Triple-ion anions and polynuclear complexing in supercritical electrolyte solutions

Abstract

Calculation of accurate mineral solubilities in hydrothermal solutions requires dissociation constants for the predominant aqueous complexes in solution, many of which may be present in appreciable concentrations only at high temperatures. Consideration of dissociation constants representing the standard molal Gibbs free energy changes resulting from the electrostatic interaction of aqueous species indicates that triple ions such as NaCl 2 − and Na 2Cl + may be major species in alkali metal halide solutions at high temperatures and low pressures where the dielectric constant of H 2O is ≲ 15. Molal triple ion dissociation constants were computed for temperatures from 400° to 800°C at pressures from 500 to 4000 bars using equations given by Fuoss and Kraus (1933b), together with values of the effective electrostatic radii of aqueous ions generated from algorithms developed by Tanger and Helgeson (1988) and Shock et al. (1990). The logarithms of the calculated second stepwise dissociation constants (log K 2) for 16 alkali metal and hydrogen halides typically decrease at 500 bars from ∼0 at 400° to ∼ −3 at 700°C, but they increase with increasing pressure from ∼ −3 at 500 bars to ∼0 at 3 kb and 700°C. Distribution of species calculations using these log K 2 values and dissociation constants for 1:1 ion pairs ( Oelkers and Helgeson, 1988), together with activity coefficients generated by adopting the revised HKF ( Helgeson, Kirkham, and Flowers, 1981) equation of state ( Tanger and Helgeson, 1988) to calculate extended term parameters in the Hückel equation ( helgeson et al., 1981) indicate that triple ions are the predominant species in low-pressure supercritical aqueous solutions at concentrations ≈ 0.8 m. Under these circumstances, polynuclear and mixed ligand complexes in natural electrolyte solutions such as KNaCl +, MgKCl 2 +, ZnNaCl 2 +, FeKCl 2 +, Na 2ClSO 4 −, etc. may have a substantial effect on the hydrothermal solubilities of minerals at high temperatures and low pressures where many geochemical processes occur.