Halogenated
flame retardants (HFRs) have been extensively used
in various consumer products and many are classified as persistent
organic pollutants due to their resistance to degradation, bioaccumulation
potential and toxicity. HFRs have been widely detected in the municipal
wastewater and wastewater treatment solids in wastewater treatment
plants (WWTPs), the discharge and agricultural application of which
represent a primary source of environmental HFRs contamination. This
review seeks to provide a current overview on the occurrence, fate,
and impacts of HFRs in WWTPs around the globe. We first summarize
studies recording the occurrence of representative HFRs in wastewater
and wastewater treatment solids, revealing temporal and geographical
trends in HFRs distribution. Then, the efficiency and mechanism of
HFRs removal by biosorption, which is known to be the primary process
for HFRs removal from wastewater, during biological wastewater treatment
processes, are discussed. Transformation of HFRs via abiotic and biotic
processes in laboratory tests and full-scale WWTPs is reviewed with
particular emphasis on the transformation pathways and functional
microorganisms responsible for HFRs biotransformation. Finally, the
potential impacts of HFRs on reactor performance (i.e., nitrogen removal
and methanogenesis) and microbiome in bioreactors are discussed. This
review aims to advance our understanding of the fate and impacts of
HFRs in WWTPs and shed light on important questions warranting further
investigation.
Reductive dehalogenation mediated by organohalide-respiring bacteria plays a critical role in the global cycling of organohalides. Nonetheless, information on the dehalogenation enantioselectivity of organohalide-respiring bacteria remains limited. In this study, we report the enantioselective dechlorination of chiral polychlorinated biphenyls (PCBs) by CG1. CG1 preferentially removed halogens from the (-)-enantiomers of the three major environmentally relevant chiral PCBs (PCB174, PCB149, and PCB132), and the enantiomer compositions of the dechlorination products depended on their parent organohalides. The assays with crude cell extracts or concentrated whole cells and the experiments with living cells showed similar enantioselectivities, in contrast with the distinct enantiomeric enrichment factors (ε) of the substrate chiral PCBs. Additionally, these results suggest that concentrated whole cells might be an alternative to crude cell extracts in tests of reductive dehalogenation activities. The enantioselective dechlorination of other chiral PCBs that we resolved via gas chromatography further confirmed the preference of CG1 for the (-)-enantiomers. A variety of agrochemicals and pharmaceuticals are chiral. Due to the enantioselectivity in biological processes, enantiomers of chiral compounds may have different environmental occurrences, fates, and ecotoxicologies. Many chiral organohalides exist in anaerobic or anoxic soils and sediments, and organohalide-respiring bacteria play a major role in the environmental attenuation and global cycling of these chiral organohalides. Therefore, it is important to investigate the dehalogenation enantioselectivity of organohalide-respiring bacteria. This study reports the discovery of enantioselective dechlorination of chiral PCBs by CG1, which provides insights into the dehalogenation enantioselectivity of and may shed light on future PCB bioremediation efforts to prevent enantioselective biological side effects.
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