Carboxymethylcellulose Mediates the Transport of ... - ACS Publications

Subscriber access provided by University of Florida | Smathers Libraries

Article

Carboxymethylcellulose Mediates the Transport of Carbon Nanotubes —Magnetite Nanohybrid Aggregates in Water-Saturated Porous Media Dengjun Wang, Chang Min Park, Arvid Masud, Nirupam Aich, and Chunming Su Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04037 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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 44

Environmental Science & Technology

1

Carboxymethylcellulose Mediates the Transport of Carbon Nanotubes—Magnetite Nanohybrid Aggregates in Water-Saturated Porous Media

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Dengjun Wang,†,* Chang Min Park,†,∆ Arvid Masud,§ Nirupam Aich,§ and Chunming Su‡,* †

National Research Council; and ‡Groundwater, Watershed, and Ecosystem Restoration Division, National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, 919 Kerr Research Drive, Ada, OK 74820, United States ∆

Department of Environmental Engineering, Kyungpook National University, Buk-gu, Daegu 41566, South Korea

§

Department of Civil, Structural and Environmental Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, United States *

Corresponding Authors: Dengjun Wang E-mail: [email protected] Phone: (580) 436-8828 Fax: (580) 436-8703 and Chunming Su E-mail: [email protected] Phone: (580) 436-8638 Fax: (580) 436-8703

1

ACS Paragon Plus Environment


32 33

TOC ART

34 35

2


Page 2 of 44

Page 3 of 44

36


ABSTRACT

37

Carbon—metal oxide nanohybrids (NHs) are increasingly recognized as the next-

38

generation, promising group of nanomaterials for solving emerging environmental issues and

39

challenges. This research, for the first time, systematically explored the transport and retention of

40

carbon nanotubes—magnetite (CNT-Fe3O4) NH aggregates in water-saturated porous media

41

under environmentally relevant conditions. A macromolecule modifier, carboxymethylcellulose

42

(CMC) was employed to stabilize the NHs. Our results show that transport of the magnetic CNT-

43

Fe3O4 NHs was lower than that of non-magnetic CNT due to larger hydrodynamic sizes of NHs

44

(induced by magnetic attraction) and size-dependent retention in porous media. Classical

45

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory can explain the mobility of NHs under

46

varying experimental conditions. However, in contrast with colloid filtration theory, a novel

47

transport feature—an initial lower and a following sharp, higher peaks occurred frequently in the

48

NHs’ breakthrough curves and the magnitude and location of both transport peaks varied with

49

different experimental conditions, due to the interplay between variability of fluid viscosity and

50

size-selective retention of the NHs. Promisingly, the estimated maximum transport distance of

51

NHs ranged between ~0.38−46 m, supporting the feasibility of employing the magnetically

52

recyclable CNT-Fe3O4 NHs for in-situ nanoremediation of contaminated soil, aquifer, and

53

groundwater.

54

3



55

Page 4 of 44

INTRODUCTION

56

Nanoscience and nanotechnology are advancing our broad societal goals in various fields

57

including, but not limited to: medicine (e.g., magnetic resonance imaging),1 agriculture (e.g.,

58

nutrient nanocapsules and nanosensors),2 energy (e.g., solar cells and supercapacitors),3 and

59

environmental remediation (e.g., photocatalysts)4, 5. They are unprecedentedly maximizing

60

human potentials towards many frontiers such as addressing water-energy-agriculture-

61

environment (WEAE) nexus.6 Recently, the focus of interest in nanomaterial (NM) research and

62

development has shifted from singular NMs to multi-component nanohybrids (NHs), which are

63

nano-/hierarchical assemblies of multiple NMs conjugated by non-covalent (hydrogen bond, van

64

der Waals (vdW), and electrostatic interactions) or covalent (molecular linkage) bonds.7-9 The

65

goal of developing NHs is to maximize the existing functionality (e.g., contaminant adsorption

66

and degradation efficiency), and achieve novel-functionality that cannot be obtained by

67

manipulating the singular NM system.7-9 Taking environmental remediation as an example, the

68

carbon nanotubes (CNT) individually show limited adsorption capacity towards metoprolol.

69

When hetero-hybridized with nanoscale zero-valent iron (nZVI), the conjugated CNT-nZVI NHs

70

exhibit significantly higher and faster adsorption and degradation efficiency for metoprolol

71

(compared to CNT) due to greater specific surface area (SSA) and Fenton-type catalytic

72

reactions in the CNT-nZVI nanostructures.10 Promisingly, compared to the single-component

73

NMs, the performance and functionality of NHs are optimized. This is particularly true for

74

carbon-family based metal oxide NHs. Specifically, the photoelectrocatalytic reactivity and

75

sensitivity of CNT/graphene oxide (GO)—FexOy/TiO2/ZnO/MnO2/SiO2/Ag for removing toxic

