<p>This study investigates the adsorption of selenium ions from aqueous solutions onto polystyrene microplastics, titanium dioxide nanoparticles, and their composite system using advanced statistical physics modeling. Experimental adsorption isotherms obtained at 288, 298, and 308&#xa0;K were analyzed using mono-energetic, bi-energetic, and tri-energetic adsorption models to elucidate the underlying adsorption mechanisms. Model evaluation based on determination coefficients, error functions, and physical consistency demonstrated that the mono-energetic adsorption model provided the most reliable description of selenium adsorption for all systems. Steric parameters, including the number of selenium ions adsorbed per functional group, functional group density, and saturation adsorption capacity, were determined and interpreted in relation to temperature effects. Polystyrene microplastics exhibited endothermic adsorption behavior, whereas titanium dioxide nanoparticles showed exothermic adsorption, while the composite system displayed intermediate, temperature-dependent behavior reflecting synergistic and antagonistic interactions between the two materials. Adsorption energies were below 40&#xa0;kJ/mol, indicating that physisorption predominates under tested conditions. Thermodynamic functions derived from the grand canonical ensemble revealed negative internal and Gibbs free energies, confirming the spontaneous and energetically favorable nature of selenium adsorption. These results provide mechanistic insight into the steric and energetic factors governing selenium adsorption on microplastics and titanium nanoparticles and contribute to understanding their role in selenium transport and diffusion in aquatic environments.</p>