Efficient Removal of Antimony (III, V) from Contaminated Water by

Article pubs.acs.org/jced

Efficient Removal of Antimony (III, V) from Contaminated Water by Amino Modification of a Zirconium Metal−Organic Framework with Mechanism Study Xingyu He,†,‡,§ Xiaobo Min,§ and Xubiao Luo*,†,‡ †

Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, P. R. China ‡ College of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, P. R. China § School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China S Supporting Information *

ABSTRACT: The adsorption performance of the amino modification of zirconium metal−organic framework (UiO66(NH2)) for the removal of antimony (Sb) from aqueous solution has been investigated. The influence of equilibrium concentration, solution pH, temperature, and contact time on the Sb adsorption were investigated by the batch method. Compared with original UiO-66, the UiO-66(NH2) adsorption capacity for Sb(III) and Sb(V) increased to 61.8 mg/g and 105.4 mg/g for Sb(III) and Sb(V), although the surface area of UiO-66(NH2) decreased from 486.31 m2/g to 113.46 m2/g. The adsorption equilibrium data of Sb on UiO-66(NH2) fitted well with the Langmuir adsorption model, and the kinetics data fitted well with the second-order adsorption model. Thermodynamic parameters indicated that adsorption processes of Sb were feasible, endothermic, and spontaneous. The adsorption isotherm parameters indicated that the Sb adsorption data fitted well with the Langmuir model. The mean adsorption energy obtained from the Dubinin−Radushkevich (D−R) isotherm model further revealed that the Sb adsorption process was chemisorption. Additionally, FTIR analysis and X-ray photoelectron spectroscopy (XPS) study revealed that the Zr−O bond and amino group played a significant role in the Sb removal. Thus, UiO66(NH2) is a promising candidate for Sb contaminated water remediation on the basis of low-cost, easy availability, high Sb adsorption capacity, nontoxicity, and high stability.

1. INTRODUCTION As one kind of hazardous contamination, antimony (Sb) has been ubiquitously distributed throughout the environment as a consequence of mining and smelting. Generally, Sb mainly occurs as inorganic forms of Sb(III) and Sb(V).1 The Sb(V) exists in Sb(OH)6− among relevant pH values range from 3 to 10 under oxidizing conditions; however, Sb(III) mainly occurs as Sb(OH)3 which is an uncharged antimonous acid in the aqueous systems.2 The toxicity of Sb(III) is 10 times higher than that of Sb(V).3 The World Health Organization (WHO) has set a guideline limit of 5 μg/L for Sb in drinking water due to the properties of their high toxicity.4 The concentration of Sb in some water system is always over the maximum permissible concentration. Hence, the effective removal of Sb from both wastewater and drinking water is essential to protect environment and human health. Different approaches, including precipitation, ion-exchange, membrane filtration, electrochemical deposition, biosorption, and adsorption,5−8 have been used widely for removal of Sb. Among these methods, adsorption is considered as a safe, simple, rapid, cost-effective, compact, and eco-friendly method © XXXX American Chemical Society

for Sb removal, especially when the Sb concentration is particularly low. So far, graphene,9 Fe−Mn binary oxide,10 and multiwalled carbon nanotubes11 were often applied in removing Sb(III). The adsorbents which were developed to simultaneously remove Sb(III) and Sb(V) were very less reported. Zhao et al.12 have synthesized an adsorbent of PVA-Fe0 with maximum adsorption capacities of only 6.99 mg/g (Sb(III)) and 1.65 mg/g (Sb(V)). Luo et al.13 reported that ZCN has the adsorption performance of 70.83 mg/g (Sb(III)) and 57.17 mg/g (Sb(V)). The adsorption capacities of these adsorbents were still remarkably low. Therefore, it is necessary to develop novel adsorbents to remove Sb with excellent adsorption performance. Metal−organic frameworks (MOFs) with pore tunability, crystalline nature, and structural diversity have attracted great interest due to their numerous potential applications such as gas adsorption,14 separation,15 chemical sensing,16−18 and linear Received: January 4, 2017 Accepted: March 22, 2017

A

DOI: 10.1021/acs.jced.7b00010 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Schematic illustration of synthesis process of UiO-66(NH2) and UiO-66.

