Earthworm-mediated partitioning of PFOA and PFOS among solid, dissolved organic, and biological phases during sludge...

Per- and polyfluoroalkyl substances (PFAS), as persistent pollutants in dewatered sludge, pose considerable environmental and health risks during final disposal. Vermicomposting is a green and low-carbon technology for sludge recovery. However, the risks of PFAS accumulation and their phase-specific migration during the vermicomposting process remain unclear. This study conducted a vermicomposting using Eisenia fetida alongside an earthworm free control to elucidate PFAS concentration dynamics and phase partitioning among solid, dissolved, and biological phases. Results showed that perfluorooctanoic acid (PFOA) concentrations surged by 933.2% over 20 days in the control treatment, whereas vermicomposting suppressed this accumulation, reducing PFOA levels by 78.31%. In contrast, perfluorooctane sulfonate (PFOS) concentrations increased in vermicompost by 26.48%, relative to the control. Phase partitioning analysis revealed pronounced mobilization of PFOA into the DOM phase, with the DOM-associated fraction increasing by 79.22% in the control. However, this process was markedly attenuated during vermicomposting, where the increase was limited to only 26.1%. Correspondingly, PFOA demonstrated substantial bioaccumulation in earthworm intestinal tissues, with a biota-to-sludge accumulation factor (BSAF) of 6.13 ± 0.42, particularly in the foregut. Conversely, PFOS showed negligible bioaccumulation (BSAF < 0.1). Microbial community succession was significantly coupled with PFAS partitioning (M² = 0.5145, P < 0.001). Earthworm-driven alterations in pH, ionic strength, and nitrogen transformation collectively constrained PFOA mobilization, whereas PFOS partitioning was governed primarily by intrinsic solid-phase affinity (>75%). Overall, vermicomposting immobilized PFOA through combined biotic uptake and earthworm-driven environmental regulation, while exerting limited influence on PFOS partitioning.