Promoted peroxymonosulfate activation into singlet oxygen over perovskite for ofloxacin degradation by controlling the oxygen defect concentration

Accepted Manuscript Promoted peroxymonosulfate activation into singlet oxygen over perovskite for ofloxacin degradation by controlling the oxygen defect concentration Panpan Gao, Xike Tian, Yulun Nie, Chao Yang, Zhaoxin Zhou, Yanxin Wang PII: DOI: Reference:

S1385-8947(18)32419-7 https://doi.org/10.1016/j.cej.2018.11.184 CEJ 20486

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

22 August 2018 18 November 2018 23 November 2018

Please cite this article as: P. Gao, X. Tian, Y. Nie, C. Yang, Z. Zhou, Y. Wang, Promoted peroxymonosulfate activation into singlet oxygen over perovskite for ofloxacin degradation by controlling the oxygen defect concentration, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.11.184

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Promoted peroxymonosulfate activation into singlet oxygen over perovskite for ofloxacin degradation by controlling the oxygen defect concentration Panpan Gaoa, Xike Tianb,*, Yulun Nieb, Chao Yangb, Zhaoxin Zhoub, Yanxin Wanga,* a

School of Environmental Studies, China University of Geosciences, Wuhan 430074,

China. b

Faculty of Materials Science and Chemistry, China University of Geosciences,

Wuhan 430074, China. Corresponding Authors * Prof. Xike Tian, E-mail: [email protected]. Tel /Fax: +86-27-67884574. * Prof. Yanxin Wang, E-mail: [email protected]. Tel / Fax: +86-27-87481030.

Abstract Recently, perovskite is becoming a promising alternative as peroxymonosulfate (PMS) activator for the remediation of organic pollutants in water. But the factor determining PMS activation efficiency of perovskite and the evolution of reactive oxygen species (ROS) remain equivocal and elusive. In this study, we proposed an oxygen defect dependent PMS activation mechanism over perovskite with the singlet oxygen (1O2) as the dominant ROS. Among the tested four perovskites, ofloxacin (OFX) degradation

1

efficiency increased with the following order: LaFeO3 < LaZnO3 < LaMnO3 < LaNiO3, which agreed well with their oxygen defect amounts based on X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) analysis. The results clearly demonstrated a good relationship among oxygen defects in LaBO3, OFX degradation efficiency and

1

O2 concentration. Moreover,

1

O2 evolution

mechanism over perovskite by decreasing the activation energy of PMS self-decomposition was proposed. The 1O2 mediated OFX degradation pathway was further studied by HPLC-MS technique and three-dimensional excitation–emission matrix fluorescence spectroscopy (3D EEMs). This work provides a new insight into PMS activation by perovskites and favors its application in actual water treatment. Keywords: peroxymonosulfate, activation, perovskite, key factor, reaction mechanism

1. Introduction Ofloxacin (OFX) is a frequently detected fluoroquinolone antibiotic in various environmental samples such as wastewater treatment plants (WWTPs) effluents, surface waters and sea waters [1-3]. Due to its resistance against biodegradation, the residuals of OFX have become a significantly emerging environmental issue. To date, many advanced oxidation processes (AOPs), such as electrochemical oxidation,

2

photocatalysis, ozonation, photoelectrocatalysis and sonophotocatalytic treatment, have been explored for the degradation of OFX [4-7]. The intrinsic drawback of high cost such as rely on light, electricity, ultrasonic, or special instrument limits their widespread application. Peroxymonosulfate (PMS) has emerged as a promising oxidizer due to its excellent stability, low cost and easy to transport, and its activation into reactive oxygen species (ROS) has received increasing interest for degrading or mineralizing refractory organic pollutants in a variety of industrial and consumer applications [8]. At present, a number of supported and unsupported metals or metal oxides have been developed as PMS activator, including MnO2, CuO-Fe3O4 and Pd/Al2O3, etc [9-13]. The magnetic metallic nanoparticles are prone to aggregation, thus their catalytic activities will be diminished. The release of metal ions such as cobalt oxides will also lead to secondary water pollution [14]. The synthesis of supported metallic catalysts typically involving complex procedures and multiple reagents often requires a relatively long preparation time. Thus, it remains necessary to expand the scope of heterogeneous catalysts for activating PMS by developing other metal/metal oxide materials. In recent years, perovskites have attracted extensive attention in various areas such as solar energy, electrochemistry, sensors and catalytic oxidation [15-18]. Perovskites

