Nitrate reduction by nanoscale zero valent iron (nFe0)-based Systems: Mechanism, reaction pathway and strategy for enhanced N2 formation

Abstract Nitrate (NO3-) removal on nanoscale zero-valent iron-based nanoparticles (nFe0 particles) represents one efficient and green technology for nitrate pollution abatement, but its development is hindered by the low product selectivity towards harmless N2. Herein we demonstrate the inferior performance of nFe0 in N2 production originates from its over-strong affinity with both the H* and the N-intermediates (denoted as N*), leading to a low N/H molar ratio and a poor mobility of N* on Fe surface that facilitates the formation of NH3 rather than N2. Increasing NO3- feeding concentration or lowing nFe0 dose can uplift the N2 selectivity up to 41.9% (vs. 19.3% by single nFe0 in removing 20 mg L-1 NO3--N), but has to sacrifice NO3- removal efficiency and yield more toxic NO2-. The nFe0 surface modification by 12.0 wt% palladium (Pd) gives rise to a N2 selectivity of 46.0% and a NO3- removal efficiency above 90% (vs. 100% by single nFe0). Theoretical calculations reveal the critical role of Pd in weakening the binding strengths of H* and N* on catalyst, which enables to reduce the H* adsorption and promote the migration of N* that increases the N*-N* encountering possibility for N2 formation. This correlation between surface chemistry and NO3- conversion may guide the design of improved Fe0-based materials for practical nitrate remediation.