A mechanistic study of amorphous CoSx cages as advanced oxidation catalysts for excellent peroxymonosulfate activation towards antibiotics degradation

Chemical Engineering Journal 381 (2020) 122768

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A mechanistic study of amorphous CoSx cages as advanced oxidation catalysts for excellent peroxymonosulfate activation towards antibiotics degradation Xiaoyong Wua,1, Wenhui Zhaoa,1, Yanhong Huanga, Gaoke Zhanga,b,

T

⁎

a

Hubei Key Laboratory of Mineral Resources Processing and Environment, School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China b State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China

HIGHLIGHTS

GRAPHICAL ABSTRACT

hollow amorphous CoS hex• The agonal cage was prepared by MOF x

derived method.

elements in CoS cage presented • The multivalence. CoS cage was very efficiency for • The PMS activation towards TC degradax

x

tion.

synergetic effect of Co /Co • The and S /S was in favor of PMS 3+

2−

2+

2− 2

activation.

ARTICLE INFO

ABSTRACT

Keywords: Tetracycline Amorphous CoSx Hollow structure Advanced oxidation process

A catalyst with multivalent of elements and a hollow structure is very promising for peroxymonosulfate (PMS) activation. In this work, a novel, hollow, and amorphous CoSx hexagonal cage was prepared by an aqueous solution assisted solvothermal method for effective PMS activation towards antibiotics degradation via an advanced oxidation process. The detailed structure of CoSx was characterized by X-ray diffraction patterns, transmission electron microscopy and X-ray photoelectron spectroscopy. As expected, the as-prepared hollow, amorphous CoSx cages presented excellent tetracycline (TC) decomposing ability by PMS activation, which was much better than those of classic Co3O4 and Fe3O4 as well as the conventional Fenton reaction of Co3O4. In addition, the effect of parameters (PMS dosage, pH, reaction temperature, TC concentration, and catalyst content) on catalytic activity was studied in detail. Meanwhile, the influence of co-existing anions (Cl−, NO3− and HCO3−) in aqueous solution on TC degradation was also investigated. A possible mechanism for PMS activation was proposed based on a quenching experiment and an electron paramagnetic resonance (EPR) test. The results confirmed that the superior catalytic performance of CoSx by PMS activation could be contributed to the hollow structure and the effective recycling of Co3+/Co2+ and S2−/S22−. The as-prepared hollow amorphous CoSx cages could provide an effective catalyst for PMS activation towards wastewater/water treatment.

Corresponding author at: Hubei Key Laboratory of Mineral Resources Processing and Environment, School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China. E-mail address: [email protected] (G. Zhang). 1 Author made equal contributions manuscript. ⁎

https://doi.org/10.1016/j.cej.2019.122768 Received 11 May 2019; Received in revised form 23 August 2019; Accepted 6 September 2019 Available online 07 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

