Extremely Effective Boron Removal from Water by Stable Metal

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Extremely Effective Boron Removal from Water by Stable Metal Organic Framework ZIF-67 Jingli Zhang, Yaona Cai, and Kexin Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05656 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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Industrial & Engineering Chemistry Research

Extremely Effective Boron Removal from Water by Stable Metal Organic Framework ZIF-67 Jingli Zhang†,‡., Yaona Cai†,‡, Kexin Liu†,‡ †

School of Environmental and Municipal Engineering, Tianjin Chengjian University, Tianjin

300384, China ‡

Tianjin Key Laboratory of Aquatic Science and Technology, Tianjin Chengjian University,

Tianjin 300384, China;

ABSTRACT: The development of adsorbents with high adsorption capacity is a great challenge. The ZIF-67 nanocrystal prepared was applied to boron removal for the first time. The results indicated that well-defined rhombic dodecahedral cobalt-based ZIF-67 synthesized at room temperature exhibited good stability. Under pH of 4 and temperature of 25oC, ZIF-67 exhibited an extraordinarily high boron adsorption capacity of 579.80 mg·g-1 at initial boron concentration of 0.5 mol·L-1, the highest boron adsorption capacity ever reported. The intraparticle diffusion and the film diffusion both were the rate controlling steps. The adsorption of boron to ZIF-67 was mainly due to electrostatic interaction, π-π stacking interaction and the coordination bonding and the boric acid molecules successfully crystallized on the ZIF-67 surface. At the fifth cycle the adsorption capacity of ZIF-67 exhibited 94.1% of the first cycle. ZIF-67 is a promising sorbent for boron removal. Keywords: Metal-organic Framework; ZIF-67; Boron; Adsorption; Regeneration

Corresponding author. Tel.: +86 22 23085117, E-mail: [email protected].



E-mail: [email protected] (Yaona Cai), [email protected] (Kexin Liu) 1

ACS Paragon Plus Environment

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1. Introduction Boron is a microelement widely distributed in earth’s hydrosphere and geosphere.1,2 In nature, boron is always found as compounds with other elements.3 Boron and its compounds are widely used in many industries such as glass, electronics, ceramics, porcelains, semiconductors, pharmaceuticals, insecticides, catalysts and cleaning products.4 The glass industry is the biggest consumer among them and consumes more than half of the total production of boron compounds. significant role in plant, animal and human being growth.

6,7

5

Boron plays a

However, both incompleteness and

increment of boron in daily intake can induce toxic effects.7 Long-term utilization of water and food sources with high-strength boron can cause health problems including neurological effects, mental deficiency, cardiovascular disease, and reproductive disorders.8 The concentration of boron in drinking water is stipulated ≤2.4 mg·L−1 by the World Health Organization and ≤1.0 mg·L−1 by European Union. Various processes such as adsorption,9,10 precipitation,11 ion exchange,12 filtration,13 and phytoremediatio,14 have been used to remove boron from water. Among those processes, adsorption has been extensively studied due to its advantages such as low cost, effective boron uptake and easy regeneration and convenient preparation. Many adsorbents for boron removal from aqueous solution have been used including Fe(Ⅲ) modified bentonite clay,9 selective ion exchange resins,15 chelating resins,16 fibers,17 activated carbon,18 oxides and hydroxides,19 layered double hydroxides,20 eggshell (agricultural waste)21 and silica-supported N-methyl-d-glucamine adsorbent.22 Among those adsorbents, resins23-27 were much studied due to large static boron isotopic separation factor and favorable recovery ability in the last decade. As well known, an important parameter to evaluate the sorbent is its adsorption capacity. Metal−organic frameworks (MOFs), constructed by the 2


