calcium alginate microparticles for highly

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Materials and Interfaces

Facile fabrication of ZIF-8/calcium alginate microparticles for highly efficient adsorption of Pb(II) from aqueous solutions Yongcun Song, Nan Wang, Li-Ye Yang, Yang Guang Wang, Di Yu, and Xiao-kun Ouyang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05879 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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Industrial & Engineering Chemistry Research

Facile fabrication of ZIF-8/calcium alginate microparticles for highly efficient adsorption of Pb(II) from aqueous solutions Yongcun Song, Nan Wang, Li–ye Yang, Yang–guang Wang, Di Yu, Xiao–kun Ouyang*

School of Food and Pharmacy, Zhejiang Ocean University, Zhoushan 316022, P. R. China *Corresponding author. Mailing address: Xiao–kun Ouyang, Ph.D Professor School of Food and Pharmacy Zhejiang Ocean University Haida South Load 1#, Lincheng Zhoushan 316022, P.R. China E–mail: [email protected]

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Industrial & Engineering Chemistry Research 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

ABSTRACT: Metal-organic frameworks (MOFs) have received special attention from scientists owing to their excellent adsorption performance. However, the difficulty in separating MOFs from adsorbed metals following use has limited their application. A zeolitic imidazole-based MOFs with broad applicability for sorption of Pb(II).In this work, a novel adsorbent employing

ZIF-8/calcium alginate microparticles was

prepared using sodium alginate (SA) and ZIF-8. This adsorbent was characterized using scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS). The performance of the ZIF-8@CA microparticles in adsorbing Pb(II) from a Pb(II) solution was investigated, and the impact of the initial Pb(II) concentration, reaction time, pH, and reaction temperature on the reaction process were investigated. The results showed that ZIF-8@CA microparticles exhibited a maximum adsorption capacity of 1321.21 mg/g at pH 5 after 120 min, and the adsorption process was found to fit the Langmuir isotherm model (R2 = 0.9856) and the pseudo-second-order kinetic model (R2 = 0.9999). These results showed that the adsorption of Pb(II) was an endothermic process. The regeneration experiment with ZIF-8@CA revealed that the removal efficiency of Pb(II) was greater than 80 % even after 5 cycles.

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1. Introduction Pb(II) is a common pollutant in water bodies1, which has serious adverse effects on the environment2, human health, and affected ecosystems3. It can interact with sulfur-containing proteins to inhibit protein metabolism and associated biological activities4. It can also cause significant damage to the central nervous system, kidneys, liver, and reproductive systems of humans5. Therefore, the removal of Pb(II) residues from water is indispensable for environmental and human health6. Currently, treatment processes for the removal of heavy metals like Pb(II) employ chemical precipitation7, ion-exchange chromatography8, membrane technology, and adsorption methods9-10. Adsorption methods have found broad application in removing heavy metals due to their simplicity, low cost, and environmental friendliness11-14. Common adsorbents used are activated carbon15, diatomite16, and zeolites17. However, the chemical properties and irregular pore structures of these adsorbents limit their adsorption capacity in the field18. Sodium alginate (SA) is a polysaccharide, which is derived from brown algae and exhibits good gelation properties19. It consists of two monomers, α-1,4-L-guluronic acid and β-1,4-D-mannuronic acid20. The surface of SA is rich in carboxyl and hydroxyl groups21, which can exchangea variety of polyvalent ions by exchanging their cations with a target metals under mild conditions21-27. Furthermore, SA can be used as a crosslinking agent when added to a CaCl2 solution, in which the sodium ions exchange with Ca2+ to generate calcium alginate (CA) microparticles28-29. However, a single CA microparticles has limited adsorption capacity and weak mechanical strength. Metal-organic frameworks (MOFs), which are porous materials consisting of metal ions or their clusters30, have attracted much attention in the field of adsorptive separation31. They have diverse structures32, large specific surface areas, tunable pore structures, high crystallinities, and recognizable organic ligands33. Zeolitic-imidazolate frameworks (ZIFs) are a type of MOF34. ZIF-8 has a tetrahedral (sodalite) topological structure, in which Zn atoms are connected via dimethyl imidazole linkers3, 35. ZIF-8 has a surface area of up to 1600 m2/g36-37. Owing to its high porosity, high chemical stability, large specific surface area, and abundant active surface, it has emerged as one 3



