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Influence of Surface Functional Groups on Deposition and Release of TiO2 Nanoparticles Zhiwei Wang, Xueye Wang, Junyao Zhang, Xueqing Yu, and Zhichao Wu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 11 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017
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Influence of Surface Functional Groups on Deposition and Release of TiO2
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Nanoparticles
3 4
Zhiwei Wang,†,* Xueye Wang,† Junyao Zhang,† Xueqing Yu,† Zhichao Wu†
5
†
6
Science and Engineering, Tongji University, Shanghai 200092, China
State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental
7 8 9 10 11
Revised Manuscript for Environmental Science & Technology (Clean version)
12
May 17, 2017
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ABSTRACT: A clear understanding of the factors governing the deposition and release
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behaviors of engineered nanoparticles (NPs), such as TiO2 NPs, is necessary for predicting
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their transport and fate in both natural and engineered aquatic environments. In this study,
16
impacts of specific chemistries on TiO2 NP deposition, as a function of TiO2 NP concentration
17
and ionic strength/valence, were investigated using self-assembled monolayers (SAMs) with
18
five different ending chemical functionalities (−CH3, −OH, −COOH, −NH2, and −CONH2).
19
The fastest deposition and maximum deposition mass were observed on −NH2, followed by –
20
COOH, –CONH2, –CH3 and –OH, showing that contact angle and zeta potential of surfaces
21
were not good indicators for predicting the deposition. Specific interactions, for instance,
22
between –COOH or –CONH2 and TiO2, significantly affected their deposition. Deposition
23
rate increased linearly with TiO2 NP concentration; however, specific deposition rate was
24
dependent on the type of SAMs. The increase of monovalent (Na+) and divalent (Ca2+) led to
25
different changes in deposition rates for the SAMs due to different functionalities. Results
26
also showed that favorable SAM (e.g., –NH2) had lowered release of NPs compared to
27
unfavorable surface (e.g., –OH). The obtained deposition and release behaviors will support
28
more accurate prediction of the environmental fate of nanoparticles.
29
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INTRODUCTION
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Titanium dioxide nanoparticles (TiO2 NPs) are increasingly used in the fields of catalysts,
32
sunscreens, cosmetics, pigments and wastewater treatment processes due to their
33
extraordinary physical and chemical properties.1-3 The extensive applications of TiO2 NPs
34
inevitably lead to their environmental release incidentally or intentionally.4,5 Recent studies
35
have revealed that TiO2 NPs exhibit toxic effects on human keratinocyte cells6, Chlorella sp.
36
(algae cells)7 and ammonium-oxidizing bacteria existing in activated sludge,8 which
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necessitates a thorough understanding of their transport and fate in the environment.
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For a release of TiO2 NPs to the aquatic environment, the interaction of TiO2 NPs with
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existing organic matters will be one of major determinants of their transport and fate. Organic
40
matters, which are composed of a large variety of compounds (e.g., carbohydrates, proteins,
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lipids, and humic and fulvic acids), have profound effects on the aggregation, mobility and
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deposition.9-11 It has been revealed that dissolved organic matter slows aggregation of
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nanoparticles and inhibits the deposition of them in saturated porous media.11-14 The
44
interactions
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Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, stating that the interaction between a
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colloid and a collector is the sum of their electrostatic and van der Walls interactions.9,15,16 In
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fact, the interactions of NPs and organic matters arise from surface chemistry, in which the
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surface functionalities of organic matters play a very important role in their specific
49
interactions17 except for the non-specific electrostatic and van der Walls interactions. That is
50
why controversial results are reported. For instance, Pettibone et al.18 have found that the
51
presence of some organic acids may destabilize nanoparticle suspensions and thus induce
between
NPs
and
organic
matters
can
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explained
by
the
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their aggregation and deposition, which is contrary to the results as mentioned above.11-14
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Unfortunately, the effects of specific chemical functionalities of organic surfaces on
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deposition and release of TiO2 NPs are largely unknown because of the complexity of the
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organic matters. Take humic acid as an example, the main functional groups include carboxyls,
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hydroxyls, carbonyls, and amides10, which makes it very difficult to examine the specific
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interactions of chemical functionalities with NPs.
