Chain-length-dependent effects of diisopropylamine on phase transfer and clay retention of perfluoroalkyl substances.
Co-contaminants can alter per- and polyfluoroalkyl substance (PFAS) mobility by modifying aqueous association, phase transfer, and retention by aquifer solids, yet how protonated amines influence these processes remains poorly constrained. Here, we investigated interactions between diisopropylamine (DIPA) and two perfluoroalkyl carboxylates with contrasting chain lengths, trifluoroacetate (TFA) and perfluorooctanoate (PFOA), using bottle tests, diffusion-ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR), retention experiments with montmorillonite, and conformer-ensemble COSMO-RS and qualitative molecular-mechanics cluster calculations. At 20-25 °C and 0.1 wt% per solute, aqueous DIPA/TFA mixtures remained optically clear, and DOSY NMR showed no convergence toward a common diffusion coefficient, consistent with weak and/or fast-exchanging association in bulk water rather than a dominant long-lived contact species. BP86/TZP COSMO-RS calculations for independently solvated ions predicted greater aqueous compatibility for DIPA+/TFA- than for DIPA+/PFOA- and a demixing tendency for the PFOA-containing conformer ensemble. In contrast, higher equimolar DIPA/PFOA loadings produced flocs or precipitates; DIPA/PFOS mixtures also precipitated. Montmorillonite retention experiments showed strong removal of DIPA, limited retention of TFA with or without DIPA, and measurable co-retention of PFOA in the presence of DIPA. These results show that favorable PFAS-amine interactions in bulk water do not necessarily translate into co-retention or reduced mobility. Instead, PFAS chain length and interfacial interactions govern whether association remains labile, as for TFA, or promotes phase transfer and mineral co-retention, as for PFOA. Thus, amine co-contaminants can differentially affect PFAS migration in clay-rich subsurface environments.