Selective separation of Congo Red from a mixture of anionic and cationic dyes using magnetic-MOF: Experimental and DFT study

La-MOF-NH2@Fe3O4 (magnetic-MOF) was used as an efficient, ultrafast, and selective adsorbent for the separation of Congo Red (CR) with 92.02% removal after 2 min. The magnetic-MOF was identified by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) surface area, Zeta Potential analysis, analysis of the magnetic hysteresis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). Kinetics, isotherms, the effect of pH, thermodynamic, and selectivity of CR adsorption were investigated. The results confirmed that the adsorption kinetics complied with the pseudo-second-order model. The adsorption isotherms were interpreted by the Langmuir isotherm model, indicating that the dye was adsorbed homogeneously on the surface of the magnetic-MOF (monolayer adsorption). Investigating the effect of pH changes on adsorption demonstrated that this magnetic-MOF has 99.2% removal efficiency at 2 pH after 2 min. The adsorption results at different temperatures suggested spontaneous and endothermic adsorption. Selective adsorption of CR was investigated in the presence of anionic dyes (Methyl Orange (MO) Sunset Yellow (SY) and Fluorescein (F)), and cationic dyes (Rhodamine B (RB), Safranin (SF) and Methylene Blue (MB)), which demonstrated that magnetic-MOF was appropriate for the selective separation of CR. The molecular chemical reactivity of the dye molecules was studied using electrophilicity index (ω), chemical potential (μ), and chemical hardness (η) based on HOMO and LUMO energy by DFT calculations. DFT calculations proved that CR was more reactive than anionic and cationic dyes. Additionally, electrostatic potential (ESP) analysis revealed that there are active sites on the CR. Thus, CR can have stronger electrostatic and host-gust interactions with magnetic-MOF. Reusability study revealed that this magnetic-MOF is appropriate for industry application because RE% was greater than 90% after five consecutive uses.