Source and degradation of plastic debris in the marine environment: A new approach based on carbonyl index and stable isotopes

Plastic pollution in marine environments has become a global concern, emphasizing the need for innovative analytical approaches to trace sources and assess degradation processes. In this study, an integrated analytical approach, combining carbonyl index (CI) and stable isotope analysis (δ¹³C and δ¹⁸O), was applied to compare polyethylene (PE) pellets retrieved from coastal environments with the corresponding industrial reference material. The study takes advantage of the PE pellet spill from the container ship Toconao, which provided both reference pellets from the original industrial batch and pellets exposed to natural marine conditions for three months, enabling a direct comparison between pristine and environmentally aged material. CI was determined using the SAUB (Specified Area Under Band) method through FTIR-ATR (Fourier Transform InfraRed - Attenuated Total Reflectance) spectroscopy, revealing an average increase of 12.4% after three months of natural exposure, consistent with the formation of new carbonyl groups due to environmental degradation. Carbon isotope analysis showed minimal variation between the standard and exposed samples, indicating preservation of the original isotopic signature during early degradation stages, thus confirming its reliability for tracing plastic sources. In contrast, oxygen isotope analysis report a linkage with CI, highlighting that photooxidative degradation affects the oxygen isotopic composition of the macromolecule, concurrently with the formation of new carbonyl groups. These results demonstrate that the combined spectroscopic-isotopic approach enables simultaneous assessment of source and degradation of plastics in the environment, providing a promising tool for large-scale monitoring of plastic origin and aging processes.