Main sources heavy metals- waste industrial enterprises, various types of power plants, factories from the mining and processing industries, as well as exhaust from automobiles and some other equipment. Most often, heavy metals enter the environment in the form of aerosols or such chemical compounds such as sulfates, sulfides, carbonates, oxides, etc.

Which heavy metals most often pollute soil? The most common heavy metals in industrial waste are mercury, lead and cadmium. Also often found among harmful emissions arsenic, zinc, iron, copper and manganese.

Heavy metals can enter the environment in insoluble and soluble forms.

Ways of soil contamination with heavy metals

The first way heavy metals pollute the soil is when it gets into the water and then spreads this water into the soil.

Another option is for heavy metals to enter the atmosphere and precipitate through dry deposition or wet deposition.

Interaction of soil with heavy metals

Soil is an adsorbent various types chemical elements, including heavy metals. Throughout long period they are in the ground, undergoing gradual decontamination. For some heavy metals, these periods can be several hundred or even thousands of years.

Heavy and other metal ions can react with soil components and are disposed of by leaching, erosion, deflation and by plants.

What methods exist for determining heavy metals in soil?

First of all, you need to understand that the composition of the soil is heterogeneous, so even on the same plot of land soil indicators can vary greatly in different parts of it. Therefore, you need to take several samples and either study each one separately, or mix them into a single mass and take a sample for study from there.

The number of methods for determining metals in soil is quite large, for example, some of them are:

• method for determining mobile forms.
• method for determining exchange forms.
• method for identifying acid-soluble (technogenic) forms.
• gross content method.

Using these techniques, the process of extracting metals from the soil is carried out. Subsequently, it is necessary to determine the percentage of certain metals in the hood itself, for which three main technologies are used:

2) Mass spectrometry with inductively coupled plasma.

3) Electrochemical methods.

The device for the appropriate technology is selected depending on what element is being studied and what its concentration is expected in the soil extract.

Spectrometric methods for studying heavy metals in soil

1) Atomic absorption spectrometry.

A soil sample is dissolved in a special solvent, after which the reagent binds to a specific metal, precipitates, dries and calcines so that the weight becomes constant. Then weighing is carried out using an analytical balance.

The disadvantages of this method include the significant amount of time required for analysis and high level researcher qualifications.

2) Atomic absorption spectrometry with plasma atomization.

This is a more common method that allows you to determine several different metals at once. Also distinguished by accuracy. The essence of the method is as follows: the sample must be transferred to a gaseous atomic state, then the degree of absorption of radiation by gas atoms - ultraviolet or visible - is analyzed.

Electrochemical methods for studying heavy metals in soil

The preparatory stage consists of dissolving the soil sample in aqueous solution. In the future, the following technologies for determining heavy metals in it are used:

• potentiometry.
• voltammetry.
• conductometry.
• Coulometry.

Heavy metals are perhaps one of the most serious soil contaminations, which threaten us with a host of undesirable and, moreover, harmful consequences.

By its nature, soil is a combination of various clay minerals of organic and inorganic origin. Depending on the composition of the soil, geographical data, as well as the distance from industrial zones, the soil may contain various types heavy metals, each of which poses a varying degree of danger to environment. Due to the fact that in different places The soil structure may also be different, redox conditions, reactivity, as well as the mechanisms for binding heavy metals in the soil are also different.

The greatest danger to the soil comes from technogenic factors. Various productions, the waste of which is particles of heavy metals, unfortunately, are equipped in such a way that even the most best filters allow elements of heavy metals to pass through, which first end up in the atmosphere and then, together with industrial waste, penetrate into the soil. This type of pollution is called technogenic. In this case great importance has the mechanical composition of the soil, carbonate content and absorption capacity. Heavy metals differ not only in the degree of impact on the soil, but also in the state in which they are found in it.

It is now known that almost all heavy metal particles can be present in the soil in the following states: as a mixture of isomorphic particles, oxidized, in the form of salt deposits, in a crystal lattice, soluble form, directly in the soil solution, and even as part of organic matter. It should be taken into account that depending on redox conditions, soil composition and carbon dioxide levels, the behavior of metal particles may change.

Heavy metals are dangerous not only because of their presence in the soil composition, but because they are able to move, change and penetrate into plants, which can cause significant harm to the environment. The mobility of heavy metal particles can vary depending on whether there is a difference between the elements in the solid and liquid phases. Pollutants, in this case elements of heavy metals, can often take on a firmly fixed form when penetrating into soil layers. In this form, metals are inaccessible to plants. In all other cases, metals easily penetrate into plants.

Water-soluble metal elements penetrate into the soil very quickly. Moreover, they not only enter the soil layer, they are able to migrate through it. From school, everyone knows that over time, low molecular weight, water-soluble mineral compounds are formed in the soil, which migrate into bottom part layer. And along with them, heavy metal compounds migrate, forming low-molecular complexes, that is, transforming into another state.

