Roles of polar groups and aromatic structures of biochar in 1-methyl-3-octylimidazolium chloride ionic liquid adsorption: pH effect and thermodynamics study.

Adsorption mechanisms of 1-methyl-3-octylimidazolium chloride ([OMIM]Cl) on rice straw-derived biochars produced at 400, 500, and 700 °C (referred as RB400, RB500, and RB700, respectively) were evaluated. Adsorption affinity followed the order of RB700 > RB400 > RB500. Electrostatic attraction and hydrogen bond controlled adsorption of [OMIM]Cl on RB400, while π-π EDA interaction between [OMIM]Cl and the aromatic rings of biochar dominated adsorption of RB500 and RB700. With increasing solution pH, -COOH and -OH on biochar became deprotonated. Consequently, [OMIM]Cl binding to these sites changed from hydrogen bond to electrostatic attraction. Adsorption capacity of [OMIM]Cl increased with increasing pH during the adsorption process. Solid concentration induced by -OH of [OMIM]Cl was higher than that of -COOH. Thermodynamics study indicated that adsorption process was spontaneous and endothermic. ∆H 0 values indicated that [OMIM]Cl adsorption on biochars was a physisorption.