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activated carbon composite: Kinetics, thermodynamics and isotherm studies
Journal of Molecular Liquids 197 (2014) 236–242
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Adsorption of Acid Yellow 99 by polyacrylonitrile/activated carbon composite: Kinetics, thermodynamics and isotherm studies Ashraf A. El-Bindary a,⁎, Mostafa A. Hussien b, Mostafa A. Diab a, Ahmed M. Eessa b a b
Department of Chemistry, Faculty of Science, University of Damietta, Damietta 34517, Egypt Department of Chemistry, Faculty of Science, University of Port Said, Port Said, Egypt
a r t i c l e
i n f o
Article history: Received 18 March 2014 Accepted 2 May 2014 Available online 16 May 2014 Keywords: Adsorption Polyacrylonitrile/activated carbon Acid Yellow 99 Kinetic Isotherm
a b s t r a c t The adsorption of Acid Yellow 99 (AY99) onto polyacrylonitrile/activated carbon (PAN/AC) composite was investigated in aqueous solution in a batch system with respect to contact time, pH and temperature. Experimental data indicated that the adsorption capacity of (PAN/AC) composite for AY99 was higher in acidic rather than in basic solutions. Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms and the isotherm constants were determined. The activation energy of adsorption was also evaluated for the adsorption of AY99 onto (PAN/AC) composite. The pseudo-first-order and pseudo-second-order model equations were used to analyze the kinetics of the adsorption process. The dynamic data fitted the pseudo-second-order kinetic model well. The activation energy, change of free energy, enthalpy and entropy of adsorption were also evaluated for the adsorption of AY99 onto (PAN/AC) composite. The thermodynamics of the adsorption indicated spontaneous and exothermic nature of the process. The (PAN/AC) composite was characterized using Fourier transform infrared spectroscopy (FTIR) and its morphology was determined by scanning electron microscopy (SEM). The results indicate that (PAN/AC) composite could be employed as low-cost material for the removal of acid dyes from textile effluents. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The industrial wastewater usually contains a variety of organic compounds and toxic pulp mills and dyestuff manufacturing discharge highly colored wastewater which have provoked serious environmental concerns all over the world [1–3]. Dyes can be classified as anionic (direct, acid and reactive dyes), cationic (basic dyes) and non-ionic (disperse dyes) [4]. The removal of color from waste textile effluents has become environmentally important [5]. The most widely used methods for removing color effluents from water include coagulation [6], electro-chemical [7], oxidation [8], ozonation [9], solvent extraction [10], adsorption [5,11], photocatalytic degradation [12], etc. but only that of adsorption is considered to be superior to other techniques. This is attributed to its low cost, easy availability of adsorbents, simplicity of design, high efficiency, ease of operation and biodegradability [13]. Consequently, kinetic studies, which provide information for the rate of removal of pollutants from solution and the adsorption equilibrium data are essential for the design of water treatment units involving an adsorption process. The basic feature of an adsorption process is surface accumulation from intermolecular penetration of material. It is now customary to
⁎ Corresponding author. Tel.: +20 1114266996; fax: +20 572403868. E-mail address: [email protected] (A.A. El-Bindary).
http://dx.doi.org/10.1016/j.molliq.2014.05.003 0167-7322/© 2014 Elsevier B.V. All rights reserved.
differentiate between two types of adsorptions. If the attraction between the solid surface and the adsorbed molecules is physical in nature, the adsorption is referred to as physical adsorption (physisorption). Generally, in physical adsorption the attractive forces between adsorbed molecules and the solid surface are van der Waals forces and they being weak in nature result in reversible adsorption. On the other hand if the attraction forces are due to chemical bonding, the adsorption process is called chemisorption. In view of the higher strength of the bonding in chemisorption, it is difficult to remove chemisorbed species from the solid surface. Activated carbon is perhaps the most widely used adsorbent in the adsorption processes due to its high specific surface area and high adsorption capacity [14–16]. In order to decrease the cost of wastewater treatment, attempts have been made in finding inexpensive adsorbents. So, the research of the recent years mainly focused on utilizing economic materials as alternatives to activated carbon; amongst them, the composite materials. A composite is a material that consists of two or more constituent materials or phases. Polymers with activated carbon are being considered as alternative low-cost adsorbents due to their specific surface area and high chemical and mechanical stability [17–20]. The aim of this study is to investigate the adsorption of Acid Yellow 99 onto polyacrylonitrile/activated carbon (PAN/AC) composite as a low cost adsorbent. Effects of different parameters such as contact time, pH, dye concentration, temperature, adsorbent concentration on both equilibrium and the rate of adsorption were studied. The kinetics, isotherms
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237
2.3. Preparation of polyacrylonitrile/activated carbon (PAN/AC) composite
Fig. 1. Structure of AY99 dye.
and thermodynamic parameters were also calculated to determine the rate constants and adsorption mechanism.
Polyacrylonitrile/activated carbon (PAN/AC) composite was prepared [21] by adding 30 g of acrylonitrile (monomer) to 20 mL of bidistilled water in a 250 mL three neck round bottom flask, then adding 0.1 g of potassium persulfate and 10 g of activated carbon to the mixture. The mixture was stirred by a magnetic stirrer and the reaction was allowed to proceed at 60–70 °C for about 24 h until gel formation. The precipitate was filtered and washed with water, ethanol, 0.1 mol L −1 HCl solution and water, respectively. The final product (composite) was left to be dried under vacuum at 50 °C for 24 h and stored on desiccator prior to use in the sorption study. The polymerization reaction is given in Scheme 1. FTIR spectrum of (PAN/AC) composite was recorded in the region 400–4000 cm−1 (Fig. 2). The band at 2921 cm− 1 corresponds to the asymmetric stretching vibration of methylene group (νCH2) and its bending vibration at 1454 cm−1. The band at 2244 cm−1 corresponds to the CN group.
2. Materials and methods 2.1. Materials
2.4. Adsorption experiments
A commercial textile dye Acid Yellow 99 (Fig. 1) was obtained from Cromatos SRL, a dye company located in Italy and was used as received without further purification. Activated carbon (particle diameter 300–500 μm) was purchased from Calgon Company, USA. Activated carbon was washed several times with bidistilled water and then dried at 120 °C for 24 h. Samples were then preserved in the desiccator over anhydrous CaCl2 for further use.
The adsorption experiments of anionic dye AY99 were carried out in batch equilibrium mode. A 0.2–0.6 g sample of (PAN/AC) composite with 50 mL aqueous solution of a 40–150 mg/L AY99 solution at various pHs (1–9) reached for 90 min was adjusted by adding a small amount of HCl or NaOH solution (1 M) using a pH meter. The optimum pH was determined and used through all adsorption processes. Experiments were conducted for various time intervals to determine whether adsorption equilibrium was reached and the maximum removal of AR57 was attained. The solution was then filtered through a Whatman (number 40) filter paper to remove any organic or inorganic precipitates formed under acidic or basic conditions and the filtrates were subjected to quantitative analyses. The equilibrium concentration of each solution was determined at the wavelength of UV-maximum (λmax) at 418 nm. Dye adsorption experiments were also accomplished to obtain isotherms at various temperatures (25–50 °C) and at a range of 40–150 mg/L dye concentrations for 90 min by using a water bath with shaker. Calibration curves were constructed to correlate concentrations to different absorbance values. Construction of this calibration curves was verified and the maximum wavelengths that corresponded to maximum absorbance for the dye were determined.
2.2. Material characterization The SEM results of the PAN/AC composite sample before and after the adsorption processes were obtained using (JEOL 5600LV) scanning microscope to observe surface modification. FTIR spectrum of PAN/AC composite sample was recorded (KBr) on a Perkin-Elmer BX Model Fourier transform infrared spectrometer. UV–visible spectrophotometer (Perkin-Elmer AA800 Model AAS) was employed for absorbance measurements of samples. An Orion 900S2 model digital pH meter and a Gallenkamp Orbital Incubator were used for pH adjustment and shaking, respectively.
Scheme 1. Schematic representation of the polymerization of acrylonitrile.
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100
100
80 2244
80
%T
1575
Removal (%)
2013
2921
1454
3399
60
60 0.2 g/L
40
0.3 g/L 0.4 g/L 0.5 g/L
20 40
0.6 g/L
0 0
1035
10
20
30
40
50
60
70
80
90 100 110 120 130
Time (min.) 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber Cm-1
Fig. 4. Effect of composite dosage on the adsorption of AY99 onto (PAN/AC) composite at dye concentration 60 mg/L, pH 1 and 25 °C.
