Simultaneous removal of U(VI) and Re(VII) by highly efficient functionalized ZIF-8 nanosheets adsorbent

Journal of Hazardous Materials 393 (2020) 122398

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Simultaneous removal of U(VI) and Re(VII) by highly efficient functionalized ZIF-8 nanosheets adsorbent

T

Jie Lia, Zheng Wua, Qingyun Duana, Xuede Lia, Ying Lib, Hamed Alsulamic, Mohammed Sh. Alhodalyc, Tasawar Hayatc, Yubing Sunb,* a

School of Resources and Environment, Anhui Agricultural University, Hefei, 230036, PR China MOE Key Laboratory of Resources and Environmental System Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, 102206, PR China c Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Editor: Danmeng Shuai

The simultaneously efficient removal of cationic and anionic radionuclides is an important and challenging topic for nuclear waste remediation as well as environmental protection. Herein, monoclinic ZIF-8 nanosheets modified with ethyleneimine polymer (denoted as ZIF-8/PEI) was achieved and used to determine the capture behaviors of both U(VI) oxycations and Re(VII) oxyanions from aqueous solution. ZIF-8/PEI assemblies showed a maximum U(VI) and Re(VII) uptake capacity of 665.3 (pH 5.0) and 358.2 mg/g (pH 3.5), respectively. Experimental, spectroscopic and theoretical calculation results directly unraveled that U(VI) adsorption onto ZIF-8/PEI assemblies was mainly ascribed to the coordination with abundant amino groups and weakly due to the Zn terminal hydroxyl groups, while anion exchange mechanism contributed predominantly to the Re(VII) sequestration. This work not only sheds light on the interaction mechanisms of simultaneous capture of U(VI) and Re(VII) but also highlights the versatile material design of cationic and anionic radionuclide immobilization in radioactive wastewater remediation.

Keywords: ZIF-8 nanosheets Polyethyleneimine U(VI) oxycations Re(VII) oxyanions Simultaneous removal

⁎

Corresponding author. E-mail address: [email protected] (Y. Sun).

https://doi.org/10.1016/j.jhazmat.2020.122398 Received 20 February 2019; Received in revised form 21 February 2020; Accepted 22 February 2020 Available online 24 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 393 (2020) 122398

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1. Introduction

MOFs (Zhao et al., 2018), including large specific surface areas exposing a lot of surface active sites, large area in close contacting with the active sites and radionuclides, further accelerating radionuclide diffusion to the contact interface, and highly open structure making buried chelating groups exposed as highly accessible active sites. Besides, we reasoned that 2D MOF nanosheets are anticipated to be a superior supporting material for the coating of PEI considering their large specific surface areas, exposed terminal functional groups and the high affinities of PEI. However, both mechanistic and quantitative understanding of such MOF-based PEI composites for the preconcentration of both U(VI) oxycations and Re(VII) oxyanions has not been reported so far. In this work, we aimed to i) achieve the coating of PEI onto ZIF-8 nanosheets (denoted as ZIF-8/PEI); ii) systematically probe the simultaneous immobilization behaviors of U(VI) oxycations and Re(VII) oxyanions by ZIF-8/PEI assemblies by using a batch adsorption technique affected by contact time, water chemistry, the coexistence of competitive U(VI)/Re(VII) and other competitive common ions; iii) investigate the feasibility of ZIF-8/PEI assemblies to remove U(VI) from intentionally contaminated natural and synthetic waters; and iv) uncover the interplay mechanisms between ZIF-8/PEI assemblies and U (VI) oxycations or Re(VII) oxyanions by using Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The findings in this study demonstrated the simultaneous elimination of U(VI) oxycations and Re(VII) oxyanions by ZIF-8/PEI assemblies and will provide a delicate method to design versatile adsorbents in the field of nuclear waste management. The highlight of this study is instantaneous removal of various radionculides from aqueous solution by ZIF-based novel adsorbents.

