2.1.1. Raw Material Extraction Site

The raw material used in this study was collected in the village of Kombissiri, located approximately 45 km south of the city of Ouagadougou (Figure 1). The geographical coordinates of the sampling site are X: 0670355 and Y: 1334702. From a geological perspective (Figure 2), the site is situated on a clayey plateau that constitutes the highest topographic relief within the internal tonalite domain.

Figure 1.Location of study areas on the map of Burkina Faso.

Figure 2. Geological map of Kombissiri.

2.1.2. Products

All solutions used in this work were prepared with ultrapure water with a resistivity of 18.2 MΩ∙cm. The equipment used to prepare the various solutions was soaked in a bath of nitric acid (5%) for at least 12 hours and rinsed with ultrapure water before use. The Pb²⁺ solutions were prepared from lead nitrate Pb ( NO 2 ) 2 .

2.2.1. Characterization of Clay

Before undertaking the Pb²⁺ lead adsorption tests, KOM clay was fully characterised using various physicochemical techniques. Brunauer-Emmett-Teller (BET) analysis and Barrett-Joyner-Halenda (BJH) isotherm analysis (Micromeritics ASAP 2020 accelerated specific surface area and porosimetry system, Norcross, GA, USA) were used to determine the specific surface area, pore volume, and pore size of KOM clay, respectively.

Elemental chemical analysis was carried out using inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Intrepid II XSP). For this purpose, 0.25 g of clay sample was digested in a microwave digestion system using a mixture of 4 mL of HF (30% w/w), 3 mL of H₂SO₄ (96% w/w), and 3 mL of HNO₃ (65% w/w).

The mineralogical composition of the powdered sample was determined by X-ray diffraction (XRD) using a Siemens D5000 diffractometer equipped with a rear graphite monochromator. The instrument was operated at an accelerating voltage of 40 kV and a current of 30 mA, using a cobalt anode emitting Kα radiation with a wavelength of λ = 1.54 Å. Data acquisition and processing were controlled by DIFFRAC AT software (version 2).

Fourier transform infrared (FTIR) spectroscopy was performed using KBr pellets containing 2 wt.% of clay powder. Infrared spectra were recorded in the 400 - 4000 cm⁻¹ range at a spectral resolution of 4 cm⁻¹ using a Shimadzu FTIR-8400S spectrometer.

The morphology of the KOM clay powders was examined by scanning electron microscopy (SEM) using a HITACHI SU8020 microscope. Observations were carried out at an accelerating voltage of 30 kV, with magnifications up to 30,000X.

2.2.2. Semi-Quantification

Semi-quantitative analysis of the different mineral phases is performed by combining the results of X-ray diffraction and chemical analysis. This combination allows the relative quantities of minerals contained in KOM clay to be evaluated using Equation (1) [10]:

T(a): oxide content “a” in the sample, Mi: mineral content (%) “i” in the sample, Pi(a): proportion of oxide “a” in the mineral “i” (this proportion is deduced from the ideal formula attributed to the mineral “i”).

2.2.3. Experimental Studies on Lead Removal

1)Adsorption kinetics

The adsorption kinetics were conducted to determine the equilibrium time between the adsorbent and the adsorbate. This is the time required for a thermodynamic equilibrium between the adsorbent and the adsorbate to be established [11]. The quantity Q t of lead Pb²⁺ adsorbed by the clay as a function of time is calculated according to Equation (2):

C₀ and C_t (mg/L) are the concentrations of lead Pb²⁺ at times t₀ and t, respectively.

V(L) and m (mg) are the volume of the solution and the dry mass of the clay used, respectively.

To evaluate the adsorption process of lead Pb²⁺ on KOM clay, the results of the effect of stirring time on adsorption were used to study two types of kinetic models (pseudo-first order and pseudo-second order). These models are represented by Equations (3) and (4), which are linear forms.

Pseudo-first-order model [11][12].

Pseudo-second-order model [11][12].

where Q_t is the amount of lead adsorbed at time t (mg/g), k₁ is the pseudo-first-order kinetic constant in min⁻¹, k₂ is the pseudo-second-order kinetic constant in g/mg∙min, and Q_e is the amount of lead adsorbed at equilibrium in mg/g.

