Cr(VI) Extraction Using Aliquat 336 in a Hollow Fiber Module Made of

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SEPARATIONS Cr(VI) Extraction Using Aliquat 336 in a Hollow Fiber Module Made of Chitosan Thierry Vincent and Eric Guibal* Laboratoire Ge´ nie de l’Environnement Industriel, Ecole des Mines d’Ale` s, 6 avenue de Clavie` res, F-30319 Ales Cedex, France

Chitosan hollow fiber modules have been prepared and tested for solvent extraction of Cr(VI) from dilute solutions using Aliquat 336 as the carrier. Chitosan membranes serve as a reactive barrier between aqueous and organic phases: both passive and active transport through the membranes are suspected to occur, depending on the experimental conditions and the characteristics of the membrane. The pH of the solution is a key parameter for the efficient extraction of Cr(VI): pH should be maintained below pH 4.5. Extraction kinetics and equilibrium are also controlled by the solvent used to dilute the carrier: kerosene is better than cumene and xylene. The reacetylation of chitosan, resulting from the reaction of acetic anhydride with the biopolymer (after water exchange with methanol as the solvent), is expected to increase its resistance to chemical degradation in very acidic solutions but decreases its extraction efficiency. Introduction The removal of metal ions from dilute or concentrated solutions has received a great deal of attention in the last 40 years, to recover high-cost metals or to decontaminate effluents.1 A number of these studies have developed sorption processes including adsorption and ion-exchange mechanisms using simple materials such as activated carbon2 or more sophisticated materials such as especially tailored resins,3 as well as biosorbents, such as fungal or bacterial biomass, and materials of biological origin, including alginate and chitosan.4-6 Solvent extraction has also received a great deal of attention, especially for the treatment of solutions with high relative metal concentrations.7-9 The major drawback of these processes is the dispersion of the solvent: this loss has environmental and economic implications. In the last 20 years, several techniques have been proposed to reduce solvent/extractant loss, including the impregnation of resins with the extractant system (extractant/diluent).10 Nondispersive solvent extraction with hollow fiber systems has been developed: the extractant (pure or diluted in a solvent) circulates inside the lumen of the hollow fiber, while the effluent to be treated is brought into contact with the shell side of the fiber.11 Hollow fiber systems are usually based on synthetic polymer fibers [poly(tetrafluoroethylene), polyacrylonitrile, polysulfone, etc.].12,13 These materials are usually weakly reactive with metal ions, acting only as porous membranes between feed and extraction cells (or extraction and stripping cells). This work examines new hollow fiber systems made of chitosan, which is known to be reactive with metal ions.5,6,14-16 Chitosan is an * To whom correspondence should be addressed. Phone: +33 (0)4 66 78 27 34. Fax: +33 (0)4 66 78 27 01. E-mail: [email protected].

aminopolysaccharide extracted by alkaline deacetylation from chitin, the most abundant biopolymer in nature after cellulose. Its high amine content makes it very reactive with metal ions through complexation, adsorption, or ion-exchange mechanisms, depending on the metal and the pH of the solution. Flat chitosan membranes have been investigated for the “reactive filtration” of Cr(VI) using a pressurized tank (Amicon-like vessel).15 Chitosan has also been studied as a liquid chelating agent for the recovery of copper in synthetic hollow fibers.17 The design of the system investigated in this study is based on a dual function: (a) the chitosan hollow fiber may be considered as an adsorbent, and the extractant which flows inside the lumen acts as an eluant to concentrate the metal and to recycle the sorbent; (b) the chitosan hollow fiber acts as a phase separation between the organic and aqueous compartments. The concept has already been successfully tested with a single chitosan hollow fiber for Cr(VI) extraction in a discontinuous system with recirculation of the extractant (Aliquat 336, tricaprylylmethylammonium chloride).18 Several amine extractants (Aliquat 336, Alamine 336, and Amberlite LA-2) were tested, giving comparable extraction efficiencies at low pH. It was found that Cr(VI) extraction followed a pseudo-firstorder kinetic equation and that extraction was mainly controlled by the length of the fiber rather than the concentration of sulfuric acid (in acidic solutions, pH < 2) and the concentration of the extractant. Different experimental conditions have been tested including the volume of recirculated extractant, the presence of trin-butyl phosphate (as the diluent/stabilizer), the solution volume, the metal concentration, and the experimental design (influence of the preimpregnation of the fiber). The influence of the extractant flow rate was also tested: this experimental parameter did not influence the extraction rate under selected experimental condi-

