Preparation of hydroxyl radicals

The combination of Fe2+ and H2O2 is called Fenton reagent in the process. It can effectively oxidize and degrade organic pollutants in wastewater, and its essence is that H2O2 produces -OH with high reactivity under the catalytic action of Fe2+.At present, the Fenton method is mainly used to produce -OH through photo-radiation, catalysts, and electrochemistry.The use of photocatalysts or photo-radiation to produce -OH suffers from the problems of low efficiency of H2O2 and solar energy utilization. In contrast, the electro-Fenton method is the continuous generation of both H2O2 and Fe2+ by electrochemistry [7], which has the advantages of high H2O2 utilization, low cost and fast reaction rate than the general chemical Fenton reagent. Therefore, the generation of -OH by electro-Fenton method will be one of the main ways.

The application of electric Fenton method to generate -OH to treat organic wastewater is mostly based on flat iron as anode, porous carbon electrode as cathode, and oxygen or air is passed to the cathode. When energized, electrochemical reactions of the same electrochemical equivalents are carried out at the cathode and anode, generating the same amount of Fe2+ and H2O2, respectively, in the same amount of time, thus enabling the subsequent chemical reaction to generate the Fenton reagent [8].

The pH of the solution has a great influence on the reaction of oxygen cathodic reduction to obtain H2O2 [9]. It has been shown that the pH of the solution not only has an effect on the cathodic reaction potential and tank voltage, but will also determine the current efficiency of the generation of H2O2, which in turn affects the efficiency of the subsequent generation of -OH and the degradation and decolorization reaction with organic pollutants.

Since the mid-1980s, research on the mechanism of the electro-Fenton method and its application in organic wastewater has been widely carried out at home and abroad.Hsiao et al[10] studied the oxidation of phenol and chlorobenzene by using graphite as a cathode, and the results showed that the oxidation of phenol and chlorobenzene by this method was more thorough than that of the photo-Fenton method. Zheng Xi [11] et al. used soluble iron as anode, porous graphite electrode as cathode, and Na2SO4 as supporting electrolyte to generate Fenton reagent at the electrolysis site, which could effectively inhibit the occurrence of cathodic and anodic side reactions at a low current density (10 mA/cm2), and the concentration of -OH generated was sufficient to efficiently degrade the dye wastewater, and the decolourization rate was up to 100%, and the removal rate of CODCr was up to 80%. In addition, the electric Fenton method combined with other methods to treat wastewater, many researchers have studied its feasibility [12], and achieved some results. Brillas et al [13] used Pt as anode and oxygenated carbon - polytetrachloroethylene as cathode, respectively, to degrade 2,4-D (dichlorophenoxyacetic acid), and the concentration of 2,4-D was as high as 90% when the mineralization degree, and the concentration of -OH was as high as 90%, and the removal of CODCr reached 80%. The mineralization degree of 2,4-D was as high as 90% at low concentration, and 2,4-D could be completely mineralized if combined with photo-Fenton method.Kusvuran et al [14] also took RR120 organic dye wastewater as the research object, and compared and analyzed the treatment effect of electro-Fenton method and other methods, and the results showed that, the degradation effect of wet air oxidation method, photoelectric Fenton method and UV/TiO2 was more satisfactory, and the degradation effect of electro-Fenton method was more satisfactory, and the degradation effect of electro-Fenton method was more satisfactory, and the degradation effect of electro-Fenton method was more satisfactory. The results showed that the degradation effect of wet air oxidation, photoelectric Fenton method, UV/TiO2 was more ideal, followed by electric Fenton method. The anode under the action of an applied electric field can directly or indirectly produce -OH with strong oxidizing activity [15]. This method is characterized by basically no secondary pollution and meets the requirements of environmental protection. For a long time, due to the limitation of electrode materials, the current efficiency of this method for degradation and treatment of organic pollutants is low and the energy consumption is large, so it is less directly applied to the actual wastewater treatment, and the research of anode materials naturally becomes the main research direction.After the 1980s, many researchers at home and abroad start from the development of electrode materials with high catalytic activity, and carry out the research on the mechanism of electrocatalytic generation of -OH and the factors affecting the degradation efficiency, and make a big breakthrough. research, made a big breakthrough, and began to be used in the treatment of special difficult biodegradable organic wastewater. For example, Song Weifeng [16] and others proposed the oxidative degradation of organic matter by two-dimensional stabilized anode (DSA for short) made of metal oxides, and achieved certain results. However, due to the small surface area of the traditional two-dimensional flat plate electrode, the mass transfer problem still cannot be solved fundamentally, the current efficiency is low, and the energy consumption is high, so it fails to be commonly used in practice. In contrast, three-dimensional electrodes have been favored by many researchers due to their increased surface-to-body ratio and better mass transfer effect, and have achieved certain results. He Chun et al [17] utilized the new technology of three-dimensional electrode electrochemical reactor to effectively remove aniline from organic wastewater. Some researchers used cheap stainless steel as the electrode material, and studied the treatment effect of two-dimensional electrode method and three-dimensional electrode method and its mechanism. Xiong Rongchun et al[18] used this method to treat rhodamine B dye wastewater, and the experimental results showed that the stainless steel electrode material had a better electrocatalytic degradation of organic pollutants, especially when the three-dimensional electrode method was used, and it could achieve an excellent water treatment effect in a shorter period of time. The results of the colorimetric method found that the stainless steel electrode material produced -OH with strong oxidizing ability during the electrocatalytic degradation process.Cui Yanping et al [19] also investigated the electrochemical oxidation process under the conditions of filler particles and ventilation air in a compound polarity three-dimensional electrolytic cell, which utilized direct oxidation at the anode, indirect oxidation of anodic -OH and cathodic generation of H2O2, thus, under the circumstance of lower energy consumption , the utilization of filled particles was sufficiently improved to achieve a better degradation effect.Duverneuil et al[20] used Ti deposited with SnO2 as an anode for the degradation study of organic wastewater, and satisfactory removal results were obtained.

