At present, traditional biological treatment techniques are mainly used for organic pollutants in wastewater. However, the effectiveness of some wastewater containing high concentrations and stable chemical structures is not ideal, such as pesticide, papermaking, printing and dyeing wastewater. The organic pollutants in these wastewater are high in concentration, toxic, and complex in composition, mostly containing stable aromatic structures that are difficult to degrade, with poor biodegradability and difficult disposal. Therefore, the removal of organic pollutants in Bio reactor organic wastewater (BROW) has become a difficult point in the field of wastewater treatment. In recent years, there has been a lot of research on the treatment methods of recalcitrant organic wastewater both domestically and internationally. Among them, compared with traditional water treatment methods, Advanced Oxidation Processes (AOPs) have attracted widespread attention for their advantages of good treatment effect, fast speed, no secondary pollution, and wide applicability. Advanced oxidation water treatment methods generally have the following characteristics: a. using a large number of vivid hydroxyl radicals with strong oxidation ability as oxidants, which can induce chain reactions of oxidation reactions; b. sufficient concentration of hydroxyl radicals can completely inorganic organic pollutants without generating secondary pollution; c. This method can oxidize organic pollutants of different concentrations in water and is also effective for certain low concentration trace organic compounds; d. This method can be used alone or combined with other methods such as biodegradation to reduce disposal costs. The detailed disposal technologies of AOPs are divided into three categories: traditional advanced oxidation method, humid air oxidation method, and electrochemical oxidation method, and their applications in the removal of organic pollutants from wastewater are discussed in detail.
1. The commonly used advanced oxidation methods currently include Fenton method, O3/UV method, O3/H2O2 method, and TiO2 photocatalytic oxidation method. The application of Fenton reagent in the oxidation removal of organic pollutants began in the 1960s, when Eisenhauer first used Fe2+/H2O2 for the removal of phenol and alkylbenzene in water treatment. Emolla et al. used the Fenton oxidation method to treat wastewater containing three antibiotics, amoxicillin, ampicillin, and clotrimazole, and found that the three antibiotics could be fully synthesized under certain conditions, with a COD removal rate of over 80%. To improve the disposal efficiency of Fenton reagent, ultraviolet (UV) light was introduced, which can reduce the amount of Fe2+and promote the synthesis of H2O2 into highly oxidizing hydroxyl radicals, which can make organic matter more abundant and inorganic. Some people have used it in the deep treatment of azo dye wastewater, and the results show that when the concentration of azo dye is 400mg/L, the UV Fenton method can achieve a decolorization rate of over 95% in the wastewater. However, this method is not highly effective for the application of solar energy, and the cost of disposal equipment is relatively high, resulting in high energy consumption during equipment operation, which limits its application. The O3/UV method was first applied by Garrison et al. to treat wastewater containing complex iron cyanide salts, and it was found that separating UV radiation from O3 can increase the oxidation rate by 10-104 times. Jia Quan used advanced oxidation methods such as O3/UV and O3/H2O2 to stop treating dye wastewater. The results showed that when the pH value was 8 and the reaction time was 2 hours, the advanced oxidation technology of O3/UV had a decolorization rate of 98.3% and a COD removal rate of 67.0% for dye wastewater. TiO2 based photocatalysts are the most commonly studied photocatalysts for wastewater treatment. The photocatalytic oxidation method for treating antibiotic wastewater has the advantages of mild reaction conditions, thorough degradation, and strong applicability. Sood et al. synthesized Bi2O3/TiO2 photocatalyst by hydrothermal method, and used the photocatalyst to synthesize simulated ofloxacin antibiotic wastewater. The experimental results showed that after 2 hours of light treatment, the synthesis rate of ofloxacin was 92%. At the same time, photocatalytic oxidation has shown good results in the treatment of pesticide wastewater, especially organophosphorus pesticide wastewater, with satisfactory degradation efficiency, COD and TOC removal rates. The photocatalytic oxidation method has certain advantages in treating organic wastewater, but the main problems are high cost of catalyst preparation, low application rate of light energy, possible production of more toxic intermediate products, and difficult catalyst recovery. Therefore, further research is needed on the application of obstacle photocatalytic oxidation method.
