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Article

Synthesis of a Novel Zwitterionic Hypercrosslinked Polymer for Highly Efficient Iodine Capture from Water

by
Jingwen Yu
1,2,3,
Luna Song
4,
Bingying Han
1,2,
Jiangliang Hu
1,2,
Zhong Li
1,2,* and
Jie Mi
1,2,*
1
State Key Laboratory of Clean and Efficient Coal Utilization, Taiyuan University of Technology, Taiyuan 030024, China
2
Key Laboratory of Coal Science and Technology, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China
3
Lu’an Chemical Group Co., Ltd., Changzhi 046204, China
4
Shanxi Institute of Energy, Jinzhong 030600, China
*
Authors to whom correspondence should be addressed.
Polymers 2024, 16(19), 2846; https://doi.org/10.3390/polym16192846
Submission received: 12 July 2024 / Revised: 27 September 2024 / Accepted: 2 October 2024 / Published: 9 October 2024
(This article belongs to the Special Issue Resilient Polymer Materials: Reduce the Risks and Effects of Natural Disasters)
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Versions Notes

Abstract

Cationic porous organic polymers have a unique advantage in removing radioactive iodine from the aqueous phase because iodine molecules exist mainly in the form of iodine-containing anions. However, halogen anions will inevitably be released into water during the ion-exchange process. Herein, we reported a novel and easy-to-construct zwitterionic hypercrosslinked polymer (7AIn-PiP)-containing cationic pyridinium-type group, uncharged pyridine-type group, pyrrole-type group, and even an electron-rich phenyl group, which in synergy effectively removed 94.2% (456 nm) of I2 from saturated I2 aqueous solution within 30 min, surpassing many reported iodine adsorbents. Moreover, an I2 adsorption efficiency of ~95% can still be achieved after three cyclic evaluations, indicating a good recycling performance. More importantly, a unique dual 1,3-dipole was obtained and characterized by 1H/13C NMR, HRMS, and FTIR, correlating with the structure of 7AIn-PiP. In addition, the analysis of adsorption kinetics and the characterization of I2@7AIn-PiP indicate that the multiple binding sites simultaneously contribute to the high affinity towards iodine species by both physisorption and chemisorption. Furthermore, an interesting phenomenon of inducing the formation of HIO2 in unsaturated I2 aqueous solution was discovered and explained. Overall, this work is of great significance for both material and radiation protection science.

1. Introduction

On 24 August 2023, the Fukushima Daiichi nuclear power plant began discharging more than 1.3 million tons of nuclear sewage into the Pacific Ocean, and will continue this behavior over the next three decades [1,2]. Japan’s nuclear sewage contains many kinds of harmful radionuclides, among which the excessive amount of 129I and 131I has aroused social concern because they can significantly induce thyroid cancer [3,4]. In particular, 129I has an ultra-long half-life (~1.57 × 107 years) [5,6]. If not appropriately handled, the hazardous radioiodine will inevitably have a serious impact on the marine ecosystem and cause great harm to human health. Therefore, it is necessary and meaningful to delve the removal of radioactive iodine in water. The solid-phase adsorption method is considered as an effective way to remove radioiodine from water due to its low cost, high efficiency, simple operation, and its use as a recyclable and reusable adsorbent [7].
As an important subgroup of porous organic polymers (POPs), ionic porous organic polymers (i-POPs) are novel and fascinating porous organic materials in which ionic moieties can be directly incorporated or post-synthetically added to the polymer backbones or side chains [8,9]. The high charge density and strong polarizability of i-POPs endow them with unique advantages in electrostatic interaction applications [10]. The recent literature reflects an upsurge in the development of i-POPs such as electrospun fiber adsorbents (N-MOF-PAN fibers) [11], imidazolium-based i-POP [12], and benzoquinone-derived porous hydrophenazine frameworks [13] for iodine removal from water. As is known to all, iodine molecules (I2) tend to form triiodide anions (I3−), which are the major components in aqueous phases [12,14,15]. All these works found that cationic binding sites are beneficial for enriching I3− through electrostatic interactions driving the ion-exchange process from Br−/Cl− to I3− [11,12,13].
With the development of the synthetic methodology, diverse ionic building blocks and i-POPs with unique structures can be rationally designed. In fact, the ionization of organic skeletons is a common phenomenon. For example, the N atoms in aromatic heterocyclic/aliphatic compounds are easily ionized through SN2 nucleophilic substitution reactions [11,12] or protonated in an acidic solution [13], which leads to quaternization and generates cationic frameworks. It should be noted that the negative halogen ions are simultaneously introduced into the frameworks of the corresponding polymers as equilibrium ions. Although the application of i-POPs for iodine capture from water has made some progress, it is still limited due to the additional negative halogen ions that are released into the aqueous phase during the adsorption process. To solve this issue, the design and development of novel i-POPs for iodine capture without the ion-exchange process in aqueous solution remain urgent tasks.
Inspired by the one-pot synthesis of viologen-based hypercrosslinked polymers via a sequential process of ionization and polymerization [16], herein, a similar strategy was applied to construct a novel i-POP with the zwitterionic scaffold. 7-azaindole (7AIn) and the excess α,α-dichloro-p-xylene (DCX) were selected as building blocks. Initially, the zwitterionic building unit was predesigned via SN2 nucleophilic substitution and the pyrrole-type H elimination reactions between 7AIn and DCX. Subsequently, Friedel–Crafts alkylation reaction was used to construct a hypercrosslinked network under the catalysis of anhydrous FeCl3. More importantly, the formation mechanism of the obtained polymer (7AIn-PiP) was proposed by analyzing the structure of the important intermediate product. To the best of our knowledge, a zwitterionic hypercrosslinked polymer for iodine adsorption from water has rarely been reported. In this paper, the utility of 7AIn-PiP for highly efficient iodine capture from water was also demonstrated. The adsorption kinetics and the recycling performance of 7AIn-PiP from saturated I2 aqueous solution were evaluated systematically. Furthermore, the iodine adsorption mechanism was comprehensively described by analyzing the characterizations of the iodine-loaded 7AIn-PiP. Ultimately, a new phenomenon of iodine adsorption by 7AIn-PiP in unsaturated I2 aqueous solution was discovered and explained.

