<p>Microplastics are widely present in the natural environment and can act as carriers for heavy metal pollutants in aquatic systems. In this study, an in situ aging approach was employed, in which polyethylene (PE) microplastic powder was confined in mesh bags and exposed to river conditions. This method was used to investigate the aging characteristics of the microplastics and their subsequent adsorption performance and mechanisms for the heavy metal cadmium (Cd²⁺). The study found that natural aging caused fragmentation of the microplastic surfaces, producing cracks, increasing roughness, and altering surface functional groups. Compared to pristine microplastics, aged PE microplastics exhibited varying degrees of increased adsorption capacity for Cd²⁺. The adsorption capacity of PE microplastics for Cd²⁺was influenced by multiple factors, including microplastic dosage, pH, salinity, background electrolytes, and different aqueous media. The adsorption process of Cd²⁺ onto pristine PE microplastics was better described by the pseudo-first-order kinetic model, whereas aged PE microplastics followed the pseudo-second-order kinetic model more closely. The Freundlich isotherm model provided a better fit for the adsorption behavior of Cd²⁺ onto PE microplastics aged for 2 and 4&#xa0;months, while the adsorption isotherm data for pristine microplastics and those aged for other durations were more consistent with the Langmuir model. The adsorption of Cd²⁺ onto aged microplastics was determined to be a multi-mechanism process.</p>