The growth of CaCO3 polymorphs in the presence of As(v): stabilization of the vaterite phase

Abstract

Calcium carbonate (CaCO₃) polymorphs are some of the most abundant minerals in natural environments on the Earth's surface. They are normally linked to fields including biomineralization, global CO₂ exchange or pollutant remediation due to the strong surface interaction with heavy metals in the environment. The aim of this work is to study the crystallization of CaCO₃ through precipitation experiments from aqueous solutions in the presence of different amounts of As(V), thus evaluating the capacity of the precipitating phases to remove As from solutions. Surprisingly, the results confirmed that, although the uptake mechanism operates relatively well, decreasing the initial concentration of arsenic in all the experiments conducted, the wonder is that the presence of this element controls the crystallization of calcium carbonate polymorphs by inhibiting the crystallization of calcite and stabilizing the vaterite, which is the least stable phase among those two. The combination of several techniques allowed us to confirm the increased uptake of As by precipitates from increased As-bearing supersaturated solutions. The gradual disappearance of calcite and the persistence of vaterite from precipitates in which As is present suggest that a potential incorporation of As into a crystalline CaCO₃ polymorph would be more likely in the vaterite. Even though the sequestering mechanism remains unclear, vaterite lattice distortions suggest that As could be absorbed inside the vaterite structure. Additionally, adsorption on the unstable polymorph would be the reason for the stabilization, by preventing its dissolution and therefore its transformation into calcite.