Understanding the Interactions of Neptunium and Plutonium Ions with

Article pubs.acs.org/JPCA

Understanding the Interactions of Neptunium and Plutonium Ions with Graphene Oxide: Scalar-Relativistic DFT Investigations Qun-Yan Wu,† Jian-Hui Lan,† Cong-Zhi Wang,† Yu-Liang Zhao,† Zhi-Fang Chai,†,‡ and Wei-Qun Shi*,† †

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 ‡ School of Radiological and Interdisciplinary Sciences (RAD-X), and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China ABSTRACT: Due to the vast application potential of graphene oxide (GO)based materials in nuclear waste processing, it is of pivotal importance to investigate the interaction mechanisms between actinide cations such as Np(V) and Pu(IV, VI) ions and GO. In this work, we have considered four types of GOs modified by hydroxyl, carboxyl, and carbonyl groups at the edge and epoxy group on the surface, respectively. The structures, bonding nature, and binding energies of Np(V) and Pu(IV, VI) complexes with GOs have been investigated systematically using scalar-relativistic density functional theory (DFT). Geometries and harmonic frequencies suggest that Pu(IV) ions coordinate more easily with GOs compared to Np(V) and Pu(VI) ions. NBO and electron density analyses reveal that the coordination bond between Pu(IV) ions and GO possesses more covalency, whereas for Np(V) and Pu(VI) ions electrostatic interaction dominates the An−OG bond. The binding energies in aqueous solution reveal that the adsorption abilities of all GOs for actinide ions follow the order of Pu(IV) > Pu(VI) > Np(V), which is in excellent agreement with experimental observations. It is expected that this study can provide useful information for developing more efficient GO-based materials for radioactive wastewater treatment. structure (EXAFS) indicates that U(VI) ions are about fivecoordinate at the equatorial plane and Th(IV) ion are about eight- or nine-coordinate with oxygen atoms.8,12 Wang et al. also studied the adsorption behavior of U(VI) on GO nanosheets and demonstrated that GO showed higher adsorption capacity of 208.33 mg/g.9,13 Romanchuk and his co-workers investigated the interactions of GO with actinides, including U(VI), Np(V), Pu(VI), Am(III), and Th(VI) as well as their sorption kinetics.10 They found that GO can effectively remove radionuclides and is far more effective in adsorbing transuranium elements from radioactive waste solutions compared to bentonite clays and activated carbon.10 Although much work has been addressed in the removal of radionuclides using GO, the adsorption mechanisms and coordination modes of radionuclides with GO have not been fully elucidated. It is still necessary to disclose the coordination modes and thermodynamic behaviors of radionuclides with GO, which might assist the design for high efficient GO-based materials toward actinide adsorption. The structural properties of the Np(V) and Pu(VI, IV) complexes as well as their hydrated forms have been examined in both gas phase and solution using density functional theory (DFT) and ab initio calculations.21−29 These studies pointed

1. INTRODUCTION Treatment of the contaminated groundwater containing radionuclides has become a major task in the cleanup of legacy nuclear sites.1 Due to the increasing nuclear industry activities, radionuclides are inevitably released into the environment and pose long-term risks to the human life and biosphere. Particular attention should be paid to transuranic elements, such as plutonium (Pu) and neptunium (Np) as well as their complexes, because those artificial actinides are mobile and highly toxic. In recent years, the removal of radionuclides from radioactive solutions with carbon materials such as carbon nanotubes,2−6 activated carbon,7 and graphene oxide8−13 has been extensively investigated. Graphene oxide (GO), as a typical carbon nanomaterial, has high surface, abundant oxygen-bearing groups and strongly reactive sites.14−17 It has been used to remove radionuclides from radioactive solutions and demonstrated to be an effective adsorbent.8−13 On one hand, GO is nontoxic and degradable and can be produced in bulk quantities.18−20 On the other hand, GO can be fully incinerated, which makes the adsorption and recovery of radionuclides convenient in real wastewater treatment. Therefore, GO has been considered to be a promising material for the removal of radionuclides from radioactive wastewater. For instance, we once reported the adsorption of Th(IV) and U(VI) by GO, and high adsorption capacities of 214.6 and 299 mg/g for Th(IV) and U(VI), respectively, were observed.8,12 Extended X-ray absorption fine © 2014 American Chemical Society

