Chemical Interactions between Nano-ZnO and Nano-TiO2

Subscriber access provided by UNIV OF UTAH

Article

Chemical interactions between nano-ZnO and nano-TiO2 in a natural aqueous medium Tiezheng Tong, Kaiqi Fang, Sara A. Thomas, John J. Kelly, Kimberly A Gray, and Jean-Francois Gaillard Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 11 Jun 2014 Downloaded from http://pubs.acs.org on June 12, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

Environmental Science & Technology

Chemical interactions between nano-ZnO and nano-TiO2 in a natural aqueous medium

Tiezheng Tong1, Kaiqi Fang1, Sara A. Thomas1, John J. Kelly2, Kimberly A. Gray1* and Jean-François Gaillard1* 1

Department of Civil and Environmental Engineering, Northwestern University, 2145

Sheridan Road, Evanston, IL, 60208 2

Department of Biology, Loyola University Chicago, 1032 West Sheridan Road,

Chicago, IL, 60660 *Corresponding author: Kimberly A. Gray and Jean-François Gaillard Email: [email protected], [email protected] Phone: (847)-467-4252, (847)-467-1376 Fax: (847)-491-4011, (847)-491-4011

ACS Paragon Plus Environment

1


Page 2 of 30

Abstract The use of diverse engineered nanomaterials (ENMs) potentially leads to the release of multiple ENMs into the environment. However, previous efforts to understand the behavior and the risks associated with ENMs have focused on only one material at a time. In this study, the chemical interactions between two of the most highly used ENMs, nano-TiO2 and nano-ZnO, were examined in a natural water matrix. The fate of nanoZnO in Lake Michigan water was investigated in the presence of nano-TiO2. Our experiments demonstrate that the combined effects of ZnO dissolution and Zn adsorption onto nano-TiO2 control the concentration of dissolved zinc. X-ray absorption spectroscopy was used to determine the speciation of Zn in the particulate fraction. The spectra show that Zn partitions between nano-ZnO and Zn2+ adsorbed on nano-TiO2. A simple kinetic model is presented to explain the experimental data. It integrates the processes of nano-ZnO dissolution with Zn adsorption onto nano-TiO2 and successfully predicts dissolved Zn concentration in solution. Overall, our results suggest that the fate and toxicity potential of soluble ENMs, such as nano-ZnO, are likely to be influenced by the presence of other stable ENMs, such as nano-TiO2.


2

Page 3 of 30

1


Introduction

2

The early 21st century is witnessing the transition of nanotechnology from discovery to

3

commercialization. Engineered nanomaterials (ENMs) are incorporated into numerous

4

commercial products ranging from pharmaceuticals and cosmetics to alternative energy

5

and electronic devices, leading to a global market projected to reach $3 trillion in 2020 1.

6

As a result, diverse ENMs (such as nano-TiO2, nano-ZnO, nano-Ag, carbon nanotubes

7

and fullerenes) will inevitably be released into natural environments

8

about their fate and potential toxicity towards living organisms. Considerable effort has

9

been exerted to understand the environmental behavior and risks associated with ENMs.

10

Information about the ecotoxicological effects of ENMs has been gathered for almost all

11

the major ENMs and a broad range of taxa (e.g., bacteria, algae, plants, invertebrate, and

12

vertebrate)

13

evaluated the fate or toxicity of a single nanomaterial at a time. Little attention, then, has

14

been paid to the potential interactions between different ENMs, let alone the effects of

15

these interactions on the ENMs toxicity potential. In addition, while contaminant

16

interactions at ENM surfaces (e.g., Trojan horse effect

17

influences of ENM surfaces on the transformation of other coexisting ENMs have yet to

18

be thoroughly explored.

19

7-11

2-6

, raising concerns

. However, these previous studies are mostly limited in scope since they

12

) have been studied, the

Nano-TiO2 and nano-ZnO, two semiconductor metal oxides, are among the most 13

20

extensively used ENMs in industry. According to studies by Muller et al.

21

Gottschalk et al. 3, the annual production volume and the predicted environmental

22

concentrations of nano-TiO2 and nano-ZnO are much higher than those of other

23

frequently studied ENMs (including nano-Ag, carbon nanotubes and fullerenes).


and

3


Page 4 of 30

24

Therefore, coupled with their wide and similar applications in personal-care products and

25

industry (e.g., both nanomaterials are used as active components in sunscreens

26

effective photocatalysts

27

investigating. Although these two nanomaterials share similar band gap and band edge

28

energies

29

solution and readily undergoes dissolution. Its toxicity is largely attributed to the release

30

of dissolved ionic zinc 18-21, although phototoxicity of nano-ZnO has also been reported 22,

31

23

32

pivotal in evaluating the environmental impacts of nano-ZnO. In contrast, nano-TiO2 is

33

chemically stable and its toxic effects are mainly due to its phototoxicity

34

previous work studying the effects of various nano-TiO2 from different commercial

35

sources and with varied morphologies on bacteria

36

greater toxicity under simulated solar irradiation than rutile but negligible toxicity was

37

observed under dark conditions.

17

16

14, 15

and

), the interactions between these two ENMs are worth

, their respective mechanisms of toxicity differ. Nano-ZnO is unstable in

. Accordingly, the level of dissolved zinc released from nano-ZnO into solution is

27-29

24-29

. In our

, we showed that anatase lead to

38

Furthermore, nano-TiO2 is a commonly used model substrate for adsorption of metal

39

contaminants, including arsenic 30, uranium 31, and zinc 32. In particular, the average local

40

coordination environment of Zn(II) at the water-TiO2 interface has been recently

41

characterized by X-ray absorption spectroscopy (XAS)

42

calculations suggest that Zn(II) is coordinated to either 4, 5, or 6 oxygen while forming

43

bidentate binuclear or bidentate mononuclear surface complexes

44

previous studies, we think that it is important to fully characterize these chemical

45

interactions and propose a quantitative model that can account for the interplay

46

between dissolution of nano-ZnO and Zn scavenging at the surface of nano-TiO2.

