Preparation of alginate chitosan hybrid gel beads and adsorption of divalent metal ions

Size: px

Start display at page:

Download "Preparation of alginate chitosan hybrid gel beads and adsorption of divalent metal ions"

Scot Harry Osborne
6 years ago
Views:

1 Chemosphere 55 (2004) Short Communication Preparation of alginate chitosan hybrid gel beads and adsorption of divalent metal ions Takeshi Gotoh a, *, Keiei Matsushima b, Ken-Ichi Kikuchi a a Department of Materials Process Engineering and Applied Chemistry for Environments, Faculty of Engineering & Resource Science, Akita University, 1-1 Tegata Gakuen-Cho, Akita , Japan b Akita Prefectural Resources Technology Development Organization, 9-3 Kosaka Kozan, Kosaka , Japan Received 1 May 2003; received in revised form 28 October 2003; accepted 4 November Abstract Naturally occurring polysaccharides such as alginic acid and chitosan have been recognized as one of the most effective adsorbents to eliminating low levels of heavy metal ions from waste water stream. The present study intended to use alginic acid and chitosan simultaneously, which are expected to form a rigid matrix structure of beads due to electrostatic interaction between carboxyl groups on alginic acid and amino groups on chitosan, and to prepare alginate chitosan hybrid gel beads. This could be achieved for the first time by using water-soluble chitosan, which was obtained by deacetylating chitin to 36 39% degree. The water-soluble chitosan dissolved in water could remain in solution in the presence of sodium alginate, and the homogeneous solution of chitosan and alginate was dispensed into a CuCl 2 solution to give gel bead particles. The resultant beads were then reinforced by a cross-linking reaction of aldehyde groups on glutaraldehyde with amine groups on the chitosan. The cross-linking reaction made the beads durable under acidic conditions. The adsorption of Cu(II), Co(II), and Cd(II) on the beads was significantly rapid and reached at equilibrium within 10 min at 25 C. Adsorption isotherms of the metal ions on the beads exhibited Freundlich and/or Langmuir behavior, contrary to gel beads either of alginate or chitosan showing a step-wise shape of adsorption isotherm. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Alginic acid; Metal recovery; Biopolymers; Adsorbent; Langmuir isotherm; Freundlich isotherm 1. Introduction Heavy metal ions must be removed from waste water streams from mineral operating industries, because they are highly toxic even at low concentrations. To remove trace levels of heavy metal ions, adsorption by natural occurring materials is one of the most effective and low * Corresponding author. Tel.: ; fax: address: tgotoh@ac5.as.akita-u.ac.jp (T. Gotoh). cost methods (Bailey et al., 1999), and utilization of bead particles has been recognized to have advantages in terms of applicability to a wide variety of process configurations and reusability for repeated runs. Alginate, which is a polysaccharide biopolymer composed of varying compositions of b-1,4 linked D- mannurinic acid (M) and L-guluronic acid (G), is well known to have a significantly high affinity to divalent metal ions. On a contact with divalent cations, an alginate solution causes gelation, which is thought to arise mainly at junctions in the G G sequence rich chain region called Ôegg box junctions (Rees and Welsh, 1977). Chitosan (poly-1,4-d-glucosamine), which is another polysaccharide biopolymer derived from chitin, also has /$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi: /j.chemosphere

