Resolving the enhancement effect of microwave-assisted pyrolysis on biochar redox properties from the structure-activity relationship

Biochar as a redox-active substance engaged in biogeochemical redox processes has received widespread attention in recent years. In this study, microwave-assisted pyrolysis (MWP) was found to have a significant enhancement effect on electron accepting capacity (EAC) and electron exchange capacity (EEC) of biochar, and the enhancement effect was further resolved by the constructed structure-activity relationship between structural compositions and redox properties. The results illustrated that the enhancement effect was mainly because MWP could significantly enhance the specific surface area (SSA), improving the formation of quinone and favouring graphite-like structures' lateral and longitudinal growth. In addition, based on the structure-activity relationship, this work also provides detailed suggestions for regulating the redox properties of biochars. For EAC, quinone had a more significant effect on EAC than electron donating capacity (EDC), and improving biochar skeleton structural properties such as La, Lc, and unsaturation degree was also effective in enhancing EAC. However, for EDC, introducing O-containing functional groups such as C-OH, C--O, or the improvement of La in the biochar structure was more practical. Especially, improving the SSA positively affected both EDC and EAC, while opposite the average pore size.