Structure–tuned, phosphorus–doped hierarchical porous biochar via green hydrothermal carbonization and activation for formaldehyde adsorption

Abstract

Carbonaceous adsorbents synthesized through traditional methods often suffer from inherent limitations, such as restricted porosity, homogeneous pore distribution, and insufficient surface functional groups, which collectively constrain their formaldehyde (HCHO) adsorption capabilities. To overcome these shortcomings, this study proposes a straightforward and efficient two-stage strategy to fabricate biochar-based adsorbents. In particular, phosphorus-doped porous carbons (MBPs) were successfully synthesized using cypress sawdust as a biomass precursor through phosphoric acid (H₃PO₄)-assisted hydrothermal carbonization (HTC), followed by KOH activation. Among the prepared samples, MBP2-240-A exhibited the highest specific surface area (1889.0 m²/g) and a micropore volume of 0.70 cm³/g, forming a hierarchical pore network that synergizes micropores and mesopores while offering abundant active adsorption sites. XPS and FTIR characterization confirmed the successful introduction of surface functional groups, such as P–O, P=O, and C–P, which enhanced the surface polarity of the carbon matrix and contributed to improved chemical adsorption capacity. Additionally, the subsequent KOH activation process further refined the pore structure and introduced additional oxygen-containing functional groups, significantly enhancing the overall adsorption performance. As a result of these synergistic effects, MBP2-240-A exhibited exceptional dynamic HCHO adsorption performance, achieving a maximum adsorption capacity of 8.75 mmol/g. Through the integration of dynamic adsorption testing and Grand Canonical Monte Carlo (GCMC) simulations, the configuration regulation of phosphorus-doped hierarchically porous biochar was systematically elucidated, providing valuable theoretical insights and practical foundations for the future design and optimization of high-efficiency HCHO adsorbent materials.