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The contents of this dissertation are primarily focused on the evaluation of molecular electrocatalysts and their intrinsic properties during electroreduction reactions such as carbon dioxide (CO2) reduction, hydrodehalogenation, and hydrogen evolution. The control of the second coordination sphere in a coordination complex plays an important role in improving catalytic efficiency. Herein, we report a zinc porphyrin complex ZnPor8T with multiple flexible triazole units comprising the second coordination sphere, as an electrocatalyst for the highly selective electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO). This electrocatalyst converted CO2 to CO with a Faradaic efficiency of 99% and a current density of –6.2 mA/cm2 at –2.4 V vs Fc/Fc+ in N,N-dimethylformamide using water as the proton source. Structure-function relationship studies were carried out on ZnPor8T analogs containing different numbers of triazole units and distinct triazole geometries; these unveiled that the triazole units function cooperatively to stabilize the CO2-catalyst adduct in order to facilitate intramolecular proton transfer. This demonstrates that incorporating triazole units that function in a cooperative manner is a versatile strategy to enhance the activity of electrocatalytic CO2 conversion. The effects of primary and second coordination spheres on molecular electrocatalysis have been extensively studied, yet investigations of third functional spheres are rarely reported. Here we report an electrocatalyst (ZnPEG8T) with a hydrophilic channel as a third functional sphere that facilitates relay proton shuttling to the primary and second coordination spheres for enhanced catalytic CO2 reduction. Using foot-of-the-wave analysis, the ZnPEG8T catalyst displayed CO2-to-CO activity (TOFmax) thirty times greater than that of the benchmark catalyst without a third functional sphere. A kinetic isotopic effect (KIE) study, in conjunction with voltammetry and UV-Visible spectroscopy, uncovered that the rate limiting step is not the protonation step of the metallocarboxylate intermediate, as observed in many other molecular CO2 reduction electrocatalysts, but rather the replenishment of protons in the proton shuttling channel. Controlled-potential electrolysis using ZnPEG8T displayed a Faradaic efficiency of 100% for CO2-to-CO conversion at –2.4 V vs Fc/Fc+. This report validates a strategy for incorporating higher functional spheres for enhanced catalytic efficiency in proton-coupled electron transfer reactions.The electrocatalytic reduction of dichloromethane (CH2Cl2) into hydrocarbons involving a main group element-based molecular triazole-porphyrin electrocatalyst H2PorT8 is reported. This catalyst converted CH2Cl2 in acetonitrile to various hydrocarbons (methane, ethane, and ethylene) with a Faradaic efficiency of 70% and current density of –13 mA/cm2 at a potential of –2.2 V vs. Fc/Fc+ using water as a proton source. The findings of this study and its mechanistic interpretations demonstrated that H2PorT8 was an efficient and stable catalyst for the hydrodechlorination of CH2Cl2 and that main group catalysts could be potentially used for exploring new catalytic reaction mechanisms.

Abstract: Oilsands bitumen produced from Alberta, Canada contains 14–20 wt.% n-pentane insoluble asphaltenes. Separation of asphaltenes from the bitumen by solvent deasphalting has the benefit of improving the properties of the deasphalted oil compared to the bitumen, for example, by decreasing its viscosity and increasing its hydrogen-to-carbon ratio. Due to the low hydrogen-to-carbon ratio and high aromatic content of the asphaltenes, the asphaltenes fraction potentially is a good feedstock for carbon fiber production. The processing path for converting asphaltenes to carbon fibers involves the following steps. First, melt-spinning is used to convert the asphaltenes into fibrous form. Then the precursor fibers are oxidatively stabilized to render them infusible for the final carbonation step, during which the fibers are subjected to high temperatures, up to 1500 °C, in an inert atmosphere. When using asphaltenes as a precursor for carbon fibers production, the production process can benefit from pretreatment of the asphaltenes to increase its softening point temperature. Low softening point is undesirable as it can lead to a lengthy stabilization process as well as potential fusing of the fibers during stabilization and carbonization steps. In this study, industrially pentane solvent deasphalted asphaltenes was pretreated in three ways to increase its softening point temperature: (i) solvent deasphalting / solvent extraction with n-pentane and n-heptane as solvent (ii) autoxidation and (iii) halogenation/dehalogenation. From this investigation, it was found that all three treatments were effective in increasing the softening point. Additionally, all three pretreatments were also effective in improving the melt spin productivity during the melt-spinning process. One exception was noted, namely, when removing the n-heptane soluble materials by rigorous solvent deasphalting, it did not improve melt spin productivity.

