In Situ X-ray Absorption Study of Surface Complexes: Selenium Oxyanions on α-FeOOH

Abstract

A novel application of x-ray absorption spectroscopy has provided structural information for ions sorbed at oxide-water interfaces. As an example, in situ extended x-ray absorption fine structure (EXAFS) measurements of adsorbed selenate and selenite ions at ah alpha-FeOOH(goethite)-water interface have been performed; these measurements show that selenate forms a weakly bonded, outer-sphere complex and that selenite forms a strongly bonded, inner-sphere complex. The selenite ion is bonded directly to the goethite surface in a bidentate fashion with two iron atoms 3.38 angstroms from the selenium atom. Adsorbed selenate has no iron atom in the second coordination shell of selenium, which indicates retention of its hydration sphere upon sorption. This method provides direct structural information for adsorbed species at solid-liquid interfaces.

The relationship between molecular structure and ion adsorption on variable charge minerals

“…Although macroscopic proton-ion adsorption stoichiometries for sulfate and selenate are similar (Fig. 2), sulfate was interpreted to be a monodentate innersphere complex (Hug, 1997;Eggleston et al, 1998), and selenate was interpreted to be either outer sphere (Hayes et al, 1987) or a bidentate inner sphere complex (Manceau and Charlet, 1994). Our results for sulfate and selenate can be interpreted as either monodentate innersphere or as outersphere complexes, but a bidentate innersphere complex is in conflict with our results.…”

“…This seems reasonable given the spectroscopic results for sulfate (Hug, 1997), selenite (Hayes et al, 1987;Manceau and Charlet, 1994), and phosphate (Tejedor and Anderson, 1990) for experimental conditions similar to those of this study (pH 4 to 6, ion adsorption below 1.5 (mol/m 2 ). Under the experimental conditions used, the surface is expected to will remain positively charged, which prevents protonation of the adsorbed oxyanion.…”

“…3 for arsenate (Waychunas et al, 1993;Waychunas et al, 1996;Sun and Doner, 1996;Fendorf et al, 1997) and selenite (Hayes et al, 1987) correspond with the spectroscopically found structures for pH values and surface coverages comparable to the ones used in our experiments. The structure of sulfate adsorbed onto hematite was studied recently by Hug (1997) with in situ ATR-FTIR.…”

“…The charges of the oxygen ligands of selenite (SeO 3 2Ϫ ) are estimated from the Se-O bond valence z/CN ϭ 4/3 ϭ 1.33. EXAFS shows (Hayes et al, 1987) that adsorbed selenite shares two of its ligands with the surface and one ligand is directed to the solution. This causes a spatial charge distribution at the mineral-solution interface (Hiemstra and van Riemsdijk, 1999).…”

“…comm. ;Waychunas et al, 1993;Hayes et al, 1987). In the present study the macroscopic proton-ion adsorption stoichiometry for the above species has been determined for pH conditions that have also been used in the spectroscopic studies mentioned above.…”