Intrinsic Roles of Conjugated and Nonconjugated Nitrogen on Graphene Aerogels in Electron Transfer Activation of Persulfates

Carbocatalysts are highly effective in driving persulfate-based advanced oxidation processes (PS-AOPs) for selective removal of organic micropollutants. Conjugated nitrogen species (e.g., graphitic, pyridinic Ns) are well-known for enhancing the catalytic activity of carbon materials. However, the mechanistic role of nonconjugated nitrogen species (e.g., amine, amide) in persulfate activation remains unexplored due to their thermal instability during pyrolysis. In this study, we synthesized three-dimensional nitrogen-doped graphene aerogels (NGAs) enriched with nonconjugated nitrogen via hydrothermal assembly and systematically compared them with their pyrolytic counterparts. Hydrothermal NGAs exhibit abundant edge-located nonconjugated nitrogen species, which enhance surface hydrophilicity and accelerate mass transport to promote efficient nonradical activation of peroxydisulfate (PDS) through outer-sphere electron-transfer process (ETP). Upon pyrolysis, these nonconjugated functionalities are transformed into conjugated nitrogen embedded within the carbon lattice, improving conductivity and creating electron-rich domains that favor peroxymonosulfate (PMS) activation via inner-sphere ETP. Techno-environmental-economic assessment further demonstrated that the hydrothermal NGA/PDS system is significantly more cost-effective, requiring one-eighth the synthesis energy and only one-third the oxidant cost of PMS-based systems. This work highlights the underappreciated role of nonconjugated nitrogen in PS-AOPs and offers new insights to carbon catalysts for wastewater treatment.