Adsorption-controlled preparation of anionic imprinted amino-functionalization chitosan for recognizing rhenium(VII)

Separation and Purification Technology 177 (2017) 142–151

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Adsorption-controlled preparation of anionic imprinted amino-functionalization chitosan for recognizing rhenium(VII) Ying Xiong ⇑, Yang Song, Qiang Tong, Peng Zhang, Yuejiao Wang, Zhenning Lou, Feng Zhang, Weijun Shan ⇑ College of Chemistry, Liaoning University, Shenyang 110036, PR China

a r t i c l e

i n f o

Article history: Received 12 October 2016 Received in revised form 16 December 2016 Accepted 17 December 2016 Available online 21 December 2016 Keywords: Selective adsorption Molybdenum(VI) Rhenium(VII) Anionic imprinting Chitosan

a b s t r a c t This study described the synthesis of the Mo(VI)-imprinted ethylenediamine (EDA) grafted chitosan (I-EDA-CS), which is a novel technique in combination with surface imprinting and polymer crosslinking. Results indicated that mass ratio of ammonium molybdate and chitosan as 1 g:25 g–3 g:25 g could be conducive to achieve higher adsorption capacity of Re(VII). The maximum adsorption capacity of Re (VII) was found to be 418.98 mg
g1 at 303 K with an initial Re(VII) concentration of 500 mg
L1, while maximum adsorption of Re(VII) on the non-imprinted absorbent (N-EDA-CS) was just 83.92 mg
g1. The maximum selectivity coefficients of the I-EDA-CS for Re(VII)/Cu(II), Re(VII)/Zn(II), Re(VII)/Mn(II), Re(VII)/Fe(II) were 1.29, 1.9, 2.31, 1.81, respectively. It revealed that the Mo(VI)-imprinted adsorbent showed superior selectivity and affinity to Re(VII) in case of the existence of competition ions. The analysis results of FT-IR and XPS confirmed that the high adsorption selectivity of the I-EDA-CS attributed to the chelation and electrostatic attraction between the amine groups and Re(VII) anionic complexes in the ‘‘cavities”. Imprinted I-EDA-CS was successfully employed for the selective adsorption of Re(VII) from industrial wastewater. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction As is known to all, rhenium as the scarce and valuable metal has been applied broadly in crucial areas. What’s more, because of the high value of rhenium, the recovery of rhenium from the effluent has both economic and environmental benefits [1]. Selective recognition ability is also a challenge, which is widely used in the fields of analysis, including the detection and extraction of rare metal ions. Thus, extraction techniques have been gradually increasing attention to the recovery of rare metal, including solvent extraction, ion-exchange, solid-phase extraction or co-precipitation, membrane filtration, and adsorption [2–6]. Most of them are expensive, can’t eliminate trace levels of rare metal ions, and have evidently disadvantages such as operating costs, incompletely recovery, energy consumption and requirements of poisonous regents [7]. In order to solve these problems, ion-imprinted polymer (IIPs) has drawn more and more attention over the past few decades. Ion-imprinting technique is an analogous method to prepare the

⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Xiong), [email protected] (W. Shan). http://dx.doi.org/10.1016/j.seppur.2016.12.028 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.

target molecules of molecularly imprinted materials [8–12]. The traditional method of molecularly imprinted polymer is that template mixed with monomer ligands into complexes, after that the complexes formed polymer by crosslinking. And finally the template ion could be removed from the copolymers. Through the ion-imprinting process, the target template ion, the ligand, geometry and the number of coordination have a significant impact in selectivity of imprinted polymer. It generates specific binding sites in the size and shape, which can match with the target template ions. Linear chain of crosslinked polymer having metal binding is a highly developed technique applied to prepare IIPs [13,14]. Biomass has been directed towards low cost biosorbents due to their sustainable sources and excellent biodegradable properties [15,16], such as chitosan, fungi, lignite, moss and peat. Among these biosorbents, chitosan (CS) have attracted significant attentions due to that it could supply a large number of amine groups and hydroxyl groups through the chelating, electrostatic interaction, or ion-exchange with the cationic metal ions. At present, ion imprinted chitosan materials have been widely used in synthesizing to adsorb the corresponding metal ions from aqueous solutions, including Cu(II)–CS [17], Cd(II)–CS [18], Ca(II)–CS [19], Ag(I)–CS [20], Ni(II)–CS [21], Sr(II)–CS [22], Pb(II)–CS [23] and Co

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143

(II)–CS [24], which indicate that ion-imprinted adsorbents have a higher adsorption capacity than not ion-imprinted adsorbents. The current metal ion imprinting technology research is limited to basic metal cation, while anion-imprinted almost has no related research. This may be more facile to prepare cation-imprinted polymer, rather than anionic-imprinting polymer. In this study, the anionic-imprinting amino-functional chitosan adsorbent for recognizing of Re(VII) was synthesized. Its adsorption infinity was higher than that of the adsorbents reported in previous studies. Adsorption mechanisms were deduced by the analyses of Fourier transform infrared spectroscopy (FTIR) and Xray photoelectron spectroscopy (XPS). Moreover, the reusability, stability and practical application of I-EDA-CS were also investigated.