Currently, nuclear power supplies cheap, efficient and sustainable energy for the world’s electricity demand, while environmental issues relevant to the nuclear fuel cycle occur including nuclear waste management and reactor accidents (Hu et al., 2019). In the radioactive liquid waste, 235uranium (235U) is prevalence because it is the dominant source of current nuclear energy and usually present as UO22+ in an aerobic atmosphere (Hu et al., 2019; Song et al., 2019; Xie et al., 2019). Also, the nuclear fission of 235U can produce a large quantity of longlived 99Tc (t1/2 = 2.13 × 105 y) (Smith et al., 2015), which primarily occurs as TcO4− and has high environmental mobility and great stability, thus raising serious concerns (Dickson et al., 2014). More importantly, Tc can greatly disturb the extraction of U during the spent fuel recycle, rendering it one of the most notorious radionuclides in nuclear waste management. Considering the high radioactivity of TcO4−, the perrhenate anion (ReO4−), the most widely used TcO4− simulant, is usually utilized to simulate the migration and transformation behaviors of TcO4− in the actual aquatic environment (Sheng et al., 2017). Among multifarious methods for the extraction of radionuclides from radioactive wastes, adsorption is well established to hold considerable promise especially when the capturers are modified with versatile functional groups for target radionuclides (Li et al., 2018a; Gu et al., 2018). It is well established nitrogen-containing functional groups showing strong affinities for uranium as a result of the powerful coordination ability of soft N atoms (Wang et al., 2013; Yuan et al., 2012). For examples, a recent study from our group reported that the saturated adsorption amount for U(VI) was significantly enhanced by the introduction of polypyrrole nanotubes onto metal-organic framework (MOF) ZIF-8 (Li et al., 2018b). Bai et al. also found that the decoration of nitrogen-containing functionalities onto MOF MIL-101 remarkably enhanced U(VI) uptake amounts, the uptake capacity of which following the order of MIL-101 < MIL-101-NH2 < MIL-101ethanediamine < MIL-101-diethylenetriamine (Bai et al., 2015). Sun et al. even proved that the nitrogen-containing functionalities in polyaniline-modified graphene oxide showed a stronger affinity for radionuclides (i.e., U(VI), Eu(III), Sr(II), and Cs(I)) in relation to oxygencontaining functionalities (Sun et al., 2013). More importantly, Chen at al. recently further demonstrated that amidoxime functionalized MOF UiO-66 could remove 94.8 % of uranium from Bohai seawater within 120 min, and the uranium adsorption capacity was 2.68 mg/g in a real seawater sample (Chen et al., 2017). Moreover, the capturers containing protonated nitrogen groups such as diisobutylamine-modified graphene oxide (Xiong et al., 2017), polyaniline functionalized titanium phosphate (Gao et al., 2015), and other different amine- and/or iminecontaining materials (Banerjee et al., 2016a; Kim et al., 2004; Lou et al., 2013; Huang et al., 2018) are also proved to show powerful adsorption affinity for ReO4− or TcO4− through electrostatic attraction. In this setting, we reasoned that branched poly(ethyleneimine) (PEI) in its molecule possessing three different kinds of amines is therefore very attractive due to its potential application for the capture of both U(VI) oxycations and Re(VII) oxyanions (Geckeler and Volchek, 1996). However, PEI molecules as well as the resulting PEI-metal complex is always water-soluble, limiting its application as an adsorbent, immobilization of PEI on a superior supporting material can be a good choice to facilitate separation and recovery while maintaining active sites for radionuclide capture (Huang et al., 2018). As a unique branch in the porous material family, MOFs have recently gained particular attention as scavengers for the radionuclide pollution management (Li et al., 2018a,b; Xiao et al., 2017; Zhu et al., 2017; Peng et al., 2018). Nowadays, the reported MOFs are mainly three-dimensional structured, and the active sites are deeply buried by organic linkers, which might inevitably affect their adsorption performance for radionuclides. Conversely, two-dimensional (2D) MOFs show the traits of both 2D layered nanomaterials (Samori et al., 2016) and