76

gases (NOx),11 organic pollutants (dyes and phenolic compounds),10,

77

(As(III)/As(V), Cd2+, Cr(VI), Pb2+, and Hg2+),12,

4

13

12

heavy metals

and radionuclides (U(VI)),14,


15

are

Page 5 of 44


78

demonstrated to be greater than those of singular NMs. This is attributed to the

79

combinative/synergistic effects within the hybridized nanostructure system.7-9

80

The superior performance and multi-functionality of carbon—metal oxide NHs make

81

them the next-generation, promising candidates for in-situ nanoremediation of contaminated air,

82

soil, aquifer, and groundwater. It is also anticipated that increasing production and use of NHs

83

will result in their release to the environment. However, to date, little is known about their fate

84

and transport in the subsurface, which will likely restrict their widespread applications in

85

resolving issues and meeting challenges within WEAE nexus. Over the past decade, the fate and

86

transport of individual NMs such as carbonaceous (CNT16-19 and graphene20-22) and iron-bearing

87

NMs (nZVI23-25 and magnetite (Fe3O4)26, 27) in the subsurface have been well-explored. It is

88

noted that colloid science principles, Derjaguin-Landau-Verwey-Overbeek (DLVO) theory28, 29

89

and colloid filtration theory (CFT)30 can describe the colloidal stability and transport of

90

individual NMs in aquatic environments. Nonetheless, deviations between experimental

91

observations and theoretical predictions occur frequently under unfavorable attachment

92

conditions or with varying properties (e.g., irregular shapes and surface modifications) of NMs.

93

For example, under unfavorable conditions, non-exponential retention31 of NMs was frequently

94

encountered in porous media (e.g., hyper-exponential retention for tubular CNT)16. Surface

95

modifications32 due to surfactant and polyelectrolyte coatings impart non-DLVO interactions

96

(steric hindrance) and change the rheological properties (viscosity) of fluid in porous media,33

97

likely yielding unanticipated transport behaviors for NMs. CFT predicts that transport of colloids

98

in porous media is controlled by random Brownian motion; and that interception and

99

gravitational sedimentation start to dominate colloid deposition when particle diameter is ≥1

100

µm.30, 34 Once released to the environment, the transport of individual NMs is dependent on the

5



101

interplay of various physicochemical conditions including: (1) NM-specific properties (e.g.,

102

size/shape/hydrophilicity/aggregation state/surface chemistry); (2) medium-specific properties

103

(e.g., grain size/shape/surface chemistry/moisture content); (3) solution chemistries (e.g.,

104

pH/ionic strength and composition (IS and IC)/presence of natural organic matter (NOM)); and

105

(4) hydrodynamic conditions (e.g., flow rate/temperature/oxygen content).35, 36 However, it is

106

unclear whether the colloid science principles used for describing the environmental behaviors of

107

individual NMs hold true for the assembled NHs, e.g., how and to what extent (e.g., qualitatively,

108

semi-quantitatively, or quantitatively) the DLVO theory and CFT explain NHs’ transport in the

109

subsurface. Particularly, NHs’ new features such as altered hydrophilicity, magnetism, and

110

DLVO-type (e.g., vdW attraction) interactions, and enhanced population heterogeneity (e.g., size

111

distribution, morphology, surface defect and roughness, and charge heterogeneity; compared to

112

individual NMs) likely impact their aggregation and transport propensities in the subsurface.9, 37

113

Assembling magnetic iron oxide (e.g., Fe3O4) NMs having high redox potential and

114

contaminant immobilization capacity38-40 with CNT having superlative mechanical-strength and

115

large SSA41 provides a powerful strategy (e.g., synergistic adsorption and redox degradation) for

116

in-situ capturing and decomposing/detoxificating both inorganic and organic pollutants. But

117

knowledge of how DLVO theory and CFT describe the transport of CNT-Fe3O4 NHs in the

118

subsurface is nonexistent. This study systematically investigated the transport of the

119

multifunctional CNT-Fe3O4 NHs in water-saturated porous media under environmentally

120

relevant conditions. An environmentally-friendly and ‘green’ (non-toxic and biodegradable)

121

macromolecule carboxymethylcellulose (CMC) was employed to stabilize the CNT-Fe3O4 NHs

122

as they were highly-hydrophobic and easily-aggregated. The CMC has long been used to

123

stabilize individual nZVI23, 42, 43 and Fe3O4 NMs44 by imparting electrosteric repulsions arising

6


Page 6 of 44

Page 7 of 44


124

from the hydrophilic, negatively charged carboxymethyl (-CH2-COOH) groups. Critical

125

parameters characterizing the NHs’ mobility in water-saturated porous media were evaluated to

126

shed light on the feasibility of using the recyclable CNT-Fe3O4 NHs for in-situ nanoremediation

127

of contaminated sites. DLVO theory and CFT were employed in combination with electrokinetic

128

property and hydrodynamic size of NHs to understand the mechanisms behind their transport and

129

retention within the porous media. Significant efforts were devoted to explore how DLVO theory

130

and CFT explain the variability in NHs’ transport under varying experimental conditions. Finally,

131

potential limitations and future research directions of DLVO theory/CFT in describing NHs’

132

transport in the subsurface were also discussed.

