## 3 Results and Discussion
### Degradation of Methylene Blue  
Prior to performing the electrofenton process, the possibility of adsorption of MB on RVC and RVC saturates with nanoparticles was verified. In none of the cases was an adsorption process observed. Mb, a cationic dye, is not adsorbed on nanoparticles because they are also positively charged at acidic pH levels. Figure 5 shows the removal efficiency values of the MB degradation process using the raw RVC electrode, RVC coated with magnetic material according to procedure A and B, called RVC/A and RVC/B. It can be seen that the raw RVC produces a progressive discolouration of the medium up to reach 100% in about 100 min, applying a current of 100 mA. On the contrary, the degradation efficiency is higher for the RVC/A electrode, occurring 40 min after the application of current. In the case of the RVC/B electrode, in the first 15 min the degradation curve practically coincides with that of the RVC/A electrode,  
Figure 4: XRD diffractogram of dip coated RVC. The observed (black dots), calculated (blue line), and difference between fitted and observed patterns (red line) are shown. Bragg positions of the Fd-3m: 227 space group are included. Figure 3: Mossbauer spectrum of coated RVC. (**a**) dip coated, (**b**) galvanostatically coated. but after this time there is a decrease in efficiency, and then after about 40 min it returns to increase. A possible explanation may be due to the low compaction of the particles on the RVC/B, as seen by SEM. At the beginning of the process, the H\({}_{2}\)O\({}_{2}\) generated degrades the MB on the outer surface of the RVC/B, but in this case, on the inner part it will be slower due to the lower surface coating. When the MB near the surface is degraded its concentration decreases and therefore its efficiency, but due to a concentration gradient the MB will diffuse from the interior to the outer surface, increasing again the concentration and therefore the degradation efficiency. Since all the RVC/B electrodes used showed similar behavior, RVC/A electrodes were selected to accurately compare the effect of magnetic nanoparticles on MB degradation with uncoated RVC electrode. Figure 6 shows the discolouration of 20 mg L\({}^{-1}\) MB solution using RVC dip coated at different applied currents. As can be seen to obtain a total degradation in a short time, currents above 100 mA must be used. These high current requirements can be explained by the methodology used. In general, the electrofenton process is limited by the low solubility of O\({}_{2}\) in the aqueous phase, as we have already commented. Furthermore, the efficiency of O\({}_{2}\) utilization is extremely low (<0.1%), and therefore a significant energy loss occurs, especially when the process is scaled. As the conditions in which we work are open cell and without forced oxygen passage, it is necessary to increase the applied current to be able to obtain a sufficient amount of peroxide. If the current is decreased the H\({}_{2}\)O\({}_{2}\) production rate slows down and therefore the removal efficiency. In addition, the diffusive effects discussed above are observed. The influence of temperature on degradation efficiency has been studied by applying an intermediate current of 50 mA (Figure 7). We can see how increasing the temperature from 25 to 50 \({}^{\circ}\)C produces an increase in removal efficiency. This increase may be due to an increase in ionic activity as well as a reduction in charge transfer resistance. However, when increasing to 70 \({}^{\circ}\)C there is a decrease in the efficiency of the process, which could be explained by a decrease in the solubility of oxygen in water [26] and therefore a lower production of H\({}_{2}\)O\({}_{2}\), or by a decrease in H\({}_{2}\)O\({}_{2}\) stability, both factors combined can produce a decrease in the amount of peroxide accessible for the degradation of MB. Figure 5: Efficiency in the discolouration of a 20 mg L\({}^{-1}\) MB solution by electrofenton, with applied currents of 100 mA.