Ro Treatment of Ind.effluent

8/8/2019 Ro Treatment of Ind.effluent

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Presented at the Conference on Desalination and the Environment, Las Palmas, Gran Canaria, November 912, 1999.European Desalination Society and the International Water Association.

0011-9164/99/$ See front matter 1999 Elsevier Science B.V. All rights reserved

Desalination 126 (1999) 219226

Treatment of an industrial effluent by reverse osmosis

Antonio Prez Padillaa*, Eduardo L. Tavanib

aInstituto de Investigaciones en Tecnologa Qumica (INTEQUI), Universidad Nacional de San Luis,CONICET, CC 290, 5700 San Luis, Argentina

Fax +54 (2652) 26711; email: [email protected] de Tecnologa de Recursos Minerales y Cermica (CETMIC), Comisin de Investigaciones Cientficas de la

Provincia de Buenos Aires, CONICET, CC49, 1897 M.B. Gonnet, Argentina

Abstract

The treatment of tanning wastewater was studied by means of reverse osmosis and ultrafiltration. Tests were carriedout on laboratory scale using membranes of polyamide (reverse osmosis) and of polysulfone (ultrafiltration). Theevaluation of the system was performed by chemical analysis, pH measurements and visible spectrophotometry. Effectsof the protein contained in the industrial effluent, the applied pressure and the feed temperature on the permeate fluxwere analyzed. The polyamide membrane used allowed us to obtain permeates with a low chromium (III) content(710mg/L) but with appreciable amounts of SO4

= (13 g/l), Cl (914g/L) and Na+ (510g/L). The presence ofchromium (III) polymers was determined in the original effluent and in the concentrates obtained by reverse osmosis.Finally, it was established that during the operation of reverse osmosis, the transport of H+(H3O

+) from the concentrateto the permeate was produced.

Keywords: Tanning wastewater; Reverse osmosis; Ultrafiltration; Chromium (III); Recovery; Leather

1. Introduction

Leather is a material that has a reasonable

mechanical resistance, good chemical stabilityand acceptable thermal behaviour. This material

is obtained by means of specific reactions among

*Corresponding author.

carboxylic groups of the protein fiber network of

animal skin (collagen) and tanning reagents. Thebasic chromium (III) sulfate [Cr(OH)(H 2O)5SO4]

is a primary tanning agent widely used at the

present time [1].

During the tanning process, large amounts ofwastewater; sludge; and solids containing

chromium, sodium, chloride and sulfate are


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A. Prez Padilla, E.L. Tavani / Desalination 126 (1999) 219226220

produced. In all these wastes the chromium is present only in the trivalent form since thetanning process not generate chromium (VI)

[1,2]. Both oxidation states of the chromium areabundant in environment. Chromium (III) is the

form most stable and its presence is necessary, in

small amounts, for human health. Chromium (VI)

is found in several commercial products and itspresence may have immediate adverse effects for

human health [2,3].Chromium (VI) is transformed rapidly to

chromium (III) under environmental normalconditions, with only contamination risks near

direct emissions. However, under very specificconditions, the oxidation of trivalent to hexa-

valent form may also occur [4,5]. Technical

regulations for disposal of wastes containing

chromium (III) are stringent and are based on theprobable presence of chromium (VI).

Wastewater is the effluent of the tannery thathas a fast interaction with the environment. The

amount of wastewater varies between 30 and 50L

per kilogram of processed skin [6,7]. From the

total amount of liquids, nearly 10% correspondsto the tanning stage (tanning wastewater) and the

remainder to the other stages of processing(dehairing, pickling, neutralization, fat-liquoring,

dyeing and washings). The greatest content ofchromium (III) is found in the tanning

wastewater.The composition of this effluent varies

according to the tanning process used and to thetype of leather to be obtained. Most of thecomponents of the tanning wastewater and their

most frequent contents are: 1625g/L sulfate,1726 g/L chloride, 1421 g/L sodium,0.62.0 g/L of chromium (III) and a residualacidity between pH 3.5 and 5.0.

