As a powerful oxidizer, perchlorate (ClO4−) has found widespread use in many energetic materials. Over the past two decades, it has increasingly drawn public attention as a pervasive and persistent water pollutant due to the improper disposal of manufacturing waste. Because human exposure to ClO4− can cause thyroid malfunction, many states in the U.S. have set the limits for ClO4− at 0.8−18 μg L−1 in drinking water. The interest in ClO4− treatment has been further fueled by the recent discovery of ClO4− on Mars, Moon, and meteorites, which collectively imply its wide distribution throughout the Solar System. Thus, ClO4− removal technologies are of great interest for water purification, disposal of hazardous materials, and human extraterrestrial exploration.Currently, the commonly used strategy for ClO4− removal from drinking water is ion exchange. While selective resins are effective in physically removing ClO4−, challenges persist, particularly with the disposal of enriched waste brine and spent reins. Catalytic reduction offers a clean and complete reduction of ClO4− to Cl−. However, the real-world application of the best ClO4− reduction catalyst (Re(hoz)2−Pd/C) is hampered by using a rare-earth metal and its short lifespan in the oxidative environment. Therefore, it is imperative to develop an effective, robust, practical, and economic heterogeneous catalyst for aqueous ClO4− reduction. In this work, we have studied the structure-stability relationship of rhenium complexes and discovered that introducing a methyl group on the oxazoline moiety could significantly enhance the overall stability of the complex. Next, we screened a series of molybdenum (Mo) precursors to identify the active Mo species for oxygen atom transfer reaction. Sodium molybdate was found to increase the catalytic activity of virgin Pd/C by 55-fold. This doctoral study has culminated in discovering a simple and straightforward way to construct Mo catalyst for aqueous ClO4− reduction. The initial turnover frequency of (L)MoOx−Pd/C (L = (NH2)2bpy) reached 165 h−1, which is the highest among all reported abiotic ClO4− reduction catalysts. Lastly, we evaluated the performance of (L)MoOx−Pd/C catalyst in synthetic waste brines mimicking ion-exchange resin regeneration. The catalyst has shown limited inhibition in concentrate salt solution and excellent stability under oxidative stress. At the same time, the challenges of deactivation by nitrate and ligand hydrogenation have been identified, and viable solutions were proposed. This doctoral research showcased the power of coordination chemistry in environmental technology innovation. It will guide the ongoing efforts to design catalysts for a wide range of oxygen atom transfer reactions.

Due to their unique properties including hydrophobicity, lipophobicity, and thermostability, per- and polyfluoroalkyl substances (PFASs) have been extensively used since the 1940s in a wide range of applications. However, concerns about the fate of PFASs have been rising because of their persistence, bioaccumulation, and toxicity, leading to worldwide efforts on PFAS regulation. Current environmental remediation efforts primarily focus on the “legacy” perfluorinated CnF2n+1−X (X = COO−, SO3−, and (CH2)m−R, where R represents highly diverse organic moieties). However, beyond the previously elucidated hydrodefluorination and decarboxylation, the degradation pathways of the legacy PFASs remain largely unknown. Additionally, although “alternative” PFAS containing –H and –Cl in the fluorinated moiety have also been systematically developed and extensively applied for decades, only a few studies have explored their degradation. In this thesis study, we first investigate the degradation of omega-hydroperfluorocarboxylates (ω-HPFCAs, H−CF2(CF2)n−1−COO−) with UV/sulfite. To our surprise, the presence of the H atom on the remote carbon makes ω-HPFCAs more susceptible than perfluorocarboxylates (PFCAs, CF3(CF2)n−1−COO−) to decarboxylation and less susceptible to hydrodefluorination. This study further systematically investigated the degradation of Clx−PFAS, including omega-chloroperfluorocarboxylates (ω-ClPFCAs, Cl−CnF2n−COO−), 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate (F-53B, Cl− (CF2)6−O−(CF2)2−SO3−) and polychlorotrifluoroethylene oligomer acids (CTFEOAs, Cl− (CF2CFCl)nCF2−COO−) under UV/sulfite treatment. After initial reductive dechlorination by hydrated electron (eaq–), multiple pathways occur, including hydrogenation, sulfonation, and dimerization. This study also identified the unexpected hydroxylation pathway that converts the terminal ClCF2− into −OOC−. The hydroxylation of the middle carbons in CTFEOAs also triggers the cleavage of C−C bonds, yielding multiple −COO− groups to promote defluorination. Based on the critical mechanistic understanding obtained from the degradation of Clx−PFAS, this study further reveals novel degradation pathways of legacy PFAS under UV/sulfite treatment via transformation product analyses of a series of legacy PFAS with various head groups and chain lengths. Beyond eaq–, several other active species could also be involved in the reaction and result in transformation products with different recalcitrance. This study renovates and further advances the mechanistic understanding of PFAS degradation in “advanced reduction” systems. It also suggests the synergy between “more degradable” molecular design and cost-effective degradation technology to achieve the balanced sustainability of fluorochemicals.

