Article
Effect of natural organic matter on the disagglomeration of
manufactured TiO2 nanoparticles
LOOSLI, Frédéric, LE COUSTUMER, Philippe, STOLL, Serge
Abstract
One major concern in the fate of nanomaterials in aquatic systems is the lack of data on
nanomaterial transformations under relevant environmental conditions. The disagglomeration
of aggregates composed of manufactured anatase titanium dioxide nanoparticles is
investigated here in the presence of alginate and Suwannee River humic acids at varying
concentrations using dynamic light scattering and electrophoretic measurements. Stability of
TiO 2 nanoparticle agglomerates at typical environmental concentrations of natural organic
matter is discussed at a pH value corresponding to the point of zero charge of TiO 2
nanoparticles. In this scenario, the surface charge of TiO 2 is neutralized, allowing the
nanoparticles to form large agglomerates. Alginate and Suwannee River humic acids exhibit a
negative structural charge under this pH condition and adsorption of both natural
polyelectrolytes on the surface of nanoparticle agglomerates leads to disagglomeration and
significant redispersion of TiO 2 nanoparticles into fragments. Results indicate that both
electrostatic forces and steric interactions play key roles during the [...]
Reference
LOOSLI, Frédéric, LE COUSTUMER, Philippe, STOLL, Serge. Effect of natural organic matter
on the disagglomeration of manufactured TiO2 nanoparticles. Environmental Science: Nano,
2014
DOI : 10.1039/c3en00061c
Available at:
http://archive-ouverte.unige.ch/unige:34259
Disclaimer: layout of this document may differ from the published version.
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Effect of natural organic matter on the
disagglomeration of manufactured TiO2
nanoparticles†
Published on 28 January 2014. Downloaded on 10/02/2014 15:43:38.
Frédéric Loosli,a Philippe Le Coustumerbc and Serge Stoll*a
One major concern in the fate of nanomaterials in aquatic systems is the lack of data on nanomaterial
transformations under relevant environmental conditions. The disagglomeration of aggregates composed
of manufactured anatase titanium dioxide nanoparticles is investigated here in the presence of alginate
and Suwannee River humic acids at varying concentrations using dynamic light scattering and
electrophoretic measurements. Stability of TiO2 nanoparticle agglomerates at typical environmental
concentrations of natural organic matter is discussed at a pH value corresponding to the point of zero
charge of TiO2 nanoparticles. In this scenario, the surface charge of TiO2 is neutralized, allowing the
nanoparticles to form large agglomerates. Alginate and Suwannee River humic acids exhibit a negative
structural charge under this pH condition and adsorption of both natural polyelectrolytes on the surface
of nanoparticle agglomerates leads to disagglomeration and significant redispersion of TiO2 nanoparticles
into fragments. Results indicate that both electrostatic forces and steric interactions play key roles during
the disagglomeration process and that the physicochemical properties of natural organic matter are
Received 22nd October 2013,
Accepted 9th January 2014
DOI: 10.1039/c3en00061c
found to influence the kinetics and importance of fragmentation in the disagglomeration process. Most
importantly, our data indicate that the presence of natural organic matter at typical environmental
concentrations induces significant disagglomeration of large submicron nanoparticle agglomerates. Such
a result constitutes an important outcome with regards to the risk associated with manufactured
rsc.li/es-nano
nanoparticles by including the possible transformations of the micron size range structures they can form.
Nano impact
One of the main problems in the ecological risk assessment of nanomaterials is the lack of important information on their environmental
(bio)physicochemical transformations. The disagglomeration of manufactured titanium dioxide nanoparticles is investigated here in the presence of
alginate and Suwannee River humic acids at realistic environmental concentrations. Under such concentration conditions, the adsorption of these
compounds is found to induce disagglomeration and significant redispersion of TiO2 nanoparticles into fragments. Such a result constitutes an important
outcome with regards to the risk associated with manufactured nanoparticles by considering one important life-cycle transformation of micron size range
structures composed of nanoparticles in aquatic systems.
