MOF-decorated track-etched membranes for the U(VI) ions sorption removal

Abstract

This study presents the development of metal–organic framework functionalized track-etched membranes (MOF@TeMs) as efficient adsorbents for uranium(VI) removal from water. Poly(N-vinylformamide) was grafted onto PET membranes by UV-induced RAFT polymerization, hydrolyzed to poly(vinylamine), and further modified with terminal alkyne groups. Chromium-based MIL-101 MOFs were post-synthetically functionalized with azide groups and covalently immobilized onto the membrane surface through copper-catalyzed azide–alkyne cycloaddition. This covalent click-immobilization strategy provided a stable integration of MOFs onto polymer track-etched membranes, which has not been demonstrated previously for uranium(VI) sorption. Comprehensive characterization, including FTIR, XPS, SEM/EDX, BET, and contact angle, confirmed each modification step and successful MOF integration. The resulting composites displayed a uranium adsorption capacity of 418 mg g^–1 at pH 6.3, with kinetics following a pseudo-second-order model and isotherm fitting best to the Freundlich equation, indicating heterogeneous sorption sites. The Dubinin–Radushkevich model yielded an adsorption energy of 14.4 kJ·mol^–1, consistent with coordination-driven interactions. Despite the relatively long equilibrium time (96 h), the membranes retained 70% of their capacity after five regeneration cycles and showed strong selectivity for U(VI) over competing cations (Co²⁺, Cr²⁺, Pb²⁺, Zr²⁺, Zn²⁺). The membrane architecture offers mechanical stability, reusability, and ease of handling, highlighting the potential of MOF-immobilized TeMs as practical sorbents for uranium remediation.

Introduction

The release of heavy metal ions into natural waters has become a serious global concern because of their widespread use in industrial processes such as metallurgy, electroplating, mining, nuclear energy, and chemical manufacturing¹. Many of these ions, including lead, cadmium, chromium, and uranium, persist in the environment due to their non-biodegradable nature and tend to accumulate in living organisms, resulting in long-term ecological and health problems. Among them, uranium is particularly critical because it combines radiological hazards with chemical toxicity. Prolonged exposure to uranium, primarily through ingestion of contaminated water, has been linked to nephrotoxicity and carcinogenic effects, making its remediation an essential component of environmental protection efforts^2,3. Areas surrounding uranium mines, nuclear facilities, and waste repositories are especially vulnerable, and untreated effluents pose a direct risk to both local populations and ecosystems⁴.

Among the various strategies explored for uranium removal, adsorption-based methods remain highly effective due to their simplicity, cost-efficiency, and ability to operate under mild conditions. In this context, track-etched membranes (TeMs) offer a unique platform for the development of selective and high-performance sorbents^5,6,7,8. These synthetic membranes are distinguished by their highly uniform and cylindrical nanochannels, which are precisely tunable in terms of diameter, density, and surface chemistry. Due to these properties, TeMs have gained prominence in diverse applications including catalysis, separation technologies, biomedical diagnostics, and energy storage systems⁹. Our research group has contributed significantly to the field through the surface engineering of TeMs, tailoring them for specific functions in catalysis, adsorption, and energy conversion processes^{10,11,12,13,14,15,16,17,18,19,20}. Given their mechanical robustness, chemical resistance, and precisely controlled pore morphology, TeMs represent ideal scaffolds for developing composite sorbents targeting uranium extraction from complex aqueous environments.

Metal–organic frameworks (MOFs), on the other hand, represent a different class of porous materials composed of inorganic nodes and organic linkers that yield extremely high surface areas and tunable functionalities²¹. Their high surface area, structural diversity, and the ability to functionalize their internal pore surfaces make them suitable candidates for a wide range of applications, including gas storage²², targeted drug delivery²³, chemical sensing²⁴ and molecular separation^25,26. In heterogeneous catalysis, MOFs offer a unique advantage due to the accessibility of catalytically active metal centers within their porous structures²⁷. In recent years, their potential in water purification, especially for heavy metals and radionuclides, has been demonstrated in many studies^28,29,30. For instance, MOFs with phosphorylurea or carboxyl-functionalized ligands have demonstrated high affinity and selectivity for U(VI) ions, even in the presence of competing ions, by exploiting the hard acid–hard base interactions between the uranyl cation and oxygen-donor sites. One notable example includes a UiO-68-based MOF functionalized with phosphorylurea groups, which exhibited outstanding U(VI) sorption performance. These findings underscore the significant potential of MOFs in nuclear waste treatment and environmental decontamination applications^31,32,33,34. Nevertheless, some drawbacks still limit their direct application, including poor mechanical stability in aqueous systems, particle aggregation, and difficulties in separation from treated water. Recent reviews have emphasized that while MOFs exhibit remarkable sorption properties, several aspects such as long-term stability in aqueous systems, recyclability over multiple cycles, and large-scale processability still require further development. In particular, many MOFs tend to lose crystallinity or undergo partial leaching in water, and the recovery of fine MOF powders after use remains difficult. Moreover, translating laboratory-scale sorption performance into practical water treatment applications demands materials that can withstand repeated regeneration steps without loss of capacity^35,36,37. Therefore, hybrid strategies that combine MOFs with mechanically stable supports, such as polymer membranes, have been increasingly explored as a way to preserve the advantages of MOFs while addressing these limitations^38,39,40.

Among the large number of MOFs investigated, MIL-101(Cr) has received special attention due to its large pore size, hydrothermal stability, and ease of post-synthetic modification. Functionalization with amino groups or carboxylates enhances the affinity of MIL-101(Cr) toward U(VI), owing to the strong interactions between the uranyl cation and electron-rich donor sites^41,42. The introduction of such amino groups, which are anchored to coordinatively unsaturated Cr(III) centers within the MOF network, increases the density of active binding sites and improves selectivity for actinides even under mildly acidic or neutral pH conditions^43,44.

In addition to nitrogen-containing groups, the uranyl cation (UO₂²⁺), as a strong Lewis acid, exhibits particularly high affinity for hard oxygen-donor ligands such as hydroxyl and carboxylate groups⁴⁴. This principle has guided the development of uranyl-binding peptides and polymers enriched with aspartate, glutamate, or terminal carboxyl groups⁴⁵. Translating this concept to MOF design, researchers have successfully enhanced uranium sorption by incorporating pendant carboxylate moieties within the MOF structure, enabling fast and selective removal of U(VI) from challenging matrices such as seawater⁴⁶. Nonetheless, despite considerable progress, further efforts are required to improve the performance of MOFs under near-neutral pH, high-ionic-strength environments, and over prolonged operational cycles.

To address these challenges, we report a novel synthesis strategy involving the covalent immobilization of MIL-101(Cr)-based MOFs onto poly(vinylamine)-grafted PET TeMs. MIL-101(Cr) was chosen for its excellent hydrothermal stability, mesoporosity, and documented uranium adsorption performance^47,48,49. The process began with the surface-initiated grafting of poly(N-vinylformamide) (PNVF), a precursor to PVAm, onto the PET membrane matrix. Subsequent alkaline hydrolysis yielded PVAm-grafted PET (PVAm@PET), introducing primary amine groups suitable for further functionalization. These were then derivatized via amidation with 2-propynoic acid, installing terminal alkyne groups on the PET surface (alkyne@PET). In parallel, azide-functionalized MIL-101(Cr) MOFs were synthesized through post-synthetic modification of amino-MIL-101(Cr), following established procedures^49,50. Finally, using copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC), the MOFs were covalently bonded to the alkyne-modified membranes, forming the final composite structure (MOF@PET). For clarity, the complete synthetic pathway and structural evolution of the membrane are illustrated in Fig. 1, providing a visual summary of each key modification step. This modular and robust functionalization strategy holds great potential for the development of next-generation membrane-based sorbents targeting not only uranium, but also other environmentally critical contaminants in complex aqueous systems.

