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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77
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Adsorption of Acid Red 57 from aqueous solutions onto polyacrylonitrile/activated carbon composite Ashraf A. El-Bindary a,⇑, Mostafa A. Diab a, Mostafa A. Hussien b, Adel Z. El-Sonbati 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
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We use (PAN/AC) composite as
Effect of initial dye concentration on adsorption of AR57 on to (PAN/AC) composite at dye concentration 60 mg/L, pH 1 and 25 °C.
adsorbent for the removal of textile dyes. XRD and FRIR studies have been carried out for characterization of the composite. Langmuir and Freundlich isotherms have been used to find out the best fit model. Kinetic and thermodynamic studies of the surfactant adsorption have been investigated. Results are of marked significance to the water treatment industries.
100 95 90
Removal (%)
85 80 75
40 mg/L 70 mg/L
70
120 mg/L
150 mg/L 65 15
30
45
60
75
90
105
120
t (min.)
a r t i c l e
i n f o
Article history: Received 27 November 2013 Received in revised form 28 December 2013 Accepted 30 December 2013 Available online 9 January 2014 Keywords: Adsorption Polyacrylonitrile SEM Acid dye Isotherms Kinetics
a b s t r a c t The adsorption of Acid Red 57 (AR57) onto Polyacrylonitrile/activated carbon (PAN/AC) composite was investigated in aqueous solution in a batch system with respect to contact time, pH and temperature. Physical characteristics of (PAN/AC) composite such as fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) were obtained. 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 AR57 onto (PAN/AC) composite. The pseudo-first-order and pseudo-second-order kinetic models were used to describe the kinetic data. 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 AR57 onto (PAN/AC) composite. The thermodynamics of the adsorption indicated spontaneous and exothermic nature of the process. The results indicate that (PAN/AC) composite could be employed as low-cost material for the removal of acid dyes from textile effluents. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Industrial effluents are one of the major causes of environmental pollution because effluents discharged from dyeing industries ⇑ Corresponding author. Tel.: +20 1114266996; fax: +20 572403868. E-mail address: [email protected] (A.A. El-Bindary). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.109
are highly colored [1]. The remaining dye molecules, even at very low concentrations in the wastewater of textile, coating and printing ink industries are common water pollutants [2]. Their presence in water is highly visible and undesirable and may significantly affect photo-synthetic activity in aquatic life due to reduced light penetration. Wastewater containing dyes from the textile industry is very difficult to treat using conventional wastewater treatment
A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77
methods, which are coagulation, ultrafiltration, ozonation, oxidation, sedimentation, reverse osmosis, flotation, precipitation, etc., due to economic considerations. In comparison to the removal methods of color, it has been well established that adsorption is the most convenient and effective technique for the removal of dyes from effluents to stable forms [3,4]. Adsorption is considered to be relatively superior to other techniques because of its low cost, simplicity of design, high efficiency, availability and ability to separate wide range of chemical compounds [5,6]. In recent years, the search for low-cost adsorbents that have dye-binding capacities has intensified. This has led many workers to search for cheaper alternates such as coal, fly ash, silica gel, wool wastes, agricultural wastes, wood wastes and clay minerals [7–9]. Due to its large surface area, high adsorption capacity and surface reactivity. Currently, activated carbon is the most common adsorbent due to its high adsorption capacity, high surface area, microporous structure and high degree of surface reactivity, but there are some problems with its use for the regeneration process. This led to a search for cheaper, easily obtainable materials for the adsorption of dye [10–12]. It is considered that one of the major challenges faced with adsorption by activated carbon is its cost effectiveness [13]. 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 [14–17]. The aim of this study is to investigate the adsorption of Acid Red 57 onto Polyacrylonitrile/activated carbon (PAN/AC) composite as a low cost adsorbent. The adsorption capacity of AR57 was also examined using the adsorption isotherm technique. The kinetic and thermodynamic parameters were also calculated to determine rate constants and adsorption mechanism. The experimental data were fitted into Langmuir and Freundlich equations to determine which isotherm gives the best correlation to experimental data.
Materials and methods Materials A commercial textile dye Acid Red 57 (Fig. 1) was obtained from Cromatos SRL, a dyes company located in Italy and was used as received without further purification. Activated carbon (particle diameter 300–500 lm) 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.
71
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. Preparation of polyacrylonitrile/activated carbon (PAN/AC) composite Polyacrylonitrile/activated carbon (PAN/AC) composite was prepared [18] by adding a 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, washed with water, ethanol, 0.1 mol/L 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 cm1 (Fig. 2). The band at 2921 cm1 that corresponding to the asymmetric stretching vibration of methylene group (mCH2) and its bending vibration at 1454 cm1. The band at 2244 cm1 corresponding to the CN group. Adsorption experiments The adsorption experiments of anionic dye AR57 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 L1AR57 solution at various pHs (1–9) reached for 90 min was adjusting by adding a small amount of HCl or NaOH solution (1 M) by using a pH meter. The optimum pH was determined and used through all adsorption processes. Experiments were conducted for various time intervals to determine when adsorption equilibrium was reached and the maximum removal of AR57 was attained. The solution was then filtered through a Whatmann (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 wavelengths of UV-maximum (kmax) at 512.5 nm. Dye adsorption experiments were also accomplished to obtain isotherms at various temperature (25–50 °C) and at arrange of 40–150 mg L1 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 was determined. Results and discussion Effect of adsorbate concentrations
Fig. 1. The structure of Acid Red 57.
