Enhanced nucleation of aragonite on zinc metal beads from hot drinking water supersaturated with calcium carbonate

Precipitation of calcium carbonate is a spontaneous but annoying phenomenon when boiling drinking water of even moderate hardness. While conventional scale inhibition methods focus on chemical additives, this study introduces a novel template-assisted crystallization (TAC)-like approach using zinc metal beads to fundamentally alter CaCO3 nucleation and growth dynamics, which is promising for scale formation control. Experiments were conducted by contacting boiled hard water immediately with beads of different material. Results showed that the presence of zinc beads with shaking could substantially reduce the sizes and change the morphology, and thus increase the number of precipitated particles in the water. This was a big contrast to glass beads or aluminum beads. The use of zinc beads would not substantially change the total amount of precipitates, or the crystalline polymorph, with most of the calcium carbonate precipitates being aragonite. Quantum chemistry calculation showed that both CO3 2- and Ca2+ ions could be facilely adsorbed onto the surface ZnO layer of bead, with adsorption energy being - 2.11---- 1.82 and - 0.34---- 0.25 eV, respectively, while the adsorption of CO3 2- ions onto glass bead surface was not thermodynamically favorable. The favoring of ion adsorption indicates that the surface atoms of zinc beads could act as nucleation templates for calcium carbonate. The grown microcrystals could be shed off from the bead surface under the water current turbulence, though some would reside on the bead surface thus hindering further functioning.