Understanding Trichloroethylene Chemisorption to Iron Surfaces Using Density Functional Theory

Abstract

This research investigated the thermodynamic favorability and resulting structures for chemical adsorption of trichloroethylene (TCE) to metallic iron using periodic density functional theory (DFT). Three initial TCE positions having the plane defined by HCC atoms parallel to the iron surface resulted in formation of three different chemisorption complexes between carbon atoms in TCE and the iron surface. The Cl-bridge initial configuration with the HCC plane of the TCE molecule perpendicular to the iron surface did not result in C-Fe bond formation. The most energetically favorable complex formed at the C-bridge site where the initial configuration had the C=C bond in TCE at a bridge site between adjacent iron atoms. In the C-bridge complex one C atom formed two sigma bonds to different Fe atoms while the second C atom formed a sigma bond with a second Fe atom. Surface complexation at the C-bridge site resulted in scission of all three C-Cl bonds, and also resulted in a shortening of the C=C bond to a distance intermediate between a double and a triple bond. Initial configurations with the C=C bond adsorbed at top or hollow sites on the iron surface resulted in formation of C-Fe sigma bonds between a single C and two adjacent Fe atoms, and the scission of only two C-Cl bonds. Bond angles and bond lengths indicated that there were no changes in bond order of the C=C bond for top and hollow adsorption. Chemisorption at the C-bridge site had an early transition state in which all three C-Cl bonds were activated from ~1.7 to ~2.2 Å, with an activation energy of 49 kJ/mol. The early transition state and the loss of all three Cl atoms upon chemisorption are consistent with most experimental observations that TCE undergoes complete dechlorination in one interaction with the iron surface. The absence of chemisorption and scission of only two C-Cl bonds at the Cl-bridge site is consistent with experimental observations that trace amounts of chloroacetylene may also be produced from reactions of TCE with iron.

Abiotic degradation of chlorinated ethanes and ethenes in water

“…Chlorinated ethenes are adsorbed on the iron surface, and sorption is the rate controlling step (Janda et al 2004). The saturation of active sites on an iron surface results in the increase of reaction half-lives and deviation from a first-order reaction (Farrel et al 2000) The weak character of sorption of chlorinated ethanes and ethenes can result in a short residence time on the metal surface and desorption of incompletely dechlorinated compounds (Zhang et al 2008). Therefore, minor amounts of less chlorinated ethylene may be formed during reductive dechlorination on metal surfaces (Orth and Gillham 1996).…”

Effect of SO on 1,1,1‐Trichloroethane Degradation by Fe0 in Aqueous Solution

“…At initial pH 6.0, the concentration of Cl − increased linearly with time to 0.036 mM (Figure 2), indicating TCA was degraded rather than adsorbed onto Fe 0 surface (Deng et al 2003; Zhang et al 2008; Lim et al 2009). As the degradation of TCA proceeded, the soluble iron ions increased quickly before 180 min reaction time and then reached 0.045 mM gradually, implying that the dissolution of Fe 0 is responsible for TCA degradation.…”

“…The reported half-lives of TCA for neutral hydrolysis at room-temperature are between 180 d and 2.8 years (Jeffers et al 1989), implying the neutral hydrolysis can be neglected during such a relatively short reaction time (420 min). At initial pH 6.0, the concentration of Cl − increased linearly with time to 0.036 mM (Figure 2), indicating TCA was degraded rather than adsorbed onto Fe 0 surface (Deng et al 2003;Zhang et al 2008;Lim et al 2009). As the degradation of TCA proceeded, the soluble iron ions increased quickly before 180 min reaction time and then reached 0.045 mM gradually, implying that the dissolution of Fe 0 is responsible for TCA degradation.…”

Understanding pH effects on trichloroethylene and perchloroethylene adsorption to iron in permeable reactive barriers for groundwater remediation

“…Although reaction rates and chemical intermediates have been measured for most commonly encountered chlorinated solvents, the reaction mechanisms themselves have not been fully resolved. These reaction mechanisms may involve either physical (Noubactep, 2011) or chemical adsorption (Zhang, et al ., 2008), and may occur via outer-sphere electron tunneling, inner-sphere electron transfer, or via reduction by atomic hydrogen produced from proton reduction (Matheson and Tratnyek, 1994; Li and Farrell, 2001; Jiao, et al ., 2009; Noubactep, 2010; Ghauch, et al ., 2011). …”