Separation of Transition Metals Using Inorganic Adsorbents Modified

The extraction and separation of transition metals has been investigated using ... to form specific metal-EDTA complexes, with stability constants lyi...
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Ind. Eng. Chem. Res. 2002, 41, 5065-5070

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Separation of Transition Metals Using Inorganic Adsorbents Modified with Chelating Ligands Yasuhiro Shiraishi,* 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, Toyonaka 560-8531, Japan

The extraction and separation of transition metals has been investigated using several inorganic adsorbents (silica gel, MCM-41, and aluminum oxide), modified with ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) as chelating ligands. The metal ions in aqueous solution were coordinated successfully by the ligands on the surface of the adsorbents under moderate conditions. Metal-ligand complexes formed on the adsorbents effectively in the region of low pH (1-2) but scarcely formed at pH > 4. Silica gel modified with EDTA (silicaED) was shown to be the most effective adsorbent for the extraction of metals and to form specific metal-EDTA complexes, with stability constants lying in the order Cu2+ > Ni2+ > VO2+ > Zn2+ > Co2+ > Mn2+. The relatively good separation of these six adjacent metals was therefore also carried out using the silicaED. The metals coordinated on the silicaED could be desorbed completely by putting the material into contact with 1 mol/L aqueous HCl solution, such that the silicaED recovered could be reused for further separation. Introduction There has been much recent interest in the production of highly purified rare metals. The separation and purification of the metals are usually carried out via liquid-liquid extraction using several organic-soluble extractants. Such processes, however, require large amounts of harmful organic solvents (e.g., benzene) as diluents and encounter the problem that a third (emulsion) phase appears in some cases during process operation. A process for the separation of metals that can be operated simply without the requirement of organic solvents is therefore required. Various investigations have been reported on the separation of metals by the use of various kinds of organic materials modified with several chelating ligands, such as cross-linked polystyrene with ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA),1 and ovanillinsemicarbazone2 and chitosan with EDTA or DTPA.3 These “chelating resins” are insoluble in organic and aqueous solutions. Therefore, when the resins are added to aqueous solution, metal ions are coordinated by the ligands on the surface of the resins. The resulting resins, loaded with metals, can then be recovered simply by decantation. For industrial applications, more inexpensive materials are preferable as the supports for the chelating ligands. In the present work, widely used inorganic adsorbents, such as silica gel, aluminum oxide, and mesoporous molecular sieve MCM-41, were used as the supporting materials. By the surface modification of these materials, inorganic adsorbents modified with EDTA and DTPA ligands were newly synthesized. In addition, the metal-chelating behavior of the ligands on the adsorbents was studied in detail by ESR and UV-vis spectrometric analyses. The extractability of the transition metals using the modified * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-6-6850-6273. Tel.: +81-6-6850-6271.

adsorbents was studied, and the applicability of the adsorbents to the extraction and separation of the metals also examined. Experimental Section 1. Materials and Synthesis. All of the reagents were purchased from Wako Pure Chemical Industries, Ltd., and used without further purification. Silica gel (surface area, 380.59 m2/g) and neutral aluminum oxide (surface area, 127.64 m2/g) were used following activation for 10 h at 393 K. MCM-41 (surface area, 533.13 m2/g; average pore diameter, 3.18 nm) was synthesized according to the literature procedure.4 EDTA and DTPA anhydrides were prepared according to the procedure of Inoue et al.5 Surface modification of the adsorbents was carried out as follows (Figure 1): each adsorbent (10 g) was added to a toluene solution (80 mL) containing (3aminopropyl)triethoxysilane (APTES, 3 mL), and the mixture was stirred for 12 h at 383 K under reflux.6 The resulting adsorbent 1 was recovered by filtration, washed with acetone and then water, and dried under vacuum. Adsorbent 1 (5 g) was then stirred with EDTA or DTPA anhydride (10 g) in ethanol/acetic acid solution (50%, 50 mL) for 12 h at 340 K under reflux. The material 2 obtained was washed with acetone/water and dried under vacuum. IR spectra of 2 showed a distinctive absorption at 1650-1700 cm-1, attributable to amide group, thus suggesting that the chelating ligands had been anchored successfully on the surface of the adsorbents. 2. Procedure and Analysis. Extraction experiments were carried out as follows: The modified adsorbent (0.1 g) was added to an aqueous HCl solution (10 mL), containing metal ions (50 ppm), and the mixture was stirred using a magnetic stirrer at a temperature of 298 ( 1 K for 1 h. The resulting adsorbent was recovered immediately by filtration. The concentrations of the metals in the aqueous solution were analyzed using an inductively coupled argon plasma atomic emission spec-

10.1021/ie020119b CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2002

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Figure 2. Variations in the distribution ratio, D, for Cu2+ as a function of equilibrium pH of the solution using the adsorbents (a) silica gel, (b) MCM-41, and (c) aluminum oxide with (open symbols) and without (solid symbols) modification by EDTA.

