Accepted Manuscript Title: Removal of cobalt ions from aqueous solution by an amination graphene oxide nanocomposite Author: Fang Fang Lingtao Kong Jiarui Huang Shibiao Wu Kaisheng Zhang Xuelong Wang Bai Sun Jin Zhen Jin Wang Xing-Jiu Huang Jinhuai Liu PII: DOI: Reference:

S0304-3894(14)00045-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.01.031 HAZMAT 15694

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

31-10-2013 12-1-2014 18-1-2014

Please cite this article as: F. Fang, L. Kong, J. Huang, S. Wu, K. Zhang, X. Wang, B. Sun, J. Zhen, J. Wang, X.-J. Huang, J. Liu, Removal of cobalt ions from aqueous solution by an amination graphene oxide nanocomposite, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.01.031 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.

Removal of cobalt ions from aqueous solution by an amination graphene oxide nanocomposite

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Zhena, Jin Wanga, Xing-Jiu Huanga,*, and Jinhuai Liua,*

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Fang Fanga,b, Lingtao Konga,*, Jiarui Huangb, Shibiao Wua,c, Kaisheng Zhanga, Xuelong Wanga,c, Bai Suna, Jin

Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines,

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Chinese Academy of Sciences, Hefei, Anhui, 230031, China.

College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui, 241000, China.

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School of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui, 230039, China.

*

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Corresponding Author

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E-mail address: [email protected]; [email protected]; [email protected] Fax: +86-551-65592420; Tel: +86-551-

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65591142.

ABSTRACT

A newly designed amination graphene oxide (GO-NH2), a superior adsorption capability to that of activated carbon, was fabricated by graphene oxide (GO) combining with aromatic diazonium salt. The resultant GO-NH2 maintained a high surface area of 320 m2/g. When used as an adsorbent, the GO-NH2 demonstrated a very quick adsorption property for the removal of Co(II) ions, more than 90% of Co(II) ions could be removed within 5 min for dilute solutions at 0.3 g/L adsorbent dose. The adsorption capability approaches 116.35 mg/g, which is one of 1

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the highest capabilities of today’s materials. The thermodynamic parameters calculated from temperaturedependent adsorption isotherms suggested that the Co(II) ions adsorption on GO-NH2 was a spontaneous process.

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Considering the superior adsorption capability, the GO-NH2 filter membrane was fabricated for the removal of Co(II) ions. Membrane filtration experiments revealed that the removal capabilities of the materials for cobalt ions

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depended on the membrane’s thickness, flow rate and initial concentration of Co(II) ions. The highest percentage

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removal of Co(II) exceeds 98%, indicating that the GO-NH2 is one of the very suitable membrane materials in

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environmental pollution management.

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Keywords: GO-NH2; Membrane; Cobalt; Adsorption; Thermodynamics; Kinetics

1. Introduction

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With the intensive industrial development of the modern society, in particular the mining industry and nonferrous metallurgy, that is accompanied by the growing pollution with ions of heavy metals of wastewaters. In these

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industries, such as metal plating facilities, mining operations, nuclear power plant, tanneries and battery manufacturing industries discharge wastewaters which contain many kinds of toxic heavy metal ions [1-6]. Some metals associated with these activities are cadmium, chromium, cobalt, lead and mercury etc. Heavy metals are not biodegradable and tend to accumulate in living organisms, causing various diseases and disorders [7-9]. Upon of these heavy metals, cobalt exists in the form of various salts in environment. It is widely used in nuclear medicine, enamels and semiconductors, grinding wheels, painting on glass and porcelain, hygrometers and electroplating; as a foam stabilizer in beer, in vitamin B12 manufacture, as a drier for lacquers, varnishes and paints, and as a catalyst for organic chemical reactions. Everyone is exposed to low levels of cobalt in air, water and food. The permissible limits of cobalt in the irrigation water and livestock watering are 0.05 and 1.0 mg/L, 2

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respectively (Environmental Bureau of Investigation, Canadian Water Quality Guidelines). In small amounts cobalt is essential for human health, because it is known to be an essential element at trace level in human beings, animals

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and plants for metabolic processes [10]. However, higher concentrations of cobalt can cause several health troubles, such as low blood pressure, lung irritations, paralysis, diarrhea, and bone defects, and may also cause mutations

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(genetic changes) in living cells [11]. Moreover, radionuclide, 60Co, which is widely used for medical and industrial

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applications, may be released from pressurized water nuclear power reactors [12]. And 60Co is considered to be one of the most serious radionuclides in the environment [13]. Therefore, the adsorption studies of Co(II) ions are

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essential for nuclear and hazardous waste management.

In the last decades, several methods, such as precipitation, reverse osmosis, coprecipitation, ion-exchange,

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membrane electrolysis, oxidation, and adsorption were employed to remove Co(II) ions from large volumes of aqueous solutions [14-17]. And one of the most effective choices for the removal of heavy metal ions from aqueous

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solutions is adsorption technology. Various materials, such as clay minerals, iron oxide nanomaterials, zeolites, Mg/Al layered double hydroxides, coal and chitosan, and carbon materials, have been used as adsorbents [18-24].

