Abstract

Rapid removal of radioactive strontium from nuclear wastewater is of great significance for environmental safety and human health. This work reports the effective adsorption of strontium ion in a stable dual-group metal-organic framework, Zr6(OH)14(BDC-(COOH)2)4(SO4)0.75 (Zr-BDC-COOH-SO4), which contains strontium-chelating groups (-COOH and SO4) and a strongly ionizable group (-COOH). Zr-BDC-COOH-SO4 exhibits very rapid adsorption kinetics (<5 min) and a maximum adsorption capacity of 67.5 mg g−1. The adsorption behaviors can be well fitted to the pseudo-second-order model and the Langmuir isotherm model. Further investigations indicate that the adsorption of Sr2+ onto Zr-BDC-COOH-SO4 would not be obviously affected by solution pH and adsorption temperature. The feasible regeneration of the adsorbent was also demonstrated using a simple elution method. Mechanism investigation suggests that free -COOH contributes to the rapid adsorption based on electrostatic interaction, while the introduction of -SO4 significantly enhanced the adsorption capacity. Thus, these results suggest that Zr-BDC-COOH-SO4 is a potential candidate for Sr2+ removal. They also introduce dual groups as an effective strategy for designing high-efficiency adsorbents.

HIGHLIGHTS

  • A dual-group (-COOH and -SO4) metal-organic framework was used to adsorb radioactive Sr2+.

  • Very rapid adsorption kinetics of <5 min was obtained for the adsorbent.

  • Large numbers of free carboxyl groups contribute to the fast adsorption.

  • Introduction of -SO4 significantly enhances the adsorption capacity.

  • The adsorbent exhibits a good anti-interference ability in terms of temperature and pH.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

Nuclear power has been widely used for its high efficiency, but every year it produces a large volume of wastewater containing radioactive ions. Among the nuclides, strontium-90 (90Sr) is one of the species of greatest concern for its long half-life (t1/2 ∼28 years) (Wang et al. 2020a). Intake of this radionuclide, even a very low amount, will cause primary harm to the human body (Sachse et al. 2012). In addition, 90Sr2+ as a beta-emitter can also destroy seawater organisms. Thus, removal from aqueous solution is becoming an important technological requirement.

The removal of Sr2+ from aqueous systems has been carried out using many common liquid-phase removal methods, including ion exchange, solvent extraction, and adsorption (Roane et al. 2003; Marinin & Brown 2000; Rahman et al. 2011; Xu et al. 2012). Adsorption has attracted a large amount of interest for its low operation cost and high removal efficiency, especially for low concentrations. Consequently, exploring effective adsorbents for 90Sr2+ would be greatly beneficial for the removal of Sr2+. The reported adsorbents involve carbon-based materials (Qi et al. 2015; Song et al. 2015), phosphates (Ivanets et al. 2020), metal oxides (Zhang et al. 2015a; Hong et al. 2017), and zeolite (Merceille et al. 2012), etc. However, the adsorption efficiency still needs to be greatly improved.

Over the past decade, metal-organic frameworks (MOFs) have emerged as a potential class of functional materials in various applications due to their excellent porosity and designable chemical property (Bavykina et al. 2020; Gan et al. 2020; Qian et al. 2020; Yang et al. 2020). Some of the MOFs have proved highly stable in water (Wang et al. 2016; Yuan et al. 2018; Wang et al. 2019; Wang et al. 2020b), thus serving as water-phase adsorbents.

For radioactive nuclides, a critical index in adsorbent evaluation is the adsorption kinetics, along with adsorption capacity. The total process of solid-liquid adsorption commonly consists of the diffusion of the guest ion from the liquid system to the surface followed by diffusion into the pores of the adsorbent, and the chemical binding by the adsorption sites. To speed up the adsorption, an effective approach is to improve the diffusion kinetics by introducing a strong interaction between the guest ion and the adsorbent, where electrostatic interaction was a recent focus. In previous reports, several ionizable groups such as -SO3H (Aguila et al. 2016) and -COOH (Mu et al. 2019) were integrated into MOFs to enhance the adsorption of Sr2+. However, along with the adsorption process, the host-guest interactions, especially for electrostatic interaction, were gradually weakened due to the coordination of the ionizable groups (e.g. -COOH, -SO3H) with the ions, leading to the slowing of diffusion kinetics. Thus, a reasonable functionalization for MOFs is needed to maintain a strong electrostatic interaction in the full adsorption process.

In this study, we put forward a dual-group strategy, where one of the groups is responsible for the continuous electrostatic interaction. For this purpose, a stable Zr-MOF (Zr-BDC-COOH-SO4) containing benzenedicarboxylate and -COOH and -SO4 groups was introduced. The free COOH is derived from the organic ligand and -SO4 is anchored to the Zr-O cluster of the MOF. Although both the -SO4 and -COOH may be capable of binding Sr, the high density of -COOH can still contribute continuously to negatively charging the surface of the MOF (-COOH → -COO + H+). The pore and surface of the MOF were characterized and its application for Sr2+ adsorption was studied systematically in terms of adsorption isotherm, kinetics, thermodynamics, and effect of pH. The adsorption mechanism as well as the contribution of the two types of groups was also investigated.