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
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Scheme 1. Schematic representation of the polymerization of acrylonitrile.
tion 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 increases and hence the percentage of adsorption goes down.
100
2244
80
2013
2921
%T
1575
1454
3399
Effect of adsorbent dosage
60
40 1035
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber Cm-1 Fig. 2. FTIR spectrum of (PAN/AC) composite.
100
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 (PAN/AC) composite 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 at 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. Effect of temperature
95
Temperature dependence of the adsorption process is associated with several thermodynamic parameters. The plot of amount
90
Removal (%)
85
100 80 75
40 mg/L
70
70 mg/L 120 mg/L 150 mg/L
65 15
30
45
60
75
90
105
120
t (min.) Fig. 3. Effect of initial dye concentration on adsorption of AR57 on to (PAN/AC) composite at dye concentration 60 mg/L, pH 1 and 25 °C.
80
60
Removal (%)
0.2 g/L
40
0.3 g/L 0.4 g/L 0.5 g/L
20
0.6 g/L
0
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 concentra-
0
10
20
30
40
50
60
70
80
90 100 110 120 130
t (min.) Fig. 4. Effect of composite dosage on the adsorption of AR57 onto (PAN/AC) composite at dye concentration 60 mg/L, pH 1 and 25 °C.
73
A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77 100
90
25 O C
Removal 80 (%)
Scheme 2. Proposed Electrostatic attraction between AR57 and PAN/AC composite.
30 O C 40 O C 50 O C
70
pH is due to the presence of excess hydroxyl ions competing with the dye anions for the adsorption sites [20,21]. Adsorption isotherms
60 15
30
45
60
75
90
Time (min.) Fig. 5. Effect of temperature on adsorption of AR57 onto (PAN/AC) composite at dye composite dosage 0.4 gm/L, dye conc. 60 mg/L and pH 1.
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 0.2 g/L, and initial concentration of 60 mg/L show that no change in the adsorption capacity at temperature from 25 to 50 °C. Effect of pH The removal of AR57 by prepared (PAN/AC) composite at different pH values was studied at initial concentrations of 60 mg/L of AR57, 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 proved to be an effective adsorbent for the removal of acid dye, AR57, via adsorption from aqueous solution at pH 1 was achieved (Fig. 6). It shows that the adsorption capacity of acid dyes AR57 onto acid activated (PAN/AC) composite increases significantly with decreasing pH. The maximum removals of AR57 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 [19]. 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 does not favor the adsorption of dye anions, due to the electrostatic repulsion. Also, lower adsorption of AR57 at alkaline
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 etc. [22]. Equilibrium studies that give the capacity of the adsorbent and adsorbate are described by adsorption isotherms, which is usually the ratio between the quantity adsorbed and that remained in solution at equilibrium at fixed temperature [23–25]. The equilibrium experimental data for adsorbed AR57 on (PAN/ AC) composite was compared using two isotherm equations namely, Langmuir and Freundlich. 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 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 [26–30],
C e =qe ¼ 1=ðqmax K L Þ þ C e =qmax
ð3Þ 1
where Ce is the equilibrium dye concentration in solution (mol L ), qe is the equilibrium dye concentration in the adsorbent (mol g1), qmax is the monolayer capacity of the adsorbent (mol g1) and KL is the Langmuir adsorption constant (L mol1). Therefore, a plot of Ce/qe vs. Ce (Fig. 7), gives a straight line of slope 1/qmax and the intercept 1/(qmaxKL). The Langmuir equation
100
1.1 1.0
80
0.9 60
0.8
Ce /qe
Removal (%) 40
pH = 1 pH = 3 pH = 5 pH = 7 pH = 9
20
25 o C 30 o C 40 o C 50 o C
0.7 0.6 0.5 0.4
0 0
15
30
45
60
75
90
t (min.) Fig. 6. Effect of pH on the adsorption of AR57 onto (PAN/AC) composite at composite dosage 0.4 g/L, dye concentration 60 mg/L and temperature 25 °C.
0.3 2
3
4
5
6
7
8
Ce Fig. 7. Langmuir plots for adsorption of AR57 onto (PAN/AC) composite.
