Macroscopic, Spectroscopic, and Theoretical Investigation for theInteraction of Phenol and Naphthol on Reduced Graphene OxideShujun Yu,† Xiangxue Wang,† Wen Yao,† Jian Wang,† Yongfei Ji,‡ Yuejie Ai,*,†,‡ Ahmed Alsaedi,§

Tasawar Hayat,§ and Xiangke Wang*,†,§

†School of Environment and Chemical Engineering, North China Electric Power University, Beijing, 102206, P.R. China‡Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, Roslagstullsbacken 15, 10691Stockholm, Sweden§NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

*S Supporting Information

ABSTRACT: Interaction of phenol and naphthol with reduced graphene oxide (rGO),and their competitive behavior on rGO were examined by batch experiments,spectroscopic analysis and theoretical calculations. The batch sorption showed thatthe removal percentage of phenol or naphthol on rGO in bisolute systems wassignificantly lower than those of phenol or naphthol in single-solute systems. However,the overall sorption capacity of rGO in bisolute system was higher than single-solutesystem, indicating that the rGO was a very suitable material for the simultaneouselimination of organic pollutants from aqueous solutions. The interaction mechanismwas mainly π−π interactions and hydrogen bonds, which was evidenced by FTIR,Raman and theoretical calculation. FTIR and Raman showed that a blue shift of CCand −OH stretching modes and the enhanced intensity ratios of ID/IG after phenolssorption. The theoretical calculation indicated that the total hydrogen bond numbers,diffusion constant and solvent accessible surface area of naphthol were higher than thoseof phenol, indicating higher sorption affinity of rGO for naphthol as compared to phenol. These findings were valuable forelucidating the interaction mechanisms between phenols and graphene-based materials, and provided an essential start insimultaneous removal of organics from wastewater.

■ INTRODUCTION

Water is one of the most essential and important componentson the earth for all living beings.1 However, water quality isdeteriorating continuously owning to the rapid growth ofcivilization, industrialization, population, and other environ-mental problems.2−4 Many organic pollutants such as dyes,pesticides, phenols, fertilizers, plasticizers, oils, greases,pharmaceuticals, etc. have been found in different waterresources.5,6 Especially, phenols have been listed as prioritypollutant by most national environmental protection agenciesand most of them are classified as the hazardous pollutantsbecause of their potential risk against human health at lowconcentrations.7 The phenols could accumulate through thefood chain and at last enter into the human body and therebythreat the human health and are dangerous to the environment.Thereby, the elimination of organic pollutants from thecontaminated water is critical to improve the disease-freehealth of our society.Sorption is one of the most widely used technologies for the

removal of organic pollutants from wastewater because of itssimple operation, low cost, high efficiency, and can be appliedin large scale in real applications.8−11 The most popular andwidely used adsorbent material was activated carbon and clay-based materials.12−16 Altenor et al.12 utilized vetiver roots to

prepared activated carbon and used as adsorbent in wastewatertreatment, the maximum sorption capacity was 408 mg/g and82.32 mg/g for methylene blue and phenol, respectively.Alkaram et al.15 reported that the maximum removal capacity ofhexadecyltrimethylammonium bromide-bentonite for phenolwas 25.57 mg/g at 25 °C and pH 10.0. Radian and Mishael16

discovered the elevated removal of pyrene to polycation-montmorillonite in the existence of humic substances, whichwas described to the sorption of pyrene-HA complexes.However, the applications of these materials were restrictedbecause of their low removal capacities or efficiencies.Graphene is the two-dimensional monolayer of sp2

hybridized carbon atoms, which are packed in the hexagonalhoneycomb lattice.17 The flat π networks, defects, wrinkles andthe oxygen-containing functional groups at the edges andsurfaces of graphene nanosheets are valuable for the highsorption of pollutants.18,19 Numerous studies revealed thatgraphene was superior adsorbent for the removal of organicchemicals in aqueous solutions because of its large and

Received: December 10, 2016Revised: February 26, 2017Accepted: February 28, 2017Published: February 28, 2017