MATERIALS AND METHODS

Materials

The chemicals zirconium sulfate hydrate (Zr(SO4)2·4H2O), 1,2,4,5-benzenetetracarboxylic acid (H4BTEC), and strontium nitrate (Sr(NO3)2) were purchased commercially from HWRK Chem. Co. Ltd. All the chemicals were used directly without further purification.

Preparation of adsorbents

The Zr-MOF-COOH-SO4 was synthesized according to a previous work (Reinsch et al. 2015). In a 100 mL round-bottom flask, H4BTEC (3.8 g, 13.4 mmol) and Zr(SO4)2·4H2O (7.1 g, 20 mmol) were mixed in deionized water (40 mL), and then sulfuric acid (2 mL) was added drop-wise into the suspension. The mixture was heated at 90 °C for 16 hours in an oil bath with a refluxing system. After being cooled to room temperature, the resulting solid was collected via filtration and further washed several times with water and acetone. Then the dried solid was treated with 3 vol% H2SO4 solution at 60 °C for 12 hours. Finally, the collected solid was again washed with water and acetone, and further dried at 100 °C for 24 hours.

Characterization of materials

The powder X-ray diffraction (PXRD) patterns of the MOFs were recorded on a D8 Advance X diffractometer equipped with Cu Kα radiation (λ = 1.54178 Å) at room temperature. The 2θ range from 5° to 50° was scanned with a step size of 0.02°. The Brunauer-Emmett-Teller (BET) specific surface area was characterized on an ASAP 2020 surface area analyzer (Micromeritics). The morphology of the sample was characterized using a SU8020 field emission scanning electron microscope (FE-SEM) and a JEM-F200 transmission electron microscope (TEM). Infrared spectra of the samples were recorded on a Nicolet iS50 FTIR spectrophotometer. The zeta potentials at various pH values were measured on a Zetasizer Nano ZS zeta potential analyzer. X-ray photoelectron spectroscopy (XPS) data was collected on an ESCALAB 250 X-ray photoelectron spectroscopy, using Al Kα X-ray as the excitation source.

Adsorption experiments

Non-radioactive Sr2+ was used in all of the experiments. In the batch adsorption process, a Zr-MOF-COOH-SO4 sample (5 mg) was added to Sr2+ aqueous solution (10 mL) and the resulting suspension was stirred at a speed of 155 rpm in a constant temperature shaker. After that, the clear filtrate was collected via a microfiltration membrane (0.22 μm) and further used to determine the concentration of Sr2+ on a inductively coupled plasma optical emission spectrometer (ICP-OES, Avio 500, PerkinElmer). The adsorption amount was calculated as follows:
formula
(1)
where C0 (mg L−1) and Ce (mg L−1) are the initial and final concentrations of Sr2+ respectively, V (L) is the volume of Sr2+ aqueous solution, m (g) is the mass of the MOF, and Q (mg g−1) is the adsorption amount for Sr2+.

RESULTS AND DISCUSSION

Characterization results

The synthesized MOF was characterized by XRD pattern, N2 adsorption-desorption isotherms, Fourier transform infrared (FTIR) spectra, XPS pattern, SEM and TEM images. The XRD patterns of the synthesized material are shown in Figure 1(a). To verify the successful synthesis of the material, the standard diffraction data was collected based on the crystal document (.cif) on Materials Studio software. It can be seen that the main diffraction peaks corresponding to the lattice planes (101), (110), (002), (112), (200), (211), (220) of the simulated pattern can be almost found in the powder XRD pattern of this MOF. These measured characteristic diffraction peaks are also consistent with those from the experimental data in the literature (Reinsch et al. 2015). Further, a refinement for the diffraction data was performed. The cell parameters of the MOF were found to be a = b = 14.0946 Å, c = 21.5662 Å and α = β = γ = 90°, almost consistent with the values from the literature (a = b = 14.1669 Å, c = 21.4206 Å and α = β = γ = 90°) (Reinsch et al. 2015). These results validate the successful preparation of this Zr-MOF.

Figure 1

Characterization results of Zr-MOF-COOH-SO4: (a) powder XRD patterns and simulated XRD patterns; (b) N2 adsorption-desorption isotherms at 77 K; (c) FTIR spectrum; (d) XPS pattern.

Figure 1

Characterization results of Zr-MOF-COOH-SO4: (a) powder XRD patterns and simulated XRD patterns; (b) N2 adsorption-desorption isotherms at 77 K; (c) FTIR spectrum; (d) XPS pattern.

