

2 ,4 -d in i t ro to lue ne in the p re s e nc e o f H 202 (42 ) . Th i s i s

be c a us e H 202 i t s e l f is a l s o a hyd roxy l r a d ic a l s c a ve nge r .

U S / u v s y s t e m U l t r a s o u n d ( u s ) c a n b e c o m b i n e d w i t h u v

t o d e s t r o y t o x i c o r ga n i c s. T h e c h e m i c a l c o n s e q u e n c e o f

pho to ly s i s a nd s o no lys i s o f a s imp le ha loa lka ne i s t he

c l e a va ge o f t he c a rbon -ha lo ge n bond in l ibe ra t ing ha lo -

ge n ions (36 ) . The p r inc ipa l p roduc t s o f w a te r unde r

s o n o l y si s a r e H 2 0 2 , H 2 , O H , a n d "H . W h e n s o n o c h e m i -

c a l de c om pos i t ion o f a que ous C C 14 s o lu t ion oc c u rs , the

ra d ic a ls s uc h a s " O H a nd .H re a c t w i th C C 14 a nd fo rm

C12, CO2, HC1, C2C16, and H OC (39, 43 , 44) .

G C /MS a na lys i s i nd ic a t e s t ha t o l e f in s s uc h a s

C H 2 = C H C I a n d C H C I = C H C I w e r e f o r m e d d u r i n g th e d e -

c o m p o s i t i o n o f CC13CH3. In a n ox ida t ive e nv i ronme n t ,

e .g . i n t he p re s e nc e o f H 202 , o l e f in s a re r e a d i ly ox id i z e d

to e pox ide s . B o th o l e f in s a nd e pox ide s a re r e a c t ive

m o n o m e r s w h i c h p o l y m e r i z e to p o l y m e r s i n t h e p re s e n c e

o f O H a n d C I O r a d ic a l s. T h e p H o f p h o t o s o n o l y s is is

ve ry a c id i c in the r a nge o f 1 . H ow e ve r , l e s s a c id i c s o lu -

t i o n s o f p H 3 w e r e f o u n d u n d e r s o n o l y s i s a n d p h o t o ly s i s

separa te ly .

T h e o r d e r o f C C I 3C H 3 d is a p p e a ra n c e a n d C I a p p e a r -

a nc e i s t he fo l low ing fo r ox ida t ion o f immis c ib l e

CCI3CH3 in water : Ph oto son olys is ---> Pho tolys is --->

Sono lys i s .

Fo r a m is c ib l e s a mple o f C C I3C H 3 in w a te r , t he e f f i -

c i e nc y fo r d i f fe re n t p roc e s s e s fo l low s the d i f fe re n t o rde r :

Ph otos ono lys is ---> So noly s is ---> Pho tolys is .

The a que ous s o lu t ions e xh ib i t l ow e r va po r p re s s u re

tha n the h ighe r c onc e n t ra t ion o f t he immis c ib l e pa i r. The

low e r va po r p re s s u re o f t he l i qu id c a us e s h ighe r c a v i t a -

t i on bubb le c o l l a ps e in t e nsi ty . S inc e the t e mpe ra tu re a nd

the p re s s u re c a n re a c h to thous a nds o f K e lv in s a nd A t -

mos ph e re s , r e s pe c t ive ly , r e s pe c t ive ly , t he inc re a s ing

c a v i t a t ion in t e ns i ty i s c ons ide re d a s t he mos t impor t a n t

fa c to r t o e nha nc e the d i s s oc ia t ion o f t he c ova le n t bonds

o f t h e o r g a n i c s a n d t h e i r e x t e n t o f d e c o m p o s i t i o n ( 3 6 , 4 5 ).

H o m o g e n e o u s S y s t e m s w i t h o u t I r ra d i a ti o n

O Y H 2 0 2 s y s t e m

B u h l e r

e t a l

( 2 7 ), F o r m

e t a l

(46 ) , a nd

Se he s t e d e t a l (47 ) ha ve re po r t e d tha t H 202 c a n in i t i a t e

t h e d e c o m p o s i t i o n o f 0 3 b y s i n g l e e l e c t r o n t r a n s f e r

w he re in the in i t i a t i ng s pe c ie s i s t he hyd rope rox ide ion

H O 2- :

H2 02 = HO 2- + H+; Ka = 1 .6 10 -12 [11]

C . P . H U A N G , C . D O N G , A N D Z. TA N G

T h e h y d r o p e r o x i d e i o n r e a ct s w i t h o z o n e t o p r o d u c e t h e

o z o n i d e i o n 0 3 - a n d h y d r o p e r o x i d e r a d i ca l H O 2 .

H O 2- + 03 - --> 03 - + H O 2;

k = 2 .8 '106 M ' l s e c -1

[121

T h e s e p r o d u c t s c a n f o r m O H r a d i c a l s t h r o u g h t h e f o l -

lowing in i t ia t ion s teps :

HO2 = H + + 02- ; Ka = 1 .6"10 5 M [13]

02 - + 03 ~ 03 - + 02 ; k = 1 .6 '109 M ' l s e c 1 [14 ]

O3- + H + ---> HO 3; k = 5.2"1010 M 'ls ec -1 [15]

HO 3 ~ .O H + 0 2 ; k = 1.1"105sec "1 [16]

O nc e the hyd roxy l r a d ic a l is fo rme d , t he fo l low ing

propa ga t ion s t e ps ge ne ra t e hyd rox y l r a d ic a l s by a u toc a ta -

ly t i c me c ha n i s m:

03 + 'HO ---> 02 + "HO2;

[17]

0 3 - + " H O 2 ~ 2 0 2 + H O

[181

T h e c h a i n m e c h a n i s m g e n e r a te s h y d r o x y l r ad i c a ls b y

c ons um ing H 202 a nd 03 . I t i s t e rmina te d by re c ombina -

t ion o f d i f fe re n t r a d ic a ls . H y drox y l r a d ic a l s a bs t ra c t a

h y d r o g e n a t o m f r o m s a t u r a t e d h y d r o c a r b o n c o m p o u n d s

o r a dd to uns a tu ra t e d o rga n ic s to fo rm a n o rga n ic c a rbon -

c e n te re d ra d ic a l i n l e s s t ha n

10 6

s e c o n d . T h e c a r b o n -

c e n te re d ra d ic a l r e a c t s qu ic k ly w i th oxyge n to y i e ld a

pe rox y l r ad ic a l w h e n oxy ge n i s p re s e n t . Th i s r a d ica l c a n

d e c o m p o s e u n i m o l e c u l a r l y to p r o d u c e a s u p e r o x i d e io n

