Sulfate-Rich Metal-Organic Framework for High Efficiency and

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Sulfate-Rich Metal-Organic Framework for High Efficiency and Selective Removal of Barium from Nuclear Wastewater Chufan Kang, Yaguang Peng, Yuanzhe Tang, Hongliang Huang, and Chongli Zhong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02887 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017

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Industrial & Engineering Chemistry Research

Sulfate-Rich Metal-Organic Framework for High Efficiency and Selective Removal of Barium from Nuclear Wastewater Chufan Kang,† Yaguang Peng,† Yuanzhe Tang,† Hongliang Huang,*,†,‡ and Chongli Zhong†,‡,§ †

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, Beijing 100029, China

‡

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin

Polytechnic University, Tianjin 300387, China §

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

100029, China Hongliang Huang, e-mail: [email protected]

ABSTRACT: Capture of radioactive barium from nuclear wastewater is of great importance for

environmental

protection.

Here,

[Zr6(OH)10.8(SO4)3.6(BDC-NH2)3(H2O)7.4]·nH2O

(Zr-BDC-NH2-SO4) was selected as an adsorbent for its high content of sulfate group, which is a strong barium-chelating group, and its binding sites are fully exposed. Zr-BDC-NH2-SO4 exhibits high adsorption capacity of 181.8 mg g-1, which is higher than those of most reported adsorbents, and showed excellent high selectivity even when the concentrations of background metal ions are 10 times of Ba2+. The breakthrough study showed good adsorption performance with fast kinetics and low outlet concentration. In addition, the great stability of

ACS Paragon Plus Environment 1


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Zr-BDC-NH2-SO4 under gamma radiation has been confirmed, making it possible to be applied in real nuclear wastewater treatment. Moreover, the adsorption of Ba2+ is irreversible and therefore it could avoid secondary pollution. Overall, this work provides a stable, efficiency adsorbent for removing radioactive barium from nuclear wastewater. KEYWORDS: radioactive Ba2+, nuclear wastewater, metal-organic framework, adsorption, sulfate group


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1. INTRODUCTION Since fossil fuels are coming to an end soon, nuclear industries are growing rapidly. Meanwhile, radionuclides arising from inadequate management and disposal of nuclear waste have been accumulating in many countries accordingly.1 133Ba from the byproduct of nuclear fuel fission and nuclear power reactor accidents is one of the most toxic radionuclides in radioactive liquid wastes, and its γ-ray energy is 356 keV.2

133

Ba is also a bone-seeking

element, which can be carcinogenic to human.3 Thus, it can cause long-term environment problems and endanger the health of human beings. Besides, the half-life of

133

Ba is 10.7

years, it needs more than 100 years for the radioactive Ba2+ to decay.2 So it must be removed completely and not be released again to cause secondary pollution.4 Hence the development of nuclear industries requires effective methods to manage and disposal radioactive Ba2+ from nuclear wastewater irreversibly and conveniently. Many approaches and measures have been taken for metal ions removal, such as ion exchange, membrane filtration, coagulation, precipitation, adsorption, etc. Compared with other treatment techniques, adsorption is a recognized method for the removal of metal ions from wastewater for its high efficiency, high treatment capacity and inexpensive cost.5 Various adsorbents, including solid humic acid,6 activated carbon,7 dolomites,8 clays,9 hydrous ferric oxides,10 titanate nanofibers,11 modified zeolites,12 etc., have been investigated for Ba2+ removal. However, each has its own disadvantages, such as low adsorption capacity, low selectivity, and some of them may bring secondary pollution. Thus, it remains a challenge to develop advanced adsorbents with high adsorption capacity, high selectivity, irreversibility and stable structure.



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Metal-organic frameworks (MOFs), a new class of porous solid materials consisting of metal ions or clusters with organic ligands, have gained wide attention in the past two decades.13 Comparing with traditional porous materials, MOFs have shown good chemical versatility.14 Nowadays, due to high surface area, high porosity and controllable structure at the molecular level, MOFs have exhibit promising potential in many fields such as gas storage,15-18 separation,19-22 catalysis,23-25 sensing,26-28 drug delivery,29-32 etc. Therefore, numerous studies have been done for liquid components adsorption with MOFs.33-35 In view of the ultralow solubility product of BaSO4 in water (Ksp ≈ 1.07×10-10), sulfate group has strong interaction with Ba2+, so that introducing sulfate group into MOFs is a good way for Ba2+ removing.36 Therefore, increasing the content of sulfate group in MOFs and making the binding sites fully exposed may be a good strategy to improve the adsorption efficiency. As a proof of concept, we aim to find a suitable MOF adsorbent for Ba2+ removing in this work.