Fig. 2. FTIR spectrum of (PAN/AC) composite.
3. Results and discussion 3.1. Effect of adsorbate concentrations The removal of dye by adsorption on the adsorbent (PAN/AC) composite was shown to increase with time and attained a maximum value at about 90 min, and thereafter, it remained almost constant (Fig. 3). On changing the initial concentration of dye solution from 40 to 150 mg/L at 25 °C, pH 1 and 0.4 g/L adsorbent dosage the amount of removed dyes was decreased. It was clear that the removal of the dye was dependent on the initial concentration of the dye because the decrease in the initial dye concentration increased the amount of dye adsorbed. This is very clear because, for a fixed adsorbent dose, the number of active adsorption sites to accommodate adsorbate ions remains unchanged but with increasing adsorbate concentration, the adsorbate ions to be accommodated increase and hence the percentage of adsorption goes down.
dosage shows that the uptake of dye per gram of adsorbent increases with increasing adsorbent dosage from 0.2 to 0.6 g/L. This is because a higher dose of adsorbent led to increased surface area and more adsorption sites are available causing higher removal of the dyes. Further increase in adsorbent dose did not cause any significant increase in % removal of dyes. This was due to the concentration of dyes reached at equilibrium status between solid and solution phase. 3.3. Effect of temperature Temperature dependence of the adsorption process is associated with several thermodynamic parameters. The plot of amount of adsorbate per amount of adsorbent of adsorption as a function of temperature (Fig. 5) shows a small increasing trend with rise in temperature from 25 to about 50 °C. Equilibrium capacity can be changed by temperature of the adsorbent for a particular adsorbate. In our case the experimental data obtained at pH 1, adsorbent dosage of 0.2 g/L, and initial concentration of 60 mg/L show no change in the adsorption capacity at temperatures from 25 to 50 °C.
3.2. Effect of adsorbent dosage
3.4. Effect of pH
The uptake of dye with change in adsorbent dosage (0.2–0.6 g/L) at adsorbate concentrations of 60 mg/L at 25 °C and pH 1 is presented in Fig. 4. Adsorption of dyes as a function of the (PAN/AC) composite
The removal of AY99 by prepared (PAN/AC) composite at different pH values was studied at initial concentrations of 60 mg/L of AY99, 25 °C and 0.4 g/L adsorbent dosage. The pH value of the solution was an important controlling parameter in the adsorption process. PAN/AC composite has been proven to be an effective adsorbent for the removal of acid dye, AY99, via adsorption from aqueous solution at pH 1 (Fig. 6). It shows that the adsorption capacity of acid dye AY99 onto acid activated (PAN/ AC) composite increases significantly with decreasing pH. The maximum removal of AY99 for contact time 90 min was carried out at pH 1. As can be seen from Scheme 2, at strongly acidic pHs, a significantly high electrostatic attraction exists between the positively charged surface of the adsorbent and anionic dye [22]. As the pH of the adsorption system increases, the number of negatively charged sites increases and the number of positively charged sites decreases. A negatively charged surface site on the adsorbent
100 95
Removal (%)
90 85 80
40 mg/L 75
70 mg/L 120 mg/L
70
150 mg/L
65 15
30
45
60
75
90
105
120
Time (min.) Fig. 3. Effect of initial dye concentration on adsorption of AY99 onto (PAN/AC) composite dosage 0.4 g/L, pH 1 and temperature 25 °C.
Scheme 2. Proposed Electrostatic attraction between AY99 and PAN/AC composite.
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239
100 1.0
0.8
25 OC
30 OC 80
40 OC
Ce /qe
Removal (%)
90
25 oC 30 oC 40 oC 50 oC
0.6
O
50 C 0.4
70 0.2
60 15
30
45
60
75
90 1.5
Time (min.)
3.0
4.5
6.0
Ce
Fig. 5. Effect of temperature on adsorption of AY99 onto (PAN/AC) composite at composite dosage 0.4 g/L, dye conc. 60 mg/L and pH 1.
does not favor the adsorption of dye anions, due to the electrostatic repulsion. Also, lower adsorption of AY99 at alkaline pH is due to the presence of excess hydroxyl ions competing with the dye anions for the adsorption sites [23,24]. 3.5. Adsorption isotherms The main factors that play the key role for the dye–adsorbent interactions are charge and structure of dye, adsorbent surface properties, hydrophobic and hydrophilic nature, hydrogen bonding, electrostatic interaction, steric effect, and van der Waal forces [25]. Equilibrium studies that give the capacity of the adsorbent and adsorbate are described by adsorption isotherms, which are usually the ratio between the quantity adsorbed and that remained in solution at equilibrium at fixed temperature [26–28]. The equilibrium experimental data for adsorbed AY99 on (PAN/AC) composite was compared using two isotherm equations namely, Langmuir and Freundlich. 3.5.1. Langmuir isotherm The Langmuir adsorption, which is the monolayer adsorption, depends on the assumption that the intermolecular forces decrease rapidly with distance and consequently predicts the existence of monolayer coverage of the adsorbate at the outer surface of the adsorbent. The isotherm equation further assumes that adsorption occurs at
Fig. 7. Langmuir plots for adsorption of AY99 onto (PAN/AC) composite.
specific homogeneous sites within the adsorbent. It then assumed that once a dye molecule occupies a site, no further adsorption can take place at that site. Furthermore, the Langmuir equation is based on the assumption of a structurally homogeneous adsorbent, where all sorption sites are identical and energetically equivalent. Theoretically, the sorbent has a finite capacity for the sorbate. Therefore, a saturation value is reached beyond which no further sorption can occur. The saturated or monolayer capacity can be represented as the known linear form of Langmuir equation [29–33], Ce =qe ¼ 1=ðqmax KL Þ þ Ce =qmax
ð1Þ
where Ce is the equilibrium dye concentration in solution (mol L−1), qe is the equilibrium dye concentration in the adsorbent (mol g−1), qmax is the monolayer capacity of the adsorbent (mol g−1) and KL is the Langmuir adsorption constant (L mol−1). Therefore, a plot of C e/q e vs. C e (Fig. 7) gives a straight line of slope 1 / qmax and the intercept 1 / (qmaxKL). The Langmuir equation is applicable to homogeneous sorption, where the sorption of each sorbate molecule onto the surface is equal to sorption activation energy. 3.5.2. Freundlich isotherm The Freundlich equation [32–34] is an empirical equation employed to describe heterogeneous systems, characterized by the heterogeneity factor 1 / n, describes reversible adsorption and is not restricted to the
100 0.86
25 oC 30 oC 40 oC 50 oC
60
0.84
log Qe
Removal (%)
80
40
pH 1 pH 3 20
0.82
pH 5 pH 7 pH 9
0 0
15
30
45
60
75
90
Time (min.) Fig. 6. Effect of pH on the adsorption of AY99 onto (PAN/AC) composite at composite dosage 0.4 g/L, dye concentration 60 mg/L and temperature 25 °C.
0.80 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
log Ce Fig. 8. Freundlich plots for adsorption of AY99 onto (PAN/AC) composite.
