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5INTERNATIONAL SCIENTIFIC-PRACTICAL CONFERENCE ACTUAL ISSUES OF AGRICULTURAL DEVELOPMENT: PROBLEMS AND
SOLUTIONS
JUNE 6-7, 2023
METHODOLOGICAL SIGNIFICANCE OF STUDYING THE TRANSFER OF DISSOLVED MICROELEMENTS THROUGH SOIL SOLUTION
Mirkozimjon Nishonov
Professor of the Department of Chemistry, Fergana State University, Candidate of
Technical Sciences https://doi.org/10.5281/zenodo.7997950
Abstract. This article discusses the question of the significance of studying the transfer of dissolved trace elements through the soil solution from the point of view of methodology. Scientific information about chemical processes occurring in soils and the causes of environmental pollution with microelements has been transformed into educational information.
Keywords: methodology, soil, chemical process, environment, microelements.
The question of the methodological significance of studying the migration of trace elements in air, water and plants was published in [1].
This article discusses the importance of studying the transfer of dissolved trace elements through the soil solution.
Thus, in soils formed in a cool humid climate, the leaching of trace elements down the soil profile is more pronounced than their accumulation, unless there is a large input of these elements into the soil. In warm dry climates, and also to some extent in humid hot climates, the upward movement of trace elements is more characteristic. However, the specific properties of soils, mainly their cation exchange capacity, control the rate of microelement migration in the soil profile.
Although many works have been devoted to the study of the movement of trace elements in the soil profile, a complete understanding of their circulation and balance has not yet been achieved. Reviews of theoretical issues related to the transport and accumulation of dissolved soil components have been published by a number of authors [6, 13]. The equilibria discussed by these researchers are applicable not only to elucidate the fundamental reactions important to understanding weathering and soil formation, but also to use in various fields of management and environmental protection. However, these models cannot be used to check the quality of thermodynamic data obtained from a particular soil without making the necessary adjustments to account for variations in soil properties, and even then a certain amount of doubt may remain.
Some detailed studies based on lysimetric experiments, as well as studies using isotope tracers, provide a lot of data on the transport of elements. However, each soil profile with well-developed horizons has its own characteristics of microelement movement.
The depletion of soils with microelements is mainly associated with the removal of the latter with infiltrating waters through the profiles of easily drained acidic soils, as well as with the absorption of microelements by plants. The other side of the balance consists of the entry of microelements with atmospheric precipitation and their accumulation in certain soil horizons. In acidic soils, some elements such as zinc, manganese, copper, iron, cobalt and boron are easily leached out. These elements, however, tend to form rather stable compounds if the soil acidity rises above 7. Other elements, such as molybdenum and selenium, are mobilized in alkaline soils, and become almost insoluble in acidic soils.
Trace element balances have been calculated for a variety of ecosystems in recent years. The difference in input and output shows that for most elements the rate of accumulation in the
INTERNATIONAL SCIENTIFIC-PRACTICAL CONFERENCE ACTUAL ISSUES OF AGRICULTURAL DEVELOPMENT: PROBLEMS AND
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JUNE 6-7, 2023
surface layer has a positive value. The leaching of manganese, iron and beryllium, as it turned out, is higher than the atmospheric input only in acidic forest soils. In the work of Kovda [19], the intensity of chemical pollution of soils was estimated; according to his calculations, the technogenic deposition of trace elements-metals on the earth's surface is 1-100 g/(ha.year). Recently, the fluxes of trace elements-metals in "technosystems" have been calculated, which gave the following values of the total deposition from the atmosphere [in g / (ha.year)]: cadmium - 29, copper - 136, manganese - 214, lead - 4708, zinc - 1098 [17]. The removal of these elements from the systems did not exceed 10% of their input, being the largest for copper and the smallest for lead. Soils are a sink of trace elements and therefore play an important role in the cycle of the latter in the environment. They have a great ability to retain many ionic compounds of trace elements. The term "sorption" used in this article refers to all phenomena at the solid-solution interface and includes the following monomolecular interactions:
1) Van der Waals forces;
2) ion-dipole interaction;
3) hydrophobic and hydrogen bonds;
4) charge transfer;
5) ion and ligand exchanges;
6) chemisorption;
7) magnetic attraction.
Soil components involved in the sorption of trace elements are:
1) oxides (aqueous, amorphous) - mainly iron and manganese and, to a much lesser extent, aluminum and silicon;
2) organic matter and living organisms;
3) carbonates, phosphates, sulfides and basic salts;
4) clay.
Among all these components, clay minerals, hydrous metal oxides, and organic matter are considered to be the most important groups that participate and compete with each other in the sorption of trace elements.
Sorption mechanisms can be based on chemical valence bonds, in which case the process is called "chemisorption". If van der Waals forces are involved in sorption, the process is called "physisorption". Both types of sorption play an important role in the fixation of uncharged complexes. Each trace element-cation can be adsorbed specifically and non-specifically, as was shown for cadmium [16].
Ion exchange reactions (also called "surface exchange") play a major role in sorption processes in general, but are almost exclusively associated with colloidal particles. Other processes can also be included in sorption, such as precipitation, the formation of minerals, absorption by meso- and microbiota and plant roots.
