Palladium(II) Modulation Enhances the Water Stability and Aqueous ⁹⁹TcO₄^–/ReO₄^– Removal Performance of Metal–Organic Frameworks

Abstract

Improving the water stability of metal-organic frameworks (MOFs) is essential for their use in water pollution treatment and environmental remediation, though it remains technically challenging. Herein, we report a novel cationic MOF constructed with [Th₆O₄(OH)₄(COO)₁₂] units and [CoN₄·Cl₂] units possessing a ftw-type topology (denoted as <b>1-Th-Co</b>). <b>1-Th-Co</b> itself exhibited poor water stability but excellent stability following a palladium(II) modulation strategy. Experimental studies reveal that Co(II) ions in <b>1-Th-Co</b> were replaced by Pd(II) ions through cation exchange in N,N-diethylformamide (yielding <b>1-Th-Pd</b>). The planar PdN₄ units in <b>1-Th-Pd</b> were responsible for improving the water stability of the framework. As a result, <b>1-Th-Pd</b> offered excellent stability, fast adsorption kinetics, and high removal ratios for ⁹⁹TcO₄^- and ReO₄^- (as a ⁹⁹TcO₄^- surrogate) in contaminated water. When used in packed columns, <b>1-Th-Pd</b> can dynamically capture ReO₄^- from groundwater. This work provides a new avenue for improving the water stability of MOFs, offering new vistas for the decontamination of aqueous solutions containing ⁹⁹TcO₄^- and ReO₄^-.