2. The wet air oxidation method was introduced in the 1950s and has received significant research attention both domestically and internationally in recent years. Japan and the United States have applied it to industrial water treatment. Wet air oxidation reaction also belongs to free radical chain reaction, and various free radicals are used as oxidants to remove organic pollutants. Wet oxidation method mixes wastewater containing organic pollutants with air or oxygen, and oxidizes the organic compounds in the wastewater under high temperature and high pressure (150-350 ℃, 0.5-20MPa) conditions. The wet air oxidation method has the advantages of complete oxidation of pollutants and minimal secondary pollution, and can effectively remove pollutants that are difficult to biodegrade. But this method also has certain limitations. The response needs to be stopped under high temperature and high pressure conditions, which can cause severe corrosion to the equipment and require a large investment in the equipment operation system. Therefore, it has certain limitations in industrial applications. In the wet oxidation reaction process, suitable catalysts can shorten the reaction time and make the reaction conditions easier to reach. The available homogeneous catalysts include transition metals, precious metals, rare earth metals and their oxides and salts. Heterogeneous catalyst recycling has attracted more attention, usually using materials such as silica gel, activated carbon, diatomaceous earth, alumina, etc. as carriers to load various types of active metals and their oxides to stop catalytic reactions. The wet air oxidation method is used to treat pesticide wastewater by continuously introducing air under high temperature and pressure conditions, which can efficiently oxidize the organic matter in the wastewater into small molecule organic matter and even completely inorganic it. Organic compounds containing phosphorus are oxidized to form phosphoric acid, while organic sulfur compounds are oxidized to form sulfuric acid. In addition to pesticide wastewater, papermaking straw pulp black liquor, coal gas wastewater, spice wastewater, etc. can also be treated by wet air oxidation method, which has good effects on the removal of organic pollutants in industrial wastewater such as pharmaceuticals, papermaking, fibers, alcohol, printing and dyeing. 3. Electrochemical oxidation and electrocatalytic oxidation are advanced oxidation methods that have attracted much attention in recent years. They apply an external electric field and generate strong oxidizing free radicals through a series of electrode reactions inside the reaction installation to stop the oxidation and degradation of organic pollutants in sewage, convert them into non-toxic or low toxicity small molecule intermediates, and ultimately completely inorganic. The development of electrodes with efficient catalytic performance is the most important aspect of electrocatalytic oxidation research. Currently, electrode materials such as carbon electrodes, non-metallic compound electrodes, and titanium based coating electrodes have been widely studied and applied both domestically and internationally. Li Hongbo used a barrier electrochemical reactor with Ti/SnO2+Sb2O3/PbO2 electrode as the anode and stainless steel plate as the cathode to simulate the degradation of isophthalic acid in wastewater. The removal rate of organic matter in isophthalic acid wastewater with an initial concentration of 250mg/L was over 80%. Some people abroad also use boron doped diamond film electrodes as anodes to treat pesticide wastewater containing chlorpyrifos with an initial COD of 450mg/L. The results show that organic matter can be completely oxidized and degraded in just 6 hours. FABIASKA et al. used boron doped diamond/stainless steel electrode data for the treatment of wastewater containing five sulfonamide antibiotics. The experimental results showed that the degradation mechanism of sulfonamide antibiotics is mainly due to the attack of hydroxyl radicals on S-N bonds and benzene rings. The electrocatalytic oxidation method has a good degradation effect on some structurally stable and difficult to degrade organic compounds. At the same time, the operation is cumbersome, the operating cost is not high, and it is easy to achieve automated control, which has good application prospects. AOPs technology has received widespread attention in prosperous countries such as Europe, America, and Japan, and has been widely applied in many industrial fields such as petrochemicals, pharmaceuticals, food, and environmental protection. However, in China, it is currently mostly limited to laboratory trials and research. Firstly, there is a lack of systematic and in-depth research on the thermodynamics, kinetics, and other aspects of AOPs processes; Secondly, due to various conditions of the reaction system such as temperature and pressure, which require high equipment requirements such as corrosion resistance, high temperature resistance, and high pressure resistance, it also increases the difficulty of process control and operation, thus hindering the further application of AOPs technology in practice.
