Exploring the Aggregation Mechanism of Graphene Oxide in the

Subscriber access provided by Kaohsiung Medical University

Environmental Processes

Exploring the Aggregation Mechanism of Graphene Oxide in the Presence of Radioactive Elements: Experimental and Theoretical Studies Yang Gao, Ke Chen, Xue Mei Ren, Ahmed Alsaedi, Tasawar Hayat, and Changlun Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02234 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018

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 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 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.

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

Graphic Abstract Aggregation mechanism of graphene oxide in the presence of radioactive elements

ACS Paragon Plus Environment


1

Exploring the Aggregation Mechanism of Graphene Oxide in the

2

Presence of Radioactive Elements: Experimental and Theoretical

3

Studies

4

Yang Gao,a,b Ke Chen,a,b Xuemei Ren,*ac Ahmed Alsaedi,d Tasawar Hayat,d and

5

Changlun Chen*acd

6

aCAS

7

Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR China

8

bUniversity

9

cCollaborative

Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of

of Science and Technology of China, Hefei 230026, PR China Innovation Center of Radiation Medicine of Jiangsu Higher Education

10

Institutions, Soochow University, Suzhou 215123, PR China

11

dNAAM

Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia

12 13

*Corresponding author. Tel: 86–551–65592788, Fax: +86–551–65591310

14

E–mail address: [email protected] (C.L. Chen); [email protected] (X.M. Ren)

15 16 17

Word counts: Main text (4500 words) + 5 big figures (4 × 600 = 2400 words) +5

18

small figure (1 × 300 = 300 words) +1 table (1 × 300 = 300 words) = 7500 words.

19 20 21 22 23 24 1


Page 2 of 34

Page 3 of 34


25 26

ABSTRACT

27

In this study, the aggregation kinetics, aggregate morphology and aggregation

28

mechanisms of graphene oxide (GO) in the presence of Cs+, Sr2+, UO22+, Eu3+, or Th4+

29

are characterized by using time-resolved dynamic light scattering, transmission electron

30

microscopy (TEM)-element mapping, re-dispersion of GO aggregate, and the density

31

functional theory (DFT) calculations. The destabilization capability of Cs+, Sr2+, UO22+,

32

Eu3+, and Th4+ and the corresponding values of critical coagulation concentrations (CCC)

33

are obtained for the first time. Polyacrylic acid is used as a dispersant to investigate the

34

reversion of GO aggregates induced by various radioactive elements. The combined

35

results of polyacrylic acid effect and TEM-element mapping show that Cs+ induces the

36

aggregation of GO through electric double layer suppression and weak binding with

37

oxygen containing functional groups. By employing DFT calculation, we find that the

38

electrostatic potential distribution and the charge transfer rather than coordination with

39

oxygen containing functional groups control the destabilizing ability of radioactive

40

elements with higher valence. A comprehensive process of experimental-theoretical

41

studies is considered to better elucidate the colloidal behavior, self-assembly process,

42

application as a novel adsorbent and environmental risk of GO.

43

2



45

 INTRODUCTION

46

The remarkable characteristics and special structure of graphene oxide (GO) increase its

47

commercial manufacture and application greatly.1,2 The preparation of GO nanosheets

48

from graphite through modified Hummers’ method introduces a great many oxygen

49

containing functional groups in forms of −COOH, C−O−C, and −COH on GO surfaces.

50

These oxygen containing functional groups are essential for the high adsorption of

51

radioactive elements (e.g., Cs+, Sr2+, UO22+, Eu3+, and Th4+).3-6 Indeed, our previous

52

study showed that the maximum adsorption capacity of GO was 0.684 mmol/g for Cs+,7

53

0.620 mmol/g for Sr2+,8 0.875 mmol/g for UO22+,9 0.953 mmol/g for Eu3+,7 and 0.253

54

mmol/g for Th4+,10 which are higher than those of most of today’s nanomaterials. This

55

will open the possible application of GO in radionuclide pollution cleanup.11 With the

56

continued increase in the large scale of production and application in various fields, it is

57

inevitable that GO will be either intentionally or unintentionally discharged into the

58

environment with negative impact on human health and ecological security.12 The

59

colloidal properties of GO play a dominant role in its application, reactivity, fate,

