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Removal of Neutral Red from aqueous solution by using modified hectorite
Desalination 267 (2011) 9–15
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Removal of Neutral Red from aqueous solution by using modified hectorite Duyuan Yue a,b, Yan Jing a, Jun Ma a, Chenglong Xia a,b, Xiaojie Yin a,b, Yongzhong Jia a,⁎ a b
Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, China Graduate University of the Chinese Academy of Sciences, Beijing, 100086, China
a r t i c l e
i n f o
Article history: Received 13 April 2010 Received in revised form 22 July 2010 Accepted 27 August 2010 Available online 24 September 2010 Keywords: Neutral Red Adsorption Modified hectorite Kinetics Thermodynamics
a b s t r a c t The object of this work was to study the modified hectorite as effective adsorbent for Neutral Red (NR) from aqueous solution. The adsorbent capacity of modified hectorite was discussed. The effects of surfactant content, adsorbent content, pH and adsorption temperature on the sorption of NR on modified hectorite were studied. Experimental results showed that the equilibrium adsorption data fitted well with Langmuir isotherm and the adsorption capacity was 393.70 mg/g for the modified cetylpyridinium bromide hectorite (CPB-Hect). Kinetic studies showed that the dynamical data fitted well with the pseudo-second-order kinetic model. For thermodynamic studies, parameters such as the Gibbs free energy (ΔG0), the enthalpy (ΔH0) and the entropy (ΔS0) indicated that the adsorption process was spontaneous and endothermic in nature. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Generally, dyes were stable to light, heat, and oxidizing agents. It had potential applications in various industries, such as textile, leather, paper, plastic, etc., to color the final products [1]. The extensive use of dyes often posed pollution problems in the form of colored wastewater. Environmental pollution caused by industrial wastewater had become a common problem in many countries. Most dyes caused damage not only to aquatic life, but also to human beings because it is toxic, mutagenic or carcinogenic [2]. Therefore, the removal of dyes from waste effluents was important. A wide range of methods, such as coagulation, electrocoagulation, flotation, chemical oxidation, filtration, membrane separation, and microbial degradation [3], have been developed for the removal of synthetic dyes from waters and wastewaters to decrease its impact in the environment. These methods might suffer from one or more limitations and could not remove the dyestuff from wastewater completely [4,5]. In contrast, adsorption was popular nowadays because of its easy operation and versatility. Previous research on the adsorption of dyes from aqueous solution revealed numerous economic adsorbents, such as coal, fly ash, wood, biogas waste slurry, waste banana pith, waste orange peel, waste red mud and agricultural wastes [6–9]. Unfortunately, the adsorption capacities of these materials were limited. Activated carbon as an adsorbent has been widely investigated for the adsorption of basic dyes [10–13], but the disposal of the used carbon was very difficult. ⁎ Corresponding author. Xinning Road 18, Xining City, China. Tel.: + 86 971 6304561; fax: + 86 971 6321767. E-mail address: [email protected] (Y. Jia). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.08.038
Recently, considerable attentions have been devoted to probing lowcost materials for the pollutant removal. A lot of works have employed natural and synthetic clays for removing the dyes from wastewater [14,15]. Hectorite has been used as an adsorbent for the removal of cationic dyes and metal ions. The inorganic cations on the clay surface (e.g. Na+, K+, and Ca2+) could be replaced by organic cations through ion exchange and the clay surface became organophilic. Baskaralingam et al. reported that the removal of acid dyes was promoted using cationic surfactant modified hectorites compared with untreated hectorite [16]. However, thermodynamic analysis of the adsorption of acid dye on modified hectorite was not carried out yet. In this paper, cetylpyridinium bromide hectorite (CPB-Hect) was used to remove Neutral Red (NR) from aqueous solution. Effects of various treated conditions, such as adsorbent content and adsorption temperature were investigated. The adsorption isotherms, kinetics and thermodynamics for NR from aqueous solution on modified hectorite were also studied. 2. Experimental 2.1. Materials Hectorite (Zhejiang Institute of Geology and Mineral Resources, China) had a cationic exchange capacity (CEC) of 90 meq/100 g. Synthetic NR (Beijing Dyestuffs Plant, China) was used without further purification. CPB was obtained from Shanghai Shanpu Chemicals Co., Ltd., China. Other agents were of analytical grade and all solutions were prepared with distilled water (the conductivity was very small and could be ignored).
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D. Yue et al. / Desalination 267 (2011) 9–15
2.2. Preparation of modified hectorite The synthesis of modified hectorite was conducted by the following procedure. The calculated amount of CPB was dissolved at 70 °C in water. The above solution was dropwise added into the dispersion of hectorite (2%). The mixture was mechanically stirred at 50 °C for 8 h, filtered and washed several times with distilled water until Br− was completely washed out. The products were dried at 70 °C for 12 h. In this paper, CPB-Hect (6%, 16%, 26%, and 36%) was synthesized with different amounts of CPB. 2.3. Adsorption experiments Batch adsorption experiments were carried out to study the effect factors on removal of NR from aqueous solution. Also, the adsorption isotherm and kinetic were studied. Adsorption experiments were carried out in a thermostatic orbital shaker. 2.3.1. The effect of the content of surfactants The effect of the content of surfactants was carried out by adding fixed amount of CPB-Hect (6%, 16%, 26%, and 36%) in 50 mL of NR solution at fixed concentration of 300 mg/L at equilibrium time. 2.3.2. The effect of pH The effect of pH was conducted by shaking 50 mL of each NR solution adsorbent for 1 h with pH values ranging from 3 to 13. The pH of the solution was adjusted with HCl or NaOH solution by using a pH meter. The removal (%) of NR with different pH values was calculated. 2.3.3. The effect of adsorbent content The effect of adsorbent content was studied with different adsorbent doses in 50 mL of NR solution at a fixed concentration of 300 mg/L at equilibrium time (about 10 h). 2.3.4. Effect of temperature Effect of temperature on dye removal was carried out in 50 mL NR solutions (300 mg/L) by adding 30 mg adsorbent at different temperatures (293, 303, 313, 323, and 333 K) until equilibrium was reached (10 h). 2.3.5. Adsorption isotherm and kinetic experiments Adsorption isotherm experiment was carried out by adding various amounts of adsorbent into 50 mL of NR solution of 300 mg/L. Kinetic studies were carried in 50 mL NR solutions (250 mg/L, 300 mg/L, and 350 mg/L) by adding 30 mg adsorbent at room temperature for predetermined intervals of time. After adsorption equilibrium was reached, the dye dispersion was centrifuged and the concentration was measured at 274 nm using an Ultraviolet–Visible spectrophotometer. Amount of dye uptake, qe (mg/g), was calculated through the following equation: qe =
ðC0 −Ce ÞV m
Fig. 1. XRD powder patterns of hectorite (A), CPB-Hect (B).
