Insight into the inhibitory roles of ionic liquids in the adsorption of levofloxacin onto clay minerals

The ionic liquids may have an impact on the interactions between fluoroquinolone antibiotics and clay minerals, which are widespread in nature. This work evaluated the adsorption properties of levofloxacin onto three clay minerals (i.e., montmorillonite, vermiculite, and kaolinite) affected by three types of soluble imidazolium-based ionic liquids with different chain lengths of cations ([C2mim]Cl, [C4mim]Cl, and [C6mim]Cl). The results demonstrated that the levofloxacin-binding capacity varied according to clay type (i.e., montmorillonite > vermiculite > kaolinite) regardless of ionic liquids-free or ionic liquids-addition. The observation was made concerning the physical-chemical characteristics of different clay minerals (e.g., physical structure, surface functional groups, specific surface area, and cation exchange capacity). Interestingly, ionic liquids suppressed levofloxacin adsorption on the three test clay minerals because of steric hindrance effects induced by the adsorbed ionic liquids as well as the site competition during co-adsorption of levofloxacin molecules and cations of ionic liquids. Additionally, the inhibitory impacts of ionic liquids in the sequence of [C6mim]Cl > [C4mim]Cl > [C2mim]Cl stemmed from steric hindrance and site competition during co-adsorption of ionic liquids and levofloxacin increased with chain lengths of ionic liquids. Another interesting finding was that the extent of inhibitory effects of [C4mim]Cl increased with the rise of pH values from 5.0 to 9.0, owing to the greater adsorption competition and steric effects derived from more adsorbed [C4mim]+ at higher pH conditions. The findings of this study are critical in assessing the effects of emerging contaminants on the fate of antibiotics in eco-environmental systems.