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coordination of metal ions and polyatomic organic bridging ligands to create open crystalline frameworks with permanent porosity, are widely studied in gas adsorption, separations and catalysis applications. Zeolitic imidazolate frameworks (ZIFs) with imidazolate linkers, the most widely-explored MOFs in the literature, are topologically isomorphic with zeolites. ZIFs are found to be a good alternative due to their high adsorption capacity, low cost and thermal stability in the aqueous environment,28 especially ZIF-8 (Zn(II)) and ZIF-67 (Co(II)).29,30 ZIF-8 shows high adsorption capacity of 247.44 mg·g-1 for boron at 45 °C and good reusability.31 In comparison with the metal ion Zn(II) in ZIF-8, the metal ion Co(II) with unsaturated electronic configuration of 1s22s22p63s23p63d7 in ZIF-67 is probably easier to form the coordination bonding with adsorbate. In this work, ZIF-67 was synthesized at room temperature and applied for boron removal. Powder X-ray diffraction (PXRD), scanning electronic microscopy (SEM) and thermogravimetric analysis were employed to characterize the structure of ZIF-67. The ZIF-67 adsorption and regeneration was investigated in batch experiments. 2. Materials and methods 2.1. Materials Ultrapure deionized water (DI water, 1.2 μg·L-1 TOC, 18.2 MΩ cm at 25 oC) was employed in this study. All reagents were of analytical grade and used without further purification. All aqueous solutions prepared were stored in polyethylene/polypropylene containers to prevent boron solution pollution from glassware. 2.2. Preparation of ZIF-67 The ZIF-67 nanocrystals were synthesized according previous report32,33 with a minor modification. In brief, 12 mmol of cobalt nitrate hexahydrate and 48 mmol of 2-methylimidazole were dissolved in 3



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0.120 L methanol, respectively. The two solutions were mixed and stirred for 24 h after 30 min sonication. And then the purple precipitate was collected by filtration and washed with ethanol for three times. Finally, the obtained powders were dried at room temperature for 24 h. 2.3. Analytical methods The synthesized ZIF-67 was characterized using a field emission scanning electron microscopy (FESEM, JEOL JSM-7800F, Japan). Powder X-ray diffraction (PXRD) patterns were recorded at room temperature by an Ulitma IV diffractometer (Rigaku, USA) with monochromatized Cu Kα radiation (40 kV, 40 mA). Diffraction data were collected in 2θ scan range from 5 to 50° with a step size of 0.02°. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Diamond 6300 TG/DTA thermal analysis instrument (USA). The boron concentration was determined by using Azomethine H. UV-vis spectrophotometric method at the wavelength of 420 nm. The specific surface area and pore volume were determined by Brunauer-Emmett-Teller (BET) method, using a Quantachrome NovaWin version 10.01. Zeta potential is determined on a Malvern Instrument Zetasizer Nano-ZS at room temperature. 2.4. Boron adsorption Adsorption of boron to ZIF-67 was investigated by batch-type operation. In a typical adsorption process, ZIF-67 sample was added into boric acid solution in 0.2 L conical flask, and then the mixture was shaken up in the water bath shaker at a set temperature. The adsorption capacity of ZIF-67 at time t, Qt (mg·g-1), is calculated as follow34

Qt 

(C0  Ct )v m

(1)

where Ct is the adsorbate concentration at time t, C0 is initial concentration, m is the ZIF-67 mass and v (L) is the solution volume. The adsorption capacity at equilibrium was denoted as Qe (mg·g-1). 4


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3. Results and discussion 3.1. Characterization The prepared ZIF-67 was characterized by SEM, XRD and TGA. The SEM image (Figure 1a) showed that the as-synthesized ZIF-67 was rhombic dodecahedron and its particle size was approximately 400-500 nm. After 11 day (Figure S1) preservation in air, the morphology of the as-synthesized ZIF-67 was unchanged, indicating that as-synthesized ZIF-67 had good stability under ambient conditions. The nitrogen adsorption–desorption isotherms of ZIF-67 (Figure 1b) showed that ZIF-67 was a typical microporous material with its specific surface area of 1776 m2·g−1, the pore volume of 0.72 cm3·g−1 and the average pore size of 1.98 nm with pore size of 1.2-3 nm. Figure 1c revealed that the XRD patterns of the as-synthesized ZIF-67 was well matched with the simulated patterns of 7.31°(011), 10.36°(002), 12.72°(112), 14.40°(022), 16.45°(013), 18.04°(222), 22.15°(114), 24.53°(233), 25.62°(224), 26.70°(134), 29.67°(044), 30.62°(334), 31.55°(244) and 32.43°(235),35 showing that the ZIF-67 nanocrystals were successfully synthesized and well-developed. A slight weight loss was noticed from 50 to 300 oC from the TGA curve in air (Figure 1d), possibly owing to the escape of guest molecules (methanol, ethanol, 2-methylimidazole and water) and gas molecules from the cavities.36 Subsequently, a significant weight loss was found in the range of 350-420 oC, which could be attributed to the decomposition of ligand.37 There was a weak weight loss at the temperature range of 250-500 oC and the significant weight loss appeared at range of 510-750 oC on the TGA curve in N2 in our previous report.32 Therefore, the as-synthesized ZIF-67 had good thermal stability in both air and nitrogen. 5



