New approaches for regulation of structure and adsorption properties of biochar based on freshwater sediments (sapropels)

Abstract

Biochar based on sapropel have been synthesized and the effect of chemical modification on their textural characteristics, acid-base and adsorption properties has been studied. It was found that preliminary treatment of raw sapropel with a weakly concentrated solution of alkali increases the fraction of micropores: from 0.006 cm3 g−1 (for BС-N) to 0.021 cm3 g−1 (for КОН-BC), and additional treatment with hydrofluoric acid leads to an increase in the fraction of micropores up 0.047 cm3 g−1 (for КОН-BC-HF) and also of specific surface area up to 211 m2 g−1 at a considerable decrease in the fraction of mineral part due to leaching of silicon-containing phases. The proposed chemical modification makes it possible not only to change the ratio of mineral and carbon parts and the pore space, but also to vary the surface properties of biochar: рНPZC, concentration and features of oxygen-containing groups. It is shown that the total content of acidic groups increases in the series BС-N (0.3290 mmol g−1) < КОН-BC (0.5567 mmol g−1) < КОН-BC-HF (1.3046 mmol g−1). Simultaneously the type of treatment affects the nature of oxygen-containing groups: the maximum amount of phenolic groups (0.4845 mmol g−1) in sample КОН-BC, the surface acidity of КОН-BC-HF is contented by the presence of a considerable amount of carboxyl groups (0.9198 mmol g−1). The analysis of the data obtained shows that the total content of acidic groups increases in the series The adsorption of Cu(II) ions from an aqueous solution on the synthesized samples of biochar was studied for the first time. Biochar obtained using the sapropel pretreated with alkali showed the maximum adsorption capacity toward copper ions of 27.9 mg g−1. Conceptual model identifying the mechanisms of adsorption copper (II) ions on biosorbent from sapropels was shown. The experimentally obtained isotherms were approximated using the Freundlich and Langmuir models. It was found that the adsorption of copper on the tested samples can be described quite adequately by the Langmuir monomolecular adsorption model (R2 ≥0.98).