Microplastics are widespread in the environment, formed through the gradual weathering and fragmentation of larger plastics into nanoplastics (NPs) and microplastics (MPs). However, the weathering process and fragmentation rate remain poorly understood. To address this, we quantitatively determined NPs (60-800 nm) and MPs (0.8-500 μm) produced from thermoplastics by accelerated sunlight simulated photodegradation in air and water environment. The initiation of fragmentation of low-density polyethylene (LDPE), polypropylene (PP) and polystyrene (PS) were determined over 240 days in air, and the fragmentation rates of virgin and additive-containing PP before and after mechanical abrasion in water were compared. The acceleration factor of the solar simulator, compared to conditions in South Korea, was calculated as 5.5.
The initiation of fragmentation by photooxidation was in order of PS (< 1 year), PP (< 2 years) and LDPE (> 3 years) in South Korea by sunlight exposure. Despite PS exhibiting the fastest initiation of fragmentation, photodegradation rates, total particle abundance, and increasing ratio (exposure/non-exposure) were comparable or lower than those of PP. The fragmentation rate of PP and additive-containing PP appeared similar after 176 days of simulated sunlight exposure followed by mechanical abrasion (equivalent to 2.7 years of outdoor exposure in South Korea). Additionally, mechanical abrasion from vortexing played a significant role in the production of MPs, whereas it had a smaller impact on the generation of NPs.
These results suggest that weathering and fragmentation by photooxidation of PP are more rapid and effective in air than in water. The initiation of fragmentation was faster in air (approx. 1.8 years) than in water (> 2.7 years). The fragmentation rate of thermoplastics determined in this study provide valuable insights for estimating secondary microplastic production through weathering, informing decision-making regarding timely plastic litter removal from the environment.