Chemical and Structural Properties of Humic Acid from Leonardites in Kerman Mines

Document Type : Research Paper

Authors

Department of Soil Science, Faculty of Agriculture, Shahid Bahonar University of Kerman, Kerman, Iran.

Abstract

Background and Objectives: Leonardites display considerable diversity in their structural and chemical properties, and a more detailed understanding of these characteristics can greatly contribute to a better appreciation of their potential applications in agriculture. Leonardite is a naturally oxidized mineral derived from lignite, distinguished by its high oxygen content and elevated levels of humic acids (40–80%), and is recognized as a primary source for humic acid (HA) extraction. The chemical structure of humic acid consists of aliphatic and aromatic chains bearing various functional groups, notably carboxyl and phenolic groups. These functional groups enable humic acid to interact effectively with metal ions and organic compounds, thereby enhancing the chemical and biological properties of soil. There is limited information regarding the chemical composition and humic acid extraction potential of leonardite deposits from the Pabdana area (Zarand) as well as the waste materials from the Zarand coal-washing plant. The present study was conducted to thoroughly investigate these resources with the aim of identifying indigenous potentials for the production of humic substances
Methodology: A total of 16 samples, including 13 leonardite; L1–L13 were collected from the Goltoot coal mine, and 3 coal-washing waste samples; C1–C3 were collected from Zarand coal washing factory. Fresh samples were air-dried, and their pH and electrical conductivity (EC) were measured in a 1:20 solid-to-liquid ratio (w/w). Humic acid extraction followed California Department of Food and Consumer Protection method (CDFA Method) California method. Briefly, 10 g of sample (<75 μm) was mixed with 0.1 M NaOH, agitated for 24 hours, filtered, acidified to pH 1–2, washed, and subsequently dried. The percentage of extracted HA was calculated as: (weight of extracted HA / weight of initial sample) × 100. An UV-Vis spectrophotometer was used to determine structural characteristic including E4/E6, E3/E5, and E2/E3. Based on results, 6 samples (C1, L1, L5, L7, L10, and L13) were selected for further evaluation. Humic acid was extracted from selected samples and their elemental CHNS analysis, pH, EC and FTIR spectroscopy (400–4000 cm⁻¹) were investigated.

 


 



Results: The results revealed the differences among the initial samples in terms of humic acid extraction yield. The highest humic acid contents were recorded in samples L1 (64.6%), L2 (60.05%), L10 (60.07%), and L11 (62.35%). The E4/E6 ratio of C1 and L13 (0.12 and 0.58) exhibited more aromatical structures, whereas samples L3 and L6, with ratios of 3.93 and 3.88 displayed predominantly aliphatic structure. The E3/E5 ratio results showed that the highest value (5.07) was observed in sample L11, while the lowest value (0.78) was measured in sample L9. The pH of extracted HA varied from 1.50 (L6) to 7.10 (L9), and EC varied from 63 in C3 to 1035 μS.cm-1 in L6. The CHNS elemental analysis revealed that there is considerable variability in the C, H, O, N content of extracted HA from leonardite and coal washing waste. The infrared spectroscopy results indicated the presence of aromatic rings, aliphatic chains, and several key functional groups—including phenolic hydroxyl, carboxyl, and carbonyl—in the extracted humic acid.
Conclusion: In conclusion, leonardites from the Pabedana region, particularly samples L1, L2, L10, and L11, constitute promising sources for humic acid extraction, characterized by high yields and diverse chemical properties that support their application in sustainable agricultural practices.
 

 

Keywords

Main Subjects

References

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Iranian Journal of Soil Research
Volume 39, Issue 3
December 2025
Pages 345-359

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