Hierarchical porous structure formation mechanism in food waste component derived N-doped biochar: Application in VOCs removal

Nitrogen-doped (N-doped) hierarchical porous carbon was widely utilized as an efficient volatile organic compounds (VOCs) adsorbent. In this work, a series of N-doped hierarchical porous carbons were successfully prepared from the direct pyrolysis process of three food waste components. The porous biochar that derived from bone showed a high specific surface area (1405.06 m2/g) and sizable total pore volume (0.97 cm3/g). The developed hierarchical porous structure was fabricated by the combined effect of self-activation (Carbon dioxide (CO2) and water vapor (H2O)) and self-template. The emission characteristics of activation gas analyzed by Thermogravimetric-Fourier transform infrared spectrometer (TG-FTIR) and the transformation of ash composition in the biochar help to illustrate the pore-forming mechanism. Calcium oxide (CaO) and hydroxylapatite were confirmed as the major templates for mesopores, while the decomposition processes of calcium carbonate (CaCO3) and hydroxylapatite provided a large amount of activation gas (CO2 and H2O) to form micropores. The materials also obtained abundant N-containing surface functional groups (up to 7.84 atomic%) from pyrolysis of protein and chitin. Finally, the porous biochar showed excellent performance for VOCs adsorption with a promising uptake of 288 mg/g for toluene and a high adsorption rate of 0.189 min-1. Aplenty of mesopores distributed in the materials effectively improved the mass transfer behaviors, the adsorption rate got a noticeable improvement (from 0.118 min-1 to 0.189 min-1) benefited from mesopores. Reusable potentials of the hierarchical porous carbons were also satisfying. After four thermal regeneration cycles, the materials still occupied 84.8%-87.4% of the original adsorption capacities.