¨ by photochemical methods in homogeneous medium

Solution of Congo red, an azoıque 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 S2 O2À /UV were used. This seems to be linked to production of radicals OH• and also SO:4À , which were gen8 erated respectively, from photolysis at 254 nm of hydrogen peroxide and potassium persulfate. The influence of parameters like addition of H2O2, S2 O2À and initial light intensity were investigated. We observed that the rate of 8 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 S2 O2À . This has allowed situating the appropriate efficiency of 8 both systems: H2O2/UV and S2 O2À /UV. 8

radicals are present in the medium [19, 20]. Moreover, the efficiency of S2O82À/UV might be enhanced by thermal oxidation process comparatively to H2O2/UV where this oxidation in absence of light was not involved. This study has shown that advanced oxidation processes such as H2O2/UV and S2O82À/ UV have been efficient in the course of decolourization of the dye. Indeed, the rate of photodecomposition of the substrate was weak in comparison to that obtained by direct UV Table 5 Comparison between efficiencies of S2O82À/UV and H2O2/UV. Values of t50%, t90% and initial rate vo [S2O82 ]o [H2O2]o 10À3 37.52 184.64 0.955 Concentration (M) t50% (min) t90% (min) vo (mg LÀ1 minÀ1) 2 Â 10À3 26.66 113.55 1.145 10À3 19.01 91.91 1.48 2 Â 10À3 8.76 45.21 2.40 Table 6 Chemical and photochemical reactions involved in the system S2O82À/UV Reactions Rate constants S2 O2À þ hn ! 2SOÀ SOÀ þ S2 O2À ! SO2À þ S2 O:8À SOÀ þ H2 O , OH þ HSOÀ SOÀ þ SOÀ ! S2 O2À OH þ OH ! H2 O2 Substrate þ OH: =SOÀ ! Intermediaries ! mineralization l = 253.7 nm 6.6 Â 105 MÀ1sÀ1 9.4 Â 103 sÀ1 3.2 Â 108 MÀ1sÀ1 5 Â 109 MÀ1 sÀ1 – DES4102.3d 6/24/2009 12:20:57 photolysis (254 nm). Moreover, experimental results have proved that this rate of photodecomposition has been improved as well as by addition of H2O2 and S2O82À as by light intensity. From the results obtained in this work, it can be concluded that the influence of various hydrogen peroxide concentrations on the photodegradation of Congo red by a monochromatic UV radiation was positive, with increasing decay rates when [H2O2] is increased. In the combined UV/H2O2 (but to a certain limit of the concentration to avoid the quenching effect) and UV/S2O82À processes, the enhancement in the photodegradation rate promoted by the presence of .OH and SO:4À clearly demonstrated, leading therefore to a better efficiency than direct UV photolysis in the decolourization process of the dye. Moreover, it has been observed that rate of Congo red photodegradation in the presence of S2 O2À was higher than that with H2O2. Furthermore, a sensitive improvement of the bleaching process has been obtained when the light intensity has been combined to the UV/H2O2 system.