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Lab-scale degradation of azo dyes with ultrasound (300 kHz), ozone and both was investigated using an aryl-azo-naphthol dye-C.I. Acid Orange 8. It was found that in all schemes color decay was faster than UV absorbance, and the rates followed pseudo-first-order kinetics except for the decay of UV-254 band by ozone. Sonication alone was sufficient for decolorization, but not for UV absorption abatement or mineralization. Ozonation was more effective than ultrasound in bleaching, but not as much for the mineralization of the dye. Combined operation of ultrasound and ozone improved the rate of bleaching and UV absorption decay and remarkably enhanced the mineralization of the dye. This was attributed to increased mass transfer of ozone in solution and its decomposition in the gas phase to yield hydroxyl radicals and other oxidative species. The effect of alpha-methyl substituent at the aryl carbon of the dye was found to decelerate the rate of degradation as a result of weakened intramolecular hydrogen bonding.

Degradability of four textile dyes was investigated in deionized water solutions during 520 kHz ultrasonic irradiation. It was found that for all dyes, the rate of color decay was first order in the visible absorption of the dye, and related to the type of functional groups that characterized the chromophore in the dye molecules. The destruction of aromatic/olefinic carbons in azo dyes was slower than that of color--to be attributed to the priority of hydroxyl radical attack on the N=N bonds, and to the formation of numerous oxidation intermediates of organic character during the course of dye degradation. Toxicity analysis of the dye solutions prior to sonolysis revealed that "reactive" dyes were non-toxic, but "basic" ones were toxic at the test concentrations employed in the study. Significant degrees of toxicity reduction were accomplished along with color and aromatic carbon degradation.

Treatability of textile dyebath effluents by two simultaneously operated processes comprising adsorption and advanced oxidation was investigated using a reactive dyestuff, Everzol Black-GSP (EBG). The method was comprised of contacting aqueous solutions of the dye with hydrogen peroxide and granules of activated carbon (GAC) during irradiation of the reactor with ultraviolet light (UV). Control experiments were run separately with each individual process (advanced oxidation with UV/H2O2 and adsorption on GAC) to select the operating parameters on the basis of maximum color removal. The effectiveness of the combined scheme was tested by monitoring the rate of decolorization and the degree of carbon mineralization in effluent samples. It was found that in a combined medium of advanced oxidation and adsorption, color was principally removed by oxidative degradation, while adsorption contributed to the longer process of dye mineralization. Economic evaluation of the system based on total color removal and 50% mineralization showed that in the case of Everzol Black-GSP, which adsorbs relatively poorly on GAC, the proposed combination provides 25% and 35% reduction in hydrogen peroxide and energy consumption relative to the UV/H2O2 system. Higher cost reductions are expected in cases with well adsorbing dyes and/or with less costly adsorbents.