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Macroscopic, spectroscopic and theoretical investigation for the interaction of phenol and naphthol on reduced graphene oxide Shujun Yu, Xiangxue Wang, Wen Yao, Jian Wang, Yongfei Ji, Yuejie Ai, Ahmed Alsaedi, Tasawar Hayat, and Xiangke Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06259 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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Environmental Science & Technology

1

Macroscopic, spectroscopic and theoretical investigation for the interaction of

2

phenol and naphthol on reduced graphene oxide

3

Shujun Yu1, Xiangxue Wang1, Wen Yao1, Jian Wang1, Yongfei Ji2, Yuejie Ai1,2*,

4

Ahmed Alsaedi3, Tasawar Hayat3, Xiangke Wang1,3*

5

1. School of Environment and Chemical Engineering, North China Electric Power

6

University, Beijing, 102206, P.R. China

7

2. Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of

8

Technology, Roslagstullsbacken 15, 10691 Stockholm, Sweden

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3. NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah

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21589, Saudi Arabia

11

ABSTRACT

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Interaction of phenol and naphthol with reduced graphene oxide (rGO), and their

13

competitive behavior on rGO were examined by batch experiments, spectroscopic

14

analysis and theoretical calculations. The batch sorption showed that the removal

15

percentage of phenol or naphthol on rGO in bi-solute systems was significantly lower

16

than those of phenol or naphthol in single-solute systems. However, the overall

17

sorption capacity of rGO in bi-solute system was higher than single-solute system,

18

indicating that the rGO was a very suitable material for the simultaneous elimination

19

of organic pollutants from aqueous solutions. The interaction mechanism was mainly

20

π-π interactions and hydrogen bonds, which was evidenced by FTIR, Raman and

1

ACS Paragon Plus Environment


21

theoretical calculation. FTIR and Raman showed that a blue shift of C=C and -OH

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stretching modes and the enhanced intensity ratios of ID/IG after phenols sorption. The

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theoretical calculation indicated that the total hydrogen bond numbers, diffusion

24

constant and solvent accessible surface area of naphthol were higher than those of

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phenol, indicating higher sorption affinity of rGO for naphthol as compared to phenol.

26

These findings were valuable for elucidating the interaction mechanisms between

27

phenols and graphene-based materials, and provided an essential start in simultaneous

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removal of organics from wastewater.

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INTRODUCTION

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Water is one of the most essential and important components on the earth for all living

31

beings.1 However, water quality is deteriorating continuously owning to the rapid

32

growth of civilization, industrialization, population, and other environmental

33

problems.2-4 Many organic pollutants such as dyes, pesticides, phenols, fertilizers,

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plasticizers, oils, greases, pharmaceuticals, etc. have been found in different water

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resources.5,6 Especially, phenols have been listed as priority pollutant by most national

36

environmental protection agencies and most of them are classified as the hazardous

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pollutants because of their potential risk against human health at low concentrations.7

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The phenols could accumulate through the food chain and at last enter into the human

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body and thereby threat the human health and are dangerous to the environment.

40

Thereby, the elimination of organic pollutants from the contaminated water is critical

41

to improve the disease-free health of our society.

42

Sorption is one of the most widely used technologies for the removal of organic 2


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pollutants from wastewater because of its simple operation, low cost, high efficiency

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and can be applied in large scale in real applications.8-11 The most popular and widely

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used adsorbent material was activated carbon and clay-based materials.12-16 Altenor et

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al.12 utilized vetiver roots to prepared activated carbon and used as adsorbent in

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wastewater treatment, the maximum sorption capacity was 408 mg/g and 82.32 mg/g

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for methylene blue and phenol, respectively. Alkaram et al.15 reported that the

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maximum removal capacity of hexadecyltrimethylammonium bromide-bentonite for

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phenol was 25.57 mg/g at 25 °C and pH 10.0. Radian and Mishael16 discovered the

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elevated removal of pyrene to polycation-montmorillonite in the existence of humic

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substances, which was described to the sorption of pyrene-HA complexes. However,

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the applications of these materials were restricted because of their low removal

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capacities or efficiencies.

