Simultaneous and Synergistic Extraction of Cationic and Anionic

In Ion Exchange Membranes; Flett, D. S., Ed.; Ellis Horwood Ltd: London, 1983; Chapter 4. There is no corresponding record for this reference. Poole, ...
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Ind. Eng. Chem. Res. 1996, 35, 4214-4220

Simultaneous and Synergistic Extraction of Cationic and Anionic Heavy Metallic Species by a Mixed Solvent Extraction System and a Novel Contained Liquid Membrane Device Zhi-Fa Yang,† Asim K. Guha,‡ and Kamalesh K. Sirkar* Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102

Many wastewaters as well as aqueous solutions used for hydrometallurgical extractions have multiple toxic heavy metals: some are present as cations; others are present as anions. This research is directed toward extracting multiple heavy metallic species simultaneously into an appropriate mixed solvent and then recovering and concentrating such heavy metals into appropriate aqueous stripping solutions. Heavy metals of interest are Cu(II), Cr(VI), Zn(II), Hg(II), etc. Such processes have been carried out in a nondispersive fashion in microporous hydrophobic hollow fiber membrane-based devices containing multiple sets of fibers. The results will be presented, and the basis of the observed synergism will be discussed. The advantages of such extraction-stripping processes will be pointed out. Introduction

2Na+(aq) + CrO42-(aq) + 2(R4N)Cl(org) a

Many industrial waste streams are contaminated with a mixture of different heavy metals. Aqueous solutions encountered in hydrometallurgical extractions are often mixtures of heavy metals. For individual removal and recovery of such heavy metals by solvent extraction, metal-specific extractants are used successively to extract and recover each metal (Lo et al., 1983) from the aqueous feed solution. If the heavy metal is present as a cation, it is extracted by an organic acidic or chelating extractant present in an organic diluent (Ritcey and Ashbrook, 1984). For example, if a chelating extractant like LIX 84, a liquid ion exchanger (represented as RH), is present in a diluent, copper can be successfully and selectively extracted via (Lee et al., 1978; Pearson, 1983)

2RH(org) + Cu2+(aq) a R2Cu(org) + 2H+(aq)

(1)

The heavy metal may be present as an anion. For example, in a highly acidic solution Cr(VI) will be primarily present as HCr2O7- (Hochhauser and Cussler, 1975). It can be extracted via ion-pair formation with a long-chain alkylamine (say, a tertiary amine represented as R3N) with a proton:

HCr2O7-(aq) + H+(aq) + 2R3N(org) a (R3NH)2CrO4(org) (2) Alternately, if the feed aqueous solution is basic, Cr(VI) will be present primarily as CrO42-. Then one can extract Cr(VI) from an aqueous solution of, say, Na2CrO4 by anion exchange with quaternary ammonium compounds (R4NCl) like Aliquat 336 in a diluent: * To whom all correspondence should be addressed. Telephone: (201)-596-8447. Fax: (201)-642-4854. E-mail: sirkar@ admin.njit.edu. † Current address: Nutrasweet and Kelco, 2025 E. Harbor Dr., San Diego, CA 92113. ‡ Current address: Department of Environmental Health, Air Pollution Control Program, County of Middlesex, 841 Georges Rd., North Brunswick, NJ 08902.

