Ind. Eng. Chem. Res. 2004, 43, 751-757
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SEPARATIONS Separation and Recovery of Cr(VI) from Simulated Plating Waste Using Microcapsules Containing Quaternary Ammonium Salt Extractant and Phosphoric Acid Extractant Syouhei Nishihama, Go Nishimura, Takayuki Hirai,* and Isao Komasawa Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Solar Energy Chemistry, Osaka University, Machikaneyama-cho 1-3, Toyonaka 560-8531, Japan
The separation and recovery of Cr(VI) from a simulated plating waste aqueous solution, containing Cr(VI), Ni(II), Cu(II), and Zn(II), have been investigated, using microcapsules consisting of styrene-divinylbenzene copolymer and containing, as the extractant, tri-noctylmethylammonium chloride (TOMAC) or bis(2-ethylhexyl)phosphoric acid (D2EHPA). The extractants were encapsulated successfully into the microcapsules by the addition of the extractants to the dispersed phase, during the polymerization process, and possess an extractability for the metals identical with that for the conventional liquid-liquid extraction system. Chromatographic operation, using a column packed with microcapsules, showed that perfect separation and recovery of toxic Cr(VI) was achieved with the TOMAC microcapsules, with further separation of the three other metals then being carried out with D2EHPA microcapsules. 1. Introduction The separation and recovery of metals have been some of the active issues to support the recent advanced technology, and liquid-liquid extraction and ion exchange have been applied as their industrial-scale operation. Along with the progress in the research for the selectivity for metals, the liquid-liquid extraction has been known to have more selectivity than the ion exchange, even when they have similar functional groups. One disadvantage of the extraction system, however, is that an organic solvent is needed to form a solution of both the extractant and the extracted species and that the organic solvent may then be lost into the aqueous phase, owing to its own solubility in the feed solution. The impregnation of the extractant onto/into a polymer resin has been investigated, for the second generation of the extraction system, to bridge the gap between the ion-exchange and liquid-liquid extraction methods. The use of the polymer resin as a supporting material of a metal extractant has the advantages that (1) a larger specific interfacial area is obtained, as compared to membrane extraction systems, (2) the separation of the resin from the resulting raffinate solution can be achieved easily by filtration or by sedimentation, (3) the recovery of metals from the dilute aqueous solution can be achieved, and (4) the industrial operation can become simple by using the packed column. There are two methods for the impregnation of the extractant to the polymer resin: (1) the resin is treated with the extrac* To whom correspondence should be addressed. Tel.: +81-6-6850-6272. Fax: +81-6-6850-6273. E-mail: hirai@ cheng.es.osaka-u.ac.jp.
tant containing organic solvent and (2) the resin containing the extractant is prepared in situ with the monomer(s) containing the extractant. In the former case, however, the disadvantage is that a part of the impregnated extractant does not take part in the extraction, because of the steric hindrance of the resin molecule.1 In the latter case, the Levextrel resin first developed by Bayer (Germany),2-5 TVEX developed by Sorbent Scientific and Educational Center (Ukraine),6,7 and microcapsules containing the extractant8-11 have been developed. The difference between the Levextrel resin and microcapsules containing extractant is that the Levextrel resin contains only extractant while the microcapsules contain both the diluent and the extractant. Recently, the styrene (St)-divinylbenzene (DVB) copolymer, which is also used for the matrix of the Levextrel resin, is to be mainly used for the matrix of the microcapsules. However, the quaternary ammonium salt type extractant has never been impregnated onto the matrix of the Levextrel-type resin because of the problems of polymerization inhibition. In contrast, a microcapsule containing quaternary ammonium salts is expected to be prepared because of the presence of diluent and thus low polymerization inhibition. Cr(VI) is well-known in the chemical industry for applications such as plating and as an alloy. The metal, however, is toxic, owing to its strong oxidizing ability, and its recovery is therefore important from industrial waste solutions. The present recovery process is based on chemical reduction with sodium sulfite.12 In this process, Cr(VI) is first reduced to Cr(III) and is then precipitated as hydrated oxides, following neutralization of the solution. This process, however, needs about 5 times the amounts of reductant and neutralizer, com-
10.1021/ie020331f CCC: $27.50 © 2004 American Chemical Society Published on Web 12/30/2003
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Figure 1. SEMs for microcapsules obtained at a stirring speed of 200 rpm.