2. Experimental

Nowadays, antibiotics have been extensively found in the groundwater system and soil and are very harmful for the ecosystem and to human beings [1–3]. To remove or degrade these harmful antibiotics to safe levels from the natural environment, many strategies, including biological degradation, physical and chemical adsorption, and photocatalysis. have been developed [4–6]. However, from the viewpoint of practical application, the cost and treatment efficiency need to be further improved [5,7]. Thus, proposing a new method with higher efficiency and lower cost for contaminated aqueous solution treatment would be promising and meaningful. Recently, the advanced oxidation process (AOPs) has emerged as an effective and economic wastewater treatment method and has attracted much attention for refractory organic pollutants removal [8–11]. Typically, more effort has been made to address the treatment of sulfate radicals (SO4%−) based AOPs for aqueous refractory pollutants due to high oxidation ability, long half-life and wide pH range [12–16]. In fact, SO4%− can be produced from peroxymonosulfate (PMS) activation by heat, UV, ultrasound, carbon based materials and transition metal ions [17,18]. Among them, transition metal ion activation for PMS has attracted much more interest from researchers and cobalt ions present the highest activation performance [19]. Nevertheless, Co2+ related homogenous PMS activation has been limited in practical application due to its harmful effects and limited tolerance towards the environment [20,21]. In this case, a heterogeneous catalyst for PMS activation has been rapidly developed. More recently, some researchers have paid particular attention to sulfide based materials for PMS or persulfate activation towards refractory organic contaminants degradation owing to their high efficiency and stability [22–24]. For instance, Fan, et al. used FeS to activate persulfate for the removal of p-chloroaniline. Their experimental results found that the existence of sulfur ions was beneficial for the recycling of Fe2+/Fe3+ in the reaction solution [23]. Xu, et al. reported that CuCo2S4 could effectively activate PMS for the degradation of bisphenol S, which was superior over conventional cobalt and copper oxides [24]. Zhu et al. also prepared graphene-supported hollow cobalt sulfide nanocrystals as a catalyst, which were very efficient for bisphenol A degradation [25]. Nevertheless, the effect of reaction conditions (e.g., PMS dosage, pH, reaction temperature, and catalyst content), coexisting anions on PMS activation as well as various antibiotics degradation and a detailed activation mechanism towards pure cobalt sulfide still needs to be further studied. Besides, for SO4·- based AOPs, the catalyst is usually crystalline, while the study of amorphous materials for PMS activation is rare but meaningful. More importantly, it has been well recognized that transition metal compounds with multivalence are greatly favored for PMS activation since the multivalent element is good for recycling ions in PMS activation [26,27]. Therefore, research into the development of amorphous CoSx particles with multivalence for PMS activation for effective antibiotics removal as well as the effects of reaction conditions and a detailed activation mechanism study, would be very promising. In this work, hollow amorphous CoSx cages have been successfully prepared by a MOF derived method and for the first time utilized as a PMS activation catalyst towards tetracycline (TC), ciprofloxacin (CIP), bisphenol A (BPA), methyl orange (MO) and rhodamine B (RhB) degradation. Particularly, cobalt and sulfur both presented multivalence in CoSx. The catalytic activity of CoSx was compared with the classical Co3O4 and Fe3O4 as well as the Fenton reaction of CoSx. Meanwhile, the degradation performance of CoSx towards TC and CIP coexisting in an aqueous solution was investigated. In addition, the effect of various parameters on the catalytic properties was also studied in detail. The degradation mechanism of CoSx by PMS activation was further clarified by the combination of quenching experiments, ESR, LC-MS and XPS tests.

