Preparation of Uniform Magnetic Chitosan Microcapsules and Their

Article pubs.acs.org/IECR

Preparation of Uniform Magnetic Chitosan Microcapsules and Their Application in Adsorbing Copper Ion(II) and Chromium Ion(III) Sai Zhang, Yifeng Zhou,* Wangyan Nie, Linyong Song, and Ting Zhang Anhui Province Key Laboratory of Environment-Friendly Polymer Materials, College of Chemistry & Chemical Engineering, Anhui University ABSTRACT: Biocompatible and biodegradable magnetic chitosan microcapsules were successfully prepared using carboxylfunctionalized polystyrene (PS) particles as core template. First, the monodisperse PS template with an average diameter of 340 nm was made by emulsifier-free emulsion polymerization. Second, the chitosan (CS) adsorbed onto the surface of the PS template and cross-linked by glutaraldehyde. After removal of the PS core, CS microcapsules could be obtained. The shell thickness of CS could be controlled between 20 and 45 nm by varying the adsorption temperature. Third, the CS microcapsules were used as a microreactor, and iron oxide nanoparticles were incorporated within CS microcapsules. The structure and morphology of the PS template, core−shell PS/CS particles, and magnetic CS microcapsules were characterized by Fourier transform infrared spectroscopy and transmission electron microscopy. The adsorption of Cu2+ and Cr3+ was investigated including the effects of adsorption time, pH value, temperature, and initial concentration by the batch method. The optimal condition for adsorption of Cu2+ and Cr3+ was 30 °C, with a pH value of 7.0, where the saturated adsorption capacity was 104 mg/g and 159 mg/g, respectively. The adsorption isotherms obeyed the Langmuir equation, and kinetics followed a pseudosecond-order model.

1. INTRODUCTION At present, environment contamination by heavy metal ions has received extensive attention. Copper and chromium are used in several industrial application and recognized as agents that present toxic effect to humans and other living beings even in very low concentration,1 but they are still hard to separate from wastewater. The main techniques that have been used on heavy metal ions reduction from wastewater are chemical precipitation, ion exchange, membrane filtration, electrolytic methods, reverse osmosis, solvent extraction, and adsorption.2,3 Adsorption techniques have been shown to be a feasible option, both technically and economically.4 The application of biopolymers as adsorbents is an emerging technique and is of interest in studies on the removal of heavy metal ions from aqueous solution.5 Biopolymers also have several advantages in comparison with other activated carbon and molecular sieve techniques, such as being hydrophilic, biocompatible, and biodegradable.6 Chitosan(CS) is an abundant biopolymer obtained after deacetylation of chitin and has been reported to have high potential for adsorption of metal ions.7 Because of its high amino content and the presence of hydroxyl groups, CS can act as a chelating agent for heavy metal ions.8,9 The major limitation of chitosan is its solubility in most dilute mineral and organic acid solutions, but cross-linking agents, such as glutaraldehyde10 or genipin,11 could be used to improve its chemical stability under acidic conditions. To improve its absorption capacity and enhance the separation rate, the design of and exploration of novel adsorbents are still necessary. Owing to low density, higher specific surface area, delivering ability, surface performance, and lower internal diffusion resistance,12−14 hollow structures of absorbents have quite a good performance compared to the traditional adsorbents used in separation process. © 2012 American Chemical Society

