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titania nanoparticle)
Desalination 275 (2011) 93–101
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation, characterization and dye adsorption properties of biocompatible composite (alginate/titania nanoparticle) Niyaz Mohammad Mahmoodi a,⁎, Bagher Hayati b, Mokhtar Arami b, Hajir Bahrami b a b
Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iran Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 27 November 2010 Received in revised form 13 February 2011 Accepted 14 February 2011 Available online 15 March 2011 Keywords: Biocompatible composite Alginate/titania nanoparticle Preparation Characterization Dye adsorption
a b s t r a c t In this paper, the preparation, characterization and dye adsorption properties of novel biocompatible composite (Sodium Alginate/titania nanoparticle) (SA/n-TiO2) were investigated. Titania nanoparticles were immobilized onto Sodium Alginate. Physical characteristics of SA/n-TiO2 were studied using Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), and wavelength dispersive X-ray spectroscopy (WDX). Two textile dyes, Direct Red 80 (DR80) and Acid Green 25 (AG25), were used as model compounds. The effect of operational parameter such as SA/n-TiO2 dosage, initial dye concentration and pH was evaluated at 25 °C. The isotherm, kinetic and thermodynamic of dye adsorption were studied. The data were evaluated for compliance with the different isotherm models. It was found that DR80 and AG25 followed with Langmuir isotherm. Adsorption kinetic of dyes was found to conform to pseudo-second order kinetics. The thermodynamic data showed that adsorption process was spontaneous, endothermic and physisorption reaction. Based on the data of present investigation, one could conclude that the SA/n-TiO2 being a biocompatible, eco-friendly and low-cost adsorbent might be a suitable alternative for elimination of dyes from colored aqueous solutions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Colored dye wastewater arises as a direct result of the production of the dye and also as a consequence of its use in the textile and other industries [1]. It is estimated that 2% of dyes produced annually are discharged in effluent from manufacturing operations, while 10% is discharged from textile and associated industries [2]. Adsorption process is widely used to remove contaminants from wastewater. Liquid–solid adsorption operations are concerned with the ability of certain solids to preferentially concentrate specific substances from solution onto their surfaces [3]. The design and efficient operation of adsorption process require equilibrium adsorption data for use in kinetic and mass transfer models. These models can then be used to predict the performance of the adsorption contact processes under a range of operating conditions. The equilibrium isotherm plays an important role in modeling for analysis and design of adsorption systems. The adsorption isotherm is also an invaluable tool for the theoretical evaluation and interpretation of thermodynamic parameters such as heats of adsorption. An isotherm may fit experimental data accurately under one set of conditions but fail entirely under another. No single model has been found to be generally applicable, a
⁎ Corresponding author. Tel.: +98 21 22969771; fax: +98 21 22947537. E-mail addresses: [email protected], [email protected] (N.M. Mahmoodi). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.02.034
fact which is readily understandable in the light of the assumptions associated with their respective derivations. The current research presents a method of direct comparison of the isotherm fit of several models to enable the best-fit and best isotherm parameters to be obtained. An accurate mathematical description of equilibrium adsorption capacities, even if empirical, is indispensable for reliable predictive modeling of adsorption systems and quantitative comparisons of adsorption behavior for different adsorbent systems or for varied conditions within any given system [4–7]. Alginate is a naturally occurring carbohydrate polymer and has a capacity to remove toxic pollutants. Many biopolymers are known to have a strong affinity for metal ions and the use of alginic acid as adsorbent for the recovery of valuable metal ions as well as removal of toxic metal ions has been studied [8,9]. Alginic acid is a biopolymer carrying carboxyl groups capable of forming complexes with metal ion. Alginic acid is derived from several genera of brown algae that are utilized as raw materials by commercial alginate producers, Nodosum being the principal source of the world's alginate supply. The intercellular mucilage in the seaweeds has been regarded as the principal site of algin [10]. One of the important properties of alginate is the ability to form hydrogels [11]. An aqueous solution of alginate is readily transformed into a hydrogel on addition of metallic divalent cations such as Ca2+. Alginate is often used for immobilization of biological entities [12,13]. Calcium alginate immobilized microbial cultures have been used for decoloration of dyes [14,15]. Also, activated carbons immobilized in calcium alginate beads are used for dye removal [14]. Sodium Alginate/acrylamide interpenetrating polymer networks
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Table 1 Adsorption capacities for the removal of Direct and Acid dyes by natural adsorbents. Adsorbent
Dye
Q0 (mg/g)
Ref.
Soy meal hull Soy meal hull Soy meal hull Soy meal hull Peat Orange peel Orange peel Orange peel Rice husk Baggase pith Baggase pith SA/n-TiO2 SA/n-TiO2
Direct Red 80 Direct Red 81 Acid Blue 92 Acid Red 14 Acid Blue 25 Direct Red 23 Direct Red 80 Acid Violet 17 Acid Yellow 36 Acid Blue 25 Acid Red 114 Direct Red 80 Acid Green 25
178.6 120.5 114.9 109.9 5–9 10.7 21.0 19.9 86.9 17.5 20.0 163.9 151.5
[19] [19] [19] [19] [20] [21] [21] [22] [23] [24] [24] In this work In this work
have been prepared and used to remove dyes [16]. Other compounds were used to remove water pollutants [17,18]. A literature review showed that dye removal using Sodium Alginate/titania nanoparticle (SA/n-TiO2) was not evaluated. Two textile dyes (Direct Red 80 (DR80) and Acid Green 25 (AG25)) were used as model compounds in this work. Removal of Direct and Acid dyes was studied by several natural adsorbents (Table 1) [19–24]. In this paper, the preparation, characterization and dye adsorption properties of biocompatible composite (SA/n-TiO2) were studied. Physical characteristics of SA/n-TiO2 were investigated. The isotherm, kinetic and thermodynamic of dye adsorption were studied.
2. Materials and methods
2.3. Physicochemical characterization of SA/n-TiO2 composite adsorbent Fourier transform infrared (FT-IR) spectra (Perkin-Elmer Spectrophotometer Spectrum One) in the range of 450–4000 cm− 1 were studied. The X-ray diffraction (XRD) spectra of SA and SA/n-TiO2 were performed using a D8 ADVANCE X-ray diffraction spectrometer (Bruker, Germany) with a Cu Kα target at 40 kV and 50 mA at a scan rate of 0.02°2θ s− 1. The morphological structure of the SA/n-TiO2 was examined by scanning electron microscopy (SEM) using LEO 1455VP scanning microscope. Wavelength dispersive X-ray spectroscopy (WDX) was employed to obtain information on the content of Ti in the composite.
2.4. Adsorption procedure The dye adsorption measurements were conducted by mixing various amounts of SA/n-TiO2 (0.15–1 g/L) for DR80 and AG25 in jars containing 200 mL of a dye solution (50 mg/L) at various pH (2– 10). The solution pH was adjusted by adding a small amount of H2SO 4 or NaOH. Adsorption experiments were performed at different dye concentrations (25, 50, 75 and 100 mg/L) using optimum amount of SA/n-TiO2 (0.5 g/L for DR80 and AG25) at pH 2, agitation speed of 200 rpm and 25 °C for 20 min. The change on the absorbance of all solution samples was monitored and determined at certain time intervals (0, 2.5, 5, 7.5, 10, 15, and 20 min) during the adsorption process. The equilibrium was established after 10 min. At the end of the adsorption experiments, the solution samples were centrifuged and the dye concentration was determined. The results were verified with the adsorption isotherms and kinetics.
