Isopiestic studies of thermodynamic properties and representation with ion-interaction model for Li2B4O7-MgCl2(B)-H2O system at 298.15 K

The isopiestic molalities and the water activities have been measured by isopiestic method and the water vapor pressures and the osmotic coefficients have been calculated from the measurements in the ion-strength range from 0.0581 mol(.)kg(-1) to 0.6320 mol(.)kg(-1) at T = 298.15 K for the system Li-Mg-Cl-borate-H2O. The formation reaction equilibria of boron species of boric acid (H3BO3) and borate anions [B(OH)(4)(-), B3O3(OH)(4)(-) and B4O5(OH)(4)(2-)] have been taken into account for the system in different total boron concentration ranges. A simplified model based on Pitzer's osmotic coefficient equations with minor modifications has been proposed by combining the same kinds of ioninteractions related to the different borate anions, which reduced the number of ion-interaction parameters from 52 in the primary model to 23 in the simplified model. The ion-interaction parameters among the groups of borate anions, Cl-, Li+ and Mg2+ for the simplified model have been estimated using the experimental data taken from this work and references. The osmotic coefficient values determined experimentally and those calculated by using the model with the parameters are in reasonable agreement. Pitzer's ion-interaction model has been extended to represent the thermodynamic properties of the complex system containing various borate anions. These studies are useful to improve the thermodynamic model for salt lake brine systems containing lithium and borates.