Langmuir 2008, 24, 4791-4799
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Low-Swelling Chitosan Derivatives as Biosorbents for Basic Dyes George Z. Kyzas,† Dimitrios N. Bikiaris,‡ and Nikolaos K. Lazaridis*,† Laboratory of General & Inorganic Chemical Technology and Laboratory of Organic Chemical Technology, DiVision of Chemical Technology, School of Chemistry, Aristotle UniVersity of Thessaloniki, GR-541 24 Thessaloniki, Greece ReceiVed December 14, 2007. In Final Form: February 6, 2008 In this study, three different chitosan microsphere derivatives were prepared as sorbents for basic dyes. Preparation was succeeded by a novel cross-linking method based on ionic gelation with tripolyphosphate and subsequent covalent cross-linking with glutaraldheyde in order to address the large amount of swelling of the powdered form of the respective derivatives. Basic blue 3G (dye) was selected as the sorbate, and chitosan microsheres grafted with acrylamide and acrylic acid were used as biosorbents. Techniques such as FTIR spectroscopy, SEM, and swelling measurements facilitated the evaluation of the materials. Sorption-desorption experiments over the whole pH range were carried out to reveal the optimum value of sorption-desorption. The Langmuir isotherm model was used to fit the equilibrium experimental data, giving a maximum sorption capacity of 0.808 mmol/g at 338 K. An intraparticle diffusion model was employed to fit the kinetic data, and the resulting diffusion coefficients were in the range of (1-10) × 10-11 m2/s. Thermodynamic analysis showed that the sorption process was spontaneous and endothermic with an increased randomness. In addition, sorption experiments were realized with a mixture of three basic dyes at various concentrations of sorbents.
1. Introduction It is obvious that the main problem in the textile industry, regarding effluents, is not only the existence but also the toxicity of most toxicogenic dyes.1 In a study sponsored by the American Dye Manufacturers Institute (ADMI), more than 50 dyes were screened for toxicity to the single-celled green algae Selenastrum capricornutum and the fathead minnow Pimephales promelas. This is probably the most comprehensive study yet performed in the United States to assess the potential hazards of dyes to aquatic life. The ADMI study showed that basic dyes are generally more toxic than acid and more reactive than direct dyes.2 Even though the relative degree of dye loss to the effluent for the main dye-fiber application of basic dyes is approximately 0-5%, their high toxicity necessitates the study of the removal of basic dyes from textile effluents.1 Nowadays, there are many low-cost, commercially available sorbents for the removal of dyes.3-6 Still, sorption capacities are not sufficiently high, and biomass-derived sorbents with higher effectiveness and performance are still needed. It has been shown that chitosan is an effective sorbent for most dye classes, reaching capacities as high as 1100 mg/g.7,8 However, because of its cationic nature, which in most cases is highly responsible for its effective sorbent abilities, and the subsequent electrostatic repulsive forces generated, the affinity * To whom correspondence should be addressed. E-mail: nlazarid@ chem.auth.gr. Tel: +32310 997807. Fax: +32310 997859. † Laboratory of General & Inorganic Chemical Technology. ‡ Laboratory of Organic Chemical Technology. (1) Blackburn, S. R. EnVriron. Sci. Technol. 2004, 38, 4905-4909. (2) Dyes and the EnVironment: Reports on Selected Dyes and Their Effects; American Dye Manufacturers Institute: New York, 1974; Vol. 2. (3) Azhar, S. S.; Liew A. G.; Suhardy, D.; Hafiz, K. F.; Hatim, M. D. I. Am. J. Appl. Sci. 2005, 11, 1499-1503. (4) Guibal, E.; Van Vooren, M.; Dempsey, B.; Roussy, J. J. Sep. Sci. Technol. 2006, 41, 2487-2514. (5) Crini, G. Prog. Polym. Sci. 2005, 30, 38-70. (6) Yi, H.; Wu, L.; Bentley, W.; Ghodssi, R.; Rubloff, G.; Culver, J.; Payne, G. Biomacromolecules 2005, 6, 2881-2894. (7) Sakkayawong, N.; Thiravetyan, P.; Nakbanpote, W. J. Colloid Interface Sci. 2005, 286, 36-42. (8) Wong, Y. C.; Szeto, Y. S.; Cheung, W. H.; McKay, G. Langmuir 2003, 19, 7888-7894.