Received: July 14, 2014 Revised: August 27, 2014 Published: October 10, 2014 10273

dx.doi.org/10.1021/jp5069945 | J. Phys. Chem. A 2014, 118, 10273−10280

The Journal of Physical Chemistry A

Article

out that Np(V)O 2+ and Pu(VI)O22+ ions prefer fivecoordinatation and Pu(IV) complexes are prone to ninecoordination in the aqueous phase.21,22,24Therefore, hydrated species [NpO2(H2O)5]+, [PuO2(H2O)5]2+, and [Pu(H2O)9]4+ were selected to interact with GO in this work. Actually, interactions between uranyl ions and organic molecules, carbon nanotubes, fullerenes, as well as GOs have also been carried out theoretically.30−35 Recently, Kumar et al. studied uranium and plutonium complexes with GO to identify the applicability of a graphene-based fissile sensor using an ab initio density functional theory (DFT) method.36 Our recent work has also theoretically addressed the coordination modes between uranyl ions and GO.35 To further investigate the affinities of GO with different actinide cations, in this work we have explored the structural and electronic properties as well as thermodynamic behaviors of Np(V) and Pu(VI, IV) ions with GO using DFT coupled with the quasi-relativistic small-core pseudopotentials method. It is expected that this work can help understanding coordination modes of Np(V) and Pu(VI, IV) complexes with GOs and further designing novel functional GOs to efficiently remove radionuclides from radioactive wastewater.

The geometry optimizations were carried out using the DFT method with Gaussian 09.38 The hybrid exchange-correlation functional B3LYP was employed,39,40 which has evolved as a practical and effective computational tool for large actinide compounds.35,41−43 For geometry optimizations, the quasirelativistic small-core pseudopotential ECP60MWB and associated ECP60MWB-SEG valence basis sets were applied for neptunium and plutonium atoms,44−46 while the 6-31G(d) basis set was used for the other light atoms H, C, and O. Harmonic vibrational frequencies were calculated to confirm the optimized structures as the local minima on the potential energy surfaces at the same level of theory. In addition, the spin contaminations of all complexes are neglected because of values close to the ideal values of S(S + 1). It has been known that geometries are less sensitive to the size of the basis set,47 while the corresponding energies can vary significantly with different basis sets.48 Therefore, single-point calculations and solvation effects at the optimized geometries were also performed for all complexes with the ECP60MWB-SEG valence basis set for actinide atoms and 6-311+G(d, p) basis set for light atoms with the B3LYP functional. Solvation effects were included by using the SMD model in Gaussian 09,38 which is the recommended choice for computing the Gibbs free energy in aqueous solution. All solution phase calculations were carried out in water. The bonding nature of Np(V) and Pu(VI, IV) complexes with GO were investigated at the B3LYP/631G(d)/RECP level of theory using natural bond orbital (NBO) analysis as implemented in Gaussian 09.49 The topological analysis of electron density have also been performed by employing quantum theory of atoms in molecules (QTAIM) with Multiwfn code.50

2. COMPUTATIONAL DETAILS As we know GO is a complex material with oxygen-containing groups, including hydroxyl (−OH), carboxyl (−COOH), carbonyl (−CO), and epoxy (−O−) groups attached on the edge as well as its surface. Due to random arrangement of the oxygen-containing functional groups and large structural disorder in the skeleton, there is no well-defined structural model of GO.37 Therefore, we mainly selected hydroxyl, carboxyl, and carbonyl groups on the zigzag side and epoxy groups on the surface to investigate the effect of functional groups on GO adsorptivity. Herein, a graphene 5 × 4 supercell with the lattice constant of 2.46 Å was chosen as a computational model. The dangling bonds of the GO fragment were saturated by adding hydrogen atoms. For simplification, we just selected monofunctionalized GO (Scheme 1) to study the structures, bonding nature, and binding energies of Np(V) and Pu(IV, VI) ions with GO.

