The clay samples used in this study are referenced: AGB, BIN and KAT. The AGB sample was collected at Grand-Morié (5˚59' north latitude and 4˚08' west longitude) in the Agnéby Tiassa region of southern Ivory Coast. The BIN sample was extracted in Bregbo (5˚18' north latitude and 3˚50' west longitude), a town in the municipality of Bingerville, in the Abidjan district of southern Ivory Coast. The KAT sample was taken from the City of Katiola (8˚08' north latitude and 5˚06' west longitude) in the Hambol region of northern Ivory Coast. The sampling was carried out based on the fact that the clays in the south of Côte d’Ivoire are very often rich in kaolinite and those in the north contain a significant amount of smectite [14] [15]. The images of the studied clays are shown in Figure 1 below.

All analyses were performed on these raw clays sieved to 63 µm and their fine fractions saturated with sodium or magnesium.

The pure clay fraction was obtained by the granulometric separation technique based on sedimentation [16]. It made it possible to obtain fine fractions of clay saturated with sodium or magnesium: BIN-Na, BIN-Mg, AGB-Na, AGB-Mg, KAT-Na and KAT-Mg.

The chemical analysis was carried out on the raw clays by atomic emission spectrometry coupled to an inductive plasma (ICP-AES type Spectro Arcos) for major elements. The study of trace elements was carried out using a mass spectrometer coupled by induction to a plasma (ICP-MS type Agilent 7800). Before analysis, the samples were mineralized in an acidic mixture (HNO₃ + HCl + HF).

The DRX study was performed on an ARL equinox 100 diffractometer operating at 45 Kv and 0.9 mA. The measurements were performed on the 63 µm sieved raw clays totally disoriented and on the oriented legs of the fine clay fractions. The monochromatic radiation used for the measurements is the copper K_α (λCu = 1.5406 Å). To differentiate “swelling” 2:1 clay minerals, such as smectites, from non-swelling minerals, oriented legs saturated with ethylene glycol were analyzed by diffractometer. Samples heated to 500˚C were also analyzed. This treatment aims to destroy the kaolinite phases in order to distinguish them from the illite phases.

The surface images of BIN, AGB and KAT clays were zoomed up to 40,000 times and observed with an Auger spectrometer type JEOL JEAMP 9500F (JEOL Ltd, Tokyo, Japan). The work was performed under a voltage of 20 kV, a low current of 1 nA and a pressure lower than 2 × 10⁻⁷ Pa. The surfaces of the samples were previously sputtered with a gold layer.

Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were carried out simultaneously using an apparatus of type SDT 650 - TA Instruments, at a rate of temperature rise of 10˚C/min from room temperature to 1200˚C, under helium scavenging. 10 mg of raw clay powder, saturated with sodium or magnesium were used for each analysis.

The measurement of the specific surface area was carried out by volumetric nitrogen adsorption at 77 K using a Tristar II surface and porosity analyser from Micromeritics. The samples of raw clays, saturated with sodium or magnesium, were previously degassed at 120˚C overnight in a VacPrep type sample preparation system under primary vacuum.

This measurement was performed by displacement of exchangeable cations by the hexaammine ion cobalt (III) [Co(NH₃)₆]³⁺ at 99%, follow-up of the dosage by spectrophotometry. This method was described by Morel [17] ; Mantin and Glaeser [18] then by Remy and Orsini [19]. In a 50 mL centrifuge tube, 20 mL of cobaltihexammine chloride solution of concentration 16.6 × 10⁻³ M is mixed with 1 g of clay. The normality of the cobaltihexammine ion in the solution is 0.05 N. The resulting mixture is rotated for 2 hours to reach equilibrium and then centrifuged at 19000 rpm for one hour. After centrifugation, the supernatant is collected and filtered at 0.45 µm. The cobaltihexammine ion present in the supernatant is determined by colorimetry using a spectrophotometer (absorption band at 472 nm). A calibration line established from five standard solutions (normality = 0.01; 0.02; 0.03; 0.04 and 0.05 N) of cobaltihexammine is used as reference.

The CEC is calculated using the following relationship (remainder method) (Equation (1)):

CEC = ( C i − C f ) ∗ V m ∗ 100 (1)

With: C_i and C_f, the initial and residual concentrations expressed in normality; V: volume of solution in mL; m: mass of sample in grams.

