Subscriber access provided by UNIV OF CONNECTICUT
Article
Enhanced Colloidal Stability of CeO2 Nanoparticles by Ferrous Ions: Adsorption, Redox Reaction, and Surface Precipitation Xuyang Liu, Jessica Renee Ray, Chelsea W. Neil, Qingyun Li, and Young-Shin Jun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es506363x • Publication Date (Web): 07 Apr 2015 Downloaded from http://pubs.acs.org on April 12, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
Environmental Science & Technology
Enhanced Colloidal Stability of CeO2 Nanoparticles by Ferrous Ions: Adsorption, Redox Reaction, and Surface Precipitation
Xuyang Liu †, Jessica R. Ray †, Chelsea W. Neil †, Qingyun Li, and Young-Shin Jun* Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, United States
E-mail: [email protected] http://encl.engineering.wustl.edu/ Submitted: December 2014 Revised: March 2015
Environmental Science &Technology
†These
authors contributed equally to the current work.
*To whom correspondence should be addressed.
ACS Paragon Plus Environment
Environmental Science & Technology
1
ABSTRACT
2
Due to the toxicity of cerium oxide (CeO2) nanoparticles (NPs), a better understanding of the redox
3
reaction-induced surface property changes of CeO2 NPs and their transport in natural and
4
engineered aqueous systems is needed. This study investigates the impact of redox reactions with
5
ferrous ions (Fe2+) on the colloidal stability of CeO2 NPs. We demonstrated that under anaerobic
6
conditions suspended CeO2 NPs in a 3 mM FeCl2 solution at pH = 4.8 were much more stable
7
against sedimentation than those in the absence of Fe2+. Redox reactions between CeO2 NPs and
8
Fe2+ lead to the formation of 6-line ferrihydrite on the CeO2 surfaces, which enhanced the colloidal
9
stability by increasing the zeta potential and hydrophilicity of CeO2 NPs. These redox reactions
10
can affect the toxicity of CeO2 NPs by increasing cerium dissolution and by creating new Fe(III)
11
(hydr)oxide reactive surface layers. Thus, these findings have significant implications for
12
elucidating the phase transformation and transport of redox reactive NPs in the environment.
1
ACS Paragon Plus Environment
Page 2 of 31
Page 3 of 31
Environmental Science & Technology
13
INTRODUCTION
14
Cerium oxide (CeO2) nanoparticles (NPs) have been enumerated in the priority list of
15
engineered nanomaterials for risk evaluation by the Organization for Economic Co-operation and
16
Development (OECD) due to their wide application in industry and daily life.1 CeO2 NPs are ideal
17
in catalysis applications, such as diesel engine catalytic converters,2 due to their oxygen storage
18
capabilities3 and their ability to readily participate in Ce4+/Ce3+ redox processes. The use and
19
production of CeO2 NPs will inevitably result in increased concentrations in natural or manmade
20
aqueous environments, such as wastewater.4 Therefore, in order to better predict the life cycle of
21
these engineered nanoparticles, it is vital to improve our understanding of their fate and transport,
22
particularly in aqueous environments.
23
Based on a 2001 report on human health risks of cerium from diesel fuels, the average
24
worldwide estimated level of cerium in soils was 20–60 ppm.5 Recent studies have also shown that
25
nanometer sized CeO2 particles are found in automobile exhaust.6,
26
warrants immediate attention to prevent harmful effects to the biosphere. The bioavailability and
27
toxicity of CeO2 NPs are largely determined by their fate and transport in the environment, which
28
is in turn affected by their surface charge and aggregation state.8, 9 While recent investigations in
29
this field have focused on the influence of solution chemistry and organic matter on the fate and
30
transport of CeO2 NPs,2, 10-16 few studies have considered the effect of redox reactions on CeO2
31
NP surface properties in the presence of redox-active ions in diverse aqueous environments.17, 18
7
This high concentration
32
When released in the environment, CeO2 NPs can coexist and interact with redox sensitive
33
elements. One important element to consider is ferrous iron (Fe2+), which is widely distributed in
34
natural aqueous systems (e.g., acid mine drainage),19 as well as in engineered systems for odor and
2
ACS Paragon Plus Environment
Environmental Science & Technology
35
corrosion control, precipitating hydrogen sulfide, and phosphorus removal.20, 21 The concentration
36
of ferrous ions used in these engineered systems can be as high as 9.85 mM for phosphorus
37
removal,22 and 1.5–3.0 mM to remove cyanide from industrial wastewater.23 In addition, in a study,
38
Fe3O4/CeO2-impregnated NPs were synthesized and used as Fenton-like catalysts for hydroxyl
39
radical generation and 4-chlorophenol degradation with the addition of H2O2.24 Ce4+ has also been
40
used in the production of silicate glasses to rid the glass of Fe2+ ions.25 Therefore, anthropogenic
41
CeO2 NPs originated from industrial wastewater can interact with ferrous ions during their
42
industrial applications, as well as during wastewater treatment processes.26, 27
43
According to the difference in the standard redox potential of Fe3+/Fe2+ (0.77V) and
44
Ce4+/Ce3+ (1.44 V), redox reactions between Fe2+ and CeO2 can occur when they coexist in
45
solution.28 However, it is largely unknown how this reaction with aqueous Fe2+ will affect the
46
surface properties and colloidal stability of CeO2 NPs in aqueous environments. In this study, we
47
systematically investigated changes in CeO2 NP surface properties when aqueous Fe2+ is present.
48
Knowledge obtained in this study can help improve our understanding of the fate and transport of
49
redox active nanomaterials during their lifetime, which will in turn give insight into the expected
50
bioavailability of these anthropogenic NPs when released into the environment.
51
EXPERIMENTAL SECTION
52
Preparation of Nanoparticle Dispersions for Aggregation and Sedimentation Tests
53
All solution preparations and the following wet experiments were conducted in an
54
anaerobic Coy chamber to prevent the influence of dissolved oxygen (DO). By distinguishing Fe2+
55
oxidation by CeO2 from oxidation by molecular oxygen, we can provide a better understanding of
56
the redox reactions and mechanisms in aqueous Fe2+–CeO2 NP systems. Depleted oxygen can
3
ACS Paragon Plus Environment
Page 4 of 31
Page 5 of 31
Environmental Science & Technology
57
occur under specific conditions, such as in anoxic wastewater treatment processes, underground,
58
or at the bottom of stratified lakes.29 Furthermore, in acidic environment, the Fe2+ oxidation rate
59
by dissolved O2(aq) equilibrated at 0.2 atm is quite slow. Thus, our anaerobic experimental
60
conditions can still be applicable in wastewater systems where Fe2+ is present and the local pH is
61
acidic. To ensure anoxic conditions, deionized (DI) water was boiled prior to use in these
62
experiments to remove DO, and then cooled to room temperature in the anaerobic chamber.
63
We applied commercially available CeO2 NPs (Sigma-Aldrich, St. Louis, MO), so that our
64
results have closer connections to real engineered NP–environmental systems. CeO2 NP
65
dispersions of 50 mg/L were created in 50 mL test tubes inside the chamber, and the test tubes
66
were indirectly ultrasonicated for 1 h before reaction using a Fisher Scientific ultrasonic cleaner
67
(model no. FS6) with a frequency of 50/60 kHz and power of 30W. For all experiments, the ionic
68
strength (IS) was maintained at 10 mM. In the reaction system, 3 mM FeCl2 and 1 mM NaCl were
69
added to the CeO2 dispersions to have an ionic strength (IS) of 10 mM and a pH of 4.8 ± 0.2. The
70
final CeO2 concentration of the mixed solution was 45 mg/L. A weakly acidic pH can occur in
71
acid mine drainage environments.30, 31 In addition, when applying iron in wastewater treatment
72
plants, the hydrolysis of iron could lead to pHs of 5 or less if no pH control is conducted., 21, 32 At
73
pH 4.8, initial ferrous iron remains largely in the ferrous state, which could have greater
74
toxicological effects than other iron species on organisms, e.g., the feeding activity and motility of
75
the mayfly larvae.33, 34 This is also the pH of the system solution after FeCl2 addition, so errors in
76
the ionic strength through pH adjustment can be minimized. The concentration of 3 mM Fe2+ used
77
in our experimental system is commonly found in natural environments (e.g. 2–4 mM in an anoxic
78
lake in Massachusetts) and wastewater treatment processes that use additive ferrous iron.22, 23, 35 In
79
the control experiment,10 mM NaCl (with no FeCl2) was added to the CeO2 dispersion and the pH 4
ACS Paragon Plus Environment
Environmental Science & Technology
80
was adjusted to pH 4.8 with dilute HCl solution to match that of the Fe2+-containing system and
81
maintain the final CeO2 concentration of the mixed solution as 45 mg/L. The pH change during
82
the reaction period was monitored (Figure S6 in the SI). In addition, these conditions give valuable
83
insight into the interactions that occur between Fe2+ and CeO2, and can be used to bolster further
84
studies of CeO2 NP stability in more complex aqueous systems.
85
During sedimentation experiments, aliquots were taken from the supernatant at elapsed
86
times using a pipette with minimal disturbance to the suspension. The concentrations of CeO2 NPs
87
were measured by UV-vis spectroscopy (Varian Inc., Cary 50 Bio UV-Vis Spectrophotometer,
88
Palo Alto, California) at a wavelength of 305 nm, where the highest absorbance was obtained
89
(Figure S1 in the Supporting Information). Sedimentation experiments were run for 93 hours in
90
triplicate. To verify the changes in CeO2 surface properties following redox reactions, after
91
reaction the zeta potentials and particle sizes (Dynamic Light Scattering, DLS) of CeO2
92
nanoparticle aggregates in the Fe2+ and control systems were measured using a Zetasizer (Nano
93
ZS, Malvern Instruments Ltd., Westborough, MA). The zeta potential was derived from the
94
original electrophoretic mobility using the Smoluchowski equation. We have characterized the
95
particle size using DLS (Table S1) and TEM (Figure S7). The hydrodynamic particle sizes by DLS
96
were collected for at least three measurements (with each measurement taken over 10 s) and
97
triplicate or more experiments were conducted for each sample. The hydrodynamic diameter is
98
defined as the diameter of the aggregate plus that of the hydration layer.
99
CeO2 NP Dissolution Experiments
100
Due to the low CeO2 solubility,36 CeO2 suspensions of 250 mg/L and 15 mM FeCl2 were
101
used in dissolution experiments in order to achieve aqueous Ce concentrations above the 25 μg/L
5
ACS Paragon Plus Environment
Page 6 of 31
Page 7 of 31
Environmental Science & Technology
102
detection limit of the inductively coupled plasma-optical emission spectrometer (ICP-OES).
103
Similar or even higher concentrations of CeO2 for dissolution experiments have been also
104
investigated by other researchers.37, 38 Because the Fe2+ concentration scales up proportionally, we
105
expect that this elevated concentration system is relevant to lower suspended CeO2
106
concentrations.37, 39, 40 Thus, the concentration we used for our dissolution tests reasonably allows
107
us to compare our results with previous CeO2 NP studies.
108
Reaction and control solutions were prepared in the anaerobic chamber as described for
109
sedimentation experiments. Triplicate batches of the reaction and control systems were separated
110
into 5 mL test tubes and put into test tube rotators in the chamber.39, 41, 42 At 1, 2, 3, 4, 5, and 6
111
hour time points, triplicate samples were taken from the two systems. Samples were placed in
112
ultracentrifuge tubes (PC Oak Ridge Tubes, Fisher Scientific) in the anaerobic chamber, capped,
113
and removed from the chamber. To separate the supernatant from the CeO2 NPs, samples were
114
then ultracentrifuged using a Thermo Scientific Sorvall WX Ultra Series Centrifuge with a T-865
115
Fixed Angle Rotor at 40,000 rpm (or 115,861 x g) for 30 minutes. In addition to centrifugation,
116
the samples were placed back inside the anaerobic chamber and filtered by a 0.2 µm filter
117
(Millipore syringe filter) to ensure the removal of all bulk CeO2 NPs and aggregates. We have
118
verified the efficacy of combining ultracentrifugation and filtration to separate CeO2 NPs in
119
preliminary tests, and this method has also been commonly used in CeO2 NP dissolution and
120
separation studies.39, 41, 42 The filtrate was collected and acidified to 1% v/v nitric acid for ICP-
121
OES measurements.
6
ACS Paragon Plus Environment
Environmental Science & Technology
122
Phase Identification of Fe(III) (Hydr)oxides on CeO2 NP Surfaces
123
We conducted replicate experiments for X-ray absorption spectroscopy (XAS) analysis.
124
After reacting for 6 hours, solutions were transferred to centrifugation tubes and capped in the
125
anaerobic chamber, then ultracentrifuged at 40,000 rpm for 30 minutes. Once centrifuged, the
126
supernatant was poured off, leaving the CeO2 NPs fixed at the bottom of the tube. The supernatant
127
was then replaced with deoxygenated DI water and the tubes were then capped and removed from
128
the chamber for an additional 30 min of ultracentrifugation to remove excess salt. After the second
129
ultracentrifugation, the DI water was poured off in the anaerobic chamber, and the solids were
130
allowed to dry in the chamber overnight to prevent any oxidation. The cerium spectra were
131
measured in transmission mode, and iron spectra were measured in fluorescence mode.
132
Experiments were conducted at Beamline 13BM-D at the Advanced Photon Source (APS),
133
Argonne National Laboratory. This station utilized a Si(111) monochromator, giving it a focused
134
beam size of 10 m by 30 m and a resolution of 1 10-4 E/E. The energy flux was 1 109 at 10
135
keV. The energy range for this station was 4.5–70 keV. The iron XANES edge was measured at
136
7.119 keV and the cerium edge was measured at 40.444 keV.