(O 2-, t he de p ro ton a te d fo rm o f t he H O z ra d ic a l ). P re -

d o m i n a n t h y d r o g e n p e r o x i d e a n d m i n o r s u p e r o x i d e c a n

a l so b e f o r m e d i f t h e c a r b o n - c e n t e r e d r a d ic a l d e c o m p o s e s

b imo le c u la r ly . The l a t te r p roc e s s i s c ons ide re d to t a ke

p la c e th rough a t e t rox ide st ruc tu re . Supe ro x ide re a c t s

q u i c k l y w i t h o z o n e , y i e l d i n g o z o n i d e a n d c o n t i n u in g t h e

c ha in re a c t ion . W h e n oz o ne i s p re s e n t , t h i s r e a c t ion i s

muc h fa s t e r t ha n s upe rox ide d i s p ropo r t iona te to hyd ro -

g e n p e r o x i d e .


7/17

A D V A N CE D C H E M I C A L O X I D A T IO N

03/01-1- system.

A t n e u t r a l t o h i g h p H , o z o n e d e c o m -

poses i n t o hydr oxy l r ad i ca l s t h r ough t he f o l l owi ng i n i t i a -

t ion s teps (9, 48) :

03 + OH --~ -O2- + 02 [19]

0 3 + H 2 0 - -~ 2 H O + 0 2

[ Ol

T he above i n i t i a l s t eps r e su l t i n t he hydr oxy l r ad i ca l s

as the m a j or ox i da t i on spec i e s . T h i s i s t he r eason t ha t a l -

k a l i n e o x i d a t i o n r a t e s b y o z o n e a r e s e v e r a l o r d e r s o f

m a g n i t u d e h i g h e r t h a n t h o s e in a c id i c m e d i a ( 4 9) . S e v -

e r a l i nves t i ga t o r s c l a s s i f i ed t he ozona t i on a t a l ka l i ne con-

d i t i on a s adv ance d ox i da t i on p r oces s ( 50) . T he ca t a l y t ic

e f f e c t o f a h y d r o x i d e i o n h a s b e e n r e c o g n i z e d f o r a l o n g

t i m e i n t h e o z o n a t i o n o f o rg a n i c p o ll u ta n t s . N i e g o w s k i

r epor t ed pH of 11 .8 a s t he op t i ma l va l ue ( 51) . Jones i n -

d i c a te d t h a t p h e n o l o x i d a t io n p r o c e e d s m o s t e f f i c i e n tl y a t

a pH of 11 .4 ( 52) .

H202/Fe2+ (Fenton s Reagen 0 system. The p o w e r f u l

o x i d i z in g p r o p e r t i e s o f a m i x t u r e o f H 2 0 2 a n d F e 2+ salts

was f i rs t obs e r ve d by F en t on a t t he end o f t he l a s t cen-

t u r y (18 ,19) . F or t y yea r s la t e r, Ha ber and W ei s s e s t ab-

l i shed t he ox i d i z i ng spec i e s a s hyd r oxy l r ad i ca l ( 53) . I n

r e c e n t y e a r s , t h e h y d r o x y l r a d i c a l h a s b e e n o b s e r v e d d i -

r e c t l y b y e l e c t r o n s p i n r e s o n a n c e s p e c t r o s c o p y ( 3 0 ) .

F r o m m u c h l it e ra t u re , o r g a n i c c o m p o u n d s o f n e a r l y a l l

t y p e s c o u l d b e o x i d i z e d b y t h i s F e n t o n ' s r e a g e n t .

F en t on ' s r eac t i on i s g i ven by E q . [ 21] ( 54 ,55) :

F e 2 + + H 2 0 2 = F e 3 + + O H + O H [2 1]

2 F e 2 + + H 2 0 2 + 2 H + = F e 3 + + 2 H 2 0

367

[231

T hi s equa t i on i nd i ca t e s t ha t F en t on ' s r eac t i on i s s t r ong l y

depen den t on so l u t i on pH. I n fac t , on l y i n ac i d i c cond i -

t io n s , i s O H t h e p r e d o m i n a n t r e a c t iv e o x i d a n t.

I n th e p r e s e n c e o f o r g a n ic c o m p o u n d s , o r g a n i c o x i d a -

t io n p r o c e e d s v i a a d d i ti o n o f O H , o r v i a h y d r o g e n a t o m

abs t r ac t ion . E q . [ 24] i s f o r O H add i t i on :

- OH + R ~ .RO H - -+ hyd r oxy l a t ed p r odu c t s [ 24]

E q . [ 25] i s f o r hydr oge n a t om abs t r ac t i on :

O H + R ~ ' R + H 2 0 ~ o x i d i z e d p r o d u c t s [ 2 5 ]

I n bo t h equa t i ons , t he ox i da t i on p r oces s i s ex t r eme l y

fast. Th e rate constants are as high as 107 - 10 m (M-~sec ) (5).

Or gan i c f r ee r ad i ca l s a r e f o r med as t r ans i en t i n t e r medi -

a t e s . T he i n t e r medi a t e r ad i ca l s a r e f u r t he r ox i d i zed by

F e 3+, oxy gen , H202, OH , o r o t he r i n t e r m edi a t e s o r ad ded

r eagen t s t o f o r m a f i na l, st ab l e , ox i d i ze d p r oduc t . T he

f o l l owi ng equa t i ons i l l us t r a t e t hese p r oces ses :

Fe 3+ + R - -~ Fe 2+ + prod ucts [26]

2 ' R - + R - R ( d i m e r i z a ti o n )

[27]

Fe 2+ + R + H + --~ Fe 3+ + R H ( redu ct ion) [28]

T he p r o duc t s o f the r eac t i on a r e f e r r i c i on , t he hyd r oxy l

r ad i ca l ( OH ) , and t he hydr o x i de i on . T he r a t e i s f i r s t o r -

de r w i t h r a t e cons t an t o f k j = 76 ( M l se c ~) ( 54 ,55) .

