Polyhydroquinone-Coated Fe3O4 Nanocatalyst for Degradation of

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Polyhydroquinone coated Fe3O4 nanocatalyst for degradation of Rhodamine B based on sulfate radicals Yanqiu Leng, Weilin Guo, Xiao Shi, Yingyun Li, and Liting Xing Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie4015777 • Publication Date (Web): 28 Aug 2013 Downloaded from http://pubs.acs.org on September 1, 2013

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Industrial & Engineering Chemistry Research

Polyhydroquinone coated Fe3O4 nanocatalyst for degradation of Rhodamine B based on sulfate radicals

Yanqiu Leng, Weilin Guo*, Xiao Shi, Yingyun Li, Liting Xing

School of Resources and Environment, University of Jinan, Jinan 250022, China

*

Corresponding author

Tel.: +86 531 8276 9233. Fax: +86 531 8276 9233. E-mail address: [email protected] (W. L. Guo)

1

ACS Paragon Plus Environment


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ABSTRACT In this study, the polyhydroquinone/Fe3O4 (PHQ/Fe3O4) was synthesized as heterogeneous catalyst to activate persulfate to effectively degrade Rhodamine B (RhB). The synthetic PHQ/Fe3O4 nanoparticles were characterized using X-ray diffraction (XRD), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET) nitrogen adsorption, and Fourier-transform infrared (FTIR) spectra. PHQ/Fe3O4 shows better catalytic performance and excellent reusability than PHQ and Fe3O4. The results indicated that PHQ/Fe3O4 maintains quinone units and the presence of the quinone moieties successfully accelerates the degradation compared to Fe3O4, owing to the role of quinone assisting the redox cycling of Fe. Effects of PHQ/Fe3O4 addition, persulfate concentration, pH and temperature on the degradation efficiency of RhB by persulfate are examined in batch experiments. Increasing the temperature may significantly accelerate the RhB degradation and the degradation is found to follow the pseudo-first-order kinetic model. On the basis of these findings, the possible mechanism of RhB degradation was proposed.

2


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1. INTRODUCTION In recent years, an innovative advanced oxidation technology (AOT) using persulfate to generate sulfate radicals (SO4− •) has gained popularity as an in situ chemical 1, 2

. SO4− • has a high oxidation potential (E0 = 2.6 V vs. NHE) that

oxidation (ISCO)

is similar to •OH (E0 = 2.73 V vs. NHE)

3, 4

. In addition, persulfate is a strong and

stable oxidizing agent (E0 = 2.01 V vs. NHE) and it has high aqueous solubility as compared to hydrogen peroxide 3. Heat, transition metal ions (such as Fe2+, Ag+, or Co2+), alkaline conditions and UV light can all excite persulfate to form SO4− • 4-6. Among those, the addition of transition metal ions appears to be an inexpensive and practical way of achieving persulfate activation, but the potential health hazards caused by the dissolved Fe2+ or Co2+ in water render such a homogeneous system with limited use 7, 8. Magnetite (Fe3O4) has been observed to have efficient catalysis in heterogeneous Fenton-like system, which is assigned to the presence of Fe(II) species in magnetite structure initiating the reaction 9. Fe3O4 has also been applied to the oxidation process of Fe3O4/persulfate owing to the similar structure of O-O bond was contained in H2O2 and persulfate 3. Unfortunately, some of these systems do not show favorable catalytic activity, which is particularly due to that Fe3+ cannot efficiently catalyze the generation of SO4− •, and the utilization efficiency of the peroxides and catalytic degradation rate of target substances is low 10, 11. It is a great challenge to improve the catalytic performance of Fe3O4. During the last two decades, quinones have been tested as redox mediators on the degradation processes of pollutants

12, 13

. The addition of quinones in Fenton system

increased the oxidation rate of contaminants, which was ascribed to their role as an electron shuttle

14

. Christopher et al.

15

and Ma et al.

16

have both reported that the

presence of quinone successfully builds up two cycles, one semiquinone/quinone cycle, another cycle of Fe(III)/Fe(II) induced by quinone. But continuous dosing of the dissolved redox mediators implies continuous expenses related to procurement of the chemical, as well as continuous discharge of this biologically recalcitrant compound12. And little research was done about the non-dissolved redox mediators. 3



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More recently, the stability, dispersibility and catalytic efficiency of Fe3O4 to H2O2 is improved by modifying their surface with polymers such as humic acid (HA) and poly(3,4-ethylene-dioxythiophene). Niu et al.

