Study of bromide salts solubility in the (m1KBr + m2CaBr2)(aq) system at T = 323.15 K. Thermodynamic model of solution behaviour and (solid + liquid) equilibria in the ternaries (m1KBr + m2CaBr2)(aq), and (m1MgBr2 + m2CaBr2)(aq), and in the quinary (Na + K + Mg + Ca + Br + H2O) systems to high concentration and temperature

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Abstract

The bromide minerals solubility in the mixed system (m1KBr + m2CaBr2)(aq) have been investigated at T = 323.15 K by the physico-chemical analysis method. The equilibrium crystallization of KBr(cr), and CaBr2·4H2O(cr) has been established. The results from solubility measurements obtained have been combined with experimental equilibrium solubility data available in the literature at T = 298.15 K to construct a chemical model that calculates (solid + liquid) equilibria in the ternary (m1KBr + m2CaBr2)(aq) system. The solubility modelling approach based on fundamental Pitzer specific interaction equations is employed. Temperature extrapolation of the mixed system model provides reasonable mineral solubilities at low (273.15 K) and high temperature (up to 373.15 K). The reference solubility data for (m1MgBr2 + m2CaBr2)(aq) system, which are available in the literature at T = (273.15, 298.15, and 323.15) K are used to evaluate mixing ion interaction parameters and to develop a model that calculates (solid + liquid) equilibria in this ternary system. The models for both ternary systems give a very good agreement with bromide salts equilibrium solubility data. Limitations of the mixed solution models due to data insufficiencies at high temperature are discussed. The mixed system models presented in this study expand the previously published temperature dependent sodium–potassium–magnesium–bromide model by evaluating potassium–calcium–bromide and magnesium–calcium–bromide mixing solution parameters and by evaluating a chemical potential of double salt 2MgBr2·CaBr2·12H2O(cr), and complete the temperature dependent thermodynamic model of solution behaviour and (solid + liquid) equilibria in quinary system (Na + K + Mg + Ca + Br + H2O). The results of Pitzer ion interaction model-based thermodynamic studies on binary, and mixed systems within the (Na + K + Mg + Ca + Br + H2O) system have been summarised. Important thermodynamic characteristics {solubilities (ms), thermodynamic solubility products (as ln K°sp), standard molar Gibbs free energy of formation (ΔfG°m), deliquescence relative humidity (DRH)} of the bromide minerals crystallizing from the saturated binary and ternary solutions are given. Model predictions on ms, ln K°sp, ΔfG°m, and DRH are compared with those available in the literature. Model calculations are in excellent agreement with the reference experimental data and recommendations.

Highlights

► We study the solubility in the mixed system (m1KBr + m2CaBr2)(aq) at T = 323.15 K. ► We construct T-variable model for (m1KBr + m2CaBr2)(aq) and (m1MgBr2 + m2CaBr2)(aq). ► The solubility modelling approach based on Pitzer equations is employed. ► We validate the model for all subsystems within quinary {Na + K + Mg + Ca + Br + H2O} system. ► Model calculations are in excellent agreement with the reference data and recommendations.

Introduction

Computer models that predict solution behaviour and (solid + liquid + gas) equilibria close to experimental accuracy have wide applicability. Such models can be powerful predictive and interpretive tools to study the geochemistry of natural waters and mineral deposits, solve environmental problems and optimise industrial processes. The specific interaction approach for describing electrolyte solutions to 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 shown that the Pitzer approach could be expanded to calculate values of solubility 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].