2. Experimental section 2.1. Synthesis of ZIF-8/PEI assemblies The procedures for the preparation of ZIF-8 nanosheets are as follows: 0.335 g of Zn(NO3)2·6H2O and 0.985 g of 2-methylimidazole were dissolved into 90 mL of deionized water, respectively, then mixed rapidly and stirred at room temperature for 24 h. The obtained white deposits were purified, dried and collected as ZIF-8 nanosheets. The ZIF-8/PEI assemblies were prepared as follows: an aqueous dispersion of ZIF-8 nanosheets 30 mL, 5000 mg/L, pH = 9.8 and a PEI solution 10 mL, 30,000 mg/L, MW = 1800) was mixed rapidly and ultrasonicated for 10 min, followed by standing for 24 h. After purification and dry, ZIF-8/PEI assemblies were obtained. Fig. 1 shows the schematic description of the approach used for the fabrication of the ZIF-8/ PEI assemblies. 2.2. Characterization X-ray powder diffractions (PXRD) (Rigaku/max 2550 diffractometer, Japan, Cu Kα radiation, λ =1.5418 Å)and scanning electron microscope (SEM, Hitachi S-4800, Japan) were used to examine the chemical composition and the microscopic structures of ZIF-8/PEI assemblies. Zeta potentials of ZIF-8 nanosheets and ZIF-8/PEI assemblies as a function of pH were determined by a Zeta Potential Analyzer (Brookhaven Instrument Corp., Holtsville, New York). FT-IR results were obtained by using a Nicolet Magana-IR 750 spectrometer in KBr pellets. XPS measurements were collected by a Thermo VG RSCAKAB 250X. 2.3. Batch sequestration experiments The batch preconcentration properties of U(VI) oxycations and Re (VII) oxyanions by ZIF-8/PEI assemblies were carried out as follows. The suspensions of 4.0 mg ZIF-8/PEI assemblies, the U(VI)/Re(VII) 2

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Fig. 1. (A) Schematic illustration of the approach used for the preparation of the ZIF-8/PEI assemblies; (B) and (C): SEM images of ZIF-8 and ZIF-8/PEI assemblies, respectively.

3. Results and discussion

stock solution, and the background electrolyte NaClO4 solution were mixed to attain the pre-set concentrations. The initial pH values were obtained by the adjustment of HCl or NaOH solutions. The suspension was kept oscillation at room temperature for 24 h and withdrawn by centrifugation. The concentrations of U(VI) and Re(VII) were detected by using the Arsenazo III spectrophotometric method at the wavelength of 650 nm, and by the SnCl2 and potassium thiocyanate spectrophotometric method at the wavelength of 396 nm, respectively. The adsorption rate (%) and capacity (Qe (mg/g)) of U(VI)/Re(VII) were expressed as Eqs. (1) and (2): Adsorption rate (%) = (C0 – Cf)/C0 × 100

(1)

Qe = (C0 – Cf) × V / m

(2)

3.1. Characterization of ZIF-8/PEI assemblies In the ZIF-8/PEI assemblies, PEI decoration onto monoclinic ZIF-8 can be proved by the increased thickness of ZIF-8 and the coarse surface of ZIF-8/PEI (Fig. 1B and C). The FT-IR spectrum of ZIF-8/PEI assemblies (Fig. 2A) shows some new absorption bands assigned to the vibration of NeH in free amines centered at ca. 1640 and 1580 cm−1 and to the stretching vibration of protonated amines located at ca. 1517 cm−1 concerning pure ZIF-8 nanosheets. Fig. 2B illustrates the zeta potentials of original ZIF-8 nanosheets and ZIF-8/PEI assemblies at different solution pH. Due to the extensive protonation of amine in PEI molecule, ZIF-8/PEI assemblies were positively charged in all of the studied pH (2.0–10.0) with respect to pure ZIF-8, which is in well conformity with the property of original PEI (pHPZC = 10.3) (Yan-Bin, 2008) and therefore further certifies the successful decoration of PEI on ZIF-8 nanosheets. Additionally, the crystal structure of ZIF-8 didn’t destroy after PEI decoration (Fig. 2C). These phenomena address the feasibility of anion uptake by ZIF-8/PEI assemblies through electrostatic attraction.

where V and m are designated as the total mixture volume (mL) and the ZIF-8/PEI mass (g), and C0 and Cf are designated as the initial and final U(VI)/Re(VII) concentrations (mg/L), respectively.

