The mechanisms involved into the inhibitory effects of ionic liquids chemistry on adsorption performance of ciprofloxacin onto inorganic minerals

The ionic liquids (ILs) can influence the adsorption of antibiotics on minerals but there was ambiguity about mechanisms. Herein, we demonstrated the adsorption characteristics of ciprofloxacin (CIP) onto ferrihydrite and montmorillonite by various soluble ILs. Five imidazolium-based ILs with different alkyl chain lengths ([C(2)mim] Cl, [C(4)mim]Cl, [C(6)mim]Cl) and counteranions ([C(4)mim][TOS] and [C(4)mim][PF6]) were employed. The results validated that ILs could suppress CIP adsorption on both minerals due to the adsorption site competition between cations and CIP molecules, steric hindrance induced by long alkyl chains of ILs, and electrostatic interaction. Interestingly, the adsorption-inhibition effects of ILs were almost unchanged with the lengthening alkyl chain for ferrihydrite, which were ascribed to the comparable adsorption capacities of the three ILs on ferrihydrite. Whereas, the effects of ILs on CIP adsorption onto montmorillonite followed the order of [C(6)mim]Cl > [C(4)mim] Cl > [C(2)mim]Cl owing to the adsorption site competition and steric hindrance effects on the ILs increased with the lengthening alkyl chain. Furthermore, the inhibitory effects of ILs on CIP adsorption onto both studied minerals were also dependent on the counteranions (kept the same cation ([C(4)mim](+))) and in the following order of [C(4)mim][PF6] > [C(4)mim][TOS] > [C(4)mim]Cl, which was mainly ascribed to the differences of anions in the ion sizes and the binding affinities to minerals. Additionally, the extent of the inhibitory impact of [C(4)mim] [TOS] on CIP adsorption onto ferrihydrite declined with increasing solution pH from 5.0 to 9.0, which was attributed to the amounts of adsorbed [TOS](- )declined with increasing pH. Contrarily, for montmorillonite, the extent increased with increasing pH, owing to increase in adsorption site competition and steric effect at higher pH conditions. Our findings emphasize the significance of emerging contaminants in understanding the antibiotic environmental behavior in eco-environmental systems.

Automated summary

This study confirms that ionic liquids can inhibit the adsorption of ciprofloxacin on ferrihydrite and montmorillonite, and elucidates the mechanisms involved, including competition for adsorption sites between ions and antibiotics, steric hindrance caused by the alkyl chains of the ionic liquids, and electrostatic interactions.