Date of Award
Doctor of Philosophy (PhD)
Manganese (Mn) oxides are ubiquitous in aquatic and terrestrial environments, and the rich redox chemistry of manganese is of interest in engineered systems, such as batteries and water oxidation catalysts. Mn oxides have large surface areas and are highly active in adsorption and redox reactions. Our research group recently reported photochemically-assisted abiotic inorganic oxidation of Mn2+(aq) to δ-MnO2 nanosheets in the presence of nitrate and without a microorganism, demonstrating that abiotic Mn oxidation processes are not negligible. My doctoral work focuses on investigating the effects of various aquatic chemistry parameters, including reactive oxygen species (ROS), peroxyl radical (ROO•), reactive halogen species (RHS), dissolved natural organic matter (DOM), nanoplastics, and co-existing cations on the photochemically-induced oxidation of Mn2+(aq) and the nucleation of Mn oxide solids. First, we investigated the effects of ROS and secondary ROO• on photochemically-induced Mn oxidation. During nitrate photolysis, photochemically-assisted abiotic inorganic oxidation of Mn2+(aq) to Mn(IV) (δ-MnO2 nanosheets) was observed. Mn2+(aq) was shown to be oxidized by superoxide radicals (O2•−) rather than hydroxyl radicals (•OH). Tert-butyl alcohol (TBA) was added as an •OH scavenger to determine the role of •OH in Mn oxidation during nitrate photolysis. However, when TBA was added, more Mn oxides were formed, which was unexpected. Although TBA is commonly assumed to transform •OH into oxidatively inert products, we found out that utilizing TBA as an •OH scavenger generated ROO•, influencing the oxidation of Mn. This study urges a closer look at the potential formation of secondary radicals during scavenged oxidative reactions in the environment. Second, we examined the effects of RHS on photochemically-induced Mn oxidation. To meet rapidly increasing clean water and energy demands, desalination and unconventional oil and gas recovery processes have been significantly exploited. These processes bring highly concentrated brine solutions to the surface, and in sunlight, the halide ions may undergo photochemical reactions to generate RHS. However, we still poorly understand the effects of halides under sunlight exposure on redox-active transition metal ions and their subsequent solid phase formation, such as the transformation of Mn2+(aq) into Mn oxide minerals. Here, we studied the previously undiscovered fast oxidation of Mn2+ by reactive halogen radicals. •OH radicals were produced by nitrate photolysis. While •OH played a limited role in the direct oxidation of Mn2+, they could react with halide ions to generate reactive halogen radicals that can oxidize Mn2+. This study advances our understanding of natural Mn2+ oxidation processes and highlights the unexpected impacts of reactive halogen radicals on redox activities with heavy metals and corresponding solid mineral formation in brine. Third, we studied the effects of DOM on photochemically-induced Mn oxidation. DOM is a complex matrix of organic matter that is abundant in natural aquatic environments. However, little is known about the oxidative roles of DOM in oxidizing Mn2+(aq) to Mn(III/IV) oxide solids. Sunlight-driven processes can initiate the degradation of DOM, accompanied by the formation of photochemically-produced reactive intermediates that can trigger the formation of other reactive radicals. In particular, in the presence of Br− and DOM, ROS and Br radicals were generated and promoted Mn oxidation. These discoveries can help us better understand the oxidative role of DOM in the formation of Mn oxide solids in the organic-rich surface water. Fourth, we investigated how Mn2+ was oxidized by the photolysis of polystyrene-based (PS-based) nanoplastics, increasing wastes of concern. So far, little is known about the oxidative roles of nanoplastics in inducing redox reactions with heavy or transition metals. In this study, we investigated how Mn2+ was oxidized by the photolysis of PS-based nanoplastics. During the photolysis of PS-based nanoplastics, the generation of ROO• was detected, and ROO• and O2•− were the ROS mainly responsible for Mn oxidation in our experimental systems. The production of ROS from nanoplastics in the presence of light endangers marine life and human health, and potentially affects the mobility of the nanoplastics in the environment. The new findings fill knowledge gaps in understanding the oxidative roles of nanoplastics in the generation of ROS. Fifth, we examined the effects of coexisting cations (i.e., Mg2+ or Ca2+) in aqueous environments on Mn oxidation and the micromorphology, chemical composition, and crystallinity of newly formed Mn oxides. In the presence of Mg2+ or Ca2+, we observed the formation of todorokite with a tunneled structure during nitrate photolysis. This was the first report of the rapid formation of todorokite at ambient temperature and pressure within several hours under environmentally relevant conditions. Notably, in the absence of Mg2+ and Ca2+, birnessite with a layered structure was formed, indicating that Mg2+ and Ca2+ played significant roles in the formation of todorokite. These findings illuminate the mechanism of todorokite formation in the environment, and this study helps design environmentally friendly processes for synthesizing Mn oxide materials.
Richard Axelbaum, Julio D'Arcy, Rohan Mishra, Kimberly Parker,
Available for download on Thursday, August 01, 2024
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