With the rapid development of global economy, pollutants containing heavy metals enter the soil through various channels, causing serious soil pollution. Heavy metal pollution in soil can affect the decline of crop yield and quality, endanger human health through the food chain, and further worsen the quality of air and water environment. Therefore, it has aroused widespread concern all over the world. At present, heavy metal pollution exists in soils all over the world. The average annual emissions of mercury, copper, lead, manganese and nickel are about 1.5 million t, 3.4 million t, 5 million t and 1.5 million t[ 1] respectively. Heavy metal pollution also exists in vegetable bases and some commodity grain bases in northern cities such as Beijing, Tianjin, Xi, Shenyang, Jinan, Changchun and Zhengzhou. .
The south is relatively light, such as Fuzhou, Ningbo, Shanghai, Wuhan, Chengdu and other places. Heavy metal pollution in soil will cause serious damage to the ecosystem. According to the situation of soil resources in China, by the end of 2000, the per capita arable land in China was only 0. 1 hm2. With the economic and social development in China in the future, such as ecological returning farmland, agricultural structure adjustment and natural disasters, soil resources will be further reduced. Therefore, how to effectively control and control soil heavy metal pollution and improve soil quality will become a very important content in ecological environment protection.
Based on the source and distribution of heavy metal pollutants in soil and the current treatment methods of heavy metal pollutants in soil, a new environmental mineralogy method for the prevention and control of heavy metal pollution in soil is proposed. The purpose is to protect the environment and improve the quality of soil environment.
1 Source and distribution of heavy metal pollutants in soil
There are many sources of heavy metals in soil. First of all, the parent material itself contains heavy metals, and the content of heavy metals in soil formed by different parent materials and soil-forming processes is very different. In addition, human industrial and agricultural production activities also cause heavy metal pollution to the atmosphere, water and soil.
1. 1 Heavy metal deposition in the atmosphere
Heavy metals in the atmosphere mainly come from a large number of harmful gases and dust containing heavy metals produced by industrial production, automobile exhaust emission and automobile tire wear. Mainly distributed around industrial and mining areas and on both sides of roads and railways. Most heavy metals in the atmosphere enter the soil through natural sedimentation [2] and rainwater sedimentation. For example, the lead pollution in Fallon, central Sweden [3] mainly comes from a large number of wastes produced by copper industrial plants, sulfuric acid plants, paint factories, mining and chemical industries in the urban area. Due to the transportation of wind, these lead particles spread from industrial waste piles to surrounding areas. The chromium pollution of a chromium-producing heavy industry factory in Nanjing has exceeded the local background value by 4.4 times. The pollution is centered on the workshop chimney, with the range of 65438 0.5km2 and the maximum extension lower limit of 65438±0.38km A sulfuric acid production plant in Russia [5] is also polluted by sulfur, vanadium and arsenic discharged from the factory chimney.
Heavy metal pollution in soil on both sides of highway and railway is mainly Pb, Zn, Cd, Cr, Co and Cu. They come from the burning of leaded gasoline, zinc-containing dust produced by automobile tire wear and so on. It is distributed in a strip shape, and the intensity of heavy metal pollution on both sides of roads and railways is gradually weakening; With the passage of time, heavy metal pollution in highway and railway soil has a strong superposition. Pollution halos of Pb, Cr and Co are formed in the soil on both sides of Nanjing section [6] of Nanjing-Hangzhou Expressway, which are distributed along the extension direction of the highway, and the pollution intensity is weakened from the highway to both sides. The lead content in the soil on both sides of Huaiyin section [7] of Ninglian first-class highway increased, but gradually decreased on both sides, and the lead content in the surface layer was higher at 0~30 cm. Heavy metals such as lead, zinc and cadmium are seriously polluted along A7 1 Expressway [8] in Sologne Province. The concentration of settled particles is 2-8 times higher than the local soil background value, while the concentration of heavy metals along the road is 7-26 times higher than that in settled particles. In Slovenia [9], lead is not only distributed on both sides of the road from Guboga to Zagrebo, but also influenced by terrace landform and prevailing wind. High lead appears in lowlands, and the lead content on the downwind side of the road is higher.
Heavy metal pollution entering the soil through natural sedimentation and rainwater sedimentation mainly centers on industrial and mining chimneys, waste rock piles and highways, and spreads around and on both sides; From urban-suburban-agricultural area, it decreases with the increase of distance from the city, especially in the suburbs of the city. In addition, it is positively related to the population density, urban land utilization rate and motor vehicle density of the city. The more developed the heavy industry, the more serious the pollution.
