The clay material that is the subject of this study comes from the locality of Moukosso in the department of Bouenza, Republic of Congo. The geographical coordinates of the sampling site are: latitude: 03˚74.963E, longitude: 9˚539.466N. This material was sampled at a depth of 2.5 meters.

The intercalation of DMSO was made in the phyllite fraction extracted from the material. 2.5 g of the fine fraction and 15 mL of the DMSO were mixed in a flask. The mixture was then refluxed for 12 hours at 150˚C. The solid material obtained was washed several times with isopropanol; then separated by centrifugation. The solid obtained is dried at 60˚C in an oven [20] .

1) Determination of cation exchange capacity

The cation exchange capacity of the studied material was determined by the ammonium acetate method [21] .

2) Thermogravimetric analysis (TGA)

The thermogravimetric analysis (TGA) was carried out by a Diamond Pyris 6000 device Perkin-Elmer TGA/DTA in the temperature range of 30˚C to 850˚C using aluminum crucibles under N₂ flow at a heating rate of 20˚C/min. The temperature accuracy of the device is ±0.5˚C. Data were processed using Origin software.

3) Fourier Transform Infrared Analysis

The infrared spectroscopy measurements were carried out on a Nicolet iS 10 Smart FT-IR device. The infrared spectrum of the material was realized in the range 4000 - 500 cm⁻¹. Data were processed using Origin software.

4) X-ray diffraction Analysis

The mineralogical analysis by X-ray powder diffraction was carried out on a Panalytical X’ Pert Pro MPD type device using the Kα line of copper with a wavelength λ =1 .5406 A 。 . The data was processed using QualX software.

In order to study the adsorption isotherms, the mass of adsorbent was kept constant and we varied the concentration of the adsorbate. In 50 mL beakers, we introduced 0.05 g of clay and 10 mL of lead chloride solution of concentration in mol/L (0.0002; 0.0003; 0.0005; 0.001; 0.002 and 0.003). The solutions are stirred using a magnetic stirrer for 60 minutes, centrifuged and filtered. After filtration, the solutions were assayed by a UV-visible spectrophotometer at 300 nm. The isotherms are obtained by plotting the adsorbed quantity curve Q_e (mg/g) as a function of the equilibrium concentration C_e (mg/L).

The cation exchange capacity of this material is 7.4 meq/100g. This value between 3 and 15 could be linked to the presence of kaolinite [22] . The low value of the CEC could explain the absence of smectites in this material [23] .

Figure 1 and Figure 2 present the TGA curves of raw and intercalated materials from Moukosso.

The thermograms obtained have the same appearance, thus showing a loss of

mass through the stepped profile. The one in Figure 1 shows several differences in level, between

· 0˚C and 100˚C, accompanied by a low mass loss of about 0.8%, would correspond to the departure of surface water from the clay soil [24] ;

· 200˚C and 250˚C with a mass loss of 1.6% would correspond to the departure of adsorbed water in the interfoliar space [24] ;

· 280˚C and 300˚C, presents a mass loss of 1.1% which could correspond to the combustion of organic residues and the loss of carbonates [25] ;

· 450˚C and 600˚C, with a mass loss of 5.7% would correspond to the dehydroxylation of kaolinite [26] . This phenomenon leads to the formation of metakaolinite, an amorphous phase [27] .

· 600˚C and 850˚C, the authors observed a progressive loss of mass, a phenomenon which could correspond to the dehydroxylation of muscovite [27] .

· On the other hand, that of Figure 2 shows two significant differences in level between:

· 0˚C and 280˚C accompanied by a strong loss of mass of approximately 17.5%, which would correspond to the departure of surface and interfoliar waters and to the decomposition of DMSO. This mass loss is much higher than that obtained with raw clay; which could confirm the success of the intercalation of this clay by DMSO. These results are in agreement with those found in the literature [28] .

· 400˚C and 850˚C with a mass loss of 7.5% which could be attributed to the dehydroxylation of kaolinite and muscovite [27] .

Figure 3 and Figure 4 present the infrared spectra of the materials studied.

For the raw material, we observe the elongation vibration bands at 3697 and 3621 cm⁻¹ which could correspond to the OH groups of the kaolinite [29] [30] . The presence of two bands instead of four (3695, 3669, 3650 and 3620 cm⁻¹) would indicate that the kaolinite is poorly crystallized [31] . The band at 3621 cm⁻¹ could also be related to the vibration of the OH group of muscovite [32] [33] . The band at 1647 cm⁻¹ would correspond to that of hygroscopic water [34] . Bands at 1140 and 1020 cm⁻¹ could correspond to Si-O stretching vibrations of kaolinite. The band at 1020 cm⁻¹ would also be that of muscovite [32] . The Al-OH deformation vibration characteristic of kaolinite appears at 906 cm⁻¹ [35] . The presence of quartz would be indicated by the bands at 813 and 703 cm⁻¹. Compared to the infrared spectrum of the raw material, two bands appear at 3550 cm⁻¹ and at 2900 cm⁻¹ in the infrared spectrum of the organomodified material. The band at 3550 cm⁻¹ would correspond to a new hydroxyl group. However, that at 2900˚C could correspond to the symmetrical stretching mode of CH₂ and CH₃ [36] ; it could also be attributed to the valence vibrations of the CH₂ and CH₃ bonds. This confirms that the clay has been intercalated by DMSO, with the presence of the characteristic bands of the S=O bond at 905 cm⁻¹ which splits the Al-OH bond of the kaolinite and of the CH3-S bond at 1008 cm⁻¹ [36] .

