Efficient reduction and adsorption of Cr(VI) using FeCl3-modified biochar: Synergistic roles of persistent free radicals and Fe(II)

Transition metal iron and persistent free radicals (PFRs) both affect the redox properties of biochar, but the electron transfer relationship between them and the coupling reduc-tion mechanism of Cr(VI) requires further investigation. To untangle the interplay between iron and PFRs in biochar and the influences on redox properties, FeCl3-modified rice husk biochar (FBCs) was prepared and its reduction mechanism for Cr(VI) without light was eval-uated. The FBCs had higher surface positive charges, oxygen-containing functional groups, and PFRs compared with pristine rice husk biochar (BC). Phenoxyl PFRs with high electron-donating capability formed in biochar. The pronounced electron paramagnetic resonance signals showed that the PFRs preferred to form at lower Fe(III) concentrations. While a high concentration of Fe(III) would be reduced to Fe(II) and consumed the formed PFRs. Adsorp-tion kinetics and X-ray photoelectron spectroscopy analysis indicated that the FBCs effec-tively enhanced the Cr(VI) removal efficiency by 1.54-8.20 fold and the Cr(VI) reduction ef-ficiency by 1.88-9.29 fold compared to those of BC. PFRs quenching and competitive reduc-tant addition experiments revealed that the higher Cr(VI) reduction performance of FBCs was mainly attributed to the formed PFRs, which could contribute to-74.0% of Cr(VI) re-duction by direct or indirect electron transfer. The PFRs on FBCs surfaces could promote the Fe(III)/Fe(II) cycle through single electron transfer and synergistically accelerate similar to 52.3% of Cr(VI) reduction. This study provides an improved understanding of the reduction mechanism of iron-modified biochar PFRs on Cr(VI) in environments. (c) 2023 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.