Optimization of dye effluent decolorization by halotolerant Halomonas strain D2 in static culture condition

Introduction:
Nowadays, the industrial effluents contain mineral and organic hazardous materials such as phenols, heavy metals, and dyes, which are carcinogenic and poisonous compounds even at low concentrations. An increase in such components especially dyestuffs in the environment makes the remediation of these pollutants valuable. Toluidine Red is a pigment containing azo groups, which is invasively used in different industries. There are several chemical, physical, biological, optical, and combinational methods for different pollutant removal from the environment. In recent years, the bioremediation technique has been considerably developed as an effective method in pollutant removal. Therefore, the goal of this study was to optimize the Toluidine Red removal from the synthetic dyes wastewater by Halomonas strain D2 in static culture conditions. In this survey, optimization and variables’ effectiveness study was performed by the response surface methodology (RSM).
Material and methods:
In this research, the effectiveness status of variables and optimum conditions were studied by the statistical approach of Design Expert V.7 software. Biodegradation of dye was investigated by 30 experiments according to the Central Composite Design (CCD) and RSM. According to previous related research performed by one factor at time (OFAT), the effects of four variables of dye concentration, NaCl salt concentration, pH, and temperature on bioremediation at 10-30 ppm, 2-10 %, 5.5-9.5 and 20-40 °C, respectively, were studied. Bioremediation experiments in the aqueous phase of the synthetic model effluent have been performed in shake flask scale and static culture condition in an incubator for 10 days. Then the analysis of the resulted data was performed by analysis of variance.
Results and discussion:
According to the results and analysis of variance, salt concentration and temperature are the most significantly effective variables on bioremediation in the studied range while pH and dye concentration were probably less significant. Furthermore, the interactive effect of dye concentration and pH, salt concentration and pH as well as salt concentration and temperature were in a 95% effectiveness possibility limit. On the other hand, the maximum dye removal was observed on the 10th day of the experiment with 62.4%. The optimum condition for bioremediation was observed at a pH of 8.5, the temperature of 35 °C, and salt and dye concentrations of 4% and 16.8 ppm, respectively. By RSM, a correlation of second order for dye removal percentage after 10 days was presented with R2 equal to 0.95. The average error of the proposed correlation and real experimental data was about 9.9%.
Conclusion:
The second-order correlation proposed in this paper can effectively predict the different operational condition effects on removal of Toluidine Red by Halomonas strain D2 that is a useful bacterium in dye biodegradation. Comparing this survey with our previous study, which was about decolorization by Gb strain, shows that the optimum condition is different for each strain type. On the other hand, D2 strain performance was higher in the basic condition in comparison with strain Gb, however, in acidic conditions, the Gb strain’s performance was better.