DFT study of cadmium removal from water using activated carbon

Density Functional Theory (DFT) calculations were used to investigate the adsorption of cadmium chloride aquo-complex (CdCl 2 [H 2 O] 4 ) on pristine and nitrogen-functionalized activated carbon AC nanoclusters. Two AC models were considered, armchair (AC-6) and zigzag (AC-7), functionalized with amine (—NH₂) or pyridine (C₅H₅N) groups at different sites. Geometric and electronic analyses were conducted to assess adsorption energies, bonding interactions, and charge transfer. Pristine AC showed weak interaction with CdCl 2 [H 2 O] 4 , while nitrogen functionalization enhanced adsorption, mainly through direct N—Cd bonding. Adsorption energies for functionalized systems ranged from −1.24 to −1.51 eV , indicating favorable stability. The most stable configuration was AC-7 with pyridine groups (AC-7-pyr-1), exhibiting both strong local bonding and long-range (>3 Å ) electrostatic interactions. Bader charge analysis revealed only modest charge redistribution upon adsorption, while the DOS results showed shifts in Cl - and Cd-based states. These changes remained localized and did not alter the overall electronic or magnetic structure of the AC. In particular, the Cl states shifted to lower energies, stabilizing near –2 eV , and the Cd levels moved from –6 eV in the isolated complex to approximately –8 eV (AC-6-ami-1) and –7.3 eV (AC-7-pyr-1). Increased hydrogen atom polarization, especially in AC-7-pyr-1, correlated with higher adsorption efficiency. These results highlight the potential of nitrogen-functionalized AC, particularly with pyridine groups, for effective and low-cost cadmium remediation in contaminated waters. • All N-functionalization processes on AC were energetically favorable. • N-functionalized ACs increased adsorption of CdCl₂[H₂O]₄. • AC-7-pyr-1 showed the most stable configuration. • Adsorption forms N-Cd bonds and releases two water molecules. • Long-range interactions contributed significantly to total bonding.