Collectable and Recyclable Mussel-Inspired Poly(ionic liquid)-Based

Feb 28, 2017 - Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada. ACS Sustainable ...
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Collectable and Recyclable Mussel-Inspired Poly(ionic liquid)-Based Sorbents for Ultrafast Water Treatment Yangyang Lu,† He Zhu,† Wen-Jun Wang,*,† Bo-Geng Li,† and Shiping Zhu*,‡ †

State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China ‡ Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada

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S Supporting Information *

ABSTRACT: We report a green method to graft directly poly(ionic liquid) onto polydopamine-modified magneticresponsive Fe3O4 nanoparticles through an organic solventfree process of self-initiated photografting and photopolymerization. PIL@PDA@Fe3O4 nanocomposite is characterized by transmission electron microscopy, attenuated total reflectioninfrared intensity, X-ray photoelectron spectroscopy, and thermal gravimetric analyzer. The nanoparticles can be used for removal of methylene blue (MB) from water. The adsorption is ultrafast, with a maximum MB adsorption capacity of 72.5 mg/g. A selective adsorption property of PIL@PDA@ Fe3O4 is also demonstrated by separation of cationic dyes from anionic dyes in water. The particles can be collected by a magnet and be regenerated by washing with salt solution after adsorption. The results show negligible change in the adsorption and desorption efficiency after 5 cycles. This work demonstrates the potential of PDA as an initiator for photografting and photopolymerization of PIL from surfaces separation applications. KEYWORDS: Ionic liquid, Polydopamine, Magnetic nanoparticle, Adsorbents

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INTRODUCTION Ionic liquids (ILs), which are entirely composed of ions under 100 °C, have attracted lots of attention in recent years.1,2 ILs are widely used in various applications, such as separation,3 catalysis,4 fuel cells,5 and batteries6 due to their special properties: negligible volatility, high polarity, remarkable electrical conductivity, incombustibility, and excellent chemical and thermal stability. Poly(ionic liquid) (PIL) or polymeric ionic liquid refers to a new class of polyelectrolyte, which contains ionic liquid units in polymer chains. PIL combines the outstanding properties of ILs and the good mechanical performance, processability of polymers. Numerous papers have been published in exploring potential applications of PIL as CO2 adsorbents,7 extractants,8 carbon precursors,9 smart surfaces,10 porous materials,11 and so on.12−14 Recently, significant efforts are focused on grafting of PIL onto a variety of substrates, such as graphene,15 silica,16,17 magnetic nanoparticles,18,19 carbon nanotubes,20 and fibers,21,22 for applications including separation, catalysis, and supercapacitors. Functional groups are often introduced onto the material surfaces to facilitate surface-initiated polymerization (SIP) of ionic liquids. Sato et al.23 reported grafting of PIL onto a premodified monodisperse silica particle by surface-initiated atom transfer radical polymerization (SI-ATRP). Pourjavadi et al.18 used magnetic nanoparticles (MNPs), which was modified © 2017 American Chemical Society

with active vinyl groups, to copolymerize IL monomer in the presence of 2,2′-azobis(isobutyronitrile) as an initiator and cross-linker to yield PIL@MNPs. However, these methods usually require complicated surface modification procedures and consume catalysts or initiators during grafting process. More recently, self-initiated photografting and photopolymerization (SIPGP) has been utilized to graft directly polymer brushes onto surfaces of various materials.24−26 Compared with SI-ATRP, SIPGP provides a robust choice for advanced SIP techniques because of its prominent advantages including no additional reactions needed to introduce initiators onto surface, as well as no catalysts consumed during polymerization process. Inspired by adhesive protein in mussel, polydopamine (PDA) has drawn significant attention from researchers because of its unique coating ability and multifunctionality. PDA, usually prepared via self-polymerization of dopamine (DA) under alkaline conditions, is proven to be capable to form thin films on inorganic and organic materials, such as metals, polymers, silica, and semiconductors via strong interaction with catechol and amine groups of the DA units.27,28 Therefore, PDA adhesion is expected to be a versatile and easy strategy for Received: January 16, 2017 Revised: February 10, 2017 Published: February 28, 2017 2829