Trichlorofluoromethane (CFC-11), dichlorofluoromethane (CFC-12), and tetrachloromethane (CT) are fully halogenated methanes that were produced as refrigerants in the early part of the 1900s and later used in many industrial processes. They are ozone-depleting agents and common groundwater contaminants. They are volatile chemicals that are moderately soluble in water. Due to their volatility when released to the environment, they are predominantly found in the atmosphere, though they also dissolve into the groundwater. In anaerobic environments, they can undergo dehalogenation reactions with several redox-active compounds. This dissertation presents results from two treatability studies from sites contaminated with CFC-11, CFC-12, and CT. Additionally, the effect of pH on the dehalogenation of CFC-11, CFC-12, and CT is examined, and a sulfidogenic enrichment culture grown in the presence of CT is characterized. The first treatability study indicates that the addition of reactive iron species (i.e., zero-valent iron or ferrous sulfide) combined with the bioaugmentation culture KB-1 Plus and lactate can facilitate the transformation of CT into non-halogenated end products. The most effective remediation strategy for CT observed during the treatability study for the second contaminated site was the addition of zero-valent iron; this facilitated the transformation of CT to chloroform (CF). CF is a non-desirable end product, and additional remediation efforts are recommended for the second contaminated site. A shift in the type of transformation products formed during the reduction of CFC-11, CFC-12, or CT by super-nucleophilic cobalamin was observed as pH increased. Mackinawite and vivianite were identified as the two precipitate phases formed in the presence of the sulfidogenic enrichment culture. Vivianite formation likely occurs via precipitation with the phosphate present in the medium, and that mackinawite forms via precipitation with hydrogen sulfide produced by the sulfidogenic bacteria present in the enrichment. Additionally, a greater decline in CT was observed in microcosms that contained active enrichment culture than in heat-killed controls, suggesting that the consortium aids in the degradation of CT, probably via mackinawite formation, as it is a reactive iron species.

Hydraulic fracturing and horizontal drilling technologies have greatly improved the production of oil and natural-gas from previously inaccessible non-permeable rock formations. Fluids comprised of water, chemicals, and proppant (e.g., sand) are injected at high pressures during hydraulic fracturing, and these fluids mix with formation porewaters and return to the surface with the hydrocarbon resource. Despite the addition of biocides during operations and the brine-level salinities of the formation porewaters, microorganisms have been identified in input, flowback (days to weeks after hydraulic fracturing occurs), and produced fluids (months to years after hydraulic fracturing occurs). Microorganisms in the hydraulically fractured system may have deleterious effects on well infrastructure and hydrocarbon recovery efficiency. The reduction of oxidized sulfur compounds (e.g., sulfate, thiosulfate) to sulfide has been associated with both well corrosion and souring of natural-gas, and proliferation of microorganisms during operations may lead to biomass clogging of the newly created fractures in the shale formation culminating in reduced hydrocarbon recovery. Consequently, it is important to elucidate microbial metabolisms in the hydraulically fractured ecosystem. The numerous nitrogen and carbon sources injected in input fluid mixtures may sustain shale-associated microorganisms, prompting a need to investigate the capacity of microbial life to enzymatically transform organic chemicals commonplace to hydraulic fracturing operations. In Chapter 2, we investigated the putative microbial metabolisms of two bacterial genera frequently identified in the first few weeks to months after hydraulic fracturing occurs (Marinobacter and Arcobacter). Using microbial culture-dependent methods (e.g., genomics, salinity range and carbon source growth testing) and microbial culture-independent methods (e.g., metagenomics) coupled to geochemical