Article pubs.acs.org/JPCA

Understanding the Bonding Nature of Uranyl Ion and Functionalized Graphene: A Theoretical Study Qun-Yan Wu,† Jian-Hui Lan,† Cong-Zhi Wang,† Cheng-Liang Xiao,† Yu-Liang Zhao,† Yue-Zhou Wei,‡ Zhi-Fang Chai,*,†,§ and Wei-Qun Shi*,† †

Nuclear Energy Chemistry Group, Key Laboratory of Nuclear Radiation and Nuclear Energy Technology and Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ Department of Nuclear Fuel Cycle and Material, School of Nuclear Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China § School of Radiological and Interdisciplinary Sciences, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: Studying the bonding nature of uranyl ion and graphene oxide (GO) is very important for understanding the mechanism of the removal of uranium from radioactive wastewater with GO-based materials. We have optimized 22 complexes between uranyl ion and GO applying density functional theory (DFT) combined with quasi-relativistic smallcore pseudopotentials. The studied oxygen-containing functional groups include hydroxyl, carboxyl, amido, and dimethylformamide. It is observed that the distances between uranium atoms and oxygen atoms of GO (U−OG) are shorter in the anionic GO complexes (uranyl/GO−/2−) compared to the neutral GO ones (uranyl/GO). The formation of hydrogen bonds in the uranyl/GO−/2− complexes can enhance the binding ability of anionic GO toward uranyl ions. Furthermore, the thermodynamic calculations show that the changes of the Gibbs free energies in solution are relatively more negative for complexation reactions concerning the hydroxyl and carboxyl functionalized anionic GO complexes. Therefore, both the geometries and thermodynamic energies indicate that the binding abilities of uranyl ions toward GO modified by hydroxyl and carboxyl groups are much stronger compared to those by amido and dimethylformamide groups. This study can provide insights for designing new nanomaterials that can efficiently remove radionuclides from radioactive wastewater.

1. INTRODUCTION With the rapid development of nuclear energy, radioactive wastewater has become of major concern and environmental challenge throughout the world. Uranium is one of the naturally occurred elements in nuclear energy programs. In the meanwhile, large amounts of uranium have been inevitably released into the environment and would cause serious health problems due to its extremely chemical and radioactive toxicity.1 Hence, the removal of mobile uranyl ions from radioactive waste solutions is of great significance in terms of environmental protection and resource recovery. Adsorption approach has been considered to be of potential application for the separation of uranium from radioactive wastewater because it is high-efficient and easily handling.2 In the past few years, the carbon materials such as activated carbon,3 carbon nanotubes (CNTs),4−8 and graphene oxide (GO)9−13 have been used as sorbents in elimination of uranium from radioactive solutions because of their large specific surface areas and strong adsorption abilities. Graphene has a planar-monolayer structure consisting of extraordinarily hexagonal sp2-bonded carbon rings. Therefore, it possesses many noteworthy physical and chemical properties © 2014 American Chemical Society

such as high thermal conductivity, large surface area, and good thermal stability.14−17 In particular, graphene oxide containing abundant oxygen-bearing groups on the surface, which are much more hydrophilic than graphene itself, can efficiently bind metal ions through sharing an electron pair of the oxygen atom. Like other carbon materials in actinide preconcentration, the advantage of GO is that the adsorption and desorption of metal ions could be readily performed and GO itself can be fully incinerated, which is quite meaningful for the minimization of radioactive wastes. Therefore, GO has been considered to be an ideal supporting material for the removal of radionuclides from radioactive wastewater.12,18 For instance, Wang et al. reported that GO showed excellent adsorption toward U(VI) and that the maximum adsorption capacity of U(VI) onto GO nanosheets was 97.5 mg·g−1 at 20 °C.9 Later, an impressive maximum adsorption capacity of 299 mg·g−1 for U(VI) with single-layered GO was achieved by our group.10 Very recently, a magnetic graphene/iron oxide composite was synthesized and Received: January 26, 2014 Revised: February 24, 2014 Published: March 4, 2014 2149

