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EXTRACTION OF HUMIC SUBSTANCES WITH SODIUM PYROPHOSPHATE SOLUTIONS Salmanova N.I.1, Ismayilov F.A.2, Huseynova E.B.3, Shiraliyeva U.I.4
1Salmanova Nazilya Iskander - associate professor;
2Ismailov FarhadArzuman - master's student, AZERBAIJAN STATE UNIVERSITY OF OIL AND INDUSTRY; 3Guseinova Elmira Bahadur - leading researcher;
4Shiralieva Ulkar Ilham - research fellow, RESEARCH INSTITUTE "GEOTECHNOLOGICAL PROBLEMS OF OIL, GAS AND CHEMISTRY",
BAKU, REPUBLIC OF AZERBAIJAN
Studying the organic matter of soils is closely linked to various methods of fractionation of humic substances. In this regard, it is evident that the interpretation of the results of the analysis of the fractional composition directly follows from the principle underlying their separation. Each method used for fractionation, extraction, and separation of humic substances must have a clear physico-chemical meaning [1].
Despite the relatively long history of studying humus using sodium pyrophosphate solutions, there is almost no data in the scientific literature characterizing the patterns of humic acid extraction with changes in the acidity (alkalinity) of the solution. However, investigating this issue is crucial, as analytical practices involve the use of solutions of this reagent with different pH values to characterize fundamentally different groups of humic substances - stable and labile. In connection with this, we conducted a study on the features of the quantitative and qualitative composition of humic substances extracted from different types of soils by sodium pyrophosphate solutions with pH values ranging from 5 to 13.
The use of sodium pyrophosphate solution as a reagent for extracting humic acids from soils was first proposed by E. Velte [2] and immediately began to be actively implemented in the practice of chemical-analytical research.
We found that the mechanism of action of sodium pyrophosphate is reduced to an irreversible exchange reaction with the formation of insoluble salts with calcium, aluminum, and iron, and soluble sodium humate, aluminum- and iron-humate salts. However, all the aforementioned authors who used sodium pyrophosphate for extracting humic acids noted a significant, fundamental difference in the use of this reagent: different pH values of the solutions.
The variation in the yield of humic acids depending on the pH of the solutions has led to significant differences in the purposes for which sodium pyrophosphate solution is applied among various authors. Some authors [3] believe that the solution of sodium pyrophosphate (at pH 10) extracts mobile humic substances, while others [4] have proposed a highly alkaline solution of pyrophosphate to extract the maximum possible number of humic substances, serving as an analogue to alkaline extraction after soil decalcification, and successfully using it to study the composition of soil humus. Following them, [3], [4] began using sodium pyrophosphate solution with alkali for the complete extraction of humic substances from coals.
A new stage in the application of sodium pyrophosphate emerged as it started to be used as a reagent for extracting the so-called labile humic substances. The methodology of using sodium pyrophosphate solutions to extract labile humic substances was most detailed in the work [2]. According to this methodology, labile humic substances are extracted by sodium pyrophosphate solutions at pH 10 and pH 7. The main argument for recommending these extractions was the relatively low alkalinity of the solutions, which led to categorizing them as relatively 'soft' and considering humic substances extracted by weakly alkaline and neutral pyrophosphate solutions as sufficiently labile, i.e., easily accessible for
biodegradation by microorganisms. Thus, a practically equivalent relationship was established between chemical activity (reactivity) and susceptibility to decomposition by microorganisms, which is a fundamental attribute of labile humic substances. However, from our point of view, this equivalence is highly questionable and weakly supported by empirical data.
At the same time, there is a wealth of data indicating that fulvic acids are decomposed by microorganisms much more easily than humic acids, and brown humic acids (BHA) are less resistant to biodegradation than black humic acids (BHA). Thus, an unquestionable fact established to date is as follows: the degree of lability, and susceptibility to microbial decomposition, is inversely proportional to the depth of organic matter humification. Consequently, the application range of sodium pyrophosphate has become quite extensive, and it has been used to extract both labile and stable forms of humic acids. The main factor determining the application of the extraction for different purposes was the chosen pH of the solution. In this regard, it seems necessary to fractionate the soil humic substance system using sodium pyrophosphate solutions at different pH values and identify patterns in the changes of chemical characteristics of humic acids with increasing alkalinity of the solution.
The research objects were selected to reveal the features of extracting humic substances by sodium pyrophosphate from samples of the most contrasting soils, which would serve as a kind of "reference" or indicative of the most important types of humus formation. Sample selection was based on the results of a series of methodological works by V. V. Ponomareva and T.A. Plotnikova, conducted by them for different soil types. [4]
In total, three types of soils were selected, fundamentally differing from each other in the following parameters: the content and ratio of the main groups in the composition of humus - humic and fulvic acids, and the composition of humic acids - the presence and ratio of brown and black HA. These humus parameters should primarily determine possible differences in the behavior of humic substances during chemical extraction. The selected samples represent the upper humus layers of the following soil types:
- chernozem-podzolic soil
- dark gray forest soil
- ordinary chernozem (Pre-Caucasian) carbonate.
