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Adsorption of estrogen contaminants by graphene nanomaterials under NOM preloading: Comparison with carbon nanotube, biochar and activated carbon Luhua Jiang, Yunguo Liu, Shaobo Liu, Guangming Zeng, Xinjiang Hu, Xi Hu, Zhi Guo, Xiao-fei Tan, Lele Wang, and Zhibin Wu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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Page 1 of 29

Environmental Science & Technology

1

Adsorption

2

nanomaterials under NOM preloading: Comparison with

3

carbon nanotube, biochar and activated carbon

4

Luhua Jiang,†,‡ Yunguo Liu,∗,†,‡ Shaobo Liu,*,§ Guangming Zeng,†,‡ Xinjiang Hu,ǁ Xi Hu,ǁ Zhi

5

Guo,†,‡ Xiaofei Tan,†,‡ Lele Wang,┴ Zhibin Wu†,‡

6 7 8 9 10 11 12 13 14

of

estrogen

contaminants

by

graphene

†

College of Environmental Science and Engineering, Hunan University, Changsha 410082, P.R.

China ‡

Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of

Education, Changsha 410082, P.R. China §

School of Metallurgy and Environment, Central South University, Changsha 410083, P. R. China

ǁ

College of Environmental Science and Engineering Research, Central South University of

Forestry and Technology, Changsha 410004, P. R. China ┴

Advanced Water Management Centre (AWMC), The University of Queensland, QLD 4072,

Australia

∗

Corresponding Author. Phone: +86 731 88649208. Fax: +86 731 88822829. E-mail: [email protected] (Y.

Liu); [email protected] (S. Liu).

ACS Paragon Plus Environment


15

ABSTRACT: Adsorption of two estrogen contaminants (17β-estradiol and

16

17α-ethynyl estradiol) by graphene nanomaterials was investigated and compared

17

with those of a multi-walled carbon nanotube (MWCNT), a single-walled carbon

18

nanotube (SWCNT), two biochars, a powdered activated carbon (PAC), and a

19

granular activate carbon (GAC) in ultrapure water and in the competition of natural

20

organic matter (NOM). Graphene nanomaterials showed comparable or better

21

adsorption ability than carbon nanotubes (CNTs), biochars (BCs) and activated carbon

22

(ACs) under NOM preloading. The competition of NOM decreased the estrogens

23

adsorption by all adsorbents. However, the impact of NOM on the estrogens

24

adsorption was smaller on graphenes than CNTs, BCs and ACs. Moreover, the

25

hydrophobicity of estrogens also affected the uptake of estrogens. These results

26

suggested that graphenes nanomaterials could be utilized to removal estrogen

27

contaminants from water as an alternative adsorbent. Nevertheless, if transferred to

28

environment, they would also adsorb estrogen contaminants leading to great

29

environment hazards.

30


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TOC:

32



33

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1. INTRODUCTION

34

Graphene, as a two-dimensional (2D) single-layer sheet of sp2-hybridized

35

conjugated carbon atoms, is a basic unit for construction of carbon allotropes such as

36

zero-dimensional

37

three-dimensional (3D) graphite.

38

attention since its discovery due to its unique structure and excellent electronic,

39

mechanical and optical properties.

40

graphene nanomaterial are expected to increase exponentially in the next decade. 7 An

41

additional unusual physical property of graphene nanomaterial compared with other

42

carbonaceous nanomaterials is a high specific surface area (SSA, 2,630 m2/g), and flat

43

morphology.

44

organic compounds. For examples, Pavagadhi et al. reported that graphene could be

45

used as an adsorbent to removal of algal toxins from water;

46

functionalized graphene oxide (GO) which could be an efficient adsorbent for

47

methylene blue (MB) removal from real wastewater. 11 Chen et al. found that reduced

48

graphene oxide (rGO) was a powerful adsorbent for removal of nitroaromatic

49

compounds from water. 12

8,9

(0D)

fullerenes, 1,2

3–6

one-dimensional

(1D)

nanotubes

or

Graphene has received the bulk of scientific

It has been reported these scale applications of

Therefore, they are expected to serve as excellent adsorbents for

10

Wu et al. prepared a

50

Estrogens, including 17β-estradiol (E2) and 17α-ethynyl estradiol (EE2), have

51

been identified to possess the serious endocrine disrupting activity among endocrine

52

disrupting chemicals (EDCs). As reported, they are capable of causing negative

53

responses to organisms even at low concentrations, such as sex reversal of males, fish

54

egg production inhibition, and collapse of local fish populations.