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© 2017 American Chemical Society 3278 DOI: 10.1021/acs.est.6b06259Environ. Sci. Technol. 2017, 51, 3278−3286

hydrophobic surface area.17,20,21 Shen and Chen22 revealed thatthe sulfonated graphene was effective adsorbent for phenan-threne (400 mg/g) and methylene blue (906 mg/g). Wang etal.23 reported that nitrogen-doped reduced graphene oxide (N-rGO) had high sorption capacity toward bisphenol A (356 mg/g) and bisphenol F (286 mg/g) mainly due to π−πinteractions. In addition, other high removal capacities fornaphthalene, nitrobenzene, and p-nitrotoluene have also beenreported.17,24 However, only single-solute sorption behaviorwas investigated in these studies, which was not meaningful forpredicting pollutant removal in real environments sincecoexistence of organic pollutants is much more common.Co-occurrence of multiple organic contaminants in natural

environments is commonplace and influences the removal ofindividual compounds via competitive or cooperative ef-fects.25−28 Yang et al.27 observed that sorption of polar 2,4-dichlorophenol and 4-chloroaniline was suppressed by non-polar naphthalene on multiwalled carbon nanotubes(MWCNTs). Ren et al.26 found the competitive sorptionbetween rhodamine 6G and dopamine onto GO because of thelimited sorption active sites. The synergistic effect was reportedbetween methyl blue (MB) and congo red (CR), whichpromoted the efficient removal of CR on MnFe2O4 andinhibited MB sorption.29 However, few studies have concernedthe competitive sorption of organic contaminants ontographene-based materials.10,26,30 To the best of our knowledge,a comprehensive experiment, spectral and theoretical study onthe interaction between aromatic compound and graphene islargely scarce, which is crucial to understand the underlyinginteraction mechanism and for simultaneous removal of organicpollutants from aqueous solutions.Herein, the interaction of phenol and naphthol with rGO was

investigated from experiments, spectroscopy analysis, andtheoretical calculations for the first time. Phenol and naphtholwere selected as typical phenols in the natural environment.The major goals of this research were (1) to investigate theinfluence of solution pH, contact time and temperature on theindividual sorption process of phenol and naphthol onto rGOfrom aqueous solutions, (2) to identify the mutual effects of thepollutants in the binary systems, and (3) to derive theinteraction mechanism of the phenols with rGO by using thespectroscopic methods (FTIR and Raman) and theoreticalcalculations. The contents are important to understand thephysicochemical behavior of phenols in the natural environ-ment and for the application of rGO in environmental pollutioncleanup.

■ EXPERIMENTAL SECTIONMaterials. The rGO was synthesized by reducing GO

according to the previous study.31 More detailed processes onthe preparation of GO and rGO were supplied in SupportingInformation (SI). The flake graphite (99.95% purity, 48 μm)was obtained from Qingdao Tianhe graphite Company(China). The phenol (≥99.5% purity) and naphthol (≥99.0%purity) were purchased from Sigma-Aldrich. All other reagentswere purchased in analytical grade and used in the experimentswithout further purification.Characterization. The rGO was characterized by using

Fourier-transform infrared spectroscopy (FTIR), Raman spec-troscopy, transmission electron microscopy (TEM), and X-rayphotoelectron spectroscopy (XPS). The TEM image wasemployed on the scanning transmission electron microscope(JEM-2000VF). The XPS spectrum was performed by an

ESCALAB 250 Xi XPS with Al Kα radiation. The FTIRspectrum was performed by a Nicolet Magana-IR 750spectrophotometer over a range from 4000 to 400 cm−1

using a KBr disc technique. The Raman spectrum was recordedwith a Renishew inVia Raman spectrometer (Renishaw) at 532nm. The potentiometric acid−base titrations were conductedunder argon gas condition using a computer-controlledautomatic titration system (DL50 Automatic Titrator, MettlerToledo) in 0.01 mol/L NaClO4 as the background electrolyte.

