The prediction of mineral solubilities in natural waters: the NaKMgCaClSO 4H 2O system from zero to high concentration at 25° C

Abstract

A chemical model of the seawater system, NaKMgCaClSO 4H 2O, is developed for predicting mineral solubilities in brines from zero to high ionic strengths. The calculated solubilities are shown to be in agreement with the experimental data from gypsum saturation ( I < 0.06 m) to bischofite saturation (I > 20 m). The model utilizes activity coefficient expressions recently developed by Pitzer and co-workers and an algorithm for rapidly identifying the coexisting phases and their composition at equilibrium. The activity coefficient expressions are parameterized using binary and ternary system solubility and osmotic data. The results indicate that a free energy model defined by binary and ternary system data will accurately predict solubilities in more complex systems. The algorithm for solving the general chemical equilibrium problem is briefly discussed. The method can be used to model systems with an arbitrary number of possible non-ideal solution phases. The iterative procedure is guaranteed to converge and has been found to be efficient and easy to implement. Calculated phase diagrams associated with the seawater system are compared to experimental data. Our calculations are within experimental accuracy whereas the prediction of other seawater models are in substantial disagreement with the data even at low concentration. The calculation of evaporation sequences is also briefly discussed and qualitatively compared to field data. The mineral assemblages predicted by this method are in substantially better agreement with core samples than the sequences predicted by phase diagram methods ( Braitsch, 1971), which do not explicitly include the Ca component.