Change in the Affinity of Ethylene Glycol Methacrylate Phosphate

Article pubs.acs.org/JPCB

Change in the Affinity of Ethylene Glycol Methacrylate Phosphate Monomer and Its Polymer Anchored on a Graphene Oxide Platform toward Uranium(VI) and Plutonium(IV) Ions Sankararao Chappa,†,‡ Ashish K. Singha Deb,§ Sk. Musharaf Ali,*,§ A. K. Debnath,∥ D. K. Aswal,∥ and Ashok K. Pandey*,†,‡ †

Radiochemistry Division, Bhabha Atomic Research Centre Trombay, Mumbai-400 085, India Homi Bhabha National Institute, Anushaktinagar, Mumbai-400 094, India § Chemical Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India ∥ Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India ‡

S Supporting Information *

ABSTRACT: The complexation behavior of the carbonyl and phosphoryl ligating groups bearing ethylene glycol methacrylate phosphate (EGMP) monomer and its polymer fixed on a graphene oxide (GO) platform was studied to understand the coordination ability of segregated EGMP units and polymer chains toward UO22+ and Pu4+ ions. The cross-linked poly(EGMP) gel and EGMP dissolved in solution have a similar affinity toward these ions. UV-initiator induced polymerization was used to graft poly(EGMP) on the GO platform utilizing a double bond of EGMP covalently fixed on it. X-ray photoelectron spectroscopy (XPS) of the GO and GO−EGMP was done to confirm covalent attachment of the EGMP via a −C−O−P− link between GO and EGMP. The extent of poly(EGMP) grafting on GO by thermal analyses was found to be 5.88 wt %. The EGMP units fixed on the graphene oxide platform exhibited a remarkable selectivity toward Pu4+ ions at high HNO3 conc. where coordination is a dominant mode involved in the sorption of ions. The ratio of distribution coefficients of Pu(IV) to U(VI) (DPu(IV)/DU(VI)) followed a trend as crosslinked poly(EGMP) (0.95) < EGMP in solvent methyl isobutyl ketone (1.3) < GO− poly(EGMP) (25) < GO−EGMP (181); the DPu(IV)/DU(VI) values are given in parentheses. The density functional theory computations have been performed for the complexation of UO22+ and Pu4+ ions with the EGMP molecule anchored on GO in the presence of nitrate ions. This computational modeling suggested that Pu4+ ion formed a strong coordination complex with phosphoryl and carbonyl ligating groups of the GO−EGMP as compared to UO22+ ions. Thus, the nonselective EGMP becomes highly selective to Pu(IV) ions when it interacts as a single unit fixed on a GO platform.

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INTRODUCTION

A chemical modification of GO is required to make it selective for given metal ions, which is important for the sequestration of target ions from the complex aqueous media. The linker groups on GO have been functionalized with cysteine, 13 ethylenediaminetriacetic acid, 3 phosphate, 14 amide, 15,16 oxalic acid, 17 and many other functional groups.2,5,6,18 In general, the carbon nanomaterial can also be used as a platform for anchoring the polymer chains either utilizing a free radical formed by the oxygen bearing functional groups under UV irradiation or any other suitable mechanism such as “grafting to” of end-functionalized polymer molecules directly with the linker groups and “grafting from” using the polymerizable double bond of monomer covalently attached to the carbon nanomaterial.19−26 The “grafting from” approach is more convenient for attaching amine/hydroxyl groups bearing

2

Graphene oxide (GO), a 2D sheet of sp -bonded carbon atoms arranged in a honeycomb lattice, has several oxygen-containing functional groups such as COOH, CO, O, and  OH on the basal plane, edges, and defects. The presence of oxygen bearing functional groups makes GO the most attractive carbon nanomaterial, as these groups act as the linkers for chemical modifications, provide hydrophilicity, and bind with heavy metal ions.1−6 The removal of actinides such as Am(III), Th(IV), Pu(IV), Np(V), and U(VI) and typical fission products Sr(II), Eu(III), and Tc(VII) by GO from solution having pH < 2 has been found to be efficient.7 The few-layered GO has been studied as the sorbent for U(VI) and other toxic metal ions from the solutions having acidity in the pH range.8−12 The major issue for using the pristine GO is that it sorbs heavy metal ions by the electrostatic interactions which makes it nonselective, and sorption becomes highly dependent upon the pH and ionic strength of the solution. © XXXX American Chemical Society

