Removal and removing mechanism of tetracycline residue from aqueous solution by using Cu-13X

Chemical Engineering Journal 273 (2015) 247–253

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Removal and removing mechanism of tetracycline residue from aqueous solution by using Cu-13X Jun-Min Lv a,b, Yu-Long Ma a,b,⇑, Xuan Chang a,⇑, Su-Bing Fan b a b

College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China State Key Laboratory Cultivation Base of Energy Sources and Chemical Engineering, Ningxia University, Yinchuan 750021, China

h i g h l i g h t s  Cu-13X had a high capacity of 2400 mg g

1

with chemically selective adsorption.

 The adsorption kinetics was studied in detail at low and high concentration of TC.  The adsorption isotherms were fitted well by the Langmuir model.  The adsorption of TC on Cu-13X was exothermic and spontaneous.  The adsorption depended on the complexation of Cu(II) with NH2 of amide group of TC.

a r t i c l e

i n f o

Article history: Received 16 January 2015 Received in revised form 15 March 2015 Accepted 17 March 2015 Available online 20 March 2015 Keywords: Adsorption mechanism Complexation Cu-13X Removal Tetracycline residue

a b s t r a c t In order to improve the removal efficiency of the reported adsorbents via freely physical adsorption, a novel adsorbent, Cu-13X, was applied to remove tetracycline (TC) residue from aqueous solution due to the chemically selective adsorption. The removal behaviors, adsorption kinetics, adsorption isotherm, adsorption thermodynamics of TC on synthesized Cu-13X were studied in batch experiments under different conditions. It was found that the adsorption capability of Cu-13X increased greatly after exchanging. The amount of Cu(II) cation in Cu-13X and the pH value were important factors. At pH 7.0 and the maximum exchange amount of Cu(II), the maximum adsorption capacity of Cu-13X for TC residue reached about 2400 mg g1. The adsorption isotherms were fitted well by the Langmuir model. The adsorption kinetics was described well by pseudo-second order equation, simultaneously was described well by the intra-particle diffusion model at high initial concentration of TC. The data calculated by adsorption thermodynamics of TC on Cu-13X showed it was exothermic and spontaneous. The probable adsorption mechanism was proposed by Fourier transform infrared (FTIR) analysis, which showed that the adsorbing TC residue on Cu-13X depended on the strong complexation of Cu(II) with NH2 radical of amide group of TC. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Tetracycline-antibiotics are the primarily antibiotics used worldwide for controlling bacterial infections in both humans and animals, and agricultural purposes [1,2]. Each year thousands tons of tetracycline-antibiotics are produced worldwide [3]. The production and consumption of tetracycline-antibiotic rank second in the world, while on top of the ranking in China [4,5]. However,

⇑ Corresponding authors at: College of Chemistry and Chemical Engineering, Ningxia University, Helanshan Rd. 539, Yinchuan 750021, China. Tel.: +86 951 2062380; fax: +86 951 2062323. E-mail addresses: [email protected] (Y.-L. Ma), [email protected] (X. Chang). http://dx.doi.org/10.1016/j.cej.2015.03.080 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

only a small part of tetracycline-antibiotic used is metabolized or absorbed in vivo, most (about 70–90%) is excreted or released in metabolite forms into the environment via urine or feces from humans and animals [6,7]. Tetracycline (TC) is an important tetracycline-antibiotic, which is frequently used in aquaculture and veterinary medicine [8]. But the mass production of TC in pharmaceutical industry makes the large amount residue of TC in the wastewater and the concentration of TC from 0.15 lg L1 to 2.37 lg L1 were detected somewhere [9–11]. TC residue in the environment induces resistant microorganisms and tends to bring a threat to the human healthy by increasing the risk of certain infections. In addition, TC residue in drinking water will directly harm the health of human body. So it has been considered to be a serious environmental pollution [12,13]. However, natural

