Environ. Sci. Technol. XXXX, xxx, 000–000
Stability and Aggregation of Metal
Oxide Nanoparticles in Natural
Aqueous Matrices
A R T U R O A . K E L L E R , † H O N G T A O W A N G , †,‡
DONGXU ZHOU,† HUNTER S. LENIHAN,†
GARY CHERR,§ BRADLEY J. CARDINALE,†
ROBERT MILLER,† AND ZHAOXIA JI|
University of California, Santa Barbara, Tongji University,
Shanghai, China, University of California, Davis, and
University of California, Los Angeles
Received October 1, 2009. Revised manuscript received
January 22, 2010. Accepted January 27, 2010.
There is a pressing need for information on the mobility of
nanoparticles in the complex aqueous matrices found in realistic
environmental conditions. We dispersed three different metal
oxide nanoparticles (TiO2, ZnO and CeO2) in samples taken from
eight different aqueous media associated with seawater,
lagoon,river,andgroundwater,andmeasuredtheirelectrophoretic
mobility, state of aggregation, and rate of sedimentation. The
electrophoretic mobility of the particles in a given aqueous media
was dominated by the presence of natural organic matter
(NOM) and ionic strength, and independent of pH. NOM adsorbed
onto these nanoparticles significantly reduces their aggregation,
stabilizing them under many conditions. The transition from
reaction to diffusion limited aggregation occurs at an
electrophoretic mobility from around -2 to -0.8 µm s-1 V-1
cm. These results are key for designing and interpreting
nanoparticle ecotoxicity studies in various environmental
conditions.
1. Introduction
The increasing use of nanomaterials in consumer products
that are exposed to environmental media has led to a need
to understand their fate and transport. In particular, metal
oxide (MeO) nanoparticles, such as TiO2, ZnO and CeO2, are
increasingly incorporated into a wide range of products (e.g.,
sunscreens, paints, coatings, catalysts). A simplified conceptual model of a typical nanoparticle life cycle considers
that MeOs move through several different aqueous pathways.
There is emerging evidence of their toxicity (1), which makes
it imperative to understand the likelihood of exposure.
How nanoparticles will travel in different water media,
such as groundwater, rivers, lakes, and seawater, is not well
understood. To a large extent this will be determined by
particle size. Most nanoparticles have been shown to
aggregate once they are hydrated, which has a significant
effect on their sedimentation rates. Several studies have
addressed the aggregation of different nanoparticles in
simpler aqueous solutions, including the effect of increasing
ionic strength (IS) and different pH levels on the size of
1
Corresponding author phone: 805-453-1822; e-mail: keller@
bren.ucsb.edu.
†
University of California, Santa Barbara.
‡
Tongji University.
§
University of California, Davis.
|
University of California, Los Angeles.
10.1021/es902987d
 XXXX American Chemical Society
aggregates (2–5). Several groups have studied the behavior
of MeO nanoparticles as they interact with natural organic
matter (NOM) under various solution chemistries (2, 4–10).
These studies provide the mechanistic understanding that
is important for predicting aggregation, including the effect
of nanoparticle surface charge and charge density (11, 12),
the shielding of nanoparticle surface charge by different
monovalent and divalent cations (3, 6, 13–15), as well as by
adsorbed NOM or other organic molecules (6, 16–18). These
interactions affect the balance between attractive and
repulsive forces, which ultimately control the aggregation of
the nanoparticles in solution, as well as their attachment to
environmental surfaces.
It is, however, the combined effect of pH, IS, ionic
composition, NOM, and other characteristics of the aqueous
media that result in either aggregation or stabilization. This
also affects nanoparticle bioavailability, and the phase (water
column or sediments) in which the bulk of the particles are
likely to reside. Toxicological studies have shown that
nanoparticle size and aggregation play an important role in
determining toxicity (19–22). Only a few studies have
addressed the aggregation of nanoparticles in complex
aqueous matrices (23–25), with a particular emphasis on
groundwater to understand the aggregation of zerovalent
iron and other treatment nanoparticles (5, 26–29).
For this study we dispersed the MeO nanoparticles in
several natural aqueous matrices. These matrices represent
a wide range of ambient water conditions, particularly with
regards to IS, NOM, and ionic composition. Our objective
was to better understand the factors that influence fate and
transport of these MeO nanoparticles in a diverse array of
real environmental media.
