THE MECHANISM OF SELENATE ADSORPTION ON GOETHITE AND HYDROUS FERRIC-OXIDE

EXAFS spectroscopy has been used to investigate the structure of selenate surface complexes sorbed on dry and wet goethite and hydrous ferric oxide. It is shown that selenate oxyanions always form inner-sphere surface complexes on both sorbents regardless of the hydration state. Selenate, like phosphate and selenite, forms binuclear bridging complexes on (hk0) planes with two singly coordinated A-type OH groups. Furthermore evidence is provided for the attachment of selenate oxyanions on (001) planes by the sharing of edges with Fe octahedra. Selenite and arsenate appear to form the same types of surface complexes as selenate at the surface of goethite and hydrous ferric oxide. It is likely that this sorption mechanism is operative for other anions including phosphate and sulfate. A consequence of this study is the conclusion that the crystallographic planes of Fe oxides (hk0 and 001) do not appear to selectivity bond either cations or anions, at least at high surface coverage. (C) 1994 Academic Press,Inc.