Adsorption of Diethyl Phthalate on Carbon Nanotubes: pH Dependence and Thermodynamics

The adsorption mechanism of diethyl phthalate (DEP) on solid particles is still unclear due to the complicated properties of adsorbents. Carbon nanotubes (CNTs) have definite structures and uniform surfaces, thus they were chosen as the model adsorbents in this study. The site energy distribution may provide important information for adsorption mechanisms, but no study has applied site energy distribution to illuminate the DEP thermodynamic adsorption on CNTs. Thus, this study focused on the thermodynamic adsorption behavior of DEP on CNTs combining the site sorption energy distribution analysis. The adsorption coefficient Kd followed the order: MG>MH≈MC. Surface area was not the only factor for DEP adsorption on CNTs. Hydrophobic interactions, H-bonds, morphology of CNTs, and DEP molecule structure may control DEP adsorption. Electron donor-acceptor interaction was limited in this system. The thermodynamic parameters changes showed that the adsorption process is exothermic and spontaneous. The site energy distribution indicated that high energy adsorption sites would be occupied first, and sequentially, the DEP molecules adsorb on the relatively low energy sites of CNTs. Nonlinear adsorption isotherms result from the heterogeneous energy distribution. This study emphasized the importance of thermodynamic adsorption for assessing the environmental behavior of organic pollutants.