Figure 1. Schematic diagram for the synthesis of adsorbents modified with EDTA (2).

trophotometer (Nippon Jarrell-Ash, ICAP-575 Mark II). ESR spectra were recorded at the X-band using a Bruker ESP300 spectrometer with 100-kHz magnetic field modulation at room temperature or 77 K and calibrated using 1,1′-diphenyl-2-picrylhydrazyl (DPPH) as the standard. Diffuse reflectance spectra of the adsorbents were recorded on a UV-vis spectrometer (Jasco Corp., V-550 with Integrated Sphere Apparatus ISV-469). Results and Discussion 1. Chelating Behavior of Modified Adsorbents. The extraction behavior of Cu2+ from aqueous HCl solution was first studied using the three adsorbents (silica gel, MCM-41, and aluminum oxide) modified with EDTA ligand. The amounts of metal ion on the adsorbents and the distribution ratios of the metal were calculated according to the following equations

q ) (Ci - Ce) × L/W

(1)

D ) q/Ce

(2)

where q is the amount of metal on the adsorbents (mol/ gadsorbent), D is the distribution ratio of the metal (L/gadsorbent), Ci is the initial metal concentration in the aqueous solution (mol/L), Ce is the the metal concentration in the aqueous solution at equilibrium (mol/L), L

Figure 3. Diffuse reflectance spectra for (a) silica gel, (b) silica gel following addition to aqueous Cu2+ solution of pH 5, (c) silicaED, and (d) silicaED following addition to aqueous Cu2+ solution of pH 1.

is the volume of the aqueous solution (L), and W is the weight of the adsorbents (g). Figure 2 shows the variation in the distribution ratio, D, for Cu2+ following extraction using the adsorbents both with modification by EDTA and without modification by EDTA (original adsorbents). For all of the unmodified adsorbents (solid symbols), the distribution ratio, D, is seen to increase linearly with increasing equilibrium pH of the solution. This is because H+ ions of the hydroxyl groups on the surface of the adsorbents are exchanged with metal ions with increasing pH of the solution, as follows

(≡X-OH)2 + M2+ + 2OH- f (≡X-O)2M + 2H2O (X ) Si or Al) (3) For each of the modified adsorbents (open symbols), the distribution ratio, D, was found to be higher than that obtained using the corresponding adsorbent without modification. To confirm the formation of a metal-ligand complex on the surface of the adsorbents, the variation in the diffuse reflectance spectra of the silica gel with and without modification by EDTA was studied. The results are summarized in Figure 3. Spectrum b of the unmodified silica gel following addition to aqueous Cu2+ solution (pH 5) showed no significant changes as compared to that for the silica gel itself (spectrum a). In contrast, spectrum d of silica gel modified with EDTA (silicaED)

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Figure 5. Difference in distribution ratio for Cu2+, ∆D, obtained using adsorbents with and without modification by EDTA (solid lines) and DTPA (dotted lines) as a function of equilibrium solution pH.

Figure 4. X-band ESR spectra for (a) aqueous Cu2+ solution (pH 1), (b) aqueous Cu2+ solution (pH 1) in the presence of EDTA, (c) silica gel following addition to aqueous Cu2+ solution (pH 5), and (d) silicaED following addition to aqueous Cu2+ solution (pH 1). Spectra a-c were recorded at 77 K, whereas spectrum d was recorded at room temperature.

