Background: Heavy metals in aquatic systems usually interfere with many beneficial uses of water. Divalent nickel is a commonly occurring toxic metal in natural ecosystems due to the effluent of refineries, electroplating, and casting industries. In aquatic environments, nickel appears as Ni (II) and Ni0. Despite the high reported toxicity for Ni (II), Ni0 is only slightly toxic. Various methods have been proposed for the treatment of aqueous solutions containing Ni (II). Photocatalytic reduction is an important process; titanium dioxide has been mostly used as a very efficient photocatalyst. Methods: In this study, the removal of divalent nickel ions in aqueous solutions was studied in the presence of remarkably low dosages of nano-titania photocatalyst. Direct imposed irradiation was utilized for treatment of solutions. Accordingly, the influence of four operational parameters, including temperature within the conventional range of 20 to 40 °C, was investigated. Design of experiments, modeling and process optimization were accomplished using central composite design of response surface methodology. Results: Reduced quadratic expression was developed for the reduction efficiency (RE), and the analysis of variance showed its capability in reproducing the data. The effectiveness of each parameter was determined. At the best found conditions of [TiO2] = 42 mg/L, pH = 9.2, T = 34 °C and after 90 minutes of treatment, about 85% removal was achieved for initial 5 mg/L nickel solutions. Pseudo first order reactions proceeded. Conclusion: Based on CCD method, the influence of individual operating parameters and their interactions were obtained. A quadratic equation predicted the variations quite well.
