Investigation of the ion exchange effect on surface properties and porous structure of clay: Application of ascorbic acid adsorption

Journal of Environmental Chemical Engineering 7 (2019) 103404

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Investigation of the ion exchange effect on surface properties and porous structure of clay: Application of ascorbic acid adsorption

T

⁎

Fatna Anouar , Abdellah Elmchaouri, Nawal Taoufik, Younes Rakhila Department of Chemistry, Faculty of Sciences and Technologies, Mohammedia, 20650, Morocco

A R T I C LE I N FO

A B S R A C T

Keywords: Smectite Cationic exchange Surface BET Porosity Ascorbic acid adsorption

As part of the contribution to the synthesis and characterization of clay materials, fits the aim of this work. It is the improvement of some physical and chemical properties of Smectites between raw and modified state. The analyze of the exchanged cation effects on these properties include specific surface BET, external and internal surfaces, pore volume, pore size was determined by the nitrogen adsorption at 77 K and the experimental data were fitted by the isotherm adsorption models for gas. Clay purification and ion exchange are the most important treatment that expends a clay characteristic, more advanced than basic materials. As a result, higher adsorption is obtained for the sodium form of smectite this is associated with the intrinsic characteristics of the material. After the cation exchange, the specific surface area has increased from 210 to 440 m² g−1, the CEC reaches a value of 101 meq/100 g ditto for the pHpzc value which moved from neutral to basic pH. However, an overall decrease in the basal distance d001 was observed for NH4+-SM and K+-SM. In the case of the Na+-SM, the d001 was slightly decreased. Moreover, the adsorption phenomena of ascorbic acid provides more information about the material surface. The four materials display a heterogeneous surface because the adsorption of ascorbic acid gets to the Freundlich model, also its kinetic fits the pseudo second order.

1. Introduction The clays have found their application in a vast field ranging from catalysis to water treatment because their advantages are enormous. First of all, their abundance in the open pit implies their low cost then their structure in phyllosilicate leaves allowing a high adsorption capacity [1]. So much effort was made to obtain new materials (modified clay) that could improve these processes and application [2]. Smectite is widely used in the decontamination application modified by ion exchange either in organic or inorganic form, for the removal of heavy metals [3], anionic and cationic dyes [4] and some toxin [5]. They are known among the best adsorbents due to their physicochemical properties, with a significant specific surface area (around 700 m² g−1 [6]), the negative and permanent surface charge and its compensation by interfolar cations allow to the molecules to be trapped. Basing on the chemical formula A0.3D2-3T4O10Z2.n H2O, smectite exhibit a high swelling capability in the presence of H2O by expanding their sheets without being exfoliated [7]. For research purposes, reference clays are sometimes used without purification [8]. Regarding the size, the most commonly used size fraction is around < 2 μm, and it is obtained by gravity sedimentation or low-speed centrifugation. Nevertheless, the further development of ⁎

the clay minerals application raises the need for purification process to determinate the optimum parameters. The community of researchers specifies that these common methods did not remove all the inorganic and organic impurities present in the clay. The effect of clay purification was evaluated by L. Jacqueline arroyo et al. (2008) using two methods based on the order of purification operations [9], they obtained a particle size < 2 μm by low-speed centrifugation, followed by removal of the impurities. This effect was applied for the evolution of the pesticide sorption and hydrolysis. On the other hand, ion exchange in clay minerals especially smectite is an effective method to modify and improve surface proprieties. Ion exchange is a reversible chemical reaction that takes place between ions held near a mineral surface by unbalanced electrical charges within the mineral framework and ions in a solution in contact with the mineral. It is dependent on the mineral crystalline structure and on the chemical composition of the solution in contact with it, it depends on the cations housed in the space between the sheets. Generally, the excess charge on the mineral is negative, and it attracts cation from the solution to neutralize this charge. Several methods were developed and improved to measure the cationic exchange capacity in soils and clays like: those allowing access to the effective CEC of the soil, and those allowing access to the potential CEC (at a given pH) the first method based on