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One possible way for simplifying the problem is to study the effect of individual chemical
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functionality using well-defined and homogeneous surface chemistry. Alkanethiols,
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HS(CH2)nX, can absorb onto surfaces of gold and silver, and spontaneously form stable and
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well-defined self-assembled monolayers (SAMs).17 Different ending functional groups at the
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terminal position of the alkanethiols, X, make it possible to control the surface properties at a
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molecular scale. Sigal et al. successfully formed SAMs of alkanethiols with different ending
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chemical groups on gold surfaces to study the contribution of surface wettability to adsorption
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of proteins and detergents.19 The surface wettability of each SAM was found to be correlated
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with the adsorption of proteins and detergents measured by surface plasmon resonance
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method.19 Contreras et al. used seven SAMs with different chemical functionalities to
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investigate their effects on adsorption of polysaccharides and proteins, and found that surface
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modification to render –OH or ethylene-glycol functionalities could mitigate the adsorption.20
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Quartz-crystal microbalance with dissipation monitoring (QCM-D) is capable of
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measuring minute changes in mass adsorbed onto a surface, and has been used extensively in
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biological studies.21-23 Recent studies have also shown that QCM-D can be applied to
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investigate deposition of nanoparticles including carbon nanotubes,24,25 graphene oxide 4
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(GO),26 fullerenes,27,28 quantum dots,29,30 cerium oxide,31 and TiO29,15 due to its high
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sensitivity and low sample volume requirements. These intensive research works demonstrate
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that QCM-D can be successfully utilized to understand interactions of various nanoparticles
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with environmentally relevant surfaces (e.g., SiO2, Fe2O3, Al2O3 and poly-L-lysine). However,
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to date, there have been no studies combining different well-defined SAMs (with different
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functionalities) and QCM-D to investigate the specific interactions between TiO2 NPs and
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functional groups of organic matters.
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In this study, we aim to elucidate the specific effects of various functional groups on
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deposition and release of TiO2 nanoparticles. QCM-D was used to investigate TiO2 NPs
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deposition equilibrium and kinetics on a variety of well-controlled SAMs representing
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common surface functionalities11 of organic matters. The influences of electrolyte
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concentrations and cation types on the deposition and release of TiO2 NPs onto/from various
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SAM-coated surfaces were also evaluated. This study is expected to provide insights into the
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deposition and release of TiO2 NPs in the aquatic environment and in water treatment
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processes through functional group-NP’s specific interaction perspectives.
89 90
MATERIALS AND METHODS
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Materials. A variety of alkanethiols, as shown in Table 1, were used to create SAMs on
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QCM-D sensors. 1-dodecanethiol, 11-hydroxy-1-undecanethiol, 11-mercaptoundecaneamide,
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11-amino-1-undecanethiol hydrochloride and 11-mercaptoundecanoic acid were purchased
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from Sigma-Aldrich (St. Louis, MO, USA). The different ending groups of the alkanethiols
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represent the common functional groups of organic matters existing in aquatic environment, 5
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e.g., −CH3, −OH, −COOH, and −NH2 and −CONH2. TiO2 nanoparticles with particle size
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reported by the manufacturer (Sigma-Aldrich, St. Louis, MO, USA) was about 25 nm.
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Reagent grade NaCl, CaCl2, NaOH, HCl, 200 proof ethanol and sodium-dodecyl-sulfate (SDS)
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were purchased from Sinopharm (Shanghai, China).
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Table 1. Characteristics of SAMs prepared in this study. XPS Terminal
Contact
group
angle (deg)
Alkanethiols
elemental
compositions (10-2) O/C
N/C
1-dodecanethiol
−CH3
100.5±1.4
3.45
/
11-hydroxy-1-undecanethiol
−OH
34.0±0.9
17.7
/
11-mercaptoundecaneamide
−CONH2
36.4±1.9
21.1
11.7
11-amino-1-undecanethiol, hydrochloride
−NH2
71.7±2.5
21.3
14.1
11-mercaptoundecanoic acid
−COOH
44.9±4.8
32.0
/
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Methods. TiO2 NPs Stock Solution. TiO2 NPs stock solution with 100 mg/L NPs was
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prepared according to the literature8,9,14 using deionized (DI) water followed by
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ultrasonication for 1 h (25 oC, 250 W, 40 kHz). A series of TiO2 suspensions (5, 10, 15, 20
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mg/L) were prepared by diluting the stock solution into a number of NaCl and CaCl2
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solutions of different ionic strengths9,14 (NaCl=10, 20, 40, 60, 80, 100 mM; CaCl2=0.5, 1.0,
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1.5, 2.0 mM) at pH 7. The resulting TiO2 suspensions were further sonicated for another 15
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min prior to use.9,14 The hydrodynamic diameter, polydispersity index (PDI), electrophoretic
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mobility (EPM), and zeta potential of TiO2 NPs were determined by a Malvern Zatasizer 4700
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(Malvern Instruments, UK) immediately after sonication. Transmission electron microscopy
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(TEM) was also employed to confirm the TiO2 NPs size distribution under investigated 6
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conditions since the hydrodynamic diameter measured by dynamic laser scattering (DLS)
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(Malvern Zatasizer 4700) may deviate from their actual size.15,24,30 The obtained results are
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well in agreement with previous studies.9,15 More information on TiO2 NPs characteristics is
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provided in Supporting Information (SI) Figures S1~S6. The DLS results were much larger
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compared to TEM measurements, suggesting that aggregation of TiO2 NPs still occurred
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under experimental conditions.
