Solution equilibria of uranyl minerals: Role of the common groundwater ions calcium and carbonate

Highlights: • Metaschoepite yields low U(VI) concentrations in CO₂-poor groundwater at near-neutral pH. • Uranophane produces low U(VI) concentrations at typical groundwater concentrations of calcium and silica. • PCO₂>0.01 and pH < 6 favor U(VI) mobility. • U(VI) attenuation is favored at near-neutral pH and when PCO₂ is minimized. -- Abstract: Understanding the factors that govern aqueous solubility of uranyl minerals is important for predicting uranium mobility in groundwater and for designing effective remediation strategies. The uranyl-containing minerals metaschoepite [UO₃∙(2H₂O)] and uranophane [Ca(UO₂)₂(SiO₃OH)₂·5H₂O] were synthesized and evaluated in batch solubility experiments conducted in the presence of common groundwater ions: calcium, bicarbonate/carbonate, and dissolved silica. Solid-phase characterization revealed the expected structural and thermogravimetric properties of metaschoepite and uranophane. Metaschoepite solubility in carbonate-free water followed a u-shaped pH dependency with minimum solubility near pH 8.5; uranium concentrations at pH ≳ 8.5 were approximately equivalent to the reference value for safe drinking water established by the EPA (30 μg/L). With increasing bicarbonate/carbonate concentration (1 mM – 50 mM) the solubility of metaschoepite increased, presumably due to the formation of uranyl-carbonate complexes. However, the experimental concentrations of uranium were lower than concentrations predicted from accepted complexation constants. For uranophane, equilibrium uranium concentrations were < 75 μg/L at typical groundwater concentrations of calcium and dissolved silica (pH > 7). The diversity of uranyl minerals that possibly form in the presence of common groundwater species: Ca-Mg-Na-K-Si-bicarbonate/carbonate-sulfate-chloride, has not been fully explored with respect to understanding potential mineral transformations and impacts on uranium solubility and mobility.