13,14

However, there

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55

are only a small amount of peer-reviewed research articles on the uptake of estrogens.

56

15–18

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adsorption of estrogens by this material has multiple important environmental

58

significances including evaluating (i) the potential of utilizing graphene nanomaterial

59

as adsorbent in pollution control, (ii) the fate and transportation of estrogens by

60

graphene nanomaterial in the environment, and (iii) the potential toxicological

61

impacts of graphene nanomaterial on the environment if this material is discharged

62

and adsorb estrogen pollutants.

Considering the predicted high-yield production of graphene, investigating

63

In natural waters, the adsorption behaviors of graphenes are likely to influence

64

by natural organic matter (NOM). NOM is ubiquitous in surface and ground waters,

65

originating

66

micro-organisms) and/or external (i.e., decomposition of plants, animal residues, and

67

terrestrial biomass) sources in waters.

68

chemically complex polyelectrolytes such as humic substances, proteins, hydrophilic

69

acids, lipids, carboxylic acids, carbohydrates, hydrocarbons, and amino acids.

70

Because of the carboxylic and phenolic moieties distributed throughout the entire

71

molecule, NOM generally carries a negative charge in in fresh waters.

72

presence of NOM may has definite impacts on the adsorption of estrogen

73

contaminants. As reported, there are two opposite impacts on organic contaminants

74

uptake: an increase in uptake because of the enhanced dispersion of adsorbent in the

75

presence of NOM and/or a decrease in uptake because of direct site competition

76

and/or pore blockage. 21 However, no attention has been paid to examine the influence

from

internal

(i.e.,

excretion

19

or

photosynthetic

products

of

NOM is a heterogeneous mixture of


20

Thus, the


77

of NOM on estrogens adsorption although graphenes, carbon nanotubes (CNTs),

78

biochars (BCs), and activated carbon (ACs) have been utilized to explore the

79

adsorption properties of estrogens. Thus, the specific objectives of this research were

80

to (i) investigate the adsorption behavior of estrogen contaminants such as E2 and

81

EE2 onto graphene materials in ultrapure water and in the presence of NOM, and (ii)

82

compare the adsorption property of graphene nanomaterial with other common

83

carbonaceous adsorbent including multiwalled carbon nanotube (MWCNT),

84

single-walled carbon nanotube (SWCNT), two BCs, powdered activated carbon

85

(PAC), and granular activate carbon (GAC).

86

2. EXPERIMENTAL SECTION

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Materials. The samples utilized in this research included two rGOs (rGO1 and

88

rGO2, laboratory preparation described in Supporting Information), CNTs (MWCNT

89

and SWCNT, Chengdu Organic Chemicals Co., Ltd.), two BCs (BC1 and BC2,

90

laboratory preparation described in Supporting Information), and ACs (PAC and GAC,

91

Sinopharm Chemical Reagent co., Ltd.). rGOs, CNTs, BCs and PAC were used as

92

obtained, while the GAC was smashed to 150-225 µm size before used.

93

E2 (≥ 98%) and EE2 (≥ 98%), two selected adsorbates in this research, were

94

purchased from Sigma-Aldrich Chemical Co. 3D molecular structures and selected

95

physical-chemical properties of the two compounds are listed in Table S1 and Fig. S1

96

in Supporting Information, respectively. Ultrapure water generated by a Milli-Q water

97

filtration system (Millipore, Billerica, MA) was applied to the preparation of all the

98

solutions.


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Humic acid (HA, Suwannee River II standard and Leonardite standard), fulvic

100

acid (FA, Suwannee River II standard and Nordic lake reference) were provided by

101

the International Humic Substances Society. NOM stock solution was prepared by

102

adding a known amount of HA and FA (weight ratio 1:1) into ultrapure water with

103

stirring 24 h. Dissolution of NOM was promoted by adding NaOH to increase the

104

solution pH to 7.

