CaCO3 Crystallization Influenced by Additives and Templates.

Abstract

Carbonate mineralization is a crucial topic for underground CO2 storage and is instrumental for understanding the fundamental thermodynamics of crystallization. This thesis studies both the nucleation process and the development of polymorphs and morphologies of CaCO3 precipitates in the presence of ammonium and on self-assembled monolayers (SAMs), in order to gain insight into how additives and templates control the mineralization. Three polymorphs of CaCO3 exist, and calcite is the most stable one while vaterite is the least stable one. However, if ammonium is present in the growth solution and its concentration is higher than a certain threshold (0.02 mol/L in this study), the formation of vaterite is strongly promoted and the vaterite crystals. Fourier transform spectroscopy studies further suggest that ammonium is incorporated into vaterite but not into calcite, implying that ammonium may serve as growth inhibitor of calcite. A semi-quantitative calculation based on experimental analysis demonstrates that the substitution of ammonium into vaterite structure is up to about 0.1 wt%. The vaterite grains induced by ammonium are polycrystals, displaying a hexagonal layered morphology. The study of the development of this type of vaterite demonstrates that vaterite has a different growth mechanism than calcite. The growth of calcite is a layer-by-layer mechanism, governed by the addition of a single calcium and carbonate ions on kink sites along the growth step. However, vaterite development lacks this organized molecular attachment scheme. Instead, it proceeds by the aggregation of nano-clusters of a couple of nanometers. As crystallization continues, the nano-clusters adjust their relative crystallographic orientations and align with each other into a more parallel arrangement. The thermodynamic and kinetic barriers of calcite nucleation were evaluated on two types of SAMs, monolayers of 16-mercaptohexadecanoic acid and 11-mercaptoundecanoic acid. In addition, whether the nucleation occurs via amorphous precursors during templating was analyzed. The experimental results reveal that (1) the two SAMs both significantly reduce the effective surface energy of calcite from about 97 mJ/m2 in solution to about 80 mJ/m2 on both SAMs; and (2) at solute activities below the solubility limit of amorphous calcium carbonate, calcite forms directly without using ACC as a precursor.