Dynamic remobilization of treated Cd from Fe-based nano-particles (nZVI, S-nZVI, and nFeS) in oxic/anoxic aqueous environments

Abstract

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 CO₃^2– 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.

Graphical abstract

Keywords

Nano–zero valent iron / Sulfided zero valent iron / FeS / Aging / Cd(II)

Highlight

	● Solution pH and DO greatly affect dynamic remobilization of Cd from Fe-NPs.
	● DO accelerated oxic corrosion of Fe-NPs and Cd(II) release.
	● Na+ and Ca2+ could decrease the stability of Fe-NPs, while CO32– suppressed Cd release.
	● Both S-nZVI and nFeS outperformed nZVI for long-term stability due to formation of CdS.

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