Date of Award
7-19-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Water scarcity represents a formidable challenge for human. Exponential population growth and industrial advancements further intensify the demand for potable water. Mineral nucleation at the nanoscale plays a central role in the strategic management of freshwater’s production and quality. In natural groundwater and surface water, the interactions between water and minerals, including earth-abundant iron (hydr)oxides and manganese (Mn) (hydr)oxides, control the mobilization and stabilization of heavy metals, harmful anions, and nutrients, thereby influencing water quality and environmental health. In an engineered context such as membrane desalination, the most widely utilized technique for desalinating seawater, the nucleation and growth of minerals like calcium carbonate, calcium sulfate, and iron (hydr)oxides can reduce operational efficiency. These minerals precipitate and form mineral fouling (scaling) on membrane surfaces, severely decreasing the performance. Scaling also increases maintenance costs and energy consumption, deteriorating the sustainability and economic viability of water supply infrastructures. To better understand the interfacial formation of mineral nanoparticles that is critical to improving freshwater production and management, this dissertation establishes three systems that focus on mineral formation at environmentally important interfaces. In System 1, we focus on water that contacts with a redox-active mineral, arsenopyrite, in managed aquifer recharge (MAR), and we examine the resulting arsenic mobilization and secondary mineral precipitation. The System 2 focuses on heterogeneous mineral scaling on functionalized surfaces in membrane desalination. The System 3 investigates photochemically-induced creation of interfaces of Mn (hydr)oxides and surface water and elucidates their abiotic formation pathways. Managed aquifer recharge (MAR) is an important engineered solution for achieving sustainable groundwater management. Unfortunately, if not operated properly, MAR can cause undesirable arsenic mobilization in groundwater. To minimize this mobilization, we need a better understanding of the dynamically evolving water chemistry and nanoscale mineral–water interfaces in MAR systems. Specifically, bicarbonate is ubiquitous in groundwater, but its effect on arsenic mobilization in MAR is not fully understood. Here, in System 1 (Chapter 2), we examined the effects of bicarbonate concentrations on the dissolution of arsenopyrite and the nanoscale secondary mineral formation in both open systems (mimicking shallow unconfined aquifers) and closed systems (mimicking deep confined aquifers). In the open system, owing to pH evolution and the subsequent formation of secondary iron (III) (hydr)oxide nanoparticles, arsenic mobilization decreased with increasing bicarbonate concentrations. However, increasing the bicarbonate concentrations from 1.0 to 10 mM did not further inhibit the arsenic mobilization, owing to surface complexation and the formation of aqueous arseno-carbonate complexes. In the closed system, arsenic mobilization and secondary mineral formation were similar for all conditions. These findings highlighted the significance of bicarbonate concentrations in affecting arsenic mobilization and secondary mineral formation in MAR system, especially in shallow unconfined aquifers. The formation of nanoparticles on desalination membrane surfaces can adversely impact the efficiency of freshwater production in membrane operations. However, quantitative in situ observations of the scale mineral nucleation that are controlled by membrane-related functional groups are still lacking. In System 2, utilizing the analytical techniques of in situ grazing incidence small angle X-ray scattering and ex situ atomic force microscopy, we investigated early-stage nucleation on surfaces with a variety of chemical functional groups. The first study (Chapter 3) focused on iron (hydr)oxide’s heterogeneous nucleation on hydroxyl-, carboxyl-, and fluoro- functionalized surfaces. Of the three surfaces, hydrophobic fluoro surfaces exhibited significantly lower nucleation rates and higher nucleation barriers than the two more hydrophilic surfaces. Fluoro surfaces also had the highest free energy barrier of nucleation. The second study (Chapter 4) examined the heterogeneous nucleation of calcium carbonate on substrates functionalized with carboxyl, hydroxyl, amino, and fluoro groups. The carboxyl surfaces had the highest calcium carbonate nucleation rate due to their hydrophilicity and affinity for calcium ions. Both studies provided fundamental kinetic and thermodynamic insights into mineral scaling that are essential for optimizing membrane processes in various applications. In surface water, sunlight can also drive nanoscale mineral formation through redox reactions triggered by photochemistry. As an example, although Mn oxide formation was previously known to be biotically facilitated owing to its fast kinetics, photochemical reactions can also facilitate abiotic formation of Mn oxides. Therefore, System 3 focused on abiotic Mn oxidation under light illumination and in the presence of anthropogenic contaminants, including disposable facemask surfaces (made of polypropylene, an organic contaminant) and cupric ions (Cu2+, a Fenton active transition metal and an inorganic contaminant). In Chapter 5, we found that the photolysis of mask layers facilitates the kinetics of Mn oxidation to Mn oxide nanoparticles through heterogeneous nucleation. The kinetics of Mn photo-oxidation were differently enhanced by the polypropylene mask layers, depending on their surface material packing density. Additionally, photoaging of the mask layers further facilitated the Mn oxidation. Superoxide radicals generated by the masks were identified as the primary reactive oxygen species (ROS) accelerating Mn oxidation. Chapter 6 revealed that, although Cu2+ was previously known to inhibit Mn oxidation via biotically driven superoxide dismutation, with light illumination, the kinetics of Mn oxidation can be promoted by Fenton chemistry that is driven by Cu2+ and the H2O2 generated from water photolysis. These results shed light on the environmental repercussions of both organic and inorganic pollutants, particularly on how Mn oxides formed on these materials can alter the behavior and distribution of trace elements, toxins, nutrients, and organic compounds in the environment. This dissertation offers critical new insights into mineral nanoparticle formation at key environmental interfaces, both natural and engineered, supporting our understanding of processes that affect freshwater production and management. It explores arsenic mobilization in managed aquifer recharge systems, showing how bicarbonate impacts arsenopyrite dissolution and secondary mineral formation in groundwater. In membrane desalination, the study identifies how mineral scaling influenced by surface functional groups affects freshwater production efficiency. Finally, it examines Mn oxide formation in natural surface waters, highlighting the impact of anthropogenic contaminants and photochemistry on the kinetics of Mn oxidation. These findings collectively provide valuable information for advancing sustainable water resource management and optimizing water treatment processes.
Language
English (en)
Chair
Young-Shin Jun