Chemical-physical parameters and microbial community changes induced by electrodes polarization inhibit PCB dechlorination in a marine sediment

Microbial reductive dechlorination of organohalogenated pollutants is often limited by the scarcity of electron donors, that can be overcome with microbial electrochemical technologies (METs). In this study, polarized electrodes buried in marine sediment microcosms were investigated to stimulate PCB reductive dechlorination under potentiostatic (-0.7 V vs Ag/AgCl) and galvanostatic conditions (0.025 mA·cm-2-0.05 mA·cm-2), using graphite rod as cathode and iron plate as sacrificial anode. A single circuit and a novel two antiparallel circuits configuration (2AP) were investigated. Single circuit polarization impacted the sediment pH and redox potential (ORP) proportionally to the intensity of the electrical input and inhibited PCB reductive dechlorination. The effects on the sediment's pH and ORP, along with the inhibition of PCB reductive dechlorination, were mitigated in the 2AP system. Electrodes polarization stimulated sulfate-reduction and promoted the enrichment of bacterial clades potentially involved in sulfate-reduction as well as in sulfur oxidation. This suggested the electrons provided were consumed by competitors of organohalide respiring bacteria and specifically sequestered by sulfur cycling, which may represent the main factor limiting the applicability of METs for stimulating PCB reductive dechlorination in marine sediments. Polychlorinated biphenyls (PCBs) are known persistent organic pollutants still lacking an effective remediation strategy. Electron donors play a crucial role in priming microbial reductive dechlorination, which lowers PCBs' toxicity. Microbial Electrochemical Technologies (METs) have been proposed to stimulate the dehalogenation of chlorinated solvents in freshwater environments. In this work, polarized electrodes were applied to a marine sediment to stimulate PCB respiration. Our results show the effectiveness of METs in stimulating reductive dechlorination processes in marine environments may be compromised by competitors of organohalide respiring bacteria and sulfur cycling sequestering the provided electrons, and suggests directions to overcome the context-specific challenges.