The permanent porosity was evaluated by N2 adsorption-desorption isotherms at 77 K, as shown in Figure 1(b). The BET specific surface area was calculated as 291.42 m2 g−1, slightly higher than that Reinsch et al. (2015) (250 m2 g−1). This improvement may be attributed to the better activation treatment.

In the FTIR spectrum (Figure 1(c)), the peaks at 1,386.6 cm−1 and 1,566.9 cm−1 are attributed to the symmetric and asymmetric stretching vibrations of OCO (or -COO-Zr), respectively. The peak at 1,131.5 cm−1 is assigned to the vibration of the S = O bond of the SO4 group and the peak at 1,698.4 cm−1 is attributed to the free -COOH in the organic ligand (Li et al. 2014; Zhao et al. 2016). Hence the existence of -SO4 and -COOH groups in the MOF can be verified.

The composition of this MOF was determined by element content analysis using XPS (Figure 1(d)). It was found that the atomic ratio of Zr and S was 6:0.75, that is, one Zr6-inorganic cluster contained 0.75 of an SO4 group. So the formula of the MOF can be determined to be Zr6(OH)14(BTEC)4(SO4)0.75. The -COOH and -SO4 contents were calculated to be ∼4.0 and ∼0.4 mmol g−1 (Zr-MOF).

Finally, the morphology of the material was observed via SEM and TEM images. From Figure 2(a) and 2(b), it can be seen that Zr-MOF-COOH-SO4 is composed of octahedral particles with a homogenous size of ∼100 nm. For a comparison, we further calculated the particle diameter based on the XRD data. The full width half maximum (FWHM) was first determined and then the particle size was calculated according to the Scherrer equation. The average value for the peaks centered at 7.5° and 12.1° was found to be 94.73 nm, consistent with the experimental results. So Zr-MOF-COOH-SO4 exhibits some features such as good porosity, nanoscale size and abundant functional groups, giving it potential for the removal of cationic Sr2+ from aqueous solution.

Figure 2

(a) SEM and (b) TEM images for Zr-MOF-COOH-SO4.

Figure 2

(a) SEM and (b) TEM images for Zr-MOF-COOH-SO4.

Adsorption of Sr2+

Adsorption kinetics

Adsorption kinetics is an important parameter to evaluate the adsorption performance of adsorbents. It may be controlled by the surface and pore properties of adsorbents, and the detailed characteristics of adsorbates. In particular, the adsorption time is especially important for radioactive 90Sr2+ to avoid its widespread diffusion in environment. Hence the adsorption amount of Zr-MOF-COOH-SO4 versus adsorption time was first investigated. As shown in Figure 3, fast and saturated adsorption occurred at only 5 min, indicating that radioactive 90Sr2+ can be removed quickly by this Zr-MOF.

Figure 3

Adsorption amount of Sr2+ versus time (conditions: C0, 100 mg L−1; T, 303 K; natural (unmodified) pH).

Figure 3

Adsorption amount of Sr2+ versus time (conditions: C0, 100 mg L−1; T, 303 K; natural (unmodified) pH).

Further, the adsorption data was fitted to several kinetics models to study the adsorption behavior of Sr2+ in Zr-BDC-COOH-SO4. Three classical isotherms, the pseudo-first-order model, the pseudo-second-order model, and the Weber and Morris intraparticle diffusion model, were used.

The pseudo-first-order model (Lagergren 1898):
formula
(2)
The pseudo-second-order model (Zhao et al. 2014):
formula
(3)
The Weber and Morris intraparticle diffusion model (Weber & Morris 1963):
formula
(4)
where Qe (mg g−1) is the equilibrium adsorption capacity for Sr2+, Qt (mg g−1) is the adsorption capacity for Sr2+ at time t (min), k1 (min−1), k2 (g min−1 mg−1), and kid (mg g−1 min−0.5) are the rate constants of the pseudo-first order model, the pseudo-second-order model, and the intraparticle diffusion model, respectively. From the fitting parameters (Table 1) especially the correlation coefficient R2, the adsorption behavior of Sr2+ onto Zr-MOF-COOH-SO4 can be well described by pseudo-second-order model. This indicates that the rate-controlling step of the adsorption process is chemical adsorption.
Table 1

Kinetics model parameters of Sr2+ adsorbed onto Zr-MOF-COOH-SO4

ModelModelling parametersValue
Pseudo-first-order Qe.cal (mg g−11.8371 
k1 (min−10.0111 
R2 0.5855 
Pseudo-second-order Qe.cal (mg g−145.05 
k2 (g min−1 mg−10.0169 
R2 0.9998 
Intraparticle diffusion kid (mg g−1 min−0.50.0793 
C 43.25 
R2 0.1727 
ModelModelling parametersValue
Pseudo-first-order Qe.cal (mg g−11.8371 
k1 (min−10.0111 
R2 0.5855 
Pseudo-second-order Qe.cal (mg g−145.05 
k2 (g min−1 mg−10.0169 
R2 0.9998 
Intraparticle diffusion kid (mg g−1 min−0.50.0793 
C 43.25 
R2 0.1727 