peak of hectorite was 6.93°, corresponding to a basal spacing of 12.7 Å (Fig. 1A). After intercalation with CPB, the typical diffraction peak of CPB-Hect moves to lower angle (5.20°), standing for a basal spacing of 16.9 Å (Fig. 1B). The increase of the basal spacing for CPB-Hect indicates that the organic cations intercalate into the interlayer space of the clay. Compared with hectorite (Fig. 2A), the bands at 2926 cm− 1, 2852 cm− 1 attributed to the C–H asymmetric and symmetric stretching vibrations of cationic surfactants were observed in CPBHect (Fig. 2B). Moreover, the bending vibrations of –CH3 (1489 cm− 1) were observed on the FTIR spectra of CPB-Hect. The intensity of –OH bending vibration (1635 cm− 1–1637 cm− 1) of H2O in CPB-Hect decrease indicated that the H2O content reduced as a result of the replacement of the hydrated cations by surfactant cations. This result showed that the surface properties of hectorite change from hydrophilic to hydrophobic by modifying with surfactants. Fig. 3 shows the SEM micrographs of hectorite and CPB-Hect. Since all the samples were firstly dispersed in ethanol before observation, the dispersibility of the samples in ethanol displays the great influence on the morphologies of hectorites. Hectorite (Fig. 3A) forms agglomerations because it could not disperse evenly in ethanol. The surface morphology of modified hectorite (Fig. 3B) was completely different, having a platy and coral like structure, since the organic modified hectorite could disperse well in ethanol. As a result of the intercalation of the organic cations into the interlayer of hectorite, the surface properties of hectorite change from hydrophilic to hydrophobic.
ð1Þ
where C0 is the initial concentration of the dye in solution (mg/L), Ce is the dye concentration at equilibrium (mg/L), m is the amount of adsorbent (g) and V is the volume of the solution (L). 3. Results and discussion 3.1. Structural characteristics The XRD (Fig. 1), FTIR (Fig. 2) and SEM (Fig. 3) of hectorite and CPB-Hect have been studied by our lab group [17]. A typical diffraction
Fig. 2. FTIR spectra of hectorite (A), CPB-Hect (B).
D. Yue et al. / Desalination 267 (2011) 9–15
11
120 B A
Removal (%)
100
B
80 60 40 20 A
0 0.00
0.04
0.08
0.12
0.16
Adsorbent mass (g) Fig. 4. Effect of adsorbent content on the adsorption of CR on hectorite (A) and CPBHect (B).
NR molecule. The Van der Waals interaction between the phenyl ring of NR and –CH2– group of modified hectorite through hydrogen bonds was another driving force for the adsorption process [18,19].
3.2. Results of the adsorption experiments 3.2.1. Effect of the content of surfactants Table 1 gave the adsorbent capacity of CPB-hectorite modified with different contents of CPB (wt.% = 6%, 16%, 26%, and 36%). From the different adsorbent capacities, it was noted that the adsorption capacity increased with the incremental quality of CPB. CPB-Hect (36%) was synthesized by the maximal CEC of hectorite. In the following experiments, CPB-Hect (36%) was to be the adsorbents. 3.2.2. Effect of adsorbent content Fig. 4 showed the effect of adsorbent content on the percentage of NR adsorption on hectorite and modified hectorite. For organic modified hectorite, the percentage of NR adsorption increased rapidly with the adsorbent content. The dye removal percentage reached 100% when 0.11 g adsorbent was used (Fig. 4B). In contrast, the dye removal percentage never exceeded 16% for hectorite (Fig. 4A). The results showed that the adsorption capacity of hectorite increased rapidly when intercalated by organic surfactant cations. It could be interpreted from present adsorption process that the hydrophobic surface of modified hectorite had more preference for the dissociated species of NR in aqueous solution. In addition, there was an electrostatic attraction between the organic cationic groups in modified hectorite and the anionic N(CH3)− group in
3.2.4. The isotherm analysis An adsorption isotherm showed how the adsorbate molecules partition between the liquid and solid phases when the adsorption process reached equilibrium conditions [20]. To determine the relationship between NR adsorbed (qe) and the equilibrium concentration of NR (Ce), the two most commonly used models were the Langmuir and the Freundlich isotherms.
100 A B
B
80
Removal (%)
Fig. 3. SEM images of hectorite (A), CPB-Hect (B).
3.2.3. Effect of pH studies The pH was known to affect the structural stability of NR and its color intensity. Hence, the effect of initial pH was studied with blank NR solution with a concentration of 300 mg/L. The solution was kept for 1 h after pH adjustment. It was found that the color was stable at initial pH around 5. Fig. 5A showed the color removal without adsorbents over an initial pH range of 3–13. The pH reduced the color above pH 7. The color change was negligible with a range of 3–6. The results indicated that the molecular form of NR in solution medium changed markedly in the pH above 7. Fig. 5B showed the effect of initial pH on the adsorption of NR with CPBHect for 300 mg/L initial dye concentration. More than 90% NR removal was observed in the pH range 3–13. In this paper, the initial pH value of NR solution was 5.
A
60
40
20
0
Table 1 Adsorption capacity of different contents of CPB in CPB-Hect. Adsorbent
CPB-hectorite
Content Adsorption
6% 85.96
2
4
6
8
10
12
Initial pH value 16% 145.02
26% 183.54
36% 211.43
Fig. 5. Effect of pH on removal of NR from aqueous solution A: without absorbent and B: with absorbent.
