- Email: [email protected]
Modification of chitosan with carboxyl-functionalized ionic liquid for anion adsorption
International Journal of Biological Macromolecules 62 (2013) 365–369
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Modification of chitosan with carboxyl-functionalized ionic liquid for anion adsorption Yanqi Wei a,b,1 , Wei Huang a,b,1 , Yuan Zhou a,b , Shuang Zhang a,b , Daoben Hua a,b,∗ , Xiulin Zhu b a Jiangsu Key Laboratory of Radiation Medicine and Protection, School of Radiation Medicine and Protection, and School for Radiological and Interdisciplinary Sciences (RAD-X), Medical College of Soochow University, Suzhou 215123, PR China b Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
a r t i c l e
i n f o
Article history: Received 3 July 2013 Received in revised form 9 September 2013 Accepted 19 September 2013 Available online 26 September 2013 Keywords: Chitosan Ionic liquid Anion adsorption
a b s t r a c t We report a novel chitosan derivative, chitosan–ionic liquid (CS–IL) conjugation for anion adsorption. Specifically, CS–IL conjugation was synthesized through the reaction of amino groups of chitosan with carboxylic groups of 1-carboxybutyl-3-methylimidazolium chloride. Due to the amphiphilic structure, CS–IL conjugation could self-assemble into nanoparticles in distilled water. This novel chitosan derivative revealed good anion adsorption performance, and the adsorption capacity of Cr2 O7 2− and PF6 − was 0.422 mmol/g and 0.840 mmol/g, respectively. The adsorption of Cr2 O7 2− and PF6 − could be improved at low pH, which was ascribed to the adsorption of protonated NH2 on chitosan. Importantly, the chitosan derivative would aggregate in the water after the adsorption and could be easily separated. The properties enable CS–IL conjugation to be used as a novel anion adsorbent for wastewater treatment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays environment pollution has been receiving more and more attention with the rapid development of industry. Anions (such as chromate, dichromate and perchlorate) are environmental contaminants that can impact the local ecosystem, groundwater, and human health [1,2]. Among them, Cr(VI) was considered as one of the most toxic species due to its mutagenic and carcinogenic [3], which was commonly used in tanning processes, electroplating, pigmentation, textile dyeing, or as a catalyst for corrosion inhibitors and wood preservatives [4,5]. At the same time, LiPF6 is also one of the hazardous pollutants, it could react with water and produce dangerous gases such as pentafluoroarsenic, pentafluorophosphate and hydrogen fluoride [6], which is the
Abbreviations: CS–IL, chitosan–ionic liquid; CmimCl, 1-carboxybutyl-3methylimidazolium chloride; BmimCl, 1-butyl-3-methylimidazolium chloride; DS, degree of amino substitution. ∗ Corresponding author at: Jiangsu Key Laboratory of Radiation Medicine and Protection, School of Radiation Medicine and Protection, and School for Radiological and Interdisciplinary Sciences (RAD-X), Medical College of Soochow University, Suzhou 215123, PR China. Tel.: +86 512 65882050; fax: +86 512 65883261. E-mail addresses: [email protected] (Y. Wei), [email protected] (W. Huang), [email protected] (Y. Zhou), [email protected] (S. Zhang), dbhua [email protected], [email protected] (D. Hua), [email protected] (X. Zhu). 1 These authors contributed equally to this work. 0141-8130/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2013.09.020
most commonly used electrolyte for lithium ion batteries due to its excellent conductivity [7]. However, the handling of PF6 − is always ignored [8]. Therefore, it is desirable to develop the materials to adsorb the hazardous anions like CrO4 2− and PF6 − . There are many methods for treating the hazardous anions, such as chemical precipitation [9,10], physical absorption [11], biosorption [3,4,12], anion exchange [13–15]. Among them, anion exchanger can remove trace quantity anions from drinking water effectively, and is considered to be ideal material for removing anions [16]. However, its preparation is complex and the recovery is difficult, which could cause secondary pollution [17]. Hence, it is still a goal to develop the simple and convenient methods to prepare and recycle anion exchanger. (1 → 4)-2-Amino-2-deoxy--d-glucan (i.e. chitosan) is a kind of low-cost natural polysaccharide, and can adsorb heavy metal ions due to the chelation of its amino and hydroxyl groups [18,19]. Furthermore, the amino groups on chitosan under acid condition (pH < 6.2) will be protonized to generate NH3 + , which could adsorb inorganic and organic anions by electrostatic interaction. For example, Ngila et al. used nanofibres electrospun from chitosan and polyacrylamide polymer blends to remove chromate and phosphate in water [20]. However, the anions were only adsorbed at low pH (<6.0), while chitosan could not be dissolved for effective adsorption in water when pH > 6.2. We also notice that ionic liquid can adsorb organic anions from water [21]. For example, Liu et al. [22] prepared an anion-exchange
366
Y. Wei et al. / International Journal of Biological Macromolecules 62 (2013) 365–369
Scheme 1. The schematic for the synthesis of CS–IL conjugation.
phase with N-methylimidazolium immobilized on silica to separate common inorganic anions. Qiu et al. [23] reported poly(ionic liquid)-grafted silica hybrid materials through surface radical chain-transfer polymerization, and the bromide counterion in ionic liquid could be exchanged with tetrafluoroborate, hexafluorophosphate, and trifluoromethanesulfonate through simple aqueous anion-exchange reaction. However, ionic liquid is usually hydrophilic and may cause secondary pollution when it is directly used as anion exchanger for wastewater treatment. Regarding chitosan and ionic liquid, Zhai et al. [24] once prepared the room temperature ionic liquids from 1-ethyl-3methylimidazolium hydroxide (EMIM·OH) and carboxymethylated chitosan (CM-chitosan) by acid–base neutralization reaction, which exhibited good ionic conductivity and thermal stability. In this study, we report here a novel chitosan derivative, chitosan–ionic liquid (CS–IL) conjugation for anion adsorption. Specifically, chitosan was modified by carboxylfunctionalized ionic liquid, 1-carboxybutyl-3-methylimidazolium chloride (CmimCl) (Scheme 1). This novel derivative could be used as anion exchanger to adsorb anions from wastewater due to the adsorption property of ionic liquid and chitosan. The hydrophilicity of ionic liquid may improve the solubility of chitosan in water, which could be expected to improve the adsorption property for wastewater treatment. To the best of our knowledge, it is the first time to report the CS–IL conjugation for anion adsorption.