of the most popular MOF materials3. Its applications in gas storage, molecular separation, and heterogeneous catalysis have received widespread attention38. ZIF-8 can also be used in the field of heavy metal adsorption39. However, powdered ZIF-8 is difficult to separate from solution following adsorption, because high-speed centrifugation is required for solid-liquid separation. Therefore, its application for the removal of Pb(II) in water has been limited. We hypothesized that if the excellent adsorption properties of ZIF-8 could be paired with the good gelling properties of SA, the difficulty of separating ZIF-8 from solution could be overcome, and the mechanical strength of the CA microparticles could be improved. Thus, microparticles comprising ZIF-8 and SA, hereafter referred to as ZIF-8@CA, would have the advantages of both of its components. To obtain ZIF8@CA microparticles, we synthesized ZIF-8 at room temperature and coated it with SA by gelation. The removal of residual Pb(II) from water by the ZIF-8@CA microparticles was systematically evaluated, and the effects of various factors on the reaction process were investigated. As a result, we found the ZIF-8@CA microparticles to be highly efficient adsorbents with high adsorption capacity, and that the sorbent could be cyclic utilizated. 2. Materials and methods 2.1 Materials Zinc nitrate hexahydrate [Zn(NO3)2·6H2O (ZH6), 99.99 %], SA [viscosity = 200 mPa∙s], 2-methylimidazole [2-Mil, ≥ 98 %], and anhydrous calcium chloride [CaCl2, ≥ 96 %] were purchased from Aladdin Industrial Corporation (Shanghai, China). Methanol [MeOH, ≥ 99.5 %], triethylamine (Et3N, 99 %), lead nitrate [Pb(NO3)2 , ≥ 99.99 %], Cupric sulfate anhydrous (CuSO4 ， ≥ 99.95 %), Zinc chloride (ZnCl2, ≥ 99.99 %) and Cadmium nitrate tetrahydrate [Cd(NO3)2·4H2O, ≥ 99.99 %] were purchased from Sinopharm Chemical Reagent Co, Ltd. (Shanghai, China). Deionized (DI) water was obtained from a Milli-Q® water cleansing system (Millipore, Svadylee, France). 2.2 Synthesis of ZIF-8 ZIF-8 was prepared according to a previously reported procedure40. Briefly, 4


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solution A was obtained by dissolving 0.2 g of ZH6 in 10 mL MeOH, and solution B was prepared by dissolving 0.1 g of 2-Mil in 10 mL DI water and 0.3 mL Et3N. Solutions A and B were stirred for 60 min at room temperature. The mixture was the centrifuged at 4500 rpm for 1 h to remove the supernatant. The remaining white precipitate was washed with methanol twice and by DI water five times, then dried under vacuum (60 ℃, 8 h) to obtain the desired product as a white solid. 2.3 Preparation of the ZIF-8@CA microparticles We determined in a prior experiment that a SA/ZIF-8 ratio of 1/0.5 resulted in the most effective removal of Pb(II) ( Table S1). Therefore, this ratio was selected for the present study. Solution C was prepared by dissolving 1 g of SA in 100 mL DI water and adding 0.5 g of ZIF-8 while stirring. The resulting mixture was stirred for an additional 60 min. Solution D was made by dissolving 1 g of CaCl2 in 50 mL DI water. Solution C was added dropwise to solution D via syringe, and the mixture was allowed to solidify for 60 min. The product was rinsed six times with DI water to obtain the ZIF-8@CA microparticles. 2.4 Adsorption experiments A 1000 mg/L stock solution of Pb(II) was prepared by dissolving Pb(NO3)2 (0.8 g) in 500 mL DI water. For subsequent experiments, the stock solution was diluted to various concentrations from 5 to 1000 mg/L. ZIF-8@CA microparticles were added to 20 mL volumes of the Pb(II) solution at the different concentrations in 100 mL conical flasks. The pH of the solution was adjusted between 2 and 7 using NaOH (0.1 mol/L) and HCl (0.1 mol/L) solutions. The conical flasks were shaken at 240 rpm (120 min) on a mechanical shaker at room temperature to facilitate the adsorption. The Pb(II) adsorption was continued until equilibrium was attained, and the residual concentration of Pb(II) in solution was determined by atomic absorption spectrometry (AAS, AA7000, Shimadzu). The equilibrium adsorption capacity (qe, mg/g) was calculated using Equation (1). qe = [(C0 – Ce)V]/m

