Photocatalytic degradation of reactive dyes by visible light and innovative Fe-doped titania catalysts

Reactive dyes are the largest class of dyes used in the textile industry. They were selected for the present study because the treatment of wastewaters containing these dyes by conventional methods is often inadequate due to their resistance to biological and chemical degradation. The use of reactive dyes for textile dyeing has increased steadily over the last few years because of theirs cost effectiveness and excellent wash and light fastness properties. However, these dyes exhibit a very low degree of fixation on the fibers, typically between 50 and 90%, which results in the release of substantial amounts of the compounds in the dyeing water. The dyeing water cannot be reused, so dye recovery is not an option with reactive dyes. The high aromaticity and low reactivity of these dyes make them highly resistant to both microbial and chemical degradation. Physical treatments are not efficient because they transfer the toxic dyes from one medium to the other, without converting them to harmless non-toxic substances. The development of novel treatment methods, called Advanced Oxidation Processes (AOPs), characterized by production of the hydroxyl radical (·OH) as a primary oxidant, could represent a solution. The application of these AOPs, alone or simultaneously, could enhance ·OH radical production leading to higher oxidation rates. In this work, the photocatalytic oxidative decolorization of various reactive dyes, Reactive Blue 4, Reactive Red 120 and Reactive Violet 5, under exposure to visible light and in the presence of new semiconductor catalysts, was investigated. The results suggest that Reactive Blue 4 is highly unstable under exposure to visible light, thus the presence of photocatalysts is not necessary for the degradation of this dye. The Reactive Red 120 cannot be treated by direct irradiation and it cannot be decolorized even in the presence of photocatalysts. The other results obtained indicate that the photocatalytic treatment under visible light can be an effective method for the removal of Reactive Violet 5 from textile effluents.