Experimental Processes for the Removal of Phenoland Naphthol. The sorption experiments of phenol andnaphthol on rGO were performed under ambient conditions byusing batch technique. Quantitative adsorbent (rGO suspen-sion, 0.1 g/L), background solution (NaClO4 solution, 0.01mol/L) and adsorbate (phenol or naphthol solution, 25 mg/L)were added into the brown glass vials, which were equippedwith the polytetrafluoroethylene-lined screw caps. The pH wasmeasured with a digital pH-meter (PHS-3C) by addingnegligible amounts of 0.01−1.0 mol/L HClO4 or NaOHsolutions. HClO4 and NaOH would not affect phenol ornaphthol sorption on rGO with varied pH. The pH of thesolution was kept below 0.1 before and after sorption. For thesorption isotherms of phenols, the temperatures werecontrolled to 298, 313, and 328 K. The competitive sorptionof phenol and naphthol on rGO was also carried out at thesame level of phenols concentration at 298 K. After the vialswere shaken for 48 h to ensure the sorption equilibrium, thesolid was separated from the liquid phase by centrifugation at5595g for 30 min. The concentration of phenol and/ornaphthol was measured by high performance liquid chromatog-raphy (HPLC). The blank experiments (without rGO) werecarried out under the same conditions to eliminate the massloss during the reaction processes. More details on the analysiswere supplied in SI. All experimental data were the average ofduplicate determinations, and the relative errors were about 5%.

Data Analysis. The kinetic data were fitted by pseudo-first-order and pseudo-second-order models, which were given asfollows:32,33

− = −q q q kln( ) ln te t e 1 (1)

+tqt k q q

t1 1

e e22

(2)

where qt (mmol/g) was the amount of adsorbed phenol and/ornaphthol at time t (h), qe (mmol/g) was the amount ofadsorbed phenol and/or naphthol after reacted completely, k1(1/h) and k2 (mmol/(g·h)) were the rate constant of pseudo-first-order and pseudo-second-order sorption, respectively.The Langmuir34 and Freundlich35 models (eq 3 and 4) were

used to fit the experimental isotherm data.

=+

bq C

bC1max e

e (3)

=qe K C nF e (4)

where Ce (mmol/L) was the final concentration of phenols inaqueous solutions after sorption equilibration, qe (mmol/g) wasthe amount of phenols adsorbed on rGO, qmax (mmol/g) wasthe Langmuir constant, indicated the maximum monolayersorption capacity, and b (L/mol) was a constant that associatedwith the sorption energy, KF (mmol1−n Ln/g) was the

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Freundlich constant when the equilibrium concentration ofphenols reach to 1, and n represented the sorption intensity.Theoretical Calculations. The geometric optimization,

sorption energies and molecular dynamics (MD) calculationsfor rGO sorption systems were performed by Vienna ab initiosimulation package (VASP) (version 5.3.5).36 The densityfunctional theory (DFT) employing projector augmented wave(PAW) method with the Perdew−Burke−Ernzerhof (PBE)functional at the generalized gradient approximation (GGA-PBE) was applied in this work.37−39 More detailed processes onthe calculations of the interactions between rGO and phenolswere provided in SI.

■ RESULTS AND DISCUSSIONCharacterization of rGO Material. The TEM image of

rGO (Figure 1A) exhibited a crumpled and wrinkled flake-likestructure. The ultrathin nature of graphene nanosheets madethem nearly invisible unless the relative clear multilayeredstacks.10 The surface functional groups of rGO weredetermined by FTIR spectroscopy. As illustrated in Figure1B, the bands at ∼3435 and 1400 cm−1 were ascribed to the−OH stretching and bending vibration. The strong band at1589 cm−1 was attributed to the aromatic CC stretchingvibration, and another strong band at 1104 cm−1 was ascribedto C−O stretching vibration.9,40 These functional groups werefurther evidenced from the high deconvolution of C 1S XPSspectrum. As shown in Figure 1C, the carbon existed mainly inthree forms, that is, nonoxygenated carbon (CC, 284.8 eV),the carbon in C−O group (286.4 eV), and the carbonyl carbon(CO, 289.5 eV). However, the carboxylate carbon (OC−O) was not determined due to its low content.41 In addition,the proportion of CC:C−O:CO obtained from theproportion of CC:C−O:CO XPS peak acreage was4.6:1.5:1 (SI Table S1). These functional groups providedabundant reactive sites for the sorption of phenols. The pH

value at the zero point charge (pHZPC) of rGO was calculatedto be 5.6 from the potentiometric acid−base titration curve(Figure 1D), which indicated that the rGO was highlypositively charged at pH < 5.6. Conversely, the surface chargeof rGO was negative at pH > 5.6.

