Presented at the Conference on Desalination Strategies in South Mediterranean Countries, cooperation betweenMediterranean Countries of Europe and the Southern Rim of the Mediterranean, sponsored by the EuropeanDesalination Society and Ecole Nationale d'Ingenieurs de Tunis, September 11–13, 2000, Jerba

0011-9164/01/$– See front matter © 2001 Elsevier Science B.V. All rights reserved

Desalination 137 (2001) 241–250

Removal of chromate anions by micellar-enhanced ultrafiltrationusing cationic surfactants

Lassâad Gzara, Mahmoud Dhahbi*Laboratoire de Physicochimie des Interfaces, BP 95, Institut National de Recherche Scientifique et Technique,

2050 Hammam-Lif, TunisiaTel. +216 (1) 430044; Fax +216 (1) 430934; e-mail: mahmoud.dhahbi@inrst.rnrt.tn

Received 1 August 2000; accepted 3 September 2000

Abstract

Micellar-enhanced ultrafiltration (MEUF) of chromate anions (CrO42–) from aqueous streams has been studied at

30°C using twice cationic surfactants (cetyltrimethylammonium bromide and cetylpyridinium chloride). Thesolution is processed by ultrafiltration, using a membrane with pore sizes small enough to block the passage of themicelles and adsorbed ions. Rejection coefficients higher than 99% are reached in optimal conditions of pressure,feed concentration in cationic surfactant, and percent filtered volume. The rejection rate depends on the ionicstrength and pH. The increasing of ionic strength decreases the retention of chromate ions and the permeatesurfactant concentration. As long as the NaCl feed concentration is less than or equal to 100 mM, more than 88% ofhexavalent chromium are retained and surfactant leakage was reduced.

Keywords: Chromate removal; Cetyltrimethylammonium bromide; Cetylpyridinium chloride; Micelles; Membraneprocess; Water treatment; Micellar-enhanced ultrafiltration

1. Introduction

The increasing contamination of urban andindustrial wastewater by toxic metal ions is a_________________________

*Corresponding author.

worrying environmental problem. Theseinorganic micro-pollutants are of considerableconcern because they are non-biodegradable,highly toxic and have a probable carcinogeniceffect. If directly discharged into the sewage

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250242

system they may seriously damage the operationof biological treatment as well as make theactivated sludge unsuitable for application toagricultural land [1].

The traditional techniques for the removal ofmetal ions from aqueous effluents are incapableof reducing concentration to the levels requiredby law (process of reduction or lime precipi-tation) or prohibitively expensive (process of ionexchange, activated carbon adsorption, electro-lytic removal). The use of membrane separationprocess in treating wastewater containing toxicmetal ions is today an attractive and suitabletechnique, and it has to be easily included inwhole process, which is the reason whymembrane separations are being used more andmore frequently. On the other hand, separationcan be carried at room temperature; the modularmembrane surface can be easily adjusted to thewastewater flows; and various industrial mem-branes are now available. In order to retainmetallic ions, reverse osmosis (or at least nano-filtration) can be used due to the size of the ionsin aqueous solutions. But the usual permeatefluxes of reverse osmosis membranes are limited[2] and require high transmembrane pressure,which makes the process expensive.

During the last decade, there has been aconstantly increasing level of interest and researchefforts in order to improve the performances ofsurfactant-based separation processes. As a result,various forms of surfactant-based separationprocesses such as micellar-enhanced ultrafiltration[3–5], micellar extraction coupled with ultra-filtration [6–8], polyelectrolyte-enhanced ultra-filtration [9,10], ion-expulsion ultrafiltration[11], and surfactant modified ultrafiltration [12],have been extensively proposed and studied toremove the metal ions and organic contaminant.

In order to remove metallic chromate anionsfrom aqueous solutions, cationic surfactant isadded to the aqueous stream containing thedissolved solutes. Above the Krafft temperatureand beyond the critical micellar concentration

(cmc), most of the surfactant is present in themicellar form [13]. Micelles are surfactantaggregates that contain about 50 to 100 surfactantmolecules. Chromate anions bind or adsorb onthe surface of the oppositely charged micelles.This solution is then passed through anultrafiltration membrane with pore sizes smallenough to block the passage of micelles. Asmicelles are rejected, the adsorbed chromateanions will also be rejected. The unbound ions,and surfactant monomers pass through theultrafiltration membrane to the permeate side.

