Journal of Hazardous Materials 297 (2015) 66–73

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Fabrication of fibrous amidoxime-functionalized mesoporous silica microsphere and its selectively adsorption property for Pb2+ in aqueous solution Yunyun Xie, Jie Wang, Mozhen Wang ∗ , Xuewu Ge ∗ CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Submicron

fibrous amidoximefunctionalized SiO2 microspheres were fabricated. • The fibrous SiO2 microspheres have hierarchical pore structure with meso- and macro-pores. • The fibrous amidoximefunctionalized SiO2 microspheres can selectively adsorb Pb2+ . • The maximum equilibrium adsorption capacity for Pb2+ could reach 284 mg/g.

a r t i c l e

i n f o

Article history: Received 3 March 2015 Received in revised form 11 April 2015 Accepted 25 April 2015 Available online 28 April 2015 Keywords: Fibrous mesoporous silica Amidoxime-functionalization Adsorption Heavy metal ions

a b s t r a c t Fibrous cyano-modified mesoporous SiO2 microspheres with specific surface area of ca. 300 m2 g−1 have been successfully fabricated respectively by in-situ synthesis and post-modification methods, based on the hydrolysis of ethyl silicate in the presence of (2-cyanoethyl)triethoxysilane at a certain condition. TEM observations show that the average diameters of the prepared fibrous cyano-modified SiO2 microspheres by these two methods are 68 and 211 nm, respectively. The N2 adsorption–desorption isotherms analysis on the fibrous SiO2 microspheres show sharp peaks in the 10–20 nm range. After the cyano groups transformed to amidoxime groups, the adsorption behavior of the fibrous amidoxime-functionalized mesoporous SiO2 microspheres for Fe3+ , Cu2+ , and Pb2+ was investigated. The results show that the prepared SiO2 microspheres can selectively adsorb Pb2+ . The maximum equilibrium adsorption capacity for Pb2+ could reach 284 mg/g. The desorption of Pb2+ in 2 M HNO3 completes within 60 min. The efficiency of the desorption is as high as 96.2%. This work provides the methods to prepare amidoxime-functionalized SiO2 microsphere with high specific surface area and total pore volume, which has the potential to be applied as an efficient adsorbent for specific heavy metal ions. © 2015 Published by Elsevier B.V.

1. Introduction Due to the rapid development of modern industry, heavy metal ions have excessively accumulated in biosphere and water,

∗ Corresponding authors. Tel.: +86 551 63600843; fax: +86 551 63601592. E-mail addresses: [email protected] (M. Wang), [email protected] (X. Ge). http://dx.doi.org/10.1016/j.jhazmat.2015.04.069 0304-3894/© 2015 Published by Elsevier B.V.

leading to the deterioration of the natural environment and a big health hazard [1,2]. Many methods, such as solvent extraction [3,4], ion exchange [5,6], flotation [7], adsorption by biomass [8] and synthetic absorbents [9–11], have been developed to remove the excessive heavy metal ions from water. Among these, synthetic sorbents are widely used due to the low cost and the diversity of the structure and properties. The key to build a sorbent with high adsorption capacity and fast adsorption rate is the appro-

Y. Xie et al. / Journal of Hazardous Materials 297 (2015) 66–73

priate selection of the functional groups and the structure of the support. Porous materials have always been chosen to be the supports for sorbents due to their nature advantage of large specific surface area [12–14]. Polshettiwar et al. [15] reported a new family of silica nanospheres with fibrous morphology in 2010, whose specific surface area can be as high as 641 m2 g−1 . Moon and Lee [16] also prepared the analogous silica nanoparticles with radial wrinkle structure. They showed that wrinkles in nanoparticles exhibited hierarchical mesoporous structures by means of N2 adsorption–desorption analysis. Thus they believed that the wrinkle structure was generated in the bicontinuous microemulsion phase of the Winsor III system. Although the formation mechanism of this kind of silica nanospheres is still in investigation [17,18], their hierarchical mesoporous structure with high surface area obviously makes them potentially be efficient support for the sorbents. On the other hand, the development of silane coupling agents gives a bright and versatile way to introduce various active groups absorbing heavy metal ions on the silica particles. Amino [19], hydroxyl [20], mercapto [21], and carboxyl [22] are common ligands to chelate with heavy metal ions in literatures. Recently, the H 2N OH C N adsorption behavior of amidoxime groups ( ) for heavy metal ions has attracted much interest since amidoxime group is an excellent amphoteric functional group containing both acidic ( OH) and basic sites ( NH2 ), and regarded as a promising ligand. It has been found that amidoxime groups have a strong tendency to form chelate complex with a wide range of transition and heavy metal ions, such as Cu2+ , Co2+ , and Hg2+ , in aqueous solution [23–25]. The amidoxime groups also can complex with uranium ions [26–28]. Amidoxime group can be formed by the reaction of cyano group and hydroxylamine at a condition of pH 7–9. Therefore, in order to introduce the amidoxime groups on a support, cyano groups are firstly introduced generally, then react with hydroxylamine. For example, Zhao et al. [29] immobilized cyano groups on amino-functionalized Fe3 O4 @SiO2 core–shell nanoparticles by coupling nanoparticles and diaminomaleonitrile using glutaraldehyde. After the amidoximation reaction, the nanoparticles exhibited enhanced sorption capacity for U(VI) from water in comparison with raw silica coated Fe3 O4 nanoparticles due to the strong chelation of amidoxime to U(VI). A simpler way to introduce cyano groups on silica is using a silane coupling agent, (2-cyanoethyl)triethoxysilane (CETEOS), during the process of the preparation of silica by sol–gel method [30,31]. Thus in this work, we successfully used two methods in the presence of CETEOS, i.e., in-situ synthesis and post-modification, to fabricate mesoporous