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Graphene is the two-dimensional monolayer of sp2 hybridized carbon atoms, which

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are packed in the hexagonal honeycomb lattice.17 The flat π networks, defects,

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wrinkles and the oxygen-containing functional groups at the edges and surfaces of

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graphene nanosheets are valuable for the high sorption of pollutants.18,19 Numerous

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studies revealed that graphene was superior adsorbent for the removal of organic

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chemicals in aqueous solutions because of its large and hydrophobic surface

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area.17,20,21 Shen and Chen22 revealed that the sulfonated graphene was effective

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adsorbent for phenanthrene (400 mg/g) and methylene blue (906 mg/g). Wang et al.23

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reported that nitrogen-doped reduced graphene oxide (N-rGO) had high sorption

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capacity toward bisphenol A (356 mg/g) and bisphenol F (286 mg/g) mainly due to 3



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π-π interactions. In addition, other high removal capacities for naphthalene,

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nitrobenzene and p-nitrotoluene have also been reported.17,24 However, only

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single-solute sorption behavior was investigated in these studies, which was not

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meaningful for predicting pollutant removal in real environments since co-existence

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of organic pollutants is much more common.

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Co-occurrence of multiple organic contaminants in natural environments is

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commonplace and influences the removal of individual compounds via competitive or

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cooperative

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2,4-dichlorophenol and 4-chloroaniline was suppressed by nonpolar naphthalene on

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multiwalled carbon nanotubes (MWCNTs). Ren et al.26 found the competitive

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sorption between rhodamine 6G and dopamine onto GO because of the limited

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sorption active sites. The synergistic effect was reported between methyl blue (MB)

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and congo red (CR), which promoted the efficient removal of CR on MnFe2O4 and

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inhibited MB sorption.29 However, few studies have concerned the competitive

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sorption of organic contaminants onto graphene-based materials.10,26,30 To the best of

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our knowledge, a comprehensive experiment, spectral and theoretical study on the

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interaction between aromatic compound and graphene is largely scarce, which is

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crucial to understand the underlying interaction mechanism and for simultaneous

83

removal of organic pollutants from aqueous solutions.

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Herein, the interaction of phenol and naphthol with rGO was investigated from

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experiments, spectroscopy analysis and theoretical calculations for the first time.

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Phenol and naphthol were selected as typical phenols in the natural environment. The

effects.25-28

Yang

et

al.27

observed

that

4


sorption

of

polar

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major goals of this research were: (1) to investigate the influence of solution pH,

88

contact time and temperature on the individual sorption process of phenol and

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naphthol onto rGO from aqueous solutions, (2) to identify the mutual effects of the

90

pollutants in the binary systems, and (3) to derive the interaction mechanism of the

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phenols with rGO by using the spectroscopic methods (FTIR and Raman) and

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theoretical calculations. The contents are important to understand the physicochemical

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behaviour of phenols in the natural environment and for the application of rGO in

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environmental pollution cleanup.

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EXPERIMENTAL

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Materials. The rGO was synthesized by reducing GO according to the previous

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study.31 More detailed processes on the preparation of GO and rGO were supplied in

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Supporting Information (SI). The flake graphite (99.95% purity, 48 µm) was obtained

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from Qingdao Tianhe graphite Company (China). The phenol (≥ 99.5% purity) and

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naphthol (≥ 99.0% purity) were purchased from Sigma-Aldrich. All other reagents

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were purchased in analytical grade and used in the experiments without further

102

purification.

103

Characterization. The rGO was characterized by using Fourier-transform infrared

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spectroscopy (FTIR), Raman spectroscopy, transmission electron microscopy (TEM),

105

and X-ray photoelectron spectroscopy (XPS). The TEM image was employed on the

106

scanning transmission electron microscope (JEM-2000VF). The XPS spectrum was

107

performed by an ESCALAB 250 Xi XPS with Al Kα radiation. The FTIR spectrum

108

was performed by a Nicolet Magana-IR 750 spectrophotometer over a range from 5



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4000 to 400 cm-1 using a KBr disc technique. The Raman spectrum was recorded with

110

a Renishew inVia Raman spectrometer (Renishaw) at 532 nm. The potentiometric

111

acid-base titrations

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computer-controlled automatic titration system (DL50 Automatic Titrator, Mettler

113

Toledo) in 0.01 mol/L NaClO4 as the background electrolyte.

