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Macrocyclic Amine-Linked Oligocarbazole Hollow Microspheres: Facile Synthesis and Efficient Lead Sorbents Yaozu Liao,* Sisi Cai, Shaojun Huang, Xia Wang,* Charl F. J. Faul*
Novel macrocyclic amine-linked oligocarbazole hollow microspheres are synthesized via a one-step oxidative method in aqueous solution. Upon altering the oxidants and acidic media, the average diameters of the obtained hollow microspheres are tunable from 0.23 to 2.0 μm. With attractive amine and carbazole functionalities, exposed surface area, thermostability, and photoluminescent properties, the amine-linked oligocarbazole hollow microspheres are directly assembled to yield heavy metal sorbents with excellent selectivity and recyclability, shown to efficiently remove lead from contaminated water.
1. Introduction The increased presence of heavy metals in water over the last three decades, resulting from rapid industrialization and technological advances, has become a global environmental and public health challenge.[1] Lead pollution stands out among these problems due to its extreme toxicity, nonbiodegradability, and large-scale effluents from industries.[2] Lead poisoning (which is particularly prevalent in children) can cause brain damage, kidney dysfunction, and central nervous system disorders.[3] Large scale and efficient removal of toxic lead from industrial and Dr. Y. Liao, S. Cai, Prof. X. Wang School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516 Jun-Gong Road, Shanghai 200093, China E-mail: [email protected]; [email protected] Dr. Y. Liao, Dr. C. F. J. Faul School of Chemistry, University of Bristol, Bristol, England BS8 1TS, UK E-mail: [email protected] Dr. S. J. Huang Research Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming 650093, China Macromol. Rapid Commun. 2014, 35, 1833−1839 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
mining effluents is undoubtedly necessary for water purification and its subsequent safe disposal. Sorption processes are one of the most promising strategies to deal with these pertinent problems. Macrocycles functionalized with thiol, amino, amine, hydroxyl, carboxylic or sulfur groups as receptors for heavy metals are being continuously developed as some of the most effective sorbents owing to their functionalities, unique conformations, and suitable cavity sizes.[4] However, current macrocycles suffer from several drawbacks, including complex synthetic procedures, insufficient active surface-binding sites, and low recyclability. The design and simple synthesis of versatile and powerful new macrocycles, specifically for lead sorption, are therefore highly desirable. Recently, we demonstrated that chemical oxidative oligomerization of fluoranthene produced a cone-like macrocyclic oligofluoranthene.[5] Stable complexes could be formed between these macrocycles and targets (Fe iron(III) or 2,4,6-trinitrophenol) owing to a synergistic effect of the extended π-conjugated electrons and an optimized 3D conical architecture. This discovery has opened a new and facile avenue to exploiting powerful macrocycle sorbents. Additionally, simply altering reaction conditions enable self-assembly of the macrocycles into tunable
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DOI: 10.1002/marc.201400415
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Scheme 1. Proposed route to the formation of macrocyclic OCB sorbents.
micro/nanostructures, thereby improving metal sorption capacity resulting from the enhanced surface area. Since electron-rich carbazole moieties can efficiently bind heavy metal ions,[6] the aim of this study is to utilize this property, and by introducing further amine functionalities and exposed surface area, to produce heavy metal sorbents. Here, we report the facile synthesis of a novel macrocyclic amine-linked oligocarbazole (OCB) via a one-step chemical oxidative oligomerization of 3-amino-9-ethylcarbazole (AEC) in a methanol/aqueous acid medium (Figure S1, Supporting Information) using ammonium persulfate (APS) as oxidant (Scheme 1). It was found that the OCB moieties assemble into hollow microspheres in aqueous solution and show high sorption affinity for lead, which make them excellent candidates to efficiently remove such heavy metal ions from contaminated water in an environmentally benign and sustainable fashion.
2. Experimental Section 2.1. Chemicals 3-Amino-9-ethylcarbazole (AEC, >95%), ammonium persulfate (APS, 98%), anhydrous ferric chloride (FeCl3, 97%), potassium dichromate (K2Cr2O7, 99.5%) and other nitrate salts, organic solvents, and acids were purchased from Sigma–Aldrich and used as received.
2.2. Synthesis of OCB Hollow Microspheres Typically, 210 mg (1.0 mmol) of AEC was dissolved in ethanol (10 mL), while an oxidant (APS, 456.6 mg, 2.0 mmol) was dissolved in acidic medium (0.5 M HCl, 10 mL). The two solutions were rapidly mixed at ambient conditions. The reaction mixture was vigorously shaken for 20 s and then left undisturbed overnight. The formed, doped hollow spheres were purified by centrifugation at 6000 rpm min−1 using deionized (DI) water at 23 °C until the supernatant became colorless. Dedoped products were obtained by further washing the doped products with 0.1 M NH3 • H2O and then DI water, resulting in a 58% yield of dedoped OCB hollow microspheres. The obtained OCB hollow
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microspheres were stable in aqueous solution but soluble in organic solvents (for chemical analyses).
