Research status of persistent organic pollutants in the environment

1.2.2. 1 Research Status of POPs in Water

It is known that there are more than 20 kinds of polycyclic aromatic hydrocarbons in surface water, which exist in water in the form of adsorption on suspended matter, dissolution in water and emulsification. Polycyclic aromatic hydrocarbons (PAHs) mainly enter the water body through urban domestic sewage and industrial wastewater discharge, surface runoff, soil leaching, oil leakage, dry and wet deposition of particles caused by long-distance atmospheric transmission and water-gas exchange. Due to the poor water solubility of PAHs, the detection rate of low-ring PAHs is usually higher than that of high-ring PAHs, especially naphthalene, phenanthrene, pyrene, fluoranthene, fluorene and other compounds, and the detection rate of high-ring PAHs is the lowest (Qing Yang Tree et al., 2004). Du Bing et al. (2004) investigated polycyclic aromatic hydrocarbons (PAHs) in the inlet and outlet water of typical sewage treatment plants in Beijing. The results show that the relative proportion of naphthalene and phenanthrene in the total PAHs at each stage is relatively stable, ranging from 60% to 70%. Zhang et al. (2007) measured polycyclic aromatic hydrocarbons (PAHs) in the influent and circulating water of municipal sewage treatment plants. The results show that the total concentrations of 16 polycyclic aromatic hydrocarbons in influent and circulating water are 1777.9ng/L and 1380. 1ng/L, respectively.

Chen Ming et al. (2006a) analyzed organochlorine pesticides in the inlet and outlet water samples of sewage treatment plants in five major cities in Beijing (Gaobeidian, Beixiaohe, Jiuxianqiao, Qinghe and Fangzhuang). The results showed that the total concentration of organochlorine pesticides in the influent of five sewage treatment plants was 10. 1 ~ 65438. Fangzhuang Sewage Treatment Plant mainly treats all domestic sewage in Fangzhuang Community, while other sewage treatment plants treat urban sewage and industrial wastewater. This shows that domestic sewage has less pollution to organochlorine pesticides. The total concentration of organochlorine pesticides in the effluent of sewage treatment plant is 8.93 ~ 70.7 ng/L. Xu Yanling et al. (2006) determined 20 organochlorine pesticides in the influent and secondary effluent of Beijing sewage treatment plant. The results showed that the total mass concentration of HCHs in influent was 65,438+0.3 ng/L, and the main component was relatively stable isomer β-HCH. Only four isomers of HCHs were detected in effluent, and the mass concentration was 65,438+0 ~ 8 ng/L.. Chen Ming et al. (2006b) analyzed the concentration of organochlorine pesticides in wastewater discharged by five typical enterprises of Yanshan Petrochemical Company, and found that there were organochlorine pollutants such as HCHs and DDT. The concentrations of organochlorine pesticides in water samples from five sampling points were 0.76 ~ 14.8 ng/L, and the contents of bhc and DDT were 0.76 respectively.

The persistent organic pollutants in groundwater mainly come from the leakage or recharge of polluted surface water, sewage irrigation, solid waste disposal sites and leaching of contaminated soil. For example, Tian Jiayi et al. (1995) monitored the water quality of sewage irrigation water and 17 underground well at 7 measuring points in Xiaoqing River. The results showed that 93 kinds of organic pollutants were detected in river water, and 56 kinds of organic pollutants were detected in groundwater. The concentrations of benzo [a] pyrene and carbon tetrachloride in groundwater exceeded China's Hygienic Standard for Drinking Water (GB5749—2006). The types, concentrations, sources, categories and logging distribution of organic pollutants detected from river water and groundwater show that Xiaoqing River sewage irrigation has caused organic pollution of coastal groundwater, and the pollution degree is related to the intensity of sewage irrigation. However, at present, there are few studies on how POPs migrate and migrate in soil system and through what mechanism they penetrate into groundwater, and there are few available materials. In the United States, the concentration of carcinogenic polycyclic aromatic hydrocarbons in groundwater is in the range of 0.2 ~ 6.9 ng/L, while the corresponding concentration in surface water is 0. 1 ~ 800 ng/L, mostly in the range of 2 ~ 50 ng/L (Menziesetal. , 1992). Polycyclic aromatic hydrocarbons (benzo [a] pyrene, benzo [g, h, i] perylene, benzo [b] fluoranthene and benzo [k] fluoranthene) are specified in the Water Quality Standard for Urban Water Supply (CJ/T206-2005). It can be seen that the pollution of polycyclic aromatic hydrocarbons in groundwater is relatively rare, and even if there is, the content level is usually not high.

