Active site on boron‑nitrogen co-doping lignin-based carbon nanotube-coated nZVI for enhanced hexavalent chromium removal by adsorption-redox behavior from groundwater

The utilization of high-efficiency and environmentally friendly materials for pollutant removal has remained a research hotspot in groundwater remediation studies. Herein, a boron (B) and nitrogen (N) co-doped lignin-based carbon nanotube-coated nano zero-valent iron (nZVI) composite (Fe⁰@LC-NB) was synthesized via a straightforward one-step pyrolysis protocol for Cr(VI) remediation from groundwater. This synthetic approach simultaneously achieves the carbonization of lignin, growth of carbon nanotubes (CNTs), formation of nZVI, and co-doping of boron and nitrogen, resulting in a tubular structure that effectively prevents the aggregation of nZVI and enhances its reactivity. The abundant active sites of B–N–C, BC, and pyridinic N generated by B/N co-doping are absent in singly-doped materials, significantly boosting the Cr(VI) adsorption capacity. Simultaneously, the electron donor-acceptor system formed by B, N, and Fe⁰ markedly increases the interfacial electron transfer rate, thereby achieving exceptional redox activity of the material. The results showed that the maximal adsorption capacity of Fe⁰@LC-NB reached 232.94 mg/g, which was 1.5 times that of un-modified lignin-based carbon nanotube-coated nZVI (Fe⁰@LC). The mechanism analysis indicates that electrostatic adsorption, redox reactions, and complexation processes were the major reasons for Cr(VI) removal. Moreover, unlike conventional nZVI materials that undergo rapid passivation in application, the protective carbon shell enables the composite to maintain outstanding performance even in complex aquatic environments, offering a cost-effective and efficient method for groundwater remediation.