Accelerated biodegradation of azo dyes by bacteria

Abstract

Faisalabad is known as a city of textiles not only in Pakistan but also all over the world. Several textile processing units are in operation in the surrounding of this city, which are releasing huge quantities of wastewater contaminated with a variety of xenobiotics. Particularly, a substantial amount of azo dyes are released continuously into wastewater streams in the surroundings of these textile units. The discharge of these contaminants is considered a potential threat to environments because of their hazardous nature. Thus, this study was undertaken to isolate bacteria possessing high potential to degrade these azo dyes from the wastewater discharged by the textile units. The wastewater samples collected from the outlets of a number of textile units showed a substantial variation in color intensity, pH and total soluble salts, implying that quality of the wastewater varies with the dyeing processes going on at the time of sampling. A total of 240 bacterial strains were isolated from these wastewater samples, and after screening five most efficient strains were selected for subsequent studies. Out of these five strains, two strains N7 (Klebsiella oxytoca) and N11 (Acinetobacter calcoaceticus) showed an excellent potential to decolorize a variety of textile dyes spiked in the liquid broth, individually as well as in mixtures. These strains also exhibited a great potential to decolorize the dyes belonging to various dye groups (i.e. reactive, direct, and disperse). The optimum conditions determined for the five selected bacterial strains for the decolorization of Reactive Black-5 were pH 7.0, temperature 35 ̊C, static condition and supplementation of the medium with 0.4% yeast extract and the dye, 100 mg L-1. Since industrial wastewater also carries a substantial amount of salts, decolorization potential of N7 and N11 were also investigated in the presence of various levels of NaCl (5-60 g L-1). It was recorded that the strains decolorized the test dyes up to 100% in the presence of low levels of NaCl, and high levels of NaCl (≥ 30 g L-1) had negative impact on the decolorization rate of test dyes. Regression analysis revealed a highly negative linear relationship between the rate of decolorization of test azo dyes [V] and salt concentration [NaCl]. Then these two strains were also tested for removal of dyes from real wastewater collected from various industrial units. Fortunately, these strains showed high effectiveness in removing the textile dyes from the real wastewater samples. The results of these studies clearly document that the isolated bacterial strains have the potential to remove a variety of dyes from the liquid culture in the presence of high salt levels in the real wastewater, which could be used to develop a biotreatment strategy for cleanup of the industrial wastewater prior to their discharge into wastewater streams of the surrounding area.