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SAM Preparation and Characterization. Five SAMs each with a different terminal
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functional group (see Table 1) were prepared on gold-coated QCM-D crystal sensors. Prior to
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self-assembly, gold-coated QCM-D crystals were cleaned sequentially in toluene, acetone,
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and ethanol twice in each solvent for 10 min each time in an ultrasonic bath.20 The crystals
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were then dried with ultrapure nitrogen gas and then placed in a cleaning chamber
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(YZD08-2C, Tangshan, China) for 30 min. The cleaned crystals were then submerged in
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ethanol for 20 min32. Afterward, the cleaned gold-coated crystals were immersed into a 1 mM
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alkanethiol solutions in ethanol for 24 h followed by sonication in ethanol for 5 min to
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remove loosely bound molecules. After that, an additional 24-h immersion was carried out to
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enhance the homogeneity of the SAMs on the crystal surface. Finally, the alkanethiol-coated
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crystals were rinsed with ethanol, dried with nitrogen, and stored under vacuum. For creating
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a more ordered and homogeneous surface for −NH2 and –COOH, the pH of
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11-amino-1-undecanethiol hydrochloride and 11-mercaptoundecanoic acid solutions were
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adjusted to 11 and 2 prior to immersion, respectively, using 0.2 M NH4OH and HCl to reduce
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electrostatic repulsion between thiol chains.
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After self-assembly, contact angle of water in air for the prepared SAMs was measured 7
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using the sessile drop method with an OCA contact angle analyzer (JC2000D, Yisu Co.
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Shanghai, China). At least six measurements were carried out for each SAM. Both DI water
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and the background solutions were used to measure the contact angles. The results were not
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evidently different, and thus only data obtained with DI water was reported. The elemental
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compositions of the prepared SAMs were analyzed by an X-ray photoelectron spectroscopy
139
(XPS) (AXIS UltraDLD, Kratos Analytical Ltd., U.K.) with an Al X-ray source and a
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monochromator. Since XPS is destructive, the SAMs measured by XPS will not be used for
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subsequent experiments. The original SAMs prepared in parallel were employed for QCM-D
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analyses. The pKa values of -OH, -CONH2, and -COOH functional groups were determined
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to be 7.09 ± 0.13, 6.54 ± 0.04, and 5.95 ± 0.23, respectively, using an automatic
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potentiometric titrator (ZDJ-4A, Shanghai Leici, China). Therefore, −OH, and −CONH2
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(close to 7) as well as −CH3 SAMs could be regarded as uncharged surfaces, while −COOH
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was negatively charged surface under experimental condition (pH=7). Since –NH3+ has a
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pKa~7.520, the terminal monolayer of −NH2 group (11-amino-1-undecanethiol hydrochloride)
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was positively charged under experimental condition.
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Deposition and Release Experiments. Deposition and release of TiO2 NPs onto/from
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prepared SAMs were investigated using QCM-D (E4, Q-Sense AB, Sweden). The system has
151
four modules, and each of them holds a 5 MHz AT-cut quartz crystal sensor. The SAM-coated
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crystals, as mentioned earlier, were mounted on the system. For all experiments, the flow rate
153
was maintained at 0.15 mL/min using a peristaltic pump (IsmaTec Pump, IDEX). Solutions
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injected into the chambers were maintained at 25 ± 1 oC. For a rigid adsorbed layer (i.e.,
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negligible energy dissipation, ∆D≤1) 20,33, the adsorbed mass is proportional to the change in 8
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frequency, which can be described by the Sauerbrey equation (Eq. 1):
∆m = C
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−∆f n n
(1)
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where ∆m is the change in deposited mass (ng/cm2), C is the mass sensitivity constant (17.7
159
ng/(Hz cm2)), ∆fn is the measured frequency shift at the nth overtone (Hz), and n is the
160
overtone number (3, 5, 7 and 9 in this study). In our study, we presented the shifts in the
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normalized third overtone frequency and dissipation (∆f3 and ∆D3) since this overtone has the
162
best signal-to-noise ratio.9,26
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For viscoelastic deposited layers that exhibit high energy dissipations (∆D>1 or ∆D/∆f>0.1)
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20,33
165
thickness (dl), shear elastic modulus (µl), and viscosity (ηl) of the deposited layer.34 All
166
experiments were carried out at 25 oC and repeated three times.
