Environmental and Kinetic Parameters for Cr(VI) Bioreduction by a Bacterial Monoculture Purified from Cr(VI)-Resistant Consortium

Hexavalent chromium, Cr(VI), is toxic to living systems. Widespread contamination of water and soil by Cr(VI) present a serious public health problem. Chromium-resistant bacteria can reduce and detoxify Cr(VI). Twelve bacteria resistant to high concentrations of Cr(VI) were isolated from soil enrichment cultures. Environmental parameters and kinetic parameters of Cr(VI) bioreduction by one monoculture isolate, identified by 16S rRNA gene sequence as Bacillus sp. PB2, were studied. The optimal temperature for growth and Cr(VI) reduction was 35°C. The isolate grew luxuriantly and substantially reduced Cr(VI) at initial pH 7.5 to 9. Maximal Cr(VI) bioreduction occurred at initial pH 8.0. Substantial Cr(VI) bioreduction was observed in salt media, but removal efficiency was inversely related to salt concentration (1–9%). Michaelis–Menten hyperbolic equation and the Lineweaver–Burk double reciprocal plot were comparatively employed to determine the km and Vmax of Cr(VI) bioreduction. A km of 82.5 μg mL−1 and Vmax of 7.78 μg mL−1 h−1 were calculated by nonlinear regression analysis of the hyperbola curve. Linear regression analysis of the double reciprocal plot revealed km and Vmax of 80.9 μg mL−1 and 10.6 μg mL−1 h−1, respectively. Time course studies displayed about 90% reduction of Cr(VI) at an initial concentration of 8,000 μg L−1 in 8 h, with an estimated t1/2 of 4 h. Data from time course analysis of the rate of Cr(VI) bioreduction fitted zero-order model, and the kinetic constant k was calculated to be 840 μg L−1 h−1. The monoculture isolate, Bacillus sp. PB2, strongly reduces Cr(VI) and could be used for bioremediation of Cr(VI)-contaminated aquatic and terrestrial environments.