Gel Containing Chitosan
Yu-Hsin Chang, Chih-Feng Huang, Wei-Ju Hsu, Feng-Chih Chang
Institute of Applied Chemistry, National Chiao Tung University, Hsinchu 30050, Taiwan
Received 4 August 2006; accepted 14 November 2006 DOI 10.1002/app.25891
Published online 28 February 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Glutaraldehyde-crosslinked alginate gel
containing chitosan (AGCC) used for the removal of Hg2þ ions from aqueous solutions. Three bead sizes were obtained and performed to study the uptake equilibrium and kinetics of Hg2þby AGCC (ca. an hour). The adsorp-tion capacity was found to be independent of adsorbent particle size indicating that sorption occurs in the whole AGCC bead. A high initial rate of Hg2þ uptake was fol-lowed by a slower uptake rate suggesting intraparticle dif-fusion as the rate-limiting step. The rate of Hg2þ uptake increases with decreasing AGCC bead size. AGCC also enhanced the rate and the capacity of Hg2þ adsorption. The maximum Hg2þ adsorption capacity of AGCC was found 667 mg/g, which is over 20 times higher than that
of alginate bead. Our results reveal the well-distributed chitosan powders in the alginate gel bead and Hg2þ ions can reach inside the chitosan bead. It indicates the feasibil-ity of using AGCC as metal adsorbent at low pH values, and allows the regeneration of adsorbent. Hg2þ ions adsorbed on AGCC bead were desorbed effectively about 95% by H2SO4at the third cycle. The use of AGCC for the
removal of Hg2þ ions from the waste streams appears to be promising. Ó 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2896–2905, 2007
Key words: biomaterials; gels; metal–polymer complexes; chitosan; mercury
INTRODUCTION
Mercury is one of the most toxic heavy metals.1,2 Many areas in the world are contaminated by mer-cury, posing serious environmental problems.3,4 Consequently, removal of mercury ions in water and wastewater is very important. Several techniques are available for this, including precipitation, ion exchange, and adsorption. Precipitation results in large volumes of mercury containing sludge, while ion exchange is effective only for wastes with low dissolved solids concentrations. Adsorbents can be designed that are specific to mercury and whose vol-ume is much less than an amorphous sludge.2Bailey et al., Babel and Kurniawan reviewed several adsorbents and their applications for metal removal and found that chitosan was capable of adsorbing moderately high amounts of Hg2þ.5,6
Chitosan is a hydrophilic, natural cationic polymer formed by the N-deacetylation of chitin,7 which is present in fungi, insects, and crustaceans. Chitosan has been known for its metal adsorption properties since 1970s,7,8 and it has been shown to effectively remove metals such as silver,8 cadmium,9 arsenic,10 gold,11,12 vanadium,13 copper,14–16 nickel,14,17 chromium,14,18 and mercury17,19 from aqueous
solutions. It was reported that the maximum adsorp-tion capacity of chitosan for Hg2þ was 815 mg/g.17 However, the result of Hg2þ removal was different from that obtained in the latter study,19 which indi-cated that an adsorption capacity of 430 mg of Hg2þ/g was achieved by chitosan. This difference occurs due to the fact that the latter study used chi-tosan, with particle size ranging from 1.25 to 2.5 mm (against 0.21–1 mm in the former study).6Because of the resistance to intraparticle mass transfer in raw chitosan, it is usually necessary to use very small particles to improve sorption kinetics. Nevertheless, small particles have proved to be inappropriate for use in column systems since they cause column clogging and serious hydrodynamic limitations. Using chitosan gel beads may be an alternative because it improves both diffusion properties and hydrodynamic behav-ior.20,21 The adsorption capacity of chitosan gel bead for Hg2þ was reported to be 294 mg/g.22 Merrifield et al. reported that the adsorption capacity of thiol-grafted chitosan gel beads for Hg2þ was 1600 mg/g, but the equilibrium time for Hg2þ adsorption on this adsorbent was as long as 1200 min.2 Otherwise, Jeon et al. reported that the adsorption capacity of aminated chitosan bead for Hg2þwas 476 mg Hg2þ/g-dry mass, and it took only 100 min to reach the equilibrium of adsorption.23 However, several chemical modifications are required for preparing these chitosan derivative adsorbents. Since the Hg2þ adsorption capacity of chitosn powder was reported to be 815 mg/g,17
Correspondence to: F.-C. Chang (changfc@mail.nctu.edu.tw). Journal of Applied Polymer Science, Vol. 104, 2896–2905 (2007)
V
immobilizing chitosan powder in a porous support material is a potential and economic means to produce an adsorbent with satisfactory kinetic and hydrody-namic properties, and high capacity for Hg2þ adsorption.