3

with a typical ABO3 structure, where A sites are larger-sized alkali and rare earth metals and the B sites are 3d transition metal ions, are capable of hosting more than 90% of metal elements in the periodic table [19, 20]. Their structural, physicochemical and electronic properties can be tuned by regulating the category and proportion of their chemical compositions, which thus provide a versatile substrate to the chemistry and materials science communities. For example, LaFeO3 and Cu-doped LaTiO3 can efficiently catalyze H2O2 for oxidation of aqueous sulfamethoxazole and Rhodamine B [21, 22]. LaMnO3 and LaTi0.15Cu0.85O3 were also used as catalysts for photocatalysis and ozonation with excellent catalytic activities [23, 24]. Although the aggregation and stability problem of perovskites were solved well, there still exist two problems retarding the further developments of perovskites to activate PMS: (a) key parameter determining perovskites in PMS activation and the involved reactive oxygen species, (b) the target perovskites should not only have desirable heterogeneous PMS activation efficiency but also possess satisfied stability. Actually, the catalytic activity of heterogeneous catalysts is intimately related with their nature of surfaces (such as the surficial atom arrangement, multivalence, specific morphology and high surface area), because it governs the activation energy, kinetic behaviors, and electron transport processes in binding with reactants and breaking the

4

chemical bonds in catalytic reaction [25-27]. Nie et al. demonstrated that the occurrence of an interfacial electron cyclic process between Ti3+/Ti4+ and Cu+/Cu2+ redox pairs was responsible for the high catalytic activity of Cu-doped LaTiO3 [22]. While for Co3O4, rather than multivalence of Co, its high surface area and exposed [220] crystal plane contributed to high efficiency for the oxidation of 1,2-dichloroethane [28]. Herein, the investigation on heterogeneous PMS activation including the key factor determining perovskite’s activity is beneficial to promote the application of this technique in practical water treatment. In this study, LaBO3 perovskites (B=Fe, Zn, Mn, and Ni) were prepared and the effect of B site metal on their PMS activation performance was evaluated by ofloxacin degradation efficiency (OFX, a toxic antibiotic with poor biodegradability). Firstly, the crystallinity, micromorphology, specific surface area of LaBO3 was characterized by XRD, SEM and BET. The amount of surface oxygen defects (ODs) in LaBO3 was further studied by XPS and EPR technique. The relationship among surface oxygen defects, OFX degradation and ROS concentration was explored and established. It was found that ODs instead of morphology and multivalence were the key parameter determining the PMS activation efficiency of LaBO3. Moreover, singlet oxygen as the dominant ROS was identified by EPR and scavenger experiments, and its evolution

5

mechanism as a result of PMS self-decomposition was also discussed. Finally, the OFX degradation pathway by singlet oxygen was proposed by HPLC-MS technique.

2. Experimental section 2.1. Materials All reagents were analytical grade and used without further purification. La, Fe, Mn, Ni and Zn nitrates and other reagents such as citric acid, Tert-butanol, methanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Peroxymonosulfate (PMS, 2KHSO5•KHSO4 •K2SO4) and ofloxacin were obtained from Aldrich. Solution pH was adjusted by a diluted aqueous solution of NaOH or HNO3. Deionized (DI) water was used throughout this study. 2.2. Catalyst preparation LaBO3 perovskites were synthesized via a previously reported sol-gel method [29]. In a typical process, 3 mmol La(NO3)3 and 3 mmol metal nitrate (Fe, Mn and Ni) were first dissolved in 20 mL deionized water. The stoichiometric amount of citric acid was added into the precursor solution as the chelating agent. The final solution was heated up to 70 ºC under gentle stirring until the formation of a gel. The gel was then dried at 110 ºC overnight. Finally, the obtained foam solid was crushed and calcined at 300 ºC for 1 h and then at 700 ºC for 5 h, respectively.