toward basic dyes is extremely low, especially at low pH values where the degree of amine protonation is higher.9 Chitosan presents a high degree of swelling in aqueous solutions. Especially in powdered form, it swells considerably and crumbles easily, thus not behaving ideally in packed-column configurations common to pump-and-treat adsorption processes, usually leading to plugging of the column. For pure chitosan, degrees of swelling as high as 2000% have been reported.10 To overcome this problem, chitosan could be cross-linked either covalently by glutaraldehyde (GA)11 or ionically by a polyanion such as tripolyphosphate (TPP).12,13 In the first case, a solid matrix is produced that should be ground to generate chitosan particles, but it still presents a high degree of swelling (e.g., higher than 600%), especially in an acidic environment.14 In the second case, micro- or nanoparticles can be formed but with reduced stability in acidic environments despite a degree of swelling as low as 50%.15 To produce chitosan microspheres directly, several methods have been developed by employing glutaraldehyde, formaldehyde, or genipin.16,17 However, in these methods chitosan microspheres are prepared mainly through a w/o emulsion procedure, which is complicated because liquid paraffin and surfactants should be added.18 Recently, Shu and Zhu investigated the possibility of three kinds of anions (tripolyphosphate, citrate, and sulfate) interacting with chitosan by turbidimetric titration.19 (9) Chao, A. C.; Shyu ,S. S.; Lin, Y. C.; Mi, F. L. Bioresour. Technol. 2004, 91, 157-162. (10) Goycoolea, F. M.; Argu¨elles-Monal, W. M.; Lizardi, J.; Peniche, C.; Heras, A.; Galed, G.; Dı´az, E. I. Polym. Bull. 2007, 58, 225-234. (11) Rinaudo, M. Polym. Int. 2008, 57, 397-430. (12) Janes, K. A.; Calvo, P.; Alonso, M. J. AdV. Drug DeliVery ReV. 2001, 47, 83-97. (13) Bodmeier, R.; Chen, H. G.; Paeratakul, O. Pharm. Res. 1989, 6, 413417. (14) Yao, K. D.; Peng, T.; Feng, H. B.; He, Y. Y. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1213-1223. (15) Lee, S. T.; Mi, F. L.; Shen, Y. J.; Shyu, S. S. Polymer 2001, 42, 18791892. (16) Berger, J.; Reist, M.; Mayer, J. M.; Felt, O.; Peppas, N. A.; Gurny. R. Eur. J. Pharm. Biopharm. 2004, 57, 19-34. (17) Sinha, V. R.; Singla, A. K.; Wadhawan, S.; Kaushik, R.; Kumria, R.; Bansal, K.; Dhawan, S. Int. J. Pharm. 2004, 274, 1-33. (18) Jameela, S. R.; Jayakrishnan, A. Biomaterials 1995, 16, 769-775.
10.1021/la7039064 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008
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Kyzas et al. Table 1. Properties of Dyes Used
Another way to produce chitosan microspheres directly is by dual cross-linking with TPP and GA.20,21 In this case, microspheres are formed in two stages by adding TPP in the first stage and GA in the second. The present investigation complements the previously published work of this team on basic dye removal from simulated aqueous solutions by sorption onto chitosan derivatives, which were prepared by homogeneous grafting and heterogeneous crosslinking with GA exhibiting a high degree of swelling of 6401190%.22 In this study, to confer on chitosan the property of a basic dye sorbent, the grafting of poly(acrylamide) and poly(acrylic acid) as side chains on chitosan was carried out. To address the process limitations of the adsorbent’s powdered form, the reactions were performed on rigid, highly cross-linked chitosan microspheres. The novelty of the study is the microspheres preparation procedure. Microspheres were formed by ionic gelation with TPP and subsequent covalent cross-linking with GA, which rendered the materials insoluble in acidic media and resilient and with a small degree of swelling. To prepare more homogeneous crosslinking microspheres, chitosan solution was added to the solution simultaneously containing both TPP and GA. According to the proposed method, the cross-linking will be within the whole matrix, avoiding surface cross-linking, which could take place in the two-stage method as a result of diffusion restrictions of GA into chitosan microspheres previously cross-linked with TPP.
