Red Mud-Based MIL-53(Fe) for the Efficient Adsorption of Methylene Blue: Preparation, Adsorption Performance, and...

This work reports the use of red mud (RM) (a widely available industrial byproduct) as the primary raw material for synthesizing cost-effective MIL-53-(Fe) (denoted as RM-MIL-53-(Fe)), which was subsequently applied for the high-efficiency removal of methylene blue (MB) from aqueous environments. The optimum preparation conditions of RM-MIL-53-(Fe) were determined by analyzing the crystal structure and adsorption performance, as follows: pH 1.85, mass ratio of RM to terephthalic acid disodium salt (Na₂BDC) 1:0.5, stirring speed 450 rpm, and temperature 80 °C. Characterization results verified the successful construction of a metal-organic framework possessing the typical MIL-53-(Fe) topology, which exhibited a notably high specific surface area of 176.5 mg·g^-1. Fitting the equilibrium adsorption data to the Langmuir model yielded a high correlation coefficient, indicating that MB uptake occurred via monolayer coverage on a uniform surface. The calculated saturation capacity reached 167.8 mg·g^-1. Thermodynamic parameters demonstrated that the MB adsorption process on RM-MIL-53-(Fe) was thermodynamically spontaneous, where chemisorption acted as the dominant mechanism, and electrostatic interaction served as the auxiliary pathway. Kinetic data fitting results exhibited a superior correlation with the pseudo-second-order kinetic model, further confirming the predominance of chemisorption in the adsorption process. Furthermore, intraparticle diffusion and liquid film diffusion were identified as the colimiting steps governing the adsorption rate. The adsorption efficacy of RM-MIL-53-(Fe) was strongly dependent on the sorbent dosage, solution pH, and coexisting ionic species. Mechanistic analysis revealed that MB retention on RM-MIL-53-(Fe) proceeded via a combination of hydrogen bonds, π-π stacking interactions, electrostatic forces, and intrapore filling.