60

transport, bioavailability, and toxicity. Therefore, a comprehensive understanding

61

colloidal properties of GO is important not only for promoting its application as a novel

62

adsorbent but also for more accurately evaluating its environmental fate, mobility, impact,

63

and risks.13-19

64

It has been found that the stability and colloidal properties of GO are affected

65

significantly by the solution chemistries such as pH, ionic strength, cation type, and 3


Page 4 of 34

Page 5 of 34


66

natural organic matter in the aquatic system.20,21 Specially, there are some reports on the

67

colloid properties of GO in the presence of metal ions (i.e., Na+, K+, Mg2+, Ca2+, Ag+,

68

Pb2+, Cu2+, Cd2+, and Cr3+).15,16,22 Generally speaking, the agglomeration tendency of GO

69

increases in the order of Me+ < Me2+ < Me3+.23,24 It is worth noting that the equivalent

70

cations such as Na+ and K+ or Mg2+ and Ca2+ show the different destabilization capability,

71

which is corporately controlled by hydration shell thickness and electronegativity of these

72

metal cations. Meanwhile, Yang et al.,15 reported that the heavy metal cations (i.e., Cr3+,

73

Pb2+, Cu2+, Cd2+ and Ag+) destabilized the GO suspension more intensively than the

74

common cations (i.e., Ca2+, Mg2+, Na+ and K+) and that the destabilizing ability of

75

bivalent heavy metal cations depended on their adsorption affinity with GO. For alkali

76

metal ions and alkaline earth metal ions, their effects on the transport of GO and

77

sulfide-reduced GO were found to obey the Hofmeister series.25 In a word, when GO

78

comes into contact with metal ions, its physiochemical properties and subsequent

79

aggregation and deposition behaviors will change significantly, depending on the nature of

80

metal ions and the interaction of GO with metal ions. Therefore, the environmental behavior

81

and toxicity of GO after being as an adsorbent especially radionuclide carrier cannot be

82

extrapolated from the available findings. Additionally, most of the previous studies mainly

83

focused on the electrokinetic and hydrodynamic properties, aggregation kinetics, and

84

microstructure transformation of GO through Zetasizer Nano instrument, time-resolved

85

dynamic light scattering (TR-DLS) and transmission electron microscopy (TEM),

86

respectively. Density functional theory (DFT) calculation is a powerful tool for 4



87

describing and predicting the change in chemical and physical properties of GO after

88

combining with cations. However, the use of DFT calculation to assist in illustrating the

89

aggregation mechanism of GO in the presence of metal ions is scarce. Moreover, very

90

little is known about how lanthanides, actinides and other fission products may

91

differently affect GO colloidal properties.

92

In order to get valuable information for more accurately predicating the environmental

93

fate and mobility and consequently assessing the environmental impact and risks after GO is

94

employed as an adsorbent for radionuclide pollution cleanup, aggregation behavior of GO

95

in the presence of Cs+, Sr2+, UO22+, Eu3+, or Th4+ is fist investigated systematically in this

96

study. Polyacrylic acid (PAA) can compete with GO for combining metal ions and

97

despoil/desorb the adsorbed metal ions from GO surface, so it is especially employed as a

98

dispersant to study its impact on GO aggregation and the re-suspension of GO aggregates.

99

Additionally, the microstructure of aggregate is characterized by TEM. More importantly,

100

the underlying aggregation mechanisms of GO with Cs+, Sr2+, UO22+, Eu3+, and Th4+ are

101

first analyzed by using DFT calculations. Through this work, we want to explore the

102

environmental behavior and risk of GO after being as radionuclide carrier.

