Langmuir 2007, 23, 12125-12130
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ATR-IR Spectroscopic Study of Antimonate Adsorption to Iron Oxide Kiri A. McComb,† Dave Craw,‡ and A. James McQuillan*,† Departments of Chemistry and Geology, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand ReceiVed April 30, 2007. In Final Form: July 26, 2007 Antimonate ions adsorb to iron oxides in mining contexts, but the nature of the adsorbed antimonate species has not frequently been investigated. In this study, ATR-IR spectroscopy was used to reveal that the adsorption of Sb(OH)6ion from aqueous solutions onto an amorphous iron oxide particle film is accompanied by changes in the Sb(OH)6spectrum and the loss of OH stretching absorptions from iron oxide surface hydroxyl groups. These spectral changes upon adsorption imply an inner-sphere surface interaction with the formation of Sb-O-Fe bonds as well as some outer-sphere adsorption. The corresponding results from solutions of antimonate in D2O confirm that chemisorption occurs. The dependence of antimonate adsorption on pH in the range from 8 to 3 follows that expected for anions on iron oxide considering its pH-dependent surface charge, with the greatest amount of adsorbed antimonate at pH 3. The study of adsorption/desorption kinetics showed a more rapid desorption of adsorbed antimonate under alkaline conditions. This trend is expected from the pH dependence of the antimonate charge and iron oxide surface charge, but it might be partly due to the fact that high pH favors hydrolysis of antimonate oligomers formed on the iron oxide surface from adsorption under acidic conditions.
Introduction Antimony has many applications in industrial products such as increasing the hardness of alloys in lead-acid batteries and in small arms bullets, functioning as a flame retardant, and being used in the clarification of glass products. Human exposure to antimony occurs mainly in the mining and extraction industries, with attendant adverse consequences.1 Trace amounts of Sb in water can be taken up by organisms in both aquatic and terrestrial environments, and antimony is very toxic, especially to organisms in aquatic environments. Water-soluble antimony exists as antimonite [Sb(III)] and antimonate [Sb(V)] species, and the ease of oxidation of Sb(III) to Sb(V) in acidic aqueous solutions has led to the search for methods to remove antimonate from industrial wastewater before its release into the environment. The common abundance of iron oxide precipitates in mine discharge waters and their generally positive surface charge under acidic conditions makes adsorption of Sb oxyanion species to iron oxides of significance for mining contexts alongside the corresponding adsorption of As oxyanion species, which are generally more abundant. Association of antimony with iron oxides in soils has been long demonstrated.2,3 Recently, Craw et al.4 and Wilson et al.5 have shown that up to 10 wt % Sb occurs in such iron oxide precipitates. Although Sb speciation in this material was not established, adsorption to iron oxide surfaces was suspected.4,5 There have been many studies of the adsorption of arsenic species to iron oxides. However, compared to adsorption studies of arsenic(V) to iron oxides, there have been relatively few investigations that have addressed the rate and mechanism of adsorption of antimony(V) to iron oxides. Okada et al.6 used * Corresponding author. E-mail:
[email protected]. Fax: +64 3 479 7906. † Department of Chemistry. ‡ Department of Geology. (1) van der Voet, G. B.; de Wolff, F. A. Toxicol. Met. 1996, 457-458. (2) Crecelius, E. A.; Bothner, M. H.; Carpenter, P. EnViron. Sci. Technol. 1975, 9, 325-333. (3) Brannon, J. M.; Patrick, W. H. EnViron. Pollut. 1985, 9, 107-126 (4) Craw, D.; Wilson, N.; Ashley, P. M. Trans. Inst. Min. Metall., Sect. B 2004, 113, B3-B10. (5) Wilson, N. J.; Craw, D.; Hunter, K. EnViron. Pollut. 2004, 129, 257-266.
Mo¨ssbauer spectroscopy to study the adsorption of 119Sb(V) ions onto hematite at pH 4 and showed that specific adsorption occurs with formation of Fe-O-Sb bonds. A further Mo¨ssbauer study by the same group7 showed that adsorption of Sb(V) onto hematite occurred mainly below pH 7 and that the antimony adsorbed at pH 2-5 was not desorbed within 5 days under alkaline conditions. Subsequently, Ambe8 carried out a study of the adsorption kinetics of 119Sb(V) to hematite at pH 4.0 and showed that the adsorption is second-order in Sb(V) concentration, suggesting that Sb(V) dimers are adsorbed. Bagby and West9 studied the adsorption of antimony [most likely Sb(V)] from aqueous wastewater solutions onto amorphous iron oxide (ferrihydrite) and, using atomic absorption analysis, found that maximum adsorption occurs at pH ≈ 4.5 after 15 min. Tighe et al.10 measured the adsorption of Sb(V) onto amorphous Fe(OH)3 and found that the amount adsorbed was high at pH 3.5 but declined at higher pH. In that study, the lack of influence of nitrate as a swamping ion on the antimonate adsorption pointed to specific adsorption. Leuz et al.11 studied the adsorption of Sb(III) and Sb(V) onto goethite suspensions and found that both Sb(III) and Sb(V) form innersphere complexes at the goethite surface with the maximum Sb(V) adsorption occurring at pH < 7. Bhakhar12 studied the removal of antimony from wastewater in the pH 5-9 range using iron oxide-coated olivine and found that the strongest adsorption occurred at the lowest pH. The present work addresses the molecular nature and properties of Sb(V) adsorbed on amorphous iron(III) oxide found typically in the environment at circumneutral pH.13 The development of attenuated total reflection infrared (ATR-IR) spectroscopic methods has led to many studies of species adsorbed to metal (6) Okada, T.; Ambe, S.; Ambe, F.; Sekizawa, H. J. Phys. Chem. 1982, 86, 4726-4733. (7) Ambe, F.; Ambe, S.; Okada, T.; Sewkizawa, H. ACS Symp. Ser. 1986, 323, 403-424. (8) Ambe, S. Langmuir 1987, 3, 489-493. (9) Bagby, E. L.; West, C. M. ACS Symp. Ser. 1995, 607, 64-73. (10) Tighe, M.; Lockwood, P.; Wilson, S. J. EnViron. Monit. 2005, 7, 11771185. (11) Leuz, A-K.; Mo¨nch, H.; Johnson, C. A. EnViron. Sci. Technol. 2006, 40, 7277-7282. (12) Bhakhar, N. Antimony Removal by Iron-Oxide Coated Olivine and Water Treatment Residual. Master’s Thesis, Royal Institute of Technology (KTH), Stockholm, Sweden, 2006.
10.1021/la7012667 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007
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oxides,14,15 including iron oxides,13,16,17 but the application of this approach to investigate the molecular nature of adsorbed antimonate species has not hitherto been reported. Dilute aqueous solutions (