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2.3. Characterization The crystal structure was studied by the powder X-ray diffraction (XRD) pattern, which was recorded on a Rigaku D/max-βB diffractometer with Cu Kα X-ray radiation (λ= 1.5432 Å). The surface morphology was observed by field emission scanning electron microscopy (FESEM, Hitachi US8010) operating at an accelerating voltage of 10 kV. Transmission electron microscope (TEM) images were taken by JEOL JEM-2010, equipped

with

selected

area

electron

diffraction

(SAED).

The

N2

adsorption−desorption isotherms were determined on ASAP 2020 physisorption apparatus at the temperature of liquid nitrogen, in which the samples were degassed at 120 °C for 24 h before measurement. X-ray photoelectron spectroscopy (XPS) was conducted on a MULTILAB2000 electron spectrometer, equipped with a monochromatized Al Kα radiation. The surface oxygen defects were detected by electron paramagnetic resonance (EPR) at room temperature on a JES-FA200 EPR spectrometer with microwave frequency 9147.785 MHz at 5 mW power. 2.4. Procedures and analysis The catalytic performance of LaBO3 perovskites was evaluated by PMS activation and OFX oxidation. All experiments were carried out in a batch reactor with a magnetic stirrer in a water bath maintained at 25 °C. In a typical experiment, 20 mg catalyst was

7

added into 100 mL 10 mg/L OFX solution. The mixture was stirred for 30 min to achieve adsorption-desorption equilibrium. The desired amount of PMS was then added into the above suspension under continuous magnetic stirring. Initial pH of OFX solution was adjusted to 3.6, 5.3, 7.0, 9.7, and 11.2 using HNO3 or NaOH to examine the effect of pH. At given time intervals, 2.5 mL of the suspension was withdrawn and filtered by a 0.45 μm membrane, and the filtrate was quickly mixed with 150 µL of a Na2S2O3 solution to quench ROS and terminate the oxidation. All the experiments were repeated three times and the data represented the average of the triplicates with a standard deviation less than 5%. The concentration of OFX was determined by high-performance liquid chromatograph (HPLC) with a UV-DAD detector. The chromatographic separation was performed by a reverse-phase C18 column (250 mm×4.6 mm, 5 μm). The mobile phase was composed of 15% acetonitrile and 85% ultrapure water acidified with 1% formic acid at 0.5 mL/min. The injection volume was 20 μL. Temperature of the column chamber maintained at 25 C and the detection wavelength was 288 nm. Electron paramagnetic resonance (EPR) experiments were performed on a JES-FA200 EPR spectrometer: temperature, 298 K; microwave frequency, 9.146 GHz; microwave power, 3 mW and modulation amplitude, 0.5 mT. 5,5-Dimethyl-1-pyrroline (DMPO) was employed as

8

the spin trapping agent in the EPR experiment for the detection of hydroxyl and sulfate radicals in aqueous solution by measuring the signals of DMPO-•OH and DMPO-SO4adducts, respectively. Besides, DMPO was also used to detect superoxide radical in methanol solution. 2,2,6,6-tetramethyl-4-piperidone (TEMP) was utilized as the spin trapping agent to detect singlet oxygen in aqueous solution. The ROS trapping experiments were performed using tert-butanol (TBA), methanol (MeOH), sodium azide (NaN3) and furfuryl alcohol (FFA) as the quenching agents. PMS concentration was measured by a spectrophotometric method based on a modified iodometric titration [30]. Dissolved oxygen (DO) generation was monitored in an airtight flask by a portable DO meter (Multi HQ40d). p-Chlorobenzoic acid (pCBA, 0.4 mM, Sigma-Aldrich) and furfuryl alcohol (FFA, 0.85 mM, Sigma-Aldrich) were used as indicators to determine the concentrations of •OH and 1O2, respectively [31, 32]. At different reaction times, 2 mL of the suspension was collected and filtered by a 0.22 μm membrane. The concentrations of pCBA and FFA in the collected filtrate were analyzed with a high-performance liquid chromatograph (HPLC, ThermoFisher Scientific U3000) equipped with a UV-DAD detector (237 nm for pCBA and 219 nm for FFA). pCBA was eluted using a mixture of acetonitrile and DI water at 50:50 (v/v). FFA was eluted using a mixture of 50% methanol and 50% DI

