What is advanced oxidation technology?

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. Through the addition, substitution, electron transfer, and bond breaking between free radicals and organic compounds, large molecules of difficult to degrade organic matter in water are oxidized and degraded into low toxic or non-toxic small molecules, and even directly degraded into CO2 and H2O, The current advanced oxidation technologies for near complete mineralization mainly include chemical oxidation, electrochemical oxidation, wet oxidation, supercritical water oxidation, and photocatalytic oxidation.
1. Chemical oxidation technology
Chemical oxidation technology is commonly used in pre-treatment of biological treatment. Generally, organic wastewater is treated with chemical oxidants 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-1990s by French scientist H J. Fenton proposed that under acidic conditions, H2O2 can effectively oxidize tartaric acid under the catalytic action of Fe2+ions and be applied to the oxidation of malic acid. For a long time, people have assumed that the main principle of Fenton is to use ferrous ions as catalysts for hydrogen peroxide, and the reaction produces hydroxyl radicals with the formula: Fe2++H2O2- Fe3++OH -+? OH, and most reactions occur under acidic conditions.
In the chemical oxidation method, the Fenton method has shown certain advantages in treating some difficult to degrade organic compounds (such as phenols and aniline). With the deepening of research on Fenton method, in recent years, ultraviolet light (UV), oxalate, etc. have been introduced into Fenton method, greatly enhancing its oxidation ability.
The chlorophenol mixture was treated using 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. However, it has disadvantages such as high processing 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, making it widely used in industrial wastewater treatment. Ozone can oxidize many organic compounds in water, but the reaction between ozone and organic compounds is selective and cannot completely decompose organic compounds into CO2 and H2O. The products of ozone oxidation are often carboxylic acid organic compounds. And the chemical properties of ozone are extremely unstable, especially in non pure water, where the oxidation and decomposition rate is measured in minutes. In wastewater treatment, ozone oxidation is usually not treated as a separate unit and some enhancement measures are usually added, such as photocatalytic ozonation, alkaline catalytic ozonation, and multiphase catalytic ozonation. In addition, the combination of ozone oxidation and other technologies is also a focus of research, such as ozone/ultrasound method, ozone/biological activated carbon adsorption method, etc.
There are literature reports that combining ozone oxidation with activated carbon adsorption can reduce the mass concentration of aromatics in wastewater to 0.002 μ G/L. The use of ozone oxidation to remove surfactants from industrial circulating water can effectively increase the purification degree of urban sewage treatment plants and improve the water quality of drainage. Yu Xiujuan et al. also achieved good results in removing organic micro pollutants from water using the ozone biological activated carbon process. Due to the low solubility of ozone in water, how to more effectively dissolve ozone in water has become a hot research topic in this technology.
2. Electrochemical catalytic oxidation method
This technology originated in the 1940s and has advantages such as wide application range, high degradation efficiency, simple energy requirements, easy implementation of automated operations, and flexible and diverse application methods. The electrochemical catalytic oxidation method can be used as a pre-treatment measure for refractory wastewater to improve its biodegradability, and can also be used as a deep treatment technology for refractory phenolic wastewater. Under optimized pH, temperature, and current intensity conditions, phenol can be almost completely decomposed.
Traditional biological and physicochemical methods have lost their advantages in dealing with high concentration, difficult to degrade, toxic and harmful phenolic wastewater. Chemical oxidation, due to its high cost, hinders its promotion and application. Electrochemical catalytic oxidation is becoming increasingly popular, but it also has some problems, such as power consumption, electrode materials mostly made of precious metals, high cost, and anodic corrosion, The micro dynamics and thermodynamics research guiding its promotion and application is still incomplete.
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 pressure conditions to oxidize organic pollutants in the wastewater. According to the presence or absence of catalysts in the treatment process, it can be divided into two categories: wet air oxidation and wet air catalytic oxidation.
3.1 Wet air oxidation method
WAO technology involves introducing air under high temperature (125-320 ℃) and high pressure (0.5-20MPa) conditions to directly oxidize and degrade high molecular weight organic matter in wastewater into inorganic or small molecule organic matter.
The wet air oxidation technology is used to pretreat the wastewater from the production of Lego. The removal rate of organic phosphorus is as high as 95%, and the removal rate of organic sulfur is as high as 90%. The wet air catalytic oxidation method, which requires high temperature and high pressure, high equipment investment, and harsh operating conditions, is difficult to be accepted by general enterprises due to its high processing efficiency and short reaction time. Therefore, in recent years, the use of catalysts to reduce reaction temperature and pressure or shorten reaction residence time has received widespread attention and research.
