Iranian Journal of Chemistry & Chemical Engineering (Feb 2021)
Decolorization Kinetics of Ponceau S Dye by Chemical Chlorination: A Comparison with Sunlight/ Chlorine and UV/Chlorine Processes
Abstract
Aqueous chlorination of Ponceau S dye (PS) was kinetically investigated through a classical method and under solar or UV radiations. For the classical method, the kinetics of PS chlorination was established by varying concentration ratios R = [Free chlorine (Cl(+I))]/[PS], pH, temperature, and bromide ions concentration. The decolorization reactions exhibited pseudo-first-order kinetics with respect to Ponceau S dye and their apparent rate constants (kapp ) increased when R increased, in fact, kapp reaches 0.205 min-1 when R=10. Important removal levels were obtained when pH= 10, 100% of decolorization obtained at only 5 minutes which means that the hypochlorite ion (ClO-) was more reactive than hypochlorous acid (HClO) for decolorizing PS dye. Otherwise, the apparent rate constant of PS disappearance increased with increasing medium temperature and led to the activation energy of 39.64 Kj/mol. When bromide ions were present in the medium, the decolorization of the PS solutions was more important than the simple chlorination when R=2.5 and at pH 7.20 following an oxidation result of bromide ions by sodium hypochlorite to hypobromite and/or hypobromous acid, the addition to the N=N double bond of dye is easier in the case of Br(+I) than Cl(+I). Under UV or solar radiations, and when R=1 and at pH 4.5, the decolorization rates of PS solutions were clearly increased in comparison with dark chlorination. Thus, the comparative order obtained in terms of apparent rate constant was kapp (Cl(+I)/UV)> kapp (Cl(+I)/Sunlight)> kapp (Cl(+I)). The disappearance efficiencies were 47.6%, 52.4% and 99.2% respectively when using dark chlorination, Cl(+I)/Sunlight and Cl(+I)/UV processes. Thus, it was clear that the advanced oxidation processes have proven to be more effective in terms of water treatment by chlorination. Thus, compared with dark chlorination, the enhanced decolorization of PS could be attributed to the formation of reactive radicals that adopt essentially radical mechanisms that are faster compared to molecular attack.
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