Sorption dynamics of six per- and polyfluoroalkyl substances in estuarine sediments: The role of particle size distribution and salinity in low organic carbon sediments

Per- and polyfluoroalkyl substances (PFAS) are a family of synthetic organofluorine compounds; many are widespread environmental contaminants. Estuarine sediments can serve as both long-term sinks, and secondary sources for PFAS, by potentially removing them from the water column through sorption, and potential remobilisation when water column chemical conditions change. Understanding sorption is therefore important for informing the environmental management of PFAS. Batch experiments were used to evaluate the sorption of six PFAS in three separate marine sediments with low total organic carbon (TOC) and varying particle size distributions. Experiments were performed at six salinities, ranging from 0 to 70 g/kg. Partition coefficients (K_d), normalised organic carbon to water partition coefficients (K_oc), and change in PFAS concentrations over time were calculated for each compound in each sediment and salinity. Sorption was generally minimal in these low TOC sediments, but sediments with a higher relative clay and fine particle content exhibited greater sorption. PFOS had the highest K_d, followed by PFHxS, PFOA, PFHxA, PFBS, and PFBA, reinforcing the importance of chain length. PFAS sorption in low TOC sediments appears to be mediated by the compounds. Longer -CF₂ chain PFAS were more likely to sorb to sediment under hydrophobic interactions, while the extent of influence of TOC and particle size will vary by sediment. While salinity is known to influence PFAS sediment sorption at the freshwater-saltwater interface, particularly at lower salinity ranges (<20 g/kg), our results suggest a modest effect across the higher salinity range (20-70 g/kg), with compound-specific variability.