Ionothermal synthesis of Cu-doped Fe3O4 magnetic nanoparticles with enhanced peroxidase-like activity for organic wastewater treatment

Advanced Powder Technology xxx (2018) xxx–xxx

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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Ionothermal synthesis of Cu-doped Fe3O4 magnetic nanoparticles with enhanced peroxidase-like activity for organic wastewater treatment Xuanlin Huang a, Cong Xu a, Jiping Ma b, Fengxi Chen a,⇑ a b

School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, PR China School of Environment & Municipal Engineering, Qingdao Technological University, Qingdao 266033, PR China

a r t i c l e

i n f o

Article history: Received 25 September 2017 Received in revised form 18 December 2017 Accepted 22 December 2017 Available online xxxx Keywords: Magnetite Cu doping Ionothermal synthesis H2O2 Peroxidase-like activity

a b s t r a c t The intrinsic peroxidase-like activity of magnetite magnetic nanoparticles (Fe3O4 MNPs) has to be improved to activate H2O2 under mild conditions for practical applications. Herein copper-doped Fe3O4 (Fe3
xCuxO4, x: 0.06–0.23) MNPs were successfully prepared by oxidative precipitation-combined ionothermal synthesis and characterized by XRD, VSM, XPS, BET, etc. The Cu2+ dopants are mainly substituted for Fe2+ ions at octahedral sites and significantly surface-enriched, which expedite the Fe3+/ Fe2+ redox cycling and enhance the H2O2-activation ability of Fe3
xCuxO4 MNPs. Kinetic study showed that the decomposition of H2O2 on Fe2.88Cu0.12O4 was much faster than that on the undoped Fe3O4 (0.584 vs. 0.153 h
1 at 25 °C) due to the lower activation energy of the former (55.3 vs. 62.1 kJ/mol). The enhanced H2O2-activation ability upon copper doping was exploited to efficiently degrade recalcitrant organic pollutants (e.g., rhodamine B) with H2O2 at pH  7 and 25 °C on Fe2.88Cu0.12O4 with good stability and reusability (16 h tested in eight cycles). Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction The intrinsic peroxidase-like activity of magnetite magnetic nanoparticles (Fe3O4 MNPs) has a wide range of potential applications in biotechnology, medicine and environmental chemistry (e.g., glucose detection, water and soil remediation) [1–5]. However, the H2O2-activation ability of Fe3O4 MNPs has to be improved for practical applications under mild conditions (e.g., pH  7 and room temperature) [2]. Transition metal doping has been used to enhance the H2O2activation ability of Fe3O4 MNPs [6]. Among various dopants, copper (Cu) is earth-abundant and low-cost. Cu2+ and Fe2+ have similar ionic radii, which is beneficial for the substitution of Fe2+ with Cu2+ to generate Cu-doped Fe3O4 (in short Cu-Fe3O4) [7,8]. In terms of the reactivity towards H2O2, both copper and iron have multiple valence states and similar redox properties (Eqs. (1)(4)) [9], leading to synergistic activation of H2O2 [10–14]. Previous studies about the applications of Cu-Fe3O4 MNPs mainly focused on the adsorption for arsenic [15,16] or antimony [17] while only one recent paper reported the catalytic activity of Fe3
xCuxO4 (0  x  0.25) nanoparticles in the degradation of Amaranth food dye by hetero-

⇑ Corresponding author. E-mail address: [email protected] (F. Chen).

geneous electro-Fenton process in alkaline medium (pH 13.4) [7]. To the best of our knowledge, the environmental application of the peroxidase-like activity of Cu-Fe3O4 MNPs under mild conditions has not been explored so far.

