Unleashing redox activity of biochar via a green thermal air oxidation process: Insights from machine learning

Abstract

The redox activity of biochar is critical in diverse biotic and abiotic processes. Herein, thermal air oxidation is proposed as a green strategy to unleash the redox activities of biochar. Results show that the electron donating capacities for biochars prepared at 300–700 °C (i.e., B300, B500 and B700) are sharply increased from ≤0.06 mmol e⁻/g to 0.53, 0.7 and 0.69 mmol e⁻/g, respectively, with the increase of thermal air oxidation temperature to 400 °C. The electron accepting capacity for B300 is increased with increasing thermal air oxidation temperatures, while the electron accepting capacities for B500 and B700 peak at thermal air oxidation temperatures of 300 and 350 °C (0.97 and 0.99 mmol e⁻/g), respectively. In addition, PFRs on B500 are more remarkably enriched after thermal air oxidation. These results suggest that thermal air oxidation highly efficiently multiplied biochar redox activity. Machine learning results further suggest that the electron donating capacity has a higher dependence on electron mobility regulated by carbon structural properties, while the electron accepting capacity is mainly limited by the number of active sites. These results are beneficial for the engineering of biochar redox activity via thermal air oxidation for potential applications in mediating redox processes and the understanding of controlling parameters for biochar redox activity.