Enhanced activation of persulfate by [email protected]2O4 nanocomposites for effective removal of lomefloxacin

Separation and Purification Technology 233 (2020) 115978

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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Enhanced activation of persulfate by AC@CoFe2O4 nanocomposites for effective removal of lomefloxacin Qiuling Maa, Li-chao Nengzib, Xinyi Zhanga, Zhuanjun Zhaoa, , Xiuwen Chenga,b, ⁎

T

⁎

a

Key Laboratory of Western China's Environmental Systems (Ministry of Education), Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, PR China b Academy of Economics and Environmental Sciences, Xichang University, PR China

ARTICLE INFO

ABSTRACT

Keywords: AC@CoFe2O4 Persulfate Lomefloxacin Mechanism

Magnetic AC@CoFe2O4 nanocomposites (decoration of activated carbon with CoFe2O4 nanoparticles) as an efficient persulfate (PS) activator were successfully prepared through a facile co-precipitation method. The crystalline, morphology and textural properties of the composites were characterized. Then the catalytic performances of as-synthesized AC@CoFe2O4 were investigated towards PS activation for the degradation of lomefloxacin (LMF). Results demonstrated that the sample prepared at a 1:1 mass ratio of AC (activated carbon) to CoFe2O4 possessed higher catalytic activity owing to the synergistic interactions between these two components. The maximum LMF degradation of 98.4% and the pseudo first-order kinetic constant of 0.03796 min−1 were achieved after 60 min reaction at the optimized operation conditions: 0.2 g∙L−1 of AC@ CoFe2O4 and 1 g∙L−1 of PS, pH of 5.0 and reaction temperature of 25 °C. The free radical quenching experiments and XPS analysis were undertaken to illustrate the proposed mechanism, which indicated that the SO4%− and HO% were the predominant radicals involved in LMF degradation. Moreover, the Co(II), Fe(II) and oxygenated functional groups participated in the PS activation process. Subsequently, several oxidation intermediates were identified and five suggested pathways were proposed to reveal the reaction mechanism, indicating a comprehensive route of LMF decomposition via the activation of PS. AC@CoFe2O4 also displayed good reusability and magnetic property, which would hold great potential in the persulfate-based treatment of antibiotic contaminated wastewater.

1. Introduction Lomefloxacin (LMF, pKa 5.64) belongs to Fluorquinolones (FQs) family, which is always employed to treat various bacterial infection including urinary tract, otitis media, bronchitis and even hematosepsis [1,2]. However, LMF with stable chemical structures is confirmed to be partially eliminated by municipal wastewater treatment plants (WWTPs) through the effluents and sludge disposal. As reported at WWTP in the capital city of Slovakia–Bratislava, LMF removal efficiencies after mechanic step, biological step and the whole WWTP were about 8.77%, 28.07% and 38.60% respectively and part of LMF remained in the sludge of WWTP [3]. According to a survey of China, LMF has been detected in the sediment collected from Yellow River, Hai River and Liao River with the concentration of 299 ng/g, which even as trace level, has potential threat not only to the natural development of microorganisms but also to eco-system and human health [4]. Thus,

new approaches for the treatment of water resident LMF are required. Advanced oxidation processes (AOPs) based on the generation of highly active radical species has attracted attentions owing to their ability to efficiently oxidize a wide range of refractory organics [5–7]. Very recently, sulfate radicals (SO4%−) involved technology shows strong capacity in decomposing emerging contaminants since it possesses higher redox potential (2.5–3.1 eV), a longer half-life and a better selectivity for oxidation in comparison to hydroxyl radicals (HO%) [8,9], which has been extensively investigated. SO4%− can be obtained from persulfate (PS) and peroxymonosulfate (PMS) through activation by heat, ultraviolet (UV), alkaline and with catalysts [10]. In consideration for the practical utilization and operation cost, the activation of PS by heterogeneous-based catalysts provides an alternative route for pollution removal in aqueous solution. Among the heterogeneous activators, cobalt-based oxides show superior catalytic activity in the activation of PS to generate SO4%−

Corresponding authors at: College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, PR China (Z. Zhao). Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, PR China (X. Cheng). E-mail addresses: [email protected] (Z. Zhao), [email protected] (X. Cheng). ⁎

https://doi.org/10.1016/j.seppur.2019.115978 Received 22 July 2019; Received in revised form 22 August 2019; Accepted 26 August 2019 Available online 29 August 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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[11–13]. Unfortunately, the leaching of potentially carcinogenic Co2+/ Co3+ is inevitable [14]. Besides, the solid-liquid separation is also hard to achieve, which may result in the secondary pollution [15]. In order to conquer these bottlenecks, magnetic CoFe2O4 is employed for activating PS as its outstanding properties of nanometer size, large surface area to volume ratio and chemical stability. Besides, CoFe2O4 can be easily recovered after the environmental remediation via external magnetic field [16]. However, the aggregation phenomenon usually occurs due to the extremely high surface energy of the nanoparticles during the crystallization, significantly decreasing the reactivity of the product [17]. Thus, to resolve these issues, the CoFe2O4 immobilization onto a support seems to be an efficient route, which can hinder the aggregation of CoFe2O4 and improve the specific surface area of catalyst. Accordingly, carbonaceous materials such as reduced graphene oxide (rGO) [18], carbon nanotubes (CNTs) [19] and activated carbon (AC) [20] have received more attentions due to their non-toxicity, good thermal stability, porous structure and large specific surface area [21]. For example, graphite felt supported Fe3O4 nanocomposite was synthesized, which could efficient decolorize acid yellow 36 with PMS added [22]. And Guo et al. have constructed Fe2O3/Co3O4/exfoliated graphite composite and used it for the effectively treatment of landfill leachate by activating PS [23]. Among various carbon materials, AC with advantages of wide availability, low cost has proved to be a promising and economical candidate [24]. Chen et al. used activated carbon fiber (ACF) as catalyst to activate PS for efficient decolorization of Orange G [25]; and AC modified with ammonia showed a significant synergistic effect of both adsorption and catalytic oxidation [26]. More importantly, AC as a support maintains the dispersion of the particles and charge separation and also plays an active role in the catalytic process by favoring the adsorptive properties of pollutants. As reported, AC@Fe3O4 was prepared as heterogeneous catalyst for efficient oxidation of tetracycline [27]; and the Fe-Ag/GAC catalyst could greatly improve the degradation of Acid Red 37 [28]. Nevertheless, to the best of our knowledge, the literature of depositing CoFe2O4 on AC surface as the heterogeneous catalyst in activation of PS for remediating LMF is very scarce. Thus, the present study was centered on the decoration of AC with CoFe2O4 nanoparticles (NPs) using a co-participate method. Techniques such as XRD, SEM, TEM, BET, VSM and XPS were applied to characterize the catalyst structure. Therefore, the integration of adsorption and degradation in LMF removal was evaluated using the AC@CoFe2O4 catalyst in the heterogeneous activation of PS. Besides, the degradation behaviors of LMF were investigated in terms of reaction kinetics, effects of reaction parameters, catalytic stability, the possible degradation pathways and degradation mechanism. It was found that a highly efficient heterogeneous persulfate-based system (AC@CoFe2O4/PS) was established under the optimal operation conditions, which might provide useful information for the practical treatment of antibiotic wastewater.

