Effects of gamma irradiation on the degradation of dyes

To investigate the degradation kinetics of aqueous solutions of dyes of several clases, we studied the effects of gamma irradiation versus dose (up to 80 krad), dye concentration, pH and oxygen content of these solutions. To study the influence of some of the above-mentioned parameters, anthraquinonic dyes have been irradiated in a wide range of doses(up to 5 Mrad). Furthermore these dyes were acted upon in order to investigate the complex reactions of molecular alteration through chromatographic separations and spectrophotometric analyses. Experimental results agreed with a first order kinetics for dye concentrations lower than 0.04 g/l, and with a zero order for higher concentrations. The pH was found to have a different influence according to the type of dye; for example we found that the degradation efficiency for anthraquinonic dyes has higher values for basic ranges. The disolved oxygen supports the degradation in comparison with de-aerated solutions, though its influence varies according to the dye type and the pH. The oxygen action is particularly evident with high doses; in fact, tests on anthraquinonic dyes with doses up to 5 Mrad showed a marked decrease in the kinetic constants caused by the oxygen disappearance. Radiochemical degradation yields (Gd), never greater than a few units, show that the radical reactions reponsible for the decolorizatfon effect, are limited to a few transfer sequences. COD decrease, on the other hand, confirms the presence of oxidation phenomena which correspond to computed radiochemical yields (Gox) markedly higher than those spectrophotometrically measured; such a difference is enhanced in the case of irradiation with aeration of solutions. The theoretical considerations are also described that were developed for outlining a general scheme involving the experimental results of both the kinetics and the radiochemical yield.