S0888-5885(96)00193-5 CCC: $12.00

2Na+(aq) + 2Cl-(aq) + (R4N)2CrO4 (3) Such metal extractions into an organic extractant present in a diluent have been carried out successfully and nondispersively using hollow fiber membrane extractors (Yun et al., 1993). Simultaneous backextraction of a metal into a very acidic stripping solution (for Cu(II)) or a very alkaline stripping solution (for Cr(VI)) has also been successfully achieved in a two-fiber-set hollow fiber contained liquid membrane (HFCLM) device in a stable fashion (Guha et al., 1994; Yun, 1992). As we studied such systems, we raised a question: is it possible to extract cationic as well as anionic metallic species in one extraction device? The results of our first attempt are illustrated in Yang et al. (1996) where we introduced two different sets of hydrophobic microporous hollow fibers in one cylindrical extraction device. In this device, the feed aqueous solution flowed through the shell side. A chelating extractant (LIX 84 in diluent) was passed through the bores of one set of fibers to extract Cu(II) from the feed solution; a basic extractant, namely, trioctylamine in a diluent was passed through the lumen of the fibers of the second set to extract Cr(VI) present as an anion via ion-pair formation with H+ present in the aqueous solution. In this study by Yang et al. (1996), we have demonstrated that indeed both Cu(II) and Cr(VI) (present as an anion) could be extracted into the respective organic extractant streams in one device; further the extractions were synergistic: Cr(VI) extraction in the anion form helped Cu(II) extraction by removing H+ (released during Cu(II) extraction) via ion-pair formation and maintaining thereby the aqueous phase pH at a desirable level. In this paper, we propose to go considerably further. We first show via batch stirred solvent extraction studies that both cationic as well as anionic forms of different heavy metallic species can be simultaneously and efficiently extracted into one organic solvent phase containing both an acidic as well as a basic organic extractant in an organic diluent. Thus, it is no longer necessary to have separate organic extractant phases © 1996 American Chemical Society

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4215

Figure 1. Setup of novel membrane-based simultaneous extraction and individual stripping device for removal and recovery of a mixture of two cationic and anionic heavy metals.

Figure 2. Schematic of the synergistic extraction of Cu(II) and Cr(VI) in a module containing three sets of hollow fibers.

to extract cationic and anionic forms of heavy metal ions in solution as was done by Yang et al. (1996). Further, we have studied the backextraction of each heavy metal into separate and appropriate aqueous backextraction (stripping) solutions consecutively. The mixed extractants possess some degree of analogy to mixed ion-exchange resin beds. In addition, we have constructed a tubular device containing three different sets of intermingled microporous hydrophobic hollow fibers (Figure 1) to simultaneously extract cationic and anionic heavy metallic species from wastewater flowing through the bore of one set of fibers into the shell-side organic solvent containing both types of extractants in an organic diluent. Two separate aqueous stripping solutions (one highly acidic for cationic species and another basic for anionic species) are passed through the bores of two additional and separate fiber sets. The shell-side organic medium acts as a contained liquid membrane (Guha et al., 1994; Sengupta et al., 1988) allowing simultaneous backextraction and concentration of the individual heavy metallic species into the two separate aqueous stripping solutions (Figures 1 and 2). An alternative not explored here would be to employ a

device with just one set of fibers to first extract both types of heavy metallic species into the mixed extractant and then employ consecutively different stripping aqueous solutions and two or more single-fiber-set backextractors to individually recover and concentrate the heavy metals in the respective aqueous stripping solutions. This work is in progress. In this particular study with the three-fiber-set module, the shell-side organic diluent contained both a chelating extractant LIX 84 for the cation Cu(II) and a basic extractant tri-n-octylamine (TOA) for the anionic species HCr2O7-, the dominant ion for Cr(VI) in the acidic feed. The aqueous stripping solution employed in one set of fibers for Cu(II) was 2 M sulfuric acid and that employed in another set of fibers for HCr2O7- was a 0.1 M NaOH solution. This paper illustrates the behavior of such systems and explores considerable advantages of such mixed extractant systems. We also seek to identify any synergy that is created in such a mixed extractant system. Although mixed extractant-based systems have been used earlier, none to our knowledge explored simultaneous extraction of two heavy metals present as cations and anions from a given solution. Juang and Chang

4216 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 Table 1. Geometrical Characteristics of the Membrane Device module type

three fiber sets

length, cm module diameter, cm no. of fibers in each seta effective area/volume, cm-1 packing density, %

41.0 1.27 420 46.9 18

a Microporous hollow fiber used (Celgard X-10, Hoechst Celanese, Charlotte, NC): i.d. ) 100 µm, o.d. ) 150 µm; porosity ) 0.3; tortuosity ) 3.5; mean pore size ) 0.03 µm.