103

 MATERIALS AND METHODS

104

Materials and Characterization. All the chemicals used in these experiments were of

105

analytical grade. GO was synthesized from the natural flake graphite with 99.95% purity

106

and an average particle diameter of 20 mm (Tianhe Graphite Co. Ltd., China) by the

107

modified Hummers’ method.26 GO was characterized by X-ray diffraction and Fourier 5


Page 6 of 34

Page 7 of 34


108

transformed infrared spectra (Figure S1 in Supporting Information). The zeta potential

109

and hydrodynamic diameter (Dh) of GO (Figure S1 in Supporting Information) were

110

determined by a Zetasizer Nano instrument (Malvern Instrument Co. Worcestershire, UK)

111

in aqueous solutions containing Cs+, Sr2+, UO22+, Eu3+, or Th4+ over a wide range of

112

concentrations. Because of the nonspherical structure of initial GO nanosheets, DLS was

113

mainly used to study the aggregation process of GO. The physicochemical properties of

114

the tested Cs+, Sr2+, UO22+, Eu3+, and Th4+ were shown in the Table S1. The size and

115

thickness distributions of GO were measured by scanning probe microscopy (SPM). The

116

surface morphology of GO aggregates was examined by TEM.

117

Batch Experiments. The initial GO concentration of 20 mg/L was used in all

118

experiments. The GO colloidal stability and aggregation kinetics were investigated in

119

well-controlled aqueous solutions by using a batch technique. The stock suspension of

120

GO, the stock solution of radionuclide analogue (i.e., Cs+, Sr2+, UO22+, Eu3+, or Th4+),

121

and Milli-Q water (18.2 MΩ·cm) were added into a glass vial (CNW Technologies) to

122

obtain the desired concentration of each component. Then the glass vials were placed on

123

a horizontal shaker. After shaking at a constant speed of 150 rpm for 24 h, those glass

124

vials were still standing on a flat surface for another 24 h to assure the complete

125

sedimentation of the large GO aggregates. Figure S2 showed that 16 h was enough to the

126

complete sedimentation of the large GO aggregates under our experimental conditions, so

127

24 h of standing time was employed in the following experiments.

128

Aggregation Kinetics Experiments. The 20 mg/L GO provided a strong DLS signal 6



129

and was therefore used in all aggregation studies. Meanwhile, there is a precedent in the

130

previous literatures for using this concentration,21 facilitating comparisons between the

131

results obtained by different investigators. The changes of Dh as a function of radioactive

132

element concentration and type were investigated by the TR-DLS to obtain the

133

destabilization ability of various radioactive elements. The aggregation kinetics of GO in

134

the presence of Cs+, Sr2+, UO22+, Eu3+, or Th4+ were quantified by attachment efficiency

135

(α), whose detailed determination procedures were provided in Supporting Information.

136

The critical coagulation concentration (CCC) values of GO for Cs+, Sr2+, UO22+, Eu3+, or

137

Th4+ were obtained via the intersection of diffusion-limited and reaction-limited fitting

138

lines. The detailed coagulation kinetics analysis processes were summarized in

139

Supporting Information.

140

DFT Calculations. In the optimized calculation, the hybrid functional B3LYP27 in

141

conjunction with 6-31G (d)28 basis set was used for H, C, and O. The quasi-relativistic

142

pseudo-potential LANL2DZ basis set29 was employed for Cs as well as Sr. The

143

Stuttgart-Dresden-Dunning basis set and the accompanying effective core potentials were

144

used for metal atoms of Eu (MWB52),30 Th (MWB60),31 and U (MWB60).31 The integral

145

equation formalism model (IEFPCM) was used to simulate the solvation effects.32 The

146

involved solvent was water unless otherwise mentioned. During the optimization, all

147

degrees of freedom were allowed to relax. Energy of all optimized configurations was

148

evaluated by bigger Def2-TZVP basis set33 for H, C, and O atoms. To adequately

149

characterize the binding, dispersion correction was included in the calculations by the 7


Page 8 of 34

Page 9 of 34


150

DFT-D3 approach of Grimme.34,35 All optimizations of electronic and geometrical

151

structures were performed by Gaussian 16 suite of program.36 The Hirshfeld charge37,38

152

analysis was adopted to evaluate changes in the electron density after Cs+, Sr2+, UO22+,

153

Eu3+, or Th4+ adsorption. To further clarify the interaction between selected cation and

154

GO, electrostatic potential (ESP) was analyzed and reduced density gradient (RDG)

155

method was carried out. The ESP mapped van der Waals (vdW) surface and the RDG

156

isosurfaces were obtained by using VMD 1.9.1 program39 based on the outputs from the

