Application of graphene oxide modified with 8-hydroxyquinoline for the adsorption of Cr (VI) from wastewater: Optimization, kinetic,. thermodynamic and equilibrium studies

In the present study, removal of Cr (VI) from aqueous solution was investigated by adsorption onto graphene oxide modified with 8-hydroxyquinoline (8-HQ-GO-Fe3O4). Graphen oxide (GO) sheets, magnetic GO-Fe3O4 nanocomposite and the 8-HQ-functionalized GO-Fe3O4 nanocomposite (8-HQ-GO-Fe3O4) were freshly synthesized and characterized by Scanning electron microscope (SEM) images, Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray (EDAX) and thermogravimetric (TGA) analysis. The response surface methodology using.R software was chosen to investigate the composition effect of input independent factors (pH, 8-HQ/GO dose, time and Cr (VI) concentration) and one dependent output response (removal efficiency). The lower P-value (1.33 x 10(-15)), higher F-value (240.4), higher R-2 (multiple R-squared: 0.994, adjusted R-squared: 0.989) and insignificant lack of fit (0.38) indicate that reduced full second-order model has highly significant for adsorption of Cr (VI) by 8-HQ-GO-Fe3O4 and it also is represented satisfactory adjustment between model and experimental data. The predicted optimal conditions by model (to involve all parameters simultaneously) for the maximum removal efficiency (95.77%) were achieved (pH, 6.58; 8-HQ/GO dosage, 98.4 g L-1; contact time, 178.4 min and Cr (VI) concentration, 1 mg L-1). Based on reported results, a high correlation coefficient (R2) was found for the Langmuir model compared with the Freundlich and the Temkin models that confirm the reasonably fit to the data. The maximum adsorption capacity calculated from the Langmuir was 11.9 mg g(-1) at 308 K The kinetics followed is pseudo second order in nature and the intraparticle diffusion was the dominating mechanism. The thermodynamic data revealed the endothermic nature, an increase in the degree of freedom of the adsorbed ions and spontaneous sorption for the adsorption process. Also the Ea value found to be 1.39 kJ mo1(-1) indicating chemical sorption. (C) 2017 Elsevier B.V. All rights reserved.