Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline

JES-01331; No of Pages 10 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 7 ) XX X–XXX

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Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline Yuwei Wu, Qinyan Yue ⁎, Yuan Gao, Zhongfei Ren, Baoyu Gao ⁎ Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China

AR TIC LE I N FO

ABS TR ACT

Article history:

In this study, bimetallic nanoscale zero-valent iron particles (nZVI), including copper/

Received 23 June 2017

nanoscale zero-valent iron particles (Cu/nZVI) and nickel/nanoscale zero-valent iron

Revised 4 October 2017

particles (Ni/nZVI), were synthesized by one-step liquid-phase reduction and applied for

Accepted 12 October 2017

oxytetracycline (OTC) removal. The effects of contact time and initial pH on the removal

Available online xxxx

efficiency were studied. The as-prepared nanoscale particles were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray

Keywords:

diffraction (XRD). Finally, the degradation mechanisms of OTC utilizing the as-prepared

Bimetallic nanoscale zero-valent

nanoparticles were investigated by using X-ray photoelectron spectroscopy (XPS) and mass

iron particles

spectrometry (MS). Cu/nZVI presented remarkable ability for OTC degradation and removed

Oxytetracycline

71.44% of OTC (100 mg/L) in 4 hr, while only 62.34% and 31.05% of OTC was degraded by Ni/nZVI

Degradation mechanism

and nZVI respectively. XPS and MS analysis suggested that OTC was broken down to form small

Hydroxyl radicals

molecules by ·OH radicals generated from the corrosion of Fe0. Cu/nZVI and Ni/nZVI have been proved to have potential as materials for application in OTC removal because of their significant degradation ability toward OTC. © 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Introduction Oxytetracycline (OTC), a member of the tetracycline family that is one of the broad-spectrum antibiotics used in veterinary medicine and aquaculture, has received increasing attention due to the spread of antibiotic resistance in microorganisms (Chen et al., 2011; Storteboom et al., 2010). It has been widely used for decades as a feed additive in farmed fish, as a growth-stimulating substance in domestic animals and as preventive therapy for bacterial diseases in plants (Chi et al., 2010). However, its difficulty being metabolized in animals results in contamination of manure or urine in the form of the parent compound or its metabolites (Heuer et al., 2008). Furthermore, as a result of its water-solubility and degradation resistance, OTC has been widely detected in soil environments,

coastal environments, and even drinking water (Li et al., 2011; Tang et al., 2015). OTC can cause inhibition of the antibody levels in fish, deoxyribonucleic acid (DNA) damage in carp, and reduction in erythrocyte counts and hemoglobin values, when it is absorbed into organisms (Chi et al., 2010; Li et al., 2011; Lunden et al., 1998; Omoregie and Oyebanji, 2002; Qu et al., 2004). Recently, nanoscale zero-valent iron (nZVI) has been extensively utilized as an environmental remediation material, especially for treatment of organic contaminants, owing to its unique advantages, including high specific surface area and great capacity for reductive reaction (Fu et al., 2013). However, the tendency of nZVI particles to agglomerate into large particles and the generation of oxide layers on the surface of particles lead to inferior reaction activity and low removal efficiency (Dong et al., 2015; Shi et al., 2016; Xiao et al., 2014). In order to overcome the

⁎ Corresponding authors. E-mail: [email protected] (Qinyan Yue), [email protected] (Baoyu Gao).

https://doi.org/10.1016/j.jes.2017.10.006 1001-0742/© 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006

