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Langmuir 2002, 18, 7688-7693
Evidence for Localization of Reaction upon Reduction of Carbon Tetrachloride by Granular Iron Daniel J. Gaspar,* A. Scott Lea, Mark H. Engelhard, and Donald R. Baer Pacific Northwest National Laboratory, Richland, Washington 99352
Rosemarie Miehr and Paul G. Tratnyek OGI School of Science & Engineering, Oregon Health & Science University, 20000 Northwest Walker Road, Beaverton, Oregon 97006 Received April 2, 2002. In Final Form: June 28, 2002 The distribution of reaction sites on iron particles exposed to water containing carbon tetrachloride has been examined by measuring the locations of reaction products. The uniformity or localization of reaction sites has implications for understanding and modeling the reduction of environmental contaminants by iron in groundwater systems. Granular iron surfaces similar to those being used for environmental remediation applications were studied using surface analysis techniques to develop an understanding of the physical and chemical structure of the surface and oxide films. Scanning Auger microscopy and imaging time-of-flight secondary ion mass spectroscopy revealed that granular iron exposed to carbon tetrachloride saturated water exhibits chloride-enriched regions with a high degree of localization. These results indicate that significant CCl4 reduction occurred at pits rather than on the passive oxide film on the metal.
* To whom correspondence should be addressed. E-mail:
[email protected].
Possible roles of the oxide film include (i) serving as an electron and ion transport barrier, causing reduction of solutes to occur primarily at pits or similar defects; (ii) behaving as a conductor or semiconductor, allowing charge to pass across the interface with some resistance; and (iii) functioning as a catalyst, where complexed FeII provides strongly reducing surface sites.8 The question of whether contaminant reduction is dominated by reactions localized at defects in the passive film or distributed across the much larger surface area of the oxide film has broad implications for ongoing work in this area. For example, a range of kinetic models have been applied to this system, most of which quantify the reductant as total particle surface area per volume of aqueous solution,9 with surface area determined by Brunauer-Emmett-Teller gas adsorption. The success of these models may be due to equivalence between the total and reactive surface areas, but it is more likely to arise because the proportion of total surface area that is comprised of reactive sites is roughly constant for iron samples of a particular type and history. A few studies of this system have used kinetic models that specify the density of reactive sites on the surface,8,10,11 but none have quantified this parameter from experimental data. The goal of this study was to look for evidence as to whether reactive sites for contaminant reduction are primarily localized at specific sites on the surface of granular iron particles in anaerobic, aqueous media. Carbon tetrachloride was chosen as the model contaminant because many aspects of its reaction with granular
(1) Smith, M. In Reduction. Techniques and Applications in Organic Synthesis; Augustine, R. L., Ed.; Marcel Dekker: New York, 1968; p 95. (2) House, H. O. Modern Synthetic Reactions; W. A. Benjamin: Menlo Park, CA, 1972; Chapter 3. (3) Hudlicky, M. Reductions in Organic Chemistry, 2nd ed.; American Chemical Society: Washington, DC, 1996; p 429. (4) Archer, W. L.; Simpson, E. L. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16, 158-162. (5) Smentkowski, V. S.; Cheng, C. C.; Yates, J. T. J. Langmuir 1990, 6, 147-158. (6) Lara, J.; Tysoe, W. T. Tribol. Lett. 1999, 6, 195-198. (7) Tratnyek, P. G. Chem. Ind. (London) 1996, 499-503.
(8) Scherer, M. M.; Balko, B. A.; Tratnyek, P. G. In Mineral-Water Interfacial Reactions: Kinetics and Mechanisms; ACS Symposium Series, Vol. 715; Sparks, D. L., Grundl, T. J., Eds.; American Chemical Society: Washington, DC, 1999; p 301. (9) Johnson, T. L.; Scherer, M. M.; Tratnyek, P. G. Environ. Sci. Technol. 1996, 30, 2634-2640. (10) Gotpagar, J.; Lyuksyutov, S.; Cohn, R.; Grulke, E.; Bhattacharyya, D. Langmuir 1999, 15, 8412-8420. (11) Arnold, W. A.; Roberts, A. L. Environ. Sci. Technol. 2000, 34, 1794-1805.
