Elucidating the Effect of Biomolecule Structure on Calcium Carbonate Crystal Formation

Anthropogenic emissions of carbon dioxide have lead to a steady increase in atmospheric concentration. This greenhouse gas has been identified as a key driver of climate change and also has lead to increased acidification of marine and terrestrial waters. Calcium carbonate precipitation at the Earth's surface is an integral linkage in the global carbon cycle, especially in regards to regulating atmospheric carbon dioxide. As concern for the effect of increasing atmospheric CO₂ levels grows, the need to understand calcium carbonate systems escalates concurrently. Calcium carbonate phases are the most abundant group of biominerals; therefore, elucidating the mechanism of biomineralization is critical to understanding CaCO₃ precipitation and may aid in the development of novel carbon sequestration strategies. The ubiquity of microorganisms leads to an extensive number of biomolecules present in the Earth's systems, and thus an extensive range of possible effects on CaCO₃ formation. Carboxylic acids are very common biomolecules and have a relatively simple structure, thus making them an ideal family of model compounds. This study examines the kinetics, thermodynamics, phase, and morphology of calcium carbonate crystals precipitated in the presence of carboxylate-containing biomolecules, including citric acid, succinic acid, and aspartic acid. The experiments utilize a unique (NH₄)₂CO₃ gas-diffusion reactor, which allows in-situ measurements of chemical conditions during the precipitation and growth of crystals. Continuous monitoring of the in-situ conditions of pCO₂, pH, [Ca²⁺], and optical absorbance provides data on the supersaturation at which nucleation occurs and the kinetics of mineral growth. The use of scanning electron microscopy and X-ray diffraction provides information on the morphology and mineralogy of precipitates. The combination of these data sets will provide an in-depth view of the ideal concentration of calcium ions required for solution saturation, as well as the effect of biomolecule structure on calcium carbonate crystals. Preliminary XRD and SEM analyses have shown vaterite, calcite, and amorphous CaCO₃ to be the primary morphologies present. This analysis indicates a lack of stability for crystals grown quickly at elevated CO₂ concentrations. The rate of precipitation of calcium carbonate solids and the subsequent decrease in free calcium concentration has been shown to be largely a function of pH.