17

prepared HA coated Fe3O4,

suggesting HA may act as an electron-transfer mediator in the chemical degradation of organic pollutants in Fenton-like system. Shin et al.

18

immobilized

poly(3,4-ethylene-dioxythiophene) on the surface of Fe3O4, exhibiting high catalytic performance for the degradation of Reactive Black 5 and Orange II. However, there is no report about the degradation of organic pollutants in water by heterogeneous Fenton-like system using polyhydroquinone coated Fe3O4 as catalyst by far. Herein, polyhydroquinone/Fe3O4 (PHQ/Fe3O4), an immobilized quinone compound, was synthesized by oxidative polymerization of hydroquinone. PHQ/Fe3O4 was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET) nitrogen adsorption, and Fourier-transform infrared (FTIR) spectra. Rhodamine B, an important representative of xanthene dyes, was selected for the degradation experiment due to its presence in the wastewater of several industries 19. Accordingly, the present study focuses on: (1) investigating the activation of persulfate by Fe3O4 for the degradation of RhB; (2) studying the role of polyhydroquinone in persulfate oxidation; and (3) proposing the possible degradation mechanism of RhB. 2. MATERIALS AND METHODS 2.1 Chemicals and Reagents. The commercially available dye Rhodamine B (Formula: C28H31CIN2O3, Formula weight: 479.02), potassium persulfate (Formula: K2S2O8, Formula weight: 270.32) and 1,4-hydroquinone (Formula: C6H6O2, Formula weight: 110.11) were purchased from Shanghai Chemical Reagent Company, China. All the other chemicals were of analytical grade and were provided from Tianjin, China, including FeCl3·6H2O, FeSO4·7H2O, trisodium citrate, sodium acetate, ethylene glycol, methanol, and H2O2. The water used in all experiments was purified by a Milli-Q system. 2.2 Preparation and characterization of PHQ/Fe3O4. Fe3O4 magnetic nanoparticles (MNPs) was synthesized according to the method reported previously 20. 4


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Typically, FeCl3·6H2O (2.6 g), trisodium citrate (1.0 g), and sodium acetate (4.0 g) were dissolved in ethylene glycol (80 mL) with magnetic stirring. The obtained yellow solution was then transferred and sealed into a Teflon-lined stainless-steel autoclave. The autoclave was heated at 200 oC for 10 h, and then allowed to cool to room temperature. The black products were washed with ethanol and deionized water three times and dried in a vacuum oven at 60 oC. PHQ/Fe3O4 was prepared through oxidative polymerization of hydroquinone method as follows

21

. Briefly, hydroquinone (1.65 g) was dissolved in 140 mL

deionized water. The solution was transferred to a three-necked flask. Then a desired amounts of Fe3O4 based on different hydroquinone/Fe3O4 mass ratios, such as 2:1, 1:1, 1:2 and 1:5, was added. After a while for mixing, 6 mL FeSO4 solution (0.02 wt%) and 3.4 mL H2O2 was added dropwise in 1 h. The mixture was stirred at 35 oC for 24 h. Solid product was collected, washed with deionized water and dried in a vacuum oven at 60 oC. XRD patterns were recorded on a Philips X’Pert Pro Super X-ray diffractometer with a Cu Kα source ( λ = 1.54178A). The Brunauer-Emmett-Teller (BET) surface area, pore volume, and pore diameter of PHQ/Fe3O4 were determined from the N2 adsorption-desorption at -196

o

C using a Micrometric ASAP 2020 system.

Fourier-transform infrared (FTIR) spectra were recorded on a Bruker VERTX-70 FTIR spectromter using KBr pellets containing 1% weight sample. It was determined in the frequency range 400-4000 cm-1 with a resolution of 4 cm-1. 2.3 Batch oxidation experiments. Stock solutions of RhB (0.8 mmol/L) and persulfate (80 mmol/L) were prepared using deionized water. The degradation experiments were conducted in 500-mL beaker flasks. Flasks were prepared in duplicates for all experiments. The reaction was initiated immediately by adding 12 mmol/L of persulfate, 10 mg PHQ/Fe3O4. An aqueous solution (100 mL) was shaken by a thermostatic reciprocating shaker at 200 rpm and 20 oC. At different elapsed times, a 4 mL sample was collected from each flask, and quenched with 1 mL methanol, an effective quenching agent for sulfate radicals 22. Then the samples were centrifuged at 8000 rpm for two minutes with a TGL-16C centrifugal (Shanghai, 5



