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Langmuir 2007, 23, 7634-7643
Chitosan Derivatives as Biosorbents for Basic Dyes Nikolaos K. Lazaridis,* George Z. Kyzas, Alexandros A. Vassiliou, and Dimitrios N. Bikiaris DiVision of Chemical Technology, School of Chemistry, Aristotle UniVersity, GR-541 24 Thessaloniki, Greece ReceiVed February 13, 2007. In Final Form: April 24, 2007 The scope of this study was to prepare and evaluate chitosan derivatives as biosorbents for basic dyes. This was achieved by grafting poly (acrylic acid) and poly (acrylamide) through persulfate induced free radical initiated polymerization processes and covalent cross-linking of the prepared materials. Remacryl Red TGL was used as the cationic dye. Equilibrium sorption experiments were carried out at different pH and initial dye concentration values. The experimental equilibrium data for each adsorbent-dye system were successfully fitted to the Langmuir, Freundlich and pH-dependent Langmuir-Freundlich sorption isotherms. Thermodynamic parameters of the adsorption process such as ∆G°, ∆H°, and ∆S° were calculated. The negative values of free energy reflected the spontaneous nature of adsorption. The typical dependence of dye uptake on temperature and the kinetics of adsorption indicated the process to be chemisorption. The grafting modifications greatly enhanced the adsorption performance of the biosorbents, especially in the case of powdered cross-linked chitosan grafted with acrylic acid, which exhibited a maximum adsorption capacity equal to 1.068 mmol/g. Kinetic studies also revealed a significant improvement of sorption rates by the modifications. Diffusion coefficients of the dye molecule were determined to be of the order 10-13 - 10-12 m2/s. Furthermore, desorption experiments affirmed the regenerative capability of the loaded material.
1. Introduction Dyeing effluent, one of the largest contributors to textile effluent and colored wastewater, has a seriously destructive impact on the environment. Several methods have been developed to remove color from dyehouse effluents, varying in effectiveness, economic cost, and environmental impact (of the treatment process itself). Among all of the treatments proposed, the sorption of dye molecules onto a substrate (sorbent) can be a very effective, low-cost method of color removal.1-3 Activated carbon is the most commonly used method of dye removal by sorption, although the performance is dependent on the type of carbon used, the characteristics of the wastewater, and the type of dye. However, the cost and regeneration difficulties of the sorbent have compelled researchers to focus on alternative low-cost sorbents.4 Among these, polysaccharides such as chitin, starch, and their derivatives (chitosan and cyclodextrin) deserve particular attention.5-7 Chitosan (poly-β-(1f4)-2-amino-2-deoxy-D-glucose) is an aminopolysaccharide, which is a cationic polymer produced by the N-deacetylation of chitin. Chitin (poly-β-(1f4)-N-acetylD-glucosamine) constitutes one of the most abundant natural biopolymers, second only to cellulose. It is mostly found in the exoskeletons of crustaceans, in the cartilage of mollusks, in the cuticles of insects, and in the cell walls of microorganisms. Because of its macromolecular structure, chitosan exhibits many characteristics that have been the cause of much recent attention * To whom correspondence should be addressed. E-mail: nlazarid@ chem.auth.gr. Tel: +32310 997807. Fax: +32310 997759. (1) San Miguel, G.; Lambert, S. D.; Graham, N. J. Chem. Technol. Biotechnol. 2006, 81, 1685-1696. (2) Crini, G. Bioresource Technol. 2006, 97, 1061-1085. (3) Bhatnagar, A.; Minocha, A. K. Indian J. Chem. Technol. 2006, 13, 203127. (4) Blackburn, S. R. EnViron. Sci. Technol. 2004, 38, 4905-4909. (5) Guibal, E.; Van Vooren, M.; Dempsey, B.; Roussy, J. Sep. Sci. Technol. 2006, 41, 2487-2514. (6) Crini, G. Prog. Polym. Sci. 2005, 30, 38-70. (7) Yi, H.; Wu, L.; Bentley, W.; Ghodssi, R.; Rubloff, G.; Culver, J.; Payne, G. Biomacromolecules 2005, 6, 2881-2894.
because the range of its applications has enormously expanded in various fields, including biotechnology, water treatment, medicine and veterinary medicine, membranes, cosmetics, and the food industry.8 Chitosan has a very high affinity for most classes of dyes such as reactive, direct, and disperse, expressing a lack of affinity only for basic dyes.4,9,10 These characteristics are mainly due to its high content of amine functional groups, which give a cationic nature to the biopolymer, as well as its hydroxyl groups. Therefore, chitosan has much potential as an inexpensive and effective adsorbent for almost all types of dyes, except basic, because of its innate cationic nature. The object of the current study was to improve the adsorption capabilities of chitosan for basic dyes. To accomplish this endeavor, chitosan was suitably modified with the aim of introducing anionic and nonanionic moieties but was also capable of forming hydrogen bonds with groups on the chitosan macromolecule. The adsorption performance of similar chitosan derivatives has not been substantially studied. Also, to increase the resistance of the final products to chemical and biological degradation, decrease their solubility at extreme pH values, and improve their mechanical properties, with the prospect of using the prepared adsorbents in packed columns, cross-linking reactions were carried out. Although heterogeneous chitosan cross-linking has been thoroughly investigated in the literature,11 especially through the use of dialdehydes, the effect of such a modification on the adsorption performance of the prepared chitosan derivatives has not been previously carried out and evaluated. The adsorption-desorption of remacryl red TGL by the prepared biopolymers was extensively studied and evaluated. 2. Materials and Methods 2.1. Materials. High-molecular-weight chitosan (Ch) was obtained from Sigma-Aldrich and purified by extraction with acetone in a (8) Rinaudo, M. Prog. Polym. Sci. 2006, 31, 603-632. (9) Gibbs, G.; Tobin, J.; Guibal, E. Ind. Eng. Chem. Res. 2004, 43, 1-11. (10) Chiou, M.; Li, H. Chemosphere 2003, 50, 1095-1105. (11) Berger, J.; Reist, M.; Mayer, J. M.; Felt, O.; Peppas, N. A.; Gurny, R. Eur. J. Pharm. Biopharm. 2004, 57, 19-34.