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However, there are still some problems that limit their practical application, such as adsorption capacity is not high enough, the adsorbents are difficult to separate, and so on [25]. Thus it is important to research a new adsorbent to solve these problems.

Graphene has been the hotspot in multidisciplinary areas in recent years due to its excellent mechanical [26], thermal [27], and electrical properties [28]. Graphene oxide nanosheets (GONS) are also regarded as suitable materials for sequestration of heavy metal ions due to their excellent adsorption performance [29,30]. Yang et al. [31] noted that Cu(II) ions adsorption on graphene oxide (GO) at pH 5.0 and T = 293 K could be aggregated with the adsorption capacity of 46.6 mg/g. Zhao et al. [32] found that the maximum adsorption capacities of Cd(II) and

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Co(II) ions on GONS at pH ∼ 6.0 and T = 303 K were about 106.3 and 68.2 mg/g, respectively. Moreover, in

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Zhao’s study [33], sulfonated graphene nanosheets were synthesized and used as adsorbents to remove naphthalene

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and 1-naphthol. The adsorption capacities of ∼2.3-2.4 mmol/g for naphthalene and 1-naphthol were the highest

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capabilities of today’s nanomaterials.

In this work, the materials GO-NH2 nanosheets were successfully synthesized with a simple method and

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characterized by SEM, AFM, BET, TGA, UV-Vis-NIR, XRD, FT-IR, XPS and Zeta potential, etc., and then the application of the material for the removal of Co(II) ions from aqueous solutions was investigated by the influence

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of different adsorption parameters, such as initial metal concentration, equilibration time, solution pH and adsorption thermodynamic parameters analysis etc. Moreover, because of the layer structure of GO-NH2

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nanosheets, very large adsorption capacities, and excellence adsorption rates, they have been fabricated filter membranes for the removal of Co(II) ions.

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2. Experimental 2.1. Materials

Graphite powder was obtained from Alfa Aesar. p-phenylenediamine, KMnO4, NaNO3, NaNO2, Co(NO3)2·6H2O, Ca(NO3)2, Mg(NO3)2, 98% H2SO4, 36.5% HNO3, 35% H2O2 and other chemicals (analytic reagent grade) were purchased from Shanghai Chemical Co. Ltd. (China). Milli-Q water (18.2 MΩ·cm) was used for all the experiments. 2.2. Synthesis of amination graphene oxide The synthesis of GO was described in our previous literature [34]. Briefly, flake graphite was strongly oxidized by using KMnO4, NaNO3 and concentrated H2SO4 under ultrasonication conditions, and then H2O2 was added to 4

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eliminate the excess MnO4− anions. GO were obtained by centrifugation at 10 000 rpm for 30 min. The detailed processes are depicted in Supporting Information S1. The N2-BET specific surface area (SSA) of the prepared GO

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was measured to be 120 m2/g, which was significantly lower than the theoretical value (∼2620 m2/g). It was

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assumed that the powder of GO could be easily aggregated together, which can result in the partial overlapping and

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coalescing of nanosheets.

The GO-NH2 was synthesized by GO combining with aromatic diazonium salt. Firstly, paraphenylenediamine

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diazonium salt was synthesized by following certain steps. 2.7 g paraphenylenediamine and excessive HCl mixture were stirred in an ice bath keeping the temperature below 278.15 K, then slowly added 1.75 g NaNO2 solution into

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mixture solution. It keeps stirring 2 hours in ice bath. Then GO was added into the mixture solution, remain stirring 2 hours in ice bath. The product was filtered and washed with Milli-Q water, methanol and diethyl ether. The N2-

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BET SSA of the prepared GO-NH2 was measured to be 320 m2/g (see Figure S5 in Supporting Information), which

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was much higher than the value of the prepared GO (120 m2/g). It was assumed that after amination the volume of

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the GO-NH2 power generated inflation due to the insert of 4-aminophenyl groups among GO sheets. 2.3. Characterization methods

Fourier transform infrared (FT-IR) spectra were recorded on a Nexus-870 spectrophotometer. Ultraviolet vision near infrared (UV-Vis-NIR) absorption spectrum was recorded by using a Solidspec-3700 spectrophotometer. Field-emission scanning electron microscopy (FE-SEM) images were taken by a Sirion 200 field-emission scanning electron microscopy. Atomic force microscopy (AFM) image was performed using a Veeco Autoprobe CP atomic force microscopy. The specific surface area and gas adsorption isotherm of the samples were tested on a Coulter Omnisorp 100CX Brunauer-Emmett-Teller (BET) by using nitrogen adsorption. Thermogravimetric analysis (TGA) was performed using a Shimadzu TGA-50H analyzer in N2 surroundings. The 5

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Raman spectrum was performed by using a HR800 Raman microscope instrument (HORIBA, Jobin Yvon, France) with the standard 532 nm He Ne 10 mW laser with a laser spot size of 1 μm. X-ray photoelectron spectrometry

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(XPS) was carried out on an ESCALab MK II using non-monochromatized Mg Kα x-ray beams as the excitation source. Binding energies were calibrated relative to the C 1s peak at 284.6 eV. Zeta potential was carried out on

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Delsa Nano C/Z.