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is applicable to homogeneous sorption, where the sorption of each sorbate molecule onto the surface has equal to sorption activation energy. Freundlich isotherm The Freundlich equation [29–31] 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 formation of the monolayer:
qe ¼ K F C 1=n e
ð4Þ
where qe is the equilibrium dye concentration on adsorbent (mol g1), Ce is the equilibrium dye concentration in solution (mol L1), KF is Freundlich constant (L g1) and 1/n is the heterogeneity factor. A linear form of the Freundlich expression can be obtained by taking logarithms of the equation
log qe ¼ log K F þ 1=n log C e
ð5Þ
Therefore, a plot of log qe vs. log Ce for the adsorption of AR57 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 ðr2L Þ and for Freundlich ðr2F Þ values are compared in Table 1. One of the Freundlich constants KF indicates the adsorption capacity of the adsorbent. The other Freundlich constants 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 to unity, this implies that adsorption process is chemical, but a value for n is above to unity, adsorption is favorable a physical process [32]. The highest value of n at equilibrium is 1.121 (Table 1), this would seem to suggest that the adsorption is physical, which
0.87
25 oC 30 oC 40 oC 50 oC
0.86
is referred the adsorption bond becomes weak [33] and conducted with van der Waals forces. 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 the solid–solution interface. The rate of removal of AR57 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 AR57 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 AR57 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 [34,35] is given as:
logðqe qt Þ ¼ log qe k1 t
ð6Þ
The pseudo-second-order kinetic model [35] is expressed as:
t=qt ¼ 1=k2 q22 þ 1=q2 t
ð7Þ
where qt is the amount of dye adsorbed (mol g1) at various times t, qe is the maximum adsorption capacity (mol g1) for pseudo-firstorder adsorption, k1 is the pseudo-first-order rate constant for the 1.8
25 oC
1.6
30 oC 40 oC 50 o C
1.4 1.2
log (qe -q t )
74
1.0 0.8 0.6 0.4 0.2 0.0 -0.2
log Q e
-0.4 10
0.85
20
30
40
50
60
70
t (min.) Fig. 9. Pseudo-first-order kinetic plot for the adsorption of AR57 onto (PAN/AC) composite at different temperatures.
0.84
0.83 0.4
0.6
0.8
0.8
0.7
log Ce
Table 1 Langmuir and Freundlich parameters for the adsorption of AR57 dye onto (PAN/AC) composite. Temperature (°C)
Langmuir isotherm qmax (mol g1)
25 30 40 50
17.64 21.08 37.18 40.01
Freundlich isotherm
KL (L mol1)
r 2L
KF (L g1)
n
r 2F
0.368 0.312 0.185 0.173
0.997 0.990 0.992 0.991
0.825 0.836 0.875 0.878
1.104 1.107 1.119 1.121
0.998 0.991 0.994 0.999
t / q t (min g mg -1)
0.6
Fig. 8. Freundlich plots for adsorption of AR57 onto (PAN/AC) composite.
0.5 0.4 0.3
25 oC 30 oC 40 oC 50 oC
0.2 0.1 0.0 -0.1 -0.2 10
20
30
40
50
60
70
80
90
100
t (min.) Fig. 10. Pseudo-second-order kinetic plot for the adsorption of AR57 onto (PAN/AC) composite at different temperatures.
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A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77 Table 2 Pseudo-first-order, pseudo-second-order for the adsorption of AR57 dye onto (PAN/ AC) composite. Temperature (°C)
25 30 40 50
Pseudo-first-order
Pseudo-second-order
qe (mol g1)
k1 (min1)
r 21
q2 (mol g1)
k2 (g mol1 min1)
r 22
0.931 0.928 0.936 0.935
4.47 4.26 3.45 3.37
0.990 0.913 0.901 0.971
153.84 153.37 150.61 150.38
0.235 0.297 0.574 0.699
0.999 0.999 0.998 0.999
adsorption process (min1), q2 is the maximum adsorption capacity (mol g1) for the pseudo-second-order adsorption, k2 is the rate constant of pseudo-second-order adsorption (g mol1 min1). The straight-line plots of log(qe qt) vs. t for the pseudo-first-order reaction and t/qt vs. t for the pseudo-second-order reaction (Figs. 9 and 10) for adsorption of AR57 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 AR57 under different temperatures were calculated from these plots and are given in Table 2. The correlation coefficients ðr 21 Þ for the pseudo-first-order kinetic model are between 0.901 and 0.990 and the correlation coefficients ðr22 Þ, for the pseudo-second-order kinetic model are between 0.998 and 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. Thermodynamic parameters 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 AR57 adsorbed onto (PAN/AC) composite at equilibrium and at different temperatures 25, 30, 40, 50 °C, have been examined to obtain thermodynamic parameters for the adsorption system. The pseudo-second-order rate constant of AR57 adsorption is expressed as a function of temperature by the following Arrhenius type relationship [36]:
ln k2 ¼ ln A Ea =RT
ð8Þ
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 mol1 K1 and T is the operated temperature. A linear plot of ln k2 vs. 1/T for the adsorption of AR57 onto (PAN/AC) composite (Fig. 11) was
Table 3 Thermodynamic parameters calculated with the pseudo-second rate constant for AR57 dye onto (PAN/AC) composite. Temperature (°C)
Kc
Ea (kJ mol1)
DG° (kJ mol1)
DH° (kJ mol1)
DS° (J mol1K1)
25 30 40 50
51.623 55.056 56.374 57.754
16.05
52.877 53.708 55.371 57.034
3.33
0.166
constructed to generate the activation energy from the slope (Ea/R). The chemical (chemisorption) or physical (physisorption) adsorption mechanism are often an important indicator to describe the type of interactions between AR57 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 mol1) are characteristics for physisorption, while higher activation energies (40–800 kJ mol1) suggest chemisorption [37]. The result obtained is +16.05 kJ mol1 (Table 3) for the adsorption of AR57 onto (PAN/AC) composite, indicating that the adsorption has a low potential barrier and corresponding to a physisorption. The other thermodynamic parameters, change in the standard free energy (DG°), enthalpy (DH°) and entropy (DS°) were determined by using following equations:
K C ¼ C A =C S
ð9Þ
DG ¼ RT ln K C
ð10Þ
ln K C ¼ DS =R DH =RT
ð11Þ
where KC is the equilibrium constant, CA is the amount of AR57 adsorbed on the (PAN/AC) composite of the solution at equilibrium (mol L1), CS is the equilibrium concentration of the AR57 in the solution (mol L1). 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. DH° and DS° 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 (DG°) confirms the feasibilty of the adsorption process and also indicates spontaneous adsorption of AR57 onto (PAN/AC) composite in the temperature range studied [38]. The small negative value of the standard enthalpy change (DH°) is (3.33 kJ mol1) indicate that
0.0035 0.0034
0.0034
1/T (K -1)
1/T (K -1)
0.0033
0.0032
0.0033
0.0032
0.0031 0.0031
0.0030 3.95 -1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
ln k 2 Fig. 11. Arrhenius plot for the adsorption of AR57 onto (PAN/AC) composite.
