Characterization of Clay Materials from Ivory Coast for Their Use as Adsorbents for Wastewater Treatment

In order to contribute to the valorisation of the clay materials of Ivory Coast in the depollution of wastewater, the physicochemical and mineralogical characterization of three clay samples taken in Agboville (AGB), Bingerville (BIN) and Katiola (KAT) was carried out. The objective of this work was to com-pare the properties of the clays in order to identify the one that is likely to have interesting adsorptive capacities to clean up contaminated water. These clays were studied by chemical analysis, X-ray diffraction, scanning electron microscopy and thermal analysis. In addition, their specific surface areas and cation exchange capacities were determined. The AGB clay is composed of 75.51% kaolinite, 14.20% illite and 9.26% quartz. The BIN clay contains 52.21% kaolinite, 6.23% illite, 17.50% quartz and 15.71% goethite. As for the KAT clay, it contains 48.08% kaolinite, 3.55% illite, 20.14% smectite, 6.11% quartz and 16.86% goethite. Their thermal behavior and microstructure are in agreement with their mineralogy. The measured specific surface areas and cation exchange capacities are consistent with literature values. The KAT clay has the highest specific surface and cation exchange capacity. In the light of the results obtained, the KAT clay appears to be more effective in depolluting water than the BIN and AGB clays.


Introduction
The issue of drinking water is a major challenge for developing countries, including Ivory Coast. Indeed, in these countries where industrialization and urbanization are rapid, the effluents from anthropic activities, rich in pollutants, are generally not well treated and are released into nature, causing enormous pollution of surface and groundwater. Among the anthropic sources of pollution, we can point out: the mining activity, the metallurgical and iron and steel industry, the fertilizers and pesticides applied in the cultivation of the grounds, the incinerators and ashes of incineration of the waste, the medical waste, the waste dumps of cities, the emissions of the factories and the explosion engine, the effluents of the sewers and the sludges of sewage… [1]. Numerous researches have shown that waters of Côte d'Ivoire are contaminated by chemical species due to anthropic activities [2] [3]. It is necessary to find inexpensive and easy to implement methods to treat these effluents before they are released into nature. Commonly adopted techniques for removing pollutants from water include chemical precipitation, lime coagulation, ion exchange, reverse osmosis and solvent extraction. These methods are less effective when the concentration of pollutants in the effluent is low. Therefore, adsorption can be an alternative technology for the removal of pollutants, which are present in very low concentrations in aquatic environments [4]. In this context, several studies have invested in the search of effective materials (adsorbent) to clean up contaminated water.
Ouédraogo et al. showed that the properties of a laterite from Burkina Faso gave it an adsorbent power in the removal of arsenic [5]. Sawdust has been proven to remove unwanted material from water [6]. Mimanne et al. proved that the interesting characteristics (large specific surface and cationic exchange capacity) of an activated carbon and a montmorillonite from Algeria allowed a good removal of Pb and Cd [7]. Several other materials such as: powdered corn cobs [8], activated carbons [9], wood powders [10], clays [11]… with interesting properties have also been proven in the decontamination of contaminated water.
Among so many materials, clays, of which the Ivory Coast has many deposits [12], presents interesting adsorption properties in relation to their mineralogy and physicochemical properties [7] [13].
The objective of this study is to characterize from the point of view of mineralogy and physicochemical properties, three samples of clays from Ivory Coast in order to better appreciate their ability to clean up contaminated water.

Clay Materials
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

Extraction of the Fine Fraction
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.

Chemical Analysis
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 (HNO 3 + HCl + HF).

X-Ray Diffraction (DRX)
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.

Scanning Electron Microscopy (SEM)
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.

Thermal Analysis
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.

Measurement of Specific Surface
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.

Measurement of the Cation Exchange Capacity (CEC)
This measurement was performed by displacement of exchangeable cations by the hexaammine ion cobalt (III) [Co(NH 3 ) 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)): 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 ca-tions (Mg 2+ , Fe 3+ , Al 3+ , K + , Ca 2+ , Na + and Mn 2+ ) 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)

Chemical Composition of 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 SiO 2 (from 41.52% to 50.87% (mass)) and Al 2 O 3 (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 (Fe 2 O 3 ) content. The Bingerville and Katiola clays are the richest in Fe 2 O 3 , 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 (TiO 2 , K 2 O, CaO, MgO, Na 2 O, MnO and P 2 O 5 ). The presence of more or less important K 2 O, 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  [23]. Moreover, mercury was not detected in all samples. This result confirms the possibility of using these clays as adsorbents for water depollution.

Total Rock
The diffractograms obtained are shown in Figure 2 and Figure 3.  30˚)). In addition to these minerals, sample BIN contains goethite (dhkl located at 2.25 Å (40.04˚)). The KAT clay also contains the minerals mentioned above, plus a smectite peak (dhkl located at 12.19 Å (7.25˚)).

Clay Fraction
Oriented deposition analysis of fine clay fractions, saturated with ethylene glycol and calcined at 500˚C (Figure 4 and Figure 5) confirmed the previously indexed peaks.  of Sagba smectites in Cameroon [24]. In their study, the smectite peak detected at 13.40 Å in the untreated sample, increases to 16.66 Å after ethylene glycol treatment and to 9.56 Å after calcination.

Mineralogical Proportion of Clays
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  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.