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2.4. Batch adsorption experiments

In order to determine the adsorption capacities of the GO-NH2 nanosheets for Co(II) ions, as well as the

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influence of the initial metal concentration, solution pH, equilibration time and temperature dependent, adsorption experiments were performed by batch equilibration technique. Solutions were prepared from Co(NO3)2·6H2O salt

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and Milli-Q water. Initial pH values of the solutions in the adsorption experiments were adjusted to 6.0, by adding

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plasma mass spectrometry (ICP-MS).

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HNO3 or NaOH. The initial and the final Co(II) ions concentrations were analyzed by using inductively coupled

All the experiments were performed using a batch technique in Erlenmeyer flasks (25 mL capacity) in NaNO3

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solutions. The desired pH of the suspensions in each flask was adjusted by adding 0.01 or 0.1 M HNO3 or NaOH. The stock suspension of GO-NH2, Co(II) solutions, NaNO3 solution and Milli-Q water were added in the test flasks to achieve the desired concentrations of different components. After the addition of the above components, the solution in the flasks achieved final volume of 10 mL. The final concentrations of the GO-NH2 and NaNO3 in the flasks were 0.3 g/L and 0.01 M, and the pH was adjusted to 6.0. For the kinetic experiments the above suspensions were withdrawn at appropriate time intervals and the supernatant liquid was separated filtration through 0.22 μm pore size membrane using a vacuum filter. For the adsorption isotherms, the suspensions were shaken for 12 h to achieve adsorption equilibration and the liquid and solid phase were separated by filtration. The adsorption percentage and the distribution coefficient (Kd) are calculated from the following equations: 6

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where, C0 is the initial concentration (mg/L), Ce is the equilibrium concentration (mg/L), m (g) is the mass of the

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adsorbent, and V (mL) is the volume of the suspension, Qe (mg/g) is the equilibrium adsorption capacity. All the

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experimental data were the averages of duplicate or triplicate determinations. The relative errors of the data were about 5%.

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2.5. Membrane filter experiments

For the membrane filtration system, polyvinylidene fluoride (PVDF) membrane filters with an 18 mm diameter

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housing, 12 mm diameter filtration area and 0.22 μm pore size, were modified with the synthesized GO-NH2 nanosheets. Prior to modification, PVDF membrane filters were cleaned by filtering with Milli-Q water. Powdered

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GO-NH2 nanosheets were weighed and then suspended in 10 mL of Milli-Q water. The suspension was then dispersed by sonication for 10 min using a water bath sonicator and immediately loaded onto a cleaned PVDF

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membrane by syringe-filtering the dispersion through the membrane filter, thus obtaining a layer GO-NH2 membrane on the PVDF surface. The adsorbent GO-NH2 membrane thickness can be determined by the concentration of the GO-NH2 suspension, the density of GO-NH2, and the area of PVDF membrane. The different layer thickness of the adsorbent GO-NH2 membrane and the flow rate of the solutions were used as specified for each experiment. The SEM images (see Figure S4 in Supporting Information) reveal that the as-prepared GO-NH2 membrane is a layered structure, and has a slightly rough surface. The N2-BET SSA of the GO-NH2 membrane was measured to be 308 m2/g (see Figure S6 in Supporting Information), which was slightly lower than the value of the prepared GO-NH2 nanosheets (320 m2/g). The amount of cobalt adsorbed was calculated from the difference between its concentration before and after filtration. The initial and final concentrations of cobalt were analyzed by 7

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using ICP-MS. 2.6. Desorption experiments

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In order to estimate the reversibility of Co(II) adsorption, desorption experiments using solutions with different pH, magnesium and calcium contents were performed. Firstly, GO-NH2 nanosheets were loaded with Co(II) ions,

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equilibrating the adsorbent with the1000 mg/L cobalt solution (3 mg GO-NH2 to 10 mL of Co(II) solutions, m/v =

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0.3 g/L). After filter with 0.22 μm pore size membrane, the solid residue was thoroughly washed several times with Milli-Q water, and dried at 120 °C. Secondly, 3 mg of the obtained solid phase was treated with the 10 mL of each

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leaching solution, on a horizontal shaker, for 24 h. Six extracting solutions of varying pH values were prepared. Four acidic solutions were prepared from nitric acid at pH = 0.81 ± 0.05, 1.52 ± 0.05, 2.82 ± 0.05 and 4.82 ± 0.05.

