Evidence for different surface speciation of arsenite and arsenate on green rust: An EXAFS and XANES study

The knowledge of arsenic speciation at the surface of green rusts (GRs), [Fe^II _(1-x)Fe^III _x (OH) ₂]_x+(CO_3, Cl,SO₄)^x-, is environmentally relevant because arsenic sorption onto GRs could contribute to arsenic retention in anoxic environments (hydromorphic soils, marine sediments, etc.). The nature of arsenic adsorption complexes on hydroxychloride green rust 1 (GR1Cl) at nearneutral pH under anoxic conditions was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy at the As K-edge. Sorption data indicate that As(V) sorbs more efficiently than As(III) at the studied As loadings (0.27 μ molm^-2 and 2.7 μ mol m ^-2). EXAFS results indicate that arsenite [As(III)] and arsenate [As(V)] form inner-sphere complexes on the surface of GR1Cl at arsenic surface coverages of 0.27 and 2.70μmol m^-2, with distinct types of As(III) and As(V) sorption complexes, which change in relative concentration as a function of arsenic loading. For As(V), the EXAFS-derived As-Fe distances (3.34 ±0.02 and 3.49±0.02 Å) suggest the presence of binuclear bidentate double-corner complexes (²C) and monodentate mononuclear corner-sharing complexes (¹V). For As(III), EXAFS-derived As-As distance (3.32±0.02 Å) and As-Fe distances (3.49±0.02 and 4.72±0.02 Å) are consistent with the presence of dimers of As(III) pyramids binding to the edges of the GR1Cl layers by corner sharing with FeO₆ octahedra. However, ²C and ¹V As(III) complexes cannot be excluded. These results improve our knowledge of the mode of As(V) and As(III) inner-sphere adsorption on green rusts, which will help to constrain sorption modeling of arsenic in soils, sediments, and aquifers.