Hydrocarbon formation in metallic iron/water systems

Chlorinated ethenes can he reduced in metallic (zero-valent) iron/water systems to produce a suite of nonchlorinated hydrocarbons. When C-13-labeled trichloroethylene is reduced, C-13-labeled hydrocarbons are produced. In the absence of chlorinated ethenes, however, lower concentrations of many of the same hydrocarbons (methane and C-2-C-6 alkanes and alkenes) are also produced. Hardy and Gillham (1996) proposed that these background hydrocarbons were due to the reduction of aqueous CO2 by metallic iron. In the present study, we examined the production of these hydrocarbons by various batch experiments. Several of the systems examined produced hydrocarbons in excess of the carbon available from aqueous CO2. In addition, carbon from aqueous C-13-labeled CO2 was not incorporated into the hydrocarbons produced. The reduction of aqueous CO2 was not a major source of carbon for the background hydrocarbons. Acid dissolution of gray cast irons containing both carbide and graphite carbon yielded hydrocarbons and a substantial amount of graphite residual. The dissolution of metallic irons containing only carbide carbon yielded total carbon conversion to hydrocarbons. Carbide carbon in the iron appears to be the most likely carbon source for the production of the background hydrocarbons. Mechanisms, analogous to the Fischer-Tropsch synthesis of hydrocarbons, are proposed for hydrocarbon production from carbide carbon. Similar mechanisms may also contribute to the formation of some of the hydrocarbons produced during the reduction of chlorinated ethenes by metallic iron.