Depassivation of Aged Fe0 by Ferrous Ions: Implications to Contaminant Degradation

Investigation of the effects of ferrous iron (Fe(II)) on the ability of aged (iron oxide coated) Fe-0 to degrade trichloroethylene (TCE) has revealed that, while neither aged Fe-0 nor Fe(II) separately were able to degrade TCE, approximately 95% of the TCE present was degraded after exposure to a mixture of aged Fe-0 and Fe(II) for 21 days. The rates of TCE degradation increased with an increase in Fe(II) concentration from 0 to 1.6 mM and then reached a relative plateau. Results of Fe(II) "adsorption" studies revealed that the equilibrium pH decreased significantly with an increase in Fe(II) concentration. Proton release during adsorption of Fe(II) to iron oxide coatings was identified as being responsible for promotion of surface dissolution and, concomitantly, enhancement in extent of TCE reduction by aged Fe-0. Results of open circuit potential analysis and Tafel plot measurement showed that the corrosion potential of aged Fe-0 (E-corr) in the presence of Fe(II) decreased to levels similar to that of Fe-0/Fe2+, while significant increase in corrosion current (I-corr) and decrease in polarization resistance (R-p were found with an increase in Fe(II) concentration. The fact that the effects of different Fe(II) concentrations on the E-corr, I-corr, and R-p was decoupled from their effects on TCE degradation by aged Fe-0 suggested that the enhancement of TCE degradation in the presence of Fe(II) was attributable to the dissolution of the Fe(III) oxyhydroxide layer coating the aged Fe-0. While the presence of Fe(II) may also lead to transformation of the Fe(III) (oxy)hydroxide coating to more crystalline phases, the rate of reduction of compounds such as TCE by Fe(II) associated with the Fe(III) (oxy)hydroxide coating is substantially slower than that mediated by Fe-0. These findings provide new insight into the molecular-scale interaction of aged Fe-0 and ferrous iron with particular implications for sustaining the reactivity of Fe-0-mediated degradation of contaminants in iron-bearing environments.