1. Introduction
Currently, the use of environmentally friendly and safe substances in agriculture is becoming relevant. One of the directions of the future is the use of humic substances. Humic substances are high-molecular compounds of natural origin, which are formed as a result of the oxidation of coal or changes in dead biomass, and are shapeless formations with a chaotic structure of dark brown color, which can dissolve or swell in water. [
1]. These compounds do not have a single chemical formula, but it is known that their main structures are aromatic rings and functional groups (hydroxyl, carboxyl, carbonyl, alkyl and methoxyl) [
2]. In addition to aromatic rings, the substance may contain polypeptide and polysaccharide fragments. Even simple compounds, such as fulvic acids, have a complex chemical structure.
Since the structure of the humic substances molecule cannot be quantitatively described by traditional methods, researchers have developed a classification method based on solubility in alkalis and acids. Thus, humic substances are classified into three categories: humins—substances insoluble neither in acids nor in alkalis; humic acids—substances insoluble in acids, but soluble in alkalis; fulvic acids—substances soluble in acids and alkalis [
3]. Humic acids are fractions of humic substances that are soluble in an alkaline medium, semi-soluble in water, and insoluble in an acidic medium. This classification parameter may vary depending on the content of humic acids, pH and ionic bonds. Due to their amphiphilic nature, humic acids form micelle-like structures called false micelles in a neutral and acidic environment. This property is used for use in water treatment plants and to increase the solubility of hydrophobic preparations in water [
4,
5]. Humic substances contain various functional groups, the number of which depends on the origin, timing, climate and environmental conditions during the extraction and production of humic substances [
6].
The various functions of humic substances mainly relate to the functional groups of phenols and carboxylic acid. These properties provide humic substances with many positive properties, such as improved plant growth and complex formation with heavy and variable metals, which means that they can remove heavy metals from the body and form chelated compounds. In addition, their antiviral and anti-inflammatory activity has been proven [
7]. It has been proven that the presence of phenols, carboxylic acids and quinones in the structure of humic substances is associated with their antioxidant, fungicidal and bactericidal activity [
8].
In the literature, a number of authors have a common opinion about the chemical composition of humic substances obtained from various sources [
9,
10]. Thus, substances prepared on the basis of humic substances contain 50% carbon, 35% oxygen and 5% hydrogen, and the remaining percentage is nitrogen and sulfur [
11]. The largest amount of carbon is contained in hard and brown coal, and its content can reach 60–65%. The ability of humic substances to bind cationic metals and complexes makes them useful in various fields. Thus, they ensure the transfer of micronutrients from soil to plants and from feed additives to the body of farm animals [
12]. In addition, humic substances reduce the content of heavy metals in soil, water and living organisms [
13].
The main source of humic substances is brown coal. Its reserves in Kazakhstan amount to 34 billion tons [
14]. It was found that brown coal differs in chemical composition compared to other sources of humic substances. It contains various trace elements that can be used in the production of animal feed additives [
15].
The main methods of isolation of humic substances include alkaline extraction with ammonia solutions or potassium/sodium hydroxides. Such methods convert humic substances into water-soluble salts, that is, potassium or sodium humates with high biological activity. This method is practically waste-free, therefore it is widely used in many countries [
16,
17].
The aim of this research was to complete a comprehensive study of the process of extraction of potassium humate from coal mining wastes of the Lenger deposit, considering the kinetic processing of experimental data, and the composition and structure of the resulting potassium humate. According to known information, these wastes have a complex organomineral composition and are a valuable component for the synthesis of humic substances. In addition, the disposal of these wastes, the volume of which is more than 6 million tons, is environmentally relevant for the local population. The scientific novelty lies in the use of technogenic resources in the form of coal mining waste for the extraction of organomineral potassium humate. The scientific interest of the authors is founded in obtaining organomineral complex compounds used in the agro-industrial sector of the economy.
2. Materials and Methods
The extraction of potassium humate wascarried out according to a well-known technique [
18], where the sample of coal mining waste is leached with an alkaline solution of potassium pyrophosphate and stirred for 1 h. The resulting suspension wascentrifuged, the solution wasdecanted and collected in a conical flask, and the insoluble residue waswashed twice with a solution of potassium hydroxide. The suspension was centrifuged after each wash by collecting the washing solution into another flask. The washed precipitate wastransferred to a flask, and a solution of potassium hydroxide wasadded and heated for 2 h in a boiling water bath. After cooling to room temperature, the contents of the flask were centrifuged for 15 min.
The effectiveness of the studied process was evaluated by the extraction degree indicator (α), which was determined by the following formula:
where
A is the initial concentration of the alkaline solution, mol/L; and
B is pulp concentration, mol/L.
The chemical analysis was carried out in accordance with the regulatory documents of the analysis: determination of the content of silicon dioxide according to the method based on the formation of a blue reduced silica-molybdenum complex, the optical density of which wasmeasured by differential photometry. The method for determining the mass fraction of iron oxide wasbased on the formation of an orange–red complex compound of divalent iron with orthophenanthroline or its analogues that is stable for several hours. Themethod for determining the mass fraction of aluminum oxide wasbased on the formation of a complex compound of trilon B with aluminum at pH 2–3 and titration of an excess amount of trilon B with zinc acetic acid at pH 5.5 with an orange xylene indicator. The method for determining the mass fraction of oxide wasbased on measuring the radiation intensity of the resonant lines of elements formed in the flame of a gas–air mixture when the analyzed solutions and comparison solutions are introduced into it. The radiation intensity of the sodium line wasmeasured at 590 nm, and potassium at 770 nm. Carbon, hydrogen and nitrogen were determined simultaneously from a single sample using a corresponding device—a CHN analyzer. When the sample was burned at a high temperature in an oxygen atmosphere, carbon, hydrogen and nitrogen were quantitatively converted into the corresponding gaseous substances (CO, HO, N2/NOX). Nitrogen oxide (NOx) formed during combustion wasreduced to N2 before the gases entered the detector. Carbon dioxide, water vapor and elemental nitrogen in the gas stream were quantified by suitable instrumental methods.
The microstructures of the initial sample and the resulting product, as well as the element–weight composition, were determined using a scanning electron microscope JSM6490 LV, and infrared spectroscopy was performed on SHIMADZU IR PRESTIGE-21 equipment. X-ray diffraction analysis was carried out on D8 Advance (Bruker) equipment, scanning speed −0.1~100 degrees/min, shooting angle 3–180 °θ. The resulting diffractograms were processed in EVA software.
Kinetic processing of experimental data was carried out in accordance with the equation:
To calculate the “apparent” activation energy of the potassium humate extraction process, the following formula was used:
Statistical processing of the received data was carried out in the STATISTICA Visual Basic (SVB)software.