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The other two solutions were neutral and alkaline, prepared from Milli-Q water and NaOH, while the last four

3. Results and Discussion

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solutions used in this study were prepared using different amounts of Ca(NO3)2 and Mg(NO3)2 salts.

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3.1. Characterization of amination graphene oxide The well-dispersible of the GO-NH2 nanosheets in aqueous solution could be attributed to their suspended hydroxyl, carboxylic and amino groups. The SEM and AFM images (shown in Fig. 1) clearly confirm that the assynthesized GO-NH2 nanosheets are micrometer level, and their thickness are less than 1 nm, indicating that formation of big single layer sheets through exfoliation and the insert of 4-aminophenyl groups among GO sheets. XRD patterns provided insight into the exfoliated structure of GO-NH2 nanosheets. In Fig. 2, XRD patterns from GO, GO-NH2, and pristine graphite were presented. The characteristic 0.34 nm (corresponding to the diffraction peak at 2θ = 26.50°) basal plane spacing for the pristine graphite completely disappeared in the GO pattern, and a strong peak arose at 2θ = 11°. The latter peak corresponds to an interlayer spacing of 0.87 nm and was attributed to 8

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the presence of oxygen containing groups. In contrast, the GO-NH2 pattern showed no peak, implying extensive exfoliation of the GO-NH2 nanosheets. Note that the XRD measurements for the GO sample were conducted using

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dried powder without sonication, while the GO-NH2 sample was prepared by precipitating the suspension containing GO-NH2 with methanol. In addition, complete disappearance of the characteristic diffraction peak for

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the GO-NH2 nanosheets indicates that the stacked fragments were relatively rare.

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FT-IR measurements were used in order to confirm the formation of amino group. Different functional groups were found in the FT-IR spectra of GO and GO-NH2 nanosheets (Fig. 3), i.e., C=O stretching mode of the

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carboxylic acid functional groups at 1730 cm-1, C-O-C group at 1250 cm-1, C-O group at 1070 cm-1, C=C at 1620 cm-1. After amination with the diazonium salt, the peaks at 1730, 1250, 1070 cm-1 are severely attenuated in the

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GO-NH2 spectrum. The peak at 830 cm-1 (N-H curving mode) confirms the presence of amino groups. As shown from Fig. 4, the chemical states of elements in the GO-NH2 nanosheets were further investigated by

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XPS. The XPS spectrum indicates a considerable degree of functionalization with different functional groups, i.e., the non-oxygenated ring C (284.6 eV), the C atom in C-O bond (286.7 eV), the carbonyl C (287.7 eV), the

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carboxylate carbon (O-C-O) (289.2 eV), the C atom in C-N bond (285.9 eV), and the N1s (399 eV). The atomic percent of C, O and N atoms can be shown as 76.4%, 18.01%, and 3.72%, respectively. From the XPS analysis, it is clear that the graphene oxide is highly functional by the diazonium salt. The specific peak area noted in Fig. 4 shows that there are many amino groups.

On the other hand, quantitative estimation for the functionalization degree of the GO-NH2 can be performed through TGA due to measurement of mass loss accompany with functional moieties removed from the GO-NH 2 in an inert environment by sufficient heating. Before TGA experiment, the sample was further washed several times with Milli-Q water, DMF, methanol, and finally diethyl ether to remove the excess of ungrafted species and other impurities, and dried at 80 °C under vacuum. As shown in Fig. 5, the TGA curve shows weight loss 14.16 % before 9

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129.92 °C, which is due to desorption of absorbed water molecules on GO-NH2. At temperatures of 129.92-266.22 °C, the 7.54% weight loss is likely due to the amino functional moieties in the sample. When temperature increases

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further (up to 800 °C), the 18.64% weight loss likely originates from the decomposition of oxygen functional groups and carbon oxidation, respectively [35, 36].

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The Zeta potential of the GO-NH2 nanosheets was investigated by measuring at room temperature. As

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indicated in the Fig. 6 the zeta potential gradually becomes negative with the increase of pH. From Fig. 6, it can be found when the pH value is more than 4, that the solution is negatively charged. Therefore, the GO-NH2 nanosheets

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are expected to form strong surface complexes with positive charge metal ions on their surfaces. 3.2. Results of batch experiments

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3.2.1. Adsorption of pH dependence

The effect of pH on the adsorption capacity can be attributed to the chemical form of heavy metals in the

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solution at a specific pH, i.e., pure ionic metal form or hydroxyl-metal form [37]. The pH effect on the adsorption of Co(II) ions onto the prepared materials GO-NH2 nanosheets was studied by evaluating the adsorption at pH

