Stepwise adsorption-oxidation removal of oxytetracycline by Zn0-CNTs-Fe3O4 from aqueous solution

Chemical Engineering Journal 375 (2019) 121963

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Stepwise adsorption-oxidation removal of oxytetracycline by Zn0-CNTsFe3O4 from aqueous solution Yunbo Liua, Ni Tana, Bingqing Wanga, Yong Liua,b, a b

T

⁎

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China Key Laboratory of Treatment for Special Wastewater of Sichuan Province Higher Education System, Sichuan, Chengdu 610066, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

adsorption-oxidation pro• Acessstepwise for the antibiotics removal was proposed.

stepwise adsorption-oxidation • The process was more efficient for OTC degradation.

The OTC removal mechanism by Zn • CNTs-Fe O was proposed. The used Zn -CNTs-Fe O could be • reused after reloading Zn . 0

3

4

0

3

4

0

A R T I C LE I N FO

A B S T R A C T

Keywords: Fenton-like Oxytetracycline Antibiotics Adsorption Oxidation Wastewater treatment

A novel strategy to remove oxytetracycline (OTC) from aqueous solution was proposed by two-step process of adsorption followed by oxidation degradation: OTC was firstly concentrated on the surface of the Zn0-CNTsFe3O4 composite by adsorption process under neutral condition. Then, the concentrated OTC on the surface of Zn0-CNTs-Fe3O4 was oxidized by Zn0-CNTs-Fe3O4/O2 Fenton-like process at pH of 3 with high solid-water ratio, which can reduce the amount of acid used for adjusting pH for Fenton reaction. Approximately 98.6% of OTC was oxidized at initial OTC concentration of 100 mg/L by the two-step process, which was higher than that by the one-step process of adsorption-oxidation at pH 3 or pH 6. The used Zn0-CNTs-Fe3O4 in two-step process could be reused after Zn0 was reloaded. The removal efficiency of OTC decreased only 6.7% after four-recycle use of the Zn0-CNTs-Fe3O4. The possible adsorption mechanism of OTC onto the Zn0-CNTs-Fe3O4 surface and the oxidation mechanism of the concentrated OTC by the Zn0-CNTs-Fe3O4/O2 process were proposed.

1. Introduction In recent years, antibiotics, as a major class of emerging contaminants, have been gained increasing concerns due to their ubiquity in water and wastewater, which can cause not only toxic risks to aquatic environments, but also the potential development of pathogenic organisms more resistant to antibacterial drugs [1,2]. Among various antibiotics, oxytetracycline (OTC), a first generation of natural ⁎

antibiotics, has received a special attention because it is widely used as an antibacterial agent and growth promoter in animal husbandry to treat many infections [3–5]. In view of its potential hazards, it is necessary to control the environmental pollution caused by OTC, especially in China where livestock and poultry farming are being developed. Advanced oxidation processes (AOPs) have been investigated for the oxidation of some antibiotics to inorganic matters by the generation of

Corresponding author at: College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China. E-mail address: [email protected] (Y. Liu).

https://doi.org/10.1016/j.cej.2019.121963 Received 1 April 2019; Received in revised form 3 June 2019; Accepted 12 June 2019 Available online 13 June 2019 1385-8947/ © 2019 Published by Elsevier B.V.

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(38–74 μm, 2.7032 g/cm3, purity > 99%) were obtained from the National Medicines Corporation Ltd., China. Ultrapure water was used in the experiments. The wastewater containing OTC was prepared by adding OTC into the secondary effluent of a municipal wastewater which was collected from a local municipal wastewater treatment plant (WWTP) (Chengdu, China). The initial OTC, TOC, COD, BOD5 and conductivity of the secondary effluent were 0 mg/L, 8.34 mg/L, 50.32 mg/L, 16.2 mg/L and 1.1 ms/cm, respectively.

highly reactive oxidant hydroxyl radicals (%OH) [6–9]. Among the AOPs, Fenton process has gained increasing interest due to its simplicity of operation and mild reaction condition [10–12]. However, the catalyst such as ferrous ions in Fenton process is difficult to recover and recycle. In order to overcome this shortcoming, Fenton-like process that uses solid iron compound as catalyst has been developed. Among the Fenton-like catalyst, Fenton-like catalysts with high performance of adsorption for organic pollutants has gained increasing interest to increase the oxidation degradation efficiency of organic pollutants [12,13]. The one-step mode, that is, the simultaneous adsorption and oxidation, was often used for the removal of organic pollutants by Fentonlike catalysts with good adsorption performance [14]. However, in the one-step mode, the condition of adsorption process and oxidation degradation process were difficult to be simultaneously optimized, which was adverse to the removal of organic pollutants. Facing this issue, a strategy of two-step mode, that is, adsorption followed by oxidation, may be efficient because the separation of adsorption process and oxidation degradation process can be performed under their respective best operating conditions. Moreover, in one-step mode, a lot of inorganic acids would be consumed because Fenton-like oxidation was usually achieved at acidic solution [15,16]. While, in two-step mode, the consumption of inorganic acids will decrease because adsorption is usually performed at neutral solution and the oxidation degradation of organic pollutants adsorbed on the Fenton catalysts requires small volume of solution. For example, a functional composite (Zn0-CNTsFe3O4) synthesized by our group was used for the oxidation degradation of antibiotics in aqueous solution [17]. The Zn0-CNTs-Fe3O4 had good adsorption performance for the refractory organic pollutants, and could mineralize refractory organic pollutants by the in situ generation of H2O2/%OH under aeration with O2. Similar to other Fenton-like oxidation system, acidic condition was beneficial for the oxidation degradation of organic pollutants by Zn0-CNTs-Fe3O4/O2 system. Based on the abovementioned assumption, the two-step mode can separate adsorption process and oxidation degradation process, and decrease the consumption of inorganic acids required for adjusting pH for Fenton reaction. During the reaction of Zn0-CNTs-Fe3O4 with O2, the Zn0 in Zn0-CNTs-Fe3O4 would be gradually consumed. However, the Zn0 can be reloaded easily on the surface of the used Zn0-CNTs-Fe3O4 by sintering process. The renewed Zn0-CNTs-Fe3O4 can be reused in the next adsorption and oxidation degradation process. Here, a conception of adsorption followed by oxidative degradation process (denoted as A-D process) for the removal of OTC was proposed. In this A-D process, OTC was firstly adsorbed on the surface of Zn0CNTs-Fe3O4, then was oxidized by the Zn0-CNTs-Fe3O4 in the presence of O2. The objectives of this paper were as follows: (1) to synthesize and characterize the Zn0-CNTs-Fe3O4; (2) to investigate the adsorption behaviors of OTC on Zn0-CNTs-Fe3O4 systematically, including the operation parameters, adsorption kinetics, adsorption isotherms, the possible adsorption mechanism and so on; (3) to discuss the oxidation degradation behaviors of the concentrated OTC on the Zn0-CNTs-Fe3O4 by the Zn0-CNTs-Fe3O4/O2 process, including the operation parameter (pH and the solid water ratio) and the possible oxidation degradation mechanism; (4) to compare two adsorption-oxidation degradation modes (one-step and two-step) for the oxidation degradation of OTC; and (5) to determine the renewability of Zn0-CNTs-Fe3O4 in the adsorption–degradation–reloading process.

