【Objective】Iron oxides play a pivotal role in the migration of heavy metals in natural environments, with humic acid (HA), a major organic component in soils and water bodies, potentially exerting significant regulatory effects on these processes. This study aims to systematically investigate the transformation kinetics of ferrihydrite (Fh) into crystalline iron oxides (lepidocrocite Lp, goethite Gt, and magnetite Mt) and its impact on cadmium (Cd) sequestration.【Method】Using transmission electron microscopy (TEM) coupled with energy-dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), this study conducted a controlled experiment involving Fh-Cd (control) and Fh-HA-Cd systems. Fe(II) was introduced as a catalyst to initiate the transformation, which was monitored over 168 h. The study examined the effects of HA (initial concentration 4.94 mmolL-1) and Cd (initial concentration 88.29–94.27 μmolL-1) on mineral phase evolution, Cd adsorption, and retention mechanisms.【Result】The results revealed that in the Fh-Cd system, Lp (48.08%) and Gt (43.49%) formed rapidly within 6 h, with Lp continuing to transform into Gt by 120 h, and a small amount of Mt (4.82%) emerging by 168 h. In contrast, in the Fh-HA-Cd system, HA significantly inhibited the transformation rate, resulting in slower formation of Lp and Gt, with a final mineral composition at 168 h of Fh (2.63%), Lp (42.79%), and Gt (54.6%), and no detectable Mt. During the transformation, the solid-phase HA concentration decreased from 4.94 mmolL-1 to 4.49 mmolL-1, and Cd concentration in the solid phase dropped sharply after 6 h before stabilizing, with the Fh-HA-Cd system exhibiting 5%–10% higher solid-phase Cd than the Fh-Cd system. TEM-high angle annular dark field and EDS analyses showed that C and Cd were initially closely associated with Fh; post-transformation, Lp and Gt retained strong Cd adsorption but exhibited significantly reduced C adsorption. EELS line scans further indicated that C was primarily retained on Fh surfaces and within defects/porosities of Lp, with carbon functional groups (C-H, C=O, C-OH) desorbing during transformation, while Cd was retained through multiple mechanisms including adsorption, structural substitution, and physical encapsulation in the newly formed iron oxides.【Conclusion】This study demonstrates that HA significantly influences Cd’s geochemical behavior by suppressing iron oxide transformation and enhancing Cd adsorption. These findings provide valuable scientific insights into the interaction mechanisms among iron oxides, organic matter, and heavy metals, offering a theoretical basis for understanding and managing Cd contamination in natural systems.