In addition, the dry and wet deposition of atmospheric mercury [10~ 12] will also cause the increase of mercury content in soil. After atmospheric mercury enters the soil through dry and wet sedimentation, it is adsorbed or fixed by clay minerals and organic matter in the soil, enriched in the surface layer of the soil, or absorbed by plants and transferred to the soil, resulting in an increase in soil mercury concentration.
1.2 use of pesticides, fertilizers and plastic films
The application of pesticides containing lead, mercury, cadmium and arsenic and unreasonable application of chemical fertilizers will lead to heavy metal pollution in soil. Generally, calcium superphosphate contains more heavy metals such as mercury, cadmium, arsenic, zinc and lead, followed by phosphate fertilizer, nitrogen fertilizer and potassium fertilizer, but the content of lead in nitrogen fertilizer is higher, among which arsenic and cadmium are seriously polluted [13]. Shanghai [14] studied vegetable fields and grain and cotton fields. After fertilization, the Cd content increased from 0. 134 mg/kg to 0.3 16 mg/kg, the Hg content increased from 0.22 mg/kg to 0.39 mg/kg, and Cu and Zn increased by 2/3. Based on the analysis of 58 soil samples in the same place in New Zealand [15] 50 years ago and now, since the application of phosphate fertilizer, cadmium has increased from 0.39 mg/kg to 0.85 mg/kg. In Argentina [16], due to the application of traditional inorganic phosphate fertilizer, the soil was polluted by heavy metals such as Cd, Cr, Cu, Zn, Ni and Pb.
Heat stabilizers used in agricultural plastic film production contain Cd and Pb, which will cause heavy metal pollution in the soil in the process of using plastic greenhouses and plastic films in large quantities.
1.3 sewage irrigation
Sewage irrigation generally refers to the use of treated urban sewage to irrigate farmland, forests and grasslands. Urban sewage includes domestic sewage, commercial sewage and industrial wastewater. Due to the rapid development of urban industrialization, a large number of industrial wastewater flows into rivers, which makes many heavy metal ions contained in urban sewage enter the soil with sewage irrigation. In terms of distribution, soil pollution near pollution sources and urban industrial areas is often serious, while soil far away from pollution sources and urban industrial areas is hardly polluted [17]. In recent years, sewage irrigation has become an important part of agricultural irrigation water. Since the 1960s, the sewage irrigation area in China has expanded rapidly, especially in the arid areas in the north, accounting for more than 90% of the total sewage irrigation area in China. Sewage irrigation areas in southern China only account for 6%, and the rest are in northwest China and Qinghai-Tibet [18]. Sewage irrigation leads to an increase in the content of heavy metals such as mercury, cadmium, chromium, arsenic, copper, zinc and lead in soil. Metal mercury, cadmium, chromium, lead, arsenic, etc. Huaiyang sewage irrigation area has gradually increased since sewage irrigation, and has exceeded the warning water level 1995 to 1997 [19]. The content of heavy metals Pb, Cd and Cr in Taiyuan sewage irrigation area far exceeds its local background value, and the accumulation is increasing year by year [20].
1.4 Sludge fertilization
Sludge contains a lot of organic matter and nutrients such as nitrogen, phosphorus and potassium, but it also contains a lot of heavy metals. With a large number of municipal sludge entering farmland, the content of heavy metals in farmland is increasing. Sludge fertilization can increase the contents of Cd, Hg, Cr, Cu, Zn, Ni and Pb in the soil. The more sludge is applied, the more serious the pollution will be. Cd, Cu and Zn will pollute rice and vegetables. Cadmium and mercury will pollute wheat and corn; With the increase of sludge, Cd, Cu, Zn, Ni and Pb in vegetables also increased [2 1]. Anthony[22] research shows that when urban sewage and sludge are used to improve soil, the contents of heavy metals such as Hg, Cd and Pb also increase significantly.
1.5 Accumulation of waste containing heavy metals
There are many kinds of wastes containing heavy metals, and their harm modes and pollution degrees are also different. The pollution range generally spreads around the abandoned pile. Through the study of heavy metal pollution in soils near Wuhan garbage dump [23], a chromium residue dump [24], urban domestic garbage dump [25] and automobile dump [26], the contents of heavy metals Cd, Hg, Cr, Cu, Zn, Ni, Pb, As, Sb, V, Co and Mn in these areas are higher than the local soil background values. Due to different types of waste, the pollution degree of heavy metals is different, such as Cd, Hg and Pb in chromium slag storage area are heavily polluted, Zn is moderately polluted, and Cr and Cu are slightly polluted.