67

silica microspheres modified with cyano groups on the surface with radial fibrous morphology. The immobilized cyano groups were then further transformed to amidoxime groups. The adsorption behavior of the as-prepared fibrous amidoxime-functionalized mesoporous silica for heavy metal ions, Fe3+ , Cu2+ and Pb2+ , was investigated, compared with that of traditional mesoporous silica MCM-41. The results show the fibrous amidoxime-functionalized mesoporous silica possess a selectively adsorption of Pb2+ ions. This work indicates the fibrous amidoxime-functionalized mesoporous silica microspheres can be potentially applied as excellent selective adsorbent for Pb2+ ions.

2. Experimental methods 2.1. Materials (2-Cyanoethyl)triethoxysilane (CETEOS, 97%) was obtained from Alfa Aesar. Analytical grade reagents including Pb(NO3 )2 , FeCl3 ·6H2 O, CuCl2 ·2H2 O, cetyltrimethylammonium bromide (CTAB), isopropyl alcohol (IPA), ethyl silicate (TEOS), cyclohexane, urea, hydroxylammonium chloride (NH2 OH·HCl), sodium carbonate anhydrous, and anhydrous ethanol, were purchased from Sinopharm Chemical Reagent Co., Ltd. Distilled water was used in all samples. All reagents were used without further purification.

2.2. Synthesis of fibrous amidoxime-functionalized mesoporous silica microspheres 2.2.1. In-situ synthesis of fibrous cyano-modified mesoporous silica microspheres CTAB (1.0 g) and urea (0.6 g) were dissolved in 30 mL of distilled water. Subsequently, 30 mL of cyclohexane and 0.92 mL of IPA were added into the solution under vigorous stirring. TEOS (2.5 mL) containing 3 mol% of CETEOS was added dropwisely. After being stirred for 30 min at room temperature, the system was heated to 70 ◦ C to react for 16 h. The product was collected by centrifugation, washed with ethanol and distilled water thrice, respectively. To remove the remaining surfactant CTAB, each sample was dispersed in the alcoholic solution of ammonium nitrate, and refluxed at 60 ◦ C for 24 h. Finally, the solid product, termed as 1-SiO2 -CN3, was collected by centrifugation, washed with ethanol thrice, and dried in a blast oven at 50 ◦ C. Fibrous silica particles were also prepared by the above method using pure TEOS, which was termed as F-SiO2 in this work.

Scheme 1. Illustration of the preparation processes and the adsorption for metal ions of the fibrous amidoxime-functionlized mesoporous silica microspheres.

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2.2.2. Fibrous cyano-modified mesoporous silica microspheres prepared by post-modification method The above prepared F-SiO2 (0.3 g) was dispersed in 25 mL of anhydrous ethanol, then 50 ␮L of aqueous ammonia was added. A certain amount of CETEOS (400 ␮L) was dropped slowly into the above mixture under magnetic stirring. The system was heated to 70 ◦ C, and kept for 12 h. The product, named as 2-SiO2 -CN400, was collected by centrifugation, washed with ethanol thrice, and then dried in a blast oven. As a comparison, the traditional MCM-41 mesoporous silica was synthesized according to the literature [31]. Briefly, 2.50 g of CTAB was dissolved in 50.0 mL of distilled water. Aqueous ammonia (14.0 mL) and ethanol (76.0 mL) were added subsequently. The mixture was vigorous stirred for 15 min, then 5.0 mL of TEOS was added to react for another 2 h. The product was collected by centrifugation, washed with ethanol, and then dried in a blast oven. The modification of MCM-41 with CETEOS was the same as described above.

2.2.3. Amidoximation of the as-prepared cyano-modified mesoporous silica microspheres Under nitrogen atmosphere, 1.50 g of NH2 OH·HCl was dissolved in 24.0 mL of distilled water under magnetic stirring at room temperature, followed by the addition of 1.10 g of Na2 CO3 . On the other hand, 0.50 g of the as-prepared cyano-modified mesoporous silica microspheres was dispersed in 24.0 mL of distilled water. Then, the dispersion was added dropwisely into the aqueous solution of NH2 OH·HCl. The system was heated to 70 ◦ C, and kept for 3 h. The product was collected by centrifugation, washed with distilled water thrice, and then dried in a blast oven at 50 ◦ C for 12 h. The final amidoxime-functionalized mesoporous silica microspheres are termed as AD-1-SiO2 -CN3, AD-2-SiO2 -CN400, and AD-MCM-41 accordingly.

