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A cationic porous organic polymer for high-capacity, fast, and selective capture of anionic pollutants
Journal of Hazardous Materials 367 (2019) 348–355
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A cationic porous organic polymer for high-capacity, fast, and selective capture of anionic pollutants Zhi-Wei Liua,b, Cong-Xiao Caoa, Bao-Hang Hana,b,
T
⁎
a
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China b Sino-Danish Center for Education and Research, Sino-Danish College, University of Chinese Academy of Sciences, Beijing 100190, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Heavy metal ions Ionic porous materials Ion exchange Pd(II) ions Water remediation
The emerging ionic porous organic materials have achieved various applications in different fields, however, there is limited study on using them to capture ionic pollutants from water. Here we demonstrate a facile method to prepare a cationic porous organic polymer via catalyst-free Schiff base reaction. The imidazolium-based polymer (ImPOP-1) was constructed through copolymerizing cationic molecules with low-cost benzidine. The as-prepared ImPOP-1 exhibits high capacity (e.g., 476.2 mg g–1 for Pd (II) and 578.5 mg g–1 for AO7−), excellent selectivity (e.g., more than 99% removal efficiency for Pd (II) in the presence of 100 times excess of SO42–), and fast kinetics (e.g., 98.6% removal efficiency within 5 min for Pd (II) ions) to the anionic pollutants including organic dyes and heavy metal ions. The excellent performance on scavenging anionic pollutants from water suggests that ImPOP-1 holds promising potential as an ion exchange material for water remediation.
1. Introduction In the past decades, on the basis of rational molecular design, tremendous porous materials with high surface area and accessible pore channels have been prepared [1], such as metal–organic frameworks
(MOFs) [2–4], covalent organic frameworks [5], [6,7], conjugated microporous polymers [8,9], polymers of intrinsic microporosity [10], and porous aromatic frameworks (PAFs) [11,12]. These porous materials have been explored in a wide range of applications including sensing [3], gas adsorption or separation [13], and heterogeneous
⁎ Corresponding author at: CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China. E-mail address: [email protected] (B.-H. Han).
https://doi.org/10.1016/j.jhazmat.2018.12.091 Received 10 July 2018; Received in revised form 10 December 2018; Accepted 22 December 2018 Available online 24 December 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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catalysis [14]. Only in the very recent years, some emerging studies suggest that the charged porous materials would bring about more advantages over the neutral analogues [15,16]. These advantages include that (1) charged skeleton of the porous materials can strengthen the host–guest repulsion or attraction that is beneficial for the applications in selective separation [17], (2) counter-ions of the charged skeleton can be easily exchanged for the adjustment of porosity [18], enhancement of adsorption behaviors [19], and introducing new functionalities (such as catalysis, ions conduction) [20,21]. Therefore, the scope of the applications of porous materials can be further extended [20,22,23]. In order to endow the porous materials with charged character, there are basically two strategies to achieve the goal. One is the postsynthesis modification (PSM) to the as-obtained porous materials. For instance, Ma and co-workers functionalized a series of PAFs with eNR3+X− (X]OH or Cl) groups via this strategy to achieve the removal of perrhenate ions or other toxic substance [22,24]. Chan and coworkers reported the incorporation of ionic polymers into MOFs to prepare the ionic MOFs as ion exchange materials [25]. However, the above-mentioned PSM strategy may suffer from the blocking of pore channels or only partial functionalization of the porous skeleton, which may weaken their application performance [16,26,27]. In addition, the complexity and high cost for preparing PAFs, and the stability concerns for MOFs may also limit the potential for their practical applications. The other strategy is the direct polymerization of molecules with target-specific charged functional groups [16],28,29]. Currently, there are some pioneering studies on the direct polymerization of ionic monomers into charged porous materials. Early in 2009, Dai and coworkers reported a series of imidazolium-based networks through the trimerization of cationic monomers with nitrile groups, in the presence of ZnCl2 at very high temperature [30]. Wang and co-workers prepared a type of imidazolium-based porous polymer through Suzuki–Miyaura reaction using expensive palladium catalyst [31]. Yuan and co-workers reported that non-porous poly(ionic liquid)s can be crosslinked into porous networks by organic acids via simple ion exchange process. However, the as-obtained material is not stable due to its ionic bonds [32]. Therefore, from the perspective of methodology, a direct, facile, and catalyst-free polymerization method is still necessary to facilitate the development of charged porous materials [15,33]. Water pollution is a global concern, since a great amount of waste water is generated from the industrial activities. The waste water contamination includes charged compounds, such as organic dyes and toxic metal ions, which mostly are very toxic and put much threat to both environment and human health [34,35]. In the very recent years, porous organic polymers (POPs) show enormous potential for the water remediation application. Due to the designable and easy-modification merits, different POPs have been prepared by incorporating task-specific functional groups into the porous network. The as-prepared porous materials display powerful removal capability to the pollutants in aquatic systems [36–38]. Among the various removal methods, ion exchange is considered to be very efficient to capture those ionic types of pollutants [39]. However, most traditional ion exchange resins suffer from low capacity or low efficiency [22]. Ionic porous materials are capable of treating the ionic pollutants, however, the applications of the recently reported ionic porous organic polymers mainly focus on the carbon dioxide capture and heterogeneous catalysis [31,40–42], there is rare study on using these ionic materials for the removal of pollutants from water [24,43,44]. Imidazolium is a cationic functional group and it will therefore endow the POPs with charged character if it is incorporated into the porous networks, there are some imidazoliumbased polymers proved to be very powerful in removing ionic pollutants [39,45]. Herein, we adopt a facile and direct polymerization method to prepare a cationic porous organic polymer. Firstly, a cationic molecule with aldehyde groups as polymerizable sites was designed and synthesized. The cationic monomers were then successfully crosslinked
Scheme 1. Synthesis routes to (a) monomer Am1 with cationic imidazolium moieties and polymerizable aldehyde groups (b) highly charged porous material ImPOP-1 via catalyst-free Schiff base reaction.
