Surfactant-Wrapped Multiwalled Carbon Nanotubes in Aquatic

Subscriber access provided by UNIV OF CAMBRIDGE

Article

Surfactant-Wrapped Multiwalled Carbon Nanotubes in Aquatic Systems: Surfactant Displacement in the Presence of Humic Acid Xiaojun Chang, and Dermont C. Bouchard Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01536 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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 31

Environmental Science & Technology

1

Surfactant-Wrapped Multiwalled Carbon Nanotubes

2

in Aquatic Systems: Surfactant Displacement in the

3

Presence of Humic Acid

4

Xiaojun Chang‡ and Dermont C. Bouchard§,* ‡

5

6

§

National Research Council Research Associate

USEPA Office of Research and Development, National Exposure Research Laboratory,

7

960 College Station Road, Athens, GA 30605, USA

8

*Corresponding author E-mail: [email protected];

9

Phone: 706-355-8333

10

Total Word Length: 5055 + 6 figures*300 = 6855 words (not including references)

11

1 ACS Paragon Plus Environment


Page 2 of 31

12

Abstract

13

Sodium dodecyl sulfate (SDS) facilitates multi-walled carbon nanotube (MWCNT) de-

14

bundling and enhances nanotube stability in the aqueous environment by adsorbing on the

15

nanotube surfaces, thereby increasing repulsive electrostatic forces and steric effects. The

16

resulting SDS-wrapped MWCNTs are utilized in industrial applications and have been widely

17

employed in environmental studies. In the present study, MWCNTs adsorbed SDS during

18

ultrasonication to form stable MWCNTs suspensions. Desorption of SDS from MWCNTs

19

surfaces was then investigated as a function of Suwannee River Humic Acid (SRHA) and

20

background electrolyte concentrations. Due to hydrophobic effects and π-π interactions,

21

MWCNTs exhibit higher affinity for SRHA than SDS. In the presence of SRHA, SDS adsorbed

22

on MWCNTs was displaced. Cations (Na+, Ca2+) decreased SDS desorption from MWCNTs due

23

to charge screening effects. Interestingly, the presence of the divalent calcium cation facilitated

24

multi-layered SRHA adsorption on MWCNTs through bridging effects, while monovalent

25

sodium reduced SRHA adsorption. Results of the present study suggest that properties of

26

MWCNTs wrapped with commercial surfactants will be altered when these materials are

27

released to surface waters and the surfactant coating will be displaced by natural organic matter

28

(NOM). Changes on their surfaces will significantly affect MWCNTs fate in aquatic

29

environments.


Page 3 of 31


30

1. Introduction.

31

As nanotechnology develops, more engineered nanomaterials are being utilized in industry

32

and studied to understand their impacts on environmental health and safety. In both industrial

33

applications and environmental research, the original properties of nanomaterials (NMs) are

34

likely to be altered as media constituents are adsorbed on their surface upon exposure to

35

biological or environmental media.1 Since many NMs are coated with surface active materials to

36

enhance suspension processing and stability, there is competition between the original coatings

37

and media constituents for adsorption sites. Displacement of the original adsorbed ligand by

38

other constituents on NM surfaces has been reported: bovine serum albumin (BSA), with a

39

higher binding affinity to gold nanoparticles (AuNPs), has been observed to displace thiolated

40

poly(ethylene glycol) on AuNP surfaces;2 Similarly, carboxylate and thiol groups displaced

41

oleylamine and oleic acid, respectively, on FePt nanoparticles surfaces due to their stronger

42

respective affinities for iron and platinum atoms.3 These changed surface characteristics, rather

43

than their original properties, dictate NM application and their environmental fate.4-6 Although

44

extensive research has been conducted on the surface properties of nanoparticles stock

45

suspensions, tracking changes in surface coatings and properties as NMs are released into

46

environmental media are less well studied.7 Lack of knowledge about the dynamics of

47

nanomaterials surface identity in the environment greatly weakens our ability to understand their

48

behaviors and to accurately predict their environmental impacts.

49

Due to high van der waal interaction energies between tubes,8 carbon nanotubes (CNTs)

50

cannot be readily dispersed in water as de-bundled individual nanotubes; in practical applications

51

and scientific studies, methodologies to suspend CNTs in aqueous systems are necessary. Other

52

than functionalization via covalent bonds,9 the most commonly used method to obtain stable 3 ACS Paragon Plus Environment


Page 4 of 31

53

CNT suspensions is to ultrasonicate CNTs in surfactant solutions.10-15 The resulting de-bundled

54

CNTs remained stable in the aqueous environment due to electrostatic and/or steric forces

55

acquired from surfactant molecules adsorbed on their surfaces. Surfactant-wrapped CNTs have

56

been widely used in studies on CNTs toxicities,16,17 transport, and fate.11,18-20 The surfactant

57

coatings can affect CNTs ability to adsorb other chemicals,21-23 however, they can also be

58

replaced by other adsorbates with a higher affinity for CNTs. For example, while employing

59

near-infrared fluorescence spectroscopy, Cherukuri et al. observed the displacement of Pluronic

60

108 surfactant coating on single-walled carbon nanotubes (SWCNTs) by rabbit serum albumin.24

61

Natural organic matter (NOM), which is ubiquitous in the environment, has shown an

62

exceptional capability for dispersing and stabilizing CNTs.12,25,26 Numerous studies have

63

investigated NOM effects on organic pollutants adsorption on CNTs surfaces. When added

64

concurrently with organic pollutants (e.g., polycyclic aromatic hydrocarbon, perfluorinated

65

compounds, pharmaceutical and personal care products, etc.) to the adsorption system, NOM

66

facilitates de-bundling and dispersion of CNTs which increases their surfaces areas, leading to an

67

increase in adsorption sites. At the same time, NOM competes with pollutants for available

68

adsorption sites, resulting in a decrease in pollutant adsorption on CNTs. Due to these opposing

69

effects, the presence of NOM in a system with de-bundled CNTs either promotes27 or inhibits

70

pollutants adsorbing on CNTs.28-32 When NOM adsorb on CNT surfaces prior to pollutants

71

introduction, the suppression of pollutants adsorption on CNTs is more profound since NOM

72

occupies available adsorption sites.29,30,33 From the referenced studies, it is clear that NOM can

73

exert significant effects on CNT surface chemistry and adsorption capacity,34 changing the

74

transport, bioavailability, and toxicity of CNTs and organic pollutants adsorbed on CNTs

75

surfaces.


Page 5 of 31


76

Upon release of surfactant-wrapped CNTs to a surface water, CNT surface chemistry may be

77

substantially changed as the initial coating is replaced by NOM. While it is clear coating of

78

CNTs is a major determinant of their environmental fate, there are no systematic studies on

79

NOM exchange with CNT surfactant coatings. To fill this gap, the present study investigated

80

displacement of surfactant on multi-walled carbon nanotubes (MWCNTs) surfaces by NOM and

81

the effects of solution chemistry during this process. Sodium dodecyl sulfate (SDS), a commonly

82

used surfactant, was employed to disperse MWCNTs in aqueous systems. The resultant SDS-

83

wrapped MWCNTs were then exposed to solutions with varying Suwannee River Humic Acid

84

(SRHA) concentration and ionic composition, and the mass and kinetics of SDS release and

85

SRHA adsorption were quantified.

