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Visualization of Silver Nanoparticle Formation on Nanoscale Zero-Valent Iron Lan Ling, Chenliu Tang, and Wei-Xian Zhang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00259 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 9, 2018

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Environmental Science & Technology Letters

Visualization of Silver Nanoparticle Formation on Nanoscale Zero-Valent Iron

1

Lan Ling,†,‡ Chenliu Tang,†,‡ Wei-xian Zhang*†,‡

2

†State Key Laboratory for Pollution Control and Resource Reuse, School of

3

Environmental Science and Engineering, Tongji University, 1239 Siping Road,

4

Shanghai, 200092, China

5

‡Shanghai Institute of Pollution Control and Ecological Security, Shanghai,

6

200092, P.R. China

7 *To whom Correspondence should be addressed, E-mail: [email protected]. Phone:

8 +86-15221378401 9 1

ACS Paragon Plus Environment


10 Abstract 11

Instant reactions of Ag(I) on nanoscale zerovalent iron (nZVI) were visualized

12 with spherical-aberration-corrected scanning transmission electron microscopy

13 (Cs-STEM) integrated with X-ray energy dispersive spectroscopy (XEDS). In

14 particular, the rapid growth of pure silver nanoparticles on nZVI was captured with

15 high-resolution STEM imaging. Dissolved silver ions are attracted to the

16 negatively-charged iron nanoparticles and quickly reduced to metallic silver on the

17 defective and electron-rich iron surface. Small (~1-2 nm) and highly reactive silver

18 nanoparticles are formed along the long chains of nZVI. Results reinforce the

19 applications of nZVI for rapid enrichment, complete separation and recovery of

20 precious metals such as Au and Ag from trace-level sources such as wastewater,

21 and demonstrate the potential of analytical electron microscopy for mapping

22 pollutant reactions on the surface of nanoparticles.

23

24

25 2


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26 ■ INTRODUCTION

27 Designing novel materials for cost-effective recovery of precious metals from

28 wastewater, groundwater and seawater has captivated scientists and engineers

29 for generations. Precious metals such as Au, Ag, Ni, Pt are ubiquitous in

30 wastewaters and natural waters, however, their concentrations are usually too low

31 to justify any large scale profitable operation. Recent progress on nanomaterials

32 rekindles optimism and also creates new demands to understand separation and 33 recovery of metals from water.1-4 For example, the global production of silver

34 nanoparticles (AgNPs), one of the highly desired nanomaterials has been 35 estimated at least 500 tons per year.5,6 Through the wide use and disposal

36 of silver-containing products, AgNPs are discharged into the environment, and

37 the transport and transformation of AgNPs in the environment result in the release

38 of persistent and toxic silver, which has received growing attentions due to its 39 potential toxicity toward human health and the environment. 7-10

40

The core-shell structured nanoscale zero-valent iron (nZVI) is a unique

3



41 example of the structure – reactivity relationship of nanomatreials, and

42 offers significant advantages of nanomaterials for cost-effective pollutant 43 transformation and recovery, especially for smelting wastewater.11-20 The

44 surface layer of iron oxides/hydroxides is negatively charged and

45 possesses high-density sites for the attraction and sorption of metal cations

46 while the metallic iron core provides the reducing power for rapid reduction

47 and enrichment of heavy metals. The nZVI extends the synergistic functions 48 of two nano-components.21-24 Previous studies show that heavy metal 49 loading in nZVI can achieve very rich levels (e.g., 2.8 g Ag/g nZVI).4 Recent

50 work has covered aspects of solution chemistry of the nZVI reactions with 51 heavy metals.11,13,18,22 In particular, rapid reactions, accelerated corrosion of 52 Fe(0), and catalytic functions of bimetallic Ag-nZVI have been reported.25,26

53

In this work, we present direct imaging on the rapid Ag enrichment with

54 nZVI under different environments (anaerobic/aerobic, with/without sulfide 55 ions, different Ag+ concentrations). The state-of-the-art X-ray energy

4


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56 dispersive spectroscopy (XEDS) integrated with an aberration-corrected 57 scanning transmission electron microscopy (Cs-STEM)15,16,27,28 is utilized

58 for elemental mapping of Ag(I)-Fe(0) reactions. The STEM-XEDS technique

59 enables direct visualization of the nanoscale structural and compositional

60 changes. These findings are corroborated with X-ray diffraction (XRD) and

61 atomic-resolution TEM. Results provide insights on the formation of silver

62 nanoparticles and further demonstrate the potential of nZVI for enrichment,

63 separation and recovery of precious metals from wastewater.