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tions.18 The affinity of Aliquat 336 for Cr(VI) anions is so high that even low extractant concentrations were sufficient to extract all of the Cr(VI) ions in the system. These different results indicated that extraction is predominantly controlled by the rate of Cr(VI) sorption on the chitosan fiber rather than by the transfer from the fiber to the organic phase. New extraction systems have been designed using hollow fiber modules that have been prepared by connecting several hollow chitosan fibers in parallel. This work is focused on the study of Cr(VI) extraction performance using such a new system; it completes the previous study by investigating complementary experimental parameters. These hollow chitosan fiber modules were tested for Cr(VI) extraction using Aliquat 336 at various pH’s, with different diluents. To increase the chemical resistance of the fibers in such drastic conditions (the mixture of sulfuric acid and Cr(VI) anions gives sulfochromic acid), the chitosan fibers were reacetylated. The extraction kinetics of hollow chitosan fiber modules made of untreated chitosan and reacetylated chitosan were compared. Experimental Section Materials. Chitosan flakes were purchased from Aber Technologies (France). Samples were characterized in advance:6 the deacetylation percentage (FT-IR spectroscopy analysis) and molecular weight (using sizeexclusion chromatography) were found to be 87% and 125 000 g mol-1, respectively. K2Cr2O7 was purchased from Carlo Erba (Italy); other reagents were provided by Fluka (Switzerland). Hollow fibers of chitosan were produced by extruding the viscous chitosan solution (in acetic acid solution) into a coagulating bath using a procedure derived from Kaminski et al.19 and Agboh and Qin.20 After external coagulation, the lumen of the fiber was emptied by withdrawing noncoagulated chitosan by means of air flow followed by water flow. The final fiber was obtained by treatment with alkali. Finally, fibers were dried at room temperature. The external diameter of the fiber was 1.1 ( 0.1 mm, and the thickness of the fiber wall was 0.1 ( 0.015 mm. Figure 1 shows scanning electron micrographs of the fibers after air-drying. Reacetylation of the chitosan fibers was performed in a two-step procedure. First, the water contained in the fiber was exchanged with methanol: the fiber was immersed in two successive methanol baths for 1 h. Finally, it was kept for 8 h in an acetic anhydride solution (3% v/v in methanol) at 80 °C. Traces of unreacted acetic anhydride were removed through extensive rinsing with methanol and then with water. The fibers were finally dried at room temperature. The hollow fiber modules were prepared by pasting all of the fibers to a connector with liquid epoxy resin. Viscous epoxy resin was spread around the connector and the fibers to make the module watertight (Figure 2). Procedures. Unless specified, 350 mL of a metal ion solution at known concentration (50 mg of Cr(VI) L-1, unless specified) with controlled pH was brought into contact with the hollow fiber (the extractant was circulated inside the lumen of the membrane). Zumer et al.21 have shown that when Alamine 336 is used, Cr(VI) extraction is much more efficient from acidic solutions, while Cr(VI) extraction is less influenced by the pH with Aliquat 336. The sulfate form of Aliquat

Figure 1. Scanning electron micrographs of chitosan hollow fiber after air-drying (hollow fiber + detail of fiber surface).

Figure 2. Hollow fiber module.

