## 3 Results and Discussion  
### Photo-Fenton  
#### 3.1.1 Determination of the Optimised Operational Conditions  
The results of the RSM are summarised in Table 5. In all the tested scenarios, the 500 W MP lamp reached a high indigo carmine removal, in some cases more than 95%, achieving even 97.5% when dosing 6.5 mg L\({}^{-1}\) of iron and 54 mg L\({}^{-1}\) of H\({}_{2}\)O\({}_{2}\), while the 40 W LP lamp reached only 49.5% with the maximum iron and H\({}_{2}\)O\({}_{2}\) dosage. The lower performance of the 40 W LP lamp may be caused by its lower power and nature. In fact, the 40 W LP lamp emits only in the UVC spectrum, and the regeneration of Fe\({}^{+3}\) to Fe\({}^{+2}\) occurs mainly between 314 and 360 nm (UVA). As such, a broad-spectrum lamp would be more effective [39]. The correlation between the two independent variables (iron and H\({}_{2}\)O\({}_{2}\)) and the response (indigo carmine removal) is modelled in Equations (6) and (7). The 500 W MP lamp followed a quadratic trend (6), and the 40 W LP lamp followed a linear trend (7). Consequently, the target compound removal on the 500 W MP lamp did not always increase as the reagent's dosage rose, whereas, with the 40 W LP lamp, the target compound removal increased as the reagent's dosage rose. Both models were carried out by an analysis of variance (ANOVA), and the summarised results are in Table 1. The F-value shows that both models are significant, and the likelihood that these results are due to noise is less than 0.5%. The fitting of the quadratic model (500 W MP lamp) was adequate, with a regression coefficient (R\({}^{2}\)) of 0.982. Nonetheless, the fitting of the 40 W LP lamp with the model was lower, with a R\({}^{2}\) of 0.72. \[\text{Y}=14.0+14.5^{\text{*}}\text{Fe}+1.3\times\text{H}_{2}\text{O}_{2}-0.02 \text{ Fe}\times\text{H}_{2}\text{O}_{2}-0.83\times\text{Fe}^{2}-0.01\times\text{H}_{2} \text{O}_{2}{}^{2} \tag{6}\]  
\[\text{Y}=5.66+12.85\times\text{Fe}+0.31\times\text{H}_{2}\text{O}_{2} \tag{7}\] \begin{table}
\begin{tabular}{l l l l l l l} \hline
**Source** & **Sum of Square** & **Degree of Freedom** & **Mean Square** & **F-Value** & **\(p\)-Value** & \\ \hline
**500 W MP Lamp** & & & & & & \\
**Quadratic** & \(1581.88\) & 5 & 316.38 & 96.68 & 0.003 & Significant \\
**A-Iron** & 870.92 & 1 & 870.92 & 266.14 & \textless{}0.0001 & \\
**B-H\({}_{2}\)O\({}_{2}\)** & 333.48 & 1 & 333.48 & 101.91 & 0.0005 & \\
**AB** & 5.64 & 1 & 5.64 & 1.72 & 0.2595 & \\
**A2** & 330.88 & 1 & 330.88 & 101.11 & 0.0006 & \\
**B2** & 97.71 & 197.71 & 29.86 & 0.0055 & & \\
**Residual** & 13.09 & 4 & 3.27 & & & Not significant \\
**Lack of fit** & 8.94 & 3 & 2.98 & & & \\
**Pure error** & 4.15 & 1 & 4.15 & & & \\ \hline
**40 W LP Lamp** & & & & & & \\ \hline
**Lineal model** & 944.07 & 2 & 472.04 & 12.54 & 0.0048 & Significant \\
**A-Iron** & 681.81 & 1 & 681.81 & 18.11 & 0.0038 & \\
**B-H\({}_{2}\)O\({}_{2}\)** & 262.26 & 1 & 262.26 & 6.96 & 0.0335 & \\
**Residual** & 2.63.59 & 7 & 37.66 & & & \\
**Lack of fit** & 261.55 & 6 & 43.59 & 21.37 & 0.1641 & Not significant \\
**Pure error** & 2.04 & 1 & 2.04 & & & \\ \hline \end{tabular}
\end{table} 
Table 1: ANOVA values for RSM of 500 W MP and 40 W LP lamps. The 3D response surface plots (Figure 2) allow a better analysis of the behaviour of the treatment when the variables change, making it easy to identify the influence of each variable on the indigo carmine removal. Results of the 500 W MP lamp (Figure 2a) showed that a slight variation in iron can significantly affect the removal of the model compound. 3 mg L\({}^{-1}\) of iron and 10 mg L\({}^{-1}\) of H\({}_{2}\)O\({}_{2}\) reached 60% while increasing the iron to 4 mg L\({}^{-1}\), keeping the same quantity of H\({}_{2}\)O\({}_{2}\), reached 70%. However, the elimination attained its maximum ratio at a concentration of 7 mg L\({}^{-1}\) of iron. Above this value, the removal of indigo carmine reained constant or even decreased. That can be attributed to a scavenging effect of the \({}^{\bullet}\)OH caused by a high photoreduction of Fe\({}^{3+}\)[40].