Efficiently removal of ciprofloxacin from aqueous solution by MIL-101(Cr)-HSO3: the enhanced electrostatic interaction

Metal-organic frameworks (MOFs) have been widely used to remove organic/toxic compounds from waste water. Ciprofloxacin (CIP) has been detected in surface and waste water, which is harmful to aquatic organisms and human body. Herein, MIL-101(Cr)-HSO3 was synthesized by solvothermal method and its structural features were characterized by XRD, SEM, FTIR, N-2 adsorption-desorption analysis at 77 K and zeta potential. Then, the CIP adsorption performance of MIL-101(Cr)-HSO3 was investigated, in which the effect of adsorbent dosage, contact time, pH and ionic strength were explored. MIL-101(Cr)-HSO3 showed the highest adsorption capacity when the adsorbent dosage was 0.1 g/L and the pH was 8.0. The observation from the effects of pH and ionic strength suggested a stronger electrostatic interactions between CIP and MIL-101(Cr)-HSO3. The pseudo-second-order model fitted the adsorption kinetics data of MIL-101(Cr)-HSO3 well. Moreover, the equilibrium adsorption data of MIL-101(Cr)-HSO3 followed the Langmuir model, indicating a mono-layer adsorption of CIP onto surface of MIL-101(Cr)-HSO3. The calculated maximum CIP adsorption capacity from Langmuir model was 564.9 mg/g, which was higher than the reported materials. Besides, the equilibrium adsorption data were fitted to the Tempkin model with r(2) = 0.9880, which also suggested a stronger electrostatic interaction between CIP and MIL-101(Cr)-HSO3. Finally, the introduced sulfonic acid group made the material more negatively charged on the surface, which benefited the adoption of CIP via stronger electrostatic interactions resulting the enhanced adsorption capacity of CIP. The results show that the MIL-101(Cr)-HSO3 is a promising candidate for removal of CIP and introducing proper functional groups on organic linker is a convenient way to obtain MOFs with better performance for a specific application.