Transport of Sulfide-Reduced Graphene Oxide in ... - ACS Publications

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Transport of Sulfide-Reduced Graphene Oxide in Saturated Quartz Sand: Cation-Dependent Retention Mechanisms Tianjiao Xia, John D. Fortner, Dongqiang Zhu, Zhichong Qi, and Wei Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02349 • Publication Date (Web): 08 Sep 2015 Downloaded from http://pubs.acs.org on September 14, 2015

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 33

Environmental Science & Technology

1

2

Transport of Sulfide-Reduced Graphene Oxide

3

in Saturated Quartz Sand: Cation-Dependent

4

Retention Mechanisms

5 Tianjiao Xia,1 John D. Fortner,2 Dongqiang Zhu,3 Zhichong Qi,1 Wei Chen1*

6 7 8

1

College of Environmental Science and Engineering/Ministry of Education Key

9

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

10

Environmental Remediation and Pollution Control, Nankai University, Tianjin 300071,

11

China

12

2

Department of Energy, Environmental and Chemical Engineering, Washington

13 14 15

University in St. Louis, St. Louis, MO 63130, USA 3

State Key Laboratory of Pollution Control and Resource Reuse/School of the Environment, Nanjing University, Jiangsu 210093, China

16 17

Manuscript prepared for Environmental Science & Technology

18 19

*To whom correspondence may be addressed: (Phone/fax) 86-22-6622-9517; (e-mail)

20

[email protected]

21

1 ACS Paragon Plus Environment


22

Page 2 of 33

ABSTRACT

23

We describe how the reduction of graphene oxide (GO), via environmentally

24

relevant pathways, affects its transport behavior in porous media. Two sulfide-reduced

25

GOs (RGOs), prepared by reducing 10 mg/L GO with 0.1 mM Na2S for 3 and 5 d,

26

respectively, exhibited lower mobility than parent GO in saturated quartz sand.

27

Interestingly, decreased mobility cannot simply be attributed to the increased

28

hydrophobicity and aggregation upon GO reduction, as the retention mechanisms of

29

RGOs were highly cation-dependent. In the presence of Na+ (a representative monovalent

30

cation) the main retention mechanism was deposition in secondary energy minimum.

31

However, in the presence of Ca2+ (a model divalent cation) cation bridging between RGO

32

and sand grains became the most predominant retention mechanism; this was because

33

sulfide reduction markedly increased the amount of hydroxyl groups—a strong

34

metal-complexing moiety—on GO. When Na+ was the background cation, increasing pH

35

(which increased accumulation of large hydrated Na+ ions on grain surface) and the

36

presence of Suwannee River humic acid (SRHA) significantly enhanced transport of

37

RGO, mainly via steric hindrance. However, pH and SRHA had little effect when Ca2+

38

was the background cation, because neither affected the extent of cation bridging that

39

controlled particle retention. These findings highlight the significance of abiotic

40

transformations on the fate and transport of GO in aqueous systems.

41


Page 3 of 33

42 43


INTRODUCTION Graphene oxide (GO) is a novel carbonaceous nanomaterial that has shown great

44

promise in many areas of applications, ranging from energy-related materials to

45

biomedicines.1 The rapidly increasing production and use of GO increases the possibility

46

of its environmental release, with unknown implications. Correspondingly, the fate,

47

transport, and negative environmental effects of GO have received much attention.2,3 One

48

important characteristic of GO is that it contains a significant amount of surface

49

O-functional groups.4-7 The high surface O-content makes GO hydrophilic and easily

50

dispersible in aqueous solutions.8-12 It has been shown that colloidal GO can be highly

51

mobile in porous media.13-20 Furthermore, mobility of GO in saturated porous media is

52

strongly dependent on ionic strength,13-15,17-19 but less sensitive to pH,15,18,19 and the

53

presence of natural organic matter generally enhances the transport of GO.16,18,19

54

Graphene oxide is likely reduction-sensitive in aquatic environments. This is

55

evidenced by a large number of studies aimed to achieve chemical reduction of GO.21-25

56

In our previous studies, we observed that GO can be reduced by low concentrations of

57

sulfide under environmentally relevant conditions; such reduction resulted in significant

58

alteration of the types and distribution of its surface O-functionality, making the reduced

59

GO (RGO) relatively less hydrophilic and more prone to aggregation than parent GO.26,27

60

Chemical reduction of GO in aquatic environments may significantly affect its transport

61

properties in subsurface, but this has not been directly investigated and the controlling

62

mechanisms will likely be complicated. For example, we found that even though sulfide

63

reduction resulted in an overall loss of surface O-content of GO, effective surface charge

64

was not significantly affected.26,27 It is noteworthy that when partially reduced, a portion 3 ACS Paragon Plus Environment


Page 4 of 33

65

of the epoxy groups of GO was converted to hydroxyl and carboxyl groups.26,27 This can

66

be a key alteration affecting the nature of interactions between GO nanosheets and porous

67

media, cations, and dissolved organic matters. It is possible that such chemical reduction

68

may significantly affect not only the retention mechanisms of GO in saturated porous

69

media, but also how transport properties of GO will respond to the changes of solution

70

chemistry conditions. Additionally, RGO can form aggregates more easily than GO,26,27

71

and this may affect particle−collector interactions and may enhance particle retention via

72

straining.

73

The overall objective of this study is to understand the effects of environmental

74

reduction of GO on its transport properties in saturated porous media. The specific goals

75

are to identify the primary mechanisms controlling the retention of sulfide-reduced GOs

76

in saturated quartz sand as affected by several important solution chemistry parameters,

77

and to link observed differences in transport properties between RGOs and GO to the

78

reduction-induced changes of surface chemistry of GO. Two Na2S-reduced GOs were

79

prepared to represent different degrees of reduction. Column experiments of the RGOs

80

and GO were conducted under varied solution chemistry conditions, including different

81

concentrations of monovalent and divalent cations (using Na+ and Ca2+ as the model

82

cations), different pH, and the presence of Suwannee River humic acid (SRHA, selected

83

as a model dissolved organic matter). Specific mechanisms (and their relative

84

contributions) controlling the retention of RGOs were analyzed and found to be highly

85

dependent on the types of cations present. The environmental implications of the findings

86

are discussed.

