BIOGEOCHEMISTRY OF URANIUM IN HIGH IONIC-STRENGTH BRINE
The long-term solubility of uranium(VI) was determined in a high magnesium (GWB) and high sodium chloride (ERDA-6) simulated brine in the presence and absence of carbonate. The solubility was ~10-6 M in GWB brine at pCH+ ≥ 7 and about 10-8 - 10-7 M in ERDA-6 at pCH+ ≥ 8 when no carbonate was present. A small effect of tetraborate, a minor constituent of the simulated brines, was also noted. As expected, uranium-carbonate species predominated when carbonate was present. The highest uranium solubility measured (10 mM dissolved carbonate) was ~ 10-4M, under WIPP-related conditions (pCH+ ~ 9). These results are well below the current 1 mM solubility used in WIPP performance assessment. A relatively small contribution of colloidal uranium was observed in these long-term solubility experiments.
The oxidation–state distribution of uranium is important in establishing the safety case for multivalent actinides in salt-based repositories. Uranium(VI) was stable over a wide range of conditions under oxic conditions but was reduced to U(IV) by zero-valent iron and microorganisms under anoxic/anaerobic conditions. The halophilic bacterial and archaeal communities that are indigenous to the WIPP are being characterized in a parallel/ongoing study. Metal-reducing microorganisms have been identified and bioreduction to form U(IV) phases occurred. Zero-valent iron, over time, also reduced U(VI) to U(IV) but we have not been able to show that an aqueous Fe2+ reaction pathway exists. In the highly reducing conditions expected in an iron-dominated subsurface in sealed salt the U(IV) oxidation state is expected to predominate.
These biogeochemical results positively support the safety case for the permanent disposal of uranium in a salt repository and demonstrate the importance of redox-active reactive barriers, such as reduced iron, in repository performance.