氧化石墨烯对水中内分泌干扰物双酚A的吸附性能

徐 婧 朱永法,2,*

(1清华大学化学系,北京100084;2南京信息工程大学,大气环境监测与污染控制高技术研究重点实验室,南京210044)

1 Introduction

The European Commission has defined an endocrine-disrupting chemical(EDC)as“an exogenous substance that causes adverse health effects in an intact organism,or its progeny,consequent to changes in endocrine function”.1EDCs can cause abnormalities in the functions of endocrine systems of wildlife and humans.2Therefore,EDCs have attracted increasing scientific and social attention in recent years.Bisphenol A(BPA),one of these EDCs,is widely used as an intermediate in the production of polycarbonates,epoxy resins,and other plastics.It is considered to be one kind of carcinogens and critical pollutants because it is harmful to organisms.3,4Although BPA is degradable under natural aerobic condition,it has been reportedly detected in wastewater,surface water,groundwater,and even drinking water.5,6Accordingly,BPA is extensively studied as the model compound for the removal from water among the phenolic EDCs.

It is important to develop advanced methods to remove BPA from aqueous solutions.Conventional methods are adsorption,7-10biological treatments,11photocatalytic degradation,12and other processes,among which adsorption has been found to be a superior and rapid removal method as it is low cost,easy to operate,and no secondary pollutants.Regarding the adsorption technique,an effective adsorbent is crucial to guaranteeing the efficiency of water treatment.Common adsorbent materials include activated carbon(AC),13carbon nanomaterials,7,14carbon nanotubes,15graphene,16,17silicas,10etc.9,18,19Among them,AC has been the most commonly used for the removal of organic contaminants.20Nevertheless,considering its low absorption rate,high content of impurities,difficulty of circulating operation,AC is not perfect enough to meet the need of industrial usages.Therefore,it is still necessary to develop new effective adsorbent materials with a high adsorption capacity,good stability,and fast adsorption rate.

Graphite oxide(GO),an oxygen-rich derivative of graphite,has been extensively investigated in recent years.It exhibits an extended layered structure with plenty of hydrophilic oxygencontaining groups(―OH,―COOH,―CHO,and epoxy groups)on the graphitic backbone.21,22Additionally,GO can be obtained from cheap natural graphite in large quantities,and shows excellent adsorption capacity for the removal of heavy metal ions,23-25dyes,26,27and antibiotics28from aqueous solutions.However,no reports regarding the application of GO on BPAremoval has been reported to the best of our knowledge.

In this paper,the removal behavior of BPA from water by GO as a function of solution characteristics,including BPA concentration,pH,ionic strength,and temperature,was investigated systematically.The adsorption capacity was evaluated and adsorption mechanism of BPAby GO was proposed.

2 Materials and methods

2.1 Synthesis of GO

All chemicals used were analytic grade reagents without further purification.GO was synthesized by a modified Hummers method.29,30Graphite(2.0 g)and NaNO3(1.0 g)were mixed with 50 mL of H2SO4(98%(w))in a 500 mL flask and stirred for 30 min in an ice bath.Then KMnO4(6.0 g)was dropped into the vigorously stirred suspension below 15°C.The ice bath was then removed,and the mixture was stirred at room temperature until it gradually became a brownish slurry,and then diluted slowly with 100 mL of water.The reaction temperature was rapidly increased to 98°C with effervescence and the colour of suspension changed to brown.After that,200 mL of water and 10 mL of H2O2(30%(w))were successively added.For purification,the mixture was centrifuged and washed with 10%HCl and then deionized water several times to remove the residual metal ions and acid.After filtration and drying under vacuum at room temperature,GO was obtained as powder.

2.2 Characterization

The samples were characterized by powder X-ray diffraction(XRD)on a Bruker D8-advance X-ray diffractometer at 40 kV and 40 mA for monochromatized CuKα(λ=0.1541 nm)radiation.The Brunauer-Emmett-Teller(BET)specific surface area of the samples was characterized by nitrogen adsorption at 77 K with a Micromeritics 3020 instrument.Zeta potential measurements were made with a Delsa Nano C zeta potential instrument(Beckman Coulter).Atomic force microscope(AFM)images were acquired in phase mode in air using Digital Instruments Shimadzu SPM-9600(The samples were prepared by drop-casting corresponding dilute dispersions onto a freshly cleaved mica surface).Fourier transform infrared(FTIR)spectra were recorded on a Thermo Nicolet Avatar 370 spectrometer between 4000 and 400 cm-1using KBr pellets.

