Preparation and Adsorption Ability of Polysulfone Microcapsules Containing Modified Chitosan Gel

TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 03/20 pp535-541 Volume 10, Number 5, October 2005

Preparation and Adsorption Ability of Polysulfone Microcapsules Containing Modified Chitosan Gel* CHEN Fei (ч

‫)׆‬, LUO Guangsheng (৭‫ڜ‬ಓ)**,

YANG Weiwei (ཷฟฟ), WANG Yujun (ฆံࢋ) State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Abstract: Chemically modified chitosan beads containing polyethyleneimine (PEI) were prepared to improve the metal ion adsorption capacity of the chitosan beads and their mechanical stability and to limit their biodegradability. The modified beads were encapsulated with the polymer material polysulfone by a novel surface coating method named the emulsion phase inversion method. The adsorption properties of the modified beads and the microstructures of the polysulfone coating layer were then analyzed. The experimental results showed that the PEI was successfully linked onto the chitosan beads. The density of the üNH2 groups in the modified beads was significantly increased, while the water content was reduced. The coating layer thickness was about 200 Pm. The modified chitosan gel beads had excellent Cu(II) adsorption capacity, with a maximum Cu(II) adsorption capacity 1.34 times higher than that of the unmodified beads. The results show that even with the polysulfone coating the adsorption kinetics of the modified beads is still better than those of the unmodified beads. The modifications improve the mass transfer performance of the chitosan beads as well as the bead stability. Key words: chitosan; coating; chemical modification; adsorption

Introduction Chitosan is the deacetylated form of one of the most abundant natural polymers, chitin. As a biopolymer, it is readily processible into membranes, hollow fibers, and beads as well as sponges from its aqueous acid solution[1,2]. Chitosan’s unique properties make it useful for a broad variety of industrial and biomedical applications, especially as a chelating agent[3,4]. The hydroxyl and amino groups enable chitosan to effectively adsorb various organic compounds, including organic  γ

Received: 2004-10-21 Supported by the National Natural Science Foundation of China (No. 20176022)

γγ To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62783870

acids, acidic drugs, polychlorinated biphenyls, proteins, and dyes[5-8]. In addition, chitosan and its derivatives are effective separation agents for adsorption and removal of transition metal ions, such as Cu(II), Cr(III), Hg(II), Cd(II), Zn(II), and Ni(II) ions[9-11]. However, chitosan cannot stand long use due to its poor mechanical stability and its biodegradability[12]. Although cross-link processing has been found to effectively improve the chitosan stability[13], the life cycles of the modified material are still limited. Moreover, the cross-linking reduces the density of the functional groups on the modified material. Therefore, alternate processing methods are needed to improve performance and stability. Recently, microencapsulation techniques have been shown to be useful for metal ion extraction by encapsulating extracting agents (liquid or solid) with

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large specific interfacial areas, higher metal selectivities, minimal use of organic solvents, and easy phase separation by filtration or sedimentation methods[14,15]. Surface coating is also a desirable method to improve material stability. Chen et al.[16,17] prepared polysulfone microcapsules containing chitosan beads using the emulsion phase inversion method which greatly improved the mechanical stability of the encapsulated chitosan gel beads. However, the mass transfer kinetics of the coated beads was reduced and the adsorption of Cu(II) ions was lower than that of the uncoated gel[17]. In this study, polyethyleneimine (PEI) is used as the functional material in the emulsion phase inversion method to improve both the adsorption capacity and the mechanical stability of chitosan beads. The mass transfer performance of polysulfone microcapsules containing modified chitosan gel was then investigated.

