Bi25FeO40: radical and non-radical mechanism

Journal of the Taiwan Institute of Chemical Engineers 100 (2019) 56–64

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Kinetic performance of peroxymonosulfate activated by Co/Bi25 FeO40 : radical and non-radical mechanism Wei Li a, Yongli Zhang a,∗, Yang Liu a, Xin Cheng a, Weihong Tang a, Chengwei Zhao a, Hongguang Guo a,b,∗∗ a b

College of Architecture and Environment, Sichuan University, Chengdu 610065, China Department of Civil & Environmental Engineering, University of Washington, Box 352700, Seattle, WA 98195-2700, United States

a r t i c l e

i n f o

Article history: Received 30 November 2018 Revised 14 February 2019 Accepted 24 February 2019 Available online 24 April 2019 Keywords: Sillenite bismuth ferrite Cobalt Peroxymonosulfate Sulfate radicals Singlet oxygen Acid orange 7

a b s t r a c t Co/Bi25 FeO40 was synthesized by a hydrothermal process and applied as a peroxymonosulfate (PMS) activator for the removal of Acid orange 7 (AO7). The morphology and physicochemical features of Co/Bi25 FeO40 were obtained and analyzed via multiple characterizations. Crucial parameters including catalyst dosage, PMS concentration, temperature, pH and AO7 concentration were evaluated thoroughly for the kinetics performance. The results revealed that the efficient PMS activation performance was demonstrated using the synthesized Co/Bi25 FeO40 . Increasing catalyst dosage, PMS concentration, temperature and pH could promote AO7 removal, while the increase of AO7 concentration led to the inhibited degradation. Structural characterization, use of chemical probes and data of ESR analyses demonstrated that sulfate radicals and singlet oxygen (1 O2 ) were identified as the major reactive oxygen species. Simultaneous pathways through radicals (sulfate radicals) and non-radical (singlet oxygen) were proposed with oxygen vacancy, active oxygen (O∗ ) and the redox pair Bi5+ /Bi3+ involved. This study could explore the application field of sillenite bismuth ferrite in non-photochemical application for environmental remediation. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Bismuth-based oxides (BO) has been manipulated in a large number of applications in environmental pollution remediation [1–3]. Among them, bismuth-based iron oxides (BIO), have raised plenty of attention in optoelectronic and solar applications, due to the advanced ferroelectric and ferromagnetic performance [4,5]. Specially, BIO has demonstrated its unmatched potential as heterogeneous catalysts for environmental decontamination of which BiFeO3 , Bi2 Fe4 O9 are well reported [6–9]. Nevertheless, compared to perovskite-type BiFeO3 , there was seldom works focused on sillenite bismuth ferrite (Bi25 FeO40 ). Bi25 FeO40 as an innate body-centered cubic crystal structure (space group I23) exhibits excellent photocatalytic activity for its appreciated band gap (< 2.8 eV) [10]. The overfast recombination of photogenerated

∗

Corresponding author. Corresponding author at: College of Architecture and Environment, Sichuan University, Chengdu 610065, China. E-mail addresses: [email protected] (W. Li), [email protected] (Y. Zhang), [email protected] (Y. Liu), [email protected] (X. Cheng), [email protected] (W. Tang), [email protected] (C. Zhao), [email protected] (H. Guo). ∗∗

holes and electrons remains one of the greatest challenges for the widespread application of Bi25 FeO40 [11–15]. In spite of the tremendous reactions with photocatalysis involved, a recent study has demonstrated the distinct chemical catalytic performance with Bi25 FeO40 as an alternative choice for its potential application [16]. Sulfate radical-based advanced oxidation processes (SR-AOP) through activated peroxymonosulfate (PMS) or persulfate (PS) have been proposed as a promising way to degrade reluctant organics in the water [17–19]. PMS, as a promising oxidizing agent, can be activated via homogeneous ways [20–27] or heterogeneous ways [28–31], among which activators with Co modification have been found most effective to degrade organic compounds [17,32,33]. Plenty of activators modified with Co have been widely reported, for instance, Co/Bi2 O3 [34], Co/graphene [35], Co/MnO2 [18] and other Co-based materials [36]. Inspired by the significant enhancement caused by the cobalt modification, the combination of cobalt modified bismuth ferrite and PMS system was put forward for the degradation of contaminant. Despite the fact that abundant literatures have attributed sulfate radicals as the dominating reactive oxygen species (ROS) regarding PMS activation, singlet oxygen has been recently expounded as an alternative reactive specie for organic pollutants decontamination through non-radical activation [37–40]. Generally

https://doi.org/10.1016/j.jtice.2019.02.033 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