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China) to remove the catalyst. The concentrations of the residual RhB were determined by monitoring decrease in absorbance at the maximum wavelength (554 nm) with UV-vis spectroscopy (Shanghai, China). 3. RESULTS AND DISCUSSION 3.1. Characterization of PHQ/Fe3O4 MNPs. The XRD pattern of PHQ/Fe3O4 is shown in Figure 1a. The peaks at values of 30.1, 35.5, 43.1, 53.5, 57.0 and 62.6 o were indexed as the diffractions of (220), (311), (400), (422), (511) and (440) of Fe3O4, respectively, being similar to the previously reported data for Fe3O4

17,23

. The

sharpness of XRD reflections clearly shows that the synthesized PHQ/Fe3O4 is highly crystalline 23. This result indicated that the crystal structure of Fe3O4 was not change after modification with PHQ. The TEM image (Figure 1b) demonstrated that PHQ/Fe3O4 were quasi-spherical in shape, and had nearly uniform distribution of particle size (~26 nm). N2 adsorption-desorption isotherms were employed to investigate the surface area and the pore structures of PHQ/Fe3O4 (Figure 1c). The BET surface area, pore volume and pore size are 63.9 m2/g, 0.188 cm³/g and 10.2 nm, respectively, which are calculated by the Barret-Joyner-Halenda (BJH) analysis. Figure 1d shows the FTIR spectra of Fe3O4, PHQ and PHQ/Fe3O4. In the spectra of Fe3O4, the broad band at 580 cm-1 corresponded to the Fe-O group; the peaks at 1628 and 3400 cm-1 were assigned to the stretching vibration of sorbed water and hydroxyl groups. The spectrum of PHQ contains bands of deformation oscillations of phenol hydroxyls at 1290 cm-1 and a wide band of valency oscillations of OH groups with maximum at 3300 cm-1; also bands of valency oscillations of C=C benzene ring with maxima at 1463, 1511, 1625 cm-1 and band-type deformation oscillations of the C-H ring at 1080 cm-1. Peaks at 808 cm-1 can be assigned to the out-of-plane vibrations of the C-H bonds of the aromatic rings 24. The spectra of PHQ/Fe3O4 showed adsorption at 580 cm-1, confirming the existence of Fe3O4. In addition, the 1124 cm-1 was assigned to the deformation oscillations of phenol hydroxyls.

6


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(a)

30

Intensity (a.u.)

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(311)

20 (440)

(220) (511)

10 (400)

(422)

0 10

20

30

40

50

60

70

2 θ / degree

7



120

(c) 3

100

Pore Volume(cm /g)

2 Quantity Adsorbed (cm /g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80 60 40

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0.3 0.2 0.1 0.0 0

20

40

60

80

100

120

Pore Diameter (nm)

20 0.0

0.2

0.4

0.6

0.8

1.0

p/p0

PHQ/Fe3O4

(d) 580

1124

1386 1625

3430

808

PHQ

1511 1080 1463 1290 1620

3300 Fe3O4

1620

3400

580

1000

2000

3000

4000

-1

Wavenumber / cm

Figure 1. XRD pattern (a), TEM (b), BET(c) of PHQ/Fe3O4 MNPs and FTIR spectra (d) of Fe3O4, PHQ and PHQ/Fe3O4 MNPs.

3.2. Effect of different PHQ and Fe3O4 ratio on the degradation of RhB. In control experiments, we confirmed that little removal of RhB (about 5%) occurs within 120 min when adding into the solution with PHQ/Fe3O4 only, indicating the 8


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effect of adsorption on RhB removal is not obvious. As shown in Figure 2, the effect of different PHQ and Fe3O4 ratio in PHQ/Fe3O4 on the degradation of RhB was investigated. The results can be explained by the following reasons. The dissolved iron resulted from dissolution of iron oxides and the surface Fe(II) initiated persulfate to form SO4−• (Eq. 1). Several studies reported that quinones increased the Fenton degradation of organic compounds, which was ascribed to their role as an electron shuttle

