To confirm the reactions involved in the plasma-based degradation process of perfluoroalkyl substances, this study analyzed the relationship between the degradation ratio of perfluorooctanoic acid (PFOA) and reductive radicals, such as hydrogen atoms (•H), at the plasma–liquid interface. In situ electron spin resonance (ESR) measurements for the detection of free radicals at liquid interfaces were performed for in-liquid Ar and He plasmas, which were also used for the PFOA degradation experiments. In situ ESR coupled with in-liquid plasma equipment using spin-trapping reagents indicated the production of hydroxyl radicals (•OH) and •H. The kinetic analysis of the concentration of radical adducts during plasma treatment allowed their production rates to be estimated. This method can be used to evaluate the amount of •OH and •H reacting at liquid interfaces using the rate constant of each radical adduct as an indicator. These measurements revealed that the He plasma supplied fewer •H than the Ar plasma, while supplying the same amount of •OH. The difference in the production of •H at the liquid interfaces between Ar and He plasmas with constant power consumption did not affect the degradation and defluorination ratio of PFOA, which were similar in the Ar and He plasma treatments. These results disproving the relationship between the •H from the plasma and the degradation reaction of PFOA are consistent with the notion that plasma-based degradation processes are primarily driven by electrons and ions.
Inoue et al. (Fri,) studied this question.
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