Thermodynamic description of the UO$_2$(NO$_3$)$_2$-NaNO$_3$-H$_2$O and UO$_2$(NO$_3$)$_2$-Mg(NO$_3$)$_2$-H$_2$O 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 UO$_2$(NO$_3$)$_2$-NaNO$_3$-H$_2$O and UO$_2$(NO$_3$)$_2$-Mg(NO$_3$)$_2$-H$_2$O were conducted at $T$ = (22 ± 0.2) °C. Corresponding solid phases UO$_2$(NO$_3$)$_2$·6H$_2$O, NaNO$_3$, and Mg(NO$_3$)$_2$·6H$_2$O 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 $^θ$UO$_2$$^{2+}$,Na$^+$, $^θ$UO$_2$$^{2+}$,Mg$^{2+}$, $^Ψ$UO$_2$$^{2+}$,Na$^+$,NO$^{3−}$, and $^Ψ$UO$_2$$^{2+}$,Mg$^{2+}$,NO$^{3−}$. The geochemical code PhreeSCALE was used together with the PhreeSCALE database and the parameter estimation software PEST.