SPECIAL
TECHNICAL
SUBMISSIONS
REPORTS
Nanoparticle Aggregation: Challenges to Understanding Transport and Reactivity
in the Environment
Ernest M. Hotze, Tanapon Phenrat, and Gregory V. Lowry* Carnegie Mellon University
Unique forms of manufactured nanomaterials, nanoparticles,
and their suspensions are rapidly being created by manipulating
properties such as shape, size, structure, and chemical
composition and through incorporation of surface coatings.
Although these properties make nanomaterial development
interesting for new applications, they also challenge the ability
of colloid science to understand nanoparticle aggregation in
the environment and the subsequent effects on nanomaterial
transport and reactivity. This review briefly covers aggregation
theory focusing on Derjaguin-Landau-Verwey-Overbeak
(DLVO)-based models most commonly used to describe
the thermodynamic interactions between two particles in a
suspension. A discussion of the challenges to DLVO posed by
the properties of nanomaterials follows, along with examples
from the literature. Examples from the literature highlighting
the importance of aggregation effects on transport and reactivity
and risk of nanoparticles in the environment are discussed.
Copyright © 2010 by the American Society of Agronomy, Crop Science
Society of America, and Soil Science Society of America. All rights
reserved. No part of this periodical may be reproduced or transmitted
in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system,
without permission in writing from the publisher.
J. Environ. Qual. 39:1909–1924 (2010)
doi:10.2134/jeq2009.0462
Published online 20 May 2010.
Received 18 Nov. 2009.
*Corresponding author ([email protected]).
© ASA, CSSA, SSSA
5585 Guilford Rd., Madison, WI 53711 USA
A
s of the submission date of this review, approximately
4700 articles on nanoparticle (NP)-related topics have been
published in the calendar year 2009 alone (Thompson Reuters,
2009). Many of these papers discuss novel particles, and applications of nanomaterials along with their use in consumer products
are expanding rapidly (Project on Emerging Nanotechnologies,
2009). For example, nanomaterials are used for environmental
clean-up and biomedical applications (Alivisatos, 2001; Liu et al.,
2005). This expansion in applications, and the incorporation of
“nano” ingredients into products that may be released during the
life cycle of those products, will increase the occurrence of manufactured nanomaterials in the environment. For example, the
leaching of silver NPs used as antimicrobial agents in cloth was
recently reported (Benn and Westerhoff, 2008). Unintentionally
or intentionally, manufactured NPs might come into contact with
biological receptors. For this reason, the risk of nanomaterials to
humans and to the environment garners attention from the scientific community, governmental agencies, and public stakeholders.
Nanoparticless in engineered or environmental systems could
be thought of as dispersions of primary particles. However, primary NPs tend to aggregate into clusters up to several microns
in size (Brant et al., 2007; Chen and Elimelech, 2007; Jiang et
al., 2009; Limbach et al., 2005; Phenrat et al., 2008). Therefore,
NP aggregation plays an important role in determining reactivity,
toxicity, fate, transport, and risk in the environment. The critical
role of aggregation in environmental implications had occasionally been ignored (Baveye and Laba, 2008), but many recent studies have begun applying colloid science principles, based around
Derjaguin-Landau-Verwey-Overbeak (DLVO) theory, to understand NP aggregation under various conditions. Manufactured
NPs challenge the limits of colloid science, however, due to their
small size, variable shape, structure, composition, and potential presence of adsorbed or grafted organic macromolecules.
Laboratory aggregation studies, which are covered in detail in this
review, have demonstrated that these challenges, acting simultaneously, will ultimately affect how NPs behave in the environment. Additionally, particles released to the environment undergo
Center for Environmental Implications of NanoTechnology (CEINT) and Deps. of Civil
& Environmental Engineering, Carnegie Mellon Univ., Pittsburgh, PA 15213-3890.
Assigned to Associate Editor Dionysios Dionysiou.
Abbreviations: CT, carbon tetrachloride; Df, fractal dimension; DLVO, Derjaguin-LandauVerwey-Overbeak; EDL, electrostatic double layer; MWCNT, multi-walled carbon
nanotubes; NOM, natural organic matter; NP, nanoparticle; NZVI, nano zero valent iron;
PZC, point of zero charge; ROS, reactive oxygen species; Velas, elastic-steric potential; VOSM,
osmotic potential; XDLVO, extended Derjaguin-Landau-Verwey-Overbeak.
1909
chemical and physical transformations and encounter a multitude of solution conditions important for aggregation, including solution pH, dissolved ions, naturally occurring organic
matter, clays, and biocolloids.
This review focuses on three questions: (i) Can basic colloid
science predict the behavior of nanomaterials in the environment? We provide a brief overview of colloid science. Several
excellent resources are available for additional information
on colloid science (Evans and Wennerstrom, 1999; Hiemenz
and Rajagopalan, 1997; Buffle et al., 1998; Elimelech and
O’Melia, 1990; O’Melia, 1980). (ii) Where are challenges to
existing theory posed by manufactured nanomaterials? We
discuss the intrinsic NP properties that raise these challenges.
(iii) How will these challenges affect studies of fate and transport, reactivity, and risk to the environment? We describe
several case studies examining environmental implications of
nanomaterial aggregation.
Colloid Science and Aggregation
Colloid science is defined as a branch of chemistry dealing with
colloids, heterogeneous systems consisting of a mechanical
mixture of dispersion (colloids) and dispersant. When particles
are dispersed, sizes range between 1 nm and 1 μm in a continuous medium. Above this size range, particles begin to sediment
out of suspension. Several theories to explain aggregation of
submicrometer spherical particles have been developed and
experimentally studied for decades, the most common being
DLVO (Derjaguin and Landau, 1941; Verwey et al., 1948)
(an alternative approach is statistical thermodynamics; see
Hermansson [1999] and Koponen [2009]). By definition, the
size range of colloidal particles overlaps the size range of manufactured nanomaterials (i.e., <100 nm). Therefore, theories in
colloid science should be applicable to manufactured nanomaterials, but, due to the novelty of nanomaterial properties, several studies have found that this is not always the case (He et
al., 2008; Kozan et al., 2008; Phenrat et al., 2008). However,
colloid science is fundamental to understanding and developing theories for nanomaterials systems. This section, therefore,
provides a very brief review of the types of aggregation and
DLVO theory.
Types of Aggregation and Aggregate Structure
Aggregation of colloidal particles occurs when physical processes bring particle surfaces in contact with each other and
short-range thermodynamic interactions allow for particle–
particle attachment to occur. For particles <100 nm in size,
Brownian diffusion controls the long-range forces between
individual nanoparticles, causing collisions between particles.
When contact occurs, it can result in attachment or repulsion. Short-range thermodynamic interactions that control
this can be understood in the context of DLVO theory. There
are two types of aggregation relevant to manufactured NPs in
the environment: homoaggregation and heteroaggregation.
Homoaggregation refers to aggregation of two similar particles (i.e., NP–NP attachment). This is observed in homogeneous suspensions of particles that are typically studied in
the laboratory to make useful correlations with DLVO theory.
Heteroaggregation refers to aggregation of dissimilar particles
1910
(e.g., NP–clay particle attachment). Real environmental systems contain natural particles (e.g., clay particles) in numbers
far greater than the number of manufactured NPs, so heteroaggregation is an important fate process for manufactured NPs.
The probability that two particles attach is given by a “sticking coefficient,” also known as the attachment efficiency (α)
(i.e., when α = 1, sticking occurs 100% of the time; when α =
0.5, sticking occurs 50% of the time). Particles attaching at first
contact (α = 1) form large dendritic aggregates, whereas particles sticking only after several collisions (α < 1) form denser
and less dendritic aggregates (Buffle and Leppard, 1995). The
concept of fractal dimension is used to describe aggregate structure. Fractal dimension (Df ) can range from 1 (a line) to 3 (a
sphere). These are limits that come from Euclidean dimension,
where 1 is a straight line (e.g., x1), 2 is a flat surface (e.g., x2),
and 3 is a volume (e.g., x3). Fractal dimensions are those that
lie between Euclidean dimensions. Ideal fractal structures are
self-similar throughout. Homoaggregation under controlled
laboratory conditions produces aggregates of reasonably predictable fractal dimension (Barbot et al., 2010; Chakraborti
et al., 2003; Hansen et al., 1999), whereas heteroaggregation
typically forms natural fractals (statistically self-similar over a
limited range of length scales), making aggregation state more
difficult to predict. The physical dimensions of the aggregates
formed can affect the reactive surface area, reactivity, bioavailability, and toxicity. Aggregate structure must therefore be
determined and considered when interpreting fate, transport,
and toxicity data.
Derjaguin-Landau-Verwey-Overbeak and Extended
Derjaguin-Landau-Verwey-Overbeak
According to the classical DLVO theory of aggregation, the
sum of attractive and repulsive forces determines attachment
(Derjaguin and Landau, 1941; Verwey et al., 1948). This
theory maintains that only two forces dominate interactions
between particles: van der Waals (vdW) attractive and electrostatic double layer (EDL) forces. Table 1 lists an expression for
the vdW attraction experienced by two spheres in suspension
(Stumm and Morgan, 1996). Expressions for vdW commonly
calculate the attraction between two spherical particles; however, due to unique NP shapes and compositions, expanded
theoretical approaches may be needed (Gatica et al., 2005).
An EDL has a charge that reflects not only the surface that it
surrounds but also the solution chemistry of the surrounding
media. Ionic strength, which is a measure of the amount of
ions present, controls the extent of the radius of the diffuse
layer from the surface. Low ionic strength means the EDL ion
cloud extends far out from the particle; high ionic strength
conditions compress the EDL. Table 1 lists an expression for
the EDL experienced by two charged spheres in suspension
(Stumm and Morgan, 1996).
Classical DLVO simplifies thermodynamic surface interactions and predicts the probability of two particles sticking together by simply summing van der Waals and electric
double-layer potentials to determine if forces are net attractive
(−VT) or net repulsive (+VT). For example, in Fig. 1, van der
Waals and electric double-layer potentials are plotted as a function of separation distance between the particles. Summing
Journal of Environmental Quality • Volume 39 • November–December 2010
Table 1. Derjaguin-Landau-Verwey-Overbeak and extended Derjaguin-Landau-Verwey-Overbeak forces in aggregation.
Force
X/DLVO†
van der Waals
attraction
DLVO
Electrostatic
double layer
DLVO
Equations‡§
Origin
s (4a + s ) ⎞
V VDW
A ⎛ 2a 2
2a 2
⎜
⎟
=−
+
+ ln
2
kT
6kT ⎜ s (4a + s ) (2a + s )
(2a + s )2 ⎟⎠
⎝
interactions of
electrons in particles
Stumm and Morgan
(1996)
charged surfaces
Delgado et al.
(2007), Stumm and
Morgan (1996)
aligning electron
spins
Phenrat et al. (2007)
entropic penalty of
separating hydrogen
bonds in water
Hoek and Agarwal
(2006), Wu et al.
