Visualizing Reversible Uranyl (UO₂ ²⁺) Capture and Release With Self-Indicating Porous Organic Polymers

Seawater uranium extraction is a promising sustainable energy pathway; however, practical application requires advanced adsorbents that combine reversible, high uranium-uptake capacity with the ability to track sorption cycles. Herein, we report self-indicating porous organic polymers (POPs), Sal-POP and Im-POP, containing uranyl (UO₂ ²⁺) ion-coordinating salphen and o-hydroxy imine cores with UO₂ ²⁺ binding energies up to -71.94 kcal mol^-1 as uranium adsorbents, and demonstrate their reversible, high uranium extraction capacities up to 0.114 g g^-1 h^-1. Enabled by the extended conjugation of salphen unit and strong UO₂ ²⁺ binding, Sal-POP readily changes color at uranium saturation from yellow to black and allows visual tracking of uranium adsorption/desorption cycles. With a high density of UO₂ ²⁺ binding sites, both Sal-POP and Im-POP demonstrate rapid, reversible, and high uranium uptake capacity, with distribution coefficients (K _d) of 1.88 × 10⁵ and 2.94 × 10⁵ mL g^-1, and removal efficiencies up to 70% and 78%. The uranyl capture is highly reversible, and the POP color changes from black to yellow upon simple elution with Na₂CO₃, thus acting as a self-indicating POP adsorbent.