105

(SUVA254) and the dissolved organic carbon (DOC), the solution was diluted to

106

desired NOM concentration (see Table S2).

19

After measurements of the specific ultraviolet absorbance

107

Adsorbents characterization. Adsorbents were characterized by several

108

instrumental analyses to examine their physical-chemical properties, including

109

elemental analysis, zeta potential meter, Fourier transform infrared spectrum (FTIR),

110

and Brunauer-Emmett-Teller (BET) method. Oxygen contents of adsorbents was

111

examined using an elemental analyzer (Vario EL III, Elementar, Germany). Zeta

112

potential measurements were performed with a zeta potential meter (Zetasizer

113

Nano-ZS90, Malvern). Besides, pH of the point of zero charge (pHPZC) of adsorbents

114

was investigated via pH equilibration technique.

115

conducted by using Nicolet 5700 Spectrometer in KBr pellet at room temperature.

116

The porosities of the samples were determined at liquid nitrogen temperature using a

117

surface area and porosity analyzer (Micromeritics, ASAP 2020 M+C) after evacuation

118

of the samples at 150 oC for 12 h. The BET equation was applied to calculate specific

119

surface area (SSA). The total pore volumes (PV) were determined from the adsorbed

120

volume of N2 near the saturation point (P/Po = 0.99). Pore size distributions (PSD) of

22

FTIR measurements were



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samples were obtained from the N2 isotherms via using the Density Functional Theory

122

(DFT) model.

123

Batch adsorption experiments. The stock solutions were prepared by

124

dissolving appropriate amount of E2 and EE2 in methanol, respectively. Constant

125

dose batch adsorption isotherms for E2 and EE2 were performed through a 250 mL

126

amber glass bottles with Teflon-lined screw caps. Two kinds of isotherm experiments

127

were carried out at room temperature (25 ± 1 oC).

128

For ultrapure water experiments, bottles with about 1 mg of adsorbents were first

129

filled with ultrapure water to nearly no free headspace. Afterward, known volumes of

130

estrogens stock solutions were directly spiked into the bottles. For the graphene

131

experiments, the bottles with samples were first half filled with ultrapure water. After

132

ultrasonicated for 30 min, the bottles nearly filled with ultrapure water before spiking

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estrogens. The volume percentage of the methanol in spiked solution was remained

134

below 0.1% (v/v) to avoid the co-solvent effect, as used by reported articles.

135

According to our group’s researches, 24 h was sufficient to reach equilibrium for

136

estrogens adsorption. 26,27 Therefore, the prepared bottles were placed in a shaker bath

137

with an agitation speed of 170 rpm for 24 h. The initial pH values in aqueous

138

solutions kept around 6.5. The desired solution pH was adjusted by negligible

139

volumes of HCl and NaOH solution through a pH meter (PHSJ-5, China).

23–25

140

The NOM influence on estrogen adsorptions by selected adsorbents was

141

investigated under preloading condition. Specifically, NOM was added four days in

142

advance before spiking estrogens, which stood for severe competition of NOM. To


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examine the effect of preloading NOM on adsorption, bottles with about 1 mg

144

samples were initially nearly filled with 3.4 mg DOC/L NOM stock solution which

145

buffered with 1 mM NaH2PO4 H2O/Na2HPO4 7H2O and adjusted pH to 7.0. Further,

146

the bottles were placed in a shaker bath with an agitation speed of 170 rpm for four

147

days. The residual NOM was examined using DOC analysis through a Shimadzu

148

TOC-VCPH analyzer (Shimadzu Co., Japan). After that, the bottles were directly

149

spiked with known volumes of estrogens stock solutions, and then continuously

150

shocked for 24 h.