In the absence of added substrate, the hydroxyl radical will oxidize a

second molecule of ferrous ion, as shown in Eq. [22]:

F e 2 + + O H = F e 3+ + OH

[22]

F en t on ' s ox i da t i ons can be c l a s s i f i ed i n t o t wo gr oups :

( 1 ) cha i n r eac t ions , i n whi ch on l y a sma l l amo unt o f r e -

duc i ng agen t i s needed , and ( 2 ) non- cha i n r eac t i ons , i n

whi ch a l l t he ox i da t i on i s e f f ec t ed by t he hydr oxy l r ad i -

ca l and t he r e i s cons i de r ab l e l os s o f t he hyd r oxy l r ad i ca l

due t o t he f o l l owi ng r eac t i on :

F e 2 + + O H ~ ( F e- O H ) 2+

[29]

T he r a t e cons t an t i s k2 = 3108 M l s e c q ) ( 54 ,55) . T he ex-

pec t ed s t o i ch i om et r y o f the r eac t i on i s g i ven i n E q . [ 23].

P r o t o n s h a v e b e e n a d d e d t o s h o w f o r m a t i o n o f w a te r :

T h e d e c o m p o s i t i o n r a t e o f h y d r o g e n p e r o x i d e r e a c h e s

t h e m a x i m u m a t a p H o f 3 .5 . T h i s p h e n o m e n o n i s a t tr i b-

u t ed t o t he p r ogr es s i ve hydr o l ys i s o f t he f e r r ic i on , whi ch

pr ov i des a r e l a t i ve l a r ge ca t a l y t i ca l l y ac t i ve su r f ace f o r


8/17

3 68 C . P . H U A N G , C . D O N G , A N D Z . T A N G

cont ac t w i t h t he H 2 0 2 . T he acce l e r a t o r i n H202 decom-

pos i t i on wi l l y i e l d more f r ee hydroxy l r ad i ca l s .

I f a s m a l l a m o u n t o f

F e E

i on i s needed , a cha i n r eac -

t i on occ ur s t h ro ugh r egene ra t i on o f F e E+ of E q . [ 6 ]. I f a ll

t h e o x i d a t i o n i s e f f e c t e d b y h y d r o x y l r a d i c a l f o r m e d b y

reac t i on 1 , non-ch a i n r eac t i ons t ake p lace . S i nce t he r e

a r e ex t r a F e 2+ p resen t , cons i de rab l e l os s o f t he hy drox y l

r ad i ca l p roceeds v i a equa t i on 2 .

A s t h e p H i s r a is e d , t h e re i s s o m e e v o l u t i o n o f o x y g e n

due t o t he r eac t i ons :

H O - O H = H O - O + H + [ 3 1

T he r eac t i on a r i s e s f rom t he na t u r e o f H202 b e c a u s e

H 2 0 2

i s an ex t r e me l y wea k ac i d (ka = 1 .55

x

10 12 at

2 0 C ). I t s h y d r o x y l g r o u p s g e n e r a l ly b e h a v e l i k e t h o s e

of a l coho l s ( 7 ) . I n t he p r esenc e o f F e 3*, 02 i s evo l ved

t h rough t he fo l l owi ng s t eps :

F e 3 + + H O - O ~ F e 2 + + H O - O [3 1]

H O - O ~ H + + ( O - O f

[321

t i on o f e l ec t r i c i ty f rom so l a r i r rad i a t ion . Up on i r r ad ia -

t i on , e l ec t rons a r e p roduced a t t he condu c t i on ban d ( cb)

a n d p o s i t i v e h o l e s a r e f o r m e d i n t h e v a l e n c e b a n d ( v b )

(56) . I f t he semi con duc t o r is in an aqueous suspe ns i on ,

t he e l ec t rons , whi ch a r e reduc i n g agen t s , can m i gra t e to

da rk su r f aces and reac t w i t h ox i d i z i ng chemi ca l s . T he

pos i t i ve ho l es , whi ch a r e ox i d i z i ng agen t s , w i l l r ema i n a t

t h e s u r f a c e o f t h e s e m i c o n d u c t o r . I n th e a b s e n c e o f o x i -

d i z a b le s u b s t a n c e s s u c h o r g a n i c c o m p o u n d s , t h e p o s i ti v e

ho l es w i l l r eac t w i t h t he semi conduc t o r and cor rode t he

so l a r ce l ls ( 57). A co a t i ng o f o rgan i c pa i n t on t he sem i -

c o n d u c t o r i s c o m m o n l y a p p l i e d t o p r o t e c t t h e s o l a r c el ls

( 5 6 , 5 7 ) . T h e o r g a n i c c o m p o u n d w i l l c o n s u m e t h e p o s i -

t i v e h o l e s w h i l e b e i n g o x i d i z e d . A n o t h e r w a y t o h a r v e s t

t h e s o l a r e n e r g y i s w it h t h e p r o d u c t i o n o f h y d r o g e n a n d

o x y g e n g a s e s f r o m w a t e r b y s e m i c o n d u c t o r m a t e r i a l i rr a -

d i a t ed wi t h sun l i gh t o r an a r t i f i c i a l l i gh t source (58) . I n

con t r a s t t o e l ec t r i c i t y gene ra t i on , t he so l a r ene rgy i s con-

v e r t e d i n t o c h e m i c a l e n e r g y , i n th e f o r m o f h y d r o g e n .

S i n c e th e s e p a r a t i o n o f h y d r o g e n f r o m t h e g a s m i x t u r e i s

d i f f i cu l t , r e sea r che r s have t r i ed t o suppres s t he ox i da t i on

o f w a t e r b y a d d i n g o r g a n i c c h e m i c a l s t o w a t e r ( 5 8 ). T h e

fo l l owi ng i l lus t r a t ion sh ows t ha t i n t he p r esence o f o r -

gan i c chemi ca l s , t he pos i t i ve ho l es w i l l be used by t he

o r g a n i c c o m p o u n d s r a t h e r t h a n w a t e r . I n f a c t t h is c o n c e p t

has been used a s cor ros i on con t ro l o f so l a r ce l ls .

n o

F e 3 + + ( O - O f ~ F e 2 + + 0 2

[331

H2 02 + ( .O-O) - ~ .HO + OH + 02 [34]

C 2

S u p e r o x i d e c a n b e p r o d u c e d t h r o u g h t h e f o l l o w i n g r e -

act ion:

O H + H 2 0 2 - -- ) H 2 0 + H O - O [351

Haber and W ei s s (53) sugges t t ha t t h i s r eac t i on con-

t r ib u t e d t o t h e m a i n t e n a n c e o f a c h a i n r e a c ti o n s e q u e n c e

unde r t he ac i d i c cond i t i ons o f t he i r st udy . Und er neu t r a l

pH, t he sum o f E qs . [ 21] and [30] g i ves E q . [ 33] . I n t h i s

supe rox i de -dr i ven F en t on ' s r eac t i on , i r on func t i ons a s

t h e t r u e c a t a l y s t as t h e r e s u l t o f th e r e c y c l i n g m e c h a n i s m

w h e r e i ro n h a s b e e n c a n c e l e d a f t e r s u m m a t i o n o f E q .

