Multi-functional magnetic hydrogels based on Millettia speciosa Champ residue cellulose and Chitosan: Highly efficient and reusable adsorbent for Congo red and Cu2+ removal

In order to obtain high-efficiency and sustainable adsorbents for both heavy metal ions and dyes, Millettia speciosa Champ cellulose and chitosan were used to prepare magnetic hydrogels with in-situ synthesis of Fe3O4 in this study. The results of characterization by vibrating sample magnetometer (VSM), Fourier transform infrared reflection (FTIR), X-ray diffraction (XRD), thermogravimetry analysis (TG and DTG), scanning electron microscope (SEM), mercury intrusion porosimetryand (MIP) and texture analysis indicated the magnetic hydrogels with high strength, high porosity and macroporous structure were successfully prepared, and they could be recovered by magnetic field. Besides, the prepared hydrogels had excellent adsorption effect on Cu2+ and Congo red (CR). The adsorption process for Cu2+ conformed to pseudo-first-order kinetics model better, whereas that for CR conformed to pseudo-second-order kinetics model better. Adsorption isotherm analysis showed that Freundlich model could better describe the adsorption process for both CR and Cu2+. The thermodynamic study results proved that the adsorption process for CR or Cu2+ was spontaneous and endothermic. Besides, the increase of pH had a negative effect on CR adsorption and a positive effect on Cu2+ adsorption. Adsorption mechanisms analysis revealed that electrostatic attraction and hydrogen bonding were involved in the CR adsorption, and amino groups acted dominantly in the process of Cu2+ adsorption. Furthermore, the prepared hydrogels exhibited excellent reusability, remaining their adsorption efficiency above 75% in five consecutive cycles. Therefore, the prepared hydrogels were expected to be promising materials for both heavy metal ions and dyes removal from waste water.