157

Multiwfn program.40

158

 RESULTS AND DISCUSSION

159

Effect of the Valence and Concentration of Radioactive Elements on GO

160

Aggregation. Figure 1A directly presents a visual proof of the GO aggregation

161

phenomenon after standing for 24 h as a function of the concentration and valence of

162

radioactive elements. One can see that GO can be well-dispersed in the absence and

163

presence of the low concentration of Cs+, Sr2+, UO22+, Eu3+, or Th4+. When their

164

concentrations reach the critical values, i.e., 12.0 mmol/L of Cs+, 0.3 mmol/L of Sr2+, 0.2

165

mmol/L of UO22+, 2.0×10−2 mmol/L of Eu3+, and 5.0×10−3 mmol/L of Th4+, visible

166

aggregation appears. The bulk aggregation phenomena are accompanied by the

167

corresponding variations of the zeta potential and Dh as a function of the concentration

168

and type of Cs+, Sr2+, UO22+, Eu3+, or Th4+ (Figures S3 in Supporting Information).

169

Figure S3A, C, E, G, and I shows that GO displays the typical electrokinetic behavior,

170

namely, the values of zeta potential increase with increasing the concentration of 8



Page 10 of 34

171

radioactive elements. And the radioactive element with higher valence is more effective

172

in changing the surface zeta potential of GO. While, the values of Dh almost keep

173

unchanging until the nuclide concentration increases to a certain threshold value, which

174

results in a very steep transition in this curve as the radioactive element concentration

175

progressively increases (Figure S3B, D, F, H, and J). These observations prove that both

176

the concentration and type of radioactive elements are pivotal factors controlling the

177

aggregation process of GO, thus influencing application of GO as a novel adsorbent for

178

radioactive element cleanup in aqueous solutions. As shown in Figure S3, the changes in

179

particle size do not seem to correlate well to those in the surface zeta potential, indicating

180

that the electrostatic interaction is not only mechanism controlling GO stability in the

181

presence of Cs+, Sr2+, UO22+, Eu3+, or Th4+.41

182

In order to further evaluate the stability of GO in the presence of radioactive elements

183

quantitatively, the aggregation kinetics of GO are determined by TR-DLS. The typical

184

aggregation kinetics profiles of GO as a function of the concentration of Cs+, Sr2+, UO22+,

185

Eu3+, or Th4+ are presented in Figure S4A-E in Supporting Information. The aggregation

186

rate accelerates with increasing the radioactive element concentration due to the

187

enhancement of charge screening. The α and CCC values of GO as a function of

188

radioactive element concentration are presented in Figure 1B. The distinct

189

reaction-limited and diffusion-limited regimes can be observed, indicating that GO

190

aggregation

191

reaction-limited region, the values of α increase quickly from 10-3 to 1 as the

follows

the

Derjaguin−Landau−Verwey−Overbeek

9


theory.

At

the

Page 11 of 34


192

concentrations of radioactive elements increase. At the diffusion-limited region, where

193

the values of α are equal to 1 and independent of the concentrations of radioactive

194

elements, the electrostatic repulsion between GO is suppressed completely and the

195

aggregation rate reaches the maximum. The concentration ranges before and after the

196

CCC are reaction-limited and diffusion-limited regions, representing different

197

aggregation processes. Once the concentration of radioactive element reaches the CCC,

198

the thermal diffusion process is the primary restriction for GO aggregation rather than the

199

energy-barrier repulsion. As shown in Figure 1B, the CCC values of GO are 21.2 mmol/L

200

of Cs+, 0.6 mmol/L of Sr2+, 3.7×10−1 mmol/L of UO22+, 3.3×10−2 mmol/L of Eu3+, and

201

6.3×10−3 mmol/L of Th4+. The CCC values of GO for radioactive elements are firstly

202

reported in our current study. For cations with equal valence, the CCC values of GO for

203

radioactive elements are obviously lower than those of GO for usual cations, i.e., Cs+

204

(21.2 mmol/L) < K+ (28.0 mmol/L) < Na+ (36.0 mmol/L), UO22+ (3.7×10−1 mmol/L)
UO22+ > Sr2+ > Cs+). Therefore,

212

charge screening contributes to the GO aggregation. Theoretically, the equivalent 10