optical properties.19,20 Recently, the MOF materials have been developed as a new class of adsorbent for the removal of various hazardous materials such as heavy metal ions,21,22 organic dyes,23 alcohols,24 and aromatic compounds25 from the environment. Cavka et al.26 had first synthesized Zr-based MOFs (UiO-66) and caused widespread attention due to its excellent chemical and thermal stability. Recently, how to improve the performance of Zr-based MOFs has become the current research focus.27 Yee et al.28 reported Zr-based MOFs functionalized with thiol for the effective mercury removal both in highly strong acid and vapor phase. Inspired by this thought, Zr-based MOFs modified with amino groups (UiO-66(NH2)) are very promising to enhance adsorption performance for Sb(III) and Sb(V), because the introduction of amino group can increase the interaction force with Sb. In this study, Zr-based MOFs (UiO-66(NH2) and UiO-66) were successfully synthesized via the simple solvothermal method and applied to the removal of both antimonite (Sb(III)) and antimonate (Sb(V)) from aqueous solution. The adsorption performance and process of the Zr-based MOFs had been investigated. Additionally, FTIR analysis and XPS study were used to elucidate the adsorption mechanism of Zr-based MOFs for Sb(III) and Sb(V).

infrared (FTIR) analysis was performed using a VERTEX 70 FTIR apparatus with KBr method. The BET surface area measurement was performed with N2 adsorption−desorption isotherms at liquid nitrogen temperature (77K) after dehydration under vacuum at 423 K for 12 h using Micromeritics TriStar II 3020. X-ray photoelectron spectroscopy (XPS) measurement was performed on a Kratos AXIS Ultra DLD using a monochromic Al−Kα (λ = 1486.7 eV). Sb analysis was performed using a ContrAA 700 atomic adsorption spectrophotometer (AAS) (Analytik Jena, Germany). 2.3. Synthesis of UiO-66(NH2) and UiO-66. Figure 1 shows the synthesis process of UiO-66(NH2) and UiO-66, and UiO-66(NH2) and UiO-66 were synthesized by hydrothermal method; the details are described in Supporting Information. 2.4. Batch Adsorption Experiments. The synthesized UiO-66(NH2) and UiO-66 were equilibrated with 20 mL different concentration of Sb(III) and Sb(V) solution. The adsorption isotherms of Sb(III) and Sb(V) were performed as batch experiments at 298, 308, and 318 K, respectively. The initial concentrations of Sb(III) and Sb(V) solution were in the range of 10−600 mg/L. All of the suspensions in the conical flasks were sealed and shaken at 160 rpm. After 24 h, the antimony concentrations were analyzed by the ContrAA 700 atomic adsorption spectrophotometer (AAS). The amount of Sb bonded on the surface of adsorbent was calculated by eq 1 as follows:

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. All chemicals used were analytical grade. p-Phthalic acid (PTA), 2-aminoterephthalic acid, C8H4K2O12Sb2·xH2O, and KSb(OH)6 were supplied by Aladdin Chemistry Co., Ltd. (Shanghai, China). Zirconium chloride (ZrCl4) were purchased from J&K Scientific Ltd. (Beijing, China). N,N-Dimethylformamide (DMF) and methanol (CH3OH) were obtained from LongXi Chemical Industry Ltd. (Ganshu, China). Water was purified with a Milli-Q water system (Bedford, USA). 2.2. Characterization. X-ray diffraction (XRD) analysis was performed using a Bruker D8-Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å). The morphology analysis was performed by scanning electron microscopy (SEM) and a transmission electron microscope (TEM) operated on a JEOL JSM 7401 and H-7650B, respectively. Fourier transform

Qe =

(C0 − Ce) × V W

(1)

where Qe (mg·g−1) is the adsorption capacity of the adsorbent at equilibrium; C0 and Ce are the initial concentration and equilibrium concentration, respectively; V (L) is the volume of the suspension; W (g) is the amount of adsorbent used in the experiment. 2.5. Adsorption Kinetics. For the adsorption kinetics experiment, 200 mg of adsorbent was added into 200 mL of 500 mg/L Sb(III) and Sb(V) solution under magnetic stirring at room temperature. Then, samples were taken out at different reaction times. The samples were filtered through a 0.22 μm B



