Enhanced performance of LaFeO3 perovskite for peroxymonosulfate activation through strontium doping towards 2,4-D degradation

Journal Pre-proofs Enhanced performance of LaFeO3 perovskite for peroxymonosulfate activation through strontium doping towards 2,4-D degradation Cheng Cheng, Shengwang Gao, Jianchao Zhu, Guoying Wang, Lijun Wang, Xunfeng Xia PII: DOI: Reference:

S1385-8947(19)32790-1 https://doi.org/10.1016/j.cej.2019.123377 CEJ 123377

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

12 August 2019 28 October 2019 3 November 2019

Please cite this article as: C. Cheng, S. Gao, J. Zhu, G. Wang, L. Wang, X. Xia, Enhanced performance of LaFeO3 perovskite for peroxymonosulfate activation through strontium doping towards 2,4-D degradation, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123377

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Enhanced performance of LaFeO3 perovskite for peroxymonosulfate activation through strontium doping towards 2,4-D degradation Cheng Cheng

a,b,

Shengwang Gao a, Jianchao Zhu a, Guoying Wang b, Lijun Wang a,

Xunfeng Xia a* a

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese

Research Academy of Environmental Sciences, Beijing 100012, PR China. b

College of Environmental Science and Engineering, Taiyuan University of

Technology, Taiyuan 030024, PR China * Corresponding author. Tel: +86-10-84915289; Fax: +86-10-84915289, E-mail: [email protected]

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ABSTRACT ABO3-type perovskite oxides with diverse active metal sites and stable texture structures have attracted much attention in heterogeneous catalysis, including catalysis of peroxymonosulfate (PMS) for wastewater treatment. Here, Sr was introduced into the A-site of LaFeO3 (LFO) perovskite to modify its structure for enhancing the catalytic performance. Specifically, La0.5Sr0.5FeO3 (LSF50) with a lower valence state of Fe and abundant oxygen vacancies exhibited excellent catalytic activity for PMS activation to degrade 2,4-dichlorophenoxyacetic acid (2,4-D), 5.7 times higher than the catalytic activity of LFO. In the presence of 0.6 g/L LSF50 and 1 mM PMS, 2,4-D (10 mg/L) could be completely removed within 60 min. Magnetic LSF50 showed a low level of metal leaching and good reusability for 2,4-D degradation. Moreover, the excellent degradation efficiency was maintained in a large range of initial pH from 5 to 11 as well as in a real water matrix such as surface water. Various reactive oxygen species (ROS) involving sulfate radical (SO4), hydroxyl radical (HO), and singlet oxygen (1O2) were generated during the catalysis. Based on electron spin resonance (ESR) studies and radical quenching experiments, SO4 played a dominant role in 2,4D degradation. A coupled PMS activation mechanism for the major free radicals and minor 1O2 was proposed for the rapid degradation of 2,4-D in LSF50/PMS system. The transformation byproducts were identified and the possible degradation pathways were proposed. This study provides a new insight for the development of efficient A-sitesmodified perovskite oxide for PMS activation in environmental remediation. Keywords: Perovskite; peroxymonosulfate activation; sulfate radical; 2,4-D

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1. Introduction Because of its nonvolatility, poor biodegradability, and endocrine disrupting behavior, 2,4-dichlorophenoxyacetic acid (2,4-D), a common herbicide for the management of various broad-leafed weeds and plant growth regulator for increasing agricultural productivity owing to its easy availability and low cost [1, 2] has been reported to contaminate both surface water and groundwater and threaten human health [3, 4]. The US EPA has categorized 2,4-D as a highly persistent chemical in aquatic environment, and the World Health Organization (WHO) recommends a maximum permissible concentration of 30 g/L of 2,4-D in drinking water [5]. Hence, it is essential to develop efficient technologies for the elimination of 2,4-D from contaminated water. Recently, advanced oxidation processes (AOPs) utilizing highly reactive radicals such as hydroxyl radical (HO) and sulfate radical (SO4) have been recognized as viable and promising technologies for environmental treatment [6, 7]. These radicals can readily react with various persistent organic contaminants and mineralize them into CO2 and H2O [8]. Specifically, sulfate radical-based AOPs (SR-AOPs) have attracted more attention because of stronger oxidation ability (Eo = 2.5–3.1 V), longer lifetime (30–40 s), and higher selectivity for oxidation of SO4 [9]. SO4 can be generated by peroxymonosulfate (PMS) and persulfate (PS) activated by heat [10], ultraviolet light (UV) [11], ultrasound [12], electro-chemistry method [13], and various catalysts involving semiconductors, transition-metal and metal-free catalysts like carbon-based

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catalyst, and integrated processes between them [14–17]. Transition-metal catalysis has the advantages of high activation efficiency and relatively low cost and energy input. However, homogeneous activation suffers from high catalyst consumption and water recontamination by these metals [18]. Thus, heterogeneous catalysts involving metal oxides such as Co3O4 [19] and CuFe2O4 [20] and zero valent metal such as Feo [21] and Zno [22] have been developed to overcome these drawbacks. Nevertheless, this method still suffers from relatively poor catalytic activity and severe metal leaching. Hence, it is essential to expand the scope of heterogeneous catalysts for PMS activation. During the past decades, perovskite materials have attracted much attention in heterogeneous catalysis because of excellent chemical properties [23]. Perovskites with a typical formula ABO3, where A sites are larger-sized alkali and rare-earth metals and B sites are transition-metal species, can accommodate 90% of metal elements in the Periodic Table. The elements can be presented solely or partially at A and/or B sites without destroying the matrix structure, generating some anion defects such as oxygen vacancies. This provides a way to tune catalytic properties by regulating the category and proportion of metal species presented [24–26]. Specifically, lanthanum-based perovskites (LaBO3, B = Co, Fe, Cu, Ni) have been systematically studied to activate PMS for the degradation of organics. LaCoO3 exhibited the best catalytic activity; however, it also suffers from the leaching of toxic Co ions with adverse effect on longterm operation [27, 28]. Several studies have been conducted to decrease the metal leaching of LaCoO3 [29] or develop modified perovskite catalysts with enhanced catalytic performance and

