Efficient U(VI) Reduction and Sequestration by Ti2CTx MXene

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Efficient U(VI) Reduction and Sequestration by Ti2CTx MXene Lin Wang, Huan Song, Li-Yong Yuan, Zijie Li, Yu-Juan Zhang, John K. Gibson, Lirong Zheng, Zhi-Fang Chai, and Wei-Qun Shi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03711 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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Environmental Science & Technology

Efficient U(VI) Reduction and Sequestration by Ti2CTx MXene 1

1,2

Lin Wang, Huan Song, Liyong Yuan,1 Zijie Li,1 Yujuan Zhang,3 John K. Gibson,4 Lirong Zheng,5 Zhifang Chai 1 and Weiqun Shi*1 1.

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. 2.

School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China. 3.

School of Materials Science and Engineering, University of Science and Technology Beijing, 100083, China. 4.

Chemical Sciences Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, California 94720, United States. 5.

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. Word counts: Main text (3854 words) +3 big Figure (1800 words) +4 small Figures (1200 words) +1 Table (300 words)= 7154 words

ACS Paragon Plus Environment


ABSTRACT: Although reduction of highly mobile U(VI) to less soluble U(IV) has been long considered an effective approach to in situ environmental remediation of uranium, candidate reducing agent are largely limited to Fe-based materials and microbials. The importance of titanium-containing compounds in natural uranium ore deposits suggests a role for titanium in uranium migration. Herein, for the first time a two-dimensional transition metal carbide, Ti2CTx, is shown to efficiently remove uranium via a sorption-reduction strategy. Batch experiments demonstrate that TiC2Tx exhibits excellent U(VI) removal over a wide pH range, with an uptake capacity of 470 mg g-1 at pH 3.0. The mechanism for U(VI) to U(IV) reduction by Ti2CTx was deciphered by X-ray absorption spectroscopy and diffraction, and photoelectron spectroscopy. The reduced U(IV) species at low pH is identified as mononuclear with bidendate binding to the MXene substrate. At near-neutral pH, nanoparticles of UO2+x phase adsorb to the substrate with some Ti2CTx transformed to amorphous TiO2. Subsequent in-depth study suggests Ti2CTx materials may be potential candidates for permeable reactive barriers in treatment of wastewaters from uranium mining. This work highlights reduction-induced immobilization of U(VI) by Ti2CTx MXene including a pH-dependent reduction mechanism that might promote applications of titanium-based materials in the elimination of other oxidized contaminants.


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INTRODUCTION As one of the central constituents of traditional nuclear fuel cycles, uranium is an underlying environmental contaminant with both chemical and radiological toxicity. 1 Uranium is easily released into soil and aquifer as a result of mining, nuclear fuel reprocessing and improper disposal of nuclear waste.

2,3

For the two dominant

oxidation states of environmental uranium, U(VI) and U(IV),

4

effects on the

hydrosphere and public health are greatly exacerbated by migration of U(VI) due to high aqueous water solubility and mobility. 5 To address the need to mitigate aqueous U(VI), solid sorbents have been evaluated, including organic materials such as functionalized graphene oxide, 9,10

nanoporous polymers

11

materials such as zeolites,

6,7

hydrothermal carbon,

8

metal organic frameworks,

and covalent organic frameworks,

13

clays,

chalcogenides 18 and phosphates. 19-21

14

12

and inorganic

metal oxides and hydroxides,

15-17

metal

In addition to adsorption, transformation from

higher oxidation state U(VI) to reduced and sparingly soluble U(IV) species is an important strategy for in situ uranium immobilization.

3,22-24

Towards this end,

microbial and chemical reductants, including bacteria, 25,26 soluble Fe(II), 27 magnetite, 28,29

iron sulfide

30,31

and zero valent Fe-based nanomaterials

22,32-36

have been

explored. Despite these efforts, there remains a need for advanced permeable reactive barrier (PRB) materials that can immobilize uranium with high removal efficiency, long-term stability, and good tolerance against harsh environmental conditions such as high acidity and radiation fields. Two-dimensional transition metal carbides and carbonitrides, referred to as MXenes, are functional materials with unique layered structures and a general composition Mn+1XnTx, where M is an early transition metal, X is C and/or N, and Tx are surface termination groups such as −O, −OH and –F.

