Lignite (brown coal) is a promising source of humic acids (HAs) and toluene-soluble extracts (bitumen "A"), which have applications as soil conditioners/biostimulants (HAs) and hydrophobic coatings or polymer additives (toluene extract). This study optimized their sequential extraction from Ukrainian lignite, evaluating yield trade-offs and structural properties. Four extraction variants were tested. Conventional toluene-first extraction (Variant 0) yielded the highest toluene extract (14.86 wt. %), but lower HAs (41.0 wt. %), while reversing the sequence (Variants 2–3) increased HA yields (47.39–51.70 wt. %) at the expense of toluene extract (1.79–5.28 wt.%). Structural analysis revealed toluene extracts had higher aromaticity (60.1–61.9% aromatic carbon) and molecular association (1.9–2.2) than HAs (48.5–60.3%; 1.6–2.0), reflecting their distinct chemistries. Alkaline pretreatment enhanced HA aromaticity but reduced toluene extract recovery. The findings enable tailored protocols for agricultural (HAs) or industrial (toluene extract) applications, supporting sustainable lignite valorization.
[1] International Humic Substances Society. http://humic- substances.org/
[2] Muscolo, A.; Sidari, M.; Nardi, S. Humic Substance: Relationship between Structure and Activity. Front Plant Sci. 2013, 4, 1–7. https://doi.org/10.3389/fpls.2013.00393
[3] Piccolo, A. The Supramolecular Structure of Humic Substances: A Novel Understanding of Humus Chemistry and Implications in Soil Science. Adv Agron. 2002, 75, 57–134. https://doi.org/10.1016/S0065-2113(02)75003-7
[4] ISO 5073:2021. Brown coals and lignites - Determination of humic acids. Geneva: International Organization for Standardization; 2021.
[5] ISO 975:2021. Brown coals and lignites - Determination of yield of benzene-soluble extract - Semi-automatic method. Geneva: ISO; 2021.
[6] Ghani, M.J.; Akhtar, K.; Khaliq, S.; Akhtar, N.; Ghauri, M.A. Characterization of Humic Acids Produced from Fungal Liquefaction of Low-Grade Thar Coal. Process Biochem. 2021, 107, 1–12. https://doi.org/10.1016/j.procbio.2021.05.003
[7] Sabar, M.A; Ali, M.I; Fatima, N.; Malik, A.Y.; Jamal, A.; Liaquat, R.; He, H.; Liu, F.J.; Guo, H.; Urynowicz, M.; et al. Evaluation of Humic Acids Produced from Pakistani Subbituminous Coal by Chemical and Fungal Treatments. Fuel 2020, 15, 278. https://doi.org/10.1016/j.fuel.2020.118301
[8] Zhang, S.; Yuan, L.; Li, W.; Lin, Z.; Li, Y.; Hu, S.; Zhao, B. Characterization of pH-Fractionated Humic Acids Derived from Chinese Weathered Coal. Chemosphere 2017, 166, 334–42. https://doi.org/10.1016/j.chemosphere.2016.09.095
[9] Sarlaki, E.; Sharif, P.A.; Kianmehr, M.H.; Asefpour, V.K. Valorization of Lignite Wastes into Humic Acids: Process Optimization, Energy Efficiency and Structural Features Analysis. Renew Energy 2021, 163, 105–122. https://doi.org/10.1016/j.renene.2020.08.096
[10] Ding, H.; Tang, L.; Nie, Y.; Ji, H. Characteristics and Interactions of Heavy Metals with Humic Acid in Gold Mining Area Soil at a Upstream of a Metropolitan Drinking Water Source. J Geochem Explor. 2019, 200, 266–275. https://doi.org/10.1016/j.gexplo.2018.09.003
