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Graphene Oxide Sorption Capacity Towards Elements over the Whole Periodic Table – a Comparative Study Kate#ina Klímová, Martin Pumera, Jan Luxa, Ond#ej Jankovský, David Sedmidubsky, Stanislava Mat#jková, and Zdenek Sofer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08088 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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The Journal of Physical Chemistry

Graphene Oxide Sorption Capacity Towards Elements over the Whole Periodic Table – a Comparative Study Kateřina Klímová a, Martin Pumera b, Jan Luxa a, Ondřej Jankovský a, David Sedmidubský a, Stanislava Matějková c and Zdeněk Sofer a,*

a

Institute of Chemical Technology, Department of Inorganic Chemistry, 166 28 Prague 6,

Czech Republic. E-mail: [email protected]; Fax: +420 224310422; Phone: +420 220444049 b

Division of Chemistry & Biological Chemistry, School of Physical and Mathematical

Sciences, Nanyang Technological University, Singapore, 637371, Singapore c

Institute of Organic Chemistry and Biochemistry AS CR, v.v.i., Flemingovo nam. 2, 166 10

Prague 6, Czech Republic

1 ACS Paragon Plus Environment


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ABSTRACT Water pollution is a worldwide environmental problem. Wastewater from industrial processes, surface and ground waters can all contain various metal ions, such as toxic Hg2+, Pb2+, Cd2+, Cu2+, As3+, Sb3+, Bi3+, etc. Consequently, efficient methods for removing impurities from such waters are in high demand. Since the large surface area of graphene oxide can make this material suitable for the uptake of various metal ions, we investigated its sorption capacity. The detail knowledge of sorption for various ions on graphene oxide surface is also crucial for graphene doping and purification. The surface sorption allowed synthesis of materials for catalysis with homogeneous distribution of catalytic active sites. The Hummers and Hoffman’s (permanganate and chlorate) methods were used to prepare two graphene oxides with different surface chemistry for investigation of sorption capacity across most of the ions within periodic table. The sorption capacity was evaluated by XRF and ICPOES, XPS, XRD and SEM-EDS. Both Hummers and Hoffman’s graphene oxides showed significant differences in sorption capacity towards various ions. For the majority of tested metal ions, our results showed that the Hummers graphene oxide had much higher sorption capacity than Hoffman’s graphene oxide. Several trends within sorption capacity across the periodic table can be observed indicating a strong influence of ion electronic structure and coordination ability as well as its acidity and redox properties on its sorption on graphene oxide surface.


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Introduction Most of the metal ions pose a high risk to both ecosystems and humans. For example, Itai-itai disease is caused by cadmium pollution, allergic reactions are caused by nickel, and jaundice is caused by long term exposure to copper.

1

A considerably high amount of these ions are

generated in industrial effluent, agricultural drainage and also in the acidic leachate from landfills.

2,3

Therefore, the major challenge is to find the most effective method or materials

for removing unwanted metal residues from wastewaters. Wastewater management uses techniques such as sorption, precipitation, ion exchange and membrane filtration.4 Among these methods, sorption using ash, active carbon, metal oxides, zeolites or chitosan as sorption materials, is most commonly used.5,6 The use of these materials is cheap, but their sorption capacity is very low.7-9 That is why it is important to search new sorption materials. Due to its large surface area (up to 2630 m2 g-1) and high number of oxygen-containing functional groups, graphene oxide is a promising candidate for efficient sorption material.

10

Graphene oxide contains epoxide, hydroxide, ketone and carboxyl functional groups on its surface.11 These groups can attach metal ions by means of electrostatic forces or coordination covalent bonding. Furthermore, because of the large size of its individual sheets, graphene oxide can be easily removed from solutions by filtration. This represents a significant advantage compared to other carbonaceous nanostructured materials like carbon nanotubes and active carbon, apart from the fact that these materials contain significantly lower concentration of oxygen functionalities.12,13 Since these functionalities play a crucial rule in enhancing the sorption activity compare to other carbon nanomaterials, the recent research focuses mainly on graphene oxide.12 The use of graphene, graphene oxide and other carbon nanomaterials have been reported by several authors14-20. This research usually focuses on the few toxic elements and also on the interference of sorption capacity with few non-toxic elements. Recently the application of graphene oxide for separation of minor and rare 3 ACS Paragon Plus Environment


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elements for industrial applications has been also reported.21 The metallic ions interlinked graphene oxide sheets can be used for assembling of various 3D structures.22 The huge importance is also capture of radioactive elements from the environments and for the spent fuel recovery. In this case the sorption of several elements like caesium, strontium, lanthanides and several others fission products is of huge importance.23 Several studies concerning on sorption of individual ions or its small groups on graphene oxide and its composites were reported recently.24-28 These process of sorption was also investigated theoretically by DFT calculations.29,30 The high affinity of graphene oxide, graphene and its derivatives towards contamination metallic all metallic ions should be considered in the synthesis strategy of high purity graphene.31 Another important factor for the investigation of metal sorption on graphene oxide is the application of graphene based materials in electrocatalysis.32,33 Since the graphene oxide exhibits high sorption activity towards a broad spectrum of metallic ions, a careful investigation of chemical composition is necessary for the evaluation of the electrocatalytic properties of graphene nanomaterials. The recent works show that graphene may contain several types of metallic and non-metallic contaminants originating from the synthesis.34,35 Such contaminants are accumulated in the graphene materials during the whole synthesis procedures and can often be related to the observed catalytic properties. The most typical example is manganese which originates from the potassium permanganate used for the synthesis of graphene oxide.36 The knowledge of sorption capacity of graphene oxide towards different ions is essential not only for its application in pollution reduction and selective ion separation, but also to avoid the unintentional contamination of graphene based materials by selection of appropriate synthetic procedure. The knowledge of sorption capacity and its mechanism is also crucial for graphene homogeneous doping by various ions for catalytic applications which require homogeneous distribution.