Fig. 1

Step-by-step schematic representation of the surface modification process and click immobilization of Cr-MIL101 on PVAm-grafted PET TeMs.

Results and discussions

Characterization of modified PET tem templates and composite PET tems

The development of MOF-functionalized track-etched membranes requires precise control over the chemical architecture of the supporting surface to ensure reproducibility, homogeneity, and nanoscale functionality. In this work, we employed surface-initiated reversible addition–fragmentation chain transfer (RAFT) polymerization to achieve uniform grafting of poly(N-vinylformamide) (PNVF) onto PET track-etched membranes (TeMs). This controlled radical polymerization technique allows for fine-tuning of polymer chain length and distribution through the use of appropriate RAFT agents such as 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPPA), thereby offering a high level of control over the resulting surface properties⁵¹.

RAFT polymerization was selected deliberately due to its ability to provide reproducible surface functionality, which is critical for subsequent covalent immobilization of MOF particles. Homogeneous grafting is particularly important in this system for two reasons: (1) to enable uniform distribution of reactive groups across the membrane surface, ensuring consistent MOF attachment, and (2) to avoid excessive polymer deposition that could lead to pore blockage in the TeM nanochannels. Such occlusion could compromise fluid transport and reduce effective surface area for uranium adsorption. Therefore, polymerization conditions were carefully optimized to maintain a balance between sufficient functional group density and preservation of membrane porosity. Additionally, oxidation of the PET surface prior to benzoyl peroxide (BP) immobilization was found to increase the number of initiator sites, thereby enhancing the overall grafting efficiency in subsequent steps⁵².

To validate each modification stage and to investigate structure–function relationships in the resulting composite membranes, a suite of complementary characterization methods was applied, including FTIR, SEM, EDX, contact angle measurements, XPS, and nitrogen adsorption-desorption porosimetry. These techniques were critical in confirming the chemical changes on the membrane surface and understanding their impact on final sorption performance.

Poly(vinylamine) (PVAm) was chosen as a key intermediate due to its water solubility, pH responsiveness, and high density of primary amine groups⁵³. However, the direct synthesis of PVAm is hindered by the instability of its monomer, which readily undergoes imine–enamine tautomerization. As a practical alternative, we used PNVF as a precursor and converted it into PVAm via alkaline hydrolysis. UV-induced graft polymerization of the NVF monomer was conducted by systematically varying parameters such as monomer concentration, solvent, and reaction time.

We initially investigated the influence of solvent type and monomer concentration on the grafting degree. In agreement with previous reports, higher alcohols yielded greater grafting efficiency due to increased hydrophobic interactions and monomer partitioning⁵⁴. As illustrated in Fig. 2a, the grafting degree followed the trend: ethanol < n-propanol < n-butanol. Concurrently, we observed that increasing NVF concentration beyond 10% led to partial degradation of the PET TeM structure, with up to 50% monomer decomposition. Therefore, all subsequent experiments were performed in n-butanol using a 10% monomer concentration.

The effect of polymerization time on the grafting degree was also examined, with reaction durations ranging from 1 to 6 h (Fig. 2b). Experiments were repeated five times for each time point to ensure reproducibility. Grafting degrees achieved at 2 and 3 h were below 5%, indicating limited polymerization activity. In contrast, a 6-hour reaction yielded a significant increase in grafting (~ 20%); however, this condition compromised membrane integrity, rendering the material brittle. Notably, the high standard deviation in grafting degree after 6 h can be attributed to mechanical damage observed in some samples during post-reaction drying.

Fig. 2

Effect of monomer concentration and solvent type on the grafting degree. (b) Time-dependent variation of the grafting degree for 10% NVF in n-butanol.

Following the optimization of NVF grafting conditions, the stepwise chemical modification of PET TeMs and Cr-MIL-101 MOFs was confirmed through Fourier-transform infrared (FTIR) spectroscopy. Figure 3a displays the FTIR spectra of pristine PET, PNVF-g-PET, and PVAm-g-PET membranes. Pristine PET exhibited characteristic absorption bands at 1716 cm^–1 (C = O stretching of ester groups), 1244 cm^–1 (C–C–O stretching), and ~ 1000 cm^–1 (aromatic C–H bending), consistent with previous reports¹⁷. Upon grafting of N-vinylformamide (PNVF), new peaks appeared at 1600 cm^–1 and 1550 cm^–1, corresponding to the C = O stretch and N–H bending of the formamide group, respectively⁵⁵. After alkaline hydrolysis, the 1600 cm^–1 band decreased in intensity, indicating successful conversion of amide groups into primary amines and formation of PVAm chains. The effect of hydrolysis parameters (NaOH concentration and reaction time) on amine group generation was also investigated. As shown in Fig. 3c, optimal amine density was obtained at 0.1 M NaOH for 10 min. Under harsher conditions (0.2 M or > 10 min), membrane degradation occurred, while milder conditions (0.05 M) led to insufficient hydrolysis.

In parallel, the synthesis and post-synthetic functionalization of Cr-MIL-101 MOFs were monitored using FTIR. As shown in Fig. 3b, the amino-functionalized MIL-101(Cr) exhibited characteristic vibrations at ~ 1582 cm^–1 (N–H bending), 1400–1500 cm^–1 (C–N deformation), and ~ 1653 cm^–1 (C = O of the terephthalic linker)⁴⁹. After azide modification, a distinct and sharp absorption band appeared at 2205 cm⁻¹, confirming the successful introduction of azide groups⁵⁶. Broad bands near 3000 cm⁻¹ were attributed to O–H stretching from MOF ligands, while the aromatic C–H bending around 616 cm^–1 remained consistent.

Fig. 3

(a) FTIR spectrum of the modified PET track-etched membrane (TeM). (b) FTIR spectra of Cr(amino)MIL-101 and Cr(azide)MIL-101 metal–organic frameworks. (c) Effect of hydrolysis time and NaOH concentration on the number of surface amino groups.

Next, alkyne groups were introduced onto the PVAm-g-PET surface via DCC-mediated amidation using propiolic acid. This reaction proceeded through the formation of an O-acylurea intermediate that facilitates nucleophilic attack by the primary amines, forming stable amide bonds. Figure 4 illustrates this functionalization route. The presence of a sharp alkyne C ≡ C stretching vibration at 2200 cm^–1 (visible in Fig. 3a) confirms the success of this surface modification, preparing the substrate for the subsequent click reaction with azide-functionalized MOFs.

Fig. 4

Schematic representation of the surface functionalization of PVAm-grafted PET with alkyne groups via DCC-mediated amidation of propiolic acid.

As illustrated in Fig. 4, the alkyne groups were covalently introduced onto the surface of PVAm-grafted PET TeMs via DCC-mediated amidation of propiolic acid. This reaction enabled selective formation of amide bonds between the carboxylic acid moieties and surface amines, resulting in stable alkyne-functionalized membranes. These reactive alkyne groups served as anchoring sites for the subsequent immobilization of azide-functionalized MIL-101(Cr) particles through copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC), a highly efficient and chemoselective “click” reaction. This strategy ensured a robust covalent attachment between the MOF and the modified membrane surface. By this way, both the structural integrity of the MOF and the accessibility of its porous architecture were preserved. The successful integration of Cr(azide)MIL101 MOFs onto the alkyne@PET TeM surface was then confirmed through detailed surface analysis by X-ray photoelectron spectroscopy (XPS). XPS is a powerful surface-sensitive technique that enables both elemental identification and chemical state analysis of atoms in the outermost layers of materials. To confirm each functionalization step and monitor the changes in surface chemistry, survey scans and high-resolution spectra were collected for all modified membrane samples.

