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

Abstract

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 [HO2−] (at constant OH− ion concentration) are observed: –d[MB]/dt = ((a′ + b′[OH–])/(1 + c′[OH–]))[MB] and –d[MB]/dt = ((a″+ b″[HO2–])/(1 + c″[HO2–]))[MB], respectively. A higher-order than linear dependence of the pseudo first-order rate constant on the nucleophiles (OH– and/or HO2–) 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 HO2– anions.