4.00
4.05
ln k C Fig. 12. Van’t Hoff plot for determination of thermodynamic parameters for the adsorption of AR57 onto (PAN/AC) composite.
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Fig. 13. (PAN/AC) Composite before adsorption of AR57 dye.
Fig. 14. (PAN/AC) Composite after adsorption of AR57 dye.
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 DH° implies that there was loose bonding between the adsorbate molecules and the adsorbent surface [39,40]. The positive value of standard entropy change (DS°) is (0.166 J mol1K1) suggest the increased randomness at the solid–solution interface during the adsorption of AR57 onto (PAN/AC) composite [41]. 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 AR57 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 AR57 show very distinguished dark spots which can be taken as a sign for effective adsorption of AR57 molecules in the cavities and pores of this adsorbent. 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 filtrated 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 [42,43]. 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 AR57 dye onto (PAN/AC) composite before recovering process was 98.63%, while after recovering process was 98.42%.
Conclusion From the present study clearly demonstrated that (PAC/AC) composite is an effective adsorbent for the removal of AR57 dye from aqueous solution and polluted water. The high adsorption capacity of AR57 onto (PAC/AC) composite in highly acidic solutions (pH 1) is due to the strong electrostatic interactions between its adsorption site and dye anion. Based on these results. For the application of Langmuir and Freundlich equations, the experimental results shows that the Freundlich model was the best. The highest value of n at equilibrium is 1.121 suggest that the adsorption is physical. The kinetic data tends to fit very well in the pseudo-second-order kinetics model with high correlation coefficients. The DG° values were negative, therefore the adsorption was spontaneous in nature. The negative value of DH° reveals that the adsorption process was exothermic in nature and a physical adsorption. The positive value of DS° implies that the increment of an orderliness between the adsorbate and the adsorbent molecules. SEM images shows well defined and characterized morphological images that are evident for the effective adsorption of AR57 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
A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77
regeneration process using Sodium hydroxide solution at pH around 12. References [1] K. Kadirvelu, M. Kavipriya, C. Karthika, M. Radhika, N. Vennilamani, S. Pattabhi, Bioresour. Technol. 87 (1) (2003) 129. [2] M. Arami, N.Y. Limaee, N.M. Mahmoodi, N.S. Tabrizi, J. Colloid Interface Sci. 288 (2005) 371. [3] K. Ravikumar, S. Krishnan, S. Ramalingam, K. Balu, Dyes Pigm. 72 (2007) 66. [4] G. Crini, Bioresour. Technol. 97 (2006) 1061. [5] V. Kumar, N.K. Goel, Y.K. Bhardwaj, S. Sabharwal, L. Varshney, Separ. Sci. Technol. 47 (13) (2012) 1937. [6] G. Bayramoglu, N. Adiguzel, G. Ersoy, M. Yilmaz, M. Yakup Arica, Water Air Soil Poll. 224 (8) (2013). art no 1640. [7] S.C. Zhu, C.G. Miao, Y. Wu, H.X. Yang, Adv. Mater. Res. 726 (2013) 629. [8] X. Jiang, Y. Sun, L. Liu, S. Wang, X. Tian, Chem. Eng. J. 235 (2014) 151. [9] G. Annadurai, R.S. Juang, D.J. Lee, J. Hazard. Mater. B92 (2002) 263. [10] G. Vijayakumar, R. Tamilarasan, M. Dharmendirakumar, J. Mater. Environ. Sci. 3 (1) (2012) 157. [11] P. Faria, J. Orfao, M. Pereira, Water Res. 38 (2004) 2043. [12] S. Sener, Chem. Eng. J. 138 (2008) 207. [13] I.D. Mall, V.C. Srivastava, N.K. Agarwal, I.M. Mishra, Chemosphere 61 (2005) 492. [14] Y. Wang, Y.-F. He, L. Zhang, R. Li, L. Wang, R.-M. Wang, Adv. Mater. Res. 773 (2013) 472. [15] B. Pan, B. Pan, W. Zhang, L. Lv, Q. Zhang, S. Zheng, Chem. Eng. J. 151 (2009) 19. [16] M. Kumar, R. Tamilarasan, Carbohydrate Polym. 92 (2013) 2180. [17] M. Kumar, R. Tamilarasan, V. Sivakumar, Carbohydrate Polym. 98 (2013) 505. [18] J. Lu, X. Zhao, J. Coll. Interface Sci. 273 (2004) 651. [19] M. Ozacar, I.A. Sengil, J. Hazard. Mater. B98 (2003) 211.
