On the removal of hexavalent chromium from wastewater: a comparative study between photocatalytic and chemical reduction processes

Common sources of chromium in wastewater are electroplating and leather industries. Hexavalent chromium is more toxic and carcinogenic compared to its trivalent counterpart. Conversion of Cr (VI) to Cr (III) in an aqueous medium both by chemical reduction using zerovalent iron and by photocatalytic reduction using UV radiation and ZnO semiconductor has been investigated using potassium dichromate as the model compound. Both the processes are effective to reduce Cr (VI) efficiently up to about 90% in 90 minutes. The process parameters were pH (varied from 1 to 6) and iron loading (0.5 to 1.0g/100mL) for chemical reduction, and ZnO loading (0 to 3g/L) and intensity of UV radiation (0 to 120W medium pressure Hg vapour lamp) for the photochemical process. Initial concentration of substrate solution was varied (40 to 125mg/L approx) in both cases. Kinetics of the chemical method was found to follow pseudo first order rate equation whereas the initial rate of the photochemical method was found to be independent of the concentration of potassium dichromate. Rate of reduction in chemical method increased with increase in loading of iron dust and with decrease in pH. Initial rate of photocatalytic reduction increased with increase in catalyst loading and intensity of UV radiation. A possible mechanism and the corresponding kinetic model have been proposed for the photo reduction and tested by experimental results.