Polyester, polyacetate and polyamide fibres are the kinds most frequently used at present to produce synthetic textiles. The dyeing of these fibre types is predominantly carried out with disperse dyes. Despite the high degree of absorption (95 - 98 %) of the water-insoluble dyes, relatively large quantities remain in solution and must be disposed of with the waste water.
This work examined the degradation of the disperse dyes by chemical and electro-chemical reduction. To this end, three representative disperse dyes (Disperse Blue 79:1, Disperse Orange 62, Disperse Red 60) were selected in agreement with the company DyStar. For analysis of the degradation products, LC-MS and GCxGC-(TOF)MS were used along with UV-VIS methods. In some cases solid-phase extraction proved unsuitable for concentrating the dye solutions because the dispersing agents contained in the solutions prevented the degradation products from being adsorbed on the stationary phase. Liquid/liquid extraction procedures were therefore developed, which made the concentration of relatively nonpolar products possible. By such extraction, followed by GCxGC-(TOF)MS, the degradation product 2,6-dichloro-4-aminoaniline was identified, which is formed during the chemical and electro-chemical reduction of the azodye Disperse Orange 62. Further degradation products could be identified by precision mass measurements. As expected, the reductive treatment of azodyes yielded aromatic amines as main degradation products. Aromatic amines also resulted from reduction of nitro or amide groups. It was shown in addition that the use of larger quantities of reducing agents and/or longer electrolysis times did not lead to increased degradation of the aromatic amines to harmless compounds. Since many of these amines are suspected of being genotoxic, it is urged that past practice be reconsidered, for it includes reductive degradation for decolourisation, either as a component of the dyeing method (reductive after-clearing) or as an anaerobic step during the waste water purification. The goal must be largely eliminating and destroying aromatic amines, as is possible by oxidative procedures.
To investigate the chemical reduction of the disperse dyes, sodium dithionite, sodium borohydride and zinc/hydrochloric acid were used. The degradation products were analysed as a function of the quantity of reducing agent, so that the course of the reaction could be examined more closely. The chemical reduction clearly ran faster than the electro-chemical reduction.
The disperse dyes in the form of the commercial products in water could be converted electro-chemically only with the assistance of a mediator system. If the dye press-cake (containing no dispersing agents) was dispersed with cationic tensides, then degradation could also be observed. Neither of these procedures, however, is suitable for a technical application. The conversion of the dye into a truly dissolved condition gave the best turnover rates in the electro-chemical treatment. Nevertheless, even at a relatively low dye concentration (0.1 g/L), it was necessary to add about 50 percent by volume of an organic solvent. The resulting secondary burden of pollutants also eliminates this method as a treatment procedure.
From the relatively low conversion rate of the electro-chemical reduction in purely aqueous systems, it can be concluded that the particle character of the incompletely dissolved disperse dye and the negatively charged dispersing agents, which coat and/or shield the dye, are unfavourable for the transmission of electrons at the cathode surface. The electro-chemical treatment of real waste water led likewise to insufficient decolourisation, although the measured particle sizes were clearly smaller (factor 30) than those in the freshly dispersed solution before the dyeing process. This suggests that the dispersing agent, which is not consumed during the dyeing process and is thus present in the real waste water in a large excess over the dye, probably has a substantial effect on the conversion rate. On the whole, the electro-chemical reduction as primary step of the degradation of water-insoluble dyes and pigments does not have the desired efficiency.
Surprisingly, the concentrations of both the disperse dyes and the vat dyes, to the extent that these were present in the waste water from the dyeing plant, could be lowered by ozonisation. The ozone consumption here was greater than what is necessary to degrade water-soluble dyes; this can be explained by the presence of ozone-degradable dispersing agents. While the fineness of the disperse after the dyeing, when the relationship of dye to dispersing agent is approximately 1:100 (before the dyeing this relationship is 50:50), makes electro-chemical reduction rather difficult, this relationship seems on the contrary even to be favourable for an oxidative treatment. It is thus conceivable that this established procedure for water-soluble dyes can also be used economically in disperse and vat dyeing.