Jean-Paul Serin

Thermal and volumetric properties of complex aqueous electrolyte solutions using the Pitzer formalism - The PhreeSCALE code

The thermal and volumetric properties of complex aqueous solutions are described according to the Pitzer equation, explicitly taking into account the speciation in the aqueous solutions. The thermal properties are the apparent relative molar enthalpy ( L ? ) and the apparent molar heat capacity ( C p , ? ). The volumetric property is the apparent molar volume ( V ? ). Equations describing these properties are obtained from the temperature or pressure derivatives of the excess Gibbs energy and make it possible to calculate the dilution enthalpy ( ? H D ), the heat capacity ( c p ) and the density (?) of aqueous solutions up to high concentrations. Their implementation in PHREEQC V.3 (Parkhurst and Appelo, 2013) is described and has led to a new numerical tool, called PhreeSCALE. It was tested first, using a set of parameters (specific interaction parameters and standard properties) from the literature for two binary systems (Na2SO4-H2O and MgSO4-H2O), for the quaternary K-Na-Cl-SO4 system (heat capacity only) and for the Na-K-Ca-Mg-Cl-SO4-HCO3 system (density only). The results obtained with PhreeSCALE are in agreement with the literature data when the same standard solution heat capacity ( C p 0 ) and volume (V0) values are used. For further applications of this improved computation tool, these standard solution properties were calculated independently, using the Helgeson-Kirkham-Flowers (HKF) equations. By using this kind of approach, most of the Pitzer interaction parameters coming from literature become obsolete since they are not coherent with the standard properties calculated according to the HKF formalism. Consequently a new set of interaction parameters must be determined. This approach was successfully applied to the Na2SO4-H2O and MgSO4-H2O binary systems, providing a new set of optimized interaction parameters, consistent with the standard solution properties derived from the HKF equations. We described the apparent properties of saline solutions with the Pitzer formalism.We implemented these calculations in the Phreeqc geochemical calculation code.PhreeSCALE is tested on several systems.We used PhreeSCALE to revise the interaction parameters for these two systems.