Alginate is a linear polysaccharides composed of (1 ? 4)-linked residues of a-L-guluronic acid (G) and b-D-mannuronic acid (M). Alginate can be found in many algal species and certain bacteria.24This polye-lectrolyte is soluble in water but precipitates in the form of a coacervate in the presence of multivalent metal ions like Ca2þ, Co2þ, Fe2þ, Fe3þ, and Al3þ.25 Alginate is one of the most extensively investigated biopolymers for metal ion removal from dilute aque-ous solutions.26,27 The interaction of cationic metals with alginate has been attributed to metal complexa-tion by the carboxylate funccomplexa-tionalities of the polyur-onic acid.28,29Alginate has been previously used as a support material for the immobilization of several enzymes and microbial cells.30 The incorporation of humic acid into calcium alginate beads was demon-strated to be useful in metal recovery.31 Several researches have shown that the Hg2þ sorption capacity of the alginate beads entrapped with fungus is higher than that of plain alginate beads.4,32 How-ever, the maximum Hg2þ sorption capacity of the fungus-immobilized alginate beads (172.41 mg/g) is not higher than that of other adsorbents.2,22,23 The combination of alginate and chitosan had introduced the composite matrix in many applications,33,34 including adsorbent for metal removal.35 Takeshi Gotoh et al. used the water-soluble chitosan to pre-pare alginate-chitosan hybrid gel beads for the adsorption of Cu2þ, Co2þ, and Cd2þ.35 By using a simple method, Huang et al. have immobilized the chitosan powders in the alginate pellets to produce the alginate/chitosan pellets to remove nickel ion and nickel cyanide complex from polluted water. However, he showed that the adsorption of Cu2þ ion by the alginate/chitosan pellets is not superior to that by alginate bead.36
Since chitosan is soluble in dilute mineral acids, except for sulfuric acid, it is thus necessary to stabi-lize it chemically for the recovery of metal ions in acidic solutions.37 The treatment of glutaraldehyde crosslinking induces new linkages between the chito-san chains allowing the polymer to be highly resist-ant to dissolution even in harsh solutions such as hydrochloric molar solutions.38 Glutaraldehyde has been used for cadmium recovery on chitosan beads.39 In this study, chitosan powder was cross-linked with glutaraldehyde and its solubility was measured before and after crosslinking. The cross-linked chitosan was then homogeneously immobi-lized in alginate gel bead for the application of removing Hg2þions from aquatic systems. The mor-phology and porosity of the resulting alginate gel
containing chitosan (hereafter called: AGCC) were characterized by scanning electron microscopy (SEM) and porosimeter. The kinetics and equilibrium characteristics of Hg2þ adsorption on AGCC were studied in batch experiments. The adsorption of Hg2þ ion by AGCC was confirmed to be signifi-cantly superior to that by alginate bead. Among reported capacity results of adsorbents with feasible adsorptive kinetic property (adsorption equilibrium time¼ 100 min), the maximum uptake capacity (667 mg Hg2þ/g-dry mass) of AGCC was to be one of the highest capacities, up to now. Besides, the Hg2þ -loaded adsorbents were characterized by X-ray energy dispersion (EDS) analysis. The reusability of AGCC was evaluated by desorption studies.