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values of 3, 4, 5, 6, and 7. The pH was adjusted to the desired values by adding negligible amounts of 0.01 or 0.1 M HNO3 or NaOH. Co(II) ions adsorption increases with increasing pH ranging from 3.0 to 7.0 as shown in Fig. 7. The relative proportion of Co(II) species is calculated from the stability constants (log β1 = 4.3, log β2 = 8.4, and log β3 = 8.4) and the results demonstrate that Co(II) presents in the form of Co2+, Co(OH)+, Co(OH)2 , and Co(OH)3- at various pH values (see Figure S7 in Supporting Information) [38]. At pH < 8.2, the predominant Co(II) species are Co2+ and Co(OH)+, Co(OH)2 precipitation begins to form at pH > 8.5. Thus, adsorption of Co(II) should be experimented at pH = 6 ~ 7 in the following a series of experiments. 3.2.2. Adsorption of time dependence The influence of contact time on the adsorption capacities of the produced materials was depicted in Fig. 8. The 10

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adsorption of Co(II) ions on the GO-NH2 is rapid over the first 5 minutes of contact time, more than 90% of Co(II) ions could be removed, and then remains constant with increasing contact time. In the following experiments, 2 h

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was selected to ascertain the adsorption equilibrium of Co(II) ions to the GO-NH2 nanosheets. To analyze the adsorption rate of Co(II) ions on the GO-NH2 nanosheets, the pseudo-first and second-order

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kinetics were thus applied to the experimental obtained.

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The pseudo-first-order kinetics can be generally described in the following equation:

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where Qe and Qt are the amounts (mg/g) of adsorbed Co(II) ions on the surface of the GO-NH2 nanosheets at equilibrium and at time t, respectively. k1 is the first order rate constant (1/min). A linear plot feature of ln(Qe-Qt)

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versus time t is achieved (see Figure S8 in Supporting Information). The very low correlation coefficient (R2 = 0.4475) suggests that kinetic adsorption is not suitable for pseudo-first-order rate equation.

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The pseudo-second-order kinetics equation was used to simulate the kinetic adsorption, presented as following:

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where K2 (g/mg h) is the pseudo-second-order rate constant of adsorption, Qt (mg/g) is the amount of Co(II) ions adsorbed on the surface of the GO-NH2 nanosheets at time t (min), and Qe (mg/g) is the equilibrium adsorption capacity. A linear plot feature of t/Qt versus t is achieved and inseted in Fig. 8. The K2 value calculated from the slope and intercept is 0.011 g/mg h. The low value of rate constant (k2) suggested that the adsorption rate decreased with the increase in time and the adsorption rate was proportional to the number of unoccupied sites [39,40]. The correlation coefficient (R2 = 0.99907) for the linear plot is very close to 1, which suggests that kinetic adsorption is very well described by a pseudo-second-order rate equation. This result indicates that the adsorption mechanism depends on the adsorbate and adsorbent and the rate-limiting step may be a chemisorption process including valence forces through sharing or exchanging of electrons [39,41]. This maybe attributes that the more suspended 11

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hydroxyl, carboxylic and amino groups on the surface of the GO-NH2 nanosheets can create intensive interaction with Co(II) ions through the electrostatic and chemical effects.

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3.3. Adsorption isotherms Equilibrium adsorption isotherms are usually used to determine the capacities of adsorbents. Fig. 9 shows Co(II)

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ions adsorption isotherms on the GO-NH2 nanosheets at three different temperatures. The experimental data are

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simulated with the Langmuir [42] (Qe = Qm · KL · Ce/1 + KL · Ce) and Freundlich [43] (Qe = KF · Ce 1/n) models (Fig. 9), (where Ce is the equilibrium concentration of metal ions in aqueous solution (mg/L), Qe is the amount of metal

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ions adsorbed on the surface of the GO-NH2 nanosheets (mg/g), Qm is the maximum amount of metal ions adsorbed per unit weight of GO-NH2 nanosheets to form a complete monolayer coverage on the surface, KL represents

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enthalpy of adsorption and should vary with temperature, and KF and n are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively). The relative parameters values calculated from the two

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models are listed in Table 1 and on comparing the linear regression values it is concluded that the Langmuir models

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are capable of representing the data more satisfactorily (R2 = 0.98164–0.99198) than the Freundlich models (R 2 =

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0.90867–0.92601). which indicates that the adsorption isotherms fit the Langmuir models, suggesting that Co(II) adsorption on the surface of the GO-NH2 nanosheets is a surface with homogeneous binding sites, equivalent adsorption energies, no interaction between adsorbed species, and monolayer coverage. The Langmuir constant KL relates to the affinity of the adsorbate for the binding sites[40], and varies with temperature. The values of KL are 0.01258 L/mg at 298 K, 0.00899 L/mg at 313 K, and 0.0098 L/mg at 328 K, respectively. Moreover, with the larger KL adsorbent has the greater adsorption capacity. The max value Qm of Co(II) ions adsorption on GO-NH2 nanosheets is 116.35 mg/g, which is one of the highest capabilities of today’s materials. For comparison, the Qm values of Co(II) ions on other adsorbents are listed in Table 2. It can be seen that the GONH2 nanosheets have much higher adsorption capacities than many other adsorbents such as graphene oxide 12

