Reductive biotransformation and decolorization of reactive anthraquinone dyes

Reactive dyes are an important class of textile dyes, particularly for cotton and cotton blend fabrics. Anthraquinone dyes, which are based on the 9,10-anthraquinone nucleus, are the second most common class of textile reactive dyes after azo dyes. In addition to the presence of dye, high salt concentration typically found in spent reactive dyebaths further complicates their management, and is currently the single most pressing environmental problem facing the textile industry.; Research was conducted to investigate the fate and biotransformation of selected reactive anthraquinone dyes in microbial communities. Reductive biological decolorization of commercially important reactive anthraquinone dyes took place in a batch aerobic enrichment culture incubated under anoxic/anaerobic conditions, in a batch methanogenic culture, in a batch halophilic enrichment culture, and in a continuous-flow fluidized bed reactor (FBR) developed using the halophilic culture and operated under anaerobic conditions, all with an externally added carbon and energy source. The identification of decolorization products of anthraquinone dyes using advanced analytical techniques confirmed that the reductive decolorization of anthraquinone dyes was achieved by reductive transformation of the anthraquinone nucleus to dihydroxyanthracene. The effect of a number of operational parameters of the continuous-flow anaerobic FBR—such as influent dye concentration, hydraulic retention time, organic loading, and reactor temperature—on its decolorization performance using a commercial reactive anthraquinone dye was investigated. The decolorization kinetics of reactive anthraquinone dyes in the microbial cultures used in this research were successfully described using a Michaelis-Menten type kinetic model. Furthermore, five repetitive decolorizations using the FBR and redyeings with the same reactive anthraquinone dyebath solution resulted in almost identical fabric color shades to that of a standard dyeing. In addition, excellent fabric color reproducibility and consistency of fabric color shades was obtained for all redyeing cycles by using biologically renovated spent dyebaths. Therefore, the continuous-flow FBR capable of decolorizing spent reactive dyebaths under hypersaline conditions has a high potential to be used as part of a closed-loop, in-plant dyebath renovation and reuse system, which will result in the minimization of fresh water consumption as well as reduction of wastewater volume and pollutant concentration in textile discharges.