Received: November 18, 2015 Revised: February 29, 2016

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DOI: 10.1021/acs.jpcb.5b11293 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

crylate were obtained from Sigma-Aldrich (Steinheim, Switzerland). Tetrahydrofuran (THF), ethanol, and N,N′-dimethylformamide (DMF) were obtained from Merck (Mumbai, India). Graphene oxide (GO) was procured from Ad-Nano Technologies, Shimoga, Karnataka, India. The specifications of GO used were the following: diameter ≈ 10 μm, thickness ≈ 0.8− 1.6 nm, purity = 99%, number of layers = 1−2, surface area 500 m2 g−1. The liquid scintillation counting of the α-activity of actinides was carried out by adding a 50−100 μL sample to 5 mL of scintillation cocktail-O having the composition 2,5diphenyl oxazole = 10 g, 1,4-di-2-(5-phenyloxazolyl) benzene = 0.25 g, and naphthalene = 100 g in 1000 mL of toluene, and 10 v/v % bis(2-ethylhexyl) phosphoric acid (HDEHP). The liquid scintillation counting was carried out in a home-built liquid scintillation counter. Synthesis of GO−EGMP. For attaching EGMP, 250 mg of GO was dispersed in 25 mL of ethanol and sonicated for 15 min. Then, 150−900 mg of EGMP was added in this solution, and kept on a shaker for 12 h at room temperature. After equilibration, GO was removed from ethanol by centrifuge and washed 3−4 times with an excess of ethanol. Finally, GO@ EGMP was dispersed in ethanol again and sonicated for 20 min to remove EGMP physically adhering to the GO, and dried at room temperature. The optimum amount of EGMP required was obtained by measuring the optimum Pu4+ ion uptake from 3 mol L−1 HNO3 as a function of EGMP amount in the reacting solutions having a fixed 250 mg of GO. Synthesis of GO−poly(EGMP). EGMP attached graphene oxide was taken and dispersed in DMF solvent. Initiator DMPA and monomer EGMP were added to this solution and kept on a magnetic stirrer for a uniform dispersion. This solution was irradiated with 365 nm UV light for 1 h in a photoreactor. This photoreactor was procured from Heber Scientific, Chennai, India (model no. HML-SW-MW-LW-888). After UV irradiation, the GO−poly(EGMP) was separated after a high speed centrifuge. The GO−poly(EGMP) was washed repeatedly with ethanol several times to remove unattached polymer from the GO, and finally washed with water and dried for use. Synthesis of Cross-Linked Poly(EGMP). EGMP, UV initiator DMPA (2 wt %), and cross-linker ethylene glycol dimethacrylate (5 mol %) were dissolved in DMF and exposed to UV light (365 nm) for 30 min. The cross-linked poly(EGMP) gel was thoroughly washed with DMF, methanol, and distilled water to remove the ungrafted components. Extraction Experiments. The required acidities of the aqueous samples containing appropriate radiotracers were adjusted with HNO3. The uptakes of alpha emitters 233U and 238,239,240 Pu in the gel samples were obtained by α-scintillation counting of the samples from the solution taken before and after equilibration with the sorbent. The α-activities of the equilibrating solutions (5−10 mL) were adjusted to obtain 5000 counts min−1 in α-scintillation counting of its 50 μL sample. A 50 μL sample of the equilibrating solution was added to 5 mL of standard organic liquid scintillation cocktail and assayed by counting in a homemade liquid scintillation counter. Liquid−liquid extractions were carried out by equilibrating 2 mL of an organic phase containing a desired concentration of extractant EGMP dissolved in methyl isobutyl ketone (MIBK) with the same volume of an aqueous phase containing required radioactivity of UO22+/Pu4+ ions at 3 mol L−1 HNO3. The equilibrations were carried out overnight at room temperature with constant shaking. After equilibration, the radioactivity of 233 U and 238,239,240Pu was measured in both the organic and

