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

摘要

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 = ((a"+ b"[HO_2~-])/(1 + c"[HO_2~-]))[MB], respectively. A higher-order than linear dependence of the pseudo first-order rate constant on the nucle-ophiles (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.