(CeO2) Nanoparticles in Wetland Mesocosms

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Environmental Processes

Size-Based Differential Transport, Uptake, and Mass Distribution of Ceria (CeO) Nanoparticles in Wetland Mesocosms 2

Nicholas K. Geitner, Jane L. Cooper, Astrid Avellan, Benjamin T. Castellon, Brittany G. Perrotta, Nathan Bossa, Marie Simonin, Steven M. Anderson, Sayako Inoue, Michael F. Hochella, Curtis John Richardson, Emily S. Bernhardt, Gregory V. Lowry, P. Lee Ferguson, Cole W. Matson, Ryan S. King, Jason M. Unrine, Mark R. Wiesner, and Heileen Hsu-Kim Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02040 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Environmental Science & Technology

Size-Based Differential Transport, Uptake, and Mass Distribution of Ceria (CeO2) Nanoparticles in Wetland Mesocosms Nicholas K. Geitnerab*, Jane L. Cooperab, Astrid Avellanad, Benjamin T. Castellonah, Brittany G. Perrottaai, Nathan Bossaab, Marie Simoninag, Steven M. Andersonag, Sayako Inoueae, Michael F. Hochella Jr.aef, Curtis J. Richardsonac, Emily S. Bernhardtag, Gregory V. Lowryad, P. Lee Fergusonab, Cole W. Matsonah, Ryan S. Kingai, Jason M. Unrineaj, Mark R. Wiesnerab, Heileen Hsu-Kimab* a

Center for the Environmental Implications of Nanotechnology, Duke University, Durham, NC, USA. b Civil and Environmental Engineering Department, Duke University, Durham, NC, USA. c Nicholas School of the Environment, Duke University, Durham, NC, USA d Civil & Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA, USA e Geosciences, Virginia Tech, Blacksburg, VA, USA f Energy and Environment Directorate Pacific Northwest National Laboratory Richland, WA, USA g Biology, Duke University, Durham, NC, USA h Environmental Science, Baylor University, Waco, TX, USA i Biology, Baylor University, Waco, Texas, USA j Plant and Soil Sciences, University of Kentucky, Lexington, KY, USA *Co-corresponding Authors: [email protected]; [email protected] TOC Art

ACS Paragon Plus Environment


1 2

ABSTRACT Trace metals associated with nanoparticles are known to possess reactivities that are

3

different from their larger-size counterparts. However, the relative importance of small relative

4

to large particles for the overall distribution and biouptake of these metals is not as well studied

5

in complex environmental systems. Here, we have examined differences in the long-term fate

6

and transport of ceria (CeO2) nanoparticles of two different sizes (3.8 vs. 185 nm), dosed weekly

7

to freshwater wetland mesocosms over 9 months. While the majority of CeO2 particles were

8

detected in soils and sediments at the end of nine months, there were significant differences

9

observed in fate, distribution, and transport mechanisms between the two materials. Small

10

nanoparticles were removed from the water column primarily through heteroaggregation with

11

suspended solids and plants, while large nanoparticles were removed primarily by sedimentation.

12

A greater fraction of small particles remained in the upper floc layers of sediment relative to the

13

large particles (31% vs 7%). Cerium from the small particles were also significantly more

14

bioavailable to aquatic plants (2% vs 0.5%), snails (44 vs 2.6 ng), and insects (8 vs 0.07 µg).

15

Small CeO2 particles were also significantly reduced from Ce(IV) to Ce(III), while aquatic

16

sediments were a sink for untransformed large nanoparticles. These results demonstrate that trace

17

metals originating from nanoscale materials have much greater potential than their larger

18

counterparts to distribute throughout multiple compartments of a complex aquatic ecosystem and

19

contribute to the overall bioavailable pool of the metal for biouptake and tropic transfer.

20 21

INTRODUCTION

22

The rapidly increasing presence of nanomaterials in commercial products and industrial

23

processes has caused significant concern over their potential environmental impact. An estimated


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10000 tons of CeO2 nanoparticles were produced in 2011, and have a wide range of applications

25

in optical devices, silicon wafer polishing, catalytic converters, and diesel fuel additives.1 In

26

addition, as the tools to trace these materials have developed, it has become increasingly clear

27

that naturally occurring or incidental nanoscale materials may play an even larger role in

28

environmental processes, with significant implications to environmental and human health.2-7

29

Processes specific to nanoscale particles may include the movement of the materials themselves

30

in ways that differ from macrocrystalline or dissolved phases,8, 9 or by co-transport of other

31

anthropogenic contaminants.10-13 In both cases of natural and engineered nanomaterials, a

32

detailed and mechanistic understanding of the fate and transport of nanoscale materials, and how

33

this compares to bulk materials, is critical to environmental modeling and risk forecasting.14, 15

34

Nanomaterials possess immense variety in characteristics including core composition,

35

surface functionality, size, and shape. Numerous studies have examined the transport of single

36

nanoparticles,16-19 while others have illuminated differences in nanoparticle attachment,

37

transport, and bio uptake due to surface chemistry in the laboratory.20 For example, gold

38

nanoparticles of smaller core diameter were taken up more by Daphnia magna than larger,

39

otherwise similar particles.21 Once in contact with biological surfaces, uptake mechanisms and

40

intra-organism transport of nanoparticles also depend strongly on size.22, 23 Modeling of

41

nanoparticle transport in the environment has also suggested that core size may be a critical

42

parameter in these processes.24 However, few studies have examined such factors of nanoparticle

43

fate in complex environments outside of the laboratory. Despite the numerous laboratory studies

44

on nanoparticle behavior in simulated environmental systems, the applicability of this knowledge

45

for real natural systems remains understudied.



