Eco-Friendly Removal of Cationic and Anionic Textile Dyes Using a Low-Cost Natural Tunisian Chert: A Promising Solution for Wastewater Treatment

Abstract

The discharge of synthetic dyes into aquatic ecosystems stands as a pointed environmental concern, with serious consequences affecting not only biodiversity and water quality but also human health. To address this challenge, this study introduces a natural Tunisian chert, a silica-rich sedimentary rock, as a promising, sustainable, and low-cost adsorbent for treating textile dye-polluted wastewater. For the first time, the adsorption capabilities of a Tunisian chert were systematically evaluated for both cationic (Methylene Blue; MB and Cationic Yellow 28; CY28) and anionic dyes (Eriochrome Black T; EBT). To assess the impacts of key operational parameters, such as pH (2–12), contact time (0–240 min), adsorbent dosage (0.02–0.25 g), and initial dye concentration (50–500 mg/L), batch mode adsorption trials were performed. The Langmuir isotherm model most accurately fits the adsorption data, yielding a maximum adsorption capacity of 138.88 mg/g for MB, 69.93 mg/g for CY28, and 119.04 mg/g for EBT, outperforming multiple conventional adsorbents. Kinetic modeling revealed that adsorption adhered to a pseudo-second-order model, with rapid equilibrium within 45–60 min, highlighting the efficiency of the Tunisian chert. Optimal dye removal was obtained at pH = 8 for cationic dyes and pH = 4 for EBT, driven by electrostatic interactions and surface charge dynamics. The current research work reveals that Tunisian chert is a low-cost and efficient adsorbent with a high potential serving for large-scale industrial applications in wastewater treatment. Using a locally abundant natural resource, this work provides a maintainable and economical approach for dye removal from polluted wastewater.

1. Introduction

The depollution of water contaminated with synthetic dyes and/or heavy metals, along with the purification of drinking water, has garnered significant global attention due to escalating environmental and public health concerns. Synthetic dyes, widely applied in industrial sectors, namely, cosmetics, food processing, paper manufacturing, textiles, leather, and plastics, are often toxic, mutagenic, or carcinogenic [1,2]. These dyes persist in aquatic environments, posing threats to ecosystems and human health [3]. Among the hazardous dyes, the present paper focuses on the study of Eriochrome Black T, an anionic dye, along with two cationic dyes, Methylene Blue and Cationic Yellow 28, due to their widespread industrial use and environmental persistence. Eriochrome Black T (EBT) corresponds to an anionic azo dye containing an -N=N- group, which is carcinogenic by nature [4]. It primarily acts in terms of being an indicator in complexometric titrations to determine total water hardness caused by elements such as calcium, zinc, magnesium, and other metal ions [5]. This synthetic dye is basically integrated in the textile industry for dyeing nylon, silk, and wool and is difficult to biodegrade [6]. This dye is hazardous, not only referring to its significant impact on the photosynthetic activity of aquatic environments when released into natural waters, but also with respect to its degradation products, like naphthoquinone, which can be carcinogenic [5]. The sulfonate groups in the structure of EBT raise potential risks to both human health and aquatic environments [7]. Methylene Blue (MB) is notably applied in wood, cotton, and silk industries. MB proves to be non-biodegradable, toxic, and carcinogenic [8]. MB belongs to the polymethine class of dyes. It is characterized by an amino auxochrome group and a positively charged structure [1]. MB begets serious risks to human health such as digestive and mental disorders, blindness, abdominal disorders, and respiratory distress [9]. It equally begets shock, nausea, diarrhea, vomiting, gastritis, cyanosis, jaundice, and an accelerated cardiac rhythm, triggering apoptosis in developing tissue cells and skin and eye sensitivity [1]. Cationic Yellow 28 corresponds to a synthetic azo dye commonly invested in leather and textile sectors. Despite its widespread application, it presents important health and environmental risks that cannot be overlooked [10]. The risks include skin irritation, allergic reactions, and respiratory issues such as bronchitis with prolonged exposure and asthma [10]. Its potential carcinogenicity raises much concern. Indeed, many research works revealed that the breakdown of Cationic Yellow 28 products can trigger cancer and mutations in living cells [10,11]. These dyes remarkably degrade the aesthetic quality of water and decrease the penetration of sunlight, disrupting the photochemical processes essential to the marine ecosystem. Therefore, the wastewater treatment containing these dyes before discharge into the environment is highly needed, owing to their harmful impacts on water quality. Currently, several methods have been established to eliminate textile dyes from effluents directly dumped into the environment. These methods include adsorption on organic supports [12,13], inorganic supports [11,14], photocatalysis [15], coagulation–flocculation [16], membrane filtration [17], and an advanced oxidation process [18]. Among these processes, adsorption proved to be the most extensively effective method for treating dye-laden contaminated water, regarding its flexibility, simplicity, affordability, and strong performance [19].

