Contrasting Size Dependence of Photochemical Lifetimes of Polypropylene and Expanded Polystyrene Microplastics in Surface Waters

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.