Cation-Inhibited Transport of Graphene Oxide ... - ACS Publications

Dec 20, 2016 - Saturated Porous Media: The Hofmeister Effects. Tianjiao Xia,. † ... University, Durham, North Carolina 27708, United States. •S Su...
0 downloads 0 Views 1MB Size
Subscriber access provided by University of Newcastle, Australia

Article

Cation-Inhibited Transport of Graphene Oxide Nanomaterials in Saturated Porous Media: The Hofmeister Effects Tianjiao Xia, Yu Qi, Jing Liu, Zhichong Qi, Wei Chen, and Mark R. Wiesner Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05007 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 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 34

Environmental Science & Technology

1

Cation-Inhibited Transport of Graphene Oxide

2

Nanomaterials in Saturated Porous Media: The Hofmeister

3

Effects

4

Tianjiao Xia,† Yu Qi,† Jing Liu,† Zhichong Qi,† Wei Chen,*† Mark R. Wiesner‡

5 6 †

7

College of Environmental Science and Engineering, Ministry of Education Key

8

Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of

9

Environmental Remediation and Pollution Control, Nankai University, Tianjin 300350, China

10 11



12

Implications of NanoTechnology, Duke University, Durham, North Carolina 27708,

13

United States

Department of Civil and Environmental Engineering, Center for the Environmental

14 15 16

Manuscript prepared for Environmental Science & Technology

17 18

* Corresponding author: (Phone/fax) 86-22-85358169; (E-mail) [email protected].

19

1

ACS Paragon Plus Environment

Environmental Science & Technology

TOC Art

Small ionic radius (strongly hydrated)

Large ionic radius (weakly hydrated)

C/C0

20

Na+, K+, Cs+

1.0 0.8 0.6 0.4 0.2 0.0

(a) GO in 20 mM M+ Na+ K+ + Cs 0

Outer-sphere complexation

21

Inner-sphere complexation

C/C0

Mg2+, Ca2+, Ba2+

1.0 0.8 0.6 0.4 0.2 0.0

10

20 PV

30

(b) GO in 0.5 mM Me

40

2+

Mg2+ Ca2+ Ba2+ 0

10

20 PV

30

40

22

2

ACS Paragon Plus Environment

Page 2 of 34

Page 3 of 34

23 24

Environmental Science & Technology

ABSTRACT Transport of negatively charged nanoparticles in porous media is largely affected by

25

cations. To date, little is known about how cations of the same valence may affect

26

nanoparticle transport differently. We observed that the effects of cations on the transport

27

of graphene oxide (GO) and sulfide-reduced GO (RGO) in saturated quartz sand obeyed

28

the Hofmeister series, that is, transport-inhibition effects of alkali metal ions followed the

29

order of Na+ < K+ < Cs+, and those of alkaline earth metal ions followed the order of

30

Mg2+ < Ca2+ < Ba2+. With batch adsorption experiments and microscopic data, we

31

verified that cations having large ionic radii (and thus weakly hydrated) interacted with

32

quartz sand and GO/RGO more strongly than did cations of small ionic radii. In

33

particular, the monovalent Cs+ and divalent Ca2+ and Ba2+, which can form inner-sphere

34

complexes, resulted in very significant deposition of GO/RGO via cation bridging

35

between quartz sand and GO/RGO, and possibly via enhanced straining, due to the

36

enhanced aggregation of GO/RGO from cation bridging. The existence of the Hofmeister

37

effects was further corroborated with the interesting observation that cation bridging was

38

more significant for RGO, which contained greater amounts of carboxyl and phenolic

39

groups (i.e., metal-complexing moieties) than did GO. The findings further demonstrate

40

that transport of nanoparticles is controlled by the complex interplay between

41

nanoparticle surface functionalities and solution chemistry constituents.

42 43

3

ACS Paragon Plus Environment

Environmental Science & Technology

44 45

INTRODUCTION Graphene-based materials, such as graphene oxide (GO) and reduced graphene oxide

46

(RGO), are a class of promising nanomaterials with potential applications in the areas of

47

electronic devices, energy storage, chemical catalysis, drug delivery and functional

48

materials, to mention a few.1 The increasingly fast growth of production and use of these

49

materials will inevitably result in their environmental release. A number of studies have

50

shown that graphene-based materials may have adverse environmental impact.2-5 Thus, it

51

is important to understand the fate and transport of these materials in the environment.

52

Several studies have been conducted to understand the transport of GO and RGO in

53

porous media.6-17 It has been reported that GO generally exhibits high mobility,6-17

54

whereas RGO can be less mobile;17 transport of GO is not very responsive to pH (5 to

55

9),7, 10, 11 but sensitive to the increase of ionic strength;7, 8, 10, 11 and the presence of natural

56

organic matter or surfactants can enhance the transport of GO, via steric hindrance or by

57

competing with GO for deposition sites on grain surfaces.10, 11, 15-17 Divalent cations have

58

particularly strong effects on the transport of GO/RGO, not only because they are more

59

effective in compressing the electrical double layer than monovalent cations, but also

60

because they can increase deposition of GO/RGO nanosheets via cation-bridging.12, 17 In

61

our previous study using GO and two RGO materials obtained by reducing GO with low

62

concentration of sulfide (designed to mimic environmentally relevant reduction of GO),

63

we further demonstrated that the significance of cation-bridging is dependent on the type

64

of surface O-functional groups.17 Specifically, even though the RGOs had much lower O-

65

contents than GO, more significant cation-bridging was observed for these two materials,

66

because the RGOs contained greater amounts of phenolic groups, a moiety that can 4

ACS Paragon Plus Environment

Page 4 of 34

Page 5 of 34

Environmental Science & Technology

67

complex strongly with Ca2+ than the epoxy groups of GO.17 Furthermore, studies on

68

aggregation properties of GO and RGO have shown that the sizes of GO/RGO aggregates

69

also depend on the interplay between divalent cations and GO/RGO surface O-

70

functionalities.18, 19 For instance, Ca2+ and Mg2+ can enhance the face-to-face interactions

71

between GO nanosheets,18, 19 but negligible binding was observed between nanosheets of

72

a RGO material obtained by solvothermally reducing GO with N-methyl-2-pyrrolidone.19

73

Such cation-induced effects on aggregation properties can in turn affect the transport

74

properties of GO/RGO, for example, through physical straining.20-23

75

Ion composition can be very complex in natural aquatic environments. Even though a

76

large number of studies have demonstrated that divalent cations can affect the transport

77

of negatively charged nanoparticles more significantly than monovalent cations (e.g.,

78

Mg2+/Ca2+ vs. Na+), very little is known about how cations of the same valence (e.g.,

79

alkali metal ions such as Na+ and K+, and alkaline earth metal ions such as Mg2+ and Ca2+)

80

may affect nanoparticle transport differently. Note that several studies have shown that

81

Ca2+ was more effective in inducing aggregation of negatively charged nanoparticles

82

(including GO and RGO) than Mg2+,18, 23-28 because Ca2+ can bind to O-functional groups

83

of nanoparticles more strongly than Mg2+.18, 23, 27 Such cation species-dependent effects

84

on particle−particle interactions likely can be extrapolated to particle−collector

85

interactions. For example, Ca2+ may result in more significant deposition of GO/RGO

86

than Mg2+, via more significant cation-bridging between grain surfaces and GO/RGO

87

nanosheets.

