Reductive dechlorination of weathered PCBs in sediments of Venice lagoon under geochemical conditions that mime those occurring in situ

Due to their high recalcitrance and hydrophobicity, PCBs are extensively accumulating in sub-surface sediments of freshwater and marine habitats, through which they enter the food chain. PCBs occurring in such sediments often undergo microbial reductive dechlorination, which results in their conversion into lower chlorinated mainly ortho-substituted biphenyls, generally less toxic and prone to bioaccumulate than parent compounds. PCB reductive dechlorination processes have been mostly observed in slurry microcosms of sediments of contaminated lakes and/or rivers spiked with exogenous PCBs and suspended in mineral media. They have been often ascribed to methanogenic bacteria and less frequently to SO4=-reducing bacteria. Recent studies on highly enriched cultures ascribed the processes to bacteria of the phylum Chloroflexi, able to use organic acids and/or H2 as electron donors and PCBs as electron acceptors. However, a few studies have been focused on weathered PCBs and only 5 of the latter were performed on marine sediments. Further, only a minority of them were performed in the presence of water from the site, i.e., under laboratory geochemical conditions that mime those occurring in situ1. Finally, nothing was know up to 5 years ago about the occurrence of similar processes in the marine sediments of Venice Lagoon, where the historical occurrence of PCBs, in particular in the Brentella Canal, was demonstrated. Therefore, a long term study was undertaken to detect and characterize the microbial reductive dechlorination processes vs. weathered PCBs in five distinct sediments of the Brentella Canal (suspended in their own water) to determine the potential of the site to undergo in situ microbial decontamination. The reductive dehalogenation of weathered PCBs was documented in all the sediments, thus indicating that the process is apparently widespread and in progress in situ. PCB dechlorination was generally slow and limited and was not significantly “primed” by the addition of exogenous PCBs, probably as a consequence of the low bioavailability of weathered pollutants. The dechlorination process was mainly directed towards the meta- and para- chlorines of the PCB molecules, and apparently proceeded through the dechlorination patterns H’ and M. Finally, a marked SO4= reduction generally accompanied PCB dechlorination, suggesting that the process is mediated by indigenous SO4=-reducing bacteria. However, preliminary results obtained from enriched cultures suggest that also other microorganisms belonging to the Chloroflexi phylum might be involved in PCB dechlorination in the Brentella Canal.