Mikhail Mironenko

Thermodynamic Properties of NaCl Solutions at Subzero Temperatures

Heat capacities at infinite dilution of NaCl (aq) for the temperature range 0 to −25°C and apparent molar volumes at infinite dilution for 0 to −15°C have been estimated from a synthesis of experimental data collected at subzero temperatures. The parameters of the Helgeson–Kirkham–Flowers (HKF) equation for Na+ (aq) have been obtained, from which the Gibbs energies of Na+ and Cl− have been calculated. The estimated values of Pitzer-equation parameters for thermal and activity-coefficient properties have been adjusted for subzero temperatures. The experimental phase diagram for the NaCl–H2O system could be reproduced with these data, demonstrating the low-temperature applicability of the HKF model to extrapolate thermodynamic properties of aqueous-solution species at infinite dilution.

Equilibrium-kinetic model of water-rock interaction

A computer model was developed for chemical interaction in water-rock systems. The model is based on the concept of partial equilibrium [1] and combines the calculation of chemical equilibria in multicomponent systems with accounting for the kinetics of the congruent dissolution of minerals as a function of pH (zeroth order kinetic reactions). The development of the process in time is simulated as a series of sequential partial equilibria, and the bulk chemical composition of the system is calculated at each time step from the chemical composition of aqueous solution at the beginning of the step and masses of minerals dissolved during time Δt. The dissolution rates of individual minerals are calculated at each time step for the given temperature, current pH value, and the degree of solution saturation with respect to minerals. Variations in the surface area of minerals due to precipitation and dissolution are accounted for. Model application is exemplified by the calculation of chemical equilibria in the water-granite system. The model may be useful for understanding the character of low-temperature interactions in water-rock systems under stagnant conditions, in particular, the multistage development of groundwater chemistry, interaction of liquid radioactive waste injected into underground repositories, etc.

Calculation of densities of aqueous electrolyte solutions at subzero temperatures

We developed a FORTRAN program based on the Pitzer equations to calculate densities of electrolyte solutions at subzero temperatures. Data from the published literature collected at -28.9, -17.8, -12.2, -6.7, 0, and 25°C were used to calculate the Pitzer-equation parameters and to evaluate model performance. Three approaches to estimating the molar volume of the solute at infinite dilution were evaluated: (1) extrapolation of apparent molar volumes to zero square-root ionic strength; (2) calculation with the Tanger and Helgeson model; and (3) global fit of the data in which the molar volume of the solute at infinite dilution was estimated along with the Pitzer-equation parameters. The last approach gave parameter estimates that reproduced the experimental data most accurately. The parameterized model predicted accurately densities of single-electrolyte and multielectrolyte solutions at -28.9, -17.8, -12.2, -6.7, 0, and 25°C. Available experimental data are generally quite poor. Accordingly, Pitzer-equation parameters estimated for subzero temperatures should be viewed as conditional until improved measurements of single-electrolyte solution densities at subzero temperatures are made.