Advanced Oxidation Processes, a technology for the treatment of toxic and difficult pollutants that began to take shape in the 1980s, is characterised by the generation of hydroxyl radicals (-OH) through the reaction, which have strong oxidative properties, and through the reaction of the free radicals are able to decompose organic pollutants effectively, or even convert them into harmless inorganic substances such as carbon dioxide and water. and water. As the advanced oxidation process has the advantages of strong oxidation, easy to control the operating conditions, and can deal with difficult wastewater that cannot be treated by biochemical methods, it has attracted the attention of countries all over the world, and has carried out the research and development work in this direction one after another. Advanced oxidation technology is mainly divided into Fenton oxidation, photocatalytic oxidation, ozone oxidation, ultrasonic oxidation, wet oxidation and supercritical water oxidation.
Commonly used advanced oxidation technology
1. Fenton oxidation
The oxidation technology system consisting of hydrogen peroxide and catalyst Fe2+ is called Fenton reagent. It is more than 100 years ago by H.J.H. Fenton invented a kind of high temperature and high pressure and the process is simple chemical oxidation water treatment technology. In recent years, research has shown that the oxidation mechanism of Fenton is due to the catalytic decomposition of hydrogen peroxide under acidic conditions, resulting in highly reactive hydroxyl radicals. Under the action of Fe2+ catalyst, H2O2 can produce two kinds of active hydroxyl radicals, thus triggering and propagating the free radical chain reaction, accelerating the oxidation of organic matter and reducing substances. Its general course is:
Fenton oxidation method is generally carried out under the condition of PH of 2~5. The advantage of this method is that the decomposition of hydrogen peroxide is fast, and thus the oxidation rate is also high. However, this method also has many problems, due to the large concentration of Fe2+ in the system, the treated water may have a colour; Fe2+ reacts with hydrogen peroxide to reduce the utilisation rate of hydrogen peroxide and its PH limitations, thus affecting the popularisation and application of the method to a certain extent.
In recent years, it has been studied to introduce ultraviolet light (UV), oxygen, etc. into the Fenton reagent, which enhances the oxidising ability of the Fenton reagent and saves the dosage of hydrogen peroxide. Since the decomposition mechanism of hydrogen peroxide is extremely similar to that of Fenton and Fenton reagent, both of which produce -OH, various improved Fenton reagents are called Fenton-like reagents. The main ones are the H2O2+UV system, the H2O2+UV+ Fe2+ system, and the Fenton system that introduces oxygen.
The application of Fenton reagent and Fenton-like reagent in wastewater treatment can be divided into two aspects: one is to oxidise the organic wastewater as a treatment method alone; the other is to combine with other methods, such as coagulation and sedimentation method, activated carbon method, etc., which can achieve good results. catalysts of the Fenton method are difficult to be separated and reused, and the reaction pH is low, which will generate a large amount of iron-containing sludge, and a large amount of Fe2+ in effluent will result in a high level of Fe2+ in the effluent. The catalyst of Fenton method is difficult to be separated and reused, the reaction pH is low, a large amount of sludge containing iron will be generated, and the effluent water contains a large amount of Fe2+, which will cause secondary pollution and increase the difficulty and cost of subsequent treatment.
In recent years, scholars at home and abroad began to study the Fe2+ fixed in the ion exchange membrane, ion exchange resin, alumina, molecular sieve, bentonite, clay and other carriers, or iron oxides, compounds instead of Fe2+, in order to reduce the dissolution of Fe2+, improve the recycling rate of catalysts, and broaden the appropriate range of pH. Daud et al. impregnation method to fix Fe3+ on kaolinite catalytic degradation of activated black 5 (RB5), the reaction pH is very low. Daud et al. immobilized Fe3+ on kaolinite by impregnation method to catalyze the degradation of reactive black 5 (RB5), and the decolorization rate of RB5 reached 99% in 150 min. Youngmin et al. chelated Fe(II) with the cross-links of chitosan (CS) and glutaraldehyde (GLA) to make a catalyst of Fe(II)-CS/GLA, and catalyzed the degradation of trichloroethene (TCE) under the neutral condition, and the degradation rate of TCE reached 95% in 5 h. In contrast to the traditional Fenton method, which was used in the neutral condition, the degradation rate of TCE reached 95%. The degradation rate of TCE reached 95% after 5 h. However, the conventional Fenton method did not degrade TCE significantly due to the iron precipitation under neutral conditions, and Plata et al. investigated the effects of catalyst dosage and light intensity on the degradation of 2-chlorophenol by photo-Fenton with the use of acicular ferrite, and the effluent contained only a small amount of iron ions.
