ARSENATE AND CHROMATE RETENTION MECHANISMS ON GOETHITE .1. SURFACE-STRUCTURE

The molecular structure of ions retained on mineral surfaces is needed to accurately model their sorption process and to determine their sta bility. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used in this study to deduce the local coordination environment of two environmental contaminants, arsenate and chromate, on the mineral goethite (alpha-FeOOH). Based on the oxyanion-Fe distances, it was co ncluded that three different surface complexes exist on goethite for b oth oxyanions: a monodentate complex, a bidentate-binuclear complex, a nd a bidentate-mononuclear complex. At low surface coverages, the mono dentate complex was favored while at higher coverages the bidentate co mplexes were more prevalent-the bidentate-binuclear complex appears to be in the greatest proportion at these highest surface coverages. The refore, modeling efforts for chromate or arsenate retention on goethit e need to consider a monodentate complex at very low coverages, both t he monodentate and bidentate complexes at intermediate coverages, and predominantly the bidentate complexes at very high coverages.