3.2. Boron adsorption 3.2.1. Effect of dosage ZIF-67 was added into boric acid solution with the dosage of 1-4 g·L-1 for 24 h at 35 oC. As shown in Figure 2a, the boron adsorption capacity decreased with the increase of ZIF-67 dosage. When the dosage of ZIF-67 increased, the equilibrium concentration Ce decreased. At higher dosages, the equilibrium adsorption capacity Qe was significantly decreased partly because of the aggregation of ZIF-67 adsorption sites. 800

a

b

Adsorption

Desorption

600 dV(d) (x10-2cm3/nmg·g)

N2 volume (cm3/g)

700

500 400 300 200 100

14 12 10 8 6 4 2 0 0

1

2

3

4

5

Pore size (nm)

0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0

c

110

ZIF-67 regenerated

d

100

ZIF-67 as-synthesized

m (%)

90

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 70 25-1000 °C -2.587 mg (-53.08%)

60

ZIF-67 simulated

50 40

0

10

20

30

40

50

0

200

2 °

400

600

800

1000

T (°C)

Figure 1. Characterization of the prepared ZIF-67: (a) SEM images at 1 day and (b) N2 adsorption-desorption isotherms (Pore size distributions at lower right corner); (c) XRD patterns; (d) TGA curve in air.

6


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3.2.2. Effect of initial pH ZIF-67 was added into a series of boric acid aqueous solutions with the different initial pH for 24 h at 35 oC. The pH of boric acid solutions was adjusted by adding 0.1 mol·L-1 hydrochloric acid or 0.1 mol·L-1 sodium hydroxide. The adsorption capacity of ZIF-67 (3 g·L-1) at various initial pH was shown in Figure 2b. The adsorption capacity increased with the increase of the solution initial pH up to 4 and then decreased gradually at higher pH values. The effects of pH on the adsorption were similar under the dosages of 1, 2, 3 and 4 g·L-1. The optimum pH value was 4 for the adsorption of boron adsorption onto ZIF-67. 340

0.470

120 0.465 100 0.460

80

(mg/g)

140

b

300

0.475

160

320

max

Qmax Ce

180

0.480

Q

a

Ce (mol/L)

200

Qmax (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


280 260 240 220 200 180 160

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2

4

Dosage (g/L)

6

8

10

pH

Figure 2. Boron adsorption in boric acid solution at temperature of 35 oC (a) ZIF-67 dosage of 1, 2, 3 and 4 g·L-1; (b) pH of 2, 3, 4, 5, 6, 8 and 10 at ZIF-67 dosage of 3 g·L-1.

3.2.3. Adsorption kinetics To further investigate the adsorption kinetics of boron to ZIF-67, ZIF-67 (3 g·L-1) was added into a series of boric acid aqueous solutions (pH: 4) with the initial boron concentrations of 0.05, 0.1, 0.2, 0.3 and 0.5 mol·L-1 for 24 h at 35 oC. The boron concentration in the supernatant was monitored at time t and Qt was calculated.

