Adsorptive removal of methylene blue dye by a ternary magnetic rGO/AK/Fe3O4 nanocomposite: Studying adsorption isotherms, kinetics, and thermodynamics

Abstract

Organic dyes have very toxic, mutagenic, and carcinogenic properties. Contamination of water by dyes can cause serious threats to aquatic systems and humankind. Therefore, water systems contaminated with organic dyes should be treated. A new cost-effective method for removing methylene blue (MB) dye using a reusable synthetic reduced graphene oxide/activated kaolin/magnetite (rGO/AK/Fe₃O₄) ternary nanocomposite was developed. The synthesis of reduced graphene oxide (rGO)-based magnetic activated kaolin nanomaterial was achieved through a simple, environmentally friendly, and single-step process. Fe₃O₄, rGO, and kaolin-based materials with distinct structures, high specific surface areas, and customizable functional groups show great potential as adsorbents. Magnetite nanoparticles (Fe₃O₄) were coated on reduced graphene oxide (rGO) nanosheets and AK surface by chemical co-precipitation method supported by ultrasonication. It is characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy to investigate the crystal structure, morphology, and functional groups present in rGO/AK/Fe₃O₄ composites, respectively. The adsorption of methylene blue dye onto the ternary nanocomposite was studied. The synthesized nanocomposite showed 66–98.2% removal of MB at pH 10, initial concentration of 15 ppm with the range of adsorbent dose 5 - 50 mg. To examine the interaction of dye molecules with the adsorbent, we employed four adsorption isotherm models: Freundlich, Langmuir, Temkin, and Dubinin-Radushkevich. The Langmuir model yielded the best fit with an R² value of 0.996 at pH 10 and 298 K. The kinetic data were analyzed with three different kinetic models at pH 10 and the linearized pseudo second-order kinetic model was found to be the best fit model for our adsorption experiment with an R² value of 0.998. Thermodynamic parameters were also evaluated. The negative values of, ΔH^o, ΔG^o and ΔS^o were calculated to be -54.8 kJ/mol, -9.91 kJmol^-1, and -150.65 J/molK at 298 K; revealing the exothermic, spontaneous, and less disordered (good adsorbent-dye affinity) process, respectively. The reusability test exhibited 98.1 % removal capacity even after the 5th cycle. Thus, the nanocomposite material showed excellent stability and reusability for removing MB dye.