The wide-scan XPS spectrum of pristine PET TeMs (Fig. 5a) exhibited dominant peaks corresponding to carbon (C1s) and oxygen (O1s), which are consistent with the chemical structure of polyethylene terephthalate¹⁴ upon grafting with poly(N-vinylformamide) (PNVF), a distinct nitrogen (N1s) peak emerged at ~ 399 eV, indicating the successful incorporation of nitrogen-bearing formamide groups (Fig. 5b)⁵⁷. Additionally, a sulfur (S2p) signal was detected, originating from the thiocarbonyl end groups of the RAFT agent (CPPA), providing direct evidence that the grafting process was mediated via RAFT polymerization.

Following alkaline hydrolysis to convert PNVF to PVAm, the N 1s peak further increased in intensity, while C1s and O1s peaks decreased slightly, reflecting the removal of formyl (-CHO) groups and the formation of primary amines (Fig. 5c). The persistence of the sulfur peak in both grafted and hydrolyzed membranes confirmed the retention of RAFT agent fragments at the polymer chain ends, thereby substantiating the controlled grafting process.

Fig. 5

XPS analysis results: (a) Survey scan of pristine PET, (b) PNVF-grafted PET and (c) PVAm-grafted PET, C1s core level spectra of pristine PET (d) and MOF-immobilized PET (e), (f) Cr2p core level spectrum of Cr(azide)MIL-101 MOF immobilized on PET.

High-resolution C1s spectra provided further chemical insights. For pristine PET (Fig. 5d), peaks at 284.8 eV (C–C), 286.4 eV (C–O), and 288.6 eV (O–C = O) were observed. After MOF immobilization, significant changes were detected in the carbon environment, with increased intensity in peaks associated with C–N and C–O functionalities, suggesting the successful incorporation of polar groups and MOF-related chemical species (Fig. 5e).

Moreover, the Cr2p core-level spectrum of the MOF-modified membrane (Fig. 5f) displayed distinct peaks at ~ 577 eV (Cr2p_3/2) and ~ 587 eV (Cr2p_1/2), characteristic of Cr(III) oxidation states in the MIL-101 framework⁵⁸. A shoulder near the main Cr 2p_3/2 peak further corroborated the presence of Cr(III) species, validating the integrity and successful immobilization of the MOF structure on the PET TeM surface⁵⁹.

Surface wettability of the modified PET TeMs was systematically evaluated using contact angle (CA) measurements. These measurements were used to assess changes in hydrophilicity resulting from successive surface modifications, including polymer grafting and MOF immobilization. As shown in Fig. 6, the contact angle values decreased progressively after each functionalization step, indicating an increase in surface hydrophilicity. Specifically, PNVF-g-PET membranes exhibited variable CA values depending on the solvent used during grafting. Among the tested conditions, the lowest contact angle was observed for membranes grafted in n-butanol at the highest NVF concentration and optimal reaction time. This observation is consistent with the higher grafting degrees achieved under these conditions, leading to denser coverage of polar functional groups.

Following immobilization of Cr(azide)MIL101 MOFs onto the alkyne-functionalized PET surface, a further decrease in contact angle was recorded. This enhancement in hydrophilicity can be attributed to the inherently polar nature of the MOF structure and the additional surface roughness introduced by MOF deposition. Collectively, these results confirm that the composite membrane surface becomes increasingly hydrophilic through each modification step, which is advantageous for applications involving aqueous media.

Fig. 6

Contact angle measurements of: (a) pristine PET TeM, (b) PNVF-grafted PET TeM (50% NVF, 4 h, EtOH as solvent), (c) PNVF-grafted PET TeM (50% NVF, 4 h, PrOH as solvent), (d) PNVF-grafted PET TeM (50% NVF, 4 h, BuOH as solvent), (e) composite Cr(azide)MIL-101 MOF@PET TeM.

Scanning electron microscopy (SEM) was employed to investigate the surface morphology and pore distribution of the PET TeM template following MOF immobilization. Representative images of the MOF@PET TeM composite membrane (degree of grafting: 10%, MOF loading degree: 2.4 ± 0.4%) are presented in Fig. 7. The top-view SEM images (Fig. 7a) reveals the presence of MOF crystallites distributed across the membrane surface, while the cross-sectional image (Fig. 7b) confirms that these MOF particles primarily reside on the surface rather than penetrating deeply into the porous matrix. This surface-localized distribution is advantageous for maximizing interfacial accessibility during sorption or catalytic applications. The effect of time on the degree of MOF immobilization on the membrane surface at the preliminary stage was also investigated using SEM-EDX mapping of the MOF@PET (Fig. 8a-d). Unfortunately, even after repeated experiments, it was unable to achieve a high degree of MOF loading exceeding 2.4%, despite increasing the loading time. Data on the change in the degree of MOF loading depending on time are presented in Fig. 8e. The analysis of the EDX spectra also indicates that the maximum saturation of MOF in terms of chromium content on the membrane surface is achieved after 3 days of immobilization (Fig. 8f).

Fig. 7

SEM images of the MOF@PET TeM composite prepared with 2.4% Cr content (a–b). SEM-EDX elemental mapping of the MOF@PET TeM composite (c–d).

It is important to emphasize that the efficiency of MOF immobilization is closely linked to the density of alkyne functional groups on the membrane, which in turn depends on the number of surface amino groups formed after hydrolysis. Each repeating unit of PNVF contains a single formamide group, which upon hydrolysis yields one primary amine group and a formate byproduct. These amines subsequently react with propiolic acid in the presence of DCC, forming stable amide linkages that introduce terminal alkyne moieties. The presence of sulfur groups observed in the XPS spectra corroborates the persistence of RAFT chain ends, while the covalently bound alkynes act as click-compatible anchor points.

Upon introduction of azide-modified MIL-101(Cr) nanoparticles, the copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) reaction facilitates a site-specific and robust covalent attachment of the MOF to the alkyne-functionalized membrane. This click reaction yields a triazole linkage, replacing the azide with an imidazole-type structure and anchoring the MOF to the polymer chain. The ligand in MIL-101(Cr) contains carboxylate groups and an azide moiety introduced at the ortho-position of aminoterephthalic acid⁶⁰, which react selectively with the alkyne units on the PET TeM. Consequently, the degree of grafting, and thus the density of reactive alkynes, directly impacts MOF deposition efficiency.

Fig. 8

SEM-EDX images of MOF@PET TeM after MOF deposition on the (a) 1st, (b) 3rd, (c) 5th, and (d) 7th days; (e) change in MOF loading degree as a function of time, and (f) chromium content on the surface of MOF@PET composites.

Brunauer–Emmett–Teller (BET) analysis was carried out to examine the porosity and pore size distribution of the prepared MOF@PET composites, as well as the powder MOF and the pristine PET template (Fig. 9). According to IUPAC classification, adsorption isotherms fall into six categories, and the isotherm obtained for MOF@PET belonged to type IV⁶¹, which is typically associated with mesoporous structures in the 2–50 nm range⁶². The hysteresis loop observed also matches the behavior usually seen in industrial mesoporous adsorbents. As expected, the unmodified PET track-etched membrane showed very low porosity⁶³, and a small surface area of only 1.72 m²/g. In contrast, the azide-modified MIL-101(Cr) powder exhibited a much higher surface area, about 649.8 m²/g, which is in good agreement with literature values⁴⁹. When roughly 2% of the MOF was immobilized on the PET surface, the surface area of the composite increased noticeably, from 1.72 to about 15.6 m²/g, which confirms that the presence of the MOF effectively contributes to the overall porosity (Table 1).