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Table 1 Langmuir and Freundlich parameters for the adsorption of AY99 dye onto (PAN/AC) composite. Langmuir isotherm qmax (mol g−1)
KL (L mol−1)
r2L
KF (L g−1)
Freundlich isotherm n
r2F
25 30 40 50
27.25 108.70 204.08 217.39
0.2277 0.0614 0.0334 0.0314
0.996 0.992 0.994 0.993
0.925 0.948 0.948 0.864
1.177 1.179 1.179 1.163
0.998 0.995 0.996 0.999
t / qt (min g mg-1)
Temperature (°C)
0.6
0.4 25oC
30oC 40oC 50oC
0.2
formation of the monolayer: qe ¼ K F :Ce
1=n
ð2Þ
10
where qe is the equilibrium dye concentration on adsorbent (mol g−1), Ce is the equilibrium dye concentration in solution (mol L−1), KF is Freundlich constant (L g−1) and 1 / n is the heterogeneity factor. A linear form of the Freundlich expression can be obtained by taking logarithms of the equation logqe ¼ log K F þ 1=n: log Ce :
ð3Þ
Therefore, a plot of log qe vs. log Ce for the adsorption of AY99 onto (PAN/AC) composite (Fig. 8) was employed to generate the intercept value of KF and the slope of 1 / n. The Langmuir and Freundlich parameters for the adsorption of AR57 are listed in Table 1. It is evident from these data that the surface of (PAN/ AC) composite is mostly made up of heterogeneous adsorption patches. The correlation coefficients for Langmuir (r2L ) and for Freundlich (r2F ) values are compared in Table 1. One of the Freundlich constants KF indicates the adsorption capacity of the adsorbent. The other Freundlich constant n is a measure of the deviation from linearity of the adsorption. If a value for n is equal to unity the adsorption is linear. If a value for n is below unity, this implies that adsorption process is chemical, but a value for n is above unity, adsorption is favorable a physical process [35]. The highest value of n at equilibrium is 1.179 (Table 1), represents favorable adsorption and therefore this would seem to suggest that the adsorption is physical, which is referred the adsorption bond becomes weak [36] and conducted with van der Waals forces rather than chemical adsorption. 3.6. Adsorption kinetic studies The study of adsorption kinetics describes the solute uptake rate and evidently this rate controls the residence time of adsorbate uptake at 25oC 30oC 40oC 50oC
2.25 2.00 1.75 1.50 1.25
log (qe-qt)
0.0
1.00
20
30
40
50
60
70
80
90
100
t (min.) Fig. 10. Pseudo-second-order kinetic plot for the adsorption of AY99 onto (PAN/AC) composite at different temperatures.
the solid–solution interface. The rate of removal of AY99 by adsorption was rapid initially and then slowed gradually until it attained an equilibrium beyond which there was significant increase in the rate of removal. The maximum adsorption of AY99 onto (PAN/AC) composite was observed at 90 min and it is thus fixed as the equilibrium time. Aiming at evaluating the adsorption kinetics of AY99 onto (PAN/ AC) composite, the pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental data, according to the below kinetic model equations. The pseudo-first-order rate expression of Lagergren [37,38] is given as: logðqe –qt Þ ¼ log qe –k1 t:
ð4Þ
The pseudo-second-order kinetic model [38] is expressed as: 2
t=qt ¼ 1=k2 q2 þ 1=q2 t
ð5Þ
where qt is the amount of dye adsorbed (mol g−1) at various times t, qe is the maximum adsorption capacity (mol g−1) for pseudo-first-order adsorption, k1 is the pseudo-first-order rate constant for the adsorption process (min−1), q2 is the maximum adsorption capacity (mol g−1) for the pseudo-second-order adsorption, k2 is the rate constant of pseudosecond-order adsorption (g mol−1 min−1). The straight-line plots of log (qe − qt) versus t for the pseudo-first-order reaction and t/qt versus t for the pseudo-second-order reaction (Figs. 9 and 10) for adsorption of AY99 onto (PAN/AC) composite have also been tested to obtain the rate parameters. The k1, k2, qe, q2, and correlation coefficients, r21 and r22 for AY99 under different temperatures were calculated from these plots and are given in Table 2. The correlation coefficients (r21) for the pseudo-first-order kinetic model are between 0.868 and 0.996 and the correlation coefficient (r22) for the pseudo-second-order kinetic model is 0.999. It is probable, therefore, that this adsorption system is not a pseudo-first-order reaction; it fits the pseudo-second-order kinetic model [21].
0.75 0.50 Table 2 Pseudo-first-order and pseudo-second-order for the adsorption of AY99 dye onto (PAN/AC) composite.
0.25 0.00 -0.25
Temperature (°C)
-0.50 -0.75 10
20
30
40
50
60
70
t (min.) Fig. 9. Pseudo-first-order kinetic plot for the adsorption of AY99 onto (PAN/AC) composite at different temperatures.
25 30 40 50
Pseudo-first-order
Pseudo-second-order
qe (mol g−1)
k1 (min−1)
r21
q2 (mol g−1)
k2 (g mol−1 min−1)
0.947 0.923 0.949 0.943
4.176 4.139 3.210 3.238
0.996 0.981 0.868 0.986
149.25 145.99 145.14 144.30
0.191 0.370 0.568 0.646
r22
0.999 0.999 0.999 0.999
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241
0.0035
0.0036 3.7. Thermodynamic parameters
0.0034
0.0035
0.0033
1/T (K-1)
1/T (K-1)
0.0034
0.0033
0.0032 0.0031
0.0032 0.0030 0.0031
0.0029
0.0030 -2.0
-1.5
-1.0
0.0028 4.2
-0.5
4.4
ln k2
4.6
4.8
5.0
5.2
5.4
5.6
ln kC
Fig. 11. Arrhenius plot for the adsorption of AY99 onto (PAN/AC) composite.
Fig. 12. van't Hoff plots for determination of thermodynamic parameters for the adsorption of AY99 onto (PAN/AC) composite.
In any adsorption process, both energy and entropy considerations must be taken into account in order to determine what process will occur spontaneously. Values of thermodynamic parameters are the actual indicators for practical application of a process. The amount of AY99 adsorbed onto (PAN/AC) composite at equilibrium and at different temperatures (25, 30, 40, 50 °C) has been examined to obtain thermodynamic parameters for the adsorption system. The pseudo-secondorder rate constant of AY99 adsorption is expressed as a function of temperature by the following Arrhenius type relationship [39]: lnk2 ¼ lnA−Ea =RT
ð6Þ
where Ea is the Arrhenius activation energy of adsorption, A is the Arrhenius factor, R is the gas constant and is equal to 8.314 J mol−1 K−1 and T is the operating temperature. A linear plot of ln k2 vs 1 / T for the adsorption of AY99 onto (PAN/AC) composite (Fig. 11) was constructed to generate the activation energy from the slope (− Ea / R). The chemical (chemisorption) or physical (physisorption) adsorption mechanism is often an important indicator to describe the type of interactions between AY99 and (PAN/AC) composite. The magnitude of activation energy gives an idea about the type of adsorption which is mainly physical or chemical. Low activation energies (5–40 kJ mol−1) are characteristics for physisorption, while higher activation energies (40–800 kJ mol−1) suggest chemisorptions [40]. The result obtained is +18.09 kJ mol−1 (Table 3) for the adsorption of AY99 onto (PAN/AC) composite, indicating that the adsorption has a low potential barrier and corresponds to a physisorption. The other thermodynamic parameters, change in the standard free energy (ΔGo), enthalpy (ΔHo) and entropy (ΔSo) were determined by using following equations: KC ¼ CA =CS
o
o
ln KC ¼ ΔS =R–ΔH =RT
where KC is the equilibrium constant, CA is the amount of AY99 adsorbed on the (PAN/AC) composite of the solution at equilibrium (mol L−1), CS is the equilibrium concentration of the AY99 in the solution (mol L− 1). The q2 of the pseudo-second-order model in Table 3 was used to obtain CA and CS. T is the solution temperature (K) and R is the gas constant. ΔHo and ΔSo were calculated from the slope and the intercept of van't Hoff plots of ln KC vs 1 / T (Fig. 12). The results are given in Table 3. The values of adsorption thermodynamic parameters are listed in Table 3. The negative value of the change of free energy (ΔGo) confirms the feasibility of the adsorption process and also indicates spontaneous adsorption of AY99 onto (PAN/AC) composite in the temperature range studied [41]. The small negative value of the standard enthalpy change (ΔHo) (−19.37 kJ mol−1) indicates that the adsorption is physical in nature involving weak forces of attraction and is also exothermic, thereby demonstrating that the process is stable energetically. At the same time, the low value of ΔHo implies that there was loose bonding between the adsorbate molecules and the adsorbent surface [42,43]. The positive value of standard entropy change (ΔSo) (0.037 J mol−1 K−1) suggests the increased randomness at the solid–solution interface during the adsorption of AY99 onto (PAN/AC) composite [44].
ð7Þ
o
ΔG ¼ –RT lnKC
ð8Þ
Table 3 Thermodynamic parameters calculated with the pseudo-second rate constant for AY99 dye onto (PAN/AC) composite. Temperature (°C)
KC
Ea (kJ mol−1)
ΔGo (kJ mol−1)
ΔHo (kJ mol−1)
ΔSo (J mol−1 K−1)
25 30 40 50
87.34 147.24 222.11 202.86
18.09
−30.27 −30.46 −30.82 −31.19
−19.37
0.037
ð9Þ
Fig. 13. (PAN/AC) composite before adsorption of AY99 dye.