following addition to aqueous Cu2+ solution (pH 1) included new absorption bands at 300-400 and 520800 nm. When aqueous Cu2+ solution (pH 1) in the presence of EDTA was analyzed, new absorption bands appeared at the same wavelength regions as in spectrum d. The new bands in spectrum d can therefore be attributed to the formation of a Cu2+-EDTA complex on the surface of adsorbents. Further confirmation for the formation of a Cu2+EDTA complex on the adsorbents was carried out by ESR analysis, with the results being summarized in Figure 4. Spectrum a, of Cu2+ in aqueous solution, shows distinctive isotropic signals at 2700-3200 G. In spectrum c, for silica gel following addition to Cu2+ solution (pH 5), signals appear at lower magnetic field than in spectrum a, thus suggesting that Cu2+ is adsorbed on the surface of the silica gel. In spectrum d, for silicaED following addition to Cu2+ solution (pH 1), signals appear at higher field than in spectra a and c. The signals in spectrum d are seen to appear at higher field than those in spectrum b for Cu2+ in aqueous solution in the presence of EDTA. This suggests that Cu2+ is coordinated successfully by the EDTA ligand anchored on the silica gel support, as shown schematically in Figure 1 (3). As shown in Figure 2, the difference in the distribution ratio, D, obtained for each of the adsorbents with and without modification by EDTA is seen to increase with decreasing pH of the solution, whereas the difference becomes nearly zero at pH > 4. In spectrum d in Figure 4, obtained by the addition of silicaED to a Cu2+ solution of pH 1, the signals attributable to Cu2+ adsorbed on the surface of silicaED (c) were not observed. However, the spectrum of silicaED following addition to a Cu2+ solution of pH 5 was found to be

almost the same as spectrum c. These findings suggest that the EDTA ligand anchored on the surface of the adsorbents acts effectively to form a Cu2+-EDTA complex at lower pH. The complexation ability of the ligand decreases with increasing pH and is essentially absent at higher pH (i.e., pH > 4), where mainly the adsorption of Cu2+ onto the silicaED surface occurs. It is wellknown that the chelating ability of free EDTA is higher in alkaline media than in acidic media. As reported by Inoue et al.,3 the chelating ability of EDTA anchored on chitosan increases with increasing solution pH, as is also the case for free EDTA. The chelating behavior of EDTA ligand anchored on inorganic adsorbent is completely different from that of free EDTA and EDTA anchored on chitosan. It is known that the surface charge of inorganic adsorbents becomes negative with increasing solution pH. The H+ of the carboxylic group of EDTA anchored on an inorganic adsorbent is therefore attracted to the negatively charged surface of the adsorbent. This charge attraction is therefore likely to suppress the complexation of Cu2+ with EDTA ligand at higher pH. 2. Chelating Ability of Modified Adsorbents. Figure 5 shows the variation in the difference of the distribution ratio for Cu2+, ∆D (L/g), obtained using the three adsorbents with and without modification by EDTA ligand. The difference, ∆D, for each of the adsorbents (solid lines) decreases with increasing equilibrium pH of the solution. The ∆D value for silica gel is seen to be higher than those for both MCM-41 and aluminum oxide, with the order being silica gel > MCM41 > aluminum oxide. By elemental analysis of the adsorbents, the nitrogen concentrations of the adsorbents were determined to be 1.26% (silica gel), 0.92% (MCM-41), and 0.38% (aluminum oxide). The quantities of EDTA ligand on the surfaces of the adsorbents were therefore estimated to be 0.30 mmol/gadsorbent (silica gel), 0.22 (MCM-41), and 0.09 (aluminum oxide), thus suggesting that relatively larger quantities of EDTA ligand are anchored on the silica gel support. Using these results, the differences in the distribution ratio per unit EDTA ligand, ∆D′ (L/mol), for the modified adsorbents were estimated, and the results are summarized in Figure 6. The value of ∆D′ is seen to fall in the order silica gel > MCM-41 > aluminum oxide. This suggests

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Figure 6. Difference in distribution ratio for Cu2+ obtained using adsorbents with and without modification by EDTA (solid lines) and DTPA (dotted lines) per unit chelating ligand, ∆D′, as a function of equilibrium solution pH.

