The Stability of U(VI) and As(V) under the Influence of pH and Inorganic Ligands

Uranium and arsenic are two pollutants commonly found in groundwater near uranium mines. However, the reactivity of U(VI) and As(V) need to be carefully investigated to better understand their fate and transport in the environment. In this study, the reactivity of U(VI) and As(V) were studied under various pH, bicarbonate, and Ca 2+ levels. In air-equilibrated systems, the reactivity of U(VI) and As(V) generally decreased with the increase in pH, as evidenced by the solubility of U(VI) and As(V) increasing along with the elevation of pH. At pH = 8, 44.70% and 37.81% of initially added U(VI) and As(V) remained soluble. The addition of 1 mM of bicarbonate increased the reactivity of U(VI) and As(V) at mild acidic to neutral pH; however, the presence of bicarbonate significantly increased the solubility of U(VI) at mild alkaline condition, as nearly all U(VI) remained soluble at pH values of 8 and 9. After the addition of Ca 2+ , the solubility of both U(VI) and As(V) decreased at mild acidic to neutral pH ranges; however, the addition of Ca 2+ markedly increased the soluble percentages of U(VI) at neutral pH, in which the condition 97.81 ± 2.95% of U(VI) remained soluble. Comparatively, only 36.13 ± 4.98% and 1.69 ± 1.08% of U(VI) were soluble at the same pH in air-equilibrated and bicarbonate systems. Our study demonstrated that U(VI) and As(V) are less reactive at neutral to alkaline conditions. Furthermore, the addition of bicarbonate and Ca 2+ can further reduce the reactivity of U(VI) and As(V) at neutral to alkaline conditions. The findings of this study contribute to a deeper understanding of the fate and transport of U(VI) and As(V) in groundwater and could aid in better designing of U(VI) and As(V) removal processes.