Hierarchical triple-channel architectures unlock scalable and high-efficient uranium extraction from seawater

Domain: physics
Published: 2025-12-01
Editorial outcome: minor_revision

Abstract
The ocean holds ~4.5 billion tons of uranium, a vast yet underexploited resource for sustainable nuclear energy. However, current seawater uranium adsorbents suffer from poor macroscale structures, limited mass transfer, and inefficient functional group utilization. Here, we report a bioinspired honeycomb-like polyamidoxime adsorbent featuring an interconnected hierarchical triple-channel architecture (HTC-PAO) that addresses these challenges. Its hierarchical structure incorporates millimeter-scale honeycomb channels, sub-millimeter transverse channels, and intrinsic micropores. The adsorbent is 100 times thicker than conventional hydrogels, enabling practical deployment in marine environments while maintaining excellent adsorption performance. Mass transfer analysis and COMSOL simulations reveal two synergistic pathways: (1) accelerated laminar flow through macrochannels (Reynolds number Re=922.50) and (2) diffusion-enhanced transport within microchannels (effective diffusivity D_eff = 6.96×10-9 vs. 1.74×10-9 m2/s), working together to maximize uranium capture and functional group accessibility. As a result, HTC-PAO achieves a uranium adsorption capacity of 14.69 mg/g over 35 days under conditions without external energy input, while also showing high ion selectivity, reusability, and cost efficiency. These findings demonstrate that the hierarchical triple-channel architecture enables highly efficient and scalable uranium extraction from seawater.

Introduction
Achieving global carbon neutrality and addressing the escalating challenges of climate change demand transformative advancements in sustainable energy systems1. Nuclear energy has emerged as a cornerstone of this transition, offering a highly efficient and low-emission alternative to fossil fuels. However, the long-term viability of nuclear energy depends heavily on the availability of uranium, its primary fuel source2,3. Currently, uranium is predominantly sourced from terrestrial uranium ores (U3O8), but these reserves are finite and projected to be depleted within the next century. This limitation poses a significant challenge to meeting the growing demand for nuclear energy, especially as more countries adopt nuclearpower to achieve decarbonization goals4,5. In this context, seawater uranium extraction represents a revolutionary technology, since the ocean contains approximately 4.5 billion tons of dissolved uranium, which is about 1000 times the known terrestrial uranium reserves6. Unlike terrestrial reserves, seawater uranium is constantly replenished through geological processes, representing an essentially inexhaustible resource7. However, the ultra-low concentration of uranium (~3.3 ppb) and the presence of competing ions such as sodium and magnesium significantly hinder economically viable extraction, making this a grand scientific and technological challenge8,9.

Over the past few decades, various methods have been developed to extract uranium from seawater, including ion exchange, adsorption, chemical precipitation, biological capture, photocatalysis, and electrochemistry10. COF-based photocatalytic and electrochemical approaches have demonstrated excellent uranium uptake in recent reports, achieving high adsorption efficiency through the synergistic combination of catalytic H2O2 generation and uranyl chelation or precipitation11,12. These methods, however, require external energy input and more elaborate material synthesis, which may limit their broader practical application and scalability13. In contrast, amidoxime-based adsorbents, particularly polyamidoxime (PAO) hydrogels, exhibit remarkable selectivity and strong affinity for uranyl ions (UO22+)14. They can be synthesized directly from nitrile-containing precursors via oximation reactions, offering cost-effective, scalable, and versatile production routes15. PAO hydrogels, with abundant amidoxime groups and excellent hydrophilicity, have been extensively studied for uranium extraction. Nevertheless, their three-dimensional crosslinked networks trap large amounts of water through hydrogen bonding16, significantly reducing water mobility and uranyl ion diffusion, thereby limiting access to active binding sites and reducing overall adsorption performance.

To address these limitations, various strategies have been explored, including hydrogel-based adsorbents in the form of ultrathin films17 (~100 μm) or superfine fibers18 (~250 nm). These designs increase the contact area between the adsorbent and seawater, enhancing uranyl ion diffusion and improving the utilization of functional groups. However, such materials often suffer from poor mechanical properties, are prone to damage, and present challenges in recovery, making them unsuitable for practical deployment in real marine environments. An alternative promising approach involves constructing uranyl ion transport channels within hydrogel materials. Techniques such as ice templating19, pore-forming agents20, and bacterial fermentation21 have been employed to enhance pore structures. While these methods have achieved some improvements in adsorption performance, the resulting adsorbents typically suffer from small pore sizes, uneven pore distribution, and limited pore connectivity, which constrain further performance enhancements. Therefore, the development of innovative materials with advanced pore architectures is essential to significantly improve adsorption capacity and operational efficiency.

Leaf veins efficiently transport water and nutrients through interconnected hierarchical networks, while natural honeycombs exemplify bulk porous structures with thin-walled, hexagonally arranged channels (Fig. 1a). Inspired by these optimized biological systems, we have developed a bulk adsorbent featuring a honeycomb-like hierarchical porous architecture for efficient and scalable uranium extraction from seawater (Supplementary Fig. 1). Its thickness (10 mm) is a hundred times greater than that of conventional ultrathin films (100 μm)17,21. This structural design significantly enhances mechanical stability, thereby facilitating deployment and recovery in marine environments. By integrating mass transfer analysis with COMSOL simulations, we demonstrate that this material features a multi-level interconnected channel network. This network comprises hexagonal longitudinal (5 mm), transverse (100 μm), and intrinsic (2 μm) channels, which enable two synergistic pathways: (1) laminar flow through macrochannels (Reynolds number Re=922.50) and (2) diffusion-enhanced transport within microchannels (effective diffusivity D_eff = 6.96×10−9 vs. 1.74×10−9 m2/s). Together, these pathways maximize uranium capture and functional group accessibility. In natural seawater, HTC-PAO achieved a uranium adsorption capacity of 14.69 mg/g after 35 days under conditions without an external energy field, with capacities 2.40 and 1.42 times those of intrinsic micropore polyamidoxime (IM-PAO) and large-pore polyamidoxime (LP-PAO), respectively. By providing a scalable and efficient solution, this study marks a significant step forward in the sustainable utilization of seawater uranium resources, offering new opportunities for securing the future of nuclear energy.

Results
Material preparation and morphology analysis
PAO was synthesized via the oximation reaction of PAN. To systematically investigate the impact of the internal channel structures of adsorbents on adsorption performance, three adsorbents with different internal channel structures were prepared. The PAO was directly crosslinked with glutaraldehyde and freeze-dried to obtain IM-PAO. LP-PAO was prepared by mixing PAO with glutaraldehyde, pouring the mixture into silicone molds with honeycomb columns (Supplementary Fig. 2a), and freeze-drying it. HTC-PAO was prepared by pouring the mixture into a brass-framed mold (Supplementary Fig. 2b, c). A directional temperature gradient was then applied to induce inward ice crystal growth toward the center, after which the sample was freeze-dried (Supplementary Fig. 3). The resulting HTC-PAO was approximately five times thicker than a coin (Supplementary Fig. 4). The material was characterized using scanning electron microscopy (SEM). The result confirmed that IM-PAO retained only the intrinsic microporous structure of PAO (Supplementary Fig. 5a). LP-PAO exhibited large honeycomb-like hexagonal longitudinal pores while maintaining the intrinsic microporous structure of PAO (Supplementary Fig. 5b, c). In contrast, HTC-PAO exhibited an interconnected triple-channel architecture, comprising hexagonal longitudinal (5 mm), transverse circular (100 μm), and intrinsic polyamidoxime (2.5 μm) channels (Fig. 1b). The wall thickness between adjacent hexagonal macroscopic channels is approximately 500 μm.

Analysis of mass transfer mechanisms
As shown in Fig. 2a, adsorption kinetics is governed by a multistep uranyl ion transport process, involving convection transfer resistance at the external surface (R_surf), the diffusion transfer resistance within the porous adsorbents (R_diff), and the apparent reaction resistance at the sorption interface (R_react)22,23. Under continuous seawater flow, uranyl ions rapidly reach the adsorbent surface and coordinate with amidoxime groups, resulting in low R_surf and R_react. Intraparticle diffusion through tortuous pores is inherently slow, making R_diff the dominant factor governing adsorption performance. Structural parameters such as external surface area (A_ext), diffusion depth (δ), tortuosity coefficient (τ), and sorption surface area (A_sorb) can be tuned to reduce all contributions to total resistance (R_total) and thus enhance overall adsorption. Supplementary Fig. 6 presents a digital photograph of HTC-PAO. Compared to conventional circular holes, hexagonal holes offer superior tessellation, effectively reducing and homogenizing wall thickness. On an equal mass basis, the honeycomb channel affords 1.55 times the external surface area (A_ext) of the triangular circular-hole design and 2.39 times that of the rectangular arrangement (detailed calculation methods are provided in the Supplementary Text 1). In addition, it exhibits a smaller maximum diffusion depth (δ_max) compared with both configurations (Fig. 2b). As a result, the honeycomb-like polyamidoxime adsorbent exhibits lower surface resistance (R_surf) and diffusion resistance (R_diff), enhancing overall mass transfer. Moreover, the geometric properties of the hexagonal holes alleviate stress concentration24, making this configuration promising for multidisciplinary applications.

As shown in Fig. 2c, IM-PAO retains only the intrinsic microporous structure without forming additional channels. This structure is characterized by a small external surface area (A_ext), long diffusion depth (δ), high tortuosity coefficient (τ), and limited sorption surface area (A_sorb), resulting in high R_surf, R_diff, and R_react. In contrast, LP-PAO features a large honeycomb pore structure while maintaining the intrinsic microporous framework. This configuration increases A_ext and reduces δ, thereby lowering R_surf and R_diff. However, its τ remains relatively high, and A_sorb remains limited. HTC-PAO, on the other hand, possesses a hierarchical pore structure that not only further reduces δ compared with LP-PAO but also decreases τ, leading to an even lower R_diff. The presence of interwall transverse pores significantly increases A_sorb, leading to lower R_react than both IM-PAO and LP-PAO. Consequently, HTC-PAO exhibits significantly lower total mass transfer resistance (R_total) compared to IM-PAO and LP-PAO. The mass transfer characteristics and adsorption performance of adsorbents with different internal channel structures were further investigated through experiments and simulations.

Material characterization and adsorption performance in simulated seawater
Fourier Transform Infrared (FTIR) spectroscopy was used to analyze PAN, PAO, IM-PAO, LP-PAO, and HTC-PAO (Supplementary Fig. 7). The FTIR spectrum of PAN exhibited a characteristic C≡N stretching vibration at 2240 cm−1. In contrast, PAO, IM-PAO, LP-PAO, and HTC-PAO displayed two distinct absorption peaks at 934 cm−1 (N─O) and 1598 cm−1 (C=N), with the C≡N band absent, confirming the successful conversion of the nitrile group in PAN to the amidoxime functional group. The amidoxime group in PAO was further confirmed by high-resolution N1s XPS, which showed two characteristic peaks of C=N and N─H in IM-PAO, LP-PAO, and HTC-PAO (Supplementary Fig. 8). Four deconvoluted peaks were detected in the high-resolution C1s spectra of IM-PAO, LP-PAO, and HTC-PAO, corresponding to C─C, C=N, C=O, and O─C=O bonds (Supplementary Fig. 9). These results provided further confirmation of the successful conversion of the C≡N group to the C=N group.

