Specific features of the sorption of methylene blue by biochars based on pine and birch carbonizates

We obtained pine and birch biochars by carbonising sawdust at 500°C for 3 hours with a heating rate to the specified temperature of 10°C/min, with a fairly good yield for slow pyrolysis of 29% and 36% and particle sizes of up to 160 and 350 μm, respectively. Two-hour alkaline activation resulted in a decrease in particle size (20–60 µm for birch and 50-150 µm for pine coals), an increase in bulk and real densities, and a slight increase in pH. Using the energy dispersive analysis, we established the carbon basis of activated biochars and identified a tendency to an increase in the carbon content and a decrease in oxygen as a result of activation, which led to reduced O/C ratio of 0.258 for birch charcoal and 0.243 for pine charcoal. The kinetic sorption curves of the methylene blue dye on all studied coal samples were correctly described by a pseudo-second order model. The adsorption rate depended on the number of adsorption centres and was controlled by the contribution of the chemical reaction. A significant share of electrostatic interactions between the cationic dye and the negatively charged biochar surface could be expected. The sorption capacity of the initial and activated birch charcoal was twice higher than the corresponding pine samples. The Langmuir adsorption model correctly described the sorption of methylene blue on the studied biochars, which was confirmed by the high values of correlation coefficients. The efficiency of sorption purification from the cationic methylene blue on activated biochars from birch and pine reached 98 and 49%, respectively, which agreed well with the values of the adsorption capacity and sorption rates on these coal samples.