Phototransformation of 6:2 fluorotelomer sulfonate promoted by Fe-montmorillonite: synergistic interfacial role of ligand-to-metal charge transfer and hydroxyl radicals.

The widespread detection and persistence of 6:2 fluorotelomer sulfonate (6:2 FTS), a common substitute for perfluorooctane sulfonate (PFOS), in aquatic environments have raised growing concerns regarding its environmental fate. As a crucial mineral in natural environments, montmorillonite is capable of mediating the transformation of various pollutants. In this study, we systematically compared the phototransformation of 6:2 FTS in systems containing Na-montmorillonite, Fe-montmorillonite, Ca-montmorillonite, and K-montmorillonite. Notably, near-complete transformation (99%) of 6:2 FTS was achieved within 6 h under light irradiation in the Fe-montmorillonite system, which significantly outperformed the transformation efficiencies observed in the other montmorillonite systems. No obvious transformation occurred in the dark control experiment, confirming that light irradiation is a necessary condition to trigger the reaction and highlighting the crucial role of photo-promotion. Irradiation promoted the leaching of interlayer iron from Fe-MMT to produce dissolved Fe3+, and the reoxidation of Fe2+ continuously replenished the pool of dissolved Fe3+ in the reaction system. Meanwhile, photoexcited Fe-MMT efficiently activated molecular oxygen to yield hydroxyl radicals (·OH). By utilizing electron paramagnetic resonance spectroscopy (EPR), quenching experiments, and Fukui function calculations, we clarified that under the combined effect of Fe3+ and ·OH, the transformation of 6:2 FTS mainly proceeds via two pathways: (1) photo-induced ligand-to-metal charge transfer (LMCT)-mediated cleavage of the C-S bond; (2) radical-mediated dehydrogenation and chain cleavage reactions. Critically, although the ·OH-initiated pathway dominates, the overall transformation rate of 6:2 FTS was markedly accelerated by the presence of Fe3+, demonstrating a pronounced synergistic effect. This study elucidates the intrinsic mechanism underlying the efficient transformation of 6:2 FTS into short-chain perfluorocarboxylic acids (PFCAs) in clay-rich aquatic environments. The findings demonstrate that under light irradiation, natural minerals such as Fe-montmorillonite can effectively facilitate the transformation of 6:2 FTS (a PFOS alternative), offering significant insights into the mineral-mediated attenuation of these emerging perfluorinated compounds.