A rapid microwave-assisted technique to synthesize micro-mesoporous biochar intercalated with Zn2+ for efficient removal of Cr(VI)

Abstract

Hierarchical micro-mesoporous biochar prepared using biomass waste (tofukasu) with the microwave-assisted method in only 200 s. In this process, ZnCl₂ provides a sintering aid and acts as an activator to construct the hierarchical structure. Due to the low melting point of ZnCl₂, the cellulose first expands and penetrates the macromolecular structure of tofukasu during the initial microwave treatment. Subsequently, the carbon molecular rearrangement was catalyzed by ZnCl₂ to produce highly aromatic biochar and fully utilized as a sintering aid. ZBC-3 biochar has a specific surface area of (788 m² g⁻¹), a mesoporous/total pore volume ratio (30.92 %), and contains rich N and O element functional groups. In addition, introducing Zn²⁺ doped into biochar assists in making it positively charged, which is conducive to the electrostatic adsorption of Cr (Ⅵ). Under the condition of pH= 2, the maximum Cr (Ⅵ) removal capacity of ZBC-3 reached up to 380.55 mg g⁻¹. The higher efficiency is attributed to the abundant micro-mesoporous structure that provides multiple charge transfer routes, high specific surface area, numerous active sites, and reduced functional groups. In addition, the cyclic experiments of ZBC-3 and different ion interference experiments still show excellent performance. Conclusively, this new approach is a promising technique for the remediation of wastewater containing heavy metals by rapid and facile synthesis of hierarchical porous biochar with rich functional groups.