336 is more efficient for Cr(VI) recovery. So, solutions of Cr(VI) in H2SO4 were used. The flow rate was controlled at 300 mL h-1, and the extractant (20 mL) was pumped and recirculated through the hollow fiber (Figure 3). Samples were collected from the aqueous phase at selected times. The withdrawn volume was sufficiently low to allow the total volume to be considered as constant. Though the flux across the membrane represents an important criterion in many studies dealing with membrane separation, in the present case the extraction is assumed to proceed through a direct sorption of Cr(VI) on the fiber, at the water/membrane interface, followed by a diffusion in the membrane and finally a reextraction at the membrane/extractant interface. According to this hypothesis on the extraction mechanism, the influence of the initial flux across the membrane has not been considered. The extractant was dissolved in kerosene, and isodecanol was added as a modifier to increase the solubil-

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Figure 3. Experimental setup.

Figure 4. Influence of the sulfuric acid concentration on the Cr(VI) extraction with chitosan hollow fiber (pH 1.63, 0; pH 1.48, 4; pH 1.28, O; pH 1.01, ]) Experimental conditions: fiber length, 4.4 m; extractants, Aliquat 336 (3%), decanol (1%); extractant volume, 20 mL; solution volume, 200 mL.

ity of the extracted complexes in the organic phase and, therefore, prevent the formation of a third phase. However, Zumer et al.21 have reported a reduction in the Cr(VI) concentration in the presence of isodecanol, when it was added as a modifier for Cr(VI) extraction by Aliquat 336 in strongly acidic solutions. The Cr(VI) concentration was measured in the aqueous phase using either inductively coupled plasma atomic emission spectrometry (Jobin-Yvon JY 2000, France) or spectrophotometry with a procedure derived from Charlot.22 The latter consists of determining the absorbance of the solution, at 350 nm, at controlled acidic pH (4 mL of sulfuric acid (1/4) was added to 1 mL of an aqueous sample, with the final volume being adjusted to 25 mL) using a spectrophotometer (Shimadzu UV-1601, Japan). The direct absorbance of the acid solutions was also measured at 350 nm with a Varian UV detector. Results and Discussion Influence of pH on Extraction Kinetics. Figure 4 shows the influence of the sulfuric acid concentration on Cr(VI) extraction using a single chitosan hollow fiber system. Increasing the concentration of the acid hardly changes the extraction kinetics. Therefore, it appears that, below pH 2, increasing the acidity (in the range pH 1-2) does not interfere with the extraction mechanism at all. Figure 5 shows the extraction kinetics at different higher pH’s for a kerosene/xylene mixture (1/ 1, v/v; Figure 5a) and xylene (Figure 5b) as the solvents. For each solvent, increasing the pH above 5-5.5 results in a significant decrease in extraction kinetics. When

Figure 5. Influence of the pH on the Cr(VI) extraction with different solvent systems: (a) kerosene + xylene (1/1, v/v) for pH 4.5 (0) and pH 5.8 (O); (b) xylene for pH 3.2 (0), pH 4.6 (O), and pH 6 (4). Experimental conditions: 32 fibers of 0.18 m; total fiber length, 5.8 m.

kerosene is added to xylene, the total extraction at equilibrium is comparable at pH 4.5 and 5.8. In the case of pure xylene as the solvent, however, the extraction kinetic curve at pH 6 falls very slowly, with an extraction yield that does not exceed 40% after 150 min of contact. Though the relative concentration decay curve continues to decrease after 150 min of contact, the trend is significantly different from that observed when kerosene is added to xylene. The comparison of Figure 5 with Figure 4 shows that, under the best extraction conditions [ca. kerosene + xylene as the diluent and a chitosan hollow fiber module (CHFM)], more than 90% of the total extraction was achieved within the first 60 min of contact at pH 4.5, while extraction did not exceed 40% at pH lower than 2 (single hollow fiber). It is difficult to directly compare experiments that were performed under different experimental conditions: single fiber (similar to fibers in series) and fiber module (fibers in parallel), total fiber length. However, this comparison gives a trend on the effect of the pH. The extraction performance is comparable at equilibrium, but the time required to reach the equilibrium increases when decreasing the pH below 4.5. There are several possible reasons that may explain the better extraction of Cr(VI) anions in acidic solutions compared with near-neutral conditions. Previous experiments on Cr(VI) sorption using chitosan derivatives have shown that Cr(VI) uptake is increased in acidic conditions.15,24 In very acidic solutions, the Cr(VI) concentration decreases in the presence of glutaraldehyde-cross-linked chitosan gel beads because of both adsorption and reduction mechanisms, while at around pH 4, the sorption efficiency decreases but reduction mechanisms can be neglected. At higher pH (around pH 6), the sorption efficiency is considerably reduced. The pKa of chitosan depends on several parameters, including the deacetylation degree, but varies between 6.2 and

