Sorption of As(V) on green rust (Fe-4(II)Fe-2(III)(OH)(12)SO4 center dot 3H(2)O) and lepidocrocite (gamma-FeOOH): Surface complexes from EXAFS spectroscopy

Green rust (Fe-4(II)Fe-2(III)(OH)(12)SO4. 3H(2)O) is an intermediate phase in the formation of iron (oxyhydr)oxides such as goethite, lepidocrocite an d magnetite; current thinking is that it occurs in many soil and sediment s ystems. Green rust: has been shown to reduce sorbed selenate and nitrate an d, therefore, might presumeably reduce sorbed arsenate to the more toxic an d mobile As(III) species. We have investigated the mechanism of As(V) sorpt ion onto green rust and its fate during oxidation of green rust to lepidocr ocite. EXAFS spectroscopy was used to determine the As speciation and coord ination environment. We find that As(V) is not reduced to the more mobile a nd toxic As(III) form following equilibrium with green rust for 24 h. It re mains adsorbed as (AsO4)(3-) by forming inner-sphere surface complexes. The same result is obtained whether As(V) is added prior to or after green rus t nucleation. Two different inner sphere surface complexes are resolved: on e results from edge-sharing between AsO4 and FeO6 polyhedra while the secon d results from and double-corner sharing between AsO4 tetrahedra and adjace nt FeO6 polyhedra. During the oxidation of green rust to lepidocrocite, the (AsO4)(3-) remains preferentially bound to green rust and only sorbs onto lepidocrocite when all of the green rust has been oxidized. Sorption onto l epidocrocite occurs via an inner-sphere complex resulting from bidentate co rner sharing between AsO4 tetrahedra and adjacent FeO6 octahedra. Copyright (C) 2001 Elsevier Science Ltd.