Magnetic Chitosan Nanocomposites: A Useful Recyclable Tool for

Nov 25, 2008 - Synthesis route of magnetic chitosan nanocomposites and their use as a facile tool for Pb2+ removal with the help of an external magnet...
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Langmuir 2009, 25, 3-8

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Letters Magnetic Chitosan Nanocomposites: A Useful Recyclable Tool for Heavy Metal Ion Removal Xiaowang Liu,*,† Qiyan Hu,‡ Zhen Fang,† Xiaojun Zhang,† and Beibei Zhang† College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, China, and Department of Pharmacy, Wannan Medical College, Wuhu 241002, China ReceiVed August 23, 2008. ReVised Manuscript ReceiVed NoVember 1, 2008 Magnetic chitosan nanocomposites have been synthesized on the basis of amine-functionalized magnetite nanoparticles. These nanocomposites can be removed conveniently from water with the help of an external magnet because of their exceptional properties. The nanocomposites were applied to remove heavy metal ions from water because chitosan that is inactive on the surface of the magnetic nanoparticles is coordinated with them. The interaction between chitosan and heavy metal ions is reversible, which means that those ions can be removed from chitosan in weak acidic deionized water with the assistance of ultrasound radiation. On the basis of the reasons referred to above, synthesized magnetic chitosan nanocomposites were used as a useful recyclable tool for heavy metal ion removal. This work provides a potential platform for developing a unique route for heavy metal ion removal from wastewater.

1. Introduction It is well known that heavy metal ions such as Pb2+, Cd2+, Hg2+, Ni2+, and Cu2+ can cause severe health problems in animals and human beings because they may specifically bind to proteins, nucleic acids, and small metabolites in living organisms. This causes either the alteration or loss of biological function and may perturb the homeostatic control of essential metals.1-3 For example, Pb2+ can obstruct heme biosynthesis, inhibit several zinc enzymes, interact with nucleic acids and tRNA to affect protein synthesis, and accumulate in the apatite structure of the bone.4-6 However, these toxic metal ions commonly exist in process waste streams from mining operations, metal-plating facilities, power generation facilities, electronic device manufacturing units, and tanneries.7 Thus, the removal of such toxic metal ions from wastewater is a crucial issue. Several methods, including chemical precipitation, ion exchange, liquid-liquid extraction, resins, cementation, and electrodialysis have been developed for the removal of the above heavy metal ions from industrial wastewater. Each method has been found to be limited by cost, complexity, and efficiency as well as by secondary waste. Using low-cost biosorbents such as agricultural wastes, clay materials, biomass, and seafood processing wastes may be an alternative wastewater technology because they are inexpensive * To whom correspondence should be addressed. E-mail: xwliu601@ yahoo.com.cn. † Anhui Normal University. ‡ Wannan Medical College.

(1) Silver, S. Microbes EnViron. 1998, 13, 177. (2) Martin, R. B. Met. Ions Biol. Syst. 1986, 20, 21. (3) Martin, R. B. In Handbook on Toxicity of Inorganic Compounds; Seiler, H. G., Sigel, H., Eds.; Marcel Dekker: New York, 1988; Chapter 2, p 9. (4) Kazakov, S. A.; Hecht, S. M. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; Wiley Interscience: Chichester, U.K., 1994; Vol. 5, p 2697. (5) Eichhorn, G. L. In Inorganic Biochemistry; Eichhorn, G. L., Ed.; Elsevier: Amsterdam, 1973; Vol. 2, p 1191. (6) Izatt, R. M.; Christensen, J. J.; Rytting, J. H. Chem. ReV. 1971, 71, 439. (7) Boddu, V. M.; Abburi, K.; Talbott, J. L.; Smith, E. D. EnViron. Sci. Technol. 2003, 37, 4449.

