Simulated aging processes induced significant changes in redox properties of biochar for chromate reduction

Biochar, a carbon-rich material, features a well-developed porous structure and abundant oxygen-containing functional groups that collectively facilitate efficient electron transfer. Upon environmental exposure, biochar generally undergoes aging processes like temperature fluctuations, water erosion, and natural oxidation, leading to significant changes in its physical and chemical properties. However, research addressing the impact of aging processes on biochar's redox behavior remain limited. In this study, we systematically investigated the effects of three simulated aging processes (i.e., chemical oxidation, acidification, and freeze-thaw cycling) on the redox properties of wheat straw-derived biochar. Electrochemical analysis suggested that the oxidized biochar (OBC) presented higher redox-active performance and partial least squares modeling revealed that enhancements in electron transfer capacity (ETC) were closely linked to the enrichment of phenolic hydroxyl and quinone groups, increases in specific surface area, and the development of conjugated π-π∗ structures. Specifically, the electron donating capacity and electron accepting capacity of (OBC) increased from 0.051 to 0.066 mmol e^-/g_c and from 0.083 to 0.151 mmol e^-/g_c, respectively. Functionally, biochar facilitated the reduction of Cr(VI) as an electron donor and it could mediate electron transfer as an electron shuttler when lactate was introduced. The combined system of biochar and lactate achieved Cr(VI) reduction rates approximately two to three times greater than those attained by either biochar or lactate alone. This synergistic enhancement was positively correlated with the ETC of biochar. This work provides critical insights into the aging-induced evolution of biochar's redox properties, highlighting its sustained and enhanced potential as a sustainable electron shuttle for long-term remediation of redox-sensitive environmental contaminants.