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above-mentioned disadvantages, the reactivity and functionality of nZVI have been enhanced through immobilization of nZVI onto support materials (Xiao et al., 2014) and deposition of a discontinuous layer of other metals (Cu, Ni, Pd etc.) onto nZVI surfaces (Chang et al., 2011; Shi et al., 2016). In most studies, nanoscale bimetallic particles have showed much higher activity than monometallic particles. Palladium (Pd) has exhibited extremely high removal efficiency as an additive in Pd/Fe bimetallic catalytic reductants, but the application is limited because of its high cost (Chang et al., 2011; Han et al., 2008). As much more economical metals, Cu and Ni were added into nZVI in this study instead of Pd. Thus, the formation of [H] on the surface of nZVI was enhanced by bimetallic Cu/nZVI and Ni/nZVI nanoscale particles (Gao et al., 2016; Zhu et al., 2010). Bimetallic nanoscale particles are increasingly used to promote the efficiency of organics removal, but research on the improvement of OTC removal using iron-based nanoparticles remains limited. This study was conducted in an effort to improve OTC removal from actual wastewater by Cu/nZVI and Ni/nZVI produced through liquid-phase reduction. The effects of significant factors on OTC removal, including initial pH and contact time, were investigated during the experiments. In addition, scanning electron microscopy (SEM) was utilized to explore the morphologies of as-prepared and exhausted nZVI, Cu/nZVI and Ni/nZVI. The crystal structures of original and modified iron-based nanoparticles were characterized by X-ray diffraction (XRD), meanwhile the chemical properties were analyzed by X-ray photoelectron spectroscopy (XPS). Subsequently, structural data on the degradation products of OTC was obtained by using mass spectrometry (MS), and the mechanisms of the degradation process were also investigated. This study investigated the transformation of Fe0 during reaction as well as the products of OTC degradation, providing insight into the mechanism of OTC removal by bimetallic nanoscale zero-valent iron particles.

Fig. 1 – Chemical structure of oxytetracycline (OTC).

To obtain nZVI, iron (II) sulfate heptahydrate (FeSO4·7H2O) (4.97 g) was dissolved in a 100-mL miscible solution with a volume ratio of absolute ethyl alcohol and distilled water of 3:7. Then the above mixture was stirred at 120 r/min in a 25°C water bath under nitrogen for 5 min, and 0.054 mol/L NaBH4 solution (50 mL) as a strong reductant was added drop-wise (2 mL/min) into the three-necked flask followed by vigorous stirring under a N2 atmosphere. The solution was shaken for another 30 min after the addition of NaBH4 was complete. After that, the synthesized nZVI particles were cleaned using distilled water and ethanol three times in turn.

1.2.1. Ni/nZVI (Cu/nZVI) bimetallic nanoparticles The bimetallic Ni/nZVI (Cu/nZVI) was synthesized by the borohydride reduction of 4.97 g FeSO4·7H2O and 0.1 g NiCl2·6H2O (0.9776 g CuSO4·5H2O) in solution containing 30 mL ethanol and 70 mL degassed reverses osmosis (RO) water. The rest of the preparation process was the same as the synthesis of nZVI described above. After the same washing sequence as above, the freshly washed nanoscale particles were dried at 70°C in vacuum for 4 hr, and stored in a nitrogen atmosphere at room temperature before use.

1. Materials and methods

1.3. Characterization of nZVI, Ni/nZVI and Cu/nZVI

1.1. Materials and chemicals

The morphological properties and characteristics of the as-prepared particles were observed utilizing a scanning electron microscope (SEM, JEOL, JSM 6700F, Japan). In addition, an energy dispersive spectroscopy (EDS, INCA, Oxford Instruments, UK) was utilized to analyze the localized compositional information of the iron-based nanoscale particles. The crystal structures of the synthesized nanoparticles were characterized by XRD (Bruker SMART APEX II, BRUKER, Germany) and chemical properties were obtained by XPS analysis using a multifunctional imaging electron spectrometer (Thermo ESCALAB 250XI, Thermo Fisher Scientific, USA).

The water used for all experiments was generated from an ultrapure water system (FFX1502-RO, Qingdao FLOM Technology Co., Ltd., China), with the exception of WaHaHa pure water used for liquid chromatography. The standard of OTC hydrochloride was purchased from Aladdin industrial corporation (purity >95%, Shanghai) and its chemical structure is shown in Fig. 1 below. FeSO4·7H2O, NaBH4 and ethanol for synthesis were obtained from Shanghai Chemical Plant Co. (China). CuSO4·5H2O was purchased from Tianjin Kermel Chemical Reagent Co. NiCl2·6H2O was purchased from Tianjin Bodi Chemical Co., Ltd.