[10] and [1].

Heterogeneous Systems with Irradiation

Photocatalytic oxidation S e m i c o n d u c t o r s s u c h a s C d S ,

CdS e , and S i hav e been use d a s so l a r ce ll s f o r t he gene ra -

Org~aies

T h r e e k i n d s o f s e m i c o n d u c t o r s c a n b e r e c o g n i z e d : ( 1 )

R- t ype , ( 2 ) O- t y pe and (3 ) RO - t ype . T he c l a s s if i ca t i on

i s m a d e o n t h e b a n d g a p p o s i t i o n o f th e s e m i c o n d u c t o r s

wi t h r e spe c t t o t he wa t e r r educ t i on (H+ / H: ) and t he wa t e r

o x i d a t io n ( O d H 2 0 ) p o t e n ti a ls . T h e O - t y p e s e m i c o n d u c -

t o r s , such a s W O3, F e203 , and M oS 2, have va l ance bands

l o c a te d b e l o w t h e E M F o f th e w a t e r o x i d a t i o n a n d c a n

o x i d i z e w a t e r t o o x y g e n . T h e R - t y p e s e m i c o n d u c t o r s ,

such a s CdT e , CdS e , and S i , have conduc t i on band l o -

c a t e d a b o v e t h e E M F o f th e w a t e r r e d u c t io n a n d c a n

r e a d i ly re d u c e w a t e r t o h y d r o g e n . T h e R O - t y p e s e m i -

conduc t o r s , such a s T i O2, CdS and S rT i O3, a combi na -

t io n o f t h e a b o v e t w o t y p e s , c a n s p l it w a t e r i n to O : a n d

H 2 . I t m u s t b e n o t e d t h a t th e p r e s e n c e o f o r g a n i c c o m -

p o u n d s c a n g r e a t ly e n h a n c e t h e d e c o m p o s i t i o n o f w a t e r

b y o n e t o t h re e o r d e r s o f m a g n i t u d e .


9/17

A D V A N CE D C H E M I C A L O X I D A T I O N 3 6 9

Stud ie s ha ve a l so show n tha t t r a c e a moun t s o f P t a nd

R u O 2 c a n d r a s t ic a l ly i m p r o v e t h e p r o d u c t i o n o f h y d r o g e n

i n a n o r g a n i c c o m p o u n d - w a t e r m i x t u r e ( 5 8 ). A p p a r e n t l y

the P t e nha nc e s t he r a t e o f w a te r r e duc t ion w h i l e t he

R uO 2 c a t a lyz e s fu r the r t he ox ida t ion o f t he o rga n ic ma t -

te rs .

K a w a i a nd Sa ka ta (59 ,60 ,61 ) ha ve de mons t ra t e d a

pho toc a t a ly t i c r e a c t ion w i th d i f f e re n t me d ia suc h a s c h lo -

r i n e a n d n i t r o g e n c o m p o u n d s . T h e d e c o m p o s i t i o n o f

s o m e o r g a n i c m o l e c u l e s a n d th e p r o d u c t i o n o f h y d r o g e n

ga s oc c u r re d a t t he sa me t ime . F ive pe rc e n t p l a t inum w a s

de pos i t e d pho toe l e c t roc h e mic a l ly on the su r fa c e o f T iO 2 .

T h e y c o n c l u d e d t h a t t h e r e a c t i o n c a n p r o c e e d a t r o o m

te mpe ra tu re i n a so lu t ion w i thou t show ing a ny the rma l

e f fe c t . A c omm e rc i a l -g ra de t i t a n ium d iox ide w a s p re -

t r e a t e d by r in s ing w i th 1M H C 104 a nd d i s t i l l e d w a te r

se ve ra l t ime s un t i l t he c onduc t iv i ty d roppe d to l e ss t ha n

10 umho /c m. The t i ta n ium d iox ide sa mple s w e re c e n t r i -

fuge d a t 10 ,000 rpm fo r 30 minu te s a nd the n d r i e d ove r -

n igh t a t 105C a nd g round to a f i ne pow de r . In tw o

s tud ie s , K a w a i a nd Sa ka da ha ve som e ve ry in t e re s t i ng

f ind ings (59 ,61 ) . The y ha ve s tud ie d the pho toc a t a ly t i c

ox ida t ion o f na tu ra l p roduc t s , e .g . g luc ose , e tha no l , c e l lu -

lose and l ignin , food stuffs , e .g . pota to , fa t ty o i l , and

he rbs , w ood , e .g . c he r ry w ood , w h i t e du tc h c love r a nd

w a te r hya c in th , g re e n a lga e , de a d a n ima l s a nd e xc re me n t

us ing T iO 2 unde r a xe no n l a mp . A s show n in Ta b le 3 , i n

ne u t ra l a nd in 5M N a O H so lu t ion , t o t a l ox ida t ion w a s a l -

mo s t poss ib l e . Th e y ha ve a lso found tha t n i t roge n a nd

c h lo r ine a re c o nve r t e d to N H 3 a nd H C 1 , r e spe c t ive ly , i n -

s t e a d o f o the r o rga n ic c he mic a l s .

