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Adsorption of tannic acid, humic acid, and dyes from water using the composite of chitosan and activated clay
Journal of Colloid and Interface Science 278 (2004) 18–25 www.elsevier.com/locate/jcis
Adsorption of tannic acid, humic acid, and dyes from water using the composite of chitosan and activated clay Min-Yun Chang a , Ruey-Shin Juang b,∗ a Department of Chemical Engineering, National United University, Miao-Li 360, Taiwan b Department of Chemical Engineering, Yuan Ze University, Chung-Li 320, Taiwan
Received 26 March 2004; accepted 18 May 2004 Available online 19 June 2004
Abstract Chitosan is a well-known excellent adsorbent for a number of organics and metal ions, but its mechanical properties and specific gravity should be enhanced for practical operation. In this study, activated clay was added in chitosan slurry to prepare composite beads. The adsorption isotherms and kinetics of two organic acids (tannic acid, humic acid) and two dyes (methylene blue, reactive dye RR222) using composite beads, activated clay, and chitosan beads were compared. With composite beads as an adsorbent, all the isotherms were better fitted by the Freundlich equation. The adsorption capacities with composite beads were generally comparable to those with chitosan beads but much larger than those with activated clay. The pseudo-first-order and pseudo-second-order equations were then screened to describe the adsorption processes. It was shown that the adsorption of larger molecules such as tannic acid (MW, 1700 g mol−1 ), humic acid, and RR222 from water onto composite beads was better described by the pseudo-first-order kinetic model. The rate parameters of the intraparticle diffusion model for adsorption onto such adsorbents were also evaluated and compared to identify the adsorption mechanisms. 2004 Elsevier Inc. All rights reserved. Keywords: Chitosan; Activated clay; Composite bead; Adsorption; Tannic acid; Humic acid; Dyes
1. Introduction Chitosan is known as an ideal natural support for enzyme immobilization because of its many characteristics such as hydrophilicity, biocompatibility, biodegradability, and antibacterial properties [1,2]. It has been used widely as an adsorbent for transition metal ions and organic species because the amino (–NH2) and hydroxy (–OH) groups on chitosan chains can serve as the coordination and reaction sites [3–7]. In addition to these, chitosan appears economically attractive since it can be obtained from deacetylation of chitin, and chitin is the second most abundant biopolymer in nature, next to cellulose [1]. Although the bead form of chitosan shows better adsorption capability than the flake form, due to its higher specific surface area [8], the weak mechanical property (highly swollen in water) and low specific gravity of the chitosan beads make them rather inconvenient for use as adsorbents in * Corresponding author. Fax: +886-3-4559373.
E-mail address: [email protected] (R.-S. Juang). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.05.029
either batch or column modes. Thus, chemical modifications of the chitosan beads are often made including carboxyalkyl substituted, aldehyde crosslinked, ligand crosslinked, and polyaminated to enhance adsorption performance, to prevent them from dissolution in strongly acidic and alkali solutions, or both [9,10]. However, such modification procedures seem to be complicated and/or are relatively expensive. In this work, chitosan beads were physically modified and activated clay particles were embedded into them. The activated clay was selected because it was also an effective adsorbent for organic matter and dyes [11]. First, the specific gravities of composite beads made from different weight fractions of activated clay were measured. The equilibria and kinetics for adsorption of tannic acid, humic acid, basic dye methylene blue, and reactive dye 222 (RR222) from water onto composite beads were compared with those of chitosan beads and activated clay. These adsorbates are selected because they are frequently encountered in process/waste streams and/or natural waters [5,12–16]. For example, tannic acid exists in many natural juices and gives them a bitter taste. The uptake of methylene blue is usually
M.-Y. Chang, R.-S. Juang / Journal of Colloid and Interface Science 278 (2004) 18–25
used as an index of adsorption performance for adsorbents. Reactive dye wastewaters have limited biodegradability in an aerobic environment and many azo dyes under anaerobic conditions decompose into potentially carcinogenic aromatic amines [12,13]. Humic substance is a general name for organic matters that are not easily decomposed by microorganisms. Major function groups of humic acid, though its chemical structure is not completely clear, include carboxylic acid, phenolic acid, alcohol acid, aldehyde acid, and methoxyl. Humic acid will react with chlorine during water treatment and sterilization and produce trihalomethanes, which cause cancer and affect the health of human beings very seriously. Hence, the adsorption of these adsorbates is of practical importance and interest.
