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 and underground waters, which usually appear as Cr(VI), and Cr(III), at sites associated with industrial activities. Cr(VI), in effluent streams with a high level of mobility and notorious mutagenic and carcinogenic toxicity; thus Cr(III) does not have much mobility in soil. So, converting it into less harmful species could be beneficial.
Cr(VI) photocatalytic reduction in aqueous media was analyzed using desperately low dosages of nanoparticles of commercial titania. A directly imposed irradiation photoreactor equipped with a supersonic source was applied. The optimization of the reduction process was done using the central composite design (CCD) experimental. The residual concentration of Cr(VI) ion was determined by colorimetrically method. In addition, the impact of other factors, including water matrix and hole scavengers, also reduction kinetics were studied.
A quadratic equation for reduction efficiency was proposed, and the adequacy of it was evaluated by a variety of statistical methods. A maximum of 80.6% reduction in aqueous samples containing an initial concentration of Cr(VI) within the investigated optimum operating condition (TiO2 dose of 33 mg/L; pH of 2.5, T=35 and t=120 min) was obtained. Results indicate that UV irradiation alone is an acceptable method for Cr(VI) reduction maybe due to H2O2 photolytic generation. The results show Cr(VI) photoreduction was greatly enhanced by about 88.2% when NO3− was used in comparison with SO42− anion. The photoreduction enhancements with the scavengers are appeared in the following order ethylene glycol > formic acid > citric acid with relevant. Maximum reduction of 96.5% for Cr(VI) was obtained in the presence of ethylene glycol hole scavenger. The results indicated that the process rate can be presented with a pseudo-first-order kinetic model.
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 inorganic or organic species can act as scavenging hydroxyl radicals- which are photo-generated- and valanceband holes that are on the photocatalysts of the semiconductor, and consistently, enhance the photocatalytic reduction of Cr(VI) ion.