I t i s no t i c e d tha t i n c on t ra s t t o w ha t w a s r e po r t e d by

Fu j ih i r a e t a l (62 ,63 ) , K a w a i a nd Sa ka da (59 ,61 ) ha ve

d e m o n s t r a t e d a r a t h e r c o m p l e t e o x i d a ti o n o f s o m e h a l o -

g e n a t e d h y d r o c a r b o n s w i t h t h e p r o d u c t i o n o f H C 1 , H F ,

C O 2 a nd H 2. Th i s is in t e re s t i ng inde e d . H ow e ve r , t he

a u tho rs p rov ide no de t a i l s o f t he r e a c t ion k ine t ic s . A l l

e xpe r ime n t s w e re c onduc te d in ne u t ra l so lu t ion o r i n 5M

N a O H m e d i a u s i n g T i O 2 c a t a l y s t a n d a 5 0 0 w a t t x e n o n

l a m p . T a b l e 4 s u m m a r i z e s th e e n d -p r o d u c t s p r o d u c e d

dur ing p ho toc a t a ly t i c ox ida t ion in uv - i r r a d i a t e d T iO 2 sus -

pe ns ions by va r ious r e se a rc he r s .

B a rbe n i e t a l (64 ,65 ) ha ve s tud ie d the de g ra da t ion o f

c h l o r in a t e d h y d r o c a r b o n s , 2 , 4 , 5 - t ri c h l o r o p h e n o x y a c e ta t e

T A B L E 3

Photocata lyt ic Dissociat ion of

O r g a nic Wa t e r M ix t ur e by TiO2-Pt.

Organic Matter End-Product

glucose CH3CHO ; C2HsOH; CH3)2CO; H2

ethanol C2H6;CH3CHO; CH3CO OH;H2

cellulose C2H6; C2H5 OH; CH3 )2CO; H2

pyruvic acid 20H; H2

gylcine NH3; H2

polyethylene CO2; H2

polyvinyl alcohol CO2; H2

polyvinyl chloride PVC) CO2; HCI; H2

proline CO2; NH 3; H2

stearic acid CO2; H2

potato CH3OH; CH3)2CO; H2

fatty oil C2H6; H2

cherry wood C2H6; CH3OH; CH3)zCO;H2

white dutch clover CH4; CH3OH; C2HsOH; NH3; H2

golden rod CH3OH; NH 3; H2

water hyacinth NH3; H2

dead cockroach NH3; H2

human excrement NH3; CH3OH

teflon HF; CO 2; H2

trichlorbenzene HCI; CO2 ; H2

trichloroethylene HCI; CO2 ; H2

a nd 2 ,4 ,5 - t r i c h lo rophe no l , u s ing T iO 2 a s pho toc a t a ly s t .

T h e y h a v e p r o p o s e d a f o r m a t i o n o f O H f r e e r a d ic a ls a s

t h e m a j o r m e c h a n i s m f o r t h e o x i d a ti o n o f c h l o r i n a t e d o r-

ga n ic s . O l l i s has e xa m ine d the pho to -ox ida t ion o f c h lo -

r ina t e d hyd roc a rbons w i th T iO 2 w i th p romis ing re su l t s

(66 ,67 ) . O l l i s a nd a ssoc i a te s ha ve r e po r t e d tha t t he o rde r

o f e a s e o f c o n v e r s i o n is c h l o r o - o le f i n s ~ c h l o r o p a r af f i n s

- -~ c h lo roa c e t i c a cid s (66 ,67 ) . M a t the w s ha s u se d a t h in

f i lm o f T iO 2 to s tudy the ox ida t ion o f some o rga n ic im-

pu r i t i e s i n w a te r . H e ha s r e po r t e d tha t t he k ine t i c s o f

pho to -ox ida t ion r e a c t ions c a n be de sc r ibe d by the La ng -

mui r t ype a ds o rp t ion e qua t ion (68 ,69 ) .

In summa ry , t he re i s s t rong e v ide nc e to sugge s t t ha t

o r g a n i c c o m p o u n d s s u c h a s t h o s e t o x i c c h e m i c a ls f o u n d

in c on ta mina te d g roundw a te r a nd w a s t e w a te r s c a n be de -

c o m p o s e d ( o x i d i z ed ) b y p h o t o c a t a ly t i c o x i d a ti o n . T h e

m a j o r p r o d u c t s w i l l b e h y d r o g e n a n d C O 2 . A c c o r d i n g t o

the r e su l t s o f K a w a i a nd Sa ka da , t he ove ra l l r e a c t ion c a n

be e xp e c t e d a s fo l low s :

T A B L E 4

O t h e r W o r k

on the Photocata lyt ic Dissociat ion of Organic Matters on

T iO2

Organic Matter End-Product Reference

acetic acid CH4 ; CO2; C2H6 ; H2 Kruatler and bard 72)

propionic acid ethane; ethylene; CO2; H2

n-butylic acid propane; CO2; H2

v-va leric acid n-butane; CO2; H2

pivalic acid isobutane; isobutylene; CO2; H2

tluene benzaldehyde; biphenyl; cresols Fujihira et al. 62, 63)

benzene phenol; bephenyl Kawai and Sakata 59-61)

acetopbenone hydroxyacetophenone; phenol

lactam imides Pavlik and Tantayanon 73)


10/17

3 7 0

CaHbOcNdCle + (2a - c )H 20 ~ aCO2 + dNH3 + er iC1

+ 0 .5(4 a+b- 2c-3 d-e)H 2 [361

C . P . H U A N G C . D O N G A N D Z . T A N G

C 7F15C O O N H 4 = C 7F15C O O + N H 4 + [39 ]

For a c omple t e pho toc a ta ly t i c ox ida t ion re a c t ion , C I w i l l

b e c o m e H C I a n d N w i l l b e c o n v e r t e d to N H 3 .

C 7F 15C O O + H + = C 7F15C O O H [40 ]

Heterogeneous System without Irradiation

Elec tro lys is has a broad def in i t ion in tha t i t inc ludes a l l

r e a c t ions oc c u r r ing a t a n e l e c t rode s u rfa c e . The re a c t ions

a re fo rc e d to ta ke p l a c e by a n e x te rna l ly impos e d vo l t a ge .

A n i m p o r t a n t d e v e l o p m e n t i n e l e c tr o c h e m i c a l o x i d at i o n

p r o c e s s i s t h e g e n e r a t io n o f h y d r o g e n p e r o x i d e b y r e d u c -

t ion o f oxyge n a t the c a thode . In the p re s e nc e o f f e r rous

ions , t he hyd roge n pe rox ide s o ge ne ra t e d c a n fo rm a hy -

d r o x y l r a d i c a l - - a w e l l k n o w n F e n t o n ' s r e a g e n t (1 8 , 1 9) .

