Elsevier Editorial System(tm) for Water Research Manuscript Draft Manuscript Number: Title: TiO2 nanoparticles aggregation and disaggregation in presence of alginates and humic acids. pH and concentration effects on nanoparticle stability. Article Type: Research Paper Keywords: Titanium dioxide nanoparticles, Alginate, Suwannee river humic acid, Nanoparticle stability, Aggregate disaggregation, Aquatic systems Corresponding Author: Dr Serge Stoll, Corresponding Author's Institution: University of Geneva First Author: Frederic Loosli Order of Authors: Frederic Loosli; Philippe Lecoustumer, Dr; Serge Stoll Abstract: The behavior of manufactured TiO2 nanoparticles is studied here in a systematic way as a function of pH and in the presence of Suwannee river humic acids and alginate, at variable concentrations, which represent two major components found in aquatic systems. TiO2 nanoparticles aggregation, disaggregation and stabilization are investigated using dynamic light scattering and electrophoretic experiments allowing the measurement and evolution determination of z-average hydrodynamic diameters and zeta potential values. Stability of the TiO2 nanoparticles is carried out by considering three pH-dependent electrostatic scenarios (below the point of zero charge of the nanoparticles, at the point of zero charge and above it). In the first scenario, when pH is below the point of zero charge of the TiO2 nanoparticles, nanoparticles exhibit a positively charged surface whereas alginate and Suwannee river humic acids are negatively charged. Fast adsorption at the TiO2 nanoparticles occurs, promotes surface charge neutralization and aggregation and, by increasing further alginate and Suwannee river humic acids, results in charge inversion and thus stabilization of TiO2 nanoparticles. In the second electrostatic scenario, at the pH of the TiO2 surface charge neutralization, TiO2 nanoparticles are rapidly forming aggregates and adsorption of alginate and Suwannee river humic acids on aggregates surface leads to the partial disaggregation of aggregates. In the third electrostatic scenario, when nanoparticles, alginates and Suwannee river humic acids are negatively charged a small amount of Suwannee river humic acids is adsorbed via hydrophobic interactions. It is found that the fate and behavior of individual and aggregated TiO2 nanoparticles in presence of environmental compounds are strongly dependent in a non linear on the electrostatic, concentration ratio, and to a less extend to steric interactions and amphiphilic compound character way and that environmental aquatic concentration ranges of humic acids and biopolymers are expected to largely modify the stability of aggregated or individual TiO2 nanoparticles. Suggested Reviewers: Mohammed Baalousha Dr. School of Geography, Earth and Environmental Science, University of Birmingham, UK
[email protected] Expert in the field of nanoparticles behavior , such as aggregation and disaggregation, and transport in water Gregory V Lowry Prof.
Center for the Environmental Implications of Nano Technology, Carnegie Mellon University, US
[email protected] Expert in the interaction processes between manufactured nanoparticles and natural organic matter Xiaojing Leng Prof. M-Director, China agricultural university, China
[email protected] Expert in the field of nanoparticle chracterization using electrophoretic measurements Martin Hassellhov Prof Environmental Nanochemistry Dep, University of Gothenburg
[email protected] Expert studying nanoscale processes in the environment, and how the reactivity of nanoscale materials is impacting the environment.
Cover Letter, For Editor only
Dr. Serge STOLL (Corresponding Author) University of Geneva, F.-A. Forel Institute Group of Environmental Physical Chemistry 10 Route de Suisse, 1290 Versoix, Switzerland tel ++ 41 22 379 0333; Email :
[email protected] Editor Water Research Prof. Mark van Loosdrecht Kluyver Inst. For Biotechnology Technische Universiteit Delft Julianalaan 67 2628 BC Delft, Netherlands
February 06/02/2013
Dear Professor van Loosdrecht,
We would like to publish in Water Research, as a regular article, a manuscript entitled: “TiO2 nanoparticles aggregation and disaggregation in presence of alginates and humic acids. pH and concentration effects on nanoparticle stability.” by Fredéric Loosli, Philippe Lecoustumer and Serge Stoll. We believe that understanding the role of humic substances and polysaccharides in the evolution, tranformation and stability of manufactured nanoparticles is an essential aspect in aquatic systems. In this manuscript, the adsorption of humic acids and alginates at the surface of titanium dioxide nanoparticles is investigated to achieve a systematic, quantitative and significant description of the physico chemical behavior of these nanoparticles. Surface charge variations and electrostatic transformation (charge neutralisation, overcharging), kinetics of disaggregation are investigated as a function of the solution pH, humic acids, alginates and nanoparticle concentration ratio. Stability diagrams are also given and clearly indicates the rapid stabilisation and dispersion of nanoparticle aggregates and isolated monomers. The manuscript has not been previously published, in whole or in part, and is not under consideration by any other journal. All authors are aware of, and accept responsibility for, the manuscript.
We are looking forward to hearing from you soon.
Yours Sincerely,
Serge Stoll (on behalf of the authors)
*Highlights (for review)
Highlights
TiO2 nanoparticles (NPs) stability is found pH dependent
Alginate and humic acids (NOM) adsorption modify the NPs TiO2 surface charge
When aggregates are initially present NOM promotes disaggregation
Electrostatic, steric and hydrophobic forces play key roles in TiO2 stability
Typical environmental NOM concentrations disperse TiO2 NPs in aquatic systems
Graphical Abstract (for review)
Graphical abstract
*Manuscript Click here to download Manuscript: Manuscript-Loosli-Lecoustumer-Stoll.docx
Click here to view linked References
TiO2 nanoparticles aggregation and disaggregation in presence of alginates and humic acids. pH and concentration effects on nanoparticle stability.
