To better understand the fate of per-and polyfluoroalkyl substances (PFAS) during conventional and advanced wastewater treatment, 42 PFAS (C3−C14 perfluorocarboxylic acids (PFCAs), C3−C10 perfluorosulfonic acids (PFSAs), per-and polyfluoroethers, and perfluoroalkyl acid (PFAA) precursors) were investigated through the treatment trains at two municipal wastewater treatment plants (WWTPs) using a targeted analysis, the total oxidizable precursor (TOP) assay, and the estimation of partitioning to sludge. Short-chain (C3−C7) PFAAs were found in higher concentrations in wastewater samples, while long-chain (≥C8) PFAAs dominated in sludge samples. PFAA concentrations were elevated by the wastewater treatment processes, particularly after biological treatment (191.3 and 185.1% increases of PFAAs at WWTP-A and B). After TOP oxidation, PFCAs, particularly short-chain ones, increased considerably (up to 311.4 and 409.3% increases of PFCAs at WWTP-A and B). The study results indicated that the transformation of precursors into shorter chain PFAAs by biological treatment and the partitioning of longer chain PFAS into sludge streams are key factors determining the fate of PFAS in WWTPs. With the increasing impact of shortchain PFAS over time, future research should focus on the evaluation of the fate and distribution of historical and emerging PFAS in WWTPs. KEYWORDS: (ultra)short-chain PFAS, PFAA precursors, municipal wastewater treatment plant, biological treatment, advanced treatment processes
The susceptibility of 19 representative per- and polyfluoroalkyl substances (PFAS) to direct photolysis and defluorination under far-UVC 222 nm irradiation was investigated. Enhanced photolysis occurred for perfluorocarboxylic acids (PFCAs), fluorotelomer unsaturated carboxylic acids (FTUCAs), and GenX, compared to that at conventional 254 nm irradiation on a similar fluence basis, while other PFAS showed minimal decay. For degradable PFAS, up to 81% of parent compound decay (photolysis rate constant (k 222 nm) = 8.19–34.76 L·Einstein–1; quantum yield (Φ222 nm) = 0.031–0.158) and up to 31% of defluorination were achieved within 4 h, and the major transformation products were shorter-chain PFCAs. Solution pH, dissolved oxygen, carbonate, phosphate, chloride, and humic acids had mild impacts, while nitrate significantly affected PFAS photolysis/defluorination at 222 nm. Decarboxylation is a crucial step of photolytic decay. The slower degradation of short-chain PFCAs than long-chain ones is related to molar absorptivity and may also be influenced by chain-length dependent structural factors, such as differences in pK a, conformation, and perfluoroalkyl radical stability. Meanwhile, theoretical calculations indicated that the widely proposed HF elimination from the alcohol intermediate (C n F2n+1OH) of PFCA is an unlikely degradation pathway due to high activation barriers. These new findings are useful for further development of far-UVC technology for PFAS in water treatment.
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