Content of heavy metals in soil and plants. List of sources used

Compounds of Cr(VI) and Cr(III) in increased quantities have carcinogenic properties. Cr(VI) compounds are more dangerous.

It enters natural waters as a result of the processes of destruction and dissolution of rocks and minerals occurring in nature (sphalerite, zincite, goslarite, smithsonite, calamine), as well as with wastewater from ore processing factories and electroplating shops, production of parchment paper, mineral paints, viscose fiber and etc.

In water it exists mainly in ionic form or in the form of its mineral and organic complexes. Sometimes found in insoluble forms: as hydroxide, carbonate, sulfide, etc.

In river waters, the concentration of zinc usually ranges from 3 to 120 μg/dm 3, in sea waters - from 1.5 to 10 μg/dm 3. The content in ore waters and especially in mine waters with low pH values can be significant.

Zinc is one of the active microelements that influence the growth and normal development of organisms. At the same time, many zinc compounds are toxic, primarily its sulfate and chloride.

The maximum permissible concentration in Zn 2+ is 1 mg/dm 3 (the limiting indicator of harm is organoleptic), the maximum permissible concentration for Zn 2+ is 0.01 mg/dm 3 (the limiting indicator of harm is toxicological).

Heavy metals already occupy the second place in terms of danger, inferior to pesticides and significantly ahead of such well-known pollutants as carbon dioxide and sulfur, and in the forecast they should become the most dangerous, more dangerous than nuclear power plant waste and solid waste. Pollution with heavy metals is associated with their widespread use in industrial production, coupled with weak purification systems, as a result of which heavy metals enter the environment, including the soil, polluting and poisoning it.

Heavy metals are priority pollutants, monitoring of which is mandatory in all environments. In various scientific and applied works, authors interpret the meaning of the concept “heavy metals” differently. In some cases, the definition of heavy metals includes elements classified as brittle (for example, bismuth) or metalloids (for example, arsenic).

Soil is the main medium into which heavy metals enter, including from the atmosphere and the aquatic environment. It also serves as a source of secondary pollution of surface air and waters that flow from it into the World Ocean. From the soil, heavy metals are absorbed by plants, which then become food for more highly organized animals.

3.3. Lead toxicity

Currently, lead ranks first among the causes of industrial poisoning. This is due to its widespread use in various industries. Workers mining lead ore, in lead smelters, in the production of batteries, during soldering, in printing houses, in the production of crystal glass or ceramic products, leaded gasoline, lead paints, etc. are exposed to lead. Lead pollution of atmospheric air, soil and water in the vicinity of such industries, as well as near major highways, poses a threat of lead exposure to the population living in these areas, and, above all, children, who are more sensitive to the effects of heavy metals.

It should be noted with regret that in Russia there is no state policy on legal, regulatory and economic regulation of the impact of lead on the environment and public health, on reducing emissions (discharges, waste) of lead and its compounds into the environment, and on completely stopping the production of lead-containing gasoline.

Due to extremely unsatisfactory educational work to explain to the population the degree of danger of the effects of heavy metals on the human body, in Russia the number of contingents with professional contact with lead is not decreasing, but is gradually increasing. Cases of chronic lead intoxication have been recorded in 14 industries in Russia. The leading industries are the electrical engineering industry (battery production), instrument making, printing and non-ferrous metallurgy, in which intoxication is caused by exceeding the maximum permissible concentration (MPC) of lead in the air of the working area by 20 or more times.

A significant source of lead is automobile exhaust fumes, as half of Russia still uses leaded gasoline. However, metallurgical plants, in particular copper smelters, remain the main source of environmental pollution. And there are leaders here. On the territory of the Sverdlovsk region there are 3 of the largest sources of lead emissions in the country: in the cities of Krasnouralsk, Kirovograd and Revda.

The chimneys of the Krasnouralsk copper smelter, built during the years of Stalinist industrialization and using equipment from 1932, annually spew 150-170 tons of lead into the city of 34,000, covering everything with lead dust.

The concentration of lead in the soil of Krasnouralsk varies from 42.9 to 790.8 mg/kg with a maximum permissible concentration of MPC = 130 μ/kg. Water samples in the water supply of a neighboring village. Oktyabrsky, fed by an underground water source, exceeded the maximum permissible concentration by up to two times.

Lead pollution of the environment affects human health. Exposure to lead disrupts the female and male reproductive systems. For women of pregnant and childbearing age, elevated levels of lead in the blood pose a particular danger, since under the influence of lead menstrual function is disrupted, premature births, miscarriages and fetal death are more common due to the penetration of lead through the placental barrier. Newborn babies have a high mortality rate.