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b,c
Frédéric Loosli , Philippe Le Coustumer , Serge Stoll
a,*
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University of Geneva, F.-A. Forel Institute, Group of Environmental Physical Chemistry, 10 route de Suisse, 1290 Versoix, Switzerland b
Université Bordeaux 3, EA 4592 Géoressources & Environnement, ENSEGID, 1 allée F. Daguin, 33607 Pessac, France c
Université Bordeaux 1, UFR STM, B.18 Av. Des facultés 33405 Talence, France
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Frédéric Loosli:
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Tel: + 41 22 379 0332 / email:
[email protected]
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Philippe Le Coustumer:
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Tel: +33 5 40 00 87 98 / email:
[email protected]
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Serge Stoll *:
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Tel: + 41 22 379 0333 / email:
[email protected]
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Fax: + 41 22 379 0302 1
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Abstract
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The behavior of manufactured TiO2 nanoparticles is studied here in a systematic way as a
31
function of pH and in the presence of Suwannee river humic acids and alginate, at variable
32
concentrations, which represent two major components found in aquatic systems. TiO2
33
nanoparticles aggregation, disaggregation and stabilization are investigated using dynamic
34
light scattering and electrophoretic experiments allowing the measurement and evolution
35
determination of z-average hydrodynamic diameters and zeta potential values. Stability of the
36
TiO2 nanoparticles is carried out by considering three pH-dependent electrostatic scenarios
37
(below the point of zero charge of the nanoparticles, at the point of zero charge and above it).
38
In the first scenario, when pH is below the point of zero charge of the TiO2 nanoparticles,
39
nanoparticles exhibit a positively charged surface whereas alginate and Suwannee river humic
40
acids are negatively charged. Fast adsorption at the TiO2 nanoparticles occurs, promotes
41
surface charge neutralization and aggregation and, by increasing further alginate and
42
Suwannee river humic acids, results in charge inversion and thus stabilization of TiO2
43
nanoparticles. In the second electrostatic scenario, at the pH of the TiO2 surface charge
44
neutralization, TiO2 nanoparticles are rapidly forming aggregates and adsorption of alginate
45
and Suwannee river humic acids on aggregates surface leads to the partial disaggregation of
46
aggregates. In the third electrostatic scenario, when nanoparticles, alginates and Suwannee
47
river humic acids are negatively charged a small amount of Suwannee river humic acids is
48
adsorbed via hydrophobic interactions. It is found that the fate and behavior of individual and
49
aggregated TiO2 nanoparticles in presence of environmental compounds are strongly
50
dependent in a non linear on the electrostatic, concentration ratio, and to a less extend to steric
51
interactions and amphiphilic compound character way and that environmental aquatic
52
concentration ranges of humic acids and biopolymers are expected to largely modify the
53
stability of aggregated or individual TiO2 nanoparticles. 2
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Keywords: Titanium dioxide nanoparticles, Alginate, Suwannee river humic acid,
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Nanoparticle stability, Aggregate disaggregation, Aquatic systems
56
1.Introduction
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Manufactured nanoparticles (NPs) represent today an important class of potential emerging
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pollutants due to their release already occurring into the environment. Contrary to natural
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aquatic colloids manufactured NPs are produced to have surface properties that have never
60
been encountered by man and other organisms in nature. Depending on physical and chemical
61
conditions prevailing in aquatic systems, and possible interaction processes with natural
62
aquatic colloids, manufactured NPs will occur either as aggregated or dispersed materials.
63
Aggregates containing NPs will sediment and become immobilized, while dispersed NPs will
64
be able to significantly diffuse in aqueous environments, and will be more mobile,
65
bioavailable and toxic. Interactions between manufactured NPs with, in particular, natural
66
organic matter such as biopolymers and humic substances, leading to surface coating, surface
67
charge modifications, aggregation, flocculation, or stabilization and dispersion, will then
68
strongly alter their dynamic properties (immobilization versus diffusion).
69
Nanoparticles are commonly defined as particles with a characteristic dimension in the 1-100
70
nm range. This class of colloidal particles includes a large number of different type of
71
compounds such as metal NPs (Au, Ag), metal oxide NPs (TiO2, ZnO, Fe2O3) and carbon
72
based NPs (nanotubes, fullerenes). They exhibit various chemical surface properties and
73
reactivities depending on their sizes, shapes and chemical compositions (Ju-Nam and Lead
74
2008). Their particular properties concern redox and catalytic properties to high performance
75
UV filtering capacity. Among oxide based NPs, titanium dioxide is one of the most produced
76
and common nanomaterial. TiO2 is used in photovoltaic, photocatalytic sensor and energy
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storage domains, plastic, fibers, foods, pharmaceuticals, cosmetics, antimicrobial applications, 3
78
and painting industry where TiO2 is primarily used as pigment and UV protective agent (Chen
79
and Mao 2007). The global production of TiO2 is of the order of thousands of tons per year
80
(Weir et al. 2012). As a result many sources of anthropogenic TiO2 are expected to enter into
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aquatic compartments such as surface waters via sewage systems. Different studies on the
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behavior of TiO2 NPs and the role of pH, ionic strength, salt valency, and surface charge on
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their aggregation and dispersion have been reported (French et al. 2009). Studies indicate that
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TiO2 NPs aggregation occurs at pH near the point of zero charge and that they are stable at
85
other pH. Moreover high ionic strength and presence of divalent salt is shown to facilitate
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aggregation process (French et al. 2009, Guzman et al. 2006).
