Massive Unknown Precursors in Wastewater Treatment Plants: A Significant Source of Ultrashort-Chain Perfluoroalkyl Substances in the Receiving River

This study investigated the occurrence, fate, and discharge of per- and polyfluoroalkyl substances (PFAS) across multiple wastewater treatment plants (WWTPs), specifically targeting ultrashort-chain perfluoroalkyl acids (PFAAs), as well as their precursors based on the total oxidizable precursor (TOP) assay. A total of 28 PFASs were detected, including four ultrashort-chain and six emerging compounds. Industrial WWTP (I2) exhibited substantially elevated total PFAS levels (1596 μg/L) in wastewater compared to those of municipal/domestic WWTP. Trifluoroacetic acid (TFA) is the dominant PFAS, with the highest concentration at 806 μg/L, contributing 22.3-92.8% for ∑PFAS. Hexafluoropropylene oxide dimer acid is the dominant emerging PFAS with a concentration up to 379 μg/L, while sodium <i>p</i>-perfluorous nonenoxybenzenesulfonate is the more frequently detected emerging PFAS. PFAS concentrations increase in most WWTPs after treatment but decrease in some WWTPs, especially M5 adding membrane bioreactor to the conventional treatment process. The 5.15-1265% increase in PFAA molar concentrations after the TOP assay reveals substantial precursors in wastewater. TFA accounts for over 90% of the total increase in PFAAs in some WWTPs. However, more than 99% of TFA increments cannot be explained from the known precursors. The annual discharge of TFA contributes 50% for the total PFAS emission flux (8993 kg/a), which should be a concern.