transform infrared (FT-IR) spectra were recorded on a Varian-1000 spectrometer; the samples were ground with KBr crystals, and the mixture was then pressed into a pellet for IR measurement. Fieldemitting scanning electron microscopy (SEM) images were taken by a HITACHI S-4700 microscope operated at an accelerating voltage of 15 kV. Energy-dispersive X-ray (EDX) analysis was carried out by a Hitachi S570 scanning electron microscope equipped with an EDAX-PV 9100 energy dispersion X-ray fluorescence analyzer. The Z average size and the polydispersity index of the nanoparticles were measured by a Malvern HPP5001 high-performance particle sizer (HPPS). The anion concentrations were determined by Varian 710-ES inductively coupled plasma (ICP), and Ar as auxiliary gas, the determination wavelength of P is 213.6 nm and Cr is 267.7 nm, respectively. The pH adjustment was operated on the Model PHS-3C pH meter with an accuracy of ±0.01 pH units, which was calibrated routinely at two points before operation by using standard buffers pH = 4.00 and 9.18.
2.3. Synthesis of CS–IL conjugation CS–IL conjugation was prepared through amidation reaction of chitosan with carboxylic acid-functionalized ionic liquid CmimCl. Specifically, CmimCl (0.3582 g, 1.75 mmol) and sulfoxide chloride (3 mL, 0.041 mol) were added to a three-necked flask, and 2 L pyridine was added as catalyst. The mixture was refluxed at 55 ◦ C for 4 h, then excess sulfoxide chloride was removed under reduced pressure, and the resulting pink liquid (acylated CmimCl) was sealed before use. Chitosan can be dissolved in ionic liquids BmimCl [27], therefore chitosan (0.3 g, 1.75 mmol repeat units) and BmimCl (15 g) were added to a 100 mL flask, and stirred at 100 ◦ C for 4–5 h until the chitosan dissolved to give completely transparent solution. Then pyridine (0.1408 mL, 1.75 mmol) was added as acid-binding agent, and the mixture was added slowly to the acylated CmimCl. After it was stirred for 48 h under dry atmosphere, the mixture was added to dialysis bag (MWCO = 3500) dialysis in distilled water thoroughly, and distilled water was replaced for many times. After dialysis, the un-reacted impurities were removed by a filter and the filtrate was collected. CS–IL conjugation was obtained by removing the water under reduced pressure. CS–IL conjugation with different substitution degree of ionic liquid was obtained by changing the ratio of CmimCl to chitosan in the experiments.
2. Experimental 2.1. Materials and reagents Chitosan (degree of deacetylation = 95.2%, determined by elemental analysis, average molecular weight = 50,000 g mol−1 ) was purchased from Golden-Shell Biochemical Co. Ltd., Zhejiang, China. 1-Methylimidazole (99%) was purchased from J & K Scientific Ltd., and dried by adding CaH2 then reduced pressure distillation. Methyl 4-chlorobutyrate (98%) was purchased from Alfa Aesar China (Tianjin) Co., Ltd. and used as received. 1-Chlorobutane (CP) was purchased from Sinopharm Chemical Reagent Co. Ltd., and purified by reduced pressure distillation avoid light. Ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl) [25] (Figure S1, Supporting Information) and functional ionic liquid 1-carboxybutyl-3-methylimidazolium chloride (CmimCl) [26] (Figure S2, Supporting Information) were synthesized according to the related references. All other chemical agents were used as received. 2.2. Characterization methods 1 H nuclear magnetic resonance (1 H NMR) spectra were obtained on a Varian INVOA-400 instrument working at 400 MHz. Fourier
2.4. Anion adsorption of ionic liquid modified chitosan (CS–IL conjugation) 0.025 g of CS–IL conjugation was dispersed in distilled water (5 mL) under ultrasonic irradiation, and then dialyzed (MWCO = 3500) in dilute aqueous solutions with 2 × 10−4 mol/L K2 Cr2 O7 or KPF6 (pH = 7.00). 1.0 mL of the solution was removed to record ICP spectrometry at the pre-set interval times. Before the measurement, calibration curves of K2 Cr2 O7 and KPF6 were constructed. The adsorption experiments of original chitosan were also performed for the comparison with CS–IL conjugation. After adsorption, the CS–IL conjugation solution was dialyzed in distilled water thoroughly, and distilled water was replaced for many times. Then it was taken out from the dialysis bag, the water was removed by a rotary evaporator. The resulting solid was collected for EDX analysis. To study the effect of pH on the anion adsorption, the KPF6 solutions with pH = 3.00 and 7.00 were used for adsorption experiments, respectively. The pH of KPF6 solution was adjusted to 3.00 or 7.00 on the pH meter with 0.1 mol/L HCl aqueous solution. In order to minimize the influence of CO2 in the atmosphere, the
Y. Wei et al. / International Journal of Biological Macromolecules 62 (2013) 365–369
367
Fig. 3. The sorption of PF6 − as a function of the time for CS–IL conjugation (DS = 18.18%) at (a) pH = 3.00 and (b) pH = 7.00.