stirred for 30 min, forming a homogeneous solution. Therefore, certain amount of AC-HCl powder was added into the mixture solution and stirred for another 2 h at room temperature. Subsequently, 3 M NaOH solution was slowly drop-wise into the obtained aqueous solution under stirring in order to adjust the solution pH to 12. And then the mixture was keeping stirring for further 3 h at 90 °C in a constant temperature heating water bath. After that, the magnetic black suspension was formed and the solid precipitate was collected and washed with DI water and ethanol for several times. At last, the resultant product was dried at 60 °C overnight and grounded. Finally, the catalysts with different CoFe2O4:AC weight ratios of 2:1, 1.5:1, 1:1, 1:2 and 1:4 were obtained, which were labeled as CoFe2O4@AC-2:1, [email protected]:1, CoFe2O4@AC-1:1, CoFe2O4@AC-1:2 and CoFe2O4@AC-1:4, respectively. For comparison, the pure CoFe2O4 NCs were fabricated using the same method without the addition of AC-HCl.

2. Experimental

q=

2.1. Materials

In addition, the intermediate products of LMF degradation were identified using high performance liquid chromatography coupled with a mass spectrometer (HPLC-MS/MS, Water Acquity Quattro Premier, USA). The column is HC-C18 (4.6 × 150 × 20 mm, Agilent, USA). The mobile phases consisted of 0.2% formic acid in water (A) and methanol (B) (25:75, v/v) at a total flow rate of 0.2 mL·min−1. The MS system operated in negative electrospray ionization (EIS+) mode at 500 °C.

2.3. Characterization The detailed characterization was listed in the supplementary file. 2.4. Adsorption and degradation tests The catalytic activity of the AC@CoFe2O4 nanocomposites was evaluated by activating PS for the degradation of LMF (pKa 5.64). For comparison, the catalytic degradation of LMF by pure CoFe2O4 NCs and AC powder with PS was individually investigated as well. All experiments were conducted in the 150 mL conical flasks containing 100 mL LMF solution (5 mg∙L−1) placed in a constant temperature rotary shaker with 160 rpm at 25 °C. In each run, 0.1 g of PS and 0.02 g of catalyst were added into the system, signaling the start of reaction. As given time intervals, certain amount of reaction solution was withdrawn by the syringe and centrifuged in 4000 rpm for 5 min. Subsequently, the collected solution was filtered by a 0.22 μm membrane filter and immediately measured in a T6 UV–vis spectrophotometer (Persee, Beijing) at the maximum adsorption wavelengths (280 nm) of LMF. During experiments, the solution pH was adjusted by H2SO4 (3 M) or KOH (3 M) and recorded in a pH analyzer (PHS-3C, General Instrument Co., Ltd., Shanghai, China). Besides, the stability of the catalyst was evaluated during the degradation experiments. After every reaction, the catalyst was collected using a magnet, rinsed thoroughly with DI water and dried at 60 °C in an oven for the next run. And for studying the degradation mechanism of LMF by AC@CoFe2O4 activating PS, typical radical scavengers such as methanol (MeOH, 5 mL) and tert-butyl alcohol (TBA, 5 mL) were added to the system before reaction started. Moreover, the LMF adsorption by AC@CoFe2O4 nanocomposites was carried out by the same procedures without the addition of PS during the batch experiments. The adsorption capacity (q, mg/g) was calculated according to the LMF concentration before and after adsorption, as shown in Eq. (S1).

The materials in the experiments were listed in the supplementary file. 2.2. Synthesis of CoFe2O4@AC nanocomposites The CoFe2O4@AC nanocomposites were prepared through a coprecipitation process with some modifications (Fig. S1). Before the synthesis, the AC powder was modified to purify the chemical. In detail, the AC powder was soaked in the HCl (5 M) for 24 h, followed by several times washed with DI water and dried at 80 °C overnight, which was noted as AC-HCl. Then at the beginning, Fe(NO3)2·9H2O (0.02 mol) and Co(NO3)2·6H2O (0.01 mol) were dissolved in 100 mL DI water with

(C0

Ct ) × V m

(S1)

3. Results and discussion 3.1. Characterization Fig. 1a showed the XRD patterns of bare AC, modified AC (AC-HCl), pure CoFe2O4 and as-synthesized AC@CoFe2O4. As displayed of bare 2