10.1021/la700423j CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007
Chitosan DeriVatiVes as Biosorbents for Basic Dyes
Figure 1. Three-dimensional molecular structure of remacryl red TGL (red, oxygen atoms; dark gray, carbon atoms; light gray, hydrogen atoms; green, chloride atoms; and blue, nitrogen atoms). Soxhlet apparatus for 24 h, followed by drying under vacuum at room temperature. The average molecular weight was estimated to be 3.55 × 105, and the degree of deacetylation was 82 wt %, as determined according to the procedures described by Rinaudo.8 Acrylamide (Aam) 97% p.a. was purchased from Sigma-Aldrich and was used without further purification. Acrylic acid (Aa) received from Merck was purified by distillation under vacuum. Potassium persulfate (KPS) obtained from Merck was used as received. Glutaraldehyde (GA, 50 wt % in water) was received and used as reagent grade from Sigma-Aldrich. All solvents were of analytical grade. Remacryl red TGL (C19H25Cl2N5O2) (Figure 1) with a molecular weight of 426.34 g/mol, which was kindly supplied by Hochest (Germany), was selected as the sorbate. 2.2. Preparation of the Biosorbents. Well-documented persulfate-induced free radical reaction procedures were utilized for the realization of the grafting reactions onto the chitosan backbone, as reported in the literature.12-15 Optimum reaction parameters, specifically, temperature, time, solvent volume, and relative concentrations of the monomer and the initiator, were used for the grafting of Aa15 and Aam.16 The exact details of the experimental procedures followed are described in Supporting Information. The final grafting percentages, determined on the basis of the percentage weight increase of the final product relative to the initial weight of chitosan (%G ) 100 × (W2 - W1)/W1 where W1 and W2 denote the weights of the initial dry chitosan and grafted chitosan after extraction and drying, respectively), were estimated to be approximately 230% poly(acrylamide) for Ch-g-Aam and 300% poly(acrylic acid) for Ch-g-Aa. All prepared products were ground to fine powders of between 75 and 125 µm. The biosorbent particles were heterogeneously chemically crosslinked with bifunctional reagent glutaraldehyde (GA), rendering the substances insoluble in acidic media and improving their resistance to chemical and biological degradation and their mechanical strength and abrasion resistance ((Ch)c, (Ch-g-Aam)c, (Ch-g-Aa)c). This reaction has been extensively reviewed in the literature for chitosan,11,16 whereas the respective copolymers’ heterogeneous reaction has not been previously studied. Whereas cross-linking usually leads to lower adsorption capacities, it is deemed necessary because of the unacceptable properties of the pristine material for use in packed columns. The experimental procedure is found in Supporting Information. The penetration of GA into the gelled bead and the fraction of amine groups actually cross-linked were not measured, nor were the mechanical properties of the gelled and dried particles. 2.3. Characterization Techniques. FTIR spectra of the biosorbents were obtained using a Perkin-Elmer FTIR spectrometer (model (12) Jayakumar, R.; Prabaharan, M.; Reis, R. L.; Mano, J. F. Carbohydr. Polym. 2005, 62, 142-158. (13) Kumbar, G. S.; Soppimath, S. K.; Aminabhavi, M. T. J. Appl. Polym. Sci. 2003, 87, 1525-1536. (14) Yasdani-Pedram, M.; Retuert, J.; Quijada, R. Macromol. Chem. Phys. 2000, 201, 923-930. (15) Yasdani-Pedram, M.; Lagos, A.; Retuert, P. Polym. Bull. 2002, 48, 9398. (16) Knaul, J. Z.; Hudson, S. M.; Creber, K. A. M. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1079-1094.