H y drog e n pe rox ide w a s ge n e ra t e d a t t he g ra ph i t e e l e c -

t rode in ac id ic cond i t ions . In the pres ence o f Fe 2+ ions ,

the ox ida t ion re a c t ion c a n t a ke p l a c e re a d i ly th rough the

hyd roxy l r a d ic a l s (54 ) . A n e l e c t ron Fe n ton ' s r e a ge n t c a n

b e p r o d u c e d b y t h e a d d i t io n o f a n a p p r o p r ia t e a m o u n t o f

F e+2 ion to the c a tho ly t e . The oxy ge n re d uc t ion po te n t i a l

i s a func t ion o f pH a nd i t i s found tha t pH 3 i s t he mos t

fa vo ra b le ( a t a c a thod ic po te n t i a l o f -0 .6 V vs . s a tu ra t e d

c a lome l e l e c t rode ) . In the p re s e nc e o f o rga n ic c om -

pounds , t he hyd roxy l r a d ic a ls fo rm e d w i l l be r a p id ly c on -

s ume d a nd the o rg a n ic c omp ound s o f i n t e re s t w i l l be

ox id iz e d (21 ,70 ,71 ) .

In a s tudy o f e l e c t roge ne ra t e d Fe n ton ' s r e a ge n t ox ida -

t io n o f p h e n o l , S u d o h

et al

a l s o c onc lude d tha t pH i s op -

t ima l a t 3 (74 -78 ) . A t pH 4 , t he de g ra da t ion o f phe no l

ha rd ly p roc e e de d e ve n w i th inc re a s ing e l e c t r i c c u r re n t .

A t p H 2 , t h e r at e s o f c a r b o n d i o x i d e p ro d u c t i o n a n d C O D

r e d u c t io n o f p h e n o l w e r e l o w e r th a n t h o s e a t p H 3 . A t

pH 1 , t he c onc e n t ra t ion s o f t he p roduc t s ( s uc h a s c a rbon

d i o x i d e ) w e r e e x t r e m e l y l ow . C O D d e c r e a s e d s lo w l y i n

c om pa r i s on w i th the de c re a s e o f phe no l c onc e n t ra t ion .

S inc e t a r ry p re c ip i t a t e s w e re obs e rv e d in the r e a c t ion s o -

l u ti o n , t h e o x i d a t io n o f p h e n o l m i g h t p r o c e e d t o p r o d u c e

t h e d i m e r o r t r i m e r o f p h e n o l .

In a r e c e n t s tudy , H ua ng a nd c ow orke rs (79 ) ha ve

de m ons t ra t e d tha t a s u r fa c t a n t , pe r f luo ro -oc ta no a te (C 8) ,

c a n b e d e c o m p o s e d e f f e c t i v e l y b y i n d i re c t e l ec t r o c h e m i -

c a l ox ida t ion (79 ) . R e s u l t s s how tha t i n 15 minu te s , t he

to t a l C 8 c onc e n t ra t ion w a s de c re a s e d f rom 100 ppm to

1 0 a n d f r o m 5 0 p p m t o 1 p p m . T h e y h a v e a l so p r o p o s e d

a r e a c ti o n p a t h w a y f o r t h e o x i d a t i o n o f C 8 b y t h e F e n -

ton ' s r e a ge n t r e a c t ion :

02 + 2H + + 2e --~ H 2 02 (c a thod ic r e duc t ion ) [37 ]

H 2 0 2 + F e 2+ --~ OH + Fe 3+ + OH [38]

C 7F15C O O H + O H ' - -~ C 7F15 . + C O 2 + H 20 [41 ]

C7F15.+ OH --+ C7F15OH (CnF2n+IOH) [42]

C nF2n+ IO H + O H ~ C nF2n(O H )2 o r

CnF2n+I(OH)

[43]

R e a c t ion [37 ] is a r e duc t ion o f oxyge n a t t he c a thode

e le c t rode ; r e a c t ion [38] i s a ge ne ra t ion o f hyd ro xy l r a d i -

ca l (Fenton 's reagent) ; reac t ion [39] is a d issoc ia t ion re -

a c t ion o f C 8 ; r e a c t ion [40 ] i s a n a c id -ba s e e qu i l ib r ium

reac t ion; reac t ion [41] is a Kolbe reac t ion; reac t ion [42]

i s a f r e e r a d ic a l r e a c t ion w i th the fo rma t ion o f pe r f luo ro -

he p ta no l ; r e a c t ion [43] s how s hy d rox y l r a d ic a l a t t a cks on

the pe r f luo rohe p ta no l t o fo rm s imp le pe r f luo ro a l c oho l s

s uc h as C3F6 OH)2, C3F7 OH), C4Fs(OH)2,or C4F9(OH).

The s e a l c oho l s a re e x t re m e ly vo la t il e w i th a bo i l i ng po in t

a round 0 C . La bo ra to ry re s u l ts c l e a r ly s how the p re s-

e nc e o f s imp le pe r f luo ro a lc oho l g roups in the ga s pha s e

o f the r e a c to r (79 ) .

C A S ES O F A D V A N C E D C H E M I C A L

O X I D A T IO N O F C H L O R O P H E N O L S

T h e o x i d a t io n o f c h l o r o p h e n o l s h a s b e e n s t u di e d b y

H u a n g a n d c o - w o r k e r s u s in g F e n t o n ' s r e a g e n t a n d p h o t o -

c a t a ly t i c ox ida t ion p roc e s s e s (80 ). F ig . 1 s how s the re -

s u lt s o f t h e d e c o m p o s i t i o n o f t h e p a r e n t c o m p o u n d s ,

mono- , d i - , t r i - , t e t r a - a nd pe n ta -c h lo rophe no i s by the

Fe n to n ' s r e a ge n t . The re s u l t s c l e a r ly ind ic a t e tha t the

de g ra da t ion o f c h lo rophe no l s i s ve ry fa s t a nd fo l low s a

f i r s t o rde r k ine t i c e xp re s s ion in t e rms o f the pa re n t c om -

poun d c onc e n t ra t ion . U n de r the e xpe r ime n ta l c ond i t ions ,

a t o t a l r e mova l o f c h lo rop he no l s i s pos s ib l e i n l e s s t ha n 5

minu te s , e xc e p t pe rha ps fo r t e t r a - a nd pe n ta -c h lo rophe -

no l s . The o rde r i n the de s t ruc t ion o f t he s e c h lo rophe no l s

is as fo l lows:


11/17

A D V A N C E D C H E M I C A L O X I D A T I O N 371

1 o 0 %

80%

o

60%

2 o ~

0%

-.-2-chlorophenol

--c3-3-dalorophenol

---4-chlorophenol

1 2 3 4 5 6 7

Tim e ( m in )

1 0 0

80

L

2 0

0%

k . . . . . . . . . . . . .. . i .. .. i . .. . . . .. . .. .

