## 3 Results and Discussion
### Thermal Analysis  
The TG curve, together with the water (\(m/z=18\)) and carbon dioxide (\(m/z=44\)) release profiles, are shown in Figure 7, top. An initial weight loss starting from 40 \({}^{\circ}\)C and centered around 130 \({}^{\circ}\)C is attributable to the removal of humidity, with a significant mass decrease. As was foreseeable, this effect is coupled to an intense peak in the signal of water (blue line in Figure 7). A second weight loss effect centered around 225 \({}^{\circ}\)C and ending at approximately 320 \({}^{\circ}\)C can also be appreciated. Within this temperature range, another intense water release peak can be observed. According to the literature, this second effect can be due to the dehydration of goethite into hematite [82]. This water release overlaps with an increase in the signal corresponding to carbon dioxide (orange line in Figure 7). This signal reaches a maximum at approximately 365 \({}^{\circ}\)C and then decreases sharply. This behavior is coherent with an initial pyrolysis of the organic matter (residues of polyphenol extracts). At temperatures above 350 \({}^{\circ}\)C, no further water release is observed. Contrarily, the pyrolysis of the organic matter appears to take place in several stages, the last starting at around 650 \({}^{\circ}\)C and finishing at 780 \({}^{\circ}\)C, which is, approximately, only a few degrees before the thermal analysis ends. The results of the TPR analysis are shown in Figure 7, bottom. Firstly, it can be seen that the mass loss plot is quite different from and more complex than the one registered under an inert atmosphere. The same applies to the gas release profiles, particularly for water. Although the two initial water release effects are very similar in both cases, up to three peaks can be seen in the TPR plot. For the sake of clarity, the five water release peaks have been marked in Figure 7, bottom. As indicated above, peak number 1 (centered around 130 \({}^{\circ}\)C) corresponds in both cases to the loss of humidity. The same applies to peak number 2 (centered around 225 \({}^{\circ}\)C), which can be attributed to the dehydration of goethite, according to the following process:  
\[2~{}FeOOH\to Fe_{2}O_{3}+H_{2}O \tag{2}\]  
Figure 6: Energy-Dispersive X-ray spectrum and semi-quantitative chemical analysis of the nZVI sample. From 350 \({}^{\circ}\)C and higher, the differences between the water release profiles in the TG and TPR experiments become quite evident. Firstly, there is a sharp increase in the signal of water between 410 and 425 \({}^{\circ}\)C, labeled number 3 in the TPR plot. This effect can be associated with a first reduction stage from hematite, Fe\({}_{2}\)O\({}_{3}\), where all the iron is in oxidation state (III), to magnetite, Fe\({}_{3}\)O\({}_{4}\), an oxide with an inverse spinel structure that consists of two Fe(III) cations and one Fe(II) cation, so that the neutrality of the crystal is maintained. Hence, Fe\({}_{3}\)O\({}_{4}\) can be more accurately formulated as \(\left[\text{Fe}^{\text{III}}\right]_{t}\left[\text{Fe}^{\text{II}}\right]_{o }\text{O}_{4}\), where the \(t\) and \(o\) subscripts refer to the occupancy of tetrahedral and octahedral sites  
Figure 7: TG (**top**) and TPR (**bottom**) plots showing water (\(m/z=18\)) and carbon dioxide (\(m/z=44\)) release profiles under inert (N\({}_{2}\)) and reducing (N\({}_{2}\):H\({}_{2}\) 90:10) atmospheres. by FeII and/or FeIII cations within the cubic close packing (ccp) of oxide anions, thus giving rise to an occupancy factor, \(\lambda\), equal to 0.5.