25-27

. In that Fenton system, Fe(III) was quickly reduced to Fe(II) by

hydroquinone (HQ) and semiquinone radicals (SQ) (Eqs. 2 and 3), which favours the generation of SO4− • through Eq. 1. ≡ Fe(II) + S 2O8 2− →≡ Fe(III) + SO4 −• + SO4 2−

(1)

HQ + ≡ Fe(III) → SQ + H + + ≡ Fe( II)

(2)

SQ + ≡ Fe( III) → BQ + ≡ Fe( II) + H +

(3)

100 Removal percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 0:1 PHQ/Fe3O4

40

1:1 PHQ/Fe3O4 1:2 PHQ/Fe3O4

20

1:5 PHQ/Fe3O4 2:1 PHQ/Fe3O4

0

0

30

60 Time (min)

90

120

Figure 2. Effect of different PHQ and Fe3O4 ratio on the degradation of RhB (PHQ/Fe3O4, 10 mg; initial concentration of RhB, 0.02 mmol/L; concentration of persulfate, 12 mmol/L; temperature, 20 oC; pH, 3.98).

It has been reported that ferrous ions can activate dissolved oxygen to form superoxide radical anion (O2• −) through Eq. 4. Fang et al.

28

confirmed that O2• − was 9



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generated by Fe3O4 in the Fe3O4/ persulfate system. The resulting benzoquinone (BQ) or SQ can rapidly react with O2• − (Eq. 5), and thus the quinone cycle is built up

27

.

This effect assists in the catalytic redox cycling of iron by reducing Fe(III) to Fe(II), promoting the SO4− • production and thus accelerate the degradation of RhB. •−

≡ Fe(II) + O2 → O2 + ≡ Fe( III)

BQ + O2

•−

→ SQ + O2

(4) (5)

The PHQ/Fe3O4 maintains quinone moieties. The presence of quinone successfully accelerates the degradation rate. Compared with different PHQ and Fe3O4 ratio in the PHQ/Fe3O4 catalysts, 1:1 PHQ/Fe3O4 favored the RhB decomposition. Therefore, 1:1 PHQ/Fe3O4 was recommended for the further study in the PHQ/Fe3O4/persulfate system.

3.3. Effect of PHQ/Fe3O4 addition on the degradation of RhB. For checking the catalytic activity of PHQ/Fe3O4 on the degradation of RhB, one set of experiments was conducted. As shown in Figure 3, the degradation efficiency increased with various dosages of PHQ/Fe3O4 in the range of 0 to 15 mg. As expected, the addition of iron oxides powder significantly improved the removal efficiently of RhB, which may be due to the PHQ/Fe3O4 act as a good provider of ≡Fe(II) for the activation of persulfate. Nevertheless, when PHQ/Fe3O4 addition was further increased to 20 mg, the degradation of RhB was not enhanced but slightly decreased, because of the high reduction potential of Fe(II) ions and the high oxidation potential of the SO4−

•

generated may initiate an even stronger interaction between Fe(II) and SO4− • (Eq. 6) 29

. Although the initial degradation rate of RhB was different at different PHQ/Fe3O4

addition (10-20mg), the final degradation was similar as a result of the same concentration of persulfate. Considering the practical application, 10 mg PHQ/Fe3O4 was adopted for the further study. Fe( II ) + SO4 −• → Fe( III ) + SO4 2 −

(6)

10


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80 60 0 mg 5 mg 10 mg 15 mg 20 mg

40 20 0

0

30

60 Time (min)

90

120

Figure 3. Effect of PHQ/Fe3O4 (1:1) addition on the degradation of RhB (initial concentration of RhB, 0.02 mmol/L; concentration of persulfate, 12 mmol/L; temperature, 20 oC; pH, 3.98).

