Photo-reductive defluorination of perfluorooctanoic acid in water

摘要

Globally distributed and highly stable, perfluorooctanoic acid (PFOA) has prompted much concern regarding its accumulation in the natural environment and its threats to ecosystems. Therefore, it is desirable to develop an effective treatment against PFOA pollution. In this study, a photo-reduction method is developed and evaluated for the decomposition of perfluorooctanoic acid (PFOA) in aqueous phase with potassium iodide (KI) as a mediator. The experiment was conducted under 254 nm irradiation at room temperature and pH 9 under anaerobic conditions. Ultraviolet photolysis of iodide solutions led to the generation of hydrated electrons (e_(aq)~-, E_(aq/e)°= -2.9 V), which contributed to the defluorination of PFOA. Defluorination was confirmed by fluoride release of 98%, indicating almost complete defluorination of PFOA. Kinetic analysis indicated that the PFOA decomposition fit the first-order model with a rate constant of 7.3 × 10~(-3) min~(-1). Besides fluoride ions, additional intermediates identified and quantified include formic acid, acetic acid, and six short-chain perfluorocarboxylic acids (C1-C6). Furthermore, small amounts of CF_3H and C_2F_6 were also detected as reaction products by using GC/MS. With observation of the degradation products and verification via an isotopic labeling method, two major defluorination pathways of PFOA are proposed: direct cleavage of C-F bonds attacked by hydrated electrons as the nucleophile; and stepwise removal of CF_2 by UV irradiation and hydrolysis. This method was applied to the decomposition of PFOA in wastewater issued from a fluo-rochemical plant and proved to be effective.