Low-temperature ZnCl₂ activation of distinct Miscanthus lignin as highly-porous biochar assembly for efficient removal of organic dyes, tetracycline and Cr(VI)
Junsheng Yu, Zongtai Tang, Lei Zhu, Bing Gao, Jing Hong, Yujia Fang, Jiacheng Kang, Dan Sun, Hao Peng, Boyang He, Mahmoud M.A. Bakr, Yanting Wang, Liangcai Peng, Hua Yu
Abstract
Efficient removal of diverse organic and inorganic pollutants from wastewater remains a critical challenge . Although lignocellulose-derived biochar is a green adsorbent, its selectivity and adsorptive capacity are still limited for large-scale application. As Miscanthus is a dedicated bioenergy crop providing abundant lignin-rich biomass, this study first extracted lignin fractions from two Miscanthus sacchariflorus accessions (Msa01 and Msa24) with distinct guaiacyl (G) and p -hydroxyphenyl (H) monomer proportions, and then generated biochar samples using ZnCl₂ as an activating agent at various temperatures. Among the total of 20 biochar samples obtained, two optimal biochar samples were obtained at 400°C, a significantly lower temperature relative to the most chemical-activations processes, but they both exhibited exceptionally high specific surface areas (>1500 m²/g) with distinct pore architectures, leading to size- and type-selective adsorptions. By comparison, the Msa01 biochar possessed a relatively smaller average pore size and volume than those of the Msa24 biochar, enabling superior uptake of the small dye methylene blue (541.9 mg/g), whereas the Msa24 biochar sample preferentially adsorbed larger organic molecules such as Reactive Blue 19 (1286.1 mg/g) and Tetracycline (731.9 mg/g). Both biochar samples also achieved effective removal of the inorganic contaminant Cr(VI) (381.2–392.1 mg/g), attributed to their exceptionally high surface areas and pore volumes. Adsorption isotherm and kinetic analyses indicated multilayer adsorption on the heterogeneous surfaces governed by active site availability, and molecular dynamics simulations further revealed that adsorption was driven by strong non-covalent interactions. In addition, two biochar samples showed excellent recyclability, retaining up to 97 % of initial adsorption capacity after five cycles. This work thus demonstrates a sustainable and energy-efficient strategy for valorizing lignin-rich agricultural residues into hierarchical porous biochar with tunable molecular-size selectivity for remediation of multiple pollutants.