Loading Actinides in Multilayered Structures for Nuclear Waste

Jun 7, 2016 - Laboratory of Nuclear Energy Chemistry and Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of. High Ene...
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Loading Actinides in Multilayered Structures for Nuclear Waste Treatment: The First Case Study of Uranium Capture with Vanadium Carbide MXene Lin Wang,†,# Liyong Yuan,†,# Ke Chen,‡ Yujuan Zhang,† Qihuang Deng,‡ Shiyu Du,‡ Qing Huang,*,‡ Lirong Zheng,§ Jing Zhang,§ Zhifang Chai,† Michel W. Barsoum,∥ Xiangke Wang,⊥ and Weiqun Shi*,† †

Laboratory of Nuclear Energy Chemistry and Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ∥ Department of Materials Science and Engineering and A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, United States ⊥ School of Environment and Chemical Engineering, North China Electric Power University, Beijing 102206, China S Supporting Information *

ABSTRACT: Efficient nuclear waste treatment and environmental management are important hurdles that need to be overcome if nuclear energy is to become more widely used. Herein, we demonstrate the first case of using two-dimensional (2D) multilayered V2CTx nanosheets prepared by HF etching of V2AlC to remove actinides from aqueous solutions. The V2CTx material is found to be a highly efficient uranium (U(VI)) sorbent, evidenced by a high uptake capacity of 174 mg g−1, fast sorption kinetics, and desirable selectivity. Fitting of the sorption isotherm indicated that the sorption followed a heterogeneous adsorption model, most probably due to the presence of heterogeneous adsorption sites. Density functional theory calculations, in combination with X-ray absorption fine structure characterizations, suggest that the uranyl ions prefer to coordinate with hydroxyl groups bonded to the Vsites of the nanosheets via forming bidentate inner-sphere complexes. KEYWORDS: MXene, vanadium carbide, actinide, sorption, uranium, DFT calculation, EXAFS

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INTRODUCTION With the increase in nuclear energy utilization, nuclear waste treatment is becoming a challenging environmental concern because the contamination of some long-lived actinides can be significant hazards even at trace amounts due to their long-term radiological and chemical toxicities.1−3 Although various techniques, including solvent extraction,4 ion exchange,5 microbial immobilization,6 and electrochemical reduction,7 have been developed for the removal and separation of radionuclides, their adsorption onto solid sorbents remains one of the more facile and effective options.8 To date, functionalized nanomaterials, such as ordered mesoporous silica,9,10 carbon-based nanomaterials (e.g., carbon nanotubes, graphene and its derivatives),11−13 metal oxide nanoparticles,14 and metal organic framework materials,15,16 have been widely explored as possible adsorbents to clean up radionuclides, heavy metal ions, and toxic organic pollutants due to their large specific surface areas, microstructural versatility, and functionalization possibilities. However, functional highly radiation-resistant materials that can separate or immobilize hazardous radionuclides while © XXXX American Chemical Society

maintaining their physicochemical and mechanical stabilities, as well as thermal conductivities, are still being sought. The recent successful synthesis of two-dimensional transition metal carbide and carbonitride materials (labeled MXenes) have garnered them great attention due to their excellent metallike electrical conductivities, thermal stabilities, ion exchange capabilities, and predicted high Seebeck coefficients.17,18 MXenes are so-called because they are obtained by etching of the A-layeralmost always the Al-layerfrom the layered ternary carbides Mn+1AXn, or MAX phases, where M is an early transition metal, A is mainly a group IIIA or IVA element, X is C and/or N, and n = 1, 2, or 3.19 In the MAX phases the A layers are less tightly bound to the structure and can thus be readily etched at room temperatures using hydrofluoric acid (HF) or hydrochloric acid (HCl) combined with lithium fluoride (LiF). Received: March 10, 2016 Accepted: June 7, 2016

