Catalytic activation of peroxymonosulfate using CeVO4 for phenol degradation: An insight into the reaction pathway

Journal Pre-proof Catalytic activation of peroxymonosulfate using CeVO4 for phenol degradation: an insight into the reaction pathway Israa Othman, Jerina Hisham Zain, Mohammad Abu Haija, Fawzi Banat

PII:

S0926-3373(20)30016-3

DOI:

https://doi.org/10.1016/j.apcatb.2020.118601

Reference:

APCATB 118601

To appear in:

Applied Catalysis B: Environmental

Received Date:

8 July 2019

Revised Date:

2 January 2020

Accepted Date:

4 January 2020

Please cite this article as: Othman I, Zain JH, Abu Haija M, Banat F, Catalytic activation of peroxymonosulfate using CeVO4 for phenol degradation: an insight into the reaction pathway, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118601

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Catalytic activation of peroxymonosulfate using CeVO4 for phenol degradation: an insight into the reaction

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pathway

, Fawzi Banat b

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[email protected]

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Israa Othman a, Jerina Hisham Zain* b [email protected], Mohammad Abu Haija* a

Department of Chemistry, Khalifa University- SAN Campus, Abu Dhabi, UAE

b

Department of Chemical Engineering, Khalifa University- SAN Campus, Abu Dhabi, UAE

*

Email of corresponding author:

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a

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Graphical Abstract:

Intensity (a.u)

O

1

O

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PMS

t=0 t=15

t=30 t=45 t=60 t=75

0

1

2

3

4

5

Retention Time (min)

1

Highlights: 

Cerium vanadate nanostructures was prepared by simple one-pot co-precipitation method.



The nanostructures resulted in complete degradation of 100 ppm phenol in 80 min via activation of PMS. Phenol degradation followed the pseudo-first-order kinetics model with high R2 value and low

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

activation energy of 23.74 kJ/mol.

The nanostructures were tested for recyclability up to 5 cycles and displayed excellent

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

recyclability and reusability.

Detailed mechanistic study revealed that non radical pathway via singlet oxygen mediated

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Abstract:

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activation of PMS govern the catalytic activity.

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

In this study, a nanostructured CeVO4 material was synthesized using a simple co-precipitation technique and investigated as a catalyst for phenol degradation via peroxymonosulfate (PMS) activation. The structural and morphological properties of the CeVO4 catalyst were characterized using XRD, XPS, FTIR, Raman spectroscopy, TGA, SEM, and TEM. The results indicated that the prepared catalyst exhibited a pure crystalline phase with a rod-like structure. The CeVO4

2

catalyst exhibited high activity towards PMS activation, resulting in complete degradation of phenol within the first 80 min at room temperature. The degradation reaction was found to follow pseudo-first-order kinetics with a low activation energy of 23.74 kJ/mol. The degradation followed a catalytic surface-mediated electron transfer route as confirmed using ESR measurements. The effects of several experimental parameters on phenol degradation were investigated, including the catalyst loading, initial phenol concentration, PMS dosage, reaction

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temperature, and pH. The regeneration and reusability of the CeVO4 catalyst were also examined

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for 5 consecutive cycles. This study puts forward CeVO4 as a heterogeneous catalyst for the

complete removal of persistent organic pollutants via PMS activation, indicating a favorable

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application in wastewater treatment.

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Keywords: PMS; CeVO4; AOP; phenol degradation; singlet oxygen

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Introduction

The wastewater produced during industrial processes typically contains organic and inorganic

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pollutants that are environmentally persistent and cannot be removed by conventional methods [1, 2]. Some of these pollutants are dyes, pesticides and organic solvents which may contain aromatic compounds, cyanides, ammonia, sulfides, and phenols [3-5]. Phenols are often used in agriculture and in general disinfection. Phenol is a persistent organic pollutant and is listed as a

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class 2 water hazard [6]. The environmental protection agency (EPA) restricted its concentration to less than 1 mg/L in the discharged inland water [7] and to 2 mg/L in drinking water [8]. Phenol’s poor biodegradability demands a tertiary treatment as the conventional primary and secondary processes are not efficient [9-11]. Techniques like membrane technology, electrochemical process and chemical oxidation are often adopted in wastewater treatment [12-

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14]. Recent studies have indicated an increase in the prospect of advanced oxidation processes (AOPs) focused on peroxymonosulfate (PMS) activation owing to the pH flexibility, their redox potential and great oxidation ability against common environmental contaminants such as pharmaceuticals, dyes, phenol, and perfluorinated compounds. The use of peroxomonosulfate (PMS) as an oxidant has attracted a lot of attention due to the presence of different reactive oxygen species produced during its activation process [15, 16].

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Various physicochemical activation strategies of PMS were carried out using ultraviolet, metal

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oxides, ultrasound, transition metals or heat[17]. Among these approaches, the activation via metal oxides as heterogeneous catalysts has drawn much attention. [15, 17, 18]. Even though

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transition metals, as homogeneous catalysts, showed high activation of PMS, their high solubility

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and toxicity made them less favorable. Conversely, heterogeneous catalysts are reusable, easy to recover and reduce the need for chemical reagents [9, 17-19]. The reaction pathway is in

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coherence with those of transition metals homogenous catalysts activation strategies wherein the conduction band (generated) electrons selectively cleave the peroxide bond to generate SO4−.

