Anthracite nanoporous carbons obtained by nitric acid intercalation followed by alkaline activation

The aim of the work is to determine the effect of nitric acid intercalation of anthracite on the porous structure and adsorption properties of activated carbons (ACs) obtained by alkaline activation. Intercalation was carried out by blowing anthracite with vapor-phase HNO3 (57 %) at 140 °C. Activation with the AC formation was performed by thermoprogrammed heating (4 deg/min) of an alkali (KOH) impregnated sample to 800 °C with isothermal holding for 1 h, cooling, washing from alkali and drying. Based on low-temperature nitrogen adsorption-desorption isotherms, the integral and differential dependences of the specific surface area (S) and pore volume (V) on pore average diameter were calculated by the 2D-NLDFT-HS method (SAIEUS program). The volumes and specific surface areas of ultramicropores, supermicropores and micropores were determined. The kinetics and isotherms of adsorption of Pb(II), 4-chlorophenol and methylene blue dye from aqueous media were measured (25 °С). Intercalation of HNO3 into anthracite transforms it into anthracite nitrate (AN) with the formation of oxygen functional groups and intra-framework ion pairs with cationic and cation-radical graphene fragments. The most intact graphene structure remains undestroyed for up to 30 min, and increasing the intercalation time results in AN oxydestruction with the formation of aroxyl radicals and products of deep oxidation up to CO and CO2. Thermal shock (800 °C) destroys the AN with the release of nitrogen dioxide and the formation of a thermolyzed material with S = 60 m2/g and the high content of structural oxygen (10.4 %). Exfoliation of the AN, similar to that of graphite nitrate, is not observed. Alkaline activation (800 °C) converts the AN into AC with a yield of 41.6 % and S ~ 2000 m2/g, which consists of 91 % of the micropore surface including 28 % of the ultramicropore surface. Activation of AN thermolyzed by thermal shock gives AС (S ~ 1500 m2/g) with a dominant portion (54.8 %) of the ultramicropore surface. Activation of the starting anthracite poorly develops the surface (S ≤ 318 м2/г). The adsorption kinetics was found to obey a pseudo-second-order equation, and the isotherms are better approximated by the Langmuir model. For each AC, the adsorption capacities increase in the order Pb(II) < dye < chlorophenol and differ by 2.3–3.6 times. The most active adsorbent is AC from anthracite nitrate, which shows the largest capacities for Pb(II) (1.80 mmol/g), dye (2.34 mmol/g) and chlorophenol (4.90 mmol/g). The increase in the specific surface area upon activation is accompanied by a proportional increase in the number of adsorption centers, but their surface concentration almost does not change. This is caused by the leveling action of KOH on the structure of ACs formed upon activation. The preliminary nitric acid intercalation is shown to increase significantly the specific surface area of the final AC (by 6.3 times) and its adsorption capacities for Pb(II) (by 7.5 times), chlorophenol (by 6.4 times), methylene blue (by 8.0 times). Thermal shock of anthracite nitrate results in a more inert carbon material, which forms AC with deteriorated (by 25–33 %) adsorption properties. The obtained anthracite carbons is concluded to be effective adsorbents for the purification of aqueous media from heavy metals, dyes and phenolic compounds.