The Effect of Salts Used in Textile Dyeing on Microbial Decolourisation of a Reactive Azo Dye

Abstract

Abstract Azo dyes account for 60 to 70% of all textile dyestuffs produced and are the most common chromophore of reactive textile dyes. Colouration of textile effluents usually can be linked to the presence of water-soluble (reactive) azo dyes. Under anaerobic conditions azo dyes can be used as terminal electron acceptors during microbial respiration, being reduced and decolourised concurrently with re-oxidation of reduced flavin nucleotides. Thus, the process of anaerobic digestion is under investigation as an effluent treatment option for the textile industry. The ability of anaerobic microbial consortia to decolourise azo dyes is well established; however, an additional factor that requires consideration is the high concentration of salts present in reactive dyeing effluent. Nitrate, sulphate, chloride and carbonate salts can all be used during reactive dyeing. This paper presents results of an investigation into the effect of nitrate and sulphate on the decolourisation of a reactive azo dye, Reactive Red 141. Nitrate was found to delay the onset of decolourisation for a period of time related to the concentration of nitrate initially present in the system. Sulphate was found to have no discernible effect on the onset or rate of decolourisation and it is proposed that the order of reduction of the compounds is nitrate > Reactive Red 141 > sulphate. Measurement of redox potential in the anaerobic system during decolourisation, with or without the addition of nitrate or sulphate, showed that strictly anaerobic conditions were conducive to rapid decolourisation of the azo dye. Keywords: Reactive dyesdecolourisationanaerobicnitratesulphate