Mapping the Reactions in a Single Zero-Valent Iron Nanoparticle

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Mapping the Reactions in a Single Zero-valent Iron Nanoparticle Lan Ling, Xiao-yue Huang, Meirong Li, and Wei-Xian Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02233 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Environmental Science & Technology

Mapping the Reactions in a Single Zero-valent Iron Nanoparticle

Lan Ling*, Xiaoyue Huang, Meirong Li, Wei-xian Zhang*

State Key Laboratory for Pollution Control School of Environmental Science and Engineering Tongji University,1239 Siping Road Shanghai, China 200092

*To whom correspondence should be addressed. Email: [email protected], Phone: +86-21-18501682197, Fax: +86-21-6598 0041.

ACS Paragon Plus Environment


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Abstract

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Nanoscale zero-valent iron (nZVI) posseses unique functionalities for metal-metalloid

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removal and sequestration. So far, direct evidence on the heavy metal-iron reactions in

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the solid phase is still limited due to low concentration of heavy metals and small size

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of nanoparticles. In this work, angstrom-resolution spectral mappings on the reactions

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of nZVI with chromate, arsenate, nickel, silver, cesium, and zinc ions are presented.

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This work was achieved with spherical aberration-corrected scanning transmission

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electron microscopy integrated with high-sensitivity X-ray energy-dispersive

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spectroscopy scanning transmission electron microscopy (XEDS-STEM). Results

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confirm that iron nanoparticles have a core-shell structure. In addition, the removal

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mechanism significantly depends on the standard potential E0 (E0 is standard potential

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w.r.t. standard hydrogen electrode at 25 °C when free ion activity is 1.). For strong

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oxidizing agents, such as Cr(VI), the removal mechanism is diffusion and

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encapsulation in the core area of the nZVI particle. For moderate oxidizers, such as

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As(V) with E0 more positive than that of iron, the removal mechanism is adsorption at

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the surface, followed by diffusion and encapsulation into the particle between the core

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and the shell. For metal cations with an E0 close to or more negative than that of iron,

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such as Cs(I) and Zn(II), the removal mechanism is sorption or surface-complex

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formation. For metal cations with E0 much more positive than that of iron, such as

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Ag(I), the removal mechanism is rapid reduction on the surface of nZVI. Meanwhile,

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metals with E0 slightly more positive than that of iron, such as Ni(II), can be

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immobilized at the nanoparticle surface via sorption and reduction. The synergetic


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effects of sorption, reduction, and encapsulation mechanisms of nZVI lead to rapid

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reactions and high efficiency for treatment and immobilization of many toxic heavy

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metals. Results also demonstrate that the XEDS-STEM technique is a powerful tool

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for studying reactions in individual nanoparticles and is particularly valuable for

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mapping trace-level elements in environmental media.



Graphical Table of Contents


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Introduction

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Heavy metal-metalloid ions, such as Cr(VI), As(V), Ni(II), Zn(II) and Ag(I) are

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nonbiodegradable, bioaccumulative and toxic, which can accumulate in living

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organisms and most of those are known to be highly toxic or carcinogenic.1,2 A major

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barrier preventing the fundamental understanding of transport, and reaction

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mechanisms of trace metal contaminants (80%) arsenic is on the outer shell (~20% volume) of nZVI, and that 60% of

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the total arsenic deposits between the iron core and the iron-oxide shell (~10%

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volume).

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After reacting with Cr(VI), oxidation of the iron nanoparticle is much more extensive

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than that with As(V), as indicated by the increase in oxygen passing through the iron

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core of the particle (Figure 2). Meanwhile, Cr and O are entrenched deeper in the iron

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particle whereas As amasses mostly at the Fe(0)–oxide interface sitting on the surface

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of the Fe(0) core. In situ time-dependent x-ray absorption spectroscopy has shown

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that the oxyanions undergo two stages of transformation upon adsorption at the nZVI

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surface. The first stage corresponds to breaking metal/metalloid–O bonds at the

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particle surface, and the second stage involves further reduction and diffusion of As

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and Cr across the thin oxide layer enclosing the nanoparticle.40 The distinctive

39,40

XEDS quantification further demonstrates that



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reactive pathways for Cr(VI) and arsenate(V) are expected from their different

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standard redox potentials [E0 Cr(VI) = +1.36 eV and As(V) = +0.56 V]20,21,40 (see

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Table S1 in the supporting information), which results in chromium diffusion into the

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core of nZVI particles whereas the enriched arsenic at the surface of the Fe(0) core

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region has limited mobility into the interior of the metal core.