with benzidine into cationic porous organic polymers, via the facile Schiff base reaction without using any catalyst. Considering the cationic character of the porous material, we performed a series of experiments to verify its capability to remove anionic pollutants in aqueous solution. Impressively, the cationic porous organic polymer (ImPOP-1, Scheme 1) exhibits high adsorption capacity, excellent selectivity, and fast kinetics to remove anionic pollutants including organic dyes, such as orange G, and inorganic metal ions, such as PdCl42–. 2. Experimental section 2.1. Materials N,N-Dimethylfomamide (DMF), toluene, dichloromethane, and methanol, sodium chloride, and sodium sulfate were purchased from Beijing Chemical Works, China. 1,3,5-Tribromobenzene (98%), 1bromo-4-(bromomethyl)benzene (99%), diisobutylaluminum hydride (1.0 M in hexane), and benzidine (98%) were purchased from J&K Chemical, China. Methylene blue (MLB+) and methyl violet (MV+) from Beijing Chemical Reagent Company were both analytically pure. Amaranth (AMR3–) was commercially available from Ourchem Company, China. Methyl blue (MB2–), acid orange 7 (AO7−), and orange G (OG2–) were purchased from Macklin, TCI, and Alfa Aesar, respectively. All chemicals were used as received, without any purification. The syringe filters (0.45 μm, Nylon 66) were purchased from Jinlong company, China. 2.2. Instrumental characterization Solid-state 13C cross-polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) measurements were performed on a Bruker Anence III 400 spectrometer. Both solution 1H and 13C NMR were performed on Bruker DMX400 NMR spectrometer. Thermogravimetric analysis (TGA) was measured using Pyris Diamond thermogravimetric/differential thermal analyzer (Perkin–Elmer Instruments, USA). The sample was heated to 900 °C at the rate of 10 °C min–1 in the nitrogen atmosphere. Ultraviolet–visible (UV–vis) spectra were recorded on UV-2600 spectrophotometer (Shimazu, Japan) and used to determine the concentration. The sample was mixed with KBr powder and then compressed to form pellets for acquiring Fourier transform infrared (FTIR) spectra, using Spectrum One FTIR spectrometer (Perkin–Elmer Instruments, USA). The porosity of the material was determined by nitrogen sorption experiment that was conducted at 77 K using a 3Flex surface area and porosity analyzer 349
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chemical structure of the monomer Am1 was confirmed by mass spectrometry, 1H NMR, and 13C NMR spectra (Supplementary Information). The commercially available and low-cost benzidine was then selected as co-monomer to react with the imidazolium monomers for the formation of imine-linked ionic porous organic polymer. The condensation process was conducted by simply heating the mixture of two monomers in DMF for 24 h in an autoclave. A dark yellow powder was obtained, after soaked in DMF and methanol. The whole polymerization process doesn’t need addition of any catalyst, so there is no concern about the impurities caused by the residue of catalyst. The thermal gravimetric analysis (TGA) (Fig. S1, Supplementary Information) indicates that the cationic porous material ImPOP-1 achieves better performance compared to the ionic porous materials cross-linked via non-covalent bonds [32], which is thermally stable up to 400 °C with minor loss of weight. The successful formation of the network was confirmed by the Fourier transform infrared spectrum (Fig. S2, Supplementary Information). The peak at around 1698 cm–1 that is characteristic to the C]O stretching signal almost disappears, which implies the high conversion of aldehyde groups. The broad peak at around 1620 cm–1 is possibly due to the overlap of C]C and C]N stretching bonds, which indicates that imine moieties are formed as the linkers in porous networks. In addition, the broad peak at around 1500 cm–1 is assigned to the C]N stretching of imidazolyl rings, which is indicative of the existence of imidazolium moieties. The backbone structure of ImPOP-1 was further confirmed by solid-state 13C CP/MAS NMR (Fig. 1). The presence of resonance peak of imine moieties at around 157 ppm and the absence of signal of aldehyde groups at around 190 ppm demonstrate that ImPOP-1 is successfully cross-linked via the Schiff base reaction. Furthermore, the peak at 52 ppm is ascribed to the methylene groups between phenyl rings and the imidazolyl rings, the broad peak from 110 to 140 ppm is owing to the overlap of signals from carbons of phenyl rings and imidazolyl rings. The peak featuring for the imidazolium carbon appears at 147.9 ppm Nitrogen adsorption–desorption measurement was performed and used to analyze the porosity of ImPOP-1. As shown in Fig. 2, ImPOP-1 possesses a BET specific surface area up to 196 m2 g–1 and pore volume up to 0.81 cm3 g–1 at P/P0 = 0.97. The sorption isotherm belongs to type IV that is indicative of mesoporous materials, the pore size distribution profile obtained from adsorption branch (Fig. 2, inset) based on NLDFT method also indicates the dominant existence of mesopores in the polymer structure, while the sharp increase in adsorbed amount of nitrogen after P/P0 = 0.90 also indicates the existence of macropores. The morphology and porous structure were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both the TEM and SEM images (Figs. S3 and S4, Supplementary Information) show that ImPOP-1 is amorphous. The amorphous structure is also supported by the X-ray diffraction (XRD) and the small-angle X-ray scattering (SAXS) measurements, from which no ordered structure was observed. (Figs. S5 and S6, Supplementary
(Micromeritics, USA). Samples were vacuumed at 120 °C for 12 h before the sorption analysis. The calculation of the specific surface area is based on the Brunauer–Emmett–Teller (BET) equation in the range of relative pressure from 0.05 to 0.30, where the C value is 43.20 and the correlation coefficient is more than 0.9999. The pore size distribution (PSD) profile is obtained based on adsorption branch using the nonlocal density function theory (NLDFT). The Pd concentration was determined using inductively coupled plasma mass spectrometry (ICP-MS, NexION 300X, Perkin-Elmer, USA) and inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 6300 Thermo Scientific, USA). Transmission electron microscopy (TEM) images were observed from a Tecnai G2 20 S-TWIN microscope (FEI, U.S.A.). Scanning electron microscopy (SEM) images and energy dispersive spectrometer (EDS) elemental mapping were obtained from Hitachi field-emission scanning electron microscope (SU-4800), with an electric voltage of 5 kV. X-ray photoelectron spectra (XPS) measurements were conducted on the Thermo ESCALAB 250Xi analyzer (Thermo Fisher Scientific Inc., USA) with 300 W Al Kα radiation. The contents of C, H, and N were determined by Flash EA 1112 elemental analyzer (Thermo Scientific, Italy), and the Br content was determined by the Hg titration method. 2.3. Synthesis of 1,3,5-Tris[3-(4-formylbenzyl)-1H-imidazol-1-yl]benzene bromide (Am1) To a 50 mL glass flask charged with 1,3,5-tri(1H-imidazol-1-yl) benzene (276 mg, 1.0 mmol) and 4-(bromomethyl)benzaldehyde (895 mg, 4.5 mmol), added DMF (5 mL). The obtained solution was stirred for 30 min. After raising the temperature to 80 °C, a white solid precipitated out within few minutes from the solution, which was kept stirring for 24 h under the nitrogen atmosphere. After cooled down to the ambient temperature, the mixture was filtrated and washed with ethanol and followed with drying under vacuum. The product was collected with yield of 85%. 1H NMR (400 MHz, DMSO-d6) δ = 10.68 (s, 1 H), 10.07 (s, 1 H), 8.80 (s, 1 H), 8.74 (s, 1 H), 8.26 (s, 1 H), 8.02 (s, 2 H), 7.82 (s, 2 H), 5.76 (s, 2 H). 13C NMR (101 MHz, DMSO-d6) δ = 193.32, 140.84, 137.21, 136.94, 130.53, 129.74, 124.33, 122.25, 116.67, 52.81. ESI-MS: calculated for [M–3Br−]3+ = 211.1 (m/z), found 211.3. 2.4. Preparation of ImPOP-1 A 5 mL autoclave was charged with Am1 solution (120 mg, 0.137 mmol) in DMF (2.0 mL) and benzidine solution (140 mg, 0.618 mmol) in DMF (2.0 mL), and then heated at 150 °C for 24 h. After cooled to ambient temperature, the solid was isolated by filtration and then soaked in DMF (50 mL) for one day, and Soxhlet extraction (methanol as solvent) was performed for one more day to further remove impurities. Freeze drying was performed to remove the residue solvents and a yellow powder was collected with yield of 74%. Elemental analysis, found: C, 64.5; H, 4.3; N, 11.4; Br, 16.58%. Calculated: C, 62.1; H, 4.4; N, 11.1; Br, 21.0%. 3. Results and discussion A facile and catalyst-free synthesis method is always attractive to promote the development of charged porous organic materials. Inspired by the easy synthesis of porous materials via Schiff base reaction achieved by Müllen and coworkers [46], we applied this type of reaction to constructing ionic porous organic polymers. The general strategy to prepare ImPOP-1 is illustrated in Scheme 1. We started with designing a molecule that contains dual functional groups, the imidazolium part for the introduction of charged character and the aldehyde group for easy polymerization via Schiff base reaction (Scheme 1). By reacting 4-(bromomethyl)benzaldehyde with 1,3,5-tri(1H-imidazol-1yl)benzene via very simple nucleophilic reaction, the positively charged molecules with retained aldehyde groups were easily obtained. The
Fig. 1. Solid-state 350
13
C CP/MAS NMR spectrum of ImPOP-1.