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syringe filter to separate the adsorbent. The Sb in the supernatant was analyzed by AAS. 2.6. Effect of pH. To elucidate the effect of the solution pH on adsorption performance, the pH value of the Sb solution was controlled and maintained first to a specific value by adding 0.1 mol/L HCl or 0.1 mol/L NaOH. Then, 50 mg of UiO66(NH2) or UiO-66 was added into 50 mL of Sb solution at a specific pH value. All of the prepared mixtures were shaken at 298 K for 24 h. After adsorption equilibration, the concentration of Sb(III) and Sb(V) in the supernatant was analyzed by AAS. 2.7. Effects of Coexisting Anions. To investigate the effect of the coexisting anions on adsorption performance, different anions, including Cl−, Br−, NO3−, CO32−, SO42−, H2PO4−, and HPO42−, were added in Sb solutions. Then all of the prepared mixtures were shaken at 298 K for 24 h. After adsorption equilibration, the concentration of Sb(III) and Sb(V) in the supernatant was analyzed by AAS.

Figure 3. N2 adsorption−desorption isotherm of UiO-66(NH2) and UiO-66.

the BET surface area of UiO-66(NH2). UiO-66(NH2) and UiO-66 exhibit a typical type IV isotherm with a distinct hysteresis H3-type loop. At the high P/P0, the UiO-66 and UiO-66(NH2) have a high N2 adsorption performance, demonstrating that accumulational pores exist in these MOFs. 3.3. Crystal Structure. The synthesized products were characterized by XRD (Figure 4). It is obvious that the

3. RESULTS AND DISCUSSION 3.1. SEM and TEM Analysis. The morphology and topography of UiO-66 and UiO-66(NH2) were analyzed by SEM and TEM. The SEM image (Figure 2a/b) shows the small

Figure 4. XRD patterns of UiO-66, UiO-66(NH2), and stimulated UiO-66. Figure 2. SEM/TEM images of (a/c) UiO-66 and (b/d) UiO66(NH2).

experimental XRD patterns of UiO-66 and UiO-66(NH2) are in good agreement with the simulated pattern of UiO-66, indicating the successful preparation of UiO-66 and UiO66(NH2).29,30 The XRD patterns of antimony containing UiO66 and UiO-66(NH2) are shown in Figure S1; no significant changes are observed in the main diffraction peaks, and this result reveals the unchanged structures and retained crystallinity, demonstrating that the excellent stability of the topology of UiO-66 and UiO-66(NH2). 3.4. FTIR Analysis. The FTIR spectra of the synthesized UiO-66(NH2) and UiO-66 are presented in Figure S2. Compared with UiO-66, the characteristic peaks of UiO66(NH2) are amine asymmetrical stretching at 3466 cm−1, amine symmetrical stretching at 3374 cm−1, N−H bending at 1622 cm−1, and C−N stretching at 1341 cm−1. This results reveal that UiO-66(NH2) modified with amine groups has been successfully synthesized. Figures 5 and 6 also show the FTIR spectra of UiO-66(NH2) and UiO-66 after Sb(III) and Sb(V) adsorption. The N−H stretching (Figure 5a), N−H bending

size, highly separated and homogeneous of the UiO-66 and UiO-66(NH2). From Figure 2a and b, we can observe the smooth UiO-66 surface, while the UiO-66(NH2) surface is very rough. This results ascribed to the presence of amino groups on the surface of UiO-66(NH2). TEM image was performed to further characterize the dispersity and diameter of UiO-66 and UiO-66(NH2). Figure 2c/d reveals the MOFs exhibited an excellent property of good dispersion and uniform size without obvious reunion and the average particle sizes of UiO-66 and UiO-66(NH2) are approximately 50 nm. 3.2. BET Analysis. The BET results (Figure 3) show the specific BET surface area of UiO-66 was 486.31 m2/g, significantly higher than that of UiO-66(NH2) (113.46 m2/ g). The amino group of UiO-66(NH2) could protrude into the space of the pores of its surface and then cause the decrease of C



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Figure 5. FTIR spectra of UiO-66(NH2) (black), Sb(III) containing UiO-66(NH2) (red), and Sb(V) containing UiO-66(NH2) (blue).