2.3. Characterizations Morphologies of the as-prepared mesoporous silica microspheres in each procedure were observed with transmission electron microscopy (TEM) (Hitachi H-7650, 100 kV) and scanning electron microscopy (SEM) (JEOL JSM6700F, 5.0 kV). Samples were firstly dispersed in ethanol. Then the dispersion was dropped onto copper grids. The samples for SEM observation were treated by spray-gold. The number average diameter (Dn ), weight average diameter (Dw ), and polydispersibility index (PDI) were calculated by the following equations with the diameters of at least 100 particles measured in the TEM images:

2.4. Adsorption performance of amidoxime-functionalized mesoporous silica microspheres for heavy metal ions It is reported that amidoxime group will be protonated when pH is lower than 6 [32], which is unfavorable for the adsorption of the metal ions due to the electrostatic repulsion effect. On the other hand, the hydrolysis of most metal ions become severe in basic solution, which results in the precipitation of the metal ions. Thus, the pH value of the solutions in all adsorption experiments in this paper is controlled to 6.3 to ensure the maximum adsorption capacity of the prepared amidoxime-functionalized SiO2 microspheres. The as-prepared amidoxime-functionalized mesoporous silica microspheres (20 mg) were ultrasonically dispersed into an aqueous solution of metal ions (50 mg L−1 , 100 mL) in a flask. The system was shaken at 25 ◦ C with a frequency of 150 rpm in a WHY-2 shaker. The adsorption kinetics was measured by sampling 2.0 mL of the solution at a given interval time. After the sampled solution was centrifuged, the concentration of metal ions in the supernatant was measured by atomic absorption spectrophotometer (4530F, Shanghai Jingke Co., Ltd.). The adsorption amount of metal ions at time t (qt ) was calculated according to the following equation: qt =

(1)

where ni is the number of microspheres with a diameter of Di . The nitrogen adsorption–desorption isotherms of the prepared silica microspheres were measured at 77.3 K on a Micromeritics Tristar II 3020 M V1.03 after the samples were outgassed at 60 ◦ C for 6 h. The specific surface area and pore size distribution were analyzed by Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) methods, respectively. The total pore volume of the samples was calculated at P/P0 = 0.97. The atom ratio of the element C and N in samples was measured by elementary analysis (EA) using a VARIO ELIII analyzer. Fourier transform infrared spectra (FTIR) were measured in the range from 4000 to 400 cm−1 with a Bruker TENSOR27 FT-IR spectrometer using KBr pellets.

(2)

where, c0 (mg L−1 ) is the initial concentration of metal ions, i.e., 50. Ct (mg L−1 ) is the measured concentration of metal ions at time t. V0 is the initial volume of the solution, i.e., 0.1 L. m (g) is the mass of the added amidoxime-functionalized mesoporous silica microspheres, i.e., 0.02. The adsorption capacity of the amidoxime-functionalized mesoporous silica microspheres (qe ) was calculated by the following Eq. (3). qe = (c0 − ce ) ×

V0 m

(3)

where, ce (mg L−1 ) is the concentration of metal ions when the system reached the adsorption equilibrium, which was measured after 12-h’ adsorption at 25 ◦ C. To investigate the desorption kinetics of metal ions, the amidoxime-functionalized mesoporous silica microspheres saturated with the metal ions were separated from the solution by centrifugation, and dried in a blast oven at 50 ◦ C. Take 20 mg of the dried microspheres to disperse in 100 mL of 2 M HNO3 solution. Then, 2.0 mL of the solution was sampled out at a given interval time, followed by replenishing the same volume of 2 M HNO3 solution.  The desorption amount of metal ions after the nth sampling qdn was calculated according to the following Eq. (4): qdn =

˙ni Di4 ˙ni Di Dw Dn = ; Dw = ; PDI = 3 Dn ˙ni ˙ni Di

(c0 − ct ) × V0 m

cn × V0 + V˙in ci−1 m

; c0 = 0, i = 1, 2, ...., n

(4)

where, cn is the measured concentration of metal ions with a unit of mg L−1 at the nth sampling. V0 is the initial volume of the solution, i.e., 0.1 L. V is the sampled volume, i.e., 0.002 L. m (g) is the mass of the added amidoxime-functionalized mesoporous silica microspheres saturated by metal ions, i.e., 0.02. 3. Results and discussion 3.1. Synthesis and morphology characterization of the fibrous amidoxime-functionalized mesoporous silica microspheres Two synthesis strategies for the fibrous amidoximefunctionalized mesoporous silica microspheres are illustrated by Routes 1 and 2 in Scheme 1. The prepared silica particles from both of Routes 1 and 2 show the fibrous morphology, as shown in Fig. 1. In Route 1, we used the mixture of TEOS and CETEOS as

