Chemical reactions and processes in freezing and thawing soils

Essentially the same chemical reactions take place in soils during freezing and thawing and in the frozen state as in unfrozen materials. These reactions are solution, hydration, substitution, oxidation-reduction, ion exchange etc., but in the cold regions they have a number of specific features. For example, solution is less intense because under lower temperatures some salts dissolve at a much slower rate. Apparently, because of low temperatures the permafrost regions contain considerable amounts of the products of chemical interaction between the dissolved substances and water molecules, that is, hydrates and crystalline hydrates. The cation exchange reactions have, probably, a predominant importance for frozen soil, because unfrozen water is a rather concentrated solution, the ions of which actively interact with the ions of the mineral surfaces. Moreover the typical processes in frozen ground are coagulation of sols and formation of colloidal compounds. These processes are predetermined by the characteristic phase transitions of water in the ground (freezing or thawing) which cause dehydration in soils and, consequently, coagulation (on reaching the threshold of coagulation) of organic-mineral compounds. The geochemical processes occurring in the cold regions also have distinct specific features of geochemical processes that play certain specific roles. For example, free water affects seasonally frozen soil intensively only in the warm period of the year. A major role therefore belongs to bound (unfrozen) water which interacts with, and is in dynamic equilibrium with, the ice and soil.

Ground water normally has a higher carbon dioxide content because with the fall of temperature the solubility of gases (including COa) increases rapidly as does the organic matter content. For example, in the soils of the tundra on Bolshaya Zemlya the free H2C03 content reaches 200 mgP1 and that of the ion HCO3" is 650 mg 1~1. Therefore, the concentration of hydrogen ions in ground water of the permafrost regions increases by several hundred times, which causes, perhaps, an acid reaction in the medium. The nature of many chemical reactions and the behaviour of soil components largely depend on the pH of the medium. An acid medium is more aggressive and chemically active; it decomposes silicates, and the hydrolysis reactions in it are stronger than those in neutral and alkaline media.

Geochemical processes in frozen ground are also influenced by the content of monatomic hydrogen (reducing) and oxygen (oxidizing). It is assumed that during phase transformations of water into ice the release of hydrogen may reach considerable values. For example, when 1 m3 of water turns into ice, 120gmol-1 of monoatomic hydrogen are released. Since the soils in the permafrost regions are highly humid, oxygen cannot easily penetrate them. Therefore in the North, the oxygen surface, which shows the distribution of free oxygen by depth in the crust, rises and reaches the surface in bogs. As a result the frozen materials in the permafrost region are predominantly a reducing medium, thus having a higher content of divalent ferrous Fe2+ with formation of its mono-oxide compounds (siderite, pyrite, vivianite, etc.). Ferrous oxide in soils colours them in bluish-grey shades, and they are usually called gley soils. They are predominantly fine-grained, reducing and acid.

The processes of breakdown of organic matter are also different. Because of lower biological and biochemical reactions, the transformation of vegetal and zoogenic remains into organic matter is slowed, and decomposition of remains (formation of humus) terminates at a less mature stage. As a result of this process, light-coloured fulvic acids are formed rather than humic acids (the product of the final stage of decomposition). In the tundra soils, the content of fulvic acids may reach 70%, while humic soils contain only 10-15 % of humus matter. Fulvic acids, like humic acids, compose a group of similar high-molecular-mass compounds, but they have less carbon and nitrogen and more oxygen and hydrogen than humic acids. Fulvic acids destroy minerals by their high acidity; they homogeneously saturate the soil and form a massive compact layer. More viscous and less mobile humic acids in the soil produce a lumpy, nutty structure typical, for example, of chernozem.

Chemical processes during a single freezing of soil

At the beginning of freezing, the water turns into ice, creating a new mineral. The gravitational, capillary and loosely bound non-saline water crystallizes at negative temperatures close to 0°C. Film water normally freezes within a wide range of negative temperature, determined from the curve of unfrozen water content. Salt waters, with mineralization of more than 30gl_1, crystallize at temperatures of about —1.5 to — 2°C whereas brines may remain liquid at — 20 °C and lower. The freezing of water usually causes a distinct differentiation of salts between the solid and the liquid phases. A part of the salts dissolved in water is enclosed in the ice, a part of the less soluble salts precipitates, and a part of the easily soluble salts is squeezed into lower water layers thus increasing their mineralization. The ice formed by freezing is several times less mineralized than the initial pore solution. Slow and gradual freezing produces the most 'pure' ice. During freezing, in accordance with the degree of solubility at negative temperatures, the most insoluble salts of CaC03 precipitate first (in the temperature range —1.5 to — 3.5°C), and then Na2S04, CaS04, etc. (at temperatures of — 7 to — 15°C), these salts forming the so-called crystal hydrates. Consequently, cryogenic layers are enriched with gypsum CaS04.2H20, mirabilite Na2S04.10H20, and calcite CaC03; in other words, they are sulphatized and carbonatized.