Heterogeneous transformations of halogenated and oxygenated chemicals have been studied for decades; however, there has been very limited progress in developing metal-ligand coordination systems for environmental applications. Transition metals coordinated with organic ligands can substantially enhance the reactivities of heterogeneous reaction systems. Similar to the evolution of metal centers in various enzymes, organic ligands used in synthetic catalytic systems can be rationally modified with diverse functionalities. In this dissertation, we systematically designed and tested a series of organic ligands bearing various functional groups at multiple positions. Two primary and highly recalcitrant water pollutants, per- and polyfluoroalkyl substances (PFAS) and perchlorate, are used as substrates to investigate the novel relationship between ligand structure and catalyst activity/stability.The second chapter of this dissertation studied ligand-enabled reductive defluorination of PFAS by metallic zinc. The addition of neutral N,N-bidentate ligands, such as substituted 2,2'-bipyridine (R2bpy), to nano-sized zinc powder (nZn0) enabled rapid and significant defluorination from linear long-chain perfluorocarboxylates (CnF2n+1–COO−), which did not degrade by the biogenic cobalt-catalyzed systems (e.g., vitamin B12). The third and fourth chapters of this dissertation developed a highly stable heterogeneous bimetallic catalyst, which incorporated an oxorhenium complex Re(O)(R-hoz)2 onto Pd/C. Modification of the original 2-(2’-hydroxyphenyl)-2-oxazoline (hoz, a bioinspired anionic N,O-bidentate ligand) resulted in substantially improved catalyst stability. A comprehensive collection of Re(O)(R-hoz)2Cl complexes bearing various functional groups with electron-donating/withdrawing and steric effects were synthesized. Upon immobilization onto the water-Pd/C interface, selected complexes exhibited unprecedented high stability during the rapid reduction of concentrated perchlorate. A novel set of ligand design rationale was obtained from this catalytic system. Both studies demonstrate the critical roles of developing organic ligands to enhance both the reactivity and stability of heterogeneous catalytic systems for the reductive degradation of recalcitrant water pollutants. The experimental results and mechanistic insights highlight the long-neglected significance of coordination chemistry for environmental engineering.

