We report a unique naturally derived activated carbon with optimally incorporated nitrogen functional groups and ultra-microporous structure to enable high CO 2 adsorption capacity. The coprocessing of biomass (Citrus aurantium waste leaves) and microalgae (Spirulina) as the N-doping agent was investigated by probing the parameter space (biomass/microalgae weight ratio, reaction temperature, and reaction time) of hydrothermal carbonization and activation process (via the ZnCl 2 /CO 2 activation) to generate hydrochars and activated carbons, respectively, with tunable nitrogen content and pore sizes. The central composite-based design of the experiment was applied to optimize the parameters of the prehydrothermal carbonization procedure resulting in the fabrication of N-enriched carbonaceous products with the highest possible mass yield and nitrogen content. The resulting hydrochars and activated carbon samples were characterized using elemental analysis, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and Brunauer−Emmett−Teller surface area analysis. We observe that while N-doping and the activation process can individually enhance the CO 2 adsorption capacity to some extent, it is the combined effect of the two processes that synergistically work to greatly increase the adsorption capacity of the N-doped activated carbon by an amount which is more than the sum of individual contributions. We analyze the origins of this synergy with both physical and chemical characterization techniques. The resulting naturally derived activated carbon demonstrates one of the highest CO 2 adsorption capacities (8.43 mmol/g) with rapid adsorption kinetics and good selectivity and reusability.

In this work, design of experiments–response surface methodology (RSM) was implemented to predict the importance of hydrothermal carbonization (HTC) key parameters and their interactions in the preparation of canola-stalk-derived hydrochar via HTC technique. According to the RSM results, temperature and reaction time were found to be the most important control factors. The possible optimum conditions were found to be 207 °C and 82 min for temperature and time, respectively, in order to achieve a hydrochar with the maximum mass yield (solid yield 53.38%), carbon recovery rate (52.66), and O/C ratio (0.69). Furthermore, the optimized hydrochar was successfully activated via potassium hydroxide (KOH), under mild activation conditions. Synthesized microporous activated carbon demonstrated the highly improved Brunauer–Emmett–Teller (BET) surface area of 474.87 m2 g–1 compared to the low BET surface area of mesoporous hydrochar (S BET of 2.69 m2 g–1). Porous activated carbon was used as an adsorbent for methylene blue removal that showed a promising dye removal capacity of 93.4 mg g–1. The morphological and chemical compositions of the solid materials were analyzed by various techniques, including elemental analysis, field emission scanning electron microscopy (FESEM), BET analysis, Fourier transform infrared (FTIR) spectroscopy, and energy-dispersive X-ray spectroscopy.

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