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stability simultaneously. Wang et al. prepared Cu-doped LaAlO3 perovskites as heterogeneous Fenton catalysts that exhibited excellent H2O2 utilization efficiency of 180% depending on the generated oxygen vacancies [30]. Duan et al. reported enhanced redox potential and electrical conductivity of modified Ba0.5Sr0.5Co0.8Fe0.2O3 perovskite for PMS activation compared with spinel cobalt oxide [31]. Lu et al. introduced LaCo0.4Cu0.6O3 for catalyzing PMS for the degradation of organic pollutants, where both radicals and non-radical species such as singlet oxygen (1O2) were generated, accelerating the phenol removal [32]. However, despite the enhanced catalytic activity, the metal leaching from B-sites hindered the further application, such as 5.9 mg/L Co leaching from Ba0.5Sr0.5Co0.8Fe0.2O3 [31] and 9.8 mg/L Co leaching from PrBaCo2O5+ [33]. In previous study, we also investigated significantly enhanced atrazine degradation by Cu-doped LaFeO3 (LFO)-activated PMS with 4.3 mg/L Cu leaching [34]. The catalytic performance of perovskites was found to be closely related to active metal species in their B-sites, while the elements in A-sites were inert for the activation reaction [35]. However, Miao et al. reported that the coordination environment and electronic structure of transition metals in B-sites were affected by the cations presented in A-sites, thus influencing the catalytic activity indirectly. They introduced La0.4Sr0.6MnO3 to activate PMS for phenol degradation and the results showed that the doping of foreign cation into A-sites enhanced the catalytic performance of LaMnO3 perovskite and decreased the leaching of Mn [36]. Nevertheless, up to now, no study has evaluated that whether the modification of Asites by doping based on LFO perovskites could both improve the catalytic activity and

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reduce the metal leaching, where Fe is frequently used for environmental remediation. Herein, in this work, a series of Sr-doped LFO perovskites La1–xSrxFeO3 (x = 0.2, 0.5, 0.8, denoted as LSFx, LSF20, LSF50, LSF80, respectively) were synthesized and first used for PMS activation towards 2,4-D degradation. Despite the fact that the degradation of 2,4-D by SR-AOPs had been widely studied by other researchers previously (Table S1 in the Supplementary Information (SI)), a highly active and stable heterogeneous catalyst based on eco-friendly Fe as active site rather than toxic Co was still required for PMS activation without the additional energy inputs. Intriguingly, LSF50 exhibited the significant enhancement of catalytic activity and stability as well as the new magnetic behavior that benefited the catalyst recovery. The operational parameters were optimized and the effects of initial pH and real water matrix on 2,4-D degradation were studied. Based on the radical quenching experiments and electron spin resonance (ESR) analysis, the PMS activation mechanism by LSF50 catalyst was proposed, where Sr doping facilitated the interfacial charge transfer and generation of ROS. The transformation intermediates were identified and the possible pathways for 2,4-D degradation were proposed. The present study provides a new idea to develop efficient heterogeneous catalysts for PMS activation in wastewater treatment. 2. Experimental Section 2.1 Reagents The reagents can be seen from Text S1 in SI. 2.2 Syntheses and characterization of LSFx LSFx perovskites were prepared by a sol-gel process with citrate as the

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complexing agents. Typically, stoichiometric amounts of La(NO3)3·6H2O, Sr(NO3)2, Fe(NO3)3·9H2O were first dissolved in water with gentle stirring. Then the mixed solution was slightly introduced into the citric acid solution with the molar ratio of citric acid to total metal ions being 2:1. A deep yellow gel was then formed with heating at 90 oC under constant magnetic stirring for 2 h. Followed by being dried at 130 oC under air atmosphere for 12 h, the gel could be transformed to a spongy precursor. After slightly milled into fine powders, the precursor was then calcined at 400 oC and subsequently at 750 oC for 4 h, respectively. The LSFx samples were obtained when the resultant product was cooled down. In addition, single A site perovskite LFO and SrFeO3 (SFO) were also prepared by the same process only in the absence of Sr(NO3)2 and La(NO3)3·6H2O, respectively. The prepared materials were then characterized by several technologies and the details were shown in Text S2. 2.3 Procedures and analysis The catalytic performance of the as-prepared perovskites was evaluated for PMS activation toward 2,4-D degradation. A typical reaction was carried out in a cylindrical glass reactor containing 50 mL of 10 mg/L 2,4-D (initial pH at 4.65) at ambient temperature in dark. LSFx catalyst was firstly dispersed into the 2,4-D solution with magnetic stirring for 30 min to achieve the adsorption-desorption equilibrium. Then PMS solution was added into the mixture with the final concentration of 1 mM to initiate the catalytic reaction. The solution pH was adjusted with 0.1 M H2SO4 or NaOH when required. At a given interval time, 1 mL of aqueous samples were withdrawn and filtered by a 0.22 m membrane, and mixed immediately with 0.5 mL of methanol to

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terminate the oxidation for HPLC analysis (Agilent 1200). The target organics were separated by an Eclipse XDB-C18 column (5µm, 4.6 × 250 mm) with a UV-DAD detector of 284 nm. The isocratic elution was composited of 75% methanol and 25% water (contained 5% acetic acid) with a flow rate of 1.0 mL/min. The concentration of residual PMS during the reaction was detected using a modified potassium iodide method [37] and the details were presented in Text S3. The concentration of dissolved metal ions was detected by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin-Elmer, Optima 8000). The values of solution pH were detected by an Orion 2-Star benchtop pH meter. The mineralization of 2,4-D was analyzed by Multi N/C 3100 analyzer (Analytikjena). The intermediates were analyzed by ultra-highperformance

liquid

chromatography/high-resolution

mass

spectrometry

(UHPLC/HRMS, Thermo Scientific Q Exactive). Details of UHPLC/HRMS measurements were described in Text S4. All experiments were carried out in triplicates and the results were presented as the mean with a standard deviation less than 5%. The reactive oxygen species (ROS) generated from PMS was analyzed on a JES-FA2000 ESR Spectrometer using DMPO as the spin-trapping agent for free radicals (SO4, HO) and TEMP for 1O2. The test conditions were shown in Text S5. 3. Results and discussion 3.1 Characterization XRD patterns of the synthesized catalysts were shown in Fig. 1. Clearly, all diffractograms showed strong diffraction peaks in the scanning range of 20  to 80 , suggesting high crystallinity of the materials. The characteristic diffraction peaks were

