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5INTERNATIONAL SCIENTIFIC-PRACTICAL CONFERENCE ACTUAL ISSUES OF AGRICULTURAL DEVELOPMENT: PROBLEMS AND
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METHODOLOGY FOR STUDYING CHEMICAL PROCESSES OCCURING IN SOILS
Mirkozimjon Nishonov
Professor of the Department of Chemistry, Fergana State University, Candidate of Technical
Sciences
https://doi.org/10.5281/zenodo.8001320
Annotation. This article discusses the importance of studying chemical processes occurring in soils 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 development of scientific and technological progress, if, on the one hand, facilitates and improves human life, on the other hand, worsens the living conditions not only of the person himself, but of a living being in general. Assessing his activities, a person always strives to realize and understand the essence of the processes taking place around him and apply appropriate measures.
Methodological culture becomes an indispensable attribute of education, professionalism in any field of human activity.[1]
It is impossible to master methodological culture without tracing the main ways of forming the methodology of scientific knowledge, without understanding the historicism of this process.
As it has been repeatedly shown by leading scientists of the world, the formation of the methodology of scientific knowledge was carried out as a result of generalizing the history of scientific and technical discoveries, analyzing the process of reflection in the dialectic of ideas. On the other hand, the ever-increasing complexity of the tasks facing science, as well as teachers of higher and secondary schools, forces us to systematize scientific knowledge, develop new methods for obtaining it, researching, generalizing, classifying, converting it into educational information, as well as its presentation. [4 ]
Today, the time has come for scientists to help not only children, pupils, students, but all segments of the population in a deep understanding of the processes that occur in nature and pollute the environment.
This article is devoted to the methodological analysis of the study of chemical processes occurring in soils and the cause of environmental pollution with microelements and the creation of methodically processed didactic materials for a full and deep understanding by university students and students of various schools of the importance of environmental protection. To fully understand the essence of the chemical processes occurring in soils, it is first necessary to familiarize yourself with the concept of soil formation.
When creating the concept of soil formation, one should abandon the common but erroneous opinion that soil is a mixture of unconsolidated substances formed during the weathering of underlying rocks. Soil is a natural body, including mineral and organic components, which has certain physical, chemical and biological properties. Soil properties are not simply the result of mixing the properties of all soil components taken individually.
Any classification of soils suffers from certain disadvantages, because the genesis and properties of soils are very complex and it is simply impossible to take into account all this
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complexity in the classification. The concepts used in different systems for classifying soils are rarely strictly equivalent. The composition of soils is extremely diverse, and although it depends on many factors, the main role is usually played by climatic conditions and the composition of bedrock.
The soil consists of three phases - solid (mineral and organic), liquid and gaseous, and all properties arise due to physical and chemical equilibria between these phases. Important factors affecting soil properties include not only the chemical composition of solid soil components, but also their mineral structure and degree of dispersion.
Although trace elements are small components in the composition of the solid phase of the soil, they play an important role in its fertility.
Soil formation from bedrock includes two stages. The first is the change in the primary mineral constituents of bedrock as a result of physical and chemical weathering processes. The second stage (pedogenesis), which consists in the formation of a soil profile based on weathered material, leads to the development of a mature zonal soil as the final result of all interacting processes. The processes of weathering and pedogenesis cannot be strictly distinguished, since they can occur simultaneously in the same place and are usually closely interconnected.
Weathering - the main process that forms the soil - has been the subject of extensive research. It was considered as a complex interaction of the lithosphere, atmosphere and hydrosphere occurring within the biosphere. The driving force behind weathering is solar energy. From a chemical point of view, weathering can be described as a set of processes of dissolution, hydration, hydrolysis, oxidation, reduction and carbonization. All these processes obey the laws of chemical thermodynamics and lead to the formation of minerals and chemical components that are relatively more stable and balanced under the conditions of a given soil. Chemical weathering leads to the destruction of the original minerals and the transition of elements from these minerals into solutions and suspensions. Very simplistically, the main weathering processes can be characterized as follows:
1. Dissolution - minerals dissolve in the water phase.
2.Hydration - minerals increase in water content
3. Hydrolysis - the reaction of minerals with water, giving new ions or insoluble components.
4. Oxidation - the entry of oxygen into chemical compounds or an increase in the valence of elements.
5. Restoration - reactions that are the reverse of oxidation.
6. Carbonatization - the conversion of compounds into carbonates caused by the absorption of carbon dioxide.
All these reactions are controlled by chemical equilibria in a given area of the earth's surface. The steady state of such a system is often depicted in the form of Eh-pH-diagrams with given chemical reactions [4,5,9,14]. Despite the fact that many questions and difficulties arise in the practical use of such diagrams, they clearly show that both these factors (Eh and pH) play a significant role in geochemical evolution.
The behavior of different elements during weathering varies greatly. The mobility of elements in weathering processes is determined, firstly, by the stability of their host minerals and, secondly, by the electrochemical properties of the elements.
