Degradation of organic pollutants in wastewater by advanced oxidation technology

At present, traditional biological treatment technology is mainly used for organic pollutants in wastewater, but the effect is not ideal for some wastewater containing high concentration and stable chemical structure of organic pollutants, such as pesticides, papermaking, printing and dyeing wastewater, etc. High concentration, high toxicity, complex components, most of them contain stable aromatic structures that are difficult to degrade, poor biodegradability, and difficult to treat. Therefore, the removal of organic pollutants in refractory organic wastewater (Bio-refractor yor-ganic wastewater, referred to as BROW) It has become a difficult point in the field of sewage treatment.

In recent years, a large number of researches on the treatment methods of refractory organic wastewater have been carried out at home and abroad. Among them, compared with traditional water treatment methods, Advanced Oxidation Processes (AOPs) is characterized by its better treatment effect, faster speed, and better treatment. The advantages of low pollution and wide application range have been widely concerned. The advanced oxidation water treatment method generally has the following characteristics: a. A large number of active hydroxyl radicals with strong oxidizing ability are used as oxidants, which can induce the chain reaction of oxidation reaction; b. A sufficient concentration of hydroxyl radicals can make organic pollutants Complete inorganicization without secondary pollution; c. This method can oxidize organic pollutants of different concentrations contained in water, and has a good effect on some low-concentration trace organic substances; d. This method can be used alone or with other Methods such as biodegradation are used in combination to reduce disposal costs.

The specific treatment technologies of AOPs are now divided into the following three categories: traditional advanced oxidation, wet air oxidation, and electrochemical oxidation, and their application in the removal of organic pollutants in wastewater is described in detail.

Traditional advanced oxidation method

At present, the commonly used advanced oxidation methods mainly include the following: Fenton method, O3/UV method, O3/H2O2 method and TiO2 photocatalytic oxidation method.

The application of Fenton reagent to the oxidative removal of organic pollutants began in the 1960s. 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 cloxacillin, and found that the three antibiotics could be completely decomposed under certain conditions, and the COD removal rate was more than 80%.

In order to improve the treatment efficiency of Fenton’s reagent, ultraviolet light (UV) is introduced into it, which can reduce the amount of Fe2+, and at the same time promote the decomposition of H2O2 into strong oxidizing hydroxyl radicals, which can make organic matter more fully inorganic. It is used in the advanced treatment of azo dye wastewater. The results show that when the concentration of azo dye is 400mg/L, the UV-Fenton method can make the decolorization rate of wastewater reach more than 95%. However, the utilization of solar energy by this method is not high, and the cost of processing equipment is relatively high, and the energy consumption of equipment operation is relatively large, which limits its application.

The O3/UV method was first applied to the treatment of wastewater containing complex ferricyanide by Garrison et al. It was found that the combination of UV radiation and O3 could increase the oxidation rate by 10-104 times. Jia Tongtong uses O3/UV, O3/H2O2 and other advanced oxidation methods to treat dye wastewater. The results show that when the pH value is 8 and the reaction time is 2h, the decolorization rate of O3/UV advanced oxidation technology to dye wastewater is 98.3%, and the COD removal rate is 98.3%. The rate was 67.0%.

TiO2-based photocatalysts are the most widely studied photocatalysts for wastewater treatment. Photocatalytic oxidation treatment of antibiotic wastewater has the advantages of mild reaction conditions, thorough degradation, and strong applicability. Sood et al. synthesized a Bi2O3/TiO2 photocatalyst by a hydrothermal method, and used the photocatalyst to decompose the simulated ofloxacin antibiotic wastewater. The experimental results showed that the decomposition rate of ofloxacin was 92% after 2 h of light treatment. At the same time, the photocatalytic oxidation method showed a good effect in the treatment of pesticide wastewater, especially organophosphorus pesticide wastewater, and the degradation efficiency, COD and TOC removal rates were all satisfactory. Photocatalytic oxidation has certain advantages in the treatment of organic wastewater, but the main problems are the high cost of catalyst preparation, low utilization of light energy, the possibility of producing more toxic intermediates, and difficulty in catalyst recovery. Therefore, the problems that hinder the application of photocatalytic oxidation methods still need to be further studied.