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We systematically investigated the sorption capacity of graphene oxide prepared by the Hummers procedure (permanganate) based methods towards all common metallic ions, which are stable in the aqueous environment. In addition the sorption capacity of graphene oxide prepared by chlorate method (Hofmann method) was also studied for comparison in order to investigate the influence of graphene oxide composition on its sorption capacity. The obtained data were compared by weight and also molar mass sorption capacity to eliminate the differences in molar mass of various ions. The results show significant differences in the sorption capacity of graphene oxides with different surface chemistry. Also the chemical properties of ions used for sorption like different valency, stability towards hydrolysis, reduction potential, ionic radii and coordination abilities are crucial for its sorption capacity.

Experimental Section Materials Graphite microparticles (99.8%), barium nitrate (>99%), ammonium chloride (99%) were obtained from Penta, Czech Republic. For the sorption experiments nitrates and chlorides of the respective ions were used. The nitrates were used for experiments in those cases where the corresponding salts were commercially available. In other cases corresponding chlorides and sulfates were used: LiNO3,

NaNO3,

KNO3,

Mg(NO3)2.4H2O,

Cr(NO3)3.9H2O,

Mn(NO3)2.4H2O,

Ni(NO3)2.6H2O,

Cu(NO3)2.4H2O,

Ca(NO3)2.4H2O,

Fe(NO3)3.9H2O, Zn(NO3)2.4H2O,

Sr(NO3)2,

Ba(NO3)2,

FeCl2.4H2O,

Co(NO3)2.6H2O,

Cd(NO3)2.4H2O,

Hg(NO3)2.H2O,

Al(NO3)3.9H2O, SnCl2.2H2O, Pb(NO3)2, SbCl3 and Bi(NO3)3.5H2O were obtained from Penta, Czech Republic. RbNO3, CsNO3, Be(NO3)2.4H2O, ZrOCl2.8H2O, HfOCl2.8H2O,



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VO(SO4)2.4H2O, TiO(SO4)2.xH2O (29 wt.% Ti; 17 wt.% H2SO4), Hg2(NO3)2.2H2O, Ga(NO3)3.xH2O, In(NO3)3.xH2O, TlNO3, GeCl4, SnCl4.5H2O, As2O3 and lanthanide nitrates (Sc, Y, La to Lu) were obtained from Sigma-Aldrich, Czech Republic.

Methods Graphene oxide preparation with the Hofmann method (HOGO).37

87.5 mL of

sulfuric acid (98%) and 27 mL of nitric acid (68%) were added to a reaction flask (Pyrex beaker with thermometer) containing a magnetic stir bar. The mixture was then cooled by immersion in an ice bath for 30 min. 5 g of graphite was then added to the mixture with vigorous stirring motion to avoid agglomeration and to obtain a homogeneous dispersion. While keeping the reaction flask in an ice bath, 55 g of potassium chlorate was slowly added to the mixture (over a 30 min period) in order to avoid a sudden increment in temperature and the formation of explosive chlorine dioxide gas. Upon the complete dissolution of potassium chlorate, the reaction flask was then loosely capped to allow the escape of the gas evolved and the mixture was continuously stirred vigorously for 96 h at room temperature. On completion of the reaction, the mixture was then poured into 3 L of deionized water and decanted. Graphene oxide was then re-dispersed in HCl (5%) solutions to remove sulfate ions and repeatedly centrifuged and re-dispersed in deionized water until a negative reaction on chloride and sulfate ions (with AgNO3 and Ba(NO3)2 respectively) was achieved. Graphene oxide slurry was then dried in a vacuum oven at 60 °C for 48 h before use. Graphene oxide preparation with the Hummers method (HUGO).38 5 g of graphite and 2.5 g of sodium nitrate were stirred with 115 mL of sulfuric acid (98%). The mixture was then cooled in an ice bath. With vigorous stirring, 15 g of potassium permanganate was then added over a period of two hours. In the subsequent four hours, the reaction mixture was allowed to reach room temperature before being heated to 35 °C for 30 min. The reaction mixture was


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then poured into a flask containing 250 mL of deionized water and further heated to 70 °C. After holding the temperature constant for 15 min, the mixture was then poured into 1 L of deionized water. The unreacted potassium permanganate and manganese dioxide were removed by the addition of 3% hydrogen peroxide. The reaction mixture was then allowed to settle and decanted. The graphene oxide obtained was then purified by repeated centrifugation and redispersing in deionized water until a negative reaction on sulfate ions (with Ba(NO3)2) was achieved. Graphene oxide slurry was then dried in a vacuum oven at 60 °C for 48 h before use. For the measurement of graphene oxide sorption capacity 400 mg of graphene oxide (HUGO and HOGO) was dispersed in 100 ml of water by ultrasonication (400 W, 60 minutes). To the suspension of graphene oxide was added 200 mL of 0.1 mol/L solution of corresponding metallic ions. The suspension was vigorously stirred for 24 hours and the graphene oxide was separated by suction filtration and repeatedly washed with water to remove unreacted metallic ions. Finally the graphene oxide with sorbet metals was dried in vacuum oven at 50 °C for 48 hours. Metallic nitrates were used when possible (Li+, Na+, K+, Rb+, Cs+, Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+, Hg22+, Al3+, Ga3+, In3+, Tl+, Pb2+, Bi3+). In order to avoid hydrolysis all nitrate solutions were acidified with 1 mL of 65 wt.% nitric acid. The corresponding chlorides (SnCl2.2H2O, SnCl4.5H2O, SbCl3, AsCl3 and GeCl4) were used as sources of Sn2+,Sn4+, As3+, Sb3+ and Ge4+ ions. AsCl3 was prepared by dissolving of As2O3 in HCl. To dissolve GeCl4, its solution with NH4Cl was added to HCl. Since titanium, hafnium, zirconium and vanadium form oxo-cations in aqueous solution, TiO(SO4)2, ZrOCl2.8H2O, HfOCl2.8H2O and VOSO4.6H2O were used as sources of TiO2+, ZrO2+, HfO2+ and VO2+ ions. The TiO(SO4)2 solution contained about 20 wt.% of