151

Analyses. After adsorption for a predetermined time, the solutions (10 mL) were

152

separated from the adsorbents by centrifugation. The estrogens concentration in

153

supernatants were measured by high-performance liquid chromatography (HPLC)

154

with fluorescence detectors (Agilent 1100 Series, USA). The analytical column was

155

C18 reverse-phase column (5 mm, 4.6 × 150 mm, Agilent). The mobile phase flow

156

rate was set at 1 mL/min, and the column temperature was kept at 30 oC, and the

157

sample injection volume was 20 µL. The mobile-phase solvent profile was 45%

158

ultrapure water acidified with 10 mM H3PO4 and 55% methanol. Then detection was

159

carried out at an excitation wavelength of 280 nm and an emission wavelength of 310

160

for both E2 and EE2. Furthermore, bottles without any adsorbents were served as

161

blanks to monitor the loss of adsorbates during the experiment, which was found to be

162

negligible. The description of the HPLC method used in this research has been

163

reported elsewhere. 28,29

164

Isotherm modeling. In this work, four common nonlinear isotherm models, such



Freundlich

(FM),

Langmuir

(LM),

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165

as

Langmuir-Freundlich

(LFM)

and

166

Polanyi-Manes models (PMM), were applied to interpret the adsorption isotherm data

167

(Table S3). The residual root-mean-square error (RMSE), chi-square test (χ2) and

168

coefficient of determination (R2) values suggested that FM showed goodness of fitting

169

the experimental data (Table S4-S5). Thus, FM parameters were utilized to evaluate

170

adsorption data further.

171

3. RESULTS AND DISCUSSION

172

Adsorbent characterization. The selected physicochemical properties of all

173

adsorbents, such as SSA, PV, PSD and oxygen content, were listed in Table 1. As can

174

been seen, rGO1 had higher oxygen content (16.69 %) than rGO2 (10.33 %); and the

175

SSA of rGO1 and rGO2 were 244 and 147 m2/g, respectively, which indicated that the

176

SSA of rGOs was considerably enhanced after exfoliation of graphite (4.5 m2/g).

177

However, the SSA gap between this obtained value and the theoretical maximum

178

value of thin graphene (single-layered graphene, 2630 m2/g)

179

aggregation tendency of graphene layers during the reduction process due to the

180

strong van der Waals force between single graphene layers. Consequently, they

181

formed bundles resulting in decreasing the SSA. Although the aggregation

182

characteristics might change the SSA, rGOs had increasing distribution of micropores

183

( PAC >

193

SWCNT > rGO1 > MWCNT > rGO2 > BC2 > BC1. As reported, micropores had

194

higher surface area than that of the mesopores and macropores.

195

pores in the rGOs, CNTs, and BCs resulted in relatively smaller SSA than those of

196

ACs. Besides, the N2 adsorption isotherms of these adsorbents are shown in Fig. S3.

197

In terms of physical structures, as illustrated by the nitrogen adsorption isotherms,

198

ACs and BCs were microporous adsorbents. However, SWCNT and rGO exhibited

199

both microporous and mesoporous properties. Furthermore, the isotherm pattern of

200

the MWCNT suggested that it is abundant in macropores, and it has less micropores

201

and mesopores than the SWCNT and rGO. It is reported that the adsorption behavior

202

in micropores depend not only on the fluid-wall interactions but also on the attractive

203

forces between fluid molecules, resulting in capillary (pore) condensation.

204

addition, the FTIR analysis (Fig. S2) illustrated the existence of C-O group at 1096

205

cm-1, C=O group at 1641 cm-1, and O-H group at 3443 cm-1 on rGOs, CNTs, BCs, and

206

ACs.

32

Thus, the larger

33

In

207

Adsorption of E2 and EE2 by graphene nanomaterials in ultrapure water.

208

Adsorption isotherms of E2 and EE2 in ultrapure water are exhibited in Fig. 1, and the



209

according Freundlich isotherm parameters are listed on Table 2. Kd values (qe/Ce), as

210

the single point adsorption descriptors, were also calculated at different equilibrium

211

concentrations such as at 0.1%, 1%, 10% and 25% of estrogens aqueous solubility,

212

respectively, and listed in Table S6. Clearly, adsorption capacities for EE2 were lower

213

than E2 for all adsorbents, as represented by KF of Freundlich model. This might be

214

ascribed to the higher hydrophobicity of E2 than EE2 as reflected by the higher

215

octanol-water distribution coefficient (LogKow) value of E2 (Table S1). To further

216

examine the effect of hydrophobic interaction in adsorption, two parameters (KF-Csw

217

and KF-LogKow ) as the solubility normalized and hydrophobicity normalized KF values,

218

respectively, were applied (Table S7). As illustrated, the difference for KF-Csw and

219

KF-LogKow between E2 and EE2 was narrowed. Nevertheless, these parameters were

220

not nearly the same after above normalization. Thus, although hydrophobic

221

interaction was influential on adsorption, it was not the sole factor affecting estrogens

222

uptake.