213

radioactive elements should induce the similar charge screening effect and have the

214

similar destabilization capability.23,24 However, the CCC values of GO for Sr2+ and UO22+

215

are distinct. Meanwhile, the overall Dh values of GO aggregates in the UO22+ solution are

216

bigger than those in the Sr2+ solution (Figure S3D and F). UO22+ is more efficient in

217

destabilizing GO than Sr2+, which indicates that the aggregation behavior of GO in the

218

equivalent radioactive elements is not only charge screening but additional mechanisms

219

are involved.15,24 The different binding energy of radioactive elements with oxygen

220

containing functional groups of GO (10.2−50.5 kcal/mol for UO22+ and 3.3−14.6

221

kcal/mol for Sr2+) may be the cause.8,42 Further evidences for the addition mechanisms

222

are investigated in the following part.

223

PAA Effect on the GO Aggregation and on the Re-dispersion of GO Aggregates.

224

PAA rich in carboxylic groups (−COOH) can combine with metal cation strongly,

225

forming stable compound.43,44 Accordingly, PAA is employed as a dispersant to study its

226

impact on GO aggregation process and the re-suspension of GO aggregate for

227

differentiating the aggregation mechanism. The effect of different concentration of PAA

228

on the aggregation kinetics of GO is shown in the Figure S5A-E, where GO, radioactive

229

elements and PAA are simultaneously added. The used concentration of radioactive

230

elements is CCC at which the α value equals to 1 (22.0 mmol/L of Cs+, 0.6 mmol/L of

231

Sr2+, 0.4 mmol/L of UO22+, 3.3×10−2 mmol/L of Eu3+, and 6.3×10−3 mmol/L of Th4+)

232

(Figure 1B). The impact of PAA concentration on the value of α is shown in Figure 1C.

233

We can see that the values of α decrease as the PAA concentration increases. Since PAA 11


Page 12 of 34

Page 13 of 34


234

has specific interaction with radioactive elements and can compete with GO for binding

235

with radioactive elements, the increase of PAA concentration decreases the effectiveness

236

of radioactive elements in destabilizing GO. The concentration of PAA that makes α

237

value of GO be 3.0×10−3 is 10 mg/L for Cs+, 117 mg/L for Th4+, 150 mg/L for UO22+,

238

200 mg/L for Sr2+, and 250 mg/L for Eu3+. That is the effect of PAA on the destabilizing

239

ability of radioactive elements decreases in the order of Cs+ > Th4+ > UO22+ > Sr2+ > Eu3+.

240

The more PAA concentrations are needed, the stronger the binding affinities of

241

radioactive elements with GO are. So we can infer that the binding affinities of

242

radioactive elements with GO decrease in order of Cs+ < Th4+ < UO22+< Sr2+ < Eu3+,

243

which is not accordance with the destabilizing ability of radioactive elements. This

244

suggests coordination between radioactive elements with oxygen containing functional

245

groups on GO surface does not play the dominant role in the aggregation of GO. In our

246

previous study, we found that the impact of PAA on common environmental cation

247

followed the order of Na+ ≈ K+ < Al3+ < Ca2+ < Mg2+ and the GO aggregates in the Na+

248

and K+ solution are hard to be re-dispersed into solution.22 Accordingly, we deduce that

249

Cs+ and Na+/K+ induce the GO aggregation via different mechanisms. Since there is no

250

specific interactions between Na+/K+ with the functional groups of GO,20 Na+/K+ induces

251

GO aggregation via electrostatic double layer (EDL) suppression.

252

In order to further evaluate the effect of the binding affinity on the aggregation process,

253

the long-term impact of PAA on GO sedimentation and on the re-dispersion of GO

254

aggregates is also investigated. Figure 2A shows the impact of the concentration of PAA 12



255

on the sedimentation process, where PAA, radioactive elements and GO are added at the

256

same time. We can see that when the PAA concentration is higher than a critical

257

threshold value, GO can be completely stabilized in solution containing radioactive

258

elements even aging for one month. The amounts of PAA needed to keep the one month

259

stability of GO in solution containing radioactive elements increase in the order of Th4+

Exploring the Aggregation Mechanism of Graphene Oxide in the

Recommend Documents