The graphical representation of ln ( Q e − Q t ) as a function of t allows the parameters of the pseudo-first-order model ( Q e _ e x p and k 1 ) to be determined.

The graph of ( t / Q t ) as a function of tallows us to determine Q e _ e x p and k 2 respectively of the pseudo-second-order model.

2)Dose effect

A series of experiments were conducted by adapting the protocols proposed by Ennajih, H. [13]. Starting with stock solutions of lead Pb²⁺ (1000 mg/L), daughter solutions of lead with a concentration of 5 mg/L were prepared.

The method consists of placing 50 mL of a lead Pb²⁺ solution with a concentration of 50 mg/L in ten 250 mL polyethylene bottles. To this, we added increasing amounts of KOM clay, ranging from 0.1 to 1 g. All experiments were conducted according to the equilibrium time found during the adsorption kinetics.

The percentage of lead or the amount of lead is calculated using Equation (5).

C₀ and C_e are the initial and final concentrations (mol/L) of lead in solution, respectively.

3)Effect of initial concentration

The effect of the initial concentration on the adsorption process was obtained by varying the initial concentration of lead (Pb²⁺) from 5 to 80 mg/L, while maintaining the adsorbent dose at 0.5 g. Adsorption experiments were performed by contacting 0.5 g of clay with 50 mL of lead (Pb²⁺) solutions with initial concentrations ranging from 5 to 80 mg/L in 250 mL polyethylene flasks at room temperature. The mixture was shaken for three hours. The solvent used to prepare the lead solutions is pure water with a resistivity of 18.2 MΩ∙cm. After three hours of agitation, the solutions are collected and centrifuged at 3000 rpm for 15 minutes using a BECKMAN J2-MI centrifuge. Then, they are filtered using nylon membranes with a 0.45 µm retention threshold.

The amount (q_e) of lead fixed by a gram of adsorbent is given by Equation (6) [14].

With:

m: Mass of the adsorbent (g).

Q e : Quantity of lead per unit mass of adsorbent (mg/g).

C 0 : Initial lead concentration (mg/L).

C e : Residual concentration of lead in the liquid phase at equilibrium (mg/L).

V: Volume of the adsorbate (L).

4)Isotherms of adsorption

To estimate the maximum adsorption capacity of the KOM clay and understand the lead (Pb²⁺) adsorption mechanism by the KOM clay, an adsorption isotherm study was conducted.

The Langmuir model assumes that the adsorption process occurs via a monolayer on the adsorbent, that all the retention sites are homogeneous, that the number of retention sites is constant, and that adsorption occurs without interference. The Langmuir isotherm equation is given by Equation (7) [6].

By plotting C e / Q e = f ( C e ) , a straight line is obtained, y=cx+d, with a slope of c = 1/ Q 0 and an ordinate at the origin of d = 1/( Q 0 K L ). K L and Q 0 can be deduced by combining K L = C/d and Q 0 = 1/C.

In addition, the Langmuir isotherm can be expressed as a separation factor ( R L ), which is given by equation 8 and indicates the nature of the adsorption process [14].

With C 0 as the initial concentration of the adsorbate in mg/L and K as the Langmuir constant.

The Freundlich isotherm is a special case of the Langmuir model where adsorption corresponds to heterogeneous multicomponent adsorption. Among other things, this model describes non-ideal physical adsorption.

The Freundlich model is described by Equation (9) [6].

By representing l n ( Q e ) = f( l n ( C e ) ), we can deduce 1/n from the slope and K F from the y-intercept. The heterogeneity condition is verified if 1 <n< 10 [14].

K F is the adsorption capacity in mg/g, C e is the equilibrium concentration of the adsorbate, and 1/n is a constant indicating the intensity of adsorption.

3.1.1. Physicochemical Characteristics

Knowledge of physicochemical characteristics is necessary to understand many adsorption phenomena [15]. The characterization results revealed that KOM clay has a specific surface area and porosity suitable for the adsorption of Pb²⁺. The specific surface area of KOM clay, determined by the BET method, is 185.713 m²/g. This value is significantly higher than the values reported in the literature for local montmorillonite clay for the adsorption of Pb²⁺ lead [16][17].

