Adsorption of methyl orange onto protonated cross-linked chitosan

2.1 Materials

Chitosan was purchased from the Sinopharm Group Chemical Reagent Limited Company (China). The degree of deacetylation was 90% and the molecular weight was 100,000 g/mol. Methyl orange (MO) was supplied by Sigma chemical company, and used as adsorbate in the tests. The chemical structures of MO and chitosan are shown in Fig. 1. MO concentrations were measured using a UV–vis spectrometer at a wavelength corresponding to the maximum absorbance for MO: 464 nm. Sodium hydroxide, glacial acetic acid, glutaraldehyde, chlorhydric acid and all other reagents were of analytical grade. pH of MO solutions was adjusted by adding 0.1 M HCl (or) NaOH solution.

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Figure 1

Chemical structures of (a) chitosan and (b) methyl orange.

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2.2 Preparation of protonated cross-linked chitosan

Chitosan (3.6 g) was dissolved thoroughly in 2.0 wt% glacial acetic acid solution (120 ml). The chitosan solution was coated in culture vessels to form uniform membranes. Subsequently, these membranes were soaked in 0.1 M NaOH solution to separate from these culture vessels, and washed with distilled water to neutral pH, and dried at 60 °C. These dry membranes were cross-linked with 2.5 wt% glutaraldehyde solution and the ratio of glutaraldehyde to chitosan was approximately 25 mL/g of dry membranes. Cross-linking reaction occurred for 23 h at 60 °C and then these cross-linked membranes were washed with distilled water to remove any free glutaraldehyde.

After the cross-linked chitosan membranes were treated with concentrated HCl for 90 min for protonation of membranes (Viswanathan et al., 2009), the protonated cross-linked chitosan membranes were washed with distilled water to neutral pH, dried at 60 °C, and were ground to obtain particles of 100-mesh size, which was used for adsorption studies.

2.3 Adsorption experiments

The adsorption of MO onto protonated cross-linked chitosan was studied by a batch method. A certain amount of adsorbent was placed in a 250 mL erlenmeyer flask, into which 50 mL MO solution was added. The adsorption experiments were performed in a temperature-controlled water bath shaker at 200 rpm. The supernatant was measured using a UV–vis spectrometer at 464 nm for its residual MO concentration. The different parameters studied included: (i) effect of adsorbent dosage: (0.06–0.27 g) at a constant temperature (20 °C), pH unadjusted; (ii) effect of pH: (1–10.5) at a constant temperature (20 °C) and different concentrations (100, 200, and 300 mg/L); (iii) effect of MO concentration: (100–500 mg/L) at a constant temperature (20 °C), pH unadjusted. (iv) effect of contact time: (5–330 min) at a constant temperature (20 °C) and different concentrations (100, 200, and 300 mg/L). (v) effect of adsorption temperature: (20, 30, and 40 °C) at different concentrations ranging from 100 to 700 mg/L.

Besides, the adsorption isotherms at different temperatures and pH values of MO solutions were investigated to understand the adsorption behavior. And the kinetics was also evaluated at different concentrations and pH values of MO solutions to investigate the adsorption characteristic.

3.1 Effect of adsorbent dosage

The effect of adsorbent dosage on MO adsorption was investigated by changing the adsorbent dosage from 0.09 to 0.27 g. The results (Fig. 2a) show that the removal percentage of MO increased as increasing adsorbent dosage. This was due to increased adsorbent surface and availability of more adsorption sites. However, even though the removal percentage toward MO enhanced, the adsorption capacity (mg of dye adsorbed/g of adsorbent) decreased with increasing adsorbent dosage. This decrease in adsorption capacity may be attributed to overlapping or aggregation of adsorption sites resulting in a decrease in total adsorbent surface area available to the dye and an increase in diffusion path length (Crini and Badot, 2008). When the adsorbent dosage exceeded 0.18 g, MO was almost removed completely. Considering the removal percentage together with adsorption capacity, 0.18 g of adsorbent dosage was chosen for further experiments.

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Figure 2

Effects of operation conditions on adsorption: adsorbent dosage (a); pH of MO solution (b); MO concentration (c); contact time (d) and adsorption temperature (e).

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3.2 Effect of pH

The pH value of MO solution plays an important role in the whole adsorption process, influencing not only the surface charge of the adsorbent, and the dissociation of functional groups on the active sites of the adsorbent, but also the chemistry of MO solution. The effect of pH value of initial MO solutions on adsorption is shown in Fig. 2b. The removal percentage decreased by 5.3%, 5.5%, and 6.7% for 100, 200, and 300 mg/L MO concentrations, respectively, when the pH value of MO solutions varied from 1.0 to 9.1. MO removal decreased significantly at these pH values higher than 9.1. These results indicated that this adsorbent has shown a relatively stable removal toward MO in a wide range of pH (1.0–9.1). According to the literatures (Zhu et al., 2009; Javed et al., 2011; Chiou and Li, 2002), the optimum pH was frequently reported to be around pH 3–6 when chitosan was used as adsorbent. In this study, a great adsorption was observed in a wide pH range of 1.0–9.1. This may be attributed to the protonation of amine groups in chitosan before adsorption. Adsorption mechanism may be as follows:

First MO is dissolved in an aqueous solution after which the sulfonate groups of MO dye (R–SO₃Na) become dissociated and converted into anionic dye ions. $$(\text{i})\text{R}-{\text{SO}}_3\text{Na}+{\text{H}}_2\text{O}\to \text{R}-{\text{SO}}_3^{-}+{\text{Na}}^{+}$$ Second the adsorption process is performed due to electrostatic attractions between the adsorbent surface and the MO anions $$(\mathit{ii}){\text{RNH}}_3^{+}+{\text{RSO}}_3^{-}\to {\text{RNH}}_3^{+}{\text{O}}_3\text{SR}$$ However, the adsorption weakened with increasing the pH value of MO solution. The decrease of MO adsorption can be explained by the competition of the abundant presence of OH⁻ ions in basic solution for adsorption sites with MO anions. Since this adsorbent showed a strong adsorption at natural pH, the pH values of MO solutions were unadjusted for further experiments without special instructions.

