Stepwise H*-Mediated and Non-H* Reduction Processes for Highly Selective Transformation of Nitrate to Nitrogen Gas Using a ZVAl-Based Material

Abstract

Improving the reduction efficiency and N₂ selectivity is important for nitrate decontamination. A novel ternary ball-milled Al-Cu-AC material is reported to achieve a highly selective reduction of nitrate to N₂. The reduction process, driven by the continuous dissolution of zero-valent aluminum (ZVAl), demonstrated a stepwise reduction scheme. The interesting shift in the electron-donating pathways was ascribed to the spontaneous change in the microenvironmental pH from neutral to alkaline. The Al-Cu-AC (1:1:5 mass ratio) material completely removed 30 mg/L of NO₃^–-N over a wide pH range (5–9), achieving over 83% TN removal and N₂-selectivity, without detectable copper leaching. The atomic hydrogen (H*)-mediated reduction occurring on the Cu component was proven to be crucial for the fast transformation from NO₃^– to NO₂^–, while the non-H* reduction process was dominated by the electrochemical reduction of NO₂^– to N₂ on the AC cathode of Al || AC microgalvanic cells formed in the material. The primary reduction route from NO₃^– to N₂ was identified as the *NOH pathway, and the superiority of the Al-Cu-AC material toward nitrate reduction was verified with actual wastewater. This study revealed how microenvironmental pH influenced the electron-donating pathways of ZVAl and provides a new approach to maximize the performance of zero-valent metals.