37

Analogous to clay

materials, MXenes have characteristics of hydrophilicity, negatively charged surfaces, flexible swelling, high ion-exchange capacity, and acid resistance, them attractive candidates for environmental remediation.


39-43

38

which make

Recently, MXenes of


multilayered V2CTx and hydrated intercalated Ti3C2Tx were demonstrated for actinide extraction, and strong affinities of MXenes for U(VI) cations were indicated by both experiment and theory.

44-47

As more than 20 MXenes have been reported, study of

U(VI) removal by MXenes beyond V2CTx and Ti3C2Tx is desirable for material screening given the diversity of layer structures, bonding and physicochemical properties that may result in new sequestration approaches. In addition, Ying et al. have reported their investigation on the reduction of strong oxidized Cr(VI) to Cr(III) by exfoliated Ti3C2Tx nanosheets, 48 which implys the potentially reductive properties of MXenes. We here report the first simultaneous adsorption and reduction of U(VI) from aqueous solution by a highly reactive Ti2CTx MXene. The objectives are as follows: (i) synthesize Ti2CTx and examine its performance for removal of U(VI) under different environmental conditions using batch sorption experiments; (ii) elucidate the reaction mechanism between U(VI) and Ti2CTx through characterizations that include X-ray absorption spectroscopy; and (iii) evaluate the potential for practical applications of Ti2CTx as a PRB material for in situ U(VI) immobilization using a simulated highly acidic mining wastewater. EXPERIMENTAL DETAILS Synthesis of Ti2CTx. Ti2AlC powders with purity >98 wt % and particle size < 19 µm (i.e., 800 mesh) were purchased from Beijing Jinhezhi Materials Co,. Ltd. 0.6 g Ti2AlC was slowly added to 16 mL of solution containing 6 M HCl and 1 M LiF (Sinopharm Chemical Reagent Co., Ltd.). The mixture was stirred at 35 ºC for 66 h, after which the product was centrifuged at 3000 rpm and washed 6 times with deionized water. The resulting multilayered Ti2CTx (M-Ti2CTx) was stored in anaerobic water at ~273 K. Super thin nanoflakes of Ti2CTx (S-Ti2CTx) were obtained by delaminating M-Ti2CTx through ultrasonication in degassed deionized water for 1 h. After centrifuging for 10 min at 2000 rpm, the supernatant was collected for further use.


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Batch Sorption Experiments. A proper amount of UO2(NO3)2. 6H2O (Sinopharm Chemical Reagent Co. Ltd.) was first dissolved in deionized water to prepare a 600 mg L-1 U(VI) stock solution. A series of U(VI) sorption experiments using Ti2CTx samples at 25 ºC under anaerobic conditions were carried out as a function of pH, contact time and initial U(VI) concentration. After Ti2CTx was mixed with degassed deionized water, NaCl and the desired amount of U(VI) in an anaerobic glove box, the pH was slowly adjusted using 0.1 M NaOH and 0.1 M HCl. The solid extractant was separated after 3 days by centrifuging at 10000 rpm for 10 min and subjected to further characterizations. The supernatants were collected using polyethersulfone syringe filters (0.22 µm, ANPEL Scientific Instrument Co., Ltd., Shanghai) and diluted with 5 wt% HNO3 before determination of uranium concentration. 0.1 M Na2CO3 was employed to extract and quantitatively determine unreduced U(VI) in Ti2CTx at different time intervals. 1g L-1 Ti2CTx was exposed, under anaerobic conditions for an initial period of 2 days, to a simulated acid mine wastewater which contained 10.8 mmol L-1 Al2(SO4)3, 0.8 mmol L-1 FeCl3, 1.08 mmol L-1 Ca(NO3)2, 0.72 mmol L-1 MgSO4, 2.88 mmol L-1 Na2SO4 and 2.88 mmol L-1 (NH4)2SO4, along with 20 mg L-1 U(VI). These solutions were then either maintained under anaerobic conditions, or exposed to air for long term experiments of up to 35 days.The removal capacity qe (mg g-1) was calculated as: qe=(C0-Ce)V/m, where C0 and Ce are initial and final equilibrium concentrations of cations, and V and m are the solution volume and the mass of solid sorbent in batch sorption tests. X-ray absorption spectroscopy. X-ray absorption fine structure (XAFS, including XANES and EXAFS) spectra of the U L3 absorption edge (~17,166 eV) were measured in transmission and fluorescence modes at beamline 1W1B of the Beijing Synchrotron Radiation Facility (BSRF). The incident X-ray beam energy was scanned using a silicon (111) double-crystal monochromator. XANES and EXAFS oscillation data were extracted, analyzed, Fourier transformed and fitted using the Athena and Artemis program, which were parts of in the IFEFFIT program package.


