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A novel functional biochar (BC) was prepared from industrial waste red mud (RM) and low-cost walnut shell by one facile-step pyrolysis method to adsorb phosphorus (P) in wastewater. The preparation conditions for RM-BC were optimized using Response Surface Methodology. The adsorption characteristics of P were investigated in batch mode experiments, while a variety of techniques were used to characterize RM-BC composites. The impact of key minerals (hematite, quartz, and calcite) in RM on the P removal efficiency of the RM-BC composite was studied. The results showed that RM-BC composite produced at 320 °C for 58 min, with a 1:1 mass ratio of walnut shell and RM, had a maximum P sorption capacity of 15.48 mg g−1, which was more than double that of the raw BC. The removal of P from water was found to be facilitated significantly by hematite, which forms Fe–O–P bonds, undergoes surface precipitation, and exchanges ligands. This research provides evidence for the effectiveness of RM-BC in treating P in water, laying the foundation for future scaling-up trials. [Display omitted] • Preparation conditions for biochar composite from red mud and walnut shell were optimized. • Batch adsorption experiments were conducted on the adsorption characteristics of P. • The maximum P adsorption capacity of biochar composite reached 15.48 mg g−1. • Fe from hematite in red mud of the biochar composite play an important role in removing P. • Adsorption mechanism includes creation of Fe–O–P bonds, surface precipitation and ligand exchange. [ABSTRACT FROM AUTHOR]