Cadmium immobilization in lake sediment using different crystallographic manganese oxides: Performance and mechanism

Cd pollution in sediments poses severe threats to environmental safety and human health. Mn oxides have potential merit for the remediation of Cd pollution in sediment but have not received enough attention. Although Mn oxides have proven effective as adsorbents for removing heavy metals from water/wastewater, the performance and the underlying mechanism of Cd immobilization in sediments by Mn oxides remain unclear. Here, three crystallographic Mn oxides delta-MnO2, gamma-MnOOH, and Mn3O4 were used as amendments to investigate their potential for the in situ immobilization of Cd in lake sediment. Experimental data showed that when the sediment samples were treated with synthesized Mn oxides at dosages of 2% and 6% (w/w) for 56 days, the TCLP (toxicity characteristic leaching procedure) leachable Cd in the sediment decreased by 43.9-66.81%, and the PBET (physiologically based extraction test) extractable Cd decreased by 45.16-99.40%. Additionally, the acidsoluble fraction of Cd was partially transformed to a residual fraction, resulting in a 27.55-35.49% decrease in acid-soluble Cd and a 25.16-30.36% increase in the residual Cd fraction. Sediment pH and oxidation-reduction potential were important factors affecting the bioavailability of Cd in the remediation process. Furthermore, scanning electron microscopy, X-ray diffractometer, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis illustrated that the interaction between the amendment and Cd mainly involved complexation with O-containing groups, ion-exchange as > OCd+, and precipitation with carbonate. The efficient remediation capacity and associated mechanism for Mn oxides provide insights for the improved restoration of heavy metal-contaminated sediment.

Automated summary

Cd immobilization in lake sediment was effectively achieved by using Mn oxides as amendments. The treatment with synthesized Mn oxides significantly reduced the leachable and extractable Cd in the sediment, and transformed the acid-soluble fraction of Cd to a residual fraction. The pH and oxidation-reduction potential of the sediment played important roles in the bioavailability of Cd during the remediation process. The interaction between the amendment and Cd mainly involved complexation, ion-exchange, and precipitation. These findings provide insights and methods for the improved restoration of heavy metal-contaminated sediment.