# Heterogeneous Electro-Fenton-like Designs for the Disposal of 2-Phenylphenol from Water
## 1 Introduction
\[\mathrm{O_{2}\;(g)+2H^{+}+2e^{-}\to H_{2}O_{2}} \tag{1}\]  
\[\mathrm{Fe^{2+}+H_{2}O_{2}\to\prescript{}{}{}^{\bullet}OH+OH^{-}+Fe^{3+}} \tag{2}\]  
\[\mathrm{Fe^{3+}+e^{-}\to Fe^{2+}} \tag{3}\]  
More recent approaches for the use of different transition metals as Co, Mn, Cu,...in several advanced oxidation processes for the removal of organic pollutants have been reported [22; 23; 24; 25]. A clear example of the efficiency of these uses can be found in spent Lithium-ion batteries that contain Li, Ni, Mn, Al, and Co, that after a simple washing\(-\)recycling process was employed as an easily recoverable catalyst for peroxy-monosulfate (PMS) activation [26; 27] with levels removal of more than 96% in more than 5 cycles of use [27]. These ions could be used instead of iron for the hydrogen peroxide decomposition and the reaction that takes place is similar to the classical Fenton process (Equation (4)) [28]. In addition, in the electrochemical process, the \(\mathrm{Me^{(n+1)+}}\) could be regenerated to \(\mathrm{Me^{n+}}\) on the cathode, avoiding the generation of sludge and increasing the efficiency of this process [28; 29]. \[\mathrm{Me^{n+}+H_{2}O_{2}+H^{+}\to Me^{(n+1)+}+\prescript{}{}{}^{\bullet}OH+H_{2 }O} \tag{4}\]  
In recent years, the evaluation of various heterogeneous catalysts with application to electro-Fenton technology have been increasing number of studies. This increase is because it allows solving several drawbacks of the conventional process. In which is remarkable the requirements for catalyst amount optimization as well as the expansion of the working pH range or the operation in a flow system [30; 31]. In this sense, iron and/or transition metals have been used to modify or functionalised cathode materials in the heterogeneous electro-Fenton process. This electrocatalytic material presents a dual benefit because the modified material could be used as a cathode and as a catalyst during the electrochemical treatment process [32; 33; 34]. To this day, studies about 2-phenylphenol degradation by advanced oxidation processes are limited in the literature, in which no one has evaluated the efficiency of electro-Fenton processes. Several of them were focused on the elimination of several compounds of the phenylphenol family by ultraviolet light (UV) and peroxide as an oxidising agent, achieving a level of 89%, while in the other isomers the levels are in a range of 71-87% [35]. TiO\({}_{2}\) and B-doped TiO\({}_{2}\) catalysts were synthesised and several pesticides (2-methyl-4-chlorophenoxyoxyacetic acid, diuron, ter-butylazine and o-phenylphenol) were eliminated by ozonation and photocatalytic processes alone or in combination under simulated solar irradiation. Having been determined combination of processes led to faster mineralization of the pollutants mixture keeping around 75% after to perform three successive cycles [36]. Recently, selective degradation of o-phenylphenol was obtained by applying N-doped TiO\({}_{2}\) photocatalysts under simulated solar light irradiation showing a degradation efficiency of around 46% [24]. In this context, this work aimed to assess the degradation of 2-phenylphenol by electro-Fenton-like process, using as a catalyst transition metal ion (nickel and zinc). Based on the preliminary results, the synthesis of new catalysts and electrocataly (used as the cathode) is carried out to open the possibility of using this degradation process in a heterogeneous mode. According to our knowledge, this research is one of the first endeavours where nickel and zinc are used as heterogeneous catalysts and electrocatalyists in Fenton processes. In this situation, the present report intends to establish the use of these transition metals as an alternative to the conventional advanced oxidation processes for 2-phenylphenol removal, using different heterogeneous designs (catalysts and electrocataly).