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(a)

component. Typically for AC-HCl sample, characteristic adsorption bands centered at 1533 cm−1 and 1379 cm−1 were detected, corresponding to the stretching vibration modes of C]C and CeO, respectively. Moreover, a sharp peak appeared at 1017 cm−1 could be assigned to CeOeC stretching mode of the carbon materials. However, this peak showed a slight shift to 1047 cm−1 in AC@CoFe2O4, which might due to the interaction of CoFe2O4 on AC. And two peaks of 669 cm−1 (CoeO bonds) and 586 cm−1 (FeeO bonds) were detected in AC@CoFe2O4, confirming the existence of CoFe2O4 in the as-prepared catalyst [29,31]. The morphologies of the AC-HCl and as-prepared AC@CoFe2O4 nanocomposites were examined by SEM and shown in Fig. 2a–c. In Fig. 2a, AC-HCl exhibited a good lamellar structure and smooth surface with the development of some micro-pores. Clearly as for AC@CoFe2O4 (Fig. 2b and c), there were large number of small size homogeneous particles covered or wrapped with the AC surface without apparent aggregation, which could be ascribed to the loading of CoFe2O4 NPs. Besides, it could be seen that the surface of compound materials became rougher than the AC sample, indicating a larger specific surface area of AC@CoFe2O4, which might facilitate the adsorption capacity. Moreover, to reveal the distribution of CoFe2O4 in the composite, the elemental mapping was performed and displayed in Fig. 2d–g. As presented, four elements of Fe, O, Co and C co-existed in the nanocomposites. And both the Co and Fe elements were highly dispersed in the images, identifying the uniform distribution of CoFe2O4 among AC@CoFe2O4. In addition, in the EDS spectrum (Fig. 2h), these existed elements were confirmed and the molar ratio of Fe/Co was estimated as 2.00, which was close to the theoretical stoichiometric value of CoFe2O4. Therefore, the information of SEM results indicated the synthesis of the hybrid material and immobilization of CoFe2O4 on the surface of AC. Then TEM and HRTEM were carried out for further determining the morphology and structure of as-fabricated AC@CoFe2O4 nanocomposites with the images displayed in Fig. 3. One could see from Fig. 3a that AC@CoFe2O4 consisted of light-colored shell and the dark colored core, which were corresponding to CoFe2O4 NPs and AC, respectively. The image also depicted that CoFe2O4 NPs with nano-size of 5–20 nm which uniformly distributed on AC surface. In Fig. 3b, it was interesting to observe a set of lattice fringes in AC@CoFe2O4. The lattice spacing of 0.26 nm presented in the picture was indexed to the (3 1 1) lattice plane of CoFe2O4, which was in keeping with the XRD results. Here we confirmed the porosity of AC and AC@CoFe2O4 samples by Nitrogen adsorption-desorption isotherms. According to the curves in Fig. 4a, the isotherms of the both samples were categorized as type IV with the obvious hysteresis loops, indicating the mesoporous structures with the presence of inconspicuous saturated adsorption platforms of AC-HCl and AC@CoFe2O4 samples. Fig. 4b displayed the corresponding pore size distribution of the samples calculated by the BJH (BarrettJoyner-Halenda) model, suggesting the porosities of AC-HCl and AC@CoFe2O4 were mainly made up of microspores. Observably seen from Table S2, the introduction of CoFe2O4 increased the surface area as well as the pore size of the as-fabricated nanocomposites (323.564 m2 g−1 and 3.627 nm) compared with AC-HCl sample (104.408 m2 g−1 and 1.178 nm). Actually, such porous structure endowed the AC@CoFe2O4 material with plentiful exposed catalytic sites and minimum diffusion resistance, which might be beneficial for pollutant adsorption and PS activation [32]. Magnetic hysteresis loops of CoFe2O4 and AC@CoFe2O4 nanocomposites were displayed in Fig. S2. When magnetic field was applied, the material showed a strong response, and the saturation magnetization (Ms) reached 62.95 emu∙g−1 for CoFe2O4 NPs. As expected, the value of Ms of AC@CoFe2O4 (44.97 emu∙g−1) was less than that of bare one, which was mainly attributed to the contribution of the nonmagnetic component (AC) in the total sample. Moreover, these products exhibited relatively high coercively might be owing to the high anisotropy feature of the composite structure. The inset pictures in Fig. S2

AC@CoFe2O4

(103)

(004)

AC

(440)

(511)

(400)

(220) (311) (222)

(110)

(002)

Intensity (a.u.)

AC-HCl

CoFe2O4 JCPDS No. 22-1086 JCPDS No. 99-0057

10

20

30

40 50 60 2 Theta (degree) 1533 C=C

(b)

70

80

90

1379 1017 C-O C-O-C

Transmittance (%)

Bare AC

AC@CoFe2O4

669 Co-O 3423 O-H

2923 -CH3-

2852 -CH2-

1626 H-O-H

1047 C-O-C

4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm )

586 Fe-O

500

Fig. 1. XRD patterns of pure AC, AC-HCl, pure CoFe2O4 and AC@CoFe2O4 (a), FT-IR spectra of AC-HCl and AC@CoFe2O4 (b).

AC, the diffraction peaks at 26.60°, 44.67°, 54.80° and 60.03° were attributed to (0 0 2), (1 0 1), (0 0 4) and (1 0 3) planes of graphite carbon (JCPDS No. 99-0057). However, there were some peaks of 26.65°, 36.57°, 39.48° and 50.17° could be indexed to quartz (JCPDS No. 85-0795), implying the existence of SiO2 in the bare AC powder. For the AC-HCl sample, the diffraction peaks were in good agreement with the standard graphite carbon. However, compared with bare AC, some characteristic peaks of impurities were discernable in the curve and the intensity of the peaks belonging to graphite carbon was enhanced, indicating the improved purity of the modified AC sample. Besides, the as-fabricated CoFe2O4 revealed a well-defined crystal structure, with XRD peaks at 30.08°, 35.44°, 37.06°, 43.06°, 56.60° and 62.59° belonging to (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1) and (4 4 0) planes of CoFe2O4 (JCPDS No. 22-1086) [29], respectively. Moreover, as could be observed that AC@CoFe2O4 exhibited the typical patterns of graphite as well as spinel ferrite CoFe2O4, which demonstrated the synthesis of AC@CoFe2O4 nanocomposites. FT-IR spectra of AC-HCl and AC@CoFe2O4 nanocomposites were presented in Fig. 1b. For these samples, the bands at 3423 cm−1 were assigned to the OeH stretching vibration of hydroxyl groups adsorbed in the catalysts. Besides, both the samples of AC-HCl and AC@CoFe2O4 exhibited the band intensities at 2923 cm−1 and 2852 cm−1, which were related to the CeH vibration of CH3 and CH2 groups in the AC component of the samples [30]. Besides, the bands at 1626 cm−1 could attributed to either HeOeH vibration or C]O stretching in AC 3