(1995) have studied the distribution of citric acid between aqueous solutions and macroporous resins impregrated with a mixture of TOA and bis(2-ethylhexyl)phosphoric acid (D2EHPA). They observed an antagonistic effect on the distribution of citric acid due to the introduction of D2EHPA into TOA. Eyal and Baniel (1982) have employed a mixture of an organic acid (e.g., oleic acid, (diethylhexyl)phosphoric acid, etc.) and a long-chain amine in a carrier organic solvent as an acid-base couple to extract a mineral acid, e.g., H2SO4, HNO3, H3PO4, or a carboxylic acid (e.g., citric acid) from an aqueous solution; they observed that such a mixed extractant allowed much easier backextraction. Such mixed extractant systems were found to be more efficient than a carrier organic solvent containing simply a long-chain amine which is known to extract mineral acids so well that the extraction is practically irreversible (Eyal and Baniel, 1982). For sour waters containing both CO2 and NH3, Mackenzie and King (1985) and Poole and King (1991) have studied the removal of CO2 by steam (or N2) stripping and ammonia by solvent extraction of NH4+ ion into organic diluent containing the cation exchanger (di-2-ethylhexyl)phosphoric acid in a sequential arrangement: stripper for CO2 first followed by solvent extraction in a separate device for NH4+ removal. Although there was synergy in such a process due to pH control, our process ensures pH control locally via simultaneous extraction of cations and anions in one device by one mixed extractant system. Mackenzie and King (1985) had also employed mixtures of Lewis base modifiers like Adogen 364, etc., with D2EHPA in various diluents for extraction of NH4+ ion. They observed reduction in ammonium ion extraction, indicating negative synergy. Grinstead et al. (1969) also used a mixed acid-base extractant system for extraction of neutral inorganic salts (e.g., NaCl, MgCl2, etc.) from aqueous solutions where the “pH is within a few units of neutrality”. The pH of our aqueous solutions was highly acidic. Experimental Section Chemicals Used. LIX 84 (anti-2-hydroxy-5-nonylacetophenone oxime) (Henkel, Tucson, AZ); tri-noctylamine (TOA), copper sulfate pentahydrate (Fluka, Ronkonkoma, NY); potassium dichromate (Aldrich, Milwaukee, WI); heptane, kerosene (Fisher, Springfield, NJ). All chemicals were ACS reagent grade except LIX 84 and TOA. Membrane Module. A three-fiber-set module was prepared by employing microporous hydrophobic Celgard X-10 hollow fibers (Hoechst Celanese SPD, Charlotte, NC). The fibers were well mixed. The geometrical characteristics of the module are provided in Table 1. Batch Extraction/Backextraction Experiments. The extent of extraction (Ei) of species i defined by

Ei )

total moles of species i in the organic phase × total moles of species i in the feed 100 (4)

was determined by stirring a given volume of aqueous feed solution containing copper sulfate and potassium chromate at particular levels with 30 mL of solvents in a 150 mL beaker. For stripping or backextraction studies, the extent of stripping is defined as Esi ) total moles of species i in the stripping aqueous phase total moles of species i in the organic phase × 100 (5) A certain volume of solvent loaded with Cu(II) and Cr(VI) was stirred with a certain volume of 10 v/v % sulfuric acid first and then after removal of aqueous phase again stirred with a 4 w/v % sodium bicarbonate solution in a 150 mL beaker. The distribution coefficient mi defined by

mi ) concentration of species i in the organic phase concentration of species i in the aqueous raffinate (6) was measured by stirring 30 mL of an aqueous feed solution containing about 1000 mg/L of Cu(II) and 200 mg/L of Cr(VI) and 30 mL of mixed extractants in a diluent (heptane) for 30 min. Continuous Membrane Module Extraction. For the experiments in the novel three-fiber-set hollow fiber membrane device, aqueous feed containing Cu(II) and Cr(VI) and the acidic and basic stripping solutions were pumped through the bore of the three sets of fibers respectively, simultaneously, and cocurrently. A mixture of TOA and LIX 84 in kerosene was kept in the shell side (Figures 1 and 2). The pumps used were Masterflex Model 7518-60 (Cole-Parmer, Chicago, IL). The flow rates were measured by graduated cylinders and a stopwatch. The aqueous and organic phase pressures were kept at 5 and 3 psi, respectively. Analytical Procedures. The concentrations of Cu(II) and Cr(VI) in the aqueous solutions were analyzed by a Thermo-Jarrel Ash Model 12 atomic absorption spectrophotometer (AAS) using an individual hollow cathode lamp and conventional flame condition with a fuel (acetylene) to air ratio of 3:9. For measurement of Cu(II), high-concentration samples were diluted to the linear calibration range of 1-40 mg/L; measurements were made at 216.5 nm with a slit width of 0.15 nm. The concentration measurement of Cr(VI) was similar to that for Cu(II) except that the linear calibration range was 1-20 mg/L, and the wavelength and slit width used were 425.4 and 0.5 nm, respectively. The pH of the aqueous phase was measured using a Corning pH meter (Model 250, relative accuracy ) 0.001 pH error, Fisher Scientific, Springfield, NJ). Results and Discussion We first present and discuss the results of batch stirred extraction studies with mixed extractants for the extraction of Cu(II) and Cr(VI) (present as an anion) from an aqueous solution. The results are discussed in terms of the distribution of coefficient mi of each metal and the aqueous equilibrium pH as a function of the