Removal of Phenol and Naphthol. A series of systematicexperiments were performed to evaluate the removal capacitiesof phenol and naphthol by the prepared rGO. Sorption kinetictests were first carried out to determine the contact timeneeded for sorption equilibrium. As can be seen from Figure2A,B, the phenol and naphthol were adsorbed rapidly at thefirst 4 h, and thereafter it proceeded at a slow rate and finallyattained saturation after 6 h of contact time. At the initial state,the phenols were adsorbed onto the rGO surface easily, and theaccumulation of molecules on the surface finally resulted in alow sorption rate in the later stage with contact timeincreased.42 The relative parameters of pseudo-first-order andpseudo-second-order models (SI Table S2) clearly confirmedthat the sorption of phenol and naphthol on rGO wasdominated by the pseudo-first-order model.The neutral or anion form of phenols was determined by

their pKa versus the solution pH values (SI Table S3). At pH <pKa, the nondissociated neutral species were dominated forphenols while the anion forms were dominant at pH > pKa.Figure 2C,D showed the effect of pH on the sorption of phenoland naphthol. The removal efficiency of phenol and naphtholincreased with solution pH increasing until it reached its pKa.The pHPZC of rGO (5.6) indicated that the surface of rGO wasmainly negatively charged at pH > 5.6. Therefore, the reducedsorption of phenol and naphthol at pH > pKa was mainly owingto the increased electrostatic repulsion between the negativelycharged rGO and the dissociated phenols.43 The dissociation ofphenols increased their hydrophilicity, so the decreasedsorption at pH > pKa was also due to the reduced hydrophobicinteractions.44 Furthermore, the dissociation of the −OH group

Figure 1. Characterization of rGO: (A) TEM image; (B) FTIR spectrum; (C) C 1s XPS spectrum; (D) potentiometric acid−base titrations.

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of the phenols was disadvantageous to form the hydrogenbonds between the surfaces of rGO and phenolic molecules,and thereby reduced the removal as well.7,44 Increased sorptionof phenol and naphthol to rGO with increasing pH before theirpKa may be due to the enhanced π−π interactions. Otherinvestigators44,45 documented that the increased sorption ofphenol, naphthol, 1,2,4-trichlorobenzene and 2,4-dinitrotolueneto carbon nanotubes (CNTs) with the increase of pH at pH <pKa. They demonstrated that the increasing pH could changethe properties of polar aromatics, such as the π-donatingstrength, and therefore improved the sorption to CNTs, but theinteraction mechanism was still unclear.Figure 2E,F showed the sorption isotherms of phenol and

naphthol on rGO at the temperatures of 298, 313, and 328 K,respectively. The removal of phenols on rGO increased withincreasing temperature, demonstrating that high temperaturewas beneficial for phenols’ sorption on rGO. The relativeparameters calculated from the Freundlich and Langmuirmodels (SI Table S4) showed that the Langmuir model fittedthe sorption isotherms better than the Freundlich model,revealing that the sorption of phenols on rGO was monolayercoverage. The qmax value of phenol sorption on rGO (2.7

mmol/g at 298 K) was slightly higher than that of naphthol(2.3 mmol/g), which was negatively related to molecular size:22.6 Å2 for phenol and 35.6 Å2 for naphthol (SI Figure S2).Such a phenomenon was also reported by other investi-gators,46,47 where the authors concluded that the pore-fillingmechanism dominated the sorption of polycyclic aromatichydrocarbons on carbon-based materials.To understand the competitive sorption behavior of two