88

Herein, we hypothesize that the effects of cations on the transport of GO and RGO in

89

saturated porous media under unfavorable deposition conditions will obey the Hofmeister 5

ACS Paragon Plus Environment

Environmental Science & Technology

90

series.29, 30 Specifically, we expect that cations having larger ionic radii (and accordingly,

91

being more weakly hydrated) will interact with both grain surfaces and GO/RGO

92

nanosheets more strongly than cations of smaller ionic radii, and thereby, will induce

93

more significant transport inhibition of GO/RGO in saturated porous media. Based on

94

observations of various cations and anions to either destabilize or stabilize proteins,29 the

95

Hofmeister series can be explained in large part by the extent that ions interact with water

96

and in particular, their degree of hydration. The Hofmeister series has been used to

97

explain the affinities of ions to clays and oxides,31-33 and is likely to be applicable also for

98

cation−GO/RGO interactions. To test the hypothesis, we examined the transport of GO

99

and a sulfide-reduced GO in saturated quartz sand, as affected by different monovalent

100

cations (Na+, K+ and Cs+) and different divalent cations (Mg2+, Ca2+ and Ba2+). We

101

intentionally chose cations from the same groups in the periodic table as the comparative

102

ions to single out their differences in ionic and hydrated radii (see Supporting

103

Information (SI) Table S1). The breakthrough curves and retained profiles of GO/RGO in

104

the presence of different monovalent or divalent cations were compared, and the

105

underlying mechanisms controlling the effects of different cations were analyzed.

106

Supplemental batch adsorption experiments were conducted and microscopic evidence

107

was collected to verify the proposed mechanisms.

108 109

MATERIALS AND METHODS

110

Materials. Graphene oxide (referred to as GO hereafter) was synthesized using a

111

modified Hummers method.34 Reduced graphene oxide (referred to as RGO hereafter)

112

was prepared by reducing GO using Na2S. The detailed procedures for the synthesis of 6

ACS Paragon Plus Environment

Page 6 of 34

Page 7 of 34

Environmental Science & Technology

113

GO and RGO are described in SI. Surface elemental compositions of GO/RGO were

114

determined by X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe, Japan).

115

Fourier transform infrared (FTIR) transmission spectra were obtained using a 110 Bruker

116

TENSOR 27 apparatus (Bruker Optics Inc., Germany).

117

Quartz sand (50–70 mesh, 0.21–0.30 mm) was purchased from Sigma–Aldrich

118

(USA). The average grain size of the sand was 0.26 mm. The sand was pretreated before

119

use.35 It was first washed with 0.1 M HCl and then with 5% H2O2. Next, it was rinsed

120

repeatedly with deionized water until neutral pH was reached. Then, it was oven-dried at

121

90 °C overnight and stored for future use.

122

Column Transport Experiments. Quartz sand was dry-packed into Omnifit

123

borosilicate glass columns (10 cm × 0.66 cm, Bio-Chem Valve Inc., USA) with 10-µm

124

stainless-steel screens (Valco Instruments Inc., USA) on both ends. Each column

125

contained approximately 3.6 g sand (dry-weight) with an average length of 6.8 cm. The

126

columns were operated in an upward direction12, 13 using syringe pumps (KD Scientific,

127

USA). The sand-packed columns were equilibrated by flushing with 100 mL deionized

128

water at a flow rate of 3 mL/h followed by 180 mL background electrolyte solution.

129

Column properties are given in SI Table S2.

130

All the influents were prepared immediately before the column experiments by

131

diluting a stock suspension of GO/RGO in an electrolyte solution and then stirring for 2 h.

132

Electrophoretic mobility (EPM) and hydrodynamic diameter (Dh) of GO/RGO

133

nanoparticles in the influents were measured with a ZetaSizer Nano ZS (Malvern

134

Instruments, UK). Ultraviolet (UV) absorbance spectra of the suspensions were recorded

135

with a UV/vis spectrophotometer (UV-2401, Shimadzu Scientific Instruments, USA). 7

ACS Paragon Plus Environment

Environmental Science & Technology

136

The scan was performed in the wavelength range of 200−600 nm. The slit width and

137

sample interval were set at 1 and 0.2 nm, respectively. Aggregation properties of the

138

RGO/GO suspensions were also examined with a JEM-2100 transmission electron

139

microscope (TEM) (JEOL Ltd., Japan), and the samples were prepared by air-drying a

140

drop of suspension onto a copper TEM grid (Electron Microscopy Sciences, USA).

141

In a typical column experiment, the influent was pumped into the column from a

142

100-mL glass syringes (SGE Analytical Science, Australia). Column effluent samples

143

were collected in 4-mL glass vials every 2–3 pore volumes (PV) to determine the

144

concentrations of GO/RGO. The concentrations of GO/RGO in the influent (C0) and

145

effluent (C) were determined by measuring the UV absorbance at 230 nm (for GO) or

146

249 nm (for RGO), based on pre-established calibration curves of RGO and GO (SI

147

Figure S1).

148

Batch Adsorption Experiments and Granular-Scale Visualization. Batch

149

adsorption experiments were conducted to determine the binding of GO and RGO to

150

quartz sand as affected by different cations. First, 5 g sand was added to each of a series

151

of 20-mL amber glass vials. Then, 15 mL of a GO/RGO suspension was added to each

152

vial. The vials were equilibrated for 3 d by tumbling at 8 rpm in the dark. Then, the vials

153

were left undisturbed overnight, and the supernatants were withdrawn to measure the

154

concentrations of GO or RGO in the suspension, using a high sensitivity total organic

155

carbon analyzer (Shimadzu Scientific Instruments, USA). The concentrations of GO or

156

RGO adsorbed to sand were calculated based on a mass balance approach. The

157

distribution coefficients, Kd (L/kg), between sand and water is defined as Kd = q/CW,

158

where q (mg/kg) and CW (mg/L) are equilibrium concentrations of GO or RGO on sand 8

ACS Paragon Plus Environment

Page 8 of 34

Page 9 of 34

Environmental Science & Technology

159

and in the suspension, respectively. The adsorption experiments were done in triplicate.

160

Two sets of Kd values were obtained, by varying the initial concentrations of GO/RGO.