1.6 acid wastewater pollution in metal mines
Mining, smelting, heavy metal tailings, smelting waste residue and heap residue in metal mines can all be leached with acid. With mine drainage and rainfall, it can be brought into water environment (such as rivers) or directly into soil, which can indirectly or directly cause heavy metal pollution in soil. 1989, heavy metals such as Hg 56t, Cd 88t, As 173t and Pb 226t were discharged into the environment by China's nonferrous metallurgy industry [27]. The range of heavy metal pollution in mine acid wastewater is generally around the mine or downstream of the river. Heavy metal pollution in different sections of rivers is often controlled by pollution sources (mines), and the downstream sections of rivers with the same pollution source are from upstream to downstream. Due to the weakening of metal element migration ability and the moderate recovery of water self-purification ability, the intensity of metal chemical pollution gradually decreases. Due to the pollution of Jiangxi Lean Jianggukou-Zhongzhou [28] Dexing copper mine, the contents of heavy metals Cu, Pb, Zn and Cr in water and soil increased and gradually decreased in Poyang Lake. Affected by mining, the concentrations of heavy metal elements Cd, Zn, Pb and As are the highest in Lolado Valley, Colorado, USA, and then gradually decrease with the distance from pollution sources. The heavy metal pollution in Lai 'an River [30] comes from a large copper mine, which leads to the concentration of heavy metals far exceeding the local background value. Heavy metal pollution varies with seasons, and the content of heavy metals in dry season is obviously higher than that in rainy season [3 1]. The slow flow rate of the river can lead to the increase of heavy metal content in this section [32].
The sources of heavy metal pollutants in soil in the same area can be single or multiple. Hu Yongding [33] pointed out that Cr, Cu, Zn and Pb are caused by garbage application, As by agricultural irrigation, Cd by agricultural irrigation and garbage application, and Hg exists in various ways. Wang Wenxiang [34] Through the study of heavy metal pollution of cultivated land in Shandong Province, it is shown that the lead content in the rapidly developing industrial areas is higher than that in the agricultural environment, and the lead content is related to the distance between highways. The backward technical equipment and low utilization rate of raw materials in township enterprises have caused serious heavy metal pollution in the surrounding soil. According to the statistics of Guizhou 1986, the towns and villages in the whole province discharged mercury147,000 kg, and some places in the soil reached 56.64 mg/kg, which was 84.5 times that of unpolluted soil. We should attach great importance to it.
Generally speaking, the higher the degree of industrialization, the more serious the pollution. The urban area is higher than the outer suburbs and rural areas, and the surface is higher than the underground. The longer the polluted area is polluted, the more heavy metals accumulate. The soil polluted by heavy metals in the atmosphere has a strong superposition, and the higher the maturity, the higher the heavy metal content.
2 Current treatment methods of heavy metal pollutants in soil
The research on soil heavy metal pollutants in foreign countries began in the 1960s and 1970s, such as Australia, the United States, Germany and other countries, especially Australia. In 1983, China made a preliminary study on the main types of soil environmental capacity, such as the ecological effects of soil heavy metals, zonal differentiation and critical content zoning.
At present, countries all over the world attach great importance to the research of heavy metal pollution control methods and have carried out extensive research work [35~39]. Generally speaking, there are four kinds of control measures:
2. 1 engineering governance methods
Engineering treatment refers to the treatment of soil heavy metal pollution by using physical or physicochemical principles. Mainly: exotic soil is to add unpolluted new soil to polluted soil; Changing soil means removing polluted soil and replacing it with new soil without pollution; Turning the soil is to turn the polluted topsoil to the lower level; Topsoil removal refers to the removal of contaminated topsoil, etc. For example, the origin of Kawaguchi's disease, a prodigy in Fuji Prefecture, Japan, is caused by long-term consumption of rice containing cadmium. Through research, they removed the topsoil of 15 cm and compacted the subsoil. Under the condition of continuous flooding, the cadmium content in rice is less than 0.4 mg/kg After removing topsoil, the cadmium content of intermittent irrigation rice will not exceed the standard, and the effect will be better if the soil exceeds 20 cm. In addition, the leaching method is to leach contaminated soil with leaching solution; Heat treatment is to heat the polluted soil to volatilize the volatile pollutants (Hg) in the soil and collect them for recycling or treatment. Electrolysis is the removal of heavy metals from soil at anode or cathode under the action of electrolysis, electromigration, electroosmosis and electrophoresis.
The advantages of the above measures are thorough and stable, but the disadvantages are complex implementation, high treatment cost and easy reduction of soil fertility.