-o-2,34ichlomphenol

-o-2,44ich10rophenol

--o-23 2 3 4 T C P ~ 2 4 5 T C P

2 3 6 T C P > 2 4 6 D C P ( fo r t r i ch l o r o p h e n o l s )

T h i s o r d e r i s s l ig h t l y d i f f e r e n t f r o m t h a t o f t h e F e n -

t o n ' s r e a g e n t p r o c e s s , e x c e p t t h a t i n b o t h p r o c e s s e s ,

2 6 D C P a n d 2 4 6 T C P a r e t h e m o s t d i f f ic u l t o n e s f o r o x i -

da t i on .

F o r t h e p u r p o s e o f c o m p a r i s o n , t h e o x i d a t io n o f t h es e

c h l o r o p h e n o l s w a s a l s o c o n d u c t e d w i t h t h e H 2 0 2 / u v s y s -

t e m ( F i g . 3 ). I t m u s t b e m e n t i o n e d t h a t th e u v l ig h t

s o u r c e u s e d i n t h i s e x p e r i m e n t w a s t h e s a m e a s t h a t u s e d

i n t h e p h o t o c a t a l y t i c o x i d a t i o n p r o c e s s r e p o r t e d a b o v e ;

t h e w a v e l e n g t h ( 3 5 0 n m ) o f th i s u v l i g h t s o u r c e is l o n g e r

t h a n o t h e r s y s te m s s u c h a s H 2 0 2/ u v a n d O J u v o f o t he r

r e s e a r c h e rs . T h e r a t e o f o x i d a t i o n i s t h e s a m e a s t h a t o f


12/17

3 7 2

C.P . HUANG, C . DONG, AND Z. TANG

=

o

=

100

8 0 ~

60

40

2O

0

0

- o - 2 ~ o r o p h e n o l

~ . - - o -3 - c h l0 r 0 p h e n 0 1

40 80 120 160 200

T i m e ( m i n )

o

100

80

60

4O

2O

0~

0

i i i i .

k - - o - 2 , Y d i c h lo r o p h e n o l

- o -2 , 4 - d i c h l o~ ophe no l

~ . o 2,5-dichlor~phenol.

I ~ . ~ ~ 2 ,6 - d i ch l o r o ph e n o l .

\ ~ , - ~ + ~ , 4 ~ c h ~ o ~ o ~

40 80 120 160 200

T i m e ( r a in )

_ o

100

80

60

4O

2O

0

0

' - ~ - 2 , 3 , 4 ~ @ [ a l o ~ p h e ~ ]

- o - 2 3 ,5 - t r ic h 1 0 r o p h e o l

. . - 2 , 3 , 6 - t f i h l or ophe no l

. .. .-a-2,4,5- td a lorophen o[

2 , 4 , 6 a d c h l o r o p h e n o l

40 80 120 160 200

T i m e ( r a in )

e~

o

100

80~

60

4 0

20

0

0

' - ~ e n o l . . . . . ' :

- o - 2 , 3 , 4 , 6 . t e t r ac h l 0r 0phe no l . '

- - o - P C P

40 80 120 160 200

T i m e ( r a in )

FIGURE 2. Destruction of Chlorophenols by UV/TiO2 Oxidation. Experimental conditions: Concentration of organic was 103 M except

2,3,4,6-tetrachlorophenol and pentachlorophen, for which concentration was 5x10 -5 M; TiO2 = 1 g/L; pH = 4.0; ionic strength = 5x10 2 M NaNO3;

room temperature.

1 0 0 I . . . , . . . , . . . , . . . , . . .

o - 2 - c h lo m p h e n o l

s 0 % - t~ - 3 4 a 1 0 r 0 ~ n o l

= ~ - - e -4 - c h ]g 6 0% r l ~ n l

k

0

. . . . .

: = , . . .

40 80 120 160 200

T i m e ( m i n )

'0 0 ] k . . . . .

, , 8 0 % [ . [ ' ~ - o -2 , 4 -d i c h lo r o p h e n o l :

[ ~ l k - - * - 2 ~ o r o ~ :

6 0 % t l ~ , - - * - 2 ,6 -d ic M o m p h e a o l.

L / ~ ' ~ - , - 3 ,4 dichlo~ot~nol

0

0 4 0

80

120 160

200

T i m e ( m i n )

100

80

6O

40

20

0

' ~ - ~ i ~ :

- o - 2 3 ~ .

- - ~ - - 2 , 4 ~ ,

40 80 120 160 200

T i m e ( m i n )

100

= 8 0

=

0

0

- o - Z 3 , 4 ~

40 g0 1211 160 200

T i m e ( m i n )

FIG URE 3. Destruction of Chlorophenols by

U V H 2 0 2

Oxidation. Experimental conditions: Concentration of organic was 10 3 M except

2,3,4,6-tetrachlorophenol and pentachlorophenol , for which concentration was 5xl 0 5 M; H202 = 0.1 M per minute; pH = 4.0; reaction volume = 1


13/17


14/17

3 7 4 C . P . H U A N G , C . D O N G , A N D Z . T A N G

n = 0 ( p h e n o l )

O H O H O H

~ A @ O H o r ~

O H

n - - I ( m o n o c h i o r o p h e n o l s )

O H O H

E l C I

n ~ 2

( d i c h i o r o p h e n o l s )

O H O H

~ C I A O ~ C I

e l ~ C I

O H

C l

O H

o r ~ H c I

n = 3 ( t r l c h l o r o p h e n o l s )

O H O H

c1_5 ~,

c I

n = 4 ( t e t r a c h l o r o p h e n o l )

O H

o r ~ C I

C I

O H

O H

O H

O H O H

C ~ [ C I B C ~ O H o r o~.,.,,Cll~.)j~ o r C ~ C '

T C l ~ C I T C I ~ I~ -El

C l C I C I O H

n = 5 ( pen taeh lo r opheno l )

O H O H O H

C ~ [ C I B C ~ . O H

o r

C ~ , ~ C |

C i ~ f C i C I ~ f c ] c i ~ f- cc i c i o H

a n d t r i h a l o g e n a t e d ( n = 3 ) p h e n o l s . A s f o r t e t r a h a l o g e n -

a t e d ( n = 4 ) a n d p e n t a h a l o g e n a t e d ( n = 5 ) p h e n o l s , o n l y

T y p e B r e a c t i o n p a t h w a y i s p o s s i b l e .