2.3. Structure Characterization and Measurements The UV–vis absorption spectra of the AEC monomers and OCB microspheres dissolved in ethanol were recorded on a Perkin Elmer Lambda-750 spectrometer. The solid state ATR/FTIR spectra of the samples were carried out on a Perkin Elmer Spectrum 100 spectrometer. The Raman spectra were obtained on a Perkin Elmer Raman Station 400F spectrometer. The proton (1H) NMR spectra were obtained using a Bruker ARX-400 spectrometer with CDCl3 as a solvent. Thermogravimetric analyses (TGA) of the dedoped OCB microspheres synthesized with variable APS concentrations in the presence of 0.5 M HCl were carried out on a Perkin Elmer TGA Pyris 1 by heating the samples from room temperature to 900 °C at a rate of 10 °C min−1. The morphologies of the samples were imaged using field emission scanning electron microscopy (FE-SEM, Quanta FEG 430) and high-resolution transmission electron microscopy (HRTEM, FEI TF30). Size distributions of the OCB microspheres synthesized with different oxidants (K2Cr2O7, FeCl3, and APS) in the medium of 0.5 M HCl were analyzed by a Beckman Coulter LS 13320 dynamic light scattering (DLS) analyzer. The samples were prepared by dispersing the as-synthesized microspheres in 5% aqueous methanol solution using a sonicator (methanol was used to lower the surface tension, thus ensuring that the microparticles dispersed better). The sheet resistance of OCB films of 0.5 ± 0.1 mm thickness was determined in the ohmic region of I–V curves with a Keithley 2400 source measurement unit in a two-probe measurement configuration. Films were prepared by casting from an OCB chloroform solution (5%). N2 adsorption and desorption studies of the degassed OCB sorbents (≈150 mg) at 150 °C for a period of 18 h under a high vacuum (0.1 Pa) were carried out using ASAP 2020 volumetric adsorption system (Micromeritics, USA) at 77 K. The Brunauer–Emmett–Teller (BET) surface area was calculated from the N2 adsorption/desorption isotherms. The fluorescence properties of AEC and OCB solutions were acquired on a RF5301PC fluorescence spectrometer. Fluorescence quenching was determined by recording the emission fluorescence spectra upon addition of different metal ions. On the basis of the fluorescence emission and UV–vis absorbance, the fluorescence quantum yield (Φ) of AEC and OCB in acetone can be calculated using Equation (1)[7,8]
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Macrocyclic Amine-Linked Oligocarbazole Hollow Microspheres: Facile Synthesis and Efficient Lead Sorbents
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Φ = (n2 Aref IΦref )/ (n2ref AI )
(1)
where n is the refractive index of the solvent, A is the absorbance at the excited wavelength, and I is the integrated area of emission. Rhodamine 101 in ethanol with an Φ value of 0.96 was used as a reference material, and acetone was used as the solvent for both AEC and OCB. 3D molecular structure was obtained by minimizing energy of OCB using a Chem3D Ultra Molecular Modelling and Analysis 2010.
2.4. Heavy Metal Ion Sorption and Desorption 2.4.1. Sorption Procedure A published heavy metal ion sorption procedure was followed.[9] OCB microspheres (50 mg) were added to 25 mL of Pb(II), Mn(II), Co(II), Cu(II), and Fe(III) aqueous solution with a concentration of C0 (mg L–1). 1.0 mL of methanol was added to lower the surface tension of the dispersion. The mixture was sonicated for 15 min and then stirred for 5 h. The microspheres were filtered from the dispersion and the solid powders were collected by heating at 50 °C overnight in a vacuum oven for the desorption experiment. The metal concentration (C) in the filtrate after sorption (mg L–1) was measured by an inductively coupled plasma/mass spectrometer (ICP-MS) system. Sorption capacity and sorption efficiency of the OCB microsphere sorbents were calculated according to Equations (2) and (3)
Q = (C 0 − C )V0 /M0
(2)
q = ⎡⎣(C 0 − C ) /C 0 ⎤⎦ × 100%
(3)
where Q is the sorption capacity (mg g−1), q is the sorption efficiency (%), C0 is the initial metal ion concentration (mg L−1), V0 is the initial volume of the metal ion solution (mL), and M0 is the mass of the sorbent (mg).
2.4.2. Desorption Procedure Twenty-five milliliters of ethylenediaminetetraacetic acid (EDTA) solution (20 mM) was added to a 50 mL conical flask containing the metal-containing sorbent powder. The mixture was then stirred for 1 h at 25 °C to release the bound metal ions into the eluent. The particles were filtered from the eluent and then left to dry in air at 50 °C for 72 h and collected for reutilization. The concentration of metal ions (C′) in the filtrate after desorption (mg L–1) was measured by ICP-MS system. Desorption efficiency was calculated according to Equation (4)
D = C dVd / ⎡⎣(C 0 − C ′ )V0 ⎤⎦ × 100%
(4)
where D is the desorption efficiency (%), Cd is the lead ion concentration in the eluent after desorption (mg L−1), and Vd is the volume of the eluent (mL). The above recycled sorbents were reused for further sorption and desorption experiments; six cycles were conducted.