Fig. 1. (A) 1 H NMR spectrum of CS–IL conjugation (DS = 10.18%); (B) FT-IR spectra of (a) CS–IL conjugation (DS = 10.18%), and (b) chitosan; (C) SEM image (scale bar: 2.00 m) and (D) particle size distributions of CS–IL conjugation (DS = 10.18%) in distilled water.
anion adsorption experiment was carried out immediately with the aqueous solution. 3. Results and discussion In order to synthesize CS–IL conjugation, ionic liquid BmimCl (Figure S1, Supporting Information) and ionic liquid CmimCl (Figure S2, Supporting Information) were first synthesized according to the literature methods [25,26]. And the chitosan derivative was synthesized by the reaction of amino groups of chitosan with carboxyl groups of ionic liquid (Scheme 1). The structures of CS–IL conjugation were characterized by 1 H NMR spectra, and a typical 1 H NMR spectrum of CS–IL conjugation is shown in Fig. 1A. Besides the characteristic peaks for chitosan, the characteristic resonances of ionic liquid were detected. The characteristic peaks at ı = 8.70 and 7.40–7.58 ppm can be attributed to the hydrogen proton characteristic peaks of imidazole ring, and characteristic peaks of 2.19 ppm, 2.53 ppm and 4.27 ppm were produced by CH2 of alkyl chain segments of ionic liquids, suggesting that CS–IL conjugation had been prepared successfully. The degree of amino
substitution (DS) of chitosan can be calculated according to the equation: DS = Ia /Ij × 100%, where Ia and Ij are the integration values of the peaks at ı = 8.70–8.86 ppm and 1.95–2.08 ppm in 1 H NMR spectroscopy, respectively. The structures of CS–IL conjugation were further confirmed by FT-IR spectra (Fig. 1B). In comparison with chitosan (Fig. 1B, trace a), the characteristic peaks occurred for the CS–IL conjugation (Fig. 1B, trace b) at 1600 cm−1 corresponding to C N stretching vibration, respectively. Because of its amphiphilic structure, CS–IL conjugation can self-assemble into micelles in the distilled water at room temperature, where the hydrophobic chitosan segments may collapse as the core and the hydrophilic ionic liquid segments form the corona shell. The typical morphology of the micelles is shown in Fig. 1C, and spherical nanoparticles were observed with a diameter of about 250 nm. The particle sizes and size distributions further demonstrate that the colloidal particles have a symmetric and narrow size distribution. It was noted that the single peak is positioned at 262.3 nm (Fig. 1D), which was consistent well with that observed from SEM image. Due to the good property of anion adsorption of chitosan and ionic liquid, CS–IL conjugation can be expected as efficient anion adsorption materials for the wastewater treatment. In order to investigate the property of anion adsorption, CS–IL conjugations and original chitosan were dialyzed in ionic aqueous solutions, i.e. 0.2 mmol K2 Cr2 O7 aqueous solution (pH ∼ 7.00) and 0.2 mmol KPF6 aqueous solution (pH ∼ 7.00), respectively. The sorption of the anions as a function of the time was depicted in Fig. 2 for CS–IL conjugation. In comparison with original chitosan (Fig. 2A, trace c and B, trace c), CS–IL conjugations have the obviously
Fig. 2. (A) The sorption of (A) PF6 − and (B) Cr2 O7 2− as a function of the time at pH = 7.00 for CS–IL conjugation with (a) DS = 18.18%, and (b) DS = 10.18%.
368
Y. Wei et al. / International Journal of Biological Macromolecules 62 (2013) 365–369
Fig. 4. The EDX spectra of CS–IL conjugation (DS = 18.18%) with (A) Cr2 O7 2− and (B) PF6 − ; and particle size distributions of CS–IL conjugation (DS = 18.18%) after the sorption of (C) Cr2 O7 2− and (D) PF6 − at pH = 7.00.
larger capacities for PF6 − (Fig. 2A, traces a and b) and Cr2 O7 2− sorption (Fig. 2B, traces a and b), which should be attributed to the adsorption of ionic liquid and the improved solubility of chitosan with ionic liquid. The sorption of PF6 − and Cr2 O7 2− could reach the maximum capacity of 0.719 mmol/g (Fig. 2A, trace a) and 0.422 mmol/g (Fig. 2B, trace a) within 90 min for CS–IL conjugation with DS = 18.18%, respectively. The difference of sorption capacity between PF6− and Cr2 O7 2− may be ascribed to the different charge density because the electrostatic interaction is essential for the ion sorption in this study: for the smaller volume of PF6 − , there is a relatively larger charge density, which may lead to the larger sorption capacity. Expectedly, the maximum capacity of anions is closely related with the substitution degree of CS–IL conjugation: the more substitution degree of ionic liquid, the more adsorption (Fig. 2), which may be attributed to the adsorption of ionic liquid at pH = 7.00. The amino groups on chitosan under acid condition (pH < 6.2) will be protonized to generate NH3 + , which can adsorb inorganic and organic anions by electrostatic interaction [28]. Due to Cr2 O7 2− can be reduced to Cr(III) in HCl aqueous solution, hence PF6 − was selected as the model ion to investigate the effect of pH on the adsorption of CS–IL conjugation. The adsorptions as a function of the time were given with different pH in Fig. 3. For original chitosan, the maximum adsorption capacity at pH = 3.00 (Fig. 3, trace c) is just slightly larger than that at pH = 7.00 (Fig. 3, trace d). Whereas, for CS–IL conjugation (DS = 18.18%) at pH = 3.00, the maximum adsorption capacity of PF6 − was markedly increased to 0.840 mmol/g (Fig. 3, trace a) from 0.719 mmol/g at pH = 7.00 (Fig. 3, trace b), which may be associated with the adsorption by NH3 + of chitosan besides ionic liquid and better solubility of CS–IL conjugation. The result indicated that the anion adsorption could be improved at low pH for CS–IL conjugation. After the complete adsorption, the resultants were characterized by EDX analyses. Fig. 4 shows the EDX spectra of CS–IL conjugation with Cr2 O7 2− and PF6 − . Cr (5.98%) element was clearly