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corresponded to LaFeO3 (JCPDS 75-0541), La0.8Sr0.2FeO3 (JCPDS 35-1480), La0.5Sr0.5FeO3 (JCPDS 82-1962), La0.2Sr0.8FeO3 (82-1963), and SrFeO2.73 (40-0906), respectively, which were cubic (for LFO) or orthorhombic (for LSF20, LSF50, LSF80, and SFO) perovskite structure. The corresponding planes for the characteristic diffraction peaks of all samples were also marked in Fig. 1. Interestingly, the diffraction peaks of SrCO3 (JCPDS 05-0418) were presented in LSF50, LSF80, and SFO, suggesting the formation of the SrCO3 besides the perovskite structure for these three materials. The intensity of SrCO3 diffraction peaks was markedly strengthened when the doped Sr was increased form x = 0.5 to 0.8. SrCO3 was formed probably by a carbonation process of segregated strontium oxide during the air-exposed calcination [38]. In addition, with the elevated substitution of Sr, the characteristic diffraction peak gradually shifted to the higher angles (Fig. S1). As the cation size of the dopant Sr2+ (1.44 Å) was larger than La3+ (1.32 Å), the peak-shift phenomenon suggested that Sr was successfully incorporated into the perovskite lattice. Moreover, the peaks intensities were also gradually decreased when the introduction of Sr increased from x = 0.2 to 0.8, indicating the suppressed growth of the large crystallites by Sr substituting. The typical morphology of the as-prepared Fe-based perovskites was shown in Fig. S2. The prepared materials were all fine powders, and the color was changed from brown to black when Sr was introduced. Intriguingly, LSF50 exhibited an obvious magnetic behavior at ambient temperature, while that of LFO was negligible. The magnetic property of LSF50 made it easy to be magnetically separated from the aqueous phase with the addition of an external magnetic field. The SEM images of the

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samples were shown in Fig. 2(a-d) and Fig. S3. The materials almost exhibited the morphology of spherical agglomerate nanoparticles. However, for Sr-doped sample such as LSF50, it was observed that it contained two different components: one is the common agglomerate nanosphere corresponding to the perovskite; another is the gathered micron-sized sheets corresponding to SrCO3, which was confirmed by XRD. To further identify the composition of LSF50 sample, EDS was performed to detect the surface elements. It was observed in Fig. S4 that the elements of La, Fe, O and Sr were homogeneously distributed on the catalyst surface and the atomic ratio of each elements was close to the nominal ratio. The peaks of impurities were not detected in EDS spectra (Fig. S4a), suggesting high purity of prepared catalysts. Fig. 3(e-i) showed TEM images of LSF50, where the sphere-like agglomeration morphology was presented with the nanoparticles average size of about 50 nm. Furthermore, a lattice space of 0.274 nm could be corresponded to the (104) plane of La0.5Sr0.5FeO3, indicating that the perovskite was prepared. The FTIR spectra of LSFx samples was shown in Fig. S5. All the spectra displayed three main adsorption bands at 3425, 1455, and ~560 cm-1, respectively. The band at 3425 cm-1 was assigned to –OH stretch vibration corresponding to the surface hydroxyl groups [39]. The band at 1455 cm-1 was due to the vibration of CO group corresponding to the carbonaceous species, which was likely related to the SrCO3 in the samples [40]. The intensities of the peaks were observably heightened with the Sr doping increasing from x = 0.2 to 0.8, which was consistent with the XRD results. The most important band of Fe-based perovskites could be observed at around 560 cm-1,

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which was ascribed to the iron-oxygen stretching vibration [(Fe–O)] of the FeO6 octahedron in LaFeO3 [41]. With the increase of Sr doping, the absorption band slightly shifted to the higher wavenumbers, also demonstrating the successful incorporation of Sr into the perovskite lattice. XPS spectra of LFO and LSF50 was shown in Fig. S6. The new peak of Sr 3d could be obviously observed in the survey spectra of LSF50. The La 3d spectrum of LSF50 was slightly shifted compared to LFO, indicating the binding energy change of La by Sr doping (Fig. S6b). The Fe 2p spectrum of two samples exhibited three main peaks at ~724, ~719 and ~710 eV, which were attributed to Fe 2p1/2, shake-up satellite, and Fe 2p3/2, respectively (Fig. S6c). The Fe 2p3/2 spectrum could be divided into four peaks at 712.4, 711.38, 710.5 and 709.68 eV. Among them the peak at 709.68 eV was attributed to Fe(II) and others were all referred to Fe(III) [42–44], indicating that the valence state of Fe in both LFO and LSF50 was a mixture. For O 1s spectrum, there were two main peaks located at 531.5 and 528.5 eV, which were ascribed to adsorbed oxygen and lattice oxygen (Fig. S6d) [34]. The spectrum could be deconvoluted more detailed into four peaks at 528.94, 529.44, 530.05, and 531.8 eV, which were assigned to the lattice oxygen ions (O2), less electron-rich oxygen species (O22 or O), hydroxyl groups or the surface-adsorbed oxygen (–OH or O2), and adsorbed molecular water and carbonates, respectively [31, 33, 45]. It was visibly observed that the spectra of Fe 2p3/2 and O 1s, especially O 1s, significantly changed after the introduction of Sr. To better understand the doping modification, the relative concentrations of various iron and oxygen species were estimated from the areas of deconvoluted peaks and the results

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were shown in Table 1. For LSF50, the proportion of Fe(II) was increased from 19.3% to 43.0%, and the proportion of O2, O22/O, –OH/O2, and CO2/H2O of O species were changed from 14.7%, 22.3%, 26.7%, 36.3% to 15.9%, 10.2%, 29.3%, and 44.6%, respectively. The increased Fe(II) content by Sr doping suggested the lower average valence state of Fe on the surface of LSF50. Generally, the doping of A site La3+ by a foreign cation Sr2+ with a lower oxidation state would result in the higher valence state of B site (Fe) based on the electrical neutrality principle. However, the valence state of Fe in LSF50 was decreased. Thus, it was expected that more oxygen vacancies would be produced in order to maintain the electrical neutrality of the material. From the oxygen species variation, the proportion of less electron-rich lattice oxygen species was reduced remarkably, suggesting that part lattice oxygen ran off. That was to say, more oxygen vacancies were generated. Therefore, the substitution of Sr led to the lower oxidation state of Fe and the formation of more oxygen vacancies, which were both beneficial for the better catalytic activity [33]. 3.2 Catalytic performance The significant enhanced catalytic performance of LFO-based perovskite for 2,4D degradation by Sr doping with the presence of PMS was investigated in Fig. 3a. It was noted that the removal of 2,4-D was both less than 5% when PMS or catalyst was solely added, which suggested that 2,4-D could be hardly degraded by PMS itself, and the adsorption removal of 2,4-D by catalysts could be negligible. In contrast, the degradation efficiency was markedly promoted by LSFx catalyzing PMS, especially when LSF50 and LSF80 were used. 2,4-D of 10 mg/L could be completely removed