Heavy metals in soil

IN lately Due to the rapid development of industry, there is a significant increase in the level of heavy metals in the environment. The term “heavy metals” is applied to metals either with a density exceeding 5 g/cm 3 or with an atomic number greater than 20. Although, there is another point of view, according to which over 40 chemical elements with atomic masses exceeding 50 are classified as heavy metals at. units Among chemical elements, heavy metals are the most toxic and are second only to pesticides in their level of danger. At the same time, the following chemical elements are considered toxic: Co, Ni, Cu, Zn, Sn, As, Se, Te, Rb, Ag, Cd, Au, Hg, Pb, Sb, Bi, Pt.

The phytotoxicity of heavy metals depends on their chemical properties: valence, ionic radius and complexability. In most cases, elements are arranged in the order of toxicity: Cu > Ni > Cd > Zn > Pb > Hg > Fe > Mo > Mn. However, this series may vary somewhat due to unequal precipitation of elements by the soil and transfer to a state inaccessible to plants, growing conditions, and the physiological and genetic characteristics of the plants themselves. The transformation and migration of heavy metals occurs under the direct and indirect influence of the complexation reaction. When assessing environmental pollution, it is necessary to take into account the properties of the soil and, first of all, the granulometric composition, humus content and buffering capacity. Buffer capacity is understood as the ability of soils to maintain the concentration of metals in the soil solution at a constant level.

In soils, heavy metals are present in two phases - solid and in soil solution. The form of existence of metals is determined by the reaction of the environment, the chemical and material composition of the soil solution and, first of all, the content of organic substances. Complexing elements that pollute the soil are concentrated mainly in its upper 10 cm layer. However, when low-buffer soil is acidified, a significant proportion of metals from the exchange-absorbed state passes into the soil solution. Strong migratory ability in acidic environment possess cadmium, copper, nickel, cobalt. A decrease in pH by 1.8-2 units leads to an increase in the mobility of zinc by 3.8-5.4, cadmium by 4-8, copper by 2-3 times. .

Table 1 Maximum permissible concentration (MAC) standards, background contents of chemical elements in soils (mg/kg)

Thus, when heavy metals enter the soil, they quickly interact with organic ligands to form complex compounds. So, at low concentrations in soil (20-30 mg/kg), approximately 30% of lead is in the form of complexes with organic matter. The proportion of complex lead compounds increases with increasing concentration up to 400 mg/g, and then decreases. Metals are also sorbed (exchangeably or nonexchangeably) by sediments of iron and manganese hydroxides, clay minerals, and soil organic matter. Metals available to plants and capable of leaching are found in the soil solution in the form of free ions, complexes and chelates.

The absorption of HMs by soil largely depends on the reaction of the environment and on which anions predominate in the soil solution. In an acidic environment, copper, lead and zinc are more sorbed, and in an alkaline environment, cadmium and cobalt are intensively absorbed. Copper preferentially binds to organic ligands and iron hydroxides.

Table 2 Mobility of microelements in various soils depending on the pH of the soil solution

Soil and climatic factors often determine the direction and speed of migration and transformation of HMs in the soil. Thus, the conditions of the soil and water regimes of the forest-steppe zone contribute to intensive vertical migration of HM along the soil profile, including the possible transfer of metals with water flow along cracks, root passages, etc. .

Nickel(Ni) - group VIII element periodic table with an atomic mass of 58.71. Nickel, along with Mn, Fe, Co and Cu, belongs to the so-called transition metals, the compounds of which have a high biological activity. Due to the structural features electron orbitals the above metals, including nickel, have a pronounced ability to form complexes. Nickel is capable of forming stable complexes, for example, with cysteine ​​and citrate, as well as with many organic and inorganic ligands. The geochemical composition of source rocks largely determines the nickel content in soils. Largest quantity Nickel is contained in soils formed from basic and ultrabasic rocks. According to some authors, the boundaries of excess and toxic levels of nickel for most species vary from 10 to 100 mg/kg. The bulk of nickel is immovably fixed in the soil, and very weak migration in the colloidal state and in the composition of mechanical suspensions does not affect their distribution along the vertical profile and is quite uniform.

Lead (Pb). The chemistry of lead in the soil is determined by the delicate balance of oppositely directed processes: sorption-desorption, dissolution-transition to the solid state. Lead released into the soil is included in a cycle of physical, chemical and physicochemical transformations. At first, the processes of mechanical movement (lead particles move along the surface and through cracks in the soil) and convective diffusion dominate. Then, as solid-phase lead compounds dissolve, more complex compounds come into play. physical and chemical processes(in particular, ion diffusion processes), accompanied by the transformation of lead compounds arriving with dust.

It has been established that lead migrates both vertically and horizontally, with the second process prevailing over the first. Over 3 years of observations in a mixed-grass meadow, lead dust applied locally to the soil surface moved horizontally by 25-35 cm, and the depth of its penetration into the soil thickness was 10-15 cm. Important role lead migration plays biological factors: plant roots absorb metal ions; during the growing season they move through the soil; When plants die and decompose, lead is released into the surrounding soil mass.

