Interaction of aqueous U(VI) with goethite, montmorillonite, and UO2(s)

Uranium contamination in subsurface environments is a matter of great concern throughout the world. Fate and transport of uranium in the subsurface can be controlled by U(VI) adsorption and reduction onto common iron (oxy)hydroxides and clay minerals. Aqueous U(VI) can also exchange uranium atoms with solids comprised of uranium which can potentially lead to changes in the morphology of the uranium-containing solids and affect their stability. First, the performance of multiple surface complexation models (SCMs) on adsorption of U(VI) onto goethite was analyzed for a broad range of input conditions. Individual models could fit the data for which they were parameterized, but they performed poorly when compared with experimental data covering a broader range of conditions. We developed a series of robust models with different levels of complexity following a systematic roadmap. Goethite-uranyl-carbonate ternary surface complexes were required in every model. A triple plane model with a dimeric goethite-uranyl complex was found to provide the best fit, but the performance of a double layer model with bidentate goethite-uranyl and goethite-uranyl-carbonate complexes was also comparable. The models that ignore the bidentate feature of uranyl surface complexation consistently performed poorly. The goodness of fit for the models that ignore adsorption of carbonate was not significantly compromised compared with their counterparts that considered carbonate adsorption. This approach of model development for a large and varied dataset improved our understanding of U(VI)-goethite surface reactions and can lead to a path for generating a single set of reactions and equilibrium constants for including U(VI) adsorption to goethite in reactive transport models. Second, multiple SCMs for U(VI) adsorption onto montmorillonite were reviewed, and their performance for a large range of input conditions was evaluated. A new SCM was developed using a large and varied composite dataset to generate a model that would provide effective simulation over the full range of input conditions. This new model also employed a state-of-the-art technique to account for the spillover charge effect when estimating edge surface potential. The new model was comprised of four edge-site U(VI) surface complexation reactions and a single cation exchange reaction. Out of the four edge-site species, one was a U(VI)-carbonate ternary surface species that was only significant at an elevated carbonate concentration. The overall performance of the new model was robust, performing better than any of the models reviewed in this study for the composite dataset. This U(VI)-montmorillonite robust model along with robust models for other such minerals can be integrated with a reactive transport model which will help better predict uranium fate and transport in environmental conditions. Third, the extent of reduction of U(VI) adsorbed to chemically reduced montmorillonite was investigated at different pH and sodium concentrations using X-ray absorption spectroscopy and chemical extractions. Nearly 100% of U(VI) was reduced to U(IV) at low sodium concentration at both pH ~3 and ~6. The extent of U(VI) reduction at high sodium concentration and pH ~6 was only 70%. Surface bound U(VI) on unreduced montmorillonite was much more easily extracted into solution with bicarbonate than surface bound U(IV) generated due to reduction of U(VI) adsorbed onto reduced montmorillonite surface. U(IV) immobilized onto the clay surface by cation exchange is more resistant to bicarbonate-induced mobilization than uranium bound to the montmorillonite edge sites. We developed a non-electrostatic surface complexation model to interpret the equilibrium adsorption of U(IV) to reduced montmorillonite as a function of pH and sodium concentration. Our findings establish the importance of structural Fe(II) in low iron content smectites in controlling uranium mobility in subsurface environments. Finally, uranium atom exchange between aqueous U(VI) and the solid associated uranium species was probed using a 236U isotope tracer at pH 7 and 1 mM dissolved inorganic carbon condition. No isotope exchange was observed between aqueous U(VI) and the UO2(s) even after 47 days of reaction. However, occurrence of isotope exchange between aqueous U(VI) and the U(VI) adsorbed onto montmorillonite was observed within timescales of 5 minutes. X-ray photoelectron spectroscopy provided evidence for the presence of U(V) and U(VI) species on the U(IV) oxide surface