Peroxymonosulfate activation on FeCo2S4 modified g-C3N4 (FeCo2S4-CN): Mechanism of singlet oxygen evolution for nonradical efficient degradation of sulfamethoxazole

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Peroxymonosulfate activation on FeCo2S4 modified g-C3N4 (FeCo2S4-CN): Mechanism of singlet oxygen evolution for nonradical efficient degradation of sulfamethoxazole ⁎

Yangju Lia,b, Jun Lia,b, Yuting Pana,b, Zhaokun Xionga,b, , Gang Yaob,c, Ruzhen Xiea, Bo Laia,b,

⁎

a

State Key Laboratory of Hydraulics and Mountain River Engineering, College of Architecture and Environment, Sichuan University, Chengdu 610065, China Sino-German Centre for Water and Health Research, Sichuan University, Chengdu 610065, China c Institute of Environmental Engineering, RWTH Aachen University, Aachen 52072, Germany b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

influencing parameters • Various SMX oxidation process were ex-

on

amined.

O was verified to be the major ROS • responsible for SMX degradation. role of sulfur species was identi• The fied. underlying mechanism of O • The evolution was clarified in details. 1

2

1

2

A R T I C LE I N FO

A B S T R A C T

Keywords: FeCo2S4 modified g-C3N4 Peroxymonosulfate Singlet oxygen Sulfamethoxazole Advanced oxidation

FeCo2S4-CN particles were used as an innovative heterogeneous catalyst to degrade sulfamethoxazole (SMX) via peroxymonosulfate (PMS) activation. The combination of 20 mg/L FeCo2S4-CN and 0.15 mM PMS exhibited an extraordinarily high catalytic activity towards SMX degradation (91.9%, 0.151 min−1). Reactive oxygen species (ROS) generated in the FeCo2S4-CN/PMS process were identified by radical scavenging tests and electron spin response (EPR) analysis. Singlet oxygen (1O2) dominated noradical pathway was verified to be the major route contributed to the degradation of SMX, which was generated by direct self-decomposition of PMS, the recombination of superoxide ions, and the interaction between g-C3N4 and PMS. Additionally, multiple characterization techniques were applied to clarify the physicochemical properties of the catalyst, which revealed that the FeCo2S4-CN contained S2−, Sn2−, S0, SO32−, and SO42−. Thus, we systematically explored the role of sulfur species. Based on these results, the plausible catalytic mechanisms and the degradation pathways of SMX were proposed. These findings shed some insight on the application of metal sulfides for the oxidative elimination of micropollutants by peroxymonosulfate activation.

⁎ Corresponding authors at: State Key Laboratory of Hydraulics and Mountain River Engineering, College of Architecture and Environment, Sichuan University, Chengdu 610065, China. E-mail addresses: [email protected] (Z. Xiong), [email protected] (B. Lai).

https://doi.org/10.1016/j.cej.2019.123361 Received 26 August 2019; Received in revised form 18 October 2019; Accepted 2 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Yangju Li, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123361

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1. Introduction

PMS activation. Graphite carbonitride (g-C3N4), emerged as an environmentally friendly organic metal-free semiconductor, which has recently applied in various fields, such as H2O2 synthesis, CO2 reduction, contaminants degradation, and H2 evolution due to its high physicochemical stability, unique optical and electronic properties, as well as facile synthesis [22]. In addition, g-C3N4 is capable of complexing metal ions since its six nitrogen lone pair can serve as electron donors, thus reducing the leach of free metal ions [11]. Thereby, g-C3N4 has been widely used as the catalyst support to overcome the drawback of iron leaching and enhance the catalytic activity of as-prepared samples. Shao et al. concluded that synergetic activation of PMS by Co3O4-g-C3N4 composites enhanced the degradation of diclofenac sodium under visible light irradiation [23]. Likewise, Feng et al. also pointed out that the removal efficiency of phenol was greatly enhanced by Fe-doped g-C3N4 composites to activate PMS due to the formation of FeIV = O species and enlarged specific surface area compared with pristine g-C3N4 [24]. Jiang et al. revealed that the the enhanced photocatalytic performance of nanocomposite CaIn2S4/g‑C3N4 might stems from the enhanced interfacial contact area between CaIn2S4 and g-C3N4, as well as the improved separation of charge carriers [22]. Considering the specific Fe–N and/or Co–N interaction, the synergistic effect of Fe and Co ions, high physicochemical stability and excellent conductivity of g-C3N4, the possible electron-donating effect of reductive sulfur, as well as the synergistic effect between g-C3N4 and FeCo2S4. Herein, we anticipated that FeCo2S4 modified g-C3N4 (FeCo2S4-CN) was an efficient catalyst for the PMS activation. However, as far as we know, no work has been conducted to choose FeCo2S4-CN composites for the heterogeneous activation of PMS to elimination antibiotics. Inspired by previous excellent work mentioned above, FeCo2S4-CN composites was synthesized and introduced to activate PMS to degrade SMX. The catalytic degradation performance of FeCo2S4-CN/PMS system under different experimental conditions was evaluated and the physicochemical properties of the as-prepared FeCo2S4-CN was also determined. The underlying mechanism of the established system was clarified in details, with an emphasis on the role of sulfur species and the synergistic effect between the g-C3N4 and FeCo2S4. Finally, the transformation pathways of SMX were tentatively proposed.

Nowadays, the presence of antibiotics in the aquatic environment is well recognized as a troublesome issue even at trace concentration, owing to their adverse effects on public health and environmental ecosystem, as well as the development of pathogen resistance to antibiotics [1]. Among antibiotics, Sulfamethoxazole (SMX) is a typical sulfonamide bacteriostatic antibiotic, which is the most frequently detected one in the aquatic environment and is widely used in human and veterinary medicine [2]. The concentration of SMX usually between ng L−1 and μg L−1 in wastewater treatment in wastewater effluents, while in the effluents of hospitals and pharmaceutical manufacturing plants, the concentration level can reach up to mg L−1 [3]. Unfortunately, most antibiotics are recalcitrant to conventional biological wastewater treatment processes for their antibacterial nature [4]. Therefore, environmentally-friendly and effective technologies are highly desirable to limit the presence of antibiotic pollutants in aquatic ecosystems. Advanced oxidation processes (AOPs) have proven to be a feasible alternative for the effective elimination of pseudo persistent and nonbiodegradable compounds in aquatic environments [5]. Among diverse AOPs, peroxymonosulfate based advanced oxidation processes (PMSbased AOPs) have received considerable attention as an effective and promising technology for the treatment of antibiotics. PMS is considered to be a substitute for chlorine-based bleaching agent, the peroxymonosulfate anion is derived from Caro’s acid or peroxysulfuric acid (H2SO5), which is widely used in the paper and pulp industry for the purpose of disinfection [6]. Moreover, it is a derivative of H2O2 where one hydrogen atom is substituted by a sulfonate group, the unique unsymmetric structure allows PMS to be more easily activated via various activation methods to generate reactive oxygen species (ROS), such as hydroxyl radical (%OH) [2,7], sulfate radical (SO4%−) [3], superoxide radical (O2%−) [8], singlet oxygen (1O2) [9], surface-bound radical [10], surface activated complex and electron shuttle [11,12]. Recently, metal oxides (MxOy) have been extensively investigated for heterogeneous activation of PMS, due to the lower energy consumption and the potential for recycling of the catalyst, especially Cobased catalysts. Biochar modified Co3O4 was introduced to activate PMS for ofloxacin degradation [13]. Yang et al. found that using MOFtemplated synthesis of CoFe2O4 nanocrystals as PMS activator can effectively degrade bisphenol A by the generated %OH and SO4%− [14]. Also it was reported that O2%− could be produced from PMS activated with CoMn2O4 to degrade sulfanilamide [15]. Meanwhile, metal sulfides (MxSy) have obtained increasing research attention, since MxSy generally possess higher electrical conductivity together with mechanical and thermal stability compared with MxOy, which are not only applied to environment decontamination but also utilized as the main materials of lithium-ion batteries and supercapacitors [16,17]. In addition, MxSy have been widely used as bifunctional catalysts in oxygen generation reactions oxygen, reduction reactions, as well as hydrogen generation/evolution reactions [18]. In contrast to the corresponding one-component sulfides, most of the reported bimetallic sulfides, such as CuFeS2, CoNi2S4, and NiCo2S4 have proven to display better electrochemical and catalytic performance, which could result from the enhanced synergistic effect of metal ions and the abundant redox reactions [19]. Very recently, Xu et al. revealed that bimetallic sulfides (CuCo2S4) performed well in activating PMS, owing to the synergetic surface redox couples [20]. Among them, FeCo2S4 nanoparticles, as the promising candidate, which has been widely investigated due to its lower intrinsic electrical resistivity and varied electronic feature associated with charge transfer [17,21]. However, the FeCo2S4 particles have poor dispersion and tend to form large particle size inevitably, leading to minimize surface active sites. What’s worse, its application is quite restricted because of the potential carcinogenic effect caused by leached cobalt iron. Hence, there still remains a huge challenge in exploring efficient and stable catalysts for

