Characterization and Adsorption Study of Thymol on Pillared Bentonite ()
1. Introduction
In recent years, material sciences have involved the studies related to the production of materials having a controlled pore structure and improvement of porous materials found in the nature. The production principle is to hold the inorganic layers a part from each other, introducing a bulky guest agent between them. When the used materials are clay minerals, the resulting materials after pillaring are called Pillared Layered Clays (PILCs). The host solid and the production conditions have rather important effects on the quality of the product. Due to its high cation exchange capacity, swelling properties, and large sheets, montmorillonite type clay have an important place in the production of pillared clays.
Any material which could enter between the layers and has a thermal stability and suitable dimensions can be used as pillaring agent [1] -[5] .
In the case of aluminum, there is a lot of information about the oligocations formed in solution that facilitates studies on its properties. Pillaring of clays with Al is thought to be an ion exchange process of the major pillaring agent [Al13O4(OH)24(H2O)12]7+, the so called keggin or Al13 ions [6] [7] were able, through the XRD analysis, to confirm the intercalation of aluminum polycations Al13 with basal spacings of about 18 Å at room temperature and which are transformed into Al2O3 oxide after calcination at 500˚C.
The intercalated species act as props (pillars) that keep the clay layers apart and prevent them from collapsing under vacuum, at higher temperatures, or under specific conditions [8] .
So, the pillaring process generates micro and or meso-porosity in the inter lamellar spaces of clays and are also called nano materials. Pillared clays have remarkable adsorption properties that are related both to the geometrical features of the porous spaces and to specific interactions of the pillars and the clay layer [8] .
Adsorption and pH-dependent ion exchange capacities of PILCs have been extensively studied [9] -[16] . These modified clay minerals (PILCs) have been used in attempts to improve controlled release formulations of pesticides [17] .
Elsewhere, the adsorption properties of PILCs on active molecules such as thymol have not yet been established not even its application to reversible encapsulation.
Thymol (2-isopropyl-5-methylphenol, IPMP) is known for its bactericidal effect (has a lethal activity of micro-organisms) [18] -[21] . It has been shown to be an efficient acaricide molecule against the Varroa destructor, an external parasitic mite that attacks honey bees [22] .
Thymol is present in thyme essential oil and volatile essential oils of diverse plants. The advantage of using thymol is based on its low cost and easy to be handled because it is not toxic. Its use is limited by the temperature of the medium; thymol is only effective at temperatures varying between 15˚C and 30˚C and has a low solubility in water 1 g/L equivalent to 6.6 × 10−3 mol/L.
In our previous work [23] , we have studied the adsorption of thymol on sodium bentonite (montmorillonite) and the maximum amount adsorbed was 177 m2∙g−1 showing a certain affinity of thymol for anionic clay sites.
The aim of this work is the study of characterization, adsorption isotherm and adsorption kinetics of thymol on pillared bentonite (modified montmorillonite clay), for multiple applications related to the technology of encapsulate volatile material in clay mineral nano-capsules.
2. Material and Method
2.1. Purification and Intercalation Method
The bentonite used in this study is a bentonite rich with montmorillonite clay type that was provided from the region of Nador (northest of Morocco) and used in a purified forme.
Bentonite clays were homoionized with a NaCl solution 2 M, with a solid: liquid ratio of 1/50 (10 g clay/500 mL NaCl). The ion exchange was realized at 25˚C and was repeated three times. After each process, the clay was washed with distilled water until no chloride ions were found. To avoid the rapid evaporation of the water, the ion exchanged samples were slowly dried at 35˚C for 24 h [24] . A portion of the sample was used to prepare pillared-bentonite.
The simplified pillaring method proposed in this paper was carried by intercaling of the pillaring agent in a aqueous sodic clay suspension 5% (%mass), ageing, washing, drying and calcination of the intercalated clay at 350˚C.