2.2. Synthesis and the measurement of point of zero charge of Zn0-CNTsFe3O4 The Zn0-CNTs-Fe3O4 was synthesized by the same methods used by our previous works [17]. The multi-walled carbon nanotubes, Polyethylene glycol 4000 and the zinc metal powder were used as raw material. Compared the previous process [17], the main synthesis conditions of Zn0-CNTs-Fe3O4 were adjusted as follows: the stirring time was 5 min after the addition of CNTs without ultrasonic shock, the separation of as-prepared precipitate (CNTs-Fe3O4) from solution by a magnet and the different mass ratio of raw materials. The detailed synthesis process of Zn0-CNTs-Fe3O4 was provided in Supplementary material. In addition, the measurement of point of zero charge (PZC) throughout the pH range from 3 to 12 was also provided in Supplementary material. 2.3. Materials characterizations and chemical analyses The element composition and microscopic morphological characteristics of samples were analyzed by a Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS) (SU8010, Hitachi, Japan). Pore distribution, pore volume and specific surface area parameters of the samples were measured with N2 adsorptiondesorption tests by a BET analysis (Quantachrome, US). The crystal structures of Zn0-CNTs-Fe3O4 were characterized by an X-ray diffraction (Bruke D8 Adv., Germany). The Fourier transform infrared spectroscopy (FTIR) spectra of the samples were recorded by a VERTEX 70 FT-IR Spectrometer and the spectra were scanned from 400 to 4000 cm−1 with a resolution of 4 cm−1. The magnetization of the samples were measured by vibrating sample magnetometer (VSMVersalab, Qutumn Desig, USA). The concentration of H2O2 was determined by UV–vis spectrophotography using potassium titanium oxalate as a color reagent at the wavelength of 400 nm. The concentration of ferrous ions was determined by 1, 10-phenanthroline spectrophotometric method at the wavelength of 510 nm. The concentration of OTC was analyzed by an Agilent 1290 Ultra-Performance Liquid Chromatography (UPLC), which equipped with acetonitrile/0.1% formic acid solution (50/50, v/v) as the mobile phase at a flow rate of 0.3 mL/min, the column temperature and the analytical wavelength parameter were set as 308 K and 355 nm, respectively [18]. The concentration of zinc ions in solution was determined by inductively coupled plasma optical emission spectrometer (ICP-OES) with axial configuration (model Optima 8000, PerkinElmer, America). 2.4. Batch adsorption experiments Batch adsorption experiments were carried out to study the adsorption performance of Zn0-CNTs-Fe3O4 for the removal of OTC. The batch experiments were performed in stoppered bottle conical flasks with different kinds of OTC solutions and under N2 atmosphere. After the quick addition of required dosages of Zn0-CNTs-Fe3O4 into OTC solutions, the flasks were placed into a constant temperature oscillator of 170 rpm. After a preset interval, a certain amount of sample was taken and filtered by 0.22 µm cellulose membrane. The residual concentration of OTC was then determined. The influence of initial pH on adsorption efficiency of OTC was

2. Materials and methods 2.1. Chemicals and materials OTC was obtained from the Damas-beta company (Chengdu, China, purity > 99%). Hydroxylated multi-walled carbon nanotubes (CNTs) was purchased from Timesnano (Chengdu, China, 10–30 μm). Polyethylene glycol (PEG) 4000 (purity > 99%) and zinc powder 2

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investigated at the Zn0-CNTs-Fe3O4 dosage of 2 g/L and the initial OTC concentration of 100 mg/L. The initial solution pH was adjusted with HCl (0.1 mol/L) and NaOH (0.1 mol/L) solutions. The effect of the presence of NaCl on OTC adsorption by Zn0-CNTs-Fe3O4 was investigated at the Zn0-CNTs-Fe3O4 dosage of 2 g/L and the initial OTC concentration from 50 mg/L to 150 mg/L. Different concentrations of NaCl (0.2–6.0 g/L) was used in the adsorption experiment. To investigate the influence of wastewater composition on the OTC adsorption, the wastewater containing 100 mg/L OTC was used in adsorption experiment at the Zn0-CNTs-Fe3O4 dosage of 2 g/L.

E=

Adsorption kinetics experiments were conducted without pH adjustment, at the adsorbent dosage varying from 1 to 3 g/L, the initial OTC concentration of 100 mg/L and the temperature of 298 K. The amount of adsorption at a certain time can be defined as qt (mg/ g) using Eq. (1):

(3)

qt = kd t 1/2 + d

(4)

1000 b⎞ K 0 = 55.5 ⎛ ⎝ 460.58 ⎠

(11) (12)

where ΔG is the Gibbs free energy change (kJ/mol), ΔH is enthalpy change (kJ/mol), ΔS0 is entropy change (kJ/(mol K)), R is the universal gas constant (8.314 J/mol·K), T is absolute temperature (K), K0 is a dimensionless associated with b (Langmuir constant (L/mg)), 55.5 is the number of moles of water per liter of solution (mol/L), 460.58 is the molecular weight of OTC [21]. 0

2.7. Oxidation degradation of OTC 2.7.1. Oxidation degradation of OTC adsorbed on the surface of Zn0-CNTsFe3O4 The adsorption of OTC onto the surface of Zn0-CNTs-Fe3O4 was performed at 298 K and 300 r/min by adding 3 g of the Zn0-CNTs-Fe3O4 into 1500 mL solution with initial OTC concentration of 100 mg/L. After 120 min of the adsorption, the solid was separated from the solution by a magnet and then dried in a vacuum drying equipment at 333 K for 24 h. The mass loss of six as-prepared samples of Zn0-CNTsFe3O4 with the mass of 0.2 g was less than 0.2 mg. The as-prepared sample was put into the aqueous solution and then O2 was aerated into solution for oxidation reaction. After solid was separated from the solution, the residual mass of OTC in solution was detected and denoted as OTC in solution. The residual OTC adsorbed on the surface of Zn0CNTs-Fe3O4 was washed out by NaOH solution because NaOH was effective in desorption of OTC from adsorbent [22]. And then OTC in eluent was detected and marked as OTC on the solid. The effect of solution pH and the ratio of solid to liquid, i.e., the mass of Zn0-CNTsFe3O4 to the volume of water on the oxidation degradation of OTC were investigated. The detailed experiments were provided in Supplementary material.

