Surface complexation of arsenic(V) to iron(III) (hydr)oxides: structural mechanism from ab initio molecular geometries and EXAFS spectroscopy
Introduction
Under moderately acidic conditions, arsenic(V) as HnAsO43−n is strongly sorbed by iron oxides and oxide hydroxides such as ferrihydrite, goethite and hematite (Fuller et al., 1993; Waychunas et al 1993, Sun and Doner 1996, Jain et al 1999. The strong sorption of As onto these minerals is invoked as an important mechanism of natural attenuation of As pollution in soil and groundwater (Livesey and Huang, 1981) and lacustrine sediments Aggett and Roberts 1986, Belzile and Tessier 1990. Sorption onto iron oxide hydroxide minerals is especially significant in controlling As solubilities in acid mine drainage and mine-tailings ponds (Carlson et al., 2002). The marine geochemistry of arsenic also indicates control by sorption onto iron oxide hydroxides Sullivan and Aller 1996, Pichler et al 1999.
A molecular understanding of the sorption of arsenic by iron oxides and oxyhydroxides is needed to predict the long-term fate of As in aqueous sediments. Previous spectroscopic studies (e.g., Waychunas et al 1993, Waychunas et al 1993, Hsia et al 1994, Sun and Doner 1996, Fendorf et al 1997, pressure-jump relaxation kinetics measurements Grossl and Sparks 1995, Grossl et al 1997 and titration measurements (Jain et al., 1999) show that arsenate adsorbs to iron oxide hydroxides by forming inner-sphere surface complexes by ligand exchange with hydroxyl groups at the mineral surface. However, the nature of the inner-sphere complex has been controversial (Fig. 1). Using EXAFS spectroscopy, Waychunas et al. (1993) argued for bidentate complexes (2C in Fig. 1) resulting from corner-sharing between AsO4 tetrahedra and edge-sharing pairs of FeO6 octahedra. The 2C complex yields an As-Fe distance near 3.26 Å. For goethite (α-FeOOH), the 2C complex would form on the {110} surfaces which are, in fact, the dominant surfaces (e.g., Boily et al., 2001). Waychunas et al. (1993) also fit their data to a second contribution corresponding to monodentate complexes (designated 1V in Fig. 1) that result from corner-sharing between AsO4 tetrahedra and FeO6 octahedra. The 1V complex gives an As-Fe distance near 3.6 Å. Manceau (1995), however, argued that Waychunas et al. (1993) incorrectly calculated the phase shifts in the EXAFS spectra and that the correct As-Fe distance for the second complex is near 2.8 Å; such a short As-Fe distance would result from bidentate edge-sharing between AsO4 tetrahedra and free FeO6 edges (designated 2E in Fig. 1). In goethite, the 2E complex would form on the {001} and {021} faces which usually comprise a small fraction of the goethite surface (e.g., Boily et al., 2001). Waychunas et al 1995, Waychunas et al 1996 argued against the 2E complex for structural reasons, problems with fits of EXAFS that include a 2E complex, and wide-angle scattering data that fail to show any distance corresponding to a 2E complex.
Fendorf et al. (1997) interpreted their EXAFS data as indicating three surface complexes: a monodentate corner sharing (1V) complex with an As-Fe distance of 3.6 Å, a bidentate corner-sharing (2C) with two As-Fe distances near 3.25 Å and a bidentate edge-sharing complex (2E) with a single As-Fe distance of 2.83–2.85 Å. Fendorf et al. (1997) proposed that the relative importance of each complex depended on the degree of surface loading. Farquhar et al. (2002) obtained EXAFS of As(V) on goethite and lepidocrocite and found peaks in the Fourier transform near 2.93 Å and 3.30–3.31 Å. They attributed the 3.30–3.31 Å peaks to (2C) complexes but were uncertain if the 2.93 Å peak represented a 2E complex.
In this paper, we attempt to resolve the controversy over the inner-sphere surface complexation mechanism: we first predict the geometries and relative energies of AsO4-FeOOH surface complexes using density functional theory calculations on analog Fe2(OH)2(H2O)nAsO4+ and Fe2(OH)2(H2O)nAsO2(OH)2+3 clusters. Secondly, we measure EXAFS spectra of AsO4 sorbed to goethite, hematite, lepidocrocite and ferrihydrite but fit the data with inclusion of effects due to multiple scattering. From these results, we are able to identify the dominant surface complex of AsO4 on iron oxides and oxide hydroxide phases and are also able to explain discrepancies between earlier experimental results.
Section snippets
Density functional calculations
Quantum mechanical calculation of cluster geometries and energies were done using the ADF 2.0 code (te Velde et al., 2001) which implements density functional theory for finite clusters and molecules using the linear combination of atomic orbital formalism. Molecular orbitals in the ADF code are constructed from Slater type atomic orbitals which consist of a Cartesian part rkrxkxykyzkz with kx+ky+kz = l (l = angular momentum quantum number) and an exponential part e−αr. Density functional
Predicted geometries and energetics of asO4 surface complexes using DFT
We calculated the optimized geometries of Fe2(OH)2(H2O)nAsO4+ and Fe2(OH)2(H2O)nAsO2(OH)2+3 clusters corresponding to 2E and 2C complexes of AsO43− and AsO2(OH)2−. We used a spin-unrestricted calculation and set the clusters to have a ferromagnetic configuration. The choice of a ferromagnetic vs. antiferromagnetic configuration for the Fe2(OH)2(H2O)8 substrate should have only a minor chemical effect. (Note that a spin-restricted calculation would be seriously in error, however, since it would
Conclusions
Adsorption of arsenate HnAsO43−n onto goethite, lepidocrocite, hematite and ferrihydrite occurs by the formation of inner-sphere surface complexes resulting from bidentate corner-sharing between AsO4 and FeO6 polyhedra (2C). The bidentate edge-sharing complexes proposed by Manceau (1995) are predicted by density functional calculations to be energetically unfavorable. Moreover, the apparent As-Fe peaks near 2.85 Å in the EXAFS spectra that have been attributed to edge-sharing complexes (Fendorf
Acknowledgements
Part of this work was supported by NERC grant GR9/03506. Synchrotron time was also provided by CLRC Daresbury Laboratory. SRR’s studentship was supported by BNFL. We are especially grateful to Dr. Ian Stewart, Bristol University for help in implementing software on the Beowulf cluster. We also thank Dr. D.M. Heasman and C. Muskett for help in data collection. We are grateful for the comments of two anonymous reviewers.
Associate editor: G. Helz
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