and capable of removing trace levels of heavy metal ions.8,9 However, to improve their absorption capacity and enhance the separation rate, the design of and exploration of novel adsorbents are still necessary. Recently, nanometer-sized hierarchically structured metal oxides have been used for wastewater treatment and have shown remarkable potential because those materials have large surface areas.10-12 From a practical point of view, there is a major drawback to the application of such nanomaterials for treating wastewater. Because the treatment of wastewater is usually conducted in a suspension of those nanostructures, it requires an additional separation step to remove such nanomaterials from solution. Removing such fine materials, especially nanostructures, from a large volume of water involves further expense. Furthermore, most studies have shown that those nanostructures have excellent adsorption capacities for toxic metal ions in water in the first cycle. The absorption ability of these nanomaterials in succeeding cycles is unclear, which is very important in practical applications. The recent successful synthesis of monodisperse magnetic nanoparticles, particularly iron oxide nanoparticles, provides a convenient tool for exploring magnetic separation techniques because of their specific characteristics. They have the capability to treat large amounts of wastewater within a short time and can be conveniently separated from wastewater; moreover, they could be tailored by using functionalized polymers, novel molecules, or inorganic materials to impart surface reactivity.13,14 For example, Xu and co-workers have shown that the bisphosphonate derivative modifies the (8) Guibal, E. Sep. Purif. Technol. 2004, 38, 43. (9) Babel, S.; Kurniawan, T. A. J. Hazard. Mater. 2003, B97, 219. (10) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. AdV. Mater. 2006, 18, 2426. (11) Zhong, L. S.; Hu, J. S.; Cao, A. M.; Liu, Q.; Song, W. G.; Wan, L. J. Chem. Mater. 2007, 19, 1648. (12) Hu, J.-S.; Zhong, L.-S.; Song, W.-G.; Wan, L.-J. AdV. Mater. 2008, 20, 2977. (13) Rocher, V.; Siaugue, J. M.; Cabuil, V.; Bee, A. Water Res. 2008, 42, 1290. (14) Banerjee, S. S.; Chen, D. H. J. Hazard. Mater. 2007, 147, 792.

10.1021/la802754t CCC: $40.75  2009 American Chemical Society Published on Web 11/25/2008

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Figure 1. Synthesis route of magnetic chitosan nanocomposites and their use as a facile tool for Pb2+ removal with the help of an external magnetic field.

magnetite nanoparticles and can remove 99 and 69% of UO22+ from water and blood, respectively.15 Deng’s group reported that Cu2+-immobilized magnetic nanoparticles could easily remove microcystins from water.16 Huang and co-workers demonstrated the potential of sugar-coated magnetic nanoparticles for fast bacterial detection and removal.17 Furthermore, vancomycin-modified magnetic nanoparticles can be extended to capture vancomycin-resistant enterococci (VRE) and other grampositive bacteria at low concentrations.18,19 In this work, magnetic chitosan nanocomposites were synthesized on the basis of aminefunctionalized magnetite nanoparticles. These nanocomposites have been used as a fast recyclable tool for Pb2+, Cu2+, and Cd2+ removal from water. The relationship among the pH value, adsorption capacity, regeneration of magnetic nanocomposites, and mechanism for absorption and desorption was studied using Pb2+ ions. It is noteworthy that these nanocomposites have a high removal efficiency of Pb2+ not only in the first cycle but also in succeeding cycles of the experiments.