Several chromium recovery techniquemethods have been proposed as the adsorption[14,15], precipitation [16] and membraneprocess, specially microfiltration [17], reverseosmosis, ultrafiltration [18–20], and recentlynanofiltration [21].

In the present study an attempt is made toremove chromate anions from aqueous solutionsby MEUF using cationic surfactant cetyltri-methylammonium bromide (CTAB) or cetyl-pyridinium chloride (CPC). The influence ofsome operating parameters on the permeate fluxand rejection of hexavalent chromium is detailed.The process is investigated as a function of ionicforce.

2. Materials and methods

2.1. Chemicals

CTAB was obtained from Fluka (purum); andCPC monohydrate was bought from Aldrich(purum). They were used as received. Potassiumdichromate (K2Cr2O7) R.P Normapur guaranteedreagent was a Prolabo product. Sodium chloride(fluka) was of analytical grade. Deionised doublydistilled water was used throughout.

2.2. Ultrafiltration

Ultrafiltration experiments were carried outwith a tangential cell system (Minitan-S

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250 243

Millipore). The inlet flux was held constant (up to0.5 m.s–1) and a drop in pressure was varied from1 to 3 bar by restricting the outlet tube (Fig. 1).Polysulfone membranes with molecular weightcut-off (MWCO) of 10,000 Da and an effectivefiltration area of 30 cm² were used (PTGCOMS10, Millipore). By totally recycling thepermeate and the retentate, a steady state withrespect to permeate quality is reached after lessthan half an hour under given temperature andpressure conditions. The data presented herewere collected under steady state conditions. Theinlet reservoir was initially filled with a 100 mlfeed solution, and 25 ml samples were collectedthroughout the run. The temperature of the feedsolution was held constant (30°C) and above theKrafft point of the surfactants using a thermostat(Grant instruments).

F

rse

wc[

For the comparison of the fluxes measured indifferent conditions to be rigorously valid, theflux with pure water was systematically checkedbetween two experiments to ensure that therewas no flux decline due to partial plugging. Incase of flux decline the cleaning procedure waspursued until the reference flux was obtained, ora new membrane was used.

2.3. Analysis

The analysis of CTAB, when it is alone, inaqueous solution is carried out through con-ductivity measurements with a conductimeterfrom PHYWE.

Chromate concentrations in the permeate weredetermined using spectrophotometer UV-visibletype Lambda-20 (Perkin-Elmer), at λ = 357.1 nm,according to the method proposed by A.Oumedgfer and O. Thomas [22]. NaOH was

ig. 1. Schematic experimental ultrafiltration system.

To evaluate the filtration efficiency inemoving the chromate anions from the feedolution, we have used the rejection rate Rxpressed as

[ ][ ]

−=

i

p

CrCr

R 1 100 (1)

here R is the percent rejection rate, [Cr]p thehromium ion concentration in the permeate, andCr]i the initial feed concentration.

used to maintain the pH of the standard solutionsand samples at a value slightly higher than eight.

CPC concentrations were measured by UVspectrophotometry at 260 nm.

3. Results and discussion

3.1. Ultrafiltration of chromate in absence ofsurfactant

Fig. 2 represents the variation of the hexa-valent chromium retention %R[Cr(VI)] andpermeate flux as a function of pressure drop forinitial chromate concentration equal to 0.1 mM atT = 30°C. It shows that chromate anionsrejections in water decrease from 20 to 5% whenpressure increase from 0.5 to 3 bar. A negligibledisappearance of hexavalent chromium from thepermeate and the retentate was measured (in-significant chromate adsorption on the memb-rane). Therefore, the observed rejection may beattributed to the presence of the membranecharge. In previous studies, Bhattacharryya et al.

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250244

Fig. 2. Permeate flux and chromate rejection rate as a function of pressure drop in absence of surfactant. T = 30°C,[Cr(VI)]i = 0.1 mM.

[23,24] have shown that the nature of the mem-brane (charged or uncharged) affects the separa-tion of inorganic ions. Those studies showed thatsolute rejection is affected by pressure, soluteconcentration, anion charge density, and inter-action of fixed membrane charge sites with ionicsolutes.

