Kinetics and thermodynamics of the adsorption of some dyestuffs and p-nitrophenol by chitosan and MCM-chitosan from aqueous solution

Kinetics and thermodynamics of the adsorption of some dyestuffs and p-nitrophenol by chitosan and MCM-chitosan from aqueous solution

Journal of Colloid and Interface Science 274 (2004) 398–412 www.elsevier.com/locate/jcis Kinetics and thermodynamics of the adsorption of some dyestu...

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Journal of Colloid and Interface Science 274 (2004) 398–412 www.elsevier.com/locate/jcis

Kinetics and thermodynamics of the adsorption of some dyestuffs and p-nitrophenol by chitosan and MCM-chitosan from aqueous solution ˙Ilhan Uzun ∗ and Fuat Güzel Department of Chemistry, Faculty of Education, Dicle University, 21280 Diyarbakir, Turkey Received 25 August 2003; accepted 6 February 2004 Available online 19 March 2004

Abstract The effect of initial concentration, temperature, and shaking rate on the adsorption of three dyestuffs [orange II (O-II), crystal violet (CV), and reactive blue 5 (RB5)] and an ideal adsorbate, p-nitrophenol (PNP), by chitosan (Sigma C-3646) and the effect of temperature on the adsorption of O-II and CV by monocarboxymethylated chitosan (MCM-chitosan) were investigated. Kinetic data obtained for the adsorption of each dyestuff and PNP by chitosan and of O-II and CV by MCM-chitosan at different temperatures were applied to the Lagergren equation, and adsorption rate constants (kads ) at these temperatures were determined. These rate constants related to the adsorption of O-II and RB5 by chitosan and of O-II by MCM-chitosan were applied to the Arrhenius equation, and activation energies (Ea ) were determined. In addition, the isotherms for adsorption, at different temperatures, of each dyestuff and PNP by chitosan and of O-II and CV by MCM-chitosan were also determined. These isothermal data were applied to linear forms of isotherm equations that they fit, and isotherm constants were calculated. Because the isotherm curves obtained for the adsorption of O-II and CV by chitosan and of CV by MCM-chitosan fit the Langmuir adsorption isotherm, b constants were applied to thermodynamic equations, and thermodynamic parameters (G, H , and S) were calculated. Lastly, chitosan and MCM-chitosan were compared with respect to the ability to take up the dyestuffs and PNP.  2004 Elsevier Inc. All rights reserved. Keywords: Chitosan; MCM-chitosan; Dyestuff adsorption from aqueous solution; Adsorption kinetics and thermodynamics; Specific surface area

1. Introduction Human beings have benefited from the natural environment and worked to make beautiful it since early time. They obtained most dyes and dyestuffs from nature. It is possible to see this even in the Stone Age. In France and Spain pictures and other objects on cave walls dating hundreds of years before Christ are proof of the use of dyes. The red bones found in these places are very interesting. It is supposed that this color derives from the iron oxide painted on corpses as a religious tradition. Thus, dyes and dyestuffs were used in that period [1]. As for our time, most dyestuffs are classified according to their solubility, coloring properties, and chemical structure. After coloring is completed, the waste ought not to be discharged into the environment without purification, because dyestuffs are carcinogenic. These compounds can contaminate underground water in trace amounts by leaking from soil and pose a very great risk to * Corresponding author.

E-mail address: [email protected] (˙I. Uzun). 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.02.022

human health over a long period. Consequently, the waste solutions ought to be purified. The aim of the present study was to investigate the effects of initial concentrations, temperature, and shaking rate on the adsorption of orange II (O-II, acidic), crystal violet (CV, basic), reactive blue 5 (RB5, reactive) and p-nitrophenol (PNP, ideal adsorbate) by chitosan and the effect of temperature on the adsorption of O-II and CV by monocarboxymethylated chitosan (MCM-chitosan), to determine the conditions for maximum removal of adsorbates and to compare the uptake capabilities of chitosan and MCM-chitosan (modified chitosan) from aqueous solution. For this purpose, some kinetic and thermodynamic equations have been used. These substances are toxic. In addition, when in contact with the eyes or skin, they cause irritation. 2. Experimental In this study, chitosan (deacetylation degree was a minimum of 85%; Sigma, Germany) and MCM-chitosan (pre-

