Linear adsorption of organic compounds on mesoporous activated carbon in bi-solute system

Knowledge of organic compounds adsorption by adsorbents is essential for evaluating the environmental fates of organic compounds and removing them from the environment. Linear adsorption, as a supplement to the traditionally nonlinear adsorption, was previously proposed for the linear sorption of organic compounds on the mesoporous surface of carbon nanotubes (CNTs) in multi-solute system. However, CNTs are not the ideal adsorbent to verify the linear adsorption mechanism, because of their partition-like phase components such as mobile graphene layers that could be responsible for the linear sorption through linear partition mechanism instead, and thus the linear adsorption theory was argued. In this study, therefore, mesoporous activated carbon (MAC), widely accepted as the model free of partition phase components, was selected as an adsorbent to investigate the adsorption of typical organic compounds in the bi-solute system for verifying whether the linear adsorption phenomenon existed or not. The isotherm of nitrobenzene on MAC was changed from nonlinear to linear with 4-nitrophenol up to 1400 mg/L, and the linear isotherm slope decreased more as 4-nitrophenol concentration increased until 4000 mg/L. It agreed with the characteristics of adsorption (i.e., competition) but not partition (i.e., noncompetition), confirming the existence of linear adsorption. The isotherm linearity was attributed to the reduction of adsorption interactions by displacement and multilayer adsorption. Moreover, linear adsorption of apolar compounds on MAC could occur with apolar or polar competitors, while for polar compounds, linear adsorption could occur with only polar competitors. The observed linear sorption and the competition of organic compounds in the bi-solute system on MAC free of partition phase components verified that the linear adsorption existed, which gives a new insight into the adsorption theory for organic compounds. The results could provide better fundamental theory of adsorption for improving the accuracy of environmental risk assessment of organic pollution and enhancing the efficiency of organic pollution control in the environment.