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3. Results and Discussion The obtained OCB hollow microspheres demonstrated typical spectroscopy features, as would be expected for materials containing quinoid (Q) and benzenoid (B) moieties:[10] UV–vis absorbance maxima at 645 and 430 nm (Figure 1a and Figure S2, Supporting Information), FTIR peaks at 1591 and 1472 cm−1 (Figure S3, Supporting Information), as well as Raman peaks at 1601 and 1555 cm−1 (Figure S4, Supporting Information), respectively originating from the Q and B moieties. No free primary amine groups were detected. These optical properties compare very favorably with those reported for poly(aniline), oligo(aniline)s, as well as conjugated microporous aniline-based structures,[11] and are in contrast to those values reported for 2,7- or 1,8-linked polycarbazole. For example, the latter polymers usually displayed a UV–vis absorbance maximum around 400 nm,[12] which signified that our OCB materials consisted of carbazole units joined through amine linkages (not C C linkages). The 1H NMR spectrum of OCB exhibited resonances at 8.78, 8.37, 7.89, 7.50, and 7.37 ppm originating from the protons in substituted carbazole moieties, and three additional re sonances at 4.83, 4.35, and 1.51 ppm that could be attributed to the –NH–, –CH2–, and –CH3 groups, respectively (Figure S5, Supporting Information). The mass spectrum (Figure 1b) of the reaction product exhibited three sharp peaks at m/z of 418.20, 624.46, and 850.58. The data obtained from the 1H NMR and elemental analyses clearly revealed that the OCB consisted of dimers (m/z: 418.20), cyclic trimers (m/z: 624.46), and macrocyclic tetramers (m/z: 850.58). Note that, the m/z: 850.58 is assigned to the tetramer [(832.54) + NH4+ (18.04)] ion. A possible oligomerization mechanism is presented in Scheme S1 (Supporting Information), which is similar to the mechanism of aniline polymerization.[13] The AEC monomer is a thermally sensitive and insulating compound. However, the obtained OCB microspheres are thermally stable up to 375 °C, retain >45 wt% char when heated to 900 °C in nitrogen (Figure S6, Supporting Information), and are electrically conductive (16.5 MΩ cm, 0.5 M HCl-doped sample), as determined by I–V measurements (Figure S7, Supporting Information). AEC and OCB acetone solutions showed strong blue and green fluorescence, respectively, when excited at 365 nm (Figure S8a, Supporting Information). The fluorescence spectrum of OCB, when compared with AEC, showed a decreased Stoke’s shift (34 vs 65 nm; Figure S8b, Supporting Information) and significantly increased quantum yields (0.45 vs 0.11), most likely owing to the extended π-conjugated macrocyclic structure formed in combination with less aggregation of the oligomers in solution.
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Figure 1. a) UV–vis spectra of AEC and OCB ethanol solutions (5 × 10−3 M); b) MALDI/TOF mass spectrum, c) SEM image, and d) TEM image of OCB hollow microspheres.
The FE-SEM and HRTEM images showed that the products synthesized with APS consisted of uniform hollow microspheres with diameters of ≈2.0 μm (Figure 1c,d). By replacing the oxidant, APS, with FeCl3 or K2Cr2O7, the average diameters of microspheres were 0.85 and 0.23 μm, respectively, as indicated by both SEM observations (Figure S9, Supporting Information) and DLS measurements (Figure 2). Note that the oxidation potentials (OP) of APS, FeCl3, and K2Cr2O7 follow the order: OPAPS = 2.0 V > OPFeCl3 = 0.77 V > OPK2 Cr2 O7 ≈ 0.1 V, which suggests that the growth process for the OCB particles produced with K2Cr2O7 is much slower than that with APS. This decrease in OP is also reflected in the decreasing particle sizes found. The results are in good agreement with previous studies of polypyrrole nanospheres.[14] Interestingly, by changing the acids (HCl, HNO3, H2SO4, and HClO4) used during the oxidative reaction, sizes and morphologies of OCB particles were also influenced (Figure S10, Supporting Information). This effect on morphology is most probably owing to slight differences in the solubility of the oxidants and monomers in the different acidic reaction mixtures. The different sized anions also affected the oligomer aggregation during the particle assembly,[14] which might further explain the observed changes. N2 adsorption/desorption measurements of typical hollow
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microspheres indicated that the BET surface areas of OCB microspheres were as high as 57 m2 g−1 (Figure S11, Supporting Information). To examine the performance of as-synthesized macrocyclic OCB hollow microspheres as sorbents for removal
Figure 2. Size distributions of the OCB microspheres synthesized with different oxidants (FeCl3, K2Cr2O7, and APS) in 0.5 M HCl.
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of heavy metals from contaminated water, solutions containing 2.0 g L−1 Pb(II), Mn(II), Co(II), Cu(II), and Fe(III), respectively, were prepared as models of contaminated water. The sorption capacities followed the order, as determined by ICP-MS (see Figure 3a): Pb(II)» Co(II)>Cu(II)>Mn(II)>Fe(III). Among the five heavy metal ions examined, the microspheres show highest sorption capacity (75.2 mg g−1) for Pb(II) with a sorption efficiency up to 75.2%. We further found that the metal sorption capacity exhibited a nonlinear increase upon increasing the metal ion concentrations (see Table S1, Supporting Information). With the very encouraging results for lead sorption, further studies on the influence of temperature and pH on sorption were conducted (2.0 g L−1 Pb(II) used in all investigations). As seen in Figure 3b, the OCB sorbents gave the best sorption capacity of 82.5 mg g−1 at 45 °C, with the optimal pH for sorption in the range from 5 to 6. In this pH range, neither precipitation of the metal hydroxide nor protonation of the amine group is expected. The exposed surfaces of the microspheres
become negatively charged, favoring Pb(II) ion sorption. For this case, a mechanism similar to that of exchange interactions published before[15] is proposed. Note that the Pb(II) sorption capacities of our OCB sorbents compare very favorably to those values (Table S2, Supporting Information) reported for Zn2GeO4–ethylenediamine hybrid nanoribbons (74.6 mg g−1),[2] sulfonated polyphenyldiamine (91.8 mg g−1),[16] longan shell (52.1 mg g−1),[17] polyvinylbutyral microbeads (86.2 mg g−1),[18] polysulfoaminoanthraquinone nanoparticles (89.6 mg g−1),[19] diethylenetriamine-bacterial cellulose (22 mg g−1),[20] meranti sawdust (34.3 mg g−1),[21] thiol-functionalized ceramic hybrids (22.4 mg g−1),[22] activated carbon (4.77 mg g−1),[23] titanosilicate ETS-10 (56.3 mg g−1),[24] and cellulose-chitosan hydrogels (28.1 mg g−1).[25] More importantly, the production of OCB microspheres is a low-cost and environmentally friendly process owing to the cheap starting materials and simple one-step aqueous synthesis. This stands in contrast to most synthetic macrocycles that are expensive to prepare and likely to cause secondary
Figure 3. a) Sorption capacities of the OCB hollow microspheres for five different metal ions at a fixed concentration of 2.0 g L−1, sorption time = 5 h; b) influence of temperature (5 h and pH 4.5) and pH (25 °C and 5 h) on the Pb(II) sorption capacity of the sorbents at an initial concentration of 2.0 g L−1; c) recyclability of the sorbents obtained by desorption experiments using EDTA as an efficient desorber at room temperature for 1 h; d) proposed 3D architecture of OCB and Pb(II) sorption.
