Perfluorooctanoic acid (PFOA) is a persistent environmental contaminant of emerging concern. Superoxide has been proposed to degrade PFOA, and it might play a role in degrading other per- and polyfluoroalkyl substances (PFAS). Superoxide stability (to reach target pollutants) can be enhanced by certain solid matrices, redox mediators, or low polarity solvents. In this project, three well-defined superoxide-producing systems were evaluated to determine the role of superoxide in PFOA degradation: KO2 in dimethyl sulfoxide, xanthine oxidase with hypoxanthine, and WOx/ZrO2 catalyst with H2O2. All three were confirmed to produce superoxide by electron paramagnetic resonance spectroscopy and by precipitation of nitroblue tetrazolium, a common superoxide probe. Positive control experiments showed that the produced superoxide degraded ~48% of bisphenol A within one day. However, none of the systems degraded PFOA, and no degradation byproducts or fluoride release were detected. Additional experiments using H2O2 and UVC light to produce hydroxyl radicals showed that hydroxyl radicals alone do not degrade PFOA. The results from these experiments demonstrate that these ROS cannot initiate PFOA degradation. However, these data do not rule out the possibility of superoxide or hydroxyl radicals participating in PFOA degradation in more complex systems that activate PFOA, (e.g. heterogeneous catalysis, mediator-based enzymatic systems) or promote concerted degradation via multiple reactive oxygen species (ROS) and reaction pathways.

Discerning the relevance of superoxide and hydroxyl radical in PFOA degradation

ACS Spring 2022

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