monomer covalently by the esterification or amidation reactions, and then growing desirable polymer chains on the GO by polymerization with the monomer having a desirable functional group. The various strategies have been used for anchoring functional groups bearing polymer chains on the GO platform for the removal of actinides and radionuclides from the aqueous streams. Poly(amidoxime) (PAO), known to have selectivity toward U(VI) at higher pH range and seawater, has been anchored on GO for the removal of U(VI) and other radionuclides.27−29 The GO/polypyrrole composites have been reported as a highly U(VI) selective sorbent.30 The GO-supported polyaniline has been studied for the preconcentration of U(VI), Eu(III), Sr(II), and Cs(I) from the aqueous solutions having acidity in a pH range.31 The major issues associated with these functionalized GO materials developed for the sorption of actinides and radionuclides are their nonselectivity in the presence of competing ions and dependency on pH and ionic strength of the aqueous media. It has been reported that the cross-linked polymer formed by ethylene glycol methacrylate phosphate (EGMP) sorbs UO22+ and Pu4+ ions preferentially from a wide variety of aqueous matrixes such as seawater and 3−4 mol L−1 HNO3.32−35 It is expected that the ion-exchange mechanism would not play any role in the sorption of UO22+ and Pu4+ ions in the poly(EGMP) from a solution having a high acidity, and the sorption of these actinides would be occurring through their complexation with the phosphoryl and carbonyl ligating groups present in the units of poly(EGMP). In a case involving multiple coordinating EGMP units, the sorption of the actinides would decrease with their physical segregation on a chemical platform. It has been observed by Qin et al. that the metal ion sorption in the copolymer of graphene oxide and poly(1-(3-aminopropyl)pyrrole) is highly dependent on the density of amino groups, which are required for the intramolecular and intermolecular complexations with metal ions.36 In the present work, the monomer EGMP and poly(EGMP) have been anchored on the GO platform to study their affinity toward UO22+ and Pu4+ ions in the solutions containing a high concentration of HNO3 (3 mol L−1) that are encountered in waste streams normally generated at the nuclear fuel reprocessing facility. The monomer EGMP has been anchored on the GO by forming a −C−O−P− link between GO and EGMP, and the poly(EGMP) chains have been formed by a UV-initiator induced EGMP grafting using polymerizable double bonds of the GO−EGMP. The choice of using GO as a chemical platform for EGMP and poly(EGMP) has been based on an assumption that it would restrict the interactions of Pu4+ and UO22+ ions to a single EGMP unit or adjacent EGMP units of a single poly(EGMP) chain. In addition to this, GO has a high mechanical strength, thermal stability, and electrons to absorb/attract metal ions onto it. Finally, the density functional theory (DFT) computations have been carried out for the complexation of UO22+ and Pu4+ ions with the EGMP molecules anchored on a GO platform in the presence of nitrate ions to study the origin of remarkable selectivity of the GO−EGMP toward Pu4+ ions.

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EXPERIMENTAL SECTION Analytical reagent grade chemicals and deionized water (18 MΩ cm−1) were used throughout the present studies. Ethylene glycol methacrylate phosphate (EGMP), α,α′-dimethoxy-αphenyl acetophenone (DMPA), and ethylene glycol dimethaB


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Figure 1. Deconvoluted core level C1s (a) and P2p (b) XPS spectra of GO−EGMP showing the appearances of an additional C1s peak corresponding to the C−O−P link and P2p corresponding to organophosphate.

