Ligand geometry matters: Comparative study of linear and annular chelating ligands in tailored magnetic chitosan for...

The chelation between metal ions and ligands is pivotal in defining the adsorption capacity and selectivity of Cu(II) adsorbents, particularly those based on chitosan (CS). However, the influence of the spatial geometric configuration of chelating ligands on Cu(II) chelation remains poorly understood. Here, we developed two amino ligand-modified CS-based adsorbents with distinct spatial ligand configurations-linear (DA-CS) and annular (MA-CS)-to elucidate their Cu(II) chelation mechanisms. Adsorption isothermal and kinetic studies revealed that the MA-CS adsorbent exhibited an adsorption capacity and rate constant approximately double those of DA-CS under identical conditions, underscoring the superior performance of MA-CS. Density Functional Theory (DFT) calculations further demonstrated that Cu(II) ions form more stable coordination bonds with nitrogen atoms in the annular MA-CS structure than in the linear DA-CS structure, with calculated adsorption energies of -28.63 eV and - 26.98 eV, respectively. Additionally, MA-CS displayed excellent thermal stability and a rougher surface morphology, achieving Cu(II) removal rates of up to 96.2 % while maintaining structural integrity and adsorption efficiency over five reuse cycles. These results establish MA-CS, with its annular ligand structure, as a durable adsorbent and optimized CS-based adsorbent for water treatment and environmental engineering applications.