Darapskite solubility in basic solutions at 25 °C: A Pitzer model for the NaNO3SO4OHH2O system

- The complex NaNO3SO4OHH2O system is described over the whole range of concentrations.
- This chemical system is of interest for nuclear waste storage applications.
- The proposed model provides new ternary Pitzer interaction parameters.
- The solubility of darapskite and other salts is correctly reproduced in highly alkaline systems.
- Results of the model suggest new darapskite solubility measurements are needed at high pH.

We propose a set of Pitzer interaction parameters to describe the chemical behavior of the Na2SO4NaNO3NaOHH2O quaternary system at 25 °C. With these parameters we have been able to adequately predict the properties of Na2SO4NaNO3H2O, Na2SO4NaOHH2O and NaNO3NaOHH2O ternary and Na2SO4NaNO3NaOHH2O quaternary systems, including the solubility of different single salts and the double salt darapskite (Na2SO4·NaNO3·H2O), a mineral that is expected to form in the matrix of some radioactive waste containers. We also successfully reproduced the osmotic coefficient, the water activity and the mean activity coefficients for the Na2SO4H2O, NaNO3H2O and NaOHH2O binary systems. We obtained the set of Pitzer parameters using a step-by-step modeling approach. First, we investigated the binary systems: the NaOHH2O and Na2SO4H2O binary system interaction parameters were established in previous studies from heat capacity and osmotic coefficient data. Among the various chemical models proposed in the literature that describe the properties of the NaNO3H2O binary system, we chose the full dissociation hypothesis, since it allowed complete description of the system with the minimal number of interaction parameters. Next, from the mineral solubility data, we determined the specific interaction parameters for ternary systems. Without further parameterization, the solubility of the various salts in the quaternary system could be reproduced accurately.