Effect of solution stoichiometry on barium sulfate and calcium carbonate nucleation and growth, with applications to drinking water softening treatment

Crystals form everywhere around us. Understanding how crystals form is therefore not only a fundamental scientific question, but also a practical challenge for society. Their formation affects how elements cycle through the environment, how we manage clean drinking water, and how we design effective pharmaceuticals and advanced materials. Since the mid-1950s many experiments and model studies have been conducted to better understand crystal formation, encompassing many types of crystal nucleation and growth kinetic rate studies. In Earth sciences, these studies are crucial to quantify fluxes in biogeochemical cycles, as minerals are important sinks for many elements. In addition, it is helpful for geological texture interpretations and to identify the physicochemical conditions during crystal formation, which may be embedded in large deposits that are observed today. Also, it is essential to determine the crystal quality and origin. In geoengineering settings, the knowledge gained from crystal kinetic rate studies assists in optimizing crystal precipitation in subsurface remediation, stabilizing CO2 sequestration, and preventing scale formation in pipelines, valves and wells in both conventional and emerging energy recovery systems, among others. It is also very relevant for the drinking water softening treatment step during drinking water production. In material science, understanding crystal formation is essential for producing effective medicine. For example, Ritonavir (an antiretroviral drug) had to be reformulated (i.e., differently produced), because a different crystal form undesirably appeared that reduced its effectiveness. A deep understanding of crystal formation requires more than idealized laboratory experiments however. In natural and engineered systems, the chemical conditions are rarely balanced. In natural settings for example, crystals form from a broad range of ionic ratios, often controlled by natural processes. For example, calcium carbonate (CaCO3) cation:anion ratios vary roughly from 0.0001 to 10000 due to weathering, atmospheric pCO2, pH and organic matter remineralization, while barium sulfate (BaSO4) cation:anion ratios vary between 0.000001 and 0.03 due to sulfate-reducing and/or sulfur-oxidizing bacteria, marine productivity, hydrothermal activity and diagenetic barium mobilization. These variations influence whether crystals grow quickly or slowly, remain stable, or carry an electric charge that alters the behavior of nanosized particles. This thesis addresses that complexity. Through in situ experiments on BaSO4 and CaCO3 mineral-forming solutions, I use dynamic light scattering and electrophoresis to study how, among other relevant physicochemical drivers, solution stoichiometry — the ratio of mineral-forming ions — controls nucleation, particle size, and charge. These techniques reveal that even subtle shifts in solution stoichiometry strongly affect crystal formation, providing insights that are directly relevant for drinking water softening, a key treatment step during drinking water production. Highlighting solution stoichiometry as a driver of crystal behavior represents the central innovation of this work. By connecting laboratory-scale observations to applications in water treatment, resource management, and environmental processes, this thesis stresses how fundamental insights into mineral formation can guide more sustainable technologies. It shows that by moving beyond ideal conditions and embracing the chemical diversity of real systems, we can improve predictions, optimize engineering applications, and better manage the resources on which we depend