Coadsorption of Trivalent Metal Ions and Anions on Strongly Acidic

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Anal. Chem. 2008, 80, 9666–9671

Coadsorption of Trivalent Metal Ions and Anions on Strongly Acidic Cation-Exchange Resins by Bridge Bonding Takanori Matsuura, Kenji Ohnaka, Mayuu Takagi, Miki Ohashi, Ko Mibu, and Akio Yuchi* Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya, 466-8555, Japan The effects of anions (P(V), P(III), P(I), Se(IV), OH-, F-, Cl-, SCN-, S(IV), and CH3COO-) on the adsorption of trivalent metal ions (Fe3+, Al3+, Ga3+, In3+, and Sc3+) to three strongly acidic cation-exchange resins (-S)- of different types (porous or gel) and different exchange capacities (4.55, 3.91, and 0.96 mmol g-1) were studied systematically. All these metal ions showed coadsorption of OH-, irrespective of the resins. In contrast, coadsorption of P(V), P(III), P(I), and Se(IV) was observed on the resins of the higher exchange capacities but not on the resin of the lowest exchange capacity. Stoichiometric analyses and spectroscopic (Mo ¨ssbauer and infrared) studies for Fe3+ demonstrated the presence of the coadsorbed species: [(-S)2Fe(OH)] and [(-S)2(Fe-O-Fe)(S-)2] for OH-, [(-S)2Fe(HPO4)Fe(S-)2] for P(V), and [(-S)2FeX]j (X- ) H2PO3-, H2PO2-, HSeO3-; j > 1) for P(III), P(I), and Se(IV). No coadsorption was observed for the other anions. These findings indicate that the bridge bonding of anions between the metal ions adsorbed on the resins of the higher exchange capacities plays a crucial role for the coadsorption. Some analytical implication was also discussed. Ion-exchange resins developed in the 1930s and commercially available in the 1940s have been used widely, and their fundamental properties have been well characterized.1,2 As is known, the Donnan membrane theory is applicable to the ion-exchange processes, and penetration of ions having the same electric charge as that of a fixed ion-exchange group is extremely unfavorable.3 Chelating resins with an anionic functional group are, however, used as the anion adsorbent4-7 and as the stationary phase of ligand-exchange chromatography and metal-affinity chromatog* To whom correspondence should be addressed. E-mail: [email protected]. (1) Helfferich, F. Ion Exchange; McGraw-Hill Book Company, Inc.: New York, 1962. (2) Korkisch, J. Handbook of Ion Exchange Resins Their Application to Inorganic Analytical Chemistry; CRC Press: Boca Raton, Florida, 1989; Vol. 1. (3) Donnan, F. G. Z. Elektrochem. 1911, 17, 572. (4) Sengupta, A. K. In Ion Exchange and Solvent Extraction; Sengupta, A. K., Marcus, Y., Eds., Dekker: New York, 2001; Vol. 14. (5) Yuchi, A.; Matsunaga, K.; Niwa, T.; Terao, H.; Wada, H. Anal. Chim. Acta 1999, 388, 201. (6) Yuchi, A.; Ogiso, A.; Muranaka, S.; Niwa, T. Anal. Chim. Acta 2003, 494, 81. (7) Yuchi, A.; Matsuo, K. J. Chromatogr., A 2005, 1082, 208.