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After reacting with Ni(II), the spherical nZVI particles are devoid of the Fe(0) core,

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leaving behind doughnut-like or horseshoe-like structures with large cavities in their

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interior (Figures 3a–3e). The reacted nZVI consists of a bright circlet containing

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metallic iron Fe(0) in the middle and two outer lower-intensity shells of iron oxides

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(Figure 3a). The iron mapping (Figure 3b) confirms the disappearance of the Fe signal

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in the core area, and the Fe footprint takes on a doughnut shape with two dimmer

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exterior layers (approximately 3–5 nm thick). The two exterior layers consist of

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oxygen and iron. The Ni mapping and elemental overlay (Figures 3d and 3e)

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reconfirm that nickel distributes over the surface of the spent particle and accumulates

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predominantly on the surface of the iron core just inside the outer shell (surface), with

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some Ni nanoparticle enriched inside the “hollow” area. The Ni nanoparticles thus

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formed range in size from 2 to 4 nm. In our previous work, electron-energy-loss

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spectroscopy demonstrated that the Ni nanoparticles contain mainly elemental Ni.42 In

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the present work, we quantify the relative abundance of Fe, O, and Ni over the spent

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particle. The Fe:O:Ni atomic ratio over the entire spent particle is 43.4:55.1:1.5,

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whereas the ratio Fe:O:Ni is 29.8:68.4:1.8 in the outer shell and 50.8:48.3:0.9 in the

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inner shell. The results of the batch experiments show that 94% of the Ni(II) is


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removed within 10 min and 100% in less than 3 h, whereas the “hollowed-out” effect

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appears after 5 min. The reduced nickel on the nZVI surface can alter the electronic

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properties of iron and accelerate the oxidization of Fe(0).42 Because the electron

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transfer from Fe(0) is often suggested as the rate-limiting step for the overall reaction,

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defects on the particle surface may offer potential breakthrough conduits for the

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nickel ions to quickly “attack” the core area, which is filled with Fe(0)—the electron

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donor for Ni(II) reduction. The close standard redox potential with iron and the

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relatively slow reaction rate may facilitate Ni diffusion into the core area for more

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Fe(0). Furthermore, the Kirkendall effect may be a mechanism for the formation of

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hollow nanoparticles.

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fast-moving Fe ions through the oxide layer and a balancing inward flow of vacancies

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in the redox reaction.43 The net directional flow of matter is balanced by an opposite

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flow of vacancies, which condense into pores. The coexistence of multiple-structure

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spent nZVI suggests that significant variations in the particle lifespan may be the case

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at the microscopic level. Factors such as physical dimension, grain size, crystallinity,

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and other structural attributes of the Fe(0) core and the oxide layer may influence the

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rate of iron oxidation.44

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After reacting with Ag(I), the characteristics of the nZVI core-shell structure become

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less noticeable, and the shape of the nZVI nanoparticles is completely distorted with

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asymmetrical boundaries and dendritic structures similar to a product of

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electrochemical reduction (Figure 3f). The iron mapping (Figure 3g) shows the

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footprint of iron, which consists of a small but dense spherical core enclosed by a

43

The voids develop because of the outward transport of



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dimmer outer layer. Ag mapping (Figure 3h) shows that the Ag outer layer condenses

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and completely covers the metallic iron particle, corresponding to the bright dendritic

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structures in the HAADF image. It may be a mixture of metallic Ag and a small

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amount of oxygen resulting from the reduction reactions. The oxygen is widely

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distributed over the whole particle although it is less concentrated in the Ag-rich area

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(Figures 3i and 3j). The relative abundance of Fe, O, and Ag over the spent particle

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gives an Fe:O:Ag atomic ratio of 89.7:1.9:8.4 in the dense core and of 3.0:5.6:91.4 in

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the outer layer. Quantification further confirms that the iron nanoparticles are covered

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by pure metallic Ag nanostructures with trace iron oxide.