2. Materials and Methods
Nanoparticles. Three nanoparticles, TiO2, ZnO, and CeO2,
were used for this experiments because they are among the
most common MeO nanoparticles in use. TiO2 was acquired
from Evonik Degussa Corp. (U.S.). ZnO and CeO2 were
received from Meliorum Technologies (U.S.). Their characteristics are presented in Table 1. While the primary size of
the three types of nanoparticles was in the range from 15 to
30 nm, the particles almost immediately aggregate even in
NanoPure water (Barnstead) to 190-230 nm.
Stock dispersions were produced by adding 1000 mg of
nanoparticles to 1.0 L of NanoPure water. The dispersed
nanoparticles were sonicated for 30 min and stored for up
to 24 h at 22 ( 2 °C.
Water Samples. Santa Barbara (CA) seawater, used for
micro- and mesocosm ecotoxicology studies, was obtained
from the Marine Science Institute (UCSB) laboratory water
system, and 0.2 µm membrane-filtered. Natural filtered
seawater from Bodega Head (CA) was also collected, since
it is used for nanoecotoxicology studies at the UC Davis
Bodega Marine Laboratory (30). Artificial seawater (31) was
also evaluated, since it is a common substitute for natural
seawater in laboratory studies. Samples were also collected
from the UCSB campus lagoon, a relatively brackish water
body. Santa Clara River (CA) water was obtained at the
Highway 23 crossing. Groundwater from Santa Paula, CA
was collected from a shallow well (2 m) used for monitoring
an agricultural site. Stormwater samples were collected from
the influent to a stormwater treatment lagoon at UCSB.
Treated effluent was received from the El Estero wastewater
treatment plant in Santa Barbara.
The freshwater mesocosm water, a common growth media
for primary producers (32), was generated by placing 1.0 L
VOL. xxx, NO. xx, XXXX / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
A
TABLE 1. Physicochemical Characteristics of the Metal Oxide Nanoparticles
properties
technique
TiO2
Evonik
4168063098
unit
CeO2
Meliorum
121008
ZnO
Meliorum
121008
primary size
TEMa
nm
27 ( 4
rods: (67 ( 8) × (8 ( 1)
(e10% polyhedra:
8 ( 1 nm)
24 ( 3
particle size in DI water
DLSa
nm
194 ( 7
231 ( 16
205 ( 14
phase and structure
XRDa
82% anatase and
18% rutile
100% ceria
cubic
100% zincite
hexagonal
shape/morphology
TEMa
semispherical
rods (e10% Polyhedra)
spheroid
surface area
BET2
51.5
93.8
42.1
6.2
7.5
9.2
m2 g-1
a
IEP
zetaPALS
EPM in 1 mM KCl
zetaPALSa
10-8 m2
V-1 s-1
2.37 ( 0.06
2.19 ( 0.04
1.83 ( 0.11
purity
TGAa
wt.%
98.03
95.14
97.27
a
wt.%
1.97
4.01
1.61
moisture content
TGA
a
Transmission and scanning electron microscopy (TEM), dynamic light scattering (DLS), X-ray powder diffraction (XRD),
isoelectric point (IEP), electrophoretic mobility (EPM), and thermogravimetric analysis (TGA) were done by the UC-CEIN at
UCLA . 2 Brunauer-Emmett-Teller analysis (BET) was conducted by Dr. Ponisseril Somasundaran’s lab at Columbia
University.
of Millipore water into a large flask, adding 90 mL of
greenhouse soil that provides organic matter and nutrients
required for algal growth, and then buffering with 0.04 g of
magnesium carbonate. The mixture was left standing for 24 h,
decanted through a 1 µm mesh filter and autoclaved for a
1 h sterilization cycle. This freshwater media was collected
before and at the end of mesocosm nanoecotoxicity experiments with phytoplankton and crustacean zooplankton. All
samples were stored at 4 °C under dark conditions until
needed.
Characterization of the Water Samples. Total organic
carbon (TOC) was measured using a Shimadzu TOC-V
instrument (Shimadzu Scientific Instruments). The pH was
measured using an Oakton pH meter (Ion 510 series, Fisher
Scientific). Conductivity, resistivity, and total dissolved solids
(TDS) were measured with a Fisher Scientific Traceable*
Conductivity, Resistivity, and TDS Meter (Fisher Scientific).
Inductively coupled plasma atomic emission spectroscopy
(ICP-AES) was used to measure 22 elements. Chloride was
determined by the argentometric method, following Standard
Methods (19th Ed., Method 4500-Cl-B). Phosphate, sulfate,
nitrite, and nitrate were measured via colorimetry (HACH
portable DR/890, HACH Company, Loveland, CO). HCO3was determined by titration via a phenolphthalein/total
alkalinity test (model WAT-MP-DR, Lamotte Chemical
Products, MD).