EXPERIMENTAL Materials
Chitosan flake produced from crab shell wastes was obtained from Kiotek Corp., Taiwan, without further purification. The chitosan flake was ground in a blender and then passed through a sieve stack con-sisting of no. 150 (0.106 mm) and no. 200 (0.075 mm) sieves. The chitosan powder (150/200 mesh) with particle size ranging from 0.075 to 0.106 mm was separated for this study. The degree of deacetylation of the powder is 92 mol %. The Mw of chitosan is 280,000, and the polydispersity index Mw/Mn is 2.8. Mercury nitrate was supplied by Merck as analyti-cal-reagent grade. Sodium alginate, glutaraldehyde, calcium chloride (CaCl2), sulfuric acid, and acetic acid were purchased from Sigma-Aldrich and used without further purification.
Chitosan crosslinking
Glutaraldehyde was used as crosslinking agent in this study. The crosslinking bath contained a 2.5 wt % glutaraldehyde solution. The ratio of glutaraldehyde to chitosan (crosslinking ratio CR: mol GA/mol NH) is 1 : 1. Crosslinking lasted for 24 h. The cross-linked chitosan particles were extensively rinsed with deionized water to remove any free glutaral-dehyde.
Dissolution test of crosslinked chitosan
Chitosan and crosslinked chitosan were tested with regard to their solubility in 5% (v/v) acetic acid and deionized water by adding 0.1 g of chitosan and crosslinked chitosan in the dilute acid and deionized water for a period of 24 h with stirring.40
Preparation of alginate gel containing chitosan AGCC(X : Y) contains alginate and chitosan with a weight ratio of X : Y. The immobilization of the crosslinked chitosan via entrapment was carried out as follows: 5 g sodium alginate was dissolved in 250 mL of deionized water and mixed with 2 g chitosan powder for preparation of AGCC(5 : 2). The sodium alginate solution containing chitosan was then added drop by drop by means of a peristaltic pump with a tube diameter of 0.5 mm into a stirred 10% CaCl2 so-lution, and the size of the droplets was controlled by applying a coaxial air stream.36,41 By doing so, the water-soluble sodium alginate is converted into water insoluble calcium alginate beads.31 The beads were washed with deionized water several times to remove CaCl2 from the bead surface and stored at 48C before use. AGCC(5 : 10) and the alginate bead were also produced by the same method.
SEM studies
The AGCC(5 : 10) with diameter of 2.7 mm was freeze-dried for SEM observation. The freeze-dried AGCC(5 : 10) was coated under vacuum with a thin layer of gold and examined by a Hitachi S-570 SEM.
Characterization of AGCC
The dry weights of alginate bead, AGCC(5 : 2), and AGCC(5 : 10) were determined by weighing the beads after drying in an oven at 708C overnight. The water contents of the wet adsorbents were measured using a gravimetric method. The loss of weight during drying was found to be 95.11, 94.93, and 94.72% for alginate bead, AGCC(5 : 2), and AGCC(5 : 10), respectively. The porosity of AGCC(5 : 10) was measured by a mer-cury porosimeter (Micromeritics Autopore II 9200). Equilibrium uptake experiments
Adsorption of Hg2þions from aqueous solutions was studied in batch systems using mercuric nitrate. Standard solution of Hg2þwas prepared at a concen-tration of 1000 mg/L. Batch equilibrium experiments were carried out using 1 g wet AGCC beads or algi-nate beads as adsorbents. A series of flasks containing 100 mL solution with various metal concentrations prepared from the standard solution and adsorbent were agitated in a rotary shaker at 200 rpm, 258C for 24 h, which is sufficient for the metal ion uptake pro-cess to reach final equilibrium.16 The loss of weight during drying was found to be 95.11, 94.93, and 94.72 for the alginate bead, AGCC(5 : 2), and AGCC(5 : 10), respectively. Therefore, the dosages of adsorbents in this equilibrium study were 0.0489 g-dry weight/100 mL, 0.0507 g-dry weight/100 mL, and 0.0528 g-dry
weight/100 mL for alginate bead, AGCC(5 : 2), and AGCC(5 : 10), respectively. The sorption experiments were conducted at a pH value close to 5 by adjusting with HCl or NaOH. After equilibrium, the solution in each flask was analyzed for metal content by a GBC Avanta S Atomic Absorption Spectrophotome-ter (AAS). Metal-free and adsorbent-free blanks were used as controls. Extent of the metal ion uptake by the adsorbent, based on dry weight, was determined by the following mass balance equation:
Q¼VðC0 CeÞ
m (1)
where Q and Ce are the adsorbent phase metal con-centration and the solution phase metal concentra-tion at equilibrium. C0is the initial metal concentra-tion, V is the solution volume, and m is the dry mass of the adsorbent. In this equilibrium study, all the parameters were calculated based on the dry weight of adsorbent.