Fig. 9

Nitrogen adsorption–desorption isotherms of (a) PET TeMs, (b) MOF MIL-101(Cr), and (c) MOF@PET composite.

Table 1 BET surface area and porosity parameters of the studied samples.

Sorption kinetics of U(VI) on MOF@TeM

Understanding the adsorption kinetics of U(VI) on functionalized composite membranes is essential for evaluating their practical applicability and elucidating the interaction mechanisms between the adsorbate and the active surface of the sorbent. In this study, we examined the time-dependent uptake behavior of uranium(VI) ions by MOF@PET TeM membranes, as well as the effect of pH, diffusion mechanisms, and kinetic models.

The pH of the solution is a key factor influencing uranium speciation and adsorbent surface charge, and consequently the sorption performance. In aqueous media, U(VI) predominantly exists as cationic species (e.g., UO₂²⁺, UO₂OH⁺, and (UO₂)₃(OH)₅⁺) in the pH range of 4–6⁶⁴. At higher pH values (> 6), both cationic and anionic uranyl species may coexist, with increasing prevalence of anionic forms such as (UO₂)₃(OH)₇^{– 65}. Therefore, the net surface charge of the adsorbent and the dominant U(VI) species must be carefully considered to maximize sorption efficiency.

As shown in Fig. 10a, the variation of surface charge of MOF@PET TeM as a function of pH reveals a point of zero charge (pHpzc) at approximately 6.5. Below this value, the surface acquires a positive charge, whereas at pH levels above 6.5, the surface becomes negatively charged. Since U(VI) species such as UO₂²⁺, UO₂OH⁺, and (UO₂)₃(OH)₅⁺ are positively charged in mildly acidic to neutral pH conditions (pH 4–6), operating at pH 6.3, which is just below the pHpzc, ensures favorable electrostatic attraction while minimizing structural degradation of the MOF under more acidic conditions. This selection is also consistent with previous studies that optimized U(VI) adsorption near the pHpzc of MOF-based sorbents for enhanced selectivity and capacity^64,65.

The effect of contact time on U(VI) adsorption was evaluated at this selected pH, and the results are presented in Fig. 10b. The sorption kinetics showed a relatively slow adsorption profile, with only ~ 10% of uranyl ions being removed from solution within the first 120 min. Equilibrium was reached after approximately 96 h, and the corresponding sorption capacity (Qₑ) was calculated to be 418.0 mg/g.

To assess the specific contribution of the Cr(azide)MIL-101 MOF component to the sorption performance, control experiments were conducted using alkyne-functionalized PET membranes that underwent the same surface modification steps, excluding the MOF immobilization. As shown in Fig. 10b, these control membranes exhibited a negligible uranium uptake (~ 3.4 mg g^–1 after 168 h), more than 100 times lower than that of the MOF-grafted composite. This result clearly indicates that the MOF domains are the dominant contributors to the overall sorption capacity. Although the support was originally grafted with PVAm chains, the subsequent alkyne functionalization likely blocked or replaced most of the free amine groups that could have otherwise contributed to uranyl coordination. Thus, the residual adsorption observed in the control membrane is minimal, confirming that the high uranium uptake in the composite can be primarily attributed to the immobilized Cr(azide)MIL-101 MOF. According to previous studies, the original PET membrane exhibits an inert nature towards various types of heavy metal ions, and its influence on the sorption capacity of the final composite is minimal^6,7.

The initial uranium concentration had a pronounced effect on the adsorption process. At low concentrations (10–300 mg/L), the uptake increased very sharply because the available sites on the MOF@PET surface were far from being occupied. With further increase above 100 mg/L, the adsorption continued to grow but at a slower rate, and then gradually approached a plateau, which is consistent with saturation of active sites (Fig. 10c).

According to the well-established classification of Giles, adsorption isotherms are divided into four main classes: S-type (cooperative adsorption, uptake increases with concentration), L-type (Langmuir-like, high affinity at the beginning followed by surface saturation), H-type (very steep at low concentration, indicating extremely strong interaction between adsorbent and adsorbate), and C-type (linear behavior, uptake directly proportional to concentration)^66,67. In our case, the adsorption curve belongs to the L-type, showing that U(VI) ions interact strongly with the MOF@PET surface initially, but the capacity is limited by the finite number of binding sites In addition, based on the IUPAC categorization, the adsorption isotherm also falls into Type II, which is usually characteristic of multilayer adsorption on porous materials⁶⁸. This means that uranium ions first form a strong monolayer on the MOF surface, and at higher concentrations, weaker multilayer adsorption can also occur.

Fig. 10

(a) Point of zero charge (pH_pzc) plot of the composite surfaces.

(b) Effect of contact time on U(VI) sorption capacity of the composite sorbents (pH = 6.3, [U(VI)] = 100 ppm) and (c) effect of initial uranium ion concentration on U(VI) adsorption capacity.

Kinetic modeling of U(VI) adsorption

To better understand the sorption dynamics and evaluate the governing mechanism, the experimental data were fitted to three commonly used kinetic models: pseudo-first-order, pseudo-second-order, and Elovich models. The pseudo-first-order model assumes that the rate of occupancy of adsorption sites is proportional to the number of unoccupied sites. The linearized form is⁶⁹:

$$\ln \left( {{q_e} - {q_t}} \right)=ln{q_e} - {k_1}t$$

(1)

where q_e and q_t are the adsorption capacities at equilibrium and time t, respectively, and k₁ is the rate constant. As shown in Fig. 11a, the data displayed moderate linearity, and the derived kinetic parameters (from Table 2) yielded relatively low correlation coefficients (R²), suggesting that this model does not adequately describe the adsorption behavior in this system.

The pseudo-second-order model is based on the assumption that chemisorption is the rate-limiting step, involving valency forces through sharing or exchange of electrons. Its linearized form is⁶⁹:

$$\frac{t}{{{q_t}}}=\frac{1}{{{k_2}{q_e}}}+\frac{1}{{{q_e}}}t$$

(2)

where ${k_2}$ is the second-order rate constant.

The plot of $t/{q_t}$ versus t (Fig. 11b) shows superior linearity compared to the first-order model, with higher R² values and better agreement between experimental and calculated ${q_e}$ values. This suggests that U(VI) adsorption is best described by a pseudo-second-order kinetic mechanism, consistent with chemisorption dominating the uptake process.

The Elovich equation is particularly useful for systems with heterogeneous surface energies and where desorption plays a role near equilibrium. Its mathematical form is⁷:

$${q_t}=~\frac{1}{\beta }\left( {ln\alpha \beta } \right)+\frac{1}{\beta }lnt$$

(3)

where $\alpha$ is the initial adsorption rate (mg g^–1 min^–1) and $\beta$ is the desorption constant (g mmol^–1).

As shown in Fig. 11c, the Elovich model also fits the data reasonably well, capturing the initial rapid adsorption followed by slower approach to equilibrium. According to Table 1, the high R² values and favorable α/β ratios⁷⁰ indicate that this model can complementarily describe the sorption process by accounting for surface heterogeneity and energetic distribution.