There are different techniques of producing hollow CS microspheres, such as layer-by-layer, cross-linking, emulsification, solvent evaporation, and spray drying. However, for most of these techniques, the size of droplets or microspheres is difficult to control and size distribution is very broad. If the size distribution of particles is narrow, physical and chemical properties are more uniform. The template method is an effective way to prepare uniform hollow microspheres and often needs a template such as spherical silica,15 micelles,16 calcium carbonate,17 or polymer particles,18 or a miniemulsion technique.19 Polystyrene (PS) particles are the traditional core templates to prepare hollow microspheres, because they are uniform, monodisperse, and easily removed. If the PS particles decorate by −COOH, the carboxylic acid groups on the surface can interact with CS. After removing the PS core, hollow CS microspheres can be obtained. There are many advantages for hollow CS microspheres used as adsorbents, but it is hard to separate them from aqueous solution by filtration or centrifugation. Magnetic adsorbents can be manipulated by an external magnetic field and hence facilitate the solid−liquid separation.20 Here, we propose a new strategy to fabricate uniform-sized, biocompatible, and biodegradable magnetic CS microcapsules, which can be potentially used to remove the heavy metal ions from wastewater (Scheme 1). Using this strategy, the monodisperse carboxyl-functionalized PS particles are used as the sacrificial cores, and a CS shell can be formed by electrostatic force between the amino groups and the carboxylic Received: Revised: Accepted: Published: 14099

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stirring and made into 1.0 wt % CS solution. A 1.0 g portion of PS powder was redispersed in 10 mL of water and dropped into the 1.0 wt % CS solution with a buret under stirring. The reaction was continued for 2 h at a definite temperature (20, 30, 40, and 50 °C). The mixture was subjected to centrifugation at 8000 rpm for 10 min and redispersed in water by sonication and centrifuged for 10 min. This dispersion−centrifugation− collection cycle was repeated four times to remove the excess CS which was not adsorbed on the surface of carboxylfunctional PS particles. The resulting core−shell particles were treated with 1.0 mL og 2.5% GA for 2 h at 40 °C. 2.4. Preparation of Magnetic CS Microcapsules. Core− shell PS/CS particles (100 mg) were washed with water and anhydrous THF three times. And then they were added to the carbonate buffer (0.2 M, pH = 9). After 12 h, they were washed with water three times. The procedure was repeated to guarantee the complete removal of water-soluble oligomer. So, the CS microcapsules were obtained. The iron oxide nanoparticles were prepared in situ synthesis within CS microspheres. The standard recipes for the preparation of magnetic CS microcapsules were listed in Table 1. The CS microcapsules were suspended in 10 mL of FeCl2·4H2O and FeCl3·6H2O solution to make the Fe2+ and Fe3+ solution infiltrating into the microcapsules. Then the microcapsules were centrifuged for 10 min at 8000 rpm and washed with water for three times, and then they were dispersed in 10 mL of 0.1 M NH3·H2O at 70 °C for 2 h. Finally, magnetic CS microcapsules were collected from the aqueous phase by magnetic separation and were washed by distilled water for three times. 2.5. Adsorption Experiment. Adsorption experiments were conducted by varying adsorption time, pH value, initial Cu2+ or Cr3+ concentration, and temperature under the aspects of adsorption isotherms and adsorption kinetics. For the adsorption time studies, 100 mg of magnetic CS microcapsules were first dispersed into 80 mL of water by ultrasound; 80.2 mg CuCl2·2H2O or 153.7 mg CrCl3·6H2O was dissolved in 20 mL of water and added in the magnetic CS microcapsules solution. And then, the mixed solution was stirred at 200 rpm and 30 °C for 3 h. A 5 mL aliquot of the solution was taken every 30 min, and the magnetic CS microcapsules were separated by centrifugation at 8000 rpm for 10 min. The pH value of solution was adjusted by HCl (0.1 mol/L). For adsorption isotherms studies, 50 mL of Cu 2+ or Cr3+ solutions (concentration varied from 50 to 300 mg/L) with 0.4 g/L adsorbent was agitated at varying temperatures of 20, 30, and 40 °C. The amount of Cu2+ or Cr3+ adsorbed onto magnetic CS microcapsules, qe (mg/g) was calculated by the following equation:

Scheme 1. Schematic for Preparation of Cross-Linked Magnetic CS Microcapsules

acid groups. The CS microcapsules can be readily produced by removal of the PS core with tetrahydrofuran (THF). The shell thickness of CS microcapsules can be controlled through the adsorption temperature. The CS microcapsules can be used as a microreactor, and Fe2+ and Fe3+ solution can infiltrate into the CS microcapsules. Then iron oxide nanoparticles could be obtained in the CS microcapsules by adding NH3·H2O. Additionally, the adsorption behavior of magnetic CS microcapsules is studied. The effects of various parameters such as adsorption time, pH value, temperature, and initial concentration on adsorption are investigated. The adsorption kinetics and isotherms for Cu2+ and Cr3+ onto magnetic CS microcapsules are also discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. All the materials were purchased from Sinopharm Chemical Reagent Co., Ltd. Styrene (St) and acrylic acid (AA) were distilled under reduced pressure. Potassium persulfate (KPS) was recrystallized from distilled water. Chitosan (CS, degree of deacetylation is 90%), glutaraldehyde (GA), tetrahydrofuran (THF), acetate acid, FeCl3·6H2O, FeCl2·4H2O, CuCl2·2H2O, CrCl3·6H2O, HCl, ammonium hydroxide (NH3·H2O) were used as received. 2.2. Preparation of Monodisperse Carboxyl-Functionalized PS Particles. The monodisperse carboxyl-functionalized PS particles were prepared via traditional emulsifier-free polymerization of St and AA. St (10.0 g) and AA (1.0 g) were added to 95 mL of water in a 250 mL three-neck round-bottom flask equipped with a mechanical stirring rod, a nitrogen inlet, and a condenser. After the mixture was deoxygenated by bubbling nitrogen gas at room temperature for 0.5 h, the flask was placed in a 75 °C water bath and stirred mechanically. An aqueous solution of KPS (0.1 g in 5 mL of water) was subsequently added to the reacting medium, all reactions being under a nitrogen atmosphere. The reaction was continued for 16 h to ensure a maximum reaction. PS powder was harvested by centrifugation of the mixture at 8000 rpm for 10 min, followed by washing with ethanol three times. 2.3. Preparation of Core−Shell PS/CS Particles. CS (0.1 g) was dissolved in 10 mL of 2.0 wt % acetate acid under

qe =

V (C 0 − C ) m

(1)

where C0 and C are the liquid-phase concentrations of Cu2+ or Cr3+ at initial and equilibrium time, respectively (mg/L); V is

Table 1. The Standard Recipes for the Preparation of Magnetic CS Microcapsule

A-1 A-2 A-3 A-4

CS microcapsules (mg)

FeCl2·4H2O (mg)

FeCl3·6H2O (mg)

infiltrating time (h)

saturation magnetization/emu (g)

100 100 100 100

200 200 400 200

0 0 0 200

4 8 4 4

5.62 6.19 7.91 8.89

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the volume of the solution (L); m is the mass of dry adsorbent used (g). 2.6. Characterizations. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 870 spectrometer. The samples were washed with ethanol and dried in a vacuum. The gained particles were mixed with KBr and pressed to a thin plate for measurement. Transmission electron microscopy (TEM; JEM, 2100, 200 kV, Hitachi) was used to observe the morphology of samples. Samples were dispersed in ethanol and then dried onto Formvar-coated copper grids before examination. The magnetic properties of magnetic CS microcapsules were measured at room temperature using a BHV-55 vibrating sample magnetometer.

Figure 2. TEM image of carboxyl-functionalize PS particles.

3. RESULTS AND DISCUSSION 3.1. Characterization of Monodisperse CarboxylFunctionalized Polystyrene Particles. To confirm the copolymerization of St and AA during the preparation of carboxyl-functional PS particles, the FT-IR spectrum of the sample PS particles is shown in Figure 1a. Absorbance peaks at

Figure 3 shows the TEM images of core−shell PS/CS particles, which are obtained at different adsorption temper-

Figure 3. TEM images of core−shell PS/CS particles for the adsorption temperature at (a) 20 °C, (b) 30 °C, (c) 40 °C, and (d) 50 °C.