observed in Fig. 4A, while P (5.98%) and F (15.39%) elements were observed in Fig. 4B, respectively. The results indicated that ion exchange of CS–IL conjugation successfully occurred with Cr2 O7 2− and PF6 − . Importantly, the aggregation would occur for CS–IL conjugation after the anion adsorption (Fig. 4C and D), which may be attributed that hydrophilic ionic liquids can be changed into hydrophobic after exchanged by larger anions [23]. The results may be beneficial for the easy separation and recycling after the adsorption. 4. Conclusion In summary, a novel chitosan derivative, CS–IL conjugation, was synthesized by a simple reaction of amino groups of chitosan with carboxyl groups of ionic liquid. The structure of CS–IL conjugation was confirmed by 1 H NMR spectrum and FT-IR spectrum. CS–IL conjugation revealed good anion adsorption performance, and the adsorption capacity of Cr2 O7 2− and PF6 − was 0.422 mmol/g and 0.840 mmol/g, respectively. The adsorption capacities were closely related with the substitution degree: the more substitution degree of ionic liquid, the more adsorption, which may be ascribed to the adsorption of ionic liquid. Furthermore, the anion adsorption could be improved at low pH (e.g. pH = 3.00), which should be attributed to the protonation of NH2 on chitosan. Compared with original chitosan, CS–IL conjugation could dissolve well for effective adsorption in a broad pH range; importantly, the conjugation could aggregate in water after anion sorption for easy separation in comparison with ionic liquid. These properties prompt CS–IL conjugation to be used as a novel anion adsorbent for wastewater treatment. To our knowledge, this is the first report to synthesize CS–IL conjugation for anion adsorption. Acknowledgements We thank Natural Science Foundation of China (91326202, 21174100), a project funded by the Priority Academic Program
Y. Wei et al. / International Journal of Biological Macromolecules 62 (2013) 365–369
Development of Jiangsu Higher Education Institutions (PAPD), and Program of Innovative Research Team of Soochow University for their financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ijbiomac.2013. 09.020. References [1] T. Narita, H. Hamana, M. Takeshita, H. Fujiwara, J. Fluorine Chem. 124 (2003) 197–201. [2] K.A. Aamer, K.L. Genson, J. Kohn, M.L. Becker, Biomacromolecules 10 (2009) 2418–2426. [3] A. Bingol, A. Aslan, A. Cakici, J. Hazard. Mater. 161 (2009) 747–752. [4] S. Bai, T.E. Abraham, Bioresour. Technol. 79 (2001) 73–81. [5] D. Park, Y.S. Yun, J.M. Park, Chemosphere 60 (2005) 1356–1364. [6] J. Nan, D. Han, M. Yang, M. Cui, X. Hou, Hydrometallurgy 84 (2006) 75–80. [7] L.O. Valoen, J.N. Reimers, J. Electrochem. Soc. 152 (2005) A882–A891. [8] S. Al-Thyabat, T. Nakamura, E. Shibata, A. Iizuka, Miner. Eng. 45 (2013) 4–17. [9] L. Pastor, D. Mangin, R. Barat, A. Seco, Bioresour. Technol. 99 (2008) 6285–6291.
369
[10] A. Bhatnagar, E. Kumar, M. Sillanpaa, Chem. Eng. J. 171 (2011) 811–840. [11] A. Bhatnagar, A.K. Minocha, Indian J. Chem. Technol. 13 (2006) 203–217. [12] A.C.A. de Lima, R.F. Nascimento, F.F. de Sousa, J.M. Filho, A.C. Oliveira, Chem. Eng. J. 185 (2012) 274–284. [13] F. Gode, E. Pehlivan, J. Hazard. Mater. 119 (2005) 175–182. [14] L.M. Blaney, S. Cinar, A.K. SenGupta, Water Res. 41 (2007) 1603–1613. [15] T. Shi, Z. Wang, Y. Liu, S. Jia, D. Changming, J. Hazard. Mater. 161 (2009) 900–906. [16] L. Ye, H. You, J. Yao, H. Su, Desalination 298 (2012) 1–12. [17] X.Q. Liu, G. Zhang, H.Q. Xing, P. Huang, X.L. Zhang, Environ. Chem. Lett. 9 (2011) 127–132. [18] W.S.W. Ngah, L.C. Teong, M.A.K.M. Hanafiah, Carbohydr. Polym. 83 (2011) 1446–1456. [19] M. Wang, L. Xu, M. Zhai, J. Peng, J. Li, G. Wei, Carbohydr. Polym. 74 (2008) 498–503. [20] R.M. Nthumbi, J.C. Ngila, B. Moodley, A. Kindness, L. Petrik, Phys. Chem. Earth 50–52 (2012) 243–251. [21] W. Bi, J. Zhou, K.H. Row, Talanta 83 (2011) 974–979. [22] H.D. Qiu, S.X. Jiang, X. Liu, J. Chromatogr. A 1103 (2006) 265–270. [23] H. Qiu, T. Sawada, S. Jiang, H. Ihara, Mater. Lett. 64 (2010) 1653–1655. [24] L. Huang, M. Zhai, J. Peng, L. Xu, J. Li, G. Wei, Carbohydr. Polym. 71 (2008) 690–693. [25] P.J. Dyson, M.C. Grossel, N. Srinivasan, T. Vine, T. Welton, D.J. Williams, A.J.P. White, T. Zigras, J. Chem. Soc. Dalton 19 (1997) 3465–3469. [26] Z.F. Fei, D.B. Zhao, T.J. Geldbach, R. Scopelliti, P.J. Dyson, Chem. Eur. J. (2004) 4886–4893. [27] H. Xie, S. Zhang, S. Li, Green Chem. 8 (2006) 630–633. [28] L. Zhu, Y. Deng, J. Zhang, J. Chen, J. Colloid Interface Sci. 364 (2011) 462–468.