To verify the CEC and to determine the cations responsible for the compensation of the surface charges of the clays, the concentrations of the displaced cations (Mg²⁺, Fe³⁺, Al³⁺, K⁺, Ca²⁺, Na⁺ and Mn²⁺) in solution are determined by ICP-AES. The sum of these contents constitutes the chemical cation exchange capacity (chemical CEC) determined by the following relation (Equation (2)):

CEC chemistry = ∑ i [ M i ] ∗ ν i ∗ V m ∗ 10 3 ∗ 100 [20]. (2)

[M_i]: concentration of the species considered in mol/L, ν_i: valence of the ion; V: volume of solution in L and m: mass of sample in g. The CEC is expressed in meq/100g. This study was carried out on raw clays.

Chemical analysis reveals that for all the chemical species studied, the same oxides are generally found but in variable proportions (Table 1). The contents of SiO₂ (from 41.52% to 50.87% (mass)) and Al₂O₃ (from 20.57% to 34.62%) are the most important, which clearly indicates the silico-aluminous nature of the clays studied. These predominant elements are followed by the iron oxide (Fe₂O₃) content. The Bingerville and Katiola clays are the richest in Fe₂O₃, with contents of 15.71% and 16.86% respectively. This high iron content in these samples is consistent with the pink and orange colors observed for BIN and KAT respectively. The AGB clay contains a low iron content (1.21%) which is a consequence of the white color of the sample. Then come the other oxides (TiO₂, K₂O, CaO, MgO, Na₂O, MnO and P₂O₅). The presence of more or less important K₂O, indicates a probable existence of illite in these clays. The existence of smectite in the Katiola clay is suggested by the relatively important presence of calcium oxide in the KAT sample. The high values of mass loss following calcination (10.51% and 17.60%) could be explained by the natural character of the clays [21].

The presence of trace elements (Table 2) can be related to the adsorption capacity of clays. The contents of metals (Hg, Pb and Cd) and arsenic in these clays

are clearly lower than the regulatory thresholds (AFNOR NF U 41,044 and WHO standard) [22] [23]. Moreover, mercury was not detected in all samples. This result confirms the possibility of using these clays as adsorbents for water depollution.

The calculation of the mineralogical percentages (Table 3) of the clays was carried out from their chemical composition, the data of qualitative mineralogical analysis and by using the following relation developed by Njopwouo (Equation (3)) [25] :

T a = ∑ i = 1 n M i ∗ P i ( a ) . (3)

where T_a = content (mass %) of oxide a in the clay

M_i = content of mineral i (%) in the clay

P_i(a) = proportion of oxide a in mineral i

Kaolinite is the main clay mineral present in these 3 samples with a proportion varying from 48.08% to 75.51%. All clays also contain illite (3.55% to 14.20%). Smectite was detected only in the KAT sample at 20.14%. The higher kaolinite content (75.51%) and the non-existence of goethite in the AGB clay are consistent with the white color of this sample.

Many works [26] [27] [28] [29] have shown that smectite type clays have large specific surfaces and consequently have much greater adsorption capacities than kaolinitic clays. The presence of iron oxyhydroxide in particular goethite also improves the adsorption capacity of clays [30] [31] [32]. In their works, Sei et al. [33] noted that goethite-rich kaolinites had a greater phosphate adsorption capacity and that this reactivity decreased significantly after deferrification.

Images of the AGB, BIN, and KAT clays (×40,000 magnification) are shown in Figure 6 below.

The images reveal large aggregates consisting of platelets of pseudo-hexagonal geometry stacked on top of each other. These randomly oriented platelets are found in all samples. According to Caillère et al., clay minerals of the kaolinite type are presented in the form of hexagonal platelets with more or less marked alterations according to the degree of crystallinity [34]. These images suggest that the kaolinite present in these clays is disordered. Work has shown that iron-rich kaolinites are most often disordered [35] [36] [37] [38]. Moreover, these results confirm those of the XRD and show that kaolinite is present in significant quantities in all the clays studied. Indeed, several researches have shown that the clayey materials of Ivory Coast contain essentially kaolinite, with in rare cases traces of swelling minerals (smectite) [14] [15] [33].

