Synthesis of Porous Red Mud/Slag-Based Spherical Geopolymers for Efficient Methylene Blue and Ni²⁺ Removal from Water

To reuse red mud and slag wastes as raw materials, a green type of porous spherical red mud/slag-based geopolymer (RSG) was synthesized by utilizing suspension curing and foaming techniques. Because methylene blue (MB) and nickel ion (Ni²⁺) were common and difficult to treat in wastewater, the adsorption characteristics of MB and Ni²⁺, as well as the phase and microstructure of the porous RSG spheres prior to and after adsorption, were thoroughly investigated. The porous RSG spheres showed a stable and mesoporous structure with a BET surface area of 31.36 m²/g. The spheres achieved the maximum removal efficiencies of 99.81% (MB) and 99.01% (Ni²⁺) at dosages of 16 and 10 g/L, respectively. The pseudo-second-order kinetic model and the Langmuir model could match the adsorption data of these spheres, with predicted maximum adsorption capacity (<i>Q</i>_max) values of 19.88 mg/g for MB and 12.39 mg/g for Ni²⁺, respectively. After three adsorption-desorption cycles, porous RSG spheres demonstrated good recycling capability with removal efficiencies of 98.10% (MB) and 54.60% (Ni²⁺). The spheres were also effective in adsorbing additional dyes (methyl orange (MO), crystal violet (CV), and malachite green (MG)) and heavy metal ions (Cd²⁺, Pb²⁺, Zn²⁺, and Cu²⁺). The spheres have potential use in water treatment.