Inter-Compartment Comparison of Weathering Extent of Microplastics Using Carbonyl Index and Its Application in Source Identification

The extent of aging in microplastics (MPs), widespread across the globe, is a critical factor in evaluating their adverse impacts and behavior. These synthetic particles undergo weathering during dispersion, primarily through photo-oxidation induced by ultraviolet (UV) light exposure, which leads to the formation of oxygen-containing functional groups and increases the potential for fragmentation into smaller-sized MPs. Time-relevant physicochemical changes of MPs can be quantified by the carbonyl index (CI), which serves not only an indicator for assessing the weathering (i.e., aging) extent of MPs, but also provides insights into the sources and/or transport pathways of MPs in different regions and compartments. In the present study, we compared the CI values of two prevalent MP polymers (PE and PP; ≥100 μm in cut-off size) transectionally collected from source regions (wastewater, river water, agricultural soils, and sand beach) to coastal region (inner- and outer-part of Incheon/Kyeonggi (I/K) bay at the Han River mouth), marginal seas (seawater of the Korean South Sea, the East China Sea, and the East Sea), the Northwestern Pacific, and the polar region (Arctic and Antarctic). Their CI values were also compared with those measured under accelerated UV light exposure in laboratory. Riverine and marine floating MPs were collected from the surface water using a manta-net, and all FT-IR spectra were obtained by the same instrument and procedure. PE in agricultural soils showed significantly higher CI values in outer soils than inner soils of greenhouse (0.32±0.16 vs. 0.25±0.16, respectively) (p<0.001). Meanwhile, much lower PE-CIs than those in soils were observed in the influent (0.13±0.10) and effluent (0.12±0.12) of sewage wastewater with no significant difference between the two wastewater (p>0.05), indicating low UV exposure. Compared to the potential two sources, the PE in downstream water of the Han River exhibited much closer CIs (0.33±0.26) to those in neighboring soils than in wastewater, suggesting the importance diffuse source in riverine MPs. Floating PE particles in coastal seawater of I/K bay exhibited the significant separation of their CIs between the inner (0.32±0.17) and outer part (0.04±0.08) of the bay (p<0.001), suggesting different sources in each region. Relatively aged PEs found in inner-bay near river mouth may have a fluvial origin associated with diffuse source, while very fresh PEs in outer-bay off the coast may have originated from the mechanical abrasion of fishing gear and/or greywater. PE-CI found in soil, river water, and inner-bay seawater corresponds to the value observed after approximately 1.2 years of natural sunlight exposure in ambient air. Unlike PE, PP exhibited less distinct separation in its CI across compartments. This is believed to be a result of the more weathering-prone PP breaking apart, leading to the formation of fresh surfaces. Our findings underscore that CI can be effectively utilized to identify the sources and/or dispersion pathways of microplastics. Additional results, including those from marginal and open seas, will be presented separately.

Acknowledgement: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2024-00356940).