NaCl enhances cesium adsorption onto microplastics in seawater: A density functional theory perspective

The interaction between microplastics (MPs) and heavy metals in aquatic environments is well-documented to be modulated by coexisting ions. However, the impact of coexisting ions on the interaction between radioactive heavy metal and MPs at the molecular scale remains poorly understood. This study explored the mechanisms by which coexisting ions affect cesium (Cs + ) adsorption on polystyrene (PS) MPs through integrated batch adsorption experiments, characterization techniques, and density functional theory calculations. Our results demonstrate that, compared to pure water systems, PS MPs exhibited significantly enhanced Cs + adsorption capacity in simulated seawater (by up to 1.76 times), with NaCl identified as the primary contributor. Kinetic and isotherm models further revealed that NaCl significantly boosts Cs + adsorption by PS MPs without altering the underlying physical multilayer adsorption mechanism. Surface characterization analysis indicated that NaCl can increase the adsorption sites for Cs + by enhancing the surface roughness of PS MPs. Spectroscopic analysis suggested that the benzene rings of PS MPs play an important role in the Cs + adsorption process. Density functional theory calculations further elucidated that the enhanced Cs + adsorption capacity onto the benzene rings of PS MPs primarily stems from NaCl-induced amplification of dispersion (van der Waals), electrostatic, and particularly polarization effects. These findings provide first atomistic view of seawater ion-facilitated Cs + adsorption on MPs, offering critical theoretical support for the ecological risk assessment of co-occurring MPs and radionuclides in marine ecosystems.