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Persulfate activation via non radical mechanism, employing transition metal-based mediators, mainly proceed through two hypotheses which are singlet oxygenation and metal-mediated electron transfer [20-22]. Despite that, literature reports indicate that the activity of the hydroxyl and sulphate radicals could be hindered by the presence of background interferences caused by

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the organic compounds and anions (Cl-, HCO3-, natural organic matter… etc.) presented in the system [23], owing to the radical trapping effect, hence limiting their application to real water treatment [24]. On the other hand, the non-radical mechanism has been explored widely due to the high resistance of the reactive oxygen species (single oxygen, 1O2 ) to some of the

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background interferences [25]. Besides, 1O2 has demonstrated an efficient degradation of pharmaceutical contaminants in real wastewater [26]. M-VO4 materials are environmentally abundant oxides with remarkable physical and chemical properties which enabled their applications in batteries, semiconductors, and catalysis[27]. Combining vanadate with rare earth metals such as Ce, La and Pr showed a great enhancement in their electrochemical properties, thermal stability, surface area, and magnetic properties [28].

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The naturally occurring cerium vanadate (CeVO4) mineral wakefieldite exhibits a tetragonal

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zircon-type structure (space group 141/amd) [29]. Due to it’s optical, electrical, magnetic and catalytic properties [30-32], CeVO4 often used as a catalyst, superhydrophobic material, gas

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sensor, counter electrode, and supercapacitor [33]. There are several reports about its efficiency

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for degrading organic pollutants such as organic dyes [34] and for the oxidative dehydrogenation of propane [35].

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In the present work, CeVO4 was prepared using a simple one-pot co-precipitation method. The prepared CeVO4 nanoparticles were characterized using XRD, FTIR, Raman, TGA, XPS, SEM-

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EDX and TEM. CeVO4 catalyst was used to activate PMS for phenol degradation. The reaction operating conditions were optimized by investigating the effect of the catalyst loading, phenol initial concentration, PMS dosage, reaction temperature, and pH. The mechanism of the reaction was proposed by identifying the intermediate species in the reaction.

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Materials and methods

Catalysts preparation. Nanostructured CeVO4 particles were prepared by using a simple onepot co-precipitation method employing high purity (99.9%) cerium nitrate [Ce(NO3)3.6H2O] and ammonium metavanadate (NH4VO3) as obtained from Sigma Aldrich. For a typical synthesis procedure, 20 mmol of ammonium metavanadate and 20 mmol of Ce(NO3)3·6H2O were

5

dissolved separately in 50 mL of de-ionized (DI) water. The ammonium metavanadate solution was warmed tilled NH4VO3 was completely dissolved. Then the cerium nitrate solution was added to the NH4VO3 solution under continuous stirring. Ammonia solution was used to maintain the pH of the solution at 9, till a brown cloudy precipitate was formed. The reaction was continued at 80°C with stirring for two hours. The obtained brown precipitate was separated by centrifuge and washed with DI water and ethanol. The product, CeVO4 catalyst, was then

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dried at 100°C in an oven for two hours.

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Catalysts characterization. The crystal structure of CeVO4 was determined using X-ray

diffraction PANalytical Empyrean Diffractometer with Cu-Kα radiation operating at 45 kV and

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was determined by the Debye-Scherrer equation:

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40 mA at 1.54 Å, in the 2θ range of 5° -70° and a step size of 0.05°. The size of the crystallite

𝐾𝜆

L = 𝐵 𝑐𝑜𝑠𝜃

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where L is the crystallite size (nm), K is Scherrer constant (0.89), λ is the wavelength of the XRD instrument, B is the peak full width at half maximum and θ is the diffraction angle [36].

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The structural analysis was carried out by using the UnitCell program [37]. The pseudo-Voigt profile function showed the best fit to the experimental data. The catalyst structure was visualized by employing VESTA software [38]. X-ray photoelectron spectroscopic (XPS) analysis was conducted using a SPECS X-Ray Photoelectron Spectroscopy instrument with a

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PHOBIOS 100/150 Delay Line Detector (DLD), with Al-Kα (1486.6 eV) dual anode as the source with the power of 385W and anode voltage of 13.85 kV. The XPS measurements were performed with a pass energy of 50 eV. The C-1s peak (at 284.5 eV) was employed as an internal reference for the absolute binding energy. The morphological structure of the catalyst was observed using scanning electron microscopy Quanta 250 FESEM equipped with EDAX

6

Apollo SDD detector for elemental analysis. The samples were coated with gold prior to the SEM measurements. The FTIR spectra were obtained by a Bruker Vertex 80v FTIR with a range of 400-4000 cm-1. Tecnai TEM, operated at 200 kV, was used for the transmission electron microscopy (TEM) analysis to determine the particle size and shape. Thermal stability analysis was conducted using NETSCH High-Temperature TGA with a heating rate of 10 °C/min in N2 atmosphere.

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The degradation of phenol. In a typical reaction, a predetermined amount of PMS was

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dissolved in a 50 mL solution of a 100 ppm phenol solution in a 100 mL beaker. Then, the

catalyst was added to the reaction mixture with continuous stirring at room temperature. Samples

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were withdrawn and filtered using 0.2 μm nylon membrane filters at specific time intervals. The

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phenol concentration in the filtrate was determined using a Shimadzu HPLC with a C18 column (Restek, 4.6 × 150 mm) and a UV detector that was set at 280 nm. The mobile phase used in the

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analysis contained a mixture of methanol (35%), water (64%), and acetic acid (1% ). Phenol degradation was monitored and the intermediates of the degradation process were observed using

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the same method [39]. The percentage of phenol removal was calculated by the equation below: Removal% =

(C° − C𝑡 ) C°

× 100

(1)

where Co and Ct are the initial and residual concentrations of phenol, respectively, at a specific time t. The degradation reactions of phenol were modeled using the following first-order kinetic

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equation: 𝐶

𝑙𝑛 𝐶𝑡 = −𝑘𝑡 𝑜

(2)

where Co and Ct are the initial and residual concentrations of phenol, respectively, at a specific time t. k is the rate constant of the degradation reaction which can be calculated from the slope of ln(Ct/Co) versus time. The effect of different reaction parameters on CeVO4 catalytic activity was

7

investigated, including catalyst loading, PMS dosage, initial phenol concentration, reaction temperature, and pH. The activation energy of the degradation reaction was determined using the Arrhenius equation: −𝐸𝑎

𝑘 = 𝐴𝑒 𝑅𝑇

(3)

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where k is the reaction rate constant, A is a pre-exponential factor, Ea is the reaction activation energy, R is the gas constant and T is the absolute temperature.

