Preparation and investigation of adsorption capacity for metal ions of amorphous chitosan

Abstract. In this work, chitosan with an amorphous structure and high surface area (increased 30 times higher than that of initial chitosan) was obtained from the decrystallization of chitosan using a reprecipitation method of chitosan from solution. The chitosan was characterized using X-ray diffraction, Scanning Electron Microscopy (SEM) and Brunauer-Emmet-Teller (BET). The adsorption capacity of the decrystallized chitosan for many metal ions has been evaluated. The results show that the adsorption capacity of decrystallized chitosan for the metal ions increased and the values obtained were 1.16 to 3.58 times higher than that of other results reported in experimental ranges.

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JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-00074 Chemical and Biological Sci. 2015, Vol. 60, No. 9, pp. 21-26 This paper is available online at Received August 26, 2015. Accepted November 25, 2015. Contact Nguyen Tien An, e-mail adress: an.tbump.vn@gmail.com 21 PREPARATION AND INVESTIGATION OF ADSORPTION CAPACITY FOR METAL IONS OF AMORPHOUS CHITOSAN Do Truong Thien1, Nguyen Tien An2, Nguyen Thi Hoa2 and Pham Thanh Khiet2 1 Institute of Chemistry, Vietnam Academy of Science and Technology 2 Thai Binh University of Medicine and Pharmacy, Thai Binh Abstract. In this work, chitosan with an amorphous structure and high surface area (increased 30 times higher than that of initial chitosan) was obtained from the decrystallization of chitosan using a reprecipitation method of chitosan from solution. The chitosan was characterized using X-ray diffraction, Scanning Electron Microscopy (SEM) and Brunauer-Emmet-Teller (BET). The adsorption capacity of the decrystallized chitosan for many metal ions has been evaluated. The results show that the adsorption capacity of decrystallized chitosan for the metal ions increased and the values obtained were 1.16 to 3.58 times higher than that of other results reported in experimental ranges. Keywords: Chitosan, decrystallization, heavy metals, adsorption. 1. Introduction Chitin is a naturally abundant biopolymer, like cellulose, that is present in the shell of crustaceans such as crab and shrimp, the cuticle of insects and also the cell wall of some fungi and microorganisms. Chitin consists of 2-acetamido-2-deoxy-(1-4)--D-glucopyranose residues (N-acetyl-D-glucosamine units) which has intra- and inter-molecular hydrogen bonds and is water-insoluble due to its rigid crystalline structure. Chitosan ideally consists of 2-amino-2-deoxy-(1-4)--D-glucopyranose residues (D-glucosamine units) and has no or a small amount of N-acetyl-D-glucosamine units, and is water-soluble as the salt with various acids on the amino group of D-glucosamine unit [1]. The possibility of extending the use of chitosan to immobilize biologically active species or to remove metal ions from wastewater has been regarded as an area worthy of further investigation. Because of its coarse porous structure and low toxicity, and the presence of free amino groups, chitosan has been considered an excellent ca didate for such purposes. The amine groups on the chit san chain have been shown to serve as a selective chelating site for transition metal ions [2, 3]. Up to now, many approaches on using chitosan for adsorption of heavy metal ions from aqueous solutions have been reported. However, the use of decrystallized chitosan with low degree of crystallinity and high surface area for these aims has not been shown. With the high surface area, the number of adsorption centers was increased with the functional groups of Do Truong Thien, Nguyen Tien An, Nguyen Thi Hoaand Pham Thanh Khiet 22 chitosan becoming more active and flexible, which facilitates complexation of chitosan with many heavy metal ions so that the adsorption capacity might be enhanced. For this reason, in this work, the decrystallized chitosan was prepared from chitosan (DA=31%). The characterizations of decrystallized chitosan were supported by NMR spectra, X-Ray diffraction and BET analysis. The adsorption capacity of decrystallized chitosan for many metal ions from an aqueous solution was also ev luated. 