Tuning Nitrate Reduction to N ₂ with Zero-Valent Aluminum Composites: pH and Activated Carbon Porosity as Key Regulators

While the ball-milled [Al-Fe-AC]^bm composite has demonstrated remarkable performance for nitrate remediation, the underlying mechanisms governing its pH-dependent reactivity and high nitrogen (N₂) selectivity remain inadequately elucidated. This work systematically decouples the synergistic roles of Al⁰, Fe⁰, and activated carbon (AC) in the [Al-Fe-AC]^bm system across a broad pH range (initial pH₀ 4.0-13.0; stabilized pH_w 7.0-12.0). We identify a critical operational pH_w threshold of approximately 10.5, beyond which the dominant reduction pathway shifts from direct electron transfer to atomic hydrogen (H*)-mediated reduction. Under acidic to circumneutral conditions, Fe⁰ corrosion elevates pH to depassivate Al⁰, enabling electron-driven nitrate reduction with high N₂ selectivity (>73%). In contrast, under strongly alkaline conditions, excessive H* generation─promoted by Al//Fe and Al//AC galvanic couples─shifts the pathway toward nonselective hydrogenation, resulting in ammonium as the predominant product, as corroborated by H* scavenging experiments and electrochemical analysis. Strong correlations between AC's specific surface area/pore volume and N₂ selectivity, combined with in situ Fourier-transform infrared (FT-IR) detection of *N₂O intermediate, demonstrate that AC's nanoconfinement promotes *NO dimerization for selective N-N coupling. This study provides a fundamental mechanistic framework for designing efficient and selective metal-carbon composites for sustainable nitrate remediation.