Fabrication of metal-organic frameworks with trimesic linkers for europium adsorption and recovery from aqueous solutions

This work reports on the effective sequestration of Eu(III) ions from aqueous solutions using three types of metal-organic frameworks (MOFs) constructed from benzene-1,3,5-tricarboxylic acid (H₃BTC) linker and Ca²⁺, Cu²⁺, or Al³⁺ metal nodes. The MOFs possessed an exceptionally high Eu adsorption capacity of 635 (91% removal), 642 (92% removal), and 628 mg g⁻¹ (90% removal) for Cu-BTC, Ca-BTC, and Al-BTC, respectively, at pH 7 for an initial Eu concentration of 350 mg L⁻¹. The MOFs were hydrolytically stable and possessed a high Eu adsorption capacity in a wide pH range of 4–8. While the pseudo-second-order model explained the adsorption kinetics, the adsorption process followed the Langmuir-Freundlich isotherms. The analysis confirmed that the Eu adsorption process was governed by chemical interactions between the carboxylic acid/carboxylate group and Eu(III) ions via complexation or ion exchange mechanism. A multi-metal adsorption study confirmed a preferential adsorption behaviour for trivalent lanthanides over divalent cations due to comparatively stronger interactions between Ln³⁺ ions and carboxylate groups. The XPS findings suggested Eu−OOC and Eu(OH)₃ formed over the MOFs' surface after Eu adsorption. These MOFs were effectively regenerated for multiple cycles using 0.1 mol L⁻¹ EDTA solution. Thus, the study demonstrated low-cost MOFs with superior regeneration capabilities for the capture of Eu(III) ions from waste solutions.