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Figure 6. Influence of the solvent on the Cr(VI) extraction with different solvent systems: kerosene (0); xylene (]); cumene (4); kerosene + xylene (1/1, v/v; O). Experimental conditions: 32 fibers of 0.18 m; total fiber length, 5.8 m; pH 4.5.

6.5; increasing the pH results in a decrease of the protonation of chitosan, making it less efficient at removing Cr(VI) anions. Cr(VI) sorption by chitosan is suspected to occur through an anion-exchange mechanism: increasing the pH thus results in a decrease in the electrostatic attraction between Cr(VI) anions and the remaining protonated amine sites.23 On the other hand, previous investigations on Cr(VI) extraction using Aliquat 336 have shown that the Cr(VI) extraction efficiency remains almost constant between pH 2 and 5 (in sulfate and chloride media), while above pH 5, the extraction significantly decreases. This decrease in the extraction efficiency is related to the speciation of Cr(VI) and especially to the decrease in fractions of HCrO4and Cr2O72- with increasing pH.24 It is consistent with the results shown in Figure 4. For pH lower than 2 in the Cr(VI) concentration range investigated in this study, Cr(VI) speciation is not really changed by decreasing the pH, and extraction performances therefore remain constant. The decrease in extraction that occurs with increasing pH may thus be explained by either the sorption step (from the aqueous phase to the sorbent) or the extraction step (from the fiber to the organic phase), as a function of metal speciation and the overall sorbent charge. In acidic solution, Cr(VI) removal is maximum at pH 2-3, but below pH 4, the lower the pH, the greater the fraction of Cr(VI) which has been reduced to Cr(III), especially with cross-linked material.23 Below pH 2, extraction decreases again because of a strong competition of counteranions (protonated amine groups are saturated by these competitor anions and are less available for Cr(VI) anionic species) and the fraction of Cr(VI) reduced to Cr(III) strongly increases. In the case of non-cross-linked beads, reduction occurs (thanks to the reducing ends of the biopolymer) but to a lower extent than in the case of cross-linked sorbent. The reducing effect of chitosan may interfere with extraction kinetics because Cr(III) species are nonextractable using Aliquat 336; thus, the extraction rate can be slowed. To investigate the extraction mechanism further, the pH was maintained at 4.5 and the influence of the diluent was tested using the same module. Influence of Diluent on the Extraction Efficiency and Kinetics. Figure 6 shows the extraction of Cr(VI) anions at pH 4.5 with different diluents using Aliquat 336 in the presence of isodecanol as the modifier. It appears that, at this pH, the equilibrium

