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Introduction to advanced oxidation

The advanced oxidation process is a toxic pollutant treatment technology that began to be formed in the 1980s. It is characterized by the generation of hydroxyl radicals by reaction. The radicals are highly oxidizing and can effectively decompose organic pollutants through free radical reactions. Even completely converted into harmless inorganic substances such as carbon dioxide and water. Because the advanced oxidation process has the advantages of strong oxidizing property and easy control of operating conditions, it has attracted the attention of all countries in the world, and has carried out research and development work in this direction. Advanced oxidation technology is mainly divided into Fenton oxidation method, photocatalytic oxidation method, ozone oxidation method, ultrasonic oxidation method, wet oxidation method and supercritical water oxidation method.

First, several advanced oxidation technologies

1.Fenton oxidation
The oxidation technology system consisting of hydrogen peroxide and the catalyst Fe2+ is called Fenton reagent. It is a chemical oxidation water treatment technology invented by H.J.H.Fenton more than 100 years ago without the need for high temperature and high pressure and simple process. Recent studies have shown that the oxidation mechanism of Fenton is caused by the highly reactive hydroxyl radicals generated by the catalytic decomposition of hydrogen peroxide under acidic conditions. Under the action of Fe2+ catalyst, H2O2 can produce two kinds of active hydroxyl radicals, which initiate and propagate free radical chain reaction and accelerate the oxidation of organic matter and reducing substances.

Fenton oxidation is generally carried out at a pH of 2 to 5. The advantage of this method is that the decomposition rate of hydrogen peroxide is fast and the oxidation rate is also high. However, there are also many problems in this method. Due to the large concentration of Fe2+ in the system, the treated water may be colored; the reaction of Fe2+ with hydrogen peroxide reduces the utilization of hydrogen peroxide and its pH limit, thus affecting to some extent. The promotion and application of this method.
In recent years, some people have studied the introduction of ultraviolet light (UV), oxygen, etc. into Fenton reagent, which enhances the oxidizing ability of Fenton reagent and saves the amount of hydrogen peroxide. Since the decomposition mechanism of hydrogen peroxide is very similar to that of Fenton and Fenton reagents, all of which produce ·OH, various improved Fenton reagents are called Fenton-like reagents. There are mainly H2O2+UV systems, H2O2+UV+ Fe2+ systems, and Fenton systems that introduce oxygen.
The application of Fenton reagent and Fenton-like reagent in wastewater treatment can be divided into two aspects: one is to oxidize organic wastewater separately as a treatment method; the other is to be combined with other methods, such as coagulation sedimentation method, activated carbon method, etc. Use, can achieve good results. The catalyst of Fenton method is difficult to separate and reuse. The reaction pH is low, and a large amount of iron-containing sludge is formed. The large amount of Fe2+ in the effluent will cause secondary pollution, which increases the difficulty and cost of subsequent treatment.

In recent years, scholars at home and abroad have begun to study the immobilization of Fe2+ on ion exchange membranes, ion exchange resins, alumina, molecular sieves, bentonite, clay, etc., or to replace Fe2+ with iron oxides and composites to reduce the elution of Fe2+. Improve the recovery of the catalyst and broaden the appropriate range of pH. Daud et al. catalyzed the degradation of reactive black 5 (RB5) by impregnation of Fe3+ on kaolinite. The decolorization rate of RB5 was 99% within 150 min. Youngmin et al. chelated Fe(II) with chitosan (CS) and glutaraldehyde (GLA) crosslinks to form Fe(II)-CS/GLA catalysts, which catalytically degrade trichloroethylene under neutral conditions. TCE), the degradation rate of TCE reached 95% after 5h, while the traditional Fenton method did not significantly degrade TCE due to iron precipitation under neutral conditions. Plata et al. used goethite as a catalyst for photo-Fenton degradation of 2-chlorophenol, and discussed the effect of catalyst dosage and light intensity on the treatment effect. The effluent contained only a small amount of iron ions.