1.4. Analytical methods 1.2. Preparation of nZVI, Ni/nZVI and Cu/nZVI Zero-valent iron nanoscale particles (nZVI) were synthesized by liquid-phase reduction according to reference (Schrick et al., 2002; Wang et al., 2006) by the following reaction:

4Fe2þ þ 2BH−4 þ 6H2 O → Fe0 ↓ þ 2BðOHÞ3 þ 7H2 ↑

ð1Þ

All the aqueous solutions of OTC for analysis were filtered by a 0.22-μm membrane filter and then analyzed by a liquid chromatograph (SPD-20A, Shimadzu, Japan) with a C18 chromatographic column (25 cm × 4.6 mm). The mobile phase was a mixture of 0.01 mol/L aqueous citric acidacetonitrile (V citric acid:V acetonitrile = 75:25) at a flow rate of 1 mL/min; the wavelength for absorbance detection was

Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006

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355 nm and the injection volume was 20 μL. The concentration of OTC in aqueous solution was determined from the linear relationship of a calibration curve of peak area versus OTC concentration.

removal. The removal efficiency of OTC using original and modified nanoscale materials was calculated using the concentration of OTC before and after reaction according to the following equation:

1.5. Batch experiments

η¼

To investigate the OTC removal efficiency using various as-prepared nanoscale particles, batch experiments were carried out with nZVI (0.01 g), Ni/nZVI (0.01 g) and Cu/nZVI (0.01 g) added respectively to solutions containing 100 mg/L OTC (50 mL). These mixed solutions were then placed in a temperature-controlled shaker (SHA-B, Jintan Youlian Instrument Research Institute, China) at 298 K with a speed of 150 r/min for 4 hr to ensure equilibrium between degradation and adsorption. Samples were taken at 10, 30, 60, 90, 120 and 240 min and filtered through 0.22-μm membranes for the purpose of OTC concentration determination. In addition, different dosages (0.1 and 0.2 g/L) were added into 50-mL flasks, in order to study the effect of particle dosage on OTC

where η (%) is the OTC removal efficiency; C0 (mg/L) is the initial OTC concentration, while Ce (mg/L) represents the OTC concentration after reaction.

ðC0 −Ce Þ  100% C0

ð2Þ

2. Results and discussion 2.1. Characterization of nZVI, Ni/nZVI and Cu/nZVI 2.1.1. Electron microscopy analysis The surface morphologies and size of synthesized bimetallic and monometallic iron-based nanoscale particles before and after reaction with OTC are shown in Fig. 2. It can be observed

Fig. 2 – Scanning electron microscopy (SEM) images of copper/nanoscale zero-valent iron particles (Cu/nZVI) (a) before and (b) after reaction, Ni/nZVI (c) before and (d) after reaction, nZVI (e) before and (f) after reaction. Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006

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that the size of spherical Cu/nZVI (Fig. 2a), Ni/nZVI (Fig. 2c) and nZVI (Fig. 3e) nanoscale particles ranged from 20 to 80 nm and clustered with each other in chains. The severely agglomerated structure of nZVI, Cu/nZVI and Ni/nZVI was due to the magnetic and electronic interactions between the nanoscale particles (Bokare et al., 2007; Xiao et al., 2014). In addition, the shape of Cu/nZVI was irregular, as found by Liang Sun (Sun et al., 2016) and its size was larger than that of Ni/nZVI, which may have influenced the OTC removal efficiency by Cu/nZVI. However, after degradation of OTC, a significant growth in the size of nZVI, Ni/nZVI and Cu/nZVI nanoscale particles was observed, as shown by the images in

a

Fe0

Intensity

Before

After

Iron oxide

10

20

30

40

50

60

70

80

2θ (degree)

b

Intensity

Fe0

Before

Iron oxide After

10

20

30

40

50

60

70

80

2θ (degree)

c

Fe0

Fig. 3b, d and f, due to the formation of products such as oxide or hydroxide generated by iron corrosion on the surface of the nanoparticles. The oxidation products were proved to be predominantly iron oxyhydroxide (FeOOH), according to the XPS analysis.

2.1.2. Energy dispersive spectrometer analysis In order to further explain the change in the tested nanoparticles due to reaction, EDS was utilized to characterize the elemental compositions of as-prepared nZVI, Cu/nZVI and Ni/nZVI. The weight ratios of the four elements (Fe, Ni, Cu and O) detected by EDS on the iron surface are shown in Table 1. As can be observed, weight ratios of 1.73% and 2.72% of Cu or Ni were successfully loaded on the nZVI, respectively. Thus, the presence of O (13.73, 14.79 and 17.20 wt.% in nZVI, Cu/nZVI and Ni/nZVI, separately) indicated that an oxide layer was formed on the surface of the synthesized nanoparticles. In addition, the decreases in the weight ratios of the inert metals (Cu, Ni) were due to the increase in the weight ratio of O. However, the decrease in the weight ratio of Fe in nZVI, Ni/nZVI and Cu/nZVI (from 86.27%, 78.34%, and 83.49% to 68.75%, 61.98%, and 60.84%, respectively) after a 240-min reaction with OTC suggested that significant iron corrosion took place in the tested nanoscale particles during degradation, as verified by subsequent XRD analysis.

