Decolourization process of an azoïque dye (Congo red) by photochemical methods in homogeneous medium

Solution of Congo red, an azoïque dye, was decoloured by direct UV photolysis at 254 nm and at pH 6.6 of the solution. It has been observed, that decolouration process of Congo red solutions, became slow when concentration increased in the medium. For example, for the lowest concentration (10 ppm), about 70% of dye decolourization was reached after 7 h of irradiation. Improvement of this process was obtained when systems like H2O2/UV and S2O82−/UV were used. This seems to be linked to production of radicals OH
• and also SO4−, which were generated respectively, from photolysis at 254 nm of hydrogen peroxide and potassium persulfate. The influence of parameters like addition of H2O2, S2O82− and initial light intensity were investigated. We observed that the rate of photooxidation of the substrate was enhanced. However, before any photochemical study, oxidation process conducted in the absence of irradiated light was investigated. The obtained results showed, mainly for low concentrations, no reaction of the dye with either H2O2 or S2O82−. This has allowed situating the appropriate efficiency of both systems: H2O2/UV and S2O82−/UV.