Advanced spectroscopic evidence for the sequestration of heavy metals via repetitive in situ synthesis of Fe oxide.

The in situ synthesis of Fe oxide is an established method for stabilizing metals and metalloids (Me) in contaminated soils. Nevertheless, the potential for enhanced Me sequestration through repeated Fe oxide application and the fundamental mechanisms of this process yet to be systemically investigated. In this study, the means by which repetitive Fe oxide synthesis enhances the immobilization of Cd, Zn, and As was investigated using batch experiments. The first synthesis resulted in surface-adsorbed Cd, Zn, and As levels of 68.7, 37.5, and 22.6 %, respectively. The second synthesis reduced the surface-adsorbed Cd and Zn levels to 53.9 % and 23.3 %, respectively, while the As level remained unchanged (22.8 %) owing to its structural incorporation and the occurrence of OH induced desorption/re-adsorption. The third synthesis further decreased the surface-adsorbed metal levels, surpassing the reduction achieved by a single synthesis with an increased Fe content. Extended X-ray absorption fine structure analysis revealed peak shifts in the Me K-edge-fitted spectra, distinguishing incorporated metals from surface-adsorbed metals. In addition, energy dispersive X-ray spectroscopy revealed decreased heavy-metal concentrations on the surfaces and cross-sections of the repeatedly synthesized Fe oxide particles, indicating encapsulation by the hydrolyzed Fe species. While conventional methods rely on single applications, repetitive Fe oxide synthesis non-specifically sequesters surface-adsorbed metals through encapsulation. This repetitive approach achieved the enhanced immobilization of weakly adsorbed metals, rendering it potentially effective in soils exhibiting different properties, and indicating its possibility for use as a viable remediation strategy.