2.1.3. Crystal structure Fig. 3 presents the XRD patterns of the nanoscale particles before and after reaction with OTC. In each pattern, the existence of zero-valent iron (α-Fe) in samples of nZVI, Ni/nZVI and Cu/nZVI was revealed by the sharp peak at 2θ = 44.6° (Petala et al., 2013; Su et al., 2011) (Zhang et al., 2016) and in addition, small amounts of by-products (such as oxides or hydroxides of Fe) were generated during synthesis. However, the content of Ni or Cu (Ni/Cu:Fe = 0.025:1 wt.%) loaded on the nZVI was too small for detection of any characteristic peaks of Cu or Ni. After reaction with OTC for 4 hr, the peak at 2θ = 44.6° disappeared. In addition, the peaks at 2θ = 30–38°, which indicate the existence of iron oxide (Sun et al., 2006), were present in every XRD pattern for the tested bimetallic or monometallic nanoparticles. The results above suggest that Fe0 was oxidized to Fe3 + during the reaction and an oxidation layer was generated on the surface, which blocked the detection of Fe0. These facts supported the SEM results and explain the phenomenon of the change in size of the iron-based nanoscale particles.

Intensity

Before

Table 1 – Energy dispersive spectroscopy (EDS) patterns of chemical composition of nanoscale zero-valent iron particles (nZVI), Cu/nZVI and Ni/nZVI before and after reaction.

Iron oxide After

Elements 10

20

30

40

50

60

70

80

2θ (degree)

Fig. 3 – X-ray diffraction (XRD) patterns of (a) Cu/nZVI, (b) Ni/nZVI and (c) nZVI before and after reaction with OTC.

O (wt.%) Fe (wt.%) Cu (wt.%) Ni (wt.%)

nZVI

Cu/nZVI

Ni/nZVI

Before

After

Before

After

Before

After

13.73 86.27 – –

31.25 68.75 – –

14.79 83.49 1.73 –

38.26 60.84 0.91 –

17.20 78.34 – 2.72

33.49 61.98 – 1.36

Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006

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100

2.2. Removal of oxytetracycline (OTC) 2.2.1. Removal of OTC using synthesized samples Removal efficiency (%)

80

To investigate the effect of contact time on OTC removal efficiency by the iron-based nanoscale particles, different dosages (0.1 or 0.2 g/L) of Cu/nZVI, Ni/nZVI and nZVI were added into OTC aqueous solution for 4 hr, with the initial OTC concentration of 100 mg/L. As shown in Fig. 4, after 120 min of reaction, OTC in solution was treated by 0.1 g/L nZVI, Ni/nZVI or Cu/nZVI, where 31.1%, 62.3% and 71.4% of OTC was degraded, respectively. When 0.2 g/L nZVI and 0.2 g/L bimetallic Ni/nZVI or Cu/nZVI was used, OTC was almost completely removed within 240 min. Meanwhile, it was found that the degradation rate of OTC using bimetallic Ni/nZVI and Cu/nZVI nanoparticles was much higher than that of nZVI after similar interaction times. During the first 60 min, the bimetallic Ni/nZVI and Cu/nZVI particles, compared with nZVI, presented a much faster growth in the reaction rate with OTC, and then almost reached degradation/adsorption equilibrium after 120 min. This was because electron transfer and the generation of H2 was promoted by the inert metals, Ni or Cu (Bokare et al., 2007; Gao et al., 2016; Zhou et al., 2010).

60

40

Cu/nZVI Ni/nZVI

20

0 1

2

3

4

5

6

7

8

9

10

pH Fig. 5 – Influence of initial pH on the removal efficiency of oxytetracycline (OTC) under different nanoparticles. Conditions: concentration of OTC 100 mg/L; temperature 298 K; rotation speed 150 r/min; Cu/nZVI = 0.1 g/L; Ni/nZVI = 0.1 g/L.