2. Experimental Section

2.1. General Remarks

2.1.1. Materials

All reagents used in this work were purchased from commercial sources without further purification unless otherwise stated. 1,2-dichloroethane (DCE) was dehydrated with CaH2 prior to use. Nonradioactive I2 was used in iodine adsorption experiments.

2.1.2. Characterization

N2 adsorption–desorption tests were performed on a JW-BK122W N2-sorption apparatus (Beijing JWGB Sci. & Tech. Co., Ltd., Beijing, China); the specific surface area of the sample was calculated based on the Bruner–Emmett–Teller (BET) equation, and the pore size distribution curve was obtained based on BJH theory. UV–Vis absorption spectra were tested on a spectrophotometer (Varian Cary 300, Palo Alto, CA, USA). Fourier-transformed infrared (FTIR) spectra were conducted on a Bruker Vertex 70 FTIR instrument, Billerica, MA, USA (KBr tablet method). 1H NMR and 13C NMR spectra were measured on a JEOL 600 MHz spectrometer using H2O and DMSO as the internal standards in DMSO-d6. High-resolution mass spectroscopies (HRMSs) were recorded using a Thermo Scientific Q Exactive mass spectrometer, Waltham, MA, USA. Solid-state 13C cross-polarization/magic-angle-spinning (CP/MAS) nuclear magnetic resonance (NMR) spectra were recorded using a Bruker Avance III 600 MHz NMR spectrometer. Electron paramagnetic resonance (EPR) spectroscopy was observed using a Bruker EMXplus-9.5/12 spectrometer. Scanning electron microscopy (SEM) images were obtained by using a Hitachi S-4800 field emission scanning electron microscope, Tokyo, Japan. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2100F transmission electron microscope with an acceleration voltage of 200 kV. Powder X-ray diffraction (PXRD) patterns were collected on a Shimadzu XRD-6000 X-ray powder diffractometer, Kyoto, Japan, with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was measured using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer. Raman spectroscopy was measured on a Horiba LabRAM HR Evolution Raman spectrometer, Kyoto, Japan (laser wavelength: 514 nm).

2.2. Synthesis of Materials

The detailed descriptions about the “preparation and characterization of I2 aqueous solutions” and “preparation of a batch of polymers and screening of the optimal adsorbent” are provided in Supplementary Materials Sections S1 and S2, respectively.

2.2.1. Synthesis of 7AIn-PiP

Under N2 protection, 7AIn (295.4 mg, 2.5 mmol) and DCX (1.7506 g, 10 mmol) were added into a dry 100 mL double-necked round-bottomed flask equipped with a reflux condenser and a magnetic stirrer. After DCE (60 mL) was injected into the flask, the mixture was stirred at 80 ℃ for 24 h. After the reaction system was cooled to room temperature, anhydrous FeCl3 (1.6220 g, 10 mmol) was added to the mixture under N2 protection as soon as possible. Then, the obtained mixture was refluxed at 80 °C for an additional 24 h. After being cooled down to room temperature again, the insoluble solid was obtained by suction filtration and continuously washed with methanol to remove the unreacted monomers, oligomers, and FeCl3 until the filtrate became colorless. The crude product was further extracted with methanol by Soxhlet extraction for 24 h, and the corresponding yellow-brown powder (1.2483 g, 95%) was obtained by vacuum drying at 90 °C for another 24 h. The schematic representation for the synthesis of 7AIn-PiP is illustrated in Scheme 1.

2.2.2. Synthesis of the Dual 1,3-Dipole

In order to precisely unveil the composition and structural information of 7AIn-PiP, we strived to obtain the new product after the SN2 nucleophilic substitution reaction.
Under a N2 atmosphere, 7AIn (295.4 mg, 2.5 mmol) and DCX (1.7506 g, 10 mmol) were added into a dry 100 mL double-necked round-bottomed flask equipped with a reflux condenser and a magnetic stirrer. Then, 60 mL of DCE was injected into the flask and the mixture was stirred at 80 °C for 24 h. After being cooled down to ambient temperature, an appropriate amount of silica gel was added into the solution, and the solvent was removed under reduced pressure. The obtained mixture was loaded and purified through silica gel column chromatography with gradient eluent (ethyl acetate:petroleum ether = 1:4 (v/v), then methanol). After evaporation of the eluate under reduced pressure, 7AIn (120.5 mg) and DCX (1.5750 g) that did not participate in the reaction were recovered. Most importantly, a new white solid product with a yield of 235.5 mg was obtained. The schematic representation for the synthesis of the dual 1,3-dipole is depicted in Scheme 2.