Thus,

[Zr6(OH)10.8(SO4)3.6(BDC-NH2)3(H2O)7.4]·nH2O

(Zr-BDC-NH2-SO4)

was

selected to investigate the adsorption performance of removing radioactive Ba2+ from nuclear wastewater. As expected, Zr-BDC-NH2-SO4 exhibits excellent adsorption performance with high selectivity and high adsorption capacity, outperforming most of reported sorbent materials. More importantly, this process is irreversible, which can efficiently avoid secondary pollution. The breakthrough performance was also investigated using a glass column to evaluate the practical applicability. The concentration of Ba2+ was reduced from 10 ppm to a very low level with a reduction of two orders of magnitude. Moreover, Zr-BDC-NH2-SO4 could maintain structural stability under gamma radiation. 2. EXPERIMENTAL SECTION


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2.1 Materials All the chemicals used in the experiments were analytical grade and used as received without further purification. 2-aminoterephthalic acid was obtained from Shanghai Macklin Biochemical Co., Ltd. Zr(SO4)2·4H2O was supplied from Energy Chemical. Formic acid was supplied from Tianjin Guangfu Technology Development. BaCl2 was obtained from Stream Chemicals and the deionized water was obtained from a Milli-Q water purification system. 2.2 Preparation of the Adsorbent Zr-BDC-NH2-SO4 was prepared according to the literature.37 2-aminoterephthalic acid (2.400 g, 13.3 mmol), Zr(SO4)2·4H2O (7.100 g, 20.0 mmol), H2O (100 mL) and formic acid (1 mL) were mixed in a round bottom flask, and the mixture was heated at 98 °C under stirring for 16 h in oil bath. After being cooled down to room temperature, the resulting powders were centrifuged and washed with water, then washed with acetone to remove solvated H2O and dried in air. The product was dried at 150 °C, and then treated in 2 vol% sulfuric acid in water (5 mL liquid per 0.100 g) at 60 °C overnight to restore the crystallinity after thermal activation. After cooling down to room temperature, the product was centrifuged and washed with water and acetone, respectively. Finally, the product was dried at room temperature. 2.3 Characterization Brunauer-Emmett-Teller (BET) surface area was measured on an Autosorb-IQ-MP (Quantachrome Instruments) using N2 adsorption at 77 K. Powder X-ray diffraction patterns (PXRD) of the samples were obtained on a D8 Advanced X diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the scan range of 2θ from 3° to 50° with a step size of 0.01°. The scanning electron microscope (SEM) was conducted by a Hitachi S-4700 field emission



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scanning electron microscope (FE-SEM). Fourier transform infrared spectra (FTIR) were measured on a Thermo Fisher Nicolet 6700 FTIR spectrophotometer in the range of 4000 400 cm-1. The elemental analysis was measured by vario EL cube (Elementar). Thermogravimetric curve was measured using a thermal gravimetric analysis (TGA) with a heating rate of 10 K min-1 in air. A Tecnai G2 F20 transmission electron microscope (FEI) equipped with an energy-dispersive X-ray spectrometer system and high-angle angular dark-field detector was used at 200 kV for high-resolution electron microscopy imaging and HAADF imaging. Gamma radiation stability was tested using BFT-II irradiation facility (60Co source-MSD Nordion irradiator) in Lab of Beijing Hongyi Sifang Radiation Technology Co., Ltd. The zeta potentials were determined in nature pH on a nano-ZS90 Zeta potential analyzer (Malvern Instruments). X-ray photoelectron spectroscopies (XPS) were measured on an ESCALAB 250 X-ray photoelectron spectroscopy. 2.4 Batch Adsorption Experiments The solutions of Ba2+ were prepared by dissolving BaCl2 in deionized water with different initial concentrations. All these solutions were used directly in the adsorption experiments. To further remove the guest molecules, the MOF was dried under vacuum at 393 K for 12 h before adsorption experiments. Activated adsorbent (0.010 g) was added into 10 mL Ba2+ solution, and then the adsorption was performed in a glass reactor shaken in a shaker at 298 K for 24 h. The liquid phase was separated from the adsorbents by filtration. The concentrations of Ba2+ were determined by inductive coupled plasma-optical emission spectrometer (ICP-OES, ThermoiCAP-6300). The adsorption capacity and removal efficiency were calculated as follows:


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qe = R=

൫C0 -Ce൯×V

(1)

m

(C0 -Ce) C0

×100 %

(2)

where qe is the adsorption capacity (mg g-1); C0 and Ce are the initial concentration and equilibrium concentrations of Ba2+ in the solution (mg L-1), respectively; V is the volume of the volume of the solution (mL); R is the removal efficiency (%); m is the mass of adsorbent (g). The effect of solution pH on the removal efficiency was studied within the pH range from 3.4 to 7.0 at 298.15 K. The solution pH was adjusted by 0.1 mol L-1 HCl or 0.1 mol L-1 NaOH. The effect of co-existing cations from nuclear wastewater was also investigated with the concentration of co-exisiting cations in Ba2+ solution at 100 ppm. 2.5 Breakthrough Experiments Breakthrough experiments were performed in a glass column with internal diameter of 10 mm and total length of 80 mm at room temperature. The column was filled with about 2.6 g Zr-BDC-NH2-SO4. The samples were extruded, ground, sieved into 60-80 mesh particles to avoid blocking the column before filling into the adsorption bed. As shown in Figure 1, the column was assembled vertically and operated continuously in down flow mode using a multi-channel peristaltic pump with the flow rate of 8 mL h-1 and the initial concentration of 0.010 g L-1. Before setting up the experiment, deionized water was used to remove bubbles in the column. Samples of outlet solution were collected at regular intervals and then examined the Ba2+ concentration by ICP-OES.



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Figure 1. Schematic diagram of experimental set up used for breakthrough experiments.

3. RESULTS AND DISCUSSION 3.1 Characterization of Adsorbent Considering that the sulfate group is a strong barium-chelating group, the high content of sulfate group in MOFs will be benefited for Ba2+ capture. Thus, Zr-BDC-NH2-SO4 was chosen as the adsorbent for Ba2+ removing from nuclear wastewater. As shown in Figure 2, the sulfate anions located around the Zr6-cluster in Zr-BDC-NH2-SO4, and the sulfate anions are coordinated to the Zr6 inorganic node by a monodentate O atom (Figure 2). However, in MOF-808-SO4, sulfate anions coordinated with Zr6 cluster with two O atoms, as shown in Figure S1. Elemental analysis results has shown that each metal cluster possesses 3.6 sulfate


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groups, so that the binding sites of Ba2+ is higher than that of MOF-808-SO4 (2.5 sulfate per Zr6 cluster).37,38 The sulfate anions in Zr-BDC-NH2-SO4 is fully exposed, indicating that Zr-BDC-NH2-SO4 will be easier to bind with Ba2+ ions compared with MOF-808-SO4. Overall, the high content and fully exposed sulfate groups in Zr-BDC-NH2-SO4 make sure that this MOF may be a good adsorbent for Ba2+ capture.

Figure 2. Structures of Zr-BDC-NH2-SO4. Color code: C, black gray; O, red; H, white; S, yellow; N, ultramarine blue; Zr, cyan.