137
Secondary Mineral Phase Precipitation on CeO2-Sputtered Substrates by Physical Vapor
138
Deposition (PVD)
139
To identify the properties of reaction products on the surface of NPs, CeO2 substrates were
140
created by sputtering CeO2 NPs on clean Si wafers using a PVD process (Kurt Lesker PVD 75,
141
Livermore, CA). The DC (diode) mode was applied under 1 mTorr argon pressure, with 100 watts
142
power input for 2000 seconds. The deposition was monitored in situ via a built-in quartz crystal
143
microbalance. Because CeO2 nanoparticles were stable, solid phase analyses were conducted under
7
ACS Paragon Plus Environment
Page 8 of 31
Page 9 of 31
Environmental Science & Technology
144
atmospheric conditions. The thickness of the CeO2–sputtered wafer was quantified by alpha-SE
145
ellipsometry (Lincoln, NE) and atomic force microscopy (AFM, Veeco, Nanoscope V) after the
146
PVD sputtering process. During experiments, the CeO2 substrate was exposed to the same solution
147
chemistry as the aqueous CeO2 NP experiments, containing either NaCl only or NaCl and Fe2+
148
ions (Figure S2). A clean Si wafer was also used as a control to compare precipitation in the system
149
containing NaCl and Fe2+ ions in the absence of CeO2. The precipitates on the CeO2-sputtered
150
substrate and Si wafer were analyzed by AFM using tapping mode. We collected height, amplitude,
151
and phase contrast information simultaneously for 5 µm × 5 µm areas, and analyzed the images
152
using the Nanoscope 7.20 software. Experimental details on the AFM setup have been reported in
153
our former studies.43, 44
154
Grazing Incidence Small-Angle X-ray Scattering (GISAXS) and SAXS Measurements
155
The surface properties and precipitates on the CeO2-sputtered substrates were also
156
characterized in situ by GISAXS. In this experiment, the solution was prepared under the same
157
solution chemistry as the CeO2 NP sedimentation tests, i.e., 3 mM FeCl2 and 1 mM NaCl at pH =
158
4.8. For the control system, a 10 mM NaCl solution was used. The CeO2 substrates were placed
159
flat at the bottom of a specially designed GISAXS cell. The solutions were injected at the top of
160
the cell, and then the cell was capped. The substrates were reacted for 1 hour. During GISAXS
161
measurements, incident X-ray beams at 18 keV were passed through the cell, where they interacted
162
with particles precipitating on the substrate surface (GISAXS). The scattered X-ray beams were
163
collected by a 2D detector. X-ray scattering data was processed by cutting along the Yoneda wing.
164
All data reduction was conducted using the GISAXS shop macro, a software package available at
165
APS beamline 12-ID. More detailed data fitting procedure descriptions are available in our
166
previous publications.43-45 Prior to size fitting, the background image of the CeO2 sputtered wafer 8
ACS Paragon Plus Environment
Environmental Science & Technology
Page 10 of 31
167
in DI water was subtracted from all 2D GISAXS scattering images. Thus, observed changes in
168
particle size were from the precipitation of Fe(III) (hydr)oxides. During the particle size fitting,
169
the shapes of the particles were assumed to be spherical, thus a form factor for polydisperse
170
spherical particles. A Schultz size distribution was also assumed for fitting.
171
RESULTS & DISCUSSION
172
Enhanced Colloidal Stability of CeO2 NPs in Fe2+ Solutions
173
First, we determined how the presence of Fe2+ affects the aggregation and subsequent
174
sedimentation of CeO2 NPs. During the 4 day monitoring period, the concentration of CeO2 NPs
175
in the supernatant was higher in the system with added Fe2+ than in the system without aqueous
176
Fe2+ (Figure 1A), indicating that the presence of 3 mM Fe2+ promoted the colloidal stability of
177
CeO2 NPs. For example, only 46% of the initial CeO2 NP’s concentration remained in the
178
supernatant in the control system while 94% of the initial CeO2 NPs remained stable after 16 h in
179
the presence of 3 mM FeCl2. In addition, the significant difference in NP sedimentation due to the
180
presence of Fe2+ can be observed visually in the solutions after stirring overnight (Figure S3,
181
Supporting Information). It is noted that the effect of double layer compression was similar in the
182
solutions because they had same 10 mM ionic strength and same anion identity (i.e., Cl-), which
183
will dominate double layer compression effects due to the positive charge of CeO2 NPs.
184
Analyses of particle sizes and zeta potentials in the two experimental systems also help to
185
illustrate the observed sedimentation trends. The zeta potential of CeO2 was 28.6 ± 1.7 mV in the
186
presence of Fe2+, while in the absence of Fe2+, the zeta potential was 8.7 ± 0.8 mV (Table S1,
187
Supporting Information). The isoelectric point, pHiep, of CeO2 NPs in 10 mM NaCl was measured
188
to be pH 4.95 (Figure S4A), which is within the range of 3.0–7.6 found in the literature. The wide
9
ACS Paragon Plus Environment
Page 11 of 31
Environmental Science & Technology
189
variation in this range can be due to the varying surface properties of the nanoparticles produced
190
through different methods.46-48 In our experimental system, the unreacted CeO2 NPs should be
191
positively charged under the test conditions.
192
The hydrodynamic size for stable suspensions (Table S1) and XRD spectra (Figure S4B)
193
for the unreacted CeO2 NPs are given in the SI. The hydrodynamic size was 137.7 ± 0.7 nm for
194
stable dispersion of unreacted CeO2 in DI water, while the particle sizes measured by TEM ranged
195
from 5–30 nm (Figure S7), consistent with the manufacture's nominal size. After 16 hours reaction,
196
the hydrodynamic diameter (DH) of CeO2 NPs in the 3 mM Fe2+ solution was 154 ±1 nm, while
197
the DH of CeO2 NPs in the 10 mM NaCl system was 3829 ± 358 nm (Table S1). The smaller size
198
of CeO2 aggregates when Fe2+ is present is in accordance with the higher suspended concentrations
199
of CeO2 NPs compared to the system without Fe2+, because the smaller aggregates will be more
200
stable in solution. In contrast, larger aggregates tend to settle out because gravitational forces are
201
predominant over Brownian motion.
202
Identification of the Reaction Products and CeO2 Dissolution
203
We hypothesize that the Fe2+-promoted stability of CeO2 NPs results from redox
204
reactions28 between Fe2+ and CeO2 NPs and the formation of Fe(III)products at the CeO2 NP
205
surfaces (eqn. (1)). Throughout the current manuscript, we expressed the oxidation state of solid
206
and sorbed species as Roman numerals and the oxidation state of aqueous species as Arabic
207
numerals. Arabic numerals were also used when both solid and aqueous species are possible. ≡CeIVO2+ Fe2+ (aq)↔ Ce3+ (aq)(or ≡CeIII2O3) + ≡Fe3+
208
10
ACS Paragon Plus Environment
(1)
Environmental Science & Technology
Page 12 of 31
209
X-ray absorption spectroscopy (XAS) was carried out to determine the Ce K-edge and Fe
210
K-edge spectra for the solid CeO2 NPs before and after reaction in the FeCl2 and control systems
211
(Figure 2). X-ray absorption near edge structure (XANES) results for Ce indicated no significant
212
change in the oxidation state after reaction. In other words, there is no CeIII contribution in both
213
initial CeO2 NPs and reacted CeO2 NPs (Figure 2A). This could be due to negligible amounts of
214
CeIII on the surface compared to bulk CeIV. Considering that the presence of surface CeIII2O3 was
215
not detected, and the aqueous Ce concentration was significantly enhanced in the presence of Fe2+
216
(as will discussed shortly after), we suggest that aqueous Ce3+ is the more predominant reaction
217
product than solid CeIII2O3. For iron, XAS results (Figure 3B) indicate that all iron on the reacted
218
CeO2 NP surface was oxidized to FeIII. Furthermore, the iron spectrum for the reacted CeO2 NPs
219
is consistent with that of the ferrihydrite standard (Figure 2B).
220
Aqueous Ce levels were monitored for the first 6 hours of reaction in the 250 mg/L CeO2
221
system (Figure 1B) because the most vigorous interfacial reaction occurs during the early stage.
222
Based on the concentration of dissolved Ce from CeO2 NPs in the supernatant, we also found that
223
the dissolved Ce concentration was 15 times higher in the presence of aqueous Fe2+ than in the
224
control system in the absence of Fe2+ over the course of the reaction period. Because the solubility
225
of Ce3+ is 25 orders of magnitude higher than CeIVO2, and it was reported that under similar
226
aqueous conditions to our experimental condition, more than 99% dissolved Ce was Ce3+,36, 49 we
227
assumed that the aqueous Ce in our system was primarily Ce3+. Thus, the increased concentration
228
of dissolved Ce could result from the reduction of CeIVO2 by Fe2+. Then, the redox reaction is also
229
likely to accelerate the precipitation of Fe(III)(hydr)oxides on the surface of CeO2 NPs while the
230
dissolved Ce3+ ions are released from the CeIVO2 NP surfaces.
11
ACS Paragon Plus Environment
Page 13 of 31
Environmental Science & Technology
231 232
Reaction Pathways: Adsorption, Redox Reaction, and Surface Precipitation of Fe(III)(Hydr)oxides on the Surfaces of CeO2 NPs and Dissolution of CeO2 NPs
233
A series of reactions could contribute to the enhanced colloidal stability of CeO2 in the
234
presence of Fe2+ ions. MgCl2 extraction revealed that the adsorbed Fe2+ concentration on the
235
surface of CeO2 NPs was 3.75 mg/L, which is equivalent to 0.05% of the total iron (Figure S5,
236
Supporting Information). The Fe2+ can be adsorbed on the CeO2 NPs by ion exchange (eq. (2)),
237
and such a process is expected to shift the zeta potential of CeO2 NPs to be more positive and
238
release H+ into the solution:50
239
Fe2+ Sorption: ≡CeOH + Fe2+ ↔ ≡CeOFe+ + H+
240
On the other hand, the adsorbed FeII can react further with CeIV on the surface of CeO2 NPs
241
rather than existing as ferrous state, due to the large enough difference in the standard redox
242
potential for Ce4+/Ce3+ (1.44 V) and Fe3+/Fe2+ (0.77 V).28 As a control experiment, we tested the
243
reaction of aqueous Ce4+ using Ce(SO4)2 and Fe2+ ions in solution and observed a significant
244
decrease of Fe2+ concentration by the Ferrozine method after 1 d (Table S2, Supporting
245
Information). Furthermore, the XANES experiments of the CeO2 NP‒Fe2+ system also revealed
246
oxidized Fe(III) on the surface of CeO2 NPs under anaerobic conditions, providing direct evidence
247
of surface redox reactions (Figure 2).
(2)
248
In the experimental systems, the pH of the reaction solution (CeO2 NPs + FeCl2) was measured
249
to be lower than either CeO2 NPs or FeCl2 solutions alone, and decreased during the first few hours
250
of reaction (Figure S6, Supporting Information). The decrease of solution pH can be attributed to
251
both the adsorption of Fe2+ through ion exchange (eqn. (2)) and the hydrolysis of Fe3+, the redox
252
reaction product, which releases more H+ into solution than is released by Fe2+ sorption. The Fe2+12
ACS Paragon Plus Environment
Environmental Science & Technology
Page 14 of 31
253
only system also has slightly decreased pH due to minor Fe2+ hydrolysis (eqn. 3a), but the rate of
254
pH decrease is much less than that of the CeO2/Fe2+ system, which undergoes the hydrolysis of
255
ferric ions (eqn. 3b).
256
Hydrolysis: Fe2+ + H2O Fe(OH)+(aq) + H+ or
(3a)
257
Fe3+ + 3 H2O Fe(OH)3 (aq) + 3 H+
(3b)
258
Once hydrolysis of ferric iron forms the monomer (eqn. 3b), then dimers and polymers can
259
form through continuous olation (hydroxo–bridging) and oxolation (oxo–bridging) reactions.44
260
Once the polymeric cluster size is larger than the critical nucleus size, stable nuclei form, and
261
Fe(III) (hydr)oxide precipitation occurs.45
262
To observe the particle formation of Fe(III) (hydr)oxides and their morphological changes
263
in Fe2+–CeO2 NP systems, AFM and GISAXS were used on a CeO2-sputtered Si wafer reacted in
264
the same solution conditions. AFM images show significant precipitation of FeIII solid phase on
265
the CeO2-sputtered Si wafer substrate after 6 h (Figure 3). The particle size of precipitated Fe(III)
266
(hydr)oxides on the CeO2 substrates increased from 4.8 ± 0.7 nm (based on 20 particle analysis)
267
to 10.6 ± 2.8 nm after 6 h, and 25.9 ± 5.4 nm after 1 day (based on 50 particle analysis each). In
268
contrast, precipitation of Fe(III) (hydr)oxide on a clean Si substrate was negligible within 1 day.
269
GISAXS results also showed Fe(III) (hydr)oxide nucleation on the surface of CeO2-sputtered
270
substrates (Figure 4). Figure 4A revealed that particles with an Rg of 1.72 nm were observed on
271
the surface of CeO2 substrates in the presence of 3 mM Fe2+ in 1 h. In contrast, no particles formed
272
in the absence of Fe2+due to no scattering increase for the CeO2 only system. SAXS images in
273
Figure 4B indicates that no particles formed in solution in the absence of CeO2 (e.g., Fe2+ only
274
system) as well. TEM analysis of CeO2 NPs revealed that nucleated Fe(III) (hydr)oxide covered 13
ACS Paragon Plus Environment
Page 15 of 31
Environmental Science & Technology
275
the surfaces of CeO2 NPs and appeared to increase the CeO2 NP particle–particle distance (Figure
276
S7). Furthermore, electron diffraction analysis of the Fe(III) (hydr)oxide products from the CeO2‒
277
Fe2+ system revealed the formation of 6-line ferrihydrite, which is consistent with XAS results. It
278
is important to note that artifacts and phase transformation might be introduced during TEM
279
sample preparation due to the drying process.43 Therefore, the morphology of the in situ Fe(III)
280
(hydr)oxide particles might be different than what is observed using TEM. However, the multiple
281
complementary techniques used in this study provide convincing and consistent evidence that the
282
precipitation of iron on the CeO2 surfaces greatly affected the surface properties and colloidal
283
stability of CeO2 NPs. Based on the above discussion, eqn. (1) can be rewritten as follows:
284
≡CeIVO2+ Fe2+ (aq)↔ ≡CeIVO2 + Ce3+ (aq) + ≡Fe(III)(hydr)oxide (e.g., ferrihydrite).