Langmuir, Vol. 23, No. 14, 2007 7635 Spectrum 1000) using KBr pellets containing the prepared materials. The resolution of each spectrum was 2 cm-1, and the number of co-added scans was 64. The spectra presented are baseline corrected and converted to absorbance mode. The surface areas of the beads were determined with a Micromeritics BET surface area analyzer, model TriStar 3000, by means of the adsorption of ultrapure nitrogen at 77 K. 2.4. Experimental Procedure. Kinetic Experiments. Batch kinetic experiments were performed by mixing a fixed amount of sorbent (1 g/L) with 50 mL of an aqueous dye solution of 0.235 mmol/L (100 mg/L) concentration. The mixture was shaken for 24 h, and during this time, samples were collected at fixed intervals. After the spectrophotometric analysis of each sample, the concentration of dye in the aqueous solution at fixed time (Ct) was calculated. Although the pH value of dye baths when using basic dyes is acidic to neutral so that better fixation of dyestuffs onto the fabric can be achieved, the dyewaste equilibration tank of dyehouses is nonetheless rather alkaline (pH ∼10).4 Thus, the pH value of the solution was adjusted to 10 by microadditions of 0.1 M NaOH. Equilibrium Experiments. The effect of the initial dye concentration was determined by placing 1 g/L of the adsorbent in contact with 50 mL of aqueous solutions containing different dye concentrations (0.047-1.175 mmol/L), and the suspensions were shaken for 24 h at pH 10. The influence of pH on the adsorption process was studied by mixing 1 g/L of sorbent with 50 mL of an aqueous dye solution of 0.235 mmol/L concentration. The pH value, ranging from 2 to 12, was kept constant throughout the adsorption process. The suspension was shaken for 24 h using a bath to control the temperature at 298 ( 1 K (Julabo SW-21C). All the sorption experiments were conducted in the presence of 0.001 mol/L NaCl as the background electrolyte because it is used as a stimulator in the dyeing processes. In general, the added salts may have two functions: (i) they may screen the electrostatic interaction of opposite charges between sorbent and dye molecules, and an increase in salt concentration could decrease the amount sorbed; (ii) they may enhance the degree of dissociation of the dye molecules and facilitate the amount of dye sorbed.10,17 Desorption Experiments. After the adsorption experiments, samples were collected and filtered using fixed pore-size membranes. Small fractions of the dye (1 to 2%) and the adsorbent (1%) were retained on the filter membrane; these small variations due to filtration were neglected. Desorption experiments were performed by mixing the collected amount of chitosan after adsorption with aqueous solutions over the pH range of 2-12. As above, after 24 h of shaking at a temperature of 298 K, the samples were collected, and spectrophotometric analysis revealed the optimum desorption pH. 2.5. Analysis. Samples were extracted using a syringe, filtered through a 50 µm pore size membrane, and then analyzed spectrophotometrically by monitoring the absorbance of the dyestuff using a UV-vis spectrophotometer (model U-2000, Hitachi). The determined λmax wavelength value for remacryl red TGL dye was 488 nm. Prior to the adsorption experiments, the effect of pH over the calibration curves of each dye was studied, but no significant deviation was observed. The amount of dye uptake at equilibrium, Qe, was calculated using the mass balance equation. The amount of dye sorbed or desorbed at different pHs was expressed as a percentage of the removal. 2.6. Theory. Kinetic Modeling. The study of sorption is important in wastewater treatment because it provides valuable information on the reaction pathways and the mechanism of sorption. In addition, predicting the solute uptake rate is of utmost importance in designing an appropriate wastewater treatment plant because it can control the residence time of solute at the solid-solution interface.18-20 The main issue when searching for an appropriate sorption mechanism is to select a mathematical model that not only fits the (17) Inbaraj, B.; Chien, J.; Ho, G.; Yang, J.; Chen, B. Biochem. Eng. J. 2006, 31, 204-215. (18) Sontheimer, H.; Crittenden, J.; Summers, S. ActiVated Carbon for Water Treatment, 2nd ed.; DVGW: Karlsruhe, Germany, 1988.
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data with satisfactory accuracy but also complies with a reasonable sorption mechanism. Generally, several steps are involved during the sorption process by sorbent particles: (i) diffusion of the dye from the solution to the boundary film surrounding the particle (bulk diffusion); (ii) diffusion from the film to the particle surface (external diffusion); (iii) diffusion from the surface to the internal sites (surface diffusion or pore diffusion); and (iv) uptake that involves several physicochemical mechanisms. Providing sufficient agitation to avoid particle and solute gradients in the batch reactor allows bulk diffusion to be neglected.20 To compare measurements from various experiments for a kinetic investigation, it is necessary to introduce a dimensionless degree of conversion. Thus, by normalizing the remaining dye concentration,Ct, with respect to some reference value, an index of sorption is defined by eq 120 X)
C0 - Ct C0 - Ce
(1)
where C0 and Ce are the initial and the equilibrium concentrations of dye in the aqueous solution (mmol L-1). External Diffusion. At the beginning (0-15 min) of the test, where the surface concentration is very small compared to the concentration in the solution and only external mass transfer resistance controls the adsorption rate,18 the solution of the mass transfer equation is given by eq 2 ln
Ct A ) -kf t C0 V
(2)
where kf is the initial external mass-transfer coefficient (m/min), A is the sorbent exchange surface (m2), and V the volume of the solution. By plotting ln Ct/C0 versus t, the kf coefficient can be determined. Intraparticle Diffusion. Once the adsorbate molecule accumulates on the outer surface of the sorbent, internal diffusion commences. In fact, it is highly likely that both external and internal diffusion occur simultaneously. However, only one mechanism will control the whole sorption process. The high complexity of the diffusion equations can be simplified to calculate the corresponding parameters. Intraparticle diffusion D, which is the sum of pore and surface diffusion, may be calculated from eq 321,22
[
(
X ) 1 - exp -
)]
4πDt 2.3d2
1/2
(3)
where D is the intraparticle diffusion coefficient and d is the particle diameter. The intraparticle diffusion coefficient can be calculated from the half-time for adsorption (i.e., by substituting X ) 0.50). Throughout the adsorption literature, many simple equations are reported for the calculation of diffusion coefficients D from kinetic adsorption data. However, the results may vary from model to model.21 Reaction Kinetics. Kinetic studies customarily utilize the following basic conversion rate equation (eq 4)20 dX ) k(T) f(X) ) k(T)(1 - X)n dt
(4)
where f(X) is a conversion-dependent function and k(T) is the reaction rate constant. Sorption has been accepted to follow Arrhenius kinetics. The analysis originally developed for isothermal homogeneous reactions was also reported to apply to sorption. Thus, f(X) could be represented by eq 5, assuming a constant system volume: (19) Ho, Y. S.; Ng, J. C. Y.; McKay, G. Sep. Purif. Method 2000, 29, 189232. (20) Lazaridis, N. K.; Karapantsios, T. D.; Georgantas, D. Water Res. 2003, 37, 3023-3033. (21) Khraisheh, M. A. M.; Al-Degs, Y. S.; Allen, S. J.; Ahmad, M. N. Ind. Eng. Chem. Res. 2002, 41, 1651-1657. (22) Tseng, R. L.; Tseng, S. K. J. Colloid Interface Sci. 2005, 287, 428-437.
