Visualizing Arsenate Reactions and Encapsulation in a Single Zero

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Visualizing Arsenate Reactions and Encapsulation in a Single Zero-Valent Iron Nanoparticle Lan Ling, and Wei-Xian Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04315 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 12, 2017

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Environmental Science & Technology

Visualizing Arsenate Reactions and Encapsulation in a Single Zero-Valent Iron Nanoparticle

Lan Ling and 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: +8621-15221378401, Fax: +86-21-6598 0041.

ACS Paragon Plus Environment

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ABSTRACT

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A nanostructure-based mechanism is presented on the enrichment, separation and immobilization

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of arsenic with nanoscale Zero-Valent Iron (nZVI). The As-Fe reactions are studied with

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Spherical Aberration Corrected Scanning Transmission Electron Microscopy (Cs-STEM). Near

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atomic-resolution (99.9%) for 30 minutes prior to

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the experiment. The iron nanoparticles for the STEM analysis were collected from batch reactors

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containing 10 ppm As(V) with pre-determined doses of iron nanoparticles (e.g., 0.2–5 g/L ).

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Batch reactors were sealed with screw caps and mixed on a shaker table at ambient temperature

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(approximately 22 °C). The collected nZVI particles were rinsed twice with high-purity nitrogen

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purged ethanol and stored in a nitrogen-purged sample vial for solid reactions before the STEM

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analysis.

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Methods. Electron microscopy characterizations are performed on a FEI Titan™ G2 80-300,

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which is a STEM (scanning transmission electron microscope) integrated with corrected electron

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optics. The system is equipped with a FEI X-FEG high brightness Schottky field emission source

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(brightness 1.8 × 109 A/cm² srad at 200 kV, beam current 1,300 pA in 0.2 nm spot), a CEOS

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hexapole/transfer doublet design aberration corrector, High-Angle Annular Dark-Field

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(HAADF) and Bright-Field (BF) detectors, an Electron Energy-Loss Spectrometer (EELS)

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(Gatan, UK), Energy Filtered TEM (EFTEM, Quantum SE/963 200 kV) and the FEI

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ChemiSTEM system.19, 20

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3D-ET and XEDS analysis are achieved with the FEI SuperX operated at a beam current of ~0.8

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nA and a probe size of ~0.2 nm at 200 KV. Each spectrum image of 512×512 pixels is acquired


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over a 10 min duration. The tilt series of XEDS images are collected from −64° to +64° with a 2°

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tilt increment. Data processing is achieved using the Inspect3D software (FEI) with a built-in

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algorithm of simultaneous iterative reconstruction technique (SIRT). 3D dataset of segmentation

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is then realized with the Amira software.23

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RESULTS AND DISCUSSION

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nZVI, an engineered nanomaterial has been increasingly used in environmental remediation and

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hazardous waste treatment.13−17, 24−27 For example, numerous reports have documented its

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effectiveness in detoxifying halogenated hydrocarbons in groundwater.28−32 Figure 1 presents

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images of nZVI (more are available in the Supporting Information, Figure S1). A fresh nZVI

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particle is spherical with diameter typically in the range of 20–100 nm. The BF and HAADF

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images (Figure 1a and 1b) provide complementary views of the core-shell structure. As shown in

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the high-resolution TEM images (Figure. 1c and 1d), nZVI has a classical core-shell structure,

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that is, an individual nanoparticle has local or nanoscale crystalline structures, comprises of an

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Fe(0) core and separates from other nanoparticles by a thin (2−5 nm) interfacial layer of iron

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oxides. Short-range lattice arrangements and some defective structures on the shell surface are

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also apparent from the high-resolution TEM micrographs (Figure 1d). The extremely thin

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dimension, disordered and defective nature of the oxide layer offer efficient electron passage and

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high reactivity via tunneling effects and defect channeling. 33 The core-shell structure bestows

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the nanoparticles with the reductive characteristics of Fe(0) and coordinative properties of iron

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oxides, offers a unique and powerful combination for the separation and sequestration of arsenic

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and heavy metal ions.13-17, 21, 22


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Figure 1. STEM images of clean nanoscale zero-valent iron (nZVI). (a) Bright Field (BF); (b)

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High-Angle Annular Dark-Field (HAADF). (c) and (d) are high-resolution TEM images.

