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) X 10(-2) L/m(2) center dot 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 kSA values should be considered approximate due to large differences in metal loadings.