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pared from chitosan) as adsorbents and O-II (Sigma, Germany), CV (Merck, Germany), RB5 (Sigma, Germany), and PNP (Fluka, Switzerland) as adsorbates were used. A kinetic study to investigate the effect of initial concentration on the adsorption of O-II, CV, RB5, and PNP by chitosan from aqueous solution was carried out first. Two-tenthsgram samples of chitosan were mixed with 50-mL samples of each dyestuff and PNP, the initial concentrations of which were know, and then mixtures were shaken. Absorbance was measured with a Shimadzu UV-120-02 spectrophotometer after different time intervals at λmax = 487 nm for O-II, λmax = 590 nm for CV, λmax = 599 nm for RB5, and λmax = 318 nm for PNP. In addition, the effects of temperature and shaking rate on the adsorption of O-II, CV, RB5, and PNP by chitosan from aqueous solution were similarly investigated. After this kinetic study, an isotherm study for each dyestuff and PNP was carried out. First, 0.2-g samples of chitosan were mixed with 50-mL samples of solutions of different initial concentration (C0 ) prepared from stock solutions of each dyestuff and PNP and shaken for their equilibrium contact times at 293 K and 150 rpm. After this shaking, the absorbance of remaining, nonadsorbed solution was measured. Then, the adsorption isotherms of each dyestuff and PNP were investigated at 333 K and 150 rpm. Chitosan was modified to increase its adsorption capability. To that end, 45.0 g chitosan (equivalent to 0.275 mol glucosamine unit) and 51.57 g monochloroacetic acid (0.55 mol) were stirred together with 75 mL pyridine as the catalyst in 750 mL ethanol and refluxed under a nitrogen atmosphere for 3 days (Scheme 1). The product gel was washed, sequentially, with 0.5 mol L−1 sulfuric acid solution and deionized water after filtration, and again stirred in 0.3 mol L−1 acetic acid solution. After filtration, it was further washed, sequentially, with water, 1 mol L−1 aqueous sodium hydroxide solution, water, 0.5 mol L−1 sulfuric acid solution, and, finally, water. It was dried in vacuo to a constant weight before use. The introduction of carboxylic groups was confirmed by its infrared spectrum. The degree of carboxymethylation, as measured by neutralization titration, was greater than 90% [2]. The effect of temperature on the adsorption of O-II (acidic) and CV (basic) by this modified chitosan from aqueous solution was investigated. Kinetic experiments were carried out at 293 and 333 K. Then, the adsorption isotherms of O-II and CV were investigated at 293 and 333 K. These

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kinetic and isotherm studies were carried out as described above for the experiments using chitosan. To compare their adsorption capabilities, 0.2-g samples of chitosan and MCM-chitosan were mixed with 50-mL samples of each dyestuff and PNP of known initial concentration (C0 : 1.2845 mmol L−1 for O-II, 0.3677 mmol L−1 for CV, 0.5812 mmol L−1 for RB5, and 1.0783 mmol L−1 for PNP) and shaken for their equilibrium contact times at 313 K and 150 rpm. Then, the absorbance of the solutions was measured. Finally, the BET surface area (S) of commercial chitosan was measured with a Micromeritics Flow Sorb II 2300 (Shimadzu Corp., Japan) and scanning electron micrographs were taken.

