Textile Azo Dye Biodecolorization Using a Facultative Thermophile Bacillus paralicheniformis SN7 Isolated from Textile Effluents

Background and Objectives: Synthetic dyes, widely used in the textile industry, are the major water contaminants. Bacterial degradation is an eco-friendly, cost- effective, highly efficient approach. Unfortunately, some of these toxic wastewaters are discharged into the environment without proper treatment, which results in environmental pollution. Today, the available water for use in industrial activities, agriculture, etc. is decreasing. By treating the wastewater of textile factories, the purified water can be returned to the production cycle. Microorganisms such as bacteria, filamentous fungi, yeasts and algae are able to decolorize azo dyes, but so far there has been no complete report on decolorization by facultative thermophilic bacteria. Facultative thermophiles are those that can grow at high temperatures as well as at relatively lower temperatures, e.g., below 50 °C.
Methodology: To isolate dye-degrading bacteria, wastewater samples were collected from different parts of a textile wastewater treatment plant located in Kashan, Iran. The different media such as tryptic soy broth (TSB), modified M9 minimal salts medium, was used to enrich the bacteria in textile wastewater. The enriched samples were used for the isolation and purification of textile azo dyes degrading bacteria as a pure culture on TSA and wastewater agar (WA) plates under aerobic conditions. In this study, the strains of facultative thermophile bacteria that can effectively degrade azo dyes were successfully isolated in the M9 media (53°C). The amount of decolorization activity of isolated bacteria was determined by using the modified M9 medium containing 150 mg l-1 using the commonly used dyes Rimazol Black 5, Reactive Red 198, Reactive Blue 21, and Reactive Yellow 15.
Optimization of decolorization conditions in selected bacteria was done using Minitab 14 software and Response Surface Methodology (RSM). These experiments were conducted in order to investigate the factors of temperature, salt concentration, inoculation, color concentration, pH, and time at different levels. The identification of the selected SN7 bacterial strain was done through 16S rRNA gene sequence analysis and conventional biochemical tests and was registered in Iran's Industrial Microorganism Collection Center (PTCC).
Findings:
The conditions for the proliferation and enrichment of dye-degrading bacteria in textile wastewater were provided through the enrichment method using TSB culture medium containing Rimazol Black and modified M9 medium containing Rimazol Black. The 47 dye degrading bacterial strains were isolated by using culture medium containing of Remazol black 12 strains of facultative thermophilic bacteria that degrade dyes in textile waste were obtained by investigating the ability of the isolated bacteria to grow at 53°C. The ability of bacteria to decolorize azo dyes was 22 to 71.5% at 48 ° C for 72 h. Among the isolated bacteria, two strains SN7 and SN10 had the most decolorization and the decolorization rate of each at 48°C after three days of incubation was 71.5% and 70% respectively. The SN7 strain was used to further investigate the dye degradation due to the ability of the SN7 strain to grow well in the medium containing the dye as well as its ability to remove it in a shorter period of time. Analysis of the 16S gene sequence of SN7 strain shows 93.99% similarity with Bacillus paralicheniformis Bac84. Therefore, this strain was named Bacillus paralisniformis SN7 and was registered with PTCC number 1907 in the Culture Collection for Research and Industrial microorganisms of Iran. The strains Bacillus paralicheniformis SN7 was chosen and was used for further characterization. B. paralicheniformis SN7 was capable of degrading reactive dyes such as of Remazol black 5, Reactive Red 198, Reactive Blue 21 and Reactive Yellow 15. The ability of this bacterium to decolorize Remazol Black 5, Reactive Red 198, Reactive Blue 21, and Reactive Yellow 15 was 71.5, 75.6, 72.2, 76.2% respectively. The optimum concentration of dye, pH, and temperature as analyzed by RSM were found to be 50 mg/l, 7, and 45°C, respectively, for decolorization Remazol black (71.5%).
Conclusion: Among the isolated bacteria, facultative thermophilic strains were isolated and B. paraliceniformis SN7 has the ability to decompose dyes. Degradation of azo dyes, unlike surface adsorption, is of considerable importance as a desirable method because the dye is completely degraded by microbial enzymes.
Some microorganisms can use dye compounds as their source of carbon and energy. Complete biological decolorization has also been observed in anoxic and anaerobic conditions. Under anoxic conditions, decolorization was higher compared to aerobic conditions when the bacteria were cultured under aerobic conditions and then under anoxic conditions. Sequential of two stages under aerobic and anoxic conditions has been proposed for decolorization of azo dye-containing wastewaters. Anoxic conditions do not contain molecular oxygen and may contain nitrate or nitrite.