A novel magnetic heterogeneous catalyst oxygen-defective CoFe2O4−x for activating peroxymonosulfate

Accepted Manuscript A novel magnetic heterogeneous catalyst oxygen-defective CoFe2O4−x for activating peroxymonosulfate

Liying Wu, Yongbo Yu, Qian Zhang, Junming Hong, Ji Wang, Yuecheng She PII: DOI: Reference:

S0169-4332(19)30646-4 https://doi.org/10.1016/j.apsusc.2019.03.034 APSUSC 41984

To appear in:

Applied Surface Science

Received date: Revised date: Accepted date:

2 January 2019 28 February 2019 3 March 2019

Please cite this article as: L. Wu, Y. Yu, Q. Zhang, et al., A novel magnetic heterogeneous catalyst oxygen-defective CoFe2O4−x for activating peroxymonosulfate, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.03.034

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ACCEPTED MANUSCRIPT

A novel magnetic heterogeneous catalyst oxygen-defective CoFe2O4-x for activating peroxymonosulfate

Liying Wu1,2, Yongbo Yu, Qian Zhang1,2, Junming Hong1,2*, Ji Wang1,2, Yuecheng She1,2.

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(1. Department of Environmental Science and Engineering, Huaqiao University, Xiamen

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361021, China；

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2. Fujian Provincial Research Center of Industrial Wastewater Biochemical Treatment (Huaqiao University), Xiamen 361021, China)

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Highlights:

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1. A novel oxygen-defective CoFe2O4-x tolerant with wide pH range was designed and synthesized.

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activation were explored.

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2. The functions of oxygen defects, transition metals, and surface hydroxyl group for the PMS

3. Bisphenol A was efficiently degraded by CoFe2O4-x/PMS with low catalyst concentration.

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Abstract:

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4. The degradation reaction of bisphenol A mainly took place on the surface of the material.

Through direct and indirect radical mechanism of peroxymonosulfate (PMS) activation, novel CoFe2O4-x catalysts were successfully developed via hydrogen calcination to overcome popular disadvantages of the dependence on pH. Bisphenol A (BPA) was selected as the model pollutant to decipher the mechanism of catalysts for peroxymonosulfate (PMS) activation. Possible degradation pathways of Bisphenol A were proposed via analysis of liquid chromatograph mass spectrometer (LC-MS). The findings indicated that catalytic activation of CoFe2O4-x was not

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ACCEPTED MANUSCRIPT dependent upon the initial pH, as the direct and indirect radicals seemed to be generated in parallel. FTIR analysis confirmed that surface hydroxyl groups were actively involved in the activation of PMS under alkaline conditions. During the reaction, the oxygen defects promoted electron transfer and participated in the redox cycle from Co3+/Fe3+ to Co2+/Fe2+ to generate 1O2 and O2•-. Hydroxyl

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(OH•) and sulfate (SO4•-) radicals were generated on the surface of CoFe2O4-x by the synergistic

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interactions among oxygen defects, transition metal, and surface hydroxyl groups. To the best of

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our knowledge, this combined mechanism of direct and indirect radical generation for advanced

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oxidation was a first-attempt study to be disclosed in public domain.

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Keywords: Oxygen defects; CoFe2O4-x; Peroxymonosulfate; Bisphenol A; Advanced oxidation

*

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Corresponding author. Department of Environmental Science and Engineering, Huaqiao

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University, Fax: +86-5926162300; (E-mail): [email protected].

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1. Introduction

Environmental pollution caused by recalcitrant organics has hazardous health effects on humans populations due to their long-term persistence. In particular, many highly toxic and

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non-biodegradable pollutants are difficult to be removed or attenuated by conventional treatment methods [1]. This led to significantly increased need for cost-effective technologies to eliminate

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organic contaminants from reclaimed waters. The development of the sulfate radical (SO4•-)-based

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advanced oxidation processes (SR-AOPs) are attractive due to their promising treatment efficiency

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in the oxidation of recalcitrant organic contaminants [2.3]. Regarding activating species, peroxymonosulfate (PMS) is a relatively stable and strong oxidant (E0=1.75 V). It can be activated

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to generate SO4•- (2.5-3.1 V) with a much stronger redox potential than a hydroxyl radical (•OH) (1.9-2.7 V) [4]. Various methods have been developed to activate PMS for SO4•- generation [5].

