Research Paper
Removal of methyl violet from aqueous solution using a stevensite-rich clay from Morocco
Khalid Elass a, ⁎ , Abderrahmane Laachach a , Abdellah Alaoui b , Mohamed Azzi c
a
Laboratory of Environmental Metrology, National School of Mineral Industry (ENIM), BP: 753 Agdal-Rabat, Morocco
b
Laboratory of Mineral treatment, National School of Mineral Industry (ENIM), BP: 753 Agdal-Rabat, Morocco
c
Laboratory of Electrochemistry and Environment, Faculty of Science Aïn Chock, Casablanca, Morocco
a b s t r a c t a r t i c l e i n f o
Article history:
Received 19 January 2011 Received in revised form 9 July 2011 Accepted 23 July 2011
Available online 31 August 2011 Keywords:
Natural clay Methyl violet Adsorption kinetics
An inexpensive and easily available Moroccan natural clay, called locally Ghassoul, was employed for adsorption of methyl violet, a cationic dye, in aqueous solution. The experiments were carried out in a batch system to optimize various experimental parameters such as pH, initial dye concentration, contact time, temperature and ionic strength. The experimental data can be well represented by Langmuir and Freundlich models. The Langmuir monolayer adsorption capacity was estimated as 625 mg/g at 298. Kinetic analyses showed that the adsorption rates were more accurately represented by a pseudo second-order model.
Intraparticle diffusion process was identified as the main mechanism controlling the rate of the dye sorption.
In addition, various thermodynamic activation parameters, such as Gibbs free energy, enthalpy, entropy and the activation energy were calculated. The adsorption process was found to be a spontaneous and endothermic process. The obtained results confirmed the applicability of this clay as an efficient and economical adsorbent for cationic dyes from contaminated water.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Industries such as textile, leather, paper, plastics, etc., use dyes in order to color their products and also consume substantial volumes of water. As a result, they generate a considerable amount of colored wastewater (Ravi et al., 2005). It is estimated that more than 100,000 commercially available dyes with over 7 × 10
5tones of dyestuff produced annually (Pearce et al., 2003; Rafatullaha et al., 2010). Their presence in water, even at very low concentrations, is highly visible and undesirable and may signi fi cantly affect photosynthetic activity in aquatic life (Gücek et al., 2005). The synthetic origin and complex aromatic structures of dyes make them stable and dif fi cult to be biodegraded (Forgacs et al., 2004).
Treatment processes for contaminated waste streams include chemical precipitation, membrane fi ltration, ion exchange, carbon adsorption and co-precipitation/adsorption (Churcley, 1998; Coro and Laha, 2001; Ferrero, 2007; Kang et al., 2000; Stephenson and Sheldon, 1996). Of the numerous techniques mentioned, the adsorption process is one of the effective techniques that have been successfully employed for color removal from wastewater, offering signi fi cant advantages like the low cost, availability, pro fi tability, ease of operation and ef fi ciency, in comparison with conventional methods especially from economical and environmental points of view (Banat et al., 2003; Gupta and Suhas, 2009).
Many adsorbents have been tested to reduce dye concentrations from aqueous solutions. Currently, the most common procedure involves the use of activated carbons (Azizian et al., 2009; Rodríguez et al., 2009; Wu et al., 2005) as adsorbents because of their higher adsorption capacities. Despite the proli fi c use of activated carbon for wastewater treatment, carbon adsorption remains an expensive process, and this fact has recently prompted growing research interest into the production of low-cost alternatives. Various workers have exploited substances such as fl y ash (Janos et al., 2003; Suna et al., 2010), activated carbon prepared from agricultural waste material (Baccar et al., 2010; El-Sheikh and Newman, 2004; Hameed, 2008; Yunus, 2006), perlite (Dogan et al., 2004; Dogan and Alkan, 2003), sepiolite (Alkan et al., 2005; Ugurlu, 2009), kaolinite (Karao ğ lu et al., 2010), bentonite (Tahir and Rauf, 2006), chitosan (Kamari et al., 2009), as potential adsorbents for dye containing waters.