2. Materials and methods 2.1. Chemicals Sources of chemicals are shown in the Supporting Information (SI) Text S1. Deionized (DI) water (> 18 MΩ·cm), produced with a UPH purification system, was applied to prepare all of the solutions. 2.2. Catalysts preparation and characterization FeCo2S4-CN composites were prepared via a modified hydrothermal procedure [25]. Detailed synthesizing steps are described in Text S2. Characterization details of as-synthesized composites were given in Text S3. 2.3. Experimental procedure Batch experiments were conducted in a 250 mL glass bottle, and the solution was constantly mixed by magnetic stirring at 300 rpm with thermostat-controlled water bath controlled at 30 ± 0.2 °C. Reactions were initiated by adding PMS (0.05–0.3 mM) and FeCo2S4-CN (10–30 mg) to the 19.7 μM of SMX stock solution without buffer (unless otherwise specified). 1 mL of aliquot sample was collected at the predetermined time intervals and the supernatant was filtered by 0.45 μm PTFE syringe filter and the residual oxidant was quenched with 1 M Na2S2O3 solution immediately prior to analysis. For the pH influence (with buffer), different buffers were introduced into the solution, with 2

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shifted from 27.6° to 27.1°, indicating a decreased gallery distance between the layers during the hydrothermal process, because strong acid (HF) was generated by the hydrolysis of NH4F which resulted in the cleavage of g-C3N4 layers, similar phenomenon can also be found in other work [30]. Fig. S2b displays the N2 adsorption–desorption isotherms of as-synthesized samples. As seen, both isotherms belong to type IV with the hysteresis loop, implying a typical mesoporous structure in accordance with the IUPAC classification [24]. The BET surface area of FeCo2S4-CN composites was 20.338 m2/g which was lower than that of pristine g-C3N4 with 97.712 m2/g and pure FeCo2S4 with 25.332 m2/g. It can be expected that these abundant pores originated from g-C3N4 may provide more exposed active sites for PMS activation. The pure FeCo2S4 particles was well dispersed onto the g-C3N4 porous layer, so the BET surface area of pristine g-C3N4 was decreased. In order to obtain further insights into the surface chemical compositions and chemical state of FeCo2S4-CN composites, XPS was introduced to analyze the freshly synthesized and used samples. The survey scan XPS spectrum (Fig. S3a) reveals that the as-synthesized particles are mainly composed of C, N, Co, Fe, and S, in which the strength of the C1s and N 1s intensity is strongest due to its highest proportion of g-C3N4 in samples. The Fe 2p spectra of fresh FeCo2S4-CN is reasonably divided into four fitted curves (Fig. S3b), the distinctive peaks at 710.7 eV, 724.2 eV, and 715.0 eV which correspond to Fe2p3/2, Fe 2p1/2, and the satellite peak pf Fe2p3/2 for Fe2+, respectively [11,31]. Obviously, the binding energy of Fe 2p3/2 (710.7 eV) higher than the value of Fe(II) phthalocyanine (709.2 eV) and lies in the range of 710.3–711.8 eV of Fe(III) valence state, demonstrating the formation Fe(III)–N moieties [32]. After the reaction, the binding energies for Fe 2p3/2 and Fe 2p1/2 of Fe2+ were increased to 712.3 and 724.7 eV, respectively. Additionally, two shakeup satellite peaks existed in 716.9 and 734.0 eV, which can be ascribed to the existence of Fe3+ [33]. The result suggested that the active component of composites was oxidized after the reaction with PMS. Fig. S3c demonstrates the XPS spectra of Co 2p region. The deconvoluted peaks at 786.3 and 802.6 eV are ascribed to satellite peaks of Co2+, the deconvoluted peaks at 798.3 and 782.5 eV are assigned to the Co 2p3/2 and Co 2p1/2, while the peak at 781.2 eV and 780.9 eV agree with Co–N bond and Co-S interaction [20,28,34]. As for the used particles, the deconvoluted peak at 782.5 eV was disappeared followed by the formation of new peak at 779.6 eV (Co3+), indicating that Co2+/ Co3+ coexist at the surface of FeCo2S4-CN composites [33]. In addition, the less intense peaks at binding 786.3 and 798.3 eV were decreased and further confirmed the oxidation state of Co. As exhibited in Fig. S3d, the O 1s spectrum of fresh FeCo2S4-CN composites comprises a broad peak at 532.2 eV, which can be assigned to the surface adsorbed water. For the used FeCo2S4-CN composites, the O 1s spectrum can be deconvoluted into two components, with binding energy peaks at 532.2 eV and 531.8 eV, respectively. The new peak appeared peak at 531.8 eV was related to the adsorbed CO2, which suggested the surface hydroxylation and mineralization of compounds [35]. Fig. S3e displays the C1s peak, which consists of two peaks at 284.8 and 288.6 eV, was attributed to the sp2 hybridized carbon atoms (N–C]N) in the lattice and the adventitious carbon species depositing on the surface of g-C3N4 for both fresh and used samples [36,37], and the characteristic peak positions almost have no change before and after use. Similarly, the N 1s XPS spectrum of fresh FeCo2S4-CN composites shows two main peaks at 398.7 eV and 399.46 eV, which assigned to the sp2 bonded N in the tri-s-triazine (N2C) and the bond of Co–N, respectively (Fig. S3f) [34,37]. After reaction, the percentage of sp2 bonded N is estimated to be 44.3%, which was higher than that of fresh samples (19.8%), it can be concluded that the conversion of both iron and cobalt species do exist on the surface of g-C3N4. As revealed from Fig. S3g, the S 2p XPS spectrum can be fitted with four peaks at 161.7, 162.7 eV, 168.9 eV, and 163.8 eV, assigning to S 2p2/3, S 2p1/2, the satellite peak, and the bond between metal and sulfur (M-S), respectively, suggesting the presence of S2− species [38]. Three new