W. Li, Y. Zhang and Y. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 100 (2019) 56–64

photosensitive processes are usually responsible for the emerging of singlet oxygen, while several literatures have reported the singlet oxygen generation under non-photochemical systems, including bismuth-based oxides [38,41–43]. In our previous study, singlet oxygen (1 O2 ), instead of hydroxyl radicals or sulfate radicals, was generated from the redox reactions as the main reactive oxygen species [41]. To our best knowledge, the combined performance concerning radical and non-radical reactions for the integration of cobalt modified Bi25 FeO40 and PMS still remains unclear. In the present study, Bi25 FeO40 modified with Co, referred to as Co/Bi25 FeO40 , was prepared via hydrothermal method to generate ROS for the degradation of acid orange 7 (AO7), which deteriorates the color index of waters, with carcinogenic and toxic potential. Characterized by FT-IR, XRD, XPS and EDX, the crystalline structure and textural property of synthesized activator were thoroughly investigated. The PMS activation performance was evaluated and optimized. The ROS in the coupled system was identified via quenching performance and electron spin resonance (ESR) analysis. This study shows the potential for environmental reluctant contaminants removal in water via the simultaneous radical and non-radical combined pathways. 2. Materials and methods 2.1. Chemicals Acid Orange 7 (C16 H11 NaO4 S·5H2 O) was purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). PMS (Oxone, 2KHSO5 ·KHSO4 ·K2 SO4 ), 5,5-dimethyl-1-pyrrpline-N-oxide (DMPO), 2,2,6,6-Tetramethyl-4-piperidinol (TEMP, 98%), t-BuOH and sodium azide (NaN3 ) were purchased from Sigma-Aldrich Co., Ltd (Shanghai, China). Cobaltous nitrate hexahydrate (Co(NO3 )2 ·6H2 O), Ferric nitrate nonahydrate (Fe(NO3 )3 ·9H2 O), Bismuth nitrate pentahydrate (Bi(NO3 )3 ·5H2 O) of analytical grade quality were purchased from Kelong Chemical Reagent Co. Ltd. (Chengdu, China). All other chemicals were of analytical grade and used as received without further purification. All the experiments were conducted with the water purified by a Millipore Reverse Osmosis (RO, 18.25 M cm) system. 2.2. Synthesis of Co/Bi25 FeO40 The activator was synthesized using a facile hydrothermal method, in which 0.06 g Co(NO3 )2 ·6H2 O, 0.32 g Fe(NO3 )3 ·9H2 O and 0.49 g Bi(NO3 )3 ·5H2 O were dissolved in 200 ml 0.5 M HNO3 with rapid magnetic stirring. After added 60 ml NaOH (2 M) dropwise, the obtained mixture was filtered and washed 3 times with 2 M NaOH. The obtained black precipitate was transferred to a Teflon liner stainless steel autoclave filled with 70 ml 14 M NaOH and 10 ml ethanol as a dispersant. Afterwards, the device was heated at 120 °C for 24 h in an electric oven. Finally, the product was filtered, then washed several times with ethanol and deionized water in sequence, and dried at 60 °C in a vacuum oven until use. The pure Bi25 FeO40 was synthesized by the above hydrothermal method where the dosage of Fe(NO3 )3 ·9H2 O was changed to 0.40 g. 2.3. Characterization of Co/Bi25 FeO40 The crystallographic and mineralogical data of the obtained catalyst were investigated using the X-ray diffraction spectrometry (XRD) patterns which were carried on a X’Pert Pro MPD diffractometer (Philips, Netherlands) using monochromatized Cu Kα radiation under 40 kV and 100 mA. A scanning ranges from 20° to 80° at room temperature was adopted. Energy dispersive X-ray spectroscopy (EDX) (JEOL, Japan) for elemental analysis was obtained. The X-ray photoelectron spectroscopy (XPS) was