7



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The adsorption capacity Qt of boron on ZIF-67 was shown in Figure 3a. The boron adsorption capacity increased rapidly at the beginning and then reached to equilibrium gradually. The equilibrium adsorption capacity Qe increased from 13.67 to 395.52 mg·g-1 when the initial boron concentration was changed from 0.05 to 0.5 mol·L-1. In order to study the adsorption mechanism and the potential rate controlling steps, three kinetic models, pseudo-second order model, intraparticle diffusion model and Boyd’s kinetic model,38 were used to fit the experimental data. The pseudo-first order model was not adopted because the pseudo-second-order kinetic model is better for the adsorption behavior in the liquid phase.39 The pseudo-second order model can be expressed as t 1 t   2 Qt k2Qe Qe

(2)

where k2 (g·mg-1h-1) is the adsorption rate constant. The linear plots of t/Qt against t based on the kinetic data were shown in Figure 3b. Boron adsorption on ZIF-67 was well described by pseudo-second order model. The pseudo-second order model based on the adsorption capacity in the solid phase was in agreement with chemisorption being the rate controlling step. That showed that there was share or exchange of electrons between boric acid and ZIF-67. The experimental data of equilibrium capacity were greater than the calculated values (Table S1), especially for high initial concentration, indicating that there probably existed other processes in addition to adsorption. In general, there are three consecutive stages in adsorption process: 1) the first stage, the adsorbate diffuses from the bulk solution to the external surface of adsorbent; 2) the second stage, the adsorbate on the external surface of adsorbent transfers in the internal pores or/and branched pores in the adsorbent particle, that is called intraparticle or pore diffusion; and 3) the third stage, the adsorbate is adsorbed at an active site on the surface of material (chemical reaction via ion-exchange, 8


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complexation and/or chelation).40 The intraparticle diffusion model was used to fit the experimental data to evaluate whether it was involved into the adsorption process of boron on ZIF-67 and the model can be expressed as

Qt  k p ,i t 0.5  C

(3)

where kp,i (mg·g-1·h-0.5) is the intraparticle diffusion rate constant at phase i, C is related to the thickness of the boundary layer and the external mass transfer in the adsorption.41 The linear plots of Qt against t-0.5 based on the experiment data showed that the boundary layer diffusion of boric acid molecules and the diffusion from solution to the external surface of ZIF-67 were performed 42 in liquid-phase adsorption (Figure 3c and Table S1). The concentration of residual boron decreased gradually and the adsorption rate slowed down to equilibrium.43 The regression line of the second stage did not pass through the origin, showing that the intraparticle diffusion was not the only controlling step for the adsorption rate.44 In order to find the rate-determining step, the kinetic data were analyzed using the Boyd kinetic model, which is represented as follows 2

  2Qt Bt        3Qe 

   

for Qt/Qe < 0.85

(4)

Bt  0.4977  ln(1 

Qt ) for Qt/Qe > 0.85 Qe

(5)

where B (h-1) is the rate coefficient. The B values were summarized in Table S1 and the linear plots of Bt versus t were shown in Figure 3d for various initial boron concentrations. If this plot is a straight line through the origin the adsorption is controlled by particle diffusion. Otherwise, it is controlled by the film diffusion. The film diffusion was the rate-limiting step for boron adsorption on ZIF-67 because the straight line did not pass through the origin.

9



3.2.4. Adsorption isotherms The isotherm experiments were carried out by adding ZIF-67 (3 g·L-1) into a series of boric acid aqueous solution with the initial boron concentration of 0.05, 0.1, 0.2, 0.3 and 0.5 mol·L-1 (mixture pH: 4) at temperatures of 25, 35 and 45 oC, respectively. The adsorptions of boron on ZIF-67 at different temperatures were shown in Figure 4a. The boron adsorption capacities increased with the decrease of temperature, indicating that the adsorption of boron on ZIF-67 was favorable at low temperatures. 1.6

a

400

b

1.4 1.2 t/Q (hg/mg) t

Qt (mg/g)

300

200

1.0 0.8 0.6 0.4

100

0.2

0

0.0

0

5

10

15

20

25

0

5

10

15

400

6.0

c

350

5.0

300

25

d

4.0 Bt

250 200

3.0

150

2.0

100

1.0

50 0

20

t (h)

t (h)

Qt (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 0

1

2

3

4

5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

t (h)

t0.5 (h0.5)

Figure 3. (a) Effect of contact time on boron adsorption on ZIF-67 (ZIF-67 dosage of 3 g·L-1, temperature of 35 oC); (b) Fitting by pseudo-second order model; (c) Fitting by intraparticle diffusion model; and (d) Fitting by Boyd’s kinetic model. ( 0.3 mol·L-1,