2. Experimental Methods Materials. FeCl3 · 6H2O, Pb(NO3)2, ethylene glycol, 1,6-hexanediamine, ethanol, sodium borate, anhydrous sodium acetate, acetum, o-phthaldialdehyde, β-mercaptoethanol, glycine, glutaraldehyde, and chitosan were purchased from the Shanghai Chemical Reagent Company. All chemicals were analytical grade and used as received without further purification. Synthesis of Amine-Functionalized Magnetite Nanoparticles. Amine-functionalized magnetite nanoparticles were synthesized via a versatile solvothermal reaction reported by Li with a slight modification.20 Typically, 1.0 g of FeCl3 · 6H2O, 6.0 g of 1,6hexanediamine, and 2.0 g of anhydrous sodium acetate were added to 40 mL of ethylene glycol to give a transparent solution via vigorous (15) Wang, L.; Yang, Z.; Gao, J.; Xu, K.; Gu, H.; Zhang, B.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2006, 128, 13358. (16) Gao, M.; Deng, C.; Fan, Z.; Yao, N.; Xu, X.; Yang, P.; Zhang, X. Small 2007, 3, 1714. (17) El-Boubbou, K.; Gruden, C.; Huang, X. J. Am. Chem. Soc. 2007, 129, 13392. (18) Gu, H.; Ho, P.-L.; Tsang, K. W. T.; Wang, L.; Xu, B. J. Am. Chem. Soc. 2003, 125, 15702. (19) Lin, Y.-S.; Tsai, P.-J.; Weng, M.-F.; Chen, Y.-C. Anal. Chem. 2005, 77, 1753. (20) Wang, L.; Bao, J.; Wang, L.; Zhang, F.; Li, Y. Chem.sEur. J. 2006, 12, 6341.

stirring. This mixture was then transferred to a Teflon-lined autoclave (50 mL) for treated at 200 °C for 6 h. The products were obtained with the help of a magnet and washed with deionized water. Finally, the nanoparticles were stored in distilled water for further processing. Synthesis of Magnetic Chitosan Nanocomposites. In a typical synthesis, 2 mL of glutaraldehyde was added to 20 mL of the above magnetic nanoparticle suspension. This suspension then was irradiated by 40 kHz ultrasonic waves at 80% output power in the air for 20 min, and carbonyl-magnetite nanoparticles were obtained. Magnetic chitosan nanocomposites were obtained by adding the above carbonyl-magnetite nanoparticles to 20 mL of chitosan solution (0.15 g) with intensive stirring. It is worth noting that both carbonylmagnetite nanoparticles and magnetic chitosan nanocomposites were separated from the resulting solution with the help of an external magnet during the synthesis procedure in order to remove unreacted glutaraldehyde and chitosan. The whole synthesis procedure is presented in Figure 1. The final magnetic chitosan nanocomposites were dried under vacuum at 40 °C overnight. Investigation of the Removal Efficiency of Pb2+, Cu2+, and Cd2+ from Water. Typically, 15 mg of magnetic chitosan nanocomposites was added to 50 mL of 10 mg/L Pb2+ solution. Then this mixture was dispersed with the help of ultrasound radiation to ensure sufficient interaction between magnetic chitosan nanocomposites and Pb2+. During the duration of this ultrasound radiation, 4 mL of the turbid solution was removed from the mixture. After the magnetic chitosan nanocomposites were removed, the atomic absorption spectrum was recorded every 10 min. The removal efficiency of Cu2+ and Cd2+ was determined similarly. Before the atomic absorption spectra of the samples were recorded, calibration curves of Pb2+, Cu2+, and Cd2+ were constructed. The reactions during the experiment are also illustrated in detail in Figure 1. Investigation of the Relationship between pH Value and Adsorption Capacity. All performances in this section are similar to the above one under different initial pH values. The pH of Pb2+ solution was adjusted with 1 M HCl and 1 M NaOH solutions and controlled from 2 to 8. Desorption Study. Desorption studies were performed by dispersing used magnetic chitosan nanocomposites into 10 mL of weakly acidic deionized water (2 drops of acetum was added) with the assistance of sonication. Then, the magnetic chitosan nanocomposites were treated with deionized water to neutralize and recondition the samples for adsorption in succeeding cycles. Characterization Methods. The structure and morphology of as-synthesized amine-functionalized magnetite nanoparticles were characterized by X-ray diffraction (XRD; XRD-6000) and transmis-

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Figure 2. (a) Typical XRD pattern of as-prepared amine-functionalized magnetite nanoparticles. (b) IR spectra of amine-functionalized magnetite nanoparticles (I), pure chitosan (II), and magnetic chitosan nanocomposites (III). (c) TEM image of amine-functionalized magnetite nanoparticles. (d) TEM image of magnetic chitosan nanocomposites.

