Depassivation of aged Fe0 by inorganic salts: implications to contaminant degradation in seawater.
In this study, aged (iron oxide coated) Fe(0) was applied to the degradation of trichloroethylene (TCE) in seawater. It was found that while the aged Fe(0) was inactive with regard to TCE degradation in Milli-Q water, more than 95% of the TCE present was degraded in real and synthetic seawater solutions after exposure to aged Fe(0) for 21 days. Results with individual salts from the synthetic seawater revealed that no significant TCE degradation was observed in the presence of Na2SO4, CaCl2, and NaHCO3. Partial TCE degradation (28.4%) was observed in 500 mM NaCl after 21 days, while a similar extent of degradation to that found in the seawater solutions was observed in 50 mM solutions of magnesium salts (MgCl2 and MgSO4). Results of open circuit potential analysis suggested that the Fe(0) corrosion potential was not a key determinant of extent of TCE reduction since the corrosion potential decreased to levels similar to that of Fe(0)/Fe(2+) in the presence of all salts examined. Lower final pH values and higher dissolved Fe(II) concentrations were observed in the presence of magnesium salts compared to other salts. Formation of the surface complex >FeOMg(+) was identified as being critical to protonation of surface sites, reductive dissolution of the passivating Fe(III) oxyhydroxide layer coating the underlying Fe(0) and enhancement in extent of TCE reduction. These findings provide insight into the molecular-scale mechanism of depassivation of aged Fe(0) by inorganic salts with particular implications for the Fe(0)-mediated degradation of contaminants in saline natural waters such as seawater and saline groundwaters.