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4217

Figure 3. Relationship between the distribution coefficients of copper extraction and the molar ratios of the extractants.

Figure 4. Relationship between the distribution coefficients of Cr(VI) extraction and the molar ratios of the extractants.

fractional amount of TOA present in a total mixture of TOA and LIX 84 in the diluent heptane. The extent of extraction of each metal is also reported as a function of mixing time. The extent of backextraction (stripping) of each metal from the metal-loaded organic solvent into the appropriate aqueous stripping solution is presented next. Finally, the results of simultaneous extraction and stripping of both heavy metals in a three-fiber-set hollow fiber contained liquid membrane permeator are presented. Figure 3 illustrates the dependence of the distribution coefficient mCu of copper for extraction from a feed aqueous solution containing 1091 ppm of Cu(II) and 249 ppm of Cr(VI) into the diluent heptane containing varying amounts of TOA with LIX 84. It is observed that mCu increases by almost of an order of magnitude when about 20% of the extractant is TOA compared to that for pure LIX 84. An almost similar increase (by an order of magnitude) is observed in the value of m for Cr(VI) when about 40% extractant is LIX 84 (Figure 4). Two explanations are useful. Consider Figure 5 which illustrates how the aqueous phase pH is affected in these extractions by the presence of the mixed extractants. Over the range of 0.2-0.8 for the quantity

Figure 5. Relationship between the aqueous pH and the molar ratios of the extractants.

[TOA]/([TOA] + [LIX84]), the pH varies only between 2.5 and 3. Compare this with the pH of around 2 for pure LIX 84 and 4.2 for pure TOA. Note further that the increase in mCu and mCr takes place also in the same composition range for the mixed extractants. Simultaneous extraction of Cr(VI) and Cu(II) may maintain the aqueous pH relatively constant and within the optimal values since H+ produced by copper extraction may be consumed by extraction of Cr(VI) by ion-pair formation (eq 2). The drastic increase in distribution coefficient for copper as the pH rises from 1.3 is well-known; data for LIX 84 in n-heptane to this effect are available in Yun et al. (1993). Additionally, any inorganic acid (e.g., H2SO4) present in the aqueous solution may be extracted by TOA; this will increase the pH and will facilitate Cu(II) extraction. The presence of Cr(VI) and its simultaneous extraction by TOA is highly beneficial in terms of extraction of Cu(II). As we see in Figure 3, when the feed contains only Cu(II), the increase in mCu due to mixed extractants is considerably less. On the other hand, extraction of copper suffers as TOA concentration increases beyond a value of 0.4 for fractional TOA in the mixed extractants. Obviously, the availability of LIX 84 decreases since any binding between LIX 84 and TOA will increase significantly as the TOA fraction increases. This is especially evident at a value of 0.8 for [TOA]/([TOA] + [LIX84]) where mCu is reduced to a value of 1.2. For Cr(VI) extraction, there is an additional benefit due to LIX 84 being present. TOA usually extracts an anion via its ion-pair formation with, say, H+. But the solubility of the extract in TOA-heptane or TOAkerosene is very low (Lo et al., 1983; Yang et al., 1992; Yun, 1992). Consequently, a precipitate may be formed during the extraction. In conventional extraction, phase modifiers are usually employed to avoid such solutions. We have observed that the precipitate which occurs during the extraction of Cr(VI) with TOA is eliminated by the addition of LIX 84. Addition of LIX 84 into TOAheptane had apparently greatly increased the solubility of the species extracted by TOA in the mixture of solvents, TOA-heptane solution. The synergistical coefficients R for Cr(VI) and Cu(II) are defined by

