Understanding the fate of cadmium (Cd) in multi-constituent systems during dynamic multi-processes is critical for its effective remediation in the environment. However, the influence of humic acids (HAs) on the transformation of iron (Fe) minerals and cadmium (Cd) dynamics remains poorly understood. This study employed microscopic characterization (X-ray diffraction, Fourier-transform infrared spectroscopy, and transmission electron microscopy) and stable Cd isotopes to investigate the effects of HA on Fe(II)-catalyzed ferrihydrite (Fh) transformation and Cd dynamics. Our results revealed that HA stabilized amorphous Fh through adsorption of its carboxyl and hydroxyl groups, inhibiting the transformation of Fh into more crystalline lepidocrocite (Lp), goethite (Gt), and magnetite (Mt). However, HA competed with Cd for the binding sites of mineral surfaces, reducing Cd retention and enhancing its mobility in solution. The Cd isotopic composition (δ114/110Cd) of Fe minerals showed the preferential adsorption of lighter Cd isotopes onto Fh initially (-0.26‰), with δ114/110Cd shifting as heavier isotopes incorporated into crystalline mineral structures (e.g., defects, pores, interlayers in Lp) over time. Our findings are vital for developing remediation and sustainable management strategies to balance carbon storage and heavy metal pollution control in organic-rich environments.

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