International audienceThe 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_ϕ) and the apparent molar heat capacity (C_(p,ϕ)). The volumetric property is the apparent molar volume (V_ϕ). 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 (ρ) 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 (V^0) 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

This article is a contribution to the modeling of the thermodynamic properties of 15 lanthanide (including lanthanum)-nitrate aqueous binary solutions, from low molalities to saturation (for Ce, Gd, Tb, and Tm) and supersaturation (for La, Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb, and Lu) with respect to the corresponding trinitrate lanthanide solid salts. A critical compilation of the experimental and previous modeling data available in the literature is presented. The modeling approach is based on the standard Pitzer formulation for strong aqueous electrolytes, with two cases considered: three (β(0), β(1), and Cφ ) or five (β(0), β(1), β(2), α2, and Cφ ) parameters. The best fits are obtained for the latter case, leading to the representation of unprecedented accuracy for the osmotic and activity coefficients over the whole range of experimental data up to the saturation/supersaturation points. On the basis of the models developed, the thermodynamic solubility product constants and the standard molar Gibbs free energy of formation of precipitating hexahydrate or pentahydrate lanthanide–nitrate solids are calculated.

Experimental CO 2 solubility data in brine at high pressures and high temperatures are needed in different technologies such as carbon dioxide storage or geothermal process. A lot of data have been acquired in single-salt solutions, whereas data for mixed-salt solutions remain scarce. In this study, new carbon dioxide solubility data in salt solutions have been measured. Two synthetic brines have been studied at 323, 373, and 423 K from 1 to 20 MPa. The brine 1 is composed of a mixture of NaCl and CaCl 2 and the brine 2 is made from a mixture of NaCl, CaCl 2 , and KCl. Measurements have been carried out by conductimetric titration. In this study, 6 isotherms presenting 48 new solubility data have been reported. These results have been obtained in an original range of temperature, pressure, and salinity. In these conditions of temperature and pressure, we verified that an increase of the temperature or the salinity involves a decrease of the CO 2 solubility. On the other hand, an increase of the pressure implies an increase of the CO 2 solubility. Then, the obtained results were compared with the values calculated using PhreeSCALE and PSUCO2 models. The comparison between experimental and calculated values revealed a good agreement.

A new set of Pitzer interaction parameters is proposed to describe the excess properties of the H−Na−K−Ca−Mg−NO 3 − H 2 O system at 298.15 K. From these parameters we reproduce the osmotic coefficient, mean activity coefficients, and density for the binary systems, and also salt solubility in the ternary subsystems. The binary interaction parameters are either selected from the literature or redetermined in this study. In the case of KNO 3 −H 2 O and HNO 3 − H 2 O binary systems, partial electrolyte dissociation has to be taken into account to represent accurately all the experimental data up to very high concentrations. Then solubility data of salts in the corresponding ternary systems are used to determine the ternary interaction parameters and the solubility products of the double salts. As KNO 3 and HNO 3 are partially dissociated, we have determined new interaction parameters involving neutral species (for instance η KNO 3 /Na + /H + or μ HNO 3 /KNO 3 /Na + ) to draw the phase diagram of quaternary systems Na−K−H−NO 3 . To complement the study, we also propose a set of volumetric Pitzer interaction parameters for all the binary systems and for the H−K−NO 3 ternary system, so solution density can be computed at 298.15 K.

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