Decolouration and mineralization of reactive dyes by H2O2/UV process

Evaluation of the efficacy of in-situ electro-synthesised H2O2 for the decolouration and mineralization of different commercially-important textile reactive dyes, in the presence of UV irradiation confirms that H2O2/UV treatment using an in-situ formation of H2O2 results in much higher decolouration rates than photolysis alone. The optimal working conditions were those that enabled maximum decolouration rates for textile dye RR 238 at the lowest electrical current (1 A) and a maximum flow-rate (150 mg/L). The best decolouration and TOC reduction was obtained with two reactive azo dyes having vinylsulphone reactive group.

The main pollution in wastewater from the textile finishing industry originates from the dyeing and finishing processes. These processes require the input of a wide-range of chemicals and dyestuffs, which are generally organic compounds of complex structures. Among textile dyes, azo dyes constitute the largest and most important class of commercial dyes. Anthraquinone dyes constitute the second most important class after azo dyes and are characterized by their fastness, which is caused by their very stable anthraquinone structures. Spent dye-baths produce a coloured effluent with complex chemical compounds which are resistant to light, heat, oxidizing agents, and biodegradation.

Evaluation of the efficacy of in-situ electro-synthesised H2O2 for the decolouration and mineralization of different commercially-important textile reactive dyes, in the presence of UV irradiation has been carried out. For this purpose, three reactive azo dyes, namely Reactive Red 238 (RR 238), Reactive Orange 16 (RO 16), and Reactive Black 5 (RB 5), and one reactive anthraquinone dye, i.e. Reactive Blue 4 (RB 4), were selected as model textile dyes. They were subjected to the best experimental conditions regarding H2O2/UV treatment. The electro-production of hydrogen peroxide under different dye concentrations was also followed. H2O2/UV treatment efficiency was assessed in terms of colour and TOC (Total Organic Carbon) removal rates.

                                  
Results based on the basis of RR 238 model dye solution confirm that H2O2/UV treatment using an in-situ formation of H2O2 results in much higher decolouration rates than photolysis alone. The optimal working conditions were those that enabled maximum decolouration rates for textile dye RR 238 at the lowest electrical current (1 A) and a maximum flow-rate (150 mg/L).

The decolouration rates and TOC reductions obtained for selected textile dyes under these conditions, at two different concentrations have shown that for each dye there is a maximum for H2O2 production and that after this point the production of H2O2 starts to decline rapidly. This phenomenon can explain the decline in decolouration rates after a certain treatment time. The scavenging mechanism cannot be excluded. This means that the concentration of H2O2 is a very important parameter, which should be optimised for each H2O2/UV application. The best decolouration degree was obtained with azo vinylsulphone dye RB 5 and the lowest with anthraquinone dichlorotriazine dye RB 4. All the studied reactive dyes could be decolourised according to the following order: RB 5 > RO 16 > RR 238 > RB 4. The structure of the dye (particularly the structure of the reactive group) has an important influence on the decolouration rate, TOC reduction and on the in situ formation of H2O2.

Do you know that…
• Under typical reactive dyeing conditions, up to 50% of the initial dye remains in the spent dye-bath.
• The largest water consumer inside EU industrial sectors is the textile finishing sector.
• More than 10.000 commercial products and 4000 different molecules are used in textile finishing processes.

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