## 3 Results and Discussion
### Preliminary Tests and Determination of the Operational Variables and Working Intervals  
In an experimental design such as the one used for the development of this work, it is of paramount importance to properly define the working range corresponding to each of the operating parameters (initial concentrations of H2O2, Fe2+, Fe3+, and polyphenols), whose effect on the response parameter (dye removal efficiency) is to be investigated. For this, it is necessary to perform a series of preliminary experiments to separately estimate the effect of each operating variable on the response variable. Given the importance of the initial concentration of hydrogen peroxide for dye removal efficiency, a first series of experiments were performed in which the initial concentrations of Fe2+, Fe3+, and polyphenols were kept constant and H2O2 concentration varied between 0 and 1.5 \(\times\) 10\({}^{-2}\) M. It can be seen that in the absence of hydrogen peroxide, dye removal is barely above 60%, whereas with [H2O2]o = 2 \(\times\) 10\({}^{-3}\) M, dye removal can be as high as 85% or 82% for chrysoidine and methylene blue, respectively. Increasing this initial concentration to 3.75 \(\times\) 10\({}^{-3}\) M results in removing approximately 92% of the dye present in the solution. A further increase of [H2O2]o to 7.5 \(\times\) 10\({}^{-3}\) M or 1.5 \(\times\) 10\({}^{-2}\) M does not result in a higher removal efficiency of any of the dyes. Rather, the removal efficiency slightly decreases in both cases. This effect, repeatedly described in the literature, could be attributed to the generation of an excess of hydroxyl radicals at overly high hydrogen peroxide levels, which could recombine and reverse the iron-mediated catalytic degradation process [83]:  
\[\cdot OH+\cdot OH\rightleftharpoons H_{2}O_{2} \tag{7}\]  
It is also possible that such radicals can be inactivated by reacting with excess hydrogen peroxide to form hydroperoxide radicals (HO2-), which have a lower oxidation potential than hydroxyl radicals, according to the reaction. \[\cdot OH+H_{2}O_{2}\rightarrow\cdot OOH+H_{2}O \tag{8}\]  
In light of the above, it was decided to use a hydrogen peroxide concentration of 2 \(\times\) 10\({}^{-3}\) M as the starting point for the experimental design, considering that an increase in [H2O2]o from 2 \(\times\) 10\({}^{-3}\) M to 3.75 \(\times\) 10\({}^{-3}\) M resulted in only a 6-7% increase in dye removal, as shown in Figure S1a. The effect of varying the concentration of polyphenols in the solution on the removal of dye is also shown in Figure S1a. For the sake of brevity, only data corresponding to chrysoidine are depicted, as those corresponding to methylene blue are very similar. Solutions containing the amount of tea extract necessary to achieve a polyphenonic concentration between 0 and 240 mg\(\cdot\)L\({}^{-1}\) were used. Looking at the results shown in the graph, this variable in principle has a very limited influence on dye-removing efficiency. For this reason, and given the initial approach to the experimental design, it was decided to start from an intermediate concentration between the two values, that is, to use a polyphenol concentration of 100 mg\(\cdot\)L\({}^{-1}\) as the central point of the design. The other two operating variables selected in this study were the initial concentrations of Fe(II) and Fe(III) cations. These cations have the dual function of catalyzing the decomposition of hydrogen peroxide into hydroxyl radicals (initial reaction of the Fenton and Fenton-like processes) and generating Fe nanoparticles through reduction by the polyphenols present in the green tea extract. Figure S1b shows the influence of both variables on the removal efficiency of dye. The initial concentrations of H\({}_{2}\)O\({}_{2}\) and polyphenols were kept constant at 2 \(\times\) 10\({}^{-3}\) M and 100 mg\(\cdot\)L\({}^{-1}\), respectively.