Upcycling biomass solid wastes into value-added N-doped biochar for enhanced VOCs adsorption

Nitrogen (N)-doped biochar derived from biomass solid wastes exhibits remarkable potential for volatile organic compounds (VOCs) control due to its high selectivity and high adsorption efficiency. In this paper, the preparation methods of N-doped biochar through pyrolysis, gasification, and hydrothermal carbonization are comprehensively reviewed, and various physical and chemical activation methods are also introduced. After nitrogen doping, the nitrogen content of the biochar increased by up to 10.7 times, and its adsorption capacity for VOCs was enhanced correspondingly by up to 10.5 times. This review also provides a detailed discussion on the mechanism for VOCs by N-doped biochar. Hydrogen bonding, electrostatic interactions, van der Waals forces, and π-stacking are considered to be the main adsorption mechanisms. Moreover, the influences of feedstock types and nitrogen doping methods on the properties of resulting N-doped biochars are revealed. The nitrogen functional groups, total pore volume, and specific surface area of biochar are the most critical factors affecting adsorption. Notably, in-situ nitrogen doping offers a simple process with high structural nitrogen content, whereas ex-situ doping provides controllable nitrogen content and greater flexibility. In the future, the integration of machine learning with life cycle assessment can guide the development of cost-effective, high-performance, and scalable N-doped hierarchical porous biochar materials. This review benefits subsequent related research in upcycling biomass solid wastes into N-doped biochar for VOCs adsorption.