Effects of coexisting goethite or lepidocrocite on Fe(II)-induced ferrihydrite transformation pathways and Cd speciation

The efficacy of ferrihydrite in remediating Cd-contaminated soil is tightly regulated by Fe(II)-induced mineralogical transformations. Despite the common coexistence of iron minerals such as goethite and lepidocrocite, which can act as templates for secondary mineral formation, the impact of these minerals on Fe(II)-induced ferrihydrite transformation and the associated Cd fate have yet to be elucidated. Herein, we investigated the simultaneous evolution of secondary minerals and Cd speciation during Fe(II)-induced ferrihydrite transformation in the presence of goethite versus lepidocrocite. The presence of goethite resulted in a more pronounced ferrihydrite transformation than lepidocrocite because goethite facilitates electron transfer. Coexisting goethite promoted the production of secondary goethite with different morphology by triggering template-directed nucleation and growth of labile Fe(III) derived from ferrihydrite and intermediate lepidocrocite, respectively. However, coexisting lepidocrocite impeded goethite formation from ferrihydrite and acted as the template to facilitate secondary lepidocrocite production. Furthermore, variations in the crystallinity of coexisting lepidocrocite influenced the particle size and crystallinity of the secondary lepidocrocite, reflecting different dominant mechanisms in secondary lepidocrocite formation. Despite partial Cd mobilization into the solution due to Fe(II)-induced ferrihydrite transformation, secondary goethite and lepidocrocite re-sequestered Cd through lattice Fe(III) substitution, indicated by an increased structural Cd proportion, expanded lattice spacing, and reduced hyperfine field intensity. Additionally, secondary goethite was more effective than secondary lepidocrocite in sequestering Cd. Coexisting goethite increased the structural Cd proportion by 3.5-fold compared to coexisting lepidocrocite, demonstrating the superior ability of coexisting goethite in enhancing Cd stability during Fe(II)-induced ferrihydrite transformation in natural soils. These findings highlight the impact of template-driven mineralogical transformation on Cd fate in polluted soils and provide crucial implications for toxic metal remediation using mineral amendments.