Effective adsorption of uranyl ions with different MoS2-exposed surfaces in aqueous solution

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Effective Adsorption of Uranyl Ions with Different MoS2 -Exposed Surfaces in Aqueous Solution Yuhui Liu , Cheng Fang , Shuang zhang , Weihong Zhong , Qianglin Wei , Yingcai Wang , Ying Dai , Youqun Wang , Zhibin Zhang , Yunhai Liu PII: DOI: Reference:

S2468-0230(19)30588-7 https://doi.org/10.1016/j.surfin.2019.100409 SURFIN 100409

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Surfaces and Interfaces

Received date: Revised date: Accepted date:

13 October 2019 28 October 2019 17 November 2019

Please cite this article as: Yuhui Liu , Cheng Fang , Shuang zhang , Weihong Zhong , Qianglin Wei , Yingcai Wang , Ying Dai , Youqun Wang , Zhibin Zhang , Yunhai Liu , Effective Adsorption of Uranyl Ions with Different MoS2 -Exposed Surfaces in Aqueous Solution, Surfaces and Interfaces (2019), doi: https://doi.org/10.1016/j.surfin.2019.100409

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Highlights: 

The MoS2 nanosheets and nanoflowers were synthesized by molten salt electrolysis.

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The adsorption properties of the MoS2 materials including equilibrium isotherms, kinetics, the effects of pH and temperature have been investigated.

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The molecular-level surface chemistry was studied by the density functional theory (DFT) calculations.

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The U(VI) group was adhered onto the (002) surfaces of MoS2 by forming a U-S bond, and the molybdenum group improved the binding energy through U-S bond.

Effective Adsorption of Uranyl Ions with Different MoS2-Exposed Surfaces in Aqueous Solution Yuhui Liu1,3, Cheng Fang2, Shuang zhang1, Weihong Zhong1, Qianglin Wei3, Yingcai Wang1, Ying Dai1, Youqun Wang1, Zhibin Zhang1,, Yunhai Liu1, 1

State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang, 330013 Jiangxi, PR China 2

College of Science, East China University of Technology, Nanchang, 330013, Jiangxi, PR China

3

School of Nuclear Science and Engineering, East China University of Technology, Nanchang, 330013, China

Abstract An understanding of the impacts regarding on two-dimensional materials on radionuclide removal is indispensable, owing to the intrinsic structure of materials can affect its properties. In this work, MoS2 nanosheets and nanoflowers were prepared by molten salt electrolysis method. The distinct adsorption behaviors of U(VI) on MoS2 was theoretically and experimentally investigated. The adsorption properties of MoS2 material including equilibrium isotherms, kinetics and effects of pH and temperature were explored. Batch experiments results show that MoS2 nanosheets and nanoflowers achieved excellent U(VI) adsorption capacities, which were 45.7 and 37.1 mg/g at 298 K, respectively. According to the computational results, U(VI) have formed more stable complexes on MoS2. Interestingly, the molybdenum group could significantly improve the binding energy through U-S bond. Therefore, the MoS2 material has potential applications in the elimination of uranyl ion from radioactive wastewater. Keywords Molten salt electrolysis; Different MoS2-Exposed Surfaces; DFT; Uranium; 

Corresponding authors at: State Key Laboratory of Nuclear Resources and Environment, East China University of Technology,

Nanchang, Jiangxi 330013, PR China. E-mail addresses:[email protected] (Zhibin Zhang), [email protected] (Yunhai Liu). 

