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

• Published in: Environmental science & technology 1997-02, Vol.31 (2), p.315-320
• Main Authors: Fendorf, Scott, Eick, Matthew J, Grossl, Paul, Sparks, Donald L
• Format: Article
• Language: English
• Subjects: Applied sciences; Biological and physicochemical properties of pollutants. Interaction in the soil; Chemistry; Contamination; environmental degradation; Exact sciences and technology; Minerals; Pollution; Scientific imaging; Soil and sediments pollution; soil chemistry; soil physics; soil science; technology; waste management
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es950653t

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 prevalentthe 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.