Solubility of silica polymorphs in electrolyte solutions, I. Activity coefficient of aqueous silica from 25° to 250°C, Pitzer's parameterisation |
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| Authors: | M Azaroual C Fouillac J M Matray |
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| Institution: | BRGM Direction de la Recherche, B.P. 6009, 45060, Orléans Cedex 2, France |
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| Abstract: | Within the framework of Pitzer's specific interaction model, interaction parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solubility measurements. The values, at 25°C, of the Pitzer interaction parameter (λSiO2(aq)?i) determined in this study are the following: 0.092 (i = Na+), 0.032 (K+), 0.165 (Li+), 0.292 (Ca2+, Mg2+), ?0.139 (SO42?), and ?0.009 (NO3?). A set of polynomial equations has been derived which can be used to calculate λSiO2(aq)?i for these ions at any temperature up to 250°C. A linear relationship between the aqueous silica-ion interaction parameters (λSiO2(aq)?i) and the surface electrostatic field (Zi/re,i) of ions was obtained. This empirical equation can be used to estimate, in first approximation, λSiO2(aq)?i if no measurements are available. From this parameterisation, the calculated activity coefficient of aqueous silica is 2.52 at 25°C and 1.45 at 250°C in 5 m NaCl solution. At lower concentrations, e.g. 2 m NaCl, the activity coefficient of silica is 1.45 at 25°C and 1.2 at 250°C. Hence, in practice, it is necessary to take into account the activity coefficient of aqueous silica (λSiO2(aq)≠1) in hydrothermal solutions and basinal brines where the ionic strength exceeds 1. A comparison of measured Marshall, W.L., Chen, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solubility behaviour in aqueous mixed electrolyte solutions to 300°C. Geochim. Cosmochim. Acta 46, 289–291.] and computed amorphous silica solubility, using this parameterisation, shows a good agreement. Because the effect of individual ions on silicate and silica polymorph solubilities are additive, the present study has permitted to derive Pitzer interaction parameters that allow a precise computation of γSiO2(aq) in the Na---K---Ca---Mg---Cl---SO4---HCO3---SiO2---H2O system, over a large range of salt concentrations and up to temperatures of 250°C. |
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| Keywords: | activity coefficient Pitzer's equation aqueous silica interaction parameters silica polymorphs solubility |
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