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Equilibrium Studies for Acid Dye Adsorption onto Chitosan Y. C. Wong,† Y. S. Szeto,† W. H. Cheung,‡ and G. McKay*,‡ Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, and Department of Chemical Engineering, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong Received February 19, 2003. In Final Form: June 26, 2003 The ability of chitosan as an adsorbent for the removal of acid dyestuff, namely, acid green 25, acid orange 10, acid orange 12, acid red 18, and acid red 73, from aqueous solution has been studied. The experimental equilibrium data for the single component dye-chitosan systems have been analyzed using the linearized forms of Langmuir, Freundlich, and Redlich-Peterson isotherms. The Langmuir isotherm was found to provide the best theoretical correlation of the experimental data for the adsorption of all five acid dyes. On the basis of the Langmuir analysis, the monolayer adsorption (saturation) capacities were determined to be 645.1, 922.9, 973.3, 693.2, and 728.2 mg of dye per gram of chitosan for acid green 25, acid orange 10, acid orange 12, acid red 18, and acid red 73, respectively. The differences in adsorption capacities may be due to the effect of molecular size and the number of sulfonate groups of each dye. The results demonstrated that monovalent and/or smaller dye molecules have superior adsorption capacities due to an increase in the dye/chitosan ratio in the system. The smaller dye molecules are able to undertake a deeper penetration of dye into the internal pore structure of the chitosan particles.
Introduction The treatment of wastewater has long been a major concern of the textile industry. Large quantities of aqueous waste and dye effluents, over 4.4 × 106 m3 per day in Mainland China,1 are discharged from the dyeing process with strong persistent color and high BOD loading that is aesthetically and environmentally unacceptable.2 Synthetic dyes are extensively used in dyeing and printing processes. Many of these dye wastes are toxic and even carcinogenic,3 and this poses a serious hazard to aquatic living organisms. As a result, many governments have established environmental restrictions with regard to the quality of colored effluents and have forced dye-using industries to decolorize their effluents before discharging. The total dye consumption of the textile industry worldwide is in excess of 107 kg per year, and an estimated 90% of this ends up on fabrics. Consequently, approximately one million kilograms per year of dyes are discharged into waste streams by the textile industry. Dye producers and users are interested in stability and fastness, and consequently, are producing dyestuffs which are more difficult to degrade after use.4 A range of conventional treatment technologies for dye removal has been investigated extensively,3,5-11 such as * Author for correspondence. E-mail:
[email protected]. † Hong Kong Polytechnic University. ‡ Hong Kong University of Science & Technology. (1) Wu, T. X.; Lin, T.; Zhao, J. C.; Hidaka, H.; Serpone, N. Environ. Sci. Technol. 1999, 33, 1379-1387. (2) Annadurai, G.; Krishnan, M. R. V. Iran. Polym. J. 1997, 6, 169175. (3) Vandevivere, P. C.; Bianchi, R.; Verstraete, W. J. Chem. Technol. Biotechnol. 1998, 72, 289-302. (4) Marc, R. Chem. Eng. News 1996, 73, 10-12. (5) Lin, S. H.; Lin, C. M. Water Res. 1993, 27, 1743-1748. (6) Ganesh, R.; Boardman, G. D.; Michelsen, D. Water Res. 1994, 28, 1367-1376. (7) Walker, G. M.; Weatherley, L. R. Water Res. 1997, 31, 20932101. (8) Chu, W.; Tsui, S. M. Chemosphere 1999, 39, 1667-1677. (9) El-Geundi, M. S. Water Res. 1991, 25, 271-273. (10) Grau, P. Water Sci. Technol. 1991, 24, 97-103.