D. Yue et al. / Desalination 267 (2011) 9–15
0.6
250
0.5
200
0.4
150
qt (mg/g)
Ce/qt (g/L)
12
0.3
250mg/L 300mg/L 350mg/L
100
50
0.2
0
0.1 60
90
120
150
0
180
20
40
60
80
100
120
t (min)
Ce (mg/L) Fig. 6. Langmuir plots for the adsorption of CR on CPB-Hect.
Fig. 7. Adsorption kinetics NR onto CPB-Hect.
The Langmuir isotherm was most widely used for the adsorption of pollutants from liquid solution. The linear form of the Langmuir isotherm was represented as Eq. (2):
assumed that adsorbent surface sites had a spectrum of different binding energies. The linear form of the Freundlich isotherm was given by the following equation [22]:
Ce 1 C = + e K b q∞ qe q∞
log qe = log KF +
ð2Þ
where q∞ (mg/g) and Kb (L/mg) were the Langmuir isotherm coefficients. The value of q∞ represented the maximum adsorption capacity. The adsorption data were analyzed according to Eq. (2). Fig. 6 represented Langmuir isotherm for modified hectorite and the values of the Langmuir constants q∞, Kb and RL with the correlation coefficient r2L were listed in Table 2. The isotherm was found to be linear over the entire concentration range with a good linear correlation coefficient (r2L N 0.9996), showing that the data correctly fitted the Langmuir isotherm. This indicated that the surface of modified hectorite was covered by a monolayer of NR molecules. Similar behaviors were also found for the adsorption of NR on activated carbon [21]. The q∞ (mg/g) for the adsorption of NR by activated carbon, organobentonites, Mn-impregnated activated were 55.17 mg/g, 175.4 mg/g, and 196.08–285.71 mg/g, respectively. The adsorption capacity of CPB-Hect was found to be 393.70 mg/g. Therefore, surfactant modified hectorite could be effectively used as an adsorbent in treatment of NR wastewater. The essential characteristics of the Langmuir isotherm could be expressed in terms of dimensionless separation parameters RL, which was indicative of the isotherm shape that predicted whether an adsorption system was favorable or unfavorable. RL was defined as Eq. (3): 1 1 + KbC0
ð4Þ
where qe was the amount of dye adsorbed (mg/g) on the adsorbent. The magnitude of the component n gave an indication of the favorability and KF (L/g) was the Freundlich constant related to the binding energy. The value of n between 1 and 10 indicated beneficial adsorption. The values of the Freundlich constants together with the correlation coefficient r2F were presented in Table 2. 3.3. Adsorption kinetics A study of adsorption kinetics was desirable as it provided information about the mechanism of adsorption, which was important for optimising the efficiency of the process. Several kinetic models such as pseudo-first-order, pseudo-second-order [23], Elovich equation and intraparticle diffusion models were applied for the experimental data. Effect of contact time and initial NR concentration on adsorption of NR by CPB-Hect were shown in Fig. 7. The amount of NR adsorbed onto CPB-Hect increased as the time increased. Most of NR molecules adsorption took place at the initial 100 min of the experiment. As the time progressed, there was a weak increase in adsorption even after a long time.
ð3Þ 2.5
where C0 (mg/L) was the initial dye concentration and Kb (L/mg) was the Langmuir constant related to the energy of the adsorption. The value of RL indicated the type of the isotherm to be either unfavorable (RL N 1), linear (RL = 1), favorable (0 b RL b 1) or irreversible (RL = 0). It could be observed in Table 2 that the value of RL in the range of 0–1 indicated the favorable uptake of the NR process. The Freundlich equation was basically an empirical equation and employed to describe the heterogeneous systems. The isotherm
-3
R2=0.947 k1=1.34x10 -3 2 R =0.962 k1=2.95x10 2 -3 R =0.916 k1=3.16x10
YA=-0.00058X+2.3679 YB=-0.00128X+2.3679 YC=-0.00158X+2.3679
250mg/L 300mg/L 350mg/L
2.4
log (qe-qt)
RL =
1 log Ce n
2.3
C
2.2 B
2.1 Table 2 Langmuir and Freundlich constants for the adsorption of NR on CPB-Hect. Adsorbent
CPB-Hect
Langmuir constants
A
2.0
Freundlich constants
q∞ (mg/g)
Kb (L/mg)
RL
r2L
KF (L/g)
n
r2F
393.70
0.0174
0.1609
0.9996
23.6990
1.9603
0.9872
20
40
60
80
100
120
t (min) Fig. 8. The pseudo-first-order kinetics for the adsorption of NR on CPB-Hect. (A, 300 mg/L; B, 350 mg/L; C, 250 mg/L).
D. Yue et al. / Desalination 267 (2011) 9–15 -3
1.5
YA =0.00799X+0.2989 R2=0.998 k1=2.39x10 -4 YB =0.00802X+0.8658 R2=0.999 k1=6.49x10 -3 2 YC=0.0438X+0.04294 R =0.998 k1=1.88x10
Table 3 The Elovich and intraparticle diffusion model parameter constants for the adsorption of NR on CPB-Hect.
250mg/L 300mg/L 350mg/L A
t/qt (g min/mg)
1.2
B
0.9
C
0.6
13
Concentration mg/L
The Elovich model α (mg/ (g min))
β (g/mg)
R2E
kp (mg/ (g min1/2))
Intraparticle diffusion model C (mg/g)
r2p
250 300 350
316.174 136.531 6.832
0.0335 0.0596 0.0349
0.9759 0.9995 0.9985
8.0886 4.5836 7.8458
127.442 66.788 12.786
0.9418 0.9829 0.9876
0.3
concluded that the mechanism of adsorption was pseudo-secondorder reaction.
0.0 40
60
80
100
120
3.3.3. The Elovich equation The linear form of Elovich equation was given as Eq. (7):
t (min) Fig. 9. The pseudo-second-order kinetics for the adsorption of NR on CPB-Hect. (A, 350 mg/L; B, 300 mg/L; C, 250 mg/L).