1. Introduction
Nowadays, manufactured nanoparticles (NPs) are part of our
everyday life. Due to their specific surface properties, high
a
Earth and Environmental Science Section, Group of Environmental Physical Chemistry,
F.-A. Forel Institute, University of Geneva, 10 route de Suisse,
1290 Versoix, Switzerland. E-mail: [email protected], [email protected];
Fax: +41 22 379 0302; Tel: +41 22 379 0333, +41 22 379 0341
b
EA 4592 Géoressources & Environnement, ENSEGID, Université Bordeaux 3,
1 allée F. Daguin, 33607 Pessac, France. E-mail: [email protected];
Tel: +33 5 40 00 87 98
c
UFR STM, Université Bordeaux 1, B.18 Av. Des facultés 33405 Talence, France
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3en00061c
This journal is © The Royal Society of Chemistry 2014
efficiency and low cost, they are employed in countless consumer goods.1,2 Unfortunately, the mass production of NPs3
and the lack of regulation concerning their production, use
and recovery have resulted in the rapid release of these
potentially harmful substances into the environment.4 Once
present in ecosystems, NPs are subject to interaction with
other entities such as natural colloids and (micro)organisms1
and undergo significant transformations during their life
cycle as they move into different environmental or biological
compartments. Despite the fact that these transformations
have received little attention to date, interactions between
NPs and natural colloids in aquatic environments are
expected to strongly influence NP stability.5 Fate, environmental impact, bioavailability, and stability, i.e. dispersion
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vs. agglomeration, of manufactured nanoparticles and the
agglomerates they can form will depend on the physicochemical properties of the medium. Indeed, pH, ionic strength,
temperature and the presence of natural organic matter
(NOM)6–9 will control transport and toxicity towards living
organisms.10,11
As a result, to achieve a better understanding of the fate,
transport and impact of manufactured NPs once released
into an aquatic system, their interaction and potential transformation in the presence of NOM have to be investigated for
risk evaluation.12–14 Natural organic matter is mainly composed of humic substances (humic acids (HA) and fulvic acids
(FA) as soluble entities) and non-humic substances such as
polysaccharides. Humic substances play a very important role
in aquatic systems. They can act as pH regulators and can also
influence a contaminant's mobility when adsorbed on their
surfaces.15 Composition, conformation and structure of HA
are still ambiguous. HA are sometimes described not only
as macromolecules adopting random coil conformation in
solution16 but also as supramolecular structures composed
of small entities linked to each other by weak interaction
forces.17 These forces are thought to result from ion–dipole
and hydrophobic interactions which confer a micelle like
structure when hydrated.18–20 HA are also described as semirigid heterogenous amphiphile macromolecules composed
of diverse functional groups such as phenol and carboxylic acid.21,22
Among non-humic substances, exopolysaccharides are an important and abundant class of organic compounds found in aquatic
environments.23 Alginate, a model polysaccharide, is a linear
block copolymer extracted from the cell walls of brown seaweed. Alginate comprises 1,4-linked β-D-mannuronic acid and
α-L-guluronic acid residues. Alginate is a semi-flexible linear
polysaccharide with a homogenous charge distribution.22
Alginate is widely used in the food industry as a thickening
agent and stabilizer and in the pharmaceutical industry as
drug carriers.24–26
NOM was found to strongly interact with NPs in suspension through electrostatic forces and steric interactions, thus
modifying significantly their transport and bioavailability.
Once the surface is coated with natural polymers or humic
substances, the NPs are stabilized against aggregation except
in the presence of divalent salts which are found to promote
aggregation via bridging mechanisms.5,27–29 NOM characterization is important and higher molecular weight fraction of
NOM was found to greatly enhance stabilization of NPs by
steric effects in comparison with lower molecular weight
NOM fraction.30 NOM adsorption was also found to be mainly
promoted by ligand exchange.9
TiO2 NPs are the most produced NPs to date.4 TiO2 NPs
are used in many consumer goods including cosmetics and
paints and as a UV protection agent31 and are thus inexorably
entering aquatic environments.4 Previous studies have demonstrated that TiO2 NP stability is governed by the physicochemical
medium parameters, i.e. pH, ionic strength and the presence of
multivalent salt as well as the NP intrinsic properties, i.e. size,
shape and crystalline form.8,32 The destabilization of TiO2 NPs,
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Environmental Science: Nano
which results in NP agglomerate formation, usually happens
at a pH close to the point of zero charge (PZC) of the metal
oxide. This is due to the predominance of van der Waals
attractive interactions over the electrostatic repulsive forces as
described by the classic Derjaguin–Landau–Verwey–Overbeek
(DLVO) colloid stability theory.33,34 Another way to induce
TiO2 NP agglomeration is to increase the ionic strength of
the solution8,35 to reduce the Debye length and potential at
the shear plan by charge screening effects. Studies indicate
that divalent cations enhance agglomeration of TiO2 NPs due
to specific adsorption, which results in NP surface charge
neutralization.8,28
It is generally assumed that agglomerated NPs are less
toxic to aquatic organisms than single NPs. It should be
noted here that most of the studies related to NP stability in
aquatic systems have focused on aggregation conditions
representative of the physicochemical properties prevailing in
freshwater.36 NPs that have been incorporated in products
and then released in aquatic systems will enter either in
their original form or in an altered form due to industrial
processes. For several reasons (autoaggregation, difficulties
to disperse powders, etc.), it is expected that a given amount
of NPs will enter aquatic environments in an agglomerated
and potentially less toxic form. However, natural processes
may considerably alter the stability of such agglomerates with
the possibility to disperse them, thus increasing diffusion
and potential toxicity of the NPs.11,37 Consequently, investigating and understanding the role of NOM on already
agglomerated NPs are of primary importance to better evaluate
the fate, impact and potential transformations of NPs in
aquatic systems.
