Oxalic acid-functionalized covalent organic frameworks for sustainable and effective europium radionuclide separation from active wastewater

Abstract

In this study, a novel covalent organic framework based on melamine and terephthalic acid (MT-COF) was synthesized, and subsequently modified with oxalic acid (MT-OX-COF) and other modifiers. A comprehensive set of characterization techniques including SEM, HR-TEM, FT-IR, XRD, EDX, XPS, BET, PL, TGA, MS, ¹H NMR, and ¹³C NMR spectroscopy were used to confirm the structure, morphology, and physicochemical properties of the COF materials. The results revealed that MT-COF formed hexagonal rods resembling roses, while MT-OX-COF displayed fibrous characteristics. The specific surface area of MT-COF was 1002.20 m² g⁻¹, which decreased to 819.98 m² g⁻¹ after modification with oxalic acid. The synthesized materials were then applied for the removal of ^152+154Eu isotopes from radioactive wastewater. Adsorption experiments were conducted under varying conditions of acidity (pH), contact time, europium concentration, ionic strength, and temperature. Optimal adsorption occurred at pH 3.5, achieving a maximum sorption capacity of 42.27 mg g⁻¹. Thermodynamic analysis indicated that the adsorption process was exothermic, while the Freundlich isotherm suggested multilayer adsorption on heterogeneous surfaces. The predominant mechanism was identified as chemisorption. Furthermore, Eu³⁺ ions were effectively desorbed from MT-OX-COF-Eu using FeCl₃, allowing the material to be reused. Overall, MT-OX-COF demonstrated excellent performance in removing europium radionuclides from real wastewater samples, outperforming several other reported adsorbents. These findings highlight MT-OX-COF as a highly efficient sorbent for the separation and recovery of radioactive isotopes.