Valorization of orange peel waste to tunable heteroatom-doped hydrochar-derived microporous carbons for selective CO adsorption and separation.

Constrained by the extortionately expensive carbon sources, low carbon yields, inadequate adsorption capacities, and corrosive chemical activating agents, the commercialization of carbonaceous CO adsorbents remains a challenging task. Herein, potassium oxalate (KCO), an activating agent with less corrosive properties, was used for the synthesis of activated carbons from inexhaustibly available "orange peel biowaste." For the first time, a comprehensive report is presented on the effect of hydrothermal treatment, hydrochar/KCO ratio, activation temperature, and melamine modification in tailoring the porosity and surface functionalization of activated carbons. The optimized sample, OPMK-900, exhibited large specific surface area 2130 m/g; micropore volume ~1.1166 cm/g, and a high pyrrolic nitrogen content ( 46.1 %). Notably, melamine played the dual role as a promoter to KCO porosity generation and a nitrogen dopant, which synergistically led to an efficient CO uptake of ~6.67 mmol/g at 273 K/ 1 bar via micropore-filling mechanism and Lewis acid-base interactions. Moreover, remarkably high IAST CO/N selectivity (105 at 273 K and 96 at 298 K) surpasses most of the biomass-derived carbons. Furthermore, the moderately high isosteric heat of adsorption (∆H ~ 38.9 kJ/mol) revealed the physisorption mechanism of adsorption with a limited energy requirement for the regeneration of the spent adsorbents.