Reduction of Chlorinated Methanes by Nano-Sized Zero-Valent Iron. Kinetics,Pathways,and Effect of Reaction Conditions

Dechlorination of three chlorinated methanes (CCl₄, CHCl₃, CH₂Cl₂) was investigated using a nano-sized iron synthesized by borohydride reduction of Fe³⁺ under anaerobic conditions. When reacted with chlorinated methanes in batch reactors, nano-sized iron rapidly transformed CCl₄ and CHCl₃, but showed negligible reactivity toward CH₂Cl₂, giving the reactivity order of CCl₄ > CHCl₃ >> CH2Cl₂. CH₄ was observed along with CH₂Cl₂ in the reduction of CCl₄ and CHCl₃, and they are presumed to be generated via concerted reductive elimination steps involving carbene and charged radical species. Pathways for CH₄ production from CCl₄ and CHCl₃ reductions are proposed. Evidence obtained from the study of several physicochemical factors including pH, initial concentration, hydrogen concentration, and metal loading indicates reduction of chlorinated methanes occurs via a direct electron transfer reduction mechanism, rather than an indirect mechanism involving reactive hydrogen species. In a comparative experiment with three types of commercial irons (Fisher, Connelly, and ARS) and CHCl3, nano-sized iron gave a surface area normalized rate constant (k _SA) value of 5.6(±0.6) × 10⁻² L/m² · h, which is one order of magnitude greater than Fisher iron and two orders of magnitude greater than ARS and Connelly irons. This comparison of k _SA values should be considered approximate due to large differences in metal loadings.