Heavy metals in soil and environment. Soil contamination with heavy metals

Heavy metals that enter the environment as a result of human production activities (industry, transport, etc.) are among the most dangerous pollutants of the biosphere. Elements such as mercury, lead, cadmium, and copper are classified as “critical group of substances - indicators of environmental stress.” It is estimated that every year metallurgical enterprises alone emit more than 150 thousand tons of copper onto the Earth’s surface; 120 - zinc, about 90 - lead, 12 - nickel and about 30 tons of mercury. These metals tend to be fixed in individual parts of the biological cycle, accumulate in the biomass of microorganisms and plants, and enter the body of animals and humans through trophic chains, negatively affecting their vital functions. On the other hand, heavy metals have a certain effect on the ecological situation, suppressing the development and biological activity of many organisms.

Zc = ΣKc - (n-1),

Heavy metals in soil

Recently, due to the rapid development of industry, there has been 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 ability to form complexes. 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 refers to 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. Cadmium, copper, nickel, and cobalt have a strong migration ability in an acidic environment. 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) is an element of Group VIII of the 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 high biological activity. Due to the structural features of electronic 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. The greatest amount of 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 physical and chemical processes come into play (in particular, processes of ion diffusion), 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. Biological factors play an important role in the migration of lead: plant roots absorb ions metals; 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 an 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. is capable of moving in 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 ​​of this ratio indicate that heavy metals are retained in the solid phase due to the sorption reaction, low values ​​due to the fact that the metals are in solution, from where they can migrate to other media 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 in recent years has also shown the important role of hydroxyl groups, iron oxides and organic matter in this process. When the level of pollution is low and the reaction of the environment is neutral, 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 differently in different types of soils. So far, little is known about the competitive relationships of cadmium with other metals in sorption processes in the soil-absorbing complex. According to research by specialists from the Technical University of Copenhagen (Denmark), in the presence of nickel, cobalt and zinc, the absorption 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. The manifestation of zinc deficiency is enhanced by the use of high doses of 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 soil is present in ionic form, where it is adsorbed by a cation exchange mechanism in an acidic environment or as a result of chemisorption in an 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.

Heavy metals in plants

According to A.P. Vinogradov (1952), all chemical elements participate to one degree or another in the life of plants, and if many of them are considered physiologically significant, it is only because there is no evidence for this yet. Entering the plant in small quantities and becoming an integral part or activator of enzymes, microelements perform service functions in metabolic processes. When unusually high concentrations of elements enter the environment, they become toxic to plants. The penetration of heavy metals into plant tissue in excess amounts leads to disruption of the normal functioning of their organs, and this disruption is stronger, the greater the excess of toxicants. Productivity drops as a result. The toxic effect of HMs manifests itself from the early stages of plant development, but to varying degrees on different soils and for different crops.

The absorption of chemical elements by plants is an active process. Passive diffusion accounts for only 2-3% of the total mass of absorbed mineral components. When the content of metals in the soil is at the background level, active absorption of ions occurs, and if we take into account the low mobility of these elements in soils, then their absorption should be preceded by the mobilization of tightly bound metals. When the content of heavy metals in the root layer is in quantities significantly exceeding the maximum concentrations at which the metal can be fixed using the internal resources of the soil, such quantities of metals enter the roots that the membranes can no longer retain them. As a result, the supply of ions or compounds of elements is no longer regulated by cellular mechanisms. On acidic soils there is a more intense accumulation of HMs than on soils with a neutral or close to neutral reaction environment. A measure of the actual participation of HM ions in chemical reactions is their activity. The toxic effect of high concentrations of heavy metals on plants can manifest itself in disruption of the supply and distribution of other chemical elements. The nature of the interaction of heavy metals with other elements varies depending on their concentrations. Migration and entry into the plant occurs in the form of complex compounds.

During the initial period of environmental contamination with heavy metals, due to the buffer properties of the soil, leading to the inactivation of toxicants, plants will experience virtually no adverse effects. However, the protective functions of soil are not unlimited. As the level of heavy metal pollution increases, their inactivation becomes incomplete and the flow of ions attacks the roots. The plant is able to convert some of the ions into a less active state even before they penetrate into the plant root system. This is, for example, chelation using root secretions or adsorption on the outer surface of roots with the formation of complex compounds. In addition, as vegetation experiments with obviously toxic doses of zinc, nickel, cadmium, cobalt, copper, and lead have shown, the roots are located in layers not contaminated with HM soils and in these cases there are no symptoms of phototoxicity.

Despite the protective functions of the root system, heavy metals enter the root under polluted conditions. In this case, protection mechanisms come into play, thanks to which a specific distribution of HMs occurs among plant organs, making it possible to protect their growth and development as completely as possible. Moreover, the content of, for example, heavy metals in the tissues of roots and seeds in highly polluted environments can vary by 500-600 times, which indicates the great protective capabilities of this underground plant organ.

