Selective Biosorption of Hg(II), Cd(II), and Pd(II) on Functionalized Chitosan (–SH/–COO−): A DFT Study with ESP/MEP and NCI/RGD Analyses

In this work, density functional theory (DFT) was used to comparatively investigate the thermodynamic and electronic factors governing the association of Cd(II), Hg(II), and Pd(II) with native chitosan (CTS) and functionalized derivatives (CTS–COOH, CTS–COO−, CTS–NH3+, and CTS–SH). Representative acid–base states were considered to approximate changes in site availability, and a uniform explicit microhydration scheme was adopted to enable controlled relative comparisons across metals and materials. Within this framework, the calculated free energies suggest metal-dependent affinity regimes: the carboxylic microenvironment favors Cd(II), the thiolated microenvironment provides the most favorable association for Hg(II), and native CTS affords the strongest calculated stabilization for Pd(II). Geometry optimizations show that most complexes retain the first hydration sphere of the metal, indicating that stabilization is dominated by outer-sphere association rather than by systematic first-sphere ligand substitution. ESP/MEP maps reveal that the heterogeneity and directionality of the electrostatic landscape govern selectivity. In contrast, NCI analysis supports a cooperative contribution of weak interactions and second-sphere organization. These results provide a comparative electronic framework to guide future experimental validation of selective metal capture by functionalized chitosan materials.