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raphy8-11 for anionic species after modification with metal ions. In these methodologies, the electric charges of the metal ions exceed that of the chelating functional group. Adsorption and separation of anions are based on both the electrostatic and Lewis acid-base interactions between the anions penetrated into the resin phase and the immobilized metal ions. Only a few and conflicting results have been reported, on the other hand, as for penetration of anions into strongly acidic cation-exchange resins (the sodium form abbreviated as -SNa). Heitner-Wirguin and coworkers indicated that, in stepwise complexation of metal ions with monovalent anions (X-), negatively charged species were adsorbed on anion-exchange resins and positively charged species on cation-exchange resins.12,13 They also suggested the presence of dimers and of some clusters in Nafion adsorbing Fe3+, based on the results of infrared and Mo¨ssbauer spectroscopy.14,15 Filatova and co-workers reported coadsorption of Fe3+ and phosphate,16 although the composition of the coadsorbed species had not been clearly identified.17,18 Yoshida et al. made a screening study on the adsorbent for phosphate using a diversity of ionexchange resins loaded with Fe3+ and reported the unsuitability of strongly acidic cation-exchange resins for this purpose.19 We had directly demonstrated the presence of three species, [(-LH)3M], [(-L)(-LH)M], and [(-L)MX], in adsorption of trivalent metal (M3+) ions on the iminodiacetate (-LH2)-type chelating resin, based on the adsorption isotherms in slight excesses of metal ions against the functional groups.20 Application of this technique to a preliminary study showed coadsorption of Fe3+ and some anions on a strongly acidic cation-exchange resin.21 In this study, the effects of the cation, the anion, and the type and the exchange capacity of the resin are systematically studied. The hydration numbers of metal ions in the resins are also (8) Davankov, V. A.; Navratil, J. D.; Walton, H. F. Ligand Exchange Chromatography; CRC Press: Boca Raton, Florida, 1988. (9) Yuchi, A.; Mizuno, Y.; Yonemoto, T. Anal. Chem. 2000, 72, 3642. (10) Porath, J. Trends Anal. Chem. 1988, 7, 254. (11) Porath, J.; Carlsson, J.; Olsson, I.; Belfrage, G. Nature 1975, 258, 598. (12) Heitner-Wirguin, C.; Cohen, R. J. Phys. Chem. 1967, 71, 2556. (13) Heitner-Wirguin, C. In Ion Exchange and Solvent Extraction; Marinsky, J. A., Marcus, Y., Eds.; Dekker: New York, 1977; Vol. 7, p 83. (14) Heitner-Wirguin, C. Polymer 1979, 20, 371. (15) Heitner-Wirguin, C.; Bauminger, E. R.; Levy, A.; de Kanter, F. L.; Ofer, S. Polymer 1980, 21, 1327. (16) Filatova, L. N.; Chepelevetskii, M. L. Zh. Neorg. Khim. 1966, 11, 1662. (17) Filatova, L. N.; Shelyakina, M. A. Zh. Fiz. Khim. 1974, 48, 1617. (18) Filatova, L. N.; Shelyakina, M. A. Zh. Fiz. Khim. 1974, 48, 2890. (19) Yoshida, I.; Takeshita, R.; Ueno, K. Nippon Kagaku Kaishi 1980, 220. (20) Yuchi, A.; Sato, T.; Morimoto, Y.; Mizuno, H.; Wada, H. Anal. Chem. 1997, 69, 2941. (21) Ohnaka, K.; Yuchi, A. Chem. Lett. 2005, 34, 868. 10.1021/ac801468f CCC: $40.75  2008 American Chemical Society Published on Web 11/17/2008

Table 1. Properties of Resins hydration numbera

average distance between M/nm resin G-4.6 P-3.9 G-1.0 a

-1

ECNa mmol g 4.55 3.91 0.96

[(-S)3M]

[(-S)2M]

[(-S)M]

MWR

H

Na

Fe

Fe-OH

Fe-P(V)

1.03 1.09 1.73

0.83 0.95 1.51

0.71 0.75 1.20

154 188 955

3.4 (C) 3.7 (D) 4.2 (A)

2.4 (D) 2.8 (B) 3.7 (A)

7.8 (C) 9 (E) 12.4 (A)

5 (E) 6.8 (D) 8.8 (B)

6 (E) 7 (E)

See text for letter in parentheses.