86

2. Materials and Methods:

87

2.1 Chemicals.

88

The unfunctionalized MWCNTs with purity of 95 wt% were purchased from Cheap Tubes

89

Inc. (Brattleboro, VT). Their inside diameters, outside diameters, and lengths are reported as 5-

90

10 nm, 20-30 nm, and 10-30 µm, respectively. Detailed characterization (e.g., XPS for

91

functionalization and ICP-AES for metal residue analysis) for the MWCNTs powder was

92

presented in our previous study.35 Analytical grade sodium dodecyl sulfate (SDS), sodium

93

chloride (NaCl), calcium chloride (CaCl2), and ScintiSafe 30% liquid scintillation cocktail for

94

14

95

labeled SDS (0.1 mCi/mL, 55 mCi/mM) was purchased from American Radiolabeled Chemicals

96

(St. Louis, Mo). Suwanee River Humic Acid (SRHA, Standard II) was purchased from the

97

International Humic Substance Society (St. Paul, MN). A representative surface water sample

98

was collected from Brier Creek GA, USA.

C isotope activity measurement were purchased from Fisher Chemicals (Fremont, CA).

14

C-



99

Page 6 of 31

2.2 Solution and Suspension Preparation.

100

2.2.1 Solutions. 1 M NaCl, 0.1 M CaCl2, and 1 g/L SDS solutions were prepared in deionized

101

water (DI) with a resistivity of 18.2 MΩ, from Aqua Solution Type I Water Purification System

102

(Aqua Solutions, Jasper, GA). The stock SDS solution was diluted to obtain 5, 10, 20, and 40

103

mg/L SDS solutions (pH ≈ 7.0).

104

spiking 50 µL of the as purchased

105

solutions. This added an additional 0.0952 mg/L SDS, causing a negligible [SDS] increase – less

106

than 4% for 2.5 mg/L SDS solution – and even lower additional fractions for solutions with

107

higher [SDS]. To prepare the SHRA stock solution, SRHA powder was first dissolved in DI

108

water and then filtered through a 0.22 µm polycarbonate filter (Osmonics Inc. Trevose, PA). The

109

working SRHA solutions (2.5 to 40 mg C/L and pH 4.8 to 5.5) were all diluted from this stock

110

solution.

14

C-labeled SDS working solutions were then prepared by 14

C-labeled SDS stock solution into 250 mL of the SDS

111

2.2.2 MWCNT Suspensions. All MWCNT suspensions used in the present study were

112

prepared using probe sonication.12 Four milligrams of MWCNT powders, as purchased, were

113

mixed with 40 mL dispersant (i.e., 14C-SDS or SRHA) working solutions, whose concentrations

114

were adjusted within 2.5 - 40 mg C/L (note: SDS concentrations used in the present study were

115

2-3 orders of magnitude lower than its critical micelle concentration).36 The mixtures were then

116

ultrasonicated with a probe sonicator (Sonic & Materials, Newton, CT) in an ice-water bath for

117

10 min at an average input energy level of ~33 W. Here, we refer to MWCNT suspensions

118

prepared in different dispersant solutions as XX mg/L YY-MWCNTs, where XX indicates

119

concentration of the dispersant and YY indicates dispersant species.

120

2.3 Characterization.

121

2.3.1 General Physicochemical Characterization for MWCNTs. 6 ACS Paragon Plus Environment

Page 7 of 31


122

Intensity-weighted hydrodynamic diameter (Dh) and electrophoretic mobility (EPM) were

123

used to represent average particle size and surface charge of dispersed MWCNTs, respectively.

124

Dh and polydispersity index (PDI) were measured by dynamic light scattering (DLS) and EPM

125

was measured by phase analysis light scattering using a Nano ZetaSizer (Malvern Instruments,

126

Worcestershire, UK) with a helium/neon laser (λ = 633 nm).12

127

2.3.2 Dispersant Concentration Determination.

128

SRHA concentrations (presented as total organic carbon, TOC, mg C/L) were monitored by a

129

calibration curve established using TOC concentrations and their corresponding ultraviolet

130

absorbance values at 300 nm, obtained from a series of diluted stock SRHA solutions. TOC for

131

the stock SRHA solution was determined using a Shimadzu carbon analyzer (TOC-VCPH,

132

Columbia, MD). 14C-SDS was employed to facilitate measurement of the SDS concentration. 14C

133

activity was determined using a liquid scintillation counter (PerkinElmer Tri-Carb 3100). For

134

samples collected from MWCNTs suspensions prepared using a certain SDS working solution,

135

there is a linear relationship between 14C activities and [SDS].

136

2.4 Adsorption and Desorption Experiments

137

2.4.1. Adsorption Isotherms. To determine the adsorption of SDS and SRHA on MWCNTs,

138

1.6 mL of MWCNTs suspensions prepared in 2.2.2 were filtered through the 0.22 µm

139

polycarbonate filters 24-hour after suspension formation via ultrasonication. Preliminary tests

140

showed the [MWCNTs] in the filtrate was below the detection limit of the UV-vis method (< 0.4

141

mg/L), indicating that the membrane retained more than 99.6% of the suspended MWCNTs.

142

Filtration of

143

both SDS and SRHA during filtration were higher than 98%. To further minimize dispersant

144

adsorption, prior to the filtration of any MWCNTs suspension, 1.6 mL of the corresponding

14

C-SDS and SRHA working solutions demonstrated that recovery efficiencies for



Page 8 of 31

145

dispersant solution was first filtered to saturate any possible adsorption sites on the filter

146

membrane and housing and the residue solution was completely drained. Dispersant adsorption

147

on the filter membrane and housing was therefore negligible during MWCNTs filtration and

148

elution. After MWCNTs suspension filtration, dispersants that adsorbed onto MWCNTs surfaces

149

were also retained on filter membranes, while SDS and SRHA concentration in the filtrates were

150

determined using the aforementioned methods. These data were fitted with both Freundlich (Eqn.

151

1) and Langmuir (Eqn. 2) adsorption models:

152

ܳ௘ = ‫ܭ‬௙ × ‫ܥ‬௘௡

153

ܳ௘ = ܳ௠ × (ଵା஻஼೐

(1)

஻஼

೐)

(2)

154

where Qe (mg C/g MWCNTs) and Ce (total organic concentration, mg C/L, for both dispersant)

155

are the equilibrium solid-phase and aqueous-phase concentrations, respectively; Kf [(mg C/g

156

MWCNTs)/(mg C/L)n] is the Freundlich adsorption coefficient; n is the nonlinearity factor; Qm

157

(mg C/g MWCNTs) is the maximum adsorption capacity; and B (L/mg C) is the affinity

158

parameter.

159

2.4.2 Dispersant Desorption by DI/Benchmark Elution. MWCNTs retained on filters in 2.4.1

160

were eluted repeatedly (14 times) with 0.8 mL DI water to determine desorption of the adsorbed

161

SDS/SRHA. Each eluate was collected and analyzed to determine the amount of desorbed

162

dispersant. For SDS-MWCNTs – after all DI elutions – each filter with retained MWCNTs was

163

sonicated in 10 mL of cocktail for 30 min in a bath sonicator, and the amount of SDS retained on

164

MWCNTs surfaces was determined by its 14C activity. For comparison to elution experiments by

165

salt and/or SRHA solutions (see 2.4.3), the DI-eluted SDS-MWCNTs experiments were referred

166

to as benchmark elution experiments.