Precipitation and adsorption are two alter-native methods for the recovery of chromium (III)contained in tanning wastewater. Chromium (III)is easily precipitated by the addition of an alkali(generally, calcium hydroxide) to the liquideffluent. In this way a supernatant free of the

metallic element and a precipitate containingchromium (III) hydroxide are obtained. From theprecipitate it is possible to recover the chromium by calcination at 600 C (as hexavalentchromium) or by acid leaching [8].

The adsorption occurs on the surface of a

substance called adsorbent and the addition ofany reagent is not required. To carry out under

favorable economic conditions, the separation ofchromium (III) by adsorption, it is necessary to

select an adequate adsorbent with high adsorption

capacity and able to remain stable at low pH.Smectite is a natural adsorbent with good

adsorption capacity, but it does not have

selectivity [9]. Thus, adsorption of chromium(III) (0.040g/g of adsorbent) and of sodium

(0.006 g/g of adsorbent) were determined on thesmectite from tanning wastewater without

dilution [10]. The alteration of this adsorbent in

contact with sulfuric acid is produced at pH ~ 1.7

[11]. Activated clay (a kaolin amorphous deriva-tive) with an adsorption capacity nearly three

times higher than the smectite is altered at loweracid concentration (the chemical attack starts at

pH ~3.0) [12].The separation (desorption) of chromium (III)

retained on the adsorbent surface requires theaddition of reagents. The facility of a cation

replacement depends on its valence, on the water

layer thickness surrounding it, and on its atomic

configuration [9]. Protons (hydronium) H+(H3O+)

have high capacity to replace to other cations, but

their use is not always convenient since someadsorbents may be altered by acid attack. In brief,

the adsorption and the desorption must be madeunder very specific operative conditions to

impede generation of new wastes.According to the two methods mentioned

above, several operations and processes must beused to separate chromium (III), but they are notcommonly utilized in the tannery. Based on thesefacts, it was considered as appropriate to studythe separation of chromium (III) contained in atanning wastewater by means of reverse osmosis


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A. Prez Padilla, E.L. Tavani / Desalination 126 (1999) 219226 221

(RO). In this method that is easier to be fulfilledthan the precipitation and the adsorption, afraction of the tanning wastewater passes throughan adequate membrane under sufficient pressureto overcome the osmotic pressure. The fractionthat passes through the membrane (permeate) isconstituted basically by water with a low contentof dissolved salts and the fraction retained by themembrane (concentrate) contains most of thedissolved salts in the original effluent [1316].

2. Experimental

The tanning wastewater was obtained from atypical tanning process. The suspended solids

were separated by ultrafiltration (UF). Table 1shows the chemical analysis of the tanning

wastewater without suspended solids.

Chemical analyses were performed by atomic

absorption/emission (AA/AE), volumetry andgravimetry. Analyses by AA/AE were made with

Jarell Ash equipment.

UF tests were carried out with Milliporeequipment using a membrane of polysulfonePellicon Cassette PTGC00005 of 10.000 NMWL,

with a surface of 0.46m2 and operable betweenpH 2.0 and 12.0.

RO tests were performed using Osmo

equipment, model 19E-HR 500, with a membrane

of polyamide Osmonics 192 HR, with a surfaceof 1.68m2 and operable between pH 2.0 and 12.0.

Table 1Chemical analysis of the tanning wastewater without

suspended solids

Component g/L

SO4=

Cl

Na+

Cr3+

17.60

24.80

17.80

0.75

The high-pressure pump and the membrane of theequipment allowed us to work with a maximumpressure of 1.3 MPa and with recycling flow of

156L/h.The cleaning of the RO membrane was carried

out with water without salts and chlorine, H3PO4solution at pH 2.02.5, NaOH solution at

pH11.011.5 and a sodium lauryl sulfate solution0.001 w/w. The cleaning operation was finished

when the initial permeate flux was recoveredusing distilled water as feed.

