Environmental Application of Acid Activated Kaolinite-Glauconite Clay Assisted by Microwave Irradiation

Crude kaolinite-glauconite clay was active with hydrochloric acid for various times under variable microwave irradiation power. The influence of activation parameters (power and/or time) on the structural and textural properties of the treated samples has been studied. The modifications were evaluated by XRD, FTIR, XRF, SEM, BET, grain size and zettametry. The XRD and IR results show that acid activation reveals only weak changes on crystallinity of samples. However, HCl activation of clay assisted by microwave modifies morphology and size of grains with a little variation of the specific surface area values. The adsorbing power of the raw and activated clay was tested with methyl orange dye and the adsorption isotherms were modeled using Langmuir and Freundlich models. This study showed that the maximum adsorbed quantity of dye passes from 3.21 mg/g for the untreated raw clay to 4.29 mg/g for the activated clay irradiated 2 min under microwave at a power of 900 W and that the Langmuir model is the most adequate to describe the adsorption process.


Introduction
Adsorption technology is the most widely used technology for waste water treatment. It was justified by several reasons including simplicity of design, rapidity, high efficiency and profitability. Although, this is a very proven technology for removing contaminants of different nature (heavy metals, organic molecules, dyes, etc.), it presents a major limitation to its application, which is the cost of the adsorbent [1][2][3][4]. In this context, natural clay minerals can be used as alternative adsorbents because they are inexpensive, friendly to the environment and available in huge quantities and notably can be easily accessible to improve their surface properties and therefore increase their adsorption capacities [5][6][7][8][9].
Glauconitic clay has been defined as green, iron potassium rich micaceous mineral of marine origin, with 2:1 dioctahedral sheets like illite structure. The octahedral sites in this clay were usually containing more Fe 3+ than Al 3+ and significant amounts of Mg 2+ and Fe 2+ . Iron is mainly present in Fe 2+ form in glauconite structure. Glauconite structure is characterized by interstratifications of expandable and non-expandable (10Å) layer [10] and low charge [11,12]. Some clays, such as, exhibit low reactivity and need so some modifications to enhance their reactivities. Several modification processes (acid activation, thermal treatment, pillaring, organic functionalization…) have been proposed in order to obtain the desired properties [7,[13][14][15][16][17].
Clays modifications via acid activation aim to involve changing in physical and chemical properties of materials, by removal of different ions plugging the surface pores and to increase the surface area as well as the pore size and to obtain solids with high number of acidic sites from these minerals. Moreover, many Si-OH groups on the surface of clay appear as result of acid treatment. Acid activation has been a traditional method employed for improving the surface properties of clay minerals [18,19]. Actually, many chemical processes use microwave techniques for heating. The use of this technique in the activation of clay minerals offers some advantages comparing with the conventional method including a higher uniform heating rate in a shorter time. Also, this technique has proven successful for preparing new promising materials for water treatment by causing structural and textural changes that enhance the products' adsorption capacity [13,[20][21][22].
In this study, we report the HCl acid activation assisted by microwave irradiation of crude kaolinite-glauconite clay. Structural and textural properties of activated clays will be evaluated then. This work will be closed by the study of adsorption of methyl orange on these obtained powders. An equilibrium adsorption isotherms' analysis to obtain the Langmuir and Freundlich constants was realized.

Starting Materials
A dark green clay mineral samples was used in this study. It was collected from Siliana region (north west of Tunisia). The clay was used in its crude form. The activating agent was hydrochloric acid (HCl, 37 %). The anionic dye used as adsorbate is the methyl orange with the molecular formula (C 14 H 14 N 3 O 3 S − Na + ) and with the molecular weight (327.34 g mol −1 ).