Page 9 of 31


167

2.4.3 SDS Desorption from MWCNTs as a function of SRHA and Salt Concentration. 5.0

168

mg/L SDS-MWCNTs retained on filter in 2.4.1 were eluted by aliquots (0.8 mL) of SRHA,

169

CaCl2, NaCl, or solutions with both SRHA and salts for 10 times, followed by 0.8 mL DI elution

170

for 4 times. SDS desorption during each elution and SDS retained on MWCNTs after all elutions

171

was determined by the method described in 2.3.2. Adsorbed SRHA during each elution was

172

determined by the difference between [SRHA] in the eluting solution and eluate. The same

173

experiments were conducted using Brier Creek (BC) water to investigate SDS desorption in the

174

presence of an actual surface water sample. For comparison, a synthetic water, i.e., a solution

175

containing the same amount of Na+ and Ca2+ as BC water but without dissolved organic carbon

176

(DOC), was also employed in the elution experiments.

177

3. Results and Discussion.

178

3.1 Physicochemical Properties of MWCNTs.

179

In the presence of SDS or SRHA, ultrasonication successfully de-bundled and stabilized

180

MWCNTs.12 MWCNTs suspended in both dispersants were negatively charged with

181

electrophoretic mobilities (EPMs) < -4.5 × 10-8 m2/V-s. Average particles sizes (Dh) for all

182

MWCNTs suspensions prepared via ultrasonication were ~220 nm; Dh was also observed to be

183

insensitive to dispersant species and concentration (Figure S1).

184

3.2 Dispersants Adsorption and Desorption.

185

During the formation of stable MWCNTs suspensions using ultrasonication, dispersants

186

adsorbed on MWCNTs reached adsorption equilibrium by 24 hours (Figure S2). For both SDS

187

and SRHA, solid-phase equilibrium concentration on MWCNTs (Qe, mg C/g MWCNTs) first

188

increased with aqueous-phase equilibrium concentration (Ce, mg C/L), then reached a plateau



Page 10 of 31

189

(Figure 1A). Fitting results for the nonlinear Langmuir and Freundlich models are presented in

190

the table inset in Figure 1A. At the same Ce, Qe for SRHA was at least four times higher than Qe

191

for SDS. The greater SRHA adsorption on MWCNTs surfaces is likely caused by structural

192

differences between SRHA and SDS: previous experimental37 and theoretical38 studies have

193

reported that linear-shaped SDS molecules lay randomly on CNTs surfaces with little protrusion

194

of the SDS hydrophobic tail into surrounding water. SRHA, with its more three-dimensional,

195

nonlinear structure,39 likely forms a thicker wrap layer on MWCNTs surfaces, which is indicated

196

by the greater SRHA adsorption. Multi-layer adsorption (i.e., subsequent SRHA adsorption on

197

SRHA layers directly adsorbed on MWCNTs) is enabled by hydrogen bond and π-π interactions

198

and is also likely to account for this higher SRHA adsorption. Multi-layer adsorption on CNTs

199

has been observed in tannic acid40 and lysozyme41 adsorption. Multi-layer adsorption of SRHA

200

on α-Fe2O3 nanoparticles surfaces has been reported by Mudunkotuwa and Grassian:1 SRHA

201

exhibited two different ATR-FTIR spectra on α-Fe2O3 nanoparticles, suggesting one type of

202

SRHA is directly adsorbed on α-Fe2O3 surfaces and a second type is adsorbed onto subsequent

203

SRHA layers.

204

Maximum adsorption capacities (Qm) from the Langmuir model fitting are 54.8 and 12.2 mg

205

C/g MWCNTs for SRHA and SDS, respectively. The two values can be interpreted as that each

206

MWCNT carbon atom can adsorb, on average, 0.054 and 0.012 carbon atoms of SRHA and

207

SDS, respectively. Such low values do not necessarily mean that only a small portion of the

208

MWCNTs surfaces was covered by these dispersants. Based upon parameters provided by the

209

manufacturer and some basic assumptions,42 a straightforward calculation (details in Supporting

210

Information) demonstrates that the carbon atoms on the outermost wall are ~5.7% of the total

211

carbon atoms in MWCNTs used in the present study. Therefore, each outermost wall carbon 10 ACS Paragon Plus Environment

Page 11 of 31


212

atom adsorbed roughly 0.96 and 0.21 carbon atoms of SRHA and SDS, respectively, suggesting

213

substantial coverage of the MWCNTs surface by either dispersant. The Freundlich model-fitting

214

shows that the nonlinear factors, n, for both dispersants were quite similar (0.326 and 0.304 for

215

SDS and SRHA, respectively), which is consistent with this parameter being mainly dependent

216

upon adsorbent surface heterogeneity and independent of adsorbate species.43

217 218 219 220 221 222

Figure 1 (A) Solid-phase equilibrium concentration (Qe) as a function of aqueous equilibrium concentration (Ce) for SRHA and SDS adsorption on MWCNTs; Table: Adsorption isotherm fitting results for Langmuir and Freundlich adsorption models. [Ce is presented as total organic carbon concentration (mg C/L) for both dispersants.] (B) [SDS] in eluate for SDS-MWCNTs eluted by DI; Inset: [SRHA] in eluate for 10 mg C/L SRHA-MWCNTs eluted by DI.

223

The higher affinity parameter B for SRHA in the Langmuir model (Table in Figure 1A)

224

suggests a stronger bonding between SRHA and MWCNTs than between SDS and MWCNTs.43

225

One approach to quantify adsorption affinity is to measure desorption of adsorbate from

226

adsorbent.1 In the present study, the SRHA- or SDS-wrapped MWCNTs retained on filter

227

membranes were eluted with DI and desorption was quantified by measuring released dispersant

228

concentration in the eluate. Sequential elution of 2.5 mg/L SDS-MWCNTs resulted in [SDS] in

229

the eluate gradually decreasing from 0.30 to 0.03 mg/L; and [SDS] in the eluate decreased from



Page 12 of 31

230

0.58 to 0.03 mg/L for the sequential elution of 5.0 mg/L SDS-MWCNTs (Figure 1B). After all

231

14 times of DI elution, 62.9 ± 0.4% and 65.1 ± 0.7% of the adsorbed SDS desorbed from 2.5 and

232

5.0 mg/L SDS-MWCNTs (Figure S3), respectively. The desorption profile (Figure S4) of 5.0

233

mg/L SDS-MWCNTs lies above the SDS-MWCNTs adsorption isotherm suggesting that

234

although desorption equilibrium was not reached during the relatively short contact time between

235

eluent (i.e., DI) and MWCNTs, a significant amount of adsorbed SDS was still released from the

236

MWCNTs during DI elution. The first DI elution of 10 mg C/L SRHA-MWCNTs yielded an

237

eluate concentration of 0.2 mg C/L. SRHA concentrations in later DI elutions were all below

238

detection limits (Figure 1B inset). The SRHA released from MWCNTs surfaces after fourteen DI

239

elutions accounted for only 3.5% of the adsorbed SRHA. The stronger SRHA-MWCNTs

240

association suggested by more facile SDS desorption than SRHA desorption is most likely due to

241

greater hydrophobic effects44 and π-π interactions45 of the more chemically heterogeneous

242

SRHA.