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Previous work has showed that nZVI can quickly remove both As(III) and As(V) from water and

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also possess large capacity (>200 mg As/g Fe) for arsenic encapsulation. 16 The reactions of

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arsenate with nZVI can be manifested with the distributions of oxygen, iron and arsenic in spent

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nZVI particles. Figure 2 presents the XEDS elemental mappings of As(Kα), Fe(Kα), O(Kα),

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XEDS line profiles, color overlays and spectra of selected areas acquired from a single nZVI

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nanoparticle reacted with arsenate. The nanoparticles were from a reactor after 48-hour reactions

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with 10 mg/L arsenate. After the reactions, the nZVI nanoparticles is slightly distorted due to


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iron dissolution and precipitation of iron hydroxides (Figure 2a). The Fe mapping (Figure 2b)

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shows footprint of iron with a dense metallic core. From Figure 2d, the ring of oxygen matches

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the lower intensity area of iron, reconfirming that the surface layer is rich in oxygen. The arsenic

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mapping (Figure 2c) on the other hand illustrates a distinctive ring of arsenic just on top of the

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Fe(0) core surface. This ring of arsenic is intense and locates exactly between the Fe(0) core and

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iron oxide shell, resulting from the reduction of As(V) and accumulation of As(0). Previous work

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with XPS showed that arsenic accumulated inside the particle is mostly the reduced As(0) and

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As(III),15, 16 we now prove that arsenic can fully penetrate the surface oxide layer.

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More detailed information on the intraparticle distributions of oxygen, iron and arsenic in a

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reacted nZVI particle is obtained from the XEDS line profiles (Figure 2a, Figure S2). The

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arsenic profiles, peak positions and heights offer valuable information on the rates of

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intraparticle mass transfer and reactions, also on the speciation [i.e., As(V), As(III) and As(0)]

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and total amount of arsenic in the nanoparticle. The iron abundance ascends rapidly within first

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4–5 nm and reaches a plateau at the depth of 8−9 nm under the particle surface. The Fe scan

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shows a sharp valley matching the location of the arsenic ring. The oxygen profile climaxes at

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about 3 nm from the water-nanoparticle interface just outside the Fe(0) core. This is consistent

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with the core-shell model that the outer surface iron is more oxidized with oxygen-rich oxides

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while the inner part is less oxidized with more Fe(II). From XEDS quantifications (Figure 2g-2j

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and Figure S2), the arsenic abundance in this thin layer is estimated at 19.44 wt% and accounts

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for more than half of the arsenic within the whole particle.


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Figure 2. High-resolution STEM-XEDS elemental mappings and XEDS quantifications of

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the Fe-As reactions. (a) HAADF image and XEDS line profiles, (b) Fe, (c) As, (d) O, (e)

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Fe+As, (f) Fe+As+O, (g-i) XEDS quantifications, and (j) XEDS spectra. Signals were collected

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from an iron nanoparticle after 48-hour reactions with 10 mg/L arsenate.


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Figure 3. 3D chemical mapping of spent nanoscale zero-valent iron after reactions with

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arsenate. (a) 3D elemental distributions of Fe (blue), O (green) and As (red). (b-j) are a series of

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mappings acquired from spent nanoscale zero-valent iron reacted with arsenate in the tilt range

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of +64° to -64°. The sample was from a batch reactor containing 10 mg As(V)/L and 0.5 g/L

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nZVI after 48 hours. A corresponding video of the 3D tomography can be found in the

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Supporting Information (Movie S1).

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The nano-structure based encapsulation can be visualized via electron tomography, which can

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generate high-resolution 3D view of pollutant distribution in a single nanoparticle. To achieve

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3D visualization, a series of projection (2D) images of a single nanoparticle are acquired from a

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wide range of angles by tilting the sample relative to the probe. A full 3D visualization is then


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established using a cross-correlation algorithm for image shift, tilt alignments and simultaneous

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iterative reconstruction (FEI Inspect3DTM, details are provided in Supporting Information). 34-39

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A video of 3D electron tomography showing arsenic in a single nZVI nanoparticle is provided in

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the Supporting Information (Movie S1). Figure 3 presents a series of projection images from

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selected angles. The XEDS tilt series and the resulting 3D tomography (Movie S1) discover a

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distinct feature of the As-nZVI reactions, that is, a thin and continuous layer of arsenic (in red

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color, ~23 Å in thickness, Figure 3) is deposited beneath the iron oxide surface layer and just on

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top of the Fe(0) core after 48-hour reactions with 10 mg/L As(V) (Na2HAsO4). The distribution

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sphere of arsenic is larger than that of the iron core but smaller than the distribution ring of

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oxygen. This technique generates extraordinary details and insights unattainable from

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conventional 2D XEDS scans as the state-of-the-art Cs-STEM can obtain atomic resolution

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structural imaging as well as high-resolution (~ 1 nm3) 3D elemental mapping. Although

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previous work has suggested that arsenic can be reduced and immobilized by zero-valent iron,

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this work, for the first time visualizes the formation of a continuous layer of arsenic within nZVI

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nanoparticles. This clearly points to a large capacity and stability of nZVI for arsenic

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encapsulation.

