Advanced oxidation technology, also known as deep oxidation technology, is based on the use of electricity, light irradiation, catalysts, and sometimes combined with oxidants to produce highly active free radicals (such as HO•) in the reaction, and then through the addition, substitution, electron transfer, bond breaking, etc. between free radicals and organic compounds, the macromolecular refractory organic matter in the water is oxidized and degraded into low-toxic or non-toxic small molecules, or even directly degraded into CO2 and H2O, close to complete mineralization. The current advanced oxidation technologies mainly include chemical oxidation, electrochemical oxidation, wet oxidation, supercritical water oxidation, and photocatalytic oxidation.
1. Chemical oxidation technology
Chemical oxidation technology is often used in pretreatment of biological treatment. Generally, chemical oxidants are used to treat organic wastewater under the action of catalysts to improve its biodegradability, or directly oxidize and degrade organic matter in wastewater to stabilize it.
1.1 Fenton reagent oxidation method
This technology originated in the mid-1890s and was proposed by French scientist H. J. Fenton. Under acidic conditions, H2O2 can effectively oxidize tartaric acid under the catalytic action of Fe2+ ions and is applied to the oxidation of malic acid. For a long time, the main principle of Fenton, which is assumed by people, is to use ferrous ions as catalysts for hydrogen peroxide. The reaction produces hydroxyl radicals in the formula: Fe2++ H2O2 --Fe3++OH-+•OH, and the reaction is mostly carried out under acidic conditions.
In the chemical oxidation method, the Fenton method shows certain advantages in treating some difficult-to-degrade organic matter (such as phenols and anilines). With the in-depth study of the Fenton method, ultraviolet light (UV) and oxalate have been introduced into the Fenton method in recent years, which greatly enhances the oxidation ability of the Fenton method.
The chlorophenol mixture was treated by UV + Fenton method, and the TOC removal rate reached 83.2% within 1 hour. The Fenton method has strong oxidation ability, mild reaction conditions, simple equipment, and a wide range of applications, but it has disadvantages such as high treatment costs, complex process conditions, and difficult process control, making it difficult to promote and apply.
1.2 Ozone oxidation method
The ozone oxidation system has a high redox potential and can oxidize most organic pollutants in wastewater. It is widely used in industrial wastewater treatment. Ozone can oxidize many organic matter in water, but the reaction between ozone and organic matter is selective, and it cannot completely decompose organic matter into CO2 and H2O. The products after ozone oxidation are often carboxylic acid organic matter. And the chemical properties of ozone are extremely unstable, especially in non-pure water, and the oxidation decomposition rate is measured in minutes. In wastewater treatment, ozone oxidation is usually not used as a separate treatment unit, and some strengthening methods are usually added, such as photocatalytic ozonation, base-catalyzed ozonation, and multiphase catalytic ozonation. In addition, the combination of ozone oxidation with other technologies is also a research focus, such as ozone/ultrasound method, ozone/bioactivated carbon adsorption method, etc.
It has been reported in the literature that the combination of ozone oxidation and activated carbon adsorption can reduce the mass concentration of aromatic hydrocarbons in wastewater to 0.002μg/L. The use of ozone oxidation to remove surfactants in industrial circulating water can effectively increase the purification degree of urban sewage treatment plants and improve the water quality of drainage. Yu Xiujuan and others have also achieved good results in removing organic micropollutants in water using the ozone-bioactivated carbon process. Due to the low solubility of ozone in water, how to dissolve ozone in water more effectively has become a hot topic in the research of this technology.
2. Electrochemical catalytic oxidation method
This technology originated in the 1940s and has the advantages of a wide range of applications, high degradation efficiency, simple energy requirements, easy automation, and flexible and diverse application methods. Electrochemical catalytic oxidation can be used as a pretreatment measure for difficult-to-degrade wastewater to improve biodegradability, and can also be used as a deep treatment technology for difficult-to-degrade phenolic wastewater. The electrolysis reaction process occurs directly in the electrocatalytic oxidation electrolytic cell. Under the conditions of optimized pH value, temperature and current intensity, phenol can be almost completely decomposed.