determined by Karl Fischer coulometry in combination with water vaporization at the variable fixed temperature, while the symmetry of Fe3+ in the resin phase is studied by Mo¨ssbauer spectroscopy, to elucidate the origin of coadsorption. EXPERIMENTAL SECTION Resins. Three strongly acidic cation-exchange resins of different types (porous and gel types) and of different exchange capacities were used. The first resin was very common and commercially available Amberlite 252 (Rohm and Haas, porous, Na-form, DVB 12%, particle size of 0.59-0.84 mm, abbreviated as P-3.9). Since there is no commercially available resin having the appropriately lower adsorption capacity, we synthesized the second resin (gel, DVB 5%, mean particle size of 0.35 mm, abbreviated as G-1.0) by the following procedure. Under the condition of 60 °C in the nitrogen atmosphere, partially hydrolyzed poly(vinyl alcohol) was dissolved in water, and a styrene solution of ethyl p-styrenesulfonate, divinylbenzene, and lauloylperoxide was added subsequently to the aqueous solution. After the mixture was stirred at 200 rpm at 70 °C in the nitrogen atmosphere, the resulting polymer was filtered out, washed sequentially with water and methanol, and dried under vacuum. The yield was 92%. The polymer was swollen in tetrahydrofuran at 60 °C and was then hydrolyzed with tetramethylammonium hydroxide. The product was filtered out and sequentially washed with tetrahydrofuran and water. Homogeneous distribution of the functional group was confirmed by mapping the sulfur atom using electron probe microanalyzer (EPMA) (Supporting Information Figure S-1). For the purpose of comparison between porous and gel types, we used commercially available Amberlyst 31WET (Rohm and Haas, gel, H-form, DVB 4%, particle size of 0.55-0.70 mm, abbreviated as G-4.6) as the third resin. All the resins were converted to the Na-form and were kept in a glovebox to show constant weights. The temperature and the relative humidity within the glovebox were controlled to 25 °C and 50% ± 2% using a saturated solution of calcium nitrate. The resins were weighed in this glovebox. Determination of Exchange Capacities. Each accurately weighed Na-form resin (1 g) was packed in a column and exposed to a 500 cm3 flow of a 1 mol dm-3 HCl solution. The bed was washed with water and subsequently exposed to a 50 cm3 flow of a 1 mol dm-3 NaCl solution. The eluent was titrated with a standard base solution. This determined the exchange capacity of the Na-form resin, ECNa/mmol g-1 (Table 1). The average distance between the metal ions was calculated in 100% occupation of the trivalent, the divalent, and the monovalent cations, and the calculation results are included in this table. Determination of Adsorption Capacities. The effects of pH on the adsorption capacities of metal ions were studied by

equilibrating each weighed resin (0.05 g) for 12 h with 20 mL of a solution containing 10-2 mol dm-3 of each metal ion (for G-4.6 and P-3.9) or containing 2.5 × 10-3 mol dm-3 of each metal ion (for G-1.0) at various acidities. Five trivalent metal ions (Fe3+, Al3+, Ga3+, In3+, and Sc3+) were selected as those having different sizes and characteristics. Completion of the equilibrium was confirmed separately. The effects of the anions were studied by similar experiments in the presence of the conjugate acid (Na salt for SCN-) of 10-4 to 10-1 mol dm-3 at a fixed pH. Ten common anions (P(V), P(III), P(I), Se(IV), OH-, F-, Cl-, SCN-, S(IV), and CH3COO-) having different sizes and properties were used for the experiments. The concentrations of metal ions and the concentrations of anions in the supernatant were determined, respectively, by EDTA titration and ion chromatography-conductometry. To validate the experimental procedure, the resin adsorbing Fe3+ and phosphoric species was washed repeatedly with nitric acid solutions at pH 1.1. The collected solutions were subjected to determination of phosphorus by the molybdenum blue method. The sum of the chemical amounts of phosphorus in the resin phase and in the aqueous phase agreed with the total loading within an experimental error. Division of the chemical amount of the adsorbed species by the mass of the Na-type resin gave the adsorption capacity, AC/mmol g-1. Since the free concentration of anions was often extremely low, the rigorous equilibrium analysis was abandoned. Determination of Water Contents. In a preliminary study,21 the water content was determined by Karl Fischer coulometry (KF), where each resin as a measurement target was directly poured into an electrolytic cell. In a subsequent study, it was demonstrated that dissociation of water from the metal ions adsorbed on the resins and release of water into the electrolytes were rather resistant both kinetically and thermodynamically and thereby led to the measurement results of the erroneously lower water content. The combination of Karl Fischer coulometry with water vaporization (WV-KF, Mitsubishi chemicals, CA-100 and VA-100) at the variable fixed temperature was accordingly adopted for determination of the water content. Parts a-e of Figure 1 show five typical heating temperature profiles in WV-KF: type A, being constant irrespective of temperature (e.g., H-form of G-1.0); type B, increasing in lower temperature and being constant in higher temperature (e.g., Na-form of P-3.9); type C, being constant in lower temperature and increasing in higher temperature (e.g., Feform of G-4.6); type D, increasing except being constant in middle temperature (e.g., Fe-OH-form of P-3.9); type E, monotonously increasing (e.g., Fe-P(V)-form of P-3.9). Heating temperature profiles for the other resins are shown in Supporting Information Figures S-2, S-3, and S-4, and their classifications are summarized in Table 1. The increase in the lower temperature range is ascribed to the enhanced water release from the resin, whereas that in Analytical Chemistry, Vol. 80, No. 24, December 15, 2008