Corresponding author. E-mail address: [email protected] (F. Anouar).

https://doi.org/10.1016/j.jece.2019.103404 Received 26 June 2019; Received in revised form 18 August 2019; Accepted 5 September 2019 Available online 06 September 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Environmental Chemical Engineering 7 (2019) 103404

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80 μm. We subjected 30 g to an attack of oxygen peroxide (30% v/v) at a temperature kept below 60 °C under stirring for 1 h. The resulting suspensions were washed several times with distilled water and dried in an oven at a temperature of 100 °C overnight. The second step consists to remove the clay carbonates by the action of nitric acid HNO3, a solution of 0.8 mol L−1 made in contact with clay fraction stirring for 4 h under pH control and maintained at a value ≥ 4.5 to avoid the attack possibility of clay mineral structure. The solids contacted with oxalic acid to 0.12 mol L−1 and stirred for 1 h at 80 °C. At this stage, the different clays cation-exchangeable are substituted by H+ ions and the excess electrolytes were removed by washing with distilled water and drying overnight at 140 °C [18].

exchange either by BaCl2 or by cobalt hexamine by Aran et al. [10]. However the second method is that of Metson [11] involved acetate ammonium method, this method remains expensive and long for this, and it improved in C. Ola et al. [12] works by saturating the adsorbent complex of the soil with CaCl2. Furthermore, the chemical reactions in ion exchange follow the law of mass action, but the reactions are restricted by the number of exchange sites on the mineral and by the bonding strength of the exchangeable cation to the mineral surface. The common metallic cations found in exchange positions in clay minerals are Ca2+, Mg2+, Na+, and K+. Generally, at low pH values, H+ replaces other cation. In this work, the effect of purification and ion exchange on the surface properties that are the surfaces area, cation exchange capacity and porosity will be approached. Those proprieties were operated by nitrogen adsorption-desorption study and parameters related to this adsorption were elicited and analyzed by specific models. Ascorbic acid or commonly known as Vitamin C has a very important role in the development of human health; it has an antioxidant effect that protects cells against damage caused by free radicals [13]. It is found in several waste industries like food industries pharmaceutical [14], cosmetic industries [15]. The detection of ascorbic acid remains difficult because of its non-stability. It is usually detected by HPLC coupled with UV–vis. Its removal is rarely discussed in the literature either on clay or activated carbon [16].

2.3. Solid modification The modification consists to exchange the clay cations by a single cation to achieve a homo cationic mineral. In this step, the cations contained in the interfoliar space are replaced by charge compensating cations. The choice was made on the following cation: Na+, K+, and NH4+, they are among cations present in the humic-clay complex [19] and they are all monovalent cations. In order to be able to conclude and make comparisons on the physical and chemical properties of the cations and not on their oxidation degree. The solids suspended in distilled water, 10 g dispersed in 100 mL. Then they brought into contact five times with solutions of chosen cation salts chloride with a concentration of 1 mol L−1, stirred for 4 h and centrifuged each time. Clay dispersions obtained after the above described treatments contain considerable quantities mainly salts, they are removed by extensive washing with distilled water; the total disappearance of chloride is verified by the silver nitrate test. The gel fraction separated and solids dried in an oven at 80 °C. We obtained four solids named: Ref-SM and K+-SM, NH4+-SM, and Na+-SM as a cationic form of Smectite. The resulting samples are analyzed by ICPMS, and the results are given in Table 1.