223

In addition, rGO2 showed higher E2 and EE2 adsorption ability than rGO1 even

224

after SSA normalization. This might be attributed to the more non-polar surface of

225

rGO2 due to the less oxygen content. As reported, oxygen containing functional

226

groups of adsorbents could reduce the accessibility for hydrophobic organic

227

compounds via decreasing the available number of adsorption sites because of the

228

formation of water clusters on the their surface.

229

found in the uptake of aromatic organic and halogenated aliphatic contaminants by

230

graphene materials.

7,35,36

34

Similar phenomenon was also

In order to further investigate the difference between rGO1


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231

and rGO2 uptake, Kd values at different equilibrium concentrations were also

232

investigated (Table S6). With increasing equilibrium concentrations, the gaps in

233

estrogens uptake between rGO1 and rGO2 narrowed, which might be ascribed to the

234

stronger hydrophobic effect and π-π interactions between estrogen contaminants and

235

the graphene surface. Thus, with increasing estrogen concentrations, a number of

236

water molecules which were clustered around the oxygen containing functional

237

groups might be displaced by estrogen molecules and then the estrogen molecules

238

could be better coated on the surface of graphene nanosheets.

239

Comparison of E2 and EE2 adsorption by rGOs, CNTs, BCs and ACs in

240

ultrapure water. Compared graphenes with other carbonaceous nanomaterials, the

241

order of both E2 and EE2 adsorption abilities was provided (PAC > SWCNT > rGO2 >

242

rGO1 > MWCNT > GAC > BC2 > BC1) as illustrated by KF. This suggested that the

243

adsorption of estrogens seriously depended on the physiochemical characteristics of

244

carbonaceous nanomaterials. To investigate the effect of surface area of all the

245

adsorbents, SSA normalization were investigated (Table 2 and Fig. 1). Two

246

parameters, KF-SSA and RSSA, as the surface area normalized KF values and the ratio of

247

KF-SSA to KF, respectively, were analyzed to quantify the effect of SSA on the

248

estrogens uptake by various adsorbents. The lower RSSA value exhibited a greater

249

impact of SSA on adsorption. As illustrated, the differences in KF-SSA were suppressed

250

after SSA normalization. In particular, the gap between BCs and other adsorbents was

251

obviously narrowed, indicating SSA was a great factor influencing estrogens uptake

252

by BCs. In addition, RSSA values of ACs and SWCNT were smaller than other



253

adsorbents, which reflected stronger decrease in the estrogens uptake by ACs and

254

SWCNT. This suggested that SSA played an important role in the uptake of estrogens.

255

Furthermore, the adsorption isotherms were further normalized on the basis of oxygen

256

content. As can be seen, notable differences still observed among these isotherms (Fig.

257

1). Interestingly, microporous adsorbents (ACs, SWCNT and rGOs) still exhibited

258

higher E2 and EE2 adsorption after SSA and oxygen content normalization, which

259

might be attributed to the desirable filling of micropores by estrogen molecules. These

260

investigations suggested that the factors affecting estrogens uptake by graphene

261

materials in ultrapure water were similar to those of CNTs, BCs, and ACs. These

262

results demonstrated that the overall adsorption behavior of these adsorbents relied on

263

their SSA, PSD, oxygen content, and hydrophobicity of estrogens.

264

Adsorption of E2 and EE2 by graphene nanomaterials under NOM

265

preloading. To study the influence of NOM on estrogens adsorption by graphene

266

nanomaterials, adsorption of E2 and EE2 by rGOs were investigated under NOM

267

preloading. The adsorption isotherms in the presence of NOM are illustrated in Fig. 2.