Y. Xie et al. / Journal of Hazardous Materials 297 (2015) 66–73

69

Fig. 1. TEM images of cyano-modified silica microspheres 1-SiO2 -CN3 (A), 2-SiO2 -CN400 (B), and F-SiO2 (C). The insets are the corresponding SEM images of the samples.

the precursor of silica particles, thus, cyano groups could be in situ introduced during the formation of fibrous silica microspheres. Fig. 1A shows the morphologies of the cyano-modified silica microspheres (1-SiO2 -CN3) prepared the in-situ synthesis method. Compared with that of pure F-SiO2 microspheres prepared with the same condition (Fig. 1C), 1-SiO2 -CN3 microspheres also have a clear fibrous morphology. However, the average diameter of 1-SiO2 -CN3 is 68 nm (PDI = 1.08), much smaller than that of F-SiO2 , i.e., 211 nm (PDI = 1.11). The N2 adsorption–desorption isotherms of 1-SiO2 -CN3 and FSiO2 displayed in Fig. 2(A-1) are of the similar shape, i.e., type IV curves according to BDDT classification with a type H3 hysteresis loops [33,34]. A rapid increase of adsorption of nitrogen can be observed when P/P0 is above 0.7, implying the presence of mesopores. It is in agreement with the pore size distribution calculated by BJH, as shown in Fig. 2(A-2). The dominant pores in both 1SiO2 -CN3 and F-SiO2 microspheres have a diameter between 10 and 20 nm. At the same time, the additional increasing N2 uptakes and H3 hysteresis at the relative pressure above 0.95 in Fig. 2(A-1) indicated that there existed a certain amount of slit like macropores, which is related to the fibrous morphology of SiO2 microspheres [16]. The diameter of macropores in 1-SiO2 -CN3 is 98 nm, as exhibited in Fig. 2(A-2), higher than that in F-SiO2 (55 nm). Both 1-SiO2 -CN3 and F-SiO2 microspheres have a relatively high specific surface area and the total pore volume of microspheres, which were calculated from the N2 absorption–desorption isotherms and are listed in Table 1. It is noted that the introduction of cyano groups with 3 mol% of CETEOS would result in a little decrease in both the specific surface area and the total pore volume of the produced SiO2 microspheres. The cyano-modified fibrous SiO2 microspheres can also be prepared by post-modification on F-SiO2 with CETEOS, as illustrated by Route 2 in Scheme 1. With this method, the obtained cyanomodified fibrous SiO2 microspheres (2-SiO2 -CN400) nearly have the same morphology as the original F-SiO2 microspheres, as shown in Fig. 1B. In addition, different from the sharp decrease in the size of microspheres induced by the addition of CETEOS in Route 1, the cyano-modified fibrous SiO2 microspheres prepared by post-modification method (Route 2) have the analogous size Table 1 The specific surface area and total pore volume of the synthesized SiO2 microspheres. Sample

F-SiO2 1-SiO2 -CN3 AD-2-SiO2 -CN400 MCM-41 AD-MCM-41

Specific surface area (m2 g−1 )

Total pore volume (cm3 g−1 )

399.2 252.4 298.6 1273.4 577.0

1.14 0.99 0.89 0.67 0.39

to the original F-SiO2 microspheres. The average diameter of 2SiO2 -CN400 microspheres in Fig. 1B is 218 nm with a PDI of 1.08. However, there is still a little loss in the specific surface area and the total pore volume (see Table 1) after the post-modification, although the N2 absorption–desorption isotherms and the corresponding pore size distribution of AD-2-SiO2 -CN400 are similar to those of F-SiO2 , as clearly exhibited in Fig. 2(A-1) and (A-2). As a comparison, the N2 absorption–desorption isotherms and the data of the pore structure characterization of the traditional mesoporous silica MCM-41 before and after amidoximation are also shown in Fig. 2(B-1),(B-2) and Table 1, respectively. Although MCM41 microspheres possesses much higher the surface area than the prepared fibrous SiO2 microspheres, they have much smaller pore size and total pore volume, especially after the amidoxiamtion, which is evidently unfavorable for the application as the efficient support for absorbents. 3.2. Efficiency of the amidoximation of cyano groups on the fibrous mesoporous silica microspheres The FTIR spectra of F-SiO2 and amidoxime-functionalized silica microspheres (AD-1-SiO2 -CN3, AD-2-SiO2 -CN400 and AD-MCM41) are displayed in Fig. 3. For F-SiO2 , the main absorption peaks of Si O stretching (1100 cm−1 ), Si OH bending (945 cm−1 and 1630 cm−1 ), Si O Si bending (800 cm−1 ), and O H stretching (∼3500 cm−1 ) can be observed. After the modification with CETEOS and the following amidoximation reaction, the absorption peaks of saturated C H stretching at 2930 cm−1 , 2860 cm−1 and 1440 cm−1 caused by the CH2 groups on CETEOS appear on the spectra of all the amidoxime-functionalized silica microspheres. The absorption of C N stretching (1650 cm−1 ) overlaps with the absorption peak of Si OH bending (1630 cm−1 ). However, it can be concluded from Fig. 3 that the conversion of C N to amidoxime group is nearly completed for 2-SiO2 -CN400 microspheres because the characteristic absorption of C N stretching at 2260 cm−1 disappears on the spectrum of AD-2-SiO2 -CN400 (Fig. 3B), but still remains on those of AD-1-SiO2 -CN3 (Fig. 3C) and AD-MCM-41 (Fig. 3D). Therefore, EA was used to estimate the efficiency of amidoximation modification. The contents of N, C, and H elements in the different silica microspheres are listed in Table 2. From the