2. Material and methods 2.1. Starting materials The origin of the material used in the present study is natural clay from Moroccan basement located in the valley of Moulouya in the Middle Atlas. It has been characterized using the XRD performed by a Siemens D-5000 diffractometer with a radiation copper (CuK =1.54 Å) fully managed by the Diffract-AT software. Further FT-IR Analysis was performed by PerkinElmer-Spectrum Two equipped (Diamond/ZnSe). The material chemical analysis was determined by an Inductively Coupled Plasma using an ICP-MS (ICP MS VARIAN 2011) spectrometer which gives the composition in term of oxide elements. The stability of our support was studied by thermogravimetric analyses (TGA). The experiments were performed under a high-purity nitrogen atmosphere with a gas flow rate of 20 mL min−1, from 20 °C to 750 °C with a heating rate of 20 °C min−1. The cationic exchange capacities (CEC) were measured by the cobalt hexamine method. The potential of zero point charge (pHpzc) is measured by adjusting between 1 and 12 the pH of a NaCl solution (0.01 mol L-1) by adding either HCl or NaOH. 50 mL of this solution was mixed to 0.15 g of the supports. The mixture has been allowed to stabilize for 24 h. After stabilization, the final pH was noted. The pHpzc was obtained from the plot of (pHf-pHi) versus pHi. this was taken as the pHpzc of the sample by Nandi [17]. All the used solutions were prepared by RP Normapur products from PROLABO chemicals

2.4. Nitrogen adsorption The adsorption-desorption isotherms of nitrogen are carried out on an apparatus ASAP 2010 Micrometrics Analyzer. To achieve the experience a sample mass of about 120 mg is introduced, it was dried and degassed in-situ at 110 °C under dynamic vacuum for 24 h, to eliminate the impurities like water, organic molecules [20] and subjected to the various pressures at 77 K required by gas expansion. The purpose of degassing is twofold [21], first to obtain a good intermediate state of the support by removing the physisorbed molecules, to evade any radical change due to ageing or surface modification. 2.5. Ascorbic acid adsorption To study the capacity of these supports for the pollutants removal, the ascorbic acid was chosen as an emergent molecule. The adsorption studies of ascorbic acid onto our adsorbents are studied in continuous mode, in Erlenmeyer the adsorbents are immersed in 50 mL of ascorbic acid after the desired time has elapsed the mixture is then separated and the supernatant is recovered by filtration using syringe filters. The remaining quantities were measured by UV–vis at 250 nm and the

2.2. Solid purification The clay used is a natural one having the cationic exchange capacity of 90 meq/100 g. It was previously ground and sieved to a size less than Table 1 Chemical composition of samples by ICP-MS Analysis.

Ref SM Na+-SM NH4+SM K+-SM

SiO2

Al2O3

Fe2O3

Na2O

K2 O

MgO

CaO

ZnO

P2O5

SO3

MnO2

SiO2/ Al2O3

56.25 54,26 53,48 54,74

10.01 9,95 9,64 10,12

4.48 4,25 4,63 4,51

1.98 5,43 0,95 1,85

0.07 0,09 0,08 4,37

0.08 0,04 0,05 0,04

0.17 0,09 0,07 0,13

0.04 0,04 0,04 0,04

0.22 0,14 0,18 0,2

0.02 0,05 0,04 0,03

0.12 0,15 0,09 0,13

5.62 5.45 5.54 5.41

2

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that of sodium clay this may be due to the compensating cation size [24].

adsorbed quantity by the following equation:

Q = (Ci − Ct )*V m

(1)

Where Ci is the initial concentration and Ct concentrations of the ascorbic acid solution at t time in (mg dm−3), respectively, V is the volume (dm−3) of an aqueous solution containing ascorbic acid, and m is the mass of dry adsorbent used (g). Concerning the determination of the optimums parameters, the influence of pH on vitamin C adsorption was studied by adjusting 50 mL of vitamin solutions to different pH values from 2 to 12 using 0.1 mol L−1 HCl or NaOH solution and agitating with 0.05 g of the adsorbent for 2 h. the effect of adsorption time and initial concentration of ascorbic acid on adsorption efficiency was performed in the 50 mL of vitamin C solutions at temperature 20 °C, m = 0.05 g and pH = 5. For kinetics, the time was varied from 0 to 200 min with initial concentration 100 mg L−1 and for equilibrium study, the solution concentrations range was from 0 to 100 mg L−1 at 120 min as a maximum equilibrium obtained time. To ensure the occurrence of the data, all the experiences were repeated three times.