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only within 60 min. While the catalytic performance of single A site materials (LFO or SFO) and LSF20 towards PMS activation were relatively lower, which were corresponding to 2,4-D degradation of 20% and 51%, respectively. The results indicated that PMS could be activated by LSFx perovskites to generate various ROS such as SO4 and HO to degrade 2,4-D efficiently. The catalytic activity was promoted with the substitution of Sr and the degradation rate significantly raised when the content of Sr was increased. For kinetics investigation, results showed that the degradation of 2,4-D followed pseudo-first-order kinetics (Fig. S7). The reaction rate constant (kobs) for LSF50 catalyst was calculated as 0.072 min-1, which was 5.7 times higher than LFO (as the plateau of degradation profiles appeared after 20 min, the kobs of LFO was calculated within 20 min). The low reaction rate by LFO was probably attributed to its inferior circulation ability of Fe(III)/Fe(II) on the catalyst surface [46, 47]. In addition, the decomposition of PMS during the reaction was detected to better investigate the catalytic performance. As shown in Fig. 3b, the decomposition rate of PMS was accelerated with the content of doped Sr was increased, which was almost resembled with the trend of 2,4-D degradation. The faster PMS decomposition meant more generated ROS for 2,4-D degradation. It was noted that the catalytic activity of LSF50 was basically the same as that of LSF80, but the impurities were less. (Fig. 1). Therefore, LSF50 was selected for the following experiments. The solution TOC was reduced by 45% during 120 min catalytic oxidation in LSF50/PMS system (Fig. S8), suggesting that part of 2,4-D and intermediates could be mineralized. Furthermore, the performance of 2,4-D degradation by LSF50 catalyzing

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PMS was compared with other heterogeneous catalysis process. As shown in Table S1, LSF50 exhibited either better catalytic performance or less metal leaching than the compared catalysts. The above results indicated that LSF50 could be a promising heterogeneous catalyst for PMS activation in environmental remediation. 3.3 Influences of several factors 3.3.1 Effect of catalyst dosage and PMS concentration The effect of catalyst dosage for 2,4-D degradation was shown in Fig. 3c. Removal of 2,4-D was observably enhanced from 37% to 79% within 90 min when the catalyst dosage raised from 0.2 to 0.4 g/L. 2,4-D could be completely degraded when the dosage of LSF50 further increased to 0.6 g/L. However, the growth rate of kobs was declined when the catalyst dosage increased from 0.6 to 1.0 g/L (Fig. 3d). The results indicated that more catalysts could provide more active sites for PMS activation when the dosage was relatively low. However, it was seemed that the active sites provided by 0.6 g/L LSF50 were enough for activation of 1 mM PMS. The effect of PMS concentration for 2,4-D degradation was depicted in Fig. 3e. The 2,4-D degradation efficiency was increased from 65% to 100% when the concentration of PMS raised from 0.25 to 1 mM. However, degradation rate constant was declined to 0.0398 min-1 when PMS concentration was further increased to 1.25 mM (Fig. S9). The results demonstrated that the overdose PMS was unprofitable for its activation. The excess PMS have the scavenging effect on the generated SO4 (Eq. (1)). The optimum dosage of LSF50 and concentration of PMS was selected to be 0.6 g/L and 1 mM, respectively.

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+  → SO2 HSO5 + SO 4 4 + SO5 + H

(1)

3.3.2 Effect of initial pH As depicted in Fig. 4a, the effect of initial pH (pHo) on degradation efficiency of 2,4-D in LSF50/PMS system was explored. 2,4-D could be completely removed within at least 90 min when pHo ranged from 5 to 11, while about 90% of 2,4-D was degraded when pHo was 3. However, the degradation rate was inhibited to some extent for all experimental groups. The highest kobs of 0.072 min-1 appeared at pHo was 4.65 (without any adjustment). It was declined both when pHo was increased to 11 and decreased to 3 (Fig. 4b). Generally, the solution pH could affect the speciation of substrates and PMS and the surface charge of catalyst consequently affect the catalytic performance in heterogeneous catalysis [19]. The pKa of 2,4-D was 2.64 [3], which indicated that 2,4D was mainly presented in deprotonation form with negative charge at pHo ranged from 3 to 11. The pKa1 of H2SO5 was  0 and its pKa2 was 9.4 [48], suggesting that PMS was mainly existed as HSO5 when pH  9.4 and SO52 when pH  9.4. The pHpzc (pH at the point of zero charge) of LSF50 was detected at around 4.4 (Fig. S10), demonstrating that when pH  4.4 the surface charge of LSF50 was positive and when pH  4.4 it was negative [49]. Accordingly, when pH ranged from 5 to 11, the electrostatic repulsion interactions between negative charged LSF50 and HSO5 or SO52 would inhibit the activation efficiency of PMS, thus the degradation rate of 2,4D was declined. In addition, the variation of solution pH during the reaction was also monitored to better investigate the impact of pH. As shown in Fig. S11, the solution pH all changed to around 6.5 except for the strong acidic or alkaline conditions (pHo was

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3 or 11) during the adsorption-desorption equilibrium stage, which was probably attributed to the buffering ability of the catalyst [49, 50]. The solution pH was all sharply declined when PMS was first added and almost kept still during the subsequent catalytic reaction. The final pH when pHo was 11 was higher than unadjusted pHo, which resulted in the stronger electrostatic repulsion interaction between catalyst and PMS and consequently slower degradation rate of 2,4-D. When pH  4.4, the positive charged catalyst was supposed to adsorb more PMS and 2,4-D by the electrostatic attraction. However, the degradation efficiency at pH 3 was restrained. In our previous study, the same phenomenon was appeared when Cu doped LaFeO3 was used for PMS activation toward atrazine degradation [34]. The severe metal leach occurred at extreme acid condition so that the crystalline of catalyst would be destroyed to some extent and the dissolved metal ions likely showed poorer catalytic activity than its solid form [34, 50]. In addition, it was also reported that H+ had scavenging on SO4and HO (Eqs. (2) and (3)) [7, 51]. The above reasons resulted in the suppressed degradation rate when pH was 3. HO• + H + + e →

