<p>Iron-based nanomaterials (Fe-NPs) have been extensively studied for heavy metal immobilization, yet knowledge of their post-treatment and long-term stability remains limited. Here, we systematically compared the remobilization of cadmium (Cd) from three widely used Fe-NPs, namely, nano–zero-valent iron (nZVI), sulfidated nano–zero-valent iron (S-nZVI), and pyrite nanoparticles (nFeS). Under both oxic and anoxic aging, solution pH strongly controlled Cd(II) speciation and the corrosion behavior of Fe-NPs. Acidic conditions (pH 4) induced substantial Fe-NP dissolution and enhanced Cd release, whereas alkaline conditions (pH 8) greatly suppressed both dissolution and Cd mobilization. During oxic aging, dissolved oxygen significantly accelerated the oxidative corrosion of Fe-NPs, thereby promoting secondary Cd release. The Cd release from nZVI became dramatically higher (up to 50.14%, even at pH 8), which sharply contrasted with the minimal release from S-nZVI (0.37%) and nFeS (0.03%). Elevated concentrations of Na⁺ and Ca²⁺ substantially reduced the stability of spent Fe-NPs, while CO₃²⁻ buffered the system and helped maintain lower dissolved Cd levels. Furthermore, mechanistic investigation, supported by X-ray diffractometer, X-ray photoelectron spectroscopy, and transmission electron microscopy analyses, revealed that nZVI partially reduced Cd(II) to Cd(0), which subsequently underwent reoxidation under oxic conditions, whereas S-nZVI and nFeS stabilized CdS by forming persistent CdS phases that effectively impeded its release. This study elucidates key factors that govern Cd remobilization and provides a theoretical basis for the long-term application of Fe-NPs in heavy-metal treatment.</p>