Date of Award

January 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

First Advisor

Feng Xiao

Abstract

Per- and polyfluoroalkyl substances (PFAS) are anthropogenic chemicals that have been produced for decades as either processing aids or individual ingredients in many industrial and commercial products, including aqueous film-forming foams (AFFFs), non-stick cookware, and fast-food packaging. PFOA and PFOS have been reported in >95% of blood samples collected during multiple U.S. national surveys at concentrations that are a risk to human health. Thermal treatment is routinely used to reactivate the spent granular activated carbon (GAC) from water purification facilities. The aims of this study were: (1) to improve our understanding of the thermal stability of per- and polyfluoroalkyl substances; (2) to investigate their decomposition mechanisms on spent granular activated carbon (GAC) during thermal reactivation; and (3) present a detailed investigation of the fate of per- and polyfluoroalkyl substances (PFAS) and one PFAS alternative (GenX) in thermal processes, focusing on the effect of GAC. We studied seven perfluoroalkyl carboxylic acids (PFCAs), three perfluoroalkyl sulfonic acids (PFSAs), and one perfluoroalkyl ether carboxylic acid (PFECA) in different atmospheres (N2, O2, CO2, and air). We found that the destabilization of studied compounds during thermal treatment followed first-order kinetics. The temperature needed for thermally destabilizing PFCAs increased with the number of perfluorinated carbons (nCF2). Decomposition of PFCAs such as perfluorooctanoic acid (PFOA) on GAC initiated at temperatures as low as 200 °C. The PFECA was even more readily decomposed than PFCA with the same nCF2. PFSAs such as perfluorooctanesulfonic acid (PFOS), on the other hand, required a much higher temperature (≥450 °C) to decompose. Volatile organofluorine species were the main thermal decomposition product of PFOA and PFOS at low to moderate temperatures (≤600 °C). Efficient mineralization to fluoride ions (>80%) of PFOA and PFOS on GAC occurred at 700 °C or higher, accompanied by near complete PFOA and PFOS decomposition (>99.9%). We demonstrate that the thermolysis of perfluoroalkyl carboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA), and GenX can occur at temperatures of 150‒200 C. Three temperature zones were discovered for PFOA, including a stable and nonvolatile zone (≤90 C), a phase-transfer and thermal decomposition zone (90‒400 C), and a fast decomposition zone (≥400 C). The thermal decomposition began with the homolysis of a C‒C bond next to the carboxyl group of PFCAs, which formed unstable perfluoroalkyl radicals. Dual decomposition pathways seem to exist. The addition of a highly porous adsorbent, such as GAC or a copolymer resin, compressed the intermediate sublimation zone of PFCAs, changed their thermal decomposition pathways, and increased the decomposition rate constant by up to 150-fold at 250 C. The results indicate that the observed thermal decomposition acceleration was linked to the adsorption of gas-phase PFCA molecules on GAC. Overall, the results indicate that (1) decomposition of PFCAs and GenX in thermal water/wastewater/sludge/waste treatments is very likely provided sufficiently high temperatures are used, and (2) the presence or addition of GAC or other highly porous materials can accelerate thermal PFAS decomposition.

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