2. Materials and methods 2.1. Chitosan and activated clay Dried cuttlebone cartilage was immersed in 5 wt% NaOH for 18 h to remove proteins and then in 5 wt% HCl for 18 h to remove CaCO3 (the weight ratio of the waste to the solution was 1/10). The resulting insoluble chitin (about 40 g) was deacetylated in 50 wt% NaOH (800 g) at 90 ◦ C for 3 h. The chitosan flakes were washed three times with deionized water (Millipore Milli-Q) and dried at 50 ◦ C in a vacuum. The degree of deacetylation and molar mass of chitosan were measured to be 97.2% and 1.85 × 106 , respectively, according to the methods described earlier [6]. Fifty grams of natural clay (Union Chem. Co., Taiwan), with a particle size of 0.02–0.06 mm, were activated by refluxing with 250 ml of 1 mol L−1 H2 SO4 at 80 ◦ C in a round-bottom flask for 2 h. The slurry was air-cooled and filtered with a glass fiber. The filter cake was repeatedly washed with deionized water until the filtrate was neutral. The surface acidity was measured to be 1.32 meq g−1 following the method of Kumar et al. [17]. It was dried in an oven at 110 ◦ C before use. Four adsorbates are selected in this work; they are reactive red RR222 (Sumitomo Chem. Co., Japan), methylene blue MB (Sigma Co.), tannic acid (Lancasseer Co., UK), and the sodium salt of humic acid (Aldrich Co.). The solutions were prepared by dissolving adsorbates in deionized water to the required concentrations without any pH adjustment. 2.2. Preparation of composite beads Chitosan flake (1 g) and activated clay (1 g) were dissolved in 1 mol L−1 acetic acid (100 ml) and were agitated by a disperser (IKA, Ultra-Turrax T25 basic) at 24,000 rpm for 10 min. The resulting viscous solution was placed in a vacuum dryer for 3 h to remove air bubbles and was then sprayed dropwise through a syringe, at a constant rate, into neutralization solution containing 15% NaOH and 95%
19
ethanol in a volume ratio of 4:1. They were left in the solution for one day. The formed composite beads were washed with deionized water until the solution became neutral. The wet composite beads after free waters were removed had an average diameter of 4.42 mm. The specific gravities of such wet beads at 30 ◦ C were measured using a pycnometer with a volume of 50 cm3 . 2.3. Procedures An amount of adsorbent (0.1 g, dry basis) was placed in 250-ml Erlenmeyer flask, into which 100 ml of adsorbate solution was added. The experiment was performed in a shaker bath for 5 days at 130 rpm and 30 ◦ C. Preliminary experiments had shown that the adsorption is complete within 3 days. After filtration with glass fibers, the aqueous sample was taken and analyzed with an UV/visible spectrophotometer (Shimadzu, U-2000). The wavelengths selected were 275, 218.5, 664.5, and 502 nm for tannic acid, humic acid, methylene blue, and RR222, respectively. The amount of adsorption at equilibrium, qe (g kg−1 ), was calculated by qe = (C0 − Ce )V /W,
(1)
where C0 and Ce are the initial and equilibrium liquid concentrations (g m−3 ), respectively, and V and W are the liquid volume (m3 ) and the weight of dried adsorbent used (kg), respectively. In adsorption kinetic studies, 1.7 g of the adsorbent (dry basis) was first placed in a vessel into which 1.7 L of the adsorbate solution was poured. Upon complete addition of the liquid solution, the mixture was agitated (300 rpm) and the experiment was started. Samples were taken at the preset time intervals. The amount of adsorption at time t, qt (g kg−1 ), was calculated by qt = (C0 − Ct )V /W,
(2)
where Ct is the liquid concentration at any time t (g m−3 ).
3. Results and discussion 3.1. Characteristics of composite beads The specific gravity of composite beads prepared from different weight fractions of activated clay is shown in Table 1. It should be noted that the composite with 50 wt% clay is used only for adsorption use in this work. When 1 g (dry basis) of chitosan flake and 1 g of activated clay were used as Table 1 Specific gravities of composite beads produced from different weight fractions of activated clay Weight fraction of activated clay Specific gravity
0
1/3
1/2
3/5
2/3
1.0055
1.0120
1.0197
1.0214
1.0299
20
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Fig. 1. Isotherms of adsorption of tannic acid, methylene blue, humic acid, and RR222 onto various adsorbents at 30 ◦ C. (The composite bead contains 50 wt% chitosan and 50 wt% clay.)
starting materials to prepare composite beads according the aforementioned procedures, the resulting composite beads contained much bound water and had a wet weight and average diameter of about 42.61 g and 4.42 mm, respectively. According to the Stokes law, the terminal velocity of the bead is a function of specific gravity difference between the bead and fluid [18]. The specific gravities of chitosan bead and composite with 50 wt% clay are 1.0055 and 1.0197, respectively (Table 1). When water is the fluid, the terminal velocity of the composite beads is 3.6 times larger than that of the chitosan beads. Therefore, addition of activated clay not only enhances the capability of chitosan to agglomerate and round gel beads, but also improves the hardness of the beads. This can facilitate the separation of the adsorbents from the solution, which is especially important for practical applications. 3.2. Adsorption isotherms Fig. 1 illustrates the equilibrium adsorption of tannic acid, methylene blue, humic acid, and RR222 onto various adsorbents. It is found that the composite bead has an adsorption ability comparable to that of the chitosan bead or activated clay, and even much higher for adsorption of methylene blue. This indicates the promise of composite beads for adsorption applications, because they possess much higher mechanical strength than chitosan beads.
The adsorption isotherm, that is, the plot of the equilibrium solid-phase concentration (qe ) versus the equilibrium liquid-phase concentration (Ce ), is often described by the two-parameter Langmuir and Freundlich equations. They are given as qe = qmon KL Ce /(1 + KL Ce ), 1/n
qe = KF Ce ,
(3) (4)
where KL is the Langmuir constant (m3 g−1 ) and qmon is the amount of adsorption corresponding to complete coverage (g kg−1 ), whereas KF (g kg−1 (g m−3 )n ) and n are the Freundlich constants. The Langmuir equation was originally developed to describe individual chemical adsorbents, and is applicable to physical adsorption (monolayer) within a low concentration range [19]. The Freundlich equation is an empirical approach for adsorbents with very uneven adsorbing surfaces. This model is applicable to adsorption of a singlesolute system within a fixed range of concentration. The Freundlich equation is generally suitable for high- and middle-concentration environments and is not suitable for low-concentration environment because it does not meet the requirements of Henry’s law [19]. The applicability of the above two isotherm equations is quantitatively judged by comparing the standard deviation (SD) between the measured and modeled results, which
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Table 2 Model parameters and standard deviations for the adsorption of tannic acid, methylene blue (MB), humic acid, and RR222 onto various adsorbents Adsorbate
Adsorbent
Tannic acid
Clay Chitosan bead Compositeb Clay Compositeb Clay Chitosan bead Compositeb Clay Chitosan bead Compositeb
MB Humic acid
RR222
Langmuira
Freundlicha
KL
qmon
SD (% )
1/n
KF
SD (%)
7.7 × 10−1
153 1533 1490 272 330 28.2 262 243 36.4 1965 1912
8.7 9.2 12.7 16.8 20.3 3.3 7.3 4.4 4.7 13.1 16.2
0.88 1.00 0.45 0.15 0.13 0.49 0.64 0.71 0.88 0.16 0.22
0.44 9.3 144 107 145 1.6 1.9 2.0 0.06 1.0 1.0
7.9 9.2 9.5 4.3 7.9 2.5 3.7 4.2 4.5 1.4 2.2
1.5 × 10−4 1.4 × 10−3 6.7 × 10−2 6.9 × 10−2 1.7 × 10−2 2.6 × 10−2 2.3 × 10−2 1.9 × 10−4 2.9 × 10−2 2.4 × 10−2
a Units: K (m3 g−1 ), q −1 −1 (g m−3 )n ). mon (g kg ), and KF (g kg L b The composite bead contains 50 wt% chitosan and 50 wt% clay.