[11] Peter, A.; Steinbach, A.; Liedl, R.; Ptak, T.; Michaelis, W.; Teutsch, G. Assessing Microbial Degradation of o-Xylene at Field-Scale from the Reduction in Mass Flow Rate Combined with Compound-Specific Isotope Analyses. J Contam Hydrol. 2004, 71, 127–154. https://doi.org/10.1016/j.jconhyd.2003.09.006
[12] Kalbitz, K.; Solinger, S.; Park, J.-H.; Michalzik, B.; Matzner, E. Controls on the Dynamics of Dissolved Organic Matter in Soils: A Review. Soil Sci. 2000, 165, 277–304. https://doi.org/10.1097/00010694-200004000-00001
[13] Niels, H. Batjes. Encyclopedia of Soil Science (Second Edition). Geoderma 2007, 139, 251. https://doi.org/10.1016/j.geoderma.2007.02.002
[14] Tan, M.; Huang, C.; Ding, S.; Li, F.; Li, Q.; Zhang, L.; Liu, C.; Li, S. Highly Efficient Extraction Separation of Uranium(VI) and Thorium(IV) from Nitric Acid Solution with di(1-Methyl- heptyl) Methyl Phosphonate. Sep. Purif. Technol. 2015, 26, 192–198. https://doi.org/10.1016/j.seppur.2015.03.027
[15] Ehsan, S.; Ali, S.P.; Mohammad, H.K.; Keyvan, A.V. Extraction and Purification of Humic Acids from Lignite Wastes Using Alkaline Treatment and Membrane Ultrafiltration. J. Cleaner Prod. 2019, 235, 712–723. https://doi.org/10.1016/j.jclepro.2019.07.028
[16] Jianguo, J.; Yong, Y.; Shihui, Y.; Bin, Y.; Chang, Z. Effects of Leachate Accumulation on Landfill Stability in Humid Regions of China. Waste Manage. (Oxford) 2010, 30, 848–855. https://doi.org/10.1016/j.wasman.2009.12.005
[17] Wen, C.W.; Chilingarian, G.V.; Fu Yen, T. Chapter 7 Properties and Structure of Bitumens. Developments in Petroleum Science 1978, 7, 155–190. https://doi.org/10.1016/S0376-7361(08)70066-9
[18] Perminova, I.V.; Frimmel, F.H.; Kudryavtsev, A.V.; Kulikova, N.A.; Abbt-Braun, G.; Hesse, S.; Petrosyan, V.S. Molecular Weight Characteristics of Humic Substances from Different Environments As Determined by Size Exclusion Chromatography and Their Statistical Evaluation. Environ. Sci. Technol. 2003, 37, 11. https://doi.org/10.1021/es0258069
[19] European Commission. EU Green Deal – Sustainable Agriculture Policy. https://ec.europa.eu/info/food-farming- fisheries/sustainability_en
[20] Luthra, S.; Mangla, S.K.; Sarkis, J.; Tseng, M.L. Resources Melioration and the Circular Economy: Sustainability Potentials for Mineral, Mining and Extraction Sector in Emerging Economies. Resour. Policy 2022, 77, 102652. https://doi.org/10.1016/j.resourpol.2022.102652
[21] Zbykovskyy, Y.; Turchanina, O.; Shvets, I.; Pinho, H.; Shvets, I.; Kaulin, V. Modelling of Iron Concentration Changes in Tap Water after Sampling. Ecological Engineering & Environmental Technology 2025, 26, 180–189. https://doi.org/10.12912/27197050/200359
[22] Dong, L.H.; Yang, J.S.; Yuan, H.L.; Wang, E.T.; Chen, W.X. Chemical Characteristics and Influences of Two Fractions of Chinese Lignite Humic Acids on Urease. Eur. J. Soil Biol. 2008, 44, 166–171. https://doi.org/10.1016/j.ejsobi.2007.07.002
[23] Tang, H.; Liu, B.; Zhang, X.; Li, L.; Zhang, Z.; Guan, Q. Extraction of Ge and Synchronous Activation of Humic Acid in Germanium Bearing Lignite via Oxidation Leaching. Fuel Process. Technol. 2025, 272, 108216. https://doi.org/10.1016/j.fuproc.2025.108216