A

DOI: 10.1021/acsami.6b02989 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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product was determined by inductively coupled plasma optical emission spectrograph (ICP-OES, Horiba JY2000-2, Japan). The residual concentrations of uranyl ions and other competing metal ions in selectivity test experiments were also measured by ICP-OES. Batch Sorption Experiments. A series of tests for the sorption of U(VI) from aqueous solutions onto multilayered V2CTx as a function of pH, contact time, initial U(VI) concentration, and competing metal cations were performed. A 42 mmol L−1 U(VI) stock solution was first prepared by dissolving appropriate amounts of uranyl nitrate hexahydrate in Milli-Q water (18.2 MΩ cm−1). U(VI) sorption experiments were performed by using a batch method with initial concentrations ranging from 5 to 120 mg L−1 at room temperature. Small quantities of 0.1 M HNO3 and 0.1 M NaOH solutions were used to adjust the solution pH. In a typical experiment, 4 mg of sorbent was added into either 10 mL of U(VI) solution or 10 mL of multi-ion test solution in a flask (viz., the solid−liquid ratio was 0.4 mg mL−1). The flasks were stirred for a specified time at room temperature, and then the sorbent was separated from the solution using a 0.22-μm nylon syringe filter (ANPEL Scientific Instrument Co., Ltd., Shanghai). Before determining the cationic concentrations, the initial solution and the supernatants, before and after the sorption, were diluted appropriately. The concentrations of U(VI) in diluted solutions were determined by spectrophotometric method. All values were measured in duplicate; the uncertainties in the concentrations was within 5%. The sorbed amount qe (mg g−1) was calculated assuming:

Because when the Al layers are etched out they are replaced by surface terminations, such as O, OH, and F, the proper chemical description of MXenes is Mn+1XnTx, where T denotes the surface terminations. Currently the number of known MXenes is >15. Examples include Ti3C2Tx, V2CTx, Nb2CTx, and Ta4C3Tx.20 These MXenes have already proven to be outstanding candidates for electrode materials in electrical energy storage such as lithium ion batteries and supercapacitors.21−24 MXenes have also been explored for water treatment quite recently due to their layered structures and abundant active sites. For example, 1 kg of alkalized Ti3C2Tx MXenes can treat 4500 kg of water to remove Pb(II) ions.25 Ultrasonically delaminated Ti3C2Tx nanosheets also exhibit high removal capacities for toxic Cr(VI) from water.26 Moreover, the adsorption and photocatalytic properties of multilayered Ti3C2Tx of a few organic molecules have been reported.27 More importantly, previous findings have demonstrated that MAX phases are of high radiation resistance and good chemical compatibility with harsh media such as molten salt and exhibit no long-term activation due to their composition of low-Z elements.28−30 These superb properties may allow MXenes to act as potentially versatile candidates to process troublesome radioactive nuclear wastes. In addition, the abundance of active sites and flexible surface chemistry of MXenes, as well as the fact that many cations spontaneously intercalate MXene, together with their predicted high thermal conductivities may open a wide range of possibilities to design a variety of novel functional materials which could be used to adsorb and separate radionulcides. However, as far as we aware, there are no experimental reports on using MXenes as sorbents for radionuclide capture. Herein, we demonstrate, for the first time, that the multilayered V2CTx MXene can be used as a potential and efficient adsorbent for uranium (U(VI)) capture from aqueous solutions. Here U(VI) is selected as a representative actinide mainly due to its central role in the traditional nuclear fuel cycle and fascinating chemistry. We also elucidated the interaction mechanisms and complex structures formed between the uranyl ions and the V2CTx nanosheets by using advanced X-ray absorption spectroscopic (XAS) techniques and density functional theory (DFT) calculations.

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qe =

(c0 − ce) × V m

(1)

where c0 and ce are the initial concentration and equilibrium concentration of the metal cations (mg L−1), respectively. V is the volume of the testing solution (mL), and m is the amount of sorbent (g). X-ray Absorption Spectroscopy Analysis. X-ray absorption near edge-structure (XANES) spectra of V K-edge (5465 eV) and extended X-ray absorption fine structure (EXAFS) of the U LIII-edge (17 166 eV) for samples and reference compounds were collected at the beamline 1W1B of the Beijing Synchrotron Radiation Facility (BSRF). The V K-edge XANES spectra of V2AlC, V2CTx (before and after U(VI) sorption), and reference compounds including V2O3 and VO2 were measured in transmission mode with the energy range of 5350−5750 eV. The U LIII-edge EXAFS spectra of U(VI) sorbed V2CTx were recorded in fluorescence mode using a lytle-type ion chamber detector with the energy range of 16 950−17 935 eV. The incident X-ray beam was modulated to desired energies by using a silicon (111) double-crystal monochromator. The U(VI) loaded V2CTx samples for EXAFS measurement were prepared under the same conditions used in typical sorption experiments. The samples were separated from the solution by using 0.45-μm nylon filter membranes (ANPEL Scientific Instrument Co., Ltd., Shanghai), and then were sealed with Kapton tape for the following EXAFS tests. To obtain Fourier transform EXAFS spectra, pre-edge background subtraction, spline-fitting, and normalization were carried out for raw EXAFS oscillations data by using Athena software. Rbkg 1.0 was used to optimize the atomic background function (μ0(E)) using the autobk utility. To extract metric parameters (neighboring atomic distances (R), EXAFS Debye−Waller factors (σ2), coordination numbers (N)) from the EXAFS, the theoretical phase shift and amplitude functions for single and double scattering paths were calculated by the program FEFF6 and optimized as implemented in the FEFFIT code using the model structure of (UO2)3(VO4)2·5H2O.31 Prior to analysis, the k3weighted EXAFS spectra were Fourier transformed over a k-space range of ∼3.7−13.5 Å−1. All the fitting operations were performed in R-space of ∼1.2−3.3 Å. Computational Details. The computational method used in this study is the same as that in our previous work.32 Briefly, Vienna ab initio simulation program (VASP)33 was used to performed all the DFT simulations for the adsorption behaviors of uranyl ion on the vanadium carbide nanosheet. A 3 × 3 supercell containing V and C