243

3.3 Displacement of SDS by SRHA on MWCNTs

244

Higher affinity of SRHA for MWCNTs than SDS, and the resulting slower SRHA desorption

245

from MWCNT surfaces suggest that SRHA will be a more efficient competitor for MWCNT

246

adsorption sites. Consequently, when SDS-wrapped MWCNTs are exposed to SRHA, SDS

247

coating will be displaced by SRHA.

248

To test the above hypothesis, 5.0 mg/L SDS-MWCNTs retained on polycarbonate membranes

249

were eluted by solutions with varying [SRHA]. SRHA was very effective at promoting SDS

250

release from MWCNTs surfaces, as depicted in Figure 2A. A significant [SRHA] effect was

251

observed in the first elution: SDS concentration in first eluate increased four-fold (from 0.579 ±

252

0.07 to 2.274 ± 0.013 mg/L, Figure S5) with [SRHA] from 0 to 40 mg C/L. During the second 12 ACS Paragon Plus Environment

Page 13 of 31


253

SRHA elution, desorbed SDS first increased with [SRHA], reaching the maximum at [SRHA] =

254

10 mg C/L, then decreased. The SDS desorption maxima in later elutions occurred at even lower

255

SHRA levels, e.g., 5 and 2.5 mg C/L for the third and fourth elutions, respectively (Figure S5).

256

After 10-0.8 mL SRHA elutions, the total SDS desorption in the presence of SRHA was much

257

higher than that desorbed in the benchmark DI elutions (59.9 ± 0.7%, Figure 2A). They were

258

insensitive to [SRHA], however, as SRHA concentration in eluent increased from 2.5 to 40 mg

259

C/L, the total desorbed SDS increased slightly – from 83.0 ± 0.6% to 91.1 ± 0.9%. A similar

260

trend was observed by Shen et al.46 in their study of lindane displacement by atrazine on

261

MWCNTs: lindane desorption spiked at a low atrazine level, then slowly increased as [atrazine]

262

further increased. The observation that most of the adsorbed SDS was released from the

263

MWCNTs regardless of [SRHA] is of great environmental importance: it indicates that even in

264

surface waters with low dissolved organic carbon (DOC) concentrations, DOC will significantly

265

enhance SDS release from MWCNTs surfaces and therefore alter the MWCNTs surface

266

properties that dictate their environmental fate.

267 268 269

Figure 2 (A) The fraction of released SDS to total SDS adsorbed on SDS-MWCNTs when 5.0 mg/L SDSMWCNTs was first eluted by SRHA solutions (0.8 mL × 10 times) and then DI (0.8 mL × 4 times). (Note: the



Page 14 of 31

270 271

[SRHA] in the top x-axis label indicates the SRHA concentration used in the previous SRHA elutions.) (B) SRHA adsorbed on MWCNTs when SRHA solution eluted 5.0 mg/L SDS-MWCNTs.

272

SRHA adsorption on MWCNTs occurred simultaneously with SDS desorption during SRHA

273

elution. In contrast to total SDS release, which was virtually independent of [SRHA], total

274

adsorbed SRHA increased significantly with [SRHA] in the eluent (Figure 2B). After 10-0.8 mL

275

elutions with 2.5 mg C/L SRHA solution, each gram of MWCNTs adsorbed 75 mg SRHA (total

276

carbon mass), while adsorbed SRHA concentration was as high as 230 mg C/g MWCNTs when

277

eluted by 40 mg C/L SRHA.

278

It is worth noting that the total adsorbed SRHA on SDS-MWCNTs in all SRHA elution

279

experiments (Figure 2B) was higher than the maximum adsorption capacity determined in the

280

previous adsorption experiment (55 mg C SRHA/g MWCNTs, see table in Figure 1A), meaning

281

that each carbon atom on the outermost layer of the MWCNTs associated with more than one

282

SRHA carbon atom during SRHA elution. Although straining effect has been reported to be an

283

important mechanism for colloids/nanoparticles retention in porous media,47 it is unlikely the

284

reason for the higher retention of SRHA observed here due to the low particle/pore size ratio48,49

285

in the system and the extremely thin MWCNTs layer formed on the filter membrane (Detailed

286

calculation and explanation is presented in Supporting Information). In addition, the filter elution

287

systems contained both SDS and SRHA, as opposed to the batch adsorption systems that

288

contained only one dispersant, and SDS-SRHA hydrophobic interactions50,51 may have resulted

289

in an increase in total SRHA retained on the SDS-MWCNT surfaces. Finally, the increase in

290

SRHA adsorption is also likely due to the inherent differences between the batch adsorption

291

system and elution system: the former is at equilibrium with a fixed amount of adsorbate while

292

the latter is at non-equilibrium with a continuous adsorbate supply.


Page 15 of 31


293

Additional DI elutions were conducted following SRHA elution to further measure SDS

294

release and the reversibility of SRHA adsorption when solution chemistry changes. Although

295

most of the adsorbed SDS (~90%) was released during SRHA elution, the amount of released

296

SDS during the following DI elution was quantifiable, and decreased from 3.6 ± 0.3 to ~1.0% as

297

[SRHA] used in previous elutions increased from 2.5 to 40 mg C/L (Figure 2A). At the same

298

time, a small fraction of SRHA that adsorbed on MWCNTs during SRHA elution desorbed

299

during DI elution: the amount of released SRHA increased from 1 to 24 mg C/g MWCNTs as

300

adsorbed SRHA increased from 75 to 232 mg C/g MWCNTs (i.e., [SRHA] increased from 2.5 to

301

40 mg C/L in eluent). The fraction of released SRHA to total adsorbed SRHA also increased

302

(Figure 3A). Higher SRHA desorption ratios in this step (e.g., 6.7% and 10.1 % for 20 and 40

303

mg/L SRHA eluted SDS-MWCNTs samples, respectively) suggest that the later adsorbed SRHA

304

during elution is more readily desorbed during DI elution than the SRHA adsorbed on MWCNTs

305

during ultrasonication.

306

Less than 5% of the initially adsorbed SDS was retained on the MWCNTs surfaces after

307

SRHA and DI elutions, while more than 90% of the adsorbed SRHA during SRHA elution

308

remained associated with MWCNTs after DI elution. For 5.0 mg/L SDS-MWCNTs eluted by

309

various SRHA solutions, final adsorbed SDS concentrations were at least one order of magnitude

310

lower than SRHA finally adsorbed on MWCNTs (Figure 3B, on carbon mass basis). In the

311

present study, concurrent SDS desorption and SRHA adsorption during elution indicates that the

312

original SDS surfactant coating on MWCNTs was displaced by SRHA, resulting in a SRHA-

313

predominant heterogeneous coating on MWCNTs. In surface waters, NOM, instead of the

314

original surfactant, will therefore be the primary material coating MWCNTs and it is the



Page 16 of 31

315

chemical and physical properties of these NOM-coated MWCNTs that will determine nanotube

316

transport, fate, and interaction with the biota.12

317 318 319 320

Figure 3 (A) The fraction of released SRHA during DI elution to total SRHA adsorbed on MWCNTs during previous SRHA elution. (B) SDS and SRHA retained on MWCNTs surfaces after all SRHA and DI elution as a function of [SRHA] used in SRHA elution.