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biodegradation is not effectual to remove the residual TC in the environment. Therefore, it is necessary to develop an effective method to remove TC from aqueous solution of environment. At present, the methods for removing TC include membrane separation, oxidation, photo-catalysis, electrochemical process, biodegradation, and adsorption. These methods all have been developed in a certain extent, however, they also have their own limits in application. For example, membrane techniques are very sensible to the amounts of organic material occurring naturally in the water matrices and the concentration of the dissolved salts. In addition, the membrane is easy to be blocked causing a higher cost [14–18]. The drawbacks such as secondary pollution or the produce of huge volumes of sludge as well as the relatively higher treatment cost of sludge in the oxidation process limit its application [18–20]. For photo-catalysis process, the light source with high energy and the high effective catalyst are required [21], and large-scale application is still limited [6]. The higher energetic consumption hinders the application of the electrochemical process [22]. The environmental conditions, such as pH value, temperature, oxygen, water and especially other antibiotics existed in environment, greatly influence the growth and degradation ability of strains in biodegradation [23]. Comparison with above methods, adsorption shows lower total cost and lower energetic consumption, making it be a practical process for the removal of TC from wastewater in situ [13]. Adsorbent mainly includes conventional adsorbent (active carbon and silica gel), biosorbent (biomass and solid waste), and nature/artificial material (clay and zeolite) [24]. Conventional adsorbent has no selective to pollutants. And for biosorbents, there are many problems, involving in the cost and mass loss of pretreatment, biodegradable or decomposable in adsorption processes, low thermal stability, difficult regeneration and disposing, unassured supply of biomass, need to be considered [24,25]. Compared with above adsorbents, nature/artificial material, especially modified zeolite/molecular sieve, is highly selective and effective, secondary pollution-free, easy regeneration for reuse, high thermostability and structure stability in adsorption process, which led the zeolite/molecular sieve to be an environmental-friendly, effective and low-cost adsorbent. TC molecule contains amphoteric functional groups, and its shape changes presenting cationic, zwitterionic and anionic three forms with the change of the pH value of solution [26]. Furthermore, the electron donating radical of TC molecule is easy to interact with two divalent metal ions such as, Cu2+, Ca2+, Mg2+ and so on [27,28], and Cu2+ has a stronger effect than Ca2+ and Mg2+ to TC adsorption [28–32]. Wang et al. [32] investigated in detail the effect of Cu(II) of solution on the adsorption of TC onto the surface of montmorillonite and the results showed that coexistence of TC and Cu(II) enhanced its adsorption on montmorillonite at environmentally relevant pH values. However, the formed complex is soluble, which will decrease the removal efficiency of TC due to the free adsorption via diffusion driven by concentration gradient although there exist strongly interactions between TC and Cu(II). Therefore, the removal of TC from aqueous solution would be enhanced if Cu(II) is fixed to a solid adsorbent. 13X molecular sieve is a common adsorbent, which is composed of silicon–oxygen tetrahedron and aluminumoxygen tetrahedron connected by oxygen atoms. The ordered pore structure and large specific surface area make it be a kind of excellent adsorbent. For example, the comparison studies of the adsorption of CO2/CH4 and CO2/N2 in 13X with that in ZIF-8 and BPL activated carbon showed that 13X had higher adsorption capacities [33]. In addition, it has excellent cation exchange capacity, suggesting that the surface properties of 13X molecular sieve could be modified to get more adsorption sites and enhance its removal ability [34]. Hence, the fixation of Cu(II) on the surface of 13X molecular sieve