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the equilibrium data fit well with the Freundlich model of adsorption for AY99 dye. The highest value of n at equilibrium (1.179) suggests that the adsorption is physical. The kinetic data tends to fit very well in the pseudo-second-order kinetic model with high correlation coefficients. The ΔGo values were negative, therefore the adsorption was spontaneous in nature. The negative value of ΔHo reveals that the adsorption process was exothermic in nature and a physical adsorption. The positive value of ΔSo implies the increment of an orderliness between the adsorbate and the adsorbent molecules. SEM images show well defined and characterized morphological images that are evident for the effective adsorption of AY99 molecules on the cavities and pores of the (PAC/AC) composite. Desorption studies were conducted and the results showed that (PAN/AC) composite can be used in adsorption of acid dyes several times by regeneration process using sodium hydroxide solution at pH around 12.
References Fig. 14. (PAN/AC) composite after adsorption of AY99 dye.
3.8. SEM analysis Scanning electron microscopy (SEM) has been a primary tool for characterizing the surface morphology and fundamental physical properties of the adsorbent surface. It is useful for determining the particle shape, porosity and appropriate size distribution of the adsorbent. Scanning electron micrographs of raw (PAN/AC) composite and adsorbed (PAN/AC) composite with AY99 dye are shown in Figs. 13 and 14, respectively. From Fig. 13, it is clear that, raw (PAN/AC) composite has considerable numbers of pores where there is a good possibility for dyes to be trapped and adsorbed into these pores. The SEM picture (Fig. 14) of (PAN/AC) composite adsorbed with AY99 show very distinguished dark spots which can be taken as a sign for effective adsorption of AY99 molecules in the cavities and pores of this adsorbent.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
3.9. Desorption studies Desorption studies help to elucidate the mechanism and recovery of the adsorbate and adsorbent. (PAN/AC) composite was washed three times with sodium hydroxide solution at pH around 12 then filtered and left to be dried at 50 °C in an oven overnight and stored on desiccator prior to reuse in the adsorption again. As the pH of desorbing solution was increased, the percent of desorption increased. As the pH of the system increases, the number of negatively charged sites increased. A negatively charged site on the adsorbent favors the desorption of dye anions due to the electrostatic repulsion [45,46]. At pH 12, a significantly high electrostatic repulsion exists between the negatively charged surface of the adsorbent and anionic dye. The removal of dye by adsorption on the adsorbent (PAN/AC) was compared before and after recovering process at the same conditions: initial concentration of dye solution 60 mg/L at about 25 °C, pH 1 and 0.4 g/L adsorbent dosage. The maximum adsorption of AY99 dye onto (PAN/AC) composite before the recovering process was 98.10%, while after recovering process was 97.50%. 4. Conclusion The present study clearly demonstrated that (PAC/AC) composite is an effective adsorbent for the removal of AY99 dye from aqueous solution and polluted water. The high adsorption capacity of AY99 onto (PAC/AC) composite in highly acidic solutions (pH 1) is due to the strong electrostatic interactions between its adsorption site and dye anion. Adsorption parameters for Langmuir and Freundlich isotherms were determined and
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46]
F. Guzel, H. Saygih, G.A. Saygili, F. Koyuncu, J. Mol. Liq. 194 (2014) 130–140. P.C.C. Faria, J.J.M. Orfao, M.F.R. Pereira, Water Res. 38 (2014) 2043–2052. Q. Qin, J. Ma, K. Liu, J. Hazard. Mater. 162 (2009) 133–139. I.D. Mall, V.C. Srivastava, N.K. Agarwal, Dyes Pigments 69 (2006) 210–223. V.K. Gupta, Suhas, J. Environ. Manag. 90 (2009) 2313–2342. B. Shi, G. Li, D. Wang, C. Feng, J. Hazard. Mater. 143 (2007) 567–574. S. Song, J. Fan, Z. He, L. Zhan, Z. Liu, J. Chen, X. Xu, Electrochim. Acta 55 (2010) 3606–3613. T.H. Kim, C. Park, J. Yang, S. Kim, J. Hazard. Mater. 112 (2004) 95–103. H. Selcuk, Dyes Pigments 64 (2005) 217–222. G.M. Nisola, E. Cho, A.B. Beltran, M. Han, Y. Kim, W.-J. Chung, Chemosphere 80 (8) (2010) 894–900. D. Suna, X. Zhang, Y. Wu, X. Liu, J. Hazard. Mater. 181 (2010) 335–342. F. Xia, E. Ou, L. Wang, Dyes Pigments 76 (2008) 76–81. K. Ravikumar, B. Deebika, K. Balu, J. Hazard. Mater. 122 (2005) 75–83. G. Vijayakumar, R. Tamilarasan, M. Dharmendirakumar, J. Mater. Environ. Sci. 3 (1) (2012) 157–170. P. Faria, J. Orfao, M. Pereira, Water Res. 38 (2004) 2043–2052. T.D. Dartins, D. Schimmel, J.B.O. dos Santos, E.A. da Silva, J. Chem. Eng. Data 58 (2013) 106–114. Y. Wang, Y.-F. He, L. Zhang, R. Li, L. Wang, R.-M. Wang, Adv. Mater. Res. 773 (2013) 472–476. B. Pan, B. Pan, W. Zhang, L. Lv, Q. Zhang, S. Zheng, Chem. Eng. J. 151 (2009) 19–29. M. Kumar, R. Tamilarasan, Carbohydr. Polym. 2171 (2013) 2171–2180. M. Kumar, R. Tamilarasan, V. Sivakumar, Carbohydr. Polym. 98 (2013) 505–513. A.A. El-Bindary, M.A. Diab, M.A. Hussien, A.Z. El-Sonbati, A.M. Eessa, Spectrochim. Acta A 124 (2014) 70–77. M. Ozacar, I.A. Sengil, J. Hazard. Mater. B98 (2003) 211–224. M. Alkan, O. Demirbas, S. Celikcapa, M. Dogan, J. Hazard. Mater. B116 (2004) 135–145. M.R. Khan, M. Ray, Bioresour. Technol. 102 (2011) 2394–2399. R. Ahmad, R. Kumar, J. Environ. Manag 91 (2010) 1032–1038. H.R. Guendy, J. Appl. Sci. Res. 6 (8) (2010) 964–972. I. Kiran, T. Akar, A.S. Ozcan, A. Ozcan, S. Tunali, Biochem. Eng. J. 31 (2006) 197–203. M. Yavuz, F. Gode, E. Pehlivan, S. Ozmert, Y.C. Sharma, Chem. Eng. J. 137 (2008) 453–461. I. Langmuir, J. Am. Chem. Soc. 38 (1916) 31–60. I. Langmuir, J. Am. Chem. Soc. 39 (1917) 1848–1906. I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361–1403. A.R. Alley, Water Quality Control Handbook, McGraw-Hill Education, Europe, London, 2000. 125–141. F. Woodard, Industrial Waste Treatment Handbook, Butterworth-Heinemann, Boston, 2001. 376–451. H.M.F. Freundlich, Z. Phys. Chem. (Leipzig) 57A (1906) 385–470. S. Tunali, A.S. Ozcan, A. Ozcan, J. Hazard. Mater. B135 (2006) 141–148. J.-Q. Jiang, C. Cooper, S. Ouki, Chemosphere 47 (2002) 711–716. S. Lagergren, Kungliga Svenska Vetenskapsakademiens Handlingar, 24, 4, 1898, pp. 1–39. Y.S. Ho, G. McKay, Chem. Eng. J. 70 (2) (1998) 115–124. R.S. Juang, F.C. Wu, R.L. Tseng, Environ. Technol. 18 (5) (1997) 525–531. H. Nollet, M. Roels, P. Lutgen, P. Van der Meeren, W. Verstraete, Chemosphere 53 (6) (2003) 655–665. M.J. Jaycock, G.D. Parfitt, Chemistry of Interfaces, Ellis Horwood Ltd., Chichester, 1981. D. Singh, Adsorpt. Sci. Technol. 18 (8) (2000) 741–748. N.K. Goel, V. Kumar, S. Pahan, Y.K. Bhardwaj, S. Sabharwal, J. Hazard. Mater. 193 (2011) 17–26. P. Sharma, M.R. Das, J. Chem. Eng. Data 58 (2013) 151–158. C. Namasivayam, D. Kavitha, Dyes Pigments 54 (2002) 47–58. F. Yuzhu, T. Viraraghavan, Adv. Environ. Res. 7 (2002) 239–247.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Adsorption of Acid Yellow 99 by polyacrylonitrile/activated carbon composite: Kinetics, thermodynamics and isotherm studies Ashraf A. El-Bindary a,⁎, Mostafa A. Hussien b, Mostafa A. Diab a, Ahmed M. Eessa b a b
Department of Chemistry, Faculty of Science, University of Damietta, Damietta 34517, Egypt Department of Chemistry, Faculty of Science, University of Port Said, Port Said, Egypt
a r t i c l e
i n f o
Article history: Received 18 March 2014 Accepted 2 May 2014 Available online 16 May 2014 Keywords: Adsorption Polyacrylonitrile/activated carbon Acid Yellow 99 Kinetic Isotherm
