Longer-chain PFCAs generate more harmful disinfection by-products in aged pipes.

Per- and polyfluoroalkyl substances (PFAS) inevitably enter drinking water distribution systems (DWDS) and may interact with corrosion-derived iron particles to affect the formation of disinfection by-products (DBPs). This study investigated the influence of perfluorocarboxylic acids (PFCAs) with varying chain lengths (C2-C8) on DBP formation in unlined cast iron pipes. Results obtained from both pipe and beaker experiments showed that increasing PFCA chain length led to increasing abundance and diversity of chlorinated DBPs, including a marked rise in low-molecular-weight species. The proportion of monochlorinated DBPs increased from 21.3% (C2) to 30.5% (C8), while trichlorinated DBPs exhibited substantial enrichment in long-chain PFCAs. At environmentally relevant PFCA concentrations in drinking water (∼100 ng/L), trihalomethanes remained relatively unchanged, but haloacetic acids (especially the highly toxic trichloroacetic acid) increased significantly with chain length. Theoretical calculations confirmed that carboxylate groups in PFCAs bound iron species to generate complexes; long-chain C-F moieties stabilized transition states via σ-π conjugation, which enhanced electron transfer, promoted Fe(II)/Fe(III) cycling and •OH generation, and thus accelerated DBP formation. This study reveals that long-chain PFCAs increase water quality risks by increasing DBP formation in iron pipes.