R)

m12 m1 + m2

(7)

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Figure 6. Effect of mixing time on the extraction of Cu(II) and Cr(VI).

where m12, m1, and m2 are respectively the distribution coefficients of copper (or chromium) extracted with an extractant mixture of TOA and LIX 84 in diluent heptane, individual extractant TOA in heptane, and individual solvent LIX 84 in heptane. The synergistical coefficients R for Cr(VI) and Cu(II) in such systems are as high as 10.9 (Figure 4) and 11.4 (Figure 3), respectively. Another advantage of using the mixture of extractants TOA and LIX 84 in heptane to extract Cr(VI) and Cu(II) is that the stripping performance of the metals may be improved; i.e., both Cr(VI) and Cu(II) could be easily stripped and concentrated into sodium bicarbonate and sulfuric acid, respectively. The emulsion and precipitation problem encountered during stripping Cr(VI) from TOA in diluent with or without phase modifiers (Lo et al., 1983; Yang et al., 1992; Yun, 1992) may also be eliminated. For the simultaneous extraction of Cu(II) and Cr(VI), a maximal extraction extent may be obtained if the molar ratio of TOA to LIX 84 was equal to 0.2 (for Cu(II)) or 0.6 (for Cr(VI)) (Figures 3 and 4). A mixture of the same molar concentration of TOA and LIX 84 was used as the extractant for further investigations even though the individual extraction efficiency of both Cu(II) and Cr(VI) ions were reduced. Figures 6 and 7 show that the extraction and stripping kinetics of Cr(VI) are much faster than those of Cu(II). It took about 10 and 20 min for Cr(VI) and Cu(II), respectively, to reach extraction equilibrium. The stripping of Cr(VI), however, reached equilibrium within 1 min, but more than 20 min was needed for the stripping of Cu(II) to reach equilibrium. The effects of aqueous/organic or organic/aqueous phase ratios on extraction and stripping of Cr(VI) and Cu(II) are illustrated in Figures 8 and 9. These results indicate that both Cr(VI) and Cu(II) could be concentrated via extraction and stripping, especially via stripping. These extraction and stripping results are also illustrated in Tables 2 and 3 for the following conditions: the synthetic feed solution contained 1091 ppm Cu(II) and 250 ppm Cr(VI), pH 4.14. The extractants consisted of 0.078 M LIX 84 and 0.078 M TOA in heptane. The mixing time for extraction and stripping was 20 min each. 10% sulfuric acid and 4-10% sodium hydrogen carbonate were used to strip Cu(II) and Cr-

Figure 7. Effect of mixing time on the stripping of Cu(II) and Cr(VI).

Figure 8. Effect of phase ratios on extraction of Cu(II) and Cr(VI).

(VI), respectively, at room temperature. These results indicated that the metals could be concentrated and completely separated by the extraction and stripping. The concentration of Cu(II) and Cr(VI) in water could be reduced to less than 30 ppm by only one batch extraction step. Two extraction steps may reduce the concentrations of the metals to the ppb level. For a given extractant ratio (e.g., TOA/LIX 84 ) 1) in the solvents mixture, Figure 10 shows that a higher concentration of the extractants will increase the extraction extent of Cu(II) and reduce the extraction efficiency of Cr(VI) due to the aqueous pH variation (Figure 10). This suggests that the molar ratio and the concentrations of the extractants play important roles in the extraction. Thus, correctly selecting the concentration and molar ratio of the mixed extractant system is the key to getting the best extraction and separation efficiency. The stirred batch extraction and stripping processes described above may well be used to remove, concentrate, and separate Cr(VI) and Cu(II) from wastewaters. However, since this process consists of one extraction step and two separate and consecutive stripping steps, there is still considerable room for this process to be

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4219

Figure 9. Effect of phase ratios on the stripping of Cu(II) and Cr(VI). Table 2. Batch Extraction of Cr(VI) and Cu(II) with 0.078 M LIX 84 and 0.078 M TOA in Heptane for Different Organic/Aqueous Phase Ratios O/Aa

pH

1/1 1/2 1/3 1/4 1/5

2.66 2.60 2.59 2.56 2.63

a

Cr(aq), Cr(org), E(Cr), Cu(aq), Cu(org), E(Cu), ppm ppm % ppm ppm % 10.1 6.48 9.98 12.1 21.6

239.9 493.5 740 987.9 1228.4

96.1 97.4 96.0 95.1 91.3

22.4 393 504.4 547.4 582.6

1068.6 1789 2725.6 3859.6 4872.4

97.9 64.0 53.8 49.8 46.6

Volume ratio.

