Density Functional Theory Studies on the Relative Reactivity of Chloroethenes on Zerovalent Iron.

Abstract

This study investigated the adsorption and dissociation of perchloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cis-DCE) on zerovalent iron. The mechanisms by which iron decomposes chlorinated solvents by catalytic cleavage of the carbon-chlorine bond, are as not yet well understood. To develop process models for the optimal design of in situ and ex situ zerovalent iron treatment systems for the removal of chlorinated solvents from drinking water supplies, it is important to understand these mechanisms, and in particular how the degree of chlorination of the contaminant affects its reactivity on the iron surface. Periodic density functional theory (DFT) and the generalized gradient approximation (GGA) were used to determine the most thermodynamically favorable site on Fe(110) for the adsorption of all three chloroethenes. Climbing image nudged elastic band (CI-NEB) method with the periodic DFT and the GGA was employed to calculate activation energies of the chloroethene compounds according to the principal dechlorination mechanism of reductive β-elimination. The dechlorination rate constants of the chloroethenes were estimated using an Arrhenius equation with theoretically calculated vibrational frequencies of the compounds. Of the adsorption sites examined, an atop site, where the chloroethene C=C bond straddles a surface iron atom, was the most energetically favorable site for the adsorption of all three chloroethenes. Electronic structure and property analyses demonstrate the strong hybridization of the π-bonding orbital between the chloroethene C=C bond and the iron surface suggesting that adsorbed chloroethenes are strongly activated on Fe(110) and are likely precursors for subsequent chloroethene dissociation on the Fe surface. Taking into account the effect of solvation indirectly, the ordering of the adsorption energies of chloroethenes from the aqueous phase onto Fe(110) is in agreement with experimental observation (PCE > TCE > cis-DCE). Chloroethenes with a higher number of chlorine atoms have lower activation energies than those with fewer number of chlorine atoms. The activation energies of PCE, TCE, and cis-DCE at their rate-limiting steps are 9.9, 16.6, and 23.8 kJ/mol, respectively. At room temperature (300 K), for example, the dechlorination rate of PCE is 14 times and 338 times faster than those of TCE and cis-DCE, respectively.