f(X) ) (1 - X)n
(5)
Most simple reactions have integer orders, n, between 0 and 3. For sorption, the value of n was customarily 1 or 2. Integrating eq 2 for boundary conditions (i) t ) 0, X ) 0 and (ii) t ) t, X ) X, and n ) 1, 2 results in pseudo-first-order (eq 6) and pseudo-second-order kinetic (eq 7) models Ct ) C0 - (C0 - Ce)(1 - e-k1t)
(
Ct ) C0 - (C0 - Ce) 1 -
1 1 + k2t
(6)
)
(7)
where k1 and k2 (min-1) are the rate constants for the pseudo-firstorder and pseudo-second-order adsorption models, respectively. Sorption Isotherms. For the effective design and operation of a wastewater treatment plant (i.e., for dye removal) processing used sorbent materials, it is necessary to quantify the ultimate capacity of the sorbent. Once the ultimate capacity is defined from adsorption equilibria, it is possible to estimate the lowest sorbent usage. To´th investigated the dependency of the characteristic function Ψ(Ce) ) (d lnCe/d ln Qe) - 1 on the liquid-phase concentration.18 If Ψ(Ce) is proportional to the concentration in the aqueous phase, then the Langmuir equation23 is, after integration, Qe )
QmaxCe (1/b) + Ce
(8)
If Ψ(Ce) is constant, then the Freundlich24 equation is, after integration, Qe ) KFCe1/n
(9)
The most promising extensions to the Langmuir and Freundlich isotherms are based on the generalized Langmuir and generalized exponential isotherms. Such an isotherm is the Langmuir-Freundlich (L-F, eq 10) one, which is essentially the Freundlich isotherm approaching a maximum at high concentration.19 Qe )
Qmax (KLFCe)β 1 + (KLFCe)β
(10)
To incorporate the pH dependence into eq 10, one possible form is25 Qe ) k(pH)
Qmax (KLFCe)β 1 + (KLFCe)β
(11)
where k(pH) is a function of pH that matches the observed pH dependence. This form of the modified L-F isotherm empirically incorporates pH into the standard isotherm as an additional independent variable. The function k(pH) can take any form that is suitable. In this case, the following expression was used k(pH) )
1 1 + fe-gpH
(12)
where Qe is the equilibrium dye concentration in the solid phase, Qmax is the maximum amount of sorption (mmol/g), b is the Langmuir sorption equilibrium constant (L/mmol), KLF is the LangmuirFreundlich constant (L/mmol)1/β, KF is the Freundlich constant representing the sorption capacity (mmol1 - 1/n L1/n/g), n is the constant depicting the sorption intensity, β is the Langmuir-Freundlich heterogeneity constant, and f and g are empirical parameters of the L-F equation. (23) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361-1368. (24) Freundlich, H. M. F. Z. Phys. Chem. 1906, 57, 384-470. (25) Decker, D. L.; Papelis, C.; Tyler, S. W.; Logsdon, M. J.; Sˇ imu˚neck, J. Vadose Zone J. 2006, 5, 419-429.