The chromium species in solution weredetermined by absorption within the visible range

(340800nm) with a Hewlett Packard 8453spectrophotometer.

3. Results and discussion

3.1. UF and RO tests

At the beginning of the RO operation, the

convective flow of the solute associated with theglobal flow of the dissolution originates an

accumulation of those species that do not passthrough the membrane (rejected species). The

accumulation of rejected species is produced nearthe membrane. Under the effect of the concen-

tration gradient so generated, the diffusion of

these substances is produced towards the interior

of the dissolution in the opposite direction to thatof the convective flow. As the operation occurs,

the concentration of the rejected speciescontinues increasing up to reaching a certain

equilibrium value. This value remains constant

and the solute forms a layer over the membrane(boundary layer). The boundary layer is formedduring the first moments of the operation and

disappears after interrupting the operationpressure.

According to the system nature and underdetermined operative conditions, a second layer

of rejected molecules may be formed on themembrane surface. This formation occurs for

some materials like proteins or silica at a certain


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concentration level. The layer near the membranesurface is very stable and the other is moredynamic. When the pump is disconnected, most

of the solute molecules disappear, although someof them remain for some time. Thus, an

additional resistance to the transport through the

membrane is added.

The use of the pressure to overcome theosmotic pressure induces the species of smallermolecular size to pass through the membrane andthus those species which are larger are

concentrated. In order to prevent anyaccumulation of particles on the membranesurface, the feed is performed by means of atangential flow with a high rate. If this action isnot sufficient, pretreatment of the feed must beperformed to remove the different particles ofsuspended solids that may affect the systembehaviour [13].

In our case, the particles of suspended solidswere almost all of them proteins released from

the collagen during the tanning process. Theprotein content in the tanning wastewater was

0.007 w/w, and its separation was performed byUF at room temperature using a transmembrane

pressure of 0.15MPa. Thus, it was possible toseparate more than 80% of the proteins contained

in the effluent.To determine the influence of the suspended

solids on the permeate flux (L/m2h), tanningwastewaters with and without proteins were used.

In each case the test was performed with aneffluent volume of 12 L, an applied pressure of

1.3 MPa and a feed temperature of 298K. The

time required to obtain 1.8 L of permeate wasmeasured, and thus it was possible to determinethe corresponding flux. This determination was

performed six times (16) resulting in a finalconcentrate volume of 1.2 L. Table 2 shows three

permeate fluxes (1, 3 and 6) obtained for tests

performed with and without pretreatment of the

effluent by UF. These values indicate that thepermeate flux was in each aliquot slightly higher

for the effluent without proteins, and this allowed

Table 2

Permeate fluxes obtained with and without pretreatment

of the effluent by UF

Permeate

flux, L/m2 h

Effluent

With pretreatment

by UF

Without

pretreatment by UF

1

3

6

0.84

0.81

0.59

0.69

0.64

0.50

us to decrease the working time to obtain the

same final permeate volume. The low proteincontent in the tanning wastewater would explain

the scarce difference of the permeate fluxes

obtained with and without pretreatment of the

effluent by UF.The permeate flux depends on the applied

pressure and on the feed temperature. In order to

study the behaviour of both variables, tests weremade using three different pressures (0.9, 1.1 and1.3 MPa) at constant temperature (288K). With

these conditions, the permeate flux changed in alinear way with the applied pressure (0.17L/m2h

at 0.9MPa, 0.34 L/m2h at 1.1 MPa and 0.50L/m2h

at 1.3MPa). When the tests were made using

three different temperatures (288, 298 and 308 K)at constant pressure (1.0 MPa), the permeate flux

changed according to a parabolic law with thefeed temperature (0.27 L/m2h at 288K;

0.54L/m2

h at 298 K and 0.71L/m2

h at 308 K).Taking into account the results mentioned

above, a new RO test was carried out using theeffluent without proteins, an applied pressure of