57

employed with a XSAM800 electron spectrometer from England Krators Scientific to identify the type of surface elements using 300 W Al Kα radiation. The binding energies for O, Bi, Co and Fe were corrected respectively by adjusting carbon C1s core level at 284.8 eV from adventitious carbon. Nicolet 6700 Fourier Transform Infrared Spectrometer (FT-IR, Thermo Fisher Scientific, United States) was employed to identify the functional groups of catalyst surface. Electron spin resonance (ESR, A300 spectrometer, Bruker) spectra were recorded at room temperature. A portable handhold dissolved oxygen meter (HACH, HQ30D) was applied to monitor the evolution of dissolved oxygen (DO). The PMS concentration was detected via the iodometric method [44]. 2.4. Experimental procedure All batch experiments were conducted in a 250-ml glass reactor with vigorous stirring at selected temperatures. Typically, 200 ml solution containing 4 mg AO7 and 0.05 g catalyst was prepared. 0.25 mM PMS was added into the solution to initiate the reaction immediately. Prior to the analysis using UV–vis spectrophotometer, samples were withdrawn and filtered by a 0.22 μm Teflon syringe filter at selected intervals of reaction. Solution pH was adjusted from 2.0 to 10.0 using 0.1 M HClO4 or NaOH. For the quenching experiments, ethanol, NaN3 or tert–butyl alcohol was added into the reaction solutions. All experiments were conducted three times, with the standard deviation shown on each data. 3. Result and discussion 3.1. Structure characteristics of Co/Bi25 FeO40 The XRD patterns of Bi25 FeO40 with/without Co modification are compared with standard card JCPDS#51-1015 in Fig. 1(a), in which indistinctive XRD peaks were observed related to cobalt modification, indicating that hybrid material (Co/Bi25 FeO40 ) might be prepared via the one-pot synthesis [36]. Moreover, the introduc˚ could replace the Fe (0.64 A), ˚ and induce tensile tion of Co (0.74 A) strain of Bi25 FeO40 lattice, which led to the gradual shift of (310) and (321) peak [45]. While the lattice parameters of Bi25 FeO40 ˚ it was calculated to be were determined as: a = b = c = 10.18 A, 10.23 A˚ for Co/Bi25 FeO40 . According to the Scherrer equation:

D = kλ/β cosθ

(1)

where k is the coefficient, β is the half height width of diffraction peak, θ is the diffraction angle, and λ is the X-ray wavelength, corresponding to the Cu Kα radiation for the (310) peak, the average perpendicular thickness to the direction of the grain was calculated to be around 85 nm. Compared with the 75 nm average crystal size of the pure Bi25 FeO40 , the increase of crystal size could be ascribed to the restricted crystal growth from the substitution of Fe by Co in coupled lattice structure [46]. FT-IR spectra of pristine/used Co/Bi25 FeO40 is shown in Fig. 1(b). The three weak absorption peaks at 40 0–60 0 cm−1 demonstrated the overlapping of Fe-O and Bi-O group vibration. Absorption peak at 579 and 447 cm−1 are ascribed to the Fe-O stretching and bending vibration of octahedral FeO6 group, respectively, and [BiO6 ] octahedral group is identified by the peaks at 528 and 457 cm−1 [47,48]. The bands at around 850, 1047, 1327 and 1384 cm−1 are due to the stretching vibrations of NO3 − on catalyst surface [47]. In accordance with the reported literature, the band at 1106 cm−1 could come from the S-O stretching of SO24− [49]. The peaks at 1527 and 1617 cm−1 are ascribed to the stretching vibration of C–H bond and C = O bond, respectively, which was caused from the synthesis process. [50]. The two weak peaks at 2850 and 2921 cm−1 were associated with the stretching vibration of surface -OH [49]. The broad absorption band at 3394 cm−1 was associated with the water molecules. Of note, NO3 − and C–H bond vanished after

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W. Li, Y. Zhang and Y. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 100 (2019) 56–64

Fig. 1. (a) XRD and (b) FT-IR patterns of Co/Bi25 FeO40 .