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nanosheets (68.2 mg/g) [25], marine green alga (46.1 mg/g) [44], crab shell (20.47 mg/g) [45], activated carbon (13.88 mg/g) [46], anaerobic granular sludge (12.34 mg/g) [47], natural zeolites (14.38 mg/g) [48], kaolinite (0.919

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mg/g) [49], al-pillared bentonite (38.6 mg/g) [50], lemon peel (25.6 mg/g) [51], and cation exchange resins IRN77 (86.17 mg/g) [52]. The high adsorption capacities suggest that GO-NH2 nanosheets are potential adsorbents for the

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removal of Co(II) ions in wastewater cleanup. The higher adsorption capacities of the GO-NH2 nanosheets toward

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Co(II) ions could be attributed to the abundant amino and oxygen-containing functional groups on the surfaces of the GO-NH2 nanosheets make the adjacent nitrogen and oxygen atoms available to bind metal ions.

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3.4. Adsorption thermodynamic parameters analysis

The thermodynamic parameters (ΔHo, ΔSo, and ΔGo) for Co(II) adsorption on GO-NH2 nanosheets can be

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calculated from the temperature dependent adsorption isotherms. It is used to define whether the process is endothermic or exothermic and spontaneous. The effect of temperature on Co(II) ions adsorption onto GO-NH2

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nanosheets at pH = 6 is given in Fig. 8. Adsorption capacity is highest at T = 298 K and lowest at T = 328 K, which shows that Co(II) ions adsorption on the GO-NH2 nanosheets is promoted at lower temperature. The standard free

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energy change (ΔGo) can be calculated from the following equation:

where R is the universal gas constant (8.314 J/mol K), T is the temperature in Kelvin. K0 is the adsorption equilibrium constant. Values of lnKo are obtained by plotting lnKd (distribution coefficient) versus Ce (see Figure S9 in Supporting Information) and extrapolating Ce to zero. The standard enthalpy change (ΔHo) and the standard entropy (ΔSo) are then calculated from the linear plot of lnK° versus 1/T for Co(II) ions adsorption on the GO-NH2 nanosheets in the following relationship:

Linear plot of lnK0 versus 1/T for the adsorption of Co(II) ions on the GO-NH2 nanosheets at 298, 313, and 328 13

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K was give (see Figure S10 in Supporting Information). Detailed processes of calculation of the thermodynamic parameters (i.e., lnK°, ΔG°, ΔH°, and ΔS°) have been described (see S6 in Supporting Information). The thermodynamic parameters of Co(II) ions adsorption on GO-NH2 nanosheets are ΔS0 = 35.49 J/mol K, ΔH0 = -

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10.77 kJ/mol, and ΔG0 = −21.3 kJ/mol at 298 K, −21.8 kJ/mol at 313 K, and −22.4 kJ/mol at 328 K, respectively

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(Table 3). The negative ΔH° value suggests that Co(II) ions adsorption on the surface of the GO-NH2 nanosheets is

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an exothermic process. Negative ΔG° values indicate that the adsorption of Co(II) ions on the GO-NH2 nanosheets is a spontaneous process.

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3.5. Results of membrane filtration experiments 3.5.1. Effect of the flow

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Because of the layer structure of GO-NH2 nanosheets and adsorbing cobalt ions quickly, they can be acted as filter membranes. Powdered GO-NH2 nanosheets were weighed and suspended in Milli-Q water. The obtained

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suspension was dispersed by sonication using a water bath sonicator and then immediately loaded onto a cleaned PVDF membrane by syringe-filtering the dispersion through the membrane filter, thus obtaining a layer of GO-NH2

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membrane on the PVDF surface. Filtration experiments were performed with flow rates of 0.5-5 mL/min whereas the thickness of the adsorbent membrane was 0.6 mm, and the initial concentration of Co(II) ions was 30 mg/L. As depicted in Fig.10 , it was observed that the lower the flow rate the higher the cobalt removal. This is due to the more contact time when the flow rate is low. 3.5.2. Effect of filter membrane thickness

The removal of Co(II) ions by GO-NH2 filter membranes of different thickness were researched. As shown in Fig. 11, by increasing the thickness of the membrane, the uptake of Co(II) ions increases. Increasing the thickness leads to increase of the available interaction sites that provide more sites for adsorption of Co(II) ions with a thicker layer and the efficiency is increased by allowing sufficient time for the adsorbate to diffuse into the adsorbent. 14

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When the thickness of the GO-NH2 filter membrane was increased from 0.2 mm to 1.2 mm, with flow rates of 1 mL/min, and the initial concentration of Co(II) ions of 30 mg/L. the percentage removal increased from 68% to

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94%. 3.5.2. Effect of Co(II) ions initial concentration

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The initial concentration of the Co(II) ions can also influence its removal rate. As shown in Fig. 12, the Co(II)

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ions solutions of different initial concentrations were used in the filtration experiments through the filter membranes of 0.6 mm thickness, with flow rates of 1 mL/min. From Fig. 12, it can be found that when the initial

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concentration of the Co(II) ions was increased from 1 to 90 mg/L, the percentage removal decreased from 89% to 21%, and by increasing the initial concentration of the Co(II) ions, the uptake of Co(II) ions decreases.

