Effect of the degree of oxidation and defects of graphene oxide on adsorption of Cu2+ from aqueous solution

Effect of the degree of oxidation and defects of graphene oxide on adsorption of Cu2+ from aqueous solution

Accepted Manuscript Title: Effect of the Degree of Oxidation and Defects of Graphene Oxide on Adsorption of Cu2+ from Aqueous Solution Authors: Ping T...

1MB Sizes 1 Downloads 22 Views

Accepted Manuscript Title: Effect of the Degree of Oxidation and Defects of Graphene Oxide on Adsorption of Cu2+ from Aqueous Solution Authors: Ping Tan, Qi Bi, Yongyou Hu, Zheng Fang, Yuancai Chen, Jianhua Cheng PII: DOI: Reference:

S0169-4332(17)31959-1 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.304 APSUSC 36504

To appear in:

APSUSC

Received date: Revised date: Accepted date:

2-5-2017 27-6-2017 29-6-2017

Please cite this article as: Ping Tan, Qi Bi, Yongyou Hu, Zheng Fang, Yuancai Chen, Jianhua Cheng, Effect of the Degree of Oxidation and Defects of Graphene Oxide on Adsorption of Cu2+ from Aqueous Solution, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.304 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of the Degree of Oxidation and Defects of Graphene Oxide on Adsorption of Cu2+ from Aqueous Solution

Ping Tana, Qi Bia, b, Yongyou Hua *, Zheng Fanga, Yuancai Chena, Jianhua Chenga

a

Ministry of Education Key Laboratory of Pollution Control and Ecological Remediation for

Industrial Agglomeration Area, School of Environment and Energy, South China University of Technology, Guangzhou 510000, PR China b

School of Civil Engineering and Transportation, , Guangzhou 510000, PR China

*Corresponding author. Tel: +86-20-39380506. Fax: +86-20-39380508. E-mail: [email protected] (Yongyou Hu)

Highlights 

A series of GO with different degree of oxidation and defects were prepared.



The adsorption of Cu2+ on GO was increased with increasing the degree of oxidation.



The adsorption of Cu2+ on GO was independent of the defects of GO.



GO series with different degree of oxidation and defects were well-described by the Langmuir model.

Abstract: Graphene oxide (GO) is a promising adsorbent for heavy metal ions from water. However, the relationship between the degree of oxidation and defects of GO and the

adsorption performance has been rarely reported. In this study, a series of GO with different degree of oxidation (GO1, GO5, GO6) and defects (GO1-GO4) were prepared by the improved Hummers method and were employed to explore the relationship between the degree of oxidation and defects of GO and the Cu2+ adsorption. The results showed that the adsorption of Cu2+ on GO was strongly dependent on the degree of oxidation and independent of the defects under various pH levels and ionic strength. The adsorption isotherms of Cu2+ on GO with different degree of oxidation and defects were well described by the Langmuir model and the maximum adsorption capacity of GO for Cu2+ increased with the improvement of the degree of oxidation but was independent of the defects, indicating that the adsorption of Cu2+ on GO was mainly proportional to the degree of oxidation but become insignificant in the structure integrity of aromatic matrixes, which might be due to the shielding effect of oxygen-containing groups. The adsorption of Cu2+ on GO with different degree of oxidation and defects reached an equilibrium state after 50 min, the adsorption kinetics followed the pseudo-second-order model and the adsorption process was controlled by the degree of oxidation.

Keywords: Graphene Oxide; Degree of Oxidation; Defects; adsorption; Copper Ion

1. Introduction Nanomaterials have attracted tremendous attention as adsorbents due to their large surface area and abundant binding sites. Among them, graphene oxide (GO), as one of the graphene derivatives, is particularly attractive. Its super large surface area (2630 m2g-1) and abundant 2

adsorption sites (hydroxyl, carbonyl, carboxyl, epoxy and π-electron system)[1-4] make GO be considered as an excellent adsorbent. Actually, previous studies have certified that GO shows strong adsorption capacity for heavy metals from aqueous solution [5-9]. Besides the super large surface area of GO and the characteristics of heavy metals themselves, the adsorption of heavy metals on GO is closely related to the structure of GO because the type and quantity of binding sites are restricted by the structure of GO. Therefore, it is very important to have an insight into the adsorption relationship between the structure of GO and the heavy metals. Up to now, the structure of GO has not been quantitatively explained. Generally speaking, in the structure of GO, the region related to the heavy metals adsorption mainly includes two parts: the oxygen-containing groups and the aromatic matrixes. The oxygen-containing groups can interact with heavy metals through electrostatic attraction or complexation[6, 10]; the aromatic matrixes can provide a large number of delocalized π-electron to bind heavy metals through the Lewis acid-base interaction[11, 12]. However, due to the different method of oxidation, the relative proportion of the two regions within GO is flexible and fickle, resulting in the structural diversity of GO and the different affinity of heavy metals. Therefore, it is very necessary to discover the relationship between affinity and structure, which may directly affect the adsorption capacity of heavy metals on GO. The primary approaches for the synthesis of GO are the chemical oxidation of graphite by using strong oxidants[13-15]. Among them, the most commonly used method is the Hummers method[15]: the graphite is oxidized by treatment with KMnO4 and NaNO3 in concentrated H2SO4. Although many oxygen-containing groups can be introduced into the 3