concentration is independent of the diluent used to dilute the extractant: the affinity of Aliquat 336 for Cr(VI) is not influenced by the diluent under the experimental conditions selected. On the other hand, the diluent significantly influences extraction kinetics. The time required to reach the same extraction levels increases in the following order: kerosene < cumene < xylene. The mixture of kerosene/xylene (1/1, v/v) is slightly slower than pure kerosene. Molinari et al.25 discuss the influence of the diluent on the extraction of Cr(VI) ions comparing several extractants diluted in several solvents, using supported liquid membranes (SLM), and conclude that the transfer through the membrane depends more on the effect of the combined extractant-diluent system than on the effect of the diluent alone. The influence of the solubility of the extractant in the diluent and the viscosity of the extractant-diluent mixture has been highlighted and modeled by Elhassadi and Do26,27 for metal extraction using SLM. However, the optimization of the process requires an exhaustive study: Mahdi,28 comparing Au(III) extraction both through solvent extraction and with SLM, has shown that the ranking of the distribution coefficient obtained in solvent extraction experiments is reversed when considering the permeation of gold anions through SLMs. Palet et al.29 obtained similar conclusions with vanadium(V) extraction: liquidliquid extraction was not affected by the choice of the diluent (cumene or isodecane), while mass transfer through the SLM was significantly decreased with cumene, especially at low carrier (Aliquat 336) concentration. Szpakowska30 also observed that transfer from SLM depended on the solvent but concluded that the effect of the polymeric support is negligible. Another possible explanation is related to the change in the wettability/hydration of the chitosan fiber because of the influence of the organic solvent: water exclusion, water transfer in the solvent, and, therefore, Cr(VI) uptake may be affected by the solvent, because of a change in the accessibility to the solvent-impregnated membrane. Influence of Chitosan Reacetylation on the Extraction Efficiency and Kinetics. The main interest of the reacetylation treatment is related to the higher stability of chitin in acidic media compared to chitosan. Resistance to hostile chemical environments may be increased by cross-linking treatments using, for example, glutaraldehyde. However, several experiments performed on cross-linked chitosan have confirmed that the cross-linking operation results in a decrease in the number of free amine groups, which are the main reactive sites. The decrease in sorption performances is especially significant when metal extraction results from complexation/binding interactions with the sorbent. When metal extraction occurs via an ion-exchange mechanism, the influence of cross-linking on extraction is less significant, especially when the sorbent is macroor mesoporous. Moreover, the cross-linking results in brittle fibers: operating the fibers becomes difficult because they break easily under mechanical stress. For this reason, the reacetylation procedure is preferable: the chitosan fibers, after reacetylation, are still flexible and workable. The degree of reacetylation was not determined; however, the number of free amine groups remains sufficiently high to maintain Cr(VI) sorption (though at a lower level than with chitosan fibers), and a rapid dissolution testing (with a hydrochloric acid solution) confirms that the reacetylated fibers are

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Figure 7. Influence of chitosan reacetylation on Cr(VI) extraction: ACHFM (0); CHFM (O). Experimental conditions for ACHFM: 36 fibers of 0.19 m; total fiber length, 6.8 m. Experimental conditions for CHFM: 32 fibers of 0.18 m; total fiber length, 5.8 m; pH 4.5.

considerably more stable than the chitosan fibers. Figure 7 compares Cr(VI) extraction kinetics using CHFM and reacetylated CHFM (ACHFM) under comparable experimental conditions (extractant volume and concentration, solution volume, metal concentration, and pH). Despite the greater total length of the fibers in the ACHFM than that in the CHFM, Cr(VI) extraction is significantly faster for the chitosan membranes than for the acetylated chitosan system. Though the equilibrium concentration is almost the same for both ACHMF and CHFM, the time required to reach the same extraction levels strongly increases when chitosan is reacetylated (2-3-fold increase). This difference in extraction performances is mainly due to two parameters related to the structure of the fiber: (a) the reacetylation of the fiber results in a decrease in the reactivity of the chitosan material for Cr(VI) anions, and (b) the reacetylation may also affect the hydrophilicity of the fiber and decrease the transfer between the aqueous phase and the sorbent. Preliminary investigations on Cr(VI) sorption using chitin and low deacetylation degree chitosan have shown that Cr(VI) is less sorbed on chitin-like materials mainly because of the lower availability and accessibility of sorption sites to metal anions. Modrzejewska and Kaminski15 also observed a decrease in Cr(VI) transport through flat membranes of acetylated chitosan in acidic solutions. To optimize the fabrication of the module, both the reactivity of the fiber and its stability must be taken into account: the appropriateness of the reacetylation treatment is conditioned by the kind of solution and the experimental conditions selected for metal recovery. Influence of the Cr(VI) Concentration on the Extraction Efficiency and Kinetics. Figure 8 shows the influence of the Cr(VI) concentration on Cr(VI) extraction kinetics using Aliquat 336 with an acetylated CHFM. Figure 8a represents the relative concentration decay curve as a function of contact time. Whatever the initial concentration, the final relative concentration tends to the same value in the range 0.01-0.03. Extraction kinetics are little influenced by the initial concentration: curves are almost overlapped for intermediary concentrations (ca. 10-25 mg L-1), while for initial concentrations of 5 and 50 mg L-1, the extraction kinetics are slightly slower. Figure 8b represents the quantity of Cr(VI) anions extracted from the solution. The equilibrium is achieved within the first 2 h of contact, independently of the initial concentration. At equilibrium, the percentage of Cr(VI) extracted from the

