Immobilization of cadmium in river sediments using sulfidized nanoscale zero-valent iron synthesized with different iron precursors: performance and mechanism

Purpose Currently, cadmium (Cd) is extensively present in sediments and is more mobile and toxic than other heavy metals. Sulfidized nanoscale zero-valent iron (S-nZVI) has been widely applied for the removal of heavy metals due to its unique core–shell structure. However, its remediation effect on Cd in sediments is still unclear. In addition, the precursors used in the synthesis also affect the reactivity of S-nZVI. Therefore, in this research, we evaluated the impacts of different iron precursors (Fe 2+ and Fe 3+ ) on the properties of nanoscale zero-valent iron (nZVI) and S-nZVI, and investigated the mechanism of S-nZVI in the immobilization of Cd-contaminated sediments. Materials and methods Synthesized nanomaterials were added to Cd-contaminated sediments. The Cd speciation was investigated using the modified Community Bureau of Reference (BCR) sequential extraction method. To evaluate the immobilization efficiency by different nanomaterials, toxicity characteristic leaching procedure (TCLP)–leachable and diethylene triamine pentaacetic acid (DTPA)–extractable Cd were measured. The changes in sediment properties and enzyme activity, as well as the mechanism of Cd immobilization by S-nZVI, were also examined. Results and discussion The BCR extraction results indicated that synthesized nanomaterials effectively transformed the acid soluble Cd into residual speciation. In comparison to the control group, the TCLP-leachable Cd was decreased by 99.36% and 98.96% in the S-nZVI (Fe 2+ ) and S-nZVI (Fe 3+ ) treated groups, respectively, while the DTPA-extractable Cd decreased by 95.31% and 94.3%, respectively, after 15 days of incubation. The sediment physicochemical properties demonstrated that the change of pH and Eh were crucial factors for Cd immobilization. Significant enhancement of urease and sucrase activity was observed. After remediation, characterization analyses of S-nZVI (Fe 2+ ) showed that Cd was successfully enriched, with CdS, CdO and CdFe 2 O 4 being the main forms. Conclusion S-nZVI has a superior remediation potential for Cd contamination in sediments. Compared with others, S-nZVI (Fe 2+ ) was the most effective for Cd immobilization. Hence, the work offers new insights into the selection of iron precursors for S-nZVI nanomaterials and their practical applications in the remediation of heavy metal contaminated sediments.