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H2SO4 to avoid hydrolysis. The solutions of ZrOCl2 and HfOCl2 were acidified with 1 mL of 35 wt.% HCl and the solution of (VO)SO4 was acidified with 1 ml 96 wt.% H2SO4. Diluted acids instead of water were used to dissolve ions with strong tendency towards hydrolysis and the resulting mixtures were subsequently acidified. The 0.1 M solution of Bi3+ was prepared by dissolution of Bi(NO3)3.5H2O in 5 wt.% nitric acid. The 0.1 M solutions of Sb3+, Sn4+ and As3+ were prepared by dissolution of the respective amounts of SbCl3, SnCl4.5H2O and As2O3 in 10 wt.% hydrochloric acid. The 0.1 M solution of Sn2+ was prepared by dissolving SnCl2.2H2O in 1 wt.% HCl. Characterization The morphology was investigated by scanning electron microscopy (SEM) with a FEG electron source (Tescan Lyra dual beam microscope). Elemental composition and mapping were performed using an energy dispersive spectroscopy (EDS) analyzer (X-MaxN) with a 20 mm2 SDD detector (Oxford Instruments) and AZtecEnergy software. To conduct these measurements, the samples were placed on a carbon conductive tape. SEM and SEMEDS measurements were carried out using a 10 kV electron beam. High resolution X-ray photoelectron spectroscopy (XPS) was performed using an ESCAProbeP spectrometer (Omicron Nanotechnology Ltd, Germany) with a monochromatic aluminium X-ray radiation source (1486.7 eV). The samples were placed on a conductive carbon tape homogeneously covered with the sample. XRF analysis was performed using an Axios sequential WD-XRF spectrometer (PANalytical, Nederland) equipped with a Rh anode end-window X-ray tube fitted with a 50 µm beryllium window. The resulting data were collected by SuperQ software and further evaluated by Omnian software. The analysed powders were pressed, without any binding agent, onto H3BO3 pellets with a total thickness of approximately 5 mm and a diameter of 40 mm. The pellets were then covered with a 4 µm supporting polypropylene (PP) film. Ty XRF


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analysis was used to determine the concentration of all elements with the exception of alkali metals and beryllium, where ICP-OES was applied. The elemental analysis was performed using SPECTRO Arcos inductively coupled plasma optical emission spectrometer (ICP-OES). The SPECTRO ARCOS features a Paschen-Runge spectrometer mount, employing the Optimized Rowland Circle Alignment (ORCA) technique with 32 linear CCD detectors. The optics is hermetically sealed and filled with argon. High optical transmission in the VUV is thus achieved, allowing the determination of non-metals as well as the use of prominent and interference free lines in this region. The wavelength range between 130 and 770 nm can be simultaneously analyzed. For the samples decomposition a procedure based on Schöniger combustion was used. A precisely weighted sample of 5 mg was wrapped in ash-free cellulose paper and burned in a flask (filled with pure oxygen) with Pt sample holder. The burning products were decomposed by prolonged ultrasonication and heating with ICP grade nitric acid. After this procedure the sample solution was diluted with deionized water (18.2 MΩ) to obtain a concentration of nitric acid of around 1% and used for analysis. Blank experiments were performed with ashfree cellulose paper in order to subtract the amount of impurities introduced by sample preparation. The calibration of spectrometer was performed with certified ICP-OES elements standards in 2% HNO3 and 2% HNO3+2% HCl matrixes; Y3+ was used as internal standard. The ICP-OES analysis was used for measurement concentration of alkali metals (Li+, Na+, K+, Rb+, Cs+) and beryllium (Be2+). Samples were analysed by X-ray powder diffraction (XRD). Data were collected with a Bruker D8 Discoverer diffractometer in Bragg–Brentano parafocusing geometry. A CuKα radiation was used. Diffraction pattern were collected between 5° and 80° of 2θ. Raman spectroscopy was conducted on an inVia Raman microscope (Renishaw, England) with a CCD detector in backscattering geometry. A DPSS laser (532 nm, 50 mW)



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with a 100x magnification objective was used for the Raman measurements. The instrument was calibrated with a silicon reference to give a peak position at 520 cm−1 and a resolution of less than 1 cm-1. Fourier transform infrared spectroscopy (FT-IR) measurements were performed on a NICOLET iS50R FT-IR spectrometer (Thermo Scientific, USA). The FT-IR measurement was performed in transmission mode using KBr pellets and DLaTGS detector in the range 4000–400 cm-1 at a resolution of 4 cm-1 (KBr beamsplitter and KBr detector window). Combustible elemental analysis (CHN-O) was performed using a PE 2400 Series II CHNS/O Analyzer (Perkin Elmer, USA). The instrument was used in CHN operating mode (the most robust and interference-free mode) to convert the sample elements to simple gases (CO2, H2O and N2). The PE 2400 analyzer automatically performed combustion, reduction, homogenization of product gases, separation and detection. An MX5 microbalance (Mettler Toledo) was used for precise weighing of the samples (1.5–2.5 mg per single sample analysis). Using this procedure, the accuracy of CHN determination is better than 0.30% abs. The internal calibration was performed using an N-fenyl urea.