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Fig. 2. SEM images of AC-HCl sample (a) and AC@CoFe2O4 (b-c), elemental mapping images of Fe (d), O (e), Co (f), C (g) and EDS for AC@CoFe2O4.

illustrated that magnetic AC@CoFe2O4 could be efficiently recovered by the external magnet, implying the catalyst could be promising for reuse in the wastewater.

nanocomposites. Fig. 5a showed the degradation curves of LMF by catalysts with different CoFe2O4:AC weight ratios in the existence of PS. As shown, all the samples exhibited relatively high degradation efficiencies of LMF (> 80%) with PS existed, reflecting the synthesis method of AC@CoFe2O4 was effective. It was clearly found that the AC@CoFe2O4-1:1 sample as catalyst activating PS exhibited the highest LMF removal rate. Besides, when the mass ratio between AC and CoFe2O4 was larger than 1:1, the LMF degradation was improved with the content of CoFe2O4 increased. However, the degradation rate of LMF declined slightly when the content of CoFe2O4 in the composite further increased (larger than 50%). Accordingly, the synergistic interactions between the CoFe2O4 and AC prevented the aggregation of the CoFe2O4 NPs, which resulted in the enhanced degradation efficiencies. However, excessive incorporation of the CoFe2O4 NPs might block the active sites and hinder the contact between AC and CoFe2O4, which therefore result in the negative effect to catalytic performance. Finally, from the experiments data, AC@CoFe2O4-1:1 was used for further study.

3.2. Catalytic PS activation for LMF degradation In this work, the adsorption behaviors of as-fabricated AC@CoFe2O4 nanocomposites, bare AC, AC-HCl as well as pure CoFe2O4 were studied over 60 min period in the LMF solution under initial concentration of 0.2 g∙L−1 and pH 4.5. As illustrated in Fig. S3a, the adsorption efficiency of LMF followed the order: AC@CoFe2O4-1:1 > AC@CoFe2O42:1 > AC@CoFe2O4-1:2 > AC-HCl > AC > CoFe2O4. And the related qt of LMF shown in Fig. S3b confirmed the results. Actually, good adsorption capacity of AC@CoFe2O4-1:1 could facilitate its degradation of LMF in the AC@CoFe2O4/PS system. Fig. S3c and displayed the effect of AC@CoFe2O4-1:1 dosage on the LMF adsorption. With the increase of catalyst dosage, the LMF adsorption was obviously enhanced, which also evidenced the superior adsorption capability of AC@CoFe2O4-1:1

Fig. 3. TEM (a) and HRTEM (b) images of as-synthesized AC@CoFe2O4. 4

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250

Removal efficiency C/C0

3

150

AC

100

50

0.2

0.4

0.6

0.8

1.0

0.0

0.45

15

30 Time (min)

45

(b)

AC

0.8 Removal efficiency C/C0

0.30

0.15 0.10

0.6

0.4 PS AC AC-HCl CoFe2O4

0.2

0.05 0

5

10

15

20

25

30

60

(b)

AC@CoFe2O4

0.35

-1 -1

0

1.0

0.40

3

(a)

0.4

-1

dV(cm nm g )

AC@CoFe2O4-1:4

0.6

Relative pressure (P P0 )

0.00

AC@CoFe2O4-1:2

0.2

0 0.0

0.20

AC@CoFe2O4-1:1

0.8

-1

Quantity adsorbed (cm g )

[email protected]:1

AC@CoFe2O4

200

0.25

AC@CoFe2O4-2:1

1.0

(a)

0.0

35

Pore diameter (nm) Fig. 4. Nitrogen adsorption-desorption isotherm of AC-HCl and AC@CoFe2O4 (a) and the corresponding pore size distribution (b).

AC@CoFe2O4

0

15

30 Time (min)

45

60

Fig. 5. Removal efficiency of LMF in aqueous solution by different as-fabricated samples (a-b). Reaction conditions: [PS]0 = 1.0 g∙L−1, [catalyst]0 = 0.2 g∙L−1, reaction temperature = 25 °C and initial pH = 4.5 (unadjusted).

Fig. 5b showed the LMF concentration declined profiles in various system. As we could see, nearly 23.8% of LMF degradation could be noticed during PS oxidation alone, indicating the single chemical oxidant (PS, E0 = 2.1 V) could not decompose LMF molecular effectively [8]. The degradation rates were significantly accelerated by adding AC and AC-HCl into the system owing to their adsorption as well as catalytic performances. Notably, the AC-HCl sample showed slightly enhanced in LMF degradation efficiency compared with bare AC. This probably because AC modified with HCl exhibited higher purity and larger specific surface area. Fig. 5b also showed that 43.5% of LMF was degraded within 60 min in CoFe2O4/PS system, indicating PS could be activated by CoFe2O4 to generate radicals for the reaction. Evidently, introduction of AC@CoFe2O4-1:1 into PS system resulted in 98.7% degradation of LMF after 60 min, implying a synergistic catalysis existed between AC-HCl and CoFe2O4. Combined with the BET and adsorption results, AC@CoFe2O4-1:1 possessed large specific surface area which helped to adsorb more pollutant and radical species. Besides, it also applied the active sites with high accessibility of reactants which therefore facilitated the LMF degradation process. The degradation performances were also compared with the catalysts reported in literatures. Cai et al. [28] reported the granular activated carbon (GAC) supported cobalt catalyst (Co-GAC) as heterogeneous catalyst for the activation of persulfate for the degradation of Acid orange 7 (AO7). In addition, Jafari et al. [4] has prepared the AC@Fe3O4 composite and applied in the persulfate activation system for TC degradation. In