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The reaction pathway of the phenol degradation was evaluated using an ESR spectrometer

(Bruker EMX X-Band Spectrometer, United States). The active species involved in the reaction

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were determined by spin trapping using DMPO (5,5-Dimethyl-1-pyrroline N-oxide) for˙O2−,

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SO4˙−or ˙OH and TEMP (2,2,6,6-Tetramethyl-4-piperidone) for1O2. The reaction intermediates were identified by LC-MS (ACQUITY UPLC H-Class System from Waters Company) equipped

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with Bruker MS, using photodiode arrays and -ESI negative and a SB-C18 column. The stability of the catalyst was further determined by quantifying the metal leaching at the end of the reaction

Elmer).

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using an inductively coupled plasma optical emission spectrophotometer (ICP-OES, Perkin

Results and discussion Structural analysis.

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The crystallinity and structure of the prepared CeVO4 were studied by XRD. The result

is presented in Figure 1 (a), which is well-matched with the Zircon phase of CeVO4 reported in the literature and indicates a well-crystallized structure [40, 41]. The zircon structure of CeVO4 can be defined by CeO8 dodecahedrons and isolated VO4 tetrahedrons as illustrated in Figure 1 (b). The sharp diffraction pattern in Figure 1 (a) can be perfectly

8

correlated with the standard pattern of CeVO4 (JCPDS No. 12-0757). No additional peaks due to impurities were observed, suggesting the formation of highly pure CeVO4 nanoparticles. The prepared CeVO4 has a tetragonal symmetry (SG: I41/amd) and the calculated lattice parameters were a= b= 7.36343 Å, and c= 6.45885 Å, which are consistent with the reported values for

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-p

ro

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CeVO4 [42, 43].

9

u2(B2g)

u3(Eg) u4(Eg)

T(B1g)

40

2q/ degree

ro -p C=O

O-H

H-O-H

50

60

(224)

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(312)

(400) (213) (420) (004) (332) (204)

(103) (321)

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(211)

30

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CeVO4 JCPDS# 12-0757

(112)

Intensity (a. u.)

(101)

20

1000

Intensity (a. u.)

400 600 800 Raman shift (cm-1)

( 200)

100 200

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u4(B1g)

R(Eg)

10

u3(B1g)

(220) (202) (301)

Intensity (a. u.)

u1(A1g)

V-O

70

4000

3500

3000

2500

2000

1500 -1 wavenumber (cm )

1000

Ce-O

500

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Figure 1. (a) XRD pattern, (b) Crystal structure, (c) Raman spectrum and (d) FTIR spectrum of pristine CeVO4 nanoparticles. The crystallite size of CeVO4 was evaluated from the lattice plane (200) in the XRD pattern using the Debye-Scherrer formula. The crystallite size of CeVO4 nanoparticles was found to be 14 nm.

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Raman spectroscopy of the CeVO4 catalyst was also employed to determine the crystal structure of CeVO4 nanoparticles by investigating the bonding state of the various atoms. At ambient conditions, CeVO4 subsists in zircon structure containing two formula units per individual primitive cell. Group theory analysis predicted 12 Raman-active modes 2A1g + 4B1g + B2g + 5Eg [44], which are categorized into internal (ν1-ν4) and external (translational, T, and rotational, R) modes of VO4 units as follows: (4)

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Г = 𝐴1 𝑔 (𝑣1 , 𝑣2 ) + 𝐵1 𝑔 (2𝑇, 𝑣3 , 𝑣4 ) + 𝐵2 𝑔 (𝑣2 ) + 𝐸𝑔 (2𝑇, 𝑅, 𝑣3 , 𝑣4 )

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Figure 1(c) presents the Raman spectrum of the prepared CeVO4 catalyst. Out of the 12

Raman active predicted peaks of CeVO4 [45], nine modes were observed in Figure 1(c) which

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clearly shows that the most intense peak is at 850 cm-1 and can be assigned to the internal

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symmetric-stretching mode ν1 (A1g). The peaks at 777 and 790 cm-1 can be attributed to the asymmetric-stretching modes ν3(Eg) and ν3(B1g), respectively [46]. Three bending modes are

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observed at 262, 378, and 460 cm-1 due to ν2(B2g), ν4(Eg), and ν4(B1g), respectively [46]. Only one rotational R (Eg) and one translational T(B1g) modes have been detected [40, 47].

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The FTIR spectrum of the prepared CeVO4 nanoparticles is displayed in Figure 1(d). The spectrum shows IR bands at about 772 cm-1 and 441 cm-1 which can be attributed to the stretching vibration of V-O and Ce-O bonds, respectively [42, 48]. The observed peak at around 3400 cm-1 is due to the OH stretching vibration of physically absorbed H2O on the catalyst

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surface [48]. Additionally, the band at 1629 cm−1 corresponds to the bending of HOH [42, 48] and the band at 2427 cm−1 can be attributed to the presence of absorbed CO2 on the surface. Hence the FTIR spectrum further confirms the chemical structure of the CeVO4 nanoparticles. A thermal analysis was performed for the CeVO4 catalyst to study its thermal stability. Figure S1 represents the TGA graph of the CeVO4 sample and indicates that CeVO4 possesses high

11

thermal stability as no significant mass loss was observed in the temperature range of 25-1000 °C. The weight-loss stage below 200 °C could be attributed to the loss of the physically adsorbed water on the samples. The minor weight losses in the range of 200–600 °C might result from the decomposition of residual materials in the sample or due to structural rearrangements. The total weight loss of the sample is less than 6.5% up to 1000 oC, which indicates the high thermal stability of the prepared CeVO4 nanoparticles.