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Fig. 2. The nitrogen adsorption–desorption isotherm of ImPOP-1 (inset, the pore size distribution profile obtained from adsorption branch using NLDFT method).
Information). The mesoporous and macroporous structure of ImPOP-1 can be clearly seen from the TEM images. 3.1. Adsorption capacity study As ImPOP-1 possesses nanoporous structure and its pore walls are decorated with cationic imidazolium moieties, it is rational to assume that ImPOP-1 owns the capability for removing anionic substance. High adsorption capacity is definitely beneficial to reduce the cost of materials for practical applications, therefore it is one of the most significant factors need investigating. We firstly chose organic dyes (Fig. S7, Supplementary Information) as model molecules to demonstrate the feasibility. Acid orange 7 (AO7−), orange G (OG2–), methyl blue (MB2–), and amaranth (AMR3–) are anionic type of dyes but different in magnitude of charge. Methylene blue (MLB+) and methyl violet (MV+) are cationic dyes with same charge and similar molecular size. As reported, the adsorption capacity is associated with the initial concentration of the dye and a higher concentration may lead to a better performance of adsorption capacity [47]. Therefore, high initial concentration of organic dye solution was adopted for the purpose of evaluating the adsorption performance. As seen in Fig. 3a, ImPOP-1 exhibits high adsorption capacity to all the anionic dyes. Especially to AO7–, the capacity is as high as 1.7 mmol g–1 (578.5 mg g–1). As for the cationic dyes, ImPOP-1 shows much lower adsorption capacity, such as the MLB+, merely 8.5 × 10–3 mmol g–1 (2.5 mg g–1). The obvious adsorption difference suggests that ImPOP-1 owns high selectivity on removing negatively charged organic dye molecules. Moreover, three anionic dyes with different magnitude of charge, namely AO7–, OG2–, and AMR3– were selected to study the influence on adsorption capacity. It is found that the adsorption amount is inversely proportional to the magnitude of charge. We reason that the adsorption process is mainly driven by the electrostatic interaction, in that case, the –1-charged AO7– is doomed to be adsorbed more than the –3-charged AMR3– for the sake of balance of charge. In the waste water from electroplating industry, some precious metal ions such as Pd and Au are anionic, exist in the form of PdCl42– or Au(CN)2–. Therefore, extraction of these precious metal ions is of both economic and environmental significance [54,55]. Encouraged by the result of high capacity to anionic organic dyes, we took a step further to study the adsorption capacity to inorganic anions. PdCl42– was then selected as the model anion due to its low toxicity compared to Au (CN)2–. The adsorption isotherm (Fig. 3b) of PdCl42– was fitted using the Langmuir model, a high correlation coefficient (0.9994) was obtained. The maximum adsorption capacity to Pd(II), based on the Langmuir model, is as high as 476.2 mg g–1. The result is superior to most of the reported materials for metal ions adsorption [22,56,57]. As far as we know, this is a record-high capacity to Pd(II) adsorption (Table 1) [49,58]. As reported previously, the pore volume actually weights over the BET specific surface area on the adsorption capacity
Fig. 3. The adsorption capacities to different anionic pollutants. (a) The maximum adsorption capacity to differently charged organic dyes. (b) The adsorption isotherm to inorganic PdCl42– ions. Table 1 Comparison of adsorption capacity for Pd(II) with selected adsorbents. Adsorbents
Adsorption capacity (mg g–1)
References
Graphene oxide Functionalized silica adsorbent Conjugate adsorbent Functionalized alumina nanopowder Chitosan derivative Chitosan derivative ImPOP-1
80.78 83.00 164.20 97.70 389.40 415.04 476.2
[48] [49] [50] [51] [52] [53] This work
[59]. Therefore, in addition to the electrostatic interaction between host polymer and guest molecules, it is supposed that the relatively large pore volume (0.81 cm3 g–1 at P/P0 = 0.97) may contribute to the high adsorption capacity towards both the organic dyes and inorganic metal ions. 3.2. Investigation on adsorption kinetics Except the capacity, the fast removal ability to target substances is also a significant factor to evaluate performance of materials. However, it is quite challenging to achieve high removal efficiency in short time, especially for the organic dyes [60–62]. Therefore, we performed experiments to verify if ImPOP-1 possesses the fast kinetics ability. Five dyes with different charges but similar molecular sizes were selected, of which MV+ and MLB+ are cationic, while the other three are anionic but different in magnitude of charge. All the initial concentration of aqueous dye solutions was adjusted to be 50 μM, and ImPOP-1 was 351
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Fig. 5. Selectivity performance towards different pollutants. (a) Selectivity towards PdCl42–, Cl– and SO42– are both 100 times (molar ratio) in excess relative to PdCl42–. (b) The charge-dependent selectivity to organic dyes. the UV–vis spectra of OG2–/MLB+ before and after separation test, the inset is the digital photograph of the dye solutions (OG2– and MLB+ solutions, OG2–/MLB+ mixture before and after separation for 5 min).
Fig. 4. The fast adsorption kinetics. (a) All three anionic dyes were removed in 30 min, while the cationic counterparts showed almost no adsorption. (b) The RE to PdCl42– is nearly 99% in 5 min, indicating superfast removal kinetics.