Results & Discussion Graphene oxide characterization Graphene oxides prepared by permanganate and chlorate method termed as HU and HO, respectively, were used for the sorption experiments. These two main routes of graphite oxidation synthesis led to significant differences in composition as concerns the formed oxygen functionalities. The results of detailed characterization performed on these two materials using XPS, XRD, FT-IR and Raman spectroscopy are shown in Figure 1. The composition obtained by elemental combustion analysis was 41.4 at.% C, 31.8 at.% O and 26.8 at.% H for HUGO and 47.5 at.% C, 28.8 at.% O and 23.7 at.% H for HOGO (+-0.1at.%).


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The higher degree of oxidation for HUGO is clearly visible from the elemental combustion analysis. Also the C/O ratio obtained from XPS survey spectra was 2.8 for HUGO and 2.9 for HOGO (Figure 1a). The main differences can be also observed on high resolution XPS spectra of C 1s peak (Figure 1b). The results of C 1s peak deconvolution are summarized in the Supporting information (Table SI1). The deconvolution of C 1s peak shows a higher concentration of hydroxyl functional groups which is in agreement with FT-IR and other analyses. The C=O vibration band of carboxylic acid at 1720 cm-1 is clearly visible on FT-IR spectra of HUGO sample (Figure 1c). In addition the absorption bands originating from OH vibration around 3300 cm-1, C=C vibration band at 1610 cm-1, C-O vibration band at 1370 cm-1 and in region 950 - 1050 cm-1 are clearly visible. In summary, the differences in HUGO and HOGO FT-IR spectra indicate the dominant presence of carboxylic acids in HUGO (vibration bad at 1720 cm-1) and hydroxyls and epoxies in HOGO (vibration bands at 1370 cm-1 and around 1000 cm-1). The Raman spectra of both GO samples are dominated by Dband and G-band at 1350 cm-1 and 1590 cm-1, respectively (Figure 1d). The ID/IG ratio indicating defects and sp3 hybridized carbon atoms is comparable reaching 0.94 for HUGO and 0.93 for HOGO. Also the X-ray diffraction in Figure 1e shows higher interlayer spacing for HUGO (0.814 nm) in comparison with less oxidized HOGO (0.689 nm). However, both values are significantly higher when compared to the starting graphite (0.34 nm). The morphology is shown in Figure 1f exhibiting a typical platelet layered structure. The permanganate oxidation route led to a high concentration of ketone and carboxylic acid functionalities that significantly enhance the sorption capacity. On the other hand, the chlorate methods of graphite oxidation yield the graphene oxide with dominant concentration of hydroxyl and epoxide groups with substantially lower coordination ability towards ions. The chemistry of graphene oxide surface has indeed a dominant effect on the sorption capacity, however several trends observed in sorption of various ions across the periodic table



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(PT) indicate a similar sorption mechanism. In the following paragraphs the sorption properties of various metal ions covering s- , p- , d- and f- blocks of PT and forming stable ions in aqueous or acidic environment are discussed in detail. The discussed sorption capacities are given in mg/g as well as mmol/g. The latter units compensate differences in molar mass of various elements and give an idea about the trends of sorption through the periodic table. The sorption capacities were investigated for graphene oxide prepared by Hummers and Hofmann method as the most typical examples of chlorate and permanganate procedures for graphene oxide synthesis.

Sorption capacity of graphene oxide prepared by Hummers method The sorption capacity of graphene oxide towards various ions exhibits notable differences. Going across PT several aspects play important rules. The sorption capacity of various elements obtained on Hummers graphene oxide is shown in Figure 2 in the mg/g of GO as well as molar sorption capacity in mmol/g of GO. The sorption capacity was measured using X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma – optical emission spectroscopy (ICP-OES).


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Figure 1. The characterization of graphene oxide prepared by

Hummers (HUGO) and

Hofmann (HOGO) method. The high resolution XPS analysis of C 1s peak of graphene oxide (A). The XPS survey spectra of graphene oxide (B) and the corresponding FT-IR spectra (C). The Raman spectra and X-ray diffraction of graphene oxide is shown on panel (D) and (E), respectively. The SEM images of graphene oxide are shown on panel (F). The scale bar corresponding to 5 µm.



