Residence time effects on arsenate adsorption/desorption mechanisms on goethite

In order to make sound decisions regarding arsenate contamination in soil a nd water environments, it is necessary to have a thorough understanding of the mechanisms of arsenate sorption and desorption over extended periods. T he major objectives of this study were to determine the effects of aging or residence time on the kinetics of arsenate sorption and desorption on goet hite, and to combine spectroscopic: x-ray absorption fine structure (XAFS) and macroscopic studies in order to determine sorption and desorption mecha nisms over time at pH 4 and 6. Sorption studies, conducted from 4 min to ap proximate to 12 mo, showed that arsenate sorption on goethite increased wit h time, Sorption was initially rapid, with over 93% arsenate being sorbed i n a 24-h period at pH 6, Similar arsenate adsorption behavior was observed at pH 4. Analysis of the samples with extended x-ray absorption fine struct ure (EXAFS) revealed that there exist two distinct atomic shells surroundin g the adsorbed As. The closest atomic shell was identified as an O atom, th e next shell out was identified as an Fe atom. The As-Fe bond distance of 3 .30 A, derived from XAFS data, is indicative of a bidentate binuclear bond forming between the arsenate atoms and the goethite surface, This is in agr eement with the findings of previous researchers. Analysis of the As EXAFS from samples incubated for various periods indicated that the molecular env ironment did not change over time. Complimentary desorption kinetic studies showed that when aging was increased, there was no significant change in t he amount of arsenate desorbed from goethite by PO43-. Initially, desorptio n was quite rapid with > 35% of the total adsorbed As being desorbed within 24 h at pH 6, After the initial rapid desorption, only a small amount of a dditional desorption occurred at longer times. A significant amount of arse nate remained bound to the goethite after 5 mo of desorption even though th e PO43- desorptive solution was three times stronger than the initial arsen ate sorptive solution, Sulfate was much less effective at promoting arsenat e desorption; at pH 6, no more than 2.5% of the total sorbed arsenate desor bed over a 5-mo period. Desorption results at pH 4 were similar to the deso rption behavior at pH 6, The XAFS analyses of PO43- desorbed arsenate sampl es showed that the molecular environment of the adsorbed arsenate did not c hange.