1 mM acetate for pH 3.5 or 5, 1 mM borate for pH 6.5, and 1 mM bicarbonate for pH 8.0 and 9.5. Cl−, SO42−, H2PO4− (2–25 mM) and HA (2–25 mg/L) were introduced into the above FeCo2S4-CN/PMS system to evaluate the influence of background water matrices on SMX degradation. Bisphenol A (BPA), atrazine (ATZ), and carbamazepine (CBZ) were chosen as probe pollutants to evaluate the oxidation capacity of FeCo2S4-CN/PMS system. To explore the possible ROS, multiple quenchers, including methanol (EtOH, 100–200 mM), tert-butyl alcohol (TBA, 100–200 mM), furfuryl alcohol (FFA, 5–10 mM), benzoquinone (BQ, 2 mM) and sodium carbonate (CO32−, 2–30 mM) were added into the system. For reusability tests, the used particles were recovered with centrifugation and washed with DI water and ethanol for three times after each run. It was further dried out at 60 °C overnight before the next run. The volume of the reaction solution and catalyst dose was the same as the first run. All the process was conducted in triplicate or over. Detailed information about the analytical method in this work are provided in the Supporting information (SI) Text S4. 3. Results and discussion 3.1. Characterization of catalyst The morphologies and microstructures of g-C3N4, FeCo2S4, and FeCo2S4-CN composites were investigated by SEM coupled with EDS. As illustrated in Fig. S1a, the pristine g-C3N4 had typical flat layer and platelet-like structure, and the EDS of g-C3N4 shows two strong peaks of C and N. It was clear that the FeCo2S4 particles possess nanoflowers constituted by nanoflakes, and the Fe (14.4 wt%), Co (30.9 wt%), and S (55.7 wt%) elements exist in the as-prepared samples. The ratio was close to 1:2:4, indicating the successful synthesis of FeCo2S4 (Fig. S1b). As for the freshly prepared FeCo2S4-CN composites, it was found that the granular FeCo2S4 particles (bright spots) were well dispersed on the layer sheets of g-C3N4, developing a uniform coating on the surface of gC3N4 layers. Accordingly, the EDS elemental spectrum of FeCo2S4-CN composites indicated the presence of Fe, Co, S, C, N, and inevasible O (Fig. S1c), which suggests that the FeCo2S4 particles was successfully supported on the pristine g-C3N4. As can be seen from Fig. S1d, there are a small amount of voids on the surface of the used FeCo2S4-CN composites and the particles have a certain degree of agglomeration, which may be attributed to the corrosion of the samples. Correspondingly, the weight ratio of O (34.41%) and S (12.28%) elements on the surface of used samples was higher than that of freshly prepared FeCo2S4-CN composites (13.76% for O and 7.26% for S). The enhancement of S may caused by the adsorbed sulfate, which resulted from the PMS induced corrosion of FeCo2S4-CN composites and SMX oxidation products. The crystal structure of as-prepared g-C3N4, FeCo2S4, and FeCo2S4CN composites was identified by XRD. As shown in Fig. S2a, two typical diffraction peaks (13.2° and 27.5°) are observed in the XRD patterns of pristine g-C3N4, which can be indexed to the interplanar repeating structural motif of tri-s-thiazine units (1 0 0) and the (0 0 2) peak of the interlayer stacking of aromatic segments, respectively [26,27]. It should be pointed out that almost all the diffraction peaks of pure FeCo2S4 are assigned to spinel-type Co3S4 (JCPDF No. 47–1738). The peaks appear at 2θ = 16.2°, 26.7°, 31.5°, 38.2 and 55.2°, which exactly matched with the crystal planes of the (1 1 1), (2 2 0), (3 1 1), (4 0 0), and (4 4 0), apart from subtly shifting to the high 2θ direction. The results strongly vindicated the partial replacement of Co ions in Co3S4 by Fe ions to yield FeCo2S4 and the crystal structure of Co3S4 has been maintained, coinciding well with the previous reports [25,28]. Owing to the low level of Fe and S element or low degree of their crystallinity, there was no obvious diffraction peaks corresponding to sulfur-bearing compounds could be observed directly by XRD analysis. The same phenomenon was also found in previous work [29]. As for the XRD patterns of FeCo2S4-CN composites contain the peaks of both FeCo2S4 and g-C3N4. In addition, the (0 0 2) peak of g-C3N4 is 3

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peaks located at 168.4, 164.5, and 169.2 eV for the used samples, which could be assigned to the S(IV), S0, and SO42−, respectively [39]. The results further indicated that some S2− was oxidized to S0 followed by the formation of Fe(II) and Co(II). Thus, the proportion of Fe(II) and Co (II) before and after reaction have no remarkable change. The FTIR spectra of fresh and used samples were presented in Fig. S4a. No obvious vibrational change was observed between fresh and used FeCo2S4-CN. Specifically, the bands at 3380–3075 cm−1 could be assigned to the hydroxyl and carboxyl groups [40]. Several strong bands were observed in the region of 985 and 1615 cm−1, which were ascribed to the characteristic breathing modes of aromatic carbon nitride heterocycles [35]. Additionally, the peak at 1242 cm−1 was associated with the Co-S bond, and the peak at 804 cm−1 corresponded to the typical bending vibration of tri-s-triazine rings [41]. The peak observed at 891 cm−1 due to deformation of the polymer skeleton [42]. It was worth noting that the peaks at 600 and 525 cm−1 were caused by the oxidation of sulfur species on the surface of the catalyst, and the intensity of the peak became stronger after reaction. Furthermore, the peak at 420 cm−1 in the spectrum of samples, which referred to the stretching vibrational mode of Co–N bond [34]. These combined results suggest that there is interaction between FeCo2S4 and g-C3N4, and the role of sulfur speices in the recycling of Fe3+/Fe2+ and Co3+/Co2+ cannot be ignored.

abilities of contaminants substituent groups [11]. Some recent studies found that 1O2 shown a high selectivity toward electronrich phenolic compounds as the nonradical pathway [43]. Therefore, the produced 1 O2 contributed to BPA removal via electrophilic attack owing to the presence of electron-donating group (phenolic hydroxyl group) in the BPA molecule. Likely, the CBZ molecule possess the of high electron density, and the olefinic double bond was verified to be the most susceptible sites, so it could also be easily degraded [44]. However, ATZ was often used as the probe compound for %OH and SO4%−, which was more sensitive to free radical systems, thus its degradation efficiency was lower than the other three pollutants [45]. The results suggest an excellent superiorityof FeCo2S4-CN/PMS system for the degradation of emerging contaminants.

