Phosphate-Functionalized Macroporous Cellulose Sponges for Uranium Recovery from Seawater

Cellulose-based adsorbents offer a sustainable platform for uranium recovery from seawater, but simple functionalization strategies that enable effective uranium capture under highly competitive marine conditions remain limited. Here, we report a post-phosphorylation strategy for introducing uranium-binding functionality into a preformed freeze-crosslinked carboxymethyl cellulose nanofiber (CMCF) sponge while retaining its interconnected macroporous architecture. Impregnation of the cellulose sponge with NH4H2PO4/urea followed by mild thermal treatment at 120 °C introduced up to 114 ± 18 mg P g-1 sponge. In seawater spiked with 10 mg L-1 U(VI), both nonphosphorylated and phosphorylated CMCF sponges showed similarly high U removal (>96%), largely obscuring the effect of phosphate functionalization. In contrast, a pronounced phosphate-dependent enhancement emerged under realistic seawater conditions: the nonphosphorylated sponge showed negligible U removal from natural seawater, whereas the post-phosphorylated P1-CMCF sponge achieved approximately 99% U removal within 24 h. SEM-EDX and XPS analyses further supported uranium accumulation at phosphate-rich cellulose interfaces and the involvement of phosphate-associated oxygen donor sites in U binding. The captured uranium was nearly quantitatively recovered using 0.1 mol L-1 NaHCO3, and adsorption performance was maintained over three adsorption-desorption cycles. These results demonstrate that simple post-phosphorylation of a preformed macroporous cellulose scaffold provides accessible uranium-binding interfaces while preserving the underlying transport architecture, offering a straightforward route to efficient and regenerable uranium recovery under chemically complex seawater conditions.