measurements from four Appalachian Basin natural-gas wells, we determined Marinobacter and Arcobacter likely play significant roles in biogeochemical cycling weeks to months after fracturing. There is evidence that Marinobacter can utilize a wide variety of nitrogen and carbon compounds including hydrocarbons, whereas Arcobacter can use a reductive TCA cycle coupled to sulfur oxidation. In Chapter 3, we tested the ability of the dominant shale-associated bacterial genera, Halanaerobium, to transform frequently used polyglycol surfactants. We used a variety of microbial and analytical chemical methods both in situ during production of a hydraulically fractured Utica-Point Pleasant natural-gas well, and in the laboratory during batch growth of Halanaerobium congolense WG10. Our results revealed that Halanaerobium can enzymatically transform alkyl polyethoxylates, polypropylene glycols, and monomeric glycols, under anaerobic conditions. In Chapter 4, we investigated microbial (de)halogenation pathways during hydraulic fracturing of natural-gas wells using a metagenomic approach. We identified genes encoding for halogenation, hydrolytic dehalogenation, and reductive dehalogenation, months after fracturing occurred. The presence of these pathways indicates the potential for microbially-generated organohalides in produced fluids as well as reduction of organohalides in wastewaters.In Chapter 5, we surveyed the microbial community at six stages of treatment in a class (II) injection well facility. The microbial community was highly similar to produced fluids from the Marcellus Shale, despite the transport of wastewaters in trucks, exposure to oxygen, and addition of chemicals in the treatment process.

Disinfection byproducts (DBPs) presence in wastewater effluents and receiving waters may impact the quality of drinking water during water reuse practices. Natural solar photolysis is one of the biogeochemical processes that may lead to decreased DBPs concentrations in water. The purpose of this dissertation is to determine the fate of chlorinated, brominated and iodinated DBPs in surface water by natural sunlight photolysis and investigate the use of solar-based advanced oxidation processes (AOPs) for removal of DBPs in water. Total organic halogen (TOX) was used to measure total chlorinated- (TOCl), brominated- (TOBr) and iodinated-DBPs (TOI) in water. The first objective was to determine the optimum protocol for TOX sample preservation conditions to ensure accurate TOX analysis throughout the following experiments. To achieve the highest TOX recovery, samples must be stored at pH 2 using nitric acid, 4 °C incubator and be analyzed within 14 days of storage. Overdosing of quenching agents such as sodium sulfite, sodium thiosulfate and ascorbic acid must be avoided to maintain stable TOX concentrations during storage. The second objective was to determine the fate of TOCl, TOBr, TOI and individual DBPs by natural sunlight in surface water. Iodinated DBPs were the most photodegradable specific halogenated DBPs, whereas chlorinated DBPs were the most resistant to sunlight photodegradation. The TOX degradation rates were generally in the order of TOI > TOBr TOCl(NH2Cl) > TOCl(Cl2) and the half-lives ranged between 2.6 and 10.7 h during solar photolysis. Typical concentrations of natural surface and wastewater containments including nitrate, nitrite and sulfite had little impact on enhancing DBPs photodegradation rates. However, natural organic matter and turbidity decreased photodegradation of DBPs by light screening. The third objective was to evaluate the use of solar-based AOPs for DBP removal in water. Both solar-TiO2 photocatalytic and solar photo-fenton processes increased DBPs photodegradation rates significantly in comparison to solar photolysis alone. TOX half-lives were reduced from hours to minutes by the two solar-based AOPs, and the rate of degradation were generally in the order of TOI > TOCl(NH2Cl) > TOBr > TOCl(Cl2). Oxidation by hydroxyl radicals is expected to be the main mechanism accountable for improved DBP degradation. Furthermore, several natural water constituents including chloride, sulfate, natural organic matter and bicarbonate decreased DBPs degradation efficiency by solar-based AOPs.