49

A k range


of ~ 2.0 – 10.0 Å-1 and a Rbkg of 1.2 were used for acquisition of the Fourier transform of k3-weighted EXAFS data. To fit parameters such as coordination number (N), atomic distance (R) and Debye-Waller factor (σ2), the theoretical phase and amplitude functions for three scattering pathways, U-O, U-U and U-Ti, were calculated based on the crystal structures of uraninite and brannerite. All fitting procedures were carried out in R space from 1.2-4.0 Å and with a fixed amplitude reduction factor (S02) of 0.74. Additional Characterization Methods. Morphologies and chemical compositions of MXene samples were measured using a Hitachi S-4800 field-emission scanning electron microscope (SEM) equipped with Horiba 7593-H energy-dispersive X-ray spectroscopy (EDS). High-resolution transmission electron microscope (TEM) images were obtained on a JEOL JEM-2100 electron microscope. A Bruker D8 Advance diffractometer (Cu Kα radiation, λ = 1.5406 Å) with a step size of 0.02º was used to collect powder X-ray diffraction (XRD) patterns. Crystallite sizes of UO2+x nanoparticles, where possible, were determined using the Scherrer equation assuming spherical particles and a K factor of 0.9 based on the reflection at 2θ 28.4°. Raman spectra were acquired using a LabRam HR Evolution Raman spectrometer (Horiba Scientific) with a 473 nm laser (Ciel model, Laser Quantum Ltd.) and a 1800 groove/mm grating (4 cm-1 spectral resolution). Zeta potentials at different pHs were collected by a Zetasizer Nano ZS90 (Malvern Instruments) dynamic light scattering (DLS) zeta potential analyzer. X-ray photoelectron spectra (XPS) were obtained using an ESCALAB 250Xi (Thermo Scientific) XPS spectrometer equipped with an Al Kα source (1486.8 eV). Residual concentrations of uranium and other metals were determined by an inductively coupled plasma optical emission spectrograph (ICP-OES, Horiba JY2000-2). RESULTS AND DISCUSSION Characterization of Ti2CTx. As shown in Figure 1A, the structure of Ti2CTx features a 2D layer formed by a single block of Ti6C octahedron with terminated


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functional groups attached to the surface Ti atoms. In addition to the most common termination by -OH/O groups, EDS results in Figure 1B indicate some -Cl and –F termination. The XRD pattern of as-synthesized M-Ti2CTx in Figure 1C shows a featured (002) peak at 7.84º, which corresponds to an interlayer spacing of 11.3 Å (Figure 1A). This interlayer spacing is larger than that of HF-etched Ti2CTx (7.5 Å), 50,51

presumably due to expansion of hydrated Li ion intercalation and chloride

termination during LiF-HCl etching.

52,53

The SEM image in Figure 1D shows a

typical multilayered structure of Ti2CTx lamellas with a thicknesses in the range of 40-80 nm and a lateral dimension of several micrometers. As for S-Ti2CTx, c-direction (002) peak was preserved while (110) peak at 61º was no longer observed (Figure S1), indicating the loss of order in non-basal directions.

38

The thickness of S-Ti2CTx is

less than 10 nm (Figure S1). The Raman spectrum of multilayered Ti2CTx in Figure 1E shows three major bands at 255 cm-1, 418 cm-1 and 604 cm-1 that correspond to vibration modes of non-stoichiometric titanium carbides. 51,54 The weak band at 1578 cm-1 corresponds to disordered carbon, suggesting the presence of some amorphous carbon in the prepared Ti2CTx.

