Surface chemical reactivity in selected zero-valent iron samples used in groundwater remediation

Permeable iron barriers have become a popular choice as a passive, cost-effective in situ remediation technology for chlorinated solvents. However, loss of reactivity over time, due to a build up of corrosion products or other precipitates on the iron surface, is a great concern. Because first-order rate constants for trichloroethylene (TCE) degradation have differed by iron pre-treatment and sonication history, X-ray photoelectron spectroscopy (XPS) was used to explore the changes in near surface chemistry of several iron samples. Both sonicated and unsonicated filings were analyzed in unwashed and groundwater-soaked conditions. Unsonicated acid-washed iron, with the highest first-order rate constant for TCE degradation, was characterized by greater surface oxygen content and was more ionic relative to the unwashed samples. The unsonicated, unwashed sample, with the lowest rate constant, exhibited a mixture of nonstoichiometric iron oxide and oxyhydroxide species. Sonication of groundwater-soaked iron removed weakly bonded iron hydroxide species and decreased the ionic character of the surface as was observed in the unwashed samples. Thus, this type of study might provide a better understanding of the chemical reactivity of selected iron samples and design better material in remediation technology.