Long-term bioremediation of cadmium contaminated sediment using sulfate reducing bacteria: Perspective on different depths of the sediment profile

Sulfate reducing bacteria (SRB) can be utilized for the in-situ remediation of heavy metal contaminated sediment, and the remediation processes occurring at different depths of the sediment profile are critical to the practical application of SRB. In this study, column reactors with sediment profile sampling devices were designed to investigate the variations in Cd speciation, labile S2− and microbial community at different depths of the sediment profile during the bioremediation of Cd contaminated sediment by SRB, over a 270 day monitoring period. In order to evaluate the remediation effect, in vitro extraction of sediment was performed with Sipunculus nudus intestinal juice to determine Cd bioavailability. Results showed that bioremediation caused acid soluble Cd to decrease in the surface sediment (0–6 cm), while reducible and residual fractions of Cd increased. The bioavailability of Cd in the surface sediment reduced by 25.1 % compared to before bioremediation, while no significant changes were observed in other sediment layers. These results indicate that the surface sediment was the main remediation range when SRB were initially added at the water–sediment interface. Furthermore, labile Cd and S2− in the surface sediment of the bioremediation column reactor exhibited a negative correlation (P < 0.1), while they were positively correlated in the sediment profile below 12 cm (P < 0.1) and throughout the whole control column reactor (P < 0.1). This demonstrates that the correlation between labile Cd and S2− could potentially be used as an indicator to reflect the remediation performance. Principal component analysis results revealed that the remediation effect was related to the different Cd fractions, labile S2− and microbial community in the sediment, among which Cd speciation was the most influential factor. Overall, this research provides comprehensive foundation for the practical application of SRB for sediment in-situ remediation.