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Poly(methacrylic acid)-modified chitosan for enhancement adsorption of water-soluble cationic dyes.


Cationic dyes known as basic dyes are widely used for dyeing of textile fibers such as cotton and polyester. These chemical materials often pose certain health hazards and environmental pollution. Therefore, environmental legislation has imposed stringent effluent limits on the concentrations of dye pollutants as chemical oxygen demand, biochemical oxygen demand, and/or color. Since many organic dyestuffs are harmful to human beings and toxic to microorganisms, removal of dyestuffs from wastewater has received considerable attention over the past decades. The most widely used methods for removing dyes from wastewater systems include physicochemical, chemical, and biological methods such as flocculation, coagulation, precipitation, adsorption, membrane filtration, electrochemical techniques, ozonation, and fungal decolorization [1]. In the aforementioned techniques, adsorption is recognized to be a promising and a cost-effective process to remove colors from aqueous solution. Many kinds of adsorbents have been developed for various applications [2-5]. Natural polymeric materials are gaining interest for application as adsorbents in wastewater treatment because of their biodegradable and nontoxic nature [6].

Chitosan is a natural, cationic, aminopolysaccharide copolymer of glucosamine and N-acetylglucosamine, obtained by the alkaline, partial deacetylation of chitin, which originates from the shells of crustaceans such as crabs and prawns [7]. Chitosan is a nontoxic, hydrophilic, biodegradable, biocompatible, mucoadhesive, and antibacterial biopolymer, which has a very diverse range of applications [8]. Acid environments lead to partial dissolution of the polymer, and to make the polymer insoluble in acidic medium, modification is done using cross-linking agents. Cross-linking enhances the resistance of chitosan against acid, alkali, and chemicals, but it reduces the adsorption capacity [9]. To improve its adsorption capacity, chemical modifications of chitosan through grafting have been explored as an interesting alternative method to develop novel hybrid materials composed of natural polysaccharide and synthetic polymers [10, 11]. The use of this polymer has been studied extensively by many researchers mainly for the removal of dyes. Lima et al. [12] proposed the use of chitosan chemically modified with succinic anhydride in the BB 9 adsorption. This chemical derivatization provides a powerful means to promote new adsorption properties, in particular, toward basic dyes in acidic medium. E1-Tahlawy and coworkers [13, 14] and Martel et al. [15] proposed the use of cyclo-dextrin-grafted chitosan as new chitosan derivatives for the removal of dyes. Uzun and Gu zel [16, 17] reported that carboxymethylated chitosan is a rather better adsorbent than raw chitosan for acidic dyestuffs, and its production is not costly. In these studies, materials often require much time and cost for modification and have low adsorptivity for dyes.

In this study, chitosan microspheres were synthesized using glutaraldehyde (GLA) as the crosslinker, according to a literature method [18] with some modifications. Then the GLA-cross-1inked chitosan (GLA-CTS) was modified with poly (methacrylic acid) through a graft-copolymerization reaction. Hence, the chitosan no longer needs to be immobilized, and the microspheres not only have high mechanical strength and good chemical stability but are also environmentally friendly. The adsorbent can be readily used without subsequent grinding and sieving, simplifying the pretreatment procedure and decreasing the yield loss. The uniform and spherical particles are easier to handle than the irregularly shaped materials and have better reproducibility. The modified chitosan was characterized using SEM, FTIR, and XPS analyses. Its adsorption capacity for the common water-soluble cationic dyes, methylene blue and malachite green, were studied. The adsorption behaviors including adsorption kinetics, isotherm, and pH-dependent performance were investigated.



Chitosan was purchased from Tianyuan Biotechnology (Wuhan, China; 84% deacetylated, the viscosity molecular weight 3.0 X [10.sup.5]). Two cationic dyes MB and MG were purchased from Sinopharm Group Chemical Reagent (Shanghai, China) and used without further purification. The chemical structures of the two dyes were shown in Fig. 1. The stock solutions were prepared by dissolving accurately weighed dyes in distilled water to the concentration of 1000 mg [L.sup.-1]. The experimental solutions were obtained by diluting the dye stock solutions in accurate proportions to different initial concentrations. All other chemicals were of analytical grade and used without further purification.