, a viscoelastic model was used to fit the ∆f and ∆D data to determine the density (ρl),
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The deposition and release behaviors of NPs can be also characterized by calculating
168
deposition rate and attachment efficiency from the frequency shifts monitored by
169
QCM-D.9,15,26 Initial deposition rates of NPs, rf and rD, are defined as rates of frequency and
170
dissipation shift in a time period (t 1 or ∆D/∆f > 0.133. The deposition rates on five
206
SAMs are shown in Figure 1b, showing similar patterns to the deposition mass (Figure 1a). 0.25
16 -NH2
14
(a)
Deposition rate (Hz/min)
Deposited mass (ng/cm2)
207 208 209 210 211 212 213 214 215 216 217
-COOH
12 10 8
-CH3
-CONH2
6 -OH
4 2 0
(b)
0.20 -NH2
0.15 -CONH2
0.10 -COOH -CH3
0.05 -OH
0.00
30
50
70 90 Contact angle (o)
110
30
50 70 90 Contact angle (o)
110
218
Figure 1. (a) Deposited mass of TiO2 at equilibrium, and (b) initial deposition rate on
219
different SAMs. Test conditions: TiO2=5 mg/L, IS=10 mM (NaCl), and pH=7. Error bars
220
represent standard deviations of triplicate measurements; where absent, bars fall within
221
symbols.
222 223
The Pearson’s product momentum correlation analysis (SPSS software, USA) was used to
224
evaluate the relationship between deposited mass and contact angle. Results showed that 11
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equilibrium deposition of TiO2 NPs did not correlate well with the water contact angle of the
226
SAMs (p > 0.05), indicating that surface hydrophobicity alone is not a strong indicator for
227
evaluating TiO2 NP deposition.
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Among the uncharged surfaces (–OH, –CONH2, –CH3), the hydrophilic surface (–OH) had
229
lower amounts of deposited TiO2 compared to the highly hydrophobic surface (–CH3),
230
consistent with previous studies36,37. Interestingly, the –CONH2 surface, with a water contact
231
angle close to that of –OH surface, showed much larger amounts of deposited TiO2. This may
232
be attributed to strong hydrogen bonding interactions of the –CONH2 and the abundant –OH
233
groups on TiO2 NPs38 and also interactions of the –CONH2 and the oxygen atoms of Ti–O
234
bond.
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The positively charged –NH2 surface had more deposited mass than the rest surfaces,
236
which is likely due to the strong electrostatic attraction between –NH2 surface and negatively
237
charged TiO2 as well as the hydrogen bonding interactions of –NH2/TiO2 NPs. Another reason
238
is attributed to the acid-base interaction between –NH3+ (pKa~7.520) and TiO2 (Figure S1,
239
isoelectric point of TiO2 is around 6)39 when –NH2 (proton acceptor) and TiO2 (proton donor)
240
are brought close together. Unexpectedly, it was found that the negatively charged and
241
hydrophilic –COOH also demonstrated larger deposition mass than –OH and –CH3. The
242
massive deposition of TiO2 NPs on the –COOH surface is attributed to the specific
243
interactions between them besides hydrogen bonding. Two oxygens of the carboxylic group
244
can bind two surface titanium ions forming a bridge-coordinating mode40,41 despite the
245
negative charge and hydrophilicity of –COOH group.
246
Initial rates of frequency shifts (rf) for all the SAMs were obtained from the initial slopes 12
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of ∆f3 (see Figure 1b). The initial deposition rates for the SAMs followed the order of –NH2 >
248
–COOH = –CONH2 = –CH3 > –OH, similar to the deposited mass on various SAMs. It again
249
suggests that the deposition behaviors had no correlations with water contact angle. The –NH2
250
surface demonstrated the greatest deposition rate, inferring that it is a favorable organic
251
surface for TiO2 NPs deposition.
252
In summary, the deposition mass and initial deposition rates of TiO2 NPs indicate that
253
specific interactions, for instance, between –COOH or –CONH2 and TiO2 NPs, play an
254
important role in TiO2 NPs deposition in addition to the nonspecific electrostatic and
255
hydrophobic interactions. Water contact angle and zeta potential of surfaces are therefore not
256
good indicators for predicting TiO2 NPs deposition. For instance, beyond expectation, organic
257
surfaces with low water contact angle and high negative zeta potential due to the assembly of
258
–COOH groups adsorb abundant TiO2 NPs.
259
Effect of TiO2 Concentration on Deposition. TiO2 NP deposition behaviors in 10 mM
260
NaCl were monitored on all the SAMs as TiO2 NP concentrations were varied from 5 to 20
261
mg/L. Deposition rates as a function of TiO2 concentration are presented in Figure 2. Results
262
showed that deposition rate (rf) increased linearly with TiO2 NP concentration (Figure 2). As
263
TiO2 NPs were injected into the QCM-D chamber in parallel flow configuration, which allows
264
for first order deposition,26 the mass deposition rates on crystals are thus proportional to TiO2
265
NP concentration.
