Formation of aqueous MgUO₂(CO₃)₃ ^2- complex and uranium anion exchange mechanism onto an exchange resin

The formation of and stability constants for aqueous Mg-UO ₂-CO₃ complexes were determined using an anion exchange method. Magnesium concentration was varied (up to 20 mmol/L) at constant ionic strength (I = 0.101, 0.202, 0.304, 0.406, and 0.509 mol/kg NaNO₃), pH 8.1, total [U(VI)] = 10.4 μmol/L under equilibrium with atmospheric CO ₂. The results indicate that only the MgUO₂(CO ₃)₃^2- complex is formed. The cumulative formation constant extrapolated to zero ionic strength is similar regardless of the activity correction convention used: logβ₁₁₃⁰ = 25.8 ± 0.5 using the Davies equation and = 25.02 ± 0.08 using specific ion interaction theory (SIT). Uranium sorption onto the exchange resin decreased in the presence of Mg putatively due to the formation of MgUO ₂(CO₃)₃^2- that had a lower affinity for the resin than UO₂(CO₃)₃^4-. Uranium sorption results are consistent with an equivalent anion exchange reaction between NO₃^- and UO₂(CO ₃)₃^4- species to retain charge neutrality regardless of Mg concentration. No Mg was associated with the anion exchange resin indicating that the MgUO₂(CO₃)₃ ^2- complex did not sorb.