Water scarcity is one of the most challenging issues in the world in the 21st century. It is estimated that there are more than one billion of people without adequate access to freshwater and facing water shortages and water deficits. People are forced to drink polluted water despite the risk of consuming pathogenic microorganisms in the water that transmit waterborne diseases such as bacterial infections, protozoal infections and viral infections. The water disinfection process is one of the most important environmental technological advances in the 20th century which inactivates microbial contaminants in drinking water. Disinfection byproducts (DBPs) are a group of chemical compounds formed from the reaction between natural organic matter and chemical disinfectants. The formation of DBPs in drinking water has caused serious health concerns since the discovery of trihalomethanes in chlorinated drinking waters in the 1970s. Many studies have evaluated factors affecting the formation of DBPs within water treatment plants. Relatively less is known about the fate of DBPs in the distribution system. The objective of this study was to evaluate the impacts of pH and temperature on the degradation of total organic chlorine (TOC1), bromine (TOBr) and iodine (TOI). In this study, we produced TOC1 (Cl2), TOBr, TOI, and TOC1 (NC2Cl) from reactions between Suwannee River fulvic acid and chlorine, bromine, iodine and chloramine, respectively. The impact of different pH values (7.0, 8.3 and 9.5) and temperatures (10°C, 20°C, 30°C, and 55°C) on the degradation of these DBPs was investigated after oxidant residuals were exhausted. The results show that halogenated DBPs degrade through based-catalyzed dehalogenation processes. The degradation of TOC1, TOBr, and TOI increased with increasing pH values. Increasing temperatures also increased the degradation kinetics of these DBPs. Iodinated DBPs were less stable than brominated DBPs, which again were less stable than chlorinated DBPs. Relatively high degradation kinetics were also found for chloraminated DBPS. In general, the relative stability of different DBPs are in the order of TOC! (Cl2)>TOBr>TOI≈TOC1 (NH2CL).

For centuries chemists have sought to improve humankind’s quality of life and address many of society’s most pressing needs through the development of chemical processes and synthesis of new compounds, often with phenomenal results. Unfortunately, there also are many examples where these chemicals have had unintended, detrimental consequences that are not apparent until years or decades after their initial use. There are numerous halogenated molecules in this category that are globally dispersed, resistant to natural degradation processes, bioaccumulative, and toxic to living organisms. Chemicals such as these are classified as persistent organic pollutants (POPs), and due to their negative environmental and health effects, they require safe, effective, and inexpensive means of remediation. This research focuses on the development and optimization of a reaction matrix capable of reductively dehalogenating several POPs. Initial experiments determined that powdered magnesium and 1% V/V acetic acid in absolute ethanol was the most effective system for degrading polychlorinated biphenyl (PCB), an extraordinarily recalcitrant environmental contaminant. Further studies showed that this matrix also was capable of degrading polychlorinated dibenzo-p-dioxins (PCDDs), polybrominated diphenyl ethers (PBDEs), and four organochlorine pesticides (OCPs); dieldrin, heptachlor, heptachlor epoxide, and chlordane. During this phase of testing, field samples contaminated with chlordane were washed with ethanol and this ethanol/chlordane solution was degraded using the same reaction matrix, thereby demonstrating this technology’s potential for “real-world” remediation projects. Finally, a set of experiments designed to provide some insight into the mechanism of dechlorination seems to indicate that two distinct processes are necessary for degradation to occur. First, the passivated iv outer layer of the magnesium must be removed in order to expose the zero-valent magnesium core. Next, an electron is transferred from the magnesium to the target molecule, causing the cleavage of the halide bond and the subsequent abstraction of either a hydrogen or proton from a solvent molecule. It is anticipated that an understanding of these fundamental chemical processes will allow this system to be tailored to a wide range of complex environmental media

