Sorption properties of U(VI) and Th(IV) on two-dimensional Molybdenum Disulfide (MoS2 ) nanosheets: Effects of pH, ionic strength, contact time, humic acids and temperature

Environmental Technology & Innovation 11 (2018) 328–338

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Environmental Technology & Innovation journal homepage: www.elsevier.com/locate/eti

Sorption properties of U(VI) and Th(IV) on two-dimensional Molybdenum Disulfide (MoS2 ) nanosheets: Effects of pH, ionic strength, contact time, humic acids and temperature Xue Li a , Qian Li a , Wensheng Linghu a , Runpu Shen a , Baoshan Zhao a , Lijia Dong a, *, Ahmed Alsaedi b , Tasawar Hayat b,c , Jin Wang d , Juan Liu d a

College of Life Science, College of Yuanpei, School of Chemistry and Chemical Engineering, Shaoxing University, Zhejiang 312000, PR China b NAAM Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia c Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan d College of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, China

highlights • • • • •

U(VI)/Th(IV) sorption is obviously affected by pH but not by ionic strength. Inner-sphere surface complexation dominated the two sorption processes. The removal efficiency of Th(IV) by MoS 2 is higher than that of U(VI). The presence of cations/anions weakly influences the adsorption. The U(VI)/Th(IV) sorption process is spontaneous and endothermic.

article

info

Article history: Received 28 October 2017 Received in revised form 14 May 2018 Accepted 3 June 2018

Keywords: MoS2 U(VI) Th(IV) Sorption HA Thermodynamic parameters

*

Corresponding author. E-mail address: [email protected] (L. Dong).

https://doi.org/10.1016/j.eti.2018.06.001 2352-1864/© 2018 Elsevier B.V. All rights reserved.

a b s t r a c t Herein, the two-dimensional MoS2 nanosheet was characterized and its performance as an adsorbent for U(VI) and Th(IV) removal from water was evaluated in batch experiments. The results showed that MoS2 nanosheets have a thin thickness and few negligible impurities. Both U(VI) and Th(IV) sorption on MoS2 were obviously enhanced by high pH and the oxygen-containing group on the surface of MoS2 increased with an increase of pH. However, the sorption was not affected by ionic strength, suggesting that inner-sphere surface complexation dominated the two sorption processes. The removal efficiency of Th(IV) by MoS2 was higher than that of U(VI) in solutions containing different cations and anions. The adsorption equilibrium for U(VI)/Th(IV) was achieved within 2 h and pseudo-second-order equation showed favorable fit. HA significantly increased the uptake of U(VI)/Th(IV) at pH <7.5/6, while decreased the sorption at pH >7.5/6. Freundlich model provided more fit to the equilibrium data than Langmuir model at pH 5.5. The maximum sorption amount of MoS2 for U(VI)/Th(IV) run up to 492.72/454.72 mg·g−1 , obviously higher than the amounts of some other materials. The U(VI)/Th(IV) sorption was a spontaneous and endothermic process. All these results suggested the potential applicable value of MoS2 nanosheets in removing U(VI) or Th(IV) from aqueous solutions. © 2018 Elsevier B.V. All rights reserved.

