A novel preparation method for <scp>Mg–Fe/Al</scp> layered hydroxides derived from municipal sludge and red mud to enhance phosphate removal from wastewater

Abstract BACKGROUND Phosphates are important nutrients for animals and plants, but excessive phosphate can lead to eutrophication of water bodies and damage the aquatic ecological environment. Red mud is a powdery aluminium industrial waste with various characteristics such as small particle size, strong alkalinity and high heavy metal content. RESULTS This study synthesized a sludge‐based biochar using municipal sludge and red mud as raw materials. The metal elements in the red mud were extracted by pyrometallurgy combined with hydrometallurgy as trivalent metal ion sources. A layered bimetallic hydroxide (LDH)‐modified biochar material (Mg–Fe/Al LDH@biochar) was synthesized by a chemical coprecipitation method. In order to solve the problem that powdered materials are unfavorable for recycling and transportation, a binder was synthesized by modifying poly(vinyl alcohol) with sodium silicate to bond powdered Mg–Fe/Al LDH@biochar into granules. The adsorption process conforms to the Freundlich isotherm adsorption model and pseudo‐second‐order kinetic model. At 35 °C, the Freundlich fitted material has a maximum adsorption capacity of 35.49 mg g −1 for phosphate. The main removal mechanisms of phosphate by the LDH‐modified biochar composite material include electrostatic interactions, ligand exchange and pore diffusion. CONCLUSION We successfully synthesized granular LDH‐modified biochar. This material exhibits good hardness, strong water resistance and high phosphate adsorption capacity, enabling effective phosphate removal from wastewater while simultaneously offering a new approach for red mud treatment. © 2025 Society of Chemical Industry (SCI).