In situ treatment of PCBs by anaerobic microbial dechlorination in aquatic sediment: are we there yet?
Graphical abstract
Highlights
► Microbial catalysts with different PCB dechlorinating activities are cultured. ► Biostimulation and bioaugmentation has been successful in the laboratory. ► Molecular tools for monitoring dechlorinating bacteria in situ are available. ► Methods to deploy these catalysts in the field are currently under development.
Introduction
Polychlorinated biphenyls (PCBs) were manufactured as inert, stable, flame-resistant and oxidation-resistant products for a variety of applications such as coolants and dielectric fluids in electrical equipment. Although their manufacture was banned in the U.S. in 1979 and subsequently worldwide in 2001, PCBs persist in the environment as a result of past disposal practices and accidents. Because PCBs are hydrophobic they partition preferentially to organic particles in the environment, which serve both as long-term reservoirs and as carriers that can distribute PCBs great distances from the original point source as a result of current and wind. Although sorbed PCBs resist migration into the water fraction, PCBs enter the food chain by ingestion and desorbtion in benthic microorganisms leading to eventual bioaccumulation and biomagnification of PCBs in organisms higher up in the food chain [1]. PCBs are listed as priority organic pollutants by the EPA (http://nlquery.epa.gov) owing to the environmental impact and health risk that they pose and there has been a long search for cost-effective and environmentally sustainable methods such as bioremediation to treat them in situ.
Section snippets
Discovery
Highly chlorinated PCBs common in many commercial Aroclors resist aerobic degradation until they are partially dechlorinated by anaerobic microbial dechlorination. The first evidence of anaerobic PCB dechlorination was based on changes in congener patterns observed downstream of a capacitor plant that released Aroclor 1242 into the Hudson River [2], which was attributed to microorganisms that could derive energy by using PCBs as electron acceptors; a process later termed dehalorespiration [3].
Biostimulation
Biostimulation of indigenous PCB dechlorinating bacteria has been achieved by halopriming with halogenated aromatic compounds. Halopriming may increase the biomass of the dehalogenating microbial catalysts, induce genes required for dechlorination, and possibly support dehalorespiration or cometabolism of additional PCB congeners. Bedard et al. [31] first described the stimulation of weathered Aroclor 1260 dechlorination in sediments by addition of 2,5,3′,4′-tetrachlorobiphenyl and subsequently
Conclusion
Currently the predominant treatment option for PCBs in sediments is dredging followed by stabilization by dewatering and landfilling, but this approach is environmentally disruptive and unsustainable. Passive capping limits exposure of PCBs to the food chain, but since PCBs remain in the environment a potential long-term risk owing to gradual or acute disruption of the cap remains. Development of a tractable microbial in situ treatment system would provide a cost-effective, and environmentally
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
This work was supported by U.S. Department of Defense, Environmental Security Technology Certification Program (ER-201215) and the National Institute of Environmental Health Science Superfund Research Program (5R01ES-016197-02). We thank Dr. R. Payne for assistance with figure preparation.
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2022, Trends in BiotechnologyCitation Excerpt :One such example would be the organohalide-respiring bacteria (OHRB) (e.g., Dehalococcoides), which have been widely used to remediate chloroethene-contaminated soil and groundwater and to generate benign ethene as a final product [8,9]. Notably, OHRB may not be able to completely dehalogenate persistent organohalides, including polychlorinated biphenyls (PCBs), triclosan (TCS), chlorophenols, and polyfluorinated compounds [10,11], requiring integration of physiochemical and biological methods (e.g., Bio-RD-PAO combining microbial reductive dehalogenation and persulfate activation and oxidation) for their extensive degradation [12–14]. These integrated methods await more experimental evidences and field test data to support their utility.