Excess of chemical elements causes toxicosis in plants. As the concentration of heavy metals increases, plant growth is first retarded, then leaf chlorosis occurs, which is replaced by necrosis, and, finally, the root system is damaged. The toxic effect of HM can manifest itself directly and indirectly. The direct effect of excess heavy metals in plant cells is due to complexation reactions, which result in enzyme blocking or protein precipitation. Deactivation of enzymatic systems occurs as a result of the replacement of the enzyme metal with a pollutant metal. When the toxicant content is critical, the catalytic ability of the enzyme is significantly reduced or completely blocked.

Plants are hyperaccumulators of heavy metals

A.P. Vinogradov (1952) identified plants that are capable of concentrating elements. He pointed to two types of plants - concentrators: 1) plants that concentrate elements on a mass scale; 2) plants with selective (species) concentration. Plants of the first type are enriched with chemical elements if the latter are contained in the soil in increased quantities. Concentration in this case is caused by an environmental factor. Plants of the second type are characterized by a constantly high amount of one or another chemical element, regardless of its content in the environment. It is determined by a genetically fixed need.

Considering the mechanism of absorption of heavy metals from soil into plants, we can talk about barrier (non-concentrating) and barrier-free (concentrating) types of accumulation of elements. Barrier accumulation is typical for most higher plants and is not typical for bryophytes and lichens. Thus, in the work of M.A. Toikka and L.N. Potekhina (1980), sphagnum (2.66 mg/kg) was named as a plant-concentrator of cobalt; copper (10.0 mg/kg) - birch, drupe, lily of the valley; manganese (1100 mg/kg) - blueberries. Lepp et al. (1987) found high concentrations of cadmium in the sporophores of the fungus Amanita muscaria growing in birch forests. In the sporophores of the fungus, the cadmium content was 29.9 mg/kg of dry weight, and in the soil on which they grew - 0.4 mg/kg. There is an opinion that plants that are concentrators of cobalt are also highly tolerant to nickel and are able to accumulate it in large quantities. These include, in particular, plants of the families Boraginaceae, Brassicaceae, Myrtaceae, Fabaceae, Caryophyllaceae. Nickel concentrators and superconcentrators have also been found among medicinal plants. Superconcentrators include melon tree, belladonna belladonna, yellow poppy, motherwort cordial, passionflower and Thermopsis lanceolata. The type of accumulation of chemical elements found in high concentrations in the nutrient medium depends on the phases of plant growth. Barrier-free accumulation is characteristic of the seedling phase, when plants do not differentiate the above-ground parts into various organs, and in the final phases of the growing season - after ripening, as well as during the period of winter dormancy, when barrier-free accumulation may be accompanied by the release of excess amounts of chemical elements in the solid phase (Kovalevsky, 1991).

Hyperaccumulating plants are found in the families Brassicaceae, Euphorbiaceae, Asteraceae, Lamiaceae and Scrophulariaceae (Baker 1995). The most famous and studied among them is Brassica juncea (Indian mustard), a plant that develops large biomass and is capable of accumulating Pb, Cr (VI), Cd, Cu, Ni, Zn, 90Sr, B and Se (Nanda Kumar et al. 1995 ; Salt et al. 1995; Of the different plant species tested, B. juncea had the most pronounced ability to transport lead aboveground, accumulating more than 1.8% of this element in aboveground organs (based on dry weight). With the exception of sunflower (Helianthus annuus) and tobacco (Nicotiana tabacum), other non-Brassicaceae plant species had a biological uptake coefficient of less than 1.

According to the classification of plants according to their response to the presence of heavy metals in their growing environment, used by many foreign authors, plants have three main strategies for growth on soils contaminated with metals:

Metal excluders. Such plants maintain a constant low concentration of metal despite wide variations in its concentration in the soil, retaining mainly the metal in the roots. Exclusive plants are capable of changing membrane permeability and metal-binding capacity of cell walls or releasing large amounts of chelating substances.

Metal indicators. These include plant species that actively accumulate metal in above-ground parts and generally reflect the level of metal content in the soil. They tolerate the existing level of metal concentration due to the formation of extracellular metal-binding compounds (chelators), or change the nature of metal compartmentation by storing it in metal-insensitive areas. Metal-accumulating plant species. Plants belonging to this group can accumulate the metal in above-ground biomass in concentrations much higher than those in the soil. Baker and Brooks defined metal hyperaccumulators as plants containing more than 0.1%, i.e. more than 1000 mg/g copper, cadmium, chromium, lead, nickel, cobalt or 1% (more than 10,000 mg/g) zinc and manganese in dry weight. For rare metals, this value is more than 0.01% in terms of dry weight. Researchers identify hyperaccumulating species by collecting plants in areas where soils contain metals in concentrations above background levels, as is the case in contaminated areas or where ore bodies are exposed. The phenomenon of hyperaccumulation raises many questions for researchers. For example, what is the significance of the accumulation of metal in highly toxic concentrations for plants? A definitive answer to this question has not yet been received, but there are several main hypotheses. It is assumed that such plants have an enhanced ion uptake system (the "unintentional" uptake hypothesis) to perform certain physiological functions that have not yet been studied. It is also believed that hyperaccumulation is one of the types of plant tolerance to high metal content in the growing environment.

