Characterization of Clay Materials from Côte d’Ivoire: Possible Application for the Electrochemical Analysis

The utilization of clay minerals as electrode modifiers is based on their unique structure and properties. In this study, clays from various regions of Côte d'Ivoire were characterized for their potential use in developing electrochemical sensors. The clay samples underwent analysis via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) mapping analysis, Fourier transform infrared spectroscopy (FTIR)


Introduction
For centuries, humans have utilized clay to produce tools in a variety of fields. Clay minerals are commonly employed in pharmacy, cosmetics, ceramics, and the manufacturing of bricks for construction purposes. In the field of chemistry, clay's applications are vast and include the development of functional solid catalysts, adsorbents, ion exchangers, and electrochemical sensors, as reported in several studies (Moraes et al., 2017 ;Supelano-García et al., 2020 ;Crapina et al., 2021 ;Maghe et al., 2012).
Clay minerals are natural materials primarily composed of hydrated aluminum silicates, commonly known as phyllosilicates, with a particle size smaller than 2 μm. Phyllosilicates belong to the silicate family (aluminosilicates) and possess a sheet-like structure. Each sheet consists of a stacking of siliceous (T-layer) and aluminous (O-layer) flat layers that are interconnected (Murray, 2006). According to their structure, clays are classified into various classes or groups, such as smectites, montmorillonite, mica (illite), kaolinite, vermiculite, and more (Shichi & Takagi, 2000). The applications of clays depend on their mineral structure, chemical composition, and physicochemical properties.
However, as regards to carbon paste electrode (CPE) modified with clay, it is a complex heterogeneous system. The use of such electrodes requires a thorough characterization to understand the phenomena of charge and mass transfer in a mixture: solids, liquid, conductors, and insulators. In addition, the origin, composition, and structure of clay included in the paste can impact the electrode response (Mousty et al, 2004;Gómez et al, 2011). Therefore, a well characterization of clay is primordial in the elaboration of clay modified carbon paste electrode.
In Côte d'Ivoire, there are numerous clay sites that have not been thoroughly studied (Konan et al., 2006). Since 1994, several papers have been published focusing on various aspects and characteristics of different clay sites (Sei et al., 2002;Andji et al., 2009;Yoboue et al., 2014;Coulibaly et al., 2014). Recently, Ivorian natural clays have been utilized for various applications based on their physicochemical properties (Coulibaly et al., 2018;Kpangni et al., 2008;Konan et al., 2007). However, to the best of our knowledge, the utilization of Ivorian clay in modified carbon paste electrodes has not been studied.
In this work, new carbon paste electrode modified with natural clay from Côte d'Ivoire was elaborated for the potential detection of organic pollutants. Clay and clay modified electrode were thoroughly characterized using microscopic and electrochemical techniques. The analytical performance of the newly developed sensor was evaluated by studying the electrochemical behavior of the ferri/ferrocyanide couple as redox probes on both the bare and modified electrodes.

Regents and Materials
All chemicals used in the experiment were of analytical grade. Graphite powder with a particle diameter (ø) of 0.1 mm was purchased from Sigma Aldrich. Potassium hexacyanoferrate (II) trihydrate (K 4 [Fe(CN)] 6 ·3H 2 O) was obtained from Scharlau Chemie S.A. Solutions were prepared using distilled water. Potassium perchlorate (KClO 4 ) with a purity of 99.5% from Merck was used as the supporting electrolyte. The firm Dp-pharma paraffin oil was employed. The experiments were conducted at a room temperature of 25°C.
Two types of electrodes were used : a carbon paste electrode and a modified carbon paste electrode, both serving as working electrodes. A saturated silver electrode (Ag/AgCl, KCl sat ) was employed as the reference electrode (RE), and a platinum wire was used as the counter electrode (CE).
Clay samples were collected from three different regions in Côte d'Ivoire, which are located in the south and midwest of the country. The Agban sample (AG) was collected at Bingerville (5.3504° N, 3.8757° W), the Adiaho sample (AD) was collected at Bonoua (5.2712° N, 3.5939° W), and the Zuenoula sample (ZU) was collected at Zuenoula (7.4240° N, 6.0520° W). The samples were collected at a depth of twenty meters for the Adiaho and Zuenoula samples, and fifteen meters for the Agban sample

