Journal of Colloid and Interface Science 431 (2014) 209–215

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Preparation of zirconium oxy ion-imprinted particle for the selective separation of trace zirconium ion from water Yueming Ren a, Pingxin Liu a, Xiaoli Liu a, Jing Feng a, Zhuangjun Fan a,⇑, Tianzhu Luan b,⇑ a Key Laboratory of Superlight Materials & Surface Technology, Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China b The First Affiliated Hospital of Harbin Medical University, Harbin 150001, PR China

a r t i c l e

i n f o

Article history: Received 13 January 2014 Accepted 29 May 2014 Available online 17 June 2014 Keywords: Metal ion imprinting Zr(IV) Sol–gel Selectivity Water environment

a b s t r a c t Zr(IV) oxy ion-imprinted particle (Zr-IIP) was prepared using the metal ion imprinting technique in a sol–gel process on the surface of amino-silica. The dosages of zirconium ions as imprinted target, (3-aminopropyl) triethoxysilane (APTES) as a functional monomer and teraethyl orthosilicate (TEOS) as a cross-linker were optimized. The prepared Zr-IIP and Zr(IV) oxy ion non-imprinted particle (Zr-NIP) were characterized. pH effect, binding ability and the selectivity were investigated in detail. The results showed that the Zr-IIP had an excellent binding capacity and selectivity in the water. The equilibrium data fitted well to the pseudo-second-order kinetic and the Langmuir model for Zr(IV) binding onto Zr-IIP, respectively. The saturate binding capacity of Zr-IIP was found to be 196.08 lmol g1, which was 18 times higher than that of Zr-NIP. The sequence of binding efficiency of Zr-IIP for various ions was Zr(IV) > Cu(II) > Sb(III) > Eu(III). The coordination number has an important effect on the dimensional binding capacity. The equilibrium binding capacity of Zr-IIP for Zr(IV) decreased little under various concentrations of Pb(II) ions. The analysis of relative selectivity coefficient (Kr) indicated that the Zr-IIP had an appreciable binding specificity towards Zr(IV) although the competitive ions coexisted in the water. The Zr-IIP could serve as an efficient selective material for recovering or removing zirconium from the water environment. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Zirconium (Zr) is one of the abundant elements in the earth’s crust, but it distributed dispersedly [1]. It is a suitable metal applied for making refractories, foundry mold, cladding nuclear fuel and controlling materials in nuclear reactors, space and metal alloys, etc. [2]. Therefore, separation and recovery of even trace amount of it from waste water can be worthwhile. Several methods such as ion exchange, solvent extraction and precipitation for the binding of this metallic ion from aqueous solution have been investigated [3]. The separation and determination processes of low levels of Zr(IV) from the water are mainly based generally on a suitable utilization of available materials and techniques [4]. However, the slow adsorption–desorption kinetics and non-ideal selective adsorption capacity are the shortcomings for the present binding materials. Therefore, it is necessary to improve the selectivity of the separated materials. ⇑ Corresponding authors. Fax: +86 451 85555058. E-mail addresses: [email protected] (Z. Fan), [email protected] (T. Luan). http://dx.doi.org/10.1016/j.jcis.2014.05.063 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

Molecular imprinted technology (MIT) is an emerging molecular recognition technique, which shapes the specific recognition sites in synthetic polymers by using templates or imprinted molecules [5]. Metal ion imprinted polymers (IIPs) are imprinting ions instead of molecules and they recognize metal ions after the imprinting. The recognition mechanism is based on MIT technique [6]. Recently, many IIPs such as Hg(II) [7], Cu(II) [8], Cr(III) [9] and Cd(II) [10] have been reported in removing some toxic heavy metal ions. But there is less selective exploration for separating the expensive rare metal of Zr(IV) in the water. Among the reported synthesis methods the metal ion imprinted polymers are prepared by different approaches including bulk, suspension and precipitation polymerization [8]. Recently, the sol–gel process is increasingly employed to synthesize the metal ion imprinted polymers. For example, Rajiv Gandhi prepared and characterized the silica gel/chitosan composite for the removal of Cu(II) and Pb(II) [11]. Arh-Hwang studied the crosslinked metal-complexed chitosans for comparative adsorptions of Cu(II), Zn(II), Ni(II) and Pb(II) ions in the sol–gel process [12]. These experiments had the desired results and the sol–gel process showed the expected advantages including low cost and simple condition. However, it is regrettable

