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Keywords
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brown coal, chemical activation, porous structure, specific surface area, surface morphology, low-temperature nitrogen adsorption, adsorption activity, benzene.
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Abstract
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In this study, a comprehensive investigation was carried out on the thermal and chemical modifications of brown coals of BPK and BR grades (Angren deposit, Uzbekistan) with the aim of establishing the regularities governing changes in their physicochemical and adsorption properties. Thermal analysis of the samples revealed the stages of carbon matrix degradation and the removal of volatile components. It was found that chemical activation leads to a significant decrease in the ash content of the coals, which is associated with the dissolution of mineral impurities during KOH treatment. Dispersion analysis demonstrated changes in particle size accompanied by increased structural fragmentation with higher KOH consumption, which was confirmed by morphological analysis using scanning electron microscopy (SEM). The samples after activation exhibited a substantial development of porosity, the formation of network-like structures, and the expansion of the channel system, indicating intensive etching of the carbon matrix. Particular attention was given to the study of the textural characteristics of the activated coals using the low-temperature nitrogen adsorption–desorption method. It was revealed that increasing the KOH ratio promotes a significant growth in the specific surface area (up to 1371.4 m²/g) and the formation of a predominantly microporous structure in BR coals, whereas BPK coals demonstrate a more developed mesoporous system. It was determined that the variation in pore size exerts a decisive influence on benzene adsorption activity. Despite the high specific surface area of the activated coals, their sorption capacity for benzene does not always increase proportionally. This discrepancy is attributed to the peculiarities of the pore structure: BR coals are dominated by narrow micropores (7–12 Å), which ensure high adsorption capacity due to capillary condensation and molecular interactions, while BPK coals contain a higher proportion of pores in the range of 10–25 Å, which limits the retention of benzene molecules.
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