Preparation of Chitosan Microspheres

The GLA-CTS microspheres were prepared according to the procedures described elsewhere [18] with some modifications: 0.75 g of chitosan was dissolved in 25 mL of 2% aqueous acetic acid and then poured into 80 mL of paraffin containing 4 mL of emulsifier (Span-80) in a 250-mL round-bottomed flask at room temperature. During the process, the suspension was stirred with a mechanical stirrer at 1500 rpm for 30 min. Then 10 mL of 7.5% GLA was added and the mixture was stirred for another 1 h at 40[degrees]C. The pH of the mixture was adjusted to 9-10 by drop-wise addition of 1.0 mol [L.sup.-1] NaOH solution. The stirring was continued for a further 2 h at 70[degrees]C. Finally, the product was filtered and washed consecutively with petroleum ether, acetone, and distilled water. The microspheres thus obtained were dried overnight in an oven at 60[degrees]C.

Modification of Microspheres

The grafting reaction was carried out in a round-bottomed flask equipped with a reflux condenser at 60[degrees]C and the mixture stirred under nitrogen gas atmosphere. 0.20 g initiator [K.sub.2][S.sub.2][O.sub.8] was added into 40.0 mL N, N-dimethylformamide/[H.sub.2]O (1/1, v/v) solution containing 0.50 g of the GLA-CTS microspheres. After 20 min, 2.00 mL of the monomer (methacrylic acid) was added and the reaction was allowed to continue for another 3.5 h. At the end of the reaction, the product was thoroughly rinsed first with acetone, and then 0.1 mol [L.sup.-1] NaOH and distilled water was added to remove unreacted monomer and homo-polymer. Finally, the poly (methacrylic acid)-modified chitosan (GLA-PMAA-CTS) microspheres were dried overnight in an oven at 60[degrees]C and stored at ambient temperature until use.

Swelling Ratio Study

The swelling ratio of GLA-PMAA-CTS was determined by immersion in distilled water at room temperature for 24 h with gentle shaking. Subsequently, the weight of the swollen microsphere ([W.sub.1]) was measured and the swelling ratio ([mu]) was calculated according to the following equation:

[mu] = [([W.sub.1] - [W.sub.0])/W.sub.1] x 100%,(1)

where [mu is the swelling ratio of the GLA-PMAA-CTS, [W.sub.0] is initial dried weight of GLA-PMAA-CTS, and [W.sub.1] is the weight of the swollen microsphere.


Morphology. The surface morphology of the dried microspheres before and after modification was examined with a scanning electron microscopy (SEM, X-650; Hitachi, Japan): a fragment of the dried microspheres was mounted on the sample mount and sputter-coated with gold. The surface of the sample was scanned at the desired magnification.

FTIR Spectroscopy. The dried microspheres were thoroughly mixed with KBr and pressed into a pellet. The IR spectra were obtained using a Fourier transform IR spectrometer (Nicolet NEXUS-470).

X-ray Photoelectron Spectroscopy. The microsphere surface before and after modification were investigated by an X-ray photoelectron spectroscopy (XPS) (VGMultilab 2000) using monochromatized Al X-ray resource. The pressure in the analysis chamber was maintained at <[10.sup.-8] Torr during each measurement. High-resolution scans for the elements were performed with the pass energy of 25 eV. All binding energies were referenced to the neutral C(ls) peak at 284.6 eV to compensate for the surface charging effects.

Batch Adsorption Studies

Adsorption of MB and MG from aqueous solution was tested in batch experiments. Effects of the dye concentrations, pH of the medium, and contact time on the adsorption and capacity were studied and the sorption kinetics was also evaluated.

The experimental procedure is as follows: 25 mg modified microspheres samples were put into conical flasks, into which 50 mL aqueous solutions of MB and MG were added separately and vibrated at 125 rpm in a shaking water bath. When the adsorption reached equilibrium after 12 h, the conical flasks were taken out and centrifuged to separate the modified microspheres and the solution. The concentrations of the free dyes in the filtrate were analyzed with a UV--visible spectrometer.

