Directing Intermediates in Photocatalytic H₂O₂ Production to Boost Uranium Extraction from Seawater

Extracting uranium from seawater is crucial for ensuring the supply of uranium resources and the sustainable development of nuclear energy. Photocatalysis-assisted extraction strategy, where photo-catalytically generated H₂O₂ reacts with uranyl ions to form metastudtite, could effectively enhance the extraction capacity and kinetics, while the mechanisms remain unclear, especially in seawater. Herein, the relationship between photocatalytic H₂O₂ production mechanisms and subsequent uranium removal was investigated by using oxidized red phosphorus-doped carbon nitride (ORP-CN) and MOF-functionalized carbon nitride (MOF-CN). Both catalysts exhibited similar H₂O₂ production rates, while ORP-CN demonstrated a 13 times higher uranium extraction efficiency. We revealed that ORP-CN follows an indirect two-step 2e^- oxygen reduction reaction (ORR) pathway, producing ·OOH radical as intermediates. Whereas MOF-CN mainly followed a direct one-step 2e^- ORR process. DFT simulations demonstrated that ·OOH radical can coordinate with uranyl carbonate more efficiently than H₂O₂ at seawater interfaces, leading to rapid metastudtite formation. This mechanistic advantage endowed ORP-CN with exceptional uranium recovery property, achieving an extraction capacity of 6.73 mg/g/day in natural seawater without sacrificial agents. Our findings directly link H₂O₂ generation pathways to uranium extraction efficiency, highlighting reactive oxygen intermediates as critical drivers in seawater uranium recovery and providing a theoretical basis for designing advanced photocatalysts.