9

water. The average molar concentration of each type of ROS was computed from the corresponding indicator curve by

where C is the average molar concentration (mM) of each type of ROS, C0 is the initial molar concentration of the indicator (mM), Ct is the molar concentration of the indicator (mM) after a reaction time of t (h), and T is the exposure time of 1 h. A Dionex Ultimate 3000 series liquid chromatography combined with Q Exactive hybrid Quadrupole-Orbitrap mass spectrometer (HPLC-MS, Thermo Scientific, Bremen, Germany) was used for the experiment and operated in the positive electronspray ionization mode (ESI+) over a mass range of 50-500 m/z. A Hypersil Gold C18 column (3 mm×150 mm, 3 μm, Thermo Fisher) was utilized for the LC separation with a mobile phase of solution A (with 0.1% formic acid) and solution B (acetonitrile) at a flow rate of 0.3 mL/min. The gradient program was as follows: 0-2 min: 5% B; 2-10 min: 5-80% B; 10-16 min: 80% B; 16-18 min: 80-5% B; 18-20 min: 5% B. The system was re-equilibrated for 10 min between runs. The mass tolerance of the precursor and fragment ions was below 5 ppm.

3. Results and discussion 3.1. Characterization of as-prepared LaBO3

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As shown in Fig. 1A, all LaBO3 exhibited the characteristic perovskite XRD patterns correspond to cubic LaFeO3 (JCPDS No. 75-0541), LaMnO3 (JCPDS No. 75-0440) and LaNiO3 (JCPDS No. 33-0711), respectively. No significant difference in terms of LaBO3 morphology and nanosize (100 nm) was found in LaBO3 based on SEM and TEM images (Fig. 2). Fig. S1 further showed nitrogen adsorption-desorption isotherms and the correspondent pore size distributions of LaBO3. The BET specific surface area of LaBO3 (B=Fe, Mn and Ni) were 16.30 m2/g, 19.54 m2/g and 9.81 m2/g, respectively. The metallic state in LaBO3 was characterized by XPS analysis in Fig. S2 and Fig. 1B-D. The peaks of La 3d5/2 and La 3d3/2 can be attributed to La3+ in the three as-prepared samples [33]. Based on XPS and hydrogen temperature-programmed reduction (H2-TPR, Fig. S3 in Supporting Information) analysis, Fe2+, Fe3+ in LaFeO3, Mn2+, Mn3+, Mn4+ in LaMnO3, and Ni2+, Ni3+ in LaNiO3 were observed [34-36]. Hence, the multivalence of B site metal rather than A site in LaBO3 was confirmed by XPS and H2-TPR technique. Therefore, the structural characteristics (e.g. crystallinity, micromorphology, specific surface area) of LaBO3 remained almost unchanged. 3.2. Surface oxygen defect for efficient PMS self-decomposition into 1O2 generation over LaBO3 The effect of B site metal on the catalytic activity of LaBO3 was evaluated by the OFX

11

degradation efficiency in PMS activation. As shown in Fig. 3, almost no OFX degradation occurred with PMS alone, suggesting that PMS itself cannot produce ROS for OFX removal. The contribution of adsorption to OFX removal was also less than 5% for all LaBO3 (Fig. S4). In comparison, the OFX degradation was greatly enhanced in the presence of LaBO3 and PMS with the order of LaFeO3 < LaMnO3 < LaNiO3. 15% and 90% of OFX was removed over LaFeO3 and LaMnO3 at 30 min, while 93% of OFX degradation was obtained over LaNiO3 at only 10 min. Meanwhile, the OFX degradation can be fitted well with pseudo-first-order kinetics (R2 > 0.99). The reaction rate constant (k) of LaNiO3 (0.239 min-1) was 3 times and 49 times that of LaMnO3 and LaFeO3 with values of 0.0803 and 0.0049 min-1, respectively. Obviously, B site metal had a great influence on its PMS activation efficiency of LaBO3. It has been reported that the redox pair such as Mn(III)/Mn(IV) in OMS-2 was reported to activate PMS into OH and SO4- radicals via the electron transfer process [35]. In our study, as depicted in Fig. 4 and Table S1, the surface atomic ratio of Fe, Mn and Ni was 5.02%, 8.85% and 6.95%, in which the ratio of reductive Fe2+, Mn2+/Mn3+ and Ni2+ was 1.0%, 4.9% and 5.0%, respectively. Hence, neither the total B site metal amount nor the ratio of reductive species was in accordance with the order of PMS activation over LaBO3, which was also inconsistent to the surface area.