(1999)
concentration of
ions between two
particles (Fig. 4)
Fritz et al. (2002),
Ortega-Vinuesa et
al. (1996), Phenrat
et al. (2008),
Romero-Cano
et al. (2001)
molecules on
particle surfaces
resist loss of entropy
due to compaction
(Fritz et al., 2002;
Ortega-Vinuesa et
al., 1996; Phenrat
et al., 2008; RomeroCano et al., 2001)
surface molecules
bridge to other
particles
Chen and Elimelech
(2007), Domingos
et al. (2009)
VEDL 64πn kT ( a + δ ) ⎡
⎛ ze Ψ d
tanh
=
kT
κ
( s + 2a ) ⎢⎣ ⎜⎝ 4kT
2
2
⎞⎤
−κ ( s −2 δ )
⎟ ⎥ exp
⎠⎦
References
1
⎡ N 2 2 ⎤2
⎢ ∑ e z i ni ⎥
⎥
κ=⎢ i
⎢ års å0kT ⎥
⎢
⎥
⎣
⎦
Magnetic
attraction
XDLVO
VM =
−8πμo M S2 a 3
⎛s
⎞
9⎜ + 2⎟
⎝a
⎠
3
Hydrophobic (Lewis
acid-base)
XDLVO
VAB
⎛ s −s⎞
= ΔG sAB
exp ⎜ 0
⎟
0
surface area
⎝ λ ⎠
Osmotic
repulsion
XDLVO
⎧V
⎫
2d ≤ s ⇒ ⎨ OSM = 0 ⎬
⎩ kT
⎭
2
a 4π 2 ⎛ 1
s ⎞ ⎪⎫
⎪⎧ V
⎞⎛
d ≤ s < 2d ⇒ ⎨ OSM =
φp ⎜ − χ⎟ ⎜ d − ⎟ ⎬
2 ⎠ ⎭⎪
v1
⎝2
⎠⎝
⎩⎪ kT
a 4π 2 ⎛ 1
1
⎪⎧ V
⎞ ⎛ s
⎛ s ⎞ ⎞ ⎪⎫
s < d ⇒ ⎨ OSM =
φp ⎜ − χ⎟ d 2 ⎜
− − ln ⎜ ⎟ ⎟ ⎬
v1
⎝2
⎠ ⎝ 2d 4
⎝ d ⎠ ⎠ ⎪⎭
⎩⎪ kT
Elastic-steric
repulsion
XDLVO
⎧V
⎫
d ≤ s ⇒ ⎨ elas = 0 ⎬
⎩ kT
⎭
2
⎧
⎛ 2 πa
⎞ ⎪⎧ s ⎡ s ⎛ 3 − s d ⎞ ⎤ ⎪⎫
⎪V
d > s ⇒ ⎨ elas = ⎜
φ p d 2ρ p ⎟ ⎨ ln ⎢ ⎜
⎟ ⎥⎬
⎠ ⎪⎩ d ⎢⎣ d ⎝ 2 ⎠ ⎥⎦ ⎪⎭
⎪⎩ kT ⎝ MW
⎛ 3−s d ⎞ ⎛
s
−6 ln ⎜
⎟ + 3 ⎜1 +
⎝ 2 ⎠ ⎝ d
Bridging
attraction
XDLVO
⎞ ⎪⎫
⎟⎬
⎠⎪⎭
NA
† DLVO, Derjaguin-Landau-Verwey-Overbeak; EDLVO, extended Derjaguin-Landau-Verwey-Overbeak.
‡ A, Hamaker Constant; a, average particle radius; χ, Flory-Huggins solvency parameter; d, thickness of “brush” polymer layer (~δ); ΔG sAB :free energy of
0
acid base interaction between particles at distance s0; δ, Stern layer thickness; ε, relative permittivity of water (=78.5 C V−1 m−1); εo, permittivity of a vacuum
−12
(=8.854 × 10 ); e, elementary charge of an electron; κ, inverse Debye length; k, Boltzmann constant; λ, decay length for acid–base interactions (=0.6
nm); Ms, saturation magnetization; μo, vacuum permeability; MW, molecular weight of polyelectrolyte; n, number concentration of ion pairs; ρp, density
of polymer; Ψd diffuse potential (~zeta potential); s, distance between interacting surfaces; s0, minimum equilibrium separation distance (=0.158 nm); T,
absolute temperature; φp, volume fraction of polymer in “brush”; v1, volume of solvent molecule; z, electrolyte valence,
§ Refer to referenced publications for details of equation derivations and constraints.
these curves (VT) demonstrates that particles can have a net
attraction in a primary or secondary minimum (well). Particles
in the primary well are considered to be irreversibly aggreHotze et al.: Nanoparticle Aggregation: Environmental Transport and Reactivity
gated, whereas particles in the secondary well are reversibly
aggregated (i.e., re-entrainment is possible if shear forces are
exerted) (Hahn et al., 2004). Figure 1 also shows something
1911
theories in colloid science, DLVO and XDLVO.
Among these challenges are shape, size, structure,
composition, and adsorbed or grafted organic
macromolecules (e.g., coatings or NOM) (Fig.
2). Although particles not classified as “nano”
(i.e., <100 nm) also present some of these challenges, manufactured NPs are often designed
with the intention of amplifying these properties.
Examining some early studies demonstrating these
challenges can inform our discussion in the previous section on implications of NP aggregation on
their transport and reactivity in the environment.
Particle Size
The small size of most NPs conflicts with a fundamental assumption of DLVO. Equations describing EDL are mathematically incorrect if this
assumption does not hold (Delgado et al., 2007).
This is because when a particle reaches a small
enough size, its surface curvature is too substantial
to assume it is flat (i.e., when the assumption of
κ*a >>1 no longer applies). Therefore, this is one of the primary challenges when considering NP aggregation in a DLVO
framework. Aside from this, vdW and EDL forces (Table 1)
scale similarly with particle size (Evans and Wennerstrom,
1999; Hiemenz and Rajagopalan, 1997; Israelachvili, 1991),
so moving from microscale to nanoscale particles should theoretically affect a change on vdW and EDL forces to a similar
degree. However, as a particle decreases in size, a greater percentage of its atoms exist on the surface. Electronic structure,
surface charge behavior, and surface reactivity can be altered as
a result. Recent studies suggest that redistribution and changing coordination environment of charge and atoms at the surfaces of NPs strongly influences their reactivity (Hochella et al.,
2008). He et al. (2008) reported that as the particle size became
smaller, the particles became more susceptible to surface charge
titration. They found an inverse correlation between particle
size and the measured point of zero charge (PZC); 12, 32, and
65 nm hematite particles had PZCs of 7.8, 8.2, and 8.8 pH
units, respectively (He et al., 2008). These hematite NPs were
synthesized using the same method so they would be identical in every aspect except for size. However, for these particles,
at pH 7 the smaller particles are less charged than larger particles, leading to lower EDL repulsion forces and subsequently
greater aggregation tendency. Indeed—given the same ionic
strength—smaller hematites have faster aggregation rates than
larger ones (He et al., 2008). This suggests that size polydispersity in manufactured NPs affects their aggregation behavior.
Polydispersion is the reality in large-scale applications. For
example, reactive nano-scale zerovalent iron (NZVI) particles
used for environmental remediation are highly polydisperse
(Liu et al., 2005; Phenrat et al., 2007; Phenrat et al., 2009a).
Unlike van der Waals attraction, long-range magnetic attraction between two NZVI particles increases with particle radius
to the sixth power (Table 1). Polydispersity and particle size
therefore greatly affected the aggregation of NZVI having particle sizes ranging from 15 to 260 nm, with aggregation being
proportional to the fraction of large particles in the dispersion
(Phenrat et al., 2009a).
Fig. 1. van der Waals force (dashed line), electrostatic double layer (EDL) force (gray
line), total Derjaguin-Landau-Verwey-Overbeak (DLVO) forces, and elastic force plotted
together to find total potential as a function of separation distance. XDLVO, extended
Derjaguin-Landau-Verwey-Overbeak. V/KT, potential energy divided by Boltzmann’s
constant and absolute temperature.
that is commonly the case with manufactured NPs having an
organic coating: Classical DLVO forces alone are not sufficient
to accurately predict aggregation behavior. Steric repulsion
forces (VT + VELAS) resulting from adsorbed polymer or polyelectrolyte coatings or natural organic matter (NOM) makes it
so these particles may only have a net attraction in a secondary minimum. Coated NPs may therefore aggregate reversibly
(Graf et al., 2009), and aggregation reversibility has significant
consequences regarding the fate, transport, bioavailability, and
effects of NPs in the environment. These additional forces are
collectively known as extended DLVO (XDLVO).
In addition to steric repulsion forces (VT + VELAS) due to NP
coatings, other XDLVO forces have been invoked to match
experimental data for various types of aggregating primary
particles (Chen and Elimelech, 2007; Fritz et al., 2002; Hoek
and Agarwal, 2006; Ortega-Vinuesa et al., 1996; Phenrat et
al., 2007; Phenrat et al., 2008; Romero-Cano et al., 2001; Wu
et al., 1999). This means that additional short-range forces are
operating on an equivalent magnitude as vdW attraction and
EDL. These forces can include bridging (Chen and Elimelech,
2007), osmotic (Fritz et al., 2002; Ortega-Vinuesa et al., 1996;
Phenrat et al., 2008; Romero-Cano et al., 2001), steric (Fritz
et al., 2002; Ortega-Vinuesa et al., 1996; Phenrat et al., 2008;
Romero-Cano et al., 2001), hydrophobic Lewis acid-base (Wu
et al., 1999) (Hoek and Agarwal, 2006), and magnetic forces
(Phenrat et al., 2007). Table 1 summarizes these forces. The relevant force type depends on the system tested, and frequently
more than one XDLVO force is important. Moreover, forces
are not completely independent of one another. In the case of
nanomaterials, understanding aggregation using a DLVO- or
XDLVO-based theory presents many challenges.
Nanoparticle Challenges to DerjaguinLandau-Verwey-Overbeak
Colloid science demonstrates that several forces with different natures and origins govern the kinetics and extent of particle aggregation. This section elaborates how manufactured
NPs present a combination of challenges to two predominant
1912
Journal of Environmental Quality • Volume 39 • November–December 2010
Fig. 2. Derjaguin-Landau-Verwey-Overbeak (DLVO) frames particle aggregation by modeling two solid spheres approaching in suspension. The
sum of van der Waals (VDW) force and electrostatic double layer (EDL) force determines if the particles attach or repulse. Nanoparticles present
challenges to DLVO frameworks because of shape, structure, composition, size, and macromolecule considerations. Not drawn to scale.
Another example of a size effects is “halo” stabilization,
where nano sized materials can control electrostatic repulsion
between two approaching micrometer-sized particles, hindering their flocculation by keeping them separated beyond the
range of DLVO attractive forces (Fig. 3). Tohver et al. (2001)
found that adding 6-nm zirconia NPs to a dispersion of unstable submicron silica particles (0.6 μm in diameter) stabilized
the mixture (Tohver et al., 2001). The same phenomenon was
observed for bimodal magnetorheological fluids consisting of
micrometer sized magnetite particles (1.45 μm in diameter)
and magnetite NPs (8 nm diameter) (Viota et al., 2007). Size
and resultant polydispersity, therefore, present a large challenge
to understanding how NP dispersions behave because these
parameters can enhance or diminish aggregation.
Chemical Composition
Colloid science captures the effect of chemical composition on
aggregation through the Hamaker constant, which governs van
der Waals attraction; saturation magnetization, which control
magnetic attraction; hydrophobicity, which controls hydrophobic interaction (Wu et al., 1999); and surface charge, which
governs electrostatic double layer (EDL) interaction (Table 1)
(Evans and Wennerstrom, 1999).
