<p>The effect of fiber shape on UO₂²⁺ adsorption performance was investigated using three polyacrylonitrile (PAN)-based fibrous adsorbents: commercial spun yarn (PAN-Y), nonwoven (PAN-NW), and electrospun nanofiber (PAN-NF). All fibers were functionalized via amidoximation, converting nitrile groups into amidoxime ligands. The PAN-NF exhibited nearly complete conversion (99.8%) of nitrile groups, compared with 96.5% and 89.5% for PAN-Y and PAN-NW, respectively. Structural analyses revealed that the nanofiber morphology was preserved after functionalization. Adsorption isotherm studies showed that nanofibrous adsorbent achieved a maximum UO₂²⁺ adsorption capacity of 99.9&#xa0;mg/g, significantly higher than adsorbents derived from PAN-Y and PAN-NW (≈30&#xa0;mg/g). The adsorption of nanofibrous adsorbent followed the Langmuir model, suggesting monolayer chemisorption onto homogeneous amidoxime binding sites, whereas yarn- and nonwoven-shaped adsorbents were better described by the Freundlich model, reflecting heterogeneous adsorption behavior. Kinetic analyses confirmed pseudo-second-order fitting for all samples, indicating chemisorption as the dominant mechanism. A 60 day field test in Caspian Sea seawater demonstrated that PAN-NF adsorbent achieved over 800-fold higher UO₂²⁺ adsorption compared with commercial fibers, along with enhanced selectivity over VO²⁺.</p> Graphical abstract <p></p>