2.1. Chemicals Sodium Alginate was supplied by Kitotak and used as received. The chemical structure of alginate was shown in Fig. 1. Anionic dyes, Direct Red 80 (DR80) and Acid Green 25 (AG25), were used in this study. The dyes were purchased from Ciba Ltd. Dyes were used without further purification. The chemical structure of dyes was shown in Fig. 2. Titania (Degussa P25) was utilized. Its main physical data are as follows: average primary particle size 30 nm, purity above 97% and with 80:20 anatase to rutile. All other chemicals were of analar grade and purchased from Merck (Germany). UV–VIS spectrophotometer CECIL 2021 was employed for absorbance measurements of samples. The maximum wavelength (λmax) used for determination of residual concentration of DR80 and AG25 at pH 2 in supernatant solution using UV–VIS spectrophotometer were 540 and 605 nm, respectively.
2.2. Preparation of SA/n-TiO2 3 g of SA flake was dissolved in 300 mL of aqueous solution. The viscous solution was stirred continuously for 12 h to fully dissolve the alginate flake. Then, 1.5 g of n-TiO2 powders was added into the solution. Subsequently, another 50 mL of distilled water was added. The slurry was stirred continuously for 24 h to obtain the final transparent solution.
3. Results and discussion 3.1. Characterization of SA/n-TiO2 composite In order to study the functional groups of SA and SA/n-TiO2, the FTIR was applied (Fig. 3). The FT-IR spectrum of SA exhibited differences from that of SA/n-TiO2. The major differences were: the wide peak at 3442 cm− 1, corresponding to the stretching vibration of carboxylic acid groups, moved noticeably to lower wave numbers (3396 cm− 1) and became broader and weaker, which indicated the strong interaction between these groups and n-TiO2 [25]. The FT-IR spectrum of SA itself showed some features of hydroxyl groups at 2151 cm− 1 [26,27]. The decrease of the band related to primary OH groups at 2151 cm− 1 is an indicative of TiO2 nanoparticle immobilization onto the SA [27]. X-ray diffraction (XRD) was employed to investigate the immobilization of TiO2 nanoparticles on SA (Fig. 4). Fig. 4a shows the XRD pattern of a sample of the n-TiO2. Fig. 4b shows the XRD pattern of a sample of the SA/n-TiO2. The direct evidence of the formation of nanoparticles on the surface of SA was given by SEM (Fig. 5). SEM images of the SA/n-TiO2 novel composite show that titania nanoparticles exist on the SA. WDX images of SA/n-TiO2 novel composite showed the content of titanium at different samples (Fig. 6). Results show that titania nanoparticles exist on the SA. To account for the adsorption behavior of the dyes on SA/n-TiO2, the Langmuir type equation related to surface coverage (θ) was used. The equation is expressed as follows: bC0 = θ = 1–θ
Fig. 1. Chemical structure of alginate.
ð1Þ
The fraction of biomass surface covered by dyes was studied by plotting the surface coverage values against dye concentration at
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Fig. 2. Chemical structure of Direct Red 80 (DR80) and Acid Green 25 (AG25).
different temperatures. The plots are presented in Fig. 7. It is seen from the figure that the surface coverage on SA/n-TiO2 increases sharply with the increase of initial dye concentration, and then increases slowly until θ value is close to 1.0. Furthermore, θ value increases with increasing temperatures in the condition of same lead concentration. These results show that higher dye concentrations and adsorption temperatures will benefit for lead ion coverage onto SA/nTiO2 with a monomolecular layer. The overall result about surface coverage indicates that SA/n-TiO2 will be very effective in removing dyes from aqueous solutions [28].
adsorption sites. However, if the adsorption capacity was expressed in mg adsorbed per gram of material, the capacity decreased with the increasing amount of composite. This may be attributed to overlapping or aggregation of adsorption sites resulting in a decrease in total adsorbent surface area available to the dye and an increase in diffusion path length [29].
3.2.1. Effect of adsorbent dosage The effect of SA/n-TiO2 dosages on dye removal was investigated by contacting 200 mL of dye solution with initial dye concentration of 50 mg/L using jar test at room temperature (25 °C) for 20 min at a constant stirring speed of 200 rpm. Different amounts of SA/n-TiO2 (0.15–1 g) were applied to remove DR80 and AG25. After equilibrium, the solution samples were centrifuged and the concentration in the supernatant dye solution was analyzed. The plot of dye removal (%) versus time (min) in different adsorbent dosage (g/L) was shown in Fig. 8. The increase in dye adsorption with composite dosage can be attributed to increased adsorbent surface and availability of more
3.2.2. Effect of dye concentration Adsorption is a mass transfer process that can generally be defined as the accumulation of material at the interface between two phases [30]. The adsorption efficiencies of DR80 and AG25 on SA/n-TiO2 were evaluated by determining the percentage decrease of the absorbance at 540 and 605 nm, respectively. The influence of varying the initial dye concentration of two dyes was assessed. The results are shown in Fig. 9. It is obvious that the higher the initial dye concentration, the lower the percentage of dye adsorbed. The amount of the dye adsorbed onto composite increases with an increase in the initial dye concentration of solution if the amount of adsorbent is kept unchanged due to the increase in the driving force of the concentration gradient with the higher initial dye concentration. The adsorption of dye by composite adsorbent is very intense and reaches equilibrium very quickly at low initial concentration. At a fixed adsorbent dosage, the amount of dye adsorbed increased with increasing concentration of solution, but the percentage of adsorption decreased. In other words, the residual dye concentration will be higher for higher initial dye concentrations. In the case of lower concentrations, the ratio of initial number of dye moles to the
Fig. 3. FT-IR spectrum of alginate and alginate/n-TiO2.
Fig. 4. XRD of n-TiO2 and SA/n-TiO2.
3.2. Effect of operational parameter on dye removal by SA/n-TiO2 composite
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Fig. 5. SEM images of (a) SA, (b) SA/n-TiO2, (c) n-TiO2, (d) AG25 adsorbed SA/n-TiO2 and (e) DR80 adsorbed SA/n-TiO2.
