Calcite solubility in Na–Ca–Mg–Cl brines

This study investigated calcite solubility approached from undersaturation in geologically relevant Na–Ca–Mg–Cl synthetic brines (35 to 200 g l− 1 TDS) at CO2 partial pressures of ∼ 0.1, 0.5 and 1.0 bar at 25 °C. Total carbon dioxide content, total alkalinity and total calcium content were measured and the carbonic acid species distribution and ion activities calculated using the Pitzer equation-based program EQPITZER. Stoichiometric (pKsp⁎) equilibrium constants ranged from 6.4 to 4.8, decreasing with increasing brine TDS. In general, good agreement was found between measured and predicted solubility data.Evaluation of the Ca2+ and CO32− activity coefficients with EQPITZER in brines < 100 g l− 1 TDS yields a mean value of the thermodynamic pKsp = 8.41 ± 0.12 in good agreement with literature data (8.48 ± 0.02). However, in more concentrated brines the values systematically decreased to 8.11 ± 0.02 in the most calcium-rich brines. The deviation was strongly correlated with calcium ion activity and was also observed in magnesium-free solutions. The error can be interpreted as the result of an uncertainty in the calculated activity of the carbonate ion, which occurs in exceedingly low concentrations in these calcium-rich, high pCO2 brines. As a result, calculated calcite saturation states may erroneously suggest in excess of a 2-fold supersaturation in the most concentrated brines. This should be a consideration in kinetic studies where precise knowledge of near-equilibrium saturation states is critical in evaluating reaction rate constants and reaction orders.