Surface structures and stability of arsenic(III) on goethite: Spectroscopic evidence for inner-sphere complexes

The adsorption and stability of arsenite [As(111)] on goethite (alpha-FeOOH) was investigated using a combination of standard batch techniques and X-ray absorption spectroscopy (XAS). The reactivity of As(111) with alpha-FeOOH at varying pH and As(111) concentration provided macroscopic evidence for strong complexation on the alpha-FeOOH surface. Extended X-ray absorption fine structure (EXAFS) spectroscopy gave an average As(lll)-Fe interatomic distance of 3.378 +/- 0.014 Angstrom, which is indicative of bidentate binuclear bridging As(111) complexes on the alpha-FeOOH surface and which is similar to other oxyanions which adsorb on alpha-FeOOH by an inner-sphere mechanism. X-ray absorption near-edge structure (XANES) analysis indicated that the As(111)-alpha-FeOOH surface complex is stable toward heterogeneous oxidation to As(V), as determined by the energy position of the X-ray absorption edge. The structural information from EXAFS was included in the description of As(111) adsorption on the alpha-FeOOH surface using a surface complexation model (the constant capacitance model). These results suggest that As(111) surface complex formation on iron(111) oxides may play an important role in the environmental behavior of arsenic.