Mobility of multiple heavy metalloids in contaminated soil under various redox conditions: Effects of iron sulfide presence and phosphate competition.

The mobility of heavy metalloids including As, Sb, Mo, W, and Cr in soil was investigated under both reducing and oxidizing conditions. The effects of soil mineralogy and the presence of competitive anions were studied as important factors affecting the mobility of these contaminants. Batch experiments conducted with the addition of oxidized and fresh FeS exhibited enhanced sorption rates for As and W under oxidizing conditions, and for Mo under reducing conditions. The inhibitory effect of phosphate on the sorption rates was most apparent for As and Mo under both oxidizing and reducing conditions, while only a small phosphate effect was observed for Sb and W. For Sb and W mobility, pH was determined to be the most important controlling factor. The results of long-term batch experiments revealed that differences in the mobility of metalloids, particularly As, were also influenced by microbial activity in the oxidizing and reducing conditions.

Density functional theory studies on the relative reactivity of chloroethenes on zerovalent iron.

The gas-phase dissociation of perchloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cis-DCE) on zerovalent iron Fe(110) was investigated using periodic density functional theory (DFT) with the generalized gradient approximation (GGA) and climbing image nudged elastic band method (CI-NEB). Activation energies and dechlorination rate constants for reductive beta-elimination of the chloroethene compounds were calculated using an Arrhenius equation with theoretically calculated vibrational frequencies for the compounds. Activation energies were found to decrease as the chlorination number increases. The reaction rate-limiting step for PCE dissociation occurs at the second chlorine cleavage, while the rate-limiting steps for TCE and cis-DCE occur at the first chlorine cleavage. 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. Energy profiles along the reaction coordinate for the dechlorination paths are presented. The relative gas-phase reactivity order among chlorothenes on Fe(110) was found to be PCE > TCE > cis-DCE. At room temperature (300 K), the PCE dechlorination rate is 14 and 338 times faster, respectively, than that of TCE and cis-DCE. Details regarding the electronic properties of the transition states of the dechlorinated compounds are reported.

Density functional theory studies of chloroethene adsorption on zerovalent iron.

Adsorption of perchloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cis-DCE) on zerovalent iron is investigated using density functional theory (DFT) to evaluate hypotheses concerning the relative reactivity of these compounds on zerovalent iron. Four different chloroethene adsorption modes on the Fe(110) surface were studied using periodic DFT and the generalized gradient approximation (GGA). Of the adsorption sites examined, the 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 provide an indication of the extent of sp2-sp3 hybridization. The strong hybridization of the pi-bonding orbital between the chloroethene C==C bond and the iron surface suggests that adsorbed chloroethenes are strongly activated on Fe(110) and are likely precursors for subsequent chloroethene dissociation on the Fe surface. When the effect of solvation is indirectly taken into account in the DFT simulations by considering the hydration energies of chloroethenes in bulkwater,the ordering ofthe adsorption energies of chloroethenes from the aqueous phase onto Fe(110) is in agreement with experimental observation (PCE > TCE > cis-DCE). Electronic properties of the adsorbed configurations of chloroethenes are also presented.