product was washed with distilled water to neutral and dry at 343 K to produce amino-functional chitosan by molybdenum ion imprinting, which is abbreviated as I-EDA-CS. To investigate the influence of crosslinking degree on the morphology and imprinted efficiency of adsorbents, parallel experiments were done including: different dosage of chitosan and different dosage of initial imprinting molybdenum ions. All these adsorbents were synthesized as described above. Non-imprinted adsorbent (N-EDA-CS) were also prepared following an identical procedure in the absence of MoO2 4 . In addition, the Re(VII) ion imprinting amino-functional chitosan adsorbent was synthesized (Re-EDA-CS), whose imprinting process was in accordance with Mo(VI) imprinting amino-functional chitosan adsorbent, just only ReO 4 as the template ion.

2. Experimental

2.3. Batch adsorption, regeneration and reuse studies

2.1. Materials

Equilibrium adsorption experiments were conducted at 303 K by shaking 10 mg of adsorbent (I-EDA-CS) and 10 mL of 20 mg
L1 Re(VII) solution for 24 h. The competitive adsorption experiments were researched by preparing a single system containing Cu2+, Fe3+, Zn2+ and Mn2+, respectively, and the initial concentration of each metal ion was 20 mg
L1. The selectivity coefficient was calculated according to Eq. (1).

The crab shell was collected from local cultivated areas. Re(VII) and Mo(VI) stock solutions were prepared by dissolving NH4ReO4 and (NH4)6Mo7O24
4H2O, respectively. Ethylenediamine (EDA) and epichlorohydrin (ECH) were purchased from Sinopharm Chemical Reagent Co. Ltd (China). All chemicals and reagents were of analytical grade. 2.2. Preparation of the I-EDA-CS The synthesis method of CS was reported in our previous work [25]. The crab shells were treated with 1 mol
L1 HCl and 50% NaOH aqueous solution, respectively. The product is specified as the crude crab chitosan, which is abbreviated hereafter as CS. The degree of deacetylation of the chitosan is 89%, which is within the normal range. Ion imprinting amino-functional chitosan adsorbent was synthesized, which the steps of the adsorbent were schematically presented in Fig 1. Firstly, 2 g ammonium molybdate was dissolved in 100 mL of water and ethanol (1:1) solution in three round bottom flask, and stirred at 298 K. Then, 30 mL ethylenediamine was further dripped slowly to the mixture at room temperature. After that, 50 mL of epichlorohydrin was added into the mixture at 333 K for 12 h. In the next step, 25 g of chitosan was added into the mixture at 343 K for 24 h to produce the intermediate I. Finally, Mo(VI) was leached from the intermediate I using certain acidity of HCl solution repeatedly until no Mo(VI) in the filtrate was detected. The

SelRe=X ¼ log

ðqe =Ce ÞRe ðqe =Ce ÞX

ð1Þ

where Ce (mg
L1) stands for the equilibrium concentration measured after adsorption, X represents Cu2+, Fe3+, Zn2+ or Mn2+, Sel represents Re(VII) adsorption selectivity. After the adsorption of Re(VII) on the I-EDA-CS adsorbent by adding 100 mg of the adsorbent in 50 mL of 300 mg
L1 rhenium ions solution at pH 4.0 for 24 h, the Re(VII)-loaded adsorbent was regenerated using 50 mL of different eluent at 303 K for 12 h. Moreover, six continuous adsorption and regeneration cycles were carried out by mixing 100 mg of the I-EDA-CS with 100 mL of 50 mg
L1 rhenium ions solution, and the Re(VII) loaded on the adsorbent was eluent by 50 mL of 15% HCl. 2.4. Characterization of adsorbent The pH of the solution was measured by S-3C model pH meter. Samples’ FTIR spectra were recorded on Nicolet 5700 FTIR spectrophotometer. Elemental analysis was done on Flash EA 1112 elemental analyzer. XPS spectra were measured by a thermo ESCALAB

Fig. 1. Preparation of I-EDA-CS.

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250 X-ray photoelectron spectrometer with Al Ka X-ray source, and were fitted using a XPSPEAK4.1 software. SEM images were recorded using a Scanning Electron Microscope (S-4800, HITACHI). Nitrogen adsorption/desorption isotherms were conducted with an automated gas sorption analyzer (Norcross, GA). The concentrations of metal ions were measured by UV-2600 UV–Visible spectrophotometer (Shimadzu, Japan) or inductively coupled plasma-optical emission spectrometry (ICP-OES) using a PE-8000 spectrometer (Pekin-Elmer, Wellesley, MA). 3. Results and discussion 3.1. Characteristics of I-EDA-CS and N-EDA-CS 3.1.1. SEM analysis The SEM images of the CS, N-EDA-CS and I-EDA-CS adsorbents were shown in Fig. 2. The surface morphologies of the CS, N-EDCS and I-EDA-CS were completely different (Fig. 2(a)). Rough porous on the surface of the crab CS were observed, which has a good agreement with our previous results [25]. Based on the analysis result in our previous work, it hinted that calcium carbonate particles were successfully removed from crab shell. However, the surface of the N-EDA-CS and the I-EDA-CS showed a smoother structure than the CS due to that the polymeric chitosan molecular chains crosslink with neighboring chitosan molecules. As shown in Fig. 2(b and c), a denser network structure was observed in the

