Development of nitrogen-doped graphene/MOF nanocomposites towards adsorptive removal of Cr(VI) from the wastewater of the Herbert Bickley treatment works

Abstract

Industrial effluents laden with Cr(VI) pose considerable health risks to human and animal life if left untreated. Hence, in this work, metal–organic framework-5/nitrogen-doped graphene (MOF-5/NGO) nanocomposites bearing Cr(VI) adsorption properties were fabricated. These unique materials were produced with increasing NGO loadings (0.05, 0.10 and 0.15 wt%) to investigate the role of NGO content on the chemical and physical properties of the composites. The FTIR spectra show the nanocomposites be to be rich in surface –OH and –COOH functional groups. Raman spectroscopy confirmed bond formation between the MOF-5 and NGO through downshift of the G- and 2D-bands. XRD analysis showed MOF-5/NGO nanocomposites, MOF-5 and the NGO to be highly crystalline while thermogravimetric analysis indicated the thermal stabilities of the composites to be enhanced. SEM studies revealed the NGO flakes to be dispersed on the MOF-5 microstructures prior to adsorption and the disappearance of microstructures post adsorption. The as-synthesized nanocomposites exhibited BET surface area in the range 384.55–395.56 m2.g−1 which is a significant increase from that of MOF-5 261.00 m2.g−1. The optimal conditions for maximum Cr(VI) adsorption were determined to be: pH = 2, contact time of 60 min, adsorbent dosage of 6 mg.L−1, initial Cr(VI) concentration of 0.5 mg/L and temperature of 30 °C. A maximum adsorption efficiency of 46.1 % was achieved by the composite MOF-5/NGO (0.05 wt%) and not significantly compromised by competing metal ions. The Cr(VI) removal on MOF-5/NGO nanocomposites is well described by the Langmuir isotherm, confirming a chemisorption process. In addition, the thermodynamic studies confirmed the adsorption process to be favorable, spontaneous and endothermic. The Cr(VI) removal % on MOF-5/NGO from real water was found to be 52.1 %. Therefore, the novel MOF-5/NGO nanocomposite can be applied for efficient Cr(VI) removal from industrial wastewater due to synergistic surface morphologies between NGO and MOF-5 units characterized by better adsorption capacity not met by pristine materials.