comparation with these works, AC@CoFe2O4 exhibited higher catalytic performances and also remained good magnetic activity which was benefited for its separation from solution. In order to investigate the effect of temperature on the removal of LMF, the experiments conducted at four temperatures (20 °C, 25 °C, 30 °C and 40 °C) were firstly conducted with results presented in Fig. 6a. As observed, higher temperature favored the degradation of LMF by PS activated with AC@CoFe2O4 owing to the thermal activation of PS and the acceleration reaction between AC@CoFe2O4 and PS. In order to estimate the thermal impacts, the activation energy (Ea) of the reaction could be calculated via Arrhenius equation (Eq. (1)).

lnk = lnA

Ea RT

(1)

Where k was the measured first-order rate constant, A was the preexponential factor, Ea was the activation energy, R was the universal gas constant and T was the temperature (K). By plotting the lnk against 1/T (Fig. S4), the Ea was determined as 21.57 kJ∙mol−1. Compared with the Co-Fe based catalysts reported in pervious literatures (Table S2), the as-fabricated AC@CoFe2O4 exhibited lower Ea, demonstrated the heterogeneous degradation of LMF with AC@CoFe2O4 could occurred at a relatively low energy. It also suggested that the AC@CoFe2O4/PS oxidation system with the advantage of energy conservation could meet the need of practical applications. 5

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1.0

1.0

0.05

(a)

0.04

0.02 0.01 0.00

0.4

10

0

1.0

o

15

30 Time (min)

45

30

-1

0.6

0.01 0.00

0.4

0.2

0.6

1.0

1.4

PS concentration (g/L)

0.2 g/L 0.6 g/L 1.0 g/L 1.4 g/L

0.2

0.0

60

0

15

30

45

60

Time (min)

1.0

0.12

(c)

0.02

0.04

(d)

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0.03

0.6

0.08 0.06 0.04 0.02 0.00

0.4

0.8

-1

0.1 g/L 0.2 g/L 0.3 g/L 0.4 g/L

Degradation efficiency C/C 0

Degradation efficiency C/C0

0.8

kobs (min )

-1

k obs (min )

0.0

25

20

Temperature ( C)

20 oC 25 oC 30 oC 40 oC

0.2

kobs (min )

0.6

0.03

0.8

0.03

Degradation efficiency C/C0

-1

kobs (min )

Degradation efficiency C/C0

0.8

0.04

(b)

0.1

0.3

0.2

0.4

Catalyst concentration (g/L)

0.2

0.0

0

15

30 Time (min)

45

0.01

0.00

0.6

0.4

3

4.5

5

7

pH value

9

11

pH=3 pH=4.5 pH=5 pH=7 pH=9 pH=11

0.2

0.0

60

0.02

0

15

30 Time (min)

45

60

Fig. 6. Effects of reaction parameters on the degradation of LMF: reaction temperature (a); PS concentration (b); catalyst dosage (c) and initial pH value (d). Reaction conditions: [PS]0 = 1.0 g∙L−1, [catalyst]0 = 0.2 g∙L−1, reaction temperature = 25 °C and initial pH = 4.5 (unadjusted).

The effects of PS concentration on LMF degradation by AC@CoFe2O4 nanocomposites were studied in the range of 0.2–1.4 g∙L−1 here. As seen from Fig. 6b, the LMF degradation efficiencies of 79.4%, 86.3%, 90.3% and 91.5% were measured with the PS concentrations of 0.2 g∙L−1, 0.6 g∙L−1, 1 g∙L−1 and 1.4 g∙L−1, respectively. Clearly, there was a significant trend that the degradation process would be accelerated by increasing the PS concentration, which was in accordance with Chen’s study [17]. Specifically, when the [PS]0 was lower than 1 g∙L−1, the decomposition efficiencies of LMF as well as its rate constants were improved with the increase of initial PS dosage. Nevertheless, the improvement of LMF decomposition became milder with further increase of PS dosage (1.4 mg∙L−1) which was probably because of the self-quenching of SO4%− (Eqs. (2) and (3)) [33,34]

SO·4 + SO·4

SO·4 + S2 O28

S2 O82

SO24 + S2 O·8

SOE =

(C 0,LMF Ct,LMF)/C0,LMF (PS0 PSt )/PS0

(4)

The value of SOE for AC@CoFe2O4/PS system after 60 min reaction was 1.58. Compared with literature [36], the AC@CoFe2O4/PS system exhibits obvious higher oxidant efficiency under the given conditions. Fig. 6c displayed the LMF degradation curve with 1 g∙L−1 PS and different amounts of AC@CoFe2O4. As seen, the LMF degradation rate could be improved by increasing the catalyst dosage, similar to the effect of PS loading. It was observed that the LMF could be degraded to 84.6% within 60 min reaction at the catalyst dosage of 0.1 g∙L−1, while higher catalyst concentration (0.3 g∙L−1) corresponded to 96.7% degradation efficiency after only 30 min. This observation could be explained that the rising AC@CoFe2O4 dosage increased the number of active sites which therefore led to the higher degradation rate of LMF. And also, there was the linear relationship between the calculated reaction rate and the AC@CoFe2O4 dosage, which could be ascribed as Eq. (5).

(2) (3)

kobs =

Moreover, the consumption of PS after the degradation of LMF by AC@CoFe2O4 activating PS was determined via spectrophotometric method using potassium iodide. It was confirmed that 62.27% of PS was consumed after degradation test. Thus, the specific oxidant efficiency (SOE) of this system could be calculated via Eq. (4) [35].

