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[Display omitted] • The effect of hydration shells on uranium adsorption kinetics is deeply investigated. • The external diffusion can be facilitated due to fast water exchange. • The increasing temperature would reduce the apparent friction during mass transfer. • The adsorption equilibrium time can be shortened from 32 days down to 18 days. Identifying mass transfer is essential for boosting adsorption kinetics. The internal and external diffusions serve as rate-limiting steps. The liquid film diffusion is commonly identified as external diffusion, while the mass diffusion across the ubiquitous hydration shell is generally ignored. Charged functional groups can bind water molecules strongly via electrostatically induced hydration. Herein, three representative polar polymers, including cation polymer (PDMAPAA), anion polymer (PSS) and zwitterion polymer (PSBMA), are crosslinked with polyamioxime (PAO) to synthesize radially aligned PAO-PDMAPAA, PAO-PSS, PAO-PSBMA, respectively, by a bidirectional freeze-casting. We focus on the effect of hydration structure and hydration dynamics on marine uranium extraction performance. It is found that the external diffusion in PAO-polar polymer adsorbents can be facilitated due to fast water exchange in dynamic hydration shell, leading to a reduced adsorption equilibrium time compared with the pristine PAO. However, the high hydration degree causes strong viscous shear and high dehydration barrier, which impede the complexation of UO 2 2+ with binding sites of PAO. Although the hydration degree follows the order of PAO-PSBMA > PAO-PDMAPAA > PAO-PSS, PAO-PDMAPAA delivers the shortest equilibrium time required. In addition, the increasing temperature would weaken viscous shear in static hydration shell, which equivalently reduces the apparent friction of hydrated uranyl ions. Consequently, light-irradiated GO@PAO-PDMAPAA adsorbent, in which GO acts as a photothermal mediator, yields a shorter equilibrium time (∼18 days) than PAO-PDMAPAA together with a high uranium adsorption capacity of 12.24 mg-U/g-Ads in natural seawater. These findings provide valuable guidance for enhancing uranium adsorption kinetics associated with hydration shell. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effect of hydrogel structure parameters on uranium sorption kinetics is studied. • Nano-structured hydrogels with interconnected micropores facilitate mass transfer. • Freeze-assisted soaking endows hydrogel with strong shrinkage resistance in seawater. • The adsorption equilibrium time can be shortened to 11 days in natural seawater. Synthetic hydrogels, which are 3D networks composed of crosslinked hydrophilic polymer chains, have been created using various crosslinked ways for uranium capture due to full exposure of chelate functional groups. However, the conventional honeycomb-like hydrogels exhibit non-connected micropore structure, which locks abundant water molecules and hence is unfavorable for mass transfer. Herein, we demonstrate a simple synthesis of the nano-structured hydrogel with activated nanogels as nano-crosslinkers (denoted as PGP hydrogel), and probe into the relationship between hydrogel structural parameters and uranium adsorption kinetics. It is found that the PGP hydrogel with a low crosslinking density of ∼0.02 mol/m3 possesses interconnected micropores, thereby facilitating diffusion of uranyl ions (UO 2 2+) in 3D polymeric networks. Furthermore, a freeze-assisted soaking strategy is proposed to endow PGP hydrogel (denoted as target hydrogel) with strong shrinkage resistance in natural seawater, resulting in a ∼40 % increase in water permeance to reach 369 Lm−2h−1MPa−1. Correspondingly, the adsorption equilibrium time is shortened to ∼11 days. Although the uranium uptake capacity of PGP hydrogels with and without freeze-assisted soaking treatment remain basically unchanged (∼9.73 mg-U/g-Ads), a high uranium adsorption rate of 0.88 mg/g/d for the target hydrogel surpasses most currently available polymer adsorbents. The presented strategy is generalizable to a broader range of applications involving the adsorption of economically valuable ions in seawater or brine lake. [ABSTRACT FROM AUTHOR]