Scanning electron microscope and EDS analysis of metal distribution
The isothermal adsorption of Hg2þ ion by 2.7 mm AGCC(5 : 10) was conducted for the concentration (500 ppm) of the Hg2þ ion solution at pH 5. After 24 h, the AGCC(5 : 10) beads were separated from the Hg2þ ion solution. AGCC(5 : 10) beads were then washed several times with deionized water and air dried for several days at room temperature and stored for further observations. Examination of the beads by Hitachi S-2500 SEM was made after coating them with a thin layer of gold. The distribution of Hg2þ ions inside the Hg2þ-loaded beads was exam-ined using the SEM with an attachment of X-ray energy dispersion (EDS) analyzer.42
Transient uptake experiments
Batch experiments for determination of the kinetics of Hg2þ adsorption on wet adsorbents were carried out using a continuously stirred 500-mL glass beaker.16 A motor was used to drive a 4-blade impeller with a di-ameter of 6 cm. The following experimental condi-tions were kept constant for the kinetic experiments: volume of mercuric nitrate solution ¼ 300 mL, tem-perature ¼ 258C, stirring speed ¼ 200 rpm. 5.11 g wet alginate bead, 4.93 g wet AGCC(5 : 2), or 4.73 g wet AGCC(5 : 10) was added in 300 mL solution, respectively, to make the dosage of adsorbent ¼ 0.25 g-dry weight/300 mL. The sorption experiment was conducted at a pH value close to 5 by adjusting with HCl or NaOH. The adsorbent particles in the solution were uniformly dispersed in the reactor. The initial metal concentration, C(0), was varied to investigate its
effect on the adsorption kinetics. During the kinetic experiments, samples were withdrawn at fixed time intervals and analyzed for metal content as described above. By plotting C(t)/C(0) against time (min), where C(t) is the solution metal concentration and C(0) is the initial metal concentration, the experimen-tal results are demonstrated.