Lead poisoning is extremely dangerous for young children - it affects the development of the brain and nervous system. Testing of 165 Krasnouralsk children aged 4 years and older revealed a significant delay in mental development in 75.7%, and mental retardation, including mental retardation, was found in 6.8% of the children examined.

Preschool-age children are most susceptible to the harmful effects of lead because their nervous systems are in the developing stages. Even at low doses, lead poisoning causes a decrease in intellectual development, attention and ability to concentrate, a lag in reading, and leads to the development of aggressiveness, hyperactivity and other problems in the child’s behavior. These developmental abnormalities can be long-lasting and irreversible. Low birth weight, stunting and hearing loss also result from lead poisoning. High doses of intoxication lead to mental retardation, coma, convulsions and death.

A white paper published by Russian experts reports that lead pollution covers the entire country and is one of numerous environmental disasters in the former Soviet Union that have come to light in recent years. Most of the territory of Russia experiences a load from lead deposition that exceeds the critical load for the normal functioning of the ecosystem. In dozens of cities, lead concentrations in the air and soil exceed the values corresponding to the maximum permissible concentrations.

The highest level of air pollution with lead, exceeding the maximum permissible concentration, was observed in the cities of Komsomolsk-on-Amur, Tobolsk, Tyumen, Karabash, Vladimir, Vladivostok.

The maximum loads of lead deposition, leading to the degradation of terrestrial ecosystems, are observed in the Moscow, Vladimir, Nizhny Novgorod, Ryazan, Tula, Rostov and Leningrad regions.

Stationary sources are responsible for the discharge of more than 50 tons of lead in the form of various compounds into water bodies. At the same time, 7 battery factories discharge 35 tons of lead annually through the sewer system. An analysis of the distribution of lead discharges into water bodies in Russia shows that the Leningrad, Yaroslavl, Perm, Samara, Penza and Oryol regions are leaders in this type of load.

The country needs urgent measures to reduce lead pollution, but for now Russia's economic crisis is overshadowing environmental problems. In a long-running industrial depression, Russia lacks the means to clean up past pollution, but if the economy begins to recover and factories return to work, pollution could only worsen.

10 most polluted cities of the former USSR

(Metals are listed in descending order of priority level for a given city)

1. Rudnaya Pristan (Primorie region)	lead, zinc, copper, manganese+vanadium, manganese.
2. Belovo (Kemerovo region)	zinc, lead, copper, nickel.
3. Revda (Sverdlovsk region)	copper, zinc, lead.
4. Magnitogorsk	nickel, zinc, lead.
5. Glubokoe (Belarus)	copper, lead, zinc.
6. Ust-Kamenogorsk (Kazakhstan)	zinc, copper, nickel.
7. Dalnegorsk (Primorsky Territory)	lead, zinc.
8. Monchegorsk (Murmansk region)	nickel.
9. Alaverdi (Armenia)	copper, nickel, lead.
10. Konstantinovka (Ukraine)	lead, mercury.

4. Soil hygiene. Waste disposal.

The soil in cities and other populated areas and their surroundings has long been different from natural, biologically valuable soil, which plays an important role in maintaining ecological balance. The soil in cities is subject to the same harmful effects as the urban air and hydrosphere, so significant degradation occurs everywhere. Soil hygiene is not given enough attention, although its importance as one of the main components of the biosphere (air, water, soil) and a biological environmental factor is even more significant than water, since the quantity of the latter (primarily the quality of groundwater) is determined by the condition of the soil, and it is impossible to separate these factors from each other. The soil has the ability of biological self-purification: in the soil, the breakdown of waste that enters it and its mineralization occurs; Ultimately, the soil compensates for the lost minerals at their expense.

Heavy metals (HM) include more than 40 chemical elements of D.I. Mendeleev’s periodic table, the mass of atoms of which is over 50 atomic mass units (amu). These are Pb, Zn, Cd, Hg, Cu, Mo, Mn, Ni, Sn, Co, etc.

The established concept of “heavy metals” is not strict, since HMs often include non-metallic elements, for example As, Se, and sometimes even F, Be and other elements whose atomic mass is less than 50 amu.

There are many trace elements among HMs that are biologically important for living organisms. They are necessary and indispensable components of biocatalysts and bioregulators of the most important physiological processes. However, the excess content of heavy metals in various objects of the biosphere has a depressing and even toxic effect on living organisms.

Sources of heavy metals entering the soil are divided into natural (weathering of rocks and minerals, erosion processes, volcanic activity) and technogenic (extraction and processing of minerals, fuel combustion, influence of vehicles, agriculture, etc.) Agricultural lands, in addition to pollution through atmosphere, HMs are also polluted specifically, through the use of pesticides, mineral and organic fertilizers, liming, and the use of wastewater. Recently, scientists have paid special attention to urban soils. The latter are experiencing significant man-made pressure, part of which is HM pollution.

