Soil water management-induced transformation of iron phases in kaolinite-loaded amorphous zero-valent iron enhances cadmium immobilization

Kaolinite-loaded amorphous zero-valent iron composite (K@AZVI) is a promising material for cadmium-contaminated soil remediation. Still, the impact of iron (Fe) oxides on the surface of K@AZVI on the immobilization of cadmium (Cd) in soils caused by varying water management remains unclear. Hence, this study investigated the transformation of Fe oxides in K@AZVI and their effects on Cd morphology under varying soil water management (drying, wetting, and flooding). The results indicated that under K@AZVI treatment, the pe +pH (pe = Eh (oxidation-reduction potential, mV) / 59.2) values decreased from 11.59 to 9.83 and 5.61 with increased soil water management. Meanwhile, the content of crystalline Fe oxides (Fe_c) decreased significantly by 34.02%-72.76%, while the content of amorphous Fe oxides (Fe_o) increased by 29.01%-55.20%. This transformation from crystalline to amorphous Fe oxides significantly enhanced the immobilization of Cd, resulting in a 30.30%-64.59% reduction in the available Cd content compared to the control. In addition, the proportion of low active Cd was enhanced from 32.94% to 56.10%, further indicating that K@AZVI exhibited superior immobilization in flooded soil. Characterization and calculations showed that water management significantly improved the immobilization of Cd by K@AZVI by regulating the crystallinity of Fe oxides and enhancing their electron transfer rate and adsorption capacity. Therefore, investigating the morphological transformation of Fe under varying water management will help us to understand the mechanism of Fe redox-mediated Cd transport and transformation in the soil system.