Solubility constraints on uranium concentrations in groundwaters of the Tono uranium deposit, Japan

Abstract

Natural analogue / Uranium deposit / Long-term geochemical condition / Uranium solubility / Safety assessment Summary.A key concept underpinning most safety assessments of geological disposal systems for high-level nuclear wastes is that the maximum aqueous concentrations of actinide elements released from the waste will be limited by the solubility of the corresponding tetravalent oxide or hydrous oxide. The validity of this concept for U is evaluated in the present study using hydrochemical and mineralogical data from the Tono uranium (U) deposit in Gifu Prefecture, Japan. In addition, the long-term stability of U mineralization in the Tono deposit is assessed in terms of plausible range of U solubilities in paleogroundwaters migrating through the deposit.A thermodynamic evaluation of the aqueous-speciation and solubility behaviour of U indicates that U concentrations in the coexisting groundwaters greatly exceed the solubility of both uraninite [UO 2 (c)] and coffinite [USiO 4 (c)]. The dissolved U concentrations appear instead to be controlled by the solubility of amorphous hydrous U(IV) oxide [UO 2 (am)], which is metastable with respect to uraninite. Mineralogical and microbiological investigations suggest that pH-E h conditions similar to those observed presently in groundwaters of the lower Toki Lignite-bearing Formation (pH ≈ 8-10; E h ≈ −250-−410 mV) have been buffered, possibly since the present geological structure formed about 5-0.7 Ma, by microbial sulphate reduction and precipitation/dissolution of calcite. If so, corresponding U concentrations controlled by UO 2 (am) solubility are calculated to have been in the range of 10 −6.1 and 10 −8.7 mol kg −1 depending on the possible environmental conditions, i.e., E h , pH and carbonate concentration.

Determination of redox potential of sulfidic groundwater in unconsolidated sediments by long-term continuous in situ potentiometric measurements

“…For effective environmental control, it is critical to determine the fate and transport of metals in water bodies (Walter 1997;Smedley and Edmunds 2002;Iwatsuki et al 2004). Redox conditions, particularly sulfidic conditions, play a major role in the removal of metal impurities (e.g.…”

Kinetics of Triscarbonato Uranyl Reduction by Aqueous Ferrous Iron: A Theoretical Study

“…Carbonate has a significant effect on uranium chemistry in the environment by enhancing its solubility proportionally to carbonate concentration , and increasing it in systems where no significant hexavalent uranium is present. , Except in certain biologic mechanisms, carbonate is known to inhibit uranium reduction to U(IV) completely . Even under reducing conditions, uranium can incorporate itself into carbonates or ferric oxides as a hexavalent species. − From a thermodynamic standpoint, complexation of U(VI) by carbonate drives its reduction potential to significantly more negative values, eliminating a wide range of otherwise useful target reductant species.…”

“…10 Carbonate has a significant effect on uranium chemistry in the environment by enhancing its solubility proportionally to carbonate concentration 11,12 and increasing it in systems where no significant hexavalent uranium is present. 13,14 Except in certain biologic mechanisms, 15 carbonate is known to inhibit uranium reduction to U(IV) completely. 16 Even under reducing conditions, uranium can incorporate itself into carbonates or ferric oxides as a hexavalent species.…”

Long-term continuous in situ potentiometrically measured redox potential in anoxic groundwater with high methane and iron contents

“…Characterization of the dominant redox processes in groundwater, which may be oxygen reduction, nitrate reduction, manganese reduction, iron reduction, sulfate reduction, or methanogenesis, is critical in determining the fate and transport of toxic metals, radionuclides, and nutrients (Korom 1992;Walter 1997;Smedley and Edmunds 2002;Iwatsuki et al 2004;Höhn et al 2006). A number of methods have been developed for this purpose, relying on groundwater composition (redox-sensitive compounds), electrochemical measurement (oxidation-reduction potential), and volatile fatty acid and hydrogen concentrations (Chapelle et al 1995;Christensen et al 2000;McMahon et al 2009).…”