Carbon Paste Electrode
The carbon paste electrode CPE was prepared by mixing 1 g of graphite powder and 300 μL of paraffin oil using mortar and pestle until homogenous paste was obtained. The paste was then incorporated into the electrode cavity and polished on smooth paper. A platinum wire provided the electrical contacts. The electrode surface could be renewed by simple extrusion of a small amount of paste from the tip of the electrode. Before each use of CPE, it was rubbed with a piece of paper until a smooth surface was observed.

Clay preparation
Each collected clay sample was carefully dried in the shade for several days to remove any moisture. Once dried, the samples were ground and sieved using a 100 μm sieve to obtain a consistent particle size. In order to perform structural characterization of the clay, the samples underwent a pretreatment process. This involved washing and decantation to obtain a more uniform product for subsequent analytical experiments.
Separation was performed on different clay granulometric particle sizes by sedimentation, decantation, centrifugation, and ultracentrifugation according to Stocke's law. This law expresses the limit speed of sedimentation as a function of the diameter (D) of a solid particle of specific mass γs in a liquid of specific mass and viscosity (Nasri et al, 2016). In principle, sedimentometry is a test that completes the particle size analysis by sieving for fractions below 80μm. Its purpose is the determination of sand, silt, and clay content.

Modified Electrode Preparation
The modified carbon paste electrode (MCPE) was prepared using a similar procedure. First, weighed amounts of clay and graphite powder were thoroughly mixed with paraffin oil. Various proportions of clay to graphite powder (w(clay)/w(G)) were used until a uniformly mixed paste was obtained. The resulting paste was then packed tightly into the electrode's cavity through vigorous packing. To maintain the electrode's performance, the surface could be renewed easily by extruding a small amount of paste from the electrode's tip. Additionally, prior to each use, the electrode surface was carefully rubbed with a piece of paper until a smooth surface was achieved.

Characterization of Clay
The density of the clay samples was determined using a pycnometer. Various techniques were employed to evaluate the properties of the clays, including physico-chemical composition studies, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). These techniques allow comprehensive information about the physico-chemical properties, mineral composition, morphological characteristics, and chemical composition of the clay samples.
X-ray diffraction (XRD) was utilized to analyze the structure and composition of clay minerals. The analysis was performed using CuK α radiation (λ = 1.5406 Å) on a Brüker D8 Advance diffractometer, with a generator current of 40 mA and voltage of 40 kV. Data collection was carried out over a 2θ angular range of 5-60° with a step size of 0.02°s -1 .
Scanning electron microscopy (SEM) was employed for morphological characterization of the clay samples. The measurements were conducted using a Hirox SH 4000 model SEM in Europe.
To identify functional groups present in the clay samples, Fourier-transform infrared (FTIR) spectroscopy was performed using a Thermofisher Scientific Nicolet 380 spectrometer. The transmission mode was used, where a mixture of potassium bromide (KBr) pellets and a small quantity of the ground sample (a few mg) was prepared. The acquisitions were made between 4000 and 400 cm -1 , using 64 scans with a resolution of 2 cm -1 .
Chemical analysis of the clay samples was conducted using X-ray fluorescence (XRF). A Thermo Fisher Scientific Energy Dispersive (EDXRF) apparatus was utilized, with a maximum voltage of 40 kV and a maximum energy of 40 keV.

Characterization of the Electrode
Cyclic voltammetric measurements were performed using a computer-controlled potentiostat (PalmSens, Ecochemie, Netherlands) and PSTrace software. A conventional three electrodes cell (10 mL) consisting of a carbon paste electrode (CPE) as working electrode, a Ag/AgCl,KCl sat as reference electrode and a Pt wire as counter electrode, was used. The solutions pH was measured using a digital pH meter (Hanna Instruments, USA). Each individual experiment was performed at least three times and the results were averages.