For high-concentration, difficult-to-degrade, toxic and harmful phenol-containing wastewater, traditional biological and physical methods have lost their advantages, and chemical oxidation methods are hindered by their high cost. Electrochemical catalytic oxidation methods are increasingly favored by people, but they also have some problems, such as power consumption, electrode materials are mostly precious metals, high cost and anode corrosion, and the micro-dynamics and thermodynamics research guiding their promotion and application is still imperfect.
3. Wet oxidation technology
Wet oxidation, also known as wet combustion, is an effective method for treating high-concentration organic wastewater. Its basic principle is to introduce air under high temperature and high pressure conditions to oxidize organic pollutants in wastewater. According to whether there is a catalyst in the treatment process, it can be divided into wet air oxidation and wet air catalytic oxidation.
3.1 Wet Air Oxidation
The first company to develop and industrialize wet air oxidation (WAO) was Zimpro in the United States. The company has applied the WAO process to the treatment of toxic and harmful industrial wastewaters such as olefin production waste washing liquid, acrylonitrile production wastewater and pesticide production wastewater. WAO technology is to introduce air under high temperature (125-320℃) and high pressure (0.5-20MPa) conditions to directly oxidize and degrade high molecular organic matter in the wastewater into inorganic or small molecular organic matter.
The removal rate of organic phosphorus and organic sulfur is as high as 95% and 90% respectively when pre-treating dimethoate production wastewater using wet air oxidation technology. Zimpro's WAO process has high treatment efficiency and short reaction time, but because the technology requires high temperature and high pressure, the required equipment investment is large, and the operating conditions are harsh, it is difficult for general enterprises to accept it. Therefore, the wet air catalytic oxidation method, which uses a catalyst to reduce the reaction temperature and pressure or shorten the reaction residence time, has received extensive attention and research in recent years.
3.2 Wet Air Catalytic Oxidation
Catalytic Wet Air Oxidation (CWAO) is a method of adding a suitable catalyst to the traditional wet oxidation process to enable the oxidation reaction to be completed under milder conditions and in a shorter time. This can reduce the temperature and pressure of the reaction, improve the oxidation decomposition capacity, accelerate the reaction rate, shorten the residence time, and thus reduce equipment corrosion and operating costs. The key issue of wet air catalytic oxidation is the high-activity and easily recyclable catalyst. CWAO catalysts are generally divided into three categories: metal salts, oxides, and composite oxides. According to the form of the catalyst in the system, wet air catalytic oxidation can be divided into homogeneous wet catalytic oxidation and heterogeneous wet catalytic oxidation.
(1) Homogeneous wet catalytic oxidation. In the homogeneous wet catalytic oxidation method, since the catalyst (mostly metal ions) is a soluble transition metal salt, these salts exist in the wastewater in the form of ions. At the ionic or molecular level, they catalyze the oxidation reaction of organic matter in the water by initiating the free radical reaction of the oxidant and continuously regenerating it. In the homogeneous wet catalytic oxidation method, since the catalyst works independently at the molecular or ionic level, the molecular activity is high, resulting in a better oxidation effect. However, since the catalyst in the homogeneous wet catalytic oxidation method exists in the form of ions, it is difficult to recover and reuse from wastewater, and it is easy to cause secondary pollution.
(2) Heterogeneous wet catalytic oxidation method. Heterogeneous wet catalytic oxidation is to add an insoluble solid catalyst to the reaction system. Its catalytic action is carried out on the catalyst surface. The specific surface area of the catalyst has a great influence on the degradation rate of organic matter. Due to the different composition types of solid catalysts and the properties of wastewater, the effect of wet catalytic oxidation is also different. In the heterogeneous wet catalytic oxidation method, since the solid catalyst does not dissolve and does not flow, it is easier to activate, regenerate and recycle, so its application prospects are very broad.
