The effect of solution chemistry on nucleation of nesquehonite

Abstract

Several series of nesquehonite nucleation experiments (62 experiments in total) were conducted in aqueous solutions having Mg²⁺/CO₃²⁻ activity ratios (here referring to log(a_Mg²⁺/a_CO3²⁻)) ranging between −0.96 to 2.89, but different saturation states (Ω ranging between 2.49–6.90) and solution pH. The goal was to understand the effect of solution chemistry on the nucleation of nesquehonite. Our experimental results show that induction-time estimates from our precipitation experiments with similar Mg²⁺/CO₃²⁻ activity ratios are consistent with classical nucleation theory (CNT), while the surface energy derived from CNT varies with Mg²⁺/CO₃²⁻ activity ratios. The induction times of nesquehonite nucleation are scattered noticeably when the saturation state of solution is low (Ω < 4), and the nuclei surface energy, derived from the relationship between induction time and saturation state of solution, increases with increasing Mg²⁺/CO₃²⁻ activity ratios. These observations can be explained by the different absorption behaviors of Mg²⁺ and CO₃²⁻ and/or reduced Gibbs free energies through better screening of the electric double layer. A surface energy model involving solution composition is developed that combines surface complexation with electrostatic models. This new model takes into account how surface charge may affect surface energy. This model implies that the highest surface energy may occur around the point of zero charge (p.z.c), where the nucleation is fastest (or conversely, where the induction time is shortest) under low saturation states, but not under high saturation states. An accelerated attachment kinetic of monomers is also expected at the p.z.c. where high energy surface requires surface absorbed ions to have higher reactivity. This study provides deeper insight into mechanisms of nesquehonite nucleation in nature, and guidelines for accelerating the precipitation rates of nesquehonite.