RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Sequestration of soluble uranium(VI) in the form of uranyl UO₂²⁺ by clay minerals such as chlorite is potentially a major sink for U in U-contaminated environments. We have used batch sorption/desorption experiments combined with U L_III-edge X-ray absorption near-edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning and transmission electron microscopy, synchrotron-based microdiffraction, and surface complexation modeling to investigate the dominant sorption process(es) governing uranyl uptake by chlorite. Uranium(VI) sorption is independent of ionic strength, suggesting dominantly inner-sphere sorption, which was supported by selective chemical extraction results. The maximum sorption loadings were 0.28 μmol U g⁻¹ chlorite (at pH 4) and 6.3 μmol U g⁻¹ chlorite (at pH 6.5 and 10). Uranium(VI) uptake as a function of solution composition followed the trends (at pH 6.5): CO₃–Ca-free system >CO₃–Ca-bearing system >CO₃-bearing system; (at pH 10): CO₃–Ca-bearing system >CO₃–Ca-free system ≈CO₃-bearing system. Desorption experiments based on selective chemical extractions indicated that (1) there is little or no weakly bound U(VI) or U(VI)-bearing precipitates, (2) 60–80% of U(VI) inner-sphere sorption complexes are desorbed following a 0.1 M HCl step over 1 week, and (3) 100% desorption of adsorbed U(VI) is accomplished by a 1.0 M HCl step over 1 week. Fits of the EXAFS spectra of the short-term sorption samples indicate that UO₂²⁺ forms inner-sphere sorption complexes with carbonate (when present) at [Fe(O,OH)₆] octahedral sites in a bidentate, edge-sharing manner. EXAFS-derived structural parameters were used to constrain the type(s) of U(VI)-bearing surface species and were combined with observed batch sorption trends as input for a diffuse double-layer surface complexation model (SCM). This model successfully predicts U(VI) sorption over a range of U(VI) concentrations, pH values, and solution compositions, although it under-predicts U(VI) sorption by up to 10% at the highest U(VI) sorption loadings and at low pH in the CO₃–Ca-bearing system. After long-term exposure of chlorite to U(VI) under anaerobic conditions at 90 °C, XANES spectra of these samples indicate 25% U(IV) in the CO₃–Ca-free system and CO₃-bearing system samples, whereas no U(IV) was detected in the CO₃–Ca-bearing system sample. Analysis of the EXAFS spectra, TEM images, and EDS spectra indicated the presence of X-ray amorphous nanoparticulate UO₂. The presence of Ca in solution prohibited U(VI) reduction in our long-term sorption experiments.