a b s t r a c t The adsorption of Acid Yellow 99 (AY99) onto polyacrylonitrile/activated carbon (PAN/AC) composite was investigated in aqueous solution in a batch system with respect to contact time, pH and temperature. Experimental data indicated that the adsorption capacity of (PAN/AC) composite for AY99 was higher in acidic rather than in basic solutions. Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms and the isotherm constants were determined. The activation energy of adsorption was also evaluated for the adsorption of AY99 onto (PAN/AC) composite. The pseudo-first-order and pseudo-second-order model equations were used to analyze the kinetics of the adsorption process. The dynamic data fitted the pseudo-second-order kinetic model well. The activation energy, change of free energy, enthalpy and entropy of adsorption were also evaluated for the adsorption of AY99 onto (PAN/AC) composite. The thermodynamics of the adsorption indicated spontaneous and exothermic nature of the process. The (PAN/AC) composite was characterized using Fourier transform infrared spectroscopy (FTIR) and its morphology was determined by scanning electron microscopy (SEM). The results indicate that (PAN/AC) composite could be employed as low-cost material for the removal of acid dyes from textile effluents. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The industrial wastewater usually contains a variety of organic compounds and toxic pulp mills and dyestuff manufacturing discharge highly colored wastewater which have provoked serious environmental concerns all over the world [1–3]. Dyes can be classified as anionic (direct, acid and reactive dyes), cationic (basic dyes) and non-ionic (disperse dyes) [4]. The removal of color from waste textile effluents has become environmentally important [5]. The most widely used methods for removing color effluents from water include coagulation [6], electro-chemical [7], oxidation [8], ozonation [9], solvent extraction [10], adsorption [5,11], photocatalytic degradation [12], etc. but only that of adsorption is considered to be superior to other techniques. This is attributed to its low cost, easy availability of adsorbents, simplicity of design, high efficiency, ease of operation and biodegradability [13]. Consequently, kinetic studies, which provide information for the rate of removal of pollutants from solution and the adsorption equilibrium data are essential for the design of water treatment units involving an adsorption process. The basic feature of an adsorption process is surface accumulation from intermolecular penetration of material. It is now customary to
⁎ Corresponding author. Tel.: +20 1114266996; fax: +20 572403868. E-mail address: [email protected] (A.A. El-Bindary).
http://dx.doi.org/10.1016/j.molliq.2014.05.003 0167-7322/© 2014 Elsevier B.V. All rights reserved.
differentiate between two types of adsorptions. If the attraction between the solid surface and the adsorbed molecules is physical in nature, the adsorption is referred to as physical adsorption (physisorption). Generally, in physical adsorption the attractive forces between adsorbed molecules and the solid surface are van der Waals forces and they being weak in nature result in reversible adsorption. On the other hand if the attraction forces are due to chemical bonding, the adsorption process is called chemisorption. In view of the higher strength of the bonding in chemisorption, it is difficult to remove chemisorbed species from the solid surface. Activated carbon is perhaps the most widely used adsorbent in the adsorption processes due to its high specific surface area and high adsorption capacity [14–16]. In order to decrease the cost of wastewater treatment, attempts have been made in finding inexpensive adsorbents. So, the research of the recent years mainly focused on utilizing economic materials as alternatives to activated carbon; amongst them, the composite materials. A composite is a material that consists of two or more constituent materials or phases. Polymers with activated carbon are being considered as alternative low-cost adsorbents due to their specific surface area and high chemical and mechanical stability [17–20]. The aim of this study is to investigate the adsorption of Acid Yellow 99 onto polyacrylonitrile/activated carbon (PAN/AC) composite as a low cost adsorbent. Effects of different parameters such as contact time, pH, dye concentration, temperature, adsorbent concentration on both equilibrium and the rate of adsorption were studied. The kinetics, isotherms
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237
2.3. Preparation of polyacrylonitrile/activated carbon (PAN/AC) composite
Fig. 1. Structure of AY99 dye.
and thermodynamic parameters were also calculated to determine the rate constants and adsorption mechanism.
Polyacrylonitrile/activated carbon (PAN/AC) composite was prepared [21] by adding 30 g of acrylonitrile (monomer) to 20 mL of bidistilled water in a 250 mL three neck round bottom flask, then adding 0.1 g of potassium persulfate and 10 g of activated carbon to the mixture. The mixture was stirred by a magnetic stirrer and the reaction was allowed to proceed at 60–70 °C for about 24 h until gel formation. The precipitate was filtered and washed with water, ethanol, 0.1 mol L −1 HCl solution and water, respectively. The final product (composite) was left to be dried under vacuum at 50 °C for 24 h and stored on desiccator prior to use in the sorption study. The polymerization reaction is given in Scheme 1. FTIR spectrum of (PAN/AC) composite was recorded in the region 400–4000 cm−1 (Fig. 2). The band at 2921 cm− 1 corresponds to the asymmetric stretching vibration of methylene group (νCH2) and its bending vibration at 1454 cm−1. The band at 2244 cm−1 corresponds to the CN group.
2. Materials and methods 2.1. Materials
2.4. Adsorption experiments
A commercial textile dye Acid Yellow 99 (Fig. 1) was obtained from Cromatos SRL, a dye company located in Italy and was used as received without further purification. Activated carbon (particle diameter 300–500 μm) was purchased from Calgon Company, USA. Activated carbon was washed several times with bidistilled water and then dried at 120 °C for 24 h. Samples were then preserved in the desiccator over anhydrous CaCl2 for further use.
The adsorption experiments of anionic dye AY99 were carried out in batch equilibrium mode. A 0.2–0.6 g sample of (PAN/AC) composite with 50 mL aqueous solution of a 40–150 mg/L AY99 solution at various pHs (1–9) reached for 90 min was adjusted by adding a small amount of HCl or NaOH solution (1 M) using a pH meter. The optimum pH was determined and used through all adsorption processes. Experiments were conducted for various time intervals to determine whether adsorption equilibrium was reached and the maximum removal of AR57 was attained. The solution was then filtered through a Whatman (number 40) filter paper to remove any organic or inorganic precipitates formed under acidic or basic conditions and the filtrates were subjected to quantitative analyses. The equilibrium concentration of each solution was determined at the wavelength of UV-maximum (λmax) at 418 nm. Dye adsorption experiments were also accomplished to obtain isotherms at various temperatures (25–50 °C) and at a range of 40–150 mg/L dye concentrations for 90 min by using a water bath with shaker. Calibration curves were constructed to correlate concentrations to different absorbance values. Construction of this calibration curves was verified and the maximum wavelengths that corresponded to maximum absorbance for the dye were determined.
2.2. Material characterization The SEM results of the PAN/AC composite sample before and after the adsorption processes were obtained using (JEOL 5600LV) scanning microscope to observe surface modification. FTIR spectrum of PAN/AC composite sample was recorded (KBr) on a Perkin-Elmer BX Model Fourier transform infrared spectrometer. UV–visible spectrophotometer (Perkin-Elmer AA800 Model AAS) was employed for absorbance measurements of samples. An Orion 900S2 model digital pH meter and a Gallenkamp Orbital Incubator were used for pH adjustment and shaking, respectively.
Scheme 1. Schematic representation of the polymerization of acrylonitrile.
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100
100
80 2244
80
%T
1575
Removal (%)
2013
2921
1454
3399
60
60 0.2 g/L
40
0.3 g/L 0.4 g/L 0.5 g/L
20 40
0.6 g/L
0 0
1035
10
20
30
40
50
60
70
80
90 100 110 120 130
Time (min.) 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber Cm-1
Fig. 4. Effect of composite dosage on the adsorption of AY99 onto (PAN/AC) composite at dye concentration 60 mg/L, pH 1 and 25 °C.