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The GO-NH2 filter membranes of different thickness (0.2-1.2 mm) were used to filter the Co(II) ions solutions with different flow rate (0.5-5 mL/min) and the different initial concentrations of Co(II) (1-90 mg/L), the results are

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shown in Fig. 13. We can notice that the removal rate of Co(II) ions increases with the increase of the membrane thickness, and decreases with the increase of the flow rate or the initial concentration of Co(II). The highest

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percentage removal of Co(II) exceeds 98%. Comparing with batch mode, the prepared adsorbent displayed the main advantage of separation convenience when a filter membrane was used. This is because the Co(II) ions are forced to interact with the active adsorbing sites on the large surface-area composite GO-NH2 filter membranes (308 m2/g, see Figure S6 in Supporting Information) during the penetration. 3.6. Desorption

In order to evaluate the reversibility of cobalt ions adsorption onto GO-NH2 nanosheets, desorption characteristics were also determined. Cobalt desorption from GO-NH2 nanosheets depends on the composition of the extracting solution (Table 4). The amounts desorbed from acidic solutions were highest, although little increases of final pH values, due to the buffering property of GO-NH2, were noticed (Table 4). The desorption ratio gets 15

Page 15 of 40

97.17% at pH = 0.81, suggesting the prepared GO-NH2 nanosheets can be repeatedly reused. Under alkaline extracting conditions, adsorbed metal remained more stable. The desorbed amount of cobalt decreased continuously with increasing pH, with only trace amounts being desorbed above pH = 7. Furthermore, the amount of Co(II)

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desorbed slightly increased with the increase of the Ca2+ and Mg2+ concentration. The Ca2+ and Mg2+ are frequent

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competing ions in nature water, which make application of GO-NH2 nanosheets for the removal of Co(II) ions still

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little effective in hard waters.

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4. Conclusions

In our research, we designed the new amination graphene oxide (GO-NH2) nanosheets in order to combine more

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heavy metal ions for the first time. These materials have all advantages of graphene and superior water-solubility. It shows very excellence adsorption properties (i.e., very large adsorption capacity and quick adsorption rate) for

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Co(II) ions from aqueous solutions due to the more suspended hydroxyl, carboxylic and amino groups on its surface which can create intensive interaction with Co(II) ions through the electrostatic and chemical effects. XRD,

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XPS, FT-IR, SEM, BET and AFM were analyzed, and the results indicate that the GO-NH2 nanosheets have been successfully prepared. The TGA analysis shows that the GO-NH2 nanosheets have good thermostabilities. And the Zeta potential gave a good proof that the solution of the GO-NH2 nanosheets is negative charge when the pH value is more than 4, which is very suitable to be used to adsorb heavy metal cations. In the batch mode, it is found that the adsorption capability is initial metal concentration, solution pH, equilibration time and temperature dependent. The highest adsorption capacity approaches 116.35 mg/g, which is one of the highest capabilities of today’s materials. The thermodynamic parameters calculated from the temperature-dependent isotherms indicate that the adsorption reaction of Co(II) ions on the GO-NH2 nanosheets is a spontaneous process. Filtration experiments revealed that the GO-NH2 nanosheets membranes can quickly remove cobalt ions from the aqueous solutions, and 16

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the removal rate was more than 98%.Therefore, the GO-NH2 nanosheets are very suitable materials for metal ions pollution cleanup in the natural environment. With the development of the preparation technology of graphene, the

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cost of graphene will be reduced and graphene or graphene composites will be used for real work. Graphene and graphene composites maybe bring a revolution of water pollution treatments in the near future.

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Acknowledgments

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This work was supported by the National Key Scientific Program-Nanoscience and Nanotechnology (Grant No. 2011CB933700), the National Natural Science Foundation of China (21177131, 61273066, 11205204, 21105001,

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21077106, 61104205), and China Postdoctoral Science Foundation funded project (20110490834).

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Appendix A. Supplementary data

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References

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Supplementary data related to this article can be found at doi: ---.

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Figure Captions:

Fig. 1 SEM (a) and AFM (b) images for GO-NH2, (c) section analysis of (b). Fig. 2 XRD patterns for (a) pristine graphite, (b) GO, and (c) GO-NH2. Fig. 3 FT-IR spectra of (a) GO, (b) GO-NH2. Fig. 4 XPS spectra analysis of GO-NH2, (a) survey of XPS data, (b) higher resolution data of the C1s. 22

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Fig. 5 TGA of GO-NH2 in N2 atmosphere (10 °C/min). Fig. 6 The Zeta potential of the GO-NH2.

