Photoreduction of Perfluorooctanoic Acid (PFOA) in Isopropanol Aqueous Solution

Perfluorinated compounds (PFCs) were used as an industrial raw material since 1950s, Perfluorooctanoic acid (PFOA) was one of these PFCs. PFOA has properties of persistant organic pollutants (POPs) and the characteristics of environmental fate and bio-toxicology had investigated widely in recent years. Based on above, this study invesgated the photo-reduction (UV) of PFOA under various pH, electron donor (isopropanol, IPA) concentration and amount of catalyst (titanium dioxide, TiO2) in anaerobic aqueous solution.
The experimental result showed that decomposition rate of PFOA was about 20% after 24 hours reaction time in all conditions of no titanium dioxide added. Moreover, higher pH value and isopropanol concentraton implied higher fluoride ion yield in aqueous solution. According to the MASS analysis, the formation of short chain perfluorocarboxylic acids (PFCAs) were main byproduct under no IPA condition, and the reaction mechanism was significantly different with IPA aqueous solution. Especially, the intermediate compound of m/z=235 [C4F9O]- was inferred in IPA aqueous solution. In no IPA condition, direct photolysis was suggested the reaction mechanism to form shorter chain PFCAs. However, the byproduct of shorter chain PFCAs would induce into radical chain reaction and yield more fluoride ion in IPA aqueous solution, especially in alkaline condition.
In catalytic photo-reduction PFOA, the result exhibited that the 0.5 g/L of titanium dioxide (optimum dose) presented significantly higher PFOA removal rate than no TiO2 condition. The PFOA removal rate could be ranged from 57% to 73% during 24 hours reaction time under various IPA concentrations in acidic condition. However, in alkaline condition PFOA did not display same high removal rate. The reason could refer that electrical repulsion between TiO2 and PFOA caused by same negative surface charges in alkaline condition. Formation of short chain PFCAs were main byproduct under no IPA condition, and the reaction mechanism was also significantly different with IPA aqueous solution. The intermediate compound of m/z=235 [C7F14HCOO]- of reduced PFOA was inferred in IPA aqueous solution. PFOA react with electron hole which generated by UV/TiO2 to produce shoter chain PFCAs under no IPA added. In IPA aqueous solution, IPA reacts with electron hole to incur radical chain reaction and reduced PFOA. Summarizing, the highest PFOA removal efficiency of 73% during 24 hours reaction time was under the condition of 0.5 g/L TiO2, 2 wt% IPA and pH 3 in aqueous solution.