the trickling filter, activated sludge, chemical coagulation, carbon adsorption, and photodegradation processes. Adsorption can handle fairly large flow rates, producing a high-quality effluent that does not result in the formation of harmful substances, such as ozone and free radicals, during the photodegradation process using UV. The adsorption of acid dyes has been studied using peat,12 activated carbon,7 pith,13-16 fuller’s earth,17,18 and wood.19-21 Other dye-adsorbent systems have also demonstrated commercial potential, including hardwood,9,21 pith,13 waste red mud,22-24 and agricultural byproducts.15,16,25,26 Yoshida et al.27-29 studied the sorption of (11) Lucarelli, L.; Nadtochenko, V.; Kiwi, J. Langmuir 2000, 16, 1102-1108. (12) Poots, V. J. P.; McKay, G.; Healy, J. J. Water Res. 1976, 10, 1061-6. (13) McKay, G.; Elgeundi, M.; Nassar, M. M. Water Res. 1987, 21, 1513-1520. (14) Namasivayam, C.; Prabha, D.; Kumutha, M. Bioresour. Technol. 1998, 64, 77-79. (15) Namasivayam, C.; Radhika, R.; Suba, S. Waste Manage. 2001, 21, 381-387. (16) Namasivayam, C.; Kavitha, D. Dyes Pigm. 2002, 54, 47-58. (17) Atun, G.; Hisarli, G.; Sheldrick, W. S.; Muhler, M. J. Colloid Interface Sci. 2003, 261, 32-39. (18) McKay, G.; Otterburn, M. S.; Aga, J. A. Water, Air, Soil Pollut. 1985, 24, 307-322. (19) Poots, V. J. P.; McKay, G.; Healy, J. J. Water Res. 1976, 10, 1067-70. (20) Asfour, H. M.; Nassar, M. M.; Fadali, O. A.; Elgeundi, M. S. J. Chem. Technol. Biotechnol. 1985, 35, 28-35. (21) Asfour, H. M.; Fadali, O. A.; Nassar, M. M.; Elgeundi, M. S. J. Chem. Technol. Biotechnol. 1985, 35, 21-27. (22) Namasivayam, C.; Arasi, D. Chemosphere 1997, 34, 401-417. (23) Namasivayam, C.; Yamuna, R. T.; Arasi, D. Environ. Geol. 2001, 41, 269-273. (24) Namasivayam, C.; Yamuna, R. T.; Arasi, D. Sep. Sci. Technol. 2002, 37, 2421-2431. (25) Marshall, W. E.; Champagne, E. T. J. Environ. Sci. Health, Part A: Environ. Sci. Eng. Toxic Hazard. Subst. Control 1995, 30, 241-261. (26) Marshall, W. E.; Johns, M. M. J. Chem. Technol. Biotechnol. 1996, 66, 192-198. (27) Yoshida, H.; Okamoto, A.; Kataoka, T. Chem. Eng. Sci. 1993, 48, 2267-2272. (28) Yoshida, H.; Takemori, T. Water Sci. Technol. 1997, 35, 29-37. (29) Yoshida, H.; Fukuda, S.; Okamoto, A.; Kataoka, T. Water Sci. Technol. 1991, 23, 1667-1676.
10.1021/la030064y CCC: $25.00 © 2003 American Chemical Society Published on Web 08/19/2003
Acid Dye Adsorption onto Chitosan
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Table 1. Physical and Chemical Characteristics of Selected Dyestuffs generic name
abbrev
commercial name
purity (%)
chromophore
fw
λmax (nm)
C.I. acid green 25 C.I. acid orange 10 C.I. acid orange 12 C.I. acid red 18 C.I. acid red 73
AG25 AO10 AO12 AR18 AR73
acid green 25 orange G crocein orange G new coccine brilliant crocein MOO
75 80 70 75 70
anthraquinone monoazo monoazo monoazo diazo
622.6 452.4 350.3 604.5 566.5
642 475 482 506 510
Table 2. Constants for Calculations of Dye Concentrations of Selected Dyes
Figure 1. 3D molecular structures of the five selected dyestuffs.