atures. Obviously, all the particles exhibit the core−shell structure. The average diameter of core−shell PS/CS particles, 380 nm of 20 °C, 385 nm of 30 °C, 370 nm of 40 °C, and 360 nm of 50 °C (measured and calculated the average value from Figure 3a,b,c,d) are all bigger than the PS template (340 nm). When adsorption temperature increases, the thermal motion of CS and PS is also strong. So, more CS could be adsorbed on the PS and the average diameter at 30 °C is bigger than that at 20 °C. However, the adsorption reaction is exothermic, so increasing the temperature results in a decrease of the CS adsorption on the PS core and the average diameter decreases. 3.3. Characterization of Magnetic CS Microcapsules. Here, we choose the core−shell PS/CS particles obtained at 30 °C to prepare magnetic CS microcapsules. After removing the PS core by THF, CS microcapsules were used as a microreactor to load iron oxide nanoparticles via the chemical coprecipitation method. Figure 4a−d shows the magnetic CS microspheres prepared at different reaction conditions. It is clear that the hollow-structured microspheres are formed, but the CS shell is not rigid enough to keep the original shape and partially collapsed. The diameter of hollow CS particles is 240 ± 20 nm which is smaller than the size of PS template, and this is ascribed to the shrink of the CS shell during the gradual drying process in air. The shell thickness of magnetic CS microspheres is about 40 nm.

Figure 1. The FI-IR spectra of (a) carboxyl-functionalize PS particles and (b) core−shell PS/CS particles.

3030, 1605, 1495, 1448, 758, and 702 cm−1 correspond to the phenyl groups; the peaks at 2920 and 2850 cm−1 correspond to the methylene and methenyl groups. The characteristic peak at 1705 cm−1 is assigned to the carbonyl group of carboxylfunctionalized PS particles. The morphology and size of carboxyl-functionalized PS particles are investigated by TEM (Figure 2). From the TEM image, we can see that all PS particles are spherical with an average diameter of 340 nm and a very narrow size distribution. 3.2. Characterization of Core−Shell PS/CS Particles. We use the carboxyl-functionalized PS particles as the template to absorb CS and study the effect of absorption temperature on the structure of core−shell PS/CS particles. The FT-IR spectrum of core−shell PS/CS particles is shown in Figure 1b. The strong absorbance peaks at 3030, 1603, 1495, 1453, 758, and 696 cm−1 correspond to the phenyl groups, the characteristic peak at 1705 cm−1 is assigned to the carbonyl group of PS particles. A new peak appeared at 1640 cm−1, indicating the formation of Schiff’s base structure. So, CS was adsorbed on the PS surface, and core−shell PS/CS particles are obtained. 14101

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3.4. The Effect of Adsorption Time. Figure 6 shows the effect of adsorption time on the adsorption capacity (qe, mg/g) of Cu2+ and Cr3+ by CS, core−shell PS/CS particles and magnetic CS microcapsules. The adsorption amounts increased with the increasing adsorption time by each adsorbent. In the case of Cu2+, the maximum adsorption is attained in 120 min, while for Cr3+ it takes 150 min, after that the adsorption reached equilibrium. The adsorption time of 150 min was found to be sufficient to reach equilibrium. The adsorption capacity of magnetic CS microcapsules of Cu2+ is 104 mg/g and that of Cr3+ is 154 mg/g, which is larger than that for CS and core−shell PS/CS particles. It has been reported that the adsorption capacity for Cr3+ or Cu2+ is below 100 mg/g by other bioadsorbents based on chitosan.8,21,22 So, the magnetic CS microcapsules have some advantage compared with other structure of chitosan bioadsorbents. 3.5. The Effect of pH Value. The pH value of the aqueous solution is an improved parameter in adsorption processes. In this study, the effect of pH value on the adsorption of Cu2+ and Cr3+ onto magnetic CS microcapsules was studied in the pH value range of 3−7 and shown in Figure 7. It is observed that the saturated adsorption capacity increases with an increase of solution pH value. Amine and hydroxyl groups are highly reactive with metal ions, which is due to nitrogen of amino group and the oxygen of hydroxyl group hold free electron doublets and can react with metal ions.23 The adsorption mechanism of heavy metal ions by CS has been reported and was contributed to the weak complexation of the hydroxyl and the N atom in the −NH2 groups24 and the possible mechanisms are schematically shown in Figure 7c (take Cu2+ for example). However, at acid conditions the amine groups will be completely covered with hydronium ions, which compete strongly with metal ions for adsorption sites. With the increase of pH value, the concentration of H3O+ ions decrease and facilitate the adsorption of metal ions via electrostatic interaction. 3.6. Adsorption Isotherms. The adsorption isotherm can indicate how the adsorbent molecules distribute between liquid and solid phase when the adsorption process reaches an equilibrium state. It is well-known that the adsorption capacity could enhance with the increasing initial metal ions’ concentration. The influence of initial Cu 2+ and Cr 3+ concentration on the adsorption capacity of magnetic CS microcapsules is shown in Figure 8. It is clear that the adsorption capacity increases with increasing initial Cu2+ or