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used as a novel sorbent for U(VI) with high adsorption capacity of 69.49 mg·g−1.11 Although the adsorption of U(VI) with GO-based materials has been investigated experimentally, the structures of the uranyl/GO complexes and the corresponding thermodynamics properties are still not clear. Therefore, it is necessary to carry out theoretical investigation for better understanding the chemical behaviors of the uranyl ion with GO at the molecular level. Several theoretical studies have been carried out on the adsorption of the uranyl ion onto organic molecules, carbon nanotubes, and fullerenes using quantum mechanical calculations based on density functional theory (DFT).19−23 Very recently, Kumar et al. has studied the uranium and plutonium complexes with G/GO to examine the applicability of a graphene-based fissile sensor using ab initio DFT method.24 Moreover, the adsorption of uranyl ion onto various mineral surfaces have been investigated by both classical and quantum mechanical simulation techniques.25−30 However, to the best of our knowledge, the adsorption of uranyl ion onto GO has still not been investigated theoretically. In the present work, to further understand the binding characters of uranyl ion onto GO, we carried out the theoretical investigation on the interaction mechanisms between uranyl ion and GO using DFT method. The hydroxyl, carboxyl, and amido groups are the simple ligands that can efficiently bind toward uranyl ion.31−36 Therefore, here we selected hydroxyl (−OH), carboxyl (−COOH), amido (−CONH2), and dimethylformamide (−CONMe2) as representative groups at the edge of the graphene to model and understand the bonding nature between uranyl ion and GO, which might help design new nanomaterials to efficiently remove radionuclides from radioactive wastewater.

which have evolved as a practical and effective computational tool for large actinide compounds.47−49 For geometry optimizations, the quasi-relativistic small-core pseudopotential ECP60MWB and associated ECP60MWB-SEG valence basis sets were applied for uranium,50−52 while the 6-31G(d) basis set was used for the other light atoms H, C, N, and O. The quasi-relativistic small-core pseudopotential replaces 60 core electrons for uranium, while the remaining 32 electrons were represented by the associated valence basis set. It was shown that the small-core ECPs are much more reliable for investigations of the thermochemistry of uranium fluorides than large-core ECPs.53 Harmonic vibrational frequencies were calculated to confirm the optimized structures as the local minima on the potential energy surfaces. Single-point calculations and solvation effects at the optimized geometries were also performed for all complexes with the ECP60MWB valence basis set for uranium and the 6-311+G(d, p) basis set for light atoms. M06 functional by Truhlar and co-workers is found to be more suitable for studying the [UO2(H2O)5]2+ systems.54 Hence, single-point calculations and solvation effects were computed by two density functional methods including B3LYP and M06-2X for comparison. Solvation effects were included by using SMD model in Gaussian09 program, which is the recommended choice for computing the Gibbs free energy in aqueous solution.55 All solution phase calculations were carried out in water. The atomic charge distributions and bonding nature between the uranyl ions and GO were investigated at the B3LYP/6-31G(d)/RECP level of theory using natural bond orbital (NBO) analysis as implemented in the Gaussian09 package.56

3. RESULTS AND DISCUSSION 3.1. Binding Geometries. We have optimized 22 complexes formed between [UO2(H2O)5]2+ and graphene modified by oxygen-containing groups at B3LYP// ECP60MWB-SEG/6-31G(d) level of theory. The hydroxyl, carboxyl, amido, and dimethylformamide groups were selected to simulate the representative groups at the edge of GO. Moreover, the complexation strength of uranyl ion with a single monofunctional (M), ortho (OB)-, and meta (MB)-bifunctional GO have been compared. In addition, the neutral (at low pH) and anionic GO (deprotonated state at high pH) modified by hydroxyl and carboxyl groups are also considered. We labeled the anionic GO complexes as X′(A), X = M, OB, and MB, and A= −OH and −COOH as shown in Figures 1 and 2, respectively. For the amino and dimethylformamide functionalized GO complexes, the uranyl ion coordinated to the oxygen atom are labeled as M(B), OB(B), and MB(B), B = −CONH2 and −CONMe2, while the uranyl ion coordinated to the nitrogen atom are labeled as M′(B), OB′(B), and MB′(B), B= −CONH2 and −CONMe2 as shown in Figures 3 and 4. The [UO2(H2O)5]2+, which has been confirmed to be the most stable state, has been optimized at the same theory level. In order to describe the structural parameters clearly, the simplified geometries of the optimized complexes in the gas phase are shown in Figures 1−4, while their entire frameworks are provided in Figures S2−S5, Supporting Information. Furthermore, the selective geometrical parameters correlated with uranium are listed in Tables S1−S4, Supporting Information. Compared to the bare [UO2(H2O)5]2+, the bond distances between uranium and the axial oxygen (U− Oax) and uranium and oxygen atom of water (U−Ow) increase for all uranyl/GO complexes, which indicate that the U−Oax