various dyes on chitosan fibers, and the use of chitin has been assessed in various studies.30-34 In this paper, the ability of chitosan to remove acid dyes, namely acid green 25 (AG25), acid orange 10 (AO10), acid orange 12 (AO12), acid red 18 (AR18), and acid red 73 (AR73), by adsorption has been studied. The equilibrium sorption capacities of dyes on chitosan have been studied using the adsorption isotherm technique. The experimental data were fitted into Langmuir, Freundlich, and Redlich-Peterson equations. Experimental Methods and Materials Materials. Adsorbent. The adsorbent used in this research is a powdered form of chitin purchased from Sigma Chemical Company. This chitin is described by the supplier as a practical grade material extracted from crab shells. All raw chitin was dried at 75 °C in an oven for 6 h and then was sieved into a discrete particle size range from 355 to 500 µm. Absorbates. Five different commercially available textile dyestuffs were used in the study, including four azo dyes (AO10, (30) Kim, C. Y.; Choi, H. M.; Cho, H. T. J. Appl. Polym. Sci. 1997, 63, 725-736. (31) McKay, G.; Blair, H. S.; Gardner, J. R. J. Appl. Polym. Sci. 1982, 27, 3043-3057. (32) Shimizu, Y.; Kono, K.; Kim, I. S.; Takagishi, T. J. Appl. Polym. Sci. 1995, 55, 255-261. (33) Knorr, D. J. Food Sci. 1983, 48, 36-41. (34) dos Anjos, F. S. C.; Vieira, E. F. S.; Cestari, A. R. J. Colloid Interface Sci. 2002, 253, 243-246.
name of dye
constant, k
acid green 25 (AG25) acid orange 10 (AO10) acid orange 12 (AO12) acid red 18 (AR18) acid red 73 (AR73)
16.650 22.019 23.098 25.928 40.907
AO12, AR18, and AR73) and one anthraquinone dye (AG25). Those selected dyestuffs are commonly used in dye houses nowadays and are regarded as dye contaminants in the discharged effluent. All dyestuffs were obtained from Aldrich Chemical Co. and used without any further purification process. The characteristics and chemical structures of the selected dyestuffs are listed in Table 1 and Figure 1, respectively. Methods. Preparation of Chitosan. The sieved chitin was deacetylated to chitosan by using 48% sodium hydroxide solution (w/w) at 100 °C under a nitrogen atmosphere for a 1-h period. Then, the product was washed with deionized water completely, dried at 70 °C in an oven overnight, and sieved again into several particle size ranges: 125-250, 250-355, and 355-500 µm. The fractions were further dried in a vacuum oven for 1 day and stored in desiccators. The degree of deacetylation was characterized by a 1H NMR method and was found to be 53%. Concentration Measurement and Calibration. To calculate the concentration of the sample from each experiment, a calibration curve of each dye was first prepared. For each dye, five different concentrations were prepared and the absorbance was measured using a Perkin-Elmer UV/VIS Spectrophotometer Lambda 18 over a range from 400 to 700 nm. The calibration checks were carried out in duplicate. Then, the maximum absorbance of each dye was plotted against a range of dye concentrations. From these results, the concentrations of the dye samples can be calculated with the following equations and the constants for calculations which are summarized in Table 2.
AG25: dye concentration (mmol/L) ) maximum absorbance/kAG25 (1) AO10: dye concentration (mmol/L) ) maximum absorbance/kAO10 (2) AO12: dye concentration (mmol/L) ) maximum absorbance/kAO12 (3) AR18: dye concentration (mmol/L) ) maximum absorbance/kAR18 (4) AR73: dye concentration (mmol/L) ) maximum absorbance/kAR73 (5) Equilibrium Sorption Studies. A fixed mass of chitosan (DD ) 53%, 0.2000 g) was weighed into 120 mL conical flasks and brought into contact with 100 mL of dye solutions with predetermined initial dye concentrations. The initial pH of the solutions was adjusted to 4.00 ( 0.1 by the addition of 20% by volume of citric acid buffer, made up of citric acid and sodium hydroxide. The flasks were sealed and agitated for 24 h at 200 rpm in the thermostatic shaker bath and maintained at the temperature 25 ( 1 °C until equilibrium was reached. At time t ) 0 and equilibrium, the dye concentrations of the solutions were measured with a UV/VIS spectrophotometer. These data were used to calculate the adsorption capacity, qe, of the
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adsorbent. Finally, diagrams of adsorption capacity, qe, against equilibrium concentration, Ce, were plotted for various systems. Calculation of Dye Concentration of Isotherm Studies. The dye concentration at equilibrium, qe, was calculated from
(C0 - Ce)V qe ) m
to Henry’s law. Therefore, a linear expression of the Langmuir equation is
Ce aL 1 ) + C qe KL KL e
(6)
where qe ) the dye concentration in the sorbent at equilibrium (mmol of dye/g of chitosan), C0 ) the initial dye concentration in the liquid phase (mmol of dye/dm3), Ce ) the liquid-phase dye concentration at equilibrium (mmol of dye/dm3), V ) the total volume of dye solution used (dm3), and m ) the mass of sorbent used (g).
