Effects of metal ions on the reactivity and corrosion electrochemistry of Fe/FeS nanoparticles

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 (Pd²⁺, Cu²⁺, Ni²⁺, Co²⁺, and Mn²⁺) 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 Mn²⁺ and Cu²⁺ decreased TCE reduction rates, while Pd²⁺, Co²⁺, and Ni²⁺ 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 (k_obs) 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, k_obs was best described by the exchange current density for activation of hydrogen, whereas k_obs for aged Fe/FeS correlated with electrochemical descriptors of electron transfer.