【作者】 杨佘维；

【导师】 胡勇有；

【作者基本信息】 华南理工大学 ， 环境工程， 2013， 博士

【摘要】 本文采用真空紫外线（VUV，λ=185nm）和高频超声（US，f=600kHz）构建耦合体系在空气氛围下对水中的全氟辛基磺酸钾（PFOS）进行脱氟。考察了反应时间、溶液pH值、反应温度和PFOS初始浓度等因素对VUV、US、VUV-US体系对PFOS脱氟的影响；采用响应曲面法（RSM）分析了影响因子排序并确定了运行调控策略；采用ESI/MS与HPLC/MS/MS等分析了各体系的中间产物，以揭示VUV-US对PFOS的脱氟途径及VUV对US的强化机制；选用半经验分子轨道程序（MOPAC）计算了PFOS与全氟羧酸(PFCAs，C_nF_2n+1COOH)分子结构参数、前线电子密度与前线轨道能量；最后考察了外加氧化剂K₂S₂O₈对PFOS在VUV-US中脱氟的影响效果。结果表明，当PFOS浓度为10mg/L，反应温度为283K，反应氛围为空气，pH未调的情况下，反应240min，VUV、US和VUV-US对PFOS的脱氟率分别为5.74%、76.46%和88.47%，VUV-US耦合体系脱氟率比单独的VUV和US处理分别提高82.73%、12.01%，VUV对US的强化程度随反应条件不同而不同。当溶液pH在5-7时，VUV-US比US对PFOS的脱氟率平均提高了11.54%，温度为283-298K时，平均提高了13.03%，PFOS初始浓度为10-30mg/L时，平均提高了11.65%。证实了VUV与US具有明显协同作用，VUV引入强化了US体系在空气氛围下对PFOS的深度脱氟，VUV的引入拓宽了US运行条件的适应范围。响应面分析的结果表明回归方程拟合程度高，VUV-US对PFOS脱氟的影响因子作用大小的排序为：PFOS初始浓度>反应温度>初始pH值。在初始pH值在4-5的范围与PFOS脱氟率表现出弱正相关性，在5-6的范围表现出弱负相关关系；在设计的范围内，反应温度（283-293K）与PFOS初始浓度（10-20mg/L）则对PFOS脱氟呈显著的负相关性。VUV-US体系的调控策略是：当PFOS初始浓度较高且难以调控时，可降低反应温度或调节pH约为4.5-5.0之间保持体系较高的脱氟率；若当反应温度较高且难以调控时，降低PFOS的初始浓度是关键的调控策略；当pH恒定或难以调控时，则可降低其中PFOS初始浓度或者反应温度，或同时降低两个因素来保持体系脱氟率。PFOS在VUV、US与VUV-US过程中的主要中间产物均为短链的全氟羧酸（PFCAs）、全氟磺酸盐、全氟烷烃等。全氟羧酸类中间产物在VUV-US体系中存在浓度比US体系更低。动力学拟合结果表明PFCAs在VUV-US体系中反应速率更快且到达最高浓度时间更短。氟元素物料衡算的结果表明，其他中间产物在VUV-US体系中存在浓度也较US更低，VUV不仅促进了PFOS的脱氟也提高了其中间产物的分解效率。空化热解是PFOS在VUV-US中的主要反应机制，而紫外光解与水解是次要反应机制。MOPAC计算结果表明离子态的PFOS及PFCAs能量高于对应的分子态，因此C₈F₁₇SO₃^-、 C_nF_2n+1COO-比C₈F₁₇SO₃H、C_nF_2n+1COOH更易于降解；PFOS在分子态与离子态断键位置相同，首先断裂C-S键后进一步分解；而PFCAs分子态与离子态首先断键的位置不同，由于PFCAs具有较低电离常数，主要存在形态为离子态，因此PFCAs光解或热解反应以脱羧反应为主。由于短链PFCAs具有更高的电子跃迁能，比长链PFCAs稳定性更高。因此，短链PFCAs除了较强亲水性外，高分子稳定性也是致使其在US体系降解速率下降的原因之一。K₂S₂O₈难以有效氧化降解PFOS，UV/K₂S₂O₈能氧化分解PFOS，PFOS脱氟率随着K₂S₂O₈投加量的增加而增大，硫酸自由基（SO₄^-）的氧化是PFOS在UV/K₂S₂O₈体系中的主要脱氟途径；K₂S₂O₈引入VUV-US体系虽带来了SO₄^-，增加了反应的途径，但K₂S₂O₈与生成SO₄^2-均不利PFOS的超声热解，由于SO₄^-与PFOS的氧化还原反应速率较低，SO₄^-的引入不能弥补K₂S₂O₈的负面效果，K₂S₂O₈未能促进PFOS在VUV-US体系中的脱氟。更多还原

【Abstract】 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.更多还原