2.3 Batch adsorption experiments

BPA((CH3)2C(C6H4OH)2,molecular weight 228.29)was purchased from the Beijing Chemical Works,China.It is a hydrophobic compound with a low solubility in water.BPA is in the molecular form at pH＜8.0 and starts the first deprotonation at around pH 8.0 and the second one at around pH 9.0.It is mostly ionized to monovalent or divalent anions after the deprotonation.13BPA possesses hydroxyl groups which can generate hydrogen bonding with adsorbents.Herein,BPA was dissolved in ethanol as a stock solution(1000 mg·L-1)and was further diluted with a large amount of water to the required concentrations before used.

All adsorption experiments were performed in sealed 250 mL glass conical bottles in a shaking water bath at a shaking speed of 200 r·min-1at the appropriate temperature.Every bottle contained 10 mg of GO and 100 mL of BPA solution in theappropriate concentration.

An adsorption kinetic study was obtained with an initial BPA concentration of 10 mg·L-1at 25 °C,pH 6.0 to determine the time required for adsorption to reach equilibrium.The concentrations of BPA were measured at different time intervals from 5 to 300 min.

Adsorption isotherm of BPA on GO was carried out at 25°C,pH 6.0,with different initial BPA concentrations ranging from 2 to 50 mg·L-1.

The effect of pH on the adsorption of BPA was examined with an initial BPA concentration of 10 mg·L-1in a pH range of 2.0-11.0 at 25°C.The solution pH was regulated by adding 0.1 mol·L-1HCl or NaOH solution.

The effect of the ionic strength on the adsorption of BPA was examined by adding NaCl to 10 mg·L-1BPA solutions with concentrations ranging from 0.02 to 0.5 mol·L-1at 25 °C,pH 6.0.

The effect of the temperature on the adsorption of BPA was examined with an initial BPA concentration of 10 mg·L-1at pH 6.0 at different temperatures of 15,25,35,and 45°C,respectively.

After adsorption experiments,the suspensions were centrifuged at 12000 r·min-1for 10 min,and the supernatant was filtered through 0.45 μm membrane.The concentration of BPA was examined by a high-performance liquid chromatography(HPLC,Lumtech)system with a Venusil XBP-C18 column(Agela Technologies Inc.)and using a UV absorbance detector(K2501)operated at 280 nm.The mobile phase was 1.0 mL·min-1of 70%methanol and 30%deionized water.

3 Results and discussion

3.1 Properties of graphene oxide

XRD patterns for graphite and GO were obtained and shown in Fig.S1(Supporting Information).Graphite exhibited a very sharp(002)diffraction peak at 26.4°.The 2θvalue corresponded to an interlayer spacing of about 0.34 nm.Oxidation treatment caused a decrease in the(002)peak intensity of graphite,and a(001)diffraction peak of GO appeared at 11.0°.This(001)peak demonstrated the typical loose-layer-like structure due to the intercalating oxygen-containing groups.The corresponding interlayer spacing of GO was about 0.80 nm which was evidently larger than that of graphite.The interlayer spacing might be dependent on the preparing method and the amount of water remained in the gallery space of GO.24

N2adsorption-desorption isotherms for graphite and GO were determined in Fig.S2a(Supporting Information).Graphite and GO both showed the type IV isotherms classified by IUPAC with hysteresis loops observed in the relative pressure(p/p0)range of 0.45-1.00,suggesting the mesoporous property.31Owing to the severe aggregation of GO sheets during desiccation,GO exhibited a relatively low BET specific surface area of 32 m2·g-1,which was still higher than that of graphite(15.9 m2·g-1).The pore size distribution of GO was more remarkable than that of graphite(Fig.S2b(Supporting Information)).The width of the pores in GO sheets was mainly 3.6 nm.Since the largest length of BPA molecule was 0.94 nm,13the pore size of GO was big enough for BPA molecules to access to the functional groups on the surface of mesostructures.Thus,GO could be a good candidate as adsorbent.

The stability of GO dispersions was examined by the zeta potential analysis.The zeta potentials of GO dispersions were always negative throughout the whole pH range,which indicated the presence of negative charges on the surface of the GO sheets(Fig.S3(Supporting Information)).The zeta potentials were below-30 mV for pH＞5.0 and can reach-40 mV for pH 10.0.Zeta potentials lower than-30 mV generally imply sufficient mutual repulsion which can ensure the stability of a dispersion,as well known in colloidal science.33Therefore,the excellent stability of GO dispersions was mainly attributed to the electrostatic repulsion among the GO sheets.