The amount adsorbed was calculated based on the difference of the dye concentrations and the weight of the modified microspheres by the following equation:

[q.sub.e] = [([C.sub.0] - [C.sub.e])V]/[W](2)

where [q.sub.e] is the amount of dyes adsorbed onto unit amount of the sorbents (mg [g.sup.-1]), [C.sub.0] is the initial dye concentration (mg [L.sup.-1]), [C.sub.e] is the equilibrium dye concentration (mg [L.sup.-1]), V is the volume of dye solution (L), and W is the dry weight of the adsorbents (g).

The adsorption temperature was 25[degrees]C. The initial dye concentration was 500 and 250 mg [L.sup.-1] for MB and MG, respectively. For adsorption experiments, the concentration ranges used were 50-650 and 50-500 mg [L.sup.-1] for both MB and MG, respectively.


Preparation and Properties of Modified Chitosan Microspheres

In this experiment, the chitosan was cross-linked with GLA to improve the mechanical strength, to prevent their dissolution in acid solutions (pH <2), and to enhance the sorption performance. After which, the chitosan microspheres were modified using poly(methacrylic acid).

Figure 2 shows the SEM micrographs of the microspheres before and after surface modification. It is indicated that the GLA-PMAA-CTS microspheres' surface has been roughened.


FTIR spectra of the microspheres before and after modification are shown in Fig. 3. The FTIR spectrum of chitosan itself (Fig. 3a) shows characteristic peaks of amide groups: amide I and amide II bands at 1647 and 1598 [cm.sup.-1], respectively. The intense band at around 3420 [cm.sup.-1] should be assigned to the stretching vibration of O--H and/or N--H, as well as to intermolecular hydrogen bonding within the polysaccharide [19]. The absorption bands at 1154 (antisymmetric stretching of the C--O--C bridge), 1078, and 1031 [cm.sup.-1] (skeletal vibrations involving the C--O stretching) are characteristic of chitosan's saccharide structure [20]. Figure 3b shows the spectrum of GLA-CTS microspheres. Comparing with the chitosan itself, the new peak at 1640 [cm.sup.-1] represents the formation of the imine (C = N) linkage which confirms that cross-linking has taken place. In Fig. 3c, two new peaks at 1557 and 1399 [cm.sup.-1] are observed, which can be assigned to C = O asymmetric and symmetric stretching in the carboxylate ions [21].


XPS spectra have widely been used to identify the existence of a particular element and to distinguish the different forms of the same element in a material. Figure 4a and b shows typical wide-scan spectra. Three peaks for Cls, Nls, and Ols are observed. For the GLA-CTS, the atomic ratios (calculated from the peak area) of C:O is 77.46:21.07, while that for the GLA-PMAA-CTS is 45.05:46.43. The change in the ratio of oxygen atom to carbon atom is attributed to the introduction of poly(methacrylic acid). The C(ls) spectra of the GLA-CTS and GLA-PMAA-CTS are also investigated. As shown in Fig. 4c, three components at 284.6, 286.2, and 287.6 eV in the high-resolution XPS spectra in the Cls region of the GLA-CTS are observed, which is corresponded to C--H, C--O, or C--N and CON[H.sub.2], respectively [22]. However, a new peak at 288.7 eV is found for the GLA-PMAA-CTS, as shown in Fig. 4d, which is assigned to the carboxylic acid groups [23]. The relative peak areas of the fitted carbon of the GLA-CTS and GLA-PMAA-CTS are given in Table 1. From Table 1. it can be seen that after PMAA surface modification, the peak at 288.7 eV contains ~5.7% of the total intensity. The XPS results indicated that poly (methacrylic acid) was successfully grafted onto the GLA-CTS surface. From the FTIR and XPS analyses, it can be indicated that a larger number of carboxyl groups were introduced, which provided many active sites for dyes adsorption.

TABLE 1. Area ratios of C(1s) spectra for the GLA-CTS and

 Peak area ration (%)

 C--H (284.6 eV) C--O (or C--N) CON[H.sub.2] COOH
 (286.2 eV) (287.6 eV) (288.7eV)

GLA-CTS 65.8 25.0 9.2 --

PMAA-CTS 63.7 24.8 5.7 5.7

In the graft copolymerization reaction, potassium persulfate was used as an initiator. When an aqueous solution of persulfate was heated, it decomposed to yield sulfate radical along with the other free radical species. These free radical species reacted directly with hydroxyl groups [24] or amine groups to produce radicals on the polymer backbone. The mechanism of graft copolymerization of methacrylic acid onto GLA-CTS can be explained in terms if Eqs. 3-10 [25].

