Since World War II, chemical pesticides have played a great role in ensuring agricultural production and protecting human health. At present, the annual output of pesticides in the world has exceeded 5 million tons, and it is still growing at a rate of 5% per year. Nevertheless, chemical pesticides are still far from meeting the objective needs.
The rapid development of pesticides is inseparable from its role in ensuring agricultural production increase. In the environment where crops and livestock live, there are about 50,000 kinds of fungi, which can cause 1500 kinds of diseases. There are about 30,000 kinds of weeds in the world, of which 1800 or more can cause economic losses. In addition, there are 654.38+100000 species of insects that can cause various hazards. It is estimated that 50% of the world's total grain output will be swallowed up by diseases, insects and weeds if pesticides are not used. Using pesticides can recover the loss of 15%. The annual losses caused by diseases, insects and weeds in the world are estimated to reach $80 billion. According to the statistics of relevant departments in China, due to the prevention and control of diseases, pests and weeds, China can save grain150-200 billion kilograms and lint 500-600 million tons every year, and the economic benefits of cash crops such as melons, fruits and yancha are more remarkable due to the use of pesticides. In view of the above reasons, human beings have actually been forced to use pesticides, especially in some countries in Asia, Africa and Latin America where the level of drug use is low. In order to solve the food problem, the amount of pesticides will increase greatly. In some developed countries, the use of pesticides is also increasing.
In addition, pesticides also play a great role in medical care. For example, from 1955 to 10 in/965, synthetic pesticides saved150,000 people, far exceeding those saved by antibiotics.
At present, there are more than 65,438+0,200 kinds of pesticides that have been registered and patented in foreign countries, of which about 500 kinds have been put on the market and 60,000 kinds of pesticides.
With the increase of population, the grain output will increase accordingly. The main way to increase grain output is undoubtedly to increase the output per unit area. An important technical measure to increase yield is to use pesticides.
There is no doubt about the great benefits of pesticides, but with the extensive use of pesticides, it has also caused some adverse consequences. In 1960s, people realized that organochlorine pesticides such as DDT not only killed pests, but also killed natural enemies of pests, beneficial birds and pollinators, which destroyed the relative balance of mutual restriction and interdependence in the biological world and caused the rampant of new pests. In addition, long-term use of the same kind of pesticides makes pests resistant, which also increases the dosage of control drugs and the number of control times, greatly increasing the control funds. According to the preliminary investigation abroad, there are about 300 species of pests that have developed drug resistance, among which agricultural pests account for more than 100.
In addition to the above adverse consequences, there is a more important problem, which is pesticide pollution. Direct or indirect harm to human health and biology.
This section will introduce the classification of pesticides, the environmental problems caused by pesticides, and the migration, degradation and residue of pesticides in the soil environment.
I. Classification of pesticides
Pesticide in a broad sense refers to chemicals used in agriculture. Include pesticides, fungicides, herbicides and other chemicals used in agriculture. In a narrow sense, it refers to chemicals that prevent and control insects, germs, weeds, mites and rodents that harm plants and agricultural and forestry products, chemicals that can regulate plant growth, and additives and synergists that increase the effectiveness of these chemicals.
At present, there are about 250 kinds of pesticides commonly used in agriculture, including pesticides 100, herbicides 50, fungicides 50, nematicides 20 and other compounds 30.
Most chemicals used to control diseases, insects and weeds are prepared by factories, and a few of them are naturally found in plants or cultivated by microorganisms. The former, such as DDT, BHC, dimethoate and trichlorfon, are commonly known as chemical pesticides; The latter, such as pyrethrins and rotenone, are generally called plant pesticides; In addition, such as kasugamycin, Jinggangmycin and so on. , commonly known as biological pesticides.
Diseases, insects, weeds and other pests are very different in morphology, behavior, physiological metabolism and so on. Therefore, the control targets of pesticides are different. According to different control objects, pesticides can be divided into insecticides, acaricides, fungicides, nematicides, herbicides, rodenticides, molluscacides, plant growth regulators and other drugs.
Pesticides pollute the environment in many ways, including air, water, soil and crops. Pesticides entering the environment migrate, transform and enrich among environmental elements through the food chain, which will eventually do harm to organisms and human bodies.
1. Pollution of pesticides to the air The pollution of pesticides in the air mainly comes from the floating chemicals produced when spraying pesticides for various purposes, as well as the evaporation, volatilization and diffusion of residual pesticides on crop surfaces, soil surfaces and water bodies. In addition, the exhaust gas from pesticide plant is also one of the reasons why pesticides pollute the air.
Under the action of wind, pesticide floating objects in the atmosphere can cross mountains and seas and reach every corner of the world. It is reported that organochlorine pesticides have been found in the southern part of the earth, the Arctic Circle and the highest peak of the Himalayas.
The pollution of pesticides in the air has the following characteristics:
(1) The pollution of pesticides to the air depends on the use of pesticides. For example, DDT is the main air pollution when DDT pesticides are widely used;
② The degree of pesticide pollution in the air varies from place to place;
③ The pesticide residues in the air increase and decrease regularly with the application time.