3.2. Chemical composition analysis The analysis of the chemical composition is done by inductively coupled plasma ICP-MS. The solid samples were dissolved in an acidic bath using a 63% concentrated nitric acid. The ICP results are reported in Table 1. Referring to the composition analysis, all samples contain an important percentage of Silicon and Aluminum in form oxide. The molar ratio which represents the maximum substitution Si4+ by Al3+, is higher than the usual value found in bentonites (SiO2/Al2O3 = 2.7), this difference indicates the presence of free quartz in the clay fraction [25]. The sum of the other oxides grants a percentage of about 10%, which shows that our clay is not pure. In the table, as expected, the presence of an important quantity of Na2O is noted in the sample exchanged by Na+ cation. The same remark is done for the sample exchanged by K+ for K2O, and however, the molecules which can indicate the presence of exchanged NH4+ isn’t referd, just a very slight decrease in the majority of oxide percentages in the sample exchanged by NH4+ is remarked [26]. The notion of the ionic and atomic radius (table2) is to be considered with caution in view of the geometry of the NH4+ ion. It was demonstrated that the ammonium ion can establish hydrogen bonds with the surface oxygen and that this particular configuration makes it possible to multiply the interactions with the sheet [27]. Therefore, This description of the interactions between the NH4+ cation and the clay sheet will also in the sense of a limitation of the opening of the interfoliar space and thus of the diminution interfoliar hydration [28].

3. Results and discussions 3.1. X-ray characterization of clay powders (XRD)-* Fig. 1 shows the diffractograms of the modified clays by cation exchange in comparison with their reference clay. The XRD diagrams identify the various minerals that constitute each sample. The distances measured on the reflections (001) can be assigned by their values to different existent species of clay minerals. The diffractograms for these Smectites in its different forms shows that even after purification, the impurities were not completely eliminated [22]. There still some Quartz, Mica and Anatase. Probably is due to their fine size which made their separation difficult. Also for some supports like NH4+-SM, new phase called Mascagnite (NH4)2SO4 is formed, this can be assigned to the facility of chlorite dissolution related to sulfate ions. The samples exchanged generally have the same structural characteristics as the reference sample. These results were compared to those of different investigators [23]. The basal distance of the clay is measurable by the reflection 001 which is located at an equidistance of 2θ ≈ 6° corresponding to d001 = 1.4713 nm the characteristic distance of Smectites. This beam is very important in reference sample more than the modified samples, it is displaced to 2θ ≈ 5° for Na+-SM and NH4+-SM and to 2θ ≈ 8° for K+-SM in which the intensity increase in this sense RefSM > Na+-SM > NH4+-SM > K+-SM. The X-ray diffraction has shown that hydration of ammoniated and potassic clay is lower than

3.3. Cationic exchange capacity (CEC) The cationic exchange capacities were measured by the cobalt hexamine method. After an exchange of clay cation with the Co (NH3)63+ ions [29], the CEC estimated is based on the determination of the remaining Co in solution by dosage in UV–vis at λ = 457 nm [30]. The CEC of the resulting samples is presented in Table 2 with some physical characteristics concerning the atomic radius of the exchanged cation. It is noted in Table 2 that the values of CEC concerning the samples exchanged with K+ ions and NH4+ varied slightly or remained the same. On the other hand, for the sample exchanged by Na+ the value of CEC increased importantly from 90 to 101 meq /100 g. The small atomic radius can probably explain this variation because the largest value of CEC corresponds to the smallest atomic radius. Various works concern intercalated clays are based on clays exchanged by sodium ion because it is an ion easily and quickly washable it allows a good dispersion in the solution. Else, The CEC is directly related to the edge properties of the four Smectites, including the average layer diameter and low layer loads [31]. 3.4. Determination of zero point charge (pHpzc) The potential of zero point charge (pHpzc) of the sample presents the pH in which the negative and the positive charges are equals. It is found to be 5.9, 6.4, 7, 9 for respectively for NH4+-SM, K+-SM, Ref-SM, Table 2 CEC of modified Smectites and atomic radius of the exchanged cation.

