Biochar as a novel carbon-negative electron source and mediator: electron exchange capacity (EEC) and environmentally persistent free radicals (EPFRs): a review

Abstract Biochar is the pyrolysis product of biomass that has been explored as an effective adsorbent for its high specific surface area, large porosity, and numerous surface functional groups. Recently, there is a renewed interest in biochar thanks to the recognition of its strong electron exchange capacity (EEC), which enables it to store and shuttle electrons to break down contaminants. Environmentally persistent free radicals (EPFRs) are an inherent property of biochar, and have been found to be an important contributing factor of the biochar EEC due to its strong electron donating capacity. This review provides a compressive analysis of recent advances in the EEC of biochar, focusing primarily on the mechanisms of EEC formation, factors affecting it and biochar applications in environmental cleanup. Pyrolysis temperature and properties of feedstock materials are two dominant factors governing the EEC of biochar. The temperature is important because it affects the characteristics of oxygen functional groups (OFGs) in biochar and high pyrolysis temperature (greater than .600 °C) will graphitize carbon which shuttles electrons but does not directly participate in redox reactions. The impact of feedstock on biochar mainly derives from the differences in feedstock compositions which contain different OFGs. Following the elucidation of the EEC formation and stability, applications of biochar as an electron mediator are given, and main knowledge gaps and research needs to further advance the understanding and applications of biochar EEC are discussed.