In this study, the photocatalytic degradation of Reactive Blue 81 (RB81) using synthesized NiO-doped ZnO–ZrO2 nanoparticles under UV irradiation was investigated. Then, the products were characterized by Scanning electron microscope (SEM), X-ray diffraction (XRD), and diffuse reflectance spectroscopy (DRS). The removal rate of RB81 using ZnO–ZrO2 after 180 min of irradiation was 96.7%. Nickel oxide (NiO) was used as an additive to ZnO–ZrO2 for improvement of RB81 degradation via photocatalysis process. Photodegradation of RB81 was achieved to 100% using ZnO–ZrO2–NiO nanoparticles with ratio of 1:2:0.3 after 180 min of irradiation. There was a red shift in absorption bands (from 410 nm to 435 nm) observed in increasing of NiO to ZnO–ZrO2 nanoparticle, that it might lead to a higher photocatalytic activity under visible light. Response surface methodology (RSM) was used for optimization of experimental and these results were obtained: solution pH = 3, ZnO–ZrO2–NiO dosage = 15 mg/L, and the initial RB81 concentration = 5 mg/L. The photodegredation of RB81 followed pseudo-first order kinetic according to the Langmuir–Hinshelwood model