2.3. I2 Adsorption Experiments of 7AIn-PiP in I2 Aqueous Solution

2.3.1. I2 Adsorption Experiments of 7AIn-PiP in Saturated I2 Aqueous Solution

The detailed information about the “adsorption kinetics of 7AIn-PiP in saturated I2 aqueous solution” is provided in Supplementary Materials Section S3. To further investigate the I2 adsorption mechanism of 7AIn-PiP in saturated I2 aqueous solution, the resulting mixture was filtered under ambient pressure when the saturation adsorption was reached, and the filter cake was dried in open air to obtain the iodine-loaded 7AIn-PiP (named as I2@7AIn-PiP), which was characterized with XPS, Raman, and FTIR spectra to confirm the main forms of iodine in the polymer skeleton.

2.3.2. Recycling Tests of 7AIn-PiP in Saturated I2 Aqueous Solution

To evaluate the reusability of 7AIn-PiP, I2@7AIn-PiP was extracted with ethanol by Soxhlet extraction for 48 h (fresh ethanol was replaced every 12 h), and the obtained sample was vacuum-dried at 90 °C for 24 h, reusing for the next round of adsorption experiment. The cyclic tests were performed three times.

2.3.3. I2 Adsorption Experiments of 7AIn-PiP in Unsaturated I2 Aqueous Solution

Firstly, 126.9 mg of I2 was completely dissolved in 500.0 mL of deionized water to prepare 1.0 mmol L−1 of I2 aqueous solution. Next, 50.0 and 10.0 mL of stock solutions were diluted to a volume of 100 mL by deionized water to obtain I2 aqueous solutions with the concentrations of 0.5 and 0.1 mmol L−1, respectively. The UV–Vis spectra of these three I2 aqueous solutions were determined. Finally, at room temperature, 5.0 mg of 7AIn-PiP was added to 10 mL of I2 aqueous solution with different concentrations (1.0, 0.5, and 0.1 mmol L−1) and stirred at a speed of 500 rpm. The UV–Vis spectra of these solutions were recorded after 30 min.

3. Results and Discussion

3.1. Characterization and Analysis of Materials

3.1.1. Pore Properties and I2 Adsorption Results of a Batch of Polymers

As indicated in Supplementary Materials Section S2, it was found that using DCX can increase specific surface areas and total pore volume. The existence of benzyl cross-linked bridges and the extended structures resulting from the self-condensation of DCX might prevent the ionic building blocks reacting directly with each other. The more DCX is used, the larger the specific surface area and total pore volume of the corresponding polymer. With the increase in DCX/7AIn molar ratio, the specific surface area increased from 14 to 814 m2 g−1. If the molar ratio of 7AIn to DCX was 1:2, Polymer-1 displayed the lowest specific surface area and total pore volume because the charge interaction and low degree of crosslinking mean that intermolecular packing easily occurred among the charged intermediate products [9]. If no 7AIn was added, the self-polymerization of DCX endowed Polymer-6 with the largest specific surface area and total pore volume. However, neither Polymer-1 nor Polymer-6 showed a satisfactory I2 adsorption performance. Notably, Polymer-2 exhibited the highest I2 removal rate among these polymers, indicating that the 7AIn/DCX molar ratio of 1:4 is the optimum proportion, as lower or higher molar ratio resulted in poor adsorption properties. In what follows, we renamed Polymer-2 as 7AIn-PiP. The above results demonstrate that the I2 removal rate is determined by both the nitrogen-containing affinity site and high specific surface area.

3.1.2. Characterization of 7AIn-PiP

In order to identify the functional groups on 7AIn-PiP, FTIR and 13C CP/MAS NMR spectra were performed. Figure 1a shows that the signal at about 3423 cm−1 corresponds to N–H stretching vibration on the 7AIn ring, which may be overlapped by the signal of O–H stretching vibration of water. The peaks at around 2923 and 2850 cm−1 can be ascribed to the C–H stretching vibration of the methylene group. The bands at approximately 1610 and 1503 cm−1 are attributed to the stretching vibration of aromatic rings (7AIn ring and benzene ring). In addition, as shown in Figure 1b, the broad peaks in the region of 155–105 ppm correspond to the aromatic carbon, while the methylene carbon originating from DCX is located at around 37 ppm. According to the above analysis, both FTIR and 13C CP/MAS NMR spectra demonstrate that 7AIn reacts efficiently with DCX to incorporate a certain amount of the methylene group through Friedel–Crafts alkylation reaction, confirming the successful preparation of 7AIn-PiP. Furthermore, the appearance of a strong paramagnetic signal in the ESR spectrum (Figure 1c) reveals that charges actually exist in the skeleton of 7AIn-PiP.
The morphology of powdered 7AIn-PiP was investigated by SEM and TEM. The observed images reveal that 7AIn-PiP exhibits an irregular wrinkled morphology. The porous spaces were formed by the agglomeration of the membrane-like structures (Figure 2a,b). To further demonstrate the crystallinity of 7AIn-PiP, PXRD was conducted. As shown in Figure 2c, the diffraction peak in the pattern is not obvious, but is relatively broad, indicating that 7AIn-PiP displays an amorphous phase without long-range order, which is consistent with the results of SEM and TEM.
The pore property of 7AIn-PiP was characterized by using a standard N2 adsorption–desorption experiment at 77 K up to 1 bar (sample was activated under a vacuum at 120 °C for 12 h). According to the International Union of Pure and Applied Chemistry (IUPAC) classification of physical adsorption isotherms, 7AIn-PiP displays a mixture of type I and type II N2 adsorption isotherms with a type H4 hysteresis loop (Figure 3a), suggesting that 7AIn-PiP mainly contains micropores and mesopores. Moreover, the obtained pore size distribution curve (Figure 3b) for 7AIn-PiP exhibits abundant microporosity and a partial mesoporous character, which is in accordance with the sharp rise curve and mild capillary condensation in the isotherms.