sion electron microscopy (TEM; JEOL-2010). Infrared (IR) spectra of amine-functionalized magnetite nanoparticles, pure chitosan, and magnetic chitosan nanocomposites were obtained on a Vectortm 22 Fourier transform infrared (FTIR) spectrometer (Bruke, Germany). The magnetic properties of the magnetic nanoparticles and nanocomposites were investigated using a vibrating sample magnetometer with an applied field of between -5000 and 5000 Oe at room temperature. The concentration of Pb2+ in all samples was measured using atomic absorption spectrometry (Thermo SOLAAR M6). The content of chitosan was determined by spectrophotometric assay (U-3100, Hitachi) based on the method reported by Chen previously.21

3. Results and Discussion The structure of amine-functionalized magnetite nanoparticles was characterized by X-ray diffraction. Figure 2a shows a typical XRD pattern of magnetite nanoparticles, and the peaks match well with pure magnetite reflections (JCPDS card no. 75-1610), which is consistent with the Li result.20 Curve I in Figure 2b shows the IR spectrum of the magnetic nanoparticles, indicating that many 1,6-hexanediamine molecules were immobilized on the surface of the nanoparticles. The peaks at 1629, 3446, and around 2852-2932 cm-1 are ascribed to an N-H stretching vibration of free 1,6-hexadiamine and the C-H stretching model of the alky chain. The major peaks for pure chitosan in Figure 2b (II) can be assigned as follows: 3452 cm -1 (O-H and N-H stretching vibrations), 1641 cm-1 (N-H deformation vibration), and 1397 cm-1 (C-H symmetric blending vibration).22 The IR spectrum of the magnetic chitosan nanocomposites demonstrated that a layer of chitosan was formed on the surface of carbonylmagnetite nanoparticles because their peaks are similar to those of pure chitosan. Figure 2c shows the TEM image of the aminefunctionalized magnetite nanoparticles, from which nearly (21) Chang, Y.-C.; Chen, D.-H. J. Colloid Interface Sci. 2005, 283, 446. (22) Li, N.; Bai, R. Ind. Eng. Chem. Res. 2005, 44, 6692.

monodisperse nanoparticles with a diameter of about 25 nm are observed. The TEM image of magnetic chitosan nanocomposites (Figure 2d) shows that the final products exhibit slight aggregation as a result of surface modification by the attachment of chitosan. The amount of chitosan was obtained on the basis of the reaction of amino groups of solids with excess o-phthaldialdehyde and the subsequent quantitative determination of unreacted ophthaldialdehyde by reaction with glycine. The content of chitosan could be estimated to be 5.6 mg/100 mg of Fe3O4, which is much lower than the value obtained from the increased weight after chitosan binding (5.8 mg). The main reason for this is that a small number of -NH2 groups were reacted with -CHO during the magnetic chitosan nanocomposites’ synthesis process. Magnetic measurements of amine-functionalized magnetic nanoparticles and magnetic chitosan nanocomposites were investigated with a vibrating sample magnetometer (VSM) at room temperature in the applied magnetic field sweeping from -5 to 5 kOe. The hysteresis loops of the magnetic nanoparticles and the magnetic nanocomposites are shown in Figure 3. It can be seen that both of them show ferromagnetic behavior. The magnetic saturation values of these are 80 and 74 emu/g, respectively. The decrease in magnetic saturation may be attributed to the increased mass of glutaraldehyde and chitosan on the surface of the magnetic nanoparticles. Such an excellent magnetic property means that as-prepared nanocomposites have strong magnetic responsivity and can be separated easily from the solution with the help of an external magnetic force. The investigation procedure of Pb2+ removal efficiency is also shown in Figure 1. To make magnetic chitosan nanocomposites and Pb2+ interact sufficiently and quickly, ultrasound radiation was employed to disperse magnetic chitosan nanocomposites into 50 mL of 10 mg/L Pb2+ solution. We removed 4 mL of the above mixture and recorded its atomic absorption

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Figure 3. Hysteresis loops of (a) amine-functionalized magnetic nanoparticles and (b) magnetic chitosan nanocomposites.