161

The GO and RGO adsorbed to sand grains were also visualized with a laser scanning

162

confocal microscope (Leica TCS SP5, Germany). The adsorbed GO or RGO was

163

detected by fluorescence, while the sand surfaces were visualized by UV light.36

164

DLVO Calculations and Analysis of Attachment Efficiencies. The particle–

165

collector interaction energy profiles under different solution chemistry conditions were

166

calculated using the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory (see detailed

167

equations and parameters in SI). (Note that in theory the equations described in SI are

168

only applicable for spherical particles and 1:1 electrolytes. Thus, the DLVO calculations

169

are only approximate, intended to qualitatively illustrate the relative effects of different

170

cations on the depth of secondary minimum well.) Additionally, theoretical attachment

171

efficiencies, α, were calculated using the Maxwell model and were compared with the

172

attachment efficiencies observed in the column experiments (the detailed equations are

173

given in SI).

174 175 176

RESULTS AND DISCUSSION Characteristics of GO and RGO. Selected physicochemical properties of GO and

177

RGO are summarized in Table 1. Graphene oxide contained abundant O-functionality

178

such as epoxy/hydroxyl (C-O-C/C-OH) (29.32%), carbonyl (C=O) (7.59%) and carboxyl

179

(O-C=O) (3.85%), as indicated by the deconvoluted peaks of C 1s spectra that correspond

180

to carbon atoms with different chemical states (SI Figure S2). In comparison, RGO had a

181

much higher C/O ratio, indicating the loss of O-functional groups during reduction. 9

ACS Paragon Plus Environment

Environmental Science & Technology

182

Notably, a higher carboxyl content was observed for RGO than for GO, likely from the

183

conversion of the epoxy groups.37 Furthermore, the FTIR spectra (SI Figure S3) show the

184

increase of hydroxyl groups (O−H bending band at ~1385 cm-1) of RGO, compared with

185

those of GO. The higher contents of carboxyl and phenolic groups (two metal-

186

complexing moieties) of RGO likely will make it more sensitive to the effects of divalent

187

cations.

188

Electrokinetics and Aggregation Properties of GO and RGO as Affected by

189

Cation Species. The effects of cations on the electrokinetics and aggregation properties

190

of GO/RGO suspensions varied significantly with the type of cation species in the

191

background solution (SI Table S3, Figure 1, and SI Figure S4). Less negative EPM

192

values of GO/RGO were observed at low concentrations of divalent cations (0.1 and 0.5

193

mM) than at high concentrations of monovalent cations (10 and 20 mM). Similarly,

194

divalent cations were also more effective in causing aggregation of GO/RGO nanosheets

195

than were monovalent cations, as indicated by the Dh values (Table S3 and Figure 1) and

196

the TEM images (Figure S4). The greater effects of divalent cations are consistent with

197

the literature.5, 21, 23, 24, 38-40

198

Interestingly, even cations of the same valence had markedly different effects on the

199

electrokinetics and aggregation properties of GO/RGO. Specifically, the capability of

200

cations to neutralize the negative surface charge of GO/RGO generally followed the order

201

of Cs+ > K+ > Na+, and Ba2+ > Ca2+ > Mg2+, as indicated by the EPM values (Table S3

202

and Figure 1). The effects of cations on aggregation also followed the same orders, as

203

indicated by the Dh values (Table S3 and Figure 1) and the different extents of

204

aggregation shown by the TEM images (Figure S4). These trends correlated well with the 10

ACS Paragon Plus Environment

Page 10 of 34

Page 11 of 34

Environmental Science & Technology

205

Hofmeister series, that is, the effects of cations on electrokinetics and aggregation

206

increase with the ionic radii of the cations. Cations with smaller ionic radii have higher

207

hydration numbers and larger hydrated radii, whereas cations with larger ionic radii have

208

weaker hydration shells, and can more easily be detached from their hydration layer.21, 41

209

Thus, Cs+ (with the most labile hydration sphere among the three monovalent cations)

210

has the tendency to form inner-sphere complexes, whereas K+ and Na+ can only form

211

outer-sphere complexes.21, 42, 43 Similarly, the poorly hydrated Ba2+ and Ca2+ can form

212

inner-sphere complexes, whereas Mg2+ forms mainly outer-sphere complexes.21, 44

213

Cations that form inner-sphere complexes with GO/RGO surfaces can more effectively

214

neutralize the negative surface charge of the nanosheets. Furthermore, by forming

215

complexes with carboxyl and phenolic groups of GO/RGO, the poorly hydrated cations

216

can result in more significant aggregation of nanosheets by serving as the bridging agents.

217

Note that the differences in Dh as affected by cations of the same valence were more

218

profound than the differences in EPM, indicating the important role of cation bridging in

219

the aggregation of GO/RGO.

220

Transport of GO and RGO as Affected by Different Monovalent Cations. The

221

effects of monovalent cations on the transport of both GO and RGO were strongly

222

dependent on cation species (Figure 2). In particular, Cs+ exerted much greater transport

223

inhibition effects than K+ and Na+. For example, at 10 mM Na+ or K+ breakthrough of

224

GO reached ~90% after 17 PV, whereas at 10 mM Cs+ breakthrough only reached ~44%

225

(Figure 2a). At the higher cation concentration tested (20 mM) the differences were even

226

larger, in that little breakthrough was observed in the presence of Cs+, whereas

227

breakthrough reached 83% and 76% in the presence of Na+ or K+ (Figure 2c). 11

ACS Paragon Plus Environment

Environmental Science & Technology

228

Furthermore, between Na+ and K+, the latter inhibited the transport of GO to a larger

229

extent, especially at the higher cation concentration (20 mM). Similar patterns were

230

observed for RGO (Figures 2b and 2d). Note that RGO exhibited lower mobility than GO

231

under the same solution chemistry conditions. This was attributable to the greater surface

232

hydrophobicity, less negative surface charge, and larger particle size of RGO aggregates

233

than GO aggregates, as demonstrated in our previous study.17

234

The observation that different monovalent cations inhibited the transport of GO and

235

RGO to different extents can be understood by considering the mechanisms via which

236

cations affect the transport of negatively charged nanoparticles in quartz sand. First,

237

cations compress the thickness of electrostatic double layer, decrease the electrostatic

238

repulsion between nanoparticles and sand, and deepen the secondary minimum energy

239

well.7, 45, 46 Cations with smaller hydrated radius (e.g., Cs+) can result in larger electronic

240

shielding on the negatively charged surfaces of quartz sand due to stronger electrostatic

241

forces; this is evident by comparing the DLVO particle–collector interaction energy

242

profiles as affected by different monovalent cations (SI Figure S5). Second, accumulation

243

of large hydrated cations (e.g., Na+ and K+) on the surface of quartz sand may interfere

244

with particle deposition through steric hindrance,17 whereas cations with smaller hydrated

245

radius (e.g., Cs+) would have smaller effects. Third, Cs+ can serve as a bridging agent by

246

forming inner-sphere complexes with surface functional groups of both GO/RGO and

247

quartz sand, and therefore, significantly enhance the deposition of GO and RGO. In

248

comparison, Na+ and K+ only form outer-sphere complexes, and cannot serve as bridging

249

agents. Fourth, as mentioned above cations of different hydrated radii also affected the

250

particle sizes differently, in that larger aggregates were formed in the presence of weakly 12

ACS Paragon Plus Environment

Page 12 of 34

Page 13 of 34

Environmental Science & Technology

251

hydrated cations (see the Dh values in Table S3 and the TEM images in Figure S4).