55

From the plot of zeta potentials in Figure 1F it is

apparent that the point of zero charge (PZC) for Ti2CTx is near pH 2.75. This finding is important for metal cation sorption in view of the positive effect of electrostatic interactions, especially in acidic media. Batch Sorption Experiments. Results for U(VI) removal by multilayered Ti2CTx are presented in Figure 2. Figure 2A shows a uranium sorption edge at very low pH of 1.5 to 3.0, above which sorption increases dramatically. Since pHPZC of Ti2CTx is about 2.75 (Figure 1F), the U(VI) sequestration behavior as a function of pH is well explained by surface charge condition of Ti2CTx. For solution pH below 2.5, electrostatic repulsion between the positively charged surface of Ti2CTx and uranyl cations should result in a decrease in uranium removal. For higher pH, from 3.0 to near-neutral, the negatively charged surface of Ti2CTx should efficiently absorb cationic U(VI), which results in sequestration of greater than 90%. The slight decrease



in U(VI) removal at pH > 8.0 can be ascribed to the formation of anionic U(VI) solution species such as UO2(OH)3− and (UO2)3(OH)7−,56,57 which are unfavorable for sorption on a negatively-charged Ti2CTx surface. To corroborate surface charge-induced removal, U(VI) was equilibrated with 0.01 M Na2CO3, to form stable UO2(CO3)34− at pH 10.94, before contacting with Ti2CTx; the result was a drastic decrease in U(VI) removal, to only 7% (Table S1). Figure 2A also reveals that the removal of U(VI) by M-Ti2CTx is essentially independent of the counter-ions such as NO3−, Cl− and ClO4−. Figure 2B shows the removal rate of U(VI) by M-Ti2CTx for three initial U(VI) concentrations as a function of contact time. The residual concentration of uranium decreased rapidly for C0 = 100 mg L-1, reaching the detection limit within 24 h. Higher C0, 200 and 300 mg L-1, resulted in slower removal rates, presumably due to depletion of active Ti2CTx sorption sites; equilibriums were nonetheless nearly achieved within about 48h. The kinetic data were analyzed by two models for the case of C0 = 200 mg L-1, as shown in Figure 2C. The experimental data were fitted quite better by pseudo-second-order kinetic model, with a correlation coefficient of 0.999 (Supporting Information and Table S2), indicating that removal of U(VI) is controlled by a chemical sorption process. An ion-exchange mechanism involving replacement of H+ in hydroxyl terminated groups by UO22+ was supported by a rapid decrease in solution pH (∆pH = 0.4~0.5) upon adding Ti2CTx to a uranyl solution. Such H+ release has similarly been reported for immersion of MXene samples in highly concentrated alkali metal salt solutions.

58

In addition, previous studies have

demonstrated a strong affinity of surface [Ti–O] −H+ groups on MXene towards uranyl and other heavy metal cations through deprotonation ion-exchange. 39,44,45 As is apparent from Figure 2D, U(VI) removal exhibits a nearly linear relationship with C0, with almost 100% elimination up to C0 = 160 mg L-1. At higher C0, U(VI) removal reached a plateau saturation capacity of ~ 470 mg g-1. This U(VI) removal behavior is very analogous to that of other absorption substrates that are highly