The Zr-BDC-NH2-SO4 was synthesized under green synthesis conditions conveniently according to the literature. 37 The powder XRD pattern was presented in Figure 3a, which is basically in accord with the simulated XRD pattern, indicating that the MOF is well synthesized. The BET data of the adsorbent were measured by the N2 adsorption-desorption isotherms at 77 K shown in Figure 3b. The BET surface area is evaluated to be 374 m2 g-1, which is closed to the reported value.37 The SEM image exhibited in Figure 3c shows that Zr-BDC-NH2-SO4 has tetrahedron morphology. To determine the composition of the adsorbent, elemental analysis were also performed (Table S1). The results demonstrate that the sample is composed of 21.16 % C, 2.31 % N, 2.78 % H and 5.98 % S, which suggest the



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successful attachment of sulfate group to the MOF. The FTIR spectrum of the adsorbent shown in Figure 3d also gives the proof. The peak obtained at 1126 cm-1 represents the S=O stretching bands, the peaks at 1392 cm-1 and 3387 cm-1 are the characteristic peaks of C-N stretching

vibrations and

N-H,

respectively.39,40

As shown in

Figure

S2,

the

thermogravimetric analysis indicates that Zr-BDC-NH2-SO4 is stable up to 400 °C, and then follows the decomposition. The slight decrease at the first step below 130 °C is due to the release of solvent molecules. All these observations demonstrate the successful synthesis of Zr-BDC-NH2-SO4.

Figure 3. PXRD patterns (a) N2 adsorption-desorption isotherm (b) SEM image (c) and FTIR spectra (d) of Zr-BDC-NH2-SO4.


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3.2 Batch Adsorption Performance Since the radioactive isotopes are toxicity, nonradioactive Ba2+ were used in our adsorption experiments instead of radioactive Ba2+ .41 3.2.1 Adsorption Kinetics The efficiency of the adsorbent on Ba2+ adsorption was studied by the adsorption kinetics tests. As shown in Figure 4a, the adsorption capacity increases rapidly during the initial 30 min and reaches equilibrium over about 420 min. The removal efficiency can reach 99 % after equilibrium. In order to further investigate Ba2+ adsorption kinetics characteristic, the experimental data were fitted by pseudo-first-order and pseudo-second order models.42 As shown in Figure S3 and Table S2, the pseudo-second-order model fits the adsorption data better than pseudo-first-order model does, indicating that the rate-limiting step may be a chemisorption process.43 100

(b) 200

80

150

-1

Qe (mg g )

(a)

Removal efficiency (%)

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60

100

40 20

Zr-BDC-NH2-SO4

50

UiO-66-NH2 0

0 0

300

600 900 t (min)

1200 1500

0

300

600 900-1 1200 1500 Ce (mg L )

Figure 4. Ba2+ adsorption kinetics of Zr-BDC-NH2-SO4 at the initial concentration of 10 mg L-1 (a). Ba2+ adsorption isotherms of Zr-BDC-NH2-SO4 and UiO-66-NH2 (b).

3.2.2 Adsorption Isotherms The adsorption isotherms of Ba2+ on adsorbents are depicted in Figure 4b. Since the UiO-66-NH2 (Zr6O4(OH)4(BDC-NH2)6) exhibits the similar chemical constitution with



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Zr-BDC-NH2-SO4 37 except that there’s no sulfate group in UiO-66-NH2, the Ba2+ uptake capacity of UiO-66-NH2 has also been explored to compare the effect of sulfate group. As can be seen in Figure 4a, the removal efficiency can exceed 99 % at the concentration of 10 mg/L. The adsorption capacity of Zr-BDC-NH2-SO4 increases as the initial concentration increases until it reaches equilibrium, however, UiO-66-NH2 shows almost no adsorption, demonstrating that the adsorption could be improved obviously by introducing sulfate group. Further, the adsorption isotherm experimental data were also fitted by Langmuir and Freundlich isotherm models.44 As can be seen in Figure S4 and Table S3, the adsorption data are more in agreement with the Langmuir isotherm model, which indicates that the adsorption of Ba2+ can be regarded as monolayer adsorption. The saturated adsorption capacity is calculated to be 181.8 mg g-1. As listed in Table 1,7-9,36,41,45-48 the adsorption capacity of Zr-BDC-NH2-SO4 is significantly larger than most of other adsorbents reported previously like activated carbon, dolomite, montmorillonite clay, titanate nanotube and MOF-808-SO4. The results show that Zr-BDC-NH2-SO4 is fairly effective in removing Ba2+ from nuclear wastewater. It is remarkable that the adsorption capacity of Zr-BDC-NH2-SO4 for Ba2+ is one of the highest among all of the adsorbents. In order to confirm that the removal efficiency of Ba2+ by MOFs is better than that by sulfate ions, additional control experiments were performed. The same amount of sulfate ions (Na2SO4) was used as a control group to explain the advantage of the MOFs. As can be seen in Figure S5, the removal efficiency of Zr-BDC-NH2-SO4 can exceed 99 %, however, Na2SO4 shows lower removal capacity compared with Zr-BDC-NH2-SO4. The good performance of Zr-BDC-NH2-SO4 is related to the fact that the accumulation effect of MOFs


