Immobilization of lead and cadmium in agricultural soil by bioelectrochemical reduction of sulfate in underground water

In situ immobilization of lead (Pb) and cadmium (Cd) in agricultural soil with sulfide (S2-) is an effective remediation strategy. Considering irrigating farmland with underground water, sulfate (SO42-) in underground water is an ideal S2- source for Pb and Cd immobilization. Herein, a bioelectrochemical remediation system was developed to immobilize Pb and Cd by coupling a bioelectrochemical sulfate-reducing reactor (SRR) with a soil immobilization reactor (SIR). In the SRR, SO42- in underground water was efficiently reduced to S2- by autotrophic sulfate-reducing bacteria (SRB) using hydrogen as an electron donor via electrochemical water splitting reactions. Inputting 20 mA electricity, 30 mg/L S2- was produced and 77.4% of bioavailable Pb was converted to organic matter bound and residual bound. While, 49.0% of bioavailable Cd was converted to carbonates bound and Fe-Mn oxide bound. The bioelectrochemical remediation system was also efficient for three other soils with different properties. In addition, the most dominant SRB in the SRR was identified with 16S rRNA sequencing. Furthermore, phytotoxicity of the treated soil was proved to be decreased than the original soil and SO4 2- in the treated soil could be continually reduced to S2- for immobilizing Pb and Cd. Our study provides a promising technology to remediate heavy metal polluted soil.

Automated summary

The study developed a bioelectrochemical remediation system for immobilizing lead and cadmium in agricultural soil, where sulfate in underground water was efficiently reduced to sulfide by autotrophic sulfate-reducing bacteria. The system was effective for soils with different properties, and the most dominant SRB in the system was identified through 16S rRNA sequencing.