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In situ electron paramagnetic resonance (EPR) spectra is extremely sensitive to oxygen vacancy defects, which could be divided into two resonances segments, low field (LF) (g=2.70) and high field (HF) (g=2.00), corresponding to the resonance absorption of cycloidal spin structure (Pcyc) and the defect induced free spins (Pdef), respectively [37]. As shown in Fig. 5A, the LF and HF resonance signals of LaNiO3 were much larger than those of LaFeO3. Since the HF resonance signals (Pdef) are supposed to be correlated with the free spins induced by oxygen-vacancy related defects [38], much stronger HF resonance signal intensity indicated that LaNiO3 should have more surface oxygen defects. The amount of oxygen defects in LaBO3 was further compared in Fig. 5B-D. The broad peak of O1s in LaBO3 was deconvoluted into four peaks: lattice oxygen at about ~528 eV, surface oxygen at ~530.2 eV, defect oxygen at ~531.1 eV and adsorbed oxygen at ~532.3 eV [39, 40]. It is found that the percentage of defect oxygen was 9.2%, 12.7% and 21.8% for LaFeO3, LaMnO3 and LaNiO3, respectively, which agreed well with the order of OFX degradation efficiency: LaFeO3 < LaMnO3 < LaNiO3. For example, LaFeO3 had the lowest amount of oxygen defects and only 15% of OFX was removed, while OFX was quickly and completely degraded over LaNiO3 due to the highest amount of oxygen defects. To further confirm the intrinsic relationship between PMS activation efficiency and oxygen defects amount, the effect

13

of Zn doping on the enhancement of LaFeO3 for PMS activation was explored in Fig. S5. Similar to the above observations, the OFX degradation rate over LaFe1-xZnxO3 increased with the Zn amount due to the generated surface oxygen defect by in situ Zn substitution. It is because that the replacement of Fe(III) by Zn(II) could cause the missing of oxygen atoms, which could produce highly active oxygen vacancies. The results confirmed that the surface oxygen defects was the crucial factor for efficient PMS activation. LaNiO3 was chosen as the representative to investigate the involved reactive oxygen species from PMS activation by EPR and scavenger experiments. As shown in Fig. 6A, almost no DMPO-O2- was generated and the weak signals of DMPO-OH and DMPO-SO4- adducts were detected. However, the strong 1:1:1 triplet signal for the characteristic peaks of 1O2 was observed. The results indicated that 1O2 should be the dominant reactive oxygen species responsible for OFX degradation. Moreover, Fig. 6B confirmed the inhibition of methanol to OFX degradation (~22.1%) was slightly higher than that of tert-butyl alcohol (~18.9%), indicating the presence of 

OH rather than SO4-. The OFX degradation was remarkably inhibited by 90% due to

the addition of NaN3. Hence, both 1O2 and OH were generated while 1O2 was the dominant ROS in LaBO3/PMS system. Since the 1O2 can be quantified using furfuryl

14

alcohol (FFA) as the probe compound [31, 32], its average concentration was 0.17, 0.34 and 0.6 mmol/L for LaFeO3, LaMnO3, LaNiO3 (Table S2), which agreed well with the trend of OFX degradation efficiency. Furthermore, the PMS decomposition rate over LaBO3 also followed the same order of LaFeO3 < LaMnO3 < LaNiO3 as shown in Fig. 7. An intimate relationship was observed among OFX degradation, PMS decomposition, 1O2 concentration and the amount of surface oxygen defects in LaBO3 (Fig. 8). It meant that LaNiO3 had the highest amount of surface oxygen defect and PMS activation efficiency while LaFeO3 had the lowest PMS activation performance due to the limited surface oxygen defect. Therefore, the amount of surface oxygen defect was the key factor determining the PMS activation efficiency by LaBO3 perovskites. HSO5- + SO5=  ODs  HSO6- + SO4= (ODs: oxygen defects in LaBO3)