The study of the permeate flux variationaccording to the pressure drop (Fig. 2) showedthat the flux varies linearly with the pressure andobeys to the DARCY’s law (Jv = Lp∆P). Themembrane permeability (Lp), is equal to64.27 L.h–1.m–2.bar–1, when aqueous solutionscontain chromate salts was used. This perme-ability is, at the experimental error range(estimated at 10%), equal to the permeability ofthe same membrane when pure water was used.It implies that the presence of chromate anionsdoes not generate some additive resistance,generally manifested when solutes were filteredthrough the membrane, due to the concentrationpolarisation phenomena.

3.2. Ultrafiltration of surfactant

Since the ultrafiltration process is usually

carried out in the moderate drop pressure rangeand gives relatively high flux, it is widely adoptedfor the separation of colloid and macromolecules.However, in spite of the many advantages ofultrafiltration process, flux decline remains themost serious and inherent obstacle for theefficient application of membrane separationprocess. Flux decline is caused by several factorssuch as concentration polarisation, fouling,adsorption of surfactant, gel layer formation, andpore plugging. Therefore, not only the separationefficiency of metal ions and the optimisation ofprocess variables but also the flux behaviours insurfactant-based ultrafiltration should beinvestigated systematically.

The study of the permeate flux variationaccording to the surfactant concentration in thefeed (Fig. 3) revealed that the permeate fluxdecreases from 73 l.h–1.m–2 to 32 l.h–1.m–2, whenthe concentration in CTAB grows from 10–6 M to6.10–4 M. Beyond this concentration, the fluxincreases, then stabilises towards a value of50 l.h–1.m–2, when the concentration in surfactantvaries between 9.10–4 (~cmc) and 4.10–3 M. Fromthis value the flux decreases and reaches35 l.h–1.m–2 for 6.10–2 M of CTAB.

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250 245

Fig. 3. Permchromate. T

For [moleculesthe size ofdiameter. easily cropartly rethindered athe permeless than tby chargea solute spmembraneretentate surface theffect canmonomersbulk solutthe vicinit

If [CTformed inthe membincreases intensity. al. [25,26sodium do

eate flux and permeate CTAB concentration as a function of initial CTAB concentration in absence of = 30°C, ∆P = 1.5 bar.

CTAB]i < cmc, all the surfactant are under the form of free monomers, which is largely smaller than the poreIn these conditions, monomers shouldss the barrier, and yet the surfactant isained. The surfactant monomer iss it passes through the membrane into

ate since the permeate concentration ishe cmc; this hindrance may be caused or steric effects. The concentration ofecies being rejected by an ultrafiltration becomes higher in the region of thesolution adjacent to the membranean in the bulk retentate solution. This generate a diffusive flow, of surfactant, from the membrane surface to theion due to the formation of micelles iny of the membrane.AB]i is towards the cmc, micelles are both bulk solution and the vicinity ofrane. As a result, the permeate fluxwith the decrease in the diffusive flowSimilar effect was shown by Azoug et] in the case of the ultrafiltration ofdecylsulfate – Cd2+ system. They had

attributed this fouling effect to the formation ofsmall micelles with both sodium and cadmium atlow Na+/Cd2+ ratio.

Finally, when [CTAB]i is higher than thecmc, the flux of solvent through the membranedecreases due to an increase in the overallresistance to flow caused by the gel layer.

Concerning the permeate concentration ofsurfactant [CTAB]p, the global trend is anincrease, but discontinuities may be noted. For[CTAB]i < cmc, values towards 50% surfactantrejection are reached. Due to the small size ofsurfactant monomers compared with the memb-rane pore size, the sieving effect can in principlebe neglected. This rejection can be attributed tothe adsorption of surfactant at the membrane andthe membrane charge, as discussed below. Then,when [CTAB]i > cmc, [CTAB]p increases with[CTAB]i and never exceeds the cmc value.

Whatever the concentrations in the feed, thesurfactant concentration in the permeate is lowerthan cmc. Consequently pollution by organics isweak. As it was shown by Sadaoui et al. [27] bymeasuring Total Organic Carbon <15 mg.l–1,Chemical Oxygen Demand < 500 mg.l–1 and

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250246

Biological Oxygen Demand < 2 mg.l–1 forpermeate containing 0.8 mM of CTAB and0.1 mg.l–1 of chromate, which is the most con-centrated among the satisfactory permeate.