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pollution since the multistep syntheses are largely conducted in organic solvent systems and noble metal catalysts are often required.[4] Further investigations of binary [Pb(II)/Co(II), Pb(II)/ Cu(II), Pb(II)/Mn(II), Pb(II)/Fe(III)] and ternary [Pb(II)/ Fe(III)/Co(II), and Pb(II)/Cu(II)/Fe(III)] systems showed that Pb(II) was still much more selectively and efficiently adsorbed than other heavy metal ions. However, we found that the sorption capacities for Pb(II) decreased by 20%–50%, when Fe(III) was present (in both binary and ternary systems). Comparison of individual fluorescence spectra for Pb(II), Mn(II), Co(II), Cu(II), and Fe(III) indicated the highest fluorescence quenching for Fe(III) compared with the other heavy metal ions (Figure S8c, Supporting Information). The strong oxidative and electron-deficient Fe(III) likely accepts both p- and π-electrons from OCB as compared with the fluoranthene and triphenylene oligomers previously reported,[5,8] resulting in a complex with low fluorescence. This competitive interaction could also explain why decreased sorption of Pb(II) was observed in the presence of Fe(III). Recyclability of the OCB hollow microspheres was examined by desorption experiments using the wellknown EDTA as an efficient desorber. During the desorption process, the microspheres were first immersed in a solution with 2.0 g L−1 (≈9 mM) of Pb(II) for 5 h, and then exposed to a 20 mM of EDTA solution. The desorption efficiency of the adsorbed Pb(II) on the microspheres is as high as 93.8% within 1 h, and can be enhanced to 96.5% by further extending desorption to 3 h. The OCB sorbents were reused six times during our investigations (Figure 3c), with recyclability much better than that of most organic sorbents previously reported.[20–23] We have shown that the OCB microspheres exhibited excellent sorption and selectivity for Pb(II) as well as recyclability. This sorption preference is mainly attributed to two main reasons: on the one hand, Pb(II) sorption on the OCB microspheres involves van der Waals electrostatic interactions and ion exchange with amine and carbazole groups present on the sorbent surface. As depicted in Figure 3d, macrocyclic OCB shows a chair-like 3D conformation at the minimal energy level; a relatively high accessible surface area would be excepted. The amine and carbazole groups would homogeneously distribute on the surface of the sorbent as well as within its inner porous structure, ensuring easy access for Pb(II) to be trapped at the sorption sites. On the other hand, Pb(II) is one of softest acids in terms of electronegativity, electrode potential, and ionic size. In contrast, OCB microspheres contain a high concentration of –NH–, –N=, and –NEt– groups, typically behaving as soft bases. According to the hard and soft acids and bases (HSAB) theory described by Pearson,[26] the soft acid Pb(II) in solution will favorably adsorb on the soft base OCB microspheres (especially when compared with
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the hard acids Mn(II) and Fe(III) as well as borderline acids Cu(II) and Co(II)). This conclusion is in accordance with results from most amine and thiol-based sorbents, as confirmed by FTIR, XRD, NMR, and XPS analyses.[27]
4. Conclusions We demonstrated the simple synthesis of macrocyclic amine-linked oligocarbazole (OCB) hollow microspheres through a one-step mild chemical oxidative route in aqueous media. The average diameters of the obtained microspheres could be tuned upon alternating the oxidants and acidic media used. OCB hollow microspheres, with attractive amine/carbazole functionalities and exposed surface area, were directly assembled to yield heavy metal sorbents with excellent selectivity and recyclability, shown to efficiently remove lead from contaminated water. This strategy has the potential to overcome practical issues and challenges associated with other macrocyclic metal sorbents, including multistep syntheses, organic solvent, and noble metal catalysts, which could cause further secondary pollution. We anticipate that OCB hollow microspheres may serve as low cost and sustainable sorbents for real-life contaminated water samples.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: The authors thank the National Natural Science Foundation of China (51203090, 51373100, and 51363012), Innovation Program of Shanghai Municipal Education Commission (13YZ074), National Natural Science Foundation of Shanghai (12ZR1446700), the University of Bristol and Marie Curie Fellowship (326385) for financial support and thank Dr. Yue Wang for valuable discussions. Note: The last author’s name was amended after initial online publication. Received: July 24, 2014; Revised: August 8, 2014; Published online: September 16, 2014; DOI: 10.1002/marc.201400415 Keywords: lead sorbents; macrocycles; oligocarbazole; water treatment
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Macromol. Rapid Commun. 2014, 35, 1833−1839 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communication
Macrocyclic Amine-Linked Oligocarbazole Hollow Microspheres: Facile Synthesis and Efficient Lead Sorbents Yaozu Liao,* Sisi Cai, Shaojun Huang, Xia Wang,* Charl F. J. Faul*
Novel macrocyclic amine-linked oligocarbazole hollow microspheres are synthesized via a one-step oxidative method in aqueous solution. Upon altering the oxidants and acidic media, the average diameters of the obtained hollow microspheres are tunable from 0.23 to 2.0 μm. With attractive amine and carbazole functionalities, exposed surface area, thermostability, and photoluminescent properties, the amine-linked oligocarbazole hollow microspheres are directly assembled to yield heavy metal sorbents with excellent selectivity and recyclability, shown to efficiently remove lead from contaminated water.