32


. XAS data and ab-initio

32

. Based on these

4

Page 5 of 30


47

In the current study, the chemical interactions between nano-TiO2 and nano-ZnO in

48

aqueous solutions were investigated in Lake Michigan water (LMW), a proxy for surface

49

freshwaters. The kinetics of nano-ZnO dissolution were monitored in the presence of

50

nano-TiO2, and the Zn(II) species in the particulate fraction were characterized and

51

quantified by XAS. Finally, a kinetic model was developed to explain the experimental

52

results. It combines the processes of nano-ZnO dissolution with Zn(II) adsorption onto

53

nano-TiO2 to predict the dissolved Zn concentrations observed. To the best of our

54

knowledge, this study represents one of the first investigations on the interactions

55

between different ENMs in a natural aqueous medium, and our results indicate the

56

importance of considering the role of adsorbing surfaces when dealing with the

57

environmental fate of ENMs that undergo significant dissolution.

58

Materials and methods

59

Nanomaterials characterization and exposure medium

60

AEROXIDE® P25 (P25) is the representative nano-TiO2 used in this study and was

61

kindly donated by Evonik Industries (Germany). Nano-ZnO was purchased from Sigma

62

Aldrich (Cat. 544906). P25 and nano-ZnO stock solutions (both at 1 g/L) were prepared

63

in Milli-Q water (Millipore, 18 MΩ·cm), and a 30 min-sonication in ultrasonic bath

64

(Health-Sonics, 110W, 42 kHz) was employed before diluting the stock solutions to the

65

desired concentrations for the following experiments. Dissolution of ZnO nanoparticles in

66

the stock solution resulted in an equilibrium concentration of dissolved zinc ([Zn]dis) of 7

67

mg/L, limited by the Zn solubility in Milli-Q water at equilibrium with atmospheric CO2

68

(398 ppm). This dissolved Zn contribution in the nano-ZnO stock solution did not affect

69

our experimental conclusions but was included in our theoretical model as described


5


Page 6 of 30

70

below. The primary sizes and morphologies of both P25 and nano-ZnO were visually

71

observed by transmission electron microscopy (TEM, Hitachi HD-2300), and powder X-

72

ray diffraction (XRD) was used to confirm their chemical composition and to estimate

73

crystallite sizes using the Scherrer equation (Figure S1 and Table S1). XRD was

74

performed on a Rigaku X-ray diffractometer by varying the diffraction angle from 20° to

75

70° (2θ) 27. In addition, hydrodynamic diameters and zeta potentials were measured using

76

a ZetaPALS analyzer (Brookhaven Instruments). The characterization results are

77

summarized in Table S1 and S2, both of which are available in the Supporting

78

Information.

79

All the experiments were conducted in Lake Michigan water (LMW) in order to assess

80

how these ENMs would behave in a freshwater matrix. LMW was collected from the

81

Evanston drinking water plant and immediately transported to our laboratory where it

82

was stored at 4°C after filtration through a 0.2 µm pore size PES membrane (NALGENE,

83

Cat. 597-4520). The chemical composition of LMW was characterized and summarized

84

in Table S3. In LMW, the solubility limit for Zn released from ZnO - Zincite - is 0.67

85

mg/L of total [Zn]dis as calculated using PHREEQC

86

MINTEQ.v4.

87

Adsorption experiments

33

and the thermodynamic database

88

Adsorption experiments were conducted at room temperature (21±1°C) by mixing

89

nano-ZnO with different concentrations of P25. Sampling was conducted at five time

90

intervals: 10 min, 3 h, 6 h, 1 d and 2 d after the onset of the experiments. The aqueous

91

phase was separated using high-speed centrifugation at 15700 g (13000 rpm) for 30

92

minutes followed by filtration through a 3KDa membrane (Nanosep®, Pall Life Sciences).


6

Page 7 of 30


93

The zinc remaining in the filtrate was analyzed as the dissolved zinc by using flame

94

atomic absorption spectrometry (FAAS, AAnalyst 100, Perkin-Elmer) with a detection

95

limit of 13 µg/L Zn. Recovery efficiencies for the processing of 0.5 mg/L and 1 mg/L

96

solutions of dissolved Zn yielded 101.0 ± 2.1% and 100.0 ± 0.9%, respectively, showing

97

that no loss of the analyte occurred during the filtration step. Dissolved Zn concentration

98

in LMW was below the detection limit of FAAS, and no significant loss of Zn was

99

observed due to adsorption to experimental container walls. Stock solutions of ZnCl2

100

were also prepared to conduct zinc adsorption experiments in the presence of P25

101

suspended in LMW, as a means of comparison. At least three independent experiments

102

were performed in order to assess the reproducibility of the results.

103

X-ray absorption spectroscopy

104

X-ray absorption spectroscopy (XAS) was performed to determine the speciation of

105

zinc in suspended particles. In detail, 20 mg/L of P25 suspended in LMW was amended

106

with an aliquot of the ZnCl2 stock solution to obtain a total concentration of 1 mg/L Zn.

107

Aliquots of suspended nano-ZnO were also added using the same approach to yield total

108

concentration of 1, 2, and 3 mg/L ZnO. 500 mL samples were prepared in acid-washed

109

Teflon bottles in order to obtain enough material for XAS analysis. After a one-day

110

incubation with constant shaking (100 rpm) at room temperature, the solids remaining

111

suspended in solution were collected on 0.2µm membranes (polycarbonate, Millipore).

112

The samples were then sandwiched between two strips of Kapton tape (Dupont). When

113

Zn salt is added into LMW, the solubility limit of Zn is controlled by the precipitation of

114

Zn(OH)2 rather than ZnO, since ZnO does not form at low temperature. This results in a

115

total dissolved Zn concentration of 4.12 mg/L, based on PHREEQC


33

thermodynamic

7


Page 8 of 30

116

calculations. Hence, when dissolved zinc was added to yield a total Zn concentration of 1

117

mg/L, all the Zn remained dissolved in LMW, as verified by conducting dissolved zinc

118

measurements using FAAS. Thus, the zinc collected in the dissolved Zn/P25 mixture

119

represents only zinc bound to P25. XAS spectra of powder standards including nano-ZnO,

120

γ-Zn(OH)2, ZnCO3 (smithsonite) were used as references to assess the speciation of Zn.