case of the I-EDA-CS. This result may be attributed that the MoO2 4 as template ion acts as a role of cross-linker, which could form a compact structure as a result of the decrease of the space and distance between molecular chains of chitosan. Eventually, the compact and smooth surface of chitosan was retained after further crosslinking. In addition, the width of cavities size is 100– 200 nm, and the depth of the cavities is around 140 nm observed from AFM micrographs (Fig. 2(d)). 3.1.2. FTIR analysis The differences of the CS, N-EDA-CS, I-EDA-CS in FTIR spectra were displayed in Fig. 3. In the FTIR spectrum of the CS, the strong band at 3427 cm1 showed the overlapping of AOH or ANH2 stretching vibration due to an ocean of hydroxyl and amino groups in the chitosan. The peaks appearing at 1665 cm1, 1441 cm1, 1074 cm1 and 1027 cm1 were attributed to NAH stretching vibration, ACH3 stretching vibration in the acetyl, CAN bending vibration, and CAO stretching vibration, respectively. Compared with that of the CS, the spectrum of the I-EDA-CS appeared blue shifting that the sharp peak at 3427 cm1 and 1591 cm1 changed into 3451 cm1 and 1618 cm1 respectively. The reason , may be NH2ACACANH2 grafting onto the surface of the chitosan lead to the changes of electron clouds environment for the N atoms. Compared the spectra of the I-EDA-CS with the CS, it showed an additional sharp peak at 2350 cm1, demonstrating the successful crosslinking a protonation of ethylenediamine on the ACAOH by

Fig. 2. SEM micrographs of CS (a), N-EDA-CS (b), I-EDA-CS (c); and AFM micrographs of I-EDA-CS (d).

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I-EDA-CS adsorbed Re(VII)

908 I-EDA-CS adsorbed Re(VII)

I-EDA-CS

2350

I-EDA-CS

N-EDA-CS 1618 1393

CS

3451

N-EDA-CS

+

1591

N -H 1074

1665 1441 3427

4000

3500

3000

2500

2000

1500

1027 magnification

1000

500

2500

2400

2300

2200

2100

2000

-1

Wave number (cm )

-1

Wave number (cm )

Fig. 3. FTIR spectra of the CS, N-EDA-CS, I-EDA-CS before and after Re(VII) adsorption.

epichlorohydrin. Compared the spectra of the I-EDA-CS to the NEDA-CS, no obvious change of the absorption peak indicated that the structure of the original polymer was not destroyed. 3.1.3. Elementary analysis and BET analysis The ultimate goal of elemental analysis of CS, I-EDA-CS, N-EDACS was to obtain the constituent of the chitosan adsorbent before and after imprinting. The elemental analysis for carbon (C), hydrogen (H), and nitrogen (N) were presented in Table 1. After introducing ethylenediamine on the chitosan, the nitrogen content exhibited a slight increasing from 7.49% (CS) to 7.88% (N-EDACS), which confirmed that the chitosan was successfully modified by ethylenediamine. However, it was observed that the nitrogen contents of the I-EDA-CS drop to 6.77%, that is to say, oxygen contents go up to 8.71%. The increasing content of oxygen is mainly due to the existence of MoO2 4 in the I-EDA-CS. It can be estimated that the average content of Mo(VI) in the I-EDA-CS is 2.18%. Moreover, the residual Mo(VI) on the surface of the I-EDA-CS is just estimated as 0.3% by XPS analysis (Fig. 6). It could be concluded that molybdenum ions not totally removed from the chitosan polymer and embedded into the reticular structure of the adsorbent with strong covalent bonds in the form of MoO2 4 . Samples’s specific surface area was measured and calculated by Nitrogen adsorption/desorption isotherm according to Brunauer– Emmett–Teller (BET), and listed in Table 1. The surface area of the CS, N-EDA-CS, I-EDA-CS was calculated as 0.93 m2
g1, 0.08 m2
g1, 1.47 m2
g1, respectively. Compared with the N-EDA-CS, the I-EDA-CS surface area had an obviously rise, because a denser network structure formed in the case of MoO2 4 as a cross-linker, which was consistent with SEM analysis of the I-EDA-CS. 3.2. Adsorption experiments 3.2.1. Imprinting efficiency In order to obtain suitable mass ratio between template ions and chitosan, adsorbents with various quality of chitosan or template ions were carried out to investigate imprinting efficiency Table 1 Content of carbon, hydrogen, nitrogen and surface area of the I-EDA-CS, N-EDA-CS and CS.

