【作者】 王政华；

【导师】 施周；

【作者基本信息】 湖南大学 ， 市政工程， 2008， 博士

【摘要】 多氯联苯(PCBs)是一类广泛分布的环境污染物,其理化性质稳定,且抗生物降解,但它可以被紫外光所降解。本文在较全面地综述了国内外有关PCBs光降解的研究概况的基础上,研究了PCBs在表面活性剂溶液和环糊精溶液中的紫外光降解情况,分析了PCBs的光降解途径和降解动力学,建立了PCBs在表面活性剂溶液中的光降解反应动力学模型。本文还采用表面活性剂和环糊精溶液对受PCBs污染的天然土壤进行了清洗,并对洗脱液的紫外光降解情况进行了研究。最后论文还从量子化学的角度对PCBs的光降解行为进行了理论上的分析。通过PCBs在表面活性剂溶液中的光降解实验发现,PCBs在表面活性剂溶液中的光降解反应与其在水溶液中的光降解反应相比得到了强化,且在阴离子表面活性剂溶液中的光降解反应速率和量子产率均略高于非离子表面活性剂溶液中。表面活性剂溶液中PCBs初始浓度影响其光降解反应速度,浓度较低时光解反应速度较快,量子产率较高,且光化学反应速率常数(y)与PCBs初始浓度(x)之间存在y=axb的关系。实验还发现表面活性剂溶液中的溶解氧分子对光降解反应速率的影响不大,这预示PCB在表面活性剂溶液中可能是在单线态下发生C-Cl键断裂而直接导致光解脱氯,对此仍需进一步的研究来确认。实验还建立了表面活性剂溶液中PCBs光降解反应动力学模型,模型的模拟曲线与实验数据能较好地吻合。多氯联苯在羟丙基-β-环糊精(HPCD)溶液中可有效地为紫外光所分解,通过对各个时段光解试样的紫外-可见吸收光谱的检测,发现随光解时间的增加其吸收光谱中的κ波段逐渐发生红移。利用气相色谱、气相色谱-质谱联用仪对试样做进一步检测确定光解产物为含氯量较少的多氯联苯,并且反应物与其产物之间存在很好的质量平衡关系。此结果表明在HPCD溶液中从邻位氯原子开始的逐步脱氯反应为所试PCBs的主要光解途径。PCBs在HPCD溶液中的光降解反应为假一级反应,试验测得2,2’,4,4’CB和2,4,4’CB的光降解反应速率常数和量子产率分别为:0.00816 s-1和0.0183、0.00911 s-1和0.0124。由于环糊精为天然化合物,环境毒性很低,所以在环境修复中仍具有良好的应用价值。利用表面活性剂和环糊精溶液可以有效地将受污染土壤中的PCBs洗脱出来。当以阴离子表面活性剂十二烷基硫酸钠(SDS)作为清洗剂时,SDS浓度为10g/L,清洗液容量(mL)与土壤质量(g)比为20:1时的清洗效果较好。但是以HPCD作为清洗剂时,难以找到最佳的HPCD浓度和清洗液容量,随着HPCD浓度和清洗液容量的升高PCBs的洗脱量增加。采用SDS和HPCD溶液清洗受污染土壤的过程均可在较短的时间内达到平衡。试验还对SDS、Brij35和HPCD三种洗脱液进行了紫外光降解,其中的PCBs可被有效地降解,且浊度较高、溶液中的颗粒粒径较大时对洗脱液的光解反应速度影响较大,pH值对洗脱液的光解反应速度影响较小。最后,论文还利用量子化学软件探讨了PCBs分子的双面夹角、生成热、前线轨道能量和总能量与其光降解性质之间的关系,并采用半经验方法(PM6)逐步分析了2,2’,4,4’CB的光降解途径,从理论上进一步证明了含邻位氯原子的PCB分子受到紫外光辐照发生降解反应时其邻位氯原子会优先脱除的结论。更多还原

【Abstract】 Polychlorinated biphenyls (PCBs) are wildly distributed environmental contaminants. Though they are physically and chemically stable and generally recalcitrant to biodegradation, they are photodegradable. On the basis of comprehensively reviewing references on PCBs photolysis both in china and abroad, the ultraviolet irradiation of PCBs dissolved in surfactant and cyclodextrin solutions was studied. Pathways and kinetics of PCB photodecomposition have been evaluated and kinetic models for the PCB degradation were then established. Washing of PCB contaminated soils by using of surfactant and cyclodextrin solutions following by UV degradation of the soil washings were also studied. Finally, the photolysis behaviors of PCBs were discussed from the viewpoint of quantum chemistry.It was found that the photodechlorination of PCBs in surfactant solutions was enhanced relative to that in water. The photolysis rates in anionic surfactant solutions were higher than those in nonionic surfactant solutions. The initial concentrations of PCBs in surfactant solutions also affected the photolysis rates and quantum yields. The lower the initial PCB concentrations, the faster the photolysis rates and the higher the quantum yields. The relationship between photolysis rates (y) and initial concentrations (x) follows y=axb. It was also found that the molecules of oxygen dissolved in solutions had few effects on photolysis rates, which indicated that cleavage of C-Cl at singlet state would directly contribute to PCB photodechlorination. Further experiments were necessary to confirm this result in the solution of complete oxygen depletion. Kinetic models were built for the photodegradation of PCBs in surfactant solutions, and found them fit to the experimental data quite well.PCBs could be photodecomposited in hydroxypropyl-β-cyclodextrin (HPCD) solutions by UV irradiation. By measuring UV-VIS spectra of photolyzed samples, bathochromic shift ofκband of the spectra was found as photolysis time increased. Less chlorinated PCB congeners were identified as the photoproducts of the tested PCB by using a GC-ECD and a GC-MS. Excellent mass balance existed between the tested PCB and its products. These results indicated that stepwise dechlorination initiated at ortho chlorine was the main photodegradation pathway of the tested PCB congener. Photodechlorination of PCBs in HPCD solutions followed pseudo-first-order kinetics. The photolysis rates and quantum yields of 2,2’,4,4’CB and 2,4,4’CB were 0.00816 s-1 and 0.0183, 0.00911 s-1 and 0.0124, respectively. Since the cyclodextrins were natural compounds with little environmental toxicity, they were valuable in environmental remediations.PCBs could be effectively washed out from field contaminated soil by using surfactant and cyclodextrin solutions. When applying anionic surfactant, sodium dodecyl sulfate (SDS), at concentration of 10 g/L and the ratio of solution volume (mL) to soil mass (g) was 20:1, the washing efficiency reached the top. When using HPCD as washing solution, it was hard to obtain the optimal concentration and ratio of washing solution volume to soil mass. Increasing the concentration and volume of HPCD solution would result in increasing amount of PCB washed from soils. The washing balance was quickly reached when SDS and HPCD solutions were applied. PCBs in nonionic surfactant Brij35, as well as SDS and HPCD soil washings can be degraded effectively by UV irradiation. The higher turbidity of the soil washing solution and larger particles in it would deteriorate the photolysis of PCB in soil washings, but the pH has little effect on the PCB photolysis rates.At the end, the relationships between photochemical behavior and dihedral angles, heats of formation, energies of frontier molecular orbitals, and total energy of PCB molecules were addressed. The photolysis pathway of 2,2’,4,4’CB was illustrated theoretically by using semi empirical computational methods (PM6). From the points of quantum chemistry, the stepwise dechlorination with ortho chlorine being preferentially removed was further proved to be the main mechanism of PCB photodegradation by UV irradiation.更多还原