Cyanide has numerous applications in industry such as chelating agent, electroplating, pharmaceuticals, and mining[1, 2]. This extensive use of cyanide results in the generation of a huge amount of cyanide waste and increases the cyanide spill risk to the environment[3, 4]. Thus, cyanide must be treated before discharging. Different protocols such as adsorption, complexation, and oxidation are used for abating cyanides[1, 2, 5–7]. The procedures other than oxidation give highly concentrated products in which toxic cyanides still exist[8, 9].
Highly powerful, economically method is the photocatalytic oxidation of cyanide, which has been demonstrated in several studies[10–17]. However, an inexpensive photocatalyst is needed for the economical removal of large quantities of cyanide. ZnO is one of the most promising materials for executing this task, as an alternative to the widely used, relatively expensive titania (TiO2). Although researchers recognized comparable photodegradation mechanisms with both ZnO and TiO2, they proved that ZnO was the superior photocatalyst in degrading pesticide carbetamide, herbicide triclopyr, pulp mill bleaching wastewater, 2-phenylphenol, phenol, blue 19, and acid red 14. This superiority of ZnO photocatalytic activity is because it has more active sites, higher reaction rates, and is more effective in generating hydrogen peroxide.
Due to its direct, wide bandgap of 3.37 eV, ZnO has a wide range of applications in optoelectronic devices such as light-emitting diodes, photodetectors, and p-n homojunctions. The large exciton binding energy of 60 meV, compared to that of GaN (approximately 25 meV), enhances the luminescence efficiency of the emitted light even at room temperature and higher. The visible photoluminescence (PL) emission at approximately 2.5 eV (approximately 495 nm), originated from intrinsic defects, makes ZnO suitable for applications in field emission and vacuum fluorescent displays.
Many techniques including chemical vapor deposition, pulsed laser deposition, molecular beam epitaxy, sputtering, hydrothermal synthesis, and oxidation of metallic zinc powder[27, 28] have been used to prepare ZnO in different forms and structures for various applications. Nanoparticulate form enhances the catalytic activity due to its large surface area and the presence of vacancies and uncoordinated atoms at corners and edges. The photocatalytic activity is also improved by bandgap engineering, as a result of the quantum confinement effect[29–31].
A well-controlled synthesis process at room temperature is needed for the economical use of ZnO in catalytic applications such as water treatment and other environmental applications. Herein, we are reporting, for the first time to the best of our knowledge, a direct, simple, room-temperature synthesis method for ZnO nanoparticles using cyclohexylamine (CHA), as a precipitating agent, and zinc nitrate hexahydrate, as a source of zinc, in both aqueous and ethanolic media. The synthesized ZnO nanoparticles were examined as a photocatalyst for the degradation of the highly toxic cyanide anion [CN-(aq)] in the aqueous medium at room temperature. The kinetics for cyanide photodegradation were investigated with respect to ZnO concentration of weight percentage.