2.2.2. Effect of pH on OTC removal efficiency To further investigate the influence of iron corrosion on OTC removal efficiency, the effect of initial pH (from 2 to 9) on the removal efficiency using various nZVI-based nanoscale particles was determined, as shown in Fig. 5. As can be seen, the optimal origin pH for OTC removal was in the range from 5 to 6. When the initial pH of the OTC solution was 5 or 6, the removal efficiency of OTC was almost 90% after reacting with Ni/nZVI or Cu/nZVI for 4 hr, respectively. Thus, iron corrosion was initiated at low pH, resulting in generation of abundant H2 and hydrogen radicals. However, the poor removal efficiency OTC at the initial pH of 2–3 using Ni/nZVI and Cu/nZVI for 240 min indicated that direct reduction and degradation on OTC by Fe0 was not the dominant reaction

80

40 60

30

40

20

20

0.2g/L Ni/nZVI

0.2g/L Cu/nZVI

0.1g/L Ni/nZVI

0.1g/L Cu/nZVI

0.1g/L nZVI

0 0

50

100

150

200

Zeta potential (mV)

Removal efficiency (%)

100

mechanism. Additionally, the concentration of OTC dropped only slightly in alkaline environments. At high pH, OTC was in a negatively charged form (Hanay and Turk, 2015), while the nZVI surface was also negative, as shown in Fig. 6, leading to electrostatic repulsion between OTC and the iron-based nanoscale particles. Besides, the active sites on the nZVI surface would be blocked by the rapid formation of oxide layers on the surface of the nanoscale particles due to iron corrosion and as a result, the removal of OTC would be inhibited. Consequently, the removal efficiency decreased severely at initial pH 8 and 9. As shown in Table 2, the changes in pH after reaction with Cu/nZVI and Ni/nZVI were investigated. The final pH of the solution increased, and when the initial pH was over 7, significant changes in pH values failed to take place at the end of reaction. It has been reported that the increase of solution pH may be related to the oxidization effect nZVI mechanism

Cu/nZVI Ni/nZVI

10 0 -10

250

Time (min)

Fig. 4 – Degradation of oxytetracycline (OTC) using iron-based nanoscale particles. Conditions: concentration of OTC 100 mg/L; temperature 298 K; rotation speed 150 r/min; nZVI = 0.1 g/L; Cu/nZVI = 0.1 or 0.2 g/L; Ni/nZVI = 0.1 or 0.2 g/L.

-20 -30 5

6

7

8

9

pH

Fig. 6 – Zeta potential of modified nZVI at different pH values.

Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006

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Table 2 – Final pH values of solution under different initial pH. Initial pH

Final pH Cu/nZVI

Ni/nZVI

2.16 4.55 6.45 7.04 7.54 7.79 8.23 8.45

2.10 4.09 6.00 6.86 7.45 8.00 8.38 8.62

2 3 4 5 6 7 8 9

2.3.2. Analysis of the degradation products

as follows (Bokare et al., 2007; Chen et al., 2011; Li et al., 2017; Lien and Zhang, 2001): Fe þ 2H2 O → Fe2þ þ H2 þ 2OH−

ð3Þ

2Fe2þ þ 2H2 O → 2Fe3þ þ H2 þ 2OH−

ð4Þ

2.3. Removal mechanism 2.3.1. XPS analysis of iron-based particles The mechanism of OTC removal by Cu/nZVI or Ni/nZVI was verified by XPS analysis. According to the results of the batch experiments above, Cu/nZVI was proved to have the highest removing capacity, and maintained high reactivity during the reaction. It has been hypothesized that Cu or Ni, as an inert metal added into nZVI, not only can prevent the oxidization of Fe0 but also can help to degrade the OTC. Thus, in order to describe the removal mechanism, the surface characteristics of nZVI, Cu/nZVI and Ni/nZVI before and after reaction with OTC were analyzed by XPS. The peaks of Fe 2p at binding energies of 710.7 ± 0.2, 712.5 ± 0.2 and 724.5 ± 0.2 eV, represented Fe2O3, FeOOH and Fe (III) respectively, as reported in previous works (Chen et al., 2011; Lien and Zhang, 2001; Zhou et al., 2010). As shown in Fig. 1, the area of Fe0 peaks at 707 ± 0.2 eV in Fig. 7a, b and c (Li and Zhang, 2007; Taha and Ibrahim, 2014; Xiao et al., 2015) followed the order of Cu/nZVI > Ni/nZVI > nZVI, which suggested that Cu and Ni as an additive could promote the nZVI stability and prevent the oxidation of iron. Specifically, no peak for Fe0 could be observed after reaction with OTC as shown in Fig. 7d, e and f and in addition, the area of the FeOOH peak increased a lot, especially in Fig. 7d. This could indicate that Fe0 is oxidized to Fe3+ and Fe2O3 and iron oxyhydroxide (FeOOH) formed on the iron surface during OTC reduction with nZVI (Su et al., 2011), which was confirmed by SEM and explained by Eqs. (5)–(7) (Bokare et al., 2007; Li and Zhang, 2007; Su et al., 2013): Fe3þ 2H2 O → FeOOH þ 2Hþ