285
(4)
Enhanced Stability by Iron (Hydr)oxide Surface Precipitation
286
First, we analyzed the change in stability of CeO2 NPs using classical Derjaguin–Landau–
287
Verwey–Overbeek (DLVO) interactions, comprised of electrostatic repulsion forces and van der
288
Waals attractions. The electrostatic interaction increases with the absolute value of surface
289
potential (eqns. (S1-3) in Section S4 of the Supporting Information). Nanoparticles with higher
290
zeta potential have higher electrostatic repulsions each other, and therefore, are more stable in
291
suspension. Hence, the increased zeta potential due to the adsorption of Fe2+ and precipitation of
292
Fe(III)(hydr)oxides on the CeO2 NP surfaces can lead to more significant electrostatic repulsions.
293
In addition, the values of the Hamaker constant for iron oxide phases (magnetite, maghemite, and
294
hematite) in the literature range from 1.3 to 4.5 × 10-20 J.51 Although the Hamaker constant for
295
ferrihydrite is not available, the highest reported value for an iron oxide related phase (4.5 × 10-20
296
J) is still smaller than that of CeO2 (5.6–6.0×10-20 J).2, 52 A smaller Hamaker constant signifies that 14
ACS Paragon Plus Environment
Environmental Science & Technology
Page 16 of 31
297
there are less attractive forces between Fe(III) (hydr)oxide–Fe(III) (hydr)oxide than that of the
298
CeO2 NP–CeO2 NP. The precipitated Fe(III)(hydr)oxides on the surface of CeO2 will most likely
299
increase the electrostatic repulsion and decrease the van der Waals attractions.
300
Non-DLVO interactions also possibly play a significant role in the interactions of CeO2
301
NPs after being reacted with Fe2+. We hypothesize that the enhanced Ce stability in the presence
302
of Fe2+ may also result from Fe(III) (hydr)oxide precipitation changing the CeO2 NP surface
303
hydrophilicity. Due to the distinctive electronic structure of rare earth atoms, the CeO2 surface has
304
intrinsically less hydrophilic properties.53 In a cerium atom, the outer full octet of electrons in the
305
5s2p6 shell shields the unfilled 4f orbitals. As a result, CeO2 tends to not exchange electrons and
306
form hydrogen bonds with surrounding water molecules, making it less hydrophilic than other
307
metal oxides.53 The precipitated Fe(III) (hydr)oxides could, therefore, make the NP surface more
308
hydrophilic than the CeO2 surface.53 As a result, the coated NPs would be more stable in the
309
aqueous phase. To test the extent of hydrophilicity changes of CeO2 NPs by Fe(III) (hydr)oxide
310
surface coatings, we conducted surface angle measurements. It appeared that the surface of CeO2-
311
sputtered wafers became more hydrophilic after the precipitation of Fe(III) (hydr)oxides within a
312
3 day reaction period. (Figure S8, Supporting Information)
313
ENVIRONMENTAL IMPLICATIONS
314
In this study, we found that redox reactions between CeO2 NPs and Fe2+ lead to the
315
formation of 6-line ferrihydrite on the CeO2 surface, which enhanced the colloidal stability by
316
increasing the zeta potential and hydrophilicity of CeO2 NPs. The findings of this work suggest
317
longer retention periods and farther transport distances of CeO2 NPs in aquatic environments
318
containing Fe2+ ions. This study calls for immediate attention to the significant effects of redox
15
ACS Paragon Plus Environment
Page 17 of 31
Environmental Science & Technology
319
reactions and surface precipitation on the fate, transport, and bioavailability of engineered NPs in
320
aquatic environments. These nanoparticles could act as heterogeneous nucleation sites and
321
adsorption sites when released into the environment, incorporating toxic elements and molecules
322
into a “hybrid” engineered/natural nanoparticle composite.54 Redox reactive elements, e.g., Fe2+,
323
Mn2+, and contaminants, such as As, Cr, and U, can be adsorbed on and react with redox reactive
324
engineered NPs such as CeO2 NPs.55, 56 In addition, the positively charged CeO2 can adsorb and
325
aggregate with negatively charged natural colloids or polymers. In the presence of Fe2+, the new
326
hybrid Fe(III) hydroxide coated CeO2 nanoparticles become more positively charged and may take
327
longer to be destabilized by the natural colloids than in the absence of Fe2+ ions. In addition, the
328
interactions of adsorbed natural polymers with Fe2+ (e.g., complexation with natural polymers)
329
can make the redox reaction of CeO2 more complicated.57 The findings of the current study provide
330
an important starting point for investigating the long term influences of Fe2+ on CeO2 fate, transport,
331
and toxicity. These reactions have a significant impact on the transport and transformation of both
332
NPs and contaminants.
333
Consequential changes in the physicochemical properties of CeO2 NPs can affect the
334
toxicity by altering factors such as the surface charge, particle size, production of reactive oxygen
335
species (ROS), dissolution of reactive ions, hydrophilicity, and surface functionality or coatings.
336
For instance, positively charged CeO2 NPs were found to penetrate C. elegans cell membranes
337
more easily than the neutral and negatively charged CeO2 NPs, making them more toxic.8 Because,
338
in general, dissolved Ce3+ is far more toxic than CeO2 NPs,27 the presence of Fe2+can also enhance
339
their toxicity due to increased dissolution. Conversely, the surface of CeO2 NPs may become
340
passivated with time by Fe(III) (hydr)oxide coatings. This would retard further dissolution and
341
could help mitigate CeO2 NP toxicity. CeO2 NP redox processes with Fe2+ and other redox active 16
ACS Paragon Plus Environment
Environmental Science & Technology
Page 18 of 31
342
species could also interact with natural organic matter in wastewater treatment plants, potentially
343
affecting their toxicity to microorganisms in wastewater treatment plants.14, 39 In addition to CeO2
344
NPs, many engineered NPs may be subject to surface reactions and precipitation due to ubiquitous
345
redox reactive elements in the environment. These interactions need to be considered
346
comprehensively while evaluating the fate of engineered CeO2 NPs and assessing their risk in
347
aquatic environments.
348
Supporting Information Available
349
Supporting information includes experimental descriptions, UV-Vis spectra, DLS data, zeta
350
potential measurements, TEM images, an ED pattern, and pH monitoring data. This material is
351
available free of charge via the Internet at http://pubs.acs.org.
352
Acknowledgments
353
This work is supported by the National Science Foundation’s Environmental Chemical Science
354
Program (CHE-1214090) and Washington University’s Faculty Startup. JRR was supported by the
355
Environmental Protection Agency STAR Fellowship and CWN was supported by the Mr. and Mrs.
356
Spencer T. Olin Fellowship. We would like to thank Dr. Seonke Seifert of the Advanced Photon
357
Source Sector 12-ID-C at Argonne National Laboratory, supported by the U.S. Department of
358
Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-
359
06CH11357 and the Institute of Materials Science and Engineering and Nano Research Facility at
360
WUStL for experimental support. XAS work was performed at GeoSoilEnviroCARS (Sector 13),
361
Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported
362
by the National Science Foundation-Earth Sciences (EAR-1128799) and Department of Energy-
363
GeoSciences (DE-FG02-94ER14466). We thank Dr. Matt Newville and Dr. Tony Lanzirotti for 17
ACS Paragon Plus Environment
Page 19 of 31
Environmental Science & Technology
364
their help with XAS experiments. We appreciate the assistance and constructive suggestions from
365
our colleagues in the Environmental NanoChemistry Lab (ENCL) at WUStL.
18
ACS Paragon Plus Environment
Environmental Science & Technology
366
TOC Art
367
19
ACS Paragon Plus Environment
Page 20 of 31
Page 21 of 31
Environmental Science & Technology
368
LIST OF FIGURES
369
Figure 1.(A) Sedimentation kinetics of CeO2 NPs at 10 mM IS and pH 4.8 in the presence and
370
absence of aqueous Fe2+ ions. The percentages were obtained from suspended nanoparticle
371
concentration normalized by the initial suspended concentration measured by UV-Vis. The error
372
bars represent the standard deviation of CeO2 concentration from triplicate experiments. (B)
373
Dissolved Ce concentrations from CeO2 NPs in the presence and absence of Fe2+ at pH 4.8.
374
Figure 2. XAS spectra for CeO2 NPs reacted in the presence of 3 mM FeCl2 (A: Ce K-edge and
375
B: Fe K-edge). The increase in energy of the K-edge position compared to the FeCl2 standard
376
indicates oxidation of Fe(II) to Fe(III). XAS results showing (A) cerium K-edge and (B) iron K-
377
edge spectra for reacted samples and standards. Ce K-edge results show no detectable Ce(III) on
378
the surface. Fe K-edge results show the most similarity between ferrihydrite and the Fe(III) phase
379
formed on the CeO2 NPs after the 6 h reaction.
380
Figure 3. Representative AFM images for the precipitation of iron oxide particles on (A) CeO2-
381
sputtered Si substrates and (B) pure Si control substrates. Height scale (HS) is 10 nm unless
382
otherwise noted.
383
Figure 4.1D reduced grazing incidence small angle X-ray scattering (GISAXS) data of
384
Fe3+precipitation on the surfaces of CeO2 sputtered wafers ( , Figure 4A) and data from the CeO2
385
NP control system ( , Figure 4A). SAXS raw data of the Fe2+ only system is depicted in Figure
386
4B. Figure 4A revealed that particles were observed in CeO2 + Fe2+ system with Rg = 1.72 nm (at
387
1 hr), and no particles formed in the absence of Fe2+ because there was no scattering increase for
388
the CeO2 only system. Figure 4B (Fe2+ only system) indicates that no particles formed without
389
CeO2 as well. In Figure 4B, no background subtraction was performed due to no increase of 20
ACS Paragon Plus Environment
Environmental Science & Technology
390
intensity throughout the reaction and some signal around 0.035 Å-1 results from the beamline setup
391
rather than actual particle contribution.
21
ACS Paragon Plus Environment
Page 22 of 31
Page 23 of 31
Environmental Science & Technology
CeO2 conc. (%)
100
3 mM FeCl2 + 1 mM NaCl 10 mM NaCl
80 60 40 20 0
A 0
20
40
60
Time (h)
393
80
100
Dissolved Ce conc. (mg/L)
392
2.0 1.5 1.0 0.5 0.0
B 0
1
2
3
4
5
Time (h)
394 395
Figure 1
22
ACS Paragon Plus Environment
6
Environmental Science & Technology
Page 24 of 31
B
Normalized absorbance (a.u.)
A CeO2 standard
CeO2 control 2+ CeO22++Fe Fe(II) CeO
5700
396 397
5750 5800 Energy (eV)
Hematite
Normalized absorbance (a.u.)
Ce(III) Nitrate standard
5850
Goethite Lepidocrocite
Ferrihydrite CeO Fe(II) CeO Fe2+ 2+ 2+
7100
398
Figure 2
23
Fe(II) chloride
ACS Paragon Plus Environment
7150 7200 Energy (eV)
7250
Page 25 of 31
Environmental Science & Technology
399
400
Figure 3
401
24
ACS Paragon Plus Environment
Environmental Science & Technology
Page 26 of 31
4
1
A
Intensity (counts)
Intensity (counts)
10
3
10
Rg = 1.72 nm
2
10
1
10
0
CeO2 + Fe(II) CeO Fe2+ CeO22 CeO
10 10
-1 6
2
0.01
B 4 2
0.1 4 2
0.01
3 4 56
2
3 4 5
2
0.1
0.01
-1
q_xy (Å )
402 403
Figure 4
25
ACS Paragon Plus Environment
3
4 5 6
0.1 -1 q_xy (Å )
2
3
4
Page 27 of 31
Environmental Science & Technology
404
REFERENCES
405
1.
OECD Nanosafety at the OECD: The First Five Years 2006-2010; Organization for Economic Cooperation and Development: Paris, France, 2011.
2.
Liu, X.; Chen, G.; Su, C., Influence of Collector Surface Composition and Water Chemistry on the Deposition of Cerium Dioxide Nanoparticles: QCM-D and Column Experiment Approaches. Environ. Sci. Technol. 2012, 46, (12), 6681-6688.
3.
Migani, A.; Neyman, K. M.; Illas, F.; Bromley, S. T., Exploring Ce3+/Ce4+ cation ordering in reduced ceria nanoparticles using interionic-potential and density-functional calculations. J. Chem. Phys. 2009, 131, (6), 064701-064707.
4.
Liu, X.; Chen, G.; Su, C., Influence of Collector Surface Composition and Water Chemistry on the Deposition of Cerium Dioxide Nanoparticles: QCM-D and Column Experiment Approaches. Environ. Sci. Technol. 2012, 46, (12), 6681-6688.
5.
Evaluation of Human Health Risk from Cerium Added to Diesel Fuel; Health Effects Institute: 2001.
6.
Gantt, B.; Hoque, S.; Willis, R. D.; Fahey, K. M.; Delgado-Saborit, J. M.; Harrison, R. M.; Erdakos, G. B.; Bhave, P. V.; Zhang, K. M.; Kovalcik, K.; Pye, H. O. T., Near-Road Modeling and Measurement of Cerium-Containing Particles Generated by Nanoparticle Diesel Fuel Additive Use. Environ. Sci. Technol. 2014, 48, (18), 10607-10613.
7.
Nanomaterials EPA is Assessing. http://www.epa.gov/nanoscience/quickfinder/nanomaterials.htm
8.
Collin, B.; Oostveen, E.; Tsyusko, O. V.; Unrine, J. M., Influence of Natural Organic Matter and Surface Charge on the Toxicity and Bioaccumulation of Functionalized Ceria Nanoparticles in Caenorhabditis elegans. Environ. Sci. Technol. 2013, 48, (2), 1280-1289.