Table 2 The weight percentage of elements, the content of cyano and amidoxime groups, and EAD of the synthesized SiO2 microspheres. Sample

N (%)

C (%)

H (%)

F-SiO2 AD-1-SiO2 -CN3 AD-2-SiO2 -CN400 MCM-41 AD-MCM-41

0.09 4.62 2.67 0.02 2.34

0.53 10.14 4.73 1.42 6.46

1.07 2.72 1.56 2.05 2.27

nCN (mmol/g)

nAD (mmol/g)

EAD

2.10 0.50

0.57 0.67

21% 57%

1.14

0.26

19%

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Y. Xie et al. / Journal of Hazardous Materials 297 (2015) 66–73

A-1

F-SiO2

dV/dlog(D) Pore Volume (cm3/g)

Quantity Adsorbed (cm3/g STP)

1000

1-SiO2-CN3

800

AD-2-SiO2-CN400

600

400

200

0 0.0

F-SiO2

2.5

AD-2-SiO2-CN400

2.0 1.5 1.0 0.5 0.0

0.2

0.4

0.6

0.8

1.0

1

10

100

Pore Diameter (nm)

Relative Pressure (P/P0)

1000

10

B-1

MCM-41 AD-MCM-41

800

dV/dlog(D) Pore Volume (cm3/g)

Quantity Adsorbed (cm3/g STP)

A-2

1-SiO2-CN3

600

400

200

B-2 8

6

4 MCM-41 AD-MCM-41

2

0

0 0.0

0.2

0.4

0.6

0.8

1.0

1

10

100

Pore Diameter (nm)

Relative Pressure (P/P0)

Fig. 2. N2 adsorption–desorption isotherms of silica microspheres with fibrous morphology (F-SiO2 , 1-SiO2 -CN3, AD-2-SiO2 -CN400) (A-1) and traditional mesoporous silica (MCM-41, AD-MCM-41) (B-1). The corresponding pore size distributions of the silica microspheres analyzed by BJH method from the adsorption branch of the isotherms are listed in A-2 and B-2, respectively.

EA results, the content of amidoxime group (nAD , mmol/g) and cyano groups (nCN , mmol/g) in the amidoxime-functionalized silica microspheres, as well as the efficiency of amidoximation (EAD ), can be calculated according to the following equations, and the results are also listed in Table 2.

2nAD + nCN =

(N% of amidoxime − functionalized silica) − (N% of silica) × 1000 14

(5) (nAD + nCN ) × 3 =

(C% of amidoxime − functionalized silica) − (C% of silica) × 1000 12

(6) EAD =

Transmittance %

D C 2930 cm-1 2860 cm

-1

2260 cm

-1

B 1650 cm-1

A

4000

1630 cm-1

3500

3000

2500

2000

1500

1000

500

-1

wavenumber (cm ) Fig. 3. FTIR spectra of F-SiO2 (A) and amidoxime-functionalized silica microspheres: AD-2-SiO2 -CN400 (B); AD-1-SiO2 -CN3 (C), and AD-MCM-41 (D).

nAD × 100% nCN + nAD

(7)

where, “N% of amidoxime-functionalized silica” means the weight percentage of N element in amidoxime-functionalized SiO2 microspheres measured by EA analysis. And so on, for the rest expressions in the numerators of Eqs. (5) and (6). The number 14 and 12 are the molar masses of N and C, respectively. 1000 is the conversion factor from the unit mol to mmol. As Table 2 indicates, the EAD of cyano-modified fibrous SiO2 microspheres is higher than that of MCM-41. It should be attributed to the much smaller pores and pore volume of MCM-41 compared with fibrous mesoporous SiO2 microspheres (see Fig. 2 and Table 1), which is unfavorable for the diffusion of the reaction agents in microspheres. It also should be noted that the cyano-modified fibrous 2-SiO2 -CN400 microspheres prepared by post-modification method have higher specific surface area than the 1-SiO2 -CN3 microspheres prepared by in-situ synthesis method, although they have approximate same total pore volume (see Table 1). Therefore, the former have much higher EAD than the latter.