252

Accordingly, retention via straining23, 47-49 can be more significant in the presence of

253

poorly hydrated cations such as Cs+. It is commonly assumed that particles may be

254

intercepted if the ratio of diameters of particle to collector (dp/dc, where dp and dc are

255

diameters of particle and collector, respectively) is above 0.002–0.003.7, 50 Based on the

256

dp/dc values in Table S3, straining was likely a viable retention mechanism for GO/RGO

257

in the presence of Cs+.

258

To understand the relative contributions of different retention mechanisms, step-

259

wise flushing using electrolyte solutions of decreasing ionic strength was carried out. The

260

observed release of retained GO/RGO during the flushing (SI Figure S6) reflects the

261

contribution of the secondary minimum.51 The results indicate that when the background

262

cation was Na+ or K+ deposition at the secondary minimum energy well was the most

263

important retention mechanism for both GO and RGO, accounting for 64−69% of the

264

overall retention in the presence of Na+, and 44−70% in the presence of K+ (Table 2). In

265

comparison, deposition at the secondary minimum was a minor deposition mechanism in

266

the presence of Cs+, accounting for only 6−7% of the overall deposition. The fact that Cs+

267

affected the transport of RGO more profoundly than that of GO was consistent with its

268

cation-bridging effects. As mentioned earlier, RGO contained greater amounts of surface

269

carboxyl and phenolic groups than did GO. These metal-complexing moieties can

270

magnify the particle−collector and particle−particle bridging effects of Cs+, leading to

271

more significant deposition through cation-bridging and possibly through straining. Note

272

that the experimentally observed attachment efficiencies in the presence of Cs+ are

273

significantly greater than the respective theoretical values calculated with the Maxwell 13

ACS Paragon Plus Environment

Environmental Science & Technology

274

model (based on the DLVO interaction energies) (SI Table S4). This further corroborates

275

the cation-bridging role of Cs+.

276

Transport of GO and RGO as Affected by Different Divalent Cations. The three

277

divalent cations also influenced the transport of GO/RGO differently. Overall, the

278

transport inhibition effects followed the order of Ba2+ > Ca2+ > Mg2+, however, the

279

specific differences between different cations depended on the concentrations of cations,

280

and varied between GO and RGO (Figure 3). At a cation concentration of 0.1 mM the

281

transport-inhibition effects of the three cations were not too significant – even in the

282

presence of Ba2+ breakthrough of GO/RGO reached ~80%. Much more significant

283

inhibition was observed at the higher cation concentration tested (0.5 mM), wherein little

284

breakthrough of GO was observed in the presence of Ba2+ and little breakthrough of RGO

285

was observed in the presence of Ca2+ or Ba2+. It is noteworthy that for GO the transport

286

inhibition effects of Ca2+ were comparable to those of Mg2+, and both were substantially

287

weaker than those of Ba2+ (Figure 3c); nonetheless, for RGO Ca2+ inhibited the transport

288

to a similar extent as did Ba2+, and much more significantly than did Mg2+ (Figure 3d).

289

In our previous study, we demonstrated with column flushing experiments that in the

290

presence of Ca2+ deposition at the secondary minimum energy well is an insignificant

291

mechanism for the retention of GO/RGO under experimental conditions similar to those

292

involved in this study, and cation bridging between GO/RGO and sand grains becomes

293

the most predominant retention mechanism.17 (The shallow secondary minimum wells

294

(Table S4 and Figure S5) and the significant underestimation of attachment efficiencies

295

using the Maxwell model and DLVO theory (Table S4) are consistent with this argument.)

296

Thus, we propose that the significantly different effects on the transport of GO/RGO 14

ACS Paragon Plus Environment

Page 14 of 34

Page 15 of 34

Environmental Science & Technology

297

among Mg2+, Ca2+, and Ba2+ stemmed from their different complexing strength. As

298

mentioned above, cations with small ionic radii have large hydrated radii, and can only

299

form outer-sphere complexes, whereas cations with large ionic radii tend to form inner-

300

sphere complexes.21 It has been reported that Ba2+ has a strong propensity for forming

301

inner-sphere complexes.43 Even though Ca2+ is smaller in ionic radius than Ba2+, it can

302

also form inner-sphere complexes.21 The smallest cation of the group, Mg2+, however,

303

forms mainly outer-sphere complexes.21 Accordingly, Ba2+ could result in stronger

304

particle−collector bridging (by forming complexes with surface functional groups of both

305

GO/RGO and quartz sand) than Ca2+ and Mg2+, and Ca2+ would be a stronger bridging

306

agent than Mg2+. Similarly, the particle−particle bridging effects of the cations would

307

also follow the order of Ba2+ > Ca2+ > Mg2+, leading to more significant straining in the

308

presence of Ba2+ than Ca2+ than Mg2+. The comparable effects of Ca2+ and Mg2+ on the

309

transport of GO, as well as the comparable effects of Ba2+ and Ca2+ on the transport of

310

RGO, further corroborate this argument – because RGO contained greater amounts of

311

carboxyl and phenolic groups (i.e., metal-complexing moieties) than GO, its transport

312

was affected more remarkably by the stronger complexing cations (i.e., Ba2+ and Ca2+).

313

To further understand the relative significance of particle−collector bridging versus

314

particle−particle bridging by the three cations, we analyzed the retained particle profiles

315

of the columns used in the transport experiments in the presence of divalent cations.50, 52

316

Interestingly, hyper-exponential retained profiles were only observed for RGO in the

317

presence of 0.5 mM Ba2+ or Ca2+ (Figure 4), an observation consistent with the large

318

particle sizes of RGO under these conditions (see the Dh values in Table S3). Thus, the

319

retained particle profiles indicate that significant straining only occurred for RGO in the 15

ACS Paragon Plus Environment

Environmental Science & Technology

320

presence of relatively high concentrations of cations capable of forming inner-sphere

321

complexes, and under the experimental conditions of this study the significant effects of

322

divalent cations (in particular, Ba2+ and Ca2+) on the transport of GO and RGO were

323

mainly exerted through particle−collector bridging.

324

Further Evidence for Cation-Dependent Effects of Particle− −Collector Bridging.

