| The interactions of trichloroethylene (TCE) with elemental iron (Fe°) in the gas phase (Fe°/H2O film/gaseous TCE) was investigated for acid-washed (oxide free) and unamended (partially oxide covered) Fe°. Elemental Fe° was found to remove TCE from the gas phase by adsorption onto the surface, and/or reduction to less chlorine containing organics (dichloroethylene, vinyl chloride, and ethylene), while Fe° is oxidized to Fe (II) and/or Fe (III) containing compounds.;A first-order kinetic expression was derived based on equilibrium partitioning of TCE from the gas phase to the liquid film (aqueous) close to the surface, linear adsorption of TCE, and assuming that the chemical reduction reaction (TCE → DCE) was rate limiting. Based on the similarity and magnitude of activation energies (acid-washed Fe°) for aqueous and gaseous phase reactors, i.e., aqueous = 54.7 (+/-1.89) kJ/mole; gas phase = 53.9 (+/-1.13) kJ/mole, it was concluded that mass transfer was not limiting, i.e., E app > 42 kJ/mole, and that the chemical reaction assumption was valid. In addition, the first-order rate constants for aqueous and gas phase reactions were similar (ks x 104 (L/m2.hr = 1.83, 4.13, and 8.08 (aqueous) and 3.10, 9.98, and 13.5 (gaseous) at 15°C, 24°C, and 35°C respectively).;The adsorption of TCE onto Fe° was strongly retarded by water. The adsorption coefficient for TCE was greatly reduced as the relative humidity (RH) increased, and this was particularly noticed at RH values less than monolayer coverage. The moisture content was a critical factor in the reaction(s) of TCE with the Fe° surface and a critical RH was needed for the reduction of TCE to take place. At RH less than critical, only adsorption of TCE occurred, while above the critical RH reduction of TCE occurred, as evidenced by the formation of reduction by-products, such as ethylene and vinyl chloride. Critical RH values were found to be 72% for acid-washed Fe° and 92% for unamended Fe°.;The surface composition of Fe° was a factor in the TCE removal rate. The rate of TCE removal was slower on an oxidized surface (ks x 103 (L/m2.hr) (anaerobic) = 1.03 (acid-washed) and 0.50 (unamended) Fe°)). The presence of oxygen irreversibly inhibited the TCE reduction rate (ks x 103 (L/m 2.hr) (aerobic) = 0.68 (acid-washed) and 0.32 (unamended) in initial region) and caused a split in the reduction rate of TCE, or a fast first-order reaction was followed by a slower first-order reaction after 10 days. |