The morphological structure of GO was characterized by AFM.The AFM image(Fig.S4a(Supporting Information))showed that the GO sheets were almost transparent with a flake-like shape.The partial overlap of the GO sheets caused different brightness values on the surface of GO.The sizes of the GO sheets were ranging from several tens of nanometers to several micrometers.The cross-section analysis of GO(Fig.S4b(Supporting Information))indicated that the average height of the GO sheets was about 1.0 nm which meant that GO existed mainly in a single-layer state in aqueous solution.Similar results had also been reported in other AFM studies of GO.23,34

3.2 BPA adsorption kinetics

Adsorption kinetics was investigated for a better understanding of the dynamics of adsorption.The concentration of BPA in aqueous solution without GO did not change after 300 min(Fig.S5(Supporting Information)).This blank experiment indicated that BPA cannot be decomposed by itself.The effect of contact time on the adsorption of BPA by GO was shown in Fig.1.The adsorption achieved equilibrium in a short time of about 30 min,suggested that GO showed very rapid adsorptionrate and high industrial application value.On the basis of the above result,the contact time of 2 h was selected for a sure establishment of the adsorption equilibrium in further adsorption studies.

Fig.1 Effect of contact time on the adsorption of BPAby GO

Table 1 Kinetic parameters for the adsorption of BPAby GO

The adsorption capacity of GO for BPA was calculated according to the following equation:

whereC0andCerepresent the initial and equilibrium concentrations of BPA aqueous solution(mg·L-1),Vis the volume of the solution(L),andmis the mass of the adsorbent(g).

To investigate the adsorption kinetics of BPA by GO,two conventional kinetic models(pseudo-first-order and pseudosecond-order)were adopted to simulate the experimental data.The pseudo-first-order model can be expressed as35

whereqeandqtare the amounts of BPA adsorbed on GO at equilibrium and at various timet(mg·g-1),respectively,andk1is the rate constant of the pseudo-first-order model of adsorption(min-1).The values ofqeandk1can be determined from the intercept and slope of the linear plot of ln(qe-qt)versus t.

The pseudo-second-order model includes all the steps of adsorption including external film diffusion,adsorption,and internal particle diffusion,which is described as36,37

whereqeandqtare defined as in the pseudo-first-order model andk2is the rate constant of the pseudo-second-order model for adsorption(g·mg-1·min-1).The slope and intercept of the linear plot oft/qtagainsttyield the values ofqeandk2.Furthermore,the initial adsorption rateh(mg·g-1·min-1)can be determined fromh=k2q2e.

Table 1 presented the kinetic parameters for the removal of BPA by GO.The correlation coefficientR2value for the pseudosecond-order model exceeded 0.99,which was much higher than that of the pseudo-first-order model.Moreover,the calculated adsorption capacity(qe,cal)obtained from the pseudosecond-order model also coincided well with the experimental adsorption capacity(qe,exp).These results indicated that the pseudosecond-order kinetic model provided a better correlation in contrast to the pseudo-first-order model for the adsorption of BPA on GO.

3.3 BPA adsorption isotherms

The adsorption isotherm models are usually used to fit experiment data and help to explore the adsorption mechanism more deeply.It can be seen in Fig.2 that the adsorption capacity of GO increased with the increasing equilibrium concentration of BPA and reached saturation progressively,as the increase in BPA concentration could accelerate the diffusion of BPA molecules onto the GO sheets.The Langmuir and the Freundlich isotherms are the most frequently used models to describe the equilibrium data of adsorption from aqueous solution.

The Langmuir isotherm assumes monolayer coverage of the adsorption surface and no subsequent interaction among adsorbed molecules.The expression for the Langmuir isotherm is38

whereqeis the adsorbed BPA amount per gram of GO(mg·g-1),Cerepresents the equilibrium concentration of BPAin solution(mg·L-1),KLis the Langmuir constant(L·mg-1),which is related to the affinity of the binding sites,andqmrepresents the maximum adsorption capacity of the adsorbents(mg·g-1).The values ofqmandKLare calculated from the slope and intercept of the linear plot ofCe/qeagainstCe.

The Freundlich isotherm is derived to model multilayer adsorption and adsorption on heterogeneous surfaces.It can be described as39

whereqeandCeare defined as in the Langmuir isotherm;KFandnare the Freundlich constants that represent the adsorption capacity and the adsorption strength,respectively.The magnitude of 1/nquantifies the favourability of adsorption and the degree of heterogeneity of the surface of GO.Ifn＞1,suggesting favourable adsorption,then the adsorption capacity increases and new adsorption sites form.40KFandncan be obtained from the intercept and slope of the linear plot of lnqeversuslnCe.

