Fine-tuning of ZIF-67 pore sizes via ligand exchange: optimal active site interactions for iodine capture

Abstract

Radioactive iodine isotopes such as ¹²⁹I and ¹³¹I exhibit high volatility and a tendency to bioaccumulate in marine organisms, ultimately affecting human health through the food chain. Addressing the environmental challenges posed by persistent and mobile radioactive iodine species, this study develops a cost-effective adsorbent through strategic ligand engineering. ZIF-67-IM was synthesized by partial ligand substitution of 2-methylimidazole (2-MIM) with imidazole (IM) ligands in ZIF-67, and N₂ adsorption–desorption results show the micro porosity of defect ZIF-67-IM materials with large surface areas. For liquid-phase iodine (non-radioactive ¹²⁷I₂) capture, ZIF-67-IM exhibits an adsorption capacity of 864.9 mg g⁻¹ and a removal efficiency of 96.1%.

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Introduction

Radioactive iodine isotopes (¹²⁹I with a 1.57-million-year half-life and ¹³¹I with a half-life of 8.0 days) represent significant environmental and health hazards as volatile uranium fission byproducts.¹ Current remediation approaches primarily involve wet scrubbing and solid-phase adsorption, with the latter offering superior advantages, including higher capacity, lower cost, and minimized secondary pollution.² Conventional adsorbents, such as silver-exchanged zeolites and activated carbon, suffer from intrinsic limitations, including sparse active sites and suboptimal adsorption performance. In contrast, metal–organic frameworks (MOFs) have emerged as promising candidates for radioactive iodine capture, as evidenced by Zhang et al.'s work, which demonstrates ZIF-90-III's exceptional adsorption capacity (1826.0 mg g⁻¹ in iodine/cyclohexane solution, representing a 3.4-fold enhancement over pristine ZIF-90).³ This breakthrough highlights the significant potential of MOF-based materials in nuclear waste management, motivating the further development of advanced adsorbents capable of effectively capturing both ionic and neutral iodine species.

Faced with two core challenges in the current environmental field—carbon emissions and radionuclide pollution, various functional materials have been extensively applied to address these pressing issues.^4–7 MOFs, traditionally conceptualized as ideal crystals with regular topology, ordered porosity, and periodic unit cell arrangement, inevitably develop structural defects during actual crystallization processes.^8–10 Notably, these defects critically determine the material's functional properties. Extensive research confirms that deliberate defect engineering can profoundly modify key characteristics, including catalytic activity, adsorption capacity, and structural stability. Zhang et al.¹¹ synthesized a defect-containing Ce-MOF (40BA) and demonstrated its critical role in enhancing phosphorus (P) adsorption performance. Wang et al.¹² designed defective UiO-66 using carboxyl-functionalized ionic liquids, which effectively improved the NH₃ adsorption capacity. Specifically, the introduction of missing-linker defects generates coordinatively unsaturated metal sites, which enhance mass-transfer fluxes within the channels.¹³ This understanding has spurred the targeted fabrication of defects, alongside advanced characterization techniques and mechanistic studies to unlock active sites and broaden material applications.^14–16 Recent studies highlight substantial adsorption enhancements in engineered zeolitic imidazolate frameworks (ZIFs).^17,18 Incorporating unsubstituted imidazole ligands during ZIF-8 synthesis induces linker defects and reduces steric hindrance by eliminating methyl groups.^19,20 This structural modification significantly improves iodine capture efficiency while preserving crystalline integrity during exposure to iodine vapor. The efficient iodine-capture MOFs universally feature large cavities, high surface areas, and electron-rich surfaces—particularly enhanced by aromatic rings and functional groups on organic linkers that promote host–guest interactions.²¹

Guided by these fundamental principles, this study developed a defect-engineered ZIF-67 material through precisely controlled ligand substitution and methyl group modification of the imidazole ring structure. The strategic introduction of structural defects serves to simultaneously expand the pore dimensions and increase the exposure of active surface sites, thereby optimizing the material's interaction with iodine molecules. The liquid-phase iodine adsorption behavior of the modified ZIF-67 is comprehensively demonstrated in Fig. 1a, which reveals the enhanced adsorption performance resulting from these targeted structural modifications.

