Adsorption isotherms, kinetic modeling and thermodynamics of toxic crystal violet from aqueous solution on eco-friendly L-cysteine-reduced graphene oxide

Abstract

Here, a facile green synthesis approach was used for the preparation of rGO that uses thiol-free amino acid (L-cysteine) as a normal reduction agent under mild conditions. The structural, chemical, optical, thermal, morphological and topographical characteristics of resultant materials were studied by means of Fourier transform infrared (FTIR), X-ray diffraction (XRD), Energy dispersive x-ray (EDX), UV-visible spectroscopy(Uv-Vis) atomic force microscopy (AFM), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA) and transmittance electron microscopy (TEM). All these characterization techniques clearly confirmed that the GO has been successfully reduced into rGO using L-cysteine as reducing agent. The synthesized rGO was used as an adsorbent for removing crystal violet dye (CV) from aqueous solution by adsorptive elimination. Effect of the parameters such as contact time, rGO dosage, temperature and initial pH have been studied to achieve the optimum operating conditions for the most efficient removal of CV dye from the aqueous solution. The results revealed that the maximal adsorption capacity of rGO was found to be 99.838 mg g−1 at 308 K. The adsorption data were investigated by using Langmuir, Freundlich and Temkin isotherm models to describe the best match to experimental results for CV adsorption on rGO. The adsorption data concurs with Langmuir isotherm indicating monolayer adsorption type. Based on the values of thermodynamic functions, the adsorption process of CV onto rGO was described as spontaneous, endothermic and primarily physical in nature. Moreover, the kinetic study showed that the adsorption process follows pseudo second order kinetics model (R2 =0.9943).