87 4 ACS Paragon Plus Environment

Page 5 of 33

88


MATERIALS AND METHODS

89

Materials. Quartz sand (50–70 mesh, 0.21–0.30 mm) was purchased from

90

Sigma–Aldrich (St. Louis, MO); the average grain size was 0.26 mm. The sand was

91

pretreated before use.28 It was first washed with 0.1 M HCl and then with 5% H2O2. Next,

92

it was rinsed repeatedly with deionized (DI) water until neutral pH was reached. Then, it

93

was oven-dried at 90 °C overnight and stored for future use. Suwannee River humic acid

94

was purchased from the International Humic Substance Society (St. Paul, MN), and was

95

reported to be composed of 52.6% C (wt:wt), 4.3% H, 42.0% O, and 1.2% N.18 The

96

distribution of functional groups, determined with 13C-nuclear magnetic resonance, was

97

carboxylic (15%), aromatic (31%), aliphatic (29%), and carbonyl (6%). In this paper the

98

concentrations of SRHA are expressed as mg SRHA per liter of solution.

99

Preparation and characterization of GO and RGOs. Graphene oxide was

100

synthesized using a modified Hummers method.29 The detailed procedures are described

101

in Supporting Information (SI). To obtain sulfide-reduced GOs, an aqueous suspension of

102

~10 mg/L GO in 10 mM Tris buffer was first purged with N2 for 40 min to remove

103

dissolved O2. The pH of the suspension was adjusted to neutral using 0.1 M HCl. Then, a

104

stock solution of Na2S was added to give a Na2S concentration of 0.1 mM in the

105

suspension. Next, a full volume of the suspension was added to 200-mL glass vials, and

106

the suspension was mixed on an orbital shaker incubated at 30.0 ± 0.5 °C for 3 or 5 d.

107

Then the suspension was filtered through 0.22-µm membrane filters. The RGO material

108

retained on the filter was collected and added to approximately 200 mL DI water, and

109

was ultra-sonicated at 100 W for 30 min. The rinsing procedure was repeated three times.

110

Finally, the suspension was filtered through 0.45-µm membrane filters to remove large 5 ACS Paragon Plus Environment


111

RGO aggregates. The sample undertaken the 3-d reduction is termed RGO3 and that

112

undertaken the 5-d reduction is referred to as RGO5.

113

Page 6 of 33

Surface elemental compositions of RGO/GO samples were determined by X-ray

114

photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe, Japan). Fourier transform

115

infrared (FTIR) transmission spectra were obtained using a 110 Bruker TENSOR 27

116

apparatus (Bruker Optics Inc., Germany). Raman spectra were recorded with a Renishew

117

inVia Raman spectrometer (RM2000, UK). Physical dimensions of the samples were

118

characterized by atomic force microscopy (AFM) (MMAFM/STM, D3100M, Digital

119

Ltd.).

120

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

121

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

122

10-µm stainless-steel screens (Valco Instruments Inc., Houston, TX) on both ends. Each

123

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

124

cm. The columns were operated in an upward direction using syringe pumps (KD

125

Scientific, Holliston, MA). The sand packed columns were equilibrated by flushing with

126

100 mL DI water at a flow rate of 3 mL/h followed by 180 mL background electrolyte

127

solution. Column properties are given in SI Table S1.

128

All the influents were prepared immediately before the column experiments by

129

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

130

The particle size distribution and ζ potential of RGO/GO nanoparticles in the influents

131

were measured by dynamic light scattering (DLS) and electrophoretic mobility,

132

respectively, using a ZetaPALS (Brookhaven Instruments, Holtsville, NY). Aggregation

133

properties of the RGO/GO suspensions were examined with a JEM-2100 transmission 6 ACS Paragon Plus Environment

Page 7 of 33


134

electron microscope (TEM) (JEOL), and the samples were prepared by air-drying a drop

135

of suspension onto a copper TEM grid (Electron Microscopy Sciences). UV absorbance

136

spectra of the suspensions were obtained with a UV/vis spectrophotometer (UV-2401,

137

Shimadzu Scientific Instruments, Columbia, MD). The scan was performed in the

138

wavelength range of 200−600 nm. The slit width and sample interval were set at 1 and

139

0.2 nm, respectively. No apparent changes in aggregation state was observed for any

140

influents in the duration of respective column experiments.

141

In a typical column experiment, the influent (containing approximately 13 mg/L of

142

RGO/GO) was pumped into the column from a 100-mL glass syringes (SGE Analytical

143

Science, Victoria, Australia). Column effluent samples were collected in 4-mL glass vials

144

every 2–3 pore volumes (PV) to determine the concentrations of RGO/GO. The

145

concentrations of RGO/GO in the influent (C0) and effluent (C) were determined by

146

measuring the UV absorbance at 249 nm (for RGOs) or 230 nm (for GO) (SI Figure S1),

147

based on a pre-established calibration curve of RGO/GO (SI Figure S2).30 In the presence

148

of SRHA (5 mg/L), the concentrations of RGO/GO were determined using the method of

149

Chen et al.,31 by obtaining the calibration curve of UV absorbance of RGO/GO as a

150

function of RGO/GO concentration in the presence of 5 mg/L SRHA (SI Figure S3).

151

Selected experiments were repeated to ensure data reproducibility (SI Figure S4).

152

DLVO Calculations. The particle–collector interaction energy profiles under

153

different solution chemistry conditions were calculated using the

154

Derjaguin−Landau−Verwey−Overbeek (DLVO) theory and extended DLVO theory (see

155

detailed equations and parameters in SI). The interaction energy profiles and the



156

associated values of the maximum energy barriers and secondary minimum depth are

157

given in SI Figures S5−S9 and Tables S2 and S3.

Page 8 of 33

158 159 160

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

161

and GO are summarized in Table 1. In general, the data show varied degrees of reduction

162

upon sulfide treatment of GO – the C/O ratios of RGOs are 40% and 63% higher than

163

that of GO. Sulfide reduction resulted in significant changes in the distribution of surface

164

O-functionality, as indicated by the deconvoluted peaks of C1s spectra that correspond to

165

carbon atoms with different chemical states (Table 1 and SI Figure S10). For example, a

166

decrease of the C−O carbon atoms and increase of carboxyl (O−C=O) carbon atoms were

167

observed upon sulfide reduction. Reduction-induced changes in surface O-functional

168

groups are also evident from the decrease of the epoxy groups (C−O−C stretching band at

169

~1050 cm-1) and the increase of hydroxyl groups (O−H bending band at ~1385 cm-1) in

170

FTIR spectra (SI Figure S11).32,33 It has been proposed that reduction of GO starts from

171

the ring-opening of the epoxy groups that forms hydroxyl groups,34,35 and given the

172

relatively mild reducing conditions in this study, dehydroxylation was likely incomplete,

173

resulting the observed increase of hydroxyl groups. Furthermore, Raman spectra (SI

174

Figure S12) show that the IG/ID values of RGOs are larger than that of GO (Table 1),

175

indicating restoration of the sp2 network from sulfide reduction.36 The AFM images (SI

176

Figure S13) show that sulfide reduction did not affect the areal dimensions and thickness

177

of GO nanosheets significantly.