Aggregation and Sedimentation Studies. Before an
experiment, the stock dispersion was sonicated for 10 min.
The stock dispersion was then added to the water samples
to achieve the target concentrations. Particle size at different
stages of the aggregation process were determined via
dynamic light scattering (DLS) using a BI-200SM (Brookhaven
Instruments, Holtsville, NY). The dynamic aggregation
process was monitored using a UV-vis spectrophotometer
(BioSpec 1601, Shimadzu, MD), measuring the sedimentation
of the nanoparticles in different waters at various MeO
concentrations via time-resolved optical absorbency (CeO2
at 321 nm, TiO2 and ZnO at 378 nm; UV-vis spectra in
Supporting Information (SI) Figure S-5). Optical absorbency
was measured every 6 min for 360 min. The experiments
B
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. xxx, NO. xx, XXXX
were run in duplicate or triplicate, and the results presented
are the average of the runs.
ICP-AES Analysis. As needed, the samples were digested
using 40 mL of a saturated (NH4)2SO4 solution to which 60
mL of H2SO4 were added slowly to dissipate the heat of
reaction. For every 4 parts of the sample solution (volumetrically), 6 parts of the acid solution was used. The mixture
was heated at 150 °C for 1 h using capped vials. It was then
diluted by at least a factor of 10 to reduce the acid content
below 5%. The samples were then analyzed via ICP-AES (iCAP
6300, Thermo Scientific, Waltham, MA).
Zeta Potential Measurements. The electrophoretic mobility of the various particles was measured using a
Brookhaven ZetaPALS (Holtsville, NY). The electrophoretic
mobilities of the MeO nanoparticles were measured at a
nanoparticle concentration of 10 mg L-1, to minimize
aggregation.
3. Results
3.1. Composition of Freshwater and Saltwater Samples.
The composition of the various samples is presented in Table
2. The pH of these samples was confined to a relatively narrow
range. The TOC varied by 4 orders of magnitude, somewhat
skewed by the freshwater mesocosm solution which was very
high in organic matter. For the rest of the paper we consider
TOC a surrogate for NOM. As expected, there was also a
significant range in IS (0.003-0.7 eq L-1), and a wide range
of concentrations for mono- and divalent cations. The ratio
between mono- and divalent cations shifted from less than
1 to almost 7, from freshwater to seawater, which has
important implications for the counterions in the double
layer around the nanoparticles (33). Overall the concentration
of trace elements was very low, and in particular dissolved
Zn, Ce, and Ti concentrations were nondetectable in most
samples, except the freshwater mesocosm media.
3.2. Electrophoretic Mobility of Metal Oxides in Ambient Waters. The electrophoretic mobilities (EPM) of the MeO
nanoparticles were measured at a nanoparticle concentration
of 10 mg L-1. In simple solutions with low IS and no NOM,
the TiO2 nanoparticles would be negatively charged at the
TABLE 2. Characteristics of Ambient Water Samples Used in These Studies
Santa
Bodega
Barbara artificial
Bay
seawater seawater seawater
units
pH
TOC
UV254
conductivity
resistivity
TDS
SO42ClNO3NO2PO43K+
Na+
Ca2+
Mg2+
µM C
cm-1
µS
mΩ
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg/L
CaCO3
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
ionic strength
eq. L-1
molar ratio
mono/divalent
Al
As
B
Ba
Ce
Cu
Fe
I
Li
Mn
P
Rb
S
Si
Sr
Ti
Zn
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
mg L-1
HCO3-
UCSB
lagoon
Santa
Paula
Santa Clara estero mesocosm
groundwater river water effluent wastewater
storm
runoff
mesocosm
freshwater
8.05
54.0
0.006
37,400
0.000
O.R.
3,133
19,333
0.45
0.03
7.99
55.9
0.004
36,300
0.000
O.R.
2,550
17,667
0.45
0.01
8.08
131.8
0.002
39,300
0.000
O.R.