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The distinctive reactive pathways for Cr(VI), As(V), Ni(I) and Ag(I) may result from

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their different standard redox potentials, reaction speed, different ionic forms and

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coordination.2,16,18,21,23,27–30 The oxyanions undergo two stages of transformation upon

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adsorption at the nZVI surface, including Cr-O and As-O bonds breaking, further

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reduction and diffusion across the oxide layer.40 The standard redox potential of Ni(II)

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is slightly more positive than that of Fe(0). Our previous work with high-resolution

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x-ray photoelectron spectroscopy demonstrates that Ni(II) is immobilized at the

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nanoparticle surface by both sorption and reduction and that the oxidized iron

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nanoparticles cannot reduce the Ni(II),45,46 thereby facilitating the diffusion of Ni into

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the Fe(0) core and accelerating the “hollowing out” of the Fe(0) core. In comparison,

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Ag(I) has a standard redox potential that is significantly more positive than that of Fe,

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so Ag(I) can be reduced by Fe(II). The surface and morphological characteristics of

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nZVI change with the progression of Ag+ electrochemical reduction, metallic Ag


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deposition, and iron ionization, dissolution, and hydroxide precipitation. As a strong

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oxidizing agent, Ag is quickly reduced to Ag(0) and deposits onto the surface of the

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nZVI particles. Meanwhile, nZVI is oxidized to Fe(II) and Fe(III), and the spent

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particle is covered with iron oxide.

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Sorption and precipitation of nZVI. After reacting with Cs(I), the spent nZVI

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particles become brighter in HAADF images and still preserve the core-shell structure

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with a bright core and a dim shell (Figure 4f). Of the three major elements in the

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system, Cs has the largest atomic number of 55, O = 8, and Fe = 26, so it is expected

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to be brighter than the surrounding area in HAADF images. Cs, Fe, and O mappings

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(Figures 4b–4d) show the footprints of Fe, O, and Cs, respectively. The metallic core

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of the spent nZVI particle is much denser whereas the signal intensity for iron gets

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dimmer near the exterior surface. From Figure 4d, the ring of oxygen matches the

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lower-intensity area of iron, reconfirming the remaining core-shell structure. The

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volume occupied by cesium is slightly greater than that occupied by oxygen, which

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may suggest the sorption or surface complex of cesium, which sits just above the

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particle surface. Cs mapping (Figures 4c and 4e) and the color overlay of the three

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elements illustrate that Cs is uniformly distributed over the whole zero-valent iron

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core just on the iron oxide shell, which makes the particle appear brighter in HAADF

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images.

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After reacting with Zn(II), HAADF images show that the spent nZVI particle retains

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its core-shell structure and is decorated with tiny agglomerates. In the system, Zn

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(atomic number 30) is in the same row of the periodic table as Fe, and so has a similar



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contrast for the same thickness in HAADF images. Thus, the agglomerates do not

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appear brighter than the nZVI particle. Fe and O mappings (Figures 4h and 4i) show

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the footprints of iron and oxygen and illustrate the retained core-shell structure. Zn

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mapping (Figure 4g) and the color overlay of the three elements (Figure 4j) show

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clearly that Zn forms clumps and sticks loosely to the zero-valent iron surface. This

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may explain why the maximum Zn(II) removal capacity is reportedly more than one

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order of magnitude greater than the theoretical uptake capacity afforded by surface

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adsorption.47

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Cesium is considered the most active metal in the alkali family, which consists of the

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elements in group 1 (IA) of the periodic table. No reduction is involved in the

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sequestration process, as expected from the significantly more negative standard

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redox potential of cesium (E0 Cs = −2.923 eV) compared with iron (E0 Fe=−0.41 V).43

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Cesium exists in solution as hydrated Cs+ cations, and its speciation is independent of

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pH and redox potential.48 Cesium does not hydrolyze or form precipitates or sparingly

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soluble

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hydrosphere-complexing agents nor intrinsic colloids. Sorption of cesium is rather

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inefficient owing to the large size and low charge of the Cs ion. The maximum Cs(I)

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removal rate with nZVI is 38.6% at pH 6. Cs(I) uptake is caused by nonspecific

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sorption or surface complexation with the iron oxide shell.

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In addition, no reduction is involved in the Zn(II) sequestration process with nZVI

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because the standard redox potential of zinc (E0 Zn = −0.76 eV) is more negative than

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that of iron (E0 Fe=−0.41 V).41 Zn(II) adsorption is strongly influenced by the pH of

compounds,

and

neither

does

it

form

complexes


with

typical

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the solution. At elevated pH, bulk precipitation of zinc hydroxide may occur. At the

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early stage of the reaction (

Mapping the Reactions in a Single Zero-Valent Iron Nanoparticle

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