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[20] M. Alkan, O. Demirbas, S. Celikcapa, M. Dogan, J. Hazard. Mater. B116 (2004) 135. [21] M.R. Khan, M. Ray, A.K. Guha, Bioresour. Technol. 102 (2011) 2394. [22] R. Ahmad, R. Kumar, J. Environ. Manag. 91 (2010) 1032. [23] H.R. Guendy, J. Appl. Sci. Res. 6 (8) (2010) 964. [24] I. Kiran, T. Akar, A.S. Ozcan, A. Ozcan, S. Tunali, Biochem. Eng. J. 31 (2006) 197. [25] M. Yavuz, F. Gode, E. Pehlivan, S. Ozmert, Y.C. Sharma, Chem. Eng. J. 137 (2008) 453. [26] I. Langmuir, J. Am. Chem. Soc. 38 (1916) 31. [27] I. Langmuir, J. Am. Chem. Soc. 39 (1917) 1848. [28] I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361. [29] A.R. Alley, Water Quality Control Handbook, McGraw-Hill Education, Europe, London, 2000. p. 125–141. [30] F. Woodard, Industrial Waste Treatment Handbook, Butterworth-Heinemann, Boston, 2001. p. 376–451. [31] H.M.F. Freundlich, Z. Phys. Chem. (Leipzig) 57A (1906) 385. [32] S. Tunali, A.S. Ozcan, A. Ozcan, T. Gedikbey, J. Hazard. Mater. B135 (2006) 141. [33] J.-O. Jiang, C. Cooper, S. Ouki, Chemosphere 47 (2002) 711. [34] S. Lagergren, Kungliga Svenska Vetenskapsakademiens. Handlingar 24 (4) (1898) 1. [35] Y.S. Ho, G. McKay, Chem. Eng. J. 70 (2) (1998) 115. [36] R.S. Juang, F.C. Wu, R.L. Tseng, Environ. Technol. 18 (5) (1997) 525. [37] H. Nollet, M. Roels, P. Lutgen, P. Van der Meeren, W. Verstraete, Chemosphere 53 (6) (2003) 655. [38] M.J. Jaycock, G.D. Parfitt, Chemistry of Interfaces, Ellis Horwood Ltd, Onichester, 1981. [39] D. Singh, Adsorp. Sci. Technol. 18 (8) (2000) 741. [40] N.K. Goel, V. Kumar, S. Pahan, Y.K. Bhardwaj, S. Sabharwal, J. Hazard. Mater. 193 (2011) 17. [41] R.S. Juang, F.C. Wu, R.L. Tseng, Environ. Technol. 18 (1997) 525. [42] C. Namasivayam, D. Kavitha, Dyes Pigm. 54 (2002) 47. [43] F. Yuzhu, T. Viraraghavan, Adv. Environ. Res. 7 (2002) 239.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Adsorption of Acid Red 57 from aqueous solutions onto polyacrylonitrile/activated carbon composite Ashraf A. El-Bindary a,⇑, Mostafa A. Diab a, Mostafa A. Hussien b, Adel Z. El-Sonbati 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
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We use (PAN/AC) composite as
Effect of initial dye concentration on adsorption of AR57 on to (PAN/AC) composite at dye concentration 60 mg/L, pH 1 and 25 °C.
adsorbent for the removal of textile dyes. XRD and FRIR studies have been carried out for characterization of the composite. Langmuir and Freundlich isotherms have been used to find out the best fit model. Kinetic and thermodynamic studies of the surfactant adsorption have been investigated. Results are of marked significance to the water treatment industries.
100 95 90
Removal (%)
85 80 75
40 mg/L 70 mg/L
70
120 mg/L
150 mg/L 65 15
30
45
60
75
90
105
120
t (min.)
a r t i c l e
i n f o
Article history: Received 27 November 2013 Received in revised form 28 December 2013 Accepted 30 December 2013 Available online 9 January 2014 Keywords: Adsorption Polyacrylonitrile SEM Acid dye Isotherms Kinetics
a b s t r a c t The adsorption of Acid Red 57 (AR57) onto Polyacrylonitrile/activated carbon (PAN/AC) composite was investigated in aqueous solution in a batch system with respect to contact time, pH and temperature. Physical characteristics of (PAN/AC) composite such as fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) were obtained. 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 AR57 onto (PAN/AC) composite. The pseudo-first-order and pseudo-second-order kinetic models were used to describe the kinetic data. 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 AR57 onto (PAN/AC) composite. The thermodynamics of the adsorption indicated spontaneous and exothermic nature of the process. The results indicate that (PAN/AC) composite could be employed as low-cost material for the removal of acid dyes from textile effluents. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Industrial effluents are one of the major causes of environmental pollution because effluents discharged from dyeing industries ⇑ Corresponding author. Tel.: +20 1114266996; fax: +20 572403868. E-mail address: [email protected] (A.A. El-Bindary). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.109
are highly colored [1]. The remaining dye molecules, even at very low concentrations in the wastewater of textile, coating and printing ink industries are common water pollutants [2]. Their presence in water is highly visible and undesirable and may significantly affect photo-synthetic activity in aquatic life due to reduced light penetration. Wastewater containing dyes from the textile industry is very difficult to treat using conventional wastewater treatment
A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77
methods, which are coagulation, ultrafiltration, ozonation, oxidation, sedimentation, reverse osmosis, flotation, precipitation, etc., due to economic considerations. In comparison to the removal methods of color, it has been well established that adsorption is the most convenient and effective technique for the removal of dyes from effluents to stable forms [3,4]. Adsorption is considered to be relatively superior to other techniques because of its low cost, simplicity of design, high efficiency, availability and ability to separate wide range of chemical compounds [5,6]. In recent years, the search for low-cost adsorbents that have dye-binding capacities has intensified. This has led many workers to search for cheaper alternates such as coal, fly ash, silica gel, wool wastes, agricultural wastes, wood wastes and clay minerals [7–9]. Due to its large surface area, high adsorption capacity and surface reactivity. Currently, activated carbon is the most common adsorbent due to its high adsorption capacity, high surface area, microporous structure and high degree of surface reactivity, but there are some problems with its use for the regeneration process. This led to a search for cheaper, easily obtainable materials for the adsorption of dye [10–12]. It is considered that one of the major challenges faced with adsorption by activated carbon is its cost effectiveness [13]. 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 [14–17]. The aim of this study is to investigate the adsorption of Acid Red 57 onto Polyacrylonitrile/activated carbon (PAN/AC) composite as a low cost adsorbent. The adsorption capacity of AR57 was also examined using the adsorption isotherm technique. The kinetic and thermodynamic parameters were also calculated to determine rate constants and adsorption mechanism. The experimental data were fitted into Langmuir and Freundlich equations to determine which isotherm gives the best correlation to experimental data.