9.
Thill, A.; Zeyons, O.; Spalla, O.; Chauvat, F.; Rose, J.; Auffan, M.; Flank, A. M., Cytotoxicity of CeO2 nanoparticles for Escherichia coli. Physico-chemical insight of the cytotoxicity mechanism. Environ. Sci. Technol. 2006, 40, (19), 6151-6156.
10.
Zhang, W.; Crittenden, J.; Li, K.; Chen, Y., Attachment Efficiency of Nanoparticle Aggregation in Aqueous Dispersions: Modeling and Experimental Validation. Environ. Sci. Technol. 2012, 46, (13), 7054-7062.
11.
Keller, A. A.; Wang, H.; Zhou, D.; Lenihan, H. S.; Cherr, G.; Cardinale, B. J.; Miller, R.; Ji, Z., Stability and Aggregation of Metal Oxide Nanoparticles in Natural Aqueous Matrices. Environ. Sci. Technol. 2010, 44, (6), 1962-1967.
406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438
12. Petosa, A. R.; Ohl, C.; Rajput, F.; Tufenkji, N., Mobility of nanosized cerium dioxide and polymeric capsules in quartz and loamy sands saturated with model and natural groundwaters. Water Res. 2013, 47, (15), 5889-5900. 26
ACS Paragon Plus Environment
Environmental Science & Technology
Page 28 of 31
439
13.
440 441
Buettner, K. M.; Rinciog, C. I.; Mylon, S. E., Aggregation kinetics of cerium oxide nanoparticles in monovalent and divalent electrolytes. Colloids Surf., A 2010, 366, (1-3), 7479.
445
14. Quik, J. T. K.; Lynch, I.; Hoecke, K. V.; Miermans, C. J. H.; Schamphelaere, K. A. C. D.; Janssen, C. R.; Dawson, K. A.; Stuart, M. A. C.; Meent, D. V. D., Effect of natural organic matter on cerium dioxide nanoparticles settling in model fresh water. Chemosphere 2010, 81, (6), 711-715.
446
15.
Van Hoecke, K.; De Schamphelaere, K. A. C.; Van der Meeren, P.; Smagghe, G.; Janssen, C. R., Aggregation and ecotoxicity of CeO2 nanoparticles in synthetic and natural waters with variable pH, organic matter concentration and ionic strength. Environ. Pollut. 2011, 159, (4), 970-976.
16.
Barton, L. E.; Auffan, M.; Bertrand, M.; Barakat, M.; Santaella, C.; Masion, A.; Borschneck, D.; Olivi, L.; Roche, N.; Wiesner, M. R.; Bottero, J.-Y., Transformation of Pristine and Citrate-Functionalized CeO2 Nanoparticles in a Laboratory-Scale Activated Sludge Reactor. Environ. Sci. Technol. 2014, 48, (13), 7289-7296.
17.
Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S. I.; Seal, S., Graphene based materials: Past, present and future. Prog. Mater. Sci. 2011, 56, (8), 1178-1271.
18.
Yu, P.; Hayes, S. A.; O'Keefe, T. J.; O'Keefe, M. J.; Stoffer, J. O., The Phase Stability of Cerium Species in Aqueous Systems: II. The Systems. Equilibrium Considerations and Pourbaix Diagram Calculations. J. Electrochem. Soc. 2006, 153, (1), C74-C79.
442 443 444
447 448 449 450 451 452 453 454 455 456 457 458 460
19. Stumm, W.; Lee, G. F., Oxygenation of Ferrous Iron. Ind. Eng. Chem. 1961, 53, (2), 143146.
461
20.
Nielsen, A. H.; Hvitved-Jacobson, T.; Vollertsen, J., Effects of pH and iron concentrations on sulfide precipitation in wastewater collection systems. Water Environ. Res. 2008, 80, (4), 380-384.
21.
Wang, Y.; Tng, K. H.; Wu, H.; Leslie, G.; Waite, T. D., Removal of phosphorus from wastewaters using ferrous salts - A pilot scale membrane bioreactor study. Water Res. 2014, 57, 140-150.
459
462 463 464 465 466
469
22. Ivanov, V.; Kuang, S.; Stabnikov, V.; Guo, C., The removal of phosphorus from reject water in a municipal wastewater treatment plant using iron ore. J. Chem. Technol. Biot. 2009, 84, (1), 78-82.
470
23.
Park, D.; Kim, Y. M.; Lee, D. S.; Park, J. M., Chemical treatment for treating cyanidescontaining effluent from biological cokes wastewater treatment process. Chem. Eng. J. 2008, 143, (1–3), 141-146.
24.
Xu, L.; Wang, J., Magnetic Nanoscaled Fe3O4/CeO2 Composite as an Efficient Fenton-Like Heterogeneous Catalyst for Degradation of 4-Chlorophenol. Environ. Sci. Technol. 2012, 46, (18), 10145-10153.
467 468
471 472 473 474 475
27
ACS Paragon Plus Environment
Page 29 of 31
Environmental Science & Technology
476
25.
Schreiber, H. D.; Lauer Jr, H. V.; Thanyasiri, T., Oxidation-reduction equilibria of iron and cerium in silicate glasses: Individual redox potentials and mutual interactions. J. Non-Cryst. Solids 1980, 38-39, (PART 2), 785-790.
26.
Limbach, L. K.; Bereiter, R.; Muller, E.; Krebs, R.; Galli, R.; Stark, W. J., Removal of Oxide Nanoparticles in a Model Wastewater Treatment Plant: Influence of Agglomeration and Surfactants on Clearing Efficiency. Environ. Sci. Technol. 2008, 42, (15), 5828-5833.
27.
Dahle, J.; Arai, Y., Effects of Ce(III) and CeO2 Nanoparticles on Soil-Denitrification Kinetics. Arch. Environ. Contam. Toxicol. 2014, 1-9.
477 478 479 480 481 482 483 485
28. Bard, A. J.; Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications John Wiley & Sons, INC.: New York, 2001.
486
29.
Wilson, N.; Webster-Brown, J., The fate of antimony in a major lowland river system, the Waikato River, New Zealand. Appl. Geochem. 2009, 24, (12), 2283-2292.
30.
Bigham, J. M.; Schwertmann, U.; Carlson, L.; Murad, E., A poorly crystallized oxyhydroxysulfate of iron formed by bacterial oxidation of Fe(II) in acid mine waters. Geochim. Cosmochim. Ac. 1990, 54, (10), 2743-2758.
484
487 488 489 490 492
31. Stumm, W.; Morgan, J., Aquatic Chemistry: Chemical Equilibria and Rate in Natural Waters. 3rd ed.; Wiley: New York, 1996.
493
32.
Liu, X.; Gong, W.; Liu, L., Treatment of sulfate-rich and low pH wastewater by sulfate reducing bacteria with iron shavings in a laboratory. Water Sci. Technol. 2014, 69, (3), 595600.
33.
Rousch, J. M.; Simmons, T. W.; Kerans, B. L.; Smith, B. P., Relative acute effects of low pH and high iron on the hatching and survival of the water mite (Arrenurus manubriator) and the aquatic insect (Chironomus riparius). Environ. Toxicol. Chem. 1997, 16, (10), 2144-2150.
34.
Gerhardt, A., Effects of subacute doses of iron (Fe) on Leptophlebia marginata (Insecta: Ephemeroptera). Freshwater Biol. 1992, 27, (1), 79-84.
35.
Díez, S.; Noonan, G. O.; MacFarlane, J. K.; Gschwend, P. M., Ferrous iron oxidation rates in the pycnocline of a permanently stratified lake. Chemosphere 2007, 66, (8), 1561-1570.
491
494 495 496 497 498 499 500 501 502 504
36. Dahle, J. T.; Livi, K.; Arai, Y., Effects of pH and phosphate on CeO2 nanoparticle dissolution. Chemosphere 2015, 119, 1365-1371.
505
37.
Hoecke, K. V.; Quik, J. T. K.; Mankiewicz-Boczek, J.; Schamphelaere, K. A. C. D.; Elsaesser, A.; Meeren, P. V. d.; Barnes, C.; McKerr, G.; Howard, C. V.; Meent, D. V. D.; Rydzynski, K.; Dawson, K. A.; Salvati, A.; Lesniak, A.; Lynch, I.; Silversmit, G.; Samber, B. D.; Vincze, L.; Janssen, C. R., Fate and Effects of CeO2 Nanoparticles in Aquatic Ecotoxicity Tests. Environ. Sci. Technol. 2009, 43, (12), 4537-4546.
38.
Rico, C. M.; Morales, M. I.; McCreary, R.; Castillo-Michel, H.; Barrios, A. C.; Hong, J.; Tafoya, A.; Lee, W.-Y.; Varela-Ramirez, A.; Peralta-Videa, J. R.; Gardea-Torresdey, J. L.,
503
506 507 508 509 510 511
28
ACS Paragon Plus Environment
Environmental Science & Technology
Page 30 of 31
Cerium Oxide Nanoparticles Modify the Antioxidative Stress Enzyme Activities and Macromolecule Composition in Rice Seedlings. Environ. Sci. Technol. 2013, 47, (24), 14110-14118.
512 513 514 515
39.
Rogers, N. J.; Franklin, N. M.; Apte, S. C.; Batley, G. E.; Angel, B. M.; Lead, J. R.; Baalousha, M., Physico-chemical behaviour and algal toxicity of nanoparticulate CeO2 in freshwater. Environ. Chem. 2010, 7, (1), 50-60.
40.
Rico, C. M.; Hong, J.; Morales, M. I.; Zhao, L.; Barrios, A. C.; Zhang, J.-Y.; Peralta-Videa, J. R.; Gardea-Torresdey, J. L., Effect of Cerium Oxide Nanoparticles on Rice: A Study Involving the Antioxidant Defense System and In Vivo Fluorescence Imaging. Environ. Sci. Technol. 2013, 47, (11), 5635-5642.
41.
Baalousha, M.; Ju-Nam, Y.; Cole, P. A.; Gaiser, B.; Fernandes, T. F.; Hriljac, J. A.; Jepson, M. A.; Stone, V.; Tyler, C. R.; Lead, J. R., Characterization of cerium oxide nanoparticles— Part 1: Size measurements. Environ. Toxicol. Chem. 2012, 31, (5), 983-993.
42.
Baalousha, M.; Le Coustumer, P.; Jones, I.; Lead, J. R., Characterisation of structural and surface speciation of representative commercially available cerium oxide nanoparticles. Environ. Chem. 2010, 7, (4), 377-385.
43.
Ray, J. R.; Lee, B.; Baltrusaitis, J.; Jun, Y.-S., Formation of Iron(III) (Hydr)oxides on Polyaspartate- and Alginate-Coated Substrates: Effects of Coating Hydrophilicity and Functional Group. Environ. Sci. Technol. 2012, 46, (24), 13167-13175.
44.
Hu, Y.; Lee, B.; Bell, C.; Jun, Y.-S., Environmentally Abundant Anions Influence the Nucleation, Growth, Ostwald Ripening, and Aggregation of Hydrous Fe(III) Oxides. Langmuir 2012, 28, (20), 7737-7746.
45.
Jun, Y.-S.; Lee, B.; Waychunas, G. A., In Situ Observations of Nanoparticle Early Development Kinetics at Mineral−Water Interfaces. Environ. Sci. Technol. 2010, 44, (21), 8182-8189.
46.
De Faria, L. A.; Trasatti, S., The Point of Zero Charge of CeO2. J. Colloid Interf. Sci. 1994, 167, (2), 352-357.
47.
Kosmulski, M., The pH-Dependent Surface Charging and the Points of Zero Charge. J. Colloid Interf. Sci. 2002, 253, (1), 77-87.
48.
Kosmulski, M., The pH-dependent surface charging and points of zero charge: V. Update. J. Colloid Interf. Sci. 2011, 353, (1), 1-15.
49.
Cornelis, G.; Ryan, B.; McLaughlin, M. J.; Kirby, J. K.; Beak, D.; Chittleborough, D., Solubility and Batch Retention of CeO2 Nanoparticles in Soils. Environ. Sci. Technol. 2011, 45, (7), 2777-2782.
516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547
50. Hunter, R. J., Foundations of Colloid Science. second ed.; Clarendon Press: Oxford, U.K., 2001; Vol. 210, p 125-125. 29
ACS Paragon Plus Environment
Page 31 of 31
Environmental Science & Technology
549
51. Faure, B.; Salazar-Alvarez, G.; Bergström, L., Hamaker constants of iron oxide nanoparticles. Langmuir 2011, 27, (14), 8659-8664.
550
52.
Li, K.; Zhang, W.; Huang, Y.; Chen, Y., Aggregation kinetics of CeO2 nanoparticles in KCl and CaCl2 solutions: measurements and modeling. J. Nanopart. Res. 2011, 1-9.
53.
Azimi, G.; Dhiman, R.; Kwon, H. M.; Paxson, A. T.; Varanasi, K. K., Hydrophobicity of rare-earth oxide ceramics. Nature Materials 2013, 12, (4), 315-320.
548
551 552 553 555
54. Grassian, V. H.; Hamers, R. J. Workshop report on "Nanomaterials and the Environment: The Chemistry and Materials Perspective"; Arlington, 2011.
556
55.
Donald, L. M.; Katherine, W.-D., Redox Chemistry and Natural Organic Matter (NOM): Geochemists? Dream, Analytical Chemists? Nightmare. In Aquatic Redox Chemistry, American Chemical Society: 2011; Vol. 1071, pp 85-111.
56.
Neil, C. W.; Lee, B.; Jun, Y.-S., Different Arsenate and Phosphate Incorporation Effects on the Nucleation and Growth of Iron(III) (Hydr)oxides on Quartz. Environ. Sci. Technol. 2014, 48, (20), 11883-11891.
554
557 558 559 560 561 562 563
57. Rose, A. L., Effect of Dissolved Natural Organic Matter on the Kinetics of Ferrous Iron Oxygenation in Seawater. Environ. Sci. Technol. 2003, 37, (21), 4877-4886.