2.1. Preparation of amorphous CoSx cages To synthesize amorphous CoSx, the ZIF-67 was first prepared as previously reported [28]. Typically, certain amount of cobaltous nitrate and 2-methylimidazole were respectively dissolved in 40 mL of methanol. Next, the cobaltous nitrate solution was added dropwise into the 2-methylimidazole solution with strong magnetic stirring and then aged for 12 h at room temperature. Finally, ZIF-67 was obtained by centrifugation and washed several times with methanol. The obtained ZIF67 particles were dispersed in 10 mL ethanol for further use. Secondly, 0.2 mL of the as-prepared ZIF-67 suspension was mixed with 80 mL ethanol with fierce stirring for 15 min. Then, 3 g of thioacetamide was added into the above solution and mixed with magnetic stirring for another 15 min. After that, the solution was transferred to an autoclave and heat treated at 120 °C for 5 h. The final amorphous CoSx was obtained by centrifuging, washing and drying at 60 °C in a vacuum oven overnight. In addition, it has been well recognized that Co3O4 and Fe2O3 are the classic active catalysts in SO4%− based AOPs [22,23]. So to demonstrate the effective catalyst performance of the as-prepared CoSx, Co3O4 and Fe2O3 were also prepared as comparison samples. The Co3O4 was synthesized by the hydrothermal reaction of Co(C2H3O2)2·4H2O and NH3 solution at 180 °C for 12 h. The Fe2O3 was also fabricated by a hydrothermal reaction of NaOH and FeCl3·6H2O at 160 °C for 24 h. 2.2. Characterization The structure of CoSx was confirmed by X-ray diffraction (XRD) (X’Pert PRO diffractometer). The element surface state of samples was determined by X-ray photoelectron spectra (XPS) (Thermo ESCALAB 250XI). The detailed microstructures of samples were obtained by transmission electron microscopy (TEM) (Talos F200S). The surface information of samples was acquired by N2 adsorption/desorption isotherms (Micromeritics ASAP 2020). The active species that existed in the PMS degradation process were measured by an electron paramagnetic resonance (EPR) spectrometer (A300-10/12) using 5,5-dimethyl-1-pyrroline Noxide (DMPO) in methanol as a solution. The intermediates produced by TC degradation were evaluated by HPLC-MS (Thermo Finnigan LCQ-Deca-XP). 2.3. Catalytic activity The catalytic activities of samples were investigated by PMS activation towards various organic contaminants (TC, BPA, CIP, MO and RhB) degradation at room temperature. Representatively, 20 mg of catalyst was added into 100 mL of 30 mg/L TC solution, and then fiercely stirred in the dark for 30 min to keep an adsorption–desorption equilibrium. The initial pH value of the YC solution was adjusted by 1 mol/L HCl and NaOH solutions. Then, certain amounts of PMS were added into the above suspension solution to start the reaction. At specific time intervals, about 5 mL of solution was withdrawn and immediately 1 mL of methanol was added to quench the reaction. The concentration of TC in the solution was checked using a UV–visible spectrophotometer (UV-1100). The concentrations of BPA, CIP, MO and RhB were also evaluated by UV–visible spectrophotometry but with various absorption peaks. 3. Results and discussion 3.1. Characterization of samples The shape and structure determination of CoSx was carried out by SEM and TEM. As shown in Fig. 1a, the sample presented uniform hollow hexagonal cages with a diameter of ca.500 nm, a structure 2

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which can provide a large specific surface area as compared to solid particles and also favors mass transfer during PMS activation towards contaminant degradation. The inserted SEAD patterns indicated an amorphous structure of the sample. It also can be clearly seen in Fig. 1bd that the elements of Co and S were uniformly distributed in the hollow hexagonal cages and the amorphous cages consisted of CoSx. To get more information about the elemental distribution and composition in the cages, the HAADF-EDS lines of the sample were measured (Fig. 1e). The shape of the HAADF-EDS lines over CoSx was consistent with a hollow cage morphology. Further studies of the samples’ structure were conducted by XRD and BET analysis. Fig. S1a shows the XRD patterns of CoSx and a comparison with Fe3O4 and Co3O4 samples. No obvious diffraction peaks were observed for the CoSx sample, indicating that the structure of CoSx was amorphous which was in good agreement with the SAED result presented in Fig. 1a. As for Fe2O3 and Co3O4, the diffraction peaks were all well indexed to the corresponding crystals and no other impurity peaks were found. The BET and pore size distribution of the samples were conducted by an N2 adsorption-desorption measurement as shown in Fig. S1b. The specific surface area of CoSx was 15.3 m2/g, which was much higher than that of Co3O4 (6.7 m2/g). Besides, all of the profiles were fitted to the type-IV isotherm and an H3 hysteresis loop, implying the mesoporous structure of CoSx [29], which was consistent with the above TEM results, and the corresponding pore size was mainly located in the range of 100–150 nm. The unique hollow structure could provide more active sites and channels, which is beneficial for contaminant adsorption, mass transport and PMS activation [30].

