Understanding competitive Cu2+ and Zn2+ adsorption onto functionalized cellulose fiber via experimental and theoretical approach

Amidoxime groups were successfully introduced to develop a novel amidoxime-functionalized cellulose fiber (AO-Cell) for absorptive removal of heavy metal ions in wastewater. The chemical structure , and the competitive adsorption of Cu 2+ and Zn 2+ by AO-Cell were investigated by experiments study, Density functional theory (DFT) and molecular dynamic (MD) simulation. The results showed the N and O atoms in the amidoxime group can spontaneously interact with Cu 2+ and Zn 2+ through sharing long pair electrons to generate stable coordination structure, which was the dominant adsorption mechanism. Besides, the enlarged surface area, improved hydrophilicity and dispersion offered by AO-Cell facilitate the adsorption process by increasing the accessibility of absorption sites. As results of these synergetic modification, AO-Cell can remain effective in a wide pH range (1–6) and reach adsorption equilibrium within 60 min. At optimal conditions, the achieved theoretical adsorption capacity is as high as 84.81 mg/g for Cu 2+ and 61.46 mg/g for Zn 2+ in the solution with multiple ions. The competition between Cu 2+ and Zn 2+ in occupying the absorption sites arises from the difference in the metallic ion affinity and covalent index with the adsorbent as demonstrated by the MD analysis. Importantly, AO-Cell demonstrated favorable recyclability after up to 10 adsorption-desorption cycles.