Plasma–liquid interfacial reaction in decomposition of perfluoro surfactants

Perfluorooctanoic acid (PFOA: C7F15COOH) in water was decomposed using either plasma generated inside oxygen bubbles or sulfate radical anions. During the PFOA decomposition processes, perfluorocarboxylic acids (PFCAs: CnF2n+1COOH) with shorter carbon chains were generated. The decomposition processes were deduced based on a zero-dimensional simulation study using experimentally obtained reaction rate constants. During the plasma treatment, the dominant reaction was found to be the direct decomposition of the PFCAs to gaseous products such as CO2 without the generation of shorter carbon-chain PFCAs in water. It is proposed that the thermal cleavages of the C–C bonds of the PFCA molecules occurred successively at the plasma–liquid interface because they adsorbed on the gas–liquid interface where they could contact the plasma. In contrast, in most of the reactions with sulfate radical anions in bulk water, the number of carbon atoms in the carbon chain decreased one by one as mentioned previously in the literature.