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The nature of the trace element distribution is a parameter that is very sensitive to changes in the weathering environment. The so-called chemical nature of an element reflects mainly its electronegativity and ion size. Some characteristic properties of microelements can explain why certain microelements tend to form a stable association with a macroelement in various geochemical conditions. Elements with ionic potentials below 3 predominantly exist as free ions, while elements with ionic potentials of 3-12 tend to form hydrolyzed or complex forms. Easily mobile elements usually give smaller hydrated ions in aqueous solutions compared to low-mobility elements. In addition, the free energy required for the formation of these ions is usually lower than the energy of formation of ions of less mobile elements.
In the formation of the soil profile, in addition to the above reactions included in the weathering process, a number of additional specific reactions are involved. These reactions, although extremely varied, include the following elementary acts:
1. Receipt of organic and mineral substances into the soil.
2. Loss of these substances by the soil.
3. Spatial movement of these substances in the soil, both vertical and horizontal.
4. Transformations of organic and mineral substances in the soil.
These processes can be constructive or destructive. The nature of the resulting soil is mainly controlled by six factors:
1. Climate (temperature, rainfall).
2.Vegetation and other soil biota.
3. Parent rock (properties of minerals).
4. Topography (open or closed systems).
5.Time.
6. Anthropogenic activity (depletion, pollution, reclamation). Soil classifications are often based on factors that dominate the soil-forming process. Climate is usually the dominant factor, but quite often the composition of the parent rock and the nature of the vegetation also play this role.
In the early stages of weathering and soil formation, the composition of trace elements in the soil is usually inherited from the parent rock. Over time, however, the state of microelements in the soil begins to differentiate under the influence of dominant soil-forming processes. The fate of trace elements mobilized upon dissolution of minerals or host compounds depends on the properties of ions and complexes formed in soil solutions. They can be:
1. Washed out (leached) from the soil.
2.Besieged.
3.Included in minerals
4.Adsorbed by soil components.
5. Adsorbed or absorbed by organic matter.
Thus, various interactions between the solid and gaseous phases of the soil, living matter and soil solution govern the dynamic equilibrium between soil components. The biological, chemical, and physical characteristics of natural systems such as soils are subject to strong changes with depth. They also show significant seasonal variations in important variables such as pH and Eh. There are no methods for reliable in siti measurement of these fundamentally important chemical parameters, and there are no data on their distribution and variations at the
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level of individual pores and particles in the soil. This imposes significant limitations on our knowledge of the chemical aspects of the behavior of soil systems.
Recently, a number of studies of the kinetics of reactions in soil systems have been carried out. Some general principles, mechanisms and models of processes were described (including the Geochem computer program) [4-6.20-22]. The main object of modeling and calculations were the chemical forms of trace elements, which control their mobility and biological availability, as well as determine the residence time of microelements coming with pollution in the soil layer. Sparks [4.20] showed that, based on thermodynamic data, only the final state of a system can be predicted from the properties of the initial nonequilibrium state. However, kinetic data make it possible to penetrate into the essence of the pathways and mechanisms of chemical reactions in complex multiphase soil systems. Chemical reactions that bring all kinds of ions into solution can be described by thermodynamic equations. In an equilibrium state, the rates of reactions in the forward and reverse directions are equal, and the composition of the soil phases (solid, liquid, and gaseous) remains constant.
Chemical equilibria were studied in different types of soils. Very detailed mathematical models for certain soil conditions are given in [7.6]. Although many articles have been published on the behavior of trace elements in soils, its chemical aspects are still not well known. The variety of ionic particles formed by microelements and their different ability to form complex compounds with organic and inorganic ligands enable any element to pass into solution in a fairly wide range of pH and Eh. Any element can also be precipitated and/or adsorbed quite easily even with small changes in the equilibrium conditions. In soils, solubility equilibria can vary greatly over distances measured by several centimeters (and even millimeters) in both the vertical and horizontal directions. Therefore, such equilibrium states are local and may be different in different parts (points) of the soil system. Since the behavior of the components that cause dissolution and precipitation is very dynamic, many minerals and amorphous substances can be out of equilibrium under soil conditions.
Diagrams of the stability of ionic forms of microelements as a function of pH and Eh are given in many manuals [8,9,13]. Under natural soil conditions, pH values are most often between 5 and 7, and Eh between +0.5 and -0.1, with the exception of waterlogged soils with very reducing conditions. The properties of the ionic forms of each element are different, and the pH ranges for the precipitation of their aqueous oxides are not the same. However, one conclusion can be drawn: usually the proportion of mobile forms is maximum at low pH and low redox potential. Therefore, it should be expected that with an increase in the pH of the soil substrate, the solubility of most microelements will decrease. Indeed, the concentrations of trace elements in soil solutions in alkaline and neutral soils are lower than in light acidic soils.
Sposito [5.21] investigated the applicability of thermodynamic methods to the study of soil solutions. He also described possible cation exchange reactions involving microelements.
