Selective and efficient removal of ciprofloxacin from water by bimetallic MOF beads: Mechanism quantitative analysis and dynamic adsorption

MOFs play an important role in the adsorption of antibiotics, but the drawbacks of MOF powders limit their practical applications. Moreover, the dominant adsorption mechanism and the quantitative assessment of MOF powders are less studied. Herein, bimetallic ZIF particles were embedded in chitosan beads via an in-situ growth process (ZnCo-ZIF@CS beads), which could selectively remove ciprofloxacin (CIP) from a complex environment. The maximum adsorption capacity was 348.90 mg/g, superior to that of other advanced adsorbents. ZnCo-ZIF@CS performed efficiently in various actual water samples and showed excellent anti-interference and recycling abilities (only a decrease of 12.74 % after 9 cycles). CIP adsorption could still reach 85.30 % after continuous flow treatment for 600 min. Interestingly, ZnCo-ZIF@CS had a good protective effect on rice seedling growth and alleviated CIP stress in rice seedlings due to the low metal leaching at pH 4.0–11.0. Macroscopic experiments, XPS, 2D-FTIR-COS and DFT results revealed that hydrogen bonding (HB), complexation (CB), electrostatic attraction (EA) and π-π interactions were involved in CIP adsorption. Different interactions dominated CIP adsorption in different pH regions. At pH < 5.0, EA and CB were the main driving forces, accounting for 42–52 % and 27–57 % of the total adsorption; at 5.0 < pH < 9.0, the contribution of EA decreased to < 15 % while CB was still dominant, accounting for ∼ 50 % of the total adsorption, and other effects (HB and π-π) increased to approximately 29–39 %; at pH > 9.0, EA, CB, HB together with π-π caused 1–27 %, 9–31 % and 66–72 % of the total adsorption, respectively. The reaction sequence of functional groups in different CIP solutions and at various pH values was also probed by 2D-FTIR-COS. This study further provides clear evidence for developing robust MOF-based adsorbents and for the mechanism quantitative analysis of pollutant adsorption by MOFs.