3,167
19,333
0.75
0.03
8.90
522.6
0.145
23,733
0.000
15,967
1,400
1,011
2.47
0.00
7.90
842.4
0.242
3,997
0.000
2,643
1,900.0
172.6
2.92
0.00
8.33
163.8
0.049
4,507
0.000
2,997
260.00
125.1
4.35
0.00
7.68
378.0
0.203
2,780
0.000
1,865
306.67
366.7
4.50
0.10
7.07
691.8
0.268
206.0
0.005
137.40
33.70
46.7
0.75
0.03
7.09
1,564.0
0.840
591.0
0.002
394.00
110.00
116.7
1.33
0.03
8.38
5,283
2.872
372.0
0.003
247.00
56.70
103.3
2.10
0.08
221.00
227.00
229.00
162.0
182.7
201.3
257.3
16.0
120.7
61.0
0.40
377.80
10,620
398.00
1361.40
7.07 ×
10-1
0.45
339.80
9,726
376.00
1225.40
6.39 ×
10-1
0.40
365.90
10,010
378.40
1191.00
6.79 ×
10-1
0.77
190.40
5,031
213.10
581.00
2.15 ×
10-1
0.27
14.28
293.70
447.10
224.90
9.09 ×
10-2
0.56
3.11
50.18
110.70
34.08
1.84 ×
10-2
1.71
35.04
377.90
102.70
49.81
3.17 ×
10-2
4.45
6.41
31.94
8.13
3.57
3.18 ×
10-3
5.73
17.70
36.33
46.78
13.03
9.63 ×
10-3
15.00
22.28
55.81
17.66
11.37
7.18 ×
10-3
7.1
7.2
7.6
7.6
0.6
0.5
3.8
4.4
1.2
3.3
N.D.
0.02
4.51
N.D.
N.D.
N.D.
N.D.
N.D.
0.15
0.01
0.10
0.18
890.2
2.31
7.53
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
0.15
N.D.
752.6
2.28
0.21
N.D.
N.D.
N.D.
0.02
4.23
0.01
N.M.
N.D.
N.D.
N.D.
0.15
N.D.
0.10
0.16
838.4
2.71
7.22
N.M.
N.M.
N.D.
N.M.
2.12
N.M.
N.M.
N.M.
N.M.
N.M.
0.12
N.M.
N.M.
0.10
386.5
2.38
3.57
N.M.
N.M.
N.D.
N.M.
2.15
N.M.
N.M.
N.M.
N.M.
N.M.
0.11
N.M.
N.M.
N.D.
588.7
8.86
4.46
N.M.
N.M.
1.28
N.M.
0.37
N.M.
N.M.
N.M.
N.M.
N.M.
0.03
N.M.
N.M.
N.D.
96.96
10.16
0.79
N.M.
N.M.
N.D.
N.D.
0.63
N.D.
N.M.
N.D.
N.D.
N.D.
N.D.
N.D.
0.57
N.D.
107.10
7.72
0.93
N.M.
N.M.
N.D.
N.D.
0.37
N.D.
N.D.
N.D.
N.D.
0.59
N.D.
N.D.
1.45
N.D.
11.58
8.48
0.04
0.12
N.D.
0.01
N.D.
0.16
N.D.
N.M.
N.D.
N.D.
N.D.
N.D.
N.D.
1.66
N.D.
41.70
6.16
0.28
N.M.
N.M.
2.82
0.00
0.55
0.11
N.D.
N.D.
0.96
2.84
N.D.
0.01
4.85
N.D.
21.78
11.87
0.10
0.01
0.01
TABLE 3. Electrophoretic Mobilities (µm s-1 V-1 cm) of Metal Oxide Nanoparticles in Different Ambient Waters
Santa
Barbara
seawater
artificial
seawater
UCSB
lagoon
Santa
Paula
ground
water
Santa
Clarariver
treated
effluent
mesocosm
effluent
UCSB
stormwater
freshwater
mesocosm
TiO2 -0.04 ( 0.09 -0.05 ( 0.15 -0.82 ( 0.09 -1.11 ( 0.03 -1.24 ( 0.13 -1.39 ( 0.05 -1.89 ( 0.04 -2.09 ( 0.05 -2.40 ( 0.05
CeO2 -0.05 ( 0.24 -0.04 ( 0.09 -0.75 ( 0.16 -1.05 ( 0.05 -1.13 ( 0.11 -1.36 ( 0.05 -1.91 ( 0.05 -2.01 ( 0.05 -2.39 ( 0.09
ZnO -0.04 ( 0.26 -0.27 ( 0.52 -1.54 ( 0.17 -1.21 ( 0.03 -1.63 ( 0.07 -1.13 ( 0.04 -1.75 ( 0.04 -1.69 ( 0.07 -2.22 ( 0.06
pH range of these ambient waters, the ZnO nanoparticles
would be positively charged except in the lagoon water, and
the CeO2 nanoparticles would be near their isoelectric point.