different compounds on the surface of rGO, the competitiveremoval of phenol and naphthol was investigated underdifferent pH and contact time (Figure 2A,D). Clearly, thesorption of phenol and naphthol on rGO were decreased inbinary system over the wide pH range, which was consistentwith the removal of aromatic organic pollutants ontoMWCNTs.27,47 The rGO has a limited number of sorptionsites for both phenol and naphthol molecules to occupycompetitively, thereby resulted in the decreased sorption in thebinary systems. The competitive sorption isotherms of phenoland naphthol on rGO at 298 K were shown in Figure 2E-F.The competitive Langmuir model was used to fit theexperimental isotherm data (SI Figure S3). For the competitivesorption, the qmax values decreased from 2.7 to 1.5 mmol/g for

Figure 2. Effect of time (A and B), pH (C and D), and temperature (E and F) on phenol and naphthol sorption onto rGO. C0 = 25 mg/L, I = 0.01mol/L NaClO4, m/V = 0.1 g/L. Sorption isotherms of phenol (E) and naphthol (F) in a single system at different temperature and in binary systemat T = 298 K on rGO at pH 6.5 ± 0.1. The solid lines represent the Langmuir model. The dashed lines represent the Freundlich model.

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phenol and from 2.3 to 1.6 mmol/g for naphthol, revealing ahigher sorption configurations of rGO for naphthol thanphenol. Furthermore, it was well-known that sorption ofphenols was controlled by a combination of hydrophobicinteractions, π−π interactions and hydrogen-bonding inter-actions.7,43,44 The strength of π−π interactions, hydrophobicinteractions and hydrogen-bonding interactions relied on thesolute π-polarity ability (π*), octane-water distributioncoefficient (Kow) and hydrogen-bonding acceptor ability (βm),respectively, which were closely link with the aromatic ringnumber. According to logKow, π* and βm values of phenol(logKow = 1.46, π* = 0.37 and βm = 0.33) and naphthol (logKow

= 2.84, π* = 0.47 and βm = 0.33), it was reasonable thatnaphthol had very strong competitive sorption effect to phenolowing to its extra benzene-rings.44,48 However, the qmax valuesof phenol and naphthol (i.e., 1.5 mmol/g for phenol and 1.6mmol/g for naphthol) were higher than that of phenol singlesystem (2.7 mmol/g) and that of naphthol single system (2.3mmol/g), suggesting that the sorption capacities of rGO wereincreased for the coexisting of multicomponents. The increasedamounts of phenol and naphthol on rGO at binary systemcould be attributed to the intramolecular interactions betweenthe phenols themselves. Due to the hydrogen bond interactionsbetween hydroxyl groups in naphthol and oxygen-containingfunctional groups in rGO, the hydroxyl groups of naphtholwere preferentially attracted to rGO surfaces and left thehydrophobic benzene rings to face the water molecular insolution. The unoccupied benzene ring of naphthol couldsupply new sorption active sites, thus a second layer of phenolwould be adsorbed to the initially adsorbed naphthol molecules

by hydrophobic as well as π−π interactions between benzenerings. As shown in SI Figure S4, significant sorption of phenolby naphthol confirmed the molecule−molecule attractionsbetween different solutes. The same reaction mechanism wasapplied to explain the competitive sorption of aromatic organiccompounds on MWCNTs and rGO.10,47,49 These findingsindicated that the rGO was very suitable materials for thesimultaneous elimination of organic pollutants from aqueoussolutions.

■ DISCUSSION ON REMOVAL MECHANISM

Spectroscopic Techniques. To understand the interactionmechanism of rGO with phenol and naphthol, the rGOsamples after pollutant sorption were characterized by FTIRand Raman techniques. Figure 3A showed the FTIR spectra ofrGO before and after the sorption of phenol and naphthol. Itwas obvious that the stretching vibration of CC band wasshifted from 1589 to 1585 cm−1 after phenol sorption and to1576 cm−1 after naphthol sorption. It was in line with theprevious observations that π−π conjugative effect and hydro-phobic interactions occurred between rGO and phenols.10,26

McDermott and McCreery50 pointed out that the graphitebasal plane in the vicinity of the edges was usually electron-rich,whereas the regions in the graphene surface center weretypically electron-depleted. Therefore, π−π electron donor−acceptor interaction was occurred between the π-electron-richphenyls of phenols and the π-electron-deficient matrix ofgraphene nanosheets. In addition, the hydroxyl groups wereelectron-donating functional groups, which could enhance theπ-donating strength of host aromatic ring.44 Thereby, the −OH

Figure 3. Characterization of rGO before and after the sorption of phenol and naphthol: (A) FTIR spectra; (B) Raman spectra.