Adsorption 1. Introduction The treatment of uranium radioactive wastewater from mining operations and nuclear fuel cycle is of great significance to the sustainable development of nuclear power [1-3]. MoS2, as a special anisotropic layered compound, can form two-dimensional permeation channels, which are useful for ion adsorption and transport by the fine regulation of the preparation process [4-6]. In addition, the surface and edges of MoS2 are rich in sulfur groups. Compared to the adsorbents (i.e., oxygen-containing adsorbents, graphene oxides, metal-organic frameworks and nanoscale zerovalent iron) that have been well studied [7-9], the understanding of sulfur-containing adsorbents is very limited. However, sulfur-containing adsorbents on behalf of another group of adsorbents with strong binding ability toward uranyl ion via Lewis soft-soft interactions. This character can be considered to design sulfur-containing adsorbents for efficient uranyl ion removal. MoS 2 is a representative member of transition-metal dichalcogenides that distinguished with sulfur-rich surface and exhibits great potential for uranyl ion removal. MoS2 has good chemical and thermal stability, high dispersion and good hydrophilicity, which are conducive to the adsorption of U(VI). At present, inorganic adsorption materials have high comprehensive adsorption performance for radionuclides and organics [10-15]. Novel-nanomaterials have large enough oxygen-containing functional groups on its basal plane and on the edges in the form of epoxy, hydroxyl, and carboxyl groups [16-19]. However, there are certain issues such as low yield of MoS2, complex preparation process, low adsorption performance and lack of a mechanistic elaboration. Therefore, it is necessary to prepare MoS2 capable of high adsorption performance, rich in micromorphology and able to retain typical crystal structure by advanced modification. Song et al. [20] illustrated that Hg2+ has priority to react with the MoS2 surface. The surface oxidation occurs only when there are enough reactive sites for the simultaneous Hg2+ adsorption and surface oxidation. There are significant investigations reported for the development and utilization of MoS2-based nanomaterials as heavy metal adsorbents. Yuvaraja et al. [21] synthesized MoS2-MnFe2O4 and employed it for the reduction of

Cr(VI) in a microwave-induced catalytic system. Molybdenum disulfide is used for not only heavy metal adsorption but also the adsorption of radionuclides from aqueous solutions. Wang et al. [22] prepared MoS2 nanosheets in aqueous solution in 2018. MoS2 nanosheets were used as adsorbents to remove U(VI) from water. The adsorption equilibrium reached at pH=5.5 and t=2 h. The maximum adsorption capacity of molybdenum disulfide on U(VI) was 492.7 mg/g. When the pH was less than 7, the adsorption of U(VI) on MoS 2 increased with the increase of pH because MoS2 has negative charges on its surface and more U(VI) is then adsorbed on the active site of MoS2. When the pH increases to a certain value, UO22+ will gradually form (UO2)3(OH)7−, UO2(CO3)22- or UO2(CO3)34−. In 2018, Hua et al. [23] grafted diethyl phosphate and maleic anhydride onto molybdenum disulfide to adsorb U(VI). Under the conditions of pH=4 and temperature 298.15 K, the adsorption equilibrium was established within 60 min, and the maximum adsorption capacity reached 448.4 mg/g. Considering the special properties and wide application of MoS 2, researchers have conducted a significant amount of research on its preparation. There are some methods to prepare MoS2 with different sizes and morphologies, which can be roughly divided into physical and chemical methods. The common physical methods include the laser method [24] and arc method [25]. Chemical methods include the gas-phase method [26], liquid-phase method [27] and solid-phase method [28]. However, the traditional methods mentioned above have the shortcomings of long heating time and weak lattice degree. Therefore, we have developed a high-temperature electrochemical method, which has the advantages of low cost and high efficiency [29]. The molten salt electrolysis is becoming a well-established method. MS3 (molten salt shielded synthesis/sintering process) has been used for the synthesis of different ternary transition metal compounds (MAX phases, such as Ti3SiC23, Ti2 AlN4, MoAlB5), binary carbides (TiC) and the sintering of titanium [30]. The availability of high-quality powders, coupled with low energy requirements and cost savings, may eliminate one of the bottlenecks in the industrial applications. Zou et al. [31] prepared a two-dimensional transition metal carbide Ti2CTx by the treatment of Ti2AlC in molten fluoride salt. Based on our previous works [32, 33], the