However, the actual charge was generally negative or almost
neutral in the presence of the various electrolytes and organic
molecules (Table 3). While the EPM of the MeOs in simple
water matrices is generally different for the three MeOs (SI
Figure S-2), it is striking that the EPM in a given natural
water is almost independent of the type of MeO nanoparticle,
with no clear trend for a given MeO to have the highest or
lowest EPM.
The EPM measurement in seawater had a much larger
uncertainty than in other waters. Measurements in seawater
ranged from slightly positive to slightly negative, and were
time-dependent, as the nanoparticles formed larger aggregates with different charges on the surface. The difference
between artificial and natural seawater was small.
The EPM of these MeOs in these natural waters is
controlled to a large extent by NOM, modulated by IS (Figure
1). As the surface of the MeO surface is coated with NOM,
the charge on the particle is dominated more and more by
the charge of the NOM, which at this range of pH varies from
-20 to -30 mV. Thus, as TOC increases, the charge on the
nanoparticles becomes even more negative (Figure 1a). As
IS increases, the nanoparticle charge is neutralized more
effectively (Figure 1b). Contrary to the clear relationship
between pH and EPM in DI water, even after adjusting the
effect of pH by subtracting the point of zero charge there was
no clear trend between pH and EPM in these complex water
matrices (SI Figure S-3).
3.3. Sedimentation and Aggregation in Various Waters.
The stability of the three MeO nanoparticles in the aqueous
samples we tested varied widely. As expected, in the low
TOC and high IS conditions of seawater, the rate of
sedimentation was very high for the three metal oxide
nanoparticles (Figure 2a), with a strong dependence on
nanoparticle concentration. At the higher initial nanoparticle
concentrations in seawater, the concentration of suspended
particles decreased more than 80% in less than 100 min; this
rate decreased rapidly as nanoparticle aggregates were
VOL. xxx, NO. xx, XXXX / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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C
FIGURE 1. Electrophoretic mobility of TiO2, ZnO and CeO2 in nine different waters as a function of (a) TOC; and (b) IS.
FIGURE 2. Sedimentation of TiO2, ZnO, and CeO2 in seawater at 4 different initial nanoparticle concentrations (10, 50, 100, and 200
mg L-1).
FIGURE 3. Dynamic light scattering studies of the aggregation of TiO2, ZnO and CeO2 at 10 mg L-1 (a) in seawater; and (b) in
mesocosm freshwater.
removed from solution, lowering the concentration of
dispersed nanoparticles. Thus we might expect MeO nanoparticles entering seawater to be removed from the water
column in a few hours. Aquatic organisms in the water
column may only be affected if the loading is frequent or
continuous; benthic organisms may be at higher risk of
exposure, although the exposure would be in the form of
relatively large aggregates (>1 µm) of nanoparticles (Figure
3). TiO2 and CeO2 aggregated within tens of minutes to
micrometer size particles in seawater even at relatively low
(10 mg L-1) concentrations (Figure 3a). At higher particle
concentrations (SI Figure S-4a), the aggregation rate was
much faster due to the increased probability of collisions
between particles. While it is possible that the concentrations
of dispersed nanoparticle aggregates will be less than 10 mg
L-1, the initial sources may discharge at these levels. A
completely different behavior was observed when the MeOs
were dispersed in a high TOC, low IS solution, such as the
freshwater used in the mesocosm studies (Figure 3b and 4).
In this case, the size of the aggregates remained stable at
slightly above 300 nm for the three MeO particles (Figure
3b). The availability of significant amounts of organic
molecules that can adsorb onto the particle surfaces provides
a barrier to aggregation. Since the attachment efficiency
(34, 35) of these particles to each other is so small, even an
D
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. xxx, NO. xx, XXXX
increase in particle concentration has no effect on the
aggregation rate (SI Figure S-4b). This translates into stable
dispersions of MeO in this media, with a very low rate of
sedimentation (Figure 4). Thus, under these conditions
aquatic organisms within the water column (e.g., fish, algae,
filter-feeders), and benthic filter-feeding invertebrates, will
be exposed to small nanoparticle aggregates for a longer time.
Eventually, within a few days the particles will sediment out,
exposing soft-sediment benthic organisms as well.
