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. 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 degrees 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 p-complexes, which effectively inhibits the reactive quenching of eaq -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 eaq -. 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 eaq -, which in turn drives the decomposition and defluorination of PFOA.