| 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.The environmental pollution and health risks caused by the improper disposal of electric and electronic waste (e-waste) have become urgent global issues. Severe and widespread contaminations of polychlorinated biphenyls (PCBs) are commonly found in drylands or paddy fields in Taizhou, a major hotspot of e-waste recycling in China. In this research, comparative study was performed on PCBs transformation in polluted agricultural soil under drying and flooded conditions. Uncontaminated soils spiked with PCB31 (2,3,4’-trichlorobiphenyl) and PCB153 (2,2’,4,4’,5,5’-hexa-chlorobiphenyl) were also investigated to verify the transformation characteristics of highly and less chlorinated congeners under both conditions. In a 180-day incubation period, total PCBs in contaminated soil decreased 63.9% in paddy conditions but only 33.1% in dryland conditions; dissipation rate of highly chlorinated congeners in flooded treatment was 39% higher than drying treatment. Furthermore, PCB 153 spiked soil exhibited a 19% higher removal rate in flooded conditions than drying ones. In contrast, degradation rate of PCB 31 under drying conditions was 33% higher. It was proposed that drying conditions in dryland or fallowed paddy field favoured the aerobic biodegradation of less chlorinated PCBs, whereas paddy conditions benefitted anaerobic reductive dechlorination process of highly chlorinated congeners, which was crucial for completely mineralisation of PCBs.This study also investigated the amount of PCB residue in local farmlands. Biotransformation of PCBs was further studied under different water management conditions in paddy field with or without rice cultivation, with a special focus on the alternating flooded and drying processes. It was found that paddy field improved the attenuation of PCBs, especially for highly chlorinated congeners. In the microcosm experiment,45% or more of the initial total PCBs was removed after sequential flood-drying treatments, compared to less than 10% in the sterilized control and 20% in the constant-drying system. Variation in the quantity of PCBs degrading and dechlorinating bacterial groups were closely related to the alteration of anaerobic-aerobic conditions. These results suggested that alternating anoxic-oxic environment in paddy field led to the sequential aerobic-anaerobic transformation of PCBs, which provided a favorable environment for natural PCB attenuation.Perchloroethene (PCE) was utilized as alternative electron acceptor to enrich and isolate PCBs dechlorinators in paddy soil. Sediment-free culture PS showed extensive PCBs dechlorination capability, which was mediated by Process N. Phylogenetical analysis showed that the PCBs dechlorinators in the culture PS shared 99% similarity with Dehalococcoides mccartyi sp. CG1.PCB-respiring Dehalococcoides mccartyi strains could be boosted to a higher cell density in a short culturing time with PCE as the alternative electron acceptor, therefore, PCE was utilized as the first growth substrate and PCBs was then added to the D. mccartyi culture after PCE was completely dechlorinated to dichloroethenes (DCEs). PCE could significantly accelerate and enhance PCBs dechlorination. However, in case of the co-existence of PCBs and chloroethenes (PCE or TCE) in the culture, both dechlorination activity of PCBs and chloroethenes were repressed. As long as little PCE left in the dechlorinating culture, PCB-respiring Dehalococcoides would not utilize PCBs. Similarly, high concentration of TCE-the intermediate products of PCE dechlorination-could also inhibit PCBs dechlorination activity, unless the amount of TCE which co-existed with PCBs dropped below 0.3 mM in the culture. |
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