Hydroxyl-radical based advanced oxidation processes can increase perfluoroalkyl substances beyond drinking water standards: Results from a pilot study

Advanced oxidation processes (AOPs) are popular technologies employed across the U.S. for wastewater reclamation and drinking water treatment of recalcitrant chemicals. Although there is consensus about the ineffectiveness of AOPs to treat perfluoroalkyl substances (PFASs; not polyfluoro compounds by definition here), there is a lack of field data demonstrating their impact on the transformation of unknown PFAS precursors during groundwater treatment. In this study, the fate of PFASs in seven pilot-scale AOPs, including four different technologies (UV/H₂O₂, UV/Cl₂, UV/TiO₂, and O₃/H₂O₂), was assessed at four drinking water systems across New York State (NYS), USA. Seven of 18 PFASs were detected in the influent at concentrations ranging from below method detection to 64 ng/L. Across all systems, all detected PFASs showed an increase in concentration after treatment presumably due to unknown precursor transformation with specific increases for perfluorobutane sulfonate (PFBS), perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid (PFHpA), perfluorohexane sulfonate (PFHxS), perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), and perfluorononanoic acid (PFNA) averaging 405 (range: 0 – 1220) %, 1.0 (-7 – 9) %, 3.8 (0 – 9.5) %, 3.3 (-11 – 13) %, 14 (0 – 48) %, 13 (3 – 25) %, and 2 (0 – 5.2) %, respectively. The increase in PFAS concentration was dependent on UV and oxidant dose, further confirming that transformation reactions were occurring due to AOPs similar to a total oxidizable precursor assay. At one of the sites, PFOA levels exceeded the current NYS drinking water standard of 10 ng/L after, but not before treatment, highlighting the importance of considering the potential impact of AOP on treated water quality when designing treatment systems for regulatory compliance. The increase in PFAS concentration in the AOP systems positively correlated (r = 0.91) with nitrate levels in groundwater, suggesting that onsite septic discharges may be an important source of PFAS contamination in these unsewered study areas. Results from this pilot-scale demonstration reveal that hydroxyl radical-based AOPs, although ineffective in treating PFASs, can help to reveal the true extent of PFAS contamination in source waters.