Kinetics and thermodynamics of cyanide removal by ZnO@NiO nanocrystals

ZnO, NiO and ZnO@NiO nanocrystals were successfully synthesized and characterized by FTIR, XRD and SEM methods. The average particles sizes of ZnO, NiO and ZnO@NiO were 32, 50 and 48 nm, respectively. The nanocrystals were examined as sensors for cyanide removal. The cyanide sensing test revealed that, compared with the pure ZnO, NiO, the ZnO@NiO nanocrystals exhibited highly improved sensing performances. The ZnO@NiO nano crystals were found to have better capacity for iron cyanide than sodium cyanide. The effects of significant parameters such as contact time, pH (2-12), nanocrystal dose (0.02-0.4 g) and cyanide concentration (5-50 mg/L) on the removal of cyanide by nanocrystals were explored. At an optimum pH<5, over 90% removal of 20 mg/L cyanide was obtained for nanocrystal dose of 0.2 g after 30 min contact time for iron cyanide by ZnO@NiO nano crystals. Cyanide removal was followed by pseudo second order kinetic model for ZnO@NiO nano crystals (k(2)=4.66x10(-2) and R-2=0.999). The values of standard entralpy change of 7.87 kJ/mol and standard free energy change of -18.62 kJ/mol at 298 K suggest the adsorption of cyanide on nanocrystals is an endothermic and spontaneous process. ZnO@NiO nanocrystal is an efficient sensor for removal of cyanide from water and wastewater.