2.4. Theoretical calculations

3.2. Immobilization behaviors of U(VI) oxycations and Re(VII) oxyanions

Theoretical calculations were applied to probe the coordination modes of U(VI)/Re(VII) on ZIF-8/PEI assemblies using DFT techniques at the molecular level. To save computational efficiency, a single cluster model of ZIF-8 and one PEI unit was selected and the most stable structure this assembly is optimized (Fig. S1). All the structures studied were optimized by the M06-2X function with the Gaussian 09 software package. The 6−31 G (d, p) basis set was used for light atoms (C, H, N, O) and the Stuttgart − Dresden 1-electron and 19-electron ECPs (SDD) basis set was used for Zn, U and Re atoms. The Solvation Model based on Density model (SMD) was employed to perform the Self Consistent Reaction Field effect (SCRF) in a solution environment. The adsorption energy (Ead) was calculated as Eq. (3):

Fig. 3A illustrates the time profiles of U(VI)/Re(VII) adsorption onto ZIF-8/PEI assemblies at an initial concentration of 100 mg/L. It reveals that the adsorption equilibrium of U(VI) occurred with 60 min at pH 3.5 and 5.0, while it only took approximately 5 and 20 min to reach the adsorption equilibriums of Re(VII) at pH 3.5 and 6.0. These adsorption phenomena disclose faster adsorption kinetics for removing ReO4−. Note the adsorption kinetics of ZIF-8/PEI assemblies for ReO4− is much faster than those of other MOF-based adsorbents. For example, the adsorption equilibrium of ReO4− onto SLUG-21 (Fei et al., 2011) and UiO-66-NH3+ (Banerjee et al., 2016b) required more than 24 h. Moreover, the experimental data were also simulated by the nonlinear pseudo-first-order (PFO) and pseudo-second-order (PSO) equations. The PSO model was more favored than the PFO model to simulate the adsorption date with larger R2 and chi-square test (χ2) values as well as

Ead=EZIF-8/PEI+EU(VI)/Re(VII)-EZIF-8/PEI-U(VI)/Re(VII)

(3) 3

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U+Re

A 904

1364 amine III

U

UO2

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Zeta potential (mV)

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30 20 10 0 -10

Re

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906 Re-O

1640 and 1580 amine I and II

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ZIF-8

4000

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3500

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1000

500

U Re

Blank 15

20

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30

2 theta (degree)

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Fig. 2. (A) FT-IR spectra of the original ZIF-8 nanosheets and ZIF-8/PEI assemblies before and after U(VI)/Re(VII) adsorption; (B) Zeta potentials of original ZIF-8 nanosheets and ZIF-8/PEI assemblies as a function of solution pH; and (C) XRD patterns of ZIF-8/PEI assemblies before and after U(VI)/Re(VII) adsorption.

Re(VII) uptake was impeded at pH > 3.0. Based on this adsorption phenomenon, the formation of cation UO2ReO4+ complex could be excluded (Zagorodnyaya et al., 2015). One reasonable explanation for the decrease of Re(VII) uptake is that the weak outer-sphere Re(VII) adsorption was influenced by the strong inner-sphere U(VI) complexation. Additionally, the inhibiting effect of NO3− ions derived from UO2(NO3)2 could be another reason. On all accounts, the immobilization of both U(VI) oxycations and Re(VII) oxyanions by ZIF-8/PEI assemblies was achieved, which is critical merit for practical application.

higher approximation degree between the theoretical and calculated qe values (Table S1). Therefore, the rate-determining step of U(VI)/Re(VII) adsorption onto ZIF-8/PEI assemblies is likely a consequence of adsorption rather than mass transport (Li et al., 2017, 2018c). The effect of solution pH on the U(VI)/Re(VII) uptake by ZIF-8/PEI assemblies was studied in the single and mixed systems (Fig. 3B). In the case of single U(VI) system, U(VI) sequestration by ZIF-8/PEI assemblies is inefficient at pH 2.0–3.0, which is probably the result of sufficient protonation of PEI amines, given the loss of coordination affinity for UO22+ (Wang et al., 2017). Then, the uptake rate of U(VI) increases sharply to nearly 100 % at pH 3.0–5.0 with the complete U(VI) sequestration maintained until pH 8.0. One reasonable explanation is that the content of free amines increases with increasing solution pH, thus leading to the efficient U(VI) uptake. Finally, U(VI) uptake was suppressed at pH > 8.0. The electrostatic repulsion between negatively charged U(VI) species (i.e.,UO2(CO3)34- and UO2(CO3)22-) (Fig. S2) and negatively charged surface of ZIF-8/PEI assemblies could be the reason. Approximate 80 % of Re(VII) was removed by ZIF-8/PEI assemblies at pH 3.0–4.0 due to the existence of main ReO4− oxyanions in the aqueous solutions (Lou et al., 2013), whereas the Re(VII) uptake reduced with increasing pH, which is in accordance with the results of Re (VII) capture on PANI/Ti(HPO4)2 (Gao et al., 2015) and GO-PEI (Huang et al., 2018). More importantly, synergistic sequestration of U(VI) and Re(VII) on ZIF-8/PEI assembies was observed, which could be attributed to a possible scenario that U(VI) oxycations and Re(VII) oxyanions were removed by free and protonated amines, respectively. In the case of a mixed system, the presence of Re(VII) has no discernible influence on U(VI) uptake under the studied pH range, while

