ZIF-8/Chitosan hybrid nanoparticles with tunable morphologies as superior adsorbents towards both anionic and cationic dyes for a broad range of acidic and basic environments

A series of nanohybrids using ZIF-8 in the presence of chitosan (CS) at various compositions (0.5–15 wt%) were synthesized. Thorough characterization exhibited that the morphology of the nanoparticles in terms of surface charge, particle size, specific surface area, and pore volume is significantly dominated by the CS content in the hybrid nanoparticles. Amongst the nanoparticles synthesized, a hybrid nanoparticle containing 2 wt% CS, named CS-ZIF-2, represented the largest positive zeta potential and smallest particle size. Moreover, adsorption experiments indicated that CS-ZIF-2 had considerable adsorption capacity against anionic dye (Congo Red, CR) compared with the individual ZIF-8 and CS samples suggesting its synergistic adsorption behavior. The maximum adsorption capacity of CS-ZIF-2 towards anionic dye was determined to be 500 mg/g, which was ∼20% more than single ZIF-8 nanoparticle. Despite the positive charge of ZIF-8 and its CS-ZIF nanohybrids, they exhibited considerably large maximum adsorption capacity towards cationic dye, e.g., ∼350 mg/g for malachite green. Such behavior was correlated to distinctive adsorption mechanisms other than electrostatic forces, such as π-π interaction and hydrogen bonding between dye molecules and adsorbents. Simultaneous adsorption of both cationic and anionic dyes over CS-ZIF-2 using a solution containing a mixture of two dyes confirmed the high adsorption capacity of this sample. Adsorption experiments at various pH levels showed that CS-ZIF-2 nanohybrid could be an efficient adsorbent for both anionic and cationic dyes at both acidic and basic conditions. Recycling experiments revealed that CS-ZIF-2 retained its adsorption capacity towards both cationic and anionic dyes after five-time recycling.