Water-soluble manganese porphyrins are catalysts for the decoloration of azo dyes by hydrogen peroxide under mild conditions (pH 8.0 and 25°C). The decoloration rate depends on the structure of the manganese porphyrin and the added ligands.

The effect of three surfactants, dodecylbenzenesulfonate (DBS), n-hexadecyltrimethylammonium bromide (HTAB), and polyethylene glycol mono-p-octylphenyl ether (PEOPE), on the decoloration rates of C.I. Acid Orange 7, C.I. Basic Green 4, and C.I. Basic Orange 33 by peroxide bleaching agents such as sodium percarbonate and hydrogen peroxide is examined. The decoloration rate of Acid Orange 7 by peroxide bleaching agents accelerates in the presence of DBS, HTAB and PEOPE, especially near the cmc, with the maximum rate being observed in the presence of HTAB. However, the decoloration rate of Basic Orange 33 by hydrogen peroxide accelerates only when DBS is present near the cmc. There is no or less enhanced decoloration in the presence of HTAB or PEOPE. Interestingly, the decoloration rate of Basic Green 4 by hydrogen peroxide accelerates with increasing HTAB concentration, and in contrast, the rate decreases when DBS is present. These results indicate that decoloration rates by peroxide bleaching agents largely depend on interactions between the dyes and surfactants.

Sulfonated hydroxyarylazo and aminoarylazo dyes were reacted with sodium percarbonte in water at pH 10.6 and at various temperatures. We have determined the initial rate and the apparent activation energy of the decoloration reaction. The results suggest a greater susceptibility to attack of the hydrazone form than the azo form. Aminoarylazo dyes are more resistant to degradation relative to similar hydroxyarylazo dyes, which contain a hydroxyl group that can be ionized in alkaline solutions. They are more resistant and have greater apparent activation energy than the others.

We have investigated the initial rate of decoloration of some natural colorants by sodium percarbonate in solution and the influence of pH and temperature on the decoloration reaction. The natural colorants were flavonol, chrysin, carthamin, car minic acid, and shikonin. Orange II (CI acid orange 7), a typical synthetic azo dye, was used for comparison. The maximum rate constant was in the neighborhood of pH 11.5 for all cases, and increased in the following order flavonol < chrysin < car thamin < orange II < carminic acid < shikonin. The decoloration rate increased as the temperature rose. The apparent activation energy of the decoloration reaction seemed to decrease in the order above.