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that the selectivity of those IIPs is not very satisfactory. Moreover, Mo(VI) oxy ion imprinted particle (Mo-IIP) with high selectivity was prepared in a sol–gel process in our previous work [13]. In this paper, Zr(IV) ion imprinted particle (Zr-IIP) with high adsorption capacity and selectivity for Zr(IV) was synthesized in a sol–gel process. Zr(IV) was the target ion. SiO2, TEOS and APTES were selected as the supporter, cross-linking agent and functional monomer, respectively. The optimal dosages of target and the functional monomer were investigated. The prepared samples were characterized by scanning electron microscopy (SEM), a standard Brunauer–Emett–Teller (BET) analysis, Fourier transform infrared spectrometer (FTIR) and thermogravimetric analysis (TGA) in detail. The effect of pH, the binding virtue and the selective recognition of Zr(IV) onto Zr-IIP and Zr-NIP were estimated by the static experiments. Cu(II), Sb(III) and Eu(III) were selected as the latent interferon due to their similar ionic radius. The relative selectivity coefficient (Kr) was studied in the mixture system. The effect of Pb(II) ions on Langmuir isotherm of Zr-IIP for Zr(IV) was studied in the mixture system. Zr-IIP developed in this work can serve as an efficient selective material for enriching and determining Zr(IV) from the aqueous environment. All the discussions may be beneficial to the further improvement and a theoretical direction for its potential application in practice. 2. Experimental 2.1. Materials and chemicals

2.4. Batch binding experiments Single batch binding experiments were conducted to test the binding kinetics, isotherms and selectivity of Zr(IV) onto the Zr-IIP. All binding experiments were carried out in 150 mL flasks with 50 mL binding solution containing. After the addition of 0.05 g of the prepared particles each, the flasks were shaken at 200 rpm and 25 ± 2 °C for 12 h (having achieved the binding equilibrium). The pH of the mixture was remained at about 4.5. For the binding kinetics experiment, the specimens were sampled at defined time intervals from 2 to 720 min with 55 lmol L1 Zr(IV) initial concentration. The binding isotherms were investigated over various initial concentrations ranging from 22 to 1100 lmol L1 of Zr(IV) solution. The selectivity binding experiments were conducted by preparing single solution of Zr(IV), Cu(II), Sb(III) and Eu(III) with each initial concentration of 165 lmol L1. The effect of Pb(II) ions on Langmuir isotherm of Zr(IV) onto Zr-IIP were conducted by preparing mixture solution of Zr(IV)/Pb(II). The initial concentrations of Zr(IV) were 55 lmol L1and that of Pb(II) ranged from 0 to 110 lmol L1. In order to test the reproducibility, the experiments were carried in triplicates and the reproducibility was found to be within ±5%. After binding equilibrium, the saturated samples were separated by centrifugation and determined by ICP. The binding capacity (q) and efficiency (E) were calculated by the following equations [15]:

q¼

ðC 0  C e ÞV W

ð1Þ

E¼

C0  Ce  100% C0

ð2Þ

Ethanol, carbinol, ZrOCl28H2O, ammonia, acetic acid (AcOH), Ethylenediaminetetraacetic acid disodium salt (EDTA), Ethyl silicate (TEOS), (3-aminopropyl) triethoxysilane (APTES) and all other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd (Taijin, China). All the chemicals used were of analytical grade and obtained commercially. Distilled water used throughout the experiments was obtained from laboratory purification system.

where q (lmol g1) is the adsorbate binding capacity, C0 and Ce (lmol L1) are the initial and equilibrium adsorbate concentrations in the solution, respectively, V (L) is the volume of the solution, W (g) is the amount of the samples added to the solution, E (%) is the removal rate.