3.3. Ultrafiltration of chromate in the presenceof surfactant

This part of the study includes experimentswith both the surfactant (CTAB or CPC) andmetallic ion. The pH of all solutions, preparedfor UF experiments, are measured; and theirvalues were found towards 6.5. At those pH,chromate anions coexist at two different formsCrO4

2– and HcrO4–. Henceforth, those two forms

of hexavalent chromium will be designed byCr(VI). Where [Cr(VI)] = [CrO4

2–] + [HCrO4–].

The permeate flux and chromate rejection, at1.5 and 3 bar transmembrane drop pressure, areplotted as a function of initial surfactant feedconcentration [CTAB]i in Fig. 4 at initialchromate concentration equal to 2.10–4 M.

It shows a rejection rate higher than 80%, atsurfactant concentrations moderately below thecmc, where micelles are absent. This is con-sidered as due to a higher surfactant concentration

in the gel layer adjacent to the membrane wheremicelles are present. Those results plead in favourof the concentration polarisation. Fillipi et al. [29]reported analogous results on the rejection ofzinc cations using anionic surfactant (SDS) atconcentrations moderately below the cmc.

Fig. 5 represents the variation of permeateflux as a function of a drop in pressure fordifferent chromate concentration ranged from0.1 to 1 mM and at 20 mM CTAB concentration.It shows that the permeate flux depends linearlyon the pressure drop according to the Poiseuillerule when pressure is ranged from 0 to 2 bar;beyond, the Poiseuille rule is not verified. Thiscould be due to an increase of the polarisationlayer thickness generated by a low circulationvelocities of the feed on the membrane.Analogous behaviours have been reported bySadaoui et al. [27], Poiseuille’s rule was verifiedat high circulation velocities. Regarding theinfluence of the chromate concentration upon theflux, it may be seen that the increase in thechromate concentration results in a slightdecrease in the permeate flux. This result may beexplained as follows: When chromate is present,mixed micelles are formed and exchanges take

Fig. 4. PermT = 30°C.

eate flux and chromate rejection rate as a function of initial CTAB concentration. [Cr(VI)]i = 2⋅10–4 mM,

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250 247

Fig. 5. Permeate flux as a function of pressure drop for different initial chromate concentrations. T = 30°C, [CTAB]I = 20 mM.

place between Cr(VI) and Br– in the neighbour-hood of the polar heads. The increase of themetal concentration leads the increase of theCTA[Cr(VI)] micelles concentration. Since thelatter are smaller than CTAB micelles, they mayenter the membrane pores, thus causing aplugging and a correlative decrease of the flux.

Table 1 reports chromate concentration in thepermeate and rejection rate as a function ofchromate concentration in the feed, for threeinitial CTAB concentrations (8, 20 and 60 mM)and at different drop pressure. It appears that: For a given pressure and fixed [CTAB]i, the

increase of chromate concentration in the feedincreases the permeate hexavalent chromiumconcentration.

For a given pressure and fixed [Cr(VI)]i, theincrease in [CTAB]i is accompanied by a risein chromate rejection.

For a given [Cr(VI)]i and fixed [CTAB]i, thegrowth in pressure grows the chromaterejection.

For all surfactant/chromate systems, rejectionexceeds 99.5%.The increase of the micelles concentration

makes grow sites on which the metallic ions aregoing to be adsorbed. The increase of the pressure

increases the gel layer thickness, which cause anincrease in concentration of micelles in the gellayer and an increase in chromate rejection.

Rejection of chromate in the presence of CPCsurfactant (20 and 60 mM) was studied at a 3 barpressure drop and different feed chromateconcentration. CPC concentration in the permeatewas determined. Results were summarised inTable 2. It shows that the increase in the initialchromate concentration decreases [CPC]p andincreases [Cr(VI)]p, whereas rejection remainsapproximately constant towards 99.5. Thepermeate corresponding to a feed chromateconcentration ranged between 200 and 300 timeshigher than the norm, is purified under acceptableoperating conditions.