3.2. Catalytic oxidation of SMX

[SMX]t ⎞ − ln⎛ = k obs. t [SMX] 0⎠ ⎝

3.3. Influencing parameters on SMX oxidation A series of experiments were performed to investigate the impacts of several relevant parameters, including solution pH, temperature, FeCo2S4 content, sulfidation conditions, FeCo2S4-CN dosage, PMS concentration, inorganic anions, and NOM on SMX removal. In all the cases, the degradation process of SMX followed pseudo-first-order kinetics, and the pseudo-first-order rate constant (kobs) was acquired by linear regression of ln [SMX] to time (Eq. (1)). ⎜

The SMX removal in various aqueous systems were evaluated and the results are shown in Fig. 1(a). The adsorption of SMX on g-C3N4, FeCo2S4, and FeCo2S4-CN were negligible. PMS alone induced only 14.3% SMX removal directly within 15 min, while 91.9% SMX removal was achieved in the case of FeCo2S4-CN/PMS system, which was higher than the sum of those in FeCo2S4/PMS (60.1%) and g-C3N4/PMS (17.8%) systems, indicating that there are synergistic effects between FeCo2S4 and g-C3N4, which may be attributed to the excellent conductivity of g-C3N4. In addition, we further compared the surface area normalized SMX degradation rate constant ksa (Table S1). The ksa value of FeCo2S4-CN/PMS system (7.42 × 10−3 g·min−1·m−2) was 2.77 times and 98.0 times larger than that of FeCo2S4/PMS system (1.97 × 10−3 g·min−1·m−2) and g-C3N4/PMS system (7.57 × 10−5 g·min−1·m−2), respectively. Interestingly, in the FeCo2O4-CN/PMS system, 54.5% SMX can be removed after 15 min, which was also lower than the FeCo2S4-CN/PMS system, since the S2− of the catalysts possess lower electronegativity, thus promoting the cycling of Fe3+/Fe2+ and Co3+/Co2+ [19]. Meanwhile, the degradation of SMX through homogeneous catalytic oxidation of PMS by leached metal ions was just only 35.2% (0.068 mg/L Co, 0.03 mg/L Fe). 73.9% of SMX can be eliminated in 15 min by Co3S4-CN/PMS system under the same condition. Moreover, we monitored the metal ion leaching of different systems. In comparison to 0.068 mg/L of Co and 0.03 mg/L of Fe were leached from FeCo2S4-CN, the metal leaching from FeCo2S4 and Co3S4-CN were much higher, about 0.12 mg/L of Co and 0.067 mg/L of Fe, 0.083 mg/L of Co for FeCo2S4 and Co3S4-CN, respectively (Fig. S4b). Previous study have revealed that bimetallic spinel sulfides could be rather stable compared to one-component metal sulfides [19]. Thus FeCo2S4-CN was superior to Co3S4-CN, and the FeCo2S4/PMS system may be dominated with a homogeneous process. All these results above suggested that FeCo2S4-CN has high activity for PMS activation and SMX elimination. To further explore the catalytic oxidation properties of FeCo2S4-CN/ PMS system, we examined the developed system for the oxidative degradation of diverse organic pollutants, including BPA, ATZ, and CBZ. As depicted in Fig. S5, all the selected target pollutants have obtained satisfactory removal efficiency. The removal efficiencies of BPA, ATZ, CBZ, and SMX reached 100%, 79.7%, 89.0% and 91.9% in 15 min, respectively. The catalytic oxidation ability of the FeCo2S4-CN/PMS system was strongly related to the electron donating/withdrawing

⎟

(1)

3.3.1. Impact of solution pH It is well known that the solution pH remarkably affects the speciation of the substrate and oxidant, thereby influencing the performance of heterogeneous catalytic oxidation processes [43,46]. Therefore, impact of initial pH was explored for FeCo2S4-CN/PMS system induced oxidation of SMX at initial pH 3.5–9.5. As illustrated in Fig. 1b, the SMX removal was inhibited at acid condition and only about 44.5% and 70.0% SMX were removed at pH 3.5 and 5.0, respectively. However, neutral condition was favorable for SMX removal, the enhanced removal efficiency of SMX was obtained at pH 6.5 (91.9%). By contrast, at pH 8.0, 80.3% of SMX removal can be achieved within 15 min, and it further decreased to 69.7% with the pH increased to 9.5. In parallel, the kobs of SMX removal at different initial pH followed the same trend. The kobs obtained at pH 6.5 (0.151 min−1) was 4.31-times and 2.19-times at initial pH of 3.5 (0.035 min−1) and 5.0 (0.069 min−1), respectively (Fig. S6). Since most of the actual wastewater possess a pH of 5–9, implying the established system has excellent potential in antibiotic degradation. As we all know, the hydrolysis of PMS will generate abundant H+ and further influence solution initial pH. Thus, the pH variation during the treatment process was examined. As expected, when the initial solution pH was 3.5, 5.0, 6.5, 8.0, and 9.5, as soon as PMS was introduced into the solution, the pH immediately dropped to 2.8–4.6 (Fig. S7a). Therefore, we further conducted the experiments controlled by buffer. As shown in Fig. 1c, the SMX removal was almost the same with the experiments performed without buffer. About 87.5%, 90.6%, and 70.3% of SMX was removed at pH 3.5, 5.0, and 6.5, respectively. However, the SMX removal was obviously decreased in the alkaline condition (60% for pH 8.0, 51.5% for pH 9.5) relative to the experiments free of buffer control, which further indicated that acid conditions favored PMS induced oxidation. Herein, the species-specific reactions among SMX, FeCo2S4-CN, and PMS were utilized to account for the pH-dependent variation of SMX removal. Since the pHpzc of FeCo2S4-CN was around 5.9 (Fig. S8a), thus, the surface of FeCo2S4-CN was positive charged at the initial pH of 3.5 and 5.0, while pH of 6.5, 8.0, and 9.5 was negative charged. Meanwhile, the SMX with pKa1 = 1.8 and pKa2 = 5.6 [47], were present as anionic species under the investigated pH range 3.5–9.5 (seen in Fig. 4

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Fig. 1. (a) Degradation of SMX in various systems, (b) impact of initial pH on the removal of SMX without buffer, (c) impact of initial pH on the removal of SMX with buffer, (d) impact of reaction temperature on SMX degradation, and (e) Plot of ln(kobs) versus 1/(T × 10−3) in FeCo2S4-CN/PMS oxidation. Reaction conditions: [SMX] = 19.7 μM, wt% = 15, [PMS] = 0.15 mM, [cata] = 20 mg/L, stirring speed = 300 rpm, T = 30 °C, and initial solutionpH = 6.5 (except for the impact of initial pH).

hydroxide complexes would be formed at alkaline condition, resulting in the passivation of catalysts and inducing adverse effect on catalytic oxidation of SMX, and the self-decomposition of PMS at a higher pH would be accelerated in base activated PMS may reducing the interaction between FeCo2S4-CN and PMS, thereby hampering the SMX removal efficiency. However, Ji et al. found that non-activated PMS

S8b and Table S2), the function of FeCo2S4-CN surface adsorbed PMS was enhanced at initial pH 3.5 and 5.0, and the interaction between FeCo2S4-CN and SMX could be enhanced. With the enhancement of pH value, the electrostatic repulsion between FeCo2S4-CN and PMS/SMX was reinforced, as well as the proportion of anionic form of SMX was increased, especially at alkaline condition, besides, the cobalt and iron 5

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contaminants. Herein, humid acid (HA), a typical case of NOMs, was taken into account in the catalytic oxidation of SMX. As depicted in Fig. 2d, slight adverse effect occurred in the presence of 2–25 mg/L HA, the SMX removal only decreased by 2.7%, 5.2%, and 7.4%, respectively. This phenomenon may be attributed to the abundant hydroxyl and carboxyl group of HA, which could retard the reaction between PMS and catalyst to produce less ROS [10]. In addition, Wenk and his colleagues have pointed out that phenolic moieties of NOM molecules could inhibit photochemical transformation of aniline compound in SMX by excited triplet state. Thus, the reduction of SMX radical cation by phenolic moieties in NOM molecules might also induce adverse effect of SMX removal [54], yet 84.5% of SMX removal was still achieved in the presence of 25 mg/L HA. The above results implied that not only the nonradical pathway indeed occur in the FeCo2S4-CN/PMS system, but also the role of radical pathway in accounting for the elimination of SMX could not be ignored.