266
Among the SAMs, specific deposition rates (∆rf/∆C), i.e., the slopes of rf as a function of
267
TiO2 concentration in Figures 2a~e, are different (Figure 2f). This indicates that TiO2 NP
268
deposition kinetics under various TiO2 concentrations are strongly dependent on the type of 13
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SAMs. The specific deposition rates (∆rf/∆C) for the SAMs are in consistent with those in
270
Figure 1b. The –NH2 surface had the largest ∆rf/∆C while the –OH surface had the lowest
271
specific deposition rates. The results demonstrate that –NH2 SAM is a favorable surface for
272
TiO2 NP deposition compared to the rest SAMs. In addition, it again confirmed that specific
273
interactions between –COOH or –CONH2 and TiO2 NPs significantly affected the deposition
274
of TiO2 NPs under various TiO2 concentrations since the changes of ∆rf/∆C among the SAMs
275
also did not correlate well with hydrophobic and electrostatic interactions. 0.50
0.50 0.40
0.30
0.30
0.20
0.20
0.10
0.10
0.00
0.00
0
10 20 TiO2 concentration (mg/L)
0
0.30 0.20 0.10 0.00
0.40
(d)
0.30 0.20 0.10 0.00 0 10 20 TiO2 concentration (mg/L)
0 10 20 TiO2 concentration (mg/L)
0.50
0.025
(e)
∆r/∆C (Hz/(min·mg/L)
0.40
10 20 TiO2 concentration (mg/L)
0.50
(c)
Deposition rate(Hz/min)
Deposition rate(Hz/min)
0.50 0.40
(b)
Deposition rate(Hz/min)
(a)
0.40
Deposition rate(Hz/min)
276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299
Deposition rate(Hz/min)
269
0.30 0.20 0.10 0.00
0.020
(f)
0.015 0.010 0.005 0.000
0 10 20 TiO2 concentration (mg/L)
300
Figure 2. Deposition rates for (a) –OH, (b) –CONH2, (c) –COOH, (d) –NH2 and (e) –CH3 as
301
a function of TiO2 concentration; (f) Specific deposition rate (∆rf/∆C) for the functional
302
groups, i.e., slopes of Figures (a)~(e). Test conditions: IS=10 mM (NaCl), and pH=7. Error 14
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bars in Figures (a)~(e) represent standard deviations of three runs; where absent, bars fall
304
within the symbols.
305 306
Effect of Ionic Strength and Valence on Deposition. Effect of Ionic Strength (IS) on
307
Deposition. Figure 3 presents initial deposition rates (rf) on all SAMs as a function of ionic
308
strength (NaCl) at pH 7. It can be observed from Figure 3d that the TiO2 NPs deposition rate
309
on −NH2 SAM (i.e., under favorable conditions) remained almost constant (0.25 Hz/min) in
310
solutions with low NaCl IS (≤ 60 mM), suggesting that TiO2 NPs deposition under favorable
311
conditions is independent of solution chemistry. However, as IS further increased, TiO2 NPs
312
initial deposition rates decreased rapidly. The decrease in deposition rate is consistent with
313
colloid deposition theory27,29 and can be explained by the formation of larger aggregates
314
(Figure S5 and Figure S6) with lower diffusivities at higher IS.24,28 With the increase of IS,
315
TiO2 NPs’ electrical double-layer42 was compressed and the zeta potential of TiO2 NPs at 60
316
mM NaCl appeared to be neutral (Figure S5), resulting in the reduced attractive electrostatic
317
forces (even repulsive) between −NH2 and NPs.
318
For negatively charged −COOH (Figure 3c), no obvious trend for the evolution of
319
deposition rate was observed. This may be attributed to the specific interactions between
320
−COOH and TiO2 NPs besides hydrogen bonding. Two oxygens of the carboxylic group can
321
bind two surface titanium ions to form a bridge-coordinating mode40,41 despite the negative
322
charge and hydrophilicity of –COOH group. For the non-specific interactions, at high IS,
323
diffusive transport of TiO2 NPs due to formation of large aggregates is hindered. However, the
324
repulsive electrostatic forces were reduced (even attractive electrostatic forces appeared)
325
between −COOH and NPs after TiO2 NPs’ electrical double-layer42 is compressed at high IS. 15
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This electrostatic force changes compromised the decreased diffusive transport of TiO2 NPs to
327
the SAM surface.
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The deposition of TiO2 NPs on the uncharged surfaces (–OH, –CONH2, –CH3) are not
329
associated with electrostatic interactions. For –OH, and –CONH2, the hydrogen bonding
330
interactions may play an important role while, for –CH3, the hydrophobic interactions might
331
dominate the deposition process. For –OH (Figure 3a) and –CH3 SAMs (Figure 3e), the
332
deposition rate initially increased with the increase of IS, then leveled off, and finally
333
decreased with further increase of IS. This suggests that the deposition rate on the neutral
334
SAMs is governed by two factors, i.e., the enhanced deposition with relatively large particles
335
(large mass per particle) and the lowered diffusion coefficients of aggregated particles.