In table 3.14 and 3.15 present the distribution of HM in various objects of the biosphere and the sources of HM entry into the environment.

Table 3.14

Element	Soils	Fresh waters	sea waters	Plants	Animals (in muscle tissue)
Mn	1000	0,008	0,0002	0,3-1000	0,2-2,3
Zn	90 (1-900)	0,015	0,0049	1,4-600	240
Cu	30 (2-250)	0,003	0,00025	4-25	10
Co	8 (0,05-65)	0,0002	0,00002	0,01-4,6	0,005-1
Pb	35 (2-300)	0,003	0,00003	0,2-20	0,23-3,3
Cd	0,35 (0,01-2)	0,0001	-	0,05-0,9	0,14-3,2
Hg	0,06	0,0001	0,00003	0,005-0,02	0,02-0,7
As	6	0,0005	0,0037	0,02-7	0,007-0,09
Se	0,4 (0,01-12)	0,0002	00,0002	0,001-0,5	0,42-1,9
F	200	0,1	1,3	0,02-24	0,05
B	20 (2-270)	0,15	4,44	8-200	0,33-1
Mo	1,2 (0,1-40)	0,0005	0,01	0,03-5	0,02-0,07
Cr	70 (5-1500)	0,001	0,0003	0,016-14	0,002-0,84
Ni	50 (2-750)	0,0005	0,00058	0,02-4	1-2

Table 3.15

Sources of environmental pollution TM

End of table. 3.4

HMs reach the soil surface in various forms. These are oxides and various salts of metals, both soluble and practically insoluble in water (sulfides, sulfates, arsenites, etc.). In the emissions of ore processing enterprises and non-ferrous metallurgy enterprises - the main source of environmental pollution with heavy metals - the bulk of metals (70-90%) are in the form of oxides.

Once on the soil surface, HMs can either accumulate or dissipate, depending on the nature of the geochemical barriers inherent in a given area.

Most of the HMs arriving on the soil surface are fixed in the upper humus horizons. HMs are sorbed on the surface of soil particles, bind to soil organic matter, in particular in the form of elemental organic compounds, accumulate in iron hydroxides, form part of the crystal lattices of clay minerals, produce their own minerals as a result of isomorphic replacement, and are in a soluble state in soil moisture and gaseous state in the soil air, are an integral part of the soil biota.

The degree of mobility of heavy metals depends on the geochemical situation and the level of technogenic impact. The heavy particle size distribution and high content of organic matter lead to the binding of HMs in the soil. An increase in pH values increases the sorption of cation-forming metals (copper, zinc, nickel, mercury, lead, etc.) and increases the mobility of anion-forming metals (molybdenum, chromium, vanadium, etc.). Increasing oxidative conditions increases the migration ability of metals. As a result, according to their ability to bind the majority of HMs, soils form the following series: gray soil > chernozem > soddy-podzolic soil.

The duration of residence of polluting components in the soil is much longer than in other parts of the biosphere, and soil contamination, especially with heavy metals, is almost eternal. Metals accumulate in the soil and are slowly removed through leaching, plant consumption, erosion and deflation (Kabata-Pendias and Pendias, 1989). The period of half-removal (or removal of half of the initial concentration) of HM varies greatly for different elements, but is quite long periods of time: for Zn - from 70 to 510 years; for Cd - from 13 to 110 years; for Cu - from 310 to 1500 years and for Pb - 2 - from 740 to 5900 years (Sadovskaya, 1994).

Soil contamination with heavy metals has two negative aspects. Firstly, entering through food chains from the soil into plants, and from there into the body of animals and humans, heavy metals cause serious diseases in them - an increase in the incidence of the population and a reduction in life expectancy, as well as a decrease in the quantity and quality of harvests of agricultural plants and livestock products.

Secondly, accumulating in large quantities in the soil, HMs are capable of changing many of its properties. First of all, changes affect the biological properties of the soil: the total number of microorganisms decreases, their species composition (diversity) narrows, the structure of microbial communities changes, the intensity of basic microbiological processes and the activity of soil enzymes decreases, etc. Heavy contamination with heavy metals leads to changes in more conservative characteristics soil, such as humus status, structure, pH of the environment, etc. The result of this is a partial, and in some cases, complete loss of soil fertility.

In nature, there are areas with insufficient or excessive content of HMs in soils. The abnormal content of heavy metals in soils is due to two groups of reasons: biogeochemical characteristics of ecosystems and the influence of technogenic flows of matter. In the first case, areas where the concentration of chemical elements is higher or lower than the optimal level for living organisms are called natural geochemical anomalies, or biogeochemical provinces. Here, the anomalous content of elements is due to natural causes - the characteristics of soil-forming rocks, the soil-forming process, and the presence of ore anomalies. In the second case, the territories are called man-made geochemical anomalies. Depending on the scale, they are divided into global, regional and local.