DOI: 10.1021/acssuschemeng.7b00150 ACS Sustainable Chem. Eng. 2017, 5, 2829−2835

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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Synthesis Route of PIL@PDA@Fe3O4 MNPs

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modification of all kinds of material surfaces. In 2016, Hafner et al.29 grafted six different types of polymers onto PDA-modified planar silicon substrates via SIPGP without any additive catalyst or initiator. It was proposed that the hydrogens of catechol and amino groups in the DA units could be abstracted via UV irradiation to generate radicals, to facilitate polymerization of various monomers. This novel PDA-based SIPGP technology is of great benefit in simplifying the traditional tedious SIP processes for various material surfaces. The water pollution caused by organic compounds has become a serious environmental problem all over the world. Among the organic contaminations, there are about tens of thousands of different dyes used in industries, with a total annual consumption over 7 × 105 tons.30 Most synthetic dyes are toxic and even carcinogenic to human beings, and also highly resistant to degradation by lights and oxidants.31 There have been many techniques developed and applied for removing dyes from water. Adsorption is regarded as the most efficient, versatile and economic method, especially if it is combined with smart separation and appropriate reuse of adsorbents. PIL can be an ideal candidate for adsorbing dyes because it strongly interacts with ionic dyes by ionic bonds. However, it is difficult to regenerate and reuse PIL after adsorption. In this work, for the first time, we report the grafting of PIL onto PDA-modified Fe3O4 nanoparticles via SIPGP. PDA@ Fe3O4 MNPs are used as UV photoinitiator for the direct SIPGP of IL to yield PIL@PDA@Fe3O 4 MNPs. The synthesized PIL@PDA@Fe3O4 MNPs show an ultrafast adsorption and high adsorption capacity of methylene blue (MB), and are capable to selectively remove MB from MB/ Acid Orange-7 (AO7) aqueous solution. The nanoparticles can be easily regenerated and reused for 5 times without loss of the adsorption efficiency. Compared with other reported methods of grafting PIL onto material surfaces, the innovation of this work is that we proposed a versatile approach to quickly graft PIL. There are no organic solvents involved in the entire synthesis procedure, no tedious chemical modification steps. This approach is efficient, cost-effective and environmental friendly.

