New insight into enhanced permanganate oxidation by lignocellulose-derived biochar: The overlooked role of persistent free radicals

Permanganate (Mn(VII)) is a traditional reagent used for water purification, but it is mild to deal with refractory organic contaminants of emerging concern. There is great interest in combination with effective and low-cost biochar to improve reaction kinetics of Mn(VII). Until recently, it still unclear how biomass composition and carbon structure of biochar influence the Mn(VII) oxidation performance. Herein, we prepared a series of biochar via pyrolyzing different sources of biomass, and their introduction enhanced 24 % of Mn(VII) oxidation of diclofenac (DCF) to 47.3 %∼100 % within 20 min. Particularly, Mn(VII)/walnut shell biochar (SBC) system achieved the highest reaction rate constant of 0.3817 min⁻¹, 5.8 times faster than that by UVA-LED-activated Mn(VII). Physicochemical properties of biochar were found to be highly dependent on the organic compositions of biomass. According to quantitative structure−activity relationship (QSAR) studies, graphitization degree of biochar was recognized to be the decisive factor, facilitating the electron transfer from organics to Mn(VII)-biochar complexes. Lignin-abundant biomass was more conducive to producing highly-graphitized biochar with superior activity. Mn(III), identified as the sole reactive Mn intermediate in Mn(VII)/biochar systems, made the secondary contribution to contaminants removal. Impressively, Mn(III) formation was positively correlated with persistent free radicals (PFRs) intensity of biochar. Manipulation experiments and theoretical calculations corroborated that PFRs generated on pyrolyzed biomass and biopolymers (cellulose, hemicellulose and lignin), could donate electrons for Mn(VII) decomposition, regulating Mn(III) production via the synergy of PFRs’ concentrations and types. Overall, this work offered new insights into the contribution of lignocellulose-derived biochar to Mn(VII) oxidation and contaminants removal.