N (%) C (%) H (%) Surface area (m2
g1)

CS

N-EDA-CS

I-EDA-CS

7.49 41.15 7.01 0.93

7.88 39.96 7.06 0.08

6.77 34.45 6.72 1.47

according to the loading capacity of the adsorbents. It proceeded as exposition in the experimental section. The equilibrium adsorption capacity of Re(VII) on the I-EDA-CS with different mass ratio of template ions (ammonium molybdate) and chitosan from 1 g:25 g–10 g:25 g was shown in Fig. 4(a). It was found that an equilibrium adsorption capacity of the I-EDA-CS for Re(VII) was lower than that of beyond the scope of 1 g:25 g–3 g:25 g. Upon increasing the mass ratio of ammonium molybdate and chitosan in the process of synthesis, the number of specific cavities increased gradually. Later, it reached a relative stable value as all the template ions were embedded in the formation of imprinted sites and fixed. With an increase of ammonium molybdate content in the synthesis process, a coordination complex formed because excessive molybdenum ions leaded to a multitude of crosslinking sites occupied by molybdenum ions. By contrast, excessive crosslinking sites would lead to a decrease of specific cavities within the scope of 4 g:25 g–10 g:25 g.

3.2.2. Effect of template ion on the adsorption of Re(VII) Molybdenum ions can form different complex due to the influence of pH, and its existence state in solution had been studied in our previous work [25]. In this study, acidity environment of synthesis process is pH 6.2, so the form of the template ion is MoO2 4 . In addition, MoO24  is sp3 hybridization state and ionic radius of 3.23 Å, which is similar to the ionic radius of ReO 4 (3.30 Å) [26]. According to the above discussion, the strengthen of the adsorption infinity for ReO 4 on the Mo(VI)-imprinting adsorbent could be interpreted by the gradually increasing amount of matching specific cavities, which may be caused by close ionic radius of molybdenum and rhenium ion. In a word, I-EDA-CS synthesized by Mo(VI)-imprinting actually has a high adsorption infinity for molybdenum ion, and the maximum adsorption capacities of Mo (VI) on the I-EDA-CS was 601.72 mg
g1, that is, it was hardly separated Re(VII) from Mo(VI) by using the I-EDA-CS. However, in view of the high price of rhenium, the major purpose of this study is to improve the adsorption capacities of Re(VII) with relatively low cost. In order to contrast the effect of template ion, such as MoO2 4 or ReO 4 , on the adsorption of Re(VII), Re-imprinting amino-functional chitosan was also synthesized, which was abbreviated as Re-EDA-CS. Re-EDA-CS had the same trend with that of the I-EDA-CS. As shown in Fig. 4(b), the maximum adsorption capacities of Re(VII) on the Re-EDA-CS and the I-EDA-CS were 445.56 mg
g1 and 418.98 mg
g1 at pH 4, respectively. The adsorption quantity of the I-EDA-CS and the Re-EDA-CS for rhenium(VII) was essentially same. As a consequence, adsorption of

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100

(a)

360

-1

(b)

400

445.56 mg⋅g

80

418.98 mg⋅g-1

40

-1

60

300 qe(mg⋅g )

-1 qe(mg⋅g )

A%

350

340

330

20

200

100

I-EDA-CS Re-EDA-CS 0

320 1:25

2:25

3:25

4:25

5:25

0

10:25

0

20

40

60

Mass ratio of template ions and chitosan

80

100

120

140

160

-1

Ce(mg⋅g )

Fig. 4. (a) Adsorption capacities of the I-EDA-CS at different mass ratio of ammonium molybdate and chitosan, (b) comparison of adsorption capacities of the I-EDA-CS and Re-EDA-CS for Re(VII) (solid-to-liquid ratio = 1 g:1 L; temperature = 303 K; shaking time = 24 h; pH = 4).

Re 100

(a)

Cu

(b)

80

Cu

100 80

Re

60

60 40

A%

40 20

20 pH

Re 0

8

p

pH

Cu

H6

pH

4

Fe

3 pH

pH

6 pH

0

Re

4

pH

Fe

3

pH

Zn

1M

Cu

8

pH

Mn

1

A%

Mn

1

1M

Zn

Fig. 5. Adsorption behavior of (a) the I-EDA-CS and (b) N-EDA-CS for various metal ions as a function of hydrochloric acid concentration. Initial concentration of metal ions = 20 mg
L1, solid-to-liquid ratio = 1 g:1 L; temperature = 303 K; shaking time = 24 h.

rhenium(VII) via the I-EDA-CS by Mo(VI)-imprinting is a kind of brilliant choice, due to that rhenium is more expensive than molybdenum.

Intensity

O1s

C1s

N1s Re4d3

N1s

Re4f7/2

Mo4s

I-EDA-CS I-EDA-CS adsorbed Re(VII)

600

500

400

300

200

100

Binding Energy(eV) Fig. 6. XPS spectra of the I-EDA-CS before and after Re(VII) adsorption.

