Biochar as an effective material for acetone sorption and the effect of surface area on the mechanism of sorption

Walnut shells and apricot pits were used to produce non-activated, air-activated and steam-activated biochar. The specific surface area decreased in the order steam-activated (500–727 m 2.g-1), air-activated (59–514 m2.g-1) and non-activated biochars (1.71–236 m2.g-1). The results indicated that water steam created a multi-layer block structure with a well-developed porous structure, especially at 900 °C, while activation with air resulted in a more fragmented structure with a higher amount of coarse pores, leading to lower specific surface values. Acetone sorption experiments were performed in order to determine the acetone sorption capacity and to evaluate the acetone sorption kinetics of the biochars, as well as to identify the possible mechanism of sorption. The maximum sorption capacity estimated from the adsorption isotherms up to a relative pressure of 0.95 ranged from 60.3 to 277.3 mg g−1, and was highest in the steam-activated biochar with the largest surface area. The acetone adsorption isotherms were fitted with different adsorption models, where the Fritz-Schlunder model showed the best fitting results. The adsorption kinetics was evaluated using two kinetics models - pseudo first order and pseudo second order. The results indicated that the biochars with a large surface area exhibited physical sorption through van der Waals forces as the dominant mechanism, while acetone sorption on samples with a smaller surface area can be attributed to a mixed dual sorption mechanism, which combines physical sorption and chemisorption on oxygen functional groups. The perfect reusability of the biochars was confirmed by four consecutive adsorption-desorption cycles.