Tracking the photooxidation products of primary plastic pellets (nurdles) in seawater

Marine plastic debris often undergoes photooxidation through sunlight exposure, potentially releasing photooxidized products to the surrounding seawater that may impact ecosystem health and carbon cycling. However, the molecular size and composition of leached products as dissolved organic carbon (DOC) remain poorly understood. In this study, we investigated photooxidation dynamics of two common plastic types - polyethylene (PE) and polycarbonate (PC) pellets, or nurdles, in seawater through 8-week laboratory simulations, representing 3 years of natural sunlight exposure along the Texas coast. This exposure facilitated the leaching of photooxidized products as DOC to seawater, with PE nurdles releasing more DOC than PC nurdles by an 18-fold difference due to the difference in their molecular structure, which introduced further differences in their light and oxygen exposure during the incubation. No nano-sized plastic particles (1 nm-0.75 μm) were detected based on the analyses using ultrafiltration (3 k Da cut-off). High-resolution mass spectrometry analysis further revealed that sunlight exposure increased the number of molecular formulas in the plastic-derived DOC, with their stoichiometry reflecting the polymers' chemical compositions. The oxygen-to‑carbon ratio in PE-derived DOC increased over time, while the hydrogen-to‑carbon ratio in PC-derived DOC decreased, indicating an enrichment of oxygenated compounds, likely due to an increase in carboxyl groups on nurdle surface during the later stages of photooxidation. Collectively, these findings highlight the role of photooxidation in driving the release of small soluble organic molecules, not nano-sized particles from plastics, providing new insights into the photooxidation products and their impacts on marine ecosystems. SYNOPSIS: We systematically investigated the production and composition of dissolved organic carbon in seawater from polyethylene and polycarbonate nurdles by photooxidation. All the dissolved organic molecules produced were truly dissolved, and they became more oxygenated as photooxidation proceeded.