1. Introduction The increased presence of heavy metals in water over the last three decades, resulting from rapid industrialization and technological advances, has become a global environmental and public health challenge.[1] Lead pollution stands out among these problems due to its extreme toxicity, nonbiodegradability, and large-scale effluents from industries.[2] Lead poisoning (which is particularly prevalent in children) can cause brain damage, kidney dysfunction, and central nervous system disorders.[3] Large scale and efficient removal of toxic lead from industrial and Dr. Y. Liao, S. Cai, Prof. X. Wang School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516 Jun-Gong Road, Shanghai 200093, China E-mail: [email protected]; [email protected] Dr. Y. Liao, Dr. C. F. J. Faul School of Chemistry, University of Bristol, Bristol, England BS8 1TS, UK E-mail: [email protected] Dr. S. J. Huang Research Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming 650093, China Macromol. Rapid Commun. 2014, 35, 1833−1839 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
mining effluents is undoubtedly necessary for water purification and its subsequent safe disposal. Sorption processes are one of the most promising strategies to deal with these pertinent problems. Macrocycles functionalized with thiol, amino, amine, hydroxyl, carboxylic or sulfur groups as receptors for heavy metals are being continuously developed as some of the most effective sorbents owing to their functionalities, unique conformations, and suitable cavity sizes.[4] However, current macrocycles suffer from several drawbacks, including complex synthetic procedures, insufficient active surface-binding sites, and low recyclability. The design and simple synthesis of versatile and powerful new macrocycles, specifically for lead sorption, are therefore highly desirable. Recently, we demonstrated that chemical oxidative oligomerization of fluoranthene produced a cone-like macrocyclic oligofluoranthene.[5] Stable complexes could be formed between these macrocycles and targets (Fe iron(III) or 2,4,6-trinitrophenol) owing to a synergistic effect of the extended π-conjugated electrons and an optimized 3D conical architecture. This discovery has opened a new and facile avenue to exploiting powerful macrocycle sorbents. Additionally, simply altering reaction conditions enable self-assembly of the macrocycles into tunable
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DOI: 10.1002/marc.201400415
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Scheme 1. Proposed route to the formation of macrocyclic OCB sorbents.
micro/nanostructures, thereby improving metal sorption capacity resulting from the enhanced surface area. Since electron-rich carbazole moieties can efficiently bind heavy metal ions,[6] the aim of this study is to utilize this property, and by introducing further amine functionalities and exposed surface area, to produce heavy metal sorbents. Here, we report the facile synthesis of a novel macrocyclic amine-linked oligocarbazole (OCB) via a one-step chemical oxidative oligomerization of 3-amino-9-ethylcarbazole (AEC) in a methanol/aqueous acid medium (Figure S1, Supporting Information) using ammonium persulfate (APS) as oxidant (Scheme 1). It was found that the OCB moieties assemble into hollow microspheres in aqueous solution and show high sorption affinity for lead, which make them excellent candidates to efficiently remove such heavy metal ions from contaminated water in an environmentally benign and sustainable fashion.
2. Experimental Section 2.1. Chemicals 3-Amino-9-ethylcarbazole (AEC, >95%), ammonium persulfate (APS, 98%), anhydrous ferric chloride (FeCl3, 97%), potassium dichromate (K2Cr2O7, 99.5%) and other nitrate salts, organic solvents, and acids were purchased from Sigma–Aldrich and used as received.
2.2. Synthesis of OCB Hollow Microspheres Typically, 210 mg (1.0 mmol) of AEC was dissolved in ethanol (10 mL), while an oxidant (APS, 456.6 mg, 2.0 mmol) was dissolved in acidic medium (0.5 M HCl, 10 mL). The two solutions were rapidly mixed at ambient conditions. The reaction mixture was vigorously shaken for 20 s and then left undisturbed overnight. The formed, doped hollow spheres were purified by centrifugation at 6000 rpm min−1 using deionized (DI) water at 23 °C until the supernatant became colorless. Dedoped products were obtained by further washing the doped products with 0.1 M NH3 • H2O and then DI water, resulting in a 58% yield of dedoped OCB hollow microspheres. The obtained OCB hollow
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microspheres were stable in aqueous solution but soluble in organic solvents (for chemical analyses).