structure, morphology and surfaces of MoS2 can be well-controlled by molten salt electrolysis. In this study, a molten salt electrolysis method was developed. The effect of interfacial potential difference on interlayer van der Waals force and dielectric relaxation effect promoted the formation of different MoS2 surfaces. The advantages of molten salt electrolysis are as follows: (i) The time of molten salt electrolysis is 1 hour, while that of hydrothermal method is 24 hours; (ii) Molybdenum disulfide with different crystal structure and micro morphology can be prepared by controlling current density, temperature and electrolyte; (iii) No further sintering or post-treatment is required. Far-reaching significance for different MoS2 surfaces to adsorb U(VI) in aqueous solution can be envisioned, and it is expected to expand the application of two-dimensional materials in the field of radionuclide treatment. 2. Experimental 2.1. Materials and synthesis All regents and solvents were analytical reagent grade and used without further purification. The U(IV) stock solution with a given concentration of 42 mmol L -1 (10 g/L) was prepared by dissolving specific UO2(NO3)2·6H2O (99.99%) into 250 mL Milli-Q water. MoS2 nanosheets and nanoflowers were prepared and used in this study. MoS2 nanosheets and nanoflowers prepared by galvanostatic electrolysis were carried out on Mo electrodes in LiCl-KCl-(NH4)6Mo7O24-KSCN melt. After electrolysis, the black precipitates were washed with distilled water and absolute ethanol and finally dried under vacuum at 60 C for 2 h. The synthesis process is shown in Scheme 1. 2.2. Characterization An Autolab potentiostat/galvanostat, which is controlled with the Nova 1.8 software package, was employed to collect all electrochemical measurements (Metrohm PGSTAT302N). The crystal structures of the samples were analyzed by XRD (Rigaku D/max-TTR-III diffractometer) using Cu-Kα radiation at 40 kV and 150 mA. The surface composition and valence state of MoS2 were recorded via high resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) (Thermo Axis Supra). X-ray source type was water-cooled Al target source. The maximum X-ray power of monochrome Al target was more than 450 W.

Monochromated was a large water-cooled quartz crystal with 500 mm Loran circle. The optimum energy resolution and sensitivity (counting intensity of C 1s) was less than 0.82 ev@60 kcps. SEM was used to analyze the microstructure of MoS2, and EDS (JSM-6480A; JEOL Co., Ltd) was used to analyze the microtone chemical of MoS2. TEM and HRTEM (America FEI, 300 kV) were carried out to analyze the crystal structure of MoS2. The specific surface areas of the MoS2 were obtained from Brunauer-Emmett-Teller (BET) measurements. The Zeta potential values were measured with a Particle Metrix flowing current potential analyzer (Stabino, Germany). 2.3. Computational methods To understand the adsorption mechanism of U(VI) with MoS 2 at the molecular level, the structures of U(VI)/MoS2 complexes were optimized using the density functional theory (DFT) [34, 35]. The calculations have been used to investigate the adsorption mechanism of uranyl ion onto MoS2 nanosheets by using the Vienna Ab initio Simulation Package (VASP) [36, 37]. The projector augmented wave (PAW) method [38] was employed to calculate the electron-electron interaction. The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) function was used to describe the electron exchange-correlation potential [39]. The cut-off energy for the plane-wave basis was set as 500 eV. All of the structures were fully relaxed until the difference of the total energy was less than 10-4 eV and the convergence criterion of force on each atom was 0.01 eV/Å. The Monkhorst-Pack k-points meshes of 3×3×1 were employed to sample the Brillouin zone for geometry. The lattice parameters of the unit cell were derived from the crystal structure of MoS2: a=b=3.16 Å, c=12.30 Å, α=β=90°, and γ=120°. The (002) surface was cleaved off, and a 15 Å vacuum layer was added above it. The 4×4×1 supercell of MoS2 was chosen as a computational model, and the valence electrons included Mo 3d, S 2p, U 4f and O 2s. The GGA+U (U=4.0) method was used to deal with the difficulty that the outermost f orbital electrons of U are in the extended and localized states [40]. The POSD and DOS were obtained for adsorbed sites of Mo 3d, S 2p, O 2s and U 4f orbitals for (001) facet MoS2 adsorption.