2.1. Materials. High-molecular-weight chitosan (Ch) was obtained from Sigma-Aldrich and purified by extraction with acetone in a Soxhlet apparatus for 24 h, followed by drying under vacuum at room temperature. The average molecular weight was estimated to be 355 kDa, and the degree of deacetylation was 82 wt %, as determined according to the procedures described by Rinaudo.23 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. Glutaraldheyde (GA, 50 wt % in water) and sodium tripolyphosphate (TPP) were received from Sigma-Aldrich. All reagents were of analytical grade. The basic dyes basic blue 3G, remacryl red TGL, and basic yellow 37 were kindly supplied by Hochest (Germany) and were used without further purification. The main target dyestuff was basic blue 3G with a purity of 52.28% w/w, which was taken into account for all calculations. Necessary information regarding each dye is shown in Table 1. The molecular size of the dyes was estimated by employing the BioMedCAChe 5.02 program by Fujitsu. The maximum absorption wavelength for the dye solutions was determined by running full-range wavelength scans. 2.2. Preparation of Chitosan Microspheres. Microspheres of cross-linked chitosan were prepared by initially dissolving chitosan (1.41 × 10-6 moles ≈ 0.5 g) in 50 mL of an aqueous 2 v/v% CH3COOH solution. The solution was added dropwise from a pipet into an aqueous solution of GA (5 × 10-2 M), which also contained TPP (1.36 × 10-3 moles) at pH 6, as adjusted with an aqueous HCl solution. The formed gelled microspheres were stirred overnight at room temperature in the aforementioned solution. Then, they were filtered and purified by extraction with water in a Soxhlet apparatus for 24 h. Freeze drying of the prepared microspheres resulted in an extremely brittle material as a result of the extensive porous structure form (Figure 1a). Thus, conventional drying (oven-dried microspheres) was carried out at 323 ( 1 K, which led to nonporous but strong, rigid biosorbent microspheres (Figure 1b,c). The size of the final microspheres could be controlled by adjusting the size of the drop (e.g., by altering the concentration of chitosan or adding surfactants to the chitosan solution, noted hereafter as (Ch)CM). The grafting reactions were carried out in aqueous suspensions of microspheres in a manner similar to that for homogeneous reactions reported in the literature.24-26 Porous microspheres, with respect to solid ones, will be preferred for grafting purposes because of their higher surface area and subsequently their higher degree of grafting. However, this is strongly influenced by the mechanical strength of porous microspheres. Porous but brittle sorbent material cannot be easily applied in real wastewater treatment processes because of structure collapse, leading to plugging of the packed-column configuration. Therefore, in the present study solid cross-linked microspheres were employed in the grafting procedure. The cross-
(19) Shu, X. Z.; Zhu, K. J. Int. J. Pharm. 2000, 201, 51-58. (20) Wang, L. Y.; Gu, Y. H.; Zhou, Q. Z.; Ma, G. H.; Wan, Y. H.; Su, Z. G. Colloids Surf., B 2006, 50, 126-135. (21) Durkut, S.; Elc¸ in, Y. M.; Elc¸ in, A. E. Artif. Cells, Blood Substitutes, Biotechnol. 2006, 34, 263-276. (22) Lazaridis, N. K.; Kyzas, G. Z.; Vassiliou, A. A.; Bikiaris, D. N. Langmuir 2007, 23, 7634-7643.
(23) Rinaudo, M. Prog. Polym. Sci. 2006, 31, 603-632. (24) Jayakumar, R.; Prabaharan, M.; Reis, R. L.; Mano, J. F. Carbohydr. Polym. 2005, 62, 142-158. (25) Yasdani - Pedram, M.; Retuert, J.; Quijada, R. Macromol. Chem. Phys. 2000, 201, 923-930. (26) Yasdani-Pedram, M.;Lagos, A.; Retuert, J. Polym. Bull. 2002, 48, 9398.