Fig. 2. The Freundlich fit for isotherms of adsorption of tannic acid, methylene blue, humic acid, and RR222 onto various adsorbents at 30 ◦ C.
is defined as SD (%) = 100
N
(qe,exp − qe,cal )/qe,exp
2
1/2 (N − 1)
.
1
(5) According to the SD values (Table 2), it is found that the Freundlich equation is more suitable for the description of adsorption isotherms of methylene blue and RR222 onto various adsorbents. On the other hand, the isotherms of tannic acid and humic acid can be well fitted by the Langmuir and
Freundlich equations; however, the latter is still slightly better. In this regard, Fig. 2 shows the typical Freundlich plots. Although the Langmuir equation is not a suitable model for the present systems, its theoretical background makes the quantity qmon quite useful and convincible for comparison in liquid-phase adsorption. This quantity is often called the adsorption capacity. It is found that the adsorption capacity of tannic acid on composite beads is 1490 g kg−1 , which is much higher than those obtained on organoclay and activated carbon, 110 and 25 g kg−1 , respectively [14,15]. On the other hand, the adsorption capacity of methylene
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blue on composite bead is 330 g kg−1 , which is slightly smaller than that of 404 g kg−1 using activated carbon [20] but much larger than those of 134.2 and 19.9 g kg−1 using diatomaceous clay [21] and kaolinite [22], respectively. The adsorption capacity of dye RR222 on activated clay is 36.4 g kg−1 , much smaller than those using chitosan beads (1965 g kg−1 ) and composite beads (1912 g kg−1 ). Moreover, chitosan flake has an adsorption capacity of 276 g kg−1 [8], indicating that adsorption can be significantly improved when chitosan is made in the form of beads. Activated clay has a relatively low adsorption ability for humic acid (adsorption capacity 28.2 g kg−1 only), while the capacities for chitosan and composite beads increase up to 262 and 243 g kg−1 , respectively. The slightly lower capacity of composite beads than of chitosan beads results from the reduced adsorption sites caused by the combination of two active substances. Another possible reason is that the structures of humic acid are complicated and the steric hindrance must be considered. Compared to activated carbon (capacity 11.9 g kg−1) [23] and nonporous carbon (capacity 100 g kg−1 ) [24], the composite bead has higher adsorption ability for humid acid. 3.3. Adsorption kinetics Fig. 3 illustrates the time profiles of the amount of adsorption on various adsorbents. Two simplified kinetic models including pseudo-first-order and pseudo-second-order equa-
tions are analyzed [25,26]. The pseudo-first-order model is given by dqt /dt = k1 (qe − qt ),
(6)
where k1 is the pseudo-first-order rate constant (min−1 ) and qe is the pseudo-equilibrium adsorption corresponding to the initial liquid concentration C0 . After integrating Eq. (6) with the conditions qt = 0, t = 0 and qt = qt , t = t, we have ln(qe − qt ) = ln qe − k1 t.
(7)
On the other hand, the pseudo-second-order model is expressed as dqt /dt = k2 (qe − qt )2 ,
(8)
where k2 is the pseudo-second-order rate constant (kg g−1 min−1 ). Similarly, the following equation can be obtained after integration: t/qt = 1/k2 qe2 + (1/qe )t. (9) The aforementioned two models basically include all steps of adsorption such as external film diffusion, adsorption, and internal particle diffusion, so they are pseudomodels. The parameters in these two models are determined from the linear plots of ln(qe − qt ) vs t and (t/qt ) vs t, respectively. The results are shown in Figs. 4 and 5. The validity of each model is checked by comparing the SD values (Table 3) as similarly defined in Eq. (5). It is found
Fig. 3. Time profiles of amounts of tannic acid, methylene blue, humic acid, and RR222 adsorbed onto various adsorbents with a dosage of 1 kg m−3 at 30 ◦ C. (The composite bead contains 50 wt% chitosan and 50 wt% clay.)
