Bidirectional reactant flux coupling in hollow hierarchical covalent organic framework enabling efficient uranium...

Photocatalytic uranium extraction can surpass the intrinsic capacity limits of adsorption by converting soluble uranyl into insoluble uranium peroxides via H₂O₂ generation. However, this process requires simultaneous local enrichment of H₂O₂ and UO₂²⁺ beyond the solubility product constant, which is difficult to achieve in continuously flowing and ultra-dilute seawater. Here we report a hollow hierarchical covalent organic framework (COF) microcavity reactor, HH-COF-(CN/AO)_x, that spatially decouples H₂O₂ generation from uranyl capture to overcome this thermodynamic barrier via a bidirectional reactant flux coupling strategy. A cyano-functionalized inner layer produces and stores H₂O₂ in a central cavity, whereas an amidoxime-rich outer layer selectively enriches uranyl ions. The convergence of outward H₂O₂ flux and inward uranyl adsorption establishes a persistent high-concentration interface that supports continuous formation of insoluble uranium peroxide. The optimized HH-COF-(CN/AO)_0.35 achieves 25.1 mg g^-1 uranium uptake in natural seawater, 3.9 times that of the non-hollow analog, which demonstrates a generalizable strategy for manipulating reactant fluxes in ultra-dilute environments for realizing effective resource extraction from water environment.