Solubility study and point of zero charge of studtite (UO2O2·4H2O)

• The variation of the studtite solubility with pH was determined.
• A studtite solubility product log Ks0 = −2.7 is proposed.
• The point of zero charge of studtite was determined to be 4.0.

The knowledge of the characteristics of the U(VI)-peroxide chemical system is of importance because of the relevance of the uranyl peroxide solid phases studtite (UO2O2·4H2O) or metastudtite (UO2O2·2H2O) as uranyl secondary solid phases during the leaching of the spent nuclear fuel as well as to the formation in alkaline waters of soluble nanoscale uranyl peroxo cage clusters. In this work, the solubility of studtite was determined for the first time as a function of pH (from 3 to 11) by means of studtite undersaturation experiments. The results obtained showed a V-shaped solubility curve, which was modelled considering the uranyl complexes formation constants (database of the Nuclear Energy Agency, NEA) and recently published formation constants of uranium hydroxo-peroxo-complexes. The best fit of the model to the experimental solubility data was obtained with log Ks0 = −2.7 ± 0.2 for the studtite solubility reaction:UO2O2·4H2O(s)+2H+⇆UO22++4H2O+H2O2On the other hand, the so-called immersion methodology was used in order to determine the pHpzc of studtite, which resulted to be 4.0 ± 0.2. This value would corroborate a sorption mechanism based on electrostatic interactions described for previously published sorption data of cations (Cs+, Sr2+) on studtite.