NATURE OF SOIL NANOSTRUCTURE

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Опубликовано в библиотеке: 2021-10-18
Источник: Science in Russia, №6, 2012, C.34-40

by Gennady FEDOTOV, Dr. Sc. (Biol.), Acad. Gleb DOBROVOLSKY, Institute of Ecological Soil Science, Lomonosov Moscow State University

 

Tens of thousands of years it was natural to cultivate the soil--a unique habitation medium for living organisms, but humankind does not know all about it even today. We are dealing here with an extremely complex object: to understand all processes connected with it, the researcher has to know organic, inorganic, analytical, physical and colloidal chemistry, enzymology, microbiology, zoology, mineralogy and a number of other subjects. Emergence of new nanotechnological methods of soil scientists allows to differently approach some problems, they are interested in. One of them is a structure and nature of humus.

 
стр. 34

 

Electron-microscopic image of particles of turfy-podzol soil silty fraction:

 

1--mineral particles;

 

2--jelly-like humic matrix;

 

3--polymer drawing rods;

 

4--polymer backing.

 

 

BACKGROUND

 

The soil, as any natural object, is studied at different levels of its organization. In particular, specialists are interested in the composition, structure and properties of colloids, determining a majority of its characteristics. A number of big scientists studied this subject--first of all, Acad. (USSR AS) Konstantin Gedroitz (1872-1932), Swiss agrochemist Georg Vigner (1883-1936), Swedish soil scientist Sante Emil Mattson (1886-1945). Works of Gedroitz demonstrated: the thin-dispersion part of soils, rather small in mass, plays the most important role in their absorbing ability, in the formation of agrochemical, agrophysical and melioration properties, in the soil-formative process as a whole.

 

Later on, the use of an X-ray-phase analysis, spectral and thermal methods, optical and electron microscopy in the process of studies of the thin-dispersion part of soils allowed to fully reveal the nature of a majority of their chemical, physico-chemical and even microbiological properties (for example, adsorptions of microorganisms on the surfaces of clayey minerals). Researchers established chemical and mineralogical composition of colloids, but they failed to understand their structure for a long time.

 

Especially difficult was a problem of the nature of organic substance, of the composition and structure of the so-called humic acids, contained mainly in colloidal fractions of the soil. Generalizing the results of his

 
стр. 35

 

 

Electron-microscopic photos of gel films, spontaneously escaping from turfy-podzol soil (a) (region 6x6 µm) and chernozem (b):

 

1--mineral particles;

 

2--humic matrix;

 

3--organic dendrites,

 

escaped from humic matrix.

 

works, the famous national soil scientist, Acad. Ivan Tyurin (1892-1962) concluded: "In humus, along with compounds, known from the chemistry of vegetable and animal substances, there exist also specific compounds, arising from the first as a result of a special category of processes, which are characteristic of humus, and ... are not characteristic of living nature." This point of view obtained wide recognition, and there appeared a definition of humic substances as a complex of specific highly molecular nitrogen-containing aromatic compounds of acidic nature in a majority of scientific monographs and manuals. In addition, it became evident for specialists: the latter make a part of soil gels and, proceeding from their complexity and heterogeneity, can exist in the form of different associates and ensembles, forming in the process of unification of several macromolecules.

 

Today it is generally recognized that functioning of the soil as a system with a definite set of properties is mainly guaranteed by colloids, which in the form of gels cover and bind its components. The basis of this "glue" is a humic jelly, reinforced by mineral particles.

 
стр. 36

 

MICROSCOPIC "TREES" IN SOIL GELS

 

During work with the X-raying electron microscope, we witness an interesting picture of transformation of such gel in the process of its drying: in the organic matrix of humic jelly, half-transparent for a sample of electrons passing through, there settle dark (impenetrable for an "electron beam") mineral particles of different sizes, while the organic matrix itself, preparing the sample, in the process of drying undergoes shrinking, forms drawing rods, connecting it with a polymer base, the sample is located on. Thus, in the process of removal of water, it behaves as a typical polymer jelly.

 

It must be noted that during humidification of dry soils their properties change due to transformation of the abovementioned gels, absorbing water and, like a majority of polymers, increase in size. Earlier researchers took their base--humic jelly--as a homogeneous substance, but modern data on the behavior of polymer systems change habitual notions.