(2)

1 1 1 1 = · + qt q2 k2 q22 t

(10)

0

where c0 and ct are initial and remaining concentrations of OTC (mg/L), respectively. M is the mass of adsorbent (g), and V is the volume of the solution (L). The pseudo-first-order, pseudo-second-order kinetic and intra-particle diffusion models were used to analyze the data acquired from adsorption kinetics experiments, as given in Eqs. (2), (3) and (4), respectively.

k1 t 2.303

ΔG 0 = −RT ln K0

ΔG 0 = ΔH 0 - T ΔS 0

(1)

log (qe − qt ) = log qe -

(9)

where Qmax represents the maximum adsorption capacity (mg/g), the b is the Langmuir constant associated with the energy of adsorption (L/ mg), the KF is the Freundlich constant which relates to adsorption capacity of adsorbent, n represents the Freundlich constant which indicates the degree of adsorption feasibility, β is a constant related to the adsorption energy (mol2/kJ2); and ε is the adsorption potential. E is the free energy of adsorption (kJ/mol). The thermodynamic parameters of the adsorption can be given by the following equations [20]:

2.5. Adsorption kinetics experiments

(c - c ) × V qt = 0 t M

1 (2β )1/2

where qe is the amount of OTC adsorbed at equilibrium, which is calculated using Eq. (1) by replacing qt and ct with qe and ce (ce is the concentrations of OTC at equilibrium (mg/L), respectively, k1 (min−1) is the rate constant for the pseudo-first-order model, q2 (mg/g) and k2 (g/(mg min)) are the equilibrium adsorption capacity and rate constant of the pseudo-second-order model, respectively. kd is the rate constant of intra-particle diffusion (mg/(g·min1/2)) and d is the intercept that is correlated to thickness of the boundary layer. 2.6. Adsorption isotherms experiments The experiments of adsorption isotherms were conducted without pH adjustment, at initial OTC concentrations varying from 10 to 150 mg/L and the Zn0-CNTs-Fe3O4 dosage of 2 g/L. In order to acquire the thermodynamic parameters of the adsorption, the adsorption experiments were conducted at 298 K, 308 K, 318 K and 328 K, respectively. The Langmuir isotherm model, the Freundlich isotherm model and Dubinin-Radushkevich (D-R model) [19] were used to analyze the equilibrium data obtained from the experiments of adsorption isotherms, as given in Eqs. (5)–(9), respectively.

(6)

2.7.2. Control experiments for the OTC degradation in Zn0-CNTs-Fe3O4/O2 system In order to investigate the main oxidation species for the degradation of OTC in Zn0-CNTs-Fe3O4/O2 system, the control experiments were conducted at pH of 3, 0.2 g Zn0-CNTs-Fe3O4 or Zn0-CNTs was added into 100 mL solution containing initial OTC concentration of 100 mg/L and O2 was aerated into solution with the flow rate of 200 mL/min for 60 min, respectively. The residual concentration of OTC and H2O2 were determined, respectively. In order to investigate the contribution of %OH radical and O2%− radical for OTC degradation, tert-butyl alcohol (TBA) and p-benzoquinone (BQ) were added into the Zn0-CNTs-Fe3O4/O2 system because TBA and BQ were often used as a quencher for %OH radical and O2%− radical [23]. The degradation efficiency (de) of OTC could be calculated by Eq. (13):

ln qe = ln Qmax − βε 2

(7)

de =

ε = RT ln(1 + (1/ ce ))

(8)

where mt, mi and mo were the total mass of OTC, OTC in solution and

ce 1 ce = + qe Qmax b Qmax logqe = log KF +

1 log ce n

(5)

3

mt - m i - m o mt

(13)

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fresh Zn0-CNTs-Fe3O4 and renewed Zn0-CNTs-Fe3O4 had microporous/ mesoporous structure [24]. The average pore sizes of the fresh Zn0CNTs-Fe3O4 and renewed Zn0-CNTs-Fe3O4 were 30.21 nm and 31.79 nm, respectively. There was no obvious difference in average pore size between the fresh Zn0-CNTs-Fe3O4 and renewed Zn0-CNTsFe3O4, indicating that the structure of Zn0-CNTs-Fe3O4 could be recovered after the used Zn0-CNTs-Fe3O4 was reloaded by Zn0. The BET specific surface area of the renewed Zn0-CNTs-Fe3O4 (75.13 m2/g) was slightly lower than that of the fresh Zn0-CNTs-Fe3O4 (89.39 m2/g), which might be caused by the residual Zn (II) on the used Zn0-CNTsFe3O4 after A-D process, which was also loaded on the renewed Zn0CNTs-Fe3O4 during the course of reloading Zn0. This could be confirmed by the aforesaid EDS results that the content of Zn atom in renewed Zn0-CNTs-Fe3O4 was lower than that in fresh Zn0-CNTs-Fe3O4. Fig. 2c shows the XRD patterns of the fresh Zn0-CNTs-Fe3O4, used Zn0-CNTs-Fe3O4 after A process, the used Zn0-CNTs-Fe3O4 after A-D process and the renewed Zn0-CNTs-Fe3O4. It can be seen that the characteristic diffraction peaks of Zn0 (PDF #04-0831), ZnO (PDF #702551), CNTs (PDF #26-1083) and Fe3O4 (PDF #89-3854) all appeared in XRD spectrum of the fresh Zn0-CNTs-Fe3O4. The weak characteristic diffraction peaks of Fe3O4 might be resulted from the low content of Fe3O4 or the cover of Fe3O4 by Zn0 or CNTs in Zn0-CNTs-Fe3O4. These results suggested that zinc and Fe3O4 were successfully loaded on the surface of CNTs. The peak intensity of Zn0, ZnO, CNTs and Fe3O4 of the used Zn0-CNTs-Fe3O4 were similar to that of the fresh Zn0-CNTs-Fe3O4, suggesting that the adsorption process did not damage the crystal structure of substances in Zn0-CNTs-Fe3O4. Compared with the fresh Zn0-CNTs-Fe3O4, the peak intensity of ZnO, CNTs and Fe3O4 did not change in the used Zn0-CNTs-Fe3O4 after A-D process and the peak intensity of Zn0 was vanished. This result indicated that Zn0 in Zn0CNTs-Fe3O4 was consumed during the oxidation process, while the crystal structure of substances in Zn0-CNTs-Fe3O4 could be recovered by reloading Zn0, which were consistent with the EDX analysis. The room temperature magnetization curves of fresh Zn0-CNTsFe3O4 and used Zn0-CNTs-Fe3O4 after A-D process were shown in Fig. 2d. Almost no magnetic hysteresis loop was observed, suggesting the superparamagnetic properties of the Zn0-CNTs-Fe3O4. The saturation magnetization (Ms) values of used Zn0-CNTs-Fe3O4 (53.18 emu/g) was slightly higher than that of fresh Zn0-CNTs-Fe3O4 (45.27 emu/g), which might be owing to the increase of relative content of Fe3O4 in Zn0-CNTs-Fe3O4 caused by the dissolution of Zn0 during the A-D process. The good magnetic performance indicated that fresh Zn0-CNTsFe3O4 and used Zn0-CNTs-Fe3O4 could be easily separated and recovered from solution.