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1. Introduction Water contamination induced by heavy metals is increasingly focused on owing to their toxicity and harm to environment and organisms. Up to now, many technologies have been developed to remove these toxic metal ions from aqueous solutions (Hu et al., 2016). Hereinto, adsorption is considered as an effective and promising water-treatment technique with low cost, ease of operation, flexibility, simple design (Shao et al., 2009; Sun et al., 2015, 2016, 2017; Yao et al., 2018; Yu et al., 2018). Among various adsorbents, functional nanomaterials are extensively used for the sorption of metal ions, such as carbon material (carbon nanotube, carbon nanofibers, graphene oxide), titanate material (titanate nanotube), and magnetic material (Fe3 O4 ) (Zhao et al., 2014; Sun et al., 2015, 2016; Hu et al., 2017a; Yao et al., 2017; Chen et al., 2018). However, there is still room for exploring higher efficient adsorbents. More recently, molybdenum disulfide (MoS2 ) nanosheets, as a two-dimensional (2D) material, have received widespread attention as an alternative to graphitic carbon owing to its high-energy capacity (∼1200 mAh g−1 ) (Chang and Chen, 2011a,b; Chang et al., 2011; Xiao et al., 2011), which is much higher than that of commercial graphite (372 mAh g−1 ) (Cahen et al., 2008) and graphene nanosheets (600–900 mAh g−1 ) (Geim and Novoselov, 2007; Yoo et al., 2008; Wang et al., 2010). Furthermore, considering the abundance of exposed sulfur, MoS2 could be an efficient adsorbent for the removal of metal ions though its strong Lewis acid and base soft–soft interactions (Wang and Mi, 2017). Several cases have proved the adsorption potentials of MoS2 . For example, MoS2 was proved to be a kind of superb adsorbent for removing Hg2+ from water (Jia et al., 2017a). Some modified composite based on MoS2 also showed high sorption ability for several metals, including Ag+ , Co2+ , Pb2+ , Li+ , even and Hg0 vapor and so on (Gash et al., 1998; Chen et al., 2013; Zhang et al., 2016a; Zhao et al., 2016; Aghagoli et al., 2017). Nevertheless, interactions between MoS2 and other more toxic metal ions in aqueous solutions need to be studied to illustrate the sorption ability and related mechanism of these MoS2 nanosheets. Generally, the mechanism of the interaction between MoS2 and adsorbed heavy metal ions was proved to have three types. Ion exchange is the first possible mechanism because MoS2 nanosheets usually exhibit negative surface charge with H+ or Li+ as a counterion and finally form metal-sulfur bonding (Zhi et al., 2016; Jia et al., 2017b). Inner layer metal-S complexation is the second potential mechanism. One case showed that Hg2+ could replace H+ ions to complex with one or two sulfur atoms at a high or low Hg-to-MoS2 ratio (Ai et al., 2016). Pb2+ was also found the similar complexation phenomenon with sulfur atoms (Chen et al., 2013). The third potential mechanism is outer layer electrostatic attraction. Co2+ was found that the outer layer electrostatic attraction was dominant in the sorption process (Aghagoli et al., 2017). It is note that there may be one or more mechanisms to occur in the sorption. As it is known that the sorption capability of an adsorbent can be strongly affected by the chemical and physical conditions of aqueous solutions. Various environmental factors, such as pH, ionic strength, contact time, solid contents, and all kinds of coexistent ions, can influence the sorption ability, the kinetics and the isotherms of the adsorbent. Therefore, it is necessary to investigate these effects in the future to probe into the interaction mechanisms between toxic metal ions and MoS2 nanosheets. In this paper, we explored the macroscopic and microscopic characteristics of MoS2 nanosheets and its application in uptake and removal of two poisonous radionuclides from aqueous solutions, i.e., Uranium (U) and Thorium (Th). In practice, both uranium and thorium are often discharged into water environment, and then may enter the biosphere with the water cycle. Once it enters the living bodies provoke the inner irradiation, and result in appearance of cancer (Sheng et al., 2008, 2014). Therefore, it is meaningful to investigate both the two metals interactions with the 2D MoS2 nanosheets. Here, SEM, TEM, AFM, Zeta potentials, XRD, FT-IR, and EDS were used to characterize the adsorbent, i.e., MoS2 nanosheets. And in batch experiments, the adsorption kinetics, isotherms, thermodynamics and effects of solution pH, ionic strength, solid content, coexisting ions on the sorption of both U and Th were evaluated. This study firstly checks the sorption capacity of MoS2 nanosheets for U/Th, and explores interaction theory between them. The results will provide the theoretical foundation for the further application of MoS2 in removing radionuclides from waste water. 2. Materials and methods 2.1. Preparation of materials All chemical reagents used in this research were analytical-grade from Nanjing XFNANO Materials Tech. Co. Ltd. and without further purification. The specific method of synthesis of MoS2 was as the following procedure. The sodium molybdate (90 mg Na2 MoO4 ·2H2 O) and thioacetamide (180 mg C2 H5 NS) were dissolved in deionized water (60 mL) and formed a transparent mixed solution. Then the solution was transferred into Teflon-lined stainless-steel autoclave (100 mL) and heated at 240 ◦ C for 24 h. After being cooling and centrifugation, the products were repeatedly washed with deionized water and 100% ethanol for several times to remove impurities. Finally, the MoS2 nanosheets were got after drying in a vacuum oven at 60 ◦ C for 12 h. And the samples were preserved until being used. The UO2 (NO3 )2 ·6H2 O was dissolved into distilled water under nitrogen gas conditions to obtain the stock solution of U(VI). A stock solution of Th(NO3 )4 was prepared by dissolving ThO2 in HNO3 . Both stock solutions were diluted to the required concentrations. Humic acid (HA) was extracted from the soil in Hua-Jia county, Gansu province, China. Its physicochemical property has been fully studied previously and it was proved to have strong interaction with adsorbate and adsorbent (Reiller et al., 2005; Sheng et al., 2010).