framework of GO, the degree of oxidation of graphite is inadequate, which is not conducive to the adsorption of heavy metals on oxygen-containing groups on the surface of GO. In order to improve the degree of oxidation, the modified Hummers’ method (Hummers’ method with additional KMnO4) has been developed to synthesize GO with higher degree of oxidation[16, 17]. However, the modified Hummers’ method is based on the expense of the aromatic matrixes in GO to improve the degree of oxidation. This method can raise the amount of oxygen-containing functional groups, but reduce the amount of delocalized π-electrons in GO, which go against the adsorption of heavy metals on GO. More worrying, the synthesis processes not only require a high reaction temperature (92-98℃), but also involve the generation of the toxic gases such as NO2 and N2O4 due to the presence of NaNO3 in the oxidation process. Recently, tour research group[18] improved the Hummers method (the improved method) by excluding NaNO3, increasing the amount of KMnO4 and performing the reaction in a 9:1 (by volume) mixture of H2SO4/H3PO4, and synthesize GO with greatly enhanced degree of oxidation. The advantages of the improved method are simple, controllable, high yield as well as the generation of no toxic gases due to without adding NaNO3 into the reaction. More importantly, the phosphoric acid can act as protective agent for keeping the structure integrity of the aromatic matrixes in GO, the complete aromatic matrixes can store more delocalized π-electron interacted with metal ions through the Lewis acid-base reaction. On the opposite, the appropriately increase in the amount of oxidant (KMnO4) can also results in the structure destruction of the aromatic matrixes in GO. It is well known that the adsorption performance of GO is greatly dependent on its structural features. The effect of oxidant (KMnO4) and 4

protective agent (H3PO4) on structure is a pair of adjustable contradictions, which would directly affect the adsorption of heavy metals on GO. Thus, it is crucial to study the effect of the aromatic matrixes and the oxygen-containing groups on adsorption because it is favorable for comprehending interaction mechanism between GO and heavy metals. However, previous works have mainly inclined to qualitative elucidation and few study has involve in quantitative interpretation. In this study, a series of GO with different degree of oxidation and defects were prepared via an improved Hummers method[18] by adjusting the dose of KMnO4 (oxidant) and H3PO4 (protective agent). The GO series have been characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron micrographs (SEM), Atomic force microscopic (AFM) and Zeta potential. Cu2+ was chosen as a model contaminant in study of the adsorption performance of GO. The effect of pH, ionic strength, metal ion concentration and contact time on adsorption of Cu2+ by the GO series have been investigated, the relationship between the degree of oxidation and defects of GO and the adsorption performance has been described.

2.Materials and Methods 2.1. Materials Graphite powder (<20 μm) was purchased from Sigma-Aldrich. Phosphoric acid (H3PO4) and 5

potassium permanganate (KMnO4) were obtained from Guangzhou Chemical Reagent Factory. All other chemicals used in the experiments were of analytical grade.

2.2. Preparation of GO series with different degree of oxidation and defects

GO was prepared by the improved Hummers method[18]. Briefly, graphite powder (3.0 g) was added to the mixture of concentrated H2SO4 and H3PO4. Then, KMnO4 was added gradually with stirring and cooling in order to maintain the temperature below 20 °C. After that, the mixture was heated to 50 °C and stirred for 12 h. Finally, the mixture was cooled to room temperature and poured onto ice (400 ml) with 30% H2O2, which turned the color of the solution from dark brown to yellow. The mixture was filtered, the remaining solid was washed with 10% HCl aqueous solution to remove metal ions. Then, it was purified by dialysis for three weeks using a dialysis membrane to remove the remaining metal species. The resulting product was dried under vacuum and a dark brown GO powder was obtained. With variation in the dose of KMnO4 (oxidant) and H3PO4 (protective agent) in each preparation process, a series of GO with different degree of oxidation and defects have been successfully obtained. The detailed preparation conditions are listed in Table 1.

2.3. Characterization of GO series

Fourier transform infrared (FT-IR) spectra were recorded using a Fourier transform infrared spectrometer (Type Vector 33; Bruker Co.; Germany). Raman spectra were obtained using a 6

Raman spectrometer (Type LabRAM Aramis; Horiba Jobin Yvon Co. Ltd.; France) with a 632.8 nm excitation wavelength. X-ray diffraction (XRD) patterns were obtained with an X-ray diffraction spectrometer (Type D8 ADVANCE; Bruker Co. Ltd.; German) at a voltage of 40 kV and a current of 40 mA using Cu Kα radiation (λ = 0.15418 nm). The X-ray photoelectron spectra (XPS) were obtained from an X-ray photoelectron spectrometer (Type Axis Ultra DLD; Kratos Co. Ltd.; UK). Scanning electron micrographs (SEM) were obtained using a FEI Sirion 200 scanning electron microscope. Atomic force microscope (AFM) images were obtained using a scanning probe microscope (Type Asylum Research Cypher; Oxford Instruments Co. Ltd.; American). Zeta potential analyses were carried out on a zeta potential instrument (Type Nano-Z; Malvern Co. Ltd.; UK).