Fig. 1. X-ray Diffractograms of exchanged smectites. 3

Samples

CEC/(meq/100 g)

ionic radius/nm

Ref- SM Na+- SM NH4+-SM K+- SM

90 101 89 92

— 0.098 0.145 0.133

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Fig. 2. Infrared spectrum of reference and modified Smectites.

Fig. 4. Isotherms of nitrogen adsorption desorption.

Na+-SM. The pHpzc increase in the sodium smectite however, it decrease in the two others cases [32].

presented graph has a shape similar to that of original clays, a decrease at 50 °C until 150 °C corresponds to the removal of hygroscopic water and then bound water, the phenomenon is generally exothermic. The second decrease from 200 °C to 500 °C corresponds to a slight dehydroxylation of structure accompanied by OH removal. Then the water constitution is eliminated between 450 °C and 550 °C and finally, the carbonates which are present only in the reference smectite and are eliminated between 570 °C and 700 °C, this three phenomenon are endothermic. It can be observed that the cation exchange decreases with mass loss [34].

3.5. Fourier transform infrared spectroscopy analysis (FT-IR) According to the FT-IR spectrum of the four Smectites, the 1050 cm−1 band have been assigned to SieO stretching as the intense band. In addition, the 540 cm−1 bands were assigned to the bending vibrations SieOeAl. The bands observed at 3457 cm−1 and 1641 cm−1 were assigned to the stretching vibrations of the molecules adsorbed by the water and to the exchangeable cation. On the spectrum, the transmittance related to interfoliar water was observed at about 3600 cm-1 with different percentage corresponding to the compensating cation. In fact, a high transmission was observed for the cations inserted in the space between the sheets for the K+ ion, and it decreases while going towards Na+ and the Ref-SM. Moreover, water molecules have three fundamental vibration modes: asymmetric stretching, symmetric stretching and HeOeH bending [33]. The stretching vibrations of the surface hydroxyl groups Si-Si−OH and Al-Al−OH were also observed at 3620 and 980 cm−1 respectively (Fig. 2).

3.7. Adsorption-desorption isotherms The limits use of adsorbents solid for an exact application depends on their physical and -chemical proprieties and structural characteristics quoting specific surface and porosity distribution [35]. The isotherms presented at Fig. 4, were performed by ASAP 2010 Micromeritics and are symbolized by the diagrams of adsorbed volume V/ (cm3 g−1), which represent the quantity of adsorbed matter on the solids versus relative pressure P/P0. Fig. 4 shows the curve of adsorbed volume versus relative pressure (P/P0) corresponding to the adsorption/desorption phenomenon of nitrogen in Smectites. In these curves, the adsorbed volume increases and decreases with the relative pressure according to three variable zone. In general, each zone in the isotherms corresponds to a fixation mode. In the first step (1) and for low pressures (0.00 < P/P0 ≤ 0.3), adsorption up takes onto preferential sites. Then, the second step (2) corresponds to a filling of micro pores and adsorption on the monolayer. the third step (3), adsorption moves to the multilayer and filling of small mesopores by capillary condensation (Fig. 5). The curves of Nitrogen adsorption-desorption isotherms of those supports indicate a shape similar to isotherms type IV from the BDDT (Brunauer-Deming-Deming-Teller) classification [36,37]. Isotherm type IV is similar to materials in which mesopores and micropores are presents [38]. For the H3 type loops, the hysteresis loop is moderately narrow, and the desorption curve is very close to the adsorption curve. As a result, the Na+-SM support has the best adsorption capacity, it is about 500 cm3 g−1 compared to the others support and the notion of the atomic radius is even now present. In the entire range P/P0 > 0.6, whether in adsorption or desorption, the order of adsorption capacities is: K+-SM < Ref-SM < NH4+-SM < Na+-SM. While in the range of P/P0 < 0.6 there is an inversion and the order is as follows RefSM < K+-SM < NH4+-SM < Na+-SM. The discernment and interpretation of the nitrogen adsorption phenomenon enable us to determine the specific surface area by the