In this work, we attempt to investigate the potential of an abundant Moroccan clay, called locally Ghassoul, as an adsorbent of a cationic dye from aqueous solutions. This mineral clay (also known as Rassoul or Ghasoul) comes from the only deposit in the world, located at the East side of the Middle Atlas Mountains, in the Moulouya valley, approxi- mately 200 km away from Fes (Benhammou et al., 2009). For several centuries, Ghassoul clay has been used in natural cosmetic products (soap, shampoo, skin conditioner). Currently, it is marketed for its detergent and grease-removing properties.
The geochemical characterization and the origin of the Ghassoul clay formation were studied by Chahi et al. (1997, 1999). The mineralogical composition of Ghassoul clay shows that the raw Ghassoul clay consists
⁎
Corresponding author. Tel.: + 212 537 68 02 30; fax: + 212 537 77 10 55.
E-mail address:
khelass@gmail.com(K. Elass).
0169-1317/$
–see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.clay.2011.07.019
Contents lists available at SciVerse ScienceDirect
Applied Clay Science
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 / c l ay
responsible via Coulombic attractions of the strong retention of cationic dyes (Bouna et al., 2010).
This Moroccan clay has been the subject of several recent studies in order to develop new industrial applications. Bejjaoui et al. (2010) were interested in the development of cordierite ceramics by mixing andalusite and Ghassoul clay. Benhammou et al. (2005a,b, 2007) studied the adsorption of heavy metals in aqueous solution on raw and on organic- and inorganic-modi fi ed Ghassoul. El Ass et al. (2010) studied the adsorption of Basic dye in aqueous solution on raw Ghassoul. Elmchaouri and Mahboub (2005) studied the synthesis and characterization of Al-pillared stevensite.
In the present study, we aimed to evaluate the potentiality of this Moroccan clay for the removal of a cationic dye (methyl violet) from aqueous solutions. Therefore, batch experiments were performed. The effects of contact time, initial dye concentration, ionic strength, solution pH, and temperature were investigated. Equilibrium and kinetic analysis were conducted to determine the factors controlling the rate of adsorption and to fi nd out the possibility of using this material as low- cost adsorbent for dye removal.
2. Materials and methods
2.1. Adsorbents and dye solution
Ghassoul clay was treated before using in the experiments as follows:
a distilled water suspension of the clay was dispersed for approximately 4 h and then cleaned several times with de-ionized water. The fi ne fraction was collected by repeated dispersion, sedimentation and siphoning techniques (El Ass et al., 2010). The solid sample was dried at 105 °C for 24 h, ground then sieved by 140 μ m sieve.
Methyl violet was dried at 70 – 80 °C for 5 h to remove moisture and then was dissolved in distilled water. Methyl violet solution was allowed to stand for 1 – 2 days until the absorbance of the solutions remained unchanged (Dogan and Alkan, 2003). The structure of this dye is shown in Fig. 1 (Hameed, 2008).
2.2. Adsorption measurements
Adsorption studies were performed by the batch technique to obtain rate and equilibrium data. All adsorption experiments were performed at temperature 25 °C and dye solution pH 7.5, except those in which the effects of temperature and pH were investigated.
An adsorbent dose of 1 g/L was kept constant for all of the adsorption experiments.
120 min was fi nally selected for all of the equilibrium tests. The solution and solid phase were separated by centrifugation at 4000 rpm for 15 min. A 10-ml aliquot of the supernatant was removed and analyzed for methyl violet by a Shimadzu 160 UV – visible spectrophotometer at the wave length of 585 nm. The adsorption capacity of dyes was then calculated using the relation:
q
e= C
o− C
em ⋅ V
where C
0and C
e(mg/L) are the liquid-phase concentrations of dye at initial and equilibrium time, respectively. V (L) is the volume of the solution, and m (g) is the mass of adsorbent used.
For kinetic studies, 0.5 g of Ghassoul clay was added into 0.5 L of methyl violet solution with different initial concentrations (200 – 1000 mg/L). The dye adsorption amounts were determined by analyzing the solution at appropriate time intervals. The effect of temperature on the adsorption kinetic was carried out by performing the adsorption experiments at various temperatures (25, 45, and 55 °C).
