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Purpose Technological progress and high market demand contributed to a significant interest in the production of fertilizers based on humic acids. The aim of this study was to evaluate the possibilities of using humic acids obtained from lignite in the production of new commercial products. For this purpose, it is necessary to determine the quality standard requirements for such material. Properties of humic acids depend on source of origin as well as method of its extraction. Materials and methods The humic acids were extracted from polish deposit of lignite-Sieniawa Lubuska by alkaline extraction using for this purpose six kinds of extractants: 0.1 M NaOH and 0.25 M NaOH, 0.1 M KOH and 0.25 M KOH, and 0.1 M Na 4 P 2 O 7 and 0.25 M Na 4 P 2 O 7 . The humic samples were used in solid powder form and characterized by UV-Vis spectroscopy, 13 C NMR spectroscopy, fluorescence spectroscopy, and thermal analysis. Results and discussion The determining factor influencing a degree of humic acids extraction from lignite and their structure is type of extractant. The largest efficiency of extraction (about 50%) was obtained with the use of NaOH solutions. All examined humic acids were generally characterized by simple and heterogeneous molecularly structure with low molecular weight and low aromatic polycondensation. Therefore, it can be concluded that humic acids extracted with NaOH and KOH solutions are less condensed than those extracted with Na 4 P 2 O 7 solutions. It can suggest that humic acids obtained from lignite using solutions of Na 4 P 2 O 7 are characterized by a low transformation degree and greater amount of carboxyl groups. Conclusions Low rank coal can be successfully used in agriculture as a rich source of humic acids. Reagent used in their extraction, apart from high efficiency should have a neutral impact on their structure. Studies on the physicochemical properties of humic acids can be helpful in predicting behaviors of such fertilizer components in the environment and in inventing new products taking the principles of sustainable development into consideration.

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