Carboxyl functional poly(ionic liquid)s confined in metal–organic frameworks with enhanced adsorption of metal ions from water

Development of porous adsorption materials tethered with functional groups was favorable to establish a more efficient, stable and broad-spectrum metal ions removal strategy from water. In this work, carboxyl functional poly(ionic liquid)s@MOF composites were prepared through in situ polymerization of ILs monomers inside MOF pores. Their thermophysical property, structure and morphology were characterized by different techniques. These PILs@MOF composites were proposed as universal and efficient adsorbents to remove selected rare earth metal ions (La 3+ , Sm 3+ , Nd 3+ ) and heavy metal ions (Pb 2+ , Cd 2+ ) from water medium. Effect of pH value and composite dosage on the adsorption efficiency were studied in detail. The optimized adsorption efficiency as high as 99.8 % could be found for the selected metal ions by PIL@MIL-101. In addition, the isotherms, kinetics and thermodynamic parameters   of the adsorption process were also determined, and these results imply that the Langmuir model and pseudo-second-order model fitted well with the experimental data. Thermodynamic parameters revealed spontaneous and exothermic nature of the adsorption process. A plausible mechanism for the adsorption process was suggested by the electrostatic interaction and coordination between the carboxyl group and metal ions with the aid of FTIR and XPS measurements. Finally, recycling experiments were performed to demonstrate the recyclability and stability of PIL@MOF composites undergoes at least five adsorption–desorption runs. The current research presented a series of poly(ionic liquid)s@MOF composites as a universal adsorption platform that demonstrated great potential for removing metal ions with remarkable efficiency.