Mercury intrusion porosimetry (MIP) measurements showed that the pore volume of HTC-PAO was approximately 1.5 times that of IM-PAO and LP-PAO (Fig. 3a). Analysis of the mercury intrusion-extrusion curves revealed that for IM-PAO (Supplementary Fig. 10a) and LP-PAO (Supplementary Fig. 10b), the curves overlapped, indicating that mercury could not exit once it entered the material. In contrast, the extrusion curve of HTC-PAO (Supplementary Fig. 10c) was lower than its intrusion curve, demonstrating partial mercury release. This result suggests that HTC-PAO exhibits superior mass transfer performance and improved internal pore connectivity. As shown in Supplementary Fig. 11, the specific surface area of HTC-PAO (1.93 m2 g−1) was 1.95 and 1.25 times that of IM-PAO and LP-PAO, respectively, providing more available adsorption sites. Correspondingly, HTC-PAO exhibited markedly higher uranium adsorption capacity (1.80 times and 1.27 times that of IM-PAO and LP-PAO, respectively), confirming that the enlarged pore volume and surface area play a critical role in the observed adsorption enhancement. The seawater transport capacity of IM-PAO, LP-PAO, and HTC-PAO was tested by bringing their surfaces into contact with a Rhodamine B solution (Fig. 3b). Due to the introduction of the interconnected triple-channel architecture, the seawater transport speed in HTC-PAO was 7.8 mm/s, which is 2.04 and 1.3 times faster than IM-PAO and LP-PAO, respectively. The experimental results are consistent with the theoretical analysis of the mass transfer process. For details on the specific experimental process, please refer to the methods. Water contact angle measurements confirmed that HTC-PAO exhibited superior hydrophilicity, improving the compatibility between the adsorbent surface and water molecules in the solution (Supplementary Fig. 12). This enhanced interaction with uranyl ions in the aqueous solution, thereby promoting the adsorption process. Additionally, compared to thin-film adsorption materials (Supplementary Fig. 13), hexagonal large-channel bulk adsorption materials exhibit superior mechanical properties (Fig. 3c).

To evaluate the uranium adsorption capacity of IM-PAO, LP-PAO, and HTC-PAO, three adsorbents were immersed in uranium-simulated seawater at a concentration of 8 ppm. The adsorption performance was found to be pH-dependent, as pH affected both the surface charge of the adsorbent and the speciation of uranium. As illustrated in Fig. 3d, the uranium adsorption capacity of IM-PAO, LP-PAO, and HTC-PAO exhibited a pH-dependent variation. The adsorption capacity initially increased and then decreased as the pH was raised from 3 to 9, with the peak uranium uptake occurring at pH 6. Under acidic conditions (pH < 4.0), uranyl ions (UO22+) predominantly exist in a positively charged state. In this environment, the adsorbent surface also carries a positive charge, leading to strong electrostatic repulsion between the adsorbent and uranyl ions. This repulsion prevents effective interactions between the adsorbent’s functional groups and uranyl ions, resulting in low uranium adsorption efficiency. As pH increases, the adsorbent surface becomes less positive, reducing electrostatic repulsion and facilitating stronger interactions with uranyl ions, which enhances adsorption. However, at pH levels above 6.0, uranyl ions form negatively charged carbonate complexes, such as UO2(CO3)22−, UO2(CO3)34−, and UO2(CO3)64−. These complexes repel the negatively charged adsorbent surface, decreasing electrostatic attraction and reducing uranium adsorption capacity at higher pH values. Notably, HTC-PAO demonstrated superior uranium adsorption capacity compared to IM-PAO and LP-PAO across all pH ranges, underscoring the critical role of its hierarchical porous structure in improving sorption performance.

To clarify the adsorption mechanism, the kinetics of uranium adsorption by IM-PAO, LP-PAO, and HTC-PAO in simulated seawater were investigated. The adsorption performance of HTC-PAO for uranium (VI) was evaluated by immersing the adsorbent in aqueous solutions with uranium concentrations of 4, 8, and 16 ppm at pH 6 for a duration of 48 h. The residual uranium concentration was quantified using UV–visible absorption spectroscopy, with arsenazo (III) employed as the colorimetric reagent. After 48 h of contact, the saturated uranium adsorption capacities of HTC-PAO were determined to be 391.2, 782.4, and 1100.24 mg/g for the 4, 8, and 16 ppm solutions, respectively (Fig. 3e). Additionally, the adsorption performances of IM-PAO and LP-PAO were also evaluated (Supplementary Fig. 14). The uranium adsorption kinetics for IM-PAO, LP-PAO, and HTC-PAO, measured at various time intervals, are summarized in Supplementary Table 1. The saturated adsorption capacities of IM-PAO and LP-PAO were much lower than the HTC-PAO under the same conditions. The observed kinetic differences highlighted the advantages of using adsorbents with hierarchical porous structures, as these materials promote efficient filling of internal channels and rapid ion diffusion, thereby facilitating the adsorption process.

The chemical composition of uranium-loaded HTC-PAO was investigated via X-ray photoelectron spectroscopy (XPS) to verify the adsorption process (Fig. 3f). For comparative analysis, the chemical compositions of IM-PAO and LP-PAO were also examined (Supplementary Fig. 15). EDS analysis revealed a uniform distribution of uranium in HTC-PAO, along with the presence of additional elements including carbon (C), nitrogen (N), and oxygen (O), as illustrated in Supplementary Fig. 16. Figure 3g displayed the characteristic doublet peaks of U4f5/2 (392.36 eV) and U4f7/2 (381.65 eV) in the XPS spectrum of uranium-loaded HTC-PAO, thereby confirming the adsorption of uranium on the HTC-PAO surface relative to the unadsorbed state. Likewise, IM-PAO and LP-PAO exhibited corresponding characteristic peaks, providing further evidence of uranium adsorption on their surfaces post-adsorption (Supplementary Fig. 17). Supplementary Fig. 18 presented digital images of HTC-PAO before and after uranium adsorption, showing a distinct color change from pale yellow to orange-red upon uranium uptake.

The adsorption kinetics of IM-PAO, LP-PAO, and HTC-PAO were analyzed using both pseudo-first-order and pseudo-second-order models (Fig. 3h and Supplementary Fig. 19). For all three adsorbents, the pseudo-second-order model yielded higher R2 values than the pseudo-first-order model (closer to 1), indicating a better fit. This suggests that the adsorption process is governed primarily by chemisorption rather than physisorption, essentially involving coordination between PAO functional groups and uranyl ions. The kinetic fitting parameters are summarized in Supplementary Table 2. In an 8 ppm uranium solution, the equilibrium adsorption capacities (qe) of IM-PAO, LP-PAO, and HTC-PAO were 433.97, 612.07, and 779.28 mg/g, respectively. Notably, HTC-PAO achieved an adsorption capacity 1.80 times greater than IM-PAO and 1.27 times higher than LP-PAO, highlighting the superior uranium uptake enabled by its hierarchical porous structure. Uranium concentrations in simulated seawater were varied within the range of 4 to 150 ppm, and the adsorption isotherms for IM-PAO, LP-PAO, and HTC-PAO were systematically analyzed. The uranium adsorption data were well fitted by the Langmuir isotherm model, indicating that the adsorption process is primarily monolayer chemisorption, which is consistent with the adsorption kinetics analysis. The theoretical maximum adsorption capacity (qm) of HTC-PAO was calculated to be 1343.6 mg/g (Fig. 3i and Supplementary Table 3), significantly higher than that of IM-PAO (712.16 mg/g) and LP-PAO (1024.38 mg/g) (Supplementary Fig. 20). These results demonstrate that HTC-PAO possesses markedly enhanced uranium adsorption performance in simulated seawater, underscoring its strong potential for practical application in marine uranium extraction.

Mathematical modeling and transient analysis of adsorption kinetics
To elucidate the effects of honeycomb channels and hierarchical porous architecture on mass transfer and uranyl adsorption, COMSOL Multiphysics was employed to simulate the uranium adsorption from seawater. Simulation models of IM-PAO, LP-PAO, and HTC-PAO were constructed under dynamic flow conditions, with seawater entering from the top and exiting from the bottom (Supplementary Fig. 21). The detailed boundary conditions are provided in the Supplementary Text 2, and the model parameters are listed in Supplementary Tables 4–6. A coupled dual-physics model was developed, comprising two modules: (1) the laminar flow module, which describes seawater flow and thereby characterizes the transport of uranyl ions to the adsorbent surface; and (2) the transport of dilute species module, which simulates diffusion of uranyl ions within both seawater and the porous adsorbent, incorporating an adsorption term to represent coordination with functional groups at the adsorption sites. Detailed material parameters are provided in Supplementary Table 7. By coupling these two modules, the intrinsic relationships among flow velocity, concentration distribution, and adsorption capacity were quantitatively analyzed.

The computational domain was meshed using two schemes: hexahedral meshes for the adsorbent and free tetrahedral meshes for the surrounding seawater. The hexahedral meshes ensured high mesh quality and numerical stability, improving computational efficiency while maintaining accuracy. In contrast, the tetrahedral meshes provided greater geometric adaptability and supported local refinement, allowing accurate resolution of concentration gradients. Additional local mesh refinement in the central adsorption region further optimized the balance between simulation accuracy and computational cost. To ensure the accuracy of the results, a mesh convergence study was conducted by comparing the results from the coarse (128,304 elements), medium (693,073 elements), and fine (890,922 elements) meshes. Key output parameters, including the velocity field, concentration profile, and adsorption capacity, showed less than 1% variation between the medium and fine meshes, indicating good convergence (Supplementary Table 8). As a result, the chosen mesh with approximately 693,000 elements was selected adequate for ensuring reliable simulation results.

As shown in Fig. 4a, the honeycomb channels in LP-PAO and HTC-PAO effectively reduced flow bypass and increased internal flow velocity, thereby enhancing uranyl diffusion into the adsorbent. This is supported by the concentration contour plots in Fig. 4b. Compared with IM-PAO, LP-PAO exhibited a 15.56% increase in diffusion depth. Meanwhile, HTC-PAO achieved a 99.48% increase in diffusion depth compared to LP-PAO. These results demonstrate that the hierarchical porous design significantly improves mass transfer efficiency. Furthermore, the simulation-predicted uranium adsorption capacities show good agreement with experimental data, as illustrated in Fig. 4c. The maximum deviation for IM-PAO, LP-PAO, and HTC-PAO was less than 17%, and the average error remained below 7% (Supplementary Table 9), confirming the accuracy and robustness of the simulation model. Using the simulation model, the time-dependent values of R_surf, R_diff, and R_react were determined based on the simulated uranyl adsorption kinetics and the spatial concentration distributions within the adsorbent matrix (Fig. 4d–f). Among the three resistance components, R_diff accounted for the majority of the overall mass transfer resistance, highlighting the critical role of internal diffusion in limiting adsorption performance. Notably, HTC-PAO exhibited a significantly lower R_diff than IM-PAO and LP-PAO, supporting the proposed mechanism that its hierarchical triple-channel architecture enhances mass transfer efficiency by reducing both the effective diffusion depth (δ) and tortuosity factor (τ). In addition, HTC-PAO also shows a markedly reduced R_react attributed to its larger specific surface area, which is consistent with the mass transfer analysis. In contrast, the R_surf of HTC-PAO, IM-PAO, and LP-PAO are comparable, suggesting that under the influence of the flow field, uranyl ions can easily reach the surface of the adsorbent, and the impact of the difference in material morphology is weakened.