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aqueous phases by subjecting 50 μL samples from each phase to the liquid scintillation countings. For the liquid−liquid extraction systems, the D values were obtained as the mean ratios (average of three experiments) of the radioactivity of 233 U/238,239,240Pu in the organic phase to that in the aqueous phase. The liquid−solid extraction was carried out by equilibrating 30 mg of GO−EGMP/GO−poly(EGMP)/cross-linked poly(EGMP) gel in 1 mL of aqueous sample containing 233 U/238,239,240Pu in 3 mol L−1 HNO3 for overnight with constant shaking. The solid part was separated from the liquid by centrifugation. The extraction of 233U/238,239,240Pu was monitored by the liquid scintillation counting as described above. The distribution coefficients (D) for the GO−EGMP/ GO−poly(EGMP)/cross-linked poly(EGMP) gel−aqueous phase system were obtained by

D=

(C0 − Ce) V × W Ce

COMPUTATIONAL METHOD Density functional theory (DFT) computations were performed by means of the Turbomole suite of programs.37 The geometrical structure of the graphene oxide (GO) was used considering one epoxy group on the surface, one carboxyl group on the edge sp2 carbon, and one hydroxyl attached with the edge sp2 carbon, on a 5 × 5 armchair edged graphene unit cell composed of 58 carbon atoms. The model GO structure used in the present calculation has been arrived at on the basis of the available models38,39 and DFT guided stability analyses given in the Supporting Information. Geometrical optimizations of the GO, GO−EGMP (the phosphate unit has been covalently connected through a phosphoesteric bond with the hydroxyl group on the graphene sheet),40 and UO22+ and Pu4+ complexes with GO−EGMP in the presence of nitrate anions were carried out using the hybrid Becke’s three-parameter nonlocal hybrid exchange functional in combination with Lee, Yang, and Parr’s correlation functional (B3-LYP) employing a split-valence plus polarization (SVP) basis set.41,42 The SVP basis set is equivalent to 6-31G*. The single point energies of the studied systems were calculated with the B3LYP functional using the triple-ζ valence plus polarization (TZVP) basis set. All calculations relating Pu4+ ions were performed using its quintet spin state. The relativistic effective core potential (ECP) was used, where 60 electrons are kept in the core of U and Pu.43 The solvent effect in the energetics was incorporated employing the conductor-like screening model (COSMO).44 The MOLDEN visualization package has been used for visualizing the molecular structures.45 The binding energy (ΔE) of the complexation reaction (eq 2) of UO22+ and Pu4+ ions with GO−EGMP in the presence of nitrate ions was calculated using eq 3.

(1)

where C0 and Ce represent the radioactivity of 233U/238,239,240Pu initially and after equilibration in the aqueous phase and W and V are the weight of the solid sample (GO−EGMP/GO− poly(EGMP)/cross-linked poly(EGMP) gel) and the volume of the aqueous phase, respectively. XPS Experiments. For X-ray photoelectron spectroscopy (XPS) analysis, a Mg Kα (1253.6 eV) X-ray source and a DESA150 electron analyzer (M/s. Staib Instruments, Germany) were employed for recording the spectra. The binding energy scale was calibrated to the Au-4f7/2 line of 83.95 eV. The XPS spectra are fitted using XPSPEAK41 software. The elemental composition (Ci) of C, O, and P was obtained from XPS data using the following relation

Mn + + L + n NO3− → ML(NO3)n

Ii /Si Ci = ∑ (Ii /Si)

(2)

ΔE = EML(NO3)n − (E Mn+ + E L + nE NO3−); n = 2/4 for UO2 2 + /Pu 4 +

where Ii represents the intensity of the C, O, or P peaks and is determined by finding the total area under the core level peak using the least-squares fitting of the Gaussian line shape. Si is the atomic sensitivity factor and has values of 0.296, 0.711, and 0.486 for C-1s, O-1s, and P-2p peaks, respectively.