Page 9 of 33


178

In Table 2 ζ potential and average particle size (Zave) values of RGO/GO

179

nanoparticles under various aqueous chemistries are presented. Aggregation state of

180

RGOs and GO under selected solution chemistry conditions is illustrated with the TEM

181

images (SI Figure S14). Under given aqueous chemistry conditions, the ζ potential values

182

of both RGO3 and RGO5 are not significantly different from the respective value of GO,

183

even though reduction of GO resulted in considerable changes in the type and abundance

184

of O-functional groups. This was possibly attributable to the relatively high pKa range

185

(9–10) of phenolic groups on the surfaces of carbonaceous materials.37 However, the Zave

186

values follow the order of RGO5 > RGO3 > GO consistently over conditions studied.

187

Enhanced aggregation of RGOs compared with GO was likely due to the increased

188

hydrophobicity of nanosheets upon sulfide reduction (as evidenced by the increased C/O

189

ratios and Raman spectra);26,27 in other words, RGO nanosheets with more hydrophobic

190

graphitic lattice tended to undergo layer-to-layer aggregation in aqueous solutions.

191

Furthermore, increasing cation concentration resulted in more significant aggregation of

192

RGO3 and RGO5 than GO (SI Figure S15). This was particularly evident for Ca2+,

193

because RGOs contained greater amounts of surface –OH groups, allowing aggregation

194

of nanosheets via a cation bridging mechanism as reported for negatively charged

195

particles.3,8,38-42

196

Effect of Monovalent Cation on Transport. Sodium (used as a model monovalent

197

cation) had more significant effects on the transport of RGOs than GO. The breakthrough

198

profiles of the two RGOs and GO over four different NaCl concentrations (Figure 1)

199

show that at 5 mM Na+ transport of the two RGOs were only slightly weaker than that of

200

GO. With the increase of Na+ concentration, the transport of RGOs became increasingly 9 ACS Paragon Plus Environment


201

more inhibited compared with the transport of GO. At 35 mM Na+, essentially no

202

breakthrough of RGOs was observed, even though the transport of GO was still

203

significant (C/C0, the ratio of effluent concentration to influent concentration, reached

204

76% after 28 PV).

205

Page 10 of 33

Increasing cation concentration compresses double layer thickness and reduces

206

double layer repulsion between nanoparticles and grain surfaces;8,43,44 additionally, the

207

secondary energy minimum between particles and collector becomes deeper.15,45-48 The

208

DLVO profiles of RGOs and GO (SI Figures S5 and S8) show that in this study

209

increasing Na+ concentration had much larger effects on the depth of secondary minimum

210

(Φsec) of RGOs than GO. At 5 mM Na+ the differences in Φsec value between the two

211

RGOs and GO are very small (also see Table S2). However, the differences become

212

increasingly larger with the increase of Na+ concentration (SI Figure S15). This trend is

213

consistent with the much more inhibited transport of RGOs than GO at a Na+

214

concentration of 10 mM and above. Moreover, because of the relatively larger particle

215

sizes of RGOs compared to GO (refer to the Zave values in Table 2 and Figure S15) at

216

elevated Na+ concentrations (e.g., 20 and 35 mM), straining could have played an

217

important role in the retention of RGOs. It is commonly assumed that particles may be

218

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

219

diameters of particle and collector, respectively) is above 0.002–0.003.15,49-51 Based on

220

the dp/dc values in Table 2, straining was likely a viable retention mechanism for RGOs at

221

35 mM Na+ (or even 20 mM Na+). Furthermore, the retained particle concentration

222

profiles of RGO3 and RGO5 are more hyper-exponential in shape than that of GO (SI

223

Figure S16), which is consistent with the more significant straining for the RGOs.52 The 10 ACS Paragon Plus Environment

Page 11 of 33


224

considerably smaller particle sizes in effluents, in comparison to the respective sizes in

225

influents, may also be indicative of straining (SI Figure S17).

226

To understand the relative contributions of different retention mechanisms at high

227

Na+ concentrations, step-wise flushing using electrolyte solutions of decreasing ionic

228

strength was carried out for Column 11 (RGO3 at 35 mM Na+) and Column 12 (RGO5 at

229

35 mM Na+). The observed release of retained RGO during the flushing (SI Figure S18)

230

reflects the contribution of the secondary minimum.44 Results show that even at 35 mM

231

Na+ deposition at the secondary minimum energy well was still the most important

232

retention mechanism for RGOs—accounting for 76% overall retention of RGO3 and 75%

233

of RGO5—and the contribution of straining due to increased aggregation was relatively

234

small.

235

Effects of Divalent Cation on Transport. Calcium (a model divalent cation) also

236

influenced the transport of RGO much more significantly than that of GO, which became

237

more significant with the increase of Ca2+ concentration (Figure 2). At 0.1 mM Ca2+, the

238

breakthrough profile of RGO5 overlapped with that of GO in the initial phase and over

239

93% breakthrough was reached after approximately 10 PV; at 0.3 mM Ca2+ the maximum

240

breakthrough of RGO5 only reached 55%, much lower than the 93% of GO; at 0.5 mM

241

Ca2+ essentially no breakthrough of RGO5 was observed, whereas the breakthrough of

242

GO reached near 86%.

