Study On Degradation Of Perfluorooctanoic Acid By UV-Fenton

Perfluorinated compounds (PFCs) have many excellent characteristics such as high surface-active effect, high thermal and chemical stability and high light transparency. Thus they have been widely used as surface treatment agents in emulsion, fire retardants, paper coatings and carpet cleaners. These compounds have caused serious health concerns such as potential toxicity on human organs, reproductive and immune systems because of its persistence and bioaccumulation. Perfluorooctane sulfonates (PFOS) and its salts and perfluorooctane sulfonyl chloride are classified as persistent organic pollunants (POPs) on the fourth conference of the Stockholm Convention. Perfluorooctanoic acid (PFOA) and PFOS are produced when most of these perfluorinated precursor degradation.Hydroxyl radical (·OH) oxidation as one of the advanced oxidation technologies is commonly used in the treatment of organic pollutants. But many researchers consider that·OH cannot oxidize PFCs on account of its abstract hydrogen process hindered. So they think that·OH has no affinity to these compounds. This work aimed at solving this problem. We used UV irradiation enhanced the Fenton's oxidation capacity in order to degradation PFCs.In this work, we used Fenton's reagents which combined with UV irradiation of 254 nm to degrade PFOA. The optimized conditions were as follows: the initial Fe²⁺ concentration 2.0 mM, the initial H₂O₂ concentration 20.0 mM and the initial solution pH 3.0. It was found that defluorination ratio of PFOA at the initial period of the reaction increased rapidly. The defluorination and degradation ratio reached more than 40% and 90% within the first two hours, respectively. The degradation products were short-chain perfluorinated carboxylic acids and F^- as determined by IC and LC-MS, and there was no HCOOH formed.In addition, this work discussed the PFOA degradation mechanism by UV-Fenton system. Through the experimental results of the role of hydrogen peroxide (H₂O₂), identification of active radicals and the molar ratio of Fe²⁺/Fe (total) in the reaction solution, we propose the possible mechanisms as below: when there is H₂O₂ in the system,·OH accelerate decarboxylation process of PFOA, and if H₂O₂ completely consumed, the ferric ion in the system receive one electron from PFOA which leading to decarboxylation. The subsequent degradation processes are similar of the two ways decarboxylation.