Figure 4. Photographs of the magnetic chitosan nanocomposite colloidal solution containing 10 mL/L Pb2+ (a) before and (b) after magnetic separation by an external magnetic field. The black powders attracted to the side of the vial by a magnet are the magnetic chitosan nanocomposites.

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Figure 6. Adsorption rate of Pb2+, Cd2+, and Cu2+ by magnetic chitosan nanocomposites.

Figure 7. Effect of initial pH on the removal of Pb2+ by magnetic chitosan nanocomposites.

Figure 8. Removal efficiency of Pb2+ in different cycles by magnetic chitosan nanocomposites.

Figure 5. Adsorption rate of Pb2+ by (a) magnetic chitosan nanocomposites, (b) amine-functionalized magnetite nanoparticles, and (c) carbonyl-magnetite nanoparticles.

spectrum every 10 min after the removal of the magnetic chitosan nanocomposites via the help of a magnet. Figure 4 shows photographs of the Pb2+ solution with dispersed magnetic chitosan nanocomposites (a) before and (b) after magnetic separation using an external magnetic field. This Figure also demonstrates the facile, fast separation process of the magnetic chitosan nanocomposites during the experiments. The results of the investigation of Pb2+ removal are shown in Figure 5 (curve a). We can see that the concentration of Pb2+ can be changed from 10 to 0.54

mg/L after 10 min of ultrasound radiation. The removal efficiency of lead is about 94.6%. The concentration of Pb2+ can be decreased further by increasing the duration of sonication. The concentrations of Pb2+ are about 0.53 and 0.52 mg/L after 20 and 30 min of ultrasound radiation, respectively. However, if we continually enhance the duration of ultrasound radiation, then the concentration of Pb2+ shows no obvious alteration. The concentration of Pb2+ is also about 0.52 mg/L after 40 min of ultrasound radiation. To confirm that the absorption effect is from the chitosan immobilized on the surface of magnetic nanoparticles, the absorption ability of amine-functionalized magnetite nanoparticles and carbonyl-magnetite nanoparticles was also investigated. Curves b and c of Figure 5 indicate that both of them have absorption capacity, with that of the former being higher than

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Figure 9. Schematic illustration of the mechanism for lead ion absorption and desorption.

that of the latter. The main reason is that both of them have large surface areas that can supply a large number of activity points for heavy metal ion absorption. However, the surface of the former has many -NH2 groups that can coordinate with lead ions,23 but the surface of the latter ones only has -CHO, which has poor coordination ability with metal ions. It is worth mentioning that in all samples no residual adsorbents were observed during experimentation, as evidenced by the fact that we did not detect any Fe in the concentrated resulting wastewater after treating with HCl. This is another important characteristic of magnetic separation techniques. It is well known that chitosan can absorb heavy metal ions through chelation by the amide groups on glucosamine.24 The absorption ability of magnetic chitosan nanocomposites for Cu2+ and Cd2+ is also investigated because they commonly exist in wastewater. Figure 6 shows the results, from which we can see that the final concentrations of Cu2+ and Cd2+ are about 1.09 and 0.79 mg/L, respectively. This conclusion means that the order of the adsorption capacity for the three metal ions is Pb2+ > Cd2+ > Cu2+, which is consistent with Sakairi’s results.25 The effect of pH on the adsorption of Pb ions by the magnetic chitosan nanocomposites is illustrated in Figure 7. It was found that the adsorption capacity increased upon increasing the solution pH; if the pH is increased further, then the adsorption capacity will be greatly reduced. When the initial pH varied from 2 to 4, the removal efficiency of Pb2+ increased from 36.8 to 95.3%, but when the initial pH increased from 4.0 to 6.0, the adsorption efficiency of Pb2+ decreased a little. If we enhance the pH from 6.0 to 8.0, the absorption ability decreases greatly. There are two main reasons for this phenomenaon. First, from the viewpoint of chitosan, it is known that the nonprotonated chitosan, having an unshared electron pair on the nitrogen atom, is capable of forming donor bonds with coordination unsaturated heavy metals, which can be demonstrated as

