Reductive Dechlorination of Carbon Tetrachloride Using Iron(II) Iron(III) Hydroxide Sulfate (Green Rust)

Abstract

The reductive dechlorination of CCl4 and CHCl3 in the presence of the synthetic sulfate form of green rust (GRSO 4 ), FeII 4FeIII 2(OH)12SO4 yH2O, at pH ∼ 8 and room temperature was investigated. Reduction of CCl4 produces CHCl3 and C2Cl6 as main chloroaliphatic products, while GRSO 4 is oxidized to magnetite (Fe3O4). The formation of C2Cl6 indicates a coupling reaction between trichloromethyl radicals in the suspension. Chloroform was much less susceptible than CCl4 to reductive dechlorination by GRSO 4 showing reduction rates approximately 100 times less than for reduction of CCl4. The transformation of CCl4 by GRSO 4 can be described by pseudo-first-order reaction kinetics with respect to formation of chloride. At room temperature the rate expression is given as: d[Cl-]/dt ≅ −d[CCl4]/dt = r·k obs[Fe(II)]GR, where k obs is in the range (0.47 × 10-5)−(2.18 × 10-5) s-1 for CCl4 concentrations above its aqueous solubility. This narrow range may be due to the constant CCl4(aq) concentration owing to buffering of the CCl4(aq) concentration by free phase CCl4(l) thereby indicating that the reaction takes place in solution. Experiments with initial CCl4 concentrations below its aqueous solubility support this theory. The reaction kinetics are compared with similar reactions where iron(0) is used as reductant of CCl4. The first-order rate constants for transformation of CCl4 with zerovalent iron and GRSO 4 , respectively, are found to be in the same range. Thus, GRs formed during corrosion of iron(0) under nonacid conditions may considerably contribute to the total reduction of CCl4 measured in iron(0) systems.

The effect of silica on the degradation of organohalides in granular iron columns

“…This may in part result from formation of a protective layer, consisting of a hydrated silica gel/metal oxide crust that forms a physical barrier between the iron and the contaminants. (Klausen et al, 1995;Sivavec and Horney, 1997;Erbs et al, 1999;Williams and Scherer, 2001;Lee and Batchelor, 2002a,b;O'Loughlin et al, 2003). The presence of dissolved silica species has been shown (Anderson and Benjamin, 1985;Hansen et al, 1994;Mayer and Jarrell, 1996) to alter the types of iron (hydr)oxide phases that may form.…”

Incorporation of Radioactive Contaminants into Pyroaurite-Like Phases by Electrochemical Synthesis

“…Pyroaurite-type mineral precipitation under reducing conditions may act as a sink for certain heavy metals and phosphate by their incorporation into the hydroxide layers and interlayers, respectively (Myneni et al, 1997). In anoxic soils and sediments, pyroauritetype minerals may also reduce nitrite or nitrate to ammonium and dechlorinate chlorinated organic compounds, at rates similar to or exceeding microbial reduction rates (Hansen et al, 1994Erbs et al, 1999). Thus, pyroaurite-type phases are considered a possible important reductant for nitrate and heavy metals in subsoils, sediments, or aquifers where microbially mediated reduction rates are slow.…”

Performance Evaluation of In-Situ Iron Reactive Barriers at the Oak Ridge Y-12 Site

“…Only small patches of green rust were observed by SEM in the February 1999 core samples and not detected by XRD. Researchers report that the precipitation of green rust is a useful component of reactive barrier systems in the remediation of contaminants (Erbs et al, 1999;Gu et al, 1999).…”