64 ■ MATERIALS AND METHODS

65

Procedures used in the preparation and batch experiments of nZVI reactions

66 with Ag(I) have been published previously27-29 (more details are provided as

67 Supporting Information). A sulﬁde solution was prepared by dissolving 0.1 mM

68 Na2S (Sigma-Aldrich) solution in 0.05 mM NaNO3 in an anaerobic chamber 69 according to the reported methods.30-32 Batch experiments were carried out in 70 40 mL polyethylene bottles, with 0.1-0.5 g•L-1 nZVI added to 30 mL 2 5



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71 µg•L-1-250 mg•L-1 Ag(I) solutions. The bottles were sealed with screw caps

72 and mixed on a shaker table (180 rpm) at room temperature (22 ± 1°C).

73 Before the addition of nZVI, the solutions were purged with high-purity

74 nitrogen (>99.999%) for 30 minutes to eliminate the influence of dissolved

75 oxygen (DO).The suspensions were separated from water with a magnet

76 and rinsed twice with anhydrous ethanol (>99.9%)，and then stored in a

77 nitrogen glovebox before S/TEM microscopy and XRD characterizations.

78 Electron microscopy characterizations were performed with a FEI Titan 79 G260−300 operated at 200 kV and a ChemiSTEMTM system. To minimize

80 the effects of specimen drift, a drift-correction mode was applied during the

81 acquisition of XEDS mappings. Cu and C deconvolutions were set during

82 data

process.

The

Cliff–Lorimer

equation

was

used

for

XEDS

83 quantifications. More details on the Cs-STEM methods can be found 84 elsewhere. 15,16,27,28

85 ■ RESULTS AND DISCUSSION 6


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86 Microscopic

observations

Spherical

aberration-corrected

scanning

87 transmission electron microscopy (Cs-STEM) integrated with X-ray energy

88 dispersive

spectroscopy

(XEDS)

enables

direct

and

high-resolution

89 observation on the compositional and structural changes of nanoparticles

90 and 2- and 3-dimensional visualization of pollutants reactions with

91 nanoparticles.

15,16,27,28

As shown in Figure 1, HAADF image and XEDS

92 mappings of spent nZVI after 15-second reactions with Ag(I) confirm that

93 the spent nZVI nanoparticles are still spherical in shape with sizes ranging

94 from 20 to 100 nm, and preserve the core-shell structure. According to XEDS

95 quantifications, the spent nZVI particle in Figure 1 are partly oxidized, and the

96 nZVI particle is consist of more than 70% Fe(0) and less than 30% iron oxides.

97 This Ag(0)-on-Fe(0) configuration endows the nanoparticles a rich and more

98 efficient electron source for reduction, as well as high-density adsorptive 99 sites and the high-pH surface for metal cations precipitation.28,33 Similar to

100 the fresh nZVI (Figure S1), the reacted nZVI particles retain a chain

7



101 structure due to the magnetic attractions and colloidal aggregation. The

102 surface is decorated with newly formed and much brighter nanoscale

103 particles. The newly-formed particles have sizes in the range from 2 to 5 104 nm.34 From previous work, the oxide shell of nZVI is a chemically

105 heterogeneous and defective structure, which offers efficient electron 106 passage and high reactivity via tunneling effects and/or defect channels.35-37

107 The newly-formed silver nanoparticles (2~5 nm) radiate from the nZVI

108 surface, and some are embedded in the oxide layer (more figures of nZVI

109 with defective shells are provided in Figure S2 and S3). Figures 1b-d show

110 the Fe Kα, O Kα, and Ag Lα elemental mappings, respectively. The Ag Lα

111 mapping and the colour overlay of Fe Kα, O Kα, and Ag Lα signals (Figure

112 1d, f) confirm that the brighter clusters in the HAADF image are metallic

113 silver. Compared to the freshly prepared nZVI particles, the outline of spent

114 nZVI slightly blurs, that is, the oxide shell has undergone structural and

115 componential changes (Figure 1d). Distribution of O is found to be

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116 considerably broader than that of fresh nZVI. Notably, there are several

117 distinct stripes on the surface (area I, II, III, Figure S2). The atomic ratio of

118 iron and oxygen (Fe:O) in these areas obtained with XEDS quantification is

119 around 1.664, indicating that these stripe-like structures may be iron oxides.