It should also be noted that montmorillonite clay has relatively small average pore diameters (2.433 nm), almost at the limit of microporosity, as well as a large pore volume (0.101 cm³/g) [17]. Figure 3 shows the pore size distribution curves for KOM clay.

Figure 3. Pore size distribution curves of montmorillonite clay.

3.1.2. Chemical Analysis

Table 1 presents the results of the elemental chemical analysis of KOM. Analysis of these data shows that silica and alumina are the major oxides in the KOM sample, with iron, potassium, calcium, and magnesium oxides present in small quantities.

This composition is consistent with the chemical compositions of montmorillonite clays, which have been the subject of numerous studies on the removal of inorganic pollutants [2][17][18].

Table 1. Chemical composition of KOM clay in mass percent.

3.1.2. X-Ray Diffraction Analysis

Figure 4 shows the X-ray diffractogram of the KOM sample. Examination of the diffractogram (Figure 4) reveals that the KOM sample is composed of quartz (SiO₂), montmorillonite ((Na,Ca)_0.3(Al,Mg)₂Si₄O₁₀(OH)₂∙nH₂O), Illite (K_0.7Al₂(Al_0.7Si_3.3)O₁₀(OH)₂), kaolinite (Al₂Si₂O₅(OH)₄), and goethite (FeO(OH)).

Figure 4.Diffractogram of montmorillonite clay KOM.

3.1.3. Infrared Analysis of the KOM Sample

Figure 5 shows the infrared spectrum of the KOM sample. The characteristic bands of the functional groups in the infrared spectrum were assigned based on the infrared tables provided in the literature (Table 2). The infrared spectrum shows three spectral regions. The first group is noted between 3650 and 3400 cm⁻¹, a second between 1650 and 900 cm⁻¹, and a third around 800 to 550 cm⁻¹.

3.1.4. Semi-Quantification

The semi-quantitative analysis of the different mineral phases present in the KOM sample was determined using XRD coupled with IR, and the data were recorded in Table 3. These results show that the KOM sample is composed of Quartz (SiO₂) (27%); Montmorillonite((Na,Ca)_0.3(Al,Mg)₂Si₄O₁₀(OH)₂∙nH₂O) (25%); Illite (K_0.7Al₂(Al_0.7Si_3.3)O₁₀(OH)₂) (21%); kaolinite (Al₂Si₂O₅(OH)₄) (18%), and goethite (FeO(OH)) (6%).

Figure 5.Infrared spectrum of the KOM sample.

Table 2. Interpretation of infrared spectra of KOM clay.

Table 3. Mineralogical composition of KOM montmorillonite clay in mass percent.

3.1.5. Morphology of the KOM Sample

The morphology of KOM clay was investigated by scanning electron microscopy (SEM), and the corresponding micrographs are presented in Figure 6. The SEM images reveal the presence of large, compact agglomerates composed of stacked pseudo-hexagonal platelets. These platelets exhibit a random spatial orientation and are consistently observed across all samples. Such microstructural features indicate a poorly ordered kaolinite phase within the clay matrix. Previous studies have demonstrated that iron-rich kaolinites commonly exhibit structural disorder [22], which is consistent with the detection of iron oxide phases in KOM clay. Moreover, the pronounced cohesiveness observed in the KOM sample suggests the formation of micrometric agglomerates constituted of individual lamellar particles whose dimensions exceed those typically associated with kaolinite. This morphological characteristic further corroborates the presence of montmorillonite as a dominant or significant phase in the KOM clay [21].

Figure 6.SEM image of montmorillonite clay KOM.