243

Interestingly, the much more remarkable transport inhibition of Ca2+ on RGO5 than

244

on GO cannot be explained with the DLVO theory. The differences in the DLVO profiles

245

between RGO5 and GO—in particular, deepened secondary minimum well—are small

246

(SI Figure S6), even at 0.5 mM Ca2+ (note that the Φsec value of RGO5 at 0.5 mM Ca2+ is 11 ACS Paragon Plus Environment


Page 12 of 33

247

only -0.0327 KBT, even smaller than the value of RGO5 at 5 mM Na+). More importantly,

248

the contribution of secondary minimum to the overall retention of RGO5—determined by

249

flushing the columns with low-ionic strength solution and DI water—was only 8.0% for

250

Column 16 (0.3 mM Ca2+) and 7.5% for Column 18 (0.5 mM Ca2+). This was largely in

251

contrast with the important contribution of secondary minimum when Na+ was the

252

background cation. Likewise, straining was probably not an important cause for the more

253

significant transport inhibition of RGO by Ca2+. The Zave values of RGO5 are 585.7 nm at

254

0.3 mM Ca2+ and 908.8 nm at 0.5 mM Ca2+ (Table 2), both are smaller than the Zave value

255

of GO at 35 mM Na+ (1113 nm). Since the much larger GO could pass through the

256

column relatively easily (Figure 1d), it was unlikely the degree of straining was

257

significant for the transport of RGO5 in the presence of 0.3 or 0.5 mM Ca2+.

258

Based on the discussion above, we propose that the much stronger transport

259

inhibition effects of Ca2+ on RGO5 than GO was likely attributable to the stronger

260

particle−collector bridging effects of Ca2+ with RGO5. Cation bridging has been

261

recognized as an important retention mechanism for negatively charged

262

nanoparticles.8,40,43,53 Compared with GO, RGO5 contained a greater amount of surface

263

–OH group, which is a strong metal-complexing moiety, both in its neutral and

264

deprotonated forms.54 Thus, in the presence of Ca2+, RGO5 could bind to the sand grains

265

through Ca2+ bridging (i.e., Ca2+ serves as the bridging agent between surface

266

O-functional groups of sand grains and –OH groups of RGO5) more strongly than GO.

267

The stronger binding of RGOs than GO to sand grains in the presence of Ca2+ was

268

confirmed with adsorption experiments (see SI for detailed procedures), which showed


Page 13 of 33


269

that adsorption coefficients of GO, RGO3, and RGO5 to sand were 7.59, 11.5, and 13.8

270

L/kg, respectively.

271

Effects of pH on Transport. The effects of pH on the transport of RGOs (tested

272

with RGO5) were largely dependent on the type of cations present in the background

273

solution. Over a pH range of 5 to 9, transport of RGO5 increased drastically with

274

increasing pH when Na+ was the predominant cation; but counterintuitively, increasing

275

pH had very little effect on the transport of RGO5 when Ca2+ was the predominant cation

276

(Figure 3). For example, at 20 mM Na+, the maximum breakthrough of RGO5 only

277

reached 11% at pH 5, but at pH 7 and 9 the maximum breakthrough reached 61% and

278

76%, respectively. At 5 mM Na+, the effect of pH was less remarkable, but still exhibited

279

the same trend. In comparison, at 0.3 mM Ca2+ the breakthrough curves of RGO5 at pH

280

4.5, 5.5 and 6.5 nearly overlap.

281

Interestingly, at any given Na+ concentration ζ potential did not change appreciably

282

with pH (Table 2), and the DLVO profiles (SI Figures S7 and S9) show that pH had very

283

small effects on the depth of secondary minimum. Even though small decrease in Zave

284

with increasing pH was observed at 5 and 20 mM Na+ (likely due to the deprotonation of

285

O-functional groups), it likely had negligible effect on transport because straining was

286

not a major retention mechanism. As Na+ is a weak complexing agent that does not

287

induce strong bridging (as can Ca2+), varying pH could not have affected the deposition

288

of RGO5 through bridging. Based on the fact that the pH effect became more significant

289

with the increase of Na+ concentration (Figures 3a−c), we propose that pH affected the

290

transport of RGO5 mainly by affecting the electrostatic double layer properties of sand

291

grains. That is, the surface charge of sand grains became more negative with increasing 13 ACS Paragon Plus Environment


Page 14 of 33

292

pH (the ζ potential values are -38 mV, -50 mV, and -55 mV at pH 5, 7, and 9,

293

respectively55), and this resulted in increased accumulation of Na+ near the grain surface.

294

In aqueous solutions, Na+ ions exist as densely hydrated ions, and accumulate on grain

295

surfaces via outer-sphere complexation. The accumulation of large hydrated Na+ ions

296

likely hindered the interaction of RGO5 with grain surface (mainly by steric hindrance),

297

and therefore, facilitated the transport of RGO. The higher the Na+ concentration, the

298

more significant the accumulation of hydrated Na+ ions on grain surface, and

299

consequently, the more pronounced transport enhancement.

300

The fact that pH had little effect on the transport of RGO5 when Ca2+ was the

301

predominant cation is consistent with the above mentioned cation-bridging mechanism by

302

Ca2+. Increasing pH could also enhance the accumulation of Ca2+ near the grain surface,

303

in a similar manner as for Na+. However, the increased accumulation of Ca2+ (in

304

particular, those adsorbed on the grain surface) might also enhance the deposition of

305

RGO5 through enhanced cation bridging.

306

Effects of SRHA on Transport. The presence of SRHA enhanced the transport of

307

RGO5, even under solution chemistry conditions extremely unfavorable for transport;

308

however, the significance of transport enhancement by SRHA was also largely dependent

309

on the type of cations present in the background solution (Figure 4). When Na+ was the

310

background cation, transport-enhancement effect of SRHA was very significant. For

311

example, in the absence of SRHA the transport of RGO5 at 35 mM Na+ was completely

312

inhibited, but in the presence of 5 mg/L SRHA, breakthrough of RGO5 reached 85%

313

after 18 PV (Figure 4b). Because the presence of SRHA had negligible effects on ζ

314

potential and sizes of RGO5 nanoparticles (Table 2), SRHA likely enhanced the transport 14 ACS Paragon Plus Environment

Page 15 of 33


315

of RGO5 via steric hindrance.3,38,56-59 That is, adsorption of SRHA to RGO5 and to the

316

surfaces of quartz sand interfered with the interaction between RGO5 and quartz sand,

317

and thus, inhibiting the deposition of RGO5. To further understand the relative

318

importance of SRHA coating on quartz sand versus SRHA adsorption on RGO5,

319

additional experiments were conducted by first saturating the sand with SRHA solution

320

before flushing RGO5 suspension through the column.60 In this case, only the sand was

321

coated with SRHA, whereas adsorption of SRHA to RGO5 was negligible. The transport

322

of RGO5 through the SRHA-saturated columns was significantly enhanced compared

323

with the transport of RGO5 in the absence of SRHA but still considerably lower than the

324

transport of RGO5 in the presence of SRHA (Figures 4b−c). Judging from the relative

325

positions of the breakthrough curves, it was likely that both SRHA adsorption on sand

326

grains and adsorption to nanoparticles contributed to the transport-enhancement effects of

327

SRHA.

