Cyclic oxygen vacancy reconstruction in MnO2 polymorphs builds electron reservoirs to drive ionic charge redistribution for stepwise UO22 + reduction

Effective uranium (UO₂²⁺) recovery from aqueous waste is vital for resource sustainability and environmental safety. Although oxygen vacancies enhance the redox activity of UO₂²⁺, their long–term stability and ability to regenerate remain unclear. Herein, we engineered α–, β–, γ–, and δ–MnO₂ through controlled NaH₂PO₂ reduction to create tunable oxygen vacancies. During the modulation of oxygen vacancies, partial phase transitions were induced, with α–MnO₂ and γ–MnO₂ maximizing structural adaptability and performance. Defect–rich α– and γ–MnO₂ exhibit significantly accelerated UO₂²⁺ reduction kinetics, retaining over 90.0 % of their initial activity after multiple redox cycles. Correspondingly, α–MnO₂–OVs and γ–MnO₂–OVs achieve maximum UO₂²⁺ adsorption capacities of 581.0 and 492.0 mg g⁻¹, respectively. In situ XRD reveals that oxygen vacancies act as reversible active sites for UO₂²⁺ adsorption and reduction in real rare–earth leachates, regenerating through lattice–oxygen recombination in the UO bond during redox cycling, thereby maintaining high catalytic activity. DFT shows that oxygen vacancies create unsaturated Mn sites that form Mn–O–U bridges, directing electrons from Mn 3d into U 5 f orbitals. The near–zero U density of states at the Fermi level confirms this Mn–O vacancy network acts as a localized electron reservoir, lowering the energy barrier for sequential UO₂²⁺ reduction. This work highlights cyclic oxygen vacancy engineering as an effective approach for efficient actinide separation and sustainable environmental remediation.