Size-dependent long-term weathering converting floating polypropylene macro- and microplastics into nanoplastics in...

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^-1) > 300-500 μm (1.25 d^-1) > 1000-2000 μm (0.780 d^-1) and 5000-7000 μm (0.900 d^-1). 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^-15 M) > 300-500 μm (4.87 × 10^-15 M) > 500-1000 (3.61 × 10^-15 M) and 5000-7000 μm (3.73 × 10^-15 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.