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The Langmuir isotherm assumes a monolayer coverage of adsorbate over a homogeneous adsorbent surface, and the sorption of each molecule onto the surface has equal sorption activation energy. The Freundlich isotherm is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of adsorption over the surface, and multilayer reversible sorption can be expressed. The main characteristics of the Langmuir isotherm can be expressed by a dimensionless constant separation factor or equilibrium parameter RL, which is given by the following expression26
Kc )
CSe CAe
(17)
where CSe and CAe are the equilibrium concentrations of dye molecules in the solution and on the adsorbent, respectively. Modeling of equilibrium and kinetic data was performed by a nonlinear regression method that involves the Levenberg-Marquardt method.28
3. Results and Discussion 1 RL ) (1 + bC0)
(13)
where b is the Langmuir constant. The value of RL indicates the shape of the isotherm to be either unfavorable (RL > 1) or linear (RL ) 1) or favorable (0 < RL < 1) or irreversible (RL ) 0). Certain properties of adsorbents are involved in all adsorber designs. These are adsorbent densities and void fractions, isotherms, and data on mass-transfer kinetics and fixed-bed dynamics. Obviously, the cost of the adsorbent must also be taken into account. Capacity governs the capital cost of an adsorption unit. It not only dictates the amount of adsorbent required but also establishes the volume of the adsorber vessels.27 Mass-transfer kinetics is a catchall term related to intraparticle mass-transfer resistance. It is important because it controls the cycle time of a fixed-bed adsorption process. Fast kinetics, or a high rate of diffusion, implies a desirably sharp breakthrough curve. Such a curve means that the effluent concentration remains level until the adsorbent is almost saturated and then rises sharply. Slow kinetics leads to a distended breakthrough curve in which the effluent concentration begins fluctuating soon after an adsorption run. The effect of a distended breakthrough curve can be overcome by adding adsorbent at the product end or by increasing the cycle time, which reduces the throughput per unit of adsorbent. Both of these options increase the amount of adsorbent required. To compensate for slow diffusion, it is also possible to use small particles, but there is a corresponding sacrifice due to the increased pressure drop. The intraparticle diffusion D enables engineers to assess the diffusional time constant d2/D. This time constant, in turn, is used with elapsed time, t, to define the dimensionless time, Dt/d2. In general, the response of a spherical particle to a sudden change in composition is linked to the dimensionless time by the function X ) f(1 - e-Dt/d2). Thus, when searching for an effective (fast) adsorbent, it is usually safe to choose one having a large diffusivity, a small diameter, or both.27 Thermodynamics. By using the equilibrium constant b obtained for each temperature from the Langmuir model, the Gibbs free energy (∆G°) can be calculated according to eq 14 ∆G° ) -RT ln K
(14)
where K corresponds to b in the Langmuir equation. The standard enthalpy change (∆H°) and standard entropy change (∆S°) of the process could be calculated using eq 15 ∆G° ) ∆H° - T∆S°
(15)
If the plot of ∆G° versus T is linear, the values of ∆H° and ∆S° could be calculated. Desorption thermodynamics were obtained from the integrated van’t Hoff equation ln Kc ) -
∆H° ∆S° + RT R
3.1. Characterization of the Prepared Biosorbents. Homogeneous graft copolymerization of the vinyl monomers onto the chitosan backbone was carried out in an aqueous solution using KPS as the initiator. The exact mechanism of such reactions has not been well clarified. Several research groups have put forward different mechanisms, even for the same reaction system.13,29,30 A possible mechanism is derived from the current literature.12,14,30,31 Applying heat to KPS in an aqueous solution causes it to decompose to sulfate anionic radicals (SO•4 -). These anionic radicals, which are attracted by the amino groups of chitosan (bearing in mind that polysaccharides are predominantly oxidized through C2-C3 bond cleavage32 and that chitosan can act as a weak reducing agent), react together with chitosan and form the active sites on the biomacromolecular backbone (Scheme 1). The monomer is grafted onto these sites, which is subsequently followed by propagation. The chemical cross-linking of the chitosan backbone was carried out through the use of bifunctional reagent glutaraldehyde. Although cross-linking can also occur between the hydroxyl groups of the polysaccharide and the cross-linking agent, especially in an acid-catalyzed environment, it mainly occurs through the amino groups because of their greater reactivity. It is generally accepted that the cross-linking mechanism is a Schiff’s base reaction between the dialdehyde and the free amine on the chitosan backbone (Scheme 1).11 However, there is further evidence suggesting that the reaction actually takes place between the amino groups of chitosan and a hemiacetal.16 The crosslinking reaction rendered the products practically insoluble, even in extremely acidic media. Their color became darker because of the formation of chromophore groups (-CdN-). Furthermore, the subsequently gelled particles were elastic and resilient to shear forces. The dry particle sizes of the cross-linked copolymers remained virtually unaltered. However, because of the grafted groups and the smaller degree of cross-linking (due to the decreased reactivity of the grafted products toward GA), the final modified biopolymers swelled considerably more than the unmodified. The grafted copolymers, before the cross-linking reactions, exhibited different solubility behaviors than chitosan itself. Whereas chitosan dissolves at pH values lower than about 4, the copolymer grafted with poly(acrylamide) could dissolve at pH values as high as 6.5. However, the copolymer grafted with poly(acrylic acid) was insoluble over the whole pH range. The decreased solubility of the latter copolymer can be explained by considering the probable inter- and/or intramolecular hydrogen bonding or internal ammonium salt formation between the
(16)
where Kc is the equilibrium constant for desorption calculated from eq 17 (26) Hall, R. K.; Eagleton, L. C.; Acrivos, A.; Vermeulen, T. Ind. Eng. Chem. Fundam. 1966, 5, 212-223. (27) Knaebel, K. S. Chem. Eng. 1999, April, 92-101.
(28) Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431-441. (29) Maddavinia, G. R.; Pourjavadi, A.; Hosseinzadeh, H.; Zohuriaan, M. J. Eur. Polym. J. 2004, 40, 1399-1407. (30) Prashanth, K. V. H.; Tharanathan, R. N. Carbohyd. Polym. 2003, 54, 343-351. (31) Peppas, N. A. Hydrogels in Medicine and Pharmacy; CRC Press: Boca Raton, FL, 1987; Vol. 3, p 109. (32) Doba, T.; Rodehed, C.; Ranby, B. Macromolecules 1984, 17, 25122519.