Fig. 1 (a) The process of iodine adsorption over ZIF-67-IM in the liquid phase; (b) PXRD patterns; (c) TG curves; (d) N₂ adsorption–desorption isotherms, and (e) pore size distributions.

Experimental

Chemicals

Cobalt(II) nitrate hexahydrate (≥98.5%); 2-methylimidazole (≥98%); iodine (≥99.8%); dichloromethane (≥99.5%); imidazole (IM, ≥99%); cyclohexane (≥99%). All reagents were used without further treatment.

Preparation of ZIF-67

The nano-sized ZIF-67 crystals were synthesized via an optimized aqueous-phase method at ambient temperature.²² In a typical synthesis, 900 mg (3 mmol) of Co(NO₃)₂·6H₂O was dissolved in 6 mL of deionized water to form a clear cobalt precursor solution. Concurrently, 11 g (134 mmol) of 2-methylimidazole was precisely weighed and ultrasonically dissolved in 40 mL of deionized water. The cobalt nitrate solution was then rapidly introduced into the 2-methylimidazole aqueous solution under vigorous stirring. The resulting mixture was maintained at room temperature with continuous stirring for 6 hours to ensure complete crystallization, followed by thorough washing and drying procedures to yield the final ZIF-67 product.

Preparation of ZIF-67-IM

The defect-engineered ZIF-67 (denoted as ZIF-67-IM) was synthesized through a controlled ligand-mixing approach. During preparation, an aqueous solution containing 21 mg of imidazole and 11 g of 2-methylimidazole was prepared as the organic ligand mixture. A separate aqueous solution of Co(NO₃)₂·6H₂O (900 mg, 3 mmol) served as the metal ion source and crystallization initiator. The two solutions were rapidly combined at ambient temperature under vigorous mixing conditions. The resulting purple suspension underwent sequential purification steps, including centrifugation, filtration, and extensive washing, followed by organic solvent activation to yield the final defective ZIF-67 crystalline powder.

Iodine adsorption experiment

This study employed stable ¹²⁷I as a non-radioactive surrogate for radioactive isotopes (¹²⁹I and ¹³¹I) due to their identical chemical properties. To minimize solvent interference, nonpolar cyclohexane was selected as the solvent for the iodine carrier.

The determination of iodine adsorption kinetics involves introducing 10 mg of activated adsorbent into a 5 mL iodine cyclohexane solution in different concentrations. After adsorption begins, samples are collected at regular intervals to measure the UV-vis absorbance of the solution. The concentrations of iodine were calculated based on the standard curve, and further calculations were performed for the removal rate of iodine and the adsorption capacity of the adsorbents, as shown in formulas (S1–S3). To improve measurement accuracy and reduce interference from the sampling process on subsequent adsorption, a parallel experimental method was adopted, and multiple independent adsorption quantitative determination experiments were conducted simultaneously. The determination of iodine adsorption isotherm involves adding 10 mg of activated adsorbent to 5 mL of iodine cyclohexane solution with varying concentrations (1000–6000 mg L⁻¹) and allowing it to stand at room temperature. After 48 hours of adsorption reaching equilibrium, the absorbance of the solution was measured, and the iodine solution concentration and equilibrium adsorption capacity at the adsorption equilibrium were calculated according to Formula S3. Subsequently, the adsorption mechanism was elucidated through model-fitting analysis of the obtained kinetic data and adsorption isotherm data.