The pillaring solution containing [Al13O4(OH)24(H2O)12]7+ cations was obtained by adding 250 ml of AlCl3 (0.4 M) stepwise to 550 ml of NaOH (0.4 M). The final neutralization ratio which is defined as [OH]Tot/[Al(III)]Tot was 2.4 and the solution was agitated for 12 h at room temperature. The resultant solution of pH = 4.5 was added to the clay suspension 2% (1 g clay/100 ml H2O) and stirred for 6 h at room temperature. The pillared clay form was then centrifuged, filtered, and dried at 60˚C in air. Calcination was performed at 350˚C for 6 h. The degree of intercalation of the pillaring cations was determined by XRD, by analyzing variations of d (001) in oriented clay-aggregate specimens.
2.2. Characterization
The natural samples purified and modified clay are subjected to analysis and identification by X-ray diffraction (XRD), infrared spectroscopy (IR) and Thermal analysis.
X-ray diffractograms were recorded in a Shimadzu XRD diffractometer D6000 stations working on the monochromatic copper Kα1 radiation (1.54 Å) (Figure 1).
The textural characteristics of clays before and after pillaring were determined from N2 adsorption/desorption isotherms at 77˚K using micrometrics ASAP 2000 volumetric adsorption-desorption apparatus and surface Area and Pore size Analyzer (Figure 2 and Figure 3). The surface areas were calculated using the multi-point BET method, and a relative pressure (P/P0) between 0.00095 and 0.9917 was applied.
Infra Red (I.R) spectra were acquired using a Shimadzu Fourier Transform spectrometer over a range varying from 400 to 4000 cm−1 with a resolution of 2 cm−1, and the samples were prepared in the form of a dispersion in a vial KBr (1/200 by weight) (Figure 4 and Figure 5).
Thermal analysis was carried out in a SHIMATZU D6000 coupled to a DC ampler and temperature controller. Data from DTA-TG were obtained in all cases at a heating rate of 5˚C/min between 30˚C and 1000˚C and under N2 atmosphere (Figure 5 and Figure 6).
2.3. Equilibrium Studies
Adsorption of thymol by the pillared bentonite fractions was carried out in batch at different initial concentrations (5 to 760 mg/L). Increasing amounts of clay fractions of 0.05, 0.1 and 0.15 g, were dispersed in 20 ml of each initial solution (C0) of thymol, and equilibrated in an overhead shaker at room temperature (19˚C) for 3 h. The particles were allowed to settle and separated by centrifugation.
After a centrifugation at 10000 rpm for 20 min speed, the amount adsorbed by clay was calculated from the initial concentration and the final concentration of products determined by absorbance measurement using double beam UV/v is spectrophotometer (Shimadzu, Model UV 6500, Japan) at 273.5 nm.
Figure 1. X-ray diffractograms for the indicated samples.
Figure 2. Nitrogen adsorption-desorption isotherm for pillared & purified sodium bentonite.
Figure 3. FTIR spectra of B-Na purified and pillared in the range of 4000 - 1800 cm−1.
(1)
with: 
is the initial concentration (mg/L), 
is the equilibrium concentration, 
(ml) is the total volume of the sample, 
(mg) is the mass of pillared clay used and 
is amount adsorbed (mg per grams) of pillared clay (mg/g). The product under consideration adsorption isotherm is obtained by drawing the curve:
(2)
For the adsorption experience, the mass of thymol that was lost during the balancing of the solution was sup-
Figure 4. FTIR spectra of B-Na purified and B-Al-PILCs in the range of 1800 - 400 cm−1.
Figure 5. TG analysis of the pillared and purified sodium bentonite.
posed to be adsorbed by pillared clay.
The desorption experiments were carried out after an adsorption step in the same conditions as described above. After adsorption and phase separation by centrifugation, the supernatant solutions were discarded. Volume of solution remaining in the pellets was calculated by weight and the amount of non adsorbed thymol was calculated.
Figure 6. DTG curves of the pillared & purified sodium bentonite.
Account was taken of the thymol remaining in the solution of the moist clay pellet in calculations of the fraction desorbed. The sequential (adsorption/desorption) run were conducted to determine the mobilization factor [25] following the equation:
(3)
where:
: the adsorption:desorption ratio;
: the amount of solute adsorbed (mg/g);
: the amount of solute desorbed (mg/g).
