Extended X-ray Absorption Fine Structure Analysis of Arsenite and Arsenate Adsorption on Maghemite
Résumé
Arsenic sorption onto maghemite potentially contributes to arsenic retention in magnetite-based arsenic removal processes because maghemite is the most common oxidation product of magnetite and may form a coating on magnetite surfaces. Such a sorption reaction could also favor arsenic immobilization at redox boundaries in groundwaters. The nature of arsenic adsorption complexes on maghemite particles, at near-neutral pH under anoxic conditions, was investigated using X-ray absorption fine structure (XAFS) spectroscopy at the As K-edge. X-ray absorption near edge structure spectra indicate that As(III) does not oxidize after 24 h in any of the sorption experiments, as already observed in previous studies of As(III) sorption on ferric (oxyhydr)oxides under anoxic conditions. The absence of oxygen in our sorption experiments also limited Fenton oxidation of As(III). Extended XAFS (EXAFS) results indicate that both As(III) and As(V) form inner-sphere complexes on the surface of maghemite, under high surface coverage conditions (~0.6 to 1.0 monolayer), with distinctly different sorption complexes for As(III) and As(V). For As(V), the EXAFS-derived As−Fe distance (~3.35 ± 0.03 Å) indicates the predominance of single binuclear bidentate double-corner complexes (2C). For As(III), the distribution of the As−Fe distance suggests a coexistence of various types of surface complexes characterized by As−Fe distances of ~2.90 (±0.03) Å and ~3.45 (±0.03) Å. This distribution can be interpreted as being due to a dominant contribution from bidentate binuclear double-corner complexes (2C), with additional contributions from bidentate mononuclear edge-sharing (2E) complexes and monodentate mononuclear corner-sharing complexes (1V). The present results yield useful constraints on As(V) and As(III) adsorption on high surface-area powdered maghemite, which may help in modeling the behavior of arsenic at the maghemite−water interface.