The characteristics of molasses-based reductive removal of Cr(VI) from groundwater by Bacillus sp. Cr(VI) chemical reduction and bio-reduction by molasses

This study was implemented to compare between the effect of molasses on chemical reduction and bioreduction of Cr(VI) in groundwater and further investigate its potential in enhancing bioreduction of Cr(VI) by indigenous strains from a Cr(VI)-contaminated site. High concentration of molasses (5.7 g/L) showed strong potential for chemical reduction of Cr(VI) at neutral pH under abiotic condition. Low concentration of molasses (1 g/L), which had limited chemical reduction potential for Cr(VI), could significantly enhance Cr(VI) bioreduction and was demonstrated stronger than sucrose which is the main stream product for molasses production. The strongest strain isolated from the site soils belonged to Bacillus sp. which was not reported for Cr(VI) reduction based on molasses as the sole carbon source. The Cr(VI) reduction behavior of the strain using molasses was further investigated. The reaction rate constant decreased with the increase of initial Cr(VI) concentration due to the toxic effect of Cr(VI). The greatest Cr(VI) bioreduction performance was observed at pH of 7.0, while the reduction rate was distinctly inhibited under acidic or alkaline conditions. The peak reduction rate constant was obtained at the temperature of 35 °C. Decrease of the rate constant was observed with the increasing concentration of SO42-, showing competitive inhibition of reduction by ions with a structure similar to CrO4-. The results of this study provide some useful information to support application of molasses in remediation of Cr(VI)-contaminated groundwater.