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Modification of chitosan with carboxyl-functionalized ionic liquid for anion adsorption Yanqi Wei a,b,1 , Wei Huang a,b,1 , Yuan Zhou a,b , Shuang Zhang a,b , Daoben Hua a,b,∗ , Xiulin Zhu b a Jiangsu Key Laboratory of Radiation Medicine and Protection, School of Radiation Medicine and Protection, and School for Radiological and Interdisciplinary Sciences (RAD-X), Medical College of Soochow University, Suzhou 215123, PR China b Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
a r t i c l e
i n f o
Article history: Received 3 July 2013 Received in revised form 9 September 2013 Accepted 19 September 2013 Available online 26 September 2013 Keywords: Chitosan Ionic liquid Anion adsorption
a b s t r a c t We report a novel chitosan derivative, chitosan–ionic liquid (CS–IL) conjugation for anion adsorption. Specifically, CS–IL conjugation was synthesized through the reaction of amino groups of chitosan with carboxylic groups of 1-carboxybutyl-3-methylimidazolium chloride. Due to the amphiphilic structure, CS–IL conjugation could self-assemble into nanoparticles in distilled water. This novel chitosan derivative revealed good anion adsorption performance, and the adsorption capacity of Cr2 O7 2− and PF6 − was 0.422 mmol/g and 0.840 mmol/g, respectively. The adsorption of Cr2 O7 2− and PF6 − could be improved at low pH, which was ascribed to the adsorption of protonated NH2 on chitosan. Importantly, the chitosan derivative would aggregate in the water after the adsorption and could be easily separated. The properties enable CS–IL conjugation to be used as a novel anion adsorbent for wastewater treatment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays environment pollution has been receiving more and more attention with the rapid development of industry. Anions (such as chromate, dichromate and perchlorate) are environmental contaminants that can impact the local ecosystem, groundwater, and human health [1,2]. Among them, Cr(VI) was considered as one of the most toxic species due to its mutagenic and carcinogenic [3], which was commonly used in tanning processes, electroplating, pigmentation, textile dyeing, or as a catalyst for corrosion inhibitors and wood preservatives [4,5]. At the same time, LiPF6 is also one of the hazardous pollutants, it could react with water and produce dangerous gases such as pentafluoroarsenic, pentafluorophosphate and hydrogen fluoride [6], which is the
Abbreviations: CS–IL, chitosan–ionic liquid; CmimCl, 1-carboxybutyl-3methylimidazolium chloride; BmimCl, 1-butyl-3-methylimidazolium chloride; DS, degree of amino substitution. ∗ Corresponding author at: Jiangsu Key Laboratory of Radiation Medicine and Protection, School of Radiation Medicine and Protection, and School for Radiological and Interdisciplinary Sciences (RAD-X), Medical College of Soochow University, Suzhou 215123, PR China. Tel.: +86 512 65882050; fax: +86 512 65883261. E-mail addresses: [email protected] (Y. Wei), [email protected] (W. Huang), [email protected] (Y. Zhou), [email protected] (S. Zhang), dbhua [email protected], [email protected] (D. Hua), [email protected] (X. Zhu). 1 These authors contributed equally to this work. 0141-8130/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2013.09.020
most commonly used electrolyte for lithium ion batteries due to its excellent conductivity [7]. However, the handling of PF6 − is always ignored [8]. Therefore, it is desirable to develop the materials to adsorb the hazardous anions like CrO4 2− and PF6 − . There are many methods for treating the hazardous anions, such as chemical precipitation [9,10], physical absorption [11], biosorption [3,4,12], anion exchange [13–15]. Among them, anion exchanger can remove trace quantity anions from drinking water effectively, and is considered to be ideal material for removing anions [16]. However, its preparation is complex and the recovery is difficult, which could cause secondary pollution [17]. Hence, it is still a goal to develop the simple and convenient methods to prepare and recycle anion exchanger. (1 → 4)-2-Amino-2-deoxy--d-glucan (i.e. chitosan) is a kind of low-cost natural polysaccharide, and can adsorb heavy metal ions due to the chelation of its amino and hydroxyl groups [18,19]. Furthermore, the amino groups on chitosan under acid condition (pH < 6.2) will be protonized to generate NH3 + , which could adsorb inorganic and organic anions by electrostatic interaction. For example, Ngila et al. used nanofibres electrospun from chitosan and polyacrylamide polymer blends to remove chromate and phosphate in water [20]. However, the anions were only adsorbed at low pH (<6.0), while chitosan could not be dissolved for effective adsorption in water when pH > 6.2. We also notice that ionic liquid can adsorb organic anions from water [21]. For example, Liu et al. [22] prepared an anion-exchange
366
Y. Wei et al. / International Journal of Biological Macromolecules 62 (2013) 365–369
Scheme 1. The schematic for the synthesis of CS–IL conjugation.
phase with N-methylimidazolium immobilized on silica to separate common inorganic anions. Qiu et al. [23] reported poly(ionic liquid)-grafted silica hybrid materials through surface radical chain-transfer polymerization, and the bromide counterion in ionic liquid could be exchanged with tetrafluoroborate, hexafluorophosphate, and trifluoromethanesulfonate through simple aqueous anion-exchange reaction. However, ionic liquid is usually hydrophilic and may cause secondary pollution when it is directly used as anion exchanger for wastewater treatment. Regarding chitosan and ionic liquid, Zhai et al. [24] once prepared the room temperature ionic liquids from 1-ethyl-3methylimidazolium hydroxide (EMIM·OH) and carboxymethylated chitosan (CM-chitosan) by acid–base neutralization reaction, which exhibited good ionic conductivity and thermal stability. In this study, we report here a novel chitosan derivative, chitosan–ionic liquid (CS–IL) conjugation for anion adsorption. Specifically, chitosan was modified by carboxylfunctionalized ionic liquid, 1-carboxybutyl-3-methylimidazolium chloride (CmimCl) (Scheme 1). This novel derivative could be used as anion exchanger to adsorb anions from wastewater due to the adsorption property of ionic liquid and chitosan. The hydrophilicity of ionic liquid may improve the solubility of chitosan in water, which could be expected to improve the adsorption property for wastewater treatment. To the best of our knowledge, it is the first time to report the CS–IL conjugation for anion adsorption.