Freundlich Isotherm. The Freundlich36 equation is an empirical equation employed to describe heterogeneous systems, in which it is characterized by the heterogeneity factor 1/n. When n ) 1/n, the Freundlich equation reduces to Henry’s law. Hence, the empirical equation can be written
qe ) KFC1/n e
Theory Equilibrium Model. To optimize the design of a sorption system to remove the dyes, it is important to establish the most appropriate correlation for the equilibrium curves. These equilibrium sorption capacity curves can be obtained by measuring the sorption isotherm of the acid dyes onto the chitosan. The experimental data of the amount of sorbate adsorbed on the sorbent are substituted into an equilibrium isotherm model to determine the best-fit model for the sorption system. Using this relationship, any variation in the concentration of dye on the chitosan with the concentration of dye in solution is correlated. Langmuir Isotherm. Langmuir35 proposed a theory to describe the adsorption of gas molecules onto metal surfaces. The Langmuir adsorption isotherm has found successful application to many other real sorption processes of monolayer adsorption. Langmuir’s model of adsorption depends on the assumption that intermolecular forces decrease rapidly with distance and consequently predicts the existence of monolayer coverage of the adsorbate at the outer surface of the adsorbent. The isotherm equation further assumes that adsorption takes place at specific homogeneous sites within the adsorbent. It is then assumed that once a dye molecule occupies a site, no further adsorption can take place at that site. Moreover, the Langmuir equation is based on the assumption of a structurally homogeneous adsorbent where all sorption sites are identical and energetically equivalent. Theoretically, the sorbent has a finite capacity for the sorbate. Therefore, a saturation value is reached beyond which no further sorption can take place. The saturated or monolayer (as Ct f ∞) capacity can be represented by the expression
qe )
KLCe 1 + aLCe
(7)
where qe is the solid-phase sorbate concentration at equilibrium (mmol/g), Ce is the aqueous phase sorbate concentration at equilibrium (mmol/dm3), KL is the Langmuir isotherm constant (dm3/g), and aL is the Langmuir isotherm constant (dm3/mmol). Therefore, a plot of Ce/qe versus Ce gives a straight line of slope aL/KL and intercept 1/KL, where KL/aL gives the theoretical monolayer saturation capacity, Q0. The Langmuir equation is applicable to homogeneous sorption where the sorption of each sorbate molecule onto the surface has equal sorption activation energy. The Langmuir equation obeys Henry’s law at low concentration. When the concentration is very low, aLCe is far smaller than unity. This implies qe ) KLCe; hence, it is analogous (35) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361-1403.
(8)
(9)
where qe is the solid-phase sorbate concentration at equilibrium (mmol/g), Ce is the liquid-phase sorbate concentration at equilibrium (mmol/dm3), KF is the Freundlich constant (dm3/(mg1-1/n g)), and 1/n is the heterogeneity factor. A linear form of the Freundlich expression can be obtained by taking logarithms of eq 9.
ln qe ) ln KF +
1 ln Ce n
(10)
Therefore, a plot of ln qe versus ln Ce enables the constant KF and the exponent 1/n to be determined. This isotherm is another form of the Langmuir approach for adsorption on an “amorphous” surface. The amount of adsorbed material is the summation of adsorption on all sites. The Freundlich isotherm is derived by assuming an exponential decay energy distribution function inserted into the Langmuir equation. It describes reversible adsorption and is not restricted to the formation of the monolayer. Redlich-Peterson Isotherm. Redlich and Peterson37 incorporate three parameters into an empirical isotherm. The Redlich-Peterson isotherm model combines elements from both the Langmuir and Freundlich equations, and the mechanism of adsorption is a hybrid one and does not follow ideal monolayer adsorption. The Redlich-Peterson equation is widely used as a compromise between Langmuir and Freundlich systems. For further application of the isotherms for use in kinetic or mass transport models, it is important to have the most accurate correlating equation.
qe )
KRCe 1 + aRCβe
(11)
where qe is the solid-phase sorbate concentration at equilibrium (mmol/g), Ce is the liquid-phase sorbate concentration at equilibrium (mmol/dm3), KR is the Redlich-Peterson isotherm constant (dm3/g), aR is the Redlich-Peterson isotherm constant (dm3/mg1-1/β), and β is the exponent, which lies between 1 and 0. The application of this equation has been discussed elsewhere, and its limiting behavior is summarized here:
where β ) 1 qe )
KRCe 1 + aRCe
(12)
It becomes a Langmuir equation. (36) Freundlich, H. M. F. Z. Phys. Chem. 1906, 57, 385-471. (37) Redlich, O.; Peterson, D. L. J. Phys. Chem. 1959, 63, 10241026.