1.3 MPa and a feed temperature of 298 K. Thetest was performed with an effluent volume of

24 L and was finished when the concentratevolume was 2.4L. From each 1.8L of permeate

obtained, an aliquot of 0.15L was taken forchemical analysis. In total, 12 aliquots of


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Table 3

Chemical analyses and pH of permeates and pH of the respective concentrates

Aliquot Permeate Concentrate pH

SO4=, g/L Cl , g/L Na+, g/L Cr 3+, mg/L pH pH

1

6

12

0.9

1.9

3.3

8.5

10.9

13.7

5.3

8.4

9.7

10.4

7.5

6.8

4.42

4.61

4.72

4.50

4.67

4.75

permeate (112) were analyzed. The characteri-

zation was completed with the pH measurementof each permeate aliquot and of the respective

concentrate. Table 3 shows the chemical analysesand the pH of three permeate aliquots (1, 6 and

12) and the pH of the concentrates. The low

chromium (III) content in all permeate aliquots

shown in Table 3, similar to the valuesmentioned by other authors [1316], confirms

that the RO method is a valid alternative for thetreatment of the tanning wastewater.

3.2. Identification of chromium (III) species

present in the tanning wastewater and in the

concentrates obtained by RO

The separation of species by RO depends on

the difference of molecular size of the feed

components to be treated, among other aspects[15]. The medium conditions (pH and

concentration of soluble species) are modified asconsequence of this separation, which may affect

the molecular size (structural changes) of somecomponent. Under the experimental conditions

used in this work, chromium (III) is the most propitious component to present structural

changes [1721]. According to these considera-

tions, the physicochemical characterization of

different solutions (chromium (III) content,storage temperature and time) was made in order

to obtain evidence to explain the results attainedby RO.

The absorption spectrum of a diluted solution

of chromium (III) nitrate recently prepared andmaintained at room temperature has two bands at

408 and 575nm. The anion nitrate is a weakligand, and it is very difficult that in diluted

solutions it may penetrate into the coordination

sphere of the chromium (III) complex. At the

same time, the possibility of hydrolytic polymeri-zation of this cation (structural changes) in fresh

solutions and maintained at room temperature isnegligible. The above-mentioned facts suggest

the only presence of water groups into the sphereof the complex [1821]. Then the bands at 408

and 575nm show the presence of a chromium(III) mononuclear complex [Cr(H2O)6

3+].

The absorption spectrum of a solution recently prepared from a commercial tanning salt

(Chromosal-BA Bayer), with a chromium (III)concentration similar to the one of nitrate and

maintained at room temperature, has two bands at432 and 594nm. A shift of the absorption

maxima towards longer wavelengths was

attributed to the presence of hydroxide groupsand sulfate groups into the sphere of thecomplex. The hydroxide groups produce

chromium (III) polymerization, and the sulfategroups (with a ligand strength higher than the one

of nitrate) act as monodentate and bidentate

ligands [1721]. Species in solution are

polynuclear (dimer), and they remain withoutstructural changes for some time when the

commercial salt is dissolved in cold water [17].


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The tanning reagents of mineral origin reactwith carboxylic groups of the collagen betweenpH 2.9 and 4.1. A tanning improvement may be

achieved when the process is performed at atemperature of 10 to 20 C higher than the room

one [22,23]. The pH of a fresh solution of the

commercial tanning salt (150g/L) and maintained

at room temperature was ~2.7. When the pH andthe temperature of this solution were similar to

the usual values of the tanning process, certainstructural changes were produced, and at the end

the maxima of absorption bands decreased. Thesechanges originated from hydroxide groups that

act as ligands among central atoms of chromium(III), which leads to a decrease in the initial

sulfate concentration into the sphere of the

complex. Hydroxide groups (as bridge or as

monodentate ligand) produce a shift of theabsorption bands lower than the sulfate groups

