Biological removal of heavy metals and radionuclides. Heavy metals are the most dangerous elements that can pollute the soil. Cleaning the soil from heavy metals.
UDC 546.621.631
SORPTION PURIFICATION OF SOILS FROM HEAVY METALS1
A.I. Vezentsev, M.A. Trubitsyn,
L.F. Goldovskaya-Peristaya, N.A. Volovicheva
Belgorodsky state university, 308015, Belgorod, st. Pobeda, 85
The results of a study of the ability of clays in the Belgorod region to absorb Pb (II) and Cu (II) ions from water and buffer soil extracts are presented. During the experiment, the optimal clay:soil ratio was established, at which the removal of heavy metals from the soil is most effective.
Keywords: clay sorbents, soil, sorption activity, montmorillonite, heavy metals.
The industrial use of heavy metals is very diverse and widespread. That is why phytotoxicity and harmful accumulation in soils are usually observed near enterprises. Heavy metals accumulate in the upper humus horizons of the soil and are slowly removed through leaching, consumption by plants, and erosion. Humus and alkaline soil conditions promote the absorption of heavy metals. Toxicity of heavy metals such as copper, lead, zinc, cadmium, etc. for agricultural crops in natural conditions is expressed in a decrease in the yield of commercial crops in the fields.
There are several methods for remediation of soils contaminated with heavy metals and other pollutants:
Removal of the contaminated layer and its burial;
Inactivation or reduction of the toxic effect of pollutants using ion exchange resins, organic substances that form chelate compounds;
Liming, application of organic fertilizers that absorb pollutants and reduce their entry into plants.
Application mineral fertilizers(for example, phosphate, reduces the toxic effects of lead, copper, zinc, cadmium);
Growing crops that are resistant to pollution.
Currently, in world practice, for the ecological refining of fertile soils, mineral aluminosilicate adsorbents are increasingly used: various clays, zeolites, zeolite-containing rocks, etc., which are characterized by high absorption capacity, resistance to environmental influences and can serve as excellent carriers for fixation on the surface of various compounds during their modification.
Materials and research methods
This work is a continuation of previously conducted studies of clays from the Gubkinsky district of the Belgorod region, as potential sorbents for the purification of fertile soils from heavy metals.
1 The work was supported by a grant from the Russian Foundation for Basic Research, project No. 06-03-96318.
In this work, clays of the Kyiv formation of the Sergievsky deposit of the Gubkinsky region, different in material composition and properties: K-7-05 (middle layer) and K-7-05 SW (lower layer), were used as sorbents. Soil samples K-8-05 and No. 129, selected on the territory of the Gubkinsko-Starooskolsky industrial region, were used as cleaning objects. Preliminary studies have shown that the clays of the Sergievskoye deposit absorb copper and lead ions from model aqueous solutions well. Therefore, further studies were carried out with water and buffer extracts from the soil.
The aqueous extract was prepared according to standard methods. The essence of the method is to extract water-soluble salts from the soil with distilled water at a soil-to-water ratio of 1:5. The concentration of metal ions was determined by the photocolorimetric method on a KFK-3-01 device using the appropriate methods for each metal.
The buffer extract from the soil was prepared according to the standard method of the Central Institute of Agrochemical Services agriculture(TsINAO) using an ammonium acetate buffer solution with a pH of 4.8. This extractant has been adopted by the agrochemical service to extract microelements available to plants. The initial concentration of mobile forms of copper and lead available to plants in the buffer extract was determined by atomic absorption spectrometry.
Sorption of copper and lead ions was carried out at a constant temperature (20 °C), under static conditions for 90 minutes. The sorbent: sorbate ratio was: 1: 250; 1:50; 1:25; 1:8 and 1:5.
Discussion of results
A study of the aqueous extract, which was prepared for 4 hours, showed that the concentration of water-soluble copper compounds is insignificant and amounts to 0.0625 mg/kg (in terms of Cu2 ions). No water-soluble lead compounds were detected.
The initial concentration of heavy metal ions in buffer extracts from soils was: for soil K-8-05: Cu2+ 2.20 mg/kg, Pb2+ 1.20 mg/kg; for soil No. 129: Cu2+ 4.20 mg/kg, Pb2+ 8.30 mg/kg.
The results of determining the degree of purification of soil K-8-05 with clays K-7-05 (middle layer) and K-7-05 SW (bottom layer) are presented in Table 1.