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reducing, such as zero-valent iron nanoparticles, 32,57 implying the possibility of U(VI) reduction by Ti2CTx following the initial ion-exchange stage. To assess this reduction hypothesis, analysis of the U(VI) and U(IV) fractions present after different adsorption intervals was performed using a Na2CO3 extraction method. 22,59 Figure S2 shows that the U(VI) fraction in the totally removed uranium decreased from 26.5% to 3.6% when the contact time increased from 2 h to 48 h, indicating continuing reduction of U(VI) to U(IV). Experiments to assess competing removal of U(VI) and Th(IV) showed that ambient conditions of N2 versus air significantly affected U(VI) uptake but not Th(IV) uptake (Figure S3), suggesting different removal pathways. Since no oxidation state below Th(IV) is accessible under these experimental conditions, it is concluded that non-reductive chemical sorption is dominant for Th(IV). The dramatic decrease of uranium removal capacity in air is attributed to inhibition of U(VI) reduction in the presence of O2. Oxidized anionic ReO4− was employed to further investigate reduction by Ti2CTx at pH 4.0 in anaerobic condition. The Re(VII) removal capacity is only 20 mg g-1 (Figure S4), presumably due to electrostatic repulsion and restricted uptake of Re(VII) around the edges of M-Ti2CTx (Figure S5). The above batch sorption experiments indicate that the initial ion-exchange stage is critical for the uptake capacity and subsequent reduction performance of Ti2CTx. Identification of Uranium Speciation and Reaction Products. To evaluate the redox state information in U-loaded Ti2CTx, U L3 edge XANES spectra were measured, as shown in Figure 3. U(IV)/U(VI) energy shift ~ 1.4 eV was found by comparison of the white line position (i.e. the energy of the maximum in the strongest adsorption resonance) in XANES spectra for reference materials UO2 and UO2(NO3)2.6H2O (UNH), which is in accord with literature values for U(IV) and U(VI).

60

The XANES white line shapes and peak positions for all U-loaded Ti2CTx

samples were very analogous with that of UO2, indicating reduction of U(VI) to U(IV) over a pH range of at least 3.0 to 8.0. U(VI) reduction was established by both



multilayered and super thin Ti2CTx materials. For further comparison, XANES spectrum of sorbed uranium in another Ti-based MXene (M-Ti3C2Tx) was also collected, as Ti3C2Tx and Ti2CTx have similar surface groups and different atomic layers.

37

Similar to previous study,

46

the white line position for U-loaded Ti3C2Tx

was found to be consistent with U(VI) references (Figure 3). In addition, a resonance shoulder at ~17192 eV, which is ascribed to multi-scattering by axial uranyl O atoms, is in good agreement with the UO2(OH)2 reference. These results suggest typical non-reductive sorption of U(VI) by Ti3C2Tx. The different reductive abilities of Ti3C2Tx and Ti2CTx can be rationalized by their different layer structures: the bigger the number of atomic layers is, the more stable the MXene will be. 61 The local coordination environments of U in U-loaded Ti2CTx were further evaluated by analysis and fitting of the k3-weighted EXAFS data and their Fourier transforms (FT), as shown in Figure 4. Low amplitudes for k > 7 Å in spectra for S-Ti2CTx-pH3.0 and M-Ti2CTx-pH3.0 (Figure 4A) reveal a lack of long-range order in reaction products under acidic conditions.

27

At higher pH (M-Ti2CTx-pH5.0 and

M-Ti2CTx-pH8.0), EXAFS amplitudes at k > 7 Å increased, and the spectra became increasingly analogous to that of UO2, suggesting the formation of more crystalline and low-oxidation-state uranium oxides. The fits to the main FT peak of the EXAFS data yielded a U−O distance of 2.32-2.33 Å for all U-loaded Ti2CTx samples (Figure 4B and Table 1), indicating similar first-shell coordination of U. The derived U−O distance in U-loaded Ti2CTx was shorter than in UO2 (2.37 Å), but was consistent with that in UTi2O6 (2.33 Å). 62 A Ti atom at 3.37 Å (Table 1) was fitted in the second coordination shell of U, with no U−U interaction in R space observed for pH 3.0, suggesting formation of mononuclear U(IV) species on the substrate. Consistent with our results, Latta et al. reported a stable U(IV)-Ti binuclear coordination geometry with a U-Ti distance of 3.36-3.43 Å.

63,64

The presence of a U−U interaction at

3.83-3.84 Å for M-Ti2CTx-pH5.0 and M-Ti2CTx-pH8.0 indicates that at higher pH polynuclear U(VI) species are sorbed or precipitated on the Ti2CTx surface, followed