2.2 Biological treatment methods
Biological treatment refers to using some habits of organisms to adapt, inhibit and improve heavy metal pollution. It mainly includes: animal control is to use some lower animals in the soil, such as earthworms and mice, to absorb heavy metals in the soil; Microbial treatment is to reduce the toxicity of heavy metals in soil by using the absorption, precipitation, oxidation and reduction of some microorganisms in soil. For example, an enzyme produced by Citrobacter can make U, Pb and Cd form insoluble phosphate. Prokaryotes (bacteria, actinomycetes) are more sensitive to heavy metals than eukaryotes (fungi), and Gram-positive bacteria can absorb Cd, Cu, Ni, Pb and so on [44]. Plant management is to remove heavy metals from soil by using the characteristics that some plants can tolerate and accumulate some heavy metals excessively; The number of plant absorption, leaching and ineffective States of heavy metals will only depend on the number of effective States, and the relationship between the concentration of heavy metal solution in soil and its effective States follows the Freundlich adsorption equation [4 1]. Hyperaccumulator plants can absorb and accumulate a large number of heavy metals, and more than 400 species have been found so far. The contents of Cr, Co, Ni, Cu and Pb accumulated by hyperaccumulator plants are generally above 0. 1%, and the contents of Mn and Zn accumulated are generally above 1% [40]. Indian mustard can absorb zinc, cadmium, copper and lead. , and can grow under the conditions of 250 mg/kg copper, 500 mg/kg lead and 500 mg/kg zinc, and turn yellow at 200 mg/kg cadmium [42]. Indian mustard can enrich Cr6+, Cd, Ni, Zn and Cu by 58, 52, 365, 438+0, 65, 438+07 and 7 times respectively [45]. Agropyron cristatum can absorb Cu [43]; Alpinia plants in Britain can absorb high concentrations of copper, cobalt, manganese, lead, selenium, cadmium and zinc.
The advantages of biological control measures are simple implementation, low investment and little damage to the environment, but the disadvantage is that the control effect is not obvious.
2.3 Chemical treatment methods
Chemical treatment is to put modifiers and inhibitors into polluted soil, increase the content of soil organic matter, cation substitutes and clay particles, change the physical and chemical properties such as pH, Eh and conductivity, and make heavy metals in soil undergo oxidation, reduction, precipitation, adsorption, inhibition and antagonism, thus reducing the bioavailability of heavy metals. Among them, precipitation method means that metal cations in soil solution form metal precipitation when the medium changes (pH, OH-, SO42-, etc.). ) reduce the pollution of heavy metals in soil; If steel slag is put into the soil, it can be easily oxidized into iron oxide in the soil, which can adsorb and precipitate cadmium, nickel and zinc ions, thus fixing metals. The large-scale lime improvement experiment in Zhangshi sewage irrigation area of Shenyang showed that the cadmium content in seeds decreased by 50% per hectare [1500 ~1875 kg]. Organic matter method means that humic acid in organic matter can complex heavy metal ions to form insoluble complexes, thus reducing the pollution of heavy metals in soil; Adsorption method means that heavy metal ions can be adsorbed and fixed by bentonite, zeolite and clay minerals, thus reducing the pollution of heavy metals in soil.
The advantage of chemical treatment measures is that the treatment effect and cost are moderate, but the disadvantage is that it is easy to be activated again.
2.4 Agricultural governance methods
Agricultural management is to change some farming management systems according to local conditions to reduce the harm of heavy metals and plants that do not enter the food chain to polluted soil. It mainly includes: controlling soil moisture means adjusting its oxidation-reduction potential (Eh) by controlling soil moisture to achieve the purpose of reducing heavy metal pollution; Choosing chemical fertilizer refers to choosing chemical fertilizer that can reduce heavy metal pollution in soil without affecting soil fertilizer supply; Increasing the application of organic fertilizer means that organic fertilizer can fix all kinds of heavy metals in soil to reduce the pollution of heavy metals in soil; Selecting crop varieties refers to choosing pollution-resistant plants and not planting plants that enter the food chain on the soil polluted by heavy metals; If ramie is replanted in the soil containing cadmium 100 mg/kg, the cadmium content in the soil will decrease by 27.6% on average after five years [46]; Planting corn, rice, soybeans, wheat, etc. According to local conditions, the heavy metal content absorbed by rice roots accounts for [35]58%~99% of the total crop absorption, while the heavy metal content absorbed by maize stems and leaves accounts for 20%~40% of the total crop absorption, and maize seeds absorb the least. The distribution law of heavy metals in crops is root >: stem and leaf > seed [47]. Heavy metal pollution in soil is also an important factor leading to the destruction of ecosystem. Rational use of agricultural ecosystem engineering measures can also maintain soil fertility, improve and prevent soil heavy metal pollution, improve soil quality, and coordinate with natural ecological cycle and system. If trees, flowers, grass or cash crops (such as castor) can be planted on both sides of roads in polluted areas, turf or ornamental trees can not only beautify the environment, but also purify the soil; Ricinus communis can be used as the raw material of soap. Agricultural improvement can also be carried out, that is, seeds (rice and corn) are cultivated in polluted areas and then planted in non-polluted areas; Or planting non-edible crops (sorghum, corn), extracting alcohol from the harvested straw, pressing the residue into fiberboard, extracting furfural, or making biogas from the residue as energy.
