An investigation on the effects of several anions on the dissolution of synthetic uraninite (UO2)

• [F−] found to have a significant effect on dissolution of UO2
• NO3− increased the rate of UO2 dissolution.
• Medium–high concentrations of PO43 − and SO42 − hindered UO2 dissolution.
• Fe species in solution have a significant influence on UO2 dissolution.

With the increase in the demand for energy, there is an increased interest in nuclear power sourced from a number of uranium minerals, primarily tetravalent UO2. During the leaching of uranium from its ores, a number of gangue minerals are leached which lead to a number of different ions in the leach slurry. The current work undertaken focused on the effect of anions that typically leach from gangue minerals found in uranium bearing ore bodies on the dissolution of synthetic UO2. The anions studied were F−, Cl−, Br−, SO42 −, PO43 − and NO3−. Cl− and Br− were found to have no significant influence on UO2 dissolution over the concentration range studied (0–1.8 g/L Cl−; 0–4 g/L Br−), whilst F− was found to have a detrimental effect on UO2 dissolution at low–medium concentration (0–0.3 g/L) and a positive effect on UO2 dissolution when present in medium–high concentrations (0.5–1 g/L), when compared to the dissolution obtained in the presence of the other halides studied. Of the oxo-anions studied NO3− had a positive effect on UO2 dissolution over the entire concentration range studied and PO43 − had a negative effect when present at medium–high concentrations. This was due to precipitation of FePO4 at medium–high PO43 − concentrations. SO42 − had a negative effect when present at medium–high concentrations. This was most likely due to the formation of less effective FeIII based complexes, where the FeIII species present in solution play an important role in the UO2 oxidation/dissolution mechanism. Results obtained on the influence of all anions investigated simultaneously are also presented and discussed.