2.4. Batch adsorption experiment

GO powder (1 g) was dispersed in 1000 ml of deionized water under vigorous stirring and ultrasonic processing for 4 h to obtain GO suspension (1 mg/ml). Batch adsorption experiments were performed in a conical flask. The pH of the solution was adjusted to constant values by the addition of drops of HCl or NaOH solution (0.01~0.1mol/L). The effect of pH on adsorption was studied by adjusting the initial pH from 2.0 to 5.5, which was chosen on the basis of the Cu2+ solubility product constant resulting in no precipitation of Cu2+. The effect of ionic strength ranging from 0 to 0.1mol/L was investigated. The adsorption equilibrium was conducted by varying the initial Cu2+ concentrations ranging from 5 to 150mg/L. The adsorption kinetics study was carried out by sampling a certain volume of the solution at various time intervals. All the experimental solutions were shaken for 5 h at 200 rpm. The solid phases were filtered through a 0.45 μm filter membrane and the 7

concentration of Cu2+ in the filtrate was determined by a Perkin-Elmer Analyst 700 atomic absorption spectrophotometer (Type PinAAcle 900T, PERKINELMER Co., Ltd.; USA). All of the experiments were conducted in triplicate and the experimental data were the average of duplicate determinations. The adsorption capacity was calculated according to the following equation:

qe 

( C0 Ce )V m

(1)

Where C0 and Ce represent the initial and equilibrium concentrations of Cu2+ (mg/L), respectively; V represents the total volume of Cu2+ solution (L); m represents the weight of GO (g).

3. Results and Discussion 3.1. Characterizations of GO series 3.1.1 FTIR analysis

The FTIR spectra of graphite and GO1–GO6 are shown in Fig. 1. The peaks of graphite appeared at 1620 cm-1 corresponding to the stretching vibrations of C=C bonds. It is worth noting that the peaks of graphite exhibited a broad, strong band at approximately 3400 cm-1 related to hydroxyl (-OH), which could be attributed to the presence of residual water molecules in graphite. Compared to graphite, the new characteristic peaks of GO1–GO6 were generated after oxidation of strong oxidant, including -OH vibrations (3400 cm-1), C=O stretching vibration (1730 cm-1), C-O-C vibrations (1250 cm-1). The results indicated that GO with different features contained the same type of groups. However, the FTIR measurements 8

could only be qualitative description. Therefore, it is necessary to use other means to quantify the structure of GO1-GO6.

3.1.2 Zeta potential analysis

Fig.2 shows the zeta potentials of GO series as a function of pH. In all test pH ranges, the zeta potentials of GO were more negative with increasing pH, the trend of which was in accordance with the literatures[6, 19]. It might due to that a mass of oxygen-containing groups with negative charges were introduced to the surface or edge of GO after oxidation and these groups could be ionized to make GO be more negative when pH increased from 2 to 5.5. In test pH ranges, the trends of the zeta potentials of GO1-GO4 were almost consistent, suggesting that the amount of negatively charged groups on surface of GO1-GO4 was almost the same. In addition, the zeta potentials of GO1-GO4 were more negative than that of GO5-GO6 under the same pH value, indicating that the amount of the oxygen-containing groups on the surface of GO1-GO4 was more than that of GO5-GO6. These results indicated that the zeta potential of GO was influenced by the degree of oxidation.

3.1.3 XPS analysis

XPS can offer direct evidence for the elemental composition of GO1–GO6. The wide scan XPS spectra are shown in Fig.3 and the devolved C1s peaks are supplied in supplementary information Fig. S1. In order to eliminate the influence of water molecules, the samples were dried under a vacuum for 6 h at 90°C before the XPS test, because the temperature does not lead to the loss of carbon and oxygen. As shown in Fig. S1, oxygen-containing functional groups were observed on GO series, including O-C=O, C=O, C–O and C–C groups, which 9

was similar to the previously reported literatures [20-22]. Moreover, the C/O atomic ratios of GO1–GO6 were 2.00, 1.96, 2.00, 1.97, 2.59 and 4.64, respectively, the relative content of oxygen atoms of GO1–GO6 increased with an increase in the amount of KMnO4, suggesting that the order of the degree of oxidation was GO1≈GO2≈GO3≈GO4>GO5>GO6. The results indicated that the degree of oxidation of GO was affected by the amount of the strong oxidant and was independent of the protective agent.

3.1.4 Raman spectroscopy analysis

Raman spectroscopy is a widely used tool for the characterization of carbon products due to that conjugated and carbon double bonds can result in high Raman intensities[23]. It is well known that the G band is associated with the vibration of sp2 carbon atoms in a graphitic 2D hexagonal lattice; the D band is related to the vibrations of sp3 carbon atoms of structural defects, amorphous carbon and edges, which can break the symmetry and selection rules of the material[24]. Therefore, the intensity ratio of the D band to the G band (ID/IG) is often used as a measure of the system’s disorder. As shown in Fig.4, the ID/IG ratios of GO1–GO4 were 1.09, 1.30, 1.42 and 1.50, respectively, suggesting that the area of sp3 carbon atoms zones in oxidized zones increased from GO1 to GO4. The result indicated that the defects of GO increased with decreasing the dose of H3PO4 because the addition of H3PO4 could produce GO with less defects and more intact graphitic basal planes, which was similar with the previously reported literatures[25, 26]. The result further suggested that the order of the defects of GO was GO1
suggesting the defects (the area of sp3 carbon atoms zones) of GO was proportional to the oxidant dose. The result indicated that the order of the defects was GO1>GO5>GO6. Therefore, the order of the defects was GO6
3.1.5 XRD analysis