Blanks containing no methyl violet were used for each series of experiments. Each experimental point was an average of three indepen- dent adsorption tests.
3. Results and discussions
3.1. Characterization of Ghassoul clay
The sample clay used in this study comes from a tertiary formation of Jbel Ghassoul located at the East of the middle Atlas, in Morocco.
Mineralogical identi fi cation was performed by X-ray diffraction (XRD) using a Siemens D5000 diffractometer (Cu K α radiation, λ = 1.5418 Å).
The XRD pattern of raw Ghassoul clay (Fig. 2a) showed that the dominant phase is the stevensite with the presence of quartz and dolomite. The XRD pattern of the fi ne fraction (Fig. 2b) show (i) an increase in the abundance of stevensite, indicating that the clay fraction of the Ghassoul clay consists mainly by phyllosilicates (ii) signi fi cant decrease in the abundance of quartz and disappearance of dolomite. The chemical composition of the raw Ghassoul clay and its fi ne mineral fraction are presented in Table 1. These results show that the fi ne fractions are principally made up of SiO
2(58.16 wt.%) and MgO (27.44 wt.%). Al
2O
3is notably concentrated in the fi ne clay mineral fraction (4.48 wt.%). The large amount of CaO (12.13 wt.%) measured in the raw clay may be ascribed partly to the dolomite recognized in XRD patterns, but particularly to gypsum. Indeed, gypsum has been reported to be present in the Rhassoul clay deposit by several authors (Benhammou, 2005). The surface area of fi ne mineral fraction, measured by the N
2-BET method, is 137 m
2/g. This high value, compared with other clays, reveals the existence of a high porosity responsible for the strong capacity of this material to fi x some cations (Benhammou et al., 2005a). the pH of zero point charge, pH
ZPC= 2.0 the same result was found by Benhammou (2005a,b) and Bouna et al. (2010). The cation exchange capacity is 79 meq/100 g, relatively large, compared to the value for natural stevensite (41 meq/l00 g), reported by Takahashi et al.
(1997).
3.2. Adsorption studies 3.2.1. Effect of solution pH
pH is one of the most important factors which control the adsorption
capacity of dye on clay surfaces. Change of pH affects the adsorptive
process through dissociation of functional groups on the adsorbent
Fig. 1.The structure of methyl violet.
surface active sites. The effect of solution pH on the equilibrium up-take capacity of Ghassoul clay was studied at 500 mg/L initial methyl violet concentration between pH value of 3 and 10. The initial pH was adjusted with NaOH or HCl solutions. As shown in Fig. 3, the amount of adsorbed dye was found to increase with an increase in pH, which can be explained by the electrostatic interaction of cationic dye molecules with the negatively charged surface. Indeed at pH 2 – 12, stevensite particles are formed of negatively charged layers responsible via Coulombic attractions of the strong retention of cationic dyes by contrast to the anionic dyes Bouna et al. (2010).
The lower sorption at low pH can be explained by the fact that at acidic pH, H
+may compete with dye cations for the adsorption sites of adsorbent. From 4.5 to 7.5 pH, adsorption is due to the electrostatic attraction between the negatively charged sites of the adsorbents and the positively charged dye molecules (=N + (CH3)2). As the pH
increases from 7.5 to 10, the number of negatively ionizable sites increases and the association of dye cations with more negatively charged Ghassoul surface could more easily take place. Similar observations have been reported by other workers for adsorption of methylene blue onto diatomite (Al-Ghouti et al., 2009) and adsorption of methylene blue on pyrophyllite (Sheng et al., 2009).