3.1.3. Characterization of the Dual 1,3-Dipole

In order to confirm the structure of the new obtained product after the SN2 nucleophilic substitution reaction, 1H NMR (Figure 4a), 13C NMR (Figure 4b), and FTIR (Supplementary Materials Section S4) were performed. It is worth noting that there is no signal of H atoms attached to the pyrrole-type N in the 1H NMR spectrum. Instead, the characteristic peaks in the FTIR spectrum and the number and types of H/C atoms in the 1H/13C NMR spectra could be matched with an interesting dual 1,3-dipole. The relevant data of liquid 1H NMR and 13C NMR are presented as follows: 1H NMR (600 MHz, DMSO-d6) δppm: 8.79 (d, J = 6.0 Hz, 2H), 8.74 (d, J = 7.8 Hz, 2H), 7.91 (d, J = 3.6 Hz, 2H), 7.64 (dd, J = 7.8; 6.0 Hz, 2H), 7.54 (s, 4H), 6.96 (d, J = 3.6 Hz, 2H), 6.12 (s, 4H). Calibration (H2O-3.36 ppm, DMSO-2.50 ppm). 13C NMR (150 MHz, DMSO-d6) δppm: 138.84 (C1), 137.64 (C2), 136.63 (C3), 134.43 (C4), 130.30 (C5), 128.82 (C6), 126.92 (C7), 116.57 (C8), 103.85 (C9), 57.17 (C10). Calibration (DMSO-39.51 ppm).
As shown in Figure 5a, HRMS further confirms the accuracy of molecular weight: calcd for [M + H]+ m/z = 339.16042, found m/z = 339.15939. The characteristic fragmentation pathway of the dual 1,3-dipole is shown in Figure 5b. Initially, the dual 1,3-dipole receives energy from the ion source, loses an electron, and thus becomes a positively charged molecular ion. Subsequently, the resulting molecular ion undergoes the homolytic cleavage of benzyl C–N bond (α-cleavage), forming a benzylic carbocation. Lastly, the obtained benzylic carbocation immediately transforms into a more stable tropyllium ion and further loses two acetylene molecules, producing a cyclopropenyl cation. In the positive-ion mode, the strongest peak (base peak, relative abundance 100%) with a mass-to-charge ratio (m/z) of 170.08374 [M + 1] and the molecular ion peak with a mass-to-charge ratio (m/z) of 339.15939 [M + 1] obviously appear in Figure 5a. Meanwhile, it is worth noting that there is also a weak peak with a mass-to-charge ratio (m/z) of 118.05257 [M + 1] in the spectrum, which is the value of the other part of the molecular ion after the α-cleavage. These results demonstrate the rationality of the fragmentation pathway and the accuracy of the structure of the dual 1,3-dipole.

3.1.4. Formation Mechanism of the Dual 1,3-Dipole and Polymer 7AIn-PiP

Based on the above structural information, a possible formation mechanism of the dual 1,3-dipole and 7AIn-PiP is proposed. As illustrated in Scheme 3, in the first stage, the pyridine-type N of 7AIn attacks the benzyl on one side of DCX, forming an ionic C–N covalent bond (quaternization reaction) [17,18,19,20]. By virtue of the formation of the pyridinium-type chloride salt, the electron-withdrawing ability of the positively charged pyridinium-type N+ is stronger than that of electroneutral pyridine-type N, which effectively weakens the electron cloud density between the N–H bond of the pyrrole-type moiety. Accordingly, the H atom attached to the pyrrole-type N is easily captured by the Cl− to form the by-product HCl, which escapes from the reaction system under the heating condition. It is notable that 1,3-dipole A was not obtained by column chromatography because the strong electron-withdrawing ability of the positively charged pyridinium-type N+ makes a difference once again; the electron cloud density between the C–Cl bond of 1,3-dipole A is lower than that of the DCX molecule, meaning that the Cl atom on 1,3-dipole A is more easily replaced by another 7AIn molecule. After going through a similar process (SN2 nucleophilic substitution and pyrrole-type H elimination reactions) as described above, the dual 1,3-dipole B is generated. It is also worth noting that not all 7AIn molecules participate in SN2 nucleophilic substitution reactions due to the limitation of its own structure; the pyrrole-type moiety has a certain steric hindrance effect on the nucleophilic attack of the pyridine-type moiety. In the second stage, a zwitterionic hypercrosslinked organic network (7AIn-PiP) with permanent porosity can be formed by a one-pot FeCl3-promoted Friedel–Crafts alkylation reaction, including the co-condensation among the dual 1,3-dipole B (ionic building unit), residual 7AIn (electroneutral building unit), and the excess DCX (crosslinker), in addition to the self-condensation of DCX. Meanwhile, the by-product HCl is still released from the reaction system under the heating condition.