(1)

(2)

(3)

O=O*  1O2

(4)

VODs + PMS  1O2 (to decrease the reaction barrier)

(5)

O2 + OFX  O2 + Degradation products

1

(6)

15

According to the literature and our experimental results, the evolution mechanism of 1

O2 during the PMS activation by LaBO3 was further studied and discussed. It has

been reported that the spontaneous decomposition of peroxycarboxylic acid takes place by two reaction pathways: to produce the corresponding carboxylic acids or oxygen [41]. On spin-conservation grounds, both mechanisms are expected to give 1

O2, rather than ground-state 3O2. Since reaction 1 is the rate-limiting step for efficient

decomposition of PMS (reaction 2-4) [42, 43], a surface oxygen defect involved reaction mechanism for PMS activation was proposed. Due to the unsymmetrical nature, PMS is very reactive to strongly nucleophilic particles such as [O-O-SO3H]- as a result of PMS hydrolysis. As depicted in Fig. S6, the reaction activation energy was 48.4, 22.6 and 15 kJ/mol for LaFeO 3, LaFe0.6Zn0.4O3, LaMnO3, respectively. LaNiO3 should have the lowest activation barrier since no negative effect of temperature was found on the OFX degradation (Fig. S6D). It is consistent with their amount of surface oxygen defects and OFX degradation efficiency. He et al. also reported that the active surface oxygen defect could lower the reaction energy barrier of benzene oxidation in air atmosphere and the formaldehyde oxidation can be conducted at ambient temperatures over Pt/TiO2 [44]. But with the increase of the reaction temperature, limited OFX was degraded without perovskite (Fig. S7B).

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Hence, surface oxygen defects play a key role in enhancement of PMS activation efficiency by decreasing the reaction barrier of PMS spontaneous decomposition. Fig. 9 showed the catalytic PMS decomposition, O2 evolution and OFX abatement as a function of reaction time. Obviously, the efficiency of PMS decomposition and OFX degradation were both determined by the amount of surface oxygen defects in LaBO3. Corresponding, LaNiO3 has the maximum oxygen evolution (DO=6 mg/L) at 10 min. Hence, 1O2 should be firstly generated from the PMS self-decomposition over LaBO3 and then 1O2 react with OFX leading to the O2 generation and OFX degradation. Since 1O2 is the dominant ROS in LaBO3/PMS system, it is necessary to investigate the degradation pathway of OFX by 1O2. As demonstrated in Fig. 10 and Table 1, methyl of piperazinyl substituent was firstly attacked by 1O2 to form TP1 (m/z=348). It was transformed into TP2 (m/z=364) and then TP5 (m/z=322) due to the -OH addition, oxazinyl substituent demethylation and decarboxylation. Moreover, the ring in piperazinyl substituent is opened due to the cleavage of C-N bond, resulting in the transformation of TP2 to TP7 (m/z=305) and (TP10, m/z=219). Of course, TP3 (m/z=304) from TP1 and TP8 (m/z=300) from TP2 are also generated because of hydroxylation and decarboxylation process. In general,

1

O2 mainly attacks the

piperazinyl substituent (hydroxylation, demethylation and ring-opening), the oxazinyl

17

substituent (demethylation) and the carboxyl group (decarboxylation) [4, 7, 45, 46]. It is noteworthy that the major byproducts identified here are similar even same with the reports in •OH, SO4•- and O2•- processes [47-50]. Besides that, the efficient destruction

of

OFX

by

LaBO3

was

confirmed

by

three-dimensional

excitation–emission matrix fluorescence spectroscopy (3D EEMs) since the intensity of characteristic peak at Ex/Em = 200-300/420-520 nm and Ex/Em = 300-360/420-520 nm decreased and even disappeared with reaction time (Fig. 11). Furthermore, about 50% of TOC content in OFX solution was removed as depicted in Fig. S8. Hence, OFX was completely degraded by

