Contributory roles of concentration and pH in CaCO3 growth inhibition by additive Ca2+ ions

CaCO3 particles grow excessively upon chemical precipitation in the absence of impurities or growth inhibitors. Additive Ca2+ ions have been shown to preferentially adsorb on CaCO3 precipitates, effectively inhibiting their growth and promoting the crystallization of pure calcite without an observable intermediate phase. This phenomenon can be adapted towards the synthesis of small calcite particles from a conventional chemical precipitation method. Complementing such effort, this study discusses the influence of additive Ca2+ ions concentration and solution pH on the extent of CaCO3 growth inhibition. Equal volumes of equimolar CaCl2 and Na2CO3 solutions were mixed in a tubular reactor at a constant flowrate. The precipitates were continuously dispersed in Ca(OH)2 solution, where Ca2+ ions irreversibly adsorb on their surfaces. Compared to conditions where additive Ca2+ ions are absent, this method can produce more than 90% decrease in particle size. The results show the degree of growth inhibition increases as the concentration of additive Ca2+ ions increase. However, it is limited by increasing volume of precipitates. This study also reveals an unusual role of media pH. Here, growth inhibition that leads to the synthesis of monodisperse submicron CaCO3 particles is only observed in high alkaline pH conditions. This is due to the hydration of additive Ca2+ ions in low pH conditions. While additive Ca2+ ions adsorb on CaCO3 precipitates in pH conditions above the isoelectric point (pH ≈ 9), their ability to limit CaCO3 growth diminishes when pH < 12.