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71

Adsorption capacity (mg/g)

120 Pb2+ Cu2+ Fe3+

100 80 60 40 20

1 -4

1

C M

-4 A D

-M

M C M

2

-C N 40

0

-C N 3 2

A D -2 -S iO

2

A D -1 -S iO

FSi

O

0

Fig. 4. Adsorption capacities of F-SiO2 , MCM-41, and the corresponding amidoximefunctionalized SiO2 for Pb2+ , Cu2+ , and Fe3+ ions individually. (Conditions: adsorbent dosage = 20 mg, T = 298 K, pH 6.3, [Pb2+ ]0 = [Cu2+ ]0 = [Fe3+ ]0 = 50 mg L−1 , V0 = 100 mL, t = 12 h).

3.3. Adsorption behavior of the fibrous amidoxime-functionalized mesoporous SiO2 microspheres for the heavy metal ions 3.3.1. The adsorption capacity of the fibrous amidoxime-functionalized mesoporous SiO2 microspheres for different heavy metal ions The adsorption capacities of F-SiO2 , MCM-41 and the corresponding amidoxime-functionalized SiO2 microspheres (AD-1SiO2 -CN3, AD-2-SiO2 -CN400, and AD-MCM-41) for different heavy metal ions (Pb2+ , Cu2+ , and Fe3+ ) are shown individually in Fig. 4. All of the SiO2 microspheres exhibit a poor adsorption property for Fe3+ . The adsorption capacities for Cu2+ of all SiO2 microspheres are also at a low level, although they are better than that for Fe3+ . In previous work [35], it was reported that amidoximated porous acrylonitrile (AN)/methyl acrylate (MA) copolymer beads showed better adsorption capacity for Fe3+ than for Cu2+ and Pb2+ in an aqueous solution with pH 3 and an initial ion concentration of 5 mmol/L. Therefore, the poor adsorption capacity for Fe3+ in this work should be mainly caused by high pH value (6.3) in the adsorption experiments, at which most of Fe3+ ions have hydrolyzed since the solubility product constant (Ksp ) of Fe(OH)3 (2.79 × 10−39 ) is much smaller than those of Cu(OH)2 (2.2 × 10−20 ) and Pb(OH)2 (1.43 × 10−15 ) [36]. The adsorption capacities for Cu2+ seems to have little relationship with the amidoximation modification. However, the adsorption capacities of the fibrous SiO2 microspheres for Pb2+ would be greatly improved after the amidoximation. The adsorption capacity of AD-2-SiO2 -CN400 for Pb2+ is as high as 97.4 mg g−1 , which should be attributed to the highest EAD of AD2-SiO2 -CN400 (see Table 2). Similarly, AD-MCM-41 has the lowest EAD , resulting in the poor adsorption property for all the three metal ions. The results implied that the amidoxime groups may have a highly selective adsorption capacity for Pb2+ . The higher adsorption capacity for Pb2+ should be related with the electron structure and the radius of ions. Since there is no reports about the complexing constant of amidoxime group with metal ions, we can consider the cumulative formation constants for metal comH2N

plexes with ethanolamine ( H2N

(

NH2

OH

) or ethylenediamine

) as the reference to the interaction between metal OH H 2N C N ions and amidoxime groups ( ). It can be found that

Fig. 5. Adsorption capacities of AD-2-SiO2 -CN400 in a solution containing all three heavy metal ions, Pb2+ , Cu2+ , and Fe3+ . (Conditions: adsorbent dosage = 20 mg, T = 298 K, pH 6.3, [Pb2+ ]0 = [Cu2+ ]0 = [Fe3+ ]0 = 50 mg L−1 , V0 = 100 mL, t = 12 h).

both ethanolamine and ethylenediamine have a better combination with metal ions with a larger ion radius (Hg2+ ) than Cu2+ [36]. Since Hg and Pb are in the same periodic in periodic table of the elements. The ion radius of Pb2+ (0.119 nm) is also close to that of Hg2+ (0.102 nm), much higher than that of Cu2+ (0.073 nm). Analogously, the amidoxime groups have a higher adsorption capacity for Pb2+ than for Cu2+ . The selective adsorption of AD-2-SiO2 -CN400 microspheres in the presence of all three metal ions Pb2+ , Cu2+ , and Fe3+ , was also examined. 20.0 mg of SiO2 microspheres was added into 100 mL of the aqueous solution in the presence of all of three metal ions at 298 K. The concentration of each metal ion was 50 mg L−1 . After 12 h, the absorption capacities for the metal ions are measured, and exhibited in Fig. 5. The results confirmed that the amidoxime-functionalized SiO2 microspheres exhibit a highly selective adsorption for Pb2+ . 3.3.2. Adsorption and desorption kinetics of AD-2-SiO2 -CN400 for Pb2+ ions The typical adsorption kinetics for Pb2+ of AD-2-SiO2 -CN400 had been investigated to discuss about the adsorption mechanism of the prepared fibrous amidoxime-functionalized SiO2 microspheres. The result is shown in Fig. 6A. Adsorption kinetics has been analyzed by using two common semi-empirical models, i.e., the pseudo-first-order and pseudo-second-order equations, which are based on the equilibrium adsorption capacity. pseudo − first − order equation: ln (qe − qt ) = ln qe − k1 t pseudo − second − order equation:

t 1 t = + qt qe k2 q2e

(8) (9)

where qe and qt are the adsorption amount (mg/g) of metal ions at the equilibrium and the time t min, respectively. k1 (min−1 ) and k2 (min−1 mg−1 g) are the corresponding adsorption rate constant. It is seen from Fig. 6A that the adsorption for Pb2+ ions has the highest rate within the first 15 min, and reaches the equilibrium adsorption capacity after 120 min. The fitting plots of ln (qe − qt ) versus t and t/qt versus t based on Fig. 6A are shown in Fig. 6B and C, respectively. The dynamic parameters (qe , k1 , and k2 ) in Eqs. (8) and (9) can be obtained from the slopes and intercepts of the fitted curves, which are listed in Table 3. The corresponding linear regression correlation coefficients (R2 ) are also given in Table 3. The R2 in pseudo-second-order model reaches 0.9957, closer to 1 than that in pseudo-first-order model (0.9290). Thus it can be assumed that the adsorption behavior of AD-2-SiO2 -CN400 for Pb2+ ions follows a pseudo-second-order kinetics. Moreover, the calculated equilibrium adsorption capacity, qe,cal , based on the pseudo-second-order

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Y. Xie et al. / Journal of Hazardous Materials 297 (2015) 66–73

Table 3 The dynamic parameters for the adsorption of AD-2-SiO2 -CN400 for Pb2+ ions. C0 (mg L−1 )

Pseudo-first-order

50

Pseudo-second-order

k1 (min−1 )

qe,cal (mg g−1 )

R2

qe,cal (mg g−1 )

k2 × 103 (g mg−1 min−1 )

qe,exp (mg g−1 )

R2

0.027

82.9

0.9290

111.7

0.353

97.4

0.9957

6

120

A

4 ln (qe-qt)

80

qt mg/g

B

5

100

60 40

pseudo-first-order kinetic

3 2 1

20 0 0 0

40

80

120

160

0

200

50

100 t (min)

t (min) 2.0

150

200

100

D

C 80 Adsorption retain (mg/g)

1.6

t/qt

1.2

0.8 pseudo-second-order kinetic

0.4

60 40 20 0

0.0 0

40

80

120

160

200

0

20

40

t (min)

60

80

100

120

t (min)

Fig. 6. (A) The dynamic adsorption curve of AD-2-SiO2 -CN400 for Pb2+ (Conditions: adsorbent dosage = 20 mg, T = 298 K, pH 6.3, [Pb2+ ]0 = 50 mg L−1 , V0 = 100 mL); (B) the fitting plot of dynamic adsorption curve by pseudo-first-order kinetics model; (C) the fitting plot of dynamic adsorption curve by pseudo-second-order kinetics model; and (D) the dynamic desorption curve of AD-2-SiO2 -C400 saturated with Pb2+ in 100 mL of 2 M HNO3 solution. (adsorbent dosage = 20 mg, T = 298 K).

3.3.3. Adsorption isotherm for Pb2+ The adsorption isotherm of AD-2-SiO2 -CN400 for Pb2+ ions is shown in Fig. 7, where the equilibrium adsorption capacity (qe , mg g−1 ) is taken as a function of the equilibrium concentration of Pb2+ in the test solutions (ce , mg L−1 ). The adsorption isotherm curve can be classified to be H type for the adsorption at solid–liquid interface [37], because the adsorption capacity for Pb2+ is obviously beyond zero when ce is very low, which indicates that this sorbent has a strong affinity with Pb2+ due to the existence of amidoxime groups. The adsorption capacity of AD-2-SiO2 -CN400 increases with the concentration of metal ions, and finally reaches a maximum of 284 mg/g.

350

280

qe (mg/g)

kinetics model is 111.7 mg g−1 , also in agreement with the experimental data (97.4 ± 8.8 mg g−1 ). The desorption kinetics in 2 M HNO3 solution is shown in Fig. 6D. It is clearly seen that all of the adsorbed Pb2+ can be extracted out after 60 min. The efficiency of the extraction is as high as 96.2%.

210

140

70

0 0

30

60

90

120

150

ce (mg/L) Fig. 7. Adsorption isotherm of AD-2-SiO2 -CN400 for Pb2+ . (Conditions: adsorbent dosage = 20 mg, T = 298 K, pH 6.3, V0 = 100 mL, t = 12 h, [Pb2+ ]0 = 5–200 mg/L).