325

To further verify that the extents of particle−collector bridging by different cations are

326

correlated to the ionic radii of the cations (and thus their capability of forming inner-

327

sphere complexes), we examined the adsorption of GO and RGO to quartz sand as

328

affected by the presence of different cations. Figure 5 clearly shows that for a given

329

nanomaterial (i.e., GO or RGO) and a given cation concentration, the adsorption affinity

330

of GO/RGO—as indicated by the sand–water distribution coefficients (Kd)—was highly

331

dependent on the type of cations present in the solution (the Kd values are summarized in

332

SI Table S5). For the reaction systems containing monovalent cations, the strongest

333

adsorption was observed in the presence of Cs+, followed by K+ and then by Na+. The

334

order is consistent with the Homeister effects mentioned above (and the subtitle

335

difference between GO and RGO might also be related to the size-dependent affinity of

336

cations to hydrophobic vs. hydrophilic surfaces53). Similarly, for the reaction systems

337

containing divalent cations, the strongest adsorption occurred when the cation present

338

was Ba2+, followed by Ca2+ and then by Mg2+. The confocal images of GO/RGO

339

adsorbed to quartz sand (Figure 6) further corroborate the Kd data, in that for both the

340

monovalent cations and divalent cations greater amounts of GO/RGO (the fluorescent

341

green areas on the images) were observed on quartz sand (the dark areas on the images)

342

in the presence of cations with larger ionic radii. 16

ACS Paragon Plus Environment

Page 16 of 34

Page 17 of 34

343

Environmental Science & Technology

The cation-dependent effects on the binding of GO/RGO to quartz sand were highly

344

consistent with the cation-inhibition effects on transport. For example, Figure 5 shows

345

that much stronger adsorption of GO was observed in the presence of 20 mM Cs+ than in

346

the presence of the same amount of K+ or Na+, correlating well with Figure 2, which

347

shows that the maximum breakthrough of GO in the presence of 20 mM Cs+ reached only

348

10%, much lower than that in the presence of K+ (76%) or Na+ (84%). Similarly, much

349

stronger adsorption of RGO was observed in the presence of 0.5 mM Ba2+ or Ca2+ than in

350

the presence of Mg2+ (Figure 5), consistent with the transport data in Figure 2 showing

351

very low breakthrough of RGO in the presence of 0.5 mM Ba2+ or Ca2+ in comparison to

352

the 68% maximum breakthrough of RGO in the presence of 0.5 mM Mg2+. The striking

353

similarities between the effects of cations on adsorption and on transport provide

354

convincing evidence that cations capable of forming inner-sphere complexes can

355

significantly influence the transport of GO/RGO via cation-bridging, and the extents of

356

the effects correlate well with the ionic radii of the cations, as described by the

357

Hofmeister series. Note that the Hofmeister series might be (partially) reversed at high

358

pH.22, 53 Further studies are needed to understand how cations of the same valence may

359

affect transport of GO/RGO under more basic solution chemistry conditions.

360

Environmental Implications. Natural aquatic environments contain many different

361

ions. The findings of this study further demonstrate that these ions—which may vary

362

significantly in charge density, size, and complexing capability— can affect transport of

363

nanoparticles very differently. An important observation of this study was that the

364

specific mechanisms through which cations inhibit transport of negatively charged

365

GO/RGO nanosheets and the extents of the effects depended on the complex interplay 17

ACS Paragon Plus Environment

Environmental Science & Technology

366

between the properties of the cations and the type and distribution of surface functional

367

groups of the nanomaterials. It is probably reasonable to expect that more intriguing

368

effects will be observed for more complex porous media, as besides of quartz sand,

369

cations can also interact strongly with other constituents of porous materials (e.g., clay

370

minerals and oxides). More studies are needed to fully understand the transport of

371

nanoparticles as a function of nanomaterial physicochemical properties and solution

372

chemistry parameters.

373 374

Acknowledgments. This project was supported by the National Natural Science

375

Foundation of China (Grant 21237002), the Ministry of Science and Technology of

376

China (Grant 2014CB932001), and the National Science Fund for Distinguished Young

377

Scholars (Grant 21425729).

378 379

Supporting Information Available: Procedures used to prepare GO and RGO, and

380

calculations of DLVO interaction energies and attachment efficiencies; tables

381

summarizing ionic and hydrated radii of cations studied, column and influent properties,

382

DLVO energy profiles and attachment efficiencies, and distribution coefficients of

383

GO/RGO to quartz sand; figures showing UV/Vis calibration curves of GO/RGO, XPS

384

and FTIR spectra of GO and RGO, TEM images of GO/RGO suspensions, DLVO energy

385

profiles, and column-flushing results. This information is available free of charge via the

386

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

387 388

Notes—The authors declare no competing financial interest. 18

ACS Paragon Plus Environment

Page 18 of 34

Page 19 of 34

Environmental Science & Technology

389 390

REFERENCES

391

1. Chen, D.; Feng, H.; Li, J. Graphene oxide: Preparation, functionalization, and

392

electrochemical applications. Chem. Rev. 2012, 112 (11), 6027−6053.

393

2. Peijnenburg, W. J. G. M.; Baalousha, M.; Chen, J.; Chaudry, Q.; von der Kammer, F.;

394

Kuhlbusch, T. A. J.; Lead, J.; Nickel, C.; Quik, J. T. K.; Renker, M. A review of the

395

properties and processes determining the fate of engineered nanomaterials in the

396

aquatic environment. Crit. Rev. Env. Sci. Technol. 2015, 45 (19), 2084−2134.

397 398

3. Schirmer, K.; Auffan, M. Nanotoxicology in the environment. Environ. Sci.: Nano 2015, 2 (6), 561−563.

399

4. Klaine, S. J.; Koelmans, A. A.; Horne, N.; Carley, S.; Handy, R. D.; Kapustka, L.;

400

Nowack, B.; von der Kammer, F. Paradigms to assess the environmental impact of

401

manufactured nanomaterials. Environ. Toxicol. Chem. 2012, 31 (1), 3−14.

402

5. Zhao, J.; Wang, Z.; White, J.; Xing, B. Graphene in the aquatic environment:

403

Adsorption, dispersion, toxicity and transformation. Environ. Sci. Technol. 2014, 48

404

(17), 9995−10009.

405 406 407

6. Feriancikova, L.; Xu, S. Deposition and remobilization of graphene oxide within saturated sand packs. J. Hazard. Mater. 2012, 235−236, 194−200. 7. Lanphere, J. D.; Luth, C. J.; Walker, S. L. Effects of solution chemistry on the

408

transport of graphene oxide in saturated porous media. Environ. Sci. Technol. 2013, 47

409

(9), 4255−4261.

19

ACS Paragon Plus Environment

Environmental Science & Technology

410

8. Liu, L.; Gao, B.; Wu, L.; Morales, V. L.; Yang, L.; Zhou, Z.; Wan, H. Deposition and

411

transport of graphene oxide in saturated and unsaturated porous media. Chem. Eng. J.

412

2013, 229, 444−449.

413

9. Lanphere, J. D.; Rogers, B.; Luth, C. J.; Bolster, C. H.; Walker, S. L. Stability and

414

transport of graphene oxide nanoparticles in groundwater and surface water. Environ.

415

Eng. Sci. 2014, 31 (7), 350−359.

416

10. Qi, Z.; Zhang, L.; Wang, F.; Hou, L.; Chen, W. Factors controlling transport of

417

graphene oxide nanoparticles in saturated sand columns. Environ. Toxicol. Chem.

418

2014, 33 (5), 998−1004.