114

Experimental processes for the removal of phenol and naphthol. The sorption

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experiments of phenol and naphthol on rGO were performed under ambient

116

conditions by using batch technique. Quantitative adsorbent (rGO suspension, 0.1

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g/L), background solution (NaClO4 solution, 0.01 mol/L) and adsorbate (phenol or

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naphthol solution, 25 mg/L) were added into the brown glass vials, which were

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equipped with the polytetrafluoroethylene-lined screw caps. The pH was measured

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with a digital pH-meter (PHS-3C) by adding negligible amounts of 0.01-1.0 mol/L

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HClO4 or NaOH solutions. HClO4 and NaOH would not affect phenol or naphthol

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sorption on rGO with varied pH. The pH of the solution was kept below 0.1 before

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and after sorption. For the sorption isotherms of phenols, the temperatures were

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controlled to 298 K, 313 K and 328 K. The competitive sorption of phenol and

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naphthol on rGO was also carried out at the same level of phenols concentration at

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298 K. After the vials were shaken for 48 hours to ensure the sorption equilibrium, the

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solid was separated from the liquid phase by centrifugation at 5595 g for 30 min. The

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concentration of phenol and/or naphthol was measured by high performance liquid

129

chromatography (HPLC). The blank experiments (without rGO) were carried out

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under the same conditions to eliminate the mass loss during the reaction processes.

were conducted under argon

gas

6


condition

using a

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More details on the analysis were supplied in SI. All experimental data were the

132

average of duplicate determinations, and the relative errors were about 5%.

133

Data

134

pseudo-second-order models, which were given as follows32,33:

135

ln (qe − qt ) = lnqe − k1t

(1)

136

t 1 1 = + t 2 qt k 2 q e qe

(2)

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where qt (mmol/g) was the amount of adsorbed phenol and/or naphthol at time t (h),

138

qe (mmol/g) was the amount of adsorbed phenol and/or naphthol after reacted

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completely, k1 (1/h) and k2 (mmol/(g·h)) were the rate constant of pseudo-first-order

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and pseudo-second-order sorption, respectively.

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The Langmuir34 and Freundlich35 models (eq. 3-4) were used to fit the experimental

142

isotherm data. qe =

143

analysis.

The

kinetic

data

were

fitted

by

bqmaxCe 1 + bCe

pseudo-first-order

and

(3)

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q e = K F C en

145

where Ce (mmol/L) was the final concentration of phenols in aqueous solutions after

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sorption equilibration, qe (mmol/g) was the amount of phenols adsorbed on rGO, qmax

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(mmol/g) was the Langmuir constant, indicated the maximum monolayer sorption

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capacity, and b (L/mol) was a constant that associated with the sorption energy, KF

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(mmol1-n Ln/g) was the Freundlich constant when the equilibrium concentration of

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phenols reach to 1, and n represented the sorption intensity.

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Theoretical calculations. The geometric optimization, sorption energies and

(4)

7



152

molecular dynamics (MD) calculations for rGO sorption systems were performed by

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Vienna ab-initio simulation package (VASP) (version5.3.5).36 The density functional

154

theory (DFT) employing projector augmented wave (PAW) method with the

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Perdew-Burke-Ernzerhof (PBE) functional at the generalized gradient approximation

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(GGA-PBE) was applied in this work.37-39 More detailed processes on the calculations

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of the interactions between rGO and phenols were provided in SI.

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RESULTS AND DISCUSSION

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Characterization of rGO material. The TEM image of rGO (Figure 1A) exhibited a

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crumpled and wrinkled flake-like structure. The ultrathin nature of graphene

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nanosheets made them nearly invisible unless the relative clear multilayered stacks.10

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The surface functional groups of rGO were determined by FTIR spectroscopy. As

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illustrated in Figure 1B, the bands at ~ 3435 and 1400 cm-1 were ascribed to the -OH

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stretching and bending vibration. The strong band at 1589 cm-1 was attributed to the

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aromatic C=C stretching vibration, and another strong band at 1104 cm-1 was ascribed

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to C-O stretching vibration.9,40 These functional groups were further evidenced from

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the high deconvolution of C 1S XPS spectrum. As shown in Figure 1C, the carbon

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existed mainly in three forms, i.e., nonoxygenated carbon (C=C, 284.8 eV), the

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carbon in C-O group (286.4 eV), and the carbonyl carbon (C=O, 289.5 eV). However,

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the carboxylate carbon (O=C-O) was not determined due to its low content.41 In

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addition, the proportion of C=C:C-O:C=O obtained from the proportion of

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C=C:C-O:C=O XPS peak acreage was 4.6:1.5:1 (Table S1). These functional groups

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provided abundant reactive sites for the sorption of phenols. The pH value at the zero 8


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point charge (pHZPC) of rGO was calculated to be 5.6 from the potentiometric

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acid-base titration curve (Figure 1D), which indicated that the rGO was highly

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positively charged at pH < 5.6. Conversely, the surface charge of rGO was negative at

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pH > 5.6.