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Impact, fate and reactivity of NPs and their possible interaction with natural organic matter in
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aquatic system is thus a topic of high interest (Nowack and Bucheli 2007) for environmental
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risk evaluation (Auffan et al. 2009, Colvin 2003, Wiesner et al. 2006). Natural organic matter
90
(NOM), composed of humic substances (HSs) and non humic substances such as
91
polysaccharides, is ubiquitous in the environment. HSs are mainly present as fulvic acids
92
(FAs) and humic acids (HAs). HAs are heterogeneous macromolecules from a conformational
93
and chemical point of view with mainly carboxylic acid and phenolic functional groups. HAs
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are principally composed of fibrils generated by the interconnection of basic molecules of 20
95
nm diameter with some isolated larger (50-300 nm) globular macromolecules (Baalousha et
96
al. 2005). Alginate is a polysaccharide linear block copolymer extracted from the cell walls of
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brown seaweed that comprises 1,4-linked β-D-mannuronic acid and α-L-guluronic acid
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residues. Alginate is widely used in the pharmaceutical and food industry. In food industry
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alginate is mainly employed as thickening agent and stabilizer whereas as efficient drug
100
carriers in biomedecine (Liu et al. 2008). Alginate was also utilized in the recovery of heavy
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metal ions and Ngomsik showed the efficiency of Ni2+ removal from wastewater with alginate
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microcapsules containing extractant and magnetic NPs (Ngomsik et al. 2006). 4
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A substantial amount of work has been reported to have a better understanding of the effect of
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NOM on NPs stability and bioavailability. Aggregation processes of metal, metal oxide and
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carbon based NPs in presence of FAs, HAs and alginate clearly indicate the importance of
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both electrostatic and steric stabilization when NOM are adsorbed on their surfaces (Chen and
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Elimelech 2008, Domingos et al. 2009, Keller et al. 2010, Ottofuelling et al. 2011). Moreover,
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effect of salt valence and presence of divalent salt facilitate bridging processes of NPs
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(fullerenes, hematite and TiO2) even once coated with NOM by formation of NOM-divalent
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cation complexes (Chen and Elimelech 2007, Chen et al. 2006, Domingos et al. 2010).
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Another important role of NOM adsorption on NPs is the enhancement of their photocatalytic
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properties and capacity to adsorb heavy metal and hydrophobic organic compounds (Chae et
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al. 2012, Chen et al. 2012, Papageorgiou et al. 2012, Yang and Xing 2009). Adsorption of
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NOM on mineral surfaces takes mainly place by ligand exchange between the carboxylic acid
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and phenolic groups of NOM even if other mechanisms such as hydrophobic interaction,
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hydrogen bonding, anion exchange or cation bridging are playing a role (Pallem et al. 2009,
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Yang et al. 2009).
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The objective of this study consists to investigate the stability of TiO2 NPs by considering two
119
important but different organic compounds found in aquatic systems, namely HAs and
120
alginates. Conformational structures of alginate and HAs are different. Alginate is a semi-
121
flexible linear polyelectrolyte with an almost constant charge density along its chain whereas
122
HAs are heterogeneous and semi-rigid macromolecules. In this study, the influence of pH
123
solution, which will play a key role on the electrostatic, NOM/NPs concentration ratio and
124
NOM structure, is investigated to have a better overview of the possible behavior of TiO2 NPs
125
when released in complex aquatic environments with regards to aggregation and dispersion.
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The influence of these two organic compounds on TiO2 NPs stability is systematically studied
127
by adjusting the pH and NOM concentration and by the measurements of z-average 5
128
hydrodynamic diameters and electrophoretic mobilities at a constant electrolyte background.
129
Focus is also made on the disaggregation processes of TiO2 NP aggregates, which is a central
130
aspect in the NPs dispersion in aquatic systems, and relation between the specific structural
131
and chemical properties of NOM and the stabilization of TiO2 NPs. Effect of NOM
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concentration and structure on the kinetic of aggregate disaggregation and variation of TiO2
133
NPs stability is investigated at the point of zero charge of TiO2 NPs and recording the
134
aggregate fragment sizes as a function of NOM concentration and time.
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2.Materials and methods
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2.1.Materials
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TiO2 anatase NPs (Nanostructured & Amorphous Material Inc) with a nominal particle
138
diameter of 15 nm were purchased as a 170 g/L TiO2 suspension in water with a specific
139
surface area of 240 m2/g. Suwannee river humic acid (SRHA) (standard II, International
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Humic Substances Society) 500 mg/L pH 9.8, and low viscosity alginate (A2158, Sigma
141
Aldrich) 1 g/L pH 6.2 stock solutions were stirred overnight and filtrated through a 0.45 μm
142
cellulose acetate filter (VWR). Hydrochloric acid (1 M HCl, Titrisol®, Merck) and sodium
143
hydroxide (1 M NaOH, Titrisol®, Merck) were used to adjust the solutions pH. Sodium
144
chloride (NaCl, 99.5%, Acros Organics) was employed to adjust the final ionic strength to
145
0.001 M. All the solutions were prepared to the target experimental concentrations with
146
deionized Milli Q water (R > 18 MΩ cm). Experiments were performed in 25 mL
147
polypropylene (PP) tubes 25 × 90 mm (Millan) with a crosshead single 8 × 10 mm magnetic
148
stirrer (VWR).
149
2.2. Zeta potential and size distribution measurements
150
A Zetasizer Nano ZS (Malvern Instrument) was used to determine the zeta (ζ) potential values
151
as well as size distributions of the TiO2 NPs, alginate, SRHA solutions and their 6
152
corresponding mixtures. For each situation triplicate measurements were performed and each
153
sample was measured 3 times to determine the zeta potential values and the size distributions.
154
For the zeta potential determination 15 sub-runs, with a delay of 5 s between them, to relax
155
and stabilize the system, whereas 12 sub-runs of 10 s with a 5 s delay, for the size distribution
156
measurements, were made. For the ζ potential calculations the Smoluchowski approximation
157
model, i.e. when particle are large as compared to the double layer thickness, was applied. 50
158
mg/L solutions for the TiO2 NPs and mixtures and a 100 mg/L solution for the NOM (alginate
159
and SRHA) were used. Solutions were vigorously stirred before analysis.
160
2.3. Aggregate size evolution determination
161
The TiO2 aggregate z-average hydrodynamic diameter determination in presence of alginate
162
and SRHA was performed in 50 mL PP tubes 29 × 115 mm (VWR) under agitation with a
163
crosshead single 8 × 10 mm magnetic stirrer (VWR). Equilibrium time before measurement
164
was set to 45 min for alginate and 24 hours for SRHA after each successive addition of NOM.
165
Variation of aggregate z-average hydrodynamic diameters and zeta potential values as a
166
function of alginate and SRHA mass concentrations were recorded with a Zetasizer Nano ZS
167
(Malvern Instrument) for an initial TiO2 mass concentration of 50 mg/L and a background
168
NaCl salt concentration equal to 0.001 M. The pH of all solutions was, before starting the
169
experiments, adjusted to the pH of interest.
