Cathodic decolourisation of reactive dyes in model effluents released from textile dyeing

Abstract

Reactive dyes represent the most important class of dyes used in the colouration of cellulose fibre based textiles. Due to a limited degree of dyestuff fixation and competitive hydrolysis of the reactive anchor group in the alkaline dyebath substantial amounts of hydrolysed dyestuff are released at the end of the dyeing process. Assuming a rather high degree of dyestuff fixation with 90%, still 20,000 tonnes of hydrolysed dyestuff are released per year thus forming heavily coloured effluents. A number of techniques have been proposed to treat such waste water streams, e.g. coagulation, precipitation, absorption, chemical destruction and electrochemical decolourisation. A clean alternative presented in this article utilises the direct cathodic reduction of the azo-chromophores, which up to now has been proposed for treatment of concentrated dyestuff solutions. A distinct problem arises from the much low concentrations of reactive dyes in exhausted dyebaths, which has been investigated in this study. As technically relevant representatives three bifunctional reactive dyes Levafix Amber CA, Levafix Fast Red CA and Levafix Blue CA, which can form a trichromy have been studied. The cathodic electron transfer has been analysed with cyclic voltammetry on a hanging mercury drop electrode. Potentiometric titration with Fe(II)-triethanolamine complexes was used to determine the reducing equivalents for complete dyestuff reduction. A multi-cathode electrolyser was applied to study the cathodic dyestuff decolourisation in batch experiments as basis for extrapolation to full scale operation. Successful reductive decolourisation could be obtained for all three investigated dyes. In case of Leavfix Amber CA the original colour recovered some hours after the experiment had been terminated due to air oxidation of the reduced dyestuff. Due to the low dyestuff concentrations present in the dyebath current efficiency was determined with 2.8–8.0% and energy consumption was estimated with of 5.6 kWh/m3 of wastewater to achieve 80% dyestuff decolourisation. The results demonstrate the potential of the technique however also indicate that careful selection of all dyes with regard to their electrochemical behaviour is required for successful implementation of this technology.