Equilibrium isotherms studies of methylene blue adsorption by MIL-101(Cr) modified with zinc oxide nanoparticles

Emerging pollutants from water are a significant concern in modern society, and surface adsorption has been identified as a promising method to eliminate them. Preparation of an effective adsorbent is crucial for the adsorption method. In this study, a new adsorbent of Metal organic frameworks, MIL-101(Cr)@ZnO, was synthesized by modifying MIL 101(Cr) with zinc oxide nanoparticles using the hydrothermal method. The adsorbent was characterized using XRD, FTIR, FESEM, EDX and BET analysis. The adsorbent's efficiency was evaluated by studies of adsorption equilibrium isotherms under optimal conditions pH = 9, amount of adsorbent 0.0035 g, volume 50 ml, time 60 minutes, at different temperatures (60, 40, 35 °C) and concentrations of methylene blue (6, 8, 9, 10, 11 mg/L). The equilibrium adsorption data were analyzed using Langmuir, Freundlich, Temkin, Alovich, and Dubinin-Radoshkevich(DR) two-parameter equilibrium isotherms models. The obtained results showed that the order of dye adsorption of the studied isotherms is as follows: Dubinin-Radoshkevich>Temkin>Freundlich>Alovich>Langmuir. Additionally, the order showed that the adsorption of methylene blue occurs in a multilayered form. The isotherm of Dubinin-Radoshkevich with the highest correlation coefficient (R2=0.9767) and low average free energy (E=0.975 J/mol) showed that physical adsorption is the dominant process of adsorption. The maximum adsorption capacity of Langmuir isotherm was determined at the temperature of 313 K with the value of qmax=258.7143 mg/g, the separation factor at the same temperature (RL=0.4364) indicating favorable adsorption. The Freundlich adsorption intensity and correlation coefficient values were n=1.3825 and R2=0.879, respectively. The heat of adsorption in the Temkin's model was calculated to be BT=57.192 J/mol, and the highest adsorption capacity was obtained from Elovich's isotherm qmax=333.3333 mg/g. Further, the amount of thermodynamic values showed that the adsorption process of the pollutant is a spontaneous and endothermic process. The present study's evidence indicates that the adsorption of methylene blue follows a physical process.