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2.2. Characterization TEM image of the MoS2 sample was obtained using a transmission electron microscope (JEM-1011, Japan) instrument. SEM and EDS measurements and elemental distribution mappings were carried out using a field emission scanning electron microscope (JSM-6360LV, Japan). Atomic force microscopy (AFM) samples were prepared by drop casting the MoS2 suspension in ethanol onto freshly cleaved mica surfaces and dried under room temperature. AFM images were recorded using Multimode-V microscope in contact mode. The powder X-ray diffraction (XRD) analysis was performed with Cu Kα radiation (λ = 0.154 nm) on a Rigaku X-ray diffractometer. The XRD pattern was identified by comparison to the JCPD standards. To locate Zeta potential, the suspension of MoS2 and NaClO4 was adjusted to an appropriate pH value by adding negligible amount of HClO4 or NaOH solutions, and then was measured by a Zeta potential analyzer (Zetasizer Nano ZS, Malvern Co., UK). The FT-IR spectra of MoS2 was recorded with a FT-IR spectrometer (NEXUS, America) in the range of wavelength 4000–400 cm−1 to characterize its surface functional groups. 2.3. Adsorption experiments The sorption experiments of U(VI)/Th(IV) on MoS2 were carried out at three temperatures (293 K, 313 K, and 333K). Specifically, the stock suspensions of U(VI) or Th(IV), MoS2 , NaClO4 , KClO4 , LiClO4 , NaNO3 , NaCl, HA, and distilled water were mixed in polyethylene tubes in order to get the desired concentrations. The ratio of the MoS2 amount to the solution volume was set as 0.30 g L−1 . The pH of sorption solutions was mediated through adding 0.01 or 0.1 M NaOH or HClO4 solutions that was negligible in volumes. After the suspensions were stirred for 24 h, the solid and liquid phases were separated by centrifugation at 9000 rpm for 30 min. It needs to note that the sorption of both metal ions on the tube wall was proved to be negligible according to the tests. The concentration of U(VI) or Th(IV) in the supernatant was tested by spectrophotometry at wavelength 670 nm using U(VI)-chlorophosphonazo(III) complex or at wavelength 650 nm using the Th-arsenazo(III) complex. The percentage of U(VI) or Th(IV) sorption, distribution coefficient (Kd ), and sorption amount (qe ) onto MoS2 were calculated from the following equations: U(VI) or Th(IV) sorption % = (C0 − Ce )/C0 × 100%

(1)

Kd = (C0 − Ce )/Ce × V/m

(2)

qe = (C0 − Ce ) × V/m

(3) −1

where C0 represents the initial U(VI)/Th(IV) concentration (mg L ), Ce is the final or equilibrium U(VI)/Th(IV) concentration (mg L−1 ), m and V is the MoS2 mass (g) and the suspension volume (L), respectively. All experimental data were subjected to the averages of duplicate or triplicate experiments. 3. Results and discussion 3.1. Characterization of MoS2 nanosheets The physicochemical properties and surface structure of MoS2 play an important role in understanding the interaction mechanism between heavy metals and MoS2 . Here, SEM, TEM, AFM, Zeta potentials, XRD, FT-IR, and EDS were test to characterize MoS2 nanosheets. The SEM image (Fig. 1a) shows that the MoS2 has a graphite exfoliation-like layered structure, illustrating that the MoS2 is a typical 2D structure containing small flat sheets and irregular wrinkles. The TEM image of MoS2 sample is shown in Fig. 1b. The overlapped layer structure is observed and each layer is visible to light, which indicates the thin thickness of MoS2 nanosheets (Splendiani et al., 2010; Coleman et al., 2011; Yin et al., 2017). The representative AFM image and corresponding cross-section analysis of MoS2 nanosheets (inset) are shown in Fig. 1c. The AFM image illustrates that most MoS2 nanosheets have a thin thickness. The height profiles illustrate that the existence of MoS2 stacks with an average thickness of ∼2.0 nm, which implies the presence of a few layers of MoS2 nanosheets. Fig. 1d shows that the zeta potential of MoS2 samples as a function of medium pH, which indicates that the point of zero charge (pHpzc ) was ∼3.75. From Fig. 1e, one can see that the most intense diffraction peaks at 2θ = ∼14◦ , demonstrating the strong crystalline structure of MoS2 . Fig. 1f shows that the FT-IR spectrum, which represents functional groups on MoS2 . The peak at ∼3450 cm−1 is assigned to the stretching vibration of Mo-OH groups. The peaks at ∼1650 and ∼1400 cm−1 are subjected to the bend vibration of MoS2 water. Fig. 2 shows SEM elemental mapping analysis, EDS results, and elemental distribution mapping of MoS2 samples. As shown in Fig. 2a & b, it is obvious that the MoS2 sample contain some non-target elements (Al, Cu, and Zn). However, compared with the elemental distribution percentage of Mo and S atoms (∼92.23%), the percentage of Al, Cu, and Zn atoms is far lower (∼7.77%). The result indicates that MoS2 samples used in this experiment contains negligible impurities, which will have few influences on the sorption system. Interestingly, there are significant differences among elemental distribution percentage of the five atoms. For example, the distribution percentage of Mo atom (58.2%) is obviously larger than the one of S atom (34.0%). The distribution percentages of Cu atom (4.5%) and Zn atom (3.1%) are larger than the one of Al atom (0.2%). However, all the five atoms show homogeneous distribution.