added into the solutions and then followed with vigorous stirring for accelerating the adsorption process. At an appropriate interval, the concentration was determined by UV–vis spectroscopy. As seen from Fig. 4a, ImPOP-1 exhibits distinct charge-dependent adsorption towards the dyes. There is almost no UV–vis signals of the anionic dyes detected in the solution after 30 min. The RE reaches to more than 99% just in a short time for all three anionic dyes, despite of the relatively slower AMR3– as compared to the other two. These findings indicate that ImPOP-1 possesses fast adsorption kinetics to the anionic organic dyes. The adsorption kinetics to inorganic metal ions was investigated as well. ImPOP-1 was added into the PdCl42– solution (50 ppm, 0.2 mM). After stirring for a desired period of time, the upper layer of the solution was taken out and the concentration of Pd was analyzed using ICP-MS. As shown in Fig. 4b, the equilibrium was established very soon, and 98.6% and 98.7% of the PdCl42– ions can be removed in 5 and 10 min, respectively. We adopted the Lagergren pseudo-first-order model (Fig. S8, Supplementary Information) and pseudo-second-order model (Fig. S9, Supplementary Information) for analysis of fast kinetics [63], the adsorption for dye AO7– and metal PdCl42– are both fitted well with the pseudo-second-order model (Table S1, Supplementary Information). The fast adsorption ability is ascribed to the relatively large pore size and hierarchical pore structure (Fig. 2, inset), which is beneficial to the
mass flow and access to the cationic active sites. 3.3. Selectivity towards the anionic pollutants 3.3.1. Selectivity investigation in mixed solution Cationic and anionic organic dyes are often coexisting in some waste water from textile-related industries, therefore separation to these differently charged dyes is important to water treatment. As seen in Figs. 3a and 4a, ImPOP-1 owns charge-dependent selectivity to differently charged organic dyes, whose selectivity is mainly based on the electrostatic interaction. However, all the adsorption experiments were conducted in individual aqueous solution. Herein, we investigated if ImPOP-1 can selectively remove target pollutants in the presence of interfering ions. The AO7–/MLB+ and OG2–/MLB+ stand for the two mixed dye solutions for the separation test. In the case of OG2–/MLB+ (Fig. 5b, inset), a dark brown mixture was formed after mixing MLB+ with oppositely charged OG2−. The material ImPOP-1 was added into the dye mixture solution, and then filtered after stirring for 5 min. The color of OG2–/MLB+ changed obviously from dark brown to blue, implying the successful separation of the two oppositely charged molecules. The separation efficiency was further confirmed by the UV–vis spectroscopy. The slight decrease in signal at 664 nm characteristic to MLB+ molecules indicates that most of the cationic MLB+ molecules 352
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are retained in the solution after separation. However, the anionic dye OG2– was almost totally removed, judging from the dramatic decrease in the spectra peak at around 480 nm. In addition, similar result was obtained for AO7–/MLB+ mixture (Fig. S10, Supplementary Information). Worth noting, the adsorption performance of MLB+ from an individual aqueous solution is different from that the mixture of AO7–/ MLB+ solution. The former solution showed almost no adsorption at all towards MLB+ (Fig. 4a), while the latter solution showed a slight decrease in concentration. This is probably due to the adsorption of the complex formed by the two oppositely charged dyes, which is supported by the change in the peak in the UV–vis spectra of the dye mixtures (Figs. S5 and S6, Supplementary Information). In addition, the selectivity to PdCl42– ions in the presence of high concentration of anions is also important factor for removing or extracting precious metal ions. The common anions Cl– and SO42– are usually coexisting with PdCl42–, therefore selected as the competing ions to investigate the selectivity. The initial concentration of the PdCl42– was adjusted to be 25 ppm (0.1 mM), Cl– and SO42– were 100 times (molar ratio) in excess relative to PdCl42–. The final concentrations of Pd were analyzed using ICP-MS. The result shows that the competing ions has very limited influence on RE (Fig. 5a), it is still able to remove 99% of the anionic pollutants. The high selectivity towards PdCl42– reflects the Hofmeister effect [64]. The Hofmeister effect has been demonstrated in a wide range of molecules and it is an empirical law that less hydrophilic anions with large size is favorable to be attached to the host materials. Comparing the hydrophilicity and the size of PdCl42– with the competing anions, the experiment results are well consistent with the Hofmeister effect [65]. 3.3.2. Selectivity investigation via column breakthrough tests In the practical operation, the column-chromatographic separation method is usually conducted by passing the contaminated water through the column packed with adsorbent. We therefore performed the column breakthrough tests to investigate if ImPOP-1 is able to selectively separate the dyes efficiently. A plastic pipette was packed with ImPOP-1 to form a column with height of ca 2.5 cm and diameter of ca 3.5 mm. First, the removal performance was verified by passing AO7– solution (25 ppm, 4 mL) through the column in 4 min, the UV–vis result shows that the spectral signal from AO7– can’t be detected (Fig. S11, Supplementary Information), which suggests more than 99% removal of AO7– dye. Moreover, we further performed the column breakthrough test to separate the mixture of two oppositely charged dyes. Two types of mixture, AO7–/MLB+ and OG2–/MLB+, were separately passed through the column and the performance was then evaluated using UV–vis spectroscopy. As seen from Fig. 6, both AO7–/MLB+ and OG2–/ MLB+ mixtures can be easily separated by ImPOP-1 during the breakthrough test. In the case of OG2–/MLB+, the residue of OG2– signal may be ascribed to the relatively slow adsorption kinetics compared to AO7–, which is evidenced from the previous removal efficiency experiments (Fig. 4a).
Fig. 6. The separation performance through the column breakthrough tests. (a) The UV–vis spectra of AO7–/MLB+ before and after column treatment, the inset is the digital photograph of the dye solutions (AO7– and MLB+ solutions, AO7–/ MLB+ mixture before and after separation). (b) The UV–vis spectra of OG2–/ MLB+ before and after column treatment, the inset is the digital photograph of the dye solutions (OG2– and MLB+ solutions, AO7–/MLB+ mixture before and after separation).
moieties with the metal Pd, and for the organic dyes, the π–π stacking between the phenyl groups from the host polymer and the phenyl groups from the guest dyes is also highly possible to facilitate the adsorption of guest molecules. In order to unravel the existence form of the Pd species after adsorption into the polymer network, X-ray photoelectron spectroscopy was therefore conducted. As seen from Fig. 7(b), the peak at 337.7 eV is assigned to the Pd–Cl, which suggests that most of the Pd exist in the form of PdCl42–, while the small peak at 336.0 eV assigned to Pd(0) suggests that the Pd(II) was partially reduced in the polymer network [66,67].
4. Adsorption mechanism In order to elucidate the sorption mechanism for anionic pollutants, we firstly conducted the SEM energy dispersive spectrometer (EDS) element mapping characterization. As seen from Fig. 7(a), the decrease in Br– anions and the appearance of the Pd featuring for PdCl42– and S featuring for AO7– suggest that the sorption process is dominantly caused by the ionic exchange that is driven by the electrostatic force. Furthermore, the solid-state NMR result shows that the peak featuring for imidazolium carbon shifts from 147.9 to 149.0 ppm after adsorption (Fig. 1 and Fig. S12, Supplementary Information). This shift indicates the interaction between the imidazolium rings with the PdCl42–. However, the residue of Br– anions suggests the incompletion of the ion exchange process. For the PdCl42– ions, it is highly possible to form the coordination bond for the nitrogen from the imine and imidazolium
5. Conclusions In summary, we have successfully designed and prepared an imidazolium-based cationic porous organic polymer, ImPOP-1, via the facile Schiff base reaction without use of any catalyst. ImPOP-1, which possesses porous and cationic character, is one of the rare examples that ionic porous organic polymers serves as the ion exchange materials for capturing ionic pollutants. It exhibits high adsorption capacity, superfast kinetics, and great selectivity towards the anionic pollutants, including inorganic metal ions and organic dyes. Worth noticing, the adsorption to PdCl42– is a record-high value. In addition, the good performance on column breakthrough test suggests that ImPOP-1 is a 353
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Fig. 7. (a) EDS-mapping of the original ImPOP-1, after adsorption of AO7– dye, and after adsorption of PdCl42–. (b) Pd 3d XPS spectra of ImPOP-1 after adsorption of PdCl42–.
promising material for realization of industrial applications. By the SEM-EDS, XPS, and solid-state CP/MAS NMR, we unravel that the adsorption of guest molecules is mainly an anion-exchange process. Our synthesis method featured in high simplicity would further promote the development of ionic porous organic materials, and the impressive experiment results on capturing anionic pollutants would give some hints on using ionic porous materials for future water treatment.
[4] [5]
[6] [7]
Conflict of interest
[8]
The authors declare no competing financial interest.
[9]
Acknowledgement
[10]
The financial support from the National Natural Science Foundation of China (Grant no. 21574032) is acknowledged.