564 565
30
ACS Paragon Plus Environment
Article
Enhanced Colloidal Stability of CeO2 Nanoparticles by Ferrous Ions: Adsorption, Redox Reaction, and Surface Precipitation Xuyang Liu, Jessica Renee Ray, Chelsea W. Neil, Qingyun Li, and Young-Shin Jun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es506363x • Publication Date (Web): 07 Apr 2015 Downloaded from http://pubs.acs.org on April 12, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
Environmental Science & Technology
Enhanced Colloidal Stability of CeO2 Nanoparticles by Ferrous Ions: Adsorption, Redox Reaction, and Surface Precipitation
Xuyang Liu †, Jessica R. Ray †, Chelsea W. Neil †, Qingyun Li, and Young-Shin Jun* Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, United States
E-mail: [email protected] http://encl.engineering.wustl.edu/ Submitted: December 2014 Revised: March 2015
Environmental Science &Technology
†These
authors contributed equally to the current work.
*To whom correspondence should be addressed.
ACS Paragon Plus Environment
Environmental Science & Technology
1
ABSTRACT
2
Due to the toxicity of cerium oxide (CeO2) nanoparticles (NPs), a better understanding of the redox
3
reaction-induced surface property changes of CeO2 NPs and their transport in natural and
4
engineered aqueous systems is needed. This study investigates the impact of redox reactions with
5
ferrous ions (Fe2+) on the colloidal stability of CeO2 NPs. We demonstrated that under anaerobic
6
conditions suspended CeO2 NPs in a 3 mM FeCl2 solution at pH = 4.8 were much more stable
7
against sedimentation than those in the absence of Fe2+. Redox reactions between CeO2 NPs and
8
Fe2+ lead to the formation of 6-line ferrihydrite on the CeO2 surfaces, which enhanced the colloidal
9
stability by increasing the zeta potential and hydrophilicity of CeO2 NPs. These redox reactions
10
can affect the toxicity of CeO2 NPs by increasing cerium dissolution and by creating new Fe(III)
11
(hydr)oxide reactive surface layers. Thus, these findings have significant implications for
12
elucidating the phase transformation and transport of redox reactive NPs in the environment.
1
ACS Paragon Plus Environment
Page 2 of 31
Page 3 of 31
Environmental Science & Technology
13
INTRODUCTION
14
Cerium oxide (CeO2) nanoparticles (NPs) have been enumerated in the priority list of
15
engineered nanomaterials for risk evaluation by the Organization for Economic Co-operation and
16
Development (OECD) due to their wide application in industry and daily life.1 CeO2 NPs are ideal
17
in catalysis applications, such as diesel engine catalytic converters,2 due to their oxygen storage
18
capabilities3 and their ability to readily participate in Ce4+/Ce3+ redox processes. The use and
19
production of CeO2 NPs will inevitably result in increased concentrations in natural or manmade
20
aqueous environments, such as wastewater.4 Therefore, in order to better predict the life cycle of
21
these engineered nanoparticles, it is vital to improve our understanding of their fate and transport,
22
particularly in aqueous environments.
23
Based on a 2001 report on human health risks of cerium from diesel fuels, the average
24
worldwide estimated level of cerium in soils was 20–60 ppm.5 Recent studies have also shown that
25
nanometer sized CeO2 particles are found in automobile exhaust.6,
26
warrants immediate attention to prevent harmful effects to the biosphere. The bioavailability and
27
toxicity of CeO2 NPs are largely determined by their fate and transport in the environment, which
28
is in turn affected by their surface charge and aggregation state.8, 9 While recent investigations in
29
this field have focused on the influence of solution chemistry and organic matter on the fate and
30
transport of CeO2 NPs,2, 10-16 few studies have considered the effect of redox reactions on CeO2
31
NP surface properties in the presence of redox-active ions in diverse aqueous environments.17, 18
7
This high concentration
32
When released in the environment, CeO2 NPs can coexist and interact with redox sensitive
33
elements. One important element to consider is ferrous iron (Fe2+), which is widely distributed in
34
natural aqueous systems (e.g., acid mine drainage),19 as well as in engineered systems for odor and
2
ACS Paragon Plus Environment
Environmental Science & Technology
35
corrosion control, precipitating hydrogen sulfide, and phosphorus removal.20, 21 The concentration
36
of ferrous ions used in these engineered systems can be as high as 9.85 mM for phosphorus
37
removal,22 and 1.5–3.0 mM to remove cyanide from industrial wastewater.23 In addition, in a study,
38
Fe3O4/CeO2-impregnated NPs were synthesized and used as Fenton-like catalysts for hydroxyl
39
radical generation and 4-chlorophenol degradation with the addition of H2O2.24 Ce4+ has also been
40
used in the production of silicate glasses to rid the glass of Fe2+ ions.25 Therefore, anthropogenic
41
CeO2 NPs originated from industrial wastewater can interact with ferrous ions during their
42
industrial applications, as well as during wastewater treatment processes.26, 27
43
According to the difference in the standard redox potential of Fe3+/Fe2+ (0.77V) and
44
Ce4+/Ce3+ (1.44 V), redox reactions between Fe2+ and CeO2 can occur when they coexist in
45
solution.28 However, it is largely unknown how this reaction with aqueous Fe2+ will affect the
46
surface properties and colloidal stability of CeO2 NPs in aqueous environments. In this study, we
47
systematically investigated changes in CeO2 NP surface properties when aqueous Fe2+ is present.
48
Knowledge obtained in this study can help improve our understanding of the fate and transport of
49
redox active nanomaterials during their lifetime, which will in turn give insight into the expected
50
bioavailability of these anthropogenic NPs when released into the environment.
51
EXPERIMENTAL SECTION
52
Preparation of Nanoparticle Dispersions for Aggregation and Sedimentation Tests
53
All solution preparations and the following wet experiments were conducted in an
54
anaerobic Coy chamber to prevent the influence of dissolved oxygen (DO). By distinguishing Fe2+
55
oxidation by CeO2 from oxidation by molecular oxygen, we can provide a better understanding of
56
the redox reactions and mechanisms in aqueous Fe2+–CeO2 NP systems. Depleted oxygen can
3
ACS Paragon Plus Environment
Page 4 of 31
Page 5 of 31
Environmental Science & Technology
57
occur under specific conditions, such as in anoxic wastewater treatment processes, underground,
58
or at the bottom of stratified lakes.29 Furthermore, in acidic environment, the Fe2+ oxidation rate
59
by dissolved O2(aq) equilibrated at 0.2 atm is quite slow. Thus, our anaerobic experimental
60
conditions can still be applicable in wastewater systems where Fe2+ is present and the local pH is
61
acidic. To ensure anoxic conditions, deionized (DI) water was boiled prior to use in these
62
experiments to remove DO, and then cooled to room temperature in the anaerobic chamber.
63
We applied commercially available CeO2 NPs (Sigma-Aldrich, St. Louis, MO), so that our
64
results have closer connections to real engineered NP–environmental systems. CeO2 NP
65
dispersions of 50 mg/L were created in 50 mL test tubes inside the chamber, and the test tubes
66
were indirectly ultrasonicated for 1 h before reaction using a Fisher Scientific ultrasonic cleaner
67
(model no. FS6) with a frequency of 50/60 kHz and power of 30W. For all experiments, the ionic
68
strength (IS) was maintained at 10 mM. In the reaction system, 3 mM FeCl2 and 1 mM NaCl were
69
added to the CeO2 dispersions to have an ionic strength (IS) of 10 mM and a pH of 4.8 ± 0.2. The
70
final CeO2 concentration of the mixed solution was 45 mg/L. A weakly acidic pH can occur in
71
acid mine drainage environments.30, 31 In addition, when applying iron in wastewater treatment
72
plants, the hydrolysis of iron could lead to pHs of 5 or less if no pH control is conducted., 21, 32 At
73
pH 4.8, initial ferrous iron remains largely in the ferrous state, which could have greater
74
toxicological effects than other iron species on organisms, e.g., the feeding activity and motility of
75
the mayfly larvae.33, 34 This is also the pH of the system solution after FeCl2 addition, so errors in
76
the ionic strength through pH adjustment can be minimized. The concentration of 3 mM Fe2+ used
77
in our experimental system is commonly found in natural environments (e.g. 2–4 mM in an anoxic
78
lake in Massachusetts) and wastewater treatment processes that use additive ferrous iron.22, 23, 35 In
79
the control experiment,10 mM NaCl (with no FeCl2) was added to the CeO2 dispersion and the pH 4
ACS Paragon Plus Environment
Environmental Science & Technology
80
was adjusted to pH 4.8 with dilute HCl solution to match that of the Fe2+-containing system and
81
maintain the final CeO2 concentration of the mixed solution as 45 mg/L. The pH change during
82
the reaction period was monitored (Figure S6 in the SI). In addition, these conditions give valuable
83
insight into the interactions that occur between Fe2+ and CeO2, and can be used to bolster further
84
studies of CeO2 NP stability in more complex aqueous systems.
85
During sedimentation experiments, aliquots were taken from the supernatant at elapsed
86
times using a pipette with minimal disturbance to the suspension. The concentrations of CeO2 NPs
87
were measured by UV-vis spectroscopy (Varian Inc., Cary 50 Bio UV-Vis Spectrophotometer,
88
Palo Alto, California) at a wavelength of 305 nm, where the highest absorbance was obtained
89
(Figure S1 in the Supporting Information). Sedimentation experiments were run for 93 hours in
90
triplicate. To verify the changes in CeO2 surface properties following redox reactions, after
91
reaction the zeta potentials and particle sizes (Dynamic Light Scattering, DLS) of CeO2
92
nanoparticle aggregates in the Fe2+ and control systems were measured using a Zetasizer (Nano
93
ZS, Malvern Instruments Ltd., Westborough, MA). The zeta potential was derived from the
94
original electrophoretic mobility using the Smoluchowski equation. We have characterized the
95
particle size using DLS (Table S1) and TEM (Figure S7). The hydrodynamic particle sizes by DLS
96
were collected for at least three measurements (with each measurement taken over 10 s) and
97
triplicate or more experiments were conducted for each sample. The hydrodynamic diameter is
98
defined as the diameter of the aggregate plus that of the hydration layer.
99
CeO2 NP Dissolution Experiments
100
Due to the low CeO2 solubility,36 CeO2 suspensions of 250 mg/L and 15 mM FeCl2 were
101
used in dissolution experiments in order to achieve aqueous Ce concentrations above the 25 μg/L
5
ACS Paragon Plus Environment
Page 6 of 31
Page 7 of 31
Environmental Science & Technology
102
detection limit of the inductively coupled plasma-optical emission spectrometer (ICP-OES).
103
Similar or even higher concentrations of CeO2 for dissolution experiments have been also
104
investigated by other researchers.37, 38 Because the Fe2+ concentration scales up proportionally, we
105
expect that this elevated concentration system is relevant to lower suspended CeO2
106
concentrations.37, 39, 40 Thus, the concentration we used for our dissolution tests reasonably allows
107
us to compare our results with previous CeO2 NP studies.
108
Reaction and control solutions were prepared in the anaerobic chamber as described for
109
sedimentation experiments. Triplicate batches of the reaction and control systems were separated
110
into 5 mL test tubes and put into test tube rotators in the chamber.39, 41, 42 At 1, 2, 3, 4, 5, and 6
111
hour time points, triplicate samples were taken from the two systems. Samples were placed in
112
ultracentrifuge tubes (PC Oak Ridge Tubes, Fisher Scientific) in the anaerobic chamber, capped,
113
and removed from the chamber. To separate the supernatant from the CeO2 NPs, samples were
114
then ultracentrifuged using a Thermo Scientific Sorvall WX Ultra Series Centrifuge with a T-865
115
Fixed Angle Rotor at 40,000 rpm (or 115,861 x g) for 30 minutes. In addition to centrifugation,
116
the samples were placed back inside the anaerobic chamber and filtered by a 0.2 µm filter
117
(Millipore syringe filter) to ensure the removal of all bulk CeO2 NPs and aggregates. We have
118
verified the efficacy of combining ultracentrifugation and filtration to separate CeO2 NPs in
119
preliminary tests, and this method has also been commonly used in CeO2 NP dissolution and
120
separation studies.39, 41, 42 The filtrate was collected and acidified to 1% v/v nitric acid for ICP-
121
OES measurements.
6
ACS Paragon Plus Environment
Environmental Science & Technology
122
Phase Identification of Fe(III) (Hydr)oxides on CeO2 NP Surfaces
123
We conducted replicate experiments for X-ray absorption spectroscopy (XAS) analysis.
124
After reacting for 6 hours, solutions were transferred to centrifugation tubes and capped in the
125
anaerobic chamber, then ultracentrifuged at 40,000 rpm for 30 minutes. Once centrifuged, the
126
supernatant was poured off, leaving the CeO2 NPs fixed at the bottom of the tube. The supernatant
127
was then replaced with deoxygenated DI water and the tubes were then capped and removed from
128
the chamber for an additional 30 min of ultracentrifugation to remove excess salt. After the second
129
ultracentrifugation, the DI water was poured off in the anaerobic chamber, and the solids were
130
allowed to dry in the chamber overnight to prevent any oxidation. The cerium spectra were
131
measured in transmission mode, and iron spectra were measured in fluorescence mode.
132
Experiments were conducted at Beamline 13BM-D at the Advanced Photon Source (APS),
133
Argonne National Laboratory. This station utilized a Si(111) monochromator, giving it a focused
134
beam size of 10 m by 30 m and a resolution of 1 10-4 E/E. The energy flux was 1 109 at 10
135
keV. The energy range for this station was 4.5–70 keV. The iron XANES edge was measured at
136
7.119 keV and the cerium edge was measured at 40.444 keV.
