Letter pubs.acs.org/journal/estlcu
Sequestration of Arsenate in Zero-Valent Iron Nanoparticles: Visualization of Intraparticle Reactions at Angstrom Resolution Lan Ling and Wei-xian Zhang* State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, China 200092 S Supporting Information *
ABSTRACT: Reactions of arsenate [As(V)] in a single nanoscale zero-valent iron (nZVI) particle are mapped by spherical aberration-corrected scanning transmission electron microscopy integrated with X-ray energy dispersive spectroscopy. Near-atomic-resolution distributions of arsenic, iron, and oxygen demonstrate the diffusion and reactions of arsenic in nZVI. The reactions occur primarily in the surface layer, especially at the oxide−metallic iron [Fe(0)] interface. As(V) is attracted to the surface via electrostatic interactions and embedded in the nanoparticle via chemical reduction to form a thin (1.0−1.2 nm) layer of elemental arsenic inside the shell. Results offer unprecedented details about the intraparticle reaction mechanisms and demonstrate intrinsic advantages of nZVI for arsenic encapsulation, treatment, and remediation.
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INTRODUCTION Removing arsenic from contaminated water has long been a vexing problem in water treatment.1,2 Severe health implications, including a variety of cancers, have been traced to longterm arsenic exposure.1−3 In particular, trace levels (Fe−OH) with arsenate ions. The iron nanoparticles also have a strong tendency to aggregate to form large structures, partly because of magnetic interactions (Figure S1a,b of the Supporting Information). Figure 2 presents EDS elemental mappings at three resolutions [As(Lα), Fe(Lα), and O(Kα)], and color overlays were acquired from one nZVI nanoparticle. The nanoparticle was from a reactor after 24 h reactions with 1.3 mM arsenate (1.3 mM arsenate was removed with 5 g/L nZVI in 3 min). The physical size of the nZVI particle is highlighted by the HAADF image (panel 1 of Figure 2). Fe mappings (panels 2, 8, and 14 of Figure 2) show the footprint of iron. The metallic core is much denser, while the iron signal intensity becomes dimmer near the exterior surface. From panels 3, 9, and 15 of Figure 2, the ring of oxygen matches the lower-intensity area of iron, reconfirming the surface layer is rich in oxygen. The oxygen distribution also points to the wide-ranging oxygen presence, with the image covered extensively with oxygen (panel 9 of Figure 2). The area of arsenic (panels 4, 10, and 16 of Figure 2) is slightly larger than that of iron. This may suggest the sorption or surface complex of arsenate, which sits just atop the iron surface. It also shows that there are two distinctive rings of arsenic with a band of oxygen between the two rings (panels 16−18 of Figure 2). The outer sphere of the arsenic ring is more dispersed and can be attributed to As(V) adsorbed to iron oxide or formation of iron oxide−As(V) surface complexes,10,19,20 while the inner ring of arsenic is more intense, resulting from reduction of As(V) and accumulation of As(III) and As(0) on the Fe(0) surface. The observed iron, arsenic, and oxygen distribution patterns are consistent with results from XPS characterizations.10,11 Panels 6, 12, and 18 of Figure 2 are color overlays of the three elements. The overlays (panel 18 of Figure 2) offer spatial extents of the three elements at angstrom resolution (∼1.0 Å) within a single nZVI. The mappings reveal unprecedented details about the elemental distributions in the surface layer. Because the shell layer has an average composition of FeOOH, there are two oxygen atoms for every iron; thus, the surface layer is inherently dominated by oxygen. It also suggests that the nature of the core−shell structure remains largely unchanged after the reactions. Arsenic predominantly accumulates at the interface (∼3 nm from the surface) between iron oxide and the Fe(0) core and forms a thin (1.0−1.2 nm) layer of elemental arsenic. Thermodynamically, the intraparticle diffusion of arsenic is highly favored because the core region has the highest concentration of Fe(0), the electron donor for arsenic immobilization. The distributions of the three elements
Figure 1. STEM images of nanoscale zero-valent iron (nZVI) after 24 h reactions with 1.3 mM arsenate. Panels a, c, and e are high-angle annular dark-field (HAADF) images, and panels b, d, and f are brightfield STEM (BF STEM) images.
Consistent with previous work,10,11 nZVI particles formed by the borohydride synthesis route are spherical, consisting of a metallic iron core [e.g., nanocrystalline Fe(0)] encapsulated by a thin iron oxide/hydroxide layer with particle sizes in the range of 20−100 nm. As shown in Figure 1a, after reactions, the spent nZVI particle still preserves the core−shell structure with a bright core, corresponding to Fe(0), and a lower-intensity shell of iron, oxygen, and arsenic. Panels a, c, and e of Figure 1 are high-angle annular dark-field (HAADF) images, which are sensitive to variations in the atomic number of elements in the specimen (i.e., the Z contrast).14−17 Iron atoms are brighter than oxygen atoms of the same thickness in a HAADF image. A close-up of the HAADF images (Figure 1c,e) indicates that after reactions, a thin and slightly dark ring (or layer) (1.0−1.2 nm) emerges between the (dark gray) oxide layer and (bright) Fe(0) core. As presented later with additional evidence, this new layer is the reaction product, reduced arsenic. Phase-contrast BF STEM imaging (Figure 1b,d,f) offers additional testament that the spent particle comprises a dense core surrounded by a thin shell exhibiting distinctly less contrast than the interior. The higher-magnification (Figure 1d,f) images show near-atomic-resolution features of shortrange periodic fringes in the shell; however, the shell as a whole 306
dx.doi.org/10.1021/ez5001512 | Environ. Sci. Technol. Lett. 2014, 1, 305−309
Environmental Science & Technology Letters
Letter
Figure 3. STEM−EDS elemental line scans of O (red), As (green), and Fe (blue) of Fe−As reactions. Signals collected from iron nanoparticles after 24 h (a−d) and 48 h (e−h) reactions with 1.3 mM arsenate.