Hotze et al.: Nanoparticle Aggregation: Environmental Transport and Reactivity
Particles with a high Hamaker constant have greater aggregation tendency compared with particles with a low Hamaker
constant at the same solution and surface chemistry. Because
the origin of van der Waals attraction is permanent or induced
dipoles, a bulk material with electronic or molecular structure
that favors generation of permanent or induced dipoles normally has a large Hamaker constant value. For example, the
Hamaker constants of gold, silver, and polystyrene are 45.3,
39.8, and approximately 9.8 × 10−20 J, respectively (Hiemenz
and Rajagopalan, 1997). van der Waals attraction forces are
approximately five times stronger for gold particles in comparison with polystyrene (see equation in Table 1). In addition to
controlling van der Waals attraction, some materials can originate forces that challenge the DLVO framework, including
magnetic and hydrophobic interactions that promote aggregation. For example, ferromagnetic and ferrimagnetic materials,
such as zerovalent iron and magnetite, can cause long-range
magnetic attraction without an applied magnetic field (Table
1) (Elena et al., 2009; Phenrat et al., 2007). This magnetic
force contributes to abnormally rapid aggregation. Moreover,
carbon-based nanomaterials, such as C60 or CNTs, have hydrophobic surfaces in aqueous environments. Interaction of allcarbon surfaces with water molecules involves a significant
entropic penalty driving an aggregation process that might
1913
Complex Crystal Structures
Complex NP crystal structures can present
challenges to colloid science because changes
in the crystal structure can alter Hamaker constants and surface charges in ways not explainable by theory. Additionally, defects in the
crystal lattice can alter surface charge (Evans
and Wennerstrom, 1999; Israelachvili, 1991).
The crystal structure of materials with the same
atomic composition can play a large role in
determining surface charge. Nano-sized TiO2,
for example, has three different polymorphs:
rutile, anatase, and brookite, each with a unique
crystal structure. The zeta potential is approxiFig. 3. Without nanoparticles present, microscale particles aggregate because net forces are
mately −20 mV (French et al., 2009) when
attractive. Nanoparticles (NPs) create a “halo” effect that increases electrostatic double layer
anatase and brookite structures are predomirepulsive forces and causes net forces between microscale particles to be unattractive.
nant and is approximately −35 mV (Lebrette et
be understood through the use of XDLVO frameworks that
al., 2004) when the rutile phase is predominant
model hydrophobic Lewis acid–base interactions (Wu et al.,
(both measured at pH 7.5). This is consistent with findings
1999). These types of XDLVO frameworks have yet to be
that the dielectric constant can be altered by annealing pure
developed for carbon-based nanomaterials.
anatase-phase TiO2 to the rutile phase (Kim et al., 2005). The
Unique NP composition and morphology (e.g., core-shell
Hamaker constant is a function of dielectric constant (Tabor
NPs) and environmental transformations of NPs will chaland Winterton, 1969); therefore, the Hamaker constant also
lenge DLVO explanations of aggregation. Particles with a high
can depend on the crystal structure of NPs. This is important
surface potential have lower aggregation tendencies compared
to consider for NP systems containing mixtures of phases or
with particles with a low surface potential given the same
where crystal structures are altered for applications, such as
solution chemistry. Nanoparticle chemical composition alters
photocatalysis with TiO2.
surface potential by dissociation or ion adsorption by surface
Shape
atoms (Evans and Wennerstrom, 1999; Israelachvili, 1991).
Surface charge of uncoated NPs is therefore governed in part
In DLVO modeling, one of the primary assumptions is that
by the type of atoms at the surface of the particles. However,
particles are spherical. This assumption is reasonable for ideal
the core of core-shell particles, which are common, can influlatex particles and some ideal colloidal systems. Manufactured
ence the charge of the surface atoms. For example, uncoated
NPs, however, come in a variety of nonspherical shapes.
Fe0/Fe3O4 core-shell NPs have a zeta potential near −32 mV (in
Triangles, icosahedrons, ellipses, rhombohedrons, spindle,
1 mmol L−1 NaHCO3 at pH 7.5), but as the Fe0 core of the parrod, and tube shapes are possible (AgNPs [Moon et al., 2009],
ticles oxidizes to magnetite, the surface charge becomes more
Fe2O3NPs [Wang et al., 2009], and Ag2S [Ma et al., 2007]),
like that of magnetite (Phenrat et al., 2007). Gold NPs (synthereby complicating traditional DLVO and XDLVO.
thesized via citrate reduction) have a charge of around −55 mV
Both vdW and EDL forces are affected by changes in
(Kim et al., 2009), presumably due to citrate acid adsorbed on
shape. Several researchers have investigated these changes
the surface. Multiwall carbon nanotubes (MWCNTs) treated
(Bhattacharjee and Elimelech, 1997; Montgomery et al., 2000;
with a mixture of sulfuric acid and nitric acid (3:1) together
Vold, 1954). At a separation distance smaller than the mean
with ultrasonication have zeta potential around −55 mV (Kim
diameter (∼v1/3, where v is the volume of particles), the attracand Sigmund, 2004) due to carboxylation of the surface of the
tion between anisometric particles, such as plates, rectanguMWCNTs (Smith et al., 2009). Brant et al., Cheng et al., and
lar rods, and cylinders, is larger than for spherical particles of
Chen et al. have shown that stable fullerene clusters with negaequal volume because a greater number of atoms are in close
tive charge of approximately −40 to −80 mV (from pH 2 to
proximity (Vold, 1954). The attraction between spheres, rods
12) were formed from mixing fullerene clusters in water for a
(and cylinders), and platelets varies as s−1, s−2, and s−3, respecperiod of several weeks (Brant et al., 2005; Chen and Elimelech,
tively, where s is separation distance (Vold, 1954). For cylindri2009; Cheng et al., 2004). The nature of these charges is under
cal particles, parallel orientation is energetically favored over a
investigation. These examples demonstrate that core-shell
perpendicular orientation at small distances. For rectangular
structures, the presence of coatings, and chemical transformarods or platelets, an orientation with the largest faces oppotions impart charges to NP surfaces. These charge sources, or
site each other is the most favorable energetically. Electrostatic
combinations thereof, are not accounted for in colloid science.
double-layer forces of cylindrical particles are theoretically a
Understanding the fate of NPs in the environment therefore
function of particle orientation (Vold, 1954). These results
requires a fundamental comprehension of the types of transimply that shape can theoretically control aggregation under
formations that the NPs may undergo during manufacturing,
some favored orientations. For example, Kozan et al. (2008)
under commercial use, and after release into the environment
studied the aggregation of tungsten trioxide (WO3) nanowire
(as discussed later in this review).
suspensions of different types (uneven, single, or bundled in
1914
Journal of Environmental Quality • Volume 39 • November–December 2010
diameter) and dimensions (diameter of 40–200 nm and nominal lengths of 2, 4, 6, and 10 μm) in various polar solvents
and compared the aggregate morphology of the nanowires with
that of spherical WO3 NPs (40 nm in diameter) (Kozan et al.,
2008). The aggregates of spherical WO3 NPs were more compact and more spherical in shape (Df ~ 2.6 and Rg ~3.8 μm)
than aggregates of nanowires with diameter of 200 nm (Df ~
2.3 and Rg ~2.2 μm). Moreover, aggregation of the nanowires for 7 d in a quiescent environment resulted in no significant Rg increase but an increase in Df (Kozan et al., 2008).
This reorganization is related to the theoretical prediction that
anisometric particles are likely to aggregate under a secondary
minimum (Vold, 1954). The degree of secondary minimum
aggregation is dependent on shape and is most favorable for
platelets, less favorable for rods and cylinders, and least favorable for spherical NPs (Vold, 1954). Loose aggregates formed
in the secondary minimum might gradually restructure into
denser aggregates in the primary minimum. Nanoparticless
designed with a myriad of unique shapes (Cai and Sandhage,
2005; Kawano and Imai, 2008; Sanchez-Iglesias et al., 2009)
therefore challenge colloid science to understand their aggregation behavior in the environment because of the model complexity of understanding these shapes in a DLVO framework.
Although nonconventional theories, such as surface element
integration (Bhattacharjee and Elimelech, 1997), may account
for interfacial forces in irregular shapes (e.g., ellipsoids), these
theories are difficult to apply to the unique (e.g., helical) and
diverse types of nanomaterial shapes.
Surfactants and Macromolecular Surface Coatings
Nanomaterials are commonly manufactured with surface
coatings (e.g., surfactants, polymers, and polyelectrolytes) to
enhance dispersion stability or to provide specific functionality
(He et al., 2007; Hezinger et al., 2008; Hydutsky et al., 2007;
Mayya et al., 2003; Phenrat et al., 2008; Saleh et al., 2008;
Saleh et al., 2005; Zhang et al., 2002). Table 2 summarizes
some examples of surface-coated NPs and their applications.
Adsorbed or covalently bound surfactants prevent aggregation
and enhance dispersion stability of NPs by increasing surface
charge and electrostatic repulsion or by reducing interfacial
energy between particle and solvent (Rosen, 2004). Coulombic
attractions (Rosen, 2004; Vaisman et al., 2006; Wang et al.,
2004), hydrophobic interactions (Vaisman et al., 2006), and
surfactant concentration (Vaisman et al., 2006) affect the
adsorbed surfactant mass and layer conformation and hence
the ability of a surfactant to stabilize a NP against aggregation. Table 3 summarizes the properties of surfactants that
affect their ability to stabilize NPs against aggregation. Lowmolecular-weight surfactants reversibly adsorb on particle surfaces (Rosen, 2004), so, without a strong interaction between
the surfactant and NP surface, the surfactant may desorb or be
displaced by higher-molecular-weight natural polymers such as
NOM when the particles are released into the environment.
Therefore, changes in the stability of surfactant-coated NPs are
expected on release to the environment.
Polymers are organic macromolecules consisting of repetitions of smaller monomer chemical units (Fleer et al., 1993).
Table 2. Examples of surface coated nanoparticles and their applications.
Particle type
Quantum dots
Surface modifier
amphiphilic alkyl-modified
polyacrylic acid
Purpose
alter particle hydrophilicity and modify quantum
dot interactions with other biological compounds
References
Luccardini et al. (2006)
Titanium dioxide
polyacrylic acid
stabilize dispersions and modify surface
chemistry allowing attachment of an antibody
for a substrate selective photocatalytic system
Kanehira et al. (2008)
Fe0/Fe-oxide
poly(styrene sulfonate), polyaspartate,
carboxymethyl cellulose, or triblock
copolymers
inhibition of aggregation, decreasing adhesion
to solid surfaces, increasing mobility in the
subsurface, and targeting specific pollutants
He et al. (2009), Kanel et al. (2008),
Phenrat et al. (2008), Phenrat et
al. (2009c), Phenrat et al. (2009a),
Saleh et al. (2005)
Carbon nanotubes
various kinds of anionic, cationic,
nonionic surfactants and polymers
individually stabilized, single-walled carbon
nanotubes
Moore et al. (2003)
Silver
daxad 19 (sodium salt of a highmolecular-weight naphthalene
sulfonate formaldehyde condensate)
stabilize dispersions silver nanoparticles
Sondi et al. (2003)
Table 3. Examples of surfactant properties and their relationship to particle stabilization.
Property
Surfactant
chain length
Molecular weight
Findings
A linear relationship exists between the surfactant chain length and the logarithm of
the dispersion concentration (cdc), where cdc is defined as the lowest concentration of a
surfactant necessary to disperse hydrophobic particles.
Stabilization of CNT† with nonionic surfactants increased with molecular weight (e.g., Brij
78 [MW = 1198 g mole−1] stabilized 4.3% of CNT, while Brij 700 [MW = 4670 g mole−1] could
stabilize 6.4% of CNT)
Type of ionic
head groups
At similar molecular weight, cationic surfactants such as dodecyltrimethylammonium
bromide (MW = 308 g mole−1) and cetyltrimethylammonium bromide (MW =
364 g mole−1) were slightly more effective in stabilizing CNT than anionic sodium
dodecylbenzenesulfonate (MW = 348 g mole−1).
