Dissolution of studtite [UO₂(O₂)(H₂O)₄] in various geochemical conditions

Highlights: • The mechanisms of studtite dissolution by [H⁺] and [OH⁻] are proposed. • Carbonate ligand and H₂O₂ in groundwater accelerate the dissolution of studtite. • Dissolved oxygen is not considered to significantly effect studtite dissolution. • Dominant uranium species in 10⁻³ M HCO₃⁻ solution are confirmed using TRLFS analysis. - Abstract: This study determined the dissolution rate of studtite, (UO₂)O₂(H₂O)₄, which can be formed by reaction between H₂O₂ and UO₂²⁺ that leaks from spent nuclear fuel (SNF) in deep geological repositories. The batch dissolution experiments were conducted using synthesized studtite under different solution conditions with varying pHs and concentrations of HCO₃⁻ and [H₂O₂] in synthetic groundwater. The experimental results suggested that carbonate ligand and H₂O₂ in groundwater accelerated the dissolution of studtite and uranium (U) release. Above 10⁻⁵ M of H₂O₂ initial concentration, the released uranium concentration in solution decreased, possibly as a result of reprecipitation of studtite due to reaction between uranium and H₂O₂. The results will be useful to assess the comprehensive transport of uranium from both nuclear waste and SNF stored in deep geological repositories.