[17].The pH of the tanning wastewater was ~4.3,

and the absorption spectrum of this effluent

without suspended solids, obtained under equal

operation conditions than the above-mentionedspectra, has two bands at 414 and 582nm. Both

maxima were increased when the pH wasdecreased with successive additions of sulfuric

acid. The shift of the absorption maxima obtainedwhen lowering the medium pH (from 4.3 up to

2.7), while the sulfate concentration did not present changes, was used as evidence of the

participation of hydroxide groups in the

structural changes produced in soluble chromium

(III) complexes. Despite the reversible character

of the shift, in none of the cases was it possible toobtain an absorption spectrum of the tanningwastewater similar to the absorption spectrum of

a fresh tanning solution. This behaviour was

attributed to the different chemical composition

of both liquids.The transport of H2O from the concentrate

(the tanning wastewater at the test start) towardsthe permeate produced an increase in the

chromium (III) concentration of the concentrate.At the same time, the pH of each permeate waslower than the pH of the respective concentrate.

According to these pH values, it is possible toassume that during RO test, the transport of

H+(H3O+) was also produced in a similar

direction (from the concentrate to the permeate).

The increases of both parameters (concentrationof cation and pH) are aspects that favor the

hydrolytic polymerization of chromium (III)[18,19].

At pH >4.7 a chromium (III) complex saltstarted its precipitation (16% of Cr2O3), and as

the RO test occurred, the amount of precipitateincreased. The addition of sulfuric acid impeded

the precipitation progress and made possible the

start of the redissolution of the solid phase

already formed. However, the redissolution wasnot complete, and an important part remained

retained in the filter placed previous to the high- pressure pump. The chemical analysis of the

concentrate supernatant so obtained is indicated

in Table 4. The formation of the precipitate

during the RO test and its subsequentredissolution with sulfuric acid was a new

evidence of the participation of hydroxide groupsin the chromium (III) polymerization. The

absorption spectra of concentrates obtained in thesuccessive stages of the RO test maintained the

band maxima at wavelengths lower than thosementioned for the commercial tanning salt

recently dissolved.

The aspects above analyzed indicate that the

chromium (III) forms polynuclear complexes in

the tanning wastewater and in the concentratesobtained by RO. These chromium (III) poly-nuclear complexes have sizes larger than the

other ionic species (SO4=, Cl , Na+) present in the

system [1721]. Consequently, chromium (III)

polymers would be the components of the systemwith higher difficulty to pass through the

polyamide membrane used, and this wouldexplain the results obtained.


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Table 4

Chemical analysis of the concentrate supernatant

obtained in this work by RO

Component g/L

SO4=

Cl

Na+

Cr3+

35.90

16.67

14.35

4.09

4. Conclusions

The polyamide membrane used allowed us to

perform an efficient separation of chromium (III)

contained in the tanning wastewater, but the

other components of the system (SO4=, Cl and

Na+) could not be separated satisfactorily.

It was determined that a low protein content(0.007 w/w) in the tanning wastewater had little

influence on the permeate flux. On the otherhand, it was established that the pressure changes

applied to overcome the osmotic pressureproduced variations of the permeate flux higher

than the temperature changes of the feed.

The presence of chromium (III) polymers in

the tanning wastewater and in the concentratesobtained by RO was identified. These chromium

(III) polymers have a more complex molecularstructure than the other ionic species present in

the system which originates a size difference

among effluent components, and this would

contribute to explain results obtained in thiswork.

It was determined that a transport of H+(H3O+)

from the concentrate towards the permeate was

produced. This ionic transfer was analyzed indetail since the medium acidity affects the

chromium (III) polymerization (molecularstructure).

Acknowledgements

The authors thank N.A. Lacour (CICCITEC)and J.A. Rodrguez (UNSLINTEQUI) for their

collaboration in the development of this work.

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[13] M. Aloy and B. Vulliermet, J. Soc. Leather Technol.Chem., 82 (1998) 140.

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