3.3. Adsorption isotherms Adsorption isotherms of U(VI) and Re(VII) on ZIF-8/PEI were showed in Fig. 4A and B, respectively. The uptake amounts of U(VI) oxycations or Re(VII) oxyanions increase with the increase of initial concentration at lower loading conditions, finally yielding the maximum adsorption capacity of 665.3 mg/g for U(VI) at pH 5.0 and 358.2 mg/g for Re(VII) at pH 3.5, which was significantly higher than graphene oxide (86.2 mg/g for U(VI) at pH 3.5) (Sun et al., 2013) and polyaniline/titanium phosphate (47.6 mg/g for Re(VII) at pH 3.0) (Gao et al., 2015). The isothermal uptake curves are further simulated by widely used Langmuir and Freundlich models (Li et al., 2018a, d). Fig. 4 and Table S2 unravel a preferable simulation by Langmuir model, disclosing a monolayer radionuclide uptake on ZIF-8/PEI surfaces. The capacity for captured U(VI)/Re(VII) in this study are much higher comparing to the literature reports (Table 1). Fig. 4A also illustrates that ionic strength (0.01 and 0.1 mol/L NaClO4) has no discernible impact on U(VI) sequestration, further

Fig. 3. Systematic adsorption investigations, C0 = 100 mg/L, m/V = 0.4 g/L and I = 0.01 M NaClO4. (A) Adsorption kinetics; (B) effect of solution pH. 4

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Fig. 4. Adsorption isotherms of U(VI) (A) and Re(VII) (B) on ZIF-8/PEI, C0U(VI) = 2-300 mg/L, C0Re(VII) = 2-200 mg/L, m/V = 0.4 g/L.

tribasic PO43− (Sheng et al., 2017; Banerjee et al., 2016b). This trend can be understood based on the large quantity of three different types of amines in ZIF-8/PEI assemblies. Besides U(VI) and Re(VII), radioactive wastewaters also contain other radioactive metal ions (e.g., Co2+, Sr2+, Eu3+, and Ce4+) and transition metal ions (e.g., Ni2+ and Zn2+) existing as typical NO3− salts. Fig. 5B reveals the preferable uptake of U(VI) over other competing cations at both pH 3.5 and 5.0 by ZIF-8/PEI assemblies. Such merit was also reported on SBA-15 modified with dihydroimidazole (Yuan et al., 2012), MIL-101 modified diethylenetriamine (Bai et al., 2015) and GO modified with PEI (Huang et al., 2018) for U(VI) uptake in similar situations. (Huang et al. (2017)) observed that chitosan functionalized GO also showed certain adsorption ability for lanthanide ions due to the –NH2, −COOH and −OH groups. Therefore, nitrogencontaining functionalities are superior to oxygen-containing groups for the selective U(VI) uptake. Besides, the high concentration of NO3− should be the reason for the decrease of Re(VII) uptake.

