Competitive Adsorption of Pb(II), Zn(II), and Cr(III) on a Chitosan–Graphene Oxide Biocomposite: Experimental and DFT Insights

A chitosan–graphene oxide (Cs/GO) biocomposite was evaluated for Pb(II), Zn(II), and Cr(III) removal using adsorption experiments, adsorbent-free controls, and density functional theory (DFT) calculations. The composite exhibited a predominantly amorphous, mesoporous structure with a specific surface area of 180.0 m2 g–1 and oxygen- and nitrogen-containing functional groups. Pb(II) uptake showed an apparent plateau from the first sampling point at 5 min. In competitive solutions containing 100 mg L–1 of each metal at pH 4.5, apparent distribution coefficients after 60 min followed Pb(II) > Cr(III) > Zn(II), with Pb/Cr and Pb/Zn separation factors of 8.9 and 27.5, respectively. Adsorbent-free controls showed 99.34% Cr(III) loss at pH 7.0, compared with 5.22% at pH 4.5, demonstrating the importance of distinguishing adsorption from pH-induced metal loss. DFT calculations incorporating six explicit water molecules around each metal reference ion, with ωB97X-D single-point energy evaluations, favored Pb(II) association but did not reproduce the experimental Cr(III)/Zn(II) ordering. Electronic-state analysis also revealed functional-dependent charge transfer in the Cr-containing model. The results support preferential Pb(II) removal by Cs/GO under the tested competitive conditions and highlight the importance of solution controls, hydration models, and electronic-state assessment when relating calculated metal binding to experimental adsorption behavior.