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Removal of phenol and naphthol. A series of systematic experiments were

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performed to evaluate the removal capacities of phenol and naphthol by the prepared

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rGO. Sorption kinetic tests were first carried out to determine the contact time needed

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for sorption equilibrium. As can be seen from Figure 2A-B, the phenol and naphthol

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were adsorbed rapidly at the first 4 h, and thereafter it proceeded at a slow rate and

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finally attained saturation after 6 h of contact time. At the initial state, the phenols

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were adsorbed onto the rGO surface easily, and the accumulation of molecules on the

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surface finally resulted in a low sorption rate in the later stage with contact time

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increased.42 The relative parameters of pseudo-first-order and pseudo-second-order

187

models (Table S2) clearly confirmed that the sorption of phenol and naphthol on rGO

188

was dominated by the pseudo-first-order model.

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The neutral or anion form of phenols was determined by their pKa versus the solution

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pH values (Table S3). At pH < pKa, the nondissociated neutral species were

191

dominated for phenols while the anion forms were dominant at pH > pKa. Figure

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2C-D showed the effect of pH on the sorption of phenol and naphthol. The removal

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efficiency of phenol and naphthol increased with solution pH increasing until it

194

reached its pKa. The pHPZC of rGO (5.6) indicated that the surface of rGO was mainly

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negatively charged at pH > 5.6. Therefore, the reduced sorption of phenol and 9



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naphthol at pH > pKa was mainly owing to the increased electrostatic repulsion

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between the negatively charged rGO and the dissociated phenols.43 The dissociation

198

of phenols increased their hydrophilicity, so the decreased sorption at pH > pKa was

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also due to the reduced hydrophobic interactions.44 Furthermore, the dissociation of

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the -OH group of the phenols was disadvantageous to form the hydrogen bonds

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between the surfaces of rGO and phenolic molecules, and thereby reduced the

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removal as well.7,44 Increased sorption of phenol and naphthol to rGO with increasing

203

pH before their pKa may be due to the enhanced π-π interactions. Other

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investigators44,45 documented that the increased sorption of phenol, naphthol,

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1,2,4-trichlorobenzene and 2,4-dinitrotoluene to carbon nanotubes (CNTs) with the

206

increase of pH at pH < pKa. They demonstrated that the increasing pH could change

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the properties of polar aromatics, such as the π-donating strength, and therefore

208

improved the sorption to CNTs, but the interaction mechanism was still unclear.

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Figure 2E-F showed the sorption isotherms of phenol and naphthol on rGO at the

210

temperatures of 298, 313 and 328 K, respectively. The removal of phenols on rGO

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increased with increasing temperature, demonstrating that high temperature was

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beneficial for phenols' sorption on rGO. The relative parameters calculated from the

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Freundlich and Langmuir models (Table S4) showed that the Langmuir model fitted

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the sorption isotherms better than the Freundlich model, revealing that the sorption of

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phenols on rGO was monolayer coverage. The qmax value of phenol sorption on rGO

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(2.7 mmol/g at 298 K) was slightly higher than that of naphthol (2.3 mmol/g), which

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was negatively related to molecular size: 22.6 Å2 for phenol and 35.6 Å2 for naphthol 10


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(Figure S2). Such a phenomenon was also reported by other investigators46,47, where

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the authors concluded that the pore-filling mechanism dominated the sorption of

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polycyclic aromatic hydrocarbons on carbon-based materials.