3.6. Thermogravimetric analyses (TGA) Fig. 3 displays the results obtained using the ATG Analysis. The

Fig. 3. ATG analysis of exchanged smectites. 4

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sample by sodium. While the smallest value is obtained for the sample exchanged by potassium cation Overall, more the CBET is bigger; the interactions are stronger. CBET reports a value corresponding to mesopores (3 < CBET < 1000) also a positive CBET indicates that micro porosity is present and the applicability field of the BET law has been well defined [43]. To estimate separately the surface fractions carried by the larger micropores or pores and the external surface, the t method by de Boer was used. The t-method is based on the analysis of the isotherm drawn in the diagram (t,V), t being the static thickness of the adsorbed coatings. The thickness of the layer is calculated from the Harkins and Jura equation [44] and as represented in Fig. 6: 1

13.99 ⎞ t = ⎜⎛ ⎟ − log P P0 ⎠ 0.34 ⎝

formation of a mono molecular layer on the surface. While through capillary condensation, the pore size is correlated by t-plot developed by Lippens and de Boer [39] and pore size distribution was modelled by the BJH method [40,41]. 3.8. The measure of specific surface The surface area of materials is estimated from the amount of nitrogen adsorbed in relation with pressure at the boiling temperature of liquid nitrogen and at normal atmospheric pressure. The information is interpreted depending on the model of Brunauer, Emmett and Teller (BET method) [42] to measure the surface area. The experimental adsorption curves are approximated by the BET equation until capillary condensation begins in the pores, it is calculated in the relative pressure range of 0-0.3 which is linear. Brunauer, Emmet and Teller have developed a multilayer model extending the Langmuir theory. The resulting BET equation is:

V

(

P0

1−P

P0

)

=

1 C−1 P . P0 + = A. P P0 + B Vm C Vm C

Stot = 15.47K1 (m² g−1) Sext = 15.47 K2 (m² g

Vm Nσ VM

(5)

−1

)

Sint= Stot - Sext (m² g−1)

(6) (7)

The initial section of t-plot is linear explains that monolayer coverage has occurred on the pore walls in the same manner as on the open surface [45]. The hysteresis phenomenon confirms the presence of mesopores. The intersection between the x-axis (between 4 and 10 nm) and the yaxis, leads to a volume adsorbed by micropores. The slope of the second part leads to the external surface whose micropores, mesopores and outside particles correspond to the Sext monolayer. In comparison of the total surfaces with the BET surface, The surface of the micropores represents percentages greater than 30%. Our supports are mixed between micro and mesopores. The data used for the reference isotherm are those of Gregg & Sing for nitrogen adsorption, it is chosen according to an energy analogy on the constant BET [46].

(2)

Where P and P0 are the equilibrium and the saturation pressure of adsorbate at the temperature of adsorption, V is the adsorbed gas quantity and Vm is the monolayer adsorbed gas quantity. C is the BET constant. Fig. 4 shows the linear plot of the BET equation. The BET method allows explaining the physical adsorption of gas molecules on a solid surface and allows the measurement of the specific surface area by the following equation:

SBET =

(4)

The initial slope (K1) of the diagrams (t, V) corresponds to the total surface area (Stot) that can be covered by a monolayer of nitrogen molecules with a thickness of 3.54 Å and at a higher pressure. The slope (K2) of the diagrams (t, V) corresponds to the adsorption of nitrogen on an adsorbed film thicker than 3.54 Å and shows the contribution of macropores and the external surface to the adsorption (Sext). The surfaces (m2 g−1), i. e. Stot and Sext, are calculated from the slopes using equations [5] [4]. The internal surface, Sint, was estimated by the difference between Stot and Sext, it is the surface that can only be covered with a unimolecular nitrogen layer. Stot, Sext and Sint derivatives for adsorbents are presented in Table 3:

Fig. 5. Representation of specific surface plot (BET equation).

