Formation of Aqueous MgUO_2(CO_3)_3~2 Complex and Uranium Anion Exchange Mechanism onto an Exchange Resin

摘要

The formation of and stability constants for aqueous Mg-UO_2-CO_3 complexes were determined using an anion exchange method. Magnesium concentration was varied (up to 2O mmol/L) at constant ionic strength I = 0.101, 0.202, 0.304, 0.406, and 0.509 mol/kg NaN0_3), pH 8.1, total [U(VI)] = 10.4 μmol/L under equilibrium with atmospheric CO_2. The results indicate that only the MgUO_2(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: Iogβ113~0 = 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 MgUO2(CO3)32~ that had a lower affinity for the resin than UO_2(CO_3)_3~(4-). Uranium sorption results are consistent with an equivalent anion exchange reaction between NO_3~- and UC_3(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 MgUO_2(CO_3)_3~(2-)complex did not sorb.