Uranium Isotopic Fractionation Induced by U(VI) Adsorption Onto Common Aquifer Minerals

Abstract:

Mining and processing of uranium (U) ore for nuclear energy and weapons has led to U contamination of groundwater. Reduction of soluble, mobile U(VI) to UO₂ decreases uranium groundwater concentrations and is an important driver of natural and stimulated attenuation. ²³⁸U/²³⁵U measurements can be used to monitor and perhaps quantify U(VI) reduction; biological reduction of U(VI) has been shown to produce a ~1.0‰ isotopic fractionation in both laboratory and field settings, with the reduced product enriched in ²³⁸U. However, adsorption of U(VI) onto minerals may complicate the use of 238U/235U in this application; adsorption of U(VI) onto Mn oxides induces an isotopic fractionation of 0.2‰ with the sorbed U(VI) depleted in ²³⁸U. At present, the isotopic shift produced by adsorption of U(VI) onto other minerals has not yet been explored.

This study measures U isotopic fractionation during adsorption onto goethite, birnessite, quartz, illite, and complex aquifer materials. In addition, the effect of U speciation on fractionation is also examined by adsorption of uranyl (UO₂²⁺), uranyl carbonato (such as UO₂(CO₃)₂²⁻ and UO₂(CO₃)₃⁴⁻), and calcium uranyl carbonato (Ca₂UO₂(CO₃)₃(aq) and CaUO₂(CO₃)₃²⁻) ions to goethite and birnessite. Experiments are carried out with a multi-stage, batch approach, in which a U(VI)-bearing solution is exposed to three stages of adsorption, and the final solution is analyzed by a double-spike MC-ICP-MS method. This increases our ability to resolve among sorbents the extent of fractionation. Early results suggest that uranium adsorption to different minerals produces different isotopic fractionations, with quartz producing little to no fractionation (<0 .05‰), while goethite produces a 0.16‰ isotopic shift (adsorbed U(VI) depleted in ²³⁸U).