Humic substances in nature are represented as products of the transformation of organic residues and are the most stable form of organic carbon compounds outside of living organisms.  Humic acids formed from different natural sources may differ in their elemental composition, the number of functional ionic groups, the degree of condensation of molecules, the ratio of hydrophobic and hydrophilic fragments, and molecular weight, which affects their physico-chemical properties. Humic compounds have a number of useful properties: ion-exchange, sorption, and surfactant, which is why humic substances are currently recognized as one of the promising areas of «green» chemistry. Humic substances are used as renewable, economically profitable and environmentally friendly sources of raw materials for the production of chemically important products such as surfactants and biologically active substances, sorbents, metal corrosion inhibitors, antioxidants, the basis for the production of medicines, biologically active additives. The theory of industrial methods for the production of humic substances from natural raw materials is an important branch of plant chemistry. However, the theoretical foundations of the chemical technology of processing moss and peat can be attributed to a relatively little-studied area. One of the reasons for this is the lack of elaboration of theoretical concepts about the chemical nature of the transformation of individual components of plant raw materials into humic substances. Therefore, reliable determination of the number of functional groups in the structure of macromolecules of humic substances found in moss and peat is an urgent task in order to study the mechanism of transformation of moss into peat and, as a result, to study the reactivity of these natural materials.

The use of natural materials capable of ion exchange is economically more profitable than the use of synthetic substances, which explains the increased interest in natural ion exchange and sorption materials [1].

Substances that exhibit the ability to ion exchange are called ionites. Ionites are divided into mineral (inorganic) and organic.

Inorganic natural ionites include zeolites, clay minerals, feldspar, and various micas. Inorganic synthetic materials include silica gels, permutites, and insoluble oxides and hydroxides of certain metals (aluminum, chromium, and zirconium).

Organic natural ionites are obtained by chemical treatment of coal, cellulose and lignin. Organic natural ionites are humic acids of soils and coals.

Organic and inorganic ionites are a three-dimensional framework that includes charge-bearing groups of atoms called potential-determining ions. The frame has a positive or negative charge, compensated by the opposite charge of mobile ions – counterions, which can be replaced by other ions with a charge of the same sign.

The exchange capacity of ionites of any structure does not depend on the size of their grains, since the entire grain volume is more or less accessible for ion exchange reaction. However, in many cases porous ionites have significant advantages over massive gel ones. Since the porosity of ionites is an important factor that helps accelerate the ion exchange process, it is advantageous to use moss and peat as ion exchange materials [1,2].

Sphagnum or peat moss is a greenish-white spore-bearing perennial plant from the family of sphagnum mosses. The structure of moss consists of cellulose, humic acids, triterpene compounds (sitosterol, sitostanol), protein substances, lignin, hemicellulose (hydrophilic part), mineral components and sphagnol [1]. The bactericidal properties of sphagnum are due to the presence in the moss of a special phenol-like substance sphagnol, which inhibits the growth and vital activity of pathological microflora: E. coli, vibrio cholerae, staphylococcus aureus, salmonella.

The structural substances of moss contain large amounts of hydrophilic groups. The hydrophilicity of sorbents contributes to the fact that water is easily sorbed in the structure of the material, which can reduce the buoyancy of the sorbent.

The hydrophobic components of sorbents are lipids, moss bitumen. A significant number and variety of functional active groups (-OH, -COOH) in the solid components of moss (mainly in humic substances) determine its high ion exchange and sorption capacity [2].

Sphagnum moss has high gas and moisture absorbing properties, as well as bacteriostatic and bactericidal effects. In this regard, moss is used in many countries as a substitute for cotton wool, as well as an independent remedy for the treatment of infected (purulent) wounds.

Sphagnum is used in horticulture, indoor floriculture. It gives the soil the necessary lightness, hygroscopicity and friability. Sphagnum retains moisture well in the earthen coma, prevents the drying of the upper soil layer during prolonged drought. Possessing bactericidal medicinal properties, sphagnum prevents rotting of the root system.