3. Results and discussion 3.1. Characterization of MoS2 samples The crystal structure of MoS2 was prepared by molten salt electrolysis. The morphologies of MoS2 nanosheets and nanoflowers were obtained by controlling the electrolytic parameters including temperature and current density. The influence of electrolysis process parameters on the formation of MoS 2 is described in detail (in S6 ESI†). The XRD results are shown in Figs. 1a and 1b. As seen in Fig. 1a, MoS2 nanosheets were exposed to (002) crystal surface. In addition, MoS2 nanoflowers were exposed to many crystal planes such as (002), (101) and (110). Representative SEM images of the products show that MoS2 particles possessed predominantly a unique surface as depicted in Fig. 2a. SEM analysis shows an average size of 500 nm and a thickness of ∼2 nm for the nanosheets of MoS2. Fig. 2b shows the EDS scan results, suggesting that Mo and S elements were well-distributed. In Figs. 2c and 2d, a high-resolution TEM image and its corresponding selected-area electron diffraction (SAED) pattern show the diffraction spot of the (002) zone atomic lattice spacing of ~0.62 nm [41]. Both the SAED pattern and HRTEM images reveal that the (002) MoS2 nanosheet has a crystalline structure of a phase with growth direction along the (001) zone axis. The images of the as-synthesized product suggest that the sample consisted of MoS2 nanosheets. In addition, the other sample was characterized by SEM-EDS, and the results show that the sample had MoS2 nanoflowers (Figs. 3a and 3b). The nanoflowers structure of MoS2 was confirmed by the ordered lattice fringes of 0.27 nm, which corresponded to the interlayer spacing of the (101) plane (Fig. 3c). The SAED patterns confirmed that the crystal can be indexed as a hexagonal MoS2 (Fig. 3d). The adsorption isotherms of MoS2 nanosheets and nanoflowers are of III type according to IUPAC classification [42]. Loops of this type are given by non-rigid aggregates of plate-like particles but also if the pore network consists of macropores which are not completely filled with pore condensate. The N2-BET surface areas of the MoS2 nanosheets and nanoflowers were measured as 2.817 and 9.166 m2/g, respectively (S7 in ESI†). The results show that the nanoflowers have larger specific surface area than nanosheets. At present, researchers tend to prepare adsorbents with

larger specific surface areas [43], mainly relying on increasing adsorption sites to efficiently adsorb U(VI) in waste. In addition, increasing the defects of MoS2 surface will also increase the adsorption capacity of U(VI) [20]. This is another differentiating feature between MoS2 nanosheets and nanoflowers. Furthermore, the oxidation states were elucidated from the XPS analysis. Figs. 4c and 4d show the XPS spectra of MoS2 nanosheets and nanoflowers, respectively. As shown in Fig. 4c, there were two strong peaks located at 229.8 eV and 232.9 eV, which can be attributed to Mo 3d 5/2 and Mo 3d3/2 binding energies, respectively, and these peaks indicate that the dominant oxidation state is Mo4+ [44]. In addition, the S 2s peak was detected at 227.1 eV. As shown in Fig. 4d, the peaks located at 162.6 eV and 163.8 eV can be indexed to S 2p3/2 and S 2p1/2 binding energies, respectively, which indicate that S2- is the dominant oxidation state [45]. 3.2. Effect of pH on the adsorption capacity Generally, the influence of the external acidity and alkalinity values mainly affects the surface charges of the adsorbents and the existing forms of the ions. Therefore, it is important to examine the relationship between the external pH value and adsorption capacity. According to the morphological distribution of uranyl ion hydrolysis, the acidity and basicity of the experiment are limited to pH less than 6.0. As shown in Fig. 5, the adsorption capacity of MoS2 nanosheets and nanoflowers increased with the increase of pH value when the pH was between 3.0 and 6.0. At pH 6.0, the maximum adsorption capacity was 45.7 mg·g-1 and 37.1 mg·g-1, respectively. In addition, the zeta potential of the MoS 2 nanosheets and MoS2 nanoflowers before and after adsorbing U(VI) were determined over a range of pH value from 2 to 8. As shown in Fig. 5c, the zeta potential of MoS2 nanosheets increased after adsorbing U(VI) ions as the pH increased, and the point of zero charge (pHpzc) was estimated to be approximately 2.91 and 2.07, respectively. The result in Fig. 5d reveals that the zeta potential of MoS2 nanoflowers after adsorbing U(VI) ions showed a pHpzc=2.03, which was less than that for MoS2 nanosheets. Considering these results with those shown in Figs. 5c and 5d, it can be concluded that the surface of MoS 2 is negatively charged. With the increase of pH, MoS2 is deprotonated and becomes favorable to binding to uranyl ions in the aqueous solution. Therefore, the adsorption capacity