137
Secondary Mineral Phase Precipitation on CeO2-Sputtered Substrates by Physical Vapor
138
Deposition (PVD)
139
To identify the properties of reaction products on the surface of NPs, CeO2 substrates were
140
created by sputtering CeO2 NPs on clean Si wafers using a PVD process (Kurt Lesker PVD 75,
141
Livermore, CA). The DC (diode) mode was applied under 1 mTorr argon pressure, with 100 watts
142
power input for 2000 seconds. The deposition was monitored in situ via a built-in quartz crystal
143
microbalance. Because CeO2 nanoparticles were stable, solid phase analyses were conducted under
7
ACS Paragon Plus Environment
Page 8 of 31
Page 9 of 31
Environmental Science & Technology
144
atmospheric conditions. The thickness of the CeO2–sputtered wafer was quantified by alpha-SE
145
ellipsometry (Lincoln, NE) and atomic force microscopy (AFM, Veeco, Nanoscope V) after the
146
PVD sputtering process. During experiments, the CeO2 substrate was exposed to the same solution
147
chemistry as the aqueous CeO2 NP experiments, containing either NaCl only or NaCl and Fe2+
148
ions (Figure S2). A clean Si wafer was also used as a control to compare precipitation in the system
149
containing NaCl and Fe2+ ions in the absence of CeO2. The precipitates on the CeO2-sputtered
150
substrate and Si wafer were analyzed by AFM using tapping mode. We collected height, amplitude,
151
and phase contrast information simultaneously for 5 µm × 5 µm areas, and analyzed the images
152
using the Nanoscope 7.20 software. Experimental details on the AFM setup have been reported in
153
our former studies.43, 44
154
Grazing Incidence Small-Angle X-ray Scattering (GISAXS) and SAXS Measurements
155
The surface properties and precipitates on the CeO2-sputtered substrates were also
156
characterized in situ by GISAXS. In this experiment, the solution was prepared under the same
157
solution chemistry as the CeO2 NP sedimentation tests, i.e., 3 mM FeCl2 and 1 mM NaCl at pH =
158
4.8. For the control system, a 10 mM NaCl solution was used. The CeO2 substrates were placed
159
flat at the bottom of a specially designed GISAXS cell. The solutions were injected at the top of
160
the cell, and then the cell was capped. The substrates were reacted for 1 hour. During GISAXS
161
measurements, incident X-ray beams at 18 keV were passed through the cell, where they interacted
162
with particles precipitating on the substrate surface (GISAXS). The scattered X-ray beams were
163
collected by a 2D detector. X-ray scattering data was processed by cutting along the Yoneda wing.
164
All data reduction was conducted using the GISAXS shop macro, a software package available at
165
APS beamline 12-ID. More detailed data fitting procedure descriptions are available in our
166
previous publications.43-45 Prior to size fitting, the background image of the CeO2 sputtered wafer 8
ACS Paragon Plus Environment
Environmental Science & Technology
Page 10 of 31
167
in DI water was subtracted from all 2D GISAXS scattering images. Thus, observed changes in
168
particle size were from the precipitation of Fe(III) (hydr)oxides. During the particle size fitting,
169
the shapes of the particles were assumed to be spherical, thus a form factor for polydisperse
170
spherical particles. A Schultz size distribution was also assumed for fitting.
171
RESULTS & DISCUSSION
172
Enhanced Colloidal Stability of CeO2 NPs in Fe2+ Solutions
173
First, we determined how the presence of Fe2+ affects the aggregation and subsequent
174
sedimentation of CeO2 NPs. During the 4 day monitoring period, the concentration of CeO2 NPs
175
in the supernatant was higher in the system with added Fe2+ than in the system without aqueous
176
Fe2+ (Figure 1A), indicating that the presence of 3 mM Fe2+ promoted the colloidal stability of
177
CeO2 NPs. For example, only 46% of the initial CeO2 NP’s concentration remained in the
178
supernatant in the control system while 94% of the initial CeO2 NPs remained stable after 16 h in
179
the presence of 3 mM FeCl2. In addition, the significant difference in NP sedimentation due to the
180
presence of Fe2+ can be observed visually in the solutions after stirring overnight (Figure S3,
181
Supporting Information). It is noted that the effect of double layer compression was similar in the
182
solutions because they had same 10 mM ionic strength and same anion identity (i.e., Cl-), which
183
will dominate double layer compression effects due to the positive charge of CeO2 NPs.
184
Analyses of particle sizes and zeta potentials in the two experimental systems also help to
185
illustrate the observed sedimentation trends. The zeta potential of CeO2 was 28.6 ± 1.7 mV in the
186
presence of Fe2+, while in the absence of Fe2+, the zeta potential was 8.7 ± 0.8 mV (Table S1,
187
Supporting Information). The isoelectric point, pHiep, of CeO2 NPs in 10 mM NaCl was measured
188
to be pH 4.95 (Figure S4A), which is within the range of 3.0–7.6 found in the literature. The wide
9
ACS Paragon Plus Environment
Page 11 of 31
Environmental Science & Technology
189
variation in this range can be due to the varying surface properties of the nanoparticles produced
190
through different methods.46-48 In our experimental system, the unreacted CeO2 NPs should be
191
positively charged under the test conditions.
192
The hydrodynamic size for stable suspensions (Table S1) and XRD spectra (Figure S4B)
193
for the unreacted CeO2 NPs are given in the SI. The hydrodynamic size was 137.7 ± 0.7 nm for
194
stable dispersion of unreacted CeO2 in DI water, while the particle sizes measured by TEM ranged
195
from 5–30 nm (Figure S7), consistent with the manufacture's nominal size. After 16 hours reaction,
196
the hydrodynamic diameter (DH) of CeO2 NPs in the 3 mM Fe2+ solution was 154 ±1 nm, while
197
the DH of CeO2 NPs in the 10 mM NaCl system was 3829 ± 358 nm (Table S1). The smaller size
198
of CeO2 aggregates when Fe2+ is present is in accordance with the higher suspended concentrations
199
of CeO2 NPs compared to the system without Fe2+, because the smaller aggregates will be more
200
stable in solution. In contrast, larger aggregates tend to settle out because gravitational forces are
201
predominant over Brownian motion.
202
Identification of the Reaction Products and CeO2 Dissolution
203
We hypothesize that the Fe2+-promoted stability of CeO2 NPs results from redox
204
reactions28 between Fe2+ and CeO2 NPs and the formation of Fe(III)products at the CeO2 NP
205
surfaces (eqn. (1)). Throughout the current manuscript, we expressed the oxidation state of solid
206
and sorbed species as Roman numerals and the oxidation state of aqueous species as Arabic
207
numerals. Arabic numerals were also used when both solid and aqueous species are possible. ≡CeIVO2+ Fe2+ (aq)↔ Ce3+ (aq)(or ≡CeIII2O3) + ≡Fe3+
208
10
ACS Paragon Plus Environment
(1)
Environmental Science & Technology
Page 12 of 31
209
X-ray absorption spectroscopy (XAS) was carried out to determine the Ce K-edge and Fe
210
K-edge spectra for the solid CeO2 NPs before and after reaction in the FeCl2 and control systems
211
(Figure 2). X-ray absorption near edge structure (XANES) results for Ce indicated no significant
212
change in the oxidation state after reaction. In other words, there is no CeIII contribution in both
213
initial CeO2 NPs and reacted CeO2 NPs (Figure 2A). This could be due to negligible amounts of
214
CeIII on the surface compared to bulk CeIV. Considering that the presence of surface CeIII2O3 was
215
not detected, and the aqueous Ce concentration was significantly enhanced in the presence of Fe2+
216
(as will discussed shortly after), we suggest that aqueous Ce3+ is the more predominant reaction
217
product than solid CeIII2O3. For iron, XAS results (Figure 3B) indicate that all iron on the reacted
218
CeO2 NP surface was oxidized to FeIII. Furthermore, the iron spectrum for the reacted CeO2 NPs
219
is consistent with that of the ferrihydrite standard (Figure 2B).
220
Aqueous Ce levels were monitored for the first 6 hours of reaction in the 250 mg/L CeO2
221
system (Figure 1B) because the most vigorous interfacial reaction occurs during the early stage.
222
Based on the concentration of dissolved Ce from CeO2 NPs in the supernatant, we also found that
223
the dissolved Ce concentration was 15 times higher in the presence of aqueous Fe2+ than in the
224
control system in the absence of Fe2+ over the course of the reaction period. Because the solubility
225
of Ce3+ is 25 orders of magnitude higher than CeIVO2, and it was reported that under similar
226
aqueous conditions to our experimental condition, more than 99% dissolved Ce was Ce3+,36, 49 we
227
assumed that the aqueous Ce in our system was primarily Ce3+. Thus, the increased concentration
228
of dissolved Ce could result from the reduction of CeIVO2 by Fe2+. Then, the redox reaction is also
229
likely to accelerate the precipitation of Fe(III)(hydr)oxides on the surface of CeO2 NPs while the
230
dissolved Ce3+ ions are released from the CeIVO2 NP surfaces.
11
ACS Paragon Plus Environment
Page 13 of 31
Environmental Science & Technology
231 232
Reaction Pathways: Adsorption, Redox Reaction, and Surface Precipitation of Fe(III)(Hydr)oxides on the Surfaces of CeO2 NPs and Dissolution of CeO2 NPs
233
A series of reactions could contribute to the enhanced colloidal stability of CeO2 in the
234
presence of Fe2+ ions. MgCl2 extraction revealed that the adsorbed Fe2+ concentration on the
235
surface of CeO2 NPs was 3.75 mg/L, which is equivalent to 0.05% of the total iron (Figure S5,
236
Supporting Information). The Fe2+ can be adsorbed on the CeO2 NPs by ion exchange (eq. (2)),
237
and such a process is expected to shift the zeta potential of CeO2 NPs to be more positive and
238
release H+ into the solution:50
239
Fe2+ Sorption: ≡CeOH + Fe2+ ↔ ≡CeOFe+ + H+
240
On the other hand, the adsorbed FeII can react further with CeIV on the surface of CeO2 NPs
241
rather than existing as ferrous state, due to the large enough difference in the standard redox
242
potential for Ce4+/Ce3+ (1.44 V) and Fe3+/Fe2+ (0.77 V).28 As a control experiment, we tested the
243
reaction of aqueous Ce4+ using Ce(SO4)2 and Fe2+ ions in solution and observed a significant
244
decrease of Fe2+ concentration by the Ferrozine method after 1 d (Table S2, Supporting
245
Information). Furthermore, the XANES experiments of the CeO2 NP‒Fe2+ system also revealed
246
oxidized Fe(III) on the surface of CeO2 NPs under anaerobic conditions, providing direct evidence
247
of surface redox reactions (Figure 2).
(2)
248
In the experimental systems, the pH of the reaction solution (CeO2 NPs + FeCl2) was measured
249
to be lower than either CeO2 NPs or FeCl2 solutions alone, and decreased during the first few hours
250
of reaction (Figure S6, Supporting Information). The decrease of solution pH can be attributed to
251
both the adsorption of Fe2+ through ion exchange (eqn. (2)) and the hydrolysis of Fe3+, the redox
252
reaction product, which releases more H+ into solution than is released by Fe2+ sorption. The Fe2+12
ACS Paragon Plus Environment
Environmental Science & Technology
Page 14 of 31
253
only system also has slightly decreased pH due to minor Fe2+ hydrolysis (eqn. 3a), but the rate of
254
pH decrease is much less than that of the CeO2/Fe2+ system, which undergoes the hydrolysis of
255
ferric ions (eqn. 3b).
256
Hydrolysis: Fe2+ + H2O Fe(OH)+(aq) + H+ or
(3a)
257
Fe3+ + 3 H2O Fe(OH)3 (aq) + 3 H+
(3b)
258
Once hydrolysis of ferric iron forms the monomer (eqn. 3b), then dimers and polymers can
259
form through continuous olation (hydroxo–bridging) and oxolation (oxo–bridging) reactions.44
260
Once the polymeric cluster size is larger than the critical nucleus size, stable nuclei form, and
261
Fe(III) (hydr)oxide precipitation occurs.45
262
To observe the particle formation of Fe(III) (hydr)oxides and their morphological changes
263
in Fe2+–CeO2 NP systems, AFM and GISAXS were used on a CeO2-sputtered Si wafer reacted in
264
the same solution conditions. AFM images show significant precipitation of FeIII solid phase on
265
the CeO2-sputtered Si wafer substrate after 6 h (Figure 3). The particle size of precipitated Fe(III)
266
(hydr)oxides on the CeO2 substrates increased from 4.8 ± 0.7 nm (based on 20 particle analysis)
267
to 10.6 ± 2.8 nm after 6 h, and 25.9 ± 5.4 nm after 1 day (based on 50 particle analysis each). In
268
contrast, precipitation of Fe(III) (hydr)oxide on a clean Si substrate was negligible within 1 day.
269
GISAXS results also showed Fe(III) (hydr)oxide nucleation on the surface of CeO2-sputtered
270
substrates (Figure 4). Figure 4A revealed that particles with an Rg of 1.72 nm were observed on
271
the surface of CeO2 substrates in the presence of 3 mM Fe2+ in 1 h. In contrast, no particles formed
272
in the absence of Fe2+due to no scattering increase for the CeO2 only system. SAXS images in
273
Figure 4B indicates that no particles formed in solution in the absence of CeO2 (e.g., Fe2+ only
274
system) as well. TEM analysis of CeO2 NPs revealed that nucleated Fe(III) (hydr)oxide covered 13
ACS Paragon Plus Environment
Page 15 of 31
Environmental Science & Technology
275
the surfaces of CeO2 NPs and appeared to increase the CeO2 NP particle–particle distance (Figure
276
S7). Furthermore, electron diffraction analysis of the Fe(III) (hydr)oxide products from the CeO2‒
277
Fe2+ system revealed the formation of 6-line ferrihydrite, which is consistent with XAS results. It
278
is important to note that artifacts and phase transformation might be introduced during TEM
279
sample preparation due to the drying process.43 Therefore, the morphology of the in situ Fe(III)
280
(hydr)oxide particles might be different than what is observed using TEM. However, the multiple
281
complementary techniques used in this study provide convincing and consistent evidence that the
282
precipitation of iron on the CeO2 surfaces greatly affected the surface properties and colloidal
283
stability of CeO2 NPs. Based on the above discussion, eqn. (1) can be rewritten as follows:
284
≡CeIVO2+ Fe2+ (aq)↔ ≡CeIVO2 + Ce3+ (aq) + ≡Fe(III)(hydr)oxide (e.g., ferrihydrite).
