Controlling calcium carbonate crystallisation

Abstract

The work reported here has focused on short chain carboxylic acids and their respective carboxylate species as potential modifiers of CaCO3 crystallisation. At a macro-scale, the morphology of CaCO3 crystals crystallised in the presence of malonic acid has been probed using a dynamic non-equilibrium open system: the vapour diffusion method, as a model for biomineralising systems. Using this method a systematic investigation of the effects of solution pH, pCO2, [Ca2+] and [malonic acid] on CaCO3 nucleation and growth has been undertaken. These results have been placed in context with literature work investigating CaCO3 crystallisation using the Kitano method. It was determined that the reaction environment is extremely important with respect to the system kinetics and therefore the final crystal habit, as the two crystallisation procedures produced different morphologies for similar constituent compositions. With Ca2+ concentrations of below 10 mM, crystals with a base morphology of a rhombohedron with curved faces were produced from solutions that were over 0.4 wt% (3.8 x 10-2 mol dm-3) malonic acid, while for solutions with Ca2+ concentrations of 30 mM and over, faceted spherical agglomerations of crystals were formed. It was found that solution pH is crucial in defining initial crystal nucleation symmetry, with calcite being the preferred form for low initial solution pH, irrespective of the source of protons. Further, while both [Ca2+] and [malonic acid] are important in defining crystal morphology, pCO2 has a minor effect on the crystallisation mechanism. Analysis of the crystal morphologies produced, and varying the pH of the crystallising solution via a mineral rather than malonic acid indicated that there is a strong association between the growing crystal and the malonic acid additive defining and manipulating the crystal growth. New protocols were developed to probe the interactions of short chain carboxylates with CaCO3 crystals at a molecular level. Protocols using Attentuated Total Reflection Fourier Transform Infra-Red Spectroscopy (ATR FT-IR) and Atomic Force Microscopy (AFM) were developed. These techniques allow the in situ crystallisation surface to be probed in real time, thereby opening up the possibility of probing the intermolecular interactions and interfacial control of crystallisation.