Jemison submitted a thesis for a MS Degree at the University of Illinois at Urbana in 2015. He reported that 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 by the author 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.
The author concluded that the adsorption of U(VI) to goethite, birnessite, illite, aquifer material, and quartz produced an average isotopic fractionation of -0.17‰ with adsorbed U(VI) isotopically lighter than aqueous U(VI). The minerals tested in this study all produced similar isotopic fractionations with uranyl carbonato species adsorbed to quartz, illite, aquifer material, goethite, and birnessite producing fractionations that were within uncertainty of each other.
Aqueous speciation had a larger effect with uranyl carbonato and calcium uranyl carbonato complexes producing a greater isotopic fractionation than uranyl hydroxyl species. Uranyl hydroxyl ions adsorbing to quartz did not produce any isotopic fractionation. U isotope ratios are increasingly being used as indicators of U redox reactions, and the effects of adsorption processes on 238U/235U data must be accounted for. Based on the present study, δ238U of U(VI) adsorbed to silicate and oxide solids can be assumed to be about -0.17‰ less than coexisting water under most conditions.
The author's results suggested that adsorption cannot cause fractionations as large as reduction, but generally causes significant fractionation that have a second-order effect. This study represents the first direct measurement of isotopic fractionation induced by adsorption of U(VI) species relevant to seawater. Paleo-redox studies of the ancient ocean could be affected by U(VI) adsorption to ferromanganese oxides if the amount of ferromanganese oxides produced was very large at certain times.
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