Removal of Bi, Cd, Co, Cu, Fe, Ni, Pb, and Zn from an Aqueous Nitrate

Removal of Bi, Cd, Co, Cu, Fe, Ni, Pb, and Zn from an Aqueous ... as in the cases of SIR and SX; (ii) the pH value at 50% metal removal, pH50, by SIF ...
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Ind. Eng. Chem. Res. 2003, 42, 4050-4054

SEPARATIONS Removal of Bi, Cd, Co, Cu, Fe, Ni, Pb, and Zn from an Aqueous Nitrate Medium with Bis(2-ethylhexyl)phosphoric Acid Impregnated Kapok Fiber Hai T. Huynh and Mikiya Tanaka* Research Institute for Green Technology, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan

Bis(2-ethylhexyl)phosphoric acid (D2EHPA) impregnated fiber (SIF) using a commercial oil sorbent made of kapok fiber as the support has been prepared by a wet method, and its ability to remove metal ions of Bi(III), Cd(II), Co(II), Cu(II), Fe(III), Ni(II), Pb(II), and Zn(II) from singlemetal nitrate solutions has been evaluated in comparison with D2EHPA-impregnated resin using XAD7HP as the support (SIR) and solvent extraction with D2EHPA dissolved in a nonpolar organic solvent (SX), where the amount of D2EHPA in each system was the same. The results are summarized as follows: (i) the removal of each metal by SIF proceeds via cation exchange as in the cases of SIR and SX; (ii) the pH value at 50% metal removal, pH50, by SIF increases in the order Fe(III) < Bi(III) < Zn(II) < Pb(II) < Cd(II) < Cu(II) < Co(II) < Ni(II), and this order agrees with those by SIR and SX; (iii) the pH50 value for each metal increases in the order SIF < SIR , SX; (iv) the loading capacity of Zn(II) by SIF is 1.5 times higher than that by SIR; (v) SIF can be used many times after regeneration by acid treatment as in the case of SIR. The removal rates for Cd(II), Cu(II), Pb(II), and Zn(II) decrease in the order SX > SIF > SIR. In conclusion, SIF is more advantageous than SIR and SX in terms of the amount of D2EHPA necessary for achieving a certain removal percentage and than SIR in terms of the removal rate; thus, the application of SIF to metal-bearing wastewater treatment is highly expected. Introduction Metal ions such as Bi(III), Cd(II), Co(II), Cu(II), Fe(III), Ni(II), Pb(II), and Zn(II) are often present in wastewater from industries such as mining, metallurgy, and surface finishing at a concentration from a few to a few hundred milligrams per cubic decimeter. Those effluents would cause serious environmental problems; thus, numerous investigations have been carried out on removing the metals from industrial wastewater using several methods such as chemical precipitation, liquidliquid extraction (solvent extraction), solid-liquid extraction (adsorption and resin ion exchange), and reverse osmosis.1-4 Some of these methods are expensive and have limitations. Currently, the usual treatment technology of metal-bearing wastewater is chemical precipitation; however, this method often creates secondary problems with sludge generation.5 Solvent extraction and resin ion exchange involve similar chemical principles, each offering advantages and drawbacks.6,7 As examples of the general advantages, solvent extraction is effective for the recovery of metal ions at high concentration with high selectivity and is suitable for continuous operation, while resin ion exchange is effective for the removal of metal ions at low concentration and simplifies the equipment and * To whom correspondence should be addressed. Tel.: +81-29-861-8486. Fax: +81-29-861-8458. E-mail: [email protected].

operation. As examples of the general drawbacks, solvent extraction is associated with the loss of organic compounds (extractant and diluent) into the aqueous phase, while resin ion exchange has a low selectivity for metal ions except in the cases using chelating resins, which show slow complexation with metal ions and are expensive. The development of solvent-impregnated resins as a link between solvent extraction and resin ion exchange has become an important direction in separation science and technology. Solvent-impregnated resins have been shown to be effective sorbents for the removal of metal ions at low concentration.8-12 Our previous study13 has shown that an oil sorbent made of kapok fiber (natural fiber) can sorb an extractant used in metal solvent extraction and serve as the support for the impregnated metal sorbent. The objective of this study is to explore the ability of removing Bi(III), Cd(II), Co(II), Cu(II), Fe(III), Ni(II), Pb(II), and Zn(II) from an aqueous nitrate medium with bis(2-ethylhexyl)phosphoric acid (D2EHPA) impregnated kapok fiber (SIF) in comparison with the D2EHPAimpregnated resin using XAD7HP as the support (SIR) and solvent extraction with D2EHPA dissolved in a nonpolar organic solvent (SX). D2EHPA was selected as the extractant because this is a typical organophosphorus acid and has been extensively studied for solvent extraction of numerous metal ions.14-16