The present study focuses on the disagglomeration process
of manufactured TiO2 NPs in the presence of alginate and
Suwannee River humic acids. The effect of NOM concentration and physicochemical properties is studied by considering
the fragmentation process of TiO2 NP agglomerates at pH =
pHPZC,TiO2. Alginate and Suwannee River humic acids belong
to two distinct models and classes of natural organic matter.
The disagglomeration process and its importance in determining TiO2 NP stability are examined by measuring in a systematic way the evolution of the electrophoretic mobility and
hydrodynamic diameters of the TiO2 fragments with respect
to time and NOM concentration.
2. Materials and methods
2.1. Materials
Anatase TiO2 NPs (Nanostructured & Amorphous Materials
Inc., USA) were obtained as a 170 g L−1 suspension of 15 nm
(nominal diameter) NPs in water. Stock suspensions of 1 g L−1,
pH 6.2 of alginate (A2158, Sigma Aldrich, Switzerland) and
500 mg L−1, pH 9.8 of Suwannee River humic acids (Standard II,
International Humic Substances Society, USA) were stirred for
24 hours and filtered through a 0.45 μm cellulose acetate filter
(VWR, Switzerland). Low viscosity alginate and SRHA average
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molecular weight are equal to 50 kDa and 1066 Da, respectively.38,39
Sodium chloride (NaCl, 99.5%, Acros Organics, Switzerland)
was employed to adjust the final ionic strength to 0.001 M.
Sodium hydroxide (1 M NaOH, Titrisol®, Merck, Switzerland)
and hydrochloric acid (1 M HCl, Titrisol®, Merck, Switzerland)
were used after dilution to adjust the pH. All suspensions
were prepared with deionized (R > 18 MΩ cm) Milli-Q water
(Millipore, Switzerland). Experiments were performed in 50 mL
polypropylene tubes, 29 × 115 mm (VWR, Switzerland), with a
crosshead single 8 × 10 mm magnetic stirrer (VWR, Switzerland).
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2.2. Zeta potential and size distribution measurements
Zeta (ζ) potential values and hydrodynamic z-average diameters
were determined with a Zetasizer Nano ZS (Malvern Instruments,
UK). Triplicate measurements were performed on each sample.
Determination of the ζ potential values and size distribution were achieved with 15 and 12 runs, respectively. The
Smoluchowski approximation model was used to calculate the
ζ potential values40–42 from the electrophoretic mobility
measurements. The characterization of the TiO2 NPs as well
as the ζ potential values and z-average diameters as a function
of NOM concentration was made at 50 mg L−1 TiO2 mass concentration at a constant 0.001 M salt concentration, whereas NOM
characterization was performed at 100 mg L−1 alginate and
SRHA mass concentration. Before starting, but also during the
experiments if necessary, the pH of all suspensions was adjusted
to the value of interest (HQ 40D, Hach Lange, Switzerland).
2.3. TEM image analysis
A Hitachi A7650 transmission electron microscope (TEM)
working at 120 kV was used for image analysis. The samples
were prepared by dropping 20 μL of 100 mg L−1 TiO2 suspensions onto a 200 square mesh copper grid coated with a thin
film of carbon (CF200-Cu Electron Microscopy Sciences,
USA). The drops were then left to dry prior to TEM analysis.
When analyzing samples containing alginate and SRHA, a
1 minute treatment was performed with osmium tetroxide
(4% in water, Electron Microscopy Sciences, USA) as a
staining agent in order to enhance the NOM contrast.43 The
acquisition time and the image resolution were set to 5 s and
3284 × 2600 pixels, respectively.
