Precipitation and in-situ surface modification of calcium carbonate in synthetic seawater: Polymorph control, crystallization kinetics, and hydrophobic vaterite preparation

This work aims at achieving the high-value application of calcium in the meantime of seawater decalcification. By utilizing synthetic seawater as a calcium source and Na2CO3/NaHCO3 solution as the supplier of carbonate, aragonite or vaterite with a high purity exceeding 99% was successfully synthesized. The process was performed without adding chemical additive or introducing irradiating ultrasound which is usually necessary in the existing methods to control the polymorph. Individual and combined effects of coexisting ions in synthetic seawater on the crystal polymorph and crystallization kinetics were investigated systematically for the first time in the present work. It was found that the presence of Mg2+, K+ or SO42-in synthetic seawater was favorable for the precipitation of aragonite, and Mg-free synthetic seawater favored the formation of vaterite CaCO3. In addition, hydrophobic spherical vaterite particles were successfully prepared by in-situ surface modification using alka-nolamide 6502 as a surfactant. The results of this study demonstrated a promising approach to enhance the utilization of calcium during the seawater decalcification process.