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Fig. 5. Plots of pseudo-second-order kinetic model for adsorption of tannic acid and methylene blue onto various adsorbents. (The experimental conditions are the same as those indicated in Fig. 3.)
tested in this work. It originates from Fick’s second law and is simply written as [25,26] qt = ki t 1/2 ,
Fig. 4. Plots of pseudo-first-order kinetic model for adsorption of tannic acid, humic acid, and RR222 onto various adsorbents. (The experimental conditions are the same as those indicated in Fig. 3.)
that adsorption on activated clay follows the pseudo-firstorder model. However, the adsorption on chitosan beads depends on the nature of the adsorbates, i.e., a pseudofirst-order model for RR222 and humic acid, and a pseudosecond-order model for tannic acid. On the other hand, the adsorption of large molecules such as tannic acid (MW, 1700 g mol−1 ), humic acid, and RR222 using composite beads is better described by the pseudo-first-order model. Also, the adsorption of smaller molecules such as methylene blue (MW, 320 g mol−1 ) obeys the pseudo-second-order model. Because the above two models cannot identify the adsorption mechanisms, the intraparticle diffusion model is
(10)
where ki is the rate constant of intraparticle diffusion (g kg−1 min−1/2) and is determined from the linear plot of qt vs t 1/2 (Fig. 6). The rate constant ki is shown in Table 3, which is usually used to compare mass transfer rates. It is found that adsorption of all adsorbates onto activated clay follows the intraparticle diffusion model best among various adsorbents. For tannic acid, the values of ki are 19.7, 14.1, 11.5, and 0.52 g kg−1 min−1/2 with chitosan beads, composite beads, chitosan flakes, and activated clay as adsorbents, respectively. The intraparticle diffusion model is better fitted for activated clay and/or chitosan flakes (small SD values) because they are denser solids and have more apparent mass transfer blockage than other adsorbents. The intraparticle diffusion rates (ki ) of tannic acid, humic acid, and RR222 for composite beads are slightly smaller than those for chitosan beads because the tight combination of activated clay and chitosan increases the mass transfer resistance to some extent [9].
4. Conclusions The elastic composite beads were prepared by embedding equal weights of activated clay into chitosan to improve
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Fig. 6. Plots of intraparticle diffusion model for adsorption of tannic acid, methylene blue, humic acid, and RR222 onto various adsorbents. (The experimental conditions are the same as those indicated in Fig. 3.) Table 3 Rate constants and standard deviations for the adsorption of tannic acid, methylene blue (MB), humic acid, and RR222 onto various adsorbents Adsorbate
Adsorbent
Tannic acid
Clay Chitosan bead Compositeb Chitosan flake Clay Compositeb Clay Chitosan bead Compositeb Chitosan bead Compositeb
MB Humic acid
RR222
First-ordera
Second-ordera
Intraparticle diffusiona
k1
qe
SD (%)
k2
qe
SD (%)
ki
SD (%)
1.2 × 10−2 1.1 × 10−2 1.2 × 10−2 1.0 × 10−2 1.3 × 10−2 8.5 × 10−3 1.1 × 10−2 7.2 × 10−3 8.0 × 10−3 9.6 × 10−3 1.1 × 10−2
138 1472 1395 753 266 368 26.4 245 232 1937 1889
4.1 8.6 4.8 6.9 6.1 25.4 2.7 5.8 4.4 6.5 4.5
3.9 × 10−3 1.9 × 10−4 2.2 × 10−4 1.4 × 10−4 2.7 × 10−4 9.7 × 10−5 4.6 × 10−3 2.5 × 10−4 4.2 × 10−4 8.4 × 10−5 1.0 × 10−4
130 1512 1371 726 242 340 21.4 220 219 1924 1947
10.5 2.5 6.1 12.2 10.7 6.8 9.2 11.6 9.1 7.6 8.1
0.52 19.7 14.1 11.5 7.7 17.2 0.51 3.6 3.1 23.1 21.7
1.4 8.1 5.8 4.6 1.2 21.2 2.8 6.6 4.7 7.1 5.3
a Units: q (g kg−1 ), SD (%), k (min−1 ), k (kg g−1 min−1 ), and k (g kg−1 min−1/2 ). e i 1 2 b The composite bead contains 50 wt% chitosan and 50 wt% clay.
the mechanical strength and specific gravity. The adsorption of tannic acid, humic acid, methylene blue, and reactive dye RR222 onto such composite beads was examined to evaluate their chemical characteristics (adsorption capacity, kinetics). The composite beads had adsorption capacities for tannin acid, humic acid, and RR222 comparable to those of chitosan beads and/or activated clay. The adsorption isotherms of all four adsorbates at 30 ◦ C could be better fitted by the Freundlich equation (standard deviation < 10%).
The adsorption of large molecules such as tannic acid (MW, 1700 g mol−1 ), humic acid, and RR222 onto composite beads was better described by the pseudo-first-order model, whereas that of smaller molecules such as methylene blue (MW, 320 g mol−1 ) followed the pseudo-second-order model. In contrast to chitosan beads, intraparticle diffusion model analysis indicated that the mass transfer resistance with composite beads slightly increased because of their denser solid structure. For example, the rate constants of intraparticle diffusion ki for adsorption of tannic acid, humic
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acid, and RR222 were 14.1 vs 19.7 (composite vs chitosan beads), 3.1 vs 3.6, and 21.7 vs 23.1 g kg−1 min−1/2 , respectively, at 30 ◦ C.