3.2. I2 Adsorption Property of 7AIn-PiP

3.2.1. Evaluation of I2 Adsorption Property of 7AIn-PiP in Saturated I2 Aqueous Solution

The time-dependent UV–Vis absorption spectra of I2 aqueous solution upon adding 7AIn-PiP are shown in Figure 6a. The capture of iodine species by 7AIn-PiP can achieve adsorption equilibrium within 30 min. The corresponding I2 saturation adsorption capacities and I2 removal rates at different characteristic absorption wavelengths are presented in Figure 6b. In order to assess the I2 adsorption property of 7AIn-PiP in saturated I2 aqueous solution, the I2 removal rates of recently reported materials under the similar test conditions are summarized in Supplementary Materials Section S5. Benefitting from the integration of the abundant adsorption sites and pore structures, 7AIn-PiP demonstrates a high I2 adsorption efficiency from water.

3.2.2. Analysis of Adsorption Kinetics of 7AIn-PiP in Saturated I2 Aqueous Solution

Initially, we used the pseudo-second-order kinetic model to fit the adsorption data. The corresponding fitting plots and correlated parameters based on this model are shown in Figure 7a–c and Table 1. The correlation coefficients (R2) are close to 1, and the experimental values (qe,exp) are close to the calculated adsorption capacities (qe,cal) at different characteristic absorption wavelengths, implying that the whole adsorption process of 7AIn-PiP can be reliably revealed by the pseudo-second-order kinetic model with a good fitting effect. The pseudo-second-order kinetic model assumes that the adsorption process is controlled by chemisorption [21], but a good fitting result does not mean that the I2 adsorption process is completely driven by chemisorption [22,23], which may be affected by both chemisorption and physisorption.
The pseudo-second-order kinetic model is not suitable for explaining the diffusion (mass transfer) process of solute on the adsorbent. In order to further describe the actual rate-controlling step, the intra-particle diffusion kinetic model was also used to fit the adsorption data. As can be seen from Figure 7d–f and Table 2, the intercepts of the linear equations of all fitting curves are not 0, implying that the adsorption rate is influenced by both liquid-film diffusion and intra-particle diffusion. All fitting curves at different characteristic absorption wavelengths can be divided into two stages. In the initial stage of the adsorption process, the slopes of the curves are large, denoting that the adsorption rate is fast. There is a large concentration gradient between the surface of 7AIn-PiP and the solute in the solution, thus providing a large driving force of mass transfer. The solutes rapidly diffuse to the outer surface of 7AIn-PiP and are adsorbed by the adsorption sites. The adsorption amount increases with the extension of time. However, in the later stage of the adsorption process, the curves have small slopes, signifying that the adsorption rate decreases. The solutes further enter the internal pores of 7AIn-PiP and are absorbed by the adsorption sites on the inner surface of the pores. When the adsorption sites are occupied and the concentration of the solute decreases, the adsorption process gradually reaches the equilibrium state.

3.2.3. Characterization of I2@7AIn-PiP

As shown in the XPS spectrum of I2@7AIn-PiP (Figure 8a), two types of polyiodide peaks I3− (618.9 and 630.4 eV) and I5− (620.5 and 632.0 eV) generated by the splitting of I 3d3/2 and I 3d5/2 orbital levels [11,24,25,26,27] can be observed. Moreover, the species of iodine within 7AIn-PiP were also confirmed by Raman spectrum (Figure 8b). Three characteristic peaks at 110, 140, and 161 cm−1 can be assigned to the vibration modes of polyiodides, mainly I3− and I5− anions. Of these, the bands at 110 and 140 cm−1 are attributed to the symmetric and asymmetric stretching vibrations of I3−, respectively, while the peak at 161 cm−1 is assigned to the stretching vibration of I5− [28,29,30,31] (consistent with I5− in the L or V configuration [32]). The results of XPS and Raman spectra demonstrate that I3− and I5− coexist inside the framework of 7AIn-PiP. Furthermore, comparing the FTIR spectra of 7AIn-PiP and I2@7AIn-PiP (Figure 8c) reveals that the characteristic bands at 2923 and 2850 cm−1 attributed to the C–H stretching vibration of the methylene group weaken significantly after adsorption, providing further evidence of the interaction between the guest iodine species and the host polymer skeleton.

3.2.4. Possible I2 Adsorption Mechanism of 7AIn-PiP in Saturated I2 Aqueous Solution

According to the above analysis, the possible I2 adsorption mechanism of 7AIn-PiP from water can be proposed as follows. The major component I3− can interact with three kinds of cationic binding sites: (1) I3− can be directly adsorbed through electrostatic force, which comes from the available cationic pyridinium-type group in the zwitterionic skeleton [11,12,13]; meanwhile, H+ in the solution can be captured by the pyrrole-type N− site. (2) It is well known that the pyridine-type N is easily protonated in an acidic environment [33,34]. After protonation, the pyridine-type group could be converted into another kind of cationic pyridinium-type group. Then, the new obtained cationic binding site also adsorbs I3− by electrostatic force; (3) the electron-rich benzene ring from DCX as another functional site interacts with H+ through cation–π interaction [35,36], forming cationic–π complex binding site, which attracts I3− by electrostatic force as well [37]. In addition to the electrostatic force, there is also hydrogen bonding interaction (N–H•••I) between the pyrrole-type N–H group and iodine species [38,39]. The above-mentioned I3− adsorption modes and the inevitable pore capture belong to the category of physisorption, laying the foundation for the recycling of 7AIn-PiP. Moreover, the adsorbed I3− (Lewis base) can interact with I2 (Lewis acid) through electron transfer to form I5− in the skeleton [32], further promoting the enrichment of I2. The discovery of I5− indicates that chemisorption also contributes to the enrichment process of the guest iodine species. To sum up, the I2 adsorption process of 7AIn-PiP in saturated I2 aqueous solution is affected by both physisorption and chemisorption. The corresponding schematic representation is depicted in Scheme 4.

