Isopiestic Determination of the Osmotic Coefficients of an Aqueous MgCl₂ + CaCl₂ Mixed Solution at (25 and 50) °C. Chemical Equilibrium Model of Solution Behavior and Solubility in the MgCl₂ + H₂O and MgCl₂ + CaCl₂ + H₂O Systems to High Concentration at (25 and 50) °C

Abstract

This paper, written in celebration of Professor Robin H. Stokes' 90th birthday anniversary, describes isopiestic measurements and the development of a thermodynamic model for the highly important MgCl 2 + CaCl 2 + H 2 O system at (25 and 50) °C. The isopiestic method has been used to determine the osmotic coefficients of the mixed solutions from low to high concentrations. Sodium chloride and sulfuric acid solutions have been used as isopiestic reference standards. The isopiestic results obtained have been combined with all other experimental thermodynamic quantities available in the literature (osmotic coefficients, water activities, calcium and magnesium chloride mineral's solubilties) to construct a chemical model that calculates solute and solvent activities and solid-liquid equilibria in the MgCl 2 + H 2 O and MgCl 2 + CaCl 2 + H 2 O systems. The mixed solution model created here used CaCl 2 + H 2 O system parametrization established in our previous study (Christov and Moller, GCA 2004b, 68, 3717). The model incorporates the concentration-dependent specific interaction equations of Pitzer for aqueous solutions (J. Phys. Chem. 1973, 77, 268). The model for the binary MgCl 2 + H 2 O system is validated by comparing activity coefficient predictions with those given in the literature and not used in the parametrization process. The model predictions on the effect of mixing on the deliquescence relative humidity in MgCl 2 + CaCl 2 + H 2 O solutions saturated with highly soluble MgCl 2 • 6H 2 O(cr), CaCl 2 • 6H 2 O(cr), and 2MgCl 2 • CaCl 2 • 2H 2 O (cr) sea-salt minerals are also given.

Isopiestic investigation of the osmotic coefficients of MgBr2(aq) and study of bromide salts solubility in the (m1KBr+m2MgBr2)(aq) system at T=323.15K. Thermodynamic model of solution behaviour and (solid+liquid) equilibria in the MgBr2(aq), and (m1KBr+m2MgBr2)(aq) systems to high concentration and temperature

“…Construction of a comprehensive thermodynamic model for bromide brine system that describes solution behaviour and bromide minerals solubility to high temperature can be a powerful predictive tool to solve the geochemical and environmental problems. This paper continues our series concerning experimental solubility and isopiestic measurements, and parameterization of comprehensive XTP-variable thermodynamic (solid + liquid) equilibria models of highly soluble chloride, and hydroxide [10][11][12][13][14][15][16]42] and bromide [19][20][21][22]43,44] minerals precipitating within (H + Li + Na + K + Mg + Ca + Al + Fe(II) + Fe(III) + OH + Cl + Br + HSO 4 + SO 4 + H 2 O) system. Other important applications of the bromide model presented here include design and assessment of nuclear and acid mine waste disposal strategies, development of high concentration halide leaching processes and water desalination, as well as production of evaporate minerals and utilization of waste solutions during treatment of natural deposits.…”

“…The osmotic coefficients of the MgBr 2 (aq) solutions were determined by the isopiestic method described in detail in many publications [51,12,19]. Sodium chloride solutions were used as isopiestic reference standards.…”

“…Model validation involves comparison of model predictions with data not used in parameter evaluation process. In constructing the temperature variable model for MgBr 2 (aq) and (m 1 KBr + m 2 MgBr 2 )(aq) systems, the solubility approach described previously is used [4,6,7,[9][10][11][12][13][14][15][16]. Following this approach, activity data for unsaturated solutions are used when available to evaluate solution parameters for low and moderate bromide concentration.…”

“…They are the binary solution model parameters, b (0) , b (1) , and C u , for the Mg-Br ion interactions (see table 3 Following the approach in our previous studies [10][11][12][13] to avoid overlapping effect of pure electrolytes and mixing solution parameters, the model for the binary MgBr 2 (aq) system was developed using only the data for this binary system. The mixed solutions model was parameterized on the basis of the data only for the (m 1 KBr + m 2 MgBr 2 )(aq) system.…”

“…The specific interaction approach for describing electrolyte solutions at high concentration introduced by Pitzer [1,2] represents a significant advance in physical chemistry that has facilitated the construction of accurate thermodynamic models. It was showed that the Pitzer approach could be expanded to calculate solubilities accurately in complex brines and to predict the behaviour of natural fluids [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Comprehensive thermodynamic models that accurately predict the solubility of bromide aqueous chemistry and bromide minerals as a function of composition from low to high temperature are critical for understanding many important geochemical processes [17][18][19][20].…”