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.