Acid Activation
Using the microwave irradiation method, the raw clay was activated with a hydrochloric acid (HCl) solution of 3 M concentration. The ratio [solid (clay) / liquid (acid solution)] used is 1/10. The suspensions prepared were irradiated in the microwave at different powers (630 and 900 W) and at different times. After irradiation, the samples were centrifuged. The solids were washed several times with demineralized water to remove traces of chloride and then dried at 70°for approximately 24 h. The materials obtained after drying were ground into a fine powder further subjected to characterizations. In the text, we note respectively the acid activated clays and the samples heated without acid as follows: Gpt and Rpt where G is clay, R is clay reference, p is the power (we note 6 if the power used is 630 W and we note 9 if the power is 900 W) and t is the activation time in minutes.

Adsorption
The concentration of dyes in the samples was determined during adsorption experiments by using UV-Visible spectrophotometer (PerkinElmer model LAMBDA 20) at 464nm. Adsorption tests were carried out by mixing 0.1 g of clay with 50ml of methyl orange solution at various concentrations. The flasks were stirred for 2 h on a mechanical shaker at 2700 rpm. After the completion of the contact time, the suspension was centrifuged at 4000 rpm for 15 min. The amounts of released dye were determined from the mass balance equation [23]: Where C 0 and C eq are respectively the initial and equilibrium concentrations of dye solution (mg/L) respectively; V s is the suspension volume (L); and m is the amount of clay used (g).

Textural and Structural Characterization Techniques
The identification of the different phases in the crude and activated clays was examined by X-ray diffraction technique by using X-ray diffractometer D8 ADVANCE BRUKER with Cukα (λ = 1.54Ǻ) radiation over a range of angles from 5 to 80°. The FTIR spectrum of the clay mineral was recorded on a KBr disk, which contains 1 % sample by mass, using a Perkin Elmer (model 783) spectrophotometer. For each sample, scans were measured in transmittance mode over the 4000-400 cm −1 range. The Chemical compositions of the studied materials were obtained by X-ray fluorescence (XRF) technique. The clay samples were prepared using the socalled fusion bead method. A specific amount of the analytical powder was first calcinated at 1050°C, and then added to lithium tetraborate. The mixture was introduced to the beader following a heating cycle. Finally, the transparent bead was analyzed using a BRUKER S8 TIGER. The zeta potential was measured by the Malvern Zetasizer Nano-ZS. Nitrogen adsorption-desorption isotherms at 77 K were performed in a Micromeritics ASAP 3000 sorptometer. Samples were outgassed at 150°C in N 2 flow with the residual pressure of 10 −5 mm Hg. Surface area values and pore size distributions were obtained from the adsorption branch of the isotherm by using the BET method. The morphology of the samples is analyzed by scanning electron microscopy (SEM) by using FEG 650. The granulometric distribution of samples was measured by means of a Laser granulometer Microtrac S3500. Figure 1 displays the X-ray diffractogram of the starting material. This diffractogram exhibits the characteristic reflections of illite (10.23 Å) and of kaolinite (7.11 Å; 3.56 Å). It also shows that the associated minerals are the quartz (3.33 Å), the calcite (3.03 Å) and the dolomite (2.85 Å) [24]. The chemical composition of starting clay, expressed as wt% of oxide, is given in Table 1. The relatively high percentage of iron oxide compared to aluminum oxide and the presence of K 2 O suggest that we are in the presence of a ferrous type mineral of illite called "glauconite" [18]. On the other hand, the high contents of SiO 2 , MgO and CaO can be explained by the presence of quartz and carbonates associated with the clay. The infrared spectrum of clay ( Fig. 2) reveals the presence of all characteristic vibration of phyllosilicates and these of associated minerals signaled by XRD [25].

