Kinetics of Trichloroethene Reduction by Zerovalent Iron and Tin: Pretreatment Effect, Apparent Activation Energy, and Intermediate Products CHUNMING SU* ,† AND ROBERT W. PULS ‡ National Research Council and U.S. Environmental Protection Agency, National Risk Management Research Lab, 919 Kerr Research Drive, Ada, Oklahom a, 74820 The degradation of trichloroethene (TCE) at 2 mg L -1 in headspace free aqueous solution by zerovalent iron (Fe 0 ) and tin (Sn 0 ) was studied in batch tests at 10, 25, 40, and 55 °C and HCl-treated Fe 0 and Sn 0 at 25 and 55 °C. Surface area normalized pseudo-first-order rate constants (k SA )ranged from 0.44 × 10 -3 to 4.3 × 10 -3 h -1 m -2 L for Fisher Fe 0 , 0.029 × 10 -3 to 0.27 × 10 -3 h -1 m -2 L for Peerless and Master Builders Fe 0 , and 0.011 × 10 -3 to 1.31 × 10 -3 h -1 m -2 L for Fisher and Aldrich Sn 0 . The Aldrich Fe 0 was the least reactive with k SA values ranging from 0.0016 × 10 -3 to 0.011 × 10 -3 h -1 m -2 L. The HCl-washing increased metal surface area and observed rate constant (k) values but generally decreased k SA values. The calculated apparent activation energy (E a ) using the Arrhenius law for the four temperature levels ranged from 32.2 to 39.4 kJ mol -1 for the untreated Fe 0 metals and 40.5-76.8 kJ mol -1 for the untreated Sn 0 metals. Greater temperature effect was observed for Sn 0 than for Fe 0 . Our results indicate that TCE reduction by Fe 0 and Sn 0 is likely controlled primarily by chemical reaction-limited kinetics rather than by mass transport of the TCE to the metal surface. Both reductive -elimination reaction and hydrogenolysis reaction are likely involved in the reduction of TCE by both Fe 0 and Sn 0 . Introduction Recent investigations on reduction of chlorinated solvents by zerovalent iron (Fe 0 ) have shown promising potential for applying the technology to the in situ remediation of contaminated groundwater (1, 2). Numerous feasibility studies, pilot tests, and field scale demonstration projects have been initiated usinggranular Fe 0 as the reactive medium in permeable reactive barriers (2, 3). Tin (Sn 0 ) has also been studied in the batch test and has been found to produce HCl and CO2 when it reacts with CCl4 in water (4). The most important factors influencing the reductive dechlorination reaction are the reactivity of individual chemical contami- nants and the available concentration of Fe 0 surface area. Other factors such as Fe 0 surface condition, pH, initial contaminantconcentration,and flowormixingrateareminor contributors to the kinetic control (5, 6). Temperature is important in affecting the rates as well as in providing unique insights into the reaction mechanisms. Chemical processes tend to exhibit larger temperature dependence than do physical processes. For example, diffusion-controlled reactions in solution have rather low activation energies (Ea < 21 kJ mol -1 ), whereas surface- controlled dissolution reactionsofmost silicate mineralshave Ea values usually in the range 42-84 kJ mol -1 (7), and Ea values for chemisorption on surfaces are often of the order of84kJmol -1 (8). Sivavec and Horney (9) reported Ea values (15-18kJmol -1 ) for the reduction ofchlorinated ethenes by severalcommercialFe 0 metals over a temperature range from 10 to 34 °C, indicative of a diffusion-limited reduction rate. No detailed information was given on the types ofFe 0 metals and name(s) of chlorinated ethenes for which the Ea values were measured. Scherer et al. (10) reported Ea ) 55.9 ( 12.0 kJmol -1 for the reaction ofCCl4 with Fluka Fe 0 ,and Ea ) 40.5 ( 4.1 kJ mol -1 for hexachloroethane with Fluka Fe 0 over a temperature range of 4-45 °C. These values are indicative ofchemicalreaction rate controlratherthan diffusion control (10).More studies are needed to resolve these discrepancies. Furthermore, the effect oftemperature on TCEreduction by Sn 0 has not been reported. More information on the temperature effect on metal-enhanced dechlorination is necessary for characterizing kinetic control on the perfor- mance of remediation technologies based on zerovalent metals. Another approach to derive reaction mechanisms is through identification of the reaction products and inter- mediates. Recent studies have generated more information on the distribution of intermediate products of chlorinated solvents reduction by Fe 0 . Orth and Gillham (11) found the principal degradation products of TCE reacted with Fisher Fe 0 to be ethene and ethane, with minor amounts of VC, all three DCE isomers, and other C1-C4 hydrocarbons. They suggested that a cumulative six electron transfer occurs to produce ethene when the TCE molecule resides at the Fe 0 surface.Matheson and Tratnyek(12)suggested direct electron transfer from the Fe 0 surface to the CCl4 molecule in the reduction processes. Asequential hydrogenolysis of PCE f TCE f cis-DCE f VC f ethene was proposed by Schreier and Reinhard (13).Roberts et al.(14)andCampbelletal.(15) proposed an additional pathway where reduction of vicinal polychlorinated ethenes by Fe 0 occurs via reductive -elim- ination. Reaction pathways including both hydrogenolysis and reductive -elimination were able to account for most of the observed products (15). These studies were generally performed at a single (room) temperature level such that there is little information available on the effects of tem- perature on both degradation rate and product distribution of chlorinated solvents reduction. The objectives of this study were to (1) compare the reactivity of different types of Fe 0 and Sn 0 toward TCE; (2) study the effect of acid pretreatment of Fe 0 and Sn 0 on TCE degradation kinetics; (3) investigate temperature effect on TCEreduction rate;and (4)examine the distribution ofmajor reaction byproducts. Experimental Section Chemicals and Materials. We used TCE (99+%, Aldrich, Milwaukee,WI),Fisher electrolytic Fe 0 (99%,100mesh,Fisher Scientific, Fair Lawn, NJ, Cat. No. I60-3), Aldrich Fe 0 (325 mesh, 97%, hydrogen reduced, Cat. No. 20930-9), Peerless Fe 0 (Peerless MetalPowders &Abrasive,Detroit,MI),Master Builders Fe 0 (Master Builders Inc., Cleveland, OH), Fisher powdered Sn 0 (Cat. No. T129-500), Aldrich powdered Sn 0 (325 mesh, 99.8%, Cat. No. 26563-2), Aldrich granular Sn 0 (30 mesh, ACS reagent), and HCl-washed Fe 0 and HCl-washed Aldrich powdered Sn 0 . Preliminary tests showed that HCl- *Correspondingauthorphone (580)436-8638; fax (580)436-8703; e-mail: su.chunming@epa.gov. † National Research Council. ‡ U.S. EPA. Environ. Sci. Technol. 1999, 33, 163-168 10.1021/es980481a CCC: $18.00  1998 American Chemical Society VOL. 33, NO. 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 163 Published on Web 11/20/1998