Reusability, selectivity, and adsorption performance in natural seawater
Reusability is a crucial parameter for assessing the economic viability and long-term stability of adsorbent materials. To evaluate the regeneration potential of HTC-PAO, its uranium adsorption capacity was determined after multiple adsorption-desorption cycles. Uranium-loaded HTC-PAO was regenerated by immersion in an elution solution consisting of 1 M Na2CO3 and 0.1 M H2O2 for 1 h. Cyclic testing revealed consistent performance characteristics, with the adsorbent sustaining a uranium adsorption capacity of 671.55 mg/g and achieving a desorption rate of 91.3% over five operational cycles (Supplementary Fig. 22 and Supplementary Table 10). The sustained reusability of HTC-PAO demonstrates its capacity for long-term uranium extraction without significant performance degradation, thereby reducing operational costs. The mechanism of the desorption process is as follows: uranium desorption is achieved through complexation chemistry, where concentrated carbonate solutions produce [UO2(CO3)3]4− complexes. The elution efficiency is enhanced when hydrogen peroxide is incorporated into the carbonate system, resulting in the formation of [UO2(O2)(CO3)2]4− species with superior stability. The addition of hydrogen peroxide enhances the formation of this complex, which is thermodynamically more stable than the uranyl tris-carbonato complex, with stability four orders of magnitude greater. This increased stability facilitates the release of uranyl ions from the adsorbent surface, allowing efficient desorption. The gradual attenuation of the uranium adsorption capacity during the reusability tests of HTC-PAO is attributed to two main factors. First, some of the binding sites on the adsorbent become occupied by tightly bound uranyl ions, which are difficult to elute due to the strong coordination between the uranyl ions and the amidoxime groups. These tightly bound ions are not easily displaced by the elution solution. Second, the elution process itself may cause partial damage to the binding sites, reducing their ability to effectively capture uranyl ions in subsequent cycles. Over time, this degradation of active sites leads to a decrease in the adsorbent’s overall capacity.

For the selectivity assessment, the competitive adsorption behavior of HTC-PAO was investigated within a simulated seawater matrix, incorporating coexisting ions25 such as U, V, Mn, Co, Ni, Cu, Pb, Zn, and Cr (Supplementary Table 11). Notably, vanadium, exhibiting comparable charge and ionic radius to uranium, competed for adsorption sites on the surface of HTC-PAO. Despite this competition, uranium demonstrated the highest selective adsorption capacity among the ions tested (Fig. 5a). These results emphasized the exceptional selectivity of HTC-PAO for uranium, thereby positioning it as a promising candidate for the targeted and efficient extraction of uranium from complex, naturally occurring seawater matrices. To simulate natural seawater conditions, a recirculating marine water system was constructed for evaluating HTC-PAO’s uranium recovery efficiency, with the system configuration presented in Supplementary Fig. 2326. In this experimental setup, 10 mg of HTC-PAO was placed between two layers of sponge in the seawater test tube, and the flow rate of the seawater was adjusted to ensure consistent and uniform flow during the adsorption process. This configuration allows for more precise monitoring and facilitates the effective collection of the adsorbent after the uranium extraction test. It also prevents material displacement, allowing continuous circulation of natural seawater through the system. After 35 days of continuous adsorption, the adsorption capacity of HTC-PAO reached 14.69 mg/g, which was 2.40 times and 1.42 times that of IM-PAO and LP-PAO (Fig. 5b). Adsorption kinetics analysis (Fig. 5c) showed that HTC-PAO not only had superior uranium adsorption capacity, but also significantly faster adsorption rate compared with IM-PAO and LP-PAO. HTC-PAO reached the equilibrium adsorption level of IM-PAO after 6 days and achieved LP-PAO’s equilibrium level by day 12. Therefore, the hierarchical porous channels of HTC-PAO promotes the rapid diffusion of uranyl ions, significantly speeds up the adsorption process, and maximizes the adsorption capacity of the polyamide oxime hydrogel adsorbent. Without external energy input, HTC-PAO exhibits a remarkably high uranium adsorption capacity among known macroscale polyamidoxime adsorbents in natural seawater (Fig. 5d and Supplementary Table 12).

Methods
Synthesis of PAO
PAO was synthesized according to existing literature. First, 5.6×g of NH2OH⋅HCl was completely dissolved in 100 mL of DMF. To achieve a neutral pH in the solution, 4.12 grams of sodium carbonate (Na2CO3) and 1.6 grams of sodium hydroxide (NaOH) were meticulously incorporated, followed by an hour-long stirring at a controlled temperature of 45°C. Then, the mixture was combined with 4×g of PAN and stirred at 65°C for 24 h. Afterward, 30 mL of DMF solution containing 2.8×g of NH2OH⋅HCl, 0.8×g of Na2CO3, and 2.06×g of NaOH was added. The resulting mixture was reacted at 65°C for another 24 h. After the reaction reached completion, the liquid phase was separated using high-speed centrifugation (1.08×104×g, 25 min) to obtain the supernatant. The supernatant was dropped into anhydrous ethanol using a dropper with a rubber tip and mechanically stirred for 24 h to completely remove the residual dimethylformamide. The mixture was filtered and then vacuum dried at 40°C to obtain PAO powder.

Preparation of HTC-PAO
To commence, introduce 1.2×g of PAO powder into 12 mL of a 0.3 M NaOH solution and ensure thorough mixing; subsequently, incorporate 400 μL of glutaraldehyde solution and blend meticulously until homogeneity was achieved. Next, pour the mixture into a silicone mold and place a brass frame around the edges of the mold. Then, place it in the freezer for 12 h. Remove the material from the mold and freeze-dry it for 24 h to obtain a large block of HTC-PAO honeycomb material. For the control sample LP-PAO, the preparation steps were the same as for HTC-PAO, except that after the mixture is poured into the silicone mold, it was placed directly in the refrigerator to freeze, but the mixture was directly poured into the silicone box for freezing, and then demolded and dried. To ensure smooth demolding, the following precautions were taken: (1) demolding was performed only after the freeze-drying process was fully completed, as the fully dried sample possesses sufficient mechanical strength to withstand stress; (2) the sample was carefully and slowly separated from the mold starting from the peripheral edges, applying a uniform force parallel to the channel direction. Before conducting uranium adsorption experiments, HTC-PAO was treated with alkali by soaking it in a 20 mM NaOH solution at 60°C for about 30 min.

Characterization
The diverse array of functional groups present within the polymers was characterized through the application of Fourier transform infrared spectroscopy (FT-IR, PerkinElmer Frontier). Elemental analysis was conducted utilizing X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi), a sophisticated technique renowned for its precision and depth of insight. Morphological analysis was conducted using a tungsten filament scanning electron microscope (Axia ChemiSEM). Structural characterization of HTC-PAO, LP-PAO, and IM-PAO was performed via mercury intrusion porosimetry (AutoPore V9620) to evaluate their pore size distributions. The hydrophilic properties of the material were assessed utilizing a sophisticated contact angle apparatus (Dataphysics OCA50). Uranium concentrations were quantified employing a UV–visible spectrophotometer (UV1800PC) alongside inductively coupled plasma mass spectrometry (ICP-MS; PerkinElmer NexION 300X).

Seawater transportation test
3 mL of Rhodamine B dye solution was mixed uniformly with 120 mL of seawater to prepare the stained seawater, which was then poured into a Petri dish. Adsorbents IM-PAO, LP-PAO, and HTC-PAO, each with a thickness of 10 mm, were fixed at the bottom exterior of another Petri dish, with the honeycomb channels oriented perpendicular to the ground. A spirit level was used to ensure that all three adsorbents were aligned on the same horizontal plane. The Petri dish containing the stained seawater was placed on a lift stage and leveled. The lift stage was slowly raised until the seawater surface just contacted the three adsorbents. After 1 s of contact, the stained seawater was withdrawn. Once the staining had stabilized, the adsorbents were vertically sectioned, and the average penetration distance of the stained seawater was measured using a vernier caliper.

Uranium adsorption in simulated seawater
Simulated seawater was prepared using deionized water, 128.161×g of NaCl, and 964.85 mg of NaHCO3, resulting in 5000 mL of solution. The measured ionic concentrations showed 438.607 mM for NaCl and 2.297 mM for NaHCO3 in the solution. For preparation of 8 ppm uranium-containing simulated seawater, 40 mL of concentrated uranyl nitrate solution (1000 ppm) was incorporated into the mixture. The pH was adjusted using NaOH (1 M) and HCl (1 M) solutions. The pH-dependent adsorption performance was investigated between pH 3.0 and 9.0, utilizing 5 mg of pretreated adsorbent in 5000 mL of 8 ppm uranium-spiked simulated seawater for each experimental condition. Shake at a temperature of 25°C and a rotational speed of 140 rpm. The uranium extraction performance was evaluated through a 48-h experimental duration using uranyl-containing simulated seawater. To evaluate the uranium adsorption capacity of HTC-PAO, alkali-treated HTC-PAO was immersed in 5000 mL of simulated seawater containing an uranium concentration of 8 ppm and a pH level of 6. Subsequently, experiments were conducted to assess uranium adsorption. The amount of uranium adsorbed was calculated using Eq. (1)26:

qt = (C0 − Ct) × V / m (1)

In the equation, C0 (mg/L) and Ct (mg/L) denote the initial uranium concentration prior to adsorption and the instantaneous uranium concentration at time t, respectively. V (L) represents the volume of simulated seawater containing uranium, m indicates the dry weight of the adsorbent, and qt (mg/g) signifies the amount of uranium adsorbed at time t.

Adsorption kinetics
Simulated seawater solutions with varying concentrations of uranium (4, 8, 16, 32, 64, and 128 ppm) at a pH level of 6 were meticulously prepared to investigate the adsorption kinetics of the HTC-PAO adsorbent. In summary, a precise quantity of 5 mg of alkali-treated HTC-PAO was introduced into 5000 mL of simulated seawater. The experiment was conducted under controlled conditions at a rotation speed of 140 rpm and a temperature maintained at 25°C. Uranium concentration was measured at four-hour intervals. The acquired data were subsequently subjected to fitting procedures utilizing both the pseudo-first-order (Eq. (2)) and pseudo-second-order (Eq. (3)) models27:

ln(qe − qt) = lnqe − k1t (2)

t / qt = 1 / (k2qe2) + t / qe (3)

In the equations, t represents the contact time (min), while k1 and k2 denote the rate constants. The variable qe (mg/L) indicates the total amount of uranium adsorbed by the adsorbent, whereas qt (mg/L) signifies the amount of uranium adsorbed at time t. Additionally, t (min) refers to the duration of contact between the adsorbent and the solution.

The equilibrium isotherm data were analyzed and fitted to the Langmuir model (as delineated in Eq. (4)) and the Freundlich model (illustrated in Eq. (5)):

Ce / qe = Ce / qm + 1 / (k3qm) (4)

lgqe = lgk4 + 1/n lgCe (5)

In the equations, Ce represents the equilibrium concentration (mg/L), while qm denotes the maximum adsorption capacity (mg/g). Additionally, k3 signifies the equilibrium constant associated with the binding strength (L/mg). The value of qe can be ascertained through the following relationship:

In the equation, k4 is an approximate indicator of the adsorption capacity, and 1/n is a function representing the adsorption intensity during the adsorption process.

