| Polychlorinated biphenyls (PCBs), one of the persistent organic pollutants, show high toxicity and high environment persistence and can transport at long distances. The contamination of PCBs has caused high risk to the ecological safety and human health. Though PCBs can be reduced by photo-degradation and biodegradation in the nature, the impact of natural organic matter (NOM) which is universal in the environment is still unclear.The primitive recycling of electronic waste (e-waste) is the main cause for the regional contamination of PCBs in China. In this study, one e-waste recycling site in Taizhou was taken as an example. Bioassay and chemical analysis were performed to determine the contamination levels and ecological risk of the surface sediments from Nanguan River. Source analysis and correlation analysis were carried out afterwards to find the key factor for the risk. Natural attenuation of PCBs in water-sediment was studied by lab simulation as well as field investigation. Coupled degradation mechanism by the photochemical process of NOM was studied by adding NOM and specific molecule probes under simulated sunlight. Humic respiration bacteria were enriched from the sediment and added into the reaction solutions of PCBs. Coupled degradation mechanism by the quinone respiration process of NOM was discussed according to the degradation results and the polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) results. In the same time, the application of electrochemistry MS (EC-MS) and quantum chemistry calculation were explored and transformation pathways of PCBs in the natural attenuation processes were accordingly proposed. The main results of this research are as follows:1) The disassembly process of electronic waste was the main cause for the contamination in the surface sediment and polychlorinated biphenyls (PCBs) were mainly responsible for the ecological risk of the sediments. Attenuation of the commercial PCB mixture Aroclor1254was observed in the lab study. The total PCBs turned out to be degraded after90days in water-sediment systems with the loss of14.8～28.8%. Besides, high content of organic matter and solar irradiation accelerated the degradation of PCBs. Significant attenuation of PCBs was also observed in the real environment according to the field investigation. Degradation percentages of the total PCBs were in range of40.3～70.0%, with a significant correlation with the organic matter contents in the sediments.2) Addition of NOM accelerated the photo-degradation of2,4’,5-trichlorobiphenyl (PCB-31) and2,2’,4,4’,5,5’-hexachlorobiphenyl (PCB-153) significantly, with a pseudo-first-order rate constant of0.0933h-1and0.0413h-1respectively in presence of5mg/L humic acid (HA). The content of aromatic moieties could make a difference on the degradation of PCBs. The photo-degradation products of PCBs were hydroxylated PCBs and polychlorinated benzoic acid identified by GC-MS. The results from electrochemistry MS (EC-MS) study showed the binding of di hydroxy-PCBs with NOM. According to the quantum chemistry calculation the para-Cl was supposed to be more reactive. Reactive oxygen species (ROS) involved in the photo-sensitized degradation of PCBs were determined by ESR and molecule probe methods, among which·OH and intra-1O2were mostly important. By using molecule probes and modeling the steady-state concentrations of·OH and intra-1O2were calculated, approximately1.75×1O-17mol/L and5.79×10-10mol/L respectively.3) The humic respiration bacteria enriched from the sediment showed acceleration to the anaerobic degradation of PCB-153. The loss percentages of PCB-153after15days reached43.5%and56.2%in1mmol/L of antraquinona2,6-disulfonato (AQDS), and200mg/L HA solutions respectively. The degradation products were identified to be2,2’,4,5,5’-Pentachlorobiphenyl (PCB-101) and2,2’,5,5’-Tetrachlorobiphenyl (PCB-52), the para-dechlorination products. PCB-101was suggested to be the main chemical reduction product of PCB-153according to the result of EC-MS. The para position was also the preferential reaction position from the viewpoint of stereo-hindrance effect. The degradation of PCB-153mostly depended on the quinone respiration. According to the fact that PCB-153could be reduced by AH2QDS chemically and the results from PCR-DGGE study for microorganisms, it is suggested that co-metabolism dechlorination was the major mechanism for the degradation of PCB-153by humic respiration bacteria as well as the electron shuttling mechanism. In general, the role of natural organic matter (NOM) in the attenuation of PCBs in water-sediment systems was studied from the aspects of its photochemical process and quinone respiration in this research. The results will help us to understand the natural attenuation mechanisms better and also provide a new way for the remediation of PCB contamination. |
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