Alkaline hydrogen peroxide as a degradation agent of methylene blue-kinetic and mechanistic studies

Methylene blue degradation has been studied spectrophotometrically in alkaline and alkaline peroxide solutions. In both systems, the reaction proceeds via successive N-demethylation and deamination steps determined applying TLC and HPLC techniques. Disappearance of the dye obeys first-order kinetics under the excess of all other reactants. The rate expression for the hydrolytic process (in the absence of hydrogen peroxide) is as follows: -d[MB]/dt = (b[OH(-)]/(1 + c[OH(-)]))[MB]. Analogous forms of the reaction rate dependence on [OH(-)] (at constant hydrogen peroxide concentration) and on [HO(2) (-)] (at constant OH(-) ion concentration) are observed: -d[MB]/dt = ((a' + b'[OH(-)])/(1 + c'[OH(-)]))[MB] and -d[MB]/dt = ((aaEuro(3)+ baEuro(3)[HO(2) (-)])/(1 + caEuro(3)[HO(2) (-)]))[MB], respectively. A higher-order than linear dependence of the pseudo first-order rate constant on the nucleophiles (OH(-) and/or HO(2) (-)) concentrations results from the competitive formation of the ion pair of the cationic dye with the chloride of the supporting electrolyte and adducts of the dye with OH(-) and/or HO(2) (-) anions.