2. Pesticide pollution to water. Pesticides in water mainly come from the application of pesticides in farmland, the erosion of pesticides in soil by water flow and the discharge of wastewater from pesticide plant into water.
The United States, Britain, Japan and other countries found that all major rivers were polluted after using organochlorine pesticide 10. China has a similar situation.
Due to the different physical and chemical properties of various water bodies, the degree of pesticide pollution is also different. According to the detection results of organochlorine pesticides in different natural waters in Japan, the pollution sequence is as follows:
Rain > river > seawater > tap water > groundwater.
3. Pesticide pollution to soil The pesticides in the soil mainly come from: ① direct application; (2) Seed soaking, seed dressing and other pesticide application methods are adopted to enter the soil; (3) Pesticides floating in the atmosphere fall to the ground and enter the soil with rainfall and dustfall.
The pollution degree of pesticides to soil depends on the types and properties of pesticides.
Pesticide residues in soil are related to soil type, organic matter content, pH value, type and quantity of metal ions, water content, aeration, vegetation type and coverage, and type and quantity of microorganisms.
The disappearance mechanism of pesticides in soil is generally related to the gasification of pesticides (the process of changing substances from liquid to gas, including evaporation and transpiration), underground infiltration, oxidative hydrolysis and the role of soil microorganisms.
The residue period of pesticides in soil and the residue of organochlorine pesticides in different soils are shown in Table 7.7 and Table 7.8.
Note: * It takes time to digest 95%, * * it takes time to digest 75%- 100%, * * it takes time to digest more than 95%, and a is half-life.
It is reported that 65,438+00,000 people die of pesticide poisoning and 400,000 patients worldwide every year. Developing countries are seriously polluted by pesticides, and 370,000 pesticide poisoning incidents occur every year.
The main way for pesticides to enter the human body is through food intake. Long-term consumption of food contaminated with pesticides will cause the accumulation of some pesticides in human body, which will do harm to human health.
At present, it is considered that the harm of pesticides to human health is: ① the influence on nerves; ② Carcinogenesis; ③ Effect on liver; ④ Induced mutation; ⑤ Chronic poisoning.
In addition, pesticides will also cause pollution and harm to aquatic organisms, birds, animals and plants.
Three. Migration, degradation and residue of pesticides in soil environment
After entering the soil, pesticides have a series of chemical, physical and chemical reactions with solid, gas and liquid substances in the soil. Through the above reaction, the pesticides in the soil have the following three functions: first, the adsorption of the soil makes the pesticides remain in the soil; Secondly, pesticides migrate in air and water in soil and are absorbed by plants; Thirdly, pesticide residues in soil are gradually reduced after chemical, photochemical and biochemical degradation. Let's briefly introduce these three aspects.
1. Adsorption of pesticides by soil
After pesticides enter the soil, they are adsorbed on the surface of soil particles through physical adsorption and chemical adsorption, and then the fluidity and toxicity of pesticides change. In a sense, the adsorption of soil is the purification and detoxification of toxic substances by soil. But this effect is unstable and limited.
The intensity of pesticide adsorption by soil depends on soil characteristics and pesticide properties. On the one hand, the properties of various soils vary greatly, such as organic matter content, turtle load type, specific surface area of clay minerals, pH value and so on. , and the adsorption capacity of pesticides is very different; On the other hand, the properties of pesticides are very different, so the adsorption is also different. Practice has proved that all pesticides with R3N—-,-—CONH2, -OH, -NH2, -OCOR and other functional groups can enhance the adsorption strength, especially the compounds with -NH2 have stronger adsorption capacity.
2. Gas migration and water migration of pesticides in soil.
The gas migration of pesticides mainly refers to the volatilization of pesticides, and the magnitude of volatilization mainly depends on the solubility and vapor pressure of pesticides, as well as the temperature and humidity of soil and the texture and structure of soil.
The diffusion speed of pesticides in soil solution is very slow, and the diffusion speed of water vapor is 10 thousand times higher than it.
There are two main ways of water migration of pesticides: directly dissolving in water and adsorbing on the surface of soil solid particles for mechanical migration with water movement. The gas migration ability and water migration ability of pesticides in soil can be compared by volatilization index and leaching index (Table 7.9). These two indexes are relative values, and the volatile index and leaching index of DDT which are the most difficult to migrate are defined as 1.0. Compared with other pesticides, the greater the index, the stronger the migration ability.
3. Degradation and residue of pesticides in soil
Organic pesticides are gradually decomposed under the action of chemistry and biochemistry, and finally transformed into inorganic substances. This process is called the degradation of chemical pesticides. It is called low-residue pesticide because of its fast degradation speed and short residue time in soil. The degradation rate is slow and the residue time in soil is long, which is called high residue pesticide.
The degradation of pesticides in soil mainly includes photochemical degradation, chemical degradation and biological degradation.
Photochemical degradation refers to the decomposition of pesticides on the soil surface caused by solar radiation and ultraviolet energy.
Chemical degradation can be divided into catalytic reaction and non-catalytic reaction. Non-catalytic reactions include hydrolysis, oxidation, isomerization and ionization.
Biodegradation has many forms, including dechlorination, dealkylation, hydrolysis, ring opening, oxidation and reduction.