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Furthermore, to study the chemical states and surface compositions of CeVO4 nanoparticles,

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XPS analysis was performed and the obtained results are depicted in Figure 2(a-d). The typical XPS survey spectrum presented in Figure 2(a) shows peaks that are assigned to Ce4p, C1s, V2p,

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O1s, and Ce3d. The appearance of C1s peaks might be ascribed to possible carbon adsorption on

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the sample due to the air exposure. Figure 2(b) represents the high-resolution Ce3d spectrum composing of two multiplets due to the spin-orbit coupling of 3d5/2 (879.6 and 883.6 eV) and

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3d3/2 (898.0 and 902.0 eV). The spin-orbit splitting was found to be around 18.4 eV, with I 3d5/2/I 3d3/2 intensity ratio fixed to 1.5. Four peaks assigned to the pairs of spin-orbit doublets can be

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assigned to the Ce3+ oxidation states [49-51].

12

O,1s

C,1s

V,2p3/2

CeVO4

Ce 3d3/2

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O,auger

Ce 3d5/2

Intensity (a.u.)

(a )

Intensity (a.u.)

(b )

Ce,3d5/2 Ce,3d3/2

Ce,4p

200

400

600

800

1000

870

Binding Energy (eV)

880

890

900

910

-p

Binding Energy(eV)

(d )

O1s

Intensity (a.u.)

re

V2p3/2

Intensity (a.u.)

(c )

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V,2p1/2

510

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V2p1/2

515

520

525

Binding Energy (eV)

525

528

531

534

537

Binding Energy (eV)

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Figure 2. XPS spectra: (a) survey spectra, (b) Cerium (Ce 3d), (c) Vanadium (V 2p) and (d) Oxygen (O 1s) of CeVO4 nanoparticles. Figure 2(c) depicts the XPS spectrum of V2p, with two peaks at 515.6 and 523.5 eV, corresponding to the two characteristic states V2p3/2 and V2p1/2, which can be assigned to the pentavalent state (V5+) of vanadium [49, 50]. The high-resolution XPS spectrum of O1s is

13

represented in Figure 2(d). The O1s peak is quite asymmetric and can be de-convoluted into two peaks positioned at 528.0 and 531.5 eV, indicating the presence of two oxygen species on the CeVO4 surface. The peak at 528 eV can be attributed to the oxygen species of lattice oxygen of CeVO4, whereas the 531.5 eV peak may correspond to the physically adsorbed water on the surface of CeVO4. FE-SEM and TEM analysis were carried out to further confirm the nanostructure and

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morphology of the synthesized CeVO4 particles. The obtained images are represented in Figure

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3. As evident from the SEM image shown in Figure 3(a), the CeVO4 particles were found to agglomerate and the particles displayed irregular rod-like nanostructures with an average length

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of 14 nm and a width of 5 nm. In order to further probe the chemical composition and elemental

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distribution of the synthesized CeVO4 nanoparticles, EDX elemental mapping was performed. Figure 3(b) shows the EDX result of the CeVO4 nanoparticles with strong signals of Ce, V and

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O only. In addition, quantitative analysis revealed that the atomic ratio of Ce:V:O in the sample was 20:21:59, which agrees with the theoretical stoichiometric ratio of 1:1:4 for Ce:V:O in

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pristine CeVO4. The mapping images display a uniform distribution of Ce, V and O in the sample as can be seen in Figure 3(b). More detailed information about the structure and crystallinity of the nanoparticles was obtained from HRTEM analysis as presented in Figure 3 (c and d). The TEM image in Figure 3(c) shows the presence of well-dispersed, rod-like

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nanostructures with approximately 14 nm length and about 5 nm width in accordance with the SEM images. The HRTEM image in Figure 3(d) provides supplementary understanding of the CeVO4 nano-rods crystal structure consistent with the selected region in Figure 3(c), illustrating well-defined lattice fringes parallel to one another with similar d spacing of 0.37 nm, which is in accordance with the interplanar spacing of the (200) lattice plane of the CeVO4 structure [52].

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Figure 3. (a) FESEM image, (b) EDX profile (inset elemental mapping), (c) TEM images (inset

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SAED pattern) and (d) HRTEM images of as-synthesized CeVO4 nanoparticles. Phenol degradation reactions. Preliminary experiments were conducted on the phenol degradation using PMS alone, CeVO4

alone, and CeVO4 with PMS. The results of phenol degradation are depicted in Figure 4 (a). To emphasize the importance of the catalyst performance, the self-adsorption of the CeVO4 catalyst and the self-oxidation of PMS were tested beforehand. It is evident from Figure 4 that PMS

15

alone (without CeVO4) was not effective for the degradation of phenol. The figure indicates that only 2% removal of phenol was observed with PMS alone after about 180 min which may due to the absence of an oxidizing species. The CeVO4 catalyst alone (without PMS) displayed passive performance with less than 20 % removal of phenol in 180 min. Remarkably, the co-existence of PMS and CeVO4 catalyst achieved a complete degradation of phenol (100 %) within 80 min,

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suggesting the PMS activation by the CeVO4 catalyst.

0.8

ro

0.5

1.0

ln (C/Co)

0.6

C/Co

CV + PMS CV PMS CV + H2O2

R2= 0.9834

-0.5

re

0.4

-p

0.0

(b) 0.0 0

20

40

60

80

100

140

160

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Time (min)

120

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0.2

-1.0

180

200

(b)

-1.5 0

20

10

30

40

Time (min)

Figure 4. (a) Phenol degradation in different systems and (b) Pseudo-first-order kinetics for CV

(b)

+ PMS phenol degradation system [Co = 100ppm, catalyst loading = 1.0 g/L, PMS loading = 2.0 g/L]

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CeVO4 nanoparticles were effective in the decomposition of PMS into active species leading

to faster mineralization of phenol in water. The oxidation occurs via the heterogeneous activation of PMS by singlet oxygenation and mediated electron transfer as per the proposed non-radical mechanism in comparison to the most common radical mechanism [53-57]. The detailed reaction pathway will be discussed in the mechanism section based on the ESR and LC-MS results.