Results and discussion

Characterization

The crystal structure integrity of the defect-engineered ZIF-67-IM material was confirmed by powder X-ray diffraction (PXRD) analysis. As evidenced in Fig. 1b, all diffraction peaks show perfect correspondence with both simulated ZIF-67 patterns and experimental ZIF-67 references in terms of peak positions and relative intensities. This comprehensive matching unambiguously confirms that the mixed-ligand strategy with a modest imidazole (IM) content (molar ratio of IM : Co = 1 : 10) preserves the parent ZIF-67 framework's crystal structure while introducing defects. Thermogravimetric (TG) analysis (Fig. 1c) reveals that ZIF-67-IM exhibits a two-stage decomposition profile analogous to pristine ZIF-67. The initial gradual weight loss (∼10%) occurring between room temperature and 502 °C corresponds to the evaporation of physisorbed species (residual solvents and water molecules), while the framework structure remains intact. A distinct thermal transition occurs at 502 °C, where rapid weight loss signifies framework decomposition. Notably, ZIF-67-IM demonstrates enhanced thermal stability compared to ZIF-67 (decomposition starting temperature: 475 vs. 300 °C), suggesting that the strategic incorporation of imidazole defects reinforces the structural integrity by optimizing lattice energetics without compromising the framework architecture. Nitrogen physisorption measurements at 77 K (Fig. 1d and e) revealed a characteristic Type I isotherm for ZIF-67-IM, exhibiting complete reversibility and confirming its microporous nature. The identical adsorption–desorption curves and narrow pore size distribution (0.8–1.2 nm) suggest randomly distributed defects without structural correlation. Remarkably, the defective material achieved a BET surface area of 1590 m² g⁻¹ and 45% enhanced pore volume [Table S1, 0.93 cm³ g⁻¹vs. 0.74 cm³ g⁻¹ (ZIF-67)], while maintaining identical pore characteristics and size-selective adsorption capability. Combined with PXRD data, these results demonstrate that controlled imidazole incorporation simultaneously preserves the ZIF-67 framework architecture while significantly improving pore structure.

The morphology and structural features of both ZIF-67 and ZIF-67-IM were systematically investigated using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). As demonstrated in Fig. 2a and b, both materials exhibit well-defined rhombic dodecahedral morphology with particle sizes predominantly distributed between 300–600 nm, consistent with literature reports.²³ High-resolution TEM analysis (Fig. 2c–f) confirms that the defect engineering strategy preserves the fundamental structure of ZIF-67.

Fig. 2 SEM images of (a) ZIF-67 and (b) ZIF-67-IM. TEM images of (c) and (d) ZIF-67 and (e) and (f) ZIF-67-IM.

Adsorption kinetics and isotherms

UV-vis spectroscopy analysis (Fig. S1a) established the adsorption profile of iodine/cyclohexane solutions, with maximum absorbance observed at λ = 525 nm. A linear calibration curve (Fig. S1b) was constructed according to the Lambert–Beer law to quantify iodine concentrations. Subsequent adsorption performance was evaluated using Formulas S1–S3 to determine removal efficiency and adsorption capacity. As shown in Fig. 3a, both ZIF-67 and ZIF-67-IM exhibited rapid iodine capture kinetics during the initial 8 h, attributed to their abundant active sites and accessible pore volume. The adsorption rate gradually decreased after 12 h as available sites became saturated, reaching equilibrium at 48 h. The adsorption capacities of ZIF-67 and ZIF-67-IM in a 2000 mg L⁻¹ iodine cyclohexane solution were calculated to be 881.5 mg g⁻¹ and 864.9 mg g⁻¹, respectively, with removal rates of 97.9% and 96.1%. Both materials demonstrated exceptional iodine removal efficiency from cyclohexane solutions, confirming their potential for radioactive iodine capture applications.

Fig. 3 (a) Adsorption kinetics of iodine by ZIF-67 and ZIF-67-IM in the cyclohexane solution; (b) iodine adsorption isotherms; (c) non-linear fitting of Pseudo-first-order and (d) Pseudo-second-order; (e) Langmuir model fitting curves of iodine adsorption isotherm and (f) Freundlich model fitting curves.

The room-temperature adsorption isotherms reveal concentration-dependent iodine uptake behavior for both ZIF-67 and ZIF-67-IM (Fig. 3b), with capacities reaching 2593.2 mg g⁻¹ and 2578.3 mg g⁻¹, respectively. The adsorption capacity of ZIF-67-IM is competitive to those of the reported MOF-based composites but inferior to those of the highly cross-linked polymers and hybrid aerogel materials (Table 1). This exceptional performance originates from synergistic effects involving imidazole-mediated chemical interactions, optimal pore-size matching, and nanocrystalline structural advantages. Comparative analysis shows ZIF-67 exhibits marginally superior adsorption in the 1000–6000 mg L⁻¹ solutions, while both materials demonstrate identical performance at 6000 mg L⁻¹, suggesting ZIF-67-IM's particular efficacy for high-concentration solutions. The defective material's enhanced surface area facilitates greater active site exposure, potentially explaining its improved performance in concentrated solutions through more efficient iodine molecule capture.