Notably, activated charcoal refers to the extensively applied material with respect to its high adsorption capacity on organic materials. However, it is expensive and remains difficult to regenerate. Therefore, numerous works attempted to use low-cost adsorbent compounds like natural clays [20,21,22], zeolites [22], cherts [11], and diatomites [23]. As a low-cost adsorbent material, chert was extensively used for removing dyes from wastewater regarding its outstanding properties and worldwide occurrence. The Tunisian chert employed in this work mainly comprises quartz (SiO₂) [11]. It is located at the southern base of Jebel el Âli (Kef Ensour), approximately 20 km west of Gafsa town. Tunisian cherts represent a worthwhile natural wealth that is extremely beneficial in multiple areas, referring to their physical and chemical features. Indeed, these materials are regarded as resources whose valorization is pivotal for national economic growth [24]. Therefore, the chief objective of the current research paper was to assess the potential use of Tunisian chert as an emerging cost-effective and efficient adsorbent material for eliminating anionic and cationic dyes, like Eriochrome Black T (EBT), Methylene Blue (MB), and Cationic Yellow 28 (CY28), from aqueous media. Operational parameters influencing adsorption, including initial dye concentrations, pH, adsorbent weight, and contact time, were studied. Data adsorption was examined through various isotherm and kinetic models. This research represents a novel investigation into the removal of cationic dyes using chert-based materials.

2. Materials and Methods

2.1. Adsorbent

The Tunisian chert employed in this study is mainly composed of quartz (SiO₂), accounting for 76.54% of the material [11,24]. Due to its physicochemical properties, chert stands as an outstanding natural resource carrying a high economic potential. Mahjoubi et al. [11] described raw chert as a sedimentary rock with a silica (SiO₂) content of 76.54%, along with impurities such as aluminum oxides (Al₂O₃s), magnesium oxides (MgO), iron oxides (Fe₂O₃s), and carbonates (CaCO₃s). XRD analysis highlights the presence of quartz and calcite as its primary crystalline phases, while FTIR analysis identifies characteristic Si–O and Si–O–Si vibrations, confirming its silica framework [11,24,25].

Morphologically, SEM analysis indicated a heterogeneous particle distribution with the presence of both large and small particles. BET analysis demonstrated a comparatively low specific surface area of 9.78 m²/g. Notably, the point of zero charge (pHzpc) for raw chert amounts to 6.5, which influences its surface charge and interaction with dyes under different pH conditions [11].

2.2. Chert Deposit

In Tunisia, cherts represent a significant natural resource with valuable physicochemical properties, making them important for various industrial applications as they contribute notably to the country’s economy. The Gafsa Basin, rich in recoverable minerals including cherts, is the central focus of the current study. The sampling site is located approximately 20 km west of Gafsa town on the southern flank of Jebel El Âli (Kef Ensour) [11,24]. Regarding large-scale applicability, the chert used in this study is naturally abundant in the Gafsa Basin, Tunisia, which helps minimize raw material costs. The extraction and processing of cherts are relatively low impact compared to synthetic adsorbents, although energy consumption for grinding and sieving should be considered. Overall, the use of chert presents a cost-effective and environmentally favorable option for adsorption applications. Therefore, future studies could include a more detailed life cycle assessment to quantify the environmental footprint and optimize large-scale implementation.

2.3. Point of Zero Charge (pHpzc) Determination

The point of zero charge (pHpzc) of chert (0.1 g) was determined using 50 mL of NaCl solution (0.01 M). The initial pH (pH_i) was adjusted between 2 and 12 through adding 0.1 M HNO₃ or 0.1 M NaOH. After 48 h of equilibration, the final pH (pH_f) was recorded, and the variation (ΔpH = pH_i − pH_f) was plotted against pH_i. The pHpzc was considered as the pH value at which ΔpH equals zero [11].

2.4. Adsorbate

The reactive dyes EBT and MB were purchased from Sigma-Aldrich, while CY28, with a purity of 99.99%, was obtained from the Tunisian textile industry The features and chemical structures of dyes are represented in

Table 1. Features and chemical structures of dyes.

Name	Eriochrome Black T	Methylene Blue	Cationic Yellow 28
Commercial name	Eriochrome Black T	Methylene Blue	Maxilon Golden Yellow GL 200%
Chemical formula	C20H12N3O7SNa	C16H18N3ClS	C21H27N3O5S
Molecular weight	461.38 g/mol	319.85 g/mol	433.52 g/mol
Class	Azo dye	Polymethine dye	Azo dye
Nature	Anionic	Cationic	Cationic
λmax (nm)	550	665	438
Color type	Brownish black	Blue	Yellow
Chemical structure

.