the reaction, while S-O bond appears suggesting the dissociation of PMS. Overall, the limited variation of the spectra demonstrates the excellent stability of catalyst. Furthermore, as illustrated in Fig. S1, the EDX elemental mapping of catalyst denoted that Bi, Fe and Co were distributed homogeneously among the catalyst. The atomic percentages of Bi, Fe and Co as determined by EDX were 6.66%, 7.25% and 1.69%, as illustrated in Fig. S2. The surface chemistry of Co/Bi25 FeO40 was further investigated using XPS analysis. The full-scan XPS spectrum of fresh and used Co/Bi25 FeO40 unveiled the presence of Co, Bi, O and Fe, as illustrated in Fig. S3. The core-level spectra of these constituents before or after the reactions were deconvoluted in Fig. 2 (bismuth and oxygen) and Fig. S4 (cobalt and iron). The two peaks at 158.8 eV and 164.1 eV in Bi 4f region are referred to Bi3+ and Bi5+ , respectively (Fig. 2(a) and (b)) [16]. The O1s XPS results of sample are

shown in Fig. 2(c) and (d). The peaks located at 529.5, 530.0 and 530.9 eV are ascribed to Fe-O bond, Co-O bond and Bi-O bond, respectively, and the binding energy for O 1 s electrons in samples is 531.3 eV caused by surface hydroxyl [51,53]. Meanwhile, the Co 2p peaks at binding energy of 780.3 and 795.4 eV are typical peaks of Co3+ , compared to Co2+ located at 782.5 and 796.7 eV, with two satellites peaks at 804.7 eV and 785.8 eV (Fig. S4a and Fig. S4b) [36,51]. The Fe 2p core-level spectrum (Fig. S4c and Fig. S4d) shows three peaks at 710.4 eV, 717.5 and 724.4 eV, which are attributed to Fe3+ , compared to the peak at 712.5 eV for Fe2+ [52,53]. Specifically, the peaks at 710.4 and 724.4 eV are derived from Fe3+ irons in octahedral sites [54]. And the peak at 717.5 eV was supposed to be the satellite peak of Fe3+ [55]. Based on the above results, it is demonstrated that the Co/Bi25 FeO40 composites have been successfully synthesized via the facile in situ one-pot approach.

W. Li, Y. Zhang and Y. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 100 (2019) 56–64

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Fig. 2. XPS spectra of the fresh Co/Bi25 FeO40 : Bi 4f (a); O 1s (c) and the used Co/Bi25 FeO40 : Bi 4f (b); O 1s (d).

3.2. Performance evaluation As can be seen in Fig. S5, AO7 can hardly be degraded by single PMS or adsorption (<1%). The removal rate could reach 41.8% in Bi25 FeO40 /PMS system within 60 min. Nevertheless, the degradation of AO7 could achieve 98.2% removal rate at 60 min by Co/Bi25 FeO40 /PMS system, which exhibited a great potential on PMS activation. Pseudo-first-order kinetic model is adopted to fit the obtained results, with the fitting parameters concerning various conditions summarized in Table 1. Fig. 3(a) showed the degradation rate of AO7 was 45.5%, 68.3%, 98.2%, 99.5%, 98.2% respectively with catalyst dosage increasing from 0.05–1.00 g/L. As listed in Table 1, the Kapp was enhanced by almost 24 times. It was demonstrated that higher dosage of catalyst can provide a higher quantity of active sites (i.e., redox sites and oxygen vacancy sites) for generation of ROS from Co/Bi25 FeO40 /PMS system to degrade AO7 [56]. As illustrated in Fig. 3(b), increasing the initial concentration of AO7 could inhibit the removal efficiency of AO7. When the initial concentration of AO7 was 10 mg/L, nearly complete degradation was achieved in 30 min, while only 65.5% of AO7 was removed for that was 60 mg/L. At higher initial concentration of AO7, more ROS was desired to meet the demand of degradation of AO7, leading to the decrease of Kapp from 0.126 to 0.019 min−1 . Besides, the degradation intermediates could also make the competitive effect with the target [56]. PMS dosage on AO7 degradation was employed and the results are shown in Fig. 3(c). The degradation efficiency of AO7 were increased from 61.1% to 97.5% with the concentration of PMS at 0.10–2.50 mM. In detail, initial increase followed the reduce of Kapp was obtained when PMS concentration was higher than 1.00 mM.