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Fig. 1. SEM image (a), TEM image (b), height cross-section profile (inset) and corresponding AFM image (c), Zeta potentials (d), XRD (e), and FT-IR spectrum (f) of MoS2 samples.

3.2. Effects of pH, ionic strength, solid content, and co-existing ions Fig. 3a & b illustrate the pH and ionic strength effects on U(VI) and Th(IV) sorption onto MoS2 , respectively. The responses of U(VI) and Th(IV) to pH and ionic strength are similar. Their sorption capabilities dramatically increased before pH 7.0 (U(VI)) or pH 6.0 (Th(IV)) and slightly with further increases in pH. This phenomenon may be attributed to electrostatic attraction. The surface of MoS2 was generally negative and the zeta potential decreased with an increase of pH (Fig. 1d). Thus, the increase negative charge reduces to a stronger electrostatic interaction between MoS2 and U(VI) and Th(IV), which enhances more UO22+ or Th4+ to be adsorbed by the active sites on the MoS2 surface. Besides, due to the competition between UO22+ or Th4+ and excess H+ presented in solution at low pH, the sorption capacity is low. When the pH increases to a high 2− 4− 4+ value, UO22+ gradually formed (UO2 )3 (OH)− gradually formed Th(OH)4 7 , UO2 (CO3 )2 , or UO2 (CO3 )3 (at pH > ∼7.0), or Th precipitation (at pH > ∼6.0) (Zhang et al., 2016b; Dong et al., 2017; Wang et al., 2017). These anions and precipitation are difficult to be retained on the negatively charged surfaces of sorbents because of electrostatic repulsion. Thereby, there is the slight increase of the adsorption at high pH values. In order to further explore the effects of pH on the elemental distribution in the sorption system, and based on the similar reflections of U(VI) and Th(IV) to solution pH, only SEM elemental mapping analysis and EDS spectrum of Th(IV)-MoS2 is displayed in Fig. 4. As shown in Fig. 4a–c, more Th(IV) is adsorbed onto the surface of MoS2 with an increase of pH. Fig. 4d–f demonstrates that the distribution percentages of Mo and S gradually decrease with the three-increasing pH (Mo: 54.2%– 39.7%, S: 28.8%–22.5%), while the distribution percentages of Th and O increase (Th: 2.6%–15.5%, O: 14.5%–22.9%). These changes of element content with increasing pH are line with EDS element mappings, which also show similar tendency.

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Fig. 2. Special SEM image (a), EDS results (b) and EDS element mapping (S, Mo, Al, Cu, and Zn panels) of MoS2 samples. EDP: Elemental distribution percentage.

