NITRIC ACID MODIFICATION OF COALS AS A METHOD FOR IMPROVING THE ADSORBTION CAPACITY OF CARBON ADSORBENTS
DOI:
https://doi.org/10.32782/naturaljournal.16.2026.14Keywords:
fossil coal, alkaline activation, activated carbon, adsorption, 4-chlorophenol, methylene blue, lead cationsAbstract
Fossil coals of various rank represent a significant group of precursors for the production of activated carbons (ACs) used in the removal of ecotoxicants. In view of the practical importance of ACs, research aimed on improvement of their synthesis methods and adsorption characteristics remains highly relevant. One promising approach is the direct changes of AC properties through oxidative treatment of the precursor, specifically the nitric acid modification of the raw coal. Literature analysis indicates that nitric acid treatment of final AC destroys its nanoporous structure, whereas the modification of the coal precursor itself results in the opposite effect: it increases the specific surface area and enhances adsorption activity. In this work, the influence of low-temperature nitric acid modification of different rank coals on the adsorption properties of ACs prepared via alkaline activation at a low KOH/coal weight ratio (1 g/g) was quantitatively evaluated for the first time. Coal samples spanning a wide range of ranks with carbon content Cdaf = 80.0–95.2% were investigated. Modification was carried out using an HNO3 solution (50%) at 25±2°C for 24 h. ACs were obtained by activating coals with KOH (800°C, 1 h), while AC(M) samples were prepared from the modified coal under identical conditions. It were determined ACs yields, specific surface area SBET, and adsorption capacities towards 4-chlorophenol (CPh), methylene blue (MB), and Pb(II) cations, which are typical ecotoxicants. It was established that under unified synthesis and activation conditions, the coal rank is the primary factor determining the porosity parameters and adsorption capacity of the resulting ACs. The dependences of SBET and the ACs capacities on the carbon content of the precursor were obtained. The samples derived from modified coals exhibit a higher specific surface area (by 1.13–1.34 times) compared to ACs derived from the raw coals. Modification results in an increase in adsorption capacity for CPh (by 1.14–2.54 times), MB (up to 1.60 times), and Pb(II) (by 1.34–1.76 times). The highest capacitiy was demonstrated by the AC based on modified long-flame coal: 4.43 mmol/g (CPh), 0.81 mmol/g (MB), and 1.02 mmol/g (Pb). Specific adsorption capacities, which are proportional to the surface concentration of adsorption sites, were calculated. This parameter for CPh varies non-monotonically, reaching a maximum for medium rank coals modified. The specific capacities for MB and Pb(II) remain nearly constant in the range of Cdaf = 80–88% and decrease sharply for ACs from anthracites. Based on a comparison of the obtained results with literature data, it is concluded that AC samples derived from modified low-rank coal are highly efficient adsorbents for the removal of organic ecotoxicants and heavy metals from aqueous media.
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