3. RESULTS AND DISCUSSION 3.1. Hydrates of Np(V) and Pu (VI, IV) Ions. As is wellknown, the chemical oxidation states Np(V) and Pu(VI, IV) are preferred in aqueous solution with different hydration forms. Np(V) and Pu(VI) ions prefer five-coordination [NpO2(H2O)5]+ and [PuO2(H2O)5]2+,21,22,24 whereas ninecoordination species [Pu(H2O)9]4+ are preferred for Pu(IV) ion.24,51 According to the total electronic energies obtained at the B3LYP/6-31G(d)/RECP level of theory (Table 1), we found that the ground state of [NpO 2 (H 2 O) 5 ] + and [PuO2(H2O)5]2+ is triplet and [Pu(H2O)9]4+ is quintet which agree with the results reported in previous work.24,36 The actinide complexes with GO are confirmed to be the same ground electronic states with the bare actinide hydrated ions, which were also confirmed by studying the hydrates of the actinide ions.26 Additionally, the calculated solvation energies of Np(V), Pu(VI), and Pu(IV) hydrated ions are −123.4, −247.6, and −763.4 kcal/mol (Table 1), respectively.

Scheme 1. Structures of GO, X = −OH, −COOH, and −CO groups, Y = −O− Group

Table 1. Electronic Configurations, Main Forms in Aqueous Phase, and the Relative Electronic Energies (kcal/mol) to Each Ground State for Np(V) and Pu(VI, IV) Ions As Well As Their Solvation Energies (Esol, kcal/mol) at the Ground States

electronic configuration form E(singlet state) E(triplet state) E(quintet state) Esol(ground state)

Np(V)

Pu(VI)

Pu(IV)

[Rn]5f2 [NpO2(H2O)5]+ 21.6 0.0 − −123.4

[Rn]5f2 [PuO2(H2O)5]2+ 32.3 0.0 − −247.6

[Rn]5f4 [Pu(H2O)9]4+ 75.5 31.4 0.0 −763.4

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Figure 1. HOMO and LUMO diagrams of G(OH), G(COOH), G(CO), and G(−O−).

3.2. Geometries. In order to investigate the interaction mechanisms between Np(V) and Pu(IV, VI) ions and GO, we selected a graphene fragment functionalized by hydroxyl, carboxyl, and carbonyl groups on the edge and epoxy group on the surface to model GO (Scheme 1). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) diagrams of four types of GOs at the B3LYP/ 6-31G(d) level of theory are presented in Figure 1. It can be clearly seen that the functional groups affect the electron density distributions, especially the HOMO and LUMO of GOs, which can result in different binding abilities of GO with actinide ions. Optimized structures of Np(V) and Pu(IV, VI) ions with GO modified by hydroxyl, carboxyl, carbonyl, and epoxy groups are shown in Figures 2−4, respectively. It can be clearly seen that

Figure 3. Structures of Pu(VI) ions with GO modified by hydroxyl, carboxyl, carbonyl, and epoxy groups at B3LYP/ECP60MWB-SEG/631G(d) level of theory.

Figure 2. Structures of Np(V) ions with GO modified by hydroxyl, carboxyl, carbonyl, and epoxy groups at B3LYP/ECP60MWB-SEG/631G(d) level of theory.

Figure 4. Structures of Pu(IV) ions with GO modified by hydroxyl, carboxyl, carbonyl, and epoxy groups at B3LYP/ECP60MWB-SEG/631G(d) level of theory.