amorphous CoSx cages under different conditions. The CoSx sample presented negligible adsorption for TC. With only OMS ca.18% TC was degraded, indicating that PMS can self-decompose to produce active radicals for TC degradation. Notably, in the presence of PMS and CoSx particles, nearly 90% TC could be degraded in 5 min and 100% degradation could be achieved in 20 min, suggesting that PMS activation by CoSx for TC degradation had occurred. To examine the effects of CoSx towards TC degradation by PMS activation, the TC removal ability of Co3O4 and Fe2O3, as well as the Fenton reaction of CoSx, were also tested for comparison. It can be clearly seen in Fig. 2a that the Co3O4 and Fe2O3 comparison samples showed similar TC degradation ability, i.e. ca. 50% of TC was decomposed in 30 min, indicating that the PMS activation for TC degradation was much more efficient by CoSx than by classic Co3O4 and Fe2O3. As for the Fenton reaction, it exhibited only 42% TC removal, meaning that the PMS activation towards TC removal was much more active than that of the Fenton reaction over CoSx. Besides, the TC degradation followed a pseudo-first-order kinetics model. The degradation rate constant Kobs of CoSx was 0.151 min−1, which was about 8.8, 8.8 and 9.6 times faster than those of Co3O4 and Fe2O3 induced by PMS activation and the Fenton reaction of CoSx, respectively. The reason for this superiority of PMS activation by CoSx will be explained in Section 3.2.5. Therefore, the above discussion verifies that the as-prepared hollow CoSx was an excellent activator for PMS towards TC degradation, and the following discussion will use CoSx as a representative sample to study the effect of other impacts on the PMS activation for TC degradation. 3.2.2. Effect of various parameters on TC removal by CoSx To get the optimal degradation capability by PMS activation over CoSx, the effect of PMS dosage, TC concentration, pH, catalyst concent, and reaction temperature should be considered. Fig. 3 shows the influence of the reaction parameters on the TC degradation by PMS activation over CoSx. First, the effect of PMS content was studied in Fig. 3a. It is explicit that without PMS, only a slight amount of TC was removed, which should be due to the adsorption of the catalyst.

3.2. Catalytic performances of samples 3.2.1. Catalytic comparison In this work, the catalytic performance of samples was mainly evaluated by degradation of TC aqueous solution via PMS activation. Fig. S2 presents the corresponding catalytic activity of hollow

Fig. 1. TEM image (the insert is SEAD) (a), element mapping (b-d), and HAADF-EDS lines (e-g). 3

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Fig. 2. The TC degradation of various samples by PMS activation as well as the Fenton reaction of CoSx (a) and the corresponding kinetic curves (b).

However, with the addition of PMS, the TC degradation reached 90% in 15 min with 0.1 g/L of PMS, and as the PMS content increased, the TC degradation could achieve 100% in 20 min. In addition, with the rise of PMS content, the TC degradation rate was increased, which should be

because as the PMS content increased, more PMS could be activated and subsequently more active radicals could be produced for TC degradation. Second, the impact of TC concentration is presented in Fig. 3b. When the TC concentration was 20 and 30 mg/L, the TC could

Fig. 3. The effect of PMS content (a), TC concentration (b), pH (c), catalyst dosage (d) and reaction temperature (e) on the TC degradation by hollow amorphous CoSx cages. 4

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be completely degraded in 20 min. As the TC concentration increased to 40 mg/L, the TC degradation ability of CoSx decreased about 10%, which should be assigned to the fact that as the TC concentration increased, and more active sites on the surface of CoSx could be occupied by TC and affect the TC degradation adversely. The third impact factor was the pH in the reaction solution. It is well known that pH value can greatly affect the catalytic performance for the AOPs [31,32]. Fig. 3c shows that TC could be effectively degraded in the pH range of 5–9 by CoSx, meaning that CoSx was very active in weakly acidic and alkaline conditions in this work, which is a great advantage for practical applications. However, when the pH value was 3, the TC degradation ability of CoSx decreased about 20%. This decrease should be because in acidic conditions, PMS is more stable and harder to be activated than in neutral and alkaline conditions, so less active radicals could be produced for TC removal [33]. Furthermore, PMS produced radicals could be partially scavenged by H+ so that the TC degradation ability declined [18]. The fourth impact factor is the catalyst dosage. It is explicit in Fig. 3d that as the CoSx dosage increased, the TC degradation became better, a trend similar to that of PMS content. With the increase of catalyst dosage, more active sites could be provided and more PMS could be activated so that the TC removal property could be improved. The final parameter is reaction temperature. The reaction temperature is also very important for the catalytic performance. Generally, the higher the reaction temperature, the better the contaminant removal performance would be. Fig. 3e presents the TC degradation ability with respect to various reaction temperatures. It is clear that when the reaction temperature increased from 15 °C to 45 °C, the TC degradation went up from 89% to 100% in 20 min. This increase should be due to the endothermic quality of the PMS activation reaction [34]. Based on the above parameters studied, the PMS content, TC concentration, pH, CoSx dosage, and reaction temperature for the following discussion were 0.3 g/L, 30 mg/L, 5, 0.2 g/L and 25 °C, respectively. 3.2.3. Effect of anions on TC removal During the PMS activation process