increased with the increase of temperature. It clearly revealed that the Sb adsorption processes on UiO-66(NH2) and UiO-66 were endothermic in nature. The equilibrium data were employed to fit with Langmuir, Freundlich, and the Dubinin−Radushkevich (D-R) isotherm models (eqs 2, 3, and 4) to analyze the nature of Sb(III) and Sb(V) adsorption process. The Langmuir31 and Freundlich32 isotherm models were shown in the following equations, respectively:

vibration (Figure 5b), and Zr−O vibration (Figure 5e and Figure 6d) shifted marginally after antimony ions adsorption, while the benzene skeleton vibration (Figure 5c,d and Figure 6a−c) adsorption peak remained the same. The results demonstrated that chemical interactions occurred between the Sb and Zr−O bond, the amino group on Zr-based MOFs. 3.5. Adsorption Isotherms. Figure S3 presents the Sb adsorption equilibrium of the adsorbent increased gradually with the increase of the initial Sb concentrations. Compared with UiO-66, UiO-66(NH2) had more excellent adsorption capacities, and the adsorption capacities of UiO-66(NH2) were 61.18 mg/g and 105.4 mg/g for Sb(III) and Sb(V), respectively, while UiO-66 had an adsorption performance of Sb(III) and Sb(V) of only 53.2 mg/g and 99.5 mg/g, respectively. Figure S4 shows the pH decreased after the adsorption of Sb on UiO-66(NH2) and UiO-66, revealing the deprotonation of adsorbate after adsorption. The adsorption thermodynamics were carried out at different temperatures, including 298, 308, and 318 K, respectively (Figures 7 and 8). The Sb adsorption capacities of the UiO-66(NH2) and UiO-66

Ce Ce 1 = + Qe Q max KLQ max

log Q e =

1 log Ce + log K f n

(2)

(3)

−1

where Qmax (mg·g ) is the maximum antimony adsorption capacity, Ce (mg·L−1) is the equilibrium antimony concentration in sample solution, Qe (mg·g−1) is the amount of antimony adsorbed at equilibrium, KL (L·mg−1) is the Langmuir constant related to the energy of adsorption; Kf D



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Figure 6. FTIR spectra of UiO-66 (black), Sb(III) containing UiO-66 (red), and Sb(V) containing UiO-66 (blue).

Figure 7. Effect of temperature on the adsorption of Sb(III) on UiO-66(NH2) (a) and UiO-66 (b), respectively. The inset is the Langmuir isotherm.

Figure 8. Effect of temperature on the adsorption of Sb(V) on UiO-66(NH2) (a) and UiO-66 (b), respectively. The inset is the Langmuir isotherm.

(mg−(n+1)/n·L1/n·g−1) is the Freundlich constant related to the sorption capacity, and n is a constant related to sorption intensity.

The corresponding parameters calculated from the Langmuir (the insets in Figures 7 and 8) and Freundlich (Figures S5 and S6) isotherm models were listed in Table 1 and Table S1. As E



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Table 1. Langmuir and Freundlich Isotherm Parameters of Sb(III) and Sb(V) Adsorption on UiO-66(NH2) Langmuir parameters Sb(III)

Sb(V)

Freundlich parameters

T (K)

Qmax.cal (mg/g)

KL (L/mg)

R2

n

Kf (mg−(n+1/n·L1/n·g−1)

R2

298 308 318 298 308 318

64.89 66.84 71.58 110.86 117.37 154.32

0.0344 0.0422 0.0394 0.0254 0.0388 0.0657

0.9985 0.9986 0.9995 0.9862 0.9952 0.9986

3.64 3.53 3.15 3.61 3.72 5.17

12.20 12.22 11.09 19.06 23.32 48.75

0.9493 0.8750 0.8500 0.9956 0.8732 0.6823

As shown in Table 3, the equilibrium data are well-fitted with the D−R isotherm model (Figures S7 and S8). The free energy (E; kJ/mol) is defined by Eq (eq 5):

shown in the Table 1 and Table S1, the Langmuir model was fitted better than the Freundlich model due to a higher correlation coefficient (R2 ≥ 0.9983). Additionally, the theoretically maximum adsorption capacities of UiO-66(NH2) calculated from the Langmuir model at 298 K were 64.89 mg/g for Sb(III) and 110.86 mg/g for Sb(V), which were agreement well with the experimental values of 61.18 mg/g and 105.4 mg/ g for Sb(III) and Sb(V), respectively. The results demonstrated that Sb adsorption on the UiO-66(NH2) and UiO-66 was monolayer adsorption. The UiO-66(NH2) exhibited a larger adsorption performance than many previously reported adsorbents (Table 2), which were conducted by the same procedure as the UiO-66(NH2) at 298 K.

