Metal-organic and Zeolitic imidazole frameworks as cationic dye adsorbents: physicochemical optimizations by parametric modeling and kinetic studies

Adsorption of methylene blue (MB) was explored using different metal-organic frameworks (MOFs) and zeolitic imidazole framework (ZIFs) to optimize the physicochemical parameters by parametric modeling and to study the kinetics and equilibrium of the adsorption process. The frameworks were synthesized using three different metals (Fe, Zn, Co) and two linkers (2-Methylimidazole, terephthalic acid). Based on screening analysis, ZIF-8 (Zn) with a cubic morphology exhibited the highest adsorption capacity of 43.1 mg g−1 toward MB. The adsorption capacities of the rest studied MOFs were 25.0 mg g−1 for MIL-53 (Fe), 22.7 mg g−1 for MIL-88B (Fe), 18.4 mg g−1 for Ni-MOF, 16.8 mg g−1 for ZIF-67, and 15.5 mg g−1 for MIL−101 (Fe). The effect of contact time, initial solution pH, the dosage of the utilized framework, and MB initial concentration and their bimodal interactions on dye removal were elucidated by modeling the process according to Box–Behnken design (BBD). The model explained an incremental MB removal by pH, contact time, and adsorbent mass and inversely by dye concentration. Parametric optimization of the model revealed that the highest dye removal was achieved at a contact time of 53.4 min, MOF dose of 0.68 g L−1, pH = 8.5, and an MB initial concentration of 25 mg L−1. Nonlinear Khan isotherm and Pseudo-first-order and Pseudo-second-order kinetic models described better MB removal. These findings revealed that the adsorption could be described as a monolayer phenomenon under low dye concentrations values, e.g., 25 mg L−1, while it switches to multilayer at higher MB concentrations around 100 mg L−1.