Halogenated chemicals represent a large and toxic class of environmental pollutants. Although regulation of certain halogenated organics resulted in decreasing production of these chemicals in the United States since the 1970s, others were yet unknown when the regulations were written and production is increasing. In many cases, halogenated organics are persistent in the environment, bioaccumulative, and bioactive, causing toxicological concerns. As such, environmental scientists have studied the processes by which these chemicals can be broken down, and the products that form in these breakdown reactions. In some cases, the toxic effects associated with halogenated organic pollutants can be ameliorated by complete dehalogenation, while incomplete dehalogenation or other transformations can result in the production of harmful compounds. The mechanisms of these transformations are in most cases not yet well understood, but a fundamental understanding of these reactions helps in the development of effective remediation strategies, and informs the fundamental chemistry inherent to these reactions. Concern about halogenated environmental pollutants has led to investigations of a number of means of dehalogenation including biological attenuation. Microbially mediated dehalogenation represents a major transformation pathway for halogenated pollutants in the environment. Metal-containing cofactors have been implicated in these processes including cobalamin (vitamin B12), factor F430, and hemitin. These cofactors are responsible for the reductive dehalogenation of environmental pollutants. These reactions can proceed by a various intermediates, but one of particular interest is the formation of radicals. Radicals have at least one unpaired electron, and as such are highly reactive and transient intermediates. These features can make them difficult to study but their powerful reactivity underscores their importance in environmental transformations. Radical intermediates are often proposed but rarely fully understood in a range of environmental systems. In this thesis, the role of radicals in dehalogenation reactions is explored with particular attention to cobalamin-mediated and nickel-mediated reactions. The mechanism of cobalamin-mediated dechlorination has been studied extensively and evidence for both outer-sphere (radical based) and inner sphere (nonradical based) mechanisms has been presented. In this thesis the literature concerning cobalamin-mediated dehalogenation is reviewed in detail (Chapter 1) and a mechanistic study on the role of radicals in cobalamin-mediated dechlorination of chloroethylenes reconciles previously seemingly contradictory data (Chapter 2). Similarly, both radical and nonradical pathways have been invoked in nickel-mediated dehalogenation of a variety of substrates. Nickel-mediated dehalogenation has not been studied as extensively as cobalt-mediated reactions and the understanding is complicated by the fundamental chemistry of nickel complexes. In order to better understand the chemistry of reduced nickel complexes, particularly their reaction with halogenated organics, a series of nickel complexes was synthesized and characterized (Chapter 3). The relationship between reduced transition metal complexes and their ligands is inextricably linked to whether and how radical intermediates are formed in these systems. The reactivity of two reduced nickel complexes precursors show that these complexes are highly sensitive to slight changes in ligand structure (Chapter 4).

The objective of this work was to study the dehalogenation of environmental pollutants mediated by rhodium hydrides. The product distribution and mechanism of dehalogenation was explored and the information obtained can possibly be applied to improve future remediation strategies. In Chapter 2, the dehalogenation of chlorinated and fluorinated ethylenes was explored using (PPh 3 ) 3 RhCl and Et 3 SiH, and counter-intuitively, vinyl fluoride was dehalogenated 6 times faster than vinyl chloride. This study established substrate scope and preferences for the Et 3 SiH and (PPh 3 ) 3 RhCl catalytic system. In Chapter 3, the mechanism for dehalogenation of chlorinated and fluorinated ethylenes was elucidated using H 2 as the reducing agent with the pre-catalyst (PR 3 ) 3 RhCl. These results were compared to those from using Et3SiH as the reducing agent. Dehalogenation using (PPh 3 ) 3 RhCl and either H 2 or Et 3 SiHsupport an insertion/β-chloride elimination mechanism; however the two systems display distinct differences. Based on these differences, the dominant pathway for Et 3 SiH is proposed to involve rhodium(I), while the H 2 system is proposed to primarily involve rhodium(III). In Chapter 4, a heterogeneous catalytic system using Rh/Al 2 O 3 as the catalyst and H 2 as the reducing agent was investigated. Consistent with the homogenous system of (PPh 3 ) 3 RhCl and H 2 , the data from this system also supports an insertion/β-Cl-elimination mechanism as the dominant degradation pathway. Ultimately, the goal of this work was to facilitate the preparation of engineered pump-and-treat strategies that will function to effectively degrade environmental pollutants to benign products with no halogen substituents.