Self-assembly
structure
Sodium dodecyl sulfate, anionic surfactant, forms cylindrical micelles as helices or double
helices and hemicellar structures on CNT. Random adsorption of surfactants on CNT
hypothesized to enhance stabilization of the CNT dispersions.
References
Dederichs et al. (2009)
Moore et al. (2003)
Moore et al. (2003)
O’Connell et al. (2002), Richard
et al. (2003), Yurekli et al. (2004)
† CNT, carbon nanotube.
Hotze et al.: Nanoparticle Aggregation: Environmental Transport and Reactivity
1915
They can be synthetic (e.g., polyethylene glycol) or naturally
occurring (e.g., biomacromolecules, such as proteins and polysaccharides) (Brewer et al., 2005; Jain et al., 2005; Liu et al.,
2006). They also are surface active agents but are generally of
higher molecular weight than surfactants discussed previously.
Unlike surfactants, the repeating monomer units can adsorb
to NPs at multiple places along the polymer chain, making
polymer adsorption stronger than surfactant adsorption and
relatively irreversible. For example, Kim et al. (2009) modified NZVI by physisorption of various types of strong and
weak polyelectrolytes and found that <30% by mass of polymer desorbed from the NPs over a period of 4 mo (Kim et al.,
2009). Polyelectrolytes are charged polymers. Their charge is
derived from ionizable groups built into the polymer structure
(e.g., from -COO−, -SO4−, or -SO3− groups) (Fleer et al., 1993).
Adsorbed polymers and polyelectrolytes provide electrosteric
repulsions that prevent NP aggregation. These repulsive forces
have osmotic (VOSM) and elastic-steric (Velas) components due
to their charge and large molecular weight, respectively (Fig.
4). The magnitude of the electrosteric repulsion is governed
by the surface excess (adsorbed mass of polymers per unit surface area of a NP), polymer molecular weight, polymer density, adsorbed layer thickness, and solution composition (see
expressions in Table 1 and examples in Tables 4 and 5) (Fritz
et al., 2002; Ortega-Vinuesa et al., 1996; Phenrat et al., 2008;
Romero-Cano et al., 2001).
Surfactant–NP and polymer–NP interactions are extremely
complex and often kinetically controlled rather than being
in thermodynamic equilibrium. The science describing or
predicting adsorbed mass and adsorbed layer conformation
is immature, and the adsorbed mass and layer conformation
depends on NP properties and solution conditions (pH, ionic
strength, ionic composition). Therefore, predicting the effects
of adsorbed surfactants, polymers, or polyelectrolytes on NP
behavior in the environment is challenging.
Effect of Nanoparticle Aggregation on
Applications and Implications
for the Environment
Understanding NP aggregation under well controlled solution
conditions in the laboratory is difficult because NP properties
present challenges for colloid science. However, ultimately we
would like to comprehend the fate, transport, reactivity, and
risks associated with manufactured NPs released into the environment. How do these intrinsic challenges in understanding
aggregation affect our understanding of NP fate, transport, and
effects in more complicated environmental settings? Figure 5
Fig. 4. Osmotic plus elastic repulsive forces (VOSM + Velas) of polyelectrolytes are dependent on surface layer thickness.
summarizes some of the environmental interactions of NPs
that affect or are affected by aggregation and are discussed next.
Effect of Solution Chemical Composition on
Aggregation: pH and Ionic Solutes
pH and dissolved ionic solutes play crucial roles on aggregation
of NPs in engineered and natural systems. These parameters can
be controlled in laboratory settings but may vary spatially and
temporally in natural systems. Spatial heterogeneity of minerals in the subsurface alters the concentration of ionic species
present in different systems (e.g., groundwater versus surface
water). Variation in aquatic environment pH stems from the
same phenomenon. A better understanding of how pH and dissolved ionic solutes affect the behavior of organic macromolecule–coated NPs in the environment is needed. Surface charge
titration and EDL screening are the two primary ways that
pH and ionic solutes promote NP aggregation. Most NP surfaces have surface functional groups (e.g., hydroxide and oxide
groups) that are titratable by H+ or OH−. Excess of H+ (low
pH) normally results in a positively charged particle surface,
Table 4. Examples of polymer properties and their relationship to particle stabilization.
Polymer property
Adsorbed layer thickness and surface excess
Molecular weight
Condition of polymer adsorption: heat treatment
Condition of polymer adsorption: solvent property
1916
Findings
Correlate to dispersion stability of polymer modified Fe0–Fe
oxide nanoparticles.
Dispersion stability of carbon nanotubes increased as the
molecular weight of Pluronic increased.
Adsorption of polyacrylic acid on TiO2 enhanced at 150°C
versus 20°C. This resulted in the enhanced dispersion stability.
More polyacrylic acid adsorbed on TiO2 in dimethylformamide
than in water.
References
Phenrat et al. (2008)
Moore et al. (2003)
Kanehira et al. (2008)
Kanehira et al. (2008)
Journal of Environmental Quality • Volume 39 • November–December 2010
Table 5. Examples of particle stabilization and destabilization by natural organic matter and biomolecules.
System
TiO2 (positively charged)
at pH 2 to 6 with 1 mg L−1 FA†
TiO2 (negatively charged)
pH > PZC with 1 mg L−1 FA
Findings
Disaggregation of TiO2, resulting in a smaller particle size (3 nm) than the
original bare TiO2 obtained from the manufacturer (10 nm).
Flocculation of TiO2 due to bridging FA was observed. Aggregation decreased
with increasing ionic concentration. High ionic concentration screened the
negative charges of FA and TiO2, resulting in greater FA adsorption onto TiO2
promoting stabilization, not bridging.
C60 or CNT with HA (CaCl2
concentration <40 mmol L−1)
Humic acid as low as 1 mg L−1 increased dispersion stability of C60 up to 5 mg L−1.
Humic acid concentrations increased the dispersion stability of CNTs from 5 to
300 mg L−1.
Unadsorbed HA aggregated through intermolecular bridging via calcium
complexation. Aggregates subsequently bridged C60, resulting in flocculation.
Gold nanoparticles coated with BSA were stable as colloids at the PZC of bare
AuNPs, suggesting steric stabilization due to adsorbed BSA layers.
Enhanced aggregation of lysozyme–AuNP assemblies at physiological pH
at very low lysozyme concentrations (16 nmol L−1). Au nanoparticles were
found embedded in the lysozyme matrix.
C60 with HA (CaCl2 concentration
>40 mmol L−1)
AuNPs with BSA
AuNPs with lysozymes
References
Domingos et al. (2009)
Domingos et al. (2009)
Chappell et al. (2009),
Chen and Elimelech (2007)
Chen and Elimelech (2007)
Brewer et al. (2005)
Zhang et al. (2009)
† AuNP, gold nanoparticle; BSA, bovine serum albumin; CNT, carbon nanotubes; FA, fulvic acid; HA, humic acid; PZC, point of zero charge.
Fig. 5. Hypothesized effects of the environment on nanoparticle aggregation and how aggregation state could alter nanoparticle (NP) environmental interactions. (A) pH and ions. Increasing acidity can lead to charge neutralization at the point of zero charge. Increasing ionic strength
reduces the size of the electrostatic double layer and repulsive forces. Certain ions (e.g., Ca2+) can bridge functional groups on the surface of
nanoparticles. (B) Heteroaggregation. Macromolecules present in the environment coat nanomaterials leading to a steric effect. Larger macromolecules trap NPs in a mesh or gel causing stabilization or destabilization. Nanoparticles also associate with other particles such as clays or
biocolloids. Nanoparticles flow through porous media depending on their aggregation state. (C) Biological interactions. Cells activate phagocytosis mechanisms depending on if a particle is aggregated or not. Nanoparticles containing metals aggregate at the surface of organisms and
release metal ions at varying rates. (D) Transformations. Oxidants or sunlight degrade particle coatings or the particles themselves. Aggregated
photoactive NPs only have the outside particles active.
Hotze et al.: Nanoparticle Aggregation: Environmental Transport and Reactivity
1917
whereas excess of OH− (high pH) normally yields a negatively
charged surface. For each particular system, the pH at which
the H+ and OH− concentration causes suspended particles to
have a neutral charge is called the PZC. As pH moves toward
this point, the EDL repulsion decreases, and aggregation is promoted by vdW attraction (Fig. 5A). Charge reversal (i.e., from
negatively charged to positively charged particles) is also possible. This change would dramatically affect NP behavior because
most surfaces in the environment are negatively charged at near
neutral pH. As for the ionic species effect, high ionic concentration decreases the Debye length (κ−1) (see Table 1 for calculation of EDL using DLVO), resulting in the decrease of the
extent of EDL repulsion (Fig. 5A). The ionic concentration
where the repulsive energy barrier is completely screened and
rapid aggregation occurs is known as the critical coagulation
concentration. Table 6 summarizes factors governing nanomaterial critical coagulation concentration and dispersion sensitivity toward electrolytes. In many cases, surface titration and EDL
screening effects occur in NP dispersions as they occur for large
colloidal particles. However, there are notable exceptions where
the pHPZC depends on the NP size. Table 7 summarizes parameters altering nanoparticle PZC.
Heteroaggregation and Deposition
The environment is a complex system with multiple primary
molecules and particles as well as solution compositions.
Straightforward DLVO theory is difficult to apply to these systems, and typically XDLVO forces are brought into models to
improve results (Wu et al., 1999). At best, DLVO + XDLVO
provides qualitative information about aggregation and deposi-
tion, but this information is useful for understanding the influence of various parameters on aggregation and deposition.
Nanoparticle Interaction with Naturally Occurring Organic Matter,
Biomolecules, and Biomacromolecules
As with synthetic surface coatings used on NPs (e.g., polyelectrolytes), naturally occurring organic macromolecules (e.g.,
NOM) in the environment can significantly alter the aggregation behavior of NPs. Even without fully defined structure,
due to its macromolecular nature, NOM is expected to prevent
aggregation and deposition presumably due to electrosteric stabilization (Amirbahman and Olson, 1993; Amirbahman and
Olson, 1995). Natural organic matter consists of mainly fulvic
and humic substances. Fulvic compounds typically make up 40
to 80% of NOM compounds, and, due to their small size (∼1
kDa), they can coat the surfaces of particles. Fulvic compounds
alter the surface charge and have been shown to stabilize with
an effect similar to surfactants (Domingos et al., 2009). In fact,
NOM may stabilize MWCNTs better than surfactants commonly used for laboratory suspensions of MWCNTs (Hyung et
al., 2007). However, enhanced dispersion stability due to electrosteric forces and destabilization due to bridging have been
experimentally observed, and it is not often possible to predict
these interactions. Natural organic matter attaches to the surface of particles in a variety of ways. For example, irreversible
adsorption onto the surfaces of iron oxide via ligand exchange
between carboxyl/hydroxyl functional groups of humic acid
and iron oxide surfaces (Gu et al., 1994) or hydrophobic interactions with carbon based nanomaterials (Hyung et al., 2007)
have been reported. Whether or not attachment results in stabi-
Table 6. Factors governing nanomaterial critical coagulation concentration and dispersion sensitivity toward electrolyte concentration (ionic
strength).
Parameter
Type of ionic
species
Particle size
Type of particles
Particle shape
Surface modifier
Findings
References
Regardless of particle type, CCC† for divalent ions (e.g., CaCl2 or MgCl2)
Chen and Elimelech (2007), Evans and
is lower than for monovalent ions (e.g., NaCl).