Soil, unlike other components of the natural environment, not only geochemically accumulates pollution components, but also acts as a natural buffer that controls the transfer of chemical elements and compounds into the atmosphere, hydrosphere and living matter.

Various plants, animals and humans require a certain composition of soil and water for their life. In places of geochemical anomalies, aggravated transmission of deviations from the norm in mineral composition occurs throughout the food chain.

As a result of disturbances in mineral nutrition, changes in the species composition of phyto-, zoo- and microbiocenoses, diseases of wild plant forms, a decrease in the quantity and quality of crops of agricultural plants and livestock products, an increase in morbidity in the population and a decrease in life expectancy are observed (Table 3.15). The mechanism of toxic action of HM is presented in Table. 3.16.

Table 3.15

Physiological disorders in plants with excess and deficiency of HM content in them (according to Kovalevsky, Andrianova, 1970; Kabata-pendias,

Pendas, 1989)

Element	Physiological disorders
Element	in case of shortage	in case of excess
Cu	Chlorosis, wilt, melanism, white curled crowns, weakened panicle formation, impaired lignification, dry tops of trees	Dark green leaves, as in Fe-induced chlorosis; thick, short or barbed wire-like roots, inhibition of shoot formation
Zn	Interveinal chlorosis (mainly in monocots), growth arrest, rosette leaves of trees, purple-red dots on leaves	Chlorosis and necrosis of leaf tips, interveinal chlorosis of young leaves, stunted growth of the plant as a whole, damaged roots that look like barbed wire
Cd	-	Brown leaf edges, chlorosis, reddish veins and petioles, curled leaves and brown underdeveloped roots
Hg	-	Some inhibition of sprouts and roots, chlorosis of leaves and brown spots on them
Pb	-	Reduced photosynthesis rate, dark green leaves, curling of old leaves, stunted foliage, brown short roots

Table 3.16

Mechanism of action of HM toxicity (according to Torshin et al., 1990)

Element	Action
Cu, Zn, Cd, Hg, Pb	Effect on membrane permeability, reaction with SH - groups of cysteine and methionine
Pb	Changing the three-dimensional structure of proteins
Cu, Zn, Hg, Ni	Formation of complexes with phospholipids
Ni	Formation of complexes with albumin
	Enzyme inhibition:
Hg2+	alkaline phosphatase, gluco-6-phosphatase, lactate dehydrogenase
Cd2+	adenosine triphosphatases, alcohol dehydrogenases, amylases, carbonic anhydrases, carboxypeptidases (pentidases), glutamate oxaloacetate transaminases
Pb2+	acetylcholinesterase, alkaline phosphatase, ATPase
Ni2+	carbonic anhydrase, cytochrome oxidase, benzopyrene hydroxylase

The toxic effect of HMs on biological systems is primarily due to the fact that they easily bind to sulfhydryl groups of proteins (including enzymes), suppressing their synthesis and thereby disrupting metabolism in the body.

Living organisms have developed various mechanisms of resistance to HMs: from the reduction of HM ions into less toxic compounds to the activation of ion transport systems that effectively and specifically remove toxic ions from the cell into the external environment.

The most significant consequence of the impact of heavy metals on living organisms, which manifests itself at the biogeocenotic and biosphere levels of organization of living matter, is the blocking of the oxidation processes of organic matter. This leads to a decrease in the rate of its mineralization and accumulation in ecosystems. At the same time, an increase in the concentration of organic matter causes it to bind HM, which temporarily relieves the load on the ecosystem. A decrease in the rate of decomposition of organic matter due to a decrease in the number of organisms, their biomass and the intensity of vital activity is considered a passive response of ecosystems to HM pollution. Active resistance of organisms to anthropogenic loads manifests itself only during the lifetime accumulation of metals in bodies and skeletons. The most resistant species are responsible for this process.

The resistance of living organisms, primarily plants, to elevated concentrations of heavy metals and their ability to accumulate high concentrations of metals can pose a great danger to human health, since they allow the penetration of pollutants into food chains. Depending on the geochemical conditions of production, human food of both plant and animal origin can satisfy human needs for mineral elements, be deficient or contain an excess of them, becoming more toxic, causing diseases and even death (Table 3.17).