Characterization of Clays
It is widely recognized that natural clay often contains impurities and exhibits heterogeneity. Table 1 presents the results pertaining to the particle size distribution of the three clay samples under investigation. These findings indicate that the collected samples consist of a mixture of particles with varying sizes. However, it is important to note that clay minerals, as natural materials, typically have particle sizes smaller than 2 μm (Uddin, 2017). Accordingly, the clay content, defined as the fraction below 2 μm, is determined to be approximately 29.66% for sample AD, 57.667% for sample AG, and approximately 18% for sample ZU. Table 1, which displays the different fractions of the materials, shows that the studied samples comprise a combination of sand, silt, and clay components. However, it is important to note that these results alone do not allow us to conclusively determine the nature of the samples. The observed density values suggest that the samples likely consist of a mixture of different types of clay minerals. To gain a more accurate understanding of the composition, further analysis and characterization techniques are required.
Thus, while the obtained density values are consistent with certain clay mineral structures, additional investigations are needed to precisely identify the nature and composition of the clay minerals in the samples.
The X-ray diffraction (XRD) patterns of the samples are presented in Figure 1AD, Figure   Several authors, including Suzuki et al. and Caillère et al. (Zuzuki et al., 2013;Caillère et al., 1982), have described kaolinite particles at the nanoscale as having a laminar structure of pseudo-hexagonal platelets. The morphology depicted in Figure 2 aligns with the typical appearance of kaolin, characterized by heterogeneous layered sheets of varying sizes. Additionally, a significant quantity of quartz mineral, which serves as an impurity in kaolin, is observed.
These findings are consistent with previous studies conducted on clay samples from Côte d'Ivoire by various authors (Konan et al., 2007;Meite et al., 2020;Kouadio et al., 2022;Konan et al., 2010). These studies consistently reported that the clay samples predominantly consist of kaolinite. Therefore, the SEM results support the XRD findings and confirm the prevalence of kaolinite as the primary mineral phase present in all the studied clays. The elemental analysis conducted with Energy Dispersive X-ray Spectroscopy (EDS) provides valuable information about the chemical composition of the clay samples and highlights the variations between the three samples. Figure 3 presents the percentage composition of various components, including Si, Al, Ti, Fe, K, Mg, Na, O, Ca, and C. These results indicate the relative abundance of these elements in the clay samples. The presence of silicon (Si) and aluminum (Al) is expected, as they are major constituents of clay minerals. The percentage of Si and Al can provide insights into the type and composition of clay minerals present in the samples.
Other elements such as titanium (Ti), iron (Fe), potassium (K), magnesium (Mg), sodium (Na), oxygen (O), calcium (Ca), and carbon (C) may also be detected in varying amounts, depending on the specific characteristics of the clay samples and their sources. The presence and quantities of these elements contribute to understanding the overall chemical composition and potential impurities in the clay samples.
The differences observed in the percentage composition of the elements among the three clay samples suggest variations in their chemical makeup. These differences may be attributed to variations in the mineralogical composition, geological origin, and processing of the clay samples.