[24] Ruan, R.; Zhang, Z.; Lan, T.; Wang, Y.; Li, W.; Chen, H.; Peng X. The Role of Soil Pore Structure on Nitrate Release from Soil Organic Matter and Applied Fertilizer under Three Fertilization Regimes. Soil Tillage Res. 2025, 1, 248. https://doi.org/10.1016/j.still.2024.106396
[25] Wang, Y.; Xi, B.; Li, Y.; Dang, Q.; Zhang, C.; Zhao, X. Insight into the Fate of Metal Ions in Response to the Refined Classification and Transformation Order of Dissolved Organic Matter Components during Municipal Solid Waste Composting. Environ. Res. 2023, 223, 115468. https://doi.org/10.1016/j.envres.2023.115468
[26] Xue, S.; Li, X.; Fu, Y.; Zhu, P.; Liu, J.; Kou, W.; Huang, D.; Gao, Y.; Wang, X. New Insights into Organic Carbon Mineralization: Combining Soil Organic Carbon Fractions, Soil Bacterial Composition, Microbial Metabolic Potential, and Soil Metabolites. Soil Tillage Res. 2024, 244, 106243. https://doi.org/10.1016/j.still.2024.106243
[27] Peuravuori, J.; Žbánková, P.; Pihlaja, K. Aspects of Structural Features in Lignite and Lignite Humic Acids. Fuel Process. Technol. 2006, 87, 829–839. https://doi.org/10.1016/j.fuproc.2006.05.003
[28] Park, T.J.; Zafar, R.; Hur, J. The Role of Humic Substances’ Hydrophobicity in Heterogeneous Adsorption onto Microplastics: Insights from Two-Dimensional Correlation Hydrophilic Interaction Chromatography. Environ. Technol. Innovation 2025, 38, 104136. https://doi.org/10.1016/j.eti.2025.104136
[29] Nascimento, F.H.; Masini, J.C. Porous Polymer Monolithic Columns to Investigate the Interaction of Humic Substances with Herbicides and Emerging Pollutants by Affinity Chromatography. Anal. Chim. Acta 2024, 1288, 342183. https://doi.org/10.1016/j.aca.2023.342183
[30] Zhylina, M.; Shishkin, A.; Miroshnichenko, D.; Sterna, V.; Ozolins, J.; Ansone-Bertina, L.; Klavins, M.; Goel, G.; Goel, S. Granulation and Pyrolysis of Agricultural Residues for an Enhanced Circular Economy. Results Eng. 2025, 26, 104919. https://doi.org/10.1016/j.rineng.2025.104919
[31] Dhaliwal, S.S.; Naresh, R.K.; Mandal, A.; Singh, R.; Dhaliwal, M.K. Dynamics and Transformations of Micronutrients in Agricultural Soils as Influenced by Organic Matter Build-Up: A Review. Environ. Sustainability Indic. 2019, 1–2, 100007. https://doi.org/10.1016/j.indic.2019.100007
[32] Havelcová, M.; Mizera, J.; Sýkorová, I.; Pekař, M. Sorption of Metal Ions on Lignite and the Derived Humic Substances. J. Hazard. Mater. 2009, 161, 559–564. https://doi.org/10.1016/j.jhazmat.2008.03.136
[33] Sun, Y.; Bai, D.; Lu, L.; Li, Z.; Zhang, B.; Liu, Y.; Zhuang, L.; Yang, T.; Chen, T. The Potential of Ferrihydrite-Synthetic Humic-Like Acid Composite to Remove Metal Ions from Contaminated Water: Performance and Mechanism. Colloids Surf., A 2023, 658, 130771. https://doi.org/10.1016/j.colsurfa.2022.130771
[34] Qiao, H.; Liu, Z.; Peng, X.; Xian, H.; Cheng, K.; Yang, F. Significance of Humic Matters-Soil Mineral Interactions for Environmental Remediation: A Review. Chemosphere 2024, 365, 143356. https://doi.org/10.1016/j.chemosphere.2024.143356
