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

Abstract

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 nonmetalheterocyclic 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.

Nitrogen-doped biochars as adsorbents for mitigation of heavy metals and organics from water: a review

“…The authors reported that Cr(VI) was reduced to Cr(III) on the biochar's surface (Chu et al 2020). Cr reduction and adsorption by N-doped biochars were also reported by Zhu et al (2020b). The authors demonstrated that persistent free radicals on N-doped biochar, which acted as electron donors, were responsible for Cr(VI) reduction to Cr(III).…”

“…Similarly, Zhu et al mixed the stalk biomass in methanol with different N precursors such as urea, ammonium nitrate, melamine, yeast powder, and biopolymers of peptone (Zhu et al 2020b). After evaporating the solvent, the solids were pyrolyzed at 450 °C for 90 min to obtain N-doped biochars.…”

“…The higher electronegativity of N (compared to C) in biochar results in increased π-electron density favoring π-π electron donor-acceptor interaction (Zhu et al 2020b). The graphitic-N can act as an electron donor to facilitate a complexation reaction with heavy metals.…”

“…Besides, N having a lower electronegative than the oxygen atom may introduce extra functional groups and positive charges on the biochar matrix's edges. Nitrogen also enhances hydrophobicity in biochar (Zhu et al 2020b). These chemical properties of N-doped biochars, along with their porous characteristics, make them a promising industrial adsorbent for the remediation of pollutants.…”

“…The process of N-doping on biochar surfaces typically results in the formation of various types of nitrogen functionalities, including pyridinic-N, pyrrolic-N, amino-N, graphitic-N, oxidized-N, and others. However, for specific adsorption processes such as adsorption of anionic species, graphitic type nitrogen species are ideal due to their positive charge (Zhu et al 2020b). Similarly, for the adsorption of CO 2 , amine and azo groups might enhance the adsorptive performance (Zhang et al 2017).…”