The research on organochlorine pesticides in groundwater environment is also relatively less. Table 1.3 lists the detection of some organochlorine pesticides in groundwater in three areas. It can be seen that compared with the groundwater in Bangladesh 1995, the contents of heptachlor and DDT in the groundwater in the Pearl River Delta region are generally lower, and the contents of organochlorine pesticides in the groundwater in the Pearl River Delta region (Huangguan Xingdeng, 2008) are much lower than those in Su Xichang (China Geo University (Beijing), 2006), but only 4,4'-DDT is in the groundwater in these two regions. Another reason is that China explicitly stipulated to stop using organochlorine pesticides such as DDT, toxaphene, hexachlorobenzene and heptachlor after 1982, while Bangladesh stopped using these organochlorine pesticides after 1993 (Matinetal, 1998).

Table 1.3 Contents of organochlorine pesticides in groundwater in different regions (unit: ng/L)

1.2.2.2 Research Status of Persistent Organic Pollutants in Soil

Because of its low solubility and strong hydrophobicity, POPs can be strongly distributed in soil organic matter, and soil becomes its important destination. It can be transferred into the atmosphere, surface water and groundwater through volatilization, diffusion and mass flow, and it can pose a threat to human health through biological enrichment and food chain. Therefore, the residue of POPs in soil and its migration and transformation law have become the focus of domestic and foreign scholars.

There are certain kinds and quantities of POPs in soils in different areas of China, and PAHs pollution in soils in industrialized areas is particularly serious. Liu Ruimin et al. (2004) compared the polycyclic aromatic hydrocarbons (PAHs) content in soil in Tianjin urban area with some foreign cities, and found that the overall level of PAHs content in soil in areas with higher industrialization and urbanization level was higher. Generally speaking, the main source of polycyclic aromatic hydrocarbons pollution in urban areas is combustion, and in some cases, oil and mineral pollution are also one of the main sources of pollution. Ge et al. (2006) investigated the residue of 15 polycyclic aromatic hydrocarbons in agricultural soil (0 ~ 20 cm) around a large mining enterprise in Nanjing. The results showed that the detection rate of polycyclic aromatic hydrocarbons was 100%, the total residue ranged from 312.2 to 27580.9 μ g/kg, and the main component was polycyclic aromatic hydrocarbons with more than 4 rings.

Sewage irrigation is the main way of soil POPs pollution in China. Long-term sewage irrigation will lead to a significant increase in the content of POPs in soil in sewage irrigation area, which will seriously exceed the environmental standards. Peng Hua et al. (2009) conducted a preliminary study on polycyclic aromatic hydrocarbons (PAHs) pollution in soils of typical agricultural areas in Henan Province, and the results showed that PAHs pollution in soils of sewage irrigation areas was the most serious. Qu Jian et al. (2006) studied the content of polycyclic aromatic hydrocarbons in the upstream soil of Shenfu Irrigation District. The results showed that the content of polycyclic aromatic hydrocarbons in soil was 787 ~ 24570 μ g/kg, which was significantly higher than that in clean water irrigated soil. Zhang Jing et al. (2007) studied Shenfu Sewage Irrigation Area, and the results showed that the PAHs content in paddy soil was between 365,438+09.5 ~ 362.8 μ g/kg. Xiao Ru et al. (2006) studied the distribution characteristics of 16 polycyclic aromatic hydrocarbons in three soil profiles with sewage irrigation history: Shenfu Irrigation Area, Hunpu Irrigation Area and Qingyuan Control Point. The results show that the amount of polycyclic aromatic hydrocarbons detected in Shenfu irrigation area and Hunpu irrigation area is obviously larger than that in Qingyuan irrigation area. Song et al. (1997) showed that sewage irrigation during rice growing period can obviously increase the total amount of polycyclic aromatic hydrocarbons in soil, and the behavior of polycyclic aromatic hydrocarbons in soil is related to the physical and chemical properties of pollutants. Canitar (2008) studied 16 polycyclic aromatic hydrocarbons in soil (0 ~ 20 cm) in Beijing-Tianjin sewage irrigation area. The results showed that the total amount of polycyclic aromatic hydrocarbons in Tianjin soil was 1304 ~ 3369 μ g/kg, and that in Beijing soil was 2687 ~ 49 16 μ g, mainly 2 ~ 4 polycyclic aromatic hydrocarbons, and the total amount of polycyclic aromatic hydrocarbons was significantly related to soil organic carbon. The research of Zhang Zhihuan et al. (2004) showed that the content of polycyclic aromatic hydrocarbons in soil in Tianjin sewage irrigation area was significantly higher than that in non-sewage irrigation area, and the content of PAHs above 4 rings was significantly higher.

