## 3 Results and Discussion
### Texture and Morphology of the nZVI Samples  
The N\({}_{2}\) adsorption isotherm registered at 77 K, together with the pore size distribution plot of the nZVI sample, are depicted in Figure 3. From the N\({}_{2}\) adsorption isotherm (Figure 3, top), it can be concluded that the isotherm belongs to Type IV of the BDDT  
Figure 2: HPLC-DAD profile of green tea leaf waste (**top**) and structures of the primary polyphenols identified (**bottom**). **1. ** Catechin; **2. ** Gallocatedin; **3. ** Epicatechin; **4. ** Epigallocatechin-3,5-digallate; **5. ** Kaempfer-rutinoside. Retention times are shown in **blue**. classification. This type of isotherm is characterized by a pronounced hysteresis loop. This is typical for mesoporous solids. It has been established that the appearance of the hysteresis cycle is because the mesopore filling process is controlled by the capillary condensation phenomenon and percolation properties of the solid. Figure 3: N\({}_{2}\) adsorption isotherm (**top**) and pore size distribution (**bottom**) of the nZVI sample. In particular, the hysteresis loop that appears in the isotherm corresponds to the H\({}_{3}\) type. This suggests that nZVI is a material with relatively high porosity development and with a significant presence of mesopores, as can be inferred from the absence of a pronounced plateau in the isotherm and the existence of a final section with a high slope. From the data of the adsorption isotherm, it is possible to calculate the volume of the micro- and mesopores (V\({}_{\text{mi}}\) and V\({}_{\text{me}}\), respectively). V\({}_{\text{mi}}\) = 0.027 cm\({}^{3}\)/g and V\({}_{\text{me}}\) = 0.238 cm\({}^{3}\)/g were obtained as described in Section 2.5.1. It is therefore confirmed that the sample has a marked mesoporous character. By applying the Dubinin-Radushkevich (DR) equation, the micropore volume (W\({}_{0}\)) may also be calculated. For the nZVI studied here, W\({}_{0}\) equals 0.031 cm\({}^{3}\)/g, a value that is in good agreement with the one calculated by directly reading the adsorption isotherm. The specific surface area (S\({}_{\text{BET}}\)) of the sample was calculated by applying the well-known Brunauer, Emmett, and Teller (BET) equation. A value of 130 m\({}^{2}\)/g was obtained. The DR model also makes it possible to calculate the micropore surface area, which provides information on the contribution of micropores to the specific surface area of the sample. In this case, the micropore surface area was 102 m\({}^{2}\)/g. Considering the noticeable difference between this value and the one obtained by applying the BET equation, as well as the large value of V\({}_{\text{me}}\) indicated above, it can be concluded that, for the nZVI sample under study, the contribution of mesopores to surface area and porosity is remarkable. Although most of the pristine nZVI previously reported in the literature exhibit values of S\({}_{\text{BET}}\) that range from 5 up to 61 m\({}^{2}\)/g [69], some authors have reported values of specific surface areas that reach 143 m\({}^{2}\)/g [70], i.e., similar or slightly larger than the ones reported here. Nevertheless, it is worth noting that these latter values were synthesized through the reduction of Fe\({}^{3+}\) with a conventional reductant such as sodium borohydride, instead of using a greener approach as in this work. Figure 3, bottom, shows the cumulative pore volume (red) and the pore size distribution (blue) in the region of the narrow and mid-size mesopores (3-32 nm), calculated by applying the DFT method. A wide variety of mesopore sizes can be observed in this plot. The total mesopore volume calculated as the area under the curve (AUC) of the cumulative pore volume (red line) equals 0.221 cm\({}^{3}\)/g, which is in very good agreement with that obtained by reading the adsorption isotherm (i.e., 0.238 cm\({}^{3}\)/g). Scanning electron microscopy (SEM) was used to analyze the particle morphology of the nZVI sample. Figure 4 shows the SEM images of both samples.