Table 1
Degree of purification of buffer extract from soil K-8-05, mass, %
Sorbent: sorbate ratio Clay K-7-05 (middle layer) Clay K-7-05 SW (bottom layer)
Cu2+ Pb2+ Cu2+ Pb2+
1: 250 45,5 33,3 54,5 33,3
1: 50 70,5 45,8 68,2 58,3
1: 25 72,3 58,3 79,5 58,3
1: 8 86,4 75,0 87,3 83,3
1: 5 95,5 83,3 95,5 83,3
The results presented in Table 1 show that with an increase in the sorbent: sorbate ratio from 1: 250 to 1: 5, the degree of purification of the buffer extract from copper ions with K-7-05 clay increases from 45.5 to 95.5%, and from lead ions - from 33.3 to 83.3%.
The degree of purification of the buffer extract with clay K-7-05 YuZ with the same increase in the ratio increased from 54.5 to 95.5% (for Cu2+) and from 33.3 to 83.3% (for Pb2+).
For your information, the initial concentration of copper ions was greater than that of lead ions. Consequently, purification of the buffer extract from copper ions with these clays is more effective than from lead ions.
Table 2
Degree of purification of buffer extract from soil No. 129 with K-7-05 clay (middle layer), wt. %
Sorbent: Cu2+ sorbate ratio
1: 250 39,3 66,7
Note: the experiment was not done with clay K-7-05 SW due to the lack of a sufficient amount of sample.
The results presented in Table 2 show that the degree of purification of the buffer extract from soil No. 129 with clay K-7-05 with an increase in the sorbent: sorbate ratio from 1: 250 to 1: 5 increases from 39.3 to 93.0% (for copper ions) and from 66.7 to 94.0% (for lead ions).
It should be noted that in this soil the initial concentration of copper ions was lower than that of lead ions. Therefore, we can assume that the efficiency of purification from copper ions of this soil is no worse than that of soil K-8-05.
To clarify the mechanism of sorption of heavy metals, we assessed the composition and state of the ion-exchange complex of clayey rocks in the Belgorod region. It was found that the cation exchange capacity of the studied samples varies from 47.62 to 74.51 meq/100 g of clay.
A comprehensive study of the acid-base properties of clays was carried out. Determination of active acidity confirmed that all clays are alkaline in nature. At the same time, the pH of the salt extract of these same samples is in the range of 7.2-7.7, which indicates that these clays have a certain amount of exchangeable acidity. Quantitatively, this value is 0.13-0.22 mmol-eq/100 g of clay and is due to the insignificant content of sufficiently mobile exchangeable protons. The amount of exchangeable bases varies within a fairly wide range of 19.6 - 58.6 mmol-eq/100 g of clay. Taking into account the data obtained, a hypothesis was formulated that the sorption capacity of the studied clay samples for heavy metals is largely determined by ion exchange processes.
From the work carried out, the following conclusions can be drawn.
With an increase in the sorbent: sorbate ratio from 1: 250 to 1: 5, the degree of soil purification increases: from 40 to 95% (for copper ions) and from 33 to 94% (for lead ions) when using clay from the Sergievskoe deposit (K-7- 05) as a sorbent.
The studied clays are a more effective sorbent with respect to copper ions than with lead ions.
It has been established that the optimal clay:soil ratio is 1:5. With this ratio, the degree of soil purification is:
For copper ions about 95% (wt.)
For lead ions about 83.% (wt.)
References
1. Bingham F.T., Costa M., Eichenberger E. Some issues of toxicity of metal ions. - M.:Mir, 1993. - 368 p.
2. Galiulin R.V., Galiulina R.A. Phytoextraction of heavy metals from contaminated soils // Agrochemistry. - 2003. - No. 3. - P. 77 - 85.
3. Alekseev Yu.V., Lepkovich I.P. Cadmium and zinc in plants of meadow phytocenoses // Agrochemistry. - 2003. - No. 9. - P. 66 - 69.
4. Dayan U., Manusov N., Manusov E., Figovsky O. On lack of interdependency between the abiotic and anthropoeic factors /// International Scientific Journal for Alternative Energy and Ecology ISJAEE, 2006.-No. 3(35). - P. 34 - 40.
5. Vezentsev A.I., Goldovskaya L.F., Sidnina N.A., Dobrodomova E.V. Zelentsova E.S. Determination of kinetic dependencies of sorption of copper and lead ions by rocks of the Belgorod region // Scientific bulletins of BelSU. Series Natural Sciences. - 2006. - No. 3 (30), issue 2. - P.85-88
6. Goldovskaya-Peristaya L.F., Vezentsev A.I., Sidnina N.A., Zelentsova E.S. Study of the gross content and content of mobile forms of cadmium in soils of the Gubkinsky-Starooskolsky industrial region // Scientific bulletins of BelSU. Series "Natural Sciences". - 2006. - No. 3(23), issue 4. - P.65-68.