1

O2 and mineralized to some extent in

LaNiO3/PMS system. Importantly, it is worthy to note that LaNiO3 as the most efficient catalyst also had an excellent stability toward OFX degradation with PMS. There was no deactivation of OFX removal efficiency in five successive cycles and XRD, XPS spectra showed no significant difference of crystalline and O1s distribution between the used and fresh LaNiO3 catalyst (Fig. 12). The oxidation capacity of 1O2 was not affected by the initial solution pH and actual water matrix (Fig. 13). Although the initial solution pH had a negligible effect on OFX degradation except at pH 11.2 under PMS self-decomposition (Fig. S7A), OFX can be removed completely over LaNiO3 at a pH

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range of 3.6-11.2. The negative effect of co-existing anions and natural organic matter (NOM) on LaNiO3 was also not significant. OFX was still efficiently removed in the presence of NO3-, SO42-, Cl-, PO43- and HCO3 - except the removal rate was inhibited to some extent in actual water matrix. Hence, the 1O2-mediated LaNiO3/PMS activation system provided a new alternative for the practical water treatment.

4. Conclusions In summary, an oxygen defect dependent PMS activation mechanism over LaBO 3 was developed, which was responsible to the efficient OFX degradation. A strong positive correlation among OFX degradation, PMS decomposition, 1O2 concentration and the surface oxygen defects of LaBO3 was established. The oxygen defect was proven to be the key parameter determining the PMS activation efficiency over perovskites. The enhanced 1O2 evolution from PMS self-decomposition was due to the decrease of activation energy by surface oxygen defect in LaBO3. LaNiO3 as the most efficient PMS activator exhibited an excellent stability and its activity was not significantly affected by the solution pH and water characteristics. Hence, the findings in this study not only provide a valuable insight into PMS activation by perovskites but also favor its practical application in water treatment.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 41773126, No. 41120124003), the Ministry of Education of China Priority Development Projects of SRFDP (No.20120145130001), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 41521001) and the “Fundamental Research Funds for the Central Universities”.

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Figure and Table captions Fig. 1. XRD patterns of as-prepared LaBO3 (B= Fe, Mn and Ni) perovskites (A); XPS spectra of Fe 2p in LaFeO3 (B), Mn 2p in LaMnO3 (C) and Ni 3p in LaNiO3 (D). Fig. 2. SEM and TEM images of as-prepared LaBO3 (B=Fe, Mn and Ni) perovskites. Fig. 3. Variation of OFX concentration with reaction time under different conditions (A) and pseudo-first-order kinetic modeling (B). Reaction conditions: [OFX]=10 mg/L, [catalyst]=0.2 g/L, [PMS]=0.5 g/L, pH 7.0, T=25ºC. Fig. 4. Atomic ratios of B site metal in LaBO3 perovskites and the percentage of reductive species. Fig. 5. EPR spectra (A) and XPS spectra of O 1s (B-D) in different LaBO3 (B=Fe, Mn and Ni) perovskites. Fig. 6. EPR spectra of different reactive oxygen species using DMPO and TEMP as trapping reagent (A) and effect of different scavengers on the OFX degradation (B). Fig. 7. PMS decomposition in LaBO3/PMS system. Reaction conditions: [OFX]=10 mg/L, [catalyst]=0.2 g/L, [PMS]=0.5 g/L, pH 7.0, T=25ºC. Fig. 8. The internal relationship among the amount of surface oxygen defects and OFX degradation, PMS decomposition and 1O2 concentration. Fig. 9. The efficiency of PMS decomposition, OFX degradation and corresponding oxygen evolution as a function of reaction time over LaBO3. Fig. 10. Proposed OFX degradation pathway in LaNiO3/PMS system. Fig. 11. 3D EEMs of the OFX solution after adsorption for 0 min (A), 30 min (B) and after catalytic degradation for 10 min (C), 30 min (D) over LaNiO 3. Reaction conditions: [OFX]=10 mg/L, [catalyst]=0.2 g/L, [PMS]=0.5 g/L, pH 7.0, T=25ºC. Fig. 12. Stability of LaNiO3 during the repetitive degradation of OFX with PMS (A) 24

and the comparison of XRD patterns and O 1s region of XPS spectra (B-D) before and after reaction. Fig. 13. Effect of initial solution pH (A) and co-existing anions (B) on the OFX degradation in LaNiO3/PMS system. The insert is the OFX degradation in real water matrix. Table 1. Accurate mass measurements of by-products of OFX in LaNiO3/PMS system.

