Per- and polyfluoroalkyl substances (PFAS) in urbanized coastal zones: spatiotemporal distribution, phase partitioning dynamics, and risk assessment in Shenzhen, China

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants of global concern. This study investigated the occurrence of 29 PFAS compounds across dissolved phase, suspended particulate matter (SPM), and surface sediment in four coastal zones of Shenzhen, China-a megacity undergoing intensive development. Results revealed widespread PFAS contamination, with total concentrations ranging from 4.306-53.716 ng/L (dissolved phase), 0.30-22.259 ng/L (SPM), and 0.561-20.354 ng/g (sediment). Short-chain perfluoroalkyl carboxylates (PFCAs, C4-C6) dominated aqueous phases (58.5% of Σ₂₉PFAS), while long-chain PFCAs (≥C7) prevailed in SPM (68.1%) and sediments (69.4 %), reflecting chain-length-dependent partitioning. Spatial analysis identified the Pearl River Estuary (PRE) as a PFAS hotspot, with levels 2.5-3.5 times higher than adjacent bays, probably linked to fluvial inputs from industrial and urban sources. Historical data for the PRE further revealed contrasting temporal trends: PFOS concentrations declined markedly from 33.91 ng/L in 2012 to 2.06 ng/L in 2020, consistent with global phase-out measures, whereas PFOA levels peaked in 2020 (27.73 ng/L) before decreasing to 1.88 ng/L in 2023, likely reflecting its initial use as a PFOS substitute and subsequent regulatory control. Sediment-water partitioning coefficients (log K_d) increased by 0.33-0.35 log units per CF₂ moiety, highlighting hydrophobic interactions as the key driver. Ecological risk assessment indicated low-to-moderate risks (RQs: 0.003-0.155), with PFOS in sediments near airports and industrial zones of PRE posing localized concerns. As the first comprehensive and systematic study on PFAS in the Shenzhen coastal area, this work provides novel insights into the distribution, partitioning behavior, and ecological risks of these persistent environmental contaminants. The findings underscore the need for source-specific management strategies to mitigate PFAS impacts in coastal urban environments.