2.3. Structure Characterization and Measurements The UV–vis absorption spectra of the AEC monomers and OCB microspheres dissolved in ethanol were recorded on a Perkin Elmer Lambda-750 spectrometer. The solid state ATR/FTIR spectra of the samples were carried out on a Perkin Elmer Spectrum 100 spectrometer. The Raman spectra were obtained on a Perkin Elmer Raman Station 400F spectrometer. The proton (1H) NMR spectra were obtained using a Bruker ARX-400 spectrometer with CDCl3 as a solvent. Thermogravimetric analyses (TGA) of the dedoped OCB microspheres synthesized with variable APS concentrations in the presence of 0.5 M HCl were carried out on a Perkin Elmer TGA Pyris 1 by heating the samples from room temperature to 900 °C at a rate of 10 °C min−1. The morphologies of the samples were imaged using field emission scanning electron microscopy (FE-SEM, Quanta FEG 430) and high-resolution transmission electron microscopy (HRTEM, FEI TF30). Size distributions of the OCB microspheres synthesized with different oxidants (K2Cr2O7, FeCl3, and APS) in the medium of 0.5 M HCl were analyzed by a Beckman Coulter LS 13320 dynamic light scattering (DLS) analyzer. The samples were prepared by dispersing the as-synthesized microspheres in 5% aqueous methanol solution using a sonicator (methanol was used to lower the surface tension, thus ensuring that the microparticles dispersed better). The sheet resistance of OCB films of 0.5 ± 0.1 mm thickness was determined in the ohmic region of I–V curves with a Keithley 2400 source measurement unit in a two-probe measurement configuration. Films were prepared by casting from an OCB chloroform solution (5%). N2 adsorption and desorption studies of the degassed OCB sorbents (≈150 mg) at 150 °C for a period of 18 h under a high vacuum (0.1 Pa) were carried out using ASAP 2020 volumetric adsorption system (Micromeritics, USA) at 77 K. The Brunauer–Emmett–Teller (BET) surface area was calculated from the N2 adsorption/desorption isotherms. The fluorescence properties of AEC and OCB solutions were acquired on a RF5301PC fluorescence spectrometer. Fluorescence quenching was determined by recording the emission fluorescence spectra upon addition of different metal ions. On the basis of the fluorescence emission and UV–vis absorbance, the fluorescence quantum yield (Φ) of AEC and OCB in acetone can be calculated using Equation (1)[7,8]
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Φ = (n2 Aref IΦref )/ (n2ref AI )
(1)
where n is the refractive index of the solvent, A is the absorbance at the excited wavelength, and I is the integrated area of emission. Rhodamine 101 in ethanol with an Φ value of 0.96 was used as a reference material, and acetone was used as the solvent for both AEC and OCB. 3D molecular structure was obtained by minimizing energy of OCB using a Chem3D Ultra Molecular Modelling and Analysis 2010.
2.4. Heavy Metal Ion Sorption and Desorption 2.4.1. Sorption Procedure A published heavy metal ion sorption procedure was followed.[9] OCB microspheres (50 mg) were added to 25 mL of Pb(II), Mn(II), Co(II), Cu(II), and Fe(III) aqueous solution with a concentration of C0 (mg L–1). 1.0 mL of methanol was added to lower the surface tension of the dispersion. The mixture was sonicated for 15 min and then stirred for 5 h. The microspheres were filtered from the dispersion and the solid powders were collected by heating at 50 °C overnight in a vacuum oven for the desorption experiment. The metal concentration (C) in the filtrate after sorption (mg L–1) was measured by an inductively coupled plasma/mass spectrometer (ICP-MS) system. Sorption capacity and sorption efficiency of the OCB microsphere sorbents were calculated according to Equations (2) and (3)
Q = (C 0 − C )V0 /M0
(2)
q = ⎡⎣(C 0 − C ) /C 0 ⎤⎦ × 100%
(3)
where Q is the sorption capacity (mg g−1), q is the sorption efficiency (%), C0 is the initial metal ion concentration (mg L−1), V0 is the initial volume of the metal ion solution (mL), and M0 is the mass of the sorbent (mg).
2.4.2. Desorption Procedure Twenty-five milliliters of ethylenediaminetetraacetic acid (EDTA) solution (20 mM) was added to a 50 mL conical flask containing the metal-containing sorbent powder. The mixture was then stirred for 1 h at 25 °C to release the bound metal ions into the eluent. The particles were filtered from the eluent and then left to dry in air at 50 °C for 72 h and collected for reutilization. The concentration of metal ions (C′) in the filtrate after desorption (mg L–1) was measured by ICP-MS system. Desorption efficiency was calculated according to Equation (4)
D = C dVd / ⎡⎣(C 0 − C ′ )V0 ⎤⎦ × 100%
(4)
where D is the desorption efficiency (%), Cd is the lead ion concentration in the eluent after desorption (mg L−1), and Vd is the volume of the eluent (mL). The above recycled sorbents were reused for further sorption and desorption experiments; six cycles were conducted.
