Mechanistic insight into the bifurcated pH-dependent decontamination trends in zero-valent aluminum systems

Zero-valent aluminum (ZVAl) shows promise for environmental remediation, yet its decontamination performance under circumneutral conditions remains debated. To unravel this paradox, we employed Se(IV) and nitrate (NO₃⁻) as model pollutants to probe how contaminant-specific modulation of Al₂O₃ transformation dictates ZVAl performance across pH₀ 2.0–11.0. Results revealed that Se(IV) sequestration exhibited U-shaped kinetics with minimum efficiency at pH₀ 4.0–7.0 (5.8–14.2 %), whereas NO₃⁻ reduction displayed an inverted U-shaped trend with maximum efficiency (> 95 %) obtained in the same pH range. Mechanistic studies demonstrated that these contrasting trends originate from pollutant-mediated Al₂O₃ evolution. Specifically, Lewis-basicity-dependent competition for Al₂O₃ surface sites dictates whether hydration or dissolution dominates activation. Se(IV), acting as a strong Lewis base, inhibited Al₂O₃ hydration by competing with OH⁻ for Al₂O₃ surface sites and/or forming passivating [tbnd]Al-O-SeO₃²⁻ complexes^, thereby self-limiting its own removal. Conversely, NO₃⁻ with weak Lewis basicity negligibly affected Al₂O₃ hydration to form porous Al(OH)₃, which can enhance electron transfer by reducing interfacial charge transfer resistance. This work resolves the long-standing pH limitation paradox in ZVAl systems by elucidating the role of Al₂O₃ evolution, thereby offering a mechanistic framework for the design of adaptive metal-based remediation technologies.