[11] [12]
Appendix A. Supplementary data
[13]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.12.091.
[14] [15]
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
A cationic porous organic polymer for high-capacity, fast, and selective capture of anionic pollutants Zhi-Wei Liua,b, Cong-Xiao Caoa, Bao-Hang Hana,b,
T
⁎
a
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China b Sino-Danish Center for Education and Research, Sino-Danish College, University of Chinese Academy of Sciences, Beijing 100190, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Heavy metal ions Ionic porous materials Ion exchange Pd(II) ions Water remediation
The emerging ionic porous organic materials have achieved various applications in different fields, however, there is limited study on using them to capture ionic pollutants from water. Here we demonstrate a facile method to prepare a cationic porous organic polymer via catalyst-free Schiff base reaction. The imidazolium-based polymer (ImPOP-1) was constructed through copolymerizing cationic molecules with low-cost benzidine. The as-prepared ImPOP-1 exhibits high capacity (e.g., 476.2 mg g–1 for Pd (II) and 578.5 mg g–1 for AO7−), excellent selectivity (e.g., more than 99% removal efficiency for Pd (II) in the presence of 100 times excess of SO42–), and fast kinetics (e.g., 98.6% removal efficiency within 5 min for Pd (II) ions) to the anionic pollutants including organic dyes and heavy metal ions. The excellent performance on scavenging anionic pollutants from water suggests that ImPOP-1 holds promising potential as an ion exchange material for water remediation.
1. Introduction In the past decades, on the basis of rational molecular design, tremendous porous materials with high surface area and accessible pore channels have been prepared [1], such as metal–organic frameworks
(MOFs) [2–4], covalent organic frameworks [5], [6,7], conjugated microporous polymers [8,9], polymers of intrinsic microporosity [10], and porous aromatic frameworks (PAFs) [11,12]. These porous materials have been explored in a wide range of applications including sensing [3], gas adsorption or separation [13], and heterogeneous
⁎ Corresponding author at: CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China. E-mail address: [email protected] (B.-H. Han).
https://doi.org/10.1016/j.jhazmat.2018.12.091 Received 10 July 2018; Received in revised form 10 December 2018; Accepted 22 December 2018 Available online 24 December 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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catalysis [14]. Only in the very recent years, some emerging studies suggest that the charged porous materials would bring about more advantages over the neutral analogues [15,16]. These advantages include that (1) charged skeleton of the porous materials can strengthen the host–guest repulsion or attraction that is beneficial for the applications in selective separation [17], (2) counter-ions of the charged skeleton can be easily exchanged for the adjustment of porosity [18], enhancement of adsorption behaviors [19], and introducing new functionalities (such as catalysis, ions conduction) [20,21]. Therefore, the scope of the applications of porous materials can be further extended [20,22,23]. In order to endow the porous materials with charged character, there are basically two strategies to achieve the goal. One is the postsynthesis modification (PSM) to the as-obtained porous materials. For instance, Ma and co-workers functionalized a series of PAFs with eNR3+X− (X]OH or Cl) groups via this strategy to achieve the removal of perrhenate ions or other toxic substance [22,24]. Chan and coworkers reported the incorporation of ionic polymers into MOFs to prepare the ionic MOFs as ion exchange materials [25]. However, the above-mentioned PSM strategy may suffer from the blocking of pore channels or only partial functionalization of the porous skeleton, which may weaken their application performance [16,26,27]. In addition, the complexity and high cost for preparing PAFs, and the stability concerns for MOFs may also limit the potential for their practical applications. The other strategy is the direct polymerization of molecules with target-specific charged functional groups [16],28,29]. Currently, there are some pioneering studies on the direct polymerization of ionic monomers into charged porous materials. Early in 2009, Dai and coworkers reported a series of imidazolium-based networks through the trimerization of cationic monomers with nitrile groups, in the presence of ZnCl2 at very high temperature [30]. Wang and co-workers prepared a type of imidazolium-based porous polymer through Suzuki–Miyaura reaction using expensive palladium catalyst [31]. Yuan and co-workers reported that non-porous poly(ionic liquid)s can be crosslinked into porous networks by organic acids via simple ion exchange process. However, the as-obtained material is not stable due to its ionic bonds [32]. Therefore, from the perspective of methodology, a direct, facile, and catalyst-free polymerization method is still necessary to facilitate the development of charged porous materials [15,33]. Water pollution is a global concern, since a great amount of waste water is generated from the industrial activities. The waste water contamination includes charged compounds, such as organic dyes and toxic metal ions, which mostly are very toxic and put much threat to both environment and human health [34,35]. In the very recent years, porous organic polymers (POPs) show enormous potential for the water remediation application. Due to the designable and easy-modification merits, different POPs have been prepared by incorporating task-specific functional groups into the porous network. The as-prepared porous materials display powerful removal capability to the pollutants in aquatic systems [36–38]. Among the various removal methods, ion exchange is considered to be very efficient to capture those ionic types of pollutants [39]. However, most traditional ion exchange resins suffer from low capacity or low efficiency [22]. Ionic porous materials are capable of treating the ionic pollutants, however, the applications of the recently reported ionic porous organic polymers mainly focus on the carbon dioxide capture and heterogeneous catalysis [31,40–42], there is rare study on using these ionic materials for the removal of pollutants from water [24,43,44]. Imidazolium is a cationic functional group and it will therefore endow the POPs with charged character if it is incorporated into the porous networks, there are some imidazoliumbased polymers proved to be very powerful in removing ionic pollutants [39,45]. Herein, we adopt a facile and direct polymerization method to prepare a cationic porous organic polymer. Firstly, a cationic molecule with aldehyde groups as polymerizable sites was designed and synthesized. The cationic monomers were then successfully crosslinked
Scheme 1. Synthesis routes to (a) monomer Am1 with cationic imidazolium moieties and polymerizable aldehyde groups (b) highly charged porous material ImPOP-1 via catalyst-free Schiff base reaction.