0

3.2.3. Comparison of adsorption behavior of I-EDA-CS to N-EDA-CS Acidity of aqueous solution plays a significant role in the chemical adsorption of metal ions and has an important influence on properties of the adsorbent surface. The adsorption behavior of the I-EDA-CS and the N-EDA-CS were examined in acidity range from pH 8 to 1 mol
L1, and the results were exhibited in Fig. 5. With the decrease of acidity from 1 mol
L1 to pH 4, adsorption quantity of ReO 4 increased. It was also observed that about 94.11% adsorption of Re(VII) on the I-EDA-CS was achieved at pH 4, while the maximum adsorption rate of the N-EDA-CS only was just 45.70% for Re(VII). The strengthen of the adsorption infinity for ReO 4 on the Mo(VI)-imprinting adsorbent could be interpreted by the gradually increasing amount of matching specific cavities, because MoO2 4 as template ion could decrease of the space and distance between molecular chains of chitosan to form matching specific cavities after being eluted. As the pH was higher than 4.0, the adsorption of rhenium ions significantly decreased because adsorption sites on the surface of the adsorbent were negatively charged at high pH, which decreased electrostatic attraction between ReO 4 and the I-EDA-CS. According to the data from Fig. 5, Sel values of the I-EDA-CS and the N-EDA-CS for the Re(VII) versus Cu(II), Fe(III), Zn(II) and Mn(II) adsorption were calculated and listed in Table 2. The I-EDA-CS showed a better selectivity (Sel  3.46) for Re(VII) than that of

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the N-EDA-CS (Sel  0.48). Especially, copper ions had slight adsorption on the I-EDA-CS. This is an encouraging result, because they usually coexist in actual ore solutions. At higher acid concentration, the competition from H+ may lead to the lower adsorption of Cu(II), Fe(III), Zn(II) and Mn(II), while adsorption of Re(VII) also decreased since there was competition with the Cl because of adding of hydrochloric acid. The increased selectivity coefficient at pH 3.0 may be attributed to the form of the ion-templated polymer, and generated the electrostatic attraction and chelation between the amine groups and ReO 4 . The results have revealed a clear increase of Re(VII) adsorption with the increase of pH, which is believed to reflect the protonation-deprotonation equilibrium of ANH+3 on its adsorption modes in the substrate. Moreover, part of the amine groups deprotonated at higher acid concentration, which may cause the combination between ReO 4 and N+ACACAN on the surface of the cavities. It was inferred that the high adsorption selectivity of the I-EDA-CS was due to the chelation and electrostatic attraction between the amine groups and ReO 4 in the ‘‘cavities”, which formed in the imprinting process. 3.3. Adsorption mechanism 3.3.1. Fourier transform infrared spectroscopy analysis Infrared spectrum analysis was used to obtain the information of feasible interaction between the functional groups on adsorbent and metal ions in solution. According to the discussion for Section 3.1.2, it was found that the I-EDA-CS contains CAN, NAH groups, which manifested that the I-EDA-CS has chemical ingredient with N and C ligand atom. Fig. 2 showed the FTIR spectra of the fresh adsorbent I-EDA-CS and the I-EDA-CS adsorbed Re(VII), and subtle changes were observed between them. The stretching vibrations peak of ANH2 at 3427 cm1 weakened completely, which proves the action of ANH2 with rhenium ion. Moreover, the intensity of sharp peak at 2350 cm1 corresponding to the N+AH vibrations disappeared after adsorption of Re(VII) on the I-EDA-CS, which indicates N+AH groups play a major role in the process of adsorption. It also showed an additional sharp peak at 908 cm1 attributed to the NAO vibrations, which likely due to that ReO 4 was successfully matched with N+ACACAN on the I-EDA-CS. In a word, the bands changed in the case of the I-EDA-CS after loading rhenium ions, which could be the electrostatic attraction and chelation between the amine groups and ReO 4 complexes in the ‘‘cavities” on the surface of the I-EDA-CS. The adsorption mechanism is shown as Eq. (2) and Fig. 8.