EXPERIMENTAL SECTION

Materials. Carboxylated Fe3O4 magnetic beads (0.1−0.2 μm), dopamine hydrochloride (≥98%), tris(hydroxymethyl)aminomethane (≥99%), 3-sulfopropyl methacrylate, potassium salt (≥98%), methylene blue, acid orange 7 (97%), crystal violet, and alizarin red S were purchased from J&K Chemical. Tributylhexylphosphonium bromide was purchased from Wuhu Nuowei chemistry Co. Ltd. All chemicals were used as received. Characterization. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Advance 400 MHz spectrometer, using CDCl3 as solvent. Transmission electron microscopy (TEM) images were obtained on a JEM-1200EX Electron Microscope (JEOL, 80 kV). Fourier transform infrared (FT-IR) spectroscopy curves were recorded on a Nicolet 6700 FT-IR spectrometer in the range of 4000−400 cm−1. X-ray photoelectron spectroscopy (XPS) spectra were recorded using Thermo Fisher Scientific Escalab 250Xi instrument. UV−vis spectra were obtained on TU-1901 double beam UV−vis spectrophotometer (PERSEE). Thermal gravimetric analysis (TGA) was carried out with air atmosphere using Pyris 1 TGA instrument in the temperature range 100−850 °C with a ramp of 10 °C·min−1. The ζpotentials of MNPs were determined using a Nano-ZS90 zetasizer at 25 °C. Synthesis of IL Monomer. The ionic liquid monomer was synthesized according to the literature.32 Tributylhexylphosphonium bromide (25.0 g, 0.068 mol) was dissolved in water (30 mL) and 3sulfopropyl methacrylate (18.4 g, 0.088 mol) was added into the solution. The mixture was stirred for 24 h at room temperature. The resulting product was extracted by dichloromethane (50 mL) for three times, followed by washing with water (30 mL) for three times. The dichloromethane was then removed with a rotary evaporator and the product was dried under vacuum at room temperature. 1H NMR (CDCl3, 400 MHz): 0.90−0.93 (t, 12H, −CH3), 1.26 (m, 4H, −CH2−), 1.47 (m, 16H, −CH2−), 1.85 (t, 3H, −CH3), 2.20−2.35 (m, 11H, −CH2), 2.80−2.88 (m, 2H, −CH2), 4.19 (t, 2H, −CH2), 5.46 (dd, 1H, CH2=), 6.02 (dd, 1H, CH2=). Synthesis of PDA@Fe3O4 MNPs. Fe3O4 MNPs of 25 mg were dispersed in 250 mL of dopamine (1 mg/mL) tris solution (pH = 8.5, 10 mM Tris−HCl buffer). The mixture was stirred for 2 h at room temperature. The product was collected by a magnet, and the residual reactants were removed by ultrasonic washing with water for several times. The product was dried under vacuum at 40 °C overnight. Synthesis of PIL@PDA@Fe3O4 MNPs. In a typical experiment, 10 mg of PDA@Fe3O4 MNPs were dispersed in 1.5 g of IL monomer in a quartz cell by ultrasonication. The mixture was bubbled with nitrogen for 10 min to remove oxygen. Self-initiated photografting and photopolymerization (SIPGP) was performed under ultraviolet (Detianyou Technology, DTY-2010) with a maximum wavelength at 2830

DOI: 10.1021/acssuschemeng.7b00150 ACS Sustainable Chem. Eng. 2017, 5, 2829−2835

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Figure 1. TEM images of (a) Fe3O4 MNPs, (b) PDA@Fe3O4 MNPs, and (c) PIL-10h@PDA@Fe3O4 MNPs.

Fe3O4 MNPs had a mean diameter of 143.8 ± 12.9 nm (Figure 1a). It can be seen in the TEM images that Fe3O4 MNPs were wrapped by a well-defined PDA layer with an average thickness of 23.2 ± 2.5 nm (Figure 1b). After SIPGP of IL for 10 h, the average thickness of the polymer layer increased to 29.4 ± 3.2 nm, with the PIL layer thickness about 6.1 nm. The ζ-potentials of Fe3O4, PDA@Fe3O4, and PIL-10h@PDA@Fe3O4 MNPs were measured in water under room temperature. The ζpotentials of Fe3O4 and PDA@Fe3O4 were −50.9 and −28.8 mV, respectively, whereas decreased to −151.9 mV after grafting of PIL layer. The PDA and PIL layers were also confirmed by Fourier transform infrared (FT-IR) spectroscopy (Figure S1) and X-ray photoelectron spectroscopy (XPS) (Figure S2). For Fe3O4 MNPs, the adsorption peak around 584 cm−1 was attributed to the vibration of FeO function group and that at 3380 cm−1 corresponded to surfaced-adsorbed water.35 The additional peaks at 3420 cm−1 (OH stretching), 2922 cm−1 (CH stretching), 1620 cm−1 (phenylic CC stretching and NH bending), and 1240 (CN stretching) proved the presence of PDA layer on Fe3O4 MNPs.33,36 In Figure S1, curve c, the polymerization of IL was confirmed by the peaks at 1385 and 1331 cm−1 of SO3− groups.36 Moreover, as shown in XPS spectra, the appearance of N 1s at a binding energy of 400.1 eV and the decrease of O/C mole ratio (Figure S2) also provided evidence to the coating of PDA. As compared to the wide scan spectra of PDA@Fe3O4 MNPs before SIPGP of IL, the appearance of S 2p and P 2p at 167.2 and 132.1 eV, respectively, as well as the disappearance of N 1s at 400.1 eV, further suggested the successful modification of IL. Besides, the decrease of O/C mole ratio gave an additional support to the claim because the O/C mole ratio of IL monomer is lower than that of PDA. The amounts of grafted PDA and PIL on Fe3O4 MNPs were determined by TGA (Figure S3a). As temperature increased, pristine Fe3O4 MNPs experienced oxidation to Fe2O3 between 270 to 570 °C, causing a theoretical 3.45% gain in weight.37 The weight loss from 570 to 750 °C was due to combustion of carbon on the particle surface. A difference of 15.43 wt % between Fe3O4 and PDA@Fe3O4 MNPs at 850 °C was observed, which was resulted from the degradation of PDA. The grafting yield of PIL on PDA@Fe3O4 MNPs was estimated by the weight loss between PDA@Fe3O4 and PIL@PDA@ Fe3O4 MNPs. As SIPGP employed UV light for photopolymerization, the grafting yield of polymer increased with the polymerization time.29 In this work, different UV radiation periods of 2, 6, 10, 14, 18 h were applied to prepare five PIL@ PDA@Fe3O4 MNPs having various grafting yields of PIL. The grafting yield data are presented in Table S1. A linear