170
2.4. TEM image analysis
171
Image analysis was also realized with a 120 kV Hitachi A7650 Transmission Electron
172
Microscope (TEM). TEM samples were prepared by dropping 20 μL of solutions containing a
173
TiO2 mass concentration equal to 100 mg/L on a 200 square mesh copper grid coated with a
174
thin film (30-50 nm) of pure carbon (Electron Microscopy Sciences, CF200-Cu). The
175
different drops were left to dry for a minimum of one hour before TEM analysis. For mixtures
7
176
analysis, i.e. TiO2 in presence of alginate and SRHA, one minute treatment with osmium
177
tetroxide as staining agent was realized to enhance the contrast of NOM. The image
178
resolution was set to 3284 × 2600 pixels with an acquisition time equal to 5 sec.
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3. Results and discussion
180
3.1. Material characterization
181
TiO2 NPs. The TiO2 NPs surface charge pH dependence and the resulting stability of the NPs
182
in solution were determined through pH titration curves for pH values in the range 2-11 and a
183
background salt concentration of 0.001 M (adjusted with NaCl). As shown in Fig. 1a, TiO2
184
NPs exhibit a stable and positive zeta potential (+40.0 ± 3.1 mV) from pH 2 to 5. Then the ζ
185
potential value decreases to the Point of Zero charge (PZC) at pH 6.2 ± 0.1 in good agreement
186
with the literature (Parks 1965, Preocanin and Kallay 2006). By increasing further the pH, the
187
zeta potential becomes negative and is found to stabilize at -44.2 ± 1.2 mV at pH 9.4. The z-
188
average diameter evolution with pH is also determined and presented in Fig. 1b. The NPs size
189
is found to increase from 52 ± 9 nm in the pH 2-5 domain to reach a maximum value at the
190
PZC with values greater than 10 µm indicating strong aggregation when the NPs surface
191
charge is neutralized. TEM images of TiO2 NPs stabilized at pH < pHPZC,TiO2, as well as TiO2
192
NPs aggregates at the PZC are presented in Fig. 2a and 2b respectively. Fig. 2a indicates the
193
presence of small aggregates composed of few particles in good agreement with DLS
194
measurements. By further increasing the pH, the z-average diameter is found to rapidly
195
decrease to reach stable values again at 57 ± 7 nm. As shown by the gray domains in Fig. 1
196
TiO2 NPs destabilization is found to occur in a zeta potential domain comprised between +30
197
and -30 mV.
198
Alginate. Titration curves of a 100 mg/L alginate solution with a background ionic strength
199
of 0.001 M (NaCl) were performed from pH 11 to 3 with HCl at variable concentrations. 8
200
Alginate exhibits negative zeta potential values and constant z-average values on the whole
201
domain of pH. As shown in Fig. 3a, a ζ potential plateau of about -30.3 ± 1.5 mV is found
202
between pH 11 and 5.75. Then, at lower pH values, the zeta potential increases due to the
203
continuous protonation of the carboxylic moieties of the α-L-guluronate and β-D-
204
mannuraonate alginate monomers to reach a value of -13.0 ± 1.1 mV at pH 3. No PZC is thus
205
observed here in good agreement with the α-L-guluronate and β-D-mannuraonate pKa values
206
of respectively 3.6 and 3.2 (Stefansson, 1999). Owing to the low pKa value of carboxylic
207
groups (pKa < 4) alginate is hence stabilized against aggregation upon pH variations at low
208
monovalent electrolyte concentration. The z-average diameter of alginate was found constant
209
from pH 3 to 11 and equal to 178 ± 21 nm (Fig. 3b).
210
Suwannee river humic acids. To get an insight into the zeta potential variation of the SRHA
211
macromolecules in solution as a function of pH, the corresponding titration curves were
212
determined with the measurement of the electrophoretic mobilities. 100 mg/L SRHA
213
solutions with a background NaCl salt concentration (10-3 M) were used and the pH was
214
adjusted from pH 11 to pH 3 with HCl at variable concentrations. As shown in Fig. 4a, SRHA
215
exhibits a strong negative structural charge from -69.0 ± 2.4 mV at pH 11 to -30.2 ± 0.8 mV
216
at pH 3 mainly due to the presence of carboxylic acid and phenolic functional groups. The
217
zeta potential decreases continuously owing to the heterogeneity of the functional groups. As
218
shown in Fig. 4b, SRHA z-average diameter is found constant on the whole pH domain with a
219
z-average diameter value of 379 ± 19 nm. Such a value, in comparison with few nanometers
220
range values obtained by transmission electron microscopy (Baalousha et al. 2005), is related
221
to the aggregation of the heterogeneous single nanometric entities that gives large-scale
222
supramolecular structures in solution.
9
223
3.2. TiO2 NPs stability in presence of alginate and SRHAs
224
As previously shown in Fig. 1 stability of TiO2 NPs is strongly pH dependant whereas
225
alginate and SRHA exhibit a negative surface charge in a large pH domain (Fig. 3 and Fig. 4).
226
According to this observation three main different electrostatic scenarios can be defined. In
227
the first one, when pH < pHPZC,TiO2, TiO2 NPs are positively charged with zeta potential
228
values greater than +30 mV. As a result they are stabilized against aggregation owing to
229
electrostatic repulsions between the NPs. On the other hand alginate and SRHA are negatively
230
charged and thus strong electrostatic adsorption of NOM is expected at the oppositely charged
231
TiO2 NPs surface hence modifying the surface charge and resulting stability of the TiO2 NPs.
232
In the second electrostatic scenario, when pH = pHPZC,TiO2, TiO2 NPs are neutral and strong
233
aggregation is observed; attractive Van-der-Waals interactions will be the predominant
234
interaction phenomena, and the adsorption of negatively charged alginate and SRHA is
235
expected to restabilize the TiO2 NPs by enhancement of the electrostatic repulsions and steric
236
effects. Finally, in the third scenario, when pH > pHPZC,TiO2, TiO2 NPs are negatively charged
237
as well as alginate and SRHA, and NOM adsorption is expected to be limited due to
238
electrostatic repulsions. Thus no significant changes in TiO2 stability should be observed.
