Photocatalytic reduction of hexavalent chromium Cr(VI) ions in aqueous media was studied using remarkably low dosages of commercial titania nanoparticles. A direct imposed irradiation photo-reactor equipped with ultrasonic source was utilized. Temperature variation, within conventional range of 15 – 45 °C, was considered. Design of experiments, modeling and process optimization were provided using central composite design (CCD) of response surface methodology. Accordingly, a reduced quadratic expression was developed to predict the reduction efficiency versus four influencing parameters, and analysis of variance showed the high model agreement with experimental data. At the found optimized conditions of [TiO2] = 33.1 mg/L, pH = 2.50, T = 36 °C and t = 120 min, about 81% efficiency was achieved for reduction of 5 mg/L of initial Cr(VI). The process was revealed proceeding through parallel photolysis and photocatalysis branches simultaneously. Based on differential method of analysis, the relevant kinetic model jointed with the Arrhenius equation was introduced. Energy (electrical and thermal) consumption evaluations revealed that treatment at higher temperatures leads to significant cost reduction. Furthermore, a criterion was proposed for the assessment of photocatalytic processes.