oxidation of SMX in the pH range from 4.5 to 8.0 was favorable for the removal of SMX, due to the higher reactivity between anionic form of SMX and PMS [48]. Thus, the removal efficiency of SMX in the pH range investigated is still acceptable, except for strong acidic condition (pH = 3.5, without buffer). It should be noted that the use of buffer would cause adverse impact on contaminant removal to some degree. Therefore, no buffer was used for the following experiments. 3.3.2. Impact of temperature The removal of SMX was rapidly increased from 61.2% to 99.9% as the temperature was raised from 10 °C to 40 °C, and the kobs value was also elevated from 0.059 min−1 to 0.294 min−1 (Fig. 1d). The activation energy (Ea) for the SMX degradation in FeCo2S4-CN/PMS system was calculated by Arrhenius equation (Eq. (2)). The Ea value for SMX degradation was evaluated as 39.16 kJ mol−1 (Fig. 1e), which was much higher compared with the Ea value of diffusion-controlled reactions (in the range of 10–13 kJ mol−1), indicating that the apparent reaction rate of SMX degradation was mainly dominated by the inherent chemical reaction rate on the as-synthesized catalyst surface rather than the rate of mass transfer [49]. In addition, the Ea value of FeCo2S4-CN/PMS system was lower than what have previously reported for PMS activation via different heterogeneous catalysts, e.g., 40.67 kJ mol−1 on CuCo2S4 [20], 45.8 kJ/mol on BC (biochar)-Co3O4 [13], 50.59 kJ mol−1 on Co3O4-Bi2O3, and 61.7–75.5 kJ mol−1 on Co3O4/SiO2 [50,51]. The results implied that FeCo2S4-CN shown excellent potential for efficient removal of pollutant at mild ambient temperature, and the increased temperature resulted in the improved PMS self decomposition to generate more ROS.

lnk obs = lnA −

Ea RT

3.3.4. Impact of preparation conditions and other operating conditions In order to better evaluate the performance of FeCo2S4-CN/PMS system, experiments were carried out to further explore the impact of others key relevant parameters, such as FeCo2S4 content, sulfidation conditions, FeCo2S4-CN dosage, and PMS dosage on the SMX degradation in the FeCo2S4-CN/PMS system and the results are summarized in Text S5 of the Supporting Information. The optimal conditions of FeCo2S4 content and sulfur-containing precursor were obtained at 15 wt % with thioacetamide (TAA). In addition, the increase of catalyst dose and the neutral condition is conducive to PMS activation. However, the excessive PMS concentration (over 0.15 mM) lead to a decrease of SMX removal efficiency.

(2)

3.4. Identification of primary ROS in FeCo2S4-CN/PMS process

Here, kobs is the rate constant, R is the universal gas constant (8.314 J mol−1 K−1), and A is the pre-exponential factor.

3.4.1. Radical scavenging experiments It is well acknowledged that the role of ROS can be determined by the typical approach, quenching experiments and EPR tests. Thus, various scavengers were introduced into the FeCo2S4-CN/PMS system to verify the dominant ROS. EtOH was frequently used as a probe for both %OH (k2(%OH, EtOH) = 1.2–2.8 × 109 M−1 s−1) and SO4%− (k2(SO4%−, EtOH) = 1.6–7.7 × 107 M−1 s−1), while TBA was an effective quenching agent for %OH (k2 = 3.8–7.6 × 108 M−1 s−1)) and less sensitive to SO4%− (k2 = 4.0–9.1 × 105 M−1 s−1)) [55]. As demonstrated in Fig. 3a, the application of 100 mM EtOH or TBA showed a trivial impact on SMX oxidation, and about 80.7% and 86.9% of SMX could be eliminated, implying that only small amount of %OH and SO4%− were produced. This can be confirmed by further increasing EtOH and TBA to 200 mM which still induce insignificant adverse impact on SMX removal, decreasing the SMX removal efficiency from 91.9% to 73.4% and 83.5%, respectively. This phenomenon intrinsically different from the typical SO4%−-dominated system of Cobased/PMS and %OH-dominated system of Fe-based/H2O2 where the oxidations could be instantly terminated [56,57]. The results further demonstrated that both of %OH and SO4%− were generated, while neither %OH nor SO4%− was the dominant reactive species for the degradation of SMX. Since the nonradical degradation behavior was also found to occur in PMS-based AOPs, we further applied Furfuryl alcohol (FFA) and Lhistidine (L-his) with a moderate concentration (5 mM-10 mM) to explore the existence of 1O2 ((k2 (FFA, 1O2) = 1.2 × 108 M−1 s−1), (k2 (Lhis, 1O2) = 3.2 × 107 M−1 s−1)) [9,58]. In the presence of 5 mM and 10 mM FFA, SMX degradation by FeCO2S4-CN/PMS system was almost hampered (Fig. 3b) with only 33.5% and 11% removal in 15 min, respectively. Additionally, L-his was further employed as a chemical probe for 1O2. A minor amount of L-his (5 mM) caused a significant decline in SMX removal, which decreased from 91.9% to 19.7%. Correspondingly, the kobs decreased from 0.151 to 0.0038 min−1 (Fig. S9). When L-his was further increased to 10 mM, the SMX removal

3.3.3. Impact of inorganic anions and NOM In practical operations, the co-existing compounds in wastewater would have appreciable influence on the removal of target contaminants. Thus, we particularly examined the impact of inorganic ions, such as chloride (Cl−), bicarbonate (HCO3−), and phosphate (H2PO4−) ions (at a level of 2–25 mM), as well as humic acid (2–25 mg/L) on the elimination of SMX. Fig. 2a illustrates the SMX removal with the introduction of H2PO4−, it was found that H2PO4− had negligible adverse effects on the removal of SMX. With the addition of 25 mM H2PO4−, the removal efficiency of SMX only decreased from 91.9% to 81.3%, which may be owing to the deactivation of catalyst surface induced by phosphate, and the insignificant quenching effect of H2PO4− in nonradical dominated progress have also pointed out in previous study [52]. Similarly, the inhibitory effect was also observed in the case of HCO3− (Fig. 2b), the removal of SMX was decreased from 91.9% to 82.5%, 80.5%, and 76.8% with the HCO3− concentration increased from 2 to 10 mM and 25 mM, due to the consumption of the small amount of the generated SO4%−, %OH, and O2%−. Besides, the solution was adjusted to alkaline condition resulted from the buffer effect, which was unfavorable for the elimination of pollutants, the phenomenons were highly consistent with findings of Duan et al. [52]. Comparatively, the addition of Cl− showed almost no influence on the SMX degradation (Fig. 2c). On the one hand, Cl− at low concentration have no promoting effect, on the other hand, Cl− at high concentration have no obvious inhibitory effect, because it was reported that nonradical-dominated oxidation did not have the ability to oxidize Cl− into low reactivity reactive species (e.g. HOCl, Cl2 and Cl%) [53], thus the oxidation of SMX hardly disturbed by the presence of Cl−. The versatile natural organic matters (NOMs) can usually function as radical scavenger via competitive reactions with ROS in AOPs, resulting in the minimization of decomposition efficiency towards target 6

