New insights into the selective adsorption mechanism of cationic and anionic dyes using MIL-101(Fe) metal-organic framework: Modeling and interpretation of physicochemical parameters

Abstract

In the current study, iron-based metal organic framework (MOF) MIL-101(Fe) was successfully prepared via a facile solvothermal method. The as–synthesized MIL-101(Fe) was characterized by XRD, FE-SEM, FTIR, TGA and zeta potential techniques, and then employed as an adsorbent for methyl orange (MO) and methylene blue (MB) dyes. The adsorbed quantities of MO (1067 to 831 mg/g) were higher than those of MB (402 to 353 mg/g) indicating the high selectivity of MIL-101(Fe) towards the anionic dye at all temperatures (20–60 °C). Adsorption processes of MO and MB followed the pseudo-second order kinetics and the Langmuir equilibrium model. The interaction mechanism at a molecular level was analyzed and deeply interpreted via the advanced multilayer adsorption model. Steric parameters indicated that MO molecular aggregation (n) was 0.95–1.33 thus signifying the presence of multi–docking and multi–interactions mechanisms. The aggregated number of MB was superior to unity (i.e., n = 1.17–1.78) suggesting a vertical adsorption position and a multi-interactions mechanism at all operating temperatures. The density of MIL-101(Fe) active sites (DM = 77.33–52.38 mg/g for MB and 149.91–107.07 for MO) and the total adsorbed dye layers (Nt = 3.12–2.49 for MB and 5.36–3.67 for MO) resulted in improving the adsorption capacities of MO dye. The adsorption energies ranged from 8.89 to 33.73 kJ/mol and they displayed that MO and MB uptake processes were exothermic controlled by physical interactions at all temperatures. Regeneration results indicated that this adsorbent can be reutilized without a significant loss in its removal efficiency after five adsorption-desorption cycles. Overall, the adsorption capacity, chemical stability, and regeneration performance of MIL-101(Fe) support its application as a very promising adsorbent for the removal of organic hazardous pollutants from water.