(1)

where C0 denotes the initial Pb(II) concentration (mg/L), Ce is the equilibrium Pb(II) concentration (mg/L), V is the volume of the Pb(II) solution (L), and m is the weight of 5



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the ZIF-8@CA microparticles (g) (12.5 mg dry bulb = 0.5 g wet bulb, m=0.0125 g). The adsorption capacity (qt, mg/g) of the ZIF-8@CA microparticles for Pb(II) at time t was calculated with Equation (2). qt = [(C0 – Ct)V]/m

(2)

where Ct denotes the concentration of Pb(II) solution at a given time t (mg/L). Equation (3) was used to calculate the removal of Pb(II) from solution after adsorption equilibrium was reached. R (%) = [(C0 – Ce)/C0] × 100

(3)

2.5 Characterization of ZIF-8 and ZIF-8@CA microparticles FT-IR analysis was performed on a Nicolet 872 FT-IR spectrophotometer (Thermo Nicolet, Bruker, Germany) using KBr pellets. SEM was performed to obtain the surface morphologies of the materials on a CR2032 scanning electron microscope (Hitachi, Japan). Elemental analysis of the material surface was performed by XPS on an Axis Ultra DLD X-ray photoelectron spectrometer (Kratos, England), and the crystal structures of the functionalized nanoparticulate materials were analyzed by XRD on a D8 Advance diffractometer (Bruker, Germany). 2.6 Regeneration experiment In the regeneration experiment, the Pb(II)-loaded ZIF-8@CA microparticles were added to a 100 mL conical flask containing 0.01 mmol/L HCl solution and were shaken on a mechanical shaker for 200 min at room temperature. The desorbed ZIF-8@CA microparticles were washed with DI water five times and dried in a vacuum freezer. The desorbed ZIF-8@CA microparticles were reused five times for absorption experiments. 3. Results and discussion 3.1 Characterization analysis 3.1.1 Morphology

6


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


Fig.1. SEM images of ZIF-8 (a, b) and ZIF-8@CA microparticles (c, d). The SEM images of ZIF-8 in Fig. 1 show that the material surface was covered with many particles (Fig. 1a). These particles are smooth and irregularly spherical with large pores between the particles (Fig. 1b). After loaded with sodium alginate, ZIF8@CA microparticles were obtained, its structure was enlarged by about 10 times than the simple ZIF-8’s structure, and the irregular spherical particles becomes irregular cubic structures (Fig. 1c) , which were composed of sheet-like structures, and the surface is rougher (Fig. 1d). 3.1.2 Functional groups

1636

2918

702

CA

Transmittance

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3444

ZIF-8

3201

3600

1423

ZIF-8@CA

993 1591 1145 3000 2400 1800 1200 600 -1 Wavelength (cm )

