N-doped carbon featuring dual active sites of Fe3C and Fe0: Mechanisms for peroxydisulfate activation in nitrite removal.

Elevated concentrations of nitrite (NO₂^-) in water pose considerable health risks, including methemoglobinemia and carcinogenesis, yet this issue has been relatively underexplored. This study successfully synthesized Fe-N co-doped carbon (Fe₃C@Fe-CN-3) by incorporating iron carbide (Fe₃C) and Fe₀ into N-doped carbon, thereby enhancing the activation of peroxydisulfate (PS) for NO₂^- removal. The impacts of diverse materials and the experimental parameters (different initial pH, PS concentration and ions) on activation efficiency were investigated. The results demonstrate that a molar ratio (1.2) of Fe to N in the raw material led to the formation of Fe₃C, whereas reducing this ratio to 0.8 resulted in the formation of iron nitride (Fe₄N). Fe₃C@Fe-CN-3, synthesized with a molar ratio of 1.2, exhibited the highest removal rate of NO₂^- (73.8 %) at a PS concentration of only 0.2 mM, with a rate constant (K_obs) of 0.049 min^-1. To achieve a basic NO₂^- removal rate (99.5 %), a molar ratio of at least 7:20 between NO₂^- and PS is required. The non-radical pathway involved in NO₂^- removal was confirmed through delayed experiments and electron suppression tests. Meanwhile, Electron paramagnetic resonance (EPR) and radical scavenging experiments confirmed that hydroxyl radicals (OH·) and superoxide anion radicals (O₂^-·) play key roles in the removal process. While Fe₃C@Fe-CN-3 showed excellent reusability, the removal rate was significantly reduced when natural water was employed as the reaction medium, emphasizing the imperative to enhance its selective removal capabilities.