Materials and methods Materials A commercial textile dye Acid Red 57 (Fig. 1) was obtained from Cromatos SRL, a dyes company located in Italy and was used as received without further purification. Activated carbon (particle diameter 300–500 lm) 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.
71
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. Preparation of polyacrylonitrile/activated carbon (PAN/AC) composite Polyacrylonitrile/activated carbon (PAN/AC) composite was prepared [18] by adding a 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, washed with water, ethanol, 0.1 mol/L 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 cm1 (Fig. 2). The band at 2921 cm1 that corresponding to the asymmetric stretching vibration of methylene group (mCH2) and its bending vibration at 1454 cm1. The band at 2244 cm1 corresponding to the CN group. Adsorption experiments The adsorption experiments of anionic dye AR57 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 L1AR57 solution at various pHs (1–9) reached for 90 min was adjusting by adding a small amount of HCl or NaOH solution (1 M) by using a pH meter. The optimum pH was determined and used through all adsorption processes. Experiments were conducted for various time intervals to determine when adsorption equilibrium was reached and the maximum removal of AR57 was attained. The solution was then filtered through a Whatmann (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 wavelengths of UV-maximum (kmax) at 512.5 nm. Dye adsorption experiments were also accomplished to obtain isotherms at various temperature (25–50 °C) and at arrange of 40–150 mg L1 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 was determined. Results and discussion Effect of adsorbate concentrations
Fig. 1. The structure of Acid Red 57.
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
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A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77
Scheme 1. Schematic representation of the polymerization of acrylonitrile.
tion 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 increases and hence the percentage of adsorption goes down.
100
2244
80
2013
2921
%T
1575
1454
3399
Effect of adsorbent dosage
60
40 1035
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber Cm-1 Fig. 2. FTIR spectrum of (PAN/AC) composite.
100
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 (PAN/AC) composite 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 at 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. Effect of temperature
95
Temperature dependence of the adsorption process is associated with several thermodynamic parameters. The plot of amount
90
Removal (%)
85
100 80 75
40 mg/L
70
70 mg/L 120 mg/L 150 mg/L
65 15
30
45
60
75
90
105
120
t (min.) Fig. 3. Effect of initial dye concentration on adsorption of AR57 on to (PAN/AC) composite at dye concentration 60 mg/L, pH 1 and 25 °C.
80
60
Removal (%)
0.2 g/L
40
0.3 g/L 0.4 g/L 0.5 g/L
20
0.6 g/L
0
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 concentra-
0
10
20
30
40
50
60
70
80
90 100 110 120 130
t (min.) Fig. 4. Effect of composite dosage on the adsorption of AR57 onto (PAN/AC) composite at dye concentration 60 mg/L, pH 1 and 25 °C.
73
A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77 100
90
25 O C
Removal 80 (%)
Scheme 2. Proposed Electrostatic attraction between AR57 and PAN/AC composite.
30 O C 40 O C 50 O C
70
pH is due to the presence of excess hydroxyl ions competing with the dye anions for the adsorption sites [20,21]. Adsorption isotherms
60 15
30
45
60
75
90
Time (min.) Fig. 5. Effect of temperature on adsorption of AR57 onto (PAN/AC) composite at dye composite dosage 0.4 gm/L, dye conc. 60 mg/L and pH 1.