Fe2þ + H2 O2 ! Fe3þ +  OH + OH
(k1 = 63—76 M
1 s
1 )

ð1Þ

Fe3þ + H2 O2 ! Fe2þ + Hþ + HOO (k2 = 0.001—0.01 M
1 s
1 ) ð2Þ Cu2þ + H2 O2 ! Cuþ + Hþ + HOO (k3 = 4.6  102 M
1 s
1 )

ð3Þ

Cuþ + H2 O2 ! Cu2þ +  OH + OH
(k4 = 1.0  104 M
1 s
1 )

ð4Þ

Magnetite-based particles were usually prepared in molecular solvents (e.g., water and ethylene glycol) via hydrothermal or solvothermal synthesis [18–20]. Cu-Fe3O4 made via these methods usually contained metallic copper as impurity due to the reduction of Cu2+ by a reducing agent in the system (e.g., ethylene glycol [15,17], NaBH4 [7] or Na2SO3 [16]). Ionothermal synthesis of Fe3O4 MNPs was carried out by co-precipitation of Fe3+ and Fe2+ ions or oxidation-precipitation of Fe2+ ions in an ionic deep eutectic solvent (DES) comprising the mixture of choline chloride and urea in a 1:2 M ratio (in short ChCl:2urea, melting point: 12 °C) [4,21].

https://doi.org/10.1016/j.apt.2017.12.025 0921-8831/Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article in press as: X. Huang et al., Ionothermal synthesis of Cu-doped Fe3O4 magnetic nanoparticles with enhanced peroxidase-like activity for organic wastewater treatment, Advanced Powder Technology (2018), https://doi.org/10.1016/j.apt.2017.12.025

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X. Huang et al. / Advanced Powder Technology xxx (2018) xxx–xxx

In the present work, it was found that the newly discovered oxidative precipitation-combined ionothermal synthesis [4] was very suitable for isomorphic substitution of Fe2+ with Cu2+ since its oxidative environment effectively avoids the formation of metallic copper during the operation and generated Cu-Fe3O4 MNPs with enhanced peroxidase-like activity to activate H2O2 under mild conditions for the degradation of recalcitrant organic pollutants.

was calculated from adsorption data in the relative pressure range from 0.05 to 0.30. The total pore volume, Vt, was assessed from the adsorbed amount at the relative pressure of 0.99 by converting it to the corresponding volume of liquid adsorbate. The conversion factor between the volume of gas and liquid adsorbate is 0.0015468 for N2 at 77 K when they are expressed in cm3/g and cm3 STP/g, respectively.

2. Experimental

2.4. Decomposition of H2O2 on Cu-Fe3O4 MNPs

2.1. Chemicals FeSO47H2O, CuCl22H2O, KOH, choline chloride, urea, RhB and H2O2 (30 wt%) were of analytic grade and purchased from Sinopharm Chemical Reagent.

The decomposition of H2O2 was investigated by allowing the mixture of the catalyst (undoped Fe3O4 or Cu-Fe3O4-2, 0.5 g/L) and H2O2 (40 mM) to react at 25 °C for various times. At each selected interval, 0.5 mL of aliquot was sampled to analyze the concentration of H2O2 by titration with KMnO4 (0.5 mM).

2.2. Preparation of Cu-Fe3O4 MNPs

2.5. Degradation of RhB with H2O2 on Cu-Fe3O4 MNPs

Three Cu-Fe3O4 MNPs with the nominal Cu/Fe molar ratio of 1:40, 1:20 and 1:10 were prepared and designated as Cu-Fe3O41, -2 and -3, respectively (Table 1). In a typical procedure, the DES was first prepared by mixing 2.793 g (20 mmol) of choline chloride and 2.402 g (40 mmol) of urea in a 60 mL Teflon liner at 50 °C. Then 0.834 g (3.0 mmol) of FeSO47H2O and 0.292 g (5.2 mmol) of KOH were added and stirred at 600 rpm for 30 min, followed by adding the second portion of KOH (0.210 g, 3.8 mmol) and CuCl22H2O (0.026 g, 0.15 mmol). After stirring at 80 °C for another 30 min, the Teflon liner was sealed in a stainless steel autoclave and heated at 110 °C for 4 h. During the heat treatment, the oxidants in the system (such as O2 from the air in the liner free space) partially oxidize Fe2+ to generate Cu-Fe3O4-2 MNPs. The product was magnetically recovered, washed to pH  7 with deionized water and ethanol and dried in a 50 °C oven for 4 h. For comparison, undoped Fe3O4 MNPs were also prepared according to this procedure but no copper source was added.