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Fig. 2. Impacts of (a) H2PO4−, (b) HCO3−, (c) Cl−, and (d) HA on SMX degradation. Reaction conditions: [SMX] = 19.7 μM, wt% = 15, [PMS] = 0.15 mM, [cata] = 20 mg/L, stirring speed = 300 rpm, T = 30 °C, and initial solution pH = 6.5.

hardly oxidized by PMS [61]. As depicted in Fig. S11b, the addition of low-concentration β-Carotene (20 mg/L) could impressively suppress the SMX degradation and only 8.9% SMX removal can be achieved, which further implying that the inhibition of SMX degradation resulted from 1O2 quenching by β-Carotene. Furthermore, Zhou et al. have pointed out that the presence of 1O2 could decay quickly to triplet oxygen (3O2), resulting in the increase of dissolved oxygen (DO) [62]. Based on above results, we further examined the change of DO under different circumstances. As illustrated in Fig. 3c, the DO concentration decreased from 8.25 mg/L to 7.32 mg/ L with the continuous stirring for 15 min. In the absence of PMS and FeCo2S4-CN, the DO concentration finally decreased to 7.22 mg/L and the decreased rate was more higher than the FeCo2S4-CN/system, while once PMS and FeCo2S4-CN were introduced into the solution at 15 min, the DO concentration was increased from 7.22 mg/L to 7.39 mg/L as time went on. This phenomena was highly consistent with previous work, providing another more convincing evidence for the existence of 1 O2 [9].

decreased from 91.9% to 11.4% and the kobs further decreased to 0.0015 min−1. Hence, the above results indicated that 1O2 was generated and most accounted for the SMX degradation in FeCo2S4-CN/PMS system. Noticeably, although FFA and L-his shown a high reactivity with free radicals (i.e., k2 (L-his, %OH) = 5 × 109 M−1 s−1 and k2 (FFA, % OH) = 1.5 × 1010 M−1 s−1), the inhibition effect of EtOH was negligible. Thus, the reaction of L-his or FFA with the radicals could be rule out, both L-his and FFA could be considered as the exclusive 1O2 quenchers in the established system. However, previous work have pointed out that FFA or L-his is a reducing agent which may induce rapid PMS consumption and cannot clearly identify the quenching effect toward 1O2 [59,60]. In order to affirm whether the inhibitory effect of L-histidine and FFA was primarily caused by the rapid PMS depletion, the decay of PMS in different systems was monitored. As illustrated in Fig. S11a, PMS alone could nearly not be consumed by SMX, whereas the presence of FeCo2S4-CN could consume PMS to a certain extent (78.7%). Likewise, a higher PMS decomposition was experienced in the presence of FFA or L-his (5 mM), 87% and 89% of PMS was consumed in 15 min, indicating that the quenchers can indeed accelerate PMS depletion, therefore, it may restrain the SMX removal to some extent. Hence, the β-Carotene, as a 1 typical scavenger of O2 (k2 (β-Carotene, 1 O2) = 2–3.0 × 1010 M−1 s−1) was further introduced into the quenching tests to verify the degradation mechanism since it was

3.4.2. EPR analysis EPR technique with DMPO or TEMP as spin trap agents were conducted to further confirm the above hypothesis. As shown in Fig. 3d, the typical peaks with the intensity ratio of 1:2:2:1 were observed in FeCo2S4-CN/PMS system, which could be ascribed to DMPO-%OH (with 7

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Fig. 3. (a) The quenching effect of free radicals on SMX degradation, (b) the quenching effect of singlet oxygen on SMX degradation, (c) the change of DO under different conditions, (d) EPR spectra obtained in different system using DMPO and (e) TEMP as spin-trapping agents. Reaction conditions: [SMX] = 19.7 μM, wt % = 15, [PMS] = 0.15 mM, [cata] = 20 mg/L, stirring speed = 300 rpm, T = 30 °C, initial solution pH = 6.5, [DMPO] = 50 mM, [TEMP] = 20 mM.

hyperfine coupling constants of αN = 15.05G and αH = 14.21G). However, the peak intensity was insignificantly and the characteristic DMPO-SO4%− could not be found, which unambiguously demonstrated that SO4%− and %OH were not the main ROS in the FeCo2S4-CN/PMS system. In addition, as the interaction time prolonged, the characteristic

peaks assigned to 5,5-dimethyl-1-pyrrolidone-2-oxyl (DMPOX, with hyperfine coupling constants of αN = 7.3 G, αH = 3.9 G), with the intensity ratio 1:2:1:2:1:2:1 was observed, which could be indexed to the direct oxidation of DMPO caused by nonradical species [63]. By comparison, no DMPOX signals were observed in PMS alone system. 8

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Zhou and his colleagues revealed that SO32− could function as the activator of PMS to induce ROS generation (Eqs. (7) and (8)) [64]. In order to verify this process, EPR spectra used DMPO as spin-trapping agent was acquired, and there was obvious DMPO-%OH EPR signal was observed when PMS and SO32− presented simultaneously (Fig. S12a), but neither DMPO-%OH nor DMPO-SO4%− signal was observed in the S2−/PMS system. Meamwhile, we found that the removal efficiency of SMX was slightly enhanced to 20% with the addition of 1.272 mg SO32− compared with the sole PMS system (14.3%), while the S2− was retardant with PMS (Fig. S12b). Thus, it was concluded that the S2− was inefficient for PMS activation, while SO32− was able to activate PMS to produce ROS, despite its contribution to ROS generation was negligible. Moreover, the promoted SMX removal rate was also observed with the introduction of revelatively low dosage of Fe3+ due to the existence of reductive sulfur species (Fig. S12c).

Therefore, EPR tests with 2,2,6,6-Tetramethylpiperidine (TEMP) as the trapping agents were performed to gain more evidences for the generated ROS via nonradical pathway. As illustrated in Fig. 3e, an inappreciable 1:1:1 triplet signal characteristic of TEMP-1O2 (with hyperfine coupling constants of αN = 17.24G) was observed in PMS alone system, and the intensity of TEMP-1O2 signals significantly enhanced accompanied by the addition of FeCo2S4-CN, suggesting the generation of abundant 1O2 and it function as the dominant ROS in FeCo2S4-CN/ PMS system. The results of EPR tests were well in accordance with the quenching experiments, indicating that 1O2 was mainly responsible for the degradation of SMX. 3.5. Mechanism of PMS activation by FeCo2S4-CN Previous studies have pointed out that 1O2 can be produced from multiple pathways, such as the combination of PMS/PS and carbonyl moiety (C]O) on carbonaceous materials, the self-decomposition of PMS/PS, photoexcitation of oxygen molecules by the energy-transfer process, the formation of quinones intermediates and external addition of quinones could supply carbonyl groups, as well the evolution of O2%− by Eq. (3). In the present work, the 1O2 generation may result from the direct oxidation of PMS, the recombination of O2%− or the interaction between g-C3N4 and PMS. In order to confirm our speculations, a series of experiments were conducted. 2O2%− + 2H2O → 1O2 + H2O2 + 2OH−

