Calcium Carbonate Mineralization: Understanding Nucleation Processes in Artificial Seawater.

Supervisor: Alejandro Fernandez Martinez (ISTerre (Institut des Sciences de la Terre, Grenoble)
Calcium carbonate is one of the most important components of the global carbon cycle and, as such, plays a vital role in maintaining the health and chemical equilibrium of the oceans. Despite plenty of literature that studies the formation processes of CaCO3, there is very limited work that examines the first stage of precipitation, i.e. nucleation, and how different environmental factors might affect it. Most existing work has been performed on the growth stage, or in the precipitation process at large, without necessarily focusing on the very first stage of the nucleation period. Whether looking at biotic precipitation present in biominerals, or abiotic precipitation of CaCO3 throughout the water column and in calcareous cements, the principles of nucleation govern a key role in how the final crystal will turn out (speed, shape, size, composition, etc.) In this thesis the focus is put on understanding the physical-chemistry principles of inorganic nucleation. Specifically, the kinetics and thermodynamics of CaCO3 nucleation in a homogenous medium were studied as a function of salinity and initial solution Mg/Ca ratios. Synthesis of CaCO3 was done using an artificial seawater protocol which included a cationic stock solution of MgCl2, CaCl2, and SrCl2 as well as an anionic stock solution of NaCl, Na2SO4, NaHCO3, Na2CO3, and KCl. The nucleation rate was calculated using the induction time derived from ultraviolet–visible spectroscopy. Structural data was collected using Fourier Transform Infrared Spectroscopy, compositional data was collected using Inductively coupled plasma atomic emission spectroscopy, and thermodynamics were studied by calculating interfacial energy values based on classical nucleation theory. The results showed that salinity had insignificant effects on the nucleation rate, interfacial energy, and on the mineralogy. Mg/Ca ratios had similarly insignificant effects on the nucleation rate but showed a positive exponential increase in interfacial energy with increasing Mg content. Structurally, there was evidence of polymorph changes occurring with increasing Mg content towards primarily aragonite precipitation. Due to limited nucleation literature present, the reasons behind these results are still not very well understood. Further studies looking at the aggregation mechanics, as well as the role of other factors such as temperature, pH, pCO2, and inorganic and organic additives must be conducted to understand the principles of CaCO3 nucleation further.