2.2. Instrumentations

3. Results and discussion

The surface micrograph and element distribution of the prepared samples were assessed by a S4800HSD scanning electron microscope (Japan). The surface area and the pore parameters of the samples were measured by an ASAP 2020 multipoint Brunauer–Emett–Teller apparatus (USA). FTIR spectra were recorded using an AVATAR 360 FTIR spectrometer with a spectral range of 4000–400 cm1 (USA). Thermogravimetric analysis was performed by a NETZSCH STA 409 PC/PG (China). The concentrations of Zr(IV) were detected by ICPS-750 (Perkin Elmer).

3.1. Dosages selection of imprinted Zr(IV) and functional monomer

2.3. Preparation of particles Firstly, the uniform silica was synthesized by TEOS hydrolysis with ammonium hydroxide according to the report by Stöber et al. [14]. Secondly, 0.400 g (dry weight) of the prepared silica was added into the carbinol and dispersed by the ultrasound for 20 min. Then 2 mL APTES, 0.677 g ZrOCl28H2O, 4 mL TEOS and 1 mL acetic acid were added into the above mixture in sequence, and the suspension (pH 4.5) was stirred for 12 h at room temperature. Thus Zr(IV) oxy ion-imprinted particle performed and then dried at 80 °C for 12 h. Finally, to remove Zr(IV) template, the imprinted particles were washed for 12 h with a mixture of methanol and acetic acid (9:1, v/v) by Soxhlet extraction. The products were washed for 3 times by EDTA and then dried under vacuum at 80 °C. The dry particles were stored in a sealed bottle for use. As a reference, Zr-NIP was prepared by the same protocol without the template.

Fig. 1 shows the possible reaction mechanism of Zr-IIP preparation. In this procedure, TEOS acting as a cross-linker reagent was easy to hydrolyze under the existence of acetic acid as catalyst. One end of APTES was cross-linked with the silica during the hydrolysis process. The other end was the amino group and it acted as the functional monomer. The memory sites were formed by the coordination key between Zr(IV) ions and the amino groups. Then the template ions of Zr(IV) were removed by elution of EDTA. Finally, the recognition sites onto the Zr-IIP were shaped. The coordination number might have an important effect on the dimensional binding capacity. The suitable dosages of the imprinted target ion and the functional monomer were crucial factors that effected on the quantity of the imprinted sites. The binding efficiency of Zr(IV) ions onto ZrIIP was studied at different molar ratio of Zr(IV) ions to amino groups. As seen in Fig. 2, the binding efficiency of Zr(IV) was only 33% at the molar ratio of Zr(IV) to amino groups for 1:2 under the same binding condition. Amino groups could not coordinate with enough Zr(IV) ions to form the imprinted sites. The binding efficiency of Zr(IV) reached the maximum value of 55% at the molar ratio of Zr(IV) to amino groups for 1:4. It illustrated that enough imprinted ions are supplied and the maximum quantity of Zr(IV) sites formed onto the Zr-IIP at this dosages. It seemed that four coordination compounds of Zr(IV) were the main imprinted sites in the synthetic process of Zr-IIP. However, the binding efficiency

Y. Ren et al. / Journal of Colloid and Interface Science 431 (2014) 209–215

Hydrolysis

SiO2

TEOS

Imprinting

AcOH TEOS

211

n

EDTA Elution

SiO2

SiO2

Fig. 1. Schematic representation of the imprinting process for Zr-IIP.

100

80

1:4

E%

60

1:8 1:6 40

1:10

1:2

20

0

molar ratio (Zr(IV) to amino groups) Fig. 2. Removal efficiency of Zr(IV) onto Zr-IIP with different molar ratio of Zr(IV) to amino groups. Sorbent: 1.0 g L1; C0: 164 lmol L1; pH 4.5; T: 25 °C; t: 12 h.

