*3.3.1. Electrochemistry*

Currently, efforts have been made at developing more effective technologies to remove per‐ sistent organic pollutants. Advanced oxidation processes are based on the in situ production of reactive hydroxyl radicals (OH**·**) which non‐selectively react with most organics, being able to degrade even resistant compounds. Although, OH**·** radicals are extremely reactive and can‐ not exist for a longtime, they can be used to decompose almost all organic to inorganic com‐ pounds [89, 90]. Combination process such as ozone, ultraviolet (UV) light, a semiconductor photocatalyst, hydrogen peroxide, ultrasound, Fenton reagent, photo‐Fenton are widely studied to generate hydroxyl radicals (OH**·**). Recently, electrochemical advanced oxidation processes (EAOPs) are a promising derivative strategy that comes from AOPs. The easiest EAOPs method is the anodic oxidation (AO), where the organics can be directly oxidized at the anode surface by electron transfer and/or indirectly oxidized by OH**·** barely physi‐ sorbed at the anode surface whit the presence of agents at the bulk solution such as oxidizing reagents, O3 , chlorine species, persulfates or H2 O2 . On the other hand, when AO accomplish with cathodic electrogeneration of H2 O2 , the process is a cathodic oxidation. When H2 O2 is electrochemically produced in the presence of Fe2+ at the bulk of reactions, as well as the OH**·**, it brings about the electro‐Fenton process (EF). The phenomena originate a variety of tech‐ niques, for example, peroxi‐coagulation (PC), Fered‐Fenton, electrochemical peroxidation and sonoelectro‐Fenton, or combine systems which include biological, chemical coagulation, electrocoagulation (EC) and membrane processes [90].

Phenol belongs to the recalcitrant pollutants commonly treated by conventional physico‐ chemical and biological methods, so advanced oxidation (AO) represents an actual process for treatment of wastewater containing toxic persistent organic compounds. Pimentel et al. [91] applied a variant of advanced oxidation techniques to remove phenolic pollutants. They stud‐ ied the oxidative degradation of aqueous phenol solutions in acidic medium by electro‐Fenton technique using a carbon felt cathode and platinum anode in order to evaluate the mineraliza‐ tion efficiency, results evidenced that pH 3 enhance hydrogen peroxide electrochemical pro‐ duction, the most effective catalysts was ferrous iron ion at optimal concentration of 0.1 mM. Phenol oxidation by hydroxyl radical follows a pseudo‐first‐order kinetic with a rate constant of 0.037 min‐1 . Additionally, phenol hydroxylation generate maleic, fumaric, succinic and gly‐ colic acids in the beginning of the reaction; benzoquinone, catechol and hydroquinone as intermediate and oxalic and formic acids as final products. The total mineralization of phenol and its reactions intermediates put in context the effectiveness of the electro‐Fenton process. If the process is combined, it could arises higher efficiencies as demonstrated Wang et al. [92], when combining electrocatalytic process and membrane bioreactor (MEBR), increases 11% the quality of the phenol removal, compared to the conventional and sum of the two individ‐ ual processes; as result of the synergetic enhancement effect in one reactor. Also it was found that one of the degradation products is the benzoquinone (2,6‐di.*tert*‐butyl‐*p*‐benzoquinone).

2,4‐dichlorophenol degradation using as photocatalyst a GO/Ag<sup>3</sup>

important challenges in the photocatalysis process.

362 Phenolic Compounds - Natural Sources, Importance and Applications

**3.3. Others**

*3.3.1. Electrochemistry*

important to note that in both investigations was used visible light which is one of the most

**Figure 6.** COD removal percentage obtained at the end of photolysis and photocatalysis tests using as catalysts (graphite oxide) GrO and GO on the 4‐CP degradation. Adapted from Bustos‐Ramírez et al. [78]; BioMed Central. 2015.

In general, the revised investigations revealed that the combination of graphene materials with different semiconductors particles improve the degradation efficiency of the different phenolic compounds from water with respect to the individual particles. Besides, the graphene oxide showed an important photocatalytic activity capable of degrading the phenolic compounds.

Currently, efforts have been made at developing more effective technologies to remove per‐ sistent organic pollutants. Advanced oxidation processes are based on the in situ production of reactive hydroxyl radicals (OH**·**) which non‐selectively react with most organics, being able to degrade even resistant compounds. Although, OH**·** radicals are extremely reactive and can‐ not exist for a longtime, they can be used to decompose almost all organic to inorganic com‐ pounds [89, 90]. Combination process such as ozone, ultraviolet (UV) light, a semiconductor photocatalyst, hydrogen peroxide, ultrasound, Fenton reagent, photo‐Fenton are widely studied to generate hydroxyl radicals (OH**·**). Recently, electrochemical advanced oxidation processes (EAOPs) are a promising derivative strategy that comes from AOPs. The easiest EAOPs method is the anodic oxidation (AO), where the organics can be directly oxidized

PO<sup>4</sup>

(5 wt%) composite. It is

Following with the electrochemical tendency process, Vasudevan [93] studied the peroxi‐ electrocoagulation method using mild steel as anode and graphite as cathode, obtaining 92% of removal from an initial phenol concentration of 2.5 mg/L and pH 2. The electro‐coagula‐ tion (oxidation of sacrificial anode), amalgamates advantages from the separate procedures. Coagulants introduced without corresponding sulfate or chloride ions are more efficient to remove contaminants from waste, mainly when eliminate competitive anions and use a highly pure coagulant, it can be obtained lower metals residuals and less sludge as by‐prod‐ ucts if used metal salts. Moreover, when electrochemical reactors operate at high cell poten‐ tial under acidic pH, the anodic process occurs in the potential region of water discharge and consequently hydroxyl radicals (OH**·**) are produced. This confirms that, ferrous ion gener‐ ated in electrocoagulation function as coagulation materials and catalytically creates OH**·** radicals according to the conditions. Therefore, EAOPs can be even more effective than their chemical analogous, showing higher removal rates and greater reductions in organic toxic wastewater. EAOPs offer, the availability of higher amounts of H<sup>2</sup> O2 at the reaction begin‐ ning in the chemical processes; mainly in the presence of aromatic compounds; which is the case of phenol and phenolic molecules, demonstrated to induce faster initial removal rates for organic pollutants. Additionally, while increases the efficiency of the process the waste by‐products diminish during the oxidation reaction to remove phenol compounds. Thus, EAOPs challenge needs to consider the implementation of high H2 O2 quantities since the reaction initiation, as well as take into account aspects related to the investment costs, design of electrochemical cell; meaning less expensive hardware and electrodes materials and more versatile systems. In case of the light‐assisted source, also should be considered, the UV lamps or photoreactors for natural sunlight capture. Even more, the operational costs include electrical energy for the electrochemical cell, plant operation, reagents and mainte‐ nance [90].

Nowadays, there is a growing interest to establish a great deal of attention to develop new strategies based on nanomaterials in conjunction with single and/or hybrid AOPs to remove or recover phenol species. One of the advantages of nanomaterial is the high surface area, where the volume/mass ratio will significantly improve the adsorption properties. Some nanomaterials studied are semiconductors, nanoclays, nanocatalyst, nanoclusters, nanorods, nanocomposites; for example, TiO<sup>2</sup> , palladium, Fe3 O2 , Cerium oxide and magnetic chitosan, along or combined, CoxFe3 ‐xO<sup>4</sup> , CoFe2 O4 magnetic nanoparticles, BiAg<sup>x</sup> Oy . From this point, can be synthesized nanoparticles, nanomembranes and nanopowders able to apply on the AOP technology [94].
