Classification, biochemical characteristics and wastewater characteristics of 1 organophosphorus pesticides
1. 1 organophosphorus pesticides can be roughly divided into chemical structures.
(1) Phosphates, such as trichlorfon and glyphosate, are easy for biochemical treatment. For example, wastewater such as trichlorfon and monocrotophos produced in Nantong pesticide plant is directly diluted into biochemistry, and the COD removal rate can reach about 85% [1].
(2) Monothiophosphate, such as methyl parathion, methyl pyrimidinyl phosphorus, profenofos, etc. Because it contains sulfur, it smells and cannot be degraded by microorganisms. When it is mixed with biodegradable substances, it can be partially degraded into orthophosphoric acid.
(3) Dithiophosphates, such as dimethoate and malathion, cannot be degraded by microorganisms because of their special sulfur smell. When they are mixed with biodegradable substances, they are rarely degraded into orthophosphoric acid.
It can be seen from the above that phosphorothioate organophosphorus pesticides are the key and difficult point of this kind of pesticides pretreatment, and only through pretreatment and degradation can they further enter the biochemical pool for biochemical treatment.
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2.2 Organic phosphorus pesticide wastewater * * * sex composition
Through the composition analysis of organophosphorus wastewater, it can be known that more than 95% of wastewater is not pesticides, but intermediates and degradation products at different stages (Figure 2). More content:
Three pretreatment methods of organophosphorus pesticide wastewater
In recent years, the treatment of organophosphorus wastewater is basically around the decomposition and removal of organic sulfur and phosphorus in wastewater, which can be roughly divided into physical treatment and chemical treatment. Physical treatment methods include adsorption, extraction, gas stripping, flocculation and precipitation, and chemical treatment methods include oxidation, reduction and hydrolysis.
3. 1 physical treatment
3. 1. 1 adsorption
Adsorption is a process in which one substance adheres to the surface of another. At present, the widely used adsorbents are macroporous resin, activated carbon, fly ash and bentonite. Among them, the use of macroporous resin and activated carbon is limited because of their high prices, and there are problems of activation and regeneration. Although the adsorption effect of fly ash is not as good as that of the former, it is simple and low cost, and it can achieve the effect of treating waste with waste, so it is widely used at present. As reported in literature [2], the phosphorus adsorption rate can reach 97% when using quaternary ammonium salt modified fly ash to treat organophosphorus wastewater.
3. 1.2 extraction
Extraction: An extractant which is insoluble in water but can well dissolve pollutants is used to make it fully contact with wastewater, and the purpose of separating and purifying wastewater is achieved by using the solubility difference of pollutants in water and solvents. Complex extraction and liquid membrane extraction are commonly used. In the treatment of profenofos wastewater, the complexing agent formed by TBP and cyclohexane was used to extract and recover chlorophenol from water, and the recovery rate of chlorophenol could reach 98%. Shenyang institute of chemical technology used liquid membrane method to extract phenol-containing wastewater, and achieved good results [3].
3. 1.3 air stripping and blowing
Gas stripping and stripping is a process of blowing gas into wastewater to turn dissolved gas or volatile substances into gas, thus purifying wastewater. Li Nan Group of Hunan Province adopted stripping method to recover ammonia nitrogen from dimethoate phoxim wastewater, and the removal rate of ammonia nitrogen could reach 85%, which greatly improved the biodegradability of wastewater.
3. 1.4 Flocculation and sedimentation
Flocculation and sedimentation is a method of adding flocculant to destroy the stability of suspended particles in wastewater, eliminate the repulsion between particles, make particles contact and adsorb together, and then be captured by flocculant bridge to form large particles separated from water. This method is simple and low cost, and is widely used in wastewater treatment. Existing flocculants mainly include inorganic flocculants and organic flocculants. Inorganic flocculants mainly include aluminum sulfate, polyaluminum chloride and polyferric sulfate, and organic flocculants mainly include polyacrylamide and formaldehyde-dicyandiamide.
3.2 chemical treatment
3.2. 1 chemical oxidation method
Chemical oxidation methods mainly include electrocatalytic oxidation, Fenton oxidation and wet oxidation.