239
Positively charged TiO2 NPs in presence of negatively charged alginate and
240
SRHA (pH < pHPZC,TiO2)
241
Alginate-TiO2 mixtures. Experiments at pH = 4.5 were conducted to get an insight on NOM
242
capacities to modify the surface charge of the TiO2 NPs. As shown in Fig. 5, in the case of
243
zeta potential variation of TiO2 NPs as a function of alginate mass concentration, before
244
addition of alginate, TiO2 NPs exhibit a zeta potential value of +34.1 ± 1.4 mV with a z-
245
average diameter of 48 ± 1 nm. During alginate addition, three stability domains are observed.
246
For alginate mass concentration below 1.1 mg/L, the TiO2 NPs are still strongly positively
10
247
charged with a zeta potential greater than +28 mV and z-average value smaller than 200 nm.
248
Even if TiO2 NPs are stabilized against strong aggregation due to the electrostatic repulsions,
249
the z-average diameter increases in this domain, as a consequence of alginate adsorption.
250
Then, in the second domain (grey area in Fig. 5), alginate mass concentration in the range 1.1-
251
2.4 mg/L strongly influences the surface charge of the TiO2 NPs and destabilizes the TiO2
252
NPs. Full charge neutralization and IsoElectic Point (IEP) of TiO2 NPs is then observed for an
253
alginate mass concentration of 1.68 ± 0.01 mg/L with a corresponding charge ratio of alginate
254
over TiO2 found here equal to 1.25 ± 0.21. At charge neutralization, the z-average diameter is
255
found to correspond to the maximum of the size distribution curve with a value of 1082 ± 90
256
nm (Fig. 5b). After the IEP, charge inversion is observed and both zeta potential and z-
257
average diameter values rapidly decrease until an alginate mass concentration equal to 2.4
258
mg/L, which corresponds to a zeta potential value of about -28 mV. Finally, for alginate mass
259
concentration greater than 2.4 mg/L, the system is stabilized and full charge inversion is
260
observed at 5 mg/L with a zeta potential value of -34.0 ± 0.7 mV corresponding to a z-average
261
diameter of 105 ± 1 nm.
262
SRHA-TiO2 mixtures. Three important domains in Fig. 6 are also observed here. For SRHA
263
mass concentration below 1.5 mg/L, the NOM is continuously adsorbed at the TiO2 NPs
264
surface, the z-average diameter slightly increases to 247 ± 19 nm at 1.5 mg/L, whereas the
265
zeta potential value at this concentration is found to be +19.5 ± 2.6 mV. Then, significant
266
destabilization occured for SRHA concentration comprised between 1.5 mg/L and 4 mg/L as
267
shown in Fig. 6 (grey area). This second domain corresponds to zeta potential values ranging
268
from +20 mV to -10 mV. IEP is obtained at 2.80 ± 0.25 mg/L corresponding to a charge ratio
269
of SRHA over TiO2 equal to 1.40 ± 0.23 and a maximum z-average diameter value of 2196 ±
270
254 nm. Finally, for SRHA concentrations above 4 mg/L the TiO2 NPs are found to be
11
271
stabilized and complete charge inversion is achieved above 8 mg/L leading to a z-average
272
diameter plateau value at 95 ± 2 nm.
273
When comparing the effects of the SRHA and alginate on TiO2 NPs stability some significant
274
differences between alginate and SRHA are notifiable. On the one hand, the aggregation
275
domain (destabilization) of TiO2 NPs is larger in the presence of alginate than with SRHA if
276
ones consider the zeta potential values. Such observation can be related to the architecture
277
difference between alginate and SRHA. Indeed the alginate is a linear semi-flexible
278
polyelectrolyte whereas SRHA is a more globular heterogeneous and semi-rigid
279
macromolecule which can exhibit important steric hindrance in comparison to alginate. On
280
the other hand, the alginate is more efficient than SRHA for charge neutralization when
281
considering the charge ratio to reach the IEP. It is again the consequence of the alginate
282
flexibility and charge distribution which is expected to facilitate its adsorption at the TiO2
283
surface.
284
TiO2 NPs aggregates in presence of negatively charged alginate and SRHA (pH =
285
pHPZC,TiO2)
286
In this second electrostatic scenario the pH is adjusted to pHPZC,TiO2 = 6.2. In such conditions,
287
TiO2 NPs are uncharged and are forming large aggregates due to predominance of attractive
288
Van-der-Waals forces. On the other hand NOM are negatively charged and their adsorption at
289
the TiO2 aggregates should result in a negatively charged surface for the TiO2-NOM system
290
and thus partial restabilization of TiO2 NPs is expected.
291
Alginate-TiO2 mixtures. At a pH value of 6.2, alginate has a zeta potential value of -28.9 ±
292
3.5 mV and a z-average diameter of 154 ± 34 nm (Fig. 3). As shown in Fig. 7, for successive
293
alginate additions and after an equilibrium time of 45 min between each measurements, a
294
significant negative surface charge acquisition and decrease of the zeta potential are observed. 12
295
For an alginate mass concentration of 3 mg/L or greater a plateau at around -30 mV is
296
reached. Partial but significant disaggregation of the TiO2 NP aggregates is also observed to
297
reach a z-average diameter plateau at around 1 μm for alginate concentrations ≥ 3 mg/L.
298
SRHA-TiO2 mixtures. SRHA exhibits a zeta potential value of -50.8 ± 5.6 mV and a z-
299
average diameter of 378 ± 14 nm (Fig. 4). In presence of SRHA, after an equilibrium time of
300
one day between successive SRHA additions, the zeta potential value decreases as the
301
concentration of SRHA is increasing to finally stabilize to -40 mV for a SRHA concentration
302
of 10 mg/L (Fig. 8a). The size of TiO2 aggregates is also decreasing via disaggregation to
303
reach a plateau value at 300 nm for [SRHA] ≥ 5 mg/L as shown in Fig. 8b.
304
TiO2 NPs disaggregation kinetics
305
The kinetics of disaggregation are investigated here by analyzing the evolution of zeta
306
potential and z-average diameter values as a function of time after addition of given amounts
307
of NOM at t = 0.