2.5. Adsorption Experiment

Raw chert adsorption was performed in batch experiments. A series of flasks (100 mL) involving 50 mL of dye solution with a range of initial concentrations (50–500 mg/L) were undertaken. Raw chert of different weights (0.025–0.25 g) was incorporated into 50 mL of each dye solution with pH ranging from 2 to 12. The solution pH was tuned with NaOH or HNO₃ solutions, and residual dye concentrations were estimated to deploy the Beckman DU 800 UV-Vis spectrophotometer at 550, 665, and 438 nm for EBT, MB, and CY28, respectively. The adsorption isotherms were examined at multiple initial dye concentrations. The dye removal efficiency (DR %) and adsorption capacity of each dye at equilibrium (q_e; mg/g) were estimated, relying upon the equations below [11,24]:

$$\mathbf{D}\mathbf{R}\mathbf{\%}={\displaystyle \frac{({\mathbf{C}}_{\mathbf{0}}-{\mathbf{C}}_{\mathbf{e}})}{{\mathbf{C}}_{\mathbf{0}}}}\times \mathbf{100}$$

(1)

$${\mathbf{q}}_{\mathbf{e}}={\displaystyle \frac{({\mathbf{C}}_{\mathbf{0}}-{\mathbf{C}}_{\mathbf{e}})}{\mathbf{m}}}\times \mathbf{V}$$

(2)

where C₀ (mg/L) denotes the initial concentration of dye, C_e (mg/L) indicates the equilibrium dye concentration, V (L) refers to the volume of the dye solution, and m (g) corresponds to the weight of chert.

3. Results

3.1. Effect of Initial Dye Concentration

The effect of the dye’s initial concentration was investigated through a concentration range of 50 to 500 mg/L at 25 °C. As illustrated in Figure 1, the EBT uptake value on the adsorbent surface gradually rises to 118 mg/g by increasing the EBT content in the solutions to 250 mg/L and then was stabilized upon reaching a concentration of 500 mg/L. The observed result may be explained by an enhanced adsorption driving the dye force, such as van der Waals interactions, which influence the adsorbent’s surface sites at high EBT concentrations [26,27].

For MB adsorption, as the initial dye concentration rises from 50 mg/L to 500 mg/L, the adsorbed amount rises correspondingly from 23 mg/g to 137 mg/g. Basically, this indicates that the rise in concentration improves the dye–adsorbent interaction, providing the needed driving force to overcome the dyes mass transfer resistance. The increase in the adsorption capacity with a higher initial MB dye concentration is attributed to the greater accessibility of MB dye molecules for interactions with the adsorbent’s binding sites [20,21,28]. At elevated MB dye concentrations, the active sites become occupied, and the adsorption capacity becomes close to a fixed value, implying that all sites are adsorbed at a 300 mg/L concentration.

Regarding the adsorption of CY28 on the studied material, with increasing initial dye concentration, the adsorbed amounts rise from 21 mg/g to 69 mg/g. Reduced initial dye concentrations allow for greater accessibility to active adsorption sites for dye uptake. However, at elevated concentrations, the accessible sites become taken, and thus, CY28 elimination relies on the initial concentration [11]. Because all sites become occupied, the adsorption capacity becomes close to a constant value at a 150 mg/L concentration [11].

3.2. Effect of Contact Time: Kinetic Study

The uptake of MB, CY28, as well as EBT by raw chert was investigated at various intervals, as shown in Figure 2 The findings revealed that large quantities of dyes were initially adsorbed rapidly, owing to the abundant available sites on the surface of chert. Furthermore, once these sites became saturated, the adsorption rate decreased. Equilibrium time was reached in 60 min for MB and 45 min for EBT and CY28. Consequently, almost all the active sites were occupied. The required contact time for equilibrium is less than the one stated by Islem et al., 2018 [29], who reported a 120 min requirement to eliminate CY28 dyes by natural clay. Rashidi et al., 2023 [26], asserted that the equilibrium time for EBT adsorption onto raw Montmorillonite was reached after 20 min for a dye concentration of 100 mg/L. Djomgoue et al., 2012 [30], argued that EBT removal with magnetic clay was achieved after 20 min. For MB adsorption onto natural Saudi Red Clay, Khan Mohammad Ilyas demonstrated that the equilibrium was reached in the first 40 min following the onset of contact [22].

The rapid adsorption noted in the first 5 min, which might be ascribed to the accessibility of the adsorbent’s negatively, promotes the rapid electrostatic adsorption of the cationic dye. It was equally inferred that the kinetic behavior of EBT, MB, and CY28 adsorption onto the surface of chert involved a rapid first step, during which the dyes were adsorbed onto the outer surface of chert. However, in the next step, the process decelerated as a result of resistance, indicated through dye molecular diffusion into the pores of the adsorbent [11,22,31].

