Investigation of ultrasonication and/or mechanical stirring on the reactive crystallization of calcium carbonate at lower temperature and higher supersaturation condition

Abstract

Calcium carbonate (CaCO3, CC) distributes diversely among the abiotic and biogenic minerals/biominerals. It has six phases (three anhydrous crystalline, two hydrates and one amorphous). Phase selectivity of the synthetic CC is of considerable interest and determines its physicochemical properties and destinated applications.In the present study the reactive crystallization process of CC has been subjected to the treatment of different disturbances, i.e., mechanical stirring (ms), ultrasonication (us), and composed ms + us at lower temperature (0 ± 1℃) and higher supersaturation condition (initial concentration of Ca2+ is 0.1 M), in comparison with the quiescent condition. In order to gain insight on the role of these disturbances in controlling the crystallization process, nature, crystal habits of precipitated CC and their consecutive phase transformation, samples were collected at different time intervals (30 s, 1 min, 5 min, 10 min, 20 min, 45 min and 60 min) for investigation. Results reveals that these disturbing ways play a significant role on the CC crystallization process and can stabilize the amorphous calcium carbonate (ACC) phase for prolonged periods of time (1–60 min) in aqueous solution, which will be a powerful way for large-scale fabrication of pure ACC nanoparticles (about 100 nm) without addition of any chemicals. Longer disturbing time tends to transform the pure and spherical ACC nanoparticles competitively into spherical vaterite particles and rhombohedral calcite particles through the self-assembly of nanoparticles aggregation or Ostwald ripening mechanism, respectively. The investigation also suggested that the reactive crystallization of CC with the three disturbances undergoes a non-classical crystallization pathway to crystallize CC particles, in comparison with the quiescent reference condition. To the best of our knowledge, the report on reactive crystallization of CC crystallites as a function of different disturbances is first of its kind in the literature.