Oxidative Remediation, Droplet-Accelerated Chemistry, and Transformation Product Analysis of Halogenated Micropollutants by High-Resolution Mass Spectrometry

<p dir="ltr">The use of persistent halogenated chemicals, such as per- and polyfluorinated alkyl substances (PFAS) and flame retardants, in consumer products results in chronic exposure to compounds with known adverse health effects. The presence of these persistent pollutants in low concentrations in the environment also leads to adverse impacts on environmental systems. In order to protect human and environmental health, it is necessary to develop strategies for chemical destruction of micropollutants, as well as characterize their transformation products in the environment to inform evolving exposure and toxicity.</p><p dir="ltr">Per- and polyfluorinated alkyl substances are extremely persistent, and have known carcinogenic, endocrine, and immune effects in humans. Ultraviolet photo-oxidation is a promising technology for the oxidation of concentrated PFAS waste streams. However, UV photo-oxidation typically does not fully mineralize PFAS, but instead produces short-chain perfluoroalkyl carboxylic acids, which are toxic and more mobile in aqueous environments compared to their parent compounds. One potential way to increase the efficiency of chemical reactions is through aerosolization, which has been shown to accelerate reaction kinetics by up to six orders of magnitude. PFAS are naturally surface-active in water due to their hydrophobic perfluoroalkyl chain, resulting in enrichment at the air-water interface of aerosolized microdroplets. Therefore, aerosolization can simultaneously concentrate PFAS and potentially accelerate the kinetics of photo-oxidation. In this dissertation, we observed the rate of vacuum ultraviolet photo-oxidation of 6:2 fluorotelomer carboxylic acid, 6:2 FTCA, in microdroplets and bulk solution. We observed almost an order of magnitude difference between the observed rate in microdroplets versus bulk using the same UV power input. This observed microdroplet accelerated kinetics of PFAS oxidation is novel, and is potentially useful as a remediation technology for concentrated waste streams, such as those produced from industrial waste or runoff from PFAS-containing firefighting foams.</p><p dir="ltr">Another potential means of oxidizing PFAS is through the use of TAML catalysts, which are iron-centered, biomimetic catalysts designed for water treatment. We evaluated the use of TAML catalysis for PFAS oxidation, and found preliminary evidence of defluorination of an unsaturated fluorotelomer carboxylic acid at pH 7 with hydrogen peroxide as the oxidant. TAML catalysis is also useful for the degradation of many classes of organic pollutants. Analytical methods are needed to quantify the kinetics of TAML-catalyzed oxidation of micropollutants. We evaluated resonance Raman spectroscopy as an alternative to UV-Vis spectroscopy for the kinetic monitoring of chromophoric micropollutant oxidation. We successfully extracted kinetic data for specific vibrational modes over time which is not possible for UV-Vis spectroscopy, and we evaluated several post-processing techniques that promoted data reproducibility.</p><p dir="ltr">Lastly, flame retardants are often added to consumer products such as furniture foams to inhibit flammability. When these products are burned in residential fires, the flame retardants decrease combustion efficiency, but the impacts of flame retardants on the resultant combustion aerosol is not well-studied. In this dissertation, we characterized intact flame retardants emitted from combustion of commercial furniture foams, two of which were marketed for use with children. We observed flame retardants in both the aerosol and gas phase emitted through combustion, which was a novel finding. We also observed that the decreased combustion efficiency resulted in higher aerosol mass loadings and a higher black carbon content compared to flame retardant-free foams.</p><p dir="ltr">Overall, the chapters of this dissertation each aim to improve understanding of the transformations of persistent micropollutants, whether it be from combustion processes or advanced oxidation technologies. Many of these chapters also focus on microdroplet acceleration, it mechanisms, and methods for quantifying it. Lastly, each chapter elucidates novel findings that can be used to understand and reduce human and environmental exposure to persistent micropollutants.</p>