<p>Plastic products have significantly enhanced convenience in daily life; however, their degradation through weathering and environmental exposure leads to the formation of microplastics. These microplastics can serve as carriers for pollutants, such as heavy metals, through adsorption and desorption processes, posing potential risks to living organisms. This study focuses on the adsorption and desorption characteristics of nickel (Ni) on two representative microplastics—Polystyrene (PS) and Polylactic Acid (PLA)—before and after three aging processes: freeze–thaw cycling, alternating dry–wet conditions, and alkali treatment. Following these aging treatments, both microplastics exhibited increased specific surface area, pore size, and crystallinity, along with the emergence of oxygen-containing functional groups on their surfaces. Adsorption experiments indicated that nickel adsorption kinetics aligned more closely with the proposed second-order model, while adsorption isotherms were best described by the Langmuir model. Aged microplastics demonstrated higher adsorption capacities compared to their unaged counterparts, with adsorption capacity ranking as follows: alkali aging &gt; alternating dry–wet aging &gt; freeze–thaw cycling. Furthermore, PLA exhibited a greater adsorption capacity than PS. Among the aging processes, alkali treatment resulted in the highest nickel desorption rates, whereas freeze–thaw cycling and alternating dry–wet aging produced similar desorption outcomes. These findings contribute to a deeper understanding of microplastic aging mechanisms and their implications for heavy metal adsorption and desorption in environmental systems.</p>