Wennerstrom (1999), Saleh et al. (2008)
Values of CCC increase with increasing particle size (e.g., CCC values of Fe2O3 with
He et al. (2008)
diameter of 12, 32, and 65 nm have CCCs of 45, 54, and 70 mmol L−1 NaCl, respectively).
The CCC of carbon nanomaterials such as fullerene appears to be higher than that of
Chen and Elimelech (2007),
metal oxide such as Fe2O3 [e.g., CCC of C60 [dp = 60 nm] is around 120 mmol L−1 NaCl,
He et al. (2008)
whereas that of Fe2O3 [dp = 65 nm] is 70 mmol L−1 NaCl). This is presumably due to
stronger van der Waals attractions of Fe2O3 in comparison to C60, making the stability
of metal oxide nanoparticles more sensitive to screening effects.
The CCC of carbon nanotubes is lower than C60 particles. Van der Waals attraction is
greater and EDL repulsion is lower for rod-like shapes than for spherical shapes.
Due to steric repulsions polymeric surface modification decreases the sensitivity
of aggregation induced by ionic species. NZVI (dp = 162 nm) modified by guar-gum
prevented aggregation at ionic concentrations up to 0.5 mmol L−1. TiO2 modified
by PAA prevented aggregation in ionic concentrations up to 0.5 mol L−1 NaCl.
Chen and Elimelech (2007),
Saleh et al. (2008)
Kanehira et al. (2008)
† CCC, critical coagulation concentration; EDL, electrostatic double layer; NZVI, nano zero valent iron; PAA, polyacrylic acid.
Table 7. Parameters altering nanoparticle point of zero charge.
Parameter
Chemical composition
Surface modification
Particle size
Particle transformation
Findings
PZCs of metal NPs,† metal oxides, and carbon nanomaterials have a wide range
and need to be measured for each material and preparation method.
Surface modification using anionic polyelectrolytes shifts PZC toward lower
values (e.g., ~3 pH units for Fe0/Fe-oxide nanoparticles).
PZCs for hematite nanoparticles of diameter 12, 32, and 65 nm are ~7.5, 7.8,
and 8.5, respectively.
Aged Fe0/Fe oxide nanoparticles have lower PZC than fresh NPs by ~3 pH
units, presumably due to surface oxidation and hydrolysis.
References
Brant et al. (2005), Kim et al. (2009),
Kosmulski (2009), Sirk et al. 2009
Kim et al. (2009), Sirk et al. (2009)
He et al. (2009)
Kim et al. (2009)
† NP, nanoparticle; PZC, point of zero charge.
1918
Journal of Environmental Quality • Volume 39 • November–December 2010
lization or destabilization depends on several factors, including
type of NOM, NOM concentration, and solution chemistry
(Chen and Elimelech, 2007; Chen et al., 2006; Diegoli et al.,
2008; Domingos et al., 2009). Stabilization usually results from
NOM forming a charged stabilizing layer on the outside of the
particle. Destabilization, on the other hand, results from particles being bridged by larger NOM molecules, such as rigid
biopolymers (Fig. 5B, left). This occurs for relatively large-sized
NOM (~10–100 kDa) and can dominate interactions between
nanoparticles (Buffle et al., 1998). Large-molecular-weight biomolecules and biomacromolecules, including proteins, polypeptides, and amino acids (McKeon and Love, 2008; Moreau
et al., 2007; Vamunu et al., 2008), affect aggregation of NPs in
aquatic environments similarly. Table 5 summarizes reports of
NP stabilization and destabilization afforded by the presence of
NOM and biomacromolecules reported in literature.
sion stability of engineered NPs via heteroaggregation (Fig.
5B). Heteroaggregation with bacteria can prevent aggregation
or promote disaggregation of NPs. Cerium oxide NPs were
reported to attach the surface of bacteria at the maximum
adsorption capacity of 15 mg of CeO2 m−2 due to electrostatic
attraction (Thill et al., 2006). Holden et al. (2009c) showed
that TiO2 NPs were disaggregated in the presence of microorganisms (unpublished data). Heteroaggregation of NZVI
and bacteria also increases dispersion stability when compared with dispersions containing no bacteria (Li et al., 2010).
Heteroaggregation with bacteria, NOM, or mobile colloids
(e.g., clay particles) could enhance NP stability, but heteroaggregation to large enough particles could also destabilize NP
dispersions. A complete understanding of the impacts of heteroaggregation on NP behavior requires more experimentation.
Nanoparticle Interaction with Inorganic and Biocolloids
Aggregation of NPs affects their fate and transport in ground
and surface waters (Fig. 6). In the subsurface, aggregation affects
deposition processes; in surface waters, aggregation drives sedimentation processes. Homoaggregation of NPs mostly leads to
Nanoparticles released to the environment can interact with
inorganic colloids (e.g., clays, aluminoscilicates and iron oxyhydroxides) and biocolloids (e.g., bacteria), altering the disper-
Effect of Aggregation on Deposition and Sedimentation
Fig. 6. Nanoparticles are hypothesized to transport differently in the environment depending on their homo- or heteroaggregation state.
Homoaggregated particles are more likely to attach to surfaces and be removed in porous media. Homoaggregation can also lead to the formation of aggregates that are large enough to be settled out of surface waters by gravitational forces. Heteroaggregation may lead to surface particle
coatings that enhance transport in porous media. Heteroaggregation can also lead to stabilization or destabilization effects in surface waters (e.g.,
with bacteria or natural organic matter).
Hotze et al.: Nanoparticle Aggregation: Environmental Transport and Reactivity
1919
high deposition rates in porous media because of increased collisions between NP clusters and aquifer materials via sedimentation and interception as opposed to relying only on Brownian
motion. For example, NZVI injected into the subsurface at high
particle concentrations (1–10 g L−1) aggregate rapidly, and their
mobility in the subsurface suffers as a result (Kanel et al., 2008;
Phenrat et al., 2007; Saleh et al., 2007) (i.e., they do not transport far from their injection point).
There are numerous studies that suggest prevention of
aggregation improves transport in porous media. Polymeric
surface modification has been used to decrease NZVI aggregation and therefore deposition (Kanel et al., 2008; Kim et al.,
2009; Phenrat et al., 2009c; Phenrat et al., 2008; Saleh et al.,
2008). He et al. (2009) and Tiraferri and Sethi (2009) observed
enhanced mobility of NZVI modified with adsorbed carboxymethyl cellulose and guar gum (He et al., 2009; Tiraferri and
Sethi, 2009). Phenrat et al. (2009a) demonstrated that NZVI
transport was limited specifically by NP aggregation during
transport in porous media by comparing the transport of nonaggregating hematite NPs and aggregating NZVI of the same
size and surface chemistry. When NP deposition is interpreted
by classical filtration theory (i.e., assuming no aggregation
during particle transport in porous media), the effect of aggregation is neglected and can lead to error in the reported attachment coefficient. This makes comparisons between different
experiments and systems difficult.
In contrast to homoaggregation, heteroaggregation with
mobile, natural colloidal particles (e.g., clay) and with natural particles that have low density (e.g., microorganisms) can
enhance transport of NPs in porous media by decreasing the
particle collision rate. For example, the presence of 20 mg
L−1 NOM enhanced NZVI transportability in porous media
(Johnson et al., 2009). Clay particles were also used as NZVI
supports to enhance NZVI mobility (Hydutsky et al., 2007),
which indicates that heteroaggregation between clay and NZVI
enhances NZVI mobility.
In surface waters, homoaggregation can enhance sedimentation if aggregates or agglomerates reach a size where gravity
forces acting on the particles becomes significant compared
with Brownian diffusion (O’Melia, 1980). Nanoparticles with
low dispersion stability therefore tend to accumulate in the
sediment near their source (Fig. 6). Heteroaggregation, with
low-density natural colloids, may facilitate stabilization or disaggregation of NPs, which would increase their residence times
in water bodies (Fig. 6). Stabilizing or destabilizing NPs against
aggregation can have far-reaching effects. For example, cerium
oxide NPs stabilized with surfactant remained dispersed as colloids and were not effectively removed by an activated sludge
system (Limbach et al., 2008). Additionally, the removal efficiency of functionalized versus nonfunctionalized fullerene
NPs by alum coagulation was compared. Increased steric stabilization by surface functionalization prevented aggregation
and attraction with alum flocs (Hyung and Kim, 2009). In
contrast, surface coatings, such as Tween 20 on SiO2 NPs,
have been shown to enhance their flocculation and removal
during water treatment, whereas uncoated SiO2 NPs were not
removed (Jarvie et al., 2009). The ability to predict the aggregation effects of adsorbed or grafted organic macromolecular
coatings on NP surfaces is not possible with current DLVO
1920
or XDLVO theory and is an important area of environmental
nanotechnology research.
Effect of Aggregation on Nanoparticle Reactivity
and Effect of Reactivity on Aggregation State
Aggregation can alter NP reactivity, and NP aggregation state
can be altered by chemical or photochemical environmental processes (Fig. 5D). Several types of NPs are photoactive
(e.g., C60, quantum Dots, and TiO2) and can produce reactive
oxygen species (ROS). A relationship between Df and photoactivity indicates that the structure of the aggregates formed has a
role in determining ROS production and subsequent reactivity.
Fullerene (C60) is a photoactive carbon-based class of NP that
forms crystalline aggregates in water with high Df (i.e., closely
packed aggregates). This aggregation promoted triplet–triplet
annihilation and self-quenching due to close contact between
cages and severely reduced the quantum yield of C60 (Hotze
et al., unpublished data). Fullerol (hydroxylated fullerene),
which has a lower theoretical quantum yield than C60 produced
greater amounts of ROS than C60 under identical solution conditions because fullerol formed aggregates with lower Df (less
closely packed fractal aggregates), which decreased the amount
of triplet–triplet annihilation and increased ROS yield relative
to C60 (Hotze et al., unpublished data).
Aggregation has been shown to decrease the rate of dissolution of lead sulfide (PbS) NPs (Liu et al., 2008). The dissolution rate of 240 nm aggregates of PbS NPs (4.7 × 10−10 mol m−2
s−1) was an order of magnitude lower than for monodisperse
14-nm primary PbS NPs (4.4 × 10−9 mol m−2 s−1) and also
lower than for micrometer-sized particles of the same materials. In addition to aggregation decreasing available surface area
for dissolution, they attributed the retarded dissolution to inhibition in the highly confined space between the particles in
the aggregates. In confined space, water molecules arrange differently than in the bulk, increasing viscosity at the interface.
Enhanced viscosity in confined space decreases the diffusion
rate of water to the surface and therefore the dissolution rate.
They also hypothesized that the overlap of the diffusion layer
between two adjacent particles in an aggregate can reduce the
concentration gradient (i.e., the driving force for dissolution)
from NP surfaces to bulk solution.
Another example of aggregation decreasing NP reactivity
is the dechlorination of carbon tetrachloride (CT) by 9-nmdiameter magnetite NPs (Vikesland et al., 2007). The CT
dechlorination rate decreased as the degree of aggregation
of the magnetite NPs increased. By promoting aggregation
through addition of different concentrations of monovalent
or divalent cations, an inverse relationship between aggregate size (measured by DLS) and the CT dechlorination rate
was established.