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In addition, transition-metal-based catalysts have strong catalytic activity for PMS activation [6]. Heterogeneous metallic oxide catalysts have a stable structure and good potential of recycling

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compared with homogeneous catalysts. Bimetallic oxides display higher reactivity and economic feasibility for catalysis than their corresponding single metal-containing oxide due to synergistic

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interactions between different metal ions [7]. From the perspective in kinetics, the degradation reaction on the material surface is rate-controlling, and the basic groups on the surface of catalysts were considered kinetically favorable to the catalytic oxidation and the degradation of organic contaminants [8.9]. Recently, significant attentions of bimetal catalysts have been paid in the field of catalytic materials [10]. However, detailed mechanism is still remained open for discussion, as two pathways of direct and indirect radical generation have been proposed by various studies. For the 3

ACCEPTED MANUSCRIPT direct radical pathway, the formation of SO4•- and •OH activated by the bimetal catalysts was proposed as follows (M represents metal ions) [11]:

(1)

 M(n 1)  HSO5  Mn   SO 5  H 

(2)

SO 4  H 2O  SO 24  OH H 

(3)

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 Mn   HSO5  M(n 1)  SO 4  OH

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The catalytic efficiency of direct generated radicals could be affected by the pH, significantly

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influencing the capacity of radical generation [12]. Prior studies also have shown that optimal degradation generally took place under neutral or acidic conditions [13]. An increase of solution

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pH leads to the increase in the amount of negative surface charge on the catalyst. This would

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inhibit the static interactions between catalysts and PMS, which were unfavorable to trigger oxidation reaction [14]. Moreover, for the indirect radical pathway, surface hydroxyl groups and

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oxygen defects are identified as active oxygen species, and they play a crucial role in oxidation

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reaction [15]. For example, Fe(III) and Co(II) have extraordinary capability to form Fe(III)-OH, Co(II)-OH complexes on the surface of catalysts via H2O dissociation [eqs (4)-(6)] [16]. These

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complexes could bond with HSO5- through hydrogen bond, and the H-O and O-O bond would

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crack after accepting the electrons discharged from the oxidation of M2+ to M3+. During this process, SO4•- is produced [eq (7)] [15].

(4) (5) (6)

e  CoOH  HSO 5-    CoO  SO 4-  H 2O -

According to this mechanism of free radicals generating from surface hydroxyl groups, the

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(7)

ACCEPTED MANUSCRIPT catalysts revealed good catalytic properties under both acidic and alkaline conditions. Yang et al. [17] synthesized ferromagnetic CoFe2O4 that contain massive hydroxyl groups, exhibiting a significant effect on the environmentally friendly activation of PMS for degradation of 2,4-dichlorophenol at alkaline conditions. The electron transfer capability of the catalyst influences

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the effect of the catalyst on the PMS [18]. Moreover, oxygen defects in pristine metal oxides may

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improve the electronic conductivity of metal oxides [19]. Some studies also indicated the

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generation of singlet oxygen (1O2) due to the presence of oxygen defects [20]. Apparently, considering the direct and indirect radical generation in parallel would be a possible

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alternative to overcome such pH dependence of PMS activation. Thus, combined the possible

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synergistic effects of oxygen defects, transition metals, and surface hydroxyl groups, we rationally designed a novel oxygen-defective CoFe2O4-x catalyst for PMS activation to overcome the effect

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of pH upon catalytic capability.

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Here, the stable oxygen-defective CoFe2O4-x nanoparticles were first obtained by thermal treatment in H2 atmosphere. The characteristics of loaded oxygen defects were investigated via X-ray

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diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and

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X-ray photoelectron spectrometry (XPS). To confirm the PMS activation mechanism that involves oxygen defects and hydroxyl groups to be arranged on CoFe2O4-x surfaces, different designated processes to degrade model pollutant BPA were investigated. Phosphate-supplemented experiment was applied to explore the effect of surface hydroxyl groups. Quenching tests were implemented to determine the reactive radicals produced in the CoFe2O4-x/PMS system. FTIR analysis was implemented to determine that surface hydroxyl groups were involved in the surface reaction. Vibrating sample magnetometry (VSM) analysis was carried out to confirm the magnetic

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ACCEPTED MANUSCRIPT properties of CoFe2O4-X nanocomposite. Fingerprint analysis via LC-MS was performed to propose the possible pathways of BPA degradation. In summary, this first-attempt study aimed to uncover the possible interactive roles of oxygen defects and surface hydroxyl group in PMS activation. This study also proposed more constructed mechanism to decipher the correlations

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between direct and indirect radical generation for BPA degradation.

