Potential for Congo red removal by biochar derived from Miscanthus used for lead-zinc tailings remediation

• Miscanthus biochar (MBC) had a good capacity to remove Congo red (CR). • The maximum removal capacities of MBC for CR were 162.9 ∼187.6 mg·g -1 (298∼318 K). • The rate of CR removal was determined by liquid film and intra-particle diffusions. • CR removal relied on chemical actions (e.g., H-bonding, π-π stacking, ion exchange). • The NPI value of the treated CR solution was below 0.7. The reutilization of contaminated biomass from phytoremediation of mine tailings remains a critical challenge to avoid secondary pollution. Herein, Miscanthus harvested from a lead-zinc tailings remediation site was valorized into biochar (MBC) at 700°C and applied for the removal of Congo red (CR) from aqueous solutions. MBC exhibited a high specific surface area of 209.6 m²·g⁻¹ and demonstrated a maximum Langmuir adsorption capacity of 187.6 mg·g⁻¹ at 318 K. The removal process was best described by the pseudo-second-order kinetic model and was predominantly governed by liquid film and intra-particle diffusion. Thermodynamic analysis confirmed that CR removal was a spontaneous and endothermic process. Mechanistic investigations revealed that CR removal was driven by a synergistic combination of electrostatic attraction, π-π interactions, hydrogen bonding, pore filling, and redox reactions, with a significant catalytic contribution from inherent metal species (ZnO). Notably, the safety assessment of the treated effluent confirmed compliance with surface water quality standards (NPI=0.507), and the biochar retained >80% removal efficiency after three regeneration cycles. This study presents a novel, sustainable strategy for converting hazardous phytoremediation residues into high-performance adsorbents, providing a dual benefit for mine tailings management and textile wastewater treatment.