285
(4)
Enhanced Stability by Iron (Hydr)oxide Surface Precipitation
286
First, we analyzed the change in stability of CeO2 NPs using classical Derjaguin–Landau–
287
Verwey–Overbeek (DLVO) interactions, comprised of electrostatic repulsion forces and van der
288
Waals attractions. The electrostatic interaction increases with the absolute value of surface
289
potential (eqns. (S1-3) in Section S4 of the Supporting Information). Nanoparticles with higher
290
zeta potential have higher electrostatic repulsions each other, and therefore, are more stable in
291
suspension. Hence, the increased zeta potential due to the adsorption of Fe2+ and precipitation of
292
Fe(III)(hydr)oxides on the CeO2 NP surfaces can lead to more significant electrostatic repulsions.
293
In addition, the values of the Hamaker constant for iron oxide phases (magnetite, maghemite, and
294
hematite) in the literature range from 1.3 to 4.5 × 10-20 J.51 Although the Hamaker constant for
295
ferrihydrite is not available, the highest reported value for an iron oxide related phase (4.5 × 10-20
296
J) is still smaller than that of CeO2 (5.6–6.0×10-20 J).2, 52 A smaller Hamaker constant signifies that 14
ACS Paragon Plus Environment
Environmental Science & Technology
Page 16 of 31
297
there are less attractive forces between Fe(III) (hydr)oxide–Fe(III) (hydr)oxide than that of the
298
CeO2 NP–CeO2 NP. The precipitated Fe(III)(hydr)oxides on the surface of CeO2 will most likely
299
increase the electrostatic repulsion and decrease the van der Waals attractions.
300
Non-DLVO interactions also possibly play a significant role in the interactions of CeO2
301
NPs after being reacted with Fe2+. We hypothesize that the enhanced Ce stability in the presence
302
of Fe2+ may also result from Fe(III) (hydr)oxide precipitation changing the CeO2 NP surface
303
hydrophilicity. Due to the distinctive electronic structure of rare earth atoms, the CeO2 surface has
304
intrinsically less hydrophilic properties.53 In a cerium atom, the outer full octet of electrons in the
305
5s2p6 shell shields the unfilled 4f orbitals. As a result, CeO2 tends to not exchange electrons and
306
form hydrogen bonds with surrounding water molecules, making it less hydrophilic than other
307
metal oxides.53 The precipitated Fe(III) (hydr)oxides could, therefore, make the NP surface more
308
hydrophilic than the CeO2 surface.53 As a result, the coated NPs would be more stable in the
309
aqueous phase. To test the extent of hydrophilicity changes of CeO2 NPs by Fe(III) (hydr)oxide
310
surface coatings, we conducted surface angle measurements. It appeared that the surface of CeO2-
311
sputtered wafers became more hydrophilic after the precipitation of Fe(III) (hydr)oxides within a
312
3 day reaction period. (Figure S8, Supporting Information)
313
ENVIRONMENTAL IMPLICATIONS
314
In this study, we found that redox reactions between CeO2 NPs and Fe2+ lead to the
315
formation of 6-line ferrihydrite on the CeO2 surface, which enhanced the colloidal stability by
316
increasing the zeta potential and hydrophilicity of CeO2 NPs. The findings of this work suggest
317
longer retention periods and farther transport distances of CeO2 NPs in aquatic environments
318
containing Fe2+ ions. This study calls for immediate attention to the significant effects of redox
15
ACS Paragon Plus Environment
Page 17 of 31
Environmental Science & Technology
319
reactions and surface precipitation on the fate, transport, and bioavailability of engineered NPs in
320
aquatic environments. These nanoparticles could act as heterogeneous nucleation sites and
321
adsorption sites when released into the environment, incorporating toxic elements and molecules
322
into a “hybrid” engineered/natural nanoparticle composite.54 Redox reactive elements, e.g., Fe2+,
323
Mn2+, and contaminants, such as As, Cr, and U, can be adsorbed on and react with redox reactive
324
engineered NPs such as CeO2 NPs.55, 56 In addition, the positively charged CeO2 can adsorb and
325
aggregate with negatively charged natural colloids or polymers. In the presence of Fe2+, the new
326
hybrid Fe(III) hydroxide coated CeO2 nanoparticles become more positively charged and may take
327
longer to be destabilized by the natural colloids than in the absence of Fe2+ ions. In addition, the
328
interactions of adsorbed natural polymers with Fe2+ (e.g., complexation with natural polymers)
329
can make the redox reaction of CeO2 more complicated.57 The findings of the current study provide
330
an important starting point for investigating the long term influences of Fe2+ on CeO2 fate, transport,
331
and toxicity. These reactions have a significant impact on the transport and transformation of both
332
NPs and contaminants.
333
Consequential changes in the physicochemical properties of CeO2 NPs can affect the
334
toxicity by altering factors such as the surface charge, particle size, production of reactive oxygen
335
species (ROS), dissolution of reactive ions, hydrophilicity, and surface functionality or coatings.
336
For instance, positively charged CeO2 NPs were found to penetrate C. elegans cell membranes
337
more easily than the neutral and negatively charged CeO2 NPs, making them more toxic.8 Because,
338
in general, dissolved Ce3+ is far more toxic than CeO2 NPs,27 the presence of Fe2+can also enhance
339
their toxicity due to increased dissolution. Conversely, the surface of CeO2 NPs may become
340
passivated with time by Fe(III) (hydr)oxide coatings. This would retard further dissolution and
341
could help mitigate CeO2 NP toxicity. CeO2 NP redox processes with Fe2+ and other redox active 16
ACS Paragon Plus Environment
Environmental Science & Technology
Page 18 of 31
342
species could also interact with natural organic matter in wastewater treatment plants, potentially
343
affecting their toxicity to microorganisms in wastewater treatment plants.14, 39 In addition to CeO2
344
NPs, many engineered NPs may be subject to surface reactions and precipitation due to ubiquitous
345
redox reactive elements in the environment. These interactions need to be considered
346
comprehensively while evaluating the fate of engineered CeO2 NPs and assessing their risk in
347
aquatic environments.
348
Supporting Information Available
349
Supporting information includes experimental descriptions, UV-Vis spectra, DLS data, zeta
350
potential measurements, TEM images, an ED pattern, and pH monitoring data. This material is
351
available free of charge via the Internet at http://pubs.acs.org.
352
Acknowledgments
353
This work is supported by the National Science Foundation’s Environmental Chemical Science
354
Program (CHE-1214090) and Washington University’s Faculty Startup. JRR was supported by the
355
Environmental Protection Agency STAR Fellowship and CWN was supported by the Mr. and Mrs.
356
Spencer T. Olin Fellowship. We would like to thank Dr. Seonke Seifert of the Advanced Photon
357
Source Sector 12-ID-C at Argonne National Laboratory, supported by the U.S. Department of
358
Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-
359
06CH11357 and the Institute of Materials Science and Engineering and Nano Research Facility at
360
WUStL for experimental support. XAS work was performed at GeoSoilEnviroCARS (Sector 13),
361
Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported
362
by the National Science Foundation-Earth Sciences (EAR-1128799) and Department of Energy-
363
GeoSciences (DE-FG02-94ER14466). We thank Dr. Matt Newville and Dr. Tony Lanzirotti for 17
ACS Paragon Plus Environment
Page 19 of 31
Environmental Science & Technology
364
their help with XAS experiments. We appreciate the assistance and constructive suggestions from
365
our colleagues in the Environmental NanoChemistry Lab (ENCL) at WUStL.
18
ACS Paragon Plus Environment
Environmental Science & Technology
366
TOC Art
367
19
ACS Paragon Plus Environment
Page 20 of 31
Page 21 of 31
Environmental Science & Technology
368
LIST OF FIGURES
369
Figure 1.(A) Sedimentation kinetics of CeO2 NPs at 10 mM IS and pH 4.8 in the presence and
370
absence of aqueous Fe2+ ions. The percentages were obtained from suspended nanoparticle
371
concentration normalized by the initial suspended concentration measured by UV-Vis. The error
372
bars represent the standard deviation of CeO2 concentration from triplicate experiments. (B)
373
Dissolved Ce concentrations from CeO2 NPs in the presence and absence of Fe2+ at pH 4.8.
374
Figure 2. XAS spectra for CeO2 NPs reacted in the presence of 3 mM FeCl2 (A: Ce K-edge and
375
B: Fe K-edge). The increase in energy of the K-edge position compared to the FeCl2 standard
376
indicates oxidation of Fe(II) to Fe(III). XAS results showing (A) cerium K-edge and (B) iron K-
377
edge spectra for reacted samples and standards. Ce K-edge results show no detectable Ce(III) on
378
the surface. Fe K-edge results show the most similarity between ferrihydrite and the Fe(III) phase
379
formed on the CeO2 NPs after the 6 h reaction.
380
Figure 3. Representative AFM images for the precipitation of iron oxide particles on (A) CeO2-
381
sputtered Si substrates and (B) pure Si control substrates. Height scale (HS) is 10 nm unless
382
otherwise noted.
383
Figure 4.1D reduced grazing incidence small angle X-ray scattering (GISAXS) data of
384
Fe3+precipitation on the surfaces of CeO2 sputtered wafers ( , Figure 4A) and data from the CeO2
385
NP control system ( , Figure 4A). SAXS raw data of the Fe2+ only system is depicted in Figure
386
4B. Figure 4A revealed that particles were observed in CeO2 + Fe2+ system with Rg = 1.72 nm (at
387
1 hr), and no particles formed in the absence of Fe2+ because there was no scattering increase for
388
the CeO2 only system. Figure 4B (Fe2+ only system) indicates that no particles formed without
389
CeO2 as well. In Figure 4B, no background subtraction was performed due to no increase of 20
ACS Paragon Plus Environment
Environmental Science & Technology
390
intensity throughout the reaction and some signal around 0.035 Å-1 results from the beamline setup
391
rather than actual particle contribution.
21
ACS Paragon Plus Environment
Page 22 of 31
Page 23 of 31
Environmental Science & Technology
CeO2 conc. (%)
100
3 mM FeCl2 + 1 mM NaCl 10 mM NaCl
80 60 40 20 0
A 0
20
40
60
Time (h)
393
80
100
Dissolved Ce conc. (mg/L)
392
2.0 1.5 1.0 0.5 0.0
B 0
1
2
3
4
5
Time (h)
394 395
Figure 1
22
ACS Paragon Plus Environment
6
Environmental Science & Technology
Page 24 of 31
B
Normalized absorbance (a.u.)
A CeO2 standard
CeO2 control 2+ CeO22++Fe Fe(II) CeO
5700
396 397
5750 5800 Energy (eV)
Hematite
Normalized absorbance (a.u.)
Ce(III) Nitrate standard
5850
Goethite Lepidocrocite
Ferrihydrite CeO Fe(II) CeO Fe2+ 2+ 2+
7100
398
Figure 2
23
Fe(II) chloride
ACS Paragon Plus Environment
7150 7200 Energy (eV)
7250
Page 25 of 31
Environmental Science & Technology
399
400
Figure 3
401
24
ACS Paragon Plus Environment
Environmental Science & Technology
Page 26 of 31
4
1
A
Intensity (counts)
Intensity (counts)
10
3
10
Rg = 1.72 nm
2
10
1
10
0
CeO2 + Fe(II) CeO Fe2+ CeO22 CeO
10 10
-1 6
2
0.01
B 4 2
0.1 4 2
0.01
3 4 56
2
3 4 5
2
0.1
0.01
-1
q_xy (Å )
402 403
Figure 4
25
ACS Paragon Plus Environment
3
4 5 6
0.1 -1 q_xy (Å )
2
3
4
Page 27 of 31
Environmental Science & Technology
404
REFERENCES
405
1.
OECD Nanosafety at the OECD: The First Five Years 2006-2010; Organization for Economic Cooperation and Development: Paris, France, 2011.
2.
Liu, X.; Chen, G.; Su, C., Influence of Collector Surface Composition and Water Chemistry on the Deposition of Cerium Dioxide Nanoparticles: QCM-D and Column Experiment Approaches. Environ. Sci. Technol. 2012, 46, (12), 6681-6688.
3.
Migani, A.; Neyman, K. M.; Illas, F.; Bromley, S. T., Exploring Ce3+/Ce4+ cation ordering in reduced ceria nanoparticles using interionic-potential and density-functional calculations. J. Chem. Phys. 2009, 131, (6), 064701-064707.
4.
Liu, X.; Chen, G.; Su, C., Influence of Collector Surface Composition and Water Chemistry on the Deposition of Cerium Dioxide Nanoparticles: QCM-D and Column Experiment Approaches. Environ. Sci. Technol. 2012, 46, (12), 6681-6688.
5.
Evaluation of Human Health Risk from Cerium Added to Diesel Fuel; Health Effects Institute: 2001.
6.
Gantt, B.; Hoque, S.; Willis, R. D.; Fahey, K. M.; Delgado-Saborit, J. M.; Harrison, R. M.; Erdakos, G. B.; Bhave, P. V.; Zhang, K. M.; Kovalcik, K.; Pye, H. O. T., Near-Road Modeling and Measurement of Cerium-Containing Particles Generated by Nanoparticle Diesel Fuel Additive Use. Environ. Sci. Technol. 2014, 48, (18), 10607-10613.
7.
Nanomaterials EPA is Assessing. http://www.epa.gov/nanoscience/quickfinder/nanomaterials.htm
8.
Collin, B.; Oostveen, E.; Tsyusko, O. V.; Unrine, J. M., Influence of Natural Organic Matter and Surface Charge on the Toxicity and Bioaccumulation of Functionalized Ceria Nanoparticles in Caenorhabditis elegans. Environ. Sci. Technol. 2013, 48, (2), 1280-1289.
9.