increases with the increase of pH because the adsorption process is an electronic reforming process of different reactants through the adsorbent surface. Generally, higher absolute value of zeta potential may represent the greater electrostatic repulsion force between the particles with better physical stability [46]. The Zeta potential results show that the structural stability of MoS2 nanosheets was better than that of MoS2 nanoflowers. 3.3. Adsorption kinetics Kinetic studies were performed at room temperature and at pH 6.0 for U(VI). As presented in Figs. 6a-c, the adsorption rate of U(VI) on the nanosheets was faster than that on the nanoflowers within the first 30 min, and the adsorption equilibrium for the nanosheets and nanoflowers was reached after 120 and 180 min, respectively. At the initial stage, the fast adsorption rate could be attributed to the large number of active sites (S ions), which was mainly located on the outer surface of the adsorbent, and thus the adsorption process was completed in a short time ( 60 min). With increasing the adsorption amount of U(VI), the repulsive forces between the adsorbed species were enhanced, and the adsorption resistance against free U(VI) is intensified. The pseudo-first-order model and pseudo-second-order model were used to explain the adsorption mechanisms of U(VI) onto MoS2 nanosheets and MoS2 nanoflowers, respectively. The details of the equation are given in ESI†. Kinetic parameters from all kinetic models were calculated and listed in Table S1. According to the R2 values, the pseudo-second-order model fits the experimental kinetic data better than the pseudo-first-order model, suggesting that the adsorption of U(VI) onto the MoS 2 nanosheets and MoS2 nanoflowers is mainly controlled by a chemically reactive process. Consequently, kinetic analysis shows that U(VI) adsorption onto MoS2 nanosheets and nanoflowers could generally be divided into three stages [47]: (a) rapid adsorption on the surface of MoS2 nanosheets and nanoflowers (0-30 min), which is attributed to the introduced Mo and S groups and the adsorption process controlled by chemical adsorption; (b) slow adsorption rise (30-150 min) due to the limited active sites; and (c) adsorption-desorption balance (150-210 min). 3.4. Adsorption isotherms

The adsorption isotherms of U(VI) are presented in Figs. 6d and 6e. As the initial concentration of uranium increased from 10 mg L -1 to 80 mg L-1, the adsorption of U(VI) onto MoS2 nanosheets and nanoflowers increased until reached a level of saturation. The adsorption isotherms reveal the interactive behaviors between the adsorbent and adsorbate, which have been simulated by utilizing well-established fundamental models. The curve fitting methods of the Langmuir, Freundlich and Dubin-Radushkevich models were used, and the fitting parameters are summarized in Table S2. Details of various isotherm models can be explored in ESI†. Comparison of the regression coefficient (R2) of the three models in Table S2 shows that the Langmuir model provides a better fit with the adsorption data than the Freundlich and Dubin-Radushkevich models, which indicates a monolayer adsorption process of U(VI) onto the adsorbent. Based on the above-mentioned results, the MoS2 nanosheets and nanoflowers exhibit the potential to removal U(VI) from aqueous solutions with a maximum monolayer adsorption capacity of 47.7 and 43.0 mg·g-1, respectively. It is well understood that the extraction of U(VI) depends on the active functional groups (S and Mo) on the surfaces of MoS2 nanosheets as well as MoS2 nanoflowers. 3.5. Thermodynamics analysis The effect of temperature on the adsorption isotherm was investigated under isothermal conditions in the temperature range of 293.15-317.15 K and at pH 6.0 for U(VI). The van’t Hoff equation was applied to examine the spontaneity and energetics involved in the interaction between the MoS 2 and U(VI). Details of the equation are given in ESI†. The thermodynamic parameters were calculated from the intercepts and slopes of the plots in Figs. 6f-h, and the results are shown in Table S3. The negative values of △G indicate that the sorption of U(VI) onto the MoS 2 nanosheets and nanoflowers is spontaneous under the experimental conditions. The positive value of △H suggests that the sorption process of U(VI) onto both the MoS2 nanosheets and MoS2 nanoflowers is an endothermic reaction, and the positive value of △S indicates increased randomness at the MoS2/solution interface. The results show that the adsorption of U(VI) by MoS2 is a process of spontaneous endothermic entropy increase [48].