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To understand the competitive sorption behavior of two different compounds on the

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surface of rGO, the competitive removal of phenol and naphthol was investigated

223

under different pH and contact time (Figure 2A-D). Clearly, the sorption of phenol

224

and naphthol on rGO were decreased in binary system over the wide pH range, which

225

was consistent with the removal of aromatic organic pollutants onto MWCNTs.27,47

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The rGO has a limited number of sorption sites for both phenol and naphthol

227

molecules to occupy competitively, thereby resulted in the decreased sorption in the

228

binary systems. The competitive sorption isotherms of phenol and naphthol on rGO at

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298 K were shown in Figure 2E-F. The competitive Langmuir model was used to fit

230

the experimental isotherm data (Figure S3). For the competitive sorption, the qmax

231

values decreased from 2.7 to 1.5 mmol/g for phenol and from 2.3 to 1.6 mmol/g for

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naphthol, revealing a higher sorption configurations of rGO for naphthol than phenol.

233

Furthermore, it was well known that sorption of phenols was controlled by a

234

combination of hydrophobic interactions, π-π interactions and hydrogen-bonding

235

interactions.7,43,44 The strength of π-π interactions, hydrophobic interactions and

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hydrogen-bonding interactions relied on the solute π-polarity ability (π*),

237

octane-water distribution coefficient (Kow) and hydrogen-bonding acceptor ability

238

(βm), respectively, which were closely link with the aromatic ring number. According

239

to logKow, π* and βm values of phenol (logKow = 1.46, π* = 0.37 and βm = 0.33) and 11



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naphthol (logKow = 2.84, π* = 0.47 and βm = 0.33), it was reasonable that naphthol

241

had very strong competitive sorption effect to phenol owing to its extra

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benzene-rings.44,48 However, the qmax values of phenol and naphthol (i.e., 1.5 mmol/g

243

for phenol and 1.6 mmol/g for naphthol) were higher than that of phenol single

244

system (2.7 mmol/g) and that of naphthol single system (2.3 mmol/g), suggesting that

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the sorption capacities of rGO were increased for the coexisting of multi-components.

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The increased amounts of phenol and naphthol on rGO at binary system could be

247

attributed to the intra-molecular interactions between the phenols themselves. Due to

248

the hydrogen bond interactions between hydroxyl groups in naphthol and

249

oxygen-containing functional groups in rGO, the hydroxyl groups of naphthol were

250

preferentially attracted to rGO surfaces and left the hydrophobic benzene rings to face

251

the water molecular in solution. The unoccupied benzene ring of naphthol could

252

supply new sorption active sites, thus a second layer of phenol would be adsorbed to

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the initially adsorbed naphthol molecules by hydrophobic as well as π-π interactions

254

between benzene rings. As shown in Figure S4, significant sorption of phenol by

255

naphthol confirmed the molecule-molecule attractions between different solutes. The

256

same reaction mechanism was applied to explain the competitive sorption of aromatic

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organic compounds on MWCNTs and rGO.10,47,49 These findings indicated that the

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rGO was very suitable materials for the simultaneous elimination of organic

259

pollutants from aqueous solutions.

260

Discussion on removal mechanism

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Spectroscopic techniques. To understand the interaction mechanism of rGO with 12


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phenol and naphthol, the rGO samples after pollutant sorption were characterized by

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FTIR and Raman techniques. Figure 3A showed the FTIR spectra of rGO before and

264

after the sorption of phenol and naphthol. It was obvious that the stretching vibration

265

of C=C band was shifted from 1589 cm-1 to 1585 cm-1 after phenol sorption and to

266

1576 cm-1 after naphthol sorption. It was in line with the previous observations that

267

π-π conjugative effect and hydrophobic interactions occurred between rGO and

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phenols.10,26 McDermott and McCreery50 pointed out that the graphite basal plane in

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the vicinity of the edges was usually electron-rich, whereas the regions in the

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graphene surface center were typically electron-depleted. Therefore, π-π electron

271

donor-acceptor interaction was occurred between the π-electron-rich phenyls of

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phenols and the π-electron-deficient matrix of graphene nanosheets. In addition, the

273

hydroxyl groups were electron-donating functional groups, which could enhance the

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π-donating strength of host aromatic ring.44 Thereby, the -OH could improve the

275

sorption ability of phenols to the surfaces of rGO via π-π interaction. Similarly, Chen

276

et al.51 documented that π-π interaction lead to stronger removal of the amino- and

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hydroxyl-replaced aromatic compounds than the nonpolar aromatic compounds to

278

CNTs. In addition, small shifts in the -OH bond were observed after sorption, from

279

3435 to 3428 cm-1 after phenol sorption and from 3435 to 3430 cm-1 after naphthol

280

sorption. Jin et al.9 also reported that -OH bond of rGO changed after the sorption, i.e.,

