The knowledge of arsenic speciation at the surface of green rusts (GRs), ((Fe(1-x)FexIII)-Fe-II(OH)(2)](x+) (CO3, Cl, SO4)(x-), is environmentally relevant because arsenic sorption onto US could contribute to arsenic retention in anoxic environments (hydromorphic soils, marine sediments, etc.). The nature of arsenic adsorption complexes on hydroxychloride green rust I (GR1CI) 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(Ill) at the studied As loadings (0.27 mu mol m(-2) and 2.7 mu mol m(-2)). EXAFS results indicate that arsenite [As(1101 and arsenate [As(V)] form inner-sphere complexes on the surface of GR1Cl at arsenic surface coverages of 0.27 and 2.70 mu mol m(-2), with distinct types of As(Ill) 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 angstrom) suggest the presence of binuclear bidentate double-corner complexes (C-2) and monodentate mononuclear corner-sharing complexes (V-1). For As(III), EXAFS-derived As-As distance (3.32 +/- 0.02 A) and As-Fe distances (3.49 +/- 0.02 and 4.72 +/- 0.02 A) are consistent with the presence of dimers of As(Ill) pyramids binding to the edges of the GR1Cl layers by corner sharing with FeO6 octahedra. However, 2 C and V-1 As(Ill) 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.
Wang, Yuheng Morin, Guillaume Ona-Nguema, Georges Juillot, Farid Guyot, Francois Calas, Georges Brown, Gordon E., Jr.