3D Imaging Photocatalytically Degraded Micro- and Nanoplastics.

Microplastics and nanoplastics have been an emerging global concern, with hazardous effects on plant, animal, and human health. Their small size makes it easier for them to spread to various ecosystems and enter the food chain; they are already widely found in aqueous environments and within aquatic life, and have even been found within humans. Much research has gone into understanding micro-/nanoplastic sources and environmental fate, but less work has been done to understand their degradation. Photocatalytic degradation is a promising green technique that uses visible or ultraviolet light in combination with photocatalyst to degrade plastic particles. While complete degradation, reducing plastics to small molecules, is often the goal, partial degradation is more common. We examined microscale polyethylene (125-150 µm in diameter) and nanoscale polystyrene (~300 nm in diameter) spheres both before and after degradation using multiple imaging techniques, especially scanning electron microscopy and electron tomography. Electron tomography is able to image the 3D exterior and interior of the nanoplastics, enabling us to observe within aggregates and inside degraded spheres, where we found potentially open interior structures after degradation. These structures may result from differences in degradation and aggregation behaviour between the different plastic types, with our work finding that polyethylene microplastics typically cracked into sharp fragments, while polystyrene nanoplastics often fragmented into smoother, more curved shapes. These and other differences, along with interior and 3D surface images, provide new details on how the structure and aggregation of polyethylene microplastics and polystyrene nanoplastics changes when degraded, which could influence how the resulting worn particles are collected or treated further.