3.6. Adsorption mechanism The adsorption mechanism was analyzed by measuring the XPS spectra of MoS2 before and after adsorption (Fig. 7a), and all photoelectron and Auger peaks are labeled on these spectra. The C 1s peak indicated the contamination on these surfaces. This peak is used to reference all binding energies relative to C 1s at 285.0 eV. There are no spectral overlaps for the narrow O 1s, U 4f, Mo 3d, S 2p and Mo 4p peaks. A BE of about 973 eV showed that one of the O KLL Auger lines. XPS spectra wide scan for measuring the region of MoS2 nanosheets and nanoflowers after U(VI) adsorption are shown in S8 (ESI†). After adsorption, a broad-spectrum scan showed a peak of U 4f, suggesting that a large amount of uranyl ion binds to MoS2 nanosheets and nanoflowers (Fig. 7a, trace a and b). Furthermore, the XPS peak intensities of the U(VI) adsorbed onto MoS2 nanosheets were much stronger than the corresponding intensities for MoS2 nanoflowers, which indicated that the adsorbed amounts of U(VI) onto MoS2 nanosheets should be higher than those onto MoS2 nanoflowers. These results were agreed with the previous DFT calculations and experimental results. In addition, the high-resolution spectrum of U 4f has two peaks (U 4f7/2 at 382.59 eV and U 4f5/2 at 393.45 eV) [49-52], further indicating that the uranyl ion adsorbed on the surface of MoS2 nanosheets and nanoflowers is essentially U(VI). The valence state of uranium reabsorbed by narrow scan high resolution U 4f peak (fig. 7b) is U(VI)-X (X is the number of electrons obtained) [53]. The position for these peaks was given in Table S4. It can be seen from the table that the peaks shifted of S 2p and Mo 3d are lower after MoS2 adsorbs U(VI), and the transfer banding energy is about 3 eV (Figs. 7c and 7d). The MoS2 nanosheets is lower than MoS2 nanoflowers, and the transfer banding energy is about 1 eV. The shift of the peak of S 2p to lower binding energy may be due to the coordination of sulfur with uranyl ion. The peaks of Mo were also transferred to lower binding energies (Fig. 7d), which may be due to the formation of covalent bonds between molybdenum and uranyl ions. The valence bands on the low-energy side for MoS2 and MoS2 adsorption U(VI) (Fig. 7e) represented a more pronounced structure. The valence bands have been decomposed into five separate peaks. A similar decomposition has been used in the case of MoS2 [54]. Valence band transfer of 1.8 eV after U(VI) adsorption on MoS2 nanosheets. The MoS2 nanosheets

is 0.7 eV below the Fermi level. The charge transfer occurs within the surface layer of MoS2. It also implies that U(VI) does not interact with MoS2 much deeper than a surface layer. Accounting for the observed 1.8 eV shift, we find that the MoS2 still contains excess electrons. A more quantitative analysis of charge transfer effects has been addressed by computational studies [55,56]. To further investigate the adsorption mechanisms and reveal MoS2 in U(VI) removal, the (i) differential charge density (Δρ), (ii) Bader charge analysis and (iii) partial densities of states (PDOS) of adsorption configurations were analyzed. It is found that there is only partial charge transfer to the uranium adsorbates on MoS2. This conjecture is confirmed by DFT results. In the present work, the stable complex of uranyl ions and (002) MoS2 was optimized and is shown in Fig. 8a. The bond distance between U(VI) and (002) MoS2 was 2. 47 Å, and the bond length of U-S was 2.97 Å. To further understand the interaction between uranyl ion and MoS2 material, we took the optimized configuration as an example. The density of states (DOS) of Mo-3d, S-2s, U-4f and O-1s were calculated and are shown in Fig. 8b, and they can be seen from the overlap of the Fermi surfaces. The results of band analysis show that the adsorption is mainly caused by the bonding of U-4f electrons with S-2s electrons and Mo-3d electrons. Fig. 8b depicts that the DOS value of Mo (DOS=19.5) was larger than that of S (DOS=10.9), so the contribution of Mo is greater than that of S when UO 22+ was adsorbed. The valence of Mo and S will increase, the valence of Mo would probably change from +4 to +4.5, and the valence of S changed from -2 to -1.6, which would accommodate the transferred electrons together. In addition, transfer from MoS2 to UO22+ charge was about -0.93e (S9, ESI†). Specifically, adsorption complexes with an abundant electron transfer between the metal atom and S could result in higher metal binding energies. To more accurately judge the contribution of electronic orbits to the energy band, we carried out the partial density of states (PDOS) of Mo-3d, S-2s, S-2p and U-4f (shown in Fig. 8c). There was a large overlap between U (U-4f) and Mo-3d (or S-2p, S-2s) orbitals, indicating the strong chemisorption and the formation of U-S and U-Mo bonds between uranyl ion and MoS2. It is worth mentioning that Mo-3d contributes more chemisorption to uranyl ion than S-2p. To understand the complexity of U-f electrons, GGA+U algorithm was used for the calculation [57]. As shown in

