XAS Study of Iron and Arsenic Speciation during Fe(II) Oxidation in the Presence of As(III)

• Published in: Environmental science & technology Vol. 39; no. 24; pp. 9478 - 9485
• Main Authors: Thoral, S, Rose, J, Garnier, J. M, van Geen, A, Refait, P, Traverse, A, Fonda, E, Nahon, D, Bottero, J. Y
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.12.2005
• Subjects: Applied sciences; Arsenic; Arsenic - chemistry; Chemical Physics; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental science; Exact sciences and technology; Experiments; Ferric Compounds - chemistry; Groundwater; Groundwaters; Hydrogen-Ion Concentration; Iron - chemistry; Kinetics; Models, Chemical; Natural water pollution; Oxidation; Oxidation-Reduction; Physics; Pollution; Pollution, environment geology; Spectrometry, X-Ray Emission - methods; Water Pollution, Chemical - prevention & control; Water treatment and pollution; Oxidation reduction; XANES spectrometry; Arsenic; Pollutant behavior; Iron II; Geochemistry; Mobility; EXAFS spectrometry; Arsenites; Iron Hydroxides oxides; Aquifers; Trace element; Oxidation; Water pollution; Structural analysis; Ground water
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es047970x

The speciation of As and Fe was studied during the oxidation of Fe(II)−As(III) solutions by combining XAS analysis at both the Fe and As K-edges. Fe(II) and As(III) were first hydrolyzed to pH 7 under anoxic conditions; the precipitate was then allowed to oxidize in ambient air for 33 h under vigorous stirring. EXAFS analysis at the As K-edge shows clear evidence of formation of inner-sphere complexes between As(III) and Fe(II), i.e., before any oxidation. Inner-sphere complexes were also observed when Fe became sufficiently oxidized, in the form of edge-sharing and double-corner linkages between AsIIIO3 pyramids and FeIIIO6 octahedra. XAS analyses at the Fe K-edge reveal that the presence of As(III) in the solution limits the polymerization of Fe(II) and the formation of green rust and inhibits the formation of goethite and lepidocrocite. Indeed, As(III) accelerates the Fe(II) oxidation kinetics and leads to the formation of nanosized Fe−As subunits of amorphous aggregates. These observations, rather than a presumed weaker affinity of As(III) for iron oxyhydroxides, might explain why As(III) is more difficult to remove than As(V) by aerating reducing groundwater.