It is known that soil has the ability to bind (sorb) technogenic lead entering it. Sorption is believed to include several processes: complete exchange with cations of the soil absorbing complex (nonspecific adsorption) and a series of reactions of lead complexation with donors of soil components (specific adsorption). In soil, lead is associated mainly with organic matter, as well as with clay minerals, manganese oxides, and iron and aluminum hydroxides. By binding lead, humus prevents its migration into adjacent environments and limits its entry into plants. Of the clay minerals, illites are characterized by a tendency to sorption of lead. An increase in soil pH during liming leads to even greater binding of lead in the soil due to the formation of sparingly soluble compounds (hydroxides, carbonates, etc.).

Lead, present in the soil in mobile forms, is fixed by soil components over time and becomes inaccessible to plants. According to domestic researchers, lead is most firmly fixed in chernozem and peat-silt soils.

Cadmium (Cd) The peculiarity of cadmium, which distinguishes it from other HMs, is that in the soil solution it is present mainly in the form of cations (Cd 2+), although in soil with a neutral reaction environment it can form sparingly soluble complexes with sulfates and phosphates or hydroxides.

According to available data, the concentration of cadmium in soil solutions of background soils ranges from 0.2 to 6 μg/l. In areas of soil pollution it increases to 300-400 µg/l. .

It is known that cadmium in soils is very mobile, i.e. capable of moving into large quantities from the solid phase to the liquid phase and back (which makes it difficult to predict its entry into the plant). The mechanisms that regulate the concentration of cadmium in the soil solution are determined by sorption processes (by sorption we mean adsorption itself, precipitation and complexation). Cadmium is absorbed by soil in smaller quantities than other HMs. To characterize the mobility of heavy metals in soil, the ratio of metal concentrations in the solid phase to that in the equilibrium solution is used. High values This ratio indicates that heavy metals are retained in the solid phase due to the sorption reaction; low values ​​are due to the fact that the metals are in solution, from where they can migrate to other environments or enter into various reactions (geochemical or biological). It is known that the leading process in the binding of cadmium is adsorption by clays. Research recent years also showed the important role of hydroxyl groups, iron oxides and organic matter in this process. At low level pollution and neutral reaction environment, cadmium is adsorbed mainly by iron oxides. And in an acidic environment (pH=5), organic matter begins to act as a powerful adsorbent. At lower pH values ​​(pH=4), adsorption functions shift almost exclusively to organic matter. Mineral components cease to play any role in these processes.

It is known that cadmium is not only sorbed by the soil surface, but is also fixed due to precipitation, coagulation, and interpacket absorption by clay minerals. It diffuses inside soil particles through micropores and other ways.

Cadmium is fixed in different ways in soils different types. So far, little is known about the competitive relationships of cadmium with other metals in sorption processes in the soil-absorbing complex. According to specialist research Technical University Copenhagen (Denmark), in the presence of nickel, cobalt and zinc, the uptake of cadmium by the soil was suppressed. Other studies have shown that the processes of cadmium sorption by soil are damped in the presence of chlorine ions. Saturation of soil with Ca 2+ ions led to an increase in cadmium sorption. Many bonds of cadmium with soil components turn out to be fragile; under certain conditions (for example, an acidic reaction of the environment), it is released and goes back into solution.

The role of microorganisms in the process of dissolution of cadmium and its transition to a mobile state has been revealed. As a result of their vital activity, either water-soluble metal complexes are formed, or physicochemical conditions are created that are favorable for the transition of cadmium from the solid phase to the liquid phase.

The processes occurring with cadmium in the soil (sorption-desorption, transition into solution, etc.) are interconnected and interdependent; the supply of this metal to plants depends on their direction, intensity and depth. It is known that the amount of cadmium sorption by soil depends on the pH value: the higher the soil pH, the more cadmium it sorbs. Thus, according to available data, in the pH range from 4 to 7.7, with an increase in pH by one unit, the sorption capacity of soils with respect to cadmium increased approximately threefold.

Zinc (Zn). Zinc deficiency can manifest itself both on acidic, highly podzolized light soils, and on carbonate soils, poor in zinc, and on highly humus-rich soils. Increases the manifestation of zinc deficiency high doses phosphorus fertilizers and strong plowing of the subsoil to the arable horizon.

The highest gross zinc content is in tundra (53-76 mg/kg) and chernozem (24-90 mg/kg) soils, the lowest in soddy-podzolic soils (20-67 mg/kg). Zinc deficiency most often occurs on neutral and slightly alkaline carbonate soils. IN acidic soils zinc is more mobile and available to plants.

