ABIOTIC AND BIOTIC FACTORS AFFECTING CONTAMINANT TRANSFORMATION AT IRON OXIDE SURFACES (Cosponsored with the Division of Geochemistry) Organized by D.E. Giammar, M.L. McCormick and E.J. O Loughlin Symposia Papers Presented Before the Division of Environmental Chemistry American Chemical Society Chicago, IL March 25-29, 2007 EFFECT OF AGING ON THE STRUCTURE AND REACTIVITY OF NANOPARTICLES OF IRON/IRON OXIDES Vaishnavi Sarathy 1, James T. Nurmi 1, Paul G. Tratnyek 1, James E. Amonette 2, Donald R. Baer 2 and Chan Lan Chun 3 1 Department of Environmental and Biomolecular Systems Oregon Health & Science University, Portland, OR 2 Environmental Molecular Sciences Laboratory Pacific Northwest National Laboratory, Richland, WA 3 Department of Chemistry, University of Minnesota, Minneapolis, MN sarathy@ebs.ogi.edu Introduction The current interest in iron/iron oxide nanoparticles for remediation applications can be partly attributed to their presumed ease of injection and faster reactivity. However, several processes can limit their effectiveness for degrading contaminants in groundwater include aging, aggregation and deposition. Under environmental conditions, nanoparticles with Fe(0) cores persist because they are passivated by a shell of oxide. The nature, structure and thickness of the oxide shell evolve with time due to reaction of the shell material with the core and with the external medium. Since the oxide shell usually mediates the reaction with contaminants, aging of the shell can affect the long-term performance of iron/iron oxide nanoparticles in remediation applications 1. To study the reactivity of nanoparticles, we have focused on carbon tetrachloride (CT) as a model contaminant and probe reactant. Reduction of CT is known to proceed via two pathways, one producing the toxic chloroform (CF) and the other yielding relatively benign products like formate and carbon monoxide. Previous studies by our research group on nano iron/iron oxides (aged for a year) and studies by other research groups using nano-magnetite have found that the presence of magnetite in these materials 383
tends to correlate with low yields of chloroform (Y CF ) 2-4. Therefore, since aging presumably alters the composition of the oxide film, we hypothesized that aging would affect Y CF. Studies by Liu and Lowry 5 have also investigated effects of aging on the Fe(0) content of and H 2 production by nano-iron and they observed that Fe(0) content decreases with age. In this study, we report on the short- and long-term aging of a nano-sized iron produced by reduction of iron oxides in a H 2 atmosphere in anoxic water and the resulting effects on the chemistry of the oxide-shell and reactivity of the particle with CT. We used a 3-pronged approach, where (i) the structure of the iron particles was characterized using spectroscopy and microscopy (X-ray photoelectron spectroscopy [XPS], X-ray diffraction [XRD] and transmission electron microscopy [TEM]), (ii) the kinetics and pathway of reaction were studied by batch experiments with CT and (iii) changes in corrosion potential of the iron oxide particles were determined using electrochemical (chronopotentiometric) measurements. Materials and Methods Experiments were performed with a precursor to RNIP-10DS (Toda Kogyo Inc., Japan) that had not been exposed to water (dry Fe) and a sample of RNIP that had been stored as an aqueous slurry for approximately a year. The iron in the latter was vacuum dried with acetone prior to use. Batch experiments were performed in vials with a constant amount of iron (~0.9 g/l) and filled with deoxygenated de-ionized water, with no headspace. These vials were equilibrated for different times ranging from 1 hr to 30 d, after which CT was spiked to achieve a starting concentration of 4 µm and the vials were rotated at 24 rpm on a rotary shaker. CT and CF were measured by GC-ECD. Replicate experiments were carried out at each pre-exposure time. Iron samples before and after experiments were examined by XPS, XRD and TEM. Specimens that had been pre-exposed to solution for the described times before reaction were dried in acetone after reaction with CT for ease of handling and storage before other measurements were performed. For analysis, specimens were prepared under a N 2 atmosphere by dispersing them on zero-background slides. After evaporation of the ethanol, specimens were exposed to the atmosphere and analyzed from 5-80 2θ by Cu-Kα radiation using a Philips X Pert MPD PW3040/00 instrument operating at 40 kv and 50 ma. The fractions of iron and magnetite were estimated from the relative intensities of the 200 and 440 reflections of iron and magnetite, respectively, after normalization using the I/Ic values of the two phases. A detailed description of the design and electrochemical properties of the powder disk electrode (PDE) used in this study was published previously 2. Linear sweep voltammery (LSV) was performed with a three-electrode cell, as described previously 6. For the chronopotentiometric (CP) experiments, a two-electrode cell (BAS, West Lafayette, IN) was used. Along with the working electrodes made of RNIP-10DS packed into a powder disk electrode (PDE), an Ag/AgCl electrode was used as the reference. 384