2. COMPUTATIONAL DETAILS In order to understand the nature of the interaction between uranyl ion and GO, a graphene 5 × 4 supercell with the lattice constant of 2.46 Å shown in Figure S1 of the Supporting Information was chosen as a computational model. We have modeled the GO with mono-, ortho-, and meta-oxygencontaining groups at the edge of graphene. GO is a complex material with oxygen-containing groups including hydroxyl, carboxyl, epoxy, and carbonyl groups attached at the edge as well as its surface. Indeed, there is not a well-defined structural model of GO,37 so we just select the hydroxyl and carboxyl groups on the zigzag side as a GO model to investigate the effects of the different functional groups. The dangling bonds of the graphene fragment were saturated by adding hydrogen atoms. A large amount of previous studies on uranyl ion have been investigated by quantum mechanics and molecular dynamics simulations.38−43 These studies pointed out that the penta-coordinated [UO2(H2O)5]2+ is the dominant species in aqueous solution. In this work, we investigated the structural and electronic properties of the uranyl/GO complexes and explored the complexation reactions of [UO2(H2O)5]2+ with GO by DFT method. The binding energies of uranyl ion with the GO are computed as shown below: GO + [UO2 (H 2O)5 ]2 + → [UO2 (H 2O)n ]2 + /GO + (5 − n)(H 2O)

(n = 3, 4)

(1)

The geometry optimizations were carried out using DFT method with the Gaussian09 program44 and the hybrid exchange-correlation function B3LYP was employed,45,46 2150

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Figure 1. Structures of the uranyl/G(OH) complexes optimized using the B3LYP method. The representative structural parameters are labeled in the snapshots.

Figure 2. Structures of the uranyl/G(COOH) complexes optimized using the B3LYP method. The representative structural parameters are labeled in the snapshots.

penta-coordination modes. The U−Oax bond lengths for M′(OH) are slightly longer compared to those of the M(OH). The average U−Ow bond length follows the similar trend, which is increased by 0.030 Å from M(OH) to M′(OH). However, the U−OG distance between the uranium atom and the oxygen atom of GO decreases from 2.586 Å in M(OH) to 2.184 Å in M′(OH). The longer U−Oax and U−Ow bonds and shorter U−OG bond in M′(OH) than those of M(OH) indicate a stronger interaction lies in between the negatively charged GO and positively charged uranyl ion.

and the U−Ow bonds are weakened due to interactions of uranyl ion with GO. 3.1.1. Hydroxyl (−OH) Functionalized Graphene Complexes. Six complexes of uranyl ion binding to hydroxyl functionalized graphene were obtained as shown in Figure 1, and the selective geometrical parameters concerning the uranium atom are provided in Table S1 of the Supporting Information. For monohydroxyl functionalized GO, the adsorbed uranyl ion in both neutral and monoanionic GO complexes are in 2151

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Figure 3. Structures of the uranyl/G(CONH2) complexes optimized using the B3LYP method. The representative structural parameters are labeled in the snapshots.

We have considered the penta-coordination mode between uranyl ion and neutral or dianionic ortho-hydroxyl functionalized graphene. Nevertheless, only the tetra-coordinated mode was obtained for the UO22+/GO2− complex, OB′(OH). Two water molecules coordinate to uranyl ion with the U−Ow bond length of 2.503 and 2.552 Å in OB′(OH), respectively, while the third water molecule deviates from uranyl ion with the U− Ow distance of 3.550 Å. The deviated water molecule belongs to the second coordination shell, which can enhance the geometry stability of the complex by two hydrogen bonds. One hydrogen bond is 1.670 Å between the oxygen atom of the third water and the hydrogen atom of the neighboring water, and the other hydrogen bond is 1.952 Å between the hydrogen atom of the deviated water and the oxygen atom of the anionic GO. The two U−OG bond lengths of OB(OH) (2.523 and 2.530 Å) are much longer than those of the OB′(OH) (2.186 and 2.228 Å), whereas the U−Ow bond distances in the former are slightly shorter. These results indicate that the relatively stronger interaction appears between uranyl ion and the dianionic GO. The uranyl ions are tetra-coordinated with the meta-hydroxyl functionalized GO. Three water molecules and one hydroxyl group are bound to uranium atom in MB(OH), whereas the other hydroxyl group is far away from the uranium atom with the distance of 4.094 Å. One hydrogen bond appears between the oxygen atom of the departed hydroxyl group and the hydrogen atom of the neighboring water molecule with the distance of 1.829 Å. The central uranium atom are tetracoordinated to the oxygen atom of two water molecules and two hydroxyl groups for dianionic GO complex, MB′(OH). The remaining water molecule is 3.768 Å away from the uranium atom. Like OB′(OH), two hydrogen bonds also appear around the remaining water molecule in MB′(OH). One distance is 1.630 Å between the oxygen atom of the departed water molecule and the hydrogen atom of the nearest water molecule, and the other appears between the hydrogen atom of the deviated water molecule and the oxygen atom of

