Use of active consortia of constructed ternary bacterial cultures via mixture design for Congo Red decolorization enhancement

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Abstract

Azo dyes are widely used in textile dyeing which contains toxic and carcinogens compounds. So they must be treated before their discharge into the receptor medium. In this aim, this work is focused on the aerobic decolorization of water soluble azo dye Congo Red (CR) by bacterial consortium (Sphingomonas paucimobilis, Bacillus sp. and Staphylococcus epidermidis). The effect of different combinations of these three strains cultivated in the pure culture in mineral salts medium (MSM) on the decolorization of Congo Red (cell density fixed at OD600 = 1 with the addition of 750 ppm of the dye) was studied using equilateral triangle diagram and mixture experimental design to assess color and COD removal during species growth. Using software analysis (Minitab 14.0), the formulation of pure culture in MSM can be optimized for several responses and the best formulation can be obtained. The results suggested that the highest decolorization and chemical oxygen demand (COD) rates were 100% and 98%, respectively. Very high regression coefficient between the variables and the responses: decolorization and COD removal were R2 = 32.69% and R2 = 52.25%, respectively indicating an excellent evaluation of experimental data by polynomial regression model. FTIR (Fourier transform infrared) and UV–vis analysis confirmed the biodegradation of Congo Red by the bacterial consortium. The produced metabolites were extracted and their phytotoxicity study suggested their low toxicity.

Introduction

Azo dyes are the most commonly used in textile, cosmetic, paper-making, and food industry [1], [2], [3], [4]. Over 7 × 105 t and approximately 10,000 different dyes and pigments are produced annually worldwide, about 10% of which may be found in wastewater [5].

Azo dyes are synthetic organic compounds widely used in textile dyeing. This chemical class of dyes, which is characterised by the presence of at least one azo bond (Ndouble bondN) bearing aromatic rings, dominates the worldwide market of dyestuffs with a share of about 70% [5]. They are designed to convey high photolytic stability and resistance towards major oxidising agents [6]. The release of azo dyes into the environment in effluent from textile dyeing plants become a major concern in wastewater treatment, since they are highly recalcitrant to conventional wastewater treatment processes. The recalcitrance of azo dyes has been attributed to the presence of sulfonate groups and azo bonds, two features generally considered as xenobiotic [7]. In addition, some azo dyes or their metabolites may be mutagens or carcinogens [8]. As a result, it is of great interest to develop effective means to treat dye-bearing wastewater. Biotreatment usually has the advantage of low cost and high efficiency.

Several combined anaerobic and aerobic microbial treatments have been suggested to enhance the degradation of azo dyes [9]. Microbiological decolorization is an environmental-friendly and cost-competitive alternative to the chemical decomposition process [10]. Most studies on azo dye biodegradation have focused on bacteria and fungi [11], [12]. Chang et al. [13] showed that mutant strain of Escherichia coli was able to decolorize azo dye C.I. Reactive red 22 to 8.2 mg dye g cell−1 h−1 and Rhodobacter sphaeroides was able to decolorize Methyl orange [14], [15]. However, the results based on a number of experiments did not only fail to reach the best combination while spending a lot of experimental materials, but will also affect the progress of the study.

In this research, we used the experimental design (Minitab 14.0) to optimize the formulation of the predominant strains isolated from textile wastewater plant, in order to effectively explain the biodegradation of Congo Red. After biodegradation, the chemical oxygen demand (COD) and color removal were measured. The relationships between the different combinations and products were analyzed through the Minitab to select the optimal bacterial combination.

Section snippets

Chemicals

Congo Red (C32H22N6O6S2Na2, C.I. No. 22120) (Fig. 1) was purchased from Sigma–Aldrich. All chemicals used were of the highest purity available and of analytical grade.

Microbial strains

The strains of Sphingomonas paucimobilis (14 × 107 cfu), Bacillus sp. (4.2 × 108 cfu), Staphylococcus epidermidis (2.6 × 106 cfu), were isolated from the activated sludge of the textile wastewater treatment plant in KsarHellal, Tunisia. Cultivable Gram-negative and Gram-positive bacteria on Mac-conkey plates were isolated and identified.

Model establishment

Through linear regression fitting, the regression models of two responses (COD (%) and decolorization (%)) were established. The regression model equations are as follows:Ydecolorization%=95.142S1+98.077S2+94.140S3+(3.068)S1×S2+(3.409)S1×S3+(0.320)S2×S3,R2=32.69%;P=0.567YCOD=46.23S1+66.403S2+85.769S3+50.022(S1×S2)+0.165(S1×S3)+117.848(S2×S3),R2=52.25%;P=0.836where S1: S. paucimobilis; S2: Bacillus sp.; and S3: S. epidermidis.

Effect of formulation on color and COD removal

Recently, the mixture design is widely used. It can

Conclusion

In this paper, the mixture design method was used for the optimization of bacteria combination for the decolorization in biodegradation processes to explore a new optimization method of mixed culture. This study showed that the decolorization by mono and combined mixed cultures S. epidermidis and Bacillus sp. contributed for the major decolorization activity of the dye biotreatment. Moreover, Bacillus sp. contributed for the majority of COD removal.

The establishment of the regression model and

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