Ethylene diaminetetraacetic acid (EDTA)-functionalized graphene was synthesized from Nigerian coal using a chemical exfoliation method and the graphene was applied for the removal of Congo red dye from aqueous solutions. The synthesized coal graphene and the raw coal were characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM)-energy-dispersive X-ray (EDX) spectroscopy, measurement of pHpzc (pH of point of zero charge), and Boehm titrations. The SEM data revealed surface roughness which is enhanced in the prepared graphene while the EDX revealed an increase in carbon content, the main constituent of graphene, from about 26% in the raw coal to about 80% in the prepared graphene. Various adsorption variables, such as pH, contact time, concentration of Congo red, and temperature, were varied for the removal of the dye using raw coal and the synthesized coal graphene. The Liu isotherm gave the best fit of the equilibrium data than the Langmuir, Freundlich, and Dubinin-Radushkevich models. The maximum adsorption capacities of the raw coal and synthesized coal graphene at 25°C are 109.1 mg/g and 129.0 mg/g, respectively. The Avrami fractional-order kinetic model was the best model for description of the kinetic data. The model had the lowest values of standard deviation than the pseudo-first-order and pseudo-second-order models. The adsorption process of the two materials occurred via two stages as proved by intraparticle diffusion model. The adsorption process of the Congo red removal was spontaneous, feasible, and endothermic. The study conclusively revealed the graphene nanomaterial to be a viable adsorbent for textile wastewater treatment.
Keywords: Avrami model; Coal; Congo red; Ethylene diaminetetraacetic acid; Graphene; Liu model.
© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.