Figure 4. TEM images of magnetic CS microcapsules for (a) A-1, (b) A-2, (c) A-3, and (d, e) A-4.

From Figure 4e (high magnification image of A-4), we can clearly see that the iron oxide nanoparticles with an average diameter about 10 nm are enwrapped in the CS microspheres, with few on the surface. Figure 5a shows the magnetic properties of magnetic CS microspheres. The magnetic CS microspheres show ferromagnetic behavior at room temperature. The saturation magnetization of magnetic CS microspheres is 5.62 emu/g of A-1, 6.19 emu/g of A-2, 7.91 emu/g of A-3, and 8.89 emu/g of A-4. The magnetic CS microspheres with high saturation magnetization are easily separated from aqueous solution by an external magnetic field and hence facilitate solid−liquid separation. Figure 5 panels b and c show photographs of the magnetic CS microcapsules (A-4) solution before and after 6 h magnetic separation by an external magnetic field. As can be seen from the photographs, the magnetic CS microcapsules are completely attracted to the magnet after 6 h. So, the magnetic CS microcapsules could be separated from solutions by the facile separation process, and A-4 is used to adsorb metal ions.

Figure 5. (a) The magnetization curve of magnetic CS microcapsules at room temperature and photographs of the magnetic CS microcapsules (A-4) solution (b) before and (c) after 6 h magnetic separation by an external magnetic field. 14102

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Figure 6. Effect of adsorption time on the adsorption of (a) Cu2+ and (b) Cr3+ by CS, core−shell PS/CS particles and magnetic CS microcapsules.

Figure 7. Effect of pH value on adsorption capacity of (a) Cu2+ and (b) Cr3+ by magnetic CS microcapsules; (c) the possible mechanism for adsorption of Cu2+.

Figure 8. The influence of initial (a) Cu2+ and (b) Cr3+ concentration on the adsorption capacity of magnetic CS microcapsules.

Figure 9. Langmuir isotherm of (a) Cu2+ and (b) Cr3+ on magnetic CS microcapsules.

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Table 2. Constants of Langmuir Isotherms for Adsorption of Cu2+ and Cr3+ on Magnetic CS Microcapsules Cu2+

Cr3+ 2

temperature (°C)

qe,exp (mg/g)

qmax (mg/g)

b

R

qe,exp (mg/g)

qmax (mg/g)

b

R2

20 30 40

101 104 96

110 112 107

0.0331 0.0386 0.0248

0.9982 0.9987 0.9831

151 159 154

175 185 182

0.0191 0.0181 0.0182

0.9967 0.9915 0.9872

where k1 is the rate constant of first-order (min−1), qe and qt are adsorption capacity (mg/g) in the equilibrium and at time t (min), respectively. The pseudo-second-order equation was represented by t 1 t = + 2 qt qe k 2qe (6)

Cr3+ concentration, because the higher initial Cu2+ or Cr3+ concentration is, the greater is the driving force at the solid− liquid interface. It is also seen that the magnetic CS microcapsules have the biggest adsorption capacity at 30 °C. With an increase of temperature, the thermal motion of CS chains is strong and the functional groups (amino groups and hydroxyl groups) show a relatively high activity. So, the adsorption reaction happens easier, and the adsorption capacity increases. At the same time, the adsorption process is exothermic;7,22 the adsorption capacity decreases with the increasing of adsorption temperature. So there the biggest adsorption capacity is at 30 °C. The widely used Langmuir model has been found to successfully fit the process. The equation can be expressed as qe =