These results suggest that the oxygen-containing functional groups for MoS2 increase at high pH values, which may be another reason for high removal percentage of Th(IV) at higher solution pH values. It is well known that if the sorption is strongly affected by pH but not by the ionic strength, the sorption process dominated by inner-sphere surface complexation; the opposite situation implies that the sorption mainly dominated by outer-sphere surface complexation or ion exchange (Hu et al., 2017b; Chen et al., 2018). As shown in Fig. 3a & b, U(VI) or Th(IV) sorption is no dependent on ionic strength, implying that the sorption of them on MoS2 is mainly dominated by inner-sphere surface complexation. Fig. 3c shows the sorption of U(VI) and Th(IV) on MoS2 . The uptake proceeds at a high rate in the first 2 h and reaches a removal percentage of ∼115% for U(VI) and ∼140% for Th(IV) after 2 h. This result indicates that MoS2 is high efficient in removing both metal ions from aqueous solutions, and the removal efficiency of Th(IV) is higher than that of U(VI). It was generally regarded that the adsorption kinetic of metal ion at the solid/water interfaces could be well described by the pseudo-second-order kinetic model (Wang et al., 2016; Yao et al., 2018), therefore, the model was used to fit the adsorption of U(VI) and Th(IV) on MoS2 . The linear form of the pseudo-second order kinetic is calculated as follows (Ho and McKay, 1999): t qt

=

1 2kq2e

+

t qe

(4)

where qt (mg g−1 ) represents the sorption capacities of U(VI) or Th(IV) at time t (h), k (g (mg h)−1 ) is the rate constant of this model. The linear plot of t /qt versus t of U(VI) or Th(IV) is shown in Fig. 3d. The sorption rate of Th(IV) becomes significantly higher than the one of U(VI) with an increase of reaction time, which suggests that the efficiency of Th(IV) sorption by MoS2 is higher than the one of U(VI). The corresponding sorption kinetics parameters are summarized in Table S1. From this table, we can see that the sorption kinetics of U(VI) or Th(IV) on MoS2 can be well-described by the pseudo-second-order kinetics model due to both the two higher correlation coefficients (nearly close to 1). This model is generally based on the assumption that the rate-limiting step may be chemical sorption or chemisorption involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate (Ho and McKay, 2000; Jia et al., 2017a). Thereby, the interaction between U(VI) or Th(IV) and MoS2 may be dominated by chemical process. Fig. 3e shows the effect of solid content on the captured capacity and sorption percentage of U(VI) or Th(IV) on MoS2 nanosheets. At lower solid dosage, the MoS2 particles disperse well in the solution, which makes all of surface sites are completely exposed for the two ions binding, leading to a higher captured capacity of MoS2 . Compared with U(VI), the Th(IV) has lower valence, which may result in higher captured capacity of MoS2 for Th(IV). However, with the increase of solid content, the particles of MoS2 tend to collide with each other, causing the formation of aggregation and the decrease

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Fig. 3. Effects of pH and ionic strength on U(VI) (a) and Th(IV) (b) removal onto MoS2 (U(VI) or Th(IV) initial concentration = 50 mg L−1 , m/V = 0.3 g L−1 , adsorption time = 15 h, T = 293 K); U(VI) and Th(IV) removal on MoS2 as a function of contact time (c) and the fitting of pseudo-second-order kinetic model (d) (U(VI) or Th(IV) initial concentration = 50 mg L−1 , m/V = 0.3 g L−1 , pH = 5.5 ± 0.1, I = 0.01 M NaCl, T = 293 K); U(VI) and Th(IV) removal on MoS2 as a function of solid content (e) (U(VI) or Th(IV) initial concentration = 50 mg L−1 , adsorption time = 15 h, pH = 5.5 ± 0.1, I = 0.01 M NaCl, T = 293 K); Effect of co-existing ions on U(VI) and Th(IV) removal onto MoS2 (f) (U(VI) or Th(IV) initial concentration = 50 mg L−1 , m/V = 0.3 g L−1 , adsorption time = 15 h, pH = 5.5 ± 0.1, I = 0.01 M, T = 293 K).

of the particle disperse in solution. This phenomenon may reduce the availability of binding sites so as to the decrease of the captured capacity of MoS2 for U(VI)/Th(IV) (Yang et al., 2010). As shown in Fig. 3e, the removal percentage increases from ∼30% to ∼140% as the solid content increases from 0.1 to 1.0 g L−1 . The higher total number of surface sites for the two metal ions binding results in an increase of their removal percentage at higher solid dosage. It is noted that the removal efficiency of MoS2 towards Th(IV) maintains stable at m/V > 0.8 g L−1 , which is different from U(VI). Considering reducing effluent purification cost, the optimal content for MoS2 to decontaminate U(VI) or Th(IV) at pH 5.5 is 1.0 or 0.8 g L−1 . Fig. 3f illustrates the removal percentage for U(VI) or Th(IV) as a function of different co-existing ions. For the removal percentage of U(VI)/Th(IV), there are no significant differences among the three electrolyte cations (K+ , Mg2+ , Na+ ) or anions − (Cl− , ClO− 4 , NO3 ), which suggests that the sorption of U(VI) or Th(IV) on MoS2 is chemical sorption/inner-sphere surface complexation rather than ion exchange/ physical sorption. Furthermore, the removal percentages of Th(IV) on MoS2 in the