R2

0.00685 min

= 0.9011

1

+ m(AC@CoFe2 O4

1: 1)0 × 0.2875 L/(g min), (5)

The decomposition of LMF was investigated at different pH0 of 3, 5, 7, 9 and 11 which represented acidic, weakly acidic, neutral, weakly alkaline and alkaline conditions, respectively (Fig. 6d). Intriguingly, the AC@CoFe2O4/PS system achieved the highest LMF degradation 6

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efficiency at pH 5.0 with a corresponding rate constant of 0.03796 min−1. It could be also observed that the degradation efficiency of LMF at unadjusted pH of 4.5 reached to 89.2% after 60 min reaction, which yielded a kobs of 0.0361 min−1. These data indicated that weakly acidic condition was favorable for the catalytic degradation of LMF by AC@CoFe2O4 and PS. However, in the extremely acidic (pH = 3) or alkaline (pH = 11) condition, the catalytic rate exhibited significantly decrease. Generally, it could be explained by the following aspects: (i) the adsorption of the organics by AC@CoFe2O4 could be influenced by the initial pH. In the acidic solutions, the catalyst might adsorb more LMF which might prevent the decomposition of LMF on the surface of nanocomposites [37]. It was also reported that excessive H+ can play the role of a scavenger for hydroxyl and sulfate radicals based on Eqs. (6) and (7). Thus, the LMF degradation efficiencies could be decreased in acidic conditions [38,39]. (ii) In alkaline condition, OH– could react with SO4%− and HO% in the solution and lead to the depletion of radicals (Eqs. (8)–(11)), which subsequently hindered the oxidation in LMF degradation [37]. (iii) The solution pH was also in close relationship with the surface charge. When pH0 < pHzpc, the positive surface charge would be obtained which assisted to adsorbed more SO4%− to oxidize organics [40]. According to literatures, the pHzpc of some catalysts with the same structures as AC@CoFe2O4 were listed in Table S3. From this, the pHzpc of AC@CoFe2O4 was inferred to be 5 ~ 6. Moreover, for investigating a more pragmatic approach in this study, the solution pH of 4.5 (unadjusted) was selected in the subsequent experiments.

+

H+

+e

OH· + H+ + e

HSO4

H2 O

(8)

SO·4 + H2 O

HSO4 + HO·

(9)

HO· + HO· S2 O82 + H2 O2

H2 O2 2H+ + 2SO24 + O2

Degradation efficiency C/C 0

4th

3rd

2nd

0.4

0.2

0

30

60

90

120

150

180

210

240

Time (min)

100

(7)

SO24 + HO·

1st

0.6

0.0

(6)

SO·4 + HO

(a)

0.8

Degradation efficiency (%)

SO·4

1.0

(10)

98.6%

(b) 84.8%

80 60

55.9%

40

28.4% 20 0

No scavenger

(11)

89.6%

TBA

MeOH

BQ

Fe(II)-EDTA

Fig. 7. LMF degradation in consecutive runs using recycled AC@CoFe2O4 (a); and effect of radical scavengers on the degradation of LMF (b). Reaction conditions: [PS]0 = 1.0 g∙L−1, [catalyst]0 = 0.2 g∙L−1, reaction temperature = 25 °C and initial pH = 4.5 (unadjusted).

3.3. Reusability of AC@CoFe2O4 nanocomposites For the practical application of a catalyst, it was essential to evaluate the stability of it. Thus, Fig. 7a showed the catalytic performance of the AC@CoFe2O4 nanocomposites which were repeatedly used for four times. As illustrated, the sample was still highly active with a removal efficiency of LMF about 79.0% within 60 min reaction at the end of the fourth cycle. The result demonstrated the satisfactory stability and reusability of AC@CoFe2O4, which showed superior potential in environmental remediation. However, the observed slight decrease in catalyst activity might attribute to two aspects. On the one hand, the intermediate products formed in the previous run could accumulate on the surface of the sample and thus hinder the further catalytic reaction. On the other hand, the possible loss of the catalyst might lead to the decrease in degradation efficiency of LMF.

revealed that both the oxidizing radical species of SO4%− and HO% were involved in the reaction system and participated in the LMF degradation process by AC@CoFe2O4 activating PS. However, LMF degradation efficiency did not exhibit significant decrease when the BQ was added to the system, which indicated that %O2– did not play the important role in LMF degradation. Moreover, a little concertation of H2O2 might be generated in the reaction system. Thus, it could be inferred that H2O2 was generated during the reaction, but it was not the main active species in LMF degradation. As known, the oxidization might induce microphase separation of the catalysts, which subsequently led to the decrease in catalytic performance. Thus, the high-scale XPS spectrum of C 1s, Co 2p, Fe 2p and O 1s of the AC@CoFe2O4-1:1 before and after the catalytic reaction were displayed in Fig. 8 In the C 1s XPS spectra (Fig. 8a), four components could be detected in the fresh sample. As seen, the main peak at 284.5 eV was assigned to carbon atoms with sp2 carbon hybridization. And the peaks with the binding energies (BEs) of 285.2 eV, 286.8 eV and 289.2 eV were corresponded to reference CeO, C]O and O]CeO species, respectively [44]. This result was in accordance with the FT-IR result, which confirmed the components of AC sample. However, the BE values which corresponded to CeO and O]CeO slightly shifted and the area of characteristic peaks (285.2 eV, C]O) changed after degradation test. The phenomenon indicated the catalyst might adsorb the intermediates of LMF on surface during degradation process and the C

3.4. Reaction mechanism According to the studies [17,41], two main types of reactive radicals, namely SO4%− and HO% were generated in the metal-based PS activation system. It was also reported that the %O2– might be generated during the persulfate activation process. Thus, scavengers of BQ (quencher of %O2–), TBA (quencher of HO%), MeOH (quencher of HO% and SO4%−) and Fe(II)-EDTA (quencher of H2O2) were utilized to further distinguish the roles of these radicals in the degradation process [42,43]. As presented in Fig. 7b, the LMF degradation efficiency decreased from 98.4% to 55.9%, 28.4%, 89.4% and 84.4% with the addition of TBA, MeOH, BQ and Fe(II)-EDTA, respectively. It was clearly 7

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Intensity (a.u.)

