Optimization of Cr(VI) bioremediation in contaminated soil using indigenous bacteria

Bench-scale soil column experiments were carried out to evaluate the effectiveness of Cr(VI) bioremediation process in soils by using indigenous bacteria with the addition of bacteria nutrient media. Effects of particle size, spray intensity, initial Cr(VI) concentration, circulation mode and soil depth on Cr(VI) remediation were studied. Results show that soils after 6 d remediation with spray intensity controlled in the range of 29.6–59.2 mL/min could well fulfill the requirement of concrete aggregate and roadbed material usage, for the leaching toxicity concentration of the Cr(VI) in treated soils under the chosen condition is far less than 5 mg/L. The leaching toxicity and fractions of both hexavalent chromium and trivalent chromium from remediated soils were determined and compared with that of untreated soil. The results show that water soluble Cr(VI) declines from 1520.54 mg/kg to 0.68 mg/kg, exchangeable Cr(VI) decreases from 34.83 mg/kg to 0.01 mg/kg and carbonates-bonded Cr(VI) falls from 13.55 mg/kg to 0.68 mg/kg. Meanwhile, a corresponding increase in carbonate-bonded Cr(III), Fe and Mn oxides-bonded Cr(III) and organic matter-bonded Cr(III) are found. It reveals that indigenous bacteria can leach out water soluble Cr(VI), exchangeable Cr(VI) and carbonates-bonded Cr(VI) from contaminated soil followed by converting into carbonate-bonded Cr(III), Fe and Mn oxides-bonded Cr(III), organic matter-bonded Cr(III) and residual Cr(III).