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HUGO

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symbol mg/g GO

H

mmol/g GO +

Li

2+

Be

0.03

1.5

0.04

0.17

+

Mg2+

Fe3+

Al3+

5.8

4.72

55.5

10.1

0.25

0.19

Na

K

+

Ca

2+

B

0.99 Sc

3+

Ti0

2+

VO

2+

Cr

3+

Mn

2+

2+

C

N

Si

P

0.37

Fe

Co

2+

2+

Ni

Cu

2+

Zn

2+

Ga

3+

4+

3+

Ge

As

9.8

7.46

19.5

4.82

58.1

12

4.72

53.7

3.31

17.2

33.9

8.67

118

355

111

0.25

0.19

0.43

0.1

1.14

0.23

0.09

0.96

0.06

0.29

0.53

0.13

1.69

4.89

1.48

+

2+

2+

3+

4+

Rb

Sr

Y

3+

Zr0

2+

29.3

47.3

18.4

110

0.34

0.54

0.21

1.21

2+

3+

2+

+

Cs

Ba

La

Hf0

48.5

209

32.6

247

0.37

1.52

0.23

1.38

Ag Nb

Mo

Tc

Ru

Rh

Pd

+

Cd

W

Re

Os

Ir

Pt

3+

3+

3+

Pr

Nd

31.6

28.4

59.9

0.23

0.20

0.28

3+

Pm

3+

Gd

3+

Tb

3+

Dy

3+

Ho

3+

Sm

Eu

46.8

34.5

14.8

32.6

32.9

53.6

0.31

0.23

0.09

0.21

0.20

0.33

Sb

3+

30.9

46.8

802

939

0.44

0.27

0.41

6.75

7.71

Au

2+

Er

3+

+

Tl

Pb

2+

3+

Bi

74.0

30.6

235

304

0.37

0.15

1.13

1.46

Hg2

Ce

Sn

47.2

Hg Ta

In

2+

Sn

2+

193

2441

0.96

18.7

3+

3+

Lu

3+

Tm

Yb

27.2

42.4

54.1

80.6

0.16

0.25

0.31

0.46

Figure 2. The sorption capacity of HUGO graphene oxide towards various ions determined by XRF and ICP-OES (Li+, Na+, K+, Rb+, Cs+ and Be2+). The error of measurement is below 2% of measured value.

For the alkali metals we can clearly see the increasing of molar sorption capacity from 0.04 mmol for Li+ up to 0.37 mmol for Cs+. Considering the coordination ability of alkali metals (1st group of PT) which is in general relatively low, we can observe surprising values in particular for heavy alkali metals such as Rb+ and Cs+ with relatively high sorption capacity. In this case, the higher sorption capacity is mainly associated with the ionic radii and higher coordination abilities of larger ions. The ability of Cs+ and Rb+ ions to be coordinated with 14 ACS Paragon Plus Environment

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oxygen functionalities is well known from the organic chemistry, where it is used for the crown ethers synthesis.27 The similar trend can be also seen also for the alkaline earths metals (2nd group), however thanks to the higher valency the absolute values of sorption capacity are significantly higher for 2+ ions compared to 1+ ions. The ability of multivalent ions to be coordinated by graphene oxide is well known and it has been broadly used for the synthesis of graphene oxide nanostructures.39 The sorption capacity increases from 0.17 mmol for Be2+ to about 0.19 mmol for both Mg2+ and Ca2+ and further to 0.54 mmol for Sr2+ and eventually to 1.52 mmol for Ba2+. The effect of increasing ionic radii can be also enhanced by the specific chemistry of graphene oxide. Since the graphene oxide typically contains sulfur moieties mainly in the form of sulfuric acid esters, the presence of Ba2+ ions induces hydrolysis of such esters and simultaneous precipitation of BaSO4 (see X-ray diffraction pattern in Figure 3A). Indeed the similar effect was observed for Sr2+ forming an insoluble SrSO4 (see XRD pattern in Figure 3B). The obtained results indicate that the main effect on the sorption capacity will be associated with the ionic radii, however impurities like sulfates also play a crucial role.

Figure 3. X-ray diffraction pattern of HUGO after sorption of Sr2+ (A) and Ba2+ (B) including, respectively, the reference diffraction lines corresponding to SrSO4 and BaSO4.



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A significantly different situation is encountered for d-block elements, which can exhibit different oxidations states, form oxo-cations and possess a sufficient redox potential to induce changes in graphene oxide chemical composition. This is related to an inherent instability of some oxygen functionalities such as epoxies which sensitive even towards relatively weak reduction agents like Fe2+. From the most abundant elements chromium forms trivalent ions, manganese, cobalt, nickel, copper, zinc, cadmium and mercury form divalent elements. Iron forms stable divalent and also trivalent cations. In addition, mercury can form dimeric cation Hg22+ which can easily disproportionate to Hg2+ and elemental mercury. Vanadium(IV) forms a stable vanadyl cation VO2+ in aqueous solutions which imposes several significant differences regarding the sorption capacity of this element. A very high molar sorption capacity of graphene oxide towards vanadium in the form of vanadyl cation originates from its redox properties. The reaction of vanadyl cation with graphene oxide leads to its partial reduction with simultaneous oxidation of vanadium to V5+ which immediately undergoes hydrolysis. Vanadium is subsequently adsorbed on the surface of graphene oxide in the form of vanadium(+V) acids. The partial reduction of graphene oxide is clearly visible on XPS spectra (Supporting information Figure S1a and S1b) where the C/O ratio increases to 3.6 and the position of V 2p3/2 at 517.0 eV also indicates a presence of vanadium(+V) (Figure S1c).40 The sorption capacity towards Cr3+ is 0.23 mmol, 0.09 mmol for Mn2+, 0.96 mmol for Fe2+, 0.99 mmol for Fe3+, 0.06 mmol for Co2+, 0.29 mmol for Ni2+, 0.53 mmol for Cu2+ and 0.13 mmol for Zn2+. The sorption capacity of Cr3+ relatively low in comparison with other trivalent ions like Sc3+ and Ga3+. This originates from inert singlet d3 configuration (t2g3eg0) of Cr3+. Chromium(+III) ions are thus usually highly kinetically stable and reveal markedly weaker Lewis acidity compared to Al3+ or Sc3+.