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Figure 1. Effects of heating temperature on water contents of resins determined by the WV-KF method (a-e) and by TG-DTA (f). Resin: (a) H-form of G-1.0; (b) Na-form of P-3.9; (c) Fe-form of G-4.6; (d) Fe-OH-form of P-3.9; (e and f) Fe-P(V)-form of P-3.9.

the higher temperature range is ascribed to the partial decomposition of the resin. The water content at plateau can thus be used to evaluate the hydration number for the profile types A-D. In the case of the profile type E, the resin was also subjected to thermogravimetry-differential thermal analysis (TG-DTA). A typical example for the Fe-P(V)-form of P-3.9 is shown in Figure 1f (Supporting Information Figure S-5 for the others). The TG profile at temperature >120 °C was almost consistent with the WV-KF profile (Figure 1e); the water content in the TG profile was slightly lower than that in the WV-KF profile, due to the delayed evaporation of water against the temperature increase in the TG profile. The DTA showed two negative peaks in temperatures of 50 and >300 °C, whereas none at 210 °C. The water content in the WV-KF profile at the heating temperature of 210 °C was thus used as a rough measure for the type E. Spectroscopy. The infrared spectra were recorded by the KBrdisk method after grinding the resin with an agate mortar and pestle. The Mo¨ssbauer spectra were recorded with a constant acceleration spectrometer. Ground resins of 10-20 mg were pressed and subjected to measurement at room temperature. The 9668

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spectrometer was calibrated from the hyperfine splitting of a metallic iron absorber, and isomer shifts were reported from the center of the iron spectrum. RESULTS AND DISCUSSION Coadsorption of M3+ and OH-. The effects of pH on the adsorption capacities of the five trivalent metal ions were studied using P-3.9. The results for Fe3+ are shown in Figure 2a (see Supporting Information Figure S-6a-d for the other metal ions). With an increase in pH, the AC value increased and reached ECNa/3 at pH 1; this indicates adsorption in the form of [(-S)3Fe]. At pH > 1.5, the AC value increased and approached to a limiting value of ECNa/2. The electroneutrality of the resin phase suggests the adsorbed species of [(-S)2Fe(OH)]j, which includes the bridged species such as [(-S)2Fe(OH)2Fe(S-)2] and [(-S)2(Fe-O-Fe)(S-)2]. The average Fe-Fe distance for P-3.9 was expected to be 0.95 nm ([(-S)2M] in Table 1 with Fe(OH)2+ as M). The bridging interaction was not excludable by taking into account the hydration shell around Fe3+ and the flexibility of the pendant ion-exchange group. The suffix “j” was thus added. Such

Figure 2. Adsorption capacities of Fe3+ and anions on strongly acidic cation-exchange resins. Anion, resin: (a) OH-, P-3.9; (b) OH-, G-1.0; (c) P(V), P-3.9; (d) P(V), G-1.0; (e) P(III), P-3.9; (f) P(III), G-1.0. b: Fe3+; O: anion.

a change in AC value was also found for other trivalent metal ions, such as Al3+, Ga3+, In3+, and Sc3+. The pH values representing the AC value of ECNa × 5/12, which was equivalent to the 50% presence of the hydrolyzed species in the resin phase, were as follows: 2.0 for Fe3+, 3.6 for Al3+, 2.4 for Ga3+, 2.3 for In3+, and 3.6 for Sc3+. The pH values for Fe3+ and Ga3+ were comparable to or slightly smaller than the first acid-dissociation constants of the metal ions (pKa,1, pKa,2, pKa,3: 2.56, 3.63 and 3.81 for Fe3+, 5.4, 4.58, and 5.62 for Al3+, 2.6, 3.5, and 4.4 for Ga3+, 4.42, 3.92, and 4.56 for In3+, and 4.9, 5.8, and 6.7 for Sc3+).22 As for Al3+ and In3+, rather facile second hydrolysis may modify the correlation. The affinities of the metal ions to OH- in the aqueous solutions accordingly reflect the coadsorption tendency. The effects of the type and ECNa of the resins on the adsorption of Fe3+ were studied using the other resins of G-4.6 (Supporting Information Figure S-6e) and G-1.0 (Figure 2b), both of which showed the presence of [(-S)3Fe] and coadsorbed species with OH-. The average Fe-Fe distance for G-1.0 was expected to be 1.51 nm (Table 1), which suggests the difficulty of the bridging interaction in the coadsorbed species as in the case of P-3.9. The adsorbed species in G-1.0 was tentatively assigned as [(-S)2Fe(OH)]. (22) Kragten, J. Atlas of Metal-Ligand Equilibria in Aqueous Solution; Ellis Horwood: Sussex, 1978.