The solubility of trace elements in soils is of great importance for their bioavailability and migratory capacity. Heavy soils - both alkaline and neutral - retain trace elements well and therefore supply them slowly to plants. However, such a slow release leads to a lack of some trace elements for plant development. Light soils, on the contrary, can be a source of readily available elements, but for a relatively short period of time. Such soils will quickly lose their supply of available microcomponents of plant nutrition.
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One of the first studies of soil solutions by Hodgson et al. [8.10] showed that they contain appreciable amounts of trace elements in the form of complex compounds, mainly with organic ligands. Inorganic ligands can also be important, but only for certain elements and under certain conditions.
Trace element concentrations in soil solutions are highly variable, from soil to soil and over time. Very large fluctuations are observed under the influence of the following factors:
1) time;
2) vegetation;
3) activity of microorganisms;
4) waterlogging;
5) heterogeneity of the solid phase of the soil
6) time;
7) vegetation;
8) the activity of microorganisms;
9) waterlogging;
10)heterogeneity of the solid phase of the soil In addition, the methods used to obtain solutions from soils vary greatly, and therefore it is difficult to adequately determine the average concentrations of trace elements. Nevertheless, the order of trace element concentrations measured in various soil solutions indicates fairly good agreement.
During rainfall, evaporation and transpiration of plants, the concentrations of trace elements in soil solutions can change by more than 10 times, while the observed variations in the main ions (Ca, Mg, K, Na, NO3-, and PO43-) are much less.
Dissolved major ions greatly influence the amount of dissolved trace elements. Solutions in most soils contain an excess of calcium: over 90% of the total concentration of cations. Therefore, calcium is the most important cation that determines the degree of dissolution of trace elements in the soil. However, there are examples when complexation protects cationic trace elements from precipitation in the presence of Ca2+ and in soil solutions with relatively high pH. Thus, elevated concentrations of dissolved metal ions against the norm and rapid uptake of trace elements by plants are usually associated with complex formation. This also agrees with the observations that half of the calcium in soil solutions is present, as a rule, in the form of organic complexes [11]. The anionic composition of soil solutions is also important for the state of microelements. However, little is known about the association of microelements with anionic particles of soil solutions.
The different ability of cations to form complex compounds with ligands can be explained in terms of the rules of coordination chemistry. It is possible to predict which cations are better bound into complexes with a particular ligand. Metal ions such as Be2+, Cr3+ and Co3+ will easily react with PO43-, CO32-, NO3-, organic amines, etc. A group of elements including Ni2+, Co2+, Cu2+, Zn2+, Cd2+, Pb2+ and Sn2+ can it is easy to contact with C1-, Br-, NO2- and NH3, while the Hg2+, Ag+ T1+ cations, which have similar chemical properties, bind predominantly with I-, CN-, CO, S, P and As.
In the soil water phase, the most abundant ligands are organics and water molecules, so hydrolysis and complexation with organics are the most common reactions in soil solutions. These reactions are pH sensitive and show a correlation with the size and charge of the cations. Elevated ionic potentials usually indicate a greater degree of hydration in the solution and hence
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easier precipitation. According to the degree of mobility in the redox conditions of soils, cations can be represented in the following order in accordance with the pH intervals for the precipitation of aqueous oxides: Bi3+ > Sn4+ > Ge3+ > Zr4+ > Sb3+. However, data on the activity of heavy metals obtained in pure systems can only be applied to soils as indicative due to the complex formation of ions, the formation of solid solutions. and surface phenomena.
The solubility of trace elements in soils obviously depends on complex formation. Most dissolved forms of trace elements, especially free cations, have low solubility, and only a small fraction of them are present in the aqueous phase. Calculations carried out by Kabata-Pendias [12] showed that the total content of trace elements-cations generally ranges from 10-100 |ig/L in normal soil solutions, while in polluted soils it can be much higher. When soluble micronutrient compounds are added to soil, their concentrations in equilibrium solutions increase with increasing dose added. In the experiments carried out by Cotteny et al. [9.7], the relative proportion of dissolved metal from the added additive at the conditional high rate of its entry into light sandy soil was 39% of the dose of 1000 mg Zn/kg, 50% of 5000 mg Cu/kg, 30 % of 5120 mg Cd/kg and 26% of 2695 mg Pb/kg.
Thus, the analysis of literature data allows us to draw some methodological conclusions:
1. Revision of historically established concepts, reassessment of factors for studying the causes and mechanisms of environmental pollution shows that the significance of the relationships of microelements with individual phases of soils and their affinity for each constituent part of the soil is the key to a better understanding of the principles governing the behavior of microelements in soils.
2. The emergence of information about the main forms of soil organization has led to the formation of conceptual systems of chemistry and to the hyperbolization of some ideas (the concept of soil formation, the processes of soil formation, weathering, pedogenesis, dissolution, transfer, sorption, adsorption.
3. Microelements enter the soil layers in various ways, including: directly deposited from the atmosphere, leaching, decomposition of aboveground parts of plants, use of waste, use of pesticides, from river water and sediments (during dredging).
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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