Kinetic aspects of Cr(VI) bioremediation of water and soil contaminated from chromite mine overburden dumps

Open-cast mining of chromite ore from the deposits of Sukinda, Orissa, is known as one of the ten most polluted sites of world due to natural oxidation of chromite from the stockpiled overburden dumps affecting the nearby areas with soluble Cr(VI) species. Native microorganisms which are adapted to toxic metal stress condition can play a significant role in bioremediation of Cr(VI) from contaminated mine soil and water. An attempt was made to isolate indigenous bacteria from the chromite mine soil to examine its potential in reduction of chromium (VI). A bacterial strain viz. CSB-4, identified as Bacillus sp. based on standard biochemical tests and 16SrRNA gene sequencing, was tolerant to as high as 2000 mg L–1 Cr(VI) concentration. The strain was capable of reducing Cr(VI) to Cr(III) in different growth media. Under the optimized conditions of pH ~7.0, 100 mg L–1 Cr(VI), 35°C temperature and stirring speed of 100 rpm, CSB-4 reduced more than 90% of Cr(VI) in 140 h. The time course reduction data showed best fit to exponential rate equation with the rate constants in the range 2.62×10–2 – 6.4×10–3 h–1 for 10–500 mg L–1Cr(VI) in water. An activation energy of 99 kJ mol–1 was acquired in the temperature range 25–35oC. These results were validated for the reduction of hexavalent chromium of chromite overburden soil containing 500 mg Cr(VI) kg–1 along with other metal oxides. It has been found that Bacillus sp. (4.05×107 cell mL–1) reduced more than 98% of Cr(VI) of the sample in 10 h at 20% pulp density and 7.0 pH. Kinetic parameters were also investigated for the bioremediation of Cr(VI) of the soil sample