Thermodynamic description of the UO2(NO3)2-NaNO3-H2O and UO2(NO3)2-Mg(NO3)2-H2O systems. Solubility experiments and full dissociation Pitzer models.

The present work improves the understanding on uranium(VI) nitrate systems in acidic, moderate to high ionic strength conditions. Undersaturation solubility studies with the systems UO2(NO3)2-NaNO3-H2O and UO2(NO3)2-Mg(NO3)2-H2O were conducted at T = (22 ± 0.2) °C. Corresponding solid phases UO2(NO3)2·6H2O, NaNO3, and Mg(NO3)2·6H2O were identified via X-ray powder diffraction. Experimental results are compared with literature data available for the ternary systems at T = 25 °C and indicate a strong dependence on temperature. Based on solubility and isopiestic data, an updated Pitzer model assuming the full dissociation of all involved salts is derived, including new Pitzer interaction parameters θUO22+,Na+, θUO22+,Mg2+, ΨUO22+,Na+,NO3-, and ΨUO22+,Mg2+,NO3-. The geochemical code PhreeSCALE was used together with the PhreeSCALE database and the parameter estimation software PEST.