3.3.1. Pseudo-first-order equation The pseudo-first-order kinetic model was one of the most widely used for the sorption of a solute from liquid solution [24] and was represented as Eq. (5): k t logðqe −qt Þ = log qe − 1 2:303
ð5Þ
where qe(mg/g) was the mass of dye adsorbed at equilibrium, qt (mg/g) was the mass of dye adsorbed at time t, K1 was the first-order reaction rate constant (L/min). The rate constants as shown in Fig. 8 were obtained from the straight line plots of log(qe − qt) against t. 3.3.2. Pseudo-second-order equation The pseudo-second-order model was based on the assumption of chemisorption of the adsorbate on the adsorbent [25]. This model was given as Eq. (6): t 1 1 = + t qt qe k2 q2e
ð6Þ
where k2 (g/(mg min)) was the equilibrium rate constant of pseudosecond-order equation. The straight line plots of t/qt against t were tested to obtain the rate parameters (Fig. 9). The correlation coefficients (r2) of the pseudo-second-order model were higher than the other model. The experimental qe values studied for CPB-Hect were close to qe values calculated from the pseudo-second-order kinetic model. It could be 210
qt (mg/g)
1 1 lnðαβÞ + ln t β β
3.3.4. Intraparticle diffusion equation The dye adsorption was governed usually by either the liquid phase mass transport rate or the intraparticle mass transport rate. When the diffusion (internal surface and pore diffusion) of dye molecules inside the adsorbent was the rate-limiting step, the adsorption data could be presented by the following equation: qt = Kp t
1=2
120
B C
ð8Þ
+C
where the parameter kp was the diffusion coefficient value, t was the time and qt was the amount of dye adsorbed. According to this model, plot of qt versus t1/2 should be linear if intraparticle diffusion was involved in the adsorption process. If the line passed through the origin, intraparticle diffusion was the rate controlling step. If not, the intraparticle diffusion was not the only rate-limiting step. Intraparticle diffusion rate constants could be obtained from the amount of dye adsorbed versus t1/2 plots as shown in Fig. 11. It could be observed in Table 3 that the correlation coefficients of this diffusion model were
240 200
150
ð7Þ
where α(mg/(g min)) was the initial adsorption rate constant and the parameter β(g/mg) was related to the extent of surface coverage and activation energy for chemisorption. The values of α and β could be calculated from the plot of qt against lnt (Fig. 10) and the constants were given in Table 3.
A
250mg/L 300mg/L 350mg/L
180
qt =
qt (mg/g)
20
250mg/L 300mg/L 350mg/L
A
160 120
B C
90
80
60
40 0
30 3.0
3.5
4.0
4.5
5.0
lnt Fig. 10. The Elovich equation kinetics for the adsorption of NR on CPB-Hect. (A, 250 mg/ L; B, 300 mg/L; C, 350 mg/L).
4
6
8
10
12
t1/2 Fig. 11. The intraparticle diffusion kinetics model for the adsorption of NR on CPB-Hect. (A, 250 mg/L; B, 300 mg/L; C, 350 mg/L).
14
D. Yue et al. / Desalination 267 (2011) 9–15
2.4
(124.61 J/(mol K)) corresponded to an increase in the degree of freedom of the adsorbed species.
2.0 Y=-4300.187X+14.934 R2=0.999
lnKC
1.6
4. Conclusions The results of these studies clearly indicated that compared with hectorite, the adsorption capacity for CPB-Hect was calculated to be 393.7 mg/g and was greatly improved. The adsorption isotherm data were fitted well with Langmuir isotherm while the kinetic data were evaluated by the pseudo-second-order kinetic model. The negative values of ΔG0 revealed that the adsorption process was spontaneous. The positive values of ΔH0 and ΔS0 showed the endothermic nature and an increase in disorder of NR molecules in the adsorption process, respectively.
1.2 0.8 0.4 0.0 0.0030
0.0031
0.0032
0.0033
0.0034
1/T (K-1) Fig. 12. Plot of ln KC versus 1/T for estimation of thermodynamic parameters.
Acknowledgements
above 0.97, which indicated that the adsorption of NR on modified hectorite could be followed by an intraparticle diffusion. These lines did not pass through the origin, showing that intraparticle diffusion was not the only rate-limiting mechanism.
This work was financially supported by the General Project of Natural Science Foundation of China (No. 20976184) and the Science and Technology Brainstorm Project of Qinghai Province (No. 2007G112).