268

For comparison, isotherms of graphenes in ultrapure water are also exhibited in the

269

same figure. Freundlich isotherm parameters in NOM solution are also summarized in

270

Table 2. KF-NOM and RNOM, as KF values of adsorption in NOM solution and the ratio

271

of KF-NOM to KF, respectively, were used to quantify the influence of NOM on the

272

estrogens adsorption by different adsorbents. The lower RNOM values suggest a greater

273

reduction of adsorption ability because of the presence of NOM. The percent

274

reduction (RKd ) in Kd value in the presence of NOM, as compared to Kd in ultrapure


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275

water, were also investigated (Table S6). Previous studies reported that NOM with

276

higher aromatic content and lower polarity tend to sorb on aromatic moieties of

277

graphene materials presumably through π-π interactions and/or hydrophobic

278

interactions. 37–39 Adsorption of E2 and EE2 by graphene nanomaterials reduced in the

279

presence of NOM, which might suggest the direct competition between NOM

280

contaminants with E2 and EE2 for the available uptake sites on aromatic moieties of

281

rGOs. In addition, as indicated by lower RNOM values of rGO2, influence of NOM on

282

adsorption capacity of rGO2 were stronger than rGO1. Meanwhile, rGO1 showed

283

comparable adsorption capacity under NOM preloading condition and in ultrapure

284

water as exhibited by RNOM values (68.9% and 60.5%) of rGO1 for E2 and EE2,

285

respectively. These observations might be ascribed to (i) the electrostatic repulsive

286

force between acidic charged rGO1 surface (pHPZC=4.8, Table 1) and negatively

287

charged NOM molecules resulting in less NOM aligning on the rGO1 surface as

288

reflected by Table S8, (ii) the better dispersed rGO1 nanosheets due to the polarity of

289

surface oxides which led to decreasing the effect of NOM on estrogens adsorption in

290

water, and (iii) the reduced hydrophobicity of rGO2 because NOM preloading

291

introduced more polar functionalities and negative charges on the rGO2 as illustrated

292

by lower pHPZC of rGO2 after NOM preloading (Table 1). The N value of Freundlich

293

model suggests the heterogeneity of the surface; and a higher N value exhibits a

294

homogeneous surface with narrow distributions of adsorption site.

295

the N values of rGO2 was 37% for E2 and 12% for EE2, while the increase in N

296

values of rGO1 was 27% and 25%, respectively. Small enhancements in the N values


40

The increase in


Page 16 of 29

297

indicated the NOM aligning did not notably alter the surface heterogeneity of

298

graphene nanomaterials. Moreover, the RNOM as a result of NOM preloading in E2

299

uptake was higher than the EE2 for rGOs, which might be ascribed to the stronger

300

hydrophobic effect of E2 molecule than EE2 molecule to graphene nanomaterials.

301

Comparison of E2 and EE2 adsorption by rGOs, CNTs, BCs and ACs under

302

NOM preloading. Adsorption of estrogens by rGOs, CNTs, BCs, and ACs in the

303

presence of NOM was also compared (Table 2 and Fig. 2). The order of E2 adsorption

304

capacity indicated by KF-NOM was PAC > SWCNT > rGO1 > rGO2 > MWCNT >

305

GAC > BC2 > BC1; the order of EE2 adsorption capacity indicated by KF-NOM was

306

PAC > rGO2 > SWCNT > rGO1 > MWCNT > GAC > BC2 > BC1. These findings in

307

the presence of NOM illustrated that in terms of engineering application, graphene

308

nanomaterials showed comparable or better adsorption ability than CNTs, BCs, and

309

ACs, thus they could considered as alternative adsorbents for eliminating estrogen

310

contaminants from aqueous solution; and in terms of environmental implication, if

311

transferred to environment, they would enrich estrogen contaminants leading to great

312

environment hazards.

313

Past researches reported that the attractive interactions between NOM and CNTs, 19,41–43

314

BCs and ACs are largely driven by π-π and hydrophobic interactions.

315

addition, the adsorption capacity, especially of ACs, is also determined by SSA of

316

porous structure available for adsorption.

