Dozens-fold improvement of biochar redox properties by KOH activation

Abstract

Recently redox-active biochar has been widely used as an electron shuttle in many redox systems to accelerate diverse reactions. However, low value of electron exchange capacity (EEC) and a lack of effective modification methods prevent it from further applications. Thus, how to efficiently enhance biochar’s redox properties becomes a hot challenge. In this study, we developed a two-step KOH activation method to modify pine wood biochar and obtained a significant improvement in biochar’s EEC, with a maximum increase of 50-fold in electron donor capacity (EDC) to 1.07 mmol e-/g biochar, and 30-fold in electron acceptance capacity (EAC) to 5.23 mmol e-/g biochar, respectively. Considering the gradually decreasing yield with increasing KOH dosage, six was the optimal KOH-to-biochar mass ratio for preparing high EEC char in large quantities, with a yield of 46.36% and a maximum EEC value of 2.80 mmol e-/g raw biochar. Results showed that KOH activation could increase O-containing functional groups, convert them to unstable forms, and also promote biochar’s aromatization, which contributed to the EEC increase. In addition, a significant linear correlation (p < 0.05) between biochar’s EDC and specific surface area (SSA) was observed, which provided a reference value for exploring modification methods targeted to biochar’s redox properties and also made it a clue how to refine a low-quality biochar into a value-added product with high redox properties.