3.4. Thermodynamic studies
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KC =
CA CS
ð9Þ
0
ΔG = −RT ln KC ln KC =
ð10Þ
ΔS0 ΔH 0 − R RT
ð11Þ
where KC was the equilibrium constant, CA was the amount of dye adsorbed per unit mass of the adsorbent (mg/g), CS was the equilibrium concentration (mg/L) of the dye solution, T was the temperature in Kelvin and R was the gas constant. The q∞ calculated from the Lamgmuir isotherm was used to obtain CA and CS [26]. ΔH0 and ΔS0 were calculated from the slope and intercept of van't Hoff plots of lnKC versus 1/T (Fig. 12). The adsorption capacity, KC, ΔH0, ΔS0 and ΔG0 at different temperatures for the initial dye concentration of 300 mg/L were listed in Table 4. The Gibbs free energy changes during the adsorption process were all negative, indicating that the adsorption process of NR on modified hectorite was spontaneous. The value of the enthalpy change (35.752 kJ/mol) indicated the adsorption was physical in nature involving forces of attraction and was also endothermic. The increase of the adsorption capacity with temperature increase also indicated the endothermic nature of the adsorption. The positive value of ΔS0
Table 4 Thermodynamic parameters for the adsorption of NR on CPB-Hect. Adsorbent
Temperature (K)
Adsorption capacity (mg/g)
KC
ΔGο (kJ/mol)
ΔHο (kJ/mol)
ΔSο (J/mol∙K)
CPB-Hect
293 303 313 323 333
212.22 276.30 326.39 366.292 396.624
1.26 2.15 3.34 5.16 7.33
− 0.563 − 1.925 − 3.141 − 4.407 − 5.515
35.752
124.161
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Removal of Neutral Red from aqueous solution by using modified hectorite Duyuan Yue a,b, Yan Jing a, Jun Ma a, Chenglong Xia a,b, Xiaojie Yin a,b, Yongzhong Jia a,⁎ a b
Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, China Graduate University of the Chinese Academy of Sciences, Beijing, 100086, China
a r t i c l e
i n f o
Article history: Received 13 April 2010 Received in revised form 22 July 2010 Accepted 27 August 2010 Available online 24 September 2010 Keywords: Neutral Red Adsorption Modified hectorite Kinetics Thermodynamics
a b s t r a c t The object of this work was to study the modified hectorite as effective adsorbent for Neutral Red (NR) from aqueous solution. The adsorbent capacity of modified hectorite was discussed. The effects of surfactant content, adsorbent content, pH and adsorption temperature on the sorption of NR on modified hectorite were studied. Experimental results showed that the equilibrium adsorption data fitted well with Langmuir isotherm and the adsorption capacity was 393.70 mg/g for the modified cetylpyridinium bromide hectorite (CPB-Hect). Kinetic studies showed that the dynamical data fitted well with the pseudo-second-order kinetic model. For thermodynamic studies, parameters such as the Gibbs free energy (ΔG0), the enthalpy (ΔH0) and the entropy (ΔS0) indicated that the adsorption process was spontaneous and endothermic in nature. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Generally, dyes were stable to light, heat, and oxidizing agents. It had potential applications in various industries, such as textile, leather, paper, plastic, etc., to color the final products [1]. The extensive use of dyes often posed pollution problems in the form of colored wastewater. Environmental pollution caused by industrial wastewater had become a common problem in many countries. Most dyes caused damage not only to aquatic life, but also to human beings because it is toxic, mutagenic or carcinogenic [2]. Therefore, the removal of dyes from waste effluents was important. A wide range of methods, such as coagulation, electrocoagulation, flotation, chemical oxidation, filtration, membrane separation, and microbial degradation [3], have been developed for the removal of synthetic dyes from waters and wastewaters to decrease its impact in the environment. These methods might suffer from one or more limitations and could not remove the dyestuff from wastewater completely [4,5]. In contrast, adsorption was popular nowadays because of its easy operation and versatility. Previous research on the adsorption of dyes from aqueous solution revealed numerous economic adsorbents, such as coal, fly ash, wood, biogas waste slurry, waste banana pith, waste orange peel, waste red mud and agricultural wastes [6–9]. Unfortunately, the adsorption capacities of these materials were limited. Activated carbon as an adsorbent has been widely investigated for the adsorption of basic dyes [10–13], but the disposal of the used carbon was very difficult. ⁎ Corresponding author. Xinning Road 18, Xining City, China. Tel.: + 86 971 6304561; fax: + 86 971 6321767. E-mail address: [email protected] (Y. Jia). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.08.038
Recently, considerable attentions have been devoted to probing lowcost materials for the pollutant removal. A lot of works have employed natural and synthetic clays for removing the dyes from wastewater [14,15]. Hectorite has been used as an adsorbent for the removal of cationic dyes and metal ions. The inorganic cations on the clay surface (e.g. Na+, K+, and Ca2+) could be replaced by organic cations through ion exchange and the clay surface became organophilic. Baskaralingam et al. reported that the removal of acid dyes was promoted using cationic surfactant modified hectorites compared with untreated hectorite [16]. However, thermodynamic analysis of the adsorption of acid dye on modified hectorite was not carried out yet. In this paper, cetylpyridinium bromide hectorite (CPB-Hect) was used to remove Neutral Red (NR) from aqueous solution. Effects of various treated conditions, such as adsorbent content and adsorption temperature were investigated. The adsorption isotherms, kinetics and thermodynamics for NR from aqueous solution on modified hectorite were also studied. 2. Experimental 2.1. Materials Hectorite (Zhejiang Institute of Geology and Mineral Resources, China) had a cationic exchange capacity (CEC) of 90 meq/100 g. Synthetic NR (Beijing Dyestuffs Plant, China) was used without further purification. CPB was obtained from Shanghai Shanpu Chemicals Co., Ltd., China. Other agents were of analytical grade and all solutions were prepared with distilled water (the conductivity was very small and could be ignored).
10
D. Yue et al. / Desalination 267 (2011) 9–15
2.2. Preparation of modified hectorite The synthesis of modified hectorite was conducted by the following procedure. The calculated amount of CPB was dissolved at 70 °C in water. The above solution was dropwise added into the dispersion of hectorite (2%). The mixture was mechanically stirred at 50 °C for 8 h, filtered and washed several times with distilled water until Br− was completely washed out. The products were dried at 70 °C for 12 h. In this paper, CPB-Hect (6%, 16%, 26%, and 36%) was synthesized with different amounts of CPB. 2.3. Adsorption experiments Batch adsorption experiments were carried out to study the effect factors on removal of NR from aqueous solution. Also, the adsorption isotherm and kinetic were studied. Adsorption experiments were carried out in a thermostatic orbital shaker. 2.3.1. The effect of the content of surfactants The effect of the content of surfactants was carried out by adding fixed amount of CPB-Hect (6%, 16%, 26%, and 36%) in 50 mL of NR solution at fixed concentration of 300 mg/L at equilibrium time. 2.3.2. The effect of pH The effect of pH was conducted by shaking 50 mL of each NR solution adsorbent for 1 h with pH values ranging from 3 to 13. The pH of the solution was adjusted with HCl or NaOH solution by using a pH meter. The removal (%) of NR with different pH values was calculated. 2.3.3. The effect of adsorbent content The effect of adsorbent content was studied with different adsorbent doses in 50 mL of NR solution at a fixed concentration of 300 mg/L at equilibrium time (about 10 h). 2.3.4. Effect of temperature Effect of temperature on dye removal was carried out in 50 mL NR solutions (300 mg/L) by adding 30 mg adsorbent at different temperatures (293, 303, 313, 323, and 333 K) until equilibrium was reached (10 h). 2.3.5. Adsorption isotherm and kinetic experiments Adsorption isotherm experiment was carried out by adding various amounts of adsorbent into 50 mL of NR solution of 300 mg/L. Kinetic studies were carried in 50 mL NR solutions (250 mg/L, 300 mg/L, and 350 mg/L) by adding 30 mg adsorbent at room temperature for predetermined intervals of time. After adsorption equilibrium was reached, the dye dispersion was centrifuged and the concentration was measured at 274 nm using an Ultraviolet–Visible spectrophotometer. Amount of dye uptake, qe (mg/g), was calculated through the following equation: qe =
ðC0 −Ce ÞV m
Fig. 1. XRD powder patterns of hectorite (A), CPB-Hect (B).