the neighboring hydroxyl group with the distance of 1.738 Å. Moreover, unlike OB′(OH), there is another hydrogen bond between the oxygen atom of the other hydroxyl group and the hydrogen atom of the near water molecule with the distance of 1.860 Å. Undoubtedly, these hydrogen bonds are beneficial for stabilizing the formed complexes. Additionally, compared to the bond distances between the uranium atom and the oxygen atom of the GO (U−OG) in MB(OH) and MB′(OH), the U− OG bond lengths are shorter in the latter. These results also indicate that uranyl ion prefers to bind with anionic GO. 3.1.2. Carboxyl (−COOH) Functionalized Graphene Complexes. Seven stable structures of the [UO2(H2O)5]2+ binding to carboxyl functionalized GO are obtained, all are the penta-coordinated modes as shown in Figure 2. M(COOH)-4w has one monodentate carboxyl group (trans structure) and four water molecules coordinated with uranium atom, whereas for M(COOH) the central uranium atom are five-coordinated with the bidentate carboxyl group (cis structure) and three water molecules. Compared to the average distances of the U−Ow bond and U−OG bond in M(COOH)4w and M(COOH), the average U−Ow bond length (2.583 Å) in the former is slightly longer than that of the latter (2.512 Å), and the U−OG bond length (2.227 Å) in the former is much shorter than that of the latter (2.236 and 2.667 Å). In addition, M′(COOH) also has a bidentate carboxyl group, including two U−OG bonds between the uranium atom and the oxygen atom of carboxyl group with the distance of 2.368 and 2.392 Å, respectively. Moreover, the average U−O G bond in M′(COOH) is much shorter than that in M(COOH), while it is much longer than that in M(COOH)-4w. For the ortho-carboxyl functionalized graphene, both carboxyl groups are monodentate coordinated with uranyl ion. The average distance between the uranium atom and the oxygen atom of the carboxyl group increases from 2.253 Å in OB′(COOH) to 2.377 Å in OB(COOH). The average distance between the uranium atom and the oxygen atom of the water molecule (U−OG) exhibits the reverse trend, which decreases 2152

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Figure 4. Structures of the uranyl/G(CONMe2) complexes optimized using the B3LYP method. The representative structural parameters are labeled in the snapshots.

complexes, the U−OG bond lengths are 2.435 and 2.390 Å in OB(CONH2), respectively, which are much shorter than U− NG bonds (2.634 and 2.681 Å) in OB′(CONH2), and the average U−Oax and U−Ow bond distances in OB(CONH2) are close to those of OB′(CONH2). Additionally, there is also one hydrogen bond with the distance of 1.566 Å between the oxygen atom of amido group and hydrogen atom of the neighboring water molecule. Two complexes of uranyl ion with meta-amido functionalized GO have been obtained. Two oxygen atoms of two amino groups coordinate with uranium atom in MB(CONH2), while one oxygen atom of amino group and one nitrogen atom of the other amino groups coordinate with uranyl ion in MB′(CONH2). The distances between the uranium atom and oxygen atom of the amino group (U−OG) are 2.242 and 2.429 Å in MB(CONH2), respectively, and it decreases to 2.226 Å in MB′(CONH2). The U−NG bond distance is 2.747 Å in MB′(CONH2), which is much longer than U−OG bond length in all amido functionalized uranyl/GO complexes. Although the UOax bond lengths for both meta-amino group complexes are almost the same, the average U−Ow bond distance is 0.047 Å longer in MB(CONH2) compared to MB′(CONH2). These results indicate that uranyl ion tends to bind with the oxygen atom of the amino group. 3.1.4. Dimethylformamide (−CONMe2) Functionalized Graphene Complexes. Only three uranyl ion complexes with dimethylformamide functionalized GO are optimized as minima (Figure 4). The stable ortho-CONMe2 coordinated complexes are not obtained, probably due to the dimethylformamide group with larger steric hinder effect. Uranyl ion binds to the mono-CONMe2 functionalized GO with the distance of 2.355 Å between the uranium and the oxygen atom of the CONMe2 group in M(CONMe2). The U−OG bond lengths are 2.356 and 2.418 Å in MB(CONMe2), respectively, while it decreases to 2.261 Å in MB′(CONMe2). The distances of U− NG bonds are 2.757 and 2.793 Å in MB′(CONMe2), respectively, which are much longer than U−OG bond lengths of all dimethylformamide functionalized GO complexes. Moreover, uranyl ion prefers penta-coordinated modes for M(CONMe2) and MB(CONMe2), and it tends to be tetra-

