Depassivation of Aged Fe0 by Divalent Cations: Correlation between Contaminant Degradation and Surface Complexation Constants

The dechlorination of trichloroethylene (TCE) by aged Fe-0 in the presence of a series of divalent cations was investigated with the result that while no significant degradation of TCE was observed in Milli-Q water or in solutions of Ba2+, Sr2+, or Ca2+, very effective TCE removal was observed in solutions containing Mg2+, Mn2+, Co-2+,Co- Fe2+, Ni2+, Zn2+, Cu2+, or Pb2+. The rate constants of TCE removal in the presence of particular cations were positively correlated to the log K representing the affinity of the cations for hydrous ferric oxide (HFO) surface sites though the treatments with Co2+ and Ni2+ were found to provide particularly strong enhancement in TCE degradation rate. The extent of Fe(II) release to solution also increased with increase in log K, while the solution pH from both experimental measurement and thermodynamic calculation decreased with increasing log K. While the peak areas of Fe and O XPS spectra of the passivated ZVI in the presence of Ba2+, Sr2+, and Ca2+ were very close to those in Milli-Q water, very significant increases in surface Fe and O (and OH) were observed in solutions of Mg2+, Mn2+, Co2+, Fe2+, Ni2+, Zn2+, Cu2+ and Pb2+, revealing that the surface oxide layer dissolution is consistent with the recovery of aged Fe-0 with respect to TCE degradation. The depassivation process is proposed to involve (i) surface complexation of cations on surface coatings of aged Fe0, (ii) dissolution of the hydrated surface as a consequence of magnetite exposure, and (iii) transport of electrons from underlying Fe-0 via magnetite to TCE, resulting in TCE dechlorination and, for some cations (Co2+, Ni2+, Cu2+, and Pb2+), reduction to their zero or (+1) valence state (with potential for these reduced metals to enhance TCE degradation).