3.4. Salt effect

One of the major drawbacks in surfactant-based ultrafiltration is the surfactant leakageproblem, especially in the high retentate concen-tration range. Monomeric surfactant moleculeswhich cannot be retained by the membrane existinherently at the concentration close to the cmc.The surfactant leakage to the permeate anddischarge to the ecosystem cause secondary

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250248

Table 1Chromate permeate concentration and rejection rate as a function of chromate concentration in the feed, pressure drop,and feed CTAB concentration

Feed solutions Permeate

[CTAB]i, mM [Cr(VI)]i, mM [Cr(VI)]p × 10³ mM %R [Cr(VI)]∆P = 1 bar ∆P = 1.5 bar ∆P = 1 bar ∆P = 1.5 bar

8 0.1 0.77 0.48 99.23 99.528 0.2 0.67 0.85 99.66 99.588 0.4 1.54 0.88 99.62 99.788 0.6 2.69 1.54 99.55 99.748 0.8 4.04 1.35 99.50 99.838 1 5 3.08 99.50 99.69

∆P = 1.5 bar ∆P = 3 bar ∆P = 1.5 bar ∆P = 3 bar

20 0.1 0.54 0.02 99.46 99.9820 0.2 0.44 0.79 99.78 99.6120 0.4 0.52 1.60 99.87 99.6020 0.6 1.19 0.96 99.80 99.8420 0.8 1.67 1.98 99.79 99.7520 1 2.5 2.02 99.75 99.80

∆P = 1 bar ∆P = 1.5 bar ∆P = 1 bar ∆P = 1.5 bar

60 0.1 0.31 0.10 99.69 99.9060 0.2 0.44 0.21 99.78 99.8960 0.4 0.58 0.54 99.86 99.8760 0.6 1.02 0.98 99.83 99.8460 0.8 0.60 0.96 99.93 99.8860 1 1.67 1.40 99.83 99.86

Table 2MEUF results in presence of CPC at 3 bar drop pressure

[Cr(VI)]p × 10³ mM [CPC]p, mM %R(Cr)

[Cr(VI)]i, mM [CPC]i=20, mM [CPC]i=60, mM [CPC]i=20, mM [CPC]i=60, mM [CPC]i=20, mM [CPC]i=60, mM

0.1 0.58 1.54 1.5 3.08 99.42 98.460.2 0.77 1.73 1.41 2.58 99.62 99.130.4 1.35 1.54 1.56 2.39 99.66 99.620.6 2.88 2.50 1.75 2.37 99.52 99.580.8 3.65 2.12 2.05 2.16 99.54 99.741 5.00 4.62 2.8 1.85 99.5 99.54

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250 249

pollution instead of the removed metal ions. Andthe leakage of the surfactant is also undesirablefrom the economic viewpoint unless the leakedsurfactant is recycled to the feed stream.

It is well known that the presence of saltsdecreases the cmc of ionic surfactants, due to theelectrostatic shielding effect: the repulsive forcesbetween the head groups are normally fightingagainst the aggregation, which becomes easier inthe presence of electrolyte.

So, we studied, in this section, the salt additioneffect upon chromate rejection and surfactantleakage. Fig. 6 represents the variation ofchromate rejection and CPC permeate concen-tration as a function of NaCl concentration in thefeed.

It revealed that the CPC permeate con-centration decreases from 1 to 0.15 mM, whenthe concentration of NaCl grows from 1 to500 mM, whereas chromate rejection remainshigher than 90%, even in presence of 100 mM ofNaCl. Beyond this concentration, rejectiondecreases and reaches 46% for 0.5 M of NaCl.

Chromate rejection rate higher than 88% wasobtained, indicating that excellent separationscan be still be attained by MEUF at 100 mM

NaCl. Permeate surfactant concentration lowerthan 0.2 mM (five times less than the cmc) wasreached. Reducing the cmc results in a smallermonomeric surfactant concentration in thepermeate, eliminating an additional surfactantremoval “polishing” step that may be requiredotherwise.

4. Conclusion

Removal of chromate ions from water byMEUF using cetylpyridinium chloride and cetyl-trimethylammonium bromide was investigated.

Hexavalent chromium was rejected atsurfactant concentrations where theoretically nomicelles exist. The observed rejection of Cr(VI)in the vicinity of the cmc of the surfactant is dueto the presence of concentration polarisation andmembrane charge effects. Concentration polari-sation results in the formation of micelles nearthe membrane surface below the surfactant cmc.

For all surfactant/chromate systems, studiedat surfactant concentration above the cmc,rejection reached 99.5%.

Fig. 6. Chroma60 mM, T = 30°

te rejection rate and permeate CPC concentration as a function of initial NaCl concentration. [CPC]I =C, [Cr(VI)]i = 0.1 mM, ∆P = 3 bar.