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Fig. 1. XRD patterns of as-prepared LaBO3 (B= Fe, Mn and Ni) perovskites (A); XPS spectra of Fe 2p in LaFeO3 (B), Mn 2p in LaMnO3 (C) and Ni 3p in LaNiO3 (D).

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Fig. 2. SEM and TEM images of as-prepared LaBO3 (B=Fe, Mn and Ni) perovskites.

27

Fig. 3. Variation of OFX concentration with reaction time under different conditions (A) and pseudo-first-order kinetic modeling (B). Reaction conditions: [OFX]=10 mg/L, [catalyst]=0.2 g/L, [PMS]=0.5 g/L, pH 7.0, T=25ºC.

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Fig. 4. Atomic ratios of B site metal in LaBO3 perovskites and the percentage of reductive species.

29

Fig. 5. EPR spectra (A) and XPS spectra of O 1s (B-D) in different LaBO3 (B=Fe, Mn and Ni) perovskites.

30

Fig. 6. EPR spectra of different reactive oxygen species using DMPO and TEMP as trapping reagent (A) and effect of different scavengers on the OFX degradation (B).

31

Fig. 7. PMS decomposition in LaBO3/PMS system. Reaction conditions: [OFX]=10 mg/L, [catalyst]=0.2 g/L, [PMS]=0.5 g/L, pH 7.0, T=25ºC.

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Fig. 8. The internal relationship among the amount of surface oxygen defects and OFX degradation, PMS decomposition and 1O2 concentration.

33

Fig. 9. The efficiency of PMS decomposition (A), OFX degradation (B) and corresponding oxygen evolution (C) as a function of reaction time over LaBO3.

34

Fig. 10. Proposed OFX degradation pathway in LaNiO3/PMS system.

35

Fig. 11. 3D EEMs of the OFX solution after adsorption for 0 min (A), 30 min (B) and after catalytic degradation for 10 min (C), 30 min (D) over LaNiO 3. Reaction conditions: [OFX]=10 mg/L, [catalyst]=0.2 g/L, [PMS]=0.5 g/L, pH 7.0, T=25ºC.

36

Fig. 12. Stability of LaNiO3 during the repetitive degradation of OFX with PMS (A) and the comparison of XRD patterns and O 1s region of XPS spectra (B-D) before and after reaction.

37

Fig. 13. Effect of initial solution pH (A) and co-existing anions (B) on the OFX degradation in LaNiO3/PMS system. The insert is the OFX degradation in real water matrix.

38

Table 1. Accurate mass measurements of by-products of OFX in LaNiO3/PMS system. Mass (m/z)

Error

Elemental

Proposed

formula

structure

Formula weight Theoretical Experimental

(ppm)

362.1511

362.1504

-1.825

361

C18H20FN3O4

366.1096

366.1093

-0.792

365

C16H16FN3O6

364.1303

364.1296

-1.992

363

C17H18FN3O5

348.1354

348.1353

-0.318

347

C17H18FN3O4

346.1198

346.1192

-1.620

345

C17H16FN3O4

322.1198

322.1196

-0.499

321

C15H16FN3O4

305.0932

305.0942

3.240

304

C15H13FN2O4

304.1456

304.1447

-2.899

303

C16H18FN3O2

300.1143

300.1131

-3.937

299

C16H14FN3O2

274.0986

274.0994

2.804

273

C14H12FN3O2

39

219.0564

219.0568

1.496

218

40

C11H7FN2O2

Graphical Abstract:

41

Highlights

Key factor determining perovskite in PMS activation was firstly investigated OFX degradation efficiency decreased with the order of LaNiO3 > LaMnO3 > LaFeO3 Oxygen defect decreased with the same order of LaNiO3 > LaMnO3 > LaFeO3 Amount of surface oxygen defect plays a key role in PMS activation over LaBO3 Evolution of 1O2 from PMS over LaBO3 was proposed by reducing activation barrier

42