336
Further increase of IS resulted in a limited transport of particles and thus a decreased
337
deposition. However, no increase stage was observed for –CONH2 SAM, indicating that the
338
influential IS range was different from –OH though the interactions between SAM and NPs
339
were both related to hydrogen bonding38. This may be attributed to strong hydrogen bonding
340
interactions of the –CONH2 and the abundant –OH groups on TiO2 NPs
341
interactions between –CONH2 and the oxygen atoms of Ti–O bond. Another reason might be
342
related to different critical deposition concentrations (CDC)27,28 for different SAMs.
38
and also special
343
Through normalizing deposition rates under unfavorable conditions (–OH, −COOH, –
344
CONH2 and –CH3) to favorable deposition rates, deposition efficiencies (ɑD) of TiO2 NPs
345
were determined, with results shown in Figure 4. Increases in NaCl concentration led to
346
higher ɑD on the unfavorable SAMs, which can be explained by DLVO theory.26 For –OH, ɑD
347
values of TiO2 NPs increased until 40 mM NaCl and further increases in IS did not produce 16
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any obvious change. The CDC of TiO2 NPs on –OH SAM was thus determined to be 40 mM
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from the intersection of extrapolations from the favorable and unfavorable regimes as
350
indicated by dotted lines in Figure 4. However, it is about 80 mM (roughly determined based
351
on the general changing trend) for –CONH2, −COOH and –CH3 SAMs. The differences in
352
CDC values for –OH SAM and –CONH2, −COOH and –CH3 SAMs suggest that CDC values
353
are dependent on specific surface chemistries. Deposition rate(Hz/min)
0.3
(a) –OH 0.2
0.1
0.3
(b) –CONH2 0.2
0.1
0.0
0.0
0 20 40 60 80 100 NaCl concentration (mM)
0 20 40 60 80 100 NaCl concentration (mM) 0.30
0.50
Deposition rate(Hz/min)
Deposition rate(Hz/min)
(c) –COOH
(d) –NH2
0.40
0.20
0.30 0.20
0.10
0.10 0.00
0.00
0
0 20 40 60 80 100 NaCl concentration (mM)
20 40 60 80 100 NaCl concentration (mM)
0.40
Deposition rate(Hz/min)
354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377
Deposition rate (Hz/min)
348
(e) –CH3
0.30 0.20 0.10 0.00 0 20 40 60 80 100 NaCl concentration (mM)
378
Figure 3. Deposition rates for (a) –OH, (b) –CONH2, (c) –COOH, (d) –NH2 and (e) –CH3 as
379
a function of NaCl concentration (TiO2 concentration 10 mg/L, and pH=7). Error bars
380
represent standard deviations of three runs; where absent, bars fall within the symbols.
381
17
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1.2 Deposition efficiency (ɑ)
Deposition efficiency (ɑ)
Environmental Science & Technology
(a) –OH
1.0 0.8 0.6 0.4 0.2 0.0
0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 NaCl concentration (mM)
Deposition efficiency (ɑ)
Deposition efficiency (ɑ)
1.2
(c) –COOH
1.0
(b) –CONH2
1.0
0 20 40 60 80 100 NaCl concentration (mM)
382
0.8 0.6 0.4 0.2
1.4 1.2
(d) –CH3
1.0 0.8 0.6 0.4 0.2 0.0
0.0 0
383
1.2
0 20 40 60 80 100 NaCl concentration (mM)
20 40 60 80 100 NaCl concentration (mM)
384
Figure 4. Deposition efficiency of (a) –OH, (b) –CONH2, (c) –COOH, and (d) CH3 as a
385
function of NaCl concentration (TiO2 concentration 10 mg/L, and pH=7) via normalizing
386
deposition rates to favorable SAM, i.e., –NH2. Error bars represent standard deviations of
387
three runs; where absent, bars fall within the symbols. The dotted lines are used to
388
schematically indicate the changing trend of deposition efficiency.