3.3. Effect of pH

The pH of the dye solution corresponds to a pivotal parameter in examining the adsorption phenomena on chert. To assess the influence of pH on the removal efficiency and to detect the optimal pH, uptake experiments were run across a broad pH range (2, 4, 6, 8, 10, and 12), with 100 mg of adsorbent at 25 °C, and using initial dye concentrations corresponding to the optimal values determined in Section 3.1 (250 mg/L for EBT, 300 mg/L for MB, and 150 mg/L for CY28). This approach was adopted to evaluate the influence of pH under the most representative conditions for each dye. The levels of pH solutions were adjusted by adding HNO₃ or NaOH.

As seen in Figure 3, the disposal efficiency of cationic dyes (MB and CY28) increased from 71% to 92% for MB and from 50% to 92% for CY28 as the pH shifted from 2 to 12. However, pH = 8 was identified as being optimal, yielding a high adsorption efficiency for both cationic dyes. Above pH = 7, the negatively charged surface of the adsorbent (chert) exhibited a pronounced electrostatic interaction with cationic dye molecules. These observations are equally attributed to the neutral surface charge (pHpzc), a crucial property that dictates the pH at which the adsorbent exhibits an electrically neutral surface [32]. In this study, the pHpzc of chert proved to be 6.5 (Figure 4). Below pHpzc, the adsorbent’s surface is charged positively, favoring the attracting of anions, whereas beyond pHpzc, it shifts to a negative charge, favoring cation adsorption [33]. The dominating positive charges on the chert at acidic pH levels are suggestive that electrostatic repulsion is the main mechanism of adsorption for cationic dye species. With an increase in pH, the uptake of MB and CY28 on chert was improved. This phenomenon results from the enhanced electrostatic forces between cationic dyes and the negatively charged surface of the adsorbent. Removal efficiency at an acidic pH was lower due to an excess in H⁺, which destabilizes the cationic dye molecules and competes with them for adsorption sites. Similar findings were reported for the adsorption of cationic dyes on clays [20,22,29,34].

Conversely, the adsorption study of the anionic dye, EBT, on chert displayed a remarkable decrease in terms of the removal efficiency at pH = 12. Above pH = 7, the removal of EBT diminished, which is assigned to the presence of OH⁻ interfering with the interaction between EBT anions and the functional moieties on the adsorbent surface. The removal efficiency of EBT improved as the initial pH decreased from 12 to 2. The highest removal efficiency (81.85%) was observed at a strongly acidic pH = 4. This outcome aligns with findings obtained in other studies regarding the optimal adsorption of EBT on natural clays at acidic pH levels ranging from 2 to 4 [26,35,36].

In summary, with an increasing pH, the surface charge of the adsorbent becomes more negative, thereby enhancing the adsorption of cationic species. Under basic conditions, removal is favored due to the electrostatic attraction between the negatively charged surface of chert and the cationic dye molecules (MB and CY28) at pH 8. In contrast, for the adsorption of the anionic dye (EBT), increasing the pH reduces the positive charges while increasing the negative ones, leading to an electrostatic repulsion that hinders adsorption. These results can be explained by the point of zero charge (pHpzc), which is a crucial characteristic of any adsorbent, as it determines the pH at which the surface carries a net neutral charge. The pHpzc of chert in an aqueous solution was determined to be 6.5. At acidic pH values, the chert surface carries a positive charge, and in the presence of cationic dyes (MB and CY28), the dominant adsorption mechanism is electrostatic repulsion. As the pH increases, the adsorption of MB and CY28 onto chert also increases, which can be attributed to the stronger electrostatic attraction between the cationic dye species and the negatively charged chert surface. Adsorption is further reduced under acidic conditions due to the excess of H⁺ ions, which destabilize the cationic dye and compete with dye molecules for adsorption sites. Above the pHpzc, the surface of chert is negatively charged, which repels negatively charged dye molecules, thereby reducing the adsorption of anionic dyes (EBT) at basic pH values. Consequently, the adsorption of EBT on chert is favored under acidic conditions, particularly at pH 4.

3.4. Effect of Adsorbent Mass

The investigation of the adsorbent weight aimed to determine the optimal amount that maximizes the dye removal efficiency. Experimental trials were carried out using the weight of adsorbents varying from 0.025 g to 0.25 g. As illustrated in Figure 5, the removal efficiency for all dyes rises using adsorbent amounts from 0.025 g to 0.1 g. The maximum removal efficiencies were 93% for MB, 94% for CY28, and 94% for EBT and were reached at an adsorbent mass of 0.1 g. This can be attributed to the extended surface area and the availability of more adsorption binding sites for all dye molecules. Increasing the adsorbent dosage beyond 0.1 g did not result in a significant enhancement in dye removal efficiency. Consequently, 0.1 g was identified as the optimum adsorbent mass for the following experiments. The results are consistent with previous studies on the removal of cationic and anionic dyes using kaolin and other natural clays [20,29,32].