H 2O

(2)

+  → SO 4 +H +e

HSO 4

(3)

3.4 Reusability and stability The reusability of LSF50 was explored through four sequential tests as the catalyst could be easily collected by a magnet (Fig. S2f). As shown in Fig. 5, the degradation of 2,4-D was declined to 82% after 4 reaction cycles. However, it was noted that the degradation efficiency was even decreased to 85% in the first run and the degradation

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profiles were nearly similar between the subsequent cycles. The results were mainly ascribed to the corrosion on the surface of LSF50 catalyst during the reaction [5]. The leached Fe from LSF50 was detected at 0.15 mg/L by ICP-OES, which was attributed to about 1% of the solid form catalyst. The concentration of the leached metal was much lower than the B site Cu-doping LaFeO3 in our previous study, where the dissolved Cu reached 4.3 mg/L [34]. To better understand the change of texture structure, XRD patterns and FTIR of used LSF50 were detected as shown in Fig. S12. The XRD patterns of used LSF50 showed that the main characteristic diffraction peaks of perovskite La0.5Sr0.5FeO3 were maintained while the peaks of SrCO3 were almost vanished. The FTIR results also showed that the adsorption band of SrCO3 was almost disappeared. Although it was reported that SrCO3 exhibited inertial catalytic activity for PMS activation [40, 45], the results in the present study showed that SrCO3 probably had a beneficial effect on the stability of LSF50. The prior corrosion of SrCO3 prevented the perovskite phase which was the main active site from being destroyed. Nevertheless, LSF50 catalyst still exhibited the decent reusability and stability for PMS activation. 3.5 ROS and possible mechanism 3.5.1 ROS identification The generated ROS in LSF50/PMS system were identified by ESR experiments. DMPO was first used as the spin-trapping agent for free radicals (SO4, HO) and the results were shown in Fig. 6a. There was no signal peak when PMS was solely added. The weak signal peaks were present when PMS and LFO were added at the same time.

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The intensities of peaks were notably increased when LSF50 was used. The observed ESR spectra could be identified as DMPO-OH and DMPO-SO4 based on the hyperfine splitting constants (DMPO-OH: H = N = 1.487 mT; DMPO-SO4: H = 1.009 mT, N = 1.394 mT, H = 0.14 mT, H = 0.06 mT). The results indicated that SO4 and HO were generated in LFO-based perovskite activating PMS system and the catalytic activity of LSF50 was much higher than LFO. TEMP was used as spin trapping agent to detect the generation of the 1O2. As shown in Fig. 6b, a typical three-line ESR spectrum was observed even when PMS was solely added (N = 1.71 mT), which was attributed to TEMPO. TEMPO with strong electrophile was generally generated from the oxidation of TEMP by 1O2 [52]. The similar results were also reported by Zhou et al. and Yin et al [53, 54]. The ESR results showed that all of SO4, HO and 1O2 were generated in LSF50/PMS system. Therefore, the contribution of different ROS to 2,4-D degradation was further explored by the quenching experiments. Several quenchers including tert-butyl alcohol (TBA), ethanol (EtOH), and NaN3 were used for the quenching of HO, both HO and SO4, and 1O2, respectively, based on their different reaction rate with these ROS [55]. For example, EtOH reacted with HO and SO4 both at high reaction rates (kSO 4 = 5.6 × 107M -1s -1, kHO = 1.9 × 109M -1s -1), whereas TBA was significantly less reactive with SO4 (4.0 × 105M -1s -1) than HO (3.3 × 109M -1s -1) [56, 57]. In addition, 2-propanol was also used to investigate whether the generated radicals were surface-bound or free. 2-propanol possessed a similar reactivity to EtOH (kSO 4 = 3.5 × 107M -1s -1, kHO = 1.9 × 109M -1s -1), while its dielectric constant (20.18)

18

was lower than EtOH (25.3) [58]. It indicated that 2-propanol had higher affinity to the catalyst surface. In other words, 2-propanol could quench both of surface-bound and free radicals while EtOH could only quench free radicals [59]. The quenching tests results were shown in Fig. 6c. It was noted that the removal of 2,4-D was significantly suppressed when the addition of EtOH, 2-propanol, and NaN3, where the corresponding degradation efficiency was 23%, 19% and 10%, respectively. The inhibition from TBA was much weaker with degradation efficiency of 87% for 2,4-D, which suggested that SO4 played a dominant role for 2,4-D degradation. The inhibition from 2-propanol was just slightly stronger than EtOH, indicating that the generated radicals (both SO4 and HO) were mainly in bulk. The suppression of removal efficiency by NaN3 was much stronger than alcohols, suggesting that 1O2 was probably participated in the 2,4D degradation. Then the PMS decomposition in the presence of these scavengers was also investigated for a deeper understanding of the quenching mechanism. As shown in Fig. 6d, the decomposition of PMS was slightly declined when TBA or 2-propanol was added, while it was significantly increased with the addition of EtOH or NaN3, especially for the latter. The inhibition from TBA or 2-propanol was probably attributed to their low dielectric constants, which were 12.47 and 20.18, respectively [58]. The relatively high affinity for the catalyst surface could retard the contact with PMS. The enhanced decomposition of PMS by EtOH was resulted from that EtOH could quickly consume the generated radicals, thereby promoting the activation of PMS. It was noteworthy that PMS was almost completely decomposed within 10 min when NaN3 was present. The results suggested that NaN3 could rapidly react with PMS, thus the