degradation, anions, which commonly exist in water solutions, would probably act as scavengers and react with the produced active radicals [35]. Therefore, the effect of anions on the TC degradation property should be investigated. Fig. 4 displays the influence of Cl−, NO3− and HCO3− on the TC removal property. It is clear in Fig. 4a that with the addition of Cl− in the TC solution, the TC removal ability of CoSx declined a little. This decrease should be due to the scavenger role of Cl− for SO4%− (Eqs. (1)) and also some reactive species, less active than SO4%−, would be produced such as Cl2 (1.36 V) and Cl2%− (2.09 V) as shown in Eqs. (2)–(4) [36]. SO4·− + Cl− → SO42− + Cl·

Cl· + H2O → ClOH·− + H+(2) ClOH·− + Cl− → Cl2·− + OH−(3) Cl2·− + Cl· → Cl2 + Cl−(4) Fortunately, only slight inhibition by Cl− was observed in this work. As for the effect of NO3− (Fig. 4b), it presented negligible inhibition for TC removal. Fig. 4c demonstrated the influence of HCO3−. When the HCO3− concentration was small, no obvious effect was seen, however, as the HCO3− concentration in the TC solution was further increased to 10 mM or more, the TC degradation ability was significantly inhibited, and the TC degradation ability reduced from 100% to 30% in 20 min. It is well known that HCO3− can not only be a buffer, but it is also a SO4%− scavenger and that HCO3− can scavenge the active SO4%− and OH% species (Eqs. (5) and (6)) to produce much weaker oxidation species [37]. When 50 mM HCO3− was added into the TC solution, the pH value of the solution changed from 5 to 8. Fig. 3c shows CoSx presented similar TC degradation ability in the pH range of 5–9. So after HCO3− addition, the dramatic decrease of the TC degradation ability should mainly be due to the scavenger role of HCO3− toward SO4%− species. HCO3− + SO4·− → SO42− + HCO3· −

·

HCO3 + OH → H2O + toward

(1)

CO3·−

contaminant

Fig. 4. Effect of Cl− (a), NO3− (b), and HCO3− (c) on the TC degradation ability over CoSx. 5

(5) (6)

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3.2.4. Catalytic universality and stability In order to check the catalytic universality of CoSx in contaminant removal, the degradation towards CIP, BPA, MO, and RhB by PMS activation was also evaluated as shown in Fig. 5a. The CIP and BPA could be degraded up to 90% in 20 min, while MO and RhB could be completely removed in 20 min, all of which indicates that CoSx is a very active catalyst for PMS activation towards various pollutants. Furthermore, to examine its effectiveness, the degradation of CIP and TC coexisting solution by CoSx was also conducted (Fig. 5). The two characteristic absorption peaks belonging to TC and CIP are displayed in Fig. 5b, and after the PMS was added into the CIP and TC co-existing solution in the presence of CoSx, the absorption intensities for these two peaks significantly decreased as time elapsed. From the corresponding degradation lines presented in Fig. 5c, it can clearly be seen that after 20 min, TC was completely removed and 90% of CIP was degraded. The degradation ability of each was equal to the sole degradation conditions. Besides, the catalytic stability of CoSx was also investigated as demonstrated in Fig. S3. After 4 runs, the degradation ability declined only ca.6%, implying good removal stability of the sample. Based on the above discussion, it can be seen that the as-prepared hollow amorphous CoSx cages are an excellent catalyst for PMS activation towards various contaminant’s degradation, even in a wide pH range and when other anions or pollutants coexist in solution. As compared to recent studies about TC degradation by PMS activation based AOPs [38–40], the asdeveloped amorphous CoSx cages in this work have outstanding TC degradation performance. Moreover, from the viewpoint of practical application, the effect of various reaction parameters, coexisting anions on TC degradation, and possible TC degradation pathways were also systematically investigated.