Zinc in the soil is present in ionic form, where it is adsorbed by the cation exchange mechanism in acidic or as a result of chemisorption in alkaline environment. The most mobile ion is Zn 2+. The mobility of zinc in soil is mainly affected by pH and the content of clay minerals. At pH<6 подвижность Zn 2+ возрастает, что приводит к его выщелачиванию. Попадая в межпакетные пространства кристаллической решетки монтмориллонита, ионы цинка теряют свою подвижность. Кроме того, цинк образует устойчивые формы с органическим веществом почвы, поэтому он накапливается в основном в горизонтах почв с высоким содержанием гумуса и в торфе .

It's no secret that everyone wants to have a dacha in an ecologically clean area, where there is no urban gas pollution. The environment contains heavy metals (arsenic, lead, copper, mercury, cadmium, manganese and others), which even come from car exhaust gases. It should be understood that the earth is a natural purifier of the atmosphere and groundwater; it accumulates not only heavy metals, but also harmful pesticides and hydrocarbons. Plants, in turn, take in everything that the soil gives them. Metal, settling in the soil, harms not only the soil itself, but also plants, and as a result, humans.

Near the main road there is a lot of soot, which penetrates the surface layers of the soil and settles on the leaves of plants. Root crops, fruits, berries and other fertile crops cannot be grown in such a plot. The minimum distance from the road is 50 m.

Soil filled with heavy metals is bad soil; heavy metals are toxic. You will never see ants, ground beetles or earthworms on it, but there will be a large concentration of sucking insects. Plants often suffer from fungal diseases, dry out and are not resistant to pests.

The most dangerous are mobile compounds of heavy metals, which are easily formed in acidic soil. It has been proven that plants grown in acidic or light sandy soil contain more metals than those grown in neutral or calcareous soil. Moreover, sandy soil with an acidic reaction is especially dangerous; it accumulates easily and is just as easily washed out, ending up in groundwater. A garden plot, where the lion's share is clay, is also easily susceptible to the accumulation of heavy metals, while self-cleaning occurs long and slowly. The safest and most stable soil is chernozem, enriched with lime and humus.

What to do if there are heavy metals in the soil? There are several ways to solve the problem.

1. An unsuccessful plot can be sold.

2. Liming is a good way to reduce the concentration of heavy metals in the soil. There are different ones. The simplest one: throw a handful of soil into a container with vinegar; if foam appears, then the soil is alkaline. Or dig a little into the soil, if you find a white layer in it, then acidity is present. The question is how much. After liming, check regularly for acidity; you may need to repeat the procedure. Lime with dolomite flour, blast furnace slag, peat ash, limestone.

If a lot of heavy metals have already accumulated in the ground, then it will be useful to remove the top layer of soil (20-30 cm) and replace it with black soil.

3. Constant feeding with organic fertilizers (manure, compost). The more humus in the soil, the less heavy metals it contains, and toxicity decreases. Poor, infertile soil is not able to protect plants. Do not oversaturate with mineral fertilizers, especially nitrogen. Mineral fertilizers quickly decompose organic matter.

4. Surface loosening. After loosening, be sure to apply peat or compost. When loosening, it is useful to add vermiculite, which will become a barrier between plants and toxic substances in the soil.

5. Washing the soil only with good drainage. Otherwise, heavy metals will spread throughout the area with water. Fill with clean water so that a layer of soil of 30-50 cm is washed for vegetable crops and up to 120 cm for fruit bushes and trees. Flushing is carried out in the spring, when there is enough moisture in the soil after winter.

6. Remove the top layer of soil, make good drainage from expanded clay or pebbles, and fill the top with black soil.

7. Grow plants in containers or a greenhouse where the soil can be easily replaced. Observe, do not grow the plant in one place for a long time.

8. If the garden plot is near the road, then there is a high probability that there is lead in the soil, which comes out with car exhaust gases. Extract lead by planting peas between plants; do not harvest. In the fall, dig up the peas and burn them along with the fruits. The soil will be improved by plants with a powerful, deep root system, which will transfer phosphorus, potassium and calcium from the deep layer to the upper layer.

9. Vegetables and fruits grown in heavy soil should always be subjected to heat treatment or at least washed under running water, thus removing atmospheric dust.

10. In polluted areas or areas near the road, a continuous fence is installed; the chain-link mesh will not become a barrier against road dust. Be sure to plant deciduous trees behind the fence (). As an option, multi-tiered plantings, which will play the role of protectors from atmospheric dust and soot, will be excellent protection.

The presence of heavy metals in the soil is not a death sentence; the main thing is to identify and neutralize them in a timely manner.

heavy metal plant soil

The content of HMs in soils depends, as has been established by many researchers, on the composition of the original rocks, the significant diversity of which is associated with the complex geological history of the development of the territories (Kovda, 1973). The chemical composition of soil-forming rocks, represented by rock weathering products, is predetermined by the chemical composition of the original rocks and depends on the conditions of supergene transformation.