317

and EE2 of rGOs were higher than that of CNTs, BCs, and ACs, which suggested that

318

the influence of NOM preloading condition on rGOs was lower than that on CNTs,

44,45

In

As illustrated in Table 2, RNOM for E2


Page 17 of 29


319

BCs, and ACs. NOM preloading could reduce the pHPZC of all samples as exhibited in

320

Table 1, which facilitated formation of water clusters through adsorbing water to

321

hydrophilic sites causing less available for the hydrophobic estrogens. The order of

322

pHPZC after NOM preloading was CNTs < rGOs < BCs~ACs, which could explain the

323

higher influence of NOM preloading on BCs and ACs than rGOs. Besides, as reported

324

by Newcombe et al., pore blockage, especially for micropore, caused by NOM can be

325

also expected to decrease the adsorption capacity through restricting access to

326

adsorption sites for the target contaminants. 46 Comparison with ACs, rGOs possessed

327

much less microporous structure, which resulting in less hindrance of estrogens access

328

to the adsorption sites. As presented by Table S8, the adsorbed mount of NOM on

329

CNTs was higher than that of rGOs after four days preloading, due to lower oxygen

330

content which was prone to adsorption through π-π interactions and hydrophobic

331

interactions. This indicated that less adsorption sites was taken placed by NOM on

332

rGOs as compared to CNTs, which resulted in less effect of NOM preloading on rGOs.

333

Furthermore, it has been reported that CNTs are more hydrophobic compared with

334

rGOs as reflected by Table 1 and prefer to aggregation as compact bundles due to the

335

unavoidable van der Waals interactions along the length axis.

336

compact bundle structure for graphene nanomaterials resulted in lower influence of

337

NOM preloading condition on rGOs comparison with CNTs. Similar phenomenon

338

was also found in the adsorption of aromatic organic and halogenated aliphatic

339

contaminants.

340

decrease in adsorbate equilibrium concentrations. This suggested that NOM

7,35,36

47,48

The much less

Furthermore, as indicated by Table S6, RKd increased with



341

molecules preferentially took up high energy adsorption sites; and then the

342

replacement of binded NOM molecules would be harder with enhancing estrogen

343

concentrations, considering that the NOM concentration of 3.4 mg DOC/L (Table S2)

344

selected to preloading was 2-3 orders of magnitude higher than estrogens at the low

345

concentration ranges. Moreover, as exhibited by increasing Freundlich N values

346

(Table 2), surface heterogeneity of all samples reduced during NOM preloading.

347

Nevertheless, there were no significant trends observed in the enhancing N values.

348

Among all samples, ACs was affected the most under NOM preloading. As indicated

349

by RKd values in Table 2, the reductions of estrogens adsorption by ACs in the

350

presence of NOM was maintained at a high level (>50%) at all the different

351

equilibrium concentrations. This clear decrease might be ascribed to the microporous

352

structure of ACs, where NOM preloading might led to a combination of pore

353

condensation and site competition for estrogens uptake.

354

ASSOCIATED CONTENT

355

Supporting Information

356

Preparation of reduced graphene and biochar. Selected properties of estrogens (Table

357

S1), characteristics of NOM solution (Table S2), nonlinear adsorption isotherm

358

models (Table S3), nonlinear simulations of adsorption isotherms in ultrapure water

359

and NOM solution (Table S4 and S5), single point adsorption coefficients (Kd) for

360

different levels of E2 and EE2 water solubilities of ultrapure water and NOM solution

361

isotherms (Table S6), the solubility normalized and hydrophobicity normalized KF

362

values (KF-Csw and KF-LogKow ) of estrogens adsorption in ultrapure water (Table S7),


Page 18 of 29

Page 19 of 29


363

the NOM adsorption capacities after four days preloading (Table S8), the 3D

364

molecular configurations of E2 and EE2 (Fig. S1), FTIR spectrums of the rGOs,

365

CNTs, ACs, and BCs (Fig. S2), and nitrogen adsorption isotherms of the rGOs, CNTs,

366

ACs, and BCs (Fig. S3). This material is available free of charge via the Internet at

367

http://pubs.acs.org.

