Unveiling the Mechanistic Role of Surface Nitrogen and Sulfur in Boosting the Dechlorination Performance of Zero-Valent Iron

Abstract

Nitridated zero-valent iron (N-ZVI) and its sulfidated counterpart (S–N-ZVI) are promising materials for groundwater remediation. The dechlorination performance of N-ZVI and S–N-ZVI is intricately linked to the specific N and S surface speciation, yet their roles in tuning the physicochemical characteristics, dechlorination reactivity, and electron selectivity of both particles remain unclear. In this study, we synthesized ZVIs using varied N and S agents, leading to the formation of different surface N species (iron nitrides (Fe_xN_y), pyridinic, and graphitic nitrogen) and sulfur species (FeS and FeS₂). The trichloroethylene (TCE) dechlorination rate showed a linear correlation with Fe_xN_y content, indicating Fe_xN_y-mediated ZVI dechlorination. Hydrogen production capacity was, however, linearly correlated with pyridinic N. Electron paramagnetic resonance (EPR) analysis revealed that pyridinic N enhanced proton transfer processes, thereby facilitating atomic hydrogen generation. This was further supported by the reduced H/D kinetic isotope effects (KIEs) in N-ZVI (2.07) and S–N-ZVI (∼1) compared to unmodified ZVI (3.06) and noticeable mitigation of surface passivation in N-ZVI and S–N-ZVI at pH 9. FeS and FeS₂ species minimized the hydrogen evolution reaction and removed the proton transfer limitation in TCE dechlorination. This magnifies the effect of Fe_xN_y and contributes to a synergistic interplay between nitridation and sulfidation in enhancing the dechlorination kinetics.