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2. Materials and methods

2.1. Chemicals

Peroxymonosulfate (potassium monopersulfate triple salt), ferric nitrate (Fe (NO3)3•9 2O),

citric acid, phosphate, Methanol (MeOH), tert-butyl alcohol

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cobalt nitrate (Co (NO3)2•3

2O),

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(TBA), and phenol were in analytical grade and obtained from Sinopharm Chemical Reagent Co.,

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Ltd. BPA was purchased from Aladdin Reagent (Shanghai, China).

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2.2. Sample preparation

Oxygen-defective CoFe2O4-x was synthesized by using hydrogen reduction method. First, the

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pristine CoFe2O4 was prepared through the sol-gel method. The following reagents were dissolved

2O,

10 mM Fe

and 20 mM citric acid. The obtained colloidal solution was heated at 65 °C for 4 h

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(NO3)3•9

2O,

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in a certain amount of water under vigorous stirring: 5 mM Co (NO3)2•6

and dried at 90 °C. Then, the xerogel was calcined at 300 °C for 3 h to obtain pristine CoFe2O4.

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Finally, the pristine CoFe2O4 was reduced in a fixed-bed reactor in a 100% hydrogen atmosphere,

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and the CoFe2O4-x was further fabricated.

2.3. Characterization

Surface morphology of the pristine CoFe2O4 and CoFe2O4-x was characterized on a SEM (Tescan mira3 Zeiss Merlin Compact) and TEM (FEI Tecnai G2 F20). The crystal structure was analyzed by XRD (Smar/Smart La). XPS (Thermo Scientific Escalab 250Xi) was performed to determine the change in element content of the pristine CoFe2O4 and CoFe2O4-x. The VSM (PPMS Dynacool) was used to measure the magnetic properties of the above-obtained catalysts. The dissolved quantity of 7

ACCEPTED MANUSCRIPT Co and Fe was detected using atomic absorption spectrophotometry (AAS, AA-6880 SHIMADZU, Japan).

2.4. Experimental procedures

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Batch experiments were carried out in 100 mL glass vessels with 30 mg L-1 BPA solution at room

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temperature ( 5 ± 1 °C). A designated dosage of PMS and solid catalyst were dispersed in the BPA

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solution under magnetic stirring to initiate the reaction. Quenching reagents were all prepared in 100 mM for MeOH, TBA, and phenol. The pH was adjusted by using a diluted aqueous solution

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of NaOH or H2SO4 (1 mM), and the initial solution p was 7.7. The transient dynamics of pH

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levels during the reaction were also monitored by a pH meter. At predetermined time intervals, 1 mL solutions were withdrawn and immediately quenched with an equal volume of methanol.

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Parallel samples were filtered through 0.  μm syringe filters prior to analysis of

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high-performance liquid chromatography (HPLC, Waters ACQUITY Arc). The HPLC was equipped with a PDA 998 detector (detection wavelength: 30 nm) and a Waters CORTECS C18

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column (46 mm × 5 mm, 3.5 μm). The flow rate of the mobile phase consisted of ultrapure water

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and methanol (15%:85%, v:v) was 0.6 mL min-1. The possible BPA degradation intermediates in the CoFe2O4-x/PMS system were identified by ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS, Agilent 1 90/6545) with a C18 column (50 mm ×  .1 mm × 1.7 μm). The mobile phase involved water and methanol solvents with a flow rate of 0.3 mL min-1. MS analyses were carried out in negative electrospray ionization (ESI-) mode in the m/z range of 50–800.

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3. Results and discussion

3.1 Structure characteristic of catalysts As shown in Fig.1 for XRD patterns of pristine CoFe2O4 and CoFe2O4-x nanoparticles, the

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well-defined peaks occurring at 30.10◦, 35.63◦, 43.64◦, 53.54◦, 57.35◦ and 62.56◦ belonging to the (220), (311), (400), (422), (511), and (440) planes of cubic spinel structure (JCPDS 22-1086). This

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result confirmed the presence of spinel structure CoFe2O4 nanoparticle, and the absence of any other

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impurity was detected. CoFe2O4-x had no phase change compared with the pristine one. However,

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XRD peaks were slightly weak and relatively broad. This phenomenon was possibly due to the

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defects in the crystal structure.