E=

adsorbent and adsorbate UiO-66(NH2) and Sb(III) UiO-66(NH2) and Sb(V) UiO-66) and Sb(III) UiO-66 and Sb(V)

adsorption capacity (mg/g) adsorbent Sb(III)imprinted raw diatomite goethite (αFeOOH) raw EPa Mn-MEPb graphene Fe−Mn binary oxide PVA-Fe0 ZCNc UiO-66 UiO-66(NH2)

pH

Sb(III)

Sb(V)

25

30

5

2−8 3−10

35.20 61.20

30 15

6 7

2−8 2−8 3−10

54.40 76.50 7.46 197.8

60 60 120 300

8 8 9 10

600 50 20

12 13 this study this study

4−10 1.0−13 1.5−12

6.99 70.83 53.5

1.5−12

61.8

1.65 57.17 99.5 105.4

20

−4

β (mol/g) −9

E (kJ/mol)

R2

11.82

0.9853

8.35 × 10

3.58 × 10

2.50 × 10−3

5.74 × 10−9

9.33

0.9899

1.20 × 10−3 2.69 × 10−3

4.24 × 10−9 6.49 × 10−9

10.87 8.78

0.9880 0.9953

ΔG° = −RT ln K

(6)

ΔH ° ΔS° + (7) RT R where T (K) is the temperature in Kelvin, R (8.314 J·mol−1· K−1) is the universal gas constant, and K is the dimensionless equilibrium coefficient. K can be estimated from the Langmuir constant (KL)39 as follows ln K = −

a c

qm (mol/g)

The E (kJ/mol) value illustrates the nature of the Sb adsorption mechanism. According to the relative parameters calculated from the D−R isotherm model, as shown in Table 3, these free energy values (11.82, 9.33, 10.87, and 8.78 kJ/mol) are at the energy range of 8−16 kJ/mol, demonstrating that Sb(III) and Sb(V) adsorption onto the Zr-based MOFs are chemisorption in nature.34 3.6. Thermodynamic Calculations. The thermodynamic parameters for antimony adsorption including: the free energy change ΔG0 (kJ·mol−1), enthalpy change ΔH0 (kJ·mol−1), and entropy change ΔS0 (kJ·mol−1) can be calculating from the following equations (eqs 6 and 7):35−38

reference

3−8

(5)

Table 3. Calculated D−R Model Parameters

Table 2. Comparison of UiO-66(NH2) with Other Adsorbents Applied for the Removal of Antimony (298 K)

contact time (min)

1 2β

Raw expanded perlite. bManganese oxides-modified expanded perlite. Zirconium oxide (ZrO2)−carbon nanofirbers.

K = KL × Cw −1

where Cw is the water concentration (1 × 10 mg·L ). According to eq 7, the ΔH0 and ΔS0 parameters can be calculated from the slope and intercept, respectively (Figures S9 and S10). The calculated thermodynamic parameters, including ΔG0, ΔH0, and ΔS0, were given in Table 4 and S2. For Sb(III) adsorption on UiO-66(NH2) and UiO-66, the ΔH0 values were calculated to be 5.27 and 3.66 kJ/mol, respectively, which revealed the adsorption processes were the endothermic reaction between 298 and 318 K. The positive ΔS0 values (104.51 and 97.15 J/mol·K, respectively) are caused by a decrease in degrees of freedom of the adsorbed species. While

The equilibrium data also can be analyzed by the D−R isotherm model to distinguish the chemisorption from physisorption of Sb adsorption process. The equation (eqs 4) of the D−R isotherm33 is expressed as follows: ln qe = ln qm − βε 2

(8) 6

(4)

where qe is the amount of metal ions adsorbed per unit weight of adsorbent (mol/g); qm is the maximum adsorption capacity (mol/g); β is the activity coefficient related to the mean free energy of adsorption (mol2/kJ2); and ε is the Polanyi potential (ε = RT ln(1 + 1/Ce)). F