In a typical procedure, 0.025 g of Cu-Fe3O4-2 and 50 mL of RhB stock solution (10 lM) were firstly stirred in a flask at 150 rpm and room temperature (25 ± 2 °C) for 15 min. The initial concentration of RhB (C0) was then measured. The degradation of RhB was initiated by rapidly adding 0.230 g of H2O2 (30%) into the solution. At each selected time interval, about 3 mL sample was taken by a syringe, passed through a filter membrane and then analyzed the concentration of RhB remaining in the solution. The concentration of RhB was determined by measuring its absorbance at 554 nm using UV–vis spectrophotometer. Multiple measurements of the catalytic activity were performed with the relative standard deviation below 5%, and the results were expressed as mean values with error bars.

2.3. Characterization

XRD patterns of the undoped and Cu-coped Fe3O4 are similar (Fig. 1a). Eight characteristic peaks were assigned according to cubic inverse spinel structure of magnetite (JCPDS No. 65-3107). Two extra small peaks at 32.04° and 52.76° are discernible in curve d for Cu-Fe3O4-3 (marked by asterisks), indicating the formation of some impurity as the initial Cu/Fe molar ratio increases to 1:10. These two peaks cannot be matched with the standard diffraction data of CuO (JCPDS No. 65-2309). They may be due to the incomplete conversion of some FeOOH intermediates to Fe3O4 as more copper salts were added in the feed. The strongest (3 1 1) diffraction peak gradually shifts to higher angles as the copper content increases, e.g., from 35.44° for undoped Fe3O4 to 35.56° for CuFe3O4-3 (Fig. 1b). Accordingly, the lattice parameter gradually reduces from 8.394 Å for undoped Fe3O4 to 8.367 Å for Cu-Fe3O43 (Table 1). The smaller lattice parameters of Cu-Fe3O4 are consistent with partial substitution of Fe2+ by Cu2+ in octahedral sites of the spinel structure [7]. Cu2+ has higher preference for octahedral

Powder X-ray diffraction (PXRD) patterns were collected on Bruker D8 Advance Diffractometer with Cu Ka radiation (k = 0.15406 nm) at 30 kV and 20 mA. The bulk Cu content was determined by flame atomic absorption spectroscopy (AAS, Thermo Solaar M6). TEM images were taken on JEOL JEM-2100 Transmission Electron Microscope. Magnetic properties were measured on VSM JDAW-2000D vibrating sample magnetometer at 299.3 K. XPS analyses were conducted on X-ray photoelectron spectrometer (ESCALAB 250XI XPS Thermo Company) using a monochrome Al Ka radiation (hm = 1486.6 eV) as the excitation source operating at 150 W and 500 lm. The binding energy (BE) was calibrated by the C1s peak of adventitious carbon at 284.6 eV. The Thermo Avantage v5.52 software was used to perform peak-fitting and calculate the surface atomic ratio. N2 physisorption analysis was performed on Quantachrome Autosorb-6 at 77 K. The BET specific surface area

3. Results and discussion 3.1. Characterization of Cu-Fe3O4 MNPs

Table 1 Compositions and properties of Fe3
xCuxO4 MNPs. Sample

Undoped Fe3O4 Cu-Fe3O4-1 Cu-Fe3O4-2 Cu-Fe3O4-3

Cu/Fe ratio Feed

Product

0 1:40 1:20 1:10

0 1:49 1:24 1:12

Cu content (wt%)

Formula

SBET (m2/g)

Vt (mL/g)

a0 (Å)a

D (nm)b

0 1.72 3.40 6.34

Fe3O4 Fe2.94Cu0.06O4 Fe2.88Cu0.12O4 Fe2.77Cu0.23O4

125.0 58.0 89.4 116.5

0.315 0.300 0.326 0.382

8.394 8.392 8.373 8.367

28.3 26.5 20.3 14.0

pffiffiffiffiffiffi The lattice parameter, a0, was calculated from the (3 1 1) peak by a0 ¼ 11d311 . The average crystallite size, D, was estimated according to the Scherrer equation D = Kk/(bcos h), where k is the Cu Ka wavelength, 2h is the position of the (3 1 1) diffraction peak, b is its pure breadth free of the instrumental broadening and K is a constant (0.9 for spherical particles). a

b

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1)

X. Huang et al. / Advanced Powder Technology xxx (2018) xxx–xxx

(440

(511 )

(422 )

0) ( 40

d: Cu-Fe3O4-3

*

Intensity (a.u.)