321

3.4 Cation Effects on SDS Desorption

322

To investigate the effects of cations on SDS desorption, CaCl2 and NaCl solutions were used

323

to elute 5.0 mg/L SDS-MWCNTs. Divalent calcium cation was highly efficient at inhibiting

324

adsorbed SDS release from MWCNTs surfaces: as [Ca2+] in the eluent increased from 0 to 8

325

mM, the fraction of desorbed SDS decreased from 59.9 ± 0.7% to 28.9 ± 0.3% (Figure 4A).

326

Even at a very low [Ca2+] level (1 mM), the released fraction (39.4 ± 0.2%) dropped to 2/3 of

327

that released in the benchmark DI elution experiment. Cation electronic screening effect, leading

328

to less negatively charged MWCNTs surfaces, is likely to be the reason for this reduced SDS

329

desorption. Interestingly, Ca2+ inhibition effects were not permanent and the subsequent DI

330

elution readily released ~15% to 30% of the adsorbed SDS (Figure 4A). Despite the fact that the


Page 17 of 31


331

SDS release during calcium salt elution varied with [CaCl2], total SDS desorption during all

332

CaCl2 and DI elutions was not statistically different – 53.4% with the 95% confidence level of

333

1.8% – and was still ~20% lower than total released SDS in the benchmark experiment.

334

Consistent with the amount of desorbed SDS, SDS retained on MWCNTs surfaces after CaCl2

335

and DI elution was also independent of [Ca2+] (3.40 mg C/g MWCNTs with the 95% confidence

336

level of 0.19 mg C/g MWCNTs, Figure 4B) and ~20% higher than that obtained in benchmark

337

elutions (2.84 ± 0.05 mg C/g MWCNTs). The above results suggest that reducing ionic strength

338

partially mitigates Ca2+’s repression of SDS release from MWCNTs surfaces.

339

Only a slight decrease in SDS desorption was observed when using 20 mM NaCl to elute 5.0

340

mg/L SDS-MWCNTs (53.2 ± 0.6% vs. 59.9 ± 0.7% in the benchmark DI elutions, Figure 4A and

341

Table S3), which reveals that the mono-valent Na+ is much less effective at inhibiting SDS

342

desorption from MWCNTs than di-valent Ca2+, likely due to Na+’s weaker screening effect.

343

Total desorbed SDS and SDS retained on MWCNTs, after all NaCl and DI elution, were both

344

comparable to their corresponding values in benchmark DI elutions (Figure 4B and Table S3).

345

Unlike the partially reversible Ca2+ inhibition on SDS desorption, inhibition of SDS desorption

346

by Na+ can be totally reversed by removing Na+.



Page 18 of 31

347 348 349 350

Figure 4 (A) The fraction of released SDS to total SDS adsorbed on MWCNTs when 5.0 mg/L SDS-MWCNTs eluted by salt solution and DI. (Note: the top x-axis label indicates salt type and concentration used in the previous salt elutions.) (B) SDS retained on MWCNTs surfaces after salt and DI elutions.

351

3.5 Ionic Effects on the Displacement of SDS by SRHA

352

A series of solutions containing SRHA and CaCl2/NaCl were used to elute 5.0 mg/L SDS-

353

MWCNTs. At a fixed SRHA level, the presence of Ca2+ reduced total SDS desorption (Figure

354

5A, Tables S1 and S2). Calcium cation’s inhibition of SDS release in the presence of SRHA,

355

however, was insensitive to [Ca2+] and less profound than in the absence of SRHA (Figure 5A).

356

The amount of retained SDS on MWCNTs surfaces after Ca2+-SRHA/DI elutions shared a

357

similar, but reversed trend (Figure S6). Compared to Ca2+, mono-valent Na+ was less efficient at

358

inhibiting SDS desorption in the presence of SRHA: 82.2 ± 0.4% adsorbed SDS was released by

359

NaCl-SRHA elution (20 mM and 5 mg C/L, respectively), which was only ~5% less than that

360

eluted by 5 mg C/L SRHA (Table S3). These results collectively demonstrate that SDS

361

desorption by NOM is relatively insensitive to cation concentration and type and that, in a

362

cation-NOM system, NOM controls displacement of adsorbed SDS on MWCNTs surfaces.


Page 19 of 31


363

In benchmark DI elution and all experiments with eluents that contained one single

364

component (e.g., SRHA, CaCl2, or NaCl), SDS release decreased as elution progressed. When

365

eluted with solution containing 5 mg C/L SRHA and CaCl2, however, desorbed SDS did not

366

follow this trend – the highest desorption occurred during the second or third elution (Figure S7).

367

Co-effects of Ca2+ and SRHA on SDS desorption from MWCNTs should not be treated as a

368

simple combination of the Ca2+-inhibition and SRHA-enhancement. It is more likely that there

369

are complex adsorption, desorption, competition, and replacement interactions among

370

MWCNTs, SDS, SRHA, and Ca2+.29 And this is environmentally important since many surface

371

waters contain significant concentrations of both cations and various DOC species.

372

Similar to SRHA-only elution experiments, SRHA simultaneously adsorbed on MWCNTs as

373

SDS released from MWCNTs surfaces during SRHA-salt elution. The presence of Na+ reduced

374

SRHA adsorption on MWCNTs: the total adsorbed SRHA after ten SRHA-NaCl elutions were

375

66.9 and 59.2 mg C/g MWCNTs when eluted by 5 mg C/L SRHA-20 mM NaCl and 5 mg C/L

376

SRHA-80 mM NaCl, respectively, which were lower than the amount of adsorbed SDS after

377

salt-free 5 mg/L SRHA elution (89.7 mg C/g MWCNTs). In contrast, divalent calcium cation

378

facilitated SRHA adsorption (Figure 5B): after ten SRHA-CaCl2 elutions using 5 mg C/L SRHA

379

with 1, 4, and 8 mM CaCl2, each gram of eluted SDS-MWCNTs retained 105.5, 120.4, and 126.3

380

mg C SRHA, respectively. These values were 10%, 34%, and 41% higher than those for SRHA

381

adsorbed when eluted by the same SRHA solution in the absence of Ca2+. Interestingly, no

382

statistical difference in SRHA adsorption was observed in the first elution when compared to the

383

salt-free 5 mg C/L SRHA elution. The Ca2+-caused SRHA adsorption enhancement became

384

more profound after the first two elutions (Figure 5B), suggesting that Ca2+ promoted SRHA-

385

MWCNTs association mainly through multi-layered SRHA adsorption by bridging SRHA



Page 20 of 31

386

molecules with each other, rather than by increasing direct association between SRHA and

387

MWCNTs surfaces.