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3. Results and Discussion The obtained OCB hollow microspheres demonstrated typical spectroscopy features, as would be expected for materials containing quinoid (Q) and benzenoid (B) moieties:[10] UV–vis absorbance maxima at 645 and 430 nm (Figure 1a and Figure S2, Supporting Information), FTIR peaks at 1591 and 1472 cm−1 (Figure S3, Supporting Information), as well as Raman peaks at 1601 and 1555 cm−1 (Figure S4, Supporting Information), respectively originating from the Q and B moieties. No free primary amine groups were detected. These optical properties compare very favorably with those reported for poly(aniline), oligo(aniline)s, as well as conjugated microporous aniline-based structures,[11] and are in contrast to those values reported for 2,7- or 1,8-linked polycarbazole. For example, the latter polymers usually displayed a UV–vis absorbance maximum around 400 nm,[12] which signified that our OCB materials consisted of carbazole units joined through amine linkages (not C C linkages). The 1H NMR spectrum of OCB exhibited resonances at 8.78, 8.37, 7.89, 7.50, and 7.37 ppm originating from the protons in substituted carbazole moieties, and three additional re sonances at 4.83, 4.35, and 1.51 ppm that could be attributed to the –NH–, –CH2–, and –CH3 groups, respectively (Figure S5, Supporting Information). The mass spectrum (Figure 1b) of the reaction product exhibited three sharp peaks at m/z of 418.20, 624.46, and 850.58. The data obtained from the 1H NMR and elemental analyses clearly revealed that the OCB consisted of dimers (m/z: 418.20), cyclic trimers (m/z: 624.46), and macrocyclic tetramers (m/z: 850.58). Note that, the m/z: 850.58 is assigned to the tetramer [(832.54) + NH4+ (18.04)] ion. A possible oligomerization mechanism is presented in Scheme S1 (Supporting Information), which is similar to the mechanism of aniline polymerization.[13] The AEC monomer is a thermally sensitive and insulating compound. However, the obtained OCB microspheres are thermally stable up to 375 °C, retain >45 wt% char when heated to 900 °C in nitrogen (Figure S6, Supporting Information), and are electrically conductive (16.5 MΩ cm, 0.5 M HCl-doped sample), as determined by I–V measurements (Figure S7, Supporting Information). AEC and OCB acetone solutions showed strong blue and green fluorescence, respectively, when excited at 365 nm (Figure S8a, Supporting Information). The fluorescence spectrum of OCB, when compared with AEC, showed a decreased Stoke’s shift (34 vs 65 nm; Figure S8b, Supporting Information) and significantly increased quantum yields (0.45 vs 0.11), most likely owing to the extended π-conjugated macrocyclic structure formed in combination with less aggregation of the oligomers in solution.
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Figure 1. a) UV–vis spectra of AEC and OCB ethanol solutions (5 × 10−3 M); b) MALDI/TOF mass spectrum, c) SEM image, and d) TEM image of OCB hollow microspheres.
The FE-SEM and HRTEM images showed that the products synthesized with APS consisted of uniform hollow microspheres with diameters of ≈2.0 μm (Figure 1c,d). By replacing the oxidant, APS, with FeCl3 or K2Cr2O7, the average diameters of microspheres were 0.85 and 0.23 μm, respectively, as indicated by both SEM observations (Figure S9, Supporting Information) and DLS measurements (Figure 2). Note that the oxidation potentials (OP) of APS, FeCl3, and K2Cr2O7 follow the order: OPAPS = 2.0 V > OPFeCl3 = 0.77 V > OPK2 Cr2 O7 ≈ 0.1 V, which suggests that the growth process for the OCB particles produced with K2Cr2O7 is much slower than that with APS. This decrease in OP is also reflected in the decreasing particle sizes found. The results are in good agreement with previous studies of polypyrrole nanospheres.[14] Interestingly, by changing the acids (HCl, HNO3, H2SO4, and HClO4) used during the oxidative reaction, sizes and morphologies of OCB particles were also influenced (Figure S10, Supporting Information). This effect on morphology is most probably owing to slight differences in the solubility of the oxidants and monomers in the different acidic reaction mixtures. The different sized anions also affected the oligomer aggregation during the particle assembly,[14] which might further explain the observed changes. N2 adsorption/desorption measurements of typical hollow
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microspheres indicated that the BET surface areas of OCB microspheres were as high as 57 m2 g−1 (Figure S11, Supporting Information). To examine the performance of as-synthesized macrocyclic OCB hollow microspheres as sorbents for removal
Figure 2. Size distributions of the OCB microspheres synthesized with different oxidants (FeCl3, K2Cr2O7, and APS) in 0.5 M HCl.
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of heavy metals from contaminated water, solutions containing 2.0 g L−1 Pb(II), Mn(II), Co(II), Cu(II), and Fe(III), respectively, were prepared as models of contaminated water. The sorption capacities followed the order, as determined by ICP-MS (see Figure 3a): Pb(II)» Co(II)>Cu(II)>Mn(II)>Fe(III). Among the five heavy metal ions examined, the microspheres show highest sorption capacity (75.2 mg g−1) for Pb(II) with a sorption efficiency up to 75.2%. We further found that the metal sorption capacity exhibited a nonlinear increase upon increasing the metal ion concentrations (see Table S1, Supporting Information). With the very encouraging results for lead sorption, further studies on the influence of temperature and pH on sorption were conducted (2.0 g L−1 Pb(II) used in all investigations). As seen in Figure 3b, the OCB sorbents gave the best sorption capacity of 82.5 mg g−1 at 45 °C, with the optimal pH for sorption in the range from 5 to 6. In this pH range, neither precipitation of the metal hydroxide nor protonation of the amine group is expected. The exposed surfaces of the microspheres
become negatively charged, favoring Pb(II) ion sorption. For this case, a mechanism similar to that of exchange interactions published before[15] is proposed. Note that the Pb(II) sorption capacities of our OCB sorbents compare very favorably to those values (Table S2, Supporting Information) reported for Zn2GeO4–ethylenediamine hybrid nanoribbons (74.6 mg g−1),[2] sulfonated polyphenyldiamine (91.8 mg g−1),[16] longan shell (52.1 mg g−1),[17] polyvinylbutyral microbeads (86.2 mg g−1),[18] polysulfoaminoanthraquinone nanoparticles (89.6 mg g−1),[19] diethylenetriamine-bacterial cellulose (22 mg g−1),[20] meranti sawdust (34.3 mg g−1),[21] thiol-functionalized ceramic hybrids (22.4 mg g−1),[22] activated carbon (4.77 mg g−1),[23] titanosilicate ETS-10 (56.3 mg g−1),[24] and cellulose-chitosan hydrogels (28.1 mg g−1).[25] More importantly, the production of OCB microspheres is a low-cost and environmentally friendly process owing to the cheap starting materials and simple one-step aqueous synthesis. This stands in contrast to most synthetic macrocycles that are expensive to prepare and likely to cause secondary
Figure 3. a) Sorption capacities of the OCB hollow microspheres for five different metal ions at a fixed concentration of 2.0 g L−1, sorption time = 5 h; b) influence of temperature (5 h and pH 4.5) and pH (25 °C and 5 h) on the Pb(II) sorption capacity of the sorbents at an initial concentration of 2.0 g L−1; c) recyclability of the sorbents obtained by desorption experiments using EDTA as an efficient desorber at room temperature for 1 h; d) proposed 3D architecture of OCB and Pb(II) sorption.