OTC on the solid, respectively. 2.8. Comparison test of two processes for the degradation of OTC When Zn0-CNTs-Fe3O4 was added into the solution containing OTC and O2, the simultaneous adsorption and degradation of OTC would occur, which was marked as SAD process. Specifically, 0.3 g Zn0-CNTsFe3O4 and O2 with the flow rate of 200 mL/min were added into 150 mL solution with 100 mg/L initial OTC concentration and pH 3 or pH 6. After reaction for 60 min, the mass of OTC in solution and on the solid were determined, respectively. During the A-D process, 0.3 g Zn0-CNTs-Fe3O4 was firstly added into 150 mL solution with 100 mg/L initial OTC concentration and pH 6 under N2 atmosphere. After adsorption for 60 min followed solid–liquid separation, the obtained Zn0-CNTs-Fe3O4 and O2 with a flow rate of 200 mL/min were added into solution at pH 3 with the solid to liquid ratio of 20 g/L. After 30 min, the mass of OTC in solution and on the solid were determined, respectively. 2.9. Reuse of Zn0-CNTs-Fe3O4 for the adsorptive-oxidative removal of OTC In this work, there were three processes: (1) the OTC in solution was adsorbed by Zn0-CNTs-Fe3O4 at the Zn0-CNTs-Fe3O4 dosage of 2 g/L, initial OTC concentration of 100 mg/L, initial pH of 6 and the temperature of 298 K for 60 min, which denoted as A process; (2) the used Zn0-CNTs-Fe3O4 after A process followed by reaction with O2 at the pH of 3 for 30 min and then was reused to adsorb OTC at the same condition of A process, which denoted as A-D-A process; (3) the used Zn0CNTs-Fe3O4 after A-D-A process followed by reaction with O2 at pH of 3 for 30 min, was reused to adsorb OTC at the same condition of A process, which denoted as A-D-A-D-A process. The reloading of Zn0 on the used Zn0-CNTs-Fe3O4 after A-D process was performed by following method: the used Zn0-CNTs-Fe3O4 after AD process, zinc power, and PEG were mixed with the mass ratio of 3:4:5 for 10 min. Then, the mixture was put into a tube furnace under Ar atmosphere at 773 K for 60 min. The process after A-D process followed reloading Zn0 was denoted as A-D-R process. 3. Results and discussion 3.1. Characterization of Zn0-CNTs-Fe3O4 3.1.1. SEM and EDS The typical morphology of the fresh, used and renewed Zn0-CNTsFe3O4 are shown in Fig. 1a. Porous structure and some covering on the surface of CNTs were observed in fresh Zn0-CNTs-Fe3O4. Some CNTs were exposed on the surface of the used Zn0-CNTs-Fe3O4, which might be owing to the dissolving of metal in Zn0-CNTs-Fe3O4 during the reaction process. Zn0 covering on the surface of CNTs in renewed Zn0CNTs-Fe3O4 indicated that Zn0 was successfully reloaded on the used Zn0-CNTs-Fe3O4. The EDS spectra of the fresh, used and renewed Zn0-CNTs-Fe3O4 are recorded to confirm the relative content of element. It can be seen from Fig. 1b that there were 12.94%, 0.00% and 15.22% of Zn atom in the fresh, used and renewed Zn0-CNTs-Fe3O4, respectively, indicating the consumption of Zn0 during A-D process and the successful reloading of Zn0 on the used Zn0-CNTs-Fe3O4, which were consistent with the SEM results. The relative content of Fe atom in three samples had no obviously different, suggesting the little loss of Fe during the A-D-R process.

3.1.3. FTIR spectra To further understand the adsorption behavior of OTC by Zn0-CNTsFe3O4, the FTIR spectra of OTC and Zn0-CNTs-Fe3O4 before and after adsorption are exhibited in Fig. 3, respectively. It can be seen that OTC had characteristic bands between 1100 and 1700 cm−1 region, which was consistent with literature report [25]. In detail, the peaks at 1621 cm−1 and 1590 cm−1 were attributed to C]O group and amideNH, respectively [26], while the peaks at 1456 cm−1, 1243 cm−1 and 1118 cm−1 were attributed to skeletal vibration of C]C, CeN, and CeO, respectively. Moreover, there was an asymmetrical stretching vibration of eNH2 at 3484 cm−1, For the spectra of Zn0-CNTs-Fe3O4, the peak at 1632 cm−1 was attributed to sp2-hybridized carbon, which was a characteristic peak of carbon nanotube, a broadband peak in 561 cm−1, which proved the presence of Fe-O group and a large broad band at 3370 cm−1 was attributed to the presence of O–H stretching frequency of hydroxyl groups and the remaining adsorbed water [27]. On the other hand, the appearance of peaks at 1456 cm−1, 1243 cm−1, 1118 cm−1 and 1590 cm−1 in the FTIR spectra of the used Zn0-CNTsFe3O4 sample, indicated the adsorption of OTC on Zn0-CNTs-Fe3O4. Besides, the peak at 1522 cm−1 assigning to amino groups of the amide disappeared after adsorption, which demonstrated that eNH2 of OTC

3.1.2. BET and XRD The N2 adsorption–desorption isotherms and the BJH adsorption pore size distribution of the fresh Zn0-CNTs-Fe3O4 and renewed Zn0CNTs-Fe3O4 are presented in Fig. 2a and b. The isotherms were identified as type IV with a clear H3 hysteresis loop, suggesting that the 4

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Fig. 1. SEM imagines and EDX spectra of Zn0-CNTs-Fe3O4: (a1), (a2) and (a3) are imagines of fresh Zn0-CNTs-Fe3O4, used Zn0-CNTs-Fe3O4 after A-D process and renewed Zn0-CNTs-Fe3O4, respectively; (b1), (b2) and (b3) are EDX spectra of fresh Zn0-CNTs-Fe3O4, used Zn0-CNTs-Fe3O4 after A-D process and renewed Zn0-CNTsFe3O4, respectively.

might have reacted with metal in the Zn0-CNTs-Fe3O4 [26]. The disappearance of the peaks at 1632 cm−1 in Zn0-CNTs-Fe3O4 indicated that CNTs in Zn0-CNTs-Fe3O4 was an active site for the adsorption of OTC. The peak of eNH2 at 3484 cm−1 disappeared and a blue shift of eOH at 3370 cm−1 appeared after adsorption, indicating that OTC had reacted with hydrogen atom in the Zn0-CNTs-Fe3O4 to form hydrogen bond.

and the dissolution of metal in Zn0-CNTs-Fe3O4 since the zinc ion concentration in solution at pH 1 and 2 was 119.12 mg/L and 49.65 mg/L, respectively, while very low concentration of zinc ions was found at pH 3. Fig. 4c shows that adsorption efficiency did not decrease with the increase of the concentration of NaCl, suggesting that the Zn0-CNTsFe3O4 for the adsorption of OTC had good resistance against monovalent ions, which also indicated that surface interactions in the form of outer sphere complexes might be a contribution to the adsorption of OTC [29]. The adsorption removal of OTC in WWTP effluent by Zn0-CNTsFe3O4 is illustrated in Fig. 4d. It can be seen that as for the same initial OTC concentration (50–150 mg/L), the adsorption efficiency of OTC in WWTP effluent at 60 min (99.37–76.24%) was not obviously different from that in aqueous solution (98.21–79.59%), which was shown in Fig. 4c, indicating that Zn0-CNTs-Fe3O4 had high selectivity for the adsorption removal of OTC, which was beneficial to its application for the removal of OTC in wastewater.

