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.