Table 3.17

Effect of HM on the human body (Kovalsky, 1974; Concise Medical Encyclopedia, 1989; Torshin et al., 1990; Impact on the body.., 1997; Handbook of toxicology.., 1999)

Element	Physiological abnormalities
Element	in case of shortage	in case of excess
Mn	Diseases of the skeletal system	Fever, pneumonia, damage to the central nervous system (manganese parkinsonism), endemic gout, circulatory disorders, gastrointestinal functions, infertility
Cu	Weakness, anemia, leukemia, diseases of the skeletal system, impaired coordination of movements	Occupational diseases, hepatitis, Wilson's disease. Affects kidneys, liver, brain, eyes
Zn	Decreased appetite, bone deformation, dwarfism, long healing of wounds and burns, poor vision, myopia	Decreased cancer resistance, anemia, inhibition of oxidative processes, dermatitis
Pb	-	Lead encephaloneuropathy, metabolic disorders, inhibition of enzymatic reactions, vitamin deficiency, anemia, multiple sclerosis. Part of the skeletal system instead of calcium
Cd	-	Gastrointestinal disorders, respiratory disorders, anemia, increased blood pressure, kidney damage, itai-itai disease, proteinuria, osteoporosis, mutagenic and carcinogenic effects
Hg	-	Lesions of the central nervous system and peripheral nerves, infantilism, reproductive dysfunction, stomatitis, disease Minamata, premature aging
Co	Endemic goiter	-
Ni	-	Dermatitis, hematopoietic disorder, carcinogenicity, embryotoxicosis, subacute myelo-optic neuropathy
Cr	-	Dermatitis, carcinogenicity
V	-	Diseases of the cardiovascular system

Different HMs pose a threat to human health to varying degrees. The most dangerous are Hg, Cd, Pb (Table 3.18).

Table 3.18

Classes of pollutants according to their degree of danger (GOST 17.4.1.02-83)

The issue of regulating the content of heavy metals in soil is very complicated. Its solution should be based on the recognition of the multifunctionality of the soil. In the process of rationing, soil can be viewed from different positions: as a natural body; as a habitat and substrate for plants, animals and microorganisms; as an object and means of agricultural and industrial production; as a natural reservoir containing pathogenic microorganisms. Standardization of HM content in soil must be carried out on the basis of soil-ecological principles, which deny the possibility of finding uniform values for all soils.

There are two main approaches to the issue of remediation of soils contaminated with heavy metals. The first is aimed at clearing the soil of HM. Purification can be carried out by leaching, by extracting HMs from the soil with the help of plants, by removing the top contaminated layer of soil, etc. The second approach is based on fixing HMs in the soil, converting them into forms that are insoluble in water and inaccessible to living organisms. For this purpose, it is proposed to introduce organic matter, phosphorus mineral fertilizers, ion exchange resins, natural zeolites, brown coal, liming of the soil, etc. into the soil. However, any method of fixing HMs in the soil has its own validity period. Sooner or later, part of the HM will again begin to enter the soil solution, and from there into living organisms.

Thus, heavy metals include more than 40 chemical elements, the mass of atoms of which is over 50 a. eat. These are Pb, Zn, Cd, Hg, Cu, Mo, Mn, Ni, Sn, Co, etc. There are many trace elements among HMs, which are necessary and irreplaceable components of biocatalysts and bioregulators of the most important physiological processes. However, the excess content of heavy metals in various objects of the biosphere has a depressing and even toxic effect on living organisms.

Sources of heavy metals entering the soil are divided into natural (weathering of rocks and minerals, erosion processes, volcanic activity) and technogenic (mining and processing of minerals, fuel combustion, influence of motor transport, agriculture, etc.).

HMs reach the soil surface in various forms. These are oxides and various salts of metals, both soluble and practically insoluble in water.

The environmental consequences of soil contamination with heavy metals depend on the pollution parameters, geochemical conditions and soil stability. The pollution parameters include the nature of the metal, i.e. its chemical and toxic properties, the metal content in the soil, the form of the chemical compound, the period from the moment of pollution, etc. The resistance of soils to pollution depends on the particle size distribution, the content of organic matter, acidity alkaline and redox conditions, activity of microbiological and biochemical processes, etc.

When regulating the content of heavy metals in soil, the multifunctionality of the soil should be taken into account. Soil can be considered as a natural body, as a habitat and substrate for plants, animals and microorganisms, as an object and means of agricultural and industrial production, as a natural reservoir containing pathogenic microorganisms, as part of the terrestrial biogeocenosis and the biosphere as a whole.

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)

Element	Hazard Class	MPC		UEC by soil groups			Background content
		Gross content	Extractable with ammonium acetate buffer (pH=4.8)	Sandy, sandy loam	Loamy, clayey
		Gross content	Extractable with ammonium acetate buffer (pH=4.8)	Sandy, sandy loam	pH x l< 5,5	pH x l > 5.5
Pb	1	32	6	32	65	130	26
Zn	1	-	23	55	110	220	50
Cd	1	-	-	0,5	1	2	0,3
Cu	2	-	3	33	66	132	27
Ni	2	-	4	20	40	80	20
Co	2	-	5	-	-	-	7,2

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, while low values indicate that 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; Raskin et al. 1994). 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 concentrations 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.