121

The γ-Zn(OH)2 powder was synthesized following the method described by Waychunas

122

et al. 34. In addition, 5 mg/L of nano-ZnO in the absence of P25 was prepared in LMW to

123

compare the spectra of nano-ZnO as dry powders and in solution.

124

XAS measurements were performed at the DuPont-Northwestern-Dow Collaborative

125

Access Team (DND-CAT) beamline located in Sector 5 of the Advanced Photon Source

126

at Argonne National Laboratory. A Si (111) double crystal monochromator was used to

127

vary the X-ray energy from -200 eV to +1000 eV of the absorption K edge of Zn (9659

128

eV). The XAS spectra, with the exception of the powder standards, were collected in the

129

fluorescence mode using a Vortex ME-4 silicon drift detector. Spectra of the powder

130

standards were measured in the transmission mode using Oxford ionization chambers

131

with 296 mm path lengths. For energy calibration, the X-ray absorption spectrum of a Zn

132

reference foil was measured with each sample in either transmission or fluorescence

133

mode. At least four scans were performed to obtain good counting statistics. Data

134

processing and analysis of the X-ray absorption near edge structure (XANES) and

135

extended X-ray absorption fine structure (EXAFS) spectra were performed with the

136

program Athena (version 0.9.17) 35. Spectra decompositions using linear combination fits

137

of the references were performed on the XANES (fit range from -20 eV to +100 eV


8

Page 9 of 30


138

above absorption edge) and EXAFS (fit range from 1 to 10 Å-1) parts of the data to

139

determine the percentages of zinc remaining as nano-ZnO and Zn adsorbed onto P25.

140

Results and discussion

141

Physicochemical characterization of nanomaterials

142

Transmission electron microscopy (TEM) was used to define the primary sizes and the

143

morphologies of P25 and ZnO nanoparticles (Figure 1). P25 is a spherical nanomaterial

144

with diameters of 15~25 nm (20.4 nm on average), which is in agreement with literature

145

reports 36. On the other hand, nano-ZnO exhibits contrasting morphologies, with sphere-

146

shape (63.6 nm on average) and rod-shape (156.6 nm × 47.1 nm on average)

147

nanoparticles coexisting together. Dynamic light scattering (DLS) results (Table S2)

148

reveal that both P25 and ZnO nanoparticles form aggregates well beyond their original

149

nano-scale in LMW. The aggregate sizes of P25, assessed as the hydrodynamic diameters,

150

increase from 484±31 nm to 1641±192 nm when the concentration increases from 10

151

mg/L to 100 mg/L, while those of nano-ZnO grow from 680±70 nm at 1 mg/L to

152

1103±40 nm at 50 mg/L. Zeta potential measurements indicate that both nanoparticles are

153

negatively charged in LMW (Table S2), demonstrating the presence of electrostatic

154

repulsion between P25 and nano-ZnO particles. However, this repulsion is likely

155

surpassed by an increased frequency of collision in P25/nano-ZnO mixtures, as evidenced

156

by the larger aggregate sizes for nano-ZnO/P25 mixtures than those of the individual

157

nanoparticles alone (Table S2).

158

Effects of nano-TiO2 on nano-ZnO dissolution

159

The effects of nano-TiO2 P25 on nano-ZnO dissolution were examined by preparing

160

suspensions of 0.5 mg/L and 1 mg/L of nano-ZnO in the presence of different P25


9


Page 10 of 30

161

concentrations – up to 100 mg/L of TiO2. In parallel, solutions of 0.5 mg/L Zn2+ were

162

prepared from the ZnCl2 stock solution to study the adsorption of dissolved Zn on P25.

163

Figure 2A shows that dissolved Zn decreased continuously with increasing concentration

164

of P25 ([P25]), which is in agreement with our established knowledge about nano-TiO2

165

as an effective metal adsorbent. Zinc adsorption onto P25 was fast and reached

166

equilibrium within minutes. Accordingly, we only monitored dissolved Zn adsorption for

167

up to 6 hours of reaction time.

168

Due to the dissolution of the ZnO nanoparticles, the [Zn]dis increased with time and

169

became relatively stable after 1 day (Figure 2B), and thus the experiments were extended

170

to 2 days. It is important to note that after a two-day reaction time in LMW, solutions

171

containing 0.5 mg/L and 1 mg/L of nano-ZnO represent fully- and partially-dissolved

172

cases, respectively. This was verified by conducting dissolved zinc measurements. For

173

0.5 mg/L nano-ZnO mixed with P25 (Figure 2B), we observed the same trend as in the

174

case of dissolved Zn prepared from ZnCl2, in which [Zn]dis decreased with increasing

175

[P25]. When the concentration of nano-ZnO was increased to 1 mg/L, however, the

176

variation of [Zn]dis with respect to the concentration of P25 presented a distinct feature

177

after 3 hours of reaction time (Figure 2C). Rather than a steady decrease of [Zn]dis with

178

increasing [P25], a set of plateaux was observed during which [Zn]dis was maintained at

179

about the same level until [P25] exceeded 10 mg/L. The pH of the solutions of nano-

180

ZnO/P25 mixtures remained at 8.4±0.1 as the concentration of P25 increased to 50 mg/L

181

(Figure S2), which excludes the contribution of pH in this phenomenon. In addition,

182

similar results were observed for a broader range of nano-ZnO concentrations (e.g.,

183

0.25~3 mg/L, Figure S3 in the Supporting Information).


10

Page 11 of 30


184

The presence of Zn in solution is controlled by two processes: 1) the dissolution of

185

nano-ZnO, and 2) the adsorption of Zn at the surface of nano-TiO2. This is described by

186

the following two reactions:

187 188 189

The presence of dissolved Zn plateaux during the interactions between ZnO and TiO2-

190

P25 nanoparticles in solution results from similar and relatively fast kinetic rates for ZnO

191

dissolution and Zn2+ adsorption. Reaction (2) decreases the dissolved Zn2+ concentration

192

and shifts reaction (1) to the right. The remaining nano-ZnO behaves as a Zn2+ reservoir

193

whose further dissolution compensates the loss of [Zn]dis due to its adsorption on nano-

194

TiO2. When the number of surface sites available for Zn sorption becomes large

195

compared to the overall pool of Zn available, i.e., the concentration of P25 is ≥ 20 mg/L

196

in our experiments, a decrease of [Zn]dis becomes visible.

