*3.2.1 Effect of adsorbent dose*

It is recognized that the effect of the adsorbent dose on the adsorption process is also considered to be one of the most important parameters that must be optimized, *Use of Natural Safiot Clay for the Removal of Chemical Substances from Aqueous Solutions… DOI: http://dx.doi.org/10.5772/intechopen.101605*

**Figure 8.** *Effect of adsorbent amount on the removal efficiency and adsorption capacity of basic blue 9 and basic yellow 28.*

since the mass of adsorbent has an effect on the adsorption capacity for a given initial concentration of the adsorbate under the operating conditions. The adsorption of BB9 and BY28 in single and mixture on natural safiot clay is studied by varying the mass of adsorbent from 5 to 35 mg in 50 mL solution of 20 mg/L dye concentration at a constant stirring rate of 60 minutes. From **Figure 8**, it can be observed that removal efficiency of the dye increases from 49.17% to 97.03% for BB9 and from 59.97% to 85.32% for BY28 as adsorbent dose is an increase from 5 to 35 mg. This is because of the extra number of adsorption sites accessible with an increase in the adsorbent dose. On the other hand, the dye uptake capacity reduces from 98.34 to 27.72 mg/g and from 119.9 to 24.38 mg/g for BB9 and BY28, respectively. This can be attributed to the unsaturation of adsorption sites through the adsorption reaction with increasing adsorbent dosage [38, 39]. Another important reason is that at high adsorbent dosage, the available dye molecules are deficient to completely cover the available binding sites on the natural safiot clay, which results in low solute uptake [40, 41]. Similar results have been reported previously by other researchers for the adsorption of dyes by different material [42–44]. The optimum adsorbent dose is fixed conveniently at 30 mg per 50 mL of solution dye for the following studies.

### *3.2.2 Effect of initial dye concentration*

The effect of initial concentrations of BB9 and BY28 dyes is examined at different initial concentrations ranging from 10 to 40 mg/L on the adsorption capacity and removal efficiency onto natural safiot clay. As seen from **Figure 9**, the adsorption capacity increases from 22.58 to 58.45 mg/g for BB9 and of 12.68 to 58.89 mg/g for BY28. In this case, the % removal decreases from 98.97% to 87.67% and from 87.67% to 70.08% for BB9 and BY28, respectively. These results indicate that the adsorption sites of NSC adsorbent for dyes adsorption are still unsaturated

### **Figure 9.**

*Effect of the initial dye concentration on the adsorption capacity and removal efficiency of BB9 and BY28, in single (S) and binary (B) system onto NSC.*

within the dye concentration range. In addition, increasing initial dyes' concentrations increases the number of collisions between dyes ions and the surface area of NSC adsorbent, which enhances the adsorption process [45]. Similar results had been reported by Auta and Hameed [46] for MB dye removal onto modified ball clay chitosan composite.

### *3.2.3 Effect of initial dye pH*

The initial pH of the aqueous solution is important parameter controlling the adsorption process, where it affects both the degree of ionization of the dye and the surface properties of the adsorbent. The effect of initial pH of dye solution on the percentage removal of dye is studied by varying the initial pH from 2 to 12 under constant process parameters onto NSC, and results are presented in **Figure 10**. It is revealed that pH has practically a small effect on the percentage removal of the two basic dyes in simple system. In binary system, BY28 percentage removal is increased from 34.74% to 71.14% when pH is varied from 2 to 12 and also for BB9 percentage removal increase from 90.86% to 96.75%. These results are explained by pH zero-point charge pHZPC, the pHzpc of any adsorbent is a very important characteristic that determines the pH at which the surface has net electrical neutrality. In explaining this behavior by the fact that the negative charge dominates the adsorbent's surface in the basic medium. Thus, an electrostatic attraction exists between the negative charges of OH deposited on the clay surface and the positive charges of the dyes. Moreover, an electrostatic attraction between BB9 and BY28 dyes and the positive charge on the surface of NSC at low pH are evident [47, 48].