from 2.599 Å in OB′(COOH) to 2.583 Å in OB(COOH). Additionally, one hydrogen bond appears between the oxygen atom of one carboxyl group and the hydrogen atom of the near water molecule with the distance of 1.638 Å for OB′(COOH), which strengthens the binding ability of dianionic GO toward uranyl ion. The hydrogen bond and shorter average U−OG bond in OB′(COOH), indicating that uranyl ion has a relatively stronger binding ability to the dianionic GO. Carboxyl groups are also monodentate coordinated with uranyl ion for both meta-carboxyl functionalized GOs. From Figure 2, it can be seen that (i) the U−OG bond lengths are 2.234 and 2.460 Å for MB(COOH), which are longer than those for MB′(COOH) (2.268 and 2.279 Å), (ii) the average UOax and U−Ow bond lengths are somewhat shorter in MB(COOH) compared to MB′(COOH), (iii) two hydrogen bonds appear between the oxygen atom of the carboxyl group and hydrogen atom of the neighboring water molecule, respectively, which can enhance the binding ability of the anionic GO toward uranyl ions. These results again indicate that uranyl ion prefers to bind to the anionic GO. 3.1.3. Amido (−CONH2) Functionalized Graphene Complexes. Six stable structures of uranyl ion with amido functionalized graphene are optimized, and the binding sites of the GO to uranyl ion are oxygen or nitrogen atom as shown in Figure 3. The distance between the uranium atom and the oxygen atom of amido group (U−OG) is 2.274 Å in M(CONH2), while the U−NG bond distance between the uranium atom and the nitrogen atom of amido group is 2.695 Å in M′(CONH2). Compared to the average distance between the uranium atom and the oxygen atom of water molecule (U− Ow) in M(CONH2) and M′(CONH2), the average U−Ow bond length is 2.571 Å in the former, which is 0.036 Å longer than that in the latter. In addition, one hydrogen bond appears between the oxygen atom of the amido group and the hydrogen atom of the neighboring water molecule with the distance of 1.587 Å. The oxygen atoms of two amido groups are bound to uranyl ion for ortho-amido functionalized GO complex, OB(CONH2), whereas the nitrogen atoms are coordinated with uranyl ion in OB′(CONH2). Like the monoamido functionalized graphene 2153

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Figure 5. Average of U−OG, UOax, and U−Ow bond lengths of uranyl ion with hydroxyl and carboxyl functionalized GO.

Table 1. Calculated WBIs of the U−O Bonds and Mulliken Atomic Charges on the U and O Atoms of GO for Uranyl/GO(OH) Complexes at the B3LYP/6-31G*/ECP60MWB Level of Theory WBIs complexes

U−OG

M(OH) M′(OH) OB(OH) OB′(OH) MB(OH) MB′(OH) UO2(H2O)52+

0.307 0.879 0.327 0.821 0.331 0.612

U−OG

0.328 0.743 0.520

Mulliken atomic charges

UOax

UOax

U−Ow

U−Ow

U−Ow

U−Ow

U

OG

OG

2.081 2.119 2.098 2.112 2.160 2.137 2.239

2.106 2.132 2.117 2.154 2.155 2.173 2.239

0.368 0.307 0.360 0.398 0.414 0.447 0.418

0.354 0.334 0.357 0.344 0.392 0.365 0.418

0.341 0.350 0.331 0.035 0.386 0.026 0.418

0.300 0.335

1.320 1.347 1.371 1.392 1.267 1.299 1.732

−0.721 −0.621 −0.735 −0.632 −0.678 −0.657

−0.739 −0.694 −0.688 −0.700

Table 2. Calculated WBIs of the U−O Bonds and Mulliken Atomic Charges on the U and O Atoms of GO for Uranyl/ GO(COOH) Complexes at the B3LYP/6-31G*/ECP60MWB Level of Theory WBIs complexes

U−OG

M(COOH)-4w M(COOH) M′(COOH) OB(COOH) OB′(COOH) MB(COOH) MB′(COOH)

