Sorption of Uranium(VI) onto ferric oxides in sulfate-rich acid waters

The Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was used to study the sorption of uranium(VI) on schwertmannite and goethite. Samples for EXAFS analysis were prepared under N₂ atmosphere with initial uranium(VI) concentrations of 1x10^-5 (pH 6.5) to 5x10^-5 M (pH 4.2). The ionic strength was adjusted to 0.02 N Na₂SO₄ or 0.01 N NaClO₄, respectively. The results from EXAFS spectroscopy of uranium(VI) sorbed on goethite in sulfate-rich, acid and near-neutral solutions indicate that uranium(VI) is sorbed as an inner-sphere mononuclear bidentate surface complex which is coordinated to ferric iron at a distance of 3.44 Å. The uranium(VI) sorbed onto schwertmannite in acid and sulfate-rich solution is coordinated to one or two sulfate molecules at a distance of 3.66 Å. The EXAFS results indicate the formation of mononuclear-monodentate surface complexes and partly, of binuclear-bidentate surface complexes coordinated to the structural sulfate of schwertmannite. Considering the uncertainty in assigning the sulfate either to the bulk structure or to adsorption reactions, the formation of ternary uranium(VI)-sulfate surface complexes could not be excluded. Due to the release of sulfate from schwertmannite in perchlorate solution, the uranium(VI) adsorption occurs predominantly as a mononuclear bidentate complexation with the ferric iron.