Mechanism of anion retention from EXAFS and density functional calculations: Arsenic (V) adsorbed on gibbsite

X-ray absorption fine structure spectroscopy and density functional calculations were used to determine the structural model of arsenic surface complex on gibbsite mineral. The structural environment of arsenic at the solid surface may determine its potential for remobilization and stability. Data were collected for arsenate adsorbed on gibbsite surface at pH 5.5. The X-ray absorption fine structure spectroscopy results showed that As(V) forms an inner sphere bidentate binuclear complex on the surface of Al oxyhydroxyl octaedra. Quantitative results showed an average interatomic As(V)-Al distance of 3.19 +/- 0.05 Angstrom and a coordination number of 1.3 +/- 1.0 atoms. Four different adsorption sites in which arsenate can interact with gibbsite have been studied using density functional calculations, i.e., bidentate binuclear complex, bidentate mononuclear complex, monodentate mononuclear complex, and monodentate binuclear complex. The density functional calculations confirm that the most stable structure predicted for As(V)-gibbsite system is the bidentate-binuclear complex. Copyright (C) 2001 Elsevier Science Ltd.