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where β ) 0 qe )
KRCe 1 + aR
(13)
that is, the Henry’s law equation. Equation 11 can be rearranged as follows:
Ce KR - 1 ) aRCβe qe
(14)
This equation can be converted to a linear form by taking logarithms:
(
)
Ce ln KR - 1 ) ln aR + β ln Ce qe
(15)
Figure 2. Sorption of acid dyes onto chitosan at temperature ) 25 °C, degree of deacetylation (DD) ) 53%, pH ) 4.00, and dp ) 355-500 µm.
Plotting the left-hand side of eq 15 against ln Ce to obtain the isotherm constants is not applicable because of the three unknowns, aR, KR, and β. Therefore, a minimization procedure is adopted to solve eq 15 by maximizing the correlation coefficient between the theoretical data for qe predicted from eq 15 and the experimental data. Results and Discussion Equilibrium Isotherms. Adsorption isotherms describe how adsorbates interact with adsorbents and so are critical in optimizing the use of adsorbents. Therefore, the correlation of equilibrium data by either theoretical or empirical equations is essential to the practical design and operation of adsorption systems. To optimize the design of a sorption system to remove dyes from effluents, it is important to establish the most appropriate correlation for the equilibrium curves. In the present studies, the experimental data of five dye-chitosan equilibrium isotherms, which were the sorption of acid green 25 (AG25), acid orange 10 (AO10), acid orange (AO12), acid red 18 (AR18), and acid red 73 (AR73) as shown in Figure 2, were compared using three isotherm equations, namely, Langmuir, Freundlich, and Redlich-Peterson (R-P). The equilibrium saturation capacities of acid dyes in each of the systems are demonstrated in Figure 2, showing the monolayer saturation or maximum adsorption is reached in all five dye-chitosan systems. All the dye solutions were adjusted to pH ) 4.0 using citric acid or sodium hydroxide. The dyes are known to be stable at this pH value. The possible mechanisms of the adsorption process of chitosan and acid dye are likely to be ionic interactions of the colored dye ions with the amino groups on the chitosan. In aqueous solution, the acid dye is first dissolved and the sulfonate groups of acid dye (D-SO3Na) dissociate and are converted to anionic dye ions. H2O
D-SO3Na 98 D-SO3- + Na +
(16)
Also, in the presence of H+, the amino groups of chitosan (R-NH2) became protonated.
R-NH2 + H+ h R-NH3+
(17)
The adsorption process then proceeds due to the electrostatic attraction between these two counterions,
R-NH3+ + D-SO3- h R-NH3‚‚‚O3S-D
(18)
Therefore, the difference in the degree of adsorption may
Figure 3. Langmuir isotherm linear plots for the sorption of acid red 18 and acid red 73 onto chitosan at temperature ) 25 °C, DD ) 53%, pH ) 4.00, and dp ) 355-500 µm.