3.3.2. X-ray photoelectron spectrum analysis In order to further determine of the adsorption mechanism by which the Re(VII) could act with the ethylenediamine in the ‘‘cavities” of chitosan, the high-resolution XPS spectra of the I-EDA-CS before and after the adsorption Re(VII) was demonstrated in Fig. 6. The results showed that the I-EDA-CS contains C, O and N element on the surface. In the spectrum of the I-EDA-CS adsorbed Re(VII), it was emphasized to observe that two new peaks at the binding energy (B.E.) of 263.82 eV (Re 4d3) and 46.02 eV (Re 4f7/2) appeared, which provides evidence of Re(VII) adsorbed successfully on the I-EDA-CS. It also could be in promising agreement with the experimental data from the FTIR analysis. Moreover, as shown in Fig. 7(a and b), the C 1 s spectra could be deconvoluted into three individual component groups (CAN, CAC, CAO) at 285.70, 284.10, and 284.83 eV for the I-EDA-CS, respectively. After Re(VII) adsorption, the binding energy of CAN shifted from 285.70 to 286.15 eV and the area ratio increased from 41.40% to 70.90%, indicating a possible chemical environment change of the C atom. A obvious change of area for CAN also indicated that small amount of ANH2 on the I-EDA-CS could react with Re(VII) by chelation reaction. In addition, as shown in Fig. 7(c and d), the XPS spectra of N 1s was deconvoluted into two different component peaks, the peak at 398.90 and 400.70 eV were assigned to NAC and AN+AH bonds, respectively. After Re(VII) adsorption, the binding energy of N+AH shifted from 398.90 to 399.60 eV, and the area ratio decreased from 67.70% to 8.30%, indicating a possible chemical environment change of the N and also demonstrated that AN+AH participated in the reaction. The new peak at 399.10 eV for NAO may be due to the adsorption of ReO 4 on the surface when rhenium ion reacts with reaction sites. Thus, the adsorption mechanism of Re(VII) on the I-EDA-CS may be attributed to the electrostatic attraction and chelation between the amine groups and anionic complexes in the ‘‘cavities”. 3.3.3. Structure analysis of I-EDA-CS According to the above comprehensive analysis, the adsorption of Re(VII) on the I-EDA-CS mainly depends on amino groups on the surface of the I-EDA-CS. The lone pair electrons on the N atoms could provide the empty atomic orbitals of the rhenium ions, which forms coordination compounds on the surface of the adsorbent. The formation of amino groups’ electrostatic attraction and chelation with rhenium ions can be classified as Fig. 8, in which

Table 2 Selectivity coefficients of the I-EDA-CS and N-EDA-CS adsorbents for Re(VII) at various acidity. Acid concentration 1 mol
L1

pH 1

pH 3

pH 4

pH 6

pH 8

I-EDA-CS

SelRe/Cu SelRe/Zn SelRe/Fe SelRe/Mn

0.92 1.08 0.83 0.4

0.94 0.1 0.35 0.83

1.4 1.51 3.46 1.83

1.29 1.9 2.31 1.81

– 1.75 – 1.51

– – – 1.34

N-EDA-CS

SelRe/Cu SelRe/Zn SelRe/Fe SelRe/Mn

0.91 0.8 0.5 0.28

0.03 0.95 0.37 0.01

0.35 0.28 0.48 0.42

0.6 0.12 0.25 0.18

– 0.39 – 0.39

– – – 0.94

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Y. Xiong et al. / Separation and Purification Technology 177 (2017) 142–151

(a)

C1s

Intensity

Intensity

C1s

C-O 284.83 (19.3%) C-N 286.15 (70.9%)

C-C 284.1 (9.8%)

280

285

290

295

(b)

C-O 284.83 (43.8%)

C-C 284.1 (14.8%)

280

285

Binding Energy(eV)

395

N-C 400.7 (31.3%)

400

405

410

(d)

N1s

Intensity

Intensity

+

N-H 398.9 (67.7%)

290

Binding Energy(eV)

(c)

N1s

C-N 285.7 (41.4%)

N-O 399.1 (26.4%)

395

Binding Energy(eV)

N-C 400.7 (65.3%)

+ N -H 399.6 (8.3%)

400

405

410

Binding Energy(eV)

Fig. 7. XPS spectra of (a) and (b) C1s, (c) and (d) N1s of I-EDA-CS before and after Re(VII) adsorption.

Fig. 8. The adsorption mechanism of Re(VII) on the I-EDA-CS.

ReO 4 coordinates in a regular tetrahedron space configuration. In order to confirm the feasibility and validity of the combining pattern and subunit’s structure of I-EDA-CS discussed above, the adsorption capacity of Re(VII) on the I-EDA-CS was calculated according to Eq. (2) [27,28]:

Adsorption capacity ¼

MReO4  d  1000 Msubunit  N

ð2Þ

where MReO4 (250.20 g
mol1) and Msubunit (279.18 g
mol1) is the molecule weight of perrhenate and the molar weight of the polymer’s subunit that can be observed from the Fig. 1, respectively. N is the amount of subunits required to form one adsorption site, thus, the denominator gives the molar weight of the binding sites (N = 2 in the model). The parameter d is the active fraction of the polymer. In this study, d is the degree of deacetylation of chitosan, which was 0.89. The maximum adsorption capacity based on the model observed from Fig. 8 was calculated as 400.60 mg
g1, which was

approximate to the experimental adsorption data, thus the model was more compatible to describe the adsorption model in this work. 3.4. Adsorption isotherms Adsorption isotherms are crucial for illustrating the relationship between the ions of adsorbate and the sites on the surface of adsorbent. In this study, four isotherm equations (Langmuir, Freundlich, Temkin, Dubinin–Radushkevich) were applied to fit the experimental data of Re(VIII) adsorption on the I-EDA-CS [29]. The isotherms constants of Re(VII) on the I-EDA-CS and N-EDA-CS were summarized in Table S1. The results illustrated that the Langmuir model (R2 = 0.98) exhibited the best fit rather than Freundlich (R2 = 0.92), Temkin (R2 = 0.97) and Dubinin–Radushkevich model (R2 = 0.94) for the I-EDA-CS. It also obviously observed that the determination coefficient of Re(VII) adsorption on the I-EDA-CS was higher than that of the N-EDA-CS, because uniform surface of the I-EDA-CS by imprinting promoting a monolayer adsorption