10.1021/ie020794l CCC: $25.00 © 2003 American Chemical Society Published on Web 07/11/2003

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Experimental Section Reagents and Working Solutions. Ion-exchangedistilled water was used throughout this study. Chemicals used in this study were all reagent grade except for the extractant and organic diluent. D2EHPA was the product of Daihachi Chemical Industry Co. and was used as received. The organic phase was prepared by dissolving D2EHPA in Shellsol D70 (Shell Chemicals) at desired concentrations. The aqueous solution of each metal ion was prepared by dissolving the metal nitrate in a 0.5 mol/dm3 sodium nitrate solution. The concentration of the metal ion was 1 mmol/dm3 in all of the experiments except for the loading test, which employed 1-12.5 mmol/dm3 Zn(II). Various concentrations of sodium hydroxide and nitric acid were used as the pHadjusting reagents. Support. Oil Catcher KT-65 (Kakui Co., Ltd.), made of kapok fiber, was supplied in the form of sheets. Before impregnation, the fiber was cut into 0.5 cm × 0.5 cm pieces. The macroporous resin Amberlite XAD7HP,17 supplied by Rohm and Haas Co., is a polymeric sorbent with an acrylic ester matrix. The Brunauer-EmmettTeller specific surface areas of the kapok fiber and XAD7HP both on a dry basis measured by COULTER SA3100 were 0.3 and 449 m2/g, respectively. Impregnation. SIF was prepared by a wet method as follows:10,18 the fiber was washed with methanol, dried, and contacted with 10 vol % D2EHPA dissolved in ethanol in the phase ratio of 50 cm3/g at 298 K at a shaking rate of 140 rpm overnight. The fiber was then removed by filtration and washed with an excess volume of water. Finally, SIF was dried in an oven overnight at 353 K. SIR was prepared by a dry method10,19 so that the D2EHPA concentration in SIR becomes equal to that in SIF. That is, based on the D2EHPA concentration in SIF, an appropriate amount of D2EHPA was first diluted in ethanol (40 cm3). The resulting solution was contacted with the desired amount of XAD7HP, washed with methanol, and dried, and ethanol was removed by slow evaporation under vacuum. SIR was removed and dried in an oven overnight at 353 K. The concentrations of D2EHPA held in SIF and SIR were determined by the difference in weight before and after the impregnation. In some cases, the concentrations of D2EHPA in SIF and SIR were also determined by digestion with a mixture of sulfuric and nitric acids followed by phosphorus analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES; Seiko SPS4000). The results agreed within (5%. Adsorption and Extraction. Specified amounts of the aqueous solution and the impregnated support (or organic phase) were placed in a stoppered 50-cm3 conical flask. When necessary, the pH of the aqueous solution was adjusted by adding a minute volume of the pHadjusting reagent. The contacts of the two phases were done by a horizontal shaker at a rate of 140 rpm in a water bath maintained at 298 K for more than 12 h in the equilibrium runs (examination of the pH dependency, loading property, and repeated usage) and for the required time in the kinetic runs. After shaking, the aqueous phase was separated by filtration for SIF and SIR or centrifuged for SX. The amounts of the impregnated support (or organic phase) and the aqueous phase placed into the conical flask were (i) 0.2 g (10 cm3) and 10 cm3 for the examination of the pH dependency of the removal and

Figure 1. Effect of the equilibrium pH on the metal removal with SIF. The D2EHPA concentration in SIF was 0.449 g/g of SIF.

Figure 2. Effect of the equilibrium pH on the metal removal with SIR. The D2EHPA concentration in SIR was 0.446 g/g of SIR.

the kinetic runs and (ii) 0.1 g (5 cm3) and 20 cm3 for the loading test. In the repeated test, SIF and SIR were shaken with the Zn(II) aqueous solution in the phase ratio of 200 cm3/g of SIS without pH adjustment. Then the solid phases washed with 667 cm3/g of SIS of water and dried at 353 K overnight were vigorously shaken with 2 mol/ dm3 HCl for 1 h in the phase ratio of 66.7 cm3/g of SIS to elute the adsorbed Zn(II) at room temperature (295298 K). The obtained solid phases were again washed and dried in the same manner as in the removal operation. This removal-elution cycle was repeated. The metal concentrations in the aqueous phase were determined by ICP-AES after appropriate dilution. The metal concentrations in the solid or organic phases after shaking were calculated on the basis of mass balance. The pH values in the aqueous phases were measured by a pH meter (Toa HM-60G). Results and Discussion The effect of the equilibrium pH on the metal removal percentage is shown in Figures 1-3, where the D2EHPA concentration in the organic phase for SX was determined so that the amount of D2EHPA used for the metal removal by SX became equal to those by SIF and SIR. The removal percentage for each metal ion by SIF, SIR, and SX depends on the equilibrium pH and