Intensity (a.u.)

Co 2p

(b)

C 1s

(a)

Before reaction

Before reaction

3+

After reaction

282

284 286 288 Binding Energy (eV)

775

290

Fe 2p

Intensity (a.u.)

(c)

Before reaction

After reaction

715 720 725 730 Binding Energy (eV)

805

O 1s

After reaction

2+

710

785 790 795 800 Binding Energy (eV)

Before reaction

Fe 16.9%

705

780

(d)

Intensity (a.u.)

280

After reaction

Co 9.0%

absorbed oxygen species 36.5%

735

525

528 531 534 537 Binding Energy (eV)

540

Fig. 8. XPS spectra of AC@CoFe2O4 before and after the reaction in C 1s (a), Co 2p (b), Fe 2p (c) and O 1s (d) energy regions.

state on the surface might be changed. As seen from Fig. 8b, two characteristic peaks with BEs of around 780.4 eV and 795.9 eV in the fresh one were associated with Co 2p3/2 and Co 2p1/2 and shake-up satellites of 785.1 eV were found, signifying the exclusive presence of Co2+ in this product [45]. Seen from the bottom one (after reaction), the peak positions almost had no change. Nevertheless, a new component (781.9 eV) occurred after the oxidation reaction, which was corresponded to Co3+ in octahedral sites, solidly demonstrated that there was redox reaction between Co2+ and Co3+ during the activation of PS by the AC@CoFe2O4. Meanwhile, the Fe 2p spectrum in Fig. 8c revealed three obvious peaks locating at BEs of 711.5 eV, 725.1 eV and 721.2 eV were attributed to the Fe 2p3/2 and Fe 2p1/2 and the satellite peak. Clearly seen from the used catalyst, the Fe 2p envelops could be deconvoluted into Fe2+ and Fe3+, implying the Fe species on the surface of AC@CoFe2O4 were presented in a mixed valence after activating PS [46]. In addition, Fe3+ and Fe2+ were found to account for 83.1% and 16.9%, respectively. Additionally, the XPS O 1s pattern was illustrated in Fig. 8d. Seen from the pristine one, the peaks located at 529.8 eV and 531.3 eV could be assigned to the lattice oxygen in CoFe2O4 (CoeO and FeeO) and the surface adsorbed hydroxyl groups, respectively [45]. Besides, the area of the latter was larger than the former, which could be explained as the large amounts of oxygen-containing groups. For the used catalyst, the O 1s region could be decomposed into three peaks centered at 529.8 eV, 530.7 eV and 531.3 eV. The new peak at 530.7 eV belonged to adsorbed oxygen species on the surface of AC@CoFe2O4 after LMF degradation. Besides, the proportion of hydroxyl and oxygencontaining groups in the total increased from 60.8% to 75.3%, which was attributed to the formation of Co/FeeOH or the chemically/physically adsorbed water of LMF/intermediates on the catalyst surface. As demonstrated [47], AOPs always yield a number of intermediates owing to its highly-active and non-selective characteristics between

oxidative species and target pollutants, thus leading to the creaturely toxicity and recalcitrance of parent compounds. In order to elucidate the intermediates formation of LMF degradation in AC@CoFe2O4/PS system, the HPLC-MS/MS in a positive ionization mode measurement was employed. The identified main products and intermediates were summarized in Table S4 and proposed five pathways of the degradation of LMF were illustrated in Fig. 9. As seen from pathway I of Fig. 9, LMF was attacked by SO4%− or HO% and the reactions of defluorination and OH-addition might lead to the formation of P1. Therefore, the decarboxylation, demethylation as well as dehydroxylation of P1 generated P11. Meanwhile, isatin analogs (P6 and P9) were obtained due to the attack of radicals generated in the system, which could be ascribed to pathway II. Additionally, seen from pathway III, it was remarkable that energetically the defluorination leading to the formation of P3 was favored in comparison to the ring cleavage reactions of LMF. Then P3 could be converted through oxidization to form products of P4, P2 and P7. On the other hand, the P8 product was very likely formed due to the eCO2 loss of P3. Besides, during the LMF degradation process, the eCeNe side chain bond belonging to the piperazine moiety of LMF was easier to be broken, generating P5. In this case, the intermediate suffered fluorine substituent lost, decarboxylation and demethylation, leading to the formation of P12. Besides, the by-product of P13 would be yielded via ring opening of P12. In the meanwhile, a directly CeN bond cleavage by the oxidation of SO4%− and HO% would occur, which led to the generation of P10, P14 and P15 Thus, these intermediates were decomposed into some macular with small weight, such as oxalic acid and ethylenediamine. Finally, as the reaction time of AC@CoFe2O4 activating PS prolonged, all the intermediates were further mineralized eventually to CO2, H2O, NO3–, NH4+ and F−. In addition, there were some intermediates of LMF degradation was 8

Separation and Purification Technology 233 (2020) 115978

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Fig. 9. Proposed decomposition pathways of LMF in the AC@CoFe2O4/PS system.