7. Guidelines on the determination of heavy metals in soils of farmland and crop products. - M.: TsINAO, 1992.-61p.
8. State control of water quality. - M.: IPK. Publishing house of standards, 2001. - 690 p.
SORPTION PURIFICATION OF SOILS FROM HEAVY METALS A.I. Vesentsev, M.A. Troubitsin, L.F. Goldovskaya-Peristaya, N.A. Volovicheva
Belgorod State University, 85 Pobeda Str., Belgorod, 308015 vesentsev@bsu. edu. ru
Results of research of the ability of clays of the Belgorod region to absorb ions Pb (II) and Cu (II) from water and buffer soil extracts are presented. During experiment of the optimum ratio clay: ground with most effective purification from heavy metals is established.
Key words: clay sorbents, soil, sorption activity, montmorillonite, heavy metals.
When soils and vegetation are contaminated with heavy metals, the following techniques are used::
1) Limiting the entry of heavy metals into the soil. When planning the use of fertilizers, ameliorants, pesticides, and sewage sludge, it is necessary to take into account the content of heavy metals in them and the buffer capacity of the soils used. Dose restrictions due to environmental requirements are a necessary condition greening of agriculture.
The entry of heavy metals into plants can be reduced by changing the nutritional regime, by creating competition for the entry of toxicants and fertilizer cations into the roots, and by precipitating heavy metals in the roots in the form of sparingly soluble sediments.
2) Removal of heavy metals beyond the root layer is achieved by the following methods:
Removing the contaminated layer of soil;
Backfilling the contaminated layer with clean soil;
Growing crops that absorb HMs and removing their plant matter from the field;
By washing soils with water and water-soluble (usually organic) compounds that form water-soluble complex compounds with heavy metals, products from agricultural waste are used as organic ligands;
Washing the soils with a solution for leaching HMs from the upper horizons to a depth of 70-100 cm and then depositing them at this depth in the form of poorly soluble sediments (due to subsequent washing of the soils with reagents containing anions that form sediments with heavy metals).
3) Binding of heavy metals in soil into low-dissociation compounds. Reducing the entry of heavy metals into plants can be achieved by their precipitation in the soil in the form of sediments of carbonates, phosphates, sulfides, and hydroxides; with the formation of low-dissociation complex compounds with high molecular weight. In the best way A method that provides a significant reduction in the content of heavy metals in plants is the combined application of manure and lime. The most effective measures leading to reducing the mobility of lead in soils is claying (zeolite application) and joint application of lime and organic fertilizers. The use of a full complex of chemical ameliorants (organic and mineral fertilizers, lime and organic matter) reduces the content of polyvalent metals in the soil by 10-20%.
4) Adaptive landscape farming systems as an optimization factor environmental situation when soils are contaminated with heavy metals.
Various types and crop varieties accumulate unequal amounts of HMs in plant products. This is due to the selectivity of the root systems of individual plants towards them and the peculiarity of their metabolic processes. HMs accumulate to a greater extent in the roots, less in the vegetative mass and generative organs. At the same time, certain groups of crops selectively accumulate certain toxicants. Selection of crops for cultivation on soils of a certain degree and nature of pollution is the simplest, cheapest and most in an efficient way optimizing the use of contaminated soils.
Phytoremediation
Microorganisms are not able to remove heavy metals harmful to human health (arsenic, cadmium, copper, mercury, selenium, lead, as well as radioactive isotopes of strontium, cesium, uranium and other radionuclides from soil and water. Plants are able to extract from the environment and concentrate in their tissues, various elements do not make up plant mass. special labor collect and burn, and the resulting ashes are either buried or used as secondary raw materials.
The method of cleaning the environment using plants was called phytoremediation– from the Greek “phyton” (plant) and the Latin “remedium” (to restore).
Phytoremediation- a set of methods for purifying water, soil and atmospheric air using green plants.
Story
The first simple methods of wastewater treatment - irrigation fields and filtration fields - were based on the use of plants.
The first scientific studies were carried out in the 50s in Israel, but the active development of the technique occurred only in the 80s of the 20th century.
The plant affects the environment in different ways, the main ones:
rhizofiltration - roots absorb water and chemical elements necessary for plant life;
phytoextraction - accumulation of dangerous contaminants in the plant’s body (for example, heavy metals);
· phytovolatilization - evaporation of water and volatile chemical elements (As, Se) by plant leaves;
phytotransformation:
1. phytostabilization - transfer of chemical compounds into a less mobile and active form (reduces the risk of the spread of pollution);
2. phytodegradation - degradation by plants and symbiotic microorganisms of the organic part of pollution;
· phytostimulation - stimulation of the development of symbiotic microorganisms taking part in the cleaning process. Microorganisms play the main role in the degradation of contaminants. The plant is a kind of biofilter, creating a habitat for them (providing access to oxygen, loosening the soil. In this regard, the cleaning process also occurs outside the growing season (in the non-summer period) with slightly reduced activity.