Standardization of heavy metal content

in soil and plants is extremely complex due to the impossibility of fully taking into account all environmental factors. Thus, changing only the agrochemical properties of the soil (medium reaction, humus content, degree of saturation with bases, granulometric composition) can reduce or increase the content of heavy metals in plants several times. There are conflicting data even about the background content of some metals. The results given by researchers sometimes differ by 5-10 times.

Many scales have been proposed

environmental regulation of heavy metals. In some cases, the highest content of metals observed in ordinary anthropogenic soils is taken as the maximum permissible concentration; in others, the content that is the limit for phytotoxicity is taken. In most cases, MPCs have been proposed for heavy metals that are several times higher than the upper limit.

To characterize technogenic pollution

for heavy metals, a concentration coefficient is used equal to the ratio of the concentration of the element in contaminated soil to its background concentration. When polluted by several heavy metals, the degree of pollution is assessed by the value of the total concentration index (Zc). The scale of soil contamination with heavy metals proposed by IMGRE is presented in Table 1.

Table 1. Scheme for assessing soils for agricultural use according to the degree of contamination with chemicals (Goskomhydromet of the USSR, No. 02-10 51-233 dated 12/10/90)

Officially approved MPCs

Table 2 shows the officially approved maximum concentration limits and permissible levels of their content according to hazard indicators. In accordance with the scheme adopted by medical hygienists, the regulation of heavy metals in soils is divided into translocation (transition of the element into plants), migratory water (transition into water), and general sanitary (effect on the self-purifying ability of soils and soil microbiocenosis).

Table 2. Maximum permissible concentrations (MAC) of chemicals in soils and permissible levels of their content in terms of harmfulness (as of 01/01/1991. State Committee for Nature Protection of the USSR, No. 02-2333 dated 12/10/90).

* - gross content - approximate.
** - contradiction; for arsenic, the average background content is 6 mg/kg; the background content of lead usually also exceeds the MPC standards.

Officially approved by the UEC

The ADCs developed in 1995 for the gross content of 6 heavy metals and arsenic make it possible to obtain a more complete description of soil contamination with heavy metals, since they take into account the level of environmental reaction and the granulometric composition of the soil.

Table 3. Approximate permissible concentrations (ATC) of heavy metals and arsenic in soils with different physicochemical properties (gross content, mg/kg) (addition No. 1 to the list of MPC and APC No. 6229-91).

It follows from the materials that the requirements are mainly imposed on bulk forms of heavy metals. Among the mobile ones are only copper, nickel, zinc, chromium and cobalt. Therefore, the currently developed standards no longer satisfy all requirements.

is a capacity factor, reflecting primarily the potential danger of contamination of plant products, infiltration and surface waters. Characterizes the general contamination of the soil, but does not reflect the degree of availability of elements for the plant. To characterize the state of soil nutrition of plants, only their mobile forms are used.

Definition of movable forms

They are determined using various extractants. The total amount of the mobile form of the metal is using an acidic extract (for example, 1N HCL). The most mobile part of the mobile reserves of heavy metals in the soil goes into the ammonium acetate buffer. The concentration of metals in a water extract shows the degree of mobility of elements in the soil, being the most dangerous and “aggressive” fraction.

Standards for movable forms

Several indicative normative scales have been proposed. Below is an example of one of the scales of maximum permissible mobile forms of heavy metals.

Table 4. Maximum permissible content of the mobile form of heavy metals in soil, mg/kg extractant 1N. HCl (H. Chuljian et al., 1988).

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 . 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 there is 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 30-50 cm layer of soil 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 metals in soil

Recently, due to the rapid development of industry, there has been 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 ability to form complexes. 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 refers to 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. Cadmium, copper, nickel, and cobalt have a strong migration ability in an acidic environment. 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) is an element of Group VIII of the 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 high biological activity. Due to the structural features of electronic 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. The greatest amount of 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 physical and chemical processes come into play (in particular, processes of ion diffusion), 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. Biological factors play an important role in the migration of lead: plant roots absorb ions metals; 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. is capable of moving in 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 ​​of this ratio indicate that heavy metals are retained in the solid phase due to the sorption reaction, low values ​​- due to the fact that the metals are in solution, from where they can migrate to other media 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 in recent years has also shown the important role of hydroxyl groups, iron oxides and organic matter in this process. When the level of pollution is low and the reaction of the environment is neutral, 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 differently in different types of soils. So far, little is known about the competitive relationships of cadmium with other metals in sorption processes in the soil-absorbing complex. According to research by specialists from the Technical University of Copenhagen (Denmark), in the presence of nickel, cobalt and zinc, the absorption 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. The manifestation of zinc deficiency is enhanced by the use of high doses of 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 soil is present in ionic form, where it is adsorbed by a cation exchange mechanism in an acidic environment or as a result of chemisorption in an 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 .