308
Alginate-TiO2 mixtures. As shown in Fig. 9, owing to the stabilization of the zeta potential
309
and z-average diameter to constant values, the time to reach equilibrium is equal to 45 min
310
after addition of alginate. Two different regimes of aggregate fragmentation are observed. The
311
first regime corresponds to a rapid decrease of the z-average diameter whereas the second
312
regime to its stabilization. Significant variation of zeta potential values in the first regime of
313
TiO2 NPs aggregate disaggregation (t < 45 min) is only observed at high alginate
314
concentration (≥ 3 mg/L). This suggests a continuous alginate adsorption at the TiO2
315
aggregates during aggregate fragmentation. Alginate concentration clearly influences the
316
disaggregation process and maximum disaggregation is obtained when [alginate] ≥ 3 mg/L
317
with final z-average diameters at 500 nm. It should be noted that only partial disaggregation is
13
318
observed here with the formation of fragments having z-average diameters larger than the
319
initial TiO2 NP sizes (50 nm).
320
SRHA-TiO2 mixtures. The time requested to equilibrate the system after adding a given
321
amount of SRHA, at t = 0, is one day as shown in Fig. 10. The maximum of fragmentation is
322
found for [SRHA] ≥ 5 mg/L. In this condition, the SRHA disaggregates the TiO2 NP
323
aggregates and a z-average diameter of about 250 nm is obtained. Continuous adsorption of
324
SRHA is also observed for high concentrations owing to a significant decrease of the zeta
325
potential values in the first regime of disaggregation (t < 24 h).
326
From the comparison of Fig. 9 and Fig. 10 three main results are emerging. i) At low
327
concentrations (≤ 4 mg/L) alginate is more effective in the disaggregation of the TiO2
328
aggregates than SRHA. Indeed for a concentration of 3 mg/L the z-average is found equal to
329
500 nm after alginate addition and 5.5 μm for the same SRHA concentration. Moreover, the
330
kinetics of disaggregation are greater in the case of alginate with an equilibrium time of
331
approximately 45 min whereas the addition of SRHA required one day before reaching size
332
equilibrium. The semi-flexible and linear character of alginate and thus its ability to adsorb at
333
the TiO2 surface and interpenetrate the aggregate structures is then expected to play an
334
important role in the disaggregation process. Alginate through conformational changes can
335
easily reach the inner structure of the TiO2 aggregates, whereas SRHA diffusion within
336
aggregate is probably more difficult owing to important steric hindrances. ii) When working
337
at higher concentrations (≥ 5 mg/L), SRHA is able to disaggregate the TiO2 NPs more
338
efficiently than alginate with regards to the z-average diameter values after system
339
equilibrium. For a 5 mg/L mass concentration, the z-average diameter is equal to 250 nm after
340
addition of SRHA and 500 nm in the case of alginate. Such a difference can be attributed to
341
the SRHA charge, which is higher than alginate resulting in higher electrostatic and steric
342
repulsions (Baalousha 2009, Christian et al. 2008, Pefferkorn 1995). Disaggregation kinetics 14
343
are also expected to be controlled by the compactness of the aggregates and corresponding
344
fractal dimensions. From a mechanistic point of view low fractal dimension aggregates are
345
supposed to disaggregate by splitting into fragments of similar sizes, whereas high fractal
346
dimension and thus more compact aggregates to undergo attrition, i.e. aggregates erosion.
347
Baalousha (2009) described the disaggregation kinetics of iron oxide in presence of SRHA by
348
two regimes; first disaggregation of aggregates resulting in fractions of similar size with a
349
relative high kinetic rate followed by a second regime of attrition. Pefferkorn when studying
350
disaggregation processes of latex colloids in presence of polyvinylpyridine suggested that
351
after a consequent lag time, a first regime of attrition followed by a second regime where
352
attrition as well as internal rupture of aggregates was taking place. According to our results,
353
aggregates disaggregation is also expected to result from random aggregates breakup leading
354
to a rapid decrease of the z-average diameter. Aggregates attrition resulting in the release of
355
individual NPs is however not excluded. iii) For high NOM concentrations continuous
356
adsorption occurs owing to the decrease of zeta potential values in the first regime of
357
disaggregation. Aggregate disaggregation, in particular aggregate internal break-up, is thus
358
expected to release some free surface for further NOM adsorption.
359
Negatively charged TiO2 NPs in presence of negatively charged alginate and
360
SRHA (pH > pHPZC,TiO2)
361
In this third electrostatic scenario, negatively charged TiO2 NPs and negatively charged NOM
362
are considered at pH 9.8. Electrostatic repulsions are expected to prevent and/or limit the
363
adsorption of NOM at the TiO2 NPs surface.
364
Alginate-TiO2 mixtures. In such condition alginate has a zeta potential value equal to -32.1
365
± 3.4 mV and 150 ± 15 nm as z-average diameter (Fig. 3). As shown in Fig. 11, TiO2 NPs
366
zeta potential values and z-average diameter are constant (-45.5 ± 1.4 mV and 55 ± 1 nm
15
367
respectively) as function of alginate concentration. No significant interactions are thus
368
obtained between TiO2 NPs and alginate.
369
SRHA-TiO2 mixtures. Z-average diameter and zeta potential value of SRHA are equal to
370
393 ± 19 nm and -63.0 ± 4.1 mV respectively (Fig. 4). Contrary to the previous situation, Fig.
371
12 indicates a significant, however limited, decrease of the zeta potential values as the SRHA
372
concentration increases whereas the z-average diameter is found to slightly increase. Such a
373
behavior denotes limited SRHA adsorption at the TiO2 NPs and importance of the
374
amphiphilic character of SRHA as shown by Ait Akbour (Ait Akbour and Jada 2013).
