Sorption of Uranium(VI) onto Schwertmannite - EXAFS Investigations

The ferric oxide schwertmannite is commonly formed in acid mine drainage waters (pH-range from 2.8 to 4.5) in the presence of high sulfate concentrations. Because of its large specific surface area its may influence the migration of uranium(VI) by sorption processes. The aim of this study were structural informations for uranium(VI) surface complexes given by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. We include Goethite in our investigations as a reference to compare and interpret the schwertmannite data.

Samples for EXAFS analysis were prepared under N₂ atmosphere using 200 mg of schwertmannite or goethite with initial uranium(VI) concentrations of 1x10^-5 (pH 6.5) to 5x10^-5 M (pH 4.2). The ionic strength was adjusted using 0.01 M Na₂SO₄ or NaClO₄ respectively. EXAFS spectra were recorded at room temperature in fluorescence mode at the Rossendorf beamline (ROBL) at the ESRF in Grenoble. The measured EXAFS oscillations were fitted using EXAFSPAK procedure. The theoretical phase shifts and backscattering amplitudes were calculated with FEFF8.

All distances of the equatorial oxygens are about 2.37 Å. These are significantly shorter than typical values of 2.41 Å for the aquatic UO₂ ²⁺ ion, which indicates strong interactions. An iron backscatterer is clearly found for uranium(VI) sorbed onto goethite at pH 4.2 and 6.5 in sulfate-rich solution as well as for schwertmannite at pH 4.2 in perchloric solution. Such a uranium - iron distance indicates a bidentate inner sphere complexation. In contrast, the EXAFS of uranium(VI) sorbed onto schwertmannite at pH 4.2 in sulfate-rich solutions shows only a sulfur backscatterer at a distance of 3.66 Å, indicating the formation of a ternary surface complex.