of Zr(IV) decreased with the dosages changed from 1:4 to 1:6. There was competition of the excess functional monomer which hindered the formation of the imprinting points. Furthermore, the binding efficiency of Zr(IV) increased with the increasing of the dosage ranged from 1:6 to 1:10. This was due to the extremely excess functional monomer resulting in forming the non-imprinting points. Hence, the optimum dosages of the imprinted Zr(IV) to amino groups were selected as molar ratio of 1:4 in the synthesis process of the Zr-IIP. 3.2. Characterization of prepared samples 3.2.1. SEM and BET analysis The synthesized SiO2, Zr-IIP and Zr-NIP were characterized using SEM in order to know the surface morphological image. It could be seen from Fig. 3a1 that the SiO2 micro particles displayed a highly spherical shape and a very smooth surface with the

average particle size of approximately 200 nm. The morphology of Zr-NIP (Fig. 3a2) and Zr-IIP (Fig. 3a3) are similar but different from the SiO2. The surface of Zr-NIP and Zr-IIP possessed a mass of floccules-like film with the average particle size of around 0.3 lm. Obviously, the shape of Zr-IIP underwent change after imprinting process comparing to the SiO2. It was due to a mass of imprinted polymers were formed on the surface of Zr-IIP. And such differences indirectly reflected that the Zr-IIP had been synthesized successfully. The specific surface area for Zr-IIP was observed to be 79.904 m2 g1 while that of the Zr-NIP was 21.871 m2 g1. Furthermore, the total pore volume and average pore diameter of Zr-IIP were 0.2287 cm3 g1 and 3.818 nm, and the total pore volume and average pore diameter of Zr-NIP were 0.08951 cm3 g1 and 3.828 nm, respectively. The specific surface area and the total pore volume of Zr-IIP were 3.7 and 2.6 times than that of Zr-NIP, respectively.

3.2.2. FTIR and TGA analysis FTIR spectra obtained from SiO2, Zr-IIP and Zr-NIP samples are shown in Fig. 3b. It showed that the broad band in the region of 3407 cm1 and 1640 cm1 was belong to –OH [16]. Seen from SiO2, Zr-NIP and Zr-IIP, the strong peaks around 1087 cm1 and 856 cm1 referred to the asymmetric and symmetric stretching of Si–O–Si groups, respectively. The peaks around 470 cm1 belonged to Si–O stretching vibrations [17]. This indicated that the basic skeleton structure of the Zr-IIP and Zr-NIP was SiO2. Seen from Zr-NIP and Zr-IIP, the peaks around 2950 cm1 and 2897 cm1 were ascribed to the characteristic vibration of the symmetric stretching and the asymmetric stretching peaks of methylene [18], and the characteristic peaks of amino were around 1607 cm1 [19]. These results suggested that APTES had successfully been grafted on the surface of silica and the imprinted sites of Zr(IV) formed. And it also indicated that the groups of Zr-NIP and Zr-IIP were similar.

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(a1)

(a2)

1.0 µm

SiO2

1.0 µm

Zr-NIP

(b)

wavenumber

(c)

100

856

4000 3500 3000 2500 2000 1500 1000

1.0 µm

Zr-IIP

SiO 2

weight (%)

470

470

95

90

Zr-IIP 85

Zr-NIP

470

1640 1607

1087 856 1087 856

2950 2879

3407

Zr-NIP

1640 1607

2950 2879

1087

3407 3407

Zr-IIP

1640

SiO2

Transmittance

(a3)

500

80

0

100

200

300

400

500

600

700

Temperature (oC)

(cm-1)

Fig. 3. (a) SEM micrographs, (b) FTIR spectra and (c) TGA curves of Zr-IIP and Zr-NIP.

3.3. Effect of pH Among all the factors that might influence the adsorption capacity of adsorbents, pH is one of the most important factors [20]. The effect of pH values ranging from 4.0 to 9.0 on Zr(IV) binding onto Zr-IIP was determined. As shown in Fig. 4, the binding efficiency increased sharply with pH increasing from 4.0 to 5.5 and reached the maximum of 95%, and it changed little at pH range from 5.5 to 9.0. It was conformed to the coordination character