3.2.2. Effect of ionic strength
The ionic strength of the solution is one of the factors that control both electrostatic and non-electrostatic interactions between the adsorbate and the adsorbent surface (Dogan et al., 2007). The in fl uence of ionic strength on the adsorption of methyl violet onto Ghassoul clay was investigated using NaCl solutions of concentrations ranging from 0 to 0.1 mol/L, at constant methyl violet concentration of 500 mg/L. The band in the spectrum at 585 was not shifted following addition of NaCl, the band remained almost constant. The effect of ionic strength on the adsorption ability was demonstrated in Fig. 4. As the ionic strength increased, the adsorption capacity increase. These results were similar to earlier fi ndings by other workers for adsorption of reactive blue 221 on kaolinite (Karao ğ lu et al., 2010) and maxilon blue GRL onto sepiolite (Dogan et al., 2006).
The higher adsorption capacity of methyl violet under these conditions can be attributed to the aggregation of methyl violet cations induced by the action of salt ions, i.e., salt ions force dye molecules to aggregate, increasing the extent of adsorption onto Ghassoul clay surface. A number of intermolecular forces have been suggested to explain this aggregation, these forces include: van der Waals forces; ion- dipole forces; and dipole – dipole forces, which occur between dye molecules in the solution. It has been reported that these forces increased upon the addition of salt to the dye solution (Eren et al., 2010; Sheng et al., 2009).
Fig. 2.
XRD diagrams of (a) the raw sample and (b) the separated
fine clay fraction ofGhassoul clay.
Table 1
Chemical composition of the raw Ghassoul clay and its
fine mineral fraction.Sample oxides Raw clay (wt.%)
fine fraction (wt.%)SiO
253.31 58.16
Al
2O
32.87 4.48
Fe
2O
31.44 1.92
MgO 24.64 27.44
CaO 12.13 1.88
Na
2O 1.12 0.17
K
2O 0.85 1.05
Total 96.36 95.20
400 420 440 460 480 500
0 5 10 15
qe (mg/g)
pH
Fig. 3.The effect of initial solution pH on the removal of methyl violet onto Ghassoul clay.
440 460 480 500
0 0.02 0.04 0.06 0.08 0.1 0.12
qe (mg/g)
[NaCl] mol/L
Fig. 4.
Effect of ionic strength on adsorption of methyl violet onto Ghassoul clay.
rate of the dye was very high for the fi rst 30 min and after decreases with increase in time until it approaches the equilibrium loading capacity. Equilibrium was established after about 120 min. Similar result was found for the adsorption kinetic of methyl violet from aqueous solution by perlite (Dogan and Alkan, 2003). The rapid adsorption at the initial contact time is due to the availability of the negatively charged surface of adsorbent which led to fast electrostatic adsorption of the cationic dye molecules from the solution. After- wards with the gradual occupancy of these sites, the adsorption became less ef fi cient, dye needed to diffuse to the sorbent surface.
As seen in Fig. 5, the amount of dyes adsorbed increases with time for all initial concentration. The adsorption capacity of the adsorbent at equilibrium increased from 198.64 to 629.26 mg/g with an increase in the initial methyl violet concentrations from 200 to 1000 mg/L.
3.3.2. Kinetics studies
In order to understand the behavior of the adsorbent and to examine the controlling mechanism of the adsorption process, the pseudo fi rst-order, the pseudo second order and intraparticle diffusion models were applied to test the experimental data.
The pseudo fi rst-order rate expression of Lagergren (Ho, 2004) is given as:
log ð q
e− q
tÞ = logq
e− k
12 : 303 t
where q
eand q
tare the amounts of dye adsorbed on the adsorbent at equilibrium and at time t, respectively (mg/g) and k
1is the rate constant of fi rst-order adsorption (min
−1). The slopes and intercepts of plots of log(q
e− q
t) versust were used to determine the fi rst-order rate constant k
1.
The pseudo second-order kinetic model (Kuncek and Sener, 2010) is expressed as:
t q
t= 1
k
2q
2e+ t q
ewhere k
2(g/mg.min) is the rate constant of second order adsorption.
The slopes and intercepts of plots of t/q
tversust were used to calculate the second-order rate constant k
2and q
e.
Adsorption rate constants were summarized in Table 2. The values of regression coef fi cient for pseudo-second-order model were nearly unity ( N 0.999) for all initial dye concentrations studied. The calculated q
e,cal2009) and onto sonicated sepiolite (Kuncek and Sener, 2010).