Fig.2. FT-IR spectra of CA microparticles, ZIF-8, and ZIF-8@CA microparticles. As can be seen from Fig. 2(1), sodium alginate gave rise to an absorption peak at 3444 cm-1, which was due to the O-H stretching vibration and the band at 2918 cm-1 7



was due to the O-H stretching vibration41. The peaks at 1636 cm-1 and 1423 cm-1 were caused by C-O, -COOH stretching vibrations,respectively42. In the FT-IR spectrum of the original ZIF-8 nanoparticles (Fig. 2(2)), the band at 3201 cm-1 was ascribed to the stretching vibrations of the N-H. The peak at 1591 cm-1 was attributed to the stretching of the N-C bond in dimethyl imidazoles43. Also, the vibration in the region of 1145-993 cm-1 was due to the C-O oscillation4, 27. In the FT-IR spectrum of the ZIF-8@CA microparticles (Fig. 2(3)) didn’t changed much than ZIF-8, the peak at 702 cm-1 corresponded to the Zn-O forward and backward vibrations. 3.1.3 Crystallinity

7.3 12.6 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

10.3

5

10

ZIF-8@CA

18.1 16.5

ZIF-8 15 20 25 30 35 2 Theta (Degree)

40

Fig.3. XRD profiles of the ZIF-8@CA microparticles and ZIF-8. The XRD patterns of ZIF-8 and ZIF-8@CA microparticles are shown in Fig. 3. In the profile of ZIF-8, the main peaks appeared at 7.29° (011), 10.3° (002), 12.6° (112), 16.5° (013), and 18.1° (222), which is in accordance with the ZIF-8 XRD standard peaks34. In addition, the characteristic peaks of ZIF-8 covered by CA were the same as those of ZIF-8, as shown in Fig. 3(2). The sharp, high-intensity peaks shown in Fig. 3(1) indicated the high crystallinity of the synthesized ZIF-8@CA microparticles. 3.1.4 Surface composition

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b

a

Zn2p

138.85eV 142.95eV

Intensity

Intensity

O1s (2) C1s Zn3p

(1)

(2) (1)

pb4f

1200

900 600 300 Binding Energy(eV)

Before adsorption

284.7 C-C

150 147 144 141 138 135 Binding Energy (eV)

0

C1s

d

285.6 C-O

After adsorption

285.8 C-O

Intensity

c Intensity

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


288.3 C=O

284.5 C-C

C1s

288 C=O

294 291 288 285 282 Binding Energy (eV)

294 291 288 285 282 Binding Energy (eV)

Fig.4. XPS spectra of the ZIF-8@CA microparticles (1) before and (2) after adsorption: a) Wide scan. b) Pb 4f spectra adsorption; c) and d) C1s spectra For better illustration of the elemental composition of the ZIF-8@CA microparticles surface and the adsorption mechanism, the samples were analyzed by XPS. The ZIF-8@CA microparticles were found to contain Zn, C, O and Pb(II) (Fig. 4a). The Zn was from ZIF-8 44. The Pb 4f peak (Fig. 4b), appeared at a BE of 138.8 and 142.95 eV after the adsorption of Pb(II), thereby verifying the presence of Pb(II) in the material, which confirmed that ZIF-8@CA microparticles adsorbed Pb(II). In addition, the C 1s spectrum of the ZIF-8@CA microparticles (Fig. 4c) showed three BE peaks at 284.7, 285.6, and 288.3 eV, ascribable to the C-O, C-C, and C=O bonds, respectively45. However, in Figure.4d, after ZIF-8@CA microparticles loaded Pb(II), these three peaks are shifted to 284.5, 285.8 and 288 eV, respectively, because the hydroxyl and carboxyl groups in the ZIF-8@CA microparticles complex Pb(II). 3.2 Adsorption studies 3.2.1 Impact of the ZIF-8@CA microparticles dosage In order to study the effect of the adsorbent dose on the adsorption capacity, 10, 9



12.5, 15, 17.5, 20, and 30 mg of ZIF-8@CA microparticles were added to 20 mL of 300 mg/L Pb(II) solution. As can be seen in Fig. 5, upon increasing the adsorbent dosage, the Pb(II) removal (%) increased from 90.08 % to 96.33 %, while the adsorption capacity at equilibrium (qe) decreased from 590 to 192 mg/g. This suggested that while an increase in the amount of adsorbent provided more adsorption sites, occupying a certain amount of adsorption sites and thus weakening the adsorption effect. In order to ensure the prudent use of materials, the optimal adsorbent dosage was determined to be 12.5 mg. 97

qe(mg/g)

