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Contrasting SizeDependence of Photochemical Lifetimesof Polypropylene and Expanded Polystyrene Microplastics in SurfaceWaters
Summary
Researchers found that photochemical dissolution of polypropylene and expanded polystyrene microplastics in surface waters does not scale linearly with surface-area-to-volume ratio as particle size decreases. Instead, the two polymers show contrasting size-dependent photochemical lifetimes, suggesting particle size and polymer type must both be considered when modeling plastic persistence in aquatic environments.
Microplastics are found floating on natural waters. Sunlight-driven photochemistry can dissolve buoyant microplastics, producing dissolved organic carbon (DOC). We hypothesized that plastic dissolution would increase linearly with increasing surface area (SA)-to-volume (V) ratio as plastics decrease in size. To test this, samples of expanded polystyrene (EPS) and polypropylene (PP) spanning a range of sizes were irradiated while floating on water in a solar simulator. A linear relationship between SA:V and DOC accumulation rate was significant for EPS (p < 0.0001) and PP (p = 0.0086), suggesting SA-controlled reactions. However, a power relationship with an exponent of approximately 0.5 between PP dissolution and SA:V provided a significantly better fit, suggesting that non-SA-controlled processes may limit PP photodissolution. Using these relationships, it was estimated that macroplastics ∼10 cm should take ∼250 to ∼8000 years to photochemically dissolve. However, estimated lifetimes are shorter for smaller plastics, with 1 mm EPS beads and 100 nm PP nanoplastics estimated to have lifetimes of 5.3 years and 3 to 196 days, respectively, with the range in lifetimes for PP dependent upon whether linear or power fits are applied.