375
Stability diagrams
376
To give an overview of such a complex TiO2 NPs behavior as a function of alginate and
377
SRHA concentrations, three relevant pH values (pH < pHPZC,TiO2, pH = pHPZC,TiO2 and pH >
378
pHPZC,TiO2) were considered and schematic stability diagrams reported in Fig. 13 and Fig. 14
379
respectively. They summarize the aggregate formation and disaggregation domains resulting
380
from the surface charge variation of TiO2 NPs in presence of NOM (§3.2). The domains of
381
destabilization (aggregation) are represented by cross area and the changes in TiO2 NPs
382
stability (aggregation versus disaggregation) by arrows. At pH 9.8 (blue area), TiO2 are stable
383
from an electrostatic point of view and further addition of alginate has no effect on TiO2 NPs
384
stability. On the other hand, in presence of SRHA, limited adsorption of SRHA at the TiO2
385
NPs occurs owing to hydrophobic interactions. Such an interaction is nonetheless promoting
386
the NPs stability and overcharging. At pH 6.2 (yellow area), TiO2 NPs form aggregates and
387
subsequent addition of NOM results in the partial disaggregation of TiO2 aggregates as
388
confirmed by TEM analysis in Fig. 15. At pH 4.5 (red area), the presence of NOM gradually
389
leads to charge neutralization of the TiO2 NPs and thus rapid destabilization of the system is
390
observed (i.e. aggregate formation). The destabilization range is comprised between 1.1 and
16
391
2.4 mg/L and 1.5 to 4 mg/L for alginate and SRHA respectively. By increasing further NOM
392
concentration, electrostatic repulsions due to charge inversion of NPs become predominant
393
and stabilization of the NPs takes place again. Stability diagrams clearly indicate here that
394
three important mechanisms are controlling the NPs stability; NOM adsorption and resulting
395
surface charge, aggregation and disaggregation mainly controlled by the electrostatic and to a
396
lesser extent by steric and hydrophobic interactions.
397
3. Conclusions
398
Zeta potential and z-average hydrodynamic diameter measurements were used to investigate
399
in a systematic way the interaction processes and stability of solutions containing TiO2 NPs in
400
presence of two important environmental compound models, i.e. alginate and Suwannee river
401
humic acid. To the best of our knowledge, it is the first time that such a systematic
402
comparison has been done on the impact of two different components on both individual NPs
403
and aggregates. Our results clearly indicate that aggregation, disaggregation and stabilization
404
of TiO2 NPs (isolated or aggregated) are depending on the solution pH, concentration ratio of
405
environmental compounds over TiO2 NPs and equilibrium time. Our study also suggest that
406
typical environmental concentrations of humic acids (> 4 mg/L) and alginate (> 2.4 mg/L) are
407
sufficient to stabilize TiO2 nanoparticles even if present at high concentration (50 mg/L) and
408
that they can play important roles in the disaggregation and dispersion of already formed
409
nanoparticle aggregates. In all cases the negatively charged SRHA and alginate adsorb on
410
positive and neutral NPs and charge inversion is observed. This indicates that the driving
411
force for adsorption and NPs behavior is a complex combination of electrostatic attractive and
412
repulsive interactions. Hydrophobic interactions and steric effects are also playing key roles
413
in the NPs stability in particular when amphiphilic compounds such as SRHA are considered.
414
Our results also indicates that times required to partially disaggregate NPs aggregates are
17
415
relatively short and involves different mechanisms when comparison is made between linear
416
and globular polyelectrolytes. The conclusions reported here should, however, considered as a
417
first step for several reasons since aquatic systems consist in heterogeneous systems and the
418
concomitant presence of humic compounds, biopolymers as well as monovalent and
419
multivalent salts and natural inorganic colloidal particles, may also significantly modify the
420
final stability, transport and transformation processes of manufactured nanoparticles.
421 422
Acknowledgments
423
The authors are grateful to Sabrina Lacomme for technical assistance and the contribution of
424
BioImaging Center at the University of Bordeaux 2. We also acknowledge the financial
425
support received from the Swiss National Foundation (project 200021_135240 and
426
206021_13377).
18
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Domingos, R.F., Tufenkji, N. and Wilkinson, K.J. (2009) Aggregation of Titanium Dioxide Nanoparticles: Role of a Fulvic Acid. Environmental Science & Technology 43(5), 12821286. French, R.A., Jacobson, A.R., Kim, B., Isley, S.L., Penn, R.L. and Baveye, P.C. (2009) Influence of Ionic Strength, pH, and Cation Valence on Aggregation Kinetics of Titanium Dioxide Nanoparticles. Environmental Science & Technology 43(5), 1354-1359. Guzman, K.A.D., Finnegan, M.P. and Banfield, J.F. (2006) Influence of surface potential on aggregation and transport of titania nanoparticles. Environmental Science & Technology 40(24), 7688-7693. Ju-Nam, Y. and Lead, J.R. (2008) Manufactured nanoparticles: An overview of their chemistry, interactions and potential environmental implications. Science of the Total Environment 400(1-3), 396-414. Keller, A.A., Wang, H., Zhou, D., Lenihan, H.S., Cherr, G., Cardinale, B.J., Miller, R. and Ji, Z. (2010) Stability and Aggregation of Metal Oxide Nanoparticles in Natural Aqueous Matrices. Environmental Science & Technology 44(6), 1962-1967. Liu, Z., Jiao, Y., Wang, Y., Zhou, C. and Zhang, Z. (2008) Polysaccharides-based nanoparticles as drug delivery systems. Advanced Drug Delivery Reviews 60(15), 1650-1662. Ngomsik, A.F., Bee, A., Siaugue, J.M., Cabuil, V. and Cote, G. (2006) Nickel adsorption by magnetic alginate microcapsules containing an extractant. Water Research 40(9), 1848-1856. Nowack, B. and Bucheli, T.D. (2007) Occurrence, behavior and effects of nanoparticles in the environment. Environmental Pollution 150(1), 5-22. Ottofuelling, S., Von der Kammer, F. and Hofmann, T. (2011) Commercial Titanium Dioxide Nanoparticles in Both Natural and Synthetic Water: Comprehensive Multidimensional Testing and Prediction of Aggregation Behavior. Environmental Science & Technology 45(23), 10045-10052. Pallem, V.L., Stretz, H.A. and Wells, M.J.M. (2009) Evaluating Aggregation of Gold Nanoparticles and Humic Substances Using Fluorescence Spectroscopy. Environmental Science & Technology 43(19), 7531-7535. Papageorgiou, S.K., Katsaros, F.K., Favvas, E.P., Romanos, G.E., Athanasekou, C.P., Beltsios, K.G., Tzialla, O.I. and Falaras, P. (2012) Alginate fibers as photocatalyst immobilizing agents applied in hybrid photocatalytic/ultrafiltration water treatment processes. Water Research 46(6), 1858-1872. Parks, G.A. (1965) Isolelectric points of solid oxides, solid hydroxydes and aqueous hydroxo complex systems. Chemical Reviews 65(2), 177-198. Pefferkorn, E. (1995) The role of polyelectrolytes in the stabilization and destabilization of colloids. Advances in Colloid and Interface Science 56, 33-104.