indicating that U(VI) uptake onto ZIF-8/PEI surfaces was not dominated by ion exchange in this study (Li et al., 2018b). In the case of Re(VII) sequestration, the decrease of its uptake amount in the coexistence of 0.01 and 0.1 mol/L NaClO4 directly unravels an anion exchange mechanism (Gao et al., 2015; Kim et al., 2004; Huang et al., 2018). 3.4. Ion competition studies for U(VI) and Re(VII) adsorption Superior selectivity under the interference of competing ions is equally as important as high adsorption capacity and fast kinetics in evaluating an adsorbent. The selectivity of ZIF-8/PEI assemblies toward ReO4− was tested in the presence of various amounts of common coexisting anions containing monobasic Cl− and NO3− ions, dibasic CO32− and SO42− ions, and tribasic PO43−. Fig. 5A reveals that the uptake amounts of ReO4− are nearly not influenced by the presence of equimolar competing anions except for SO42−, which is likely due to the more powerful affinity of HSO4− ions (Huang et al., 2018). This phenomenon contrasts sharply with the case of UiO-66-NH3+ (Banerjee et al., 2016b) and PAF-1 (Banerjee et al., 2016c) under the same conditions, where tribasic PO43− effectively depresses ReO4− uptake. When the concentration of each competing anion is 20 in excess, its uptake amount reduces remarkably, and less than 60 mg/g of the uptake amount remains when further increasing the amount of each anion. Banerjee et al. proposed that the ion exchange of NO3−, ClO4− and ReO4− is isoenergetic driving by the concentration gradient based on theoretical simulations (Banerjee et al., 2016b). The observed results herein disclose that ion exchange mechanism contributed to the ReO4− uptake. The adsorption trend is ReO4− ∼ SO42− > CO32− ∼ PO43− > NO3− ∼ Cl−, which is surprising considering the low charge density of ReO4− concerning dibasic CO32− and SO42− ions, and

3.5. U(VI) uptake in natural and synthetic waters Encouraged by the results described above, we were then motivated to evaluate its applicability in different water samples including milli-Q water, taper water, Dongpu lake water, synthetic groundwater, synthetic wastewater, and synthetic surface water intentionally spiked with U(VI) (100 mg/L). The detailed information about these water samples were described in supporting information. Fig. 6A illustrates that ZIF-8/PEI assemblies could efficiently remove U(VI) from all these water samples, highlighting the vast potential of ZIF-8/PEI assemblies as promising candidates in accomplishing radionuclide removal from water.

Table 1 Comparison of U(VI) and Re(VII) adsorption on ZIF-8/PEI with other adsorbents.

U(VI)

Re(VII)

Materials

Experimental conditions

Qmax (mg/g)

Ref.

MIL-101 MIL-101-NH2 UiO-66 GO–COOH/UiO-66 MOF-76 Fe3O4@ZIF-8 PPy/ZIF-8 ZIF-8/PEI PANI/Ti(HPO4)2 GO-DEA-DIBA UiO-66-NH3+ SCU-101 GO-PEI Resin D318 NU-1000 SLUG-21 ZIF-8/PEI

T =298 K, pH = 5.5 T =298 K, pH = 5.5 T =298 K, pH = 5.5 T = R.T., pH = 8.0 T = R.T., pH = 3.0 T = R.T., pH = 3.0 T =298 K, pH = 5.0 T =298 K, pH = 5.0 T =293 K, pH = 4.0 T =303 K, pH = 4.0 T = R.T. T = R.T. T = R.T., pH = 3.5 T =298 K, pH = 5.2 T = R.T. T = R.T. T =298 K, pH = 3.5

20 90 109.9 188.3 298 523.5 534 665.3 41.84 140.82 159 217 262.6 351.4 210 602 358.2

(Bai et al., 2015) (Bai et al., 2015) (Luo et al., 2016) (Yang et al., 2017) (Yang et al., 2013) (Min et al., 2017) (Li et al., 2018b) This work (Gao et al., 2015) (Xiong et al., 2017) (Banerjee et al., 2016b) (Zhu et al., 2017) (Huang et al., 2018) (Shu and Yang, 2010) (Drout et al., 2018) (Fei et al., 2011) This work

5

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Fig. 5. Effect of competing ions on the adsorption of (A) Re(VII) and (B) U(VI), C0 = 100 mg/L, m/V = 0.4 g/L and I = 0.01 M NaClO4. Fig. 6. (A) U(VI) removal by ZIF-8/PEI from various water samples spiked with 100 mg/L U (VI) at m/V = 0.4 g/L; (B) Reusability of the ZIF-8/PEI in the adsorption of U(VI) and Re (VII). m/V = 0.4 g/L, C0 =100 mg/L, pH = 5.0. A 0.1 mM NaOH solution was utilized to desorb the adsorbed Re(VII). A 0.1 M Na2CO3 solution was utilized to desorb the adsorbed U(VI).

