One-electron reduction of thionine has been studied by using the technique of nanosecond pulse radioly'sis and kinetic spectrophotometry. H, e2q as well as radicals derived from methanol, ethanol, isopropanol, THF, dioxane and t-butanol by H atom abstraction were used as reductants. The rate constants for the transfer of electrons from these radicals to thionine were directly determined from the pseudo first-order formation rates of the product, semithionine and the one-electron reduction potential of thionine estimated. The absorption spectrum of semithionine in its different conjugate acid-base forms was found to be in agreement with previously reported spectra and the decay of the species was second order. By monitoring transient absorbance changes as a function of pH, two pK, values were observed and, based on the effect of ionic strength on the second-order decay constants of the species were assigned to the equilibria described.

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Using the pulse radiolysis technique, studies on reactions of 3,4,5-trihydroxybenzoic acid [gallic acid (GA)] with radical species generated in water are reported. At pH 6.8 and 9.7, OH radicals react with GA to give an adduct initially with rate constants of the order of 1 ] 1010 dm3 mol~1 s~1. This adduct then reacts with parent GA molecules with rate constants of the order 5 ] 108 dm3 mol~1 s~1 to give phenoxyl type radical species having absorption maxima in the 350 nm region. At pH 12 and 13.6, OH/O~radicals directly bring about oxidation of GA. SpeciÐc oxidants like azidyl radical bring about one electron oxidation at neutral and alkaline pHs with almost di †usion controlled rate constants. Rate constants for the reaction of radicals Br 2 ãre found to be lower than those for OH radicals by an order of magnitude. At pH 0, both OH and Cl 2 radicals react with GA to give phenoxyl type radicals. The phenoxyl radicals formed are quite stable at higher pHs, which make GA a good antioxidant. Rate constants for the reactions of with di †erent protolytic e aq forms of GA are determined.

The comparison in the extent of % mineralization of Simulated Textile Dye Waste Water (STDWW) in three Advanced Oxidation Processes.

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