Desorption
Desorption of Hg2þ ions was achieved by using 1N H2SO4 as the desorbing agent. The AGCC(5 : 10) loaded with Hg2þ ion in the following conditions-initial concentration of Hg2þ ion: 500 mg/L, amount of AGCC(5 : 10): 1 g-wet weight (0.0528 g-dry weight); volume of adsorption medium: 100 mL; pH: 5; temperature: 258C; and adsorption time: 24 h. Then, the AGCC(5 : 10) was placed in the desorption medium and stirred at a stirring rate of 200 rpm up to 24 h. The concentrations of Hg2þions in the aque-ous phase were determined as mentioned earlier. The extent of desorption percents was calculated from the following expression43:
desorptionð%Þ
¼amount of Hg2þ ions desorbed
amount of Hg2þion sabsorbed 100 ð2Þ
RESULTS AND DISCUSSIONS
Solubility test of chitosan and crosslinked chitosan In this study, chitosan was found to be soluble in 5% (v/v) acetic acid and insoluble in deionized
water. However, it was shown that after crosslink-ing, the crosslinked chitosan was found to be insolu-ble in 5% (v/v) acidic acid and deionized water. It is well-known that the high hydrophilicity of chitosan due to a primary amine group makes chitosan easily soluble in dilute acetic or formic acid solutions to yield a hydrogel in water. Thus, it is necessary to reinforce its chemical stability by a chemical cross-linking using glutaraldehyde.44,45 The reaction of aldehyde functions with amine groups leads to the formation of imine functions and the insolubility of the polymer even at low pH.46
Properties of AGCC
AGCC, calcium alginate beads containing crosslinked chitosan powders, were prepared by the liquid curing method in the presence of Ca2þions. They are spheri-cal in shape and their diameters are within a narrow range around 1.8, 2.7, and 3.6 mm diameter, respec-tively. Both crosslinked chitosan powders and cal-cium alginate beads were shown to be very stable at low pH.4,25,32 The operational stability of the adsorb-ent under specified solution conditions is also a very important parameter in the Hg2þadsorption. Besides, to allow regeneration by acids, the sorbent materials have to be insoluble at low pH values.
By using a mercury porosimeter, the porosity of AGCC(5 : 10) was found to be 0.59. Figure 1 presents SEM photographs of AGCC(5 : 10) with diameter of 2.7 mm. The beads are highly porous, which agrees with the porosity data. The scanning electron micro-graph of AGCC(5 : 10) also reveals a uniform chitosan distribution in the bead. This is an impor-tant criterion for the proper adsorption of Hg2þ ions
on AGCC(5 : 10). Thus, immobilization of chitosan in the alginate beads could provide advantages over the chitosan powder. Besides, it was demonstrated that the chitosan and alginate could be homogeneous mixed using this preparation method.
Adsorption isotherms
The adsorptive capacity of alginate bead and AGCC for Hg2þ removal was determined through adsorp-tion isotherm studies whose parameters were calcu-lated based on the dry weight of adsorbent. Figures 2 and 3 show the relationship between the quantities of Hg2þ adsorbed per unit dry mass adsorbent (Q) and the equilibrium concentration (Ce) in terms of chitosan content and particle size of the adsorbent, respectively. All of the isotherms showed similar behavior which can be described by using the Lang-muir adsorption equation as:
Ce Q¼ Ce Qmaxþ 1 ðQmaxÞKs (3)
where Qmax, the maximum adsorption capacity at monolayer coverage, and Ksis the Langmuir adsorp-tion equilibrium constant (mL/mg) which is a mea-sure of the energy of adsorption.
The plots of the experimental Q and Ce values as specific sorption (Ce/Q) against the equilibrium con-centration (Ce) for adsorption of Hg2þon adsorbents are shown in Figures 4 and 5. These isotherms are linear over the entire concentration range studied and the correlation coefficients are extremely high (R2 > 0.99), implying strongly that the sorptions of
Hg2þclosely follow the Langmuir model of sorption for monolayer adsorption onto a surface containing finite number of identical sites.15,32 Linearized plots of (Ce/Q) versus (Ce) give Qmax and Ks values and are summarized in Table I.
Unlike the adsorption of alginate/chitosan pellets for Cu2þ,36 the adsorption capacity of AGCC for Hg2þsignificantly increased with its chitosan content (Fig. 2). This might be due to the difference between the Hg2þadsorption capacity of chitosan and that of alginate. The maximum adsorption capacity of chito-san for Hg2þ is 815 mg/g,17 while that of alginate
Figure 2 Adsorption isotherms of Hg2þ ions on adsorb-ents with diameter of 2.7 mm: alginate bead (^), AGCC(5 : 2) (), and AGCC(5 : 10) (~) (the solid curves are calculated by the Langmuir equation).
