Nitrogen reutilization from hydrothermal aqueous phase for preparing N-doped activated carbons toward efficient CO2 capture.

N-doped carbon materials are recognized for CO2 capture due to their well-developed microporous structures and abundant surface basic sites, which enhance adsorption performance. Municipal sewage sludge is a promising nitrogen-rich precursor, but a significant fraction of nitrogen is lost to the hydrothermal aqueous phase during carbonization, reducing nitrogen retention in the solid precursor and increasing the burden of wastewater treatment. To overcome this, spent coffee grounds and sewage sludge were employed as feedstocks, and three nitrogen reutilization strategies were systematically investigated: recycling of the hydrothermal aqueous phase, struvite-assisted nitrogen recovery and re-doping, and urea supplementation. Struvite crystallization reduced the ammonium concentration from 1573.73 to 30.95  mg/L, corresponding to an apparent aqueous-phase ammonium removal efficiency of approximately 98.0 %. Both aqueous-phase recycling and struvite-assisted re-doping improved nitrogen retention and generated N-doped activated carbons with hierarchical microporosity and favorable surface basicity. The resulting materials, CSAC-R and CSAC-S, exhibited CO2 uptakes of 5.73 and 5.22  mmol/g at 273  K and 1  bar, and 3.74 and 3.33  mmol/g at 298  K, respectively, outperforming the urea-supplemented sample. Structure-property analysis revealed that CO2 adsorption was governed not solely by BET surface area, but by the synergistic effects of narrow micropores, pyridinic/pyrrolic nitrogen species, and defect sites. Overall, aqueous-phase recycling was more favorable for forming narrow micropores and maximizing CO2 uptake, while struvite re-doping offered additional benefits in ammonium recovery, defect regulation, and surface functionality. This study provides a clean and efficient strategy for converting nitrogen from hydrothermal aqueous phases into functional adsorption sites in waste-derived carbon materials.