This analysis of the clay minerals reveals some noteworthy differences between the samples. In the case of ZU, the presence of calcium (Ca) is detected, while it is not detected in the AD and AG clay minerals. This indicates a variation in the elemental composition of the three samples.
Furthermore, the analysis shows that the Al/Si atomic concentration ratio is close to 1.0 for AG, which aligns with the expected chemical composition of kaolinite (Al 2 Si 2 O 5 (OH) 4 ). This suggests that the AG sample predominantly consists of kaolinite. On the other hand, AD and ZU exhibit a higher silicon (Si) content compared to aluminum (Al). This higher Si content may be attributed to the presence of a significant concentration of quartz in these clay minerals. Quartz is known for its high Si content and is commonly found as an impurity in clay samples.
Additionally, the analysis reveals the presence of several components in small quantities and varying proportions, including titanium (Ti), potassium (K), magnesium (Mg), sodium (Na), and calcium (Ca). These elements may be present as minor constituents or impurities in the clay samples. The elemental composition of the three clay samples was analyzed, and the oxide content is presented in Table 2.
The results confirm that the samples are predominantly composed of aluminum oxide (Al 2 O 3 ) and silicon oxide (SiO 2 ), indicating their classification as aluminosilicates (Nirmala & Viruthagiri, 2015). The SiO 2 /Al 2 O 3 ratios for AG, ZU, and AD are 1.72 ; 3.34 ; and 3.87, respectively. These ratios are higher compared to the typical ratio of 1.18 found in kaolinites (Lecomte-Nana et al., 2013). The elevated SiO 2 /Al 2 O 3 ratios suggest the possible presence of free quartz in a significant proportion within the clay fraction (Gourouza et al., 2013). The excess silica observed can be attributed to the presence of quartz and/or compounds such as 2/1 clay minerals like illite and muscovite (Coulibaly et al., 2020).
Additionally, the clay samples exhibit a relatively large quantity of iron oxide, indicating the presence of ferric phases. The presence of other oxides such as CaO, MgO, Na 2 O, K 2 O, and TiO 2 is also observed, albeit in low percentages.
These findings further support the aluminosilicate nature of the clay samples, highlighting the elevated SiO 2 /Al 2 O 3 ratios, the potential presence of free quartz, and the occurrence of ferric phases. The detailed oxide composition presented in Table 2 provides a comprehensive overview of the elemental composition of the clay samples. The semi-quantitative mineralogical composition of the three clay samples was determined by summing the values obtained from the qualitative mineralogical analysis. This allows for the calculation of the total chemical composition, as shown in Table 3. The calculation is based on the relationship developed by Njopwouo (). T(a) = Σ Mi x Pi (a), as referenced in Kouadio et al. (2022), where T(a) represents the content (mass %) of oxide a in the clay, Mi represents the content of mineral i (%) in the clay, and Pi(a) represents the proportion of oxide a in mineral.
By applying this relationship, the semi-quantitative mineralogical composition of the three samples was determined, providing valuable insights into their overall chemical composition. These results are found to be consistent with the findings published in the literature by Meite et al. (2020) and Kouakou et al. (2022), further validating the accuracy and reliability of the analysis conducted in this study. Table 3. Semi-quantitative mineralogical compositions of clays

