Magnetic seeded filtration (MSF) is a solid-liquid separation process based on the formation of hetero-agglomerates between target (non-magnetic) particles and added magnetic particles, followed by magnetic separation. Previous experimental studies reported high separation efficiencies for hydrophobic microplastic particles (MP) and focused mainly on the polymer type. This study investigates the influence of the particle size, morphology and aging on the separation efficiency of different polymer types. Surface morphology and particle size only marginally affect the separation of particles larger than 30μm. However, the agglomeration of particles in the lower micron range is increasingly dominated by repulsive electrostatic interactions. After oxidative treatment with Fenton's reagent, separation efficiencies for most MP remain between 55% and 96%. Exposure to UV light results in a significant decrease in separation efficiency, particularly for polystyrene, where the separation efficiency decreases from 86% to 9%. Mechanical aging, simulated by mixing MP in a sand matrix on a horizontal shaker, reduces the separation to below 50% for all polymer types. Exposure to UV light causes surface oxidation, as evidenced by the formation of carbonyl peaks in the Fourier transformed infrared spectra. Mechanical treatment results in the deposition of small silica particles on the MP surface, as revealed by electron microscopy. Both mechanisms render the polymer surface more hydrophilic and reduce the tendency to form hetero-agglomerates with (hydrophobic) magnetic seed particles. MSF is a promising technique for MP separation but also offers the possibility to probe surface properties of (environmentally aged) MPs. This study demonstrates that environmental aging can significantly affect the behavior of MP and highlights the importance of environmental aging in MP fate studies.