In-situ prepared zeolitic imidazolate framework (ZIF-8) @ carboxymethyl cellulose composite adsorbents for methylene blue removal

In this study, zeolitic imidazolate framework (ZIF-8)-based composite adsorbents (ZIF-8@CMC/Fe, ZIF-8@CMC/Al, or ZIF-8@CMC/Cr) were synthesized by the in-situ growth method and employed for the adsorption of methylene blue (MB) dye from aqueous solutions. The carboxymethyl cellulose (CMC) hydrogel beads crosslinked with metal ions (Fe³⁺, Al³⁺, or Cr³⁺) were employed as host materials. The confinement effect resulted in the formation of small, non-agglomerated ZIF-8 nanoparticles within the porous CMC hydrogel beads, which effectively enhanced the adsorption performance of ZIF-8-based composite adsorbents. The findings of the study indicated that the metal ion species exerted a notable influence on the microstructure and physicochemical characteristics of the CMC hydrogel host materials. Compared to CMC hydrogels cross-linked with Al³⁺ or Cr³⁺, CMC hydrogels cross-linked with Fe³⁺ demonstrate a high specific surface area (23.753 m²/g) and a developed pore structure, which is attributed to a high degree of cross-linking. Therefore, the ZIF-8@CMC/Fe composite adsorbent contains a higher proportion of ZIF-8, resulting in a higher maximum adsorption capacity of ZIF-8@CMC/Fe (546.6 mg/g) than that of ZIF-8@CMC/Al (344.9 mg/g) and ZIF-8@CMC/Cr (191.6 mg/g), based to Langmuir isotherm model(R^2, 0.978–0.996). The optimal adsorption of MB on ZIF-8@CMC/Fe was achieved at pH=6, with a contact time of 360 min and an adsorbent concentration of 1.0 g/L. Under these conditions, the removal efficiency of MB on ZIF-8@CMC/Fe (500 mg/L) reached 90.51 %. ZIF-8@CMC/Fe exhibits a high and stable MB adsorption capacity over a wide pH range (3–10), and maintains its excellent adsorption capacity after five adsorption-regeneration cycles. The adsorption of MB by ZIF-8@CMC/Fe is primarily attributed to interactions between the active groups and MB, including hydrogen bonding, electrostatic interactions and π-π stacking.