Fig. 2. FTIR spectrum of (PAN/AC) composite.
3. Results and discussion 3.1. Effect of adsorbate concentrations The removal of dye by adsorption on the adsorbent (PAN/AC) composite was shown to increase with time and attained a maximum value at about 90 min, and thereafter, it remained almost constant (Fig. 3). On changing the initial concentration of dye solution from 40 to 150 mg/L at 25 °C, pH 1 and 0.4 g/L adsorbent dosage the amount of removed dyes was decreased. It was clear that the removal of the dye was dependent on the initial concentration of the dye because the decrease in the initial dye concentration increased the amount of dye adsorbed. This is very clear because, for a fixed adsorbent dose, the number of active adsorption sites to accommodate adsorbate ions remains unchanged but with increasing adsorbate concentration, the adsorbate ions to be accommodated increase and hence the percentage of adsorption goes down.
dosage shows that the uptake of dye per gram of adsorbent increases with increasing adsorbent dosage from 0.2 to 0.6 g/L. This is because a higher dose of adsorbent led to increased surface area and more adsorption sites are available causing higher removal of the dyes. Further increase in adsorbent dose did not cause any significant increase in % removal of dyes. This was due to the concentration of dyes reached at equilibrium status between solid and solution phase. 3.3. Effect of temperature Temperature dependence of the adsorption process is associated with several thermodynamic parameters. The plot of amount of adsorbate per amount of adsorbent of adsorption as a function of temperature (Fig. 5) shows a small increasing trend with rise in temperature from 25 to about 50 °C. Equilibrium capacity can be changed by temperature of the adsorbent for a particular adsorbate. In our case the experimental data obtained at pH 1, adsorbent dosage of 0.2 g/L, and initial concentration of 60 mg/L show no change in the adsorption capacity at temperatures from 25 to 50 °C.
3.2. Effect of adsorbent dosage
3.4. Effect of pH
The uptake of dye with change in adsorbent dosage (0.2–0.6 g/L) at adsorbate concentrations of 60 mg/L at 25 °C and pH 1 is presented in Fig. 4. Adsorption of dyes as a function of the (PAN/AC) composite
The removal of AY99 by prepared (PAN/AC) composite at different pH values was studied at initial concentrations of 60 mg/L of AY99, 25 °C and 0.4 g/L adsorbent dosage. The pH value of the solution was an important controlling parameter in the adsorption process. PAN/AC composite has been proven to be an effective adsorbent for the removal of acid dye, AY99, via adsorption from aqueous solution at pH 1 (Fig. 6). It shows that the adsorption capacity of acid dye AY99 onto acid activated (PAN/ AC) composite increases significantly with decreasing pH. The maximum removal of AY99 for contact time 90 min was carried out at pH 1. As can be seen from Scheme 2, at strongly acidic pHs, a significantly high electrostatic attraction exists between the positively charged surface of the adsorbent and anionic dye [22]. As the pH of the adsorption system increases, the number of negatively charged sites increases and the number of positively charged sites decreases. A negatively charged surface site on the adsorbent
100 95
Removal (%)
90 85 80
40 mg/L 75
70 mg/L 120 mg/L
70
150 mg/L
65 15
30
45
60
75
90
105
120
Time (min.) Fig. 3. Effect of initial dye concentration on adsorption of AY99 onto (PAN/AC) composite dosage 0.4 g/L, pH 1 and temperature 25 °C.
Scheme 2. Proposed Electrostatic attraction between AY99 and PAN/AC composite.
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239
100 1.0
0.8
25 OC
30 OC 80
40 OC
Ce /qe
Removal (%)
90
25 oC 30 oC 40 oC 50 oC
0.6
O
50 C 0.4
70 0.2
60 15
30
45
60
75
90 1.5
Time (min.)
3.0
4.5
6.0
Ce
Fig. 5. Effect of temperature on adsorption of AY99 onto (PAN/AC) composite at composite dosage 0.4 g/L, dye conc. 60 mg/L and pH 1.
does not favor the adsorption of dye anions, due to the electrostatic repulsion. Also, lower adsorption of AY99 at alkaline pH is due to the presence of excess hydroxyl ions competing with the dye anions for the adsorption sites [23,24]. 3.5. Adsorption isotherms The main factors that play the key role for the dye–adsorbent interactions are charge and structure of dye, adsorbent surface properties, hydrophobic and hydrophilic nature, hydrogen bonding, electrostatic interaction, steric effect, and van der Waal forces [25]. Equilibrium studies that give the capacity of the adsorbent and adsorbate are described by adsorption isotherms, which are usually the ratio between the quantity adsorbed and that remained in solution at equilibrium at fixed temperature [26–28]. The equilibrium experimental data for adsorbed AY99 on (PAN/AC) composite was compared using two isotherm equations namely, Langmuir and Freundlich. 3.5.1. Langmuir isotherm The Langmuir adsorption, which is the monolayer adsorption, depends on the assumption that the intermolecular forces decrease rapidly with distance and consequently predicts the existence of monolayer coverage of the adsorbate at the outer surface of the adsorbent. The isotherm equation further assumes that adsorption occurs at
Fig. 7. Langmuir plots for adsorption of AY99 onto (PAN/AC) composite.
specific homogeneous sites within the adsorbent. It then assumed that once a dye molecule occupies a site, no further adsorption can take place at that site. Furthermore, the Langmuir equation is based on the assumption of a structurally homogeneous adsorbent, where all sorption sites are identical and energetically equivalent. Theoretically, the sorbent has a finite capacity for the sorbate. Therefore, a saturation value is reached beyond which no further sorption can occur. The saturated or monolayer capacity can be represented as the known linear form of Langmuir equation [29–33], Ce =qe ¼ 1=ðqmax KL Þ þ Ce =qmax
ð1Þ
where Ce is the equilibrium dye concentration in solution (mol L−1), qe is the equilibrium dye concentration in the adsorbent (mol g−1), qmax is the monolayer capacity of the adsorbent (mol g−1) and KL is the Langmuir adsorption constant (L mol−1). Therefore, a plot of C e/q e vs. C e (Fig. 7) gives a straight line of slope 1 / qmax and the intercept 1 / (qmaxKL). The Langmuir equation is applicable to homogeneous sorption, where the sorption of each sorbate molecule onto the surface is equal to sorption activation energy. 3.5.2. Freundlich isotherm The Freundlich equation [32–34] is an empirical equation employed to describe heterogeneous systems, characterized by the heterogeneity factor 1 / n, describes reversible adsorption and is not restricted to the
100 0.86
25 oC 30 oC 40 oC 50 oC
60
0.84
log Qe
Removal (%)
80
40
pH 1 pH 3 20
0.82
pH 5 pH 7 pH 9
0 0
15
30
45
60
75
90
Time (min.) Fig. 6. Effect of pH on the adsorption of AY99 onto (PAN/AC) composite at composite dosage 0.4 g/L, dye concentration 60 mg/L and temperature 25 °C.
0.80 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
log Ce Fig. 8. Freundlich plots for adsorption of AY99 onto (PAN/AC) composite.