pared to the amount of Cr(VI), and thus produces a lot of waste. In addition, Cr recovered via this process is difficult to reuse. Experimental studies of separation and recovery processes for Cr(VI) based on liquid-liquid extraction were therefore carried out.13-19 Other heavy metals, such as Ni(II), Cu(II), and Zn(II), occur in a waste plating solution,20 and these metals must also be recovered from the waste solution. If microcapsules, containing the extractant, can be applied to the separation of Cr(VI) from the waste plating solution, an effective and environmentally friendly extraction and recovery process might be expected to be achieved. In the present work, therefore, the separation and recovery of Cr(VI) from a simulated waste plating solution, containing Cr(VI), Ni(II), Cu(II), and Zn(II), based on extractant-containing microcapsules, have been investigated. Tri-n-octylmethylammonium chloride (TOMAC) and bis(2-ethylhexyl)phosphoric acid (D2EHPA) were employed as extractants. The extraction characteristics of Cr(VI), in a single-component Cr(VI) aqueous solution, were first investigated by conventional liquid-liquid extraction. Extraction was then carried out, using the microcapsule system, and the extraction characteristics were compared to those obtained from conventional liquid-liquid extraction. The extraction behavior of the four-metal-component system, Cr(VI), Ni(II), Cu(II), and Zn(II), with microcapsules, containing either TOMAC or D2EHPA, was also investigated. Based on the results of preliminary batch studies, the separation and recovery of the Cr(VI), from a simulated waste plating solution, were then carried out using a column packed with microcapsules containing TOMAC. The separation of the three remaining metals in the plating waste, following the recovery of Cr(VI), was also carried out using the column packed with microcapsules containing D2EHPA. 2. Experiment 2.1. Reagents and Procedures. TOMAC was supplied by Koei Kagaku Kogyo Co., Ltd., and D2EHPA was supplied by Daihachi Chemical Industry Co., Ltd. TOMAC was purified prior to use, following the method developed for the purification of Aliquat 336.21 St, DVB (55%, mixture of isomer), 2,2′-azobis(isobutyronitrile) (AIBN), arabic gum, benzene, Na2CrO4‚4H2O, NiCl2‚ 2H2O, CuCl2‚2H2O, and ZnCl2 were supplied by Wako Pure Chemical Industries, as analytical-grade reagents.
Deionized water was purified by simple distillation, prior to use. 2.2. Preparation of Microcapsules. A mixture of DVB and St was washed with an aqueous 1 mol/L NaOH solution. In the case of TOMAC extractant, 0.037 mol/L initiator AIBN, 0.2 mol/L TOMAC, and a mixture of 0.51 mol/L DVB and 3.19 mol/L St were dissolved in benzene (3.62 g of DVB and 9.98 g of St in 12 mL of benzene, totaling 30 mL), as the dispersed phase. In the case of D2EHPA, 0.02 mol/L AIBN, 0.2 mol/L D2EHPA, and a mixture of 0.28 mol/L DVB and 1.72 mol/L St were dissolved in kerosene (1.96 g of DVB and 5.38 g of St in 19 mL of kerosene, totaling 30 mL). The dispersed phase (30 mL) was added to the continuous phase (150 mL of an aqueous solution containing 0.5 wt % arabic gum) and agitated at 200 rpm under a nitrogen atmosphere at 343 K for 5 h. The microcapsules containing the TOMAC or D2EHPA, thus obtained, were washed and then dried in vacuo overnight. The resulting microcapsules were characterized by means of a scanning electron microscope (SEM; Hitachi S-2250) and a laser scattering particle-size distribution analyzer (Horiba LA-910W). Prior to SEM examination, the samples were sputter-coated with a ca. 10-nm-thick platinum layer to minimize any possible surface charging effects. 