Peat moss is widely used in many branches of the national economy. Sphagnum is used as a packaging material during transportation, for storing vegetables and fruits, as bedding and feed for livestock, in powder form it is used for filling sewage, cesspools. It is a valuable raw material for the production of ammonia, wax, paraffin, alcohol. Sphagnum moss is a sought-after thermal insulation and insulation material that is widely used in construction. Hygroscopic properties of moss make it possible to neutralize humidity fluctuations.

Sphagnum is the main producer of peat, the deposits of which are formed due to the death of stems: sphagnum grows in the upper part, while the stems of the plant gradually die off annually, forming a significant layer of brown peat [2,3].

Peat is a weakly acidic multifunctional ion exchanger consisting of various chemical components of organic and inorganic nature. It is formed as a result of plant decomposition due to the death and incomplete decomposition of marsh plants in conditions of increased moisture with a lack of oxygen [3]. Peat occupies an intermediate position between vegetable raw materials and solid fuels.

The organic part of peat is conventionally divided into several groups:

• substances extracted by organic solvents; they consist of waxes, paraffins and resins (bitumen);
• substances extracted from peat with cold and hot water, as well as those that dissolve in water after hydrolysis in the presence of mineral acids; this group of compounds includes sugars, pectin substances and polyuronides, semi-cellular and cellulose;
• humic substances extracted from peat with an alkali solution (humic and fulvic acids);
• Non-hydrolyzable substances (lignin).

Peat-forming plants include (Table.1) [4]:

Table 1

Composition of peat-forming plants

In contrast to the component composition, the elemental composition of peat-forming plants is more constant (Table 2):

Table 2

The elemental composition of peat-forming plants

The composition of peat includes the same groups of substances characteristic of peat-forming plants, as well as humic substances formed during the humification of plants.

The carbohydrate complex of peat consists of water-soluble and easily hydrolyzable substances (pentoses, uronic acids, hexoses), contained in amounts from 6.9 to 63%. The hard-to–hydrolyze substances of peat include cellulose (0.2-20%), and non-hydrolyzable substances include lignin (26%). Humic substances make up up to 70% of the organic part of peat [5].

Unlike other solid combustible fossils (lignites, brown coals, coal), peat contains a whole range of biologically active compounds found in wildlife. Being a product of partial decomposition of dead plants, peat can retain biologically active compounds of plant origin, as well as accumulate organic compounds of various classes resistant to microbiological degradation. From this point of view, peat can be considered not only as a source of biologically active substances, but also as their accumulator [6].

Density and porosity are the main physical characteristics of peat and moss that determine the condition of materials. The main characteristics of the porous structure of moss and peat include total porosity, pore size, kinetic specific surface area, active porosity, and the content of still water. At low density, peat has high porosity, which also depends on humidity and degree of decomposition. Slightly decomposed normalized peat has a porosity of 90-95%, and this indicator decreases with the decomposition of organic matter. The composition of peat also affects the amount of porosity and the nature of pores: the presence of clay and colloidal particles in peat determines very small pores, while poorly decomposed plant residues provide very large pores [6,7]. The porosity of the adsorbent is of great importance for adsorption: the higher it is (that is, the smaller the pores), the greater the specific surface area of the sorbent and the greater its adsorption activity. Porosity and density are determined by such a value as strength [7].

Peat in its natural state is characterized by high water saturation. To assess the water properties of peat, full moisture capacity is used, that is, the ability of peat to retain moisture under the influence of molecular, capillary and other bonds. This concept is applicable to peat at its maximum saturation with water in conditions of free filtration and in the complete absence of evaporation [8].

The diagnostic characteristic of peat is acidity (pH), which plays an important role in the formation of peat properties. The term «acidity» refers to the reaction of the aquatic environment of peat, which is determined by the activity of hydrogen ions (H+) and is numerically equal to the negative decimal logarithm of the activity of H+. The acidity is due to the presence of free acids (acetic, formic, oxalic, lactic). The more free acids there are in peat, the higher its acidity. There are more free acids in upland peat than in lowland peat, which has less acidity. The high content of cations in these peats contributes to the formation of salts (humates) [8,9].