Figure 4. Snapshots of the MD trajectory for the sorption process of phenol (a) and the optimized static structure for the phenol-rGO system fromthe side view (b) and the top view (c).

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could improve the sorption ability of phenols to the surfaces ofrGO via π−π interaction. Similarly, Chen et al.51 documentedthat π−π interaction lead to stronger removal of the amino- andhydroxyl-replaced aromatic compounds than the nonpolararomatic compounds to CNTs. In addition, small shifts in the−OH bond were observed after sorption, from 3435 to 3428cm−1 after phenol sorption and from 3435 to 3430 cm−1 after

naphthol sorption. Jin et al.9 also reported that −OH bond ofrGO changed after the sorption, that is, from 3444 to 3415cm−1 for 4-n-nonylphenol and from 3444 to 3428 cm−1 forbisphenol A. The substituent hydroxyl group on phenolmolecules may form hydrogen bonds with the O-containingpolar moieties on rGO. The proposed mechanism was furthersupported by Raman spectroscopy analysis (Figure 3B), the G

Figure 5. Snapshots of the MD trajectory for the sorption process of naphthol (a) and the optimized static structure for the naphthol-rGO systemfrom the side view (b) and the top view (c).

Figure 6. (A) Dynamical properties analyses of hydrogen bonds for the phenol (a) and naphthol (b) in the individual solution boxes. (B) Dynamicalproperties analyses of hydrogen bonds for the phenol (a), naphthol (b) and phenol-naphthol (c) in the mixed solution boxes.

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band (∼1580 cm−1) was related to the vibration of sp2 carbonatoms in the 2-dimensional hexagonal lattice of graphite, andthe D band (∼1350 cm−1) was assigned to the vibrations of thedefected and disordered sp3 carbon atoms.52 The weak andbroad 2D peak at ∼2700 cm−1 was an out-of-plane vibrationmode which was another indication of disorder consquence.52

The ratio of D and G band intensities (ID/IG) was a commonindex about the extent of defects on the surfaces of rGO.Noteworthy from Figure 3B, the intensity ratios of ID/IG ofrGO-phenol (0.93) and rGO-naphthol (0.95) were larger thanthat of rGO (0.91). This implied that the size of the “graphene-like” domains was smaller than those before sorption, howeverit was much more numerous in the number.53

Theoretical Calculations. The snapshots for the gradualsorption process of phenol and naphthol were shown in Figures4 and 5, respectively. The final closest interaction distances ofphenol and naphthol were 3.503 and 3.438 Å, respectively.Thus, naphthol may have stronger π−π interaction with rGOplane than phenol. This conclusion was further proved by thesorption energy (Es) calculations. The Es was calculated by theformula: Es = E[A] + E[B] − E[total], where E[total] represented thetotal energy of the target complex system, E[A] was the totalenergy of rGO, and E[B] was the total energy of the isolatedphenol or naphthol molecule. The calculated Es in SI Table S5showed that the rGO-phenols system was stable and rGO waseffective adsorbent for the removal of phenols pollutants fromnatural environment. The more positive the Es is, the morestable the system is.54 The Es of naphthol-rGO was higher thanthat of phenol-rGO indicated the naphthol-rGO system hadstronger stability, which agreed well with the competitivesorption experimental observations.Interestingly, in aqueous solutions, except for the sorption