350 nm. The polymerization time was varied to obtain various grafting yields of polymer layers on PDA@Fe3O4 MNPs. The product was then rinsed with water for several times to remove unreacted monomer. Ultrasonication was carried out to ensure only grafted PIL remaining on PDA@Fe3O4 MNPs. The resulting PIL@PDA@ Fe3O4 MNPs were collected by a magnet and dried under vacuum at room temperature for overnight. Adsorption of MB. An example adsorption experiment was conducted as followed: a portion of 5 mg PIL@PDA@Fe3O4 MNPs and 10 mL of 50 ppm MB aqueous solution were mixed and shaken for 2 min. The pH values of the solutions were adjusted by adding HCl or NaOH. The MB adsorbed PIL@PDA@Fe3O4 MNPs were collected by a magnet. The concentration of supernatant after adsorption was determined by UV absorbance at 664 nm. The adsorption capacity (q) was calculated according to

q = (C0 − C1)V /m

(1)

,where C0 and C1 were mass concentrations of MB before and after adsorption, respectively, V was the volume of MB solution and m was the mass of PIL@PDA@Fe3O4 MNPs. Regeneration and Reuse of PIL@PDA@Fe3O4 MNPs. After adsorption, PIL@PDA@Fe3O4 was washed with saturated potassium acetate solution until the adsorbed MB molecules were entirely released. The desorption efficiency (D) was calculated according to

D = C tVt /(C0 − C1)V

(2)

,where Ct and Vt were mass concentration of MB and volume of the total washing solution. The PIL@PDA@Fe3O4 was then collected and dried under vacuum at 40 °C to be used in subsequent adsorption experiments, which were conducted following the same procedure.

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RESULTS AND DISCUSSION The synthesis of PIL@PDA@Fe3O4 MNPs is illustrated in Scheme 1. First, PDA@Fe3O4 MNPs were prepared as reported.33 Catechol moieties of PDA are known to interact with Fe3+ of Fe3O4 nanoparticles.34 Fe3O4 MNPs were dispersed in an alkaline DA solution (pH = 8.5) for 2 h (Step 1), resulting in a layer of PDA on the surface of MNPs. The color of MNPs solution changed from brown to dark black because of the self-polymerization of DA. The resulting MNPs were collected by a magnet and washed thoroughly by ultrasonication to remove free PDA. Second, PDA@Fe3O4 MNPs were used to polymerize the IL monomer, tributylhexylphosphonuim 3-sulfopropyl methacrylate, by SIPGP method, resulting in PIL-covered MNPs (Step 2). The radicals were generated via UV irradiation of the abundant catechol and amino functional groups on PDA surface.29 The final product was then washed with water by ultrasonication to remove unreacted monomer, as well as physically adsorbed PIL chains. The size and shape of Fe3O4, PDA@Fe3O4, and PIL@PDA@ Fe3O4 MNPs were characterized by TEM. The commercial 2831