600

96

500

95

400

94 93

300

92

200

Removal(%)

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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91 10

15

20 25 Dose (mg)

30

Fig.5. Effect of the ZIF-8@CA microparticles dosage on the adsorption 3.2.2 Impact of the original Pb(II) concentration To study the relationship between initial Pb(II) concentration (C0) and qe, the adsorption of Pb(II) from solutions with different initial Pb(II) concentrations (10 mg/L to 1000 mg/L) was measured. As can be seen from Fig. 6a, upon increasing the initial Pb(II) concentration, qe increased, while the percent removal decreased. The sorption capacity of the ZIF-8@CA microparticles increased from 16 to 1303.28 mg/g as the Pb(II) C0 was increased from 10 to 1000 mg/L. When the original Pb(II)concentration in the solution was 10mg/L, the metal ion could be efficiently absorbed and had a very high removal ratio, whereas the qe of the sorbent was lower. Based on the observed removal ratio and adsorption capacity, all the subsequent experiments were performed with a Pb(II) C0 of 300 mg/L. The equilibrium adsorption isotherm is an all-important argument to describe the relationship between an adsorbent and adsorbate. The observed adsorption can be 10


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described by evaluating the adsorption equilibrium data of the Pb(II) removal by the ZIF-8@CA microparticles with equilibrium adsorption models. The effect of the equivalent adsorption or single-layer adsorption on the transverse interaction and spatial resistance can be described with the Langmuir model (Eq.4)46, while the Freundlich isotherm model (Eq. 5) can expound the multilayer adsorption course and reversible adsorption47. In addition, RL (Eq. 6) is used to describe whether or not the adsorption process is preferential48. Ce/qe = 1/bqm + Ce/qm

(4)

lg qe = lg KF + 1/n(lg Ce)

(5)

RL = 1/(1 + bC0)

(6)

In Equations 4-6, qm denotes the maximum adsorption capacity of the ZIF-8@CA microparticles for Pb(II) removal (mg/g), n is a constant related to the adsorption strength, b represents a constant, and KF denotes a Freundlich constant related to the adsorption capacity (mg/g). The curves obtained by fitting the adsorption data to the Langmuir isotherm model and Freundlich isotherm model are shown in Figs. 6b and 6c, respectively, and the parameters obtained are shown in the Table S2. R2 = 0.9856 was fitted by Langmuir isotherm model and R2 = 0.9564 was fitted by Freundlich isotherm model, which are more consistent with Langmuir isotherm model. The qm obtained by fitting the model is 1321.21 mg/g. It indicated that the process Pb(II) adsorption by ZIF-8@CA adsorbent was is mainly dominated by monolayer adsorption, and the adsorption capacity of each active site on the surface is uniform and identical. The value of b obtained by the Langmuir equation is positive, indicating that the theoretical saturated adsorption capacity of ZIF-8@CA microparticles on Pb(II) is greater than the adsorption capacity measured in actual experiments. The calculated RL values, all of which are between 0-1 (Table S3), indicating that the adsorption process is favorable for adsorption48.

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a 1500

100

qe (mg/g)

1200

98 96

900

94

600

92 90

300

88

0

86 0

c

0.15

3.0

0.10 Ce/q e

200 400 600 800 1000 Original concentration(mg/L)

R2=0.9856

lg (q e)

b

Recovery (%)

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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0.05

2.5

R2=0.9564

2.0

0.00 0

30

60 90 Ce (mg/L)

1.5

120 150

-2

-1

0 1 lg (Ce)

