Research Article pubs.acs.org/journal/ascecg
Facile Green Synthesis of Magnetic Porous Organic Polymers for Rapid Removal and Separation of Methylene Blue Lijin Huang, Man He, Beibei Chen, Qian Cheng, and Bin Hu* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, P. R. China S Supporting Information *
ABSTRACT: Herein, magnetic porous organic polymers composites (MOPs) with abundant free phenolic hydroxyl group were synthesized via a simple and green azo coupling reaction under mild conditions in water. Remarkably, Fe3O4@SiO2 nanoparticles were encapsulated in the disordering porous organic polymers during the coupling reaction. The prepared MOPs possessed excellent stability, high special surface areas, controllable magnetism, and high adsorption capacity (1153 mg g−1) toward cationic dye methylene blue. Moreover, the MOPs could be easily and rapidly recovered from the solution by using a magnet due to their superparamagnetic property and could be reused at least 5 times without decreasing the adsorption capacity, demonstrating a potential application in pollutants removal. KEYWORDS: Magnetic composites, Porous organic polymers, Azo coupling reaction, Dyes removal, Water treatment, Dye separation
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INTRODUCTION The growing discharge of dyes, such as methylene blue (MB), has been paid extensive attention due to its negative effects on the environment and human health.1,2 Accordingly, it is of great significance to develop effective methods for eliminating these dyes from polluted water resources. A lot of methods have been used for dye removal from wastewater, such as membranes/film separation,3,4 adsorption,5−7 and photocatalytic oxidation.8 Among them, adsorption is an efficient and simple approach for dye capture, and the adopted adsorbents is the crux. The suitable sorbents for dye uptake are expected to feature high adsorption capacity, fast adsorption kinetics, easy preparation and regeneration, cost-efficiency, chemical stability, and good selectivity. In the past decades, porous materials with high special surface areas have been selected as great candidates for dye capture. Several classes of porous materials, including porous organic polymers (POPs),9−11 metal−organic frameworks (MOFs),12−15 ordered mesoporous carbons,16 activated carbons,17 and functionalized mesoporous silica,18 have been employed for dye removal. Currently, the most commonly used adsorbents for organic dye removal are activated carbons. However, their applications are severely limited by the slow adsorption kinetics, low adsorption capacity, tough regeneration ability, and high cost. Although MOFs have been widely studied for dye removal, they still suffered from the drawbacks of relative poor chemical and water stability in practical applications. Comparatively, POPs constructed through covalent bonds show comprising chemical stability and would not get into the trouble of framework degradation or collapse. Very © 2017 American Chemical Society
recently, Zhang et al. synthesized a highly efficient magnetic porous covalent triazine-based framework composite by a facile microwave-enhanced high-temperature ionothermal method for methyl orange (MO) removal.9 Yu et al. prepared the polycationic COFs by the formation of imine bonds between the rigid triangular triamine and a 4,4′-bipyridinium-derived dialdehyde, and the prepared polycationic COFs displayed high uptake capacity toward a range of anionic dye pollutants.10 Although POPs exhibit good performance for dye capture, the preparation of POPs is still a challenge. High reaction temperature, environmentally unfriendly solvents (e.g., 1,4dioxane, dimethyl sulfoxide, 1,3,5-trimethylbenzene) and noblemetal catalysts are generally required during the preparation process of POPs, which could increase the manufacturing cost and damage the environment.19−22 In addition, the poor solubility of organic monomers in water makes the preparation of POPs more difficult in water than in organic solvents. On the other hand, the rapid separation and regeneration of POPs from aqueous solution in environmental treatment are another challenge. A combination of POPs and magnetic technology provides an effective way to achieve fast separation. However, compared with the abundance of available parent POPs, the preparation of magnetic POPs (MOPs) with high stability is still in the early stages.9,23−25 Indeed, similar to the shortcomings in POPs preparation, either organic solvent or high reaction temperature required during the synthesis makes Received: January 4, 2017 Revised: March 16, 2017 Published: March 27, 2017 4050
DOI: 10.1021/acssuschemeng.7b00031 ACS Sustainable Chem. Eng. 2017, 5, 4050−4055
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ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of MOP-2