Coadsorption of M3+ and Phosphoric Species. The effects of the total concentration of the phosphoric species (hereafter denoted as P(V)) on the adsorption capacity of M3+ on P-3.9 were studied at fixed pH values, where the coadsorption of OH- was negligible; pH 1.1 for Fe3+, 2.3-2.4 for Al3+, 1.7-1.9 for Ga3+, 1.5 for In3+ and 2.5 for Sc3+. The results for Fe3+ are shown in Figure 2c. The AC value of Fe3+ increased with an increase in total concentration of the phosphoric species and approached to ECNa/2. The adsorption of P(V) correspondingly increased and approached to ECNa/4. Such changes in AC value of the metal ions and P(V) were also found for the other metal ions (Supporting Information Figure S-7a-d). By taking into account the electroneutrality of the resin phase, the adsorbed species was estimated as [(-S)4M2(HPO4)]. Two intimate structures, [(-S)2M(HPO4)M(-S)2] (two metal ions bridged by HPO42-) and [(-S)2M]+, [(HPO4)M(-S)2]- (charges separated), are possible for this composition. The coadsorption of Fe3+ and P(V) was also observed on G-4.6 (Supporting Information Figure S-7e), but not on G-1.0 (Figure 2d). The coadsorption of P(V) requires a higher exchange capacity, which suggests involvement of the bridge bonding in [(-S)2M(HPO4)M(-S)2]. Since no coadsorption was observed on G-1.0 having the lower exchange capacity, a charge-separated Analytical Chemistry, Vol. 80, No. 24, December 15, 2008

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Figure 3. Difference infrared spectrum of the Fe-P(V)-form and Fe-form of P-3.9 (a) and infrared spectra of related compounds (b): thick curve, Na2HPO4; thin curve, NaH2PO4.

species, [(-S)2M]+, [(HPO4)M(-S)2]-, may not be formed in the resin phase. Coadsorption of M3+ and Other Anions. The effects of other anions on adsorption of Fe3+ on P-3.9 and G-1.0 were examined. No substantial change or rather a decrease in AC value, due to the complexation in the aqueous phase, was observed for F-, Cl-, SCN-, HSO3-, and CH3COO- (Supporting Information Figure S-8). On the other hand, the AC value of Fe3+ and the anions increased to ECNa/2 by the adsorption on P-3.9 as shown in Figure 2e for the phosphorus species (denoted as P(III)) and as shown in Supporting Information Figure S-9 for the phosphinic and selenious species (denoted as P(I) and Se(IV)). This indicates the coadsorbed species of [(-S)2FeX]j (X-: H2PO3-, H2PO2-, HSeO3-), which is the same composition as that in the coadsorption with OH-. In contrast, no coadsorption was observed on G-1.0 as shown in Figure 2f for P(III) and as shown in Supporting Information Figure S-9 for P(I) and Se(IV). This also indicates the essential role of the bridge bonding. Hydration Numbers. The exchange capacity and the percent water content (WNa) of the Na-form resin are, respectively, given by ECNa ) 103/(MWR + 23 + (18)nNa)

(1)

WNa ) (102)(18)(nNa)/(MWR + 23 + (18)nNa)

(2)

where MWR denotes the apparent molecular weight of the cationexchange group together with a polymer portion and nNa denotes the hydration number of Na+. On the basis of the values of ECNa and WNa, the MWR and nNa values were determined for three resins (Table 1). Once the MWR value is determined, the hydration numbers p and q of the adsorbed species [(-S)3M(H2O)p] and [(-S)2MX(H2O)q] are calculable only from the water contents, WM: p ) (WM)((MWR)(3) + MWM)/(18)(100 - WM)

(3)

q ) (WM)((MWR)(2) + MWM + MWX)/(18)(100 - WM) (4) where MWM and MWX, respectively, represent the molecular weights of the metal ion and the anion. Equation 4 is also applicable to the species [(-S)4M2(HPO4)] using MWX ) 96/2 ) 48. The hydration numbers thus calculated are summarized in 9670