Introduction The reactivity of organic oxidants on the surface of reducing metals is relevant in a wide range of contexts. For example, “dissolving metal reductions” are traditional procedures in organic synthesis,1-3 corrosion by some organic solvents is a common concern in chemical engineering,4 and dissociative adsorption is the basis for some lubricating films used in tribology.5,6 Recently, similar chemistry has found application in environmental engineering, where granular scrap iron is being used to degrade or adsorb a variety of groundwater contaminants, including chlorinated aliphatic compounds (solvents), nitroaromatic compounds (explosives), and hazardous metals (radionuclides).7 The environmental application of granular iron presents a complex interfacial system with attributes familiar to surface chemists, but with a significant degree of complexity so that the relationship between different processes is highly uncertain. For example, although the overall reaction involves oxidation of the metal and reduction of an oxidant, how much of the desired reduction of dissolved contaminants is related to the presence of hydrogen produced by the aqueous corrosion of the iron remains unclear. Equally uncertain is the role of the oxide film in mediating the transfer of reducing equivalents from the underlying zerovalent metal to contaminants adsorbed at the interface of the oxide with the aqueous phase.
10.1021/la025798+ CCC: $22.00 © 2002 American Chemical Society Published on Web 08/16/2002
Reaction Site Localization for Fe/CCl4
iron have been examined previously,12-20 it will produce reaction products that are convenient for the purposes of this study, and it is among the major contaminants of U.S. Department of Energy sites that eventually may be treatable using technologies based on reaction with iron metal. The approach taken in this study involved high-vacuum surface analytical methods to determine the condition of granular iron surfaces after reaction with CCl4 under controlled, aqueous conditions. Previous work has demonstrated the ability of ex situ methods to extract information on reactions similar to those studied in this paper.21-24 To do this, care was taken to rule out artifacts due to sample handling (e.g., postreaction contaminants or precipitates) and data interpretation (e.g., discrimination between active and inactive sites). Here, we report Auger electron spectroscopy/scanning Auger microscopy (AES/SAM), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), and X-ray photoelectron spectroscopy (XPS) measurements of the surface structure and composition of relatively pure, reagent grade granular iron after reaction with aqueous CCl4 under controlled conditions. CCl4 reacts with iron under the conditions of this study to produce primarily Cl- and partially dechlorinated organics (mainly chloroform, but also methylene chloride, etc.; for example, see ref 14). The primary reaction product we analyzed was Cl; in this study we did not attempt to determine the chemical form of the Cl but rather measured elemental Cl (XPS and AES/SAM) or Cl- (TOF-SIMS; secondary Cl- may be produced by any primary species containing Cl) remaining on the surface as an indicator of reaction. We find that Cl is localized and associated with pits in the granular iron surface. Experimental Section Sample Preparation/Sample Handling. Commercial, highpurity granular iron metal (Fluka, 99.9%) was hand-sorted to remove grains with discoloration or without a large flat edge that was conducive to standard surface analysis techniques. The resulting cohort was roughly uniform in color, size, and gross morphology. The grains were irregularly shaped, 1-2 mm long, with an average mass of approximately 7.5 mg/grain. The specific surface area of this particular cohort was not measured, but a summary of specific surface areas measured on various preparations of Fluka iron shows a range from 0.005 to 0.2 m2 g-1 and an average value of 0.05 m2 g-1.25 (12) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28, 2045-2053. (13) Scherer, M. M.; Westall, J. C.; Ziomek-Moroz, M.; Tratnyek, P. G. Environ. Sci. Technol. 