133 134

MATERIALS AND METHODS

135

Preparation of CNT-Fe3O4 NH Influents and Solution Chemistries

136

Carbon nanotubes (purity>99.9%, outer diameter=8−15 nm, inside diameter=3−5 nm,

137

length=10−50 µm, and carboxyl group content=1.85%) were purchased from the Cheap Tubes

138

Inc. (Grafton, VT). The CNT-Fe3O4 NHs were synthesized in-house, and their physicochemical

139

properties including those of purchased CNT were characterized using X-ray diffraction (XRD),

140

transmission electron microscopy (TEM), Raman spectroscopy, and thermogravimetric analysis

141

(TGA) (see Supporting Information (SI) S1). The synthesized CNT-Fe3O4 NHs were highly-

142

hydrophobic because most hydrophilic carboxyl groups on the CNT surfaces were used for

143

anchoring CNT and magnetite during synthesis.

144

The NHs were easily-aggregated due to strong vdW and magnetic attractions (magnetism

145

was confirmed using a magnet). Various surfactants (sodium dodecyl sulfate (SDS), Triton X-

146

100, and Tween 20), polyelectrolytes (polyacrylic acid (PAA), polyvinyl alcohol (PVA), and

7



147

polyvinyl pyrrolidone (PVP)), and macromolecules (CMC; nominal molecular weight=90,000

148

g/mol; Sigma-Aldrich) were tested to investigate the NHs’ colloidal stability. By monitoring the

149

UV-Vis response of absorbance for the NHs under different surfactants, polyelectrolytes, and

150

macromolecules, we found that only the CMC at the concentration of 2% well-stabilized the

151

NHs (see SI S2). Consequently, 2% CMC was chosen as the ‘modifier’ to stabilize the CNT-

152

Fe3O4 NHs for column experiments (described below).

153

Environmentally-relevant solution chemistries were selected to evaluate the mobility of

154

CNT-Fe3O4 NHs in porous media including: monovalent (0, 1, 10, and 50 mM NaCl) and

155

divalent electrolytes (0, 0.33, 1.67, and 3.33 mM CaCl2; note IS of 0, 0.33, 1.67, and 3.33 mM

156

CaCl2 is 0, 1, 5, and 10 mM, respectively); the presence of NOMs (0, 1, and 10 mg C/L

157

Suwannee River humic acid (SRHA) and fulvic acid (SRFA)) (details on preparing SRHA/SRFA

158

stock suspensions were given in SI S3), pHs (4.0, 7.3, and 10), and particle concentrations (10,

159

25, and 50 mg/L) (Table 1).45 2% CMC-stabilized CNT-Fe3O4 NH influents (via adding a pre-

160

determined volume of 5% CMC stock suspension; S3) at the desired solution chemistries were

161

freshly prepared via ultrasonication at 100 W and 42 kHz for 1 h (Branson 3510R-DTH

162

sonicator). Changes in concentration and average hydrodynamic radius (rh) of CNT-Fe3O4 NHs

163

in the influent suspensions were monitored over the time frame of influent injection experiments

164

(described below) to investigate their colloidal stability (aggregation propensity). The influent

165

concentration changes of NHs were determined spectrophotometrically at 218 nm (SI Figure S1).

166

Time-resolved dynamic light scattering (DLS) was used to monitor rh change of NHs in influents

167

using the Zetasizer Nano-ZS ZEN3600 analyzer (Malvern Instruments Inc.) at a scattering angle

168

of 173° and 25 °C (see SI S4).

169

8


Page 8 of 44

Page 9 of 44


Table 1. Physicochemical properties of column transport experiments and mass recoveries of CNT-Fe3O4 NHs (in 2% CMC) under different experimental conditions. NaCl (mM) 0 1 10 50 0 0 0 1 1 1 1 1 1 0 0 1 1

CaCl2 (mM) 0 0 0 0 0.33 1.67 3.33 0 0 0 0 0 0 0 0 0 0

SRHA (mg C/L)

SRFA (mg C/L)

pH

0 0 0 0 0 0 0 1 10 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 1 10 0 0 0 0 0 0

7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 4.0 10 7.3 7.3 7.3 7.3

Particle conc (mg/L)

Sand size (µm)

Mass recovery (%) Meff

Mret

Mtot

10 10 10 10 10 10 10 10 10 10 10 10 10 25 50 10 10

360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 280 230

88.3±0.02 79.9±2.4 37.8±0.67 20.3±0.88 57.4±2.0 22.7±4.4 16.2±0.84 86.9±1.5 97.6±3.8 83.1±2.6 92.2±1.2 35.0±0.58 89.5±0.67 98.5±1.2 104±0.49 28.6±1.8 97.4±0.36

12.8±0.25 25.0±0.49 62.2±1.1 84.2±0.18 44.2±2.2 73.1±0.9 86.9±1.4 14.2±0.28 8.0±0.28 14.9±0.72 9.7±0.77 66.1±1.8 16.1±0.49 8.0±0.36 4.0±0.30 64.9±0.89 6.1±0.42

