Effects of exogenous co-planar polychlorinated biphenyls (PCBs) on the microbial reductive dechlorination of PCBs pre-existing in an anaerobic sediment of Venice Lagoon

The occurrence of reductive dechlorination processes towards pre-existing PCBs and the possibility of enhancing them by the addition of exogenous PCBs were investigated in a contaminated sediment of Porto Marghera (Venice Lagoon, Italy) suspended, under strictly anaerobic conditions, in water collected from the same site. After a five-month lag phase, several pre-exiting hexa-, penta- and tetra-chlorinated biphenyls were slowly bio-converted into tri- and di-, ortho-substituted PCBs. The spiked co-planar, dioxin-like 3,3',4,4'-tetrachlorobiphenyl, 3,3',4,4',5- and 2,3',4,4',5-pentachlorobiphenyls, 3,3',4,4',5,5'- and 2,3,3',4,4',5-hexachlorobiphenyls were extensively transformed to lower chlorinated, mostly ortho-substituted congeners, during the 16-months experiment. The reductive dechlorination of the exogenous PCBs did not influence significantly the biotransformation onset of the sediment-carried PCBs. PCB dechlorination initiated when sulfate was completely depleted and methanogenesis started to take place, thus suggesting that sulfate-reducing bacteria started to use PCBs as electron acceptors only when sulfate was completely depleted or a possible involvement of methanogenic bacteria in the process.