(3)

where EML(NO3)n, EMn+, EL, and ENO−3 are the energies of the metal ion (Mn+)−ligand (L) complex, metal ion, GO−EGMP ligand, and nitrate anion, respectively. The thermodynamic parameters, C


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resolved. Thus, the C−O−P link between GO and EGMP would have formed utilizing either −OH or epoxy groups that could not be identified experimentally. In Raman spectra of the GO and GO−EGMP, the peaks corresponding to G and D bands were seen; see Figure S2 and Table S1 (Supporting Information). It is seen that the D/G intensity ratio remained the same (1.15) in both of the materials, indicating no creation of further defects during the covalent attachment of EGMP molecules to GO. However, a red shift in both of the bands was observed in the GO−EGMP, indicating altered interactions within the carbon networks due to the formation of the C−O−P links between EGMP molecules and GO matrix. A comparison of the FE-SEM images of GO and GO−EGMP shows no significant physical changes after the attachment of EGMP on the GO. Density functional theory (DFT) computation was performed to obtain the optimized geometric chemical structure of GO−EGMP shown in Figure 2. The poly(EGMP) chains were also anchored on the GO platform utilizing the double bond of the EGMP covalently attached to it, as shown in Scheme 1. The polymerization was initiated by UV-initiator that would lead to the possibility of growing the poly(EGMP) chain from GO−EGMP and also poly(EGMP) chains growing independently would be terminated with attachments with the double bond of GO−EGMP. There is a possibility that GO may undergo reduction during UV irradiation. However, XPS data did not show a high reduction in O content during grafting on GO−EGMP. As such, the reduction of GO to rGO would affect the uptake in the pH range and not in a high acidity range where actinide sorption occurs only through phosphate groups of EGMP units. The field emission scanning electron microscopy (FE-SEM) image showed whitening of the GO and formation of gel-like structure after the attachment of poly(EGMP); see Figure 3. The diameters of pristine GO and GO−poly(EGMP) were found to be 2−5 μm, which was significantly lower than the 10 μm diameter given in the specifications of GO. It may be because of breaking of GO during sonication used for dispersing it in the solvent. To confirm formation of poly(EGMP), the elemental mappings of C, O, and P on GO−poly(EGMP) were carried out by energy-dispersive spectroscopy (EDS) attached to FESEM, and shown in Figure 4. It is seen from the elemental mappings that P are uniformly distributed on the GO but not as dense as seen in the mappings of C and O. This seems to suggest that EGMP units on the GO were spaced. The presence of expected functional groups was confirmed from the

the binding enthalpy (ΔH), and the binding free energy (ΔG) of the complexation were evaluated from the thermal and zero point energy corrected electronic binding energy and entropy change of the process at 298 K.46

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RESULTS AND DISCUSSION To understand the coordination affinity of the segregated EGMP molecules toward UO22+ and Pu4+ ions, EGMP and its

Figure 2. Front and lateral views of the optimized geometric chemical structure of GO−EGMP.

polymer poly(EGMP) were anchored on a graphene oxide (GO) platform. The choice of GO as a chemical platform was based on the fact that EGMP could be attached covalently with the linker groups on GO with a sufficient spacing. It is evident from the literature that COOH, >CO, O, and OH groups on the GO could be utilized for its functionalization depending upon the chemical structure of molecules.1−6 It is obvious from the chemical structure of GO that most of these linker groups would not be in close proximity. It was found in the minimum energy conformation analyses of a diphosphonate ligand that the simultaneous phosphoryl oxygen and acidic oxygen coordination with f-element ions is not favorable.47 Therefore, utilization of one hydroxyl group would not affect the complexation behavior of EGMP. The EGMP molecules were covalently attached to the GO as described in the Experimental Section. X-ray photoelectron spectroscopy (XPS) of the GO and GO−EGMP was done to confirm covalent attachment of the EGMP. The comparison of C1s, O1s, and P2p peaks in the XPS spectrum of the GO−EGMP with the XPS spectrum of the pristine GO is given in Figure S1 (Supporting Information). The P2p peaks at 133.9 and 134.7 eV correspond to the phosphate group, and the additional C1s peak at 285.7 eV shown in Figure 1 is due to formation of the C− O−P groups as reported elsewhere.40 These peaks were not present in the XPS spectrum of pristine GO. Also, O1s peaks were modified in the XPS spectrum of the GO−EGMP due to the appearance of bridging oxygen but O1s peaks could not be