149

Y. Xiong et al. / Separation and Purification Technology 177 (2017) 142–151 Table 3 Comparison Re(VII) adsorption capacity with some adsorbent materials in the references. No.

Adsorbent

pH

Adsorption capacity (mg
g1)

Reference

1 2 3 4 5 6 7 8 9

Nanoscale zero-valent iron particles supported on grapheme Amino-functionalized magnetic Cu-ferrites Di-2-ethylhexylamine functionalized corn stalk Di-2-ethylhexylamine functionalized corn stalk Di-2-ethylhexylamine functionalized corn stalk 4-vinyl pyridine functionalized polystyrene microspheres Anion exchange resin D318 4-amino-1,2,4-triazole resin I-EDA-CS

6.5 1.0–4.7 5 1 0.1 3–8 5.2 2.6 3–4

85.77 41.67 141.52 163.57 98.69 252 351.4 354 418.98

[31] [32] [33] [33] [33] [34] [35] [36] This paper

Table 4 Comparison of the pseudo first- and second-order equations, Elovich equation and intraparticle diffusion equation at different temperatures for Re(VII) adsorption on the I-EDA-CS. Temperature (K)

303 K 313 K 323 K

qe (mg
g1 )

100.038 98.367 91.64

Pseudo first-order equation

Pseudo second-order equation

qe (mg
g1)

k1 (h1)

R2

qe (mg
g1)

k2 (g
mg1
h1)

R2

a (mg
g1
h1)

b (g
mg1)

R2

kp (mg
g1
h1/2)

R2

80.95 11.93 16.25

0.001 0.242 0.2653

0.52 0.57 0.89

103.09 98.81 92.42

3.88 0.002 0.001

1 1 1

4.98 10.07 15.28

0.07 0.07 0.19

0.84 0.62 0.55

14.27 7.48 5.04

0.52 0.27 0.17

3.5. Kinetics and thermodynamics

3.6. Desorption and reuse of Re(VII) The reuse of the adsorbent was the principal factor of its practicability in wastewater or effluent. The desorption efficiency of the I-EDA-CS adsorbed Re(VII) by different concentrations of NH4SCN and HCl was researched and showed in Table 5, respectively. The desorption efficiency gradually increased from 82.33% to 99.91%

Intraparticle diffusion

Table 5 Desorption of loaded-Re(VII) on the I-EDA-CS.

model [30]. The maximum adsorption capacities of Re(VII) on the I-EDA-CS and N-EDA-CS were 418.98 mg
g1 and 83.92 mg
g1 at pH 4, respectively. It was apparently observed that the adsorption capacities of the I-EDA-CS for Re(VII) was higher than that of the N-EDA-CS. In addition, the literature precedents of the adsorbents for the adsorption of Re(VII) were presented in Table 3 [31–36]. It was observed that the Mo(VI)-imprinting amino-functional chitosan (I-EDA-CS) prepared in this study has a high adsorption capacity (418.98 mg
g1) for Re(VII).

In order to better understand the controlling mechanism of adsorption process of Re(VII) on the I-EDA-CS, the pseudo-firstorder, pseudo-second-order, Elovich equation and intraparticle diffusion rate constant were studied [37–39]. The kinetic parameters and the determination coefficient (R2) from the linear equation were calculated and showed in Table 4. It could be observed that pseudo-second-order kinetic model (R2 = 1) can best represent the adsorption process of Re(VII) onto the I-EDA-CS. Moreover, the ultimately maximum adsorption for pseudo-second-order follows the order of 303 > 313 > 323 K in the case. In order to research the adsorption thermodynamics of Re(VII) on the I-EDA-CS, adsorption experiments were carried out with temperature ranging from 303 to 323 K. The standard freeenergy change (DGh), standard enthalpy change (DHh), and standard entropy change (DSh) were calculated using the equations described in our previous work [40]. The negative values of DGh for the Re (VII) were 6.75, 6.31, 4.81 kJ
mol1, respectively, which indicated that the adsorption of Re(VII) was spontaneous adsorption. Under the circumstances, DSh values (68.62 J
mol1
K1) indicated the higher randomness as a result of the adsorption of Re(VII) on the I-EDA-CS was spontaneous at low temperature due to exothermic nature (DHh, 27.36 kJ
mol1).