solution. Subsequently, m-trihydroxybenzene (1 mmol), an appropriate amount of MNPs (100 mg for MOP-1, 200 mg for MOP-2, 300 mg for MOP-3, and 400 mg for MOP-4) and Na2CO3 (3 mmol) were spiked in 30 mL of water, which was then slowly added into the above solution, and the reaction was kept for 12 h. The products were separated with an external magnet and washed sequentially with water, ethanol, and water. Finally, the product was obtained by freeze-drying. POPs were prepared by the same procedure without the addition of MNPs. Characterization of Materials. The concentration of dyes (MB/ MO) was identified by spectrophotometer (Shimadzu UV−vis spectrophotometer, UV-1800, Japan) at 665 and 464 nm, respectively. PPMS-9 vibrating sample magnetometer (VSM) (QUANTOM, USA) was employed to evaluate the magnetic properties of the materials. The morphology of the materials was obtained through transmission electron micrograph (TEM, JEM-2010 electron microscope, Tokyo, Japan). Thermodynamic analysis (TGA) was carried out on PE diamond TG/DTA 6300 (USA) in nitrogen and the heating speed was 5 °C min−1. The XRD patterns were identified by Bruker D8 diffractometer (Germany) with monochromatized Cu Kα radiation (40 kV, 40 mA). The Brunauer−Emmett−Teller (BET) surface area was collected using ASAP 2020 apparatus (Micromeritics, USA). The Fourier transform infrared spectrometer (FT-IR) spectra were analyzed on the NEXUS 870 spectroscopy (Thermo, Madison, USA). Adsorption Studies. The adsorption of dyes on MOP-2 was carried out by spiking 10 mg of MOP-2 into 10 mL of dye solution. The concentration of dyes in aqueous solution was detected by spectrophotometer. The effect of contact time on the elimination of MB by MOP-2 was investigated by adding MOP-2 into MB (50 mg L−1) solution. Supernatant was taken out at certain time intervals, and the concentration of MB was detected by spectrophotometer. All experiments were carried out in triplicate under the same conditions.
it hard to obtain MOPs under mild conditions in aqueous solution. More recently, template-free or metal-free strategies have been reported for the preparation of POPs,26−29 which are more simple and useful for practical application, as they can be prepared under very mild conditions.30−32 Liu and co-workers reported a template-free strategy for the preparation of mesoporous POPs with high yield based on a mild diazo coupling reaction in water.32 The mild reaction conditions that are free of environmentally unfriendly solvents make this method extraordinarily attractive to construct MOPs with designable flexibility by adopting different monomers. Herein, we synthesized MOPs containing a rich phenolic hydroxyl group (identified as MOP-2) by employing Fe3O4@ SiO2 magnetic nanoparticles (MNPs),33 4,4′-diaminobiphenyl, and m-trihydroxybenzene as precursors (Scheme 1). MOP-2 was obtained via in situ growth of porous networks on the surface of MNPs. Due to its good hydrophobicity, high surface areas, and suitable extensive conjugated system, the obtained MOP-2 possessed good adsorption capacity for cationic organic dye MB from water (1153 mg g−1), and its selective removal ability for MB from the mixture of MB and MO is mainly attributed to the synergistic effect of electrostatic and hydrogenbonding interaction. In addition, the good magnetic separation ability of MOP-2 contributes to a rapid adsorption of MB from water.
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EXPERIMENTAL SECTION
Chemicals and Materials. All chemicals and reagents are commercially available and used without further purification. The MNPs (∼15 nm) were obtained via the previous method.33 Synthesis of MOPs. The synthesis of MOPs was carried out according to the reported procedure25 with minor modifications. 4,4′diaminobiphenyl (1.5 mmol) was added to 100 mL of ultrapure water containing 0.7 mL of concentrated hydrochloric acid. The mixture was stirred under ice bath for 15 min, then sodium nitrite (3.1 mmol) dissolved in 30 mL water was added. The mixture was kept at 0 °C for 30 min and then adjusted to neutral by adding Na2CO3 aqueous
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RESULTS AND DISCUSSION Materials Characterization. As shown in Scheme 1, MOP-2 was prepared in water at 0−5 °C through diazocoupling reactions with high yield (∼95%). SEM, TEM, and element mapping images in Figure 1 show the morphologies 4051