peak of hectorite was 6.93°, corresponding to a basal spacing of 12.7 Å (Fig. 1A). After intercalation with CPB, the typical diffraction peak of CPB-Hect moves to lower angle (5.20°), standing for a basal spacing of 16.9 Å (Fig. 1B). The increase of the basal spacing for CPB-Hect indicates that the organic cations intercalate into the interlayer space of the clay. Compared with hectorite (Fig. 2A), the bands at 2926 cm− 1, 2852 cm− 1 attributed to the C–H asymmetric and symmetric stretching vibrations of cationic surfactants were observed in CPBHect (Fig. 2B). Moreover, the bending vibrations of –CH3 (1489 cm− 1) were observed on the FTIR spectra of CPB-Hect. The intensity of –OH bending vibration (1635 cm− 1–1637 cm− 1) of H2O in CPB-Hect decrease indicated that the H2O content reduced as a result of the replacement of the hydrated cations by surfactant cations. This result showed that the surface properties of hectorite change from hydrophilic to hydrophobic by modifying with surfactants. Fig. 3 shows the SEM micrographs of hectorite and CPB-Hect. Since all the samples were firstly dispersed in ethanol before observation, the dispersibility of the samples in ethanol displays the great influence on the morphologies of hectorites. Hectorite (Fig. 3A) forms agglomerations because it could not disperse evenly in ethanol. The surface morphology of modified hectorite (Fig. 3B) was completely different, having a platy and coral like structure, since the organic modified hectorite could disperse well in ethanol. As a result of the intercalation of the organic cations into the interlayer of hectorite, the surface properties of hectorite change from hydrophilic to hydrophobic.
ð1Þ
where C0 is the initial concentration of the dye in solution (mg/L), Ce is the dye concentration at equilibrium (mg/L), m is the amount of adsorbent (g) and V is the volume of the solution (L). 3. Results and discussion 3.1. Structural characteristics The XRD (Fig. 1), FTIR (Fig. 2) and SEM (Fig. 3) of hectorite and CPB-Hect have been studied by our lab group [17]. A typical diffraction
Fig. 2. FTIR spectra of hectorite (A), CPB-Hect (B).
D. Yue et al. / Desalination 267 (2011) 9–15
11
120 B A
Removal (%)
100
B
80 60 40 20 A
0 0.00
0.04
0.08
0.12
0.16
Adsorbent mass (g) Fig. 4. Effect of adsorbent content on the adsorption of CR on hectorite (A) and CPBHect (B).
NR molecule. The Van der Waals interaction between the phenyl ring of NR and –CH2– group of modified hectorite through hydrogen bonds was another driving force for the adsorption process [18,19].
3.2. Results of the adsorption experiments 3.2.1. Effect of the content of surfactants Table 1 gave the adsorbent capacity of CPB-hectorite modified with different contents of CPB (wt.% = 6%, 16%, 26%, and 36%). From the different adsorbent capacities, it was noted that the adsorption capacity increased with the incremental quality of CPB. CPB-Hect (36%) was synthesized by the maximal CEC of hectorite. In the following experiments, CPB-Hect (36%) was to be the adsorbents. 3.2.2. Effect of adsorbent content Fig. 4 showed the effect of adsorbent content on the percentage of NR adsorption on hectorite and modified hectorite. For organic modified hectorite, the percentage of NR adsorption increased rapidly with the adsorbent content. The dye removal percentage reached 100% when 0.11 g adsorbent was used (Fig. 4B). In contrast, the dye removal percentage never exceeded 16% for hectorite (Fig. 4A). The results showed that the adsorption capacity of hectorite increased rapidly when intercalated by organic surfactant cations. It could be interpreted from present adsorption process that the hydrophobic surface of modified hectorite had more preference for the dissociated species of NR in aqueous solution. In addition, there was an electrostatic attraction between the organic cationic groups in modified hectorite and the anionic N(CH3)− group in
3.2.4. The isotherm analysis An adsorption isotherm showed how the adsorbate molecules partition between the liquid and solid phases when the adsorption process reached equilibrium conditions [20]. To determine the relationship between NR adsorbed (qe) and the equilibrium concentration of NR (Ce), the two most commonly used models were the Langmuir and the Freundlich isotherms.
100 A B
B
80
Removal (%)
Fig. 3. SEM images of hectorite (A), CPB-Hect (B).
3.2.3. Effect of pH studies The pH was known to affect the structural stability of NR and its color intensity. Hence, the effect of initial pH was studied with blank NR solution with a concentration of 300 mg/L. The solution was kept for 1 h after pH adjustment. It was found that the color was stable at initial pH around 5. Fig. 5A showed the color removal without adsorbents over an initial pH range of 3–13. The pH reduced the color above pH 7. The color change was negligible with a range of 3–6. The results indicated that the molecular form of NR in solution medium changed markedly in the pH above 7. Fig. 5B showed the effect of initial pH on the adsorption of NR with CPBHect for 300 mg/L initial dye concentration. More than 90% NR removal was observed in the pH range 3–13. In this paper, the initial pH value of NR solution was 5.
A
60
40
20
0
Table 1 Adsorption capacity of different contents of CPB in CPB-Hect. Adsorbent
CPB-hectorite
Content Adsorption
6% 85.96
2
4
6
8
10
12
Initial pH value 16% 145.02
26% 183.54
36% 211.43
Fig. 5. Effect of pH on removal of NR from aqueous solution A: without absorbent and B: with absorbent.
