Stable Isotope applied to organic geochemistry of emerging contaminants: innovative isotopic tools for source tracing and degradation assessment

This PhD research aimed to develop and apply stable isotope methodologies for investigating emerging organic contaminants in geological and environmental matrices. The work addressed both analytical and interpretative challenges in isotope-based techniques through a strongly methodological approach. While stable isotopes have long been used to trace contaminant sources and degradation processes, the increasing diversity and chemical complexity of modern pollutants require continuous methodological innovation to ensure accurate and reproducible isotopic data. Within this framework, the objective was to design and implement new isotope-based analytical tools, supported by multi-isotopic approaches, to characterize contaminant sources and quantify transformation processes in contemporary environments. The study focused on hree main classes of emerging contaminants, marine plastics, oxygenated fuel additives, and per- and polyfluoroalkyl substances (PFAS), selected for their environmental relevance and analytical challenges. The sequence of studies reflects increasing methodological complexity, from bulk stable isotope analysis (BSIA) to compound-specific isotope analysis (CSIA). The first study applied BSIA to marine plastic pellets, integrating isotopic (δ¹³C and δ¹⁸O) and infrared data to investigate early photooxidative degradation of pellets released in seawater. Results demonstrated the stability of δ¹³C as a source tracer and a progressive δ¹⁸O depletion proportional to oxidation, providing a quantitative degradation proxy. The second study applied CSIA to oxygenated fuel additives (methyl tert-buthyl ether and ethyl tert-buthyl ether), major groundwater contaminants, using dual-isotope (δ¹³C–δ²H) analysis to describe biodegradation patterns and assess site-specific isotopic variability of nonspilled gasoline, essential for accurate degradation estimates. The third study developed the first CSIA method for perfluorocarboxylic acids (PFCAs), the most detected subclass, demonstrating the feasibility of carbon isotope analysis for these persistent compounds. A subsequent field application confirmed the method’s robustness through the first isotopic characterization of PFAS in environmental samples. Overall, this PhD expands the analytical boundaries of isotope geochemistry by establishing methodological foundations for the isotopic study of emerging contaminants. The developed tools provide new opportunities for tracing contaminant sources, quantifying degradation processes, and supporting future applications of stable isotope analysis in environmental monitoring and remediation.