Development of adsorbent models
The adsorbent models were constructed using the commercial finite element software COMSOL Multiphysics (version 6.2). The IM-PAO model was based on a simple cubic structure to resemble the experimental sample (Supplementary Fig. 21a), while the LP-PAO and HTC-PAO models were constructed based on regular hexagonal pore arrays (Supplementary Fig. 21b, c). To avoid issues such as excessively dense grids, prolonged computation time, and poor convergence caused by structural complexity, this study simplified the intrinsic micropores and lateral pores of the adsorbents into a homogeneous continuous medium. Macroscopic parameters, including permeability, porosity, and effective diffusion coefficient were employed to characterize the differences in diffusion behavior of uranium-containing seawater through adsorbents with different structures. Due to the absence of oriented secondary channels within the LP-PAO material, isotropic parameters were adopted for it. Its permeability, porosity, and effective diffusion coefficient are the same as those of IM-PAO. In contrast, as the HTC-PAO material contains internally oriented secondary channels, different parameters were assigned along the X, Y, and Z directions to fully reflect the anisotropy induced by these oriented channels. The specific parameter settings are provided in Supplementary Tables 4–7.

Numerical simulation
The simulation adsorption model consists of the Navier–Stokes equations and the convective-diffusion-reaction equations. The Navier–Stokes equations are used to simulate the fluid movement process both outside and inside the adsorbent, while the convective-diffusion-reaction equations are used to simulate the diffusion and transport process of uranyl ions in seawater and within the adsorbent, as shown in Eqs. (6)–(9).

ρf(u⋅∇)u = ∇⋅[−pI + K] (6)

1/εp ρ(u⋅∇)u 1/εp = ∇⋅[−pI + K] − μκ−1 u (7)

ρ ∇⋅u = 0 (8)

∂(εp ci) / ∂t + ∂((1−εp)ρs⋅cP,i) / ∂t + ∇⋅Ji + u⋅∇ci = Ri + Si (9)

In the equation, ρf represents the fluid density, u is the fluid velocity, p is the fluid pressure, I is the unit tensor, K denotes the viscous stress tensor, εp is the porosity, κ is the permeability, ci is the concentration of uranyl ions in the solution, ρs is the solid phase density of the adsorbent, Ji is the diffusion flux, Ri is the adsorption of uranyl ions, and Si is the source term.

The calculation of the viscous stress tensor K is presented in Eqs. (10) and (11)28, which are respectively used to calculate the flow field outside and inside the adsorbent:

K = μ(∇u + (∇u)T) (10)

1/εp K = μ(∇u + (∇u)T) − 2/3 μ(∇⋅u)I (11)

The calculation Eq. (12) of permeability κ is as follows:

κ = d2 εp3 / (18(1−εp)2) (12)

In the equation, d represents the particle size and εp represents the porosity.

The calculation Eq. (13) of the diffusion flux Ji is as follows:

Ji = −(εp D + (1−εp) De,i) ∇ci (13)

In the equation, D represents the dispersion coefficient, and De,i represents the diffusion coefficient.

The calculation Eq. (14) for the adsorption rate Ri is as follows:

∂((1−εp)ρs cP,i) / ∂t = (1−εp)ρs⋅(∂cP,i / ∂ci)⋅(∂ci / ∂t) − ci ρs (∂εp / ∂t) = Ri (14)

The calculation Eq. (15) for the concentration of uranyl ions on the adsorbent cP,i is as follows:

cP,i = (k3 ci cP,max) / (1 + k3 ci) (15)

In the equation, k3 represents the Langmuir constant during the adsorption process, cP,max represents the maximum adsorption capacity of the adsorbent for uranyl ions.

Mass transfer resistance
Mass transfer resistance refers to the obstacle encountered when a substance is transferred from the fluid to the adsorbent interior during the adsorption process. This resistance will affect the adsorption rate and efficiency. For mass transfer process, mass transfer resistance includes surface resistance (Rsurf), diffusion resistance (Rsorb) and reaction resistance (Rreact), surface resistance (Rsurf) is calculated by Eq. (16), diffusion resistance (Rsorb) is calculated by Eq. (17), reaction resistance (Rreact) is calculated by Eq. (18):

Rsurf = 1 / (Ks⋅As) = ΔC / m_dot (16)

Rsorb = (τ⋅δ) / (εp⋅Dsorb⋅As) = ΔC / m_dot (17)

Rreact = 1 / (Kr⋅As) = ΔC / m_dot (18)

In the equations, Ks represents the external mass transfer coefficient, As represents the adsorbent surface area, the τ represents the tortuosity, εp represents the porosity, δ represents the diffusion depth, Dsorb represents the diffusion coefficient, Kr represents the reaction rate constant, Camb is the concentration of uranyl ions in the environment, Csurf is the concentration of uranyl ions on the surface of the adsorbent, Csorb is the concentration of uranyl ions inside the adsorbent, and Csorb,eq is the concentration of uranyl ions in the adsorbent after adsorption equilibrium is reached.

Material reusability
Uranium was eluted from the used adsorbent using an elution solution containing 1 M Na2CO3 and 0.1 M H2O2 at room temperature for 1 h. After the elution process, the material was immersed in pristine water for a duration of 30 min. Subsequently, it underwent an alkali treatment before being introduced into simulated seawater to conduct uranium adsorption experiments. The elution efficiency was calculated using Eq. (19)29:

EE = (Cel × Vel) / ((Ci − Cf) × Vu) × 100% (19)

In the equation, EE (%) represents the elution efficiency, Cel (mg/L) is the uranium concentration in the elution solution, Vel (L) is the volume of the elution solution, Ci and Cf are the initial and final uranium concentrations, respectively, and Vu (L) is the volume of the uranium-spiked simulated seawater.

Adsorption selectivity
The adsorption selectivity of the adsorbent for uranium was evaluated through ion selectivity experiments to assess its extraction ability in the presence of interfering ions. Simulated seawater was prepared using natural seawater as the base and adding various metal salts (such as U, V, Mn, Cr, Ni, Co, Pb, Mo, Cu). The concentration of all metal ions was 100 times higher than their normal concentration in natural seawater (Supplementary Table 11). A 5000 mL solution containing 5 mg of adsorbent was used for the experiments. Uranium extraction was carried out at 25°C with stirring at 140 rpm for 48 h. The adsorption capacity was used to evaluate the uranium adsorption selectivity of IM-PAO, LP-PAO, and HTC-PAO, as well as their selectivity for other competing ions.