Below the freezing boundary, the waters are highly mineralized due to easily soluble salts expelled from the frozen layer (chlorides of calcium, magnesium, sodium and hydrocarbonates of sodium). As a result of this cryogenic concentration, the rather highly mineralized subpermafrost water (up to 200gl~1 and more) is formed (also occurring sometimes as interper-mafrost water) and gives cryopegs. These waters remain liquid at negative temperatures. The thickness of layers with cryopegs below the base of the permafrost may reach several hundreds of meters.

Moreover, frozen ground also contains gases in the form of hydrates, the so-called gas hydrates, whose crystalline lattice is built up of water molecules. Molecules of hydrate-forming gas are distributed in cavities inside the lattice. The crystalline lattice of the water itself (without gas molecules) is thermodynamically unstable and cannot exist independently. In natural conditions the structure of the crystalline lattice is often filled with molecules of methane, ethane, hydrogen sulphide, and carbon dioxide. When gas penetrates into the water lattice, it becomes rigid and the water turns into a solid. Superficially the gas hydrate is very much like ice. Undamaged crystals of gas hydrate are transparent and homogeneous. The appearance in them of microcracks and different gas inclusions indicates commencement of decomposition (Fig. 3.1). The amount of heat generated by the phase transitions of gas hydrates is about 0.5 kJg-1. Under the effects of warming and interaction with water, the gas hydrate decomposes with a hissing sound and intensively exudes gas bubbles. Negative and low positive temperatures in the permafrost regions are favourable for the formation of hydrate deposits at shallow depths.

Fig. 3.1. Structural-textural features of a man-made agglomerate 'ice-methane hydrate': (a) general view of gas-hydrate thin section; (b) contact of pure ice with hydrate-bearing ice. 1 - zone of completely decomposed agglomerate with very few microchannels and microbubbles of gas; 2 the zone of largely decomposed agglomerate with numerous gas bubbles; 3 - the zone of slightly decomposed agglomerate with typical microchannels; 4-5 - ice (4 - pure, 5 - hydrate-bearing with microchannels).

Fig. 3.1. Structural-textural features of a man-made agglomerate 'ice-methane hydrate': (a) general view of gas-hydrate thin section; (b) contact of pure ice with hydrate-bearing ice. 1 - zone of completely decomposed agglomerate with very few microchannels and microbubbles of gas; 2 the zone of largely decomposed agglomerate with numerous gas bubbles; 3 - the zone of slightly decomposed agglomerate with typical microchannels; 4-5 - ice (4 - pure, 5 - hydrate-bearing with microchannels).

Chemical processes in frozen soils

Because in the frozen state (perennial or seasonal), the water phase in soils is not visible, for a long time it was believed that the soils were in a state of complete chemical inactivity. This misrepresentation underestimates the role of unfrozen water and would automatically lead to the application of Yan't Hoff's law, which states that with a drop in temperature of 10°C the rate of chemical reactions is reduced by half. The fallacy of this has been demonstrated by many researchers who found considerable cation exchange reactions between soil minerals and bound water and who showed a much greater concentration of substances in film water compared to that in pore water. Though the absence of free water in frozen ground would seem to preclude the escape of chemical components from the unfrozen water, the mass-transport processes are sufficiently intense because of diffusion of ions in unfrozen water, to lead to adjustment of concentration of dissolved substances. In frozen ground, the process of flow is also active in unfrozen water films inducing convective transport of ions and soluble matter with migrating water. In the course of this process, phase transitions between pore ice and films of unfrozen water take place in accordance with the increase or decrease of concentration of ions. As a result there is a levelling out of the concentration of salts in unfrozen water with a continuously occurring decrease down to initial levels.

Chemical processes during repeated freeze-thaw of soil The chemical processes developing during repeated (cyclic) freezing and thawing have been studied in great detail. Unlike perennially frozen ground, the chemical reactions in seasonally thawing soils are much more intense and are obviously periodic. The interaction between soil minerals and water (both free and bound) is a pulsating process, and the phase transitions of water into ice and vice versa should result in marked intensification of chemical weathering in seasonally frozen soils. These conclusions are confirmed by data obtained on 'cold' or 'cryogenic' soils and on weathering crusts in severe climates, which indicate that the process of geochemical reactions is qualitatively identical to that in warm humid areas. Moreover, an intense chemical transformation in seasonally frozen ground commences at the very first stage of weathering under the effects of hydrolysis, leaching, oxidation, hydration, and migration of colloids, and neogenetic clay and other minerals are formed. The studies of M.A. Glazovskaya in Antarctica have shown that in the 10-15 cm surface layer of soils, if the supply of oxygen is sufficient, oxidation takes place and MnO and Fe203 accumulate, colouring the ferric and manganese extractions on rock fragments into ochre-rusty or orange-red hues. Lower down in the layer, where there are manifestations of wash-out products and carbonatization, more mobile products of weathering accumulate, such as calcium carbonate and calcium, which do not effervesce in HC1. The study of surface weathering crusts under the microscope has established the stages of decomposition of the original minerals. At the initial stage, chlorite disappears, then hornblende and biotite, i.e. the banded and stratified silicates are the first to disintegrate. Feldspars become covered with yellowish-brown fine silty aggregates, i.e. secondary clay minerals.

