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Thermal Decomposition Mechanisms of Per- and Polyfluoroalkyl Substances and Application to Analysis of Total Organofluorine
Civil and Environmental Engineering
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Per- and polyfluoroalkyl substances (PFASs) are a family of anthropogenic organic chemicals broadly defined as containing a fully or partially fluorinated alkyl chain. PFASs are ubiquitous in the environment due to their wide application in consumer products and aqueous film-forming foams (AFFFs) for over 70 years. The likelihood of exposure in combination with the potential for adverse ecological and health effects has caused tremendous concern.Thermal treatment of PFASs has attracted increasing attention. To understand thermal decomposition and mineralization of PFASs, a thorough literature review was conducted. The review evaluated thermal decomposition mechanisms, pathways, and byproducts of PFASs which are crucial to the design and operation of thermal treatment processes. Some primary decomposition mechanisms of PFASs were identified, including cleavage of intramolecular bonds through transition states, direct homolytic cleavage, radical reactions, hydrolysis, and oxidation. Mineralization tends to occur above 700 °C in oxidizing atmospheres and at lower temperatures in inert atmospheres, volatile organofluorine is formed. However, the thermal decomposition products and mechanisms for most PFASs are still not clear, especially in oxygen (i.e., combustion), which is typical for many thermal treatment processes, including incineration. To better understand thermal decomposition mechanisms and products, the fate and transformations of perfluoropropionic acid (PFPrA) and perfluorobutyric acid (PFBA) in nitrogen and oxygen at temperatures from 200 °C to 780 °C were investigated. Also, platinum was used as packing material to study potential surface catalysis effects. O2 can facilitate thermal defluorination by reacting with PFCAs directly and oxidizing pyrolysis products (e.g., fluorinated olefins and fluorocarbon radicals). The addition of platinum to the combustion process improved the defluorination efficiency of PFBA at lower temperatures while quartz promoted the mineralization of PFAS into SiF4 at higher temperatures. Finally, quantitative PFAS analysis is challenging as it is conducted with instrumentation that not all laboratories have access to. To provide a quantification method which is more widely available, I developed a methodology to determine total organofluorine based on combustion followed by ion chromatography methodology. A modified total organic carbon (TOC) analyzer, common in laboratories across disciplines, was used as a combustion furnace. I showed that TOC combined with ion chromatography are promising tools to directly measure PFAS concentrations using bench-scale mechanistic experiments. Application of solid phase extraction would enable measurements at environmentally relevant concentrations.