Exploring freezing-assisted catalyst-free photochemical recovery of low-concentration uranium

Catalyst-free photochemical uranium recovery is promising, but at low U(VI) concentrations slow uranyl peroxide precipitation can leave uranium in the dissolved phase despite H₂O₂ accumulation and apparent supersaturation, thereby limiting solid-liquid separation and uranium recovery. Here, we demonstrate that a subsequent freezing step overcomes this precipitation-initiation limitation. In a 20 mg L^-1 U(VI) solution, irradiation generated approximately 100 μmol L^-1 H₂O₂ but left U(VI) largely dissolved, whereas freezing at -20 °C achieved 96.2% U(VI) removal and 92.8% uranium recovery, with the recovered solid identified as crystalline uranyl peroxide. H₂O₂ consumption was nearly equimolar with U(VI) precipitation, confirming direct peroxide incorporation. Confocal imaging showed H₂O₂ enrichment within intercrystalline liquid-like layers, while room-temperature concentration controls confirmed that solute enrichment increased the local concentration product and promoted precipitation. DFT calculations showed that the UO₂²⁺-H₂O₂ precursor binding energy decreased from -0.328 eV in bulk water to -0.535 eV at a single ice-water interface and -0.684 eV under dual-interface confinement. Independent gradient model based on Hirshfeld partition (IGMH) analysis further revealed multipoint weak interactions with both ice interfaces that stabilized the precursor complex. Thus, freezing overcomes kinetic barriers to catalyst-free recovery of low-concentration uranium through coupled freeze concentration and ice-confined precursor stabilization.