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

Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) in polluted marine sediments is often limited by the scarcity of electron donors. In this regard, microbial electrochemical technologies (METs) can guarantee a long-lasting effect by providing a constant and controlled amount of electrons. In this experiment, METs were applied to stimulate reductive dechlorination of PCBs in marine sediment. The impact of the electrochemical stimulation on the physical-chemical features of the sediment, as pH and redox potential (ORP), and on the microbial activities was assessed under potentiostatic (-0.7 V vs Ag/AgCl) and galvanostatic conditions (0.025 mAxcm-2-0.05 mAxcm-2), using graphite rod as cathode and iron plate as anode. A novel two antiparallel circuits configuration (2AP) was used to mitigate the variations of pH. During the incubation, the pH, ORP, concentration of PCBs and sulfate and composition of the microbial community were monitored. After 159 days of incubation, the pH and ORP changed accordingly to the intensity of the electrical stimulation. The 2AP demonstrated its potential in controlling pH, maintaining average values between 6.5 and 8.5. METs primed sulfate reducing bacteria (SRB), known competitors of organohalide respiring bacteria for electron equivalents, increasing sulfate depletion activity and enriching them compared to the control condition. Conversely, PCBs reductive dechlorination was not stimulated whereas it was partially inhibited under galvanostatic conditions. The results reported herein underline the need of an optimization of the set up of the METs and operational conditions when working in environments with strong bacterial competition for available reducing equivalents, such as marine sediments.