Phytoremediation of soils contaminated with heavy metals

The presence of elevated concentrations of metals in the soil leads to their accumulation in wild flora and agricultural crops, which is accompanied by contamination of food chains. High concentrations of metals make the soil unsuitable for plant growth, thereby affecting biodiversity. Soils contaminated with heavy metals can be restored by chemical, physical and biological means. In general, they can be classified into two categories.

The ex-situ method requires the removal of contaminated soil for on-site or off-site treatment and the return of the treated soil to its original location. The sequence of ex-situ methods used to remediate contaminated soils includes excavation, detoxification, and/or degradation of the contaminant by physical or chemical means, resulting in the contaminant being stabilized, settled, immobilized, incinerated, or decomposed.

The in-situ method involves cleaning contaminated soil without excavating it. Reed et al. defined in-situ remediation technologies as degradation or transformation of the contaminant, immobilization to reduce bioavailability, and separation of the contaminant from the soil. The in-situ method is preferable to the ex-situ method due to its low cost and gentle effect on the ecosystem. Traditionally, the ex-situ method involves removing heavy metal-contaminated soil and burying it, which is not an optimal choice because burying contaminated soil off-site simply transfers the contamination problem to another location; however, there is a certain risk associated with the transport of contaminated soil. By diluting heavy metals to acceptable levels by adding clean soil to the contaminated soil and mixing them, covering the soil with an inert material can be an alternative to cleaning up the soil within the contaminated site.

Immobilization of an inorganic contaminant can be used as a remediation method for soils contaminated with heavy metals. It can be achieved by complexation of contaminants, or by increasing the soil pH through liming. Increasing pH reduces the solubility of heavy metals such as Cd, Cu, Ni and Zn in soil. Although the risk of being taken up by plants is reduced, the concentration of metals in the soil remains unchanged. Most of these traditional cleanup technologies are expensive and cause further damage to an already damaged environment. Bioremediation technologies, called phytoremediation, involve the use of green plants and associated microbiota for in-situ purification of contaminated soils and groundwater. The idea of using metal-accumulating plants to remove heavy metals and other compounds was first proposed in 1983. The term phytoremediation is composed of the Greek prefix phyto- (plant) attached to the Latin root remedium (recovery).

Rhizofiltration involves the use of plants (both terrestrial and aquatic) to adsorb, concentrate and deposit contaminants into roots from contaminated water sources with low contaminant concentrations. This method can partially treat industrial wastewater, surface runoff from agricultural land and buildings, or acidic drainage from mines and mines. Rhizofiltration can be applied to lead, cadmium, copper, nickel, zinc and chromium, which are mainly retained by roots. Advantages of rhizofiltration include its ability to be used both "in-situ" and "ex-situ" and use plant species that are not hyperaccumulators. The ability of sunflower, Indian mustard, tobacco, rye, spinach and corn to remove lead from wastewater was studied, with sunflower showing the greatest removal efficiency.

Phytostabilization is used primarily for the treatment of soils, sediments and sewage sludge and depends on the ability of plant roots to limit the mobility and bioavailability of contaminants in the soil. Phytostabilization is carried out through sorption, precipitation and complexation of metals. Plants reduce the amount of water that percolates through contaminated soil, which prevents erosion processes and the penetration of dissolved contaminants into surface and groundwater and their spread to uncontaminated areas. The advantage of phytostabilization is that this method does not require the removal of contaminated plant biomass. However, its main disadvantage is the preservation of the contaminant in the soil, and therefore the use of this cleaning method must be accompanied by constant monitoring of the content and bioavailability of contaminants.

Phytoextraction is the most suitable method for removing heavy metal salts from soils without destroying the soil structure and fertility. Some authors call this method phytoaccumulation. Because the plant absorbs, concentrates and precipitates toxic metals and radionuclides from contaminated soils into biomass, it is the best way to clean up areas with diffuse surface contamination and relatively low concentrations of contaminants. There are two main phytoextraction strategies:

Phytoextraction in the presence of chelates, or induced phytoextraction, in which the addition of artificial chelates increases the mobility and absorption of the metal contaminant;

Sequential phytoextraction, in which metal removal depends on the natural purification ability of the plants; in this case, only the number of sowing (planting) plants is under control. The discovery of hyperaccumulating species further contributed to the development of this technology. To make this technology feasible, plants must extract large concentrations of heavy metals through their roots, move them into aboveground biomass, and produce large amounts of plant biomass. In this case, factors such as growth rate, element selectivity, disease resistance, and harvesting method are important. However, slow growth, shallow spreading root system, and low biomass productivity limit the use of hyperaccumulating species for cleaning areas contaminated with heavy metals.