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Figure 3. Effect of the equilibrium pH on the metal removal with SX. The D2EHPA concentration in the organic phase was 9.00 g/dm3.

Figure 4. Effect of the equilibrium pH on the metal removal with kapok fiber.

Table 1. pH50 Values for SIF, SIR, and SX pH50 system

Bi(III)

Zn(II)

Pb(II)

Cd(II)

Cu(II)

Co(II)

Ni(II)

SIF SIR SX

0.15 0.23 0.75

1.57 1.74 2.10

1.88 2.09 2.95

2.26 2.44 3.42

2.53 2.72 3.54

3.31 3.46 4.32

3.50 3.69 4.77

increases rapidly with a rise in pH, suggesting that all of the metal ions are removed by a cation-exchange reaction. For SIF, the pH50 value (50% of the metal ion is removed at this pH value) is increased in the order Fe(III) < Bi(III) < Zn(II) < Pb(II) < Cd(II) < Cu(II) < Co(II) < Ni(II) (for Bi(III), the pH50 value is estimated by extrapolation), and this order is in agreement with those by SX and SIR (Table 1). Although the amount of D2EHPA per unit volume of the aqueous phase is the same in each system (9 g/dm3), the pH50 value for each metal increases in the order SIF < SIR , SX. Therefore, SIF is more advantageous than SIR and SX in terms of the amount of D2EHPA necessary for achieving a certain removal percentage. The main reason for the difference in the equilibrium removal efficiencies between SIF and SIR seems to lie in the fact that the D2EHPA concentration based on the surface area for SIF (2.7 g/m2 of kapok fiber) was much higher than that for SIR (1.8 mg/m2 of XAD7HP). Similarly, the difference between SIR and SX seems to be explained by the fact that the D2EHPA concentration based on the volume for SIR (480 g/dm3) was much higher than that for SX (9 g/dm3). The blank experiments have shown that the kapok fiber itself sorbs those metal ions to a slight extent (Figure 4). No difference in the sorption behavior among the metal ions of the same valency is found. The pH dependency of the sorption is slight for Cd(II), Co(II), Cu(II), Ni(II), Pb(II), and Zn(II) and great for Bi(III) and Fe(III). Like other natural fibers, the hydroxyl group in cellulose, which is the main component of the kapok fiber, seems to play an important role in the sorption of metals. For XAD7HP, none of the metal ions are sorbed at all; therefore, another reason for the difference in the removal efficiencies between SIF and SIR would be the metal sorption ability of kapok fiber. Figure 5 shows the results of the loading test of zinc as the adsorption and extraction isotherms, where the pH values of the aqueous solutions were not adjusted.

Figure 5. Adsorption and extraction isotherms of Zn(II). The D2EHPA concentrations were 0.449 g/g of SIF, 0.446 g/g of SIR, and 9.00 g/dm3 (SX).

The amount of D2EHPA per unit volume of the aqueous phase was the same in each experiment (7 mmol/dm3). The dissolution of the Zn(II)-D2EHPA complex into the aqueous phase at the high zinc loading region was not observed, which is in contrast with the Ni(II)-D2EHPA complex in our previous study.13 The saturated Zn(II) concentrations for SIF, SIR, and SX were 43 mg/g of SIF, 29 mg/g of SIR, and 0.42 g/dm3, respectively. SIF has a 50% higher loading capacity of Zn(II) than SIR. The numbers of D2EHPA molecules at the saturated regions per one zinc ion were calculated to be 2.1, 3.1, and 4.3 for SIF, SIR, and SX, respectively. Figure 6 shows the results of the repeated test. The removal efficiencies of Zn(II) at the first and seventh cycles were respectively 98% and 92% for SIF and 93% and 79% for SIR. This result is in qualitative agreement with the data of our previous study.13 During the experiment in the present study, the elution was always complete, indicating that the decrease in the removal efficiency was caused by the loss of extractant from SIF and SIR. Although the D2EHPA concentration for SIF based on the weight of the impregnated support was equal to that of SIR (0.53 g/g), the D2EHPA concentration for SIF based on the surface area of the support (3.7 g/m2 of kapok fiber) is much higher than that for SIR (2.5 mg/m2 of XAD7HP). Therefore, the dissolution of D2EHPA from SIF could be greater than that from SIR.9,11 Nevertheless, the experimental results indicate