16

Figure 4(b) shows the first-order kinetic plots for the phenol degradation reaction using PMS and CeVO4 catalyst. The data were fitted to the pseudo-first-order kinetics model with a high R2 value. The data are not in a good fit for the zero-order and pseudo-second-order kinetics model, as indicated by the regression coefficients presented in Table 1.

Table 1

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Rate constants for phenol degradation system; Reaction conditions: Co = 100ppm, catalyst loading = 1.0 g/L, PMS loading = 2.0 g/L Rate equation

R2

Zero order

0.9029

0.0153

Pseudo-first order

0.9621

0.0427

-p

ro

k

0.7364

0.1829

re

Pseudo-second order

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Effects of reaction parameters on phenol removal.

Figure 5(a) depicts the degradation of phenol at different loadings of the CeVO4 catalyst. The

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proposed kinetic model was further verified in the CeVO4/PMS system for the degradation of phenol by using different dosages of PMS and CeVO4. The k values are presented in Table 2. Figure 5(a) shows that the catalytic activity increased as the CeVO4 loading increased, which can be due to the increase in the surface area of the catalyst that provides additional active sites

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for the activation of PMS that consequently enhances the degradation process. The catalyst loading of 2 g/L completely degraded phenol in about 60 min with k = 0.427 min-1 while the loading of 0.1 g/L achieved the same result in about 105 min with k = 0.0205 min-1, as shown in Figure 5(a). As can be seen in Figure 5(b), the degradation of phenol fitted the pseudo-firstorder kinetics at different catalyst loadings. In addition, from Table 2, it is clear that the rate

17

constant ( 𝑘 ) values showed a linear increase as the catalyst loading increases from 0 to 2 g/L, indicating that the generation of active site was directly proportional to the loading of the 1.0

C/Co

0.6

0.4

-0.5

R2= 0.9901

of

0.8

R2= 0.915

0.0

ln (C/Co)

100 mg 50 mg 25 mg 10 mg 5 mg 0

R2= 0.9966

-1.0

0.2

ro

R2= 0.9965

R2= 0.9855

-1.5

(b)

-p

(b)

0.0 0

20

catalyst.

40

60

0

80

10

20

30

R2= 0.9872 40

50

Time (min)

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lP

re

Time (min)

Figure 5. Effect of CeVO4 loading (a) and First-order kinetics (b) on phenol degradation

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efficiency [ Co = 100ppm, PMS loading = 2.0 g/L, T = 25 oC, pH=6] Table 2

Pseudo-first-order rate constants for phenol degradation conducted using different catalyst loadings. Conc (g\L) 0 0.1

R2 0.915 0.9901

k(min−1) 0.001 0.0205

18

0.2 0.5 1

0.9965 0.9966 0.9872

0.0239 0.0354 0.0365

2

0.9855

0.0427

The CeVO4 nanoparticles catalytic degradation of phenol was also investigated under different initial concentrations (Co) of phenol ranging from 10 to 200 mg/L. The results are

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depicted in Figure 6 (a) which reveals that as the Co of phenol increased from 10 to 200 mg/L, the phenol degradation dropped from 100 to 63% for a catalytic dosage of 1 g/L in about 30 min.

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At 100 ppm of phenol, a degradation of 27% was achieved after 180 min with k =0.0155 min-1

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while complete degradation was observed in 30 min for 10 ppm phenol with k = 0.1877 min-1 as indicated in Figure 6(b). At lower concentrations of phenol, complete degradation of phenol

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was achieved in a shorter time due to the high concentration of singlet oxygen and low

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concentration of phenol. Whereas at higher concentrations, the reaction proceeded at a much

19

slower rate (Table 3)which is possibly due to the limited catalytic surface availability, in agreement with literature findings [39, 58].

Figure 6. Effect of initial phenol concentration (a) and First-order kinetics (b) on oxidation

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efficiency [catalyst loading = 1.0 g/L, PMS loading = 2.0 g/L, T = 25 °C, pH=6]

10 ppm 25ppm 50 ppm 100 ppm 150 ppm 200 ppm

-0.5

R2= 0.9689

re

0.8

0.0

-p

0.5

1.0

ln (C/Co)

-1.0

lP

C/Co

0.6

0.2

(b) 0.0 0

Table 3

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0.4

20

40

60

80

-1.5

R2= 0.9679

-2.0

120

R2= 0.9902 R2= 0.9592

-2.5

R2= 0.9733

-3.0

(b)

-3.5

100

R2= 0.966

0

20

40

60

80

Time (min)

Time (min)

Pseudo-first-order rate constants for phenol degradation conducted at different initial

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concentrations of phenol. Conc (g\L)

R2

k(min−1)

0.2 0.5

0.9689 0.966

0.0155 0.0248

1

0.9679

0.1877

2 3 4

0.9592 0.9902 0.9733

0.0967 0.0559 0.0457

20

As evident from Figure 4, the degradation of phenol was very negligible when no PMS was added to the system, hence the role of PMS in the degradation process was investigated. Figure 7 (a) shows the relationship between the catalytic degradation of phenol and PMS dosage from 0 to 3 g/L. The degradation of phenol was considerably affected by the PMS concentration in the reaction. Complete degradation of phenol was observed within 30 min using 3 g/L of PMS. This

of

could be explained based on the amount of singlet oxygen generated; the lower the amount of

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PMS the lower the amount of singlet oxygen produced and consequently the slower the reaction. Alternately, the effect of PMS dosage was fitted into rate equations and it matched the pseudo-

re

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first-order kinetics at each PMS dosage (Figure 7 (a)).

