Novel red mud/polyacrylic composites synthesized from red mud and its performance on cadmium removal fromaqueous solution

BACKGROUND: Red mud (RM) is a solid waste generated by the aluminum industry and has demonstrated promising physical adsorption properties for cadmium (Cd) due to its relatively large surface area. To improve the chemisorption performance of RM for Cd(II) ions, a new method utilizing in situ graft polymerization in a reversed-phase suspension system is proposed, and red mud/polyacrylic acid (RM/PAA) composites were synthesized for the first time. RESULTS: The RM/PAA composites were used to adsorb Cd(II) ions from aqueous solutions. Batch equilibrium measurements of RM/PAA were carried out to optimize the parameters. The characteristics were analyzed by scanning electron microscopy, the Brunauer- Emmett-Teller method, X-ray diffraction, Fourier transform infrared and X-ray photon spectroscopy. Themaximum adsorption capacity of RM/PAA reached 96.15 mg g(-1), whichwas much higher than that of the RM (21.70). The Cd(II) adsorption rate increased with increasing solution temperature and followed pseudo-second-order kinetics. The experimental data fitted well with the Langmuir isotherm model, indicating that the adsorption followed the monolayer adsorption model. The thermodynamic parameter values suggested that endothermic and spontaneous adsorption occurred. The surface of RM/PAA possesses abundant oxygen-containing functional groups, and because the specific surface area of the RM/PAA decreased and the amount of adsorption increased comparedwith those of RM, the surface groups contributemore to Cd(II) retention. Physical adsorption, ion exchange and chelation were possibly the predominant adsorption mechanisms. CONCLUSION: Thus, the RM/PAA composites produced from RM are considered effective and low-cost adsorbents for Cd(II) removal from aqueous solutions. This study is of great significance for waste utilization and environmental protection. (C) 2019 Society of Chemical Industry