Fig 1 XRD patterns of pristine CoFe2O4 and CoFe2O4-x nanoparticles

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As revealed in the morphology of the CoFe2O4 nanoparticles via SEM and TEM, the pristine CoFe2O4 were spherical and tended to be aggregated [Fig.2 (a-b)], and the average diameter was in scales of nanometers. The fringe distances of pristine CoFe2O4 were ca. 0.29 nm and 0.25 nm (Fig. 2(c)), and these can be indexed to the (220) and (311) fringes of cubic spinel CoFe2O4 [21]. This finding also agreed with the results of the XRD pattern. In comparison, the oxygen-defective CoFe2O4 nanoparticles were typical spherical particles without significant aggregation [Figs. 2(d-e)]. The lattice fringe image in Fig. 2f indicated that the lattice fringe spacings of CoFe2O4-x 9

ACCEPTED MANUSCRIPT were around 0.29 and 0.25 nm, which corresponded to the (220) and (311) fringes. However, the lattice fringes of CoFe2O4-x became disordered, suggesting that the CoFe2O4-x had poor crystal structure with more defects after 3 h reduction [22].

CoFe2O4-x

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pristine CoFe2O4

(d)

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(a)

（e）

(b)

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（c）

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(f)

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Fig.2 SEM images of pristine CoFe2O4 (a) and CoFe2O4-x (d) nanoparticles, TEM images of pristine

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CoFe2O4 (b-c) and CoFe2O4-x (e-f) nanoparticles. To explore the phenomenon of oxygen defects, XPS measurements were conducted (Fig.

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3(a-b)). The wide survey spectra showed that pristine CoFe2O4 and CoFe2O4-x contained Co, Fe, and O elements. The XPS result also revealed that the content of oxygen atoms in CoFe2O4

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decreased after hydrogen reduction (Table 1), indicating that oxygen defects were produced in CoFe2O4 [18.23]. Furthermore, O 1 s spectra in XPS was analyzed to confirm the generation of

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oxygen defects. As shown in Fig. 3 (c-d), O 1 s spectra of both CoFe2O4 and CoFe2O4-x could be resolved into three Gaussian peaks. The lowest peak at 529.95 eV, the second higher peak at 530.45

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eV, and the highest peak at 531.52 eV were attributed to the lattice oxygen species, oxygen vacancies, surface hydroxyl species [21], respectively. Gaussian function was used to calculate the area of O 1 s spectrum to investigate the change of oxygen defects concentration (Table 1). The results of the XPS measurements further indicated that after calcining CoFe2O4 with hydrogen, the oxygen defects concentration increased approach 3.5 times, while the content of surface hydroxyl species remained nearly identical.

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Fig. 3. XPS wide survey spectra of (a) pristine CoFe2O4 and (b) Oxygen-defective CoFe2O4-x; O 1 s spectra of (c) pristine CoFe2O4 and (d) Oxygen-defective CoFe2O4-x.

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Table 1 Proportional changes of oxygen species in XPS result Oxygen-defective CoFe2O4-x

oxygen atoms

47.05

43.65

lattice oxygen species

0.65

0.40

Oxygen defects

0.16

0.38

surface hydroxyl species

0.19

0.22

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pristine CoFe2O4

VSM technique was employed to investigate the magnetic behaviors of the pristine CoFe2O4 and CoFe2O4-x nanoparticles, and the room temperature magnetic hysteresis loops are shown in 12

ACCEPTED MANUSCRIPT Fig. 4. The saturation magnetization (Ms) of CoFe2O4-x increased from 56 emu g-1 to 82 emu g-1 compared with the pristine CoFe2O4. It was sufficient to separate nanoparticles and aqueous medium through superparamagnetic characteristic of CoFe2O4-x, as shown in the inset of Fig. 4. These all provided the supporting evidence for its potential practicability as a reusable magnetic

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catalyst.

Fig.4 Magnetic hysteresis loops of pristine CoFe2O4 and CoFe2O4-x nanoparticles at room

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temperature

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3.2 Catalytic degradation of BPA under various systems

3.2.1 Validity testing of CoFe2O4, CoFe2O4-x

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The degradation of BPA was investigated using various processes, (i.e., the PMS, pristine CoFe2O4, CoFe2O4-x, CoFe2O4/PMS, and CoFe2O4-x/PMS processes.) Fig. 5(a) presents the degradation profiles of BPA, with the same initial BPA, PMS, and catalyst concentrations of 30 mg L-1, 1 mM, and 0.05 g L-1, respectively, and the initial pH was 7.7. These control experiments demonstrated that the degradation of BPA by pristine CoFe2O4 and PMS alone was negligible under normal atmospheric temperature. The CoFe2O4-x system showed a certain removal effect on

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ACCEPTED MANUSCRIPT BPA, and the removal rate of BPA reached 35% after 30 min of reaction. This may be due to the oxygen defects [24]. High charge and oxygen-ion conductivity were involved in reaction with dissolved oxygen to produce 1O2 and O2•- in the surface of CoFe2O4-x [eqs (8)-(9)] [25]. The active species of 1O2 and O2•- could thus degrade BPA [26]. 