Use of new methods for cleaning urban soils from heavy metals
V.I. Savich, Doctor of Agricultural Sciences, Professor, S.L. Belopukhov, Doctor of Agricultural Sciences, Professor, D.N. Nikitochkin, Candidate of Agricultural Sciences, Russian State Agrarian University - Moscow Agricultural Academy named after. K.A. Timiryazev; A.V. Filippova, Doctor of Biology, Professor, Orenburg State Agrarian University
Pollution of urban soils reduces the quality of life of the population, as dust particles carried by the wind enter the human body, leading to health problems. Filtration of pollutants, or their accumulation, depends on the properties of the soil and its saturation with pollutants. Issues of cleaning urban soils were discussed by the scientific community, measures were proposed for the periodic change of urbanized soils, the use of micropreparations that bind heavy metals, etc. It should be noted that any research that can improve the quality of urban soils has its place.
Biological purification of urban soils from heavy metals has its own characteristics. Cleaning urban soils of heavy metals can be carried out by removing them from the soil by green plants. At the same time, for more enhanced development of the process, it is necessary to select growing conditions and plant species. Different plants have different resistance to certain types of pollution, which is determined by the characteristics of the metabolic processes occurring in them. So, according to E.M. Ivanova et al., when comparing the resistance to copper sulfate of three grasses - crystal grass, meadow clover and rapeseed - clover showed the greatest resistance. At the same time, the toxicity of copper to plants was largely determined by its ability to bind to the BN groups of proteins and easily change its redox state, generating reactive oxygen species and causing a state of oxidative stress.
Purpose and methodology of research. When studying the possibilities of phytoremediation, experiments were carried out to study the possibilities of the removal of heavy metals by plants.
In experiment No. 1, the purpose of the study was to identify the influence of soil composition on the development of plants grown on it, the removal of certain elements (μn, Fe, Mn, Mg) with plants, and the assessment of plants that maximally accumulate and minimally accumulate various microelements. The components of the studied soils were quartz sand, peat, zeolite impregnated with an NPK solution, sod-podzolic soil (taken from a forest park in Moscow), soil contaminated with various toxicants (taken from the side of the road). Plants of watercress, radish, meadow grass and fescue were grown on the resulting soils.
red for 1-1.5 months. Then the resulting seedlings were analyzed using chemical analysis data (the content of the elements manganese, zinc, magnesium, iron), as well as data on the length of the stems and roots of the grown seedlings (the pH values of the studied soils ranged from 6.4 to 7.1).
Research results. The maximum development of stems was observed in the variant containing 10 g of zeolite, 30 g of peat, 30 g of sand and 30 g of contaminated soil. The options most favorable for the formation of mass, length of stems and roots differ. This is apparently due both to the presence of different growth substances in the variants and to the formation of a set of physical-chemical, water-physical, structural-chemical properties of soils favorable for various individual processes.
The best development of plants in terms of their weight was observed in the variant containing 25 g of peat, 25 g of zeolite, 25 g of sand and 25 g of contaminated soil. At the same time, the optimum for the development of different plants is observed on different soils.
The removal of zinc from soils due to biological reclamation is shown in Table 1.
The removal of zinc from soils depends on the composition of the soil and the plants grown. The crop with a higher vegetative mass had greater yield. Obviously, feeding plants with nutrients will increase the removal of heavy metals by plants. At the same time, fescue and bluegrass showed the greatest removal of mg of zinc per plant. The removal of zinc in soils with the addition of peat was 46.5 + 13.4 mg/vessel, and in soils without peat - 38.4 + 14.0.
The maximum removal of zinc from contaminated soils (mg/vessel) was carried out by radishes, the minimum - by lettuce (Table 2).
1. Removal of zinc from soils by individual crops (n = 8)
Culture Zinc removal
mg/vessel 100 mg/g plant 100
Watercress 16.5±4.7 50.0
Radish 109.2±28.7 67.0
Bluegrass 22.3±5.6 82.6
Fescue 32.6±8.5 90.5
2. Removal of zinc by plants, mg/vessel 102
Option Plants
salad radish bluegrass fescue
zeolite > 10% (option 1) 7.7±6.4 75.5±3.7 18.9±2.2 42.3±26.9
zeolite< 10% (вариант 2 и 4) 15,4±6,5 112,8±39,9 20,9±6,8 22,0±4,7
The introduction of zeolite into the soil by more than 10% (25%) compared with the application of 10% zeolite led to the binding of zinc in the soil and to less zinc removal by lettuce and radish plants (mg/vessel) (for bluegrass and fescue the differences are not significant).
In experiment No. 2, the removal of lead, cadmium, iron, and zinc from soils by vetch and oat seedlings was studied. The objects of study were contaminated soils. To increase the mobility of heavy metals in soils, samples were filled with 0.001 m EDTA to 60% PV, then seedlings were grown on them for 10 days. At the end of the growing period, heavy metals were extracted from the seedlings with 0.1 N HC1 and then determined using an atomic absorption spectrophotometer. According to the data obtained, the removal of heavy metals from soils by plants differed for soils different levels pollution, as can be seen from Table 3.