3.2.5. Regeneration Property of 7AIn-PiP in Saturated I2 Aqueous Solution

As seen in Figure 9a, the UV–Vis absorption curves of I2 aqueous solutions are no longer smooth, and the characteristic peaks shift slightly after adsorption. In particular, the characteristic peak at 354 nm is almost impossible to identify. Herein, 290 and 456 nm were chosen as the characteristic absorption wavelengths for the calculation of I2 adsorption capacity. As shown in Figure 9b,c and Table 3, 7AIn-PiP can still maintain about 95% of its original I2 adsorption efficiency after three rounds of recycling tests, indicating that our prepared adsorbent 7AIn-PiP indeed possesses excellent recycling ability and exhibits good development prospect in the field of I2 adsorption from water.

3.2.6. New Adsorption Phenomenon of 7AIn-PiP in Unsaturated I2 Aqueous Solution

As shown in Figure 10a, it was found that when the initial concentration of I2 aqueous solution was reduced to 1.0 mmol L−1, the characteristic peaks at 290 and 354 nm disappear after adsorption by 7AIn-PiP, while a new peak at around 320 nm appears. In addition, when the initial concentration of I2 aqueous solution was further reduced to 0.5 and 0.1 mmol L−1, this phenomenon still exists (Figure 10b,c).
In order to reveal this interesting phenomenon, 0.5 mmol L−1 of I2 aqueous solutions before and after adsorption by 7AIn-PiP were characterized by HRMS with the negative-ion mode. As shown in Figure 11a,b, three characteristic peaks with a mass-to-charge ratio (m/z) of 126.90342/126.90355, 212.07431/212.07437, and 380.71371/380.71225 appear in the HRMS spectra of I2 aqueous solutions before and after adsorption. Of these, the peaks at 126.90342/126.90355 and 380.71371/380.71225 correspond to I− (Exact mass: 126.90502) and I3− (Exact mass: 380.71395), respectively, while the peak at 212.07431/212.07437 may be attributed to the form of HIO3•2H2O (Exact mass: 211.91817). After adsorption, the peak of I3− is obviously weakened and hardly observed, indicating that I3− is effectively adsorbed. Meanwhile, a new peak with a mass-to-charge ratio (m/z) of 159.84393 appears, corresponding to the newly generated HIO2 (exact mass:159.90212, weak acid, stable in water).
The detailed analysis is as follows: in the preparation of I2 aqueous solution, I2 and H2O weakly react to form HI and HIO [Equation (1)] [40,41]. As a result, HI completely dissociates to H+ and I−, while HIO is extremely unstable in water and is easily disproportionated to form I− and IO3− [Equations (2) and (3)] [40,41,42,43]. The generated I− further reacts with molecular I2 to form I3− [Equations (4) and (5)] [41,42,43,44], thus promoting the dissolution of solid I2. The associated ionic reaction equations are as follows:
I2 + H2O ⇌ I− + HOI + H+
3HOI ⇌ 2I− + IO3− + 3H+
(1) × 3 + (2): 3I2 + 3H2O ⇌ 5I− + IO3− + 6H+
I2 + I− ⇌ I3−
(4) × 3 − (3): IO3− + 8I− + 6H+ ⇌ 3I3− + 3H2O
When 7AIn-PiP is added into the I2 aqueous solution, a large number of I3− are adsorbed rapidly through the electrostatic interaction generated by the cationic pyridinium adsorption sites, which shifts the equilibrium [Equation (5)] to the right and triggers the comproportionation between IO3− and I−, inducing the formation of HIO2 in the aqueous solution [Equations (6)–(8)] [44]. The associated ionic reaction equations are as follows:
IO3− + I− + 2H+ ⇌ HOI + HIO2
2HOI ⇌ HIO2 + H+ + I−
(6) × 2 + (7): 2IO3− + I− + 3H+ ⇌ 3HIO2

4. Conclusions

In conclusion, a novel zwitterionic hypercrosslinked polymer (7AIn-PiP), featuring a unique dual 1,3-dipole composition, was successfully synthesized via one-pot SN2 nucleophilic substitution and FeCl3-catalyzed Friedel–Crafts alkylation reactions. In contrast to the previously reported ionic porous materials as iodine adsorbents from water [11,12,13], there is no exchangeable counterion in the skeleton of 7AIn-PiP during the iodine adsorption process. 7AIn-PiP possesses a high I2 removal rate (94.2%, at 456 nm) and fast adsorption kinetics (reaching adsorption equilibrium in 30 min) from the saturated I2 aqueous solution, mainly resulting from the synergistic effect of the constructed multiple binding sites. Moreover, the adsorption kinetic experiments show that the I2 adsorption behavior of 7AIn-PiP is fitted with the pseudo-second-order kinetic model, and the adsorption rate is influenced by both liquid-film diffusion and intra-particle diffusion. The adsorption mechanism was further studied by XPS, Raman, and FTIR spectra of I2@7AIn-PiP, clarifying that the I2 adsorption process of 7AIn-PiP is influenced by both chemisorption and physisorption. In addition, 7AIn-PiP still maintains good I2 adsorption performance (~95% I2 removal rate) after three adsorption–regeneration cycles, which is favorable for tackling the problem of removing radioactive iodine from nuclear sewage. Furthermore, we discovered and explained that the rapid adsorption of I3− by 7AIn-PiP in unsaturated I2 aqueous solution can induce the formation of HIO2. To sum up, this work provides a new strategy for the design and development of rare zwitterionic porous organic polymers and has a certain basic research significance for the removal of radioactive iodine from water.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/polym16192846/s1. Figure S1. (a) UV–Vis absorption spectra of I2 aqueous solutions with different concentrations. (b) Corresponding standard curves at 290, 354, and 456 nm, respectively. Table S1. Yields and pore properties of polymer-1, -2, -3, -4, -5, and -6. Figure S2. (a) UV–Vis absorption spectra of I2 aqueous solutions before and after adsorption by a batch of obtained polymers under the same conditions. Corresponding I2 adsorption capacity and removal rate at (b) 290, (c) 354, and (d) 456 nm, respectively. Figure S3. FTIR spectrum of the dual 1,3-dipole. Table S2. Analysis of characteristic peaks of the dual 1,3-dipole. Table S3. Summary of I2 removal rates of various adsorbents from I2 aqueous solutions. References [45,46,47,48,49,50,51,52,53,54,55,56,57,58,59] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, J.Y.; methodology, J.Y.; investigation, J.Y.; data curation, J.Y. and L.S.; writing—original draft, J.Y.; formal analysis, J.Y. and L.S.; visualization, J.Y. and L.S.; funding acquisition, L.S. and J.M.; writing—review and editing, L.S.; validation, B.H. and J.H.; resources, Z.L. and J.M.; supervision, J.M.; project administration, J.M. All authors have read and agreed to the published version of the manuscript.