Although aggregation can affect NP reactivity, NP reactivity can also affect aggregation. Clusters of C60 also undergo
photochemical oxidation and subsequent disaggregation in
water (Hou and Jafvert, 2009; Lee et al., 2009). For example, Hou and Jafvert (2009) reported that the photochemical
transformation of aqueous C60 clusters in sunlight decreased
mean aggregate size from 500 to 160 nm and produced
unidentified soluble reaction products after 65 d of exposure
Journal of Environmental Quality • Volume 39 • November–December 2010
to natural sunlight (Hou and Jafvert, 2009). Lee et al. (2009)
observed similar transformations under UV light exposure
(Lee et al., 2009). Photochemical transformations of C60
also took place during heteroaggregation with NOM (Li et
al., 2009) and enhanced the dispersion stability of NOMcoated C60 in comparison to nonilluminated samples (Li et
al., 2009). Enhanced dispersion appeared to be due to a surface erosion or dissolution-recrystallization process catalyzed
by sunlight, rather than a simple breakage of C60 from clusters (Li et al., 2009). Therefore, NPs designed to be reactive
under photoilluminated conditions (e.g., C60 or TiO2) could
have their aggregation properties altered by long-term sunlight exposure.
Chemical oxidation could act in a similar manner by altering surface coatings or the underlying NP so that the particles
gain or lose stability against aggregation (Fig. 5D). For example, ozone reacts with C60 (Fortner et al., 2007) and CNTs,
breaking the carbon cage structure and altering the stability of
the NPs against aggregation in aqueous suspension. This ozonation process has been demonstrated to render the oxidized
C60 more photoactive than its unoxidized parent aggregates
(Cho et al., 2009). Additionally, surfactants and coatings are
susceptible to these same types of oxidation processes possibly decreasing the stability of NPs in the environment due to
loss of the stabilizing coating. A better understanding of the
biological and abiotic reactions affecting NP stability against
aggregation is needed to predict their fate in the environment.
Effects of Aggregation on Biological Impacts
from Nanoparticles
Aggregation of NPs can alter their biological effects by affecting
ion release from the surface, if and where particles collect inside
an organism, and the reactive surface area of NPs (Fig. 5C).
Calculation of metal flux at cell surfaces is important for determining the bioavailability and ecotoxicity of nanomaterials. This
is especially true in the case of silver NPs where silver ion release
plays an important role in the inhibition of bacterial growth
(Taurozzi et al., 2008; Ju-Nam and Lead, 2008). Ion release
may also prove to be important for determining the impacts of
quantum dots and iron NPs, among others. Heteroaggregation
can confound understanding of metal flux because the metal
can also bind to surfaces and internal sites of naturally occurring colloids of various sizes and compositions (Buffle et al.,
2007). Solution chemistry (e.g., ionic strength and the amount
and type of NOM) controls the fractal dimension of NP aggregates, with lower Df (1.3–2.0) formed in solutions with high
NOM, and higher Df (2.3–2.7) formed in solutions without
significant NOM present (Zhang et al., 2007). Aggregate structure plays an important role in how metal ions diffuse inside of
the flocs where the kinetics of metal association and dissociation
from binding sites is slowed by orders of magnitude compared
with rates expected for free ions in solution (Buffle et al., 2007).
Understanding mechanisms by which aggregate structure affects
the dissolution and release of ions and resulting ecotoxicity (e.g.,
from silver NPs) will improve insights into the potential impacts
of such particles in the environment.
Aggregation alters the NP size distribution, in turn affecting the fate of NPs in biological systems as well as their toxicHotze et al.: Nanoparticle Aggregation: Environmental Transport and Reactivity
ity. For example, 20-nm NPs were found to deposit mostly in
the alveolar region, whereas 5- to 10-nm particles deposited
in tracheobronchial region, and particles smaller than 10 nm
accumulate mostly in the upper respiratory tract (Heyder et
al., 1986; Oberdorster et al., 2007; Swift et al., 1994). This
indicates that aggregation can alter exposure locations or
exposure pathways. Nanoparticle aggregation was also shown
to affect the mode of cellular uptake of NPs and subsequent
biological responses. For example, phagocytosis by macrophages and giant cells is a mechanism used by cells to clear
particles of a few microns or less (Oberdorster et al., 2007).
Phenrat et al. (2009b) found that aggregation affected uptake
of bare and polymer-modified NZVI particles by mammalian
glial cells. Although the primary particle sizes of bare NZVI
particles (20–100 nm) are smaller than the optimal size range
(1–2 μm) for phagocytosis, the NZVI loosely aggregated to
micrometer-sized clusters in the physiological buffer, triggering phagocytosis and subsequent uptake. Biological effects,
therefore, vary depending on whether or not a particle is
aggregated, among other confounding factors (e.g., endotoxins, adsorption of biomacromolecules, etc.). Given this fact,
understanding aggregation in biological systems is essential
for elucidating mechanisms of toxicity.
Conclusions
The ability to manipulate atoms allows scientists to create
NPs with significantly different reactivity than their microsized analogs (He et al., 2008; Kanehira et al., 2008).
Reports on NP behavior and effects to date indicate that
the aggregation behavior of the NPs will significantly affect
their fate, transport, reactivity, bioavailability, and effects in
environmental systems. Fortunately, the science of aggregation is well developed. Unfortunately, the translation of that
science into the regime of nanomaterials remains a challenge
due their unique physical and chemical properties (e.g., size,
shape, composition, and reactivity). The presence of organic
macromolecular coatings on NPs including those engineered
as part of the NP, or those that are acquired in the environment (e.g., adsorption of NOM), further complicate analysis
of NP behavior by traditional colloid science. Nanomaterial
aggregation studies, therefore, aim to take into account
how these types of novel surface properties will alter what
is already understood about aggregation science. Ultimately,
this will guide scientists’ grasp of how nanomaterials and
NPs will occur in and affect environmental systems. Much
work remains to gain a full understanding of NP aggregation
and how to apply these principles to create better applications and lower impacts of nanomaterials.
Acknowledgments
This material is based on work supported by the National Science
Foundation (NSF) and the Environmental Protection Agency (EPA)
under NSF Cooperative Agreement EF-0830093, Center for the
Environmental Implications of NanoTechnology (CEINT). Any
opinions, findings, conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily reflect the
views of the NSF or the EPA. This work has not been subjected to
EPA review, and no official endorsement should be inferred.
1921
References
Alivisatos, A. 2001. Less is more in medicine: Sophisticated forms of nanotechnology will find some of their first real-world applications in biomedical
research, disease diagnosis and, possibly, therapy. Sci. Am. 285:66–73.
Amirbahman, A., and T.M. Olson. 1993. Transport of humic matter-coated
hematite in packed beds. Environ. Sci. Technol. 27:2807–2813.
Amirbahman, A., and T.M. Olson. 1995. The role of surface conformations in
the deposition kinetics of humic matter-coated colloids in porous media.
Colloids Surf. A Physicochem. Eng. Asp. 95:249–259.
Barbot, E., P. Dussouillez, J.-Y. Bottero, and P. Moulin. 2010. Coagulation of
bentonite suspension by polyelectrolytes or ferric chloride: Floc breakage
and reformation. Chem. Eng. J. 156:83–91.
Baveye, P., and M. Laba. 2008. Aggregation and toxicology of titanium dioxide
nanoparticles. Environ. Health Perspect. 116:A152–A152.
Benn, T.M., and P. Westerhoff. 2008. Nanoparticle silver released into water from commercially available sock fabrics. Environ. Sci. Technol.
42:4133–4139.
Bhattacharjee, S., and M. Elimelech. 1997. Surface element integration: A
novel technique for evaluation of DLVO interaction between a particle
and a flat plate. J. Colloid Interface Sci. 193:273–285.
Brant, J., H. Lecoanet, M. Hotze, and M. Wiesner. 2005. Comparison of
electrokinetic properties of colloidal fullerenes (nC60) formed using two
procedures. Environ. Sci. Technol. 39:6343–6351.
Brant, J.A., J. Labille, C.O. Robichaud, and M. Wiesner. 2007. Fullerol cluster
formation in aqueous solutions: Implications for environmental release.
J. Colloid Interface Sci. 314:281–288.
Brewer, S.H., W.R. Glomm, M.C. Johnson, M.K. Knag, and S. Franzen.
2005. Probing BSA binding to citrate-coated gold nanoparticles and surfaces. Langmuir 21:9303–9307.
Buffle, J., and G. Leppard. 1995. Characterization of aquatic colloids and macromolecules: 1. Structure and behavior of colloidal material. Environ.
Sci. Technol. 29:2169–2175.
Buffle, J., K.J. Wilkinson, S. Stoll, M. Filella, and S. Zhang. 1998. A generalized description of aquatic colloidal interactions: The three-colloidal
component approach. Environ. Sci. Technol. 32:2887–2899.
Buffle, J., Z. Zhang, and K. Startchev. 2007. Metal flux and dynamic speciation at (bio)interfaces. Part I: Critical evaluation and compilation of
physicochemical parameters for complexes with simple ligands and fulvic/humic substances. Environ. Sci. Technol. 41:7609–7620.
Cai, Y., and K.H. Sandhage. 2005. Zn2SiO4-coated microparticles with biologically-controlled 3D shapes. Phys Status Solidi A 202:R105–R107.
Chakraborti, R., K. Gardner, J. Atkinson, and J. Van Benschoten. 2003. Changes in fractal dimension during aggregation. Water Res. 37:873–883.
Chappell, M.A., A.J. George, K.M. Dontsova, B.E. Porter, C.L. Price, P. Zhou,
E. Morikawa, A.J. Kennedy, and J.A. Steevens. 2009. Surfactive stabilization of multi-walled carbon nanotube dispersion with dissolved humic
substances. Environ. Pollut. 157:1081–1087.
Chen, K., and M. Elimelech. 2007. Influence of humic acid on the aggregation kinetics of fullerene (C60) nanoparticles in monovalent and divalent
electrolyte solutions. J. Colloid Interface Sci. 309:126–134.
Chen, K.L., and M. Elimelech. 2009. Relating colloidal stability of fullerene
(C60) nanoparticles to nanoparticle charge and electrokinetic properties.
Environ. Sci. Technol. 43:7270–7276.
Chen, K.-L., S.E. Mylon, and M. Elimelech. 2006. Aggregation kinetics of
alginate-coated hematite nanoparticles in monovalent and divalent electrolyte. Environ. Sci. Technol. 40:1516–1523.
Cheng, X.K., A.T. Kan, and M.B. Tomson. 2004. Naphthalene adsorption and
desorption from aqueous C60 fullerene. J. Chem. Eng. Data 49:675–683.
Cho, M., J.D. Fortner, J.B. Hughes, and J.-H. Kim. 2009. Escherichia coli
inactivation by water-soluble, ozonated C60 derivative: Kinetics and
mechanisms. Environ. Sci. Technol. 43:7410–7415.
Dederichs, T., M. Möller, and O. Weichold. 2009. Colloidal stability of hydrophobic nanoparticles in ionic surfactant solutions: Definition of the
critical dispersion concentration. Langmuir 25:2007–2012.
Delgado, A.V., F. Gonzalez-Caballero, R.J. Hunter, L.K. Koopal, and J. Lyklema. 2007. Measurement and interpretation of electrokinetic phenomena. J. Colloid Interface Sci. 309:194–224.
Derjaguin, B.V., and L. Landau. 1941. Theory of the stability of strongly
charged lyophobic sols and of the adhesion of strongly charged particles
in solutions of electrolytes. Acta Phys. Chim. URSS 14:633–662.
Diegoli, S., A.L. Manciulea, S. Begum, I.P. Jones, J.R. Lead, and J.A. Preece.
2008. Interaction between manufactured gold nanoparticles and naturally occurring organic macromolecules. Sci. Total Environ. 402:51–61.
Domingos, R.F., N. Tufenkji, and K.J. Wilkinson. 2009. Aggregation of titanium dioxide nanoparticles: Role of a fulvic acid. Environ. Sci. Technol.