*Use of Natural Safiot Clay for the Removal of Chemical Substances from Aqueous Solutions… DOI: http://dx.doi.org/10.5772/intechopen.101605*

**Figure 10.** *Effect of initial dye pH on the removal efficiency (%) of BB9 and BY28 dyes in single and binary solutions.*

Consequently, the mechanism proposed can be described by the following equation.

$$\text{Si-OH} + \text{OH}^- \leftrightarrow \text{Si-O}^- + \text{H}\_2\text{O} \tag{8}$$

$$\text{Si-OH}^- + \text{BB9}^+ \text{ or } \text{BY28} + \leftrightarrow \text{Si-O}^- + \text{BB9} \text{ or } \text{Si-OH}^- + \text{BY28} \tag{9}$$

Similarly, the montmorillonite, bentonite clay, and montmorillonite/CoFe2O4 composite adsorption capacities were studied as a function of pH, and it was observed that maximum basic blue 9 dye adsorption was in acidic pH range [49–51].

### *3.2.4 Competitive adsorption between BB9 and BY28*

BB9 and BY28 adsorption in single and binary adsorption systems onto NSC is studied and is illustrated in **Figure 11**. The removal efficiency decreased in binary systems (as compared with single dye systems), the reduction was from 96.20% to 95.57% and from 85.32% to 61.48% for BB9 and BY28, respectively. For these results it is clear in the binary system, BB9 dye is most dominant and BY28 is most recessive dye. The values of R% also show that adsorption of BB9 and BY28 is reduced by the presence of other dyes in solutions within reduction percentage of 0.66% and 27.94% for BB9 and BY28, respectively. This behavior can be explained by the competitive adsorption between BB9 and BY28 for active sites with that BB9 dye is the first to be adsorbed in the active sites and by that BB9 is more electrophilic than BY28. This result will be demonstrated by following quantum chemicals study.

### **3.3 Adsorption isotherms**

Adsorption isotherms play an important role in the determination of the maximum adsorption capacity and the identification of the type of adsorption. The results of the adsorption experiments were analyzed per the well-known models of Langmuir, Freundlich, and Dubinin–Radushkevich (D-R):

The Langmuir isotherm is valid for monolayer adsorption on surface containing a finite number of identical sites [52]. The linear form of the Langmuir isotherm can be represented by the following equation:

**Figure 11.** *Adsorption competition of BB9 and BY28 onto NSC adsorbent sites.*

*Use of Natural Safiot Clay for the Removal of Chemical Substances from Aqueous Solutions… DOI: http://dx.doi.org/10.5772/intechopen.101605*

$$\frac{C\_{\epsilon}}{q\_{\epsilon}} = \frac{1}{q\_{m}K\_{L}} + \frac{1}{q\_{m}}C\_{\epsilon} \tag{10}$$

Where Ce (mg/L) represents the equilibrium concentration of the adsorbate, qe the amount adsorbed at equilibrium (mg/g), KL (L/mg) and qm (mg/g) are the Langmuir constant and the maximum amount of adsorbate, respectively.

To confirm the favorability of the adsorption process, the separation factor RL was calculated by the following Equation [53]:

$$R\_L = \frac{1}{1 + K\_L C\_0} \tag{11}$$

where the adsorption process is unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). Here, RL values for the adsorption of dyes are less than 1 and greater than 0, indicating favorable adsorption.

Freundlich isotherm model is an empirical equation based on sorption on a heterogeneous surface or surface supporting sites of varied affinities [54]. The linearized Freundlich model is represented by the following equation:

$$\log\left(q\_{\epsilon}\right) = \log\left(K\_f\right) + \frac{1}{n}\log\left(\mathcal{C}\_{\epsilon}\right) \tag{12}$$

where Kf (mg/g) is the measurement of adsorption capacity, and 1/n is the adsorption intensity of the adsorbent.