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Scheme 1. Mechanism of the Grafting Reaction of Poly(acrylamide) and Poly(acrylic acid) onto the Chitosan Backbone and the Cross-Linking Reaction with GA
carboxyl groups of the grafted poly(acrylic acid) chains and the amino groups of chitosan. The FTIR spectra of pure chitosan and the cross-linked chitosan are presented in Figure 2. The most typical absorption bands of chitosan are situated at 1663 and 1556 cm-1, corresponding to amide I and II, respectively, and a broad band appearing at ∼3300 cm-1 as a result of the stretching vibration of O-H, the extension vibration of N-H, and interhydrogen bonds of the polysaccharide. The amide II band may be overlapped by the NH2 bending band at ∼1590 cm-1, with the presence of this group being confirmed by the absorption occurring at ∼1420 cm-1 (which may be related to the presence of NH2 groups). The absorption bands at ∼1150 (asymmetric stretching of the COC bridge), ∼1080, and ∼1030 cm-1 (skeletal vibrations involving CO stretching) are characteristic of chitosan’s saccharide structure.33,34 The cross-linking reaction can be confirmed by the appearance of a strong absorption peak at 1665 cm-1 in the spectra of the modified chitosan, corresponding to the imine moiety formed as a result of the reaction between the free amino groups of the chitosan backbone and the aldehyde groups of GA. As was expected, the FTIR (33) Fernades, A. L. P.; Morais, W. A.; Santos, A. I. P.; de Araujo, A. M. L.; dos Santos, D. E. S.; dos Santos, D. S.; Pavinatto, F. J.; Oliveira, O. N.; Dantas, T. N. C.; Pereira, M. R.; Fonseca, J. L. C. Colloid Polym. Sci. 2005, 284, 1- 9. (34) de Vasconcelos, C. L.; Bezerril, B. M.; dos Santos, D. E. S.; Dantas, T. N. C.; Pereira, M. R.; Fonseca, J. L. C. Biomacromolecules 2006, 7, 1245-1252.
spectra of the grafted chitosan copolymers are drastically altered compared with the spectra of chitosan, exhibiting a similarity with the pure homopolymersspoly(acrylamide) and poly(acrylic acid). This is due to the presence of the homopolymers (230 and 300% grafting, respectively) and the strong absorption bands of the grafting macromolecules that mask the absorbance bands of chitosan. In the spectra of chitosan grafted with poly(acrylamide), the amide carbonyl absorption band from the grafted chains appears at 1672 cm-1. The absorption bands at 1728 cm-1 in the spectra of chitosan grafted with poly(acrylic acid) corresponds to the carbonyl absorption from the grafted poly(acrylic acid), whereas the peaks at 799 and 611 cm-1 are characteristic of poly(acrylic acid). The formation of the imine moiety by the cross-linking reaction in the copolymers is not visible in the FTIR spectra as a result of masking by the strong carbonyl absorption bands of the grafted polymers. To elucidate the effect of the various modifications over the crystallinity of the prepared adsorbents, X-ray diffraction patterns were obtained for each sample. From the X-ray diffractogram of chitosan (Figure 3), two characteristic reflection falls were observed at 2θ ) 11 and 20°, typical of chitosan, which have been assigned to crystal forms I and II, respectively. The pattern is similar for cross-linked chitosan, revealing an unaffected crystallinity because the reaction was carried out in a hetero-
Chitosan DeriVatiVes as Biosorbents for Basic Dyes
Langmuir, Vol. 23, No. 14, 2007 7639
Figure 2. FTIR spectra of (A) Ch, (B) (Ch)c loaded with dye, (C) (Ch)c, (D) (Ch-g-Aam)c loaded with dye, (E) (Ch-g-Aam)c, (F) (Ch-g-Aa)c loaded with dye, (G) (Ch-g-Aa)c, and (H) remacryl red TGL.
Figure 3. X-ray diffraction patterns of the prepared materials.
Figure 4. SEM micrographs of (a) (Ch)c, (b) (Ch-g-Aam)c, and (c) (Ch-g-Aa)c.
Table 1. Kinetic Constants for the Sorption of Remacryl Red TGL onto Chitosan Derivatives pseudo-first- pseudo-secondorder model order model sorbent
k1 min-1
R2
(Ch)c 0.029 0.924 (Ch-g-Aam)c 0.065 0.834 (Ch-g-Aa)c 0.154 0.949
k2 min-1
R2
0.048 0.117 0.303
0.986 0.955 0.996
intraparticle diffusion model kfx104 Dx1013 m/min m2/s 25.94 13.99 5.35
R2
3.25 0.979 7.98 0.931 19.60 0.978
geneous environment. However, both the grafted and cross-linked copolymers exhibit one broad peak at around 2θ ) 21°, whereas the crystalline peak of chitosan at 2θ ) 11° disappears. This can probably be attributed to the weakening of hydrogen bonding interactions between the hydroxyl and amino groups of the chitosan macromolecules as a result of stereochemical hindrance of the side chains or the diminished concentrations of those groups. From this, it is believed that the grafting reaction of the macromolecular chains onto the chitosan backbone is initiated. Scanning electron micrographs of the prepared particles are presented in Figure 4. It is readily observed that the drying method caused the collapse of any porous microstructure of the particles. This is possibly due to hydrophilic interactions between the water molecules and the carboxylic, hydroxyl, and amino groups on the macromolecular chains of the prepared materials. This loss of porous structure has a significant effect on the surface area of the particles. For (Ch-g-Aa)c particles, the surface area reaches
Figure 5. Sorption kinetic data for chitosan derivatives, fitted to the pseudo-second-order equation. The inset presents data, as X values, fitted to the intraparticle model.
a minimum equal to 0.3 m2/g, whereas for (Ch-g-Aam)c and (Ch)c, the BET surface area values are 0.44 and 0.87 m2/g, respectively. These values are typical of nonporous materials. 3.2. Sorption Kinetics for Remacryl Red TGL. The kinetics of dye sorption by chitosan derivatives is depicted in Figure 5. The plots are characterized by a monotonous decreasing trend with the steep descent at the beginning of sorption (within 90 min) being succeeded by a more gradual decay (90-300 min), reaching equilibrium at 300 min. Chitosan modifications had a strong effect on both the sorption rate and loading in the order
7640 Langmuir, Vol. 23, No. 14, 2007
Lazaridis et al.
