*2.5. Statistical Analysis*

To attain a further higher reasonable dye removal efficiency with a good understanding of the roles of the significant independent parameters in the Levafix dye oxidation, a response investigated via dye percentage as dependent variable removal, a three-level factorial design, that is, the so-called Box–Behnken design, with triplicates of the central points, was applied. The selected independent variables were chosen according to the most affecting factors as follows: (i) H2O2 dose; (ii) Fuller's earth doses; and (iii) pH value. The levels and range for each parameter were selected according to a preliminary study that determined the manually optimized values. The selected levels and ranges are displayed in Table 1. Subsequently, SAS, statistical analysis software (SAS, Institute USA, Cary, NC, USA), was used to propose the full factorial experimental design matrix. Then, Matlab (7.11.0.584) software, as well as an analysis of variance (ANOVA), was chosen in order to analyze and specify the significance of the statistical technique. Moreover, Mathematica (V 5.2) software was selected in order to determine the optimal values. Finally, extra triplicates of the experiments were conducted to validate the proposed investigated model equation.

**Table 1.** Boundaries of the uncoded and coded experimental domains of the Box–Behnken factorial design with respect to their level spacing.


#### **3. Results and Discussion**

#### *3.1. Characterization of the Prepared Fuller's Earth Material*

Scanning electron microscopy (SEM) was used to explain the treated and calcinated morphologies of the Fuller's earth material; an image is displayed in Figure 2. The surface was observed to have an irregular shape, and the clay possessed lots of asymmetric, open pores. The voids aid adsorption, as well as catalytic oxidation activity, a source of various metals. Moreover, elemental analysis techniques of the Fuller's earth material using an energy-dispersive X-ray analyzer (EDX) revealed its chemical composition, which constituted different oxides. The predominant oxide in the Fuller's earth was SiO2, as well as Al2O3 and Fe2O3. The presence of Al2O3 and Fe2O3 leads to the occurrence of the Fenton reaction. Furthermore, trace amounts of CaO, MgO, K2O, SO3, and Na2O were present in the clay. A previous study [15] confirmed that the presence of Al2O3 leads to the Fenton reaction [18]. Moreover, various studies [15–18] verified that such oxides improve the adsorption tendency of Fuller's earth clay.

**Figure 2.** FE-SEM micrograph image of the prepared catalyst "Fuller's earth clay".

An oxide material analysis of the Fuller's earth clay data obtained via an EDX analysis is displayed in Table 2. The main oxide of the chemically treated and then calcined modified Fuller's earth powder was SiO2, along with Al2O3 and Fe2O3, with the presence of small amounts of the oxides Ca, Mg, K, and Na. The use of Fuller's earth, which is considered a clay, is due to its composition.

**Table 2.** Chemical oxide composition of modified Fuller's earth determined via EDX.


LOI: Loss of Ignition.

Although previous data [19] have indicated the importance of the oxides SiO2, Al2O3, and Fe2O3 in enhancing the adsorption capacity of pollutants, Al2O3 and Fe2O3 are sources of the Fenton reaction [15–18]. Such oxides might be initiated through hydrogen peroxide to generate hydroxyl radicals, which are categorized as highly reactive intermediates and possess the ability to oxidize pollutant species [13].

A Fourier transform infrared (FTIR) transmittance spectrum analysis was applied as a suggestive technique. FTIR was applied to identify the different forms of minerals present in the Fuller's earth that were introduced as a source for the Fenton oxidation test. Moreover, FTIR provided information on the chemical nature of the Fuller's earth substance, as well as some details about the interactions. As illustrated in Figure 3, the FTIR performances of the Fuller's earth clay exhibited coupled vibrations that are mainly due to the existence of numerous elements. Additionally, the main absorption-intensive bands of Fuller's earth clay appeared. Silanol, which is characterized by Si–O stretching vibrations, is located at the 1034 cm−<sup>1</sup> wavenumber. The occurrence of silanol represents the existence of quartz. Moreover, the bands at 528.4 cm−<sup>1</sup> and 779 cm−<sup>1</sup> are associated with the presence of illite, which is reflected by the Si-O-Al group. Moreover, the interlayer hydrogen bonding illustrates the possibility of hydroxyl linkage. Al-Mg-OH and Si-O-Fe bonding are reflected by the bands at 779 and 820 cm−1, respectively. This confirmed the existence of Fe and Al bonding, which is necessary for the Fenton oxidation test.

**Figure 3.** FTIR spectrum of the Fuller's earth clay.
