(29e) Adsorption and Dissociation of Chloroethenes on Zero-Valent Iron

Zero-valent iron has been used with success to remove chlorinated solvent contaminants from drinking water supplies. Controversy exists regarding the dechlorination mechanisms for chloroethenes on the zero-valent iron (Fe(0)). Although it is generally assumed that the reactivity of chloroethene solvents on iron increases with the degree of chlorination, the opposite trend has been reported by several research groups. In this study, quantum mechanics-based simulation is used to elucidate the effect of chlorination on the relative reactivity of chloroethenes on zero-valent iron. The most stable adsorption configurations for perchloroethene, trichloroethene, and cis-dichloroethene at four adsorption sites on the Fe(110) surface were investigated using periodic density functional theory (DFT) and a generalized gradient approximation (GGA). The method yielded good agreement with experimental data for the physical properties of bulk iron. Among the possible adsorption sites, the atop site, where the C=C bond of chloroethenes sits atop a central Fe atom, was found to be the most favorable site for the adsorption of all chloroethene compounds considered in this study. Geometry and electronic property analyses of the adsorption configurations provide an indication of the extent of sp2-sp3 hybridization of the carbon atoms on the Fe surface. These analyses also confirm that a strong hybridization of π-bonding orbital occurs between the carbon p orbitals and the Fe d band. The dechlorination pathways of chloroethenes on the iron surface were also analyzed. The results indicate that chloroethenes with a lower number of chlorine atoms have higher activation energies than the more highly substituted congeners. The findings reported in this study should contribute to a better understanding and an improved design of zero-valent iron media for in situ and ex situ remediation of chloroethene-contaminated sites.