Scanning Electron Microscopy (SEM)
Images of the AGB, BIN, and KAT clays (×40,000 magnification) are shown in Figure 6 below.   Figure 7 shows the thermograms of raw clays, treated with sodium or magnesium from Agboville and Bingerville. The AGB and BIN clays show two mass losses associated with endothermic events: A small loss of mass between 25˚C and 200˚C, due to the departure of hygroscopic water [14].

AGB and BIN Clays
An important loss of mass from 400˚C to 800˚C associated with an endothermic peak around 480˚C. This thermal accident corresponds to the dehydroxylation of kaolinite (transformation of kaolinite into metakaolinite), according to the following reaction: Si 2 Al 2 O 5 (OH) 4 → Si 2 Al 2 O 7 + 2H 2 O [39].
The appearance of this endothermic peak at a temperature below 530˚C, suggests that AGB and BIN are extremely disordered kaolinitic clays, according to the Smykatz-Kloss classification [40].
Around 960˚C, an exothermic peak not associated with a mass variation is attributable to a structural reorganization of metakaolinite linked to 3 reactions: Formation of a spinel structure phase and extraction of amorphous silica: This exothermic phenomenon is subject to various interpretations in the literature. Indeed, some authors exclude the formation of mullite at this temperature. However, Pialy states in his work that: "In the case of very disordered kaolinites, there would be direct formation of mullite. Moreover, very slow heating rates would favour the segregation of metakaolinite and the appearance of the spinel structure phase." [42]. The analysis of the diffractogram of the Agboville clay heated to 960˚C shows the simultaneous presence of mullite and spinel phase ( Figure 8). This result is corroborated by the thesis of Gridi-Bennadji who states that generally, the spinel phase and mullite form simultaneously below 1100˚C [43]. At 1112.57˚C, AGB-Mg shows an exothermic peak attributable to the nucleation of a second generation of mullite. This additional mullite formation peak is in agreement with the work of Maitra et al. [44] and Viswabaskaran and Gnanam [45] who affirmed in their work that the magnesium oxide (MgO) used as additive intervenes in a positive way in the formation of the mullite.

KAT Clay
The thermograms of the Katiola clay show 3 mass losses associated with thermal reactions on the DSC curves ( Figure 9). These 3 observed mass losses suggest that the mineralogical composition of this clay is a little different from those of AGB and BIN.
We observe: Two consecutive mass losses between 25˚C and 300˚C (appearing as a split) due to the departure of hygroscopic water and interfoliar water. These two losses of mass confirm the presence of clay mineral of type 2:1.
From 400˚C to 800˚C, a significant mass loss associated with an endothermic peak is observed and is attributed to the dehydroxylation of the phyllosilicates contained in the clay samples [24].
An exothermic event around 900˚C due to the recrystallization of minerals such as quartz and/or mullite [46]. This peak is of low intensity in the case of treated samples and does not appear in the case of raw clay.

Specific Surface of Clays
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, the-criterion of Rouquerol et al. [48] was used to accurately select the range of relative pressures for the calculation of the specific surfaces. The N 2 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) where P is the equilibrium pressure, P 0 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 allows to determine both C and n m . The specific surface is then determined by the relation (Equation (5)): With N A the Avogadro number (6.023 × 10 23 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 The values of surface areas are presented in Table 4.
These values vary from 18.0 to 48.5 m 2 /g; 31.4 to 53.7 m 2 /g and 31.0 to 50.1 m 2 /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 m 2 /g). However, these values are lower than those of a pure smectite (80 m 2 /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]   and from 3.8 to 19.8 m 2 /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. 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.

Conclusions
The objective of this work was to study the physicochemical properties of 3 clays collected in Agboville (AGB), Bingerville (BIN) and Katiola (KAT) with the intention of using them later as adsorbents in water treatment. To achieve this goal, the first step was to determine their chemical and mineralogical compositions. Then, the reactions and thermal transformations were studied. Finally, the specific surface area and the cation exchange capacity were measured. The chemical composition, determined by ICP-AES and ICP-MS techniques, showed that the main oxides are SiO 2 , Al 2 O 3 and Fe 2 O 3 . Iron is present in significant quantities in the BIN and KAT samples. The K 2 O content (0.41% to 1.64%) indicates the presence of illite in these clays. The relatively high CaO content (1.41%) in the KAT clay suggests the presence of smectite in the clay. In addition, the content of harmful elements (Hg, Pb, Cd and As) in the samples is below their toxicity threshold according to the AFNOR NF U 41,044 and WHO standards. Mineralogical characterisation by X-ray diffraction (XRD) confirmed the assumptions made during the chemical analysis. The study shows that these clays consist of kaolinite and illite. In addition to these clay minerals, the KAT sample consists of smectite. These clay minerals are associated with quartz and goethite (15.71% and 16.86% respectively in BIN and KAT). Thermal analysis and scanning electron microscopy confirmed that the clays are predominantly kaolinite. The specific surface area and cation exchange capacity values clearly indicate that AGB and BIN are kaolinitic clays. Whereas the sample KAT is a mixed clay, KAT shows the best values for specific surface area and cation exchange capacity. All the properties determined suggest that the clays studied have interesting adsorption capacities, in particular the Katiola clay. These clays could be used in the depollution of contaminated waters, the experimental study being the subject of our future work. Moreover, the properties of these clays could be intensified by the intercalation between their sheets, of big iron polycations (bridged clay) in order to improve their adsorption capacity.