Table 3. Batch Stripping of Cr(IV) and Cu(II) with Sulfuric Acid and Sodium Bicarbonate for Different Organic/Aqueous Phase Ratios A/Oa 1/2 1/3 1/4

pH

Cr(aq), Cr(org), Es(Cr), Cu(aq), Cu(org), Es(Cu), % ppm ppm % ppm ppm

9.51 422.7 9.29 649 b 9.75 1030c

57.1 71.0 ∼0

88.0 90.1 ∼100

1913 2903 3589d

224.2 304.8 685

89.5 90.5 84.0

a Volume ratio. b 10% NaHCO was used to strip Cr(VI). c No 3 copper(II) was observed in this solution. d No chromium(VI) was observed in this solution.

improved for continuous operation. A novel but robust three-set microporous hollow fiber membrane device described in Table 1 was fabricated and used to improve the process. The schematic flow sheet is showed in Figure 1, and the operation is described in the experimental part. The extraction and stripping of both heavy metals proceeded simultaneously in one device. Under appropriate conditions of operation in such a device, Cr(VI) and Cu(II) may be removed completely from wastewaters and separated and concentrated into acidic and basic solutions simultaneously and respectively. Preliminary results show that this device can remove and separate Cr(VI) and Cu(II) simultaneously from synthetic wastewater (Table 4). For each liquid organic

Figure 10. Effect of the solvent concentrations on extraction of Cu(II) and Cr(VI).

membrane composition, the experiments were run for more than 20 h. Initially, there was considerable extraction from the feed solution and the treated feed had low concentrations of Cu(II) and Cr(VI). After steady state was achieved, the extent of feed purification decreased considerably due to limited membrane surface area and nonoptimal extractant concentration. However, at steady state, for all the experiments listed in Table 4, mass balances were achieved for both Cr(VI) and Cu(II). Further experiments need to be carried out to get the optimal organic membrane composition. System optimization and modeling are being undertaken. Concluding Remarks (1) A mixture of the chelating extracting agent LIX 84 and the basic extracting agent TOA in the same diluent not only simultaneously extracted Cu(II) and Cr(VI) but also showed a significant synergy for simultaneous extraction of Cu(II) and Cr(VI). The synergistical coefficients were as high as 11.4 and 10.9 for Cu(II) and Cr(VI), respectively. (2) A new solvent extraction process was proposed in the present article by which a cation, e.g., copper(II), and an anion of chromium(VI), H2Cr2O7-, were not only extracted from the waste aqueous solution into a mixed extractant system present in a diluent but also separated and concentrated in different aqueous stripping solutions for possible reuse. (3) Based on the aforementioned results, a novel three-fiber-set microporous hollow fiber contained liquid membrane device was fabricated and used to remove, separate, concentrate, and recover Cr(VI) and Cu(II) from synthetic wastewaters into separate basic and acidic stripping solutions respectively and simultaneously. Preliminary results are encouraging.

Table 4. Extraction and Separation of Cr(VI) and Cu(II) via a Three-Fiber-Set HFCLM Device organic membrane compositiona

feed flow rate, mL/min

run time, h

pHout

0.1 M NaOH flowrate, mL/min

Cr(VI) in NaOH, ppm

2 M H2SO4 flowrate, mL/min

Cu(II) in H2SO4, ppm

extractant 1 extractant 2 extractant 3

2.33 1.43 1.83

20 24 30

3.82 3.47 3.45

0.28 0.36 0.49

58.0 150.0 93.0

0.12 0.10 0.29

69.0 228.0 62.0

a Extractant 1: 5 v/v % LIX 84 and 3.6 v/v % TOA in kerosene. Extractant 2: 25 v/v % LIX 84 and 18 v/v % TOA in kerosene. Extractant 3: 50 v/v % LIX 84 and 25 v/v % TOA in kerosene. Feed: Cr(VI) ) 220 ppm, Cu(II) ) 1000 ppm, pH 4.19.

