<p>The coexistence and interaction of free metal cations in the environment can significantly affect the migration of organic pollutants, leading to varied effects depending on environmental conditions. However, the mechanisms affecting the adsorption of organic pollutants in the presence of metal ions remain poorly understood due to limited molecular-level studies. This study investigated the adsorption behavior of sulfamethoxazole (SMX) on montmorillonite (MT) at different pH values (1.6, 3.0, and 5.0) in the presence of three metal cations with different valences: Na⁺, Ca²⁺, and Cr³⁺. At pH 5.0, the adsorption of SMX by MT at pH 5.0 in Ca²⁺ and Cr³⁺ systems increased significantly—by 7.25 times and 47 times, respectively, compared to those at pH 1.6. In contrast, Na⁺ had a less pronounced effect on SMX adsorption. Density functional theory (DFT) calculations indicated that as the pH value increases, the interaction between SMX, metal ions, and MT strengthens. Furthermore, the adsorption binding energy of SMX in the high-valence Cr³⁺ system (− 94.51&#xa0;kcal/mol) was significantly lower than in the low-valence Na⁺ system (− 36.55&#xa0;kcal/mol). As pH and cation valency increased, the bonding density of cation bridges also increased, leading to a more substantial enhancement in SMX adsorption. This study provides insights into the adsorption mechanism of SMX on MT in the presence of metal cations, contributing valuable understanding of the environmental behavior of organic pollutants under varying cationic conditions.</p>