EXPERIMENTAL DETAILS

Materials. The V2AlC phase material was synthesized by a solid state reaction (Supporting Information and Figure S1). The multilayered V2CTx powder was synthesized by immersing V2AlC powder (200 mesh) in a 40% concentrated HF solution at room temperature for 7 d. The solution was stirred gently every 12 h. After HF treatment, the product was filtered out and washed several times, first by deionized water and then alcohol in sequence. The V2CTx powder was collected after drying in air at room temperature for 48 h. Characterization. X-ray diffraction (XRD) patterns were obtained with a powder diffractometer (Bruker, D8-Advance, Germany) using Cu Kα radiation (λ = 1.5406 Å) and a step size of 0.02°. The microstructures of the powders were characterized by a field-emission scanning electron microscope (SEM, Hitachi S-4800, Japan), and the chemical component analysis was performed by energy-dispersive Xray spectroscopy (EDS, Horiba7593-H model). The zeta potentials of the V2CTx powders were measured by dynamic light scattering (DLS) method using a Zetasizer Nano ZS90 (Malvern Instruments, UK) over a pH range of 2.5 to 7.0. Fourier-transform infrared (FTIR) spectra were collected using a Bruker Tensor 27 spectrometer. The quantitative determination of U(VI) in solution was performed by a spectrophotometric method using Arsenazo-III as complexing agent at a wavelength of 656 nm. The contents of V and Al in the HF etched B

DOI: 10.1021/acsami.6b02989 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces atoms and terminated OH/F groups was selected as substrate to investigate the interaction between uranyl ion and nanosheet. In our theoretical simulations, the neutral system was simply considered created by invoking surface defects.34 A vacuum spacing of 15 Å was added along z-axis to avoid the interaction between neighboring slabs. In the adsorption process, the formation energy (ΔE) of hydrated uranyl ion on vanadium carbide nanosheet is calculated as ΔE = Ecomplex + nEwater − Esubstrate − Euranyl

