The absorption spectra ofspecies in aqueous solution are investigated theoretically and experimentally, and the spectrum ofspecies is investigated theoretically. The spectrum ofin perchloric acid solution was taken from 350 to 1350 nm. Peak positions and optical densities are reported with overall uncertainties of 0.3 nm and 3%, respectively. A more precise value for the extinction coefficient of the most intense line is reported (398 ± 4 M-1 cm-1 for the 980.2 nm line). The intensities and positions of the electronic transitions of these actinyl complexes are computed from relativistic quantum chemical theory involving relativistic effective core potentials, corresponding spin-orbit operators, and spin−orbit, graphical unitary group configuration interaction. Because all of the low-lying electronic states for the isolated actinyl ions have the same parity, the equatorial ligands must break the inversion symmetry. Thus, model calculations onwith one, three, and five chloride ligands were carried out; the five-ligand spectrum was quite similar to experimental solution spectra, whereas the one-ligand and three-ligand spectra were not. Calculations onwere then made in order to provide a close comparison with experimental results. Similar calculations onwere also carried out but were hampered by the difficulty in doing sufficiently extensive calculations to determine the ground electronic state with the ligands present. Comparisons were made, nevertheless, using both of the candidates for ground state. A simplified crystal-field theory is developed to show how the necessary symmetry-breaking orbital mixing, 5fφ with 6dδ, occurs selectively with 5-fold coordination.
Subsurface transport of groundwater contaminants is greatly influenced by chemical speciation, precipitation, and sorption processes. The transport of Pu potentially released from spent nuclear fuel disposal and storage sites will be dependent on its interaction with mineral surfaces and speciation in the subsurface. The sorption of dissolved Pu(V) on a natural zeolitic tuff that was equilibrated with synthetic groundwater was examined using synchrotron-based microanalytical techniques. The tuff contained trace quantities of smectites and iron and manganese oxides, which are present as fracture fill and pore space materials. Synchrotron-based micro-X-ray fluorescence (SXRF) showed that Pu is predominately associated with manganese oxides (rancieite) and smectites but not with iron oxides (hematite). In situ micro-X-ray absorption nearedge structure (XANES) spectroscopy measurements on two highly enriched regions (∼10 × 15 µm 2 ) of Pu indicated that the average oxidation state of sorbed Pu was (+V) in one region and (+VI) in the other. The observed heterogeneous speciation of the sorbed Pu demonstrates the complex nature of this process. Thermodynamic equilibrium calculations indicated that the solution was dominated by negatively charged Pu species (such as PuO 2 CO 3 -), suggesting that sorption to the negatively charged manganese oxide surfaces would be energetically prohibited. Subsequent speciation changes upon sorption to manganese oxide surfaces are discussed.
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