Enhancing biochar redox properties through feedstock selection, metal preloading and post-pyrolysis treatments

Abstract

There is growing evidence on the importance of the redox properties of biochar for many environmental applications. However, its variability and the difficulty in controlling its redox properties could be delaying the use of biochar in those areas that involve the exchange of electrons, like microbial fuel cells or contaminant degradation related to microbial electron shuttling. To help with these issues, we produced a wide range of biochars showing different redox capacities through a variety of strategies. These include optimizing production and processing parameters, feedstock selection, preloading biomass with redox-active metals and post-pyrolysis treatments. A modified Hummer’s method was the most efficient treatment, increasing the electron donating capacity from 0.244 mmol e−/gbiochar to 0.590 mmol e−/gbiochar and the electron accepting capacity from 0.169 mmol e−/gbiochar to 0.645 mmol e−/gbiochar. The characterization of the phases responsible for the redox properties, mainly surface functional groups, radicals and redox-active metals, allowed us to better understand the changes caused to biochar by the different strategies. It revealed that the most important approach to enhance redox properties is to increase the number of C–OH and C]O groups in biochar, while the methods that use redox-active metals showed higher than predicted electron donating capacities. We also measured other attributes such as surface area, pH and conductivity, with a focus on their relationship with the redox properties. By selecting the appropriate production and modification methods, we were able to produce a balanced biochar with acceptable conductivity (1.34 mS/cm) and electron exchange capacity (0.418 mmol e−/gbiochar), even though these properties usually have an inverse relationship. This work opens the possibility for the production of designer biochars with tailored properties optimized for specific applications.