3. Results and discussion Figs. 1, 2, and 3 show the effect of initial concentration, temperature, and shaking rate, respectively, on the adsorption of three dyestuffs and PNP by chitosan from aqueous solution. A small increase in the amount of O-II adsorbed occurred when its initial concentration was increased, and O-II was adsorbed faster but in smaller amounts at higher temperature and in greater amounts at higher shaking rate. These results most probably arise because physical adsorption and chemical adsorption are occurring simultaneously between O-II and chitosan. Simultaneous physical and chemical adsorption is known as sorption. Less O-II is adsorbed because of the desorption that occurs as a result of physical adsorption, whereas O-II is adsorbed faster because of the chemical adsorption on chitosan at higher temperature. Such a result was also reported by Yoshida et al. [3]. O-II is an anionic dyestuff. The pKa value of the amino group (R–NH2 ) in the structure of chitosan is 6.3, and the amino group dissociates partly into R–NH+ 3 even at pH 6.9 [4]. As an aqueous solution of O-II is acidic, the amino group in the structure of chitosan has a positive charge when it is introduced into this solution, and a chemical affinity forms between this positive charge and negative charge in the structure of O-II. In addition, there is some boundary layer resistance inhibiting the adsorption process, and higher agitation rates are able to decrease this effect [5]. A great increase in the amount of CV adsorbed also occurred when its initial concentration was increased, and less CV was adsorbed at higher temperature and more CV at higher shaking rate. These results may

Scheme 1. Route of the synthesis of monocarboxymethylated (MCM)-chitosan.

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Fig. 1. Effect of initial concentration on the adsorptions of (a) O-II, (b) CV, (c) RB5, and (d) PNP by chitosan from aqueous solution.

be attributed to the physical adsorption occurring between CV and chitosan. As is known, physical adsorption is generally a type of adsorption occurring with a polylayer. CV is a cationic dyestuff. Consequently, an important change

in the structure of chitosan does not occur when it is introduced into this solution due to the basicity of the aqueous solution of CV. Because of the physical adsorption occurring between CV and chitosan, most probably, more CV is

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401

Fig. 2. Effect of temperature on the adsorptions (a) O-II, (b) CV, (c) RB5, and (d) PNP by chitosan from aqueous solution.

adsorbed at higher shaking rate. A significant change in the amount of RB5 adsorbed does not occur when its initial concentration is increased, and more RB5 is adsorbed at higher temperature; and a significant change in the amount of RB5

adsorbed does not occur when the shaking rate is changed. These results are most likely due to the very strong chemical adsorption occurring between RB5 and chitosan. As is known, chemical adsorption is a type of adsorption occur-

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Fig. 3. Effect of shaking rate on the adsorptions of (a) O-II, (b) CV, (c) RB5, and (d) PNP by chitosan from aqueous solution.

ring with a single layer. RB5 is a reactive dyestuff. There are three –SO− 3 groups per molecule in its structure. These –SO− groups make the RB5 rather acidic. The amino group 3 in the structure of chitosan has a positive charge when it is

introduced into this solution because of the acidity of the aqueous solution of RB5, and a very strong chemical affinity forms between this positive charge and negative charges in the structure of RB5. As a result of this chemical affinity,

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Scheme 2.

(a)

(b)

(c)

(d)

Fig. 4. The molecular structures of (a) O-II, (b) CV, (c) RB5, and (d) PNP.

the resistance of the boundary layer surrounding the adsorbent weakens. Thus, most probably, the effect of the shaking rate on the adsorption of RB5 by chitosan is unimportant. A great increase in the amount of PNP adsorbed also occurs when its initial concentration is increased, and less PNP is adsorbed at higher temperature and more at a higher shaking rate. These results may be explained by the significant physical adsorption occurring between PNP and chitosan. PNP is an acidic chemical used in the synthesis of the dyestuff. Its pKa value is 7.15. The O–H bond can be broken off easily, and the nitro group causes the structure to achieve resonance stability by helping to delocalize the negative charge (Scheme 2) [6]. The amino group in the structure of chitosan has a positive charge when it is introduced into this solution because of the acidity of the aqueous solution of PNP, and a chemical interaction occurs between this positive charge and negative charge existent and delocalized in the anionic structure of PNP. However, the adsorption between chitosan and PNP is significantly physical, because PNP is a very weak acid and has resonance stability due to its anionic structure. Because of the significant physical adsorption occurring between PNP and chitosan, most probably, more PNP is ad-

