Evolution of redox activity of biochar during interaction with soil minerals: Effect on the electron donating and mediating capacities for Cr(VI) reduction

Biochar in soil is susceptible to natural aging along with soil minerals, which might alter its electrochemical properties and redox reactions with contaminants. In this study, soluble mineral salts (FeCl₃, MnCl₂, AlCl₃, CaCl₂) and clay mineral (kaolinite) were selected to investigate the impact of co-aging with soil minerals on the redox activity of peanut-shell biochar for Cr(VI) reduction. Natural aging for 3-month induced oxidation of biochar with the decrease of reducing moieties, i.e., ‒C‒OH from 26.8–43.7% to 18.4–24.1%. Co-aging with minerals except for Mn(II) further decreased the proportion of ‒C‒OH to 6.94–22.2% because of the interaction between mineral ions and biochar, resulting in the formation of mineral-biochar complex and new minerals, e.g. β-FeOOH. Due to its reductivity, Mn(II) presented the least decrease or even slight increase of ‒C‒OH while itself was oxidized to Mn(III) and Mn(IV). The decline of ‒C‒OH caused the decrease of Cr(VI) reduction rate constant from 2.18 to 2.47 × 10^-2 h^-1 for original biochars to 0.71–1.95 × 10⁻² h⁻¹ for aged ones, of which co-aging with Fe(III) showed the lowest reduction rate constant among all minerals. The electron mediating capacity of biochar also decreased after aging alone or co-aging with Al, Ca, and kaolinite, while co-aging with Fe(III) and Mn(II) facilitated the electron transfer process, increasing the rate constant by 219.3–1237% due to electron mediation through valence transformation of Fe(III)-Fe(II) and Mn(II)-Mn(III). Given the abundance of soil minerals, it was essential to consider this crucial factor for redox reactions when applying biochar for soil remediation.