the Fig.8(d), the results show that the U-f electrons are in the superposition of the localized and extended states, and the electrons at the E f are cruising electrons [58]. Taken together, our computational results reveal that the uranyl ion was adsorbed on the surface of the MoS2 by chemisorption, and the molybdenum group could significantly improve the binding energy through U-Mo bond formation. Therefore, the MoS2 material has the potential application in the field of elimination for uranyl ions. 4. Conclusion As we surmised, MoS2 nanosheets has shown superior properties compare to MoS2 nanoflowers, such as (i) higher adsorption capacity, (ii) ultrafast adsorption kinetics and (iii) great regeneration properties. MoS2 nanosheets could be efficiently conducted on U(VI) removal in pH range of 2.0-6.0. Both XPS and DFT calculations confirmed that sulfur atom was the active adsorption site and responsible for U(VI) adsorption. Moreover, the intrinsic mechanism was unraveled via PDOS analysis. The adsorption is mainly caused by the bonding of U-4f electrons with S-2s electrons. The Mo-3d electrons and Mo-3d may contribute more chemisorption to uranyl ion than S-2p. Therefore, it can be concluded that this work lays an experimental and theoretical foundation for the future explorations of molybdenum disulfide as an adsorbent material. Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21906019, 21906018, 21561002, 21866004, 21866003), the Science & Technology Support Program of Jiangxi Province (Grant NO. 2018ACB21007), the Jiangxi Program of Academic and Technical Leaders of Major Disciplines (Grant No. 20182BCB22011), the Project of the Jiangxi Provincial Department of Education (Grant No. GJJ160550, GJJ180385, GJJ180400). The authors declare that they have no competing interests.

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Scheme 1. Schematic illustration for the synthesis of MoS2 nanosheets and nanoflowers via the molten salt electrolysis. Fig. 1. XRD patterns of the (a) MoS2 nanosheets and (b) nanoflowers. Fig. 2. (a-b) SEM-EDS images; (c) HR-TEM; (d) SAED of the MoS2 nanosheets. Fig. 3. (a-b) SEM-EDS images; (c) HR-TEM; (d) SAED of the MoS2 nanoflowers. Fig. 4. (a-b) XPS spectra of Mo-3d spectra and S-2p spectra. Fig. 5. (a and c) Effect of pH and zeta potential values of MoS2 nanosheets; (b and d) MoS2 nanoflowers on the adsorption of U(VI). Fig. 6. (a-c) Adsorption kinetics curves; (d and e) adsorption isotherm curves; (f-h) thermodynamic curve of U(VI) on MoS2 nanosheets and nanoflowers. Fig. 7. XPS spectra wide scan of (a) MoS2 nanosheets and nanoflowers after U(VI) adsorption and (c) MoS2; (b) Mo-3d spectra; (c) S-2p spectra; (d) U-4f spectra. Fig. 8. (a) The most stable configuration of U(VI) adsorbed on the (002) facet MoS2; (b) the DOS of Mo-3d, S-2p, O-1s and U-4f orbitals for (002) facet MoS2 adsorption; (c) the PDOS of Mo-3d, S-2p, S-2s and U-4f orbitals for (002) facet MoS2 adsorption; (d) the f electrons of U(VI).

