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

X-ray absorption fine structure spectroscopy and density functional calcula tions were used to determine the structural model of arsenic surface comple x on gibbsite mineral. The structural environment of arsenic at the solid s urface may determine its potential for remobilization and stability. Data w ere collected for arsenate adsorbed on gibbsite surface at pH 5.5. The X-ra y 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 dista nce of 3.19 +/- 0.05 Angstrom and a coordination number of 1.3 +/- 1.0 atom s. Four different adsorption sites in which arsenate can interact with gibb site have been studied using density functional calculations, i.e., bidenta te binuclear complex, bidentate mononuclear complex, monodentate mononuclea r complex, and monodentate binuclear complex. The density functional calcul ations confirm that the most stable structure predicted for As(V)-gibbsite system is the bidentate-binuclear complex. Copyright (C) 2001 Elsevier Scie nce Ltd.