3.3 Effect of initial MO concentration

Fig. 2c shows the effect of initial MO concentration on adsorption. The removal percentage reduced with an increase in initial MO concentration at a fixed adsorbent dosage (0.18 g). This trend may be explained as follows. In the case of lower concentrations, the ratio of initial number of dye molecules to the available adsorption sites was low and subsequently more adsorption sites were available for dye molecules, thus the removal percentage increased. However, at higher concentrations, the ratio of initial number of the available adsorption sites to dye molecules was low, thus the number of available adsorption sites becomes lower and subsequently the removal of dyes decreased. Fig. 2c also shows that the adsorption capacity of the protonated cross-linked chitosan increased compared with one of the cross-linked chitosans for the same MO concentration, as expected. Protonation was able to improve the adsorption of anion dye onto chitosan effectively.

3.4 Effect of contact time

Contact time is another important variable in adsorption processes. Fig. 2d shows the effect of contact time on adsorption for various MO concentrations. The results show that with increasing MO concentration, the time required to reach equilibrium increased accordingly. For initial dye concentrations of 100, 200, and 300 mg/L, the times reaching equilibrium were 30, 100 and 270 min, respectively. At low initial concentrations, the MO adsorption by chitosan was very intense and reached equilibrium very quickly. However, during the adsorption process, the adsorbent surface was progressively blocked by dye molecules, becoming covered after some time. The hindrance enhanced with increasing dye concentration, and thus the time for adsorption equilibrium increased accordingly. When Gibbs et al. (2003) investigated the adsorption of AG 25 onto chitosan, the similar results were also observed. The time attaining equilibrium increased with increasing concentrations. Their study found that 1–2 h was sufficient to achieve complete recovery of the dye at initial concentrations below 100 mg/l. However, for the highest concentration (200 mg/l), 8 h was necessary to reach equilibrium. In this work, in order to achieve adsorption equilibrium, the data were measured in 12 h for adsorption isotherms.

3.5 Effect of adsorption temperature

A plot of the removal percentage toward MO with initial concentration from 100 to 700 mg/L at different adsorption temperatures (20, 30, and 40 °C) is shown in Fig. 2e. The results reveal that the removal percentage increased with increasing temperatures for the same MO concentration. The increase in temperature would provide a faster rate of diffusion of adsorbate molecules from the solution to the adsorbent (Bernardin, 1985). Also, a rise in temperature would produce a swelling effect together with an increase in the internal structure of chitosan. The greater the particle pore sizes were, the smaller the resistance of dye molecules into chitosan was. Therefore, the above aspects resulted in an increase in removal percentage. Similar profiles were found by Dutta et al. (2001). When they studied the adsorption of reactive and direct dyes on chitosan, they also observed that as the temperature of the solution increased, so did the extent of adsorption. However, generally speaking, the adsorption process is not usually operated at high temperatures because this would increase operation costs. Therefore, in this study, room temperature was adopted for the following experiments.

3.6 Adsorption isotherms

The equilibrium adsorption isotherm can be obtained by plotting the concentration at the surface of a solid phase versus the concentration in the liquid phase. The distribution of the adsorbate between the sorbent and the liquid phase is a measure of the position of equilibrium in the adsorption process and can generally be expressed by isotherm models. In the present investigation, the equilibrium data were analyzed according to Langmuir (Eq. (1)) and Freundlich (Eq. (2)) adsorption isotherms:

The sorption data were analyzed according to the Eq. (1) of the Langmuir isotherm and Eq. (2) of the Freundlich isotherm. The plots of C_e/q_e against C_e, and log q_e against log C_e for solutions, are shown in Figs. 3 and 4, respectively. The parameters for the two isotherm models are summarized in Table 1. It can be seen that the Langmuir model was obviously the most appropriate to describe the adsorption process due to its high correlation coefficients relative to the ones obtained from Freundlich model. The equilibrium monolayer capacities, Q₀, enhanced with increasing temperature from 20 to 40 °C, indicating that the adsorption of MO onto the protonated cross-linked chitosan was in favor of high temperatures. Also, the equilibrium monolayer capacities, Q₀ changed slightly at three different pH values, and this fact suggests that the MO adsorption onto protonated cross-linked chitosan was stable in a wide range of pH, which was in accordance with the results mentioned in Section 3.2. Table 2 lists the comparison of maximum monolayer adsorption capacity of MO on various adsorbents. From Table 2, it was found that the protonated cross-linked chitosan studied had a relatively high adsorption capacity toward MO.

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Figure 3

Langmuir models for MO adsorption onto protonated cross-linked chitosan at different temperatures (a) and at different pH values of solutions (b).

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Figure 4

Freundlich models for MO adsorption onto protonated cross-linked chitosan at different temperatures (a) and at different pH values of solutions (b).

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3.7 Adsorption kinetics

In order to investigate the adsorption characteristic further, the pseudo first-order and pseudo second-order equations were used to test the experimental data of various MO concentrations, and pH. The expressions of pseudo first-order and second-order equations were given according to Eqs. (3) and (4), respectively.

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Figure 5

Pseudo first-order models for MO onto protonated cross-linked chitosan at pH 4.5 (a), 6.7 (b) and 9.1 (c).

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Figure 6

Pseudo second-order models for MO onto protonated cross-linked chitosan at pH 4.5 (a), 6.7 (b) and 9.1 (c).

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