Comparison of adsorption isotherms of single-ringed compounds between carbon nanomaterials and porous carbonaceous materials over six-order-of-magnitude concentration range

Understanding the mechanisms controlling the differences in adsorption properties between carbon nanomaterials (CNMs) and porous carbonaceous materials-particularly, the structure-activity correlations-is critical for exploring CNMs as special adsorbents for environmental applications. In this study, batch adsorption isotherms of 4-chloronitrobenzene (planar, aromatic) to three CNMs (single-walled carbon nanotubes/SWNT, multi-walled carbon nanotubes/MWNT, and graphene) and three porous carbonaceous materials (two activated carbons/ACs, and a template-synthesized mesoporous carbon/CMK-3) were compared with those of trans-1,2-dichlorocyclohexane (nonplanar, aliphatic) to the respective adsorbents. The isotherms covered a concentration range of over six orders of magnitude. After being normalized by the adsorbent total surface area, adsorption of 4-chloronitrobenzene to CNMs was stronger than to ACs and CMK-3, likely due to strong π-π stacking between 4-chloronitrobenzene and the well-crystallized and open accessed graphitic surfaces of CNMs. In contrast, the surface area-normalized adsorption of trans-1,2-dichlorocyclohexane was stronger to ACs and CMK-3 than to CNMs, which can be attributed to the stronger micropore-filling effect with porous carbonaceous materials. The proposed adsorption mechanisms were verified by examining the adsorption properties of 1,2-dichlorobenzene and 1,4-dichlorobenzene to SWNT and one of the ACs, as well as modeling the adsorption data with Polanyi theory-based Dubinin-Ashtakhov model.