Date of Award
Doctor of Philosophy (PhD)
Interfaces in environmental systems can change the distribution of pollutants and nutrients, determine the fate and transport of nanoparticles, and affect the efficiency of engineering projects. Complex interactions, including electrostatic interactions, Van der Waals interactions, chemical interactions, and ion-surface specific interactions, can all contribute to different ion, ion-pair, and nanoparticle formations near interfaces. Although environmental interfaces play critical roles, only a limited number of studies have considered the nanoscale picture of how they control ions and nanoparticles. Thus, this dissertation addresses three research questions: (1) How does sulfate chemically interact with calcium carbonate (CaCO3) during its heterogeneous formation on quartz substrates? (2) How does CaCO3 form heterogeneously at liquid–liquid interfaces? (3) How do surface functional groups and the bulk solution change pH and ion behaviors in nanopores? Here, utilizing synchrotron-based measurement techniques and surface-enhanced Raman spectroscopy, we developed a novel in situ experimental platform to study heterogeneous CaCO3 formation at interfaces and ion concentrations in nanopores, and used that platform to gain a better understanding of the interfacial control of ion behaviors and CaCO3 formation in environmental systems.First, sulfate’s effects on the formation kinetics and phases of heterogeneously formed CaCO3 on quartz was evaluated by in situ grazing incidence X-ray scattering techniques. In the nucleation stage of CaCO3, sulfate incorporation plays a dominant role compared to sulfate adsorption. Incorporated sulfate could non-linearly increase the interfacial energy of CaCO3, while inhibiting the vaterite-to-calcite phase transformation. The increased interfacial energy and bulk free energy inhibited nucleation kinetics, but the incorporated/absorbed sulfate could promote mineral growth and inhibit Ostwald ripening by serving as a bridging agent and forming a passivation layer. We found that the amounts of sulfate-controlled CaCO3 formation were process-dependent. This new result illuminates the complexity of heterogeneous CaCO3 formation under sulfate’s impacts. Then, we applied these findings to improve Portland cement for safer geologic carbon sequestration. Harnessing enhanced heterogeneous CaCO3 growth, we utilized sulfate to promote CaCO3’s ability to fill nanopores in cement and thus form a layer that resists attack from supercritical carbon-saturated acidic brine. Reactive transport modeling and small-angle X-ray scattering measurements respectively confirmed the smaller porosity and smaller pore size in cement when sulfate was added. Second, we examined the formation pathways of CaCO3 at two liquid–liquid interfaces: isooctane–water and surfactant–water. From in situ experiments using small angle X-ray scattering at isooctane–water interfaces, the interfaces were found to promoted CaCO3 formation, unlike other hydrophobic interfaces. From surface-enhanced Raman spectroscopy, the higher pH at the interface facilitated nucleation of CaCO3. Moreover, isooctane–water interfaces uniquely provided an environment with limited water accessibility, promoting the dehydration of CaCO3 prenucleation clusters and amorphous calcium carbonate (ACC). Then, to study CaCO3 formation at confined surfactant–water interfaces, we designed a nanoemulsion system composed of isooctane, Span 80, Tween 80, and aqueous CaCO3 precursor solutions. Ultrasmall ACC can stably exist under soft nanoconfinement, even after 24 hours of reaction. Surfactant–water interfaces promoted ultrasmall CaCO3 formation, but the confinement by the nanoemulsion limited the precursor amounts, restricting the growth of ultrasmall CaCO3 particles. Third, to understand the local pH and ion concentrations in nanopores and compare them with the bulk solution values, we developed a novel nanoscale plasmonic sensing method. The nanosensor comprises gold nanorods (plasmonic particles for enhancing Raman intensities), para-mercaptobenzoic acid/ortho-mercaptonicotinic acid (probe molecules for pH and heavy metals), and mesoporous silica. Interestingly, ion-surface-specific interactions enhanced anion concentrations and suppressed cation concentrations in nanopores, further affecting the pH in both pristine and hydrophobic functionalized nanopores. On the other hand, strong chemical interactions in hydrophilic functionalized nanopores controlled the pH and heavy metal concentrations. The findings of this research will advance our understanding of CaCO3 formation in environmental systems where sulfate, nanopores, and liquid–liquid interfaces are present. The underpinning mechanisms and theoretical understanding of the ion behaviors in functionalized nanopores can benefit the development of nanoporous materials for catalysis and sensing. We expect the advanced fundamental understanding of the interfacial chemistry in this work can also benefit CO2-related subsurface projects, scale control in water treatment projects, and pollutant removal with novel nanoporous materials.
Fangqiong Ling, Srikanth Singamaneni, Carl Steefel, Fuzhong Zhang,
Available for download on Saturday, May 18, 2024