A new set of Pitzer parameters for dissolved oxygen and its implication on the U(IV)/U(VI) system.

Uranium as a major component in nuclear waste can form various mineral phases in mainly two oxidation states: +IV and +VI. For the modelling of different chemical aspects of a nuclear waste repository in salt rock, a thermodynamic database including Pitzer parameters for all relevant aqueous species is necessary. The latest THEREDA (Thermodynamic Reference Database) data release (no. 9) enables the modelling of uranium species in high saline solutions. The amount of dissolved oxygen is an important parameter for the description of redox-sensitive systems like U(IV)/ U(VI) in aqueous solution, namely when geochemical speciation codes and associated thermodynamic databases directly use oxygen as reaction partner for redox reactions. The O₂ solubility’s in aqueous solutions according to the reaction O₂(g) ↔ O₂(aq) follows Henry’s law (Henry’s law constant K_H expressed in mol/kg·101.35 kPa). The respective constant as well as a new set of Pitzer parameters for the solubility in solutions of the oceanic salt system (Na, K, Mg, Ca / Cl, SO₄, HCO₃/CO₃ - H₂O(l)) was deduced from data sets collected by IUPAC.