Legacy and Emerging Per- and Polyfluoroalkyl Substances in Sediments from a Subtropical Estuarine Ecosystem in Africa: Occurrence, Contributions of Unknown Precursors, and Ecological Risks

• First PFAS assessment of sediments in a subtropical estuary in Southeastern Africa • The predominant PFAS were PFOS and PFOA • TOP assay reveals elevated PFAS precursor amounts in Infulene and Matola estuaries • WWTP effluents and industrial discharges represent the main local sources of PFAS • The environmental risk of PFAS was not high in the Espírito Santo Estuary As legacy per- and polyfluoroalkyl substances (PFAS) are gradually phased out, emerging PFAS alternatives are increasingly being used in industry and consumer products. Despite concerns about their environmental persistence and ecological risks, data on both legacy and emerging PFAS in sensitive environments, such as estuarine habitats, remain scarce, especially in Africa. This study is the first to investigate the occurrence, sources, substitution trends, and ecological risks of 45 PFAS compounds, including 8 emerging PFAS, in sediment samples from the Espírito Santo Estuary (ESE), located on the Mozambican coast. The samples were analyzed using both targeted analysis and the Total Oxidizable Precursor (TOP) assay, enabling the detection of unknown precursors and providing a more comprehensive characterization of the PFAS load. Targeted analysis revealed a diverse array of PFAS compounds contributing to the contamination, with legacy compounds, particularly perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), frequently dominating the profiles. Total PFAS concentrations determined via the TOP assay ranged from 0.45 to 6.23 ng g -1 dry weight (dw), which were 1 to 29 times higher than the corresponding targeted PFAS concentrations (0.22 to 1.50 ng g -1 dw), indicating substantial variability in the contribution of oxidizable PFAS precursors. Most sites heavily contaminated with precursors showed low contamination levels in targeted analysis alone, underscoring the critical role of the TOP assay in achieving a more comprehensive assessment of PFAS contamination. Principal Component Analysis (PCA) and ratio analysis indicated that municipal and industrial discharges, along with domestic sewage, were likely the primary sources of PFAS in the Espírito Santo Estuary. Substitution trends suggested that legacy PFAS are still used in Mozambique, with 6:2 fluorotelomer sulfonic acid (6:2 FTSA) emerging as the predominant replacement for PFOS. Preliminary environmental risk assessment showed that PFOA and PFOS may present a low to medium risk to benthic organisms. Although the overall ecological risk in the estuary appeared low, the potential for bioaccumulation under long-term exposure and the synergistic toxicity of multiple pollutants cannot be ignored. Taken together, this study not only provides essential baseline data on PFAS contamination in a subtropical estuarine ecosystem, but also highlights the value of integrating the TOP assay into targeted analyses to more comprehensively evaluate the total PFAS burden, especially in areas influenced by point sources.