Uranium isotopic fractionation induced by U(VI) adsorption onto common aquifer minerals

Uranium groundwater contamination due to mining and processing affects numerous sites in the western United States. Bioreduction of soluble, mobile U(VI) to U(IV)-bearing solids is a common remediation strategy. Uranium isotopes (238U/235U) have been utilized to track the progress of microbial reduction, with laboratory and field studies finding a ~1‰ isotopic fractionation with the U(IV) product enriched in 238U. However, the isotopic fractionation produced by adsorption may complicate the use of 238U/235U to trace microbial reduction. A previous study found that adsorption of U(VI) onto Mn oxides produced a -0.2‰ fractionation with the adsorbed U(VI) depleted in 238U (Brennecka et. al. 2011b). The aqueous speciation of U(VI), and the characteristics of the sorbent surfaces may both produce variations in isotopic fractionation induced by adsorption. This study determined U isotopic fractionation induced by U(VI) adsorption onto goethite, birnessite, illite, quartz, and complex aquifer materials. The influence of U(VI) speciation on fractionation was also examined by measuring differences between fractionation factors accompanying the adsorption of uranyl, uranyl hydroxyl, uranyl carbonato, and calcium uranyl carbonato ions. To increase our ability to resolve fractionation, experiments were carried out with a multi-stage, batch approach, in which a U(VI)-bearing solution was exposed to three stages of adsorption. The dissolved U(VI) was analyzed by a double-spike MC-ICP-MS method. Adsorption to the studied solid phases induces an average isotopic fractionation of -0.17‰ with the adsorbed U(VI) isotopically lighter. U(VI) speciation can affect the magnitude of isotopic fractionation with uranyl carbonato and calcium uranyl carbonato complexes producing a greater isotopic fractionation than uranyl hydroxyl species. Studies using U isotope data to assess U(VI) reduction must consider adsorption as a lesser, but significant isotope-fractionating process