Comprehensive thermodynamic models that accurately predict the solubility of bromide aqueous chemistry and bromide minerals as a function of composition of precipitates and solution concentration and to high temperature are critical for understanding many important geochemical, environmental and industrial processes. For example, the investigation of the solubility diagrams of bromide salts is of practical importance with a view to explain the distribution of bromide ions in natural evapourate deposits during crystallization of salts as a result of sea water evaporation and during treatment of natural deposits [16], [17]. The behaviour of bromide as a trace element in evapourating seawater and diagenetic reactions of evapourates is of prime importance for geochemical studies in marine chemical sediments. The conservative vertical distribution in the oceans (the high concentration of 0.84 mmol · kg−1 H2O [18]), and one of the highest residency times (from 1.0 · 108 [19], to 7.9 · 108 y [20]) in the ocean makes Br the most important trace element in chemically precipitated marine Cl sediments. The geochemical study of bromide is traditionally focused on its fractionation between brines and chloride salts, especially halite (NaCl(cr)) [21], [22], [23]. The results from the evaporation experiments for distribution of Br in the systems with precipitation of sylvie (KCl(cr)), kainite (KCl·MgSO4·3H2O(cr)), carnallite (KCl·MgCl2·6H2O(cr)) and bischofite (MgCl2·6H2O(cr)) minerals are also reported in the literature [24]. Geochemists have generally used the empirical distribution coefficient to predict Br contents in chloride minerals (salt) precipitating from brine solution. The distribution coefficient DBr is defined by ratios of Br:Cl as (see [25]):DBr=[(Br/Cl)salt]/[(Br/Cl)solution].

From a practical viewpoint, it is more useful to predict Br content on the basis of thermodynamic equilibrium than to use an empirical DBr coefficient, and therefore there are many works on theoretical discussions and stoichiometric saturation in (solid solution + aqueous solution) systems [25], [26], [27], [28], [29]. All these equilibrium models are aimed at the correct determination of activity coefficients of bromide {γ(Br)} and chloride {γ(Cl)} ions in brine solutions and of the thermodynamic solubility product of solid-solution’s end-members (for example ln K°sp {KCl(cr)}, and ln K°sp {KBr(cr)} for the bromide distribution in the liquid phase and in the solid solutions {K(Cl,Br)(cr)} precipitating in mixed system (m1KCl + m2KBr)(aq)).

Recent field observations show that hydrothermal waters can be strongly enriched with bromide relative to seawater [30], [31], [32], [33]. Bottomley et al. [32] explain the elevated Br/Cl ratios in subsurface waters by evaporation beyond halite saturation. Leybourne and Goodfellow [33] suggested that “elevated Br/Cl ratios of saline waters compared to sea-water may be explained by differential uptake of Br and Cl during groundwater evolution through water–rock reaction”.

In the late 1990s, the role of halogen species, especially bromide, in the ozone layer depletion became evident. Honninger et al. [34] measured a significant emission of bromide from the giant salar de Uyuni in the Bolivian Altiplano. Risacher et al. [35] measured a significant release of bromide from surface waters and brines of Central Andes. Vogt et al. [36] described a mechanism of halogen release from sea-salt aerosols. On the basis of chemical kinetic experiments on the OH uptake by complex sea-salt and sea-type mixtures and thermodynamic modelling studies, Park et al. [37] concluded that surface composition determines to a large extent the heterogeneous reactivity of inorganic salt mixtures at high humidity conditions. It was also established that at wet conditions the surface of sea-salt aerosols is enriched by sea-salt components with lowest deliquescence. The last experimental observations [38], [39] also show that there is an asymmetrical distribution of the halide (Cl, Br) ions in solid aerosol bulk and in the liquid air–water aerosol interface: the concentration of bromide ion in the surface area is much higher than in the bulk solid aerosol. The questions which arise are: (1) what is the effect of bromide on the deliquescence of sea salt minerals and of the complex sea-salt, and (2) to what extent bromide behaves conservatively in geochemical and aerosol formation processes? Other important applications of the bromide models 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 lithium and other high soluble evapourate minerals, and utilisation of waste solutions during treatment of natural brine-type deposits.

This paper continues our series concerning experimental solubility and isopiestic measurements, and parameterization of thermodynamic (solid + liquid) equilibrium models of high soluble bromide minerals precipitating within the (Li + Na + K + NH4 + Rb + Cs + Mg + Ca + Cl + Br + SO4 + H2O) system [16], [17], [40], [41], [42], [43], [44], [45], [46]. In the previous studies, we described a temperature dependent model that accurately calculates activities in unsaturated binary solutions NaBr(aq) [41], KBr(aq) [41], MgBr2(aq) [42] and CaBr2(aq) [43], and accurately predicts (solid + liquid) equilibria in bromide binary systems, and in ternary (m1NaBr + m2KBr)(aq) [41], (m1NaBr + m2MgBr2)(aq) [44], (m1KBr + m2MgBr2)(aq) [42], and (m1NaBr + m2CaBr2)(aq) [43] systems. The only two models for bromide systems, which are not available to complete the temperature-variation thermodynamic database for quinary bromide sea-type system (Na + K + Mg + Ca + Br + H2O) are the models for ternaries (m1KBr + m2CaBr2)(aq) and (m1MgBr2 + m2CaBr2)(aq). This is the main goal of the present study.