In recent decades, anthropogenic activities of mankind have been intensively involved in the processes of migration of heavy metals in the natural environment. The amounts of chemical elements entering the environment as a result of technogenesis, in some cases, significantly exceed the level of their natural intake. For example, the global release of Pb from natural sources per year is 12 thousand tons. and anthropogenic emissions 332 thousand tons. (Nriagu, 1989). Being included in natural migration cycles, anthropogenic flows lead to the rapid spread of pollutants in the natural components of the urban landscape, where their interaction with humans is inevitable. The volume of pollutants containing heavy metals increases every year and damages the natural environment, undermines the existing ecological balance and negatively affects human health.

The main sources of anthropogenic entry of heavy metals into the environment are thermal power plants, metallurgical enterprises, quarries and mines for the extraction of polymetallic ores, transport, chemical means of protecting crops from diseases and pests, burning oil and various wastes, production of glass, fertilizers, cement, etc. The most powerful HM halos arise around ferrous and especially non-ferrous metallurgy enterprises as a result of atmospheric emissions (Kovalsky, 1974; Dobrovolsky, 1983; Israel, 1984; Geokhimiya..., 1986; Sayet, 1987; Panin, 2000; Kabala, Singh, 2001). The effect of pollutants extends over tens of kilometers from the source of elements entering the atmosphere. Thus, metals in amounts from 10 to 30% of the total emissions into the atmosphere are distributed over a distance of 10 km or more from an industrial enterprise. In this case, a combined pollution of plants is observed, consisting of the direct deposition of aerosols and dust on the surface of leaves and the root absorption of heavy metals accumulated in the soil over a long period of time of receipt of pollution from the atmosphere (Ilyin, Syso, 2001).

Based on the data below, one can judge the size of mankind's anthropogenic activity: the contribution of technogenic lead is 94-97% (the rest is natural sources), cadmium - 84-89%, copper - 56-87%, nickel - 66-75%, mercury - 58%, etc. At the same time, 26-44% of the global anthropogenic flow of these elements occurs in Europe, and the European territory of the former USSR accounts for 28-42% of all emissions in Europe (Vronsky, 1996). The level of technogenic fallout of heavy metals from the atmosphere in different regions of the world is not the same and depends on the presence of developed deposits, the degree of development of the mining and processing and industrial industries, transport, urbanization of territories, etc.

A study of the share of various industries in the global flow of HM emissions shows: 73% of copper and 55% of cadmium are associated with emissions from copper and nickel production enterprises; 54% of mercury emissions come from coal combustion; 46% of nickel - for combustion of petroleum products; 86% of lead enters the atmosphere from vehicles (Vronsky, 1996). A certain amount of heavy metals is also supplied to the environment by agriculture, where pesticides and mineral fertilizers are used; in particular, superphosphates contain significant amounts of chromium, cadmium, cobalt, copper, nickel, vanadium, zinc, etc.

Elements emitted into the atmosphere through pipes of chemical, heavy and nuclear industries have a noticeable effect on the environment. The share of thermal and other power plants in atmospheric pollution is 27%, ferrous metallurgy enterprises - 24.3%, enterprises for the extraction and production of building materials - 8.1% (Alekseev, 1987; Ilyin, 1991). HM (with the exception of mercury) are mainly introduced into the atmosphere as part of aerosols. The set of metals and their content in aerosols are determined by the specialization of industrial and energy activities. When coal, oil, and shale are burned, elements contained in these types of fuel enter the atmosphere along with smoke. Thus, coal contains cerium, chromium, lead, mercury, silver, tin, titanium, as well as uranium, radium and other metals.

The most significant environmental pollution is caused by powerful thermal power plants (Maistrenko et al., 1996). Every year, only when burning coal, mercury is released into the atmosphere 8700 times more than can be included in the natural biogeochemical cycle, uranium - 60 times, cadmium - 40 times, yttrium and zirconium - 10 times, tin - 3-4 times. 90% of cadmium, mercury, tin, titanium and zinc that pollute the atmosphere enter it when burning coal. This significantly affects the Republic of Buryatia, where energy enterprises using coal are the largest polluters of the atmosphere. Among them (in terms of contribution to total emissions) Gusinoozerskaya State District Power Plant (30%) and Thermal Power Plant-1 in Ulan-Ude (10%) stand out.

Significant pollution of atmospheric air and soil occurs due to transport. Most heavy metals contained in dust and gas emissions from industrial enterprises are, as a rule, more soluble than natural compounds (Bolshakov et al., 1993). Large industrialized cities stand out among the most active sources of heavy metals. Metals accumulate relatively quickly in urban soils and are removed extremely slowly from them: the half-life of zinc is up to 500 years, cadmium - up to 1100 years, copper - up to 1500 years, lead - up to several thousand years (Maistrenko et al., 1996). In many cities around the world, high rates of HM pollution have led to disruption of the basic agroecological functions of soils (Orlov et al., 1991; Kasimov et al., 1995). Growing agricultural plants used for food near these areas is potentially dangerous, since crops accumulate excess amounts of HMs, which can lead to various diseases in humans and animals.