Extraction of uranium from natural seawater
Natural seawater (uranium concentration approximately 3.3 ppb; 200 L) was collected from the western coast of Haikou City, Hainan Province, China. The suitability of the prepared adsorbent in real environmental conditions was evaluated using this seawater. The seawater was introduced into a circulating water system, with the adsorbent (10 mg) positioned within a sea test tube of the system. The experiment was conducted over 35 days, with samples taken every 7 days. The variations in uranium concentration within seawater were assessed utilizing Inductively Coupled Plasma Mass Spectrometry (ICP-MS). After 35 days of immersion, the adsorbents maintained their structural integrity, with no disintegration observed under visual inspection (Supplementary Fig. 18).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Due to the enormous amount of uranium resources in the ocean, uranium extraction from seawater is of great significance for the development of nuclear energy. The authors effectively improved the mass transfer process of uranyl in the adsorbent by designing a hierarchical triple-channel system, thereby enhancing the uranyl adsorption performance. This idea is interesting and valuable, and provides a positive research direction for the future engineering of uranium extraction from seawater. In my opinion, this article is suitable for publication in the journal Nature Communications, but before publication, the following issues need to be carefully and meticulously revised.\n(i) The manuscript repeatedly emphasizes that HTC-PAO is “100 times thicker than conventional hydrogels”, however, the actual thickness values of the compared materials should be specified for transparency.\n(ii) The mercury intrusion porosimetry and BET data (Figure 3a, Supplementary Figures 10-11) need more quantitative discussion linking pore volume/surface area to the observed adsorption improvements.\n(iii) The kinetics fitting (Figure 3h, Supplementary Figure 19) shows high R² values for pseudo-second-order models; the authors should discuss why this model is more appropriate than pseudo-first-order in terms of the adsorption mechanism.\n(iv) Explain clearly how the external surface area ratios 1.55× and 2.39× in Figure 2b were derived, rather than only presenting the numbers.\n(v) Please clarify how the simulation models of the three adsorbents (IM-PAO, LP-PAO, and HTC-PAO) were constructed, and specify their structural differences.\n(vi) The manuscript contains several lengthy or repetitive sentences that reduce clarity (e.g., in the Introduction and Results sections). The language and grammar of the paper should be further polished.", "reviewer_2": "In this article, Wang et al. present an interesting bioinspired adsorbent, hierarchical triple-channel polyamidoxime, for efficient uranium extraction from seawater. The study integrates material design, experimental validation, and numerical simulations to elucidate the synergistic roles of macro-, meso-, and micropores in enhancing mass transfer and uranyl adsorption performance. These findings offer a novel design perspective for the development of advanced uranium adsorbents and hold significant scientific value, so it is suitable for publication after addressing the following minor points.\n1. It is suggested to briefly explain the rationale for using a dual-physics model in the COMSOL simulations, indicate whether the models are coupled, and outline the key processes they represent.\n2. When describing the meshing process for the simulation domain, providing the mesh density, refinement strategy, and convergence criteria would enhance the transparency and reproducibility of the modeling.\n3. The seawater transport experiment is central to demonstrating the rapid liquid transport in the hierarchical channels. It would strengthen the manuscript to include a more detailed description of the setup and procedure. For example, the specific dimensions of the samples, the depth of immersion, timing, and how the penetration distance was measured.\n4. The numerical values in Figure 4b are difficult to read. A clearer presentation, perhaps with larger fonts or better contrast, would make the data more accessible to readers.\n5. The adsorption kinetics section could be more informative by including a comparative table that summarizes the fitting parameters such as correlation coefficients (R²), rate constants, and error analyses for both the pseudo-first-order and pseudo-second-order models. This would better substantiate the conclusion that the pseudo-second-order model provides a superior fit.\n6. Since the HTC-PAO monolith is demolded from a honeycomb mold, it would be useful to discuss whether any structural damage or deformation occurs during this step. Briefly describing the precautions taken during demolding to maintain structural integrity would be helpful for researchers seeking to reproduce the material.\n7. Some descriptions, such as the repeated emphasis on the hierarchical pore structure of HTC-PAO in multiple sections, could be slightly streamlined to improve conciseness and flow without affecting the clarity of the discussion.\n8. The adsorption capacity values are expressed inconsistently, sometimes as “mg-U/g-ads” and sometimes as “mg g-1。” Unifying the notation across the manuscript will improve clarity and presentation consistency.", "reviewer_3": "This manuscript reports a hierarchical triple-channel polyamidoxime (HTC-PAO) adsorbent that tackles key challenges in the engineering of uranium extraction from seawater by enhancing mass transfer. Combining experiments with simulations, the study identifies two synergistic pathways, accelerated laminar flow in macroscopic channels and diffusion enhancement in microchannels, which together improve transport efficiency and the adsorption capacity for uranyl in seawater. Overall, the work introduces a valuable and practical strategy with clear relevance to the field, and I recommend publication after minor revisions.\nSpecial comments:\n1. The manuscript should clarify how the Reynolds number and effective diffusion coefficient were calculated. It should indicate the basis for these values.\n2. A detailed description of the calculation process used to obtain the external surface area ratios (1.55× and 2.39×) should be provided.\n3. The boundary conditions applied in the mathematical model need to be specified, together with a short justification for their selection.\n4. Since three types of resistance are discussed, a brief explanation of why Rdiff is regarded as the dominant kinetic limitation in macroscopic adsorbents would make the reasoning more convincing in revised form.\n5. For clarity, the meanings of parameters such as Ks, Dp, Kr in Figure 2a should be explained in the main text. Figure 1a would benefit from a more detailed description in the manuscript.\n6. In Figure 5d, some recent important results should be compared such as: Nature Communication. 2025, 16, 3574; 2024, 15, 6700.\n7. How about the stability of the materials in real seawater?"}, "author_response": "Journal: Nature Communications\nManuscript Number: NCOMMS-25-56968A\nTitle: Hierarchical Triple-Channel Architectures Unlock Scalable and High-Efficient\nUranium Extraction from Seawater\nAuthors: Hui Wang#; Congwen Cao#; Jianming Chen#; Guanbing Zhou; Tao Liu; Yihui\nYuan*; Ning Wang*\nResponse to Reviewers’ Comments\nDear editor and reviewers:\nThank you for your valuable comments and suggestions. All of your comments\nare highly insightful and have been immensely helpful in revising and improving our\nmanuscript. Your input also provides important guidance for our ongoing research. We\nhave carefully reviewed all of the comments and have made revisions accordingly,\nwhich we hope will meet with your approval. Revised sections are marked in red text\nand underlined. Below, we provide a point-by-point response to the reviewers'\ncomments and outline the main corrections made in the paper.\nReviewer 1\nComment\nDue to the enormous amount of uranium resources in the ocean, uranium\nextraction from seawater is of great significance for the development of nuclear energy.\nThe authors effectively improved the mass transfer process of uranyl in the adsorbent\nby designing a hierarchical triple-channel system, thereby enhancing the uranyl\nadsorption performance. This idea is interesting and valuable, and provides a positive\nresearch direction for the future engineering of uranium extraction from seawater. In\nmy opinion, this article is suitable for publication in the journal Nature\nCommunications, but before publication, the following issues need to be carefully and\nmeticulously revised.\nQ1. The manuscript repeatedly emphasizes that HTC-PAO is “100 times thicker than\nconventional hydrogels”, however, the actual thickness values of the compared\nmaterials should be specified for transparency.\nResponse 1:\nWe thank the reviewer for the valuable comments. Conventional hydrogel\nadsorbents typically exhibit thicknesses on the order of tens to hundreds of micrometers.\nA representative value commonly reported in the literature is around 100 μm1,2. In this\nstudy, the thickness of our fabricated HTC-PAO adsorbent reaches 10 mm. This order-\nof-magnitude increase is critical for achieving high mechanical strength and stable\nperformance in flow systems.\nAs suggested by the reviewer, we have incorporated detailed descriptions in the\ntext and cited the corresponding references, with these changes located on page 5 (lines\n98-101), page 27 (lines 710-711), and page 28 (lines 724-726) of manuscript.\nfilms (100 μm)17,22. This structural design significantly enhances mechanical stability,\nthereby facilitating deployment and recovery in marine environments.\nuranium extraction from seawater. Desalination 539, 115965 (2022).\nDispersion for Efficient Uranium Extraction from Seawater. Advanced Science\n11, 2306534 (2024).\nQ2. The mercury intrusion porosimetry and BET data (Figure 3a, Supplementary\nFigures 10-11) need more quantitative discussion linking pore volume/surface area to\nthe observed adsorption improvements.\nResponse 2:\nWe are grateful to the reviewer for the insightful comments. In accordance with\nthe suggestion, we have revised the text to include a more quantitative discussion that\nrelates pore volume and surface area to the observed adsorption enhancements, with\nthese changes located on page 10 (lines 211-216) of the manuscript.\nAs shown in Supplementary Figure 11, the specific surface area of HTC-PAO (1.93\nm² g⁻¹) was 1.95 and 1.25 times that of IM-PAO and LP-PAO, respectively, providing\nmore available adsorption sites. Correspondingly, HTC-PAO exhibited markedly\nhigher uranium adsorption capacity (1.80 times and 1.27 times that of IM-PAO and LP-\nPAO, respectively), confirming that the enlarged pore volume and surface area play a\ncritical role in the observed adsorption enhancement.\nQ3. The kinetics fitting (Figure 3h, Supplementary Figure 19) shows high R² values for\npseudo-second-order models; the authors should discuss why this model is more\nappropriate than pseudo-first-order in terms of the adsorption mechanism.\nResponse 3:\nWe thank the reviewer for the valuable comments. For all three adsorbents, the R²\nvalues of the pseudo-second-order kinetic model were superior to those of the pseudo-\nfirst-order model (closer to 1), indicating a better fitting performance. This indicates\nthat the adsorption process is governed primarily by chemisorption rather than\nphysisorption, essentially involving coordination between PAO functional groups and\nuranyl ions. Moreover, the adsorption isotherm analysis (Supplementary Table 3)\nfurthermore supports this conclusions: the Langmuir model showed superior goodness-\nof-fit compared with the Freundlich model, confirming that uranium adsorption occurs\npredominantly through monolayer chemisorption. Taken together, these results\ndemonstrate that the pseudo-second-order model provides a more accurate description\nof the adsorption mechanism than the pseudo-first-order model.\nAs recommended by the reviewer, we have incorporated these descriptions into\nthe revised text , with these changes located on pages 12-13 (lines 284-288) and page\n13 (lines 295-298) of the manuscript.\nboth pseudo-first-order and pseudo-second-order models (Figure 3h, Supplementary\nFigure 19). For all three adsorbents, the pseudo-second-order model yielded higher R2\nvalues than the pseudo-first-order model (closer to 1), indicating a better fit. This\nsuggests that the adsorption process is governed primarily by chemisorption rather\nthan physisorption, essentially involving coordination between PAO functional groups\nand uranyl ions. The kinetic fitting parameters are summarized in Supplementary Table\n2.