Solubility of sodium chloride in superheated and supercritical water at 350-700°C and 25-250 bar

Abstract

The solubility of sodium chloride was measured in superheated and supercritical water at 350-700°C and 25-250 bar corresponding to water densities of ρH2O = 0.007-0.111 g/cm3. The measured concentrations were 0.3-115 ppm for Na and 0.8-173 ppm for Cl. The results at 400-600°C were interpreted in terms of formation of a hydrated NaCl(f) fluid species according to the reaction, NaCl(s) + nH2O(f) = NaCl*nH2O(f) and the results were fitted to a solvation model to give, log⁡cNaCl(f) = 3.50log⁡ρH2O(f) - 904.97/T + 6.94 where cNaCl(f) is the solubility of sodium chloride expressed in ppm (by weight) based on the measured Na concentration, ρH2O(f) is the density of pure water in g/cm3 and T is the temperature in Kelvin. The model used data taken at 400-600°C and 50-250 bar. The Cl/Na molal ratio was typically close to ~1.0 at temperatures <600°C consistent with the formation of NaCl(f) species whereas at higher temperatures (600-700°C) much higher Cl/Na ratios were observed or 1.6-9.0. These are considered to result from hydrolysis of NaCl(s) to form NaOH(s,lq) and HCl(f). The measured NaCl(f) solubilities were in good agreement with previous experimental studies as well as theoretical work on speciation of Na and Cl in high-temperature superheated and supercritical water at relatively low pressure. The results were used to estimate equilibrium NaCl(f) concentration in natural superheated and supercritical hydrothermal fluids considered to exist in the roots of active geothermal systems. The equilibrium NaCl(f) concentration in superheated water at shallow depth (~1 km or ~100 bar) at 400°C is predicated to be ~4 ppm whereas supercritical fluids at greater depth (~3 km or ~250 bar) at 400°C have much higher NaCl(f) concentrations of ~750 ppm. In comparison, the superheated IDDP-1 fluid discharges at Krafla had Na and Cl concentration of 0.05-1.5 ppm and 20-166 ppm, respectively (Ármannsson et al., 2014).