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[11] R.S. Juang, F.C. Wu, R.L. Tseng, Environ. Technol. 18 (1997) 525– 531. [12] S. Netpradita, P. Thiravetyanb, S. Towprayoona, Water Res. 37 (2003) 763–772. [13] T. Panswad, W. Luangdilok, Water Res. 34 (2000) 4177–4184. [14] S.K. Dentel, A.I. Jamrah, D.L. Sparks, Water Res. 32 (1998) 3689– 3697. [15] C.T. Hsieh, H. Teng, Carbon 38 (2000) 863–869. [16] M.R. Jekel, J. Water SRT Aqua 40 (1991) 18. [17] P. Kumar, R.V. Jasra, T.S.G. Bhat, Ind. Eng. Chem. Res. 34 (1995) 1440–1448. [18] M.L. Davis, D.A. Cornwell, in: Introduction to Environmental Engineering, third ed., McGraw–Hill, New York, 1998, pp. 219–222. [19] A. Meghea, H.H. Rehner, I. Peleanu, R. Mihalache, J. Radioanal. Nucl. Chem. 229 (1998) 105–110. [20] N. Kannan, M.M. Sundaram, Dyes Pigments 51 (2001) 25–40. [21] R.A. Shawabkeh, M.F. Tutunji, Appl. Clay Sci. 24 (2003) 111–120. [22] D. Ghosh, K.G. Bhattacharyya, Appl. Clay Sci. 20 (2002) 295–300. [23] J. Duan, F. Wilson, N. Graham, J.H. Tay, Desalination 151 (2003) 53– 66. [24] S. Han, S. Kim, H. Lim, W. Choi, H. Park, J. Yoon, T. Hyeon, Micropor. Mesopor. Mater. 58 (2003) 131–135. [25] R.S. Juang, F.C. Wu, R.L. Tseng, J. Colloid Interface Sci. 227 (2000) 437–444. [26] Y.S. Ho, G. McKay, J. Environ. Sci. Health A 34 (1999) 1179–1204.
Adsorption of tannic acid, humic acid, and dyes from water using the composite of chitosan and activated clay Min-Yun Chang a , Ruey-Shin Juang b,∗ a Department of Chemical Engineering, National United University, Miao-Li 360, Taiwan b Department of Chemical Engineering, Yuan Ze University, Chung-Li 320, Taiwan
Received 26 March 2004; accepted 18 May 2004 Available online 19 June 2004
Abstract Chitosan is a well-known excellent adsorbent for a number of organics and metal ions, but its mechanical properties and specific gravity should be enhanced for practical operation. In this study, activated clay was added in chitosan slurry to prepare composite beads. The adsorption isotherms and kinetics of two organic acids (tannic acid, humic acid) and two dyes (methylene blue, reactive dye RR222) using composite beads, activated clay, and chitosan beads were compared. With composite beads as an adsorbent, all the isotherms were better fitted by the Freundlich equation. The adsorption capacities with composite beads were generally comparable to those with chitosan beads but much larger than those with activated clay. The pseudo-first-order and pseudo-second-order equations were then screened to describe the adsorption processes. It was shown that the adsorption of larger molecules such as tannic acid (MW, 1700 g mol−1 ), humic acid, and RR222 from water onto composite beads was better described by the pseudo-first-order kinetic model. The rate parameters of the intraparticle diffusion model for adsorption onto such adsorbents were also evaluated and compared to identify the adsorption mechanisms. 2004 Elsevier Inc. All rights reserved. Keywords: Chitosan; Activated clay; Composite bead; Adsorption; Tannic acid; Humic acid; Dyes
1. Introduction Chitosan is known as an ideal natural support for enzyme immobilization because of its many characteristics such as hydrophilicity, biocompatibility, biodegradability, and antibacterial properties [1,2]. It has been used widely as an adsorbent for transition metal ions and organic species because the amino (–NH2) and hydroxy (–OH) groups on chitosan chains can serve as the coordination and reaction sites [3–7]. In addition to these, chitosan appears economically attractive since it can be obtained from deacetylation of chitin, and chitin is the second most abundant biopolymer in nature, next to cellulose [1]. Although the bead form of chitosan shows better adsorption capability than the flake form, due to its higher specific surface area [8], the weak mechanical property (highly swollen in water) and low specific gravity of the chitosan beads make them rather inconvenient for use as adsorbents in * Corresponding author. Fax: +886-3-4559373.
E-mail address: [email protected] (R.-S. Juang). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.05.029
either batch or column modes. Thus, chemical modifications of the chitosan beads are often made including carboxyalkyl substituted, aldehyde crosslinked, ligand crosslinked, and polyaminated to enhance adsorption performance, to prevent them from dissolution in strongly acidic and alkali solutions, or both [9,10]. However, such modification procedures seem to be complicated and/or are relatively expensive. In this work, chitosan beads were physically modified and activated clay particles were embedded into them. The activated clay was selected because it was also an effective adsorbent for organic matter and dyes [11]. First, the specific gravities of composite beads made from different weight fractions of activated clay were measured. The equilibria and kinetics for adsorption of tannic acid, humic acid, basic dye methylene blue, and reactive dye 222 (RR222) from water onto composite beads were compared with those of chitosan beads and activated clay. These adsorbates are selected because they are frequently encountered in process/waste streams and/or natural waters [5,12–16]. For example, tannic acid exists in many natural juices and gives them a bitter taste. The uptake of methylene blue is usually
M.-Y. Chang, R.-S. Juang / Journal of Colloid and Interface Science 278 (2004) 18–25
used as an index of adsorption performance for adsorbents. Reactive dye wastewaters have limited biodegradability in an aerobic environment and many azo dyes under anaerobic conditions decompose into potentially carcinogenic aromatic amines [12,13]. Humic substance is a general name for organic matters that are not easily decomposed by microorganisms. Major function groups of humic acid, though its chemical structure is not completely clear, include carboxylic acid, phenolic acid, alcohol acid, aldehyde acid, and methoxyl. Humic acid will react with chlorine during water treatment and sterilization and produce trihalomethanes, which cause cancer and affect the health of human beings very seriously. Hence, the adsorption of these adsorbates is of practical importance and interest.
