Significant conversion of divalent nickel ions in aqueous solutions, into zero valent nickel species was achieved with trace amounts of nano-size titania photocatalyst using a direct irradiating photo-reactor. Accordingly, the influence of four operational parameters, including temperature within its conventional range, was investigated. Design of experiments, modeling and process optimization were accomplished using central composite design of response surface methodology. A reduced quadratic expression was developed for reproducing the reduction efficiency (RE) data. At the best found conditions of [TiO2] = 42 mg/L, pH = 9.2, T = 34 degrees C and after 90 min reaction, about 85% RE was achieved. The reduction of substrate was ascertained with XRD, FTIR and XPS analysis. The reaction kinetic was also studied considering the parallel reaction branches of photolysis and photocatalysis. Pseudo first order reactions are corresponding. In addition, energy (electrical and thermal) consumption evaluations revealed the significant cost effective operations at elevated temperatures. Finally, based on different criteria the performance was evaluated in comparison with similar processes. (C) 2014 Elsevier B.V. All rights reserved.