mainly be attributed to the chemical structure of each dye. Both Figure 1 and Table 1 indicate acid orange 12 (AO12) has only one sulfonate acid group (monovalent) and has the smallest molecular size, which not only increases the dye/chitosan ratio in the system but also may enable a deeper penetration of dye molecules into the internal pore structure of chitosan. In the case of divalent acid orange 10 (AO10) and acid red 73, a slightly higher adsorption capacity is observed for AO10 than the capacity of AR73, suggesting that the larger dye ions do not completely or partly penetrate the particle, so that the dyes preferentially adsorb near the outer surface of the particle. A similar phenomenon was reported previously by McKay et al.31 for the adsorption of basic dye on chitin. Langmuir Isotherm. The Langmuir adsorption isotherm assumes that the adsorbed layer is one molecule in thickness. The strength of the intermolecular attractive forces is believed to fall off rapidly with distance. The sorption data were analyzed according to the linear form (eq 8) of the Langmuir isotherm. The linear plots of specific sorption Ce/qe against the equilibrium concentration Ce for two dyes, acid red 18 (AR18) and acid red 73 (AR73), are shown in Figure 3. The linearized form of the isotherms of all five dyes are found to be linear over the whole concentration range studied, and the correlation coefficients were extremely high, as shown in Table 3. These values of the correlation coefficients strongly support the fact that the dyes-chitosan sorption data closely follow the Langmuir model of sorption. However, the isotherm of AO10 was found to have the lowest correlation coefficient R2 with the Langmuir model. The isotherm constants aL and KL and the equilibrium monolayer capacities Q0 are presented in Table 3. Both sorption constants KL and sorption capacities for AG25 and AR73 are higher than
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Table 3. Langmuir Sorption Isotherm Constants for AG25, AO10, AO12, AR18, and AR73 dye
KL (dm3/g)
aL (dm3/mmol)
R2
AG25 AO10 AO12 AR18 AR73
175.4390 1.1829 33.8983 39.8406 53.4759
169.316 0.5798 12.200 34.745 41.6043
0.9999 0.9812 0.999 0.9997 0.9996
those for AR18 and AO12, and these are significantly greater than that of AO10. The Langmuir monolayer capacity Q0 represents the saturation capacity of the acid dyes, for AO10, AO12, AG25, AR18, and AR73, and these values are 922.9, 973.3, 645.1, 693.2, and 728.2 mg/g, respectively. The plot in Figure 3 is typical and demonstrates that the Langmuir equation provides an accurate description of the experimental data, which is further confirmed by the extremely high values of the correlation coefficients for all five systems. The high degree of correlation for the linearized Langmuir relationship suggests a single surface reaction with constant activation energy is the predominant sorption step and possibly the predominant rate controlling step. The general shape of this curve is also very characteristic of a Langmuir equilibrium with the high sorption capacity and sharp curvature close to saturation indicating a high degree of irreversibility.31 Freundlich Isotherm. The Freundlich equation predicts that the dye concentrations on the adsorbent will increase as long as there is an increase of the dye concentration in the liquid. The experimental data in the present systems indicate that there is a limiting value of the solid-phase concentration. By plotting the linear transformation of the Freundlich equation, Figure 4 shows the logarithmic plots of the Freundlich expression for the selected acid dyes. The figure exhibits deviation from linearity on the Freundlich linear plot for the whole concentration range. However, if the whole concentration range is divided into regions, that is, region 1, region 2, and region 3, good fits to the experimental data can be observed in several cases, especially at the lower concentration regions 1 and 2. Region 3 does not fit the Freundlich equation well. Table 4 shows the Freundlich sorption isotherm constants bF and KF and the correlation coefficients R2 for the different concentration regions. Some of the low values are due to the mathematical dependency of the subdivision of the experimental data, as, in some cases, only two or three of the experimental points fall within a particular linearized region, resulting in a high R2. Furthermore, in the third Freundlich region, the slopes are extremely low and the errors in measuring the larger liquid-phase dye concentrations, for a small number of experimental points, will result in the relatively poor correlation of data. Despite these mathematical possibilities, there seems to be no doubt mechanistically that the Langmuir isotherm provides a better description of the adsorption of acid dyes on chitosan than does the Freundlich isotherm. The Freundlich isotherm is applicable to heterogeneous surface adsorption with a uniform energy distribution. There are many examples in the literature whereby it has been divided up into sections, sometime as many as five.38 The explanation normally adopted is that there are irregular energy distributions due to different surface groups with different levels of activation energies for the range of sorption reactions. In the present work, the Freundlich analyses produced three linear regions, which (38) Fritz, W.; Schlunder, E. U. Chem. Eng. Sci. 1981, 36, 721-730.