the complexes of Np(V) and Pu(VI) ions with GO are prone to five-coordination at the equatorial plane, in which four oxygen atoms of water molecules and one oxygen atom of GO bind to actinide centers in Figures 2 and 3. It should be pointed out that the hydrogen bonding only appears in the [NpO2(H2O)4]+/G(COOH) and [NpO2(H2O)4]+/G(CO) complexes with the distance of about 2.0 Å, which could strengthen the binding ability between Np(V) and GO. As for the Pu(IV)/GO complexes in Figure 4, there are seven oxygen atoms of water molecules and two oxygen atoms of GO

coordinated with Pu(IV) ions for [Pu(H2O)7]4+/G(COOH) complex which still keeps nine-coordination. For [Pu(H 2 O) 8 ] 4+ /G(OH), [Pu(H 2 O) 8 ] 4+ /G(CO), and [Pu(H2O)8]4+/G(−O−) complexes, Pu(IV) ions prefer to bind with one oxygen atom of GO and seven oxygen atoms of water molecules in eight-coordination, and the eighth water molecule belongs to the second coordinated shell. The eighth deviated water molecule forms hydrogen bonding with the neighboring 10275



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Table 2. Selected Bond Lengths (Å) for Np(V) and Pu(IV, VI) Complexes with GO at the B3LYP/ECP60MWB-SEG/6-31G(d) Level of Theorya +

[NpO2(H2O)5] [NpO2(H2O)4]+/G(OH) [NpO2(H2O)4]+/G(COOH) [NpO2(H2O)4]+/G(CO) [NpO2(H2O)4]+/G(−O−) [PuO2(H2O)5]2+ [PuO2(H2O)4]2+/G(OH) [PuO2(H2O)4]2+/G(COOH) [PuO2(H2O)4]2+/G(CO) [PuO2(H2O)4]2+/G(−O−) [Pu(H2O)9]4+ [Pu(H2O)8]4+/G(OH) [Pu(H2O)7]4+/G(COOH) [Pu(H2O)8]4+/G(CO) [Pu(H2O)8]4+/G(−O−)

An−OG

An=Oa

An−Ow

hydrogen bonding

− 2.642 2.401 2.491 2.503 − 2.687 2.532 2.454 2.592 − 2.545 2.299/2.704 2.188 2.106

1.780 1.789(0.009) 1.795(0.015) 1.795(0.015) 1.780(0.0) 1.716 1.770(0.054) 1.765(0.049) 1.777(0.061) 1.766(0.050) − − − − −

2.566 2.592(0.026) 2.620(0.054) 2.586(0.020) 2.576(0.010) 2.465 2.567(0.102) 2.590(0.125) 2.596(0.131) 2.573(0.108) 2.451 2.561(0.110) 2.579(0.128) 2.570(0.119) 2.598(0.147)

− − 1.980 1.997 − − − − − − − 1.838/1.799 − 1.796/1.823 1.821/1.809/1.996

a

Average bond length, the differences of An−O bond length between An/GO complexes and bare hydrated actinide ions are listed in the bracket. “−” denotes no values.

water molecules. As investigated in our previous work,35 the hydrogen bonds play a significant role in the stabilization of Pu(IV)/GO complexes. The selected geometrical parameters concerning the actinide atoms are listed in Table 2. It can be clearly seen that the distances between the actinide atom (An) and the oxygen atom of GO (An−OG) make a great difference in different actinide complexes. For instance, the An−OG bond lengths are in the range of 2.401−2.642 Å, 2.454−2.687 Å, and 2.106−2.545 Å for Np(V), Pu(VI), and Pu(IV) complexes, respectively. It can be concluded that the coordination bond of Pu(IV)−OG provides the shortest bond distances, which suggests that the Pu(IV) ion can be easily bound with GO. The average distances between actinide atoms [Np(V) and P(VI) ions] and the axial oxygen atoms (AnO) are similar for Np(V) (1.780−1.795 Å) and Pu(VI) (1.765−1.777 Å) complexes with different functionalized GO, respectively. The AnO bond lengths for bare hydrated actinide ions are 1.780 and 1.716 Å for [NpO2(H2O)5]+ and [PuO2(H2O)5]2+ at the same level of theory, respectively. The disparity of the AnO bond length between An/GO complexes and bare hydrated actinide ions are smaller for Np(V)/GO complexes (0−0.015 Å) than for Pu(VI)/GO complexes (0.049−0.061 Å). This also indicates that the binding strength between Np(V) ions and GO are the smaller compared to Pu(VI) complexes. In addition, the Np O bond length is 1.780 Å in [NpO2(H2O)5]+/G(−O−) complex, which is the same with the bare [NpO2(H2O)5]+ ion. This suggests that the Np(V) ion may weakly interact with GO modified by a single surface epoxy group, which is in excellent agreement with the calculated binding energy in aqueous solution using the 6-311+G(d, p) basis set as mentioned in Energies (26.9 kcal/mol). The average distances between actinide ions and oxygen atoms of water molecules (An−Ow) are also provided in Table 2. The difference of An−Ow bond length between An/GO complexes and bare hydrated actinide ions are 0.010−0.054 Å, 0.102−0.131 Å, and 0.110−0.147 Å for Np(V)/GO, Pu(IV)/GO, and Pu(VI)/GO complexes, respectively. These results also support our finding that the binding strength of actinide complexes are in order of Pu(IV) > Pu(VI) > Np(V), as discussed in Energies. It is worthwhile to note that one C−O bond of the surface epoxy group breaks when the Pu