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The relative parameters calculated from the pseudo-firstorder (Figure S11) and pseudo-second-order (the insets in Figure 9) kinetic models are listed in Tables 5 and S3. As shown in the Tables 5 and S3, the pseudo-second-order kinetic model was fitted better than the pseudo-first-order model due to higher correlation coefficient (R2 ≥ 0.9969). Additionally, the theoretically maximum adsorption capacities of UiO66(NH2) and UiO-66 calculated from the pseudo-secondorder kinetic model were identical with the experimental value, demonstrating that antimony adsorption on the UiO-66(NH2) and UiO-66 was chemisorption. 3.8. Effect of pH. Solution pH is one of the most critical parameters affecting the surface charges of adsorbents/ adsorbates. Thus, the pH value has a great influence on the adsorption behavior of antimony onto Zr-based MOFs. As shown in Figure 10, these adsorbents had a wide pH range for Sb removal and had a maximum adsorption capacity at pH 1.5. The adsorption of Sb(III) decreased rapidly at pH < 4 with the increase of pH, while it increased gradually at pH 4−6 (Figure 10a). The adsorption of Sb(V) decreased at pH with the increase of pH (Figure 10b). Based on the results of zetapotential measurements (Figure S12), the points of zero charge (pHpzc) values of the UiO-66(NH2) and UiO-66 are 8.5 and 7.2, respectively. At pH < pHpzc, the surface of the Zr-based MOFs is positive due to the protonation reaction. However, at pH > pHpzc, the surface charge of the Zr-based MOFs is negative because of the adsorption of OH− ions through hydrogen bonding at a high pH. For Sb(III), Sb(OH)3 is the most common species that exists over a range of pH from 2.0 to 10.4. For Sb(V), Sb(OH)6− is the most common species over the pH from 2.7 to 10.4. Due to the electrostatic repulsion at strong alkalinity, the capacity of antimony adsorption is relatively low in strong alkalinity conditions. In some industries, the pH of antimony contained wastewater is very low; furthermore, the solubility of antimony is very high at this condition. The adsorbent in this study is excellent for the conditions, and similar observations were reported for the Sb adsorption on goethite.7 3.9. Effects of Coexisting Anions. Coexisting anions, such as Cl−, Br−, NO3−, CO32−, SO42−, H2PO4−, and HPO42−, are ubiquitous in environmental waters. Although they are considered environmentally friendly, these anions can compete with antimony anions for adsorption sites of the adsorbent. Therefore, investigating the effect on adsorption behavior in the presence of coexisting anions is important. The results of effects

Table 4. Thermodynamic Parameters of Sb(III) and Sb(V) Adsorption on UiO-66(NH2) at Different Temperatures thermodynamics parameters Sb(III)

Sb(V)

T (K)

ΔG0 (kJ/mol)

ΔH0 (kJ/mol)

ΔS0 J/mol·K

298 308 318 298 308 318

−25.88 −26.84 −27.98 −25.13 −27.06 −29.33

5.27

104.51

37.37

209.55

the ΔG0 values at 298, 308, and 318 K were calculated to be −25.88, −26.84, and −27.98 kJ/mol for UiO-66(NH2) and −25.29, −26.35, and −27.23 kJ/mol for UiO-66, respectively. The negative ΔG0 values suggest the adsorption processes are spontaneous adsorption reactions. The similar tendency is found in the Sb(V) adsorption; the ΔH0 values were 37.37 and 71.15 kJ/mol, respectively. The ΔS0 values were 209.55 and 315.55 J/mol·K, respectively. The ΔG0 values were −25.13, −27.06, and −29.33 kJ/mol for UiO-66(NH2) and −23.70, −26.28, and −30.04 kJ/mol for UiO-66 at 298, 308, and 318 K, respectively. The parameters (ΔG0, ΔH0, and ΔS0) revealed that the adsorption processes were endothermic and spontaneous. 3.7. Adsorption Kinetics. The Sb uptake time to reach adsorption equilibrium is an important factor for antimony water treatment in practical applications. The adsorption kinetics of antimony uptake are described in Figure 9; the results indicated that Sb(III) and Sb(V) adsorption on UiO66(NH2) and UiO-66 increased rapidly during the initial 10 min and then gradually achieved equilibrium around 20 min. The pseudo-first-order and pseudo-second-order kinetic models40 were employed to analyze the sorption kinetic data to further investigate the mechanism of adsorption. ln(Q e − Q t ) = ln Q e − k1t 1 t t = + Qt Qe h0

where h0 = K 2Q e 2

(9)