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Table 1 Langmuir and Freundlich parameters for the adsorption of AY99 dye onto (PAN/AC) composite. Langmuir isotherm qmax (mol g−1)
KL (L mol−1)
r2L
KF (L g−1)
Freundlich isotherm n
r2F
25 30 40 50
27.25 108.70 204.08 217.39
0.2277 0.0614 0.0334 0.0314
0.996 0.992 0.994 0.993
0.925 0.948 0.948 0.864
1.177 1.179 1.179 1.163
0.998 0.995 0.996 0.999
t / qt (min g mg-1)
Temperature (°C)
0.6
0.4 25oC
30oC 40oC 50oC
0.2
formation of the monolayer: qe ¼ K F :Ce
1=n
ð2Þ
10
where qe is the equilibrium dye concentration on adsorbent (mol g−1), Ce is the equilibrium dye concentration in solution (mol L−1), KF is Freundlich constant (L g−1) and 1 / n is the heterogeneity factor. A linear form of the Freundlich expression can be obtained by taking logarithms of the equation logqe ¼ log K F þ 1=n: log Ce :
ð3Þ
Therefore, a plot of log qe vs. log Ce for the adsorption of AY99 onto (PAN/AC) composite (Fig. 8) was employed to generate the intercept value of KF and the slope of 1 / n. The Langmuir and Freundlich parameters for the adsorption of AR57 are listed in Table 1. It is evident from these data that the surface of (PAN/ AC) composite is mostly made up of heterogeneous adsorption patches. The correlation coefficients for Langmuir (r2L ) and for Freundlich (r2F ) values are compared in Table 1. One of the Freundlich constants KF indicates the adsorption capacity of the adsorbent. The other Freundlich constant n is a measure of the deviation from linearity of the adsorption. If a value for n is equal to unity the adsorption is linear. If a value for n is below unity, this implies that adsorption process is chemical, but a value for n is above unity, adsorption is favorable a physical process [35]. The highest value of n at equilibrium is 1.179 (Table 1), represents favorable adsorption and therefore this would seem to suggest that the adsorption is physical, which is referred the adsorption bond becomes weak [36] and conducted with van der Waals forces rather than chemical adsorption. 3.6. Adsorption kinetic studies The study of adsorption kinetics describes the solute uptake rate and evidently this rate controls the residence time of adsorbate uptake at 25oC 30oC 40oC 50oC
2.25 2.00 1.75 1.50 1.25
log (qe-qt)
0.0
1.00
20
30
40
50
60
70
80
90
100
t (min.) Fig. 10. Pseudo-second-order kinetic plot for the adsorption of AY99 onto (PAN/AC) composite at different temperatures.
the solid–solution interface. The rate of removal of AY99 by adsorption was rapid initially and then slowed gradually until it attained an equilibrium beyond which there was significant increase in the rate of removal. The maximum adsorption of AY99 onto (PAN/AC) composite was observed at 90 min and it is thus fixed as the equilibrium time. Aiming at evaluating the adsorption kinetics of AY99 onto (PAN/ AC) composite, the pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental data, according to the below kinetic model equations. The pseudo-first-order rate expression of Lagergren [37,38] is given as: logðqe –qt Þ ¼ log qe –k1 t:
ð4Þ
The pseudo-second-order kinetic model [38] is expressed as: 2
t=qt ¼ 1=k2 q2 þ 1=q2 t
ð5Þ
where qt is the amount of dye adsorbed (mol g−1) at various times t, qe is the maximum adsorption capacity (mol g−1) for pseudo-first-order adsorption, k1 is the pseudo-first-order rate constant for the adsorption process (min−1), q2 is the maximum adsorption capacity (mol g−1) for the pseudo-second-order adsorption, k2 is the rate constant of pseudosecond-order adsorption (g mol−1 min−1). The straight-line plots of log (qe − qt) versus t for the pseudo-first-order reaction and t/qt versus t for the pseudo-second-order reaction (Figs. 9 and 10) for adsorption of AY99 onto (PAN/AC) composite have also been tested to obtain the rate parameters. The k1, k2, qe, q2, and correlation coefficients, r21 and r22 for AY99 under different temperatures were calculated from these plots and are given in Table 2. The correlation coefficients (r21) for the pseudo-first-order kinetic model are between 0.868 and 0.996 and the correlation coefficient (r22) for the pseudo-second-order kinetic model is 0.999. It is probable, therefore, that this adsorption system is not a pseudo-first-order reaction; it fits the pseudo-second-order kinetic model [21].
0.75 0.50 Table 2 Pseudo-first-order and pseudo-second-order for the adsorption of AY99 dye onto (PAN/AC) composite.
0.25 0.00 -0.25
Temperature (°C)
-0.50 -0.75 10
20
30
40
50
60
70
t (min.) Fig. 9. Pseudo-first-order kinetic plot for the adsorption of AY99 onto (PAN/AC) composite at different temperatures.
25 30 40 50
Pseudo-first-order
Pseudo-second-order
qe (mol g−1)
k1 (min−1)
r21
q2 (mol g−1)
k2 (g mol−1 min−1)
0.947 0.923 0.949 0.943
4.176 4.139 3.210 3.238
0.996 0.981 0.868 0.986
149.25 145.99 145.14 144.30
0.191 0.370 0.568 0.646
r22
0.999 0.999 0.999 0.999
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241
0.0035
0.0036 3.7. Thermodynamic parameters
0.0034
0.0035
0.0033
1/T (K-1)
1/T (K-1)
0.0034
0.0033
0.0032 0.0031
0.0032 0.0030 0.0031
0.0029
0.0030 -2.0
-1.5
-1.0
0.0028 4.2
-0.5
4.4
ln k2
4.6
4.8
5.0
5.2
5.4
5.6
ln kC
Fig. 11. Arrhenius plot for the adsorption of AY99 onto (PAN/AC) composite.
Fig. 12. van't Hoff plots for determination of thermodynamic parameters for the adsorption of AY99 onto (PAN/AC) composite.
In any adsorption process, both energy and entropy considerations must be taken into account in order to determine what process will occur spontaneously. Values of thermodynamic parameters are the actual indicators for practical application of a process. The amount of AY99 adsorbed onto (PAN/AC) composite at equilibrium and at different temperatures (25, 30, 40, 50 °C) has been examined to obtain thermodynamic parameters for the adsorption system. The pseudo-secondorder rate constant of AY99 adsorption is expressed as a function of temperature by the following Arrhenius type relationship [39]: lnk2 ¼ lnA−Ea =RT
ð6Þ
where Ea is the Arrhenius activation energy of adsorption, A is the Arrhenius factor, R is the gas constant and is equal to 8.314 J mol−1 K−1 and T is the operating temperature. A linear plot of ln k2 vs 1 / T for the adsorption of AY99 onto (PAN/AC) composite (Fig. 11) was constructed to generate the activation energy from the slope (− Ea / R). The chemical (chemisorption) or physical (physisorption) adsorption mechanism is often an important indicator to describe the type of interactions between AY99 and (PAN/AC) composite. The magnitude of activation energy gives an idea about the type of adsorption which is mainly physical or chemical. Low activation energies (5–40 kJ mol−1) are characteristics for physisorption, while higher activation energies (40–800 kJ mol−1) suggest chemisorptions [40]. The result obtained is +18.09 kJ mol−1 (Table 3) for the adsorption of AY99 onto (PAN/AC) composite, indicating that the adsorption has a low potential barrier and corresponds to a physisorption. The other thermodynamic parameters, change in the standard free energy (ΔGo), enthalpy (ΔHo) and entropy (ΔSo) were determined by using following equations: KC ¼ CA =CS
o
o
ln KC ¼ ΔS =R–ΔH =RT
where KC is the equilibrium constant, CA is the amount of AY99 adsorbed on the (PAN/AC) composite of the solution at equilibrium (mol L−1), CS is the equilibrium concentration of the AY99 in the solution (mol L− 1). The q2 of the pseudo-second-order model in Table 3 was used to obtain CA and CS. T is the solution temperature (K) and R is the gas constant. ΔHo and ΔSo were calculated from the slope and the intercept of van't Hoff plots of ln KC vs 1 / T (Fig. 12). The results are given in Table 3. The values of adsorption thermodynamic parameters are listed in Table 3. The negative value of the change of free energy (ΔGo) confirms the feasibility of the adsorption process and also indicates spontaneous adsorption of AY99 onto (PAN/AC) composite in the temperature range studied [41]. The small negative value of the standard enthalpy change (ΔHo) (−19.37 kJ mol−1) indicates that the adsorption is physical in nature involving weak forces of attraction and is also exothermic, thereby demonstrating that the process is stable energetically. At the same time, the low value of ΔHo implies that there was loose bonding between the adsorbate molecules and the adsorbent surface [42,43]. The positive value of standard entropy change (ΔSo) (0.037 J mol−1 K−1) suggests the increased randomness at the solid–solution interface during the adsorption of AY99 onto (PAN/AC) composite [44].
ð7Þ
o
ΔG ¼ –RT lnKC
ð8Þ
Table 3 Thermodynamic parameters calculated with the pseudo-second rate constant for AY99 dye onto (PAN/AC) composite. Temperature (°C)
KC
Ea (kJ mol−1)
ΔGo (kJ mol−1)
ΔHo (kJ mol−1)
ΔSo (J mol−1 K−1)
25 30 40 50
87.34 147.24 222.11 202.86
18.09
−30.27 −30.46 −30.82 −31.19
−19.37
0.037
ð9Þ
Fig. 13. (PAN/AC) composite before adsorption of AY99 dye.
