Adsorption of ofloxacin on carbon nanotubes: solubility, pH and cosolvent effects

The adsorption of ofloxacin (OFL) on carbon nanotubes (CNTs) has been investigated using the solubility, pH and cocolvent effects. In this work, solubilities of OFL and sorption of OFL on three multi-walled CNTs at different pHs and different methanol volume fractions (f(c)) of methanol/water mixture solutions were systematically measured. The solubilities of different OFL species were obtained based on the analysis of pH-dependent solubility. Cationic and anionic OFL showed much higher solubilities than zwitterionic OFL. The highest sorption was not observed at the pH with lowest OFL solubility, indicating hydrophobic interaction was not the dominant sorption mechanism. The sorption decreasing in pH range of 5-8 was consistent with cationic OFL species distribution, suggesting cation exchange may play an important role. With the increased methanol fraction, both OFL solubility and sorption decreased, which was different from hydrophobic organic contaminants (HOCs). Analysis of methanol-fraction-dependent OFL sorption suggested that cosolvent-sorbent (methanol-CNTs) interactions were much stronger than solute-cosolvent (OFL-methanol) interactions. This statement was also confirmed in sorption isotherm study as evidenced by decreased sorption and increased linearity of the isotherms in methanol than those in water.