Propagation characteristics of heavy metal pollutants

with the intensification of industrial and urban pollution and the increase of the types and quantities of agricultural chemicals, the soil heavy metal pollution in China is becoming more and more serious. The degree of pollution is increasing and the area is expanding year by year. According to the investigation of about 5.48 million hectares of soil in 32 seriously polluted areas in 24 provinces and cities in China by the environmental monitoring system of the Ministry of Agriculture, it was found that the over-standard area of agricultural products in the field accounted for 2% of the farmland area in the polluted areas, of which heavy metal pollution accounted for 8%. According to the national grain survey, the over-standard rate of heavy metals Pb, Cd, Hg and As accounted for 1% [3]. Heavy metal pollutants have poor mobility and long retention time in soil, and most microorganisms cannot degrade them. It can eventually endanger human health through water, plants and other media. 2.1.1 Heavy metals entering the soil with atmospheric sedimentation Heavy metal dust generated by human activities enters the atmosphere in the form of aerosol, and enters the soil through natural sedimentation and precipitation, resulting in soil pollution. In particular, automobile transportation has caused serious pollution along the highway. The soil on both sides of expressways in Jiangsu Province is seriously ill, and 1 meters on both sides of the expressway has become a lead pollution area. The lead pollution to the soil has reached 3cm, which is often the depth of crop growth, which directly leads to the lead content in vegetables and other crops exceeding the standard. Among 3 observation points, the lead content in vegetables exceeds the standard by as much as 6 times. Experts believe that the pollution comes from the lead emitted by automobile exhaust, the unburned tetraethyl lead residue and the dust generated by automobile tire wear entering the soil. Experts call for planting more trees and flowers on both sides of traffic trunk lines, instead of planting vegetables and food crops [4]. Xie Xuejin, the founder of China's geological and chemical exploration discipline and a senior academician of Chinese Academy of Sciences, pointed out that the aggravation of heavy metal pollution in soil in some areas has a great relationship with China's current extensive production methods. Due to the large consumption of coal and other resources, air pollution is aggravated, which leads to the increase of acid rain, thus accelerating the accumulation of heavy metals such as cadmium, mercury, lead and arsenic in the soil, resulting in the increase of "poisoned" soil. Pb, Cr and Co pollution zones are formed in the soil on both sides of Nanjing section of Ning Hang Highway, and they are distributed along the extension direction of the highway, and the pollution degree from both sides of the highway is weakened. Heavy metal pollution entering the soil through natural sedimentation and rain in the atmosphere is directly related to the developed degree of heavy industry, urban population density, land utilization rate and transportation. The order of pollution intensity is: urban-suburban-rural [5]. 2.1.2 Heavy metal sewage entering the soil with sewage irrigation can be divided into urban domestic sewage, petrochemical sewage, industrial and mining enterprise sewage and urban mixed sewage according to the source and quantity. Due to the rapid development of industry in China, the sewage from industrial and mining enterprises is discharged into sewers and mixed with domestic sewage without separate treatment, resulting in the increase of heavy metals such as Hg, Cd, Cr and Pb in sewage irrigation areas year by year. According to the national survey of sewage irrigation areas conducted by China's Ministry of Agriculture, in about 1.4 million hectares of sewage irrigation areas, the land area polluted by heavy metals accounts for 64.8% of the sewage irrigation areas, of which 46.7% are lightly polluted, 9.7% are moderately polluted and 8.4% are seriously polluted [5]. According to the "Environmental Quality Bulletin of Liaoning Province in 24", the soil environmental quality of eight main sewage irrigation areas in Liaoning Province has been polluted to varying degrees, with a polluted area of 64,6 hectares. The main pollutants in sewage irrigation area are cadmium, followed by nickel, mercury and copper. Cadmium and mercury in the soil at the depth of 7-1cm in some heavily polluted areas still exceed the standard [6]. Researcher Chen Tongbin from the Institute of Geographical Sciences and Resources, Chinese Academy of Sciences found that when floods occurred in Guangxi, Yunnan and other places, the sewage with high concentration of heavy metals in the mining minerals accumulated in the upper reaches spread down with the trend, causing pollution to hundreds of kilometers of rivers and farmland in the lower reaches, thus seriously reducing the production of rice fields in a large area or even failing to harvest them. The concentration of Hg in Zhengzhou sewage irrigation area is .242mg/kg, while the content of Hg in soil is .194 mg/kg, which will cause serious pollution [5]. According to Xu Shujun and Wei Shiqiang's analysis of soil samples from 16 cities, counties and districts in Chongqing of the Three Gorges, the contents of heavy metals as, Cd, Cu and Ni in the soil of the reservoir area all exceeded the standard, and CD exceeded the standard to the greatest extent. Heavy metals such as Hg, Co, Cr, Pb and As in the soil of Huaiyang sewage irrigation area exceeded the warning line as early as 1995, and the contents of some heavy metals in other irrigation areas also far exceeded the local background value [7]. In Yixing Sewage Irrigation District, Jiangsu Province, it was found that there were more than 12 carcinogens in rice by Academician Zhao Qiguo of Nanjing Institute of Soil Science, Chinese Academy of Sciences. The detection rate of lead in wheat, rice and flour is as high as 88.1%. According to Xu Ming, deputy director of Wuxi Center for Disease Control and Prevention, the incidence of liver cancer, stomach cancer and lung cancer in Wuxi has increased significantly in recent years. According to analysts, this is related to the once-popular rice in Yixing sewage irrigation area, while the illegal discharge of wastewater from ceramic enterprises in Yixing, Jiangsu Province is still going on [1].

