Molecular mechanisms of per- and polyfluoroalkyl substances (PFASs) removal from sludge alkaline fermentation liquid by coagulation: Role of interactions between DOM and PFASs

Per- and polyfluoroalkyl substances (PFASs), as highly recalcitrant pollutants, tend to accumulate in sludge, posing challenges for subsequent treatment and resource recovery. Sludge alkaline fermentation (SAF) produces sludge alkaline fermentation liquid (SAFL) with potential for circular-economy reuse, while simultaneously transferring PFASs from the solid phase into SAFL, thereby creating a trade-off between resource recovery and contamination. Coagulation is a critical step in SAFL separation, yet the fate of PFASs during this process remains poorly understood. This study evaluated the performance of polyaluminum chloride (PAC) and polyferric sulfate (PFS) in removing nine representative legacy and emerging PFASs from SAFL. High removal efficiencies were achieved across PFASs with varying chain lengths and headgroups, with long-chain PFASs (C ≥ 8) reaching up to 74% and emerging short-chain PFASs (C < 8) up to 55%. Spectroscopic analysis indicated that the hydrophobic interaction between PFASs and the Al/Fe based hydrolysates were the primary driving force for the direct removal of PFASs by coagulants. Dissolved organic matter (DOM) in SAFL played a facilitating role in PFASs removal, with proteins identified as the key contributors. DOM analysis revealed that additional coagulants facilitated the removal of protein-like substance and hydrophobic macromolecules that have strong affinities for PFASs. Molecular dynamics simulations suggested that Al/Fe-based hydrolysates increased electrostatic attraction and binding free energy, thereby increasing protein compactness and promoting co-precipitation removal of PFASs. These findings offer valuable insights for optimizing coagulation strategies in practical applications to effectively mitigate ecological risks in DOM-rich water contaminated with PFASs.