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21
Figure
Fig. 1 - a) Zeta potential variation of TiO2 NPs as a function of pH. The PZC is found here to be equal to pH = 6.2 ± 0.1. b) TiO2 particles z-average diameter variation as a function of pH. TiO2 NPs aggregation domain is found important and comprised between -30 mV and +30 mV (gray domain); [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 2 - a) Transmission Electron Microscopy image of dispersed TiO 2 NPs at pH 4.5 (positively charged NPs) with a 500000× magnification. Isolated and small aggregates are observed. b) TEM image of TiO2 NPs aggregate at pH = 6.2 (point of zero charge) with a 60000× magnification. Large aggregates are observed.
Figure
Fig. 3 - a) Zeta potential variation of alginate as a function of pH. No PZC is found here. b) Alginate z-average diameter variation as a function of pH. The z-average diameter is found constant with a value of 178 ± 21 nm. [Alginate] = 100 mg/L; I = 0.001 M.
Figure
Fig. 4 - a) Zeta potential variation of SRHA as a function of pH. No PZC is found here. b) SRHA z-average diameter variation as a function of pH. The z-average diameter is found constant with a value of 379 ± 19 nm. [HA] = 100 mg/L; I = 0.001 M.
Figure
Fig. 5 - a) Zeta potential variation of TiO2 NPs as a function of alginate concentration at pH 4.5. Charge neutralization (IEP) is found here for [alginate] = 1.68 ± 0.01 mg/L and a charge ratio of alginate over TiO2 equal to 1.25 ± 0.21. b) TiO2 z-average diameter variation as a function of alginate concentration at pH 4.5. Aggregation domain is found important and comprised between +28 mV and -28 mV (gray domain) and maximum aggregation is observed at the IEP. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 6 - a) Zeta potential variation of TiO2 NPs as a function of SRHA concentration at pH 4.5. Charge neutralization (IEP) is found here for [SRHA] = 2.80 ± 0.25 mg/L and a charge ratio of SRHA over TiO2 equal to 1.40 ± 0.23 . b) TiO2 z-average diameter variation as a function of SRHA concentration at pH 4.5. Aggregation domain is found important and comprised between +20 mV and -10 mV (gray domain) and maximum aggregation is observed at the IEP. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 7 - a) Zeta potential variation of TiO2 NPs as a function of alginate concentration at pH 6.2 (PZC) for successive alginate addition. b) TiO2 z-average diameter variation as a function of alginate concentration at pH 6.2 (PZC). Maximum of disaggregation is found for [alginate] ≥ 3 mg/L. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 8 - a) Zeta potential variation of TiO2 NPs as a function of SRHA concentration at pH 6.2 (PZC) for successive SRHA addition. b) TiO2 z-average diameter variation as a function of SRHA concentration at pH 6.2 (PZC). Maximum of disaggregation is found here for [SRHA] ≥ 5 mg/L. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 9 - a) TiO2 aggregate z-average diameter variation as a function of time at pH 6.2 (PZC) for alginate mass concentration of: ▲ 0.5 mg/L, ▼ 1 mg/L, ◄ 2 mg/L, ► 3 mg/L and ● 5 mg/L. b) Zeta potential variation of TiO2 aggregates in presence of alginate as a function of time at pH 6.2 (PZC). Equilibrium time for disaggregation is found to be equal to 45 min. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 10 - a) TiO2 aggregate z-average diameter variation as a function of time at pH 6.2 (PZC) for a SRHA mass concentration of: ▲ 3 mg/L, ▼ 4 mg/L, ◄ 5 mg/L, ► 7 mg/L and ● 10mg/L. b) Zeta potential variation of TiO2 aggregates in presence of SRHA as a function of time at pH 6.2 (PZC). Equilibrium time for disaggregation is found to be equal to 24 h. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 11 - a) Zeta potential variation of TiO2 NPs as a function of alginate concentration at pH 9.8 for successive alginate addition. b) TiO2 z-average diameter variation as a function of alginate concentration at pH 9.8. No specific interaction between alginate and TiO2 NPs are observed owing to the electrostatic repulsions. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 12 - a) Zeta potential variation of TiO2 NPs as a function of SRHA concentration at pH 9.8 for successive SRHA addition. b) TiO2 z-average diameter variation as a function of SRHA concentration at pH 9.8. Limited adsorption of SRHA at TiO2 surface is observed due to the amphiphilic character of SRHA. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 13 - Stability diagram for TiO2 NPs in presence of alginate at pH = 4.5 < pHPZC,TiO2, pH = 6.2 = pHPZC,TiO2 and pH = 9.8 > pHPZC,TiO2. The two only destabilization domains are represented by shaded areas. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 14 - Diagram of stability for TiO2 NPs in presence of HA for pH = 4.5 < pHPZC,TiO2, pH = 6.2 = pHPZC,TiO2 and pH = 9.8 > pHPZC,TiO2. The two only destabilization domains are represented by shaded regions. [TiO2] = 50 mg/L; I = 0.001 M.
Figure
Fig. 15 - a) TEM image of TiO2 NPs aggregate fragments at pH 6.2 in presence of alginate with a 120000× magnification. b) TEM image of TiO2 NPs aggregate fragments at pH = 6.2 in presence of SRHA with a 100000× magnification. Presence of NOM is found to result in aggregate fragmentation.