X-Ray Diffraction
In order to make the difference between the heat treatment introduced by the microwave irradiation and the acid treatment on the crude clay, starting material has been irradied alone, without acid, one the microwave at different times (1; 2 or 3mn) and various powers (630 or 900 W). The XRD patterns of this serie are shown in Fig. 3. All the patterns exhibit reflections detected for untreated material whatever the time or the power of microwave irradiation used. Furthermore, no news phases were detected in any of the patterns, indicating that the microwave heating haven't an effect on the crystallinity of material. Nevertheless, the microwave treatment of starting material in the presence of chlohydric acid reveals some changes on crystallinity of samples. Figure 4 presents diffractograms of crude clay and these of treated with chlorhydric acid (3 M) assisted by microwave irradiation at different times (1 ; 2 or 3mn) and various powers (630 or 900 W). All X-ray diffractograms registered after microwave irradiation in the presence of acid display the presence of the characteristic reflections of quartz, indicating their resistance to chlorhydric acid solution. Although, characteristic reflections of carbonates (dolomite and calcite) disappeared rapidly with acid activation treatment. On the other hand, we note that all reflections attributed to the glauconite remain at the same positions and with almost the same intensities whatever the power or time of activation used. However, it is shown that the kaolinic phase resists less and diffractograms indicate that 2 min is a sufficient time to disappear all their characteristic reflections. The base line of diffractograms is conserved with the same allure. The absence of amorphous phase indicates that the structural unit of the clay is partially affected without being destroyed.

Chemical Analysis
The chemical composition of the resulting solids is given in Table 2. Following acid treatment, the content of iron and aluminum oxide decreased with increasing the treatment time and/or the power of microwave irradiation, while the relative amount of SiO 2 increased. This indicates the partial destruction of clays structures signaled by XRD and IR analyses. It was also revealed from these results that the percentage of K 2 O decreases slightly indicating the resistance of glauconite to the acid activation. However, the highest decrease of the CaO content confirms the elimination of carbonates.    To evaluate more the decomposition and the change in dissolution characteristics of clay introduced by the activation, we used the proportion of reacted material value (α) as a measure of the decomposition [26]. The α formula is: ¼

Granulometry
Crude clay displays a granulometry lower than 100 μm (Fig. 6). The particle size of this clay presents a bimodal distribution with two medium sizes 0.85 and 2.27 μm. However, activated clays showed a high particles size which can extended to 1000 μm. The particle size increases with acid activation assisted by microwave irradiation. The highest size (around 295 μm) is obtained by an activation during 3 min under a power irradiation of 630w.  natural clay, G9-2 sample appears to be highly compact and an agglomeration phenomenon has been observed following the acid treatment assisted by microwave irradiation. These observations are in good agreement with the increase of particles sizes of activated clays signaled by granulometry.

Adsorption-Desorption Isotherms and BET Surface Area Analysis
N 2 adsorption-desorption isotherms at 77 K of crude and acid activated clay under microwave irradiation are presented in Fig. 8. Adsorption-desorption isotherms of natural or acid activated clays at various microwave irradiations exhibit the classical type II shape of adsorbents with H 3 hysteresis loop according to the IUPAC classification [28,29]. H 3 hysteresis loop is observed for slit pores or in case of particles in the form of layers. The surface area values and total volume pore of studied samples are listed in Table 4. As seen from this table, HCl activated clay assisted by different microwave irradiation (630 or 900 W) do not increase or increase little the surface area values. This can be correlated firstly to the low rate of amorphization signaled by XRD which can makes a very important contribution. Secondly, these results can be explained by the increase of particle sizes indicating by granulometry and SEM.

Zeta Potential
The zeta potential of non-activated and activated clay is shown in Fig. 9. Compared to another clays, very limited literatures were interested to zeta potential on glauconite type. Herein, both for natural or acid activated kaolinite-glauconite samples the zeta potential values are negative in all the field of pH. This indicates that surfaces of activated or non-activated samples have net negatives electrostatic charges and no isoelectric points are detected. These observations are similar to these observed by Khaled A. Selim et al. [30].