The BET method was used to compute the specific surface areas from the nitrogen adsorption isotherms at 77 K [47]. Even if the samples are mesoporous, thecriterion of Rouquerol et al. [48] was used to accurately select the range of relative pressures for the calculation of the specific surfaces. The N₂ adsorption isotherms are presented in Figure 10. They show almost the same pattern and are typical of mesoporous adsorbents such as clays. The determination of the specific surface area requires the use of the BET relationship (Equation (4)) in the appropriate range of relative pressures ( 0.05 ≲ P / P 0 ≲ 0.35 for the samples of this study):

P n a d s ( P 0 − P ) = C − 1 C n m P P 0 + 1 C n m [47]. (4)

where P is the equilibrium pressure, P₀ is the saturating vapor pressure at the temperature considered, n_ads is the number of molecules adsorbed at the equilibrium pressure and C is the characteristic constant of the gas-solid system. The plot of the line P n a d s ( P 0 − P ) = f ( P P 0 ) allows to determine both C and n_m. The specific surface is then determined by the relation (Equation (5)):

S B E T = σ ⋅ N A ⋅ n m . (5)

With N_A the Avogadro number (6.023 × 10²³ mol⁻¹) and σ is the surface area occupied by a gas molecule. In the case of nitrogen adsorption at 77 K, σ = 16.2 Ų.

The values of surface areas are presented in Table 4.

These values vary from 18.0 to 48.5 m²/g; 31.4 to 53.7 m²/g and 31.0 to 50.1 m²/g respectively for the crude, sodium-saturated and magnesium-saturated clays. In general, the fine fractions obtained by sedimentation have larger specific surface areas than the corresponding raw samples. The specific surface areas of AGB and BIN clays are in agreement with those of kaolinitic clays in many studies [33] - [49]. Although both samples contain the same clay minerals, BIN, which is richer in iron, has a larger surface area than AGB in agreement with the work of Sei et al. [33]. Indeed, iron oxyhydroxides (goethite) contribute strongly to the specific surface [50]. The higher surfaces of the KAT samples are related to the presence of smectite and goethite detected in this sample. The values obtained with the KAT sample are in good agreement with those of Mache et al. [24] compared to the study of a smectite from Sagba in Cameroon (33 - 90 m²/g). However, these values are lower than those of a pure smectite (80 m²/g) [51] and this is related to the fact that the Katiola clay is a mixture of smectite and other clay minerals such as kaolinite and illite detected in DRX. It should be noted that the presence of other mineral phases due to rock alteration can also affect the specific surface area of the samples. Gupta and Bhattacharyya [52]

studied the removal of Pb, Cd and Ni from a kaolinitic and montmorillonitic clay. For specific surface values varying from 3.1 to 18.7 m²/g for kaolinitic clays and from 3.8 to 19.8 m²/g for montmorillonitic clays, they obtained maximum adsorption capacities varying from 6.8 to 11.5 mg/g for kaolinites and from 21.1 to 31.1 mg/g for montmorillonites. As the specific surface values measured in the present study are much higher, we can assume that the clays studied will have interesting adsorption capacities.

Table 5 below shows the CEC values obtained by determination of remaining cobaltihexamine ions (CEC) and by displacement of the cations responsible for surface charge compensation (CEC_chemistry).

The CEC values of BIN and AGB clays are 6.18 and 6.67 meq/100g respectively. That of Katiola clay (KAT) is 35.47 meq/100g. The CEC determined by the cobaltihexamine method and that determined by the sum of major cations determined by ICP-AES (chemical CEC) are slightly different for BIN and AGB. Most of the cationic compensation is provided by aluminium for BIN (CEC_Al = 4.36 meq/100g). Whereas for the AGB clay, the cationic compensation is essentially assured by Ca, Mg and Na (CEC_Ca = 1.17 meq/100g; CEC_Mg = 1.93 meq/100g and CEC_Na = 1.43 meq/100g). Concerning the Katiola clay, the CEC chemistry and cobaltihexamine are practically identical. Ca and Mg are essentially the cations responsible for the charge compensation of the KAT clay (CEC_Ca = 8.18 meq/100g; CEC_Mg = 25.88 meq/100g).

The CEC of BIN and AGB are well within the range of cation exchange capacity values of a kaolinite (3 to 15 meq/100g). This confirms that AGB and BIN are mostly kaolinite. The discrepancy between the cobaltihexamine and chemical CEC values for BIN and AGB clays could be explained by the presence of other ions not measured but contributing to the cationic compensation [53]. The CEC value of the KAT clay is significantly lower than that of pure smectite (CEC = 80 to 120 meq/100g). This result confirms that the Katiola sample does not consist solely of smectite. Indeed the Katiola clay is a mixture of 3 clay minerals: kaolinite, illite and smectite. This observation is corroborated by Gourouza et al. [54] who qualified a Niger clay as mixed (semi-kaolinitic and semi-smectitic). This measurement indicates that of these three clays, KAT has the highest exchange capacity.