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The low sorption capacity of Mn2+ can be explained by low stability of high spin Mn2+ complexes (t2g3eg2) resulting in weak coordination ability of Mn2+. This is essential for the synthesis of graphene oxide since Mn2+ ions are formed as a by-product in permanganate procedure for graphene oxide procedure. High sorption capacity of Fe2+ and Fe3+ originates from high reactivity of their respective electronic configurations t2g4eg2 and t2g3eg2. Iron(+II) has a sufficient reduction potential for partial reduction of graphene oxide during its sorption, which is documented by XPS spectra indicating a slight reduction clearly visible on the changes of C 1s band (Figure SI 2). The reduction is also manifested by C/O ratio measured by XPS, which gives 2.56 for Fe2+ ion in comparison with the value 3.08 obtained for Fe3+ having no reduction ability. Similarly to VO2+ case the simultaneous oxidation of Fe2+ to Fe3+ is likely accompanied with partial hydrolysis and adsorption of the hydrolysis products on graphene oxide surface resulting in high sorption capacity towards Fe2+. The low sorption capacity of Co2+ can be explained by stable and kinetically inert singlet electron configuration e4t23 in tetrahedral environment while the increased sorption capacity for Ni2+ and Cu2+ originates from more reactive electronic configurations of tetragonal e4t24 and octahedral t2g6eg3, respectively. Moreover, the increasing effective charge causing a lowering of dorbitals energy levels for the right-hand side of d-block elements invokes a more covalent interaction with oxygen 2p orbitals and thus a formation of more stable bonds. Finally, the Zn2+ ion have low sorption capacity due to the fully occupied and highly localized d-orbitals (t2g6eg4). A systematic increase of molar sorption capacity can be observed in the 12th group ranging from 0.13 mmol for Zn2+ to 0.27 mmol for Cd2+ to 0.37 mmol for Hg2+. This is a similar behaviour that was observed also for the 2nd group as well. A significantly higher sorption capacity was indicated for dimeric Hg22+ ion. The reaction of dimeric mercury(I) ion with reactive oxygen functionalities led probably to its disproportionation into Hg2+ and elemental mercury and, consequently, to a considerable



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increase of graphene oxide net sorption capacity towards Hg22+ ions. Interestingly, the sorption capacity for Ag+ ion (0.44 mmol) was found slightly higher compared to other monovalent ions with similar ionic radii. In the group 14 we can observe a substantial increase in sorption capacity towards zirconyl and hafnyl oxocations (1.21 for ZrO2+ and finally 1.38 HfO2+) compared to titanyl ion (TiO2+) with a small sorption capacity of 0.10 mmol. The low sorption capacity towards TiO2+ is related to strongly acidic environment which is necessary to avoid hydrolysis and TiO2 formation. A slight increase of sorption capacity for HfO2+ compared to ZrO2+ suggests a possibility to use this system for Hf/Zr separation which is extremely difficult due to the similar ionic radii originating from lanthanide contraction and leading eventually to almost identical chemical behaviour of these two elements. On the base of this observation we performed the sorption experiments for the mixture containing 50 at.% of Zr and 50 at.% of Hf in the form of ZrO2+ and HfO2+ chlorides. The atomic ratio Hf:Zr in the graphene oxide after the sorption increased to 56.3 : 43.7 indicating a slight enrichment with Hf. The most surprising behaviour in the sorption capacity of HUGO was observed for the 3rd group and the following lanthanide ions series. The sorption capacity was 0.43 mmol for Sc3+, 0.21 mmol for Y3+ and 0.23 mmol for La3+. High sorption capacity towards Sc3+ originates from its behaviour as a hard Lewis acid in comparison with Y3+ and La3+. In the lanthanide series the sorption capacities ranged between 0.16 and 0.46 mmol with the exception of Gd3+ with the sorption capacity of 0.094 mmol. Slight increase in the sorption capacity was observed for the end of lanthanide group with increasing of atomic number. This may indicate an important role of ionic radii reduction due to lanthanide contraction which led to very close ionic radii of Y3+ and Lu3+. The key factor influencing the anomalous low sorption capacity towards Gd3+ is viewed in the electron configuration of this cation with half-filled 4f orbitals making them highly localized and thus less prone to form stable coordination bonds.


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Apart from the cations undergoing hydrolysis reactions on graphene oxide surface or redox reactions with its oxygen functionalities, the sorption capacity of graphene oxide is generally influenced by several factors like ionic radii, valence state as well as coordination ability originating from its electronic structure. The general trend of increasing sorption capacity can be observed dominantly only for mono- and divalent metals with empty, or completely filled valence orbitals like elements from groups 1, 2 and 12. For the p-block elements the sorption capacity of graphene oxide was only investigated towards those cations which are stable in aqueous or diluted acids environment. This applies to Al3+, Ga3+, In3+, Tl+, Ge4+, Sn2+, Sn4+, Pb2+, As3+, Sb3+ and Bi3+. To avoid hydrolysis the sorption experiments with Sn2+, Sn4+ as well as As3+, Sb3+ and Bi3+ were performed in strongly acidic environment with the concentrations of acids reaching up to 10 wt.% referred to the corresponding anion. In the 13th group the highest sorption capacity was found towards Ga3+ reaching 1.69 mmol followed by In3+ with 0.41 mmol and Al3+ with 0.37 mmol. Thallium in the oxidation state 1+ exhibits a markedly lower sorption capacity (0.15 mmol) compared to other trivalent elements in group 13. This obviously related to chemical behaviour of Tl+ being close to larger alkali metals and revealing low tendency to form coordination compounds with oxygen ligands. The high sorption capacity of graphene oxide towards Ga3+ was recently reported showing its application potential for separation of Ga from bauxite.21 The relatively high sorption capacity of In3+ in comparison with d-elements offers a possible application for separation of In from sulfides ores like zinc blende (ZnS) using graphene oxide. In the group 14 the sorption experiments were performed on Ge4+, Sn2+, Sn4+ and Pb2+ ions. The Ge4+ ion is stable only in highly acidic solutions and it was stabilized as GeCl62- (by a mixture of hydrochloric acid and ammonium chloride). The found sorption capacity was 4.89 mmol.