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typical well-defined crystal lattice distances of 0.25 and 0.30 nm, corresponding to the (311) and (220) crystalline planes of Fe3O4 (JCPDS no. 19-0629), respectively. In contrast, no spaced lattice fringe is observed on the edge of this composite, implying that the POPs are amorphous in nature. Combining the element mapping (Figure 1c) and EDX data (Figure S2), it can be confirmed that the MNPs are incorporated into POPs successfully. The magnetic property of MOP-2 was measured, and the result demonstrated that the superparamagnetism of initial MNPs was well retained and the amount of MNPs in the polymer was tunable. Excellent magnetic property assures the easy separation of MOP-2 under external magnetic field (Figure S3). The successful preparation of MOP-2 was also confirmed by FT-IR characterization. For MOP-2, the intensive characteristic adsorption peak of the asymmetric vibration of −NN− bond appears around 1398 cm−1 (Figure S4). The wide adsorption peaks around 3385 cm−1 are ascribed to −OH stretching vibration, confirming the successful coupling reaction.32 The BET surface area of MOP-2 was investigated by nitrogen adsorption−desorption measurements at 77 K. As shown in Figure S5, the special surface area of MOP-2 is 327 m2 g−1, which is much higher than that of MNPs (59 m2 g−1) and comparable to that of pure POP (∼256 m2 g−1).32 The nitrogen adsorption−desorption isotherm belongs to type IV (Figure S5), demonstrating the existence of mesoporous in MOP-2. The pore size distribution curve evaluated from BET measurement (Figure S5, inset) clearly validates the mesopore structure. The high thermal stability of MOP-2 was confirmed by TGA measurement (Figure S6). The result shows that MOP-2 is stable up to 290 °C. Overall, the porous structure along with high surface area of the obtained MOP-2 would provide abundant adsorption sites for interested analytes, the encapsulated MNPs would benefit the rapid separation by external magnetic field, and the thermal
Figure 1. TEM (a); HRTEM image (b); and element mapping images (c) of MOP-2.
and structure of MNPs and MOP-2. Amorphous agglomerate morphologies are proved by the SEM images (Figure S1). In the TEM images (Figure 1a) shown, the MNPs are embedded in a porous POPs-matrix. The lighter areas surrounding can be ascribed to the matrix of POPs, whereas the darker areas represent the MNPs. The size of MNPs encapsulated in MOPs is consistent with the size of MNPs precursor (Figure S1). Compared with bare MNPs, the MOP-2 shows a better dispersion, probably attributing to the integration of POPs. In addition, the HRTEM image (Figure 1b) clearly shows two
Figure 2. Photographs of MB aqueous solutions before (left) and after (right) adsorption (a); effect of contact time on the removal of MB (b); adsorption isotherms of MB onto MOP-2 (c). 4052
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Figure 3. Separation of MB/MO mixture under different concentration ratios (a) and photographs of MB/MO mixture solutions before (left) and after (right) separation (50 mg L−1/50 mg L−1) (b).
selectively adsorb MB and that the concentration of MO has no effect on the removal of MB. The adsorption selectivity of the prepared Fe3O4 or Fe3O4@SiO2 MNPs for MB was not studied in this work since neither MB nor MO can be adsorbed on Fe3O4 or Fe3O4@SiO2 MNPs. To further elucidate the adsorption mechanism, the adsorption capacity of MB on MOP-2 under different pH and the zeta potential of MOP-2 were investigated. As shown in Figure S8, the adsorption capacity of MB on MOP-2 increases slightly with an increase of pH from 2 to 12. Figure S9 shows that MOP-2 is positively charged at pH 2 and negatively charged at pH 4−12. The large adsorption capacity for cationic MB over the tested whole pH range indicates that electrostatic force is not the only interaction for adsorption of MB by MOP2. While for anionic MO, the negatively charged MOP-2 under neutral pH would prevent the adsorption of MO through electrostatic interaction. On the other hand, it has been demonstrated that the phenolic −OH groups in the porous materials can selectively interact with MB with nitrogencontaining aromatic ring through strong hydrogen bonding.32 It indicates the abundant phenolic −OH groups on MOP-2 are effective for MB capturing through hydrogen-bonding interaction. Therefore, it can be concluded that both electrostatic interaction and hydrogen-bond formation are contributive to the selective adsorption of MB over MO. Stability and Recycling. As MOP-2 showed good stability in water and exhibited a robust capability for cationic MB adsorption, the regeneration property was further evaluated. The release of MB from MOP-2 was achieved by soaking the sorbents in a hydrochloric acid containing methanol solution (0.1 mM). The reusability of MOP-2 for MB trapping is shown in Figure 4. The results demonstrate that the adsorption capacity varies a little even after five adsorption−desorption cycles, indicating that the sorbent had good regeneration property and stability. As shown in Figure S10, the FT-IR spectra of MOP-2 after five adsorption−desorption cycles matches well with the as-synthesized MOP-2, demonstrating a good stability of MOP-2. Moreover, the results of nitrogen adsorption−desorption isotherms of reused MOP-2 confirm that the MOP-2 maintains its high surface area (321 m2 g−1) after five adsorption−desorption cycles.