2

3

Fig.6. a) Effect of the original concentration (300mg/L) on the adsorption capacity at 298.2 K. b) Langmuir isotherm models for the Pb(II) adsorption by the ZIF-8@CA microparticles. c) Freundlich isotherm models for the Pb(II) adsorption by the ZIF8@CA microparticles. 3.2.3 Impact of time The adsorption of Pb(II) by the ZIF-8@CA microparticles was studied for times ranging from 5 to 300 min using 20 mL 300 mg/L Pb(II) solution. The change in the adsorption process is shown in Fig. 7a. In the first 120 min of adsorption, both the removal rate and the Pb(II) adsorption ability of the ZIF-8@CA microparticles increased with time. Similarly, probably because of the short contact time, the adsorption capacity of the ZIF-8@CA microparticles was far from saturation, and the Pb(II) in the solution was quickly adsorbed. After 120 min, the number of available adsorption sites on the ZIF-8@CA microparticles surface were gradually reduced, and 12


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the sorbent approached saturation, thus reducing the adsorption efficiency. After 120 min, the adsorption gradually approached equilibrium and the calculated adsorption capacity increased. After 200 min, the adsorption process achieved equilibrium, with a calculated adsorption capacity of 516 mg/g. Based on these results, the adsorption time for subsequent works was chosen to be 120 min. The adsorption kinetics data of the ZIF-8@CA microparticles for the reaction of Pb(II) were fitted using the pseudo-first-order kinetics (Eq. 7) and pseudo-second-order kinetics (Eq. 8) models. ln(qe – qt) = ln qe = k1t

(7)

t/qt = 1/k2qe2 + t/qe

(8)

where, qe denotes the Pb(II) adsorption capacity of the ZIF-8@CA microparticles at equilibrium (mg/g), qt represents the Pb(II) adsorption capacity of the ZIF-8@CA microparticles at time t (mg/g), k1 denotes the pseudo-first-order kinetic rate constant (L/min), and k2 is the pseudo-second-order kinetic rate constant (g/mg·min). The results from fitting the data with the kinetics models are shown in Figs. 7b and 7c, and the calculated and obtained kinetics parameters are shown in Table S4. As can be seen from Table S4, the R2 of the pseudo-second-order dynamics model fitting exceeded 0.9999 and higher than that of the pseudo-first-order kinetics (R2=0.9234). Furthermore, qe calculated with the pseudo-second-order model parameters was 502.91 mg/g, which closed to the experimentally determined 503.46 mg/g. Therefore, the process of Pb(II) adsorption by the ZIF-8@CA microparticles conformed to the pseudosecond-order dynamics model. These results indicated that Pb(II) adsorption by the ZIF-8@CA microparticles was closer to chemical adsorption8, 49.

13



qt (mg/g)

a 510

100

480

95 90

450

85

420

80

390 360

b

75 0

50

100 150 200 250 300 t (min)

c

6

0.3 0.2

4 t/qt

ln(qe-qt)

Removal (%)

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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R2=0.9234

2

0.1

R2=0.9999

0.0 0

0

60 t (min)

120

0

60 t (min)

120

Fig.7. a) Effect of contact time on the adsorption capacity (300 mg/L Pb(II); 298.2 K; pH=5). b) Fitting of the ZIF-8@CA adsorption data with the pseudo-first-order model. c) Fitting of the ZIF-8@CA adsorption data with the pseudo-second-order model. 3.2.4 Impact of pH To study the impact of the pH on the Pb(II) adsorption capacity of ZIF-8@CA microparticles, the 300 mg/L Pb(II) solution in individual 20 mL volumes, When pH is greater than 7, Pb(II) is converted to Pb(OH)2 precipitation, so the pH of Pb(II) solution is adjusted to 2-6 in this experiment, and the solutions were shaken for 120 min at room temperature. The Pb(II) sorption capacities at different pH values were determined, and the results are shown in Fig. 8. As can be seen, In the pH range tested, qe gradually increases from 317.31 mg/g to 510.12 mg/g with the increase of pH, as well as the percent Pb(II) removal from 67.64 % to 94.31 %. When the pH is greater than 5, qe does not change much, which was attributed to the presence of a large amount of deprotonated carboxyl groups with negative charges on the surface of the ZIF-8@CA 14


microparticles50. At acidic pH, excess protons will occupy carboxyl sites on the sorbent, preventing sorption of Pb(II). Based on these reasons, pH 5 was chosen as the optimum pH for the Pb(II) adsorption by ZIF-8@CA microparticles in this experiment.