2.3. Extraction of Cr(VI), Ni, Cu, and Zn. For the conventional liquid-liquid batch extraction studies, the organic solution was prepared by diluting TOMAC with benzene. Aqueous feed solutions for the Cr(VI) singlemetal system [5 mmol/L Cr(VI)] were prepared and adjusted to the required Cl- concentration by adding appropriate quantities of NaCl to the Na2CrO4 solution. The organic and aqueous solutions were mixed, at an organic/aqueous (O/A) volume ratio of 1:1, and shaken for equilibration at 298 K for 1 h. The metal concentrations in the aqueous phase samples were analyzed by using a Nippon Jarrell-Ash ICAP-575 Mark II emission spectrometer, and those in the organic phase were determined by material balance. The aqueous phase pH was measured by an Orion 920A pH meter equipped with a glass combination electrode. For the batch extraction using microcapsules, 1 g of microcapsules and 10 mL of an aqueous feed solution were mixed, using a magnetic stirrer. The feed aqueous solutions contained 5 mmol/L Cr(VI) for the Cr(VI) single-metal-system studies and 4.81 mmol/L (250 ppm) Cr(VI), 0.852 mmol/L (50 ppm) Ni, 0.787 mol/L (50 ppm) Cu, and 0.765 mol/L (50 ppm) Zn for the four-metal-
Ind. Eng. Chem. Res., Vol. 43, No. 3, 2004 753 Table 1. Loading Capacity and Time for Achieving the Extraction Equilibrium resin type
extractant or functional group
metal
microcapsules
TOMAC D2EHPA tri-n-octylamine bis(2,4,4-trimethylpentyl) phosphinic acid tributyl phosphate phosphonic acid group
Cr(VI) Zn(II) Zn(II) Zn(II), Cu(II), and Cd(II) UO22+ Ni(II), Ba(II), and Ca(II) Pd(II)
Levextrel resin
ion exchange
quaternary ammonium group a
loading capacity 0.054 mmol/g 0.25 mmol/g ca. 0.2 mmol/g 1.09 mequiv/ga
equilibrium time (min) 10 10 ca. 40 40
23 24
ca. 30
25
1.7 mmol/g ca. 0.0015 mmol/L
ref present work present work 1 22
Total extractant capacity.
loading capacity and the time for achieving the extraction equilibrium, are summarized in Table 1 and compared with those with the Levextrel resin and conventional ion-exchange resin. The loading capacity of the microcapsules is smaller than those of both the Levextrel resin and conventional ion-exchange resin, while the extraction rate with the microcapsules is much higher than those with other ones. The microcapsule system can, therefore, reduce the operation time, although it needs more resin than other conventional systems. 3.2. Extraction of Cr(VI) in a Single-Metal System. TOMAC (R3R′NCl) is well-known to extract metal ions via an anion-exchange mechanism, and thus the anionic species of metals are easily extracted by this extractant. Cr(VI) occurs as five anionic forms in an aqueous solution, as expressed in eqs 1-4.17 Figure 2. Size distributions for the microcapsules obtained.