The Dubinin–Radushkevich model is a more generalized model as compared with the Langmuir isotherm and often used to estimate the characteristic porosity and the apparent free energy of adsorption [55]. The linearized Dubinin–Radushkevich (D-R) isotherm model is represented by the following equation:

$$
\ln\left(q\_e\right) = \ln\left(q\_m\right) - B\varepsilon^2\tag{13}
$$

where qm is the theoretical saturation capacity (mg/g), B is the D-R constant related to the sorption energy (mol<sup>2</sup> /kJ<sup>2</sup> ), and ε represents the Polanyi potential (J/mol), which is determined by:

$$
\varepsilon = RT\text{Ln}\left(\mathbf{1} + \frac{\mathbf{1}}{\mathcal{L}\_\varepsilon}\right) \tag{14}
$$

R is the universal gas constant (8.314 J mol�<sup>1</sup> K�<sup>1</sup> ), and T is the absolute temperature (K). The mean free energy of adsorption E calculated from B using the following relation:

$$E = \frac{1}{\sqrt{2B}}\tag{15}$$

The main parameters, characterizing each model as well as the coefficients of determination (R2 ), are grouped in **Table 1**. Comparison with Freundlich and Dubinin–Radushkevich model shows the high correlation coefficient of Langmuir isotherm for both dyes in single and mixture systems. This result suggests that the dye was homogeneously adsorbed on a monolayer surface of the adsorbent.

The value of parameter 1/n of the Freundlich equation gives an indication of the validity of the adsorption of the adsorbent adsorbate system. The values of 1/n presented in **Table 1** are between 0 and 1 indicating that the adsorption of the two dyes on our prepared adsorbent material (NSC) is favorable.

The magnitude of E is useful for estimating the type of adsorption process. The found values of E for BB9 and BY28 in the single and binary system are less than 8 kJ mol�<sup>1</sup> , knowing that energy values less than 8 kJ mol�<sup>1</sup> indicate physisorption and energy values varying from 8 to 16 kJ mol�<sup>1</sup> indicate chimisorption. Therefore, the adsorption type of BB9 and BY28 onto NSC has been defined as physical adsorption (physisorption). This confirms the results following the study of the influence of pH.

### **3.4 Kinetics of adsorption**

The kinetic of adsorption is an important characteristic in evaluating the efficiency of adsorption process**.** Three kinetics models (pseudo-first order, pseudo-second order, and intraparticle diffusion) were utilized to test the experimental data and predict the controlling mechanism of dye adsorption process.

### *3.4.1 Pseudo-first-order model*

The linearized form of pseudo-first-order rate expression is given as:

$$\log\left(q\_{\epsilon} - q\_{t}\right) = \log\left(q\_{\epsilon}\right) - \frac{k\_{1}}{2.303}t \tag{16}$$

Where qe and qt are the amount of dye adsorbed on sorbent (mg/g) at equilibrium and time t, respectively, k1corresponds to the reaction rate constant of pseudo-firstorder (min�<sup>1</sup> ), and t is time (min) [56]. The values of qe and k1 were calculated from the slope and intercept of the plots of the log (qe-qt) vs. t.

### *3.4.2 Pseudo-second-order model*

Pseudo-second-order rate expression reaction model is expressed as (linearized form) [57]:

$$\frac{t}{q\_t} = \frac{1}{k\_2 q\_e^2} + \frac{1}{q\_e} t \tag{17}$$

Where k2 is the pseudo-second-order rate constant (g/mg.min). A plot of t/qt and t should give a linear relationship if the biosorption follows pseudo-second-order model. The qe and k2 can be calculated from the slope and intercept of the plot.

### *3.4.3 Intraparticle diffusion model*

The intraparticle diffusion model is based on the theory proposed by Weber and Morris [58]. The Weber and Morris equation is (18):

$$q\_t = k\_{\rm id} t^{1/2} + \mathcal{C} \tag{18}$$


*Use of Natural Safiot Clay for the Removal of Chemical Substances from Aqueous Solutions… DOI: http://dx.doi.org/10.5772/intechopen.101605*

### **Table 3.**

*Kinetics parameters for the adsorptive removal onto NSC adsorbent of BB9 and BY28 dyes and their mixture.*

Where qt is the adsorption capacity (mg/g) at time t (min), kid is the intraparticle diffusion rate constant (mg/g.min), and C (mg/g) is a constant, which provides the information regarding the thickness of the boundary layer. The values of Kid and C were calculated from the slope and intercept of the plots of the qt against t1/2.