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by in situ reduction. The fitted N(U−U) for pH 5.0 and 8.0 are 1.4 and 2.2 (Table 1), respectively, which are much smaller than for UO2 (N = 12), possibly because of a large contribution of U surface atoms, and/or structural disorder. 4 SEM images and XRD patterns in Figure 5 show more evidence to support the interpretation of the XANES and EXAFS results. At pH 3.0, the morphologies of Ti2CTx after U loading remained unchanged, with a smooth surface; only residual Ti2CTx XRD peaks were observed, suggesting a uniform surface dispersion of non-crystalline U(IV) species on the MXene. As the pH was increased, the morphologies of U-loaded M-Ti2CTx became increasingly rough, with the surface covered by many tiny nanoparticles (Figure 5C and 5D). TEM images in Figure S6 confirm that these nanoparticles grow around the surface of Ti2CTx nanosheets, although it is difficult to distinguish their phases due to very low crystallinity of the samples. These nanoparticles were subsequently identified as the formation of UO2+x phase, since the additional broadening diffraction peaks appeared in Figure 5E could be indexed to the cubic U4O9 (PDF No. 72-0125) quite well. The grain sizes of the UO2+x phase extracted from the XRD peak widths by Scherrer equation are 6 nm and 11 nm for M-Ti2CTx-pH5.0 and M-Ti2CTx-pH8.0, respectively, which are consistent with the observations of SEM images in Figures 5C and 5D. XPS measurements were performed to assess the changes of chemical species on Ti2CTx before and after U removal. The survey scan XPS in Figure 6A confirmed the presence of Ti, C, O, F and Cl in as-synthesized Ti2CTx, as well as substantial uranium after U(VI) sorption. Spectral fitting of the high-resolution U 4f7/2 and 4f5/2 peaks (Figure 6B) revealed the presence of U(IV) (centered at 380.2 eV and 391.1 eV) and U(VI) (centered at 381.8 eV and 392.7 eV).

57,65

The U(IV):U(VI) ratio was

estimated as ~3.0, indicating U(IV) as the predominant valence state after sorption. The U(IV) fraction (75%) is slightly lower than that obtained from Na2CO3 extraction method, mainly because partial oxidation occurs on the surface of U-loaded Ti2CTx in sample preparation and drying process. The fate of Ti2CTx in the U(VI) reduction



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process was also investigated. Figures 6C and 6D represent detailed XPS in the Ti 2p and C 1s regions. The binding energies of the fitted components, such as Ti−C, Ti(II), Ti−O/F, C−C, C−O and C−F species, are in very good agreement with literatures values.

54,66-68

The peaks for Ti−C and Ti(II) disappeared after reaction with U(VI),

while the area of the Ti−O and Ti−OH peaks increased (Figure S7), indicating breaking of Ti−C bonds and formation of Ti−O/OH bonds, and an increase in oxidation state to fully Ti(IV) in Ti2CTx. Furthermore, the Raman spectrum of U-loaded Ti2CTx was found to closely resemble that of TiO2 reference (Figure S8), with the major band at 150 cm-1 evidently corresponding to anatase TiO2.

51

As no

Ti-based crystalline phase was indicated by XRD (Figure 5), it is concluded that the product of oxidation of Ti2CTx is probably amorphous TiO2. Treatment of Simulated Acidic Mine Wastewater. To evaluate the efficacy of Ti2CTx for reductive removal of U(VI) under environmental conditions, simulated U(VI)-containing acidic mine wastewater was studied, with the results shown in Figures 7 and S9. It is apparent that the U(VI) removal percentage exceeded 99% when simulated waste solution containing 20 mg L-1 U(VI), 162 mg L-1 Al3+, 45 mg L-1 Fe3+, 12 mg L-1 Ca2+, 4.8 mg L-1 Mg2+, 37 mg L-1 Na+, 29 mg L-1 NH4+, and 1037 mg L-1 SO42- (solution pH ~2.55) was in contact with 1g L-1 Ti2CTx under anaerobic conditions for 2 days. The removal percentage was ~95.7% in the presence of 6 times higher concentration of co-existing ions at pH ~2.10 (Figure S9). These results indicate an excellent potential of Ti2CTx for sequestration of uranium from acid mine wastewater. Ti2CTx also exhibited long-term immobilization of reduced U(IV) species under the anaerobic environment (Figure 7), though partial release of incorporated uranium occurred upon continuous exposure to air, presumably due to oxidation of U(IV) to soluble U(VI) by dissolved oxygen. The influence of dissolved iron was assessed given that the Fe(III)/Fe(II) redox processes can significantly affect U(VI) reduction in the environment. 4,69 It was found that the presence of iron had no discernable impact on uranium sequestration under anaerobic conditions but


