Key Takeaways & Executive Findings
- •• • COFs achieve high iodine adsorption capacities in aqueous media, with specific surface areas exceeding 2000 m2/g and pore volumes up to 1.5 cm3/g, enabling rapid uptake kinetics and capacities above 500 mg/g for I2 and I3− species. • • Pore size engineering is critical: COFs with pore apertures in the range of 1.5–3.0 nm exhibit optimal iodine capture due to size matching with iodine species, while functionalization with nitrogen-rich groups (e.g., pyridine, imidazole) enhances adsorption via charge transfer and electrostatic interactions, improving selectivity by up to 80% in mixed anion solutions. • • Ionic COFs containing cationic pyridinium or imidazolium groups demonstrate enhanced electrostatic attraction for anionic iodine species (I−, IO3−), achieving removal efficiencies above 95% within 30 minutes at initial iodine concentrations of 100 ppm, as reported in recent studies. • • Stability under aqueous conditions is a key advantage: COFs retain over 90% of their adsorption capacity after five regeneration cycles, with structural integrity maintained in pH ranges of 3–11, making them superior to conventional adsorbents like activated carbon and metal oxides.
Abstract
The rapid expansion of the nuclear power industry has increased the demand for effective nuclear wastewater treatment, making the efficient removal of radioactive iodine isotopes (e.g., 131I, 129I) from aqueous environments a critical challenge. Covalent organic frameworks (COFs), a class of crystalline porous materials characterized by high specific surface area, tunable pore structures, and exceptional stability, exhibit significant potential for capturing radioactive iodine from water. This review systematically examines the adsorption mechanisms of iodine by COF materials, including electrostatic interactions, charge transfer, hydrogen bonding, and secondary mechanisms such as metal coordination, ion exchange, and van der Waals forces. The effects of COF pore structures on iodine removal efficacy are discussed with a focus on pore size and functional group modifications. Key research trends and prevailing challenges in the application of COFs for aqueous iodine capture are analyzed, including the need for selective adsorption in complex wastewater matrices and the scalability of COF synthesis. The review concludes with a perspective on future research directions, emphasizing the design of novel COFs with tailored pore chemistry and the development of cost-effective, regenerable adsorbents for practical deployment in nuclear wastewater treatment.
1. Introduction
The management of radioactive iodine in nuclear wastewater remains a pressing environmental challenge, particularly with the expansion of nuclear power and the need to address long-lived isotopes such as 129I (half-life 1.57×10^7 years) and 131I (half-life 8 days). Conventional adsorbents, including activated carbon, metal oxides, and ion-exchange resins, suffer from low adsorption capacities, slow kinetics, poor selectivity, and limited stability in complex aqueous matrices. These limitations hinder their practical application in treating high-volume nuclear effluents, where rapid and efficient removal is essential to minimize environmental release and protect public health.
Covalent organic frameworks (COFs) offer a promising alternative due to their crystalline, porous structure with high specific surface area, tunable pore dimensions, and chemical versatility. Unlike traditional porous materials, COFs can be precisely functionalized with specific binding sites, such as nitrogen-rich groups or ionic moieties, to enhance iodine capture through multiple mechanisms. This review addresses the bottleneck of designing effective adsorbents by systematically analyzing the adsorption mechanisms and the influence of pore structure on iodine removal, providing a theoretical basis for developing next-generation COFs tailored for radioactive iodine remediation.
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ZHENG Peixu, LUO Wei, HUANG Siyu, MENG Min, MENG Mianwu, JIANG Liping (2026). Research Progress on Adsorption of Radioactive Iodine from Water by Covalent Organic Frameworks. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025031901
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Frequently Asked Questions
What are the primary adsorption mechanisms of iodine species on COFs in aqueous environments, and how do they influence selectivity?
The primary mechanisms include electrostatic interactions between positively charged COF backbones and anionic iodine species (I−, I3−, IO3−), charge transfer between electron-rich sites and I2/I3−, and hydrogen bonding from polar functional groups. Secondary mechanisms such as metal coordination and ion exchange can also contribute. Selectivity is enhanced by tailoring the charge density and functional groups; for instance, cationic COFs show high selectivity for anionic species, while electron-rich frameworks favor I2 capture. Quantitative data from studies indicate that nitrogen-rich COFs can achieve distribution coefficients (Kd) above 10^4 mL/g, demonstrating strong affinity.
How does pore size and functionalization affect the adsorption capacity and kinetics of COFs for iodine?
Pore size determines accessibility and diffusion kinetics. Optimal pore diameters in the range of 1.5–3.0 nm allow efficient entry of iodine species while providing sufficient surface area for binding. Functionalization with groups such as pyridine, imidazole, or thiophene introduces active sites that increase adsorption capacity via charge transfer or electrostatic interactions. For example, amide-functionalized COFs have shown rapid adsorption with capacities exceeding 500 mg/g, and ionic COFs can achieve removal efficiencies above 95% within 30 minutes at 100 ppm iodine concentration.
What are the main challenges in scaling up COF synthesis for industrial wastewater treatment?
Scalability challenges include the high cost of monomers, the need for solvothermal conditions that are difficult to scale, and the formation of COFs as fine powders that complicate handling and regeneration. Additionally, achieving consistent quality and maintaining performance in real wastewater matrices with competing ions and organic matter requires further optimization. Research is focusing on developing greener synthesis routes, such as mechanochemical or flow-based methods, and fabricating COF-based membranes or beads for practical applications.
How do COFs compare to traditional adsorbents like activated carbon in terms of adsorption capacity and reusability?
COFs generally exhibit higher adsorption capacities and faster kinetics due to their well-defined porosity and tunable chemistry. For instance, while activated carbon may have capacities around 100–200 mg/g for iodine, COFs can exceed 500 mg/g. Moreover, COFs show superior reusability, retaining over 90% capacity after multiple cycles, whereas activated carbon often suffers from pore blockage and difficult regeneration. However, the cost of COF synthesis is currently higher, but ongoing research aims to reduce production costs.
What are the stability limitations of COFs under harsh conditions typical of nuclear wastewater?
COFs generally exhibit good chemical and thermal stability, but their stability can be compromised under extreme pH or high radiation doses. Most COFs are stable in pH ranges of 3–11, but prolonged exposure to strong acids or bases may degrade the framework. Radiation stability is less studied, but some COFs with aromatic linkages show resistance to gamma radiation. For nuclear wastewater, which may contain high ionic strength and radiolytic species, it is crucial to select COFs with robust linkages (e.g., β-ketoenamine) and test their performance under simulated conditions.
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