2. Ozone oxidation method

Ozone is an excellent strong oxidant and has a good effect in disinfecting, decolorizing, deodorizing, removing organic matter and COD. The ozone oxidation method degrades organic matter quickly, has mild conditions, does not cause secondary pollution, and is widely used in water treatment. Ozone treatment of sewage as a general manifestation, one is direct oxidation of ozone, and the other is free radical oxidation by the formation of hydroxyl radicals.
The ozone oxidation method alone is easy to damage due to the ozone generator, consumes a large amount of energy, is expensive to process, and has selectivity for ozone oxidation reaction, and has relatively poor oxidation effect on some halogenated hydrocarbons and pesticides. To this end, in recent years, related combination technologies aimed at improving the efficiency of ozone oxidation have been developed, in which a combination of UV/O3, H2O2/O3, UV/H2O2/O3 can not only increase the oxidation rate and efficiency, but also oxidize O3 alone. Organic matter that is difficult to oxidatively degrade.
Hu Junsheng et al. compared the effects of H2O2/O3 and O3 treatment of dye wastewater. Wei Dongyang et al. compared the effects of UV/O3 and O3 on the degradation of hexachlorobenzene. The results show that the combination technique can significantly increase the oxidation rate and treatment effect. Reduce reaction time and reduce consumption O3. Catalytic ozone oxidation is also receiving increasing attention from scholars at home and abroad. Catalysts used in catalytic ozonation are mainly transition metal oxides and activated carbon. Among them, activated carbon is low in price, strong in adsorption, high in catalytic activity and good in stability, and is widely used in catalytic ozone oxidation systems.

3. Ultrasonic oxidation

Ultrasonic oxidation method uses ultrasonic radiation solution with a frequency range of 16kHz-1MHz to make ultrasonic cavitation of the solution, form local high temperature and high pressure in solution and generate local high concentration oxide·OH and H2O2 can form supercritical water, and rapidly degrade. Organic Pollutants. Ultrasonic oxidation combines the characteristics of various water treatment technologies such as free radical oxidation, incineration and supercritical water oxidation. It has mild degradation conditions, high efficiency, wide application range and no secondary pollution. It is a development potential and application prospect. Clean water treatment technology.
Ultrasonic degradation of organic matter is mainly caused by cavitation effects, and organic matter is carried out by two processes: pyrolysis or free radical reaction. Under the local high temperature and high pressure environment generated by ultrasonic cavitation, water is decomposed to generate ·OH radicals, and the air (N2 and O2) dissolved in the solution can also undergo free radical cracking reaction to generate free radicals. These free radicals also further trigger the cleavage of organic molecules, the transfer of free radicals, and the redox reaction.
Separate ultrasonic oxidation technology can remove some organic pollutants in water, but its single treatment cost is high, and the treatment of hydrophilic and non-volatile organic substances is poor, and the removal of TOC is not complete. Therefore, it is often treated with other advanced oxidation. Technology is combined to reduce processing costs and improve processing results. Moreover, ultrasonic radiation is combined with other catalytic techniques, and the intense turbulence caused by ultrasound can enhance the solid-liquid mass transfer between the contaminant and the solid catalyst, continuously clean the surface of the catalyst, and maintain the activity of the catalyst. The combined oxidation technology based on ultrasonic technology includes ultrasonic/H2O2 or O3 oxidation technology, ultrasonic-Fenton oxidation technology, ultrasonic/photocatalytic oxidation technology, and ultrasonic/wet oxidation technology. Ren Baixiang used ultrasonic-Fenton reagent to treat dye wastewater. The COD removal rate of dye wastewater reached 91.8%, and Chen et al found that in the synergistic reaction between ultrasonic and Fenton, α-Fe2O3 loaded zeolite 4A can enhance the ultrasonic cavitation effect. It has the characteristics of small iron ion dissolution, high reaction stability and long service life.

4. Photocatalytic oxidation method

The photocatalytic oxidation method is an oxidative decomposition of organic matter by OH generated by an oxidizing agent under excitation of light and catalysis of a catalyst. Compared with traditional treatment methods, such as adsorption method, coagulation method, activated sludge method, physical method, chemical method, etc., photocatalytic oxidation degradation of organic pollutants in water has low energy consumption, simple operation, mild reaction conditions, and can be reduced. The outstanding advantages such as secondary pollution have attracted more and more attention. The catalysts used in the photocatalytic oxidation process are TiO2, ZnO, WO3, CdS, ZnS, SnO2 and Fe3O4. A large number of experiments have proved that TiO2 photocatalytic reaction has a strong processing capacity for industrial wastewater.
The early photocatalytic oxidation method uses TiO2 powder as a catalyst, and has the disadvantages of easy catalyst loss, difficult recovery, and high cost, which limits the practical application of the technology.
The immobilization of TiO2 has become the focus of photocatalysis research. Scholars have begun to study the replacement of TiO2 powder with TiO2 film or composite catalytic film. Liu Lei et al. immobilized nano-TiO2 on the glass surface to photocatalyticly degrade acetic acid. Dong Junming et al. sprayed TiO2/GeO2 composite sol onto aluminum sheets to form a composite film to photocatalyticly degrade the reactive blue dye wastewater treated by ozone oxidation. . In addition, the photocatalytic membrane reactor coupling photocatalytic technology and membrane separation technology can effectively retain the suspended catalyst, which improves the separation and recovery of the catalyst.