Moist air oxidation method

The wet air oxidation method appeared in the 1950s. In recent years, great research progress has been made at home and abroad. Japan and the United States have applied it to industrial water treatment. The wet air oxidation reaction also belongs to the free radical chain reaction, and various free radicals act as oxidants to remove organic pollutants. Wet oxidation method mixes wastewater containing organic pollutants with air or oxygen, and oxidizes and decomposes organic matter in wastewater under high temperature and high pressure (150-350℃, 0.5-20MPa). The wet air oxidation method has the advantages of complete oxidation of pollutants and little secondary pollution, and can effectively remove pollutants that are difficult to biodegrade. However, this method also has certain limitations. The reaction needs to be carried out under high temperature and high pressure conditions, which causes serious corrosion to the equipment and requires a large investment in the equipment operation system. Therefore, it has certain limitations in industrial application.

In the wet oxidation reaction process, suitable catalysts can make the reaction time shorter and the reaction conditions easier to achieve. Alternative homogeneous catalysts include transition metals, noble metals, rare earth metals and their oxides and salts. Heterogeneous catalysts are easy to recover and attract more attention. Usually, silica gel, activated carbon, diatomite, alumina and other substances are used as carriers, and various forms of active metals and their oxides are supported for catalytic reactions.

When the wet air oxidation method is used to treat pesticide wastewater, air is continuously introduced into it under high temperature and high pressure conditions, which can efficiently oxidize the organic matter in the wastewater to small molecular organic matter or even completely inorganic. Phosphorus-containing organic compounds are oxidized to phosphoric acid, and organic sulfur compounds are oxidized to form sulfuric acid. In addition to pesticide wastewater, papermaking straw pulp black liquor, gas wastewater, fragrance wastewater, etc. can also be treated by wet air oxidation, which is used in the removal of organic pollutants in industrial wastewater such as pharmaceutical, papermaking, fiber, alcohol, printing and dyeing. better effect.

Electrochemical oxidation method

Electrocatalytic oxidation is an advanced oxidation method that has attracted much attention in recent years. It uses an external electric field to generate strong oxidative free radicals through a series of electrode reactions in the reaction device to oxidatively degrade organic pollutants in sewage. , which are converted into non-toxic or low-toxic small molecule intermediates, and finally completely inorganic.

The development of electrodes with efficient catalytic performance is the most important content in the research of electrocatalytic oxidation. At present, carbon electrodes, non-metallic compound electrodes, titanium-based coated electrodes and other electrode materials have been widely studied and used at home and abroad. Li Hongbo used a diaphragm electrochemical reactor, Ti/SnO2+Sb2O3/PbO2 electrodes as anodes and stainless steel plates as cathodes, to degrade isophthalonitrile simulated wastewater, and the initial concentration of isophthalonitrile wastewater was 250mg/L. The removal rate of organic matter in the system is more than 80%. Some foreigners also used boron-doped diamond thin film electrodes as anodes to treat chlorpyrifos pesticide wastewater with an initial COD of 450 mg/L, and found that the organic matter could be completely oxidized and degraded in only 6 hours. FABIASKA et al. used boron-doped diamond/stainless steel electrode materials in the treatment of wastewater containing five sulfonamide antibiotics. The experimental results show that the degradation mechanism of sulfonamide antibiotics is mainly caused by hydroxyl radicals attacking S—N bonds and benzene rings.

The electrocatalytic oxidation method has a good degradation effect on some organic compounds with stable structure and difficult to degrade, and at the same time, the operation is relatively simple, the cost and operation cost are not high, and it is easy to realize automatic control, so it has a good application prospect.

AOPs technology has received extensive attention in developed countries such as Europe, America and Japan, and has been widely used in petrochemical, pharmaceutical, food, environmental protection and many other industrial fields, but it is still mostly limited to laboratory research in China. First, there is a lack of systematic and in-depth research on the thermodynamics and kinetics of the AOPs process; secondly, due to the various conditions of the reaction system, such as temperature, pressure, etc., have higher requirements on equipment, such as corrosion resistance, high temperature resistance, high pressure resistance, etc. It increases the difficulty of process control and operation, thus hindering the further application of AOPs technology in practice.

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