101±0.23 105±2.9 100±0.41 104±1.1 102±0.29 95.8±3.5 103±2.3 101±1.2 106±4.1 98.0±1.9 102±0.41 101±2.4 106±1.2 107±1.5 108±0.19 93.5±2.7 104±0.79

170 171 172 173 174

SRHA and SRFA are Suwannee River humic acid and Suwannee River fulvic acid, respectively. Conc is concentration. Meff, Mret, and Mtot are mass percentages of CNT-Fe3O4 NHs recovered from effluent, retentate, and total column, respectively. Other column transport parameters such as porosity, pore-water velocity, and dispersivity are shown in SI Table S1.

175

Porous Media

176

Ottawa sands (U.S. Silica, Berkeley Springs, WV) were chosen as representative aquifer

177

materials for column experiments. The sands were pre-sifted through 40-, 50-, 60-, and 70-mesh

178

sieves (U.S. Standard Testing Sieves), and the fractions captured by 40−50, 50−60, and 60−70

179

mesh sieves (average grain sizes of 360-, 280-, and 230-µm, respectively) were used. Before use,

180

the sieved sands were cleaned using 1 M HCl and washed thoroughly with deionized (DI) water

181

(Nanopure Diamond D11911, Barnstead International, Dubuque, IA). The colloidal particles

9



Page 10 of 44

182

from pulverized, cleaned sands were used as surrogates of sand grains for electrophoretic

183

mobility (EPM) measurements in triplicate on the Zetasizer analyzer at 25 °C. Experimentally

184

determined EPM values were converted to apparent zeta (ζ)-potentials for the sand grains using

185

the Smoluchowski equation.46

186 187

Column Experiments

188

Column experiments were conducted using glass chromatography columns (1.7-cm i.d. ×

189

10-cm long).47 Each column was dry-packed using the cleaned 360-, 280-, or 230-µm sands. The

190

packed-column was then purged with CO2 gas to remove any air remained during column

191

packing processes, and to maximize the water accessibility of the column. After purging, the

192

column was immediately saturated with DI water slowly in an upward mode. Following the

193

saturation step, a nonreactive tracer experiment was performed to assess the hydrodynamic

194

properties of column (see SI S5 and Table S1).

195

After the completion of tracer experiment, the column was pre-equilibrated with desired

196

background solution (Table 1). 2% CMC was not co-injected during the pre-equilibration

197

procedure because CMC alters column hydromechanical (rheological) properties due to its high

198

viscosity33 (described below). A two-step transport experiment was then initiated by injecting 3

199

PVs of CNT-Fe3O4 NH influents (in 2% CMC) at the desired solution chemistries (designated as

200

phase 1), followed by elution with 7 PVs of NH-free background electrolyte solution (without

201

CMC; phase 2). Column experiments with different collector sizes (360-, 280-, and 230-µm,

202

respectively) were also performed to examine the role of straining48,

203

Previous studies suggest that, under unfavorable conditions, secondary minimum starts to

204

capture colloids when solution IS is ≥10 mM NaCl50 or ≥1 mM CaCl245. To identify whether

10


49

on NHs’ retention.

Page 11 of 44


205

secondary minimum is also involved in NHs’ retention, two three-step transport experiments

206

were run at the highest NaCl (50 mM) and CaCl2 concentrations (3.33 mM) investigated in this

207

study, i.e., eluting the column with 8 PVs of DI water in phase 3 after completing the two-step

208

transport experiments. Darcy velocity was maintained at 0.44 cm/min for all experiments,

209

mimicking typical fluid velocities in coarse aquifers51 or in forced-gradient in-situ remediation52.

210

Column effluents were collected via a fraction collector. Following completion of each transport

211

experiment, the spatial distribution of CNT-Fe3O4 NHs retained in the column (dissection

212

experiments) was determined (SI S6). Column transport and dissection experiments were also

213

performed for CNT-alone (in 2% CMC) to compare the mobility between NHs and parent NMs.

214

The concentrations of CNT-Fe3O4 NHs and CNT in the effluents and retentates (retained

215

colloids collected from dissection experiments) were determined spectrophotometrically at 218

216

nm (SI Figure S1). To eliminate the interferences of CMC in NHs (and CNT) measurements,

217

two-step column transport experiments were also performed for 2% CMC-alone under the

218

experimental conditions identical to those for the transport of CNT-Fe3O4 NHs (Table 1). The

219

actual concentration of CNT-Fe3O4 NHs (and CNT) in the effluents was obtained by subtracting

220

the spectrophotometric response (at 218 nm) of CMC from that of the NHs (and CNT) at the

221

identical PV. Furthermore, to minimize the potential interferences of particle aggregation and

222

sedimentation, spectrophotometric measurements for all NHs, CNT, and CMC in the effluents

223

were controlled to be completed within 1.5 h after collecting from the fraction collector.