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x y gd ee nf e c t O2  e- o   O2

(9)

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x y gd ee nf e c 1t O2  Fe3 /Co 3  2e o   O2  Fe2 / C o2

(8)

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To further test the degradation mechanism of BPA in a CoFe2O4-x system, N2 bubbling experiment was conducted. The reaction solution was purged with nitrogen for half an hour before the reaction,

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and the nitrogen atmosphere was maintained during the reaction process to ensure that no dissolved

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oxygen was remained in the solution. The result showed that the BPA was unlikely to be degraded (Fig. 5(a)). It was concluded that the O2 participated in the reaction with the presence of oxygen

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defects. The removal rate of BPA in CoFe2O4/PMS system reached 68% after 30 min of reaction,

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possibly due to the synergistic interactions of transition metals and surface hydroxyl groups to activate PMS. The CoFe2O4-x/PMS system had excellent degradation efficiency for BPA

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degradation (98%). This was likely attributed to the coexistence of oxygen defects and surface

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hydroxyl groups in the catalyst. To further confirm this point, iron and cobalt ion dissolved in both systems was also applied for feasibility testing. Compared with the CoFe2O4-x/PMS, the metal leaching of pristine CoFe2O4/PMS seemed to be larger, but with poor degradation of BPA (Figs. 5(b) and (c)). This result suggested that the homogeneous reactions were negligible [27]. The synergistic interactions of oxygen defects and surface hydroxyl groups in the catalyst apparently played the crucial role for BPA degradation.

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Fig. 5 (a) Removal efficiency of BPA in different reaction systems within 30 min, (b) iron and (c)

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cobalt ions dissolution in CoFe2O4-x/PMS and CoFe2O4/PMS system. (conditions: [PMS]=1.0 mM,

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[catalyst]=0.05 g L-1, [BPA]=30 mg L-1, pH=6.0, T=25 °C.)

3.2.2 pH overcoming test

Operating parameters (e.g., catalyst dosage, PMS dosage, and the initial pH) all affect BPA removal efficiency. Therefore, in order to obtain the optimized conditions, the effect of these parameters on BPA removal in CoFe2O4-x/PMS system were studied. The transient profiles of degradation at different initial pH in CoFe2O4-x/PMS were shown in Fig. 6(a). The results were not

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ACCEPTED MANUSCRIPT identical as literature [28.29] indicated. Catalytic oxidation of BPA was not dependent upon the initial pH (3-11). The degradation of BPA was excellent when the solution pH value was 11 and was still dissatisfactory when the reaction solution had neutral pH. This phenomenon may be associated to the point of zero charge (pHpzc) of the catalyst and the effect of surface hydroxyl

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groups. Fig. 6(b) presents the zeta potentials of CoFe2O4-x in solution under different pH values.

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The zeta potentials varied from positive to negative with changing pH values, and the pHpzc of

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CoFe2O4-x was determined to be 5.80. Thus, the surface of CoFe2O4-x at pH < 5.80 (pH < pHPZC) was positively charged, which was more favorable for PMS (anion) approaching the surface of

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CoFe2O4-x [30]. Therefore, the interaction between PMS and CoFe2O4-x promoted the production of

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reactive species for BPA removal. At the initial pH of 11, the surface of CoFe2O4-x was negatively charged (pH > pHZPC), this would inhibit the combination of catalyst and PMS. However, the

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degradation of BPA was not significantly changed, which may be due to the formation of CoOH+

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with the existence of large amounts of surface hydroxyl groups at alkaline pHs [16]. The CoOH+ would interact with PMS to generate free radicals SO4•- and HO• with the assistance of oxygen

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defects [eqs (3)-(7)]. Fig. 7 indicates FTIR spectra of the reacted CoFe2O4-x surface at different pH

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levels. Two absorption bands observed at around 740-750 cm-1 and 1174 cm-1 for PMS alone representing S-O stretching of HSO5- and SO42- [31], respectively. However, about red-shift was observed when CoFe2O4-x was supplemented to PMS solution, suggesting that the change of S-O bond was possibly taken place. Such characteristics demonstrated that SO4•- or SO5•- could be produced from PMS oxidation [15]. Moreover, signal peaks near the wave number 3394 cm-1 were assigned to the stretching band of the surface hydroxyl groups of CoFe2O4-x. Evidently, the peak intensities of these bands also increased when the pH increased to 11. It was thus proposed that

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ACCEPTED MANUSCRIPT surface hydroxyl groups were directly involved in the activation of PMS at alkaline pHs. The

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degradation reaction mainly took place on the surface of CoFe2O4-x, not intraparticle.