stability ensures the application of MOP-2 in water treatment to some extent. Adsorption Kinetics. Because of its rich phenolic −OH functional groups, convenient fabrication, and porous structure, MOP-2 is expected to be a good alternative for MB removal based on a strong interaction between phenolic −OH groups and MB via hydrogen bonding.32 First, the relationship between incubation time and removal efficiency was studied by soaking 10 mg of MOP-2 into the aqueous solution of MB (50 mg L−1). Photographs of aqueous MB solutions before (left) and after (right) magnetic separation are shown in Figure 2a. The concentration of MB at different time intervals in supernatant was identified by UV−vis spectra. As shown in Figure 2b, the concentration of MB declines with the extension of incubation time, and the solution of MB changes from blue to colorless in 5 min. The experimental data fit well with the pseudo-second-order kinetic model (Figure S7), and the value of adsorption rate constant K2 was determined to be 0.105 g mg−1 min−1, which is much higher than that of other magnetic sorbents for MB removal under similar conditions (Table S1). The adsorption kinetics of MB onto Fe3O4, Fe3O4@SiO2, and POPs were also investigated. As can be seen in Figure 2b, no obvious adsorption of MB is observed on Fe3O4 and Fe3O4@ SiO2, and the bare POPs show a similar adsorption kinetics to MOP-2 for MB. This indicates that the MOP-2 possesses very fast adsorption kinetics for MB removal, and the good adsorption performance of MOPs for MB is mainly attributed to the component of POPs rather than Fe3O4 or Fe3O4@SiO2. Adsorption Isotherms. The adsorption isotherms of MB onto MOP-2 were also measured to determine the maximum adsorption capacity (Figure 2c). The results show that the adsorbed amount of MB increases gradually with the increase of initial concentration of MB and reaches a platform. It was found that 1153 mg of MB can be adsorbed onto 1 g of MOP2. The adsorption capacity of MOP-2 is the highest among magnetic sorbents for MB so far, including Fe3O4-PIAgCHI (470.2 mg g−1),34 Fe3O4@MIL-100(Fe) (78 mg g−1),35 chitosan and active charcoal-Fe3O4 (500 mg g−1),36 magnetic nanoporous carbon (292.4 mg g−1),37 Fe3O4/PDA (204 mg g−1),38 magnetic mesoporous carbon (608 mg g−1),39 γ-Fe2O3/ C@HKUST-1 (370.2 mg g−1),40 Fe3O4@ZIF-8 (20.2 mg g−1),41 and magnetic carbon nanospheres (45 mg g−1).42 Selectivity. To investigate the selectivity of MOP-2 for MB removal, a mixture of cationic dye MB and anionic dye MO with different concentration ratio was prepared and spiked with 10 mg of MOP-2. Figure 3 shows that MOP-2 can only
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CONCLUSIONS In conclusion, MOP-2 with free rich −OH functional groups was prepared in water with high yield by employing an azocoupling reaction. MOP-2 exhibits good dispersion, high 4053
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Figure 4. Adsorption capacity of MB obtained by MOP-2 after several adsorption−desorption cycles.
stability, and good magnetic property. Moreover, MB can be selectively adsorbed over MO from the aqueous solution by MOP-2 and released easily from MOP-2. MOP-2 can be reused five times without losing its adsorption performance. In consideration of convenient preparation, low cost, high adsorption performance, easy recovery, and good regeneration ability, MOP-2 merits good application prospects in the field of environment engineering.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00031. Materials characterization and other data. Additional information, including SEM images and EDX spectra of MOP-2, magnetization curve of different MOPs, FT-IR spectra of precursors and MOP-2, nitrogen adsorption− desorption isotherms and TG curves of MOP-2, MB removal performance across different pH range by MOP2, zeta potential of MOP-2, FT-IR spectra of MOP-2 after five recycling experiments (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 86-27-68752701. Fax: 8627-68754067. ORCID
Beibei Chen: 0000-0001-7772-5171 Bin Hu: 0000-0003-2171-2202 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (grant nos.: 21175102, 21205090, 21575107) and the National Basic Research Program of China (973 Program, 2013CB933900).
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