Electrochemical Oxidation of Hexafluoropropylene Oxide Dimer Acid (GenX): Mechanistic Insights and Efficient Treatment Train with Nanofiltration.

Hexafluoropropylene oxide dimer acid (HFPO-DA, trade name GenX) is a perfluoroalkyl ether carboxylic acid (PFECA) that has been detected in watersheds around the world. Similar to other per- and polyfluoroalkyl substances (PFASs), few processes are able to break HFPO-DA's persistent carbon-fluorine bonds. This study provides both experimental and computational lines of evidence for HFPO-DA mineralization during electrochemical oxidation at a boron-doped diamond anode with a low potential for the generation of stable organofluorine intermediates. Our density functional theory calculations consider the major operative mechanism, direct electron transfer, throughout the entire pathway. Initial oxidative attack does not break the ether bond, but leads to stepwise mineralization of the acidic side chain. Our mechanistic investigations reveal that hydroxyl radicals are unreactive toward HFPO-DA, while electrochemically activated sulfate facilitates its oxidation. Furthermore, we demonstrate that an NF90 membrane is capable of removing 99.5% of HFPO-DA from contaminated water. Electrochemical treatment of the nanofiltration rejectate is shown to reduce both energy and electrode costs by more than 1 order of magnitude compared to direct electrochemical treatment of the raw water. Overall, a nanofiltration-electrochemical oxidation treatment train is a sustainable destructive approach for the cost-effective elimination of HFPO-DA and other PFASs from contaminated water.

Differential ionic permeation of DNA-modified electrodes.

Ionic permselectivity of DNA films has been investigated by the analysis of the electrochemical response of methylene blue (MB) as a function of pH and ionic strength on DNA-modified electrodes in aqueous p-nitrophenol (p-NP) and phosphate buffers. We have observed a linear Pourbaix diagram in p-NP buffer indicating that the reduction of MB occurs with a two-electron plus one-proton reaction. Interestingly, in phosphate buffer the Pourbaix diagram is curved and this suggests that the thermodynamics of MB incorporated in the film depend also on the ratio of mono- versus divalent anions in the bulk. This result indicates that DNA films do not behave as pure ion-exclusion films, but instead there is a differential permselectivity that depends on the identity of the anions. Based on this consideration of the ionic distribution in the films, we provide a new method for the analysis of the DNA surface coverage based on AC impedance of an anionic species, ferricyanide. The methodology is of particular value in analyzing DNA hybridization and dehybridization. This approach presents an advantage compared to standard ruthenium hexammine assays since our methodology is insensitive to film morphology, and is highly sensitive to the amount of negative charge on the surface.

In situ scanning tunneling microscopy of DNA-modified gold surfaces: bias and mismatch dependence.

In situ scanning tunneling microscopy has been performed on DNA-modified gold surfaces under physiological conditions. The STM images of DNA-modified gold surfaces are strongly dependent on the applied potential and percentage of DNA duplexes containing a single base mismatch. At negative surface potentials we observe reproducible features that are attributed to DNA agglomerates where the DNA duplexes are in the upright orientation; at positive potentials, when DNA molecules lie down on the surface, the film is transparent, and only the gold surface is distinguishable. These observations indicate that DNA possesses a non-negligible local density of states which can be probed when the DNA duplex is in the upright orientation. By varying the percentage of DNA duplexes containing a single base mismatch, we have observed a dramatic change in the image contrast as a result of the perturbation induced by the mismatch on the electronic pathway inside the DNA. These results emphasize the central role of the integrity of the pi-stack for DNA charge transport. Duplex DNA is a promising candidate in molecular electronics, but only in arrangements where the orbitals can efficiently overlap with the electronic states of the electrodes and the environment does not constrain the DNA in non-native, poorly stacked conformations.