2. Materials and Methods
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Figure 1. SEM micrographs of (a) the surface of a freeze-dried (Ch)CM microsphere, (b) a (Ch)CM oven-dried microsphere, (c) the surface morphology of a (Ch)CM oven-dried microsphere, and (d) the surface morphology of a (Ch-g-Aa)CM oven-dried, grafted microsphere. linked chitosan microspheres (1 g) were dispersed in an aqueous solution of the monomer (4.06 × 10-2 moles for Aam and 5.83 × 10-2 moles for Aa) with the use of a magnetic stirrer. After stirring for 1 h, a solution of the initiator (KPS, 6.7 × 10-4 moles) was added, and stirring continued for 15 min. The above solution (30 mL) was transferred to a 100 mL stoppered flask and placed in a thermostated bath (333 ( 1 K) for 3 h under continuous stirring. The solution was constantly purged with argon at all stages. The grafted microspheres were filtered and purified by extraction with water in a Soxhlet apparatus for 24 h, leading to the removal of any unreacted monomer and initiator, byproducts, and the eventually formed (PAa) homopolymer. After being dried at 323 ( 1 K, the grafted copolymer microspheres were obtained with grafting percentages of approximately 60% for Aa and 40% for Aam (noted hereafter as (Ch-g-Aa)CM and (Ch-g-Aam)CM, respectively) with a diameter 900 ( 50 µm. The surface morphology of (Ch-g-Aa)CM is given in Figure 1d. The powder forms of the materials, prepared in our previous study,22 were denoted as (Ch)C, (Ch-g-Aam)C, and (Ch-g-Aa)C, respectively. The particle size of the microspheres should not be too small in order to keep the pressure drop through the chitosan column low. However, it must not be excessively large because the long diffusion distances would delay adsorption and desorption. In the case of activated carbon, the most applied sorbent material, microspheres with a diameter of 0.8 to 2 mm, turned out to be most efficient.27,28 2.3. Characterization Techniques. FTIR spectra of the biosorbents were obtained using a Perkin-Elmer FTIR spectrometer (model Spectrum 1000) using KBr pellets containing the prepared materials, predried for 24 h at 323 ( 1 K. The resolution of each spectrum was 2 cm-1, and the number of co-added scans was 64. The spectra (27) Bansal, Roop Chand; Goyal, Meenakshi. ActiVated Carbon Adsorption; Taylor & Francis: New York, 2005. (28) Smisek, M.; Cerny, S. ActiVe Carbon: Manufacture, Properties and Applications; Elsevier Publishing Company: Amsterdam, 1970.
presented are baseline corrected and converted to the absorbance mode. For the degree-of-swelling measurements, 0.100 g of sample was placed in at least 100 mL of a range of nitric acid and sodium hydroxide solutions, whose concentrations were adjusted to give a range of pH values between 2 and 12. After immersion for 4 h at room temperature, by which time the material was visually judged to have finished swelling, the material was removed from the solvent by filtration, and excess solvent was removed by blotting with moistened filter paper. The weight of the swollen sample was measured, and the degree of swelling (g/g) was calculated using the relation S ) (WS - W0)/W0, where WS and W0 are the weights of the swollen and dry polymers, respectively. At least three measurements were carried out for each initial pH condition, and the mean values are presented. Scanning electron microscopy observations of the prepared particles were carried out using a JEOL JMS-840A scanning microscope equipped with an energy-dispersive X-ray (EDX) Oxford ISIS 300 microanalytical system. All of the studied surfaces were coated with carbon black to avoid charging under the electron beam. 2.4. Experimental Procedure. 2.4.1. Sorption Edges. The effect of pH on the adsorption process was studied by mixing 1 g/L of sorbent with 50 mL of an aqueous dye solution of 0.148 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 in a water bath to control the temperature to 298 ( 1 K. Dye uptake was significantly higher in the alkaline environment (pH g10), so pH 10 was selected for the kinetic and equilibrium experiments. 2.4.2. Kinetics. Kinetics experiments were performed by mixing 1 g/L of sorbent with 50 mL of an aqueous dye solution of 0.148 mmol/L (53 mg/L) concentration. The mixture was shaken for 24 h in a water bath at 298 ( 1 K. Samples were collected at fixed intervals (5, 10, 20, and 30 min and 1, 2, 3, 4, 5, 6, 12, and 24 h).