2. Ozone oxidation
Ozone is an excellent strong oxidant, which has good effect in disinfection, colour removal, deodorization, removal of organic matter and COD in wastewater. Ozone oxidation degradation of organic matter fast, mild conditions, does not produce secondary pollution, widely used in water treatment. Ozone treatment of wastewater role of the broad performance of the material, one is the direct oxidation of ozone, the second is through the formation of hydroxyl radicals and free radical oxidation.
Separate ozone oxidation method due to the ozone generator is easy to damage, energy consumption, treatment costs are expensive, and its ozone oxidation reaction is selective, for some halogenated hydrocarbons and pesticides, such as oxidation effect is relatively poor. For this reason, in recent years, the development of ozone oxidation to improve the efficiency of the relevant combination of technologies, including UV / O3, H2O2 / O3, UV / H2O2 / O3 and other combinations not only to improve the rate and efficiency of oxidation, but also able to oxidise the role of O3 alone is difficult to oxidative degradation of organic matter.
Hu Junsheng et al. compared the effect of H2O2/O3 and O3 in the treatment of dye wastewater, while Wei Dongyang et al. compared the effect of UV/O3 and O3 in the degradation of hexachlorobenzene, and the results showed that the use of the combination of technologies can significantly improve the oxidation rate and treatment effect, shorten the reaction time, and reduce the amount of consumption of O3. Catalytic ozone oxidation is also receiving attention from domestic and foreign scholars day by day. The catalysts used in catalytic ozone oxidation method are mainly transition metal oxides and activated carbon, of which activated carbon is widely used in catalytic ozone oxidation system because of its low price, strong adsorption, high catalytic activity and good stability.
3. Ultrasonic oxidation method
Ultrasonic oxidation method is the use of frequency range of 16kHz-1MHz ultrasonic radiation solution, so that the solution produces ultrasonic cavitation, the formation of local high temperature and high pressure in the solution and the generation of local high concentration of oxides – OH and H2O2 can be formed in supercritical water, rapid degradation of organic pollutants. Ultrasonic oxidation method combines the characteristics of free radical oxidation, incineration, supercritical water oxidation and other water treatment technologies, degradation conditions are mild, high efficiency, wide range of applications, no secondary pollution, is a very promising development potential and prospects for the application of clean water treatment technology.
Ultrasonic degradation of organic matter is mainly in the cavitation effect, organic matter through high temperature decomposition or free radical reaction two courses. In the ultrasonic cavitation generated by the local high temperature, high pressure environment, water is decomposed to produce -OH radicals, in addition to dissolved in the solution of air (N2 and O2) can also be generated by free radical cleavage reaction free radicals. These free radicals can also further trigger the fracture of organic molecules, the transfer of free radicals and redox reactions.
Individual ultrasonic oxidation technology can remove certain organic pollutants in water, but its individual treatment cost is high, and the treatment effect on hydrophilic and difficult to volatilise organic matter is poor, and the removal of TOC is incomplete, so it is often used in conjunction with other advanced oxidation technologies to reduce the cost of treatment and improve the treatment effect. Moreover, when ultrasonic radiation is used in conjunction with other catalytic technologies, the intense turbulence caused by ultrasound can strengthen the solid-liquid mass transfer between the pollutants and the solid catalyst, continuously clean the catalyst surface, and maintain the catalyst activity. Combined oxidation technologies based on ultrasound technology include ultrasound/H2O2 or O3 oxidation, ultrasound-Fenton oxidation, ultrasound/photocatalytic oxidation, ultrasound/wet oxidation, and so on. Ren Baixiang used ultrasonic -Fenton reagent joint treatment of dye wastewater, dye wastewater COD removal rate of 91.8%, and Chen et al. found that, in the synergistic reaction of ultrasound and Fenton, loaded with α-Fe2O3 4A zeolite can strengthen the effect of ultrasonic cavitation, and has the characteristics of small iron ion dissolution, high stability of the reaction, and long service life.
4. Photocatalytic oxidation
Photocatalytic oxidation method is through the oxidant in the light of the excitation and catalyst catalytic effect of -OH oxidation decomposition of organic matter. Compared with traditional treatment methods, such as adsorption, coagulation, activated sludge, physical method, chemical method, etc., photocatalytic oxidation degradation of organic pollutants in water has the outstanding advantages of low energy consumption, easy operation, mild reaction conditions, and reduction of secondary pollution, which is increasingly valued by people. The catalysts used in photocatalytic oxidation technology are TiO2, ZnO, WO3, CdS, ZnS, SnO2 and Fe3O4. A large number of experiments have proved that TiO2 photocatalytic reaction has a strong ability to treat industrial wastewater.