(IV) ion bind to the GO, which suggests that the GO modified by epoxyl group has a strong binding ability with the Pu(IV) ion. 3.3. Frequencies and IR. Table 3 provides the calculated symmetrical and asymmetrical harmonic vibrational frequencies Table 3. Symmetrical and Asymmetrical Harmonic Vibrational Frequencies (νs and νas, cm−1) and the Corresponding Infrared Intensities (I, km/mol) of OAn O for Np(V) and Pu(VI) Complexes with GO at the B3LYP/ 6-31G*/ECP60MWB Level of Theory [NpO2(H2O)5]+ [NpO2(H2O)4]+/G(OH) [NpO2(H2O)4]+/G(COOH) [NpO2(H2O)4]+/G(CO) [NpO2(H2O)4]+/G(−O−) [PuO2(H2O)5]2+ [PuO2(H2O)4]2+/G(OH) [PuO2(H2O)4]2+/G(COOH) [PuO2(H2O)4]2+/G(CO) [PuO2(H2O)4]2+/G(−O−)

νas/I

νs/I

925/389 920/315 906/183 902/206 921/115 1035/212 927/105 940/304 910/167 933/70

854/0 842/41 830/66 829/36 856/1 916/0 837/5 858/18 821/9 855/3

Δνas

Δνs

5 19 22 4

12 23 25 −2

108 95 125 102

79 59 95 61

as well as the corresponding infrared (IR) intensities of O AnO for Np(V) and Pu(VI) complexes. From IR intensities, we find that the asymmetrical harmonic frequencies of O AnO in [NpO2(H2O)5]+ and [PuO2(H2O)5]2+ are infrared active, and the symmetrical ones are nonactive. However, the symmetrical harmonic frequencies of OAnO become infrared active when [NpO2(H2O)5]+ and [PuO2(H2O)5]2+ ions combine with GO. It can be clearly seen that except symmetrical harmonic frequency of ONpO in [NpO2(H2O)5]+/G(−O−) (2 cm−1 blue shift), all harmonic frequencies are red-shifted in Np(V)/GO and Pu(VI)/GO complexes compared to those in bare [NpO2(H2O)5]+ and [PuO2(H2O)5]2+, respectively. The Pu(VI)/GO complexes show a much larger red shift than the Np(V)/GO complexes. For example, the maximum red shift of asymmetrical harmonic frequency of OAnO is 125 cm−1 for the [PuO2(H2O)4]2+/ G(CO) complex, while the corresponding value is only 22 cm−1 10276



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for the [NpO2(H2O)4]+/G(CO) complex. This result indicates that the Pu(VI) ions bind more easily with GO compared to Np(V) ions, which is in good consistency with the trend of the average AnO bond distance. Moreover, for the actinide ions, the extent of the red shift are different after combining with the GO modified by different functional groups. For instance, the red shift of asymmetrical harmonic frequency of OAnO for [PuO 2 (H 2 O) 4 ] 2+ /G(CO) is 125 cm −1 , while for [PuO2(H2O)4]2+/G(COOH) it is 95 cm−1. Hence, the O AnO symmetric and asymmetric harmonic frequencies could provide the quantitative pictures for the experimental observation on complexing strength. IR spectra of Np(V) and Pu(VI) complexes are also provided in Figure 5, and the relative intensities as well as the extent of red shift of symmetrical and asymmetrical harmonic vibrational frequencies for OAnO can be clearly seen.