419 420 421

11. Qi, Z.; Zhang, L.; Chen, W. Transport of graphene oxide nanoparticles in saturated sandy soil. Environ. Sci. Proc. Impacts. 2014, 16 (10), 2268−2277. 12. Fan, W.; Jiang, X.; Yang, W.; Geng, Z.; Huo, M.; Liu, Z.; Zhou, H. Transport of

422

graphene oxide in saturated porous media: Effect of cation composition in mixed Na–

423

Ca electrolyte systems. Sci. Total. Environ. 2015, 511, 509−515.

424

13. Sun, Y.; Gao, B.; Bradford, S. A.; Wu, L.; Chen, H.; Shi, X.; Wu, J. Transport,

425

retention, and size perturbation of graphene oxide in saturated porous media: Effects

426

of input concentration and grain size. Water. Res. 2015, 68, 24−33.

427

14. He, J.; Li, C.; Wang, D.; Zhou, D. Biofilms and extracellular polymeric substances

428

mediate the transport of graphene oxide nanoparticles in saturated porous media. J.

429

Hazard. Mater. 2015, 300, 467−474.

430

15. Fan, W.; Jiang, X.; Lu, Y.; Huo, M.; Lin, S.; Geng, Z. Effects of surfactants on

431

graphene oxide nanoparticles transport in saturated porous media. J. Environ. Sci.

432

2015, 35, 12−19. 20

ACS Paragon Plus Environment

Page 20 of 34

Page 21 of 34

433

Environmental Science & Technology

16. Liu, L.; Gao, B.; Wu, L.; Sun, Y.; Zhou, Z. Effects of surfactant type and

434

concentration on graphene retention and transport in saturated porous media. Chem.

435

Eng. J. 2015, 262, 1187−1191.

436

17. Xia, T.; Fortner, J. D.; Zhu, D.; Qi, Z.; Chen, W. Transport of sulfide-reduced

437

graphene oxide in saturated quartz sand: Cation-dependent retention mechanisms.

438

Environ. Sci. Technol. 2015, 49 (19), 11468−11475.

439

18. Wu, L.; Liu, L.; Gao, B.; Muñoz-Carpena, R.; Zhang, M.; Chen, H.; Zhou, Z.; Wang,

440

H. Aggregation kinetics of graphene oxides in aqueous solutions: Experiments,

441

mechanisms, and modeling. Langmuir 2013, 29 (49), 15174−15181.

442

19. Chowdhury, I.; Mansukhani, N.; Guiney, L. M.; Hersam, M. C.; Bouchard, D.

443

Aggregation and stability of reduced graphene oxide: Complex roles of divalent

444

cations, pH, and natural organic matter. Environ. Sci. Technol. 2015, 49 (18),

445

10886−10893.

446

20. Chen, K. L.; Elimelech, M. Influence of humic acid on the aggregation kinetics of

447

fullerene (C60) nanoparticles in monovalent and divalent electrolyte solutions. J.

448

Colloid. Interface. Sci. 2007, 309 (1), 126−134.

449

21. Pham, M.; Mintz, E. A.; Nguyen, T. H. Deposition kinetics of bacteriophage MS2 to

450

natural organic matter: Role of divalent cations. J. Colloid. Interface. Sci. 2009, 338

451

(1), 1−9.

452

22. Yi, P.; Chen, K. L. Influence of surface oxidation on the aggregation and deposition

453

kinetics of multiwalled carbon nanotubes in monovalent and divalent electrolytes.

454

Langmuir 2011, 27 (7), 3588−3599.

21

ACS Paragon Plus Environment

Environmental Science & Technology

455

23. Chowdhury, I.; Duch, M. C.; Manuskhani, N. D.; Hersam, M. C.; Bouchard, D.

456

Colloidal properties and stability of graphene oxide nanomaterials in the aquatic

457

environment. Environ. Sci. Technol. 2013, 47 (12), 6288−6296.

458

24. Nguyen, T. H.; Chen, K. L. Role of divalent cations in plasmid DNA adsorption to

459

natural organic matter-coated silica surface. Environ. Sci. Technol. 2007, 41 (15),

460

5370−5375.

461

25. Huynh, K. A.; Chen, K. L. Aggregation kinetics of citrate and polyvinylpyrrolidone

462

coated silver nanoparticles in monovalent and divalent electrolyte solutions. Environ.

463

Sci. Technol. 2011, 45 (13), 5564−5571.

464

26. Chowdhury, I.; Duch, M. C.; Mansukhani, N. D.; Hersam, M. C.; Bouchard, D.

465

Deposition and release of graphene oxide nanomaterials using a quartz crystal

466

microbalance. Environ. Sci. Technol. 2014, 48 (2), 961−969.

467

27. Ren, X.; Li, J.; Tan, X.; Shi, W.; Chen, C.; Shao, D.; Wen, T.; Wang, L.; Zhao, G.;

468

Sheng, G.; Wang, X. Impact of Al2O3 on the aggregation and deposition of graphene

469

oxide. Environ. Sci. Technol. 2014, 48 (10), 5493−5500.

470

28. Hua, Z.; Tang, Z.; Bai, X.; Zhang, J.; Yu, L.; Cheng, H. Aggregation and

471

resuspension of graphene oxide in simulated natural surface aquatic environments.

472

Environ. Pollut. 2015, 205, 161−169.

473 474 475 476

29. Zhang, Y.; Cremer, P. S. Interactions between macromolecules and ions: the Hofmeister series. Curr. Opin. Chem. Biol. 2006, 10 (6), 658−663. 30. Zhen, Y. Hofmeister effects: an explanation for the impact of ionic liquids on biocatalysis. J. Biotechnol. 2009, 144 (1), 12−22.

22

ACS Paragon Plus Environment

Page 22 of 34

Page 23 of 34

477 478

Environmental Science & Technology

31. Stumm, W., Morgan, J. J., Aquatic Chemistry, 3rd, ed.; Wiley Interscience: New York, 1996.

479

32. Dishon, M.; Zohar, O.; Sivan, U. From repulsion to attraction and back to repulsion:

480

the effect of NaCl, KCl, and CsCl on the force between silica surfaces in aqueous

481

solution. Langmuir 2009, 25 (5), 2831−2836.

482 483 484

33. Schwierz, N.; Horinek, D.; Netz, R. R. Anionic and cationic Hofmeister effects on hydrophobic and hydrophilic surfaces. Langmuir 2013, 29 (8), 2602−2614. 34. Duch, M. C.; Budinger, G. R. S.; Liang, Y.; Soberanes, S.; Urich, D.; Chiarella, S. E.;

485

Campochiaro, L. A.; Gonzalez, A.; Chandel, N. S.; Hersam, M. C.; Mutlu, G. M.

486

Minimizing oxidation and stable nanoscale dispersion improves the biocompatibility

487

of graphene in the lung. Nano Lett. 2011, 11 (12), 5201−5207.

488

35. Mattison, N. T.; O’Carroll, D. M.; Kerry Rowe, R.; Petersen, E. J. Impact of porous

489

media grain size on the transport of multi-walled carbon nanotubes. Environ. Sci.

490

Technol. 2011, 45 (22), 9765–9775.