Scheme 1. Schematic Representation of the Grafting of Poly(EGMP) Chains on the GO Platform by UV-Initiator Induced Polymerization

D


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Figure 3. FE-SEM images showing pristine GO sheets (a) and GO−poly(EGMP) (b).

Figure 4. FE-SEM image and corresponding C, O, and P elemental mappings of GO−poly(EGMO) obtained by EDS.

(Supporting Information) shows expected peaks. The comparison of core level C1s and O1s peaks in the XPS spectra of GO, GO−EGMP, and GO−poly(EGMP) is given in Table S2 (Supporting Information). The elemental compositions of GO−EGMP and GO−poly(EGMP) obtained from XPS data were found to be 72.8% C, 24.0% O, 3.2% P and 67.6% C, 27.3% O, 5.1% P, respectively. The increase of O and P contents in the GO−poly(EGMP) with respect to the GO− EGMP confirmed formation of the polymer chains during grafting. The extent of poly(EGMP) grafting on GO was obtained by thermal analysis. TG and DTA curves of GO and GO− poly(EGMP) were obtained in a nitrogen atmosphere with a heating rate of 10 °C min−1. It is seen from the TG/DTA curves given in Figure S5a (Supporting Information) that thermolysis of pristine GO was in two regions: (i) decomposition around 100−200 °C due to oxygen containing

Table 1. Distribution Coefficients of U(VI) and Pu(IV) in the Different Forms of EGMP from the Solutions Containing 3 mol L−1 HNO3 material EGMP in MIBK (0.05 mol L−1) EGMP in MIBK (0.1 mol L−1) GO−EGMP GO−poly(EGMP) cross-linked poly(EGMP)

DU(VI)a

DPu(IV)a

0.41 ± 0.02

0.55 ± 0.03

1.34

0.44 ± 0.02

0.60 ± 0.03

1.36

1.66 ± 0.08 26 ± 1 353 ± 18

301 ± 15 640 ± 32 335 ± 17

DPu(IV)/DU(VI)

181 25 0.95

a

The units of D values of EGMP in MIBK are dimensionless and mL g−1 in other cases. The values are the average of three to four experiments having ±5% variations.

FTIR spectra given in Figure S3 (Supporting Information). The XPS spectrum of GO−poly(EGMP) given in Figure S4 E


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Figure 5. Different views of the optimized geometric chemical structures of the complexes formed by plutonium nitrate (a) and uranyl nitrate (b) with the EGMP molecule fixed on the GO platform. C, O, N, and P atoms are shown in green, red, orange, and blue, respectively.