The intraparticle diffusion equation, based on the theory proposed by Weber and Morris (1963), is expressed as follows:
q
t= k
pt
1=2+ C
where k
pis the intraparticle diffusion rate constant (mg/g.min
1/2), which can be evaluated from the slope of the linear plot of q
tversus t
1/2and C is the intercept, which represents the thickness of the boundary layer (Furusawa and Smith, 1974).
The plots of q
tagainst t
1/2(Fig. 6) were multi-linear and there were three different portions, indicating the different stages in adsorption.
The fi rst, sharper portion represents the external mass transfer. The second portion is the gradual adsorption stage where intraparticle diffusion is rate-limiting. The third portion is the fi nal equilibrium stage where intraparticle diffusion starts to slow down due to the extremely low solute concentrations in the solution (Kavitha and Namasivayam, 2007). The lines do not pass through the origin, this indicates that the intraparticle diffusion is involved in the adsorption process but not the only rate-controlling step (Wu et al., 2005). That is, some other mechanisms such as complexation or ion-exchange may also control the rate of adsorption (Ozcan et al., 2005; Poots et al., 1978). The large deviation from the origin shows that the boundary layer diffusion affects the adsorption. Similar results have been reported in adsorption of Congo Red by clay materials (Vimonses et al., 2009) and by modi fi ed hectorite (Xia et al., 2010), adsorption of basic dyes on activated carbon (Wang et al., 2010).
The values of k
pand C were determined from the slopes of the second linear portion, and the constants of intraparticle diffusion model are given in Table 3. It was observed that the R
2values are close to unity, which indicates the appropriateness of the application of this model.
The k
pvalues (2.572 – 24.160 mg/g.min
1/2) increased with the initial concentration (200 – 1000 mg/L). Additionally, the C value (168,30 – 417.21 mg/g) varied like the k
pvalues with initial concentration.
The values of C are helpful in determining the boundary thickness:
a larger C value corresponds to a greater boundary layer diffusion effect (Kannan and Sundaram, 2001). It may be concluded that surface adsorption and intraparticle diffusion were concurrently operating during the adsorption of methyl violet onto Ghassoul clay. Previous studies have showed similar results that a multi-step is involved in the adsorption process of dye on several solid adsorbents (Janos et al., 2003; Suna et al., 2010).
3.4. Adsorption isotherm and equilibrium
The equilibrium isotherm plays an important role in predictive modeling for analysis and design of adsorption systems (Rodríguez et al., 2009). The equilibrium adsorption is usually described by an
0 100 200 300 400 500 600 700
0 50 100 150 200
qt (mg/g)
t (min)
1000 mg/L 800 mg/L 600 mg/L 400 mg/L 200 mg/L
Fig. 5.
Effect of the initial concentration on the adsorption of methyl violet onto Ghassoul clay.
Table 2
Comparison of the
first-order and second-order adsorption rate constants, calculatedq
e, caland experimental q
e,expvalues for different initial dye concentrations.
C
0(mg/L) q
e (exp)mg/g
First-order kinetic model Second-order kinetic model K
1.10
2min
−1q
e (cal)mg/g
R
2q
e (cal)mg/g
K
2.10
3g/
mg.min R
2200 198.64 2.16 34.97 0.9354 200.00 2.07 0.9996
400 380.01 2.06 85.52 0.9512 384.61 0.78 0.9998
600 513.56 3.15 131.64 0.8843 526.32 0.41 0.9999
800 577.54 2.99 142.52 0.8843 588.23 0.47 0.9998
1000 629.26 2.39 192.84 0.9575 625.00 0.35 0.9999
isotherm equation characterized by the surface properties and af fi nity of adsorbent.
The equilibrium adsorption of methyl violet onto Ghassoul clay was studied as a function of methyl violet concentration (100 – 1000 mg/L).
The amount of dye adsorbed (q
e), plotted against the equilibrium concentration (C
e) for methyl violet, was given in Fig. 7. As shown in Fig. 7, the equilibrium adsorption of dye increased with the increase of initial methyl violet concentration, showing the adsorption process to be dependent on the initial dye concentration. According to Giles et al.
