Effect of ionic strength on heterogeneous nucleation of calcite during biomineralization
| dc.contributor.author | Knight, Brenna M. | en |
| dc.contributor.committeechair | Dove, Patricia M. | |
| dc.contributor.committeemember | Edgar, Kevin J. | |
| dc.contributor.committeemember | Gill, Benjamin C. | |
| dc.contributor.department | Geosciences | |
| dc.date.accessioned | 2025-01-23T20:14:23Z | |
| dc.date.available | 2025-01-23T20:14:23Z | |
| dc.date.issued | 2024-12-18 | en |
| dc.description.abstract | Biominerals often form within a matrix of biomacromolecules in high-salinity environments, yet the relationships for how macromolecules and ionic strength influence the crystallization of sparingly soluble salts (e.g., CaCO₃) are not established. Developing a physical picture of these controls is hindered by the traditional assumption that background electrolytes are inert. In this study, we investigate calcite nucleation onto two model organic matrix polysaccharides, chitosan and alginate, in a series of ionic strength solutions (65 – 600 mM NaCl). Chitosan is near-neutral (at pH 8.5) and analogous to the structural polysaccharide chitin. In contrast, alginate has a strong negative charge akin to the many anionic biopolymers in the organic matrix. By measuring the rate of calcite nucleation onto these materials and fitting classical nucleation theory to the data, we find the interfacial free energy (γ<sub>net</sub>) and the kinetic prefactor depend upon ionic strength for both polysaccharides. The thermodynamic barrier to nucleating calcite onto alginate strongly depends on ionic strength, while calcite nucleation onto chitosan shows a similar but weaker dependence. Parallel molecular dynamics (MD) simulations were conducted to examine ion (Ca²⁺, Na⁺, HCO₃⁻, Cl⁻) and water interactions with models of a carboxylated polysaccharide and a chitosan material. The MD predictions indicate that at higher ionic strength, the polysaccharide-solution interface is increasingly stabilized by progressively higher concentrations of Na⁺ and Cl⁻. Stronger Na⁺ interactions with the polysaccharide are observed in the carboxylated system. The numbers of H₂O and HCO₃⁻ in the Ca²⁺ hydration sphere decrease with increasing ionic strength, while the number of Cl⁻ increases for both polysaccharides. The evidence suggests the increase in interface stabilization by Na⁺ and Cl⁻ increases γ<sub>net</sub> through reductions in the polysaccharide-solution interfacial energy. We predict the effect of higher salinity is enhanced for alginate because Na⁺ interactions with COO⁻ groups make it more difficult for Ca²⁺ to displace near-surface water and/ or Na⁺. Relatively weak Na⁺-chitosan molecular interactions lead to a lesser dependence on ionic strength. Calcite nucleation rates were also measured onto chitosan in a series of sodium halide solutions (NaCl, NaBr, NaI) and onto alginate in a series of chloride salts (LiCl, NaCl, CsCl) at constant ionic strength. CaCO₃ nucleation in the presence of electrolytes with the strongest hydration properties presents the lowest γ<sub>net</sub>. Values of γ<sub>net</sub> increase in the order Cl⁻<Br⁻<I⁻ and Li⁺<Na⁺<Cs⁺ for nucleation onto chitosan and alginate, respectively. The findings demonstrate that background electrolytes can modulate the energy barrier to CaCO₃ nucleation through tunable effects at the polysaccharide-solution interface. | en |
| dc.description.abstractgeneral | Skeletal structures often form within an organic matrix of polysaccharides, proteins, and lipids. An ongoing challenge is to determine the roles of these biopolymers and their surroundings during the onset and growth of biominerals. In this study, we use two characterized polysaccharide materials as models for the organic matrix where calcium carbonate (CaCO3) biomineralization takes place to investigate the role of salinity in nucleation kinetics. By measuring the rate of CaCO3 nucleation onto chitosan (neutral) and alginate (carboxylated) and fitting classical nucleation theory to the data, we find the thermodynamic and kinetic controls on nucleation rate are dependent upon sodium chloride (NaCl) concentration for both polysaccharides. Although NaCl is considered an inert electrolyte, the thermodynamic barrier to nucleating calcite onto alginate strongly depends on its concentration. Chitosan shows a similar but weaker dependence. Simulations of the polysaccharide-NaCl environment show the polysaccharide-solution interface is increasingly stabilized by Na+ and Cl-, thus increasing the difficulty for a crystal nucleus to displace surface water and ions. The effect of higher salinity is enhanced for alginate due to stronger Na+ interactions with carboxyl groups. We also find the energetics of CaCO3 nucleation onto polysaccharides in the presence of electrolytes are dependent upon type of electrolyte. The relations suggest functional groups interact with background electrolytes (e.g., NaCl) to modulate activity and function at the polysaccharide-water interface during CaCO3 crystallization processes. | |
| dc.description.degree | Master of Science | en |
| dc.format.medium | ETD | en |
| dc.format.mimetype | application/pdf | en |
| dc.identifier.uri | https://hdl.handle.net/10919/124330 | |
| dc.language.iso | en | en |
| dc.publisher | Virginia Tech | en |
| dc.rights | In Copyright | en |
| dc.rights.uri | http://rightsstatements.org/vocab/InC/1.0/ | en |
| dc.subject | polysaccharides | en |
| dc.subject | kinetics | en |
| dc.subject | interface | en |
| dc.subject | electrolytes | en |
| dc.title | Effect of ionic strength on heterogeneous nucleation of calcite during biomineralization | en |
| dc.type | Thesis | en |
| dc.type.dcmitype | Text | en |
| thesis.degree.discipline | Geosciences | |
| thesis.degree.grantor | Virginia Polytechnic Institute and State University | en |
| thesis.degree.level | masters | en |
| thesis.degree.name | Master of Science | en |