in accordance with the results reported by Zhang et al. [48] in our group, in which the biological toxicity of the LMF and its intermediates during the degradation process was carried out via activated sludge inhibition experiment. Results indicated that the intermediates were further decomposed into low-toxic or non-toxic substances, which implied that LMF would be decomposed in a facile way with low toxicity in the persulfate-based process. Moreover, the proposed degradation intermediates of LMF in this study were also in keeping with some literatures with diffident systems including ozonation [2], UV light photodegradation [49] as well as photolysis [50]. Nevertheless, some of the detected compounds revealed different. As known, few studies reported the LMF degradation via the activation of PS. Thus, the study provided more detailed intermediates and a more comprehensive degradation route of LMF during the reaction of AC@CoFe2O4 activating PS. According to the obtained results above, the possible reaction mechanism of the PS activation by AC@CoFe2O4 nanocomposites for the removal of LMF was proposed and revealed in Fig. 10. Owing the large specific surface area and the enhanced adsorption capability of AC@CoFe2O4, the molecules of LMF and PS were easily adsorbed onto the catalyst. Subsequently the in-situ PS activation as well as LMF degradation might occur through a none-radical route. And also, the Co and Fe species of CoFe2O4 dispersed on the surface of the composites were the mainly reactive sites. Firstly, in the system, the Fe(III) would react with H2O in the solution to form the HO% radical species and Fe(II) (Eq. (12)). Then during the PS-based process, the Co(II) and Fe(II) reacted with PS to generate SO4%− (Eqs. (13) and (14)). In the meanwhile, it was known that AC possessed unique carrier mobility which could efficiently activate PS by the sp2 carbon and functional oxidation groups. Thus, mounts of SO4%− were formed according to Eqs.

Fig. 10. Proposed mechanism of AC@CoFe2O4 activating persulfate for LMF degradation.

(15)–(17). Besides, the SO4%− adsorbed onto the catalyst might diffuse into the solution and participated in the generation of HO%, seen Eqs. (18) and (19) [34]. Therefore, the active radicals have taken responsibility for the LMF degradation (Eq. (20)). More importantly, the regeneration of catalyst during the catalytic process was beneficial for its practical use. As described in Fig. 10 and Eq. (21), the component of CoFe2O4 could be regenerated through the oxidation-reduction reaction between Co(II)/Co(III) and Fe(II)/Fe(III). And the AC would be refreshed as well (Eqs. (22) and (23)). Thus, little loss of catalyst might be 9

Separation and Purification Technology 233 (2020) 115978

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found at each cycle, which demonstrated an environmental-friendly application for AC@CoFe2O4.

Co(II) + S2 O28

Co(III) + SO·4 + SO24

(13)

Fe(II) + S2 O82

Fe(III) + SO·4 + SO24

(14)

AC surface

COOH + S2 O28

References

(12)

Fe(II) + HO· + H+

Fe(III) + H2 O

doi.org/10.1016/j.seppur.2019.115978.

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Huang, Insights into the mechanism of nonradical activation of persulfate via activated carbon for the degradation of pchloroaniline, Chem. Eng. J. 362 (2019) 262–268. [25] J. Chen, H. Wei, T. Huang, L. Zhang, W. Li, Y. Wang, Activated carbon fiber for heterogeneous activation of persulfate: implication for the decolorization of azo dye, Environ. Sci. Pollut. Res. Int. 23 (2016) 18564–18574. [26] S. Yang, L. Li, T. Xiao, Y. Zhang, D. Zheng, Promoting effect of ammonia modification on activated carbon catalyzed peroxymonosulfate oxidation, Sep. Purif. Technol. 160 (2016) 81–88. [27] A.J. Jafari, B. Kakavandi, N. Jaafarzadeh, R.R. Kalantary, M. Ahmadi, A.A. Babaei, Fenton-like catalytic oxidation of tetracycline by AC@Fe3O4 as a heterogeneous persulfate activator: Adsorption and degradation studies, J. Ind. Eng. Chem. 45

COO· + SO·4 + HSO4

AC surface

(15)

S2 O28

O·

AC surface

+ HSO4

(16)

AC surface

OH +

AC surface

C = O + S2 O28

SO·4(adsorbed)

SO·4(free)

(18)

H2 O + SO·4

H+ + HO· + SO24

(19)

AC surface

+

SO·4

CO· + SO·4 + SO24 (17)

SO·4

+

HO·

+ LMF

Co(III) + Fe(II)

(20)

CO2 + H2 O +

(21)

Co(II) + Fe(III)

AC surface

O· + H2 O

AC surface

COO·

+ H2 O

AC surface

OH + OH

AC surface

COOH + OH

(22) (23)

Compared with experimental simulated wastewater, the effluent of WWTPs was more complex. However, persulfate-based AOPs may provide a promising strategy for deeply oxidizing organics contaminations. The magnetic AC@CoFe2O4 catalyst designed in this study not only exhibits high catalytic efficiency of PS, but overcomes the complicated recovery of catalysts in suspension, which may show great potential for removing antibiotics in realistic water environment. 4. Conclusion In summary, a magnetic AC@CoFe2O4 nanocomposites with low cost and facile synthesis method was fabricated and used as the PS activator for the degradation of LMF. Results indicated that the sample of AC@CoFe2O4-1:1 exhibited much higher catalytic performances compared with bare AC, pure CoFe2O4 and other as-prepared samples. In addition, the LMF degradation reached 98% within 60 min under the following conditions: 0.2 g∙L−1 catalyst, 1 g∙L−1 PS, solution pH of 5 and reaction temperature of 25 °C. The underlying mechanism of AC@CoFe2O4 activating PS was studied. In brief, part of LMF molecules adsorbed on the surface of catalyst as well as dissociated in the aqueous solution could be caught by the generated SO4%− and HO%, then finally were decomposed into CO2 and H2O etc. And the none-radical pathway occurred as well which took responsibility for LMF degradation. Particularly, the magnetic AC@CoFe2O4 nanocomposites showed high recyclability and could be easily separated by applied magnet, which avoided the secondary contamination of catalysts, providing a promising strategy for removing antibiotic LMF in aqueous solution. Acknowledgements This work was kindly funded by National Natural Science Foundation of China (51978319, 41771341), Fundamental Research Funds for the Central Universities (lzujbky-2017-it98), National College student innovation and Entrepreneurship training program of Lanzhou University and Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lake, Chinese Academy of Sciences. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// 10

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