that the EDTA ligand anchored on the silica gel support acts effectively to form a metal-EDTA complex. Elemental analysis of the adsorbents modified with DTPA showed the quantities of DTPA ligand on the surfaces of the adsorbents to be 1.29 mmol/g (silica gel), 1.67 (MCM-41), and 1.22 (aluminum oxide). These are significantly higher values than those obtained for the EDTA ligand on the adsorbents. When the adsorbents modified with DTPA were used for the extraction of Cu2+ from aqueous solution, the difference in the distribution ratio, ∆D, was found to be significantly lower those for EDTA, as shown in Figure 5. The ∆D′ values obtained for DTPA were lower than those for EDTA, as shown in Figure 6. These findings suggest that the chelating ability of the DTPA ligand anchored on the adsorbents is lower than that of EDTA ligand and that silicaED is the most effective adsorbent for the extraction of metals. The stability constant of the metal-DTPA complex is reported to be higher than that of metal-EDTA complex in aqueous solution.7,8 The results obtained here for the modified adsorbents, are apparently inconsistent with the data obtained in solution. The DTPA ligand is larger than EDTA ligand. Because these ligands are anchored on the rigid surfaces of the inorganic adsorbents, complexation of the metals with DTPA is therefore likely to be hindered sterically. 3. Separation of Transition Metals by Silica Gel Modified with EDTA. The silicaED adsorbent was then used for the extraction of transition metals other than Cu2+. The extractability of the metals was examined by the addition of silicaED (0.1 g) to aqueous HCl solutions (10 mL) containing each metal (50 ppm). Figure 7 shows the distribution ratio, D, for each metal as a function of the equilibrium pH of the solution. The extractabilities of the metals determined at pH 1 are seen to lie in the order Cu2+ > Ni2+ > VO2+ > Zn2+ > Co2+ > Mn2+. The order of extractabilities agrees reasonably well with the order of stability constants for the metal-EDTA complexes obtained in aqueous solution.7,8 The difference in D for each metal is seen to be relatively large at low pH but to become smaller with increasing solution pH. For chitosan modified with EDTA,3 the difference in D is also reported to decrease with increasing pH. This indicates that separations of the transition metals using the silicaED should also be carried out at lower pH, especially 1-2.

Figure 7. Distribution ratio, D, for various transition metals as a function of equilibrium solution pH using silicaED for silicaED (0.1 g) added to aqueous solutions containing each metal (50 ppm).

Figure 8. Extractability of metals from aqueous HCl solution (pH 1, 10 mL) containing six transition metals (each 50 ppm) as a function of quantity of silicaED added.

The separation of metals from aqueous solutions (pH 1) containing more than six metals was then investigated. The extractability of each metal is plotted in Figure 8 as a function of quantity of silicaED added. The order of extractability was found to be Cu2+ > Ni2+ g VO2+ > Zn2+ > Co2+ > Mn2+, which agrees with the order of the distribution ratios, D, obtained upon extraction from aqueous solutions containing each metal, as shown in Figure 7. This indicates that the EDTA ligand on the silica gel support can form specific metal-EDTA complexes in aqueous solutions containing various metals. A chromatographic separation of the above six transition metals was then carried out to estimate the practical usage of silicaED. The silicaED (1 g) was packed into a glass column (10 mm i.d. × 500 mm), and aqueous HCl solution (pH 1, 100 mL) containing the metals (each 12.5 ppm) was then introduced into the top of the column. Figure 9a shows the concentrations of the metals in each fraction eluted from the column. Although the elution of Cu2+ was hardly observed, the concentrations in the eluents of Mn2+, Co2+, and Zn2+, as well as those of VO2+ and Ni2+, were almost the same. As shown in Figure 9c, the amount of Cu2+ on the silicaED was significantly higher than the amounts of the other metals, with the amounts of the metals on the

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Figure 9. (a,b) Concentrations of metals in eluent fractions and (c,d) amounts of metals on silicaED (1 g). Parts a and c include the data for aqueous HCl solution (pH 1, 100 mL) containing six metals (each 12.5 ppm) being fed for extraction; parts b and d include the data for 1 M HCl aqueous solution (70 mL) being fed for stripping.

silicaED lying in the order Cu2+ > VO2+, Ni2+ > Mn2+, Co2+, Zn2+. The results suggest that, in the present process using silicaED, practical separations of VO2+ and Ni2+ and of Mn2+, Co2+, and Zn2+ would be difficult to achieve. However, the separations of Cu2+ from the other metals and of VO2+ or Ni2+ from Mn2+, Co2+, and Zn2+ were carried out successfully. It is necessary to recover the metals coordinated by the EDTA ligand on the adsorbents following extraction. To accomplish this, 1 mol/L HCl aqueous solution was used as a stripping solvent, which was introduced into the top of a column loaded with the metals. The concentrations of the metals in the eluent and the amounts of the metals on the silicaED are summarized in Figure 9b and d, respectively. Almost all of the metal coordinated on silicaED was found to be stripped successfully and recovered completely from the column. The elemental analysis of the resulting silicaED showed no change in the elemental composition as compared to virgin silicaED. When the extraction of Cu2+ was carried out using the resulting silicaED, no change in the distribution ratio for Cu2+ was observed as compared to that for the virgin silicaED. These results therefore suggest that the EDTA ligands of the silicaED recovered