0.766 0.255 0.551 0.436 0.720 0.771 0.720

Mulliken atomic charges

U−OG

UOax

UOax

U−Ow

U−Ow

U−Ow

U−Ow

U

OG

OG

2.151 2.155 2.150 2.154 2.175 2.177 2.184

0.318 0.379 0.381 0.296 0.360 0.371 0.393

0.349 0.371 0.369 0.357 0.337 0.344 0.351

0.394 0.376 0.355 0.372 0.358 0.343 0.406

0.297

0.612 0.520 0.614 0.738 0.444 0.700

2.152 2.157 2.157 2.110 2.191 2.181 2.171

1.395 1.463 1.404 1.377 1.351 1.522 1.431

−0.584 −0.577 −0.561 −0.571 −0.590 −0.497 −0.570

−0.675 −0.590 −0.529 −0.634 −0.588 −0.574

atom has stronger coordination ability toward oxygen atom compared to nitrogen atom. 3.2. Electronic Structures and Natural Bond Orbital (NBO) Analysis. In order to probe the bonding nature of the U−OG between uranyl ions and GO modified with hydroxyl and carboxyl groups, the molecular orbitals (MOs) of the uranyl/G(OH) and uranyl/G(COOH) complexes were calculated at the B3LYP/6-31G(d)/RECP level of theory. The highest occupied MO (HOMO), HOMO-1 and the lowest unoccupied MO (LUMO) are illustrated in Figures S6 and S7, Supporting Information, respectively. The relevant MOs of U− O bond in uranyl/GO(OH) complexes possess σ bonding character. In order to further study the bonding nature between uranyl ions and GO, the bond orders and atomic charges have been investigated by NBO analysis at the B3LYP/6-31G(d)/RECP level of theory. The calculated Wiberg bond indices (WBIs) of the U−O bonds and Mulliken atomic charges on the U and O atoms of GO for the uranyl/GO(OH) and uranyl/GO(COOH) complexes are listed in Tables 1 and 2, respectively. Similar related data for uranyl/GO(CONH2) and uranyl/

coordinated modes in MB′(CONMe2 ) with only one coordinated water molecule. As discussed above, we compared the bond characters between uranyl ion and four oxygen-containing functionalized GO, respectively. For clearly comparing the different bonding nature, the average U−OG, UOax, and U−Ow bond lengths for the hydroxyl and carboxyl functionalized GO and uranyl ion complexes are shown in Figure 5. It is clearly seen that (i) the average U−OG bond length is much shorter in the anionic GO complex compared to the corresponding neutral ones, (ii) the average U−Ow bond lengths are in the range of 2.470 Å to 2.599 Å, (iii) the average UOax bond lengths do not vary significantly, only from 1.770 to 1.795 Å. In addition, hydrogen bonds appear for complexes between uranyl ion and the orthoor meta-functionalized anionic GO. Thanks to the cooperative effect of the negatively charged GO and hydrogen bonds, uranyl cation are easier to bind with the anionic GO than the corresponding neutral one. As to the amino and dimethylformamide functionalized GO complexes, the distance between the uranium atom and the oxygen atom of the GO (U−OG) is much shorter than U−NG bond length, suggesting that uranium 2154

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Figure 6. Binding energies for the complexes in the gas and solution phase together with the change of the Gibbs free energies in the solution at the B3LYP method.

atoms, which also supported that the U−OG bonds have more covalent character. 3.3. Vibrational Frequencies and Infrared (IR) Spectra. The harmonic vibrational frequencies corresponding to symmetric (υs) and asymmetric (υas) stretching of the UO bond of uranyl ion in all complexes are provided in Tables S1− S4, Supporting Information. It can be seen that the calculated UO symmetric and asymmetric stretching modes for all complexes are red-shifted compared to the bare [UO2(H2O)5]2+ (944 and 1029 cm−1). The extent of redshift of the complexes depends on the different oxygencontaining functional groups. For instance, the scope of UO symmetric and asymmetric harmonic frequencies for hydroxyl functionalized GO complexes are 830−858 and 925−960 cm−1 respectively, which are red-shifted about 100 and 90 cm−1 compared to those of the bare [UO2(H2O)5]2+. As for the amino and dimethylformamide functionalized GO complexes, the UO symmetric and asymmetric harmonic frequencies change from 852 to 886 cm−1 and from 952 to 977 cm−1, respectively. The values of the computed harmonic frequencies are consistent with the UO axial bond length as discussed above. That is, the UO axial bond distances in all complexes get longer compared to those of the bare [UO2(H2O)5]2+ due to uranyl ion binding to the functionalized GO. In addition, the UO symmetric and asymmetric harmonic frequencies for hydroxyl functionalized uranyl/GO complexes are somewhat red-shifted compared to other three functionalized uranyl/GO complexes. The UO symmetric and asymmetric harmonic frequencies could provide the quantitative pictures for the experimental observation. IR spectra for all uranyl/GO complexes were also provide in Figures S8−S11, Supporting Information, so that experimentalists could compare them against their own experimental data. The UO vibrational intensity is weaker for OB′(OH) compared to OB(OH), as the UO vibrational intensity is weaker for OB′(COOH) than that for OB(COOH). These results also supported that there are stronger interactions between uranyl ions and anionic GO.

