Depassivation of Zero-Valent Iron through Continuous Surface Renewal with Mechanochemical Processing

Zero-valent iron (ZVI) is a solid reductant that can react with a wide array of contaminants; however, its effectiveness is significantly reduced by the surface passivation layer formed on its surface. This study introduces mechanochemical depassivation to revitalize passivated ZVI and restore its reactivity. The method is exemplified through mechanical milling to depassivate ZVI with different passivation layers formed in solutions containing Cr(VI), Ni(II), and/or trichloroethylene, as well as in a complex water matrix. The results demonstrate rapid restoration of ZVI reactivity within 10 min of milling, achieving a 7-fold increase in contaminant removal capacity after eight cycles. The method universally outperforms acid pickling and ultrasonication across all of the tested passivation scenarios. Mechanistic analysis attributes its efficacy to the ductile-brittle disparity between the metallic iron and the passivation layers: brittle passivation layers fragment under mechanical stress, while ductile iron resists structural damage. Discrete element method simulations provide further mechanical insights into the milling dynamics and identify the contact frequency as the key factor determining depassivation efficiency. The study provides a versatile, environmentally benign, and effective depassivation method for ZVI reactions that can be integrated into ZVI reuse processes for water treatment applications.