might enhance the removal of TC from aqueous solution via their strongly interactions instead of free diffusion. Up to now, there are few reports about removal of TC by modified 13X molecular sieve. In this work, Cu(II) was fixed on 13X molecular sieve by ion-exchange method with cupric nitrate to prepare modified molecular sieve (Cu-13X). Batch experiments were carried out to study its fundamental adsorption behaviors for removing of TC from aqueous solutions and the removal mechanism was also researched. 2. Materials and methods 2.1. Materials Tetracycline hydrochloride (TC, 98%) was obtained from Qiyuan Pharmaceutical Co., Ltd. of China, and stored in dark at 4 °C. NaOH (AR, 96%), hydrochloric acid (AR, 36–38%), Cu(NO3)23H2O (AR, 99%), and kaolin (CP) were commercial reagents and used as received. 2.2. Synthesis procedures 2.2.1. Synthesis of 13X molecular sieve 13X molecular sieve was prepared by hydrothermal treatment as reported [35]. The main process was as follows: sodium hydroxide and kaolin with the weight ratio of 2:1 were milled and fused in an MgO ceramic crucible at 200 °C for 4 h. The fused mixture was cooled at room temperature, ground further and added to water (10 g fused mixture/75 mL water). The slurry obtained was vigorously agitated for 2 h at 50 °C for homogenization (800 rpm), and then was crystallized at 90 °C for 8 h (300 rpm). Finally, the solid was obtained by filtration and washing thoroughly several times with deionized water until the pH reached around 8.0, drying at 105 °C overnight and then was stored for use. 2.2.2. Preparation of Cu-13X 2 g of 13X molecular sieve was added to 100 mL of different concentrations of copper nitrate solutions in 250 mL flask. The initial concentrations of copper nitrate were 0, 0.25, 0.45, 0.58, 0.75, 1.00, 1.50, 2.00, and 2.50 g per gram of 13X, respectively. Then the suspension liquid was refluxed at 80 °C under stirring for 3 h, followed by filtered and washed with hot demineralized water until no Cu(II) was detected in filtrates. The ion-exchange process was repeated for three times. Finally, the obtained solids were dried at 80 °C overnight in an air oven [36,37]. 2.3. Characterization and analysis The X-ray diffractions (XRD) of synthesized 13X and Cu-13X materials were characterized on a Rigaku D/Max 2200 diffractometer with Ni-filtered CuKa radiation at 30 kV and 20 mA. Samples were scanned from 3° to 50° (2h at 8°min1 with a scanning step of 0.02°). Scanning electron microscopy (SEM) images were obtained on a JSM7500F at 3 kV. The adsorption mechanism was studied via infrared spectrum recorded on a Fourier transform infrared spectrometer (FT-IR) of Bruker Tensor 27 with the resolution of 2 cm1. The real exchange amount of Cu(II) ion was analysized by atomic absorption spectroscopy (AAS). 2.4. Adsorption experiments 100 mL of a designed concentration of TC was added to the flask and then was adjusted to a designed pH value with less than 0.1 mL of NaOH solutions (0.10 mol L1 for 50–200 mg L1 of TC solution, and 1.0 mol L1 for 400–1000 mg L1 of TC solution) at

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a designed temperature. 0.04 g of Cu-13X modified molecular sieve with different exchange amount Cu(II) as adsorbent was then added to the above solution. Next, the flask was sealed with aluminum foil and stirred (120 rpm) continuously in the dark. About 1 mL of filtrate was collected with 0.22 lm membrane to analyze the removal efficiency using HPLC at a selected time intervals until the adsorption reached equilibrium. The adsorption capacity is calculated using Eq. (1) [38] as follows:

qe ¼ ðC 0  C e Þ  V=M

ð1Þ

1

where qe (mg g ) is the adsorption capacity in the Cu-13X at the equilibrium; C0 and Ce (mg L1) are the initial and equilibrium concentrations of solution, respectively; V (L) is the volume of the aqueous solution and W (g) is the mass of adsorbent used in the experiments. 2.5. Adsorption isotherms The adsorption experiments were carried out at different temperatures (298, 308, and 318 K). The concentrations of TC in aqueous solutions were from 100 to 1000 mg L1. In order to describe the interactive behavior between solute and adsorbent, the Langmuir and Freundlich models were applied to analyze the adsorption data. The Langmuir model supposes that adsorption happens only on the homogeneous surface to form monolayer adsorption and there are not interactions between adsorbed molecules. Freundlich model is an experiential adsorption isotherm occurring on the heterogeneous surface and can well explain the experimental results in a broader range of concentration. The Langmuir and Freundlich equations are commonly used in the adsorption of antibiotics on several adsorbent systems [26]. The equations of the models are described as follows: Langmuir isotherm:

qe ¼ qmax K L Ce=ð1 þ K L CeÞ

ð2Þ

Freundlich isotherm:

qe ¼ kf ðCeÞ1=n

ð3Þ

The linear forms of these isotherms are displayed as equations Eqs. (4) and (5), respectively.

1=qe ¼ 1=ðqmax K L Þ  ð1=CeÞ þ 1=qmax

ð4Þ

lgðqe Þ ¼ lgðkf Þ þ 1=n lgðCeÞ

ð5Þ

The pseudo-second-order model:

t=qðti Þ ¼ 1=ðk2 q2e Þ þ t=qe

ð7Þ

The intra-particle diffusion model:

qðti Þ ¼ k3 t 1=2 þ C

ð8Þ

1

1

where k1 (min ) is the first-order rate constant; k2 (mg g min1) is the second-order rate constant; k3 (mg g1 min1/2) is the intraparticle diffusion rate constant; C (mg g1) is a constant that is about the thickness of the boundary layer. qe and q(ti) (mg g1) are the amount of TC adsorbed at the equilibrium and at time t (min), respectively. 2.7. Adsorption thermodynamics Thermodynamic equilibrium constant K for the adsorption was calculated from intercept determined by plotting ln (qe/Ce) vs. qe using the method of Khan and Singh [39]. The standard free energy change DG was calculated using the equation:

DG ¼ RT ln k

ð9Þ

where DG is the Gibbs energy change, R is the molar gas constant, T is the absolute temperature, K is the same as indicted above. The enthalpy change DH and entropy change DS were calculated by according to the follow equation:

ln k ¼ DH=ðRTÞ þ DS=R

ð10Þ

3. Results and discussion 3.1. Characterization of 13X and the modified Cu-13X molecular sieve Sample S1 in Fig. 1 is the XRD pattern of the 13X molecular sieve synthesized. It was found that the tested 13X molecular sieves had a good crystallinity, and six obvious characteristic peaks at 6.1°, 10.0°, 15.4°, 23.3°, 26.7°, 31.0° proved that the 13X molecular sieve had been synthesized successfully. The XRD pattern of Cu-13X with different exchange amount of Cu(II) is showed in Fig. 1 (S2–S8). With the increasing amount of Cu(II) on the surface of 13X, the characteristic peaks of 13X remained although their intensities decreased, indicating that the framework structure of 13X molecular sieve was preferably maintained. And no characterized peaks of CuO was found, suggesting that Cu was chemically bonded onto the surface of 13X by Cu(II)

1

where qe (mg g ) is the amount of TC adsorbed on the adsorbent at the equilibrium. qmax (mg g1) is the maximum of adsorption, which is used to describe the adsorption capacity. KL is the Langmuir adsorption equilibrium constant and Ce (mg L1) is the concentration of equilibrium solution. kf is Freundlich constant. 1/ n is the adsorption intensity.

S9 S8 S7 S6

2.6. Adsorption kinetics The experiments of adsorption TC were conducted at different initial concentrations of 50, 100, 200, 400, 500, 700, 1000 mg L1, respectively. The dosage of adsorbent Cu-13X [146.94 mg g1 Cu(II)] was 0.04 g. The temperature was 308 K and the initial pH was 7.0. Blank experiment was also carried out with the unmodified 13X molecular sieve. Three adsorption kinetic models, i.e. the pseudo-first-order model, the pseudo-second order model and the intra-particle diffusion model, were applied to interpreting the experiment data. The pseudo-first-order model:

lnðqe  qðti Þ Þ ¼ ln qe  k1 t

ð6Þ

Intensity

S5 S4

S3 S2

S1

0

10

20

30

40

50

2θ /deg. Fig. 1. XRD patterns of the tested sample with different exchange amount of Cu(II). The real Cu(II) content (mg g1) was: (S1) 0, (S2) 66.12, (S3) 115.52, (S4) 146.94, (S5) 148.33, (S6) 149.07, (S7) 149.25, (S8) 149.38, respectively, and the exchanged copper salt of sample S9 was 2.50 g g1.

J.-M. Lv et al. / Chemical Engineering Journal 273 (2015) 247–253

cation form rather than loaded by CuO form. When the dosage of copper salt was above 0.58 g per gram of 13X, the actual exchange capacity of Cu(II) cation reached 146.94 – 149.38 mg g1, which was close to the theoretical exchange capacity (152.0 mg g1). Therefore, continuing to increase the amount of copper salt had little significance. When the amount of copper salt was 2.50 g per gram 13X (sample S9), the characteristic peaks of copper appeared in the XRD pattern and the skeleton structure of 13X molecular sieve decreased greatly. The SEM images of 13X and Cu-13X are shown in Fig. 2. The particle size distributed more uniform and was about 2 lm. Compared with 13X, the exchange of Cu(II) did not change the morphology of 13X and its particle size.