281

from 3444 to 3415 cm-1 for 4-n-nonylphenol and from 3444 to 3428 cm-1 for

282

bisphenol A. The substituent hydroxyl group on phenol molecules may form hydrogen

283

bonds with the O-containing polar moieties on rGO. The proposed mechanism was 13



284

further supported by Raman spectroscopy analysis (Figure 3B), the G band (∼1580

285

cm-1) was related to the vibration of sp2 carbon atoms in the 2-dimensional hexagonal

286

lattice of graphite, and the D band (∼1350 cm-1) was assigned to the vibrations of the

287

defected and disordered sp3 carbon atoms.52 The weak and broad 2D peak at ∼2700

288

cm-1 was an out-of-plane vibration mode which was another indication of disorder

289

consquence.52 The ratio of D and G band intensities (ID/IG) was a common index

290

about the extent of defects on the surfaces of rGO. Noteworthy from Figure 3B, the

291

intensity ratios of ID/IG of rGO-phenol (0.93) and rGO-naphthol (0.95) were larger

292

than that of rGO (0.91). This implied that the size of the “graphene-like” domains was

293

smaller than those before sorption, however it was much more numerous in the

294

number.53

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Theoretical calculations. The snapshots for the gradual sorption process of phenol

296

and naphthol were shown in Figures 4 and 5, respectively. The final closest interaction

297

distances of phenol and naphthol were 3.503 and 3.438 Å, respectively. Thus,

298

naphthol may have stronger π-π interaction with rGO plane than phenol. This

299

conclusion was further proved by the sorption energy (Es) calculations. The Es was

300

calculated by the formula: Es=E[A]+E[B]-E[total], where E[total] represented the total

301

energy of the target complex system, E[A] was the total energy of rGO, and E[B] was

302

the total energy of the isolated phenol or naphthol molecule. The calculated Es in

303

Table S5 showed that the rGO-phenols system was stable and rGO was effective

304

adsorbent for the removal of phenols pollutants from natural environment. The more

305

positive the Es is, the more stable the system is.54 The Es of naphthol-rGO was higher 14


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than that of phenol-rGO indicated the naphthol-rGO system had stronger stability,

307

which agreed well with the competitive sorption experimental observations.

308

Interestingly, in aqueous solutions, except for the sorption between phenols and rGO,

309

there were also inter- or intra-molecular interactions between the phenols themselves.

310

The initial physical or chemical properties of the adsorbed molecules themselves may

311

play an important role during the sorption process. Thus, furthermore, the MD

312

simulations were performed in solution box to explore the initial interactions between

313

the adsorbed molecules. The MD simulation details were shown in SI. The total

314

hydrogen bond numbers, diffusion constant, solvent accessible surface area (SASA)

315

were computed using the g_hbond, g_msd and g_sas tool of the Gromacs Program

316

package, respectively.55 From the self-diffusion constants calculated from the theory

317

(Table S6), one may draw the conclusion that the self-diffusivity in naphthol packed

318

solution box (1.54 × 10-5 cm2s-1) was a little higher than that of phenol (1.53 × 10-5

319

cm2s-1), while their mixture showed much higher self-diffusivity of 2.73 × 10-5 cm2s-1.

320

Generally, the diffusion constant reflected the diffusivity and mobility of molecule to

321

a certain extent. The higher the diffusivity, the faster it diffuses and this will

322

eventually influence the sorption process. The hydrogen bond numbers of different

323

reactants were shown in Figure 6. The hydrogen bond number in individual phenol or

324

naphthol was quite same. When the phenol and naphthol molecules were mixed, they

325

were apt to form hydrogen bonds between phenol and naphthol other than phenol or

326

naphthol themselves individually, since the formation frequency of phenol-naphthol

327

was much dense than the ones in phenol or naphthol alone. Therefore, the different 15



328

formation pattern of hydrogen bond was probably another important factor in sorption.

329

The SASA reflected the surface area of a molecule that was accessible to solvent.

330

Figure 7 showed that the hydrophilic and hydrophobic SASAs of naphthol were larger

331

than that of phenol. Since the naphthol molecule possessed one more aromatic ring

332

than the phenol molecule, which was benefit for the formation of π-π and

333

hydrophobic interactions. In addition, from the calculated curves, the naphthol system

334

was much more fluctuant than the smooth ones in the phenol system, which showed

335

the instability of the naphthol clusters. Based on the above analysis, one can conclude

336

that the interaction of phenols with rGO was mainly dominated by hydrophobic

337

interactions, π-π interactions and hydrogen bonds.

