Studies on bioreduction of cr(VI) using environmentally significant microorganisms

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Cr(VI) bioreduction was studied with fungi and bacteria. Initially Phanerochaete chrysosporium a white rot fungi was tested for bioreduction of Cr(VI). P. chrysosporium successfully reduced 10 mg L-1 of Cr(VI) at pH 6 in 160 h. In order to ease the separation of cells P. chrysosporium was entrapped in Ca-alginate and three other matrices. A maximum of 98.5 % reduction of Cr(VI) at an initial Cr(VI) concentration of 10 mg L-1 was obtained with Ca-alginate immobilized cells at pH 5. Cr(VI) reduction by P. chrysosporium involves charge based interaction with the amide and amine groups of the chitin of cell wall and intracellular reduction by metabolism dependant process. With the aim to reduce the time taken for the Cr(VI) bioreduction, Halomonas sp, a halophillic bacteria was applied for bioreduction of Cr(VI). A modified growth media for the Halomonas sp was developed. Halomonas sp survived in a highly saline environment containing 20 g L-1 of NaCl. Free cells of Halomonas sp successfully reduced up to 50 mg L-1 Cr(VI) within 48 h. The cell free extracts of Halomonas sp successfully reduced 60 mg L-1 Cr(VI) in less than 6 h duration. NADPH was found to be the suitable electron donor for the Cr(VI) bioreduction with the cell free extracts of Halomonas sp. The Cr(VI) reduction of immobilized cells of Halomonas sp was also studied. Free cells of the Halomonas sp were more efficient than the immobilized cells. Isolation of bacteria capable of reducing Cr(VI) was carried out from soil contaminated with Cr(VI). The isolated culture have the advantage as they are naturally acclimatized to Cr(VI) due to their long exposure to Cr(VI) in the environment. The 16s rDNA study showed that the organism was Bacillus cereus. The isolated Bacillus cereus was successful in reducing Cr(VI) at an initial concentration up to 50 mg L-1 at pH 6. The CFE of the Bacillus cereus successfully reduced 60 mg L-1 within 6 h in the presence of NADPH as electron donor. Packed bed reactor study was conducted in order to provide better aeration to Halomonas sp. The reactor was operated in batch mode and continuous mode. The HRT of 24 h resulted in complete reduction of 20 mg L-1 of Cr(VI). A maximum COD reduction of 84.1% was obtained..
Supervisor: K. Mohanty