Defluorination Of Aqueous Perfluorooctanesulfonate (PFOS) By Combined Process Of Vacuum Ultraviolet And High-frequency Ultrasound

This study represents the first use of ultraviolet （VUV, λ=185nm）-assisted ultrasound（US, f=600kHz） irradiation to defluorinate aqueous perfluorooctane sulfonate （PFOS） underair atmosphere. The effects of reaction time, pH, temperature and initial PFOS concentrationon the defluorination performance of VUV, US and VUV-assisted US systems were compared.The importance order of influence factors and control strategies were determined throughresponse surface method （RSM）. The intermediates formed during PFOS defluorination ineach system were detected using ESI/MS and LC/MS/MS. The possible defluorinationpathway and mechanism of synergism between VUV and US and was proposed. Thestructural parameters, electron density and frontier orbital energy of PFOS and PFCAsmolecules were calculated using semi-empirical molecular orbital software （MOPAC）. Finally,the effect and mechanism of potassium persulfate （K₂S₂O₈） on the sonochemical andphoto-sonochemical defluorination of aqueous PFOS under air atmosphere were alsoinvestigated.The results showed that the defluorination efficiency of PFOS （10mg/L） reached up to88.47%at PFOS initial concentration was10mg/L after240min at283K under VUV-assistedUS irradiation, which is82.73%and12.01%higher than that achieved under VUV （5.74%）and US （76.46%） irradiation alone, respectively. Introduction of VUV enhanced the ultrasonicdefluorination of PFOS and the enhanced extents were largely dependent on particularreaction conditions. Compared with that achieved by US irradiation alone, the defluorinationefficiency of PFOS （10mg/L） increased by an average of11.54%at initial pH of5-7,13.03%at283-298K and11.65%at initial PFOS concentrations of10-30mg/L using theVUV-assisted US system. Thus, enhanced defluorination of PFOS by VUV-assisted US wasdemonstrated. Introduction of VUV could enhance ultrasonic defluorination of PFOS andextend the application range of US under air atmosphere.Response surface analysis showed that the experimental findings were in closeagreement with the model prediction. The initial concentration of PFOS was the mostimportant factor affecting the defluorination of PFOS in the VUV-US system followed byreaction temperature and initial pH. Initial pH had weak positive correlation with defluorination efficiency of PFOS at initial pH of4-5and a weak negative correlation atinitial pH of5-6. It was apparent that the defluorination efficiency of PFOS was decreasedwith the increase of temperature （283-293K） and initial concentration of PFOS （10-20mg/L）within the range of trial design. The control strategies for performance improvements ofVUV-US system were proposed. The system could not be easily adjusted at high PFOS initialconcentration or high reaction temperature. For the case of high PFOS initial concentration,the high defluorination efficiency of PFOS could be maintained by reducing the temperatureor adjusting pH to4.5-5.0. For the case of high reaction temperature, the high defluorinationefficiency of PFOS could be maintained by lowering the initial concentration of PFOS, whichwas more effective than adjusting pH. When reaction temperature was constant and could notbe easily adjusted, it could reduce temperature or PFOS initial concentration or both tomaintain a high defluorination efficiency.Low levels of short-chain perfluorinated carboxylic acids （PFCAs）, perfluorinatedsulfonic and polyfluorinated alkenes were detected as the main intermediates in VUV, US andVUV-US systems, and the concentrations of PFCAs in VUV-assisted US system were lowerthan that in US alone system. Dynamic analysis showed that PFCAs have a faster degradationrate under VUV-US irradiation than that under US irradiation alone. In addition, the time toachieve the highest concentration of the PFCAs was shortened under VUV-US irradiationcompared to that under US alone. The results of mass balance of fluorine element indicatedthe existence of low concentration of other form of intermediates in the VUV-US system.Thus, the introduction of VUV radiation could not only improve the defluorination of PFOSbut also the further degradation of its intermediates. Pyrolysis of cavitation played a primaryrole while photolysis and hydrolysis were secondary mechanisms in PFOS defluorination inthe VUV-US system.The calculated results of MOPAC showed that ionic form of PFOS and PFCAs hadhigher energy than their corresponding molecular, therefore, C₈F₁₇SO₃^-, C_nF_2n+1COO-wereeasy to be decomposed as compared with C₈F₁₇SO₃H, C_nF_2n+1COOH. Molecular and ionicform of PFOS could break bond at the same position. C-S bond was prior to other bonds tocleave in both C₈F₁₇SO₃^-and C₈F₁₇SO₃H due to longer bond length and lower dissociationenergy. Molecular and ionic form of PFCAs could break bond at different positions. Most of PFCAs occurred in C_nF_2n+1COO-because of low ionization constants, therefore,decarboxylation reaction was the main sonolysis and photolysis mechanisms of PFCAs.Short-chain PFCAs had higher electronic transition energies, and were more stable thanlong-chain PFCAs, therefore, besides better hydrophilicity, the higher molecular stability ofshort-chain PFCAs could resist ultrasonic decomposition.K₂S₂O₈was not an efficient oxidant for degradation of PFOS, but PFOS could be slowlydefluorinated in UV/K₂S₂O₈system and the defluorination efficiency increased when theinitial amount of K₂S₂O₈was increased. The SO₄^-oxidation was mainly responsible for thedefluorination of PFOS in UV/K₂S₂O₈oxidation. Introduction of K₂S₂O₈into VUV-US couldproduce SO₄^-and increase the degradation pathways of PFOS, but the presence of the S₂O₈^2-and formation of SO₄^2-were unfavorable for sonolysis of PFOS due to the slow redoxreactions between SO₄^-and PFOS. The negative effects of K₂S₂O₈could not be offseted byadditional SO₄^-and thus introduction of K₂S₂O₈could not enhance photo-sonochemicaldefluorination of PFOS.