Composite adsorbents of aminated chitosan @ZIF-8 MOF for simultaneous efficient removal of Cu(II) and Congo Red: Batch experiments and DFT calculations

In this study, we employed a straightforward in-situ synthesis strategy to fabricate multi-site adsorption functional composites known as aminated chitosan@ZIF-8 (AmCs@ZIF-8) for the efficient removal of Cu(II) and Congo red. The AmCs@ZIF-8 adsorbed Cu(II) at pH 5 and Congo red at pH 3 with excellent removal efficiency values of 94 % and 100 % respectively. The maximum adsorption capacity for Cu(II) and Congo red are obtained as 785 mg g(-1) and 1156 mg g(-1), respectively. Interestingly, AmCs@ZIF-8 composites exhibited an excellent removal efficiency (>80 %) after reusing them for four sequential cycles. Analysis of the adsorption isotherm and kinetic data indicated that chemisorption primarily governed the adsorption of Cu(II) and Congo red by AmCs@ZIF-8. Furthermore, we elucidated the adsorption energy of Cu(II) and Congo red on the AmCs@ZIF-8 composite through density functional theory. Specifically, the adsorption energy values for Cu(II) with AmCs and ZIF-8 were -20.907 and -57.809 kcalmol(-1), and -26.514 and -16.242 kcalmol(-1) for Congo red, respectively. The efficient adsorption mechanism of Cu(II) and Congo red on the AmCs@ZIF-8 composite can be attributed to the synergistic effects of chelation, electrostatic attraction, ion exchange, and hydrogen bonding through the analysis of experimental characterization and theoretical calculation. Therefore, AmCs@ZIF-8 composites elevate the adsorption capacity of Cu(II) and Congo red in a single-component system and exhibit high removal efficiency (>90 %) in the context of the Cu(II)-Congo red binary system.