224

Additionally, selected column effluents of NHs were digested in 2.6 M HNO3 for 3 d at 25 °C,

225

and then filtered through 0.2-µm cellulose acetate membranes. Total Fe concentrations in the

226

filtered digestion solutions were determined using inductively-coupled-plasma optical-emission-

227

spectroscopy (ICP-OES; PerkinElmer Optima 3300DV, Norwalk, CT) to verify the reliability of

11



Page 12 of 44

228

spectrophotometric measurements of the NHs. Selected column effluents of NHs were also

229

measured using ICP-OES (effluents after HNO3 digestion for total Fe measurements) to identify

230

whether Fe3O4 NMs detach from the NHs or dissolve under acidic conditions (pH 4.0). Our

231

measurements (Fe content < detection limit, meaning no Fe3O4 NMs detach or dissolve)

232

substantiate the integrity of the NHs during porous media transport.

233 234

Electrokinetic Properties and Hydrodynamic Radii of CNT-Fe3O4 NHs in the

235

Influents

236

Electrophoretic mobility (EPM) and ζ-potential of CNT-Fe3O4 NHs (and CNT) in various

237

influents (Table 1) were measured using the Zetasizer analyzer at 25 °C. The ζ-potentials of

238

CNT-Fe3O4 NHs and sand grains were then used to calculate their interaction energy using

239

DLVO theory that includes vdW and electrostatic double layer (EDL) interactions. It is

240

impossible to determine Hamaker constant (A) of the hydrophobic CNT-Fe3O4 NHs (without

241

CMC modification) in aqueous solutions experimentally. Thus, three Hamaker constants, i.e.,

242

Hamaker constants of CNT (ACNT=6.00×10−20 J)53, 54 and Fe3O4 NMs (AFe3O4=33.0×10−20 J)55,

243

and an estimated Hamaker constant of the NHs (ACNT−Fe3O4=17.6×10−20 J) based on the Hamaker

244

constants of parent NMs and NHs’ elemental (C and Fe) contents, were employed to calculate

245

the interaction energy between NHs and sand grains.56,

246

energy calculation including inherent limitation of this approach were given in SI S7.

57

Details on the DLVO interaction

247

The average hydrodynamic radius (rh) of CNT-Fe3O4 NHs (and CNT) in various influents

248

and hydrodynamic particle size distribution of CNT-Fe3O4 NHs in the influents at different NaCl

249

and CaCl2 concentrations were determined using DLS (described above). Particle size

250

distribution of 2% CMC was also determined to investigate the role of CMC on the transport of

12


Page 13 of 44


251

CNT-Fe3O4 NHs. The CONTIN algorithm was used to convert intensity autocorrelation

252

functions to intensity-weighted rh using the Stokes-Einstein equation for spherical particles.58

253

Given that the NHs, CNT, and CMC are non-spherical, DLS provides the diameter of a sphere

254

that has the same average translational diffusion coefficient as the particle being measured. For

255

selected column experiments, the rh of NHs and CNT in the effluents and retentates (retained in

256

the column inlet 0−1 cm) was measured using DLS to understand size-dependent retention in

257

saturated sand.

258 259

Data Analyses

260

Parameters characterizing the mobility of CNT-Fe3O4 NHs in water-saturated porous

261

media including single-collector contact efficiency (η), attachment efficiency (α), deposition rate

262

coefficient (kd), and maximum transport distance (Lmax) were calculated using the well-defined

263

Tufenkji-Elimelech (TE) equation (SI S8).34

264 265

RESULTS AND DISCUSSION

266

Electrokinetic Properties and Hydrodynamic Radii of CNT-Fe3O4 NHs in the

267

Influents and Electrokinetic Properties of Sand Grains

268

Physicochemical properties of CNT-Fe3O4 NHs and CNT were characterized using XRD,

269

TEM, Raman spectroscopy, and TGA (Figure 1). A strong peak occurred at 26.5° for both NHs

270

and CNT in the XRD spectra, which corresponds to the d-spacing of [002] plane of graphite

271

(CNT; Figure 1a).59 In contrast, the characteristic reflections of 30.0° [220], 35.6° [311], and

272

43.4° [400] of Fe3O4 (JCPDS No. 19-0629) occurred only for the NHs, confirming the successful

273

hybridization of CNT and Fe3O4 NMs. TEM images showed that the Fe3O4 NMs (20−30 nm size)

13



Page 14 of 44

274

heterogeneously-deposited on CNT’s surface as dense aggregates (Figure 1b). This is due to the

275

large SSA and surface reactivity, and strong magnetic attraction of the Fe3O4 NMs that promote

276

partial aggregation on the CNT’s surface (see SI Figure S5). The Fe3O4 NMs were strongly

277

anchored on the CNT’s surface even after ultrasonication treatment (100 W and 42 kHz for 1 h)

278

since no Fe was released from the NHs (via ICP-OES measurements). This could be due to the

279

strong covalent interaction between CNT (e.g., carboxyl (1.85% in the pristine CNT) and