[S.sub.2][O.sub.8.sup.2-] [right arrow] 2[SO.sub.4.sup.-.](3)

2[SO.sub.4.sup.-] + [H.sub.2]O [right arrow] [HSO.sub.4.sup.-] + HO(4)

2HO [right arrow] HOOH(5)

HO + HOOH [right arrow] [H.sub.2]O + [HO.sub.2](6)





where [FORMULA NOT REPRODUCIBLE IN ASCII] and [FORMULA NOT REPRODUCIBLE IN ASCII] are the GLA-CTS microspheres and R * is the free-radical species.

The swelling ration of GLA-PMAA-CTS was 82% according Eq. 1.

Adsorption Isotherm Experiments

Studies on the adsorption isotherm are a prerequisite to understand the adsorbate--adsorbent interaction and to optimize the use of the adsorbent. Figure 5 shows the adsorption isotherms of MB and MG at 25[degrees]C on the GLA-CTS and the GLA-PMAA-CTS. The pH of the dye solution for MB and MG was 7.1 and 4.2, respectively. It was observed that the adsorption capacity increased with increase in equilibrium concentration and ultimately attained a saturated value for the two dyes.


The adsorption curves were fitted to both the Langmuir and Freundlich equations. The Langmuir isotherm model assumes monolayer adsorption on a surface with a finite number of identical sites, so that all sites are energetically equivalent and there is no interaction between the adsorbed molecules. The Freundlich expression is an empirical equation for adsorption on heterogeneous surface with a nonuniform distribution of heat of adsorption over the surface and multilayer sorption. The Langmuir and Freundlich isotherms are expressed as Eqs. 11 and 12, respectively:

[q.sub.e] = [Qb[C.sub.e]]/[1 + b[C.sub.e]] or [1]/[q.sub.e] = [1]/[Q] + [1]/[Qb[C.sub.e]](11)

[q.sub.e] = a[C.sub.e.sup.1/n] or log [q.sub.e] = log a + [1]/[n]log[C.sub.e](12)

where Q is the maximum amount of adsorption (mg [g.sup.-1]), [q.sub.e] is the adsorption capacity at equilibrium (mg [g.sup.-1]), b is the adsorption equilibrium constant (L m[g.sup.-1]), [C.sub.e] is the equilibrium concentration of substrates in the solution (mg [L.sup.-1]), a is the Freundlich constant related to adsorption capacity of adsorbent (m[g.sup.1-1/n] [g.sup.-1] [L.sup.1/n]), and n is the Freundlich exponent related to adsorption intensity. The Langmuir and Freundlich adsorption constants evaluated from the isotherms with the correlation coefficients are listed in Table 2. It showed that the Langmuir isotherm gave better fits than the Freundlich isotherm, and so it illustrates that the adsorption on the surface of the GLA-PMAA-CTS was a monolayer adsorption. According to the Langmuir equation, the maximum uptake capacities ([q.sub.m]) for MB and MG were 1000.0 and 523.6 mg [g.sup.-1], respectively.
TABLE 2. Adsorption isotherms and corresponding parameters for
dye binding by GLA-PMAA-CTS.

 Langmuir model

Dyes Q (mg b (L [R.sup.2] SD
 [g.sup.-1]) m[g.sup.-1])

MB 1000.0 0.94 0.997 1.28 X

MG 523.6 2.64 0.935 6.79 X

 Freundlich model

Dyes a (m[g.sup.1-1/n] [g.sup.-1] n [R.sup.2] SD

MB 390.3 1.814 0.937 0.084

MG 271.7 5.344 0.641 0.177

SD, standard deviation.

