Accurate determination of the As(v) coordination environment at the surface of ferrihydrite using synchrotron extended X-ray absorption fine structure spectroscopy and ab initio Debye–Waller factors

Abstract

The adsorption and desorption behavior of arsenate (As(V)) at the ferrihydrite/water interface in nature is determined by its complexation structure. However, current understanding of the coordination environment of As(V)–ferrihydrite complexes obtained from synchrotron extended X-ray absorption fine structure (EXAFS) fitting is still controversial due to the lack of precise Debye–Waller (DW) factors. We computed the DW factors of bidentate binuclear (BB) and monodentate mononuclear (MM) As(V)–ferrihydrite complexation models by using ab initio density functional theory (DFT) calculations. Using the ab initio DW factors, the EXAFS spectra of the As–ferrihydrite samples synthesized at pH 8 and 9 were better fitted with MM models than with BB models. Specifically, the BB model significantly overestimated the amplitude of the As–Fe shell in R-space, whereas the MM model perfectly simulated it. Our results supported that ferrihydrite adsorbs As(V) dominantly through MM complexes at weakly alkaline pH values. These findings are of significance for understanding the adsorption/desorption processes of As(V) at the ferrihydrite/water interface and have far-reaching implications for the geochemical behavior of As in aqueous environments.