with benzidine into cationic porous organic polymers, via the facile Schiff base reaction without using any catalyst. Considering the cationic character of the porous material, we performed a series of experiments to verify its capability to remove anionic pollutants in aqueous solution. Impressively, the cationic porous organic polymer (ImPOP-1, Scheme 1) exhibits high adsorption capacity, excellent selectivity, and fast kinetics to remove anionic pollutants including organic dyes, such as orange G, and inorganic metal ions, such as PdCl42–. 2. Experimental section 2.1. Materials N,N-Dimethylfomamide (DMF), toluene, dichloromethane, and methanol, sodium chloride, and sodium sulfate were purchased from Beijing Chemical Works, China. 1,3,5-Tribromobenzene (98%), 1bromo-4-(bromomethyl)benzene (99%), diisobutylaluminum hydride (1.0 M in hexane), and benzidine (98%) were purchased from J&K Chemical, China. Methylene blue (MLB+) and methyl violet (MV+) from Beijing Chemical Reagent Company were both analytically pure. Amaranth (AMR3–) was commercially available from Ourchem Company, China. Methyl blue (MB2–), acid orange 7 (AO7−), and orange G (OG2–) were purchased from Macklin, TCI, and Alfa Aesar, respectively. All chemicals were used as received, without any purification. The syringe filters (0.45 μm, Nylon 66) were purchased from Jinlong company, China. 2.2. Instrumental characterization Solid-state 13C cross-polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) measurements were performed on a Bruker Anence III 400 spectrometer. Both solution 1H and 13C NMR were performed on Bruker DMX400 NMR spectrometer. Thermogravimetric analysis (TGA) was measured using Pyris Diamond thermogravimetric/differential thermal analyzer (Perkin–Elmer Instruments, USA). The sample was heated to 900 °C at the rate of 10 °C min–1 in the nitrogen atmosphere. Ultraviolet–visible (UV–vis) spectra were recorded on UV-2600 spectrophotometer (Shimazu, Japan) and used to determine the concentration. The sample was mixed with KBr powder and then compressed to form pellets for acquiring Fourier transform infrared (FTIR) spectra, using Spectrum One FTIR spectrometer (Perkin–Elmer Instruments, USA). The porosity of the material was determined by nitrogen sorption experiment that was conducted at 77 K using a 3Flex surface area and porosity analyzer 349
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chemical structure of the monomer Am1 was confirmed by mass spectrometry, 1H NMR, and 13C NMR spectra (Supplementary Information). The commercially available and low-cost benzidine was then selected as co-monomer to react with the imidazolium monomers for the formation of imine-linked ionic porous organic polymer. The condensation process was conducted by simply heating the mixture of two monomers in DMF for 24 h in an autoclave. A dark yellow powder was obtained, after soaked in DMF and methanol. The whole polymerization process doesn’t need addition of any catalyst, so there is no concern about the impurities caused by the residue of catalyst. The thermal gravimetric analysis (TGA) (Fig. S1, Supplementary Information) indicates that the cationic porous material ImPOP-1 achieves better performance compared to the ionic porous materials cross-linked via non-covalent bonds [32], which is thermally stable up to 400 °C with minor loss of weight. The successful formation of the network was confirmed by the Fourier transform infrared spectrum (Fig. S2, Supplementary Information). The peak at around 1698 cm–1 that is characteristic to the C]O stretching signal almost disappears, which implies the high conversion of aldehyde groups. The broad peak at around 1620 cm–1 is possibly due to the overlap of C]C and C]N stretching bonds, which indicates that imine moieties are formed as the linkers in porous networks. In addition, the broad peak at around 1500 cm–1 is assigned to the C]N stretching of imidazolyl rings, which is indicative of the existence of imidazolium moieties. The backbone structure of ImPOP-1 was further confirmed by solid-state 13C CP/MAS NMR (Fig. 1). The presence of resonance peak of imine moieties at around 157 ppm and the absence of signal of aldehyde groups at around 190 ppm demonstrate that ImPOP-1 is successfully cross-linked via the Schiff base reaction. Furthermore, the peak at 52 ppm is ascribed to the methylene groups between phenyl rings and the imidazolyl rings, the broad peak from 110 to 140 ppm is owing to the overlap of signals from carbons of phenyl rings and imidazolyl rings. The peak featuring for the imidazolium carbon appears at 147.9 ppm Nitrogen adsorption–desorption measurement was performed and used to analyze the porosity of ImPOP-1. As shown in Fig. 2, ImPOP-1 possesses a BET specific surface area up to 196 m2 g–1 and pore volume up to 0.81 cm3 g–1 at P/P0 = 0.97. The sorption isotherm belongs to type IV that is indicative of mesoporous materials, the pore size distribution profile obtained from adsorption branch (Fig. 2, inset) based on NLDFT method also indicates the dominant existence of mesopores in the polymer structure, while the sharp increase in adsorbed amount of nitrogen after P/P0 = 0.90 also indicates the existence of macropores. The morphology and porous structure were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both the TEM and SEM images (Figs. S3 and S4, Supplementary Information) show that ImPOP-1 is amorphous. The amorphous structure is also supported by the X-ray diffraction (XRD) and the small-angle X-ray scattering (SAXS) measurements, from which no ordered structure was observed. (Figs. S5 and S6, Supplementary
(Micromeritics, USA). Samples were vacuumed at 120 °C for 12 h before the sorption analysis. The calculation of the specific surface area is based on the Brunauer–Emmett–Teller (BET) equation in the range of relative pressure from 0.05 to 0.30, where the C value is 43.20 and the correlation coefficient is more than 0.9999. The pore size distribution (PSD) profile is obtained based on adsorption branch using the nonlocal density function theory (NLDFT). The Pd concentration was determined using inductively coupled plasma mass spectrometry (ICP-MS, NexION 300X, Perkin-Elmer, USA) and inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 6300 Thermo Scientific, USA). Transmission electron microscopy (TEM) images were observed from a Tecnai G2 20 S-TWIN microscope (FEI, U.S.A.). Scanning electron microscopy (SEM) images and energy dispersive spectrometer (EDS) elemental mapping were obtained from Hitachi field-emission scanning electron microscope (SU-4800), with an electric voltage of 5 kV. X-ray photoelectron spectra (XPS) measurements were conducted on the Thermo ESCALAB 250Xi analyzer (Thermo Fisher Scientific Inc., USA) with 300 W Al Kα radiation. The contents of C, H, and N were determined by Flash EA 1112 elemental analyzer (Thermo Scientific, Italy), and the Br content was determined by the Hg titration method. 2.3. Synthesis of 1,3,5-Tris[3-(4-formylbenzyl)-1H-imidazol-1-yl]benzene bromide (Am1) To a 50 mL glass flask charged with 1,3,5-tri(1H-imidazol-1-yl) benzene (276 mg, 1.0 mmol) and 4-(bromomethyl)benzaldehyde (895 mg, 4.5 mmol), added DMF (5 mL). The obtained solution was stirred for 30 min. After raising the temperature to 80 °C, a white solid precipitated out within few minutes from the solution, which was kept stirring for 24 h under the nitrogen atmosphere. After cooled down to the ambient temperature, the mixture was filtrated and washed with ethanol and followed with drying under vacuum. The product was collected with yield of 85%. 1H NMR (400 MHz, DMSO-d6) δ = 10.68 (s, 1 H), 10.07 (s, 1 H), 8.80 (s, 1 H), 8.74 (s, 1 H), 8.26 (s, 1 H), 8.02 (s, 2 H), 7.82 (s, 2 H), 5.76 (s, 2 H). 13C NMR (101 MHz, DMSO-d6) δ = 193.32, 140.84, 137.21, 136.94, 130.53, 129.74, 124.33, 122.25, 116.67, 52.81. ESI-MS: calculated for [M–3Br−]3+ = 211.1 (m/z), found 211.3. 2.4. Preparation of ImPOP-1 A 5 mL autoclave was charged with Am1 solution (120 mg, 0.137 mmol) in DMF (2.0 mL) and benzidine solution (140 mg, 0.618 mmol) in DMF (2.0 mL), and then heated at 150 °C for 24 h. After cooled to ambient temperature, the solid was isolated by filtration and then soaked in DMF (50 mL) for one day, and Soxhlet extraction (methanol as solvent) was performed for one more day to further remove impurities. Freeze drying was performed to remove the residue solvents and a yellow powder was collected with yield of 74%. Elemental analysis, found: C, 64.5; H, 4.3; N, 11.4; Br, 16.58%. Calculated: C, 62.1; H, 4.4; N, 11.1; Br, 21.0%. 3. Results and discussion A facile and catalyst-free synthesis method is always attractive to promote the development of charged porous organic materials. Inspired by the easy synthesis of porous materials via Schiff base reaction achieved by Müllen and coworkers [46], we applied this type of reaction to constructing ionic porous organic polymers. The general strategy to prepare ImPOP-1 is illustrated in Scheme 1. We started with designing a molecule that contains dual functional groups, the imidazolium part for the introduction of charged character and the aldehyde group for easy polymerization via Schiff base reaction (Scheme 1). By reacting 4-(bromomethyl)benzaldehyde with 1,3,5-tri(1H-imidazol-1yl)benzene via very simple nucleophilic reaction, the positively charged molecules with retained aldehyde groups were easily obtained. The
Fig. 1. Solid-state 350
13
C CP/MAS NMR spectrum of ImPOP-1.