1922
43:1282–1286.
Elena, D.V., M. Coisson, C. Appino, F. Vinai, and R. Sethi. 2009. Magnetic
characterization and interaction modeling of zerovalent iron nanoparticles for the remediation of contaminated aquifers. J. Nanosci. Nanotechnol. 9:3210–3218.
Elimelech, M., and C. O’Melia. 1990. Effect of particle size on collision efficiency in the deposition of Brownian particles with electrostatic energy
barriers. Langmuir 6:1153–1163.
Evans, D.F., and H. Wennerstrom. 1999. The colloidal domain: Where physics, chemistry, biology, and technology meet. Wiley-VCH, New York.
Fleer, G.J., M.A. Cohen Stuart, J.M.H.M. Scheutjens, T. Cosgrove, and B.
Vincent. 1993. Polymers at interfaces. Chapman & Hall, London, UK.
Fortner, J.D., D.-I. Kim, A.M. Boyd, J.C. Falkner, S. Moran, V.L. Colvin, J.B.
Hughes, and J.-H. Kim. 2007. Reaction of water-stable C60 aggregates
with ozone. Environ. Sci. Technol. 41:7497–7502.
French, R.A., A.R. Jacobson, B. Kim, S.L. Isley, R.L. Penn, and P.C. Baveye.
2009. Influence of ionic strength, pH, and cation valence on aggregation kinetics of titanium dioxide nanoparticles. Environ. Sci. Technol.
43:1354–1359.
Fritz, G., V. Schadler, N. Willenbacher, and N. Wagner. 2002. Electrosteric
stabilization of colloidal dispersions. Langmuir 18:6381–6390.
Gatica, S., M. Cole, and D. Velegol. 2005. Designing van der Waals forces
between nanocolloids. Nano Lett. 5:169–173.
Graf, P., A. Mantion, A. Foelske, A. Shkilnyy, A. Masic, A.E. Thuenemann,
and A. Taubert. 2009. Peptide-coated silver nanoparticles: Synthesis,
surface chemistry, and pH-triggered, reversible assembly into particle assemblies. Chem. Eur. J. 15:5831–5844.
Gu, B., J. Schmitt, Z. Chen, L. Liang, and J.F. McCarthy. 1994. Adsorption
and desorption of natural organic matter on iron oxide: Mechanisms and
models. Environ. Sci. Technol. 28:38–46.
Hahn, M., D. Abadzic, and C. O’Melia. 2004. Aquasols: On the role of secondary minima. Environ. Sci. Technol. 38:5915–5924.
Hansen, P., M. Malmsten, B. Bergenstahl, and L. Bergstrom. 1999. Orthokinetic aggregation in two dimensions of monodisperse and bidisperse
colloidal systems. J. Colloid Interface Sci. 220:269–280.
He, F., D. Zhao, J. Liu, and C.B. Roberts. 2007. Stabilization of Fe-Pd
nanoparticles with sodium carboxymethyl cellulose for enhanced transport and dechlorination of trichloroethylene in soil and groundwater.
Ind. Eng. Chem. Res. 46:29–34.
He, F., M. Zhang, T. Qian, and D. Zhao. 2009. Transport of carboxymethyl
cellulose stabilized iron nanoparticles in porous media: Column experiments and modeling. J. Colloid Interface Sci. 334:96–102.
He, Y.T., J. Wan, and T. Tokunaga. 2008. Kinetic stability of hematite
nanoparticles: The effect of particle sizes. J. Nanopart. Res. 10:321–332.
Hermansson, M. 1999. The DLVO theory in microbial adhesion. Colloids
Surf. B Biointerfaces 14:105–119.
Heyder, J., J. Gebhart, G. Rudolf, and C. Schiller. 1986. Deposition of particles in the human respiratory tract in the size range 0, 005–15 μm. J.
Aerosol Sci. 17:811–825.
Hezinger, A.F.E., J. Tessmar, and A. Göpferich. 2008. Polymer coating of
quantum dots: A powerful tool toward diagnostics and sensorics. Eur. J.
Pharm. Biopharm. 68:138–152.
Hiemenz, P.C., and R. Rajagopalan. 1997. Principles of colloid and surface
chemistry. CRC, New York.
Hochella, M.F., S.K. Lower, P.A. Maurice, R.L. Penn, N. Sahai, D.L. Sparks,
and B.S. Twining. 2008. Nanominerals, mineral nanoparticles, and earth
systems. Science 21:1631–1635.
Hoek, E., and G. Agarwal. 2006. Extended DLVO interactions between spherical particles and rough surfaces. J. Colloid Interface Sci. 298:50–58.
Hotze, E.M., J. Labille, P. Alvarez, and M.R. Wiesner. 2008. Mechanisms of
photochemistry and reactive oxygen production by fullerene suspensions
in water. Environ. Sci. Technol. 42:4175–4180.
Hou, W.-C., and C.T. Jafvert. 2009. Photochemical transformation of aqueous
C60 clusters in sunlight. Environ. Sci. Technol. 43:362–367.
Hydutsky, B.W., E.J. Mack, B.B. Beckerman, J.M. Skluzacek, and T.E. Mallouk. 2007. Optimization of nano- and microiron transport through
sand columns using polyelectrolyte mixtures. Environ. Sci. Technol.
41:6418–6424.
Hyung, H., J. Fortner, J. Hughes, and J. Kim. 2007. Natural organic matter
stabilizes carbon nanotubes in the aqueous phase. Environ. Sci. Technol.
41:179–184.
Hyung, H., and J.-H. Kim. 2009. Dispersion of C60 in natural water and removal by conventional drinking water treatment processes. Water Res.
43:2463–2470.
Israelachvili, J.N. 1991. Intermolecular & surface forces. Academic Press,
Journal of Environmental Quality • Volume 39 • November–December 2010
New York.
Jain, T.K., M.A. Morales, S.K. Sahoo, D.L. Leslie-Pelecky, and V. Labhasetwar. 2005. Iron oxide nanoparticles for sustained delivery of anticancer
agents. Mol. Pharm. 2:194–205.
Jarvie, H., H. Al-Obaidi, S. King, M. Bowes, M. Lawrence, A. Drake, M.
Green, P. Dobson. 2009. Fate of silica nanoparticles in simulated primary wastewater treatment. Environ. Sci. Technol. 43:8622–8628.
Jiang, J., G. Oberdorster, and P. Biswas. 2009. Characterization of size, surface
charge, and agglomeration state of nanoparticle dispersions for toxicological studies. J. Nanopart. Res. 11:77–89.
Johnson, R.L., G.O. Johnson, J.T. Nurmi, and P.G. Tratnyek. 2009. Natural
organic matter enhanced mobility of nano zerovalent iron. Environ. Sci.
Technol. 43:5455–5460.
Ju-Nam, Y., and J.R. Lead. 2008. Manufactured nanoparticles: An overview of
their chemistry, interactions and potential environmental implications.
Sci. Total Environ. 400:396–414.
Kanehira, K., T. Banzai, C. Ogino, N. Shimizu, Y. Kubota, and S. Sonezaki.
2008. Properties of TiO2–polyacrylic acid dispersions with potential for
molecular recognition. Colloids Surf. B Biointerfaces 64:10–15.
Kanel, S.R., R.R. Goswami, T.P. Clement, M.O. Barnett, and D. Zhao.
2008. Two dimensional transport characteristics of surface stabilized
zero-valent iron nanoparticles in porous media. Environ. Sci. Technol.
42:896–900.
Kawano, T., and I. Hiroaki. 2008. A simple preparation technique for shapecontrolled zinc oxide nanoparticles: Formation of narrow size-distributed nanorods using seeds in aqueous solutions. Colloid Surface A
319:130–135.
Kim, B., and W.M. Sigmund. 2004. Functionalized multiwall carbon nanotube/gold nanoparticle composites. Langmuir 20:8239–8242.
Kim, H.-J., T. Phenrat, R.D. Tilton, and G.V. Lowry. 2009. Fe0 nanoparticles
remain mobile in porous media after aging due to slow desorption of
polymeric surface modifiers. Environ. Sci. Technol. 43:3824–3830.
Kim, J., D. Kim, H. Jung, and K. Hong. 2005. Influence of anatase-rutile
phase transformation on dielectric properties of sol-gel derived TiO2 thin
films. Jpn. J. Appl. Phys. 1:6148–6151.
Koponen, I.T. 2009. Cluster growth poised on the edge of break-up, II: From
reaction kinetics to thermodynamics. Physica A 388:2659–2665.
Kosmulski, M. 2009. pH-dependent surface charging and points of zero charge.
IV. Update and new approach. J. Colloid Interface Sci. 337:439–448.
Kozan, M., J. Thangala, R. Bogale, M.P. Menguc, and M.K. Sunkara. 2008.
In-situ characterization of dispersion stability of WO3 nanoparticles and
nanowires. J. Nanopart. Res. 10:599–612.
Lebrette, S., C. Pagnoux, and P. Abélard. 2004. Stability of aqueous TiO2 suspensions: Influence of ethanol. J. Colloid Interface Sci. 280:400–408.
Lee, J., M. Cho, J.D. Fortner, J.B. Hughes, and J.-H. Kim. 2009. Transformation of aggregated C60 in the aqueous phase by UV irradiation. Environ.
Sci. Technol. 43:4878–4883.
Li, Q., B. Xie, Y.S. Hwang, and Y. Xu. 2009. Kinetics of C60 fullerene dispersion in water enhanced by natural organic matter and sunlight. Environ.
Sci. Technol. 43:3574–3579.
Li, Z., K. Greden, P. Alvarez, K. Gregory, and G.V. Lowry. 2010. Adsorbed
polymer and NOM limits adhesion and toxicity of nano scale zero-valent
iron (NZVI) to E. coli. Environ. Sci. Technol. (in press).
Limbach, L.K., R. Bereiter, E. Muller, R. Krebs, R. Glli, and W.J. Stark. 2008.
Removal of oxide nanoparticles in a model wastewater treatment plant:
Influence of agglomeration and surfactants on clearing efficiency. Environ. Sci. Technol. 42:5828–5833.
Limbach, L.K., Y.C. Li, R.N. Grass, T.J. Brunner, M.A. Hintermann, M.
Muller, D. Gunther, and W.J. Stark. 2005. Oxide nanoparticle uptake
in human lung fibroblasts: Effects of particle size, agglomeration, and
diffusion at low concentrations. Environ. Sci. Technol. 39:9370–9376.
Liu, J., D.M. Aruguete, J.R. Jinschek, J.D. Rimstidt, and M.F. Hochella.
2008. The non-oxidative dissolution of galena nanocrystals: Insights into
mineral dissolution rates as a function of grain size, shape, and aggregation state. Geochim. Cosmochim. Acta 72:5984–5996.
Liu, Y., S.A. Majetich, R.D. Tilton, D.S. Sholl, and G.V. Lowry. 2005. TCE
dechlorination rates, pathways, and efficiency of nanoscale iron particles
with different properties. Environ. Sci. Technol. 39:1338–1345.
Liu, Z., W. Cai, L. He, N. Nakayama, and K. Chen. 2006. In vivo biodistribution and highly efficient tumour targeting of carbon nanotubes in mice.
Nature Nanotechnol. 2:47–52.
Luccardini, C., C. Tribet, F. Vial, V. Marchi-Artzner, and M. Dahan. 2006.
Size, charge, and interactions with giant lipid vesicles of quantum dots
coated with an amphiphilic macromolecule. Langmuir 22:2304–2310.