Among the three kinetic models evaluated, the pseudo-second-order model exhibited the highest correlation with experimental data. This indicates that U(VI) adsorption onto MOF@PET TeM is likely governed by chemisorption, involving interactions between U(VI) ions and specific active sites, such as carboxylate or azide-derived triazole groups.

These findings are consistent with literature reports on metal-ion uptake by MOF-modified surfaces, where the concentration of available functional groups and their accessibility strongly influence the adsorption rate and mechanism⁷¹. Furthermore, the Elovich model reinforces the presence of site heterogeneity, suggesting a combination of surface-driven and diffusion-mediated adsorption behaviors.

Table 2 Comparison of the parameters of different kinetic models for describing the sorption of U(VI) using MOF@PET TeM.

Fig. 11

Comparison of kinetic model fittings for U(VI) adsorption onto Cr(azide)MIL-101 MOF@PET TeM: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich, (d) Morris-Weber and (e) Boyd models.

Diffusion kinetics of uranium ion sorption describes the process of uranium ion adsorption by the sorbent surface, where the rate of the process is limited by the diffusion of ions in the liquid phase to the sorbent surface and/or in the sorbent pores. Depending on the conditions, the sorption process can be limited by external diffusion (diffusion in the solution volume) or internal diffusion (diffusion in the sorbent pores)^72,73. In this study, we used the Boyd and Weber–Morris model to describe the diffusion kinetics of uranium ion sorption. The intraparticle diffusion model is formulated based on the Weber–Morris approach and is used to determine the stage that limits the rate of the adsorption process⁷⁴ (Table 2). Generally, substances present in a solution are adsorbed by mass transfer-surface, film diffusion and diffusion through pores. It is known that if the graph of Q_t versus t_1/2 (Fig. 11d) is linear and passes through the origin, then intraparticle diffusion is the rate-controlling step⁷⁵. From the presented data, it is clear that for MOF@PET composites, the kinetic curve is bilinear (i.e., uranyl ion sorption is accompanied by two separate steps). The first, sharper step is associated with the diffusion of U(VI) ions in the boundary layer. The second step corresponds to the final equilibrium stage. In addition, the first linear section does not pass through the origin, indicating high initial sorption of U(VI) with diffusion through the solution to the outer surface of the adsorbent through a thick boundary layer. The corresponding intraparticle diffusion parameters are: $\:{k}_{id}$= 0.65 mg g^–1 min^–1/2 and C = 411.6 mg g^–1, confirming the strong boundary layer effect (Table 2).

The Boyd model is used to understand whether film diffusion is the rate-controlling step of the adsorption process⁷⁶. According to this kinetic model, the external surface around the adsorbent has a major influence on the diffusion of the adsorbate and pore diffusion controls the mass transfer when the graph is linear and passes through the origin. Figure 11e shows the graph of Boyd’s model for U(VI) adsorption: it can be concluded that the graph does not pass through the origin and therefore the rate is controlled by film diffusion. The apparent Boyd rate constant was calculated as k = 0.20 min⁻¹ (Table 2).

Adsorption isotherm modeling for U(VI) uptake

The accurate interpretation of adsorption isotherms is essential for understanding the interaction mechanisms between adsorbent surfaces and target contaminants. In this study, three widely used isotherm models; Langmuir, Freundlich, and Dubinin–Radushkevich (D–R), were employed to evaluate the U(VI) adsorption behavior of MOF@PET TeM composite membranes.

Langmuir isotherm

The Langmuir model is based on the assumption of monolayer adsorption onto a homogeneous surface with a finite number of identical binding sites. It assumes no interaction between adsorbed molecules, and that adsorption occurs uniformly until saturation is reached. The Langmuir adsorption model, originally developed to describe gas–solid adsorption phenomena, is widely used to assess and compare the adsorption capacities of various solid adsorbents in liquid-phase systems as well⁷⁷. The model assumes monolayer adsorption onto a surface with a finite number of energetically equivalent and non-interacting sites. It further proposes that dynamic equilibrium is achieved when the rates of adsorption and desorption become equal. In this context, the rate of adsorption is directly proportional to the number of available (unoccupied) active sites, whereas the rate of desorption depends on the number of occupied sites on the adsorbent surface⁷⁸. This balance is mathematically described by the linearized Langmuir isotherm equation, as presented in Eq. (4):

$$\frac{{{C_e}}}{{{q_e}}}=\frac{{{C_e}}}{{{Q_0}}}+\frac{1}{{{Q_0}b}}$$

(4)

Here, ${C_e}$ (mg L^–1) is the equilibrium concentration of the adsorbate in solution, ${q_e}$ (mg g^–1) is the amount of adsorbate adsorbed at equilibrium, ${Q_0}$ (mg g^–1) represents the maximum monolayer adsorption capacity, and b (L mg^–1) is the Langmuir constant associated with the affinity of the binding sites. Both parameters b and ${Q_0}$ are characteristics values that reflect the basic properties of the “adsorbent-adsorbate” pair presented in the Table 3. The linear plot of C_e vs. C_e/${q_e}$ dependence is shown in Fig. 12a.

Freundlich isotherm

The Freundlich model is an empirical equation that assumes adsorption occurs on a heterogeneous surface with varying site energies. It is particularly suitable for describing multilayer adsorption. The linearized form is:

$$ln{q_e}=ln{k_F}+\frac{1}{n}ln{C_e}$$

(5)

Where ${k_F}$ (mg g^–1) is the Freundlich constant indicating adsorption capacity, and n is the adsorption intensity. Values of n in the range of 1–10 indicate favorable adsorption⁷⁹. From Fig. 12b, the model yielded an n value of 0.85, and ${k_F}$ was calculated to be 5.29 mg g^–1, indicating favorable adsorption and strong interaction strength. The Freundlich isotherm fitting (Fig. 12b) yielded a higher correlation coefficient (R²) compared to the Langmuir model, suggesting that the adsorption of U(VI) onto MOF@PET TeM occurs on a heterogeneous surface with varying adsorption energies rather than through ideal monolayer formation. These findings are consistent with the composite’s chemically diverse surface, where multiple functional groups (e.g., amine, triazole, carboxylate) contribute to the uptake process through different binding affinities.

Fig. 12

Adsorption isotherm model fittings for U(VI) adsorption onto Cr(azide)MIL-101 MOF@PET TeM: (a) Langmuir, (b) Freundlich, and (c) Dubinin–Radushkevich (D–R) models.

Dubinin-Radushkevich (D–R) isotherm

Usually Dubinin-Radushkevich isotherm model applies for describing adsorption in microporous materials⁷⁰. This model proposed that adsorption has a multilayered nature, including Van der Waals forces⁶.

The Dubinin–Radushkevich (D–R) isotherm model provides insights into the sorption mechanism by differentiating between physical adsorption, ion-exchange, and chemisorption based on adsorption energy. Unlike the Langmuir model, which assumes surface homogeneity, the D–R model considers the influence of pore structure and adsorption potential, making it particularly useful for microporous adsorbents such as MOFs⁸⁰. The linear form of the D–R equation is expressed as⁸¹:

$$ln{q_e}=ln{Q_d} - \beta {\varepsilon ^2}$$

(6)

Here, qe is the equilibrium adsorption capacity (mg g^–1), Q_d is the is the theoretical maximum sorption capacity (mg g^-1), β is the activity coefficient related to mean adsorption energy (mol² kJ^{- 2})⁴ and ε is the Polanyi potential, calculated via:

$$\varepsilon =RTln\left( {1+\frac{1}{{Ce}}} \right)$$

(7)

Based on the D–R fitting shown in Fig. 12c, the calculated Q_d value was 34.8 mg g^–1, and β was 0.002 mol² kJ^–2. These values suggest a notable adsorption capacity and indicate that the MOF-modified membrane surface provides a highly accessible microporous network for U(VI) ion uptake.