multilayered V2CTx is remarkably pH-dependent. The uptake capacity increases sharply with the increase of solution pH and reaches its maximum value of 174 mg g−1 at a pH of 5.0, which is comparable with, or even higher than, conventional inorganic nanomaterials (Table S1).35−42 Sorption experiments under higher pH values are not taken into account in this work as the hydrolysis of U(VI) starts above pH ∼ 5 at the concentration of 100 mg L−1. Considering that the results shown in Figure 2A were obtained with powders containing 32 at. % unreacted V2AlC, the adsorption capacity of pure V2CTx nanosheets can reach as high as about 256 mg g−1. Zeta potential measurements (Figure S2) indicate that the V2CTx multilayers are negatively charged within the entire range of pH tested from 2.5 to 7.0. As the pH increases, the zeta potential of V2CTx becomes more negative, revealing a stronger electrostatic interaction between the sorbent and U(VI). Therefore, a high U(VI) uptake was observed. The negligible U(VI) sequestration in low pH (pH < 3) also suggests the multilayered V2CTx could be regenerated by acidic solutions. To test stability of the sorbent, three V2CTx samples after U(VI) sorption at pH values of 3.0, 4.5, and 5.0 were characterized. XRD patterns (Figure 3A and 3B) show that the featured peaks of (0002) plane for V2CTx are retained and the corresponding peak positions do not change after 4.5 h U(VI) sorption, though the order of multilayered structures reveals a little decrease (intensity decreasing) due to uranium sorption and/or crush of some sample particles during the stirring process. SEM images in Figure 3C and 3D suggest that the multilayered structures of V2CTx particles are retained after U(VI) sorption. Although there are no obvious difference in FTIR spectra (Figure S3) of V2CTx MXenes before and after U(VI) sorption, the EDS result (Figure 3E) shows the presence of significant amount of captured uranium in U(VI) sorbed sample. The oxidation states of V in various samples were also compared in Figure S4 by analyzing the corresponding XANES spectra. According to the method used in previous research work,43 we conclude that the element of V has an estimated equivalent valence of +3 in the sorbent of V2CTx. XANES spectra of V K edge for V2CTx before and after U(VI) sorption are very analogous, revealing that the valence of V has no obvious changes in the sorption process. Therefore, it can be concluded that the MXenes of V2CTx have an overall stability in the efficient U(VI) sorption process at pH range of 3.0−5.0. The sorption kinetic experiments were performed at an initial U(VI) concentration of 100 mg L−1 and a pH of 4.5 with contact time of 5−360 min. As shown in Figure 2B, the sorption kinetics can be divided into two distinct steps: a relatively fast sorption process in the initial 20 min, followed by a slower process that reaches equilibrium after about 4.5 h. To shed light on the sorption process, the experimental kinetic data were analyzed using two kinetic models (i.e., pseudo-first-order kinetic model and pseudo-second-order kinetic model, see Supporting Information and Table S2). The corresponding fitting curves by the two models are shown in Figure 2B, from which it is clear that the pseudo-second-order model fits the experimental data quite well with a much better correlation coefficient (R2> 0.998). In addition, the obtained sorption capacity from the model fitting was 143 mg g−1, which is quite close to the experimental equilibrium uptake capacity (142 mg g−1). These observations suggest that the sorption of U(VI) onto V2CTx is controlled by a chemical adsorption process. The sorption isotherm in Figure 2C shows that the U(VI) uptake has a sharp increase at the ce of U(VI) < 10 mg L−1,

(2)

where Ecomplex denotes the energies of V2CTx-uranyl complexes, Ewater is the energy of a water molecule, n is the number of water molecules, and Esubstrate and Euranyl correspond to the energies of V2CTx and the hydrated uranyl ion, respectively. To obtain the total energy of Esubstrate and Euranyl, the distance between V2CTx nanosheet and hydrated uranyl ion is fixed at 6 Å, since there is almost no interaction between the above two parts with a separation of 6 Å.32

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RESULTS AND DISCUSSION The morphology of the HF etched product was examined in a SEM. As shown in Figure 1A and 1B, the as-synthesized V2CTx

Figure 1. (A) and (B) Low- and high-magnification SEM images of V2CTx particle. (C) Powder XRD diffraction patterns of V2AlC (a) before and (b) after HF treatment.

comprises multilayered stacks of nanosheets with lateral dimensions of the order of 10 μm. The conversion of V2AlC to V2CTx is further confirmed by powder XRD patterns. Figure 1C shows that there is a large decrease in the intensity of the 002 peak of V2AlC at 2θ of 13.45° after HF etching, while a broadened peak centered at 2θ of 7.45° appears, corresponding to the (0002) plane of newly formed V2CTx MXenes with a c lattice parameter (c-LP) of 23.7 Å. The unreacted V2AlC in MXene samples may be attributed to the presence of some large particles that were not completely etched.21 ICP analysis confirmed that the V:Al ratio in HF-etched product was 2.0:0.32, suggesting at least about 68 at. % of the V2AlC had been converted into V2CTx in our sample. The as-synthesized V2CTx was then used as sorbent to capture U(VI) from aqueous solution at various conditions. Figure 2A shows that the U(VI) sorption behavior of C

DOI: 10.1021/acsami.6b02989 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. U(VI) sorption from aqueous solution onto multilayered V2CTx as a function of (A) pH, (B) contact time, (C) initial U(VI) concentration, and (D) competing metal cations. Detailed experimental parameters: (A) msorbent/Vsolution = 0.4 mg mL−1, [U]initial = 100 mg L−1; (B) pH = 4.5 ± 0.1, msorbent/Vsolution = 0.4 mg mL−1, [U]initial = 100 mg L−1; (C) [U]initial = 5−120 mg L−1, msorbent/Vsolution = 0.4 mg mL−1, pH = 4.5 ± 0.1; (D) [U]initial = [M]initial = 0.42 mmol L−1, msorbent/Vsolution = 0.4 mg mL−1.