0.05 mol·L-1,

0.1 mol·L-1,

0.2 mol·L-1,

0.5 mol·L-1) 10


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To further investigate the adsorption isotherm of boron to ZIF-67, the three isotherm models, Langmuir isotherm model,45 Freundlich isotherm model and Henry isotherm model, were employed to describe the boron adsorption. The Langmuir isotherm model, which assumes that adsorption takes place as a monolayer on a homogenous surface, can be expressed as Ce C 1   e Qe K L Qmax Qmax

(6)

where KL, the Langmuir isotherm constant, is related to the adsorption bonding energy, and Qmax is the estimated maximal adsorption capacity. Fitting results were seen in Figure 4b and the correlation coefficients at different temperature were lower than 0.2 (Table S2), suggesting that Langmuir isotherm model cannot fit the data well. Freundlich isotherm model is widely used to describe the adsorbent-adsorbate interaction and the adsorption process on heterogeneous surfaces with different functional groups. Freundlich isotherm model can be expressed as follow

1 log Qe  log K F  log Ce n

(7)

where KF is the Freundlich isother constant, and 1/n is a constant that indicates the adsorption capacity of the adsorbent and the intensity of the adsorption process. KF and 1/n indicate the magnitude of the adsorption driving force. Fitting results were seen in Figure 4c. KF and 1/n could be obtained from the intercepts and slopes from the plots of logQe versus logCe as summarized in Table 1. Considering 1/n < 1 at all tested temperatures, an affinity should exist between boron and ZIF-67, which might be the electrostatic attraction.46 The equilibrium constants derived from various isotherms such as Langmuir, Frumkin, Henry isotherms, can be used for calculation of free energy changes (ΔG) of adsorption. Henry model was employed to describe the boron adsorption isotherms on ZIF-6 and can be expressed as follow 11



Qe  K p Ce

(8)

where Kp (ml g-1) is the distribution constant. The linear plots of Qe versus Ce based on the isotherm data were shown in Figure 4d and Kp were summarized in Table 1. The data could fit the Henry isotherm model well and the correlation coefficients were larger than 0.96 for all tested temperatures.

a

35

500

30

400

25

Ce/Qe (g/L)

Qe (mg/g)

600

300 200

b

20 15

100

10

0

5 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Ce (mol/L)

Ce (mol/L)

-1.2 -1.4

600

c

d

500

-1.6 Qe (mg/g)

-1.8 logQe

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-2.0 -2.2

400 300

-2.4

200

-2.6

100

-2.8 0

-3.0 -1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Ce (mol/L)

logCe

Figure 4. (a) Boron adsorption isotherms on ZIF-67 at different temperatures (pH of 4); (b) fitting of boron adsorption isotherms on ZIF-67 by Langmuir model; (c) Freundlich model; and (d) Henry model.(

25°C,

35°C,

45°C)

The adsorption capacities of materials found in literature and this study were showed in Figure 5a (details seen in Table S3). ZIF-67 exhibited the highest adsorption capacity of 579.80 mg·g-1 at 25 oC,

12


Page 13 of 25

which was much higher than any other adsorbent ever reported. The adsorption ability of ZIF-67 was much better than that (247.44 mg·g-1) of isostructural ZIF-8, indicating that Co(II) made great contribution in boron adsorption. Figure 5b showed that the boric acid molecules crystallize on the ZIF-67 surface.

600 500 400 300 200

a

b

1 Activated Carbon 2 Fly ash 3 Mg-AI Layered double hydroxides 4 Iron oxide/hydroxide-based nanoparticles 5 MIL-96(AI) 6 UIO-66 7 Calcium alginate gel beads 8 Glucamine-functionalized hydrogel beads 9 ZIF-8 10 ZIF-67

25C 35C 45C

600 500 400

Qe(mg/g)

700

Qmax (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 200 100 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Ce(mol/L)

100 0

1

2

3

4

5

6

7

8

9

10

Figure 5. (a) Boron adsorption capacities of adsorbents; (b) ZIF-67 adsorbed with boron at 25 oC (initial boron concentration of 0.5 mol·L-1 and ZIF-67 dosage of 3 g·L-1).