46

In the present work, we examined the environmental distribution and transformations of two

47

similar CeO2 nanoparticles of different primary core size added to outdoor wetland mesocosms.

48

The first nanoparticle examined here was at the smallest range of typical nanomaterials (3.8 nm),

49

while the second was much larger (185 nm) and likely pushes the boundary between nano and

50

bulk material. Previous studies have found that 5-7 nm CeO2 nanoparticles used as diesel fuel

51

additives are transformed to single crystals in excess of 100 nm after combustion, making both

52

size ranges highly relevant for environmental exposure.25

53

This nine-month study consisted of chronic, weekly dosing of two sizes of CeO2

54

nanoparticles into the aquatic portion of wetland mesocosms, which include both submerged

55

aquatic, wetland, and dry terrestrial zones, over the course of nine months. While chronic dosing

56

was selected to mimic reality, dosing concentrations were not intended to replicate expected

57

environmental exposures, as the goal of this study was to determine material fate and transport,

58

not to monitor toxicological effects. These large and complex systems captured a wide variety of

59

environmental processes not possible in the lab. These include diel cycling of pH and dissolved

60

oxygen, a depth gradient of redox potential, high biodiversity, a food web of multi-branch

61

trophic chains, and gradual seasonal changes in conditions. The mesocosms supported a range of

62

terrestrial and aquatic plants typical of North Carolina wetlands, algal communities, a variety of

63

invertebrates, and a population of eastern mosquitofish (Gambusia holbrooki). Previous work in

64

these mesocosms has revealed surprising results including nanoparticle transformations not

65

previously seen in laboratory-scale studies,26, 27 with significant implications in toxicity and fate.

66

The objective of this study was to test the hypothesis that a simple difference in core particle

67

size distribution would result in significant variation in the fate and transport of these materials,

68

manifesting in diverging mass distributions in the wetland ecosystem. We further examined


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transport mechanisms of both nanoparticles with supplemental laboratory studies. Any observed

70

differences may then be utilized in support of a more comprehensive and predictive framework

71

of particulate matter transport in aquatic and soil systems.

72 73

MATERIALS AND METHODS

74

CeO2 Particle Characterization

75

Stocks of small CeO2 nanoparticles were obtained from Nyacol (CeO2AC-30, Ashland,

76

MA), and large particles from Alpha Aesar (NanoArc CE-6080, Tewksbury, MA). Each was

77

characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS),

78

zeta potential, and Fourier transform infrared spectroscopy (FTIR). TEM was performed with a

79

FEI Tecnai G2Twin. Samples were prepared by dilution in DI water; a few drops were then

80

applied to a Formvar-coated Cu grid (Ted Pella), and excess liquid was wicked away with filter

81

paper. SEM was carried out on an FEI Quanta 600 FEG equipped with Energy Dispersive X-ray

82

spectrometer (Bruker QUANTAX 400). The instrument was operated at 20 kV. DLS and zeta

83

potential were performed with Malvern ZetaSizer ZS (Malvern, UK). For these, nanoparticle

84

stock suspensions (as received) were diluted to 5 mg/L in freshly collected mesocosm water

85

(details below), and pH adjusted to 8.8 using 0.1 M NaOH to mimic morning mesocosm pH

86

values. These dilutions were bath sonicated for 30 minutes immediately prior to characterization.

87

FTIR (Thermo Electron Nicolet 8700) was utilized in order to examine any molecules at the

88

nanoparticle surfaces. Solid pellets were formed by mixing freeze-dried nanoparticles and an

89

inert medium, boron nitride. Spectra were recorded from 4000 to 400 cm-1 with 4 cm-1 resolution.

90

Results were recorded as an average of 64 scans.

91

Mesocosm Setup



92

The constructed wetland ecosystems, or mesocosms, are open air, controlled release

93

facilities, located in a clearing in the Durham division of the Duke Forest (North Carolina, USA).

94

The structure of the mesocosms has been previously described.28 In brief, these mesocosms

95

consist of manufactured wooden boxes with dimension of 0.81 × 3.66 × 1.2 m (H × W × D)

96

(Figure 1). The facility contains 30 of these mesocosms; this work utilized 9. These were

97

randomly dosed with either small or large CeO2 nanoparticles, or undosed controls, 3 replicates

98

each. The bottom of each mesocosm comprised a flat bottom (1 m long) combined with a sloped

99

bottom (13° inclination). This design created a permanent aquatic zone, a transition zone, and an

100

upland or terrestrial zone, collectively providing a gradient of saturation and redox conditions in

101

the system that were meant to simulate a freshwater wetland. The soil thickness throughout was

102

21 cm before rain, compaction and bio growth. The soil consisted of a commercial mix (Sands

103

and Soil, Durham, NC) designed to mimic the composition of a local wetland (64% sand, 28%

104

silt, 13% clay and 5% loss on ignition).

105

Approximately 250 L of local, untreated groundwater was added in June 2015 to each

106

mesocosm such that the water table reached halfway up the transition zone slope. Plugs of an

107

aquatic plant Egeria densa were planted 3 days later in aquatic zone. Additional species relevant

108

to local North Carolina wetlands (Lobelia elongate, Carex Lurida, Panicum Virgatum, and

109

Juncus effusus) were simultaneously planted as plugs in a uniform grid in the transition and

110

terrestrial zones. Water was subsequently added only through natural precipitation. The site

111

received average rainfall of 14.9 mm/day throughout Spring, Summer, and Fall. Small, recorded

112

volumes of water were removed 4 times over the course of the experiment to prevent overflow

113

after significant rain events. After 2 weeks of plant growth and prior to treatments, water was

114

cycled between all mesocosms to ensure maximum uniformity in initial water chemistry.