3.2. The adsorption removal of OTC by Zn0-CNTs-Fe3O4 3.2.1. Influencing factor for the adsorption removal of OTC The effect of initial pH on adsorption removal of OTC is shown in Fig. 4a. The high adsorption efficiency could be gained in a wide pH range from 2 to 8, which is favor to its practical application because the pH range of wastewater is usually between 6 and 9. The wide operating pH range suggested that Zn0-CNTs-Fe3O4 was a OTC adsorbent with strong competitive advantage compared with other reported adsorbents for OTC adsorption, such as boron nitride (BN) and the cotton linter fibers activated carbon (CLAC) bundles, their high adsorption efficiency was achieved at pH of 3.0–3.5 and 3.0–5.5, respectively [24,28]. The point of zero charge (PZC) of Zn0-CNTs-Fe3O4 was determined to be 7.5, where the difference between the equilibrium pH and the initial pH was equal or near to 0 (Fig. 4b) [24]. This indicated that the Zn0-CNTsFe3O4 was positively charged at pH < 7.5, while negatively charged at pH > 7.5. There were three forms of OTC in solution at different pH owing to the acid dissociation constants (pKa = 3.57, 7.49 and 9.44) of OTC molecule [26]. Cationic form of OTC was primarily present at pH < 3.57, zwitterion was the major form at pH between 3.57 and 7.49, and deprotonation species was existed predominantly at pH > 7.49. The maximum adsorption efficiency of OTC (99.1%) was obtained at initial pH of 2, which was lower than that at PZC of 7.5, suggesting that electrostatic interaction was not mainly responsible for the OTC removal by Zn0-CNTs-Fe3O4. The similar result was reported [24]. The low adsorption efficiency of OTC at pH > 8 might be owing to the decreased affinity of OTC for the adsorbent surface and the increased electrostatic repulsion between the negatively charged Zn0CNTs-Fe3O4 and the dissociated OTC. The low adsorption efficiency of OTC at pH < 2 might be due to the competition adsorption of proton

3.2.2. Isotherm model of OTC adsorption Adsorption isotherms plotted as adsorbed content versus aqueousphase concentration at equilibrium are shown in Fig. 5a. The Langmuir, Freundlich isotherm and Dubinin-Radushkevich models (D-R model) were used to analyze the adsorption experimental data, and the obtained model parameters are listed in Table 1. Higher correlation coefficients (R2 > 0.99) and better fitting with the Langmuir model were observed, which indicated that monolayer adsorption was involved in the OTC adsorption removal process by Zn0-CNTs-Fe3O4 [30]. The maximum adsorption capacity of OTC was calculated by the Langmuir model to be 68.39 mg/g at temperature of 298 K, which was higher than many adsorbents in reported literatures. For example, the OTC adsorption capacity was 67.00 mg/g for Bi2O3/BiOCl supported on graphene sand composite, 65.53 mg/g for polyaniline coated peanut shells, respectively [31,32]. For the parameter of D-R model, the value of E represented different adsorption mechanisms that might occur in the adsorption process. The adsorption process was governed by physical adsorption in case of E < 8 kJ/mol and chemical adsorption in case of E between 8 kJ/mol to 16 kJ/mol. As can be seen from the data 5

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Fig. 2. (a) pore size distributions of (a1) fresh Zn0-CNTs-Fe3O4 and (a2) renewed Zn0-CNTs-Fe3O4; (b) N2 adsorption/desorption isotherms of (b1) fresh Zn0-CNTsFe3O4 and (b2) renewed Zn0-CNTs-Fe3O4; (c) XRD patterns of (c1) fresh Zn0-CNTs-Fe3O4, (c2) used Zn0-CNTs-Fe3O4 after A process, (c3) used Zn0-CNTs-Fe3O4 after A-D process and (c4) renewed Zn0-CNTs-Fe3O4; (d) magnetic hysteresis loops of fresh Zn0-CNTs-Fe3O4 (d1) and used Zn0-CNTs-Fe3O4 (d2).

6

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60 min on Zn0-CNTs (48.38 mg/g) and on CNTs-Fe3O4 (63.92 mg/g) were observed from Fig. 5b. The qe of OTC by Zn0-CNTs-Fe3O4 (61.15 mg/g) was slightly higher than that by Zn0-CNTs and close to that by CNTs-Fe3O4, indicating that CNTs-Fe3O4 component in Zn0CNTs-Fe3O4 composite played a major role in OTC adsorption. The thermodynamic parameters for the adsorption of OTC on Zn0CNTs-Fe3O4 calculated by Eqs. (6)–(8) are illustrated in Table 2. Negative values of ΔG0 indicated that the process was spontaneous and feasible. The obtained positive values of ΔH0 implied that the adsorption process of OTC on Zn0-CNTs-Fe3O4 was endothermic, which was also supported by the increasing adsorption capacity of OTC with the increase of temperature. 3.2.3. Adsorption kinetics The effect of contact time on OTC removal efficiency (Fig. 6a) by Zn0-CNTs-Fe3O4 with different dosages were also investigated. As shown in Fig. 6a, rapid initial adsorption of OTC was observed within 15 min, suggesting that a fast transfer into the near surface boundary layers of Zn0-CNTs-Fe3O4. With the increase of contact time, the adsorption rate decreased and gradually achieved equilibrium within about 45 min. The kinetic adsorption data were fitted to pseudo-firstorder kinetic model and pseudo-second-order model, and the results are shown in Fig. 6b, c and Table 3, The adsorption of OTC on Zn0-CNTsFe3O4 was described well by pseudo-second-order kinetic model, with high correlation coefficient (R2 > 0.998), indicating that the adsorption rate-limiting step might be the chemisorption [33]. Moreover, the intra-particle diffusion model was used to further study the rate-limiting step and the qt was plotted versus the square root of time, t1/2 (Fig. 6d). It can be observed that all plots showed the initial linear portion and a curved portion and the linear portions on all plots did not pass through the origin, indicating that the intra-particle

Fig. 3. FTIR spectra of OTC and Zn0-CNTs-Fe3O4 before and after adsorption: OTC (a), Zn0-CNTs-Fe3O4 (b) and Zn0-CNTs-Fe3O4 after A process (c).

of OTC by Zn0-CNTs-Fe3O4 that the E ranged from 15.107 kJ/mol to 16.162 kJ/mol when temperature ranged from 298 K to 328 K, suggesting that chemical adsorption played a major role for the adsorption of OTC by Zn0-CNTs-Fe3O4. Moreover, the qe of OTC at 298 K and

Fig. 4. (a) Effect of initial pH; (b) PZC analysis of Zn0-CNTs-Fe3O4; (c) Effect of NaCl concentration and (d) adsorption removal in WWTP effluent. (In the test of (a), initial OTC concentration = 100 mg/L; The initial pH was not adjusted during the test of (c) and (d), adsorbent dosage = 2 g/L, reaction temperature = 298 K.) 7

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Fig. 5. (a) Adsorption isotherms of OTC on Zn0-CNTs-Fe3O4 at different temperatures; (b) Adsorption isotherms of OTC on two different adsorbents (adsorbent dosage = 2 g/L, initial OTC concentration = 100 mg/L, contact time = 60 min, pH was not adjusted).

concentration of reactant in solution. However, when the ratio of solid to water was over 20 g/L, the limited increase of degradation efficiency was observed and the less solution volume would decrease the amount of oxygen transferred into solution. Therefore, the ratio of solid to water of 20 g/L was chosen for the oxidation degradation of OTC.

