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.