T o f u r t h e r d e m o n s t r a t e t h e v e r s a t i l i t y o f t h e F e n t o n ' s

r e a g e n t p r o c e s s , a s a n i t a r y l a n d f i l l l e a c h a t e w a s t r e a t e d

w i t h t h i s s y s t e m . F i g . 5 in d i c a t e s t h a t s ig n i f i c a n t C O D

r e m o v a l c a n b e a c c o m p l i s h e d b y t h e H 2 0 2 / F e ~+ s y s t e m .

T h e C O D o f t h e l e a c h a te d e c r e a s e s f r o m a n o r ig i n a l

8 , 5 0 0 p p m t o 2 , 0 0 0 p p m a f t e r t r e a t m e n t w i t h 5 1 0 .2 M

H 2 0 2 a n d 2 1 0 3 M F e S O 4 a t p H 3 . T h e r e s u l t s a l s o s h o w

l --o-2-dal0r0phen01

i 8o --o--chlorophenol

60% -o-4


15/17

A D V A N C E D C H E M I C A L O X I D A T I O N 3 75

tO

6

4

2

Raw Leachate H202 treated Fe*2+H202 treated

F I G U R E 5 . CO D r e m ova l by H 202 a nd F e +2 + H 202 w i t h a nd w i t hou t

p r e c i p i ta t i on . E xpe r i me n t a l c ond i t i ons : In i t ia l CO D = 8 , 500 ppm ,

H202 = 5x1 0 2 M; FeS O4 = 2x10 3 M, pH = 3.0 (wi th H2SO4) .

t ha t t he ex t en t o f minera l i za ti on is h igh when t he

leachate is treated by the Fe nto n s reagent process (Fig. 6).

O N L U S I O N

Advanced chemica l ox ida t i on p rocesses , espec i a l l y t he

H202/Fe 2+ system s, are e ffect ive in dec om posing organic

po l l u t an t s . Much has been l earned abou t t he mechan i sm

o f t h e o x i d a ti o n o f o r g a ni c c o m p o u n d s b y A O P s y s te m s ,

espec i a l l y t hose invo lv ing O H reac t ions . Whi l e O H can

be genera t ed by m any sys t em s , i t appear s t ha t t he hom o-

geneous sys t ems us ing H 2 0 2 and catalyt ic metal ions,

spec i f ical ly Fe 2+, wi l l b e the mo st at t ract ive ones, both

f rom p rocess con t ro l and p rocess per fo rmance po in t s o f

v i ew. The OJu v p rocess a lso depends on O H to d r ive

the oxidat ion react ion. Upo n i r radiat ion wi th uv, 03 i s

conver t ed t o H202 which i s t hen conver t ed t o OH upon

uv i r radiat ion. The refore, the O3/uv system w i l l not be

compet i t i ve aga ins t t he H E O 2 / F e 2+ process. Moreover ,

g

1

8

6

4

2

o ~

2 4 6 8 I 1 2

T i m e r a i n )

2

F I G U R E 6 . A c o m p a r i s o n o n C O p r o d u c t io n b y v a r io u s c h e m ic a l

t r e a t me n t me t hods . E xpe r i me n t a l c ond i t i ons : I n i ti a l CO D = 8 , 500

ppm, H / O 2 = 5x10 2 M ; F e S O 4 = 2x10 3 M , pH = 3 . 0 ( w i t h H 2S O 4).

t he O3 /uv sys t em wi l l be l imi t ed by t he ozone t r ans fer

process betw een the gas and the l iquid phase. Photocata-

l y t ic ox ida t i on , espec i a l l y a l ong t he use o f H202 , be-

comes an ex t r emely compet i t i ve p rocess aga ins t t he

H2Oz/Fe2+ system. How ever , the process i s operat iona l ly

l imi ted i n t ha t t he deve lopm en t o f a f i xed -bed pho toca t a -

l y t ic reac to r i s ye t to be per fec t ed com merc i a l l y . The ap -

p l i ca t i on o f pho toca t a ly t i c ox ida t i on i s mos t r ead i l y

appl ied in suspensions.

R E F E R E N E S

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37:913 (1965).

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12 . R i c e , R . G . a nd G om e z - T a y l o r , M . O x i da t i on by - p r oduc t s f r om

dr inking wate r t r ea tment , in: P . M. Huck and P. Tof t ( ed) ,

Treatment o f Drink ing Water fo r O rganic Contaminants.

P e r g a m o n

P r e s s , N e w Y or k , p l 07 - 134 ( 1987).

13. Farha taz iz , P . C. and R oss , A. B. ,

Selec ted Speci f ic Rates o f

Radica ls o f Transien ts f rom Water in Aqueous So lu t ions. N a t i ona l

Bur e a u o f S t a nda r ds , Wa sh i ng t on , D . C . N S R D S - N B S 59 , ( 1977).

14 . G l a z e , W. H . D r i nk i ng w a t e r t r e a tme n t w i t h oz one . Environ. Sci.

and TechnoL

21:224 (1987) .

15 . G l a z e , W. H . , K a ng , J . W. , a nd Cha p i n , D . H . T he c he m i s t r y o f

w a t e r t r e a t me n t i nvo l v i ng oz one , hyd r oge n pe r ox i de a nd u l t r a v io l e t

radia t ion.

Ozone Sci. Technol.

9(4):335 (1987).

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G r e e nw a l d ( e d ) ,

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of fe r rous sa l t s . Proc. Chem. Soc.

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20. Eisenhauer, H. R. O xidation of phe nolic wastes. J. Water Poll.

C o n t r o l F e d 36:1116 (1964).

21. Bishop, D. F., S tem, G., Fleisch man , M., and M arshall, L. S.

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toxicity of hydrogen peroxide oxidation products of phenols.

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