Highly Stable Covalent Organic Framework for Palladium Removal from Nuclear Wastewater

Abstract

The effective management of High-Level Liquid Waste (HLLW) is critical for environmental and human health protection. The presence of platinum group metals (PGMs) in HLLW, particularly their refractory nature due to their high melting points, complicates vitrification processes. This study presents a targeted adsorption strategy using COF-42 for Pd2+ sequestration in HLLW systems. The comprehensive characterization of COF-42 and its Pd-loaded counterpart (Pd@COF-42) via PXRD, FT-IR, TGA, XPS, and SEM confirms structural robustness and successful Pd2+ incorporation. Kinetic and thermodynamic analyses reveal pseudo-second-order adsorption behavior with a maximum capacity of 170.6 mg/g, highlighting the exceptional Pd2+ affinity. Systematic optimization identifies HNO3 concentration (≤3 M) and adsorbent dosage (≤30 mg) as critical parameters governing adsorption efficiency through protonation–deprotonation equilibria. Furthermore, COF-42 exhibits superior selectivity for Pd2+ over 13 competing metal ions and maintains ~80% adsorption efficiency after four regeneration cycles. These performance metrics originate from the synergistic interplay of (1) framework flexibility enabling adaptive Pd2+ coordination, (2) hierarchical porosity facilitating ion diffusion, and (3) dense –NH/–NH2 groups acting as electron-rich chelation sites.