2 136 T. Gotoh et al. / Chemosphere 55 (2004) a high affinity for transition metal ions in a way that the amine groups on chitosan serve as a chelation site for the metal ions (Yang and Zall, 1984). By using the characteristics, chitosan gel bead particles are easily prepared by dispensing chitosan solutions to alkaline media, and the beads obtained after the following chemical crosslinking reactions were shown to remain a high affinity for heavy metal ions. On equilibrium adsorption, however, the beads were clogged with metal ions in the outer sphere and did not fully exhibit their abilities in a low concentration region of metal ions (Juang and Shao, 2002). This can be attributed to the cramped micropores (Rorrer et al., 1993), which have initially formed on the gelation event by binding of amino groups of the Ôfree polymers to metal ions, afterward presenting metal binding sites. The purpose of the present study is to explore novel sorbents which have a high affinity for heavy metal ions and fully exhibit their ability in a wide range of metal concentration. To prevent the formation of cramped micropores accompanying the beads of naturally occurring biopolymers, we intended to use both alginic acid and chitosan simultaneously whose oppositely charged functional groups are expected to restrict the mobility of the polymer chains each other on a gelation event by their electrostatic interaction and to prepare cross-linked alginate chitosan hybrid gel beads for the first time after the covalent cross-linking reaction by glutaraldehyde. To examine the potential of the beads as a sorbent for heavy metal ions, adsorption behavior of Cu(II), Co(II), and Cd(II) is also determined. 2. Materials and methods 2.1. Materials Sodium alginate ( cp) was supplied by Wako Pure Chemicals (Tokyo, Japan). Three kinds of chitosan (Chitosan 10B, 100% deacetylation; Chitosan 9B, 90% deacetylation; Chitosan 7B, 70% deacetylation) were the products of Funakoshi (Tokyo, Japan) and used without further purification. Chitin was reagent grade of Wako Pure Chemicals and ground with a homogenizing mill prior to the use for the preparation of water-soluble chitosan Preparation of water-soluble chitosan Water-soluble chitosan was prepared essentially according to the method of Kurita et al. (1989). In brief, powdered chitin (6 g) was added to 150 ml of 40 wt% NaOH and stirred at 25 C for 3 h under reduced pressure. After the addition of crushing ice (550 g) the solution was stirred at 0 C by using a mechanical impeller until gelation occurred, and the stirring was further continued for three days at room temperature. After the addition of another portion of crushing ice (420 g) and following ph adjustment to 8.7 with 6 M HCl, the solution was added dropwise to 10 l of cold acetone water (7:1) while the acetone water ratio was maintained by adding cold acetone. The solution was allowed to stand more than 30 min, and a white precipitate was separated from the solution by filtration using a glass filter and washed with cold acetone water (7:1) until the filtrate did not get turbid with silver nitrate. The precipitate was dried in vacuo and impurities insoluble in cold water was removed by centrifugation at g for 30 min. The deacetylation degree of chitosan was determined by a colloidal titration with potassium polyvinyl sulfate according to the method of Sannan et al. (1976) Preparation of alginate chitosan hybrid beads The alginate chitosan solution was prepared by slowly pouring a water-soluble chitosan solution into the same volume of alginate solution with well stirring on a magnetic stirrer, and was further stirred for 3 h under reduced pressure. The solution was sprayed through a thin nozzle ( ¼ 0.8 mm) into a 0.15 M CuCl 2 solution with nitrogen gas blown, and the resultant alginate chitosan hybrid gel beads (Cu form) were allowed to stand in the solution for 24 h at room temperature with gentle agitation on a magnetic stirrer. To the bead suspension were added twice one equivalent each and finally three equivalents of glutaraldehyde to amino groups of chitosan every 6 h at room temperature. The beads were then rinsed with deionized water and sieved between 16 mesh (0.991 mm) and 65 mesh (0.208 mm) screens. The H form of the beads, which was obtained by the immersion of the beads in 0.1 M HCl for 3 h (3 times), was used for adsorption experiments after extensive rinse with deionized water. The bead size was determined by taking digitized images for at least 800 beads and analyzing with 2- dimensional image analysis software (KS300, Carl Zeiss Japan) Adsorption kinetics of metal ions Adsorption was examined in a batch mode. The alginate chitosan hybrid beads (3 g) were immersed in 60 ml of 5 mm metal solutions whose initial ph was adjusted to 3.5 and agitated on a reciprocal shaker at 25 C. The test solutions used were aqueous solutions of CuCl 2, CoCl 2, CdCl 2. Samples were withdrawn from the solutions at indicated time for the analysis of metal concentrations by using atomic absorption spectroscopy (AA-782, Nippon Jarrell Ash, Tokyo, Japan). The amounts of metal ions adsorbed on the beads, q (mmol g 1 wet beads), were determined from the