The XRD patterns of graphite and GO1–GO6 are shown in Fig. 5. The graphite exhibited a typical sharp diffraction peak at 2θ=26.5° corresponding to an interlayer spacing of 0.34 nm. The oxidation reaction resulted in a decrease of the typical sharp diffraction peak intensity of graphite and the appearance of the diffraction peak of GO series [28]. The diffraction peak at 26.5° disappeared entirely in GO1–GO4 but still faintly existed in GO5 and GO6, indicating that incomplete oxidation was occurred in GO5–GO6 and was not appeared in GO1–GO4. Moreover, a new diffraction peak at approximately 2θ=10° was observed in GO1–GO6, which was probably caused by the intercalation of oxygen-containing functional groups in the interlayer space and consequent expansion of GO layers along the c-axis. From Table 1, the 11

interlayer spacing of GO1–GO4 was gradually reduced when the dose of H3PO4 was decreased, which was consistent with the results reported by tour[18]. In addition, comparing to GO1, owing to the decrease of the oxidant dose, the interlayer spacing of GO5 and GO6 were reduced to 0.86 and 0.75 nm, respectively, indicating that the interlayer spacing of GO was restricted by the degree of oxidation. In order to further quantitatively analyze the structure of GO, the degree of oxidation of GO1-GO6 could be calculated by an empirical equation of OG(%)=

I graphite I GO  I graphite

 100% [29],

where OD(%) is the degree of oxidation of GO; IGO and Igraphite are the integrated intensities of GO and graphite characteristic peaks in XRD patterns, respectively. The degree of oxidation of GO1-GO6 were 57.9%, 58.42%, 57.9%, 58.2%, 40.3% and 25.4%, respectively. The result further confirmed that the order of the degree of oxidation was GO1≈GO2≈GO3≈GO4>GO5>GO6.

3.1.6 Morphology analysis

Fig.6 shows the TEM images of various GO samples. The six GO samples had a two-dimensional structure with lateral sizes of several nanometers. GO1-GO4 samples were presented as few or monolayer structure while GO5 and GO6 exhibited a compact stacked multilayer structure, revealing that the increase of the degree of oxidation was beneficial to the enhanced dispersivity of GO. The AFM (Fig.S2) images indicated that GO samples were in nanometer thickness.

12

3.2 Adsorption characteristics 3.2.1 Effect of pH To evaluate the effect of pH on adsorption of Cu2+ on GO series and avoid the occurrence of Cu2+ precipitation, solubility product constants (Ksp) of Cu2+ ions was used to calculate the maximum pH at which Cu2+ ions would not occur as hydrolyzed species. The Ksp used for −

Cu(OH)2 is 6.0×10 20[30], the calculated maximum pH value for Cu2+ was 5.7. Therefore, Cu2+ started to precipitate above pH 5.7, and the GO series were responsible for the adsorption of Cu2+ when pH was less than 5.7. To ensure that the Cu2+ ions were completely adsorbed, pH 2-5.5 was selected to avoid Cu2+ precipitation conditions. Fig.7 shows that the pH of the solution played a critical role in adsorption of Cu2+ on GO. It was found that Cu2+ adsorption was strongly dependent on pH. The adsorption of Cu2+ on GO series sharply increased with increasing pH, and maintained lower level at pH 2 and higher level at pH 5.5. This phenomenon might be caused by the different physical-chemical nature of binding sites in GO series under different pH conditions. The adsorption of Cu2+ on GO series might be related to the complexation and electrostatic interaction between Cu2+ and oxygen-containing groups on the surface of GO series, and the Lewis acid-base reaction between Cu2+ and delocalized π-electron in aromatic matrixes of GO series. Owing to that the Lewis acid-base reaction was not be affected by pH, the different of Cu2+ adsorption by GO series under different pH conditions could only be caused by the diverse complexation and electrostatic attraction ability which affected by varying pH. From the results of the zeta potential of GO series, all GO samples surface was negatively charged, the charge could be more negative and prove stronger electrostatic 13

attraction with increasing pH, resulting in that the adsorption of Cu2+ on GO series increased. Besides, the increase of pH value in solution also led to the deprotonation reaction of the oxygen-containing groups on the surface of GO series, GO series allowed more ligands to form complexes with Cu2+, which lead to being more favorable for Cu2+ adsorption with increasing pH. Interestingly, the effect of the degree of oxidation on adsorption was different from that of the defects on adsorption at varying pH levels, the adsorption capacities of GO series (GO1-GO4) with same degree of oxidation were almost no difference but obviously higher than that of other GO series (GO5-GO6) with less defects (incomplete oxidation) under the same pH condition. From the results of XPS and XRD analysis of GO series, the degree of oxidation was in the order of GO1≈GO2≈GO3≈GO4, indicating the effect of pH on physical-chemical nature of oxygen-containing groups on the surface of GO1-GO4 was no difference and these oxygen-containing groups should exhibit the same binding ability for Cu2+ under various pH conditions, this reasoning was consistent with the adsorption results of GO1-GO4 in Fig.7. In addition, the degree of oxidation of GO1-GO4 was higher than that of GO5 and GO6, this result indicated that more oxygen-containing functional groups were introduced onto the surface of GO1-GO4 series and the adsorption capacities of GO1-GO4 were higher than that of GO5 and GO6 in all test pH ranges, implying that the adsorption of Cu2+ on GO1-GO4 series were more easily influenced by pH through protonation and deprotonation. These results indicated that the effect of pH on adsorption was strongly dependent on the degree of oxidation in all test pH ranges because hydrogen ions could change the physical-chemical nature of oxygen-containing groups on the surface of GO series. 14

