# Heterogeneous Electro-Fenton-like Designs for the Disposal of 2-Phenylphenol from Water
## 2 Materials and Methods
### Heterogeneous Nickel Catalyst  
Once established nickel as the best electro-Fenton-like catalyst, several heterogeneous catalysts were synthesised in order to allow the operation in a flow system process. As was described in material and methods, the nickel was included in different materials such as perlite, biochar or nanofibers by hydrothermal and adsorption treatments (Figure 3). Perlite was selected because it has been proved to be a good material for the fixation of metals such as iron by hydrothermal treatment [46]. In addition, this material has a floating characteristic that allows the catalyst to be on the liquid surface so that it can be recovered very easily [47]. Both of these characteristics are considered suitable for use in the design of a heterogeneous system electro-Fenton treatment. Biochar and nanofiber are materials with higher surface area and porosity [33; 48], that allow the retention of metal ions by adsorption that can be used as a catalyst. The morphology and structural characteristics were determined by SEM, which confirmed the high porosity of these materials that favours the contact of the catalyst with the hydrogen peroxide increasing the removal rate [37]. Attending at Figure 3, it can be observed that the perlite presents a high amount of macropores for the fixation of the metal as suggested previously; however, biochar and carbon nanofibers have a superior surface area (45 and 54 m\({}^{2}\)/g) with small-sized pores. Figure 4 shows the 2-phenylphenol removal profiles using the different catalysts tested in this study. It is clear that with catalyst supported in nanofiber and biochar the removal levels achieved after 5 min are quite similar (70-75%) which could be explained by the adsorption of the pollutant in the carbonaceous materials (biochar and nanofiber). To elucidate this hypothesis, the adsorption of 2-phenylphenol in these materials was evaluated and compared with the electro-Fenton process. It was detected a high adsorption rate on carbonaceous materials, with levels around 60% after 5 min, achieving the maximum removal (76%) after 60 min. These values are lower than obtained when the electro-Fenton process was evaluated. Desorption process was accomplished in order to elucidate if the 2-phenylphenol was adsorbed upon the carbonous materials after the adsorption and electro-Fenton process. Desorption of 2-phenylphenol from nickel catalysts with acetonitrile did not show a residual concentration, however from nanofiber and biochar used in the adsorption test was possible the total recovery of the adsorbed pollutant in the acetonitrile solution. Figure 4: Profiles of 2-phenylphenol removal (%) in different heterogeneous electro-Fenton-like and homogeneous electro-Fenton processes. These results confirm that the electro-Fenton treatment with this kind of catalyst is an efficient process for the disposal of pollutants that takes place in two steps: adsorption and oxidation. Even when adsorption has an important role, the adsorbed 2-phenylphenol was finally oxidized, resulting in a high removal from the bulk solution. This behaviour is in accordance with other previous studies in which the combination of adsorption and oxidation process had been detected. Thus, the efficiency of a hybrid process such as adsorption-Fenton oxidation using granular activated carbon showed, that this two-step process is an excellent combination for the effective removal of bisphenol A [49]. This synergistic interaction of adsorption and photocatalysis process has been considered in several studies as one of the most efficient techniques for environmental purification of wastewater [50; 51]. Recently, doped porous carbon prepared by in-situ doping after carbonization of nitrogen-containing Metal-organic framework materials showed high efficiency for removal of tetracycline hydrochloride as a catalyst of PMS advanced oxidation process. It was demonstrated that the high degradation level achieved is closely related to the adsorption and catalytic performance of the catalyst [52].