The isotherm parameters were calculated and listed in Table 2.On the basis of a comparison of the correlation coefficientR2values,the Langmuir model fit the adsorption data better than the Freundlich model.In other words,this adsorption process took place at the functional groups on the surface of GO sheets,which was regarded as monolayer adsorption.In addition,it can be calculated from the Langmuir equation thatqmwas 87.80 mg·g-1.

Fig.2 Adsorption isotherms of BPAby GO

Table 2 Isotherm parameters for the adsorption of BPAby GO

For comparison,the adsorption capacities of other common adsorbents in the literature were also summarized in Table S1.Compared with them,the adsorption capacity of GO was the highest.Considering the BET surface area of the adsorbents,the high affinity of GO to BPA was even more obvious.Besides,the adsorption rate of GO had also been compared with that of AC.10The required contact time to reach equilibrium was approximately 30 and 120 min for GO and AC,respectively.This implied that the attachment of BPA to the surface of GO was faster than that of AC.These results indicated that GO was an excellent BPAabsorbent with high capacity and rate.

3.4 Effect of the solution pH,ionic strength and temperature

The solution pH is one of the most important parameters affecting the adsorption process in aqueous solution.41Fig.3(a)showed the effect of the solution pH on BPA adsorption by GO,with the initial pH ranging from 2.0 to 11.0.With the increasing pH,the adsorption capacity of GO firstly increased and then decreased,the maximum was at pH 6.0.These phenomena can be explained by the net charge of GO and BPA at different pH values.GO was negatively charged over the whole pH range,which was shown by the zeta potential analysis(Fig.S3).BPA was in its molecular form at pH＜8.0 and started the first deprotonation at around pH 8.0 and the second at around pH 9.0.It was mostly ionized to monovalent or divalent anions after the deprotonation.13Therefore,the increase in adsorption capacity of GO in the acidic pH range might be due to the competition between hydrogen ions and BPA for the adsorption sites on GO.With increasing the pH value,there were fewer hydrogen ions in solution,which led to more binding sites available for BPA.The reduction in the adsorption capacity of GO observed in the alkaline pH range was partly owing to the repulsive electrostatic interactions established between the negatively charged GO surface and the bisphenolate anion.

It is well known that industrial sewage contains not only pollutants but also high concentrations of salts,which may affect the removal of pollutants.Thus,a study was also conducted on the effect of the solution ionic strength on the adsorption of BPA by GO.We can see that the adsorption capacity of GO was decreased in the presence of NaCl in solution(Fig.3(b)).This indicated that NaCl was more competitive than BPA for adsorption sites on GO.The binding sites of GO available for BPA were occupied by NaCl.However,the adsorption capacity was increased slightly when the NaCl concentration exceeded 0.1 mol·L-1.This phenomenon can be attributed to the saltingout effect of electrolytesviadecreasing the solubility of BPA and enhancing its adsorption on GO.8,13

The effect of the solution temperature on the adsorption of BPA by GO was also investigated under four different temperatures(Fig.3(c)).With increasing the temperature,the decline in the adsorption capacity of GO was observed.It indicated that the adsorption of BPA on GO was unfavorable at higher temperature and the adsorption reaction was an exothermic process.8

3.5 Regeneration study of GO

The recyclability of GO was determined by investigating the adsorption ability of the regenerated GO.In this part,10 mg of GO was first mixed with 100 mL of BPA solution(10 mg·L-1)for 2 h at 25°C,pH 6.0.Then the above BPA adsorbed GO was dispersed in an aqueous solution(100 mL)containing ethanol(10%(volume fraction))for elution.The desorption efficiency(η)can be determined from the following equation:

whereCtrepresents the concentration of BPA aqueous solution at a given time(mg·L-1),qeis the amount of BPA adsorbed on GO at equilibrium(mg·g-1),Vis the volume of the solution(L),andmis the mass of the adsorbent(g).

After BPA was completely desorbed from GO,it was thoroughly washed with water,filtered and dried in a vacuum.The regenerated GO was then applied to the repeated adsorption/desorption cycles to study the recyclability of GO.

The desorption efficiency was shown in Fig.4(a).It can be seen that BPA gradually desorbed from GO.After 300 min,about 91%of BPA adsorbed on GO previously had desorbed into the solution.From Fig.4(b),it was found that the adsorp-tion capacity of GO for BPA declined from 33.3 to 27.8 mg·g-1after five time cycles.Compared with the first adsorption,the fifth one decreased by about 17%which still indicated the good reusability of GO.Thus,due to its excellent recyclability,GO is qualified for practical application.