3.2 Wet air catalytic oxidation method
The catalytic wet air oxidation (CWAO) method involves adding suitable catalysts to the traditional wet air oxidation process to complete the oxidation reaction under milder conditions and in a shorter time. This can reduce the temperature and pressure of the reaction, improve the oxidation and decomposition ability, 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 availability of highly active and easily recyclable catalysts. CWAO catalysts are generally divided into three categories: metal salts, oxides, and composite oxides. According to the form of catalysts present in the system, wet air catalytic oxidation can be further divided into homogeneous wet catalytic oxidation and heterogeneous wet catalytic oxidation.
(1) Homogeneous wet catalytic oxidation method. In the homogeneous wet catalytic oxidation method, due to the fact that the catalyst (mostly metal ions) is a soluble transition metal salt, these salts exist in the form of ions in wastewater. At the ion or molecular level, they catalyze the oxidation reaction of organic compounds in water by triggering free radical reactions of oxidants and continuously regenerating them. In homogeneous wet catalytic oxidation, the catalyst acts independently at the molecular or ionic level, resulting in high molecular activity and better oxidation efficiency. However, due to the presence of catalysts in the form of ions in homogeneous wet catalytic oxidation, it is difficult to recover and reuse them from wastewater, and it is prone to secondary pollution.
(2) Heterogeneous wet catalytic oxidation method. Heterogeneous wet catalytic oxidation is the addition of insoluble solid catalysts to the reaction system, and its catalytic effect is carried out on the surface of the catalyst. The specific surface area of the catalyst has a significant impact on the degradation rate of organic compounds. Due to the different types of solid catalysts and the nature of wastewater, the effectiveness of wet catalytic oxidation also varies. In the multiphase wet catalytic oxidation method, due to the solid catalyst not dissolving, not losing, and easy activation, regeneration, and recovery, 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, which uses 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 to react under high temperature and pressure.
But its improvement and enhancement lies in utilizing the properties of water in the supercritical state, reducing the dielectric constant of water to be similar 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, eliminating the interphase mass transfer resistance in the wet oxidation process and increasing the reaction rate. Additionally, due to the higher independent activity of oxidized free radicals in the homogeneous system, The degree of oxidation also increases accordingly. Supercritical water is a good solvent for organic matter and oxygen. Organic matter undergoes homogeneous oxidation in oxygen rich supercritical water, and its reaction rate 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, allowing organic carbon and hydrogen to be completely converted into CO2 and H2O.
Supercritical water oxidation technology has received increasing 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. Currently, most commonly used catalysts are those used in wet catalytic oxidation processes. Finding catalysts with broad-spectrum catalytic performance for supercritical water oxidation technology is a challenge in promoting 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 visible or ultraviolet light. Part of the near-ultraviolet light (290-400nm) in the natural environment is easily absorbed by organic pollutants, and strong photochemical reactions occur in the presence of active substances, leading to the degradation of organic matter. However, due to the limitations of reaction conditions, photochemical oxidation degradation is often not thorough enough, and it is easy to produce various aromatic organic intermediates, which becomes a problem that photochemical oxidation needs to overcome.
Since Carey et al. first used TiO2 for photocatalytic degradation of biphenyls and chlorinated biphenyls in 1976, the research focus of photocatalytic oxidation technology has shifted to the direction of photocatalytic oxidation degradation of organic pollutants using TiO2 as a catalyst.
Due to its simple equipment structure, mild reaction conditions, easy control of operating conditions, strong oxidation ability, and no secondary pollution, as well as its high chemical stability, non toxicity, and low cost, TiO2 photocatalytic oxidation technology is a new type of water treatment technology with broad application prospects.
6. Ultrasonic oxidation method
The development of sonochemistry has attracted increasing attention to its application in water and wastewater treatment. The power source of ultrasonic oxidation is acoustic cavitation. When a sufficiently strong ultrasonic wave (15 kHz -20 MHz) passes through an aqueous solution, during the negative pressure half cycle of the sound wave, the amplitude of the sound pressure exceeds the internal static pressure of the liquid, and the cavitation nucleus in the liquid rapidly expands; During the half cycle of positive pressure of sound waves, bubbles rupture again due to adiabatic compression, with a duration of approximately 0.1 μ S. 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 micro jet with a speed of 110 m/s is generated.
The equipment used for ultrasonic oxidation is a magneto electric or piezoelectric ultrasonic transducer, which generates ultrasonic waves through electromagnetic energy transfer. Radiation plate ultrasonic instruments, probe type, and NAP reactors are commonly used in the laboratory. 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.