3 T. Gotoh et al. / Chemosphere 55 (2004) measured bulk concentrations, using the following equation: q ¼ V M ðc 0 CÞ; ð1þ where V (l) is the solution volume, M (g) is the weight of wet beads, C 0 (mm) is the initial metal concentration, and C (mm) is the bulk metal concentration at indicated time Adsorption equilibrium of metal ions The alginate chitosan hybrid beads (0.5 g) were placed in sealed bottles containing known initial concentrations of metal solutions (20 ml) at initial ph of 3.5 and agitated on a reciprocal shaker at 25 C for 3 h. The equilibrated amounts of metal ions adsorbed on the beads were double-checked from the bead phase and liquid phase concentrations. The equilibrated solutions were sampled and analyzed for the metal concentrations as described above. The equilibrated beads were recovered by filtration and briefly washed with deionized water for removal of the metal solutions present on the surface of the beads, and then were treated with 20 ml of 0.1 M HCl for 1 h in order to desorb the bead phase metal ions. The desorption was repeated four times and the eluates were combined for the analysis of metal concentrations. 3. Results and discussion 3.1. Preparation of alginate chitosan hybrid beads Alginic acid and chitosan are contradictory in terms of solubility in aqueous solutions. Alginic acid is an anionic polysaccharide and soluble in neutral and alkaline solutions, but insoluble in acidic solutions. In contrast, chitosan is a cationic polysaccharide and soluble in acidic solutions, but insoluble in alkaline solutions. Polyelectrolyte complexes are generally formed when such oppositely charged polysaccharides coexist in aqueous media (Nakajima and Shimoda, 1976). Therefore, we began with exploring conditions under which alginic acid and chitosan are simultaneously dissolved in aqueous media, according to the homogeneity of the mixture. As three different types of commercially available chitosan (deacetylation degree: 100%, 90%, and 70%) dissolved in acetic acid solutions were contacted with alginate dissolved in distilled water, white pellicular gelation instantaneously occurred at the boundary of the solutions under any conditions tested in this study varying the concentrations and ratio of the biopolymers, acetic acid concentration, ph, and ionic strength (Table 1). Chitosan with about 50% deacetylation was reported to be soluble in water and remained in solution even on alkalization (Kurita et al., 1989). So we next examined the possibility of the water-soluble chitosan coexisting with alginate in solution. In the present study, Table 1 Preparation of alginate chitosan solutions No. Alginate Chitosan Alginate (wt%) (ph) NaCl (wt%) (wt%) (ph) Acetic acid (mol l 1 ) chitosan ratio Homogeneity of mixed solution a a 1.0 1:1 ) a a 1.0 1:2 ) a a 1.0 1:5 ) a a 1.0 2:1 ) a a 1.0 5:1 ) a a 0.5 5:1 ) a a 0.2 5:1 ) a a 0.1 5:1 ) a a 0.5 1:1 ) a a 0.2 1:1 ) a a 0.1 1:1 ) a a 0.1 1:1 ) a a 0.1 1:1 ) b : b : c :1 + a Three types of commercially available chitosan varying deacetylation degree (100%, 90%, and 70%) were examined. The deacetylation degree affected the ph of the solutions. b Water-soluble chitosan of deacetylation degree of 36%. c Water-soluble chitosan of deacetylation degree of 39%.

