Optimization of Cr(VI) Photocatalytic Reduction by UV/TiO2: Influence of Inorganic and Organic Species and Kinetic Study

Background & Aims of the Study: Chromium is widely detected in surface waters andunderground waters, which usually appear as Cr(VI), and Cr(III), at sites associated withindustrial activities. Cr(VI), in effluent streams with a high level of mobility and notoriousmutagenic and carcinogenic toxicity; thus Cr(III) does not have much mobility in soil. So,converting it into less harmful species could be beneficial.Materials & Methods: Cr(VI) photocatalytic reduction in aqueous media was analyzed usingdesperately low dosages of nanoparticles of commercial titania. A directly imposed irradiationphotoreactor equipped with a supersonic source was applied. The optimization of the reductionprocess was done using the central composite design (CCD) experimental. The residualconcentration of Cr(VI) ion was determined by colorimetrically method. In addition, the impactof other factors, including water matrix and hole scavengers, also reduction kinetics werestudied.Results: A quadratic equation for reduction efficiency was proposed, and the adequacy of it wasevaluated by a variety of statistical methods. A maximum of 80.6% reduction in aqueoussamples containing an initial concentration of Cr(VI) within the investigated optimum operatingcondition (TiO2 dose of 33 mg/L; pH of 2.5, T=35 °C and t=120 min) was obtained. Resultsindicate that UV irradiation alone is an acceptable method for Cr(VI) reduction maybe due toH2O2 photolytic generation. The results show Cr(VI) photoreduction was greatly enhanced byabout 88.2% when NO3− was used in comparison with SO42− anion. The photoreductionenhancements with the scavengers are appeared in the following order ethylene glycol > formicacid > citric acid with relevant. Maximum reduction of 96.5% for Cr(VI) was obtained in thepresence of ethylene glycol hole scavenger. The results indicated that the process rate can bepresented with a pseudo-first-order kinetic model.Conclusion: The results showed, the CCD design was approximately adequate in Cr(VI)reduction, so it can be a suitable option for water quality improvement. The addition of inorganicor organic species can act as scavenging hydroxyl radicals- which are photo-generated- andvalanceband holes that are on the photocatalysts of the semiconductor, and consistently, enhancethe photocatalytic reduction of Cr(VI) ion.

کلیدواژه ها:

Photocatalytic reduction ، Hexavalent chromium ، Hole scavenger ، Kinetic ، Energy consumption ، Iran