3.2.3. Effects of Photo-Fenton Parameters

#### Effect of Fuller's Earth Dose

To examine the effect of the Fuller's earth catalyst, as a source of the Fenton reaction, on the Fenton oxidation of azo dye, experiments were undertaken to investigate the influence of clay dose on the reaction kinetics. Figure 6 displays its influence on the oxidation reaction by varying the Fuller's earth concentration over the range of 0.5 to 1.5 mg/L.

**Figure 6.** Effect of Fuller's earth concentration on photo-Fenton oxidation system (experimental conditions: azo dye Levafix Dark Blue 50 ppm; H2O2 800 mg/L; and pH 3.0).

The oxidation removal efficacy was enhanced (from 92% to 97%) by increasing the Fuller's earth dose from 0.5 to 1.0 mg/L. However, further increasing Fuller's earth to 1.5 mg/L resulted in a significant decline in dye oxidation, reaching only 72%. This could be associated with the Al and Fe ions that form in the aqueous solution since they are present in Fuller's earth and then react with H2O2 to form further OH radicals and metal ions. These non-selective ˙OH radicals attack the dye molecules and strongly oxidize them. However, a further increase in the Fuller's earth catalyst resulted in a decline in the oxidation efficiency as a result of the shadowing effect arising from the turbid solution. This phenomenon of shadowing the UV illumination is due to the Fuller's earth comprising inorganic materials that obey UV transmittance when it is in excess. Additionally, an excessive concentration of metal ions results in their acting as ˙OH radical scavengers rather than generators according to the abovementioned equations (Equation (5)). This investigation is in accordance with the studies in [24–28], in which organic materials and dyes from wastewater are treated via the Fenton reaction.

## Effect of Hydrogen Peroxide Reagent Concentration

Figure 7 displays the reactive dye Levafix Dark Blue oxidation in the presence of the Fuller's earth-based photo-Fenton system. The oxidation rates elevated when the H2O2 reagent dose was increased from 200 to 800 mg/L, until reaching practically 97% Levafix Dark oxidation. This occurred with a 20 min illumination time using the optimal value of 800 mg/L of H2O2, 1 g/L of Fuller's earth, and a solution pH of 3.0. This increase in the removal efficiency with the increase in the reagent is associated with the production of more peroxide species and hydroxyl radicals in the aqueous medium. These radicals are the horsepower of the oxidation reaction, as well as its main responsibility. Moreover, elevating the H2O2 concentration to 1000 mg/L, more than the optimal limit (800 mg/L), affects the reaction rate and results in a decline in oxidation, reaching only 74%. This is probably related to the excess reagent concentration of ion pairs on the surface that could reduce the availability of the Fuller's earth elemental sites to react with the excess H2O2 (Equations (6) and (7)). In such a situation, additional H2O2 may have a detrimental effect due to the scavenging of hydroxyl radicals that occurs at higher H2O [24]. Moreover, there are radicals other than OH radicals in the presence of excess hydrogen peroxide; such radicals inhibit the oxidation reaction (Equations (8) and (9)).

$$\text{H}\_2\text{O}\_2 + \text{ h}\nu \to 2\text{OH}^\* \tag{6}$$

$$\rm{OH}^\* + \rm{OH}^\* \rightarrow \rm{H}\_2\rm{O}\_2 \tag{7}$$

$$\text{H}\_2\text{O}\_2 + \text{OH}^\* \rightarrow \text{H}\_2\text{O} + \text{OOH}^\* \tag{8}$$

$$\text{COOH}^\* + \text{OH}^\* \rightarrow \text{H}\_2\text{O} + \text{O}\_2 \tag{9}$$

**Figure 7.** Effect of hydrogen peroxide concentration on photo-Fenton oxidation system (experimental conditions: azo dye Levafix Dark Blue 50 ppm; Fuller's earth. 1.0 g/L; and pH 3.0).
