Numerous studies have reported the formation of passivation products of nanoscale zerovalent iron (NZVI) under oxic conditions. However, inconsistent observations, particularly regarding the formation of magnetite (Fe₃O₄) and lepidocrocite (γ-FeOOH), remain unresolved. In this study, the passivation behavior of NZVI under various oxidation conditions was interpreted from a thermodynamic perspective. Fe₃O₄ was predominantly formed under O₂, H₂O₂, and NO₃- conditions, whereas γ-FeOOH was preferentially formed under HCl, persulfate, and HCO₃- conditions. By integrating pH-Eh trajectories with Pourbaix diagram analysis, passivation product formation was found to be governed not only by overall oxidizing conditions but also by pH-Eh pathways influenced by coexisting anions. In particular, γ-FeOOH formation is thermodynamically favored along pathways passing through the green rust stability region compared to pathway involving Fe₃O₄. Under pure O₂ purging conditions, Fe₃O₄ undergoes sequential transformation to γ-FeOOH, indicating that intermediate phases influence the transformation pathway and kinetics. Cr(VI) removal experiments revealed phase-dependent reactivity. Fe₃O₄ exhibitted reduction-dominated removal, whereas γ-FeOOH showed adsorption-dominated behavior. This indicates that additional electron transfer properties of passivation products control residual reactivity. These findings provide a thermodynamic framework linking phase formation to reactivity and enable improved prediction of NZVI performance under varying geochemical conditions, although the role of green rust requires further experimental validation.

Keywords: Nanoscale zerovalent iron, Passivation products, pH-Eh diagram, Mineral transformation, Oxidation conditions