Date Published: May 21, 2018
Publisher: Springer Berlin Heidelberg
Author(s): Rasha A. Hashem, Reham Samir, Tamer M. Essam, Amal E. Ali, Magdy A. Amin.
Azo dyes are complex derivatives of diazene used in food and textile manufacture. They are highly recalcitrant compounds, and account for severe environmental and health problems. Different strains of Pseudomonas species were isolated from textile wastewater effluents. The bioconversion of Remazol black B (a commonly used water soluble dye) by Pseudomonas aeruginosa was observed in static conditions. The bio-decolorization process was optimized by a multi factorial Plackett–Burman experimental design. Decolorization of 200 mg L−1 reached 100% in 32 h. Interestingly, the presence of yeast extract, magnesium and iron in the culture media, highly accelerated the rate of decolorization. Moreover, one of our isolates, P. aeruginosa KY284155, was kept high degradation rates at high pH (pH = 9), which represents the pH of most textile wastewater effluents, and was able to tolerate high concentration of dye up to 500 mg L−1. In bacteria, azo-dye degradation is often initiated by reductive azo compound cleavage catalyzed by azo-reductases. Three genes encoding azo-reductases, paazoR1, paazoR2 and paazoR3, could be identified in the genome of the isolated P. aeruginosa stain (B1). Bioinformatics analyses of the paazoR1, paazoR2 and paazoR3 genes reveal their prevalence and conservation in other P. aeruginosa strains. Chemical oxygen demand dramatically decreased and phyto-detoxification of the azo dye was accomplished by photocatalytic post treatment of the biodegradation products. We suggest applying combined biological photocatalytic post treatment for azo dyes on large scale, for effective, cheap decolorization and detoxification of azo-dyes, rendering them safe enough to be discharged in the environment.
Textile wastewater usually contains a large variety of dyes and chemicals additives used in the dyeing process, such as heavy metals, soda ash, caustic soda and acetic acid. Pollution with these dyes represents an important environmental challengeto the textile industry (Bansal and Kanwar 2013). Azo dyes are the most commonly used textile dyes, containing one or more azo groups attached to aromatic groups. They are difficult to biodegrade aerobically because of their chemical stability. Since oxygen is a more efficient electron acceptor than azo dyes, their aerobic bacterial treatment under shaking conditions is less efficient than aerobic treatment under static conditions or anaerobic ones (Stolz 2001; Mantzavinos and Psillakis 2004). More than 100,000 dyes are used in the textile industry and more than 700,000 tons of commercial dyes are produced annually (Lucas et al. 2007). About 10–15% of these dyes is discharged into textile wastewater effluent, which not only creates serious environmental hazards, but also renders textile wastewater aesthetically unacceptable. In addition, azo dyes themselves are toxic and highly carcinogenic. Consequently, it is important to treat textile wastewater before discharging it to the environment (Sudarjanto et al. 2006; Pratum et al. 2011). Reactive azo dyes are highly recalcitrant to conventional methods (biological methods) used in wastewater treatment because of the presence of strong electron-withdrawing groups that give them stability against bacterial degradation (Lucas et al. 2007; Gregorio et al. 2010). Most azo dye degrading microorganisms cleave the azo bond(s), which subsequently generates colourless aromatic amines. These amines are toxic products, but could be metabolized under aerobic conditions to less toxic ones (Mohanty et al. 2006).
The development of environmentally favourable, cost-effective techniques to treat azo dyes in textile wastewater have attracted a great attention. Reactive azo dyes are highly recalcitrant to conventional wastewater treatment because of the presence of strong electron-withdrawing groups giving them stability against bacterial degradation (Gregorio et al. 2010). In the present study, the characterization, optimization and the application of a biological or a photocatalytic or a combination thereof were investigated to enhance the decolorization and the detoxification of Remazol black B.