1 1.1

Materials and Microcapsule Preparation Materials

Chitosan with a deacetylation degree of 0.952 and an average relative molecular mass of 1.5h106 kD was purchased from Yuhuan Ocean Biochemical Co., Ltd. (China). Polysulfone (PSF) with an intrinsic viscosity of 0.56 was purchased from the Boxiong Chemical Industry (China). Ethylene glycol diglycidyl ether (EGDE) and polyethyleneimine were obtained from TCI (Tokyo Kasei Kogyo Co., Ltd, Japan). SPAN 80, SPAN 20, cupric nitrate, n,n-dimethylformamide (DMF), paraffin, epichlorohydrin, and other reagents were purchased from VAS Chemical Co., Ltd (China). All materials were used as received without any further purification. 1.2

Preparation of chitosan gel beads

To prepare unmodified chitosan gel beads (CTS), a chitosan solution was prepared by dissolving 5 g of chitosan in 100 mL, 2 wt.% acetic acid. This solution was dropped through a nozzle into a gelation medium composed of 20 wt.% sodium hydroxide, 30 wt.% methanol, and 50 wt.% deionized water. The formed chitosan beads were retained in this solution for 1 h and then rinsed with deionized water. The chitosan was then cross-linked chemically using 5% EGDE in a

water-ethanol mixing solution for 12 h at room temperature, then rinsed with ethanol and then with deionized water. The chitosan gel beads were modified by polyethyleneimine (PEI-CTS) using 50 g of cross-liked CTS washed several times with pure isopropanol, following by a reaction in 50 mL solution of 34 wt.% epichlorohydrin and 66 wt.% isopropanol for 2 h at 50ć. The CTS modified by epichlorohydrin was put into 50 mL of 30 wt.% PEI solution, and reacted with PEI for 3 h at 80ć. Finally, the PEI-CTS beads were washed with ethanol and then with deionized water. Figure 1 shows the chemical modification process. The free üNH2 group content, Camino, in the CTS and the PEI-CTS was determined using titration[18]. The water content, Cwater, in the beads was measured by weighting. 1.3

Preparation of PSF microcapsules containing chitosan beads

The encapsulation process used the emulsion phase inversion method, which was described in detail previously[16,17]. Figure 2 shows a simple schematic preparation procedure of the PSF coated CTS, PSF-CTS, and the PSF coated PEI-CTS, PSF-PEI-CTS. Emulsion A was prepared with paraffin as the continuous phase, Span 80 as the emulsifier, and PSF solution as the dispersed phase. Then the chitosan gel beads were added into the emulsion. After the desired time, another water-in-oil emulsion composed of water and paraffin was added into the mixture, which gelated all the PSF micro droplets in emulsion A. The coated beads were separated, filtered, and rinsed with 2 wt.% Tween 20 solution and then with deionized water. 1.4

SEM, FT-IR, and XRD analyses of the coating layer

The surface and cross-section morphologies of the microcapsules, as well as the coating layer thickness were characterized with a scanning electron microscope (SEM) (Hitachi S-450). The infrared spectrum of the modified chitosan beads was measured on an Shimadsu Z-2000 Fourier transform infrared (FT-IR) spectrophotometer. X-ray diffraction (XRD) experiments were performed on a RIGAKU D/Max-RB diffractometer to determine the internal structure of the modified beads.

CHEN Fei ч

‫ ׆‬et al˖Preparation and Adsorption Ability of Polysulfone MicrocapsulesĂĂ

Fig. 1

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Procedure of chemically modified CTS with PEI

1.5

Cu(II) adsorption ability

The Cu(II) ion solution was prepared by dissolving Cu(NO3)2x3H2O in deionized water with the pH adjusted to about 5.2 using HCl. The adsorption kinetics is measured in a continuous shaking water bath with a mixture of the Cu(II) ion solution and the chitosan beads. The shaking rate was 150 r/min and the temperature was 30ć or 40ć. After a predetermined period, 0.2 mL of the solution sample was removed and analyzed using spectrophotometry[19] on an Agilent 8453 UV-visible spectrophotometer. The adsorption capacity was calculated from Q v ª¬V u  C0  Ct º¼ W Fig. 2

PSF-CTS or PSF-PEI-CTS preparation procedure

(1)

where C0 (mg/L) and Ct (mg/L) are the initial concentration and the concentration at time t of Cu(II).