available adsorption sites is low and subsequently the fractional adsorption becomes independent of initial concentration [31–34]. 3.2.3. Effect of pH At lower pH more protons will be available to protonate hydroxyl groups of SA, thereby increasing electrostatic attractions between negatively charged dye anions and positively charged adsorption sites and causing an increase in dye adsorption [35,36]. This explanation agrees with our data on pH effect. It can be seen that the pH of aqueous solution plays an important role in the adsorption of anionic dyes onto SA/n-TiO2. The SA contains hydroxyl group, –OH, which is easily protonated to form –OH+ 2 , in acidic solutions. The high adsorption capacity is due to the strong electrostatic interaction between the –OH+ 2 of SA and dye anions [37]. The effect of pH on the adsorption of DR80 and AG25 onto SA/n-TiO2 is shown in Fig. 10. For two dyes, the adsorption capacity increases when the pH is decreased. Maximum adsorption of anionic dyes occurs at acidic pH (pH 2). At various pH values, the electrostatic attraction as well as the organic property and structure of dye molecules and SA/n-TiO2 could play very important roles in dye adsorption on SA/n-TiO2. At pH 2,
a significantly high electrostatic attraction exists between the positively charged surface of the adsorbent, due to the ionization of functional groups of adsorbent and negatively charged anionic dye. As the pH of the system increases, the number of negatively charged sites is increased. A negatively charged site on the adsorbent does not favor the adsorption of anionic dyes due to the electrostatic repulsion [38]. Also, lower adsorption of DR80 and AG25 dyes at alkaline pH is due to the presence of excess OH− ions destabilizing anionic dyes and competing with the dye anions for the adsorption sites. The effective pH was 2 and it was used in further studies. 3.3. Adsorption isotherm The adsorption isotherm expresses the relation between the mass of the dye adsorbed at a particular temperature, the pH, particle size and liquid phase of the dye concentration. Several isotherms were investigated. The Langmuir isotherm which has been successfully applied to many sorption processes can be used to explain the sorption of dye into Sodium Alginate. A basic assumption of the Langmuir theory is that sorption takes place at specific sites within the
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Fig. 7. Plots of surface coverage (θ) against initial concentration of dyes at different temperatures (a) DR80, and (b) AG25 (pH 2, T of 25 °C, CAd = 0.5 g/L).
RL values indicate the type of isotherm to be irreversible (RL = 0), favorable (0 b RL b 1), linear (RL = 1) or unfavorable (RL N 1) [21,22]. Also, isotherm data were tested with Freundlich isotherm that can be expressed by [41,43]:
Fig. 6. WDX images of (a) SA, (b) n-TiO2 and (c) SA/n-TiO2.
1=n
adsorbent [30,39,40]. The Langmuir equation can be written as follows [41,42]: qe = Q0 KL Ce = 1 + KL Ce
ð2Þ
where qe, Ce, KL and Q0 are the amount of dye adsorbed on SA/n-TiO2 at equilibrium (mg/g), the equilibrium concentration of dye solution (mg/L), Langmuir constant (L/g) and the maximum adsorption capacity (mg/g), respectively. The linear form of Langmuir equation is: Ce = qe = 1 = KL Q0 + Ce = Q0
ð3Þ
The essential characteristic of the Langmuir isotherm can be expressed by the dimensionless constant called equilibrium parameter, RL, defined by RL = 1 = ð1 + KL C0 Þ where C0 is the initial dye concentration (mg/L).
ð4Þ
ð5Þ
qe = KF Ce
where KF is adsorption capacity at unit concentration and 1/n is adsorption intensity. 1/n values indicate the type of isotherm to be irreversible (1/ n = 0), favorable (0 b 1/n b 1) and unfavorable (1/n N 1). Eq. (5) can be rearranged to a linear form: logqe = logKF + ð1 = nÞ logCe
ð6Þ
In order to deepen the understanding of adsorption mechanism, Dubinin–Radushkevich (D–R) isotherm model was chosen to apply on adsorption study. The D–R isotherm can be used to describe adsorption on both homogenous and heterogeneous surfaces [44]. A linear form of D–R isotherm is 2
lnqe = Lnqm −βε
ð7Þ
where β, qm, ε, R and T are a constant related to the mean free energy of adsorption (mol2/kJ2), the theoretical saturation capacity, the Polanyi potential (which is equal to RT ln(1 + (1/Ce)), the gas constant
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(8.314 J/mol K) and the absolute temperature(K), respectively. The plots of specific sorption, ln qe against ε2. For D–R isotherm equation, from the β values the mean energy of adsorption; Ea can be calculated using the relation [45]. −1 = 2
Ea = ð2βÞ
ð8Þ
Based on Eqs. (7) and (8), the isotherm constants, Ea and correlation coefficients (R2) are calculated and presented in Table 2. From Table 2, the value of Ea is 7.857 kJ/mol for DR80 and 7.071 kJ/mol for AG25. The mean energy of adsorption is the free energy change when one mole of the ions is transferred from infinity in the solution to the surface of the solid. The value of this parameter can give information about adsorption mechanism. When one mole of ions is transferred, its value in the range of 1–8 kJ/mol indicates physical adsorption [46], the value of Ea is between 8 and 16 kJ/mol, which indicates the adsorption process, followed by ion-exchange [47], while its value in the range of 20–40 kJ/mol is indicative of chemical adsorption [48]. So, it seems that physical mechanism is dominating in the adsorption process. The Tempkin isotherm is given as: qe = RT = b lnðKT Ce Þ
ð9Þ
which can be linearized as: qe = B1 lnKT + B1 lnCe
ð10Þ
Fig. 9. The effect of dye concentration on dye removal by SA/n-TiO2 (a) DR80, and (b) AG25 (pH 2, T of 25 °C, CAd = 0.5 g/L).
where B1 = RT = b
ð11Þ
Tempkin isotherm contains a factor that explicitly takes into the account adsorbing species adsorbent interactions. This isotherm assumes that (i) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent–adsorbate
Fig. 8. The effect of adsorbent dosage on dye removal by SA/n-TiO2 (a) DR80, and (b) AG25 (pH 2, T of 25 °C, C0 = 50 mg/L).
Fig. 10. The effect of pH on the adsorption of dyes on SA/n-TiO2 (C0 = 50 mg/L, pH 2, T of 25 °C, CAd = 0.5 g/L).
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Table 2 Linearized isotherm coefficients for dye adsorption onto SA/n-TiO2 composite. Langmuir Q0 DR80 163.934 AB26 151.515
Freundlich
Dubinin–Radushkevich
Tempkin
KL
RL
R2
KF
n
R2
qm
β
R2
Ea
KT
B1
R2
0.099
0.168
0.999
24.934
2.103
0.979
259.823
0.008
0.995
7.857
1.103
36.956
0.998
0.660
0.029
0.999
63.401
3.652
0.963
215.121
0.010
0.997
7.071
15.986
24.220
0.992
interactions, and that (ii) the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy [49,50]. A plot of qe versus ln Ce enables the determination of the isotherm constants B1 and KT from the slope and the intercept, respectively. K T is the equilibrium binding constant (L/mol) corresponding to the maximum binding energy and constant B1 is related to the heat of adsorption. To study the applicability of the Langmuir, Freundlich, Dubinin– Radushkevich and Tempkin isotherms for the dye adsorption onto composite, linear plots of Ce/qe against Ce, log qe versus log Ce, ln qe versus ε2 and qe versus ln Ce are plotted respectively. The Q0, KL, RL, KF, n, qm, β, Ea, KT, B1, and R2 (correlation coefficient) are given in Table 2. The data of Table 2 indicate that the Langmuir isotherm is the most appropriate for adsorption of DR80 and AG25 on SA/n-TiO2. The linear fit between the Ce/qe versus Ce and calculated correlation coefficients (R2) for Langmuir isotherm model show that the dye removal isotherm can be approximated as Langmuir model (Table 2). This means that the adsorption of cationic dyes takes place at specific homogeneous sites and a one layer adsorption onto SA/n-TiO2 surface.
where k2 is the equilibrium rate constant of pseudo-second order (g/ mg min). To understand the applicability of the intraparticle diffusion, pseudo-first order and pseudo-second order models for the dye adsorption onto SA/n-TiO2, linear plots of qt versus t1/2, log(qe–qt) versus contact time (t) and t/qt versus contact time (t) (Fig. 11) are plotted. The values of kp, I, k1, k2, and R2 (correlation coefficient values of all kinetics models) and the calculated qe ((qe)Cal.) are shown in Table 3. The linearity of the plots (R2) demonstrates that the intraparticle diffusion and pseudo-first order kinetic models do not play a significant role in the uptake of the dye by SA/n-TiO2 (Table 3). The linear fit between the t/qt versus contact time (t) and the calculated correlation coefficients (R2) for pseudo-second order kinetics model show that the dye removal kinetic can be approximated as pseudosecond order kinetics (Table 3). In addition, the experimental qe ((qe)Exp.) values agree with the calculated ones ((qe)Cal.), obtained from the linear plots of pseudo-second order kinetics (Table 3).