2. Content 2.1. Experiments * Materials -chitosan with a 31% degree of acetylation (DA= 31%) was received from the deacetylation of chitin. Sodium hydroxide and hydrochloric acid were purchased from Merck (Germany). Ni(NO3)2, CuSO4, Pb(NO3)2, HgCl2, Zn(CH3COO)2, FeSO4, Fe(NO3)3, Cr(NO3)3 and other chemicals used in the experiments were of analytical grade. * Preparation and characterization of decrystallized chitosan -chitosan (1 g) was dissolved in 500 ml of 0.1 M hydrochloric acid under constant stirring at room temperature for 24 h. A chitosan solution was obtained after filtering the mixture through a textile cloth to remove any insoluble components. Then the chitosan solution (100 mL) was added drop-by-drop into 500 ml of 90% (v/v) ethanol stirring vigorously. The forming precipitate was filtered out, washed with the excess amount of distilled water to remove any impurities and dried for obtaining decrystallized chitosan. X-ray diffraction patterns for initial chitosan and descrytallized chitosan were analysed using a Siemens D5000 (Japan) diffractometer equipped with a CuK target at 40 kV and 30 mA with a scan rate of 4/min. The diffraction angle ranged from 2 = 0 to 2 = 60. Teller (BET) surface area was measured using Micromeritics ASAP 2010 gas adsorption surface analyzer. Scanning electron microscopy (SEM) images of initial chitosan and descrytallized chitosan were collected with a HITACHI S-4800 (Japan) under an acceleration voltage of 20kV. * Adsorption capacity The solutions of salt used in this experiment were prepared by dissolving salt in di tilled water. The pH value of these solutions was adjusted using dilute sodium hydroxide or hydrogen chloride solutions. In a typical experiment, 0.10 g of decrystallized chitosan was added to 50 mL of metal ion solution (initial concentration 10 mmol/l), shaking 4 h at 30 C, then filtered out. The concentration of metal ion in aqueoussolution was determined by atomic adsorption spectrophotometer with a Perkin Elm r atomic adsorption spectrophotometer (AAS-3300). The adsorption capacity of metal ion was calculated based on the difference of metal ion concentration in aqueous solutions before and after adsorption, according to the following equation: W C)(CV Q 0   where Q is the adsorption capacity (mg/g), Co and are, respectively, the initial and solution phase metal ion concentration at equilibrium (mg/L), V is the solution volume (L), and W is the mass of sorbent (g). Preparation and investigation of adsorption capacity for metal ions of amorphous chitosan 23 2.2. Results and discussion 2.2.1. Preparation of decrystallized chitosan Chitosan is a product which resulted from the deacetylation of chitin in an alkaline medium. The origin of chitosan influences the arrangement of polymer chains and three different types of chitosan obtained by deacetylation of three kinds of chitin have been identified: -chitosan results from -chitin (shrimp and crab shells), -chitosan from -chitin (squid pen) and -chitosan from -chitin (stomach cuticles of cephalopoda), corresponding to parallel, anti-parallel, and alternated arrangements of polymer chains, respectively. In the crystalline state, chitosan has a tight structure due to the strong hydrogen linkages among the hydroxyl and amine groups. This was one of main things that limited the ability o apply chitosan [4, 5]. In order to decrease the crystallinity of chitosan or produce chitosan with amorphous chitosan, other methods were necessary. According to the methods described in experimental part, dissolving chitosan in a dilute solution and stirring vigorously, the crystal structure of chitosan was broken forming an amorphous structure (Scheme 1). After precipitation or lyophilisation, this structural state was still maintained, so decrystallized chitosan was obtained. Because the processs of obtaining amorphous chitosan were carried out completely in water, it could be concluded that the above method used for obtaining decrystallized chitosan was quite simple, cheap and easily doing. The decrystallized chitosan prepared by the above method has promising applications in many fields including film, complexion and loading medicine. Scheme 1: Illustrate the structural state of chitosan before and after decrystallization 2.2.2. X-Ray diffractions The X-Ray curves of chitosan and decrystallized chitosan were shown in Figure 1. Figure 1. X-Ray diffraction of chitosan (a) and decrystallized chitosan (b) Do Truong Thien, Nguyen Tien An, Nguyen Thi Hoaand Pham Thanh Khiet 24 Figure 1 shows that there were two strong and sharp peaks in the diffractogram