389 390
Effect of Ion Valence on Deposition. Deposition rates of TiO2 on various SAMs as a
391
function of CaCl2 concentration at pH 7 are presented in Figure 5. Since Ca2+ ions have a
392
bridging ability, they may be more aggressive in promoting aggregation of NPs than NaCl.43
393
That is why the deposition rates on –CONH2 and –CH3 SAMs decreased with the increase of
394
IS (CaCl2), with no increase phase of rf compared to NaCl. However, the –OH SAM exhibited
395
a relatively stable deposition rates, which might be due to the bridging effects between Ca2+
396
and –OH in addition to the aggregation of NPs brought by Ca2+. For the negatively charged –
397
COOH SAM, it showed an increase of deposition rate with the increase of Ca2+ strength. This
398
is mainly associated with the strong complexation effects between Ca2+ and –COOH, which is 18
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supported by literature that an increase of carboxyl groups on membrane surfaces can induce
400
severe membrane fouling by alginate in the presence of calcium ions44. The positively charged
401
–NH2 undoubtedly showed a decrease trend as the IS increased, attributed to the mitigation of
402
electrostatic interactions and also the formation of large aggregates. Deposition efficiency
403
(Figure S8) shows that CDC value for –CONH2 is 1 mM compared to 1.5 mM for –OH and –
404
CH3 SAMs. However, it is evident that the negative charged –COOH SAM has no CDC value,
405
indicating that the specific interaction (complexation) between TiO2 and –COOH might
406
dominate the deposition efficiency. 0.3
(a) –OH 0.2
0.1
0.0
(b) –CONH2 0.2
0.1
0.0 0
0.30
Deposition rate (Hz/min)
Deposition rate (Hz/min)
(c) –COOH 0.20
0.10
0.00 0 0.5 1 1.5 2 2.5 CaCl 2 concentration (mM) Deposition rate (Hz/min)
409
0.3
0 0.5 1 1.5 2 2.5 CaCl 2 concentration (mM)
407
408
Deposition rate (Hz/min)
Deposition rate (Hz/min)
399
0.5 1 1.5 2 2.5 CaCl 2 concentration (mM)
0.30
(d) –NH2 0.20
0.10
0.00 0 0.5 1 1.5 2 2.5 CaCl 2 concentration (mM)
0.30
(e) –CH3 0.20
0.10
0.00 0 0.5 1 1.5 2 2.5 CaCl 2 concentration (mM)
410
Figure 5. Deposition rates for (a) –OH, (b) –CONH2, (c) –COOH, (d) –NH2 and (e) –CH3 as
411
a function of CaCl2 concentration (TiO2 concentration 10 mg/L, and pH=7). Error bars 19
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412
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represent standard deviations of three runs; where absent, bars fall within the symbols.
413 414
Release of Deposited TiO2 NPs. In aqueous environments, rainfall, flooding events and
415
ground water-surface water exchange can result in changes in solution chemistry and might
416
cause a release of deposited TiO2 NPs. In our study, release of TiO2 NPs on various SAMs
417
was monitored in DI water (pH=7) without any TiO2 NPs in suspension. The results in terms
418
of release fractions are summarized in Table 2.
419 420
Table 2. Release fractions (%) of deposited TiO2 (TiO2: 10 mg/L) on five SAMs after
421
introducing DI water. Salt type
IS (mM)
–OH
–CONH2
–COOH
–NH2
–CH3
NaCl
10
8.80±12.4
N.D.
40.5±13.8
20.0±0.00
N.D.
20
31.2±15.5
21.2±10.6
39.1±7.84
N.D.
N.D.
40
44.0±0.00
65.5±20.5
23.5±6.90
13.7±1.16
11.0±1.97
60
62.0±26.7
72.2±6.13
68.2±20.2
57.5±6.00
45.4±0.60
80
100
87.0±22.9
97.5±3.60
100
36.7±15.0
100
100
93.1±12.0
100
52.4±0.00
28.7±7.70
0.5
68.0±0.00
N.D.
12.9±10.8
N.D.
23.0±11.9
1.0
14.4±6.28
20.0±0.00
15.4±21.7
N.D.
N.D.
1.5
N.D.
5.70±0.00
9.70±0.00
N.D.
14.3±20.2
2.0
5.90±8.30
14.1±6.78
40.2±9.20
N.D.
20.4±7.50
CaCl2
422
N.D.: not detectable.
423 424
From Table 2, it can be observed that the release fractions for –OH and –CONH2 increased
425
with the increase of NaCl IS after introducing DI water. Although the release fractions for the
426
rest SAMs had fluctuations, the release behaviors were generally increased at higher IS 20
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427
compared to lower IS. The fluctuation in release fractions might be related to the non-rigid
428
binding of TiO2 and SAM surface when IS became higher. For favorable SAM (–NH2), the
429
release fractions under various NaCl IS are less than those for the unfavorable SAMs except
430
for –CH3. This suggests that the deposited TiO2 NPs on favorable surface are more
431
irreversible compared to unfavorable surfaces. High release fractions of deposited TiO2 NPs
432
on unfavorable surfaces will increase the potential mobility of these emerging NPs in the
433
environment. However, the release of TiO2 NPs on –CH3 SAM is also less significant,
434
indicating that the deposition of NPs due to hydrophobic interactions might be more
435
irreversible.