P

2

3.9. Pore size and distribution Barrett, Joyner and Halenda’s method founded on the distribution curves of the pore volume (dVp/ d dp, cm3 g−1 nm−1) as a function of pore diameter from nitrogen adsorption-desorption data. It is proposed that pore radius are cylindrical (rp) and it is connected to the thickness of the adsorbed layer (t) and the meniscus radius (rK) given by the Kelvin equation [8]:

(3)

Where Vm: Adsorbed volume corresponds to the monolayer (cm3 g−1), VM: Molar volume of a perfect gas, N: Avogadro number, the bulk section of nitrogen σ: 0.162 nm². The monolayer volume and the BET constant are calculated from the BET equation using the slope and an intercept of the BET equation. the experiemental data are operated in order to calculate the formation energy of the monolayer. All the results are represented in Table 3. The calculated energies, ΔE, allow to notice that nitrogen adsorption on an exchanged clay minerals by cationic exchange is probably a physical adsorption involves a weak binding interactions, such as Van der Waals bonds, between the adsorbed chemical species and the adsorbent due to the nonpolar nature of nitrogen what leads to an adsorption on the external surface. According to the results shown in Table 3, as expected, a maximum value of specific surface area is obtained (440m² g−1) for the exchanged

rp = t + rK = t - 4.5/ log (P/P0)

(8)

The calculation of pore size by BJH theory is done on the desorption branch in the field where the desorption branch joins the adsorption branch (P/P0≥0.42) for the four samples. The results are shown in Table 3 corresponding to the pore size calculated from the isotherm of the adsorbed volume as a function of relative pressure. This hysteresis is significant of the presence of mesopores constituting a stable structure: it seems that the saturation of the pores at the highest relative pressure was obtained, which is reflected by the inflexion of the adsorption curve (capillary 5

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Table 3 Data of BET, t- plot and BJH model for adsorption-desorption of the system N2/M-SM at 77 K /M: exchanged cation. BET

Ref- SM Na+- SM NH4+-SM K+-SM

t-plot

CBET

ΔE/ (kJ mol

469 96 89 99

4.002 2.941 2.729 2.935

−1

)

SBET 210 440 306 148

BJH

Stot

Sext

Sint

Pore size /(Å)

196,78 423,26 313.89 172,65

49,39 257,42 76.99 12.58

147,38 165,84 236,89 160,07

31.5984 35.3482 46.1312 65.9371

Vm: Monolayer Volume, CBET constant, ΔE: Adsorption energy of the monolayer, am (N2) = 16.2 Ų, ΔHL= -5.6Kj mol−1 (Liquefaction enthalpy of nitrogen at 77 K), Stot=Sint+Sext.

Fig. 8. Effect of contact time on ascorbic acid adsorption at m = 0.05/ pH (Na+-SM) = 8, pH (NH4+-SM,K+-SM,Ref-SM) = 5/Ci = 100mgL−1.

Fig. 6. Graphical representation of t-plot.

Fig. 7. Effect of pH on ascorbic acid adsorption at m = 0.05, t = 120 min, Ci = 100 mg L−1.

Fig. 9. Effect of initial concentration on ascorbic acid adsorption at m = 0.05/ pH (Na+-SM) = 8, pH (NH4+-SM, K+-SM, Ref-SM) = 5/ t (Ref-SM, Na+SM) = 90 min, t (K+-SM, NH4+-SM) = 120 min.

condensation). Otherwise, a maximum value is obtained by K+-SM concerning pore size estimated at 6.594 nm. According to these results, more the exchanged cation is small more the support is effective and performing. Also, the pore size indicates size pore corresponding to mesoporous solids [47]. The BJH calculations gave about 5 nm as pore size before and after cation exchange. The PSD calculated with the BJH method is centered on a single value for the four samples, which is confirmed by the work of Gautier (BJH and DFT) [48,49]. From this results two hypotheses can arise it means that Na+ doesn’t occupy pores but it is localizited only in internal surface which allows N2 to enter in pores consequently an important pore size is

obtained, or the Na+ cation is easy to stitch it means while Na is inserted in the clay structure, Nitrogen can replace it in pores. These properties monitor the clay applications from catalysis Groen [50] to many other applications [51]. the nature of the material in the interfoliar space and its characterstics (ionic radius), influences the gas adsorption [52].

