Study On Defluorination Of Perfluorooctanoic Acid By Vacuum Ultraviolet And Thermal Activated Persulfate

Perfluorooctanoic acid is a persistent organic pollutant that is often detected in water.It is applied to areas that are closely related to humans such as non-stick coatings,semiconductors and fire-fighting foams because of its good thermal stability,surface activity and chemical stability.At the Asia-Pacific Preparatory Meeting of the Conference of the Parties to the International Convention on the Hazardous Chemicals(BRS COPS),held in Suzhou,China,from March 4 to March 9,2019,PFOA is clearly on the blacklist of alternative bans under the Stockholm Convention.In recent years,the effective technology of PFOA degradation has become a new research hotspot in the field of environmental protection.In this paper,perfluorooctanoic acid,a common perfluoro compound,was used as the target pollutant.Vacuum ultraviolet/Persulfate(VUV/PS)and Thermally activated persulfate(TAP)were used for defluorination of PFOA in water.For two different systems,the effects of factors such as initial pH,PS dosage,initial PFOA concentration,and temperature on the defluorination effect of PFOA were investigated,and a kinetic model was established.Response surface method(RSM)was used for analyzing the main factors of the system for defluorination of PFOA,constructing a response surface model and optimizing the defluorination conditions of the system.In addition.LC-MS was used for analyzing the byproducts during the reaction and the defluorination pathThe study found that Ultraviolet(UV)has little effect on defluorination of PFOA,and VUV can greatly promote the defluorination of PFOA.After adding persulfate(PS)to the VUV system,the defluorination efficiency of PFOA was further improved.However,when PS was replaced by residual chlorine(RC)of the same concentration,the effect of PFOA defluorination was smaller than VUV alone.The following conclusions can be obtained for VUV/PS system:the defluorination reaction of PFOA all conforms to the pseudo-first-order kinetic model.The initial pH 5.0-pH 9.0 has little effect on the defluorination efficiency of PFOA,and the effect is better under acidic pH conditions.With PFOA concentration of 15 ?M,the defluorination effect is ideal when the molar ratio of PS to PFOA is 60:1.It would further increased with increasing temperature and the system had better performance for low-concentration PFOA solutions.The negative effects of inorganic anions on the defluorination of PFOA in this system following the order:NO3->Cl->CO32-.The prediction model obtained by the BOX-behnken method of the Design Expert 10 software has a high regression rate and a good fit.The factors that affect the system following the order:PFOA initial concentration>PS initial concentration>initial pH;the interaction of two of the three factors is not significant.The error between the theoretical optimal value predicted by the software and the actual measured average value is small,indicating good practical value.There are two degradation pathways of PFOA by VUV/PS system:direct photolysis and reaction with SO4-·.The two paths are essentially the process of gradually removing CF2 in different ways,generating short-chain perfluorocarboxylic acids from C2 to C7,and finally mineralizing into carbon dioxide and fluoride ions.In addition,the following conclusions can be obtained for TAP system:the defluorination reaction of PFOA all conforms to the pseudo-first-order kinetic model;with initial PFOA concentration of 15?M,the molar ratio of PS to PFOA of 4000:1 and the reaction temperature of 80?,the defluorination effect of PFOA is ideal and the initial pH value(5-9)has little effect on the defluorination efficiency of PFOA;the system had better performance for low-concentration PFOA solutions;the negative effects of inorganic anions on the defluorination of PFOA in this system following the order:CO32->NO3->Cl-.The prediction model obtained by the BOX-behnken method of the Design Expert 10 software has a high regression rate and a good fit.The factors that affect the system following the order:temperature>PS initial concentration>initial pH;the interaction of two of the three factors is not significant.The error between the theoretical optimal value predicted by the software and the actual measured average value is small,indicating good practical value.There is only one degradation pathway of PFOA by TAP system:the reaction with SO4-·.Therefore,it is a process of gradually removing CF2,forming short chain perfluorocarboxylic acid of C2?C7,and finally mineralizing into carbon dioxide and fluorine ions.