4 138 T. Gotoh et al. / Chemosphere 55 (2004) water-soluble chitosan was prepared by deacetylation of chitin, and the deacetylation degree of the water-soluble chitosan was determined to be 36 39%, which was lower than the values (45 55%) obtained in their report. As a result, the water-soluble chitosan turned out to be remained in solution on the addition of the same quantity of sodium alginate solutions (Table 1). A homogeneous solution of 1.5 wt% each of alginate and chitosan was sprayed to a 0.15 M CuCl 2 solution with the aid of N 2 blow to prepare the bead particles. Since no soluble matters were determined in the aqueous phase released from the particles, both alginate and chitosan were thought to consist in the gel beads. After sieving on a screen, we obtained alginate chitosan hybrid gel beads (Cu form) having a mean diameter of 533 lm (Table 2). To improve the mechanical strength and resistant to swelling and erosion of the hybrid gel beads, we further cross-linked the chitosan biopolymer chains with bifunctional reagent glutaraldehyde, which is commonly used as a cross-linking reagent by the reaction of aldehyde groups on glutaraldehyde with amine groups, in the presence of Cu(II) which was doped in the matrix of the beads. The cross-linking reaction significantly reduced the bead diameter by about 30%. More importantly, the reaction rendered the gel beads resistant to acidic media: Soluble chitosan was not released from the beads by a batch contact with 0.1 M HCl for 3 h at room temperature. These results strongly suggest that the chitosan of bead matrix is covalently cross-linked in each bead by glutaraldehyde bridges, though the fraction of amine groups actually cross-linked were not measured. Table 2 summarizes some physical properties of alginate chitosan hybrid gel beads obtained in the present study. The beads shrank by 6% on a replacement of Cu(II) with H þ by conditioning with 0.1 M HCl. In the following examinations, the gel beads in H þ form were employed Kinetic adsorption of metal ions on alginate chitosan hybrid beads The adsorption kinetics of several metal ions on the cross-linked alginate chitosan hybrid gel beads were examined at initial ph of 3.5 (Fig. 1). For each of the Adsorption (mmol g -1 ) Time (min) Fig. 1. Adsorption kinetics of metal ions on covalently crosslinked alginate chitosan hybrid gel beads. Circles, Cu(II); triangles, Co(II); and squares, Cd(II). metal ions, significantly rapid increase of adsorption is observed during the first 2 min, and a steady equilibrium is already reached in 10 min. The rapid adsorption was also determined with smaller dosages of the beads (0.3 1 g wet beads) in the same way. The highest adsorption level tested in this study was determined for Cu(II). This is consistent with the results obtained for alginate beads (unpublished data). For alginate, metal ions could be adsorbed by ionexchange to carboxyl groups on alginate chains. For chitosan, amino groups on chitosan chains could serve as a chelation site for transition metal ions. In the kinetic adsorption study on alginate chitosan hybrid gel beads, where the contact metal solutions were not controlled for ph, the ph of the solutions was decreased by 0.50(Cu) 0.22(Co) according to the adsorbed level of metal ions. This indicates that metal ions are adsorbed on the hybrid gel beads by ion-exchange with hydrogen ions on the binding sites formed by carboxyl groups on alginate. The amino groups on chitosan coexisting in the gel beads are thought to mainly support the matrix structure by interacting with the carboxyl groups on alginate and by making cross-linking bridges with glutaraldehyde Table 2 Properties of cross-linked alginate chitosan hybrid gel beads Cations Cross-linking Density (g cm 3 ) Mean diameter (lm) Moisture content ()) Cu ) ND 533 ± 111 ND Cu ± ± ± H ± ± ± The beads of at least 800 for each sample were analyzed. The values are shown with standard deviation.

5 T. Gotoh et al. / Chemosphere 55 (2004) Equilibrium adsorption of metal ions on alginate chitosan hybrid beads For cross-linked chitosan gel beads, the equilibrium adsorption of metal ions was reported to exhibit a stepwise behavior that there was a significant increase in the adsorption at quite low equilibrium bulk concentrations of metal ions followed by an insensitive region which led into a large extent of adsorption with increasing metal concentrations (Rorrer et al., 1993; Juang and Shao, 2002). Fig. 2 shows the adsorption isotherms of the cross-linked alginate chitosan hybrid gel beads in a single-metal system of Cu(II), Co(II), or Cd(II). Unlike the results for chitosan beads, the adsorbed amounts of any metal ions rapidly increase in a low bulk metal concentration range and then gradually reached plateau maxima as the bulk metal concentrations increased. The ph decreased by (Cu), (Co), and (Cd) during the adsorption experiments. The maximum adsorption level for Cu(II) with the alginate chitosan hybrid gel beads (0.12 mmol g 1 wet gel), corresponding to about 70 mg g 1 dry gel, is comparable to those with chitosan gel beads (Wan Ngah et al., 2002) and alginate gel beads (unpublished data), though is about half that with raw chitosan. Adsorption (mmol g -1 ) Bulk metal concentration (mm) Fig. 2. Adsorption isotherms of metal ions on covalently crosslinked alginate chitosan hybrid gel beads. Circles, Cu(II); triangles, Co(II); squares, Cd(II); dotted lines, Langmuir isotherm calculation; solid lines, Freundlich isotherm calculation. The Langmuir isotherm (2) and Freundlich isotherm (3) equations were applied for the equilibrium adsorption: q e ¼ q maxkc e ð1 þ KC e Þ ; ð2þ q e ¼ kc 1=n e ; ð3þ where C e (mm) is the equilibrium concentration of metal ions, q e (mmol g 1 wet beads) is the amount of metal ions adsorbed on the beads at equilibrium, q max (mmol g 1 wet beads) is the maximum amount of metal ions adsorbed on the beads at equilibrium, and k (l g 1 ), n, and K (mm 1 ) are a constant related to the adsorption. The solid and dotted curves were calculated to best fit the data by the least square method. Table 3 lists the kinetic parameters for the equations. The Freundlich isotherm seems superior to the Langmuir isotherm with respect to the significant adsorption at very low metal concentration region. The step-wise shape of adsorption isotherm observed for chitosan beads was explained by Rorrer et al. (1993) with a pore-blockage mechanism that, in a low metal concentration range the micropores near the outer surface of beads are once clogged by clusters formed by metal ions which preferentially adsorb onto exposed binding sites, rendering binding sites deep within the interior of the beads inaccessible to metal ions, and in a high metal concentration range metal ions in turn shoot deep into the interior microporous matrix. This unusual behavior is probably attributed to the mobility of polysaccharide chains when the gelation occurs. When chitosan droplets are dispersed in a heavy metal solution, the polysaccharide polymers migrate from the interior to the outer sphere of the beads to form a dense matrix region by complexation with the metal ions (Mikkelsen and Elgsaeter, 1995). Contrary, in a binary composition system with oppositely charged alginate and chitosan, the electrostatic interaction between them is thought to extremely limit their mobility on the gelation event to give a sparse but rather uniform bead matrix. That is, the deficiency in mobility of alginate and chitosan due to the electrostatic interaction between carboxyl groups and amino groups on the metal-induced gelation event is thought to cause a sparse and rigid Table 3 Langmuir and Freundlich isotherm constants and correlation coefficients Metals Langmuir Freundlich q max (mmol g 1 ) K (mm 1 ) Correlation coefficient k (l g 1 ) n ()) Correlation coefficient Cu(II) Cd(II) Co(II)

