Hexavalent Chromium Reduction by Immobilized Cells and the Cell-Free Extract of Bacillus sp. ES 29

Bacillus sp. ES 29 (ATCC: BAA-696) is an efficient chromate reducing bacterium. We evaluated hexavalent chromium (Cr[VI]) reduction by immobilized intact cells and the cell-free enzyme extracts of Bacillus sp. ES 29 in a bioreactor system. Influences of different flow rates (3 to 14 mL h-1), Cr(VI) concentration (2 to 8 mg L-1), and immobilization support materials (Celite, amberlite, and Ca-alginate) on Cr(VI) reduction were examined. Both immobilized intact cells and the cell-free extract of Bacillus sp. ES 29 displayed substantial Cr(VI) reduction. Increasing flow rates from 3 to 6 mL h-1 did not affect the rate of Cr(VI) reduction, but above 6 mL h-1, the Cr(VI) reducing capacity of the immobilized intact cells and cell-free extract of Bacillus sp. ES 29 decreased. With both intact cells and the cell-free extracts, the rate of Cr(VI) reduction was inversely related to the concentration. Intact cells immobilized to Celite displayed the highest rate (k = 0.443 at 3 mL h-1) of Cr(VI) reduction. For the immobilized cell-free extract, maximal reduction (k = 0.689 at 3 mL h-1) was observed with Ca-alginate. Using initial Cr(VI) concentrations of 2 to 8 mg L-1 at flow rates of 3 to 6 mL h-1 both immobilized intact cells and the cell-free extracts reduced 84 to 98% of the influent Cr(VI). Results indicate that immobilized cells and the cell-free extracts of Bacillus sp. ES 29 could be used for large-scale removal of Cr(VI) from contaminated water and waste streams in containment systems.