197

In order to further test our explanation above, a set of 1 mg/L nano-ZnO solution was

198

aged for 1 day prior to the addition of P25. Thus, about 0.5 mg/L of dissolved zinc had

199

been already released into LMW at the onset of the adsorption experiments (Figure 3). In

200

this case, the [Zn]dis decreased readily and steadily with increasing [P25] within a short

201

time (i.e., 10 min) after the addition of P25. However, the [Zn]dis curve subsequently

202

shifted upwards and became more similar to what was observed in Figure 2C,

203

demonstrating the further dissolution of remaining nano-ZnO induced by Zn adsorption

204

to P25.

205

Zn partitioning


11


Page 12 of 30

206

XAS was performed to characterize the speciation of zinc in the particulate fraction of

207

the nano-ZnO/P25 mixtures 37. The Zn K-edge XANES (from -20 eV to +100 eV of the

208

absorption edge) and the EXAFS data (k range from 0 to 14 Å-1) of the nano-ZnO/P25 as

209

well as dissolved Zn/P25 mixtures with reference compounds used for spectral

210

decomposition

211

with 20 mg/L P25 in LMW, the XANES and the EXAFS spectra were nearly identical to

212

those of the dissolved Zn/P25 mixture. We can conclude in this case that the Zn present

213

in the particulate phase is primarily adsorbed to the surface of P25. When the nano-ZnO

214

concentration increased to 2 and 3 mg/L, however, the spectra become similar to those of

215

nano-ZnO. It should be noted that the spectrum of nano-ZnO equilibrated with LMW was

216

identical to that of the nano-ZnO powder standard. Therefore, we did not detect the

217

presence of other Zn phases, such as Zn(OH)2 or ZnCO3, in our experiments. Spectral

218

decomposition by linear combination fitting (LCF) of both the XANES and the EXAFS

219

was performed to determine the percentages of Zn species present in the nano-ZnO/P25

220

samples. The data and the results of the curve fitting with the residuals of the EXAFS

221

LCF are presented in Figure 4C, while the LCF of the XANES is included in the

222

Supporting Information (Figure S4). Only two reference spectra were needed to perform

223

the analysis: the spectrum of nano-ZnO in LMW and the spectrum corresponding to Zn

224

adsorbed at the surface of P25 (the addition of a third zinc species did not improve the fit).

225

For 1 mg/L nano-ZnO reacting with 20 mg/L P25, the best EXAFS fit was obtained

226

considering 95.5% of the sample as zinc adsorbed to P25 and only 4.5% of zinc as nano-

227

ZnO. Since only ~60% of 1 mg/L nano-ZnO dissolves in LMW after two days without

228

P25 (Figure 2C), this result is consistent with our hypothesis that the remaining nano-

38

are shown in Figure 4A and B. When 1 mg/L nano-ZnO was mixed


12

Page 13 of 30


229

ZnO was further dissolved in the presence of P25. When 2 and 3 mg/L of nano-ZnO

230

interacted with the same dose of P25, nano-ZnO contributes to 54.6% and 57.0% of the

231

total zinc in the particulate fraction, respectively. The percentages of the particulate Zn

232

species obtained from the LCF of the XANES spectra are identical to those presented

233

above (Figure S4). Therefore, the XAS results provide quantitative evidence, at the

234

molecular level, that Zn(II) species coexist as nano-ZnO and adsorbed Zn onto P25 in a

235

system where partially-dissolved nano-ZnO interacts with nano-TiO2.

236

Modeling nano-ZnO and nano-TiO2 interactions.

237

In order to explain the results obtained from the nano-ZnO/nano-TiO2 interaction

238

experiments, we developed a simple kinetic model that integrates the processes of nano-

239

ZnO dissolution and adsorption of the released zinc onto nano-TiO2.

240 241 242

The dissolution of nano-ZnO is fast and diffusion controlled as shown by David et al 39

, so it can be expressed using the Noyes-Whitney equation 40 as: d[ZnO] A = −D ([Zn]eq − [Zn]dis ) dt d

(3)

243

where [ZnO] is the concentration of nano-ZnO, [Zn]dis is the concentration of dissolved

244

zinc, [Zn]eq is the [Zn]dis at equilibrium with nano-ZnO, A is the surface area of nano-

245

ZnO, D is the diffusion coefficient of Zn2+, and d is the diffusion layer thickness 40, 41.

246

The mass balance of zinc added into the system can be expressed as

247

[Zn]total = [ZnO] + [Zn]dis + [Zn]ad = [ZnO] + [Zn]dis + [TiO2 ]qe

248

where [Zn]total is the total amount of zinc added, [Zn]ad is the amount of zinc adsorbed on

249

nano-TiO2, [TiO2] is the concentration of nano-TiO2, and qe is the amount of zinc

250

adsorbed on the nano-TiO2 per unit weight. Since zinc adsorption onto nano-TiO2 reaches

251

equilibrium very rapidly (Figure 2A), the parameter qe is obtained from an isotherm-type


(4)

13


Page 14 of 30

252

experiment in LMW and expressed as a function of [Zn]dis using the non-linear Langmuir

253

model (i.e., f([Zn]dis), see details in the Supporting Information).

254

qe =

255

For given concentrations of [Zn]total and [TiO2], we can rewrite eq (4) as:

256

[Zn]dis = f ([Zn]dis ) 12.7 + 14.6[Zn]dis

(5)

[ZnO] = [Zn]total − [Zn]dis − [TiO2 ] f ([Zn]dis ) = g([Zn]dis )

(6)

257

which indicates that [ZnO] is also a function of [Zn]dis (i.e., g([Zn]dis)).

