Residence Time Effects on Arsenate Adsorption/Desorption Mechanisms on Goethite

Abstract

In order to make sound decisions regarding arsenate contamination in soil and water environments, it is necessary to have a thorough understanding of the mechanisms of arsenate sorption and desorption over extended periods. The major objectives of this study were to determine the effects of aging or residence time on the kinetics of arsenate sorption and desorption on goethite, and to combine spectroscopic x‐ray absorption fine structure (XAFS) and macroscopic studies in order to determine sorption and desorption mechanisms over time at pH 4 and 6. Sorption studies, conducted from 4 min to ≈12 mo, showed that arsenate sorption on goethite increased with time. Sorption was initially rapid, with over 93% arsenate being sorbed in 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 structure (EXAFS) revealed that there exist two distinct atomic shells surrounding the adsorbed As. The closest atomic shell was identified as an O atom, the next shell out was identified as an Fe atom. The As–Fe bond distance of 3.30 Å, derived from XAFS data, is indicative of a bidentate binuclear bond forming between the arsenate atoms and the goethite surface. This is in agreement with the findings of previous researchers. Analysis of the As EXAFS from samples incubated for various periods indicated that the molecular environment did not change over time. Complimentary desorption kinetic studies showed that when aging was increased, there was no significant change in the amount of arsenate desorbed from goethite by PO 3− 4 Initially, desorption 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 additional desorption occurred at longer times. A significant amount of arsenate remained bound to the goethite after 5 mo of desorption even though the PO 3− 4 desorptive solution was three times stronger than the initial arsenate sorptive solution. Sulfate was much less effective at promoting arsenate desorption; at pH 6, no more than 2.5% of the total sorbed arsenate desorbed over a 5‐mo period. Desorption results at pH 4 were similar to the desorption behavior at pH 6. The XAFS analyses of PO 3− 4 desorbed arsenate samples showed that the molecular environment of the adsorbed arsenate did not change.