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can significantly improve the regional concentration of SO42- group in MOFs. Due to the large amount of sulfate group in the pore structure of Zr-BDC-NH2-SO4, the concentration of Ba2+ in solution could be largely decreased. Table 1. Adsorption Capacities for the Removal of Ba2+ by Different Adsorbents. Adsorbents

Qe（mg g-1）

References

Activated carbon Expanded perlite Dolomite Niobate Ca-clinoptilolite Ca-exchanged montmorillonite Zero-valent iron MIL-101-Cr-SO3H Na-4-mica Fungus-titanate bio-nanocomposites MOF-808-SO4 Na2Ti3O7 Calcined Hydrotalcite Zr-BDC-NH2-SO4

3.1×10-6 2.5 3.9 13.7 15.3 15.3 22.6 70.5 78.8 120 131.1 159.6 360 181.8

7 45 8 41 9 9 46 36 41 47 36 41 48 This work

3.2.3 Effect of Solution pH The effect of pH on the removal efficiency was also investigated. Figure 5 displays the influence of pH on Ba2+ adsorption. The removal efficiency of Zr-BDC-NH2-SO4 exhibit no significant change with the pH value increases from 3.4 to 7.0 and are all above 99 %. This result suggests that Zr-BDC-NH2-SO4 is suitable for Ba2+ adsorption within a wide range of pH in solutions.



100 Removal efficiency (%)

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80 60 40 20 0 3.4

4

5.37 pH

5.7

7

Figure 5. Effect of pH on the adsorption of Ba2+ by Zr-BDC-NH2-SO4.

3.2.4 Effect of Co-existing Cations The co-existing cations are common in nuclear waste water and they are important factors of Ba2+ adsorption. The effects of co-existing cations on Ba2+ adsorption efficiency were investigated in the presence of co-existing cations (Cs+ Ni2+, Zn2+, Co2+, Eu3+, La3+, Sr2+) existed in nuclear waste water. As shown in Figure 6, when the concentration of co-existing cations are 10 times higher than that of Ba2+, Zr-BDC-NH2-SO4 can still keep high adsorption efficiency for Ba2+, and the adsorption efficiency can be at least 96 %. It clearly indicates that the adsorbent not only exhibits a high adsorption capacity, but also has fairly high selectivity for Ba2+ in the presence of co-existing cations from nuclear wastewater at high concentration. It is worth noting that calcined hydrotalcite has higher adsorption capacity than that of Zr-BDC-NH2-SO4, whereas Ba2+ adsorbents based on ion exchange method usually have poor selectivity compared with Zr-BDC-NH2-SO4.


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Figure 6. Effect of co-existing cations on the adsorption of Ba2+ by Zr-BDC-NH2-SO4. Initial Ba2+ concentration=10 mg L-1, co-existing cations concentration=100 mg L-1.

3.2.5 The Property of Irreversible Removal To avoid secondary pollution, the radioactive Ba2+ which has been adsorpted needs to be sealed up and then buried deeply. Thus, removing Ba2+ by an irreversible process is necessary. To further confirm that it is an irreversible process, the MOFs after saturated adsorption were collected and washed with water to remove Ba2+ on the surface. Then the sample was immersed in water and shaken at 298 K for 2 days. The supernatant and the solids were separated. After determined by ICP analysis respectively, we found that few barium ions was detected in water and 99 % of Ba2+ still remained in Zr-BDC-NH2-SO4. The result confirms that the removal of Ba2+ by Zr-BDC-NH2-SO4 is an irreversible process. 3.2.6 The Stability of Adsorbent The stability is also an important factor for an adsorbent. The Zr-BDC-NH2-SO4 was



