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We studied the structures of UO2(CO3)34−, Ca2UO2(CO3)30, and Ba2UO2(CO3)30 using HF, MP2, and B3LYP methods. We obtained an unusual long uranyl bond for the UO2(CO3)34− complex by ECP calculations. This study shows that it is crucial to explicitly include the counterion(s) and the hydration water molecules in the calculation of the aqueous uranyl tricarbonate complex. The calculated structure of the hydrated Ca2UO2(CO3)30 has good agreement with the experiment. [Copyright &y& Elsevier]

Abstract The sorption of actinides (Th, U – Am) was studied in dependence of the solid-to-liquid (S/L) ratio (0.5–20.0 g/L) and the calcium-to-silicon (C:S) ratio. The C:S ratio was varied between 1.80 and 0.70 to simulate the changing composition of the C-S-H phases during cement degradation from high to low C:S ratios. The decrease of the calcium content in the C-S-H phases by time is accompanied by a decrease in pH in the corresponding suspensions from 12.6 to 10.2. X-ray photoelectron spectroscopy (XPS) of the C-S-H phases showed an increasing depletion of Ca on the surface with increasing C:S ratio in comparison to the composition of the solid phase as a whole. The sorption experiments were performed with the redox stable species Am(III), Th(IV) and U(VI), as well as the redox sensitive Np(V) and Pu(III). The average distribution coefficients R d for all investigated actinides are around 105 L/kg. The oxidation state of Pu retained by the C-S-H phases was investigated with high-energy resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy. Samples with C:S ratios of 0.75 and 1.65 showed that the initially added Pu(III) was oxidized to Pu(IV) in the course of the experiment. Graphical abstract Image 1 Highlights • First investigation of Pu uptake by C-S-H phases combining batch experiments with Pu M 5 -edge HR-XANES spectroscopy. • Similar R d values determined for Am(III), Pu(III), Th(IV), Np(V) and U(VI), independent of the C:S and S/L ratios. • XPS measurements indicate a depletion of Ca at the surface of the C-S-H phases. [ABSTRACT FROM AUTHOR]

Acid rock drainage (ARD) solution from an abandoned ore mine (pH 2.7, SO2−4 concentration 411 mmol/l, Fe concentration 93.5 mmol/l) was investigated by photon correlation spectroscopy, centrifugation, filtration, ultrafiltration, scanning electron microscopy, ICP–MS, AAS, ion chromatography, TOC analysis, and extended X-ray absorption fine structure (EXAFS) spectroscopy. A colloid concentration of &ges;1 g/l was found. The prevailing particle size was <5 nm. Iron, As and Pb were the metal constituents of the colloidal particles. The most probable mineralogical composition of the particles is a mixture of hydronium jarosite and schwertmannite. A small amount of a relatively coarse precipitate was formed in the ARD solution during the months after sampling. The colloid particles are obviously an intermediate in the precipitate formation process. The results suggest that the arsenate is bound to the colloids by the formation of a bidentate binuclear inner-sphere surface complex. However, the transformation of the colloidal material to the more aggregated long-term precipitate results in the incorporation of the arsenate into the interior of the Fe hydroxy sulfate crystal structures. Lead seems to occur as anglesite. [Copyright &y& Elsevier]