From the results of Raman analysis of GO series, the order of the defects of GO1-GO6 was GO6GO5>GO1>GO2>GO3>GO4), which was different from that the order (GO6
3.2.2 Effect of ionic strength

The effect of ionic strength on adsorption is presented in Fig. 8. It is well known that industrial sewage not only contain heavy metals but also involve high concentrations of salts, which will impact the binding of adsorbing species[31]. As shown in Fig. 8, it was found that the effect of NaCl concentration on adsorption was distinguishing in the presence of different concentration of NaCl. When the NaCl concentration was lower than 0.01mol/L, the adsorption capacities of Cu2+ on GO series declined slightly with an increase of NaCl concentration. However, the adsorption capacities were almost unchanged with increasing NaCl concentrations from 0.05 to 0.1 M. At first, Na+ could compete with Cu2+ for a limited number of the binding sites on the surface of GO series with increasing NaCl concentration, leading to that the adsorption capacities of Cu2+ on GO series declined slightly. However, the affinity of GO series for Cu2+ was higher than that for Na+, comparing to the adsorption of Na+ on GO series, the adsorption of Cu2+ on GO series exhibited strong competitive 15

adsorption advantage, which result in that GO series had high adsorption capacity for Cu2+ and thus kept almost constant even in the case of higher concentrations of sodium ions. Additionally, the adsorption capacities of Cu2+ on GO1-GO4 were almost the same with an increase of the defects of GO under the same concentration of NaCl, the trends of which were also consistent at varying NaCl concentration, indicating that the effect of ionic strength on adsorption was independent of the defects of GO. However, the adsorption capacities of GO1, GO5 and GO6 for Cu2+ were greatly decreased with reducing in degree of oxidation of GO at same NaCl concentration and the adsorption trends of GO series were similar at varying NaCl concentration, suggesting that the effect of ionic strength on adsorption was weakly dependent on degree of oxidation related with oxygen-containing groups, because the oxygen-containing groups could be slightly affected by ionic strength through weakly competition effect between salt ions and heavy metals.

3.2.3 Effect of initial Cu2+ concentration and adsorption isotherms

To understand the adsorption mechanism, the widely used Langmuir[32] and Freundlich[33] isotherm models were applied to analyze the adsorption experiment data. The Langmuir model assumes that adsorption occurs on a homogeneous surface by monolayer coverage with no subsequent interactions between adsorbed species and the adsorbate. This relationship can be described in the following equation: ce 1 c   e qe bqmax qmax

(2)

Where Ce (mg·L-1) is the final concentration of Cu2+ in aqueous solution after adsorption equilibration, qe (mg·g-1) is the amount of Cu2+ adsorbed on GO, qmax (mg·g-1) is the 16

maximum adsorption capacity corresponding to the complete monolayer coverage. b (L·mg-1) is the constant related to the heat of sorption. The Freundlich model is an empirical model based on multilayer adsorption on heterogeneous surfaces. The model is represented by the following equation:

ln qe  ln K F  (1/ n) ln C f

(3)

Where Cf (mg·L-1) is the final concentration of Cu2+ in aqueous solution after adsorption equilibration. KF and n are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. Fig. 9 shows the adsorption isotherms of Cu2+ on GO1–GO6. The experimental data were simulated with both the Langmuir and Freundlich models. The relevant parameters calculated from equations (2) and (3) are listed in Table 2. From the correlation coefficients (R2), the adsorption isotherms of GO1–GO6 were fitted better by the Langmuir model than by the Freundlich model. The result suggested that monolayer coverage of Cu2+ on GO with different degree of oxidation and defects was the main sorption mechanism. qmax values increased with an increase of the degree of oxidation, indicating that GO with higher degree of oxidation had stronger affinity for Cu2+.

Generally speaking, the surface regions of GO usually consist of two parts: oxidized zones and unoxidized graphitic zones, the former is made up of remnant sp2 clusters and sp3 zones, where the oxygen-containing functional groups have been introduced; the latter is mainly made up of the aromatic matrixes, where the delocalized π electron has been existed[34, 35]. In previous reports, it is vaguely believe that the adsorption of heavy metals 17

on GO includes the complexation and electrostatic interaction between oxygen-containing groups on the surface of GO and heavy metals, and the Lewis acid-base interaction between delocalized π electron of GO and heavy metals. The amount of oxygen-containing groups and delocalized π electron is determined by the degree of oxidation and defects, respectively. In fact, the adsorption of Cu2+ on GO was only controlled by the degree of oxidation rather than by the defects. As shown in Fig.9, the adsorption capacities followed the order of GO1≈GO2≈GO3≈GO4>GO5>GO6, the adsorption capacities of GO series (GO1-GO4) with same degree of oxidation did not show higher adsorption capacity with decreasing defects, indicating that the adsorption of Cu2+ on GO was not limited by the defects, which might be due to the shielding effect of oxygen-containing groups. For the adsorption of GO1, GO5 and GO6, the adsorption capacity of Cu2+ on GO gradually decreased with a reduction of the degree of oxidation, suggesting the adsorption capacity of Cu2+ on GO was mainly determined by the degree of oxidation. Thus, it can infer that the adsorption of Cu2+ on GO was mainly proportional to the degree of oxidation but become insignificant in the structure integrity of aromatic matrixes.