Funding

National Natural Science Foundation of China (21978188); Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi (2022L602).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article and Supplementary Material.

Conflicts of Interest

Author Jingwen Yu was employed by the company Lu’an Chemical Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Polymers 16 02846 sch001
Scheme 1. Schematic representation for the synthesis of 7AIn-PiP.
Scheme 1. Schematic representation for the synthesis of 7AIn-PiP.
Polymers 16 02846 sch001
Polymers 16 02846 sch002
Scheme 2. Schematic representation for the synthesis of the dual 1,3-dipole.
Scheme 2. Schematic representation for the synthesis of the dual 1,3-dipole.
Polymers 16 02846 sch002
Polymers 16 02846 g001
Figure 1. (a) FTIR, (b) 13C CP/MAS NMR (the red rectangle is the region of aromatic carbon), and (c) EPR spectra of 7Ain-PiP.
Figure 1. (a) FTIR, (b) 13C CP/MAS NMR (the red rectangle is the region of aromatic carbon), and (c) EPR spectra of 7Ain-PiP.
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Polymers 16 02846 g002
Figure 2. (a) SEM (scale bar: 2 μm) image, (b) TEM (scale bar: 200 nm) image, and (c) PXRD pattern of 7Ain-PiP.
Figure 2. (a) SEM (scale bar: 2 μm) image, (b) TEM (scale bar: 200 nm) image, and (c) PXRD pattern of 7Ain-PiP.
Polymers 16 02846 g002
Polymers 16 02846 g003
Figure 3. (a) N2 adsorption-desorption isotherms at 77 K and (b) pore size distribution curve calculated by BJH theory.
Figure 3. (a) N2 adsorption-desorption isotherms at 77 K and (b) pore size distribution curve calculated by BJH theory.
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Polymers 16 02846 g004
Figure 4. (a) 1H NMR (the letters in the picture represent different types of H atoms) and (b) 13C NMR spectra of the dual 1,3-dipole.
Figure 4. (a) 1H NMR (the letters in the picture represent different types of H atoms) and (b) 13C NMR spectra of the dual 1,3-dipole.
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Polymers 16 02846 g005
Figure 5. (a) HRMS spectrum (the red rectangles represent the characteristic peaks that need to be focused) and (b) the mass spectrometry fragmentation pathway of the dual 1,3-dipole.
Figure 5. (a) HRMS spectrum (the red rectangles represent the characteristic peaks that need to be focused) and (b) the mass spectrometry fragmentation pathway of the dual 1,3-dipole.
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Polymers 16 02846 sch003
Scheme 3. Schematic diagram of formation mechanism of the dual 1,3-dipole and polymer 7AIn-PiP.
Scheme 3. Schematic diagram of formation mechanism of the dual 1,3-dipole and polymer 7AIn-PiP.
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Polymers 16 02846 g006
Figure 6. (a) Time-dependent UV-Vis absorption spectra of saturated I2 aqueous solution (1.2 mmol L−1, 10.0 mL) upon adding 5.0 mg of 7AIn-PiP. (b) Corresponding I2 adsorption capacities and I2 removal rates at adsorption equilibrium for 7AIn-PiP.
Figure 6. (a) Time-dependent UV-Vis absorption spectra of saturated I2 aqueous solution (1.2 mmol L−1, 10.0 mL) upon adding 5.0 mg of 7AIn-PiP. (b) Corresponding I2 adsorption capacities and I2 removal rates at adsorption equilibrium for 7AIn-PiP.
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Polymers 16 02846 g007
Figure 7. Fitting plots of I2 adsorption data with (a–c) pseudo-second-order and (d–f) intra-particle diffusion kinetic models, respectively.
Figure 7. Fitting plots of I2 adsorption data with (a–c) pseudo-second-order and (d–f) intra-particle diffusion kinetic models, respectively.
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Polymers 16 02846 g008
Figure 8. (a) XPS and (b) Raman spectra of I2@7AIn-PiP. (c) FTIR spectra of 7AIn-PiP and I2@7AIn-PiP.
Figure 8. (a) XPS and (b) Raman spectra of I2@7AIn-PiP. (c) FTIR spectra of 7AIn-PiP and I2@7AIn-PiP.
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Polymers 16 02846 sch004
Scheme 4. Schematic diagram of the possible I2 adsorption mechanism of 7AIn-PiP in saturated I2 aqueous solution.
Scheme 4. Schematic diagram of the possible I2 adsorption mechanism of 7AIn-PiP in saturated I2 aqueous solution.