Results and Discussion TEM and XRD studies have previously shown that the unreacted particles have a coreshell structure with a core of Fe(0) and a shell of magnetite 2. XPS measurements confirm that the oxide shell was mostly Fe(III) and sufficiently thin that Fe(0) can be measured through the shell. Upon exposure to water, the initial highly crystalline oxide shell is transformed into a highly disordered phase and other Fe/O x /OH phases are formed that eventually grow into nano-crystallites that have a diffraction pattern consistent with magnetite. Chronopotentiometric experiments indicate depassivation of the iron oxide shell on the dry nano-iron at around 17-24 hr of exposure to water (data not shown). The timing of this transition corresponds well with the results observed from the batch experiments where there was an increase in reactivity over the first 24 hours of exposure to water and a gradual decrease thereafter up to an aging time of 30 d (Figure 1). Figure 1. Effect of pre-exposure to water on the kinetics of CT degradation (k CT ) and yield of CF (Y CF ) for RNIP that was received dry. Figure 1 shows that there is a good correlation between the k CT (pseudo first-order disappearance rate constant of CT) and Y CF over the range of pre-exposure times studied. The initial period of depassivation, while increasing the rate of CT reduction, also increases the Y CF, indicating that breakdown of the oxide film favors the CF pathway. Chronopotentiometry shows that re-passivation, or re-growth of an oxide film, starts to occur around 20-24 hr. Since Y CF decreases after the peak at 1-2 d preexposure, we can infer that repassivation involves new oxide film material, probably magnetite, and this material favors the benign dechlorination pathway of CT, producing CO and formate. 385
From XRD data on dry Fe at different aging times (Figure 2), we see an increase in the thickness of the magnetite film after around 5 d of exposure to water and then relatively little change in composition of the shell. Comparison of Figures 1 and 2 shows that the amount of magnetite appears to correlate with the yield of chloroform. Figure 2. Fraction of Fe atoms in the magnetite shell of nano iron recovered after the experiments shown in Figure 1. Material from duplicate batch experiments was combined for XRD. Long-term aging experiments performed on RNIP slurry that was a year old (data not shown) did not show the initial increase in rate or Y CF, and k CT extrapolates smoothly from the short-term pre-exposure experiments (Figure 1) to the long-term exposure experiments. Y CF for the latter experiments was constant at a low value of ~0.25 over the entire variation in pre-exposure times. Taken together, these data show the importance of the initial dissolution of the airformed oxide film over 1-2 days, leading to increased exposure of CT to new phases which appear to favor the CF pathway. Subsequently the oxide shell reforms, presumably rich in magnetite, and stabilizes over a period of five days. As indicated by XRD and TEM, new magnetite-containing material also forms at longer times producing a system containing both Fe(0) particles with an oxide coating and oxide particles. After about five days, relatively stable conditions are established and Y CF remains stable and k CT decreases slowly with time. Aging in water, therefore, alters the nature of the iron oxide shell in the short-term and corrosion results in the formation of larger oxide phases with increasing time. Both these factors affect not only kinetics, but also product branching during CT degradation. 386
Acknowledgements This work was supported by the U.S. Department of Energy (DOE) Office of Science, Offices of Basic Energy Sciences and Biological and Environmental Research. Parts of the work were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a DOE User Facility operated by Battelle for the DOE Office of Biological and Environmental Research. References 1. Johnson, T. L., Fish, W., Gorby, Y. A. and Tratnyek, P. G. J. Contam. Hydrol. 1998, 29, 377-396. 2. Nurmi, J. T., Tratnyek, P. G., Sarathy, V., Baer, D. R., Amonette, J. E., Pecher, K., Wang, C., Linehan, J. C., Matson, D. W., Penn, R. L. and Driessen, M. D. Environ. Sci. Technol. 2005, 39, 1221-1230. 3. Danielsen, K. M. and Hayes, K. F. Environ. Sci. Technol. 2004, 38, 4745-4752. 4. McCormick, M. L. and Adriaens, P. Environ. Sci. Technol. 2004, 38, 1045-1053. 5. Liu, Y. and Lowry, G. V. Environ. Sci. Technol. 2006, 40, 6085-6090. 6. Nurmi, J. T., Bandstra, J. Z. and Tratnyek, P. G. J. Electrochem. Soc. 2004, 151, B347-B353. 387