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V is the solution volume. W is the mass of the beads and Q is the adsorption capacity of the beads (mg Cu(II)/mmol chitosan).

2

Results and Discussion

2.1

Characterization of CTS and PEI-CTS

Figure 3 shows CTS and PEI-CTS SEM micrographs. The surfaces of both materials are rough with many micro-pores and micro-particles. However, there are striking differences in the surface structure of the two beads. The PEI-CTS surface is somewhat smoother than that of the CTS because the PEI was bonded not only to the surface, but also into the pores.

groups. The FT-IR spectra indicate that the PEI was successfully bonded onto the CTS.

Fig. 4 FT-IR spectra of CTS and PEI-CTS

The XRD patterns of CTS and PEI-CTS are displayed in Fig. 5. The PEI-CTS XRD pattern shows a sharp peak near 2T=30o, which does not show on the CTS pattern. Therefore, the modified CTS has some new structure or new material as was seen in Fig. 3. The PEI itself may also form nano-crystals during the modification process.

Fig. 5 XRD spectra of CTS and PEI-CTS

Fig. 3

SEM micrograph of cross-sections

Figure 4 shows the FT-IR spectra of CTS and PEI-CTS. The CTS spectrum shows the OüH and NüH bond peaks at 3400 cm–1, as well as the üNH2 bond at 1599 cm–1. After the CTS was modified with PEI, the ü CH2 ü and ü NH2 contents on the PEI-CTS were much higher than that in the unmodified CTS as indicated by the intensified peaks at 2922 cm–1 and 2881 cm–1 for the üCH2 group and at 1680-1600 cm–1 for the ü NH2 and ü NH ü

Previous work with the encapsulation mechanism has shown that the water content in the gel beads has a major influence on the coating layer thickness and its structure when all other parameters are fixed. Higher water content accelerates the formation of the PSF coating layer which increases the coating layer thickness. Therefore, the water content is one of the most important properties of the beads. Another important property is the free üNH2 content, which represents the maximum adsorption capacity of the beads. These properties are listed on Table 1 for CTS and PEI-CTS. The modified beads had the same size as

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the unmodified beads but the ü NH2 content was increased and the water content was reduced. All these measurement results further indicate that the PEI is successfully linked to the CTS. Table 1 CTS and PEI-CTS properties Particle size Water content,

üNH2 content,

(mm)

Cwater/(wt.%)

Camino/(mmoLgg–1)

CTS

2.5 r 0.2

90 r 1

5.92

PEI-CTS

2.5 r 0.2

86 r 1

11.43

2.2

Characterization of coating layer structure

The thickness and morphology of the coating layer are two key characteristics of the PSF microcapsules. Chen et al.[16] related the coating layer thickness to the coating time to determine the coating time of 20 s selected for this work. Because the water contents of the CTS and the PEI-CTS (see Table 1) were similar, only the PSF-CTS was used as an example to study the coating layer structure. Figure 6 shows the SEM micrographs of the coating layer. The layer thickness was around 200 Pm with the material density increasing from the inside to the outside of the layer. The solidification of the PSF droplets was initially faster, so the inner porosity was higher. After some time, the PSF droplet solidification process slowed, so the outer layer was smoother and denser. 2.3

Fig. 6

SEM micrographs of cross-section of coating layer

Cu(II) ion adsorption ability of the microcapsules

Chitosan gel beads have a good Cu(II) affinity and adsorption capacity at about pH 5. Therefore, all the experiments were carried out at pH 5.2 to eliminate the effect of pH on the adsorption kinetics. Figure 7 shows a photograph of both types of beads loaded with Cu(II) ions for the same experimental conditions. The photograph shows that the Cu(II) adsorption ability of the CTS ion is much less than that of the PEI-CTS since the PEI modified beads have a navy blue color (heavy gray shown in Fig. 7) after loading with the Cu(II) ions while the unmodified beads have a light blue color (light gray shown in Fig. 7). The Cu(II) adsorption rates for the two kinds of beads at 30ć were measured with the results shown in Fig. 8. The figure shows that the adsorption capacity of PEI-CTS is much larger than that of CTS and that their capacities increase with increasing Cu(II) concentration