Fig. 6, the peak attributed to Fe-O species at binding energy of 529.6 eV decreased, while the peak of Fe-O groups increased. From the Fe 2p and O 1s XPS spectra, it can be deduced that the Fe0 could transform into FeOOH after reaction. Moreover, hydroxyl radicals could be generated during the production of FeOOH, (Sun et al., 2016) which would then oxidize OTC.

ð5Þ

In order to further investigate the removal mechanism of OTC by iron-based nanoscale particles, O 1s spectra of Cu/nZVI before and after reaction were analyzed and exhibited Fig. 1g and h. The peaks at 529.9 ± 0.2, 531.6 ± 0.2 and 532.6 ± 0.2 eV corresponded to Fe-O (oxygen bonded to metal), Fe-OH (hydroxyl bonded to metal) and adsorbed water at the surface (H2O), respectively (Li et al., 2006; Su et al., 2013). In

As shown in Fig. 8, the OTC aqueous solutions were analyzed by MS analysis before and after reaction for 30 min, 2 hr and 4 hr with the purpose of further determination of the products of OTC after reacting with the original and modified nanoscale particles. Molecules with one proton less were detected in Fig. 8. As shown in Fig. 8a, a cluster of mass-to-charge ratio (m/z) signals was detected around 459, which corresponded to OTC. After the reaction with Cu/nZVI, it was indicated by mass spectra in Fig. 8b that high mass intensity representing OTC continued to be observed, and no new response stronger than OTC was discovered, due to the further degradation of OTC by-products. However, ions with m/z of 201, 219, 263 and 475 were found in the MS of the supernatant after 30 min, and they grew stronger in the MS of the aqueous solution after 2 hr of reaction (Fig. 8c). After degradation for 4 hr, the concentration of ions at the m/z of 263 declined, while ions at the m/z of 201, 219 and 475 were mainly observed in Fig. 8d. Based on the MS results, the conclusion could be drawn that the OTC was degraded into small molecules and the signals of the different molecules were lower than those of OTC, indicating that the products of degradation were further degraded into fragments during the reaction. According to Fig. 8, the potential structures of these products are shown in Appendix A. Table S1. Additionally, the increase by 16 (m/z) of molecules with the m/z of 475 probably resulted from the addition of oxygen (Fig. 9) (Chen et al., 2011, 2016; Ji et al., 2016), indicating an oxidation process. Based on the results of XPS and MS analysis, the oxidization effect of Fe0 may have led to the degradation above. Hydrogen peroxide (H2O2) was produced via reaction between the dissolved oxygen and Fe0 corresponding to Eq. (6), and thus, Fe2+ was formed and then reacted with H2O2 to produce hydroxyl radicals (·OH) through a Fenton-like reaction, Eq. (7) (Fang et al., 2011; Fu et al., 2015; Guan et al., 2015; Ju, 2012; Shemer et al., 2006; Yirsaw et al., 2016): Fe0 þ O2 þ 2Hþ → H2 O2 þ Fe2þ

ð6Þ

Fe2þ H2 O2 →  OH þ Fe3þ þ OH−

ð7Þ

2Fe3þ þ Fe0 → Fe2þ

ð8Þ

From the data above, it is possible that the ·OH produced by the corrosion of Fe0 played an essential role in the degradation of OTC, rather than the direct reduction of OTC. Besides, the degradation of tetracyclines by ·OH has been already reported. Fu et al. (2015) found that TC can be degraded into fragments by ·OH produced in the reaction between dissolved oxygen and nZVI, and Ju (2012) indicated that ·OH played a significant role in the microwave-Fenton system for OTC degradation. These experiments have provided evidence that supports the oxidation of OTC by ·OH. However, further investigation is needed in order to study the degradation factors in detail.

Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006

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50000

30000 Fe 2p3/2

a

45000 Fe(III)

40000

Fe(III) FeOOH

24000

Intensity

Intensity

d Fe 2p1/2

26000

FeOOH

Fe2O3

35000

Fe 2p3/2

28000

Fe 2p1/2

satellite of Fe2+

30000 25000

Fe2O3

22000

satellite of Fe2+

20000 18000

Fe0

20000

16000 14000

15000

12000 10000

10000 705

710

715

720

725

730

705

735

710

715

50000

Fe 2p3/2

30000

740

e

FeOOH Fe2O3

FeOOH

Intensity

satellite of Fe2+

30000 25000 20000

735

Fe(III)

Fe2O3

35000

730

Fe 2p1/2

Fe(III)

40000

Intensity

725

Fe 2p3/2

35000

b

Fe 2p1/2

45000

720

Binding energy (eV)

Binding energy (eV)

satellite of Fe2+

25000

20000

15000

Fe0

15000 10000

10000 700

705

710

715

720

725

730

735

705

740

710

715

720

725

730

735

Fe(III)

f

730

735

740

Binding energy (eV)

Binding energy (eV) 50000 Fe 2p3/2

Fe 2p3/2

35000

c

45000

Fe 2p1/2

Fe 2p1/2

Fe(III)

FeOOH

30000 Fe2O3

35000

Fe2O3

FeOOH

satellite of Fe2+

satellite of Fe2+

Intensity

Intensity

40000

30000 25000

25000

20000

20000

15000 Fe0

15000

10000

10000 705

710

715

720

725

730

735

740

705

710

715

Binding energy (eV)

60000

740

h

Fe-OH 50000

Intensity

Intensity

40000

725

60000

g Fe-OH

50000

720

Binding energy (eV)

Fe-O

30000

40000 30000

Fe-O 20000

20000

10000

10000

0

0 524

526

528

530

532

534

536

538

540

Binding energy (eV)

524

526

528

530

532

534

536

538

540

Binding energy (eV)

Fig. 7 – X-ray photoelectron spectroscopy (XPS) spectra of Cu/nZVI (a) before and (d) after reaction with OTC, Ni/nZVI (b) before and (e) after reaction with OTC, nZVI (c) before and (f) after reaction with OTC, and O 1s of Cu/nZVI (g) before and (h) after reaction.

3. Conclusions The removal efficiency of OTC from aqueous solution could be improved significantly by the nZVI particles partially coated

with an inert metal (Cu or Ni). The addition of inert metal (Cu or Ni) has been proved to promote electron transfer between particles and the generation of hydroxyl radicals, which can protect Fe0 from oxidation. As nearly 71.4% and 62.3% of OTC was removed, the removal capacity of modified nZVI particles

Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006

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Fig. 8 – Mass spectrometry (MS) spectra of 100 mg/L oxytetracycline (OTC) aqueous solution (a), with 0.1 g/L Cu/nZVI mixture after 30 min (b), 2 hr (c), 4 hr (d).

Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006

J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 7 ) XX X–XXX

Fig. 9 – Proposed structure of molecule at the mass-to-charge ratio (m/z) of 475.

was much better than that of raw nZVI (31.1%). The initial pH was found to be a vital factor in the removal efficiency of OTC utilizing iron-based nanoscale particles, and the optimal condition for removal of OTC was found to be a weakly acidic environment. In addition, based on the data of XRD and XPS, the Fe0 was oxidized into FeOOH with the generation of hydroxyl radicals (·OH). Further, the main degradation products were investigated by MS. According to these experiments, OTC was degraded into small molecules, and it is possible that oxidation by ·OH is the dominant removal mechanism for OTC, instead of reaction with [H]. The Cu/nZVI nanoparticles can be used as an effective and economical material for application in OTC removal. Supplementary data to this article can be found online at https://doi.org/10.1016/j.jes.2017.10.006.

Acknowledgments This work was supported by grants from Tai Shan Scholar Foundation (No. ts 201511003).

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Please cite this article as: Wu, Y., et al., Performance of bimetallic nanoscale zero-valent iron particles for removal of oxytetracycline, J. Environ. Sci. (2017), https://doi.org/10.1016/j.jes.2017.10.006