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In direct contrast, configurations of the XEDS mapping and electron tomography on the

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arsenate-ferric hydroxide (FeOOH) reactions (Figure 4 and 5) show that the arsenate is attracted

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rather loosely to the surface, that is, the arsenic atoms are to a large degree detached from the

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surface. The cyan color in Figure 4 and 5 reflects the nature of blue (iron) (Figure 4a) and green

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(oxygen) (Figure 4b) mixtures, suggesting that the particle body consists of Fe and O with As

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floating on top and/or near the surface. There is no intraparticle diffusion or accumulation of

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arsenic inside the oxide nanoparticle. The atomic ratios of oxygen, iron and arsenic from XEDS


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Figure 4. High-resolution STEM-XEDS elemental mappings and XEDS line profiles of

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ferric hydroxide (FeOOH)-arsenate reactions. (a) Fe, (b) O, (c) As, (d) Fe+As+O, (e)

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HAADF image, (f) XEDS line profiles (quantifications and spectra are provided in Supporting

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Information, Figure S3). The sample was from a batch reactor with 10 mg/L As(V) and 0.5 g/L

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Fe. Reaction time was 48 hrs.

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quantification implies no arsenate reduction (Figure S3).11, 40 XEDS quantitative analysis shows

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that weight percentage of arsenic is in the range of 2.48% to 2.97% on the FeOOH surface and

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average 1.75% for the whole particle. Furthermore, arsenic distributes unevenly on the FeOOH

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surface as shown in the 3D tomography (Movie S2 and Figure S3). The 3D tomography thus


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Figure 5. 3D chemical mapping on arsenate-goethite (FeOOH) reactions. (a) 3D elemental

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distribution of Fe (blue), O (green) and As (red). (b-j) are a series of mappings acquired from

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spent FeOOH reacted with arsenate in the tilt range of +64° to -64°. The sample was from a

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batch reactor with 10 mg As(V)/L and 0.5 g/L Fe after 48 hours. A corresponding video of the 3D

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tomography is provided in the Supporting Information (Movie S2).

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provides direct confirmation of a widely held postulation that arsenic is attracted to the goethite

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surface by forming surface complexes.11, 40 It further confirms that arsenic covering on the oxide

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surface is far from continuous and the amount of arsenic on iron hydroxide (FeOOH) is much

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less than the continuous layer of arsenic in nZVI.


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The observed pattern of arsenic in the nZVI nanoparticle reflects the net result of the arsenic

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diffusion and reactions within the unique nanostructure. As presented earlier, nZVI consists of

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two nanomaterials. The core is zero-valent or metallic iron, which serves as a reductant for

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arsenite and arsenate reduction. The surface layer of iron oxides is formed from spontaneous

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oxidation of Fe(0) and can be represented with an average stoichiometry of iron hydroxide

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(FeOOH).15,16 Furthermore, the nZVI shell consists of a mixed Fe(II)/Fe(III) phase close to the

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Fe(0) interface and a predominantly Fe(III) at the outer surface of nZVI, resulting in a clear

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phase junction. Thus nZVI possesses the combinatorial properties of zero-valent iron and iron

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oxides. The iron oxide layer is polar and/or charged in water, offers electrostatic attractions and

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sorption sites to both cations and anions in water while the core supplies electrons or the

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reducing capability for further diffusion, reduction and deposition of arsenic and heavy metals

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such as Ni and Cu. The intraparticle diffusion of arsenic is highly favored since the core region

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has the highest concentration of Fe(0) for the arsenic reduction and immobilization. The grain

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boundary provides additional support to the observed arsenic diffusion in nZVI particle, that is,

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arsenic atoms diffuse preferably along the defective, high energy, and non-equilibrium

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polycrystalline grain boundary of iron oxides 41,

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patterns presented in Figure 2 and 3 are also consistent fully with results from previous XPS

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characterizations.15, 16

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Atomic-resolution STEM images of nZVI and a conceptual model on the sorption, precipitation

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and reduction of arsenic by nZVI are illustrated in Figure 6. As shown in Figure 6a and 6b, the

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reacted nZVI particle retains the classical core-shell structure. Furthermore, short-range lattice

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arrangements in the shell area are also apparent from the high-resolution STEM micrographs

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with individual iron atoms clearly identified under the HAADF model. nZVI formed from

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The iron, arsenic and oxygen distribution


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sodium borohydride reduction of ferric chloride has a body-centered cubic structure (bcc) in the

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space group Oh9

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orientation in unprocessed HAADF image. Higher magnification image (inset in Figure 6a)

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provides further evidence that a dense core displayed along (111) zone axis. The bright columns

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are the iron atoms and show subtle details about the shape and faceting of the nanocrystals. The

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intensity line profile (in red color, Figure 6a) measured across the periodical arrangements

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clearly signals individual atoms (highlighted with the blue circles). A HAADF intensity peak

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matches precisely to an atomic column. This is further evidenced by the corresponding electron

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diffraction patterns.