100

80

60

E%

SiO2, Zr-NIP and Zr-IIP were evaluated by TGA analysis at a heating rate of 20 °C min1 in nitrogen atmosphere. As shown in Fig. 3c, the weight loss processes of SiO2 experienced three periods: it lost the absorbed water and bound water at temperature from 25 to 125 °C with 2.5% weight lost and the weight keep steady at temperature from 125 to 300 °C, then it lost the hydroxy on the surface of silica decomposed from 300 to 700 °C with a mass loss of about 6%. Moreover, the Zr-IIP lost 17.5% weight totally with four periods: it lost the absorbed water at temperature from 25 to 100 °C with 2% weight lost, and the Zr-IIP show a heat stability while the temperature from 100 to 190 °C. The methylene began to decompose at temperature reached 190 °C, and the amino-groups and the floccules on the surface of silica were also gradually decomposing from 190 to 500 °C. In this period, the Zr-IIP lost 10% weight. The final period began at 500 °C and caused nearly 5.5% weight losses, it was due to the decomposition of the hydroxyl. While, the Zr-NIP lost the absorbed water at temperature from 25 to 100 °C, and it began to decompose at temperature 100 °C without any stability period. Finally, the total weight lost of ZrNIP was 18%, which was much similar to that of Zr-IIP. The results proved that the composition of Zr-IIP had no difference to that of Zr-NIP, and it also illustrated that flocculent polymers were formed on the surface of SiO2 for both Zr-IIP and Zr-NIP. The result was consistent with the analysis of FTIR and SEM.

40

20

0 4

5

6

7

8

9

pH Fig. 4. Effect of pH on the binding of Zr(IV) onto Zr-IIP. Sorbent: 1.0 g L1; C0: 55 lmol L1; T: 25 °C; t: 12 h.

of Zr(IV) ions. The Zr(IV) was four values at low pH value in the aqueous solution. The Zr(OH)4 precipitation will generate along with the OH- concentration increment. The formation processes of Zr(OH)4 precipitation are as follows:

½ZrðOH2 Þ8 4þ þ H2 O ! ½ZrðOHÞðOH2 Þ7 3þ þ H3 Oþ

ð3Þ

½ZrðOHÞðOH2 Þ7 3þ þ H2 O ! ½ZrðOHÞ2 ðOH2 Þ6 2þ þ H3 Oþ

ð4Þ

The hydrolysis reaction occurs in the water. The polycondensation of hydrolysate was reacted by hydroxyl bridging action and finally formed [Zr(OH)24H2O]8+ 4 [21]. ð84xÞþ ½ZrðOHÞ2  4H2 O8þ þ 4xHþ 4 ! ½ZrðOHÞ2þx  ð4  xÞH2 O4

ð5Þ 

Zr(OH)4 will eventually be formed if the concentration of OH is high enough [22]. Zr(IV) was imprinted on the Zr-IIP by ZrO2+ with

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the pH was about 4.5 in the imprinting process. For this reason, the pH of all binding experiments was remained at about 4.5. 3.4. Binding kinetics and isotherm In order to confirm the rate-limiting of the Zr(IV) binding procedure, Lagergren pseudo-first-order model (Eq. (6)) and pseudo-second-order model (Eq. (7)) [23] are employed. Also, Langmuir isotherm model (Eq. (8)) and Freundlich isotherm model (Eq. (9)) are used to describe the results of Zr(IV) binding onto Zr-IIP and Zr-NIP. They are expressed as:

logðqe  qt Þ ¼ log qe 

k1 t: 2:303

ð6Þ

t 1 1 ¼ þ t: qt k2 q2e qe

ð7Þ

Ce Ce 1 ¼ þ : qe qm K L qm

ð8Þ

log qe ¼ log K F þ

1 log C e : n

ð9Þ

where qe (lmol g1) and qt (lmol g1) are the amount of Zr(IV) binding on the sorbent at equilibrium and t (min), respectively, k1 (min1) and k2 (g lmol1 min1) are the first order rate and the second order rate constant, respectively, k2qe2 (lmol g1 min1) is the second-order initial binding rate as t ? 0. Ce (lmol L1) is the same as in Eq. (1), qm (lmol g1) is the maximum binding capacity, KL (L lmol1) is Langmuir binding coefficient, KF ((lmol g1) (L lmol)1/n) is Freundlich binding coefficient and n is Freundlich binding constant. It was clearly shown in Fig. 5a that remarkable equilibrium of Zr(IV) binding onto Zr-IIP and Zr-NIP occurred at about 100 min and 80 min, respectively. And there were no appreciable changes of the Zr(IV) binding ability noticed after that. It seemed that the imprinted sites onto Zr-IIP were favor for the improving of Zr(IV) recognition in the water. As shown in Table 1, the pseudosecond-order model was greatly suitable to describe (R2 > 0.99) Zr(IV) binding onto Zr-IIP and Zr-NIP. The calculated qe values were in accord with the experimental results well. However, the correlation displayed in the pseudo-first-order model was very poor. It indicated that the rate-limiting of Zr(IV) binding process might be controlled by the chemisorption [24]. The initial binding rates (k2qe2) of Zr(IV) were 0.8788 and 0.0813 lmol g1 min1 for Zr-IIP and Zr-NIP, respectively. Such quick uptake rate was caused by availability of the actual binding sites onto the Zr-IIP [25]. 60

Therefore, the pseudo-second-order mechanism was predominant for the Zr(IV) binding onto Zr-IIP and the rate-limiting of the Zr(IV) binding process might be controlled by the chemisorption. As shown in Fig. 5b and Table 2, it indicated that Langmuir model of Zr(IV) binding onto Zr-IIP and Zr-NIP ideally matched with the experimental data (R2 > 0.99). Obviously, it showed that the binding occurred on a homogeneous surface of Zr-IIP and Zr-NIP by the monomolecular layer sorption [26]. Moreover, the maximum binding capacity (qm) from the Langmuir isotherm of Zr(IV) binding onto Zr-IIP was approximately three times higher than that of Zr-NIP. This showed that specific recognition sites had formed on the surface of Zr-IIP during the imprinting process. Obviously, such process had greatly improved the binding capacity of Zr(IV). 3.5. Selective binding character As shown in Fig. 6a, the binding efficiency of different competitive ions was presented. The other three metal ions including Cu(II), Sb(III) and Eu(III) were selected as the latent interferon due to their similar ionic radius. It was found that the sequence of binding efficiency of Zr-IIP for various ions was Zr(IV) > Cu(II) > Sb(III) > Eu(III). The maximum efficiency of Zr(IV), Cu(II), Sb(III) and Eu(III) could be up to 55%, 35%, 17% and 8%, respectively. However, Zr-NIP had a low binding efficiency of around 5% for these four ions. It can conclude that the Zr-IIP has obvious binding advantage for Zr(IV) and the Zr-NIP has no selectivity for these four ions. It proved that the memory sites for Zr(IV) had formed in the imprinting process. It was worth noting that the coordination number of Cu(II) is 4 or 6, which is similar to Zr(IV). And the efficiency of Cu(II) was only 20% lower than that of Zr(IV) binding onto Zr-IIP. The coordination number of Sb(III) and Eu(III) are 6 and 6 or 8, respectively. And the efficiency of Sb(III) and Eu(III) binding onto Zr-IIP is of 17% and 8% lower than that of Zr(IV). It seems to illustrate that the dimensional binding capacity is related with the coordination numbers, and the recognition imprinting points are mainly the four-coordination. The binding competitiveness and the recognition performance of Zr-IIP could be calculated according to the static adsorption experiment results. The relative selectivity coefficient (Kr) was studied in a batch system. The value of Kr can be calculated according to Eqs. 10–12 and the results are presented in Fig. 6b. Distribution and selectivity coefficients were calculated as explained below:

Kd ¼

ð10Þ

250

(a)

(b)

50

200

40

Zr-IIP Experimental

qe (µmol g -1)

qt (µmol g -1)

Ci  Cf ðV=mÞ: Cf

Zr-NIP Experimental Pseudo-First-order

30

Pseudo-Second-order

20

150 Zr-IIP Experimental Zr-NIP Experimental

100

Langmuir model Freundlich model

50

10 0

0 0

100

200

300

400

t (min)

500

600

700

800

0

200

400

600

800

1000

1200

-1

Ce (µmol L )

Fig. 5. (a) Binding kinetics and (b) isotherms of Zr(IV) binding onto Zr-IIP and Zr-NIP. Sorbent: 1.0 g L1; C0: 55 lmol L1; pH: 4.5; T: 25 °C; t: 12 h.