(1960) classi fi cation, the shape of the isotherm indicated L-behavior.
This con fi rms a high af fi nity between the clay sample and the dye molecule.
The analysis of the isotherm data by fi tting them to different isotherm models is an important step to fi nd the suitable model that can be used for design purposes. The equilibrium isotherms in this study have been described in terms of Langmuir and Freundlich isotherms. The linear forms of Langmuir(1918) and Freundlich(1906) adsorption isotherm models can be described with the following equations, respectively:
1 q
e= 1
q
max+ 1 q
maxK
L1 C
ewhere C
eis the equilibrium concentration (mg/L); q
ethe amount adsorbed per unit weight of Ghassoul clay (mg/g); q
maxthe monolayer capacity of the adsorbent (mg/g), K
Lthe Langmuir adsorption constant (L/mg).
lnq
e= lnK
F+ 1 n lnC
ewhere K
Fis the Freundlich constant, related to adsorption capacity of the adsorbent and 1/n is the adsorption intensity. Values of n N 1 represent favorable adsorption condition (Wang et al., 2010).
Langmuir and Freundlich isotherm parameters, for the removal of methyl violet onto Ghassoul clay at 25 °C, can be obtained by plotting 1/q
eversus 1/C
eand ln q
eversus ln C
e, respectively. The calculated values of the constants and the regression correlation coef fi cients (R
2) are given in Table 4. Both of the Langmuir and Freundlich adsorption
models were found to be suitable for the adsorption of methyl violet adsorption onto Ghassoul clay modeling. The maximum monolayer sorption capacity q
max, determined with the Langmuir isotherm, was 625 mg/g. A similar results were previously reported by Ozdemir et al.
(2006) for adsorption of methyl violet and methylene blue onto sepiolite; Jiang et al. (2008) for adsorption of Basic Violet 14 on bentonite and Annadurai et al. (2008) for adsorption of Remazol black 13 onto chitosan.
3.5. Thermodynamic studies 3.5.1. Effect of temperature
Temperature is an important parameter for the adsorption process.
Fig. 8 exhibits contact time versus adsorbed amount graph at initial methyl violet concentration of 500 mg/L and different temperatures (25, 45 and 55 °C). The equilibrium adsorption capacity was found to increase with increasing temperature, increasing from 459,74 mg/g at 25 °C to 493,55 mg/g at 55 °C, indicating that the mobility of dye molecules increased with temperature. Additionally, increasing the temperature reduces the viscosity of the solution and increases the rate of diffusion of dye molecules across the external boundary layer and in the internal pores of the adsorbent particle. These results are in agreement with the fi nding reported by Dogan and Alkan (2003) for the removal of methyl violet from aqueous solution by perlite and Karao ğ lu et al. (2010) for the kinetic analysis of reactive blue 221 adsorption on kaolinite.
3.5.2. Thermodynamic parameters
The thermodynamic parameters provide in-depth information regarding the inherent energetic changes associated with adsorption;
therefore, they should be properly evaluated. The second-order rate constants listed in Table 2 are used to estimate the activation energy of methyl violet adsorption on Ghassoul clay using Arrhenius equation:
lnk
2= lnA − E
aRT
where E
ais the Arrhenius activation energy (kJ/mol), A the Arrhenius factor, R the gas constant (8.314 J/mol K) and T is the solution temperature in Kelvin. The activation energy E
awas determined from the slope of the Arrhenius plot of ln k
2versus 1/T. The magnitude of activation energy gives an idea about the type of adsorption which is 0
100 200 300 400 500 600 700
2.23 4.23 6.23 8.23 10.23 12.23 14.23
qt (mg/g)
t
1/2(min)
1/21000 mg/L 800 mg/L 600 mg/L 400 mg/L 200 mg/L
Fig. 6.
Intra-particle diffusion plots for different initial dye concentrations.
Table 3
Intraparticle diffusion parameters for the adsorption of methyl violet onto Ghassoul clay.
