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

Area of Science:

• Environmental Engineering
• Materials Science
• Water Treatment Technologies

Background:

• Zero-valent iron (ZVI) is effective for contaminant remediation but suffers from surface passivation, reducing its reactivity.
• Passivation layers hinder ZVI's performance in treating pollutants like Cr(VI), Ni(II), and trichloroethylene.
• Existing methods for ZVI revitalization, such as acid pickling, have limitations.

Purpose of the Study:

• To introduce and evaluate mechanochemical depassivation as a method to restore the reactivity of passivated ZVI.
• To demonstrate the effectiveness of mechanical milling for depassivating ZVI under various conditions.
• To compare mechanochemical depassivation with conventional methods like acid pickling and ultrasonication.

Main Methods:

• Mechanochemical depassivation using mechanical milling to treat ZVI passivated in solutions with Cr(VI), Ni(II), and/or trichloroethylene.
• Testing ZVI depassivation in a complex water matrix.
• Utilizing discrete element method (DEM) simulations to analyze milling dynamics and depassivation efficiency.

Main Results:

• Mechanical milling rapidly restored ZVI reactivity within 10 minutes.
• A 7-fold increase in contaminant removal capacity was observed after eight milling cycles.
• Mechanochemical depassivation outperformed acid pickling and ultrasonication across all tested passivation scenarios.
• DEM simulations identified contact frequency as a key factor in depassivation efficiency.

Conclusions:

• Mechanochemical depassivation is a versatile, environmentally benign, and effective method for revitalizing passivated ZVI.
• The ductile-brittle disparity between ZVI and passivation layers is crucial for the milling process.
• This method can be integrated into ZVI reuse processes for enhanced water treatment applications.