Enhanced Transformation of Cr(VI) by Heterocyclic‑N within Nitrogen-Doped Biochar: Impact of Surface Modulatory Persistent Free Radicals (PFRs)

Redox processes mediated by biochar­(BC) enhanced the transformation of Cr­(VI), which is largely dependent on the presence of PFRs as electron donors. Natural or artificial dopants in BC’s could regulate inherent carbon configuration and PFRs. Until recently, the modulation of PFRs and transformation of Cr­(VI) in BC by nonmetal-heterocyclic dopants was barely studied. In this study, changes in PFRs introduced by various nitrogen-dopants within BC are presented and the capacity for Cr­(VI) transformation without light was investigated. It was found N-dopants were effectively embedded in carbon lattices through activated-Maillard reaction thus altering their charge and PFRs. Transformation of Cr­(VI) in N doped biochar relied on mediated direct reduction by surface modulatory PFRs. The kinetic rate of transformation of Cr­(VI) was increased 1.4–5 fold in N-BCs compared to nondoped BCs. Theortical calculation suggested a deficiency in surface electrons induced Lewis acid–base bonding which could acted as a bridge for electron transfer. Results of PCA and orbital energy indicated a colinear relationship between PFRs and pyrrolic N, as well as its dual-mode transformation of Cr­(VI). This study provides an improved understanding of how N-doped BC contributes to the evolution of PFRs and their corresponding impacts on the transformation of Cr­(VI) in environments.