Evidence for Different Surface Speciation of Arsenite and Arsenate on Green Rust: An EXAFS and XANES Study

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Abstract
The knowledge of arsenic speciation at the surface of green rusts (GRs), [FeII(1−x)FeIIIx (OH)2]x+ (CO3, 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 near-neutral 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 μmol m−2 and 2.7 μmol 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 2.70 μmol 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.

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