Adsorption isotherm and thermodynamics

The adsorption capacity of Zr-MOF-COOH-SO4 was investigated based on the adsorption isotherms at the initial concentration range of 10–500 mg L−1. Figure 4(a) lists three adsorption isotherms at 293, 303, and 313 K, respectively. It can be seen that the adsorption capacity decreases slightly with the increase in adsorption temperature. The maximum adsorption capacities were found to be 71.6, 67.5, and 64.5 mg g−1 at 293, 303, and 313 K, respectively. This indicates that room temperature rather than higher temperatures enable efficient removal of Sr2+. As a comparison, the adsorption performances of other reported Sr2+ adsorbents were investigated (İnan & Altaş 2010, 2011; Tel et al. 2010; Wu et al. 2012; Song et al. 2015; Tayyebi et al. 2015; Zhang et al. 2015a, 2015b; Xiao et al. 2016; Asgari et al. 2019; Mu et al. 2019; El-saied et al. 2020). As listed in Table 2, Zr-MOF-COOH-SO4 exhibits a superior adsorption capacity compared to other adsorbents including metal oxides, GO-type materials, and some modified minerals. Although the capacities of KTNflux-600 and MOF-808-SO4 (or -C2O4) are higher than that of Zr-MOF-COOH-SO4, the adsorption equilibrium time of those materials (2 or 3 hours) is considerably longer than the 5 min of this work. Thus, we suggest the material in this work may be a promising adsorbent for Sr2+.

Table 2

Comparison of maximum adsorption capacities of adsorbents

AdsorbentsQmax (mg g−1)Equilibrium time (min)Temperature (K)Reference
Wet-oxidized OMC FDU-15 7.3 298 Song et al. (2015)  
ZrO2 10.5a 150 303 Tel et al. (2010)  
Ca-Mt 13.2 ∼1,500 301 Wu et al. (2012)  
M-GO 14.3 300 R.T. Tayyebi et al. (2015)  
Zn-Mn oxide/PAN 21.4  333 İnan & Altaş (2011)  
Sb(III)/Sb2O5 23.6a ∼6 303 Zhang et al. (2015a)  
Zr-Mn oxide 30.9 ∼165 303 İnan & Altaş (2010)  
SANCHs nanocomposite 47.2 180 R.T. El-saied et al. (2020)  
SiSb-1 ∼55 ∼30 303 Zhang et al. (2015b)  
Nd-BTC 58 ∼60 298 Asgari et al. (2019)  
APTES-Mt 65.6 ∼3,000 301 Wu et al. (2012)  
Zr-MOF-SO4/COOH 67.5 303 This work 
KTNflux-600 91.1 180 R.T. Xiao et al. (2016)  
MOF-808-SO4 176.6 120 293 Mu et al. (2019)  
MOF-808-C2O4 206.3 120 293 Mu et al. (2019)  
AdsorbentsQmax (mg g−1)Equilibrium time (min)Temperature (K)Reference
Wet-oxidized OMC FDU-15 7.3 298 Song et al. (2015)  
ZrO2 10.5a 150 303 Tel et al. (2010)  
Ca-Mt 13.2 ∼1,500 301 Wu et al. (2012)  
M-GO 14.3 300 R.T. Tayyebi et al. (2015)  
Zn-Mn oxide/PAN 21.4  333 İnan & Altaş (2011)  
Sb(III)/Sb2O5 23.6a ∼6 303 Zhang et al. (2015a)  
Zr-Mn oxide 30.9 ∼165 303 İnan & Altaş (2010)  
SANCHs nanocomposite 47.2 180 R.T. El-saied et al. (2020)  
SiSb-1 ∼55 ∼30 303 Zhang et al. (2015b)  
Nd-BTC 58 ∼60 298 Asgari et al. (2019)  
APTES-Mt 65.6 ∼3,000 301 Wu et al. (2012)  
Zr-MOF-SO4/COOH 67.5 303 This work 
KTNflux-600 91.1 180 R.T. Xiao et al. (2016)  
MOF-808-SO4 176.6 120 293 Mu et al. (2019)  
MOF-808-C2O4 206.3 120 293 Mu et al. (2019)  

aThe values were obtained from the Langmuir model fitting result.

Figure 4

(a) Adsorption isotherms at different temperatures (conditions: natural pH; t, 12 hours); (b) van 't Hoff plot of Sr2+ adsorption onto Zr-MOF-COOH-SO4.

Figure 4

(a) Adsorption isotherms at different temperatures (conditions: natural pH; t, 12 hours); (b) van 't Hoff plot of Sr2+ adsorption onto Zr-MOF-COOH-SO4.