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Table 1. As described in the Experimental Section, the hydration numbers in the preliminary report were erroneously low.21 The hydration number of the H-form demonstrated the presence of discrete [H(H2O)4]+ in the resin phase. Sodium ion is less hydrated than proton, whereas Fe3+ is heavily hydrated. The hydration number of Fe3+ is larger than that (around 6)23 in an aqueous solution and is dependent on the exchange capacity. The average distance between the centers of sulfo groups (equivalent to the average distance between M atoms in [(-S)M] in Table 1: 0.75 nm for P-3.9 and 1.20 nm for G-1.0) is appreciably larger than twice the Fe-O distance of [Fe(H2O)6]3+ (0.40 nm).23 The interstitial space between [Fe(H2O)6]3+ and the functional group was occupied by excess water molecules (1.8 in G-4.6, 3 in P-3.9 and 6.4 in G-1.0) according to the exchange capacity. The hydration number of the coadsorbed species was appreciably lower than that of the Fe-form. Coadsorption enhanced the dehydration of Fe3+ and the anion. Spectroscopic Evidences and Intimate Structures. The infrared spectra were recorded for the species [(-S)3Fe] and the Fe-X-form resins (X: P(V), P(III), and P(I)). The difference spectrum of the Fe-P(V)-form and [(-S)3Fe] on P-3.9 is shown with the IR spectra of NaH2PO4 and Na2HPO4 in Figure 3 (Supporting Information Figure S-10 for the other species). Although the presence of P(V) in the resin phase was undoubtedly demonstrated, the protonation state of P(V), whether H2PO4- or HPO4-, was not unambiguously identified. The Mo¨ssbauer spectra of [(-S)3Fe] (P-3.9 and G-1.0), [(-S)2Fe(OH)]j (P-3.9 andG-1.0), and [(-S)4M2(HPO4)] (P-3.9) are shown in Figure 4. An extremely broadened singlet with an isomer shift of δ ) 0.43 mm s-1 was observed for [(-S)3Fe] on both P-3.9 and G-1.0 as previously reported.24 This spectral characteristic is consistent with the high symmetry of the hexaaqua ion of Fe3+, [Fe(H2O)6]3+. The broadening tendency may be related to the number of excess water molecules. Two doublets (an isomer shift of δ1 ) 0.39 mm s-1 and a quadrupole splitting of ∆E1 ) 0.57 mm s-1, an isomer shift of δ2 ) 0.48 mm s-1 and a quadrupole splitting of ∆E2 ) 1.66 mm s-1) were observed for [(-S)2Fe(OH)]j on P-3.9, whereas only the less split doublet was observed for [(-S)2Fe(OH)] on G-1.0. Hydrolysis of Fe3+ reduces the symmetry and induces the quadrupole splitting. The quadrupole splittings of structurally characterized (23) Ohtaki, H.; Tadnai, T. Chem. Rev. 1993, 93, 1157. (24) Johansson, Å. J. Inorg. Nucl. Chem. 1969, 31, 3273.

pH to give [(-S)2Fe(OH)] or [(-S)2Fe-O-Fe(S-)2] was accompanied with partial desorption of Fe3+. The performance was inferior to that using a chelating resin.5-7,19 When the total concentration of Fe3+ was decreased down to -3 10 mol dm-3, however, no coadsorption was observed in the total P(V) concentration range of 3 × 10-4 to 3 × 10-3 mol dm-3. The extent of contamination by phosphate in ion-exchange separation of Fe3+ is thus not so serious except the condition of relatively high concentrations.

Figure 4. Mo¨ssbauer spectra of resins: (a) [(-S)3Fe] of P-3.9; (b) [(-S)3Fe] of G-1.0; (c) [(-S)2Fe(OH)]j of P-3.9; (d) [(-S)2Fe(OH)] of G-1.0; (e) [(-S)4M2(HPO4)] of P-3.9.