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Mesocosms were then allowed 6 weeks to establish before the introduction of eastern

116

mosquitofish (Gambusia holbrooki, Triangle Pond Management, USA), 53 females and 15 males

117

per mesocosm. At the beginning of dosing, water pH cycled daily between approximately 7.0

118

and 10.0, with an average pH of 8.8 in early morning. Water conductivity was an average of 111

119

± 20 µS/cm and dissolved organic carbon an average of 11.9 ± 4.4 mg/L over the course of the

120

experiment.

121

Aquatic Zone

Transition Zone

Terrestrial Zone

1.0 m

Floc Sediment

13º

3.66 m

122 123 124

0.81 m

Upland Soil

Figure 1. Cross-sectional view of the wetland mesocosm, with height and width dimensions and zone definitions noted. The depth of each mesocosm was 1.2 m. Dotted line indicates the water table.

125 126

Dosing

127

Once per week, each mesocosm received an assigned nanoparticle dose as a freshly prepared

128

suspension in 1 L DI water. Each suspension was sonicated in a water bath for 10 minutes prior

129

to dosing. Three mesocosm boxes were assigned to small CeO2 particles, three boxes for large

130

CeO2, and three control boxes without treatment. Control boxes received 1 L DI water without

131

nanoparticles at each dosing. Both large and small CeO2 nanoparticle weekly doses consisted of

132

19.23 mg nanoparticles for 9 months from January to October 2016. Overall, a total of 0.75 g

133

CeO2 (0.61 g Ce) was added per mesocosm during the course of the 39-week experiment. Dosing

134

concentrations were determined by inductively coupled plasma mass spectroscopy (ICP-MS)



135

analysis of stock solutions, as described below. Dosing suspensions were injected into the

136

aquatic zone of the mesocosms, 2 cm below the water surface in a moving grid pattern to ensure

137

even and consistent addition across the water surface.

138 139

Sampling and Digestion

140

For each water sample, 10 mL aliquots were collected weekly, immediately acidified with

141

HNO3, and stored at room temperature. Whole plant roots or stems, as appropriate, were

142

collected quarterly (Q1-3), dried at 60°C, and homogenized by grinding. At Q3 (39 weeks), the

143

full plant biomass was similarly collected. Half of all E. densa samples were rinsed in 500 mL

144

sterilized mesocosm water by shaking for 2 minutes followed by a final rinse by flowing water to

145

remove particles from the exterior. Soil and sediment (aquatic and transition zones) samples

146

were collected by submerging polypropylene tubes into the surface and extracting intact cores.

147

Three cores were collected and composited in each zone of the mesocosm in order to better

148

capture intra-mesocosm heterogeneity. These were subsequently separated into floc, 0-1 cm, and

149

1-5 cm depths. Plant roots were removed manually to the extent possible. The floc was assumed

150

to exist in the top 0.5 cm of aquatic sediments. Each depth portion was then dried at 60°C, and

151

homogenized by grinding. Fish were sampled non-invasively by trapping (n = 5 individuals per

152

NP- treated mesocosm, n = 3 individuals per control mesocosm) and immediately euthanized

153

with tricaine methanesulfonate and frozen (Duke University IACUC protocol # A135-16-06). As

154

both snail species (Physella sp. and Lymnaea sp.) colonized the mesocosms incidentally with the

155

E. densa, up to n = 5 individuals were hand collected from each mesocosm where available.

156

Insects were collected from sediments at the end of Q3. The insects were then separated by


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species, dried at 60°C, weighed, and homogenized by grinding. This work focuses solely on the

158

final distribution of materials at 39 weeks (Q3).

159

For each water sample digestion, a 5 mL aliquot was transferred to a 50 mL polypropylene

160

tube. To this, 750 µL concentrated nitric acid (70%, trace metal grade) and 375 µL 30%

161

hydrogen peroxide were added and allowed to react overnight at room temperature. Each tube

162

was then heated to approximately 100°C on a heating block and held for 30 min. An additional

163

aliquot of 750 µL nitric acid was added to water samples from the mesocosms dosed with large

164

CeO2 nanoparticles (to ensure complete digestion of these samples). Each sample was then

165

cooled and diluted with ultra-pure water (18 MΩ-cm) resulting in approximately a 5% acid

166

concentration.

167

The same procedure was followed for plant samples consisting of 25 mg dried and ground

168

plant tissue. The digestion procedure was also followed for 25 mg dried and ground soil or

169

sediment samples.

170

Whole fish were dried at 60° C and homogenized individually with mortar and pestle.

171

Homogenates (mean of 18 dry mg) were hydrated 24 h and digested in the same manner as the

172

other samples.