(10)

−1

where Qt and Qe (mg·g ) are the amounts of antimony adsorbed on UiO-66(NH2) or UiO-66 at time t and at equilibrium, respectively; K1 (min−1) and K2 (g·mg−1·min−1) represent the adsorption rate constants; h0 (mg·g−1·min−1) is the initial adsorption rate.

Figure 9. Effect of contact time on the adsorption of Sb(III) onto UiO-66(NH2) and UiO-66, respectively (antimony concentration, 500 mg/L; initial pH, 4.3) (a). Effect of contact time on the adsorption of Sb(V) onto UiO-66(NH2) and UiO-66, respectively (antimony concentration, 500 mg/L; initial pH, 5.4) (b). The inset is the fitting plots of the pseudo-second-order kinetic model. G



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Table 5. Kinetic Parameters of Sb(III) and Sb(V) Adsorption on the UiO-66(NH2) pseudo-first-order kinetics Sb(III) Sb(V)

pseudo-second-order kinetics

K1 (min−1)

Qe,cal (mg/g)

R2

K2 (g·mg−1·min−1)

Qe,cal (mg/g)

h0 (mg·g−1·min−1)

R2

0.025 0.023

13.22 29.36

0.3984 0.5018

0.014 0.007

60.98 103.3

50.76 74.40

0.9995 0.9996

Figure 10. Effect of initial pH on the adsorption of Sb(III) on UiO-66(NH2) and UiO-66 (a). Effect of initial pH on the adsorption of Sb(V) on UiO-66(NH2) and UiO-66 (b). Initial antimony concentration, 500 mg/L.

Figure 11. Effect of competing anions on the adsorption of Sb(III) on UiO-66(NH2) and UiO-66 (a). Effect of competing anions on the adsorption of Sb(V) on UiO-66(NH2) and UiO-66 (b). Initial concentration of each anions, 0.1 M.

Figure 12. XPS spectra of UiO-66(NH2) before and after Sb(III) and Sb(V) adsorption. (a) Survey, (b) Zr 3d, and (c) N 1s.

of competing anions (such as Cl−, Br−, NO3−, CO32−, SO42−, H2PO4−, and HPO42−) are presented in Figure 11. The results demonstrated that the presence of these competing anions had no significant effect on the removal of Sb(III) and Sb(V). Cl−, Br−, NO3−, and CO32− did not significantly compete with Sb(III) and Sb(V) for adsorption sites of the UiO-66(NH2) and UiO-66. SO42− and HPO42− could inhibit the adsorption of Sb(V) due to structural similarities. But overall, it implied that

the adsorption of Sb(III) and Sb(V) onto the UiO-66 and UiO66(NH2) was mainly specific adsorption. 3.10. XPS Analyses. The elemental composition of UiO66(NH2) and UiO-66 before and after antimony adsorption was measured by XPS. As shown in Figures 12a and 13b, the peaks correspond to the Zr, O, C, and Sb are clearly presented in full scan spectrum, demonstrating that the existence of these elements (Zr, O, C, and Sb) on UiO-66(NH2) and UiO-66. H



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Figure 13. XPS spectra of UiO-66 before and after Sb(III) and Sb(V) adsorption. (a) Survey and (b) Zr 3d.