However, there are still some problems in the industrial application of electrolytic oxidation, such as the current efficiency is still low, the energy consumption is large, the efficiency of the electrocatalytic degradation reactor is low, and the mechanism of electrochemical catalytic degradation of organic pollutants needs to be further explored [21]. Strengthening the research on the above problems is the direction of the future development of this method. Since certain semiconductor materials have good photochemical properties and lively electrochemical behavior, the research application of electrodes made of semiconductor materials in organic wastewater has attracted the attention of many researchers in recent years [22].

Semiconductor catalytic materials in the electric field has a "hole" effect [23], that is, the semiconductor is in a certain strength of the electric field, the valence band electrons will cross the forbidden band into the conduction band, at the same time in the valence band to form an electrically excited hole, the hole has a strong ability to capture electrons, can capture the semiconductor particles on the surface of the organic matter or solvent in the electron redox reaction. The electrons in the organic matter or solvent on the surface of the semiconductor particles can be captured and the redox reaction occurs. In the electrocatalytic oxidation reaction occurring in aqueous solution, water molecules lose electrons on the semiconductor surface to generate strong oxidizing -OH, and at the same time, semiconductor catalysts and electrodes generated by H2O2 and other active oxidizing substances also play a synergistic role, so there are a variety of pathways to generate strong oxidizing factors in the electrocatalytic reaction system, which effectively improves the efficiency of catalytic degradation. In the semiconductor electrocatalytic reaction, both the voltage and current intensity should reach a certain value. Generally, with the increase of the applied voltage, the rate of -OH production in the system increases, and the removal efficiency of organic matter increases [24]. However, it has also been found that when the applied voltage reaches a certain value, further increase in voltage inhibits the generation of free radicals and reduces the catalytic efficiency [25].

Semiconductor electrocatalysis in organic wastewater treatment is mainly studied in the doped semiconductor electrodes and nano semiconductor material electrodes as anode to generate -OH to treat organic wastewater. Dong Hai et al [26] studied the electrocatalytic degradation reaction of phenol-containing wastewater using semiconductor electrodes made of antimony-doped SnO2 powder, and the degradation of phenol reached 90%. Under the irradiation of ultraviolet light and other irradiation, and the role of the external electric field TiO2 semiconductor will also exist within the "hole" effect, the photoelectric combination of the method of generating -OH is also known as photoelectrocatalytic method. TiO2 photoelectric combination of the effect of the conduction band electrons not only with the oxidation of the valence band hole process from the spatial location of the separation (compared with the semiconductor), the photoelectric combination of the process of oxidation of the valence band electron from the spatial location of the separation (with the semiconductor). Compared with the semiconductor particles), the simple complexation is obviously reduced, as a result, the generation efficiency of -OH on the semiconductor surface is greatly increased and the re-reduction of oxidation intermediates on the cathode is prevented, and the conduction band electrons can be led to the cathode to reduce the H+ in the water, so that there is no need to bulge the O2 as an electron capturing agent into the system [27].

Because of the above advantages, the photoelectrocatalytic technology in organic wastewater has been rapidly developed. Qing Dai et al [28] established an electrically-assisted photocatalytic system using TiO2 thin-film electrodes as working electrodes, and photoelectrocatalytic studies were carried out using chlorophenol-containing (e.g., 4-chlorophenol and 2,4,6-trichlorophenol) wastewater as the degrading objects. Cheng et al[29] used three-dimensional electrode photoelectrocatalytic degradation to treat methylene orchid wastewater, and the study showed that the decolorization rate and COD removal were 95% and 87%, respectively.Waldne et al[30] carried out a study on the degradation of 4-chlorophenol by TiO2 semiconductor photoelectrocatalysis, and achieved a better treatment effect.

At present, the research work of photoelectrochemical reaction is still mostly confined to the laboratory stage, and there are not many reports on the application of nano TiO2 semiconductor electrode photoelectrocatalytic method for the treatment of large-scale industrial organic wastewater, which is mainly due to the low reuse rate of TiO2 semiconductors and the decrease of the photocatalytic efficiency of photoelectrocatalytic reactors. Therefore, the preparation of TiO2 into highly efficient and reusable electrodes through modification and modification, such as precious metal deposition, doping metal ions, composite semiconductors, and surface photosensitizers on the surface of TiO2 material [31], has become a hot spot in the study of photoelectrocatalytic degradation of organic pollutants using TiO2 as semiconductor electrode. In addition, the practical application of this technology inevitably involves the determination of the structure and type of reactor, and the development of an industrialized photocatalytic reactor with high efficiency and reuse and low cost will also be the key to the industrialized application of nanoTiO2.