3.5. Adsorption Isotherms

The adsorption process can be characterized by a sorption isotherm, which represents the correlation between the equilibrium adsorption capacity (q_e) and the solute concentration in the solution at equilibrium (C_e).

Figure 6 exhibits the adsorption isotherms of EBT, CY28, and MB on raw chert. These isotherms reveal considerable adsorption at low concentrations with an increasing adsorption capacity as the initial concentration rises from 50 mg/L to 500 mg/L, until reaching a saturation beyond which no further increase in adsorption occurs, suggesting the full occupancy of available binding sites. Based on the arrangement established by Giles et al. [37], the adsorption isotherms are of type H, indicating strong ionic interactions between the adsorbent and the adsorbate dominating the adsorption process.

3.5.1. Application of the Langmuir Isotherm Model

According to Langmuir isotherm, it assumes homogeneous adsorption sites on the adsorbent surface with no interaction between adsorbed molecules. It indicates the maximum level of monolayer adsorption achievable on the adsorbent’s uniform surface [27]. The following equation determines the linearized form of the Langmuir isotherm [27]:

$${\displaystyle \frac{{\mathbf{C}}_{\mathbf{e}}}{{\mathbf{Q}}_{\mathbf{e}}}}={\displaystyle \frac{\mathbf{1}}{{\mathbf{Q}}_{\mathbf{m}\mathbf{a}\mathbf{x}}}} \times {\mathbf{C}}_{\mathbf{e}}+{\displaystyle \frac{\mathbf{1}}{{\mathbf{Q}}_{\mathbf{m}\mathbf{a}\mathbf{x}}\times {\mathbf{K}}_{\mathbf{L}}}} (\mathbf{\text{Nonlinear}} \mathbf{\text{form}})$$

(3)

where Q_e stands for the amount of solute adsorbed at equilibrium, C_e corresponds to the solute equilibrium concentration (mg/L) in the solution, Q_max denotes the maximum adsorption capacity (mg/g), and K_L indicates the Langmuir constant associated with free energy adsorption (L/g).

Figure 7 presents the linear plot of the Langmuir isotherm. Basically, the Langmuir isotherm is characterized by a dimensionless constant called the separation factor (R_L), which is computed with respect to the following equation [27]:

$${\mathit{R}}_{\mathit{L}}={\displaystyle \frac{\mathbf{1}}{\mathbf{1}+{\mathit{K}}_{\mathit{L}}\times {\mathit{C}}_{\mathit{i}}}}$$

(4)

where C_i refers to the initial concentration of adsorbate (mg/L) and K_L designates the Langmuir adsorption equilibrium constant (L/mg). R_L is the separation factor that expresses the adsorption condition as favorable (0 < R_L < 1), unfavorable (R_L > 1), linear (R_L = 1), and irreversible (R_L = 0).

3.5.2. Application of the Freundlich Isotherm Model

The Freundlich isotherm commonly serves to depict multilayer adsorption on a heterogeneous adsorbent surface. It relies on the assumption that the active surface sites are exponentially distributed [27]. It is expressed by the following empirical equations [27]:

$${\mathbf{Q}}_{\mathbf{e}}={\mathbf{K}}_{\mathbf{F}}\times {{\mathbf{C}}_{\mathbf{e}}}^{{\displaystyle \frac{\mathbf{1}}{\mathbf{n}\mathbf{F}}}} (\mathbf{\text{Nonlinear}} \mathbf{\text{form}})$$

(5)

$$\mathbf{ln}{\mathbf{Q}}_{\mathbf{e}}=\mathbf{l}\mathbf{n}{\mathbf{K}}_{\mathbf{F}}+{\displaystyle \frac{\mathbf{1}}{{\mathbf{n}}_{\mathbf{F}}}}\mathbf{l}\mathbf{n}{\mathbf{C}}_{\mathbf{e}} (\mathbf{\text{Nonlinear}} \mathbf{\text{form}})$$

(6)

where K_F represents the Freundlich constant, which reflects the relative adsorption capacity of the adsorbent, while n_F is a parameter that describes the heterogeneity of the adsorbent surface. As highlighted in Figure 8, the Freundlich parameters K_F and n_F were specified as departing from the intercept and slope of the plot of LnQ_e versus LnC_e, respectively.

The adsorption isotherms of all dyes (EBT, MB, and CY28) onto raw chert can be most accurately described using the Langmuir model grounded on the correlation coefficients (R²) outlined in

Table 2. Langmuir and Freundlich isotherms parameters.