A higher dosage of PMS could increase the quantity of activated molecule, and boost the capability of interaction between PMS and catalyst, leading to the enhancement of degradation of AO7, while excessive PMS would also react with ROS and the recommendation occurred between radicals could cause the decrease of required amounts of ROS (Eqs. (2) and (3)) [57,58]. 2− 2− ·− SO·− 4 + S2 O8 → SO4 + S2 O8

·− 2− SO·− 4 + SO4 → S2 O8

K = 6.1 × 105 M−1 s−1

K = 4.0 × 108 M−1 s−1

(2)

(3)

Fig. 3(d) shows the temporal evolution of UV–vis spectrum for the degradation of AO7 in the Co/Bi25 FeO40 /PMS system. A characteristic peak for AO7 of aromatic rings emerged at 310 nm. The shoulder caused by N = N group of the azo form is shown at 403 nm and the peak at 485 nm is ascribed to the transition n-ð∗ (from the hydroxyl group to the nitrogen bridge of the hydrazine form) [59]. As can be seen, the peak of aromatic rings decreased rapidly, demonstrating the preference of ROS on aromatic rings of AO7. Then, the gradual decrease of the two characteristic peaks at 484 nm and 408 nm was observed until reaching a flat, which suggested that the two tautomeric forms could be simultaneously removed [60]. According to the previous studies, the increase of the absorbance at 250 nm is ascribed to the broken bond of AO7, illustrating the formation of monoaromatic by-products with discoloration effect. The XRD patterns of pristine and re-used Co/Bi25 FeO40 were also employed to investigate the stability of Co/Bi25 FeO40 through the reactions. As shown in Fig. S6, the used activator revealed almost the same pattern as the pristine catalyst, which is in consistence with the obtained FT-IR results.

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W. Li, Y. Zhang and Y. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 100 (2019) 56–64 Table 1 Fitting parameters of pseudo-first-order kinetic model under various conditions. Co/Bi25 FeO40 (g/L)

AO7 (mg/L)

PMS (mM)

Temperature (°C)

pH

Kapp (min−1 )

R2

t1/2 (min)

5.8

0.010 0.019 0.068 0.207 0.243

0.998 0.999 0.997 0.998 0957

69.315 36.481 10.193 3.349 2.852

5.8

0.126 0.068 0.035 0.026 0.019

0.976 0.997 0.999 0.999 0.975

5.501 10.193 19.804 26.660 36.481

0.971 0.925 0.966 0.997 0.948 0.918 0.911

14.748 13.591 10.664 10.193 9.242 1.086 11.951

0.05 0.10 0.25 0.50 1.00

20

0.25

10 20 30 40 60

0.5

0.25

20

0.10 0.20 0.25 0.50 1.00 1.50 2.00

25

5.8

0.047 0.051 0.065 0.068 0.075 0.064 0.058

0.25

20

0.25

15 25 35

5.8

0.024 0.068 0.096

0.905 0.997 0.947

28.881 10.193 7.220

2.0 3.0 4.0 5.8 8.0 10.0

0.005 0.018 0.040 0.068 0.051 0.065

0.947 0.995 0.985 0.997 0.914 0.900

138.629 38.508 17.329 10.193 13.591 10.664

0.25

20

0.5

0.25

25

25

25

Fig. 3. Effects of (a) catalyst dosage, (b) AO7 concentration, (c) PMS dosage, and (d) UV–vis spectra changes during AO7 degradation via PMS activated by Co/Bi25 FeO40 .

3.3. Effects of temperature Since PMS decomposition was an endothermic reaction, the effect of temperature for the reaction system was also investigated. As shown in Fig. 4(a), the removal of AO7 could reach 98% at 35 °C within 30 min, while only 78% of AO7 was eliminated within 60 min at 15 °C. It is indicated in Table 1 that the degradation rate increases 4 times from 15 to 35 °C. The results demonstrated that

higher temperature could accelerate the decomposition of PMS and the degradation efficiency of the contaminant [60]. According to the Arrhenius equation:

ln Kapp = ln A −

Ea RT

(4)

where Ea is the apparent activation energy, A denotes the preexponential factor, R is the gas constant and T is the absolute

