Study of the physicochemical characteristics of modified carbon sorbents based on wood waste

Abstract

The use of dispersed wood in the development of methods for producing sorption materials based on it becoming increasingly economically feasible, however, to solve practical problems, materials with a certain porous structure and surface chemistry are required. This article presents a study of the physicochemical characteristics of carbon sorbents obtained by modifying sawdust (SD) of Scots pine (Pinus sylvestris). It has been established that variations in thermal modification (annealing at a temperature of 300±10°C for 35 minutes) and chemical treatment of sawdust samples of fraction 0.75-2.0 mm with a solution of 5 M nitric acid contributed to an increase in the total pore volume (from 5.787 to 8.648 cm3/g) and adsorption activity for methylene blue (from 31.2 to 47.6 mg/g). At the same time, the swelling and bulk density of the resulting micro- and/or mesoporous carbon sorbents were somewhat lower than the values for native sawdust. Changes in the composition of functional groups of all samples were monitored by Fourier transform IR spectroscopy and potentiometric titration using the Boehm method. It was revealed that the thermal modification of wood waste by annealing led to an increase in the total number of oxygen-containing groups on the surface by 2.3 times (from 2.3 to 5.33 mEq/g). The modification of the surface of the original and heat-treated sawdust by oxidation with nitric acid solutions at a temperature of 80°C for 5 h also contributed to an increase in the total number of oxygen-containing groups by more than 3 times, but with an increase in the concentration of HNO3 from 0.5M to 5M the number of carboxyl groups decreased with a simultaneous increase in the content of hydroxyl groups. The extraction of copper(II) ions occurs as a result of ion exchange on the functional groups of carbon sorbents, and the maximum sorption capacity of the studied samples depended on the method of their modification. The sorption of copper(II) ions by initial sawdust is most accurately described by the Dubinin-Radushkevich model, and the sorption of carbon sorbents modified by various methods is best described by the Langmuir model. It has been shown that the chemical modification of native and thermally treated samples with a 5 M HNO3 solution allows to obtain sorbents with the best sorption capacity with respect to Cu(II) ions.