173 174

Elemental Analysis

175

Elemental analyses of acid digested samples were performed by ICP-MS with either Agilent

176

7900 (fish, snails, invertebrates, water) or Agilent 7700 (plants, soil) instruments. Analytes

177

included Na, Mg, K, Ca, Mn, Fe, Cu, Zn, La, Ce, and Nd. La and Nd were measured in order to

178

calculate background levels of Ce, and the remaining elements were measured as standard water



179

and soil chemistry parameters. Calibration was performed across a range of 0.1 – 500 µg/L for

180

Ce, Nd, La, and Pb, up to 50,000 µg/L for the remaining analytes. Calibration for Ce in fish,

181

snail, and invertebrate samples was performed down to 0.01 µg/L. Each sample run began with 2

182

blank samples. Additional method validation involved the analysis of metals drinking water

183

standard (NIST 1643e), standard spikes, and a method blank solution of 2% nitric, 0.5%

184

hydrochloric acid every 15 samples. Method limits of detection were calculated as the mean of 3

185

blank samples + 3 standard deviations of 10 replicate analyses of blank samples. The method

186

limit of detection in water samples was found to be 1.2 and 0.22 µg/L for Ce and Nd,

187

respectively. Limits of detection were similar in fish (0.99 µg/kg Ce), snails and insects (3.1

188

µg/kg Ce), and plants and soils (0.021 µg/kg Ce and 0.016 µg/kg Nd). Duplicate digests were

189

conducted for 10.4% of the fish samples, and the mean relative percent difference [Ce] between

190

samples was 11 ± 6%.

191

Background concentrations of Ce were accounted for by observing the ratio

192

[Ce]Bkrnd/[Nd]Bkrnd in each sample type. In each sample type, this ratio varied by less than 1%

193

across samples. This method was used instead of subtracting Ce concentration values measured

194

in the control (undosed) mesocosms due to spatial heterogeneity in soils and sediments, resulting

195

in greater variability in control values. Both CeO2 nanoparticles contained Nd below detection

196

limits. We then utilized this ratio to calculate [Ce]NP within Ce treated samples by measuring

197

[Nd] in these samples:

ሾ‫݁ܥ‬ሿே௉ = ሾ‫݁ܥ‬ሿ௧௢௧ − ሾܰ݀ሿ

198 199

ሾ‫݁ܥ‬ሿ஻௞௥௡ௗ #1 ሾܰ݀ሿ஻௞௥௡ௗ

For biological compartments including fish, snails, and insects, total Ce masses were calculated following organism body burden measurements after subtracting background Ce


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concentrations as measured in control, untreated mesocosms. This was done by extrapolating the

201

mass concentration found in each organism using the total mass of the particular organism found

202

in that mesocosm. For fish, insects, and snails, this was accomplished by collecting all organisms

203

present at the end of the experiment. Insect concentrations are based on collected dragonfly

204

nymphs in the sediments, as these were present in all mesocosms and constituted the majority of

205

the harvest.

206 207

Speciation of cerium in plants and sediment

208

Ce oxidation states in samples was assessed by X-ray Absorption Near Edge Structure

209

(XANES) at Ce LIII-edge (5.723 keV) at the beamline 20-ID of Advanced Photon Source

210

facility (Chicago). Three young shoots (first 20 cm) of Egeria densa per mesocosm and

211

composite samples of the floc layer and transition sediment per mesocosm were frozen just after

212

collection. Prior to XANES analysis, these samples were freeze-dried at -55°C and 0.014mBars

213

for 3 days, ground under N2 atmosphere as one composite sample per treatment, pressed into

214

pellets, sealed with Kapton® tape, and kept under anoxic environment until analysis.

215

During the analysis, samples were held in a He cryostat to prevent beam damage. XANES

216

spectra were acquired from 5524 eV to 5684 eV at 10 eV increments, from 5684 eV to 5774 eV

217

with a 0.3 eV increments, and then up to 6000 eV using 0.05 eV increments. XANES spectra

218

were recorded in fluorescence mode using a vortex Si drift (4-element) and a Si(111)

219

monochromator. For each sample, 2 scans were acquired on at least 3 different locations

220

(minimum of 6 scans per pellet) to (i) prevent sample damage under the beam and (ii) check for

221

the homogeneity of the pellet. During each scan, the XANES of a metallic Cr sheet was recorded

222

(k edge=5898 eV) to verify beam stability. XANES spectra were calibrated in energy (if needed),



223

background subtracted with E0 defined as the maximum of the 1st XANES derivative, and

224

normalized using Athena software.29 Ce(III)-acetate and the commercial additive Nanobyk-3810

225

Ce(IV)O2 nanoparticles were used as Ce(III) and Ce(IV) references, respectively. Linear

226

combination fits were performed in order to determine relative Ce(III) and Ce(IV) content.

227 228

Sediment Depth Profile of Redox Relevant Species

229

Sediment cores were collected at the end of 9 months of dosing. Depth concentration

230

profiles of dissolved O2, Mn2+, and Fe2+ concentrations at the sediment-water interface of these

231

cores were collected via voltammetry using solid state Au-Hg amalgam microelectrodes.26 One

232

depth profile was taken from each core. Dissolved O2 was calibrated against an air-saturated

233

sample of mesocosm surface water at room temperature (~22°C), assuming [O2] = 272 µM.

234

Limits of quantification were 5 µM, 15 µM, and 15 µM for O2, Mn2+, and Fe2+, respectively.

235 236

Sedimentation and Heteroaggregation Studies

237

Nanoparticle heteroaggregation was quantified by measuring their attachment efficiency, ߙ,

238

as described previously.8 Briefly, 2 g washed glass beads were acclimated with 1 g mesocosm

239

floc in 40 mL mesocosm water for 72 hours under gentle mixing. The glass beads were then

240

allowed to settle, and the solution was decanted. The glass beads were then gently washed three

241

times in mesocosm water and dried in air; as a result of this process, the glass beads were coated

242

by organics and other residual material from mesocosm floc, as evidenced by a darker color.

