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Soil Chemistry

Selenite Adsorption Mechanisms on Pure and Coated Montmorillonite: An EXAFS and XANES Spectroscopic Study

Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon SK S7N 5A8 Canada

^* Corresponding author (derek.peak{at}usask.ca)

Selenite (SeO₃^2–) is an oxyanion of environmental importance due to its toxicity to animals at higher concentrations, notably waterfowl and grazing animals. Sorption of SeO₃^2– with mineral phases typically controls the movement and bioaccessibility of SeO₃^2– in soils and sediments. Previous studies have successfully utilized synchrotron-based Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopy to determine SeO₃^2– bonding mechanisms on Fe and Mn oxides, but the direct evidence of SeO₃^2– surface complexation mechanisms on important mineral phases such as Al hydroxide and aluminosilicate minerals is still lacking. In this study both EXAFS and XANES spectroscopy was conducted on aqueous SeO₃^2– solutions and on a variety of Al-bearing sorption samples at pH 4.5. The sorbents chosen were a hydroxyaluminosilicate (HAS) polymer, a hydroxyaluminum (HYA) polymer, montmorillonite, and both HYA and HAS coated montmorillonite. For SeO₃^2– sorption on montmorillonite, only bidentate binuclear inner-sphere complexation was observed. For the hydroxyaluminum and hydroxyaluminosilicate polymers, a mixture of outer-sphere and bidentate binuclear inner-sphere was observed. When montmorillonite was coated with either HYA or HAS polymers then adsorption behavior was intermediate between that of the mineral and the pure polymer. Since temperate soils often contain aluminum-hydroxy and aluminosilicate coated minerals rather than discrete Al hydroxide minerals and pristine clay surfaces, the adsorption mechanisms observed on these coated surfaces are more realistic of the natural environment than sorption on pure minerals.

Abbreviations: EXAFS, Extended x-ray absorption fine structure spectroscopy • HAS, hydroxyaluminosilicate • HAS-Mt, hydroxyaluminosilicate-coated montmorillonite • HYA, hydroxyaluminum • HYA-Mt, hydroxyaluminum-coated montmorillonite • pK_a, acid dissociation constant • Mt, montmorillonite • PZC, point of zero charge • PZSE, point of zero salt effect • RSF, radial structure function • XANES, X-ray Absorption Near Edge Structure spectroscopy • XAS, X-ray Absorption Spectroscopy