NANOPOROUS MATERIALS FROM LONG-FLAME COAL FOR PURIFYING WATER FROM METHYLENE BLUE DYE

Abstract

Industrial development has led to water pollution with dyes being ecotoxicants due to carcinogenic and mutagenic activity and a negative effect on photosynthesis. An effective method of water purification is adsorption by nanoporous carbon materials (CM) with a high specific surface area (≥2000 m2/g). Such CMs are obtained by thermochemolysis of coals with KOH at high KOH/coal ratios (3–7 g/g), which is technologically and environmentally unacceptable. Therefore, research focused on the synthesis of CMs at low alkali ratios (≤1 g/g) and the study of CMs ability to capture organic compounds from water are relevant. The purpose of this work is to study the transformation of long-flame coal into CMs, caused by an increase in the temperature of alkaline thermochemolysis, and to evaluate its effect on the CMs adsorption activity towards methylene blue (MB). MB is selected as a representative contaminant dye to quantify the adsorption capacity of new CMs. The CM samples were obtained by heating coal from KOH to a given temperature in the range t=350– 825°C and holding for 1 h. Based on nitrogen adsorption-desorption isotherms (77 K), it was calculated (2D-NLDFT-NS method, SAIEUS program) the total pore volume Vt, specific surface area SDFT, volumes and surface area of ultramicropores (Vumi, Sumi), supermicropores (Vsmi, Ssmi), micropores ( Vmi, Smi) and meso- and macropores (Vme+ma, Sme+ma). The designations of pores and their average diameters (D) are accepted on IUPAC recommendation. The alkaline thermochemolysis temperature was established to determine the porosity and the MB adsorption capacity. The highest specific surface areas (1514–1530 m2/g) and capacities (189–204 mg/g) are characteristics of CMs prepared at 785– 825°C. Based on the temperature dependences of the pore volume and specific surface, an increase in the values of Vumi, Vsmi, Smi and Ssmi was revealed up to 600°C. At higher temperatures, the volume Vumi decreases due to transforming ultramicropores (D ≤0.7 nm) into supermicropores (D = 0.7–2.0 nm). The kinetics of MB adsorption (25°C) follows the pseudo-second order model; adsorption equilibrium is achieved for ~3 h. The rate determining stage is the interaction of MB molecules with surface adsorption centers (ACs). Adsorption isotherms are best described by the Langmuir model. The activity of CMs prepared at 350–600°C was established to be low. Its capacities vary at narrow intervals (14–38 mg/g), although the surface increases from 11 m2/g to ~1000 m2/g. As SDFT increases to ~900 m2/g, the MB capacity is almost constant, so that in this region additional ACs are practically not formed or are spatially inaccessible. For CMs with a surface SDFT >900 m2/g, the formation of ACs is proportional to the SDFT growth. The main factors determining the CMs adsorption activity were established to be the mesopores filling and the π–π interaction of MB with the polyarene fragment of CM. Acidic functional groups do not play a significant role in the absorption of MB due to their location on the micropores surface being inaccessible to dye molecules.