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The following intraparticle diffusion model was used to estimate the intraparticle diffusion coefficient, D (m2/s) (eq 1), X)
[
(
)]
C0 - Ct 4πDt ) 1 - exp C0 - Ce 2.3d2
1/2
(1)
22,29
where d is the particle diameter (m) and C0, Ct, and Ce are the initial, transient, and equilibrium concentrations of dye in the aqueous solution (mmol/L), respectively. The diffusion coefficient, which is the sum of pore and surface diffusion, can be calculated from the half-time for adsorption (X ) 0.50). 2.4.3. Isotherms. Batch studies were conducted by shaking 50 mL of a 0.148 mmol/L (53 mg/L) concentration of an aqueous dye solution with 0.05 g of chitosan microspheres (1 g/L) for 24 h by using a thermostatic shaker bath operated at three temperature levels (298 ( 1, 318 ( 1, and 338 ( 1 K). The pH of the aqueous solutions was adjusted by using 0.1 M NaOH. The Langmuir isotherm model, which is given by eq 2, was used to describe the experimental data30 Qe )
QmaxCe (1/b) + Ce
(2)
where Qe and Qmax are the equilibrium and maximum dye concentrations in the solid phase (mmol/g), respectively, and b is the Langmuir sorption equilibrium constant (L/mmol). Thermodynamic parameters such as the Gibbs free energy (∆Go), standard enthalpy change (∆Ho), and standard entropy change (∆So) were also calculated. The Gibbs free energy (∆Go) can be calculated according to the following equation: ∆Go ) -RT ln b
(3)
The standard enthalpy change (∆Ho) and standard entropy change (∆So) of the process could be calculated by the following equation: ∆Go ) ∆Ho - T∆So
(4)
Additionally, kinetic and equilibrium experiments were performed with the high-swelling powder (Ch-g-Aa)C. The latter presented the highest sorption behavior in our previous work and was used for comparison. 2.4.4. Sorption of Dyes Mixture. To study the equilibrium sorption behavior with respect to the coexistence of other basic dyes, a series of experiments was performed with various concentrations of chitosan microspheres. The three-dye mixture solution was composed of basic blue 3G, remacryl red TGL, and basic yellow 37, the selection of which was realized by the fact that dye baths operate on a trichromic basis. The dye mixture was prepared by mixing 5 mg/L of each dye, producing 2370 ADMI color units. As communicated in the literature, the values of the ADMI units of liquids in the equalization tank of a dye house ranges from 50 to 3890 ADMI units.31 2.4.5. Desorption. After sorption experiments, samples were collected and filtered using fixed-pore-sized membranes. Desorption experiments were performed by mixing the collected amount of loaded chitosan with aqueous solutions over a pH range of 2 to 12 for 24 h at a temperature of 298 ( 1 K. 2.5. Analysis. Samples of the single solution basic blue 3G were collected at various time intervals and filtered and analyzed using a UV-vis spectrophotometer (model U-2000, Hitachi). Although a small fraction of the dye (1 to 2%) was retained on the filter membrane, this small variation due to filtration was neglected. The effect of pH over the calibration curves of each dye was studied prior to the sorption experiments because the λmax of the dye solution might shift to different wavelengths, although no significant deviation was observed. The dyestuff content of the three-dye mixture was estimated in ADMI units (American Dyestuff Manufacturer Institute). (29) Khraisheh, M. A. M.; Al-Degs, Y. S.; Allen, S. J.; Ahmad, M. N. Ind. Eng. Chem. Res. 2002, 41, 1651-1657. (30) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361-1368. (31) O’Neill, C.; Hawkes, F. R.; Hawkes, D. L.; Lourenc¸ o, N. D.; Pinheiro, H. M.; Dele´e, W. J. Chem. Technol. Biotechnol. 1999, 74, 1009-1018.
