title: EXAFS analysis of arsenite adsorption onto two-line ferrihydrite, hematite, goethite, and lepidocrocite.
abstract: The modes of As(III) sorption onto two-line ferrihydrite (Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp) have been investigated under anoxic condition using X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure spectroscopy (XANES) indicates that the absence of oxygen minimized As(III) oxidation due to Fenton reactions. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates thatAs(III)forms similar inner-sphere surface complexes on two-line ferrihydrite and hematite that differ from those formed on goethite and lepidocrocite. At high surface coverage, the dominant complex types on Fh and Hm are bidentate mononuclear edge-sharing (2E) and bidentate binuclear corner-sharing (2C), with As-Fe distances of 2.90 +/- 0.05 and 3.35 +/- 0.05 A, respectively. The same surface complexes are observed for ferrihydrite at low surface coverage. In contrast, As(III) forms dominantly bidentate binuclear corner-sharing (2C) sorption complexes on Gt and Lp [d(As-Fe) = 3.3-3.4 A], with a minor amount of monodentate mononuclear corner-sharing (1V) complexes [d(As-Fe) = 3.5-3.6 A]. Bidentate mononuclear edge-sharing (2E) complexes are virtually absent in Gt and Lp at the high surface coverages that were investigated in the present study. These results are compared with available literature data and discussed in terms of the reactivity of iron(III) (oxyhydr)oxide surface sites.
authors.affiliation: Institut de Minéralogie et de Physique des Milieux Condensés UMR 7590, CNRS, Université Paris 6 & 7, IPGP, France.
authors.forename: Georges
authors.initials: G
authors.lastname: Ona-Nguema
authors.forename: Guillaume
authors.initials: G
authors.lastname: Morin
authors.forename: Farid
authors.initials: F
authors.lastname: Juillot
authors.forename: Georges
authors.initials: G
authors.lastname: Calas
authors.forename: Gordon E
authors.initials: GE
authors.lastname: Brown
mesh_terms: D000327:Adsorption; D001151:Arsenic / Q000737:chemistry; D005290:Ferric Compounds / Q000737:chemistry*; D058085:Iron Compounds / Q000737:chemistry*; D008903:Minerals; D010084:Oxidation-Reduction; D013057:Spectrum Analysis / Q000379:methods*; D014965:X-Rays
chemical_list: D005290:Ferric Compounds; D058085:Iron Compounds; D008903:Minerals; C094886:goethite; C000499:ferric oxide; C092844:ferric oxyhydroxide; D001151:Arsenic