Cu2+ adsorption onto ion-imprinted composite hydrogels: thermodynamics and mechanism studies

In the current study, in situ free radical polymerization was employed to prepare Cu²⁺-imprinted composite hydrogel (Cu²⁺-ICH). The cross-sectional morphology of the Cu²⁺-ICH evaluated by scanning electron microscopy indicated that the copper-loaded Cu²⁺-ICH became rougher and the pore size of the gel became smaller compared to the unloaded Cu²⁺-ICH. The ability of the Cu²⁺-ICH to adsorb Cu²⁺ from aqueous solutions was assessed using batch adsorption technique. The adsorption capacity increased with the initial concentration of Cu²⁺, but decreased as the temperature rose from 298 to 318 K. Thermodynamic parameters such as Gibbs free energy (ΔG ⁰), enthalpy (ΔH ⁰), and entropy (ΔS ⁰) for the Cu²⁺ adsorption were evaluated. It was suggested that the adsorption process was a spontaneous, exothermic process that had positive entropy. Selectivity study indicated that ion imprinting technique resulted in excellent affinity of the Cu²⁺-ICH toward Cu²⁺. Finally, the adsorption mechanism was studied by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results indicated that copper adsorption was mainly through interactions with the amine and carbonyl groups.