Photochemical Degradation Of Perfluorooctylphosphonic Acid(PFOPA) In Water

Perfluorooctylphosphonic acid?PFOPA?is a class of synthetic commercial perfluorinated compounds and a newly emerging class of organic pollutants found in water,which has a chemical structure similar to that of perfluorooctanoic acid?PFOA?.It is a potential prerequisite for PFOA.It has only been found to be capable of metabolic transformation by biological action,but the pathway of degradation and transformation is still unclear,and other transformation pathways have not been reported.Therefore,in order to effectively control the pollution of PFOA in the environment,and to find other possible ways to convert PFOPA into PFOA in the environment.In this paper,the photochemical properties of PFOPA under ultraviolet lights were investigated.By investigating the influence of various factors and analyzing and identifying the degradation products,the possible degradation mechanism of PFOPA was inferred and the following results were obtained:?1?Direct photolysis of PFOPA?1?The degradation efficiency of PFOPA under 185 nm UV much higher than254 nm UV.And the photochemical degradation reactions of PFOPA follow the pseudo first-order reaction kinetics.?2?When the pH value of the solution is weakly acidic,the degradation of PFOPA is best.At this time,the ion form of PFOPA is more easily degraded than the molecular form of PFOPA.?3?The effects of different common anions in water on the direct photolysis of PFOPA showed that:Cl^-can promote the degradation of PFOPA,CO₃^2-,HCO₃^-and NO₃^-have an inhibitory effect on the degradation,while in the later stage of the reaction,the effect of SO₄^2-on PFOPA degradation is close to the control group.?4?The study of free radical trapping inhibition experiments found that HO·with oxidative activity has a slight promotion effect on its degradation,but it is not the main active species of PFOPA degradation.?5?PFOPA photolysis products are fluoride ion,PFOA and some short chain perfluorocarboxylic acids,such as TFA,PFPrA,PFBA,PFPeA,PFHxA and PFHpA.?6?According to the influence of the free radical trapping agent and the identification of indirect photolysis products,firstly,PFOPA breaks the C-P bond and through a series of reactions to form PFOA,then gradually removes-CF₂ to form a series of short-chain perfluorocarboxylic acids.In this process,it's accompanied by defluorination.?2?Indirect photolysis of PFOPA under 185 nm UV?1?TiO₂?P25,powdery?is used as a photocatalyst,which is mainly anatase type.?2?The indirect photolysis efficiency of PFOPA is greater than that of direct photolysis,and the reactions follow the pseudo first-order reaction kinetics.?3?In the reaction system,when the concentration of P25 is in the range of0.02².00 mg·L^-1,its promotion of PFOPA degradation increase with the increase of P25 concentration in solution.?4?The degradation rate of PFOPA decreased with the initial pH of the solution.?5?The studies on the effects of common anions in water show that:under 185nm UV,all anions in the 60 min before the reaction inhibit the indirect photolysis of PFOPA,and as the reaction proceeds,Cl^-and HCO₃^-gradually promote.effect.The remaining anions exhibited inhibitory effects ranging from large to small,SO₄^2-,CO₃^2-and NO₃^-.?6?The experiments on the inhibition of free radical scavengers show that oxalic acid is more effective in inhibiting the indirect photolysis of PFOPA than in isopropanol.?7?The indirect photolysis product of PFOPA is the same as its direct photolysis,including fluoride ion,PFOA and some short-chain perfluorocarboxylic acids?TFA,PFPrA,PFBA,PFPeA,PFHxA and PFHpA?.?8?According to the influence of the free radical trapping agent and the identification of indirect photolysis products,In the process of indirect photolysis of PFOPA,in addition to partial direct photolysis,the hole oxidation on the surface of P25 is the main reason for the degradation of PFOPA.?3?Indirect photolysis of PFOPA under 254 nm UV?1?TiO₂?P25,powdery?is used as a photocatalyst,which is mainly anatase type.?2?The indirect photolysis efficiency of PFOPA is greater than that of direct photolysis,and the reactions follow the pseudo first-order reaction kinetics.?3?In the reaction system,when the concentration of P25 is in the range of0.02².00 mg·L^-1,its promotion of PFOPA degradation increase with the increase of P25 concentration in solution.?4?Except for pH=3,it showed strong promotion effect.When the pH environment of the solution was weakly acidic,near-neutral and strong alkaline,there was no significant difference in the degradation efficiency of PFOPA.?5?The studies on the effects of common anions in water show that:under the254 nm UV,the inhibition of CO₃^2-and HCO₃^-is most obvious in the reaction system,and the promotion of Cl^-is the largest.SO₄^2-and NO₃^-showed inhibition at 60 min before the reaction,and gradually approached the control group as the reaction proceeded.?6?The experiments inhibited by free radical scavengers show that:the inhibitory effect of isopropanol on the indirect photolysis of PFOPA is less than that of oxalic acid.?7?The indirect photolysis product of PFOPA is the same as its direct photolysis,including fluoride ion,PFOA and some short-chain perfluorocarboxylic acids?TFA,PFPrA,PFBA,PFPeA,PFHxA and PFHpA?.?8?According to the influence of the free radical trapping agent and the identification of indirect photolysis products,the hole oxidation on the surface of P25is the main reason for the degradation of PFOPA.In addition to this there is also some direct photolysis.