The Langmuir parameters can also be used to predict affinity between the sorbate and sorbent using the dimensionless separation factor [R.sub.L], which has been defined by Hall et al. [26] as

[R.sub.L] [1]/[1+b[C.sub.0]](13)

where [R.sub.L] is the dimensionless separation factor, [C.sub.0] is the initial concentration (mg [L.sup.-1]), and b is the Langmuir constant (L m[g.sup.-1]). The value of [R.sub.L] can be used to predict whether a sorption system is "favorable" or "unfavorable" in accordance with the criteria shown in Table 3. The values of [R.sub.L] for sorption of MB and MG on GLA-CTS and PMAA-GLA-CTS are shown in Fig. 6. The [R.sub.L] values indicated that sorption was more favorable for PMAA-GLA-CTS than GLA-CTS.

TABLE 3. Characteristics of adsorption Langmuir isotherms.

Separation factor, [R.sub.L] Type of isotherms

[R.sub.L] > 1 Unfavorable
[R.sub.L] = 1 Linear
0 < [R.sub.L] < 1 Favorable
[R.sub.L] = 0 Irreversible

Much literature has reported the use of chitosan to remove dyes from aqueous solution. Table 4 shows the comparison between MB and MG removals by GLA-PMAA-CTS and others adsorbents found in the literatures [6, 27--32]. Compared with these results, the GLA-PMAA-CTS in this work had higher adsorption capacity for the two dyes.
TABLE 4. Comparison of MB and MG removals between GLA-PMAA-CTS
and others found in literatures.

Adsorbents Dye [Q.sub.m] (mg Literature

The composite of chitosan and MB 330 [6]
activated clay

Activated carbon MB 580 [27]

Poly-[gamma]-glutamic acid MB 353 [28]

Jalshakti MB 172 [29]

CS-AC sorbent MB 835 [30]

GLA-CTS MB 101.4 This work

GLA-PMAA-CTS MB 1000.0 This work

Activated carbon MG 200 [31]

Jute fiber carbon MG 137 [32]

GLA-CTS MG 33.7 This work

GLA-PMAA-CTS MG 523.6 This work

Adsorption Thermodynamics

Equilibrium adsorption constant (K) was obtained following a method used by Khan and Singh [33]. First, the adsorption isotherm data were plotted as ln([q.sub.e]/[C.sub.e]) versus [q.sub.e] and extrapolated to zero [q.sub.e]. Then, a linear regression was performed on the experimental data based on least-squares analyses, and the intercept on the y-axis gives the value of ln K. Based on the ln K values, the standard free energy changes ([DELTA]G[degrees]) for the reaction were then calculated from the relationship

[DELTA]G[degrees] = -RT In K(14)

where R is the universal gas constant (8.314 J [mol.sup.-1] [K.sup.-1] and T is the temperature in Kelvin. The Gibbs free energies of adsorption for MB and MG were calculated to be--5.43 and--6.36 kJ [mol.sup.-1], respectively. The negative values of [DELTA]G[degrees] indicated that the adsorption of the two dyes on the GLA-PMAA-CTS were spontaneous under the experimental conditions.

Adsorption Kinetics Experiments

The sorption kinetics of dyes are illustrated in Fig. 7. The pH of the dyes solution for MB and MG were 7.1 and 4.2, respectively, and were kept constant during the process by adjustment with HCl or NaOH solution. The removal rates were very rapid during the initial stages of the sorption process. After a very rapid sorption, the uptakes increased with time and reached equilibrium values at ~ 12 h for the two dyes. The three phases of the dye sorptions could be attributed to boundary layer sorption, intraparticle diffusion, and sorption equilibrium respectively. Figure 7 shows that the dye adsorption capacities of both MB and MG on the GLA-PMAA-CTS were much higher than those of the GLA-CTS.


