| Polychlorinated biphenyls(PCBs) are a typical persistent organic pollutants (POPs). Sediments are one of the dominant sinks of PCBs. Understanding PCB dechlorination in sedimends is crucial to better control the environmental risks posed by PCBs in the contaminated sediments and ultimately restore the ecosystem.Surficial sediment samples(0-5 cm) were collected from Zhushan Bay, Lake Taihu, and their basic physicochemical properties were determined. Nine PCB congeners were investigated for their dechlorination in sediment microcosms. The dechlorination experiment was set up with no carbon source group, acetic acid group and mixed fatty acids group. The overarching goal of this study was to examine the effects of different carbon sources on PCB degradation. The followings are the main findings:(1) The properties of sediments showed:heavy metals had significant agglomeration effect, heavy metal contents in Zhushan Bay were significantly higher than those in inflowing rivers; the spatial variations of anions were at moderate degree or strong degree, greatly impacted by human activities; TC and TOC contents were between 0.39?2.98% and 0.36?2.46%, respectively, and the organic nutrient may be not sufficient. Relatively low TOC may be a limiting factor for microbial growth, and the effects of co-exsisting heavy metals and anions should not be ignored.(2) After 24 weeks' incubation, the concentrations of chlorine atoms in no carbon source group reduced from 707.86nmol/g slurry to 568.88nmol/g slurry, and 19.63% of chlorine atoms from the benzene ring were removed, mainly resulted from flanked-meta and flanked-para dechlorination. The total toxic equivalency quotients (TEQs) of dioxin-like PCBs was dramatically reduced from 297pg/g to 21pg/g.(3) Additions of extra carbon sources did not significantly reduce the lag time. However, they were able to significantly increase the PCBs dechlorination rate. The lag times of the three groups were all around 3 weeks. After 15 weeks'incubation, the average concentrations of chlorine atoms in acetic acid group and mixed fatty acids group were reduced from 707.86nmol/g slurry to 591.08nmol/g slurry and 605.25nmol/g slurry respectively. In other words,16.50% and 14.50% of total chlorines were removed in 15 weeks. And the TEQs were reduced from 297pg/g to 28.2pg/g and 32.1pg/g respectively. The data were better than the data of the no carbon source group at 15th week.(4) The composition and amount of carbon source amended had different impacts on PCB dechlorination. After adding 7.5mM of acetic acid, flanked meta and flanked para removals were favored. When repeatedly adding the same amount of acetic acid, The preferences of flanked meta and flanked para dechlorination were more apparent. In contrast, after adding 7.5mM of mixed fatty acids, the removals of double-flanked meta and double-flanked para chlorines were dominant. However, the dechlorination of single-flanked meta or para chlorines was partially inhibited. When amended with 7.5mM of mixed fatty acids again, meta and para chlorine atoms were targeted regardless of their neighboring conditions.(5) The second addition of carbon sources in the rapid degradation stage (6?12 weeks), can significantly increase the rate of dechlorination. Re-addition of acetic acid increased the average rates of chlorine removal from 13.41 nmol/(g·week) to 20.30 nmol/(g·week) during 6?12 weeks and 11.54nmol/(week/g) to 17.90nmol/(g·week)during 9?15 weeks. In the meanwhile, re-addition of mixed fatty acids accelerated the dechlorination rates from 11.12nmol/(g·week) to 19.34nmol/(g·week)and 8.39nmol/(g·week)to 16.01nmol/(g·week) during 6?12 weeks and 9?15 weeks respectively. The repeated addition of carbon sources was found to effectively improve the dechlorination extent in a limited time period(12?15 weeks), which indicated an apparent dechlorination enhancement. After 24 Weeks' incubation, the TEQs were dramatically lowered to 21pg/g,12.45pg/g and 8.57pg/g in the no carbon source group, acetic acid group and mixed fatty acids group. |
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