To further elucidate the nature of the adsorption mechanism, the mean free energy of sorption (${E_{DR}}$) was calculated using the following relationship:

$${E_{DR}}=\frac{1}{{\sqrt { - 2\beta } }}$$

(8)

The calculated ${E_{DR}}$ value was 14.4 kJ mol^–1 provides important insight into the strength of interactions driving U(VI) sorption in this system. According to widely accepted criteria in the literature, when the ${E_{DR}}$ value is less than 8 kJ mol^–1, the sorption process is typically governed by physical adsorption. If the energy falls in the 8 to 16 kJ mol^–1 range, it is indicative of an ion-exchange mechanism, whereas values greater than 20 kJ mol^–1 are generally associated with chemisorption⁷⁰. Although this intermediate energy value numerically aligns with ion-exchange behavior, it is important to emphasize that the D–R model offers an indirect estimate based solely on adsorption energy, not a definitive identification of the sorption mechanism. Thus, the result should be interpreted as a supporting clue, not conclusive evidence.

Table 3 Langmuir, Freundlich and Dubinin–Radushkevich isotherm constants for the adsorption of U(VI) ions onto MOF@PET sorbents.

From a chemical standpoint, the U(VI) ion (UO₂²⁺) is a linear and strongly oxophilic cation that preferentially coordinates with electron-rich oxygen donor ligands, such as carboxylate, hydroxyl, and carbonyl groups^82,83. In the current system, such functional groups are most prominently found on the surface of the MIL-101(Cr) framework, which is constructed from trimeric Cr(III)-oxo clusters and terephthalate linkers forming a robust mesoporous architecture⁸⁴. After solvent removal during post-synthetic handling, MIL-101(Cr) is known to present coordinatively unsaturated Cr³⁺ centers and terminal oxygenated groups on its internal surface⁸⁵. These Lewis acidic sites are capable of interacting strongly with uranyl species via the formation of inner-sphere surface complexes, wherein UO₂²⁺ binds to the framework’s surface ligands through coordination^{42,85,86,87,88}. Such interactions differ from classical ion exchange, which involves electrostatic substitution of counterions without direct ligand involvement. Instead, the adsorption behavior observed here aligns with surface complexation mechanisms that are widely reported for actinide-MOF systems. Several studies have demonstrated that oxygen atoms associated with metal clusters or organic linkers within MOFs serve as anchoring points for uranyl ions, forming stable coordination environments through donor–acceptor interactions^32,86,89,90.

Furthermore, it has recently been reported that a hydrogen-bonding network can form between the oxygen atoms in the MIL-101 framework and the coordinated water molecules of the uranyl ion. This secondary interaction was shown to enhance the stability of the uranyl adsorption configuration within the MOF structure⁸⁶. In our system, a similar interaction may contribute to reinforcing the local coordination environment and improving the spatial stability of UO₂²⁺ within the porous architecture.

While minor contributions from residual functional groups on the PET substrate or partially oxidized PVAm chains cannot be entirely ruled out, control experiments clearly indicate that their role is negligible compared to the dominant adsorption sites located within the MIL-101(Cr) framework. Hence, the sorption performance of the MOF@PET TeM composite is primarily attributed to the MOF’s structural and chemical features, including accessible pores, oxygen-rich coordination environments, and high surface area.

The isotherm analysis further supports this mechanism. Among the models tested, the Freundlich isotherm provided the best fit (R² = 0.9907), suggesting adsorption occurs on a heterogeneous surface with a range of binding energies. The D–R model also exhibited a strong fit (R² = 0.9457), slightly outperforming the Langmuir model (R² = 0.9428). This indicates that both energetic heterogeneity and pore-filling effects contribute to the adsorption process, which is expected for microporous materials like MIL-101. The steric incorporation of U(VI) into the MOF’s cage-like domains, combined with multiple coordination modes, creates a hybrid interaction model involving spatial confinement, surface complexation, and potentially localized electrostatics.

In summary, although the energy value from the D–R model numerically supports an ion-exchange–like interaction strength, the actual sorption mechanism is better described as coordination-driven surface complexation. The role of residual amines appears secondary, and no direct evidence of classical ion exchange (i.e., counter-ion release) is observed. The structural features of the MOF@PET TeM composite; its microporosity, oxygen-rich surface chemistry, and immobilized MOF domains, collectively facilitate efficient U(VI) uptake under mildly acidic conditions.

Stability and selectivity of the MOF@PET composite sorbents

The stability of a sorbent’s sorption capacity is a crucial factor for its reusability and overall effectiveness. The ability to maintain adsorption performance over multiple cycles is essential for ensuring cost-effectiveness and practical applicability on industrial and environmental scales.

In this study, the MOF@PET composite was tested for five consecutive cycles. Desorption of uranium(VI) ions was performed in 0.1 M nitric acid, and after drying, the material was reused without any additional activation procedures. As shown in Fig. 13a, the sorption capacity of the MOF@PET sorbent decreased by 18.6% and 30.5% after the third and fifth cycles, respectively, compared with the first test. Nevertheless, even after five cycles, the MOF@PET sorbent retained a high sorption capacity of 290.9 mg/g.

A plausible explanation for the gradual loss in sorption efficiency is the leaching of the active MOF phase from the membrane surface. Elemental mapping of the composite using EDS (Fig. 14) confirmed that, after the fifth cycle, the chromium content on the membrane decreased from 5.7 at% to 3.78 at% compared with the initial loading.

The effect of competitive cations (Co²⁺, Cr²⁺, Pb²⁺, Zr²⁺, and Zn²⁺) on uranium(VI) uptake was also examined to evaluate the selectivity of MOF@PET. As shown in Fig. 13b, the total adsorption capacity for uranium(VI) was significantly higher than that for the other coexisting metal ions, highlighting the strong affinity and remarkable selectivity of the composite. This suggests that MOF@PET is suitable for practical water treatment applications where multiple competing ions are present. Furthermore, the distribution coefficient (K_d) values calculated from the experimental data (Fig. 13c) revealed a K_d(U) of 7.97 × 10⁷ mL/g, which is in good agreement with, and in some cases exceeds, the values reported for other uranium(VI) adsorbents^91,92.

Fig. 13

(a) Reusability test for the sorption of U(VI), (b) effect of competitive adsorption and coexisting ions and (c) corresponding sorption distribution coefficient.

Fig. 14

Elemental mapping of MOF@PET composite sorbents after several testing cycles.

To contextualize the performance of the MOF@PET TeM composite, its uranium adsorption capacity was compared with several previously reported MIL-101-based sorbents under various experimental conditions, as summarized in Table 4.

Table 4 Comparison of the U(VI) adsorption capacity of the Cr(azide)MIL101 MOF@ TeMs and other sorbents.

The maximum sorption capacity (Qₑ) of the composite developed in this study was 418 mg g^–1 at pH 6.5 and 298 K. Although this value is not the highest reported, it exceeds many comparable systems. For instance, MIL-101 functionalized with ethanediamine (MIL-101-ED) and with NH₂ groups (MIL-101-NH₂) demonstrated lower Qe values of 200 mg g^–1 and 90 mg g^–1, respectively⁴¹. Similarly, Fe₃O₄@AMCA-MIL53(Al), another MOF composite system, reached only 227.3 mg g^–1 under slightly more acidic and elevated temperature conditions⁹⁴.