The early photocatalytic oxidation method uses TiO2 powder as catalyst, which has the disadvantages of catalyst loss, difficult to recover and high cost, which limits the practical application of this technology.
The immobilisation of TiO2 has become the focus of photocatalytic research, and scholars have begun to study the replacement of TiO2 powder with TiO2 film or composite catalyst film. Liu Lei et al. immobilized TiO2 nanoparticles on glass surface for the photocatalytic degradation of acetic acid, and Dong Junming et al. sprayed TiO2/GeO2 composite sol on aluminium sheet to make a composite film for the photocatalytic degradation of ozone-treated reactive blue dyes, and both of them obtained better degradation effects. In addition, the photocatalytic membrane reactor coupling photocatalytic technology and membrane separation technology can effectively retain the suspended catalyst, which improves a new idea for the separation and recovery of catalyst.
5. Wet oxidation method
Wet oxidation method is to oxidise organic matter in wastewater into carbon dioxide and water under high temperature and high pressure by using oxidant, so as to achieve the purpose of removing pollutants. Wet oxidation method was initially proposed by the United States F.J. Zimmermann in 1958, used for paper black liquor. Subsequently, the oxidation process has been rapidly developed, the scope of application from the recovery of useful chemicals and energy to further expand to the treatment of toxic and hazardous waste.
Wet oxidation method is generally in the high temperature (150 ~ 350 ℃) high pressure (0.5 ~ 20MPa) operating conditions, in the liquid phase, with oxygen or air as an oxidant, oxidation of water in the dissolved state or suspended state of organic matter or reduced state of inorganic substances, there are generally two steps: ① oxygen in the air from the gas phase to the liquid phase of the mass transfer process; ② dissolved oxygen and the substrate of the chemical reaction between.
Wet oxidation method still has some limitations in practical application:
1) Wet oxidation is generally required to be carried out at high temperatures and high pressures, the intermediate products are often organic acids, so the equipment and materials requirements are relatively high, must be resistant to high temperatures, high pressure, and corrosion resistance, so the equipment cost is large, the system’s one-time investment is high;
2) Due to the wet oxidation reaction needs to be maintained at high temperature and high pressure conditions, it is only suitable for small flow of high concentration of wastewater treatment, for low concentration of large amounts of wastewater is very uneconomical;
3) Even at a very high temperature, the removal of certain organic substances such as PCBs, small molecules of carboxylic acids is not ideal, and it is difficult to achieve complete oxidation;
4) More toxic intermediate products may be produced during wet oxidation. The catalytic wet oxidation method developed on the basis of the wet oxidation method has become a hot spot in the research of wet oxidation method by adding catalysts to improve the oxidation capacity of the technology, lowering the reaction temperature and pressure, thus reducing the investment and operating costs and expanding the application scope of the technology. Catalytic wet oxidation method commonly used catalysts are Fe, Cu, Mn, Co, Ni, Bi, Pt and other metal elements or a combination of several elements.
6. Supercritical water oxidation method
In order to completely remove some of the wet oxidation method is difficult to remove the organic matter, the study of the waste liquid temperature to the critical temperature of water above the use of supercritical water to accelerate the reaction process of the good characteristics of supercritical water oxidation method. Supercritical oxidation technology is a new type of oxidation technology that can completely destroy the structure of organic matter proposed by American scholar Model in the mid-80s. Its principle is in the state of supercritical water in the wastewater contained in the organic matter with the oxidant quickly decomposed into water, carbon dioxide and other simple harmless small molecular compounds.
In the process of supercritical water oxidation, because supercritical water is an excellent solvent for organic matter of oxygen, so the oxidation of organic matter can be carried out in the oxygen-rich homogeneous phase, the reaction will not be limited by the transfer of interphase. At the same time the high reaction temperature makes the reaction faster.
The catalytic supercritical water oxidation technology developed on the basis of supercritical water oxidation method has stronger degradation ability and lower reaction temperature and pressure. Commonly used catalysts in catalytic supercritical water oxidation technology are MnO2, CuO, TiO2, CeO2, Al2O3, Pt and several other substances in the composition of the composite catalysts, such as Cr2O3/A12O3, CuO/A12O3, MnO2/CeO2 and so on.
Supercritical water oxidation is an emerging and promising wastewater treatment technology. After more than 20 years of development, the method has made great progress, but there are still some problems, such as: high equipment and process requirements, large one-time investment; equipment corrosion and salt deposition problems have not been completely resolved; reaction mechanism needs to be further explored. These problems have hindered the development of supercritical water oxidation technology. However, supercritical water oxidation technology has shown vitality in industrial wastewater treatment, we believe that with the continuous progress of science and technology, this method will be widely used.
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