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The high-resolution STEM images of nZVI provide remarkable details on the atomic

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arrangements within the core-shell structure and complement previous work of XPS and EXAFS. 15,

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of iron structure model in the (111) lattice plane with the lattice constant a of 2.405 Å. The

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interatomic spacing along this direction is measured at 1.70 Å, in good agreement with the

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calculated interatomic spacing along direction, that is,

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XEDS, this surface layer is enriched with arsenic as a result of the reductive deposition. The

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observed difference of peak intensity is likely caused by the substitution or replacement of iron

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for the arsenic atoms. 43, 44

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As shown in the conceptual model (Figure 6c), formation of the thin layer of arsenic between the

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Fe(0) core and iron oxide shell offers the most direct evidence on the nano-encapsulation and

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proves that the outer layer, especially the Fe(0)/oxide interface is the frontline for the As-Fe

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reactions. Evidence obtained in this work has important implications on applications of iron

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nanoparticles for arsenic separation and immobilization. The total capacity of nZVI for arsenic

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Figure 6a shows the structure of spent nZVI predominantly along (111)

In the Annular Dark Field (ADF) image (Figure 6b), the seven red points illustrate a unit-cell


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√

=1.70 Å. As observed from

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removal is the sum of surface sorption and immobilization by the Fe(0) reduction. Since the

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driving force for arsenic diffusion in nZVI is chemical reduction and successive encapsulation,

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the chemical reduction of arsenic is much greater than that of the surface sorption with capacity

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for arsenic immobilization governed primarily by and proportional to the amount of Fe(0).

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Laboratory batch experiments have confirmed that arsenic removal capacity can reach as high as

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200 mg As/g Fe.15,16

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Figure 6. Atomic-resolution STEM images of spent nanoscale zero-valent iron (nZVI) and

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a conceptual model on arsenate-nZVI reactions. (a) Unprocessed High-angle Annular Dark-

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field (HAADF) image of a portion of a nanoparticle, inset is a higher-magnification view of the

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core area. The HAADF intensity line profile (in red color) measured across the periodical

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arrangements signals individual atoms (highlighted with the solid blue circles). (b) Annular

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Dark-Field (ADF) image shows that Fe atoms in the core area are predominantly in the body-

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centered cubic (111) lattice plane. (c) A conceptual model on the sorption, precipitation and

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reduction of toxic metals with nZVI.


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Additionally, the reduced arsenic, As(0) is very stable in the presence of Fe(0). This is critical for

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the immobilization and stabilization of toxic heavy metals such as mercury, arsenic, and

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radioactive materials.22 Our laboratory experiments showed the nZVI-bound arsenic is much less

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sensitive to solution pH changes. For example, optimal pH for arsenic sorption to iron oxides is

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less than 4 with little sorption observed above pH 8. With nZVI, high efficiency (>90%) can be

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maintained over a wide range of solution pH (~3−10) (Figure S4). The presence of Fe(0) thus

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serves as an enduring shield or long-term stabilizer, which inhibits oxidation, dissolution and

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leaching of pollutants.

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Reactions with solid surfaces are the principal mechanisms controlling speciation, distribution,

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transformation, bioavailability, ultimate fate and toxicity of environmental contaminants, thus

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are fundamental in environmental, geological and colloidal chemistry.

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knowledge on the surface reactions of most environmental contaminants is still limited, largely

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impeded by the lack of effective tools for direct and in situ observations. We demonstrate that for

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the first time, reactions of arsenic with iron nanoparticles can be “seen” via high-resolution 3D

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electron tomography at near atomic resolution. This technique offers new opportunities in the

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investigation of pollutant transformation and detoxification by generating simultaneous high-

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resolution structural characterization and high-accuracy identification and quantification of

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contaminants in the solid phase. 34, 35 With further improvements and refinements, this technique

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will undoubtedly expand our understanding on the intraparticle mass transfer, catalysis,

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transformation and detoxification of environmental toxins in the heterogeneous environmental

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media.

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ASSOCIATED CONTENT


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45, 46

Nevertheless, our

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Supporting Information

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Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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Movies S1-S2, Figures S1-S4, and more experimental details are available in the Supporting

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Information.

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ACKNOWLEDGEMENTS

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Financial support from the National Natural Science Foundation of China (NSFC Grants

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21307094, 21277102 and 21677107), the Collaborative Innovation Center for Regional

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Environmental Quality and the State Key Laboratory for Pollution Control and Resource Reuse

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are acknowledged.

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AUTHOR INFORMATION

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Corresponding author

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Email: [email protected], Phone: +86-21-15221378401, Fax: +86-21-6598 0041.

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Notes

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The authors declare no competing financial interests.

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