3.2.5. Adsorption thermodynamics The feasibility of the adsorption process was evaluated by the thermodynamic parameters including Gibbs free energy change (ΔG, kJ·mol-1), enthalpy change (ΔH, kJ·mol-1), and entropy change (ΔS, kJ·mol-1 k-1), which were obtained through Eq. (9)

ln K p 

VS VH  R RT

(9)

where R (8.314 J·mol-1·K-1) is the universal gas constant, T (K) is Kelvin temperature, Kp is the distribution coefficient which can be obtained from Henry model. According to Eq. (9), the linear plot of lnKp versus 1/T was shown in Figure 6. The data fitted well with a correlation coefficient of 0.996. The parameters of the adsorption of boron on ZIF-67 were given in Table 1. All ΔG values at

13



various temperatures were negative, demonstrating that the adsorption of boron on ZIF-67 was spontaneous. When the temperature was decreased, ΔG was more negative, showing that the adsorption of boron to ZIF-67 was easier at the low temperature. The negative ΔH (-31.39 kJ mol-1) suggested that the adsorption process was exothermic reaction. The negative value of ΔS suggested the randomness decrease (boric acid molecules) at the solid/liquid interface during the adsorption of boron on ZIF-67. 5.2 5.0

lnKp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4.8 4.6 4.4 4.2 3.10

3.15

3.20

3.25

3.30

3.35



1/T ( )

Figure 6. Van’t Hoff plot.

Table 1 Thermodynamic parameters of two isotherm models for boron adsorption on ZIF-67. Freundlich model 1/n KF R2

T (oC) 25 35 45

Henry model Kp R2 (mL·g-1)

0.93 1.56 1.06

0.16 0.93 0.20 0.97 0.08 0.97

165.39 113.17 74.69

ΔG

ΔS

ΔH

(KJ mol-1) (KJ·mol-1·k-1) (KJ·mol-1) 0.97 -12.68 0.96 -12.17 0.98 -11.42

-0.06

-31.39

3.3. Adsorption mechanism The adsorption of the substrate on MOFs is mainly related to electrostatic interaction, acid-base 14


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interaction, π-π stacking interaction, hydrogen bonding and ligand binding in the liquid phase.47 Under pH of 4 and temperature of 25 oC, ZIF-67 exhibited an extraordinarily high boron adsorption capacity of 579.80 mg·g-1, the highest boron adsorption capacity ever reported. Firstly, the highest boron adsorption is mainly related to the electrostatic interaction between ZIF-67 nanocrystals and boric acid. The speciation of boric acid depends on its concentrations.48 The polyborate species prevail at concentrations higher than 20 mmol·L-1. In the 0.5 mol·L-1 boron aqueous solution, there exist B5O6(OH)4-, B3O3(OH)4-, B4O5(OH)2-, B(OH)4- and B(OH)3, these species can be interconverted,49,50 and their distribution are controlled by pH. The pristine ZIF-67 exhibited positive surface charges at pH ≤10 in water (Figure 7a). With the initial pH increased from 4 to 9, the functional groups on ZIF-67 nanocrystal surface might be deprotonated, thus resulting in the decrease of positive charge density on ZIF-67 nanocrystals. The electrostatic interaction between ZIF-67 nanocrystals and borate ions with negative charge decreased with the pH increase. Boron acid is a weaker acid (0.5 mol·L-1, pH=4.43) and B(OH)3 is not easy to transform polymeric borate ions at low pH of 2 and 3. Boron exist three different chemical states on the adsorbed MOFs

51.

