Key Takeaways & Executive Findings
- •• • Type-II Me-NBP-Cage and Et-NBP-Cage achieve >99% iodine removal from water within seconds, despite having low BET surface areas of 33.4 and 2.3 m2 g−1, respectively, demonstrating that porosity is not a prerequisite for ultrafast adsorption. • • The amorphous, small particles (2–6 μm) of type-II materials exhibit superior removal kinetics compared to crystalline, larger particles (10–80 μm) of type-IV materials, indicating that morphology and particle size critically influence adsorption performance. • • Et-NBP-Cage can be recycled by simple washing with acetonitrile, maintaining flow-through removal efficiency at the same level after five cycles, offering a cost-effective and sustainable regeneration method. • • The nonporous nature of these cages, combined with their rapid kinetics, challenges the conventional reliance on high surface area for iodine sorbents, potentially enabling new material design strategies for nuclear waste treatment.
Abstract
The rapid and efficient removal of radioactive iodine species from water is critical for nuclear waste treatment, particularly given the short half-life of 131I (8.02 days). Traditional porous inorganic materials exhibit low uptake capacities (<1 g g−1), while porous frameworks such as MOFs and COFs achieve high capacities (>5 g g−1) but suffer from slow removal kinetics, often requiring hours to capture 80% of iodine. This study introduces nonporous naphthobipyrrole-based organic cages (NBP-Cages) that demonstrate ultrafast iodine removal from water. Among the materials tested, type-II Me-NBP-Cage and Et-NBP-Cage, prepared via reprecipitation, exhibit amorphous morphology with small particle sizes (2–6 μm) and low BET surface areas (33.4 and 2.3 m2 g−1, respectively). Despite their nonporosity, these materials achieve >99% iodine removal within seconds, outperforming previously reported sorbents. The adsorption performance correlates with particle size and morphology: amorphous, small particles with effective surface gaps show superior kinetics. The materials are recyclable; for instance, Et-NBP-Cage can be regenerated by washing with acetonitrile, maintaining removal efficiency over five cycles. This work highlights the potential of nonporous organic cages as high-performance iodine sorbents, addressing the critical need for materials that combine high uptake capacity with rapid removal kinetics.
1. Introduction
The removal of radioactive iodine from aqueous streams remains a pressing challenge in nuclear waste management, particularly due to the short half-life of 131I (8.02 days), which demands rapid and efficient capture. Traditional sorbents such as silver-doped zeolites, activated carbons, and functionalized silica offer straightforward preparation but suffer from low uptake capacities (<1 g g−1). The advent of porous frameworks like MOFs and COFs has dramatically improved uptake capacities, with some COFs exceeding 10 g g−1. However, these materials typically exhibit slow removal kinetics, often requiring hours to achieve 80% capture, which is inadequate for time-sensitive applications. This bottleneck has driven the search for sorbents that combine high capacity with ultrafast kinetics.
This study introduces nonporous naphthobipyrrole-based organic cages (NBP-Cages) that achieve >99% iodine removal from water within seconds, a performance that surpasses previously reported materials. The key innovation lies in the material's morphology: amorphous, small particles with effective surface gaps, rather than porosity, dictate the rapid adsorption. This finding challenges the conventional emphasis on surface area and opens new avenues for designing high-performance iodine sorbents. The recyclability of these cages via simple solvent washing further enhances their practical applicability, offering a sustainable solution for nuclear waste treatment.
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Gen Liu, Caixia Li, Bingbin Tan, Liang Ma, Yajie Xu, Yuting Ren, Xiaoqiang Hu, Xiufeng Ji, Xinyun Zhao, Zhan Zhang (2026). Removal of iodine from water in seconds using nonporous naphthobipyrrole-based organic cages. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4053-5
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Frequently Asked Questions
What is the maximum iodine uptake capacity of the type-II NBP-Cage materials, and how does it compare to traditional porous sorbents?
The provided text does not specify the maximum uptake capacity for type-II NBP-Cages. However, it emphasizes that these materials achieve >99% removal within seconds, which is a significant kinetic advantage over traditional porous sorbents that typically require hours. The focus is on removal rate rather than capacity, but the nonporous nature suggests that capacity may be lower than high-surface-area COFs; further data would be required for a quantitative comparison.
How does the morphology of type-II materials (amorphous, small particles) contribute to the ultrafast iodine removal kinetics?
The SEM analysis reveals that type-II materials consist of small (2–6 μm) amorphous particles covered with randomly dispersed thin plates, creating effective gaps among themselves. These gaps likely facilitate rapid iodine diffusion and adsorption, whereas the tightly stacked plates in crystalline type-I and type-IV materials hinder access. This morphological feature, rather than porosity, is the primary driver of the observed ultrafast kinetics.
What is the recyclability of Et-NBP-Cage, and what is the regeneration procedure?
Et-NBP-Cage can be recycled by simply washing with acetonitrile. The flow-through removal efficiency remains at the same level after five cycles, indicating excellent recyclability under neutral conditions. This simple regeneration method is cost-effective and environmentally friendly, making the material suitable for repeated use in practical applications.
Are the type-II NBP-Cage materials stable under the conditions of iodine removal from water, and what is the mechanism of adsorption?
The text does not explicitly discuss stability under operational conditions, but the recyclability over five cycles suggests good stability. The adsorption mechanism is proposed to involve a shrinking-swelling process, as previously suggested for nonporous cages by He's group. The effective gaps on the particle surfaces likely accommodate iodine molecules, facilitating rapid uptake.
How do the adsorption kinetics of type-II NBP-Cages compare to other state-of-the-art iodine sorbents, such as COPOC cages or superphane cages?
The text states that type-II NBP-Cages remove >99% of iodine from water in seconds, which is faster than the reported 2 minutes for COPOC cages and 10 minutes for superphane cages. This positions the NBP-Cages as among the fastest iodine sorbents reported to date, addressing the critical need for materials that can capture iodine within a short timeframe.
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