OG bonds in [Pu(H2O)8]4+/G(CO) and [Pu(H2O)8]4+/ G(−O−) complexes are 0.620 and 0.795, respectively, which suggest that more covalency exists in the Pu(IV)−O bonds of Pu(IV) complexes. In addition, the natural charges on Pu atoms in Pu(IV) complexes are larger than those of Np (V) and Pu(VI) complexes, which also suggest that the Pu(IV) ion have stronger binding ability than Np (V) and Pu(VI) ions. The topological analysis of electron density of the An−OG bond have also been performed by employing QTAIM with Multiwfn code.50 This method has been used for actinide complexes and could provide consistent trends with the strength of chemical bonds.52−57 A bond critical point (BCP) is the (3, −1) saddle point on electron density curvature being a minimum in the direction of the atomic interaction line and a maximum in the two directions perpendicular to it.58 Within QTAIM, a chemical bond is defined by the presence of a line of maximum electron density along a bond path between two atoms and BCP. Therefore, the topological analysis of electron density ρ(r) and its Laplacian could provide valuable information about the properties of the bond.59−61 As a general rule, the electron density at BCP ρ(r) > 0.2 au and ∇2ρ(r) < 0 for a covalent bond, while ρ(r) < 0.1 au and ∇2ρ(r) > 0 indicates an ionic bond. As shown in Table 4, the ρ(r) of the An−OG bond are in range of 0.035−0.056 au and 0.030− 0.045 au for Np(V) and Pu(VI) complexes, respectively, and their corresponding ∇2ρ(r) are all positive, which indicate that the Np(V)−OG and Pu(VI)−OG bonds possess distinct ionic bond character. The ρ(r) of the Pu(IV)−OG bond are higher than those of the corresponding Np(V)−OG and Pu(VI)−OG bonds. The maximum ρ(r) value is 0.111 au in the [Pu(H2O)8]4+/G(−O−) complex, which indicate the Pu(IV)−OG bond to be more covalent. The topological analysis of electron density of the An−OG bond shows an excellent consistency with the results of WBIs. 3.5. Energies. The binding energies of Np(V) and Pu(VI, IV) ions with the GO were computed as below, respectively: GO + [NpO2 (H 2O)5 ]+ → [NpO2 (H 2O)4 ]+ /GO + H 2O (1) 2+

GO + [PuO2 (H 2O)5 ]

→ [PuO2 (H 2O)4 ]2 + /GO + H 2O

(2)

GO + [Pu(H 2O)9 ]4 + → [Pu(H 2O)n ]4 + /GO + (9 − n)(H 2O)(n Figure 5. IR spectra of (a) Np(V) and (b) Pu(VI) complexes with GO at B3LYP/6-31G*/ECP60MWB level of theory. The data denote the OAnO symmetric and asymmetric harmonic frequencies.

= 7, 8)

(3)

Here, the binding energies for all cases were calculated by using Gibbs free energies of the reactants and products, including the thermal contributions which were obtained in the gas phase with the 6-31G(d) basis set. In accordance with the equations above, the binding energies in the gas phase and aqueous solution for Np(V) and Pu(IV, VI) complexes with GO at the B3LYP/ECP60MWB-SEG/6-31G(d) level of theory are listed in Table 5. The binding energies decrease when the solvation effect is considered. As shown in Table 5, using the 631G(d) basis set, the binding energies of Np(V) and Pu(VI) complexes with GO in aqueous solution are positive in most cases except for the Pu(VI)/G(OH) complex. These results can be used to evaluate the relative binding ability of Np(V) and Pu(VI) ions with GO. Because of the significant dependence of energies on the size of the basis set,48 here we have also