sorbed at a higher shaking rate. The molecular structures of O-II, CV, RB5, and PNP are shown in Fig. 4. Experimental data related to the adsorption of O-II, CV, RB5, and PNP by chitosan at different temperatures were applied to the Lagergren equation [7] (Fig. 5), kads t, (1) 2.303 and the adsorption rate constants (kads ) in Table 1 were determined, where qe and q (both in mmol L−1 ) are the amounts of adsorbate adsorbed at equilibrium and time t (min), respectively. As can be seen from the kads constants in Table 1, O-II and RB5 are adsorbed faster at higher temperature, and CV and PNP are adsorbed faster at lower temperature on chitosan. The kads constants related to the adsorption at different temperatures of O-II and RB5 by chitosan were applied to the Arrhenius equation [8] (Fig. 6),

log(qe − q) = log qe −

Ea 1 , (2) 2.303R T and activation energies (Ea ) equivalent to the energies of adsorbed O-II and RB5 were determined 25.74 and 7.02 kJ mol−1 , respectively. log kads = log A −

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Fig. 5. Lagergren plots of kinetic curves related to (a) O-II, (b) CV, (c) RB5, and (d) PNP.

Fig. 7 shows the effect of temperature on the adsorption isotherms of three dyestuffs and PNP by chitosan from aqueous solution. Because the adsorption isotherms related to O-II and CV fit the Langmuir adsorption isotherms [9],

the experimental data were applied to the Langmuir linear isotherm equation (Fig. 8), Ca = Cm

bCe , 1 + bCe

(3)

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Ce Ce 1 + = , (4) Ca Cm b Cm and the Langmuir constants (Cm and b) were calculated (Table 2), where Cm (mmol g−1 ) and b (L mmol−1 ) are Langmuir constants related to the capacity and energy of adsorption, respectively. The b constants at every two temperatures were applied to the equations [10] log b = log A −

H 1 , 2.303R T

(5)

G = −RT ln b,

(6)

and H (enthalpy change for the adsorption of adsorbate by chitosan) and G (free enthalpy change for the adsorption of adsorbate by chitosan) in Table 3 were calculated, respectively. H and G were applied together to the equation [10] G = H − T S,

(7)

Table 1 Adsorption rate constants (kads ) related to the adsorption of three dyestuffs and PNP by chitosan from aqueous solution

293 313 333

O-II

CV

RB5 × 103

0.0039 0.0067 0.0138

0.0295 0.0253 0.0210

4.2368 5.3798 5.9853

and S (entropy change for the adsorption of adsorbate by chitosan) values in Table 3 were calculated. The Cm constants at every two temperatures were applied to the equation Cm (mol g−1) N (1/mol) δ (m2 ) , (8) n and the specific surface areas of chitosan in Table 3 were calculated, where N is Avogadro constant, δ is the average area occupied by a molecule of adsorbate in the completed monolayer (this value is 120 Å2 for O-II and 225 Å2 for CV), and n is the aggregation number (this value is 3 for O-II and CV) [11]. The negative H and larger Cm constant at lower temperature confirmed that more O-II and CV are adsorbed by chitosan at lower temperature. The type of isotherm obtained at every two temperatures relative to the adsorption of RB5 is more evidence of the strong chemical adsorption between RB5 and chitosan. This type of isotherm is known as an H-type isotherm according to Giles’ isotherm classification [12]. Because the adsorption isotherms of PNP fit Freundlich adsorption isotherm [13], the experimental data were applied to the Freundlich linear isotherm equation (Fig. 9), S (m2 g−1 ) =

1/n

Ca = kCe ,

kads (min−1 )

T (K)

PNP × 103 8.7889 7.3459 5.9846

405

(9)

1 log Ce , (10) n and the Freundlich constants (k and n) in Table 4 were calculated: where k (L g−1 ) and n(−) are Freundlich constants

log Ca = log k +

Fig. 6. log kads versus 1/T for the adsorptions of (a) O-II, (b) RB5 by chitosan from aqueous solution.