Table 1 Parameters of the Langmuir, Freundlich and D-R adsorption isotherm models. Isotherm

Langmuir

T (K)

b (L/mg) Qmax (mg/g) R2

Freundlich

KF (mg/g) (L/mg) n R2

D-R model

Qmax (mg/g) E (kJ/mol) R2

1/n

298

308

318

328

0.478 68.385 0.991

0.596 70.477 0.994

0.327 76.936 0.996

0.461 80.323 0.990

24.989 2.589 0.897

28.025 3.141 0.856

23.776 2.366 0.912

28.062 2.318 0.892

50.296 15.107 0.961

53.106 16.052 0.953

53.138 15.839 0.988

52.721 16.162 0.968

3.3.2. The oxidation removal of OTC in solution by different process In order to evaluate the contribution of main oxidation species to the oxidation removal of OTC in Zn0-CNTs-Fe3O4/O2 system, the oxidation removal of OTC in solution by Zn0-CNTs-Fe3O4/O2, Zn0-CNTsFe3O4/O2/TBA, Zn0-CNTs-Fe3O4/O2/BQ, and Zn0-CNTs/O2 system was investigated, respectively. Fig. 8a shows that almost no degradation of OTC happened in Zn0-CNTs-Fe3O4/O2/TBA system, while 96.56% and 92.53% degradation efficiency of OTC at 60 min were obtained in Zn0CNTs-Fe3O4/O2 system and Zn0-CNTs-Fe3O4/O2/BQ, respectively, suggesting that %OH radical was the main oxidation species for the oxidation removal of OTC in Zn0-CNTs-Fe3O4/O2 system, however, the effect of superoxide radicals in the system cannot be ignored. Fig. 8b shows that the accumulated concentration of H2O2 could reach 163.22 mg/L at 60 min in Zn0-CNTs/O2 system, while 17.67 mg/L at 60 min in Zn0-CNTs-Fe3O4/O2 system. However, compared with Zn0CNTs-Fe3O4/O2 system, OTC was almost not removed by oxidation removal in Zn0-CNTs/O2 system. Moreover, there were almost no oxidation degradation of OTC and the in situ generation of H2O2 in CNTsFe3O4/O2 system, suggesting that (1) in Zn0-CNTs-Fe3O4/O2 system, the reaction of Zn0-CNTs in Zn0-CNTs-Fe3O4 with O2 could generate a large amount of H2O2 [36,37], which could not oxidize OTC; (2) Zn0 in Zn0CNTs composite play a key role for the generation of H2O2; (3) the Fe3O4 in Zn0-CNTs-Fe3O4 had negligible role to produce H2O2 but played a role for the catalytic decomposition H2O2 to form oxidation species such as %OH radical and O2%− radical, which could oxidize OTC quickly due to its strong oxidizing ability. In order to determine the role of Fe3O4 in Zn0-CNTs-Fe3O4, the concentration of Fe2+ in Zn0-CNTs-Fe3O4/O2/OTC system at pH 3 after 30 min was detected, and the results showed that Fe2+ concentration in pH 3 solution was 0.0471 mg/L. Then the oxidation degradation of OTC in Fe2+/H2O2 system at pH 3, initial Fe2+ concentration of 0.05 mg/L, initial H2O2 concentration of 164 mg/L was investigated. It was found that only 0.95% OTC was removed and the residual H2O2 concentration of 141 mg/L was observed after 30 min, indicating that Fe2+ in solution had a little contribution to the catalytic decomposition of H2O2. Therefore, it could be concluded that the catalytic decomposition H2O2 into %OH radical was mainly contributed by Fe3O4 in Zn0-CNTs-Fe3O4.

Table 2 Thermodynamic parameters for the adsorption of OTC on Zn0-CNTs-Fe3O4. ΔH0 (J/(mol K))

23.128

ΔS0 (J/(mol K))

0.039

ΔG0 (KJ/mol) 298 K

308 K

318 K

328 K

−11.871

−10.994

−9.713

−10.985

diffusion was not the only rate-determining step and the reaction rate was controlled jointly by surface adsorption and liquid film diffusion. 3.3. The oxidation removal of OTC adsorbed on the Zn0-CNTs-Fe3O4 3.3.1. Influence of pH and the ratio of solid to water on the oxidation removal of OTC As can be seen in Fig. 7a, when the saturated Zn0-CNTs-Fe3O4 reacted with O2 in solution at different pH, some of OTC on Zn0-CNTsFe3O4 was desorbed into solution. Moreover, the higher pH of the solution was, the more OTC will be desorbed into solution, because OH− would weaken the interaction force between OTC and Zn0-CNTs-Fe3O4. It can also be seen that the degradation efficiency of OTC decreased with the increase of pH, which might be owing to that the loss of catalytic activity of Fe3O4 [34] and the insufficient H+ [35] at high pH solution decreased the continuous production of %OH/H2O2 in Zn0CNTs-Fe3O4/O2 system. The highest degradation efficiency of OTC (95.43%) was gained at pH 3. During the oxidation degradation of OTC by Zn0-CNTs-Fe3O4/O2 system, the ratio of solid to water would affect the degradation efficiency of OTC. Fig. 7b shows that high ratio of solid to water favored the oxidation degradation efficiency of OTC owing to the high 8

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Fig. 6. (a) Effect of contact time on OTC removal efficiency; (b) Pseudo first-order model; (c) Pseudo second-order model; (d) Plots of qt vs. t1/2 for intra-particle diffusion of kinetics investigation (initial OTC concentration = 100 mg/L, reaction temperature = 298 K).

3.4. Comparison of two modes of adsorption-oxidative degradation

Table 3 Parameters of three kinds of kinetic models. Absorbent dosage (g/L)

1 2 3

Pseudo first-order equation

Pseudo second-order equation

There might be two simultaneous processes: adsorption and oxidative degradation of OTC, i.e., SAD process when Zn0-CNTs-Fe3O4 was added into the solution containing OTC with O2 aeration. The oxidative degradation efficiency of OTC in the process of first adsorption followed oxidation degradation, i.e., A-D process, might be different from the SAD process. Fig. 9a shows that the mass summation of OTC in solution and on the solid was almost equal to the total mass of OTC before reaction during the SAD process at initial pH of 6, indicating that OTC was hardly degraded by SAD process at initial pH of 6, but 75.21% of the adsorption efficiency of OTC was obtained. The oxidative degradation efficiency (98.62%) by the A-D process was higher than that

intra-particle diffusion model

k1 (min−1)

R2

k2 (g/ (mg.min))