\nUranium concentrations in simulated seawater were varied within the range of 4\nto 150 ppm, and the adsorption isotherms for IM-PAO, LP-PAO, and HTC-PAO were\nsystematically analyzed. The uranium adsorption data were well fitted by the Langmuir\nisotherm model, indicating that the adsorption process is primarily monolayer\nchemisorption, which is consistent with the adsorption kinetics analysis.\nQ4. Explain clearly how the external surface area ratios 1.55× and 2.39× in Figure 2b\nwere derived, rather than only presenting the numbers.\nResponse 4:\nWe thank the reviewer for the valuable comments. The rectangular arrangement,\ntriangular circular-hole, and honeycomb channels represent different geometric arrays,\nwith their respective unit cells indicated by the shaded regions (Figure 2b). Taking the\nrectangular arrangement as the reference, the shaded areas of the rectangular, triangular\ncircular-hole, and honeycomb unit cells were calculated as 10.62, 3.28, and 2.27 mm2,\nrespectively. This corresponds to a mass equivalence of one rectangular unit to 3.24\ntriangular circular-hole units or 4.68 honeycomb units. The surface areas of a single\nrectangular, triangular circular-hole, and honeycomb unit were 178.30, 85.10, and\n91.15 mm2, respectively. On an equal mass basis, the total surface area of the triangular\ncircular-hole structure was 85.10 × 3.24 = 275.72 mm2, while that of the honeycomb\nstructure was 91.15 × 4.68 = 426.58 mm2. Accordingly, the honeycomb channels\nprovide 1.55-fold higher external surface area than the triangular circular-hole structure\nand 2.39-fold higher than the rectangular arrangement.\nAs recommended by the reviewer, we have added these descriptions, with these\nchanges located on page 7 (lines 149-154) of the manuscript and page 3 (lines 24-37)\nof the supplementary information.\nOn an equal mass basis, the honeycomb channel affords 1.55 times the external\nsurface area (𝐴 ) of the triangular circular-hole design and 2.39 times that of the\n𝑒𝑥𝑡\nrectangular arrangement (detailed calculation methods are provided in the\nSupplementary Text 1). In addition, it exhibits a smaller maximum diffusion depth (𝛿 )\n𝑚𝑎𝑥\ncompared with both configurations (Figure 2b).\nSupplementary Text 1 | Surface Areas of Channel Arrays\nThe rectangular arrangement, triangular circular-hole, and honeycomb channels\nrepresent different geometric arrays, with their respective unit cells indicated by the\nshaded regions (Figure 2b). Using the rectangular arrangement as the reference, the\n3\nshaded areas of the rectangular, triangular circular-hole, and honeycomb unit cells\nwere calculated to be 10.62, 3.28, and 2.27 mm2, respectively. This indicates that one\nrectangular unit corresponds to 3.24 triangular circular-hole units or 4.68 honeycomb\nunits on a mass-equivalent basis. The surface areas of individual rectangular,\ntriangular circular-hole, and honeycomb units were 178.30, 85.10, and 91.15 mm2,\nrespectively. Accordingly, when normalized to equal mass, the triangular circular-hole\nand honeycomb structures provide total external surface areas of 275.72 and 426.58\nmm2, respectively. Thus, the honeycomb channels offer 1.55-fold greater external\nsurface area than the triangular circular-hole design and 2.39-fold greater than the\nrectangular arrangement.\nQ5. Please clarify how the simulation models of the three adsorbents (IM-PAO, LP-\nPAO, and HTC-PAO) were constructed, and specify their structural differences.\nResponse 5:\nWe thank the reviewer for the valuable comments. The IM-PAO model was\nconstructed as a simple cubic structure resembling the actual experimental sample\n(Supplementary Figure 21a), while the models for LP-PAO and HTC-PAO were built\nbased on regular hexagonal pore arrays (Supplementary Figure 21b and c). To avoid\nissues such as excessively dense grids, prolonged computation time, and poor\nconvergence caused by structural complexity, this study simplified the intrinsic\nmicropores and lateral pores of the adsorbents into a homogeneous continuous medium.\nMacroscopic parameters including permeability, porosity, and effective diffusion\ncoefficient were employed to characterize the differences in diffusion behavior of\nuranium-containing seawater through adsorbents with different structures.\nDue to the absence of oriented secondary channels within the LP-PAO material,\nisotropic parameters were adopted for it. Its permeability, porosity, and effective\ndiffusion coefficient are the same as those of IM-PAO. In contrast, as the HTC-PAO\nmaterial contains internally oriented secondary channels, different parameters were\nassigned along the X, Y, and Z directions to fully reflect the anisotropy induced by these\noriented channels. The specific parameter settings are provided in Supplementary Table\n4-7.\nAs recommended by the reviewer, we have included these descriptions in the\nMethods section, with these changes located on pages 22-23 (lines 557-573) of the\nmanuscript.\nDevelopment of Adsorbent Models. The adsorbent models were constructed using the\ncommercial finite element software COMSOL Multiphysics (version 6.2). The IM-PAO\nmodel was based on a simple cubic structure to resemble the experimental sample\n(Supplementary Figure 21a), while the LP-PAO and HTC-PAO models were\nconstructed based on regular hexagonal pore arrays (Supplementary Figures 21b and\n21c). To avoid issues such as excessively dense grids, prolonged computation time, and\npoor convergence caused by structural complexity, this study simplified the intrinsic\nmicropores and lateral pores of the adsorbents into a homogeneous continuous medium.\nMacroscopic parameters including permeability, porosity, and effective diffusion\ncoefficient were employed to characterize the differences in diffusion behavior of\n4\nuranium-containing seawater through adsorbents with different structures. Due to the\nabsence of oriented secondary channels within the LP-PAO material, isotropic\nparameters were adopted for it. Its permeability, porosity, and effective diffusion\ncoefficient are the same as those of IM-PAO. In contrast, as the HTC-PAO material\ncontains internally oriented secondary channels, different parameters were assigned\nalong the X, Y, and Z directions to fully reflect the anisotropy induced by these oriented\nchannels. The specific parameter settings are provided in Supplementary Table 4-7.\nQ6. The manuscript contains several lengthy or repetitive sentences that reduce clarity\n(e.g., in the Introduction and Results sections). The language and grammar of the paper\nshould be further polished.\nResponse 6:\nWe thank the reviewer for these valuable comments and fully agree with the\nassessment regarding clarity and language issues. In response, we have carefully\nreviewed the manuscript, with particular emphasis on the Introduction and Results\nsections, and have revised lengthy sentences, removed repetitive expressions, and\nimproved overall readability and logical flow, as recommended, with these changes\nlocated on page 5 (lines 101-105), page 6 (lines 123-127), and page 11 (lines 241-244)\nof the manuscript.\nBy integrating mass transfer analysis with COMSOL simulations, we demonstrate\nthat this novel material features a multi-level interconnected channel network. This\nnetwork comprises hexagonal longitudinal (5 mm), transverse (100 μm), and intrinsic\n(2 μm) channels, which enable two synergistic pathways: (1) laminar flow through\nmacrochannels (Reynolds number Re = 922.50) and (2) diffusion-enhanced transport\nwithin microchannels (effective diffusivity D = 6.96 × 10⁻9 vs. 1.74 × 10⁻9 m²/s).\neff\nTogether, these pathways maximize uranium capture and functional group accessibility.\nHTC-PAO was prepared by pouring the mixture into a brass-framed mold\n(Supplementary Figure 2b and 2c). A directional temperature gradient was then applied\nto induce inward ice crystal growth toward the center, after which the sample was\nfreeze-dried (Supplementary Figure 3).\nAs pH increases, the adsorbent surface becomes less positive, reducing\nelectrostatic repulsion and facilitating stronger interactions with uranyl ions, which\nenhances adsorption.\nReviewer 2\nComment\nIn this article, Wang et al. present an interesting bioinspired adsorbent,\nhierarchical triple-channel polyamidoxime, for efficient uranium extraction from\nseawater. The study integrates material design, experimental validation, and numerical\nsimulations to elucidate the synergistic roles of macro-, meso-, and micropores in\nenhancing mass transfer and uranyl adsorption performance. These findings offer a\nnovel design perspective for the development of advanced uranium adsorbents and hold\n5\nsignificant scientific value, so it is suitable for publication after addressing the\nfollowing minor points.\nQ1. It is suggested to briefly explain the rationale for using a dual-physics model in the\nCOMSOL simulations, indicate whether the models are coupled, and outline the key\nprocesses they represent.\nResponse 1:\nWe thank the reviewer for the valuable comments. The adsorption of uranyl ions\nby the adsorbent generally involves three consecutive stages: (i) transport from\nseawater to the adsorbent surface via flow and diffusion; (ii) further diffusion within\nthe porous adsorbent to reach the adsorption sites; and (iii) chemical coordination with\nfunctional groups at the adsorption sites. In this study, the entire adsorption process was\nsimulated using a coupled laminar flow and dilute species transport model. Specifically,\nthe laminar flow module describes seawater flow, thereby characterizing the transport\nof uranyl ions to the adsorbent surface, while the dilute species transport module\nsimulates diffusion of uranyl ions within both seawater and the porous adsorbent,\nincorporating an adsorption term to represent coordination with functional groups at the\nadsorption sites. This coupled multiphysics model enables quantitative analysis of the\nintrinsic relationships among flow velocity, concentration distribution, and adsorption\ncapacity, providing a comprehensive numerical simulation of the adsorption process.\nAs recommended by the reviewer, we have added detailed descriptions and\nexplanations in the manuscript on pages 13-14 (lines 311-319).\nA coupled dual-physics model was developed, comprising two modules: (1) the\nlaminar flow module, which describes seawater flow and thereby characterizes the\ntransport of uranyl ions to the adsorbent surface; and (2) the transport of dilute species\nmodule, which simulates diffusion of uranyl ions within both seawater and the porous\nadsorbent, incorporating an adsorption term to represent coordination with functional\ngroups at the adsorption sites. Detailed material parameters are provided in\nSupplementary Table 7. By coupling these two modules, the intrinsic relationships\namong flow velocity, concentration distribution, and adsorption capacity were\nquantitatively analyzed.\nQ2. When describing the meshing process for the simulation domain, providing the\nmesh density, refinement strategy, and convergence criteria would enhance the\ntransparency and reproducibility of the modeling.\nResponse 2:\nWe thank the reviewer for the valuable comments. In our simulations, the\ncomputational domain was discretized into approximately 693,000 elements. To ensure\nthe accuracy of the results, a mesh convergence study was conducted by comparing the\nresults from the coarse (128,304 elements), medium (693,073 elements), and fine\n(890,922 elements) meshes. Key output parameters, including the velocity field,\nconcentration profile, and adsorption capacity, showed less than 1% variation between\nbetween the medium and fine meshes, indicating good convergence. As a result, the\n6\nchosen mesh density was considered adequate for ensuring reliable simulation results.\nThe corresponding explanation has been added to the revised manuscript to clarify\nthis point, with these changes located on page 15 (lines 336-343) of the manuscript and\npage 31 (line 184) of the supplementary information.\nTo ensure the accuracy of the results, a mesh convergence study was conducted by\ncomparing the results from the coarse (128,304 elements), medium (693,073 elements),\nand fine (890,922 elements) meshes. Key output parameters, including the velocity field,\nconcentration profile, and adsorption capacity, showed less than 1% variation between\nthe medium and fine meshes, indicating good convergence (Supplementary Table 7). As\na result, the chosen mesh with approximately 693,000 elements was selected adequate\nfor ensuring reliable simulation results.\nSupplementary Table 8 | Convergence study for different mesh densities\nNumber of Velocity Concentration Capacity\nelements (m/s) (ppm) (mg/g)\n128304 0.2044 4.71E-4 310.63\n693073 0.2008 2.55E-5 17.943\n890922 0.2002 2.53E-5 17.784\nQ3. The seawater transport experiment is central to demonstrating the rapid liquid\ntransport in the hierarchical channels. It would strengthen the manuscript to include a\nmore detailed description of the setup and procedure. For example, the specific\ndimensions of the samples, the depth of immersion, timing, and how the penetration\ndistance was measured.