120

Close inspection of the spent particles shows that the formed silver

121 nanoparticles mostly agglomerate and overlap unevenly on the nZVI

122 surface. With the high reactivity of nZVI at the initial stage, the mass

123 transfer of silver may be limited by the low-intensity mixing in the solution,

124 which can significantly impact the diffusion of Ag(I) to reach the nZVI

125 surface. A likely explanation is the effect of localized galvanic couples 126 between the metallic silver and iron,38,39 which can accelerate localized

127 reactions between Ag(I) and nZVI, and lead to quick and uneven silver

128 distribution of the reduced silver. Moreover, the oxide films are consisted of

129 several types of iron oxides such as Fe2O3, Fe3O4, FeOOH etc., which

130 present varied conductivity and different reactivity of the shell.

9


35,36


131 Therefore, reactions of silver with nZVI are thus affected by even the minor

132 difference of the nZVI nanoparticles, causing silver reduction to occur 133 preferentially at defects or sites with slightly better electron conductivity.40,41

134 Meanwhile, XEDS quantifications show that the element atomic ratio of Ag

135 and Fe in core area of nZVI particle (area IV, Figure S2, S4) is close to 0.

136 That is, there is no silver penetrating or diffusing across the oxide shell and

137 depositing inside the nZVI particles. Initially, silver ions are attracted to the

138 shell layer, speedily reduced to metallic Ag and deposited on the surface of

139 nZVI particle in the form of Ag nanoparticles, which trigger futher oxidation

140 of iron nanoparticles at the same time.

141

After 45-second reactions, most silver nano-structures distribute along

142 the chain and face the outside direction, and with only a few silver

143 nanoparticles embedded into the inner iron chains (Figure S5). Individual

144 silver nanoparticles, with an average size 2~5 nm, overlap and aggregate to

145 form larger clusters along the magnetic iron chain. However, the silver

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146 dendrites preferentially present at the outside direction of iron chains

147 probably due to space restriction for growth. This is consistent with

148 diffusion-limited aggregation (DLA) model, which is widely reported for the 149 dendrites growth.42,

43

The Fe distribution (Figure S5b) in chain-like

150 structure indicates that the intensity of iron becomes lower compared to the

151 fresh nZVI (Figure S1), suggesting that the iron core has been oxidized and

152 partially dissolved. However, the distributions of Fe Kα, O Kα (Figure S5b, c)

153 show that the inner iron chains still preserve core-shell configuration. The

154 behavior for the localized distribution of silver might be attributed to ‘steric

155 effect’ and fast reduction kinetics. That is, the aggregation of nZVI, fast Ag

156 reduction, and limited mixing of bulk solution during reactions, can inhibit

157 the mass transfer of silver to the surface of nanoparticles, and result in the 158 uneven reaction of nZVI with Ag(I) to some extent.41,42 Additionally, with the

159 Ag deposition, electrical conductivity of metallic silver may promote iron 160 oxidation, and deposition of Ag+ to Ag seeds on the nZVI chain, as these

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161 silver particles could work as a better conductor than iron for the electron

162 transfer (the single electron s-orbital occupancy of Ag configuration results 163 in an exceptionally good electrical conductivity of 6.30 × 105 S/cm).39

164 After 90-second reactions, the flower-like Ag nanoparticle clusters (~ 20 nm)

165 come out from the clear edges of nZVI chains. Figure 2 presents images of

166 spent nZVI after 90-second reactions, more images of spent nZVI after

167 reactions with Ag(I) of different concentrations are provided in the

168 Supporting Information (Figure S8). According to HAADF image (Figure 2a),

169 several brighter flower-like clusters are formed on the dark color nZVI chain.

170 Specifically, the Ag nanoparticles overlap with each other and form a

171 hierarchical structure. Atomic-resolution TEM image (the circled silver

172 nanoparticles in the HAADF image, Figure 2a) provides additional details on

173 the arrangement of silver atoms (Figure 2b). The seven red points in Figure

174 2b illustrate a unit cell of Ag fcc structure model in the (111) lattice plane.