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Fig. 7 The effect of pH on the amount of Co (
) absorbed on the GO-NH2 nanosheets. (m/V = 0.3 g/L, I = 0.01 M NaNO3, [Co(II)]initial = 30 mg/L)

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Fig. 8 Effects of contact time on the adsorption of Co(II) ions onto GO-NH2 nanosheets. The inset displays the

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pseudo-second-order kinetic plots for the adsorption of Co(II). (m/V = 0.3 g/L, I = 0.01 M NaNO3, [Co(II)]initial = 30 mg/L, pH = 6.0)

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Fig. 9 Co(II) ions adsorption isotherms on GO-NH2 nanosheets at three different temperatures. (m/V = 0.3 g/L, I = 0.01 M NaNO3, pH = 6.0). The solid lines are Langmuir model simulation, and the dashed lines are Freundlich

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model simulation.

Fig. 10 The effect of flow rate on the amount of Co(II) ions adsorbed on the GO-NH2 filter membrane.( membrane

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thickness: 0.6 mm, [Co(II)]initial = 30 mg/L))

Fig. 11 The effect of membrane thickness on the amount of Co(II) adsorbed on the GO-NH2 filter membranes.

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(flow rate: 1 mL/min, [Co(II)]initial = 30 mg/L)

Fig. 12 The effect of initial concentration on the amount of Co(II) adsorbed on the GO-NH2 filter membrane. (flow rate: 1 mL/min, membrane thickness: 0.6 mm) Fig.13 The effects of membrane thickness (0.2-1.2 mm), flow rate (0.5-5 mL/min), and initial concentrations of Co(II) (a, 1 mg/L; b, 30 mg/L; c, 60 mg/L; d, 90 mg/L) on the amount of Co(II) adsorbed on the GO-NH 2 filter membranes.

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Table 1 Parameters for Langmuir and Freundlich models of Co(II) ions adsorption on GO-NH2 nanosheets. Freundlich

Qm (mg/g)

KL (L/mg)

R2

KF (mg1-n·Ln/g)

T = 298 K

116.35

0.01258

0.99198

120.149

T = 313 K

108.70

0.00899

0.98321

15.039

T = 328 K

105.40

0.0098

0.98164

11.631

R2

n 2.3240

0.9139

2.0920

0.90867

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condition

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Langmuir

Experiment

0.92601

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1.9161

experimental condition adsorbent pH

T (K)

Graphene oxide nanosheets

6.0

303

Marine green alga

4.0

Crab shell

4.0

Activated carbon

6.0

Reference

(mg/g)

303

46.1

[44]

-

20.47

[45]

303

13.88

[46]

7.0

-

12.34

[47]

60.-7.0

298

14.38

[48]

298

0.919

[49]

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[25]

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Natural zeolites

adsorption capacity

68.2

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Anaerobic granular sludge

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Table 2 The comparison of Co(II) ions adsorption capacities for various adsorbents.

Kaolinite

Al-pillared bentonite

6.0

303

38.6

[50]

Lemon peel

6.0

298

25.6

[51]

Cation exchange resins IRN77

5.3

298

86.17

[52]

Amination graphene oxide

6.0

298

116.35

This study

Table 3 The obtained thermodynamic parameters of Co(II) ions adsorption on GO-NH2 nanosheets

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△S0 (J/mol)

△G0 (kJ/mol)

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T(K)

△H0 (kJ/mol)

298

-21.3 -10.77

35.49

-21.8

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313

-22.4

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328

Solutions

Initial

Final

Desorbed

pH

pH

Co(II) %

0.81

97.17

Composition HNO3 solution 1

0.92

2

HNO3 solution 2

1.52

1.73

83.68

3

HNO3 solution 3

2.82

3.25

28.34

4

HNO3 solution 4

4.82

5.16

16.15

5

Milli-Q water

6.92

6.73

0.87

6

NaOH solution

12.45

12.42

0.12

7

Ca2+ 0.01 M

5.35

5.28

1.32

8

Ca2+ 0.05 M

5.21

5.19

1.85

9

Mg2+ 0.01 M

5.32

5.30

1.67

10

Mg2+ 0.05 M

5.18

5.20

2.12

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1

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No.

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Table 4 The characteristics of leaching solutions and percents of Co(II) ions desorbed

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Graphical Abstract

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*Highlights

Highlights 1. A newly designed GO-NH2: Higher adsorption capability than that of activated carbon. 2. Very quick adsorption property: More than 90% of Co(II) can be removed within 5 min.

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4. GO-NH2 membrane can remove more than 98% Co(II) from the water.

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3. One of the highest adsorption capabilities of today’s nanomaterials for Co(II). (116.35 mg/g)

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Figure 12

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Figure 13

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