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Figure 4. X-ray diffraction patterns of HU-Sn2+ (A) and HU-Sn4+ sample (B). High resolution XPS spectra of C 1s for HU-Sn2+ (C) and HU-Sn4+ (D).

Sn2+ as a well known reducing agent brings about an irreversible reduction of graphene oxide to graphene during the sorption with a simultaneous oxidation to Sn4+ and its subsequent hydrolysis to SnO2. This is the origin of the anomalous sorption capacity observed for Sn2+ and reaching 18.7 mmol. A lower but still considerable sorption capacity, 6.75 mmol, was observed for Sn4+ and can be associated with the hydrolysis of Sn4+ induced by the graphene oxide surface even in strongly acidic environment. The presence of nanocrystalline SnO2 within the samples was proved by X-ray diffraction (see Figure 4a and 4b). The significant reduction of graphene oxide using Sn2+ in comparison to Sn4+ is also documented by XPS (see Figure 4c and 4d). The sorption capacity towards Pb2+ is also significantly higher compared to other divalent ions. Since the Pb2+ exhibits a negligible reducing power towards oxygen functionalities on graphene oxide surface, we examined the sample in detail using X-ray diffraction to resolve the origin of such sorption capacity. The results showed the 20 ACS Paragon Plus Environment

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origin of such a high sorption capacity is related to the formation of insoluble PbSO4 adsorbed on graphene oxide surface. The presence of Pb2+ ions apparently induces hydrolysis of sulfuric acid esters present on the graphene oxide surface with subsequent formation of insoluble lead sulfate (See Figure S3). A very high sorption capacity towards all investigated elements was observed for the group 15. The sorption capacity was 1.48 mmol for As3+, 7.71 mmol for Sb3+ and 1.46 mmol for Bi3+. In all cases the sorption experiments were performed in strongly acidic environment to avoid hydrolysis of these ions. Most importantly, the high sorption capacity towards As3+ ion in acidic environment is highly interesting for future applications in pollution remediation. It is unlikely that such a sorption activity is related to As3+ hydrolysis, since the hydrolysis product – arsenic acid – is soluble in water. Nonetheless, As3+ apparently caused a partial reduction of graphene oxide as manifested by both As(+III) and As(+V) compounds identified in

the

sorption

product

by

X-ray

diffraction,

namely

as

(NH4)H2AsO4

and

NH4ClAs2O3.(H2O)0.5 (Figure 5c). High sorption capacity towards Sb3+ and Bi3+ can be presumably associated with hydrolysis like in the case of Sn4+, where a very high sorption capacity was observed too. However, no crystalline hydrolytic products were observed by X-ray diffraction in the case of Bi3+ (see Figure 5a). By contrast, formation of basic chloride (Sb4O5Cl2) as shown on the diffraction pattern in Figure 5b clearly points to the hydrolysis of the original SbCl3.

Sorption capacity of graphene oxide prepared by Hofmann method In addition the sorption capacity was investigated for graphene oxide prepared by Hofmann method. The results are summarized in Figure 6. As expected, due to the significant differences in composition of oxygen functionalities, the differences in sorption capacities were indeed observed. The surface of graphene oxide prepared by Hofmann method is



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dominated by hydroxyls and epoxies exhibiting lower coordination abilities in comparison with ketone groups. Moreover chlorate methods of graphite oxidation (including Hofmann method) yield much lower concentration of carboxylic acid functionalities which play important role in sorption of metallic ions. This led to a considerably lower sorption capacity in comparison with graphene oxide prepared by permanganate methods with high concentration of ketones and carboxylic acids. The elemental distribution maps obtained by EDS are shown in the Supporting information (Figure S4).

Figure 5. X-ray diffraction patterns of HU-As3+ (A), HU-Bi3+ (B) and HU-Sb3+ (C) and reference diffraction lines of the observed phases ((NH4)H2(AsO4); (NH4)ClAs2O3(H2O)0.5 and Sb4O5Cl2).


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HOGO

symbol

H

mg/g GO mmol/g GO

Li+

Be2+

0.01

0.40

0.01

0.04

Na

+

Mg

B

2+

3+

N

Si

P

3+

Fe

Al

0.10

1.10

88.9

2.91

0.04

0.05

1.59

0.11

+

C

K 0.17

Ca 2.62

2+

Sc 1.12

3+

TiO 1.20

2+

VO 25.9

2+

Cr 2.40

3+

Mn 1.81

2+

Fe 51.5

2+

Co 3.21

2+

Ni 7.22

2+

Cu 5.83

2+

Zn 1.94

2+

Ga 17.6

3+

Ge 269

4+

As 280

0.04

0.06

0.24

0.25

0.49

0.05

0.03

0.92

0.05

0.12

0.09

0.03

0.25

3.70

3.74

+

2+

2+

3+

4+

Sb3+

Rb

Sr

Y

3+

0.86

5.42

0.22

0.10

0.06

0.25

2+

3+

+

Cs

Ba

La

1.21

22.4

0.47

0.09

0.16

0.34

Ag Zr

Nb

Mo

Tc

Ru

Rh

Pd

+

Cd

3.73

1.34

62.1

63.3

0.06

0.03

0.12

0.52

0.52

Hg Ta

W

Re

Os

Ir

Pt

Sn

6.12

2+

Hf

In

3+

Au

+

Tl

Pb

2+

3+

Bi

7.74

0.47

8.62

5.73

0.04

0.02

4.67

0.03

Hg22+

Sn2+

96.4

981

0.48

8.26

Figure 6. The sorption capacity of HOGO graphene oxide towards various ions determined by XRF and ICP-OES (Li+, Na+, K+, Rb+, Cs+ and Be2+). The error of measurement is below 2% of measured value.