Thill, A.; Zeyons, O.; Spalla, O.; Chauvat, F.; Rose, J.; Auffan, M.; Flank, A. M., Cytotoxicity of CeO2 nanoparticles for Escherichia coli. Physico-chemical insight of the cytotoxicity mechanism. Environ. Sci. Technol. 2006, 40, (19), 6151-6156.
10.
Zhang, W.; Crittenden, J.; Li, K.; Chen, Y., Attachment Efficiency of Nanoparticle Aggregation in Aqueous Dispersions: Modeling and Experimental Validation. Environ. Sci. Technol. 2012, 46, (13), 7054-7062.
11.
Keller, A. A.; Wang, H.; Zhou, D.; Lenihan, H. S.; Cherr, G.; Cardinale, B. J.; Miller, R.; Ji, Z., Stability and Aggregation of Metal Oxide Nanoparticles in Natural Aqueous Matrices. Environ. Sci. Technol. 2010, 44, (6), 1962-1967.
406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438
12. Petosa, A. R.; Ohl, C.; Rajput, F.; Tufenkji, N., Mobility of nanosized cerium dioxide and polymeric capsules in quartz and loamy sands saturated with model and natural groundwaters. Water Res. 2013, 47, (15), 5889-5900. 26
ACS Paragon Plus Environment
Environmental Science & Technology
Page 28 of 31
439
13.
440 441
Buettner, K. M.; Rinciog, C. I.; Mylon, S. E., Aggregation kinetics of cerium oxide nanoparticles in monovalent and divalent electrolytes. Colloids Surf., A 2010, 366, (1-3), 7479.
445
14. Quik, J. T. K.; Lynch, I.; Hoecke, K. V.; Miermans, C. J. H.; Schamphelaere, K. A. C. D.; Janssen, C. R.; Dawson, K. A.; Stuart, M. A. C.; Meent, D. V. D., Effect of natural organic matter on cerium dioxide nanoparticles settling in model fresh water. Chemosphere 2010, 81, (6), 711-715.
446
15.
Van Hoecke, K.; De Schamphelaere, K. A. C.; Van der Meeren, P.; Smagghe, G.; Janssen, C. R., Aggregation and ecotoxicity of CeO2 nanoparticles in synthetic and natural waters with variable pH, organic matter concentration and ionic strength. Environ. Pollut. 2011, 159, (4), 970-976.
16.
Barton, L. E.; Auffan, M.; Bertrand, M.; Barakat, M.; Santaella, C.; Masion, A.; Borschneck, D.; Olivi, L.; Roche, N.; Wiesner, M. R.; Bottero, J.-Y., Transformation of Pristine and Citrate-Functionalized CeO2 Nanoparticles in a Laboratory-Scale Activated Sludge Reactor. Environ. Sci. Technol. 2014, 48, (13), 7289-7296.
17.
Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S. I.; Seal, S., Graphene based materials: Past, present and future. Prog. Mater. Sci. 2011, 56, (8), 1178-1271.
18.
Yu, P.; Hayes, S. A.; O'Keefe, T. J.; O'Keefe, M. J.; Stoffer, J. O., The Phase Stability of Cerium Species in Aqueous Systems: II. The Systems. Equilibrium Considerations and Pourbaix Diagram Calculations. J. Electrochem. Soc. 2006, 153, (1), C74-C79.
442 443 444
447 448 449 450 451 452 453 454 455 456 457 458 460
19. Stumm, W.; Lee, G. F., Oxygenation of Ferrous Iron. Ind. Eng. Chem. 1961, 53, (2), 143146.
461
20.
Nielsen, A. H.; Hvitved-Jacobson, T.; Vollertsen, J., Effects of pH and iron concentrations on sulfide precipitation in wastewater collection systems. Water Environ. Res. 2008, 80, (4), 380-384.
21.
Wang, Y.; Tng, K. H.; Wu, H.; Leslie, G.; Waite, T. D., Removal of phosphorus from wastewaters using ferrous salts - A pilot scale membrane bioreactor study. Water Res. 2014, 57, 140-150.
459
462 463 464 465 466
469
22. Ivanov, V.; Kuang, S.; Stabnikov, V.; Guo, C., The removal of phosphorus from reject water in a municipal wastewater treatment plant using iron ore. J. Chem. Technol. Biot. 2009, 84, (1), 78-82.
470
23.
Park, D.; Kim, Y. M.; Lee, D. S.; Park, J. M., Chemical treatment for treating cyanidescontaining effluent from biological cokes wastewater treatment process. Chem. Eng. J. 2008, 143, (1–3), 141-146.
24.
Xu, L.; Wang, J., Magnetic Nanoscaled Fe3O4/CeO2 Composite as an Efficient Fenton-Like Heterogeneous Catalyst for Degradation of 4-Chlorophenol. Environ. Sci. Technol. 2012, 46, (18), 10145-10153.
467 468
471 472 473 474 475
27
ACS Paragon Plus Environment
Page 29 of 31
Environmental Science & Technology
476
25.
Schreiber, H. D.; Lauer Jr, H. V.; Thanyasiri, T., Oxidation-reduction equilibria of iron and cerium in silicate glasses: Individual redox potentials and mutual interactions. J. Non-Cryst. Solids 1980, 38-39, (PART 2), 785-790.
26.
Limbach, L. K.; Bereiter, R.; Muller, E.; Krebs, R.; Galli, R.; Stark, W. J., Removal of Oxide Nanoparticles in a Model Wastewater Treatment Plant: Influence of Agglomeration and Surfactants on Clearing Efficiency. Environ. Sci. Technol. 2008, 42, (15), 5828-5833.
27.
Dahle, J.; Arai, Y., Effects of Ce(III) and CeO2 Nanoparticles on Soil-Denitrification Kinetics. Arch. Environ. Contam. Toxicol. 2014, 1-9.
477 478 479 480 481 482 483 485
28. Bard, A. J.; Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications John Wiley & Sons, INC.: New York, 2001.
486
29.
Wilson, N.; Webster-Brown, J., The fate of antimony in a major lowland river system, the Waikato River, New Zealand. Appl. Geochem. 2009, 24, (12), 2283-2292.
30.
Bigham, J. M.; Schwertmann, U.; Carlson, L.; Murad, E., A poorly crystallized oxyhydroxysulfate of iron formed by bacterial oxidation of Fe(II) in acid mine waters. Geochim. Cosmochim. Ac. 1990, 54, (10), 2743-2758.
484
487 488 489 490 492
31. Stumm, W.; Morgan, J., Aquatic Chemistry: Chemical Equilibria and Rate in Natural Waters. 3rd ed.; Wiley: New York, 1996.
493
32.
Liu, X.; Gong, W.; Liu, L., Treatment of sulfate-rich and low pH wastewater by sulfate reducing bacteria with iron shavings in a laboratory. Water Sci. Technol. 2014, 69, (3), 595600.
33.
Rousch, J. M.; Simmons, T. W.; Kerans, B. L.; Smith, B. P., Relative acute effects of low pH and high iron on the hatching and survival of the water mite (Arrenurus manubriator) and the aquatic insect (Chironomus riparius). Environ. Toxicol. Chem. 1997, 16, (10), 2144-2150.
34.
Gerhardt, A., Effects of subacute doses of iron (Fe) on Leptophlebia marginata (Insecta: Ephemeroptera). Freshwater Biol. 1992, 27, (1), 79-84.
35.
Díez, S.; Noonan, G. O.; MacFarlane, J. K.; Gschwend, P. M., Ferrous iron oxidation rates in the pycnocline of a permanently stratified lake. Chemosphere 2007, 66, (8), 1561-1570.
491
494 495 496 497 498 499 500 501 502 504
36. Dahle, J. T.; Livi, K.; Arai, Y., Effects of pH and phosphate on CeO2 nanoparticle dissolution. Chemosphere 2015, 119, 1365-1371.
505
37.
Hoecke, K. V.; Quik, J. T. K.; Mankiewicz-Boczek, J.; Schamphelaere, K. A. C. D.; Elsaesser, A.; Meeren, P. V. d.; Barnes, C.; McKerr, G.; Howard, C. V.; Meent, D. V. D.; Rydzynski, K.; Dawson, K. A.; Salvati, A.; Lesniak, A.; Lynch, I.; Silversmit, G.; Samber, B. D.; Vincze, L.; Janssen, C. R., Fate and Effects of CeO2 Nanoparticles in Aquatic Ecotoxicity Tests. Environ. Sci. Technol. 2009, 43, (12), 4537-4546.
38.
Rico, C. M.; Morales, M. I.; McCreary, R.; Castillo-Michel, H.; Barrios, A. C.; Hong, J.; Tafoya, A.; Lee, W.-Y.; Varela-Ramirez, A.; Peralta-Videa, J. R.; Gardea-Torresdey, J. L.,
503
506 507 508 509 510 511
28
ACS Paragon Plus Environment
Environmental Science & Technology
Page 30 of 31
Cerium Oxide Nanoparticles Modify the Antioxidative Stress Enzyme Activities and Macromolecule Composition in Rice Seedlings. Environ. Sci. Technol. 2013, 47, (24), 14110-14118.
512 513 514 515
39.
Rogers, N. J.; Franklin, N. M.; Apte, S. C.; Batley, G. E.; Angel, B. M.; Lead, J. R.; Baalousha, M., Physico-chemical behaviour and algal toxicity of nanoparticulate CeO2 in freshwater. Environ. Chem. 2010, 7, (1), 50-60.
40.
Rico, C. M.; Hong, J.; Morales, M. I.; Zhao, L.; Barrios, A. C.; Zhang, J.-Y.; Peralta-Videa, J. R.; Gardea-Torresdey, J. L., Effect of Cerium Oxide Nanoparticles on Rice: A Study Involving the Antioxidant Defense System and In Vivo Fluorescence Imaging. Environ. Sci. Technol. 2013, 47, (11), 5635-5642.
41.
Baalousha, M.; Ju-Nam, Y.; Cole, P. A.; Gaiser, B.; Fernandes, T. F.; Hriljac, J. A.; Jepson, M. A.; Stone, V.; Tyler, C. R.; Lead, J. R., Characterization of cerium oxide nanoparticles— Part 1: Size measurements. Environ. Toxicol. Chem. 2012, 31, (5), 983-993.
42.
Baalousha, M.; Le Coustumer, P.; Jones, I.; Lead, J. R., Characterisation of structural and surface speciation of representative commercially available cerium oxide nanoparticles. Environ. Chem. 2010, 7, (4), 377-385.
43.
Ray, J. R.; Lee, B.; Baltrusaitis, J.; Jun, Y.-S., Formation of Iron(III) (Hydr)oxides on Polyaspartate- and Alginate-Coated Substrates: Effects of Coating Hydrophilicity and Functional Group. Environ. Sci. Technol. 2012, 46, (24), 13167-13175.
44.
Hu, Y.; Lee, B.; Bell, C.; Jun, Y.-S., Environmentally Abundant Anions Influence the Nucleation, Growth, Ostwald Ripening, and Aggregation of Hydrous Fe(III) Oxides. Langmuir 2012, 28, (20), 7737-7746.
45.
Jun, Y.-S.; Lee, B.; Waychunas, G. A., In Situ Observations of Nanoparticle Early Development Kinetics at Mineral−Water Interfaces. Environ. Sci. Technol. 2010, 44, (21), 8182-8189.
46.
De Faria, L. A.; Trasatti, S., The Point of Zero Charge of CeO2. J. Colloid Interf. Sci. 1994, 167, (2), 352-357.
47.
Kosmulski, M., The pH-Dependent Surface Charging and the Points of Zero Charge. J. Colloid Interf. Sci. 2002, 253, (1), 77-87.
48.
Kosmulski, M., The pH-dependent surface charging and points of zero charge: V. Update. J. Colloid Interf. Sci. 2011, 353, (1), 1-15.
49.
Cornelis, G.; Ryan, B.; McLaughlin, M. J.; Kirby, J. K.; Beak, D.; Chittleborough, D., Solubility and Batch Retention of CeO2 Nanoparticles in Soils. Environ. Sci. Technol. 2011, 45, (7), 2777-2782.
516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547
50. Hunter, R. J., Foundations of Colloid Science. second ed.; Clarendon Press: Oxford, U.K., 2001; Vol. 210, p 125-125. 29
ACS Paragon Plus Environment
Page 31 of 31
Environmental Science & Technology
549
51. Faure, B.; Salazar-Alvarez, G.; Bergström, L., Hamaker constants of iron oxide nanoparticles. Langmuir 2011, 27, (14), 8659-8664.
550
52.
Li, K.; Zhang, W.; Huang, Y.; Chen, Y., Aggregation kinetics of CeO2 nanoparticles in KCl and CaCl2 solutions: measurements and modeling. J. Nanopart. Res. 2011, 1-9.
53.
Azimi, G.; Dhiman, R.; Kwon, H. M.; Paxson, A. T.; Varanasi, K. K., Hydrophobicity of rare-earth oxide ceramics. Nature Materials 2013, 12, (4), 315-320.
548
551 552 553 555
54. Grassian, V. H.; Hamers, R. J. Workshop report on "Nanomaterials and the Environment: The Chemistry and Materials Perspective"; Arlington, 2011.
556
55.
Donald, L. M.; Katherine, W.-D., Redox Chemistry and Natural Organic Matter (NOM): Geochemists? Dream, Analytical Chemists? Nightmare. In Aquatic Redox Chemistry, American Chemical Society: 2011; Vol. 1071, pp 85-111.
56.
Neil, C. W.; Lee, B.; Jun, Y.-S., Different Arsenate and Phosphate Incorporation Effects on the Nucleation and Growth of Iron(III) (Hydr)oxides on Quartz. Environ. Sci. Technol. 2014, 48, (20), 11883-11891.
554
557 558 559 560 561 562 563
57. Rose, A. L., Effect of Dissolved Natural Organic Matter on the Kinetics of Ferrous Iron Oxygenation in Seawater. Environ. Sci. Technol. 2003, 37, (21), 4877-4886.
564 565
30
ACS Paragon Plus Environment