transform infrared (FT-IR) spectra were recorded on a Varian-1000 spectrometer; the samples were ground with KBr crystals, and the mixture was then pressed into a pellet for IR measurement. Fieldemitting scanning electron microscopy (SEM) images were taken by a HITACHI S-4700 microscope operated at an accelerating voltage of 15 kV. Energy-dispersive X-ray (EDX) analysis was carried out by a Hitachi S570 scanning electron microscope equipped with an EDAX-PV 9100 energy dispersion X-ray fluorescence analyzer. The Z average size and the polydispersity index of the nanoparticles were measured by a Malvern HPP5001 high-performance particle sizer (HPPS). The anion concentrations were determined by Varian 710-ES inductively coupled plasma (ICP), and Ar as auxiliary gas, the determination wavelength of P is 213.6 nm and Cr is 267.7 nm, respectively. The pH adjustment was operated on the Model PHS-3C pH meter with an accuracy of ±0.01 pH units, which was calibrated routinely at two points before operation by using standard buffers pH = 4.00 and 9.18.
2.3. Synthesis of CS–IL conjugation CS–IL conjugation was prepared through amidation reaction of chitosan with carboxylic acid-functionalized ionic liquid CmimCl. Specifically, CmimCl (0.3582 g, 1.75 mmol) and sulfoxide chloride (3 mL, 0.041 mol) were added to a three-necked flask, and 2 L pyridine was added as catalyst. The mixture was refluxed at 55 ◦ C for 4 h, then excess sulfoxide chloride was removed under reduced pressure, and the resulting pink liquid (acylated CmimCl) was sealed before use. Chitosan can be dissolved in ionic liquids BmimCl [27], therefore chitosan (0.3 g, 1.75 mmol repeat units) and BmimCl (15 g) were added to a 100 mL flask, and stirred at 100 ◦ C for 4–5 h until the chitosan dissolved to give completely transparent solution. Then pyridine (0.1408 mL, 1.75 mmol) was added as acid-binding agent, and the mixture was added slowly to the acylated CmimCl. After it was stirred for 48 h under dry atmosphere, the mixture was added to dialysis bag (MWCO = 3500) dialysis in distilled water thoroughly, and distilled water was replaced for many times. After dialysis, the un-reacted impurities were removed by a filter and the filtrate was collected. CS–IL conjugation was obtained by removing the water under reduced pressure. CS–IL conjugation with different substitution degree of ionic liquid was obtained by changing the ratio of CmimCl to chitosan in the experiments.
2. Experimental 2.1. Materials and reagents Chitosan (degree of deacetylation = 95.2%, determined by elemental analysis, average molecular weight = 50,000 g mol−1 ) was purchased from Golden-Shell Biochemical Co. Ltd., Zhejiang, China. 1-Methylimidazole (99%) was purchased from J & K Scientific Ltd., and dried by adding CaH2 then reduced pressure distillation. Methyl 4-chlorobutyrate (98%) was purchased from Alfa Aesar China (Tianjin) Co., Ltd. and used as received. 1-Chlorobutane (CP) was purchased from Sinopharm Chemical Reagent Co. Ltd., and purified by reduced pressure distillation avoid light. Ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl) [25] (Figure S1, Supporting Information) and functional ionic liquid 1-carboxybutyl-3-methylimidazolium chloride (CmimCl) [26] (Figure S2, Supporting Information) were synthesized according to the related references. All other chemical agents were used as received. 2.2. Characterization methods 1 H nuclear magnetic resonance (1 H NMR) spectra were obtained on a Varian INVOA-400 instrument working at 400 MHz. Fourier
2.4. Anion adsorption of ionic liquid modified chitosan (CS–IL conjugation) 0.025 g of CS–IL conjugation was dispersed in distilled water (5 mL) under ultrasonic irradiation, and then dialyzed (MWCO = 3500) in dilute aqueous solutions with 2 × 10−4 mol/L K2 Cr2 O7 or KPF6 (pH = 7.00). 1.0 mL of the solution was removed to record ICP spectrometry at the pre-set interval times. Before the measurement, calibration curves of K2 Cr2 O7 and KPF6 were constructed. The adsorption experiments of original chitosan were also performed for the comparison with CS–IL conjugation. After adsorption, the CS–IL conjugation solution was dialyzed in distilled water thoroughly, and distilled water was replaced for many times. Then it was taken out from the dialysis bag, the water was removed by a rotary evaporator. The resulting solid was collected for EDX analysis. To study the effect of pH on the anion adsorption, the KPF6 solutions with pH = 3.00 and 7.00 were used for adsorption experiments, respectively. The pH of KPF6 solution was adjusted to 3.00 or 7.00 on the pH meter with 0.1 mol/L HCl aqueous solution. In order to minimize the influence of CO2 in the atmosphere, the
Y. Wei et al. / International Journal of Biological Macromolecules 62 (2013) 365–369
367
Fig. 3. The sorption of PF6 − as a function of the time for CS–IL conjugation (DS = 18.18%) at (a) pH = 3.00 and (b) pH = 7.00.