Scheme 1.

Fig. 1 (002)

(a) MoS2-nanosheets

(b) MoS2-nanoflowers

(002) (101)

(110) (103) (108) (105)

MoS2(PDF#04-0836)

10

20

30

40

50

2/degree

60

70

80

Fig. 2

Fig. 3

Fig.4

Mo 4+

a Mo 3d5/2 Intensity/a.u.

Mo 4+ Mo 3d3/2 S 2s

240

237

234

231

228

225

Binding Energy/ev

Intensity/a.u.

b

S 2p3/2

S 2p1/2

175

170

S 2-

S 2-

165

Binding Energy/ev

160

222

Fig.5 50

a 40

qe(mgg-1)

30

20

10

0 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

pH 40

b

35

qe(mgg-1)

30

25

20

15

10 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

pH

10

c

MoS2-nanosheets

MoS2

pHPZC =2.91

After adsorb

Zeta potential (mV)

0

-10 pHMoS =2.07 PZC 2

-20

-30

-40 2

3

4

5

6

pH

7

8

9

10

d

MoS2-nanoflowers MoS2

pH PZC =2.03

After adsorb

Zeta potential (mV)

0

-10

-20

-30

-40 2

3

4

5

6

pH

7

8

9

Fig.6 50

a 40

qe(mgg-1)

30

MoS2 nanosheets MoS2 nanoflowers

20

10

0

0

50

100

150

200

t/(min) 42

40

b

qe(mg/g)

38

pesudo first order model pesudo second order model

36

34

32

30 40

80

120

160

200

t(min) 45

c 40

qe(mg/g)

35

pesudo first order model pesudo second order model

30

25

20

15 40

80

120

t(min)

160

200

-8.0

d -8.5

lnqe

-9.0

-9.5

MoS2 nanoflowers MoS2 nanosheets

-10.0

-10.5

-11.0 400

450

500

550

600

650

2

e

50

q(mg/g)

40 30

MoS2 nanoflowers

20

MoS2 nanosheets Langmuir models Freundlich models

10 0 0

10

20

30

40

50

60

70

80

90

Ce(mg/L) 50

f

48 46

qe(mg/g)

44 42 40 38 36 290

295

300

305

T(K)

310

315

320

g

46

qe(mg/g)

44

42

40

38

36

34 290

295

300

305

310

315

320

T(K) 7.10 7.05

h

MoS2 nanoflowers MoS2 nanosheets

7.00

lnKd

6.95 6.90 6.85 6.80 6.75 6.70 3.1x10

-3

3.2x10

-3

3.3x10

1/T

-3

3.4x10

-3

3.4x10

-3

Mo 3d

Fig.7

S 2p

C 1s

U 4f

O KLL

O 1s

Mo 3p

a

Mo 4p

a b c 1200

1000

800

600

400

200

0

Binding Energy/ev

U

U 4f5/2 sat U 4f7/2 sat

U 4f5/2

U 4f7/2

b

a b

420

410

400

390

Binding Energy/ev

380

Mo 3d5/2

Mo

S 2s

Mo 3d3/2

c

a b c 238

236

234

232

230

228

226

224

222

220

S 2p3/2

Binding Energy/ev S

S 2p1/2

d

a b c 170

165

160

Binding Energy/ev

155

e

1.9

0.5 -0.7

11.2 a

b c

25

20

15

10

5

Binding Energy/ev

0

-5

Fig.8

110 100

b

Mo S U O

90

DOS (sattes/eV)

80 70 60 50 40 30 20 10 0 -3.0

-2.5

-2.0

-1.5

-1.0

Energy (eV)

-0.5

0.0

0.5

120 110

c

Mo 3d S 2p Ss U 4f

100 90

PDOS

80 70 60 50 40 30 20 10 0 -5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Energy (eV) 40 35

s p d f

d

30

U PDOS

f extended state 25 20 15 10

f local density of states

5 0 -5

-4

-3

-2

-1

Energy (eV)

0

1

2

3

Graphical Abstract