Enhanced immobilization of Cr(VI) by a Fe⁰ -microorganisms composite system: Benchmark and pot experiments

In this study, a collaborative system of Fe⁰ and mixed anaerobic microorganisms was established for remediating chromium (Cr)-contaminated soil and restraining the translocation of Cr from soil to swamp cabbage (Ipomoea aquatica Forssk.). Solid phase characterization demonstrated that more reactive secondary minerals such as green rust, magnetite, and lepidocrocite were generated in the composite system as compared with the Fe⁰ -only system. Hence, the Fe⁰ -microorganisms composite system achieved a remarkably higher aqueous Cr(VI) removal of 85.6%, 2.9 times higher than that in the Fe⁰ -only system. After 14 d remediation, easily available Cr(VI) and Cr_total species such as water-soluble, exchangeable, and bound-to-carbonates were converted to less available Cr(III) and Cr_total species (e.g., Fe-Mn oxides-bound and organic matter-bound species) because of the production of Cr-Fe hydroxides and oxides [Cr_x Fe_1-x (OH)₃ or Cr_x Fe_1-x OOH] on the Fe⁰ surface. A pot experiment showed that Cr uptake by swamp cabbage after the composite system remediation was suppressed by 69.1%, two times higher than that after the Fe⁰ -only system remediation. Excessive Fe uptake by swamp cabbage also was efficiently inhibited by the composite system treatment due to enhanced Fe hydroxides and oxides production on the Fe⁰ surface because of biological corrosion and mineralization. These results indicated that Fe⁰ -microorganisms composite system remediation could efficiently enhance Cr(VI) immobilization and decrease its bioavailability and bioaccumulation by plants, which is a promising technology in Cr-contaminated soil remediation.