R2

Kd (mg/ (g·min1/ 2 ))

d

1.527 1.540 1.932

0.873 0.937 0.959

0.076 0.058 0.059

0.994 0.998 0.997

8.153 9.555 9.625

28.435 34.351 39.024

Fig. 7. (a) The effect of pH on OTC degradation. T = 298 K, aeration time = 30 min, ratio of solid to water = 5 g/L; (b) the effect of ratio of solid to water on OTC degradation. T = 298 K, aeration time = 30 min, reaction pH = 3. 9

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Fig. 8. (a) The degradation of OTC in different systems (solid dosage = 2 g/L, initial OTC concentration = 100 mg/L, T = 298 K, aeration time = 60 min, solution pH = 3); (b) the accumulated concentrations of H2O2 in different systems (solid dosage = 2 g/L, T = 298 K, aeration time = 60 min, solution pH = 3).

process). A reasonable explanation might be that Zn0 in Zn0-CNTsFe3O4 was exhausted in the step of aeration during A-D-A process, which decreased the production of H2O2 required for the oxidative degradation of OTC by Fenton reaction. Therefore, it was necessary to reload Zn0 in the used Zn0-CNTs-Fe3O4 during the course of cyclic utilization of Zn0-CNTs-Fe3O4. The cyclic utilization of Zn0-CNTs-Fe3O4 in the process of adsorption, oxidative degradation and reloading by Zn0 (A-D-R process) was studied. Fig. 10b shows that the adsorption efficiency only reduced by 6.26% after four cyclic utilization of Zn0-CNTs-Fe3O4 by A-D-R process, suggesting that Zn0-CNTs-Fe3O4 had obvious advantage for the adsorptive-oxidative removal of OTC by the A-D-R process. The adsorption efficiency decreased slightly when the cyclic number increased, probably because the residual Zn(II) might increase with the increase of cyclic number, which would decrease the specific surface area. The BET specific surface area of the renewed Zn0-CNTs-Fe3O4 was slightly lower than that of the fresh Zn0-CNTs-Fe3O4, which could be a proof for this explanation.

by the SAD process at pH of 3 (93.66%). Besides, the oxidative degradation of OTC by SAD process at pH of 3 needed the addition of acid to adjust pH before reaction. To the contrary, during the oxidative degradation of OTC by A-D process, the volume of solution needed to adjust pH was only ten percent of that by SAD process and the higher oxidative degradation efficiency could be obtained owing to the higher ratio of solid to water. Therefore, A-D process was potential for the oxidative degradation of OTC in wastewater. The degradation experiments of OTC in WWTP effluent by A-D process and the SAD process were carried out at the same conditions as that of the OTC in aqueous solution. As can be seen from Fig. 9b, the Zn0-CNTs-Fe3O4/O2 process was very effective in removing OTC from WWTP effluent. The oxidative degradation efficiency of OTC in WWTP effluent by A-D process was also higher than that of the SAD process. 3.5. The renewability and reuse of Zn0-CNTs-Fe3O4 Zn0-CNTs-Fe3O4 played a key role for the removal of OTC in the A-D process. If Zn0-CNTs-Fe3O4 could be cyclically used for A-D process, the resource would be saved. The direct cyclic utilization of Zn0-CNTsFe3O4 after A-D process was investigated. Fig. 10a shows that the adsorption efficiency of OTC decreased only 3.82% in the first direct cyclic utilization of the used Zn0-CNTs-Fe3O4 after A-D process (A-D-A process). This might be owing to the oxidative degradation of OTC in the step of aeration, which made many adsorption sites be exposed. However, there was almost no adsorption efficiency of OTC in the second direct cyclic utilization of the used Zn0-CNTs-Fe3O4 (A-D-A-D-A

3.6. Proposed mechanism 3.6.1. The adsorption of OTC on Zn0-CNTs-Fe3O4 The results of adsorption isotherm suggested that the Zn0-CNTsFe3O4 had very strong adsorption affinity to OTC. The XRD and EDX results confirmed that abundant active sites of CNTs, Fe and Zn0 were existed in the Zn0-CNTs-Fe3O4. The CNTs could combine with the aromatic rings and the unshared pair of electrons of nitrogen and

Fig. 9. (a) The degradation removal of OTC by different adsorptive-oxidative processes (initial OTC concentration = 100 mg/L, T = 298 K, aeration time of SAD process = 60 min); (b) The degradation removal of OTC in WWTP effluent by different adsorption-oxidation processes of Zn0-CNTs-Fe3O4. 10

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Fig. 10. The adsorption removal of OTC in different processes of cyclic utilization of Zn0-CNTs-Fe3O4. Initial OTC concentration = 100 mg/L, T = 298 K, contact time = 60 min, aeration time = 30 min.

oxygen (e.g., amino group, hydroxyl group) in OTC molecule by π-π electron-donor-acceptor interactions and n-π electron-donor-acceptor interactions, respectively [38]. The CNTs could also combine OTC by hydrophobic interaction because both CNTs and OTC had some hydrophobicity. The active Fe and Zn0 might react with OTC to form surface complex owing to their strong affinity to electron-donating groups (e.g., amino group, hydroxyl group), which could be proved by the FTIR analysis. The active Fe and Zn0 might also combine OTC by Lewis acid-base interaction because it was an important extra interaction for the adsorption of molecules with amino group [39,40]. FTIR analysis demonstrated that a lot of hydroxyl groups existed on the surface of Zn0-CNTs-Fe3O4, which could combine OTC by the form of hydrogen bond [41,42]. Moreover, electrostatic interaction could be neglected for OTC adsorption owing to that the optimal pH of 2 was lower than the pHPZC value. Therefore, it was concluded that the electron-donor-acceptor interactions, hydrophobic interaction, the coordination reaction and Lewis acid-base interaction of metal center sites (Fe and Zn0) on the adsorbent surface and the formation of hydrogen bonds were responsible for the adsorptive removal of OTC by Zn0-CNTsFe3O4 with high adsorption capacity and selectivity.

system, which made the CNTs-Fe3O4 be the major component of the used Zn0-CNTs-Fe3O4. This was also proved by the SEM, EDX and XRD analyses. When zinc powder, PEG and the used Zn0-CNTs-Fe3O4 were mixed together, Zn0 and PEG were dispersed in the CNTs-Fe3O4 particles. When the mixture was sintered over the melting point of Zn0, Zn0 was melted and PEG was decomposed into gas. Zn0 could be adhered to the surface of CNTs-Fe3O4 after the melts were cooled to room temperature. Fig. 1 shows that there was not obvious difference on the morphology, elemental composition and crystal structure between the renewed Zn0-CNTs-Fe3O4 after reloading of Zn0 and the fresh Zn0-CNTsFe3O4.