280

carbonyl groups) and Fe3O4 NMs.7 The Raman spectrum of CNT exhibited three absorption

281

bands at 1343, 1571, and 2688 cm−1 (Figure 1c), which is assigned to the D-, G’-, and G’-bands,

282

respectively.60 However, these three bands shifted to higher wavenumbers of 1357, 1582, and

283

2710 cm−1, respectively, for the NHs, yielding a larger D/G-band intensity ratio of ID/IG=0.326

284

(ID/IG=0.316 for CNT). This indicates a smaller average size of the sp2 domains of carbon for the

285

NHs,60 likely due to the enhanced aggregation of NHs (more compressed structure) induced by

286

magnetic attraction (see S1 for more discussion). The TGA profiles showed that Fe accounted

287

for 42.8% of the total elemental mass because CNT was decomposed completely when the

288

temperature was >930 °C (Figure 1d). The C and Fe elemental composition information was

289

used to calculate the Hamaker constant and density of the NHs for DLVO interaction energy and

290

transport parameter calculations (described below).

14


Page 15 of 44


a)

CNT

c)

CNT-Fe3O4

CNT-Fe3O4 CNT

Intensity (a.u.)

Intensity (a.u.)

CNT

Fe3O4

10

20

30

40

50

60

70

80

90

ID/IG=0.326

ID/IG=0.316

400

800 1200 1600 2000 2400 2800 3200

Raman shift (cm-1)

o

2Theta ( )

CNT-Fe3O4

Retained weight (%)

b)

CNT

100 80 60

42.8% 40 20

d)

0 0

0 200

400

600

800

1000

o

T ( C)

50 nm

291 292 293 294 295

Figure 1. XRD patterns (a), TEM images (b), Raman spectra (c), and TGA profiles (d) of the synthesized carbon nanotubes-magnetite nanohybrids (CNT-Fe3O4 NHs) and parent CNT component.

296

The EPMs and ζ-potentials of CNT-Fe3O4 NHs in the influents (in 2% CMC) and sand

297

grains under different experimental conditions are shown in Table S2. Both NHs and sand grains

298

were negatively charged, suggesting unfavorable conditions for column experiments. As

299

mentioned above, the NHs without CMC modification were highly-hydrophobic, making EPM

300

(and DLS) measurements impossible in aqueous solutions. However, in 2% CMC, the ζ-

301

potentials of NHs in the influents were highly negative (−39.4 to −58.7 mV) due mainly to the

15



302

adsorption of hydrophilic, negatively charged carboxymethyl groups of CMC (e.g., ζ-potential of

303

2% CMC-alone = −49.3 mV at 1 mM NaCl and pH 7.3) onto NHs’ surface. The ζ-potential of

304

NHs in the influents (in 2% CMC) was more negative than that of 2% CMC-alone at a specified

305

solution chemistry, e.g., −53.4 > −49.3 mV at 1 mM NaCl and pH 7.3 (Table S2). This

306

discrepancy could be due to the deprotonation of carboxyl and carbonyl groups of the NHs,

307

although most carboxyl groups are expected to be used for anchoring CNT and magnetite NMs

308

(described above). Consistent with reported results for individual NMs and sand grains,61-63

309

decreasing IS, and increasing NOM concentration and pH increased (more negative) the ζ-

310

potential of both NHs and sand grains. The divalent Ca2+ decreased the ζ-potentials of NHs and

311

sand grains more than monovalent Na+ at equivalent ISs by greater charge screening (both Na+

312

and Ca2+) and neutralization (Ca2+ only) effects.46

313

Table S2 also shows the variability in rh of CNT-Fe3O4 NHs in the influents (2% CMC)

314

with varying experimental conditions. Greater aggregation (larger rh) of the NHs was associated

315

with the lower ζ-potentials due to less electrostatic repulsions according to the DLVO interaction

316

prediction (described below). Similar findings have been documented for individual NMs.61, 62

317

The measured hydrodynamic diameter (2rh) of NHs and CNT (Table S2) lied within the diameter

318

(8−15 nm) and length (10−50 µm) of the CNT, consistent with the results reported previously.19

319

This is likely because, for non-spherical NHs and CNT, DLS gives the diameter of a sphere that

320

has the same average translational diffusion coefficient as the particle (NHs and CNT) being

321

measured (described above).