3. Removal of heavy metals by plants
Degree of contamination Removal, mg/100 g
Weak Increased 0.85±0.38 1.95±0.55 2.9±0.81 6.7±2.8 6.1±1.9 21.4±5.4 74±±63
4. Removal of heavy metals from soils by vetch and oat seedlings (mg/100 g of plants)
Seedlings Pb Cd Fe Zn
Vika 1.0±0.4 7.1±2.5 8.5±3.1 2.9±1.0
Oats 0.7±0.2 3.0±1.0 11.4±3.8 2.1±0.6
Vetch and oats differed in their ability to extract heavy metals from soils.
Judging by the data obtained, vetch removed more lead, cadmium, and zinc from the soil, and oats removed more iron.
A series of experiments have shown that the purification of urban soils from mobile forms of heavy metals can be carried out not only with the use of sorbents, with the precipitation of heavy metals in the form of sparingly soluble sediments, with the use of electroreclamation of soils, and very successfully with the help of phytoobjects. It is obvious that the removal of heavy metals from soils by plants (or microorganisms, fungi) depends on the degree of mobility of toxicants in the soil and increases when conditions are created for intensive plant development. Since different plants can withstand both a certain nature and degree of pollution, then for biological treatment urban soils from specific metals should be selected and selective conditions for their extraction (including changing physical and chemical properties soils and selection of ameliorant crops).
In one of the experiments, the development of seedlings was studied on soil samples taken in various areas of Moscow. The pH value of the aqueous suspension was determined in the samples; The length of the roots and stems of the seedlings and their weight were assessed. Growing plants at
optimal humidity lasted 10 days. The obtained data are shown in Table 5.
5. Development of seedlings on soils of parks and heavily polluted areas
Region Massa Roots Stems
MKAD, t. 1 Squares, t. 6, 8 0.8 1.7±0.1 2.7 5.2±1.2 7.3 11.6±1.5
As can be seen from the data presented, on heavily polluted soils near the Moscow Ring Road, plants developed much worse than in city parks.
From a theoretical point of view, adding a nutrient solution to the soil should improve the development of plants, and adding lead to the soil, on the contrary, should worsen their development. In the experiment, a nutrient solution and Pb(CH3COO)2 were added according to variants.
The addition of lead to contaminated soils led to complete inhibition of plants, and in the soils of public gardens it reduced their weight and shortened the length of roots and stems. At the same time, adding a nutrient solution to the soil improved the development of plants on contaminated soils and almost did not change the development on the soils of public gardens.
In the next experiment, the effect on the content of heavy metals in the soil of vetch, ryegrass, and white mustard plants was assessed. Despite the fact that plants absorbed a certain amount of heavy metals from the soil, the content of their mobile forms in the soil did not decrease due to the release of complexones by plants through the root system and the influence of decomposition products of organic residues on the mobility of heavy metals.
Theoretically, when KNO3 is added to the soil (when watering the soil), the development of plants should improve, and therefore, their removal of heavy metals from the soil should increase. However, this will also increase the ionic strength of the solution and, consequently, the solubility of the precipitates. The influence of plants on the solubility of sediments in the soil will also increase. In connection with the above, the gross content of heavy metals in soils during such biological reclamation should decrease, and the content of mobile forms may increase. Similar processes occur when soils are irrigated with EDTA (complexone for polyvalent metals). However, this reagent is not a source of plant nutrition, and its effect on the solubility of sediments is greater than KNO3, and less on plant development. The considered theoretical patterns are illustrated by the data in Table 6.
