Unraveling the Co-Adsorption Mechanisms of Sulfonamide Antibiotics and Cu ²⁺ on Microplastics in Aquatic Environments: Joint Effects and Molecular-Level Insights from Experiments and DFT Calculation

In aquatic environments, microplastics (MPs), sulfonamides (SAs), and heavy metals frequently coexist due to their enduring persistence and continual influx. MPs exhibit a notable capacity for adsorbing both SAs and heavy metals, but their coadsorption behavior and the underlying mechanisms remain inadequately understood. This study explores the coadsorption behavior of ten SAs and copper ions (Cu2+) on polyamides (PA) in aqueous solutions. Results reveal that PA possesses an excellent adsorption capacity with synergistic effects for SAs and antagonistic effects for Cu2+. These effects persist under varying pH values, calcium ions, and humic acid. Mechanistic investigations indicate that these joint effects are not driven by the SA–Cu2+ complexation but by changes in PA’s surface properties, functional groups, and weak interactions. Introducing Cu2+ leads to variations in the functional groups at the interaction interface, resulting in an increase negative surface charge of PA. In coadsorption, interaction distances and energies for SAs and Cu2+ are significantly modified, particularly for electrostatic and van der Waals forces, due to enhanced electron transfer. Key SA quantum chemical descriptors (εα and π) correlate with these joint effects. These findings provide a new theoretical foundation for understanding coadsorption of pollutants on MPs.