Chemistry, Physics and Technology of Surface

ISSN 2518-1238 (Online), ISSN 2079-1704 (Print)

The aim of current work is to establish the effect of coal rank (CR) on the porous structure characteristics and adsorption properties of activated carbons (ACs) prepared by potassium hydroxide activation at the same KOH/coal ratio (1.0 g/g). Coals are samples with an increasing carbon content (C^daf=70.4–95.6 %), which is selected as the CR criterion. ACs were obtained in argon in three stages: 1) thermoprogrammed heating (4 degree/min) to 800 °С; 2) isothermal exposure 1 h; 3) cooling, washing from alkali and drying. Based on low-temperature (77 K) nitrogen adsorption-desorption isotherms (Micromeritics ASAP 2020), characteristics of ACs pore structures were determined: total specific volume (V_t, cm³/g) and surface (S, m²/g) of adsorbing pores, total volume of meso- and macropores V_me+ma, volumes of micropores (V_mi) and micropores with diameter D ≤ 1 nm (V_1nm). For all ACs, adsorption capacities for methylene blue (MB) and iodine at 25 °C were obtained. For ACs from brown and long-flame coals, the kinetics and isotherms of MB adsorption were studied. The kinetic curves were approximated by models of intraparticle diffusion, pseudo-first and pseudo-second orders. Adsorption isotherms were calculated by Langmuir, Freundlich, Toth and Redlich – Peterson models. With increasing C^daf, the V_t and S values were found to vary extremely with maxima for ACs from coals with С^daf = 80.0–86.4 %. The dominant contribution to the S value is made by the micropore surface S_mi: its fraction S_mi/S varies in the range of 94.7–99.4 %. The contribution of meso- and macropores surface is small (≤ 5.3 %), so that the adsorption properties of ACs are determined by their microporous structure. With an C^daf increasing in the range of 70.4–95.6 %, the maximum adsorption capacity of MB (A_m) increases from 197 to a maximum of 241 mg/g (AC from coal with C^daf=81.0 %) and then decreases to 113 mg/g for anthracite. The iodine adsorption capacity (A_I) changes according to a similar curve with a maximum, increases from 963 to 1175 mg/g, then decreases to 502 mg/g. The MB absorption rate was determined to be limited by diffusion in micropores. The adsorption kinetics is best described by the pseudo-first order model    (k₁ = 0.029–0.030 min^–1, R² ≥ 0.976); adsorption isotherms - by the Toth model (R² ≥ 0.991). The use of other models gives significant (up to 86 %) deviations from experimental data. It was shown that the increase of ACs surface increases the A_m and A_I values but reduces the specific capacitances expressed in mg/m² and proportional to the concentration of surface adsorption centers (SACs). A general pattern was found for ACs from hard coal and anthracite - an increase in CR reduces the porosity and surface of ACs, decreases MB and iodine capacities but increases specific capacities (i.e., concentration of SACs) when going to AC from anthracite.

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