[35] Miroshnichenko, D.; Lebedev, V.; Lebedeva, K.; Cherkashina, A.; Petrushenko, S.; Bogoyavlenska, O.; Olkhovska, A.; Hrubnyk, I.; Maloshtan, L.; Klochko, N. Thermosensitive and Wound-Healing Gelatin-Alginate Biopolymer Hydrogels Modified with Humic Acids. J. Renewable Mater. 2024, 12, 1691–1713. https://doi.org/10.32604/jrm.2024.054769
[36] Pyshyev, S.; Miroshnichenko, D.; Chipko, T.; Donchenko, M.; Bogoyavlenska, O.; Lysenko, L.; Miroshnychenko, M.; Prysiazhnyi, Y. Use of Lignite Processing Products as Additives to Road Petroleum Bitumen. ChemEngineering 2024, 8, 27. https://doi.org/10.3390/chemengineering8020027
[37] Lebedev, V.; Miroshnichenko, D.; Vytrykush, N.; Pyshyev, S.; Masikevych, A.; Filenko, O.; Tsereniuk, O.; Lysenko, L. Novel Biodegradable Polymers Modified by Humic Acids. Mater. Chem. Phys. 2024, 313, 128778. https://doi.org/10.1016/j.matchemphys.2023.128778
[38] Lebedev, V.; Miroshnichenko, D.; Pyshyev, S.; Kohut, A. Study of Hybrid Humic Acids Modification of Environmentally Safe Biodegradable Films Based on Hydroxypropyl Methyl Cellulose. Chem. Chem. Technol. 2023, 17, 357–364. https://doi.org/10.23939/chcht17.02.357
[39] Miroshnichenko, D.; Lebedeva, K.; Cherkashina, A.; Lebedev, V.; Tsereniuk, O.; Krygina, N. Study of Hybrid Modification with Humic Acids of Environmentally Safe Biodegradable Hydrogel Films Based on Hydroxypropyl Methylcellulose. C - Journal of Carbon Research 2022, 8, 71. https://doi.org/10.3390/c8040071
[40] Sinitsyna, A.O.; Karnozhitskiy, P.V.; Miroshnichenko, D.V.; Bilets, D.Yu. The use of Brown Coal in Ukraine to Obtain Water- Soluble Sorbents. Naukovyi Visnyk Natsionalnoho Hirnychoho Universytetu 2022, 4, 5–10. https://doi.org/10.33271/nvngu/2022- 4/005
[41] Lebedev, V.; Miroshnichenko, D.; Bilets, D.; Mysiak, V. Investigation of Hybrid Modification of Eco-Friendly Polymers by Humic Substances. Solid state Phenomena 2022, 334, 154–161. https://doi.org/10.4028/p-gv30w7
[42] Zbykovskyy, Y.; Shvets, I.; Kaulin, V.; Shvets, I. Stabilization of Iron Content in Drinking Water After Sampling – Study of Tap Water in Donetsk Region, Ukraine. Ecological Engineering & Environmental Technology 2024, 25, 269–277. https://doi.org/10.12912/27197050/194839
[43] Pyshyev, S.; Zbykovskyy, Y.; Shvets, I.; Miroshnichenko, D.; Kravchenko, S.; Stelmachenko, S.; Demchuk, Y.; Vytrykush, N. Modeling of Coke Distribution in a Dry Quenching Zone. ACS Omega 2023, 8, 19464–19473. https://pubs.acs.org/doi/pdf/10.1021/acsomega.3c00747
[44] Starovoit, A.G.; Zbykovskyy, E.I.; Shvets, I.B. Environmental and Economic Assessment of Reagent Treatment for Wastewater Used in Coke Quenching. Coke Chem. 2021, 64, 532–535. https://doi.org/10.3103/S1068364X21110077
[45] Zbykovskyy, Y.; Shvets, I.; Turchanina, O.; Castelbranco, A. Investigation for a Sustainable Use of Fossil Coal through the Dynamics of Interaction of Smokeless Solid Fuel with Oxygen. In Proceedings of the International Conference on Industrial Engineering and Operations Management; IEOM Society International: Rome, Italy 2021; pp 171–182. https://doi.org/10.46254/EU04.20210173