Correspondence Comment on “Manipulating the Size and Dispersibility of Zerovalent Iron Nanoparticles by Use of Carboxymethyl Cellulose Stabilizers” The article “Manipulating the Size and Dispersibility of Zerovalent Iron Nanoparticles by Use of Carboxymethyl Cellulose Stabilizers,” by He et al. (1) appearing in a recent issue of Environmental Science & Technology, discusses the controllable synthesis of zerovalent iron nanoparticles using surfactants such as carboxymethyl cellulose. In the experimental procedures, the authors used deionized water to prepare the CMC solution and FeSO4 · 7H2O stock solution, which were purged with N2. According to previous research articles (2, 3) and our research experience, Fe0 undergoes redox reaction with water even if the water is purged with N2. Fe0 + 2H2O f 2Fe2+ + H2 + 2OHAs we know, when the Fe0 nanoparticle size decreases, the reactivity of nanoparticles increases; therefore, oxidation of synthesized Fe0 nanoparticles can occur more easily than bulk sized Fe0 in water. In this case, the purity of synthesized Fe0 nanoparticles would be affected by formed iron oxide nanoparticles. Mallouk (2), Lowry (4), and Choi (5) used a 30% ethanol aqueous solution to prepare Fe2+ or Fe3+ stock solution with N2 purging. In addition, for preparing stock solutions, we also found using N2 purged 70% and 90% ethanol aqueous solutions or a 100% ethanol solution was more effective in preventing oxidation of Fe0 in water. After the iron nanoparticles were synthesized, He et al. mentioned that “the resultant nanoparticle suspension was then sampled and analyzed within 1 h to obtain the mean particle size or size distribution.” In fact, the resultant nanoparticles contain excess salts in the reaction mixture prior to analysis; therefore, the resultant nanoparticles should be washed with ethanol or methanol 2-5 times in order to remove these excess salts and water, which helps to prevent immediate rusting (2, 6). He and Zhao reported that when the CMC/Fe2+ molar ratio was reduced to 0.0062, approximately 92.4% of the ZVI particles aggregated to greater than 3 µm. Their Figure 2 shows such ZVI nanoparticle aggregation. In fact, it is difficult to see single nanoparticles in the aggregate clearly and it is also difficult to distinguish whether it is the ZVI nanoparticles aggregate or iron oxide nanoparticles aggregate solely from this TEM image. Additionally, because the concentration of Fe2+ is too low, the final synthesized iron nanoparticles’ size is very small and the surface area is very high, which leads to oxidation of ZVI nanoparticles to form iron oxide nanoparticles. To investigate the composite of the aggregate in their Figure 2b, these aggregates should be characterized by XRD or TEM-EDS. In our research group, ZVI nanoparticles with a capsule structure were synthesized (our Figure 1), i.e., tens of single ZVI nanoparticles aggregated altogether, without a stabilizer. Comparing their Figure 2b with our Figure 1, it is easy and clear to see ZVI nanoparticles with a size of 2–3 nm packed into iron oxide layers. Finally, the authors wrote that the discrepancy of particle size of nanoparticles calculated by TEM and DLS “is due in part to that TEM measures only the electron dense metal
10.1021/es800195v CCC: $40.75
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2008 American Chemical Society
FIGURE 1. TEM image of synthesized ZVI nanoparticles with capsule structure: freshly prepared at 0.009 M Fe3+ in a 100% ethanol solution. The scale bar represents 20 nm. core”. In previous research, TEM also can measure the surface layer in core–shell structures, as when Carpenter et al. (7) provided a very clear transmission electron micrograph of a Fe/iron oxide core–shell structure. However, DLS is likely to show overestimated results because as particle size decreases, the surface energy increases, which will increase the degree of aggregation in aqueous suspensions. Due to this formed agglomeration of aggregates, the particle size calculated by DLS will be higher than the particle size calculated by TEM (8, 9). The discrepancy of particle size between the TEM image and DLS results is likely due to continued excited agglomeration.
Literature Cited (1) He, F.; Zhao, D. Y. Manipulating the size and dispersibility of zerovalent iron nanoparticles by use of carboxymethyl cellulose stabilizers. Environ. Sci. Technol. 2007, 41, 6216–6221. (2) Ponder, S. M.; Darab, J. G.; Mallouk, T. E. Remediation of Cr(VI) and Pb(II) Aqueous Solutions Using Supported, Nanoscale Zero-valent Iron. Environ. Sci. Technol. 2000, 34, 2564–2569. (3) Kanel, S. R.; Manning, B.; Charlet, L.; Choi, H. Removal of Arsenic(III) from Groundwater by Nanoscale Zero-Valent Iron. Environ. Sci. Technol. 2005, 39, 1291–1298. (4) 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. (5) Giasuddin, A. B. M.; Kanel, S. R.; Choi, H. Adsorption of humic acid onto nanoscale zerovalent iron and its effect on arsenic removal. Environ. Sci. Technol. 2007, 41, 2022–2027. (6) Liu, Y.; Choi, H.; Dionysiou, D.; Lowry, G. V. Trichloroethene Hydrodechlorination in Water by Highly Disordered Monometallic Nanoiron. Chem. Mater. 2005, 17, 5315–5322. (7) Carpenter, E. E.; Calvin, S.; Stroud, R. M.; Harris, V. G. Passivated iron as core-shell nanoparticles. Chem. Mater. 2003, 15, 3245– 3246. (8) Finsy, R.; Deriemaeker, L.; Gelade, E.; Joosten, J. Inversion of Static Light Scattering Measurements for Particle Size Distributions. J. Colloid Interface Sci. 1992, 153, 337–354. (9) Tseng, Y.-H.; Lin, H.-Y.; Kuo, C.-S.; Li, Y.-Y.; Huang, C.-P. Thermostability of Nano-TiO2 and its Photocatalytic Activity. React. Kinet. Catal. Lett. 2006, 89, 63–69.
Qiliang Wang and Heechul Choi* Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1-Oryong-dong, Buk-gu 500-712 Gwangju, The Republic of Korea ES800195V
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