their chelating ability and that the silicaED could be reused for further separations of the metals. Conclusion The extraction and separation of transition metals using inorganic adsorbents (silica gel, MCM-41, and aluminum oxide) modified with EDTA and DTPA chelating ligands were investigated, and the following results were obtained: (1) The metals in aqueous solution are coordinated successfully by the ligands anchored on the adsorbents. Silica gel modified with EDTA (silicaED) is the most effective adsorbent for the recovery of metals. MetalEDTA complexes formed on the adsorbents effectively at low pH 1-2, where the adsorption of the metals on the surface of the adsorbents scarcely occurs. (2) The extractabilities of the metals by silicaED agree reasonably well with the stability constants of the metal-EDTA complexes in aqueous solution, thus suggesting that the EDTA ligand on the silica gel maintains its chelating ability. The relatively good separation of transition metals (Cu2+, Ni2+, VO2+, Zn2+, Co2+, and Mn2+) was achieved at low pH’s of 1-2.

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(3) In practical separations using columns packed with silicaED, the separations of Zn2+, Co2+, and Mn2+ and of Ni2+ and VO2+ are quite difficult, whereas the separation of Cu2+ can be carried out successfully. The metals coordinated on silicaED can be successfully desorbed with 1 mol/L HCl aqueous solution, thus maintaining the chelating ability of the EDTA ligand. Nomenclature q ) amount of metal on adsorbent in eq 1, mol/g Ci ) initial metal concentration in feed aqueous solution in eq 2, mol/L Ce ) metal concentration in aqueous solution at equilibrium in eq 1, mol/L D ) distribution ratio of metal in eq 2, L/g ∆D ) difference in distribution ratio using adsorbents with and without modification byligand, L/g ∆D′ ) difference in distribution ratio per unit ligand, L/mol L ) volume of feed aqueous solution, L W ) weight of adsorbent in eq 1, g

Acknowledgment The authors are grateful to the Division of Chemical Engineering for the Lend-Lease Laboratory System. Literature Cited (1) Takeda, K.; Akiyama, M.; Kawakami, F.; Sasaki, M. Recovery of Highly-Purified Rare Earth Elements Using NewlySynthesized Chelating Resins. Bull. Chem. Soc. Jpn. 1986, 59, 2225.

(2) Jain, V. K.; Handa, A.; Sait, S. S.; Shrivastav, P.; Agrawal, Y. K. Pre-concentration, Separation and Trace Determination of Lanthanum(III), Cerium(III), Thorium(IV) and Uranium(VI) on Polymer Supported o-Vanillinsemicarbazone. Anal. Chim. Acta 2001, 429, 237. (3) Inoue, K.; Yoshizuka, K.; Ohto, K. Adsorption Separation of Some Metal Ions by Complexing Agent Types of Chemically Modified Chitosan. Anal. Chim. Acta 1999, 388, 209. (4) Cheng, C.-F.; Zhou, W.; Park, D. H.; Klinowski, J.; Hargraves, M.; Gladden, L. F. Controlling the Channel Diameter of the Mesoporous Molecular Sieve MCM-41. J. Chem. Soc., Faraday Trans. 1997, 93, 359. (5) Inoue, K.; Ohto, K.; Yoshizuka, K.; Yamaguchi, T.; Tanaka, T. Adsorption of Lead(II) Ion on Complexane Types of Chemically Modified Chitosan. Bull. Chem. Soc. Jpn. 1997, 70, 2443. (6) Liu, C. J.; Li, S. G.; Pang, W. Q.; Che, C. M. Ruthenium Porphyrin Encapsulated in Modified Mesoporous Molecular Sieve MCM-41 for Alkene Oxidation. Chem. Commun. 1997, 65. (7) Suzuki, Y.; Yokoi, S.; Katoh, M.; Minato, M.; Takizawa, N. In The Rare Earths in Modern Science and Technology; McCarthy, G. J., Ed.; Plenum Press: New York, 1980; Vol. 2, p 121. (8) Flascheka, H. A.; Barnard, A. J., Jr. The Titrations with EDTA and Related Compounds. In Comprehensive Analytical Chemistry; Wilson, C. L., Wilson, D. W., Eds.; Elsevier Publishing Co.: London, 1980; Vol. 1B.

Received for review February 8, 2002 Revised manuscript received July 2, 2002 Accepted July 29, 2002 IE020119B