3. Results and discussion
3.1. TiO2, alginate and Suwannee River humic
acid characterization
TiO2 NPs. In order to get an insight into TiO2 stability
as a function of pH, titration curves for pH values in the
range 2–11 were realized. In Fig. 1, it is shown that for a
pH value lower than 5, TiO2 NPs exhibit strong and stable
positive zeta potential values (black squares) of +40.0 ± 3.1 mV
(mean ± standard deviation). As the pH increases, the zeta
potential rapidly decreases to the point of zero charge at
pH = pHPZC,TiO2 = 6.2 ± 0.1, which is in good agreement with
This journal is © The Royal Society of Chemistry 2014
Fig. 1 Zeta potential and z-average diameters of TiO2 NPs as a function
of pH. The point of zero charge (pHPZC,TiO2) was found at pH 6.2 ± 0.1.
An important TiO2 NP agglomeration domain (gray shaded area) was
found between −30 mV and +30 mV; [TiO2] = 50 mg L−1; I = 0.001 M.
the literature.44 Further pH increase leads to charge inversion
and surface charge stabilization is observed for a pH range
of 9–11 with a ζ potential value equal to −44.2 ± 1.2 mV.
The hydrodynamic z-average diameter variation as a function
of pH is also shown in Fig. 1 (blue dots). For pH values
between 2 and 5, the TiO2 NPs are stable and the z-average
diameter is 52 ± 9 nm. Then, the z-average diameter increases
rapidly to a maximum value at pH = pHPZC,TiO2, corresponding
to the formation of large agglomerates in the micron size range.
A further pH increase rapidly leads to the decrease of TiO2
z-average diameter. A plateau corresponding to a z-average of
57 ± 7 nm is obtained at pH > 9. A domain of TiO2 destabilization is found here, as indicated by the gray shaded area in
Fig. 1, for zeta potential values between +30 mV and −30 mV.
TEM images of stable (pH < pHPZC,TiO2) and agglomerated
TiO2 (pH = pHPZC,TiO2) NPs are shown in the ESI† in Fig. S1
and S2, respectively. Out of the destabilization domain, individual
NPs as well as small NP agglomerates composed of a few NPs
are present, which is in good agreement with our dynamic
light scattering measurements (52 ± 9 nm).
Alginate. The pH was adjusted from 11 to 3 with diluted
HCl at variable concentrations. As shown in Fig. 2a, alginate
exhibits a negative surface charge in the full domain. For a
pH range of 11–5.75, ζ potential values are found to be stable
(−30.3 ± 1.5 mV), whereas at a lower pH, values increase
continuously as a result of protonation of the carboxyl functional
groups present in the α-L-guluronate and β-D-mannuronate
monomers with pKa values of 3.65 and 3.38, respectively.45
The z-average diameter is found to be constant with a mean
z-average diameter equal to 178 ± 21 nm (Fig. 2a, inset).
Suwannee River humic acids. In the 11 to 3 pH range,
SRHA have a significant negative charge and a constant z-average
diameter as can be seen in Fig. 2b. The ζ potential increases
from −69.0 ± 2.4 mV at pH 11 to −30.2 ± 0.8 mV at pH 3. No ζ
potential plateau is obtained due to the heterogeneity of SRHA
functional groups. The z-average diameter is 379 ± 19 nm
(Fig. 2b, inset). Both ζ potential and z-average diameter values
are in agreement with previous studies.46,47
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Fig. 2 a) Zeta potential of alginate as a function of pH. No PZC was
found here. z-Average diameters of alginate as a function of pH (inset).
The z-average diameter had a constant value of 178 ± 21 nm. b) Zeta
potential of SRHA as a function of pH. No PZC was found here.
z-Average diameters of SRHA as a function of pH (inset). The
z-average diameter had a constant value of 379 ± 19 nm. [Alginate] =
[SRHA] = 100 mg L−1; I = 0.001 M.
3.2. TiO2 NP disagglomeration in the presence
of alginate and SRHA
To investigate the disagglomeration process, the pH of the
TiO2 suspension was adjusted to pH = pHPZC,TiO2 in order to
form large agglomerates via surface charge neutralization
(Fig. 1). The effect of NOM on TiO2 agglomerate stability was
then studied by recording ζ potential and size distribution
variations as a function of time for different NOM concentrations. The agitation speed was set to obtain a low velocity of
mixing only to gently homogenize the suspensions.
