**2.5 Computational details**

Density functional theory (DFT) is one of the most important tools of quantum chemistry of understanding popular qualitative chemical concepts such as energy of highest occupied molecular orbital (EHOMO) and the lowest unoccupied molecular

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

**Figure 2.** *Absorption spectra of BB9, BY28, and their mixture.*

orbital (ELUMO), dipole potential (μ), hardness (η), softness (S), electrophilicity index (ω), and local reactivity descriptors such as Parr function P(r) [24, 25]. All computations are carried out with the Gaussian 09 program. The geometries of dyes, BB9 and BY28, are optimized using density functional theory (DFT) at the B3LYP/6 G-31G (d) level. Optimizations are carried out using the Berny analytical gradient optimization method. The geometries optimized are characterized by positive vibrational frequency definite Hessian matrices [26].

When the values of EHOMO and ELUMO are known, one can determine through the following expressions [27] the values of the electronic chemical potential μ, the absolute hardness η, and the softness S as:

$$
\mu = \frac{E\_H + E\_L}{2} \tag{3}
$$

$$
\eta = (E\_L - E\_H) \tag{4}
$$

The global softness (S) introduced is the inverse of the global hardness [28]:

$$\mathcal{S} = \frac{1}{\eta} \tag{5}$$

Using Parr's definition [29], the electrophilicity ω index is given by:

$$
\rho = \frac{\mu^2}{2\eta} \tag{6}
$$

Based on this idea, Domingo et al. [30] have introduced an empirical (relative) nucleophilicity index N, based on the HOMO energies obtained within the Kohn– Sham scheme and defined as:

$$N = E\_{HOMO}(\text{Nu}) - E\_{HOMO(TCE.)}\tag{7}$$

The HOMO energy of Tetracyanoethylene is �0.3351 a.u. at the same level of theory.

The electrophilic *P*<sup>þ</sup> *<sup>K</sup>* and nucleophilic *P*� *<sup>K</sup>* Parr functions, which allow for the characterization of the electrophilic and nucleophilic centers of a molecule, were obtained through the analysis of the Mulliken ASD (Atomic Spin Density) of the radical anion and the radical cation, respectively. These indices were obtained by single-point energy calculations over the optimized neutral geometries using the restricted B3LYP formalism for radical species. The results obtained will be compared with the experimental data.

The adsorption progress of the studied dyes on kaolinite surface is performed using Materials Studio (MS) 8.0 software developed by Accelrys Inc. The kaolinite crystal was optimized (a = 5.196 Å, b = 9.007 Å, c = 7.372 Å, and α = 93.029°, β = 105.983°, γ = 89.866) and cleaved along the (001) plane, a vacuum slab with 10 Å thickness was built. The final structure was enlarged to (4 � 2 � 1) to provide a large surface for the interaction of the dyes [31].

## **3. Results and discussion**

### **3.1 Characterization of natural safiot clay**

### *3.1.1 Energy-dispersive X-ray spectroscopy*

The spectrum of chemical constitution of natural safiot clay adsorbent is given in **Figure 3**. The EDX spectrum of **Figure 2** presents well-defined peaks, confirms the presence of the following chemical elements: Si, Al, Mg, Fe, K, P, S, O, Ca, C. These results confirm those found by the analysis XRF (**Table 1**), which also reveals the presence of these elements in the form of oxides: SiO2, Al2O3, Fe2O3, MgO, Na2O, CaO, K2O, TiO2. The atomic and mass percentages of the elements are summarized in **Table 2**. The predominance of silicon and oxygen peaks is clearly observed, which confirms the majority presence of kaolinite and quartz in the sample studied.

**Figure 3.** *EDX spectrum of natural safiot clay.*

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


**Table 1.**

*Isotherm constants for BB 9 and BY28 in single and binary systems.*


**Table 2.**

*Atomic and mass percentage of the natural safiot clay constituents.*

### *3.1.2 Fourier transform infrared spectroscopy*

Fourier transform infrared (FTIR) analysis was applied to determine the functional groups present on the surface of natural safiot clay and understand its adsorption mechanism. FT-IR spectra of NS clay in the range of 400 cm–4000 cm<sup>1</sup> are taken to obtain information on the nature of functional groups at the surface of the adsorbent. The spectrum of natural safiot clay is shown in **Figure 4**. The band that stretches between 3200 and 3700 cm<sup>1</sup> shows a peak with two shoulders at 3407 cm<sup>1</sup> and 3610 cm<sup>1</sup> corresponding to the vibrations of elongation of the hydroxyl group – OH linked to the water of constitution. In addition to the vibrations of deformation of the O-H bond due to the water molecules adsorbed between the sheets located at 1639 cm<sup>1</sup> . The bands that appear approximately around 3430 cm<sup>1</sup> and 1630 cm<sup>1</sup> correspond respectively to the vibrations of elongation and deformation of the OH group of the adsorbed water [32]. While the characteristic bands of carbonates are detected at 1436 cm<sup>1</sup> and 2521 cm<sup>1</sup> [33].

An intense absorption band at 900 cm–1200 cm<sup>1</sup> is centered on 1030 cm<sup>1</sup> , it characterizes the valence vibrations of the Si-O bond [34]. The bands between 795 and 748 cm<sup>1</sup> , coming from the Si-O-Al bond, also give way to a band around 778.4 cm<sup>1</sup> [35]. The absorption band located at 1030 cm<sup>1</sup> is in agreement with the X-ray

*Mineralogy*

**Figure 4.** *FT-IR spectrum of natural safiot clay.*

fluorescence indicating the presence of kaolinite in natural clay. However, the absorption bands at 423, 480, 534, 694, and 797 cm<sup>1</sup> correspond to quartz [36]. These results are in agreement with those found from XRF. They confirm the presence of quartz, carbonate, kaolinite, and dolomite in the clay studied.

### *3.1.3 Scanning electronic microscopy analysis*

Scanning electronic microscopy (SEM) technique was carried out in order to observe the morphology, structure, and distribution of the grains of our adsorbent material studied. **Figure 5a** and **b** show the SEM micrographs of natural safiot clay before and after adsorption. The scanning electron microscope image (**Figure 5a**) shows aggregates of kaolin grains in spherical form and of heterogeneous size, the interstices between the grains form pores. We also observe large irregularly shaped cavities; this confirms the heterogeneous composition of our clay revealed by the XRD (kaolinite + calcite + vermiculite). In contrast, **Figure 5b** shows that the NSC surface is more homogeneous and saturated after adsorption.