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Fig. 4. Special SEM images of Th(IV)-MoS2 in three pH values (a, b, c), EDS results in three pH values (d, e, f) and EDS element mappings (O, S, Mo, and Th panels) of Th(IV)-MoS2 in three pH values.

conditions of different co-existing ions are significantly higher than ones of U(VI), which is consistent with the effects of contact time. 3.3. Effects of HA, sorption isotherms and thermodynamic study In general, HA has strong binding capacity and high reactivity with heavy metal ions, and thus controls the transformation, migration behaviors and biological effectiveness of heavy metal ions in environment (Zhao et al., 2012). Fig. 5a & b show that the role of HA in the captured capacity of U(VI)/Th(IV) on MoS2 as a function of solution pH. It can be seen that the presence of HA enhances the uptake of U(VI)/Th(IV) at pH < 7.5/6, while inhibits their uptake at pH > 7.5/6. In fact, only a small fraction of uptake groups on HA is free to interact with metal ions due to its macromolecular structure (Strathmann and Myneni, 2005). Furthermore, the capacity of complexation between U(VI)/Th(IV) and HA is stronger than that between U(VI)/Th(IV) and MoS2 . Therefore, at low pH values, the negatively HA is liable to adsorb on the positively charged surfaces of MoS2 due to electrostatic attraction and finally enhances the U(VI)/Th(IV) sorption at pH < 7.5/6. On the contrary, at high pH values, because the electrostatic repulsion occurs between the negatively charged HA and MoS2 , the HA in solution can form dissolved complexes of HA-U(VI)/Th(IV), leading to the decrease of U(VI)/Th(IV) on MoS2 at pH > 7.5/6. Fig. 5c & d illustrate the sorption amount of U(VI)/Th(IV) at three temperatures. As a general rule, the analysis of the isotherms can provide the most important parameter for designing a desired sorption system (Sheng et al., 2014). From the figures, it is clear that the sorption isotherm for the two metal ions is the highest at 333 K and lowest at 293 K, indicating that both U(VI) and Th(IV) uptake are enhanced at a higher temperature. The result may connect with the easily dissolved properties at higher temperature (Zong et al., 2013; Wang et al., 2016). Two equilibrium models, i.e., Langmuir and Freundlich isotherm equations were selected to describe their sorption process and to explore the related mechanism. Linear form of the two isotherm models can be expressed as follows: Ce

=

1

+

Ce

(5)

qe KL qmax qmax log qe = log kF + n log Ce −1

(6) −1

where qmax (mol g ) is the U(VI)/Th(IV) maximum sorption capacity on per weight unit of MoS2 ; KL (L mol ) is the Langmuir affinity parameter; KF (mol1−n Ln g−1 ) and n represent the Freundlich affinity-capacity parameter and exponent, respectively. As shown in Fig. 5e, f, g, & h, the uptake isotherms of U(VI) or Th(IV) on MoS2 at three temperatures (293 K, 313 K, and 333 K) are simulated by linear Langmuir model and Freundlich model, respectively. The related parameters calculated from the two models are listed in Table S2. According to the values of regression correlation coefficients (R2 ), it is concluded that

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Fig. 5. The role of HA in the captured capacities of U(VI) (a) and Th(IV) (b) on MoS2 as a function of pH (U(VI) or Th(IV) initial concentration = 50 mg L−1 , m/V = 0.3 g L−1 , adsorption time = 15 h, I = 0.01 M NaCl, T = 293 K). Adsorption isotherms at three temperatures and the effect of HA on sorption amount at T = 293 K of U(VI) (c) and Th(IV) (d) on MoS2 ; fitting results of Langmuir (e, f) and Freundlich (g, h) sorption isotherms of U(VI) and Th(IV) sorption on MoS2 at three temperatures. Here, U(VI) or Th(IV) initial concentration = 50 mg L−1 , m/V = 0.3 g L−1 , adsorption time = 15 h, pH = 5.5 ± 0.1, I = 0.01 M NaCl.