Thus, it is possible various ways removal of mobile forms of heavy metals from the top layer of soil, the priority of use of which is determined by specific soil, lithological, hydrological conditions and economic opportunities. In addition
6. Effect of adding KIO, EDTA to soils and growing plants on the content of mobile forms of heavy metals in soils (n=10-30)
Options C<1 Си Ми
Yugo vetch3 EDTA Rye grass White mustard KZh)3 + vetch + ryegrass + mustard EDTA + vetch + ryegrass + mustard 1.10±0.21 0.95±0.10 0.81±0D0 0.78±0D9 1.20± 0.18 1.08±0.21 0.28±0.13 0.0 0.51±0.16 0.0 0.0 0.90±0.11 0.55±0.06 3.60 ±0.4 0.79±0.16 1.17±0.53 0.70±0.16 3.90±1D 2.72±0.8 3.60±1.1 1.70±0.5 1 ,10±0.2 323.5±47.5 167.7±18.3 332.1±38.9 230.7±43.2 237.5±36.5 212.7±35.1 113, 8±42.3 72.4±31.0 373.5±77.2 332.0±67.1 77.9±31.7
To the known methods, from our point of view, it is advisable to add the following:
1) leaching of heavy metals with solutions of complexones to a certain depth and then their precipitation there by subsequent washing of the soil with solutions containing carbonates, phosphates, having an alkaline environment;
2) removal from soils due to phytoremediation and absorption of heavy metals by fungi while creating conditions for their greater bioproductivity;
3) regulation of exchange constants in the soil - roots system; roots - the above-ground part of plants due to the nutritional regime;
4) use for phytoremediation of plant species and varieties with a greater sorption capacity of roots for heavy metals;
5) use of long-acting sorbents for the sorption of heavy metals,
taking into account the equilibrium constants in the system soil - heavy metal and sorbent - heavy metal;
6) reducing the entry of heavy metals into plants when complexing compounds from agricultural waste are added to the soil, forming stable complexes of large molecular weight with metals;
7) electroreclamation of soils while creating conditions for increasing the mobility of heavy metals;
8) creation of geochemical barriers in the soil profile that prevent their entry into plants, migration into groundwater and evaporation from soils.
The choice of strategy when using a set of measures to improve the condition of urban soils, sometimes called urban soils, is possible only by carrying out physical and chemical calculations and predicting the ongoing processes for specific soils, plants and environmental conditions.
Literature
1. Kholodova V.P., Volkov K.S., Kuznetsov V.V. Adaptation of crystal grass plants to high concentrations of copper and zinc salts and the possibility of their use for phytoremediation purposes // Plant Physiology. 2005. T. 52. S, 848-858.
2. Ivanova E.M., Volkov K.S., Kholodova V.P., Kuznetsov V.V. New promising plants in the phytoremediation of copper-polluted areas // Bulletin of RUDN University. Series "Agronomy and Livestock Husbandry". 2011. No. 2. P. 28-37.
3. Clemens D. Toxic metal accumulation. Responses to exposure and mechanisms of tolerance in plants, Biochem., 2006, v. 88, p. 1707-1719.
4. Kramer U. Metal hyper-accumulation in plants, Ann. Rev. Plant Biol., 2010, v. 10, p. 517-534.
5. Savich V.I., Belopukhov S.JI., Nikitochkin, Filippova A.V. New methods for cleaning soils from heavy metals / News of the Orenburg State Agrarian University. 2013. No. 4. S, 216-218.
The chemical composition of soils in different territories is heterogeneous and the distribution of chemical elements contained in soils across the territory is uneven. For example, being predominantly in a dispersed state, heavy metals are capable of forming local bonds, where their concentrations are many hundreds and thousands of times higher than clarke levels.
A number of chemical elements are necessary for the normal functioning of the body. Their deficiency, excess or imbalance can cause diseases called microelementoses 1, or biogeochemical endemics, which can be both natural and man-made. In their distribution, an important role is played by water, as well as food products, into which chemical elements enter from the soil through food chains.
It has been experimentally established that the percentage of HMs in plants is influenced by the percentage of HMs in the soil, atmosphere, and water (in the case of algae). It was also noticed that on soils with the same content of heavy metals, the same crop produces different yields, although the climatic conditions also coincided. Then the dependence of yield on soil acidity was discovered.
The most studied soil contaminations are cadmium, mercury, lead, arsenic, copper, zinc and manganese. Let us consider soil contamination with these metals separately for each. 2
Cadmium (Cd)
The cadmium content in the earth's crust is approximately 0.15 mg/kg. Cadmium is concentrated in volcanic (in quantities from 0.001 to 1.8 mg/kg), metamorphic (in quantities from 0.04 to 1.0 mg/kg) and sedimentary rocks (in quantities from 0.1 to 11.0 mg/kg). Soils formed on the basis of such initial materials contain 0.1-0.3; 0.1 - 1.0 and 3.0 - 11.0 mg/kg cadmium, respectively.
In acidic soils, cadmium is present in the form of Cd 2+, CdCl +, CdSO 4, and in calcareous soils - in the form of Cd 2+, CdCl +, CdSO 4, CdHCO 3 +.
The uptake of cadmium by plants decreases significantly when acidic soils are limed. In this case, an increase in pH reduces the solubility of cadmium in soil moisture, as well as the bioavailability of soil cadmium. Thus, the cadmium content in beet leaves on calcareous soils was lower than the cadmium content in the same plants on unlimed soils. A similar effect has been shown for rice and wheat -->.
The negative effect of increasing pH on cadmium availability is associated with a decrease not only in the solubility of cadmium in the soil solution phase, but also in root activity, which affects absorption.
Cadmium is rather little mobile in soils, and if cadmium-containing material is added to its surface, the bulk of it remains untouched.