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Table 1 The kinetics parameters of Zr(IV) binding onto Zr-IIP and Zr-NIP. Samples

qe,exp (lmol g1)

Pseudo-first-order model k1 (min

Zr-IIP Zr-NIP

)

1

qe,cal (lmol g

R

3

52.95 11.65

Pseudo-second-order model

2

1

5.06  10 9.21  103

0.650 0.952

k2 (g lmol1 min1)

)

4

22.35 10.17

2.6  10 3.2  104

R2

qe,cal (lmol g1)

0.992 0.997

58.14 15.94

Table 2 The Langmuir and Freundlich isotherm model parameters of Zr(IV) binding onto Zr-IIP and Zr-NIP. Samples

Langmuir model

Zr-IIP Zr-NIP

Freundlich model

qm (lmol g1)

KL (L lmol1)

R2

KF ((lmol g1) (L lmol)1/n)

1/n

R2

196.08 11.60

0.0268 0.0308

0.997 0.999

25.204 2.079

0.318 0.268

0.948 0.786

0.7

60

(a)

Zr—IIP

50

C 0 (Zr(IV)) 55 µmol L-1

(b)

C 0 (Zr(IV)) 110 µmol L-1

0.6

Zr—NIP

C 0 (Zr(IV)) 165 µmol L-1

0.5

C 0 (Zr(IV)) 220 µmol L-1

Kr

E%

40 30

0.4 0.3

20 0.2 10 0.1 0

Cu(II)

Zr(IV)

0

Eu(III)

Sb(III)

20

40

60

80

100

Ce (Pb(II)) 200

(c)

qe (µmol g -1)

160

120 C 0 (Pb(II)) 0 µmol L-1 C 0 (Pb(II)) 22 µmol L-1

80

C 0 (Pb(II)) 55 µmol L-1 C 0 (Pb(II)) 110 µmol L-1

40

0

0

200

400

600

800

1000

-1

Ce(Zr(IV)) (µmol L ) Fig. 6. (a) Binding efficiency of Cu(II), Zr(IV), Sb(III), Eu(III) onto Zr-IIP and Zr-NIP, (b) the relative selectivity coefficient (Kr) of Zr-IIP and (c) the effect of Pb(II) on Langmuir isotherm of Zr(IV) binding onto Zr-IIP. Sorbent: 1.0 g L1, pH: 4.5, T: 25 °C, t: 12 h.

where Kd, Ci and Cf represent the distribution coefficient, initial and final adsorbate concentrations (lmol L1), respectively, V and m are the volume of the solution (L) and the mass of the particles (g), respectively. The selectivity coefficient for the binding of an ion in the presence of competitor species can be obtained from equilibrium data according to Eq. (11):

k¼

K d ðtemplate metalÞ : K d ðinterferent metalÞ

where k is the selectivity coefficient of interfering metal.

ð11Þ

A comparison of the k values of the imprinted polymer with those of metal ions allows an estimation of the effect of imprinting on selectivity. In order to evaluate an imprinting effect, and a relative selectivity coefficient Kr is defined as follows:

Kr ¼

kimprinted knon-imprinted

ð12Þ

The relative selectivity coefficient (Kr) resulting from the comparison of the k values of Zr-IIP and Zr-NIP allows an estimation of the effect of imprinting on selectivity.