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Fig. 6. Linear plots of ln Kd versus Ce for U(VI) (a) and Th(IV) (b), and linear regression plots of ∆ G0 versus T for U(VI) (c) and Th(IV) (d) sorption on MoS2 at three temperatures. Adsorption time = 15 h, m/V = 0.3 g L−1 , pH = 5.5 ± 0.1, I = 0.01 M NaCl.

the Freundlich isotherm models fit the sorption data of U(VI) or Th(IV) on MoS2 better than the Langmuir isotherm models. The result indicates that the uptake process occurs at the heterogeneous surface of MoS2 , which may be determined by the distribution of sulfur atoms (Wang and Mi, 2017). Simultaneously, since an exponentially increasing sorption is usually assumed in Freundlich model (Chen and Wang, 2007), the sorption capacity of MoS2 for U(VI)/Th(IV) is predicted to be strong. Besides, the values of n obtained by the Freundlich model are lower than 1, implying that U(VI) or Th(IV) sorption is a nonlinear process taking place on MoS2 (Zhang et al., 2010). This phenomenon may be related with the special structure of interior sulfur atoms from MoS2 nanosheets (Wang and Mi, 2017). A practical comparison of the capacity of some materials for the scavenging of U(VI)/Th(IV) anions from aqueous solution has been made, and the results are shown in Table S3. It can be seen that the potentials of MoS2 nanosheets removal for U(VI)/Th(IV) is higher than that of most of reported materials. These findings show a potential application prospect of MoS2 nanosheets for scavenge heavy metal ions from wastewater in real work of environmental remediation. To further demonstrate the nature of the sorption process, the thermodynamic parameters of U(VI)/Th(IV) sorption on MoS2 , including the Gibbs free energy (∆G0 ), the enthalpy (∆H 0 ), and the entropy (∆S 0 ), were calculated as follows:

∆G0 = −RT ln K 0

(7)

ln K 0 = ∆S 0 /R − ∆H 0 /RT 0

(8) 0

where K is the distribution coefficient of uptake reaction, and the values of ln K are evaluated by plotting ln Kd versus Ce and extrapolating Ce to zero (Fig. 6a & b). The ∆S 0 is evaluated from the slope of linear plot of ∆G0 versus T (Fig. 6c & d). Then the thermodynamic parameters are listed in Table S4. The values of ∆G0 calculated by U(VI)/Th(IV) sorption isotherms decrease from −22.47/−24.13 to −27.33/−28.73 as temperature increases from 293 to 313 K. These negative values illustrate that the both the two uptake processes are spontaneous, and these values are less than -20, reflecting the chemical nature of the sorption (Bekci et al., 2007). The positive ∆H 0 informs that both the U(VI) and Th(IV) sorption reactions are endothermic. While the values of ∆H 0 are less than 40 kJ mol−1 , suggesting that there are physical forces existing in U(VI)/Th(IV) sorption process (Whitehouse, 1984). The positive ∆S 0 illustrates that U(VI)/Th(IV) sorption is an irreversible process, and the metal ions may arrange themselves in a randomly pattern on the surface of MoS2 nanosheets (Zhang et al., 2011). The thermodynamic analysis demonstrates that the U(VI)/Th(IV) sorption on MoS2 is an endothermic, spontaneous, and chemical dominated process.

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4. Conclusions In this work, two-dimensional MoS2 nanosheets were characterized and the sorption properties of U(VI) and Th(IV) on MoS2 were investigated, respectively. Most MoS2 nanosheets used in this experiment have a thin thickness and few negligible impurities. The sorption of U(VI)/Th(IV) on MoS2 was obviously affected by pH but not by ionic strength. The oxygen-containing group on MoS2 increased with an increase of pH. The sorption percentage of Th(IV) by MoS2 was higher than the one of U(VI). The adsorption equilibrium for U(VI)/Th(IV) was achieved within 2 h according to the kinetic studies and pseudo-second-order equation showed favorable fit with high R2 . The equilibrium data was satisfactorily simulated by Langmuir and Freundlich isotherm models. Freundlich model correlated better with the experimental data according to its higher value of R2 . The thermodynamic parameters implied the U(VI)/Th(IV) sorption process is spontaneous and endothermic. 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