The adsorption mode of Sr2+ onto Zr-MOF-SO4/COOH was studied through the fitting to several isotherms models.

The Langmuir isotherm model (Langmuir 1916):
formula
(5)
The Freundlich isotherm model (Freundlich 1906):
formula
(6)
where Ce (mg L−1) is the equilibrium concentration of Sr2+, b (L mg−1) and Qm (mg g−1) are the Langmuir constant and Langmuir adsorption capacity, respectively; n and KF [(L mg−1)1/n mg g−1] are related to the adsorption intensity and capacity, respectively. From the values of Qm and the correlation coefficient R2 (Table 3), the adsorption behavior agrees better with the Langmuir model than the Freundlich model, validating the homogenous distribution of adsorption sites and the monolayer adsorption of Sr2+ onto Zr-BDC-COOH-SO4.
The thermodynamic behavior of Sr2+ adsorption onto Zr-MOF-COOH-SO4 was studied. The thermodynamic parameters standard enthalpy change (ΔH°), entropy change (ΔS°), and Gibbs energy change (ΔG°) were calculated according to the thermodynamic equations:
formula
(7)
formula
(8)
formula
(9)
where Kd (L g−1) is the adsorption equilibrium constant and R is the universal constant. The relationship of lnKd and 1/T at C0 = 400 mg L−1 is illustrated in Figure 4(b); the parameters were calculated according to the fitting results. The negative values of ΔS° (−30.1 J mol−1 K−1) and ΔH° (−4.9 kJ mol−1) indicate the adsorption process is an entropy decrease reaction and beneficial from the enthalpy decrease effect. The ΔG° at 293, 303, and 313 K were determined as the positive values of 3.9, 4.2, and 4.5 kJ mol−1, caused by the low values of Kd (Qe/Ce). This is attributed to the fact that the small pore channels of the material may limit the full use of the interior adsorption sites and thereby the much larger capacity, due to the pore blockage effect. As a comparison, ΔG° was found to have a negative value at the lower concentration where the adsorption percentage of Sr2+ was larger and thereby the Kd value was significantly enhanced. As proved in the ‘Adsorption kinetics’ and ‘Adsorption mechanism’ sections, this adsorption process involves a strongly chemical reaction. Thus, even with the positive values, the adsorption of Sr2+ onto Zr-MOF-COOH-SO4 is still practicable due to the strong host-guest chemical interactions.
Table 3

Isotherms models parameter of Sr2+ adsorbed onto Zr-MOF-COOH-SO4

Adsorption modelModelling parameterValue
293 K303 K313 K
Langmuir Qm,1 (mg g−176.34 73.53 68.49 
b (L mg−10.0287 0.0247 0.0238 
R2 0.9954 0.9946 0.9905 
Freundlich kF [(mg g−1)(L mg−1)1/n12.85 7.129 8.365 
1/n 0.2922 0.3945 0.3484 
R2 0.9934 0.9698 0.9764 
Adsorption modelModelling parameterValue
293 K303 K313 K
Langmuir Qm,1 (mg g−176.34 73.53 68.49 
b (L mg−10.0287 0.0247 0.0238 
R2 0.9954 0.9946 0.9905 
Freundlich kF [(mg g−1)(L mg−1)1/n12.85 7.129 8.365 
1/n 0.2922 0.3945 0.3484 
R2 0.9934 0.9698 0.9764 

Effect of solution pH

Adsorption of ionic adsorbates is commonly controlled by solution pH, which determines the surface charge property of adsorbents and detailed existing mode of adsorbates. Therefore, the effect of pH on the adsorption as well as the optimal pH was investigated. First, the zeta potentials of Zr-MOF-COOH-SO4 in the pH range of 3.0–9.0 were measured, as shown in Figure 5(a). It can be seen that in the wide pH range, the surface of the material is always negatively charged, attributed to the ionization process of the functional groups, especially for -COOH (MOF-COOH → MOF-COO + H+). Considering the pH of the Sr2+-containing wastewater is commonly in the acidic range, we studied the adsorption behavior of Zr-BDC-COOH-SO4 in acidic solutions. The effect of pH on the adsorption behavior is shown in Figure 5(b). In the pH range of 3.7–5.3, the adsorption amount of Sr2+ depends slightly on solution pH; while at pH >5.3, the extent of Sr2+ adsorption was enhanced, which may be attributed to the enhanced electrostatic interaction between Sr2+ and the negatively charged MOF sample.

Figure 5

(a) Zeta potentials of Zr-BDC-COOH-SO4 at a pH range of 3.0–9.0; (b) adsorption capacity at a pH range of 3.7–6.2 (conditions: C0, 100 mg L−1; t, 12 hours; T, 303 K).

