Reductive Dehalogenation of Trichloroethylene with Zero-Valent Iron: Surface Profiling Microscopy and Rate Enhancement Studies J. Gotpagar, † S. Lyuksyutov, ‡ R. Cohn, ‡ E. Grulke, † and D. Bhattacharyya* ,† Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506, and The Electrooptics Research Institute, University of Louisville, Louisville, Kentucky 40292 Received March 18, 1999. In Final Form: July 9, 1999 Mechanistic aspects of the reductive dehalogenation of trichloroethylene using zerovalent iron are studied with three different surface characterization techniques. These include scanning electron microscopy, surface profilometry, and atomic force microscopy. It was found that the pretreatment of an iron surface by chloride ions causes enhancement in the initial degradation rates. This enhancement was attributed to the increased roughness of the iron surface due to crevice corrosion obtained by pretreatment. The results indicate that the “fractional active site concentration” for the reactive sorption of trichloroethylene is related to the number of defects/abnormalities present on the surface of the iron. This was elucidated with the help of atomic force microscopy. Two possible mechanisms include (1) a direct hydrogenation in the presence of defects acting as catalyst and (2) an enhancement due to the two electrochemical cells operating in proximity to each other. The result of this study has potential for further research to achieve an increase in the reaction rates by surface modifications in a practical scenario. 1. Introduction Since the original studies by Gillham and O’Hannesin 1,2 who proposed the use of reductive dehalogenation reaction for environmental remediation, several studies have been published which deal with the reaction of trichloroethylene (TCE) with zerovalent iron. Matheson and Tratnyek 3 were the first to report a detailed kinetic and mechanistic study of this reaction with several contaminants of concern. It is well-known now that though these reactions are faster compared to the natural biotic and abiotic processes, the rates are nonetheless too low to be feasible for ex situ applications. Therefore, the focus so far in this area is directed toward the in situ applications. Both in situ reactive barriers and above ground reactors have been developed for this purpose. Several test installations have already been completed at contaminated sites, and more are being planned. 4-7 However, little information is available on the exact mechanism of the reductive dehalogenation using zerovalent iron. The effective design and operation of systems involving zerovalent metals would be greatly improved by a more detailed, process- level understanding of the mechanism by which these contaminants degrade. Furthermore, improving the rates of these processes would also make the ex situ applications feasible and reduce the remediation time and would also be cost-effective. In this paper, an attempt is made to gain an understanding of the surface phenomena on the surface of iron during the reductive dehalogenation of TCE with the ultimate goal of increasing the rate of reaction. Atomic force microscopy (AFM), surface profilometry, and scan- ning electron microscopy (SEM) were used to analyze the surface features of the iron. In addition, the relation between the metal dissolution process occurring during dehalogenation of chlorinated organics to the classical crevice corrosion mechanism of iron in the presence of chloride ion is described. Only recently, promising use of AFM was suggested by Boronina et al. 8 for such envi- ronmental applications. In this study, AFM is also found to be important for the indication of crevice corrosion, as will be discussed later on. 2. Background. Role of the Metallic Surface 2.1. Role of the Metal Surface on Electron Trans- fer. Although considerable advancement has been made recently in identifying the product distribution, 9-11 to date the exact surface mechanism for TCE degradation by iron is not known. There is general agreement that electron transfer at the metal surface is required. This observation was used by Gotpagar et al. 12 and Boronina et al. 13 to develop the macroscopic model. Recent publications 3,5,13-18 have repeatedly emphasized the importance of the metal * To whom correspondence should be addressed. Phone: (606) 257-2794. Fax: (606) 323-1929. E-mail: db@engr.uky.edu. † University of Kentucky. ‡ University of Louisville. (1) Gillham, R. W.; O’Hannesin, S. F. Metal-catalyzed Abiotic Degradation of Halogenated Organic Compounds. Paper presented at the 1992 IAH Conference on Modern Trends in Hydrogeology, Hamilton, Ontario, Canada, May 10-13, 1992. (2) Gillham, R. W.; O’Hannesin, S. F. Groundwater 1994, 32, 958. (3) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28, 2045. (4) Gillham, R. W.; O’Hannesin, S. F.; Orth, W. S. Metal Enhanced Abiotic 5. Degradation of Halogenated Aliphatics: Laboratory Tests and Field Trials. Paper presented at the 1993 HazMat Central Conference, Chicago, IL, March 9-11, 1993. (5) Gillham, R. W. Prepr. Extended Abstr. Am. Chem. Soc. 1995, 35, 691. (6) Puls, R. W.; Powell, R. M. Environ. Sci. Technol. 1997, 31, 2244. (7) Yamane, C. L.; Gallinatti, J. D.; Szerdy, F. S.; Delfino, T. A.; Hankins, D. A.; Vogan, J. L. Prepr. Extended Abstr. Am. Chem. Soc. 1995, 35, 792. (8) Boronina, T. N.; Lagadic, I.; Sergeev, G. B.; Klabunde, K. J. Environ. Sci. Technol. 1998, 32, 2614. (9) Roberts, A. L.; Wells, J. R.; Campbell, T. J.; Burris, D. R. Environ. Toxicol. Chem. 1997, 16, 625. (10) Burris, D. R.; Delcomyn, C. A.; Smith, M. H.; Roberts, A. L. Environ. Sci. Technol. 1996, 30, 3047. (11) Arnold, W. A.; Roberts, A. L. Environ. Sci. Technol. 1998, 32, 3017. (12) Gotpagar, J.; Grulke, E.; Tsang, T.; Bhattacharyya, D. Environ. Prog. 1997, 16, 137. (13) Boronina, T.; Klabunde, K. J.; Sergeev, G. Environ. Sci. Technol. 1995, 29, 1511. 8412 Langmuir 1999, 15, 8412-8420 10.1021/la990325x CCC: $18.00 © 1999 American Chemical Society Published on Web 09/18/1999