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Zinc oxide nanoparticles for the photocatalytic reduction of hexavalent chromium in wastewater under sunlight

Prantik Banerjee, Pallavi Mitra, Sampa Chakrabarti, Sekhar Bhattacharjee

Common sources of chromium in wastewater are leather industries, electroplating, electronic, and timber. Hexavalent chromium is more toxic and carcinogenic than trivalent chromium. Photocatalytic reduction of Cr (VI) to Cr (III) in an aqueous medium under tropical sunlight and nanosized ZnO semiconductor catalyst has been investigated using potassiumdichromate as the model compound. Methanol was added as a hole- scavenger for enhancement of the photo-reduction. Effects of the process parameters such as nano-ZnO loading (0 to 0.6g/L), initial concentration of substrate solution (10 to 50mg/L), pH(4-9) and methanol-dosing (0 – 1.75mol/ L) have been investigated. The initial rate of the photochemical method was found to be independent of the concentration of potassium dichromate as well as concentration of methanol above an optimum value. A probable mechanism for the entire reaction and the corresponding kinetic model have been proposed for the photo reduction and proved by experimental results. Another rate equation based on the LHHW model compares well with the mechanistic rate equation.