The Effect Of Physical/chemical Technology On The Degradation And Transformation Mechanisms Of Fluoroalkylether Compounds

Poly-and perfluoroalkyl substances（PFASs）are a class of synthetic organic substances of great interest.Fluorinated alkyl chains endow PFASs with unique properties,including superior surface tension,thermal stability,and hydrophobic and oleophobic properties.Since the 1950s,PFASs have been mass-produced and widely used in clothing,insecticides,carpets,paints,shampoos,medicines,personal care products and fire-fighting foams.In recent years,PFASs have been widely detected in environmental samples and humans.Due to the characteristics of high persistence,bioaccumulation and potential biological toxicity,researchers and policy makers show great concern.Therefore,many countries have begun to produce and apply fluoroalkylether compounds to gradually replace traditional PFASs.Among them,the three most representative new substitutes for fluoroalkylether compounds include:6:2chlorinated polyfluoroalkyl ether acid（F-53B）,hexafluoropropylene oxide dimer acid（HFPO-DA）and docecafluoro-3H-4,8-dioxanonanoate（ADONA）.As one of the substitutes of perfluorooctane sulfonate（PFOS）,F-53B is widely used as an aerosolization inhibitor in metal plating industry in China,and its annual production was about 20-30 tons.At present,F-53B has been detected in the air,wastewater,surface water,municipal sewage sludge,wild animal,human serum and human umbilical cord blood.Existing evidence shows that F-53B can not only destroy the thyroid gland,but also induce neurotoxicity.HFPO-DA and ADONA as perfluorooctanoic acid（PFOA）substitutes have also been detected in soil,surface water,wild animals and human plasma.HFPO-DA has been shown to be toxic in thyroid cells,mouse liver cells and animal models,while ADONA could cause liver damage in rodents.Obviously,the widespread distribution of F-53B,HFPO-DA and ADONA poses a considerable threat to ecological security and human health.However,the current biodegradation efficiency of such compounds is not high,so it is necessary to find physical or chemical methods to efficiently degrade fluoroalkylether compounds in the aqueous solutions.The present paper investigated that the effects of different types of physical/chemical degradation technologies on the degradation and transformation mechanisms of fluoroalkylether compounds in aqueous solutions.Firstly,the hydrothermal liquefaction technology（HTL）was used to degrade fluoroalkylether compounds in the aqueous solutions and accumulated in plants.Secondly,different types of sonochemical reactors were used to degrade fluoroalkylether compounds in the aqueous solutions.Thirdly,ultraviolet（UV）was used to study the photodegradation of fluoroalkylether compounds in the aqueous solutions,focusing on the UV/Iodide system that enhanced the reductive degradation and defluorination of F-53B.Finally,the oxidative removal of fluoroalkylether compounds by sulfidated micron zero valent iron（S-m ZVI）/H₂O₂,and the degradation of F-53B by UV combined with S-m ZVI/H₂O₂ technology were studied.The degradation intermediates of F-53B were identified and the possible degradation pathways of F-53B in HTL and photodegradation technology were proposed.The main results were as followed:（1）The high temperature in HTL can directly pyrolyze fluoroalkylether compounds in the aqueous solutions and plants.The removal efficiency of F-53B by HTL was much lower than that of HFPO-DA and ADONA.But in the mixed aqueous solution of fluoroalkylether compounds,the bulk and degradation intermediates of HFPO-DA and ADONA can promote the removal of F-53B.HFPO-DA and ADONA,and can be completely removed in pure water at 300℃for only 30 minutes;while F-53B can only be removed by 61.69±2.64%by reacting in pure water at 330℃for 120minutes.The results of the HTL of Typha latifolia are consistent with those obtained in the aqueous solutions:after 120 minutes of HTL at 250℃,the removal efficiencies of HFPO-DA and ADONA in the roots and shoots of Typha latifolia was 100%,and they were not detected in the water phase,solid phase,and biological crude oil.After 120minutes of HTL at 300℃,F-53B in the shoots of Typha latifolia was completely removed,while F-53B in the roots was only removed by 76.17±1.67%.Plants significantly promoted the removal of F-53B.After adding 5 mmol of Ca（OH）₂,F-53B in the roots of Typha latifolia can be completely removed after HTL at 300℃for 120minutes.