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Fig.6 (a) Influence of initial pH on the efficiency of CoFe2O4-x / PMS system during degradation

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of BPA (b) The Zeta potential of CoFe2O4-x corresponding to different pH levels. (operation

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conditions: [PMS]=1.0 mM, [catalyst]=0.05 g L-1, [BPA]=30 mg L-1, T=25 °C.)

Fig. 7 FTIR spectra of the PMS and CoFe2O4-x alone, CoFe2O4-x in PMS solution at different pH. (conditions: [PMS]=1.0 mM, [catalyst]=0.05 g L-1, [BPA]=30 mg L-1, T=25 °C.)

3.2.3 Catalyst and PMS dosage

To evaluate the catalytic activity of CoFe2O4-x, comparative experiments was implemented at 17

ACCEPTED MANUSCRIPT pH 7.7, PMS 1.0 mM, and 30 mg L-1 BPA (Fig. 8(a)). The BPA removal increased markedly with catalyst dosage increased from 0 g L-1 to 0.05 g L-1. This was likely due to that the catalysts provided more reactive sites for PMS activation. However, BPA degradation was slightly repressed with the continued increase of catalyst dosage to 0.1 g L-1. This result likely resulted

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from self-quenching of the generated SO4•- by the excess catalyst [28]. Therefore 0.05 g L-1

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catalyst was sufficient in a CoFe2O4-x/PMS system.

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The effect of PMS concentration on BPA removal was also investigated (Fig. 8(c)). Similarly, removal efficiency was significantly increased with increasing PMS amount from 0 mM to 0.5

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mM. This might suggest that 0.5 Mm PMS was threshold concentration to trigger BPA

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degradation to be fully expressed. Furthermore, an increase in PMS concentrations beyond 1.0 mM was not resulted in further improvement in BPA removal. This finding indicated that 1.0 mM

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PMS could provide sufficiently abundant active radicals like SO4•- and HO• for BPA degradation.

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Moreover, the transient dynamics of pH were also shown in Figs. 8 (b) and (d), which indicated that pH levels were not affected by catalyst dosage, but significantly decreased to around 3 in the

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presence of PMS.

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Fig. 8 Effects of CoFe2O4-x dose on time courses of (a) BPA degradation and (b) solution pH;

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Effects of PMS dose on time courses of (c) BPA degradation and (d) pH. (conditions: [BPA]=30

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mg L-1, initial pH=7.7, [CoFe2O4-x]=0.05 g L-1, [PMS]=1 mM.)

3.3 Postulated reaction pathways via intermediate identification

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The degradation intermediates of BPA via transition metal activating PMS oxidation were tentatively identified via UPLC-Q-TOF-MS analysis. According to mass spectrograms and recent experimental studies [32.33], some aliphatic compounds and aromatic intermediates were detected

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(e.g., 2, 3, 5-trimethylh, benzyl glycolate, and benzene ethanol). Detailed information of

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intermediate products could be shown in Table 2. Evidently, the BPA degradation in the CoFe2O4-x/PMS system involved the breakage of carbon-carbon bonds, aromatic ring cleavage, elimination reactions, and hydroxylation [34]. The possible degradation routes are proposed in Fig. 9. First, the aromatic ring in BPA was attacked by reactive oxygen species, and benzyl glycolate and benzene ethanol were then generated. Next, the carbon-carbon bond-breaking would occur, and the intermediate products were transformed into n-propylbenzene and ethylbenzene. The oxidation reactions further converted these compounds into low molecular organics (e.g., 19

ACCEPTED MANUSCRIPT p-benzoquinone and acetic acid) [32]. Finally, the intermediate products were completely mineralized into CO2 and H2O during the degradation process. Table 2 Reaction intermediates identified by UPLC-Q-TOF-MS molecular Number

Chemical name

(m/z) [M-H]

-

Tentative structure

227.1078

C15H16O2

2

Benzyl glycolate

165.0557

C9H10O3

3

Benzene ethanol

149.0972

C10H14O

4

2,3,5-Trimethylhexane

127.1492

5

Benzene acetaldehyde

119.0502

C8H8O

6

n-Propylbenzene

119.0866

C9H12

7

p-Benzoquinone

107.0139

C6H4O2

8

Ethylbenzene

105.0710

C8H10

103.0553

C8H8

Benzene

93.0346

C6H6O

acetic acid

88.9880

C2H2O4

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11

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C9H20

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9

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Bisphenol-A

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1

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formula

20

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Fig.9 Proposed pathways for the BPA degradation by CoFe2O4-x /PMS system