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 0.2 g/L, and initial concentration of 60 mg/L show that no change in the adsorption capacity at temperature from 25 to 50 °C. Effect of pH The removal of AR57 by prepared (PAN/AC) composite at different pH values was studied at initial concentrations of 60 mg/L of AR57, 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 proved to be an effective adsorbent for the removal of acid dye, AR57, via adsorption from aqueous solution at pH 1 was achieved (Fig. 6). It shows that the adsorption capacity of acid dyes AR57 onto acid activated (PAN/AC) composite increases significantly with decreasing pH. The maximum removals of AR57 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 [19]. 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 does not favor the adsorption of dye anions, due to the electrostatic repulsion. Also, lower adsorption of AR57 at alkaline
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 etc. [22]. Equilibrium studies that give the capacity of the adsorbent and adsorbate are described by adsorption isotherms, which is usually the ratio between the quantity adsorbed and that remained in solution at equilibrium at fixed temperature [23–25]. The equilibrium experimental data for adsorbed AR57 on (PAN/ AC) composite was compared using two isotherm equations namely, Langmuir and Freundlich. 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 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 [26–30],
C e =qe ¼ 1=ðqmax K L Þ þ C e =qmax
ð3Þ 1
where Ce is the equilibrium dye concentration in solution (mol L ), qe is the equilibrium dye concentration in the adsorbent (mol g1), qmax is the monolayer capacity of the adsorbent (mol g1) and KL is the Langmuir adsorption constant (L mol1). Therefore, a plot of Ce/qe vs. Ce (Fig. 7), gives a straight line of slope 1/qmax and the intercept 1/(qmaxKL). The Langmuir equation
100
1.1 1.0
80
0.9 60
0.8
Ce /qe
Removal (%) 40
pH = 1 pH = 3 pH = 5 pH = 7 pH = 9
20
25 o C 30 o C 40 o C 50 o C
0.7 0.6 0.5 0.4
0 0
15
30
45
60
75
90
t (min.) Fig. 6. Effect of pH on the adsorption of AR57 onto (PAN/AC) composite at composite dosage 0.4 g/L, dye concentration 60 mg/L and temperature 25 °C.
0.3 2
3
4
5
6
7
8
Ce Fig. 7. Langmuir plots for adsorption of AR57 onto (PAN/AC) composite.
A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77
is applicable to homogeneous sorption, where the sorption of each sorbate molecule onto the surface has equal to sorption activation energy. Freundlich isotherm The Freundlich equation [29–31] 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 formation of the monolayer:
qe ¼ K F C 1=n e
ð4Þ
where qe is the equilibrium dye concentration on adsorbent (mol g1), Ce is the equilibrium dye concentration in solution (mol L1), KF is Freundlich constant (L g1) and 1/n is the heterogeneity factor. A linear form of the Freundlich expression can be obtained by taking logarithms of the equation
log qe ¼ log K F þ 1=n log C e
ð5Þ
Therefore, a plot of log qe vs. log Ce for the adsorption of AR57 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 ðr2L Þ and for Freundlich ðr2F Þ values are compared in Table 1. One of the Freundlich constants KF indicates the adsorption capacity of the adsorbent. The other Freundlich constants 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 to unity, this implies that adsorption process is chemical, but a value for n is above to unity, adsorption is favorable a physical process [32]. The highest value of n at equilibrium is 1.121 (Table 1), this would seem to suggest that the adsorption is physical, which
0.87
25 oC 30 oC 40 oC 50 oC
0.86
is referred the adsorption bond becomes weak [33] and conducted with van der Waals forces. 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 the solid–solution interface. The rate of removal of AR57 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 AR57 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 AR57 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 [34,35] is given as:
logðqe qt Þ ¼ log qe k1 t
ð6Þ
The pseudo-second-order kinetic model [35] is expressed as:
t=qt ¼ 1=k2 q22 þ 1=q2 t
ð7Þ
where qt is the amount of dye adsorbed (mol g1) at various times t, qe is the maximum adsorption capacity (mol g1) for pseudo-firstorder adsorption, k1 is the pseudo-first-order rate constant for the 1.8
25 oC
1.6
30 oC 40 oC 50 o C
1.4 1.2
log (qe -q t )
74
1.0 0.8 0.6 0.4 0.2 0.0 -0.2
log Q e
-0.4 10
0.85
20
30
40
50
60
70
t (min.) Fig. 9. Pseudo-first-order kinetic plot for the adsorption of AR57 onto (PAN/AC) composite at different temperatures.
0.84
0.83 0.4
0.6
0.8
0.8
0.7
log Ce
Table 1 Langmuir and Freundlich parameters for the adsorption of AR57 dye onto (PAN/AC) composite. Temperature (°C)
Langmuir isotherm qmax (mol g1)
25 30 40 50
17.64 21.08 37.18 40.01
Freundlich isotherm
KL (L mol1)
r 2L
KF (L g1)
n
r 2F
0.368 0.312 0.185 0.173
0.997 0.990 0.992 0.991
0.825 0.836 0.875 0.878
1.104 1.107 1.119 1.121
0.998 0.991 0.994 0.999
t / q t (min g mg -1)
0.6
Fig. 8. Freundlich plots for adsorption of AR57 onto (PAN/AC) composite.
0.5 0.4 0.3
25 oC 30 oC 40 oC 50 oC
0.2 0.1 0.0 -0.1 -0.2 10
20
30
40
50
60
70
80
90
100
t (min.) Fig. 10. Pseudo-second-order kinetic plot for the adsorption of AR57 onto (PAN/AC) composite at different temperatures.