According to a number of authors (Ilyin, Stepanova, 1979; Zyrin, 1985; Gorbatov, Zyrin, 1987, etc.), the degree of soil contamination with HMs is more correctly assessed by the content of their most bioavailable mobile forms. However, maximum permissible concentrations (MPC) of mobile forms of most heavy metals have not currently been developed. Therefore, literature data on the level of their content leading to adverse environmental consequences can serve as a criterion for comparison.

Below is a brief description of the properties of metals regarding the characteristics of their behavior in soils.

Lead (Pb). Atomic mass 207.2. The priority element is a toxicant. All soluble lead compounds are poisonous. Under natural conditions, it exists mainly in the form of PbS. Clark Pb in the earth's crust is 16.0 mg/kg (Vinogradov, 1957). Compared to other HMs, it is the least mobile, and the degree of mobility of the element is greatly reduced when soils are limed. Mobile Pb is present in the form of complexes with organic matter (60 - 80% mobile Pb). At high pH values, lead is fixed in the soil chemically in the form of hydroxide, phosphate, carbonate and Pb-organic complexes (Zinc and cadmium..., 1992; Heavy..., 1997).

The natural content of lead in soils is inherited from parent rocks and is closely related to their mineralogical and chemical composition (Beus et al., 1976; Kabata-Pendias and Pendias, 1989). The average concentration of this element in the soils of the world reaches, according to various estimates, from 10 (Saet et al., 1990) to 35 mg/kg (Bowen, 1979). The maximum permissible concentration of lead for soils in Russia corresponds to 30 mg/kg (Instructive..., 1990), in Germany - 100 mg/kg (Kloke, 1980).

High concentrations of lead in soils can be associated with both natural geochemical anomalies and anthropogenic impact. In case of technogenic pollution, the highest concentration of the element is usually found in the top layer of soil. In some industrial areas it reaches 1000 mg/kg (Dobrovolsky, 1983), and in the surface layer of soils around non-ferrous metallurgy enterprises in Western Europe - 545 mg/kg (Reutse, Kirstea, 1986).

The lead content in soils in Russia varies significantly depending on the type of soil, the proximity of industrial enterprises and natural geochemical anomalies. In soils of residential areas, especially those associated with the use and production of lead-containing products, the content of this element is often tens or more times higher than the maximum permissible concentration (Table 1.4). According to preliminary estimates, up to 28% of the country's territory has Pb content in the soil, on average, below the background level, and 11% can be classified as a risk zone. At the same time, in the Russian Federation the problem of soil contamination with lead is primarily a problem in residential areas (Snakin et al., 1998).

Cadmium (Cd). Atomic mass 112.4. Cadmium is close in chemical properties to zinc, but differs from it by greater mobility in acidic environments and better accessibility to plants. In the soil solution, the metal is present in the form of Cd2+ and forms complex ions and organic chelates. The main factor determining the content of the element in soils in the absence of anthropogenic influence is the parent rocks (Vinogradov, 1962; Mineev et al., 1981; Dobrovolsky, 1983; Ilyin, 1991; Zinc and cadmium..., 1992; Cadmium: ecological..., 1994) . Clarke of cadmium in the lithosphere 0.13 mg/kg (Kabata-Pendias, Pendias, 1989). In soil-forming rocks, the average metal content is: in clays and shales - 0.15 mg/kg, loess and loess-like loams - 0.08, sands and sandy loams - 0.03 mg/kg (Zinc and cadmium..., 1992). In Quaternary sediments of Western Siberia, the concentration of cadmium varies within the range of 0.01-0.08 mg/kg.

The mobility of cadmium in soil depends on the environment and redox potential (Heavy..., 1997).

The average cadmium content in soils around the world is 0.5 mg/kg (Sayet et al., 1990). Its concentration in the soil cover of the European part of Russia is 0.14 mg/kg - in soddy-podzolic soil, 0.24 mg/kg - in chernozem (Zinc and cadmium..., 1992), 0.07 mg/kg - in the main types soils of Western Siberia (Ilyin, 1991). The approximate permissible content (ATC) of cadmium for sandy and sandy loam soils in Russia is 0.5 mg/kg, in Germany the MPC of cadmium is 3 mg/kg (Kloke, 1980).

Contamination of soil with cadmium is considered one of the most dangerous environmental phenomena, since it accumulates in plants above the norm even with weak soil contamination (Cadmium..., 1994; Ovcharenko, 1998). The highest concentrations of cadmium in the upper soil layer are observed in mining areas - up to 469 mg/kg (Kabata-Pendias, Pendias, 1989), around zinc smelters they reach 1700 mg/kg (Reutse, Cirstea, 1986).

Zinc (Zn). Atomic mass 65.4. Its clarke in the earth's crust is 83 mg/kg. Zinc is concentrated in clayey sediments and shales in quantities from 80 to 120 mg/kg (Kabata-Pendias, Pendias, 1989), in colluvial, loess-like and carbonate loamy deposits of the Urals, in loams of Western Siberia - from 60 to 80 mg/kg.

