The fate of polyfluoroalkyl substances (precursors) in drinking-water treatment processes
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Per- and polyfluoroalkyl substances (PFAS) are a class of synthetic chemicals whose widespread use in consumer and industrial products, including aqueous film-forming foams (AFFFs), has led to extensive environmental contamination and poses considerable risks to human health. Conventional drinking water treatment systems often inadequately remove PFAS. Moreover, these processes can transform polyfluoroalkyl compounds (or precursors) into highly stable perfluoroalkyl acids (PFAAs), a critical transformation pathway historically overlooked in drinking water treatment, which creates more persistent compounds often subject to stricter regulation. While separation technologies like granular activated carbon (GAC) are commonly employed, significant challenges remain, including variable adsorption efficiencies in complex water matrices, the management of spent media, and the potential for forming hazardous products of incomplete destruction (PIDs) during thermal regeneration of PFAS-landed media. This dissertation aims to address critical knowledge gaps regarding the transformation, treatability, and ultimate destruction of precursors and perfluorinated counterparts during drinking-water treatment (coagulation, flocculation, disinfection, GAC adsorption) and the thermal regeneration and reactivation of spent GAC. The results will provide valuable insights into optimizing PFAS removal strategies, improving the sustainability of water treatment processes, and minimizing the formation of hazardous byproducts. Within conventional drinking water treatment, coagulation demonstrated limited effectiveness (4%-28%) for cationic and zwitterionic precursors in AFFFs, whereas anionic PFAS exhibited higher removal rates (up to 54%). Flocculation subsequently improved overall removal efficiency to approximately 44%. Chlorine disinfection investigations revealed complex mechanisms where AFFF-derived polyfluoroalkyl substances (precursors) transformed into more stable perfluorinated acids, predominantly via chlorine attacks on susceptible -NH- functional groups, resulting in persistent perfluorinated by-products. Chlorination degraded all tested precursors effectively; however, reaction rate constants significantly decreased (over 50%) due to interference from dissolved organic matter (DOM). Reactivity varied notably among PFAS, with nitrogen-containing and longer-chain compounds undergoing more extensive degradation. These findings provide the first comprehensive assessment of conventional water treatment processes for AFFF-contaminated water samples. Thermal regeneration analyses demonstrated effective regeneration of PFAS-laden activated carbon using conventional coil heaters (muffle or tube furnaces) or novel induction heating methods, in both air and nitrogen atmospheres. Spectroscopic analyses indicated that PFAS thermal degradation initiated via bond cleavage, forming perfluoroalkyl radicals and subsequent organofluorine PIDs (e.g., perfluoroalkenes). Critically, the generation of tetrafluoromethane (CF4) was not detected, but carbonyl fluoride (CF2O) formed exclusively under specific conditions from potassium perfluorooctanesulfonate. Incorporating additives like GAC and noble metal catalysts significantly enhanced PFAS mineralization and minimized PID formation. Overall, this dissertation advances the mechanistic understanding of precursor transformation, adsorption behaviors, and thermal degradation processes in engineered systems. These findings support optimized and sustainable treatment strategies, providing critical insights for comprehensive environmental management practices and regulatory frameworks aimed at mitigating PFAS-associated risks.
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Ph. D