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 bioconverted into tri- and di-, ortho-substituted PCBs. The spiked co-planar, dioxin-like https://www.w3.org/1998/Math/MathML" display="inline"> 3 , 3 &s; , 4 , 4 &s; -tetrachlorobiphenyl, https://www.w3.org/1998/Math/MathML" display="inline"> 3 , 3 &s; , 4 , 4 &s; , 5 - and https://www.w3.org/1998/Math/MathML" display="inline"> 2 , 3 &s; , 4 , 4 &s; , 5 –pentachlorobiphenyls, https://www.w3.org/1998/Math/MathML" display="inline"> 3 , 3 &s; , 4 , 4 &s; , 5 , 5 &s; - and https://www.w3.org/1998/Math/MathML" display="inline"> 2,3 , 3 &s; , 4 , 4 &s; , 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.