2.4. Adsorption Kinetic
For kinetic studies, 0.150 g of pillared Bentonite (B-PILC) was contacted with 200 mL of two different concentrations of thymol C1 = 78.3 mg/L and C2 = 93.84 mg/L. Using water-bath shaker at 20˚C. The agitation speed was kept constant at 500 rpm. At predetermined intervals of time, solutions were analyzed for the final concentration of thymol. The amount of adsorption 
(mg/g), at time 
(h), was calculated by:
(4)
where Ct (mg/L) is concentrations of thymol at time 
(h).
3. Results and Discussion
3.1. Characterization of Samples
3.1.1. X-Ray Diffraction
The (Figure 1) shows the XRD patterns of the sodic and the pillared bentonite obtained.
The XRD analyses show in all cases, a clear shift of the signal at 13.22 Å (corresponding to the smectitic signal for the sodic bentonite), to a value close to 19.44 Å, which represents an increase in the basal space of the pillared clay. This indicates that the chemical modification of the clays led to a successful pillaring process. The basal spacing that expands to about 19.44 Å is equal to the thickness of one clay layer (9.4 Å) plus the height of one Al13 cation (9.7 Å).
3.1.2. Textural Characteristics
These analyses were done in order to determine the influence of chemical modification of clays (purified sodium and pillared bentonite) on there structural characteristics. Before analyses the samples were automatically degassed under vacuum for 4 h at 475 K; the samples mass used are 0.1318 g and 0.1438 g of purified sodium and pillared bentonite.
The adsorption-desorption isotherms are important due to the parameters that can be established: specific surface area, porosity, pore volume, pore size distribution and average pore diameter. More, we can also obtain qualitative information regarding the structure (pores shape, interconexions etc.). To determine the textural characteristics several models are proposed [24] .
In order to determine the textural properties, the Langmuir model in a range of relative pressure smaller than 0.07 [26] , and the correction to the BET model proposed by [27] were used. The microporous areas were determined by curves using the De Boer’s method, and the micropore volumes were determined by curves using the Harkin-Jura equation.
The adsorption-desorption isotherms with N2 of purified sodium bentonite (B-Na purified) and the pillared bentonite (B-Al-PILCs), are shown in Figure 2. The comparison between the tow isotherms, lead to a type IV isotherm with H1 hysteresis for B-Na purified [28] and H3 hysteresis for B-Al-PILCs [29] . This means that in the case of sodium purified bentonite (B-Na), the distribution of pore size is regular, and the increase in the adsorption of N2 for B-Al-PILCs in comparison to the B-Na purified, as a result of the porosity generated because of the pillaring process.
The pillared material present developed mesoporosity and the hysteresis has a narrow loop with two branches which are almost vertical and parallel.This form of hysteresis is often associated with adsorbents made up of agglomerates and aggregated plane particles forming slit shape and narrow size pore distribution. Relevant data for the sodic purified and pillared bentonite obtained are presented in Table1
The increase in the superficial area and the porous volumes of the B-AL-PILCs compared to B-Na purified is remarkable [30] . This increase could be related to the high introduction of Al in the clays, and the high homogeneity and intensity of the pillaring signals reached by the methodology used in this paper. The superficial area of the B-AL-PILCs is determined mainly by the microporous area (Table 1), which is evident for of a significant textural modification of the B-Na purified.
3.1.3. FT IR Spectroscopy
FTIR spectra of B-Na purified and B-Al-PILCs clays are given in Figure 3 and Figure 4. At high frequency regions of the infrared spectra (Figure 3), bands corresponding to the water molecules present in the interlayers and the structural hydroxyl groups in the clay layers were observed in the region between 3350 - 4000 cm−1. The absorption band at 3450 cm−1, corresponds to the symmetric O-H stretching vibration of H-bonded water, (Figure 3) [31] .
Also, it can be seen that its intensity is dependent on the type and the concentration of the interlayer cations. For B-Al-PILCs, the band locates at 3643 cm−1 and the intensity is higher than that of B-Na purified 3644 cm−1. The former is ascribed to the O-H stretching vibration in hydroxy-Al cations while the latter corresponds to the hydroxyl groups involved in water-water hydrogen bands [32] . The samples show a band at 3621 cm−1 related to the OH stretching of Si-OH, corresponding to silanol groups. Such hydroxyls are either located at corners and fractures of sheets or are formed by the processes of tetrahedral inversion. For B-Al-PILCs sample, a new band is found at 3652 cm−1 (Figure 3), which is probably produced by the change of position of the SiOH group in the structure of the smectite. The original position was altered by the entrance of the Al ion in the smectite structure, which is likely to be due to Si-OH species perturbed by pillars, and designated as SiOH*.
