Effects of dry-wet alternation patterns on PFAS fate in sediments amended with microplastics

The accumulation of PFASs in sediments is influenced by both dry-wet alternation (ADW) patterns and microplastic (MP) pollution, yet their combined effects on PFAS fate in water-sediment systems remain unclear. In this study, we investigated PFAS accumulation in sediments under three ADW patterns (1ADW, 2ADW, 3ADW) and continuous submergence (CS), combined with or without polypropylene (PP) MPs (1% w/w). The results showed that, relative to CS, ADW generally inhibited the accumulation of PFOA & substitutes, whereas its effect on PFOS & substitutes was pattern-dependent, with 3ADW even showing elevated accumulation. For individual compounds, HFPO-TA and PFHxS varied most across ADW treatments; Gen-X and PFBS least. MPs presence altered this trend: MPs promoted PFAS accumulation under ADW (with the highest increment of 44.68 μg/kg for PFOS & substitutes under 1ADW), but exhibited inhibition (for PFOA & substitutes) or weak (for PFOS & substitutes) effects under CS, indicating that the direction of MP-mediated regulation is highly dependent on hydrological conditions. Based on sediment physicochemical property differences and multiple linear regression (MLR) model results, compared to CS, ADW primarily inhibited PFAS accumulation by disrupting sediment flocculent structures and altering the role of DOM, whereas MPs presence enhanced PFAS retention by intensifying surface fragmentation, modulating key parameters (e.g., Zeta potential, BIX, and DOC), and serving as an additional sorption phase, thereby altering the original inhibition pattern imposed by ADW.