\nResponse 3:\nWe thank the reviewer for the valuable comments. The specific experimental\ndetails are as follows: 3 mL of Rhodamine B dye solution was mixed uniformly with\n120 mL of seawater to prepare the stained seawater, which was then poured into a Petri\ndish. Adsorbents IM-PAO, LP-PAO, and HTC-PAO, each with a thickness of 10 mm,\nwere fixed at the bottom exterior of another Petri dish, with the honeycomb channels\noriented perpendicular to the ground. A spirit level was used to ensure that all three\nadsorbents were aligned on the same horizontal plane. The Petri dish containing the\nstained seawater was placed on a lift stage and leveled. The lift stage was slowly raised\nuntil the seawater surface just contacted the three adsorbents. After 1 second of contact,\nthe stained seawater was withdrawn. Once the staining had stabilized, the adsorbents\nwere vertically sectioned, and the average penetration distance of the stained seawater\nwas measured using a vernier caliper.\nRhodamine B was chosen as a tracer dye due to its high water solubility, clear\nvisual detectability, and negligible impact on the structural integrity of the adsorbents.\nThe 1-second contact time was selected for measurement because it provides the most\ndistinct and readily quantifiable differences in penetration depth among the different\nadsorbent structures. As suggested by the reviewer, corresponding descriptions have\nbeen added to the Methods section, with these changes located on page 21 (lines 498-\n508) of the manuscript.\nwith 120 mL of seawater to prepare the stained seawater, which was then poured into a\nPetri dish. Adsorbents IM-PAO, LP-PAO, and HTC-PAO, each with a thickness of 10\nmm, were fixed at the bottom exterior of another Petri dish, with the honeycomb\nchannels oriented perpendicular to the ground. A spirit level was used to ensure that\nall three adsorbents were aligned on the same horizontal plane. The Petri dish\ncontaining the stained seawater was placed on a lift stage and leveled. The lift stage\nwas slowly raised until the seawater surface just contacted the three adsorbents. After\n1 second of contact, the stained seawater was withdrawn. Once the staining had\nstabilized, the adsorbents were vertically sectioned, and the average penetration\ndistance of the stained seawater was measured using a vernier caliper.\nQ4. The numerical values in Figure 4b are difficult to read. A clearer presentation,\nperhaps with larger fonts or better contrast, would make the data more accessible to\nreaders.\nResponse 4:\nWe thank the reviewer for the valuable comments. In the revised manuscript, we\nhave recreated Figure 4b to significantly enhance the clarity of the data, ensuring all\ndetails are legible at publication resolution, with these changes located on page 14 (line\n320) of the manuscript.\nFigure 4b, Comparison of adsorption concentration cloud maps of IM-PAO, LP-PAO\nand HTC-PAO.\nQ5. The adsorption kinetics section could be more informative by including a\ncomparative table that summarizes the fitting parameters such as correlation\ncoefficients (R²), rate constants, and error analyses for both the pseudo-first-order and\npseudo-second-order models. This would better substantiate the conclusion that the\npseudo-second-order model provides a superior fit.\nResponse 5:\nWe thank the reviewer for the valuable comment. As suggested, we have added a\ncomparative table that summarizes the key fitting parameters for both the pseudo-first-\n8\norder and pseudo-second-order models, such as the correlation coefficients (R2) and\nrate constants, which has been included on pages 12-13 (lines 284-288) of the\nmanuscript and page 29 (lines 161-162) of the supplementary information. The table\ndata were used to evaluate the adsorption kinetics of IM-PAO, LP-PAO, and HTC-PAO,\nsupporting the conclusion that the pseudo-second-order model better describes the\nadsorption behavior.\nFor all three adsorbents, the pseudo-second-order model yielded higher R2\nvalues than the pseudo-first-order model (closer to 1), indicating a better fit. This\nsuggests that the adsorption process is governed primarily by chemisorption rather\nthan physisorption, essentially involving coordination between PAO functional groups\nand uranyl ions. The kinetic fitting parameters are summarized in Supplementary Table\n2.\nSupplementary Table 2 | Fitting parameters of adsorption kinetics data based on\npseudo-second-order and pseudo-first-order models.\nPseudo-second-order Pseudo-first-order\nC k\n0 2\nq q k\n(ppm) R2 e (g mg-1 R2 e 1\n(min-1)\n(mg/g) (mg/g)\nmin-1)\n4 0.9942 228.63 3.09E-4 0.9823 221.92 8.65E-2\nIM-\n8 0.9985 433.97 2.53E-4 0.9929 417.16 1.15E-1\nPAO\n16 0.9970 604.46 3.07E-4 0.9940 579.67 1.60E-1\n4 0.9910 293.44 2.21E-4 0.9862 284.77 8.31E-2\nLP-\n8 0.9978 612.07 2.19E-4 0.9850 588.20 1.29E-1\nPAO\n16 0.9973 860.04 1.93E-4 0.9918 824.08 1.50E-1\n4 0.9903 375.25 9.93E-4 0.9471 360.01 2.56E-1\nHTC\n8 0.9962 779.28 1.28E-4 0.9838 752.21 1.07E-1\n-PAO\n16 0.9891 1099.80 1.66E-4 0.9855 1050.55 1.63E-1\nQ6. Since the HTC-PAO monolith is demolded from a honeycomb mold, it would be\nuseful to discuss whether any structural damage or deformation occurs during this step.\nBriefly describing the precautions taken during demolding to maintain structural\nintegrity would be helpful for researchers seeking to reproduce the material.\nResponse 6:\nWe thank the reviewer for the valuable comment. The hierarchical three-channel\nstructure of the HTC-PAO adsorbent remains intact during demolding, as confirmed by\nthe SEM image in Figure 1b, which shows that the macrostructure is preserved and the\ninternal multi-level channels exhibit no noticeable collapse or tearing. To ensure smooth\ndemolding, the following precautions were taken: (1) demolding was performed only\nafter the freeze-drying process was fully completed, as the fully dried sample possesses\nsufficient mechanical strength to withstand stress; (2) the sample was carefully and\nslowly separated from the mold starting from the peripheral edges, applying a uniform\nforce parallel to the channel direction. As suggested by the reviewer, these descriptions\nhave been added to the revised manuscript on page 20 (lines 479-484).\nbut the mixture was directly poured into the silicone box for freezing, and then demolded\nand dried. To ensure smooth demolding, the following precautions were taken: (1)\ndemolding was performed only after the freeze-drying process was fully completed, as\nthe fully dried sample possesses sufficient mechanical strength to withstand stress; (2)\nthe sample was carefully and slowly separated from the mold starting from the\nperipheral edges, applying a uniform force parallel to the channel direction.\nQ7. Some descriptions, such as the repeated emphasis on the hierarchical pore\nstructure of HTC-PAO in multiple sections, could be slightly streamlined to improve\nconciseness and flow without affecting the clarity of the discussion.\nResponse 7:\nWe thank the reviewer for the valuable comments. Following your suggestions,\nwe have revised the sentences describing the hierarchical pore structure of HTC-PAO.\nRedundant expressions have been removed, lengthy sentences have been simplified,\nand overall logical coherence has been improved. The corresponding revisions have\nbeen incorporated into the manuscript on page 7 (lines 166-168) and page 15 (lines 347-\n350).\nHTC-PAO, on the other hand, possesses a hierarchical pore structure that not only\nfurther reduces δ compared with LP-PAO but also decreases τ, leading to an even lower\n𝑅 .\n𝑑𝑖𝑓𝑓\nCompared with IM-PAO, LP-PAO exhibited a 15.56% increase in diffusion depth.\nMeanwhile, HTC-PAO achieved a 99.48% increase in diffusion depth compared to LP-\nPAO. These results demonstrate that the hierarchical porous design significantly\nimproves mass transfer efficiency.\nQ8. The adsorption capacity values are expressed inconsistently, sometimes as ‘‘mg-\nU/g-ads’’ and sometimes as ‘‘mg g-1’’. Unifying the notation across the manuscript will\nimprove clarity and presentation consistency.\nResponse 8:\nWe thank the reviewer for this helpful comment. We agree that consistent notation\nis important for clarity and apologize for the oversight. The manuscript has been\ncarefully revised to uniformly express adsorption capacity values as mg/g, including all\nrelevant figures, tables, and text, ensuring consistency throughout, with these changes\nlocated on page 9 (line 180), page 12 (line 258), page 13 (line 290), page 13 (lines 299-\n300), page 18 (line 424) of the manuscript and page 29 (lines 167-168) of the\nsupplementary information.\nAfter 48 hours of contact, the saturated uranium adsorption capacities of HTC-\nPAO were determined to be 391.2 mg/g, 782.4 mg/g, and 1100.24 mg/g for the 4 ppm,\n8 ppm, and 16 ppm solutions, respectively (Figure 3e).\nIn an 8 ppm uranium solution, the equilibrium adsorption capacities (q ) of IM-\ne\nrespectively.\nThe theoretical maximum adsorption capacity (q ) of HTC-PAO was calculated to\nm\nbe 1343.6 mg/g (Figure 3i and Supplementary Table 3), significantly higher than that\nof IM-PAO (712.16 mg/g) and LP-PAO (1024.38 mg /g) (Supplementary Figure 20).\nFigure 3 | Characterization of materials and adsorption properties in simulated\nseawater. a, Mercury intrusion porosimetry analysis for IM-PAO, LP-PAO, and HTC-\nPAO. b, Seawater transport ability of IM-PAO, LP-PAO and HTC-PAO. c, Mechanical\nproperties of HTC-PAO. d, The uranium adsorption capacity of IM-PAO, LP-PAO, and\nHTC-PAO as a function of pH. e, Uranium adsorption kinetics of HTC-PAO in\nsimulated seawater. f, XPS spectra before and after uranium adsorption of HTC-PAO.\ng, High-resolution XPS spectra of U4f for HTC-PAO. h, Uranium adsorption kinetics\nand the fitting curves for IM-PAO, LP-PAO, and HTC-PAO, based on the Pseudo first-\norder and Pseudo second-order models. i, Equilibrium adsorption characteristics of\nHTC-PAO: Freundlich and Langmuir isotherm models with fitting curves.\nadsorption selectivity of IM-PAO, LP-PAO and HTC-PAO was studied by simulated\nseawater system. b, The uranium adsorption capabilities of IM-PAO, LP-PAO, and\nHTC-PAO within the context of natural seawater. c, Kinetics of Uranium Adsorption by\nIM-PAO, LP-PAO, and HTC-PAO in Natural Seawater. d, Report on the adsorption\ncapacity of existing amidoxime adsorbents for uranium in natural seawater1,17,22,23,28-\n48.\nSupplementary Table 3 | Fitting parameters of equilibrium adsorption isotherms\nof IM-PAO, LP-PAO and HTC-PAO based on Langmuir and Freundlich models.\nLangmuir model Freundlich model\nR2 𝒒 (mg/g) R2 𝒒 (mg/g)\n𝐦 𝐦\nIM-PAO 0.92532 712.16 0.68055 775.37\nLP-PAO 0.93544 1024.38 0.71298 1119.61\nHTC-PAO 0.95798 1343.6 0.77258 1475.75\nReviewer 3\nComment\nThis manuscript reports a hierarchical triple-channel polyamidoxime (HTC-PAO)\nadsorbent that tackles key challenges in the engineering of uranium extraction from\nseawater by enhancing mass transfer. Combining experiments with simulations, the\nstudy identifies two synergistic pathways, accelerated laminar flow in macroscopic\nchannels and diffusion enhancement in microchannels, which together improve\ntransport efficiency and the adsorption capacity for uranyl in seawater. Overall, the\nwork introduces a valuable and practical strategy with clear relevance to the field, and\nI recommend publication after minor revisions.\ncoefficient were calculated. It should indicate the basis for these values.\nResponse 1:\nWe thank the reviewer for the valuable comments. The Reynolds number (R ) was\ne\ncalculated as follows3:\n𝜌 𝑣𝐿\nf\n𝑅 = (1)\ne 𝜇\nIn the equation, 𝜌 is the density of seawater, 𝑣 is the flow velocity of seawater, 𝐿 is\nf\nthe characteristic length, which in this study corresponds to the pore diameter, and 𝜇\nis the dynamic viscosity of seawater.\nThe effective diffusion coefficient (𝐷 ) was determined by the equation4:\neff\n𝜀\np\n𝐷 = 𝐷 (2)\neff −1/3 p\n𝜀\np\nIn the equation, 𝜀 represents the porosity of the adsorbent (IM-PAO: 0.856, LP-PAO:\np\n0.863, HTC-PAO: 0.878), and 𝐷 is the diffusion coefficient of uranyl ions in seawater.\np\nDetailed calculation parameters are provided in Supplementary Table 4-7. As\nrecommended by the reviewer, we have included descriptions in the text, with these\nchanges located on page 4 (lines 51-63) and page 35 (lines 206-209) of the\nsupplementary information.\nSupplementary Text 3 | Reynolds Number Calculation\nThe Reynolds number (R ) was calculated as follows1:\ne\n𝜌 𝑣𝐿\n𝑓\n𝑅 = (1)\n𝑒 𝜇\nIn the equation, 𝜌 is the density of seawater, 𝑣 is the flow velocity of seawater, 𝐿 is\n𝑓\nthe characteristic length, which in this study corresponds to the pore diameter, and 𝜇\nis the dynamic viscosity of seawater. Detailed calculation parameters are provided in\nSupplementary Table 7.