Uniform and stable immobilization of metal-organic frameworks into chitosan matrix for enhanced tetracycline removal from water

Metal-organic frameworks (MOFs) have been attractive adsorbents for the treatment of antibiotic wastewater. However, in their powder form, the adsorbents suffer from the disadvantage of poor recyclability and complex operational process. Herein, we incorporated typical MOFs (ZIF-8, ZIF-67, HKUST-1, and Fe-BTC) into low-cost chitosan matrix for the tetracycline removal. Metal hydroxide(or metal oxide)/chitosan composite beads from the metal salt/chitosan solution were used as MOF precursor, which lead to the uniform and stable MOF loading into chitosan. This composition could not only shape MOFs into beads but also improve the adsorption capacity of the chitosan. As a result, ZIF-8-chitosan composite beads exhibited good adsorption performance toward antibiotic tetracycline. The adsorption processes followed pseudo-second-order model and Langmuir isotherm model. The maximum adsorption capacity could reach 495.04 mg/g, which is higher than most of the MOF-based or natural polymer-based tetracycline adsorbents. Moreover, the removal efficiency of ZIF-8-chitosan composite beads toward tetracycline was still above 90% after ten adsorption-desorption cycles. In the fixed-bed experiment, ZIF-8-chitosan column could treat ~887 bed volumes (BV) of tetracycline solution in real water sample. Both experimental characterizations and DFT calculations indicated that the adsorption mechanism involves in the electrostatic interaction, π-π stacking interaction and hydrogen-bonding interaction. This study provides a promising strategy for uniform MOF loading into the matrix and potential adsorbents for pharmaceutical wastewater treatment.