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Response to Comment on “Manipulating the Size and Dispersibility of Zerovalent Iron Nanoparticles by Use of Carboxymethyl Cellulose Stabilizers” We appreciate the comments by Wang and Choi (1) on the subject article (2). The objective of our study was to demonstrate a new strategy to control the size of zerovalent iron (ZVI) nanoparticles using carboxymethyl cellulose (CMC) as a stabilizer. Prior to our response, readers are cautioned that ZVI nanoparticles are highly labile both chemically and physically. Consequently, the properties of ZVI nanoparticles are highly dynamic in nature and are a function of synthesizing chemistry and particle aging. Unfortunately, this dynamic feature is often ignored in the open literature. As a result, the size of ZVI nanoparticles reported in the literature spans from a few to hundreds of nanometers, and the reactivity differs by orders of magnitude. Further, readers are cautioned that stabilized ZVI nanoparticles, as presented in our paper, are present as discrete particles in aqueous suspension, while nonstabilized ZVI “nanoparticles” are present as agglomerates, which can be orders of magnitude greater than the nanoscale. Yet, these aggregates of ZVI particles have been often referred to as nanoparticles in the literature. Oxidation of ZVI Nanoparticles and Uses of Ethanol. It is well-known that ZVI nanoparticles can react with water, and thus, ZVI nanoparticles are characterized with a so-called core–shell structure (3, 4). As cited by Wang and Choi (1), researchers have prepared nonstabilized ZVI nanoparticles in aqueous solutions containing various fractions of (30%100%) ethanol. However, there have been no experimental data available on how effective this approach is to reduce oxidation of ZVI nanoparticles. For our CMC-stabilized ZVI nanoparticles, we doubt that the presence of ethanol would prevent the oxidation as long as water is present because of the extremely fast initial reaction rate of the nanoparticles with water molecules. Interestingly, we observed that it takes about 7 days for 0.1 g/L of our CMC-ZVI nanoparticles to be completely oxidized in water (2). Apparently, the iron oxide shell slows down further oxidation of the nanoparticles. It should also be noted that our intention was to prepare an aqueous suspension of ZVI nanoparticles that can be directly delivered into the subsurface to degrade chlorinated hydrocarbons. For this purpose, it would not be feasible to introduce high concentrations of ethanol. Moreover, the production of hydrogen in the reduction of water that accompanies the oxidative conversion of ZVI nanoparticles does not necessarily diminish the overall dechlorination power of the nanoparticles because the resultant hydrogen is equally effective for dechlorination (5, 6). Finally, the addition of ethanol would likely impede the stabilizing function of CMC molecules. In fact, ethanol has been commonly used as an antisolvent for facilitating separation of nanoparticles from water. Wang and Choi (1) also suggested rinsing the nanoparticles with ethanol before the particles were analyzed to “remove these excess salts and water, which helps to prevent immediate rusting”. This approach would be beneficial for nonstabilized particles. In our case, it is not only nontrivial to separate the stabilized nanoparticles from water, but such practices may further compromise the integrity of the nanoparticles. Note that only a small drop of the nanoparticle suspension was needed for transmission electron microscopy 3480
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(TEM) analyses, and it was easily nitrogen-dried. For dynamic light scattering (DLS) analyses, the suspension was directly analyzed without any amendment. In both cases, no evidence was observed that the ions in the solution would affect the results. Analysis of Particle Compositions. Our Figure 2 was intended to provide a snapshot contrast of the morphologies of stabilized versus agglomerated iron particles. It was not our intention to determine the compositions of the agglomerates, i.e., the core–shell structures. The TEM used in our study was operated at an energy level below 60 KeV (2, 7), which does not appear to offer the resolution to showcase the core and shell compositions. We agree that, if needed, high-resolution TEM and/or X-ray diffraction can be employed as demonstrated by a number of researchers (1, 3, 4). Particle Size Measurements: TEM versus DLS. Both TEM and DLS have been commonly employed to measure nanoparticle size, as in our study. While TEM images are formed due to scattering and diffraction of electrons by the nanoparticles, DLS measurements are based on the diffusivity of the nanoparticles. The CMC-stabilized nanoparticles are composed of a ZVI core and an iron oxide shell with CMC molecules adsorbed thereon. Our laboratory study indicated that CMC molecules are invisible under TEM. Therefore, TEM measures only the electron-dense core and the shell but not the attached CMC. However, these CMC macromolecules are expected to retard the diffusion of the nanoparticles. Therefore, DLS inherently results in an overestimated particle size compared to TEM. In our study, the sample age for both TEM and DLS analyses was about the same and DLS analysis was conducted within 30 min. Therefore, “continued excited agglomeration” (1) may not be the primary cause for the observed size discrepancy. In fact, our laboratory data indicated that the DLS-based particle size remained nearly the same during day 1 (changed only from 17.2 ( 3.2 to 16.9 ( 2.2 nm).
Literature Cited (1) Wang, Q.; Choi, H. Comment on “Manipulating the size and dispersibility of zero valent iron nanoparticles by use of carboxymethyl cellulose stabilizers”. Environ. Sci. Technol. 2008, 42, 3479. (2) He, F.; Zhao, D. Manipulating the size and dispersibility of zero-valent iron nanoparticles by use of carboxymethyl cellulose stabilizers. Environ. Sci. Technol. 2007, 41, 6216–6221. (3) Li, X.; Zhang, X. Iron nanoparticles: the core-shell structure and unique properties for Ni(II) sequestration. Langmuir 2006, 22, 4638–4642. (4) Carpenter, E. E.; Calvin, S.; Stroud, R. M.; Harris, V. G. Passivated iron as core-shell nanoparticles. Chem. Mater. 2003, 15, 3245– 3246. (5) Liu, Y.; Majetich, S. A.; Tilton, R. D.; Sholl, D. S.; Lowry, G. V. TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. Environ. Sci. Technol. 2005, 39, 1338–1345. (6) He, F.; Zhao, D. Hydrodechlorination of trichloroethene using stabilized Fe-Pd nanoparticles: Reaction mechanism and effects of stabilizers, catalysts and reaction conditions. Appl. Catal. B: Environ. , In review. (7) He, F.; Zhao, D. Preparation and characterization of a new class of starch-stabilized bimetallic nanoparticles for degradation of chlorinated hydrocarbons in water. Environ. Sci. Technol. 2005, 39, 3314–3320.
Feng He and Dongye Zhao* Department of Civil Engineering, Auburn University, Auburn, Alabama 36849 ES8004255 10.1021/es8004255 CCC: $40.75
2008 American Chemical Society
Published on Web 03/26/2008