3.2.4 Effect of contact time and adsorption kinetics

The effect of contact time on adsorption is shown in Fig. 10. The adsorption capacities of Cu2+ on GO1–GO6 sharply increased within 25 min, then slowly rose from 25 to 50min, and gradually reached an equilibrium state after 50 min. The fast adsorption and the slow adsorption might be caused by the electrostatic interaction and complexation between Cu2+ and GO, respectively.

18

C[Cu]initial=40mg/L, pH=5.5, I=0.01M NaCl, T=298K) To explore the adsorption process, in particular the rate-controlling step, the experimental data of adsorption process were analysed using pseudo-first-order[36] and pseudo-second-order[37] kinetics. The pseudo-first-order equation is expressed as:

ln(qe  qt )  lnqe  k1t

(4)

Where k1 (min-1) is the pseudo-first-order adsorption rate coefficient. qe and qt (mg·g-1) are the values of the amount adsorbed per unit mass at equilibrium and instant of time t (min), respectively. The pseudo-second-order equation is expressed as: t 1 1   t 2 qt k2 qe qe

(5)

Where k2 (g·mg-1·min-1) is the pseudo-second-order adsorption rate coefficient. The parameter values for the adsorption kinetics calculated from equations (4) and (5) are listed in Table 3. The correlation coefficients of the pseudo-second-order kinetic model were obviously higher than that of the pseudo-first-order kinetic model, suggesting the adsorption of Cu2+ on GO1–GO6 followed the pseudo-second-order kinetic model. The qe values calculated by the pseudo-second-order model were close to the values measured experimentally, indicating that the rate-limiting step of Cu2+ adsorption was restricted by chemical adsorption[38]. From the results of isotherms, the chemical adsorption was associated with oxygen-containing groups on the surface of GO. Thus, the results indicated that GO series had a same dynamic process and the adsorption process was controlled by the degree of oxidation. 19

3. Conclusion In this study, a series of GO with different degree of oxidation and defects were prepared by the improved Hummers method with variation in the dose of KMnO4 (oxidant) and H3PO4 (protective agent), and were employed as adsorbents for removal of Cu2+. The relationship between the degree of oxidation and defects of GO and the adsorption performance was investigated. The results indicated that the adsorption of Cu2+ on GO was strongly dependent on the degree of oxidation and independent of the defects under various pH levels and ionic strength. The isothermal adsorption behavior was well described by the Langmuir adsorption model, the maximum adsorption capacities of Cu2+ on GO1–GO6 were 91.6,90.3,92.3,90.4, 78.7 and 48.8 mg/g, respectively, which were greatly enhanced with the increase of the degree of oxidation and were not significantly affected by the defects. The adsorption equilibrium was reached in a shorter time and the adsorption kinetics of Cu2+ on GO series followed the pseudo-second-order model, indicating that the adsorption process was controlled by chemisorptions.

Supporting Information Line shape analysis for the C1s XPS spectra of GO series (Fig. S1) and AFM images (Fig. S2) were supplied in supporting Information, respectively.

Acknowledgments This research was supported by the Risk and Control Principle of Water Pollution for the Drinking Water Source Area of Dongjiang River (U1401235). 20

References

[1] V. Georgakilas, J.N. Tiwari, K.C. Kemp, J.A. Perman, A.B. Bourlinos, K.S. Kim, R. Zboril, Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications, Chemical reviews, 116 (2016) 5464-5519. [2] D. Chen, H. Feng, J. Li, Graphene oxide: preparation, functionalization, and electrochemical applications, Chemical reviews, 112 (2012) 6027-6053. [3] S. Zhu, Y.-g. Liu, S.-b. Liu, G.-m. Zeng, L.-h. Jiang, X.-f. Tan, L. Zhou, W. Zeng, T.-t. Li, C.-p. Yang, Adsorption of emerging contaminant metformin using graphene oxide, Chemosphere, 179 (2017) 20-28. [4] S.-W. Nam, C. Jung, H. Li, M. Yu, J.R.V. Flora, L.K. Boateng, N. Her, K.-D. Zoh, Y. Yoon, Adsorption characteristics of diclofenac and sulfamethoxazole to graphene oxide in aqueous solution, Chemosphere, 136 (2015) 20-26. [5] W. Peng, H. Li, Y. Liu, S. Song, Comparison of Pb(II) adsorption onto graphene oxide prepared from natural graphites: Diagramming the Pb(II) adsorption sites, Applied Surface Science, 364 (2016) 620-627. [6] G. Zhao, J. Li, X. Ren, C. Chen, X. Wang, Few-Layered Graphene Oxide Nanosheets As Superior Sorbents for Heavy Metal Ion Pollution Management, Environmental Science & Technology, 45 (2011) 10454. [7] S.T. Yang, Y. Chang, H. Wang, G. Liu, C. Sheng, Y. Wang, Y. Liu, A. Cao, Folding/aggregation of graphene oxide and its application in Cu 2+ removal, Journal of Colloid & Interface Science, 351 (2010) 122-127. 21