Methods for removing contaminants from soil include either removing the contaminated layer itself, removing cadmium from the layer, or covering the contaminated layer. Cadmium can be converted into complex insoluble compounds by available chelating agents (eg ethylenediaminetetraacetic acid). .
Because of the relatively rapid uptake of cadmium from soil by plants and the low toxicity of commonly occurring concentrations, cadmium can accumulate in plants and enter the food chain faster than lead and zinc. Therefore, cadmium poses the greatest danger to human health when introducing waste into soil.
A procedure for minimizing the amount of cadmium that can enter the human food chain from contaminated soils is to grow non-food crops or crops that absorb small amounts of cadmium in the soil.
In general, crops grown on acidic soils absorb more cadmium than those grown on neutral or alkaline soils. Therefore, liming of acidic soils is effective remedy reducing the amount of absorbed cadmium.
Mercury (Hg)
Mercury is found in nature in the form of metal vapor Hg 0 formed during its evaporation from the earth's crust; in the form of inorganic salts Hg(I) and Hg(II), and in the form of an organic compound of methylmercury CH 3 Hg +, monomethyl and dimethyl derivatives CH 3 Hg + and (CH 3) 2 Hg.
Mercury accumulates in the upper horizon (0-40 cm) of the soil and weakly migrates into its deeper layers. Mercury compounds are highly stable soil substances. Plants growing on mercury-contaminated soil absorb significant amounts of the element and accumulate it in dangerous concentrations, or do not grow.
Lead (Pb)
According to experiments conducted in sandy culture conditions with the introduction of threshold soil concentrations of Hg (25 mg/kg) and Pb (25 mg/kg) and exceeding the threshold concentrations by 2-20 times, oat plants grow and develop normally up to a certain level of contamination. As the concentration of metals increases (for Pb starting from a dose of 100 mg/kg), the appearance plants. At extreme doses of metals, plants die within three weeks from the start of the experiments. The metal content in biomass components is distributed in descending order as follows: roots - above-ground part - grain.
The total input of lead into the atmosphere (and therefore partially into the soil) from motor transport in Russia in 1996 was estimated at approximately 4.0 thousand tons, including 2.16 thousand tons contributed by freight transport. The maximum load of lead occurred in the Moscow and Samara regions, followed by the Kaluga, Nizhny Novgorod, Vladimir regions and other constituent entities of the Russian Federation located in the central part of the European territory of Russia and North Caucasus. The highest absolute emissions of lead were observed in the Ural (685 t), Volga (651 t) and West Siberian (568 t) regions. And the most adverse impact of lead emissions was noted in Tatarstan, Krasnodar and Stavropol territories, Rostov, Moscow, Leningrad, Nizhny Novgorod, Volgograd, Voronezh, Saratov and Samara regions (Green World newspaper, special issue No. 28, 1997).
Arsenic (As)
Arsenic is found in environment in a variety of chemically stable forms. Its two main oxidation states are As(III), and As(V). Pentavalent arsenic is common in nature in the form of a variety of inorganic compounds, although trivalent arsenic is easily detected in water, especially under anaerobic conditions.
Copper(Cu)
Natural copper minerals in soils include sulfates, phosphates, oxides and hydroxides. Copper sulfides can form in poorly drained or flooded soils where reducing conditions occur. Copper minerals are usually too soluble to remain in free-draining agricultural soils. In metal-contaminated soils, however, chemical environment can be controlled by nonequilibrium processes leading to the accumulation of metastable solid phases. It is assumed that covellite (CuS) or chalcopyrite (CuFeS 2) may also be present in restored soils contaminated with copper.
Trace amounts of copper may occur as discrete sulfide inclusions in silicates and can isomorphously replace cations in phyllosilicates. Clay minerals that are unbalanced in charge absorb copper nonspecifically, but oxides and hydroxides of iron and manganese show a very high specific affinity for copper. High molecular weight organic compounds are capable of being solid absorbents for copper, while low molecular weight organic substances tend to form soluble complexes.
The complexity of soil composition limits the ability to quantitatively separate copper compounds into specific chemical forms. indicates -->The presence of a large mass of copper conglomerates is found both in organic substances and in Fe and Mn oxides. The introduction of copper-containing waste or inorganic copper salts increases the concentration of copper compounds in the soil that can be extracted with relatively mild reagents; Thus, copper can be present in the soil in the form of labile chemical forms. But the easily soluble and replaceable element - copper - forms a small amount of forms capable of absorption by plants, usually less than 5% of the total copper content in the soil.
Copper toxicity increases with increasing soil pH and when soil cation exchange capacity is low. Enrichment of copper through extraction occurs only in the surface layers of the soil, and grain crops with deep root systems do not suffer from this.
