Formation of aqueous MgUO2(CO3)32- complex and uranium anion exchange mechanism onto an exchange resin

The formation of and stability constants for aqueous Mg-UO2-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 mu mol/L under equilibrium with atmospheric CO2. The results indicate that only the MgUO2(CO3)(3)(2-) complex is formed. The cumulative formation constant extrapolated to zero ionic strength is similar regardless of the activity correction convention used: 1001130 = 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)(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- andUO(2)(CO3)(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.