3.2.1. Effect of Contact Time and Adsorption Kinetics

Figure 7 illustrates the amount of lead adsorbed per gram of KOM clay as a function of contact time. The results show a rapid increase in Pb²⁺ uptake during the first three hours, followed by a plateau beyond this period, indicating that adsorption equilibrium is reached after approximately 3 h. At equilibrium, the adsorption capacity (Qₜ) is about 0.5 mg∙g⁻¹, corresponding to a removal efficiency exceeding 90% for an initial Pb²⁺ concentration of 50 mg∙L⁻¹. Figure 8 further indicates that the equilibrium contact time remains unchanged for both KOM dosages (0.25 g and 0.5 g), suggesting that the adsorbent dose does not significantly influence the time required to reach equilibrium.

The kinetic data were analyzed using two commonly applied models: the pseudo-first-order kinetic model (Figure 8(a)) and the pseudo-second-order kinetic model (Figure 8(b)). Among these, the pseudo-second-order model provides the best fit to the experimental data, as evidenced by the higher coefficients of determination (R² = 0.996 and 0.998). Moreover, the experimentally determined equilibrium adsorption capacities (Qₑ, exp) are in good agreement with the calculated values (Qₑ, cal) obtained from the pseudo-second-order model (Table 4). These results indicate that Pb²⁺ adsorption onto KOM clay follows pseudo-second-order kinetics, suggesting that chemisorption mechanisms, such as inner-sphere complexation, play a dominant role in the adsorption process [12][25].

Figure 7. Effect of contact time.

Figure 8. Kinetic models of Pb²⁺ lead adsorption. (a): pseudo-first-order kinetics and (b): pseudo-second-order kinetics.

Table 4. Kinetic parameters and correlation coefficients for lead adsorption on KOM clay.

3.2.2. Dose Effect

The adsorbent dosage is a key parameter that significantly influences heavy metal removal from aqueous solutions, as it governs the adsorbent–adsorbate equilibrium of the system under investigation [26][27]. Figure 9 illustrates the effect of KOM clay dosage on Pb²⁺ removal. As the adsorbent dose increased from 0.1 to 1 g, the adsorption capacity decreased from 1.097 to 0.300 mg∙g⁻¹, while the Pb²⁺ removal efficiency increased from 54.88% to 97.30%.

The observed increase in removal efficiency with increasing adsorbent dosage can be attributed to the greater availability of active sorption sites on the clay surface [22][28]. The higher number of adsorption sites resulting from the increased amount of adsorbent enhances Pb²⁺ uptake from solution, despite the concomitant decrease in adsorption capacity per unit mass, which is commonly associated with partial site saturation and possible particle aggregation at higher dosages [29][30].

Figure 9. Effect of KOM montmorillonite clay dosage on Pb²⁺ adsorption rate = 50 mg/L; Adsorbent dosage = 0.1 - 1 g; Stirring time = 3 hours.

3.2.3. Effect of Initial Concentration

Figure 10 shows the effect of the initial concentration on the elimination of Pb²⁺. This figure shows that the amount of Pb²⁺ adsorbed increases with the initial concentration of Pb²⁺. This increase is due to a decrease in the solute’s adsorption resistance as the concentration of Pb²⁺ increases [6].

3.2.4. Isotherms of Adsorption

The results of the initial concentration effect allowed us to model the adsorption isotherms (Langmuir and Freundlich). Figure 11(a) shows the Langmuir isotherm, Figure 11(b) shows the Freundlich isotherm, and the parameters of these adsorption isotherms are recorded in Table 5. An analysis of Figure 11(a) and Figure 11(b) shows that the Langmuir model (R² = 0.9917) for the adsorption of Pb²⁺ by KOM clay is superior to the Freundlich model (R² = 0.9724). Consequently, the Langmuir isotherm best fits the experimental data. This suggests monolayer adsorption [6][14].

In the present study, the RL values are between 0 and 1. This indicates that a monolayer adsorption process is favorable. These results are also confirmed by the R² determination coefficient. Combined with the Langmuir isotherm, the equilibrium RL values indicate that the clay used has good potential for adsorbing lead (Pb²⁺) in an aqueous solution.

Figure 10. Effect of initial concentration on the adsorption rate of Pb²⁺.

(a)

(b)

Figure 11. Modeling of Pb²⁺ adsorption isotherms. (a) Langmuir model; (b) Freundlich model.

Table 5. Parameters of the isotherms and determination coefficients.