Molasses-based in situ bio-sequestration of Cr(VI) in groundwater under flow condition

The in situ biosequestration of Cr(VI) in groundwater with molasses as the carbon source was studied based on column experiments and model simulation in this study. Compared with biological reduction, molasses-based chemical reduction did not cause significant Cr(VI) removal at molasses concentration as high as 1.14 g L⁻¹. The molasses at a concentration as low as 0.57 g L⁻¹ could support biofilm-based Cr(VI) sequestration under flow conditions and showed better sequestration performances than D-glucose and emulsified vegetable oil (8 g L⁻¹). The existence of molasses (1.14 g L⁻¹) decreased the pH of the effluent from 7.5 to 6.3 and the oxidation-reduction potential from 275 mV to 220 mV in the groundwater, which was responsible for reduction and thus the sequestration of Cr(VI). Advection-dispersion-reaction model well described the process of the Cr(VI) transport with biosequestration in the column (R² ≥ 0.96). Owing to the Cr(VI) toxicity to the biofilms, the removal ratio decreased by 24% with a rise of Cr(VI) concentration from 8.6 to 43 mg L⁻¹. The prolongation of hydraulic retention time could promote the performance of Cr(VI) biosequestration. The chemical form of Cr deposited as the product of bio-reduction was confirmed as Cr(OH)₃·H₂O and other complexes of Cr(III). Our work demonstrated the efficacy of molasses for in situ sequestration of Cr(VI) under the dynamic flow condition and provide some useful information for Cr-contaminated groundwater remediation.