A Defective Zr(IV) Metal–Organic Framework for Capturing Uranium from Mildly Acidified Seawater under Continuous-Flow Conditions

Abstract With terrestrial uranium reserves declining, the vast amount of uranium in seawater represents an indispensable, sustainable resource. Exploiting this reservoir not only advances the remediation of aquatic environments from toxic metals but also secures the long-term nuclear fuel supply. Here, we report the synthesis and characterization of a robust UiO-66-type metal–organic framework with the formula [Zr6O4(OH)6.6(MTATP)4.7(H2O)2.6]·solvents (MOR-4, metal–organic resin-4), where H2MTATP = 2-((3-(methyl-thio)propyl)amino)terephthalic acid, featuring defective Zr6 clusters interconnected by amino/thioether-functionalized ligands. Batch sorption studies show rapid and efficient removal of uranyl ions (UO22+) under acidic and neutral conditions, achieving equilibrium within minutes and reducing uranium concentrations to <2 ppb, well below the World Health Organization’s drinking water threshold (30 ppb). The sorbent exhibits high selectivity for uranium in the presence of competing ions, with a modest reduction in performance observed under carbonate-rich conditions. The excellent sorption properties of the MOF are due to the strong binding of uranyl cations to the MOF’s defect sites, as revealed by a series of experimental data. Most notably, the sorbent successfully extracts uranium from mildly acidified seawater samples, demonstrating its remarkable selectivity under high-competition conditions. For practical applications, a column packed with MOR-4/Calcium Alginate beads and silica sand was used to remove and recover uranium from the acidified seawater under continuous-flow conditions. The results exceeded expectations, revealing U removal and recovery above 95% and 85%, respectively, across two sorption/desorption cycles, indicating the strong potential of the material for uranium sequestration from seawater. Furthermore, treating MOR-4 with either a strong acid (pH ∼ 0) or a base (pH ∼ 12) resulted in materials with increased defects and larger pore sizes than those of the pristine material, providing a route to isolate materials with enhanced properties.