 

Studies of soil gels by methods of electron microscopy have demonstrated: organic matrix is not homogeneous--it has nanostructure. On microphotos, in case of increasing by several tens of thousands of times, one can distinctly notice the diversity of colors of its surface, black points and relatively clarified ones. If magnified for several thousands of times, the latter (more compact and convex) in some cases form dendrites (from Gr. Soevopov--a tree), stretching from whitish inorganic particles in the gel, though sometimes these odd figures are far from mineral grains.

 

What is the described phenomenon conditioned by, what is the emergence mechanism of such organization of fine-dispersion fraction? It was obvious that we could not answer these questions, until we understood the structure of soil humus. Thus, we started studies of the whole spectrum of zonal soils. We studied samples of gels of podzols, podzol, turfy-podzol, grey forest, different types of chestnut soils, chernozem, grey earth, and red earth. It turned out that the regularities we discovered were equally typical of a majority of them.

 

Thus, by means of electronic and probe microscopes* we have established: the base of soil gels are round formations of different sizes--from many tens to several hundreds of nanometers. Raster electronic microscopy, photon-correlation spectroscopy and small-angle scattering** of neutrons helped see that such structures of organic nature are typical of solutions of humic substances (extracts from soils, river and lake waters).

 

On images, received by the tunnel microscope***, we can see that the revealed formations consist of smaller objects, sizes of which vary in different type soils: in

 

* Probe microscope--a class of microscopes for getting pictures of the surface and its local characteristics by means of special probe scanning.--Ed.

 

** Small-angle scattering--elastic scattering of electromagnetic radiation or particle beam (electrons, neutrons) on heterogeneities of substances, sizes of which significantly exceed the radiation wave length.--Ed.

 

*** Tunnel microscope--a variant of scanning probe microscope for measuring a relief of conducting surfaces with a high spatial resolution.--Ed.

 
стр. 37

 

 

Photos of gels, escaped from turfy-podzol soil, received by means of a tunnel microscope. Soil, modified by SAS (a), without SAS (b).

 
стр. 38

 

turfy-podzol soils they reach 2-5 nm, in black earth--8-12 nm. In a word, the organic matrix of soil gels, earlier regarded by specialists as a homogeneous jelly, in fact is a multitude of globules of the size of many tens to several hundreds of nanometers, in their turn emerging in case of uniting primary structures of humic substances of the size of several nanometers. Let's call the abovementioned globules clusters.

 

Studies of organic solutions by the method of small-angle scattering of neutrons have shown that they have a fractal* organization. Using the same approach to soils, we have established: their colloids are arranged in the same way. Consequently, we can affirm: the base of the latter are fractal clusters, formed from particles of humic substances.

 

SECRET OF HUMUS STRUCTURE

 

In early 1920s, the German chemist, Nobel Prizewinner (1953) Herman Schtaudinger (1881-1965) gave principally new characteristics of polymers as substances consisting of macromolecules--particles of extremely big molecular mass. This allowed to explain their properties, first of all, elasticity. It has become obvious that the chemists' knowledge help understand a lot of earlier unknown properties of humic substances. Academician Ivan Tyurin proposed a polymer theory of humus, accepted at that time by a majority of colleagues. For a long time specialists thought that the given substance, very important for fertility of lands, is a set of macromolecules. Proceeding from these positions, soil gels should be regarded as systems of fractal clusters from polymer molecules. However, in the process of humus studies, there have accumulated a lot of facts, inexplicable by means of the old theory.

 

Thus, there remained unclear the reasons of multiple changes during one season of the correlation of basic components of the soil organic substance--humic and some other facts. Finally, there was no theory of formation of polymer molecules of humic substances, confirmed by experiments. In 1985, Dmitry Orlov, professor of the soil science faculty, Lomonosov Moscow State University, Dr. Sc. (Biol.), relying on the results of electron-microscopic examinations, made a conclusion: the method allows to see only associates of humic acid molecules. This coincided with a conjecture, expressed earlier by the outstanding national specialist in the sphere of humus chemistry, Dr. Sc. (Agr.) Lyudmila Alexandrova (1908-1983).

 

The recent 10-15 years witnessed principally new opinions on the structure of humic substances. In mid-1990s, the Italian soil scientist Alessandro Piccolo from the chemico-agrarian faculty of the University of Napo-li published several works, where he, on the basis of experimental data, put forward an assumption: humic substances are not polymer molecules, where all parts are bound to each other by covalent links, they are associates relatively to low-molecular components, arising in the process of degradation of biological material, and then united and stabilized by a great number of weak (non-covalent) links.

