Growth of calcium carbonate mediated by slowly released alginate

Abstract

The presence of additives has demonstrated strong effects on the crystallization and morphology of calcium carbonate (CaCO₃). To understand the mediating function of alginate on the growth of CaCO₃, we design a novel method to add alginate molecules and inorganic ions mildly and continuously to the mineralization system: a Ca-alginate gel is used as a slow-releasing source of calcium ions and alginate molecules; the gel is gradually broken down by the diffusion of CO₂ to the solution, inducing the slow release of Ca²⁺ and alginate molecules. The slowly released alginate is involved in the nucleation and growth of CaCO₃, in the form of micro-sized lens-like particles with a vaterite polymorph and composed of fused nanoparticles. With the increasing reaction time, the lens-like CaCO₃ particles gradually develop into a hollow structure and finally turn into ring-shaped CaCO₃, in which the polymorph of CaCO₃ remains vaterite. The formation of the lens-like particles is the result of the partially-oriented aggregation of primary nanoparticles mediated by alginate. The further evolution of the morphology to ring-shaped particles is due to a dissolution–recrystallization process as well as Ostward ripening.