A comparison between the aggregate sizes in freshwater
and seawater (Figure 3a vs 3b) indicates that there is very
rapid aggregation from ∼300 nm to >500 nm in seawater
within the first two minutes of the DLS experiments. This
has significant implications for ecotoxicity studies with bare
nanoparticles in high IS media, since it is highly unlikely that
the organisms will actually be exposed to them in their original
size.
Figure 5 compares sedimentation of the three MeOs in
eight different natural aqueous matrices. The mesocosm
freshwater and seawater are the two end-points, with a
distribution of sedimentation rates in between. The MeOs
are more stable in stormwater, treated WWTP effluent, and
the residual mesocosm effluent water, which are generally
low in IS (0.001-0.01 eq L-1) and medium to high in TOC
(378-5283 µM C). The MeOs have high EPM in these waters,
FIGURE 4. Sedimentation of TiO2, ZnO and CeO2 in mesocosm freshwater at four different initial nanoparticle concentrations (in mg
L-1).
FIGURE 5. Sedimentation in different natural aqueous media of (a) TiO2; (b) ZnO; and (c) CeO2. Note that the symbols may vary for
clarity. The order of the media in the legend reflects sedimentation rate from lowest to highest.
FIGURE 6. Relationship between electrophoretic mobility (EPM) and (a) sedimentation rate of the three metal oxide particles at 10
mg L-1; (b) estimate of apparent attachment efficiency.
with a high negative charge that reduces the attachment
which for fast sedimentation conditions (e.g., lagoon, river,
efficiency. In the case of TiO2, there is a very distinct behavior
groundwater) occurs within a few minutes, while for slower
between these four waters, and the groundwater, river, lagoon
sedimentation conditions (e.g., stormwater, treated effluent)
and seawater matrices, where aggregation is very fast,
may take at least 60 min to observe a measurable sedimenresulting in rapid deposition. These last four aqueous matrices
tation rate. There is a trend of increasing sedimentation rate
are high in IS (0.1 to ∼1 eq L-1), while low to medium in TOC
as EPM decreases to near zero (Figure 6a), although the
(54-523 µM C). This behavior suggests that the critical
behavior is complex. Following (35), the initial sedimentation
coagulation concentration (34) is exceeded for TiO2 under
rates are normalized by the maximum sedimentation rate
these high IS, low TOC conditions, since the rate of
observed (for seawater in all cases) to obtain an estimate of
sedimentation does not vary significantly. While this behavior
the apparent attachment efficiency, R, for collisions between
is also observed for ZnO and CeO2, it is not as distinct as for
nanoparticles in the various media. Additional details are
TiO2. The observed pattern correlates well with the measured
provided in the Supporting Information on the calculation
EPM in these different waters.
of R.
The measured initial sedimentation rates for the three
There is a clearer relationship between EPM and R
MeO particles at 10 mg L-1 ranged from around 1 × 10-7 to
(Figure 6b). At very negative EPM, the electrostatic
1 × 10-4 s-1, considering a normalized concentration (C/Co).
repulsion is high and R is nearly zero. At EPM near zero,
The sedimentation rate is estimated from the initial 5%
there is almost no electrostatic repulsion and R is nearly
decrease in normalized particle concentration (Figure 5),
unity. The transition from reaction to diffusion limited
VOL. xxx, NO. xx, XXXX / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
E
aggregation (34) occurs from around -2 to -0.8 µm s-1
V-1 cm. Further studies using a wider range of aqueous
solutions will be necessary to better characterize the
relationship between R and EPM. These results will serve
to better design and interpret nanoparticle ecotoxicity
studies in various environmental conditions.
Acknowledgments
This work was supported in part by the National Science
Foundation and the U.S. Environmental Protection Agency
under Cooperative Agreement No. NSF-EF0830117. Any opinions, findings, and conclusions or recommendations expressed
in this material are those of the authors and do not necessarily
reflect the views of the National Science Foundation or the U.S.
Environmental Protection Agency. The research was also
partially supported by the UC Toxic Substances Research and
Training Program, Lead Campus on Nanotoxicology. We also
thank Maia Colyar and Priya Vytla for their help with the
sedimentation experiments, and Dr. Sharon Walker’s lab at UC
Riverside and Dr. Eric Hoek’s lab at UCLA for preliminary
characterization of the nanomaterials.
Supporting Information Available
Additional information on the method for calculating the
attachment efficiency, the correlation between UV254 and
TOC, the electrophoretic mobility of the three metal oxide
nanoparticles in simple solutions and in nine different natural
waters, results from dynamic light scattering studies, and
UV-vis spectra for the three metal oxide nanoparticles. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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