Formation of Aqueous MgUO2(CO3)32− Complex and Uranium Anion Exchange Mechanism onto an Exchange Resin

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Abstract
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 µmol/L under equilibrium with atmospheric CO2. The results indicate that only the MgUO2(CO3)32− 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 MgUO2(CO3)32− that had a lower affinity for the resin than UO2(CO3)34−. Uranium sorption results are consistent with an equivalent anion exchange reaction between NO3− and UO2(CO3)34− species to retain charge neutrality regardless of Mg concentration. No Mg was associated with the anion exchange resin indicating that the MgUO2(CO3)32− complex did not sorb.

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