Figure 3 Adsorption isotherms of Hg2þ ions on AGCC (5 : 10) with diameter of 1.8 mm (&) and 2.7 mm (~) (the solid curves are calculated by the Langmuir equation).
Figure 4 Langmuir isotherms of Hg2þions on adsorbents with diameter of 2.7 mm: alginate bead (^), AGCC(5 : 2) (), and AGCC(5 : 10) (~).
bead for Hg2þ is only 32 mg/g (Table I). Hence, the chitosan may play an important role in the adsorp-tion of Hg2þ ions. Although crosslinking did some-what reduce the adsorption capacity of the chitosan, but this loss of capacity may be necessary to ensure stability of the polymer.5 Moreover, it was reported that the heterogeneous crosslinking of chitosan flakes resulted in a significant decrease in sorption capaci-ties for large particle sizes, whatever the characteris-tics of the chitosan samples, while for small particle sizes the sorption capacities were comparable for raw and crosslinked materials.20The computed max-imum monolayer capacity Qmax of Hg2þ on the AGCC(5 : 10) has large value, ranging from 662 to 667 mg/g, which is over 20 times higher than that of plain alginate bead (Table I). The Hg2þ adsorption by chitosan was reported to increase with decreasing its particle size.6,17,19Therefore, in AGCC, small par-ticle size (0.075–0.106 mm) of the immobilized chito-san might result in enhancing its Hg2þ adsorption capacity. However, the high adsorption capacity of AGCC might be attributable in part to the high po-rosity of alginate bead so that all the activated sites of chitosan become accessible to the Hg2þ ions. In general, chitosan is more responsible for Hg2þ adsorption and alginate is more responsible for sup-porting structure in this bead system.
The size of sorbent particles has been shown to be a key parameter in the control of sorption perform-ances of several metal ions on chitosan. However, the influence of this experimental parameter depends on the chemistry of the metal ion and the characteristics of the sorbent.37 Therefore, different results about the influence of the size of chitosan
particles on metal adsorption have been demon-strated.6,13,15,37 It was reported that the particle size of chitosan in flaked or powdered forms influences Hg2þsorption performance, and these results can be explained by the surface limitations for chitosan par-ticles.6In this study, the Hg2þsorption capacity was found to be independent of the size of AGCC gel beads (Fig. 3, Table I). It means that sorption occurs in the whole mass of AGCC and not only on the external layer of AGCC bead. On the other hand, if equilibrium adsorption is proportional to the bead size, there is a restrictive layer at the bead surface and sorption is limited to the surface. Guibal et al. have demonstrated that particle size does not influ-ence equilibrium for chitosan beads, but increasing sorbent radius significantly decreases uptake capaci-ties for chitosan flakes.13 In this work, the sorption capacity of the alginate bead was drastically enhanced by immobilizing very small particle size of chitosan powders in alginate bead (Fig. 2). However, the capacity was confirmed to be independent of gel bead size (Fig. 3).
A greater Ks value indicates a steep initial slope of an isotherm, which in turn implies a high affinity of the adsorbent for the sorbate under dilute condi-tions.16 The Ks values of the isotherms for Hg2þ sorp-tion on the 2.7 mm adsorbents are in the order of AGCC(5 : 10)> AGCC(5 : 2) > alginate bead (Table I). This might be due to the high adsorption capacity of chitosan for Hg2þ. Otherwise, the Ks values of the isotherms for Hg2þ sorption on AGCC(5 : 10) are in the order of 1.8 mm adsorbent > 2.7 mm adsorbent (Table I). This is most likely because the smaller par-ticles have more outside surface area per weight.