491

36. Yang, X.; Zhang, Y.; Chen, F.; Yang, Y. Interplay of natural organic matter with flow

492

rate and particle size on colloid transport: Experimentation, visualization, and

493

modeling. Environ. Sci. Technol. 2015, 49 (22), 13385−13393.

494

37. Gao, X.; Jiang, J.; Nagase, S. Hydrazine and thermal reduction of graphene oxide:

495

Reaction mechanisms, product structures, and reaction design. J. Phys. Chem. C 2010,

496

114 (2), 832−842.

497

38. Schwyzer, I.; Kaegi, R.; Sigg L.; Smajda, R.; Magrez, A.; Nowack, B. Long-term

498

colloidal stability of 10 carbon nanotube types in the absence/presence of humic acid

499

and calcium. Environ. Pollut. 2012, 169, 64−73. 23

ACS Paragon Plus Environment

Environmental Science & Technology

500 501 502

39. Chen, K. L.; Elimelech, M. Aggregation and deposition kinetics of fullerene (C60) nanoparticles. Langmuir 2006, 22 (26), 10994−11001. 40. Stankus, D. P.; Lohse, S. E.; Hutchison, J. E.; Nason, J. A. Interactions between

503

natural organic matter and gold nanoparticles stabilized with different organic

504

capping agents. Environ. Sci. Technol. 2011, 45 (8), 3238−3244.

505

41. Tansel, B.; Sager, J.; Rector, T.; Garland, J.; Strayer, R. F.; Levine, L. F.; Roberts, M.;

506

Hummerick, M.; Bauer, J. Significance of hydrated radius and hydration shells on

507

ionic permeability during nanofiltration in dead end and cross flow modes. Sep. purify.

508

Technol. 2006, 51 (1), 40−47.

509

42. Newman, J. K.; Mccormick, C. L. Water-Soluble Copolymers. 52. Sodium-23 NMR

510

Studies of Ion-Binding to Anionic Polyelectrolytes: Poly(sodium 2-acrylamido-2-

511

methylpropanesulfonate), Poly(sodium 3-acrylamido-3-methylbutanoate),

512

Poly(sodium acrylate), and Poly(sodium galacturonate). Macromolecules 1994, 27

513

(18), 5114−5122.

514

43. Qu, X.; Zhang, Y.; Li, H.; Zheng, S.; Zhu, D. Probing the specific sorption sites on

515

montmorillonite using nitroaromatic compounds and hexafluorobenzene. Environ. Sci.

516

Technol. 2011, 45 (6), 2209−2216.

517

44. Pochard, I.; Denoyel, R.; Couchot, P.; Foissy, A. Adsorption of barium and calcium

518

chloride onto negatively charged α-Fe2O3 particle. J. Colloid. Interface. Sci. 2002,

519

255 (1), 27−35.

520

45. Hahn, M. W.; O’Melia, C. R. Deposition and reentrainment of Brownian particles in

521

porous media under unfavorable chemical conditions: Some concepts and

522

applications. Environ. Sci. Technol. 2014, 38 (1), 210−220. 24

ACS Paragon Plus Environment

Page 24 of 34

Page 25 of 34

523

Environmental Science & Technology

46. Cornelis, G.; Pang, L.; Doolette, C.; Kirby, J. K.; McLaughlin, M. J. Transport of

524

silver nanoparticles in saturated columns of natural soils. Sci. Total Environ. 2013,

525

463-464, 120−130.

526

47. Wang, Y.; Li, Y.; Pennell, K. D. Influence of electrolyte species and concentration on

527

the aggregation and transport of fullerene nanoparticles in quartz sands. Environ.

528

Toxicol. Chem. 2008, 27 (9), 1860−1867.

529

48. Tufenkji, N.; Miller, G. F.; Ryan, J. N.; Harvey, R. W.; Elimelech, M. Transport of

530

Cryptosporidium oocysts in porous media: Role of straining and physicochemical

531

filtration. Environ. Sci. Technol. 2004, 38 (22), 5932−5938.

532

49. Raychoudhury, T.; Tufenkji, N.; Ghoshal, S. Straining of polyelectrolyte-stabilized

533

nanoscale zero valent iron particles during transport through granular porous media.

534

Water. Res. 2014, 50, 80−89.

535

50. Bradford, S. A.; Torkzaban, S.; Walker, S. L. Coupling of physical and chemical

536

mechanisms of colloid straining in saturated porous media. Water. Res. 2007, 41 (13),

537

3012−3024.

538 539 540 541 542 543

51. Ryan, J. N.; Elimelech, M. Colloid mobilization and transport in groundwater. Colloids. Surf. A 1996, 107 (95), 1−56. 52. Jiang, X.; Tong, M.; Lu, R.; Kim, H. Transport and deposition of ZnO nanoparticles in saturated porous media. Colloids. Surf. A 2012, 401, 29−37. 53. Schwierz, N.; Horinek, D.; Sivan, U.; Netz, R.Reversed Hofmeister series—The rule rather than the exception. Curr. Opin. Colloid Interface Sci. 2016, 23, 10−18.

544

25

ACS Paragon Plus Environment

Environmental Science & Technology

Page 26 of 34

Table 1. Selected Physicochemical Properties of GO and RGO a C b (wt%) aromatic rings

epoxy/ hydroxyl

carbonyl

GO 29.23 29.32 7.59 RGO 39.71 28.28 3.53 a RGO represents sulfide-reduced GO. b Analyzed with X-ray photoelectron spectroscopy.

carboxyl 3.85 4.30

total Cb (wt%) 70.00 75.82

26

ACS Paragon Plus Environment

total Ob (wt%) 28.39 21.35

C/O ratio 2.47 3.55

Page 27 of 34

Environmental Science & Technology

Table 2. Mass Balance Expressed as Percentage of Eluted Mass during Each Flushing Step, and Mass Recovered from Column Eluted mass a (%)

Column No.

GO/ RGO

Effluent mass (%)

1

GO

75.06

3.91

1.50

12.33

7.20

Mass recovered from column (%) 5.86

98.66

Contribution of secondary minimum d (%) 66

2

GO

71.33

5.66

2.74

7.40

12.87

11.07

98.20

44

3

GO

32.04

2.69

1.99

2.54

60.73

46.39

85.66

6.9

4

RGO

68.28

3.86

3.16

15.96

8.74

8.46

99.72

69

5

RGO

66.71

3.72

1.65

19.19

8.73

8.13

99.39

70

6

RGO

26.51

2.77

2.03

2.50

66.21

48.24

82.04

6.4

7

GO

67.03

3.34

2.48

16.59

10.56

9.03

98.48

64

8

GO

53.98

7.34

4.35

17.40

16.94

15.24

98.30

56

9

GO

6.15

1.21

2.47

3.59

86.58

68.46

81.89

6.5

10

RGO

27.64

4.45

3.46

42.23

22.22

21.30

99.08

67

11

RGO

19.44

4.25

3.41

38.02

34.87

32.42

97.54

54

12

RGO

3.23

1.65

2.40

3.62

89.10

69.31

80.21

6.3

Flushing #1

Flushing #2

Flushing #3

Mass retained in column b (%)

a

Mass balance c (%)

Columns were flushed with the GO/RGO-free background solutions (flushing #1), lower concentration of background solutions (flushing #2), and deionized water (flushing #3). b Mass retained in column = 100 – effluent mass – eluted mass. c Mass balance was calculated as: effluent mass + eluted mass + mass recovered from column. d Contribution of secondary minimum to overall deposition was calculated as: (eluted mass in Flushing #2 + eluted mass in Flushing #3)/(100 – effluent mass – eluted mass in Flushing #1).