Table 1 Comparison of adsorption capacity of different materials for iodine in the liquid phase at room temperature

Materials	C 0 (ppm)	Solvent	Q max (mg g^−1)	Equilibration time (h)	Ref.
UiO-66-NH2@WCA	381	Water	248.05	72	33
ZIF-8 @PU	3260	Cyclohexane	330	96	34
JLU-MOF132	300	Cyclohexane	153	40	35
KQ-1	100	n-Hexane	131.05	12	36
Cu-2	1630	Cyclohexane	971.03	24	37
BZ-1	200	Cyclohexane	786.4	14	38
HCP-91	127	Water	2900	1	39
IPcomp-7	300	Water	4740	1	40
COF-TAPT	400	Water	2380	50	41
ZIF-67@MCF	3000	Cyclohexane	1630	48	22
ZIF-67-IM	2000	Cyclohexane	864.9	48	This work
ZIF-67-IM	6000	Cyclohexane	2578.3	48	This work

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Kinetic analysis was performed using both Pseudo-first-order (physical adsorption dominant) and Pseudo-second-order (chemical adsorption rate-limiting) models, with concentration gradient serving as the primary driving force for mass transfer.²⁴ The Pseudo-first-order model, described by Formulas S4 and S5, assumes diffusion-controlled adsorption kinetics where the rate is proportional to the concentration gradient.²⁵ In contrast, the Pseudo-second-order model (Formulas S6 and S7) considers chemical adsorption involving electron exchange or sharing as the rate-determining step. Experimental data fitting revealed superior correlation (R² = 0.999) with the Pseudo-second-order model (Table S2 and Fig. 3c, d), confirming chemical adsorption dominance in both ZIF-67 and ZIF-67-IM systems. Thermodynamic analysis employed Langmuir and Freundlich isotherm models (Formulas S8–S11) to characterize 48 h equilibrium adsorption data (Fig. 3e and f). These isotherm studies provide critical parameters including maximum adsorption capacity (Q_max) and equilibrium constants, elucidating surface characteristics and adsorbent-adsorbate interactions. The excellent agreement with Pseudo-second-order kinetics suggests covalent bonding mechanisms play a pivotal role in iodine capture by these imidazole-based MOFs.²⁶

The Langmuir adsorption model provides excellent fitting for the iodine adsorption isotherms (Table S3). This agreement strongly suggests monolayer chemical adsorption dominates the interaction between both ZIF materials and iodine in cyclohexane solution. The Langmuir constants exceeding unity for both ZIF-67 and ZIF-67-IM confirm favorable adsorption thermodynamics, with ZIF-67 demonstrating marginally stronger liquid-phase iodine affinity.²⁷ Visual observations at 8 h (Fig. S2) revealed distinct colorimetric differences, where ZIF-67-IM solutions retained darker coloration, indicating higher residual iodine concentrations. The apparent decoupling between iodine uptake and textural properties suggests charge-transfer mediated chemisorption may constitute the primary adsorption mechanism.²⁸ The equilibrium iodine adsorption capacity of ZIF-67-IM is almost equal to that of ZIF-67. Partial ligand substitution was employed to construct defect structures in ZIF-67 crystals: specifically, a portion of the 2-MIM ligands within the crystals was replaced with IM. After IM coordinates with metal centers, the missing methyl moiety in the lattice can be regarded as a defect site. While such defects enhance porosity, they disrupt the ordered microporous structure. Defect-induced disorder in the pore architecture prolongs the diffusion paths of adsorbates, resulting in an extended adsorption equilibrium time and a slight reduction in adsorption performance. These findings highlight the complex interplay between chemical functionality and porosity in governing iodine capture efficiency.