2. Materials and methods 2.1. Chitosan and activated clay Dried cuttlebone cartilage was immersed in 5 wt% NaOH for 18 h to remove proteins and then in 5 wt% HCl for 18 h to remove CaCO3 (the weight ratio of the waste to the solution was 1/10). The resulting insoluble chitin (about 40 g) was deacetylated in 50 wt% NaOH (800 g) at 90 ◦ C for 3 h. The chitosan flakes were washed three times with deionized water (Millipore Milli-Q) and dried at 50 ◦ C in a vacuum. The degree of deacetylation and molar mass of chitosan were measured to be 97.2% and 1.85 × 106 , respectively, according to the methods described earlier [6]. Fifty grams of natural clay (Union Chem. Co., Taiwan), with a particle size of 0.02–0.06 mm, were activated by refluxing with 250 ml of 1 mol L−1 H2 SO4 at 80 ◦ C in a round-bottom flask for 2 h. The slurry was air-cooled and filtered with a glass fiber. The filter cake was repeatedly washed with deionized water until the filtrate was neutral. The surface acidity was measured to be 1.32 meq g−1 following the method of Kumar et al. [17]. It was dried in an oven at 110 ◦ C before use. Four adsorbates are selected in this work; they are reactive red RR222 (Sumitomo Chem. Co., Japan), methylene blue MB (Sigma Co.), tannic acid (Lancasseer Co., UK), and the sodium salt of humic acid (Aldrich Co.). The solutions were prepared by dissolving adsorbates in deionized water to the required concentrations without any pH adjustment. 2.2. Preparation of composite beads Chitosan flake (1 g) and activated clay (1 g) were dissolved in 1 mol L−1 acetic acid (100 ml) and were agitated by a disperser (IKA, Ultra-Turrax T25 basic) at 24,000 rpm for 10 min. The resulting viscous solution was placed in a vacuum dryer for 3 h to remove air bubbles and was then sprayed dropwise through a syringe, at a constant rate, into neutralization solution containing 15% NaOH and 95%
19
ethanol in a volume ratio of 4:1. They were left in the solution for one day. The formed composite beads were washed with deionized water until the solution became neutral. The wet composite beads after free waters were removed had an average diameter of 4.42 mm. The specific gravities of such wet beads at 30 ◦ C were measured using a pycnometer with a volume of 50 cm3 . 2.3. Procedures An amount of adsorbent (0.1 g, dry basis) was placed in 250-ml Erlenmeyer flask, into which 100 ml of adsorbate solution was added. The experiment was performed in a shaker bath for 5 days at 130 rpm and 30 ◦ C. Preliminary experiments had shown that the adsorption is complete within 3 days. After filtration with glass fibers, the aqueous sample was taken and analyzed with an UV/visible spectrophotometer (Shimadzu, U-2000). The wavelengths selected were 275, 218.5, 664.5, and 502 nm for tannic acid, humic acid, methylene blue, and RR222, respectively. The amount of adsorption at equilibrium, qe (g kg−1 ), was calculated by qe = (C0 − Ce )V /W,
(1)
where C0 and Ce are the initial and equilibrium liquid concentrations (g m−3 ), respectively, and V and W are the liquid volume (m3 ) and the weight of dried adsorbent used (kg), respectively. In adsorption kinetic studies, 1.7 g of the adsorbent (dry basis) was first placed in a vessel into which 1.7 L of the adsorbate solution was poured. Upon complete addition of the liquid solution, the mixture was agitated (300 rpm) and the experiment was started. Samples were taken at the preset time intervals. The amount of adsorption at time t, qt (g kg−1 ), was calculated by qt = (C0 − Ct )V /W,
(2)
where Ct is the liquid concentration at any time t (g m−3 ).
3. Results and discussion 3.1. Characteristics of composite beads The specific gravity of composite beads prepared from different weight fractions of activated clay is shown in Table 1. It should be noted that the composite with 50 wt% clay is used only for adsorption use in this work. When 1 g (dry basis) of chitosan flake and 1 g of activated clay were used as Table 1 Specific gravities of composite beads produced from different weight fractions of activated clay Weight fraction of activated clay Specific gravity
0
1/3
1/2
3/5
2/3
1.0055
1.0120
1.0197
1.0214
1.0299
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Fig. 1. Isotherms of adsorption of tannic acid, methylene blue, humic acid, and RR222 onto various adsorbents at 30 ◦ C. (The composite bead contains 50 wt% chitosan and 50 wt% clay.)
starting materials to prepare composite beads according the aforementioned procedures, the resulting composite beads contained much bound water and had a wet weight and average diameter of about 42.61 g and 4.42 mm, respectively. According to the Stokes law, the terminal velocity of the bead is a function of specific gravity difference between the bead and fluid [18]. The specific gravities of chitosan bead and composite with 50 wt% clay are 1.0055 and 1.0197, respectively (Table 1). When water is the fluid, the terminal velocity of the composite beads is 3.6 times larger than that of the chitosan beads. Therefore, addition of activated clay not only enhances the capability of chitosan to agglomerate and round gel beads, but also improves the hardness of the beads. This can facilitate the separation of the adsorbents from the solution, which is especially important for practical applications. 3.2. Adsorption isotherms Fig. 1 illustrates the equilibrium adsorption of tannic acid, methylene blue, humic acid, and RR222 onto various adsorbents. It is found that the composite bead has an adsorption ability comparable to that of the chitosan bead or activated clay, and even much higher for adsorption of methylene blue. This indicates the promise of composite beads for adsorption applications, because they possess much higher mechanical strength than chitosan beads.