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3.4. Mechanistic study

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In the spinel structure of CoFe2O4, the octahedral site can easily accommodate Co2+, Co3+, Fe2+ and Fe3+ [27]. The intimate Fe-Co interactions are very critical for efficient heterogeneous

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activation of PMS mainly due to three reasons as shown herein. First, Fe3+ has extraordinary

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capability to form surface FeOH2+ complexes via H2O dissociation [eq. (4)] compared with Co2+ [27]. Second, Co2+ might interact with the nearby surface hydroxyl groups bound to Fe3+ [eq (6)]

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rather than directly with H2O [eq (5)] to form surface CoOH+ complexes [17]. Third, the

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generated CoOH+ bonds with HSO5- and then accepts the electrons from synergistic interaction of oxygen defects and transition metal to generate SO4•- [eq (8)]. The aforementioned assumption can be confirmed by introducing phosphate into the CoFe2O4-X/PMS system. Phosphate exerts a masking effect by strongly bonding with transition metals dispersed on the catalyst surface [35]. Only 10 mM phosphate could significantly suppress the decomposition of PMS and limit BPA degradation in the CoFe2O4-X/PMS system (Fig. 10). Apparently, the synergistic effect of surface hydroxyl groups and transition metals was essential for the activation of PMS. 21

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 CoOH  HSO5- oxygen defects    CoO  SO 4-  H2O

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Fig. 10 The inhibitory effect of phosphate on BPA degradation in CoFe2O4-x/PMS system. (reaction conditions: [PMS]=1.0 mM, [catalyst]=0.05 g L-1, [BPA]=30 mg L-1, [phosphate]=10

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mM.)

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In fact, SO4•- and •OH could be formed during the catalytic activation of PMS by transition metal oxide. To further verify the effects of reactive species on the BPA oxidation, quenching

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tests of methanol and TBA were performed in the CoFe2O4-X/PMS system. Methanol is considered

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as a scavenger for SO4•- (k=1.6-7.7 × 107 M-1s-1) and •OH (k=1.2-2.8 × 109 M-1s-1), whereas TBA was a scavenger mainly for •OH (k=3.8-7.6 × 108 M-1s-1) [9.24]. However, there was no

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remarkable inhibition on BPA removal when 100 mM methanol and TBA were added to the

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CoFe2O4-X/PMS system (Fig. 11(a)). This could be due to the reactive radicals originally produced on the surface of CoFe2O4-X. Furthermore, TBA and methanol may not accumulate on the surface of catalyst [36]. In order to verify this postulate, phenol was introduced to the system. Phenol can readily approach the solid-liquid interface and quench the SO4•- (k=8.8 × 109 M-1s-1) and •OH （(k=6.6 × 109 M-1s-1) [36]. The rate constants of the quenched reactions were in the following order: phenol > MA > TBA. This could be on account of the different permittivity (phenol: 9.78, TBA: 12.47 and Methanol: 33) of scavengers [36]. As Fig. 11 (a) showed, that the degradation of

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ACCEPTED MANUSCRIPT the BPA was strongly inhibited in the presence of phenol. Therefore, in a CoFe2O4-X/PMS system, the reactive radicals could be originally produced on the surface of CoFe2O4-X, and the degradation of BPA most likely took place on the catalyst surface. RBK5 (azo dye-Reactive Black 5) degradation experiments were also conducted by using the CoFe2O4-X/PMS system. As

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presented in Fig. 11(b), similar phenomenon as BPA degradation were observed. These results

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further confirmed the effect of CoOH+ on CoFe2O4-X surface dealing with the removal of

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contaminants.

(a)

(b)

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Fig. 11 Effect of radical scavengers on (a) BPA and (b) RBK5 degradation in the CoFe2O4-x/PMS

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system. Reaction conditions: [PMS]=1.0 mM, [catalyst]=0.05 g L-1, [BPA]=30 mg L-1, [scavengers]=100 mM.