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pollution since the multistep syntheses are largely conducted in organic solvent systems and noble metal catalysts are often required.[4] Further investigations of binary [Pb(II)/Co(II), Pb(II)/ Cu(II), Pb(II)/Mn(II), Pb(II)/Fe(III)] and ternary [Pb(II)/ Fe(III)/Co(II), and Pb(II)/Cu(II)/Fe(III)] systems showed that Pb(II) was still much more selectively and efficiently adsorbed than other heavy metal ions. However, we found that the sorption capacities for Pb(II) decreased by 20%–50%, when Fe(III) was present (in both binary and ternary systems). Comparison of individual fluorescence spectra for Pb(II), Mn(II), Co(II), Cu(II), and Fe(III) indicated the highest fluorescence quenching for Fe(III) compared with the other heavy metal ions (Figure S8c, Supporting Information). The strong oxidative and electron-deficient Fe(III) likely accepts both p- and π-electrons from OCB as compared with the fluoranthene and triphenylene oligomers previously reported,[5,8] resulting in a complex with low fluorescence. This competitive interaction could also explain why decreased sorption of Pb(II) was observed in the presence of Fe(III). Recyclability of the OCB hollow microspheres was examined by desorption experiments using the wellknown EDTA as an efficient desorber. During the desorption process, the microspheres were first immersed in a solution with 2.0 g L−1 (≈9 mM) of Pb(II) for 5 h, and then exposed to a 20 mM of EDTA solution. The desorption efficiency of the adsorbed Pb(II) on the microspheres is as high as 93.8% within 1 h, and can be enhanced to 96.5% by further extending desorption to 3 h. The OCB sorbents were reused six times during our investigations (Figure 3c), with recyclability much better than that of most organic sorbents previously reported.[20–23] We have shown that the OCB microspheres exhibited excellent sorption and selectivity for Pb(II) as well as recyclability. This sorption preference is mainly attributed to two main reasons: on the one hand, Pb(II) sorption on the OCB microspheres involves van der Waals electrostatic interactions and ion exchange with amine and carbazole groups present on the sorbent surface. As depicted in Figure 3d, macrocyclic OCB shows a chair-like 3D conformation at the minimal energy level; a relatively high accessible surface area would be excepted. The amine and carbazole groups would homogeneously distribute on the surface of the sorbent as well as within its inner porous structure, ensuring easy access for Pb(II) to be trapped at the sorption sites. On the other hand, Pb(II) is one of softest acids in terms of electronegativity, electrode potential, and ionic size. In contrast, OCB microspheres contain a high concentration of –NH–, –N=, and –NEt– groups, typically behaving as soft bases. According to the hard and soft acids and bases (HSAB) theory described by Pearson,[26] the soft acid Pb(II) in solution will favorably adsorb on the soft base OCB microspheres (especially when compared with
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the hard acids Mn(II) and Fe(III) as well as borderline acids Cu(II) and Co(II)). This conclusion is in accordance with results from most amine and thiol-based sorbents, as confirmed by FTIR, XRD, NMR, and XPS analyses.[27]
4. Conclusions We demonstrated the simple synthesis of macrocyclic amine-linked oligocarbazole (OCB) hollow microspheres through a one-step mild chemical oxidative route in aqueous media. The average diameters of the obtained microspheres could be tuned upon alternating the oxidants and acidic media used. OCB hollow microspheres, with attractive amine/carbazole functionalities and exposed surface area, were directly assembled to yield heavy metal sorbents with excellent selectivity and recyclability, shown to efficiently remove lead from contaminated water. This strategy has the potential to overcome practical issues and challenges associated with other macrocyclic metal sorbents, including multistep syntheses, organic solvent, and noble metal catalysts, which could cause further secondary pollution. We anticipate that OCB hollow microspheres may serve as low cost and sustainable sorbents for real-life contaminated water samples.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: The authors thank the National Natural Science Foundation of China (51203090, 51373100, and 51363012), Innovation Program of Shanghai Municipal Education Commission (13YZ074), National Natural Science Foundation of Shanghai (12ZR1446700), the University of Bristol and Marie Curie Fellowship (326385) for financial support and thank Dr. Yue Wang for valuable discussions. Note: The last author’s name was amended after initial online publication. Received: July 24, 2014; Revised: August 8, 2014; Published online: September 16, 2014; DOI: 10.1002/marc.201400415 Keywords: lead sorbents; macrocycles; oligocarbazole; water treatment
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