Figure 8. Influence of the Cr(VI) concentration on the Cr(VI) extraction efficiency (a) and the amount of extracted Cr(VI) (b). C0 (mg of Cr(VI) L-1): 50, 0; 25, ]; 10, O; 5, 4. Experimental conditions for ACHFM: 36 fibers of 0.19 m; total fiber length, 6.8.

solution reached 96%, 98.3%, 98.5%, and 98.6% for 5, 10, 25, and 50 mg L-1, respectively. Lo and Shiue24 have established that the ratio of Cr(VI) in the extractant to the dosage of Aliquat 336 is nearly 1/1: thus, it can be assumed that 1 mol of Aliquat 336 extracts 1 mol of Cr(VI). Under the experimental conditions selected in this study, Aliquat 336 is in excess in comparison to Cr(VI) anions and it was predictable that the same extraction level should be reached at equilibrium. On the other hand, the sorption step is suspected to control extraction kinetics rather than extraction from the fiber to the organic phase. Previous experimentations have shown that Cr(VI) sorption on chitosan derivatives is little dependent on the initial concentration.23 Several Cr(VI) species may coexist with different affinities for sorption sites (HCrO4-, Cr2O72-, CrO42-, or isopolyacids). Metal ion speciation has been shown to be a key parameter for metal ion sorption on chitosan derivatives:32 molybdate sorption in acidic solutions was related to the predominance of polynuclear hydrolyzed anionic species; similar conclusions were found by Nekova´r and Schro¨tterova´33 for molybdate extraction using Primene JM-T as the extractant. Salazar et al.31 also reported the influence of metal speciation on Cr(VI) extraction using Aliquat 336. Conclusions Hollow fiber modules prepared with chitosan (and its reacetylated form) are very efficient at removing Cr(VI) anions from dilute acidic solutions in a solvent extraction process with Aliquat 336 as the extractant. Extrac-