322 323

Mobility Comparison between CNT-Fe3O4 NHs and Individual CNT

16


Page 16 of 44

Page 17 of 44


324

Breakthrough curves (BTCs) and retention profiles (RPs) of CNT-Fe3O4 NHs and CNT

325

(both in 2% CMC) under the identical transport condition (1 mM NaCl and pH 7.3) are presented

326

in Figure 2. Total mass recoveries (Mtot) of NHs in the effluents (Meff) and retentates (Mret) under

327

different experimental conditions are given in Table 1, which confirms a high degree of

328

confidence in our experimental measurements because virtually all NHs were recovered from

329

column experiments (Mtot−NHs=93.5−108% and Mtot−CNT=99.4%). The ICP-OES results for

330

effluent Fe analyses matched (standard errors 1526 > 3.4

368

nm) (Table S2 and Figure S4). This is due primarily to the greater interception efficiency (ηI) and

369

overall single-collector contact efficiency (ηo) at larger rh, because ηI and ηo increase

18


Page 19 of 44


370

monotonically with colloid size when the diameter is ≥1 µm.34 To better understand the two-peak

371

transport feature, rh of NHs and CNT in the influents, effluents, and retentates was determined

372

(Figure 2d and Table 2). The rh of NHs was in the order: effluent < influent < retentate (e.g.,

373

1432 < 1632 < 2001 nm in 1 mM NaCl and pH 7.3; Table 2), indicating size-selective retention

374

in porous media. The size-selective retention that larger particles preferentially retain in the

375

column (retentates) due likely to greater ηI and ηo; and smaller ones elute out (effluents) has been

376

reported in colloid transport studies.47 Furthermore, for the effluents, the rh of both NHs and

377

CNT increased progressively with PV (Figure 2d), again strongly substantiating the size-

378

selective retention. It is thus logical to anticipate that smaller NHs/CNT associated with lower

379

viscous CMCs breakthrough earlier (yielding ‘peak 1’); whereas, larger NHs/CNT coated with

380

higher viscous CMCs are eluted out more retarded (i.e., ‘peak 2’). The peak location of ‘peak 2’

381

was earlier for CNT than that of NHs (4.5 vs. 5 PV; Figure 2a) further supports that larger

382

cumbersome NHs (effluent rh: CNT < NHs; Figure 2d) have a lower transport velocity (i.e., more

383

retarded breakthrough). Greater retention of NHs observed in Figure 2b is due to greater rh

384

(greater ηI and ηo) in the influents and effluents (Figure 2d). To sum up, the above results

385

underscore the interplay of viscosity variability of fluid and size-selective retention in the

386

transport of NMs (in 2% CMC) in water-saturated porous media.

19



Peak 2

1.0

Page 20 of 44

1.2

a)

c)

CNT-Fe3O4

0.8

2% CMC

1.0

CNT

0.8

C/Co [-]

C/Co [-]

0.6 Peak 1 0.4 0.2

0.6 0.4 0.2

0.0

0.0 0

1

2

3

4

5

6

7

8

9

10

0

1

2

3

4

PV 0.5

5

6

7

8

9

10

PV

b)

1600

CNT-Fe3O4

d)

CNT-Fe3O4

CNT

0.4

CNT

rh (nm)

0.3

3

S/Co (cm /g)

1200

0.2

400

0.1 0.0

800

0

1

2

3

4

5

6

7

8

9

10

0

0

1

2

3

4

5

6

7

8

9

10

PV

Depth (cm)

387 388 389 390 391 392 393 394 395 396 397 398 399 400 401

Figure 2. Breakthrough curves (BTCs; a) and retention profiles (RPs; b) of CNT-Fe3O4 NHs and individual CNT, respectively, in water-saturated sand columns at 1 mM NaCl and pH 7.3 (in 2% CMC). Breakthrough curve describes the normalized effluent concentration of NMs, C/Co (where Co is the initial influent concentration of CNT-Fe3O4 NHs or CNT) as a function of pore volume (PV); and retention profile shows the normalized solid-phase retention concentration of NMs, S/Co (where S is retention amount of CNT-Fe3O4 NHs or CNT per gram dry sand) as a function of distance from the column inlet. For column transport experiments, 3 PVs (marked by dash line in a) of CNT-Fe3O4 NH or CNT influent were injected in the column followed by elution with 7 PVs of NM-free background electrolyte solution (without CMC). Other column transport parameters were the same including: CNT-Fe3O4 NH or CNT influent concentration = 10 mg/L, influent pH = 7.3; background electrolyte = 1 mM NaCl; average sand grain size = 360 µm; and Darcy velocity = 0.44 cm/min. For comparison, the breakthrough curve of 2% CMCalone under the experimental condition (1 mM NaCl and pH 7.3) identical to that of NHs/CNT is shown in (c). Figure d shows the average hydrodynamic radius (rh) of CNT-Fe3O4 NHs and CNT

20


Page 21 of 44

402 403


in the effluents (collected from a). The error bars represent the standard deviations from duplicate experiments. Note different y-axis scales in figure.