3.4. Adsorption kinetic Several models can be used to express the mechanism of solute sorption onto a sorbent. In order to investigate the mechanism of sorption, characteristic constants of sorption were determined using intraparticle diffusion [51–53], pseudo-first order equation [54] and pseudo-second order equation [55]. The possibility of intraparticle diffusion resistance affecting adsorption was explored by using the intraparticle diffusion model as qt = kp t
1=2
+I
ð12Þ
where qc, kp and I are the amount of adsorbed dye onto adsorbent (mg/g) at time t, intraparticle diffusion rate constant and intercept, respectively. Values of I give an idea about the thickness of the boundary layer, i.e, the larger intercept the greater is the boundary layer effect. According to this model, the plot of uptake, should be linear if intraparticle diffusion is involved in the adsorption process and if these lines pass through the origin then intraparticle diffusion is the rate controlling step. When the plots do not pass through the origin, this is indicative of some degree of boundary layer control and it shows that the intraparticle diffusion is not the only rate limiting step, but also other kinetic models may control the rate of adsorption, all of which may be operating simultaneously. A linear form of pseudo-first order model (Eq. (13)) is: logðqe −qt Þ = log ðqe Þ−ðk1 = 2:303Þt
ð13Þ
where k1 is the equilibrium rate constant of pseudo-first order (1/min). The linear fit between the log(qe–qt) and contact time (t) under pH 2 can be approximated as pseudo-first order kinetics. Linear form of pseudo-second order model (Eq. 14), was illustrated as: t = qt = 1 = k2 qe + ð1 = qe Þt
ð14Þ
Fig. 11. Pseudo-second order sorption kinetics of dye on SA/n-TiO2 (a) DR80, and (b) AG25 (C0 = 50 mg/L, pH 2, T of 25 °C, CAd = 0.5 g/L).
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Table 3 Linearized kinetic coefficients for dye adsorption onto SA/n-TiO2 composite. Dye (mg/L)
(qe)Exp.
DR80 25 50 75 100 AG25 25 50 75 100
Intraparticle diffusion
Pseudo-first order
kp
I
R2
43 80 109 128
8.761 16.363 22.519 26.552
12.969 23.763 31.902 36.068
0.684 0.691 0.701 0.717
49 94 129 144
9.918 19.065 26.282 29.639
15.125 28.813 38.908 41.817
0.672 0.676 0.684 0.703
Pseudo-second order
k1
R2
(qe)Cal.
k2
R2
30 57 81 98
0.483 0.474 0.463 0.447
0.945 0.950 0.955 0.963
43 81 111 130
0.102 0.050 0.033 0.025
0.999 0.999 0.999 0.999
33 64 90 106
0.499 0.493 0.483 0.462
0.937 0.940 0.945 0.956
49 95 130 150
0.105 0.050 0.035 0.025
0.999 0.999 0.999 0.999
(qe)Cal.
3.5. Adsorption thermodynamic
4. Conclusion
Thermodynamic parameters including change in the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) are the actual indicators for practical application of an adsorption process. According to the values of these parameters, what process will occur spontaneously can be determined. The thermodynamic parameters were determined using the following equations [56]:
The preparation, characterization and dye adsorption properties of novel biocompatible composite (SA/n-TiO2) was investigated. SA/n-TiO2 were studied using FT-IR, SEM and WDX. Equilibrium and kinetic studies were done for the adsorption of Direct Red 80 (DR80) and Acid Green 25 (AG25) from aqueous solutions onto SA/n-TiO2. Results showed that titanium oxide nanoparticles were immobilized onto SA. Adsorption studies showed that SA/n-TiO2 could be effectively used as a biocompatible composite adsorbent for the removal of anionic dyes. The equilibrium data showed that data for DR80 and AG25 followed with Langmuir isotherm. The kinetics data indicated that the adsorption kinetics of dyes on SA/n-TiO2 followed the pseudo-second order. The results showed that the SA/n-TiO2 being a biocompatible, eco-friendly and low-cost adsorbent with relatively large adsorption capacity might be a suitable alternative for elimination of dyes from colored aqueous solutions.
ΔG = ΔH−TΔS
ð15Þ
Kc = CA = CS
ð16Þ
lnKc = ðΔS = RÞ−ðΔH = RT Þ
ð17Þ
where Kc, CA and CS are the equilibrium constant, the amount of dye adsorbed on the adsorbent of the solution at equilibrium (mol/L) and the equilibrium concentration of dye in the solution (mol/L), respectively. The obtained thermodynamic parameters are given in Table 4. Positive ΔH suggests that dye adsorption onto SA/n-TiO2 is an endothermic reaction. The positive value of ΔS suggests the increased randomness at the solid/solution interface during the adsorption of dyes onto SA/n-TiO2. The negative values of ΔG imply the spontaneous nature of the adsorption process. Further, the decrease in the values of ΔG with the increasing temperature indicates that the adsorption is more spontaneous at higher temperatures. Generally, the change in free energy for physisorption is between − 20 and 0 kJ/ mol, but chemisorption is in a range of −80 to 400 kJ/mol [57]. The values of ΔG obtained in this study are within the ranges of the physisorption mechanism.
