Arsenate and Chromate Retention Mechanisms on Goethite. 1. Surface Structure

TLDR

The molecular structure of ions retained on mineral surfaces is essential for accurate sorption modeling and stability assessment. Modeling chromate or arsenate retention on goethite must account for a monodentate complex at very low coverages, both monodentate and bidentate complexes at intermediate coverages, and predominantly bidentate complexes at high coverages. Extended X‑ray absorption fine structure (EXAFS) spectroscopy was employed to determine the local coordination environment of arsenate and chromate on goethite (α‑FeOOH). Three surface complexes—monodentate, bidentate‑binuclear, and bidentate‑mononuclear—were identified on goethite, with the monodentate complex favored at low coverage and the bidentate‑binuclear complex dominating at high coverage.

Abstract

The molecular structure of ions retained on mineral surfaces is needed to accurately model their sorption process and to determine their stability. 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 (α-FeOOH). Based on the oxyanion−Fe distances, it was concluded that three different surface complexes exist on goethite for both oxyanions: a monodentate complex, a bidentate-binuclear complex, and a bidentate-mononuclear complex. At low surface coverages, the monodentate complex was favored while at higher coverages the bidentate complexes were more prevalentthe bidentate-binuclear complex appears to be in the greatest proportion at these highest surface coverages. Therefore, modeling efforts for chromate or arsenate retention on goethite need to consider a monodentate complex at very low coverages, both the monodentate and bidentate complexes at intermediate coverages, and predominantly the bidentate complexes at very high coverages.