436
Interestingly, the release fractions of deposited TiO2 NPs on SAMs in the presence of
437
CaCl2 are lower compared to those in the presence of NaCl. Although the deposition of TiO2
438
NPs is mitigated due to the formation of large aggregates in the suspension with the strong
439
aggregation effects of CaCl243, the deposited TiO2 NPs are more irreversible because the Ca
440
ions can also form bridges among the deposited NPs and resist the release. It is worth noting
441
that for –NH2 SAM there is no obvious release for all CaCl2 concentrations, confirming that
442
favorable SAM can significantly avoid the release of TiO2 NPs in aquatic environments.
443 444
ENVIRONMENTAL IMPLICATIONS
445
Natural organic matter (NOM) is ubiquitous in the environment. After TiO2 NPs are
446
released in the aquatic environment, their interactions with NOM or NOM-modified solid
447
surfaces in the natural environment will significantly govern their transport and fate. The
448
present study reports the deposition and release behaviors of TiO2 NPs using well-controlled 21
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449
SAMs with different ending functionalities to simulate NOM or NOM-covered surfaces.
450
Results of this study suggest that the –NH2 functional group can result in significant TiO2 NPs
451
deposition, while the –OH functionality demonstrates lower potential of NPs deposition.
452
Solution chemistry (IS and valence) also impacts their deposition and transport in both
453
natural and engineered aquatic environments. For –OH and –CH3 surfaces, deposition of TiO2
454
NPs initially increases with NaCl IS; however, further increase of NaCl concentration hinders
455
their deposition. For –COOH SAM, the deposition of NPs is independent on the change of
456
NaCl IS. The deposition of NPs on –CONH2 and –NH2 exhibits no change at lower NaCl
457
concentrations and tends to decrease with further increase of NaCl concentration. The
458
increase of divalent ions (Ca2+) concentrations results in the decreased deposition for –
459
CONH2, –NH2 and –CH3 SAMs but increased deposition for –COOH SAM and no obvious
460
change for –OH. This indicates that transport of TiO2 NPs in aquatic environments will be not
461
only governed by IS and ionic valence but also surface functional groups.
462
Our results also demonstrate that a change of ionic strength (using DI water) leads to a
463
dramatic remobilization of TiO2 NPs, implying that release of TiO2 NPs from a surface or
464
NPs-NOM aggregates back into the aqueous phase can occur following specific changes in
465
solution chemistry16 as a result of rainfall, flooding events and/or surface water-ground water
466
exchange. The release behavior is significantly mitigated on favorable SAMs (e.g., –NH2)
467
compared to unfavorable SAMs (e.g., –OH). The deposition and release behaviors obtained in
468
our study may support more accurate prediction of the environmental fate of these
469
nanoparticles by identifying the dominant surface functional groups in aquatic environments.
470
The effects of concomitant presence of more functional groups on one surface on the 22
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471
environmental fate of NPs need further investigation. In addition, in natural water
472
environments, the concentrations of –OH and –COOH may be several orders of magnitude
473
higher than that of –NH2 group, and therefore the former groups may have more significant
474
effects on the environmental fate of NPs.
475
For engineered aquatic environments, surface modification through rendering or
476
eliminating special functional groups can be envisaged dependent on the specific purposes.
477
For instance, in order to capture the released NPs for controlling their potential toxicity in
478
water, surface modification of membranes (microfiltration/ultrafiltration) or other filter
479
materias by presenting –NH2 groups can facilitate their removal from water in engineered
480
water treatment facilities. On the contrary, for preventing the NPs depostion on solid surfaces,
481
rendering –OH group should be an effective way. For instance, it has been reported that NPs
482
can result in severe membrane fouling in membrane bioreactors (MBRs)34. Membrane
483
modification by rendering –OH groups can control the NPs-related fouling, and lead to
484
dominant deposition of NPs on activated sludge in MBRs. The deposited NPs in the system
485
can be consequently removed by extracting waste sludge. Therefore, by taking surface
486
functionalities and solution chemistry into full consideration, people can control their
487
deposition or release more efficiently, thus minimizing the negative impacts of TiO2 NPs on
488
ecosystems and better utilizing their functions for the desired purposes.
489 490
SUPPORTING INFORMATION
491
Figures S1-S8 are included. This information is available free of charge via the Internet at
492
http://pubs.acs.org. 23
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493 494
AUTHOR INFORMATION
495
Corresponding Author
496
Tel.: +86-21-65980400, Fax: +86-21-65980400. E-mail:
[email protected] 497
Notes
498
The authors declare no competing financial interest.
499 500
ACKNOWLEDGMENTS
501
We thank the National Natural Science Foundation of China (Grant 51422811) for the
502
financial support of this work.
503 504
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TOC Art
Deposited mass (ng/cm2)
16 -NH2
14 -COOH
12 10 8
-CH3
-CONH2
6 -OH
4 2 0 30
634
50 70 90 Contact angle (o)
31
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