characterized by Powder XRD before and after immersed in Ba2+ solution for 48 h, and the spectrums are shown in Figure 7a. It is obvious that the spectrum of the MOF after 48 h in Ba2+ solution retained the same peaks as the original one, which indicates high structural stability of the MOF after Ba2+ adsorption. The SEM image of the adsorbents after adsorption was measured. As can be seen in Figure S6, Zr-BDC-NH2-SO4 possesses tetrahedron morphology, which is the same as the fresh sample. This result also confirmed that Zr-BDC-NH2-SO4 is stable in Ba2+ solution. The gamma radiation stability of Zr-BDC-NH2-SO4 was also tested by irradiation facility at different doses of radiation. Then, these samples were collected and characterized by Powder XRD. As shown in Figure 7b, the PXRD patterns of sample after gamma irradiated fit well with that of the original sample, even for the one at 1000 kGy doses. The BET of the sample after gamma irradiated were also measured by N2 adsorption-desorption isotherms at 77 K, as shown in Figure S7. The BET surface area is evaluated to be 367 cm3/g for 1000 kGy doses, which is closed to that of the original sample. The result indicates that the adsorbent possess good gamma radiation stability.

(a)

(b) after adsorption before adsorption

Intensity (a.u.)

Intensity (a.u.)

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5 10 15 20 25 30 35 40 45 50 2 theta (degree)

1000 kGy 800 kGy 500 kGy 200 kGy before irradiated

5 10 15 20 25 30 35 40 45 50 2 theta (degree)

Figure 7. PXRD patterns of Zr-BDC-NH2-SO4 before and after Ba2+ adsorption (a) and gamma irradiated (b).


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In addition, the real nuclear wastewater treatments are usually carried out under acid conditions. To further confirm the stability of the adsorbents in acid conditions, the prepared MOFs were immersed in HCl solution (pH=1) for 24 h. Then the samples were collected and characterized by PXRD. As shown in Figure S8, the related peak patterns of the MOFs soaked in acid conditions are the same as the original one, indicating that the frameworks remain intact and no apparent collapse occurs after treatment. The result suggests the adsorbents have high stability in acid condition and it could be applied in real nuclear wastewater treatment. 3.3 Breakthrough Studies Column operations are important for fixed-bed adsorber designing in industrial scale.49 To describe the fixed-bed column behavior and provide a primary experiment for scaling it up to industrial scale, breakthrough studies were investigated. As shown in Figure 8, the breakthrough curve barely grew in the initial 200 h and the value of C/C0 kept very low, indicating that the outlet solution maintained a fairly low concentration (0.05-0.17 ppm), which reduced from 10 ppm with a reduction of two orders of magnitude, for at least 200 h. Once the breakthrough point was reached, the slope of the curve increased sharply, indicating that the adsorption rate is very fast. It is noteworthy that the removal efficiency could reach more than 99 % at first and exceed 98 % for about 200 h.



1.0 0.8 C/C0

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0.6 0.4 0.2 0.0 0

100

200 300 t (h)

400

500

Figure 8. Breakthrough curve in continuous adsorption experiments. Initial Ba2+ concentration=10 mg L-1.

We also noticed that adsorption capacity of barium on Zr-BDC-NH2-SO4 in breakthrough experiments is not good enough as that in batch experiments. Since the particle size of the original sample is so small that it blocked the barium solution flow. To solve the problem, the samples were extruded, ground, sieved into 60-80 mesh particles to increase the flow rate. The disadvantage of this method is that it decreased the effective contact area between the adsorbent and the adsorbate so that the adsorption capacity decreased. Moreover, the feeding rate of the barium solution in breakthrough experiment is 8 mL per hour, the contact time of it is shorter than that of batch adsorption experiment (24 h). Thus the adsorption capacity in breakthrough experiment is lower than that in batch adsorption experiments and the similar phenomenon has also been observed in other report.[50] 3.4 Adsorption Mechanism The EDS mapping elemental analysis of Zr-BDC-NH2-SO4 after Ba2+ adsorption clearly shows the presence of Ba2+ on the surface of Zr-BDC-NH2-SO4 after Ba2+ adsorption as


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shown in Figure 9a. Zeta potentials of the sample before and after adsorption were also carried out as shown in Table S4. The zeta potential of Zr-BDC-NH2-SO4 is 18.3 mV, which changed to 21.3 mV after adsorption. It is clear that the surface of the sample before and after Ba2+ adsorption was kept as positive charge, indicating that the electrostatic interaction does not play an important role in the Ba2+ adsorption process of Zr-BDC-NH2-SO4.