Dyes	Experimental qe (mg/g)	Langmuir Parameters				Freundlich Parameters
		R2	Qmax (mg/g)	KL	RL	R2	nF	KF
EBT	118.76	0.9998	119.04	0.84	0.023–0.0023	0.37	4.74	42.10
MB	137.03	0.9996	138.88	0.64	0.030–0.0031	0.77	2.96	28.57
CY28	69.25	0.9999	69.93	1.05	0.018–0.0018	0.45	5.92	29.39

. The Langmuir isotherm model matches adsorption data better than the Freundlich isotherm model due to its higher coefficient of determination. This outcome implies that the adsorption process predominantly follows a monolayer coverage mechanism, which is characteristic of physisorption. On the other side, the calculated maximum adsorption capacity (qe_cal = 119.04, 138.88, and 69.93 mg/g for EBT, MB, and CY28, respectively) approaches the experimental data (q_exp). Moreover, the value (0 < R_L < 1) demonstrates that the dyes’ adsorption on natural chert is favorable. The R_L value reflects favorable adsorption. In addition, this mechanism corresponds to a monolayer adsorption with identical and independent sites.

As plotted in Figure 9, the R_L decreases with the growing initial concentration. This result indicates that natural chert displays a good adsorption capacity for the three dyes studied.

Table 3 presents a comparative analysis of the findings from the present study with those reported in previous works on dye removal using various adsorbents. The adsorption capacity (q_e; mg/g) of natural Tunisian chert is almost higher than that of other adsorbents (except natural clay for MB adsorption, raw Montmorillonite, and tea waste for the removal of EBT), demonstrating the potential of this novel material as an excellent adsorbent for wastewater dye removal.

Table 3. Comparison of the adsorption capacity obtained in the present study with those reported for various adsorbents in previous state of the art studies.

Adsorbent	Dyes	Qmax (mg/g)	References
Natural Saudi Red Clay	Methylene Blue (MB)	50.251	[22]
Raw Clay		19.194	[38]
Activated Clay		36.5	[38]
Clay		2.04	[39]
Kaolin (Algerie)		52.76	[40]
Apricot Kernel Shells		33.67	[41]
Muscovite (Morocco)		59.47	[42]
Zeolite/Fe3O4		32.258	[43]
Natural Clay		147.71	[44]
Clinoptilolite		50	[45]
Activated Carbon		232.5	[46]
Natural Tunisian Chert		138.88	Present study
Green Alga	Cationic Yellow 28 (CY28)	27	[47]
Clinoptilolite		59	[48]
Kaolin		5.71	[49]
Calcined Eggshells		28	[50]
Natural Tunisian Chert		69.93	Present study
Bentonite Carbon Composite	Eriochrome Black T (EBT)	2.899	[51]
Polyvinyl Alcohol/Starch/ZSM-5 Zeolite		16	[52]
Raw Montmorillonite		243.9	[26]
Orange Peel		0.667	[53]
Tea Waste		151.26	[54]
Sawdust		42.49	[55]
Fava Bean Peels		5.55	[56]
Magnetic Composite Carbon		20.79	[57]
Doum fruit		24.68	[58]
Activated Carbon of Cannabis		14.025	[59]
Natural Tunisian Chert		119.04	Present study

Table 3. Comparison of the adsorption capacity obtained in the present study with those reported for various adsorbents in previous state of the art studies.

Adsorbent	Dyes	Qmax (mg/g)	References
Natural Saudi Red Clay	Methylene Blue (MB)	50.251	[22]
Raw Clay		19.194	[38]
Activated Clay		36.5	[38]
Clay		2.04	[39]
Kaolin (Algerie)		52.76	[40]
Apricot Kernel Shells		33.67	[41]
Muscovite (Morocco)		59.47	[42]
Zeolite/Fe3O4		32.258	[43]
Natural Clay		147.71	[44]
Clinoptilolite		50	[45]
Activated Carbon		232.5	[46]
Natural Tunisian Chert		138.88	Present study
Green Alga	Cationic Yellow 28 (CY28)	27	[47]
Clinoptilolite		59	[48]
Kaolin		5.71	[49]
Calcined Eggshells		28	[50]
Natural Tunisian Chert		69.93	Present study
Bentonite Carbon Composite	Eriochrome Black T (EBT)	2.899	[51]
Polyvinyl Alcohol/Starch/ZSM-5 Zeolite		16	[52]
Raw Montmorillonite		243.9	[26]
Orange Peel		0.667	[53]
Tea Waste		151.26	[54]
Sawdust		42.49	[55]
Fava Bean Peels		5.55	[56]
Magnetic Composite Carbon		20.79	[57]
Doum fruit		24.68	[58]
Activated Carbon of Cannabis		14.025	[59]
Natural Tunisian Chert		119.04	Present study