Fig. 1. (A) 1 H NMR spectrum of CS–IL conjugation (DS = 10.18%); (B) FT-IR spectra of (a) CS–IL conjugation (DS = 10.18%), and (b) chitosan; (C) SEM image (scale bar: 2.00 m) and (D) particle size distributions of CS–IL conjugation (DS = 10.18%) in distilled water.
anion adsorption experiment was carried out immediately with the aqueous solution. 3. Results and discussion In order to synthesize CS–IL conjugation, ionic liquid BmimCl (Figure S1, Supporting Information) and ionic liquid CmimCl (Figure S2, Supporting Information) were first synthesized according to the literature methods [25,26]. And the chitosan derivative was synthesized by the reaction of amino groups of chitosan with carboxyl groups of ionic liquid (Scheme 1). The structures of CS–IL conjugation were characterized by 1 H NMR spectra, and a typical 1 H NMR spectrum of CS–IL conjugation is shown in Fig. 1A. Besides the characteristic peaks for chitosan, the characteristic resonances of ionic liquid were detected. The characteristic peaks at ı = 8.70 and 7.40–7.58 ppm can be attributed to the hydrogen proton characteristic peaks of imidazole ring, and characteristic peaks of 2.19 ppm, 2.53 ppm and 4.27 ppm were produced by CH2 of alkyl chain segments of ionic liquids, suggesting that CS–IL conjugation had been prepared successfully. The degree of amino
substitution (DS) of chitosan can be calculated according to the equation: DS = Ia /Ij × 100%, where Ia and Ij are the integration values of the peaks at ı = 8.70–8.86 ppm and 1.95–2.08 ppm in 1 H NMR spectroscopy, respectively. The structures of CS–IL conjugation were further confirmed by FT-IR spectra (Fig. 1B). In comparison with chitosan (Fig. 1B, trace a), the characteristic peaks occurred for the CS–IL conjugation (Fig. 1B, trace b) at 1600 cm−1 corresponding to C N stretching vibration, respectively. Because of its amphiphilic structure, CS–IL conjugation can self-assemble into micelles in the distilled water at room temperature, where the hydrophobic chitosan segments may collapse as the core and the hydrophilic ionic liquid segments form the corona shell. The typical morphology of the micelles is shown in Fig. 1C, and spherical nanoparticles were observed with a diameter of about 250 nm. The particle sizes and size distributions further demonstrate that the colloidal particles have a symmetric and narrow size distribution. It was noted that the single peak is positioned at 262.3 nm (Fig. 1D), which was consistent well with that observed from SEM image. Due to the good property of anion adsorption of chitosan and ionic liquid, CS–IL conjugation can be expected as efficient anion adsorption materials for the wastewater treatment. In order to investigate the property of anion adsorption, CS–IL conjugations and original chitosan were dialyzed in ionic aqueous solutions, i.e. 0.2 mmol K2 Cr2 O7 aqueous solution (pH ∼ 7.00) and 0.2 mmol KPF6 aqueous solution (pH ∼ 7.00), respectively. The sorption of the anions as a function of the time was depicted in Fig. 2 for CS–IL conjugation. In comparison with original chitosan (Fig. 2A, trace c and B, trace c), CS–IL conjugations have the obviously
Fig. 2. (A) The sorption of (A) PF6 − and (B) Cr2 O7 2− as a function of the time at pH = 7.00 for CS–IL conjugation with (a) DS = 18.18%, and (b) DS = 10.18%.
368
Y. Wei et al. / International Journal of Biological Macromolecules 62 (2013) 365–369
Fig. 4. The EDX spectra of CS–IL conjugation (DS = 18.18%) with (A) Cr2 O7 2− and (B) PF6 − ; and particle size distributions of CS–IL conjugation (DS = 18.18%) after the sorption of (C) Cr2 O7 2− and (D) PF6 − at pH = 7.00.
larger capacities for PF6 − (Fig. 2A, traces a and b) and Cr2 O7 2− sorption (Fig. 2B, traces a and b), which should be attributed to the adsorption of ionic liquid and the improved solubility of chitosan with ionic liquid. The sorption of PF6 − and Cr2 O7 2− could reach the maximum capacity of 0.719 mmol/g (Fig. 2A, trace a) and 0.422 mmol/g (Fig. 2B, trace a) within 90 min for CS–IL conjugation with DS = 18.18%, respectively. The difference of sorption capacity between PF6− and Cr2 O7 2− may be ascribed to the different charge density because the electrostatic interaction is essential for the ion sorption in this study: for the smaller volume of PF6 − , there is a relatively larger charge density, which may lead to the larger sorption capacity. Expectedly, the maximum capacity of anions is closely related with the substitution degree of CS–IL conjugation: the more substitution degree of ionic liquid, the more adsorption (Fig. 2), which may be attributed to the adsorption of ionic liquid at pH = 7.00. The amino groups on chitosan under acid condition (pH < 6.2) will be protonized to generate NH3 + , which can adsorb inorganic and organic anions by electrostatic interaction [28]. Due to Cr2 O7 2− can be reduced to Cr(III) in HCl aqueous solution, hence PF6 − was selected as the model ion to investigate the effect of pH on the adsorption of CS–IL conjugation. The adsorptions as a function of the time were given with different pH in Fig. 3. For original chitosan, the maximum adsorption capacity at pH = 3.00 (Fig. 3, trace c) is just slightly larger than that at pH = 7.00 (Fig. 3, trace d). Whereas, for CS–IL conjugation (DS = 18.18%) at pH = 3.00, the maximum adsorption capacity of PF6 − was markedly increased to 0.840 mmol/g (Fig. 3, trace a) from 0.719 mmol/g at pH = 7.00 (Fig. 3, trace b), which may be associated with the adsorption by NH3 + of chitosan besides ionic liquid and better solubility of CS–IL conjugation. The result indicated that the anion adsorption could be improved at low pH for CS–IL conjugation. After the complete adsorption, the resultants were characterized by EDX analyses. Fig. 4 shows the EDX spectra of CS–IL conjugation with Cr2 O7 2− and PF6 − . Cr (5.98%) element was clearly