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Fig. 7. Effect of temperature on the adsorption isotherms of (a) O-II, (b) CV, (c) RB5, and (d) PNP by chitosan from aqueous solution.

related to the capacity of adsorbent to adsorb and the tendency of the adsorbate to be adsorbed, respectively. The amounts adsorbed (Ca ) for the same equilibrium concentrations (Ce ) at every two temperatures were determined.

These Ca values were applied to the Clausius–Clapeyron equation [8], log Ca = log A −

H 1 , 2.303R T

(11)

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Fig. 8. Langmuir linear isotherm plots related to the adsorptions of (a) O-II and (b) CV by chitosan from aqueous solution. Table 2 Langmuir constants related to the adsorption isotherms of two dyestuffs by chitosan from aqueous solution T (K)

O-II

293 333

CV

Cm (mmol g−1 )

b (L mmol−1 )

Cm × 103 (mmol g−1 )

b (L mmol−1 )

0.3295 0.3215

3.9620 2.9942

1.5457 0.7449

74.8844 70.9439

Table 3 Thermodynamic parameters related to the adsorption of two dyestuffs and PNP by chitosan from aqueous solution and the specific surface areas of chitosan Dyestuff

O-II CV PNP

293 K

333 K

H (J mol−1 )

G (J mol−1 )

S (J mol−1 K−1 )

S (m2 g−1 )

H (J mol−1 )

G (J mol−1 )

S (J mol−1 K−1 )

S (m2 g−1 )

−5,680.8 −1,096.4 −11,450.6

−3,353.8 −10,513.7 –

−7.9421 32.14 –

79.3436 0.6979 –

−5,680.8 −1,096.4 −11, 450.6

−3,036.2 −11,799.3 –

−7.9417 32.14 –

77.4172 0.3363 –

Table 4 Freundlich constants related to the adsorption isotherms of PNP by chitosan from aqueous solution k (L g−1 ) n (−)

293 K

333 K

0.0258 1.4166

0.0131 1.5492

and the H (enthalpy change for the adsorption of PNP by chitosan) value in Table 3 were calculated. The negative H and larger k constant (Table 4) at lower temperature

confirmed that more PNP is adsorbed by chitosan at lower temperature. Fig. 10 shows the effect of temperature on the adsorption of two dyestuffs by MCM-chitosan from aqueous solution. More O-II is adsorbed at higher temperature. This result may be explained on the basis of the strong chemical adsorption occurring between O-II and MCM-chitosan. MCM-chitosan (product having high possibility) resembles an amino acid structurally (Scheme 1). As is known, amino acids are less acidic than most carboxylic acids and less basic than most amines. The reason for these unexpected properties of amino

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acids is that amino acids contain a basic amino group (–NH2 ) and acidic carboxylic group (–CO2 H) in the same molecule.

Fig. 9. Freundlich linear isotherm plots related to the adsorption of PNP by chitosan from aqueous solution.