Table 4 Thermodynamic coefficients for dye adsorption onto SA/n-TiO2 composite. ΔH (kJ/mol)
ΔS (J/mol K)
DR80 25 50 75 100
7.447 5.392 4.277 3.595
45.798 35.356 28.366 22.596
− 6.200 − 5.144 − 4.176 − 3.139
− 6.658 − 5.497 − 4.459 − 3.364
− 7.116 − 5.850 − 4.743 − 3.590
− 7.574 − 6.204 − 5.026 − 3.816
AG25 25 50 75 100
64.468 19.496 7.448 4.171
253.776 93.798 45.798 27.595
− 11.157 − 8.456 − 6.200 − 4.052
− 13.694 − 9.393 − 6.658 − 4.327
− 16.232 − 10.331 − 7.116 − 4.603
− 18.770 − 11.269 − 7.574 − 4.879
Dye (mg/L)
ΔG (kJ/mol) 298 K
308 K
318 K
328 K
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation, characterization and dye adsorption properties of biocompatible composite (alginate/titania nanoparticle) Niyaz Mohammad Mahmoodi a,⁎, Bagher Hayati b, Mokhtar Arami b, Hajir Bahrami b a b
Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iran Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 27 November 2010 Received in revised form 13 February 2011 Accepted 14 February 2011 Available online 15 March 2011 Keywords: Biocompatible composite Alginate/titania nanoparticle Preparation Characterization Dye adsorption
a b s t r a c t In this paper, the preparation, characterization and dye adsorption properties of novel biocompatible composite (Sodium Alginate/titania nanoparticle) (SA/n-TiO2) were investigated. Titania nanoparticles were immobilized onto Sodium Alginate. Physical characteristics of SA/n-TiO2 were studied using Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), and wavelength dispersive X-ray spectroscopy (WDX). Two textile dyes, Direct Red 80 (DR80) and Acid Green 25 (AG25), were used as model compounds. The effect of operational parameter such as SA/n-TiO2 dosage, initial dye concentration and pH was evaluated at 25 °C. The isotherm, kinetic and thermodynamic of dye adsorption were studied. The data were evaluated for compliance with the different isotherm models. It was found that DR80 and AG25 followed with Langmuir isotherm. Adsorption kinetic of dyes was found to conform to pseudo-second order kinetics. The thermodynamic data showed that adsorption process was spontaneous, endothermic and physisorption reaction. Based on the data of present investigation, one could conclude that the SA/n-TiO2 being a biocompatible, eco-friendly and low-cost adsorbent might be a suitable alternative for elimination of dyes from colored aqueous solutions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Colored dye wastewater arises as a direct result of the production of the dye and also as a consequence of its use in the textile and other industries [1]. It is estimated that 2% of dyes produced annually are discharged in effluent from manufacturing operations, while 10% is discharged from textile and associated industries [2]. Adsorption process is widely used to remove contaminants from wastewater. Liquid–solid adsorption operations are concerned with the ability of certain solids to preferentially concentrate specific substances from solution onto their surfaces [3]. The design and efficient operation of adsorption process require equilibrium adsorption data for use in kinetic and mass transfer models. These models can then be used to predict the performance of the adsorption contact processes under a range of operating conditions. The equilibrium isotherm plays an important role in modeling for analysis and design of adsorption systems. The adsorption isotherm is also an invaluable tool for the theoretical evaluation and interpretation of thermodynamic parameters such as heats of adsorption. An isotherm may fit experimental data accurately under one set of conditions but fail entirely under another. No single model has been found to be generally applicable, a
⁎ Corresponding author. Tel.: +98 21 22969771; fax: +98 21 22947537. E-mail addresses: [email protected], [email protected] (N.M. Mahmoodi). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.02.034
fact which is readily understandable in the light of the assumptions associated with their respective derivations. The current research presents a method of direct comparison of the isotherm fit of several models to enable the best-fit and best isotherm parameters to be obtained. An accurate mathematical description of equilibrium adsorption capacities, even if empirical, is indispensable for reliable predictive modeling of adsorption systems and quantitative comparisons of adsorption behavior for different adsorbent systems or for varied conditions within any given system [4–7]. Alginate is a naturally occurring carbohydrate polymer and has a capacity to remove toxic pollutants. Many biopolymers are known to have a strong affinity for metal ions and the use of alginic acid as adsorbent for the recovery of valuable metal ions as well as removal of toxic metal ions has been studied [8,9]. Alginic acid is a biopolymer carrying carboxyl groups capable of forming complexes with metal ion. Alginic acid is derived from several genera of brown algae that are utilized as raw materials by commercial alginate producers, Nodosum being the principal source of the world's alginate supply. The intercellular mucilage in the seaweeds has been regarded as the principal site of algin [10]. One of the important properties of alginate is the ability to form hydrogels [11]. An aqueous solution of alginate is readily transformed into a hydrogel on addition of metallic divalent cations such as Ca2+. Alginate is often used for immobilization of biological entities [12,13]. Calcium alginate immobilized microbial cultures have been used for decoloration of dyes [14,15]. Also, activated carbons immobilized in calcium alginate beads are used for dye removal [14]. Sodium Alginate/acrylamide interpenetrating polymer networks
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Table 1 Adsorption capacities for the removal of Direct and Acid dyes by natural adsorbents. Adsorbent
Dye
Q0 (mg/g)
Ref.
Soy meal hull Soy meal hull Soy meal hull Soy meal hull Peat Orange peel Orange peel Orange peel Rice husk Baggase pith Baggase pith SA/n-TiO2 SA/n-TiO2
Direct Red 80 Direct Red 81 Acid Blue 92 Acid Red 14 Acid Blue 25 Direct Red 23 Direct Red 80 Acid Violet 17 Acid Yellow 36 Acid Blue 25 Acid Red 114 Direct Red 80 Acid Green 25
178.6 120.5 114.9 109.9 5–9 10.7 21.0 19.9 86.9 17.5 20.0 163.9 151.5
[19] [19] [19] [19] [20] [21] [21] [22] [23] [24] [24] In this work In this work
have been prepared and used to remove dyes [16]. Other compounds were used to remove water pollutants [17,18]. A literature review showed that dye removal using Sodium Alginate/titania nanoparticle (SA/n-TiO2) was not evaluated. Two textile dyes (Direct Red 80 (DR80) and Acid Green 25 (AG25)) were used as model compounds in this work. Removal of Direct and Acid dyes was studied by several natural adsorbents (Table 1) [19–24]. In this paper, the preparation, characterization and dye adsorption properties of biocompatible composite (SA/n-TiO2) were studied. Physical characteristics of SA/n-TiO2 were investigated. The isotherm, kinetic and thermodynamic of dye adsorption were studied.
2. Materials and methods
2.3. Physicochemical characterization of SA/n-TiO2 composite adsorbent Fourier transform infrared (FT-IR) spectra (Perkin-Elmer Spectrophotometer Spectrum One) in the range of 450–4000 cm− 1 were studied. The X-ray diffraction (XRD) spectra of SA and SA/n-TiO2 were performed using a D8 ADVANCE X-ray diffraction spectrometer (Bruker, Germany) with a Cu Kα target at 40 kV and 50 mA at a scan rate of 0.02°2θ s− 1. The morphological structure of the SA/n-TiO2 was examined by scanning electron microscopy (SEM) using LEO 1455VP scanning microscope. Wavelength dispersive X-ray spectroscopy (WDX) was employed to obtain information on the content of Ti in the composite.
2.4. Adsorption procedure The dye adsorption measurements were conducted by mixing various amounts of SA/n-TiO2 (0.15–1 g/L) for DR80 and AG25 in jars containing 200 mL of a dye solution (50 mg/L) at various pH (2– 10). The solution pH was adjusted by adding a small amount of H2SO 4 or NaOH. Adsorption experiments were performed at different dye concentrations (25, 50, 75 and 100 mg/L) using optimum amount of SA/n-TiO2 (0.5 g/L for DR80 and AG25) at pH 2, agitation speed of 200 rpm and 25 °C for 20 min. The change on the absorbance of all solution samples was monitored and determined at certain time intervals (0, 2.5, 5, 7.5, 10, 15, and 20 min) during the adsorption process. The equilibrium was established after 10 min. At the end of the adsorption experiments, the solution samples were centrifuged and the dye concentration was determined. The results were verified with the adsorption isotherms and kinetics.