3.4. Bonding Nature. In order to study the bonding nature between actinide ions and GO, the Wiberg bond indices (WBIs) of An−O bonds and natural atomic charges on actinide atoms have been investigated by NBO analysis at the B3LYP/631G(d)/RECP level of theory. As listed in Table 4, the WBIs of An−OG bonds are in the range of 0.25−0.43 and 0.20−0.35 for Np(V) and Pu(VI) complexes, respectively. These results reveal that the interactions between Np(V) and Pu(VI) atom and oxygen atoms of GO are weakly covalent and the electrostatic interaction dominates the An−O bonds. As for Pu(IV) complexes, the WBIs of Pu−OG bonds are in the range of 0.26−0.80, which are larger than those of the corresponding Np(V) and Pu(VI) complexes. For example, the WBIs of Pu− 10277



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Table 4. Wiberg Bond Indices (WBIs) of An−O Bonds and Natural Charges on Atoms as well as Electron Density (ρr, au) of An−OG Bond Critical Point and Its Laplacian (∇2ρr, a.u.) for Np(V) and Pu(IV, VI) Complexes with GO at the B3LYP/ ECP60MWB-SEG/6-31G(d) Level of Theory An−OG

An=O

An−Ow

QAn

∇2ρr(An−OG)

ρr(An−OG)

0.245 0.428 0.369 0.332 0.198 0.306 0.352 0.246 0.259 0.467/0.223 0.620 0.795

2.065 2.070 2.063 2.092 2.073 2.095 2.060 2.090 − − − −

0.284 0.274 0.294 0.306 0.276 0.266 0.268 0.273 0.267 0.280 0.278 0.265

1.444 1.357 1.365 1.333 1.370 1.310 1.315 1.338 1.541 1.495 1.542 1.532

0.130 0.252 0.200 0.198 0.114 0.178 0.222 0.148 0.160 0.297/0.113 0.431 0.514

0.035 0.056 0.044 0.041 0.030 0.039 0.045 0.033 0.042 0.077/0.027 0.091 0.111

An/GO complexes +

[NpO2(H2O)4] /G(OH) [NpO2(H2O)4]+/G(COOH) [NpO2(H2O)4]+/G(CO) [NpO2(H2O)4]+/G(−O−) [PuO2(H2O)4]2+/G(OH) [PuO2(H2O)4]2+/G(COOH) [PuO2(H2O)4]2+/G(CO) [PuO2(H2O)4]2+/G(−O−) [Pu(H2O)8]4+/G(OH) [Pu(H2O)7]4+/G(COOH) [Pu(H2O)8]4+/G(CO) [Pu(H2O)8]4+/G(−O−)

Table 5. Calculated Binding Energies (ΔG, kcal/mol) of Np(V) and Pu(IV, VI) Complexes with GO in the Gas Phase and Aqueous Solution with 6-31G(d) and 6-311+G(d, p) Basis Sets, Respectively ions GO

[NpO2(H2O)5]+

[PuO2(H2O)5]2+

[Pu(H2O)9]4+

6-31G(d)

ΔGgas

ΔGsol.

ΔGgas

ΔGsol.

ΔGgas

ΔGsol.

G(OH) G(COOH) G(CO) G(-O−) 6-311+G(d, p)

17.3 2.9 9.0 40.6 ΔGgas

22.3 22.1 24.6 55.3 ΔGsol.

−79.8 −75.2 −80.5 −62.3 ΔGgas

−4.4 9.1 2.4 12.8 ΔGsol.

−264.8 −281.1 −253.2 −274.5 ΔGgas

−23.4 −7.1 −9.3 −30.9 ΔGsol.