（2）The cavitation bubble collapses generated in sonochemistry at the high temperature can also directly pyrolyze fluoroalkylether compounds.Different types of sonochemical reactors have different sonochemical degradation rules for fluoroalkylether compounds.Economical and simple ultrasonic（US）bath did not degrade fluoroalkylether compounds.The US disruptor and US cell disruptor can effectively degrade fluoroalkylether compounds through interfacial pyrolysis.The degradation kinetics of fluoroalkylether compounds in single and mixed solutions were different.The sonochemical degradation efficiencies of fluoroalkylether compounds increased with the increase of US power density.However,with a certain US frequency,there was an optimal US power intensity to maximize the sonochemical degradation of fluoroalkylether compounds.The higher initial concentration of F-53B was not conducive to the sonochemical degradation of F-53B（≥5μg/L）.（3）UV/Iodide photoreduction system can effectively degrade F-53B.Under the irradiation of a photochemical quartz lamp,the UV/Iodide system with 0.3 m M KI can degrade more than 90%of F-53B within 45 minutes.The hydrated electrons(e^-_aq)generated by the excitation of I^-by UV can efficiently reduce and degrade F-53B.At the same time,the increase of e^-_aq under alkaline conditions can also enhance the defluorination of F-53B.Dissolved oxygen（DO）in water had no adverse effect on the photodegradation efficiency of F-53B in the UV/Iodide system,but DO significantly reduced the defluorination efficiency of F-53B.Soil DOM significantly enhanced photodegradation rate of F-53B in UV/Iodide system.When the initial concentration of F-53B is in the range of 0-20μg/L,the UV/Iodide system was more effective in treating F-53B concentrated solution（ie“economically advantageous”）.After 4 hours of UV irradiation with the photochemical quartz lamp,the degradation efficiency and defluorination efficiency of HFPO-DA were higher than those of ADONA.Perfluoroalkyl ether carboxylic acids with different structures showed different photochemical reactivity.Compared with the N₂ atmosphere,the DO significantly reduced the photodegradation efficiency of HFPO-DA and the defluorination efficiency of ADONA.UV irradiation condition was relatively more conducive to the treatment o f the aqueous solutions containing low concentrations of HFPO-DA and ADONA.Both HFPO-DA and ADONA can be photodegraded well in thehe solution p H 4-10,while photodegradation of HFPO-DA and ADONA not high at the strong acid conditions.The UV/Iodide system with 0.3 m M KI showed a significant inhibitory effect on the photodegradation of HFPO-DA and ADONA.Reducing the initial concentration of KI can alleviate this inhibitory effect.（4）The S-m ZVI/H₂O₂ technology can effectively remove F-53B within 30minutes,but cannot remove HFPO-DA and ADONA.The free radical quenching experiment with tert-Butanol（TBA）showed that·OH was not the main free radical species to degrade F-53B.When the p H value of the solution was 4,the removal efficiency of F-53B in the S-m ZVI/H₂O₂ system was the highest.The increase of H₂O₂concentration can significantly inhibit the degrade of F-53B in the S-m ZVI/H₂O₂system.The increase in the concentration of S-m ZVI can accelerate the removal eficiency of F-53B in the S-m ZVI/H₂O₂ system.Different initial concentrations of F-53B had no significant effect on the removal of F-53B in the S-m ZVI/H₂O₂ system.The results of the effect of small molecular organic acids on the oxidation technology of S-m ZVI/H₂O₂ showed that formic acid（FA）significantly promoted the removal of F-53B in the S-m ZVI/H₂O₂ system,while oxalic acid（OA）and citric acid（CA）showed the opposite effect,which significantly inhibited the removal of F-53B.Different types of soil DOM significantly inhibited the removal of F-53B in the S-m ZVI/H₂O₂ system.UV combined with H₂O₂,S-m ZVI or S-m ZVI/H₂O₂ can significantly improve the removal efficiency of F-53B.（5）Comparison among different technologies showed that F-53B,HFPO-DA and ADONA can be directly pyrolyzed by HTL technology and sonochemical technology,and HFPO-DA and ADONA were easier to be directly pyrolyzed than F-53B.Compared with HFPO-DA and ADONA,F-53B was more easily removal by UV/Iodide advanced reduction technology and S-m ZVI/H₂O₂ advanced oxidation technology.