Where k2 is the rate constant of the pseudo-second-order model (g·mg−1·min−1). The intraparticle diffusion equation is used to verify whether the diffusion mechanism is the limiting step of the adsorption process. It is represented by

qt = K pt 1/2 + C

qmax bCe 1 + bCe

Ce 1 1 = Ce + qe qmax qmax b

Where K P is the intraparticle diffusion rate constant (mg·g−1·min−1/2) and C is related to the boundary layer thickness (mg·g−1). All of the parameters mentioned above are determined and listed in Table 3. The graphical presentations for pseudo-first-

(2)

(3)

Table 3. Pseudo-first-order, pseudo-second-order, and intraparticle diffusion models parameters for the adsorption of Cu2+ and Cr3+ on the magnetic CS microspheres

where b is a constant of adsorption equilibrium, in liter per milligrams and qmax is the saturated adsorption capacity (mg/g). The Langmuir isotherm fitting line is shown in Figure 9, and the corresponding parameters are stated in Table 2. As seen from Table 2, the correlation coefficient (R2) is close to 1, and the saturated adsorption capacity (qmax) from Langmuir model is close to the experimentally obtained adsorption capacity (qe,exp). It can be concluded that the monolayer Langmuir adsorption isotherm is suitable to explain the adsorption of Cu2+ and Cr3+ onto magnetic CS microcapsules. The degree of suitability of resin toward metal ions is estimated form the values of the separation factor constant (RL), which gives indication for the possibility of the adsorption process to proceed. RL > 1 is unsuitable; RL = 1 is linear; 0 < RL < 1 is adsorption; RL = 0 is irreversible.9 The values of RL can be calculated from the relation RL =

1 1 + bCe

pseudo-first-order metal 2+

Cu Cr3+

pseudo-secondorder

intraparticles difusion

qe,exp (mg/g)

qe,cal (mg/g)

R2

qe,cal (mg/g)

R2

R2

105 154

59 102

0.9488 0.9377

125 196

0.9977 0.9993

0.9499 0.9767

order, pseudo-second-order and intraparticle diffusion are given in Figure 10. The results of pseudo-second-order kinetics show that the linear fit with extremely high correlation coefficients (R2 > 0.99) are obtained for Cu2+ and Cr3+. Moreover, the calculated qe values also close to the experimental data in the case of pseudo-second-order kinetics. These results show that the rates of adsorption conform to pseudo-second-order kinetics. This suggests that the adsorption of Cu2+ and Cr3+ onto magnetic CS microcapsules may consist of two processes: the first process is interpreted to be the instantaneous adsorption stage or external surface adsorption. The second process is interpreted to be the gradual adsorption stage where intraparticle diffusion controls the adsorption rate until finally the metal uptake reaches equilibrium.7 3.8. Adsorption from Binary Metal Systems. The competitive adsorption is carried out in the binary system. The initial concentration of each metal ion in the mixed solution was 100 mg/L. The results are shown in Figure 11. In binary system, the obtained adsorption processes are similar to the results obtained in the unitary system; but the adsorption capacity has a 43% and 38% decrease for Cu2+ and Cr3+, respectively. The decrease of adsorbed amount of Cu2+ or Cr3+ in binary solutions can be attributed to the competition

(4)

where b is the Langmuir equilibrium constant and Ce is the initial concentration of metal ion. The values of RL lie between 0.0915 and 0.4464 for Cu2+, 0.1486 and 0.5236 for Cr3+. It indicates that the adsorption of Cu2+ and Cr3+ on magnetic CS microcapsules is favorable. 3.7. Adsorption Kinetics. To examine the controlling mechanism for the adsorption process, kinetic models have been used to assess the experimental data. The pseudo-firstorder, pseudo-second-order, and intraparticle diffusion models were employed to interpret the experimental data. The pseudo-first-order equation was represented by ⎛q − q ⎞ t⎟ ln⎜⎜ e ⎟ = −k1t q ⎝ ⎠ e