1.0 0.9 0.8

Adsorption Rate

250

0.7 0.6 0.5 0.4 0.3 0.2 2

4

3.2. Adsorption performance

6

8

10

12

pH value

3.2.1. Effect of pH The effects of different initial pH value set at 2.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 11.0, respectively, on adsorption of TC on Cu-13X were studied and the results are shown in Fig. 3. The ‘‘volcano type’’ curve was found by analysis of the correlation between the adsorption amount and pH value, and the adsorption efficiency of TC reached the highest of almost 100% at pH 7.0, which was useful for the selection of adsorption conditions. On the acid or basic condition, the adsorption amount of TC on Cu-13X sharply decreased, which might be ascribed to two reasons. One reason is the effect of solution pH on TC molecule. The protonation–deprotonation transition of functional groups of TC at different solution pH results in the transformation of chemical speciation for ionizable organic compounds [26]. At pH below 3.3, the dimethyl-ammonium group is protonated, resulting in a cationic form (TCHþ 3 ). When the pH was between pH 3.3 and 5.5, the cationic ((TCHþ 3 ) gradually disappeared while the zwitterion (TCH02 ) gradually increased to be dominant due to the loss of proton from the phenolic diketone moiety. Hence, the adsorption of TC gradually increased at pH < 5.5 because the decreasing of the electrostatic repulsion between Cu(II) on Cu-13X and the cationic TCHþ 3 . When pH is above 5.5, TC changes into a monovalent anion (TCH-), or a divalent anion (TC2), due to the loss of protons from the tricarbonyl system and phenolic diketone moiety [26,40]. With the increase of solution pH, especially above 5.5, the hydroxyl groups or ammonium groups are more negative which is helpful for the complexation of TC with Cu(II) on the Cu-13X [32]. Another is that the effect of solution pH on the stability of Cu13X. Although Cu(II) ion was ion-exchanged on 13X instead of H+ ion, it is still exchangeable. When the solution pH is lower than 7.0, Cu(II) ion would be exchanged by H+ again resulting in the loss of Cu(II) on the surface of 13X to the solution and forming the soluble complex compounds and even dealumination. When the solution pH was more than 7.0, the desilication of Cu-13X framework

Fig. 3. Adsorption rate of Cu-13X with Cu(II) 146.94 mg g1 with the pH value of solution.

would happen. This resulted in the loss of Cu(II) to the solution which greatly decreasing the adsorption capacity of Cu-13X. Therefore, the solution pH is important for the adsorption of TC on Cu-13X and the adsorption amount of TC reached the highest at around pH 7.0. The solution pH was set 7.0 to the next experiments.

3.2.2. Effect of the Cu(II) exchange amount In order to investigate removal ability of TC on 13X and Cu-13X, and analyze the effect of the amount of Cu2+ exchanged. The batch experiments were conducted at initial pH 7.0 and the temperature of 308 K. The concentrations of TC aqueous solutions were 200 mg L1. Fig. 4 shows the removal efficiency of TC under different exchange amount of Cu(II) on 13X (the corresponding exchanged amount of Cu(II) on 13X of sample S1–S6 as seen in Table 1). Compared with 13X without exchanging Cu(II), the removal efficiency of 13X with exchanging Cu(II) was far greater, indicating that the exchanged Cu(II) played an important role for adsorption of TC. In addition, the removal efficiency of 13X increased with the increase of exchange amount of Cu(II). When the dosage of copper salt in per gram of molecular sieve was 0.58 g, the exchange amount of Cu2+ was 146.94 mg g1 and the removal efficiency for TC reached 99.54%. However, continually increasing the concentration of Cu(II) during the process of impregnation, the real amount of Cu(II) on Cu-13X did not obviously increased and the removal efficiency for TC was almost the same, corresponding to the above crystallinity by XRD patterns. Thus, the following experiments were carried out with the modified Cu-13X with the exchange amount of copper salt of 0.58 g per gram of molecular sieve as an adsorbent for the removal of TC.

a

Fig. 2. SEM images of (a) 13X and (b) Cu-13X with Cu(II) 146.94 mg g1.

b

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J.-M. Lv et al. / Chemical Engineering Journal 273 (2015) 247–253 1.0

500 450

0.6

400

0.08 y=0.00198x+0.000898 2 R =0.9999 0.06

350

0.04 0.02

300

0.00

-1

0.4

S1 S2 S3 S4 S5 S6

0.2

0.0 0

5

10

15

Amount TC adsorbed /mg g

Adsorption Rate

0.8

20

t /h Fig. 4. Adsorption rate with the amount of Cu(II) exchanged. The contents of Cu(II)exchanged on 13X were: (S1) 0, (S2) 66.12, (S3) 115.52, (S4) 146.94, (S5) 148.33, and (S6) 149.07 mg g1, respectively.