G(OH) G(COOH) G(CO) G(−O−)

−11.0 −24.7 −20.8 12.0

−6.3 −6.8 −4.0 26.9

−107.4 −102.4 −109.4 −91.2

−32.9 −19.6 −25.9 −15.9

−297.8 −319.4 −287.4 −309.6

−56.7 −45.2 −42.0 −64.0

−45.2, −42.0, and −64.0 kcal/mol for complexes between Pu(IV) ions and GO modified by hydroxyl, carboxyl, carbonyl, and epoxy groups, respectively. On the basis of the binding energies in aqueous solution, the adsorption abilities of GO toward Np(V), Pu(VI), and Pu(IV) ions follow the order of G(COOH) > G(OH) > G(CO) > G(−O−), G(OH) > G(CO) > G(COOH) > G(−O−), and G(−O−) > G(OH) > G(COOH) > G(CO), respectively. This can provide qualitative trends for experimental investigations.

calculated the single point energies of Np(V) and Pu(IV, VI) complexes with GO in the gas phase and aqueous solution using the significantly larger 6-311+G(d, p) basis set, which is much more suitable for energy calculations.62 The binding energies including the thermal contributions of the 6-31G(d) basis set in gas phase and aqueous solution using the 6311+G(d, p) basis set are also provided in Table 5. It can be clearly seen that the trends of the binding abilities in aqueous solution at the 6-311+G(d, p) basis set are consistent with those at the 6-31G(d) basis set except for the [Pu(H2O)7]4+/ G(COOH) complex which does not possess hydrogen bonding. The differences of the two binding energies of each actinide−GO complex in aqueous solution obtained by two basis sets are about 30 kcal/mol. Here, we selected the results in aqueous solution with the 6-311+G(d, p) basis set in the following discussions. The binding energies in aqueous solution are all negative except that for the [NpO2(H2O)4]+/G(−O−) complex (26.9 kcal/mol), which indicate that Np(V) and Pu(IV, VI) ions can be adsorbed onto the studied GO materials. However, the adsorption ability of all the studied GO for actinide ions are in order of Pu(IV) > Pu(VI) > Np(V), which is in excellent agreement with the observation by Romanchuk that GO has high affinity for Pu(IV) ions.10 For instance, the binding energies between Pu(IV), Pu(VI) and Np(V), and GO modified by hydroxyl group in aqueous solution are −56.7, −32.9, and −6.3 kcal/mol, respectively. In addition, the functional groups change the adsorption abilities of GO toward the actinide ion to a different extent. For example, the binding energies in aqueous solution are −56.7,

4. CONCLUSIONS In summary, the structures, bonding nature, and energies of Np(V) and Pu(IV, VI) complexes with GO modified by hydroxyl, carboxyl, carbonyl, and epoxy groups have been investigated using scalar-relativistic DFT. It was found that Np(V) and Pu(VI) complexes with GO are five-coordination at the equatorial plane, while Pu (IV) complexes prefer eightcoordination. Moreover, the distances between Pu(IV) ions and GO are much shorter than those of Np(V) and Pu(VI) complexes, which indicates that Pu(IV) ions can more easily interact with GO. Compared to [NpO 2 (H2 O) 5 ]+ and [PuO2(H2O)5]2+ cations, the harmonic frequencies of O AnO display a red shift in Np(V)/GO and Pu(VI)/GO complexes. Furthermore, the red shift of the Pu(VI)/GO complexes are much larger than those of the Np(V)/GO complexes, which also identifies that Pu(VI) ions may be more easily adsorbed on GO compared to Np(V) ions. NBO analyses reveal that the bond between Pu(IV) ion and GO possesses more covalency, whereas the electrostatic interaction 10278



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dominates the An−OG bond for the cases of the Np(V) and Pu(VI) ions. The topological analyses of electron density for the An−OG bond are well-consistent with the results of NBO. On the basis of the binding energies in solution, adsorption abilities of all GO for actinide ions follow the order of Pu(IV) > Pu(VI) > Np(V), which is in excellent agreement with experimental observations. This work might help with understanding the interaction mechanisms and coordination modes of Np(V) and Pu(VI, IV) complexes with GO and shed light on designing new functional GOs to efficiently remove radionuclides, especially long-lived transuranic elements, from radioactive wastewater.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-10-88233968. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 91326202, 11205169, 21101157, 21477130, 21261140335, and 91126006) and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant 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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REFERENCES

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