Potential of halophiles and alkaliphiles in bioremediation of azo dyes-laden textile wastewater: a review

Abstract

Azo dye-laden textile wastewater must be treated before release due to various health and environmental concerns. Bioremediation of textile wastewater, however, is a challenge owing to its alkaline and saline nature as mesophilic microbes, in general, are either not able to thrive or show less efficiency under such hostile environment. Thus, pre-treatment for neutralization or salinity removal becomes a prerequisite before applying microbes for treatment, causing extra economical and technical burden. Extremophilic bacteria can be the promising bioremediating tool because of their inherent ability to survive and show toxicants removal capability under such extreme conditions without need of pre-treatment. Among extremophiles, halophilic and alkaliphilic bacteria which are naturally adapted to high salt and pH are of special interest for the decolorization of saline-alkaline-rich textile wastewater. The current review article is an attempt to provide an overview of the bioremediation of azo dyes and azo dye-laden textile wastewater using these two classes of extremophilic bacteria. The harmful effects of azo dyes on human health and environment have been discussed herein. Halo-alkaliphilic bacteria circumvent the extreme conditions by various adaptations, e.g., production of certain enzymes, adjustment at the protein level, pH homeostasis, and other structural adaptations that have been highlighted in this review. The unique properties of alkaliphiles and halophiles, to not only sustain but also harboring high dye removal competence at high pH and salt concentration, make them a good candidate for designing future bioremediation strategies for the management of alkaline, salt, and azo dye-laden industrial wastewaters.

Keywords: Alkaliphiles; Azo dyes; Dye decolorization; Halophiles; Industrial effluent; Wastewater.

Fig. 1

Classification of azo dyes based on chemical composition (Benkhaya et al. ,; Ajmal et al. 2014)

Fig. 2

Effect of toxic azo dyes on human beings and environmental ecosystem (Adapted from Periyasamy ; Alzain et al. ; Kalra and Gupta ; Ali et al. ; Gita et al. ; Pan et al. 2012)

Fig. 3

Bioremediation of azo dyes studies conducted using halophilic and alkaliphilic bacteria based on analysis report of SciFinder database (Search Keywords: “Bioremediation of azo “dyes” containing textile wastewater using “alkaliphilic” “bacteria””; “Bioremediation of azo “dyes” containing textile wastewater using “halophilic” “bacteria””)

Fig. 4

Bacterial degradation of azo dyes with adaptive strategies: 1. General mechanism of bacterial degradation of azo dye (Samsami et al. 2020) 2. Adaptive strategy of alkaliphilic bacteria in highly alkaline textile wastewater (Gomes and Steiner 2004) 3. Adaptive strategy of halophilic bacteria in highly saline textile wastewater (Orellana et al. ; Kakkar and Wadhwa ; Daoud and Ali ; Eslami et al. 2016)