Elovich equation

Eluted agent

Desorption efficiency (%)

2% NH4SCN 5% NH4SCN 10% NH4SCN 15% NH4SCN 20% NH4SCN 5% HCl 10% HCl 15% HCl 20% HCl

82.33 90.84 99.35 99.63 99.91 95.23 96.34 98.81 99.12

Table 6 Performance of the I-EDA-CS in consecutive adsorption-elution cycles.

Cycle Cycle Cycle Cycle Cycle Cycle

1 2 3 4 5 6

Absorbed (mg
g1)

Eluted (mg
g1)

% Recovery

44.81 44.74 44.75 44.57 44.4 44.45

44.89 44.76 44.62 44.41 44.01 43.38

100 100 99.7 99.64 99.11 97.6

and 95.23% to 98.63% with the increase of NH4SCN and HCl, respectively. Although the desorption efficiency of NH4SCN was better than HCl, the desorption efficiency was almost no difference (>99%) when the NH4SCN concentration was higher than 10%, and low concentration (>5%) of HCl could elute about 95% Re(VII) from loaded I-EDA-CS. Hence, 10% NH4SCN or 15% HCl could be chose as an appropriate concentration for the regeneration of rhenium ions. Adsorption–desorption cycles were repeated 6 times by using the same adsorbent, which can be used to prove the reusability of Mo(VI)-imprinted adsorbent. The weakening of adsorption capacity was primarily depended upon the amount of remaining rhenium ions in the desorption process. The adsorption capacity for Re(VII) by reusing adsorbent was shown in Table 6. After 6 cycles, the adsorption capacity for Re(VII) had little change. The results suggested that the Mo(VI)-imprinted aminofunctionalization chitosan had a certain regeneration adsorption efficiency, which is economic and amicable for the environment.

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Y. Xiong et al. / Separation and Purification Technology 177 (2017) 142–151

3.7. Recovery of Re(VII) from industrial wastewater

References

In this work, industrial wastewater collected from Yangjiazhangzi Economic Development Zone Management Committee, China. The pH of the sample was 2.8. In our previous work, we have prepared a persimmon gel (APF) and successfully used in the selective removal and recovery of Mo(VI) [41]. After the adsorption of Mo(VI) by the APF, the industrial wastewater contains rhenium, copper, manganese and zinc ions in concentrations of 61.42 mg
L1, 115.28 mg
L1, 17.02 mg
L1, 8.49 mg
L1, respectively. When the wastewater was adsorbed by the I-EDA-CS one stage, the adsorption percentage of Re(VII) was 98.91% and the concentration in the rhenium raffinate was just 0.672 mg
L1. Also, the copper, manganese and zinc ions in the effluent could not be adsorbed onto the I-EDA-CS. From the above analysis, Re(VII) can be efficiently recovered with the I-EDA-CS from wastewater containing copper, manganese and zinc ions. The recovery of rhenium in this work seems to be highly satisfactory, and this may be attributable to the ideal affinity of the adsorbent prepared in this work.

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4. Conclusions In the present work, Re(VII) as ReO 4 coordination anion was successfully adsorbed by Mo(VI)-imprinted ethylenediamine (EDA) grafted chitosan (I-EDA-CS). An anionic imprinted treatment could make the adsorbent has a denser network structure, which was confirmed by the scanning electron microscopy. The quantity of imprinted sites was controlled by the amount of ammonium molybdate as template ion, which indicates that mass of ammonium molybdate and chitosan ratio of 1 g:25–3 g:25 g could conducive to achieve higher adsorption capacity of Re(VII). The maximum adsorption capacities of Re(VII) on the I-EDA-CS was 418.98 mg
g1, while the maximum uptake on the non-imprinted adsorbent (N-EDA-CS) was found to be just 83.92 mg
g1. The I-EDA-CS also showed a higher affinity for Re(VII), and had higher selectivity to recovery Re(VII) from the solution containing Cu(II), Fe(III), Zn(II) and Mn(II) than the N-EDA-CS. In addition, adsorption mechanism of Re(VII) could be the electrostatic attraction and chelation between the amine groups and ReO 4 complexes in the ‘‘cavities”. Langmuir isotherm and pseudo-second-order kinetic were used to describe the adsorption of Re(VII) on the I-EDA-CS. The effectiveness of recovery of Re(VII) from real industrial wastewaters was also tested, and it demonstrated highly encouraging results of the I-EDA-CS. In a word, ammonium molybdate as template ion is a kind of very good choice, because rhenium was more expensive than molybdenum. The innovation of the novel adsorption can achieve cost effective and environment-friendly purpose for the recovery of rhenium(VII) from industrial wastewater. Acknowledgements This project is supported by National Natural Science Foundation of China (21201094, 51674131, 21373005), Scientific Research Found of Liaoning Provincial Education Department (L2014004), Program for Liaoning Excellent Talents in University (LR2015026), Liaoning Provincial Department of Education Innovation Team Projects (LT2015012), Project supported National Science Technology Ministry (2015BAB02B03) and Education Bureau of Liaoning Province (LZ2014001). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2016.12. 028.

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