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tion results from two mechanisms including sorption of Cr(VI) anions on the fiber and a solvent extraction from the fiber to the organic phase, which flows inside the lumen of the fiber. Under the experimental conditions selected, corresponding to an excess of extractant, the extraction efficiency is independent of the initial concentration of Cr(VI) anions. The optimum pH for Cr(VI) removal is below pH 5, and the extraction efficiency is little influenced by the pH or the acid concentration below this pH value. This optimum pH range may be explained by the optimum sorption of Cr(VI) on chitosan derivatives in acidic solutions and by the optimum extraction of Cr(VI) anions with Aliquat 336 at pH lower than 5. The influence of the pH is especially significant when the solvent used for extractant dilution is xylene. Though at pH 4.5 the extraction at equilibrium is almost independent of the solvent used for Aliquat 336 dilution, extraction kinetics are significantly decreased in the following order: kerosene > cumene > xylene. For the treatment of very corrosive solutions containing high concentrations of both sulfuric acid and Cr(VI) anions, the chemical resistance of the fiber must be improved. Among several procedures including cross-linking treatment of the fibers, the reacetylation of chitosan fibers proves to be the most efficient for maintaining extraction performances plus mechanical and chemical resistances. However, after reacetylation, extraction kinetics are slowed: the optimization of the process has to take into account both chemical resistance and kinetic parameters as a function of the type of metal solution to be treated. Literature Cited (1) Brooks, C. S. Metal Recovery from Industrial Wastes; Lewis Publishers: Chelsea, MI, 1991. (2) Levya-Ramos, R.; Juarez-Martinez, A.; Guerrero-Coronado, R. M. Adsorption of Chromium(VI) from Aqueous Solutions on Activated Carbon. Water Sci. Technol. 1994, 30, 191. (3) Beauvais, R. A.; Alexandratos, S. D. Polymer-Supported Reagents for the Selective Complexation of Metal Ions: an Overview. React. Funct. Polym. 1998, 36, 113. (4) Sharma, D. C.; Forster, C. F. Removal of Hexavalent Chromium Using Sphagnum Moss Peat. Water Res. 1993, 27, 1201. (5) Udaybhaskar, P.; Iyengar, L.; Prabhakara Rao, A. V. S. Hexavalent Chromium Interaction with Chitosan. J. Appl. Polym. Sci. 1990, 39, 739. (6) Guibal, E.; Milot, C.; Roussy, J. Metal Anion Sorption by Chitosan Beads: Equilibrium and Kinetic Studies. Ind. Eng. Chem. Res. 1998, 37 (4), 1454. (7) Rydberg, J.; Musikas, C.; Choppin, G. R. Principles and Practices of Solvent Extraction; Marcel Dekker: New York, 1992. (8) Marcus, Y.; Kertes, A. S. Ion Exchange and Solvent Extraction of Metal Complexes; Wiley: New York, 1969. (9) Lo, T. C.; Baird, M. H. I.; Hanson, C. Handbook of Solvent Extraction; Wiley: New York, 1983. (10) Cortina, J. L.; Miralles, N.; Aguilar, M.; Sastre, A. M. Distribution Studies of Zn(II), Cu(II) and Cd(II) with Levextrel Resins Containing Di(2,4,4-trimethylpentyl)phosphonic Acid (Lewatit TP807′84). Hydrometallurgy 1996, 40, 195. (11) Ortiz, M. I.; Galan, B.; Irabien, J. A. Kinetic Analysis of the Simultaneous Nondispersive Extraction and Back-Extraction of Chromium(VI). Ind. Eng. Chem. Res. 1996, 35 (4), 1369. (12) Ortiz, I.; Galan, B.; Irabien, A. Membrane Mass Transport Coefficient for the Recovery of Cr(VI) in Hollow Fiber Extraction and Back-Extraction Modules. J. Membr. Sci. 1996, 118, 213. (13) Alonso, A. I.; Galan, B.; Gonzalez, M.; Ortiz, I. Experimental and Theoretical Analysis of a Nondispersive Solvent Extraction Pilot Plant for the Removal of Cr(VI) from a Galvanic Process Wastewaters. Ind. Eng. Chem. Res. 1999, 38 (4), 1666. (14) Baba, Y.; Hirakawa, H. Selective Adsorption of Palladium(II), Platinum(IV) and Mercury(II) on a New Chitosan Derivative Possessing Pyridyl Group. Chem. Lett. 1992, 1905.