500

96

450

90

400

84 78

350

72

300

Removal (%)

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


qe (mg/g)

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66

250

2

3

4 pH

5

6

Fig.8. Effect of the pH on the adsorption capacity. Pb(II) concentration: 300 mg/L, T = 298.2 K. 3.2.5 Impact of temperature In order to study the impact of different temperatures on the adsorption reaction, Pb(II) adsorption experiments were carried out at the following temperatures: 298.2, 303.2, and 308.2 K. First, 20 mL of a 300 mg/L Pb(II) solution at pH 5 was shaken 120 min. Fig. 9 shows the impact of different temperatures on qe and % Pb(II) removal by the ZIF-8@CA microparticles. As can be seen, the adsorption capacity increased from 510.19 to 515.84 mg/g, and the Pb(II) removal by the adsorbent increased from 93.01 % to 94.43 %. Therefore, both qe and Pb(II) removal rate increased with temperature. This indicated that the adsorbent reaction was possibly endothermic, so the rise in temperature was favorable for adsorption.

15



520

95 94

qe (mg/g)

515

93 92

510

91

Removal (%)

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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90 505 297

300

303 T(K)

306

89 309

Fig.9. Effect of temperature on Pb(II) adsorption by ZIF-8@CA microparticles when C0 = 300 mg/L The thermodynamic parameters of Gibbs free energy (ΔG, kJ/mol), enthalpy (ΔH, kJ/mol), and entropy (ΔS, kJ/mol) were calculated using the following equation (Eq. 9):

G   RT ln

qe H S   RT (  ) Ce RT R

(9)

where T denotes the absolute temperature of the solution (K) and R denotes the gas constant. ΔG is negative and ΔH is positive at a given temperature, the adsorption process is a spontaneous endothermic reaction. In this case, the adsorption process could be improved by an appropriate increase in the temperature. Furthermore, a ΔS value above zero indicates an increase in entropy51. Results of the thermodynamic parameter calculations are summarized in Table S5. 3.2.6 Comparison of Pb(II) adsorption capacities of ZIF-8@CA, CA microparticles, and ZIF-8 To compare the adsorption capacities of the three sorbents, 20 mL of 300 mg/L Pb(II) solution (pH= 5) was added to each of three 100 mL conical flasks, followed by either ZIF-8@CA microparticles (12.5 mg), CA (12.5 mg), or ZIF-8 (12.5 mg). The flasks were shaken at room temperature for 120 min, and the qe of each sorbent was determined. The results are shown in Fig. 10. The values of qe among the adsorbents decreased in the order ZIF-8@CA microparticles > ZIF-8 > CA. 16


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ZIF-8@CA

500 400

qe (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

CA

ZIF-8

200 100 0

Fig.10. Pb(II) adsorption capacities of ZIF-8@CA microparticles, CA microparticles, and ZIF-8 3.2.7 Comparison with adsorbents reported in literature Table S6 shows a comparison of the adsorption capacities of materials reported in literature with that of the ZIF-8@CA microparticles. As can be seen, ZIF-8@CA microparticles exhibited a higher qe than those of the previously reported materials. This indicated that the ZIF-8@CA microparticles were a good adsorbent for removing Pb(II). 3.3 Regeneration analysis The reusability of an adsorbent significantly affects the environment and economy. Therefore, in order to study the reusability of ZIF-8@CA microparticles, regeneration experiments were performed. As can be seen in Fig. 11, over the course of five regeneration cycles, the Pb(II) adsorption capacity of the ZIF-8@CA microparticles decreased from 453.44 to 392.79 mg/g. After five cycles, the Pb(II) adsorption capacity of the ZIF-8@CA microparticles was 392.79 mg/g, and the removal rate was 81.83 %, which indicated that the ZIF-8@CA adsorbent retained good adsorption properties. Based on these results, we concluded that the ZIF-8@CA microparticles would be an economical and effective adsorbent with the ability to be regenerated.