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Fig. 2. The nitrogen adsorption–desorption isotherm of ImPOP-1 (inset, the pore size distribution profile obtained from adsorption branch using NLDFT method).
Information). The mesoporous and macroporous structure of ImPOP-1 can be clearly seen from the TEM images. 3.1. Adsorption capacity study As ImPOP-1 possesses nanoporous structure and its pore walls are decorated with cationic imidazolium moieties, it is rational to assume that ImPOP-1 owns the capability for removing anionic substance. High adsorption capacity is definitely beneficial to reduce the cost of materials for practical applications, therefore it is one of the most significant factors need investigating. We firstly chose organic dyes (Fig. S7, Supplementary Information) as model molecules to demonstrate the feasibility. Acid orange 7 (AO7−), orange G (OG2–), methyl blue (MB2–), and amaranth (AMR3–) are anionic type of dyes but different in magnitude of charge. Methylene blue (MLB+) and methyl violet (MV+) are cationic dyes with same charge and similar molecular size. As reported, the adsorption capacity is associated with the initial concentration of the dye and a higher concentration may lead to a better performance of adsorption capacity [47]. Therefore, high initial concentration of organic dye solution was adopted for the purpose of evaluating the adsorption performance. As seen in Fig. 3a, ImPOP-1 exhibits high adsorption capacity to all the anionic dyes. Especially to AO7–, the capacity is as high as 1.7 mmol g–1 (578.5 mg g–1). As for the cationic dyes, ImPOP-1 shows much lower adsorption capacity, such as the MLB+, merely 8.5 × 10–3 mmol g–1 (2.5 mg g–1). The obvious adsorption difference suggests that ImPOP-1 owns high selectivity on removing negatively charged organic dye molecules. Moreover, three anionic dyes with different magnitude of charge, namely AO7–, OG2–, and AMR3– were selected to study the influence on adsorption capacity. It is found that the adsorption amount is inversely proportional to the magnitude of charge. We reason that the adsorption process is mainly driven by the electrostatic interaction, in that case, the –1-charged AO7– is doomed to be adsorbed more than the –3-charged AMR3– for the sake of balance of charge. In the waste water from electroplating industry, some precious metal ions such as Pd and Au are anionic, exist in the form of PdCl42– or Au(CN)2–. Therefore, extraction of these precious metal ions is of both economic and environmental significance [54,55]. Encouraged by the result of high capacity to anionic organic dyes, we took a step further to study the adsorption capacity to inorganic anions. PdCl42– was then selected as the model anion due to its low toxicity compared to Au (CN)2–. The adsorption isotherm (Fig. 3b) of PdCl42– was fitted using the Langmuir model, a high correlation coefficient (0.9994) was obtained. The maximum adsorption capacity to Pd(II), based on the Langmuir model, is as high as 476.2 mg g–1. The result is superior to most of the reported materials for metal ions adsorption [22,56,57]. As far as we know, this is a record-high capacity to Pd(II) adsorption (Table 1) [49,58]. As reported previously, the pore volume actually weights over the BET specific surface area on the adsorption capacity
Fig. 3. The adsorption capacities to different anionic pollutants. (a) The maximum adsorption capacity to differently charged organic dyes. (b) The adsorption isotherm to inorganic PdCl42– ions. Table 1 Comparison of adsorption capacity for Pd(II) with selected adsorbents. Adsorbents
Adsorption capacity (mg g–1)
References
Graphene oxide Functionalized silica adsorbent Conjugate adsorbent Functionalized alumina nanopowder Chitosan derivative Chitosan derivative ImPOP-1
80.78 83.00 164.20 97.70 389.40 415.04 476.2
[48] [49] [50] [51] [52] [53] This work
[59]. Therefore, in addition to the electrostatic interaction between host polymer and guest molecules, it is supposed that the relatively large pore volume (0.81 cm3 g–1 at P/P0 = 0.97) may contribute to the high adsorption capacity towards both the organic dyes and inorganic metal ions. 3.2. Investigation on adsorption kinetics Except the capacity, the fast removal ability to target substances is also a significant factor to evaluate performance of materials. However, it is quite challenging to achieve high removal efficiency in short time, especially for the organic dyes [60–62]. Therefore, we performed experiments to verify if ImPOP-1 possesses the fast kinetics ability. Five dyes with different charges but similar molecular sizes were selected, of which MV+ and MLB+ are cationic, while the other three are anionic but different in magnitude of charge. All the initial concentration of aqueous dye solutions was adjusted to be 50 μM, and ImPOP-1 was 351
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Fig. 5. Selectivity performance towards different pollutants. (a) Selectivity towards PdCl42–, Cl– and SO42– are both 100 times (molar ratio) in excess relative to PdCl42–. (b) The charge-dependent selectivity to organic dyes. the UV–vis spectra of OG2–/MLB+ before and after separation test, the inset is the digital photograph of the dye solutions (OG2– and MLB+ solutions, OG2–/MLB+ mixture before and after separation for 5 min).
Fig. 4. The fast adsorption kinetics. (a) All three anionic dyes were removed in 30 min, while the cationic counterparts showed almost no adsorption. (b) The RE to PdCl42– is nearly 99% in 5 min, indicating superfast removal kinetics.