In the dry solid state, amino acids exist as dipolar ions, a form in which the carboxyl group is present as a carboxylate ion, –CO− 2 , and the amino group is present as an aminium group, –NH+ 3 (Scheme 3). Dipolar ions are also called zwitterions [6]. MCM-chitosan, similarly to the intramolecular acid–base reactions of amino acids, gives the intramolecular acid–base reaction in Scheme 4. As a result, the amino group in the structure of MCM-chitosan is charged positively. This amino group charged positively because the effect of the zwitterion increases when it is placed into the acidic solution of O-II, and a very strong chemical affinity forms between this positively charged amino group and the –SO− 3 group in the structure of O-II. Consequently, more O-II is adsorbed by MCM-chitosan at higher temperature because of the chemical adsorption between O-II and MCMchitosan. More CV is adsorbed at lower temperature. This result may be attributed to the physical adsorption occurring between CV and MCM-chitosan. The amino group in the structure of MCM-chitosan is charged positively because of the intramolecular acid–base reaction as stated above. Consequently, adsorption between CV and MCM-chitosan is a physical adsorption. Experimental data related to the adsorption of O-II and CV by MCM-chitosan at different temperatures were applied to the Lagergren equation (Fig. 11), and the kads constants in Table 5 were calculated. As can be seen from Table 5, O-II is adsorbed faster at higher temperature and CV is adsorbed faster at lower temperature by MCMchitosan. The kads constants related to the adsorption of O-II

Fig. 10. Effect of temperature on the adsorptions of (a) O-II and (b) CV by MCM-chitosan from aqueous solution.

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by MCM-chitosan at different temperatures were applied to the Arrhenius equation, and Ea , equivalent to the energy of O-II adsorbed, was determined as 1.56 kJ mol−1 . Fig. 12 shows the effect of temperature on the adsorption isotherms of two dyestuffs by MCM-chitosan from aqueous solution. The type of isotherm obtained at every two temperatures relative to the adsorption of O-II is more evidence of the strong chemical adsorption between O-II and MCMchitosan. Because the adsorption isotherms of CV fit the Langmuir adsorption isotherm, the experimental data were applied to the Langmuir linear isotherm equation (Fig. 13), and the Langmuir constants (Cm and b) in Table 6 were cal-

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culated. The b constants at every two temperatures were applied to Eqs. (5) and (6), and the thermodynamic parameters H (enthalpy change for the adsorption of CV by MCMchitosan) and G (free enthalpy change for the adsorption of CV by MCM-chitosan) in Table 7 were calculated, respectively. These parameters were applied together to Eq. (7), and the thermodynamic parameter S (entropy change for the adsorption of CV by MCM-chitosan) in Table 7 was calculated. The Cm constants at every two temperatures were also applied to Eq. (8), and the specific surface areas of MCM-chitosan in Table 7 were calculated. The negative H Table 5 Adsorption rate constants (kads ) related to the adsorption of two dyestuffs by MCM-chitosan from aqueous solution kads (min−1 )

T (K)

Scheme 3.

293 333

O-II

CV

0.0050 0.0054

0.0094 0.0088

Scheme 4.

Fig. 11. Lagergren plots of kinetic curves related to the adsorptions of (a) O-II and (b) CV by MCM-chitosan from aqueous solution.

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Fig. 12. Effect of temperature on the adsorption isotherms of (a) O-II and (b) CV by MCM-chitosan from aqueous solution. Table 6 Langmuir constants related to the adsorption isotherms of CV by MCMchitosan from aqueous solution Cm × 104 (mmol g−1 ) b (L mmol−1 )

293 K

333 K

4.1377 123.0263

3.9888 39.4868

Table 7 Thermodynamic parameters related to the adsorption of CV by MCMchitosan from aqueous solution and the specific surface areas of MCM-chitosan H (J mol−1 ) G (J mol−1 ) S (J mol−1 K−1 ) S (m2 g−1 )

293 K

333 K

−23,050.7 −11,723.0 −38.6611 0.1868

−23,050.7 −10,177.1 −38.6595 0.1801

and larger Cm constant at lower temperature confirmed that more CV is adsorbed by MCM-chitosan at lower temperature. Data obtained as a result of the experiments performed to compare the abilities of chitosan and MCM-chitosan to adsorb were applied to the equation Fig. 13. Langmuir linear isotherm plots related to the adsorption of crystal violet by MCM-chitosan from aqueous solution.

