## 3 Results and Discussion
### Chemical Characterization and Crystal Structure of the nZVI Samples  
X-ray diffraction analysis was carried out to study the samples presented in this section. The XRD pattern is shown in Figure 5.  
The XRD patterns are coherent with the presence of crystalline Fe\({}^{0}\). Thus, a strong diffraction peak at 20 = 44.85\({}^{\circ}\) for the lattice plane of (1 1 0), and a second weaker peak at 20 = 65.2\({}^{\circ}\) for the lattice plane of (2 0 0), can be easily appreciated. This confirms the generation of zero-valent iron nanoparticles because of the reduction of Fe\({}^{2+}\) and Fe\({}^{3+}\) by the polyphenols that are present in the green tea leaf extract. In addition, extra reflections corresponding to the XRD patterns of goethite (FeOOH) and hematite (Fe\({}_{2}\)O\({}_{3}\)) have been recorded. This is in good agreement with the fact that Fe\({}^{0}\) can be at least partially re-oxidized by the aqueous media in which the synthesis was carried out. This is following other results previously published in the literature [55; 74; 75; 76].  
Other authors report similar XRD patterns but propose that the nanoparticles may consist of a Fe\({}^{0}\) core that would be surrounded by iron oxide in a higher oxidation state (Fe\({}_{2}\)O\({}_{3}\)) or by iron oxyhydroxic (FeOOH) [77; 78; 71].  
From the XRD pattern, a semi-quantitative analysis of the different crystalline phases in the sample can be obtained. The results of this analysis, together with the main characteristics of such phases, are summarized in Table 1.  
Figure 4: SEM (**left**) and STEM (**right**) images of the nZVI under different magnifications.  
The Energy Dispersive X-ray (EDX) spectrum of the nZVI sample is shown in Figure 6. The semi-quantitative chemical analysis of the sample is shown as an insert in the figure. From the EDX spectrum, it can be concluded that nZVI is constituted by iron; however, the presence of relatively large amounts of other elements, mainly carbon and oxygen that are present in the carbonyl/carboxyl and phenolic moieties of polyphenols, reveals the presence of organic matter in the sample, corresponding to the dry residue of the polyphenol extract. The presence of small amounts of other oligo-elements such as phosphorus, chlorine, and sulfur, all of them essential for vegetal life, corroborates this hypothesis. Hence, it can be concluded that the surface of the nZVI sample under study appears to be coated with a layer of organic compounds initially present in the green tea leaf extract, mainly polyphenols. As a result, the elemental analysis reveals a relatively low iron content (6.12 wt% in mass, 1.67 in mol %) due to the presence of iron hidden beneath carbon and oxygen, which cannot be measured using EDS analysis [55; 79; 80; 81]. The latter, however, does not hinder the ability of Fe to act as a catalyst in heterogeneous Fenton reactions, as the system is kept under constant stirring and the residual polyphenols that might deposit onto the nZVI particles are immediately removed. Only when the nanoparticles are filtered and dried does the remnant organic matter cover the nZVI.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \
\begin{tabular}{c} **Crystal** \\ **Phase** \\ \end{tabular} & **Formula** & **Percent** & **Crystal System** &
\begin{tabular}{c} **Space** \\ **Group** \\ \end{tabular} & **a** & **b** & **c** & **Alpha** & **Beta** & **Gamma** \\ \hline \(\alpha\)-Iron & \(\alpha\)-Fe & 63.2 & Cubic & Im-3m (229) & 2.8608 & 2.8608 & 2.8608 & 90 & 90 & 90 \\ Goethite & FeOH & 23.9 & Orthorhombic & Pbnm (62) & 4.6080 & 9.9560 & 3.0215 & 90 & 90 & 90 \\ Hematite & Fe\({}_{2}\)O\({}_{3}\) & 12.9 & Rhombohedral & R-3c (167) & 5.4200 & 5.4200 & 5.4200 & 55.12 & 55.12 & 55.12 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Semi-quantitative analysis and crystal characteristics of the nZVI sample.  
Figure 5: XRD pattern of the nZVI sample.