Secondly, the π–π stacking interaction, a weaker interaction than electrostatic interaction, occurred between the 2-methylimidazole ligand and polymeric borate ions containing six membered rings such as B3O3(OH)4-, B5O6(OH)4- and B4O5(OH)42- (Figure 7b).52 Thirdly, the coordination bonding between open metal ion sites in ZIF-67 and borate ions through OH groups. As a result, the concentration of boron acid adsorbed on ZIF-67 becomes higher and the boric acid molecules successfully crystallized on the ZIF-67 surface (Figure 5b). The pristine ZIF-8 exhibited negative surface charges at pH < 9.153 and there is an electrostatic repulsion between ZIF-8 and boric acid molecules. In addition, the characteristics of ZIF-8 (Figure 7c) are different to its isostructural

15



ZIF-67 (Figure 7d). The cell lengths of ZIF-67 are longer than that of ZIF-8 and the cell of ZIF-67 is larger than that of ZIF-8 (Table S4), indicating that the larger pore size is conducive to the entry of adsorbents and pore volume of ZIF-67 can accommodate more adsorbents. The adsorption capacity of ZIF-67 was much higher than that (247.44 mg·g-1) of isostructural ZIF-8.

50

Zeta potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

40

a b

30 20 10 0 -10 2

4

6

8

10

12

pH

c

d

Figure 7. (a) Zeta potentials of ZIF-67 at various pH; (b) Polymeric borate ions B3O3(OH)4-, B4O5(OH)42- and B5O6(OH)

4

-

from left to right; simulation structure of (c) ZIF-8 and (d) ZIF-67

(Task: locate, Method: metropolis, Quality: customized, Force field: Universal).

3.4. Regeneration and recycling The regeneration and reusability of the ZIF-67 are crucial for practical application, especially for

16


Page 17 of 25

cost control. The reusability of ZIF-67 was evaluated by a multiple cycle test. The desorption experiment was carried out as follows. The exhausted ZIF-67 was added into hydrochloric acid solutions (0.1 L , pH: 4), and then the mixtures were stirred at 35 oC for 4 h with hydrochloric acid renewed once at 2 h. The regenerated ZIF-67 was washed with ethanol (2×0.01 L) and dried at room temperature for 24 h. The adsorption conditions in every cycle were the same. At the fifth regeneration cycle, the adsorption capacity of ZIF-67 could still exhibit 94.1% of the original capacity (Figure 8a), showing that the acid-washing was a feasible and effective method to remove boron from the used ZIF-67. ZIF-67 had no significant change from its XRD pattern (Figure 1c) and morphology (Figure 8b) at the fifth regeneration cycle. Therefore, ZIF-67 is a stable, effective and recyclable sorbent for boron removal.

100

Recovery Rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

80 60 40 20 0

1

2

3

4

5

Cycle

Figure 8. (a) Recycling of ZIF-67 on boron adsorption; (b) SEM images of the ZIF-67 desorption.

4. Conclusions Well-defined rhombic dodecahedral cobalt-based ZIF-67 (size: 400-500 nm) was synthesized at room temperature. ZIF-67 exhibited the highest uptake capacity of 579.80 mg·g-1 at the optimal pH of 4, which was much higher than any other adsorbent ever reported. The adsorption kinetics

17



Page 18 of 25

analysis indicated that intraparticle diffusion and film diffusion were the rate-limiting steps. Adsorption of boron on ZIF-67 was a spontaneous and exothermic process. The adsorption of boron to ZIF-67 was mainly due to electrostatic attraction, π–π stacking interaction and the coordination bonding between open metal ion sites in ZIF-67 and polymeric borate ions and then the molecules successfully crystallize on the ZIF-67 surface. The adsorbent was easily regenerated by using hydrochloric acid and the boron adsorption capacity exhibited 94.1% of the first cycle at the fifth cycle. The high uptake capacity and the good stability of ZIF-67 indicated that it is a promising adsorbent for boron removal.

Supporting Information SEM image of ZIF-67 at 11-day preservation in air, parameters of kinetic models, thermodynamic parameters of Langmuir model, experimental conditions of the boron adsorption capacities of the various adsorbents and cell parameters for simulation structures of ZIF-67 and ZIF-8.

Acknowledgments This work was supported by the National Natural Science Foundation of China (grant number 51078265) and by the Research Fund of Tianjin Key Laboratory of Aquatic Science and Technology (TJKLAST-PT-2016-07).

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Abstract Graphics

Regeneration

High boron adsorption

ZIF-67

H3BO3 ⇋

25


Extremely Effective Boron Removal from Water by Stable Metal

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