Y. Xie et al. / Journal of Hazardous Materials 297 (2015) 66–73

4. Conclusions Submicron fibrous cyano-modified mesoporous SiO2 microspheres have been successfully fabricated by in-situ synthesis and post-modification method, respectively, in the presence of CETEOS. The N2 adsorption–desorption isotherms analysis indicates that the prepared fibrous SiO2 microspheres possess both mesopores (10–20 nm) and macropores (50–100 nm) and have a similar pore size distribution. The SiO2 microspheres prepared by post-modification method (2-SiO2 -CN400) have a relatively higher specific surface area and total pore volume than those prepared by in-situ synthesis (1-SiO2 -CN3), although the specific surface area and total pore volume of these two samples are lower than those of fibrous SiO2 formed by pure TEOS (F-SiO2 ). The immobilized cyano groups on 1-SiO2 -CN3 and 2-SiO2 -CN400 were then further transformed to amidoxime groups. The adsorption behavior of the as-prepared fibrous amidoxime-functionalized mesoporous silica (AD-1-SiO2 -CN3 and AD-2-SiO2 -CN400) for Fe3+ , Cu2+ and Pb2+ was investigated, and compared with that of amidoxime-functionalized traditional mesoporous silica (AD-MCM-41). The results show that the fibrous amidoxime-functionalized mesoporous SiO2 microspheres can selectively adsorb Pb2+ . The adsorption capacity for Pb2+ of AD-2-SiO2 -CN400 reaches as high as 97.4 mg g−1 at an initial ion concentration of 50 mg L−1 , better than that of AD-1-SiO2 -CN3. Moreover, the desorption of Pb2+ in 2 M HNO3 solution can be completed after 60 min. The efficiency of the desorption is as high as 96.2%. This work indicates the fibrous amidoxime-functionalized mesoporous silica microspheres have the potential to be applied as an excellent adsorbent for specific heavy metal ions. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51103143, 51173175, and 51473152) and the Fundamental Research Funds for the Central Universities (WK2060200012, 2014; WK3450000001, 2015). References [1] J. Harte, C. Holdren, R. Schneider, C. Shirley, Toxics A to Z: A Guide to Everyday Pollution Hazards, University of California Press, Berkeley and Los Angeles, California, 1991. [2] B. Volesky, Z.R. Holan, Biosorption of heavy metals, Biotechnol. Prog. 11 (1995) 235–250. [3] S.K. Wadhwa, M. Tuzen, T.G. Kazi, M. Soylak, Graphite furnace atomic absorption spectrometric detection of vanadium in water and food samples after solid phase extraction on multiwalled carbon nanotubes, Talanta 116 (2013) 205–209. [4] T. Golan, G. Dahan, Z. Ludmer, N. Brauner, A. Ullmann, Heavy metals extraction with the SRPTE process from two matrices-industrial sludge and river sediments, Chem. Eng. J. 236 (2014) 47–58. [5] Y. Treekamol, M. Schieda, L. Robitaille, S.M. MacKinnon, A. Mokrini, Z.Q. Shi, S. Holdcroft, K. Schulte, S.P. Nunes, Nafion® /ODF-silica composite membranes for medium temperature proton exchange membrane fuel cells, J. Power Sources 246 (2014) 950–959. [6] Z. Qu, L.L. Yan, L. Li, J.F. Xu, M.M. Liu, Z.C. Li, N.Q. Yan, Ultraeffective ZnS nanocrystals sorbent for mercury(II) removal based on size-dependent cation exchange, ACS Appl. Mater. Interfaces 6 (2014) 18026–18032. [7] I. Svanedal, S. Boija, M. Norgren, H. Edlund, Headgroup interactions and ion flotation efficiency in mixtures of a chelating surfactant, different foaming agents, and divalent metal ions, Langmuir 30 (2014) 6331–6338. [8] S.L. Luo, X.J. Li, L. Chen, J.L. Chen, Y. Wan, C.B. Liu, Layer-by-layer strategy for adsorption capacity fattening of endophytic bacterial biomass for highly effective removal of heavy metals, Chem. Eng. J. 239 (2014) 312–321. [9] T.M. Mututuvari, C.D. Tran, Synergistic adsorption of heavy metal ions and organic pollutants by supramolecular polysaccharide composite materials from cellulose, chitosan and crown ether, J. Hazard. Mater. 264 (2014) 449–459. [10] H.C. Gao, Y.M. Sun, J.J. Zhou, R. Xu, H.W. Duan, Mussel-inspired synthesis of polydopamine-functionalized graphene hydrogel as reusable adsorbents for water purification, ACS Appl. Mater. Interfaces 5 (2013) 425–432. [11] P. Xu, G.M. Zeng, D.L. Huang, C.L. Feng, S. Hu, M.H. Zhao, C. Lai, Z. Wei, C. Huang, G.X. Xie, Z.F. Liu, Use of iron oxide nanomaterials in wastewater treatment: a review, Sci. Total Environ. 424 (2012) 1–10.

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