*

( 22 2)

( 11 1)

( 22 0)

)

( 31

a

c: Cu-Fe3O4-2

b: Cu-Fe3O4-1

a: Undoped Fe3O4

JCPDS 65-3107

10

20

30

40

50

60

70

80

1)

2θ (°)

( 31

b

Intensity (a.u.)

d: Cu-Fe3O4-3

c: Cu-Fe3O4-2

b: Cu-Fe3O4-1

a: Undoped Fe3O4

33

34

35

2θ (°)

36

37

38

Fig. 1. (a) XRD patterns of the undoped and Cu-doped Fe3O4 MNPs. (b) The magnified (3 1 1) diffraction peaks.

sites at lower contents and has smaller ionic radius than Fe2+ in octahedral coordination (0.73 vs. 0.78 Å). In addition, Fig. 1b reveals that the linewidth of the (3 1 1) peak increases as the Cu content increases due to the smaller crystallite size (Table 1). TEM images in Fig. 2 reveal that the Cu(II) addition has no obvious influence on the morphology and size of the Cu-Fe3O4 particles, which mainly consist of aggregated spherical nanoparticles due to high surface energy and magnetic dipole interaction. The particle sizes of Fe3O4 and Cu-Fe3O4-1, -2 and -3 were measured from the TEM images to be approximately 11.4 ± 3.3, 14.5 ± 1.1, 14.2 ± 2.3 and 14.9 ± 1.1 nm. Both the undoped and Cu-coped Fe3O4 show type IV N2 adsorption-desorption isotherms with type H3 hysteresis loops in the high P/P0 range due to the presence of interparticle pores in nanoparticle aggregates (Fig. 3) [22]. As shown in Table 1, the undoped Fe3O4 has larger SBET than Cu-Fe3O4 (125.0 vs. 58.0– 116.5 m2/g), which may be related to different surface composition and properties (XPS results below indicate that the surface of CuFe3O4 is significantly copper-enriched). SBET and Vt of Cu-Fe3O4 gradually increase from Cu-Fe3O4-1 to Cu-Fe3O4-3 due to the decrease in the crystallite size. The Cu contents of Cu-Fe3O4-1, -2 and -3 were determined by AAS to be 1.72 wt%, 3.40 wt% and 6.34 wt%, respectively. The chemical formula were correspondingly expressed as Fe3
xCuxO4 (x = 0.06, 0.16, and 0.23). The final Cu/Fe molar ratios in Fe3
xCuxO4 are close to the initial values in the feed (Table 1), implying that the oxidative precipitation-combined ionothermal synthesis is

Fig. 2. TEM images of (a, b) undoped Fe3O4, (c, d) Cu-Fe3O4-1 (e, f) Cu-Fe3O4-2 and (g, h) Cu-Fe3O4-3 MNPs.

beneficial for the incorporation of copper into Fe3O4.Fig. 4 shows room-temperature magnetization curves for the undoped Fe3O4 and Cu-Fe3O4-2. Considering that Cu2+ may occupy both tetrahedral and octahedral sites in the spinel structure [7,16,23], Cu2þ 2
3þ 2þ 3þ Fe3O4 is generally presented as ðCu2þ y Fe1
y Þ½Cub
y Fe1
b Fe1þy O4 , 2+ where b is the total Cu content (0.12 for Cu-Fe3O4-2) and y is the Cu2+ content at tetrahedral sites. The magnetic moment l per formula unit increases with y according to l = (4
3b + 8y), where l(Fe2+), l(Fe3+) and l(Cu2+) are 4lB, 5lB and 1lB, respectively. Since Cu-Fe3O4-2 has a much smaller saturation magnetization than the undoped Fe3O4 (48.0 vs. 73.4 emu/g), the y value in Cu-Fe3O4-2 is negligible. Therefore, Cu2+ ions in Cu-Fe3O4-2 mainly substitute Fe2+ ions at octahedral sites. The elemental composition and oxidation states of iron and copper species on the surface of Cu-Fe3O4-2 were investigated by XPS. The XPS survey (Fig. S1) confirms the presence of Fe, Cu and