S2− + Fe(III)/Co(III) → S0↓ + Fe(II)/Co(II) 2−

S

+ 3H2O → SO3

2SO3

2−

+ 2H2O →

HSO3− + SO4%−

HSO5−

2−

−

+ 6e + 6H

2SO42−

→ SO4%− %

+

+ H2O → OH + H

(5)

−

+

(6)

+ H2O

(7)

+ 4e + 4H + SO3%−

(4)

+

(8)

(3) 3.5.3. Identify the role of g-C3N4 Since the g-C3N4 was recognized as the excellent and novel semiconductor, Shao et al. found that the Co3O4-g-C3N4 hybrids greatly enhanced the diclofenac sodium (DCF) removal in the presence of PMS [23]. Likely, in our work, the outstanding performance of FeCo2S4-CN in the presence of PMS may be attributed to the synergistic effect between FeCo2S4 and g-C3N4. This assumption was also verified by electrochemical analysis. The Tafel plots of FeCo2S4 and g-C3N4 possess a corrosion current 3.07 × 10−8 A and 3.88 × 10−7 A, respectively. On the contrary, the corrosion current of FeCo2S4-CN composite (5.45 × 10−6) was much higher than either of them (Fig. 5a). Meanwhile, the Nyquist plots of the FeCo2S4-CN displays a smaller semicircle diameter than the both FeCo2S4 and g-C3N4, thereby the combination of FeCo2S4 and g-C3N4 contribute to the enhanced charge transfer (seen in Fig. 5b). Moreover, the obvious strengthened TEMP-1O2 signal corresponding to the FeCo2S4-CN/PMS system relative to the FeCo2S4/PMS system in Fig. S12d provided additional evidence for the improvement of electron transfer resulted from g-C3N4. Given the electron-rich structure of g-C3N4, which contains heptazine rings with pyridinic nitrogen groups, six nitrogen lone pair electrons can act as electron donors, thus inducing the reduction of the PMS. Likely, the C atoms adjacent to N atoms can engaged as electron acceptor and PMS can function as electron donor due to the presence of peroxy bond of PMS [65], thus resulting in the oxidation of PMS and the generation of SO5%−, which further combine with H2O molecule to generate 1O2, as described by Eqs. (9) and (10) [66]. The electron transfer from PMS to g-C3N4 was evidenced by the remarkably enhanced current for the g-C3N4 electrode in the presence of PMS in the Tafel plots analysis (Fig. S13a), which provided another evidence for the oxidation of PMS. Likely, as illustrated in Fig. S13b, the concentration of residual PMS in FeCo2S4-CN/PMS system (0.032 mM) was lower than that in FeCo2S4/PMS system (0.058 mM). This phenomenon supported the observation of high-efficiency consume of PMS caused by the electron transfer from PMS to the electron deficient C atoms induced the the superior ROS generation, and similar results have been found in other work [67].

3.5.1. Verify the existence of superoxide radical The presence of O2%− can be verified by selective scavenger sodium carbonate (k2(O2%−, CO32−) = 5 × 108 M−1 s−1) [43]. Since the concentration of %OH was minor in this system, the inhibitory effect caused by the addition of carbonate could be mainly attributed to the scavenging of O2%−. As can be seen in Fig. 4a, the presence of 2 mM carbonate decreased the SMX removal from 91.9% to 79.2% in FeCo2S4-CN/PMS system. Further increase the carbonate concentration to 10 and 30 mM, 77.5% and 70% SMX removals were obtained, respectively. Additionally, BQ (benzoquinone), another more effective radical scavenger for O2%− (k2(O2%−, BQ) = 0.9–1 × 109 M−1 s−1) was selected to further confirm the adverse effect on the FeCo2S4-CN/ PMS system. As expected, the introduction of 2 mM BQ inhibited the degradation of SMX from 91.9% to 62.8%. Correspondingly, as shown in Fig. 4b, the DMPO-trapped EPR spectra for O2%− was observed. Furthermore, the hampered removal of SMX (61.5%) in the N2-saturated FeCo2S4-CN/PMS system was observed (Fig. 4c). The above results proved that O2%− did existed in the FeCo2S4-CN/PMS system and it was involved in the formation of 1O2. 3.5.2. Evaluate the role of sulfur species According to the XPS analysis of as-obtained FeCo2S4-CN samples, a series of sulfur species, including S2−, Sn2−, S0, SO32−, and SO42− were obtained, suggested that the S2− has been subjected to a certain degree of oxidation based on the Eqs. (4)–(6). Previous work have revealed that the lower electronegativity of sulfur was favorable to the redox active sites [19,20]. Thus, as determined by XPS analysis of used samples, the proportion of Fe(III) and Co(III) almost no change compared with the fresh samples. In the present work, electrochemical analysis was employed to confirm this hypothesis. As illustrated in Fig. 5a, in contrast to the Tafel profiles of FeCo2O4 (1.03 × 10−8 A), FeCo2S4 displayed a higher corrosion current,which was determined to be 3.07 × 10−8 A, implying a superior electron transfer capability of FeCo2S4. Moreover, the typical Nyquist plots shows that the FeCo2O4 had a higher semicircle diameter than the FeCo2S4 (Fig. 5b), further indicates that FeCo2S4 had a greater overall rate of charge transfer than FeCo2O4, since the replacement of oxygen with sulfur would result in a promoted cycling of redox couples [20].

HSO5− + g-C3N4 → SO5%− + H+ + e− 2SO5 9

%−

+ H2O

→ 2HSO4− +

1

1.5 O2

(9) (10)

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Fig. 4. (a) Determination of superoxide radicals by different scavengers, (b) spin-trapping EPR spectra for O2%− in different system, and (c) SMX removal in the N2saturated FeCo2S4-CN/PMS system. Reaction conditions: [SMX] = 19.7 μM, wt% = 15, [PMS] = 0.15 mM, [cata] = 20 mg/L, stirring speed = 300 rpm, T = 30 °C, initial solution pH = 6.5, and [DMPO] = 50 mM.

a certain degree, and the activation process can be reasonably expressed by Eqs. (11)–(17). Specifically, as there are a certain amount of Co(II) and Fe(II) on the surface of g-C3N4, which are capable of

3.5.4. The existence of radical pathway Both the quenching tests and EPR measurements demonstrateed that the radical pathway was also contributed to the removal of SMX to

Fig. 5. (a) Tafel plots and (b) electrochemical impedance spectroscopic analysis of various as-prepared samples. 10

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Fig. 6. Schematic illustration of the mechanism of PMS activation on FeCo2S4-CN.

capturing H2O lead to the formation of M(II)-OH− (M = Co and Fe) (Eq. (11)). Afterward, M(II)-OH− would react with HSO5− to generate SO4%− by the cleavage of O–O bond, and the produced SO4%− would further react with H2O/OH− to form %OH (Eqs. (12)–(17)). Generally, both the radical pathway (minor route) and nonradical (major route) process contributed to the excellent removal of SMX. Based on the above discussion, the evolution mechanisms of ROS in FeCo2S4-CN/PMS system are proposed in Fig. 6. Specifically, it can be concluded in the following four pathways: (i) the direct self-decomposition of PMS, (ii) the recombination of O2%− with H2O, (iii) the interaction between g-C3N4 and PMS (i.e, the oxidation and reduction of PMS), (iv) the Fe and Co induced radical pathway. M(II) + 2H2O → M(II)-OH− + H+ −

‾

(11)

M(II)-OH + HSO5 → MO + H2O + SO4 +

MO + 2H → M(III) + H2O +

+

‾

M(III) + HSO5 → M(II) + SO5 M(II)