Phytoevaporation involves the use of plants to remove contaminants from the soil, transform them into a volatile form, and transpirate them into the atmosphere. Phytoevaporation is used primarily to remove mercury, transforming the mercury ion into less toxic elemental mercury. The disadvantage is that mercury released into the atmosphere is likely to be recycled back through deposition and then re-entered into the ecosystem. American researchers have discovered that some plants growing on selenium-rich substrates produce volatile selenium in the form of dimethyl selenide and dimethyl diselenide. There are reports that phytoevaporation has been successfully applied to tritium, a radioactive isotope of hydrogen), which decays to stable helium with a half-life of about 12 years. Phytodegradation. In organic matter phytoremediation, plant metabolism is involved in contaminant recovery by transforming, decomposing, stabilizing, or evaporating contaminants from soil and groundwater. Phytodegradation is the decomposition of organic substances absorbed by a plant into simpler molecules that are incorporated into plant tissue.

Plants contain enzymes that can break down and convert weapons waste, chlorinated solvents such as trichlorethylene and other herbicides. Enzymes are usually dehalogenases, oxygenases and reductases. Rhizodegradation is the breakdown of organic compounds in soil through microbial activity in the root zone (rhizosphere) and is a much slower process than phytodegradation. The given phytoremediation methods can be used in a comprehensive manner. So, from the literature review it is clear that phytoremediation is currently a rapidly developing area of research. Over the past ten years, researchers from many countries around the world have obtained experimental confirmation, including in the field, of the promise of this method for purifying contaminated environments from organic, inorganic contaminants and radionuclides.

This environmentally friendly and inexpensive method of cleaning up contaminated areas is a real alternative to traditional methods of restoring disturbed and contaminated lands. In Russia, the commercial application of phytoremediation for soils contaminated with heavy metals and various organic compounds, such as petroleum products, is at an early stage. Large-scale research is needed aimed at searching for fast-growing plants that have a pronounced ability to accumulate contaminants from among cultivated and wild species characteristic of a particular region, experimental confirmation of their high phytoremediation potential, and studying ways to increase it. A separate important area of research is studying the issue of recycling contaminated plant biomass in order to prevent re-contamination of various components of the ecosystem and the entry of contaminants into food chains

CONTENTS

Introduction

1. Soil cover and its use

2. Soil erosion (water and wind) and methods of combating it

3. Industrial soil pollution

3.1 Acid rain

3.2 Heavy metals

3.3 Lead toxicity

4. Soil hygiene. Waste disposal

4.1 The role of soil in metabolism

4.2 Ecological relationships between soil and water and liquid waste (wastewater)

4.3 Limits of soil load with solid waste (household and street garbage, industrial waste, dry sludge after sewage sedimentation, radioactive substances)

4.4 The role of soil in the spread of various diseases

4.5 Harmful effects of the main types of pollutants (solid and liquid wastes) leading to soil degradation

4.5.1 Neutralization of liquid waste in soil

4.5.2.1 Neutralization of solid waste in soil

4.5.2.2 Garbage collection and removal

4.5.3 Final removal and rendering harmless

4.6 Disposal of radioactive waste

Conclusion

List of sources used

Introduction.

A certain part of the soil, both in Russia and throughout the world, leaves agricultural use every year for various reasons, discussed in detail in the UIR. Thousands or more hectares of land suffer from erosion, acid rain, improper cultivation and toxic waste. To avoid this, you need to become familiar with the most productive and inexpensive reclamation measures (For the definition of reclamation, see the main part of the work) that increase the fertility of the soil cover, and above all with the negative impact on the soil itself, and how to avoid it.

These studies provide insight into the harmful effects on soil and have been conducted through a number of books, articles and scientific journals dealing with soil issues and environmental protection.

The problem of soil pollution and degradation has always been relevant. Now we can add to what has been said that in our time the anthropogenic influence has a strong impact on nature and is only growing, and the soil is one of the main sources of food and clothing for us, not to mention the fact that we walk on it and will always be in close contact with her.

1. Soil cover and its use.

Soil cover is the most important natural formation. Its importance for the life of society is determined by the fact that soil is the main source of food, providing 97-98% of the food resources of the planet's population. At the same time, the soil cover is a place of human activity on which industrial and agricultural production is located.

Highlighting the special role of food in the life of society, V.I. Lenin pointed out: “The real foundations of the economy are the food fund.”