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Fig. 3. N2 adsorption-desorption isotherms of the undoped and Cu-doped Fe3O4 MNPs.

Fig. 5. XPS spectra for (a) Fe 2p and (b) Cu 2p of Cu-Fe3O4-2 MNPs. Shake-up satellites were marked by the letter S.

Fig. 4. Room-temperature magnetization curves of the undoped Fe3O4 and CuFe3O4-2 MNPs. Inset at the right bottom is the enlargement near the origin.

O on the surface. The Fe 2p XPS spectrum of Cu-Fe3O4-2 (Fig. 5a) shows two main peaks at about 710.6 eV (Fe 2p3/2) and 724.6 eV (Fe 2p1/2) and a shoulder at 718.9 (shake-up satellite of Fe 2p3/2) [11,12,24–27]. The Fe 2p3/2 peak is narrower and stronger than the Fe 2p1/2 peak, which is consistent with fourfold degeneracy of Fe 2p3/2 due to the spin-orbit coupling while Fe 2p1/2 has only twofold degeneracy. Through the deconvolution of the Fe 2p3/2 line, two distinct peaks centered at 710.2 eV and 711.6 eV were obtained, indicating that both Fe2+ and Fe3+ exist at the surface of

Cu-Fe3O4-2. Similarly, the Fe 2p1/2 line could also be fitted as Fe2+ at 723.2 eV and Fe3+ at 724.8 eV. The surface Fe2+/Fe3+ ratio of Cu-Fe3O4-2 was estimated from the corresponding peak areas of Fe 2p3/2 to be 0.28 [11,24,25,27], which is much lower than the theoretical value (0.5) of Fe3O4 due to partial substitution of Fe2+ by Cu2+ and partial oxidation of surface Fe2+ ions in the air. The Cu 2p XPS spectrum of Cu-Fe3O4-2 (Fig. 5b) reveals two main peaks at 933.0 eV (2p3/2) and 953.1 eV (2p1/2) with the corresponding shake-up satellites at about 942.8 eV and 963.3 eV, respectively. The shake-up satellites indicate the presence of Cu2+ in Cu-Fe3O4-2 [7,28]. No Cu+ or metallic Cu species was resolved by the deconvolution of the Cu 2p3/2 peak at 933.0 eV, indicating that only Cu2+ ions exist at the surface of Cu-Fe3O4-2. The surface Cu/Fe atomic ratio determined by XPS is 10 times the bulk value determined by AAS (0.4 vs. 0.04), which indicates that Cu2+ ions were significantly surface-enriched. Since Cu2+ mainly occupies octahedral sites in Cu-Fe3O4-2, its surface enrichment is consistent with the previous finding that octahedral sites in the spinel structure are almost exclusively exposed at the surface [29]. 3.2. H2O2-activation ability of Cu-Fe3O4 MNPs As shown in Fig. 6a, <20% of H2O2 (40 mM) was decomposed after heating for 3 h at 55 °C without the addition of a catalyst.

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Fig. 6. Catalytic decomposition of H2O2 at various temperatures on the undoped Fe3O4 (a and b) and Cu-Fe3O4-2 (c and d) MNPs.

