The efficacy of bacterial species to decolourise reactive azo, anthroquinone and triphenylmethane dyes from wastewater: a review.

The industrial dye-contaminated wastewater has been considered as the most complex and hazardous in terms of nature and composition of toxicants that can cause severe biotic risk. Reactive azo, anthroquinone and triphenylmethane dyes are mostly used in dyeing industries; thus, the unfixed hydrolysed molecules of these dyes are commonly found in wastewater. In this regard, bacterial species have been proved to be highly effective to treat wastewater containing reactive dyes and heavy metals. The bio-decolourisation of dye occurs either by adsorption or through degradation in bacterial metabolic pathways under optimised environmental conditions. The bacterial dye decolourisation rates vary with the type of bacteria, reactivity of dye and operational parameters such as temperature, pH, co-substrate, electron donor and dissolved oxygen concentration. The present paper reviews the efficiency of bacterial species (individual and consortia) to decolourise wastewater containing reactive azo, anthroquinone and triphenylmethane dyes either individually or mixed or with metal ions. It has been observed that bacteria Pseudomonas spp. are comparatively more effective to treat reactive dyes and metal-contaminated wastewater. In recent studies, either immobilised cell or isolated enzymes are being used to decolourise dye at a large scale of operations. However, it is required to investigate more potent bacterial species or consortia that could be used to treat wastewater containing mixed reactive dyes and heavy metals like chromium ions.