PMS

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C/Co

0.4

Jo

0.2

(a)

0.0

0

20

40

60

80

100

120

Time (min)

140

pH=1.5 pH=3.0 pH=6.0 pH=9.0 pH=12.0

0.8

0.6

C/Co

0.8

0.6

pH

1.0

lP

1.0

0.4

0 25 mg 50 mg 100 mg 150 mg 160

180

0.2

(b) 0.0 0

20

40

60

80

100

120

Time (min)

Figure 7. Effect of (a) PMS dosage and (b) solution's initial pH value on the catalytic activation of PMS using CeVO4towards phenol degradation; Reaction conditions: Co = 100ppm, catalyst loading = 1.0 g/L, T = 25 oC, pH=6.

21

The efficiency of PMS in AOPs depends on the initial pH of the solution. Therefore, the effect of different pH values on phenol degradation was investigated. The results are presented in Figure 7(b) which shows that as the pH increased from 3 to 12, the phenol removal decreased indicating that the degradation reaction favors acidic conditions. However, at pH=1.5, the reaction proceeded at a slower rate than the one at pH=3. This can be attributed to the increase in the concentration of H+ presented in the solution at pH=1.5 which consequently increases the

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possibility of the hydrogen bond formation between the H+ and the peroxo linkage in PMS;

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weakening the O-O bond and scavenging the active species (singlet oxygen) in the solution [15, 59].

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Additionally, PMS was found to be more stable at extremely acidic medium which makes it

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harder to be activated [60]. At the pH range between 3 and 9, more singlet oxygen can be generated when they interact with water and OH- [61]. At extremely alkaline solutions (pH=12),

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CeVO4 form hydroxide complexes which have lower oxidation potentials and negatively charged surfaces that elevate the repulsion between the catalyst and PMS anions which reduces CeVO4

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activity [62, 63].

The effect of the reaction temperature was investigated at 25, 35 and 45 oC and the results are depicted in Figure 8 (a). At 45 oC, complete phenol degradation was achieved in 40 min. In addition, the reaction rate constant at each temperature was determined based on first-order

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kinetics and the Arrhenius equation was used to calculate the activation energy of the reaction. Figure 8 (a) further illustrates that the rate of the reaction elevated significantly at higher temperatures, suggesting the endothermic nature of the degradation reaction. Using the Arrhenius equation, the activation energy for phenol degradation is found to be 23.74 kJ/mol. The calculated Ea of the degradation reaction is greater than the value for the diffusion-controlled

22

reaction (10 to 13 kJ/mol) [64], signifying that the observed rate of the degradation was attributed to the rate of surface-mediated intrinsic chemical reactions rather than the rate of mass transfer [64]. Henceforth, higher temperatures would enhance PMS activation by CeVO4 nanoparticle to generate more reactive singlet oxygen and thereby improve the catalytic degradation efficiency. The activation energies of various catalytic systems reported for the degradation of phenol via the activation of PMS are listed in Table 4. As reported earlier, the

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catalyst has a great influence on the activation energy of the degradation reaction. However, the

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data depicted in Table 4 suggested that the CeVO4 nanoparticle system exhibited lower

activation energy than most of the reported catalysts, indicated the promising efficiency of the

1.0

-2.6

y= -2855.2x+6.3108 R2=0.9461

ln (k)

Phenol Resorcinol Methyl Orange Acrylamide Methyl Violet (6B)

0.8

lP

-2.8

0.8

re

1.0

-p

CeVO4 system for catalytic degradation of phenol.

-3.0

ur na

-3.2

C/Co

0.6

C/Co

0.6

0.4

-3.4 0.0031

0.0032

0.4

0.0033

0.0034

1/T (K-1)

0.2

0.2

T=25 oC T=35 oC T=45 oC

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0.0 0

10

20

30

40

Time (min)

50

60

(b)

(a) 0.0 70

80

0

20

40

60

80

100

120

Time (min)

Figure 8. (a) Effect of the reaction’s initial temperature on the catalytic activity of CeVO4 toward phenol degradation and the activation energy plot and (b) degradation performance of PMS and CeVO4 on different pollutants: Resorcinol, methyl orange, Acrylamide and methyl

23

violet compared to the degradation of phenol; [Reaction conditions: Co = 100ppm, catalyst loading = 1.0 g/L, PMS loading = 2.0 g/L, T=25 °C, no pH adjustment] Table 4 Activation energies of different catalysts with PMS for phenol degradation reaction. Ea (kJ/mol)

Reference

SrCo0.6Ti0.4O3−δ

77.5

[65]

Co3O4/SBA-15

67.4

[66]

Fe3O4 /Mn3O4 /rGO

25.49

CoMoO4

69.8

CeVO4

23.74

[67]

This work

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ro

[15]

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Degradation efficiency of different pollutants.

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Catalyst

To investigate the applicability of CeVO4 and PMS system for the catalytic degradation of

lP

different organic pollutants, four different pollutants were also investigated. The degradation of each of resorcinol, acrylamide, methyl violet (6B) and methyl orange was tested under the same

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experimental conditions. Figure 8 (b) shows that CeVO4 /PMS system is not phenol oriented and it successfully degraded all the above organic pollutants. The regeneration and reusability are important criteria for practical applications of a

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heterogeneous catalyst. The performance of the regenerated CeVO4 nanoparticles was tested for 5 consecutive cycles at the same reaction parameters. After each cycle, the catalyst was recovered by filtration followed by washing with DI water. The obtained results are presented in Figure 9 which shows that, even after 5 consecutive regeneration cycles, the reused catalyst exhibited a high catalytic activity as indicated by complete degradation of phenol within 60 min. These results further specify the good repeatability and robustness of the CeVO4 catalyst. In

