Functional biochar fabricated from red mud and walnut shell for phosphorus wastewater treatment: Role of minerals

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.