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61

Fig. 4. Effect of temperature (a), pH (b) and scavenger (c) for AO 7 degradation in Co/Bi25 FeO40 /PMS system; (d) ESR spectra of DMPO-·SO4 − adduct and TEMP-1 O2 adduct. (AO7 = 20 mg/L, PMS = 0.25 mM, catalyst dosage = 0.25 g/L).

temperature, Kapp is the reaction constant under different temperature. Based on the related kinetic results (inset of Fig. 4(a)), the activation energy of AO7 degradation was calculated as 51.30 kJ / mol (R2 = 0.995), which was relatively lower than the activation energies obtained for the degradation of AO7 in other systems [61–63].

3.4. Effects of pH To investigate the effect of pH on degradation of AO7 in Co/Bi25 FeO40 /PMS system, various pH conditions were conducted from 2.0 to 10.0. As depicted in Fig. 4(b) and Table 1, Kapp was obtained as 0.005, 0.018, 0.040, 0.068, 0.051 and 0.065 min−1 at pH 2.0, 3.0, 4.0, 5.8, 8.0 and pH 10.0, respectively. At pH 10.0, 95.0% of the contaminant could be degraded within 60 min, nevertheless, that could only reach 29.0% when pH 2.0 was employed. The results imply that Co/Bi25 FeO40 /PMS system has a better capacity in alkaline conditions that could owe to the complex reasons. Since the pKa1 /pKa2 of AO7 was 1.0/11.4, the aggregates of molecular AO7 might be inclined to disaggregate at alkaline circumstance, which can be profitable for the reaction [64]. Furthermore, given that H+ -dependency could accelerate the stabilization effect of H+ on the HSO5 − which could hinder the interaction between PMS and catalyst, meantime, the excessive OH− could activate PMS to generate extra ROS (i.e. sulfate radicals and superoxide radicals), higher removal rate of AO7 was expected at alkaline solutions [24,49].

3.5. Mechanism of PMS activation In order to further investigate the ROS in the Co/Bi25 FeO40 /PMS system, scavenging tests were detected by using several selected scavengers, with the results shown in Fig. 4(c). Tert-butanol (TBA), ethanol (EtOH), CHCl3 and NaN3 were used as scavenger owing to the distinct quenching performance for hydroxyl radicals, sulfate radicals ·O− and singlet oxygen. [37,41,65–67]. When 125 mM TBA 2 was employed, practically no discernible inhibition was observed, compared to the relative sound inhibition observed in the EtOH system, demonstrating the typical role of sulfate radicals instead of ·OH in the Co/Bi25 FeO40 /PMS system. The existence of ·O− was 2 verified via the inferior removal rate for dosing CHCl3 . Compared to the insignificant inhibition effect caused by EtOH, the degradation performance was rather inhibited in the presence of 5 mM NaN3 , and the inhibition was even strengthened when higher NaN3 (125 mM) was present, which demonstrated that singlet oxygen could contribute the critical role in the coupled system. In order to further identify the ROS in the system, ESR spectroscopy with the spin-trapping reagent of DMPO or TEMP was conducted in the system (Fig. 4(d)). While DMPO can specifically react with hydroxyl radicals and sulfate radicals to generate characteristic products, TEMP is generally considered as an excellent probe for singlet oxygen [41,68]. As shown in Fig. S7, no distinct signal was observed in single DMPO system, while the 1:2:2:1 signal peaks with weak intensity observed in DMPO/PMS system were attributed to hydroxyl radicals. The similar phenomenon was also reported which was ascribed to the strong hydrolysis process of HSO5 − as shown in Eqs. (4) and (5). [69]. Meantime, seven

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Scheme 1. Proposed mechanism of AO7 degradation with the Co/Bi25 FeO40 /PMS system.

main peaks ascribed to the DMPX adduct were detected after adding DMPO into the Co/Bi25 FeO40 /PMS system. Previous works have illustrated that the DMPOX adducts could be produced by the oxidation of DMPO by ·SO4 − . Then the highly unstable DMPO·SO4 − adduct was transformed to DMPOX (Fig. 4(d)) [70–72]. The existence of DMPOX indicated that sulfate radicals were produced during the catalytic PMS activation. As denoted in Fig. 4(d), a typical three-line ESR spectrum with equal intensity was observed, which coincided with the above conclusion that singlet oxygen was formed in Co/Bi25 FeO40 /PMS system [40]. Although similar peaks were observed in single PMS system (Fig. S8), the signal were much weaker to be prominent, which came from the self-decomposition of PMS [16].