Fig. 7 Photograph of CTS and PEI-CTS beads after Cu(II) ion loading

in the water solution. Figure 8 also shows that the curve of Ce Qe is a linear function of Ce for the two kinds of beads, indicating that the adsorption capacity of the beads can be described by the Langmuir adsorption equation. The constants of the Langmuir adsorption equation evaluated from the experimental data using the least squares method are listed in Table

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2. The maximum adsorption capacity, Qmax, of PEI-CTS was about 1.34 times that of the unmodified CTS.

would reduce the mass transfer even more. The adsorption kinetic curves indicate that the mass transfer rate of PEI-CTS was fast with less than 200 min required to reach equilibrium. For PSF-PEI-CTS, about 400 min was required to reach a similar adsorption capacity. The mass transfer rates of CTS and PSF-CTS were lower than that of PEI-CTS and PSF-PEI-CTS with equilibrium not reached for CTS and PSF-CTS even after 400 min. The maximum capacities of PEI-CTS and PSF-PEI-CTS were higher than that of CTS and PSF-CTS, so the mass transfer driving force was increased, which increased the mass transfer rate.

Fig. 9 Adsorption kinetics of CTS, PEI-CTS, PSF-CTS, and PSF-PEI-CTS Fig. 8

Cu(II) adsorption rate of PEI-CTS and CTS

Table 2 Regression results of Langmuir isotherm constant KL Isotherm equation * e

Ce Q

2.05  0.03C

aL

Lgmmol–1 Lgmg–1

Saturation capacity Qmax mggmmol–1

* e

PEI-CTS

0.49

0.015

32.67

0.34

0.014

24.29

(R=0.995) * e

Ce Q

2.97  0.04C

CTS

* e

(R=0.994) * The units of Ce and Qe are same as in Fig. 8b.

The polysulfone coating layer is expected to reduce the mass transfer rate. The influence of the coating layer on the mass transfer performance was evaluated by measuring the adsorption kinetics of CTS, PEI-CTS, PSF-CTS, and PSF-PEI-CTS. About 0.15 mmol of adsorbent (based on the CTS volumn) was mixed into 100 mL of 40 Pmol/L aqueous solution at 40oC. Figure 9 shows the experimental results. The adsorption rate of PSF-CTS and PSF-PEI-CTS was reduced due to the mass transfer resistance of the PSF coating layer. Increase of the coating layer thickness

3

Conclusions

Chitosan beads were chemically modified with PEI and a surface coating was applied to the beads using PSF as the coating polymer to improve the poor mechanical stability and the biodegradability of chitosan beads and to improve their adsorption of metal ions. SEM, FT-IR, and XRD analyses showed that the PEI was successfully linked onto CTS. The modification significantly increased the üNH2 density on the PEI-CTS while the water content was slightly reduced. The coating layer was about 200 Pm thick, which improves the mechanical stability and slows the biodegradability but reduce the mass transfer rate. The chitosan gel beads modified with PEI have excellent Cu(II) adsorption capacity with a maximum Cu(II) adsorption capacity 1.34 times higher than the unmodified beads. Even after coating with PSF, the absorption capacity and adsorption kinetics of the PSF-PEI-CTS were still higher than those of the CTS. Therefore, these modifications improved both the mass

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‫ ׆‬et al˖Preparation and Adsorption Ability of Polysulfone MicrocapsulesĂĂ

transfer performance of the PSF-PEI-CTS and the stability of the beads.

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[11] Becker T, Schlaak M, Strasdeit H. Adsorption of nickel(II), zinc(II) and cadmium(II) by new chitosan derivatives. Reactive and Functional Polymers, 2000, 44: 289-298.

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