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

Nitrate (NO3-) removal on nanoscale zero-valent iron-based nanoparticles (nFe(0) particles) represents one efficient and green technology for nitrate pollution abatement, but its development is hindered by the low product selectivity towards harmless N-2. Herein we demonstrate the inferior performance of nFe(0) in N-2 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 N-2. Increasing NO3- feeding concentration or lowing nFe(0) dose can uplift the N-2 selectivity up to 41.9% (vs. 19.3% by single nFe(0) in removing 20 mg L-1 NO3--N), but has to sacrifice NO3- removal efficiency and yield more toxic NO2-. The nFe(0) surface modification by 12.0 wt% palladium (Pd) gives rise to a N-2 selectivity of 46.0% and a NO3- removal efficiency above 90% (vs. 100% by single nFe(0)). 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 N-2 formation. This correlation between surface chemistry and NO3- conversion may guide the design of improved Fe-0-based materials for practical nitrate remediation.

Automated summary

The research demonstrates that nanoscale zero-valent iron-based nanoparticles have low N-2 selectivity in nitrate removal, but after surface modification with Pd, the N-2 selectivity and NO3- removal efficiency can be improved. The theoretical calculations reveal that Pd plays a crucial role in weakening the binding strengths of H* and N* on the catalyst surface, promoting the formation of N-2.