2.1. Chemicals Sodium Alginate was supplied by Kitotak and used as received. The chemical structure of alginate was shown in Fig. 1. Anionic dyes, Direct Red 80 (DR80) and Acid Green 25 (AG25), were used in this study. The dyes were purchased from Ciba Ltd. Dyes were used without further purification. The chemical structure of dyes was shown in Fig. 2. Titania (Degussa P25) was utilized. Its main physical data are as follows: average primary particle size 30 nm, purity above 97% and with 80:20 anatase to rutile. All other chemicals were of analar grade and purchased from Merck (Germany). UV–VIS spectrophotometer CECIL 2021 was employed for absorbance measurements of samples. The maximum wavelength (λmax) used for determination of residual concentration of DR80 and AG25 at pH 2 in supernatant solution using UV–VIS spectrophotometer were 540 and 605 nm, respectively.
2.2. Preparation of SA/n-TiO2 3 g of SA flake was dissolved in 300 mL of aqueous solution. The viscous solution was stirred continuously for 12 h to fully dissolve the alginate flake. Then, 1.5 g of n-TiO2 powders was added into the solution. Subsequently, another 50 mL of distilled water was added. The slurry was stirred continuously for 24 h to obtain the final transparent solution.
3. Results and discussion 3.1. Characterization of SA/n-TiO2 composite In order to study the functional groups of SA and SA/n-TiO2, the FTIR was applied (Fig. 3). The FT-IR spectrum of SA exhibited differences from that of SA/n-TiO2. The major differences were: the wide peak at 3442 cm− 1, corresponding to the stretching vibration of carboxylic acid groups, moved noticeably to lower wave numbers (3396 cm− 1) and became broader and weaker, which indicated the strong interaction between these groups and n-TiO2 [25]. The FT-IR spectrum of SA itself showed some features of hydroxyl groups at 2151 cm− 1 [26,27]. The decrease of the band related to primary OH groups at 2151 cm− 1 is an indicative of TiO2 nanoparticle immobilization onto the SA [27]. X-ray diffraction (XRD) was employed to investigate the immobilization of TiO2 nanoparticles on SA (Fig. 4). Fig. 4a shows the XRD pattern of a sample of the n-TiO2. Fig. 4b shows the XRD pattern of a sample of the SA/n-TiO2. The direct evidence of the formation of nanoparticles on the surface of SA was given by SEM (Fig. 5). SEM images of the SA/n-TiO2 novel composite show that titania nanoparticles exist on the SA. WDX images of SA/n-TiO2 novel composite showed the content of titanium at different samples (Fig. 6). Results show that titania nanoparticles exist on the SA. To account for the adsorption behavior of the dyes on SA/n-TiO2, the Langmuir type equation related to surface coverage (θ) was used. The equation is expressed as follows: bC0 = θ = 1–θ
Fig. 1. Chemical structure of alginate.
ð1Þ
The fraction of biomass surface covered by dyes was studied by plotting the surface coverage values against dye concentration at
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95
Fig. 2. Chemical structure of Direct Red 80 (DR80) and Acid Green 25 (AG25).
different temperatures. The plots are presented in Fig. 7. It is seen from the figure that the surface coverage on SA/n-TiO2 increases sharply with the increase of initial dye concentration, and then increases slowly until θ value is close to 1.0. Furthermore, θ value increases with increasing temperatures in the condition of same lead concentration. These results show that higher dye concentrations and adsorption temperatures will benefit for lead ion coverage onto SA/nTiO2 with a monomolecular layer. The overall result about surface coverage indicates that SA/n-TiO2 will be very effective in removing dyes from aqueous solutions [28].
adsorption sites. However, if the adsorption capacity was expressed in mg adsorbed per gram of material, the capacity decreased with the increasing amount of composite. This may be attributed to overlapping or aggregation of adsorption sites resulting in a decrease in total adsorbent surface area available to the dye and an increase in diffusion path length [29].
3.2.1. Effect of adsorbent dosage The effect of SA/n-TiO2 dosages on dye removal was investigated by contacting 200 mL of dye solution with initial dye concentration of 50 mg/L using jar test at room temperature (25 °C) for 20 min at a constant stirring speed of 200 rpm. Different amounts of SA/n-TiO2 (0.15–1 g) were applied to remove DR80 and AG25. After equilibrium, the solution samples were centrifuged and the concentration in the supernatant dye solution was analyzed. The plot of dye removal (%) versus time (min) in different adsorbent dosage (g/L) was shown in Fig. 8. The increase in dye adsorption with composite dosage can be attributed to increased adsorbent surface and availability of more
3.2.2. Effect of dye concentration Adsorption is a mass transfer process that can generally be defined as the accumulation of material at the interface between two phases [30]. The adsorption efficiencies of DR80 and AG25 on SA/n-TiO2 were evaluated by determining the percentage decrease of the absorbance at 540 and 605 nm, respectively. The influence of varying the initial dye concentration of two dyes was assessed. The results are shown in Fig. 9. It is obvious that the higher the initial dye concentration, the lower the percentage of dye adsorbed. The amount of the dye adsorbed onto composite increases with an increase in the initial dye concentration of solution if the amount of adsorbent is kept unchanged due to the increase in the driving force of the concentration gradient with the higher initial dye concentration. The adsorption of dye by composite adsorbent is very intense and reaches equilibrium very quickly at low initial concentration. At a fixed adsorbent dosage, the amount of dye adsorbed increased with increasing concentration of solution, but the percentage of adsorption decreased. In other words, the residual dye concentration will be higher for higher initial dye concentrations. In the case of lower concentrations, the ratio of initial number of dye moles to the
Fig. 3. FT-IR spectrum of alginate and alginate/n-TiO2.
Fig. 4. XRD of n-TiO2 and SA/n-TiO2.
3.2. Effect of operational parameter on dye removal by SA/n-TiO2 composite
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Fig. 5. SEM images of (a) SA, (b) SA/n-TiO2, (c) n-TiO2, (d) AG25 adsorbed SA/n-TiO2 and (e) DR80 adsorbed SA/n-TiO2.
