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

Abstract

The reductive dechlorination of CCl 4 and CHCl 3 in the presence of the synthetic sulfate form of green rust (GR SO 4 ), Fe II 4 Fe III 2 (OH) 12 SO 4 y H 2 O, at pH ∼ 8 and room temperature was investigated. Reduction of CCl 4 produces CHCl 3 and C 2 Cl 6 as main chloroaliphatic products, while GR SO 4 is oxidized to magnetite (Fe 3 O 4 ). The formation of C 2 Cl 6 indicates a coupling reaction between trichloromethyl radicals in the suspension. Chloroform was much less susceptible than CCl 4 to reductive dechlorination by GR SO 4 showing reduction rates approximately 100 times less than for reduction of CCl 4 . The transformation of CCl 4 by GR SO 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 - ]/d t ≅ −d[CCl 4 ]/d t = r · k obs [Fe(II)] GR, where k obs is in the range (0.47 × 10 - 5 )−(2.18 × 10 - 5 ) s - 1 for CCl 4 concentrations above its aqueous solubility. This narrow range may be due to the constant CCl 4 (aq) concentration owing to buffering of the CCl 4 (aq) concentration by free phase CCl 4 (l) thereby indicating that the reaction takes place in solution. Experiments with initial CCl 4 concentrations below its aqueous solubility support this theory. The reaction kinetics are compared with similar reactions where iron(0) is used as reductant of CCl 4 . The first-order rate constants for transformation of CCl 4 with zerovalent iron and GR SO 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 CCl 4 measured in iron(0) systems.