The large amount of cotton that continues to be used for textiles means that reactive dyes are still quantitatively the most significant group of dyes. Dying cellulose textiles with reactive dyes is always accompanied by hydrolysis of the dye. The degree to which reactive dyes are fixed to the cellulose fibre is between 70 and 95 %. The dye hydrolysates negatively affect the wet fastness of the dyed textile. For this reason they have to be removed from the textile in an extensive washing process, which means a high consumption of water, energy and time. The resulting colour in the wastewater prevents both direct water recycling in textile finishing and discharge into a sewer. Decolourisation of the wastewater is thus necessary. If it is possible to attain a sufficient level of decolourisation, the water can be reused in the finishing process; this recycling means a great reduction in the necessary chemicals, energy and time.
This work describes the oxidative degradation of selected reactive dyes. The chemical oxidative degradation with peroxodisulphate and an enzymatically catalysed oxidative reaction (Baylase® RP) are examined. To this end, certain commercial reactive dyes with typical chromophores and anchor groups were chosen; these were provided to us in the form of trade products with an exact declaration of the content of auxiliary substances. Three of these dyes, called Black C, Yellow D and Red F, were used in extensive investigations of the intermediates and products of oxidative degradation.
After textile finishing, the dyes in the wastewater are present in the hydrolysed form, so that a procedure for the quantitative hydrolysis of the dyes had to be worked out. It was possible to hydrolyse the anchor groups without cleaving the chromophoric system.
The redox potential of peroxodisulphate depends on the pH value of the solution and is large enough to oxidize chloride to chlorine or hypochlorite. In the beginning of the work the question arose as to whether oxidation in the presence of chloride, an important component in dye-baths or wash-waters, leads to contamination with chlorinated aromatic compounds. For this study, 1-naphthol was used as a model compound; an excess of chloride was added to it, and the mixture was treated oxidatively. At pH values of 11 and above, no chlorinated aromatic compounds were found among the oxidation products. The results were quite different in acidic milieu, where a large number of chlorinated aromatic compounds were formed. To prevent additional dangers for the environment, oxidative degradation with peroxodisulphate thus has to take place in alkaline solution. All further studies were carried out in the presence of the amount of base needed to neutralise the protons arising during the reaction with peroxodisulphate.
After the oxidation took place, the concentration of the intermediates was low. To improve detection, the products were first enriched by solid-phase extraction; this also separated them into polar and moderately polar fractions. The main moderately polar compounds were characterised by reversed-phase chromatography with a UV/VIS diode array detector or by mass spectrometry (LC-QTof with ESI). Precision masses, with in most cases deviations of = 5 ppm from the theoretical value (W-mode with phosphoric acid as an internal standard), led in combination with plausibility considerations to proposals for the structures. These results were supported in some cases by NMR spectra (¹H- and 13C-NMR) of isolated degradation products (preparative HPLC).
The oxidative treatment of the hydrolysates led to intermediates in low concentrations. Only in a few cases was further reaction of the intermediates slower than the degradation of the hydrolysates themselves; in these cases products were observed to increase in concentration, at least for a short time.
At a point where the values required in supplement 38 of the German Waste Water regulation (Abwasserverordnung) with 7 m-1 (λ = 436 nm), 5 m-1 (λ = 525 nm) and 3 m-1 (λ = 620 nm) were satisfied, no hydrolysed dyes were detectable. The moderately polar intermediates formed were also detectable only in traces. Mainly polar products with low molecular weights were found.
The structures of the intermediates based on MS and NMR data make the following generalisations about the degradation pathway possible:
• cleavage of the azo link and formation of OH, NH₂ or NO₂ groups in its stead • loss of sulphate-ester and sulphonic-acid groups • loss of hydroxy-ethyl groups • formation of isoxazole compounds where the basic structure allows it
Comparison of the oxidation methods examined (with peroxodisulphate and Baylase® RP) showed two main differences, the speed of decolourisation on the one hand and the number of observed degradation products on the other. Peroxodisulphate was proved to be very effective for rapid decolourisation of dye hydrolysates: complete decolourisation, even of heavily coloured samples, could be achieved within a few minutes. Baylase® RP turned out to be slower, decolourisation sometimes taking hours. The reason for this can be found in the different pathways of decolourisation, radical vs. enzymatic mechanisms.
The two oxidative methods differ in the number of degradation products formed. In spite of a ten-fold greater amount of dye, oxidation with peroxodisulphate led to only a few detectable degradation products. The enzymatic system generates a huge number of products, which can be used to elucidate the dyes’ decomposition route. Studies with Baylase® RP and a higher concentration of dye (1 g L-1) showed fewer degradation products, as with the chemical oxidation with peroxodisulphate. It is possible that the surprisingly small number of products is due less to the oxidation method than to some mass-spectrometric discrimination.