Electrochemical Characterization of Clay Modified Electrode
The ferri/ferrocyanide couple is an ideal electrochemical probe which is widely used on different electrodes for the study of surfaces (Promph et al, 2015;Vogt et al, 2016). In order, to compare the electrochemical properties of bare carbon paste electrode, clay paste electrode and clay modified carbon paste electrode, the redox couple ferricyanide/ferrocyanide was chosen.
Preliminary studies were performed on clay paste electrodes (ClPE) in potassium hexacyanoferrate solution at pH 7. Figure 5 shows a typical Cyclic voltammetry (CV) recorded at a ClPE electrode between -0.4 V and 0.6V.
The absence of redox peaks observed on the clay paste electrodes (ClPE) in the cyclic voltammetry (CV) analysis indicates that the ferri/ferrocyanide redox couple is inactive on the ClPE. This suggests either a lack of reduction of Fe (CN) 3− or the absence of subsequent re-oxidation of Fe(CN) 4− on the ClPE surface. It is possible that the ClPE electrode itself is inactive in this electrochemical system.
The conductivity of clays has been reported to depend on various factors, such as heating temperature, clay pore structure, and soil mineralogy. These factors can influence the electrochemical behavior and conductivity of the clay paste electrodes.    Vol. 12, No. 1;2023 59 have highlighted the relationship between clay conductivity and these factors. Therefore, the lack of activity observed on the ClPE electrodes could be attributed to their specific conductivity properties influenced by these factors.
The electrochemical behavior of clay-modified electrodes was tested by cyclic voltammetry (CV) in potassium hexacyanoferrate solution. Figure 6 represents the responses obtained between 0.0 V and +0.7 V (vs. AgCl/Ag) on CPE, and CPE modified by 5% of clay AD (ADCPE), 5% of clay AG (AGCPE) and 5% of clay ZU (ZUCPE) respectively in a solution containing 1 mM [Fe(CN) 6 ] 3/4 (1:1) at 20 mV/s. The system ferri/ferrocyanide showed a different behavior on CPE and MCPEs. The oxidation and reduction peak currents observed increase for the modified CPEs versus unmodified electrode (see Table 4). A slight decrease in (Epa) was observed for ADCPE while it increases for AGCPE and ZUCPE. The value of ratio between the anodic and cathodic peak currents (Ipa/Ipc) is superior to 1; this shows a quasi-reversible of the system because the ideal reversible process is characterized by the ratio Ipa/Ipc approaches unity. Intensity of peaks on CPE system was markedly lower than those on MCPE, which can indicative of hindered diffusion on CPE and the improvement of the electron transfer rate on MCPE. Another characteristic parameter is the separation of the peak potentials ΔEp.
The theoretical value for ΔEp in reversible process is 60 mV, and it is independent of scan rate. Here, with a difference of 100 mV, ΔEp characterizes a slow electron transfer kinetics due to several factors, such as uncompensated solution resistance and non-linear diffusion (Xiao et al., 2014;Aristov & Habekost, 2015).   The electrochemical behavior of clay-modified electrodes, namely ADCPE, AGCPE, and ZUCPE, was investigated by altering the clay content in the carbon paste. It has been reported in previous studies (Lubna et al., 2022;Salih et al., 2017;Eslami et al., 2014) that the clay content in carbon paste can significantly influence the voltammetric responses and sensor properties. In this study, the proportion of clay in the carbon paste was varied from 3% to 20% to assess its impact on the electrochemical performance. Figure 7 illustrates the responses of the redox probe as a function of the clay percentage in the carbon paste. By systematically altering the clay content, the aim was to understand the relationship between clay concentration and the resulting electrochemical behavior. The behavior of the ferri/ferrocyanide redox couple varied with the concentration of clay in the carbon paste. The highest anodic current peaks were obtained when the clay content was 5% for AG and ZU, and 10% for AD. However, beyond these concentrations, a significant decrease in current peaks was observed. This decrease can be attributed to a reduction in the carbon content of the electrode material. As shown in Figure 5, the clays used in this study appear to be non-conductive materials. It has been reported that a high clay content can lead to the saturation of the electrode surface and consequently reduce the oxidation current of the reactant (Salih et al., 2017).
Nevertheless, the properties of these clays can be modified to enhance the sensitivity of the sensors. In order to avoid potential saturation of the electrode, it was determined that a clay concentration of 5% for AG and ZU, and 10% for AD would be suitable for future studies. These concentrations strike a balance between maximizing the anodic current peaks and maintaining the electrode's performance

Conclusion
This study investigated the potential of composite materials based on clay and carbon paste for the development of electrochemical sensors. The three clays obtained from Côte d'Ivoire (Adiaho, Agban, & Zuenoula) were thoroughly characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), EDS-mapping analysis, and Fourier transform infrared spectroscopy (FTIR). The characterization results consistently identified kaolinite as the dominant component in all three clay samples.
The electrochemical behavior of the ferri/ferrocyanide redox couple was then evaluated on clay-modified carbon paste electrodes using cyclic voltammetry. The results demonstrated that the modification of the electrodes with varying amounts of clay in the carbon paste had a significant impact on the response of the analyte.
Based on these findings, it can be inferred that the modified electrodes utilizing Ivorian clays, hold potential for future electroanalytical investigations. These clay-based composite materials offer new opportunities for the development of sensitive and reliable electrochemical sensors. Further research and optimization of the clay content in the carbon paste are warranted to fully harness the capabilities of these modified electrodes and explore their application in various electroanalytical techniques.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.