388 389 390 391

Figure 5 (A) The fraction of released SDS to total SDS adsorbed on MWCNTs when 5.0 mg/L SDS-MWCNTs eluted by solutions containing CaCl2 and/or SRHA (0.8 mL × 10 times). (B) SRHA adsorbed onto MWCNTs surfaces at each elution step when eluted by 5.0 mg C/L SRHA with varying CaCl2 and NaCl levels.

392

3.6 DOC Displacement of SDS in a Surface Water.

393

Brier Creek (BC) is a coastal plain river in the Savannah River basin in Georgia.52 BC water,

394

containing various cations and DOC species (detailed information presented in Table S4), was

395

used to investigate desorption of SDS from MWCNTs when exposed to an actual surface water.

396

Compared to synthetic water that contained similar cation components to BC water (but without

397

DOC) and resulted in 61.7 ± 0.3% of the adsorbed SDS release, BC water elutions led to more

398

SDS desorption (82.0 ± 0.5%, Figure 6). Similarly to SRHA, the DOC species in BC water,

399

quantified by their UV absorbance at 254 nm, adsorbed on MWCNTs surfaces during BC water

400

elution (inset of Figure 6) which suggests displacement of SDS by DOC in the surface water.

401

Results of the elution experiment using a SRHA-CaCl2 solution with similar TOC and ionic

402

strength levels to BC water are presented as green bars in Figure 6 for comparison. The two sets


Page 21 of 31


403

of experiments share a similar SDS desorption trend, indicating SRHA is a good model for

404

naturally-occurring organic materials in the water column that can adsorb on MWCNTs surfaces.

405

Taken together, these data indicate that MWCNT coatings like SDS will be replaced by DOC in

406

surface waters.

407 408 409 410 411

Figure 6 Released SDS from MWCNTs when eluted with synthetic water (containing same Ca2+ and Na+ as Brier Creek water but without DOC, red bars), Brier Creek (BC) water (blue bars), and SRHA-CaCl2 solution (green bars). Inset: Adsorbed DOC during BC water elution (represented as the difference of UV absorbance at 254 nm between each eluent and eluate).

412

4. Environmental Implications.

413

SDS is a commercial surfactant commonly used to disperse CNTs in aqueous media. SRHA,

414

with higher affinity towards MWCNTs due to hydrophobic and π-π interactions, can displace the

415

SDS coating and most of the adsorbed SDS will be released in the presence of SRHA. At the

416

same time, SRHA adsorption occurs on MWCNTs, resulting in a heterogeneous coating

417

containing SDS and SRHA directly adsorbed on MWCNTs, and a secondary coating composed

418

of weakly associated SRHA that is more easily desorbed. Although cations can inhibit SDS from

419

desorbing from MWCNTs surfaces in the absence of SRHA, their effects are significantly

420

lessened in the presence of SRHA. Di-valent and mono-valent cations affect SRHA adsorption

421

during displacement quite differently: Ca2+ enhances it by promoting multi-layered adsorption

422

through bridging effects, while Na+ reduces it. 21 ACS Paragon Plus Environment


Page 22 of 31

423

When surfactant-wrapped CNTs are released to surface waters, they encounter humic

424

materials, proteins, extracellular polymeric substance, organic pollutants, and various

425

components present in the water column. Many of these have strong affinities toward CNT

426

surfaces,53 resulting in displacement of the original surfactant coating. According to results from

427

the present study, a homogeneous surfactant coating on CNTs will be replaced by a

428

heterogeneous coating after contact with various constituents in environmental waters.

429

Formation of this complex coating on CNTs is of significant environmental importance: CNTs

430

surface identity controls their interactions with other environmental surfaces, alters CNTs

431

transport and fate in the environment, and determines what the cell “sees” prior to and during

432

NM uptake54,55 as well as organisms’ response to CNTs exposure. In addition, displacement of

433

CNTs coatings in the water column influence their ability to further adsorb other contaminants,

434

changing their uptake pathways, bioavailability, and toxicities. Since these heterogeneous

435

coatings of naturally occurring materials play a major role in determining CNTs fate in the

436

aquatic environment, their nature must be considered in experimental design and conceptual

437

models developed in CNT environmental studies.

438

Supporting Information

439

Calculation of the ratio of carbon atoms of the outermost layer to the total carbon atoms

440

contained by the employed MWCNTs. Hydrodynamic diameter and size distributions of

441

MWCNTs determined by dynamic light scattering. Monitoring of SDS concentration in the

442

filtrate during the experimental period. The released SDS as the fraction of adsorbed SDS on

443

MWCNTs during each DI elution. Comparison between the SDS-MWCNTs adsorption isotherm

444

and desorption profile during DI elution of 5.0 mg/L SDS-MWCNTs. Size exclusion and

445

straining effects are excluded for higher SRHA adsorption during SRHA elution. Released SDS 22 ACS Paragon Plus Environment

Page 23 of 31


446

as a function of [SRHA] in eluent in the first to sixth SRHA elution. Released SDS during each

447

elution from 5 mg/L SDS-MWCNTs eluted by various solution. SDS retained on MWCNTs

448

surfaces after SRHA/CaCl2 and DI elutions. SDS desorption as elution progressed.

449

Characterization of Brier Creek Surface water. This material is available free of charge via the

450

internet at http://pubs.acs.org.

451

Disclaimer

452

This paper has been reviewed in accordance with the USEPA’s peer and administrative

453

review policies and approved for publication. Mention of trade names or commercial products

454

does not constitute endorsement or recommendation for use.

455

References.

456

(1)Mudunkotuwa, I. A.; Grassian, V. H., Biological and environmental media control oxide

457

nanoparticle surface composition: The roles of biological components (Proteins and amino

458

acids), inorganic oxyanions and humic acid. Environ.-Sci. Nano 2015, 2, 429-439.

459

(2)Tsai, D.; DelRio, F. W.; MacCuspie, R. I.; Cho, T. J.; Zachariah, M. R.; Hackley, V. A.,

460

Competitive adsorption of thiolated polyethylene glycol and mercaptopropionic acid on gold

461

nanoparticles measured by physical ccharacterization methods. Langmuir 2010, 26, 10325-

462

10333.

463

(3)Bagaria, H. G.; Ada, E. T.; Shamsuzzoha, M.; Nikles, D. E.; Johnson, D. T., Understanding

464

mercapto ligand exchange on the surface of FePt nanoparticles. Langmuir 2006, 22, 7732-7737.

465

(4)Cheng, X.; Tian, X.; Wu, A.; Li, J.; Tian, J.; Chong, Y.; Chai, Z.; Zhao, Y.; Chen, C.; Ge, C.,

466

Protein corona influences cellular uptake of gold nanoparticles by phagocytic and nonphagocytic

467

cells in a size-dependent manner. ACS Appl. Mater. Interfaces 2015, 7, 20568-20575.



Page 24 of 31

468

(5)Ge, C.; Du, J.; Zhao, L.; Wang, L.; Liu, Y.; Li, D.; Yang, Y.; Zhou, R.; Zhao, Y.; Chai, Z.;

469

Chen, C., Binding of blood proteins to carbon nanotubes reduces cytotoxicity. Proc. Natl. Acad.

470

Sci. 2011, 108, 16968-16973.