complexes are 0.63 mm s-1 for the hydroxo iminodiacetate complex,25 0.53-0.91 mm s-1 for the dioxo-bridged squarate complexes,26 and 1.61 mm s-1 for the oxo-bridged EDTA complex.27 The larger quadrupole split species is thus assigned as the oxo-bridged species [(-S)2(Fe-O-Fe)(S-)2]. The sum (0.74 nm) of the Fe-Fe distance (0.34 nm)28 in this oxo-bridged species and twice the Fe-O(water) distance is comparable to the average metal-metal distance of [(-S)2M] on P-3.9 (0.95 nm, M: Fe(OH)2+ in Table 1). This suggests adsorption of the oxo-bridged species by four functional groups. In contrast, the average metal-metal distance on G-1.0 (1.51 nm) is much larger than the sum of the oxo-bridged species. This suggests formation of only the hydroxo complex (Fe-OH)2+, which gives the less split doublet, on G-1.0 having a lower density of functional groups. The hydration numbers of [(-S)2Fe(OH)] on G-1.0 are appreciably larger than those of the oxo-bridged species. Only one doublet (an isomer shift of δ1 ) 0.42 mm s-1 and a quadrupole splitting of ∆E1 ) 0.55 mm s-1) was observed for [(-S)4M2(HPO4)]. Formation of a charge-separated species [(-S)2M]+, [(HPO4)M(-S)2]- should induce two doublets. This spectroscopic result well supports the bridging structure of [(-S)2M(HPO4)M(-S)2]. Analytical Implication. This system was evaluated as an adsorbent of phosphate. Recovery of phosphate was more than 90% by the use of 10-2 mol dm-3 Fe3+ at pH 1. The phosphate adsorbed by this prior method, however, could not be recovered without contamination by Fe3+. Elution of phosphate by increasing (25) Krishnamurthy, M.; Hambright, W. P.; Morris, K. B.; Thorpe, A. N.; Alexander, C. C. J. Inorg. Nucl. Chem. 1969, 31, 873. (26) Wrobleski, J. T.; Brown, D. B. Inorg. Chim. Acta 1979, 35, 109. (27) De Araujo, F. T.; Cufresne, A.; De Lima, C. G.; Knudsen, J. M. Chem. Phys. Lett. 1970, 7, 333. (28) Ozarowski, A.; McGarvey, B. R.; Drake, J. E. Inorg. Chem. 1995, 34, 5558.

CONCLUSIONS There are three possible chemical phenomena in the ternary system of a cation-exchange resin (including a chelating resin), a metal ion, and an anion: adsorption of the metal ion on the resin irrespective of the anion, complex formation between the metal ion and the anion irrespective of the resin, and coadsorption of the metal ion and the anion on the resin. Which occurs depends on the strengths of the interaction between the resin and the metal ion and the interaction between the metal ion and the anion, as well as on their balance. Since there are strong interactions between chelating resins and metal ions, coadsorption occurs provided that the polymer complex has a vacant site for the anion to directly interact with the metal ion. Charge neutralization is not essential as evidenced by formation of K+, [(-L)ZrF2]- in the iminodiacetate-type chelating gel.7 There are, on the other hand, only weak interactions between strongly acidic cation-exchange resins and metal ions. The interaction of the metal ion and the anion in the aqueous phase thus prevails as found for Fe3+-F-, Cl-, and HSO3- in this study (Supporting Information Figure S-8). Although Heitner-Wirguin found coadsorption in the Cu2+-Cl- and Co2+-Cl-, SCN-, and NO3- systems,12,13 it occurred only in the condition of the extremely high concentrations of both the metal ion and the anion: CM/mol dm-3, 10-1 to 100.6; CX/mol dm-3, 10-0.7 to 100.3. In these concentrated solutions, the complexes like CoCl- exist as the major species in the aqueous phase and are simply adsorbed on the cation-exchange resin. Such type of coadsorption was suppressed by lowering their concentrations as adopted in the present work: CM/mol dm-3, 10-2.6 to 10-2; CX/mol dm-3, 10-3.5 to 10-2. Even under such conditions, P(V), P(III), P(I), and Se(IV) showed coadsorption, which is ascribed to the bridge bonding in the resins of the higher exchange capacities. ACKNOWLEDGMENT This work was supported by a Grant-in-Aid of Scientific Research from the Ministry of Education, Culture, Science and Technology, Japan (No. 16550135). SUPPORTING INFORMATION AVAILABLE Additional figures showing mapping of sulfur atoms on G-1.0, effects of temperature on water contents of resins determined by WV-KF, TG-DTA of resins, adsorption isotherms, and difference infrared spectra of resins. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review July 15, 2008. Accepted October 20, 2008. AC801468F

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