19

validation of 1O2 formation by NaN3 was not suitable, which was also reported by the previous study [60]. Therefore, furfuryl alcohol (FFA) was used as the target substrate for the identification of 1O2. As shown in Fig. S13, degradation of FFA by solely PMS was only 5% within 90 min while 36% of FFA was degraded in LSF50/PMS system. The results suggested that 1O2 was most generated by LSF50 catalyzing PMS rather than the self-decomposition of PMS (Eq. (4)) [61]. However, considering the reaction rate between 2,4-D with HO or SO4 was higher than that with 1O2 [62], the contribution of 1O2 to 2,4-D was relatively limited than radicals. 2  1 HSO5 + SO2 5 → HSO4 + SO4 + O2

(4)

3.5.2 Activation mechanism Generally, the interfacial reaction types involved the inner-sphere and the outersphere mechanism in heterogeneous catalysis and it could be distinguished by exploring the effect of ionic strength [58]. Increasing ionic strength could compress the thickness of electric double layers consequently influence the electrostatic interactions, that was, outer-sphere interactions, while the inner-sphere was not affected [48]. As depicted in Fig. 7, the added NaClO4 showed no obvious influence on the degradation of 2,4-D, indicating the interactions between LSF50 and PMS probably went through the innersphere mechanism. In fact, the inner-sphere mechanism usually proceeded via the complexation between active metals with surface hydroxyl groups (–OH), which were characterized by FTIR in Fig. S5 and XPS in Fig. S6d [63]. It was also reported that H2PO4– could behave as quencher for –OH due to its high affinity for –OH [42]. Thus, the effect of H2PO4– on 2,4-D degradation in LSF50/PMS system was investigated to

20

study the role of –OH. As shown in Fig. 7, degradation of 2,4-D was significantly inhibited with the addition of H2PO4–, where only 10% of 2,4-D was removed. The results implied that –OH could be replaced by phosphate consequently influenced the catalytic activity of LSF50 catalysts, suggesting that –OH of LSF50 might play a vital role on the degradation of 2,4-D. The generation of ROS and 2,4-D degradation occurred mainly on the surface of catalyst. Therefore, XPS spectra of fresh and used LSF50 were compared to better understand the change in chemical states during the catalytic reaction for the identification of primary functional sites. As shown in Fig. 8a, there was no obvious change of La 3d spectrum after reaction, suggesting that La was not participated in the PMS activation. In fact, it was reported that La as the A site usually just provided structural support to anchor perovskite structures [23]. The spectrum of Sr 3d for the fresh and used LSF50 was shown in Fig. 8b. It could be fitted into four peaks located at 135.13, 133.69, 133.03, and 131.87 eV. The two peaks at higher binding energy were attributed to Sr 3d3/2, while the peaks at lower binding energy were attributed to Sr 3d5/2 [64, 65]. For the spectra of Sr, the peaks at 131.87 and 133.03 eV were corresponded to Sr 3d5/2 in the perovskite phase and the surface strontium carbonate, respectively [66, 67]. As depicted in Fig. 8c, the proportion of Fe(II) decreased from 43.0% to 31.0% after the activation reaction (Table 1), which suggested that electron was transferred from Fe(II) to HSO5 to generate SO4. The results indicated that Fe behaved as active site for PMS activation. Besides, as analyzed above, the lower oxidation state of Fe in LSF50 than LFO was beneficial for the better catalytic activity.

21

In addition, the change of O 1s during the reaction was also analyzed. As shown in Fig. 8d and Table 1, the proportion of O2– and O22–/O– were increased from 15.9% and 10.2% to 19.0% and 30.4%, respectively. The increased proportion of O22–/O– signified that the concentration of oxygen vacancies was declined after reaction, suggesting that oxygen vacancies were participated in PMS activation. In fact, oxygen vacancies as an electron-rich anion defect were likely to donate electron back to adsorbed HSO5 to promote the formation of SO4 [30]. Additionally, our previous study reported that oxygen vacancies could behaved as strong surface Lewis acid sites thus enhanced the chemical bonding between catalyst with PMS [34]. Oxygen vacancies could act as an additional bridge for electron transfer from transition metal of low valance state to HSO5, resulting in the facilitated charge transfer rate [33], which could be evidenced by EIS. As shown in Fig. S14, the diameter of the semicircle from LSF50 was smaller than that of LFO, suggesting that LSF50 catalyst possessed the faster interfacial electron transfer and a higher surface reaction rate [68]. Furthermore, the possible synergistic effect for 2,4-D degradation in LSF50/PMS system was explored with comparative experiments. As depicted in Fig. S15, the degradation of 2,4-D was negligible when La2O3 was used for PMS activation. And it also had no obvious effects on 2,4-D removal when the mixtures of SFO and La2O3 were used compared with the sole addition of SFO. These results suggested that the element of La was inert for PMS activation and the external interaction between La and Sr was feeble. Therefore, the enhanced catalytic performance of LSF50 was probably attributed to the intrinsically excellent activity of Fe and the forceful assistance of

22

oxygen vacancies generated by Sr doping, as discussed above. Based on the above analysis, the mechanism for the enhanced catalytic performance of LSF50 for PMS activation was proposed and illustrated in Fig. 9. On the one hand, –OH bonded with active metal (Fe(II)) first reacted with HSO5 to form a complex Fe(II)(HO)OSO3 through inner-sphere complexation (Eq. (5)). Then one electron transferred from Fe(II) to bonded HSO5 resulting in the cleavage of O=O to generate SO4 (Eq. (6)). On the other hand, the abundant oxygen vacancies of LSF50 could behave as Lewis acid sites to promote the chemical bonding with PMS and facilitate the charge transfer between active metal and HSO5 for SO4 generation, resulting in the enhanced redox cycle ability of Fe(II)/Fe(III) (Eqs. (7)–(8)). During this process, oxygen vacancies (Vo••) could be transformed to the oxygen ion in a normal oxygen site (Oo×) [33]. Then 1O2 was generated through the reaction between Oo× and HSO5 via Eq. (9) [36]. Meanwhile, peroxymonosulfate radical (SO5) which possessed a lower oxidation potential was also produced and could further self-react to generate PS molecules and SO4 (Eqs. (10)–(11)) [69, 70]. SO4 could further react with H2O to form HO (Eq. (12)). Therefore, low oxidation state of Fe by Sr doping and the enhanced charge transfer by oxygen vacancies synergistically contributed to the high catalytic activity of LSF50 for PMS activation, consequently leading to the promoted oxidation of 2,4-D. ≡ Fe(II)–OH– + HSO5 →

≡ Fe(II)–(HO)OSO3 + OH (5)