added into the solutions and then followed with vigorous stirring for accelerating the adsorption process. At an appropriate interval, the concentration was determined by UV–vis spectroscopy. As seen from Fig. 4a, ImPOP-1 exhibits distinct charge-dependent adsorption towards the dyes. There is almost no UV–vis signals of the anionic dyes detected in the solution after 30 min. The RE reaches to more than 99% just in a short time for all three anionic dyes, despite of the relatively slower AMR3– as compared to the other two. These findings indicate that ImPOP-1 possesses fast adsorption kinetics to the anionic organic dyes. The adsorption kinetics to inorganic metal ions was investigated as well. ImPOP-1 was added into the PdCl42– solution (50 ppm, 0.2 mM). After stirring for a desired period of time, the upper layer of the solution was taken out and the concentration of Pd was analyzed using ICP-MS. As shown in Fig. 4b, the equilibrium was established very soon, and 98.6% and 98.7% of the PdCl42– ions can be removed in 5 and 10 min, respectively. We adopted the Lagergren pseudo-first-order model (Fig. S8, Supplementary Information) and pseudo-second-order model (Fig. S9, Supplementary Information) for analysis of fast kinetics [63], the adsorption for dye AO7– and metal PdCl42– are both fitted well with the pseudo-second-order model (Table S1, Supplementary Information). The fast adsorption ability is ascribed to the relatively large pore size and hierarchical pore structure (Fig. 2, inset), which is beneficial to the
mass flow and access to the cationic active sites. 3.3. Selectivity towards the anionic pollutants 3.3.1. Selectivity investigation in mixed solution Cationic and anionic organic dyes are often coexisting in some waste water from textile-related industries, therefore separation to these differently charged dyes is important to water treatment. As seen in Figs. 3a and 4a, ImPOP-1 owns charge-dependent selectivity to differently charged organic dyes, whose selectivity is mainly based on the electrostatic interaction. However, all the adsorption experiments were conducted in individual aqueous solution. Herein, we investigated if ImPOP-1 can selectively remove target pollutants in the presence of interfering ions. The AO7–/MLB+ and OG2–/MLB+ stand for the two mixed dye solutions for the separation test. In the case of OG2–/MLB+ (Fig. 5b, inset), a dark brown mixture was formed after mixing MLB+ with oppositely charged OG2−. The material ImPOP-1 was added into the dye mixture solution, and then filtered after stirring for 5 min. The color of OG2–/MLB+ changed obviously from dark brown to blue, implying the successful separation of the two oppositely charged molecules. The separation efficiency was further confirmed by the UV–vis spectroscopy. The slight decrease in signal at 664 nm characteristic to MLB+ molecules indicates that most of the cationic MLB+ molecules 352
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are retained in the solution after separation. However, the anionic dye OG2– was almost totally removed, judging from the dramatic decrease in the spectra peak at around 480 nm. In addition, similar result was obtained for AO7–/MLB+ mixture (Fig. S10, Supplementary Information). Worth noting, the adsorption performance of MLB+ from an individual aqueous solution is different from that the mixture of AO7–/ MLB+ solution. The former solution showed almost no adsorption at all towards MLB+ (Fig. 4a), while the latter solution showed a slight decrease in concentration. This is probably due to the adsorption of the complex formed by the two oppositely charged dyes, which is supported by the change in the peak in the UV–vis spectra of the dye mixtures (Figs. S5 and S6, Supplementary Information). In addition, the selectivity to PdCl42– ions in the presence of high concentration of anions is also important factor for removing or extracting precious metal ions. The common anions Cl– and SO42– are usually coexisting with PdCl42–, therefore selected as the competing ions to investigate the selectivity. The initial concentration of the PdCl42– was adjusted to be 25 ppm (0.1 mM), Cl– and SO42– were 100 times (molar ratio) in excess relative to PdCl42–. The final concentrations of Pd were analyzed using ICP-MS. The result shows that the competing ions has very limited influence on RE (Fig. 5a), it is still able to remove 99% of the anionic pollutants. The high selectivity towards PdCl42– reflects the Hofmeister effect [64]. The Hofmeister effect has been demonstrated in a wide range of molecules and it is an empirical law that less hydrophilic anions with large size is favorable to be attached to the host materials. Comparing the hydrophilicity and the size of PdCl42– with the competing anions, the experiment results are well consistent with the Hofmeister effect [65]. 3.3.2. Selectivity investigation via column breakthrough tests In the practical operation, the column-chromatographic separation method is usually conducted by passing the contaminated water through the column packed with adsorbent. We therefore performed the column breakthrough tests to investigate if ImPOP-1 is able to selectively separate the dyes efficiently. A plastic pipette was packed with ImPOP-1 to form a column with height of ca 2.5 cm and diameter of ca 3.5 mm. First, the removal performance was verified by passing AO7– solution (25 ppm, 4 mL) through the column in 4 min, the UV–vis result shows that the spectral signal from AO7– can’t be detected (Fig. S11, Supplementary Information), which suggests more than 99% removal of AO7– dye. Moreover, we further performed the column breakthrough test to separate the mixture of two oppositely charged dyes. Two types of mixture, AO7–/MLB+ and OG2–/MLB+, were separately passed through the column and the performance was then evaluated using UV–vis spectroscopy. As seen from Fig. 6, both AO7–/MLB+ and OG2–/ MLB+ mixtures can be easily separated by ImPOP-1 during the breakthrough test. In the case of OG2–/MLB+, the residue of OG2– signal may be ascribed to the relatively slow adsorption kinetics compared to AO7–, which is evidenced from the previous removal efficiency experiments (Fig. 4a).
Fig. 6. The separation performance through the column breakthrough tests. (a) The UV–vis spectra of AO7–/MLB+ before and after column treatment, the inset is the digital photograph of the dye solutions (AO7– and MLB+ solutions, AO7–/ MLB+ mixture before and after separation). (b) The UV–vis spectra of OG2–/ MLB+ before and after column treatment, the inset is the digital photograph of the dye solutions (OG2– and MLB+ solutions, AO7–/MLB+ mixture before and after separation).
moieties with the metal Pd, and for the organic dyes, the π–π stacking between the phenyl groups from the host polymer and the phenyl groups from the guest dyes is also highly possible to facilitate the adsorption of guest molecules. In order to unravel the existence form of the Pd species after adsorption into the polymer network, X-ray photoelectron spectroscopy was therefore conducted. As seen from Fig. 7(b), the peak at 337.7 eV is assigned to the Pd–Cl, which suggests that most of the Pd exist in the form of PdCl42–, while the small peak at 336.0 eV assigned to Pd(0) suggests that the Pd(II) was partially reduced in the polymer network [66,67].
4. Adsorption mechanism In order to elucidate the sorption mechanism for anionic pollutants, we firstly conducted the SEM energy dispersive spectrometer (EDS) element mapping characterization. As seen from Fig. 7(a), the decrease in Br– anions and the appearance of the Pd featuring for PdCl42– and S featuring for AO7– suggest that the sorption process is dominantly caused by the ionic exchange that is driven by the electrostatic force. Furthermore, the solid-state NMR result shows that the peak featuring for imidazolium carbon shifts from 147.9 to 149.0 ppm after adsorption (Fig. 1 and Fig. S12, Supplementary Information). This shift indicates the interaction between the imidazolium rings with the PdCl42–. However, the residue of Br– anions suggests the incompletion of the ion exchange process. For the PdCl42– ions, it is highly possible to form the coordination bond for the nitrogen from the imine and imidazolium
5. Conclusions In summary, we have successfully designed and prepared an imidazolium-based cationic porous organic polymer, ImPOP-1, via the facile Schiff base reaction without use of any catalyst. ImPOP-1, which possesses porous and cationic character, is one of the rare examples that ionic porous organic polymers serves as the ion exchange materials for capturing ionic pollutants. It exhibits high adsorption capacity, superfast kinetics, and great selectivity towards the anionic pollutants, including inorganic metal ions and organic dyes. Worth noticing, the adsorption to PdCl42– is a record-high value. In addition, the good performance on column breakthrough test suggests that ImPOP-1 is a 353
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Fig. 7. (a) EDS-mapping of the original ImPOP-1, after adsorption of AO7– dye, and after adsorption of PdCl42–. (b) Pd 3d XPS spectra of ImPOP-1 after adsorption of PdCl42–.
promising material for realization of industrial applications. By the SEM-EDS, XPS, and solid-state CP/MAS NMR, we unravel that the adsorption of guest molecules is mainly an anion-exchange process. Our synthesis method featured in high simplicity would further promote the development of ionic porous organic materials, and the impressive experiment results on capturing anionic pollutants would give some hints on using ionic porous materials for future water treatment.
[4] [5]
[6] [7]
Conflict of interest
[8]
The authors declare no competing financial interest.
[9]
Acknowledgement
[10]
The financial support from the National Natural Science Foundation of China (Grant no. 21574032) is acknowledged.
[11] [12]
Appendix A. Supplementary data
[13]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.12.091.
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