The adsorption isotherm, that is, the plot of the equilibrium solid-phase concentration (qe ) versus the equilibrium liquid-phase concentration (Ce ), is often described by the two-parameter Langmuir and Freundlich equations. They are given as qe = qmon KL Ce /(1 + KL Ce ), 1/n
qe = KF Ce ,
(3) (4)
where KL is the Langmuir constant (m3 g−1 ) and qmon is the amount of adsorption corresponding to complete coverage (g kg−1 ), whereas KF (g kg−1 (g m−3 )n ) and n are the Freundlich constants. The Langmuir equation was originally developed to describe individual chemical adsorbents, and is applicable to physical adsorption (monolayer) within a low concentration range [19]. The Freundlich equation is an empirical approach for adsorbents with very uneven adsorbing surfaces. This model is applicable to adsorption of a singlesolute system within a fixed range of concentration. The Freundlich equation is generally suitable for high- and middle-concentration environments and is not suitable for low-concentration environment because it does not meet the requirements of Henry’s law [19]. The applicability of the above two isotherm equations is quantitatively judged by comparing the standard deviation (SD) between the measured and modeled results, which
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21
Table 2 Model parameters and standard deviations for the adsorption of tannic acid, methylene blue (MB), humic acid, and RR222 onto various adsorbents Adsorbate
Adsorbent
Tannic acid
Clay Chitosan bead Compositeb Clay Compositeb Clay Chitosan bead Compositeb Clay Chitosan bead Compositeb
MB Humic acid
RR222
Langmuira
Freundlicha
KL
qmon
SD (% )
1/n
KF
SD (%)
7.7 × 10−1
153 1533 1490 272 330 28.2 262 243 36.4 1965 1912
8.7 9.2 12.7 16.8 20.3 3.3 7.3 4.4 4.7 13.1 16.2
0.88 1.00 0.45 0.15 0.13 0.49 0.64 0.71 0.88 0.16 0.22
0.44 9.3 144 107 145 1.6 1.9 2.0 0.06 1.0 1.0
7.9 9.2 9.5 4.3 7.9 2.5 3.7 4.2 4.5 1.4 2.2
1.5 × 10−4 1.4 × 10−3 6.7 × 10−2 6.9 × 10−2 1.7 × 10−2 2.6 × 10−2 2.3 × 10−2 1.9 × 10−4 2.9 × 10−2 2.4 × 10−2
a Units: K (m3 g−1 ), q −1 −1 (g m−3 )n ). mon (g kg ), and KF (g kg L b The composite bead contains 50 wt% chitosan and 50 wt% clay.
Fig. 2. The Freundlich fit for isotherms of adsorption of tannic acid, methylene blue, humic acid, and RR222 onto various adsorbents at 30 ◦ C.
is defined as SD (%) = 100
N
(qe,exp − qe,cal )/qe,exp
2
1/2 (N − 1)
.
1
(5) According to the SD values (Table 2), it is found that the Freundlich equation is more suitable for the description of adsorption isotherms of methylene blue and RR222 onto various adsorbents. On the other hand, the isotherms of tannic acid and humic acid can be well fitted by the Langmuir and
Freundlich equations; however, the latter is still slightly better. In this regard, Fig. 2 shows the typical Freundlich plots. Although the Langmuir equation is not a suitable model for the present systems, its theoretical background makes the quantity qmon quite useful and convincible for comparison in liquid-phase adsorption. This quantity is often called the adsorption capacity. It is found that the adsorption capacity of tannic acid on composite beads is 1490 g kg−1 , which is much higher than those obtained on organoclay and activated carbon, 110 and 25 g kg−1 , respectively [14,15]. On the other hand, the adsorption capacity of methylene
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blue on composite bead is 330 g kg−1 , which is slightly smaller than that of 404 g kg−1 using activated carbon [20] but much larger than those of 134.2 and 19.9 g kg−1 using diatomaceous clay [21] and kaolinite [22], respectively. The adsorption capacity of dye RR222 on activated clay is 36.4 g kg−1 , much smaller than those using chitosan beads (1965 g kg−1 ) and composite beads (1912 g kg−1 ). Moreover, chitosan flake has an adsorption capacity of 276 g kg−1 [8], indicating that adsorption can be significantly improved when chitosan is made in the form of beads. Activated clay has a relatively low adsorption ability for humic acid (adsorption capacity 28.2 g kg−1 only), while the capacities for chitosan and composite beads increase up to 262 and 243 g kg−1 , respectively. The slightly lower capacity of composite beads than of chitosan beads results from the reduced adsorption sites caused by the combination of two active substances. Another possible reason is that the structures of humic acid are complicated and the steric hindrance must be considered. Compared to activated carbon (capacity 11.9 g kg−1) [23] and nonporous carbon (capacity 100 g kg−1 ) [24], the composite bead has higher adsorption ability for humid acid. 3.3. Adsorption kinetics Fig. 3 illustrates the time profiles of the amount of adsorption on various adsorbents. Two simplified kinetic models including pseudo-first-order and pseudo-second-order equa-
tions are analyzed [25,26]. The pseudo-first-order model is given by dqt /dt = k1 (qe − qt ),
(6)
where k1 is the pseudo-first-order rate constant (min−1 ) and qe is the pseudo-equilibrium adsorption corresponding to the initial liquid concentration C0 . After integrating Eq. (6) with the conditions qt = 0, t = 0 and qt = qt , t = t, we have ln(qe − qt ) = ln qe − k1 t.
(7)
On the other hand, the pseudo-second-order model is expressed as dqt /dt = k2 (qe − qt )2 ,
(8)
where k2 is the pseudo-second-order rate constant (kg g−1 min−1 ). Similarly, the following equation can be obtained after integration: t/qt = 1/k2 qe2 + (1/qe )t. (9) The aforementioned two models basically include all steps of adsorption such as external film diffusion, adsorption, and internal particle diffusion, so they are pseudomodels. The parameters in these two models are determined from the linear plots of ln(qe − qt ) vs t and (t/qt ) vs t, respectively. The results are shown in Figs. 4 and 5. The validity of each model is checked by comparing the SD values (Table 3) as similarly defined in Eq. (5). It is found
Fig. 3. Time profiles of amounts of tannic acid, methylene blue, humic acid, and RR222 adsorbed onto various adsorbents with a dosage of 1 kg m−3 at 30 ◦ C. (The composite bead contains 50 wt% chitosan and 50 wt% clay.)