Adsorption Isotherms
Methyl orange adsorptions isotherms of the natural and prepared samples have been evaluated. To identify the interaction between adsorbent and adsorbate, the adsorption process was studied as a function of equilibrium methyl  orange concentration (Ce) (Fig. 10). According to GILES classification [31] isotherms seem to be of the L type, revealing that methyl orange was rapidly adsorbed and has a good affinity for kaolinite -glauconite clays surfaces. The exam of isotherms shows that only for G6-1sample the form L of isotherm is conserved like the form of crude clay. For all other activated samples, all isotherms are similar in shape and they reveal the presence of two clear plateaus. This can be correlated to the existence of two types of reactive sites. These observations of two plateaus in adsorption isotherms were also obtained in some previous studies concerning the adsorption of quinalizarin onto acid activated palygorskite [32] and about the adsorption of organic molecules on silica surface [33] either about a sulphate adsorption on a volcanic ash soil [34]. According to the isotherms, the higher adsorption capacity was detected for G9-2 sample and Qm is 4.29 mg/g ( Table 5). This result is not correlated with values of specific surface area of sample indicating that this phenomenon was related essentially to the nature of the active sites created after acid activation assisted by microwave irradiation.
Previously some researchers have investigated several adsorbents materials similar to our materials for the removal of some dyes or heavy metals from aqueous solutions. According to the data summarized in Table 6, the maximum adsorption capacity (Qm) of both glauconite or kaolinite clays Fig. 9 Zeta potential of nonactivated and activated kaoliniteglauconite Fig. 10 Adsorption isotherms of dye onto crude clay before and after activation is relatively low compared with those of other clays. The adsorption capacities of activated clay were higher than that of non-activated materials. It can be stated from this that our findings materials are good and promising for an environmental application.

Modelling Adsorption Isotherms
To describe solid/liquid adsorption process, two empirical models are tested, which are the Langmuir and the Freundlich isotherms ( Fig. 11; Table 7). Langmuir model assumes that adsorption occurs at specific homogeneous sites within the adsorbent and has found successful application in many studies of monolayer adsorption. The obtained adsorption data were fitted by the linearized Langmuir equation [40]: Where Q ad is the equilibrium capacity of adsorbate on adsorbent (mol.g −1 ), C eq is the equilibrium concentration of adsorbate solution (mol.L −1 ), Q m is the maximum adsorption capacity of the adsorbent (mol.g −1 ), and K is the Langmuir constant (L.mol −1 ) related to the adsorption energy.
The Freundlich isothermis an empirical equation employed to describe heterogeneous adsorption surface with sites that have different energies of adsorption and are not equally available. The Freundlich isotherm is more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model. Its linearized form can be written as follow [4]: Where: K f ðL:g À1 Þ and n (dimensionless) are the freundlich adsorption isotherm constants, being indicative of the extent of adsorption and the degree of non-linearity between solution concentration and adsorption respectively.
The correlation coefficients R 2 , calculated from the linear Langmuir and Freundlich equations, and the characterstic coefficients of each model are presented in Table 7. We can see from this table that for non-activated kaolinite-glauconite the Langmuir model appears better than the Freundlich model to describe methyl orange adsorption process. However, these correlations are not satisfactory enough for the activated clay. N. Frini-Srasra and E. Srasra (2009) have attributed these observations in isotherms shape to the existence of two kinds of adsorption sites: the stronger and the weaker sites.

Conclusions
In conclusion, acid activation of Tunisian Kaoliniteglauconite clay was successfully made in microwave by irradiation assistance. This method has the advantage of a short time of manipulation in comparison with the traditional heating activation method. It has shown that microwave irradiation alone, without acid, seems to have no effect on the crystal structure of crude clay. However, an effect on the textural and structural properties of crude clay after acid activation under microwave was detected. The level of effects depends to the power and time of microwave irradiation. The acid activation under microwave irradiation does not increase the specific surface area but increases well the adsorbing power of crude clay and provide promising materials for adsorption applications. Under the same conditions, the Q max value increases from 3.21 mg/g for the untreated clay to 4.29 mg/g for G9-2. This increase results from the existence of new active sites created by acid activation. The adsorption  isotherms were modeled using Langmuir and Freundlich models. The Langmuir model is the most adequate to predict the adsorption process.