important to check for the existence of active radicals in PMS activation induced catalytic activity. To confirm the existence of the SO4%−, ·OH and O2%− radicals, EPR analysis was employed using DMPO as a spin trapper. As shown in Fig. 6a, when only PMS was added in solution without the CoSx catalyst, no obvious signals referring to radicals were observed. While, when PMS and the catalyst coexisted in solution, 6line and 4-fold signals could be cleared viewed, which indicate the existence of SO4%− and %OH, respectively. As for the O2%− radicals, corresponding signals were also exhibited in Fig. 6b with the presence of PMS and catalyst, so the SO4%−, ·OH and O2%− radicals were in fact produced in PMS activation over the CoSx catalyst. To further study the effect of active radicals on the catalytic performance, trapping experiments were done as illustrated in Fig. 7. In this work, tert-butyl alcohol (TBA) was used to check for the existence of %OH with a rate k = 7.6*108 M−1 s−1 which was much faster than that of SO4%− (k = 9.1*105 M−1 s−1), and methanol (MeOH) was utilized to determine the effect of both SO4%− and %OH with corresponding rates of k = 9.7*108 M−1 s−1 and 3.2*106 M−1 s−1, respectively [43]. Meanwhile, 4-benzoquinone (BQ) was employed to estimate the presence of O2%−. As presented in Fig. 7a, when MeOH was added into the TC solution, the catalytic activity of CoSx dramatically decreased and with incremental MeOH addition, more degradation ability towards TC was inhibited. As TBA was added, only slight decrease for TC degradation was exhibited (Fig. 7 b), indicating that SO4%− radicals played an essential role in TC degradation but only a little by ·OH. In addition, when 2 mg BQ was introduced into the TC solution, no obvious decrease in TC degradation was presented as compared to that of the initial TC solution without scavenger. However, as BQ content increased to 20 mg, about 22% of the degradation ability was inhibited, implying that O2%− contributed to the TC degradation to some extent. Therefore, SO4%−, ·OH and O2%− radicals all made contributions to the TC degradation, and SO4%− made the main contribution followed by O2%− and %OH. It is well recognized that SO4%−, ·OH and O2%− radicals are produced by PMS activation through the multivalence of elements in a

3.2.5. Catalytic mechanism It is well known that during the PMS activation process, active radicals such as SO4%−, ·OH and O2%−, will be produced to attack contaminants and then react for degradation [41,42], so it is very

Fig. 5. Catalytic performance of CoSx as a function of different contaminants (a), absorption spectra of CIP and TC coexisting in solution at various PMS activation times (b) and the corresponding degradation ability (c). 6

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Fig. 6. EPR spectra of DMPO-SO4%− and DMPO-%OH (a) and DMPO-O2%− (b) in CoSx existed systems.

Fig. 7. TC degradation ability of CoSx with respect to various scavengers (MeOH (a), TBA (b), and BQ (c)).

Fig. 8. XPS spectra of fresh and used CoSx: Co 2p (a) and S 2p (b).