4. Supercritical water oxidation technology
Supercritical water oxidation technology is an enhancement and improvement of wet air oxidation technology. It was successfully developed by the American MODAR Company in 1982. Its principle is to use supercritical water as a medium to oxidize and decompose organic matter. It also uses water as the main liquid phase and oxygen in the air as the oxidant, and reacts under high temperature and high pressure.
However, its improvement and enhancement lies in the use of the properties of water in the supercritical state. The dielectric constant of water is reduced to a value close to that of organic matter and gas, so that gas and organic matter can be completely dissolved in water, the phase interface disappears, and a homogeneous oxidation system is formed, which eliminates the interphase mass transfer resistance existing in the wet oxidation process, increases the reaction rate, and because the independent activity of the oxidized free radicals in the homogeneous system is higher, the degree of oxidation is also increased. Supercritical water is a good solvent for organic matter and oxygen. Organic matter is homogeneously oxidized in oxygen-rich supercritical water, and the reaction speed is very fast. At 400-600℃, the structure of organic matter can be destroyed in a few seconds, and the reaction is complete and thorough, so that organic carbon and hydrogen are completely converted into CO2 and H2O.
Supercritical water oxidation technology has attracted more and more attention due to its rapid reaction and thorough oxidation. How to reduce the temperature and pressure of the reaction or shorten the reaction residence time through catalysts is a research hotspot in this field. At present, most of the commonly used catalysts are catalysts used in wet catalytic oxidation processes. Finding catalysts with broad-spectrum catalytic properties for supercritical water oxidation technology is a difficulty in the promotion of this technology.
5. Photocatalytic oxidation technology
Photocatalytic oxidation technology is developed on the basis of photochemical oxidation technology. Photochemical oxidation technology is a reaction process in which organic pollutants are oxidized and degraded under the action of visible light or ultraviolet light. Some near-ultraviolet light (290-400nm) in the natural environment is easily absorbed by organic pollutants. When active substances are present, strong photochemical reactions occur, thereby degrading organic matter. However, due to the limitations of reaction conditions, photochemical oxidation degradation is often not thorough enough, and it is easy to produce a variety of aromatic organic intermediates, which has become a problem that photochemical oxidation needs to overcome.
Since Carey et al. first used TiO2 to photocatalytically degrade biphenyl and chlorobiphenyl in 1976, the research hotspot of photocatalytic oxidation technology has shifted to the direction of photocatalytic oxidation degradation of organic pollutants using TiO2 as a catalyst.
Due to the simple structure of photocatalytic oxidation equipment, mild reaction conditions, easy control of operating conditions, strong oxidation ability, no secondary pollution, and the high chemical stability, non-toxicity and low price of TiO2, TiO2 photocatalytic oxidation technology is a new water treatment technology with broad application prospects.
6. Ultrasonic oxidation method
The development of sonochemistry has attracted more and more attention to its application in water and wastewater treatment. The power source of ultrasonic oxidation is acoustic cavitation. When ultrasonic waves (15 kHz-20 MHz) of sufficient intensity pass through aqueous solution, the sound pressure amplitude exceeds the static pressure inside the liquid in the negative pressure half cycle of the sound wave, and the cavitation nucleus in the liquid expands rapidly; in the positive pressure half cycle of the sound wave, the bubble ruptures due to adiabatic compression, and the duration is about 0.1μs. At the moment of rupture, a local high temperature and high pressure environment of about 5000 K and 100 MPa is generated, and a strong impact microjet with a velocity of 110 m/s is generated.
The equipment used for ultrasonic oxidation is a magnetoelectric or piezoelectric ultrasonic transducer, which generates ultrasonic waves through electromagnetic transduction. The most commonly used in the laboratory are radiation plate type ultrasonic instruments, probe type and NAP reactors. Ultrasonic oxidation reaction conditions are mild, usually carried out at room temperature, with low equipment requirements, and is a pollution-free green treatment technology with broad application prospects.
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