27

ACS Paragon Plus Environment

Environmental Science & Technology

(a) EPM of GO/RGO in M

+

0

-1

EPM (10-8 m2/Vs)

EPM (10-8 m2/Vs)

0

-2

Na+ + K + Cs

-3

-4

-1

-2

GO in 20 mM M+

(c) Dh of GO/RGO in M

RGO in 10 mM M+

GO in 2+ 0.1 mM Me

RGO in 20 mM M+

+

GO in 2+ 0.5 mM Me

RGO in RGO in 2+ 2+ 0.1 mM Me 0.5 mM Me

2+

1400

(d) Dh of GO/RGO in Me 2+

Na K+ Cs+

1200 1000

Dh (nm)

Dh (nm)

1000

2+

Mg 2+ Ca Ba2+

-3

+

1200

(b) EPM of GO/RGO in Me2+

-4 GO in 10 mM M+

1400

Page 28 of 34

800 600

Mg 2+ Ca Ba2+

800 600

400

400

200

200 0

0 GO in 10 mM M+

GO in 20 mM M+

RGO in 10 mM M+

RGO in RGO in GO in GO in 2+ 2+ 0.1 mM Me2+ 0.5 mM Me2+ 0.1 mM Me 0.5 mM Me

RGO in 20 mM M+

Figure 1. Electrophoretic mobility (EPM) and hydrodynamic diameter (Dh) of GO/RGO as affected by different monovalent cations (M+) (plots a and c) and divalent cations (Me2+) (plots b and d).

28

ACS Paragon Plus Environment

Page 29 of 34

Environmental Science & Technology

(a) GO in 10 mM M+

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

10

20

30

(b) RGO in 10 mM M+

1.0

C/C0

C/C0

1.0

40

0

10

PV

0.6

0.6

C/C0

C/C0

0.8

0.4

0.4

0.2

0.2

0.0

0.0 10

20 PV Na+

40

30

(d) RGO in 20 mM M+

1.0

0.8

0

30

PV

(c) GO in 20 mM M+

1.0

20

0

40

10

20

30

40

PV K+

Cs

+

Figure 2. Effects of monovalent cations (M+) on transport of GO and RGO: (a) GO in 10 mM M+ (Columns 1–3); (b) RGO in 10 mM M+ (Columns 4–6); (c) GO in 20 mM M+ GO(Columns 7–9); and (d) RGO in 20 mM M+ (Columns 10–12).

29

ACS Paragon Plus Environment

Environmental Science & Technology

(b) RGO in 0.1 mM Me2+

2+

(a) GO in 0.1 mM Me

1.0

0.8

0.8

0.6

0.6

C/C0

C/C0

1.0

0.4

0.4

0.2

0.2

0.0

0.0 0

10

20

30

40

0

10

PV

0.6

0.6

C/C0

C/C0

1.0 0.8

0.4

40

0.4

0.2

0.2

0.0

0.0 10

30

20 PV

(d) RGO in 0.5 mM Me2+

2+

0.8

0

20 PV

(c) GO in 0.5 mM Me

1.0

Page 30 of 34

30

0

40

10

20

30

40

PV 2+

Mg2+

Ca

Ba2+

Figure 3. Effects of divalent cations (Me2+) on transport of GO and RGO: (a) GO in 0.1 mM Me2+ (Columns 13–15); (b) RGO in 0.1 mM Me2+ (Columns 16–18); (c) GO in 0.5 mM Me2+ (Columns 19–21); and (d) RGO in 0.5 mM Me2+ (Columns 22–24).

30

ACS Paragon Plus Environment

Page 31 of 34

Environmental Science & Technology

(a) GO in 0.1 mM Me

300

2+

2+

250

250

µg-RGO/g-Sand

µg-RGO/g-Sand

(b) RGO in 0.1 mM Me

300

200 150 100 50

200 150 100 50

0

0 0

2

4

0

6

(c) GO in 0.5 mM Me

6

2+

2+

(d) RGO in 0.5 mM Me

300 250

µg-RGO/g-Sand

250

µg-RGO/g-Sand

4

Distance from Inlet (cm)

Distance from Inlet (cm)

300

2

200 150 100

200 150 100 50

50

0

0 0

2

4

0

6

2

4

6

Distance from Inlet (cm)

Distance from Inlet (cm)

2+

Ca2+

Mg

2+

Ba

Figure 4. Retained profiles of GO and RGO for transport in the presence of divalent cations (Me2+): (a) GO in 0.1 mM Me2+ (Columns 13–15); (b) RGO in 0.1 mM Me2+ (Columns 16–18); (c) GO in 0.5 mM Me2+ (Columns 19–21); and (d) RGO in 0.5 mM Me2+ (Columns 22–24).

31

ACS Paragon Plus Environment

Environmental Science & Technology

20

Kd (L/kg)

15

10

5

C m a 2+ M R B G a 2+ O 0. 5 R m G M O 0. M 5 g 2+ R m G O M 0. C 5 a 2+ m M B a 2+

g m M

G

O

0. 5

0. 5 O

G

O G

m M

M

C 0. 5

20 O

G R

2+

+

s

+

m M

K

a N 20

m M R

G

O

20

R

G

O

20 O

m M

s C

K

m M

m M 20 G

O G

G

O

20

m M

N

a

+

+

+

+

0

Figure 5. Adsorption coefficients (Kd) of GO/RGO to sand as affected by cation species.

32

ACS Paragon Plus Environment

Page 32 of 34

Page 33 of 34

Environmental Science & Technology

Figure 6. Confocal images of GO and RGO (fluorescent green areas) adsorbed to sand surface (dark areas) showing different extents of adsorption as affected by different cations: (a) GO in 20 mM NaCl; (b) GO in 20 mM KCl; (c) GO in 20 mM CsCl; (d) 33

ACS Paragon Plus Environment

Environmental Science & Technology

RGO in 20 mM NaCl; (e) RGO in 20 mM KCl; (f) RGO in 20 mM CsCl; (g) GO in 0.5 mM MgCl2; (h) GO in 0.5 mM CaCl2; (i) GO in 0.5 mM BaCl2; (j) RGO in 0.5 mM MgCl2; (k) RGO in 0.5 mM CaCl2; and (l) RGO in 0.5 mM BaCl2.

34

ACS Paragon Plus Environment

Page 34 of 34