328

An interesting (and surprising) observation in this set of experiments was that SRHA

329

was much less effective in mitigating the transport-inhibiting effects of divalent cation

330

(i.e., Ca2+) than those of monovalent cation (i.e., Na+). This appears to be related to the

331

different transport-inhibiting mechanisms between Ca2+ and Na+. As discussed earlier,

332

Na+ enhanced the deposition of RGO mainly by deepening the secondary minimum well,

333

and adsorption of SRHA could greatly weaken this deposition mechanism through steric

334

hindrance. Ca2+, however, enhanced the retention of RGO primarily by serving as a

335

bridging agent between RGO and sand grains. Thus, even when quartz sands were coated

336

with SRHA, binding of RGO could still occur through bridging. In this case, the bridging

337

would be in the form of RGO−Ca−SRHA (instead of RGO−Ca−sand), because SRHA is 15 ACS Paragon Plus Environment


338

also rich in surface O-functional groups and able to complex with divalent

339

cations.39,40,56,61,62

340

Page 16 of 33

Environmental Implications. Given its reactive nature, GO likely can be reduced

341

in aquatic environments by naturally occurring reductants such as sulfide, among others,

342

resulting in increased hydrophobicity with corresponding changes in surface O-functional

343

groups. As demonstrated here and in our previous study, reduction of GO will likely

344

reduce its colloidal stability and consequently, mobility in the subsurface. While the

345

reduced mobility is more or less expected, the underlying mechanisms are not

346

straightforward and in some cases counterintuitive. The most important observation in

347

this study was that because RGOs contained relatively higher amounts of surface

348

hydroxyl groups than GO, their transport behaviors deviated more significantly from the

349

DLVO theory in the presence of Ca2+ (i.e., 0.3 mM and above). Additionally, the sulfide

350

reduction-induced changes in the types and distribution of GO surface O-functional

351

groups significantly altered the underlying mechanisms via which solution chemistry

352

(e.g., pH and dissolved organic matter) influenced the transport of GO, leading to

353

strikingly different effects depending on the type of predominant cations in the solution.

354

Overall, these findings clearly highlight the significance of identifying, understanding,

355

and delineating relevant GO transformation pathways as they relate to the transport, fate,

356

and effects, thus ultimate sustainability, of this novel class of carbon nanomaterials. Note

357

that the GO products used in this study were of relatively uniform sizes, and more

358

research is needed to understand how size effects of GO may interplay with GO surface

359

O-functionalities to affect the overall transport properties.

360 16 ACS Paragon Plus Environment

Page 17 of 33


361

Supporting Information. Procedures used to synthesize GO and prepare SRHA

362

solutions, calculation of DLVO and extended DLVO interaction energy, determination of

363

binding affinities of RGO/GO to sand, column properties, determination of RGO/GO

364

concentrations with UV/Vis spectrometry, data reproducibility results, particle–collector

365

interaction energy profiles, XPS, FTIR and Raman spectra of RGOs and GO, AFM and

366

TEM images, retained particle profiles, changes of particle sizes during transport,

367

column-flushing results and mass balance. This material is available free of charge via

368

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

369 370

Acknowledgments. This project was supported by the Ministry of Science and

371

Technology of China (Grant 2014CB932001), and the National Natural Science

372

Foundation of China (Grants 21425729 and 21237002).

373 374

REFERENCES

375

1.

376

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

377

2.

378

3815−3835.

379

3.

380

Adsorption, dispersion, toxicity and transformation. Environ. Sci. Technol. 2014, 48 (17),

381

9995−10009.

382

4.

383

Structural evolution during the reduction of chemically derived graphene oxide. Nature

384

Chem. 2010, 2 (7), 581−587.

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

Hu, X.; Zhou, Q. Health and ecosystem risks of graphene. Chem. Rev. 2013, 113 (5),

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

Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B.



Page 18 of 33

385

5.

Bai, H.; Li, C.; Wang, X.; Shi, G. On the gelation of graphene oxide. J. Phys. Chem.

386

C 2011, 115 (13), 5545–5551.

387

6.

388

nitrogen doping and reduction of graphene oxide. J. Am. Chem. Soc. 2009, 131 (43),

389

15939–15944.

390

7.

391

versus kinetics. J. Phys. Chem. C 2011, 115 (24), 11991–11995.

392

8.

393

Colloidal properties and stability of graphene oxide nanomaterials in the aquatic

394

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

395

9.

396

Deposition and release of graphene oxide nanomaterials using a quartz crystal

397

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

398

10. Konkena, B.; Vasudevan, S. Understanding aqueous dispersibility of graphene oxide

399

and reduced graphene oxide through pKa measurements. J. Phys. Chem. Lett. 2012, 3 (7),

400

867−872.

401

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

402

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

403

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

404

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

405

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

406

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

Li, X.; Wang, H.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H. Simultaneous

Lu, N.; Yin, D.; Li, Z.; Yang, J. Structure of graphene oxide: Thermodynamics

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

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


Page 19 of 33


407

13. Fan, W.; Jiang, X.; Yang, W.; Geng, Z.; Huo, M.; Liu, Z.; Zhou, H. Transport of

408

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

409

electrolyte systems. Sci. Total Environ. 2015, 511, 509–515.

410

14. Feriancikova, L.; Xu, S. Deposition and remobilization of graphene oxide within

411

saturated sand packs. J. Hazard. Mater. 2012, 235−236, 194−200.

412

15. Lanphere, J. D.; Luth, C. J.; Walker, S. L. Effects of solution chemistry on the

413

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

414

(9), 4255−4261.

415

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

416

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

417

Eng. Sci. 2014, 31 (7), 350−359.

418

17. Lin, L.; Gao, B.; Wu, L.; Morales, V. L.; Yang, L.; Zhou, Z.; Wan, H. Deposition

419

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

420

2013, 229, 444–449.

421

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

422

graphene oxide nanoparticles in saturated sand columns. Environ. Toxicol. Chem. 2014,

423

33 (5), 998−1004.

424

19. Qi, Z.; Zhang, L.; Chen, W. Transport of graphene oxide nanoparticles in saturated

425

sandy soil. Environ. Sci. Proc. Impacts 2014, 16 (10), 2268−2277.

