Formation of Low-Molecular-Weight Organic Compounds During Anoxic Corrosion of Zero-Valent Iron

摘要

Speciation of carbon during the anoxic corrosion of steel is poorly known, whereas its knowledge would be of great importance in connection with assessments of the safe disposal of ~(14)C-containing irradiated steel in repositories for radioactive waste. The chemical form of the ~(14)C-bearing organic compounds determines routes of migration from engineered barrier systems and their reactivity at disposal sites. Batch-type corrosion experiments with unirradiated iron powders reported in this study show for the first time that both reduced and oxidized carbon species are present in corroding iron-water systems in anoxic conditions. Methane and volatile C_(2)–C_(4) alkanes and alkenes were produced during the course of corrosion whereas formate, acetate, and oxalate were released to solution in the early stage of the corrosion process. Evidence is provided that reduced and oxidized hydrocarbons were produced by two different processes. Formation of reduced hydrocarbons occurred at the surface of iron particles by a Fischer-Tropsch-type mechanism, whereas oxidized hydrocarbons were produced in the course of oxidative pretreatment of iron particles and released instantaneously from the surface in contact with alkaline solution. Results from this study have implications for safety assessments of radioactive waste disposal sites as they suggest predominant formation of alkanes and alkenes during anoxic steel corrosion and instantaneous release of only a small fraction of carbide carbon as oxidized hydrocarbons.