Preparation of a novel CSM@ZIF-67 composite microsphere to facilitate Congo red adsorption from dyeing wastewater

ABSTRACTChitosan (CS) is commonly used as an adsorbent for wastewater treatment because of its low cost, strong adsorption properties, and high availability of raw materials required for its production. However, CS exhibits limited adaptability to pH, poor mechanical properties, and high swelling in aqueous media; these limitations restrict its widespread use. To address these issues, herein, zeolitic imidazolate framework-67 (ZIF-67) is loaded onto crosslinked CS microspheres (CSM) to prepare CSM@ZIF-67, a composite adsorbent. Next, the CSM@ZIF-67 is applied to the treatment of Congo red (CR) dye, which is typically present in printing and dyeing wastewater. The results demonstrate that the in situ synthesis of metal-organic frameworks (MOFs) on CSM improve the dispersion of MOFs and preserve the morphology of the MOFs. The adsorption equilibrium of CSM@ZIF-67 is reached within 150 min, and its adsorption capacity is as high as 538.4 mg/g at a pH of 9 and temperature of 25 °C. The CR adsorption process is consistent with the pseudo-second-order kinetic and Langmuir isotherm models, thus revealing that chemisorption is the primary rate-limiting step, and the pollutants are adsorbed on the adsorbent surface in a monolayer. Experiments on material cycling and regeneration performance reveal that the removal efficiency of CSM@ZIF-67 remains above 90%, even after five rounds of adsorption. CSM@ZIF-67 has abundant functional groups and adsorption sites and can efficiently remove CR through mutual interactions between the metal coordination effect, π-π conjugation, hydrogen bonding, and electrostatic interactions.