As the matter of fact, Anipsitakis et al. [37] found that CoOH+ complexes were the rate-limiting step for Co2+-mediated PMS activation. CoFe2O4 could efficiently regenerate CoOH+, and this could be responsible for the remarkable increase of catalyst activity. That is, the presence of CoFe2O4-x on the activation of PMS is crucial for the oxidative activity due to the presence of massive oxygen defects. With the oxidation of ≡Co2+ to ≡Co3+ (eq (10)), the electrons are discharged. The H-O and O-O bonds in ≡CoOH+-(OH) OSO4- would be cracked after accepting 23

ACCEPTED MANUSCRIPT the electrons and producing SO4•- [15]. In this process, the oxygen defects can effectively promote electronic transfer. As aforementioned, the following activation mechanism (Fig. 12) on the surface of CoFe2O4-x can be proposed. In the preliminary stage, ≡Fe3+ captured H2 and further formed ≡CoOH+ on the

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surface of the catalyst [eqs (4)-(6)]. Afterward, ≡CoOH+ reacted with HSO5- to generate SO4•- [eqs

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(6)-(7)]. Meanwhile, ≡Fe2+ and ≡Co2+ also participated in the reactions [eqs (11)-(12)] for the

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generation of SO4•-, ≡Fe3+ and ≡Co3+ [38]. The latter can be backward reduced by PMS in the presence of oxygen defects, thereby generating less active SO5•- [eq (13)]. Furthermore, HO•

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would be further produced by SO4•- and H2O [eqs (3)]. In the presence of oxygen defects, oxygen

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reacted with Co3+/Fe3+ to produce O2•-, 1O2 and reduced Co3+/Fe3+ to Co2+/Fe2+ [eqs (8)-(9)]. Lastly, these produced radicals in CoFe2O4-x/PMS system efficiently degraded BPA to

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intermediates [Eq (14)]. It should be emphasized herein that in the traditional transition

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metal/PMS system, the reduction of ≡Fe3+/≡Co3+ to ≡Fe2+/≡Co2+ would be not kinetically favorable. However, the formation of oxygen defects could promote the redox cycle of Co3+/Co2+

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and Fe3+/Fe2+ [eq (13)] [19].

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(11)

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(12) 5

4(

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,

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)

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intermediates

(13) (14)

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Fig. 12 Postulated mechanism of the heterogeneous activation of PMS by CoFe2O4-X

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4. Conclusion

In this paper, CoFe2O4-x with abundant oxygen defects was synthesized, XRD analysis also

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confirmed the spine structure of catalysts. SEM, TEM, and XPS indicated that the number of oxygen atoms in the CoFe2O4-x decreased after hydrogen reduction, and apparently more oxygen defects were produced. The oxygen defects could react with dissolved oxygen to produce 1O2 and

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O2•−. The synergistic interactions of oxygen defects, surface hydroxyl groups, and transition metals

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could activate PMS to produce SO4•- and •OH for BPA removal. Results of TBA, phenol, and MeOH quenching experiments indicated that the main active species were solely generated on the surface of catalyst. The CoFe2O4-x with excellent magnetism can be easily recycled. The mechanism underlying BPA degradation by CoFe2O4-x/PMS system could be proposed as follows. 1) 1O2 and O2•- were formed at the oxygen defects sites in the presence of O2. 2) Co2+ might interact with the nearby surface hydroxyl groups to form surface CoOH+ complexes and further bonded with HSO5- to generate OH• and SO4•-. 3) Oxygen defects in the CoFe2O4-x promoted 25

ACCEPTED MANUSCRIPT electronic transfer of the redox cycle from Co3+/ Fe3+ to Co2+/ Fe2+, which is feasible for the generation of OH• and SO4•-. 4) Results of LC-MS analysis demonstrated that BPA could be

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completely mineralized to H2O and CO2.

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Acknowledgement This work was financially supported by Fujian province Science and Technology project Foundation (2017I01010015), Xiamen Technology project Foundation (3502Z20173050, 3502Z20173052, 3502Z20183023), Quanzhou Technology project Foundation (2016Z074,

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2018Z002). ubsidized Project for Postgraduates’ Innovative Fund in cientific Research of

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Huaqiao University（17013087060）.

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ACCEPTED MANUSCRIPT Graphical abstract: Novel CoFe2O4−x with abundant oxygen defects that were successfully developed via hydrogen calcination were used for peroxymonosulfate (PMS) activation. The oxygen defects promoted electronic transfer and participated in the redox cycle from Co3+/Fe3+ to Co2+/Fe2+ to generate 1O2 and O2•−. Hydroxyl (OH•) and sulfate (SO4•−) radicals were generated on the surface of CoFe2O4-x by the synergistic action of oxygen defects, transition metal, and surface hydroxyl groups for the

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degradation of BPA.

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