TiO2 NP disagglomeration in the presence of alginate. As
shown in Fig. 3, the presence of alginate induces the
disagglomeration of the TiO2 aggregates and two distinct
regimes are found. In the first regime, an important decrease
of z-average diameters is observed, which is due to the fragmentation of TiO2 agglomerates during the first 45 min after
alginate addition. Then, a plateau is reached, corresponding
to a second regime. In the second regime, the system is at a
long-time state, according to particle sizes and ζ potential
values, and no further disagglomeration is observed as indicated by the stabilization of z-average diameters and ζ potential values. Increasing the alginate concentration also enhances
the importance of the disagglomeration process with maximum efficiency obtained for alginate concentrations ≥3 mg L−1.
In such conditions, the corresponding final z-average diameters are equal to 500 nm. Thus, only a partial, but significant,
TiO2 NP disagglomeration occurs with z-average diameters
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Fig. 3 a) z-Average diameters of TiO2 agglomerates as a function of
time at pHPZC,TiO2 (6.2 ± 0.1) for different alginate mass concentrations.
b) Zeta potential of TiO2 agglomerates in the presence of alginate as a
function of time at pHPZC,TiO2. Equilibrium time for alginate induced
disagglomeration was 45 min. [TiO2] = 50 mg L−1; I = 0.001 M.
for high alginate concentrations an order of magnitude greater
than the individual size of TiO2 NPs (500 nm vs. 50 nm).
Another interesting point to mention, at high concentrations in
the first regime (Fig. 3b), is the continuous decrease of ζ potential
values resulting from the continuous adsorption of alginate.
The effect of alginate concentration on TiO2 disagglomeration
at t = 60 min (in the long-time state plateau) is represented in
Fig. 4. It is shown that maximum disagglomeration is achieved
with an alginate concentration ≥3 mg L−1. A TEM image of
TiO2 agglomerate fragments in the presence of alginate is
given in Fig. S3.†
TiO2 NP disagglomeration in the presence of SRHA. As
shown in Fig. 5, two regimes are also present and the time
needed to reach system long-time state after SRHA addition
is now equal to 24 h. Maximum TiO2 disagglomeration is
obtained for SRHA concentrations ≥5 mg L−1. Once the system
is at the long-time state, the z-average diameters are equal
to 250 nm, i.e. smaller than with alginate. Disagglomeration
is therefore much important with SRHA than with alginate.
SRHA is also continuously adsorbed onto the surface of TiO2
NPs as denoted by the decrease of the ζ potential values in
the first regime for high SRHA concentrations. The TiO2
z-average diameter at the long-time state depends on the
amount of SRHA added as observed in Fig. 6 where the
variation of TiO2 z-average diameters with respect to SRHA
concentration 48 h after SRHA addition is presented. A TEM
image of TiO2 agglomerate fragments in the presence of
SRHA is given in Fig. S4.†
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Fig. 4 Final z-average diameters of TiO2 NPs as a function of alginate
mass concentration at pHPZC,TiO2 (6.2 ± 0.1). Measurements were made
60 min after the addition of alginate, i.e. after equilibrium time for NP
disagglomeration. Maximum disagglomeration occurs at [alginate]
≥3 mg L−1, which corresponds to a z-average diameter of 500 nm.
[TiO2] = 50 mg L−1; I = 0.001 M.
Fig. 5 a) z-Average diameters of TiO2 agglomerates as a function of
time at pHPZC,TiO2 (6.2 ± 0.1) for different SRHA mass concentrations.
b) Zeta potential of TiO2 agglomerates in the presence of SRHA as a
function of time at pHPZC,TiO2. Equilibrium time for SRHA induced
disagglomeration is 24 h. [TiO2] = 50 mg L−1; I = 0.001 M.
Influence of NOM properties on TiO2 disagglomeration.
Two important differences in TiO2 NP disagglomeration
are observable when comparing the effects of alginate
(Fig. 3 and 4) and SRHA (Fig. 5 and 6). Firstly, the time needed
to reach system long-time state is 45 min in the case of
alginate, whereas it is 24 h for SRHA. The disagglomeration
process is expected to be controlled by both adsorption of
NOM onto the TiO2 agglomerates and its ability to reach the
inner structure of the agglomerates to produce fragments. As
shown by Pefferkorn, the interpenetration of polyelectrolyte
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Fig. 6 Final z-average diameters of TiO2 NPs as a function of SRHA
concentration at pHPZC,TiO2 (6.2 ± 0.1). Measurements were made
48 hours after the addition of SRHA, i.e. after equilibrium time for NP
disagglomeration. SRHA induced disagglomeration and maximum
disagglomeration occurred at [SRHA] ≥5 mg L−1, which corresponds to
a z-average diameter of 250 nm. [TiO2] = 50 mg L−1; I = 0.001 M.