R% =

C0 − Ce 100, C0

(12)

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Fig. 14. The SEM micrographs of chitosan (Sigma C 3646).

and percentage removal (R%) of dyestuffs and PNP was calculated (Table 8). As can be seen in Table 8, MCM-chitosan removed more O-II and less CV than chitosan did for the reasons stated earlier. MCM-chitosan removed more RB5 than chitosan did, but not as much as O-II, because RB5 is

a rather acidic dyestuff. The amino group in the structure of chitosan has sufficient positive change in an aqueous solution of this dyestuff. Consequently, MCM-chitosan did not remove that much more RB5. MCM-chitosan removed less PNP than chitosan did. MCM-chitosan is more acidic than

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Table 8 Percentage removal (R%) of three dyestuffs and PNP by chitosan and MCM-chitosan from aqueous solution Adsorbate O-II CV RB5 PNP

Adsorbent Chitosan

MCM-chitosan

11.96 9.65 12.10 7.37

99.25 3.05 34.92 0.92

chitosan and makes the cleavage of the O–H bond in the structure of PNP difficult by preventing the PNP from being charged negatively. The BET specific surface area of commercial chitosan was measured as 0.65 m2 g−1 with a Micromeritics Flow Sorb II 2300 (Shimadzu Corp., Japan). Because CV is only physically adsorbed by chitosan, a more accurate specific surface area was measured for chitosan. The density of commercial chitosan is reported as 0.15–0.30 g mL−1 by the manufacturer. Because the pore volume of chitosan measured with the Micromeritics Autopore II 9220 Mercury Porosimeter apparatus was not sufficient to do a precise measurement, the porosity and pore size distribution of commercial chitosan could not be measured. Fig. 14 shows the scanning electron micrographs of chitosan. As is known, scanning electron microscopy (SEM) is one of the most widely used surface diagnostic tools. With this technique a narrowly focused beam of high-energy electrons was projected across the surface of chitosan. Interaction of the beam with the surface generated a shower of secondary and backscattered electrons, which were collected by a detector. The scanning electron micrograph is a rigid combination of the cross section and mean free path of the scattering event which are strongly dependent on the energy of the beam of electrons and on material variations such as type and density. Thus, the true interpretation of a scanning electron micrograph is not always straightforward. However, chitosan has a heterogeneous surface and macropores as seen from its a scanning electron micrographs. Its specific surface area confirms that chitosan has macropores.

4. Conclusion For maximum adsorption yield on the basis of the experimental results obtained: (1) The adsorption of O-II, CV, and PNP by chitosan from aqueous solution must be studied at high concentration, low temperature, and high shaking rate. (2) The adsorption of RB5 by chitosan from aqueous solution must be studied at high temperature.

(3) The adsorption of O-II by MCM-chitosan from aqueous solution must be studied at high temperature. In addition, because MCM-chitosan removes more O-II than does chitosan, chitosan must be modified with ClCH2 COOH. The modification of chitosan with ClCH2 COOH is not very costly. (4) The adsorption of CV by MCM-chitosan from aqueous solution must be studied at low temperature. However, because MCM-chitosan removes less CV than does chitosan, the modification of chitosan with ClCH2 COOH is not necessary for adsorption of CV from aqueous solution. (5) Because MCM-chitosan removes more RB5 than does chitosan, chitosan must be modified with ClCH2 COOH for the adsorption of RB5 from aqueous solution. (6) Because MCM-chitosan removes less PNP than does chitosan, the modification of chitosan with ClCH2 COOH is not necessary for the adsorption of PNP from aqueous solution. It can easily be said that chitosan and MCM-chitosan can be used together with other adsorbents in studies of dyestuff adsorption related to the environment, because chitosan is, at the very least, a good adsorbent compared with most adsorbents in the adsorption of particularly heavy metals and acidic dyestuffs from aqueous solution [14]. In addition, chitosan is cheaper than most adsorbents and is found in abundance in nature. MCM-chitosan is, at the very least, a better adsorbent than chitosan for the adsorption of acidic dyestuffs from aqueous solution, and its production is not costly.

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