Remediation of trichloroethylene (TCE) and other chlorinated solvents is of great concern due to their toxicity and their persistence in the environment. Iron has been used extensively in the past decade as a subsurface reactive agent for the remediation of dense, nonaqueous-phase liquids (DNAPLs). Permeable reactive barrier walls (PRBW) have been installed at many sites around the country to treat contaminated plumes resulting from the presence of DNAPL pools. The use of zero-valent metals, such as iron, to effectively reductively dechlorinate DNAPLs has been employed as the reactive material in these PRBWs (Gillham et al., 1993). However, limited work has been conducted to compare the kinetics of TCE degradation related to various manufacturing sources of iron and the pretreatment the iron receives prior to subsurface installation. Determination of iron reactivity through kinetic studies makes it possible to compare different types of iron and the effects that pretreatment has on reactivity. This research utilized rate studies, scanning electron microscopy, and BET surface area analysis for iron particles that were obtained from several sources. Peerless Metal Powders and Abrasive, Inc., Connelly-GPM, Inc., and Alfa Aesar Inc., produced the iron particles using various manufacturing techniques, and nanoscale iron was synthesized in our laboratory. By utilizing zero-headspace batch vial experiments and gas chromatography, changes in TCE concentration were determined. The data obtained produced linear first order rate plots from which dehalogenation rate constants were obtained. The rate constants were normalized by iron mass, solution volume, and surface area. The pretreatment techniques employed in this study, including ultrasonication and acid washing, demonstrated a beneficial effect by removing oxide precipitates from the iron surface, thus increasing the reactivity of the iron. Mass loading studies revealed how physical factors, associated with the experimental setup, could influence reaction rates. Surface area studies confirmed that the smaller iron particles, such as the nanoscale iron, have a greater surface area per unit mass. The large mass and volume normalized rate constant, kMV, obtained for the nanoscale iron was a result of this high surface area. However, the calculated surface area normalized rate constant, kSA, for the nanoscale iron was significantly lower than those for the granular iron samples tested. It was concluded that differences in surface area normalized rate constants, between different iron particle types, could be attributed to inherent characteristics of the iron, such as composition and crystal structure.

A nearly stoichiometric, (1×1) Cr₂O₃ (101̲2) surface was prepared from a single crystal of α-Cr₂O₃. The five-coordinate cations exposed at the stoichiometric surface dissociatively adsorb molecular oxygen to form a (1×1), terminating chromyl (Cr=O) layer that is stable to >1100 K. TDS and AES were used to investigate the reactivity of the halo-alkanes CFCl₂CH₂Cl, CF₂ClCH₂Cl, CF₃CH₂Cl, and CF₂CH₂F, in addition to the halo-alkenes CFCl=CH₂ and CF₂=CH₂. The halo-alkanes CFCl₂CH₂Cl, CF₂ClCH₂Cl, and CF₃CH₂Cl undergo 1,2-dihalo elimination similar to the Zn-catalyzed dehalogenation of vicinal dihalides to form alkenes. Some acetylene is also formed. The halo-alkenes CFCl=CH₂ and CF₂=CH₂ decompose to yield acetylene. Halogen removed from the molecules remains bound to the surface following TDS experiments and eventually terminates the surface chemistry due to site blocking of the cations. Reactivity is directly related to the chlorine content of the molecules investigated. Only CFCl₂CH₂Cl was reactive on a chromyl-terminated surface.