3.5.3. Application of the Dubinin–Radushkevich (D–R) Isotherm Model

This model is a simplified approach serves to describe the nature of adsorption on either homogeneous or heterogeneous adsorbent surfaces and to identify the adsorption type involved, whether physical or chemical [60]. The nonlinear and linear forms of this model are expressed by the following two equations (Equations (7) and (8)) [60]:

$${\mathbf{q}}_{\mathbf{e}}={\mathbf{q}}_{\mathbf{m}}\mathbf{e}\mathbf{x}\mathbf{p} (-\mathsf{\beta }{\mathsf{\epsilon }}^{\mathbf{2}})$$

(7)

$$\mathbf{L}\mathbf{n}\mathbf{q}\mathbf{e}=\mathbf{L}\mathbf{n}\left({\mathbf{q}}_{\mathbf{m}}\right)-\left(\mathsf{\beta }{\mathsf{\epsilon }}^{\mathbf{2}}\right)$$

(8)

where β denotes the D–R constant corresponding to the sorption energy (mol²/kJ²), and ε is the Dubinin–Radushkevich constant provided by the equation [60]:

$$\mathsf{\epsilon }=\mathbf{R}\mathbf{T}\mathbf{L}\mathbf{n}\left(\mathbf{1}+{\displaystyle \frac{\mathbf{1}}{{\mathbf{C}}_{\mathbf{e}}}}\right)$$

(9)

where R denotes the constant universal gas (J·mol⁻¹·K⁻¹), T indicates the temperature (K), C_e (mol/L) expresses the equilibrium concentration, and q_m corresponds to the maximum adsorption capacity.

The adsorption mechanism is specified by the value of the mean adsorption energy (E), which elucidates the physical or chemical nature of the adsorption process [60]. The mean adsorption energy E (J·mol⁻¹) is calculated as follows [60]:

$$\mathbf{E}={\displaystyle \frac{\mathbf{1}}{\sqrt{-\mathbf{2}\mathsf{\beta }}}}$$

(10)

Energy values are lower than 8 kJ/mol indicate physisorption, while values between 8 and 16 kJ/mol suggest that the adsorption process corresponds to chemisorption. In contrast, values higher than 16 kJ/mol suggest that the adsorption process may be dominated by ion exchange [60].

The obtained results (

Table 4. Dubinin–Radushkevich model parameters.

Dyes	R2	β (mol2/kJ2)	Qmax (mg/g)	E (kJ/mol)
EBT	0.9938	4.219	128.17	0.34
MB	0.9981	0.104	137.88	2.18
CY28	0.9998	3.121	69.66	0.40

) reveal that the adsorption energy values “E” of the three studied dyes onto raw chert, as determined by the Dubinin–Radushkevich (D–R) model (Figure 10), are 0.34, 2.18, and 0.40 kJ/mol, respectively, for EBT, MB, and CY28. These values are all below 8 kJ/mol, indicating that the process of adsorption is characteristic of physisorption.

3.6. Kinetics of Adsorption

Kinetic studies of adsorption processes provide useful and relevant data on adsorption efficiency. Adsorption kinetics data can be assessed using various mathematical models, such as pseudo-first-order, pseudo-second order, and intra-particle diffusion models.

The pseudo-first-order equation is identified as follows [22]:

$${\displaystyle \frac{\mathbf{d}\mathbf{q}\mathbf{t}}{\mathbf{d}\mathbf{t}}}=\mathbf{k}({\mathbf{q}}_{\mathbf{e}}-{\mathbf{q}}_{\mathbf{t}}) (\mathbf{\text{Nonlinear}} \mathbf{\text{form}})$$

(11)

$$\mathbf{L}\mathbf{n}\left({\mathbf{q}}_{\mathbf{e}}-{\mathbf{q}}_{\mathbf{t}}\right)=\mathbf{L}\mathbf{n}\left({\mathbf{q}}_{\mathbf{e}}\right)-{\mathbf{k}}_{\mathbf{1}}\mathbf{t} (\mathbf{\text{Nonlinear}} \mathbf{\text{form}})$$

(12)

where q_t and q_e refer to the amount of adsorbed dye at time t and at the equilibrium, respectively (mg/g); k₁ represents the constant rate adsorption for the pseudo-first order (min⁻¹).

k₁ was calculated based on the linear relationship established by plotting ln(q_e − q_t) versus time in Figure 11. These values are summarized in

Table 5. Kinetic parameters for the adsorption of dyes by raw chert.