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Figure 2. (a) Adsorption behavior of PIL-10@PDA@Fe3O4 MNPs toward MB. The initial concentrations of MB and MNPs were 10 ppm and 0.5 mg/mL. The adsorption temperature was 25 °C and the adsorption time was 2 min. (b) Adsorption kinetics of MB on PIL-10h@PDA@Fe3O4. The initial concentrations of MB and MNPs were 50 ppm and 0.5 mg/mL, respectively. The correlation line was calculated from the pseudo-secondorder kinetic equation, with qe = 59.4 mg/g and k2 = 1.16 g·mg−1·min−1. (c) Fitting curves for the pseudo-second-order kinetics. (d) Effect of grafting yield of PIL on MB adsorption capacity.

Figure 3. (a) Images of the selective adsorption of MB from MB/AO7 mixture by PIL-10@PDA@Fe3O4 MNP. Adsorption conditions: the initial concentration of MB and AO7 in mixture solution C0 = 5 ppm, temperature 25 °C. (b) UV−vis spectra of MB, AO7 and the mixture of dyes before and after adsorption. The samples of the initial MB and AO7 were diluted 1:4 and the samples taken before and after adsorption were diluted 1:2. (c) Images of the selective adsorption of CV from CV/ARS mixture by PIL-10@PDA@Fe3O4 MNP. Adsorption conditions: the initial concentration of CV and ARS in mixture solution C0 = 5 ppm, temperature 25 °C. (b) UV−vis spectra of CV, ARS and the mixture of dyes before and after adsorption. The samples of the initial CV and ARS were diluted 1:4 and the samples taken before and after adsorption were diluted 1:2.

relationship between the grafting yield of PIL and polymerization time was observed (Figure S3b). We supposed that IL monomer was highly polar and it could diffuse into PDA layer

so that the polymerization was initiated both on surface and in the PDA layer. The polymerization rate remained constant until the initiator groups were exhausted, which resulted in the linear 2832

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resulting in blackish and purple solutions, respectively. The PIL-10@PDA@Fe3O4 MNPs were placed into the two mixtures. The cationic dyes were fully adsorbed after 2 min shaking. The MNPs were then collected by a magnet. The color of the MB/AO7 solution turned to yellow, corresponding to the color of AO7, suggesting that the cationic dye (MB) was selectively adsorbed onto MNPs, whereas the anionic dye (AO7) remained in water. Similarly, the color of the CV/ARS solution turned to pink, corresponding to the color of ARS, indicating the selectively adsorption of cationic dye as well. The results were then further confirmed by UV−vis spectra. Figure 3b shows that the peak of 664 nm, which was the maximum absorbance wavelength of MB, disappeared after the adsorption. However, the peak of maximum absorbance wavelength of AO7 at 484 nm remained the same after adsorption, suggesting little adsorption toward AO7. Similarly, the maximum CV absorbance peak at 583 nm disappeared after adsorption, whereas ARS at 519 nm remained the same. The high selectivity in the competitive adsorption makes PIL@ PDA@Fe3O4 MNPs a promising candidate for separating cationic dyes from anionic dyes. The recyclability and reusability of adsorbents are the important considerations in adsorbent applications for the sake of cost effectiveness and environmental protection. Stimuli-responsive polymers can facilitate separation processes by incorporation of external triggers, such as CO2, temperature, and pH.8,40−49 The obtained PIL@PDA@Fe3O4 MNPs could be collected easily and rapidly by applying an external magnet. The total time for one cycle was less than 3 min, that is, 2 min for adsorption and several seconds for magnetic separation. The reusability of the magnetic-responsive PIL@PDA@Fe3O4 MNPs was demonstrated by washing the adsorbed dyes with salt solution and water, followed by vacuum drying overnight at room temperature. The same PIL could be regenerated by washing with tributylhexyl phosphonium bromide salt solution. We used potassium acetate salt as different elution agent to regenerate the composite, because of its low cost for water treatment. The regenerated MNPs showed no difference in the adsorption capacity for MB. The morphology of PIL@PDA@ Fe3O4 MNPs after regeneration was also confirmed by TEM (Figure S4b.) The adsorption capacity of MNPs showed a slight decrease of 5.0% after 5 cycles, whereas the desorption efficiency (D) remained unchanged (Figure 4). We then investigated the long-term stability of PIL@PDA@Fe3O4 MNPs. The PIL-10h@PDA@Fe3O4 MNP samples stocked