The research of organochlorine pesticides is mainly concentrated in the surface soil, but there is little research on the content of organochlorine in soil profile. The existing research results show that the migration ability of organochlorine pesticides in soil profile is poor, and they are mainly accumulated in surface soil. For example, the research results of Li Changliang et al. (2008) show that the maximum value of ∑HCH in the contaminated site of an enterprise is 27 1.72mg/kg, which is mainly concentrated in surface soil (0-20~40cm) and subsurface soil (20-40 cm), and the content of HCH in surface soil and subsurface soil is much higher than that in deep soil (40-6540). The research results of Nana Zhao et al. (2007) showed that the DDT content in the surface soil around the workshop was as high as 104mg/kg, and in the deep soil, the concentration decreased rapidly with the increase of depth, and the maximum depth of the content gradient varied between 0.2-3m, and the detection value was small in the soil below 3m. And the maximum concentration at the depth of 10 m is not more than 8.89mg/kg. From their research results, it can be seen that HCH and DDT mainly accumulate in the soil surface layer, and their downward migration ability is poor. Long-term sewage irrigation is likely to lead to the cumulative pollution of organochlorine pesticides in the surface soil. Gong et al. (2002) showed that the detection rate of 8 organochlorine pesticides such as DDT in farmland soil in Tianjin suburban sewage irrigation area was 65,438+000%, and the main organochlorine pollutants were β-HCH and 4,4'-DDE, among which the highest residue of β-HCH reached 65,438+02.87 mg/kg, which was the most polluted dry land in the whole year. Sun Libo et al. (2006) studied the soil profile of a sewage irrigation area, and the results showed that the contents of HCH and DDT in all soil samples did not exceed the soil environmental quality standard (GB15618-1995). Compared with the local soil background value, the contents of HCH and DDT in a few soil samples have an upward trend, exceeding the local soil background value. The contents of the two pollutants in the upper soil (0 ~ 60 cm) were significantly higher than those in the lower soil (60 ~ 100 cm).

Summarizing the data published at home and abroad in recent years, the residue of PAHs in soil is shown in table 1.4, and the residue of OCPs is shown in table 1.5.

Table 1.4 Detection of Polycyclic Aromatic Hydrocarbons in Soil at Home and Abroad

sequential

Table 1.5 Residues of organochlorine pesticides in shallow soil at home and abroad (unit: μ g/kg)

To sum up, scholars at home and abroad have done a lot of research on POPs in soil, but most of them are in the background investigation stage of POPs pollution level, and mainly focus on the study of surface soil, and there are few reports on the migration and change of POPs in deep soil. Most of the research work is concentrated in an independent environment, and the surface, soil and groundwater are not studied as a linked system. In addition, the research scope is too narrow, mostly confined to the POPs residues in agricultural soil and different types of vegetable fields, while the research on POPs in soil in sewage irrigation areas is mainly concentrated in Tianjin and Shenyang, and there is almost no systematic research in other places, and the POPs pollution in soil produced by reclaimed water irrigation is even less in relevant research fields at home and abroad. As mentioned above, the shortage of water resources in China is increasingly prominent, and sewage irrigation will lead to the deterioration of soil environmental quality and seriously endanger human health. Therefore, in recent years, reclaimed water has gradually replaced sewage for irrigation of farmland to save water resources. However, due to the insufficient utilization and research of urban reclaimed water in China, and the fact that reclaimed water still contains a large number of persistent toxic substances, whether it will cause soil pollution has attracted scholars' attention, but the research in this field is very weak and there are few related reports. Therefore, it is of great significance to study whether reclaimed water irrigation will cause POPs pollution to soil and groundwater.