Important factors influencing the mobility of Zn in soils are the content of clay minerals and pH. When the pH increases, the element passes into organic complexes and binds to the soil. Zinc ions also lose mobility, entering the interpacket spaces of the montmorillonite crystal lattice. Zn forms stable forms with organic matter, so in most cases it accumulates in soil horizons with a high humus content and in peat.

The reasons for the increased zinc content in soils can be both natural geochemical anomalies and technogenic pollution. The main anthropogenic sources of its receipt are primarily non-ferrous metallurgy enterprises. Soil contamination with this metal has led in some areas to its extremely high accumulation in the upper soil layer - up to 66,400 mg/kg. In garden soils, up to 250 or more mg/kg of zinc accumulates (Kabata-Pendias and Pendias, 1989). The MPC of zinc for sandy and sandy loam soils is 55 mg/kg; German scientists recommend a MPC of 100 mg/kg (Kloke, 1980).

Copper (Cu). Atomic mass 63.5. Clark in the earth's crust is 47 mg/kg (Vinogradov, 1962). Chemically, copper is a low-active metal. The fundamental factor influencing the value of Cu content is its concentration in soil-forming rocks (Goryunova et al., 2001). Of the igneous rocks, the largest amount of the element accumulates in basic rocks - basalts (100-140 mg/kg) and andesites (20-30 mg/kg). Cover and loess-like loams (20-40 mg/kg) are less rich in copper. Its lowest content is observed in sandstones, limestones and granites (5-15 mg/kg) (Kovalsky, Andriyanova, 1970; Kabata-Pendias, Pendias, 1989). The metal concentration in clays of the European part of the territory of the former USSR reaches 25 mg/kg (Malgin, 1978; Kovda, 1989), in loess-like loams - 18 mg/kg (Kovda, 1989). Sandy loam and sandy soil-forming rocks of the Altai Mountains accumulate an average of 31 mg/kg of copper (Malgin, 1978), in the south of Western Siberia - 19 mg/kg (Ilyin, 1973).

In soils, copper is a weakly migratory element, although the content of the mobile form can be quite high. The amount of mobile copper depends on many factors: the chemical and mineralogical composition of the parent rock, the pH of the soil solution, the content of organic matter, etc. (Vinogradov, 1957; Peive, 1961; Kovalsky, Andriyanova, 1970; Alekseev, 1987, etc.). The largest amount of copper in the soil is associated with oxides of iron, manganese, hydroxides of iron and aluminum, and, especially, with montmorillonite and vermiculite. Humic and fulvic acids are capable of forming stable complexes with copper. At pH 7-8, the solubility of copper is the lowest.

The average copper content in world soils is 30 mg/kg (Bowen, 1979). Near industrial sources of pollution, in some cases, soil contamination with copper up to 3500 mg/kg can be observed (Kabata-Pendias and Pendias, 1989). The average metal content in the soils of the central and southern regions of the former USSR is 4.5-10.0 mg/kg, the south of Western Siberia - 30.6 mg/kg (Ilyin, 1973), Siberia and the Far East - 27.8 mg/kg (Makeev, 1973). The maximum permissible concentration of copper in Russia is 55 mg/kg (Instructive..., 1990), the maximum permissible concentration for sandy and sandy loam soils is 33 mg/kg (Control..., 1998), in Germany - 100 mg/kg (Kloke, 1980).

Nickel (Ni). Atomic mass 58.7. In continental sediments it is present mainly in the form of sulfides and arsenites, and is also associated with carbonates, phosphates and silicates. The Clarke of the element in the earth's crust is 58 mg/kg (Vinogradov, 1957). Ultrabasic (1400-2000 mg/kg) and basic (200-1000 mg/kg) rocks accumulate the largest amount of metal, while sedimentary and acidic rocks contain it in much lower concentrations - 5-90 and 5-15 mg/kg, respectively (Reutse , Cîrstea, 1986; Kabata-Pendias, Pendias, 1989). Their granulometric composition plays a great role in the accumulation of nickel in soil-forming rocks. Using the example of soil-forming rocks of Western Siberia, it is clear that in lighter rocks its content is the lowest, in heavy rocks it is the highest: in sands - 17, sandy loams and light loams - 22, medium loams - 36, heavy loams and clays - 46 (Ilyin, 2002) .

The nickel content in soils largely depends on the supply of this element to the soil-forming rocks (Kabata-Pendias and Pendias, 1989). The highest concentrations of nickel are usually observed in clayey and loamy soils, in soils formed on basic and volcanic rocks and rich in organic matter. The distribution of Ni in the soil profile is determined by the content of organic matter, amorphous oxides and the amount of clay fraction.