250 0 247

Several models for adsorption kinetic were used to describe the experimental results. According to the pseudo-first order equation, they didn’t show any linear relationships between ln (qe – qt) and adsorption time t, verifying that the adsorption kinetic on Cu13X did not follow the pseudo-first order equation. Then, pseudo-second order was applied to analyze the experimental data at seven initial TC concentrations. The results are given in Fig. 5 and Table 2. It was found that all the correlation coefficients (R2) of the linear form of pseudo-second order model were much closer to 1.00 and the calculated qe was agreement with the experimental result, indicating that the pseudo-second order model was more suitable for the adsorption process of TC on Cu-13X and the chemical adsorption was the rate-controlling step [41]. In order to identify the diffusion mechanism and the rate-controlling step influencing the adsorption process, intra-particle diffusion mode was used to fit the experimental date. According to the Eq. (8), when the plot of qt versus t1/2 shows a good linear relationship and passes through the origin of coordinates, then the intra-particle diffusion was the only rate-controlling step in the adsorption process. If the linear is fitted better, but do not passes through the origin, intra-particle diffusion is the rate-controlling step but not the only. If the qt–t1/2 curve shows multiple linear, suggesting that there occur multiple steps in the adsorption process [42,43]. In this work, the correlation coefficients R2 of the linear form of intra-particle diffusion model for the high initial concentration from 400 to 1000 mg L1 TC also reached above 0.9, while the fitting line didn’t pass through the origin, indicating that the intraparticle diffusion was the rate-controlling step in the adsorption process of TC on Cu-13X, but not the only and it had an important effect on the adsorption rate. On the other hand, the R2 values of the qtt1/2curves for the low initial concentration were low and the curves present multiple segment linear relationships, suggesting that the intra-particle diffusion was not the rate-controlling step of the adsorption process.

209 190 0

1

30

40

20

30

40

2

3

4

5

-1

100 mg L 6

7

8

9

0.06 y=0.00808x+0.000174 2 R =0.9999 0.04

115

0.02 0.00

110

20

0.035 y=0.00399x+0.000286 0.030 2 0.025 R =0.9999 0.020 0.015 0.010 0.005 0.000 0 1 2 3 4 5 6 7 8 9

120

3.3. Adsorption kinetics

10

10

228

125

-1

200 mg L 0

0

1

-1

0

2

2

4

3

6

4

50 mg L

8

5

6

7

8

9

t /h Fig. 5. Adsorption kinetics of TC and linear fitted by Pseudo-second-order kinetics model (inset).

3.4. Adsorption isotherm Adsorption isotherms such as Langmuir and Freundlich model were investigated to describe adsorption phenomena and to determine the interactions between TC and Cu-13X. The parameters of isotherm models were calculated according to plots of 1/qe versus 1/Ce and log qe versus log Ce, and the fitting parameters and the regression coefficient R2 were listed in Table 2. The regression coefficients R2 of the linear equation for Langmuir model were close to 1 suggested the experimental data were fitted well by the Langmuir-type isotherm. That is, the adsorption of TC on Cu-13X is monolayer adsorption. Commonly, physical adsorption is nonselective multilayer adsorption, while the essence of chemical adsorption is the formation of chemical bond and is selective monolayer adsorption. Hence, the adsorption of TC on Cu-13X might mainly be the chemical monolayer adsorption. The TC maximum adsorption capacity on Cu-13X shown in Table 3 reach 2428 mg g1, which is higher than that on materials reported [13], indicating Cu-13X is an effective adsorbent to remove TC from aqueous solution. 3.5. Adsorption thermodynamics The adsorption thermodynamics equilibrium constant K and thermodynamic parameters for the adsorption of TC on Cu-13X were calculated using the method reported [40] and listed in

Table 1 The real Cu(II) content in ion-exchanged Cu-13X by AAS. Sample

S1

S2

S3

S4

S5

S6

S7

S8

S9

Cupric nitrate/g g1 13X Real Cu(II) content/mg g1 13X

0 0

0.25 66.12

0.45 115.52

0.58 146.94

0.75 148.33

1.00 149.07

1.50 149.25

2.00 149.38

2.50 –

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Table 2 The fitted parameters of adsorption kinetics. Pseudo-second-order 1

qe/mg g

K2  10

123.76 250.63 505.05 1007.88 1282.27 1696.29 2381.01

3752.24 556.63 43.67 1.85 0.96 0.65 0.53

1

/g mg

1

min

R

2

0.9999 0.9999 0.9999 0.9931 0.9950 0.9864 0.9883

Table 3 The fitted parameters of adsorption isotherms.