The conformity between the experimental data and the predicted model is based on the values of the correlation coefficients (R2 ), hence the value R2 closest to unity will indicate the adequate model to correctly describe the kinetics of adsorption of the dye.

**Table 3** summarizes the rate constants and correlation coefficients (R2 ) of the three kinetic models. The fitting of the kinetic data in the pseudo-second-order equation showed excellent linearity with high correlation coefficient (R2 > 0.999), and the good agreement between the experimental and calculated equilibrium adsorption for the pseudo-second-order model confirms that this one describes correctly the adsorption kinetics. Similar results have been observed in the adsorption of basic dyes onto Moroccan Clay [59] and in the adsorption of Methylene Blue (MB) by montmorillonite clay [60].

### **3.5 Quantum chemical studies**

### *3.5.1 Global reactivity descriptors*

The global chemical reactivity descriptors, energy gap (ΔE), dipole moment (μ), hardness (η), softness (S), nucleophilicity (N), and electrophilicity index (ω) witch

### *Mineralogy*


### **Table 4.**

*Quantum chemical parameters of the studied dyes calculated at B3LYP/6 G-31G (d).*

calculated from HOMO and LUMO energies and are obtained at the level of theory B3LYP/6 G-31G(d) and summarized in **Table 4**.

Energy gap (ΔEgap = EHOMO – ELUMO): The energy gap between the HOMO and LUMO is very important in determining the chemical reactivity of the molecule dyes toward the adsorption on the adsorbent surface. On the other hand, the decrease in the value of ΔEgap increases the reactivity of the molecule, which facilitates adsorption and increases the adsorption efficiency. It can be seen from **Figure 12** that the BB9 dye shows a lower ΔEgap (ΔEgap = 1.163 eV) compared with the BY28 dye, which has a difference of 3.198 eV, which clearly means that the molecule of BB9 is more reactive than BY28, Therefore, the BB9 dye will be adsorbed firstly. This conclusion is in agreement with the experimental results.

Dipole moment (μ): The dipole moment (μ) is another important electronic parameter, provides information on the polarity of the whole molecule. The high

### **Figure 12.**

*Highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) density of BB9 (a) and BY28 (b) BY DFT at the B3LYP/6 G-31G(d).*

*Use of Natural Safiot Clay for the Removal of Chemical Substances from Aqueous Solutions… DOI: http://dx.doi.org/10.5772/intechopen.101605*

molecular polarity probably gives rise to great chemical reactivity. It is clearly established in the literature that molecules with high dipole moments are more reactive, and their action results in a significant elimination efficiency. In our case the high dipole moment value of BB9 (11.351 D) probably increases the adsorption between the BB9 dye and the surface of natural clay compared with BY28 (7.573 D), which explains the adsorption efficiency higher than BB9 when compared with BY28 and confirms the experimental results.

**Hardness (η) and softness (S):** The stability and reactivity of a molecule are determined by the calculation of two important parameters: the global hardness (η) and the softness (S). The resistance of a molecule to deformation is determined by the chemical hardness, a hard molecule has a high energy gap. In addition, a soft molecule has a low energy gap. It is important to note that electronic systems with hard molecules have the least tendency to react while systems with soft molecules have a higher tendency to react.

The high percentage of elimination of a molecule is linked to a low value for chemical hardness and a high value for softness. In the present work, the values of the global hardness (η) and the softness (σ) presented in **Table 5** clearly show that the BB9 dye has the lowest value of the hardness (η = 1.163 eV) and the higher value of the softness (S = 0.859 eV), which explains their significant elimination percentage compared with BY28, these results are in good agreement with the experimental results.

Global electrophilicity index (ω): The global electrophilicity index (ω) represents the capacity of the dyes to accept electrons. More reactive nucleophilic is characterized by lower value of ω, and conversely more reactive electrophilic is characterized by a higher value of ω. From **Table 5**, we notice that the electrophilicity value of BB9 (ω = 6.178) is greater than that of BY28 (ω = 2.48); this indicates that the molecule of BB9 is more electrophilic than that of BY28. Consequently, BB9 will be adsorbed first followed by BY28.