The most important property of the soil cover is its fertility, which is understood as the totality of soil properties that ensure the yield of agricultural crops. Natural soil fertility is regulated by the supply of nutrients in the soil and its water, air and thermal regimes. The role of soil cover in the productivity of terrestrial ecological systems is great, since soil nourishes land plants with water and many compounds and is an essential component of the photosynthetic activity of plants. Soil fertility also depends on the amount of solar energy accumulated in it. Living organisms, plants and animals inhabiting the Earth record solar energy in the form of phyto- or zoomass. The productivity of terrestrial ecological systems depends on the thermal and water balance of the earth's surface, which determines the variety of forms of exchange of matter and matter within the geographic envelope of the planet.

Analyzing the importance of land for social production, K. Marx identified two concepts: land-matter and land-capital. The first of these should be understood the earth that arose in the process of its evolutionary development without the will and consciousness of people and is the place of human settlement and the source of his food. From the moment when land, in the process of development of human society, becomes a means of production, it appears in a new quality - capital, without which the labor process is unthinkable, “... because it gives the worker... a place on which he stands... , and its process - the scope of action...”. It is for this reason that the earth is a universal factor in any human activity.

The role and place of land are different in various spheres of material production, primarily in industry and agriculture. In the manufacturing industry, construction, and transport, the earth is the place where labor processes take place regardless of the natural fertility of the soil. Land plays a different role in agriculture. Under the influence of human labor, natural fertility turns from potential into economic. The specificity of the use of land resources in agriculture leads to the fact that they act in two different qualities, as an object of labor and as a means of production. K. Marx noted: “By the mere new investment of capital in plots of land... people increased land-capital without any increase in the matter of the earth, i.e., the space of the earth.”

Land in agriculture acts as a productive force due to its natural fertility, which does not remain constant. With rational use of land, such fertility can be increased by improving its water, air and thermal conditions through reclamation measures and increasing the content of nutrients in the soil. On the contrary, with irrational use of land resources, their fertility decreases, resulting in a decrease in agricultural yields. In some places, cultivation of crops becomes completely impossible, especially on saline and eroded soils.

At a low level of development of the productive forces of society, the expansion of food production occurs due to the involvement of new lands in agriculture, which corresponds to the extensive development of agriculture. This is facilitated by two conditions: the availability of free land and the possibility of farming at an affordable average level of capital costs per unit area. This use of land resources and farming is typical of many developing countries in the modern world.

During the era of scientific and technological revolution, there was a sharp distinction between the farming system in industrialized and developing countries. The former are characterized by the intensification of agriculture using the achievements of scientific and technological revolution, in which agriculture develops not due to an increase in the area of cultivated land, but due to an increase in the amount of capital invested in the land. The well-known limitation of land resources for most industrialized capitalist countries, the increasing demand for agricultural products throughout the world due to high rates of population growth, and a higher culture of agriculture contributed to the transfer of agriculture in these countries back to the 50s on the path of intensive development. The acceleration of the process of intensification of agriculture in industrialized capitalist countries is associated not only with the achievements of scientific and technological revolution, but mainly with the profitability of investing capital in agriculture, which concentrated agricultural production in the hands of large landowners and ruined small farmers.

Agriculture developed in other ways in developing countries. Among the acute natural resource problems of these countries, the following can be identified: low agricultural standards, which caused degradation of soils (increased erosion, salinization, decreased fertility) and natural vegetation (for example, tropical forests), depletion of water resources, desertification of lands, especially clearly manifested in African countries. continent. All these factors related to the socio-economic problems of developing countries have led to chronic food shortages in these countries. Thus, at the beginning of the 80s, in terms of provision per person with grain (222 kg) and meat (14 kg), developing countries were inferior to industrialized capitalist countries, respectively, several times. Solving the food problem in developing countries is unthinkable without major socio-economic transformations.

In our country, the basis of land relations is the national (national) ownership of land, which arose as a result of the nationalization of all land. Agrarian relations are built on the basis of plans according to which agriculture should develop in the future, with financial and credit assistance from the state and the supply of the required number of machines and fertilizers. Paying agricultural workers according to the quantity and quality of work stimulates a constant increase in their living standards.

The use of the land fund as a whole is carried out on the basis of long-term state plans. An example of such plans was the development of virgin and fallow lands in the east of the country (mid-50s), thanks to which it became possible to introduce more than 41 million hectares of new areas into arable land in a short period of time. Another example is a set of measures related to the implementation of the Food Program, which provides for accelerating the development of agricultural production based on improving farming standards, extensive land reclamation activities, as well as the implementation of a broad program of socio-economic reconstruction of agricultural areas.

The world's land resources as a whole make it possible to provide food for more people than is currently available and will be the case in the near future. At the same time, due to population growth, especially in developing countries, the amount of arable land per capita is decreasing.

Heavy metals in soil

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

	Hazard Class		UEC by soil groups
	Extractable with ammonium acetate buffer (pH=4.8)	Sandy, sandy loam	Loamy, clayey
pH xl< 5,5	pH xl > 5.5

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

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.).

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.