24

addition to that, there was no significant leaching of either Ce or V (< 5 ppm using ICP OES) from the CeVO4 catalyst during the catalytic reaction. Figure S2 compares the Raman, XRD, FT-IR and Figure S3 compares SEM/EDX data for CeVO4 before and after the reaction. The recovered catalyst was washed and dried prior to the analysis. The obtained results suggest no notable changes in the composition and morphology of the catalyst after the reaction. The EDX data agrees with the ICP-OES results which confirm the

of

stability of CeVO4 catalyst. On the other hand, the FT-IR showed an increase in the intensity of

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O-H peak (3400 cm-1) which could be due to the adsorption of phenol onto the CeVO4 surface. The appearance of C=C bond around 1630 cm-1 in addition to the shift in O-H bond are both a

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clear indication of the non-radical pathway via phenol adsorption on the catalyst surface. Similar

Jo

ur na

lP

re

observations were previously reported in the literature [68].

25

1.0

1st

2nd

3rd

4th

5th

0.8

ro

C/Co

of

0.6

-p

0.4

0.0 20 40 60

0

20 40 60

Time (min)

Time (min)

0

20 40 60 Time (min)

0

20 40 60 Time (min)

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Time (min)

0

lP

0 20 40 60 80

re

0.2

Figure 5. Regeneration cycles of CeVO4 for phenol degradation reaction [Co = 100ppm, catalyst loading = 1.0 g/L, PMS loading = 2.0 g/L, T=25 oC, no pH adjustment] Identification of reactive species and plausible mechanism

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Phenol degradation via PMS activation by heterogeneous catalysts has been reported

previously by various research groups [69-71], which followed different mechanistic pathways as (a) radical-mediated (SO4•- and •OH radicals) catalytic PMS decomposition [15, 72-74] (b) nonradical mediated pathway where PMS activation is carried out by electrons from the organic substrate via an electron transfer mediator [69]. The quenching experiments using the scavenging

26

agents for these radicals can be employed as a typical test to identify the reactive species in the PMS activation process. Tert-butyl alcohol (TBA) is employed as a characteristic scavenger for OH radicals (k =3.8-7.6×108 M-1s -1), whereas methanol (MeOH) is exploited for scavenging both SO4·- (k=3.2×106 M−1s −1) as well as OH radicals (k =9.7×108 M−1s −1) [70]. NaN3 is reported 1.0

1.0

(b)

(a)

as

0.8

0.8

0.05 0.04 0.03

0.4

0.02

0

0.2 Quenching Condition

TBA MeO

0.0

0

0.0 20

efficient quencher

40

60

Time (min)

80

20

40

60

80

100

120

140

Time (min)

for •OH

lP

0

re

0.2

-p

0.01

No scavenger NaN3

10 mM 5 mM 0 mM No scavenger

ro

0.4

0.6

C/Co

k (g L-1min-1)

C/Co

0.6

of

an

(k=1.2×1010 M−1 s −1 ), SO4 •− (k=2.51×109 M−1 s −1) and non-radical oxidation process 1O2

Jo

ur na

(k=2×109 M−1 s −1) [75].

Figure 6. Effect of (a) NaN3 and (b) 10mM of different scavengers on PMS oxidation for phenol degradation [Co = 100ppm, catalyst loading = 1.0 g/L, PMS loading = 2.0 g/L, T=25 oC, pH=6].

27

Referring to Figure 4, negligible phenol degradation was achieved when the catalyst was removed from the reaction system which shows that the reaction is completely derived by the catalyst. The proposed reaction pathway was further investigated by scavenging the active species and quenching the reaction using NaN3, MeOH and TBA. The results are depicted in Figure 10(b) which clearly indicates that TBA and MeOH had almost no inhibition effect on phenol degradation, whereas a considerable decrease in the phenol degradation was observed in

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the presence of NaN3. These results confirm that the reaction proceeds via a non-radical path.

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Compared to the initial degradation pathway, the oxidation rate of phenol at a given condition decreased by 98% on the addition of 10 mM NaN3 and nearly by 20% on the addition of 10 mM

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MeOH and 10 mM TBA, respectively. Furthermore, upon the addition of excess NaN3, a change

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in degradation level was perceived linear to the concentration of the added scavenger. The degradation efficiency decreased from the original 100% at 30 min to 81, 54 and 28% on the

lP

addition of 1 mM, 5 mM and 10 mM of NaN3, respectively, as evident from Figure 10(a). This indicates that the reaction is mainly driven by the non-radical pathway via singlet oxygen.

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To further confirm the non-radical reaction pathway of the CeVO4/PMS system, ESR measurements were performed using spin trapping agents DMPO to determine ˙O2−, SO4˙−or ˙OH as well as TEMP for 1O2. The results are presented in Figure 11 which shows no signal of DMPO-OH and DMPO-SO4 adduct observed in the case of DMPO, DMPO/PMS and

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DMPO/PMS-CeVO4 systems indicating that ˙O2−, SO4˙−or ˙OH radicals are not generated in the CeVO4/PMS system. Although, in the case of TEMP/PMS-CeVO4 system, a characteristic triplet signal with a relative intensity of 1:1:1 of TEMPO adduct is observed and can be assigned to 1O2 mediated oxidation of TEMP [76]. The above results are in concurrence with the radical quenching experiments, which further confirm the presence of 1O2 as the most reactive species in

28

the activation of PMS using CeVO4, proposing a non-radical pathway as a prevailing catalytic reaction for phenol degradation[77].

CV+PMS (TEMP)

of

TEMP only CV+PMS (DMPO)

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Intensity (a. u.)