that UO22+ and Pu4+ ions did not sorb in the pristine GO at 3 mol L−1 HNO3. Therefore, the sorption of UO22+ and Pu4+ ions would have occurred only through a complexation with the EGMP moiety attached on GO. It was observed from the extraction experiments that Am3+, Eu3+, Sr2+, and Cs+ ions did not sorb in the GO−EGMP significantly (3−5%) from the aqueous solutions having 3 mol L−1 HNO3. It is seen from Table 1 that the D values of Pu4+ and UO22+ ions in EGMP/MIBK increased with the EGMP concentration, but the ratio of DPu(IV)/DU(VI) remained almost the same. It is interesting to observe that the D values of EGMP are not promising in a liquid−liquid extraction system for actinides. However, the cross-linked poly(EGMP) gel has a high affinity toward UO22+ and Pu4+ ions similar to Diphonix chelating resin developed for the actinide ions separation.47 It is also seen from Table 1 that the value of DPu(IV) is highest in the GO− poly(EGMP) with a considerably lower DU(VI) value. The D values given in Table 1 for the liquid−liquid and liquid−solid systems could not be compared directly due to a difference in their units. However, the ratio of DPu(IV)/DU(VI) could be compared for understanding the Pu4+ selectivity pattern in different systems. The ratio of DPu(IV)/DU(VI) followed a trend as cross-linked poly(EGMP) (0.95) < EGMP in MIBK (1.3) < GO−poly(EGMP) (25) < GO−EGMP (181); the DPu(IV)/ DU(VI) values are given in parentheses. This seems to suggest that there is no selectivity between Pu4+ and UO22+ when EGMP units are in very close proximity as in the case of crosslinked poly(EGMP) or EGMP dissolved in a solution (free form). However, the EGMP molecules and poly(EGMP) chains exhibit a high selectivity toward Pu4+ when fixed on the GO platform. It has been observed that 65% of Pu4+ was deloaded from the GO−EGMP by a first equilibration in 1 mol L−1 oxalic acid,

Table 2. Structural Parameters of Uranyl Nitrate and Plutonium Nitrate Complexes with GO−EGMP bond distances (Å) bond description

GO−EGMP−Pu(NO3)4

GO−EGMP−UO2(NO3)2

MOPO MOCO MONO3,biden

2.4695 2.4467 2.4279

2.5715 2.4999 2.5172

MONO3,monoden

2.2619

2.1842

functional groups on GO and (ii) second largest mass loss at 300 °C that continued until 800 °C due to pyrolysis of the carbon skeleton of GO. In addition to this, the GO− poly(EGMP) had another thermolysis region at 300−500 °C corresponding to poly(EGMP) on GO; see Figure S5b. The total mass losses from 100 to 800 °C were found to be 10.03 and 17.16 wt % for GO and GO−poly(EGMP), respectively. In the region corresponding to poly(EGMP), the mass loss was found to be 5.88 wt %, which was indicative of the amount of poly(EGMP) grafted on GO. The amount of poly(EGMP) grafted (5.88 wt %) was considerably lower than that obtained for GO−poly(NIPA) (15−17 wt %) using a similar thermal analysis method.48 This was attributed to the fact that tris(hydroxy methyl)amiomethane was anchored first to GO via ring opening of epoxy groups on the basal plane of GO to increase −OH sites for binding the poly(NIPA) chains. The extraction property of free EGMP molecules was studied in the solvent methyl isobutyl ketone (MIBK), selected for a solubility reason. It is reported in the literature that the GO adsorbs actinides from the solutions having acidity in a pH range where the ion-exchange mechanism dominates.7 The sorption experiments carried out in the present work showed

Table 3. Calculated Binding Energies and Thermodynamic Parameters in kcal mol−1 for the Uranyl Nitrate and Plutonium Nitrate Complexation with GO−EGMP complexation reaction

UO2 Pu

2+

4+

+ GO−EGMP +

+ GO−EGMP +

2NO3−

4NO3−

→ UO2 (NO3)2 EGMP

→ Pu(NO3)4 EGMP

ΔGgas

ΔE

ΔH

ΔG

−440.41

−96.11

−89.83

−12.27

−1693.51

−373.78

−368.65

−308.38

F


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excess of nitrate ions. The findings of the present work suggest a new possibility of developing the ion-selective functionalized GO by appropriately preorganizing the functional groups.