AGCC(5 : 10) saturated with Hg2þ solutions were cross section analyzed by SEM. The location of sorbed mercury is determined with an X-ray EDS analyzer. Figure 6 shows the distribution pattern of the Hg2þ ions taken along the lines crossing the AGCC(5 : 10) beads. A uniform distribution of the Hg2þions throughout the beads was observed. This distribution demonstrates the formation of open pores and channels during the formation process of the AGCC(5 : 10) beads, which allow the penetration of solution into the inner part of the beads during experiments. Furthermore, the homogeneous
Figure 5 Langmuir isotherms of Hg2þ ions on AGCC(5 : 10) with diameter of 1.8 mm (&) and 2.7 mm (~).
TABLE I
Langmuir Isotherm Constants and Correlation Coefficients
Adsorbent Langmuir
Type Size (mm) Qmax(mg/g) Ks(L/mg) R2
Alginate bead 2.7 32 0.066 0.999 AGCC(5 : 2) 2.7 300 0.100 0.997 AGCC(5 : 10) 2.7 667 0.126 0.998 AGCC(5 : 10) 1.8 662 0.170 0.999
distribution of chitosan on the entire structure of the AGCC(5 : 10) is demonstrated as well. The results demonstrate that Hg2þ ions are able to reach chito-san immobilized in the entire structure of AGCC(5 : 10). Similar results have been reported for protonated alginate beads.27 Using SEM and X-ray EDS analyzer, it was demonstrated that after forma-tion of the AGCC(5 : 10) beads, the porous alginate bead has homogenously immobilized chitosan, where Hg2þ metals can be sorbed (Fig. 2, Fig. 6). In addition, these observations agree with the high adsorption capacity of AGCC(5 : 10).
Adsorption kinetics
By plotting C(t)/C(0) against time (min), where C(t) is the solution metal concentration and C(0) is the initial metal concentration, the experimental results are shown in Figures 7 and 8. Each plot shows the experiment results at an agitation rate of 200 rpm for initial concentrations of 10 and 50 ppm, respectively. Metal uptake by gel particles follows two-step kinetics. The extraparticle association (surface bind-ing) occurs first and rapidly. Further metal uptake is controlled by diffusion through the particle pores. The rate-limiting step is diffusion inside the alginate gel beads.47
Figure 6 EDS analysis of Hg2þion distribution in AGCC.
Figure 7 Influence of chitosan content of alginate bead on Hg2þ sorption kinetics using (a) 10 ppm and (b) 50 ppm mercury nitrate solution. The experiments were conducted for medium sized (diameter, 2.7 mm) alginate bead (^), AGCC(5 : 2) (), and AGCC(5 : 10) (~) at pH ¼ 5, agita-tion rate¼ 200 rpm, and temperature ¼ 258C.
The rates of Hg2þ adsorption by different adsorb-ents of the same size (diameter, 2.7 mm) are shown in Figure 7. Interestingly, the uptake rates for Hg2þ sorption are in the order of AGCC(5 : 10) > AGCC (5 : 2) > alginate bead. Addition of chitosan to algi-nate microsphere can result in significant enhance-ment of its uptake rate for Hg2þ. This might be attributable to the high adsorption capacity of chito-san for Hg2þand high porosity of alginate beads.
As would be expected, decreasing the particle size of the AGCC(5 : 10) results in rapid increase in the initial sorption rate as shown in Figure 8. The de-pendence of the uptake rate on size is probably because the smaller particles have more outside sur-face area per weight, resulting in the observed faster initial uptake.9 It was also reported that a decrease
in the size of immobilized biomass alginate bead leads to an increase in the rate of metal biosorp-tion.48
Increasing the initial concentration leads to an increase in uptake rate because of the higher driving force (Figs. 7 and 8). Besides, the time to reach equi-librium of Hg2þadsorption by AGCC(5 : 10) is only 60 min. The Hg2þ adsorption capacity and the time to reach equilibrium are presented in Table II, which also shows, for purposes of comparison, similar data from the literature. To increase the uptake capacity of mercury ions, several chemical modifications of chitosan beads which are crosslinked with glutaral-dehyde were performed.2,22,23 Among reported capacity result of adsorbents with feasible adsorptive kinetic property (adsorption equilibrium time ¼ 100 min), the maximum uptake capacity (667 mg Hg2þ/ g-dry mass) of AGCC(5 : 10) was to be one of the highest capacities, up to now. In general, both high uptake capacity and fast time to equilibrium were demonstrated as the excellent characteristics of AGCC(5 : 10).
