Insight into the adsorption of magnetic microspheres with large mesopores: Tailoring mesoporous structure and ethylenediamine functionalization for ultrahigh Congo red removal

In this study, ethylenediamine (EDA)-functionalized magnetic microspheres with large mesopores (MMMs) were fabricated and explored for the use as highly efficient adsorbents to remove Congo red (CR). The size of mesoporous channels and loading of EDA functional groups were adjusted to optimize the CR adsorption performance of resulting adsorbents. It was found that higher specific surface area (SBET) was favorable for greater CR adsorption, while increasing EDA loading resulted in CR adsorption capacity with a change of first increasing and then declining. The optimal adsorbent FSN-2 with 1.71 wt% N content possessed an ultrahigh maximum CR adsorption capacity of 1662 mg/g, ascribed to its high SBET of 530.91 m2/g and large average pore size of 8.78 nm. The experimental data were fitted best by using the Langmuir model and pseudo-second-order model, suggesting that the adsorption feature be monolayer and chemosorption. The thermodynamic parameters, i.e.ΔG, ΔH and ΔS, were also estimated, indicating the CR adsorption onto FSN-2 was spontaneous and endothermic in nature. High adsorption capacities were recorded in a wide pH range of 5.0 – 9.0, while the addition of Cl-, SO42- and NO3– had negligible effect on its CR removal. Moreover, FSN-2 showed good reusability and high stability in adsorption–desorption cycles, as well as excellent CR removal capacity in simulated actual wastewater. The adsorption mechanisms of CR onto FSN-2 mainly involved electrostatic attraction and hydrogen bonding. This work provides a new insight into the fabrication of highly efficient magnetic adsorbents by optimizing the porous structure and functional group loading for potential practical application in wastewater treatment.