Fig. 7. (A) N 1s, (B) O 1s, (C) Re 4f and (D) U 4f high resolution spectra. T = 298 K, m/V = 0.4 g/L, I = 0.01 M NaClO4 and C0 = 100 mg/L.

shows new absorption bands appeared at ca. 904 cm−1 assigned to chemically coordinated UO22+ (Li et al., 2018b) and to bending of amine III groups (1364 cm−1), and the disappearance of amine I bending after U(VI) immobilization (Piron and Domard, 1998), further confirming the coordination of U(VI) with amines. The FT-IR spectrum of Re(VII)-laden ZIF-8/PEI has a strong ReO4− absorption peak located at ca. 906 cm−1 (Huang et al., 2018) with no obvious change on other peaks except that the NH3+ stretching band has a slight redshift, evidencing the reaction between protonated amines and ReO4−. XPS technique can trace the roles that various functional groups played in the U(VI)/Re(VII) uptake by ZIF-8/PEI assemblies. Fig. S3 shows the wide scan XPS spectra of pure ZIF-8/PEI assemblies and U (VI)/Re(VII)-laden ZIF-8/PEI. The N 1s peaks centered at ca. 400 eV can be assigned to various nitrogen-containing groups of ZIF-8/PEI. A U 4f signal centered at 381.4 and 392.2 eV with an apparent reduction in the N 1s spectrum area after U(VI) sequestration. The high-resolution N 1s peaks can be resolved into three individual component peaks centered

3.6. Recyclability of ZIF-8/PEI assemblies The viability of a waste remediation technique remarkably improves if an adsorbent is recyclable, which can alleviate both the financial and energy burdens of legacy waste remediation. A 0.1 mmol/L NaOH or 0.1 mol/L Na2CO3 were utilized to regenerate Re(VII)-laden ZIF-8/PEI or U(VI)-laden ZIF-8/PEI, respectively, under 6 h. Fig. 6B reveals the excellent adsorption − desorption regeneration ability toward U(VI) (89.6-88.5 %) or Re(VII) (64.5-60.2 %) overall four cycles. Additionally, the dominant diffraction peaks of ZIF-8/PEI are still obvious after U(VI) or Re(VII) immobilization (Fig. 2C), disclosing that its crystal structure didn’t destroy obviously. These properties are critical advantages for practical applications. 3.7. Interaction mechanisms Fig. 2A illustrates that the FT-IR spectrum of U(VI)-laden ZIF-8/PEI 6

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Fig. 8. (A) Optimized geometry of U(VI)-laden ZIF-8/PEI; (B) ADCH charge analysis of U(VI)-laden ZIF-8/PEI; (C) color-filled RDG mapped isosurface of U(VI)-laden ZIF-8/PEI; (D) optimized geometry of Re(VII)-laden ZIF-8/PEI; (E) ADCH charge analysis of Re(VII)-laden ZIF-8/PEI; and (F) color-filled RDG mapped isosurface of Re(VII)-laden ZIF-8/PEI.

respectively (Fig. 7D). To further shed light on different U(VI) and Re(VII) immobilization behaviors onto ZIF-8/PEI assemblies, the DFT calculations were also conducted to probe the coordination modes of U(VI)/Re(VII) with ZIF8/PEI. After DFT calculations, the most stable configurations and representative structural parameters of U(VI)/Re(VII)-laden ZIF-8/PEI are shown in Fig. 8. The Ead of U(VI)- and Re(VII)-laden ZIF-8/PEI were 36.57 and 4.33 kcal/mol, respectively, disclosing stronger interaction occurred in U(VI) uptake than Re(VII) capture by ZIF-8/PEI assemblies. The Ead of U(VI) oxyanions toward ZIF-8/PEI (36.57 kcal/mol) is much higher than that of U(VI) oxyanions toward HOOC-GOs·(12.1 kcal/mol) (Sun et al., 2013) and S-doped graphene (9.78 kcal/mol) (Chen et al., 2018), which discloses that ZIF-8/PEI assemblies can be potential

