Decomposition of Environmentally Persistent Perfluorinatied Acids by Use of Metal Complex Photocatalysis

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The decomposition of environmentally persistent perfluorocarboxylic acids (PFCAs, typically perfluorooctanoic acid ; PFOA) in water by UV-visible light irradiation, by H_2O_2 with UV-visible light irradiation, by a tungstic heteropolyacid (polyoxometalate) photocatalyst, and by persulfate was examined. In the case of long-chain PFCAs such as PFOA, direct photolysis proceeded slowly to produce CO_2, F^- ions and short-chain PFCAs. Compared to the direct photolysis, H_2O_2 was less effective in PFCA decomposition. On the other hand, the heteropolyacid photocatalyst led to efficient PFCA decomposition and the production of F^- ions and CO_2. The photocatalyst also suppressed the accumulation of short-chain PFCAs in the reaction solution. PFOA in the concentrations of 1.35 mM was completely decomposed by the catalyst within 24 h of irradiation from a 200-W xenon-mercury lamp, with no accompanying catalyst degradation. Gas chromatography/mass spectrometry (GC/MS) measurements showed no trace of environmentally undesirable species such as CF_4, which has a very high global-warming potential. When persulfate was used as a photochemical oxidant, the PFCA decomposition proceeded more rapidly. PFOA at a concentration of 1.35 mM was completely decomposed by a photochemical system using 50 mM S_2O_8^<2-> and 4 h of irradiation and the initial PFOA decomposition rate was 11 times higher than with photolysis alone.