3.10. Effect of exchanged cation on ascorbic acid Adsorption The efficiency of our supports is represented here by studing their 6

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Fig. 10. Freundlich isotherm of ascorbic acid adsorption.

Fig. 11. Pseudo-second-order model of ascorbic acid adsorption.

Fig. 12. Adsorption mechanism of ascorbic acid on Smectites depending pHpzc.

7

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After adsorption, the acidic function bands appeared, the CH bond at 2400 cm-1 and the C]O bond around 1680 cm-1. It is a qualitative method that indicates the presence of the adsorbed molecule. In this case, the ascorbic acid was adsorbed successfully on Na + -SM and NH4 + -SM (Fig. 13). 5. Conclusion It appears that the purification of natural clays is difficult but fundamental for preparing materials with physic-chemical properties much more interesting than those of raw materials. The surfaces properties whose specific surface BET, the external and internal surface have been improved by cation exchange. Mineralogically, the porosity remains identical after cationic exchange. these results conclude that the surfaces developed by the clay minerals depend on several factors and parameters (preparation, purification, exchange). For the four adsorbents, the adsorption of ascorbic acid fits the Freundlich model while the kinetics study discribed by the pseudo second order. The mechanism depend strongly on pH this is correspond to the best adsorbent which the sodium smectite (bigger pHpzc).

Fig. 13. Infrared spectrum of reference and modified Smectites after ascorbic acid adsorption.

adsorptive behavior representing the adsorbed quantity compared to different parameters: pH, Initial concentration and time variation. The pH variation plays a very important role because the affinity between adsorbent and adsorbate charges influences directly on the adsorption by either favouring or disfavoring the process, this is related directly to the notion pHpzc. The optimization figures indicate an interesting adsorption for Na+SM. The high pHpzc value of the sodium smectite (pHpzc = 9) gives a large field to ascorbic acid and Na+-SM to be in contact (Fig. 9). The most effective carrier is Na+-SM, it gives maximum adsorbed amounts of ascorbic acid with adsorption efficiency of 80%–90%. The adsorption phenomenon of ascorbic acid depends directly on the pH, the maximum elimination corresponds to the optimum pH value of 5 (corresponds to or approaches the pKa value of ascorbic acid) [53]. From the optimization experiments, the following optima give the best adsorption are corresponding to pH = 5 for the supports except for Na +-SM which grains pH = 8 (Fig. 6). By switchng the materials, the optimal pH for adsorption changes like the experiences of Mansoor anbia who adsorbed ascorbic acid onto modified nanoporous carbon they find that the optimal pH is between 7 and 11 [54]. The effect of contact time and initial concentration are also reported in Figs. 7 and 8, which demonstrate an increase in the removal quantities of ascorbic acid within the increase of the studied parameter. A maximal adsorbed quantity is reached in 90 min for Na+-SM and RefSM and 120 min for NH4+-SM and K+-SM for initial optimal concentration of 60 mg L−1. the Freundlich model describes well the adsorption of the ascorbic acid with a correlation coefficient equal to R² = 0.91 for the Ref-SM and 0.99 for Na+-SM which suppose that the adsorption phenomena of ascorbic acid is heterogeneous and take place on sites with various energies so it is multilayer adsorption (Fig. 9). Pseudo first order and the pseudo second order model (Fig. 10) are two complementary models while the pseudo second order describes the adsorption process of ascorbic acid.

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4. Plausible mechanism of ascorbic acid adsorption Knowing that for pH values above 4.2, the anionic form of ascorbic acid is predominant. The area favourable to ascorbic acid adsorption increases from K+-SM to Na+-SM, for this reason the ascorbate ion has a long margin for being in contact with surface area. therefore, modification by sodium ion remains the most appropriate to homogenize Smectites. Depending on the mechanism, the choice of the appropriate support can be made with reference to the working bath (depending on the pH of the effluent). Fig. 11 typically in phyllosilicates [55] (Fig. 12). 8

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