The low frequency regions of the infrared spectra of the B-Na purified and B-Al-PILCs are very similar, but the latter shows a very small low intensity band at 550 - 450 cm−1 originated from Si-O bending and Al-O stretching vibration and there were no changes in Si-O bending, but a small increase in Al-O stretching in
Table 1 . Selected textural properties of the investigated samples.
SBET: specific surface area; V0.98: total pore volume; Vmic: micropore volume; Dmax: the pore diameter where the maximum of derivative cumulative volume curves is reached; Dmed: median value of pore diameter.
intensity by pillaring. This situation was supported by the increase in Al content of pillared samples around 657 cm−1 (Figure 4). The lattice vibration at 657 cm−1 can be ascribed to the Al-O bond of tetrahedrally coordinated Al cations in the center of the Al13 pillars [33] . The band center of B-Al-PILCs locates at 1556 cm−1 with a higher intensity compared with that of B-Na purified at 1560 cm−1. This should be attributed to an increase of water content in B-Al-PILCs, resulted from the intercalation of hydroxy-Al cations into the clay interlayer space. The infrared spectra of B-Na purified presented bands in the region of 1687 - 1640 cm−1 attributed to hydrating water. The aluminum pillaring agent had caused a decrease in the free silica peak intensity which occurred at 840 and 916 cm−1 [34] .
3.1.4. Thermal Analysis (DTA-TG)
The thermal curves are depicted in Figure 5 and Figure 6 in the 30˚C - 1000˚C range. The curves correspond to the starting purified sodium and pillared bentonite, after equilibrating in a desiccator, at room temperature. The general feature of thermal curves clearly reveals two steps: 1) in the 30˚C - 340˚C and 2) in the 300˚C - 650˚C temperature ranges.
The first step with purified sodium and pillared clay has been ascribed to physisorption of the hydrating water, whereas the second step is due to dehydroxylation of silicate structure.
Sometimes, this step occurs dissociated into two which is not well visible here but is clearly shown in the corresponding DTG and, moreover, in the TGA curve (Figure 5), denoting dehydroxylation of the silicate structure in two different environments. For Al pillared materials, the net isomorphic substitution in the clay with different bonding strengths between the oligocations and the surrounding oxygen (or hydroxyl) ions can be observed [35] . Dehydroxylation continues between 337˚C and 650˚C and is also detected to approximately 620˚C in an important step. This step is related with the stability of the pillars, since an important decrease in the basal spacing values occurs at this temperature, indicating the collapse of the clay structure. Therefore, the thermogravimetric analyses are in agreement with the aforementioned thermal stability of the synthesized pillared clay up to 650˚C.
3.2. Adsorption, Desorption Studies of Thymol on B-Al-PILCs
3.2.1. Adsorption Isotherms, Solid/Liquid Ratio and Concentrations Effects
Under ideal saturated conditions, the solid liquid ratio should not influence the amount of organic or inorganic molecules adsorbed per unit of adsorbent. However, some interested studies have shown that both organic and inorganic contaminant adsorption is dependent on solid-liquid ratio to some degree [36] .
The adsorption of thymol was studied at different initial concentrations (5 to 760 mg/L). Figure 7 shows the result for effect of initial concentration and for various solid/liquid ratios on adsorption of thymol onto B-ALPILCs. The amount of thymol adsorbed
, plotted against the equilibrium concentration 
on B-AL-PILCs, is given in Figure 7.
