Photocatalytic reduction of hexavalent chromium, Cr(VI), ions in aqueous solutions was studied using remarkably low doses of commercial titania nanoparticles at conventional temperature range of 15–45 C. A direct imposed irradiation photo-reactor, equipped with ultrasonic source was utilized. Design of experiments, correlating and process optimization were performed using central composite design of response surface methodology. Accordingly, a reduced quadratic expression was developed to predict the reduction efficiency as a function of involved parameters. Analysis of variance shows the agreement of the provided model with experimental data. At the found optimum conditions of titania concentration: 33.1 mg/L, pH: 2.50, temperature: 36 C and reaction time: 120 min; 81 % efficiency was achieved for reduction of initial 5 mg/ L of Cr(VI). The process revealed proceeding through parallel branches of photolysis and photocatalysis, and only UV irradiation was promising for reaction progress. 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 provides a significant cost reduction. Meantime, a criterion was proposed for a more valid assessment of photocatalytic processes.
