Reduction of Polyfluorinated Compounds

Abstract

Reduction of perfluoroalkylphenylcyanides and perfluorodecalin was investigated by cyclic voltammetry and interpreted by intramolecular dissociative electron-transfer models, taking into account the dependence of the diffusion coefficient on the molecular volume of the substrate. In both cases, reduction is governed by the first electron transfer and the following chemical reaction corresponding to F- expulsion. In the case of perfluoroalkylphenylcyanides, at the first reduction stage, one electron is exchanged for each fluorinated carbon atom. For perfluorodecalin, the first stage involves a transfer of four electrons, leading to the formation of a diene, in agreement with literature data on the electrochemical behavior of fluorinated olefins. Comparison of the different parameters obtained (standard reduction potentials and cleavage rate constants) with existing thermodynamic values allows us to observe the effect of lowering the C−F bond dissociation energy on the cleavage rate constant when going from primary to tertiary fluorinated carbon atoms. Moreover, deviations from the existing model become more important when the number of electron-withdrawing groups increases, indicating interactions between the radical and fluoride ion fragments. In the special case of perfluorodecalin, the slowness of the electron transfer could be attributed to the nature of the σ* orbital that receives the extra electron.

Treatment technologies for aqueous perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA)

“…For example, PFOS can be reduced by elemental iron, Fe(0), in water under hightemperature and high-pressure conditions (e.g., 350°C, 20 MPa) [175]. Unsaturated per-and poly-fluoroorganics (i.e., those containing aromatic, benzylic, olefinic, and tertiary functional groups) will readily reductively defluorinate [176][177][178][179][180][181]. Fluoroorganics containing only secondary and primary C-F bonds are difficult to defluorinate due to low reduction potentials (E < -2.7 V) [182,183].…”

“…Subsequent eaq reductions with partially defluorinated intermediates (C n F 2n IX -, C n F 2n -1 X -, or C n F 2n HX -) should be faster than the initial defluorination step [180]. After partial reductive defluorination of PFOX, the ionic headgroup is cleaved.…”

Reductive degradation of perfluoroalkyl compounds with aquated electrons generated from iodide photolysis at 254 nm

“…This is consistent with the observed lower reduction potentials (i.e., values range from -0.16 to -1.36 V) for perfluoroolefins. 42 Combellas et al 43 investigated the effects of the carbon tail on the electrochemical reduction potentials of C4, C6, and C8 perfluoroalkylphenylcyanides, which clustered around E = -1.3 V, indicating no significant tail-length effects. Thus, the higher current yields observed during the electrochemical reduction of PFOS are likely due to further reduction of defluorinated intermediates.…”

Thermodynamic and kinetic characteristics of intermediates of electrode reactions: A comparative investigation of a number of alkylaryl and alkyl halide radicals by the laser photoemission methods

“…. possess, as is known [90], an elevated electrochemical activity as compared with their analogues containing no fluorine.…”