The equilibrium adsorption of thymol increases with the increase of initial thymol concentration and the decrease of the solid/liquid ratio showing the adsorption process to be dependent on the initial concentration and the content of the solid adsorbent. Initially, the adsorption isotherms of thymol molecules show a rising part whose slope increases when the amount of solid decreases, suggesting a strong affinity of the molecules for the surface sites. The amount of adsorption reaches a limiting value of around 319 mg∙g−1 for low solid/liquid ratio (0.2%). The pH of the thymol solution will effect on the ability of adsorption. The pH of clay suspension tends to be neutral, i.e.: pH 7.5. The thymol (pKa 10.6) in this condition is mainly in a neutral form, therefore, it will stick to the surface of the negatively clay.
The high adsorption capacity for thymol uptake presented by pillared clay may be caused by adsorption by Van Der Waals interactions and by hydratation forces with polar groups on the pillared bentonite (Al-OH and Si-OH) with (OH groups and benzene group) of thymol on the basal plane and on the edges of the layers silicates, knowing that in pH < pKa, the thymol is in neutral form. It is evident that the intercalation by pillaring process, makes greatly influences the adsorption of thymol by the surface orientation of pillared clay, and leads to large areas of the pillared clay surface being exposed allowing the adsorption of thymol.
Tow models of isotherms were tested for their ability to describe the experimental results, namely the Langmuir and Freundlich isotherms.
The Langmuir adsorption model [37] , (Figure 9) is based on the assumption that maximum adsorption cor-
Figure 7. Equilibrium adsorption of thymol on pillared bentonite at different solid/liquid ratio.
responds to a saturated mono-layer of solute molecules on the adsorbent surface, with no lateral interaction between the sorbed molecules. The linear expression of the Langmuir model (Figure 8) is given by Equation (5)
(5)
where 
(mg/g) and 
(mg/L) are respectively the amount of adsorbed thymol per unit mass of adsorbent and thymol concentration at equilibrium, 
is the maximum amount of the thymol per unit mass of adsorbent to form a complete monolayer on the surface bound at high
, and 
is a constant related to the affinity of the binding sites (L/mg).
The Langmuir constants 
and 
were determined from the slope and intercept of the plot and are presented in Table2
The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor 
that is given by Equation (6) [38] :
(6)
where 
is the highest initial concentration of adsorbate (mg/L), and 
(L/mg) is Langmuir constant.
The value of 
indicates the shape of the isotherm to be either unfavorable
, linear
, favorable
, or irreversible
. The 
value is <1 indicates that the adsorption model is not conformed.
The Freundlich isotherm (H. Freundlich, 1906) is an empirical equation employed to describe heterogeneous systems. The linear form of Freundlich equation is expressed:
(7)
where 
and n are Freundlich constants with 
(mg/g) (L/mg), (1/n) is the adsorption capacity of the sorbent and 
giving an indication of how favorable the adsorption process. The magnitude of the exponent, 1/n, gives an indication of the favorability of adsorption.
The Value of 
and 
are calculated from the intercept and slope of the plot (Figure 9) and listed in Table2
The plot of amount adsorbed 
against the equilibrium concentration
. (Figure 9) shows that
Figure 8. Langmuir adsorption isotherm of thymol on pillared bentonite.
Figure 9. Freundlich adsorption isotherm of thymol on B-AL-PILL.
the adsorption obeys the Freundlich [39] model [40] [41] .
The Temkin isotherm [42] has generally been applied in the linear form (Figure 10):
(8)
where
, 
is the Temkin constant related to heat of sorption (J/mol); A is the Temkin isotherm constant (L/g), 
the gas constant (8.314 J/mol˚K) and 
the absolute temperature (˚K). Therefore, by plotting 
vs 
(Figure 9) the constants A and B can be determined. The constants A and B are listed in Table2
As seen in Table 2, the Freundlich isotherm fits quite well with the experimental data.
Examination of the plot suggests that the linear Freundlich isotherm is a good model for the sorption of the thymol onto B-AL-PILL.
Table 2. Isotherm parameters for adsorption of thymol by B-AL-PILL.
Figure 10. Temkin adsorption isotherm of thymol on pillared bentonite.
