Reduction of Polychlorinated Ethanes and Carbon Tetrachloride by Structural Fe(II) in Smectites

Abstract

Ferrous iron associated with clay minerals can be important for the reductive transformation of organic contaminants in anoxic soils and groundwaters. We investigated the reactivity of structural Fe(II) in ferruginous smectite for the reduction of a series of polychlorinated alkanes (hexa-, penta-, 1,1,1,2-and 1,1,2,2-tetrachloroethane, and carbon tetrachloride (CCl4)) in laboratory batch reactors. Evaluation of reaction kinetics, product distribution, and C-isotope fractionation suggest that polychlorinated ethanes containing three alpha-Cl atoms reacted via reductive beta-elimination to the corresponding ethenes while CCl4-reduction leads predominantly to the formation of chloroform. Reduction kinetics followed a typical biphasic behavior characteristic of the presence of two types of Fe(II) species exhibiting different reactivity in the octahedral sheet of smectites, and reaction rate constants were pH-independent. Dehydrochlorination reactions of chloroethanes containing at least one beta-H atom were found to compete with or even dominate over the reduction reaction with increasing suspension pH. Reference experiments in homogeneous solution and with non-reduced smectite performed in the pH range of 5.5-8.5 suggest that the HCl-elimination is not catalyzed at mineral surfaces. From the observed slow transformation of chloroalkanes, we hypothesize that structural Fe(II) in smectites will be important mainly as a reductant in the subsurface once iron(hydr)oxides have been reductively dissolved.

Carbon and chlorine isotopologue fractionation of chlorinated hydrocarbons during diffusion in water and low permeability sediments

“…This proportionality implies no information loss when considering only one of the two proportional isotopologue pairs. Due to the low abundance of the heavy carbon isotope ( 13 C 1.07% vs. 12 C 98.93%), in the present study only the most abundant TCE and 1,2-DCA carbon isotopologue pairs containing one and zero heavy carbon isotope, respectively ( 131 TCE/ 130 TCE and 101 1,2DCA/ 100 1,2DCA) are considered, which is coherent with previous studies (Elsner and Hunkeler, 2008;Neumann et al, 2009;Jin et al, 2013).…”

Tc(VII) reduction kinetics by titanomagnetite (Fe3−xTixO4) nanoparticles

“…Attribution of faster observed Tc(VII) reduction rates to an increase in structural Fe(II)/Fe(III) ratio is similar to that reported in previous contaminant reduction studies Neumann et al, 2008Neumann et al, , 2009, and in electrochemical parlance can be likened to decreasing the open circuit potential (E OCP ) built into the solid. For example, a systematic relationship between redox reactivity and stoichiometry was established for the magnetite-maghemite solid solution series Neumann et al, 2008Neumann et al, , 2009.…”

“…For example, a systematic relationship between redox reactivity and stoichiometry was established for the magnetite-maghemite solid solution series Neumann et al, 2008Neumann et al, , 2009. In addition to increasing relative amounts of Fe(II) and decreasing E OCP , localization of reducing equivalents to the surface of Fe 3Àx Ti x O 4 nanoparticles increases their accessibility.…”

Evaluation of redox-active iron sites in smectites using middle and near infrared spectroscopy

“…While microbes can reduce a large share of the structural Fe in phyllosilicates (Kostka et al, 1999b), most studies concerned with Fe redox reactivity addressed Fe(II) oxidation and concomitant reduction of contaminants, that is nitroaromatic compounds (Hofstetter et al, 2003(Hofstetter et al, , 2006Neumann et al, 2008), polychlorinated ethanes (Neumann et al, 2009), uranium (Ilton et al, 2004), technetium (Peretyazhko et al, 2008). Few general trends describing Fe reactivity have been established to date, for example, that rates of contaminant reduction were almost pH independent over a narrow, ambient range (pH 6.5-8).…”