Efficient removal of ionic liquids from aqueous media using ZSM-5 zeolites: A tunable mechanism combining micropore filling and electrostatic interaction

Removal of aqueous phase ionic liquid (IL) compounds has drawn increasing attention due to their potential human health and ecological impact. This study addresses the contribution of various adsorbent-adsorbate interactions at solid/liquid interface and how these interactions can be manipulated controlling the adsorption of ILs onto zeolites, with a particular focus on hydrophobic partition and electrostatic interaction. For this purpose, a variety of zeolites with different pore structure and aluminum contents were used to probe the adsorption mechanisms for the removal of imidazolium-based ILs. Nonporous SiO2, ordered mesoporous silica MCM-41 and SBA-15 were used for comparison. Batch adsorption results along with characterization of the adsorbents indicate that micropore filling effect is a common mechanism for the adsorption of ILs on different types of zeolites. Hydrophobic partition also contributes to the IL adsorption on zeolites with low or null aluminum content. Aluminum-containing zeolites (ZSM-5-11 and ZSM-5-36) with negatively charged framework have much higher adsorption capacities than aluminum-free zeolites owing to the additional electrostatic attractive interaction. Moreover, IL saturated ZSM-5-11 can be regenerated and reused, and displayed stable adsorption behavior for ten consecutive adsorption-regeneration cycles, highlighting the potential of ZSM-5 zeolite for practical applications.