Adsorption and modeling of humic acid by Cu-doped metal-organic framework meta-ZIF-8

This work explores humic acid (HA) removal using ZIF-8, along with Cu doping introduced to boost its adsorption efficiency. The structural properties and morphology of the synthesized ZIF-8 and Cu-ZIF-8 were characterized by FE-SEM, XRD, FTIR spectroscopy, pHpzc, and N 2 adsorption-desorption techniques. The comparative adsorption efficiency by ZIF-8 and Cu-ZIF-8 indicated an ∼8% improvement in HA removal. Based on the model development and optimization, HA adsorption was highest at Cu-ZIF-8 dosage = 0.75 g L -1 , HA concentration: 25 mg L -1 , and pH= 4. Kinetics of adsorption for HA removal obeys the intraparticle diffusion (IPD) model and the Sips isotherm model at equilibrium. The maximum adsorption capacity for Cu-ZIF-8 was 115.4 mg g -1 , which compared favorably to many other adsorbents in the literature. Thermodynamic analysis revealed that HA adsorption onto Cu-ZIF-8 is spontaneous, exothermic, and governed by physisorption. Coexisting ions in real water samples influenced HA adsorption by Cu-ZIF-8, with moderate ionic strength enhancing removal and excessive ionic strength reducing efficiency. The level of metal ions (Zn and Cu) leached from the Cu-ZIF-8 structure was well below drinking water standards. A dramatic decline in adsorption efficiency highlighted the limited adsorbent regenerability, indicating the need for further investigation. Cost analysis showed that Cu-ZIF-8 entails higher synthesis costs than pristine ZIF-8, while both remain economically viable for scale-up. Electrostatic attraction, hydrogen bonding, π–π interactions, and pore entrapment contribute to HA uptake by Cu-ZIF-8, as supported by FTIR spectral and literature results. XRD confirms that Cu-ZIF-8 retains its crystal structure after HA adsorption, indicative of strong stability. • Cu-doped ZIF-8 revealed enhanced humic acid adsorption by versus undoped ZIF-8 • The maximum equilibrium adsorption capacity was 115.4 mg/g based on the Sips model • HA uptake followed IPD kinetics at 0.75 g/L Cu-ZIF-8, 25 mg/L HA, & pH 4 • Adsorption includes pore entrapment, hydrogen, bonding, electrostatic & π–π interactions