available adsorption sites is low and subsequently the fractional adsorption becomes independent of initial concentration [31–34]. 3.2.3. Effect of pH At lower pH more protons will be available to protonate hydroxyl groups of SA, thereby increasing electrostatic attractions between negatively charged dye anions and positively charged adsorption sites and causing an increase in dye adsorption [35,36]. This explanation agrees with our data on pH effect. It can be seen that the pH of aqueous solution plays an important role in the adsorption of anionic dyes onto SA/n-TiO2. The SA contains hydroxyl group, –OH, which is easily protonated to form –OH+ 2 , in acidic solutions. The high adsorption capacity is due to the strong electrostatic interaction between the –OH+ 2 of SA and dye anions [37]. The effect of pH on the adsorption of DR80 and AG25 onto SA/n-TiO2 is shown in Fig. 10. For two dyes, the adsorption capacity increases when the pH is decreased. Maximum adsorption of anionic dyes occurs at acidic pH (pH 2). At various pH values, the electrostatic attraction as well as the organic property and structure of dye molecules and SA/n-TiO2 could play very important roles in dye adsorption on SA/n-TiO2. At pH 2,
a significantly high electrostatic attraction exists between the positively charged surface of the adsorbent, due to the ionization of functional groups of adsorbent and negatively charged anionic dye. As the pH of the system increases, the number of negatively charged sites is increased. A negatively charged site on the adsorbent does not favor the adsorption of anionic dyes due to the electrostatic repulsion [38]. Also, lower adsorption of DR80 and AG25 dyes at alkaline pH is due to the presence of excess OH− ions destabilizing anionic dyes and competing with the dye anions for the adsorption sites. The effective pH was 2 and it was used in further studies. 3.3. Adsorption isotherm The adsorption isotherm expresses the relation between the mass of the dye adsorbed at a particular temperature, the pH, particle size and liquid phase of the dye concentration. Several isotherms were investigated. The Langmuir isotherm which has been successfully applied to many sorption processes can be used to explain the sorption of dye into Sodium Alginate. A basic assumption of the Langmuir theory is that sorption takes place at specific sites within the
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Fig. 7. Plots of surface coverage (θ) against initial concentration of dyes at different temperatures (a) DR80, and (b) AG25 (pH 2, T of 25 °C, CAd = 0.5 g/L).
RL values indicate the type of isotherm to be irreversible (RL = 0), favorable (0 b RL b 1), linear (RL = 1) or unfavorable (RL N 1) [21,22]. Also, isotherm data were tested with Freundlich isotherm that can be expressed by [41,43]:
Fig. 6. WDX images of (a) SA, (b) n-TiO2 and (c) SA/n-TiO2.
1=n
adsorbent [30,39,40]. The Langmuir equation can be written as follows [41,42]: qe = Q0 KL Ce = 1 + KL Ce
ð2Þ
where qe, Ce, KL and Q0 are the amount of dye adsorbed on SA/n-TiO2 at equilibrium (mg/g), the equilibrium concentration of dye solution (mg/L), Langmuir constant (L/g) and the maximum adsorption capacity (mg/g), respectively. The linear form of Langmuir equation is: Ce = qe = 1 = KL Q0 + Ce = Q0
ð3Þ
The essential characteristic of the Langmuir isotherm can be expressed by the dimensionless constant called equilibrium parameter, RL, defined by RL = 1 = ð1 + KL C0 Þ where C0 is the initial dye concentration (mg/L).
ð4Þ
ð5Þ
qe = KF Ce
where KF is adsorption capacity at unit concentration and 1/n is adsorption intensity. 1/n values indicate the type of isotherm to be irreversible (1/ n = 0), favorable (0 b 1/n b 1) and unfavorable (1/n N 1). Eq. (5) can be rearranged to a linear form: logqe = logKF + ð1 = nÞ logCe
ð6Þ
In order to deepen the understanding of adsorption mechanism, Dubinin–Radushkevich (D–R) isotherm model was chosen to apply on adsorption study. The D–R isotherm can be used to describe adsorption on both homogenous and heterogeneous surfaces [44]. A linear form of D–R isotherm is 2
lnqe = Lnqm −βε
ð7Þ
where β, qm, ε, R and T are a constant related to the mean free energy of adsorption (mol2/kJ2), the theoretical saturation capacity, the Polanyi potential (which is equal to RT ln(1 + (1/Ce)), the gas constant
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(8.314 J/mol K) and the absolute temperature(K), respectively. The plots of specific sorption, ln qe against ε2. For D–R isotherm equation, from the β values the mean energy of adsorption; Ea can be calculated using the relation [45]. −1 = 2
Ea = ð2βÞ
ð8Þ
Based on Eqs. (7) and (8), the isotherm constants, Ea and correlation coefficients (R2) are calculated and presented in Table 2. From Table 2, the value of Ea is 7.857 kJ/mol for DR80 and 7.071 kJ/mol for AG25. The mean energy of adsorption is the free energy change when one mole of the ions is transferred from infinity in the solution to the surface of the solid. The value of this parameter can give information about adsorption mechanism. When one mole of ions is transferred, its value in the range of 1–8 kJ/mol indicates physical adsorption [46], the value of Ea is between 8 and 16 kJ/mol, which indicates the adsorption process, followed by ion-exchange [47], while its value in the range of 20–40 kJ/mol is indicative of chemical adsorption [48]. So, it seems that physical mechanism is dominating in the adsorption process. The Tempkin isotherm is given as: qe = RT = b lnðKT Ce Þ
ð9Þ
which can be linearized as: qe = B1 lnKT + B1 lnCe
ð10Þ
Fig. 9. The effect of dye concentration on dye removal by SA/n-TiO2 (a) DR80, and (b) AG25 (pH 2, T of 25 °C, CAd = 0.5 g/L).
where B1 = RT = b
ð11Þ
Tempkin isotherm contains a factor that explicitly takes into the account adsorbing species adsorbent interactions. This isotherm assumes that (i) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent–adsorbate
Fig. 8. The effect of adsorbent dosage on dye removal by SA/n-TiO2 (a) DR80, and (b) AG25 (pH 2, T of 25 °C, C0 = 50 mg/L).
Fig. 10. The effect of pH on the adsorption of dyes on SA/n-TiO2 (C0 = 50 mg/L, pH 2, T of 25 °C, CAd = 0.5 g/L).
N.M. Mahmoodi et al. / Desalination 275 (2011) 93–101
99
Table 2 Linearized isotherm coefficients for dye adsorption onto SA/n-TiO2 composite. Langmuir Q0 DR80 163.934 AB26 151.515
Freundlich
Dubinin–Radushkevich
Tempkin
KL
RL
R2
KF
n
R2
qm
β
R2
Ea
KT
B1
R2
0.099
0.168
0.999
24.934
2.103
0.979
259.823
0.008
0.995
7.857
1.103
36.956
0.998
0.660
0.029
0.999
63.401
3.652
0.963
215.121
0.010
0.997
7.071
15.986
24.220
0.992
interactions, and that (ii) the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy [49,50]. A plot of qe versus ln Ce enables the determination of the isotherm constants B1 and KT from the slope and the intercept, respectively. K T is the equilibrium binding constant (L/mol) corresponding to the maximum binding energy and constant B1 is related to the heat of adsorption. To study the applicability of the Langmuir, Freundlich, Dubinin– Radushkevich and Tempkin isotherms for the dye adsorption onto composite, linear plots of Ce/qe against Ce, log qe versus log Ce, ln qe versus ε2 and qe versus ln Ce are plotted respectively. The Q0, KL, RL, KF, n, qm, β, Ea, KT, B1, and R2 (correlation coefficient) are given in Table 2. The data of Table 2 indicate that the Langmuir isotherm is the most appropriate for adsorption of DR80 and AG25 on SA/n-TiO2. The linear fit between the Ce/qe versus Ce and calculated correlation coefficients (R2) for Langmuir isotherm model show that the dye removal isotherm can be approximated as Langmuir model (Table 2). This means that the adsorption of cationic dyes takes place at specific homogeneous sites and a one layer adsorption onto SA/n-TiO2 surface.
where k2 is the equilibrium rate constant of pseudo-second order (g/ mg min). To understand the applicability of the intraparticle diffusion, pseudo-first order and pseudo-second order models for the dye adsorption onto SA/n-TiO2, linear plots of qt versus t1/2, log(qe–qt) versus contact time (t) and t/qt versus contact time (t) (Fig. 11) are plotted. The values of kp, I, k1, k2, and R2 (correlation coefficient values of all kinetics models) and the calculated qe ((qe)Cal.) are shown in Table 3. The linearity of the plots (R2) demonstrates that the intraparticle diffusion and pseudo-first order kinetic models do not play a significant role in the uptake of the dye by SA/n-TiO2 (Table 3). The linear fit between the t/qt versus contact time (t) and the calculated correlation coefficients (R2) for pseudo-second order kinetics model show that the dye removal kinetic can be approximated as pseudosecond order kinetics (Table 3). In addition, the experimental qe ((qe)Exp.) values agree with the calculated ones ((qe)Cal.), obtained from the linear plots of pseudo-second order kinetics (Table 3).