Figure 5

(a) Zeta potentials of Zr-BDC-COOH-SO4 at a pH range of 3.0–9.0; (b) adsorption capacity at a pH range of 3.7–6.2 (conditions: C0, 100 mg L−1; t, 12 hours; T, 303 K).

Regeneration investigation

From the point of view of practical application, an adsorbent should be reusable to control the cost. Therefore, the regeneration of Zr-BDC-COOH-SO4 was investigated. A two-step washing method using Na2SO4 aqueous solution (500 mg L−1) and HNO3 solution (pH = 2) was used. In detail, an Sr2+-loaded sample was successively immersed in the two solutions and stirred for 12 hours to wash away the adsorbed Sr2+. After that, the sample was slightly washed with deionized water and further dried. As shown in Figure 6, after three uses, the adsorption capacity of the sample could still reach ∼70% of that of the fresh sample. This decrease may be attributed to the strong host-guest binding force and the incomplete desorption of Sr2+ from the adsorbent. Even so, the capacity after three uses is still higher than those of materials like Sb(III)/Sb2O5 (Zhang et al. 2015a) and M-GO (Tayyebi et al. 2015).

Figure 6

The reusability of Zr-BDC-COOH-SO4 using Na2SO4 and HNO3 as the eluents (conditions: natural pH; t, 12 hours; T, 303 K; C0, 100 mg L−1).

Figure 6

The reusability of Zr-BDC-COOH-SO4 using Na2SO4 and HNO3 as the eluents (conditions: natural pH; t, 12 hours; T, 303 K; C0, 100 mg L−1).

Adsorption mechanism

First, the adsorption of Sr2+ onto Zr-MOF-COOH-SO4 was confirmed through XPS. As shown in Figure 7(a), the signature of Sr 3d can be clearly found in the XPS pattern of the sample after adsorption, validating the existence of the Sr element in the sample. The powder XRD pattern of the sample after adsorption was also measured. An obvious decrease in the long-range order can be seen in Figure 7(b), which is attributed to the fact that the introduction of Sr2+ affects the coordination of the Zr-O cluster and the SO42− ion. Meanwhile, the main distinct diffraction peaks remained, indicating that the framework of the MOF remained relatively complete. Similar phenomena can be found in the high-temperature activation process of this MOF (Reinsch et al. 2015). This conclusion can also be verified by the SEM image. As shown in Figure 7(c), compared with the original MOF (Figure 2(a)), there are no obvious changes in either surface or morphology.

Figure 7

Mechanism analysis for Sr2+ adsorption onto Zr-BDC-COOH-SO4: (a) XPS patterns; (b) powder XRD patterns; (c) SEM image of Sr2+-loaded sample: (d) FTIR spectra; (e) adsorption capacity of Zr-BDC-COOH-SO4, UiO-66-(COOH)2, and MIL-121 at C0 = 100 mg L−1; (f) possible adsorption mechanism based on coordination interaction and electrostatic interaction.

Figure 7

Mechanism analysis for Sr2+ adsorption onto Zr-BDC-COOH-SO4: (a) XPS patterns; (b) powder XRD patterns; (c) SEM image of Sr2+-loaded sample: (d) FTIR spectra; (e) adsorption capacity of Zr-BDC-COOH-SO4, UiO-66-(COOH)2, and MIL-121 at C0 = 100 mg L−1; (f) possible adsorption mechanism based on coordination interaction and electrostatic interaction.

The detailed adsorption mechanism was further analyzed by FTIR, as shown in Figure 7(d). After adsorption, the peak assigned to S = O shifted from 1,131.5 cm−1 to 1,126.4 cm−1 and the sign of free -COOH also showed a change from 1,698.4 cm−1 to 1,704.3 cm−1, verifying the binding between Sr2+ and -SO4 and -COOH. Furthermore, the degree of contribution of these groups was evaluated through a comparison with UiO-66-(COOH)2 (Zr-BDC-COOH) and MIL-121 (Al-BDC-COOH). The contents of free -COOH in these MOFs are 5.5 and 6.7 mmol g−1 respectively, comparable with that of Zr-BDC-COOH-SO4 (4.0 mmol g−1). At the same condition (100 mg L−1), the adsorption capacities of the two MOFs were measured as 23.2 and 19.0 mg g−1, lower than that of Zr-BDC-COOH-SO4 (40.0 mg g−1) (Figure 7(e)). This highlights the important contribution of -SO4 groups.

FInally, the surface charge property of the Sr2+-loaded sample was analyzed. Zr-BDC-COOH-SO4 was immersed in the Sr2+ solution (pH0 = 6.0, pHf = ∼4.0) for 12 hours and then the solid was collected and further dried. The zeta potential of the sample at pH = 4.0 was measured as −3.5 mV, indicating the surface of this Zr-MOF was always negatively charged in the total adsorption process. As a result, a strong electrostatic interaction existed in the adsorption process, including the diffusion step of Sr2+, which was the main reason for the rapid kinetics. Therefore, in the Zr-BDC-COOH-SO4, the high density of -COOH was beneficial for the rapid adsorption and capture of Sr2+, but the introduction of -SO4 improved the adsorption capacity. The possible adsorption mechanism is illustrated in Figure 7(f).