relationship between the grafting yield and polymerization time.29 After the obtained PIL@PDA@Fe3O4 MNPs were thoroughly characterized, we investigated their adsorption behavior toward MB. As shown in Figure 2a, the dark blue water turned colorless and became transparent after 2 min of adsorption, suggesting that PIL@PDA@Fe3O4 MNPs could be used as an efficient adsorbent for the removal of MB from water. We also examined PDA@Fe3O4 MNPs without the modification of PIL as a control experiment, which gave weak adsorption ability toward MB. The ultrafast MB adsorption could be attributed to synergistic effects of electrostatic interaction (main interaction), hydrogen bonding, and π−π interaction between PIL/PDA and MB. The adsorption behavior of PIL@PDA@Fe3O4 MNPs with MB as a function of time was studied. Compared to the previous work,33,38 the adsorption rate of PIL@PDA@Fe3O4 MNPs was much faster. It took less than 2 min to reach the adsorption equilibrium (Figure 2b). A pseudo-second-order kinetic model given by Langergren and Svenska was applied to describe the adsorption kinetics: t t 1 = + qt qe k 2qe 2

(3)

where t is the adsorption time (min), k2 is the pseudo-secondorder rate constant (g·mg−1·min−1), qt and qe are the adsorption capacities at time t and equilibrium time, respectively. The model correlated the data very well (Figure 2c). The high k2 value of 1.16 g·mg−1·min−1 revealed the fast adsorption rate. The adsorption ability of PIL@PDA@Fe3O4 MNPs heavily depended on the grafting yield of PIL. PIL@PDA@Fe3O4 MNPs having five IL content levels were studied (Figure 2d). The MB adsorption capacity of PIL@PDA@Fe3O4 MNPs increased rapidly from 2 and 6 h to 10 h. However, with the longer polymerization time of 14 and 18 h, the MB adsorption capacity increased slowly. We assumed that a higher density of PIL chains was obtained with the longer polymerization time, which caused steric hindrance among PIL chains. Therefore, some active adsorption functional groups became inaccessible by MB molecules. The morphology of PIL-10@PDA@Fe3O4 MNPs after adsorption remained the same as the pristine MNPs, which was confirmed by TEM, indicating no damage to the MNPs (Figure S4a). We then investigated the effect of pH on the adsorption of MB (Figure S5). It is known that solution pH affects adsorption process when the target product is ionizable.39 In this case, the solution pH slightly influenced the adsorption capacity of MB onto MNPs when the solution was alkaline. However, the adsorption capacity of MB showed a dramatic decrease at pH < 7, which was probably attributed to the strong electrostatic interactions between proton and sulfonate groups of PIL, thus blocking the adsorption of MB molecules to the polymer. Meanwhile, the solutions at low pH dissolved Fe3O4 and damaged the structure of MNPs. We also considered the effect of solution temperature on the adsorption of MB (Figure S6). The results showed no obvious difference of the adsorption capacities in a range of 0 to 55 °C. The selective adsorption of PIL@PDA@Fe3O4 MNPs toward cationic dyes (MB, CV) and anionic dyes (AO7, ARS) was also studied. As shown in Figure 3a,c, the two dyes of MB/AO7 and CV/ARS were mixed in a 1:1 mass ratio,