There are many kinds of heavy metal solid wastes that enter the soil with solid wastes, and their components are complex. Among them, mining and industrial solid waste pollution is the most serious. In the process of stacking or treatment, heavy metals in this kind of waste are easy to move due to sun exposure, rain and water washing, and spread to the surrounding soil in a radial and funnel shape. China's solid waste piles pollute about 5, hectares [2], and the waste batteries do great harm to the soil. Zhejiang Geological Survey and Research Institute conducted a survey on Meishan Town, where the storage battery enterprises are most concentrated in Changxing County. The results show that the contents of heavy metals cadmium and lead in Meishan Town of Changxing County have exceeded the national standards. Its pollution source is the disorderly stacking of waste battery materials. Soil pollution makes the lead content of roses in the rose planting base in Shangshi Village, Lincheng Town, Changxing County exceed the standard, which makes it difficult to sell. Not only roses, but also rice, tea, peaches, plums and other agricultural products in Shangshi Village have been tested for exceeding the standard, and the market prospect is bleak [8]. However, due to the massive accumulation of solid wastes, the average lead content in the soil of old residential areas in Nanjing reached 141.6mg/kg, far exceeding the soil background value of 24.8mg/kg [8]. With the development of electronic industry, waste dry batteries, lithium batteries, storage batteries and other electronic wastes have become important sources of soil pollution. According to the calculation, the rotten mercury-containing battery in section 1 can make 1 square meter of land useless, and the waste residue littered by a large number of ceramic enterprises in Yixing, Jiangsu Province is also an important reason for the excessive content of heavy metals in local soil. On March 26th, 27, according to the confirmation of the relevant departments in Zhuzhou City, Hunan Province, 14 children were poisoned by lead from the waste piled up by a lead smelting enterprise in Qijiang Township, Chaling County. The Environmental Protection Bureau of Zhuzhou City collected samples in the vicinity of the waste piled up with lead for testing, and found that the lead content in soil, vegetables and rice exceeded the standard, and some of them exceeded the standard seriously [9]. The study on the contents of heavy metals in the soil near the garbage dump in Wuhan and the road slag dump in Hangzhou found that the contents of heavy metals such as CD, Hg, Cr, Cu, Zn, Pb and As in these areas were higher than the local background values [1]. Some solid wastes containing heavy metals were applied to farmland in large quantities as fertilizers because they contained certain nutrients, which caused the heavy metal content in farmland soil to exceed the standard [5]. Phosphogypsum belongs to the waste of chemical fertilizer industry. Because it contains a certain amount of orthophosphoric acid and different forms of phosphorus-containing compounds, it can improve acidic soil, so it is applied to the soil in large quantities, resulting in an increase in the contents of Cr, Pb, Mn and As in the soil [5]. One year after Yanshan petrochemical sludge was applied to farmland in Beijing, the concentrations of Hg and CD exceeded the standard [5]. According to Fang Shibo and Pan Jianjun's investigation of soil pollution in Nanjing, the soil pollution in the suburbs of Nanjing is mainly mercury and zinc, and the vicinity of Jiangning County is a heavily polluted area. The pollution causes are the disorderly stacking of waste from industrial and mining enterprises, the agricultural use of domestic garbage, and the application of various fertilizers and pesticides [11]. 143 villagers near an arsenic products factory in Binzhou City, Hunan Province were found to have urine arsenic exceeding the standard, and 249 people were hospitalized. According to the investigation by Liao Xiaoyong, Chen Tongbin and others, the water, vegetables, soil and grains near the arsenic factory were all polluted by arsenic to varying degrees. As a result of arsenic pollution, about 5 hectares of rice fields and vegetable fields in this area were abandoned for cultivation, and the arsenic content of Chinese cabbage, radish and spinach planted on polluted soil was seriously exceeding the standard. The reason is that 2.2 Status Quo of Soil Organic Pollution Organic pollutants in soil mainly come from organic pesticides and "three industrial wastes", and the common ones are organic pesticides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and dioxins (PCDDs) in organic halides, and organic compounds such as oil pollutants and phthalates. In addition, the pollution of agricultural film to soil is also quite serious. Some pollutants are widely used in production and life because of their unique thermal stability, chemical stability and insulation properties, which often cause serious cumulative consequences, especially some kinds with hormonal effects, which have interference or negative effects on the reproductive function of human beings and other animals. The elimination and treatment of their toxic effects is a major environmental issue facing mankind. 2.2.1 Organic Pesticides China is a big country that produces and uses pesticides, and the amount of pesticides used every year reaches 5,-6, tons, of which about 8% pesticides directly enter the environment, and the land area where pesticides are used every year is more than 28 million hectares [26]. There are more than 12 kinds of pesticides [16], most of which are organic pesticides. Most pesticides applied in the field will directly enter the soil environment. In addition, the residual pesticides in the atmosphere and pesticides sprayed on crops will also fall into the soil after being leached by rain. Sewage irrigation and surface runoff are also the causes of soil pesticide pollution. The average application of pesticides per hectare in China is 13.9kg, which is about twice as high as that in developed countries, and the utilization rate is less than 3%. Organic pesticides that cause large-scale soil pollution can be divided into organochlorine pesticides, organophosphorus pesticides, carbamate pesticides and phenoxyalkanoate pesticides according to their chemical properties. The first two kinds of pesticides are very toxic, and organochlorine pesticides are not easy to degrade in soil, which causes heavy pollution to the soil. Although organophosphorus pesticides are easy to degrade in soil, they are also widely polluted due to large usage. The latter two kinds of pesticides are less toxic, easy to degrade in soil and have little pollution to soil.