Figure 3. Characterizations of V2CTx sorbent after U(VI) sorption. (A) and (B) Wide and small angle XRD patterns of U(VI) sorbed samples at different solution pHs. XRD pattern of orginal V2CTx is also given for comparison. (C) and (D) SEM images of U(VI) sorbed sample at pH 4.5. (E) EDS results of U(VI) sorbed sample at pH 4.5.

corresponding to ca. 70 mg g−1 of sorption capacity, after which the increase of U(VI) sorption starts slowing down somewhat. To understand the underlying sorption mechanisms, three

models, viz. Langmuir, Freundlich, and Dubinin-Radusckevich (D-R), were used to fit the experimental results (Supporting Information and Table S3). It can be clearly seen that the D

DOI: 10.1021/acsami.6b02989 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces U(VI) sorption onto V2CTx follows the Freundlich and D-R models quite well (Figure 2C). A reasonable explanation is that the MXenes of V2CTx are stacked with multilayer structures and also terminated with various functional groups such as −OH, −O, and −F, thus leading to the presence of heterogeneous adsorption sites, which is probably why the Freundlich model describes the sorption isotherm better. The value of the parameter n in Freundlich model is about 2.5, which is larger than 1, indicating the favorable adsorption behavior of U(VI). In addition, a mean free energy of 11.97 kJ mol−1 calculated from D-R model fitting also suggests that the interaction between U(VI) and V2CTx follows an ion-exchange mechanism, which is a typical chemisorption process.44,45 The slight decrease of solution pH after U(VI) sorption in our experiments maybe another evidence for the presence of ionexchange reaction. To quantify the ion selectivity of V2CTx, U(VI) uptake was also performed from aqueous solutions containing a range of competing metal cations (i.e., Co2+, Ni2+, Zn2+, Sr2+, La3+, Nd3+, Sm3+, Gd3+, and Yb3+) at two pH values of 4.5 and 5.0. As can be seen in Figure 2D, U(VI) sorption capacities on V2CTx nanosheets at pHs of 4.5 and 5.0 are 0.41 and 0.58 mmol g−1 (corresponding to 98 and 138 mg g−1), respectively. That is, the U(VI) uptake is not obviously lowered by the tested competing cations. The sorption capacities for these competing ions, however, are all as low as less than 0.1 mmol L−1. From these results we can calculate a selectivity coefficient, SU/M, defined as

SU/M = Kd U /Kd M U

Figure 4. Snapshots of uranyl inner- and outer-sphere adsorption configurations on V2CTx nanosheets. (A) Uuranyl outer-sphere adsorption configuration on bare V2C nanosheet. (B) and (C) Monodentate and bidentate uranyl inner-sphere adsorption configurations on V2C(OH)2 nanosheet. (D) Bidentate uranyl inner-sphere adsorption configuration on V2CF2 nanosheet.

deprotonated O atoms (O1 in Figure 5A) form. There is an obvious hybridization between U d orbital and the deprotonated O s orbital in the −17 to −21 eV range, and the U d and p orbitals and the deprotonated O p orbital in the −8 to −3 eV range (Figure 5Ba and 5Bb). Compared with O atoms of hydroxyls far from U atom (O2 in Figure 5A, Figure 5Bc), the peak of the deprotonated O s orbital located at −22 to −21 eV shifts up to −21 to −18 eV. Meanwhile, the peak of the deprotonated O p orbital located at −9 eV disappears. All of the above calculations suggest that each uranyl ion preferentially combines with two hydroxyl groups attached to V atoms. The deprotonation of hydroxyl groups in the adsorption process also implies the presence of an ion-exchange reaction mechanism. This result is consistent with the finding of D-R model fitting in the sorption isotherm section. To further confirm the sorption mechanism, the local coordination environment of U(VI) sorbed onto V2CTx was carefully examined by EXAFS measurement for [U]initial of 100 and 200 mg L−1, and at pHs that ranged from 4.2 to 5 (Figure 6). It is found that spectra A, B, and C show almost the same oscillation modes and the same intense Fourier transforms (FT) that define peaks in 1−4 Å range (not phase corrected), from which it is reasonable to conclude that in the tested pH range and [U]initial herein, the local coordination between U(VI) and the V2CTx sorbent is unchanged. Reasonable fits of these spectra result in the metric parameters listed in Table 2. It is clear that the U(VI) ions sorbed onto V2CTx at different solution pH and initial U(VI) concentrations have almost the same fitting parameters, which further confirms that the local atomic coordinates remain unchanged. Specially, the U−Oax distances of ∼1.79−1.80 Å obtained are typical of uranyl compounds,51 and the value of U−Oax EXAFS Debye−Waller factors (σ2 ∼ 0.003 Å2) are consistent with reported values for uranyl (0.001−0.003 Å2).52,53 Five oxygen atoms at a range of 2.3−2.5 Å (two at ∼2.32 Å, and the other three at ∼2.45 Å) comprise the first equatorial shell of the U(VI) ions bonded by the V2CTx sorbent and water. These obtained U−O distances are also in agreement with those in carnotite, K2(UO2)2(VO4)2·3H2O.54 Furthermore, a V atom at ∼3.2 Å was fitted as the second equatorial shell of the U(VI) ions, and the U−V distance is

