Photocatalytic decomposition of a perfluoroether carboxylic acid by tungstic heteropolyacids in water

The photochemical decomposition of perfluoroether carboxylic acid C2F5OC2F4OCF2COOH (EEA), which is being introduced in industry as a surfactant alternative to environmentally persistent and bioaccumulative perfluorocarboxylic acids, was investigated by use of water-soluble tungstic heteropolyacids H3PW12O40 and H4SiW12O40, and the results were compared with those of conventional techniques, UV–visible light irradiation in the presence of H2O2, treatment with Fenton's reagent, and heterogeneous TiO2 photocatalyst. H4SiW12O40 induced little photochemical decomposition of EEA. Irradiation with UV–visible light in the presence of H2O2 and treatment with Fenton's reagent also led to little decomposition of EEA. Alternatively, irradiation of EEA at wavelengths of >290 nm in the presence of H3PW12O40 under an oxygen atmosphere efficiently decomposed EEA to F− and CO2. The reaction followed pseudo-first-order kinetics, and the decomposition rate was almost identical with that of the corresponding perfluorocarboxylic acid, that is, C5F11COOH. The proposed reaction mechanism involves redox reactions between EEA, the heteropolyacid catalyst, and oxygen, followed by CC bond cleavage between C2F5OC2F4OCF2 and COOH (induced by the photo-Kolbe mechanism), and subsequent formation of C2F5OCF2COOH and trifluoroacetic acid. EEA was also photocatalytically decomposed by TiO2 with a rate constant comparable with that by H3PW12O40. Although the reactivity of TiO2 toward perfluorocarboxylic acids was reported to be very low, EEA, which has ether linkages in the perfluoroalkyl chain, was clearly decomposed to F− and CO2. This fact suggests that oxidative active species other than OH radicals significantly participate in the reaction.