3.4. Effect of persulfate concentration on the degradation of RhB. Effect of persulfate concentrations (0-20 mmol/L) on the degradation of RhB (0.02 mmol/L) was studied without pH adjustment. In Figure 4, the absorbance of RhB by PHQ/Fe3O4 was found too slight to be neglected in the absence of persulfate. The results clearly showed that the degradation of RhB increased as the initial persulfate concentration increased, because persulfate was the origin of driving force for the forming of SO4− •. It should be pointed out that the degradation did not increase further when the initial persulfate concentration was higher than 12 mmol/L. Excess addition of persulfate leads to produce sulfate anions without producing active SO4− •. It is also reported that radical-radical reactions may be prior to the radical-organic reactions and SO4− • can also be scavenged by S2O82- (Eqs.7 and 8) 3. SO4 −• + SO4 −• → S 2O8 2− SO4 −• + S 2O8 2 − → SO4 2 − + S 2O8 −•

(7) (8)

11




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80 60 0 mmol/L 4 mmol/L 8 mmol/L 12 mmol/L 20 mmol/L

40 20 0

0

30

60 Time (min)

90

120

Figure 4. Effect of persulfate concentration on the degradation of RhB (PHQ/Fe3O4 (1:1), 10 mg; initial concentration of RhB, 0.02 mmol/L; temperature, 20 oC; pH, 3.98).

3.5. Effect of pH on the degradation of RhB. The pH value of the solution was usually an important parameter affecting oxidative degradation of organic pollutants. Therefore, the influence of pH on the degradation of RhB in the system was explored by adjusting the solution pH to 2.12, 3.98, 5.22, 7.19, 9.30 and 11.06. After 120 min of reaction time, the final pH value was decreased to 3.19, 3.80, 4.31, 4.36, 7.66, and 9.89, respectively. It can be seen from Figure 5 that the degradation rate of RhB was highest at pH 3.98, and the degradation rate was low at neutral and alkaline conditions.This results quite agree with Li’s works 30. This might be due to the rapid decay of SO4− • resulting from the reactions with hydroxyl ions (Eq. 9) 31. SO4 −• + OH − → SO4 2− + • OH

(9)

12


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pH=2.12 pH=3.98 pH=5.22 pH=7.19 pH=9.30 pH=11.06

80 60 40 20 0

0

30

60 Time (min)

90

120

Figure 5. Effect of pH on the degradation of RhB (PHQ/Fe3O4 (1:1), 10 mg; initial concentration of RhB, 0.02 mmol/L; concentration of persulfate, 12 mmol/L; temperature, 20 oC).

3.6. Effect of temperature on the degradation of RhB. In order to investigate the effect of temperature on the degradation of RhB, four different values between 30 and 60 oC were performed. The degradation reaction of RhB was significantly influenced by temperature. As shown in Figure 6, the higher the temperature, the faster the RhB degradation. Under the tested conditions, RhB degradation approximately followed the pseudo-first-order kinetics, which may be expressed as ln(ct/c0) = kt + y, where y is a constant, t is reaction time (min), k is the apparent rate constant (min− 1), and c0 and ct are RhB concentrations (mmol/L) at time of t = 0 and t = t, respectively 17. The degradation rate of k were 0.029, 0.072, 0.122, 0.270 min-1 at 30, 40, 50, 60 oC, respectively. In addition, the activation energy (Ea) of the degradation reaction could be obtained based on the experimental data using the Arrhenius equation, k = A exp (−Ea/RT), where A is the frequency factor, Ea is the activation energy, R is the universal gas constant and T is the temperature in Kelvin

22

. The plot of ln k versus

1/T was linear (R = 0.947), and Ea was 73.50 kJ mol−1. It can be deduced that the temperature was conducive to the removal rate.

13



100 80 0

60

-2 -4

40

30oC 40oC

20 0

ln(c/c 0)

Removal percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50oC 60oC

0

30

-6

0

30

60

90

120

Time (min)

60 Time (min)

90

120

Figure 6. Effect of temperature on the degradation of RhB (PHQ/Fe3O4 (1:1), 10 mg; initial concentration of RhB, 0.02 mmol/L; concentration of persulfate, 12 mmol/L; pH, 3.98).

3.7. Plausible mechanism. To check whether modification of PHQ improve the ability of PHQ to enhance the performance of Fe3O4, experiments were conducted with 10 mg Fe3O4 and 12 mmol/L persulfate in the presence of 10 mg free PHQ. Figure 7 shows RhB removal kinetics by different systems. The degradation rate of k were 0.0091, 0.0132, 0.0219 min-1 in Fe3O4, Fe3O4 and free PHQ, and PHQ/Fe3O4 systems, respectively. Compared with the Fe3O4, the presence of PHQ in the magnetite structure favored the RhB decomposition. This may be due to the fact that persulfate can be activated by Fe3O4 to generate SO4− •, and the presence of PHQ which act as an electron shuttle increased the degradation of RhB.