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the equilibrium data fit well with the Freundlich model of adsorption for AY99 dye. The highest value of n at equilibrium (1.179) suggests that the adsorption is physical. The kinetic data tends to fit very well in the pseudo-second-order kinetic model with high correlation coefficients. The ΔGo values were negative, therefore the adsorption was spontaneous in nature. The negative value of ΔHo reveals that the adsorption process was exothermic in nature and a physical adsorption. The positive value of ΔSo implies the increment of an orderliness between the adsorbate and the adsorbent molecules. SEM images show well defined and characterized morphological images that are evident for the effective adsorption of AY99 molecules on the cavities and pores of the (PAC/AC) composite. Desorption studies were conducted and the results showed that (PAN/AC) composite can be used in adsorption of acid dyes several times by regeneration process using sodium hydroxide solution at pH around 12.
References Fig. 14. (PAN/AC) composite after adsorption of AY99 dye.
3.8. SEM analysis Scanning electron microscopy (SEM) has been a primary tool for characterizing the surface morphology and fundamental physical properties of the adsorbent surface. It is useful for determining the particle shape, porosity and appropriate size distribution of the adsorbent. Scanning electron micrographs of raw (PAN/AC) composite and adsorbed (PAN/AC) composite with AY99 dye are shown in Figs. 13 and 14, respectively. From Fig. 13, it is clear that, raw (PAN/AC) composite has considerable numbers of pores where there is a good possibility for dyes to be trapped and adsorbed into these pores. The SEM picture (Fig. 14) of (PAN/AC) composite adsorbed with AY99 show very distinguished dark spots which can be taken as a sign for effective adsorption of AY99 molecules in the cavities and pores of this adsorbent.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
3.9. Desorption studies Desorption studies help to elucidate the mechanism and recovery of the adsorbate and adsorbent. (PAN/AC) composite was washed three times with sodium hydroxide solution at pH around 12 then filtered and left to be dried at 50 °C in an oven overnight and stored on desiccator prior to reuse in the adsorption again. As the pH of desorbing solution was increased, the percent of desorption increased. As the pH of the system increases, the number of negatively charged sites increased. A negatively charged site on the adsorbent favors the desorption of dye anions due to the electrostatic repulsion [45,46]. At pH 12, a significantly high electrostatic repulsion exists between the negatively charged surface of the adsorbent and anionic dye. The removal of dye by adsorption on the adsorbent (PAN/AC) was compared before and after recovering process at the same conditions: initial concentration of dye solution 60 mg/L at about 25 °C, pH 1 and 0.4 g/L adsorbent dosage. The maximum adsorption of AY99 dye onto (PAN/AC) composite before the recovering process was 98.10%, while after recovering process was 97.50%. 4. Conclusion The present study clearly demonstrated that (PAC/AC) composite is an effective adsorbent for the removal of AY99 dye from aqueous solution and polluted water. The high adsorption capacity of AY99 onto (PAC/AC) composite in highly acidic solutions (pH 1) is due to the strong electrostatic interactions between its adsorption site and dye anion. Adsorption parameters for Langmuir and Freundlich isotherms were determined and
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46]
F. Guzel, H. Saygih, G.A. Saygili, F. Koyuncu, J. Mol. Liq. 194 (2014) 130–140. P.C.C. Faria, J.J.M. Orfao, M.F.R. Pereira, Water Res. 38 (2014) 2043–2052. Q. Qin, J. Ma, K. Liu, J. Hazard. Mater. 162 (2009) 133–139. I.D. Mall, V.C. Srivastava, N.K. Agarwal, Dyes Pigments 69 (2006) 210–223. V.K. Gupta, Suhas, J. Environ. Manag. 90 (2009) 2313–2342. B. Shi, G. Li, D. Wang, C. Feng, J. Hazard. Mater. 143 (2007) 567–574. S. Song, J. Fan, Z. He, L. Zhan, Z. Liu, J. Chen, X. Xu, Electrochim. Acta 55 (2010) 3606–3613. T.H. Kim, C. Park, J. Yang, S. Kim, J. Hazard. Mater. 112 (2004) 95–103. H. Selcuk, Dyes Pigments 64 (2005) 217–222. G.M. Nisola, E. Cho, A.B. Beltran, M. Han, Y. Kim, W.-J. Chung, Chemosphere 80 (8) (2010) 894–900. D. Suna, X. Zhang, Y. Wu, X. Liu, J. Hazard. Mater. 181 (2010) 335–342. F. Xia, E. Ou, L. Wang, Dyes Pigments 76 (2008) 76–81. K. Ravikumar, B. Deebika, K. Balu, J. Hazard. Mater. 122 (2005) 75–83. G. Vijayakumar, R. Tamilarasan, M. Dharmendirakumar, J. Mater. Environ. Sci. 3 (1) (2012) 157–170. P. Faria, J. Orfao, M. Pereira, Water Res. 38 (2004) 2043–2052. T.D. Dartins, D. Schimmel, J.B.O. dos Santos, E.A. da Silva, J. Chem. Eng. Data 58 (2013) 106–114. Y. Wang, Y.-F. He, L. Zhang, R. Li, L. Wang, R.-M. Wang, Adv. Mater. Res. 773 (2013) 472–476. B. Pan, B. Pan, W. Zhang, L. Lv, Q. Zhang, S. Zheng, Chem. Eng. J. 151 (2009) 19–29. M. Kumar, R. Tamilarasan, Carbohydr. Polym. 2171 (2013) 2171–2180. M. Kumar, R. Tamilarasan, V. Sivakumar, Carbohydr. Polym. 98 (2013) 505–513. A.A. El-Bindary, M.A. Diab, M.A. Hussien, A.Z. El-Sonbati, A.M. Eessa, Spectrochim. Acta A 124 (2014) 70–77. M. Ozacar, I.A. Sengil, J. Hazard. Mater. B98 (2003) 211–224. M. Alkan, O. Demirbas, S. Celikcapa, M. Dogan, J. Hazard. Mater. B116 (2004) 135–145. M.R. Khan, M. Ray, Bioresour. Technol. 102 (2011) 2394–2399. R. Ahmad, R. Kumar, J. Environ. Manag 91 (2010) 1032–1038. H.R. Guendy, J. Appl. Sci. Res. 6 (8) (2010) 964–972. I. Kiran, T. Akar, A.S. Ozcan, A. Ozcan, S. Tunali, Biochem. Eng. J. 31 (2006) 197–203. M. Yavuz, F. Gode, E. Pehlivan, S. Ozmert, Y.C. Sharma, Chem. Eng. J. 137 (2008) 453–461. I. Langmuir, J. Am. Chem. Soc. 38 (1916) 31–60. I. Langmuir, J. Am. Chem. Soc. 39 (1917) 1848–1906. I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361–1403. A.R. Alley, Water Quality Control Handbook, McGraw-Hill Education, Europe, London, 2000. 125–141. F. Woodard, Industrial Waste Treatment Handbook, Butterworth-Heinemann, Boston, 2001. 376–451. H.M.F. Freundlich, Z. Phys. Chem. (Leipzig) 57A (1906) 385–470. S. Tunali, A.S. Ozcan, A. Ozcan, J. Hazard. Mater. B135 (2006) 141–148. J.-Q. Jiang, C. Cooper, S. Ouki, Chemosphere 47 (2002) 711–716. S. Lagergren, Kungliga Svenska Vetenskapsakademiens Handlingar, 24, 4, 1898, pp. 1–39. Y.S. Ho, G. McKay, Chem. Eng. J. 70 (2) (1998) 115–124. R.S. Juang, F.C. Wu, R.L. Tseng, Environ. Technol. 18 (5) (1997) 525–531. H. Nollet, M. Roels, P. Lutgen, P. Van der Meeren, W. Verstraete, Chemosphere 53 (6) (2003) 655–665. M.J. Jaycock, G.D. Parfitt, Chemistry of Interfaces, Ellis Horwood Ltd., Chichester, 1981. D. Singh, Adsorpt. Sci. Technol. 18 (8) (2000) 741–748. N.K. Goel, V. Kumar, S. Pahan, Y.K. Bhardwaj, S. Sabharwal, J. Hazard. Mater. 193 (2011) 17–26. P. Sharma, M.R. Das, J. Chem. Eng. Data 58 (2013) 151–158. C. Namasivayam, D. Kavitha, Dyes Pigments 54 (2002) 47–58. F. Yuzhu, T. Viraraghavan, Adv. Environ. Res. 7 (2002) 239–247.