Sustainable synthetic approach and applications of ZnO/r-GO in the adsorption of toxic Pb2+ and Cr6+ ions

[Display omitted] •Tectona grandis leaves extract based synthesis of ZnO NPs.•Sono-chemical preparation of ZnO/r-GO and its characterisation using several analytical methods.•Utilisation of ZnO/r-GO in the removal of toxic Pb2+ and Cr6+ ions from contaminated water via adsorption.•Isothermal and kinetic studies. The current work presents a sustainable method for the synthesis of ZnO/r-GO material using Tectona grandis leaf extract. Leaf extract was used to synthesise ZnO nanoparticles (ZnO NPs). To synthesise ZnO/r-GO, reduced graphene oxide (r-GO) and ZnO NPs were incorporated under ultrasonic conditions. The characteristic bonds onto ZnO/r-GO were identified as OH, CO, C-Zn, Zn-O, and others. The ZnO/r-GO was analysed using several analytical methods and has been used to eliminate hazardous Pb2+ and Cr6+ ions from contaminated water via adsorption. Maximum decontamination has been achieved through optimization. The removal of 68.3 and 60.9 % at 60 min, 67.9 and 63.9 % at 100 mg dosage, and 98 and 88.5 % at optimal pH was recorded for Pb2+ and Cr6+ ions. The adsorption equilibrium findings were validated using various isotherm and kinetic models. Pb2+ and Cr6+ had the highest adsorption capacities of 16.261 and 10.718 mg/g, respectively. The pseudo second order rate constants (k2) were found to be 2.738 and 3.415 g/mg/min for Pb2+ and Cr6+ ions. The regeneration study also revealed the potential of the ZnO/r-GO and, after the first cycle, the % removal has been found to be 77.9 and 73.2 % for Pb2+ and Cr6+ ions, respectively.