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

摘要

Sequestration of soluble uranium(VI) in the form of uranyl UO_2~(2+)by clay minerals such as chlorite is potentially a majorsink for U in U-contaminated environments. We have used batch sorption/desorption experiments combined with U LIII-edgeX-ray absorption near-edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectros-copy, scanning and transmission electron microscopy, synchrotron-based microdiffraction, and surface complexation model-ing to investigate the dominant sorption process(es) governing uranyl uptake by chlorite. Uranium(VI) sorption isindependent of ionic strength, suggesting dominantly inner-sphere sorption, which was supported by selective chemical extrac-tion results. The maximum sorption loadings were 0.28 μmol U g-1chlorite (at pH 4) and 6.3 μmol U g-1chlorite 6.5and 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 systemioCO3-bearingsystem. Desorption experiments based on selective chemical extractions indicated that (1) there is little or no weakly boundU(VI) or U(VI)-bearing precipitates, (2) 60-80% of U(VI) inner-sphere sorption complexes are desorbed following a 0.1 MHC1 step over 1 week, and (3) 100% desorption of adsorbed U(VI) is accomplished by a 1.0 M HC1 step over 1 week. Fitsof the EXAFS spectra of the short-term sorption samples indicate that UO22+ forms inner-sphere sorption complexes withcarbonate (when present) at [Fe(O,OH)6] octahedral sites in a bidentate, edge-sharing manner. EXAFS-derived structuralparameters were used to constrain the type(s) of U(VI)-bearing surface species and were combined with observed batch sorp-tion 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) sorp-tion by up to 10% at the highest U(VI) sorption loadings and at low pH in the CO3–Ca-bearing system. After long-term expo-sure of chlorite to U(VI) under anaerobic conditions at 90 °C, XANES spectra of these samples indicate 25% U(IV) in theCO3–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 nanoparticulateUO2. The presence of Ca in solution prohibited U(VI) reduction in our long-term sorption experiments.