243 244

For experimental measurement of ߙ, 0.4 g of these beads were added to 30 mL unfiltered mesocosm water with pH adjusted to 8.8 using 0.1 M NaOH. Under mixing with a magnetic stir


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bar, 10 mL of either large or small CeO2 nanoparticles was added to final suspension

246

concentrations of 10 mg/L CeO2. Aliquots were collected over the course of a 60-minute mixing

247

experiment, and held static for 30 seconds, resulting in the settling of glass bead/CeO2

248

heteroaggregates and separation from suspended, free nanoparticles. The suspended free

249

nanoparticle concentration was quantified by absorbance (305 nm for small particles, 610 nm for

250

large) A(t) for various mixing times t in a 1-cm pathlength cuvette (BMG SpectroStar Nano).

251

The same heteroaggregation experiment was performed under favorable attachment conditions

252

(ߙ = 1) by spiking solutions with 6 mM NaNO3, thus effectively eliminating electrostatic

253

stabilization of the nanoparticles. The resulting measurements were fit to a first order kinetic equation where the slope of

254

஺(௧ୀ଴)

versus t scales with the product ߙߚ‫ ܤ‬. 9 We assumed that B, the initial concentration of

255

ln

256

background particles, and ߚ, the collision frequency, were both constant across experiments.

257

Attachment efficiency ߙ was determined by the linear slope value during unmodified attachment

258

conditions (ߙߚ‫ )ܤ‬divided by the same slope in the favorable attachment scenario (1 × ߚ‫)ܤ‬.9

஺(௧)

259

Sedimentation was assessed at the lab scale, simulating mesocosm conditions. 100 g of acid

260

washed glass beads (d=320 µm) were placed at the bottom of 750 mL glass bottles together with

261

500 mL of freshly collected mesocosm water. After 1 day of stabilization, nanoparticles were

262

injected with 100 µg/L final concentration. Glass beads were found to help prevent resuspension

263

of particles from lower regions of the bottle without affecting sedimentation rates. Samples were

264

taken at 5 cm depth from the water surface to measure the concentration of suspended

265

nanoparticles over time, and analyzed with ICP-MS as described above.

266



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267

Statistical Analysis

268

Where applicable, data are reported as the mean of all samples across 3 replicate mesocosms

269

or 3 laboratory replicates. In all cases, error bars and reported uncertainties are standard

270

deviations. In tests of statistical significance, a standard t-test of equal variance was employed

271

where significance was defined as p < 0.05.

272 273

RESULTS AND DISCUSSION

274

Nanoparticle characterization

275

The large and small CeO2 nanoparticles in pH 8.8 mesocosm water (to approximately mimic

276

the mesocosm water pH in the mornings) were drastically different in size (hydrodynamic

277

diameters less than 20 nm for small CeO2; >100 nm for large CeO2), while they both had similar

278

magnitude of negative surface charge, as indicated by measured zeta potentials (Table 1). FTIR

279

analysis confirmed manufacturer specifications of an acetate coating on small CeO2

280

nanoparticles, with characteristic peaks at 1400 cm-1 and 1530 cm-1 from the carbonyl group

281

(Supporting Information Figure S1).30 Analysis of the large CeO2 particles indicated the presence

282

of chitosan. Identified peaks included 1660 cm-1, attributed to C=O, 1570cm-1, attributed to NH

283

groups, and 1422 cm-1, attributed to CH·OH.31

284 285 286

Table 1. Summary of nanomaterial characterization. DLS and Zeta potential measurements were carried out in mesocosm water. Size – TEM (nm)

Size – DLS, pH 8.8 (nm)

Zeta Potential, pH 8.8 (mV)

Weekly Particle Dose (mg)

Total Ce Dose (g)

Small CeO2

3.8 ± 1.1

15.8 ± 2.4

-23.3 ± 0.67

19.23

0.61

Large CeO2

185 ± 60

281 ± 55

-25.8 ± 0.65

19.23

0.61


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287 288

Distribution of CeO2 Nanoparticles Across Compartments of the Mesocosms

289

The analysis of Ce in various compartments of the wetland mesocosms after the 9 month

290

experiment indicated that most of the CeO2 particles accumulated in the sediment, soil, and floc

291

layers (Figure 2). Recovery of Ce relative to the total added Ce was 91 ± 18% for large and 134

292

± 31% for small CeO2 nanoparticles. The greater than 100% recovery for the small CeO2

293

particles was possibly due to greater than expected heterogeneity in floc and sediment samples.

294

Regardless, both particles had low residual aquatic concentrations over long timescales, and were

295

also found primarily in various compartments of sediment and soil, compromising 94% and 98%

296

for small and large particles, respectively.

297 1000 900 6.2

Surface Water

800 23.3

Cerium (mg)

700

E. densa (Internal)

239.9

600

Upland Soil

4.3 2.1

500

Transition Sediment

186.7

400

255.8

Floc

39.9

300 163.8

Aquatic Sediment

200 229.0

100

Deep Sediment

157.8 48.5

0

Large 298 299 300 301 302 303

Small

Figure 2. Mass of Ce in major compartments of the wetland mesocosms after 9 months of weekly dosing with either large or small CeO2 nanoparticles. Aquatic sediment was defined as the top 1 cm, while deep sediment was from a depth 1-5 cm of sediment in the aquatic zone. The dotted line indicates the total amount of Ce (610 mg) added as CeO2 nanoparticles during this period. Each grouping represents the average (±1 std. dev.) for 3 replicate mesocosms. Compartments contributing less than 0.5% of the total are not shown for clarity.