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Figure 6. Removal of Zn(II) during the repeated usage of SIF and SIR. The D2EHPA concentrations were 0.449 g/g of SIF, 0.446 g/g of SIR, and 9.00 g/dm3 (SX).

rates are increased in the order SIR < SIF , SX. Despite the fact that the specific surface area of XAD7HP is much greater than that of the kapok fiber, the removal rate of SIR is smaller than that of SIF, suggesting that the intraparticle diffusion resistance in SIR greatly contributes to its smaller removal rate. The removal rate of SX is much higher than that of SIF, probably because of the larger interfacial area formed by the shaking in SX. For SX of Cd(II), Cu(II), and Pb(II), the maxima were observed at the initial stage of metal removal. For Pb(II), after reaching the maximum, the extraction percentage was slightly decreased to the equilibrium value. For Cd(II) and Cu(II), the extraction percentages were also slightly decreased after the maximum; however, they were afterward increased to the equilibrium values. For Zn(II), a maximum was not found. These phenomena may be related to the difference in the rates of the dissolution of D2EHPA into the bulk aqueous phase and the interfacial reactions for the formation of the metal-D2EHPA complex and its dissociation; however, determination of the true reason is beyond the scope of the present paper. Conclusions

Figure 7. Time courses of the relative removals of Cd(II) and Pb(II). The D2EHPA concentrations were 0.525 g/g of SIF, 0.531 g/g of SIR, and 10.5 g/dm3 (SX).

The ability of SIF using a commercial oil sorbent made of kapok fiber as the support to remove metal ions of Bi(III), Cd(II), Co(II), Cu(II), Fe(III), Ni(II), Pb(II), and Zn(II) from the single-metal nitrate solutions has been evaluated in comparison with those of SIR and SX. The results are summarized as follows: (i) The removal of each metal by SIF proceeds via cation exchange as in the cases of SIR and SX. (ii) The pH50 by SIF increases in the order Fe(III) < Bi(III) < Zn(II) < Pb(II) < Cd(II) < Cu(II) < Co(II) < Ni(II), and this order agrees with those by SIR and SX. The pH50 value for each metal increases in the order SIF < SIR , SX when the amount of D2EHPA in each system is the same. (iii) The loading capacity of Zn(II) by SIF is 1.5 times higher than that by SIR. (iv) SIF can be used many times after regeneration by acid treatment as in the case of SIR. (v) The removal rates for Cd(II), Cu(II), Pb(II), and Zn(II) decrease in the order SX . SIF > SIR. Therefore, SIF is more advantageous than SIR and SX in terms of the amount of D2EHPA necessary for achieving a certain removal percentage and than SIR in terms of the removal rate. Thus, the application of SIF to metal-bearing wastewater treatment is highly expected. Acknowledgment

Figure 8. Time courses of the relative removal Cu(II) and Zn(II). The D2EHPA concentrations were 0.525 g/g of SIF, 0.531 g/g of SIR, and 10.5 g/dm3 (SX).

that SIF as well as SIR can be used repeatedly, although gradual losses of D2EHPA seem to be accompanied. Figures 7 and 8 are the results of the kinetic runs, showing the relative removal of the metal ions as a function of shaking time when the initial pH was 6.0. Here, the relative removal is the removal at a certain time divided by the equilibrium removal. The removal

The authors thank the Ministry of the Environment, Japan, and the Japan Science and Technology Cooperative for their financial support. Literature Cited (1) Grosse, D. W. A Review of Alternative Treatment Processes for Metal Bearing Hazardous Waste Streams. J. Air Pollut. Control Assoc. 1986, 36 (5), 603-614. (2) Janson, C. E.; Kenson, R. E.; Tucker, L. H. Treatment of Heavy Metals in Wastewater. Environ. Prog. 1982, 1 (3), 212216. (3) Koivula, R.; Lehto, J.; Pajo, L.; Gale, T.; Leinonen, H. Purification of Metal Plating Rinse Waters with Chelating Ion Exchangers. Hydrometallurgy 2000, 56 (1), 93-108.

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Received for review October 7, 2002 Revised manuscript received May 23, 2003 Accepted May 26, 2003 IE020794L