1997, 31, 2385-2391. (14) Balko, B. A.; Tratnyek, P. G. J. Phys. Chem. B 1998, 102, 14591465. (15) Bonin, P. M. L.; Odziemkowski, M. S.; Gillham, R. W. Corros. Sci. 1998, 40, 1391-1409. (16) Hung, H.-M.; Hoffmann, M. R. Environ. Sci. Technol. 1998, 32, 3011-3016. (17) Johnson, T. L.; Fish, W.; Gorby, Y. A.; Tratnyek, P. G. J. Contam. Hydrol. 1998, 29, 377-396. (18) Novak, P. J.; Daniels, L.; Parkin, G. F. Environ. Sci. Technol. 1998, 32, 1438-1443. (19) Bonin, P. M. L.; Jedral, W.; Odziemkowski, M. S.; Gillham, R. W. Corros. Sci. 2000, 42, 1921-1939. (20) Gerlach, R.; Cunningham, A. B.; Caccavo, F. J. Environ. Sci. Technol. 2000, 34, 2461-2464. (21) Castle, J. E. Surf. Interface Anal. 1986, 9, 345-356. (22) Astrup, T.; Stipp, S. L. S.; Christensen, T. H. Environ. Sci. Technol. 2000, 34, 4163-4168. (23) Qiu, S. R.; Lai, H. F.; Roberson, M. J.; Hunt, M. L.; Amrhein, C.; Giancarlo, L. C.; Flynn, G. W.; Yarmoff, J. A. Langmuir 2000, 16, 2230-2236. (24) Adib, K.; Camillone, N.; Fitts, J. P.; Rim, K. T.; Flynn, G. W.; Joyce, S. A.; Osgood, R. M. Surf. Sci. 2002, 497, 127-138. (25) Alowitz, M. J.; Scherer, M. M. Environ. Sci. Technol. 2002, 36, 299-306.
Langmuir, Vol. 18, No. 20, 2002 7689 The particles were treated in seven groups of four particles each. Two groups were used as controls: one was not treated at all, and the other was rinsed but not exposed to CCl4. The rinsing procedure (performed on the latter control and the five groups that were subsequently treated with CCl4) consisted of rinsing with deionized (DI) water (Millipore, 18 MΩ cm) and then with acetone and finally drying under a mild (1 Pa) vacuum. This procedure was intended to remove soluble salts and minimize adventitious contamination from handling, without significantly altering the amount or form of the passive film on the surfaces. Particles were treated by immersion in CCl4-saturated water (nominally 825 mg L-1) for exposure times ranging from 24 to 168 h (1-7 days). The solutions were unbuffered, and thus the pH in these experiments reflects iron corroding in water similar to natural groundwater. The pH is expected, therefore, to increase gradually to, and remain at, 8.7, maintained there by precipitation of iron hydroxides.26 This length of exposure was chosen to ensure significant reaction based upon earlier experiments with more dilute CCl4 solutions.9,12 Using a wide time window ensured that the particle-to-particle differences in reactivity, due to surface area, passive oxide layer differences, contaminants, and so forth, would not dominate the extent of reaction over a number of samples. After exposure to CCl4, samples were stored in sealed vials under ambient atmosphere until analyzed. Surface Analysis. AES/SAM measurements were made using a Physical Electronics model 680 Nanoprobe, with a field emission electron source, a cylindrical mirror analyzer, and a multichannel plate detector. Most secondary electron images (SEMs), Auger spectra, and Auger elemental maps were taken using a 10 nA, 10 kV electron beam with a beam size of ∼30 nm. Scanning Auger maps were generated using a 2-point process where the peak intensity minus the background intensity for each element of interest was calculated at each x-y position. Auger spectra were collected using 1 eV steps. Quantitative analysis of the data was done using sensitivity factors for peak-to-peak amplitudes of derivative spectra. The spectra were processed with 9-point smoothing followed by 5-point derivatization prior to quantification. XPS measurements were made using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe. This system uses a focused monochromatic Al KR X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 16-element multichannel detection system. The X-ray beam used was a 90 W, 100 µm diameter beam rastered over a 1.4 mm × 0.2 mm rectangle on the sample. The incident X-ray beam is normal to the sample, and the X-ray detector is located 45° away from normal. Survey scans were collected using a pass energy of 117.4 eV. For the Ag 3d5/2 peak, these conditions produce a full width at half-maximum (fwhm) better than 1.6 eV. The highenergy resolution data were collected using a pass energy of 23.5 eV for the C 1s, O 1s, and Fe 2p photopeaks. For the Ag 3d5/2 peak, these conditions produce a fwhm of