304 305

There were also significant size-based differences in the fate of these particles. First,

306

accumulation of Ce in the floc was significantly greater for small particles (256 ± 57 mg) relative

307

to the large particles (39.9 ± 4.2 mg) (p=0.03). Instead, large particles were found to accumulate

308

in deeper (1-5 cm) layers of aquatic sediment, with 157 ± 110 mg detected for large particles vs

309

48.5 ± 24 mg for small, though not statistically significant due to variation between mesocosms

310

(p=0.2). The presence of large CeO2 nanoparticle aggregates in deeper sediment was confirmed

311

by SEM and EDS (SI Figure S2). Further transport of large CeO2 particles below 5 cm may

312

account for some of the Ce mass not detected. Approximately 2% of small particles were

313

detected in upland soil, while large particles in that compartment were below detection limits.

314

The transport of small particles to upland soil is likely due to free particles still present in the

315

water column during occasional flooding events.

316

Small particles also had significantly greater (p=0.002) total association with E. densa

317

compared to the large particle dosings. This is in agreement with previous studies which found

318

greater plant uptake in mesocosms dosed with nanoscale Ag and ZnO compared with mesocosms

319

receiving larger scale materials of similar composition.32 We hypothesized that the difference in

320

E. densa association may be due to greater attachment efficiencies of these smaller particles,

321

resulting in greater initial interactions with plant surfaces. This hypothesis is explored further

322

below.

323

Finally, by subtracting rinsed from unrinsed E. densa Ce concentrations, we were able to

324

approximate internalized particles and those externally adsorbed to plant or associated

325

periphyton surfaces. These results (Figure 3A) suggested that Ce originating from small CeO2

326

nanoparticles were taken up significantly more by these aquatic plants, with internal Ce mass


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327

more than twice external concentrations (23.3 ± 1.6 mg interior vs 9.5 ± 0.9 mg exterior,

328

p=0.01). Large particles, in contrast, were approximately twice as likely to be found on the

329

exterior than the interior of E. densa (3.0 ± 0.5 mg interior vs 6.0 ± 3.0 mg exterior, p=0.04). We

330

argue that this is not an artificial result due to size differences in rinsing efficiency (i.e. greater

331

shear experienced by larger particles), as such differences would actually be expected to give the

332

opposite result.

333

The uptake of Ce in the aquatic animal compartments of the mesocosms at 9 months was a

334

small fraction of the overall nanoparticle fate (Figure 3). However, understanding differences in

335

bioavailability of small relative to large CeO2 is critical to understanding the ecological

336

importance of nanoscale materials. In fish, no significant (p > 0.05) difference was observed in

337

total internalized mass of Ce after dosing with small and large CeO2 nanoparticles. However,

338

significant differences were observed in both snail (p=0.018) and insect (p < 0.0001) Ce masses.

339

In insects, 8.0 ± 1.1 µg Ce was found when dosed with small particles. However, total mass in

340

insects was over 120 times lower for those exposed to large particles, with just 0.065 ± 0.02 µg

341

Ce detected. Snails comprised the lowest total masses (though highest organism concentrations

342

of Ce) of the three compartments. Ce masses in the snails of mesocosms dosed with small and

343

large particles were quantified at 44 ± 18 and 2.6 ± 0.4 ng, respectively, a 17-fold difference. In

344

aggregate, these results suggest a higher bioavailability of the Ce originating from smaller

345

nanoparticles to particular animal compartments in these mesocosms. However, no difference in

346

accumulation was observed in fish. We hypothesize that this may be related to organism

347

localization within a mesocosm as well as feeding habits. Verification of this hypothesis is left

348

for future work.

349



A

Interior

35

Page 18 of 29

Exterior

Total Mass Ce (mg)

30 25 20 15 10 5 0 Small

B

1 E+1

0.62 mg/kg

Large Small

Large

0.67 mg/kg

10.8 mg/kg

Total Mass Ce (µg)

1 E+0

17.7 mg/kg

1 E-1

0.4 mg/kg

1 E-2 4.1 mg/kg

1 E-3 Fish

350 351 352 353

Snails

Figure 3. The calculated total mass of Ce from nanoparticles detected in E densa (A) and fish, snails, and insects (B) in the mesocosms for small (blue) and large (green) ceria nanoparticles. Data labels in B indicate the concentration of Ce detected per dry weight of organism, corrected for background concentrations.

354 355

Insects

Cerium Speciation


Page 19 of 29

356


We also closely examined the speciation of Ce across the mesocosms over time. The relative

357

abundance of Ce(III) and Ce(IV) has particular implications in understanding observed

358

differences in Ce biouptake as well as nanoparticle toxicity and reactivity,1, 33 and so is an

359

important environmental endpoint. The XANES spectra for Ce in various compartments over

360

time (Figure 4) highlight several important differences. Quality factors of the fits can be found in

361

Table S1. First, in mesocosms receiving small CeO2, 57% of the Ce on or within E. densa was in

362

the Ce(III) form (Figure 4B), a drastic contrast to the original small CeO2 particles that

363

comprised fully of Ce(IV) (Figure 4A). For the E. densa exposed to large particles, a much

364

smaller percentage of Ce (10%) was observed as the reduced form of Ce (Figure 4B).