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Polymers 16 02846 g009
Figure 9. (a) UV–Vis absorption spectra of saturated I2 aqueous solutions after adsorption by 7AIn-PiP during recycling tests. Corresponding iodine adsorption capacity and iodine adsorption efficiency at (b) 290 and (c) 456 nm, respectively.
Figure 9. (a) UV–Vis absorption spectra of saturated I2 aqueous solutions after adsorption by 7AIn-PiP during recycling tests. Corresponding iodine adsorption capacity and iodine adsorption efficiency at (b) 290 and (c) 456 nm, respectively.
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Polymers 16 02846 g010
Figure 10. UV–Vis absorption spectra of I2 aqueous solutions with different concentrations [(a) 1.0, (b) 0.5, and (c) 0.1 mmol L−1] before and after adsorption by polymer 7AIn-PiP.
Figure 10. UV–Vis absorption spectra of I2 aqueous solutions with different concentrations [(a) 1.0, (b) 0.5, and (c) 0.1 mmol L−1] before and after adsorption by polymer 7AIn-PiP.
Polymers 16 02846 g010
Polymers 16 02846 g011
Figure 11. HRMS spectra of I2 aqueous solution (0.5 mmol L−1, 10.0 mL) before (a) and after (b) adsorption by 5.0 mg of 7AIn-PiP (the red rectangles represent the same peaks before and after adsorption, and the green rectangle represents the new peak after adsorption).
Figure 11. HRMS spectra of I2 aqueous solution (0.5 mmol L−1, 10.0 mL) before (a) and after (b) adsorption by 5.0 mg of 7AIn-PiP (the red rectangles represent the same peaks before and after adsorption, and the green rectangle represents the new peak after adsorption).
Polymers 16 02846 g011
Table 1. Parameters of pseudo-second-order kinetic model at 290, 354, and 456 nm, respectively.
Table 1. Parameters of pseudo-second-order kinetic model at 290, 354, and 456 nm, respectively.
Characteristic
Absorption
Wavelength λmax (nm)		Experimental Adsorption
Capacity
at Equilibrium qe, exp (g g−1)		Parameters of Pseudo-Second-Order Model
qe, cal
(g g−1)		k2
(g g−1 min−1)		R2
2900.5420.54512.1251.00000
3540.5430.54612.5060.99999
4560.5740.5823.5890.99996
Table 2. Parameters of intra-particle diffusion kinetic model at 290, 354, and 456 nm, respectively.
Table 2. Parameters of intra-particle diffusion kinetic model at 290, 354, and 456 nm, respectively.
Characteristic
Absorption
Wavelength λmax (nm)		The First StageThe Second Stage
kp
(g g−1 min−0.5)		C
(g g−1)		R2kp
(g g−1 min−0.5)		C
(g g−1)		R2
2900.013820.495460.999470.001720.533180.90717
3540.011640.501570.954370.001430.535130.93978
4560.029880.462060.949660.007030.53540.95255
Table 3. Iodine adsorption capacity and iodine adsorption efficiency of 7AIn-PiP during recycling tests in saturated I2 aqueous solution.
Table 3. Iodine adsorption capacity and iodine adsorption efficiency of 7AIn-PiP during recycling tests in saturated I2 aqueous solution.
AdsorbentThe Mass of
Adsorbent (mg)		Iodine Adsorption Capacity of
1 g Adsorbent (g g−1)		Iodine Adsorption
Efficiency (%)
290 nm456 nm290 nm456 nm
Original 7AIn-PiP5.00.5420.574100100
After the 1st regeneration5.10.5310.56398.098.1
After the 2nd regeneration5.20.5180.54895.695.5
After the 3rd regeneration5.20.5160.54595.294.9
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MDPI and ACS Style

Yu, J.; Song, L.; Han, B.; Hu, J.; Li, Z.; Mi, J. Synthesis of a Novel Zwitterionic Hypercrosslinked Polymer for Highly Efficient Iodine Capture from Water. Polymers 2024, 16, 2846. https://doi.org/10.3390/polym16192846

AMA Style

Yu J, Song L, Han B, Hu J, Li Z, Mi J. Synthesis of a Novel Zwitterionic Hypercrosslinked Polymer for Highly Efficient Iodine Capture from Water. Polymers. 2024; 16(19):2846. https://doi.org/10.3390/polym16192846

Chicago/Turabian Style

Yu, Jingwen, Luna Song, Bingying Han, Jiangliang Hu, Zhong Li, and Jie Mi. 2024. "Synthesis of a Novel Zwitterionic Hypercrosslinked Polymer for Highly Efficient Iodine Capture from Water" Polymers 16, no. 19: 2846. https://doi.org/10.3390/polym16192846

APA Style

Yu, J., Song, L., Han, B., Hu, J., Li, Z., & Mi, J. (2024). Synthesis of a Novel Zwitterionic Hypercrosslinked Polymer for Highly Efficient Iodine Capture from Water. Polymers, 16(19), 2846. https://doi.org/10.3390/polym16192846

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