Figure 9. EDS element mapping analysis on Zr-BDC-NH2-SO4 after Ba2+ adsorption (a). Wide-scan XPS spectra (b); S 2p scan XPS spectra (c) and FTIR spectra (d) of Zr-BDC-NH2-SO4 before and after Ba2+ adsorption.

In order to gain a better understanding of the mechanism for Ba2+ adsorption, XPS and FTIR spectrums before and after adsorption were measured, as shown in Figure 9b-9d. In the wide-scan XPS spectra, an obvious peak of the sample after adsorption at binding energy around 783.26 eV reveals the presence of Ba2+ ions after Ba2+ adsorption. It also can be seen that in the S 2p scan, the peak at 168.97 eV shifted significantly to 170.97 eV after adsorption, confirming the strong binding interaction between Ba2+ with SO42- in Zr-BDC-NH2-SO4. As shown in the FTIR spectrum, the peaks at 1126 cm-1 and 1040 cm-1, which represent the exist



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of sulfate,51 shifted to 1120 cm-1 and 1073 cm-1 also gives the proof of the strong binding interaction between Ba2+ and SO42- in MOF. To further investigate the interaction mode between Ba2+ with SO42- group on Zr-BDC-NH2-SO4, the sample after saturated adsorption was determined by ICP-OES. As shown in Table S5, as the temperature increases from 303 K to 333 K, the molar ratio of Zr/Ba decreases, which indicates that the content of barium in the sample increases, confirming that chemical interaction plays a key role in this process.52 To investigate whether the sulfate anions would drop from the framework after Ba2+ adsorption, the element contents of the sample after immersed in barium solution were also determined by element analysis. As shown in Table S6, the contents of C and S are 17.18 % and 4.82 %, respectively. And the value of C/S is 3.56, which is similar to the value of C/S before adsorption (C/S=3.54). The result suggests that the sulfate anions would not drop from the framework after Ba2+ adsorption. 4. CONCLUSION In summary, Zr-BDC-NH2-SO4 was selected as an adsorbent for Ba2+ removal due to its high content of sulfate group. As expected, Zr-BDC-NH2-SO4 exhibits high adsorption capacity and fairly high selectivity for Ba2+. Moreover, considering that the raw materials of Zr-BDC-NH2-SO4 are commercial available and Zr-BDC-NH2-SO4 could be synthesized under green synthesis conditions, it could be synthesized low-costly and conveniently. Furthermore, it is suitable for scaling up to industrial scale. Besides, Zr-BDC-NH2-SO4 has the ability to adsorb barium ions irreversible with high stability even after gamma irradiated. Therefore, Zr-BDC-NH2-SO4 could be a suitable adsorbent for removing Ba2+ from nuclear


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wastewater. This work also indicates that increasing the content of sulfate group in MOFs is an efficiency method for barium removing in the management and disposal of nuclear wastewater. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publication website. Structures of Zr-BDC-NH2-SO4 and MOF-808-SO4; elemental analysis and TGA spectra of Zr-BDC-NH2-SO4; adsorption kinetics data fitting and adsorption isotherm data fitting; removal efficiency for barium of Zr-BDC-NH2-SO4 and Na2SO4; SEM image of Zr-BDC-NH2-SO4

after

Ba2+

adsorption;

N2

adsorption-desorption

isotherms

of

Zr-BDC-NH2-SO4 before and after gamma irradiated; XRD pattern of Zr-BDC-NH2-SO4 after soaking in HCl solution; zeta potentials of Zr-BDC-NH2-SO4; element molar ratio in Ba2+ loading samples at various temperatures; elemental analysis of Zr-BDC-NH2-SO4 after Ba2+ adsorption. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (NO. 21536001 and 21606007) and National Key Projects for Fundamental Research and Development of China



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Table of Contents


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Sulfate-Rich Metal-Organic Framework for High Efficiency and

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