[8] Y. Bian, Z.Y. Bian, J.X. Zhang, A.Z. Ding, S.L. Liu, H. Wang, Effect of the oxygen-containing functional group of graphene oxide on the aqueous cadmium ions removal, Applied Surface Science, 329 (2015) 269-275. [9] H. Raghubanshi, S.M. Ngobeni, A.O. Osikoya, N.D. Shooto, C.W. Dikio, E.B. Naidoo, E.D. Dikio, R.K. Pandey, R. Prakash, Synthesis of graphene oxide and its application for the adsorption of Pb+2 from aqueous solution, Journal of Industrial and Engineering Chemistry, 47 (2017) 169-178. [10] G. Zhao, X. Ren, X. Gao, X. Tan, J. Li, C. Chen, Y. Huang, X. Wang, Removal of Pb(II) ions from aqueous solutions on few-layered graphene oxide nanosheets, Dalton Transactions, 40 (2011) 10945. [11] Z.H. Huang, X. Zheng, W. Lv, M. Wang, Q.H. Yang, F. Kang, Adsorption of Lead(II) Ions from Aqueous Solution on Low-Temperature Exfoliated Graphene Nanosheets, Langmuir, 27 (2011) 7558-7562. [12] M. Machida, T. Mochimaru, H. Tatsumoto, Lead(II) adsorption onto the graphene layer of carbonaceous materials in aqueous solution, Carbon, 44 (2006) 2681-2688. [13] B.C. Brodie, On the Atomic Weight of Graphite, Philosophical Transactions of the Royal Society of London, 149 (2009) 249-259. [14] L. Staudenmaier, Verfahren zur Darstellung der Graphitsäure, Berichte Der Deutschen Chemischen Gesellschaft, 32 (2010) 1394-1399. [15] W.S. Hummers, R.E. Offeman, Preparation of Graphitic Oxide. J Am Chem Soc 80:1339, Journal of the American Chemical Society, 80 (1958). [16] M. Hirata, T. Gotou, S. Horiuchi, M. Fujiwara, M. Ohba, Thin-film particles of graphite 22

oxide 1: : High-yield synthesis and flexibility of the particles, Carbon, 42 (2004) 2929-2937. [17] C.J. Madadrang, H.Y. Kim, G. Gao, N. Wang, J. Zhu, H. Feng, M. Gorring, M.L. Kasner, S. Hou, Adsorption behavior of EDTA-graphene oxide for Pb (II) removal, Acs Applied Materials & Interfaces, 4 (2012) 1186. [18] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved Synthesis of Graphene Oxide, Acs Nano, 4 (2010) 4806. [19] Z. Yang, H. Yan, H. Yang, H. Li, A. Li, R. Cheng, Flocculation performance and mechanism of graphene oxide for removal of various contaminants from water, Water Research, 47 (2013) 3037. [20] X. Liu, Y. Huang, S. Duan, Y. Wang, J. Li, Y. Chen, T. Hayat, X. Wang, Graphene oxides with different oxidation degrees for Co(II) ion pollution management, Chemical Engineering Journal, 302 (2016) 763-772. [21] X. Liu, J. Li, X. Wang, C. Chen, X. Wang, High performance of phosphate-functionalized graphene oxide for the selective adsorption of U(VI) from acidic solution, Journal of Nuclear Materials, 466 (2015) 56-64. [22] X. Liu, X. Wang, J. Li, X. Wang, Ozonated graphene oxides as high efficient sorbents for Sr(II) and U(VI) removal from aqueous solutions, Science China Chemistry, 59 (2016) 869-877. [23] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'Homme, I.A. Aksay, R. Car, Raman spectra of graphite oxide and functionalized graphene sheets, Nano letters, 8 (2008) 23

36-41. [24] A. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Physical review B, 61 (2000) 14095. [25] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS nano, 4 (2010) 4806-4814. [26] A.L. Higginbotham, D.V. Kosynkin, A. Sinitskii, Z. Sun, J.M. Tour, Lower-defect graphene oxide nanoribbons from multiwalled carbon nanotubes, ACS nano, 4 (2010) 2059-2069. [27] L. Cancado, K. Takai, T. Enoki, M. Endo, Y. Kim, H. Mizusaki, A. Jorio, L. Coelho, R. Magalhaes-Paniago, M. Pimenta, General equation for the determination of the crystallite size L a of nanographite by Raman spectroscopy, Applied Physics Letters, 88 (2006) 163106-163106-163103. [28] V. Chandra, J. Park, Y. Chun, J.W. Lee, I.-C. Hwang, K.S. Kim, Water-dispersible magnetite-reduced graphene oxide composites for arsenic removal, ACS nano, 4 (2010) 3979-3986. [29] H. Yan, X. Tao, Z. Yang, K. Li, H. Yang, A. Li, R. Cheng, Effects of the oxidation degree of graphene oxide on the adsorption of methylene blue, Journal of hazardous materials, 268 (2014) 191-198. [30] L.V. Gurgel, L.F. Gil, Adsorption of Cu(II), Cd(II) and Pb(II) from aqueous single metal solutions by succinylated twice-mercerized sugarcane bagasse functionalized with triethylenetetramine, Water Research, 43 (2009) 4479. 24

