Coordinatiom of actinides in aqueous sulfate solution

Structural information on the sulfate coordination of actinides in aqueous solution is actually rather scarce. At the other hand there is often a high sulfuric acid concentration in the environment mines containing pyrite, because pyrite, exposed to aerobic conditions, undergoes a weathering followed by sulfuric acid release. An artificial source of sulfate in natural environment is in situ leaching, so-called solution mining, where sulfuric acid is injected into the ore deposit, as has been used especially for uranium mining in sandstone formations. Due to the lack of information on the coordination of sulfate with actinides in aqueous solution, we investigated Th(IV), Pa(V), U(IV), U(VI), Np(IV), Np(V) and Np(VI) complexes in aqueous sulfate solution by L3-edge EXAFS, high energy x-ray scattering (HEXS), and UV-Vis spectroscopy using samples of 10-50 mM of actinide and total sulfate concentrations, 0.05 ≤ [SO42-] ≤ 10 M. In contrast to the coordination of actinides with carbonate where bidentate coordination always prevails, the coordination with sulfate comprises monodentate and bidentate linkage with a wide variety of combinations. In general, with increasing [SO42-]/[Ann+] ratio the bidentate coordination becomes dominant in solution. As example, at low [SO42-]/[UO22+] ratio, where the UO2SO4(aq) species prevails, the sulfate coordinates in a monodentate and only to a less extend in bidentate fashion. At high [SO42-]/[UO22+] ratio, where UO2(SO4)22- species prevails, bidentate sulfate coordination with the species [UO2(SO4)2bid]2- becomes dominant [1]. In oxidation state IV, up to five coordinating sulfate groups have been observed, mostly with predominant bidentate coordination as for example in the high-charged complex [U(SO4bid)3(SO4mon)2]6- [2]. Pa(V) shows in high excess of sulfate a coordination of monodentate and bidentate sulfate, e.g. the complex [PaO(SO4bid)2(SO4mon)3]7- [3]. Np(IV) and Np(VI) follows the tendency of U(IV) and U(VI), whereas Np(V) is weakly coordinated by sulfate, in accordance with its low formation constant. The attempt to preserve solution species in crystal structures led in most of the cases to a rearrangement of bidentate sulfate groups in solution to monodentate–bridging coordination in solid state. The coordination of the solution complexes was optimized by DFT calculation and the results were compared with the experimental observations.

[1] C. Hennig, K. Schmeide, V. Brendler, H. Moll, S. Tsushima, A.C. Scheinost, Inorg. Chem. 46, 5882, 2007.
[2] C. Hennig, W. Kraus, F. Emmerling, A. Ikeda, A.C. Scheinost, 47, 2987, 2008
[3] C. Le Naour, D. Trubert, M.V. Di Giandomenico, C. Fillaux, C. Den Auwer, P. Moisy,C. Hennig, Inorganic Chemistry 44, 9542, 2005