(3)

M

where Kd and Kd are distribution ratios of U(VI) and competing ions in sorbent and solution, respectively. At pHs of 4.5 and 5.0, SU/M for U(VI) is >10, indicating that V2CTx selectively absorbs U(VI) over the other test metal cations. To further understand the adsorption mechanism, a series of finite-sized computational models were anaylzed by the first principle calculations. A pentacoordinated uranyl species of [UO2(H2O)5]2+ was used in the simulations as it is the dominant species of U(VI) in the aqueous solution under the tested pH and U(VI) concentration.46 Because the exposed vanadium surfaces were terminated by OH/F group after exfoliation in the synthesis process, substrates including bare V2C and functionalized V2CTx (T = OH and F, x = 2) were considered. Four adsorption configurations, as shown in Figure 4, were simulated. The corresponding formation energies and various bond distances are presented in Table 1. On the basis of these results, the most energetically favorable adsorption configuration is the bidentate inner-sphere coordinated to a −OH functionalized V2C surface (Figure 4C). In this configuration, penta-coordinated uranyl species removes two coordinated water ligands and binds to two surface O atoms deprotonated from hydroxyl groups in activated V sites, forming a bidentate coordinated complex we label as VO2−UO2(H2O)3. The calculated U−Oeq 1 distances in VO2−UO2(H2O)3 (2.28 and 2.31 Å, Table 1) are shorter than the bond distances between U atom and water ligands (2.39−2.52 Å),47−50 suggesting the presence of a stronger binding strength between uranyl ions and MXene surface. The charge density distribution and the projected density of states (PDOS) are shown in Figure 5. From the charge density distribution along (0−1 0) plane (Figure 5A), it can be clearly seen that the bonds between the U atom and the two E

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Table 1. Calculated Bond Lengths and Formation Energies for the Uranyl Adsorption Configurations Based on the DFT Simulationsa configuration

d (U−Oax) (Å)

A B C D

1.78/1.78 1.82/1.84 1.83/1.81 1.77/1.77

d (U−Oeq 1) (Å)

d (U−Oeq 2) (Å)

d (U−V) (Å)

ΔE (eV)

2.28 2.28/2.31 2.66/2.73

2.48/2.47/2.45/2.57/2.46 2.51/2.51/2.48/2.58 2.55/2.43/2.57 2.38/2.37/2.40

4.31 3.82 3.23 3.85

−9.09 −9.59 −10.38 −5.91

a

Oax, the axial O of uranyl species; Oeq 1, the O/F atom at adsorption site; Oeq 2, the O atom of water ligands. The adsorption configurations of A−D in the table correspond to the structures of Figure 4.

Figure 5. (A) Charge density distribution and (B) PDOS for bidentate inner-sphere adsorption configuration of VO2−UO2(H2O)3. (A) is (0−1 0) plane of VO2−UO2(H2O)3. (Ba) U atom; (Bb) O1 atom in A; (Bc) O2 atom in A.

