BACKGROUND: A bioelectrochemical process for trichloroethene (TCE) dechlorination was developed. In this new process, a solid-state electrode polarized to -450 mV versus the standard hydrogen electrode (SHE), in combination with a redox mediator (i.e., methyl viologen, MV) is employed as an electron donor for the microbial reductive dechlorination of TCE. In this study we compared the performance of the process with the redox mediator immobilized at the surface of electrodes or dissolved in the bulk liquid, using both a culture highly enriched in Desulfitobacterium spp., capable of dechlorinating TCE to cis-dichloroethene (cis-DCE), and a culture highly enriched in Dehalococcoides spp. capable of dechlorinating cis-DCE to ethene. RESULTS: Short-term potentiostatic (-450 mV versus SHE) experiments showed that TCE or cis-DCE was dechlorinated both in the presence of soluble (500 mu mol L-1) and immobilized MV. However, TCE or cis-DCE dechlorination rates with MV-modified electrodes were remarkably lower than with soluble MV. Both cultures produced significant amounts of H-2 in the presence of electrically reduced, soluble MV, whereas no H-2 was produced when the mediator was immobilized at the electrode surface, regardless of the potential applied to the electrode, in the range -425 to -500 mV versus SHE. CONCLUSIONS: The process, operated with modified electrodes, supports the microbial dechlorination of TCE to ethene. Immobilization not only allows retention of the mediator within the system, but also increases process efficiency by preventing bioelectrochemical H-2 formation. On the other hand, strategies to increase dechlorination rates with modified electrodes need to be developed. (C) 2009 Society of Chemical Industry