7

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catalyst. So, to study the valence status of elements for catalysts, high resolution XPS spectra of prestine and used CoSx were carefully evaluated and are exhibited in Fig. 8. In Fig. 8a, before the PMS activation process, the Co 2p XPS spectrum could be fit to three pairs of peaks. The first pair peaks located at 779.5 and 794.6 eV should be ascribed to Co3+, the second pair peaks situated at 782.0 and 797.6 eV should be attributed to Co2+ and the final pair of peaks at 785.2 and 802.5 eV should be assigned to satallites of Co. After TC degradation, the Co 2p XPS spectrum also could be reassigned to three pairs but with various peak areas. The relative intensity of peaks belonging to Co2+ significantly increased as compared to that of the pristine one, indicating that after TC degradation, a large amount of Co3+ was transformed into Co2+. As for the S 2p spectra in Fig. 8b, before the reaction, the S 2p spectrum could be fit with four peaks located at 162.7, 163.7, 164.7 and 169.2 eV, which belonged to S2−, S22−, Sn2− and SOx, respectively [44]. After the reaction, it could still be fit with four peaks but with various peak areas. The peak area belonging to S2− decreased and the ones corresponding to S22− and SOx were dramatically increased, implying that some S2− had oxidized into S22− and SOx. Based on these XPS analyses, the probable PMS activation and active species production process could be proposed as follows (Fig. 9): First, Co (II) could be oxidized into Co(III) via activation of PMS (HSO5−) to produce SO4%− species (Eq. (7)). Meanwhile, Co (II) could be regenerated by the reduction of PMS (Eq. (8)); Besides, SO4%− species could react with H2O or OH− to generate ·OH (Eq. (9)) and O2%− could be produced by Eqs. (10)–(12); Finally, the produced SO4%−, O2%−, and %OH would attack TC to degrade it and then produce CO2 and H2O or intermediates (Eq. (14)). More essentially, S2- could also reduce Co(III) into Co(II), as referred to Eq. (13). So the existence of multivalent of sulfur in CoSx was beneficial for Co (II)/Co(III) recycling and greatly favored PMS activation for TC degradation [20,30]. Co (II) + HSO5− → Co(III) + SO4·− + OH−

(7)

Co (III) + HSO5− → Co(II) + SO5·− + H+

(8)

SO4·− + H2O/OH− → SO42− + ·OH

(9)

HSO5− +

H2O → H2O2 + HSO4

·OH + H2O2 → HO2·

−

Fig. 10. The possible degradation process of TC solution by PMS activation over CoSx.

HO2· → H+ + O2·−

(12)

Co(III) + S2− → Co(II) + S22− /SOx

(13)

−

−

SO4· /O2· /·OH + TC → CO2 + H2O + intermediates

(14)

To further investigate the degradation mechanism of CoSx by PMS activation, LC-MS was employed to verify the TC degradation intermediates and a possible corresponding pathway is proposed in Fig. 10 based on previous reports [45–47]. The starting m/z value of 445 should be assigned to the original TC; after the PMS activation produced the SO4%−, O2%−, and %OH active species, the TC was oxidized in two various ways, through deamidation, hydroxylation and ring open reaction. Finally, some of the intermediate products would be decomposed into H2O and CO2.

(10) (11)

Fig. 9. Schematic diagram of PMS activation by CoSx towards TC degradation. 8

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4. Conclusions [13]

Hollow amorphous CoSx cages have been successfully prepared by a MOF derived method. Owing to the large specific surface area and multivalence of elements in CoSx, the CoSx cages exhibited excellent TC degradation ability by PMS activation. Furthermore, with an increase of PMS dosage, catalyst content and reaction temperature, the TC degradation performance improved but decreased with increasing TC concentration. Meanwhile, CoSx was active in the pH range 5–9. In addition to TC, CoSx could also degrade BPA, CIP, MO and RhB and was even effective when TC and CIP coexisted in aqueous solution. The EPR and trapping experiments found that the SO4%− species was the main active radical for TC degradation followed by O2%− and %OH. The S2−/ S22−/Sn2− in CoSx was beneficial for Co(II)/Co(III) recycling so that PMS could be effectively activated for high performance of TC degradation. The as-synthesized hollow amorphous CoSx cages would provide an effective catalyst for PMS activation towards wastewater treatment.

[14] [15] [16] [17] [18] [19] [20]

Acknowledgements

[21]

X.Y. Wu and W.H. Zhao made equal contributed to this paper. G.K. Zhang and X.Y. Wu proposed the project; W.H. Zhao conducted the experiments; X.Y. Wu wrote the paper. Y.H. Huang and G.K. Zhang discussed the results. This work was supported by the NSFC (No. 51472194 and 51602237) and the NSF of Hubei Province (2016CFA078).

[22] [23] [24]

Appendix A. Supplementary data

[25]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122768.

[26]

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