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somewhat accelerated initial release of U(VI) under aerobic conditions (Figure 7). Given the higher redox potential for Fe(III)/Fe(II) (+0.77 V) versus U(VI)/U(IV) (+0.27 V),

70

this effect is attributed to enhanced formation of soluble U(VI) from

U(IV) when in competition with the Fe(III)/Fe(II) redox couple. Release of U(VI) reached equilibrium for exposure of more than 24 days. At this equilibrium, the sorbent color changed from black to greyish white, and the corresponding XRD pattern could be indexed to anatase, together with a small amount of rutile (Figure S10). The retention of > 40% sorbed uranium under these conditions is attributed to formation of stable surface complexes between TiO2 and U(VI). 71,72 Environmental Significance. Despite the relatively low terrestrial abundance of titanium, studies have shown that uranium is readily adsorbed on titanium-rich mineral surfaces,

72,73

such that titanium-based functional materials for

environmental remediation of uranium are clearly relevant. The results demonstrate simultaneous adsorption and reduction of U(VI) by Ti2CTx MXene over a wide pH range. Depending on the reaction pH, the reduced uranium species are mononuclear U(IV) complexes anchored to =TiO sites of amorphous TiO2 (U(IV)-A-TiO2), and/or UO2+x nanoparticles. The observation of U(IV)-A-TiO2 provides new insights into the immobilization pathway, and the inhibition of migration of U(IV) under acidic condition. Under anaerobic geological conditions, U(IV)-A-TiO2 may be a potential precursor to brannerite, which is common in uranium deposits. Notably, Ti2CTx-transformed TiO2 exhibits a considerable retention rate for low concentration uranium when re-exposed to air, indicating effective prevention against release. Nevertheless, TiC2Tx shows a weak reducibility toward anion species of U(VI) due to the electrostatic repulsion, which may limit its application in some environmental conditions. Further investigations should aim to control the surface charge of Ti2CTx via surface modification, grafting and composite strategies, which should enhance reduction and immobilization of oxidized anion species of U(VI), Re(VII), Tc(VII), Cr(VI) and other oxidized metal anions.



Corresponding Authors: Tel: +86-10-88233968; FaX: +86-10-88238494; E-mail: [email protected] (W. Shi).

Acknowledgements This work was supported by the Natural Science Foundation of China (Grants no. 11675192, 21577144, 21790373 and 21790370) and the Science Challenge Project (TZ2016004). Research of JKG was supported by the Center for Actinide Science and Technology, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0016568. We are grateful to the staff of Beijing Synchrotron Radiation Facility (BSRF) for EXAFS and XANES measurement. We thank Wuqing Tao and Shuangxiao Li for optimizing the synthesis method of Ti2CTx.

Supporting Information. Details of sorption kinetics fitting, characterization of S-Ti2CTx, U(IV) and U(VI) fraction analysis, competing removal of U(VI) and Th(IV), Re(VII) removal by Ti2CTx and characterizations, TEM images, high-resolution O 1s XPS and Raman spectra of Ti2CTx-U, removal of U(VI) from simulated acidic mine wastewater at different concentrations of competing metal cations, and XRD pattern of oxidation product of Ti2CTx in air. The supporting information is available free of charge via the Internet at http//pubs.acs.org.

References (1) Yusan, S.; Akyil, S. Sorption of uranium(VI) from aqueous solutions by akaganeite. J. Hazard. Mater. 2008, 160, 388-395. (2) Stewart, B. D.; Mayes, M. A.; Fendorf, S. Impact of Uranyl-Calcium-Carbonato Complexes on Uranium(VI) Adsorption to Synthetic and Natural Sediments. Environ. Sci. Technol. 2010, 44, 928-934. (3) O'Loughlin, E. J.; Kelly, S. D.; Cook, R. E.; Csencsits, R.; Kemner, K. M. Reduction of Uranium(VI) by mixed iron(II/iron(III) hydroxide (green rust):


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Figure captions:

Figure 1. (A) Schematic illustration of hydroxylated Ti2C. (B) powder XRD patterns of Ti2CTx and its precursor material Ti2AlC. (C–F) SEM image (C), Raman spectrum (D), EDS results (E) and zeta potentials versus solution pH (F) of Ti2CTx particles.