Adsorbent	Experimental qe (mg/g)	Pseudo-First Order			Pseudo-Second Order			Intra-Particle Diffusion
		R2	qe (mg/g)	K1 (min−1)	R2	qe (mg/g)	K2 (g/mg.min)	R2	Kid
EBT	118.36	0.98	6.37	0.0347	0.99	117.64	0.038	0.95	1.17
MB	137.30	0.98	32.87	0.0593	0.99	138.88	0.0069	0.87	5.90
CY28	70.75	0.99	23.67	0.0691	0.99	70.92	0.011	0.92	3.94

. The experimental data deviated considerably from linearity. This is supported by the low q_e and low correlation values (Table 5). Kinetics data were further applied to the Ho and McKay’s pseudo-second-order model. This model equation resting on the adsorption equilibrium is expressed as follows [22]:

$${\displaystyle \frac{{\mathbf{d}\mathbf{q}}_{\mathbf{t}}}{\mathbf{d}\mathbf{t}}}={\mathbf{k}}_{\mathbf{2}}\times {({\mathbf{q}}_{\mathbf{e}}-{\mathbf{q}}_{\mathbf{t}})}^{\mathbf{2}} (\mathbf{\text{Nonlinear}} \mathbf{\text{form}})$$

(13)

$${\displaystyle \frac{\mathbf{t}}{{\mathbf{q}}_{\mathbf{t}}}}={\displaystyle \frac{\mathbf{1}}{{\mathbf{k}}_{\mathbf{2}}\times {\mathbf{q}}_{{\mathbf{e}}^{\mathbf{2}}}}} +{\displaystyle \frac{\mathbf{1}}{{\mathbf{q}}_{\mathbf{e}}}}\mathbf{t} (\mathbf{\text{Nonlinear}} \mathbf{\text{form}})$$

(14)

where k₂ represents the constant rate adsorption of the pseudo-second-order model (g.mg⁻¹ min⁻¹). The k₂ value was calculated based on the linear relationship established by plotting t/q_t versus time in Figure 12.

As shown in Table 5, the R² values corresponding to the pseudo-second-order kinetic model are notably closer to unity compared to those of the pseudo-first-order model, suggesting a better fitting. The findings confirmed that the second-order kinetic model provides an accurate interpretation of the system under study, as it is consistent with the experimental outcomes, suggesting that the rate of adsorption is primarily governed by the availability of active sites on the adsorbent’s surface [61,62].

The intra-particle diffusion model is basically employed to identify the rate-limiting step in adsorption processes. It assumes negligible film diffusion, thereby designating intra-particle diffusion as the sole controlling rate mechanism [29]. This model, in its nonlinear form, is expressed as follows [29]:

$${\mathbf{q}}_{\mathbf{t}}={\mathbf{K} }_{\mathbf{i}\mathbf{d}}\times {\mathbf{t}}^{\mathbf{0.5}}+\mathbf{C}$$

(15)

where q_t represents the adsorbed amount at time t; t^0.5 is the square root of time; and K_id refers to the constant rate for intra-particle diffusion.

Figure 13 elucidates that adsorption plots are nonlinear across the entire time interval and may be divided into two separate linear zones, verifying the multi-stage nature of the adsorption process. The sequential phases of external mass transfer and intra-particle diffusion suggest that the molecules of dyes are first transferred to the surface of chert particles by rapid film diffusion. Subsequently, the chert molecules diffuse into the chert particles via intra-particle diffusion through the pores. The regression results show a deviation from the origin (C ≠ 0), which suggests the simultaneous involvement of film diffusion and intra-particle diffusion mechanisms. Additionally, complexation or ion exchange contributes equally to the control of the rate of adsorption.

4. Conclusions

Based on the results obtained in the current research, the practical and economic potential of employing natural chert in the dye treatment industry is evident. The influence of certain experimental parameters (contact time, initial dye concentration, solution pH, and adsorbent weight) was investigated through a batch adsorption technique.

The uptake of cationic and anionic dyes (MB, CY28, and EBT) is largely dependent on the pH. Moreover, the optimal pH for achieving maximum removal of cationic dyes is approximately eight, whereas for the anionic dye (EBT), it is around four. The adsorbed amount of dye rises with the rising initial concentration and then becomes closer to a fixed value. The adsorbed amount of dyes on the natural chert rose strongly with the increasing adsorbent mass up to 0.1 g. The kinetic study of the dyes’ adsorption onto chert revealed that the adsorption process is quite rapid, achieving equilibrium after 45 min for EBT and CY28 and 60 min for MB. Investigating the adsorption kinetics made it possible to specify the reaction order. Three kinetic models, namely, the Lagergren kinetics model (pseudo-first order), the Blanchard model (pseudo-second-order model), and intra-particle diffusion model, have been investigated. By comparing the regression coefficients of the fitted curves, it can be concluded that the adsorption kinetics of EBT, MB, and CY28 on the adsorbent matrices align with second-order kinetics of 117.64, 138.88, and 70.92 mg/g, respectively. The findings indicate that the obtained isotherms have an H-type appearance, suggesting powerful ionic interactions between the adsorbent and adsorbate. The adsorption isotherms of all dyes (EBT, MB, and CY28) on chert are suitably described using the Langmuir model for the theoretical maximum adsorption capacities of 119.04, 138.88, and 69.93 mg/g for EBT, MB, and CY28, respectively. According to the Dubinin–Radushkevich model, adsorption occurs through physical sorption.