Per- and polyfluoroalkyl substances (PFAS) comprise a large class of chemically stable compounds causing ubiquitous pollution. Their detrimental effects to humans and the environment are exacerbated by their mobility in aquatic systems. Effective and efficient methods are necessary to chemically destroy these aqueous contaminants. This thesis study focuses on understanding structure-reactivity relationships of aqueous PFAS within the UV/sulfite system, developing and optimizing this system for effective and efficient PFAS destruction, and identifying next-generation PFAS design for rapid and complete defluorination. A photochemical system equipped with a 254 nm Hg lamp is used to irradiate an aqueous solution amended with a photosensitizer (i.e., sulfite, SO32–), spontaneously generating reactive hydrated electrons (eaq–) and sulfite radicals (SO3⦁–), in order to probe the reactivity with PFAS. A systematic investigation using the UV/sulfite system reveal critical structure–reactivity relationships for legacy (e.g., carboxylates, sulfonates, and telomer carboxylates) and emerging (e.g., ether carboxylates) aqueous PFAS. Decay kinetics, transformation products, and defluorination (i.e., percent C–F bond cleavage) results highlight distinct reaction pathways. Quantum chemical calculations on bond dissociation energies and reaction simulations provide mechanistic interpretation of experimental results in order to elucidate destruction pathways. System parameters, including solution pH and photosensitizer concentration, are optimized to achieve the deepest and most efficient defluorination. Kinetic studies reveal competition between reductive and oxidative defluorination mechanisms directly influencing overall system performance. Increased reactivity and deeper defluorination is achieved under increasingly basic conditions, where the reaction pathways with SO32– are thermodynamically mediated. Effective and efficient PFAS treatment within the UV/sulfite system is highly dependent on structure. Perfluorocaboxamides (PFCAms), containing the distinct amide functional group, can be rapidly destroyed and deeply defluorinated within the UV/sulfite system. By taking advantage of a novel defluorination mechanism upon reactivity with SO3⦁–, PFCAms exhibit higher reactivity resulting in deeper defluorination in modest basic solution in the presence of UV-irradiated SO32–. Furthermore, N-substituted PFCAms observe even faster reactivity and resistance to hydrolysis, demonstrating the possibility of designing future PFAS for rapid and complete defluorination.

Per and polyfluoroalkyl substances (PFAS) have been notoriously known for their high environmental persistence. Driven by government regulations, PFAS manufacturers have gradually shifted to relatively short-chain PFAS and emerging PFAS alternatives. Among the developed PFAS treatment technologies, UV/Sulfite (UV/S) process has unique advantages in treating all chain-length PFAS because the reaction occurred in a homogeneous environment. However, the UV/S process still faced challenges from (i) incomplete defluorination (e.g., 40%~70%) after reduction reactions, (ii) relatively slow reaction kinetics, and (iii) unknown feasibilities for the emerging PFAS alternatives.To address the incomplete PFAS defluorination from UV/S reduction (Red) process, we demonstrated for the first time that incorporating the subsequent advanced oxidation (Ox) process further cleaved the residue C−F bonds in reduction products to elevate the overall treatment performance. The integrated Red and Ox process reported >97% defluorination for major PFAS categories (e.g., CnF2n+1−COO−/SO3− and CnF2n+1−CH2CH2−COO−/SO3−). To resolve the relatively slow reaction kinetics for persistent short-chain perfluoroalkyl sulfonate in UV/S, we achieved accelerated PFAS degradation by adding iodide (UV/S+I). The highlight of the UV/S+I system is the substantially improved degradation kinetics (e.g., up to four times increase) and significantly enlarged treatment capacity for concentrated PFAS wastes (e.g., 300 mg/L PFAS mixture), with fewer chemical dosages and shorter reaction time. Using ether-based PFAS (e.g., Nafion-related ether sulfonates, PFESAs) as emerging PFAS alternatives, we further investigated the role of critical structural features (e.g., linear vs. branched and introduction of C−H bond) in PFAS degradation potential in UV/S+I and evaluated the feasibility of integrated Red and Ox process. The results showed that the introduction of branched structure (e.g., −CF3), C−H bond and −COO− group improved PFAS degradation potential. Previously proposed the integrated Red and Ox process achieved >98% defluorination for these emerging PFESAs structures. Finally, we comprehensively evaluated membrane separation performance for a wide spectrum of PFAS compounds. The results showed that perfluoroalkyl sulfonamides (FOSAms) are distinctively different from the other PFAS compounds, with noticeably higher transmission percentages in the permeate solution in NF270 filtration. We also proposed enhancement strategies for FOSAms removal, including switching to “tighter” membrane NF90, elevating solution pH to ~9, and fully peroxidation of FOSAms to the well-rejected PFAS acids.