Discerning the inefficacy of hydroxyl radicals during perfluorooctanoic acid degradation

Perfluorooctanoic acid (PFOA) is a recalcitrant contaminant of emerging concern, and there is growing interest in advanced oxidation processes to degrade it. However, there is ambiguity in the literature about the efficacy of hydroxyl radicals ((OH)-O-center dot) for degrading PFOA. Here, we resolve this controversy by comparing PFOA degradation by UV photolysis (254 nm, 6 x 10(-6) E/L.s) versus UV + H2O2, which produces (OH)-O-center dot. We optimized (OH)-O-center dot production in a UV + H2O2 system using nitrobenzene (NB) as a (OH)-O-center dot probe, but even under optimized conditions (i.e., 5 g/L H2O2), no significant difference occurred in PFOA removal by UV photolysis (21.1 +/- 0.4%) versus UV + H2O2 (19.7 +/- 0.7%) after 1-day treatment. Both treatments also resulted in similar daughter by-product concentrations and defluorination efficiencies (9.5 +/- 1.7% for UV photolysis and 6.8 +/- 1.0% for UV + H2O2), which indicates that (OH)-O-center dot is ineffective towards PFOA degradation and infers that other degradation mechanisms that are independent of (OH)-O-center dot production should be explored. (C) 2020 Elsevier Ltd. All rights reserved.