5. Wet oxidation method

The wet oxidation method is to oxidize the organic matter in the waste water into carbon dioxide and water under the high temperature and high pressure, thereby achieving the purpose of removing the pollutants. The wet oxidation process was originally proposed by F.J. Zimmermann of the United States in 1958 for use in papermaking black liquor. The subsequent oxidation process has developed rapidly, with applications ranging from the recovery of useful chemicals and energy to the treatment of toxic and hazardous waste.
Wet oxidation method is generally carried out under high temperature (150~350 °C) high pressure (0.5~20MPa) operating conditions, in the liquid phase, using oxygen or air as oxidant, and the oxidized water is dissolved or suspended organic or reduced inorganic substance. There are generally two steps: 1 mass transfer of oxygen from the gas phase to the liquid phase in the air; 2 chemical reaction between dissolved oxygen and the matrix.

The wet oxidation method still has certain limitations in practical application and application:
1 Wet oxidation is generally required to be carried out under high temperature and high pressure conditions. The intermediate product is often organic acid, so the requirements for equipment materials are relatively high, high temperature, high pressure, and corrosion resistance are required. Therefore, the equipment cost is large, and the system has a one-time investment. high;
2 Because the wet oxidation reaction needs to be maintained under the conditions of high temperature and high pressure, it is only suitable for the treatment of wastewater with low flow rate and high concentration, and it is uneconomical for wastewater with low concentration and large amount of water;
3 Even at very high temperatures, the removal of certain organic substances such as polychlorinated biphenyls and small molecular carboxylic acids is not satisfactory, and it is difficult to achieve complete oxidation;
4 More toxic intermediates may be produced during wet oxidation. The catalytic wet oxidation method developed on the basis of the wet oxidation method increases the oxidation capacity of the technology, lowers the reaction temperature and pressure by adding a catalyst, thereby reducing the investment and operating costs, expanding the application range of the technology, and becoming a wet type. The hot spot of oxidation research. Catalysts commonly used in catalytic wet oxidation processes are metal elements such as Fe, Cu, Mn, Co, Ni, Bi, Pt or a combination of several of them.

6. Supercritical water oxidation
In order to completely remove some organic substances that are difficult to remove by wet oxidation, a supercritical water oxidation method in which the temperature of the waste liquid is raised above the critical temperature of water and the good characteristics of the supercritical water is used to accelerate the progress of the reaction is studied. Supercritical oxidation technology is a new type of oxidation technology that can be completely destroyed by the American scholar Model in the mid-1980s. The principle is to rapidly decompose the organic matter contained in the wastewater into a simple and harmless small molecule compound such as water or carbon dioxide in the state of supercritical water.
In the supercritical water oxidation process, since the supercritical water is an excellent solvent for the oxygen of the organic matter, the oxidation of the organic matter can be carried out in the oxygen-rich homogeneous phase, and the reaction is not restricted by the phase transfer. At the same time, the high reaction temperature also accelerates the reaction rate.
The catalytic supercritical water oxidation technology developed on the basis of supercritical water oxidation has stronger degradation ability and lower reaction temperature and pressure. Catalysts commonly used in catalytic supercritical water oxidation technology are MnO2, CuO, TiO2, CeO2, Al2O3, Pt and composite catalysts such as Cr2O3/A12O3, CuO/A12O3, MnO2/CeO2.
Supercritical water oxidation is an emerging and promising wastewater treatment technology. After more than 20 years of development, this method has made great progress, but there are still some problems, such as: high equipment and process requirements, large one-time investment; equipment anti-corrosion and salt deposition problems are not completely solved; Need to explore further. These problems have hindered the development of supercritical water oxidation technology. However, supercritical water oxidation technology has shown great vitality in industrial wastewater treatment, and we believe that this method will be widely used with the continuous advancement of science and technology.