Adsorption mechanism

Raman spectroscopy confirmed iodine capture by both adsorbents.²⁹ Following I₂ adsorption, ZIF-67 exhibited new peaks at 110.0 cm⁻¹ and 142.0 cm⁻¹, corresponding to the symmetric and asymmetric stretching vibrations of I₃⁻, respectively.³⁰ Similarly, ZIF-67-IM showed new peaks at 106.0 cm⁻¹ and 148.0 cm⁻¹ (Fig. 4a). These results indicate partial conversion of I₂ to I₃⁻ in the liquid phase, suggesting the formation of charge transfer complexes between iodine molecules and the adsorbents.³¹ Compared with defective materials, I₂-ZIF-67 exhibits closer peak spacing and more significant fluctuations. This difference indicates that the charge-transfer interaction between ZIF-67 and iodine is slightly stronger, corroborating that defect-induced pore-structure disorder results in a moderate decrease in adsorption performance. XPS and FTIR analyses further elucidated the adsorption mechanism. FTIR spectra (Fig. 4b) revealed significant shifts in the characteristic C=N stretching vibration of the imidazole group post-adsorption. Specifically, this peak shifted from 1154.0 cm⁻¹ to 1138.0 cm⁻¹ for ZIF-67 and from 1144.0 cm⁻¹ to 1115.0 cm⁻¹ for ZIF-67-IM, implicating imine bonds and imidazole groups as active adsorption sites.^22,32 XPS survey spectra (Fig. S3) confirmed iodine capture through the appearance of I 3d and I 4d peaks, with the sharper I 3d peak in I₂-ZIF-67 indicating slightly superior capture ability in 2 mg mL⁻¹ iodine cyclohexane solution compared to I₂-ZIF-67-IM. While elemental iodine (I₂) typically exhibits binding energies at 621.6 eV and 633.1 eV, the observed peaks at 618.1 eV and 629.7 eV (Fig. 4c) correspond to I₃⁻, confirming the predominant conversion of I₂ to I₃⁻ during adsorption.^42,43 Analysis of the Co 2p region (Fig. 4d) showed binding energies increasing by 0.3–0.4 eV (from 780.5 eV and 781.0 eV) after iodine uptake, signifying altered chemical environments around Co atoms and enhanced core electron binding forces.³⁶ For ZIF-67 (Fig. 4e), the N 1s binding energy increased by 0.4 eV post-adsorption, indicating reduced electron density around N atoms due to electron donation to iodine. In ZIF-67-IM (Fig. 4f), the disappearance of the N 1s peak at 404.7 eV suggests partial positive charging of imidazole nitrogen atoms from electron transfer to iodine.^22,44 These findings collectively demonstrate the vital role of nitrogen-rich sites in facilitating liquid-phase iodine adsorption via electron transfer mechanisms.⁴⁵

Fig. 4 (a) Raman spectrum of I₂-ZIF-67 and I₂-ZIF-67-IM. ZIF-67 and ZIF-67-IM before and after adsorption of I₂. (b) Infrared spectra; high-resolution XPS spectra of (c) I 3d; (d) Co 2p; (e) N 1s in ZIF-67; (f) N 1s in ZIF-67-IM.

Conclusions

This study successfully engineered defective ZIF-67-IM through a ligand exchange strategy, exhibiting favorable liquid-phase iodine capture performance with an adsorption capacity of 864.9 mg g⁻¹ and 96.1% removal efficiency. Unlike previous observations, a comparative analysis between ZIF-67 and its modified counterpart revealed that textural properties play a secondary role in iodine adsorption. Although such defects enhance porosity, the well-organized microporous structure was somewhat disordered. Defect-induced disorder in the pore architecture extends the diffusion paths of adsorbates, resulting in a longer adsorption equilibrium time and a slight decrease in adsorption performance. However, XPS spectra indicate additional charge-transfer interactions between I and N atoms, which may enhance the capture capacity of radioactive iodine in defective ZIF-67-IM materials. These findings may offer some insight for the design of adsorbents targeting radioactive iodine species.

Author contributions

Le Chen: conceptualization, investigation, formal analysis, writing – original draft; Jianjun Yin: investigation, data curation, formal analysis, writing – original draft; Lin Nie: investigation, data curation; Junyan Qian: data curation, validation; An Xie: data curation; Pengxiang Qiu: conceptualization, writing – review & editing; Junfeng Qian: conceptualization, resources; funding acquisition; Qun Chen: software, project administration, supervision; Zhihui Zhang: conceptualization, visualization, writing – review & editing, funding acquisition, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: extra experimental details, SEM-EDS, UV-vis spectra, Raman spectra, and iodine adsorption kinetics as well as equilibrium. See DOI: https://doi.org/10.1039/d5nj03807c.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (12175024) and the Qinglan Project of Jiangsu Province. Graduate Research and Innovation Projects of Jiangsu Province (SJCX24_1629) is also acknowledged.

Notes and references

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