Migration And Transformation Of Per-and Polyfluoroalkyl Substances (PFASs) During Heat Treatment

Per-and polyfluoroalkyl substances（PFASs）are a class of synthetic organic compounds with very high stability,which are neither hydrophilic nor lipophilic,and have unique properties.Therefore,they are mass-produced and widely used in daily necessities.Most PFASs will be turned into municipal waste for incineration or pyrolysis with the end of daily necessities.These wastes containing PFASs can produce harmful gases through incineration or pyrolysis treatment,and their emissions will pollute the environment,enhance the greenhouse effect,and can exist in the atmosphere for a long time,affecting the atmosphere circulation.By studying the thermal behavior of PFASs,the harmful greenhouse gases that may be emitted by the thermal decomposition of PFASs can be predicted and warned.Adding Ca（OH）₂for mineralization not only generate environmentally friendly Ca F₂,but also greatly reduce the generation of harmful gases and thermally stabilize PFASs.In this article,five PFASs with different functional groups,including perfluorooctane sulfonate（PFOS）,Perfluorooctane sulfonamide（PFOSA）,6:2 Chlorinated polyfluoroalkyl ether sulfonic（F-53B）,Perfluorooctanoic acid（PFOA）,and 1H,1H,2H,2H-perflfluoro-decan-1-ol（8:2 FTOH）were selected as the research objects.Using the thermogravimetric/differential scanning calorimeter（TGA/DSC）analysis,examine their thermal decomposition characteristics（such as:weightlessness,endothermic peak conditions and thermal decomposition temperature,etc.）.They were heat-treated at different temperatures（100-900°C）using a thermal decomposition device,and gas products were detected using the gas chromatography/mass spectrometry（GC/MS）.The effects of functional groups,temperature and calcium hydroxide on the thermal decomposition gas products of PFASs and their thermal decomposition mechanism were studied.The qualitative and quantitative analysis of X-ray diffraction（XRD）was used to study the fluoride fixation efficiency of calcium hydroxide to PFASs,and the change rule of solid products at different temperatures were analyzed.The main conclusions are as follows:1.The TGA/DSC curve was studied and the results showed that PFOS and PFOSA,F-53B,PFOA and 8:2 FTOH started to decompose at 430°C,418°C,350°C,85°C and 55°C,respectively.The order of thermal stability was PFOS>F-53B>PFOSA>8:2 FTOH>PFOA.The thermal decomposition process of PFOS,PFOSA and F-53B consists of 2 stages,mainly due to its decomposition.The thermal decomposition characteristics of PFOA and 8:2 FTOH are similar to the single-step thermal decomposition process,mainly due to their own volatilization and decomposition.2.The types of gas products after thermal decomposition were studied.The results show that the pyrolysis products of PFASs with different functional groups were quite different.PFOS,PFOSA and F-53B were mainly thermally decomposed into perfluorocarbon gases C_nF_2n+2（n=4,5,6,7,8）,Hydrofluorocarbon(C₈H₃F₁₃,C₆H₃F₉and C₆HF₁₃),and Chlorofluorocarbons(C₆Cl₂F₁₂,C₄Cl₂F₈,C₂Cl₂F₄and C₂Cl F₅),respectively.PFOA was mainly decomposed into perfluorocarbon gases C_nF_2n+2（n=4,5,6,7）and C₆HF₁₃.8:2 FTOH was mainly thermally decomposed into FTOHs,C_nF_2n+2（n=6,7,8）and perfluorocarbon gases.Temperature had a obviously effect on pyrolysis products.With the increase of temperature,the types of gas products increase,and can produce smaller molecular gas products.High temperature can make PFASs decompose strongly.The addition of calcium hydroxide will significantly reduce the types of components that form gas products,generate new gas products,enhance the reaction capacity,and accelerate the solid-phase reaction.The thermal decomposition of the five kinds of PFASs were through the first-order reaction and the second-order reaction.5 PFASs were thermally decomposed into first-level pyrolysis products,and macromolecular products in the first-level pyrolysis products can also be pyrolyzed into products of smaller molecules,but the specific pyrolysis methods are different.3.The composition of the solid product and the change rule of its content were studied.The results showed that the composition of the solid product produced by the 5 kinds of PFASs and Ca（OH）₂after heat treatment were not much different,but the composition content were different.Among them,Ca（OH）₂had better fluorine mineralization effect on PFOS and F-53B,and more Ca F₂can be generated.PFOSA,PFOA and 8:2 FTOH have poor fluorine mineralization ability and generate less Ca F₂,and other solid products occupy a certain proportion.Ca（OH）₂had different fluorine fixation effects on PFASs.At 400-900°C,Ca（OH）₂had better fluorine fixation effect on PFOS and F-53B,which can reach more than 90.0%.At 600°C,Ca（OH）₂had a good fluorine fixation effect on F-53B and the fluorine transformation ratio was 95.0%.