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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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Environmental Science & Technology

1

Influence of Surface Functional Groups on Deposition and Release of TiO2

2

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

1

ACS Paragon Plus Environment


13

ABSTRACT: A clear understanding of the factors governing the deposition and release

14

behaviors of engineered nanoparticles (NPs), such as TiO2 NPs, is necessary for predicting

15

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

2


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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

37

necessitates a thorough understanding of their transport and fate in the environment.

38

For a release of TiO2 NPs to the aquatic environment, the interaction of TiO2 NPs with

39

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,

41

lipids, and humic and fulvic acids), have profound effects on the aggregation, mobility and

42

deposition.9-11 It has been revealed that dissolved organic matter slows aggregation of

43

nanoparticles and inhibits the deposition of them in saturated porous media.11-14 The

44

interactions

45

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, stating that the interaction between a

46

colloid and a collector is the sum of their electrostatic and van der Walls interactions.9,15,16 In

47

fact, the interactions of NPs and organic matters arise from surface chemistry, in which the

48

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

3


be

explained

by

the


52

their aggregation and deposition, which is contrary to the results as mentioned above.11-14

53

Unfortunately, the effects of specific chemical functionalities of organic surfaces on

54

deposition and release of TiO2 NPs are largely unknown because of the complexity of the

55

organic matters. Take humic acid as an example, the main functional groups include carboxyls,

56

hydroxyls, carbonyls, and amides10, which makes it very difficult to examine the specific

57

interactions of chemical functionalities with NPs.

58

One possible way for simplifying the problem is to study the effect of individual chemical

59

functionality using well-defined and homogeneous surface chemistry. Alkanethiols,

60

HS(CH2)nX, can absorb onto surfaces of gold and silver, and spontaneously form stable and

61

well-defined self-assembled monolayers (SAMs).17 Different ending functional groups at the

62

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

65

of proteins and detergents.19 The surface wettability of each SAM was found to be correlated

66

with the adsorption of proteins and detergents measured by surface plasmon resonance

67

method.19 Contreras et al. used seven SAMs with different chemical functionalities to

68

investigate their effects on adsorption of polysaccharides and proteins, and found that surface

69

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

75

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

84

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

92

QCM-D sensors. 1-dodecanethiol, 11-hydroxy-1-undecanethiol, 11-mercaptoundecaneamide,

93

11-amino-1-undecanethiol hydrochloride and 11-mercaptoundecanoic acid were purchased

94

from Sigma-Aldrich (St. Louis, MO, USA). The different ending groups of the alkanethiols

95

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

97

reported by the manufacturer (Sigma-Aldrich, St. Louis, MO, USA) was about 25 nm.

98

Reagent grade NaCl, CaCl2, NaOH, HCl, 200 proof ethanol and sodium-dodecyl-sulfate (SDS)

99

were purchased from Sinopharm (Shanghai, China).

100 101

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

/

102

Methods. TiO2 NPs Stock Solution. TiO2 NPs stock solution with 100 mg/L NPs was

103

prepared according to the literature8,9,14 using deionized (DI) water followed by

104

ultrasonication for 1 h (25 oC, 250 W, 40 kHz). A series of TiO2 suspensions (5, 10, 15, 20

105

mg/L) were prepared by diluting the stock solution into a number of NaCl and CaCl2

106

solutions of different ionic strengths9,14 (NaCl=10, 20, 40, 60, 80, 100 mM; CaCl2=0.5, 1.0,

107

1.5, 2.0 mM) at pH 7. The resulting TiO2 suspensions were further sonicated for another 15

108

min prior to use.9,14 The hydrodynamic diameter, polydispersity index (PDI), electrophoretic

109

mobility (EPM), and zeta potential of TiO2 NPs were determined by a Malvern Zatasizer 4700

110

(Malvern Instruments, UK) immediately after sonication. Transmission electron microscopy

111

(TEM) was also employed to confirm the TiO2 NPs size distribution under investigated 6


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112

conditions since the hydrodynamic diameter measured by dynamic laser scattering (DLS)

113

(Malvern Zatasizer 4700) may deviate from their actual size.15,24,30 The obtained results are

114

well in agreement with previous studies.9,15 More information on TiO2 NPs characteristics is

115

provided in Supporting Information (SI) Figures S1~S6. The DLS results were much larger

116

compared to TEM measurements, suggesting that aggregation of TiO2 NPs still occurred

117

under experimental conditions.