Table 2. Fitting Parameters Extracted from EXAFS Spectra sample

shell

CNa

R/Åb

σ2/Å2c

pH = 4.5 [U] = 100 ppm

U−Oax U−Oeq 1 U−Oeq 2 U−V U−Oax U−Oeq 1 U−Oeq 2 U−V U−Oax U−Oeq 1 U−Oeq 2 U−V

2f 2.0 3.1 0.9 2f 1.9 2.9 0.9 2f 2.0 3.0 0.9

1.80 2.32 2.45 3.21 1.80 2.33 2.45 3.21 1.79 2.32 2.44 3.19

0.003 0.003 0.010 0.006 0.003 0.003 0.010 0.006 0.003 0.004 0.006 0.009

pH = 5.0 [U] = 100 ppm

pH = 4.2 [U] = 200 ppm

Figure 6. (Left) Raw U LIII-edge k3-weighted EXAFS spectra and the best theoretical fits of U sorbed to V2CTx under different solution pH and initial U(VI) concentration. (A) pH 4.5, [U]initial = 100 mg L−1; (B) pH 5.0, [U]initial = 100 mg L−1; (C) pH 4.2, [U]initial = 200 mg L−1. (Right) Corresponding nonphase shift corrected Fourier transforms.

Rfactord 0.001

0.002

0.001

a Coordination number. N ± ∼ 20%. bInteratomic distance. R ± ∼ 0.03 Å. cDebye−Waller factor. dGoodness-of-fit parameter. fFixed parameter.

Additionally, it is possible to theoretically estimate the maximum adsorption capacity of uranium for the configuration of VO2−UO2(H2O)3 (Figure 4C) according to our previous study.32 Considering the spatial effect, each uranyl ion can occupies about six O sites on V2C(OH)2 nanosheets and only two of them can coordinate with the uranyl ion. Thus, the number of active sites for uranyl ions on per gram V2C(OH)2 nanosheets can be estimated to be 0.00225 mol, which suggests that the maximum uptake capacity of V2C(OH)2 nanosheets for uranium can reach as high as 536 mg g−1. This theoretical adsorption capacity is twice higher than the uptake capacity we obtained from experiments (174 mg g−1). Hence, more research about material optimization (e.g., etching, intercala-

consistent with those in the (UO2)3(VO4)2·5(H2O) crystal,31 in which vanadate groups bind U(VI) ions in a bidentate fashion. In addition, the corresponding bond distances and coordination number obtained from fitting EXAFS spectra (Table 2) also agreed quite well with those obtained from the adsorption configuration of C in Figure 4 (Table 1). Therefore, we can conclude that the EXAFS fitting results confirm the coordination structure of VO2−UO2(H2O)3 complex obtained by calculation in the former section, that is, the uranyl ion favors to adsorb onto hydroxylated vanadium carbide nanosheets with a bidentate inner-sphere adsorption configuration. F

DOI: 10.1021/acsami.6b02989 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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tion, activation, and delamination) are desired to further improve the uranium uptake performance of V2CTx MXene.

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CONCLUSION We report here a first study of U(VI) capture from aqueous solutions using multilayered V2CTx MXene. The maximum uptake capacity for U(VI) is comparable with, or even larger than, that of the conventional inorganic nanomaterials. The sorption of U(VI) on MXenes could be described by a heterogeneous adsorption model due to the presence of heterogeneous adsorption sites (such as −OH, −O, and −F) in the sorbent. The interaction mechanisms between the U(VI) and V2CTx nanosheets were studied at the molecular level. Both DFT calculations and EXAFS results indicate that the U(VI) is adsorbed onto the V-based MXene by the formation of bidentate adsorption configurations, with hydroxyl groups attached to V atoms. The deprotonation of the hydroxyl group after bonding with U(VI) suggests the adsorption process follows an ion-exchange mechanism. It is expected that the findings in this work will greatly enrich our fundamental understanding of the interaction mechanisms between actinide ions and MXenes, and therefore further lay new avenues for promoting the applications of MXenes for nuclear waste treatment and radionuclide removal from the environment in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02989. Additional information for reagents, synthesis of V2AlC, fitting results of sorption kinetics and isotherm data, stability test of V2CTx, comparison of adsorption capacities, zeta potential dependence on pH of V2CTx, and XANES spectra comparison of V-containning samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-10-88233968; fax: +86-10-88235294; e-mail: [email protected]. *Tel: +86-574-86686062; e-mail: [email protected]. Author Contributions #

Authors L.W. and L.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grants 21577144, 91326202, and 91226202) and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant XDA030104). We thank the staff of BSRF for EXAFS and XANES measurements.

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REFERENCES

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DOI: 10.1021/acsami.6b02989 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b02989 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX