*3.2.2 Effect of Fe2+ concentration*

The amount of ferrous ions is one of the primary parameters that influences the Fenton and photo-Fenton processes. In a study [16], it was observed that the extent of degradation increases with increasing initial Fe2+ concentration, promoted by more OH radicals, as presented by **Figure 12** [16].

Contrary, the excessive Fe2+ ion produced larger amount of Fe3+ ions (reaction in Eq. 7) that further allowed them to react with hydroxide ions to form Fe(OH)3 precipitate, as also seen in Eq. (13) [17–21]. The precipitate formation created turbid solution that could inhibit the light entering into the solution. This situation depleted the number of OH radical formed, that further declined the degradation. This finding was in a good agreement with the other observations elsewhere [17–20]. However, the optimal values of Fe2+ concentration was varied among the reports, that were 5 mg/L [16], 56 mg/L [17], 40 mg/L [18], 130 mg/L [20], and 120 mg/L [21].

$$\text{Fe}^{3+} + \text{3}^{-} \text{OH} \blackrightarrow \text{Fe}(\text{OH})\_{\text{3(solid)}} \tag{15}$$

#### *3.2.3 Effect of initial pH*

The solution pH plays an important role in the efficiency of the photo-Fenton reaction, since it greatly influences the speciation of Fe, H2O2 and LAS. The relationship between pH alteration and the effectiveness of LAS degradation as

**Figure 11.** *Effect of H2O2 concentration on the LAS degradation effectiveness through photo-Fenton process [16].*

*Photo-Processes as Effective and Low-Cost Methods for Laundry Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.94336*

**Figure 12.** *Effect of Fe2+ concentration on the LAS degradation effectiveness [16].*

reported by a study [16], is displayed as **Figure 13**. It can be observed a trend, that the LAS degradation is less efficient at very low pH, and the efficiency of the degradation improves considerably when the pH is increased up to 3. The higher pH than 3 causes a sharp decrease in the degradation.

At very low level of pH, hydrogen ions (H<sup>+</sup> ) were present in large amount, that could protonate H2O2 to form protonated hydrogen peroxide or H3O2 + [17–20], as shown by Eq. (16).

$$\text{H}\_2\text{O}\_2 + \text{H}^\* \rightarrow \text{H}\_3\text{O}\_2^\* \tag{16}$$

The protonated hydrogen peroxide can inhibit the hydroxyl radical generation, resulting in small number of OH radicals, that further led to the lower photodegradation. An other reason proposed is that Fe2+, found in abundant, may form a stable complex with H2O2, which neutralized the Fe2+ catalyst [16]. The neutral catalyst

**Figure 13.** *Effect of pH on the LAS degradation effectiveness [16].*

could only generate few amount of OH, that significantly declined of the photodegradation. Further, increasing pH up to 3, provided smaller amount of H+ than at pH 1, so that the protonation of H2O2 could be prevented, and further enhances the number of the OH radicals formed. In addition, at pH 3, the complex of Fe2+ with H2O2 should be decomposed allowing Fe2+ to catalyze H2O2 maximally, and much OH radicals could be provided [17–21]. These explained clearly the highest photodegradation occurred at pH 3.

When the pH was increased up to 7, the number of hydroxide ion (<sup>−</sup> OH) were enriched, allowing Fe3+ ions to deposit as Fe(OH)3 (Eq. 15). As an effect, the sufficient Fe2+ catalyst did not remain in the solution. This caused lower decomposition of H2O2 and reduced the efficiency of the Fenton's process. Also, studies have shown that at higher pH, the oxidative potential of OH radical decreased and H2O2 was believed to be less stable [16, 18–19]. All the mentioned conditions obviously reduced the produced of OH radicals, and hence the amoxicillin degradation. The finding optimum pH (= 3) agreed with several other studies [17–21].
