Free Cyanide (FCN) is a harmful product released into the environment by several metallurgical industries. Various concentrations of Zinc Oxide (ZnO) nanoparticles have been synthesized by green method using Eucalyptus globulus leaves and tested for FCN removal. The optical and physico-chemical characteristics of the synthesized ZnO nanoparticles were investigated. X-ray diffraction has shown a pure crystallite Zincite form with average size 9.8 nm. Their bandgaps were obtained by UV-VIS analysis with values between 3.32 and 3.43 eV. Intense and narrow peaks were detected by photoluminescence studies at 374 nm. Fourier transform infrared (FTIR) spectroscopy have shown various functional chemical bending between the following groups O–H, C–H, C–N, and C=C which explain the ZnO nanoparticles formation mechanism. Spherical surface morphologies were revealed by scanning electron microscopy (SEM) image analysis. Photocatalytic efficiency of ZnO in removing FCN from synthetic wastewater was investigated under solar irradiation and UV light with a power 30 W. The maximum degradation percentage of 3 g CN− L−1 was 45% and 98 % in 20 min in the presence of solar irradiation and UV light. Therefore, the green synthesis of ZnO nanoparticles used in photocatalytic process is an eco-friendly and sustainable method to decontaminate high level of FCN in wastewater.