observed in Fig. 4A, while P (5.98%) and F (15.39%) elements were observed in Fig. 4B, respectively. The results indicated that ion exchange of CS–IL conjugation successfully occurred with Cr2 O7 2− and PF6 − . Importantly, the aggregation would occur for CS–IL conjugation after the anion adsorption (Fig. 4C and D), which may be attributed that hydrophilic ionic liquids can be changed into hydrophobic after exchanged by larger anions [23]. The results may be beneficial for the easy separation and recycling after the adsorption. 4. Conclusion In summary, a novel chitosan derivative, CS–IL conjugation, was synthesized by a simple reaction of amino groups of chitosan with carboxyl groups of ionic liquid. The structure of CS–IL conjugation was confirmed by 1 H NMR spectrum and FT-IR spectrum. CS–IL conjugation revealed good anion adsorption performance, and the adsorption capacity of Cr2 O7 2− and PF6 − was 0.422 mmol/g and 0.840 mmol/g, respectively. The adsorption capacities were closely related with the substitution degree: the more substitution degree of ionic liquid, the more adsorption, which may be ascribed to the adsorption of ionic liquid. Furthermore, the anion adsorption could be improved at low pH (e.g. pH = 3.00), which should be attributed to the protonation of NH2 on chitosan. Compared with original chitosan, CS–IL conjugation could dissolve well for effective adsorption in a broad pH range; importantly, the conjugation could aggregate in water after anion sorption for easy separation in comparison with ionic liquid. These properties prompt CS–IL conjugation to be used as a novel anion adsorbent for wastewater treatment. To our knowledge, this is the first report to synthesize CS–IL conjugation for anion adsorption. Acknowledgements We thank Natural Science Foundation of China (91326202, 21174100), a project funded by the Priority Academic Program
Y. Wei et al. / International Journal of Biological Macromolecules 62 (2013) 365–369
Development of Jiangsu Higher Education Institutions (PAPD), and Program of Innovative Research Team of Soochow University for their financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ijbiomac.2013. 09.020. References [1] T. Narita, H. Hamana, M. Takeshita, H. Fujiwara, J. Fluorine Chem. 124 (2003) 197–201. [2] K.A. Aamer, K.L. Genson, J. Kohn, M.L. Becker, Biomacromolecules 10 (2009) 2418–2426. [3] A. Bingol, A. Aslan, A. Cakici, J. Hazard. Mater. 161 (2009) 747–752. [4] S. Bai, T.E. Abraham, Bioresour. Technol. 79 (2001) 73–81. [5] D. Park, Y.S. Yun, J.M. Park, Chemosphere 60 (2005) 1356–1364. [6] J. Nan, D. Han, M. Yang, M. Cui, X. Hou, Hydrometallurgy 84 (2006) 75–80. [7] L.O. Valoen, J.N. Reimers, J. Electrochem. Soc. 152 (2005) A882–A891. [8] S. Al-Thyabat, T. Nakamura, E. Shibata, A. Iizuka, Miner. Eng. 45 (2013) 4–17. [9] L. Pastor, D. Mangin, R. Barat, A. Seco, Bioresour. Technol. 99 (2008) 6285–6291.
369
[10] A. Bhatnagar, E. Kumar, M. Sillanpaa, Chem. Eng. J. 171 (2011) 811–840. [11] A. Bhatnagar, A.K. Minocha, Indian J. Chem. Technol. 13 (2006) 203–217. [12] A.C.A. de Lima, R.F. Nascimento, F.F. de Sousa, J.M. Filho, A.C. Oliveira, Chem. Eng. J. 185 (2012) 274–284. [13] F. Gode, E. Pehlivan, J. Hazard. Mater. 119 (2005) 175–182. [14] L.M. Blaney, S. Cinar, A.K. SenGupta, Water Res. 41 (2007) 1603–1613. [15] T. Shi, Z. Wang, Y. Liu, S. Jia, D. Changming, J. Hazard. Mater. 161 (2009) 900–906. [16] L. Ye, H. You, J. Yao, H. Su, Desalination 298 (2012) 1–12. [17] X.Q. Liu, G. Zhang, H.Q. Xing, P. Huang, X.L. Zhang, Environ. Chem. Lett. 9 (2011) 127–132. [18] W.S.W. Ngah, L.C. Teong, M.A.K.M. Hanafiah, Carbohydr. Polym. 83 (2011) 1446–1456. [19] M. Wang, L. Xu, M. Zhai, J. Peng, J. Li, G. Wei, Carbohydr. Polym. 74 (2008) 498–503. [20] R.M. Nthumbi, J.C. Ngila, B. Moodley, A. Kindness, L. Petrik, Phys. Chem. Earth 50–52 (2012) 243–251. [21] W. Bi, J. Zhou, K.H. Row, Talanta 83 (2011) 974–979. [22] H.D. Qiu, S.X. Jiang, X. Liu, J. Chromatogr. A 1103 (2006) 265–270. [23] H. Qiu, T. Sawada, S. Jiang, H. Ihara, Mater. Lett. 64 (2010) 1653–1655. [24] L. Huang, M. Zhai, J. Peng, L. Xu, J. Li, G. Wei, Carbohydr. Polym. 71 (2008) 690–693. [25] P.J. Dyson, M.C. Grossel, N. Srinivasan, T. Vine, T. Welton, D.J. Williams, A.J.P. White, T. Zigras, J. Chem. Soc. Dalton 19 (1997) 3465–3469. [26] Z.F. Fei, D.B. Zhao, T.J. Geldbach, R. Scopelliti, P.J. Dyson, Chem. Eur. J. (2004) 4886–4893. [27] H. Xie, S. Zhang, S. Li, Green Chem. 8 (2006) 630–633. [28] L. Zhu, Y. Deng, J. Zhang, J. Chen, J. Colloid Interface Sci. 364 (2011) 462–468.