Adsorption behavior and mechanisms of norfloxacin onto porous resins and carbon nanotube

Four adsorbents (Hypercrosslinked resin MN-202, aminated polystyrene resin MN-150, macroporous resin XAD-4, and carboxylated multiwall carbon nanotube C-MWCNT) were used to remove norfloxacin from aqueous solutions. Despite the different surface properties and pore structures of the adsorbents, similar patterns of pH-dependent adsorption showed that the adsorption amounts of norfloxacin increased with the increase of zwitterionic form, implying the importance of zwitterionic form. Norfloxacin adsorption to the three resins exhibited similar ionic strengths dependence consistent with the positive contribution of hydrophobic effect and the negative contribution of electrostatic interaction. The amounts of norfloxacin adsorbed decreased much more in the presence of CaCl2 on the surface of C-MWCNT than that of NaCl because of the dominant contribution of electrostatic interaction and H-bond. The adsorption kinetics of the four adsorbents followed the pseudo-second order model while the adsorption isotherm data fit well to the Langmuir models, indicating the coverage of the surface by a mono-molecular layer. The apparent temperature effect on the adsorption process demonstrated that hydrophobic effect was dominant mechanisms responsible for norfloxacin removal by XAD-4, while, micropore-filling was likely main mechanisms for the adsorption by MN-202 and MN-150, with the aid of hydrophobic effect, electrostatic interaction, H-bond and π–π electron–donor–acceptor interactions.