Desorption and regeneration
Desorption studies will help to elucidate the nature of adsorption process and to recover the Hg2þions from AGCC.40 Moreover, it also will help to regenerate the AGCC, so that it can be used again to adsorb Hg2þ ions. Desorption experiments were performed by using 1N H2SO4solution as the desorption agent. De-sorption ratio for Hg2þ ions from the AGCC(5 : 10) loaded with 500 mg/L of Hg2þ ions was calculated by using eq. (2) and given in Figure 9. More than 95% of the adsorbed Hg2þ ions were desorbed with 1N H2SO4 solution. Adsorption/desorption cycles were repeated three times. When H2SO4 solution is used as the desorption agent, the Hg2þ ions were released from the solid surface into the desorption medium. Therefore, AGCC(5 : 10) can be used repeat-edly without significantly loosing their adsorption capacities for the Hg2þions.
TABLE II
The Hg2þAdsorption Capacity and the Time to Reach Equilibrium of Chitosan Containing Adsorbents Derived
from Literature and Experimental Data
Adsorbent Qmax (mg/g) Time to reach equilibrium (min) Paper source Chitosan bead 294 [22] Thiol-grafted chitosan bead 1,600 1,200 [2] Aminated chitosan bead 476 100 [23] AGCC(5 : 10) 667 60 This study Alginate bead 32 60 This study
Figure 8 Influence of adsorbent size on Hg2þ sorption kinetics using (a) 10 ppm and (b) 50 ppm mercury nitrate solution. The experiments were conducted for AGCC(5 : 10) with diameter of 3.6 mm (l) and 1.8 mm (&) at pH ¼ 5, agitation rate¼ 200 rpm, and temperature ¼ 258C.
CONCLUSIONS
Alginate and chitosan are all environmental friendly bioadsorbents for heavy metals. The chitosan flake was ground and sieved to collect the chitosan powder with particle size ranging from 0.075 to 0.106 mm. The collected chitosan powder was then crosslinked with glutaraldehyde to overcome solubil-ity. It was confirmed that the crosslinked chitosan is insoluble in acidic and neutral media. A viscous so-dium alginate solution containing certain amount of the crosslinked chitosan powders was directly dis-pensed drop wise into the calcium chloride solution to form spherical alginate gels. Three particle sizes with average diameters of 1.8, 2.7, and 3.6 mm were prepared. The SEM results reveal the uniform distri-bution of chitosan powders in the alginate gel bead.
Alginate gels containing chitosan, AGCC, have been successfully used as adsorbing agents for re-moval of Hg2þ from aqueous medium. The equilib-rium was well described by Langmuir adsorption isotherms. And the adsorption of mercury ions was almost completed in 60 min at 200 rpm. Addition of chitosan to alginate microsphere results in significant enhancement of not only its adsorption capacity but also its uptake rate for Hg2þ. Decreasing the bead size of the AGCC results in rapid increase in the ini-tial sorption rate for Hg2þ. However, the uptake capacity is independent of bead size. The SEM-EDX results demonstrate that Hg2þ ions are able to reach the chitosan immobilized in the entire structure of alginate beads.
The adsorbent, AGCC, can be regenerated and reused by acid treatment. The adsorbent was reused in three adsorption/desorption cycles with negligible decrease (up to 95% recovery) in sorption capacity. This adsorbent can be applied easily using existing treatment technologies.
These results suggest that AGCC is a strong candi-date as an adsorbent for the removal of Hg2þ from wastewaters.
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