L. Gzara, M. Dhahbi /Desalination 137 (2001) 241–250250

The effect of addition of salt could sub-stantially improve rejection of surfactant, whereaschromate rejection remains higher than 88%.Micellar-enhanced ultrafiltration is a suitableseparation process for aqueous solutionscontaining hexavalent chromium in presence of100 mM of NaCl.

References

[1] P. Madoni, D. Davoli, G. Gorbi and L. Vescovi,Water Res., 30 (1996) 135.

[2] X. Chai, G. Chen, P.L. Yue and Y. Mi, J. Membr.Sci., 123 (1997) 235.

[3] R.O. Dunn, J.F. Scamehorn and S.D. Christian,Colloid Surf., 35 (1989) 49.

[4] A. Hafiane, I. Issid and D. Lemordant, J. ColloidInterf. Sci., 142 (1991) 167.

[5] J.F. Scamehorn, S.D. Christian, D.A. El-Sayed,H. Uchiyama and S.S. Younis, Sep. Sci. Technol.,29 (1994) 809.

[6] M. Hebrant, S.-G. Son, C. Tondre, P. Tecilla andP. Scrimin, J. Chem. Soc., 88 (1992) 209.

[7] C. Tondre, S.-G. Son, M. Hebrant, P. Scrimin andP. Tecilla, Langmuir, 9 (1993) 950.

[8] M. Hebrant, A. Bouraine, C. Tondre, A. Brembillaand P. Lochon, Langmuir, 10 (1994) 3994.

[9] M. Tuncay, S.D. Christian, E.E. Tucker, R.W. Taylorand J.F. Scamehorn, Langmuir, 10 (1994) 4688.

[10] M. Tuncay, S.D. Christian, E.E. Tucker, R.W. Taylorand J.F. Scamehorn, Langmuir, 10 (1994) 4693.

[11] D.K. Krehbiel, J.F. Scamehorn, R. Ritter, S.D.Christian and E.E. Tucker, Sep. Sci. Technol. 27,(1992) 1775.

[12] G. Morel, N. Ouazzani, A. Garciaa and J. Lachaise,J. Membr. Sci., 134 (1997) 47.

[13] M.J. Rosen, Surfactant and Interfacial Phenomena,2nd ed., Wiley, 1995, pp. 2331.

[14] J. Pradhan, S. Nath and R. Sing Thakur, J. ColloidInterf. Sci., 217 (1999) 137.

[15] G.S. Gupta and Y.C. Sharma, J. Colloid Interf. Sci.,168 (1994) 118.

[16] C. Visvanathan, R. Ben Aim and S. Vigneswaran,Desalination, 71 (1989) 265.

[17] B. Keskinler, U. Danis, A. Cakici and G. Akay,Sep. Sci. Technol., 32(11) (1997) 1899.

[18] S.D. Christian, J.F Scamehorn, S.N. Bhat, D.A.Elsayed and E.E. Tucker, AIChE J., 34 (1988) 189.

[19] M. Renault, F. Aulas and M. Rumeau, Chem. Engr.J., 23 (1982) 137.

[20] Z. Sadaoui, Thesis, Aix-Marseille University,France, 1995.

[21] A. Hafiane, D. Lemordant and M. Dhahbi, Desali-nation, 130 (2000) 305.

[22] A. Oumedjbeur and O. Thomas, Analysis, 4 (1989)212.

[23] D. Bhattacharyya, A.B. Jumawan and R.B. Grieves,Sep. Sci. Technol., 14 (1979) 441.

[24] D. Bhattacharyya, D.P. Schaaf and R.B. Grieves,Cand. J. Chem. Engr., 54 (1976) 185.

[25] C. Azoug, Z. Sadaoui, F. Charbit and G. Charbit,Cand. J. Chem. Engr., 75 (1997) 743.

[26] Z. Sadaoui, C. Azoug, G. Charbit and F. Charbit, J.Envir. Engr., (1998) 695.

[27] Z. Sadaoui, C. Azoug, G. Charbit and F. Charbit, J.Chem. Engr. of Japan, 30 (1997) 799.

[28] D. Doulia, V. Gekas and G. Trägårdth, J. Membr.Sci., 69 (1992) 251.

[29] B.R. Fillipi, L.W. Brant, J.F. Scamehorn and S.D.Christian, J. Colloid Interf. Sci., 213 (1999) 68.