Bacterial Isolates in hexavalent chromium reduction:Applications form synthetic to real systems

Abstract

Industrialization and mining activities across the globe have contributed to the increasing level of toxicity. Chromite mines of India produces 19% of the total chromite ore being produced in the world. Chromium is the seventh abundant element on the earth’s crust. The varying degree of toxicity has been found due to the presence of soluble Cr(VI) particularly in the overburden soil samples of the chromite mines area which is about 300-500mg Cr(VI)/kg. The level of Cr(VI) in final effluents wastewaters needs to be reduced to the permissible limit <0.05mg/L (as per USEPA) using appropriate technology before it is discharged into the soil. There are certain microorganisms isolated from the mine samples and overburden soil dumps that have turned out to be effective in the bioremediation of Cr(VI). In this work, the reduction capabilities of the novel bacterial isolates: CSB-9 and CSB-10, isolated and screened from the mine samples of Baula-Nuasahi (Orissa) that serve out to be effective in reducing hexavalent chromium to its trivalent form. CSB-9 was identified as gram- positive, motile, thick rods having capsular layer. CSB-10 was also identified as gram positive, motile, endospore forming thin rods. The shape and size of the rods of CSB-10 were smaller in comparison to CSB-9. The optimum pH and temperature for the growth of the organisms was 7.0 and 35ºC respectively. The generation time of CBS-9 was deduced as 4.58 h -1 , whereas the generation time of CSB-10 was 4.77 h -1 . The organisms were incubated with varying concentration of Cr(VI). CSB-9 was able to tolerate 100 ppm of Cr(VI), having a growth rate of 2.66 h -1 . After 96 h, cell death was recorded due to faster growth and nutrient depletion in the medium. In case of CSB-10, it was observed that the organisms grew in the presence of Cr(VI). But the shape and size of the cells reduced and a capsule layer was seen around the cells. After 96 h, endospore formation was higher in the culture medium. The protein estimation of the cell free extract obtained was done by Lowry’s method. The bacterial isolate CSB-9 gave 90% reduction in 120 hours at optimum pH 7.0, 35ºC with 1% (v/v) culture, whereas CSB-10 gave 90% reduction with 2% (v/v) culture. Adaptation of the bacterial isolates was carried on overburden soil (OBS) and Sukinda chromite concentrate (SCC). The bacterial isolates showed magnificent growth in both the mine samples. The reduction experiments carried out to test the potential ability of bacterial isolates are also summarized.