17



350

80

280

60

210

40

140

Removal(%)

100

420

qe(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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20

70 1

2

3 Recycles

4

5

0

Fig.11. Reusability of ZIF-8@CA microparticles for Pb(II) adsorption (300 mg/L; 298.2 K) 3.4 Adsorption selectivity The selective adsorption of different metal ions by ZIF-8@CA microparticles would be beneficial to the environment and recycling56. Pb(NO3)2, CuSO4，ZnCl2, and Cd(NO3)2·4H2O were dissolved in DI water to obtain a mixed solution of lead, copper, chromium, and zinc each metal at a concentration of 0.5 mmol/L. The solution also contained NO3-, SO42-, and Cl- anions. The results are shown in the Fig.12. The results showed that the removal rate of Pb(II) by ZIF-8@CA microparticles was as high as 97.21 %. In addition, the removal rates for Cd(II) and Cu(II) were above 70 %, which indicated that the material are widely used. The qe of the ZIF-8@CA microparticles for Zn(II) was comparatively low, which may have been due to the fact that the material itself contains Zn(II), so it is repellent to Zn(II).

18


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100

120

80

qe (mg/g)

90

60

60

40 30

Removal (%)

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


20 0 Pb(II)

Cu(II)

Cd(II)

Zn(II)

Fig.12. Adsorption selectivity of ZIF-8@CA microparticles at 298.2 K and 0.5 mmol/L per metal 3.5 Adsorption mechanism The adsorption mechanism of ZIF-8@CA on Pb(II) is mainly because the adsorbent is rich in carboxyl groups and has a negative charge on its surface. Electrostatic interaction between a positively charged Pb(II) and a negatively charged adsorbent allows lead to be successfully adsorbed by ZIF-8@CA14. The adsorption process is mainly based on single-layer adsorption, and the adsorption capacity on the surface of each adsorption site is uniform. The adsorption process of Pb(II) by ZIF8@CA is an ion exchange mechanism. Its adsorption mechanism is shown in Fig.13.

Fig.13. Mechanism of adsorption of Pb(II) by ZIF-8@CA 4. Conclusion 19



In this study, ZIF-8 was synthesized using Et3N at room temperature, then mixed with SA and cured with CaCl2 to produce ZIF-8@CA microparticles. Characterization of the adsorbent by SEM, FT-IR spectroscopy, XRD, and XPS confirmed its successful synthesis. The ZIF-8@CA microparticles were applied for the removal of Pb(II) from an aqueous solution. The adsorption reaction reached equilibrium after 120 min. At pH 5, parameter optimization afforded a maximum adsorption capacity as high as 1331.21 mg/g. ZIF-8@CA microparticles demonstrated a higher adsorption capacity than those reported for other adsorbents. Based on the adsorption kinetics and the Langmuir adsorption isotherm model, we concluded adsorption by ZIF-8@CA microparticles was endothermic. Furthermore, experimental comparison of ZIF-8, CA microparticles, and ZIF-8@CA microparticles revealed that the ZIF-8@CA microparticles had the highest adsorption capacity. Regeneration analysis also showed that the ZIF-8@CA microparticles not only effectively removed Pb(II) from water, but they could also be reused 5 times. Supporting Information Tables S1 to S6. This material is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21476212), Science and Technology department of Zhejiang Province (2018C02038) and Bureau of Science and Technology of Zhoushan (2017C41005). Notes The authors declare that there are no conflicts of interest. References 1. Lu, J.; R. N. Jin; C. Liu; Y. F. Wang; X. K. Ouyang, Magnetic carboxylated cellulose nanocrystals as adsorbent for the removal of Pb(II) from aqueous solution. Int J Biol Macromol 2016, 93, 547-556.

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