 

Such formations in chemistry are called "supramo-lecular compounds". These complex substances contrary to mixtures have a definite composition and structure. They are built according to the principle "guest-host", and their big molecules are stabilized by forces of Vander-Vaals*, hydrophobic** and other interactions.

 

Initially soil scientists perceived the assumption of Piccolo with a considerable mistrust, but further experiments have confirmed the correctness of his concept, and though a lot of specialists even today did not give up usual macromolecular ideas on the structure of humic substances, lately the supramolecular approach begins to dominate.

 

In the course of original studies we singled out several levels of humus organization in soils. First, molecules, appearing as a result of decomposition of biological residues. Then supermolecules, forming among the first at the expense of non-covalent links. From them are formed fractal clusters--the following phase. Their unification leads to the formation of soil gels.

 

According to our data, branches near dendritic associations of supermolecules of humic substances penetrate into emptiness of "neighbors", thus creating a more compact contact with them at the expense of a great number of weak intermolecular interactions. As a result, there emerge systems, called suprapolymers, as their properties are mainly similar to those of real polymers, thus misleading researchers.

 

CONTROL OVER SOIL PROPERTIES

 

After answering the question about the structure of soil humus, we had to answer the second--on the causes of emergence of nanostructures in the organic matrix of soil gels. Due to experiments we have established: to initiate such organization of the given material can ions of iron, calcium, aluminum, as well as some surface-

 

* Fractal--a geometric figure, in which each part is like the figure as a whole. The tree can serve as a good illustration of fractal organization: from trunk to branches, from each of them to smaller ones, then more and more thin branches.--Ed.

 

* Vander-Vaals forces arise during polarization of molecules and formation of dipoles. Opened by Yan Vander-Vaals in 1869.--Ed.

 

** Hydrophobic interactions--strong attraction in water between non-polar particles.--Ed.

 
стр. 39

 

 

Diagram of fractal cluster prior to interacting with modificators (a), and after (b, c).

 

active substances (SAS). It goes without saying that such components exist in real soils too.

 

Introduction of the above-listed modificators into the turfy-podzol soil led to the expected result. Surface-active substances (used in food industry and ecologically safe) contributed to the noticeable growth of supermol-ecules of humic substances--from 2-5 to 10 nm and more. But this is only the beginning of a complex process. The use of a raster electronic microscope has demonstrated: initially there are formed nanoparticles of a new phase in gels, while on increasing the modificator concentration, there starts their unification into relatively big dendrites ("trees"), sometimes of the size of tens of microns! In other words, the agent introduced by the experimenter starts a mechanism of self-organization in soil gels.

 

Besides, humic supermolecules in the make-up of fractal associates are also transformed. Evidently, they are becoming less hydrophilous, i.e. intensity of their interaction with water is decreasing. And this results in breakage of the stability of clusters, "built" from them, as contacts between more hydrophobic (striving to avoid contacts with water) "participants of the process" turn out to be thermodynamically more advantageous. Consequently, segregation at the nanolevel takes place in soil gels: under low concentration of the modificator, super-molecules, interacting with it, gather together and stand apart inside the matrix from hydrophilous humic substances. Due to lesser content of water in them, shrink-age of the given areas during drying out of gels in considerably lesser than in the surrounding mass: they appear over the surface and look like lighter points or regions on electron-microscopic images.

 

On increasing concentration of the modificator, the number of such hydrophobic regions increases too, they begin interacting between themselves, creating different, including dendritic, forms. The more of the agent is introduced into the system, the bigger become regions occupied by them. Then comes the moment, when a bigger part of supermolecules of humic substances is reorganized, while the number of hydrophilous part decreases till complete disappearance.

 

Thus, our data are a step to understanding the nature of humus and soil gels. Besides, there appeared a serious possibility to restore structurally degraded soils and improve properties of their fertile horizons. Today we have 16 patents for use of modificators, which help increase water-stability of soil structure under norms of their introduction of 10-15 kg/hectare.


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© Gennady FEDOTOV, Gleb DOBROVOLSKY () Источник: Science in Russia, №6, 2012, C.34-40

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