471

(6)Hu, W.; Peng, C.; Lv, M.; Li, X.; Zhang, Y.; Chen, N.; Fan, C.; Huang, Q., Protein corona-

472

mediated mitigation of cytotoxicity of graphene oxide. Acs Nano 2011, 5, 3693-3700.

473

(7)Lowry, G. V.; Gregory, K. B.; Apte, S. C.; Lead, J. R., Transformations of nanomaterials in

474

the environment. Environ. Sci. Technol. 2012, 46, 6893-6899.

475

(8)Girifalco, L. A.; Hodak, M.; Lee, R. S., Carbon nanotubes, buckyballs, ropes, and a universal

476

graphitic potential. Phys. Rev. B 2000, 62, 13104-13110.

477

(9)Sun, Y. P.; Fu, K.; Lin, Y.; Huang, W., Functionalized carbon nanotubes: Properties and

478

applications. Acc. Chem. Res. 2002, 35, 1096-1104.

479

(10)Rastogi, R.; Kaushal, R.; Tripathi, S. K.; Sharma, A. L.; Kaur, I.; Bharadwaj, L. M.,

480

Comparative study of carbon nanotube dispersion using surfactants. J. Colloid Interface Sci.

481

2008, 328, 421-428.

482

(11)Bouchard, D.; Zhang, W.; Powell, T.; Rattanaudompol, U., Aggregation kinetics and

483

transport of single-walled carbon nanotubes at low surfactant concentrations. Environ. Sci.

484

Technol. 2012, 46, 4458-4465.

485

(12)Chang, X.; Henderson, W. M.; Bouchard, D. C., Multiwalled carbon nanotube dispersion

486

methods affect their aggregation, deposition, and biomarker response. Environ. Sci. Technol.

487

2015, 49, 6645-6653.

488

(13)Dassios, K. G.; Alafogianni, P.; Antiohos, S. K.; Leptokaridis, C.; Barkoula, N.-M.; Matikas,

489

T. E., Optimization of sonication parameters for homogeneous surfactant-Assisted dispersion of

490

multiwalled carbon nanotubes in aqueous solutions. J. Phys. Chem. C 2015, 119, 7506-7516.


Page 25 of 31


491

(14)Geng, H.; Kim, K. K.; Lee, K.; Kim, G. Y.; Choi, H. K.; Lee, D. S.; An, K. H.; Lee, Y. H.;

492

Y., C.; Lee, Y. S.; Kim, B.; Lee, Y., Dependcne of material quality on performance of flexible

493

transparent conducting films with single-walled carbon nanotubes. Nano 2007, 02, 157-167.

494

(15)Chen, J.; Tran, T. O.; Ray, M. T.; Brunski, D. B.; Keay, J. C.; Hickey, D.; Johnson, M. B.;

495

Glatzhofer, D. T.; Schmidtke, D. W., Effect of surfactant type and redox polymer type on single-

496

walled carbon nanotube modified electrodes. Langmuir 2013, 29, 10586-10595.

497

(16)Wang, X.; Xia, T.; Duch, M. C.; Ji, Z.; Zhang, H.; Li, R.; Sun, B.; Lin, S.; Meng, H.; Liao,

498

Y.; Wang, M.; Song, T.; Yang, Y.; Hersam, M. C.; Nel, A. E., Pluronic F108 coating decreases

499

the lung fibrosis potential of multiwall carbon nanotubes by reducing lysosomal injury. Nano

500

Lett. 2012, 12, 3050-3061.

501

(17)Zhang, L.; Lei, C.; Chen, J.; Yang, K.; Zhu, L.; Lin, D., Effect of natural and synthetic

502

surface coatings on the toxicity of multiwalled carbon nanotubes toward green algae. Carbon

503

2015, 83, 198-207.

504

(18)Tian, Y. A.; Gao, B.; Ziegler, K. J., High mobility of SDBS-dispersed single-walled carbon

505

nanotubes in saturated and unsaturated porous media. J. Hazard. Mater. 2011, 186, 1766-1772.

506

(19)Lu, Y.; Yang, K.; Lin, D., Transport of surfactant-facilitated multiwalled carbon nanotube

507

suspensions in columns packed with sized soil particles. Environ. Pollut. 2014, 192, 36-43.

508

(20)Chang, X.; Bouchard, D. C., Multiwalled carbon nanotube deposition on model

509

environmental surfaces. Environ. Sci. Technol. 2013, 47, 10372-10380.

510

(21)Yang, K.; Jing, Q.; Wu, W.; Zhu, L.; Xing, B., Adsorption and conformation of a cationic

511

surfactant on single-Walled carbon nanotubes and their influence on naphthalene sorption.

512

Environ. Sci. Technol. 2010, 44, 681-687.



Page 26 of 31

513

(22)Hilmer, A. J.; McNicholas, T. P.; Lin, S.; Zhang, J.; Wang, Q.; Mendenhall, J. D.; Song, C.;

514

Heller, D. A.; Barone, P. l. W.; Blankschtein, D.; Strano, M. S., Role of adsorbed surfactant in

515

the reaction of aryl diazonium salts with single-walled carbon nanotubes. Langmuir 2012, 28,

516

1309-1321.

517

(23)Gasser, M.; Rothen-Rutishauser, B.; Krug, H. F.; Gehr, P.; Nelle, M.; Yan, B.; Wick, P., The

518

adsorption of biomolecules to multi-walled carbon nanotubes is influenced by both pulmonary

519

surfactant lipids and surface chemistry. J. Nanobiotechnol. 2010, 8, 31.

520

(24)Cherukuri, P.; Gannon, C. J.; Leeuw, T. K.; Schmidt, H. K.; Smalley, R. E.; Curley, S. A.;

521

Weisman, R. B., Mammalian pharmacokinetics of carbon nanotubes using intrinsic near-infrared

522

fluorescence. Proc. Natl. Acad. Sci. 2006, 103, 18882-18886.

523

(25)Hyung, H.; Fortner, J. D.; Hughes, J. B.; Kim, J. H., Natural organic matter stabilizes carbon

524

nanotubes in the aqueous phase. Environ. Sci. Technol. 2007, 41, 179-184.

525

(26)Kennedy, A. J.; Gunter, J. C.; Chappell, M. A.; Goss, J. D.; Hull, M. S.; Kirgan, R. A.;

526

Steevens, J. A., Influence of nanotube preparation in aquatic bioassays. Environ. Toxicol. Chem.

527

2009, 28, 1930-1938.

528

(27)Pan, B.; Zhang, D.; Li, H.; Wu, M.; Wang, Z.; Xing, B., Increased adsorption of

529

sulfamethoxazole on suspended carbon nanotubes by dissolved humic acid. Environ. Sci.

530

Technol. 2013, 47, 7722-7728.

531

(28)Wang, X.; Tao, S.; Xing, B., Sorption and competition of aromatic compounds and humic

532

acid on multiwalled carbon nanotubes. Environ. Sci. Technol. 2009, 43, 6214-6219.

533

(29)Zhang, S.; Shao, T.; Karanfil, T., The effects of dissolved natural organic matter on the

534

adsorption of synthetic organic chemicals by activated carbons and carbon nanotubes. Water Res.

535

2011, 45, 1378-1386.


