This letter details the early events in the reductive defluorination of perfluoroalkyl substances (PFASs) and presents a straightforward methodology to predict the reduction behavior of the perfluoroalkyl acids (PFAAs) using electronic structure calculations. Electron attachment to linear perfluorocarboxylic acids (PFxAs) generally occurs at the α-carbon and is energetically not correlated to chain length, contrary to the linear perfluoroalkane sulfonates (PFxSs) where electrons generally insert into other positions. Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), two widely studied and scrutinized PFAAs, are therefore predicted to be reduced through diverging pathways. Our protocol can predict the standard reduction potentials of PFAAs, provides a rational basis to probe reaction intermediates, establish free energy relationships, and accounts for PFASs’ inherent structural diversity beyond the linear substrates.

We have determined the identity of the complexes extracted into the ALSEP process solvent from solutions of nitric acid. The ALSEP process is a new solvent extraction separation designed to separate americium and curium from trivalent lanthanides in irradiated nuclear fuel. ALSEP employs a mixture of two extractants, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N,N,N′,N′-tetra(2-ethylhexyl)diglycolamide (TEHDGA) in n-dodecane, which makes it difficult to ascertain the nature of the extracted metal complexes. It is often asserted that the weak acid extractant HEH[EHP] does not participate in the extracted complex under ALSEP extraction conditions (2–4 M HNO₃). However, the analysis of the Am extraction equilibria, Nd absorption spectra, and Eu fluorescence emission spectra of metal-loaded organic phases argues for the participation of HEH[EHP] in the extracted complex despite the high acidity of the aqueous phases. The extracted complex was determined to contain fully protonated molecules of HEH[EHP] with an overall stoichiometry of M(TEHDGA)₂(HEH[EHP])₂·3NO₃. Computations also demonstrate that replacing one TEHDGA molecule with one (HEH[EHP])₂ dimer is likely energetically favorable compared to Eu(TEHDGA)₃·3NO₃, whether the HEH[EHP] dimer is monodentate or bidentate.