Table 2. Equilibrium Constants for the Sorption of Remacryl Red TGL onto Chitosan Derivatives Freundlich model sorbent (Ch)c (Ch-g-Aam)c (Ch-g-Aa)c
pH-dependent L-F
T K
KF mmol1 - 1/n L1/n/g
n
R2
Qmax mmol/g
KLF (L/mmol)1/β
β
R2
298 318 338 298 318 338 298 318 338
0.559 0.643 0.732 0.998 0.996 0.994 0.996 0.998 0.997
2.213 2.160 2.065 1.919 2.120 2.159 1.948 1.905 1.858
0.979 0.989 0.993 0.983 0.986 0.988 0.972 0.977 0.983
0.575 0.639 0.656 0.858 0.902 0.934 1.253 1.309 1.431
4.634 3.338 4.790 5.958 5.679 5.730 14.477 19.084 25.934
0.869 0.759 0.825 0.950 0.798 0.758 0.958 0.955 0.905
0.998 0.999 0.998 0.999 0.999 0.999 0.997 0.998 0.998
(Ch-g-Aa)c > (Ch-g-Aam)c > (Ch)c. The residual dye content was 0.064 mmol/L for (Ch)c but decreased to 0.037 and 0.010 mmol/L for (Ch-g-Aam)c and (Ch-g-Aa)c, respectively. Table 1 presents the initial mass-transfer coefficient, the parameters for the two kinetic models, and the intraparticle diffusion coefficients. A high correlation obtained between the experimental data and the kinetic models, although more precise, was obtained for the pseudo-second-order model. The foregoing kinetic models were used to describe successful experimental data for the sorption of dyes onto chitosan10 and activated carbons.22,35-37 However, the intraparticle model, presented as the inset in Figure 5, succeeded in predicting the experimental data. The resulting diffusion coefficients for (Ch)c, (Ch-g-Aam)c, and (Chg-Aa)c were 3.25 × 10-13, 7.98 × 10-13, and 1.96 × 10-12 m2/s, respectively. The calculated Biot number was significantly higher than 100, giving evidence that sorption is mainly controlled by intraparticle difusion.23 The rate of adsorption and the magnitude of D are dependent upon the nature of the adsorption process. For physisorption processes, the magnitude of D ranges from 10-6 to 10-9 m2/s, and for chemisorption systems, from 10-9 to 10-17 m2/s. This may be explained by the stronger bonds holding the molecules more tightly to the sorbent walls, thus lowering the rate of molecular migration.38 3.3. Sorption Isotherms for Remacryl Red TGL. Figure 6a presents the concentration of dye in the solid phase versus its concentration in the liquid phase after 24 h of mixing for chitosan derivatives at three different temperatures (298, 318, and 338 K). The data showed an increase in the amount of dye sorbed when the initial dye concentration was increased. The amount of dye adsorbed increased following an increase from 298 to 338 K, indicating the endothermic nature of the process. This was more significant for (Ch)c and (Ch-g-Aa)c. The shapes of the curves clearly indicate that the isotherms for all three systems are L type according to the classification of the sorption isotherm in solution by Giles et al.39 These isotherm types, commonly referred to as Langmuir-type isotherms, are characterized by a high degree of adsorption at very low concentrations. The data for all the of prepared materials were fitted by the three isotherm models, and the fitting results are presented in Figure 6a (Langmuir model) and in Table 2. The correlation coefficient, R2, reveals that Freundlich and pH-dependent L-F isotherms provide adequate theoretical correlation. However, the latter should be evaluated by pH-dependent equilibrium experiments (sorption (35) Preethi, S.; Sivasamy, A.; Ramamurthi, V.; Swaminathan, G. Ind. Eng. Chem. Res. 2006, 45, 7627-7632. (36) Kavitha, D.; Namasivayam, C. Bioresource Technol. 2007, 98, 14-21. (37) Senthilkumaar, S.; Kalaamani, P.; Subburaam, C. V. J. Hazard. Mater. 2006, B136, 800-808. (38) Walker, G. M.; Weatherley, L. R. Water Res. 1999, 33, 1895-1899. (39) Giles, C.; MacEwan, T.; Nakhawa, S.; Smith, D. J. Chem. Soc. 1960, 3973-3993.