In contrast, 35% of H2O2 (40 mM) was decomposed on undoped Fe3O4 after 3 h at 25 °C due to Fe3O4-catalyzed decomposition of H2O2. In comparison, 60% of H2O2 (40 mM) was decomposed on Cu-Fe3O4-2 after only 1.5 h at 25 °C (Fig. 6c), confirming that the Cu doping significantly enhances the decomposition of H2O2 at 25 °C. To determine the apparent activation energy (Ea), the catalytic decomposition of H2O2 was also conducted at other temperatures and fitted with the first-order kinetic model by plotting ln(C0/C) versus time (Fig. 6b and d), which generally exhibited good linear relationship as reported earlier [30–32]. The ks1 value obtained from the slope of the regression equation (R2: 0.966–0.999) increases as the decomposition temperature increases (Table 2). By plotting lnks1 against 1/T according to the Arrhenius plot, lnks1 = lnA
Ea/RT (Fig. 7), where R is the universal gas constant (8.314 J/(mol K)) and A is the pre-exponential factor, the Ea values were calculated to be 62.1 kJ/mol for undoped Fe3O4 and 55.3 kJ/mol for Cu-Fe3O4-2. The lower Ea value of Cu-Fe3O4-2 corroborates its

Table 2 Catalytic decomposition of H2O2 on undoped Fe3O4 and Cu-Fe3O4-2 MNPs. Catalyst

ks1 (h
1)

Undoped Fe3O4

0.153 0.342 0.709 1.531 0.584 1.122 2.207 4.502

Cu-Fe3O4-2

@25 °C @35 °C @45 °C @55 °C @25 °C @35 °C @45 °C @55 °C

Ea 1000 ln ks1 ¼ ln A
1000R T

Ea (kJ/mol)

y =
7.466x + 23.156 (R2 = 0.999)

62.1

y =
6.648x + 21.726 (R2 = 0.998)

55.3

Fig. 7. Arrhenius plots of H2O2 decomposition on the undoped Fe3O4 and Cu-Fe3O42 MNPs.

stronger H2O2-activation ability and supports the promotional effect of the Cu doping on the catalytic decomposition of H2O2. The environmental application of the enhanced H2O2-activation ability upon Cu doping was demonstrated by the degradation of RhB with H2O2 at pH 6.9 and 25 °C on Cu-Fe3O4-2 MNPs. RhB is a widely used colorant in textile and printing industries and known as a water pollutant with a considerable threat to the environment as it is harmful to humans and animals [33]. As shown in Fig. 8a. The degradation efficiencies of the RhB solution after 2 h (in short

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Fig. 8. Catalytic degradation of RhB with H2O2 over Cu-doped Fe3O4 catalysts. (a) C/C0-t plots and (b) ln(C0/C)-t plots. Experimental conditions: RhB solution (50 mL, 10 lM), catalyst (0.5 g/L) and H2O2 (40 mM) at pH 6.9 and 25 °C.

2 h DEs) over undoped Fe3O4, Cu-Fe3O4-1, Cu-Fe3O4-2 and CuFe3O4-3 are 42.5%, 74.6%, 95.2% and 97.5%, respectively. The degradation kinetics of RhB was analyzed according to pseudo-first-

Fig. 9. Recyclability of Cu-Fe3O4-2 for catalytic degradation of RhB with H2O2. Experimental conditions for each run: RhB solution (50 mL, 10 lM), catalyst (0.5 g/ L) and H2O2 (40 mM) at pH 6.9 and 25 °C.

order reaction model assuming that the concentration of H2O2 is constant throughout the reaction since it is in large access with regard to pollutants [34–39]. The straight lines in the plots of ln (C0/C) vs. time (Fig. 8b) confirm the suitability of this kinetic model. The rate constant, ks1, was obtained from the slope of the straight line by linear regression (R2: 0.960–0.997). The corresponding ks1 values over undoped Fe3O4, Cu-Fe3O4-1, Cu-Fe3O4-2 and Cu-Fe3O4-3 are 0.0038, 0.0111, 0.0256 and 0.0274 min
1, respectively. Since the catalytic degradation of RhB are controlled by available surface active sites that are proportional to the specific surface area of the catalysts, the ks1 values were normalized by the specific surface area per unit volume (SSAV = SBET  catalyst loading, m2 L
1) to minimize the influence of the specific surface area of different catalysts on the degradation of RhB for direct comparison of their intrinsic catalytic activities [11,34]. The corresponding ks1/SSAV for undoped Fe3O4, Cu-Fe3O4-1, Cu-Fe3O4-2 and CuFe3O4-3 were calculated to be 6.08  10
5, 3.83  10
4, 5.73  10
4 and 4.70  10
4 L m
2 min
1, respectively. Both the 2 h DE and ks1/SSAV values indicate that the Cu2+ doping significantly improved the catalytic performance of Cu-Fe3O4 at pH 6.9 and 25 °C. Among the three Cu-Fe3O4 catalysts, Cu-Fe3O4-2 had the highest intrinsic catalytic activity. The lower activity of Cu-Fe3O43 with higher copper content may be partly ascribed to the formation of some phase impurity with increasing the Cu content.

