Sorption and structural characteristics of biochars based on poplar saw-dust

Abstract

The sorption of methylene blue dye on biochar obtained from poplar sawdust was studied. The coal was obtained by carbonization at 500°C for 3 hours with a heating rate of 10°C/min to this temperature; the product yield was 39%. It was found that the surface characteristics of biochar strongly depend on the type of raw material and the type of processing. When modified with 2 M KOH solution, the density increases due to a decrease in the size of biochar particles, the O/C ratio decreases from 0.167 to 0.150. Modification with alkali contributes to an increase in the specific surface area by 6 times, the volume of mesopores by 4 times, and the pore diameter remains virtually unchanged. Treatment of biochar with a 2 M KOH solution led to a number of changes in its IR spectrum, reflecting the structural and chemical modifications key to its adsorption properties. It was found that the number of hydroxyl groups (-OH) on the surface of the modified charcoal increases, while the aromatic component increases. KOH treatment activates the carbon, creating micropores through reactions that form and subsequently wash out intermediate compounds. Methylene blue adsorption on the studied biochar is a complex process involving physical and chemical interactions. The applicability of the Langmuir and Freundlich adsorption models for approximating the experimental dye adsorption isotherms was estimated. It was found that the adsorption of methylene blue by biochars is adequately described by the Langmuir equation (R² = 0.99). The shape of the obtained isotherms allows us to assume that the sorption of the dye is limited by the formation of a monolayer, adsorption occurs on the active centers of the carbon surface of both acidic and basic nature, to a greater extent on the surface of modified biochar.