The level of nickel concentration in the top layer of soil also depends on the degree of technogenic pollution. In areas with a developed metalworking industry, very high accumulation of nickel is found in soils: in Canada its gross content reaches 206-26000 mg/kg, and in Great Britain the content of mobile forms reaches 506-600 mg/kg. In soils of Great Britain, Holland, Germany, treated with sewage sludge, nickel accumulates up to 84-101 mg/kg (Kabata-Pendias, Pendias, 1989). In Russia (according to a survey of 40-60% of soils on agricultural land), 2.8% of the soil cover is contaminated with this element. The share of soils contaminated with Ni among other HMs (Pb, Cd, Zn, Cr, Co, As, etc.) is actually the most significant and is second only to lands contaminated with copper (3.8%) (Aristarkhov, Kharitonova, 2002). According to land monitoring data from the State Station of Agrochemical Service “Buryatskaya” for 1993-1997. on the territory of the Republic of Buryatia, an excess of the maximum permissible concentration of nickel was registered on 1.4% of the lands from the surveyed agricultural area, among which the soils of the Zakamensky (20% of the land - 46 thousand hectares are contaminated) and Khorinsky districts (11% of the land - 8 thousand hectares are contaminated).

Chromium (Cr). Atomic mass 52. In natural compounds, chromium has a valence of +3 and +6. Most of the Cr3+ is present in chromite FeCr2O4 or other spinel minerals, where it replaces Fe and Al, to which it is very close in its geochemical properties and ionic radius.

Clarke of chromium in the earth's crust - 83 mg/kg. Its highest concentrations among igneous rocks are typical for ultramafic and basic rocks (1600-3400 and 170-200 mg/kg, respectively), the lowest for medium rocks (15-50 mg/kg) and the lowest for acidic rocks (4-25 mg/kg). kg). Among sedimentary rocks, the maximum content of the element was found in clayey sediments and shales (60-120 mg/kg), the minimum in sandstones and limestones (5-40 mg/kg) (Kabata-Pendias, Pendias, 1989). The metal content in soil-forming rocks of different regions is very diverse. In the European part of the former USSR, its content in the most common soil-forming rocks such as loess, loess-like carbonate and cover loams averages 75-95 mg/kg (Yakushevskaya, 1973). Soil-forming rocks of Western Siberia contain on average 58 mg/kg Cr, and its amount is closely related to the granulometric composition of the rocks: sandy and sandy loam rocks - 16 mg/kg, and medium loamy and clayey rocks - about 60 mg/kg (Ilyin, Syso, 2001) .

In soils, most chromium is present in the form of Cr3+. In an acidic environment, the Cr3+ ion is inert; at pH 5.5, it almost completely precipitates. The Cr6+ ion is extremely unstable and is easily mobilized in both acidic and alkaline soils. The adsorption of chromium by clays depends on the pH of the medium: with increasing pH, the adsorption of Cr6+ decreases, and Cr3+ increases. Soil organic matter stimulates the reduction of Cr6+ to Cr3+.

The natural content of chromium in soils depends mainly on its concentration in soil-forming rocks (Kabata-Pendias and Pendias, 1989; Krasnokutskaya et al., 1990), and the distribution along the soil profile depends on the characteristics of soil formation, in particular on the granulometric composition of genetic horizons. The average chromium content in soils is 70 mg/kg (Bowen, 1979). The highest content of the element is observed in soils formed on basic and volcanic rocks rich in this metal. The average content of Cr in soils of the USA is 54 mg/kg, China - 150 mg/kg (Kabata-Pendias, Pendias, 1989), Ukraine - 400 mg/kg (Bespamyatnov, Krotov, 1985). In Russia, its high concentrations in soils under natural conditions are due to the enrichment of soil-forming rocks. Kursk chernozems contain 83 mg/kg of chromium, soddy-podzolic soils of the Moscow region - 100 mg/kg. In the soils of the Urals, formed on serpentinites, the metal contains up to 10,000 mg/kg, in Western Siberia - 86 - 115 mg/kg (Yakushevskaya, 1973; Krasnokutskaya et al., 1990; Ilyin, Syso, 2001).

The contribution of anthropogenic sources to the supply of chromium is very significant. Chromium metal is primarily used for chrome plating as a component of alloy steels. Soil contamination with Cr is noted due to emissions from cement factories, iron-chromium slag dumps, oil refineries, ferrous and non-ferrous metallurgy enterprises, the use of industrial wastewater sludge in agriculture, especially tanneries, and mineral fertilizers. The highest concentrations of chromium in technogenically contaminated soils reach 400 or more mg/kg (Kabata-Pendias, Pendias, 1989), which is especially typical for large cities (Table 1.4). In Buryatia, according to land monitoring data carried out by the State Station of Agrochemical Service “Buryatskaya” for 1993-1997, 22 thousand hectares are contaminated with chromium. Excesses of MPC by 1.6-1.8 times were noted in Dzhidinsky (6.2 thousand hectares), Zakamensky (17.0 thousand hectares) and Tunkinsky (14.0 thousand hectares) regions.