(15) Modrzejewska, Z.; Kaminski, W. Separation of Cr(VI) on Chitosan Membrane. Ind. Eng. Chem. Res. 1999, 38 (12), 4946. (16) Guibal, E.; Jansson-Charrier, M.; Saucedo, I.; Le Cloirec, P. Enhancement of Metal Ion Sorption Performances of Chitosan: Effect of the Structure on the Diffusion Properties. Langmuir 1995, 11 (2), 591. (17) Tomida, T.; Katoh, M.; Inoue, T.; Minamino, T.; Masuda, S. Transient Analysis of Mass-Transfer Rate in Recovering Metal Ions Using a Microporous Hollow Fiber Membrane and a WaterSoluble Chelating Polymer. Sep. Sci. Technol. 1998, 33 (15), 2281. (18) Vincent, T.; Guibal, E. Non-Dispersive Liquid Extraction of Cr(VI) by TBP/Aliquat 336 Using Chitosan-Made Hollow Fiber. Solvent Extr. Ion Exch. 2000, 18 (6), 1941. (19) Kaminski, W.; Eckstein, W.; Modrzejewska, Z.; Sroda, Z. Chitosan Flat and Hollow-Fiber Membranes. In Chitin World; Karnicki, Z. S., Wojtasz-Pajak, A., Brzeski, M. M., Bykowski, P. J., Eds.; Wirtschaftsverlag NW: Bremerhaven, Germany, 1995. (20) Agboh, Q. C.; Qin, Y. Chitin and Chitosan Fibers. Polym. Adv. Technol. 1997, 8, 355. (21) Zumer, M.; Modic, R.; Zupan, J. Extraction of Chromium(VI) by Means of Aliquat 336 and Alamine 336. Vestn. Slov. Kem. Drus. 1974, 21, 105. (22) Charlot, G. Dosages Absorptiome´ triques des Ele´ ments Mine´ raux; Masson: Paris, 1978. (23) Bosinco, S.; Guibal, E.; Roussy, J.; Le Cloirec, P. Adsorption of Hexavalent Chromium on Chitosan Beads: Sorption Isotherms and Kinetics. Miner. Process. Extr. Metall. Rev. 1998, 19, 277. (24) Lo, S. L.; Shiue, S. F. Recovery of Cr(VI) by Quaternary Ammonium Compounds. Water Res. 1998, 32 (1), 174. (25) Molinari, R.; Drioli, E.; Pantano, G. Stability and Effect of Diluents in Supported Liquid Membranes for Cr(III), Cr(VI), and Cd(II) Recovery. Sep. Sci. Technol. 1989, 24 (12 and 13), 1015. (26) Elhassadi, A. A.; Do, D. D. Effect of a Carrier and its Diluent on the Transport of Metals Across Supported Liquid Membranes (SLM). I. Solubility Mechanisms. Sep. Sci. Technol. 1986, 21, 267. (27) Elhassadi, A. A.; Do, D. D. Effect of a Carrier and its Diluent on the Transport of Metals Across Supported Liquid Membranes (SLM). II. Viscosity Effect. Sep. Sci. Technol. 1986, 21, 285. (28) Mahdi, A. Estudio de la Recuperacion de Oro de Disoluciones Clorhidrı´cas y Cianuradas Mediante Extraccio´n Lı´quidoLı´quido y Membranas Lı´quidas Soportadas. Ph.D. Thesis, Universita´t Polite`cnica de Catalunya, Barcelona, Spain, 1998. (29) Palet, C.; Mun˜oz, M.; Hidalgo, M.; Valiente, M. Transport of Vanadium(V) through a Tricaprylmethylammonium Solid Supported Liquid Membrane from Aqueous Acetic/Acetate Solutions. J. Membr. Sci. 1995, 98, 241. (30) Szpakowska, M. Coupled Transport of Copper through Different Types of Liquid Membranes Containing Acorga P-50 as Carrier. J. Membr. Sci. 1996, 109, 77. (31) Salazar, E.; Ortiz, M. I.; Urtiaga, A. M.; Irabien, J. A. Equilibrium and Kinetics of Cr(VI) Extraction with Aliquat 336. Ind. Eng. Chem. Res. 1992, 31 (6), 1516. (32) Guibal, E.; Milot, C.; Roussy, J. Influence of Hydrolysis Mechanisms on Molybdate Sorption Isotherms Using Chitosan. Sep. Sci. Technol. 2000, 35 (7), 1021. (33) Nekova´r, P.; Schro¨tterova´, D. Extraction of Molybdenum(VI) by Primene JMT. Solv. Extr. Ion Exch. 1999, 17 (1), 163.

Received for review September 20, 2000 Revised manuscript received November 30, 2000 Accepted December 18, 2000 IE000833Y