(7)

(5) 14104

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Figure 10. (a) Pseudo-first-order, (b) pseudosecond-order, and (c) intraparticle diffusion models for the adsorption of Cu2+ and Cr3+ on magnetic CS microcapsules.

the desorbing solution for Cu2+ or Cr3+-loaded by magnetic CS microcapsules.26 The capacities of the magnetic CS microcapsules for Cu2+ and Cr3+ in the adsorption−desorption− adsorption cycles are shown in Figure 12. It can be seen that

Figure 11. Effect of adsorption time on the adsorption capacity of (a) Cu2+ and (b) Cr3+ by magnetic CS microcapsules in the binary system.

between Cu2+ and Cr3+. In binary solutions, it has been reported that the synergistic effect may be take place with several functional groups, available in bioadsorbents based on chitosan.25 But, the adsorption sites of magnetic CS microcapsules do not increase, so the adsorbed amounts of heavy ions in binary solutions do not increase compared with that in unitary solutions. When the two metals are present, the adsorption capacity of Cr3+ is still bigger than that of Cu2+, which is in accord with the unitary metal adsorption. 3.9. Desorption and Reuse. For potential application, it is important to examine the possibility of desorbing the metal ions adsorbed on magnetic CS microcapsules. The mechanism of adsorption Cu2+ and Cr3+ by magnetic CS microcapsules is the weak complexation of the hydroxyl and the N atom in the −NH2 groups. Usually, a HCl aqueous solution is selected as

Figure 12. Adsorption−desorption cycles of Cu2+ and Cr3+ on the magnetic CS microcapsules.

the adsorption capacity of magnetic CS microcapsules decreases with the increase of the time for reuse. For desorption conduct with HCl solution, it is found that the Cu2+ and Cr3+ adsorbed on magnetic CS microcapsules are easily desorbed. The adsorption capacity of magnetic CS microcapsules is a 14% and 19% decrease for Cu2+ and Cr3+ after the third cycle and the desorption efficiencies reach about 84% and 82% after the first cycle, respectively. These results show that the magnetic CS microcapsules can be successfully regenerated and repeatedly used in Cu2+ and Cr3+ adsorption. 14105

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4. CONCLUSION Magnetic CS microcapsules, which are biologically compatible, biodegradable, and with high adsorption capacity, were successfully fabricated using PS particles as template. Our novel strategy provides a general method to synthesize monodisperse magnetic microcapsules from natural polysaccharides, and the magnetic CS microcapsules are highly efficient in removing heavy metal ions from aqueous solution. The thickness of the CS shell is determined by the adsorption temperature. The magnetic CS microcapsules show ferromagnetic behavior at room temperature, and the highest saturation magnetization is 8.89 eum/g. The magnetic CS microcapsules are highly effective in removing heavy ions. The adsorption of Cu2+ and Cr3+ is best at 30 °C while the pH value is 7.0, where the saturated adsorption capacity is 104 mg/g and 159 mg/g, respectively. The adsorption isotherms of Cu2+ and Cr3+ obey the Langmuir equation, and the kinetic data follow a pseudo-second-order model. The adsorption−desorption cycle results demonstrate that the regeneration and subsequent use of magnetic CS microcapsules would enhance the economics of practical applications for the removal of Cu2+ and Cr3+ from wastewater.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-551-5107614-632. Fax +86-551-5108963. Address: No.3, Feixi Road, Hefei, Anhui, China 230039. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for joint support by the 211 Project of Anhui University, Anhui Provincial Natural Science Foundation (1208085QB38) and The Foundation for Outstanding Young Talent in University of Anhui Province (2012SQRL024).

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REFERENCES

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dx.doi.org/10.1021/ie301942j | Ind. Eng. Chem. Res. 2012, 51, 14099−14106