### *Mineralogy*


**Table 5.**

*Theoretical prediction of reactive sites using Parr function for BB9 and BY28 dyes.*

### *3.5.2 Local molecular reactivity*

The local reactivity site of the studied dyes has been analyzed by evaluating Parr functions (PF). The PF is used to obtain the detail information of local reactivity of each atom in the molecule. Domingo proposed the Parr functions P(r) [61], which are given by the following equations:

*Use of Natural Safiot Clay for the Removal of Chemical Substances from Aqueous Solutions… DOI: http://dx.doi.org/10.5772/intechopen.101605*

$$P^{+(r) = \rho\_r^m(r)}\qquad\text{for nucleophile attack}\tag{19}$$

$$P^{-}(r) = \rho\_{\varepsilon}^{\text{rc}}(r) \qquad \text{for electrophilic attack} \tag{20}$$

With these electrophilic and nucleophilic Parr functions are at hand, the local electrophilicity ω<sup>K</sup> and the local nucleophilicity NK indices will be redefined as follows:

$$
\alpha\_K = \alpha \mathbf{P}\_K^+ \tag{21}
$$

$$\mathbf{N}\_{K} = \mathbf{N} \mathbf{P}\_{K}^{-} \tag{22}$$

**Table 5** shows that at the DFT level, the most susceptible site to a nucleophilic attack for BB9 is located on sulfur, nitrogen, and benzene ring. In the case of an electrophilic attack, the most reactive site is on Cl39. For BY28 the more susceptible sites to nucleophilic attacks are nitrogen and C8 atoms, while N26 and C13 are the most susceptible sites for electrophilic. The results indicated that the BB9 dye has more and strong electrophilic sites than BY28; consequently, BB9 has a high affinity for NSC than BY28.

### *3.5.3 MC and MD simulations*

In this study, Monte Carlo simulations were performed to study the adsorption and orientation of dyes on charged surfaces based on (001) kaolinite surface and all-atom models. The most stable low-energy adsorption configurations of the studied dyes are shown in **Figure 13**. It is clear that the three dyes examined BB9, BB41, and BY28 are adsorbed almost parallel to the plane to maximize surface and contact coverage. These adsorption configurations indicate that there are strong interactions between the studied dyes and the kaolinite atoms. This facilitates their adsorption to the surface of the kaolinite (001) by blocking a maximum of sites and ensuring a great influence on the removal efficiency.

The outputs and descriptors calculated by the Monte Carlo simulation are presented in **Table 6**. The parameters presented in **Table 6** include total energy, in kcal mol�<sup>1</sup> , of the substrate–adsorbate configuration. As can be seen from **Table 6**, BB9 gives the maximum adsorption energy in negative value found during the simulation process. High values of adsorption energy indicate that BB9 molecule will give the highest removal efficiency and strong interaction between a kaolinite substrate and the studied dye. These results are in good agreement with experimental findings.

To further confirm our results, we have performed the energy fluctuation curves as obtained from MD simulations; the equilibration of the system is confirmed by the stable mean values of energy fluctuations, as shown in **Table 7**. The mean square displacement (MSD) and the diffusion coefficient were calculated after 100,000 steps. The obtained data included in **Table 7** show that the diffusion coefficient of the free water molecules was more pronounced (5.85 � <sup>10</sup>�<sup>6</sup> cm2 /s) than the water with BB9 and BY28 molecules. Much smaller diffusion coefficients obtained for water with BB9 were caused by the strong interaction between (water + BB9) and the kaolinite surface, which decreased the mobility of the water [62].

### **Figure 13.**

*The most stable low-energy configuration for the adsorption of the dyes on kaolinite surface obtained through the Monte Carlo simulations.*


### **Table 6.**

*Outputs and descriptors calculated by the Monte Carlo simulations for the lowest adsorption configurations of tested dyes on kaolinite (001) surface (in kcal/Mol).*

*Use of Natural Safiot Clay for the Removal of Chemical Substances from Aqueous Solutions… DOI: http://dx.doi.org/10.5772/intechopen.101605*


### **Table 7.**

*Calculated diffusion coefficient of free water and water with dyes in kaolinite surface.*