The results given above are in agreement with the data on cold tundra and taiga soils, where non-gley cryogenic soils (brown soil, podzol, Al-Fe humus) dominate and poorly drained gley soils are in the minority. The chemical elements in non-gley soils are arranged by V.O. Targul'yan according to migration capacity as follows: Si>Fe>Ti>Al. The silicate forms, appearing as a result of hydrolysis, are fairly mobile in an acidic medium and are evacuated from the soil profile. Iron, titanium and aluminium in an acidic medium have low solubility and normally remain in the soil as oxides (Fe203, TiOz, A1203) and hydroxides (Al(OH)3, Fe(OH)O). In the course of humification in the permafrost regions fulvic acid appears as one of the most aggressive and mobile forms of humus. This acid moves downwards with the soil solution and destroys the hydroxides and silicate min erals by forming different kinds of organic-mineral compounds (oxalates, chelates, fulvates and adsorbed organic-mineral compounds).

Fulvates and oxalates, as the more mobile compounds, are removed from the soil profile, whereas chelates and adsorbed organic-mineral compounds soon lose their mobility and remain in the illuvial horizon. During this process the brown-coloured Al-Fe-humus coarse silty-clay horizons appear. At the same time, the real humus horizons and horizons of Al-Fe-humus and titanium compounds can be formed. The compounds of titanium, aluminium, iron (Ti-Al-Fe) and humus are accumulated in these alluvial horizons and this process, called, by Targul'yan 'tialferrisation', is typical of cryogenic soil formation. The leached horizon A2, naturally, is depleted of Fe and A1 hydroxides and oxides; therefore a relatively (not absolutely) higher Si02 content is observed as well as a higher shade of colouring due to decomposition and removal of dark-coloured compounds and minerals.

The chemical and physico-chemical processes are somewhat different in gley soils (poorly drained or boggy) typical of the north of the European part of the former USSR and of the Siberian coastal lowlands. Fine-grained (dust) matter is dominant in these soils with reducing conditions and acid reaction of the medium. The profiles of gley soils do not normally show distinct illuvial horizons but in gley and gley-podzol soils, on heavy sandy-silty-clay, for example, the content of Fe203 and A1203 is reduced concurrently with relative silica enrichment. The higher mobility of the iron is a result of its transition under reducing conditions into the monoxide form Fe(OH)2 which does not precipitate from solution until the pH reaches about 5-6. The blue-grey monoxides of iron colour the profile of gley soil with the typical grey and blue-grey hues. This phenomenon is also promoted by the presence of fulvic acids which are the immature forms of humus and which are not brown (as humic acids), but light grey, thus making the colour of gley soils less intense than that of non-gley soils.

The chemical differentiation of the products of weathering, which is closely associated with the mobility of the chemical elements, is particularly important in geochemical processes in the permafrost regions and especially in soils with cyclic freezing and thawing. More mobile chemical elements are intensively removed by underground and surface runoff; other elements, on the contrary, are practically immobile and accumulate in watershed areas and on slopes increasing their relative concentration. For example, potassium, calcium, magnesium, sulphate and chlorine ions in the permafrost regions are very mobile and migrate in all waters in a truly dissolved state. The silicate form of silicon migrates mostly as mono- and polysilicic acids, which are removed in solutions by ground water. A certain amount of silicic acids (up to 40%) can be transported as gels and colloids in combination with organic matter. Non-silicate Si02 in the permafrost regions is practically immobile, as is illustrated by intensive formation of podzol soils. The low mobility of silica is attributed to the extremely low solubility of Si02 in the highly acidic medium typical of tundra and taiga soils. Up to 70-90% of aluminium migrates in the permafrost regions as colloids and as complex compounds with humic acids. Iron (Fe2+ and Fe3+) has low mobility outside the permafrost regions. In cold humid conditions, 90-98% of the total iron content migrates as highly mobile colloids which are high-molecular organic-mineral complex compounds of the chelate type. Under the northern conditions some other microcomponents (Ti, Zn, Cu, Ni, etc.) also become more mobile and are transported not as simple ions but as colloids or as complex ions with a larger radius, which are formed with participation of high-molecular-mass organic matter.

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    What are the chemical reactions that take place during freezing?
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