Unconsumed dyes in wastewaters still impose serious aesthetic and environmental problems because of their colour and their high chemical oxygen demand. Especially reactive azo dyes have poor biodegradation under aerobic conditions. Aromatic amines formation under anaerobic conditions is well documented as well. Recent studies indicate that ultrasound as advanced oxidation process might be an alternative for the destruction of many organic compounds including textile dyes [1-3].

The decolouration and degradation of C.I. Reactive Black 5, a worldwide used azo dye, by low (20 kHz) and high frequency (279 kHz/817 kHz) ultrasonic irradiation, without the addition of any oxidant was studied. Characterization of the used ultrasound systems was performed using calorimetric measurements and oxidative species monitoring using Fricke and iodine dosimeter in order to find the best experimental conditions - a compromise between acoustic efficiency and radical formation. Emphasis was given to the effect of different operating conditions on radical formation rate, decolouration and degradation efficiency, as well as on the toxicity profile of such treatment.

Sonochemical decolouration was substantially depressed by the addition of tert-butanol as a radical scavenger that suggests radical-induced reactions in the solution. Acute toxicity to marine bacteria Vibrio fischeri was measured before and after ultrasound irradiation. Degradation efficiency was estimated by TOC measurements.

Degradation products determination was performed using ion chromatography, HPLC and LC-MS-MS at Institute of Chemical Process Engineering and Plant Design in Cologne. The authors are very grateful to prof. Astrid Rehorek for providing these measurements and for useful discussions.

[1] H.M. Pinheiro, E. Touraud, O. Thomas, Aromatic amines from azo dye reduction: status

review with emphasis on direct UV spectrophotometric detection in textile industry wastewaters, Dyes and Pigments 61 (2004) 121-139.

[2] A. Rehorek, M. Tauber, G. Gübitz,Application of power ultrasound for azo dye

degradation, Ultrasonics Sonochemistry, 11, 3-4 (2004) 177-182.

[3] S. Vajnhandl, A. Majcen Le Marechal, Ultrasound in textile dyeing and the

decolouration/mineralization of textile dyes, Review, Dyes and Pigments 65 (2005) 89-101.
[4] S. Vajnhandl, A. Majcen Le Marechal, Case Study of the Sonochemical Decolouration
of

Textile Azo Dye Reactive Black 5, accepted paper at Journal of Hazardous Materials

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