Ma, D., X. Hu, H. Zhou, J. Zhang, and Y. Qian. 2007. Shape-controlled
Hotze et al.: Nanoparticle Aggregation: Environmental Transport and Reactivity
synthesis and formation mechanism of nanoparticles-assembled Ag2S
nanorods and nanotubes. J. Cryst. Growth 304:163–168.
Mayya, K.S., B. Schoeler, and F. Caruso. 2003. Preparation and organization
of nanoscale polyelectrolyte-coated gold nanoparticles. Adv. Funct. Mater. 13:183–188.
McKeon, K.D., and B.J. Love. 2008. The presence of adsorbed proteins on
particles increases aggregated particle sedimentation, as measured by a
light scattering technique. J. Adhes. 84:664–674.
Montgomery, S., M. Franchek, and V. Goldschmidt. 2000. Analytical dispersion force calculations for nontraditional geometries. J. Colloid Interface
Sci. 227:567–584.
Moon, S.Y., T. Kusunose, and T. Sekino. 2009. CTAB-assisted synthesis of
size- and shape-controlled gold nanoparticles in SDS aqueous solution.
Mater. Lett. 63:2038–2040.
Moore, V.C., M.S. Strano, E.H. Haroz, R.H. Hauge, R.E. Smalley, J. Schmidt,
and Y. Talmon. 2003. Individually suspended single-walled carbon
nanotubes in various surfactants. Nano Lett. 3:1379–1382.
Moreau, J.W., P.K. Weber, M.C. Martin, B. Gilbert, I.D. Hutcheon, and J.F.
Banfield. 2007. Extracellular proteins limit the dispersal of biogenic
nanoparticles. Science 316:1600–1603.
O’Connell, M.J., S.M. Bachilo, C.B. Huffman, V.C. Moore, M.S. Strano, E.H.
Haroz, K.L. Rialon, P.J. Boul, W.H. Noon, C. Kittrell, J.P. Ma, R.H.
Hauge, R.B. Weisman, and R.E. Smalley. 2002. Band gap fluorescence
from individual single-walled carbon nanotubes. Science 297:593–596.
O’Melia, C.R. 1980. ES&T features. Aquasols: The behavior of small particles
in aquatic systems. Environ. Sci. Technol. 14:1052–1060.
Oberdorster, G., V. Stone, and K. Donaldson. 2007. Toxicology of nanoparticles: A historical perspective. Nanotoxicology 1:2–25.
Ortega-Vinuesa, J., A. Martín-Rodríguez, and R. Hidalgo-Alvarez. 1996. Colloidal stability of polymer colloids with different interfacial properties:
Mechanisms. J. Colloid Interface Sci. 184:259–267.
Phenrat, T., H.-J. Kim, F. Fagerlund, T. Illangasekare, R.D. Tilton, and G.V.
Lowry. 2009a. Particle size distribution, concentration, and magnetic attraction affects transport of polymer-modified Fe0 nanoparticles in sand
columns. Environ. Sci. Technol. 43:5079–5085.
Phenrat, T., T. Long, G. Lowry, and B. Veronesi. 2009b. Partial oxidation (“aging”) and surface modification decrease the toxicity of nanosized zerovalent iron. Environ. Sci. Technol. 43:195–200.
Phenrat, T., Y. Liu, R. Tilton, and G. Lowry. 2009c. Adsorbed polyelectrolyte
coatings decrease Fe0 nanoparticle reactivity with TCE in water: Conceptual model and mechanisms. Environ. Sci. Technol. 43:1507–1514.
Phenrat, T., N. Saleh, K. Sirk, H. Kim, R. Tilton, and G. Lowry. 2008.
Stabilization of aqueous nanoscale zerovalent iron dispersions by anionic polyelectrolytes: Adsorbed anionic polyelectrolyte layer properties and their effect on aggregation and sedimentation. J. Nanopart.
Res. 10:795–814.
Phenrat, T., N. Saleh, K. Sirk, R.D. Tilton, and G.V. Lowry. 2007. Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions.
Environ. Sci. Technol. 41:284–290.
Project on Emerging Nanotechnologies. 2009. Nanotechnology consumer
products inventory. Available at http://www.nanotechproject.org (verified 22 Apr. 2010). Project on Emerging Nanotechnologies, Washington, DC.
Richard, C., F. Balavoine, P. Schultz, T.W. Ebbesen, and C. Mioskowski. 2003.
Supramolecular self-assembly of lipid derivatives on carbon nanotubes.
Science 300:775–778.
Romero-Cano, M., A. Martin-Rodriguez, and F. de las Nieves. 2001. Electrosteric stabilization of polymer colloids with different functionality.
Langmuir 17:3505–3511.
Rosen, M.J. 2004. Surfactants and interfacial phenomena. 3rd ed. Wiley-Interscience, New York.
Saleh, N., H.-J. Kim, T. Phenrat, K. Matyjaszewski, R.D. Tilton, and G.V.
Lowry. 2008. Ionic strength and composition affect the mobility of
surface-modified Fe0 nanoparticles in water-saturated sand columns. Environ. Sci. Technol. 42:3349–3355.
Saleh, N., T. Phenrat, K. Sirk, B. Dufour, J. Ok, T. Sarbu, K. Matyiaszewski,
R.D. Tilton, and G.V. Lowry. 2005. Adsorbed triblock copolymers deliver reactive iron nanoparticles to the oil/water interface. Nano Lett.
5:2489–2494.
Saleh, N., K. Sirk, Y. Liu, T. Phenrat, B. Dufour, K. Matyjaszewski, R.D. Tilton, and G.V. Lowry. 2007. Surface modifications enhance nanoiron
transport and DNAPL targeting in saturated porous media. Environ.
Eng. Sci. 24:45–57.
Sanchez-Iglesias, A., M. Grzelczak, B. Rodriguez-Gonzalez, R.A. AlvarezPuebla, L.M. Liz-Marzan, and N.A. Kotov. 2009. Gold colloids with
unconventional angled shapes. Langmuir 25:11431–11435.
1923
Sirk, K.M., N.B. Saleh, T. Phenrat, H.-J. Kim, B. Dufour, J. Ok, P.L. Golas, K.
Matyjaszewski, G.V. Lowry, and R.D. Tilton. 2009. Effect of adsorbed
polyelectrolytes on nanoscale zero valent iron particle attachment to soil
surface models. Environ. Sci. Technol. 43:3803–3808.
Smith, B., K. Wepasnick, K.E. Schrote, H.-H. Cho, W.P. Ball, and D.H. Fairbrother. 2009. Influence of surface oxides on the colloidal stability of
multi-walled carbon nanotubes: A structure-property relationship. Langmuir 25:9767–9776.
Sondi, I., D.V. Goia, and E. Matijevic. 2003. Preparation of highly concentrated stable dispersions of uniform silver nanoparticles. J. Colloid Interface Sci. 260:75–81 .
Stumm, W., and J.J. Morgan. 1996. Aquatic chemistry: Chemical equilibria
and rates in natural waters. 3rd ed. John Wiley & Sons, New York.
Swift, D., Y. Cheng, Y. Su, and H. Yeh. 1994. Ultrafine aerosol deposition in
the human nasal and oral passages. Ann. Occup. Hyg. 38:77–81.
Tabor, D., and R. Winterton. 1969. The direct measurement of normal and
retarded van der Waals forces. Proceed. R. Soc. London A. 312:435–450.
Taurozzi, J.S., H. Arul, V.Z. Bosak, A.F. Burban, T.C. Voice, M.L. Bruening, and V.V. Tarabara. 2008. Effect of filler incorporation route on the
properties of polysulfone-silver nanocomposite membranes of different
porosities. J. Membr. Sci. 325:58–68.
Thill, A., O. Zeyons, O. Spalla, F. Chauvat, J. Rose, M. Auffan, and A.M.
Flank. 2006. Cytotoxicity of CeO2 nanoparticles for Escherichia coli:
Physico-chemical insight of the cytotoxicity mechanism. Environ. Sci.
Technol. 40:6151–6156.
Thompson Reuters. 2009. Web of knowledge. Available at http://WebofKnowledge.com (verified 22 Apr. 2009). Thompson Reuters, Philadelphia, PA.
Tiraferri, A., and R. Sethi. 2009. Enhanced transport of zerovalent iron
nanoparticles in saturated porous media by guar gum. J. Nanopart. Res.
11:635–645.
Tohver, V., J.E. Smay, A. Braem, P.V. Braun, and J.A. Lewis. 2001. Nanoparticle halos: A new colloid stabilization mechanism. Proc. Natl. Acad. Sci.
USA 98:8950–8954.
Vaisman, L., H.D. Wagner, and G. Marom. 2006. The role of surfactants in dispersion of carbon nanotubes. Adv. Colloid Interface Sci.
128–130:37–46.
Vamunu, C.I., P.J. Hol, Z.E. Allouni, S. Elsayed, and N.R. Gjerdet. 2008.
Formation of potential titanium antigens based on protein binding to
1924
titanium dioxide nanoparticles. Int. J. Nanomedicine 3:69–74.
Verwey, E., J. Overbeek, and K. van Nes. 1948. The theory of the stability
of liophobic colloids: The interaction of sol particles having an electric
double layer. Elsevier, Amsterdam.
Vikesland, P.J., A.M. Heathcock, R.L. Rebodos, and K.E. Makus. 2007. Particle size and aggregation effects on magnetite reactivity toward carbon
tetrachloride. Environ. Sci. Technol. 41:5277–5283.
Viota, J.L., F. González-Caballero, J.D.G. Durán, and A.V. Delgado. 2007.
Study of the colloidal stability of concentrated bimodal magnetic fluids.
J. Colloid Interface Sci. 309:135–139.
Vold, M.J. 1954. Van der Waals’ attraction between anisometric particles. J.
Colloid Sci. 9:451–459.
Wang, G.-H., W.-C. Li, K.-M. Jia, B. Spliethoff, F. Schüth, and A.-H. Lu.
2009. Shape and size controlled -Fe2O3 nanoparticles as supports for
gold-catalysts: Synthesis and influence of support shape and size on catalytic performance. Appl. Catal. A 364:42–47.
Wang, W., B. Gu, L. Liang, and W.A. Hamilton. 2004. Adsorption and
structural arrangement of cetyltrimethylammonium cations at the silica
nanoparticle-water interface. J. Phys. Chem. B 108:17477–17483.
Wu, W., R.F. Giese, and C.J. van Oss. 1999. Stability versus flocculation of
particle suspensions in water-correlation with the extended DLVO approach for aqueous systems, compared with classical DLVO theory. Colloids Surf. B Biointerfaces 14:47–55.
Yurekli, K., C.A. Mitchell, and R. Krishnamoorti. 2004. Small-angle neutron
scattering from surfactant-assisted aqueous dispersions of carbon nanotubes. J. Am. Chem. Soc. 126:9902–9903.
Zhang, D., O. Neumann, H. Wang, V.M. Yuwono, A. Barhoumi, M. Perham, J.D. Hartgerink, P. Wittung-Stafshede, and N.J. Halas. 2009. Gold
nanoparticles can induce the formation of protein-based aggregates at
physiological pH. Nano Lett. 9:666–671.
Zhang, Y., N. Kohler, and M. Zhang. 2002. Surface modification of superparamagnetic magnetite nanoparticles and their intracellular uptake.
Biomaterials 23:1553–1561.
Zhang, Z., J. Buffle, and D. Alemani. 2007. Metal flux and dynamic speciation
at (bio)interfaces. Part II: Evaluation and compilation of physicochemical parameters for complexes with particles and aggregates. Environ. Sci.
Technol. 41:7621–7631.
Journal of Environmental Quality • Volume 39 • November–December 2010