surface (Figure 3d). The iron abundance increases most rapidly within the first 4−5 nm, corresponding to the shell layer thickness. After 48 h, the iron profile is smooth and monotonous with a steady gradient and with the Fe content in the nanoparticle reduced by slightly more than 10% (Figure 3h). Previous work with a high-resolution STEM found that over time the metallic iron at the center of the nZVI particles can be completely dissolved to form a doughnutlike structure (the “hollow out” effect).15 From the oxygen profiles (Figure 3b,f), there is a broad oxygen peak spanning the shell layer. The oxygen profile climaxes ∼3 nm from the water−nanoparticle interface just outside of the center of the surface layer in the 24 h sample (Figure 3a,b). The peak is slightly out of the middle of the surface layer because the outer surface iron is more oxidized with oxygen-rich oxides and more Fe(III) while the inner part is less oxidized with more Fe(II). Furthermore, the oxygen peak moves inward and resides just outside the Fe(0) core after 48 h reactions (Figure 3e,f), suggesting continued iron oxidation and oxygen invasion. More significantly, two peaks are evident on the arsenic profile after 24 h reactions (Figure 3a,c). The peak positions and heights are the sum of three arsenic species: As(V), As(III), and As(0). Presumably, As(V) sits mostly on the outer surface, while As(0) accumulates at the Fe(0)−oxide interface. As mentioned earlier, a thin dark band is visible from the highresolution HAADF image (Figure 3a,e), which matches up with
Figure 2. STEM−EDS elemental mappings of Fe−As reactions at three resolutions: (1, 7, and 13) HAADF images, (2, 8, and 14) Fe, (3, 9, and 15) O, (4, 10, and 16) As, (5, 11, and 17) Fe and As, and (6, 12, and 18) Fe, As, and O. Signals collected from an iron nanoparticle after 24 h reactions with 1.3 mM arsenate.
provide direct and conclusive evidence that the surface of the Fe(0) core is the main “battlefield” for the As−Fe reactions. Figure 3 presents EDS line profiles, which provide the relative atomic abundance of oxygen, iron, and arsenic across the surface layer after 24 h (Figure 3a−d) and 48 h (Figure 3e− h) reactions. The backgrounds (Figure 3a,e) are HAADF images with the arrow illustrating the scan trajectory. Each step (scan pixel width) in the figure represents 0.318 nm (3.18 Å). EDS counts of iron increase rapidly from the water−particle interface and reach a plateau Fe− OH) with an arsenate ion. This constitutes the outer ring observed on the arsenic mapping. The surface-bound As(V) continues to diffuse through the surface layer. Further penetration or diffusion of As(V) is induced by its reduction to As(III) and As(0), and the latter accumulates at the Fe(0)− oxide interface and forms the surface and smaller peak presented on the EDS line profile (Figure 3e,g). A conceptual model summarizing the observations is provided in Figure S4 of the Supporting Information. Results in Figures 2 and 3, to the best of our knowledge, offer the most detailed information about the reactions in nZVI so far. This work reconfirms to our earlier reports about the parallel oxidation of As(III) to As(V) and reduction of As(III) to As(0) in nZVI.10,11 The core−shell nature of nZVI offers a high degree of materials utilization efficiency as arsenic can enter, diffuse, and react inside the nanoparticles. The Fe(0) core provides a strong driving force for the diffusion of As(V) and As(III) into the interior and the reduction and embedding of As(V) and As(III) in the solid structure instead of being retained as surface-bound species. More importantly, with arsenate converted to elemental arsenic, nZVI exhibits an arsenic storage capacity much greater than that of iron oxides compared on a mass or surface area basis. Furthermore, with Fe(0) in the core area, As(0), the product of As(V) reduction, is more stable than the surface-bound arsenate, which desorbs with an even modest shift in the solution pH. In short, the intraparticle encapsulation offers an ultimate scheme for capturing toxic contaminants such as arsenic and heavy metal wastes (e.g., mercury, lead, and chromium) and for site remediation and toxic waste disposal.
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ASSOCIATED CONTENT
S Supporting Information *
Details about the experiments and Figures S1−S6. This material is available free of charge via the Internet at http:// pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +86-21-65985885. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Grants 21277102 and 21307094) and the Science and Technology Commission of 308
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Letter
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