Figure 5 shows us the diffractogram of the raw clay of Moukosso.

The qualitative analysis showed the following mineral phases in angular positions (degree): muscovite (8.88; 17.80; 19.86; 20.86; 24.89; 25.52; 26.81; 27, 87; 31.25…), kaolinite Al₂Si₂O₅(OH)₄ (12.38; 19.86; 21.32; 24.76; 34.92; 36.94; 39.48…), anatase TiO₂ (25 .32; 36.94; 37.74; 47.96) and quartz SiO₂ (20.86; 26.64; 36.55; 39.47; 40.29; 42.45; 45.8, etc.). These results were also observed by [37] [38] . In contrast, quantitative analysis revealed 58.9% quartz, 29.7% muscovite, 8.9% kaolinite and 2.4% anatase.

The spectrophotometric dosage of lead chloride made it possible to find the

wavelength at the maximum of adsorption (λ_max) of the Pb²⁺ ions. The results are shown in Figure 6.

Observation of this curve shows that the wavelength at the adsorption maximum is λ max = 300   nm . This value will be used in the remainder of this study.

The authors determined the quantity adsorbed by the untreated fine fraction and by the organomodified fine fraction from the formulas:

Q e = ( C o − C e ) ⋅ V m (1)

C e = C o × D o e D o i (2)

With:

C_o: initial concentration of lead in aqueous solution

C_e: equilibrium concentration (mol/L) of lead in aqueous solution

m: sorbent mass

V: volume of solution

D_oi: initial optical density of the solution

D_oe: equilibrium optical density of the solution

a) Influence of stirring time on the quantity adsorbed by the fine fraction

Figure 7 shows the evolution of the quantity of Pb²⁺ ions adsorbed by the fine fraction as a function of contact time.

The analysis of this figure shows a rapid adsorption of Pb²⁺ ions by the clay between 5 and 15 minutes of adsorbate-adsorbent contact, followed by a slow increase until equilibrium. This curve shows a balance between 25 and 30 minutes. This equilibrium time could correspond to the maximum efficiency of the adsorption of Pb²⁺ ions on this material. These results are in agreement with those found by [18] [39] [40] .

b) Influence of stirring time on the quantityadsorbed by the organomodifiedclay

Figure 8 shows the evolution of the adsorbed quantity of Pb²⁺ ions by organomodified clay as a function of stirring time.

There is a strong adsorption of Pb²⁺ ions by the clay between 5 and 15 minutes of agitation, followed by a slow increase between 15 and 30 minutes, which could be related to the saturation of the pores. The clay intercalated by DMSO has a greater adsorption capacity than that of the fine fraction, which would lead to maximum adsorption at the start of contact with the Pb²⁺ ions.

The adsorption isotherms (Figure 9 and Figure 10) are obtained by plotting the curve of the quantity adsorbed at equilibrium of Pb²⁺ ions by the fine fraction and by the organomodified fraction as a function of the equilibrium concentration of Pb²⁺ ions.

The analysis of these figures shows that these adsorption isotherms exhibit the same behavior. These isotherms would be of type S presenting at low concentration, concavities facing upwards [41] [42] . The maximum adsorbed quantities of Pb²⁺ ions are 25.32 mg/g with the fine fraction and 34.45 mg/g with the organomodified fine fraction. The quantity adsorbed by the organoclay is greater than that obtained with the fine fraction; which could be explained by the fact that DMSO increases the interlayer distance of kaolinite and therefore promotes adsorption [20] .

The authors have adopted the classic models of Langmuir and Freundlich adsorption isotherms, which allow by linear regression to obtain the values ​​of the constants (Q_max, K_L, 1/n and K_F) in order to better interpret the adsorption results obtained.

a) Langmuir model

Figure 11 and Figure 12 give the Langmuir model obtained respectively from the fine fraction and the organomodified clay. This model is obtained by drawing the line:

1 Q e = 1 Q max ⋅ K L ⋅ C e + 1 Q max (3)

with:

Q max : monolayer capacity of the adsorbent (mg/g)

K L : Langmuir adsorption constant

1 Q max : origin coordinate

1 Q max ⋅ K L slope

The linearization of the Langmuir equations made it possible to determine the parameters grouped in Table 1.

The parameter Q_max (mg/g) which expresses the maximum adsorption capacity of the monolayer of the material is 22.471 mg/g for the fine fraction and 41.493 mg/g for the organomodified clay. This high maximum quantity in the organomodified clay would confirm the success of the intercalation of DMSO in this clay. The K_L coefficient, which is the equilibrium constant, is equal to 1.07 × 10⁻² L/mg for the fine fraction and 5.9 × 10⁻³ L/mg for the organomodified clay. These values being low would induce a fairly strong adsorption of lead particles by the two matrices [43] .

b) Freundlich model

Figure 13 and Figure 14 present the Freundlich model obtained from the fine fraction and the organomodified clay by drawing the straight line:

ln Q e = ln K F + 1 n ln C e (4)

The linearization of the Freundlich equations made it possible to determine the parameters grouped together in Table 2.

K_F and 1/n are Freundlich constants related to the temperature, the nature of the adsorbate and the adsorbent. In both cases, the determined K_F coefficient is the same and equal to 4.09 L/mg. It is the same for 1/n which is close to 0.62; which would indicate a strong adsorption of Pb²⁺ ions by the studied materials [29] .

In view of the values of the coefficients (Q_max, K_L, 1/n et K_F) obtained, we

deduce that the Langmuir model may be adequate to model the adsorption isotherm of lead by the clay studied.