Synergistic impact of humic acid on the photo-reductive decomposition of perfluorooctanoic acid

Abstract

Perfluorooctanoic acid (PFOA) is a stable perfluoro-surfactant due to the contribution of electronegative fluorine atoms and the high strength of C-F bonds (116 kcal/mol). However, UV irradiation of iodide at 254 nm produces hydrated electrons (e_(aq)^–) that react with PFOA to initiate its photo-reductive decomposition. In this study, we investigate the role of humic acid (HA) during the decomposition of PFOA at λ_(max) = 254 nm as mediated by the formation of e_(aq)^–. Results show that, in UV/iodide/HA system, the decomposition and defluorination ratios of PFOA at 1.5 h were increased to 67.5% and 23.5%, respectively, compared with iodide-photolysis alone results as 8.7% and 3.3%, respectively, at room temperature (25°C) and pH 10.0 under anaerobic conditions. Kinetic analysis indicated that the PFOA decomposition fit the first-order kinetics with a rate constant of 0.822 h^(–1). Reaction mechanism accounting for the effects of HA on the photo-reductive decomposition of PFOA is proposed that the aromatic rings of HA react with I_2 to form π-complexes, which effectively inhibits the reactive quenching of e_(aq)^– by I_2. After adduct formation and subsequent oxidation of phenolic hydroxy groups by I_2, I^– is regenerated, which allows for the continuous photo-production of e_(aq)^–. The quinone moieties in HA effectively act as electron shuttles leading to the reduction of I_2 back to 2 I^–. The reaction sequence involving HA provides a catalytic cycle that leads to the continuous production of e_(aq)^–, which in turn drives the decomposition and defluorination of PFOA.

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