Fig.3 Effects of the solution pH(a),ionic strength(b),and temperature(c)on the adsorption of BPAby GO

Fig.4 (a)Desorption efficiency(η)of GO and(b)re-adsorption of GO

Fig.5 Schematic of hydrogen bonding and π-π interaction between GO and BPA

3.6 Adsorption mechanism

GO contained hydrophobic graphene basal planes with aromatic rings and hydrophilic groups such as hydroxyl(―OH)and carboxyl(O＝C―O).BPA also had both hydrophobic phenyl groups and hydrophilic hydroxyl groups.Due to the similar composition of BPA and GO,it was expected that two kinds of adsorbent-adsorbate interactions might be responsible for the adsorption of BPA on GO.10One possible interaction was hydrogen bonding between the hydrophilic groups of GO and BPA.The other wasπ-πinteraction between the benzene rings contained in both BPA and GO.42Fig.5 showed the schematic of hydrogen bonding andπ-πinteraction between BPA and GO.The interactions between BPA and GO were also investigated using the FTIR spectra of GO and GO after BPA adsorption.

FTIR spectroscopy has been used as a useful tool in identifying the presence of certain functional groups on the surface of a solid.In Fig.6,the FTIR spectrum of GO showed broad absorption at 3418 cm-1,which corresponded to―OH groups.The peak at 1727 cm-1indicated the existence of C＝O bonds in carboxylic acid and carbonyl moieties.The peak at 1622 cm-1may be from skeletal vibrations of aromatic C＝C bonds.The peak around 1383 cm-1belonged to carboxyl O＝C―O bonds.The peak at 1223 cm-1was from epoxy C―O―C bonds.The peak around 1048 cm-1referred to alkoxy C―O bonds.43The FTIR spectrum of GO after BPA adsorption displayed that the stretching frequency of hydroxyl groups shifted from 3418 to 3433 cm-1.This indicated that there might be hydrogen bonding between hydroxyl groups of BPA and hydrophilic groups of GO.10The band corresponding to aromatic groups at 1622 cm-1broadened obviously compared to that of GO.This proved that there might beπ-πinteraction between the phenyl groups of BPA and the graphene planes with linked aromatic rings of GO.44Therefore,the result of the FTIR spectroscopy was a new evidence in proving the presence of hydrogen bonding andπ-πinteraction between GO and BPA.

Fig.6 FTIR spectra of GO(a)and GO after BPAadsorption(b)

4 Conclusions

GO showed excellent adsorption capacity and high adsorption rate for BPA.The maximum adsorption capacity of GO for BPA estimated from the Langmuir isotherm was 87.80 mg·g-1at 25°C.Besides,the required contact time to reach the adsorption equilibrium was about 30 min.The effect of the experimental conditions on the adsorption properties of GO in aqueous solution has been demonstrated.The kinetics and isotherm data were fit well with the pseudo-second-order kinetic model and the Langmuir isotherm,respectively.The neutral pH and low temperature of the solution were favorable for the adsorption,whereas the presence of NaCl in the solution was unfavorable.GO has good recyclability,can retain about 83%of their adsorption ability after five adsorption-desorption cycles.The large adsorption affinity of GO for BPA seems to be attributed to its hydrophobic graphene planes with aromatic rings and hydrophilic oxygen-containing groups,which can generateπ-πinteraction and hydrogen bonds with two benzene rings and two hydroxyl groups of BPA,respectively.The adsorption mechanism was also proved by the FTIR spectra of GO and GO after BPA adsorption.Although the adsorption capacity of GO for BPA is lower than that of graphene,16GO has its own available superiorities,like large quantity production,hydrophilic surface with plenty of oxygen-containing groups,and good dispersion in water.Therefore,GO is a potential adsorbent for water treatment.

Supporting Information:XRD patterns of graphite and GO(Fig.S1).Nitrogen adsorption-desorption isotherms and the pore size distribution of graphite and GO(Fig.S2).Zeta potential analysis of GO dispersion(Fig.S3).AFM image and cross-section analysis of GO(Fig.S4).The blank experiment of BPA concentration change without GOversustime(Fig.S5).Adsorption capacity of BPA by GO in comparison to other literature values(Table S1).This information is available free of chargeviathe internet at http://www.whxb.pku.edu.cn.

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