\nSupplementary Text 4 | Effective Diffusion Coefficient Calculation\nThe effective diffusion coefficient (𝐷 ) was determined by the equation2:\n𝑒𝑓𝑓\n𝜀\np\n𝐷 = 𝐷 (2)\neff −1/3 p\n𝜀\np\nIn the equation, 𝜀 represents the porosity of the adsorbent, and 𝐷 is the diffusion\n𝑝 𝑝\ncoefficient of uranyl ions in seawater. Detailed calculation parameters are provided in\nSupplementary Table 4-7.\nturbulent pipe flows at moderate Reynolds numbers. Physics of Fluids 36, 045138\n(2024).\n388 (1959).\narea ratios (1.55× and 2.39×) should be provided.\nResponse 2:\nWe thank the reviewer for the valuable comments. The rectangular arrangement,\ntriangular circular-hole, and honeycomb channels represent different geometric arrays,\nwith their respective unit cells indicated by the shaded regions (Figure 2b). Taking the\nrectangular arrangement as the reference, the shaded areas of the rectangular, triangular\ncircular-hole, and honeycomb unit cells were calculated as 10.62, 3.28, and 2.27 mm2,\nrespectively. This corresponds to a mass equivalence of one rectangular unit to 3.24\ntriangular circular-hole units or 4.68 honeycomb units. The surface areas of a single\nrectangular, triangular circular-hole, and honeycomb unit were 178.30, 85.10, and\n91.15 mm2, respectively. On an equal mass basis, the total surface area of the triangular\ncircular-hole structure was 85.10 × 3.24 = 275.72 mm2, while that of the honeycomb\nstructure was 91.15 × 4.68 = 426.58 mm2. Accordingly, the honeycomb channels\nprovide 1.55-fold higher external surface area than the triangular circular-hole structure\nand 2.39-fold higher than the rectangular arrangement.\nAs recommended by the reviewer, we have added these descriptions, with these\nchanges located on page 7 (lines 149-154) of the manuscript and page 3 (lines 24-37)\nof the supplementary information.\nOn an equal mass basis, the honeycomb channel affords 1.55 times the external\nsurface area (𝐴 ) of the triangular circular-hole design and 2.39 times that of the\n𝑒𝑥𝑡\nrectangular arrangement (detailed calculation methods are provided in the\nSupplementary Text 1). In addition, it exhibits a smaller maximum diffusion depth (𝛿 )\n𝑚𝑎𝑥\ncompared with both configurations (Figure 2b).\nSupplementary Text 1 | Surface Areas of Channel Arrays\nThe rectangular arrangement, triangular circular-hole, and honeycomb channels\nrepresent different geometric arrays, with their respective unit cells indicated by the\nshaded regions (Figure 2b). Using the rectangular arrangement as the reference, the\nshaded areas of the rectangular, triangular circular-hole, and honeycomb unit cells\nwere calculated to be 10.62, 3.28, and 2.27 mm2, respectively. This indicates that one\nrectangular unit corresponds to 3.24 triangular circular-hole units or 4.68 honeycomb\nunits on a mass-equivalent basis. The surface areas of individual rectangular,\ntriangular circular-hole, and honeycomb units were 178.30, 85.10, and 91.15 mm2,\nrespectively. Accordingly, when normalized to equal mass, the triangular circular-hole\nand honeycomb structures provide total external surface areas of 275.72 and 426.58\nmm2, respectively. Thus, the honeycomb channels offer 1.55-fold greater external\nsurface area than the triangular circular-hole design and 2.39-fold greater than the\nrectangular arrangement.\ntogether with a short justification for their selection.\nResponse 3:\nWe thank the reviewer for the valuable comment. In the mathematical model, we\napplied three boundary conditions: inlet, outlet, and symmetry boundaries\n(Supplementary Figure 21). The inlet was set with a flow velocity of 0.13 m/s and a\nuranyl ion concentration of 8 ppm. This velocity was selected to replicate the actual\nconditions of adsorption experiments, where the shaker speed was maintained at 120\nrpm, resulting in a corresponding simulated seawater flow velocity of approximately\n0.13 m/s, a value that also lies within the typical range of natural seawater flow\nvelocities (0–3 m/s). The outlet boundary was configured with 0 Pa, ensuring fully\ndeveloped flow at the exit without reverse flow, which aligns with physical realism.\nDue to the periodic array nature of the honeycomb channels, symmetry boundaries were\nimposed in two directions, enabling the simulation of a single representative unit rather\nthan the entire complex array, thereby significantly reducing computational cost while\nmaintaining result accuracy.\nAs recommended by the reviewer, we have incorporated the corresponding\nrevisions in the text on page 13 (lines 307-311) of the manuscript and page 3 (lines 38-\n49) of the supplementary information.\nSimulation models of IM-PAO, LP-PAO, and HTC-PAO were constructed under\ndynamic flow conditions, with seawater entering from the top and exiting from the\nbottom (Supplementary Figure 21). The detailed boundary conditions are provided in\nthe Supplementary Text 2, and the model parameters are listed in Supplementary Tables\n4–6.\nSupplementary Text 2 | Boundary Conditions for the Mathematical Model\nIn the mathematical model, boundary conditions included an inlet, an outlet, and\nsymmetry boundaries (Supplementary Figure 21). At the inlet, a flow velocity of 0.13\nm/s and a uranyl ion concentration of 8 ppm were specified. The velocity was chosen\nto replicate the experimental adsorption conditions, where a shaker speed of 120 rpm\ncorresponds to a simulated seawater velocity of approximately 0.13 m/s. This value also\nfalls within the typical range of natural seawater flow velocities (0–3 m/s). At the outlet,\nthe pressure was fixed at 0 Pa to ensure fully developed flow and to avoid reverse flow,\nconsistent with physical realism. Given the periodic arrangement of the honeycomb\nchannels, symmetry boundaries were imposed along two directions, allowing the\nsimulation of a single representative unit. This approach reduced computational cost\nsubstantially while preserving the accuracy of the results.\ndiff\nregarded as the dominant kinetic limitation in macroscopic adsorbents would make the\nreasoning more convincing in revised form.\nResponse 4:\nWe thank the reviewer for the valuable comments. Due to the continuous seawater\nflow, uranyl ions can rapidly migrate to the adsorbent surface, resulting in a small R .\nsurf\nMeanwhile, PAO exhibits a fast reaction with uranyl ions, leading to a low R .\nreact\nDiffusion is inherently a slow kinetic process, and its time scale is proportional to the\nsquare of the diffusion path length5. This means that even a slight increase in path length\nwill cause the required time to rise exponentially. Uranyl ions must travel through\ntortuous pores within the adsorbent and are eventually captured by internal adsorption\nsites. Therefore, the overall adsorption performance of the adsorbent is mainly\ncontrolled by the diffusion process of uranyl ions within the material. The key to\nimproving adsorption performance lies in shortening the internal diffusion path of the\nions and constructing a highly developed pore structure to minimize R .\ndiff\nAs recommended by the reviewer, these explanations have been incorporated into\nthe revised manuscript on pages 6-7 (lines 141-144).\nUnder continuous seawater flow, uranyl ions rapidly reach the adsorbent surface\nand coordinate with amidoxime groups, resulting in low 𝑅 and 𝑅 .\n𝑠𝑢𝑟𝑓 𝑟𝑒𝑎𝑐𝑡\nIntraparticle diffusion through tortuous pores is inherently slow, making 𝑅 the\n𝑑𝑖𝑓𝑓\ndominant factor governing adsorption performance.\nQ5. For clarity, the meanings of parameters such as Ks, Dp, Kr in Figure 2a should be\nexplained in the main text. Figure 1a would benefit from a more detailed description in\nthe manuscript.\nResponse 5:\nWe thank the reviewer for the valuable comments. To improve clarity, we have\nadded explicit explanations of the parameters shown in Figure 2a: K (external mass\ns\ntransfer coefficient), D (diffusion coefficient of uranyl ions in seawater), and K\np r\n(reaction rate constant). These details have been incorporated into the Mass Transfer\nResistance subsection of the Methods section. In addition, a more detailed description\nof Figure 1a has been provided in the manuscript on pages 24-25 (lines 619-622).\nMass transfer resistance. Mass transfer resistance refers to the obstacle encountered\nwhen a substance is transferred from the fluid to the adsorbent interior during the\nadsorption process. This resistance will affect the adsorption rate and efficiency. For\nmass transfer process, mass transfer resistance includes surface resistance (𝑅 ),\n𝑠𝑢𝑟𝑓\ndiffusion resistance (𝑅 ) and reaction resistance (𝑅 ), surface resistance (𝑅 )\n𝑠𝑜𝑟𝑏 𝑟𝑒𝑎𝑐𝑡 𝑠𝑢𝑟𝑓\nis calculated by equation (16), diffusion resistance (𝑅 ) is calculated by equation\n𝑠𝑜𝑟𝑏\n(17), reaction resistance (𝑅 ) is calculated by equation (18):\n𝑟𝑒𝑎𝑐𝑡\n1 𝐶 −𝐶\n𝑎𝑚𝑏 𝑠𝑢𝑟𝑓\n𝑅 = = (16)\n𝑠𝑢𝑟𝑓 𝐾 ·𝐴 𝑚̇/𝐴\ns\n16\n𝜏· 𝛿 𝐶 −𝐶\n𝑅 =\n𝑠𝑜𝑟𝑏\n=\n𝑠𝑢𝑟𝑓 𝑠𝑜𝑟𝑏\n(17)\n𝑠𝑜𝑟𝑏 𝜀 ·𝐷 ·𝐴 𝑚̇/𝐴\np\n1 𝐶 −𝐶\n𝑠𝑜𝑟𝑏 𝑠𝑜𝑟𝑏,𝑒𝑞\n𝑅 = = (18)\n𝑟𝑒𝑎𝑐𝑡 𝐾 ·𝐴 𝑚̇/𝐴\nr\nIn the equations, 𝐾 represents the external mass transfer coefficient, A represents the\ns\nadsorbent surface area, the 𝜏 represents the tortuosity, 𝜀 represents the porosity,\n𝛿 represents the diffusion depth, 𝐷 represents the diffusion coefficient, 𝐾\n𝑠𝑜𝑟𝑏 𝑝 𝑟\nrepresents the reaction rate constant, 𝐶 is the concentration of uranyl ions in the\n𝑎𝑚𝑏\nenvironment, 𝐶 is the concentration of uranyl ions on the surface of the adsorbent,\n𝑠𝑢𝑟𝑓\n𝐶 is the concentration of uranyl ions inside the adsorbent, and 𝐶 is the\n𝑠𝑜𝑟𝑏 𝑠𝑜𝑟𝑏,𝑒𝑞\nconcentration of uranyl ions inside the adsorbent after adsorption equilibrium is\nreached.\nQ6. In Figure 5d, some recent important results should be compared such as: Nature\nCommunication. 2025, 16, 3574; 2024, 15, 6700.\nResponse 6:\nWe thank the reviewer for the valuable suggestion. These two recent studies\nrepresent important advances in the field of uranium adsorption. Jin et al. reported an\nindirect electrochemical approach using a self-standing COF electrode to achieve\nsimultaneous electrocatalytic H O generation and uranyl chelation–precipitation6.\n2 2\nGao et al. demonstrated that structural regulation of COFs enables a combination of\nphotocatalytic H O production with a nanodot growth–elution cycle, achieving ultra-\n2 2\nefficient uranium enrichment7. Both studies integrate photo/electrocatalysis with uranyl\nadsorption, achieving high-performance uranium uptake with significant academic and\nengineering relevance. Figure 5d presents the adsorption performance of amidoxime-\nfunctionalized polymer adsorbents under conditions without external energy input.\nAccordingly, these references have been cited and briefly discussed in the Introduction\nto contextualize the significance and contribution of the present work on page 3 (lines\n58-61) and page 27 (lines 692-699) of the manuscript.\nOver the past few decades, various methods have been developed to extract\nuranium from seawater, including ion exchange, adsorption, chemical precipitation,\nbiological capture, photocatalysis, and electrochemistry10. COF-based photocatalytic\nand electrochemical approaches have demonstrated excellent uranium uptake in recent\nreports, achieving high adsorption efficiency through the synergistic combination of\ncatalytic H₂O₂ generation and uranyl chelation or precipitation11,12.\nand challenges. Science China Chemistry 68, 3923-3926 (2025).\nstudtite nanodots growth-elution cycle. Nature Communications 15, 6700 (2024).\nwastewater with a self-standing covalent-organic framework electrode. Nature\nCommunications 16, 3574 (2025).\nResponse 7:\nWe thank the reviewer for the valuable comments. The three adsorbents all\nexhibited good stability in natural seawater. After 35 days of immersion, they\nmaintained their structural integrity with no visible disintegration (Supplementary\nFigure 18).\nAs recommended by the reviewer, corresponding descriptions have been added to\nthe manuscript on page 26 (lines 654-656).\nAfter 35 days of immersion, the adsorbents maintained their structural integrity,\nwith no disintegration observed under visual inspection (Supplementary Figure 18).\nReferences\nuranium extraction from seawater. Desalination 539, 115965 (2022).\nDispersion for Efficient Uranium Extraction from Seawater. Advanced Science\n11, 2306534 (2024).\nReynolds numbers. Physics of Fluids 36, 045138 (2024).\n(1959).\n1980.\nwastewater with a self-standing covalent-organic framework electrode. Nature\nCommunications 16, 3574 (2025).\nstudtite nanodots growth-elution cycle. Nature Communications 15, 6700\n(2024).\n18"}, {"round": 2, "reviewer_comments": {"reviewer_1": "All my concerns have been addressed in the revised file. So I recommend accepting the manuscript for publication in Nature Communications.", "reviewer_2": "All issues have been resolved,the manuscript is recommended for acceptance.", "reviewer_3": "The authors revised the manuscript carefully. I recommend for publidcation in current form."}, "author_response": ""}]

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