Kinetics of Na- and K- uranyl arsenate dissolution

We integrated aqueous chemistry analyses with geochemical modeling to determine the kinetics of the dissolution of Na and K uranyl arsenate solids (UAs_(s)) at acidic pH. Improving our understanding of how UAs_(s) dissolve is essential to predict transport of U and As, such as in acid mine drainage. At pH 2, Na_0.48H_0.52(UO₂)(AsO₄)(H₂O)_2.5(s) (NaUAs_(s)) and K_0.9H_0.1(UO₂)(AsO₄)(H₂O)_2.5(s) (KUAs_(s)) both dissolve with a rate constant of 3.2 × 10^-7 mol m^-2 s^-1, which is faster than analogous uranyl phosphate solids. At pH 3, NaUAs_(s) (6.3 × 10^-8 mol m^-2 s^-1) and KUAs_(s) (2.0 × 10^-8 mol m^-2 s^-1) have smaller rate constants. Steady-state aqueous concentrations of U and As are similarly reached within the first several hours of reaction progress. This study provides dissolution rate constants for UAs_(s), which may be integrated into reactive transport models for risk assessment and remediation of U and As contaminated waters.