of chitosan at 2θ at 10 and 19.5. The peak at 2 θ about 22 was attributed to the allomorphic tendon form of chitosan, which resulted in a strong decrease in sorption capacities. Meanwhile, no clear sharp peak was found in the diffractogram of decrystallized chitosan. In addition, the peak intensity in the diffractogram of initial chitosan was higher than that of decrystallized chitosan. These indicate that the crystal structure of the chitosan had been destroyed and replaced by an amorphous structure after the process of d crystallization of chitosan. In the amorphous state, functional groups typical for chitosan such as amine (-NH2) and hydroxyl (-OH) groups become more active and flexible, these lead the chitosan to easily react with other reagents to form derivatives of chitosan or complexes with many metal ions. 2.2.3. SEM analysis of decrystallized chitosan The morphological structures of initial chitosan and decrystallized chitosan have been examined by means of SEM and shown in Figure 2. As seen from Figure 2, the structure of chitosan after descrystallization was more porous than that of initial chitosan. Because of the porous structure, the surface area of chitosan increased strongly which would facilitate the adsorption of metal ions onto the surface of chitosan. Figure 2. SEM photographs of initial chitosan (a) and decrystallized chitosan (b) 2.2.4. BET characterization of decrystallized chitosan The BET surface areas for initial chitosan and decrystallized chitosan were measured from N2 adsorption isotherms and the obtained results are shown in Figure 3. The values were computed and the results show that the surface area of initial chitosan and decrystallized chitosan were 0.17m2.g-1 and 5.52m2.g-1, respectively. The decrystallized chitosan had a high surface area that would make its functional groups, such as the amine (-NH2) and hydroxyl (-OH) groups, become more active and flexible so that these functional groups would be easily available for interaction with metal ions and as a result decry t llized chitosan shows promise for adsorption of metal ions. Preparation and investigation of adsorption capacity for metal ions of amorphous chitosan 25 (a) (b) Figure 3. BET surface area plot of initial chitosan (a) and decrystallized chitosan (b) 2.2.5. Adsorption capacity of metal ions by decrystallized chitosan The maximum adsorption capacities (Qmax) of decrystallized chitosan for metal ions were summarized in Table 1 and compared with the results reported on adsorption properties of chitosan [6-10]. The results in Table 1 show that the adsorption capacities of decrystallized chitosan for many metal ions were almost all higher than that of other reported chitosan. This could be explained as follows: the ability of adsorption of chitosan to metal ions strongly depends on its surface area and the mobility of groups such as hydroxyl and amine. As the surface area of the material increased, the binding sites of metal ions with chitosan would also increase. Therefore, the adsorption capacity of decrystallized chitosan for the metal ions increased. In addition, in the porous state, the distance that these functional groups in the chitosan molecular chain increased would reduce the formation of ydrogen linkages among these groups. So, functional groups such as amine (-NH2) and hydroxyl (-OH) groups became more active and flexible and as result the interaction of the groups with metal ions was enhanced. Table 1. Comparison of maximum adsorption capacities for metal ions of the decrystallized chitosan with other reported chitosan Metal ions Qmax1 (mg.g -1 ) Qmax2 (mg.g -1 ) PCS Qmax2/Qmax1 Cu (II) 130 (pH = 6) [6] 295.0 (pH = 6) 2.27 Pb (II) 238 (pH = 6) [7]728.6 (pH = 6) 3.06 Ni (II) 67.0 (pH = 6) [7]212.4 (pH = 6 - 6.5) 3.18 Hg (II) 1127 (pH = 6 - 6.5) [8] 1266 (pH = 6 - .5) 1.16 Zn (II) 78.6 (pH = 6) [7]281.4 (pH = 6.5) 3.58 Cr (III) 26.0 (pH = 5) [9]67.6 (pH = 5 - 6) 2.60 Fe (II) 63.8 (pH = 6 - .5) [10] 92.4 (pH = 6) 1.45 Fe (III) 90.1 (pH = 6 - .5) [10] 119.3 (pH = 6 - .5) 1.32 Do Truong Thien, Nguyen Tien An, Nguyen Thi Hoaand Pham Thanh Khiet 26 3. Conclusion Decrystallized chitosan (with an amorphous structure and high surface area) was obtained from a decrystallization of chitosan using the reprecipitation method. Adsorption capacity of the decrystallized chitosan for many metal ions was evaluated. 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