Zerovalent iron nanoparticles (Fe(0) NPs or nZVI) synthesized by reductive precipitation in aqueous solution (Fe/FeO) differ in composition and reactivity from the NPs obtained by reductive precipitation in the presence of a S-source such as dithionite (Fe/FeS). To compare the redox properties of these types of NPs under a range of environmentally relevant solution conditions, stationary powder disk electrodes (PDEs) made from Fe/FeO and Fe/FeS were characterized using a series of complementary electrochemical techniques: open-circuit chronopotentiometry (CP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV). The passive films on these materials equilibrate within minutes of first immersion and do not show further breakdown until >1 day of exposure. During this period, the potentials and currents measured by LPR and LSV suggest that Fe/FeS undergoes more rapid corrosion and is more strongly influence by solution chemical conditions than Fe/FeO. Chloride containing media were strongly activating and natural organic matter (NOM) was mildly passivating for both materials. These effects were also seen in the impedance data obtained by EIS, and equivalent circuit modeling of the electrodes composed of these powders suggested that the higher reactivity of Fe/FeS is due to greater abundance of defects in its passive film.

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Nano-zerovalent iron (nZVI) formed under sulfidic conditions results in a biphasic material (Fe/FeS) that reduces trichloroethene (TCE) more rapidly than nZVI associated only with iron oxides (Fe/FeO). Exposing Fe/FeS to dissolved metals (Pd(2+), Cu(2+), Ni(2+), Co(2+), and Mn(2+)) results in their sequestration by coprecipitation as dopants into FeS and FeO and/or by electroless precipitation as zerovalent metals that are hydrogenation catalysts. Using TCE reduction rates to probe the effect of metal amendments on the reactivity of Fe/FeS, it was found that Mn(2+) and Cu(2+) decreased TCE reduction rates, while Pd(2+), Co(2+), and Ni(2+) increased them. Electrochemical characterization of metal-amended Fe/FeS showed that aging caused passivation by growth of FeO and FeS phases and poisoning of catalytic metal deposits by sulfide. Correlation of rate constants for TCE reduction (kobs) with electrochemical parameters (corrosion potentials and currents, Tafel slopes, and polarization resistance) and descriptors of hydrogen activation by metals (exchange current density for hydrogen reduction and enthalpy of solution into metals) showed the controlling process changed with aging. For fresh Fe/FeS, kobs was best described by the exchange current density for activation of hydrogen, whereas kobs for aged Fe/FeS correlated with electrochemical descriptors of electron transfer.

The corrosion inhibition properties of 2-methylimidazoline (MIMD) and N-methyl-2-methylimidazoline (MMIMD) were studied by density functional theory (DFT) and electrochemical methods. The interaction of imidazoline molecules with the iron atoms was studied by DFT as a model for a corrosion inhibition process. DFT results explain why the imidazoline moieties favor the perpendicular adsorption, while their protonated species adsorb in parallel position over the metal surface. The electrochemical results show that although both perform as good inhibitors, MMIMD is better than MIMD because the former contains a methyl substituent in the ring which facilitates a strong adsorption on the metal surface. Furthermore, the delocalization (N1C2N3) region of the compounds that are involved in the corrosion inhibition process was analyzed by theoretical calculation, showing that the higher occupied molecular orbitals of the compounds have adequate symmetry to interact with the lower unoccupied molecular orbital of the metal. This theoretical prediction is in good agreement with the electrochemical results that MMIMD has a greater effect on corrosion inhibition than MIMD, because the methyl group in the imidazoline ring considerably increases the formation of the inhibitor film over the metallic surface and as a result a low Gibbs free energy value was obtained for the MMIMD film.