E

C

Ce(IV)

Small CeO2 NP

Ce(III)

42% Ce(IV) 57% Ce(III)

Floc large 6 mo 65% Ce(IV) 42% Ce(III)

Floc large 9 mo 85% Ce(IV) 8% Ce(III)

Energy (eV)

Floc small 3 mo

E. densa small 9 mo 42% Ce(IV) 57% Ce(III)

E. densa large 9 mo 96% Ce(IV) 10% Ce(III)

Normalized absorption

Normalized absorp�on


Transition sediment large 9 mo 20% Ce(IV) 79% Ce(III)

D

Energy (eV)

365 366 367 368 369 370

Transition sediment small 9 mo 6% Ce(IV) 79% Ce(III)

Energy (eV)

Energy (eV)

B


Ce(IV)

Floc large 3 mo Normalized absorp�on Normalized absorption

Large CeO2 NP


Titre

A

32% Ce(IV) 67% Ce(III)

Floc small 6 mo 10% Ce(IV) 90% Ce(III)

Floc small 9 mo 0% Ce(IV) 109% Ce(III)

Energy (eV)

Figure 4.: X-ray absorption near edge structure (XANES) spectra for (A) Ce(IV)O2 or Ce(III)-acetate reference compounds used for the combination fitting of the spectra of (B) Egeria densa exposed to CeO2 small or large at 9 mo, or for the floc at 3, 6, or 9 mo exposed to (C) large CeO2 or (D) small CeO2, and (D) the surficial sediment exposed to either small or large CeO2. Dotted lines indicate peak positions. Fitting results are shown on the plot.



371 372

Further, Ce in flocs of the large CeO2 mesocosms underwent very limited reduction (Figure

373

4C), while in the small CeO2 mesocosms the Ce in the flocs were completely reduced to Ce(III)

374

by the end of the experiment (9 months, Figure 4D). In fact, the speciation of Ce at 3 months in

375

the floc is similar to the background Ce speciation found in the control mesocosm (data not

376

shown). The fraction of Ce(III) found in the flocs of the large CeO2 mesocosms was mostly

377

likely the native Ce in the wetland soil and the relative contribution of this native Ce decreased

378

as CeO2 continued to be added during the 9 month experiment. Finally, the Ce detected in the

379

top layer of the transition sediments (Figure 4E) at the 9 month time point indicate that

380

approximately 20% of large CeO2 remains as Ce(IV) while the rest has been reduced.

381

The differential reduction of Ce(IV) to Ce(III) is thus dependent on particle properties, the

382

compartment to which they accumulate within the mesocosm, and the residence time of the

383

particles in each compartment. There are multiple possible explanations for these stark

384

differences in Ce speciation over time. First, the clear size effect. It is expected that, due to

385

lattice expansion at small crystal sizes, the smaller particles may have a higher percentage of

386

defects and/or oxygen vacancies, giving them a higher propensity for reduction to Ce(III).34 The

387

small particles’ surface/volume ratio was also much larger than those of the large particles. They

388

were therefore expected to be more reactive, as atoms in the bulk of the material do not directly

389

participate in redox reactions.35

390

Second, the coatings of the two particles differed. The small CeO2 particles were coated

391

with a low molecular weight organic acid (acetate) that may not have protected the particles from

392

Ce reduction reactions while the large CeO2 were coated with high molecular weight molecules

393

(long chitosan chains) can could be more protective of surface reactions. Coating also affects the


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394

affinity of the nanoparticles for surfaces. Measurements of surface affinity discussed below, are

395

also consistent with a lower rate of reduction of the Ce in the case of a lower surface affinity,

396

seem to mirror results found for smaller rates of cerium nanoparticle reduction with lower

397

surface affinities for the case of activated sludge reported previously.36 However, differences in

398

surface coatings are expected to be short lived compared to the time scale of the experiment, as

399

neither stabilizer was covalently bound to the surface. Therefore, over time these surfaces are

400

expected to exchange with various components of the local environment.37-39

401

Finally, differences in fate of the particles based on their size, as detailed in Figures 2 and 3,

402

likely plays a secondary role. Accumulation in different mesocosm compartments results in

403

different local chemical environments that would alter the extent of CeO2 transformation. This is

404

highlighted by measurements of redox conditions as a function of depth, which found 50-75% O2

405

saturation just above the sediment surface, but 0% saturation immediately below the sediment-

406

water interface (SI Figure S3). Also observed were increasing concentrations of Mn2+ and Fe2+ as

407

a function of depth. We also observed similar profiles in mesocosms dosed with small and large

408

CeO2 nanoparticles. The presence of O2 at the surface of the sediment is not detrimental to the

409

presence of Ce(III), since the reduction of Ce(IV) is likely (EhCe4+/Ce3+ = +1.5V, pH 8)40 in oxic

410

environments for the pH range found in the mesocosms (approx. 7 to 10). The drastic drop of

411

oxygen on the top layer of the sediment can explain the further reduction of large CeO2 NP on

412

that compartment, where the proportion of Ce(III)/Ce(IV) originating from small or large CeO2

413

tended to be similar. Therefore, differences in Ce oxidation state were primarily due to particle

414

size and reactivity, not varying mesocosm conditions. However, Ce from both nanoparticles in

415

the anoxic sediment environment were reduced in its majority. This reduction in soil

416

environments is in agreement with previous studies of the reduction of small CeO2 nanoparticles



417

in soil growth media.41 While no dissolution was observed in mesocosm water and sediments in

418

the laboratory, it should be noted that under different environmental conditions, the release of Ce

419

ions may significantly alter transport and bioavailability of smaller particles.

420

Nanoparticle Transport Mechanisms

421

Following the observation of significant differences in overall fate of small and large CeO2

422

nanoparticles, we further investigated their respective transport mechanisms. Specifically, we

423

examined their sedimentation (Figure 5) and heteroaggregation in mesocosm water.

