The Study On Photolysis Of Polychlorinated Biphenyls (PCBs) Dissolved In Surfactant And Cyclodextrin Solutions

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.