[31] C.-H. Weng, C. Huang, Adsorption characteristics of Zn (II) from dilute aqueous solution by fly ash, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 247 (2004) 137-143. [32] I. Langmuir, THE ADSORPTION OF GASES ON PLANE SURFACES OF GLASS, MICA AND PLATINUM, J.am.chem.soc, 143 (1918) 1361-1403. [33] X. Mi, G. Huang, W. Xie, W. Wang, Y. Liu, J. Gao, Preparation of graphene oxide aerogel and its adsorption for Cu 2+ ions, Carbon, 50 (2012) 4856-4864. [34] H. Yan, H. Wu, K. Li, Y. Wang, X. Tao, H. Yang, A. Li, R. Cheng, Influence of the surface structure of graphene oxide on the adsorption of aromatic organic compounds from water, Acs Appl Mater Interfaces, 7 (2015) 6690-6697. [35] Y. Han, T. Xue, Y. Zhen, K. Li, Y. Hu, A. Li, R. Cheng, Effects of the oxidation degree of graphene oxide on the adsorption of methylene blue ☆, Journal of Hazardous Materials, 268 (2014) 191. [36] A. Vernadakis, Zur Theorie der sogenannten Adsorption gelöster Stoffe, Zeitschrift Für Chemie Und Industrie Der Kolloide, 2 (1907) 15-15. [37] Y.S. Ho, G. Mckay, Pseudo-second order model for sorption processes, Process Biochemistry, 34 (1999) 451-465. [38] P. Tan, J. Sun, Y. Hu, Z. Fang, Q. Bi, Y. Chen, J. Cheng, Adsorption of Cu(2+), Cd(2+) and Ni(2+) from aqueous single metal solutions on graphene oxide membranes, Journal of Hazardous Materials, 297 (2015) 251-260.

25

Fig. 1. FTIR spectra of graphite and GO1–GO6.

Fig.2. The pH dependence of zeta potentials of GO1-GO6.

26

Fig. 3. Wide scan XPS spectra of graphite and GO1–GO6.

Fig. 4. Raman spectra of graphite and GO1–GO6.

27

Fig. 5. XRD patterns of graphite and GO1–GO6.

28

Fig. 6. TEM images of GO series.

29

Fig. 7. The effect of pH on the adsorption of Cu2+ on GO1–GO6. (m/v=0.1g/L, C[Cu]initial=40mg/L, I=0.01M NaCl, T=298K)

Fig. 8. The effect of ionic strength on the adsorption of Cu2+ on GO1–GO6. (m/v=0.1g/L, C[Cu]initial=40mg/L, pH=5.5, T=298K) 30

Fig. 9. Adsorption isotherms of Cu2+ on GO1-GO6. (m/v=0.1g/L, pH=5.5, I=0.01M NaCl, T=298K)

Fig. 10. The effect of contact time on the adsorption of Cu2+ on GO1-GO6. (m/v=0.1g/L,

31

Table 1 Preparation conditions and physiochemical parameters of graphite and GO1–GO6. graphite GO1

GO2

GO3

GO4

GO5

GO6

Oxidation time at 323 K (h)

0

12

12

12

12

12

12

KMnO4 dose (g)

0

18

18

18

18

9

4.5

H3PO4 dose (ml)

0

40

20

10

0

40

40

ID/IG ratio

-

1.09

1.30

1.42

1.50

1.01

0.49

Interlayer spacing (nm)

-

0.97

0.92

0.91

0.89

0.86

0.75

2.00

1.96

2.00

1.97

2.59

4.64

C/O atomic ratio from XPS

Table 2 Parameters values for Freundlich and Langmuir isotherm models of Cu2+ sorption onto GO1–GO6 at 298K. Freundlich isotherm

Langmuir isotherm

Adsorbents KF (mg·g -1)

n

R2

qmax (mg·g -1)

b(L·mg -1)

R2

GO1

44.90

4.81

0.790

92.68

0.69

0.995

GO2

44.06

4.92

0.839

91.41

0.79

0.991

GO3

42.93

4.64

0.831

93.28

0.86

0.997

GO4

44.39

4.88

0.826

91.32

0.69

0.999

GO5

36.96

4.67

0.793

79.68

0.84

0.992

GO6

26.96

6.18

0.711

49.33

0.97

0.993

32

Table 3 Parameters values for pseudo-first-order and pseudo-second-order models of Cu2+ sorption on GO1-GO6 at 298K. Adsorbents

GO1

GO2

GO3

GO4

GO5

GO6

Pseudo-first-order

qe (mg.g-1)

60.17

65.96

60.42

65.30

60.66

20.74

model

k1 (min-1)

0.0627 0.0883 0.0651 0.0768 0.0896 0.0722

R2

0.855

0.840

0.962

0.914

0.942

0.936

Pseudo-second-order

qe (mg.g-1)

93.63

92.25

94.07

92.00

80.13

49.41

model

k2 (g mg-1 min-1)

0.0029 0.0032 0.0030 0.0036 0.0045 0.0106

R2

0.995

33

0.991

0.993

0.992

0.997

0.993