The C 1s has signals at a binding energy of 285.01 eV,41 and it has no change after the Sb adsorption. Two peaks at the binding energy of 530.7 and 540.0 eV are attributed to Sb 3d5/2 and Sb 3d3/2,42 demonstrating that Sb(III) and Sb(V) were adsorbed on the UiO-66(NH2) and UiO-66.43 As shown in Figure 12c, the signals at binding energy of 399.62 and 400.91 eV are assigned to N 1s, and the peaks have shifted marginally after Sb adsorption, indicating amino played a significant role in both Sb(III) and Sb(V) adsorption. As shown in Figure 12b and 13b, the Zr 3d peak could be decomposed into four peaks.44 The peaks at 182.9 and 185.3 eV and the peaks at 183.5 and 185.9 eV are assigned to Zr−O bonds and Zr−Zr bonds, respectively. It is worth mentioning that the peaks of Zr−O bonds shifted marginally after both Sb(III) and Sb(V) adsorption, which are consistent with the above FTIR analysis. Zr−O bonds had played a key role in the Sb adsorption for UiO-66(NH2) and UiO-66. The difference with UiO-66 is that amino groups are also very critical for Sb on UiO-66(NH2) adsorbents. 3.11. Adsorption Mechanism by UiO-66(NH2). From the pH value decrease after Sb(III) and Sb(V) adsorption, it can be concluded that the adsorbed antimony species on UiO66(NH2) can be H2SbO3− and Sb(OH)6− because the H+ ion released from the antimony species leaded to the decrease of pH values of the aqueous solution. Second, the adsorption interactions mainly depend on both electrostatic forces and coordinated action. Amino groups of UiO-66(NH2) promote the adsorption of antimony through electrostatic interaction with H2SbO3− and Sb(OH)6−. H2SbO3− would form a monodentate mononuclear complex with one Zr−OH group in the adsorption process, while Sb(OH)6− could form a bidentate binuclear complex with two Zr−OH groups through coordinated action. 3.12. Application to Real Samples. To investigate Sb(III) and Sb(V) removal performance of UiO-66(NH2) in actual wastewater, metallurgical wastewater was collected and spiked with 10 mg/L of Sb(III) and Sb(V), respectively. The wastewater was treated at an initial pH of 7.0 according to the adsorption kinetic experiment method. The UiO-66(NH2) exhibited 99.16% and 99.10% removal efficiency toward Sb(III) and Sb(V), respectively. These results demonstrated that UiO66(NH2) has high removal efficiencies for Sb(III) and Sb(V) in complicated wastewater; therefore, UiO-66(NH2) is a promising adsorbent for the removal of Sb(III) and Sb(V).

4. CONCLUSION This study demonstrated that UiO-66(NH2) and UiO-66 were promising adsorbents to treat Sb contaminated water. These adsorbents had a wide pH range for the Sb removal. UiO66(NH2) showed notably higher adsorption performance in batch as well as real application adsorption. The adsorption processes were feasible, endothermic, and spontaneous reactions, and the experimental data fitted well with the Langmuir model. The kinetic parameters indicated that the adsorption of Sb ions onto UiO-66 and UiO-66(NH2) were described well by the pseudo-second-order kinetic model. The Dubinin−Radushkevich (D−R) isotherm model further confirmed the chemisorption of the Sb adsorption process. Additionally, strong acidic conditions as well as the coexisting anions had no significant influence on the Sb adsorption capacity. The FTIR analysis and XPS study demonstrated that the Zr−O bond and amino group had played a significant role in Sb adsorption. UiO-66(NH2) will be a promising candidate for Sb removal in the future for its significant advantages in the Sb removal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00010. Synthesis of UiO-66(NH2) and UiO-66, XRD patterns (Figure S1), FTIR spectrum (Figure S2), adsorption performance (Figures S3−S4), Freundlich plots (Figures S5−S6), D−R isotherm plots (Figures S7−S8), ln K vs 1/T (Figures S9−S10), log(Qe − Qt) vs t (Figure S11), pH−zeta potential (Figure S12), XPS spectra (Figure S13−S14) for Sb(III) and Sb(V) adsorption on UiO66(NH2) and UiO-66. Langmuir, Freundlich, thermodynamic, and kinetic of Sb(III) and Sb(V) adsorption on UiO-66 (Tables S1−S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 79153372; fax: +86 791 395373. E-mail: [email protected]. ORCID

Xubiao Luo: 0000-0002-3935-1268 Funding

This work was financially supported by Natural Science Foundation of China (51178213, 51238002, 51272099, I



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51308278), the National Science Fund for Excellent Young Scholars (51422807). Notes

The authors declare no competing financial interest.

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Efficient Removal of Antimony (III, V) from Contaminated Water by

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