in Eqs. (7)–(10) [16,37]. Meantime, considering the lattice distortion caused by the doping of the cobalt element, it can be considered that the release of the active oxygen may be exacerbated, thereby further promoting the production of singlet oxygen and the degradation rate of the contaminant. Furthermore, given the disparate XPS results of metal elements ions (i.e. Bi5+ , Fe3+ , Co3+ ) for fresh and re-used catalyst (Fig. 2(a) and (b) and Fig. S4), PMS can be partially activated by the metal elements ions which can further contribute the production of sulfate radicals and 1 O2 via Eqs. (11)–(13) [16,55].

Bi5+ + O2− → Bi3+ + Ovac + 0.5O2

(7)

− HSO− 5 + H2 O ↔ H2 O2 + HSO4

(5)

Ovac → O∗

(8)

H2 O2 → HO · +HO ·

(6)

− 1 O∗ + HSO− 5 → HSO4 + O2

(9)

Since the singlet oxygen could decay rapidly to triplet oxygen (3 O2 ), resulting in the increase of dissolved oxygen, further tests to monitor the DO in the blank condition or the coupled system were conducted [40,41]. As demonstrated in Fig. S9, the DO concentration decreased with the agitation of the magnetic stirring from the saturation in both conditions. And the decrease of DO in the blank system was more rapid than that in the presence of PMS and Co/Bi25 FeO40 . Besides, once the PMS and Co/Bi25 FeO40 were added into the blank solution at 60 min, the DO concentration increased to the comparable level, which further demonstrated the in-situ generation of 3 O2 from the decay of 1 O2 in Co/Bi25 FeO40 /PMS system. The XPS analysis for the pristine and reused Co/Bi25 FeO40 shown in Fig. 2(c) and (d) indicated that the intensity of Bi-O bonds was weakened distinctly compared to the pristine material. Meanwhile, chemisorbed oxygen increased significantly after reactions. In detail, during the synthesis process, parts of [Bi3+ O3 ] umbrella group was substituted by [Fe3+ O4 ] tetrahedral group via the transportation of Bi3+ ions to the oxygen hole [16]. Peculiarly, according to the previous studies, the transportation of Bi3+ to the oxygen hole can lead to the increase distance between Bi3+ and other oxygen atoms. [16,73] Moreover, the outermost lone pair electrons (6s2 ) of Bi3+ can also lead to the expansion of lattice structure of Co/Bi25 FeO40 . Through the substitution of Bi5+ by Bi3+ , oxygen vacancy can be generated subsequently, which will be released to form active oxygen (O∗ ) to generate 1 O2 , which is listed

1

O2 →

3

O2

(10)

·− (n−1)+ + H+ Mn+ + HSO− 5 → SO5 + M

(11)

·− n+ M(n−1)+ + HSO− + OH− 5 → SO4 + M

(12)

− 1 2SO·− 5 + H2 O → 2HSO4 + 1.5 O2

(13)

As elucidated in Fig. S10, the decomposition of PMS was also examined in the presence or absence of Co/Bi25 FeO40 and AO7. No decomposition of PMS was observed in the control test and single PMS with AO7, whereas the presence of Co/Bi25 FeO40 led to the significant decomposition of PMS, which match the 1 O2 generation behavior. Based on the above conclusion, the degradation mechanism of AO7 using PMS activated by Co/Bi25 FeO40 was proposed in Scheme 1. This scheme shows that AO7 could be degraded through sulfate radicals and singlet oxygen generated from active oxygen and metallic elements. These reactions highlight the existence and roles of the radical and nonradical pathways that cause the degradation of AO7.