Table 2 shows the linear Freundlich sorption isotherm constants, coefficients of determination (R2) and error values. Based on the R2 values, (correlation coefficient R2 > 0.90). The linear form of the Freundlich isotherm appears to produce a reasonable model for sorptions in all three ratios, implying the presence of the highly energetic sites were the molecules of thymol were adsorbed. After the point of inflection of the experimental data, the Freundlich isotherm predicted that the equilibrium adsorption capacity should keep increasing exponentially with increasing equilibrium concentration in the liquid phase. However, the experimental adsorption isotherm for thymol presented a plateau at higher equilibrium concentration, implying the saturation of adsorption sites and the maximum filling of the pores. Thus, Freundlich model should not be used for extrapolation of this data to higher concentration (problem of high-saturation), [43] .
3.2.2. Desorbability
The adsorption and desorption isotherm of thymol on pillared clays was presented in Figure 11.
The thymol can be adsorbed on pillared clays through ligand exchange, hydrogen bonding, and electrostatic, hdrophobic, and Van Der Waals interactions.
In this study, the pillared clay and thymol interaction was partly non-reversible, in water and in the same codtions of equilibrium adsorption:
The adsorption and desorption isotherm show a low desorption amount with 
that’s mean that the percentage of desorption amount is about 5%, it is probably caused by the hydrophobicity of thymol makes a strong interactions with the clay, and low with water.
3.3. Adsorption Kinetics
3.3.1. Pseudo First and Second Order
Kinetic study of adsorption is important because it gives the times necessary to reach the equilibrium for adsorption and desorption that are required data for obtaining valuable isotherms and parameter characteristics of molecular displacements and reaction near or on the adsorbent surface (which bring the solute in the adsorbed state).
Kinetic values of adsorption were determined by analyzing adsorptive uptake of the thymol from aqueous solutions by pillared bentonite (B-AL-PILL), in two different concentrations C1 = 78.3 mg/L and C2 = 93.84 mg/L.
All the kinetically experiments were carried out at initial pH (pH: 7.5), and the plots were drawn by calculating the average of closed each other values of the experimental data. Then, equilibrium contact times for thymol used were determined from graph drawing residual concentrations no adsorbed of adsorbate versus times.
The type curves of the kinetics of thymol by pillared bentonite (B-AL-PILL) retention are reproduced in
Figure 11. Equilibrium adsorption-desorption of thymol on pillared bentonite.
(Figure 12).
The modeling of adsorption kinetics of thymol on pillared bentonite (B-AL-PILCs) was investigated by two common models, namely, the Lagergren pseudo-first order model [44] and Pseudo second order model [45] [46] . Lagergren proposed a method for adsorption analysis which is the pseudo first order kinetic.
For a batch contact time process where the rate of sorption of thymol on to the pillared clay (B-AL-PILCs) surface is proportional to the amount of thymol sorbed from the solution phase, the first-order kinetic equation may be expressed as:
(9)
After integration and applying boundary conditions, viz that the initial conditions are 
at
, equation becomes:
(10)
A linear plot of 
against time with good correlation coefficient allows one to obtain the rate constant, indicating that Lagergren’s equation is appropriate to thymol sorption on pillared clay B-AL-PILCs (Figure 13). The Lagergren’s first-order rate constant 
and 
determined from the model are presented in Table 3 along with the corresponding correlation coefficient.
The pseudo-second-order kinetics may be expressed as [47] [48] :
(11)
where: 
and 
are the adsorption capacity at equilibrium and at time
, respectively (mg∙g−1), 
is the rate constant of pseudo first-order adsorption (L∙min−1), 
is the rate constant of pseudo second-order adsorption (g∙mg−1∙min−1). The plot of 
vs. t should give a linear relationship from which 
and 
can be determined from the slope and intercept of the plot, respectively.
The equilibrium adsorption capacity
, and the second-order constants 
(g/mgh) can be determined experimentally from the slope and intercept of plot 
versus 
(Figure 13).
The conformity between experimental data and the model predicted values was expressed by the correlation coefficients (
, values close or equal to 1). A relatively high 
value indicates that the model successfully
Figure 12. Adsorption kinetics of thymol on B-AL-PILL.
Figure 13. Pseudo-second-order kinetic for adsorption of thymol on pillared bentonite.
Table 3. Comparison of the pseudo-first-order, pseudo second order adsorption rate constants.
describes the kinetics of thymol adsorption.