Interestingly, the MOF@PET TeM composite displayed a higher uranium uptake (418 mg g^–1) compared to the powder form of Cr(azide)MIL-101 MOF (314 mg g^–1) under similar conditions⁴⁹. This enhancement may be attributed to several factors, including improved dispersion of MOF particles on the membrane surface, reduced aggregation, and better accessibility of sorption sites due to the structural openness of the track-etched PET support. Moreover, the prolonged equilibrium time (96 h) may facilitate deeper uranyl diffusion into MOF pores, further enhancing overall uptake.

The highest adsorption value in the comparison belongs to MIL-101-AO (amidoxime-functionalized) with 613.5 mg g^–1, which benefits from the well-documented high uranyl affinity of amidoxime groups⁹³. In contrast, while our MOF@PET TeM composite presented a lower capacity (418 mg g^–1), it offers distinct advantages in terms of structural integrity, ease of handling, and potential for device integration. Notably, the membrane-based configuration provides a mechanically stable and freestanding sorbent that can be easily retrieved from solution without centrifugation or filtration, even using simple tools such as tweezers. This facilitates not only batch sorption but also potential implementation in continuous flow or fixed-bed systems. Moreover, the uniform distribution of MOF particles on the porous and chemically functionalized PET substrate enables efficient mass transfer and minimizes particle loss, making the composite highly attractive for practical uranium separation applications.

Conclusion

The removal of uranium (VI) from aqueous environments remains a critical environmental challenge. In this work, we present a novel approach that combines metal-organic frameworks (MOFs) with polymeric track-etched membranes (TeMs) to develop an efficient and stable composite sorbent. The immobilization of Cr(azide)MIL-101 MOFs onto a PET TeM substrate was achieved through a carefully optimized surface modification strategy involving RAFT grafting, hydrolysis, and click chemistry. Each step was confirmed through detailed surface and structural characterizations. The resulting MOF@PET TeM composites exhibited a high U(VI) adsorption capacity of 418 mg g^–1, achieved under mildly acidic conditions. Adsorption kinetics followed a pseudo-second-order model, while isotherm analyses indicated a heterogeneous surface with microporous contributions. Based on energy estimations and structural considerations, the sorption mechanism is best described as surface complexation, likely involving coordination of UO₂²⁺ ions to oxygen-donor sites within the MIL-101(Cr) framework. While the overall performance is promising, a clear limitation is the long equilibrium time (96 h), which highlights the need for further optimization of mass transport properties. Future work may aim to reduce kinetic constraints by increasing MOF loading, tailoring pore structure, or introducing hierarchical porosity.

Importantly, the membrane architecture presents significant advantages over powder-based sorbents, particularly in terms of ease of handling, physical robustness, and potential applicability in flow-through or modular filtration systems. This practical usability offsets the slower sorption kinetics and underscores the membrane’s translational potential for real-world remediation scenarios. Furthermore, the click-based integration approach preserved the functional integrity of both the MOF and the polymeric support, suggesting that this strategy could be extended to the immobilization of other functional porous materials for selective removal of a broad range of heavy metals or radionuclides. Overall, our results demonstrate that MOF-functionalized TeMs are both feasible and versatile. Overall, our results show that MOF-functionalized TeMs can effectively capture uranium(VI) and provide a solid basis for developing reusable and scalable membrane-based adsorbents for environmental applications.

Methods

Compounds

All chemicals were of analytical grade and used without further purification. Benzophenone (BP), sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂), N-vinylformamide (NVF), hydrochloric acid (HCl), 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPPA), propiolic acid (PA), dicyclohexylcarbodiimide (DCC), tert-butyl nitrate (tBuONO), azidotrimethylsilane (TMSN₃), 2-aminoterephthalic acid, chromium(III) nitrate nonahydrate (Cr(NO₃)₃·9 H₂O), copper(II) acetate monohydrate (Cu(OAc)₂·H₂O), sodium L-ascorbate, butanol (BuOH), ethanol (EtOH), dimethylformamide (DMF), dichloromethane (DCM), tetrahydrofuran (THF), and tert-butyl alcohol (t-BuOH) were purchased from Sigma-Aldrich (Schnelldorf, Germany), uranyl nitrate hexahydrate (UO₂(NO₃)₂·6H₂O) was obtained from Sisco Research Laboratories Pvt. Ltd., India. Deionized (DI) CPPA was used as the RAFT agent for controlled polymerization. All aqueous solutions were prepared using deionized water (resistivity: 18.2 MΩ·cm, Aquilon D-301 system).

Modification of PET tem polymer template

Polyethylene terephthalate (PET) Hostaphan^® RNK films (Mitsubishi Polyester Film, Germany) were used as the base polymer for TeM fabrication. Irradiation was carried out at the Institute of Nuclear Physics (Kazakhstan) using⁸⁴Kr¹⁵⁺ ions with controlled energy and fluence, followed by chemical etching in 2.2 M NaOH to create cylindrical nanopores. The resulting PET TeMs exhibited an average pore diameter of 333.5 ± 14.9 nm with a pore density of 1 × 10⁸ ions cm^–2. To introduce surface –COOH groups, PET TeMs were oxidized in 500 mM H₂O₂ at pH 3 for 90 min per side under 254 nm UV light (190 W). The carboxylic acid concentration was determined using the method detailed elsewhere¹⁴ which showed a COOH density of 9.0 ± 0.1 µM g^–1 for oxidized membranes, compared to 3.0 ± 0.04 µM g^–1 in untreated samples. Following oxidation, membranes were immersed in a 5% (w/v) BP solution in DMF for 24 h at room temperature to immobilize the photoactive initiator, facilitating subsequent graft polymerization¹⁷.

Grafting of PNVF onto PET TeMs and hydrolysis to PVAm

PNVF grafting was performed via UV-induced RAFT polymerization. Oxidized PET samples (2 cm²) were placed in sealed glass vials containing NVF (10, 30, or 50% w/v) and CPPA in various solvents (EtOH, n-PrOH, i-PrOH, and BuOH). Polymerization was initiated under 295 nm UV light (Ultra-Vitalux 300 W, Osram, Germany) at a 10 cm distance for 2–6 h. After polymerization, membranes were rinsed with deionized water and dried. The grafting degree (GD) of PNVF-g-PET TeMs was calculated gravimetrically using Eq. (9):

$$GD\% =\frac{{\left( {{m_2} - {m_1}} \right)~}}{{{m_1}}} \times 100\%$$

(9)

where ${m_1}$and ${m_2}~$are the weights before and after grafting, respectively. Subsequent hydrolysis of the PNVF-grafted PET was performed in NaOH solutions (0.05–0.2 M) at room temperature for 1–20 min. Amino group density of PVAm-g-PET TeMs was determined using the protocol described in⁹⁶.

Functionalization of PVAm-g-PET TeMs with alkyne groups (alkyne@PET TeMs)

PVAm-g-PET TeMs were functionalized with terminal alkynes through DCC-mediated amidation. The dried membranes were immersed in 10 mL DCM containing 0.058 mmol DCC and 0.37 mL (5.9 mmol) PA. The reaction proceeded at room temperature. DCC activates the carboxyl group of PA, enabling its coupling with primary amines on the membrane surface, forming stable amide bonds and installing alkyne functionality.