+ HSO5‾

→ M(III) + SO5

of catalysts are vital for wastewater treatment. Bearing this in mind, successive experiments were conducted to evaluate the possibility of FeCo2S4-CN reuse. As displayed in Fig. S14a, the FeCo2S4-CN could be continuously used to catalyze PMS for the degradation of SMX with acceptable efficiency in the second run, and approximately 80% of SMX could be eliminated after 15 min. However, the removal efficiency gradually decreased to 67.8% in the third run. Additionally, the concentration of leached Fe and Co ions from the FeCo2S4-CN/PMS system in 15 min were monitored to further identify the stability of catalyst during the three cycles. As depicted in Fig. S14b, the concentrations of both leached Fe and Co ions were decreased accompanied by the increased runs. Specifically, the leached concentration of Fe and Co decreased from 0.03 to 0 mg/L and 0.068 to 0.022 mg/L, respectively. Based on the XPS analysis of fresh and used samples, we know that the S, Fe, and Co have suffered a certain degree of oxidation. Thus, it may be deduced that the loss of dopants and the surface-active sites caused by the adsorption of intermediates impaired the synergistic effect between FeCo2S4 and g-C3N4, resulting in the deactivation of the catalyst. Because of the poor electron transfer capacity caused by the oxidation of reductive S2−, which was unfavorable for the elimination of target pollutant. In order to evaluate the mineralization ability of the FeCo2S4-CN/ PMS system, the TOC analysis was also performed. As depicted in Fig. S14c, TOC decreased with time, and the final TOC removal efficiency was near 26.1%, which was higher than the mineralization efficiency of FeCo2S4-CN/PMS (26.1%), Co3S4/PMS system (20.24%), FeCo2O4/PMS (18.6%), PMS (1.25%), FeCo2S4/PMS (21.0%), FeCo2S4-CN (0.50%), and g-C3N4 system (0.51%). Generally, the excellent decontamination

%−

(12) (13)

%− %−

+H

+

(14)

+H

+

(15)

SO4%− + H2O → HSO4‾ + OH−

(16)

SO4%− + OH− → SO42‾ + OH−

(17)

3.6. Reusability and TOC analysis From the view of practical application, the reusability and stability 11

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Fig. 7. The proposed SMX degradation pathway in the FeCo2S4-CN/PMS system.

biisoxazol]-3′-yl)diazenyl)sulfonyl)aniline (m/z 348), 4-hydroxy-N-(5methylisoxazol-3-yl)benzenesulfonamide (m/z 255), N-(5-((4-methylbenzyl)thio)-1,3,4-thiadiazol-2-yl)-1-naphthamide (m/z 392) (seen in Table S3 and Figs. S15–21). On the basis of the degradation products structures elucidated, the possible degradation pathways of SMX in the FeCo2S4-CN/PMS system were proposed as illustrated in Fig. 7. The amine group (–NH2) on benzene ring of SMX could be easily attacked by ROS (SO4% O2%−, %OH, O2%−, and 1O2), leading to the formation of nitro derivative TP-1(NO2SMX), which was easily identified in AOPs based on direct ROS attacking on reactive site N in both radical and nonradical processes [2,68]. The nitro derivative SMX could be further electrophilic substituted by the non-selective %OH, resulting in the generation of TP-2. Additionally, the hydroxylation product TP-6 was initially formed by the ROS attack on the reactive site N of the –NH2 in the benzene ring,

ability is accompanied by satisfactory TOC removal efficiency, which further implied the mineralization efficiency of PMS-based AOPs could be remarkably enhanced by activation of FeCo2S4-CN. 3.7. Transformation products and SMX degradation pathway The intermediates of SMX obtained from FeCo2S4-CN/PMS system were analyzed by LC-QTOF-MS/MS. Based on the measurement of accurate mass of the transformation products and their main fragments. Totally, seven transformation products (TPs) were identified during SMX degradation, including N-(5-methylisoxazol-3-yl)-4-nitrobenzenesulfonamide (m/z 284), N-(4-hydroxy-5-methylisoxazol-3yl)-4-nitrobenzenesulfonamide (m/z 300), 5-methyl-3-((5-methylisoxazol-3-yl)diazenyl)isoxazol-4-ol (m/z 209), 1-butyl-3-((4-ethylphenyl) amino)pyrrolidine-2,5-dione (m/z 275), 4-(((4,4′-dimethyl-[3,5′12

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this product was also identified in CuFe2O4/hydroxylamine system [8]. Moreover, it was proposed that the interaction of the ROS with the –NH2 group results in the formation of N-centered radical, thus, the TP3 may derive form the coupling of the intermediate of TP-2 with the oxazole ring via recombination of the N-centered radical derived from –NH2 group. Likewise, the dimer TP-5 could also be regarded as the coupling of oxazole ring dimer (β-cleavage intermediate) with the 4aminobenzene sulfonate (ε-cleavage intermediate) through the recombination of the N-centered radical. The formation of TP-4 could be ascribed to the existence of strong oxidation of the ROS, inducing the opening of the isoxazole ring and then it could rearrange to a carbonyl at the N–C bond and the unsaturated N could be coupled with small molecular alkyl groups as well as the anilino group. The TP-4 further gives rise to TP-7 via the successive isoxazole ring opening accompanied by the polymerization of small molecular alkanes and benzene ring. Finally, partial by-products could be further mineralized to CO2 and H2O.

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4. Conclusions In this work, we conducted a comprehensive study to investigate PMS activation on FeCo2S4-CN for oxidation of SMX. The as-synthesized FeCo2S4-CN was superior to other metal-based catalysts (Co3S4-CN, FeCo2O4-CN, and FeCo2S4), and 91.9% of SMX removal can be achieved in 15 min. EPR spectra and competitive radical tests implied the simultaneous generation of nonradical and radical species (i.e., 1O2, O2%−, %OH, and SO4%−), and 1O2 was identified to be the major ROS responsible for SMX degradation in FeCo2S4-CN/PMS system. In the proposed mechanism, 1O2 was generated from a direct self-decomposition of PMS, the recombination of superoxide radicals, and the interaction between g-C3N4 and PMS. It was also found that there was synergistic effect between FeCo2S4 and g-C3N4. Importantly, the role of sulfur species in the catalytic degradation processes was clearly elucidated, the lower electronegativity of sulfur was favorable to the redox active sites. The S2− was inefficient for PMS activation, while SO32− exhibited catalytic activity in activation of PMS to produce ROS. This work paves a new avenue for the development of high-performance metal sulfide-based catalysts toward the PMS-mediated advanced oxidation water purification. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 51878423) and Sichuan Province Science and Technology Innovation Seedling Project (No. 2019084). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123361. References [1] Alam G. Trovó, Raquel F.P. Nogueira, Ana Agüera, Amadeo R. Fernandez-Alba, Carla Sirtori, Sixto Malato, Degradation of sulfamethoxazole in water by solar photo-Fenton. Chemical and toxicological evaluation, Water Res. 43 (2009) 3922–3931. [2] J. Du, W. Guo, H. Wang, R. Yin, H. Zheng, X. Feng, D. Che, N. Ren, Hydroxyl radical dominated degradation of aquatic sulfamethoxazole by Fe(0)/bisulfite/O2: Kinetics, mechanisms, and pathways, Water Res. 138 (2018) 323–332.

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