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 (GRSO4), FeII4FeIII2(OH)12SO4yH2O, at pH ∼ 8 and room temperature was investigated. Reduction of CCl4 produces CHCl3 and C2Cl6 as main chloroaliphatic products, while GRSO4 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 GRSO4 showing reduction rates approximately 100 times less than for reduction of CCl4. The transformation of CCl4 by GRSO4 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·kobs[Fe(II)]GR, where kobs 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 GRSO4, 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.