3.4. Adsorption kinetic Several models can be used to express the mechanism of solute sorption onto a sorbent. In order to investigate the mechanism of sorption, characteristic constants of sorption were determined using intraparticle diffusion [51–53], pseudo-first order equation [54] and pseudo-second order equation [55]. The possibility of intraparticle diffusion resistance affecting adsorption was explored by using the intraparticle diffusion model as qt = kp t
1=2
+I
ð12Þ
where qc, kp and I are the amount of adsorbed dye onto adsorbent (mg/g) at time t, intraparticle diffusion rate constant and intercept, respectively. Values of I give an idea about the thickness of the boundary layer, i.e, the larger intercept the greater is the boundary layer effect. According to this model, the plot of uptake, should be linear if intraparticle diffusion is involved in the adsorption process and if these lines pass through the origin then intraparticle diffusion is the rate controlling step. When the plots do not pass through the origin, this is indicative of some degree of boundary layer control and it shows that the intraparticle diffusion is not the only rate limiting step, but also other kinetic models may control the rate of adsorption, all of which may be operating simultaneously. A linear form of pseudo-first order model (Eq. (13)) is: logðqe −qt Þ = log ðqe Þ−ðk1 = 2:303Þt
ð13Þ
where k1 is the equilibrium rate constant of pseudo-first order (1/min). The linear fit between the log(qe–qt) and contact time (t) under pH 2 can be approximated as pseudo-first order kinetics. Linear form of pseudo-second order model (Eq. 14), was illustrated as: t = qt = 1 = k2 qe + ð1 = qe Þt
ð14Þ
Fig. 11. Pseudo-second order sorption kinetics of dye on SA/n-TiO2 (a) DR80, and (b) AG25 (C0 = 50 mg/L, pH 2, T of 25 °C, CAd = 0.5 g/L).
100
N.M. Mahmoodi et al. / Desalination 275 (2011) 93–101
Table 3 Linearized kinetic coefficients for dye adsorption onto SA/n-TiO2 composite. Dye (mg/L)
(qe)Exp.
DR80 25 50 75 100 AG25 25 50 75 100
Intraparticle diffusion
Pseudo-first order
kp
I
R2
43 80 109 128
8.761 16.363 22.519 26.552
12.969 23.763 31.902 36.068
0.684 0.691 0.701 0.717
49 94 129 144
9.918 19.065 26.282 29.639
15.125 28.813 38.908 41.817
0.672 0.676 0.684 0.703
Pseudo-second order
k1
R2
(qe)Cal.
k2
R2
30 57 81 98
0.483 0.474 0.463 0.447
0.945 0.950 0.955 0.963
43 81 111 130
0.102 0.050 0.033 0.025
0.999 0.999 0.999 0.999
33 64 90 106
0.499 0.493 0.483 0.462
0.937 0.940 0.945 0.956
49 95 130 150
0.105 0.050 0.035 0.025
0.999 0.999 0.999 0.999
(qe)Cal.
3.5. Adsorption thermodynamic
4. Conclusion
Thermodynamic parameters including change in the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) are the actual indicators for practical application of an adsorption process. According to the values of these parameters, what process will occur spontaneously can be determined. The thermodynamic parameters were determined using the following equations [56]:
The preparation, characterization and dye adsorption properties of novel biocompatible composite (SA/n-TiO2) was investigated. SA/n-TiO2 were studied using FT-IR, SEM and WDX. Equilibrium and kinetic studies were done for the adsorption of Direct Red 80 (DR80) and Acid Green 25 (AG25) from aqueous solutions onto SA/n-TiO2. Results showed that titanium oxide nanoparticles were immobilized onto SA. Adsorption studies showed that SA/n-TiO2 could be effectively used as a biocompatible composite adsorbent for the removal of anionic dyes. The equilibrium data showed that data for DR80 and AG25 followed with Langmuir isotherm. The kinetics data indicated that the adsorption kinetics of dyes on SA/n-TiO2 followed the pseudo-second order. The results showed that the SA/n-TiO2 being a biocompatible, eco-friendly and low-cost adsorbent with relatively large adsorption capacity might be a suitable alternative for elimination of dyes from colored aqueous solutions.
ΔG = ΔH−TΔS
ð15Þ
Kc = CA = CS
ð16Þ
lnKc = ðΔS = RÞ−ðΔH = RT Þ
ð17Þ
where Kc, CA and CS are the equilibrium constant, the amount of dye adsorbed on the adsorbent of the solution at equilibrium (mol/L) and the equilibrium concentration of dye in the solution (mol/L), respectively. The obtained thermodynamic parameters are given in Table 4. Positive ΔH suggests that dye adsorption onto SA/n-TiO2 is an endothermic reaction. The positive value of ΔS suggests the increased randomness at the solid/solution interface during the adsorption of dyes onto SA/n-TiO2. The negative values of ΔG imply the spontaneous nature of the adsorption process. Further, the decrease in the values of ΔG with the increasing temperature indicates that the adsorption is more spontaneous at higher temperatures. Generally, the change in free energy for physisorption is between − 20 and 0 kJ/ mol, but chemisorption is in a range of −80 to 400 kJ/mol [57]. The values of ΔG obtained in this study are within the ranges of the physisorption mechanism.
Table 4 Thermodynamic coefficients for dye adsorption onto SA/n-TiO2 composite. ΔH (kJ/mol)
ΔS (J/mol K)
DR80 25 50 75 100
7.447 5.392 4.277 3.595
45.798 35.356 28.366 22.596
− 6.200 − 5.144 − 4.176 − 3.139
− 6.658 − 5.497 − 4.459 − 3.364
− 7.116 − 5.850 − 4.743 − 3.590
− 7.574 − 6.204 − 5.026 − 3.816
AG25 25 50 75 100
64.468 19.496 7.448 4.171
253.776 93.798 45.798 27.595
− 11.157 − 8.456 − 6.200 − 4.052
− 13.694 − 9.393 − 6.658 − 4.327
− 16.232 − 10.331 − 7.116 − 4.603
− 18.770 − 11.269 − 7.574 − 4.879
Dye (mg/L)
ΔG (kJ/mol) 298 K
308 K
318 K
328 K
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