Table 3. Thermodynamic Constants for the Sorption and Desorption of Remacryl Red TGL onto Chitosan Derivatives sorption sorbent (Ch)c
T K
298 318 338 (Ch-g-Aam)c 298 318 338 (Ch-g-Aa)c 298 318 338
desorption
-∆G° ∆H° ∆S° -∆G° -∆H° ∆S° kJ/mol kJ/mol kJ/mol K kJ/mol kJ/mol kJ/mol K 21.77 23.46 25.10 21.86 24.41 26.45 23.96 26.33 29.31
3.40 3.01 3.50 5.75 12.22 5.12 6.10 16.02 5.03
0.083
3.44
2.59
0.003
0.115
5.64
10.50
0.016
0.134
5.61
14.04
0.027
edges). Figure 6b depicts the RL values at 298 K plotted versus initial dye concentration, suggesting favorable sorption (0 < RL < 1). After the determination of the Freundlich parameters (KF, n) and kinetic parameters (kf, D), the design characteristics and breakthrough curve can be easily estimated. Hand, Crittenden, and Thacker18 presented constant pattern solutions, in a form that is user-oriented, to the plug-flow homogeneous surface diffusion model for fixed bed. Additionally, Table 3 presents thermodynamic parameters. The negative values of ∆G° show that the sorption of dye onto chitosan derivatives was spontaneous under the experimental conditions, without requiring an induction period. An increasing trend in the ∆G° values revealed an increasing trend in the degree of spontaneity. The positive values of ∆H° of sorption reflect its endothermic nature, which corresponds to the results of two processes: (a) desorption of the water molecules previously adsorbed on the dye and (b) adsorption of dye molecules on the adsorbent. Each dye molecule has to displace more than one molecule of solvent. The net result corresponds to the endothermic process.40 The positive values of ∆S° confirm that the adsorption of the dyes is a combination of the two aforementioned simple processes. However, another possible interpretation of the positive changes in enthalpy and entropy could be the release of numerous water molecules from the adsorbent: the adsorption of hydrated anions onto a hydrophilic polymer network inevitably disturbs the order of water molecules in the closest environment and releases them to the external liquid. Therefore, adsorbed molecules are probably attracted because of long-distance electrostatic interactions between oppositely charge groups. During the formation of ionic bonds, counterions should gain a higher degree of freedom and increase the entropy. The sorption of the dyes increasing as the temperature increases may also suggest that the number of active surface centers available for sorption increases with temperature.41 Furthermore, the increase in the uptake of (40) Singh, B. K.; Rawat, N. S. J. Chem. Technol. Biotechnol. 1994, 61, 5765.
Chitosan DeriVatiVes as Biosorbents for Basic Dyes
Langmuir, Vol. 23, No. 14, 2007 7641
Figure 7. Effect of pH on the sorption and desorption of remacryl red TGL onto chitosan derivatives.
Figure 6. (a) Sorption isotherms, fitted to the Langmuir model (R2 > 0.993), for chitosan derivatives at different temperatures. (b) Separation factor (RL) versus the initial dye concentration at 298 K.
dyes with temperature could also be a result of the enhanced rate of intraparticle diffusion of sorbate because the diffusion is an endothermic process.42 In general, the enthalpy change due to chemical adsorption (>20 kJ/mol) is considerably larger than that due to physical adsorption ( 0.990). The maximum adsorption capacities, on the basis of the Langmuir analysis, for remacryl red TGL were 0.479 for (Ch)c, 0.727 for (Ch-g-Aam)c, and 1.068 mmol/L for (Ch-g-Aa)c. The grafting modifications improved the sorption rates of the adsorbents, with adsorbent-bearing carboxylic groups (Ch-g-
sorbate AR-114 BR-22 BB-69 eosin yellow malachite green crystal violet MB MO AB-80 AR-114 AY-117 RB-5 RR-189 remacryl red TGL remacryl red TGL remacryl red TGL
Qmax (mg/g)
D (m2/s)
ref
22 75.4 160 31.49 136.58 27.99 19.59 9.43 158.4 103.7 185.7 550 1840 204.22 309.82 510.74
7.06 × 10 1.37 × 10-12 1.07 × 10-12 1.12 × 10-12 8.46 × 10-13 1.22 × 10-10 1.56 × 10-8 1.54 × 10-8 1.05 × 10-14 1.00 × 10-14 1.43 × 10-14 1.0 × 10-13
46 46 46 47 47 47 48 48 49 49 49 9 10 this study this study this study
-13
3.25 × 10-13 7.98 × 10-13 1.96 × 10-12
Aa)c exhibiting much faster rates for all dyes in comparison to the rest. Kinetic data followed a pseudo-second-order equation, and the rate was controlled by intraparticle diffusion. The diffusion coefficients were determined to be in the range of 3.25 × 10-13 to 1.96 × 10-12 m2/s. The desorption of the loaded material improved as the pH value of the desorption eluent decreased. (Ch-g-Aa)c showed a greater degree of desorption for all pH values, followed by (Chg-Aam)c and (Ch)c. When in an acidic environment, all adsorbents desorbed over 80% of the adsorbed dye. The negative ∆G° values show that the sorption of dye onto each chitosan derivative was spontaneous under the experimental conditions, whereas the positive values of the enthalpy change indicate that the sorption is endothermic. Positive values of the entropy change show the affinity of the sorbent for the dyes. Increased randomness at the biomass/solution surface occurred. The prepared adsorbents exhibited impressive adsorption capacities, kinetics, and regenerative abilities. However, their potential applicability as sorbents in large-scale implementations is hindered by their substantial swelling, which ultimately limits their practical use in commercial packed columns. Therefore, the next logical step would be to devise a feasible solution to overcome this drawback, which at the same time does not significantly compromise the overall adsorption performance. Acknowledgment. The financial support received for this study from the Greek Ministry of Education through the research program Pythagoras II is gratefully acknowledged. Supporting Information Available: Experimental preparation. This material is available free of charge via the Internet at http://pubs.acs.org. LA700423J
