Uranyl-chlorite sorption/desorption: Evaluation of different U(VI) sequestration processes

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. ne 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 mu mol U g(-1) chlorite (at pH 4) and 6.3 mu mol 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 approximate to 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 degrees 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. (C) 2009 Elsevier Ltd. All rights reserved.