6 140 T. Gotoh et al. / Chemosphere 55 (2004) structure of matrix of the alginate chitosan hybrid beads. In conclusion, the alginate chitosan hybrid gel beads were prepared for the first time and demonstrated to very rapidly adsorb heavy metal ions. The beads are expected to be a good candidate for an excellent adsorbent of heavy metal ions in waste water stream. Acknowledgements The authors wish to thank R. Iwabuchi and S. Kon for excellent technical support. References Bailey, S.E., Olin, T.J., Bricka, R.M., Adrian, D.D., A review of potentially low cost sorbents for heavy metals. Water Res. 33, Juang, R.-S., Shao, H.-J., A simplified equilibrium model for sorption of heavy metal ions from aqueous solutions on chitosan. Water Res. 36, Kurita, K., Koyama, Y., Nishimura, S.-I., Kamiya, M., Facile preparation of water-soluble chitin from chitosan. Chem. Lett., Mikkelsen, A., Elgsaeter, A., Density distribution of calcium-induced alginate gels. A numerical study. Biopolymers 36, Nakajima, A., Shimoda, K., Complex formation between oppositely charged polysaccharides. J. Colloid Interface Sci. 55, Rees, D.A., Welsh, E.J., Secondary and tertiary structure of polysaccharides in solutions and gels. Angew. Chem. Int. Edit. Engl. 16, Rorrer, G.L., Hsien, T.-Y., Way, J.D., Synthesis of porous-magnetic chitosan beads for removal of cadmium ions from waste water. Ind. Eng. Chem. Res. 32, Sannan, T., Kurita, K., Iwakura, Y., Studies on chitosan. 2. Effect of deacetylation on solubility. Makromol. Chem. 177, Wan Ngah, W.S., Endud, C.S., Mayanar, R., Removal of copper(ii) ions from aqueous solution onto chitosan and cross-linked chitosan beads. Reac. Func. Polym. 50, Yang, T.C., Zall, R.R., Absorption of metals by natural polymers generated from seafood processing wastes. Ind. Eng. Chem. Prod. Res. Dev. 23,

Environment Protection Engineering REMOVAL OF HEAVY METAL IONS: COPPER, ZINC AND CHROMIUM FROM WATER ON CHITOSAN BEADS

Environment Protection Engineering Vol. 3 No. 3 4 KATARZYNA JAROS*, WŁADYSŁAW KAMIŃSKI*, JADWIGA ALBIŃSKA**, URSZULA NOWAK* REMOVAL OF HEAVY METAL IONS: COPPER, ZINC AND CHROMIUM FROM WATER ON CHITOSAN