Y. Ren et al. / Journal of Colloid and Interface Science 431 (2014) 209–215 Table 3 The effect of Pb(II) on the Langmuir isotherm model parameters of Zr(IV) binding onto Zr-IIP. C0(Pb(II)) (lmol L1)

qm (lmol g1)

KL (L lmol1)

R2

SD

0 22 55 110

187.60 156.25 135.14 131.58

0.0269 0.0143 0.0152 0.0132

0.997 0.968 0.977 0.970

0.058 0.809 0.810 1.115

The metal ion of Pb(II) as a common interference ion was selected in the experiment [27]. The initial concentrations of Zr(IV) were 55 lmol L1, 110 lmol L1, 165 lmol L1 and 220 lmol L1, respectively. And the initial concentrations of Pb(II) were 25 lmol L1, 61 lmol L1 and 122 lmol L1, respectively. It can be seen from Fig. 6b, the competition of Pb(II) increased with increasing concentration of Pb(II) ions. But the relative selectivity coefficient (Kr) decreased dramatically. It is worth mentioned that the higher initial concentration of Zr(IV) resulted in the more downtrend of Kr values. This means that the higher concentration of Zirconium ion has and the better selectivity it will possess. The imprinted sites played an important role during the interference process from other competing metal ions. The effect of Pb(II) ions on Langmuir isotherm of Zr-IIP for Zr(IV) was shown in Fig. 6c, and the parameters of Langmuir isotherm model were presented in Table 3. Seen from Fig. 6c, the maximum binding capacity (qm) from the Langmuir isotherm of Zr-IIP was 187.60 lmol g1. And it reduced to 31.35 lmol g1, 52.46 lmol g1 and 56.02 lmol g1 when the concentration of Pb(II) ions were 22 lmol L1, 55 lmol L1 and 110 lmol L1, respectively. It is due to the competitive adsorption between different ions. The equilibrium binding capacity of Zr(IV) increased sharply with the increasing concentration of Zr(IV) ions and then it changed slowly when the initial concentration of coexisting ion is low. Seen from Table 3, the Langmuir model of Zr(IV) binding onto Zr-IIP matched with the experimental data well (R2 > 0.96) despite the competitive ions existed. This means that the binding was abided by the monomolecular layer sorption hypothesis. It also can been seen that the equilibrium binding capacity for Zr(IV) reduced at 16.7%, 27.9% and 29.8% with initial concentration of Pb(II) ions increasing, respectively. The decreases of equilibrium binding capacity were more than 10% because the binding mechanism of Pb(II) might be similar to that of zirconium. But the decreases were no more than 30% due to the existence of imprinting sites. It proved that the Zr-IIP showed excellent selectivity for Zr(IV) in the competitive water phase. Zr-IIP prepared in this work held the desired binding selectivity ability so that it could separate Zr(IV) from the water phase effectively. Most importantly, the method of preparing Zr-IIP were cheap and environmentally friend comparing with traditional polymerization method.

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kinetics showed that the initial adsorption rate (k2qe2) was 0.8788 lmol g1 min1. The binding kinetics followed the pseudo-second-order kinetic model closely. Langmuir isotherm was fitted for the experimental data of zirconium binding onto Zr-IIP. The maximum binding capacity was 196.08 lmol g1, which was almost 18 times higher than that of Zr-NIP. The sequence of binding efficiency of various ions onto Zr-IIP was Zr(IV) > Cu(II) > Sb(III) > Eu(III). The coordination numbers is one of the important factors to be related to the dimensional binding capacity. The relative selectivity coefficient (Kr) decreased with the increasing of the ion concentration interference. The existing Pb(II) ions of 122.0 lmol L1 in the solution had no effect on Zr(IV) binding onto Zr-IIP. All the results clearly confirmed that Zr-IIP would have a potential use in recovering or removing Zr(IV) in the water environmental. Acknowledgments We appreciate the financial support of the National Natural Science Foundation of China (Nos. 51108111, 51178134, 51378141, 21301038 and 21203040), Fundamental Research Funds for the Central Universities (HEUCF201403009), Heilongjiang Natural Science Foundation (E201125), and Heilongjiang Education Office Foundation (12513044). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

4. Conclusions

[23]

Zr(IV) oxy ion-imprinted particle (Zr-IIP) was successfully synthesized in a sol–gel process on the surface of amino-silica by the surface metal ions imprinting technique. And the optimum ratio of the prepared material for Zr-IIP is obtained. Reaction

[24] [25] [26] [27]

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