Facet and crystallinity-dependent transformation of lepidocrocite impacts cadmium redistribution

In redoximorphic soils and sediments, cadmium (Cd) fate is governed by Fe(II)-induced transformation of metastable iron oxides. Lepidocrocite (Lep), a ubiquitous intermediate in such environments, commonly exhibits plate-like (P-Lep) or rod-like (R-Lep) morphologies with distinct exposed facet ratios and crystallinity. These structural variations engender differential Fe(II) adsorption affinities and electron transfer capacities, thereby influencing mineralogical transformation pathways and associated Cd redistribution. Herein, we investigated the transformation of Cd-adsorbed P-Lep and R-Lep under Fe(II) concentrations of 0.2-5.0 mM using synchrotron radiation X-ray diffraction, Mössbauer spectroscopy, high-resolution transmission electron microscopy and Cd speciation extraction. At 0.2-1.0 mM Fe(II), R-Lep readily transformed into magnetite due to its enhanced Fe(II) adsorption affinity, whereas at 2.0-5.0 mM, P-Lep exhibited preferential transformation arising from its superior electron transfer capacity. Mineralogical analysis revealed that P-Lep and R-Lep transformed into magnetite primarily through dissolution-reprecipitation and topotactic transformation, respectively. Notably, magnetite formed via topotactic transformation exhibited superior Cd immobilization capacity, whereas the homoepitaxial growth of Lep facilitated Cd migration. These findings provide a mechanistic foundation for predicting Cd mobility in redox-fluctuating environments, facilitating targeted remediation strategies utilizing iron oxides.