In the chernozem-podzolic soil, humic acids (HA) are mainly present in the form of free or associated with mobile forms of sesquioxides. It is essential to emphasize that chemically, the humic acids in chernozem-podzolic soil are brown (BHA). The fractional composition of fulvic acids is similar: the most represented fractions transition directly to alkaline extraction (FA-1), and to a lesser extent - the 3rd fraction (FA-3), and the 2nd fraction (FA-2) are completely absent. The dark gray forest soil is characterized by the presence of both brown and black humic acids (BHA and BHA) in the humus horizon. Ordinary chernozem is characterized by a relatively low humus content, which is a provincial feature of pre-Caucasian chernozems. However, the qualitative composition of humus is very characteristic and typical of the steppe type of humus formation. The humus composition is overwhelmingly dominated by BHA, with virtually no BHA transitioning to direct alkaline extraction, i.e., free or associated with sesquioxides. The majority of chernozem humic acids are associated with calcium.
The methodology for experimenting with extracting humic substances from soils using sodium pyrophosphate solutions at different pH values was as follows. Decimolar sodium pyrophosphate solutions were adjusted to the desired pH using NaOH or H2SO4 with an accuracy of ±0.05 pH units. A weakly acidic reaction of the solution (pH 5) was chosen as the initial starting point, and a mixture of pyrophosphate solution with a 0.1 M NaOH solution was selected as the final point.
In chernozem soil, weakly acidic sodium pyrophosphate solutions (up to a pH just above 7) extract almost exclusively fulvic acids, which significantly dominate the composition of organic carbon (Corg). However, with an increase in the alkalinity of the solution above pH 7, there is a sharp increase in the proportion of humic acids (HA), which persists until the
10
end. Thus, the ratio of CHA/CFulvic will be greater than one at any pH value in the alkaline range, which generally corresponds to the group composition of humic acids in this soil. Analyzing the extraction of humic acids from chernozem, one notable feature distinguishes it from extractions from other soils: a consistent presence of fulvic acids in all pyrophosphate extractions, which almost does not change depending on pH. Therefore, it can be assumed that fulvic acids in chernozem are almost completely extracted by pyrophosphate even at low pH values. The reason for such mobility and susceptibility of chernozem fulvic acids to dissolution in pyrophosphate undoubtedly requires further study.
In general, for all soil types, during the extraction of humic substances with sodium pyrophosphate at pH 5-6, there is a minimum ratio of CHA/CFulvic, as almost exclusively fulvic acids are extracted. After pH 7, there is a gradual increase in the proportion of humic acids, reaching a maximum at pH 10. This maximum is maintained in the 10-12 pH range, and at pH > 12, there is active extraction of humic acids with a large proportion of fulvic acids, causing a sharp decrease in the CHA/CFulvic ratio.
Now, let's consider the most important question for which this experiment was conducted: how does the depth of humification change, and what is the nature of the extracted humic acids? The depth of humification is evaluated based on the optical density index.
Patterns in the change of the optical density index of humic acids extracted from soils with sodium pyrophosphate solutions at different pH values are evident for all soil types. There is a clear increase in the optical density index with increasing alkalinity of the solutions, reaching maximum values at pH 9-11 (depending on the soil type), and then a noticeable decrease due to the extraction of less optically dense humic acids during the dissociation of phenolic hydroxyls in a strongly alkaline environment. This pattern is observed for humic acids and extracts in general but is weakly pronounced for fulvic acids.
Results of determining the optical density index of humic acids extracted from soils at pH values recommended by the methods - [4] and [5] - are presented in table.
Table 1. For comparison, the optical density indices of humic acids in a 0.1 MNaOH extract are
provided.
Soil 0,1 M Na4P2O7 pH 7 0,1 M Na4P2O7 pH10 0,1 M Na4P2O7 +0,1 M NaOH pH 12 0,1 n NaOH pH 12
Chemozem-podzolic soil 10,8±0,2 11,3±0,1 9,5±0,1 9,3±0,2
Dark gray forest soil 15,4±0,2 19,9±0,2 15,7±0,2 7,4±0,2
Ordinary chernozem 23,5±0,2 28,3±0,3 25,0±0,2 5,7±0,2
Analyzing these results, one can conclude that the most optically dense are the humic acids (HA) extracted from the soil by sodium pyrophosphate at pH 10. The least optically dense and, consequently, possessing the least depth of humification, are the humic acids from direct alkaline extraction, or HA-1. Thus, all pyrophosphate extractions, due to their interaction mechanism with the soil, extract more humified substances than the alkali solution except for soils where humic acids of the 2nd fraction are completely absent. In these cases, the optical density indices in pyrophosphate and alkaline extractions should be equal or, in extreme cases, very close.
Using the idea of unifying the conditions for extracting humic acids with sodium pyrophosphate, we still believe that, under laboratory conditions with a constant "room" temperature within the range of 18-22°C, this is quite sufficient to obtain satisfactory and reproducible results.
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