D. Yue et al. / Desalination 267 (2011) 9–15
0.6
250
0.5
200
0.4
150
qt (mg/g)
Ce/qt (g/L)
12
0.3
250mg/L 300mg/L 350mg/L
100
50
0.2
0
0.1 60
90
120
150
0
180
20
40
60
80
100
120
t (min)
Ce (mg/L) Fig. 6. Langmuir plots for the adsorption of CR on CPB-Hect.
Fig. 7. Adsorption kinetics NR onto CPB-Hect.
The Langmuir isotherm was most widely used for the adsorption of pollutants from liquid solution. The linear form of the Langmuir isotherm was represented as Eq. (2):
assumed that adsorbent surface sites had a spectrum of different binding energies. The linear form of the Freundlich isotherm was given by the following equation [22]:
Ce 1 C = + e K b q∞ qe q∞
log qe = log KF +
ð2Þ
where q∞ (mg/g) and Kb (L/mg) were the Langmuir isotherm coefficients. The value of q∞ represented the maximum adsorption capacity. The adsorption data were analyzed according to Eq. (2). Fig. 6 represented Langmuir isotherm for modified hectorite and the values of the Langmuir constants q∞, Kb and RL with the correlation coefficient r2L were listed in Table 2. The isotherm was found to be linear over the entire concentration range with a good linear correlation coefficient (r2L N 0.9996), showing that the data correctly fitted the Langmuir isotherm. This indicated that the surface of modified hectorite was covered by a monolayer of NR molecules. Similar behaviors were also found for the adsorption of NR on activated carbon [21]. The q∞ (mg/g) for the adsorption of NR by activated carbon, organobentonites, Mn-impregnated activated were 55.17 mg/g, 175.4 mg/g, and 196.08–285.71 mg/g, respectively. The adsorption capacity of CPB-Hect was found to be 393.70 mg/g. Therefore, surfactant modified hectorite could be effectively used as an adsorbent in treatment of NR wastewater. The essential characteristics of the Langmuir isotherm could be expressed in terms of dimensionless separation parameters RL, which was indicative of the isotherm shape that predicted whether an adsorption system was favorable or unfavorable. RL was defined as Eq. (3): 1 1 + KbC0
ð4Þ
where qe was the amount of dye adsorbed (mg/g) on the adsorbent. The magnitude of the component n gave an indication of the favorability and KF (L/g) was the Freundlich constant related to the binding energy. The value of n between 1 and 10 indicated beneficial adsorption. The values of the Freundlich constants together with the correlation coefficient r2F were presented in Table 2. 3.3. Adsorption kinetics A study of adsorption kinetics was desirable as it provided information about the mechanism of adsorption, which was important for optimising the efficiency of the process. Several kinetic models such as pseudo-first-order, pseudo-second-order [23], Elovich equation and intraparticle diffusion models were applied for the experimental data. Effect of contact time and initial NR concentration on adsorption of NR by CPB-Hect were shown in Fig. 7. The amount of NR adsorbed onto CPB-Hect increased as the time increased. Most of NR molecules adsorption took place at the initial 100 min of the experiment. As the time progressed, there was a weak increase in adsorption even after a long time.
ð3Þ 2.5
where C0 (mg/L) was the initial dye concentration and Kb (L/mg) was the Langmuir constant related to the energy of the adsorption. The value of RL indicated the type of the isotherm to be either unfavorable (RL N 1), linear (RL = 1), favorable (0 b RL b 1) or irreversible (RL = 0). It could be observed in Table 2 that the value of RL in the range of 0–1 indicated the favorable uptake of the NR process. The Freundlich equation was basically an empirical equation and employed to describe the heterogeneous systems. The isotherm
-3
R2=0.947 k1=1.34x10 -3 2 R =0.962 k1=2.95x10 2 -3 R =0.916 k1=3.16x10
YA=-0.00058X+2.3679 YB=-0.00128X+2.3679 YC=-0.00158X+2.3679
250mg/L 300mg/L 350mg/L
2.4
log (qe-qt)
RL =
1 log Ce n
2.3
C
2.2 B
2.1 Table 2 Langmuir and Freundlich constants for the adsorption of NR on CPB-Hect. Adsorbent
CPB-Hect
Langmuir constants
A
2.0
Freundlich constants
q∞ (mg/g)
Kb (L/mg)
RL
r2L
KF (L/g)
n
r2F
393.70
0.0174
0.1609
0.9996
23.6990
1.9603
0.9872
20
40
60
80
100
120
t (min) Fig. 8. The pseudo-first-order kinetics for the adsorption of NR on CPB-Hect. (A, 300 mg/L; B, 350 mg/L; C, 250 mg/L).
D. Yue et al. / Desalination 267 (2011) 9–15 -3
1.5
YA =0.00799X+0.2989 R2=0.998 k1=2.39x10 -4 YB =0.00802X+0.8658 R2=0.999 k1=6.49x10 -3 2 YC=0.0438X+0.04294 R =0.998 k1=1.88x10
Table 3 The Elovich and intraparticle diffusion model parameter constants for the adsorption of NR on CPB-Hect.
250mg/L 300mg/L 350mg/L A
t/qt (g min/mg)
1.2
B
0.9
C
0.6
13
Concentration mg/L
The Elovich model α (mg/ (g min))
β (g/mg)
R2E
kp (mg/ (g min1/2))
Intraparticle diffusion model C (mg/g)
r2p
250 300 350
316.174 136.531 6.832
0.0335 0.0596 0.0349
0.9759 0.9995 0.9985
8.0886 4.5836 7.8458
127.442 66.788 12.786
0.9418 0.9829 0.9876
0.3
concluded that the mechanism of adsorption was pseudo-secondorder reaction.
0.0 40
60
80
100
120
3.3.3. The Elovich equation The linear form of Elovich equation was given as Eq. (7):
t (min) Fig. 9. The pseudo-second-order kinetics for the adsorption of NR on CPB-Hect. (A, 350 mg/L; B, 300 mg/L; C, 250 mg/L).