Figure 4. Recyclability of PIL-10@PDA@ Fe3O4 MNPs for the adsorption of MB. 2833

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(6) Zhao, L.; Hu, Y. S.; Li, H.; Wang, Z. X.; Chen, L. Q. Porous Li4Ti5O12 Coated with N-Doped Carbon from Ionic Liquids for LiIon Batteries. Adv. Mater. 2011, 23, 1385−1388. (7) Tang, J. B.; Sun, W. L.; Tang, H. D.; Radosz, M.; Shen, Y. Q. Enhanced CO2 absorption of poly(ionic liquid)s. Macromolecules 2005, 38, 2037−2039. (8) Lu, Y. Y.; Yu, G. Q.; Wang, W. J.; Ren, Q. L.; Li, B. G.; Zhu, S. P. Design and Synthesis of Thermoresponsive Ionic Liquid Polymer in Acetonitrile as a Reusable Extractant for Separation of Tocopherol Homologues. Macromolecules 2015, 48, 915−924. (9) Fellinger, T. P.; Thomas, A.; Yuan, J. Y.; Antonietti, M. 25th Anniversary Article: ″Cooking Carbon with Salt″: Carbon Materials and Carbonaceous Frameworks from Ionic Liquids and Poly(ionic liquid)s. Adv. Mater. 2013, 25, 5838−5854. (10) Azzaroni, O.; Brown, A. A.; Huck, W. T. S. Tunable wettability by clicking into polyelectrolyte brushes. Adv. Mater. 2007, 19, 151− 154. (11) Zhao, Q.; Zhang, P. F.; Antonietti, M.; Yuan, J. Y. Poly(ionic liquid) Complex with Spontaneous Micro-/Mesoporosity: TemplateFree Synthesis and Application as Catalyst Support. J. Am. Chem. Soc. 2012, 134, 11852−11855. (12) Mecerreyes, D. Polymeric ionic liquids: Broadening the properties and applications of polyelectrolytes. Prog. Polym. Sci. 2011, 36, 1629−1648. (13) Yuan, J. Y.; Mecerreyes, D.; Antonietti, M. Poly(ionic liquid)s: An update. Prog. Polym. Sci. 2013, 38, 1009−1036. (14) Lu, Y. Y.; Wang, W. J.; Li, B. G. Poly(ionic liquid)s and their applications in natural product separation. CIESC J. 2015, 67, 416− 424. (15) Kim, T. Y.; Lee, H. W.; Stoller, M.; Dreyer, D. R.; Bielawski, C. W.; Ruoff, R. S.; Suh, K. S. High-Performance Supercapacitors Based on Poly(ionic liquid)-Modified Graphene Electrodes. ACS Nano 2011, 5, 436−442. (16) He, X. Y.; Yang, W.; Pei, X. W. Preparation, characterization, and tunable wettability of poly(ionic liquid) brushes via surfaceinitiated atom transfer radical polymerization. Macromolecules 2008, 41, 4615−4621. (17) Yu, L.; Zhang, Y. T.; Wang, Y. M.; Zhang, H. Q.; Liu, J. D. High flux, positively charged loose nanofiltration membrane by blending with poly (ionic liquid) brushes grafted silica spheres. J. Hazard. Mater. 2015, 287, 373−383. (18) Pourjavadi, A.; Hosseini, S. H.; Doulabi, M.; Fakoorpoor, S. M.; Seidi, F. Multi-Layer Functionalized Poly(lonic Liquid) Coated Magnetic Nanoparticles: Highly Recoverable and Magnetically Separable Bronsted Acid Catalyst. ACS Catal. 2012, 2, 1259−1266. (19) Zheng, X. Y.; He, L. J.; Duan, Y. J.; Jiang, X. M.; Xiang, G. Q.; Zhao, W. J.; Zhang, S. S. Poly(ionic liquid) immobilized magnetic nanoparticles as new adsorbent for extraction and enrichment of organophosphorus pesticides from tea drinks. J. Chromatogr. A 2014, 1358, 39−45. (20) Wu, B. H.; Hu, D.; Kuang, Y. J.; Liu, B.; Zhang, X. H.; Chen, J. H. Functionalization of Carbon Nanotubes by an Ionic-Liquid Polymer: Dispersion of Pt and PtRu Nanoparticles on Carbon Nanotubes and Their Electrocatalytic Oxidation of Methanol. Angew. Chem., Int. Ed. 2009, 48, 4751−4754. (21) Feng, J. J.; Sun, M.; Xu, L. L.; Li, J. B.; Liu, X.; Jiang, S. X. Preparation of a polymeric ionic liquid-coated solid-phase microextraction fiber by surface radical chain-transfer polymerization with stainless steel wire as support. J. Chromatogr. A 2011, 1218, 7758− 7764. (22) Meng, Y.; Pino, V.; Anderson, J. L. Role of counteranions in polymeric ionic liquid-based solid-phase microextraction coatings for the selective extraction of polar compounds. Anal. Chim. Acta 2011, 687, 141−149. (23) Sato, T.; Morinaga, T.; Marukane, S.; Narutomi, T.; Igarashi, T.; Kawano, Y.; Ohno, K. J.; Fukuda, T.; Tsujii, Y. Novel Solid-State Polymer Electrolyte of Colloidal Crystal Decorated with Ionic-Liquid Polymer Brush. Adv. Mater. 2011, 23, 4868−4872.