R2

3.06 17.43 30.92 76.17 85.00 92.81 106.13

116.66 209.47 343.90 327.37 354.81 603.98 1096.68

0.4213 0.5272 0.7856 0.9708 0.9749 0.9787 0.9922

401.71 398.11 392.07

1.89 1.97 1.84

0.9219 0.9230 0.8276

Table 4. With the increasing of adsorption temperature, the values of K decreased, suggesting that the adsorption is an exothermic process, which was accordance with the negative enthalpy change DH. The more negative value of DH (26.77 kJ mol1) indicated that the adsorption of TC on Cu-13X might be a chemical adsorption, which agreed with the results of adsorption kinetics and thermodynamics. The values of standard Gibbs free energy change (DG) were negative, demonstrating that the adsorption process was spontaneous. The negative entropy change DS showed that the system became in order when TC was adsorbed on the surface of Cu-13X and the system degree of freedom decreased.

13X

TC

4000 3500

1500

500

-1

Wavenumber /cm

Fig. 6. Infrared spectrum of the tested samples.

N

HO

3.6. Proposed adsorption mechanism

O

OH

To further identify the adsorption mechanism of TC on Cu-13X modified molecular sieve, infrared spectrums of the tested Cu-13X samples were recorded (Fig. 6). When 13X molecular sieve was exchanged with Cu(II) cation, some changes were found: (1) the band around 3481 cm1 assigned to OH radical of 13X molecular sieve shift to 3443 cm1; (2) a new band around 907 cm1 assigned to Cu–O bond appeared; (3) the band of 568 cm1 and 468 cm1 assigned to Si–O bond shifted to 551 cm1 and 455 cm1, respectively; (4) the band of 676 cm1 assign to Al–O bond shifted to 689 cm1. These changes suggested that Cu(II) ion was chemically interaction with the O2 anion of SiOAl instead of H+ indicating CuO bond was formed. When adsorbed TC on Cu-13X at pH 7.0, the similar changes were also found: (1) the band of 548 cm1 enhanced which was ascribed to the formation of Cu–N bond; (2) the band of 1523 cm1 assigned to N–H in-plane blend vibration shifted to 1499 cm1; (3) the bands of 1141 cm1 (assigned to Aryl-C@O), 522 cm1 (assigned to @C–C@O) and 341 cm1 (O@C– N) shifted to 1129 cm1, 527 cm1 and 339 cm1, respectively. And the above facts indicated that Cu(II) ion interacted with N atom of NH2 radical of A-ring and affected the adjacent bond, which evidencing the formation of the copper ammonia complex compound. It is agreed that there are strong interaction between

Table 4 Calculated thermodynamic parameters. T/K

K

DG/kJ mol1

DS/J mol1 K1

DH/kJ mol1

298 308 318

690.92 485.91 360.97

16.20 15.84 15.57

33.45

26.77

339

548 527 1000

341

0.9999 0.9999 0.9904

455

0.151 0.156 0.157

468

2389 2413 2428

Cu/13X

522

R2

689

n

551

kf

676 568

R2

907

KL/L mg1

1129

1499

Freundich

qmax/mg g1

1523

Langmuir

3443

298 308 318

C/mg g1

TC-Cu/13X

Transmittance /%

T/K

K3/g mg1 min1

3481

50 100 200 400 500 700 1000

Intra-particle diffusion 4

1141

C0/mg L1

H

O

OH

O Si

H

OH OH

Al

δ Cu

δN

O

O

O

Fig. 7. Proposed adsorption mechanism of tetracycline on Cu-13X.

TC and metals at pH between 4.0 and 8.0 [33]. Therefore, it could be included that at pH 7.0, the adsorption of TC on Cu-13X depended on the strong complexation of Cu(II) with NH2 radical of amide group in TC, which is accordance with the results of adsorption isotherm and thermodynamics, and the proposed adsorption mechanism is drawn in Fig. 7. 4. Conclusions In this study, a novel adsorbent, Cu-13X, was prepared by ionexchange method. The Cu bonding onto the surface of 13X by exchanged Cu(II) cation form rather than loaded by CuO form had higher chemically selective and higher adsorption efficiency for TC. The exchange amount of Cu(II) is important for the adsorption of TC. When increasing the amount of copper salt, the exchange amount of Cu(II) reached the maximum, so did the removal efficiency. And the pH value of solution was also found as an important factor, the maximum adsorption was reached at pH 7.0. The adsorption kinetics researches indicated that the chemical adsorption played a dominant role, while at high initial concentration of TC the intra-particle diffusion was the rate-