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4. Conclusions •

•

•

•

Co/Bi25 FeO40 was successfully synthesized through a hydrothermal method and efficient PMS activation performance was demonstrated using Co/Bi25 FeO40 ; Increasing catalyst dosage, PMS concentration, temperature and pH could promote AO7 removal, while the increase of AO7 concentration led to the inhibited degradation; Sulfate radicals and singlet oxygen were identified via using chemical probes and ESR analysis; Simultaneous pathways through radicals (sulfate radicals) and non-radical (singlet oxygen) were proposed with oxygen vacancy, active oxygen (O∗ ) and the redox pair Bi5+ /Bi3+ involved.

Declarations of interest None. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (NO. 51508354, 51878422), Science and Technology Projects of Sichuan Province (2018 HH0104), and Science and Technology Bureau of Chengdu (2017-GH02-0 0 010-HZ) for the financial support. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.02.033. References [1] Tian F, Zhao H, Li G, Dai Z, Liu Y, Chen R. Modification with metallic bismuth as efficient strategy for the promotion of photocatalysis: the case of bismuth phosphate. ChemSusChem 2016;9:1579–85. [2] Zhao S, Dai Z, Guo W, Chen F, Liu Y, Chen R. Highly selective oxidation of glycerol over Bi/Bi3.64 Mo0.36 O6.55 heterostructure: dual reaction pathways induced by photogenerated 1 O2 and holes. Appl Catal B 2019;244:206–14. [3] Dai Z, Qin F, Zhao H, Ding J, Liu Y, Chen R. Crystal defect engineering of aurivillius Bi2 MoO6 by Ce doping for increased reactive species production in photocatalysis. ACS Catal 2016;6:3180–92. [4] Wang J, Neaton J, Zheng H, Nagarajan V, Ogale S, Liu B, Viehland D, Vaithyanathan V, Schlom D, Waghmare U. Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 2003;299:1719–22. [5] Zhao T, Scholl A, Zavaliche F, Lee K, Barry M, Doran A, Cruz MP, Chu YH, Ederer C, Spaldin NA, Das RR, Kim DM, Baek SH, Eom CB, Ramesh R. Electrical control of antiferromagnetic domains in multiferroic BiFeO3 films at room temperature. Nat Mater 2006;5:823–9. [6] Spinicci R, Faticanti M, Marini P, De Rossi S, Porta P. Catalytic activity of LaMnO3 and LaCoO3 perovskites towards VOCs combustio. J Mol Catal A: Chem 2003;197:147–55. [7] Barbero BP, Gamboa JA, Cadús LE. Synthesis and characterisation of La1−xCax FeO3 perovskite-type oxide catalysts for total oxidation of volatile organic compounds. Appl Catal B 2006;65:21–30. [8] Wang HC, Xu HM, Zeng CC, Shen Y, Lin YH, Nan CW. Visible light photocatalytic activity of bismuth ferrites tuned by Bi/Fe ratio. J Am Ceram Soc 2016;99:1133–6. [9] Sun HB, Ai YJ, Li D, Tang ZK, Shao ZX, Liang QL. Bismuth iron oxide nanocomposite supported on graphene oxides as the high efficient, stable and reusable catalysts for the reduction of nitroarenes under continuous flow conditions. Chem Eng J 2017;314:328–35. [10] Zhang L, Zhang X, Zou Y, Xu Y-H, Pan C-L, Hu J-S, Hou C-M. Hydrothermal synthesis, influencing factors and excellent photocatalytic performance of novel nanoparticle-assembled Bi25 FeO40 tetrahedrons. CrystEngComm 2015;17:6527–37. [11] Sun A, Chen H, Song C, Jiang F, Wang X, Fu Y. Magnetic Bi25 FeO40 -graphene catalyst and its high visible-light photocatalytic performance. RSC Adv 2013;3:4332–40. [12] Jing Q, Feng X, Zhao X, Duan Z, Pan J, Chen L, Liu Y. Bi/BiVO4 chainlike hollow microstructures: synthesis, characterization, and application as visible– light-active photocatalysts. ACS Appl Nano Mater 2018;1(6):2653–61. [13] Ren L, Lu SY, Fang JZ, Wu Y, Chen DZ, Huang LY, Chen YF, Cheng C, Liang Y, Fang ZQ. Enhanced degradation of organic pollutants using Bi25 FeO40 microcrystals as an efficient reusable heterogeneous photo-Fenton like catalyst. Catal Today 2017;281:656–61.

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