between phenols and rGO, there were also inter- orintramolecular interactions between the phenols themselves.The initial physical or chemical properties of the adsorbedmolecules themselves may play an important role during thesorption process. Thus, furthermore, the MD simulations wereperformed in solution box to explore the initial interactionsbetween the adsorbed molecules. The MD simulation detailswere shown in SI. The total hydrogen bond numbers, diffusionconstant, solvent accessible surface area (SASA) werecomputed using the g_hbond, g_msd, and g_sas tool of theGromacs Program package, respectively.55 From the self-diffusion constants calculated from the theory (SI Table S6),one may draw the conclusion that the self-diffusivity innaphthol packed solution box (1.54 × 10−5 cm2s−1) was alittle higher than that of phenol (1.53 × 10−5 cm2s−1), whiletheir mixture showed much higher self-diffusivity of 2.73 × 10−5

cm2s−1. Generally, the diffusion constant reflected the diffusivityand mobility of molecule to a certain extent. The higher thediffusivity, the faster it diffuses and this will eventually influencethe sorption process. The hydrogen bond numbers of differentreactants were shown in Figure 6. The hydrogen bond numberin individual phenol or naphthol was quite same. When thephenol and naphthol molecules were mixed, they were apt toform hydrogen bonds between phenol and naphthol other thanphenol or naphthol themselves individually, since the formationfrequency of phenol-naphthol was much dense than the ones inphenol or naphthol alone. Therefore, the different formationpattern of hydrogen bond was probably another importantfactor in sorption. The SASA reflected the surface area of amolecule that was accessible to solvent. Figure 7 showed thatthe hydrophilic and hydrophobic SASAs of naphthol werelarger than that of phenol. Since the naphthol moleculepossessed one more aromatic ring than the phenol molecule,which was benefit for the formation of π−π and hydrophobicinteractions. In addition, from the calculated curves, thenaphthol system was much more fluctuant than the smoothones in the phenol system, which showed the instability of thenaphthol clusters. Based on the above analysis, one canconclude that the interaction of phenols with rGO was mainlydominated by hydrophobic interactions, π−π interactions andhydrogen bonds.

Environmental Implications. Phenols were regarded aspriority contaminants because they are harmful to organisms atlow levels and can be toxic when present elevatedconcentrations and are suspected to be carcinogens.7 Hence,it was regarded particularly important and urgent to eliminatethe phenols from industrial effluents before discharging into theaqueous solution. Graphene is expected to have excellentsorption capacity toward phenols organic compounds, and hasthe potential to be applied as a superior adsorbent inwastewater and drinking water treatments.19 For the firsttime, we systematic studied the sorption of phenol andnaphthol on rGO and demonstrated their interaction processby MD simulation and DFT calculations. The batchexperimental results proved that the sorption of phenols onrGO was highly dependent on solution chemistry. Whencompared with phenol, naphthol exhibited a higher sorptionenergy with rGO. In binary phenol−-naphthol system,naphthol presented a greater inhibition of the removal ofphenol. The competitive sorption of phenols on rGO inmultiple organic contaminants systems was mainly dependenton their chemical properties and the experimental conditions.The findings were crucial to assess the removal of coexistingaromatic organic pollutants on rGO and offered an indication

Figure 7. Dynamical properties analyses of solvent accessible surface area (SASA, nm2) for the phenol (A) and naphthol (B) in solution boxes.

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of future directions to synthesize new kinds of nanomaterialsfor the simultaneous elimination of organic pollutants fromwastewater.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.est.6b06259.

Additional preparation of GO and rGO. More detailedprocesses and information on MD simulation and DFTcalculation. The relative parameters of model simulation(PDF)

■ AUTHOR INFORMATIONCorresponding Authors*(X.K.W.) Phone/fax:86-10-61772890; e-mail: xkwang@ncepu.edu.cn.*(Y.J.A.) aiyuejie314@126.com.ORCIDYongfei Ji: 0000-0002-6759-7126Xiangke Wang: 0000-0002-3352-1617NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge the National Natural Science Foundation ofChina (91326202, 21225730, 21577032, and 21403064), theScience Challenge Project (JCKY2016212A04), the Funda-mental Research Funds for Central Universities (JB2015001).X. Wang acknowledged the CAS Interdisciplinary InnovationTeam of Chinese Academy of Sciences.

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