75
A.A. El-Bindary et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 70–77 Table 2 Pseudo-first-order, pseudo-second-order for the adsorption of AR57 dye onto (PAN/ AC) composite. Temperature (°C)
25 30 40 50
Pseudo-first-order
Pseudo-second-order
qe (mol g1)
k1 (min1)
r 21
q2 (mol g1)
k2 (g mol1 min1)
r 22
0.931 0.928 0.936 0.935
4.47 4.26 3.45 3.37
0.990 0.913 0.901 0.971
153.84 153.37 150.61 150.38
0.235 0.297 0.574 0.699
0.999 0.999 0.998 0.999
adsorption process (min1), q2 is the maximum adsorption capacity (mol g1) for the pseudo-second-order adsorption, k2 is the rate constant of pseudo-second-order adsorption (g mol1 min1). The straight-line plots of log(qe qt) vs. t for the pseudo-first-order reaction and t/qt vs. t for the pseudo-second-order reaction (Figs. 9 and 10) for adsorption of AR57 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 AR57 under different temperatures were calculated from these plots and are given in Table 2. The correlation coefficients ðr 21 Þ for the pseudo-first-order kinetic model are between 0.901 and 0.990 and the correlation coefficients ðr22 Þ, for the pseudo-second-order kinetic model are between 0.998 and 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. Thermodynamic parameters 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 AR57 adsorbed onto (PAN/AC) composite at equilibrium and at different temperatures 25, 30, 40, 50 °C, have been examined to obtain thermodynamic parameters for the adsorption system. The pseudo-second-order rate constant of AR57 adsorption is expressed as a function of temperature by the following Arrhenius type relationship [36]:
ln k2 ¼ ln A Ea =RT
ð8Þ
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 mol1 K1 and T is the operated temperature. A linear plot of ln k2 vs. 1/T for the adsorption of AR57 onto (PAN/AC) composite (Fig. 11) was
Table 3 Thermodynamic parameters calculated with the pseudo-second rate constant for AR57 dye onto (PAN/AC) composite. Temperature (°C)
Kc
Ea (kJ mol1)
DG° (kJ mol1)
DH° (kJ mol1)
DS° (J mol1K1)
25 30 40 50
51.623 55.056 56.374 57.754
16.05
52.877 53.708 55.371 57.034
3.33
0.166
constructed to generate the activation energy from the slope (Ea/R). The chemical (chemisorption) or physical (physisorption) adsorption mechanism are often an important indicator to describe the type of interactions between AR57 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 mol1) are characteristics for physisorption, while higher activation energies (40–800 kJ mol1) suggest chemisorption [37]. The result obtained is +16.05 kJ mol1 (Table 3) for the adsorption of AR57 onto (PAN/AC) composite, indicating that the adsorption has a low potential barrier and corresponding to a physisorption. The other thermodynamic parameters, change in the standard free energy (DG°), enthalpy (DH°) and entropy (DS°) were determined by using following equations:
K C ¼ C A =C S
ð9Þ
DG ¼ RT ln K C
ð10Þ
ln K C ¼ DS =R DH =RT
ð11Þ
where KC is the equilibrium constant, CA is the amount of AR57 adsorbed on the (PAN/AC) composite of the solution at equilibrium (mol L1), CS is the equilibrium concentration of the AR57 in the solution (mol L1). 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. DH° and DS° 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 (DG°) confirms the feasibilty of the adsorption process and also indicates spontaneous adsorption of AR57 onto (PAN/AC) composite in the temperature range studied [38]. The small negative value of the standard enthalpy change (DH°) is (3.33 kJ mol1) indicate that
0.0035 0.0034
0.0034
1/T (K -1)
1/T (K -1)
0.0033
0.0032
0.0033
0.0032
0.0031 0.0031
0.0030 3.95 -1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
ln k 2 Fig. 11. Arrhenius plot for the adsorption of AR57 onto (PAN/AC) composite.
4.00
4.05
ln k C Fig. 12. Van’t Hoff plot for determination of thermodynamic parameters for the adsorption of AR57 onto (PAN/AC) composite.
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Fig. 13. (PAN/AC) Composite before adsorption of AR57 dye.
Fig. 14. (PAN/AC) Composite after adsorption of AR57 dye.
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 DH° implies that there was loose bonding between the adsorbate molecules and the adsorbent surface [39,40]. The positive value of standard entropy change (DS°) is (0.166 J mol1K1) suggest the increased randomness at the solid–solution interface during the adsorption of AR57 onto (PAN/AC) composite [41]. 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 AR57 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 AR57 show very distinguished dark spots which can be taken as a sign for effective adsorption of AR57 molecules in the cavities and pores of this adsorbent. 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 filtrated 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 [42,43]. 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 AR57 dye onto (PAN/AC) composite before recovering process was 98.63%, while after recovering process was 98.42%.
Conclusion From the present study clearly demonstrated that (PAC/AC) composite is an effective adsorbent for the removal of AR57 dye from aqueous solution and polluted water. The high adsorption capacity of AR57 onto (PAC/AC) composite in highly acidic solutions (pH 1) is due to the strong electrostatic interactions between its adsorption site and dye anion. Based on these results. For the application of Langmuir and Freundlich equations, the experimental results shows that the Freundlich model was the best. The highest value of n at equilibrium is 1.121 suggest that the adsorption is physical. The kinetic data tends to fit very well in the pseudo-second-order kinetics model with high correlation coefficients. The DG° values were negative, therefore the adsorption was spontaneous in nature. The negative value of DH° reveals that the adsorption process was exothermic in nature and a physical adsorption. The positive value of DS° implies that the increment of an orderliness between the adsorbate and the adsorbent molecules. SEM images shows well defined and characterized morphological images that are evident for the effective adsorption of AR57 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
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