*3.2.1 Effect of H2O2 concentration*

The concentration of H2O2 is a critical variable in the degradation through Fenton and photo-Fenton processes. Many researchers have observed the influence of H2O2 concentration on the LAS degradation by Fenton and photo-Fenton methods. One example data is taken and exhibited in **Figure 11** [16]. It is seen that the low concentration of H2O2 did not generate enough •OH in solution, giving less effective degradation. Increasing H2O2 concentration improved the LAS degradation due to more •OH available. Addition of H2O2 above the optimum level lead to a decrease in the LAS degradation, that is caused by the depletion of the •OH amount due to free radical scavenging by the excess H2O2 to produce hydroperoxy radicals (•O2H). Then the hydroperoxy radical will further react with OH radical to form water and O2 [17–20]. The reactions are exhibited by Eq. (13) and Eq. (14) below:

$$\bullet \bullet \mathrm{H} + \mathrm{H}\_{2}\mathrm{O}\_{2} \bullet \bullet \bullet \mathrm{O}\_{2}\mathrm{H} + \mathrm{H}\_{2}\mathrm{O} \tag{13}$$

$$\bullet \bullet \mathrm{O}\_{2}\mathrm{H} + \bullet \mathrm{OH} \blackrightarrow \mathrm{H}\_{2}\mathrm{O} + \mathrm{O}\_{2} \tag{14}$$

It is obvious that there is an optimum H2O2 concentration to achieve the maximum percentage of LAS removal, although the values of the concentration range varies for different conditions. In a study the optimum photo-Fenton condition was mediated by a [H2O2]/[Fe2+] ratio = 40 [16]. The effect of mode of reagent addition was also studied giving ratio of 10 [17]. Similar results were obtained in other studies, that were 1.4 [18], 7.6 [20], and 11 [21].