(a)

CV (DMPO)

-p

PMS (DMPO)

3300

3400

re

DMPO only

3500

3600

3800

3700

ur na

Intensity (a.u)

lP

Magentic field (G)

(b)

PMS

t=0 t=15

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t=30

0

1

t=45 t=60 t=75

2

3

4

5

Retention Time (min)

29

Figure 11. (a) ESR spectra of different systems in DMPO and TEMP and (b) HPLC graphs of phenol degradation by CeVO4, via PMS activation [Co = 100ppm, catalyst loading = 1.0 g/L, PMS loading = 2.0 g/L, T=25 oC, pH=6]. The detection of the reaction intermediates was performed by HPLC throughout the catalytic process by identifying the intermediate products that were formed during the degradation of

of

phenol. Figure 11(b), depicts the process of the degradation, where the phenol peak at a retention time (RT) of 4.5 min decreases with increasing the reaction time and completely

ro

disappears within 80 min of the reaction time. A new peak appeared at a RT 2.5 min due to a

-p

reaction intermediate within the first 15 min of catalytic reaction along with a shoulder at 2.19 min and increased in intensity till 30 min of the reaction time, after which the peak intensity

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decreased indicating the degradation of the reaction intermediate. By increasing the reaction time, three new peaks appeared at RT 1.5, 1.2 and 0.68 min, indicating the presence of secondary

lP

degraded products. The peaks were identified as p-benzoquinone, hydroquinone, maleic acid,

solutions.

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malonic acid, and acetic acid, respectively, distinguished by RT values of their standard

To further confirm the existence of these intermediates in the degradation process, the samples were analyzed by LC-MS and the results are depicted in Figure S4. Based on the results obtained from the scavenging studies and the intermediate identification,

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the degradation mechanism for phenol by PMS activation follows a non-radical pathway via singlet oxygen generation and can be schematically represented in Figure 12 and Figure 13. During PMS activation by CeVO4 the first step is the adsorption of PMS on to the surface of CeVO4 nanocatalysts and subsequent charge transfer happens between the catalysts and PMS molecules resulting in the generation of singlet oxygen. The singlet oxygen is then is

30

successively desorbed from the surface of the CeVO4 nanoparticles into the aqueous solution containing phenol for degrading phenol. Similar observations are being reported by other groups [78-80]. In the process of PMS activation, Ce3+ in CeVO4 is oxidized to Ce4+ by the defect sites in the crystal lattice of the CeVO4 and V5+ is reduced to V4+ by the electrons present simultaneously. Aforementioned, a redox reaction can rapidly occur between V4+ with strong reducing capacity and Ce4+ with strong oxidizing ability, generating the original V5+ and Ce3+,

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respectively. The formation of these types of redox reaction centers in CeVO4 is widely studied

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in-depth and reported in the literature previously [49, 81-83]. The generation of oxygen

vacancies also favors the interfacial electron transfer mechanism, as reported earlier by other

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research groups [80, 84, 85]. Henceforth the redox centers and oxygen defects present at the

re

catalyst interface play a major role in the generation of singlet oxygen.

In the second step, the singlet oxygen interacts with phenol and initiates the degradation to

lP

hydroquinone which is oxidized to p-benzoquinone. In the third step, the p-benzoquinone further undergoes ring-opening reaction and degrade to organic acids and breaks down into simpler

ur na

acids in the fourth step, and then complete mineralization occurs to CO2 and H2O in the last step. The degradation mechanism was found to be consistent with those reported in the literature [21,

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23, 69, 70].

31

CV (defects) + PMS O 2 + phenol

of

1

O2

O 2 + degradation products

-p

1

O2

ro

1

O=O*

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ur na

lP

re

Figure 12. Possible mechanism of PMS activation by CeVO4.

Figure 13. Plausible degradation mechanism of phenol by CeVO4, via PMS activation. Conclusions This study demonstrates the high catalytic performance of CeVO4 catalyst towards phenol degradation via PMS activation. The CeVO4 nanoparticles, synthesized through a simple one-pot

32

co-precipitation method, exhibited good crystallinity and displayed a rod-like structure. The catalytic degradation process was found to depend on various operating parameters such as the initial concentration of phenol, loading of the catalyst, PMS dosage, pH and reaction temperature. Detailed kinetic studies revealed that the catalytic degradation of phenol fitted with the pseudo-first-order kinetics, with an Ea= 23.74kJ/mol. The catalytic degradation mechanism and the species involved were further investigated by quenching the radicals using scavengers

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like TBA, MeOH, and NaN3, which showed that the degradation followed a non-radical pathway

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as confirmed by the ESR results. The regeneration and reusability experiments of the catalyst over 5 consecutive cycles indicated the good reusability of the catalyst. The catalytic system was

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efficient at removing a wide range of organic pollutants indicating that the catalytic performance

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of the CeVO4/PMS system was not phenol dependent. The reaction intermediates were investigated using LC-MS and confirmed the presence of p-benzoquinone, hydroquinone, maleic

lP

acid, malonic acid, and acetic acid during the phenol degradation process. Combining a simple synthesis method with excellent catalytic CeVO4 properties is a suitable cost-effective,

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environmentally friendly technique that can be employed in water treatment applications.

Author contribution

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I.O and J.HZ conceived and planned the experiments and carried out the experiments. J.HZ contributed to sample preparation. I.O, J.HZ, M.A and F.B contributed to the interpretation of the results. I.O wrote the initial draft of the manuscript, and further modified by J. HZ. All authors provided critical feedback and helped shape the research, analysis and manuscript.

33

Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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ACKNOWLEDGMENT

The authors are grateful to Khalifa University (KU) - for providing support for conducting this

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research under grant LTR14013. The authors would like to thank Dr. Kaustava Bhattacharyya, Bhabha Atomic Research Centre, for XPS measurements. The authors would like to

Jo

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Waterloo, for the ESR studies.

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acknowledge Dr. Ali Elkamel, Dr. Rahul Deshpande and Prof. David Cory, University of

34

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