CONCLUSION

In this work, Zr-BDC-COOH-SO4 with dual functional groups was introduced for the adsorptive removal of Sr2+ from wastewater. Due to the anchored -SO4 and high density of -COOH groups, the material exhibited an ideal adsorption capacity and very rapid adsorption kinetics. In addition, an excellent lack of sensitivity towards temperature and solution pH was demonstrated. Compared with those MOFs with single type of adsorption site, the dual-group MOF shows a promising application when the surface charge property and active adsorption sites can be well controlled. Therefore, this work not only demonstrates that Zr-BDC-COOH-SO4 is a potential adsorbent for radioactive Sr2+ in nuclear wastewater treatment, but also highlights a new method for designing adsorbents by introducing dual functional groups.

ACKNOWLEDGEMENTS

This work was supported by Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2019L0960) and LuLiang Key Research and Development Projects (No. GXZDYF2019084).

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

REFERENCES

Aguila
B.
Banerjee
D.
Nie
Z.
Shin
Y.
Ma
S.
Thallapally
P. K.
2016
Selective removal of cesium and strontium using porous frameworks from high level nuclear waste
.
Chemical Communication
52
,
5940
5942
.
Bavykina
A.
Kolobov
N.
Khan
I. S.
Bau
J. A.
Ramirez
A.
Gascon
J.
2020
Metal-organic frameworks in heterogeneous catalysis: recent progress, new trends, and future perspectives
.
Chemical Review
120
,
8468
8535
.
El-saied
H.-A.
Shahr El-Din
A. M.
Masry
B. A.
Ibrahim
A. M.
2020
A promising superadsorbent nanocomposite based on grafting biopolymer/nanomagnetite for capture of 134Cs, 85Sr and 60Co radionuclides
.
Journal of Polymers and the Environment
28
,
1749
1765
.
Freundlich
H. M. F.
1906
Über Die adsorption in Lösungen (Adsorption in solution)
.
Zeitschrift für Physikalische Chemie
57
,
385
470
.
Gan
L.
Chidambaram
A.
Fonquernie
P. G.
Light
M. E.
Choquesillo-Lazarte
D.
Huang
H.
Solano
E.
Fraile
J.
Viñas
C.
Teixidor
F.
Navarro
J. A. R.
Stylianou
K. C.
Planas
J. G.
2020
A highly water-stable meta-carborane-based copper metal-organic framework for efficient high-temperature butanol separation
.
Journal of the American Chemical Society
142
,
8299
8311
.
Hong
H.-J.
Kim
B.-G.
Hong
J.
Ryu
J.
Ryu
T.
Chung
K.-S.
Kim
H.
Park
I.-S.
2017
Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism
.
Chemical Engineering Journal
319
,
163
169
.
Ivanets
A.
Milyutin
V.
Shashkova
I.
Kitikova
N.
Nekrasova
N.
Radkevich
A.
2020
Sorption of stable and radioactive Cs(I), Sr(II), Co(II) ions on Ti-Ca-Mg phosphates
.
Journal of Radioanalytical and Nuclear Chemistry
324
,
1115
1123
.
Lagergren
S.
1898
Theorie der Sogenannten Adsorption Geloster Stoffe (Theory of so-called adsorption soluble substances), Kungliga Svenska Veten-skapsakademiens
.
Handlingar
24
,
1
39
.
Langmuir
I.
1916
The constitution and fundamental properties of solid and liquids. Part I. Solids
.
Journal of the American Chemical Society
38
,
2221
2295
.
Li
B.
Zhang
Y.
Krishna
R.
Yao
K.
Han
Y.
Wu
Z.
Ma
D.
Shi
Z.
Pham
T.
Space
B.
Liu
J.
Thallapally
P. K.
Liu
J.
Chrzanowski
M.
Ma
S.
2014
Introduction of π-complexation into porous aromatic framework for highly selective adsorption of ethylene over ethane
.
Journal of the American Chemical Society
136
,
8654
8660
.
Merceille
A.
Weinzaepfel
E.
Barre
Y.
Grandjean
A.
2012
The sorption behaviour of synthetic sodium nonatitanate and zeolite A for removing radioactive strontium from aqueous wastes
.
Separation and Purification Technology
96
,
81
88
.
Qian
Q.