175 The interatomic spacing along this direction is measured at 2.36 Å, in good

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176 agreement with the calculated interatomic spacing along silver (111) 177 direction.34 This observation is also consistent with the XRD results. Weak

178 Fe signal in the HAADF image (Figure 2a, 2c) and oxygen distribution

179 (Figure 2d) show the wide range of oxygen presence, indicating that the

180 nZVI particles have undergone extensive oxidization. Further, distribution

181 pattern of Ag, Fe, O (Figure 2c-f) provides conclusive and direct evidence

182 that the reduced silver nanoparticles have aggregated into flower-shaped

183 clusters, which prefer to grow out from the edge of the nZVI chains. Fast

184 silver-nZVI reaction is largely governed by both the diffusion kinetics of the 185 Ag+ ions in solution and the sticking probability of the Ag+ ions onto the

186 growing Ag metal surface. The silver clusters therefore preferentially

187 deposit at sharp edges due to certain space superiority of iron chains in

188 competition for silver ions. Meanwhile, some Ag NPs may release from

189 nZVI and then captured on other Ag NPs by Oswald ripening or oriented

190 attachment. In addition, Figure S9 presents images of spent nZVI after

13



191 6-hour reactions with oxygen present. The experiment with oxygen present 192 is also be conducted with 0.1 gL-1 nZVI loading and 10 ppm silver solution,

193 and the nZVI is recycled in the “reaction-separation-reuse” system for 5

194 times. Figure S9 shows that after 6-hour reactions at aerobic condition, the

195 silver nanoparticles remain solidly captured by fully oxidized nZVI particles. 196 The sulfurization of the silver in the process of Ag+ enrichment with nZVI is 197 considered with different HS2- concentrations (S/Ag ratio=0.1, 0.2 and 1.0) and in 198 different adding ways (add HS2- and nZVI together/add nZVI after HS2-) (More

199 details are provided in Supporting Imformation). Results show that in the 200 presence of HS- at low S/Ag ratio with some Ag+ ions precipitation with HS-, the 201 Ag+ ions are reduced to Ag nanoparticles attached to the iron/iron oxides chains

202 (Figure S10-S12), while the nZVI chains and Ag nanoparticles are partly

203 sulfurized (Figure S10e, S11e, S12e). For high S/Ag ratio (S/Ag=1.0) (Figure S13,

204 S14), the morphology of Ag nanoparticles attached to the iron/iron oxides chains

205 is changed resulting from sulfurization. Thus the enriched silver can be easily

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206 separated from water with a magnet due to the magnetic properties of iron

207 oxides (Figure S15).

208 Removal of Ag(I) from water

Rapid removal of Ag(I) from water can be

209 achieved with nZVI even at high levels of Ag(I) (e.g., at 300 mg•L-1) (Figure S15). 210 At a constant nZVI dose of 0.1 g•L-1 and a wide-range Ag(I) concentration (2, 211 10, 20, 150 µg•L-1; 2, 50, 100, 250 mg•L-1), removal efficiency is constantly

212 above 99% within 10 minutes. It is also found that the removal rate

213 decreases with the increasing initial Ag(I) concentration. The maximal Ag(I) 214 capacity, obtained at an initial concentration of 300 mg•L-1, is 2.8 g Ag/1 g

215 nZVI. After reactions, the residues can be easily separated from water with

216 a magnet due to the magnetic properties of iron oxides (a photo of this

217 experiment is provided in the inset of Figure S15).

218 XRD analysis

From XRD analysis, Ag(I) is predominately reduced to

219 metallic Ag, consistent with microscopic characterizations. The structure of

220 fresh nZVI nanoparticles and residues are characterized with XRD (Figure

15



221 S16). A distinct broad diffraction pattern at 44.9° is observed in the fresh

222 iron nanoparticles, corresponding to body-centered cubic (bcc) metallic 223 iron.33 After 30-minute reactions, the presence of magnetite (Fe3O4) and/or

224 maghemite (γ- Fe2O3) suggests extensive iron oxidization. Peaks at 2θ of

225 38.06°, 44.28°, 64.38°, 77.4°, 81.4° are indexed to the (111), (200), (220),

226 (311) and (222) planes of face-centered cubic (fcc) silver, and demonstrate 227 Ag(I) is predominately reduced to metallic Ag.34

228

Results from the STEM imaging suggest an exceedingly efficient method for

229 the reduction, enrichment, separation and recovery of dissolved silver from water.

230 It also shows a novel approach for the synthesis of stabilized silver nanoparticles.

231 The addition of nZVI creates a virtually perfect environment for quick reduction

232 and separation of silver with low redox potential (e.g.,

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