Surface Structures and Stability of Arsenic(III) on Goethite:  Spectroscopic Evidence for Inner-Sphere Complexes

The adsorption and stability of arsenite [As(III)] on goethite (α-FeOOH) was investigated using a combination of standard batch techniques and X-ray absorption spectroscopy (XAS). The reactivity of As(III) with α-FeOOH at varying pH and As(III) concentration provided macroscopic evidence for strong complexation on the α-FeOOH surface. Extended X-ray absorption fine structure (EXAFS) spectroscopy gave an average As(III)−Fe interatomic distance of 3.378 ± 0.014 Å, which is indicative of bidentate binuclear bridging As(III) complexes on the α-FeOOH surface and which is similar to other oxyanions which adsorb on α-FeOOH by an inner-sphere mechanism. X-ray absorption near-edge structure (XANES) analysis indicated that the As(III)−α-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(III) adsorption on the α-FeOOH surface using a surface complexation model (the constant capacitance model). These results suggest that As(III) surface complex formation on iron(III) oxides may play an important role in the environmental behavior of arsenic.