The method involves measuring the absorbance of the samples after filtration in sets of 10 or 30 wavelengths depending on the accuracy required to generate the CIE (Commission International de l’Eclairage) Tristimulus values X, Y, and Z. These are converted by the use of published tables to values called Munsell values. From the Munsell values, the Adams-Nickerson color difference (DE) is calculated from an equation. The DE values of a series of APHA platinumcobalt standards are plotted against the corresponding ADMI values to give a calibration plot, and the DE values of samples are read against this plot to obtain the ADMI values of the samples. According to the U.S. Pollutant Discharge System, the permitted level is 300 ADMI units.31-33
3. Results and Discussion 3.1. Characterization of Chitosan Microspheres. The interaction of the polycationic chitosan and the nontoxic multivalent counterion TPP resulted in the formation of a gel. The nature and extent of the ionic interactions are sensitive to factors such as the molecular weight and charge density of both electrolytes. Because the intrinsic pK of chitosan is about 6.3, when dissolved in an acidic aqueous solution it presents -NH3+ sites. TPP, being a weak polyprotic acid-like phosphoric acid, exhibits Ka values decreasing from Ka1 to Ka5 in a quantity of several orders of magnitude. Because of this, when dissolved in water, TPP dissociates to both OH- and TPP ions and P3O105-, HP3O104-, and H2P3O103- can coexist in the TPP solution. The OH- or TPP ions competitively interact with the -NH3+ binding site of chitosan by either deprotonation or ionic cross-linking. When the solution pH is adjusted to 0.973)
( )
ADMI ) ADMIf + ADMIB0 exp -
( )
ADMIR0 exp -
m + kB
( )
m m + ADMIY0 exp (5) kR kY
where ADMIf, ADMIB0, ADMIR0, and ADMIY0 are the final, initial blue, initial red, and initial yellow ADMIs and kB, kR, and kY are the mass rates of blue, red, and yellow dyes, respectively.
Table 2. Equilibrium Constants and Thermodynamic Parameters for the Sorption of Basic Blue 3G onto Chitosan Microspheres thermodynamics of chitosan microspheres
Langmuir model sorbent (Ch)CM (Ch-g-Aam)CM (Ch-g-Aa)CM (Ch-g-Aa)C
T K
Qmax mmol/g
b L/mmol
R2
298 318 338 298 318 338 298 318 338 298
0.306 0.337 0.355 0.495 0.509 0.520 0.722 0.784 0.808 1.388
8.844 11.007 13.388 9.688 13.094 21.490 23.374 29.489 41.479 34.472
0.998 0.998 0.998 0.998 1.000 0.997 0.998 0.998 0.998 0.997
-∆Go kJ/mol 22.54 24.60 26.70 22.57 24.74 27.50 22.74 25.06 28.12
∆Ho kJ/mol
∆So kJ/mol‚K
8.68
0.105
14.26
0.123
17.47
0.135
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Figure 6. Effect of pH on the sorption and desorption of basic blue 3G onto chitosan microspheres (0.278 mmol/L initial dye concentration, 1 g/L sorbent concentration, T ) 298 K) (open symbols, sorption; solid symbols, desorption).
Parameters were estimated from single dye-sorption experiments and were kept constant, except ADMIf, which was freely floated (data not shown). 3.5. Effect of pH on Sorption/Desorption, the Sorption Mechanism, and Diffusivity. The hydrogen ion concentration of the solution can give insight into the dominant mechanism of sorption. The effect of pH on the sorption/desorption of basic blue 3G by chitosan derivatives is presented in Figure 6. Sorption (%) begins (at pH 2) from low values of ∼10, 20, and 40% and approaches (at pH 10) high values of ∼70, 80, and 90% for (Ch)CM, (Ch-g-Aam)CM, and (Ch-g-Aa)CM, respectively. Although the (Ch)CM and (Ch-g-Aam)CM responses presented an abrupt ascent at pH 8 for (Ch-g-Aa)CM, this was more gradual. At higher pH values, less protonated amino groups of chitosan will be available, thereby decreasing the electrostatic repulsion with the positively charge dye molecules. The dye molecules can then be more easily sorbed through other mechanisms, such as the formation of hydrogen and van der Waals bonds. Especially for (Ch-g-Aa)CM, deprotonation of the carboxylic groups creates negatively charged sorption sites leading to electrostatic attractions with positively charged dye molecules. The