Page 27 of 31


536

(30)Deng, S.; Bei, Y.; Lu, X.; Du, Z.; Wang, B.; Wang, Y.; Huang, J.; Yu, G., Effect of co-

537

existing organic compounds on adsorption of perfluorinated compounds onto carbon nanotubes.

538

Front. Environ. Sci. En. 2015, 9, 784-792.

539

(31)Cho, H.-H.; Huang, H.; Schwab, K., Effects of solution chemistry on the adsorption of

540

ibuprofen and triclosan onto carbon nanotubes. Langmuir 2011, 27, 12960-12967.

541

(32)Ji, L.; Chen, W.; Bi, J.; Zheng, S.; Xu, Z.; Zhu, D.; Alvarez, P. J., Adsorption of tetracycline

542

on single-walled and multi-walled carbon nanotubes as affected by aqueous solution chemistry.

543

Environ. Toxicol. Chem. 2010, 29, 2713-2719.

544

(33)Hüffer, T.; Schroth, S.; Schmidt, T. C., Influence of humic acids on sorption of alkanes by

545

carbon nanotubes – Implications for the dominant sorption mode. Chemosphere 2015, 119,

546

1169-1175.

547

(34)Linard, E. N.; van den Hurk, P.; Karanfil, T.; Apul, O. G.; Klaine, S. J., Influence of carbon

548

nanotubes on the bioavailability of fluoranthene. Environ. Toxicol. Chem. 2015, 34, 658-666.

549

(35)Henderson, W. M.; Bouchard, D.; Chang, X.; Al-Abed, S. R.; Teng, Q., Biomarker analysis

550

of liver cells exposed to surfactant-wrapped and oxidized multi-walled carbon nanotubes

551

(MWCNTs). Sci. Total Environ. 2016, 565, 777-786.

552

(36)Dominguez, A.; Fernandez, A.; Gonzalez, N.; Iglesias, E.; Montenegro, L., Determination of

553

critical micelle concentration of some surfactants by three techniques. J. Chem. Educ. 1997, 74,

554

1227-1231.

555

(37)Yurekli, K.; Mitchell, C. A.; Krishnamoorti, R., Small-angle neutron scattering from

556

surfactant-assisted aqueous dispersions of carbon nanotubes. J. Am. Chem. Soc. 2004, 126, 9902-

557

9903.



Page 28 of 31

558

(38)Patel, N.; Egorov, S. A., Dispersing nanotubes with surfactants: A microscopic statistical

559

mechanical analysis. J. Am. Chem. Soc. 2005, 127, 14124-14125.

560

(39)Baalousha, M.; Motelica-Heino, M. l.; Coustumer, P. L., Conformation and size of humic

561

substances: Effects of major cation concentration and type, pH, salinity, and residence time.

562

Colloid Surf. A-Physicochem. Eng. Asp. 2006, 272, 48-55.

563

(40)Lin, D.; Xing, B., Tannic acid adsorption and its role for stabilizing carbon nanotube

564

suspensions. Environ. Sci. Technol. 2008, 42, 5917-5923.

565

(41)Du, P.; Zhao, J.; Mashayekhi, H.; Xing, B., Adsorption of bovine serum albumin and

566

lysozyme on functionalized carbon nanotubes. J. Phys. Chem. C 2014, 118, 22249-22257.

567

(42)Laurent, C.; Flahaut, E.; Peigney, A., The weight and density of carbon nanotubes versus the

568

number of walls and diameter. Carbon 2010, 48, 2994-2996.

569

(43)Tsai, D.; Davila-Morris, M.; DelRio, F. W.; Guha, S.; Zachariah, M. R.; Hackley, V. A.,

570

Quantitative determination of competitive molecular adsorption on gold nanoparticles using

571

attenuated total Reflectance–fourier transform infrared spectroscopy. Langmuir 2011, 27, 9302-

572

9313.

573

(44)Yang, K.; Zhu, L.; Xing, B., Adsorption of polycyclic aromatic hydrocarbons by carbon

574

nanomaterials. Environ. Sci. Technol. 2006, 40, 1855-1861.

575

(45)Zhu, D.; Pignatello, J. J., Characterization of aromatic compound sorptive interactions with

576

black carbon (charcoal) assisted by fraphite as a model. Environ. Sci. Technol. 2005, 39, 2033-

577

2041.

578

(46)Shen, X.; Wang, X. K.; Tao, S.; Xing, B., Displacement and competitive sorption of organic

579

pollutants on multiwalled carbon nanotubes. Environ. Sci. Pollut. Res. 2014, 21, 11979-11986.


Page 29 of 31


580

(47)Bradford, S. A.; Torkzaban, S.; Walker, S. L., Coupling of physical and chemical

581

mechanisms of colloid straining in saturated porous media. Water Res. 2007, 41, 3012-3024.

582

(48)Matthess, G.; Pekdeger, A.; McCarty, P. L., Survival and transport of pathogenic bacteria

583

and viruses in ground water. In Ground Water Quality, Ward, C. H.; Giger, W.; McCarty, P.,

584

Eds. Wiley-Interscience: 1985; pp 472-482.

585

(49)Bradford, S. A.; Bettahar, M.; Simunek, J.; Van Genuchten, M. T., Straining and attachment

586

of colloids in physically heterogeneous porous media. Vadose Zone J. 2004, 3, 384-394.

587

(50)Otto, W. H.; Britten, D. J.; Larive, C. K., NMR diffusion analysis of surfactant–humic

588

substance interactions. J. Colloid Interface Sci. 2003, 261, 508-513.

589

(51)Koopal, L. K.; Goloub, T. P.; Davis, T. A., Binding of ionic surfactants to purified humic

590

acid. J. Colloid Interface Sci. 2004, 275, 360-367.

591

(52)Knightes, C. D.; Sunderland, E. M.; Barber, M. C.; Johnston, J. M.; Ambrose, R. B.,

592

Application of ecosystem-scale fate and bioaccumulation models to predict fish mercury

593

response times to changes in atmospheric deposition. Environ. Toxicol. Chem. 2009, 28, 881-

594

893.

595

(53)Salvati, A.; Pitek, A. S.; Monopoli, M. P.; Prapainop, K.; Bombelli, F. B.; Hristov, D. R.;

596

Kelly, P. M.; Aberg, C.; Mahon, E.; Dawson, K. A., Transferrin-functionalized nanoparticles

597

lose their targeting capabilities when a biomolecule corona adsorbs on the surface. Nat. Nano

598

2013, 8, 137-143.

599

(54)Walczyk, D.; Bombelli, F. B.; Monopoli, M. P.; Lynch, I.; Dawson, K. A., What the cell

600

“sees” in bionanoscience. J. Am. Chem. Soc. 2010, 132, 5761-5768.

601

(55)Oberdörster, G.; Elder, A.; Rinderknecht, A., Nanoparticles and the brain: Cause for

602

concern? J. Nanosci. Nanotechnol. 2009, 9, 4996-5007.



Page 30 of 31

603


Page 31 of 31


73x47mm (150 x 150 DPI)

ACS Paragon Plus Environment

Surfactant-Wrapped Multiwalled Carbon Nanotubes in Aquatic

Recommend Documents