at 398.1 eV (amide), 399.0 eV (free amine) and 400.4 eV (protonated amine) (Sun et al., 2013; Huang et al., 2018) (Fig. 7A). The binding energy of amine peaks in U(VI)-laden ZIF-8/PEI displayed a 0.25 eV shift with respect to that in original ZIF-8/PEI. A similar change has also observed on europium adsorption by PANI@GO composites (Sun et al., 2013), thereby supporting the involvement of free amines in the U(VI) coordination. Additionally, there is no obvious change on the O 1s signal after U(VI) uptake (Fig. 7B) except only an intensity reduction in the −OH peaks, suggesting that terminal Zn hydroxyl groups of ZIF8 didn’t play an indispensable role in the capture of U(VI) in relation to the amine groups. Furthermore, Fig. 7C reveals that the adsorbed Re (VII) oxyanions have one dominant species while the adsorbed U(VI) oxycations contain two structures centered at 381.3 and 383.5 eV, 7

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Acknowledgment

candidates for U(VI) uptake. Fig. 8A reveals that three amine N atoms coordinate with one U(VI) oxycation. The calculated bond distances of UeN (2.61, 2.65 and 3.15 Å) in the U(VI)-laden ZIF-8/PEI structure is comparable to the DFT (2.59 Å) and EXAFS results (3.4 and 2.6 Å) of U (VI) capture by PEI molecule (Huang et al., 2018). The result of atomic dipole moment corrected Hirshfeld (ADCH) charge analysis also discloses that U(VI) oxycation was coordinated N atoms (Fig. 8B), which evidences that amine groups of PEI molecule play some more roles than terminal Zn hydroxyl groups of ZIF-8 in U(VI) uptake. The reduced density gradient (RDG) method was also used to investigate the interaction between U(VI) oxycations and ZIF-8/PEI assemblies. Fig. 8C illustrates that strong attraction interaction occurred between U(VI) oxycations and amine N atoms, which could be regared as coordinate covalent bonds. In terms of Re(VII) immobilization, the DFT calculations illustrate that hydrogen bonding and/or electrostatic attraction contributes to the Re(VII) uptake by ZIF-8/PEI assemblies (Figs. 8D-F), which also supports the experimental data. Therefore, the inhibiting effect of U(VI) coordination on the Re(VII) uptake was the consequence of the weaker interaction between Re(VII) and ZIF-8/PEI than that between U(VI) and ZIF-8/PEI.

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3.8. Environmental implication In the nuclear fuel cycle, 1.0 kg spent fuel included ∼0.95 kg 238U, 0.009 kg 235U and 0.55 g 99Tc, thus U(VI) and Tc(VII) are co-exhibited in the spent fuel. Our finding showed that the no significant change in removal of U(VI) on ZIF-8/PEI was observed in the presence of Re(VII). In addition, Re(VII) adsorption was hardly influenced by various competitive anions (e.g., NO3−, Cl−, CO32- and PO42-). These findings indicated that ZIF-based materials were the promising candidates for the highly efficient removal of various radionuclides in actual environmental cleanup. These observations have substantial implications for the management of radionuclides in the environment, and for the prediction of their mobility. 4. Conclusions In conclusion, ZIF-8/PEI assemblies are attractive capturers for cationic UO22+ and anionic ReO4− uptake from contaminated water systems. The strong UO22+ and ReO4− uptake capabilities are elucidated by spectroscopic and theoretic results. This study offered a promising method to improve the capture ability of 2D MOFs for wastes containing both radionuclide oxycations and oxyanions through the decoration of functional groups according to the waste properties. CRediT authorship contribution statement Jie Li: Conceptualization, Methodology, Investigation, Writing original draft. Zheng Wu: Validation, Formal analysis, Visualization, Software. Qingyun Duan: Validation, Formal analysis, Visualization. Xuede Li: Resources, Writing - review & editing, Supervision, Data curation. Ying Li: Resources, Writing - review & editing, Supervision, Data curation. Hamed Alsulami: Writing - review & editing. Mohammed Sh. Alhodaly: Writing - review & editing. Tasawar Hayat: Writing - review & editing. Yubing Sun: Writing - review & editing, Supervision, Data curation. Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Simultaneous removal of U(VI) and Re(VII) by highly efficient functionalized ZIF-8 nanosheets adsorbent”. 8

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