368 369

Notes

370

The authors declare no competing financial interest.

371

ACKNOWLEDGEMENTS

372

This work was supported by the National Natural Science Foundation of China

373

(Grants Nos. 51521006, 51609268 and 51608208), the Hunan Provincial Innovation

374

Foundation for Postgraduate (Grant No. CX2016B135), and the Key Project of

375

Technological Innovation in the Field of Social Development of Hunan Province,

376

China (Grant Nos. 2016SK2010 and 2016SK2001).



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Fig. 1. Adsorption isotherms in ultrapure water (A) E2 mass basis, (B) EE2 mass basis, (C) E2 SSA normalized, (D) EE2 SSA normalized, (E) E2 oxygen content normalized, and (F) EE2 oxygen content normalized.


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Fig. 2. Adsorption isotherms in NOM solution (A) E2 mass basis, (B) EE2 mass basis.



Page 28 of 29

Table 1. Selected physiochemical properties of all adsorbents. Adsorbent

rGO1 rGO2 MWCNT SWCNT BC1 BC2 PAC GAC

*

SSA m2/g

PV** cm3/g

244 167 175 557 85 142 1255 1354

0.155 0.109 0.664 1.043 0.057 0.185 0.757 0.778

DFT pore volume distribution*** Vmicro (50 nm) %

25.1 18.8 7.0 16.1 13.4 11.5 37.0 42.6

69.2 73.9 65.4 77.8 75.7 69.8 61.9 56.4

4.7 7.3 27.6 6.1 10.9 18.7 1.1 1.0


Oxygen content %

pHpzc Pristine

NOM preloading

16.69 10.33 2.99 4.94 24.27 21.95 28.01 21.92

4.8 5.6 7.5 6.7 3.3 4.1 3.2 4.1

4.5 4.3 5.5 4.5 3.1 3.8 2.8 3.5

Page 29 of 29


Table 2. Freundlich isotherm parameters of estrogens adsorption in ultrapure water and NOM solution. Adsorbent

rGO1 rGO2 MWCNT SWCNT BC1 BC2 PAC GAC

KF (mg/g)/(mg/L)N E2 EE2 64.39 77.86 43.54 103.81 6.56 9.19 132.73 29.30

38.68 61.41 22.68 74.66 1.52 2.85 114.05 12.21

KF-SSA (mg/m2)/(mg/L)N E2 EE2 0.26 0.47 0.25 0.19 0.08 0.06 0.11 0.02

0.16 0.37 0.13 0.13 0.02 0.02 0.09 0.01

KF-NOM (mg/m2)/(mg/L)N E2 EE2 44.36 43.76 16.33 47.75 2.60 3.84 58.66 9.70

23.39 33.94 9.93 29.68 0.72 1.27 49.59 5.18

RSSA (‰)

RNOM (%)

E2

EE2

E2

EE2

4.0 6.0 5.7 1.8 12.2 6.5 0.8 0.7

4.1 6.0 5.7 1.7 13.2 7.0 0.8 0.7

68.9 56.2 37.5 46.0 39.6 41.8 44.2 33.1

60.5 55.3 43.8 39.8 47.4 44.6 43.5 42.4

In ultrapure water N R2 E2 EE2 E2 EE2 0.698 0.611 0.651 0.505 0.747 0.728 0.392 0.759


0.524 0.698 0.690 0.476 0.844 0.851 0.326 0.496

0.9942 0.9958 0.9943 0.9991 0.9796 0.9971 0.9938 0.9795

0.9956 0.9965 0.9691 0.9935 0.9980 0.9939 0.9679 0.9501

In NOM solution N R2 E2 EE2 E2 EE2 0.889 0.836 0.795 0.601 0.895 0.776 0.522 0.762

0.656 0.782 0.778 0.670 0.910 0.993 0.469 0.590

0.9982 0.9975 0.9983 0.9973 0.9728 0.9986 0.9967 0.9882

0.9855 0.9951 0.9919 0.9973 0.9872 0.9918 0.9873 0.9797

Adsorption of Estrogen Contaminants by Graphene ... - ACS Publications

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