(1) electro-catalytic oxidation treatment technology
Electrocatalytic oxidation treatment technology is an advanced electrochemical oxidation process, which uses an applied electric field to achieve the expected purpose of removing pollutants from wastewater or recovering useful substances through a series of designed chemical reactions, electrochemical processes or physical processes in a specific electrochemical reactor. In the reaction process, direct oxidation and indirect oxidation are generally carried out at the same time. At present, the widely used electrocatalytic oxidation technologies include activated carbon and inert metals (silver, platinum, titanium, etc.). ) and inert metals coated with oxide film, such as PbO2 _ 2, SnO2 _ 2 and Sb _ 2O _ 5, are used as anode and iron plate is used as cathode, which can remove pollutants and purify water through direct and indirect action of electrodes [4]. Hunan Li Nan Group applied this technology to the treatment of thiophosphate wastewater and methylpyrimidine phosphorus wastewater, and the COD removal rate reached 45%, which greatly improved the biodegradability.
(2) Fenton oxidation method
Fenton process is an advanced oxidation process. The combination of Fe2+ and H2O2 produces highly active hydroxyl radicals, which can effectively treat most refractory organic wastewater. Compared with other advanced oxidation processes, it has the advantages of simple operation and rapid reaction. Because of the use of hydrogen peroxide, the cost is still relatively high, which limits the wide application of this method. For example, Li Rongxi used Fenton method to degrade triazophos pesticide wastewater from Hunan Tianyu Chemical Pesticide Co., Ltd., and the COD removal rate was as high as 95%[5]. In order to improve the efficiency of Fenton reagent, it is reported that UV/Fenton and ultrasonic (microwave) /Fenton method can improve the COD removal rate 10% ~ 20% [6].
(3) Wet oxidation method
Wet oxidation, WAO for short, is a method that decomposes dissolved and suspended organic matter or reducing inorganic matter in water in liquid phase with air and oxygen as oxidants at high temperature and high pressure, and greatly removes COD/BOD/SS. This method is completely oxidized. For example, when treating wastewater containing thiophosphate, it can be completely inorganic. However, this method has high requirements for equipment, harsh reaction conditions and high equipment cost, so it is not widely used in China [7].
Chemical reduction method
Iron/carbon micro-electrolysis belongs to electrochemical reduction technology. Micro-cells formed by iron/carbon system are used to treat refractory pollutants in water. The mechanism of micro-electrolysis mainly includes: (1) adsorption of iron filings; (2) Reduction of iron; (3) Reduction of Fe2+and hydrogen; (4) Flocculation of Fe2+/Fe3+. Kuang Lei, Yang Geng and others used this method to treat the wastewater from the production of chloroethane, an intermediate of organophosphorus pesticides. The removal rates of COD, sulfide and total phosphorus in treated water are as high as 90.2%, 99.4% and 95.0% respectively, and the biodegradability of wastewater is obviously improved, which creates conditions for entering biochemistry [8].
Hydrolysis method
Hydrolysis of organophosphorus pesticides can be divided into alkaline hydrolysis and acid hydrolysis [9]. The mechanism of alkaline hydrolysis is that OH- attacks P atoms and Sn2 substitution occurs. Triglycerides are easily hydrolyzed into diesters under alkaline conditions, but it is difficult to continue hydrolysis, so they usually stay in the first-order hydrolysis stage. The mechanism of hydrolysis reaction under acidic conditions is generally believed that the oxygen atom of the ester is protonated first, then the carbon atom is attacked to produce Sn2 substitution reaction, and after continuous substitution, it is finally hydrolyzed into inorganic phosphorus. Chemical hydrolysis method is feasible in theory and effective in practical application, especially suitable for treating high concentration organophosphorus pesticide wastewater. If isocarbophos is hydrolyzed under acidic conditions, the removal rates of organophosphorus, sulfide, NH3-N and total phosphorus are all above 90%, and the removal rate of COD is above 50% [10].
4 conclusion
There are many kinds of organophosphorus wastewater. According to the structure, the intermediate of * * * has the same wastewater. However, due to the difference in the condensation of the other half of pesticides, different wastewater should be treated in different ways. Treating high-concentration organophosphorus pesticide wastewater by any method alone runs through the classroom discussion. Strengthening the teaching effect and making students master some basic concepts and methods of composite materials are also important for cultivating college students' innovative ability.