Adsorption is very important in the processes of sorption of chemical elements from solution on the surface of soil particles. Adsorption is a kinetic reaction based on the rules of equilibrium thermodynamics. The forces involved in the adsorption of ionized particles on a charged surface are electrostatic in nature; they are described by Coulomb's law, according to which unlike charges attract and like charges repel. When adsorbed on soil particles, the equilibrium concentration of an element can be described by the Langmuir or Freundlich adsorption isotherm equations [6]. The surface charge of soil substances is associated primarily
INTERNATIONAL SCIENTIFIC-PRACTICAL CONFERENCE ACTUAL ISSUES OF AGRICULTURAL DEVELOPMENT: PROBLEMS AND
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JUNE 6-7, 2023
with the substitution of ions, which is found mainly in colloids. At low acidity values, positively charged surfaces predominate, and at high acidity values, negatively charged ones. Therefore, colloids in most soils carry a negative charge and can be neutralized by cations in the surrounding solution. In the case of an excess of cations, the electrical neutrality of the system is maintained by the exchange of one cation for another, that is, cations adsorbed on the solid phase can be replaced by other cations, often including a hydrogen ion. As a result of dehydration and recrystallization, the bond strength of adsorbed metal ions increases, which is observed on the surface of colloids, especially in alkaline soils.
The ability of the solid phase of soils to exchange cations, the so-called cation-exchange capacity, is one of the most important properties of soils that control the circulation of microelements. The adsorption capacity determines the number of ions required to occupy all adsorbable positions on the surface of a unit mass of soil. The excess of the amount of adsorbed cations over their amount in solution is interpreted as the buffer capacity of the soil.
The cation-exchange capacity of various soils varies over a wide range and can be 1-100 meq/100 g of soil. The surface properties of soil particles are the most important factor determining the adsorption capacity for cationic trace elements. Although adsorption processes in general are not connected by simple relationships with the value of the cation exchange capacity, the amounts of adsorbed cations correspond to this value. Usually, the solid phase of soils with a larger surface area also has a larger cation exchange capacity, large adsorption and buffer capacities.
The affinity of cations for adsorption, i.e., for anionic exchange positions, is closely related to the ionic potential (equal to the ratio of charge to radius). In some systems, metal ions (zinc, cadmium, manganese) occupy almost the same proportion of the cation exchange capacity of various minerals [15]. Other cations, on the contrary, may have a higher substitution energy than the others and will therefore be selectively fixed by sorption sites. It has been found that the selectivity of adsorption indicates the possible formation of coordination complexes of heavy metals when they displace protons from the hydroxyl OH and carboxyl COOH groups [4]. Such specific sorption is clearly seen in the example of heavy metals, which have a high affinity for organic matter and oxide surfaces, since their substitution energy is greater than that of alkali and alkaline earth elements. This phenomenon is very important for the processes of nutrient supply to plants and soil pollution. The forms of occurrence and localization of trace elements-metals in soils depend on their chemical forms inherited from the parent rock, or on those in which they enter the soil. Microelements-metals coming with atmospheric dust are usually in the mineral form - in the form of oxides and sulfides. When they come from burning coal, vitreous particles predominate. If trace elements enter the soil during irrigation with wastewater, their forms depend on the source of entry and the method of treatment of wastewater.
Soils are heterogeneous mixtures of various organic and organo-mineral substances, clay minerals, oxides of iron, aluminum and manganese and other solid components, as well as various soluble substances. Therefore, the mechanisms of heavy metal binding in soils vary in many ways depending on the composition of soils, their reactivity, and redox conditions. Thus, the elements can form different compounds depending on the soil component they are associated with and what are the areas of the reacting surfaces of the phases, on the presence of external or internal positions in the crystal structure with different binding energies [18].
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In order to evaluate the forms of presence or binding forms of heavy metals in solids, various analytical methods have been developed using sequential extraction. One of the first sequential extraction methods for determining the forms of an element of interest in soils was proposed by Tessier et al. [23]. Other - kinetic - separation methods have also been developed. All these methods are based on the assumption that the following forms of heavy metals are present in soils:
1) water-soluble (for example, in soil solution);
2) exchange;
3) bound into organic compounds;
4) captured in oxides of iron and manganese;
5) own minerals (for example, carbonates, phosphates and sulfides of heavy metals);
6) bound in the structure of silicates (i.e., in an insoluble residue).
Soluble and exchangeable forms are the mobile fraction of metals in the soil. Other forms are more or less fixed. The mobilization of metals from these forms or the transformation of the mobile fraction of metals into the immobile one is usually a very slow process controlled mainly by kinetic factors [17]. The question of the conditions under which intrinsic microelement compounds (carbonates, phosphates, silicates, and some others) are present in soils is still open. The adsorption capacity of some soil components for trace elements can be very high, so significant amounts of these elements can be bound by adsorption before their own compounds begin to form.
Thus, the analysis of the literature data and the work performed allow us to draw some scientific and methodological conclusions:
1. Revision of historical concepts, reassessment of factors in the study of the causes and mechanisms of environmental pollution shows that the study of the transfer of dissolved trace elements through the soil solution helps to better understand the principles governing the behavior of trace elements in soils.
2. It seems more likely that these elements are bound by soil minerals, such as iron and manganese oxides, carbonates and clay minerals, through isomorphic substitution or fixation in free structural positions.
3. Trace elements ( cadmium, cobalt, chromium, copper, lead, zinc) can still form separate solid phases under specific conditions when they are present in soils in elevated concentrations due to pollution.
4. Scientific information about the chemical processes occurring in soils and the causes of environmental pollution with microelements converted into educational information serve as didactic material on chemistry, ecology, soil science and agronomy for students of higher educational institutions and secondary school students, as well as for the population of the planet earth.
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INTERNATIONAL SCIENTIFIC-PRACTICAL CONFERENCE ACTUAL ISSUES OF AGRICULTURAL DEVELOPMENT: PROBLEMS AND
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