426

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

427

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

428

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



Page 20 of 33

429

21. Bose, S.; Kuila, T.; Mishra, A. K.; Kim, N. H.; Lee, J. H. Dual role of glycine as a

430

chemical functionalizer and a reducing agent in the preparation of graphene: An

431

environmentally friendly method. J. Mater. Chem. 2012, 22 (19), 9696–9703.

432

22. Chandra, V.; Park, J.; Chun, Y.; Lee, J. W.; Hwang, I.; Kim, K. S. Water-dispersible

433

magnetite-reduced graphene oxide composites for arsenic removal. ACS Nano 2010, 4 (7),

434

3979–3986.

435

23. Chen, W.; Yan, L.; Bangal, P. R. Chemical reduction of graphene oxide to graphene

436

by sulfur-containing compounds. J. Phys. Chem. C 2010, 114 (47), 19885–19890.

437

24. Fan, Z.; Wang, K.; Wei, T.; Yan, J.; Song, L.; Shao, B. An environmentally friendly

438

and efficient route for the reduction of graphene oxide by aluminum powder. Carbon

439

2010, 48 (5), 1686–1689.

440

25. Fernandez-Merino, M. J.; Guardia, L.; Paredes, J. I.; Villar-Rodil, S.;

441

Solis-Fernandez, P.; Martinez-Alonso, A.; Tascon, J. M. D. Vitamin C is an ideal

442

substitute for hydrazine in the reduction of graphene oxide suspensions. J. Phys. Chem. C

443

2010, 114 (14), 6426–6432.

444

26. Fu, H.; Qu, X.; Chen, W.; Zhu, D. Transformation and destabilization of graphene

445

oxide in reducing aqueous solutions containing sulfide. Environ. Toxicol. Chem. 2014, 33

446

(12), 2647− 2653.

447

27. Wang, F.; Wang, F.; Zhu, D.; Chen, W. Effects of sulfide reduction on adsorption

448

affinities of colloidal graphene oxide nanoparticles for phenanthrene and 1-naphthol.

449

Environ. Pollut. 2015, 196, 371–378.


Page 21 of 33


450

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

451

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

452

2011, 45 (22), 9765–9775.

453

29. Duch, M. C.; Budinger, G. R. S.; Liang, Y. T.; Soberanes, S.; Urich, D.; Chiarella, S.

454

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

455

Minimizing oxidation and stable nanoscale dispersion improves the biocompatibility of

456

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

457

30. Wang, G.; Wang, B.; Park, J.; Yang, J.; Shen, X.; Yao, J. Synthesis of enhanced

458

hydrophilic and hydrophobic graphene oxide nanosheets by a solvothermal method.

459

Carbon 2009, 47 (1), 68–72.

460

31. Chen, G.; Liu, X.; Su, C. Distinct effects of humic acid on transport and retention of

461

TiO2 rutile nanoparticles in saturated sand columns. Environ. Sci. Technol. 2012, 46 (13),

462

7142–7150.

463

32. Li, M.; Cushing, S. K.; Zhou, X.; Guo, S.; Wu, N. Fingerprinting photoluminescence

464

of functional groups in graphene oxide. J. Mater. Chem. 2012, 22 (44), 23374–23379.

465

33. Szabó, T.; Berkési, O.; Forgó, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dékány, I.

466

Evolution of surface functional groups in a series of progressively oxidized graphite

467

oxides. Chem. Mater. 2006, 18 (11), 2740–2749.

468

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

469

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

470

114 (2), 832–842.

471

35. Kim, M. C.; Hwang, G. S.; Ruoff, R. S. Epoxide reduction with hydrazine on

472

graphene: A first principles study. J. Chem. Phys. 2009, 131 (6), 064704. 21 ACS Paragon Plus Environment


Page 22 of 33

473

36. Paredes, J. I.; Alonso, A. M.; Yamazaki, T.; Matsuoka, K.; Tascon, J.; Kyotani, T.

474

Structural investigation of zeolite-templated, ordered microporous carbon by scanning

475

tunneling microscopy and Raman spectroscopy. Langmuir 2005, 21 (19), 8817–8823.

476

37. Álvarez-Merino, M. A.; Fontecha-Cámara, M. A.; López-Ramón, M. V.;

477

Moreno-Castilla, C. Temperature dependence of the point of zero charge of oxidized and

478

non-oxidized activated carbons. Carbon 2008, 46, 778−787.

479

38. Chen, K. L.; Mylon, S. E.; Elimelech, M. Aggregation kinetics of alginate-coated

480

hematite nanoparticles in monovalent and divalent electrolytes. Environ. Sci. Technol.

481

2006, 40 (5), 1516–1523.

482

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

483

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

484

5370−5375.

485

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

486

natural organic matter: Role of divalent cations. J. Colloid Interface Sci. 2009, 338 (1),

487

1−9.

488

41. Zhang, J.; Terracciano, A.; Meng, X. Comment on "Colloidal properties and stability

489

of graphene oxide nanomaterials in the aquatic environment". Environ. Sci. Technol.

490

2014, 48 (2), 1359–1359.

491

42. Park, S.; Lee, K. S.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. Graphene

492

oxide papers modified by divalent ions-enhancing mechanical properties via chemical

493

cross-linking. ACS Nano 2008, 2 (3), 572−578.


Page 23 of 33


494

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

495

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

496

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

497

44. Ryan, J. N.; Elimelech, M. Colloid mobilization and transport in groundwater.

498

Colloids Surf. A 1996, 107, 1–56.

499

45. Franchi, A.; O’Melia, C. R. Effects of natural organic matter and solution chemistry

500

on the deposition and reentrainment of colloids in porous media. Environ. Sci. Technol.

501

2003, 37 (6), 1122−1129.

502

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

503

porous media under unfavorable chemical conditions: Some concepts and applications.

504

Environ. Sci. Technol. 2004, 38 (1), 210−220.

505

47. Tufenkji, N.; Elimelech, M. Deviation from the classical colloid filtration theory in

506

the presence of repulsive DLVO interactions. Langmuir 2004, 20 (25), 10818−10828.

507

48. Tufenkji, N.; Elimelech, M. Breakdown of colloid filtration theory: Role of the

508

secondary energy minimum and surface charge heterogeneities. Langmuir 2005, 21 (3),

509

841–852.

510

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

511

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

512

3012–3024.