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Fig. 5. Plots of pseudo-second-order kinetic model for adsorption of tannic acid and methylene blue onto various adsorbents. (The experimental conditions are the same as those indicated in Fig. 3.)
tested in this work. It originates from Fick’s second law and is simply written as [25,26] qt = ki t 1/2 ,
Fig. 4. Plots of pseudo-first-order kinetic model for adsorption of tannic acid, humic acid, and RR222 onto various adsorbents. (The experimental conditions are the same as those indicated in Fig. 3.)
that adsorption on activated clay follows the pseudo-firstorder model. However, the adsorption on chitosan beads depends on the nature of the adsorbates, i.e., a pseudofirst-order model for RR222 and humic acid, and a pseudosecond-order model for tannic acid. On the other hand, the adsorption of large molecules such as tannic acid (MW, 1700 g mol−1 ), humic acid, and RR222 using composite beads is better described by the pseudo-first-order model. Also, the adsorption of smaller molecules such as methylene blue (MW, 320 g mol−1 ) obeys the pseudo-second-order model. Because the above two models cannot identify the adsorption mechanisms, the intraparticle diffusion model is
(10)
where ki is the rate constant of intraparticle diffusion (g kg−1 min−1/2) and is determined from the linear plot of qt vs t 1/2 (Fig. 6). The rate constant ki is shown in Table 3, which is usually used to compare mass transfer rates. It is found that adsorption of all adsorbates onto activated clay follows the intraparticle diffusion model best among various adsorbents. For tannic acid, the values of ki are 19.7, 14.1, 11.5, and 0.52 g kg−1 min−1/2 with chitosan beads, composite beads, chitosan flakes, and activated clay as adsorbents, respectively. The intraparticle diffusion model is better fitted for activated clay and/or chitosan flakes (small SD values) because they are denser solids and have more apparent mass transfer blockage than other adsorbents. The intraparticle diffusion rates (ki ) of tannic acid, humic acid, and RR222 for composite beads are slightly smaller than those for chitosan beads because the tight combination of activated clay and chitosan increases the mass transfer resistance to some extent [9].
4. Conclusions The elastic composite beads were prepared by embedding equal weights of activated clay into chitosan to improve
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Fig. 6. Plots of intraparticle diffusion model for adsorption of tannic acid, methylene blue, humic acid, and RR222 onto various adsorbents. (The experimental conditions are the same as those indicated in Fig. 3.) Table 3 Rate constants and standard deviations for the adsorption of tannic acid, methylene blue (MB), humic acid, and RR222 onto various adsorbents Adsorbate
Adsorbent
Tannic acid
Clay Chitosan bead Compositeb Chitosan flake Clay Compositeb Clay Chitosan bead Compositeb Chitosan bead Compositeb
MB Humic acid
RR222
First-ordera
Second-ordera
Intraparticle diffusiona
k1
qe
SD (%)
k2
qe
SD (%)
ki
SD (%)
1.2 × 10−2 1.1 × 10−2 1.2 × 10−2 1.0 × 10−2 1.3 × 10−2 8.5 × 10−3 1.1 × 10−2 7.2 × 10−3 8.0 × 10−3 9.6 × 10−3 1.1 × 10−2
138 1472 1395 753 266 368 26.4 245 232 1937 1889
4.1 8.6 4.8 6.9 6.1 25.4 2.7 5.8 4.4 6.5 4.5
3.9 × 10−3 1.9 × 10−4 2.2 × 10−4 1.4 × 10−4 2.7 × 10−4 9.7 × 10−5 4.6 × 10−3 2.5 × 10−4 4.2 × 10−4 8.4 × 10−5 1.0 × 10−4
130 1512 1371 726 242 340 21.4 220 219 1924 1947
10.5 2.5 6.1 12.2 10.7 6.8 9.2 11.6 9.1 7.6 8.1
0.52 19.7 14.1 11.5 7.7 17.2 0.51 3.6 3.1 23.1 21.7
1.4 8.1 5.8 4.6 1.2 21.2 2.8 6.6 4.7 7.1 5.3
a Units: q (g kg−1 ), SD (%), k (min−1 ), k (kg g−1 min−1 ), and k (g kg−1 min−1/2 ). e i 1 2 b The composite bead contains 50 wt% chitosan and 50 wt% clay.
the mechanical strength and specific gravity. The adsorption of tannic acid, humic acid, methylene blue, and reactive dye RR222 onto such composite beads was examined to evaluate their chemical characteristics (adsorption capacity, kinetics). The composite beads had adsorption capacities for tannin acid, humic acid, and RR222 comparable to those of chitosan beads and/or activated clay. The adsorption isotherms of all four adsorbates at 30 ◦ C could be better fitted by the Freundlich equation (standard deviation < 10%).
The adsorption of large molecules such as tannic acid (MW, 1700 g mol−1 ), humic acid, and RR222 onto composite beads was better described by the pseudo-first-order model, whereas that of smaller molecules such as methylene blue (MW, 320 g mol−1 ) followed the pseudo-second-order model. In contrast to chitosan beads, intraparticle diffusion model analysis indicated that the mass transfer resistance with composite beads slightly increased because of their denser solid structure. For example, the rate constants of intraparticle diffusion ki for adsorption of tannic acid, humic
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acid, and RR222 were 14.1 vs 19.7 (composite vs chitosan beads), 3.1 vs 3.6, and 21.7 vs 23.1 g kg−1 min−1/2 , respectively, at 30 ◦ C.
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