Fast and Selective Adsorption of Cu ²⁺ from IL Aqueous Solution with Carboxyl-Functionalized Polymers of Intrinsic Microporosity Beads: Adsorption Behavior and Mechanism

While ionic liquids (ILs) hold significant promise for diverse applications, their sustainable recycling or regeneration is hindered by the persistent challenge of removing metal-ion impurities such as Cu²⁺. To address this, we attempted to develop a novel polymer adsorbent, carboxyl-functionalized polymers of intrinsic microporosity (PIM-COOH) beads, for the selective separation and elucidating their underlying adsorption mechanisms. The beads not only inherited a hierarchical structure with the macropores, mesopores, and micropores, facilitating the rapid adsorption of ILs and Cu²⁺ within 30 min (>95% at their equilibrium capacity), but also possessed the enriched -COOH accessible sites to selectively adsorb Cu²⁺. A high molar adsorption capacity of Cu²⁺ (∼0.74 mmol/g) relative to that of ILs (∼0.05 mmol/g) at 313 K was achieved, giving a separation factor of 26.9. Mechanistic studies revealed that the hierarchical pores govern the similar mass transfer and adsorption kinetics for both species, whereas the functional sites dictate their distinct adsorption capacities and thermodynamic behaviors. Both theoretical and experimental analyses suggested that IL adsorption proceeds mainly through physical interactions (ion exchange and π-π stacking), while Cu²⁺ adsorption involves a combined physicochemical mechanism dominated by ion exchange and coordination.