424

Heteroaggregation studies were performed on floc treated glass beads in order to more closely

425

mimic the surface chemistry of the upper sediment layers. The zeta potential of these beads in

426

pH 8.8 mesocosm water was -18.3 ± 1.1 mV. The sedimentation kinetics clearly show much

427

more rapid settling of large particles. Heteroaggregation mixing studies resulted in ߙ values of

428

0.72 ± 0.08 for small and 0.11 ± 0.04 for large CeO2 nanoparticles, indicating much greater

429

heteroaggregation of small particles with floc treated glass beads in mesocosm water. The

430

difference in surface coating may also play a role in the different values of ߙ, though these

431

differences are likely mitigated by the complex water chemistry and presence of organic matter

432

through sorption of organic matter on particle surfaces. However, autoaggregation is likely the

433

dominant process in these experiments. Collision frequencies between nanoparticles and larger

434

background particles (diameter on the order of several µm) decrease rapidly with increasing

435

nanoparticle size. 41 Settling rates increase as diameter to the second power.8, 9, 42 Based on

436

Stokes settling alone, the time for the large and small particles to settle 5 cm (the sample height

437

in these experiments) would be on the order of 103 and 106 minutes respectively. Thus, settling

438

of individual nanoparticles based on their initial size alone cannot explain the removals observed

439

in these experiments. We may therefore expect heteroaggregation or autoaggregation to play an


Page 22 of 29

Page 23 of 29


440

important role in reducing the concentration of CeO2 particles over time. While

441

heteroaggregation may have been favored for the smaller particles, heteroaggregation and

442

subsequent settling, plus perhaps settling in the absence of heteroaggregation, favored more rapid

443

removal of the larger CeO2 nanoparticles (Figure 2). Together, differences in sedimentation and

444

heteroaggregation clearly showed a stark contrast in transport mechanisms. These were

445

manifested as significant differences in mesocosm fate. Specifically, large particles avoided

446

heteroaggregation and sedimented to the deeper layers of aquatic sediment (Figure 2), while

447

smaller particles remained in suspension and heteroaggregated with background surfaces,

448

including floc. Smaller particles exhibiting more rapid surface attachment and resulting higher

449

bioavailability is in agreement with previous mammalian studies.43 This also provides an

450

additional potential mechanism for differences in nanoparticle speciation. Previous studies of

451

CeO2 nanoparticles in activated sludge found that particles with higher values of ߙ were more

452

rapidly reduced to Ce(III) under aerobic conditions.36 This was due to more rapid attachment to

453

reducing bacteria, as was potentially the case in mesocosms. These results also provide possible

454

insight for other exposure scenarios. For example, environments with higher organic matter

455

content are expected to result in lower rates of heteroaggregation for both particles,8 and

456

therefore both would be expected to have longer water lifetimes.



457 458 459 460

Figure 5. The sedimentation of an initial 100 µg/L aqueous suspension in mesocosm water of small (blue circles) or large (green squares) CeO2 nanoparticles monitored over time

461

Environmental Implications

462

In this 9-month study with complex simulated wetland ecosystems, we found significant

463

differences in the fate and transport of two nanoparticles of different size, with significant

464

implications in understanding the environmental transport of nanomaterials, and of chemical and

465

particulate environmental transport in general. While the two CeO2 test materials were otherwise

466

highly similar, including core composition and zeta potential, Ce originating from small particles

467

was much more bioavailable, with significantly greater association with aquatic plants and

468

macrofauna. Small particles accumulated in the upper floc layers of aquatic sediment, while

469

larger particles settled in much deeper layers of sediment. These differences are consistent with

470

relatively simple laboratory experiments of sedimentation and heteroaggregation, in which small

471

particles possessed values of ߙ approximately 6.5 times greater than large particles. Instead of

472

heteroaggregating with other particles in the mesocosm surface water, large particles rapidly

473

self-aggregated and settled out of solution.


Page 24 of 29

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474


Small CeO2 nanoparticles were also more significantly transformed from Ce(IV) to Ce(III)

475

by surface chemical reduction. This is most likely due to their higher reactivity by virtue of their

476

small size, in addition to potential greater association with reducing microbes. Ce originating

477

from the large particles did not experience the same extent reduction, as the much larger size of

478

the particles rendered them less redox active.

479

Overall, trace elements such as Ce originating from smaller particles were much more likely

480

to enter biological compartments due to transport properties as well as increased surface area for

481

transformation reactions (e.g., reductive dissolution in the case of Ce). Larger particles

482

transformed more slowly and settled quickly, making the upper sediment layers a sink for

483

untransformed nanoparticles. We may therefore further expect long-range transport of large

484

particles in flowing systems to be dominated by the motion of sediments, while smaller particles

485

are more likely to move with the fluid and more loosely bound flocs. This work highlights that

486

particle size not only influences the transport and transformation potential of trace elements

487

originating from nanoparticles, but also measurably influences eventual biouptake and trophic

488

transfer.

489 490 491

Acknowledgments This material is based upon work supported by the National Science Foundation (NSF) and

492

the Environmental Protection Agency (EPA) under NSF Cooperative Agreement EF-0830093

493

and DBI-1266252, Center for the Environmental Implications of NanoTechnology (CEINT).

494

Any opinions, findings, conclusions or recommendations expressed in this material are those of

495

the author(s) and do not necessarily reflect the views of the NSF or the EPA. This work has not

496

been subjected to EPA review and no official endorsement should be inferred.



497

A portion of this research was performed using resources of the Advanced Photon Source,

498

an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of

499

Science by Argonne National Laboratory, and was supported by the U.S. DOE under Contract

500

No. DE-AC02-06CH11357, and the Canadian Light Source and its funding partners. We thank

501

Dale L Brewe at APS (BL 20-ID) for his support.

502

Supporting Information

503

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

504

FTIR spectra, electron microscopy images, EDS spectra, XANES fitting parameters, and soil

505

chemistry depth profiles.

506

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507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530

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