Diffusion-adsorption synergistic mechanism in the adsorption of hierarchical porous carbon for antibiotics removal from water

Hierarchical porous carbon (HPC) has gained extensive application in recent decades in the fields including adsorption, catalysis, and electrochemistry . The incorporated meso/macro pores are assumed to facilitate mass transport into micropores in HPCs. However, understanding the synergistic mechanisms of guest molecules diffusing and adsorbing within the multi-scale porous structures remains a challenging task till now, especially in aqueous systems. In this study, HPCs (HPC3–1.9 and HPC5–1.9) with connected meso-micro pores were produced, and the adsorption behavior and dynamic process of representative antibiotic, nalidixic acid (NA), in their multiple-leveled nanopores were explored by molecular dynamics (MD) simulation. The results reveal that the accessibility of micropores is dramatically enhanced by the hierarchical porous structure, and the microporous surfaces of the studied HPCs are preferred effective sites for the adsorption of NA molecules from water. Surface diffusion of adsorbed molecules is directly observed from the molecular trajectories during the adsorption process , which is identified to be an essential pathway for guest molecules transferring from mesopores into micropores. The interfacial barrier aroused by the confined water molecules may hinder the solution diffusion of NA molecules into micropores. Adsorption energy analysis clarifies that the smaller-sized micropore surface exerts enhanced van der Waals interactions with NA molecules than the mesopore surface, thus this energy difference drives the surface diffusion of adsorbed NA molecules. The results of this study provide an insightful understanding of the mass transfer mechanisms within hierarchical porous structures, offering valuable references for the structural optimization of effective adsorbents. • The accessibility of micropores is enhanced by the hierarchical porous structure. • Surface diffusion of adsorbed molecules into micropore is directly observed. • Surface diffusion is verified to be an essential way for mass transfer into micropores in HPCs. • The enhanced van der Waals interactions with micropore surface drive the surface diffusion.