On the induction time of CaCO₃: effect of ionic strength

This research investigated the induction times of CaCO3 as a function of the saturation andionic strength for synthetic solutions based on the Gulf of Oman seawater analysis. The investigationaimed to determine at elevated ionic strength levels, (i) the CaCO3 phase incorporatedin the Stiff and Davis saturation index and (ii) the mechanism of nucleation involved. Inductiontime experiments were performed with synthetic concentrates having different ionic strengthsof 0.054, 1.12, 1.34, 1.61 mole/l. Results showed that S&DSI incorporate the solubility of calciteat low ionic strength but vaterite for high ionic strength water. This expectation was confi rmedwith microscopic analysis of the formed crystals at the end of the induction time experiments(24 h) as vaterite was found in the solution instead of calcite. These fi ndings indicate that vateriteand not calcite maybe the precipitating phase in seawater. The study of the mechanism of nucleationshowed three different trends describing three different nucleation mechanisms; namelyhomogenous, heterogeneous and a one characterized by an intermediate surface tension. Thecalculations of the apparent surface energy showed values ranging from 15-94 mJ/m2 whichis consistent with literature data for vaterite. Results suggest that for the level of saturationreached in SWRO systems, the nucleation mechanism will most probably be heterogeneous.