Asinger
P. A.
Lee
M. J.
Han
G.
Rodriguez
K. M.
Lin
S.
Benedetti
F. M.
Wu
A. X.
Chi
W. S.
Smith
Z. P.
2020
MOF-based membranes for gas separation
.
Chemical Review
120
,
8161
8266
.
Rahman
R. O. A.
Ibrahim
H. A.
Hung
Y.
2011
Liquid radioactive waste treatment: a review
.
Water
3
,
551
565
.
Reinsch
H.
Bueken
B.
Vermoortele
F.
Stassen
I.
Lieb
A.
Lillerud
K.-P.
De Vos
D.
2015
Green synthesis of zirconium-MOFs
.
CrystEngComm
17
,
4070
4074
.
Roane
J. E.
Devol
T. A.
Leyba
J. D.
Fjeld
R. A.
2003
The use of extraction chromatography resins to concentrate actinides and strontium from soil for radiochromatographic analyses
.
Journal of Environmental Radioactivity
66
,
227
245
.
Sachse
A.
Merceille
A.
Barre
Y.
Grandjean
A.
Fajula
F.
Galarneau
A.
2012
Macroporous LTA-monoliths for in-flow removal of radioactive strontium from aqueous effluents: application to the case of Fukushima
.
Microporous Mesoporous Materials
164
,
251
258
.
Wang
B.
Lv
X.-L.
Feng
D.
Xie
L.-H.
Zhang
J.
Li
M.
Xie
Y.
Li
J.-R.
Zhou
H.-C.
2016
Highly stable Zr(VI)-based metal-organic frameworks for the detection and removal of antibiotics and organic explosives in water
.
Journal of the American Chemical Society
138
,
6204
6216
.
Wang
K.
Huang
H.
Zhou
X.
Wang
Q.
Li
G.
Shen
H.
She
Y.
Zhong
C.
2019
Highly chemically stable MOFs with trifluoromethyl groups: effect of position of trifluoromethyl groups on chemical stability
.
Inorganic Chemistry
58
,
5725
5732
.
Wang
K.
Wang
Q.
Wang
X.
Wang
M.
Wang
Q.
Shen
H.-M.
Yang
Y.-F.
She
Y.
2020b
Intramolecular hydrogen bond-induced high chemical stability of metal-organic frameworks
.
Inorganic Chemistry Frontiers
7
,
3548
3554
.
Weber
W. J.
Morris
J. C.
1963
Kinetics of adsorption on carbon from solution
.
Journal of the Sanitary Engineering Division, American Society of Civil Engineers
89
,
31
40
.
Wu
P.
Dai
Y.
Long
H.
Zhu
N.
Li
P.
Wu
J.
Dang
Z.
2012
Characterization of organo-montmorillonites and comparison for Sr(II) removal: equilibrium and kinetic studies
.
Chemical Engineering Journal
191
,
288
296
.
Xiao
X.
Hayashi
F.
Shiiba
H.
Selcuk
S.
Ishihara
K.
Namiki
K.
Shao
L.
Nishikiori
H.
Selloni
A.
Teshima
K.
2016
Platy KTiNbO5 as a selective Sr ion adsorbent: crystal growth, adsorption experiments, and DFT calculations
.
Journal of Physical Chemistry C
120
,
11984
11992
.
Xu
C.
Wang
J.
Chen
J.
2012
Solvent extraction of strontium and cesium: a review of recent progress
.
Solvent Extraction and Ion Exchange
30
,
623
650
.
Yang
S.
Peng
L.
Syzgantseva
O. A.
Trukhina
O.
Kochetygov
I.
Justin
A.
Sun
D. T.
Abedini
H.
Syzgantseva
M. A.
Oveisi
E.
Lu
G.
Queen
W. L.
2020
Preparation of highly porous metal-organic framework beads for metal extraction from liquid streams
.
Journal of the American Chemical Society
142
,
13415
13425
.
Yuan
S.
Feng
L.
Wang
K.
Pang
J.
Bosch
M.
Lollar
C.
Sun
Y.
Qin
J.
Yang
X.
Zhang
P.
Wang
Q.
Zou
L.
Zhang
Y.
Zhang
L.
Fang
Y.
Li
J.
Zhou
H.-C.
2018
Stable metal-organic frameworks: design, synthesis, and applications
.
Advanced Materials
30
,
1704303
.
Zhang
L.
Wei
J.
Zhao
X.
Li
F.
Jiang
F.
Zhang
M.
2015a
Strontium(II) adsorption on Sb(III)/Sb2O5
.
Chemical Engineering Journal
267
,
245
252
.
Zhang
L.
Wei
J.
Zhao
X.
Li
F.
Jiang
F.
2015b
Adsorption characteristics of strontium on synthesized antimony silicate
.
Chemical Engineering Journal
277
,
378
387
.
Zhao
X.
Liu
D.
Huang
H.
Zhang
W.
Yang
Q.
Zhong
C.
2014
The stability and defluoridation performance of MOFs in fluoride solutions
.
Microporous Mesoporous Materials
185
,
72
78
.
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