C
0(mg/L) k
pmg/g.min
1/2C mg/g R²
200 2.572 168.30 0.9948
400 11.989 271.94 0.9912
600 18.077 375.28 0.9378
800 17.175 437.10 0.9802
1000 24.160 417.21 0.9845
0 100 200 300 400 500 600 700
0 100 200 300 400
qe (mg/g)
Ce (mg/L)
Fig. 7.
Adsorption isotherm for methyl violet onto Ghassoul clay.
Table 4
Langmuir and Freundlich parameters for the adsorption of methyl violet onto Ghassoul clay.
Langmuir constants Freundlich constants
q
max(mg/g) K
L(L/mg) R
2n K
FR
2625 0.158 0.9988 5.35 229.09 0.9902
mainly physical or chemical. Nollet et al. (2003) suggested that the physisorption process normally had activation energy of 5 – 40 kJ/mol, while chemisorption had a higher activation energy (40 – 800 kJ/mol).
The value of activation energy (5.22 kJ/mol) given in Table 4 shows that the adsorption of methyl violet onto Ghassoul clay was a physisorption process. Similar result was reported for adsorption of cationic methyl violet and methylene blue dyes onto sepiolite (Dogan et al., 2007).
The thermodynamic parameters such as change in standard free energy ( Δ G°), enthalpy ( Δ H°) and entropy ( Δ S°) were determined by using the following equations:
ln k
c= Δ S∘
R − Δ H∘
RT Δ G∘ = Δ H ∘− T Δ S∘
where R (8.314 J/mol K) is the gas constant, T (K) the absolute temperature and k
c(L/g) is the standard thermodynamic equilibrium constant de fi ned by q
e/C
e. By plotting a graph of ln k
cversus 1/T, the values Δ H° and Δ S° can be estimated from the slopes and intercepts.
The results are given in Table 5. The positive value of Δ H°
con fi rmed that the adsorption process was endothermic. Moreover, The positive value of Δ S° showed the af fi nity of Ghassoul clay for methyl violet and the increasing randomness at the solid-solution interface during the adsorption process. The Δ G° values were negative at all of the tested temperatures, con fi rming that the adsorption of methyl violet onto Ghassoul clay was spontaneous and thermody- namically favorable. These results are in agreement with the fi nding reported by Tahir and Rauf (2006); Ahmad et al. (2009) and Wang et al. (2010).
4. Conclusion
Based on the results obtained from this study, it appears that the Ghassoul clay constitutes a good adsorbent for removing cationic dye from aqueous solution. The equilibrium up-take capacity increased with the increase of pH in the range of 3 – 10. The results showed that the adsorption system could be explained by the electrostatic attraction between the negatively charged surface and the positively charged dye molecule. The addition of salt had a positive effect on the adsorption capacity of Ghassoul clay. The adsorption rate increased with the increasing initial dye concentration and the adsorption
kinetic was very well described by the pseudo-second-order kinetic model. The intraparticle diffusion was the rate-limiting step for the adsorption process. The calculated thermodynamic parameters show the spontaneous and endothermic natures of the adsorption process which is favored at higher temperatures. The low value of the activation energy shows that dye adsorption process by the Ghassoul clay may involve a physical sorption. The adsorption isotherm was fi tted by the Langmuir and Freundlich models. The maximum sorption capacity was determined with the Langmuir isotherm and found to be 625 mg/g at 25 °C. The adsorption capacity of other adsorbents for MV obtained by some other investigators is presented in Table 6. A comparison of these values with the one obtained in this study showed that Ghassoul clay used in this research exhibited a higher capacity for MV adsorption from aqueous solutions. Adsorption of cationic dyes on Ghassoul clay can be considered as a simple, fast and economic method for their removal from water and wastewater.
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200 250 300 350 400
0 50 100 150 200
qt (mg/g)
Time (min)
25 °C 45 °C 55 °C
Fig. 8.
Effect of contact time for the adsorption of methyl violet onto Ghassoul clay at various temperatures.
Table 5
Thermodynamic parameters for the adsorption of methyl violet onto Ghassoul clay.
T (K) Thermodynamic parameters
ΔG°