for 3 months were re-evaluated for their adsorption capacity of MB and selective adsorption ability. The results showed no obvious differences, suggesting that the MNPs were stable during long-term storage (Table S2).

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CONCLUSION In summary, we developed a facile method to synthesize magnetic-responsive poly(ionic liquid) sorbents for water treatment. The study demonstrated that PDA@Fe3O4 MNPs could be directly used in self-initiated photografting and photopolymerization of ionic liquid to yield PIL@PDA@Fe3O4 MNPs. The synthetic route was simple and free of organic solvents, which are often required in the traditional methods. Because of high anion charge density of the grafting PIL, the resulted PIL@PDA@Fe3O4 MNPs exhibited ultrafast adsorption behavior toward cationic dyes, such as MB, and highly selective adsorption of MB from the mixture of MB and AO7 by electrostatic interactions. Moreover, the adsorbents can be easily collected and regenerated by washing with salt solution and maintain high adsorption and desorption efficiency after 5 cycles. This work represents a significant development in employing PDA-modified nanoparticles as a photoinitiator for the polymerization of IL monomers and in applying the resulted PIL@PDA@Fe3O4 MNPs for separating cationic dyes from anionic dyes.

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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.7b00150. Details about FT-IR, XPS, TGA, TEM data of MNPs, the grafting yield of PIL onto MNPs, and the effect of pH and temperature on adsorption capacity of MNPs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*S. Zhu. E-mail: [email protected]. *W.-J. Wang. E-mail: [email protected]. ORCID

Shiping Zhu: 0000-0001-8551-0859 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Grants 21420102008 and 21376211), and Chinese State Key Laboratory of Chemical Engineering at Zhejiang University (Grants SKL-ChE-14D01 and SKL-ChE-15T03).

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b00150 ACS Sustainable Chem. Eng. 2017, 5, 2829−2835

Letter

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DOI: 10.1021/acssuschemeng.7b00150 ACS Sustainable Chem. Eng. 2017, 5, 2829−2835