To obtain further insight into the mechanism of the adsorption of MB and MG on the GLA-PMAA-CTS, a pseudo-second-order mechanism was investigated. The pseudo-second-order equation was given by [34]

[d[q.sub.t]]/[dt] = [k.sub.2] [([q.sub.e] - [q.sub.t].sup.2](15)

where [k.sub.2] (g [mg.sup.-1] [min.sup.-1]) is the constant of pseudo-second-order rate, [q.sub.e] (mg [g.sup.-1]) is the adsorption capacity at equilibrium, and [q.sub.t] (mg [g.sup.-1]) is the adsorption capacity at time t (min). Separating the variables in Eq. 14 and integrating gives

[t]/[q.sub.t] = [1]/[k.sub.2][q.sub.e.sup.2] + [t]/[q.sub.e] = [1]/[v.sub.0] + [t]/[q.sub.e](16)

where [v.sub.0] represents the initial adsorption rate (mg [g.sup.-1][min.sup.-1]). The equilibrium adsorption capacity [q.sub.e] and the pseudo-second-order rate [v.sub.0] can be experimentally determined from the slope and the intercept of the plot t/[q.sub.t] against t. The corresponding parameters and regression coefficients for the model results are given in Table 5.
TABLE 5. Kinetic parameters of the pseudo-second-order equation
for MB and MG adsorption.



[v.sub.0] (mg [g.sup.-1] 54.1 3.29 168.4 19.9

[q.sub.e] (mg [g.sup.-1]) 99.1 34.2 1,001.9 473.9

[R.sup.2] 0.999 0.999 0.999 0.999

As shown in Table 5, the good fit ([R.sup.2] = 0.999) was obtained for both dyes, and their calculated equilibrium adsorption capacities [q.sub.e, cal] were consistent with the experimental data. These suggested that the pseudo second-order adsorption mechanism was predominant and that the overall rate of the dye adsorption process appeared to be controlled by the chemisorption process [35, 36]. Moreover, from the value of [v.sub.0], it was obvious that the initial adsorption rate of GLA-PMAA-CTS was higher than that of GLA-CTS. The higher rate had significant practical importance, as it would facilitate smaller reactor volumes ensuring efficiency and economy. On the other hand, the equilibrium adsorption capacity [q.sub.e] of GLA-PMAA-CTS for MB and MG were 1001.9 and 473.9 mg [g.sup.-1]. Compared with 99.1 and 34.2 mg [g.sup.-1] of the GLA-CTS, the increases were about 10-and 14-fold for MB and MG, respectively.

Influence of Initial pH

The effects of the pH of the sample solutions on the adsorption of dyes were evaluated by adjusting the pH by HCl or NaOH. The effect of initial pH on biosorption percentages of dyes was examined over a range of pH values from 2 to 11 and from 2 to 10 for MB and MG, respectively. As shown in Fig. 8, the dye adsorption for the two dyes was minimum at the initial pH 2. The dyes sorption increases with the increase in the initial pH from pH 2 to 6 and from 2 to 4 for MB and MG, respectively, and thus the dye adsorption was not significantly altered. For this reason, the pH 7.1 and 4.2 for MB and MG, respectively, was selected for future experiments. It was observed that the adsorption capacities for the cationic dyes increased significantly after the GLA-CTS microspheres had been modified with PMAA. The solution pH affected the activity of the functional groups (carboxyl) as well as the competition of cationic dyes for the binding sites. At lower pH value, the GLA-PMAA-CTS microspheres surface became more positively charged, thus increasing the electrostatic repulsive force between the modified microspheres and cationic dyes [37]. In contrast, as the pH value increased, the microspheres surface was more negatively changed, which promoted the uptake on the modified microspheres.



From this work it is seen that the GLA-PMAA-CTS could be an excellent adsorbent for cationic dyes. The adsorption capacities of the GLA-PMAA-CTS for MB and MG were enhanced significantly in comparison before the modification. The presence of PMAA on the micro-spheres surface was verified by FTIR and XPS. The kinetics of adsorption for both cationic dyes followed the pseudo-second-order equation demonstrating that the overall rate of the adsorption process was controlled by a chemisorption process. The experimental data were well fitted to the Langmuir isotherm model. This study shows that grafting effective groups on the chitosan might result in a biosorbent with high adsorption capacity for water-soluble cationic dyes.


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Yun Xing, (1) Xiaomei Sun, (2) Buhai Li (1), (2)

(1) College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China

(2) Key Laboratory of Analytical Chemistry of the State Ethnic Affairs Commission, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, China

Correspondence to: Buhai Li; e-mail:

DOI 10.1002/pen.2l253

Published online in Wiley InterScience (

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Date:Feb 1, 2009
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