system experiments. In the case of the four-metalsystem studies, NaCl was not added for the adjustment of the Cl- concentration. In the case of the column chromatography studies, microcapsules, 10 g of TOMAC microcapsules (27.2 cm height) or 4 g of D2EHPA microcapsules (16.2 cm height), were packed into a column of 1 cm diameter. An aqueous feed solution containing the four metals of pH ) 5.0 was fed to the top of the column at a flow rate of 10 mL/h for the TOMAC system or at 20 mL/h for the D2EHPA system, using a peristaltic pump. 3. Results and Discussion 3.1. Preparation of Microcapsules Containing Extractant. Extractant-containing microcapsules can be prepared by adding TOMAC or D2EHPA to the dispersed phase during the polymerization process. The microcapsules containing TOMAC can be produced with the high concentration of each monomer, while they can hardly be produced under the same conditions with the D2EHPA system. Typical SEM images of the microcapsules obtained, at an agitation speed of 200 rpm, are shown in Figure 1. With TOMAC as the extractant, spherical microcapsules of ca. 50 µm diameter are obtained. With the D2EHPA extractant, however, microcapsules of ca. 1-2 µm are obtained and then aggregate. Such an aggregation was also observed in the acidic phosphinate system.11 Figure 2 shows the size distribution of the microcapsules obtained. In the case of the TOMAC system, microcapsules with a narrow size distribution (ca. 30-150 µm) are again seen to be obtained, whereas the size distribution for the D2EHPA system is much wider (ca. 1-500 µm). The basic properties of the present microcapsules, such as the
H2CrO4 a H+ + HCrO4-; K1 ) 1.58 × 10-1 (1) HCrO4- a H+ + CrO42-; K2 ) 3.16 × 10-7 (2) 2HCrO4- a Cr2O72- + H2O; K3 ) 3.31 × 10 HCr2O7- a H+ + Cr2O72-; K4 ) 1.17
(3) (4)
Lo et al. have reported that the extraction equilibrium formulations for Cr(VI) with TOMAC are as expressed by eqs 5-7.12
R3R′NCl + HCrO4- a (R3R′N)HCrO4 + Cl- (5) 2R3R′NCl + Cr2O72- a (R3R′N)2Cr2O7 + 2Cl- (6) 2R3R′NCl + CrO42- a (R3R′N)2CrO4 + 2Cl- (7) Figure 3a shows the effect of the extraction efficiency for Cr(VI) versus equilibrium pH value, as obtained by conventional liquid-liquid extraction. The extraction efficiency, in the range of pH > 7, decreases with an increase in the concentration of the chloride ion in the feed aqueous solution. This indicates that the dissociation of TOMAC is suppressed by the addition of Cl-, into the aqueous phase, and thus the extraction efficiency is reduced, owing to the decrease in the dissociated ligand of TOMAC (R3R′N+). The extraction efficiency, from the aqueous solution of the same Clconcentration, decreases dramatically around pH ) 7 with an increase in the equilibrium pH value. Figure 3b shows the effect of the pH value on the mole ratio of the differing species Cr(VI) in an aqueous solution, as
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Figure 3. Effect of the aqueous pH value on (a) the extraction efficiency for Cr(VI) in the single-metal system by TOMAC in the conventional liquid-liquid extraction system and (b) the mole ratio variation of Cr(VI) species in the aqueous solution. For part a, [ TOMAC]feed ) 0.1 mol/L, and for parts a and b, [Cr(VI)]feed ) 0.005 mol/L.
Figure 4. Effect of the aqueous pH value on the extraction efficiency for Cr(VI) in the single-metal system using microcapsules containing TOMAC. [Cr(VI)]feed ) 5 mmol/L and MC ) 1 g.
calculated using the equilibrium constants from eqs 1-4. These results thus indicate that the change in the relative concentrations of the Cr(VI) species in the aqueous phase thus affects the extraction efficiency obtained with TOMAC. As shown in Figure 3, Cr(VI) may be expected also to be effectively recovered from an aqueous solution in the acidic pH region, by employment of the microcapsule system. The extraction of Cr(VI) by microcapsules containing TOMAC was thus then also carried out. Figure 4 shows the effect of the equilibrium pH value on the extraction efficiency of Cr(VI), obtained in the microcapsule system. In previous work, the extraction of rare-earth metals, using microcapsules, was found to proceed according to the same schematic as that in the conventional liquid-liquid extraction system.11 The present work also confirms that the extraction behavior, in the
Figure 5. Effect of the mass of microcapsules on the extraction efficiency in the four-metal system using (a) TOMAC microcapsules and (b) D2EHPA microcapsules. [Cr(VI)]feed ) 244.3 ppm, [Ni]feed ) 55.2 ppm, [Cu]feed ) 46.8 ppm, and [Zn]feed ) 47.9 ppm.