338

Environmental implications. Phenols were regarded as priority contaminants

339

because they are harmful to organisms at low levels and can be toxic when present

340

elevated concentrations and are suspected to be carcinogens.7 Hence, it was regarded

341

particularly important and urgent to eliminate the phenols from industrial effluents

342

before discharging into the aqueous solution. Graphene is expected to have excellent

343

sorption capacity toward phenols organic compounds, and has the potential to be

344

applied as a superior adsorbent in wastewater and drinking water treatments.19 For the

345

first time, we systematic studied the sorption of phenol and naphthol on rGO and

346

demonstrated their interaction process by MD simulation and DFT calculations. The

347

batch experimental results proved that the sorption of phenols on rGO was highly

348

dependent on solution chemistry. When compared with phenol, naphthol exhibited a

349

higher sorption energy with rGO. In binary phenol-naphthol system, naphthol 16


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350

presented a greater inhibition of the removal of phenol. The competitive sorption of

351

phenols on rGO in multiple organic contaminants systems was mainly dependent on

352

their chemical properties and the experimental conditions. The findings were crucial

353

to assess the removal of coexisting aromatic organic pollutants on rGO and offered an

354

indication of future directions to synthesize new kinds of nanomaterials for the

355

simultaneous elimination of organic pollutants from wastewater.

356

ASSOCIATED CONTENT

357

Supporting Information. Additional preparation of GO and rGO. More detailed

358

processes and information of MD simulation and DFT calculation. The relative

359

parameters of model simulation. This information is available free of charge via the

360

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

361

AUTHOR INFORMATION

362

Corresponding Authors. Tel(Fax):86-10-61772890; e-mail: [email protected]

363

(X.K. Wang); [email protected] (Y.J. Ai).

364

Notes

365

The authors declare no competing financial interest.

366

ACKNOWLEDGMENTS

367

The authors acknowledged the National Natural Science Foundation of China

368

(91326202, 21225730, 21577032 and 21403064), the Science Challenge Project

369

(JCKY2016212A04), the Fundamental Research Funds for Central Universities

370

(JB2015001). X. Wang acknowledged the CAS Interdisciplinary Innovation Team of

371

Chinese Academy of Sciences. 17



372

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25



530 531

Figure 1. The characterization of rGO: (A) TEM image; (B) FTIR spectrum; (C) C 1s

532

XPS spectrum; (D) potentiometric acid-base titrations.

26


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533 534

Figure 2. Effect of time (A and B), pH (C and D) and temperature (E and F) on phenol

535

and naphthol sorption onto rGO. C0 = 25 mg/L, I = 0.01 mol/L NaClO4, m/V = 0.1 g/L.

536

Sorption isotherms of phenol (E) and naphthol (F) in a single system at different

537

temperature and in binary system at T = 298 K on rGO at pH = 6.5 ± 0.1. The solid

538

lines represent the Langmuir model. The dashed lines represent the Freundlich

539

model. 27



540 541

Figure 3. The characterization of rGO before and after the sorption of phenol and

542

naphthol: (A) FTIR spectra; (B) Raman spectra.

543 544

Figure 4. The snapshots of the MD trajectory for the sorption process of phenol (a)

545

and the optimized static structure for the phenol-rGO system from the side view (b)

546

and the top view (c).

28


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547 548

Figure 5. The snapshots of the MD trajectory for the sorption process of naphthol (a)

549

and the optimized static structure for the naphthol-rGO system from the side view (b)

550

and the top view (c).

29



551 552

Figure 6. (A) Dynamical properties analyses of hydrogen bonds for the phenol (a) and

553

naphthol (b) in the individual solution boxes. (B) Dynamical properties analyses of

554

hydrogen bonds for the phenol (a), naphthol (b) and phenol-naphthol (c) in the mixed

555

solution boxes.

30


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556 557

Figure 7. Dynamical properties analyses of solvent accessible surface area (SASA,

558

nm2) for the phenol (A) and naphthol (B) in solution boxes.

559

31



560

TOC

561

Competitive sorption of phenol and naphthol on reduced graphene oxide (rGO).

562

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