chains within agglomerates plays a key role in the kinetic
process of disagglomeration.48 Alginate is a semi-rigid linear
biopolymer with a homogenous charge distribution, whereas
SRHA is a semi-rigid globular macromolecule.22 Thus, the ability
to adsorb alginate on the TiO2 agglomerate surface and penetrate inside the aggregates through conformational changes
(reptation) is greater for alginate in comparison to SRHA which
is subject to more important steric hindrance. Secondly, alginate
is found to be less efficient than SRHA for disagglomeration
when comparing the final NOM–TiO2 fragment sizes at high
NOM concentrations (Fig. 7). Indeed, SRHA exhibits a higher
ζ potential value and thus, once adsorbed onto the TiO2
agglomerate, restabilizes the NPs more efficiently owing to
greater electrostatic repulsion forces. Here, NP surface charge is
found to be an essential ingredient in the disagglomeration
processes.
In the present study, TiO2 disagglomeration is expected to
mainly result from random disagglomeration leading to a
rapid decrease in z-average diameters. Nonetheless, even if
attrition, i.e. the production of small fragments, has not been
identified using DLS analysis, such a fragmentation mechanism cannot be excluded.
The stability diagram which is presented in Fig. 8 gives
an overview of the disagglomeration processes and highlights the main differences between alginate (purple area)
and SRHA (orange area). Agglomeration occurs at the PZC
before addition of NOM with agglomerates in the micron
range. NOM addition induces partial disagglomeration, as
indicated by the black arrow, at relatively low concentrations
which are representative of NOM concentrations found in
aquatic systems. Indeed, to obtain NP fragments of 1 μm,
concentrations of alginate and SRHA should be equal to
2 mg L−1 and 4.25 mg L−1, respectively (grey vertical lines).
The increase of the NOM concentration results in smaller
fragment sizes and maximum disagglomeration is obtained
for concentrations ≥3 mg L−1 for alginate and ≥5 mg L−1
for SRHA.
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Fig. 7 Disagglomeration of TiO2 NPs in the presence of alginate and SRHA at pH = 6.2 = pHPZC,TiO2. Influence of adsorption and interpenetration
on disagglomeration kinetic and NOM physicochemical properties on final fragment size (TiO2/NOM ratio numbers are only illustrative).
Fig. 8 Stability of TiO2 agglomerates in the presence of alginate (purple area) and SRHA (orange area) at pH = pHPZC,TiO2. Agglomeration is
represented by hatched areas. In both cases, the presence of NOM promotes TiO2 disagglomeration at NOM environmental concentrations.
[TiO2] = 50 mg L−1; I = 0.001 M (TiO2/NOM ratio numbers are only illustrative).
3. Conclusions
The stability of TiO2 agglomerates was investigated by
determining the evolution of the ζ potential and z-average
diameter values as a function of alginate and Suwannee River
humic acid concentrations. Typical environmental NOM
concentrations were used here, and it was clearly shown that
NOM significantly modifies the stability of TiO2 NP agglomerates
(50 mg L−1 TiO2 suspensions) by promoting their disagglomeration.
The physicochemical properties of NOM strongly influenced
the kinetics and rates of TiO2 disagglomeration which was
found to be mainly governed by electrostatic repulsive forces
and steric interactions. Disagglomeration was found to occur
on a relatively short time scale and different mechanisms were
discussed when comparing the two NOM models. Since
disagglomeration processes are expected to be influenced by
the fractal dimensions of agglomerates and bonding energies
between the NPs, one important aspect to be developed in
Environ. Sci.: Nano
future work concerns the agglomerate fractal dimension
importance and detailed mechanisms of disagglomeration in
the presence of NOM. Overall, our results clearly indicate the
importance of NOM on the transformation of agglomerates
composed of NPs and underline the need to reevaluate, in
some circumstances, the fate, transport and impact of
nanomaterial assemblies.
Acknowledgements
The authors are grateful to Daniel Azmoon and Nadia Von
Moos for stimulating discussions and to Sabrina Lacomme
and Etienne Gontier for technical assistance and the contribution of BioImaging Center at the University of Bordeaux 2.
We also acknowledge the financial support received from the
Swiss National Foundation (project 200021_135240) and
nanoMILE FP7 Project.
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