MICROBIAL DECOLORIZATION OF A REACTIVE AZO-DYE UNDER ANAEROBIC CONDITIONS

Water-soluble azo dyes are used extensively in the textile industry an d are known to be problematic with respect to the removal of colour fr om textile waste waters. Under anaerobic conditions azo dyes can be ut ilised as terminal electron acceptors in microbial respiration, and ar e reduced and decolourised concurrently with re-oxidation of reduced f lavin nucleotides. The microbial decolourisation of an azo dye (C.I. R eactive Red 141) was investigated with respect to the kinetic order of azo reduction and rate-controlling factors of the reaction. Decolouri sation of C.I. Reactive Red 141 was found to be first order with respe ct to dye concentration, although increasing the initial dye concentra tion in the serum bottles resulted in decreasing k values of -0.441/h (100 mg/l of C.I. Reactive Red 141), -0.316/h (150 mg/l) and -0.252/h (200 mg/l). The presence of labile carbon in the anaerobic systems was found to be essential in order to obtain an acceptable rate of decolo urisation. The k value obtained for decolourisation of the azo dye wit hout a supplemental carbon source (glucose) was -0.012/h, in compariso n to a k value of -0.441/h when supplemented with glucose (1 g/l). The presence of nitrate in the anaerobic system was found to inhibit deco lourisation, while the presence of sulphate was found to have no disce rnible effect on the rate of decolourisation. A low redox potential (- 450 to -500 mV) was found to be conducive to rapid decolourisation of C.I. Reactive Red 141. AC.I. Reactive Red 141 degradation product was positively identified as 2-aminonaphthalene-1,5-disulphonic acid, conf irming that azo reduction was responsible for decolourisation of the a zo dye. A toxicity assay was performed which showed that C.I. Reactive Red 141 was inhibitory to the anaerobic microbial community at concen trations > 100 mg/l, but that prior exposure of the biomass to the dye increased the resistance to previously inhibitory dye concentrations.