and complete deloading was achieved during a second equilibration in 1 mol L−1 oxalic acid. After deloading, Pu4+ could be again reloaded in the GO−EGMP after washing with water with the same efficiency. This seems to suggest that the GO−EGMP is reusable for Pu4+ ion sorption. To understand the selectivity of a single EGMP molecule toward Pu4+ ions, the DFT computations were performed for the complexation of Pu4+ and UO22+ in the presence of nitrate ions. The details of DFT guided stability analyses of the geometric chemical structures are shown in Figures S6−S8 and Table S3 (Supporting Information). The phosphoryl (PO) and carbonyl (CO) ligating groups of EGMP molecules covalently fixed on the GO platform are involved in the complexation with metal ions along with nitrate anions, as shown in Figure 5. Pu4+ ion formed a 8-coordinated complex structure where two nitrates bonded in bidentate and the other two in monodentate mode, as shown in Figure 5a. UO22+ ion was found to complex in a 7-coordinate structure where one nitrate coordinated in bidentate and another in monodentate fashion; see Figure 5b. Pu4+ showed smaller M−O bond distances for both the phosphoryl and carbonyl groups than that of uranyl ions, as shown in Table 2. This seems to indicate that Pu4+ ion formed a stronger coordination complex with the EGMP−GO as compared to UO22+ ions. This fact was also corroborated from the gas phase complexation free energy between the actinide ions and the GO−EGMP given in Table 3. The calculation was performed in the aqueous phase where the metal ion complexes with ligand and nitrate ions in the aqueous solvent (ε = 80) continuum according to a reaction shown in Table 3. It was observed that the binding energy (ΔE) was more for Pu4+ ion by −277.67 kcal mol−1 than that for UO22+ ion. The free energy of complexation (ΔG) which could be correlated with the experimental D values of metal ions in the aqueous solution has also been found to be higher by −296.11 kcal mol−1 for Pu4+ ion with respect to UO22+ ion. For both Pu4+ and UO22+ ions, the ΔG values were decreased from the enthalpy of complexation (ΔH) due to the negative contribution of entropy change. Thus, the origin of high selectivity of single EGMP units on GO toward Pu4+ ions appears to be associated with stable 8coordinated complex structure formation involving a single bidentate EGMP unit and four mono/bidentate nitrate ions. Contrary to this, UO22+ forms a lower stability complex with single EGMP and nitrate ions. UO22+ ions require multiple EGMP units for formation of a stable complex leading to its quantitative sorption in pure cross-linked poly(EGMP) gel.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b11293. Figures showing XPS spectra for GO and GO−EGMP, Raman spectra of GO and GO−EGMP, FTIR spectra and TG/DTA curves of GO and GO-poly(EGMP), and optimized structures of surface functionalized GO− EGMP and sp3 carbon based edge −OH functionalized GO−EGMP and tables containing a comparison of D and G band Raman spectra and calculated binding energies and thermodynamic parameters for the uranyl nitrate and plutonium nitrate complexation with sp3 carbon based edge −OH functionalized GO−EGMP (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +91-22-25594566. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Akshay Dhayagude, RPCD, BARC, for help in Raman spectroscopy and Siddhart Kolay, Chemistry Division, BARC, for thermal analyses. S.C. thanks Department of Atomic Energy, India, for providing a fellowship for the Doctoral studies at Homi Bhabha National Institute, Mumbai. The authors are thankful to Dr. K. T. Shenoy, Head Chemical Engineering Division, and Dr. P. K. Pujari, Head, Radiochemistry Division, for their keen interest in the present work.

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CONCLUSIONS The complexation behavior of EGMP molecule and poly(EGMP) chains anchored on a rigid graphene oxide platform toward UO22+ and Pu4+ ions was found to be quite different from that in a pure polymeric form (cross-linked poly(EGMP) gel) having several ligating groups in close proximity. It was unusual to observe that the nonselective EGMP exhibited a remarkable selectivity toward Pu4+ ion when interacted as a single unit. The poly(EGMP) chains on GO (GO−poly(EGMP)) have a 2 times higher distribution coefficient for Pu4+ ions as compared to GO−EGMP and cross-linked poly(EGMP) gel under similar conditions. The DFT computation modeling has attributed high selectivity of GO−EGMP toward Pu4+ ion to higher stability of Pu(NO3)4EGMP complex with respect to UO2(NO3)2EGMP complex in the presence of an G


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