14


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3.0

10mg Fe 3 O 4

2.5 ln(C0/C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10mg Fe3 O + 10mg PHQ 4 10mg PHQ/Fe3 O 4

2.0 1.5 1.0 0.5 0

30

60 90 Time (min)

120

Figure 7. RhB degradation in different systems (initial concentration of RhB, 0.02 mmol/L; concentration of persulfate, 12 mmol/L; temperature, 20 oC; pH, 3.98).

Scheme 1. Role of PHQ in PHQ/Fe3O4/persulfate system

The catalytic efficiency in PHQ/Fe3O4 system was higher than that of free PHQ-Fe3O4 system (Figure 8). It could be inferred that the increased degradation of RhB in the former system was resulted from the gradual bonding of PHQ on Fe3O4 surface. PHQ coated on Fe3O4 surface played an important role on the redox cycling of Fe. The role of PHQ is summarized in Scheme 1. The catalyst maintains quinone unit. The presence of quinone successfully builds up two cycles, one semiquinone/quinone cycle, another cycle of Fe(III)/Fe(II) induced by quinine (Eqs.1-5)

14,15,28

. As the

cycles increased, the quinone analogues accumulated more and more in the system, 15



leading to form more and more Fe(II) and SO4−•, so the degradation was accelerated. It is well in agreement with the results that the quinone-containing compounds serve as an electron shuttle to enhance the redox reaction rates 30, 31. Therefore, the presence of redox mediators, such as quinones, play a crucial role in steering oxidation into the desired pathways.

3.8. Reusability studies. It is known that the reusability of the catalyst is crucial for its practical application. The catalyst was recovered from the reaction mixture at the end of each process, then washed by distilled water, dried in the vacuum oven and stored at ambient temperature. As can be seen from Figure 8, the degradation rates of used catalysts is similar to the fresh catalysts. The results indicates that PHQ/Fe3O4 is stable in this study. In addition, quinone units assists in the catalytic redox cycling of iron by reducing Fe(III) to Fe(II), promoting the SO4− • production and thus accelerate the reusability of the catalyst. While for Fe3O4/persufate system, the major reason for the decline of removal percentage is the conversion of the ≡ Fe(II) to ≡ Fe(III) on the surface of Fe3O4, which leads to less activity of the catalyst 3. Therefore, the good catalytic efficiency, the long term stability PHQ/Fe3O4 and convenient recycling without any regeneration made the catalyst attractive. 100 Removal percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 First cycle Second cycle Third cycle

20 0 0

80

160 240 Time (min)

320

400

Figure 8. Different regeneration times on the degradation of RhB (PHQ/Fe3O4 (1:1), 10 mg; initial concentration of RhB, 0.02 mmol/L; concentration of persulfate, 12 mmol/L; temperature, 20 oC; pH, 3.98).

16


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4. CONCLUSIONS A route to prepare PHQ/Fe3O4 nanocomposites is proposed in the current study. The synthetic compounds were highly efficient to decompose persulfate to produce large amount of SO4− •. PHQ/Fe3O4 shows better catalytic performance and excellent reusability than free PHQ and Fe3O4. In aqueous system, the effects of persulfate concentration, PHQ/Fe3O4 dosage, pH and temperature were studied for the PHQ/Fe3O4/persulfate system. The degradation of RhB was slightly dependent on the operational parameters. Results indicated that the PHQ/Fe3O4 maintains quinone units and the presence of quinone successfully accelerates the degradation compared to Fe3O4, owing to the role of quinone assisting the redox cycling of Fe. Increasing the temperature may significantly accelerate the RhB degradation and the degradation is found to follow the pseudo-first-order kinetic model. Finally, the reusability of the catalyst was also studied and the results indicates that PHQ/Fe3O4 is stable. The studied system showed that the radicals produced from the activation of persulfate by PHQ/Fe3O4 could degrade RhB efficiently. Therefore, the proposed method has great potential in the degradation of printing and dyeing wastewater and other refractory organic pollutants.

ACKNOWLEDGMENTS The authors would like to thank the financial support from the National Nature Science Foundation of China (Grant No. 41172222).

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Polyhydroquinone-Coated Fe3O4 Nanocatalyst for Degradation of

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