Fig. 10. TEM images of Cu-Fe3O4-2 MNPs after the eighth run.

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To check the recyclability of Cu-Fe3O4-2, the used catalyst was recovered by decanting the decolorized RhB solution out of the flask with the assistance of a magnet after each run [40]. Without any treatment of the recovered catalyst, another 50 mL of RhB stock solution (10 mM) was then added to the flask and degraded with H2O2 under the same conditions as described in Section 2.5. As can be seen from Fig. 9, Cu-Fe3O4-2 maintains its high catalytic activity after it has been continuously running for 16 h in eight runs. The 2 h DE and ks1 values are 94.2–96.3% and 0.0218– 0.0266 min
1, respectively, in the first five runs (Fig. 9). Afterwards the 2 h DE and ks1 values slightly reduce to 84.7–88.7% and 0.0132–0.0167 min
1, respectively, in the sixth to eighth runs, which may be related to particle aggregation or catalyst loss during multiple sampling and supernatant discharges in recycling experiments. The recyclability tests indicate that Cu-Fe3O4-2 is quite stable under the current experimental conditions. The stabile activity of Cu-Fe3O4-2 could be attributed to its structural stability and negligible loss of iron and copper during the degradation of RhB under mild conditions (pH  7 and 25 °C). The XRD pattern (Fig. S2) confirms that the crystal structure of Cu-Fe3O4-2 is stable after the reaction. The TEM images of CuFe3O4-2 after the eighth run (Fig. 10) show that primary nanoparticles became a bit more aggregated. However, there is no obvious change in the size of primary nanoparticles after the reaction (Fig. 2e–f vs. Fig. 10). No iron or copper leached into the solution after the reaction was detected by AAS (detection limits: 0.03 ppm for iron and 0.01 ppm for copper). Additional experiments showed that the degradation of RhB almost stopped after removal of Cu-Fe3O4-2 from the RhB solution by magnetic separation. Therefore, the catalytic degradation of RhB with H2O2 is not due to iron or copper ions leached into the solution. Instead, it occurs on the surface of Cu-Fe3O4-2. 4. Conclusions Cu-doped Fe3O4 magnetic nanoparticles (Fe3
xCuxO4 MNPs, x: 0.06–0.23) were successful prepared by oxidative precipitationcombined ionothermal synthesis. The substitution of Fe2+ with Cu2+ at octahedral sites was supported by XRD, VSM and XPS. XPS indicated that Cu2+ ions were surface-enriched. The surface Cu2+ ions expedite the regeneration of Fenton active species Fe2+ by reduction of Fe3+ with HOO radicals or Cu+ and thus intensify the decomposition of H2O2. Kinetic study showed that Fe2.88Cu0.12O4 MNPs decomposed H2O2 much faster than the undoped Fe3O4 (0.584 vs. 0.153 h
1 at 25 °C) due to the lower apparent activation energy of the former (55.3 vs. 62.1 kJ/mol). The enhanced peroxidase-like activity of Fe3
xCuxO4 MNPs could be applied to activate H2O2 under mild conditions for the degradation of recalcitrant organic pollutants. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 21571146). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apt.2017.12.025. References [1] L. Gao, J. Zhuang, L. Nie, J. Zhang, Y. Zhang, N. Gu, et al., Intrinsic peroxidaselike activity of ferromagnetic nanoparticles, Nat. Nanotechnol. 2 (2007) 577– 583.

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