Synthesis and immobilization of Cr-MIL101-N₃ MOFs via click chemistry

Amino-functionalized MIL-101(Cr) MOFs were synthesized hydrothermally by mixing 5.2 mmol 2-aminoterephthalic acid and 5.0 mmol Cr(NO₃)₃·9 H₂O. The mixture was stirred for 3.5 h at room temperature and then sealed in a Teflon-lined autoclave at 130 °C for 24 h⁵⁶. Azide functionalization was carried out using post-synthetic modification, following previously reported methods⁴⁹. Following azide modification, the resulting Cr-MIL101-N₃ particles were recovered by centrifugation, washed thoroughly with ethanol and deionized water, and subsequently dried under vacuum at 60 °C for 12 h to remove residual solvents. This mild drying step was applied as a pre-activation treatment to ensure the removal of pore-occluding solvent molecules.

For immobilization, alkyne@PET membranes were added to a mixture of t-BuOH: H₂O (1:1 v/v) containing Cu(OAc)₂·H₂O (0.025 mmol) and sodium L-ascorbate (0.178 mmol). The reaction was conducted in a sealed system at 35 °C for 72 h to complete the Cu(I)-catalyzed azide–alkyne cycloaddition. After completion of the click reaction, the MOF-grafted PET membranes were thoroughly washed with deionized water and ethanol to remove unreacted components and loosely bound species. To ensure removal of residual solvents and to expose the porous domains of the immobilized MOFs, the membranes were subsequently dried under vacuum at 60 °C for 12 h prior to adsorption experiments.

Characterization of prepared samples

The physical and chemical properties of both the unmodified and functionalized PET track-etched membranes (TeMs) were thoroughly characterized using a combination of complementary analytical techniques.

Pore structure and size were evaluated by porometry calculations based on the Hagen–Poiseuille equation, which estimates flow-related pore characteristics from fluid transport behavior⁹⁷. This method was applied to both pristine and modified TeMs to assess any alterations in the pore geometry resulting from surface functionalization and MOF immobilization.

Fourier-transform infrared spectroscopy (FTIR) was employed to identify chemical functional groups introduced at each modification stage. Spectra were recorded using an Infra LUM FT-08 spectrometer equipped with an ATR accessory (GladiATR, PIKE Technologies), over the spectral range of 400–4000 cm^–1. The evolution of key vibrational bands was monitored to confirm successful monomer grafting, hydrolysis, and MOF immobilization.

Water contact angle (CA) measurements were performed to assess surface wettability changes upon each modification. Measurements were taken using a DSA100 goniometer (Krüss GmbH, Hamburg, Germany) at ambient temperature. For each membrane type, five droplets of deionized water (10 µL each) were deposited, and the average contact angle was calculated using the Young–Laplace fitting method within the Drop Shape Analysis software. A decrease in contact angle was interpreted as an increase in surface hydrophilicity due to the introduction of polar groups or porous materials.

Scanning electron microscopy (SEM) was used to visualize surface morphology and pore structure. SEM images were obtained using a Phenom ProX Desktop SEM (Thermo Scientific, MA, USA) at magnifications ranging from 10,000× to 50,000×, under an accelerating voltage of 20 kV. In addition, elemental analysis was conducted using a Hitachi TM3030 SEM (Hitachi Ltd., Tokyo, Japan) coupled with a Bruker XFlash MIN SVE energy-dispersive X-ray spectroscopy (EDX) system, operated at 15 kV. Elemental mapping was used to confirm the presence and distribution of key elements such as Cr (from the MOF), N (from amine groups), and S (from the RAFT agent).

X-ray photoelectron spectroscopy (XPS) was employed to determine surface elemental composition and chemical states. Measurements were carried out using a Thermo Scientific K-Alpha spectrometer (Waltham, MA, USA) equipped with a monochromatic Al Kα X-ray source (photon energy: 1486.6 eV). Survey spectra were collected with a pass energy of 200 eV (1.0 eV step), and high-resolution core-level spectra were recorded with a pass energy of 30 eV (0.1 eV step). A dwell time of 100 ms was used throughout. All spectra were acquired at a 90° take-off angle under ultra-high vacuum (pressure ≤ 2 × 10^–9 torr). A spot size of 400 μm was used for all analyses. The elemental composition and chemical environment of surface atoms (e.g., Cr 2p, N 1s, C 1s, O 1s) were interpreted to validate each functionalization step.

Nitrogen adsorption–desorption isotherms were obtained using a V-Sorb 2800P analyzer (Gold APP Instrument, Xi’an, China) to assess the specific surface area and pore volume of the final MOF@PET composites. The data were analyzed using the Brunauer–Emmett–Teller (BET) method to determine surface area and pore size distribution, allowing comparison with known microporous or mesoporous structures.

Bath absorption experiments

Batch adsorption experiments were conducted to evaluate the uranium (VI) sorption performance of the synthesized MOF@PET composite membranes under controlled equilibrium conditions. Prior to sorption testing, the surface charge behavior of the composites was characterized by determining the point of zero charge (pH_pzc). For this purpose, 1 × 1 cm² membrane samples were immersed in 0.01 M NaCl solutions adjusted to different pH values ranging from 4 to 8. The pH adjustments were made dropwise using 1.0 M HCl and 1.0 M NaOH. After equilibrium was reached, the final pH was recorded using a calibrated pH meter (HANNA HI2020-02, HANNA Instruments, Smithfield, USA).

The uranium adsorption kinetics were studied using a standard batch method at an initial U(VI) concentration of 100 ppm and a solution pH of 6.5. Composite membranes (2 × 2 cm²) were added to 15 mL of uranium solution in disposable polypropylene vials (Isolab, Eschau, Germany). The samples were agitated at 100 rpm using an orbital shaker (IKA KS 3000 IS control, IKA, Königswinter, Germany) and incubated at room temperature for varying durations, ranging from 24 to 168 h. At predetermined time intervals, aliquots of the solution were withdrawn and filtered for analysis. The residual uranium concentration in each sample was measured by inductively coupled plasma–mass spectrometry (ICP–MS, Thermo Fisher Scientific XSeries 2, Bremen, Germany).

The amount of U(VI) and other coexisting ions adsorbed by the composite membrane at equilibrium as well as distribution coefficient ${K_d}$ were calculated using Eqs. (10) and (11):

$$Qe=\frac{{\left( {{C_0} - ~{C_e}} \right) \times V}}{m}$$

(10)

$${K_d}=\frac{{\left( {{C_0} - ~{C_e}} \right) \times V \times 1000}}{{{C_e} \times m}}$$

(11)

where $Qe$ is the amount of U (VI) adsorbed by MOF@PET TeM (mg/g), ${C_0}$ is the feed concentration (mg/L), ${C_e}$ is the concentration of U(VI) in aliquots (mg/L), V is the volume of the solution (L), and m is the weight of MOF@PET TeM (g). This calculation approach is consistent with previously published methodologies^93,94,95.

To assess the selectivity behavior of the MOF@PET adsorbent, a combination of competitive cations coexisting in solution (Co²⁺, Cr³⁺, Pb²⁺, Zr²⁺ and Zn²⁺) with a concentration of 60 mg/L (10 mg/l for each pollutant) was combined with 5.01 mg of adsorbent in 15 mL of solution.

To explore the possibility for reusability of sorbent, desorption experiments were performed: the uranium sorbed MOF@PET composites were washed with deionised water for removal of any unsorbed uranium and then stirred at the room temperature with 50 mL of HNO₃ (0.1 M) within 1 h and dried in vacuum oven over night. The experimental conditions for reusability tests: sorption time − 96 h, U(VI) concentration – 100 mg/l, pH – 6.3).