regenerative potential for the prepared sorbents was examined by performing desorption experiments over the whole pH range. An almost linear curve was obtained, giving high desorption percentages in acidic environments (pH 2, 74-88%) and low desorption percentages in alkaline environments (pH 12, 22-37%). Thus, the exact opposite trend of the sorption process with pH was followed. The three different amine groups of the dye are protonated in an acidic environment. In particular, the aromatic amine is mostly protonated in all of the pH ranges studied because of its strong basic nature. As seen in the FTIR spectra of (Ch)CM, the adsorption of the dye results in a significant shift of the hydroxyl adsorption band around 3200 cm-1. This is evidence of hydrogen bond formation between the hydroxyl groups of chitosan and the chlorine atom of dye molecules (Figure 7a). Furthermore, the adsorption band of the PdO groups at 1158 cm-1 is shifted to 1151 cm-1, probably because of the ionic interactions between the positively charged dye molecule and the free negatively (45) De Castro Danta, T. N.; Dantas Neto, A. A.; de A. Moura, M. C. P.; Barros Neto, E. L.; de Paiva Telemaco, E. Langmuir 2001, 17, 4256-4260. (46) Li, H.; Teppen, B. J.; Hohnston, C. T.; Boyd, S. A. EnViron. Sci. Technol. 2004, 38, 5433-5442. (47) Al-Degs, Y. S.; El-Barghouthi, M. I.; El-Sheikh, A. H.; Walker, G. M. Dyes Pigm. 2008, 77, 16-23. (48) ten Hulscher, Th. E. M.; Gornelissen, G. Chemosphere 1996, 32, 609626.
Figure 7. Main ionic interactions and hydrogen bonding between (a) dye and the cross-linked chitosan backbone, (b) (Ch-g-Aam)CM and dye-specific, and (c) (Ch-g-Aa)CM and dye-specific and desorption. (Wavy bonds represent the ionic interactions, and hashed bonds represent the hydrogen bonds.)
charged groups of TPP (-P-O-). In (Ch-g-Aam)CM, a significant shift of the amide-carbonyl peak from 1661 to 1667 cm-1 confirms the interactions with the dye molecules through intermolecular hydrogen bonds with the dye of the amide group. Moreover, because of the aforementioned interaction with the dye, the adsorption band of the PdO groups is shifted from 1156 to 1151 cm-1. The pKa of chitosan’s amine group is 6.5, which means that its protonated form exists below pH 8.5, with almost all amine groups being protonated at values lower than 4.5. These exert repulsive forces on the positively charged dye molecules (Figure 7b), leading to low sorption levels, whereas desorption is favored. At higher pH values the protonated amine groups are significantly decreased in number, resulting in improved adsorption behavior, mainly through intermolecular bonds between the dye and the hydroxyl groups of chitosan and electrostatic interactions between the positively dye molecules and the free
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with acrylic acid ((Ch-g-Aa)cM), which is clear evidence that dye molecules are diffused up to the center of the microspheres. With the initial concentration of the dye solution being 0.738 mmol/L (265 mg/L), the relative sorbed dye quantity was 80% at 100 µm from the surface of the microspheres and 60% in the center with respect to the concentration 5 µm from its surface (100%). Obviously, the intraparticle diffusion was effective throughout the sorbent particle, offering a smooth distribution of dye in the section of the microspheres. Figure 8b shows the kinetic profiles for the sorption of dye, whereas an initial steep descent is followed by a flat plateau. By transforming data according to eq 1 and employing nonlinear regression analysis (R2 > 0.950), the inset of Figure 8b was generated. The resulting diffusion coefficients were 5.69 × 10-11, 7.81 × 10-11, 9.56 × 10-11, and 0.12 × 10-11 m2/s for (Ch)CM, (Ch-g-Aam)CM, (Ch-g-Aa)CM, and (Ch-g-Aa)C, respectively. According to this, the diffusivities in microsphere derivatives are higher than those of the respective powder derivatives (1 to 2 orders of magnitude), which could be attributed to the strong dependence of the diffusion coefficient from the second-order particle diameter (eq 1).