≡ Fe(II)–(HO)OSO3 → ≡ Fe(III)–OH– + SO 4

23

(6)

+  + OO× Fe(II) + HSO5 + V O →Fe(III) + SO4 + H

(7)

Fe(III) + HSO5 + H + + OO× →Fe(II) + SO 5 + H 2O

(8)

OO× + HSO5 →HSO4 + 1O2

(9)

2 2SO 5 → S2O8 + O2   2(SO 5 ) → SO4 + SO4 + O2

SO 4 + H2O →

+  SO2 4 + HO + H

(10) (11) (12)

3.6 Possible pathways The transformation byproducts of 2,4-D degradation in LSF50/PMS system were identified by UHPLC/HRMS (Figs. S16–S21) and the results were shown in Table S2, which included 2,4-dichlorophenol (2,4-DCP; A), 4-chloro-1,3-benzenediol (4chlororesorcinol, 4-CR; B), 2-chlorohydroquinone (2-CHQ; C), 2-chlorophenol (2-CP; D), hydroquinone (HQ; E), 1,2,4-benzontriol (F), and 4,6-dichlororesorcinol (4,6-DCR; G). Then the possible pathways were proposed based on the detected results and previous reports [5, 69, 71]. As shown in Fig. 10, the CO bound on the phenoxy group of 2,4-D was firstly attacked by the ROS and the lateral chain was broke to generate 2,4-DCP and glycolic acid. Subsequently, 2,4-DCP could be converted to 4-CR and 2CHQ by the dechlorination-hydroxylation process followed single electron transfer mechanism [4, 34]. Meanwhile, a radical addition product could be formed by the attack of SO4 or HO through a radical adduct formation mechanism and then converted to 4-CR and 4,6-DCR [4, 5]. The generated 4-CR and 2-CHQ could undergo further dechlorination and/or hydroxylation to form 2-CP, HQ, and 1,2,4-benzontriol. Then HQ could be converted to benzoquione (BQ; H) [69, 71]. Subsequently, the short-chain

24

carboxylic acids including oxalic acid and acetic acid were generated through the oxidative ring opening reactions by radicals and finally degraded to CO2 and H2O [5, 69]. 3.7 Practical application The application potential of LSF50/PMS system was explored by the degradation experiments in different practical water matrix. As shown in Fig. 11a, tap water, surface water (from Yangtze river, Wuhan section), and 2nd effluent water were used. The degradation of 2,4-D was slightly inhibited in tap water and surface water, suggesting the excellent potential of practical application for LSF50/PMS system. However, the degradation efficiency was obviously declined in 2nd effluent water, where only 34% of 2,4-D was removed. Generally, the condition of 2nd effluent water was more complicated than tap water and surface water. The involved alkalinity, natural organic matters (NOM) and inorganic salts both affected the oxidation of 2,4-D by generated ROS, especially radicals. Therefore, the effects of common inorganic anions (Cl– and HCO3–) and NOM (used as humic acid, HA) on 2,4-D degradation were further investigated. As shown in Fig. 11b, the removal of 2,4-D was both suppressed to some extent with the addition of Cl–, HCO3– and HA. Cl– and HCO3–, which usually behaved as radical scavengers, could exhibit detrimental effect on the radical-based reaction [6]. Cl– and HCO3– could compete with substrate for the generated SO4 to form some less reactive radical species such as Cl, ClOH and CO3/HCO3 [49, 72]. For HA, though it could also act as radical scavenger for HO and SO4, the quinones/semiquinones functional groups

25

would also activate PMS for the degradation of organics [73]. These combined effects resulted in the less inhibitory on 2,4-D degradation, which was also reported by Guan et al [49]. 4. Conclusion In summary, Sr doped perovskites La1-xSrxFeO3 were prepared by a complexing sol-gel method and were first introduced to activate PMS for 2,4-D degradation. LSF50 exhibited a lower valence state of Fe and magnetic behavior by Sr doping. The degradation rate of 2,4-D by LSF50 catalyzing PMS was 5.7 times higher than that by LFO. 2,4-D of 10 mg/L could be completely removed in the presence of 0.6 g/L LSF50 and 1 mM PMS within 60 min. LSF50 also possessed good reusability and practical application potentiality. The degradation efficiency maintained 82% after four successive cycle reaction. The initial pH ranged from 5 to 11 and surface water matrix showed negligible inhibitory effect on the degradation efficiency of 2,4-D. SO4, HO and 1O2 were both detected in LSF50/PMS system by ESR. The radicals quenching experiments showed that SO4 played a dominant role for 2,4-D degradation. The lower oxidation state of Fe facilitated the process of surface metal catalyzing PMS. The surface hydroxyl groups were also determined as main active sites. The large amount of oxygen vacancies generated by Sr doping promoted the chemical bonding with PMS consequently accelerated the interfacial charge transfer. Then, based on the UHPLC/HRMS results, the reaction byproducts were identified and the possible oxidation pathways of 2,4-D were also proposed. The method of activating PMS by Asites modified perovskite may bring great benefits for the application of advanced

26

oxidation in the treatment of herbicide-contaminated water. Acknowledgements This work was financially supported by Project on the Integrated of Water Quality Target Management Based on Watershed Control Units (2017ZX07301-003). Appendix A. Supplementary data Supplementary data to this article can be found online at doi: References [1] X. Li, M. Zhou, Y. Pan, Enhanced degradation of 2,4-dichlorophenoxyacetic acid by pre-magnetization Fe-C activated persulfate: influential factors, mechanism and degradation pathway, J. Hazard. Mater. 353 (2018) 454–465. [2] E. Brillas, J.C. Calpe, J. Casado, Mineralization of 2,4-D by advanced electrochemical oxidation processes, Water Res. 34 (2000) 2253–2262. [3] C. Liang, Y.Y. Guo, Y.R. Pan, A study of the applicability of various activated persulfate processes for the treatment of 2,4-dichlorophenoxyacetic acid, Int. J. Environ. Sci. Technol. 11 (2013) 483–492. [4] A. Serra-Clusellas, L. De Angelis, C.H. Lin, P. Vo, M. Bayati, L. Sumner, Z. Lei, N.B. Amaral, L.M. Bertini, J. Mazza, L.R. Pizzio, J.D. Stripeikis, J.A. RengifoHerrera, M.M. Fidalgo de Cortalezzi, Abatement of 2,4-D by H2O2 solar photolysis and solar photo-Fenton-like process with minute Fe(III) concentrations, Water Res. 144 (2018) 572–580. [5] H. Chen, Z. Zhang, M. Feng, W. Liu, W. Wang, Q. Yang, Y. Hu, Degradation of 2,4-dichlorophenoxyacetic acid in water by persulfate activated with FeS