513

50. Fang, J.; Xu, M.; Wang, D.; Wen, B.; Han, J. Modeling the transport of TiO2

514

nanoparticle aggregates in saturated and unsaturated granular media: Effects of ionic

515

strength and pH. Water Res. 2013, 47 (3), 1399−1408.



Page 24 of 33

516

51. Wang, D.; Zhang, W.; Hao, X.; Zhou, D. Transport of biochar particles in saturated

517

granular media: Effects of pyrolysis temperature and particle size. Environ. Sci. Technol.

518

2013, 47 (2), 821−828.

519

52. Jiang, X.; Tong, M.; Lu, R.; Kim, H. Transport and deposition of ZnO nanoparticles

520

in saturated porous media. Colloids Surf. A 2012, 401, 29−37.

521

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

522

kinetics of multiwalled carbon nanotubes in monovalent and divalent electrolytes.

523

Langmuir 2011, 27 (7), 3588–3599.

524

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

525

York, 1996.

526

55. Kaya, A.; Yukselen, Y. Zeta potential of clay minerals and quartz contaminated by

527

heavy metals. Can. Geotech. J. 2005, 42 (5), 1280−1289.

528

56. Chowdhury, I.; Cwiertny, D. M.; Walker, S. L. Combined factors influencing the

529

aggregation and deposition of nano-TiO2 in the presence of humic acid and bacteria.

530

Environ. Sci. Technol. 2012, 46 (13), 6968−6976.

531

57. Chowdhury, I.; Duch, M. C.; Gits, C. C.; Hersam, M. C.; Walker, S. L. Impact of

532

synthesis methods on the transport of single-walled carbon nanotubes in the aquatic

533

environment. Environ. Sci. Technol. 2012, 46 (21), 11752−11760.

534

58. Hong, B. J.; Compton, O. C.; An, Z.; Eryazici, I.; Nguyen, S. T. Successful

535

stabilization of graphene oxide in electrolyte solutions: Enhancement of

536

biofunctionalization and cellular uptake. ACS Nano 2012, 6 (1), 63−73.


Page 25 of 33


537

59. Saleh, N. B.; Pfefferle, L. D.; Elimelech, M. Influence of biomacromolecules and

538

humic acid on the aggregation kinetics of single-walled carbon nanotubes. Environ. Sci.

539

Technol. 2010, 44 (7), 2412−2418.

540

60. Yang, H.; Kim, H.; Tong, M. Influence of humic acid on the transport behavior of

541

bacteria in quartz sand. Colloids Surf. B 2012, 91, 122−129.

542

61. Janjaroen, D.; Liu, Y.; Kuhlenschmidt, M. S.; Kuhlenschmidt, T. B.; Nguyen, T. H.

543

Role of divalent cations on deposition of Cryptosporidium parvum oocysts on natural

544

organic matter surfaces. Environ. Sci. Technol. 2010, 44 (12), 4519−4524.

545

62. Mylon, S. E.; Chen, K. L.; Elimelech, M. Influence of natural organic matter and

546

ionic composition on the kinetics and structure of hematite colloid aggregation:

547

Implications to iron depletion in estuaries. Langmuir 2004, 20 (21), 9000−9006.

548 549



Page 26 of 33

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

aromatic rings

epoxy/ hydroxyl

carbonyl

carboxyl

GO RGO3 RGO5

32.61 40.12 43.56

23.78 21.14 21.47

8.52 7.83 5.31

1.22 2.98 4.38

total Cb (wt%) 66.13 72.06 74.72

total Ob (wt%) 31.47 24.49 21.87

C/O ratio

IG/ID c

2.10 2.94 3.42

1.03 1.11 1.20

a

RGO represents sulfide-reduced GO; the suffix “3” or “5” indicates reduction time (d). Analyzed with X-ray photoelectron spectroscopy. c Analyzed with Raman spectrometry.

b


Page 27 of 33


Table 2. Selected Properties of RGO/GO Suspensions under Different Solution Chemistry Conditions (mV)

EPM b (10 m2/V·s)

Zave c (nm)

dp/dc d

-43.2 ± 1.3 -42.1± 0.4 -42.2 ± 0.5 -34.8 ± 0.7 -33.6 ± 0.8 -31.8 ± 1.1 -28.0 ± 0.6 -26.0 ± 0.4 -24.1 ± 1.0 -26.9 ± 0.7 -25.4 ± 0.6 -22.5 ± 1.0 -23.3 ± 1.4 -21.3 ± 0.6 -18.8 ± 0.7 -16.6 ± 0.8 -10.3 ± 0.7 -10.6 ± 1.2 -47.6 ± 1.1 -49.3 ± 0.9 -51.5 ± 0.7 -42.8 ± 0.7 -43.8 ± 2.0 -45.9 ± 1.6 -23.5 ± 0.9 -24.7 ± 0.4 -25.8 ± 0.7 -16.0 ± 1.0 -15.4 ± 0.9 -16.9 ± 1.2

-3.37 ± 0.10 -3.29± 0.03 -3.30 ± 0.04 -2.72 ± 0.05 -2.62 ± 0.06 -2.48 ± 0.09 -2.19 ± 0.05 -2.03 ± 0.03 -1.88 ± 0.08 -2.10 ± 0.05 -1.98 ± 0.05 -1.76 ± 0.08 -1.82 ± 0.11 -1.66 ± 0.05 -1.47 ± 0.05 -1.30 ± 0.06 -0.80 ± 0.05 -0.83 ± 0.09 -3.72 ± 0.09 -3.85 ± 0.07 -4.02 ± 0.05 -3.34 ± 0.05 -3.42 ± 0.16 -3.59 ± 0.12 -1.84 ± 0.07 -1.93 ± 0.03 -2.02 ± 0.05 -1.25 ± 0.08 -1.20 ± 0.07 -1.32 ± 0.09

238.5 254.0 261.6 376.6 400.2 439.6 557.4 656.6 778.2 1112.5 1377.4 1652.6 237.3 307.1 350.5 585.7 609.3 908.8 236.8 237.9 231.4 259.6 238.5 241.9 733.1 634.3 612.7 644.5 634.5 641.5

0.0009 0.0009 0.0010 0.0014 0.0015 0.0017 0.0021 0.0025 0.0030 0.0043 0.0053 0.0064 0.0009 0.0012 0.0013 0.0023 0.0023 0.0035 0.0009 0.0009 0.0009 0.0010 0.0009 0.0009 0.0028 0.0024 0.0024 0.0025 0.0024 0.0025

-25.3 ± 0.6

-1.98 ± 0.05

676.6

0.0026

35 mM NaCl+5 mg/L SRHA pH 5.7

-21.3 ± 0.9

-1.66 ± 0.07

1545.2

0.0059

RGO5

0.5 mM CaCl2+5 mg/L SRHA pH 5.3

-11.5 ± 1.0

-0.90 ± 0.08

904.2

0.0035

34 e

RGO5

35 mM NaCl SRHA-saturated column

-21.4 ± 1.1

-1.67 ± 0.09

1612.7

0.0062

35 e

RGO5

0.5 mM CaCl2 SRHA-saturated column

-11.4 ± 1.0

-0.89 ± 0.08

888.9

0.0034

Column No.

