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Developing sustainable strategies for immobilizing heavy metal contaminants in subsurface environments is critical for advancing environmental remediation technologies. Elucidating the interactive mechanisms between Cu²⁺, clays, and polymers is crucial for the development of advanced barrier materials for copper contamination. In this research molecular dynamics (MD) was employed to simulate the swelling behavior of carboxymethyl cellulose (CMC)-modified hexagonal montmorillonite (MMT) under CuCl₂ concentrations ranging from 0 to 100 mM. Addition of CMC was shown to expand the interlamellar distance from 17.6 (unmodified MMT) to 67.02 Å (100 mM) in general. Analysis of density distributions, RDF, adsorption isotherms, and adsorption kinetics suggested a dual role of CuCl₂ in MMT swelling. At CuCl₂ concentrations of 20–60 mM, CMC facilitated intercalation and cation bridging, resulting in a maximum interlamellar distance of 79.51 Å at 20 mM. Beyond 60 mM, swelling was reduced due to charge screening, with interlamellar distance decreasing to 54.52 Å at 100 mM. Notably, the MMT edge surface exhibited selective adsorption of Na⁺ over Cu²⁺ due to the lower charge, smaller hydrodynamic radius and partial formation of inner-sphere complexes of Na⁺, preserving edge-specific interactions even at high Cu²⁺ concentrations. Comparison with DLVO theory highlighted the much lower concentrated ion distribution near basal surfaces calculated from MD, attributed to the assumptions of the continuum model not taking ion size into account. These findings provided a quantitative understanding of the interactions between CMC, MMT, and Cu²⁺ ions, offering insights into optimizing polymer-clay composites for environmental remediation.