The advantages of agricultural control measures are simple operation and low cost, but the disadvantages are long period and no obvious effect.
3. A new method of treating heavy metal pollutants in soil with natural minerals.
In addition to clay minerals, there are a large number of natural minerals such as iron, manganese and aluminum oxides, hydroxides, silica, carbonates and organic sulfides in the soil. In the research on the prevention and control of heavy metal pollutants in soil at home and abroad, people have always emphasized the self-purification ability of soil, but the self-purification ability of soil can not be separated from the adsorption, desorption, fixation and release of heavy metals by mineral species in soil. The purification ability of specific minerals in soil truly reflects the purification ability and capacity of soil itself. The content of toxic and harmful elements in soil is not the only sign to directly judge the quality of soil environment and even the ecological effect of soil. The key problem is to reveal the environmental balance relationship between these heavy metals and various inorganic substances in the soil. In order to seek the remediation methods of groundwater and soil organic pollution, the modification of various clay minerals in soil is directly studied at home and abroad, that is, organic surfactants are used to replace a large number of exchangeable inorganic cations in natural clay minerals to form organic clay minerals, which can effectively intercept or fix organic pollutants, prevent further pollution of groundwater and limit the migration and diffusion of organic pollutants in soil environment. However, it is particularly important to point out that in the process of clay mineral modification, the fixed heavy metals are also replaced, resulting in the destruction of the established environmental balance in the soil system, and the total amount of heavy metal pollutants desorbed and released in the soil environment is greatly increased. So far, heavy metal pollutants in soil come from both active heavy metals in soil and heavy metals released by replacement when clay minerals are modified. Take natural Fe-Mn-Al oxides and hydroxides as examples [49, 50], among which magnetite, hematite, goethite, pyrolusite, hard manganese ore and bauxite are also becoming one of the key research objects of natural mineral pollution purification methods in the world [5 1]. We think that the surfaces of natural Fe-Mn-Al oxides and hydroxides have obvious chemical adsorption characteristics, and manganese oxides and hydroxides also have relatively perfect pore characteristics. In particular, iron and manganese are rare but common valence-changing elements in nature, and their oxides and hydroxides often show certain redox effects. Therefore, natural Fe-Mn-Al oxides and hydroxides have the potential function of purifying heavy metal pollutants and can be effective substances for adsorbing and fixing heavy metal pollutants in soil environment.
To sum up, some achievements have been made in the present situation and treatment of heavy metal pollution in soil at home and abroad, but there are also some theoretical and technical problems, such as the study on the balance of adsorption and desorption, fixation and release of heavy metals in soil, the systematic study on the morphological characteristics, transformation and migration of heavy metals in soil, and the timely treatment of secondary pollutants in soil.
Soil heavy metal pollution should start from the source and control the pollution source. Heavy metal pollution in soil has reached a serious level, so we should fully understand its long-term, hidden, irreversible and unable to completely decompose or disappear. Soil quality is a strategic issue of sustainable economic development and all-round social progress, which directly affects soil quality, water quality, crop growth, agricultural output, agricultural product quality and so on, and causes harm to human health through the food chain. The pollutants discharged in industrial production have not been completely controlled, especially today when chemical fertilizers and pesticides are used blindly in agricultural production. The total accumulation of inorganic and organic pollutants in rivers, lakes, oceans, groundwater and land is increasing day by day, which makes the environmental quality of land extremely fragile. Once the absorption capacity of soil to these pollutants, especially heavy metals, reaches saturation, the potential destructive damage of these pollutants to the productivity of cultivated land may be imminent, and some people call it a "time bomb" in agricultural production. In this sense, land management and protection is not only the supervision of the total amount of cultivated land, but also the protection and improvement of cultivated land quality. Protecting soil quality is to protect the productivity of cultivated land, and it is also one of the powerful measures to improve the efficiency of land use. For China, a large agricultural country with a large population, it is more necessary and urgent to carry out land quality investigation and evaluation, carry out experimental research on soil heavy metal pollutants, and develop methods and technologies to control cultivated land pollution.