For the s-block elements similar trends in molar sorption capacity like for HUGO were observed, however, the sorption capacity was generally about one order in magnitude lower. In the case of d-elements several similar trends were also observed, however, there are some exceptions due to the different chemical composition of oxygen functionalities. Similarly, like in the case of HU-GO, the sorption of Ba2+ and Sr2+ ions was accompanied by the formation of insoluble BaSO4 and SrSO4 adhering to the surface of graphene oxide. For the d-block elements, the main differences in trends of sorption capacities were identified for groups 3 and 12. For the group 12 no increase of molar sorption capacity with 23 ACS Paragon Plus Environment


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ionic radii was observed. Similarly, almost constant sorption capacity was found in the 3rd group indicating the negligible role of hard/soft acid/base behaviour of ions on their sorption on graphene oxide prepared by chlorate methods. These results show an important role of carboxylic acid functionalities in sorption of such ions. Like in HUGO the sorption capacity towards Cr3+, Mn2+ and Co2+ was very low due to their electron configurations imposing a kinetic inertness and localization. The high sorption capacity towards Fe2+ and Fe3+ as well as VO22+ is similar to HUGO and is related to coordination sphere electronic structure of these ions and to their redox properties. Low sorption capacity towards Ag+ originates from its negligible coordination ability towards oxygen ligands comparable to other monovalent ions (alkali metals and thallium). High sorption capacity towards Hg22+ results from its redox properties and possible disproportionation to Hg2+ and elemental mercury. The trends in p-block elements sorption capacities are also similar to those observed on HUGO. For the 13th group elements the highest sorption capacity was found for Ga3+ while thallium exhibited a negligible sorption capacity in line with its similar chemical similarity to alkali metals. In the groups 14 and 15 high sorption capacities were observed towards elements undergoing hydrolysis on graphene oxide surface even in highly acidic environment. The presence of hydrolytic products was identified by X-ray diffraction showing nanocrystalline SnO2 originating from sorption of SnO2 and As2O3 from sorption of As3+. The presence of As2O3 is unusual since it is a water soluble compound and its presence indicates its possible intercalation within graphene oxide layers. The extremely high sorption capacity towards Sn2+ can be explained in terms of SnO2 formation as a hydrolytic product of graphene oxide reduction since Sn2+ is relatively strong reducing agent. This was confirmed by XPS as well as X-ray diffraction (Figure S5). The high sorption capacity towards Pb2+ originates from formation of PbSO4 which is driven by hydrolysis of sulfuric acid ester moieties present on graphene oxide surface (Figure S6). The sorption products of both Ge4+ and Sb3+ are


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amorphous and could not be identified by X-ray diffraction. The sorption capacity towards Bi3+ was significantly lower in comparison with HUGO indicating a crucial rule of carboxylic acids in its sorption on graphene oxide. The homogeneity of elements distribution was investigated for selected elements by EDS mapping similarly like for HUGO. The homogeneous distribution of sorbet elements was observed for most of the d-elements and selements. The phase separation was dominantly observed for the elements, where other phases were identified by X-ray diffraction. The unhomogeneities in the elements distribution were observed for Sn2+ and Sn4+ as well as arsenic and lead where phases like SnO2, As2O3 and PbSO4 were identified by X-ray diffraction. The EDS maps are shown in Supporting information (Figure S7).

CONCLUSION We have performed a complex study of sorption of various ions on the graphene oxide prepared by permanganate (Hummers) and chlorate (Hofmann) method. The results confirmed the ability of graphene oxide act as an effective sorbent for removal of toxic ions from water solutions. Significant differences in the chemistry of oxygen functionalities present on graphene oxide surface prepared by different oxidation methods led to high differences in sorption capacities. In particular permanganate based methods are favourable to the formation of ketone and carboxylic acid functionalities which brought about an increase of sorption capacity due to their coordination abilities. Moreover the electronic structure, redox properties and acidity of the involved ions play also an important role in their sorption capacities. The ions with hard acid behavior show high sorption capacities due to their strong affinity towards oxygen functionalities as well as due to their tendency towards hydrolysis. In addition impurities such as sulfates present in graphene oxide can strongly influence the sorption capacity due to the formation of insoluble sulfates. The results show the necessity of



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careful analysis of sorption products on carbon nanomaterials in order to unveil the right origin of sorption mechanisms. These results manifest an extraordinary application potential of graphene oxide based materials for environmental remediation, waste water treatment, decontamination and other emergency applications. The homogeneously distributed sorbed elements over graphene or graphene oxide can be applied also for construction of electrochemical catalysis, sensing devices and microelectronic elements as well as building 3D structures.41-45

ASSOCIATED CONTENT Supporting Information. Additional figures and table including the results of C1s peak deconvolution for starting graphene oxide, XPS spectra of graphene after sorption of VO22+, Fe2+ , Fe3+, Sn2+ and Sn4+ ions, X-ray diffractograms of graphene oxide after Pb2+ sorption and elemental distribution maps after sorption obtained by EDS. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Zdeněk Sofer: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENT: The work was supported by Czech Science Foundation (GACR No. 15-09001S and GACR No. 16-05167S) and by specific university research (MSMT No. 20-SVV/2016). M.P. was supported by Tier2 grant. 26 ACS Paragon Plus Environment

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