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controlling step in the adsorption process of TC on Cu-13X, but not the only and it had an important effect on the adsorption rate. The adsorption isotherm showed it was a monolayer adsorption, which was in accordance with the results of adsorption kinetics. Relating with the adsorption kinetics and thermodynamics, the further identification of adsorption mechanism indicated again that the high removal efficiency of TC on Cu-13X is at least contributed to the strong complexation of Cu(II) cation with TC molecule. The maximum adsorption capacity of TC on Cu-13X calculating based on experiments reached about 2400 mg g1, which is higher than that on almost all of adsorbents reported in literatures and shows great potential in the field of removing antibiotics. Acknowledgement The authors would like to thank the National Natural Science Foundation of China (NSFC) for financial support (No. 21467023; 21166020). References [1] R. Li, Y. Zhang, C. Lee, L. Liu, Y. Huang, Hydrophilic interaction chromatography separation mechanisms of tetracyclines on amino-bonded silica column, J. Sep. Sci. 34 (2011) 1508–1516. [2] X. Ding, S. Mou, Ion chromatographic analysis of tetracyclines using polymeric column and acidic eluent, J. Chromatogr. A 897 (2000) 205–214. [3] E. Michalova, P. Novotna, J. Schlegelova, Tetracyclines in veterinary medicine and bacterial resistance to them, Vet. Med. 49 (3) (2004) 79–100. [4] X. Xie, Q. Zhou, Z. He, Y. Bao, Physiological and potential genetic toxicity of chlortetracycline as an emerging pollutant in wheat (Triticum aestivum L.), Environ. Toxicol. Chem. 29 (2010) 922–928. [5] G. Cheng, Interaction of tetracycline with aluminum and iron hydrous oxides, Environ. Sci. Technol. 39 (2005) 2660–2667. [6] R. Daghrir, P. Drogui, Tetracycline antibiotics in the environment: a review, Environ. Chem. Lett. 11 (2013) 209–227. [7] Q. Zhou, M. Zhang, C. Shuang, Z. Li, A. Li, Preparation of a novel magnetic powder resin for the rapid removal of tetracycline in the aquatic environment, Chin. Chem. Lett. 23 (2012) 745–748. [8] J.J. López Peñalver, C.V. Gómez Pacheco, M. Sánchez Polo, J. Rivera Utrilla, Degradation of tetracyclines in different water matrices by advanced oxidation/reduction processes based on gamma radiation, J. Chem. Technol. Biotechnol. 88 (2012) 1096–1108. [9] Y. Tao, W. Mai, Present status of biological chemical industry and pollution treatment, Henan Chem. Ind. 2002 (2002) 4–7. [10] A. Pena, M. Paulo, L. Silva, M. Seifrtová, C. Lino, P. Solich, Tetracycline antibiotics in hospital and municipal wastewaters: a pilot study in Portugal, Anal. Bioanal. Chem. 396 (2010) 2929–2936. [11] T. Deblonde, C. Cossu-Leguille, P. Hartemann, Emerging pollutants in wastewater: a review of the literature, Int. J. Hyg. Environ. Health 214 (2011) 442–448. [12] M.A.F. Locatelli, F.F. Sodré, W.F. Jardim, Determination of antibiotics in Brazilian surface waters using liquid chromatography-electrospray tandem mass spectrometry, Arch. Environ. Contam. Toxicol. 60 (2011) 385–393. [13] M. Liu, L. Hou, S. Yu, B. Xi, Y. Zhao, X. Xia, MCM-41 impregnated with A zeolite precursor: synthesis, characterization and tetracycline antibiotics removal from aqueous solution, Chem. Eng. J. 223 (2013) 678–687. [14] J. Radjenovic, M. Petrovic, F. Ventura, D. Barcelo, Rejection of pharmaceuticals in nanofiltration and reverse osmosis membrane drinking water treatment, Water Res. 42 (2008) 3601–3610. [15] S. Li, X. Li, D. Wang, Membrane (RO-UF) filtration for antibiotic wastewater treatment and recovery of antibiotics, Sep. Purif. Technol. 34 (2004) 109–114. [16] I. Koyuncu, O. Arikan, M. Wiesner, C. Rice, Removal of hormones and antibiotics by nanofiltration membranes, J. Membr. Sci. 309 (2008) 94–101.

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