Kinetics and mechanism of color removal of methylene blue with hydrogen peroxide catalyzed by some supported alumina surfaces

The catalyzed kinetics of the oxidative mineralization of the cationic dye methylene blue, phenothiazonium, 3,7-bis(dimethylamino)-chloride, with hydr ogen peroxide were studied both in buffered and unbuffered solutions. The s upported alumina catalysts used were in the form of copper(II), cobalt(II), manganese(II), and nickel(II)-ions. Also, some copper(II)-complexes were u sed, e.g. copper(II)-ammine ([Cu(amm)(4)](2+)), copper(II)-ethylenediamine ([Cu(en)(2)](2+)) and copper(II)-monoethanolamine ([Cu(mea)(2)](2+)). The r eaction is first order with respect to methylene blue. On the other hand, t he order with respect to hydrogen peroxide is concentration range dependent . This range depends strongly on the catalyst used. At lower [H2O2], the or der was 1 which then decreases with increasing [H2O2] passing through 0 at the maximum rate and finally becomes negative. This phenomenon is parallel to the formation of a colored intermediate on the surface of the catalyst. This suggests that the intermediate has an inhibiting effect on the rate of color removal. Moreover, the rate of the reaction was found to be strongly dependent on the pH of the solution and its ionic strength. It increases w ith increasing both pH and the concentration of added potassium chloride. A lso, the rate of reaction is inhibited in presence of sodium dodecylsulfate anionic surfactant. The repeated use of the different catalysts showed tha t their catalytic activities are almost unaffected. A reaction mechanism wa s proposed with the formation of free radicals as reactive intermediates. ( C) 2000 Elsevier Science Ltd. All rights reserved.