Size-dependent long-term weathering converting floating polypropylene macro- and microplastics into nanoplastics in coastal seawater environments

In this study, we systematically developed the long-term photoaging behavior of different-sized polypropylene (PP) floating plastic wastes in a coastal seawater environment. After 68 d of laboratory accelerated UV irradiation, the PP plastic particle size decreased by 99.3 ± 0.15%, and nanoplastics (average size: 435 ± 250 nm) were produced with a maximum yield of 57.9%, evidencing that natural sunlight irradiation-induced long-term photoaging ultimately converts floating plastic waste in marine environments into micro- and nanoplastics. Subsequently, when comparing the photoaging rate of different sized PP plastics in coastal seawater, we discovered that large sized PP plastics (1000-2000 and 5000-7000 μm) showed a lower photoaging rate than that of small sized PP plastic debris (0-150 and 300-500 μm), with the decrease rate of plastic crystallinity as follow: 0-150 μm (2.01 d) > 300-500 μm (1.25 d) > 1000-2000 μm (0.780 d) and 5000-7000 μm (0.900 d). This result can be attributed to the small size PP plastics producing more reactive oxygen species (ROS) species, with the formation capacity of hydroxyl radical •OH as follows: 0-150 μm (6.46 × 10 M) > 300-500 μm (4.87 × 10 M) > 500-1000 (3.61 × 10 M) and 5000-7000 μm (3.73 × 10 M). The findings obtained in this study offer a new perspective on the formation and ecological risks of PP nanoplastics in current coastal seawater environments.