Microbial-Catalyzed Reductive Dechlorination of Polychlorinated Biphenyls in Hudson and Grasse River Sediment Microcosms: Determination of Dechlorination Preferences and Identification of Rare Ortho Removal Pathways

Abstract

Biodegradation of polychlorinated biphenyls (PCBs) is an important transformation and detoxification route in the environment. To better understand the influence of PCB congener compositions on dechlorination, sediments from two rivers, Hudson and Grasse, and two PCB mixtures (PCB 5/12, 64/71, 105/114, and 149/153/170 in Mixture 1 and PCB 5/12, 64/71, 82/97/99, and 144/170 in Mixture 2) were used for this microcosm study. The Grasse River sediment microcosms exhibited more extensive dechlorination than the Hudson River sediment microcosms. The extent of dechlorination was predominantly controlled by sediment itself, not by the PCB compositions. Rare ortho dechlorination, targeting mono-ortho PCB congeners was observed in Grasse sediment, indicating a potential for full dechlorination of some PCBs in this sediment. The identified ortho dechlorination pathways were PCB 28 (24-4-CB) to PCB 15 (4-4-CB) and PCB 25 (24-3-CB) to PCB 13(3-4-CB). The relative abundances of Dehalococcoides were much higher in both sediments spiked with PCBs. An apparent increase of Dehalococcoides 16S rRNA genes coincided with the commencement of dechlorination. The dechlorination preferences were identified using a modified data analysis approach focusing on chlorine neighboring conditions. In both sediments, the overall dechlorination preferred meta > para > ortho. Specially, ortho-/double-flanked meta-chlorines were primarily targeted followed by single-/double-flanked para-chlorines.

Effects of Ferric Oxyhydroxide on Anaerobic Microbial Dechlorination of Polychlorinated Biphenyls in Hudson and Grasse River Sediment Microcosms: Dechlorination Extent, Preferences, Ortho Removal, and Its Enhancement

“…The sediments were kept in filled 3-gallon plastic buckets in the dark at 4°C until used. The geochemical properties, including background PCBs, metals (e.g., Fe, Al), acid volatile sulfide, total phosphorous, total residue as percent solids, inorganic anions, and organic carbon content of the two sediments were determined previously ( Xu et al, 2016 ).…”

“…Thirteen PCB congeners were used in this study based on criteria previously described ( Karcher et al, 2004 ; Xu et al, 2016 ). These 13 individual PCB congeners, related to 10 tracker pairs, were classified into two mixtures.…”

“…PCB Mixture 2 contained congeners PCB 5/12, 64/71, 82(234-23-CB)/97(245-23-CB)/99(245-24-CB), 144(2346-25-CB)/170. The structures of selected PCB congeners, their theoretical first step dechlorination pathways and the possibly associated dechlorination processes were discussed previously ( Xu et al, 2016 ).…”

“…The PCB-spiked dry sediment substrates containing 500 mg/kg of each PCB mixture were prepared as described previously ( Xu et al, 2016 ). Modified reducing anaerobic media (RAMM) using 2 mM L -cysteine⋅HCl as the reducing agent was prepared and 1% (vol/vol) of Wolfe’s vitamin solution was added ( Shelton and Tiedje, 1984 ; Yan et al, 2006 ).…”

“…Grasse sediment. We reported on rare ortho dechlorination in the Grasse sediment ( Xu et al, 2016 ). These lead to a hypothesis that Fe and carbon content may be the important factors for PCB dechlorination in these sediments.…”