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

Elevated concentrations of nitrite (NO 2 − ) 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 (Fe 3 C@Fe-CN-3) by incorporating iron carbide (Fe 3 C) and Fe 0 into N-doped carbon, thereby enhancing the activation of peroxydisulfate (PS) for NO 2 − 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 Fe 3 C, whereas reducing this ratio to 0.8 resulted in the formation of iron nitride (Fe 4 N). Fe 3 C@Fe-CN-3, synthesized with a molar ratio of 1.2, exhibited the highest removal rate of NO 2 − (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 NO 2 − removal rate (99.5 %), a molar ratio of at least 7:20 between NO 2 − and PS is required. The non-radical pathway involved in NO 2 − removal was confirmed through delayed experiments and electron suppression tests. Meanwhile, Electron paramagnetic resonance (EPR) and radical scavenging experiments confirmed that hydroxyl radicals ( O H · ) and superoxide anion radicals ( O 2 − · ) play key roles in the removal process. While Fe 3 C@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.