microcapsule system, is identical with that obtained in the conventional liquid-liquid extraction and that the extraction efficiency decreases with an increase in the concentration of chloride ion in the feed aqueous solution and with an increase in the pH value. In the microcapsule system, however, the extraction efficiency for pH > 7, from an aqueous solution containing zero NaCl, decreases with increasing pH but remains at 100% in the conventional liquid-liquid extraction system. This may result because the amount of TOMAC used in the microcapsule system is less than that used in the conventional liquid-liquid extraction. Thus, Cr(VI) can be extracted perfectly into the microcapsules in the acidic pH region, for all chloride concentrations, as expected from the results of the conventional liquidliquid extraction. The present microcapsule system is thus useful for both the extraction and recovery of toxic Cr(VI) from the aqueous solution and is especially environmentally friendly because it requires no organic solvent during the extraction processing. 3.3. Separation and Recovery of Cr(VI) from a Simulated Plating Waste Solution. The separation and recovery of Cr(VI) from a waste plating solution were then carried out. The waste plating solution contains Ni, Cu, and Zn at 10-40 ppm concentratons,20 together with 30-200 ppm of Cr(VI).12 In this work, an aqueous solution, containing 250 ppm of Cr(VI) and 50 ppm of each heavy metal, was used to simulate the waste plating solution. Zero NaCl was added to the feed solution, owing to the high extraction efficiency of Cr(VI), obtained at low Cl- concentration, and the initial pH value was adjusted to 5.0. Figure 5a shows the extraction efficiencies, obtained for each metal in the four-metal system, using microcapsules containing TOMAC. Here the extraction of Cr(VI) increases with an increase in the quantity of microcapsules, for quantities greater than 0.1 g. In the case of the three other heavy metals, the extraction efficiencies are much smaller, though with Cu also increasing with an increase in the microcapsule quantity for quantities
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Figure 6. Frontal analysis for a simulated waste plating solution, with the column packed with TOMAC microcapsules. [Cr(VI)]feed ) 281.7 ppm, [Ni]feed ) 59.5 ppm, [Cu]feed ) 56.7 ppm, and [Zn]feed ) 53.3 ppm.
greater than 0.1 g, whereas all three metals are not extracted at all by the conventional liquid-liquid extraction. The increased extraction efficiencies obtained using microcapsules may result from the absorption of the metal ions on the surfaces of the microcapsules. Figure 5b shows the extraction efficiencies obtained for the metals with microcapsules containing D2EHPA. Here the extraction of Zn progresses more dominantly, with Cu behaving similarly as with TOMAC and the extraction of Ni and Cr(VI) hardly progressing at all. These tendencies also follow the extractabilities observed for D2EHPA in the liquid-liquid extraction system. Following the batch studies, the separation and recovery of metals from a waste plating solution were then carried out using the column packed with microcapsules. Considering the batch study results, as shown in Figure 5, Cr(VI) should be recovered first by the use of TOMAC microcapsules, and the separation of the other three metals may then be carried out using D2EHPA microcapsules. The simulated waste plating solution was, therefore, treated first with a column packed with microcapsules containing TOMAC. Figure 6 shows a typical example of the frontal analysis, obtained for the plating waste solution passing down the column. This shows that, for an effluent weight < 190 g (Bed Volume < 8.90), all of Cr(VI) is extracted, and thus no Cr(VI) is contained in the effluent solution. Differing portions of the other three metals, especially Cu, are also seen to be absorbed onto the surface of the microcapsules, as occurred in the batch studies. For effluent weights greater than 190 g, the extraction of Cr(VI) no longer proceeds such that the Cr(VI) concentration in the effluent solution increases immediately. An elution of the metals from the column was then carried out. Figure 7a shows the elution of the metals using 1.2 mol/L HCl. Effective stripping of Ni, Cu, and Zn is seen to be carried out at a small amount of HCl (
