Sulfidation of (n)ZVI is relevant to in situ chemical reduction (ISCR) using (n)ZVI because the (n)ZVI creates subsurface conditions that are favorable microbial sulfate reduction. The resulting sulfide reacts with the oxides associated with ZVI to form iron sulfides, which may substantially alter the reactivity of the original (n)ZVI, and consequently affect long-term treatment performance. In the present study, we have used electrochemical impedance spectroscopy (EIS) to characterize sulfidation of nZVI. EIS has been widely applied to characterize solid-water interfacial redox processes in metal corrosion, the fundamental chemistry of which is similar to nZVI sulfidation. To perform EIS on these materials, a powder disk electrode (PDE) packed with nZVI was employed in a three-electrode system. The PDE was exposed to electrolyte containing various concentrations of sulfide under anaerobic conditions, and EIS data was collected periodically in order to characterize the kinetics. At all sulfide concentrations, there was an initial period where little change can be seen in the Nyquist and Bode plots, followed by a subsequent period that consistently shows a feature that evolved with increasing exposure time. The length of the initial lag phase decreased with increasing sulfide concentration, likely indicating that the two-phase transition was due to surface species transforming from adsorbed sulfide to iron sulfide precipitates. The kinetics of iron sulfide formation were extracted from EIS data of the second phase. Equivalent circuit modeling of the EIS data was performed to further assess the effects of sulfidation on Fe(0) reactivity. At low sulfide concentration, nZVI corrosion was inhibited, as suggested by significantly increasing charge transfer resistance as a function of exposure time, possibly due to the formation of an FeS barrier. At high sulfide concentration, however, the nZVI corrosion was much less inhibited, possibly due to localized corrosion induced by high sulfide concentration.

Sulfidation of nano-zerovalent iron (nZVI): Characterization by electrochemical impedance spectroscopy (EIS)

246th National Meeting (2013)

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