GO(CONMe2) complexes are provided in Tables S5 and S6, Supporting Information. As shown in Table 1, although the WBIs of U−Oax bond for all uranyl/GO(OH) complexes are somewhat smaller (about 2.0−2.2) than those of bare [UO2(H2O)5]2+, U−Oax bonds still remain with double bond character. The WBIs of U−OG bonds are about 0.31−0.33 for neutral GO complexes, which indicate that these bonds possess weaker covalent character. The WBIs of U−OG bonds for anionic GO complexes reach about 0.88, suggesting these U− OG bonds contain more covalent character. The WBIs of U− Ow bonds for all uranyl/GO(OH) complexes are in the range of 0.30 to 0.45, also indicating weaker covalent character. Moreover, the Mulliken atomic charges on the uranium atoms for all uranyl/GO(OH) complexes are almost the same, whereas the Mulliken atomic charges on the oxygen atoms for neutral GO(OH) are larger than those of the corresponding anionic GO(OH). These results suggest that electrostatic interactions dominate the U−O bonds for neutral GO/uranyl complexes. Like uranyl/GO(OH) complexes, the WBIs of U− Ow bonds for all uranyl/GO(COOH) complexes are in the range of 0.30 to 0.45 (Table 2), which indicates the weaker covalent character. Additionally, the WBIs for most of the U− OG bonds indicate that U−OG bonds have more covalent character for anionic GO(COOH) complexes compared to neutral GO(COOH) complexes, except for the complex M(COOH)-4w. Unlike uranyl/GO(OH) complexes, the Mulliken atomic charges on uranium atoms are in the range of 1.35 to 1.52, while those on oxygen atoms show little difference between the neutral and anionic GO(COOH) complexes. As for the uranyl/GO(CONH2) and uranyl/ GO(CONMe2) complexes, the WBIs of U−NG bonds are in the range of 0.25 to 0.35, which are smaller than those of the U−OG bonds (Tables S5 and S6, Supporting Information). These results give a hint that U−OG bonds have more covalent character than U−NG bonds. Furthermore, the Mulliken atomic charges on nitrogen atoms are larger than those of oxygen 2155

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3.4. Binding Energy of Complexes. To examine the binding ability of four oxygen-containing functionalized GO with uranyl ion, the binding energies (ΔBE) are evaluated using eq 1. The SMD solvation model, which is the recommended choice for computing the Gibbs free energy, was chosen to mimic the solvent effect.55 The binding energies of uranyl ion and GO in both the gas and aqueous phase together with the changes of the Gibbs free energy in solution (ΔGsol) by B3LYP and M06-2X are provided in Tables S7 and S8, Supporting Information, respectively. The binding energies for both methods show very similar trends; here, we selected the results of B3LYP for the following discussions. As can be seen from Figure 6, the binding energies are very sensitive to the environment. The binding energies for all complexes are negative in the gas phase, whereas they become positive for some complexes when the solvent effect is considered. In addition, the tendency of binding energies in solution is in accordance with the change of the Gibbs free energies in solution. From the results of ΔGsol, uranyl ion is favorable to combine with the anionic GO in solution, which are in excellent agreement with our experimental observation.10 The higher adsorption capacity of GO was found in higher pH values due to the GO in deprotonated states.10 As shown in Table S7, Supporting Information, ΔGsol are −13.36 and −34.67 kcal/ mol for the complexation reactions of mono- and orthohydroxyl UO22+/GO−/2− complexes, respectively, and ΔGsol are −13.19, −15.75, and −21.65 kcal/mol for those of anionic carboxyl functionalized UO22+/GO−/2− complexes, respectively. The relatively stronger binding energy appears in the complexes between uranyl ion and GO modified by hydroxyl and carboxyl groups with shorter U−OG bond distance. ΔGsol is −3.89 kcal/mol for the complexation reactions of M(COOH)4w with monodentate carboxyl group (trans conformer), while it increases to 3.61 kcal/mol for M(COOH) with bidentate carboxyl group (cis conformer). Senent reported that the cis conformer of acetic acid is 5.38 kcal/mol (1882.7 cm−1) higher in energy than trans one at the MP4/cc-pVTZ level of theory,57 which is in good agreement with our computed binding energies of M(COOH)-4w and M(COOH) complexes. As to the amino and dimethylformamide functionalized GO, the uranyl ion is favorable for the oxygen-atom coordinated modes except for MB(CONH2) with small positive ΔGsol of 2.96 kcal/mol. For instance, ΔGsol are −4.04 and −7.70 kcal/ mol for the complexation reactions of M(CONH2) and OB(CONH2) in Table S7, Supporting Information, respectively, and ΔGsol are −7.97 and −1.17 kcal/mol for those of M(CONMe2) and MB(CONMe2), respectively. These results indicate that uranyl ion easier tend to combine with the oxygen atom compared to nitrogen atom, which are again in agreement with our structural results. Nevertheless, ΔGsol for the complexation reaction of MB′(CONMe2) is maximum (51.15 kcal/mol) among all complexes, which is probably caused by the cooperative results of one coordinated water molecule and the two nitrogen atoms coordination sites. In total, ΔGsol are negative for some complexation reactions of uranyl ion combining the GO modified by four oxygencontaining groups, and ΔGsol are relatively negative for uranyl ion complexation reactions concerning hydroxyl and carboxyl functionalized anionic GO compared to those for amido and dimethylformamide functionalized GO. For instance, the maximum ΔGsol is −34.67 kcal/mol for the hydroxyl functionalized GO complex OB′(OH), while the maximum value is only −7.97 kcal/mol for dimethylformamide function-

