Redox-driven dissolution of clay minerals by uranium under high pressure CO2 conditions

Geologic sequestration of supercritical CO2 is one technology proposed to mitigate global warming. Increased acidity of brine due to CO2 injection could lead to mineral dissolution of cap rock and well seals and mobilization of contaminants (e.g., U, Pb, As). In this study we examined the dissolution of nontronite NAu-2, an Fe(III)-rich clay mineral, and partially-reduced nontronite (R-NAu-2) in a synthetic brine (0.33 M Na2SO4) under high pressure CO2 conditions (PT = 9.66 bar, PCO2 ≥ 8.66 bar CO2, T = 20 °C) and in 1.40 M H3PO4–0.50 M H2SO4. Uranyl(VI) or biogenic uraninite(IV) was added as a redox-active contaminant and reaction kinetics were measured over a 15 d period. Unaltered nontronite [3.4% Fe(II)] dissolved very little under high pressure CO2 conditions. However, chemically-reduced nontronite [48% Fe(II)] dissolved more rapidly (half-life of 78.4 d under high pressure CO2 conditions, 17.8 h in H3PO4–H2SO4). Structural Fe(II) in reduced nontronite [R-NAu-2 Fe(II)] was preferentially dissolved compared to structural Fe(III) in unaltered nontronite [NAu-2 Fe(III)]. No redox reactions were observed between R-NAu-2 Fe(II) and U(VI). In contrast, uraninite was oxidized by NAu-2 Fe(III) faster and to a greater extent under high pressure CO2 conditions as compared to ambient pressure conditions (PT = 1.0 bar, 95:5% N2:H2). Redox reactions between uraninite and NAu-2 Fe(III) enhanced the dissolution of both clay and U, indicative of potential risks associated with geologic carbon sequestration.