Figure 4. Freundlich isotherm linear plots for the sorption of acid orange 10 onto chitosan at temperature ) 25 °C, DD ) 53%, pH ) 4.00, and dp ) 355-500 µm. Table 4. Freundlich Sorption Isotherm Constants for AG25, AO10, AO12, AR18, and AR73 at Different Concentration Ranges dye
region
bF
KF (dm3/(mg1-1/n g))
R2
AG25
(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)
2.4818 0.0715 0.0272 1.32 0.517 0.1105 0.2791 0.2324 0.027 1.087 0.4305 0.0455 2.6293 0.4333 0.0298
41606 1.0299 1.0046 0.651 0.7755 1.2667 312.249 2.81 2.6172 35.784 2.5053 1.1194 6125.4 3.0328 1.2471
0.9681 0.8508 0.7352 0.996 0.9995 0.9011 0.9251 0.9199 0.577 0.9957 0.992 0.7874 0.9886 0.9622 0.6627
AO10 AO12 AR18 AR73
R2 for single line 0.7503 0.8201 0.5321 0.7632 0.6821
could be considered as sorption onto various surface groups, such as amino, acetyl, or hydroxyl, or a purely physical sorption. However, the correlation coefficients in Table 4 indicate the data are not well correlated compared to the Langmuir correlation coefficients, when only one line was used to represent the whole range of experimental data. Redlich-Peterson Isotherm. The Redlich-Peterson isotherm contains three parameters, and the isotherm equation includes features of the Langmuir and the Freundlich isotherm equations. The parameters of the Langmuir and Freundlich models can be determined using the linear forms of the equations. However, the parameters of the Redlich-Peterson isotherms cannot be determined by simple linearization because this three parameter model equation cannot be solved from the linear equation. Therefore, the parameters of the equations were determined by minimizing the distance between the experimental data points and the theoretical model predictions with the “Solver” add-in function of the Microsoft Excel program. The sorption behavior of AO10, AO12, AG25, AR18, and AR73 onto chitosan can be described by the RedlichPeterson isotherm equation. The linearized forms of the Redlich-Peterson isotherm plots for the sorption of the five dyes onto chitosan are presented in Figure 5. Examination of the data shows that the Redlich-Peterson model describes the sorption of AG25, AR18, and AR73 on chitosan extremely well over the concentration ranges studied and gives moderate fits for AO10 and AO12. The Redlich-Peterson isotherm constants aR, KR, and β and the correlation coefficients R2 for the Redlich-Peterson isotherm are listed in Table 5.
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Figure 5. Redlich-Peterson isotherm linear plots for the sorption of AG25 and AR18 onto chitosan at temperature ) 25 °C, DD ) 53%, pH ) 4.00, and dp ) 355-500 µm.
Figure 6. Different isotherm equation plots for the sorption of acid green 25 onto chitosan at temperature ) 25 °C, DD ) 53%, pH ) 4.00, and dp ) 355-500 µm.
Table 5. Redlich-Peterson Sorption Isotherm Constants for AG25, AO10, AO12, AR18, and AR73 dye
b
KR (dm3/g)
aR (dm3/mg1-1/β)
R2
AG25 AO10 AO12 AR18 AR73
0.9963 0.6081 0.9108 0.9816 0.9074
101.824 22.435 25.2658 38.0287 1377.996
98.5366 26.4819 8.2203 32.0983 1119.255
0.9992 0.9375 0.9232 0.9944 0.9886
By comparing the results presented in Tables 3-5, it can be seen that the Langmuir sorption isotherm can accurately describe the sorption of acid green 25 (AG25), acid red 18 (AR18), and acid red 73 (AR73) onto chitosan in this study and that the experimental sorption data of acid orange 10 can fit the composite Freundlich isotherm model. Figures 6 and 7 show plots comparing the theoretical Langmuir, Freundlich, and Redlich-Peterson isotherm equations with experimental data, for the acid green 25 and the acid orange 10 sorption systems. Adsorption Characteristics. The monolayer saturation adsorption capacities Q0 in milligrams per gram or millimoles per gram of chitosan can be evaluated from the Langmuir constants KL and aL using eq 19:
Q0 )
KL aL
Figure 7. Different isotherm equation plots for the sorption of acid orange 10 onto chitosan at temperature ) 25 °C, DD ) 53%, pH ) 4.00, and dp ) 355-500 µm.