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of

atrazine

by

magnetic

porous

copper

ferrite

catalyzed

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Table Table 1. The relative amounts of Fe, O and Sr species on the surface of LFO, LSF50 and used LSF50 obtained from the XPS results. Sample

Fe2+ (%)

Fe3+ (%)

O2

O22/ O

O2/ OH

CO2/H2O

Sr (P)a

SrCO3

LFO

19.3%

80.7%

14.7%

22.3%

26.7%

36.3%





Fresh LSF50

43.0%

57.0%

15.9%

10.2%

29.3%

44.6%

38.4%

61.6%

Used LSF50

31.0%

69.0%

19.0%

30.4%

20.2%

30.4%

45.6%

54.4%

a: P represented perovskite phase

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Figure caption Fig. 1. XRD patterns of LSFx samples. Fig. 2. SEM and TEM images of LSF50. Fig. 3. (a) Degradation of 2,4-D, (b) PMS decomposition by different LSFx catalysts, effect of (c) catalyst dosage and (e) PMS concentration on 2,4-D degradation and (d, f) corresponding plots of -ln(C/C0) versus reaction time in LSF50/PMS system. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM (except (e)), [catalyst] = 0.6 g/L (except (c)), [pHo] = 4.65. Fig. 4. (a) Effect of pH0 on 2,4-D degradation and (b) corresponding kobs. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM, [catalyst] = 0.6 g/L. Fig. 5. Recycling study of LSF50. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM, [catalyst] = 0.6 g/L, [pHo] = 4.65. Fig. 6. ESR spectra using (a) DMPO and (b) TEMP as spin trapping agent, (c) radical quenching experiments and (d) corresponding PMS decomposition by different quenchers. Conditions: [PMS] = 5 mM (in (a) and (b)), [DMPO] = [TEMP] = 50 mM, [2,4-D] = 10 mg/L, [PMS] = 1 mM (in (c) and (d)), [catalyst] = 0.6 g/L (in (c) and (d)), [EtOH] = [TBA] = [2-propanol] = 100 mM, [NaN3] = 10 mM , [pHo] = 4.65 (in (c) and (d)). Fig. 7. Effect of ionic strength (represented as NaClO4) and KH2PO4 on 2,4-D degradation in LSF50/PMS system. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM, [catalyst] = 0.6 g/L, [NaClO4] = [KH2PO4] = 10 mM, [pHo] = 4.65. Fig. 8. XPS spectra of fresh and used LSF50 catalyst: (a) La 3d; (b) Sr 3d; (c) Fe 2p;

39

(d) O1s. Fig. 9. Proposed activation mechanism of LSF50/PMS system. Fig. 10. Proposed pathways for degradation of 2,4-D in LSF50/PMS system. Fig. 11. Effects of (a) water matrix and (b) coexisting anions on 2,4-D degradation in LSF50/PMS system. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM, [catalyst] = 0.6 g/L, [Cl–] = [HCO3–] = 10 mM, [HA] = 10 mg/L.

40

Fig. 1. XRD patterns of LSFx samples.

41

Fig. 2. SEM and TEM images of LSF50.

42

Fig. 3. (a) Degradation of 2,4-D, (b) PMS decomposition by different LSFx catalysts, effect of (c) catalyst dosage and (e) PMS concentration on 2,4-D degradation and (d, f) corresponding plots of -ln(C/C0) versus reaction time in LSF50/PMS system. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM (except (e)), [catalyst] = 0.6 g/L (except (c)), [pHo] = 4.65.

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Fig. 4. (a) Effect of pH0 on 2,4-D degradation and (b) corresponding kobs. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM, [catalyst] = 0.6 g/L.

Fig. 5. Recycling study of LSF50. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM, [catalyst] = 0.6 g/L, [pHo] = 4.65.

44

Fig. 6. ESR spectra using (a) DMPO and (b) TEMP as spin trapping agent, (c) radical quenching experiments and (d) corresponding PMS decomposition by different quenchers. Conditions: [PMS] = 5 mM (in (a) and (b)), [DMPO] = [TEMP] = 50 mM, [2,4-D] = 10 mg/L, [PMS] = 1 mM (in (c) and (d)), [catalyst] = 0.6 g/L (in (c) and (d)), [EtOH] = [TBA] = [2-propanol] = 100 mM, [NaN3] = 10 mM , [pHo] = 4.65 (in (c) and (d)).

45

Fig. 7. Effect of ionic strength (represented as NaClO4) and KH2PO4 on 2,4-D degradation in LSF50/PMS system. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM, [catalyst] = 0.6 g/L, [NaClO4] = [KH2PO4] = 10 mM, [pHo] = 4.65.

46

Fig. 8. XPS spectra of fresh and used LSF50 catalyst: (a) La 3d; (b) Sr 3d; (c) Fe 2p; (d) O1s.

47

Fig. 9. Proposed activation mechanism of LSF50/PMS system.

Fig. 10. Proposed pathways for degradation of 2,4-D in LSF50/PMS system.

48

Fig. 11. Effects of (a) water matrix and (b) coexisting anions on 2,4-D degradation in LSF50/PMS system. Conditions: [2,4-D] = 10 mg/L, [PMS] = 1 mM, [catalyst] = 0.6 g/L, [Cl–] = [HCO3–] = 10 mM, [HA] = 10 mg/L.

Graphical Abstract

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Highlights  La0.5Sr0.5FeO3 exhibited enhanced catalytic performance and stability to activate PMS for 2,4-D degradation.  A coupled PMS activation mechanism for the major free radicals involving SO4 and HO and minor 1O2 is proposed.  A lower valence state of Fe and abundant oxygen vacancies were responsible for the excellent catalytic property.  Possible degradation pathways of 2,4-D were proposed.

Declaration of Interest Statement The authors declare that they have no known competing financial interests or

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personal relationships that could have appeared to influence the work reported in this paper.

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