3.6.4. The mechanism of adsorptive-oxidative removal process Base on the characterization and experimental results, a novel conception of an adsorptive-oxidative process for the removal of OTC was proposed (Fig. 11). The OTC in solution was firstly adsorbed onto the surface of Zn0-CNTs-Fe3O4 by electron-donor-acceptor interaction, hydrophobic interaction, the coordination reaction and Lewis acid-base interaction as well as the formation of hydrogen bonds. Then the saturated Zn0-CNTs-Fe3O4 was separated by the magnetic separation process and added into a solution at pH of 3 with high ratio of solid to water, O2 was aerated into the solution. In this case, the OTC adsorbed on the surface of Zn0-CNTs-Fe3O4 was oxidized and mineralized by %OH radicals from the Fenton-like catalytic reaction of Zn0-CNTs-Fe3O4 with O2, which released a large number of adsorption sites for the next adsorption of OTC. During the reaction of Zn0-CNTs-Fe3O4 with O2, the consumption of Zn0 would lead to the inactivation of used Zn0-CNTsFe3O4 and release zinc ion into solution. However, the activity of the used Zn0-CNTs-Fe3O4 could be renewed by reloading Zn0. The residual zinc ion in solution could be recovered in the form of Zn(OH)2 by base precipitation process due to the low solubility of Zn(OH)2, with a solubility product constant of 1.2 × 10−17 at 298 K. Compared with conventional one-step adsorptive-oxidative Fentonlike process, this novel two-step adsorptive-oxidative removal process could adsorb organic pollutants in neutral solution and oxidize organic pollutants with high concentration, which increased the degradation efficiency of organic pollutants due to its good mass transfer performance. This novel two-step adsorptive-oxidative removal process had a significant superiority for the degradation of refractory organic pollutants, especially at low concentration. Based on the proposed mechanism for the adsorption-oxidation removal process, a technology to work under continuous mode could be proposed as follows: (1) In a push-flow adsorption tank, Zn0-CNTsFe3O4 and the wastewater flow into the head of adsorption tank and flow out at the end of the adsorption tank, then the mixed solution flow into the magnetic separator for the solid-liquid separation; (2) The clear

3.6.2. The oxidative removal of OTC in Zn0-CNTs-Fe3O4/O2 system The XRD and EDX results proved the existence of CNTs, Zn0 and Fe3O4 in the Zn0-CNTs-Fe3O4. SEM results showed the overlap containing Zn0 on the surface of CNTs. Therefore, if Zn0-CNTs-Fe3O4 was added into the solution, corrosion cells would be formed with Zn0 as anode and CNTs as cathode. When O2 was aerated into the solution, the following electrode reaction would occur in solution [17,43]:

Zn + O2 + 2H + + 2e - → H2 O2

(14)

Anode Zn − 2e− → Zn2 +

(15)

Cathode O2 + 2H+ + 2e− → H2 O2

(16)

The production of H2O2 might be due to the good selectivity of CNTs for the two electrons reduction of O2. Then the produced H2O2 would be decomposed by Fe3O4 for the formation of oxidizing species, such as %OH radical (Eq. (13)), because Fe3O4 was a good heterogeneous Fenton-like catalyst [15,17,44].

Fe3 O4 + H2 O2 → ·OH

(17)

As well known, the degradation and mineralization of OTC by %OH radicals would happen owing to the strong oxidizing ability of %OH radicals with high standard potential of 2.80 V, which had been widely applied for the degradation of refractory organic pollutants [45]. 3.6.3. The reloading of Zn0 on the used Zn0-CNTs-Fe3O4 As aforementioned, Zn0 would be consumed in Zn0-CNTs-Fe3O4/O2 11

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Fig. 11. Proposed mechanism for the adsorption-oxidation removal process.

solution flow into the clean-water pools and the solids were transferred to aeration oxidation tank; (3) The mixture effluent from aeration tank flow into the magnetic separator and a settling tank in turn, and finally flow into clean-water pools, the solids from magnetic separator after reloading Zn0 return to the head of adsorption tank.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.121963. References

4. Conclusions

[1] J.L. Wang, R. Zhuan, L.B. Chu, The occurrence, distribution and degradation of antibiotics by ionizing radiation: an overview, Sci. Total Environ. 646 (2019) 1385–1397. [2] M. Kumar, S. Jaiswal, K.K. Sodhi, P. Shree, D.K. Singh, P.K. Agrawal, P. Shukla, Antibiotics bioremediation: perspectives on its ecotoxicity and resistance, Environ. Int. 124 (2019) 448–461. [3] F. Zhang, Q. Yue, Y. Gao, B. Gao, X. Xu, Z. Ren, Y. Jin, Application for oxytetracycline wastewater pretreatment by Fenton iron mud based cathodic-anodicelectrolysis ceramic granular fillers, Chemosphere 182 (2017) 483–490. [4] J. Chen, Y. Hu, W. Huang, Y. Liu, M. Tang, L. Zhang, J. Sun, Biodegradation of oxytetracycline and electricity generation in microbial fuel cell with in situ dual graphene modified bioelectrode, Bioresour. Technol. 270 (2018) 482–488. [5] Y. Wang, P. Ni, S. Jiang, W. Lu, Z. Li, H. Liu, J. Lin, Y. Sun, Z. Li, Highly sensitive fluorometric determination of oxytetracycline based on carbon dots and Fe3O4 MNPs, Sens. Actuators B 254 (2018) 1118–1124. [6] J.L. Wang, L.J. Xu, Advanced oxidation processes for wastewater treatment: formation of hydroxyl radical and application, Crit. Rev. Environ. Sci. Technol. 42 (2012) 251–325. [7] J.L. Wang, S.Z. Wang, Activation of persulfate (PS) and peroxymonosulfate (PMS) and application for the degradation of emerging contaminants, Chem. Eng. J. 334 (2018) 1502–1517. [8] J.L. Wang, Z.Y. Bai, Fe-based catalysts for heterogeneous catalytic ozonation of emerging contaminants in water and wastewater, Chem. Eng. J. 312 (2017) 79–98. [9] C. Chen, T.F. Ma, Y.N. Shang, B.Y. Gao, B. Jin, H.B. Dan, Q. Li, Y.W. Li, Y. Wang, X. Xu, In-situ pyrolysis of Enteromorpha as carbocatalyst for catalytic removal of organic contaminants: Considering the intrinsic N/Fe in Enteromorpha and nonradical reaction, Appl. Catal. B: Environ. 250 (2019) 382–395. [10] J.T. Tang, J.L. Wang, Metal organic framework with coordinatively unsaturated sites as efficient Fenton-like catalyst for enhanced degradation of sulfamethazine,

The as-prepared Zn0-CNTs-Fe3O4 could not only adsorb OTC in solution at a wide pH range from 2 to 8, with the maximum adsorption capacity of 68.39 mg/g and high selectively for OTC, but also oxidize OTC adsorbed on its surface under the O2 aeration. The activity of the used Zn0-CNTs-Fe3O4 could be recovered by reloading Zn0. Based on above excellent performance of Zn0-CNTs-Fe3O4, the novel process of adsorption, degradation and recovery (A-D-R) for the removal of OTC in aqueous solution was very efficient to remove OTC, especially at low or trace concentration. Compared to the one-step process of adsorption and oxidation, the proposed A-D process could remove OTC from aqueous solution under neutral condition, had higher oxidative degradation efficiency, which is promising to the oxidative removal of refractory organic pollutant at low or trace concentration in wastewater.

Acknowledgement The research was financially supported by the National Natural Science Foundation of China (No. 51878427) and the Educational Commission of Sichuan Province of China (No. 18ZA0397). 12

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