404 Table 2. Average hydrodynamic radius (rh) of CNT-Fe3O4 NHs in the influents (in 2% CMC), effluents, and retentates at different NaCl and CaCl2 concentrations. NaCl (mM)

CaCl2 (mM)

Sand grain diameter (µm)

rh (nm) Influent

Effluent a

Retentate b

∆rh c

0 1 10 50 0 0 0

0 0 0 0 0.33 1.67 3.33

360 360 360 360 360 360 360

994±226 1632±143 1687±167 2025±290 1988±237 2238±399 2728±349

836±48 1432±152 1455±182 1734±324 1691±182 1825±297 1958±350

1096±41 2001±182 2285±234 2789±188 2338±146 2875±221 3568±438

260 569 830 1055 647 1050 1610

405 406 407 408 409 410

a

411

Mobility of CNT-Fe3O4 NHs under Varying Physicochemical Conditions

rh/r50 d (−) 5.52×10−3 9.07×10−3 9.37×10−3 1.13×10−2 1.10×10−2 1.24×10−2 1.52×10−2

Effluents collected at 1.5, 2, 2.5, and 3 PVs (Figures 2−3) were used for rh measurements. Retentates retained at the column inlet (0−1 cm) were used for rh measurements. c rh difference of NHs in the effluents and retentates. d r50 is the average sand grain radius (180 µm). The rh/r50 values were calculated to test the effect of straining on the retention of CNT-Fe3O4 NHs in column experiments. b

412

Environmentally-relevant physicochemical conditions were chosen to investigate the

413

mobility of CNT-Fe3O4 NHs (in 2% CMC) including: NaCl (Figure 3a−b), CaCl2 (Figure 3c−d),

414

SRHA (Figure 3e−f), and SRFA concentrations (Figure 3g−h), pHs (Figure S6a−b), particle

415

concentrations (Figure S6c−d), and collector sizes (Figure S6e−f). The DLVO interaction

416

energies between CNT-Fe3O4 NHs and sand grains under different experimental conditions are

417

shown in SI Figure S7 and Table S3. Before interpreting the mobility of NHs under different

418

experimental conditions, it is important to understand the colloidal stability of NH influents. In 2%

419

CMC, the NH influents were stable (influent concentration and rh stayed unchanged; Figure S2)

420

over the time frame of influent injection experiments. Recalling that without CMC, partial

21



Page 22 of 44

421

aggregation of NHs was pronounced due to magnetic attraction (Figure S5), our findings

422

emphasize the critical role of 2% CMC in stabilizing the NH aggregates (e.g., NHs exist as stable

423

aggregates because rh varies with solution chemistries). No further aggregation and gravitational

424

sedimentation of NHs occurred in the influents during influent injection experiments, again

425

confirming the strong stabilizing effect of 2% CMC. Nonetheless, both CFT and TE equation

426

predict that gravitational sedimentation is appreciable when colloid diameter is ≥1 µm.30, 34 This

427

is likely because high viscous and negatively charged CMCs (described above) counterbalance

428

the role of gravity in causing particle sedimentation.

429

Similarly, the two-peak transport feature occurred frequently in the NHs’ BTCs with

430

altered C/Co values and peak locations (Figures 3 and S6). Decreasing IS, and increasing NOM

431

concentration, pH, particle concentration, and collector size (with the exception of 230-µm sand;

432

Figure S6e−f) increased the mobility of NHs (see Meff and Mret in Table 1), consistent with

433

DLVO theory prediction (Figure S7 and Table S3). Lower mobility of NHs under different

434

experimental conditions was again associated with less negative ζ-potential and larger rh due to

435

greater ηI and ηo. The size-selective retention also occurred during NHs transport (rh order:

436

effluent < influent < retentate; Table 2); and this effect became more pronounced (increased ∆rh)

437

at higher NaCl/CaCl2 concentrations. To better unravel the size-selective retention of NHs in

438

porous media, hydrodynamic particle size distribution of NHs in the influents (in 2% CMC) was

439

determined (SI Figure S8). Our results indicate a greater degree of particle size heterogeneity

440

(wider particle size distribution) among NHs populations, likely accounting for the greater size-

441

selective retention of NHs at higher NaCl/CaCl2 concentrations. Other potential explanations for

442

the greater size-selective retention at higher NaCl/CaCl2 concentrations include deeper attractive

443

secondary minimum (Table S3) and more pronounced straining (Table 2) given that hyper-

22


Page 23 of 44


444

exponential RPs occurred at high NaCl/CaCl2 concentrations (Figure 3b, d). The fact that

445

retained NHs were released upon injecting DI water during the three-step transport experiments

446

(SI Figure S9a) substantiates the important role of secondary minimum in NHs’ retention.

447

However, the extent of NHs’ release was much less in Ca2+ than in Na+ due likely to more

448

pronounced straining due to larger rh of NHs in the influents, effluents, and retentates (Table 2

449

and Figure S9b), i.e., Ca2+ bridges the -CH2COOH and -COOH groups of CMC-coated CNT-

450

Fe3O4 NHs. Straining is demonstrated to play a critical role in colloid retention and cause hyper-

451

exponential RPs when the diameter ratio of colloid vs. collector is ≥0.008.49 Our calculations

452

show that when the size ratio of NHs vs. collector was ≥0.00937 (≥10 mM NaCl and ≥1.67 mM

453

CaCl2; Table 2), straining started to yield the hyper-exponential RPs (Figure 3b, d). The slight

454

difference (0.008

Carboxymethylcellulose Mediates the Transport of ... - ACS Publications

Recommend Documents