RGO/ GO a

1 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

GO RGO3 RGO5 GO RGO3 RGO5 GO RGO3 RGO5 GO RGO3 RGO5 GO RGO5 GO RGO5 GO RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5

31

RGO5

32

RGO5

33

Background solution 5 mM NaCl pH 5.8 5 mM NaCl pH 5.8 5 mM NaCl pH 5.7 10 mM NaCl pH 5.6 10 mM NaCl pH 5.7 10 mM NaCl pH 5.7 20 mM NaCl pH 5.4 20 mM NaCl pH 5.5 20 mM NaCl pH 5.6 35 mM NaCl pH 5.6 35 mM NaCl pH 5.5 35 mM NaCl pH 5.5 0.1 mM CaCl2 pH 5.3 0.1 mM CaCl2 pH 5.2 0.3 mM CaCl2 pH 5.0 0.3 mM CaCl2 pH 5.1 0.5 mM CaCl2 pH 5.1 0.5 mM CaCl2 pH 5.0 DI water pH 5.0 DI water pH 7.0 DI water pH 9.0 5 mM NaCl pH 5.0 5 mM NaCl pH 7.0 5 mM NaCl pH 9.0 20 mM NaCl pH 5.0 20 mM NaCl pH 7.0 20 mM NaCl pH 9.0 0.3 mM CaCl2 pH 4.5 0.3 mM CaCl2 pH 5.5 0.3 mM CaCl2 pH 6.5 20 mM NaCl+5 mg/L SRHA pH 5.6

ζ potentialb

-8



Page 28 of 33

a

RGO represents sulfide-reduced GO; the suffix “3” or “5” indicates reduction time (d). Values after ± sign represent standard deviation of five replicates. c Hydrodynamic diameter of RGO/GO nanoparticles based on dynamic light scattering analysis. d dp/dc represent ratio of Zave of RGO/GO aggregates to average diameter of sand grains. e Column was pre-saturated with SRHA before injecting RGO suspension. b


Page 29 of 33


(a) 5 mM NaCl

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

5

(b) 10 mM NaCl

1.0

C/C0

C/C0

1.0

10 15 20

25

30

0

5

10

PV

0.6

0.6

C/C0

C/C0

0.8

0.4

0.2

0.0

0.0 10

15 PV

30

25

30

0.4

0.2

5

25

20

(d) 35 mM NaCl

1.0

0.8

0

20

PV

(c) 20 mM NaCl

1.0

15

25

30

0

5

10

15

20

PV

GO

RGO3

RGO5

Figure 1. Effects of Na+ on transport of RGOs and GO: (a) 5 mM NaCl (Columns 1–3); (b) 10 mM NaCl (Columns 4–6); (c) 20 mM NaCl (Columns 7–9); (d) 35 mM NaCl (Columns 10–12).



(a) 0.1 mM CaCl2

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

5

10 15 20 25 30 35

(b) 0.3 mM CaCl2

1.0

C/C0

C/C0

1.0

Page 30 of 33

0

5 10 15 20 25 30 35 PV

PV (c) 0.5 mM CaCl2

1.0

C/C0

0.8 0.6 0.4 GO RGO5

0.2 0.0 0

5

10 15 20 25 30 35 PV

Figure 2. Effects of Ca2+ on transport of RGO5 and GO: (a) 0.1 mM CaCl2 (Columns 13 and 14); (b) 0.3 mM CaCl2 (Columns 15 and 16); (c) 0.5 mM CaCl2 (Columns 17 and 18).


Page 31 of 33


(a) DI water

1.0

(b) 5 mM NaCl

1.0 0.8

C/C0

0.8 C/C0

0.6 0.4

0.6 0.4

pH 5.0 pH 7.0 pH 9.0

0.2 0.0 0

5

pH 5.0 pH 7.0 pH 9.0

0.2 0.0

10 15 20 25 30 35

0

5

PV (c) 20 mM NaCl

1.0

pH 4.5 pH 5.5 pH 6.5

0.8 C/C0

C/C0

0.8

(d) 0.3 mM CaCl2

1.0

pH 5.0 pH 7.0 pH 9.0

0.6

0.6

0.4

0.4

0.2

0.2

0.0

10 15 20 25 30 35 PV

0.0 0

5

10 15 20 25 30 35

0

5

PV

10 15 20 25 30 35 PV

Figure 3. Effects of pH on transport of RGO5: (a) DI water (Columns 19–21); (b) 5 mM NaCl (Columns 22–24); (c) 20 mM NaCl (Columns 25–27); (d) 0.3 mM CaCl2 (Columns 28–30).



(a) 20 mM NaCl

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

5

10

15

20

(b) 35 mM NaCl

1.0

C/C0

C/C0

1.0

25

30

Page 32 of 33

0

5

PV

10

15

20

25

30

PV

(c) 0.5 mM CaCl2

1.0

C/C0

0.8 0.6 RGO5 RGO5 + SRHA RGO5 (SRHA-saturated column)

0.4 0.2 0.0 0

5

10

15

20

25

30

PV Figure 4. Effects of 5 mg/L Suwannee River humic acid (SRHA) on transport of RGO5: (a) 20 mM NaCl (Columns 9 and 31); (b) 35 mM NaCl (Columns 12, 32, and 34); (c) 0.5 mM CaCl2 (Columns 18, 33, and 35).


Page 33 of 33


TOC Art GO

Deepened secondary minimum RGO C/O

GO

S2-

RGO

OH

H

H O

O

Ca

O-

O

Enhanced cation-bridging

Ca O H

OH


Transport of Sulfide-Reduced Graphene Oxide in ... - ACS Publications

Recommend Documents