The correlation coefficient 
for the pseudo first-order adsorption model (Figure 14) has a high value
, so the adsorption rate 
is proportional to the first order of
.
3.3.2. Adsorption Mechanism
In order to gain insight into the mechanisms and rate controlling steps affecting the kinetics of adsorption, the kinetic experimental results were fitted to the Weber’s intra-particle diffusion [39] [49] . The kinetic results were analyzed by the intra-particle diffusion model to elucidate the diffusion mechanism. In this model, the evolution of the amount adsorbed versus the time is expressed as:
(12)
where, C is the boundary layer diffusion effects, 
is the rate constant for intra-particle diffusion. A plot of 
vs 
giving straight line confirms intra-particle diffusion sorption which can be evaluated from the slope of the linear plot of qt versus 
[50] as shown in (Figure 15). The intercept of the plot reflects the boundary
Figure 14. Pseudo-fist-order kinetic for adsorption of thymol on pillared bentonite.
Figure 15. Intraparticle diffusion plots for adsorption of thymol on pillared bentonite.
layer effect. The larger the intercept, the greater and the contribution of the surface sorption in the rate controlling step.
Another type of intra-particle diffusion model is expressed as [49] [50] :
(13)
A linearised form of the equation is given as [51] :
(14)
where, 
is the fraction of the amount adsorbed, 
is the contact time (min), “a” is the gradient of linear plots; 
is the intra-particle diffusion rate constant (min−1).
This model was used to analyze the experimental data. The plot of log 
vs. 
is shown on (Figure 16). The regression equations and R2 values for intra-particle diffusion model are shown on (Table 4).
The values of the constants “
” and “
”, are shown on Table4 The values of “
” and 
were calcu-
Figure 16. Intraparticle diffusion plots II for adsorption of thymol on pillared bentonite.
Table 4. Comparison of the Intra-particle diffusion model I & II of adsorption.
lated from the slopes and intercepts of the plot respectively. The values of “a” depicts the adsorption mechanism and 
may be taken as a rate factor [52] .
A relatively high R2 value of Intraparticle diffusion model I indicates that the model successfully describes the mechanism of adsorption kinetics.
If the regression of 
versus 
is linear and passes through the origin, then intra-particle diffusion is the slow rate-limiting step. However, the linear plots (Figure 14) at each concentration did not pass through the origin. This indicates that the intra-particle diffusion was not only a rate controlling step. So the multiple nature observed in the intra-particle diffusion plot suggests that intra-particle diffusion is not solely (25 min for equilibrium) rate controlling. External mass transfer of thymol molecules on to pillared clay is also significant in the sorption process, especially at the initial reaction period (80% of amount adsorbed in the first 5 min).
4. Conclusions
The resultant pillared bentonite complexes were characterized using FTIR, with a combination of XRD, and textural analysis. Results obtained from FTIR spectra showed that there were changes in the clay structure with pillaring. The XRD patterns show that the basal spacings of the complexes increase to 19 Å and it was observed that basal spacing, surface area values and the properties against temperature effects of bentonites were improved by pillaring.
A Moroccan bentonite pillared by Al13 [OH/Al = 2.4] possessed an important capacity for adsorbing thymol. The quantity of thymol adsorbed was 319.5 mg/g at pH 7.54 and showed high irreversibility (desorption) in water.
The Freundlich isotherms were found to be applicable for the adsorption equilibrium data of thymol on pillared bentonite. The pseudo first order equation gave a better fit to the sorption process than the pseudo second order equation.
The intraparticle diffusion plot confirmed that the sorption process was a controlled particle diffusion. The amount of thymol released (desorbed) from the pillared clay was negligible in water. Mechanisms involved in the adsorption, which explain the high thymol uptake and irreversibility, were the molecule fixed by the adsorbed clay on polar groups (Al-OH and Si-OH) (remaining) sites onto the basal plane and on the layer silicate edges. It is believed that due to these properties, pillared clay, shows much potential as an adsorbent for thymol.
Acknowledgements
The authors are sincerely thankful to CNRST-Morocco (PROTARS p23/66) for its financial support and Professor A. EL MEDIOUNI (FLSH-Oujda) for correcting the English of the article.
NOTES
*Corresponding author.