Wood-Derived Black Carbon (Biochar) as a Microbial Electron Donor and Acceptor

Research on the environmental impacts of black carbon has focused largely on sorption. Besides being a strong geosorbent, black carbon is redox-active and may facilitate abiotic and microbial transformation. Using a wood-derived black carbon (biochar) and the bacterium Geobacter metallireducens (GS-15), we showed that air-oxidized biochar served as an electron acceptor to enable acetate oxidation, and that chemically or biotically reduced biochar served as an electron donor for nitrate reduction. The bioavailable (to GS-15) electron storage capacities (ESCs) of the biochar, estimated on the basis of acetate oxidation and nitrate reduction, were 0.85 and 0.87 mmol e – /g, respectively, comparable to the ESCs of humic substances and other biochars measured electrochemically. We propose that black carbon should be regarded as a rechargeable reservoir of bioavailable electrons in anaerobic environments. The redox cycling of biochar in natural and engineered systems and its impact on microbial processes and contaminant fate merit further investigations.