Humic Acids as a Specific Class of Organomineral Polymers
Yu.G.Putsykin, A.A. Shapovalov, A.L. Stepanova, Leonid A. Troshaninb
Research Institute of Plant Protection Chemicals, Ugreshskaya str. 31, Moscow
aMoscow State University, Dep. of Soil Science, Moscow, Russia, 119992
bRosbulat GmbH, Kapersbugweg 16, D-61350 Bad Homburg v.d.H., Germany; ,
Keywords: humic acids, minerals, interactions, microbial transformation, polymerizatin-depolymerization
The deposits of brown coal are practically unlimited sources of humic acides (HA) – the substance which can be utilize for resolving many agricultural and ecological problems such as stimulation of root system development, plants grows rate, soil fertility, protection against environmental pollution and soil remediation process. The tentative of isolation, purification and application of HA in agriculture needs to get a reliable information about their chemical structure and properties. The idea that HA are the substances whose structure is controlled by the static properties disagree with their key role in processes of the development and functioning of soils which is known to be a highly stable and self-reproducing systems. Last time it was shown that HA are well-ordered and labile organomineral structures (2-4), which are capable of a reversible polymerization and depolymerization.
The goal of this research are estimate molecular weight of HA from brown coals (deposits: Gusinoozersky - Buriatia; Berezovsky - western Siberia, and Rio-Major – Italy) at different pH value and HA content; the study of structure and quantity of mineral compounds associated with HA and conditions in which their formed stable aggregates with clay minerals.
2. MATERIAL AND METHODS
HA were isolated from the samples of brown coals by traditional method – after alkali extraction with continuos precipitation in acid media using aproton solvent as dimethylsulphoxide for estimation on the obtained results different extraction conditions. Extraction of HA carried out by shaking of dispergated coal with dimethylsulphoxide in ratio 1:20 at ambient temperature during 1 h with continuos treatment in centrifuge (14 000 rate/min) at 10 min. Supernatant dilute by acetonitril in 1:10 ratio. The siege isolate and drying in vacuum at
50 0C. Dried siege dissolve in dimethylsulphoxide at 1:10 and all procedures were repeated. As a control of HA extraction, all siege was checking by a full dissolving in alkali water solution. Inorganic compounds associated with HA were treated by piroliz in HNO3 and H2O2 at ratio 1:1 during 4 h in titanium autoclave at 240 0C. The composition of elements were tested by the method of nuclear-adsorbtion spectrometry with induction bound plasma. The composition of mineral fragments associated with HA from brown coals was tested by treatment of 2-4% alkaline solution allowed isolate a highly disperse indissolved fractions. HA content in this fractions was tested via boiling (2-4 h in 5% alkaline solution). The mineral compounds tested by IR-spectroscopy after pretreatment with concentrated H2O2 during 2 h with boiling up to completely removal of organic components.
The molecular weight of isolated HA estimated by ultracentrifuge which allowed obtain characteristics of molecular weight of high-molecular compounds in solution (5). The tests carried at different pH value and HA content. For this purpose HA as a sodium salts were placed in buffer solution with different pH and equal ionic power. The fractional analysis of HA was carried out in ultracentrifuge with analytical rotor type AN-G with six cells v = 0,45 ml supply with scanning optical system allowed to study sedimentation process in dynamic. The procedure carried out at ambient temperature with 4800 rate/min at 330 and 410 nm.
3. RESULTS AND DISCUSSIONS
The study of inorganic compounds associated with HA isolated from different type of brown coals shown practically the equal quantity (reached to 1%) of magnesium, aluminium, sodium, potassium, calcium and silicium in all samples studied. The molecular weight of HA in water solution depends of their concentration and pH value. The figures 1 and 2 showed changes of HA weight in different pH at concentration 40 mg/l.
In fig.1 the curves of HA molecular weight isolated from both deposits has the same character. Molecular weight increase with grows of pH value. In neutral pH (fig.2) a part of HA transfer to low molecular weight (monomers 1000 oxygen units). In alkaline conditions this changes has quantitative character. Such type of results may be explained by aggregation of low-molecular fractions by the hydrogen bonds and electostatic interactions which is typical for colloidal systems. By ours opinion we can conclude about much more power interactions which can proved by clear demarcate of little number of HA fractions with different molecular weights dependent of HA concentration. In case of low power interactions the curve of different inorganic fractions distribution should have another view.
Fig. 1. Relationship between sedimentation koefficient (S), HA molecular weight (D)
in water solution (40 mg/l) and pH value. Deposits: Rio Maior (1) and Berezovskiy (2)
Fig. 2. The ratio of HA molecular weights in different pH value at concentration
Fig. 3 show an integral curve of optical density changes in HA solution (40 mg/l at pH 8,0), isolated from Rio-Maior deposit, obtained by the method of analitical centrifuge cell scanning after 30 min before beginning of the procedure. There is a clear evidence between fractions of HA with different molecular weight. An indirect evidence of HA depolymerization in alkaline media are the possibility of soil microorganisms to utilize HA at high pH as a carbon source. In acid media the rate of microbial transformation of HA is very slow, but at pH = 8.0 microbial activity increase due to dissociation of HA to the fragments degradable for soil microorganisms. So that HA can be considered as labile structures capable for reversible polymerization-depolymerization which is not normal for the class of organic polymers but is ordinary for inorganic compounds.
the border of solution in cell cell’s bottom
Fig. 3. An integral curve of optical density (D) changes in HA solution
The study of clay minerals associated with HA after alkali extraction from brown coals include 30% of HA which can be transfer to solution upon their prolong agitation in concentrated alkali. The mineral compounds isolated from such fractions by treatment with concentrate hydrogen peroxide at high temperature identified as montmrellonite. We make an attempt to develop artificially substantial layer of HA on the surface of montmrellonite. Such structure was obtained only by their precipitation on clay minerals from saturated solution of calcium-humat.
It may be conclude that monomers of humic acids produced via the microbiological destruction of plant residues are capable of an ordered polymerization on the surface of some clay minerals and formation of HA. Further binding of HA into organomineral globules is likely to be provided by the development of calcium (magnesium) bridges. However, on moving away from the mineral surface, the layer structure becomes more friable. HA from the above layers may easily react with alkali; some fragments of this layer may leave their positions, migrate in the soil, and interact with the surface of other clay mineral particles; as a result, humic soil globules are produced.