4220 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996

Acknowledgment We acknowledge the research funds provided by the EPA Northeast Hazardous Substance Research Center at NJIT, Newark, NJ. We express our thanks to Hoechst Celanese SPD, Charlotte, NC, for providing the microporous hollow fibers and to Henkel, Tucson, AZ, for providing LIX 84. Nomenclature A/O ) phase volume ratio, aqueous phase/organic phase mi ) distribution coefficient of species i, defined by eq 6, i ) 1, 2 m12 ) distribution coefficient of species 1 or 2 extracted by a mixture of extractants Ei ) extent of extraction of species i (%), defined by eq 4 Esi ) extent of stripping (%), defined by eq 5 LIX 84 ) anti-2-hydroxy-5-nonylacetophenone oxime O/A ) phase volume ratio, organic phase/aqueous phase R ) synergistic extraction coefficient, defined by eq 7 TOA ) tri-n-octylamine

Literature Cited Eyal, A.; Baniel, A. Extraction of strong mineral acids by organic acid-base couple. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 334. Grinstead, R. R.; Davis, J. C.; Lynn, S.; Charlesworth, R. K. Extraction by phase separation with mixed ionic solvents. Ind. Eng. Chem. Prod. Res. Dev. 1969, 8, 218. Guha, A. K.; Yun, C. H.; Basu, R.; Sirkar, K. K. Heavy metals removal and recovery by contained liquid membrane permeator. AIChE J. 1994, 40 (7), 1223-1237. Hochhauser, A. M.; Cussler, E. L. Concentrating chromium with liquid surfactant membranes. AIChE Symp. Ser. 1975, 71 (152), 136. Juang, R. S.; Chang, H.-L. Distribution equilibrium of citric acid between aqueous solutions and tri-n-octylamine-impregrated macroporous resins. Ind. Eng. Chem. Res. 1995, 34, 1294.

Lee, K.; Evans, D. F.; Cussler, E. L. Selective copper recovery with two types of liquid membranes. AIChE J. 1978, 24, 860. Lo, T.-C.; Baird, M. H. I.; Hansen, C. Handbook of Solvent Extraction; Wiley-Interscience: New York, 1983. Mackenzie, P. D.; King, C. J. Combined solvent extraction and stripping for removal and isolation of ammonia from sour wastes. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 1192. Pearson, D. Supported liquid membranes for metal extraction from dilute solutions. In Ion Exchange Membranes; Flett, D. S., Ed.; Ellis Horwood Ltd: London, 1983; Chapter 4. Poole, L. T.; King, C. J. An improved extractant for separation of ammonia from sour wastes by conbined stripping and extraction. Solvent Extr. Ion Exch. 1991, 9 (1), 103. Ritcey, G. M.; Ashbrook, A. W. Solvent Extraction; Elsevier: Amsterdam, The Netherlands, 1984; Part I. Sengupta, A. K.; Basu, R.; Sirkar, K. K. Separation of solutes from aqueous solutions by contained liquid membranes. AIChE J. 1988, 34, 1698-1708. Yang, Z. F.; Yu, S. Q.; Chen, J. Y. Extraction of penicillin G with aliphatic amines in organic solvents of different polarities. J. Chem. Technol. Biotechnol. 1992, 53, 97-103. Yang, Z. F.; Guha, A. K.; Sirkar, K. K. Novel membrane-based synergistic metal extraction and recovery processes. Ind. Eng. Chem. Res. 1996, 35 (4), 1383-1394. Yun, C. H. Removal of pollutants and recovery of toxic heavy metals from wastewater using microporous hollow fiber modules. Doctoral Dissertation, Stevens Institute of Technology, Hoboken, NJ, 1992. Yun, C. H.; Prasad, R.; Guha, A. K.; Sirkar, K. K. Hollow fiber solvent extraction removal of toxic heavy metals from aqueous waste streams. Ind. Eng. Chem. Res. 1993, 32 (6), 1186-1195.

Received for review April 2, 1996 Revised manuscript received July 15, 1996 Accepted July 15, 1996X IE960193S

X Abstract published in Advance ACS Abstracts, September 15, 1996.