M2+ + nRNH2 T M(RNH2)n2+

(1)

It is found that the amino groups of chitosan may react with H+ according to (23) Battistuzzi, G.; Borsari, M.; Menabue, L.; Saladini, M. Inorg. Chem. 1996, 35, 4239. (24) Bailey, S. E.; Olin, T. J.; Bricka, R. M.; Adrian, D. D. Water Res. 1999, 33, 469. (25) Liu, X. D.; Tokura, S.; Haruki, M.; Nishi, N.; Sakairi, N. Carbohydr. Polym. 2002, 49, 103. (26) Anthonsen, M. W.; Varum, K. M.; Hermansson, A. M.; Smidsrod, O.; Brant, D. A. Carbohydr. Polym. 1994, 25, 13.

RNH2 + H+ T RNH3+

(2)

On the basis of the above equations, we can get that:

M2+ + nRNH3+ T M(RNH2)n2+ + H+

(3)

On the basis of eq (3), an increase in pH enhances the formation of the metal-chitosan complexes, thus, the absorption capacity of magnetic chitosan nanocomposites can be increased by enhancing the pH value. However, if a continued increase in pH occurs, chitosan will aggregate due to hard protonation of the amino groups in chitosan.26 It affects chitosan combination to Pb2+, so the adsorption capacity will descend. Second, it has been found that heavy metal ions may form as many polynuclear species in water. For example, when pH > 6, hydrolysis of Pb2+ will occur and it will exist as Pb2 (OH)3+ and Pb3 (OH)4+. Both of hydrolysis and polymerizing of metal ions will reduce the absorption capacity of the nanocomposites. Desorption studies were examined by dispersing used magnetic chitosan nanocomposites to 10 mL weak acetum solution and sonicated the mixture for 10 min. The concentration of Pb2+ in the eluent is about 32.32 mg/L. Then, the magnetic chitosan nanocomposites were treated with deionized water to neutralize and were explored for Pb2+ removal in the succeeding cycles. We have repeated above procedure up to six cycles. The removal efficiency in each cycle is shown in Figure 8. The removal efficiency is reduced gradually in the later cycles; however the removal efficiency is still above 93% in the final cycle. The mechanism for lead absorption and desorption is illustrated in Figure 9 in detail. In Pb2+ ions solution, Pb2+ ions are grabbed by two -OH groups and one -NH2 groups of chitosan. If the nanocompoistes are separated at this stage, metal ions will remove from solution. However, under acidic condition, H+ can cause the protonation of amino groups, which means that part of sites occupied by metal ions may replace by H+. After being washed with deionized water to neutralize, the absorption capacity of the magnetic chitosan nanocomposite can be reconditioned. Thus, the recycled magnetic chitosan nanocomposites have high capacity for Pb2+ removal in each cycle.

4. Conclusions In summary, the magnetic chitosan nanocomposites have been synthesized on the basis of amine-functionalized magnetite nanoparticles. These nanocomposites provide a very efficient, fast, and convenient tool for removing Pb2+, Cu2+,

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and Cd2+ from water. Because the interaction between chitosan and heavy metal ions is reversible, such metal ions can be released from chitosan under weakly acidic condition with the help of ultrasound radiation. Thus, as-prepared magnetic chitosan nanocomposites can be used as a recyclable tool for heavy metal ion removal. Acknowledgment. This work was supported by the College Natural Science Foundation of Anhui Province (nos.

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KJ2008B168 and KJ 2008B167), NSFC (no. 20701002), and the Education Department of Anhui Province (no. 2006KJ006TD). Supporting Information Available: Calibration curves of Pb2+, Cu2+, and Cd2+. This material is available free of charge via the Internet at http://pubs.acs.org. LA802754T