Solubility and Speciation Calculations for U, Pu, Np, and Th in Natural Groundwaters: Theory, Thermodynamic Data Files, and Initial Applications

摘要

The present document deals with theoretical calculations of the solubility and speciation of different actinide elements under the most realistic groundwater conditions possible. It is intended to clarify, from a thermodynamic point of view, the assumptions underlying the physical dispersion models and if necessary to revise them, but it cannot be a complete and definitive work; improved and expanded thermochemical data banks for actinide elements, along with an increased understanding of the control mechanisms for groundwater chemistry, will from time to time require a revision of the model assumptions and results. In general, the following conclusions can be drawn: The solubility and speciation of U, Pu and Np depend to a massive extent on the redox potential. For temperatures between 25 and 70 exp 0 C their solubilities vary in the E/sub H/ range -300 to +600 mV between 10 exp -13 and 10 exp -5 mol/l. In a reductive milieu (E/sub H/ between -200 and 0 mV) the dissolved Np and Pu concentrations probably will hardly exceed 10 exp -10 mol/l; for uranium we can find values between 10 exp -10 and 10 exp -7 mol/l. Thorium solubilities between 10 exp -9 and 10 exp -10 mol/l can be expected under identical conditions. Quantitatively, only a very few dissolved actinide species are significant, namely hydrolysis products and carbonate complexes. Ligands such as sulfate, chloride or phosphate can affect solubility under special conditions. The calculated solubilities depend directly on the characterization of the solubility-limiting solid phase and the dominant dissolved species. The differences between parallel studies can be attributed largely to the different sets of data considered. The selection of these data records is not trivial and in some cases gives rise to scientific controversy. (ERA citation 08:045741)