In this article, the bromide minerals solubility in the mixed system (m1KBr + m2CaBr2)(aq) system have been investigated at T = 323.15 K by the physico-chemical analysis method. The results from solubility measurements obtained have been combined with experimental equilibrium solubility data available in the literature at T = 298.15 K to construct a chemical model that calculates (solid + liquid) equilibrium in the ternary (m1KBr + m2CaBr2)(aq) system. The reference solubility data for (m1MgBr2 + m2CaBr2)(aq) system available from T = (273.15 to 323.15) K are used to evaluate mixing parameters of Mg–Ca, and Mg–Ca–Br interactions and to develop a model that calculates (solid + liquid) equilibria in this ternary system. The solubility modelling approach based on fundamental Pitzer specific interaction equations is employed. Temperature extrapolation of the model for the mixed system (m1KBr + m2CaBr2)(aq) provides reasonable values of mineral solubility at low (273.15 K) and high temperature (up to 373.15 K). The models for both ternary systems give very good agreement with bromide salts equilibrium solubility data. Limitations of the mixed models due to data inadequacies at high temperature are discussed. The results of Pitzer model-based thermodynamic studies on binary, and mixed systems within the quinary (Na + K + Mg + Ca + Br + H2O) system have been summarised. Important thermodynamic characteristics {solubilities (ms), thermodynamic solubility products (as ln K°sp), standard molar Gibbs free energy of formation (ΔfG°m), deliquescence relative humidity (DRH)} of the bromide minerals crystallizing from the saturated binary and ternary solutions are given. Model predictions on ms, ln K°sp, ΔfG°m, and DRH are compared with those available in the literature.

Section snippets

Experimental

The solubility data in the (m1MgBr2 + m2CaBr2)(aq) system are available in the literature within the temperature range from T = (273. 15 to 323.15) K [47]. The equilibrium solubility data in the mixed (m1KBr + m2CaBr2)(aq) system were found in the literature only at T = 298.15 K [47]. The ternary systems solubility data are critical for developing a temperature variable ion-interaction model, which describe (solid + liquid) equilibria in the (Na + K + Mg + Ca + Br + H2O) system. To extend with temperature the

Modelling approach

The model presented here incorporates the concentration-dependent specific interaction equations of Pitzer [1], [2] for aqueous solutions. Since the Pitzer representation of the aqueous phase is based on the excess Gibbs free energy, all the activity expressions are consistent, allowing different kinds of data [e.g., activity {(water activity (aw); osmotic (φ), and activity (γ±) coefficients}, voltage, and solubility measurements] to be used in the parameter evaluations and other thermodynamic

Evaluation of parameters in the system (m1KBr + m2CaBr2)(aq) system

The solubility data for ternary system (m1KBr + m2CaBr2)(aq) are used to evaluate temperature variable ψK,Ca,Br mixing parameter. The data obtained in this study at T = 323.15 K (table 1) are combined with those available in the literature to develop the (solid + liquid) equilibria model for the mixed system. The activity data for mixed solutions cannot be found in the literature. The reference solubility data are available only at T = 298.15 K. For the ternary system (m1KBr + m2CaBr2)(aq), the data given

Conclusions

Several important conclusions can be made on the basis of activity {figure 4(a) to (d) and table 4} and DRH/MDRH calculations {figures 6(a) to (d) and 7, and table 4} presented here and in our previous studies:

  • (i)

    The excellent agreement between DRH model predictions and raw data in binary bromide systems {figure 6(a) to (d)} proves that over the wide range of temperatures from T = (273.15 to 373.15) K the deliquescence behaviour of bromide minerals is determined in the highest degree by their

Acknowledgments

This work has been supported by National Science Fund of the Bulgarian Ministry of Science and Education (Grant No. DO 02-243).

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