Figure 2. U(VI) removal from aqueous solution by multilayered Ti2CTx as a function of pH (A), contact time (B and C) and initial U(VI) concentration (D). Detailed experimental parameters: (A) msorbent/Vsolution = 0.1 g L-1, C0= 33.7 mg L-1; (B-D) pH = 3.0±0.1, msorbent/Vsolution = 0.4 g L-1, C0 = 5~400 mg L-1, I = 0.01mol L-1 NaCl.

Figure 3. U L3 edge XANES spectra of U-loaded samples and U(IV) and U(VI) references.

Figure 4. (A) U L 3 edge k3-weighted EXAFS spectra (solid lines) and the best theoretical fits (dots lines) of U-loaded samples under different solution pH. (B) Corresponding non-phase shift corrected Fourier transforms. (a) S-Ti2CTx, pH 3.0; (b) M-Ti2CTx, pH 3.0; (c) M-Ti2CTx, pH 5.0; (d) M-Ti2CTx, pH 8.0; (e) UO2 reference.

Figure 5. SEM images (A-D) and XRD patterns (E) of U-loaded Ti2CTx MXenes. (A) S-Ti2CTx, pH 3.0; (B) M-Ti2CTx, pH 3.0; (C) M-Ti2CTx, pH 5.0; (D) M-Ti2CTx, pH 8.0.

Figure 6. XPS spectra of multilayered Ti2CTx before and after U(VI) removal. (A) Survey

scan; (B) High-resolution U 4f region; (C) High-resolution Ti 2p region; (D) High-resolution C 1s region.

Figure 7. Treatment of simulated U(VI) containing acidic mine wastewater as a function of time under anaerobic or aerobic conditions. C0 = 20 mg L-1, msorbent/Vsolution = 1 g L-1. C1=10.8 mmol L-1 Al2(SO4)3, 1.08 mmol L-1 Ca(NO3)2, 0.72 mmol L-1 MgSO4, 2.88 mmol L-1 Na2SO4



and 2.88 mmol L-1 (NH4)2SO4. [Fe3+] = 0.8 mmol L-1.


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Figure 1



Figure 2


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Figure 3

Figure 4



Figure 5


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Figure 6



Figure 7


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Table 1. Fitting parameters extracted from least-squares fitting analysis of EXAFS spectra. sample

path

CNa

R(Å)b

∆E(eV)c σ2(Å2)d

R-factore

S-Ti2CTx-pH3.0

U-O

9.2±0.9

2.32±0.01

1.7

0.018

U-Ti

1.2±0.5

3.37±0.02

U-O

9.8±1.0

2.33±0.01

U-Ti

1.1±0.4

3.37±0.02

U-O

8.9±0.8

2.33±0.01

U-Ti

0.6±0.4

3.37±0.03

0.01

U-U

1.4±1.0

3.83±0.02

0.007

U-O

7.8±0.6

2.33±0.01

U-U

2.2±0.7

3.84±0.03

U-O

8f

2.36±0.01

U-U

12f

3.88±0.01

M-Ti2CTx-pH3.0

M-Ti2CTx-pH5.0

M-Ti2CTx-pH8.0

UO2

0.014 0.008

1.9

0.016

0.016

0.009 2.5

3.1

0.014

0.012

0.01

0.007

0.004 5.5

a

0.011

0.01

0.006

Coordination number. bRadial distance. cEnergy shift relative to the calculated Fermi level. dDebye-Waller factor. eGoodness-of-fit indicator. fFixed during fitting.



Table of Content:


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Figure 1 87x52mm (300 x 300 DPI)



Figure 2 124x103mm (300 x 300 DPI)


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Figure 3 189x246mm (300 x 300 DPI)



Figure 4 93x58mm (300 x 300 DPI)


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Figure 5 126x117mm (300 x 300 DPI)



Figure 6 119x95mm (300 x 300 DPI)


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Figure 7 216x184mm (300 x 300 DPI)



Table of Content 31x11mm (300 x 300 DPI)


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Efficient U(VI) Reduction and Sequestration by Ti2CTx MXene

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