258

Combined with eq (3), the differential of eq (6) leads to:

259

d[ZnO] d[ZnO] d[Zn]dis d[Zn]dis A = = g'([Zn]dis ) = −D ([Zn]eq − [Zn]dis ) dt d[Zn]dis dt dt d

260

and therefore,

261

A 1 d[Zn]dis = −D ([Zn]eq − [Zn]dis ) dt d g'([Zn]dis )

262

Although the TEM observations show that the ZnO nanoparticles are present under

263

various morphologies (Figure 1), we used a spherical approximation of the nano-ZnO

264

aggregates with the same initial radius r0 and surface area A0. In addition, we assume that

265

nano-ZnO aggregates undergo dissolution independently, and thus the number of ZnO

266

aggregates (N) remains constant. Similar assumptions were made in the recent study by

267

David et al 39. After dissolution r0 and A0 become r and A, respectively. Accordingly, we

268

have the following relationships:

269

[ZnO] N ρπ r 3 r 3 r [ZnO] 13 = = ⇒ = ( ) [ZnO]0 N ρπ r0 3 r0 3 r0 [ZnO]0

270

A 4π r 2 r 2 [ZnO] 23 = = =( ) A0 4π r0 2 r0 2 [ZnO]0

(7)

(8)

(9)

(10)


14

Page 15 of 30


271

where ρ is the density of ZnO (5.606 g/cm3) and [ZnO]0 is the initial concentration of

272

nano-ZnO that is equal to [Zn]total.

273

Furthermore, the change in the diffusion layer thickness (d) of nano-ZnO during the

274

dissolution process is incorporated into our model as proportional to the size of nano-ZnO

275

aggregate 42,

276

d = α r = α r0 (

277

where α is a constant.

278

From eq (8)~(11), we can write

279

d[Zn]dis =− dt

280

[ZnO] 13 ) [ZnO]0

(11)

1 3

DA0

α r0 [ZnO]0

1 3

[ZnO] ([Zn]eq − [Zn]dis )

we define another constant K = −

DA0

α r0 [ZnO]0 281 282

1 3

1 g '([Zn]dis )

(12) ,

(13) ,

and combined with eq (6), eq (12) is converted to: 1 1 d[Zn]dis = −K[g([Zn]dis )] 3 ([Zn]eq − [Zn]dis ) = h([Zn]dis ) dt g'([Zn]dis )

(14)

283

Eq (14) is a differential equation that was solved numerically using Mathematica 9

284

(Wolfram Research). The constant K was determined by least squares fitting of the

285

experimental data corresponding to the dissolution of nano-ZnO without any addition of

286

P25. The original [Zn]dis (at t = 0) is equal to the dissolved zinc concentration coming

287

from the nano-ZnO stock solution (i.e., a contribution of 3.5 µg/L for the 0.5 mg/L nano-

288

ZnO case and 7 µg/L for the 1 mg/L nano-ZnO case). A series of simulations were

289

performed

290

dissolution/adsorption experiments and the results are shown in Figure 5 and S6

to

calculate

the

concentrations

of

dissolved


Zn

during

the

15


Page 16 of 30

291

(Supporting Information). The experimental and simulated results for t=10 minutes are

292

not included in Figure 5 since the separation time of the particles from the solution,

293

including centrifugation followed by filtration, takes about 40 minutes, and therefore

294

exceeds the reaction time. As shown in Figure 5, our model successfully reproduces the

295

trend observed in the measured [Zn]dis curves for both fully- (0.5 mg/L) and partially-

296

dissolved (1 mg/L) nano-ZnO mixed with P25. In particular, it explains the presence of

297

the [Zn]dis plateaux as shown in Figure 2C. We notice that the simulated [Zn]dis curves for

298

1 mg/L nano-ZnO mixed with P25 (Figure 5B) reaches the plateaux later than what was

299

observed in Figure 2C. It might be due to a decrease in the adsorption capacity of P25

300

(per unit weight) because of P25/nano-ZnO co-aggregation, which is not considered

301

herein. If one plots all the experimental data points (56 in total) against the simulation

302

results a slope of 1.006 ± 0.014 is obtained by linear regression (Figure 6). Hence, this

303

simple model provides a reasonable description of the chemical processes occurring

304

during the interactions of these two ENMs in solution. Overall, our study shows that

305

[Zn]dis released from nano-ZnO is synergistically controlled by both the dissolution of

306

nano-ZnO and the adsorption of Zn onto nano-TiO2. Our modeling approach can be

307

extended to predict the fate of ENMs undergoing dissolution and subsequent metal

308

adsorption onto either other stable ENMs or environmental particles. In order to do so,

309

the metal adsorption capability of those adsorbing surfaces (described as the parameter qe

310

in the current model) needs to be determined.

311

Environmental implications

312

A recent review article by Lowry et al. 7 emphasizes that nanomaterials are subjected

313

to various physical, chemical, and biological transformations in natural environments,


16

Page 17 of 30


314

which determine their fate, transport and corresponding toxicity potential. As a variety of

315

ENMs are commonly used in industry and consumer products, they will inevitably be

316

discharged into the environment, creating mixtures of ENMs. In the current study, we

317

focused on the interactions between nano-ZnO and nano-TiO2, two extensively used

318

ENMs that are the most likely to be released into aquatic environments. We demonstrate

319

that the presence of nano-TiO2 influences the concentration of dissolved zinc released

320

from nano-ZnO in solution, which may consequently alter the exposure and

321

bioavailability of nano-ZnO to aquatic organisms.

322

Previous studies have shown that inorganic (e.g., sulfide 45

43

and phosphate

44

) and

323

organic (e.g., citric acid

) ligands participate in the environmental transformation of

324

nano-ZnO by forming Zn-ligand complexes or precipitates. Our results, on the other hand,

325

indicate that other stable ENMs or adsorbing surfaces, such as nano-TiO2, also contribute

326

to determining the fate of nano-ZnO in natural waters. When the concentration of nano-

327

ZnO is below its solubility limit, nano-ZnO will completely dissolve within a few days.

328

The zinc released will form surface complexes with either nano-TiO2 or other

329

environmental particles, decreasing the level of dissolved zinc in water. When the

330

concentration of nano-ZnO exceeds its solubility limit (which represents either a future

331

scenario or a spill event), nano-ZnO particles/aggregates can exist; however the presence

332

of other stable ENMs in solution and surface sites on background particles will enhance

333

dissolution.

334

Since dissolved zinc ions released from nano-ZnO are considered to be the primary

335

cause for nano-ZnO toxicity 18-21, our results indicate that interactions between nano-ZnO

336

and nano-TiO2 may play a mediating role in ENM toxicity. According to chemical


17


Page 18 of 30

46

337

equilibrium-based models such as the free-ion activity model (FIAM)

338

ligand model (BLM) 47, the formation of Zn(II)-TiO2 surface complexes will decrease the

339

level of the free Zn ion in solution, consequently reducing the bioavailability and toxicity

340

attributed to nano-ZnO. This possibility is evidenced by Yang et al. who observed an

341

diminished toxicity of Cd2+ to green alga Chlamydomonas reinhardtii in the presence of

342

nano-TiO2

343

of zinc ions in solution may remain unchanged due to the continued dissolution of nano-

344

ZnO. A “Trojan horse effect” may occur if the metal-carrying nanoparticles are able to

345

cross biological membranes and be internalized into living organisms. For example, it has

346

been found that nano-TiO2 increases the toxicity of Cu2+

347

and retention of Cd and Zn in Daphnia magna 50, as a result of the metal ion scavenging

348

by and ingestion of nano-TiO2 by Daphnia magna. Overall, when multiple nanomaterials

349

are present in aqueous solutions, the environmental behavior and risks of a soluble ENM

350

- such as nano-ZnO - are affected by the presence of other stable ENMs – such as nano-

351

TiO2. These effects are currently ignored since most toxicity studies focus on the

352

behavior of a single ENM.

353

Acknowledgement

48

and the biotic

. In contrast, when nano-ZnO is only partially dissolved, the bioavailability

49

and facilitates the biouptake

354

This research was supported by the National Science Foundation (Grant No. CBET-

355

1067751 to K.A.G. and J.-F.G. and Grant No. CBET-1067439 to J.J.K.) and the Institute

356

for Sustainability and Energy (ISEN) at Northwestern University (a graduate research

357

grant to K. F. and T. T.). We thank Dr. Qing Ma and Dr. Jinsong Wu for their technical

358

assistance, and Prof. Teri W. Odom for the use of ZetaPALS analyzer. Portions of this

359

work were performed at the DND-CAT Synchrotron Research Center located at Sector 5


18

Page 19 of 30


360

of the APS. DND-CAT is supported by the E.I. DuPont de Nemours & Co., The Dow

361

Chemical Company, the U.S. National Science Foundation through Grant DMR-9304725,

362

and the State of Illinois through the Department of Commerce and the Board of Higher

363

Education Grant IBHE HECA NWU 96. XRD and TEM were performed in the J. B.

364

Cohen X-ray diffraction facility and NUANCE center at Northwestern University,

365

respectively.

366

Supporting Information available

367

Additional tables, figures and text can be found in the Supporting Information. This

368

material is available free of charge via the Internet at http://pubs.acs.org.


19


Page 20 of 30

369

References

370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412

1. Roco, M. C.; Mirkin, C. A.; Hersam, M. C., Nanotechnology research directions for societal needs in 2020: Summary of international study. J Nanopart Res 2011, 13, (3), 897-919. 2. Benn, T. M.; Westerhoff, P., Nanoparticle silver released into water from commercially available sock fabrics. Environ Sci Technol 2008, 42, (18), 7025-7026. 3. Gottschalk, F.; Sonderer, T.; Scholz, R. W.; Nowack, B., Modeled Environmental Concentrations of Engineered Nanomaterials (TiO2, ZnO, Ag, CNT, Fullerenes) for Different Regions. Environ Sci Technol 2009, 43, (24), 9216-9222. 4. Kaegi, R.; Ulrich, A.; Sinnet, B.; Vonbank, R.; Wichser, A.; Zuleeg, S.; Simmler, H.; Brunner, S.; Vonmont, H.; Burkhardt, M.; Boller, M., Synthetic TiO2 nanoparticle emission from exterior facades into the aquatic environment. Environ Pollut 2008, 156, (2), 233-239. 5. Kim, B.; Park, C. S.; Murayama, M.; Hochella, M. F., Discovery and characterization of silver sulfide nanoparticles in final sewage sludge products. Environ Sci Technol 2010, 44, (19), 7509-7514. 6. Kiser, M. A.; Westerhoff, P.; Benn, T.; Wang, Y.; Perez-Rivera, J.; Hristovski, K., Titanium nanomaterial removal and release from wastewater treatment plants. Environ Sci Technol 2009, 43, (17), 6757-6763. 7. Lowry, G. V.; Gregory, K. B.; Apte, S. C.; Lead, J. R., Transformations of nanomaterials in the environment. Environ Sci Technol 2012, 46, (13), 6893-6899. 8. Nel, A.; Xia, T.; Madler, L.; Li, N., Toxic potential of materials at the nanolevel. Science 2006, 311, (5761), 622-627. 9. Auffan, M.; Rose, J.; Bottero, J. Y.; Lowry, G. V.; Jolivet, J. P.; Wiesner, M. R., Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective. Nat Nanotechnol 2009, 4, (10), 634-641. 10. Wiesner, M. R.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P., Assessing the risks of manufactured nanomaterials. Environ Sci Technol 2006, 40, (14), 4336-4345. 11. Navarro, E.; Baun, A.; Behra, R.; Hartmann, N. B.; Filser, J.; Miao, A. J.; Quigg, A.; Santschi, P. H.; Sigg, L., Environmental behavior and ecotoxicity of engineered nanoparticles to algae, plants, and fungi. Ecotoxicology 2008, 17, (5), 372-386. 12. Auffan, M.; Rose, J.; Proux, O.; Masion, A.; Liu, W.; Benameur, L.; Ziarelli, F.; Botta, A.; Chaneac, C.; Bottero, J. Y., Is there a Trojan-horse effect during magnetic nanoparticles and metalloid cocontamination of human dermal fibroblasts? Environ Sci Technol 2012, 46, (19), 10789-10796. 13. Mueller, N. C.; Nowack, B., Exposure modeling of engineered nanoparticles in the environment. Environ Sci Technol 2008, 42, (12), 4447-4453. 14. Gulson, B.; McCall, M.; Korsch, M.; Gomez, L.; Casey, P.; Oytam, Y.; Taylor, A.; McCulloch, M.; Trotter, J.; Kinsley, L.; Greenoak, G., Small amounts of zinc from zinc oxide particles in sunscreens applied outdoors are absorbed through human skin. Toxicol Sci 2010, 118, (1), 140-149. 15. Rampaul, A.; Parkin, I. P.; Cramer, L. P., Damaging and protective properties of inorganic components of sunscreens applied to cultured human skin cells. J Photoch Photobio A 2007, 191, (2-3), 138-148.


20

Page 21 of 30

413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457


16. Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W., Environmental applications of semiconductor photocatalysis. Chem Rev 1995, 95, (1), 69-96. 17. Burello, E.; Worth, A. P., A theoretical framework for predicting the oxidative stress potential of oxide nanoparticles. Nanotoxicology 2011, 5, (2), 228-235. 18. Franklin, N. M.; Rogers, N. J.; Apte, S. C.; Batley, G. E.; Gadd, G. E.; Casey, P. S., Comparative toxicity of nanoparticulate ZnO, bulk ZnO, and ZnCl2 to a freshwater microalga (Pseudokirchneriella subcapitata): The importance of particle solubility. Environ Sci Technol 2007, 41, (24), 8484-8490. 19. Li, M.; Lin, D. H.; Zhu, L. Z., Effects of water chemistry on the dissolution of ZnO nanoparticles and their toxicity to Escherichia coli. Environ Pollut 2013, 173, 97102. 20. Li, M.; Zhu, L. Z.; Lin, D. H., Toxicity of ZnO nanoparticles to Escherichia coli: Mechanism and the influence of medium components. Environ Sci Technol 2011, 45, (5), 1977-1983. 21. Rousk, J.; Ackermann, K.; Curling, S. F.; Jones, D. L., Comparative toxicity of nanoparticulate CuO and ZnO to soil bacterial communities. PLoS One 2012, 7, (3), e34197. 22. Li, Y.; Zhang, W.; Niu, J. F.; Chen, Y. S., Mechanism of photogenerated reactive oxygen species and correlation with the antibacterial properties of engineered metaloxide nanoparticles. ACS Nano 2012, 6, (6), 5164-5173. 23. Ng, A. M. C.; Chan, C. M. N.; Guo, M. Y.; Leung, Y. H.; Djurisic, A. B.; Hu, X.; Chan, W. K.; Leung, F. C. C.; Tong, S. Y., Antibacterial and photocatalytic activity of TiO2 and ZnO nanomaterials in phosphate buffer and saline solution. Appl Microbiol Biot 2013, 97, (12), 5565-5573. 24. Bar-Ilan, O.; Chuang, C. C.; Schwahn, D. J.; Yang, S.; Joshi, S.; Pedersen, J.; Hammers, R. J.; Peterson, R. E.; Heideman, W., TiO2 nanoparticle exposure and illumination during zebraﬁsh development: Mortality at parts per billion concentrations. Environ Sci Technol 2013, 47, 4726-4733. 25. Bar-Ilan, O.; Louis, K. M.; Yang, S. P.; Pedersen, J. A.; Hamers, R. J.; Peterson, R. E.; Heideman, W., Titanium dioxide nanoparticles produce phototoxicity in the developing zebrafish. Nanotoxicology 2012, 6, (6), 670-679. 26. Miller, R. J.; Bennett, S.; Keller, A. A.; Pease, S.; Lenihan, H. S., TiO2 nanoparticles are phototoxic to marine phytoplankton. PLoS One 2012, 7, (1), e30321. 27. Tong, T.; Binh, C. T. T.; Kelly, J. J.; Gaillard, J.-F.; Gray, K. A., Cytotoxicity of commercial nano-TiO2 to Escherichia coli assessed by high-throughput screening: Effects of environmental factors. Water Res 2013, 47, 2352-2362. 28. Tong, T.; Shereef, A.; Wu, J.; Binh, C. T. T.; Kelly, J. J.; Gaillard, J.-F.; Gray, K. A., Effects of material morphology on the phototoxicity of nano-TiO2 to bacteria. Environ Sci Technol 2013, 47, 12486-12495. 29. Binh, C. T. T.; Tong, T.; Gaillard, J.-F.; Gray, A. K.; Kelly, J. J., Common freshwater bacteria vary in their responses to short-term exposure to nano-TiO2 in natural surface waters. Environ Toxicol Chem 2013, In press. 30. Pena, M. E.; Korfiatis, G. P.; Patel, M.; Lippincott, L.; Meng, X. G., Adsorption of As(V) and As(III) by nanocrystalline titanium dioxide. Water Res 2005, 39, (11), 2327-2337.


21


458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502

Page 22 of 30

31. Comarmond, M. J.; Payne, T. E.; Harrison, J. J.; Thiruvoth, S.; Wong, H. K.; Aughterson, R. D.; Lumpkin, G. R.; Muller, K.; Foerstendorf, H., Uranium sorption on various forms of titanium dioxide - Influence of surface area, surface charge, and impurities. Environ Sci Technol 2011, 45, (13), 5536-5542. 32. He, G. Z.; Pan, G.; Zhang, M. Y.; Waychunas, G. A., Coordination structure of adsorbed Zn(II) at water-TiO2 interfaces. Environ Sci Technol 2011, 45, (5), 1873-1879. 33. Parkhurst, D.; Appelo, C. A. J. User’s Guide to PHREEQC (version 2) - A computer program for speciation, batch-reaction, one- dimensional transport, and inverse geochemical calculations; U.S. Geological Survey Water-Resources Investigations Report 99-4259: 1999. 34. Waychunas, G. A.; Fuller, C. C.; Davis, J. A., Surface complexation and precipitate geometry for aqueous Zn(II) sorption on ferrihydrite I: X-ray absorption extended fine structure spectroscopy analysis. Geochim Cosmochim Ac 2002, 66, (7), 1119-1137. 35. Ravel, B.; Newville, M., Athena, Artemis, Hephaestus: Data analysis for X-ray absorption spectroscopy using IFEFFIT. J Synchrotron Radiat 2005, 12, 537-541. 36. Battin, T. J.; Kammer, F. V. D.; Weilhartner, A.; Ottofuelling, S.; Hofmann, T., Nanostructured TiO2: Transport behavior and effects on aquatic microbial communities under environmental conditions. Environ Sci Technol 2009, 43, (21), 8098-8104. 37. Gaillard, J. F.; Webb, S. M.; Quintana, J. P. G., Quick X-ray absorption spectroscopy for determining metal speciation in environmental samples. J Synchrotron Radiat 2001, 8, 928-930. 38. Da Silva-Cadoux, C.; Zanella, L.; Gaillard, J. F., Selecting reference compounds for determining chemical speciation by X-ray absorption spectroscopy. Journal of Analytical Atomic Spectrometry 2012, 27, (6), 957-965. 39. David, C. A.; Galceran, J.; Rey-Castro, C.; Puy, J.; Companys, E.; Salvador, J.; Monne, J.; Wallace, R.; Vakourov, A., Dissolution Kinetics and Solubility of ZnO Nanoparticles Followed by AGNES. J Phys Chem C 2012, 116, (21), 11758-11767. 40. Dokoumetzidis, A.; Macheras, P., A century of dissolution research: From Noyes and Whitney to the Biopharmaceutics Classification System. Int J Pharm 2006, 321, (12), 1-11. 41. Morel, F. M. M.; Hering, J. G., Principles and applications of aquatic chemistry. John Wiley & Sons, Inc.: New York, NY, 1993. 42. deAlmeida, L. P.; Simoes, S.; Brito, P.; Portugal, A.; Figueiredo, M., Modeling dissolution of sparingly soluble multisized powders. J Pharm Sci-Us 1997, 86, (6), 726732. 43. Ma, R.; Levard, C.; Michel, F. M.; Brown, G. E.; Lowry, G. V., Sulfidation mechanism for zinc oxide nanoparticles and the effect of sulfidation on their solubility. Environ Sci Technol 2013, 47, (6), 2527-2534. 44. Lv, J. T.; Zhang, S. Z.; Luo, L.; Han, W.; Zhang, J.; Yang, K.; Christie, P., Dissolution and microstructural transformation of ZnO nanoparticles under the influence of phosphate. Environ Sci Technol 2012, 46, (13), 7215-7221. 45. Mudunkotuwa, I. A.; Rupasinghe, T.; Wu, C. M.; Grassian, V. H., Dissolution of ZnO nanoparticles at circumneutral pH: A study of size effects in the presence and absence of citric acid. Langmuir 2012, 28, (1), 396-403.


22

Page 23 of 30

503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520


46. Cambell, P. G. C., Interactions between trace metals and aquatic organisms: A critique of the free-ion activity model. In Metal Speciation and bioavailability, Tessier A.; R., T. D., Eds. John Wiley & Sons Ltd: 1995; pp 45-97. 47. Paquin, P. R.; Gorsuch, J. W.; Apte, S.; Batley, G. E.; Bowles, K. C.; Campbell, P. G. C.; Delos, C. G.; Di Toro, D. M.; Dwyer, R. L.; Galvez, F.; Gensemer, R. W.; Goss, G. G.; Hogstrand, C.; Janssen, C. R.; McGeer, J. C.; Naddy, R. B.; Playle, R. C.; Santore, R. C.; Schneider, U.; Stubblefield, W. A.; Wood, C. M.; Wu, K. B., The biotic ligand model: a historical overview. Comp Biochem Phys C 2002, 133, (1-2), 3-35. 48. Yang, W. W.; Miao, A. J.; Yang, L. Y., Cd2+ Toxicity to a green alga Chlamydomonas reinhardtii as influenced by its adsorption on TiO2 engineered nanoparticles. PLoS One 2012, 7, (3), e32300. 49. Fan, W. H.; Cui, M. M.; Liu, H.; Wang, C. A.; Shi, Z. W.; Tan, C.; Yang, X. P., Nano-TiO2 enhances the toxicity of copper in natural water to Daphnia magna. Environ Pollut 2011, 159, (3), 729-734. 50. Tan, C.; Fan, W. H.; Wang, W. X., Role of titanium dioxide nanoparticles in the elevated uptake and retention of cadmium and zinc in Daphnia magna. Environ Sci Technol 2012, 46, (1), 469-476.


23


Page 24 of 30

Figure 1. TEM micrographs of nano-TiO2 P25 (left) and nano-ZnO (right).


24

Page 25 of 30


Figure 2. [Zn]dis remaining in solution when (A) 0.5 mg/L of dissolved Zn – prepared from a stock solution of ZnCl2, (B) 0.5 mg/L nano-ZnO, and (C) 1 mg/L nano-ZnO was mixed with P25 at different concentrations. Note that 0.5 mg/L and 1 mg/L of nano-ZnO represent fully- and partially-dissolved nano-ZnO in Lake Michigan water after a two-day reaction period, respectively.


25


Page 26 of 30

Figure 3. [Zn]dis remaining in solution when 1 mg/L of nano-ZnO was aged for one day prior to the addition of P25.


26

Page 27 of 30


Figure 4. The Zn K-edge XANES (A) and EXAFS (B) spectra of the nano-ZnO/P25 and dissolved Zn/P25 mixtures as well as the powder standards for qualitative comparison; (C) EXAFS spectra (solid lines) and linear combination fits (open dots) of the nanoZnO/P25 mixtures with different nano-ZnO concentrations. The P25 concentration is 20 mg/L. The residues of the fittings are shown below the spectral decompositions of the corresponding fittings.


27


Page 28 of 30

Figure 5. Experimental and modeling results of [Zn]dis from 0.5 mg/L (A) and 1 mg/L (B) of nano-ZnO mixed with different concentrations of P25. The modeling results of 1 day (1d) and 2 day (2d) overlap each other and are thus not distinguishable.


28

Page 29 of 30


Figure 6. Comparison of the experimental and modeling results for [Zn]dis when nano-ZnO and P25 interact in solution. The red solid line, green curves and blue curves represent the best linear fit, the 95% confidence interval, and the 95% prediction interval of the fit, respectively.


29


Page 30 of 30

TOC


30

Chemical Interactions between Nano-ZnO and Nano-TiO2

Recommend Documents