4. Conclusions
Figure 8. (a) Profile distribution of dye by SEM and EDS analysis (considering the chlorine atom of the dye) in the sorbent microsphere (SEM of the transversal incision of (Ch-g-Aa)cM presenting the internal microsphere structure) at an initial dye concentration of 0.738 mmol/L (265 mg/L). (b) Sorption kinetic data; inset data are modeled by the intraparticle diffusion model (0.148 mmol/L initial dye concentration, 1 g/L sorbent concentration, T ) 298 K, and pH 10).
negatively charged groups of TPP in the adsorbent. Grafting of poly(acrylamide) leads to a decrease in the amino group concentration in the sorbent. Thus, less-protonated amino groups exert less-repulsive forces. Additional sorption sites are also introduced because the amide groups participate in hydrogen bonds with the dye molecule. Ultimately, the aforementioned processes result in enhanced adsorption characteristics compared to those of the ungrafted sorbent. The spectra of (Ch-g-Aa)CM presents significant variations due to the sorption of the dye. Specifically, a new strong peak at 1572 cm-1 appears and is attributed to the asymmetric deformation of the carboxylate ion. The negatively charged carboxylate ions and the positively charged dye molecules strongly interact with one another, and this interaction is the prime reason behind the high sorption performance of (Ch-g-Aa)CM (Figure 7c). Increasing the pH of the solution increases the degree of deprotonation of the sorbent’s carboxylic acid groups, leading to a greater number of anionic groups, which causes the significant observed improvement of dye sorption. The absorption band of the PdO group at 1160 cm-1 is shifted to 1156 cm-1, revealing further interactions of the positively charged dye molecule and the free negatively charged groups of TPP. At low pH values, the protonation of the carboxylic acid groups, leading to lower electrostatic interactions with the dye, and the increased protonation of chitosan’s amine groups, increasing the repulsive forces, result in the effective release and desorption of the dye from the sorbent. The concentration profile of the dye distribution in the loaded microspheres was analyzed by EDS analysis of the dye’s chlorine atom and is shown in Figure 8a. This Figure presents the transversal incision (SEM) of a cross-linked microsphere grafted
The present investigation complements the previously published work on basic dye removal from aqueous solutions by sorption onto chitosan derivatives. In the first study, three types of chitosan powder derivatives, namely, (Ch)C, (Ch-g-Aam)C, and (Ch-g-Aa)C, were prepared by homogeneous grafting and heterogeneous cross-linking with GA. In this study, the corresponding chitosan microspheres, namely, (Ch)CM, (Ch-g-Aam)CM, and (Ch-g-Aa)CM, were prepared by cross-linking with TPP and GA followed by grafting with poly(acrylamide) and poly(acrylic acid) groups. The comparison of experimental results between microspheres and powder derivatives is summarized below: • At pH 7, the degrees of swelling of microspheres are 43, 48, and 69% whereas for powders they are 640, 1050, and 1190% for derivatives of the form of (Ch), (Ch-g-Aam), and (Ch-g-Aa), respectively. • Microspheres present lower sorption capacities (109.91, 177.79, and 259.33 mg/g for (Ch)CM, (Ch-g-Aam)CM, and (Chg-Aa)CM, respectively) than does (Ch-g-Aa)C powder (498.52 mg/g), but still to a remarkable level. Equilibrium data, with endothermic behavior, were successfully fitted to the Langmuir model. • Microspheres present higher diffusion coefficients (5.69 × 10-11, 7.81 × 10-11, and 9.56 × 10-11 m2/s for (Ch)CM, (Chg-Aam)CM, and (Ch-g-Aa)CM, respectively) than does (Ch-gAa)C powder (0.12 × 10-11 m2/s). Kinetic data were successfully fitted to a simple diffusion model. • Optimum pH for sorption was found to be alkaline (pH 10) whereas for desorption (over 80%) it was very acidic (pH 2). • Microspheres reduce the dye content from a trichromatic mixture below the limit of 100 ADMI units, according to the pollutant discharge system. • The previous findings indicate that a feasible solution to the drawback of the high degree of swelling of chitosan powder derivatives was devised by chitosan microspheres. In particular, (Ch-g-Aa)CM has all of the sorption characteristics of effective application in large-scale implementations for basic dye removal from aqueous solutions. Acknowledgment. The financial support received for this study from the Greek Ministry of Education through the research program Pythagoras II is gratefully acknowledged. LA7039064