of some combination of these three factors. Table 6 shows that dye adsorption capacities follow the inverse order of the dye molecular weights. This suggests that molecular size may be the significant parameter. The software package, CS Chem3D Ultra, was used to generate 3 D models of the five structures. The molecular diameters and the linear dimensions in the x directions and y directions in Figure 1 were determined and are listed in Table 6. None of these three parameters give the correct order of the adsorption capacities. However, the Connolly molecular surface areas of the dye molecules were determined from the CS Chem3D Ultra model Modeler output, which takes into account the spatial orientation of the various functional groups and structures. These values will give a fairly accurate prediction of the area occupied by one acid dye molecule attaching itself more or less in a flat manner onto the surface. This method does correlate the adsorption capacities in the expected order for all five acid dyes. In terms of the mechanism, this result suggests that the dyes are covering the chitosan in this manner, that is, lying on the surface in this orientation. If the attachment of the dye was at one point only, on the basis of the reaction between one -NH3+ and one -SO3D, then it would be expected that the dye molecule would be more spatially oriented and either the molecular diameter or the linear dimension parameters, x or y, would have provided the correct correlation.
(19)
The Q0 values for the five acid dyes are presented in Table 6 in terms of milligrams of dye per gram of chitosan and millimoles of dye per gram of chitosan. In addition, several possible factors which could be important characteristics of the adsorption process are also listed in Table 6. Since the capacities do not show a direct molar dependence, then the mechanism is not based solely on the formation of the R-NH3+‚‚‚O3S-D on a one -NH3+ to one -SO3D basis. Furthermore, the order of the capacities is not controlled by the number of charges on the dye molecule, as can be seen by the relative order of the capacities AG25 < AR18 < AR73 compared to the relative order of the number of charges AG25 ) AR73 < AR18. Therefore, it would appear that the differences in capacities may be a function of solution pH, affecting the degree of protonation of the amino groups, or a function of the chemical groups in the dye or the size of the dye molecules, or a function
Table 6. Possible Factors Characterizing the Sorption Process dimension (Å) acid dye AG25 AO10 AO12 AR18 AR73
Q0
mol wt (g)
mol diam (Å)
x
y
mol area (Å2)
charge
mg/g
mmol/g
624.6 454.4 351.3 607.5 556.5
15 12 12 14 18
14.9 11.5 12.6 14.5 13.4
11.6 9.5 8.8 8.4 16.6
456.136 325.87 274.339 369.765 387.649
-2 -2 -1 -3 -2
645.1 922.9 973.3 693.2 728.2
1.03 1.54 2.66 1.11 1.25
7894
Langmuir, Vol. 19, No. 19, 2003
However, a flat or layered attachment of the molecule seems to be the more favored orientation of the dye molecules on the chitosan surface. Therefore, in addition to ionic bonding, there seems to be a strong possibility that some of the dye atoms or molecular components such as N, S, O, and benzene rings are hydrogen bonding with -CH2OH groups in the polysaccharide chain of the chitosan molecule. Conclusions The performance of the chitosan as an adsorbent to remove acid dyes, namely, acid green 25, acid orange 10, acid orange 12, acid red 18, and acid red 73, from aqueous solution has been investigated. Equilibrium isotherms and kinetic models have been measured to assess the capacities of chitosan for the dyestuffs for the sorption process. The experimental isotherm data were analyzed using Langmuir, Freundlich, and Redlich-Peterson equations for each individual dye. On the basis of the Langmuir isotherm analysis, the monolayer adsorption capacities were determined to be 645.1, 922.9, 973.3, 693.2, and 728.2 mg per gram of chitosan for acid green 25, acid orange 10,
Wong et al.
acid orange 12, acid red 18, and acid red 73, respectively. The differences in capacities may be due to the differences in the particle size of the dye molecules and the number of sulfonate groups on each dye. The results have demonstrated that monovalent and smaller dye molecular sizes have superior capacities due to the increase in dye/ chitosan ratio in the system, enabling a deeper penetration of dye molecules to the internal pore structure of chitosan. By comparing the correlation coefficients determined for each linear transformation of the isotherm analysis, the Langmuir isotherm equation was found to provide the best prediction for the sorption of all five acid dyes for the entire concentration range. Acknowledgment. We gratefully acknowledge the support of the Research Grant Council of Hong Kong SAR. One of the authors (W.H.C.) would like to acknowledge the financial support of the Innovation and Technology Fund of Hong Kong SAR, Green Island Cement Co. Ltd., and Hong Kong University of Science and Technology. LA030064Y