alized GO complex M(CONMe2). Compared to the changes of the Gibbs free energies in solution phase for complexation reactions of uranyl ion with GO modified by different oxygencontaining groups, we can easily find that GO is probably a suitable material for the removal of uranyl ion from radioactive waste solutions.

4. CONCLUSIONS To conclude, the binding geometries and energies of complexes between uranyl ion and graphene modified by four oxygencontaining groups have been investigated using DFT. The oxygen-containing groups include hydroxyl (−OH), carboxyl (−COOH), amido (−CONH2), and dimethylformamide (−CONMe2) groups. We have obtained 22 stable structures of uranyl/GO complexes. Compared to the bare [UO2(H2O)5]2+, the corresponding symmetric and asymmetric stretching vibrational frequencies of all complexes show red shift, which indicate weaker axial UO bond. For hydroxyl and carboxyl functionalized GO, the U−OG bond length is somewhat shorter in the anionic GO complexes than that of the neutral ones. The computed WBIs of the U−OG bonds also support this point. Moreover, hydrogen bonds appear in the anionic GO modified by the hydroxyl and carboxyl groups, which play a significant role in the stabilization of complexes. These results indicate that uranyl ion prefers to be adsorbed on the anionic hydroxyl and carboxyl GO, which are in good agreement with our experimental observations. Our computed changes of the Gibbs free energy in solution also support this point, that is, the changes of the Gibbs free energy are negative for the complexes between uranyl ion and the anionic GO modified by the hydroxyl and carboxyl groups except MB′(COOH). In addition, the distance between uranium atom and nitrogen atom of GO is much longer relative to that of U−OG bond for amino and dimethylformamide functionalized GO complexes. Furthermore, on the basis of the changes of the Gibbs free energy in solution, the coordination ability of uranyl ion is much stronger when binding with the graphene modified by hydroxyl and carboxyl groups than by CONH2 and CONMe2 groups. This study indicated that graphene modified by hydroxyl and carboxyl groups would be a promising candidate for application in radioactive wastewater and may open up a new field for uranyl ion separation with GO.

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ASSOCIATED CONTENT

* Supporting Information S

Optimized structures of the complexes between uranyl ion and graphene oxide, the selective structural parameters, harmonic vibrational frequencies, molecular orbitals, NBO analyses, IR spectra, the binding energies, and the complete Gaussian 09 reference (ref 44). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(W.-Q.S.) Fax: 86-10-88235294. Tel: 86-10-88233968. Email: [email protected]. *(Z.-F.C.) E-mail: [email protected] Notes

The authors declare no competing financial interest. 2156

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ACKNOWLEDGMENTS

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REFERENCES

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This work was supported by the National Natural Science Foundation of China (Grant Nos. 91326202, 11205169, 21101157, 21261140335, and 91126006) and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant Nos. XDA030104). The results described in this work were obtained on the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences.

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