3.3.1. Pseudo-first-order equation The pseudo-first-order kinetic model was one of the most widely used for the sorption of a solute from liquid solution [24] and was represented as Eq. (5): k t logðqe −qt Þ = log qe − 1 2:303
ð5Þ
where qe(mg/g) was the mass of dye adsorbed at equilibrium, qt (mg/g) was the mass of dye adsorbed at time t, K1 was the first-order reaction rate constant (L/min). The rate constants as shown in Fig. 8 were obtained from the straight line plots of log(qe − qt) against t. 3.3.2. Pseudo-second-order equation The pseudo-second-order model was based on the assumption of chemisorption of the adsorbate on the adsorbent [25]. This model was given as Eq. (6): t 1 1 = + t qt qe k2 q2e
ð6Þ
where k2 (g/(mg min)) was the equilibrium rate constant of pseudosecond-order equation. The straight line plots of t/qt against t were tested to obtain the rate parameters (Fig. 9). The correlation coefficients (r2) of the pseudo-second-order model were higher than the other model. The experimental qe values studied for CPB-Hect were close to qe values calculated from the pseudo-second-order kinetic model. It could be 210
qt (mg/g)
1 1 lnðαβÞ + ln t β β
3.3.4. Intraparticle diffusion equation The dye adsorption was governed usually by either the liquid phase mass transport rate or the intraparticle mass transport rate. When the diffusion (internal surface and pore diffusion) of dye molecules inside the adsorbent was the rate-limiting step, the adsorption data could be presented by the following equation: qt = Kp t
1=2
120
B C
ð8Þ
+C
where the parameter kp was the diffusion coefficient value, t was the time and qt was the amount of dye adsorbed. According to this model, plot of qt versus t1/2 should be linear if intraparticle diffusion was involved in the adsorption process. If the line passed through the origin, intraparticle diffusion was the rate controlling step. If not, the intraparticle diffusion was not the only rate-limiting step. Intraparticle diffusion rate constants could be obtained from the amount of dye adsorbed versus t1/2 plots as shown in Fig. 11. It could be observed in Table 3 that the correlation coefficients of this diffusion model were
240 200
150
ð7Þ
where α(mg/(g min)) was the initial adsorption rate constant and the parameter β(g/mg) was related to the extent of surface coverage and activation energy for chemisorption. The values of α and β could be calculated from the plot of qt against lnt (Fig. 10) and the constants were given in Table 3.
A
250mg/L 300mg/L 350mg/L
180
qt =
qt (mg/g)
20
250mg/L 300mg/L 350mg/L
A
160 120
B C
90
80
60
40 0
30 3.0
3.5
4.0
4.5
5.0
lnt Fig. 10. The Elovich equation kinetics for the adsorption of NR on CPB-Hect. (A, 250 mg/ L; B, 300 mg/L; C, 350 mg/L).
4
6
8
10
12
t1/2 Fig. 11. The intraparticle diffusion kinetics model for the adsorption of NR on CPB-Hect. (A, 250 mg/L; B, 300 mg/L; C, 350 mg/L).
14
D. Yue et al. / Desalination 267 (2011) 9–15
2.4
(124.61 J/(mol K)) corresponded to an increase in the degree of freedom of the adsorbed species.
2.0 Y=-4300.187X+14.934 R2=0.999
lnKC
1.6
4. Conclusions The results of these studies clearly indicated that compared with hectorite, the adsorption capacity for CPB-Hect was calculated to be 393.7 mg/g and was greatly improved. The adsorption isotherm data were fitted well with Langmuir isotherm while the kinetic data were evaluated by the pseudo-second-order kinetic model. The negative values of ΔG0 revealed that the adsorption process was spontaneous. The positive values of ΔH0 and ΔS0 showed the endothermic nature and an increase in disorder of NR molecules in the adsorption process, respectively.
1.2 0.8 0.4 0.0 0.0030
0.0031
0.0032
0.0033
0.0034
1/T (K-1) Fig. 12. Plot of ln KC versus 1/T for estimation of thermodynamic parameters.
Acknowledgements
above 0.97, which indicated that the adsorption of NR on modified hectorite could be followed by an intraparticle diffusion. These lines did not pass through the origin, showing that intraparticle diffusion was not the only rate-limiting mechanism.
This work was financially supported by the General Project of Natural Science Foundation of China (No. 20976184) and the Science and Technology Brainstorm Project of Qinghai Province (No. 2007G112).
3.4. Thermodynamic studies
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KC =
CA CS
ð9Þ
0
ΔG = −RT ln KC ln KC =
ð10Þ
ΔS0 ΔH 0 − R RT
ð11Þ
where KC was the equilibrium constant, CA was the amount of dye adsorbed per unit mass of the adsorbent (mg/g), CS was the equilibrium concentration (mg/L) of the dye solution, T was the temperature in Kelvin and R was the gas constant. The q∞ calculated from the Lamgmuir isotherm was used to obtain CA and CS [26]. ΔH0 and ΔS0 were calculated from the slope and intercept of van't Hoff plots of lnKC versus 1/T (Fig. 12). The adsorption capacity, KC, ΔH0, ΔS0 and ΔG0 at different temperatures for the initial dye concentration of 300 mg/L were listed in Table 4. The Gibbs free energy changes during the adsorption process were all negative, indicating that the adsorption process of NR on modified hectorite was spontaneous. The value of the enthalpy change (35.752 kJ/mol) indicated the adsorption was physical in nature involving forces of attraction and was also endothermic. The increase of the adsorption capacity with temperature increase also indicated the endothermic nature of the adsorption. The positive value of ΔS0
Table 4 Thermodynamic parameters for the adsorption of NR on CPB-Hect. Adsorbent
Temperature (K)
Adsorption capacity (mg/g)
KC
ΔGο (kJ/mol)
ΔHο (kJ/mol)
ΔSο (J/mol∙K)
CPB-Hect
293 303 313 323 333
212.22 276.30 326.39 366.292 396.624
1.26 2.15 3.34 5.16 7.33
− 0.563 − 1.925 − 3.141 − 4.407 − 5.515
35.752
124.161
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