The environment and plant nutrition can influence copper phytotoxicity. For example, copper toxicity to lowland rice was clearly observed when the plants were watered with cold rather than warm water. The fact is that microbiological activity is suppressed in cold soil and creates those reducing conditions in the soil that would facilitate the precipitation of copper from solution.
Copper phytotoxicity occurs initially from an excess of available copper in the soil and is enhanced by soil acidity. Since copper is relatively inactive in the soil, almost all copper that enters the soil remains in the upper layers. The addition of organic substances to copper-contaminated soils can reduce toxicity due to the adsorption of the soluble metal by the organic substrate (in this case, Cu 2+ ions are converted into complex compounds less accessible to the plant) or by increasing the mobility of Cu 2+ ions and leaching them from the soil in the form of soluble organocopper complexes.
Zinc (Zn)
Zinc can be present in the soil in the form of oxosulfates, carbonates, phosphates, silicates, oxides and hydroxides. These inorganic compounds are metastable in well-drained agricultural land. Sphalerite ZnS appears to be the thermodynamically dominant form in both reduced and oxidized soils. Some association of zinc with phosphorus and chlorine is evident in reduced sediments contaminated with heavy metals. Therefore, relatively soluble zinc salts should be found in metal-rich soils.
Zinc is isomorphously replaced by other cations in silicate minerals and can be occluded or coprecipitated with manganese and iron hydroxides. Phyllosilicates, carbonates, hydrated metal oxides, and organic compounds absorb zinc well, using both specific and nonspecific binding sites.
The solubility of zinc increases in acidic soils, as well as during complex formation with low molecular weight organic ligands. Reducing conditions can reduce the solubility of zinc due to the formation of insoluble ZnS.
Phytotoxicity of zinc usually occurs when plant roots come into contact with a solution in the soil that contains excess zinc. Transport of zinc through soil occurs through exchange and diffusion, with the latter process being dominant in soils low in zinc. Metabolic transport is more significant in high-zinc soils, in which soluble zinc concentrations are relatively stable.
The mobility of zinc in soils increases in the presence of chelating agents (natural or synthetic). The increase in soluble zinc concentration caused by the formation of soluble chelates compensates for the decrease in mobility caused by the increase in molecular size. Plant tissue zinc concentrations, total uptake, and toxicity symptoms are positively correlated with the zinc concentration in the solution bathing the plant roots.
Free Zn 2+ ion is predominantly absorbed by the root system of plants, therefore the formation of soluble chelates promotes the solubility of this metal in soils, and this reaction compensates for the reduced availability of zinc in chelated form.
The initial form of metal contamination influences the potential for zinc toxicity: the availability of zinc to the plant in fertilized soils with equivalent general content of this metal decreases in the order ZnSO 4 >sludge>garbage compost.
Most experiments on soil contamination with Zn-containing sludge did not show a decrease in yield or their obvious phytotoxicity; nevertheless, their long-term introduction with high speed can damage plants. A simple application of zinc in the form of ZnSO 4 causes a decrease in crop growth in acidic soils, while its long-term application in almost neutral soils goes unnoticed.
Zinc reaches toxic levels in agricultural soils typically from surface zinc; it usually does not penetrate deeper than 15-30 cm. The deep roots of certain crops can avoid contact with excess zinc due to their location in uncontaminated subsoil.
Liming of soils contaminated with zinc reduces the concentration of the latter in field crops. Additions of NaOH or Ca(OH) 2 reduce the toxicity of zinc in vegetable crops grown on high-zinc peat soils, although in these soils the uptake of zinc by plants is very limited. Iron deficiency caused by zinc can be eliminated by adding iron chelates or FeSO 4 to the soil or directly to the leaves. Physically removing or burying the zinc-contaminated top layer may avoid toxic effects of the metal on plants altogether.
Manganese
In soil, manganese is found in three oxidation states: +2, +3, +4. For the most part, this metal is associated with primary minerals or with secondary metal oxides. In the soil, the total amount of manganese ranges from 500 to 900 mg/kg.
The solubility of Mn 4+ is extremely low; trivalent manganese is very unstable in soils. Most of the manganese in soils is present in the form of Mn 2+, while in well-aerated soils most of it in the solid phase is present in the form of oxide, in which the metal is in oxidation state IV; in poorly aerated soils, manganese is slowly restored by the microbial environment and passes into the soil solution, thus becoming highly mobile.
The solubility of Mn 2+ increases significantly at low pH values, but the uptake of manganese by plants decreases.
Manganese toxicity often occurs where total manganese levels are moderate to high, soil pH is quite low, and soil oxygen availability is low (i.e., reducing conditions exist). To eliminate the effects of these conditions, soil pH should be increased by liming, efforts should be made to improve soil drainage, and water supply should be reduced, i.e. generally improve the structure of a given soil.