Due to the variety of the above properties, peat is widely used in a wide variety of fields of activity.

Peat is used as a fuel in the energy industry. In its pure, unprocessed form, peat has lower energy efficiency than coal, but it also has a number of advantages: peat contains less sulfur and harmful impurities and its use is less harmful to the environment; it is also much cheaper. By exposing peat to high temperatures, peat coke is obtained from peat, and activated carbons are obtained from them. [10].

Peat is widely used in agriculture. In its pure form, peat is an excellent environment for the growth of any plants. Peat is rich in humic acids, which are plant growth stimulants. Due to the fact that peat retains harmful substances, it heals the soil and reduces the nitrate content in plants.

The structure of peat allows it to be used as an excellent filter and sorbent for industrial wastewater.

Peat is an excellent adsorbent, filter and gas absorber, therefore it is indispensable in the elimination of various environmental accidents [11]. Moss and peat, which are natural ion-exchange and sorption materials due to the presence of humic substances in them, are of great interest in ecology.

Humic substances are natural compounds formed during the decomposition of biological residues under the influence of microorganisms and abiotic environmental factors [12].

The process of formation of humic substances is commonly referred to as humification. In natural conditions, the process of humification proceeds with the active participation of fungi, microorganisms and invertebrates in the soil, peat deposits, and natural waters. In a simplified way, humification can be described as a set of complex chemical and biochemical reactions, the product of which is the formation of organic compounds resistant to further transformation – humic substances. The process of humification is catalyzed by enzymes formed during the vital activity of the consults. The resulting humic substances are sorbed on decomposing plant material, thereby blocking further humification. The structure of the humic substances formed is determined by the hydrothermal conditions of the process. For example, the «loose» structure of humic substances occurs in the case of anaerobic humification conditions, i.e. in conditions of excessive moisture [12, 13].

Humification is the second largest process of transformation of organic matter after photosynthesis. Some of the dead remains are mineralized to CO2 and H2O, the rest is converted into humic substances [13].  Unlike synthesis in a living organism, the formation of humic substances is not guided by the genetic code, but follows the principle of natural selection – the most biodegradable structures remain. The result is a stochastic, probabilistic mixture of molecules in which none of the compounds is identical to the other. Thus, humic substances are a very complex mixture of natural compounds that do not exist in living organisms. Humic substances are the connecting link between living and inanimate matter. Being the end product of the transformation of plant cell wall components, humic substances accumulate in soils (up to 90% of soil organic matter), peat (up to 50%), brown coals (up to 60%), and mosses [13].

Humic substances contain three fractions:

• humin is insoluble in neither acids nor alkalis;
• humic acids — soluble in alkalis and insoluble in acids (at pH < 2);
• fulvic acids are soluble in both alkalis and acids [14].

Humin is an organic substance that is part of bio-bony bodies and is a combination of humic acids and fulvic acids that are strongly bound to minerals, as well as some non-specific organic compounds (cellulose, chitin, lignin). Humin is the starting material for the formation of humic acids [15,16].

The sum of humic and fulvic acids is called «humic acids». Humic acids are actively involved in natural chemical processes, as they are the most mobile and reactive part of humic substances. Humic acids perform a set of important biospheric functions: structuring, accumulation of nutrients, regulation of geochemical metal fluxes in aquatic and soil ecosystems [15, 16].

Humic acids present in moss and peat are among the most important natural ion exchangers and sorbents with significant applied importance in environmental protection. The unique physico-chemical properties of these substances, such as their high ion exchange and sorption capacity, make them an indispensable tool in combating air, water and soil pollution. Due to the presence of a large number of functional groups, humic acids are able to effectively bind and remove various pollutants, which makes their use extremely relevant in modern environmental technologies.

The high economic feasibility of using moss and peat in comparison with synthetic analogues further emphasizes their importance. A wide range of possible applications, from medical purposes to waste disposal, demonstrates the versatility of these natural materials. Nevertheless, in order to fully unlock the potential of moss and peat, additional research is needed aimed at in-depth assessment of their reactivity and transformations during the transition from moss to peat.