118

SAM Preparation and Characterization. Five SAMs each with a different terminal

119

functional group (see Table 1) were prepared on gold-coated QCM-D crystal sensors. Prior to

120

self-assembly, gold-coated QCM-D crystals were cleaned sequentially in toluene, acetone,

121

and ethanol twice in each solvent for 10 min each time in an ultrasonic bath.20 The crystals

122

were then dried with ultrapure nitrogen gas and then placed in a cleaning chamber

123

(YZD08-2C, Tangshan, China) for 30 min. The cleaned crystals were then submerged in

124

ethanol for 20 min32. Afterward, the cleaned gold-coated crystals were immersed into a 1 mM

125

alkanethiol solutions in ethanol for 24 h followed by sonication in ethanol for 5 min to

126

remove loosely bound molecules. After that, an additional 24-h immersion was carried out to

127

enhance the homogeneity of the SAMs on the crystal surface. Finally, the alkanethiol-coated

128

crystals were rinsed with ethanol, dried with nitrogen, and stored under vacuum. For creating

129

a more ordered and homogeneous surface for −NH2 and –COOH, the pH of

130

11-amino-1-undecanethiol hydrochloride and 11-mercaptoundecanoic acid solutions were

131

adjusted to 11 and 2 prior to immersion, respectively, using 0.2 M NH4OH and HCl to reduce

132

electrostatic repulsion between thiol chains.

133

After self-assembly, contact angle of water in air for the prepared SAMs was measured 7



134

using the sessile drop method with an OCA contact angle analyzer (JC2000D, Yisu Co.

135

Shanghai, China). At least six measurements were carried out for each SAM. Both DI water

136

and the background solutions were used to measure the contact angles. The results were not

137

evidently different, and thus only data obtained with DI water was reported. The elemental

138

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

140

monochromator. Since XPS is destructive, the SAMs measured by XPS will not be used for

141

subsequent experiments. The original SAMs prepared in parallel were employed for QCM-D

142

analyses. The pKa values of -OH, -CONH2, and -COOH functional groups were determined

143

to be 7.09 ± 0.13, 6.54 ± 0.04, and 5.95 ± 0.23, respectively, using an automatic

144

potentiometric titrator (ZDJ-4A, Shanghai Leici, China). Therefore, −OH, and −CONH2

145

(close to 7) as well as −CH3 SAMs could be regarded as uncharged surfaces, while −COOH

146

was negatively charged surface under experimental condition (pH=7). Since –NH3+ has a

147

pKa~7.520, the terminal monolayer of −NH2 group (11-amino-1-undecanethiol hydrochloride)

148

was positively charged under experimental condition.

149

Deposition and Release Experiments. Deposition and release of TiO2 NPs onto/from

150

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

152

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

154

injected into the chambers were maintained at 25 ± 1 oC. For a rigid adsorbed layer (i.e.,

155

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

157

−∆f n n

(1)

158

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

161

normalized third overtone frequency and dissipation (∆f3 and ∆D3) since this overtone has the

162

best signal-to-noise ratio.9,26

163

For viscoelastic deposited layers that exhibit high energy dissipations (∆D>1 or ∆D/∆f>0.1)

164

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),

167

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



225

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.

228

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.

235

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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247

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



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)


0.50 0.40

(b)


(a)

0.40


276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299


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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303

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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326

This electrostatic force changes compromised the decreased diffusive transport of TiO2 NPs to

327

the SAM surface.

328

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


Page 17 of 31


any obvious change. The CDC of TiO2 NPs on –OH SAM was thus determined to be 40 mM

349

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



(c) –COOH

(d) –NH2

0.40

0.20

0.30 0.20

0.10

0.10 0.00

0.00

0


20 40 60 80 100 NaCl concentration (mM)

0.40


354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377


348

(e) –CH3

0.30 0.20 0.10 0.00 0 20 40 60 80 100 NaCl concentration (mM)

378


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


1.2 Deposition efficiency (ɑ)

Deposition efficiency (ɑ)


(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)



1.2

(c) –COOH

1.0

(b) –CONH2

1.0


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


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


Page 18 of 31

Page 19 of 31


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



(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



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


411

a function of CaCl2 concentration (TiO2 concentration 10 mg/L, and pH=7). Error bars 19



412

Page 20 of 31

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


Page 21 of 31


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



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


Page 22 of 31

Page 23 of 31


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



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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505

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30


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633


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


110

Nanoparticles - ACS Publications - American Chemical Society

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