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Humic substances (hereinafter "HS") are formed in soils, peats, coals and other natural bodies. They accumulate nutrients and energy, participate in the migration of cations, reduce the negative effect of toxic substances, affect the development of organisms and the thermal balance of the planet.
They are stable, high molecular weight, polydisperse, contain various functional groups, amino acids, polysaccharides, benzoid fragments.
There are several groups of HS:
All these groups of humic acids are usually spoken of in the plural (for example, humic acids), since their composition and properties vary depending on the source of HS, but even in preparations obtained from the same source (one type of soil, peat, coal), they are heterogeneous, polydisperse and represented by a large set of similar structures, but non-identical molecules.
Humic substances perform many functions in the biosphere, of which the following are the most important.
On the method of extraction of humic acid from peat F. Ahard wrote as follows: "Extracts from peat obtained by caustic alkali, I saturated with vitriolic acid. The mixture darkened and seemed dark brown, almost black, the sediment sank to the bottom." This method is used almost to this day to isolate HS from any natural bodies. In other words, HS is extracted with alkali solutions, then humic acids and himatomelanic acids are precipitated with acid, while fulvic acids and non-specific substances remain in the solution.
The content of various chemical elements in these substances has been well studied. The carbon content in mass fractions ranges from 40 to 60%, depending on the origin and source of HS. Nitrogen is always there, it was proved by the Russian scientist R. Hermann in the middle of the last century, but it is not enough – 3-5%. Hydrogen usually contains 3-6%, and oxygen – 33-37%. Be sure to include sulfur – up to 0.7-1.2% and phosphorus – up to 0.5%. There are always different metals, although it is difficult to say whether they are mandatory for HS or just an impurity, since it is not easy to clean HS. For example, fine-crystalline quartz SiO2, fine-crystalline goethite FeOOH were found in the products of HS, which have to be recognized as obvious impurities.
Any HS contains a large set of functional groups, they are multifunctional. Their molecules contain carboxyl groups –COON, phenolic –OH, quinone =C=O, amino groups –NH2, etc. Firstly, their number is large, and secondly, they are distributed unevenly across molecules of different sizes, and even molecules of the same size may differ in the content of functional groups. Moreover, HS molecules differ in the number of amino acid residues included in their composition (there are 17-20 in total), in the number of carbohydrate residues and the nature of their location.
The content of functional groups, expressed in mM ⋅ kg-1 according to M. Schnitzer, varies in humic acids within the following limits: – COON – 1500-5700, acidic – OH – 2100-5700, slightly acidic and alcoholic – OH – 200-4900, quinoid – C=O – 100-5600, ketonic – C=O – about 1700, – OSN3 – 300-800. In addition, groups –NH2 play an important role. The variety of acidic functional groups is so great that L.I. Glebko proposed not to identify them, but only to divide them according to the apparent dissociation constants, pK = −lgK, where K is the dissociation constant.
Table 1. Average elemental composition of humic acids from various natural bodies, %
Source of humic acids |
C |
H |
N |
O |
Stone coals |
66,0 |
4,4 | 1,8 | 27,8 |
Brown coals |
66,0 |
4,6 | 1,3 | 27,1 |
Peat |
59,0 |
5,5 | 2,2 | 33,3 |
Sapropels |
58,0 |
6,3 | 4,8 | 30,9 |
Podzols |
53,0 |
4,4 | 4,2 | 38,4 |
Chernozems |
58,0 |
4,3 | 4,1 | 33,6 |
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*D.S. Orlov, Moscow State University