Metal Organic Framework with Coordinatively Unsaturated Sites as

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Metal organic framework with coordinatively unsaturated sites as efficient Fenton-like catalyst for enhanced degradation of sulfamethazine Juntao Tang, and Jianlong Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00092 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Environmental Science & Technology

Metal organic framework with coordinatively unsaturated sites as efficient Fenton-like catalyst for enhanced degradation of sulfamethazine Juntao Tang a, Jianlong Wang a,b *

a

Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET,

Tsinghua University, Beijing 100084, P.R. China b

Beijing Key Laboratory of Radioactive Wastes Treatment, Tsinghua University,

Beijing 100084, P.R. China

*

Corresponding author:

E-mail: [email protected]

1

ACS Paragon Plus Environment


TOC Art

2


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Abstract In this paper, a novel Fenton-like catalyst, metal organic framework MIL-100(Fe) with

FeII/FeIII

mixed-valence

coordinatively

unsaturated

iron

center

(CUS-MIL-100(Fe)), was synthesized, characterized and used for the degradation of sulfamethazine (SMT). The catalytic performance of CUS-MIL-100(Fe) was investigated on the basis of various parameters, including initial pH, H2O2 concentration, catalyst dosage and initial SMT concentration. The results showed that CUS-MIL-100(Fe) could effectively degrade SMT, with almost 100% removal efficiency within 180 min (52.4% mineralization efficiency), under the reaction conditions of pH 4.0, 20 mg L-1 SMT, 6 mM H2O2, and 0.5 g L-1 catalyst. Moreover, CUS-MIL-100(Fe) displayed a higher catalytic activity than that of MIL-100(Fe) for SMT degradation. Combined with the physical-chemical characterization, the enhanced catalytic activity can be ascribed to the incorporation of FeII and FeIII CUS (coordinatively unsaturated metal sites), the large specific surface area as well as the formation of mesopores. Furthermore, CUS-MIL-100(Fe) exhibited a good stability and reusability. The possible catalytic mechanism of CUS-MIL-100(Fe) was tentatively proposed.

Keywords: Fenton-like catalyst; metal-organic framework; coordinatively unsaturated metal sites; sulfamethazine

3



1. Introduction Advanced oxidation processes (AOPs), as highly effective, promising and environmentally-friendly technologies, have been extensively utilized in the removal of various hazardous or recalcitrant organic pollutants from wastewater [1-3]. Among the AOPs that generally rely on the generation of highly reactive free radicals such as hydroxyl radical (•OH), Fenton reaction (Fe2+/Fe3+ + H2O2) is particularly preferable due to its higher •OH radical generation rate, mild reaction conditions, simple operation, inexpensive materials and low maintenance [4,5]. Nevertheless, several inherent drawbacks of homogeneous Fenton reaction, such as the narrow working pH range, iron-containing sludge production and impossible regeneration of catalyst, remarkably impede its further application on a large scale [6]. To overcome such problems associated with homogeneous Fenton reaction, heterogeneous Fenton reaction using insoluble solid catalysts has been developed because of its high stability, wider pH range, and facile recovery [7-9]. To date, various iron-based catalysts, such as zero-valent iron [10], iron-oxygen compounds [8,11,12], Fe-based metallic glasses [13,14] and Fe-immobilized materials [15-17], have been proved to be effective for catalytic activation of H2O2. Likewise, most of these heterogeneous Fenton catalysts usually exhibit favorable catalytic activity at acidic or neutral condition. Besides, iron leaching cannot be completely avoided during the operation, resulting in a significant deactivation of catalysts. Accordingly, it is extremely desirable to explore highly efficient and stable heterogeneous Fenton-like catalysts 4


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with wide working pH range. As an emerging category of multifunctional porous materials, Fe-based metal organic frameworks (MOFs) have received substantial attention in recent years and have offered a potential platform for applications in gas adsorption/separation [18,19], drug delivery [20], and catalysis [21,22]. These materials are usually self-assembled from metal ions coordinated to polyfunctional organic ligands and possess a well-defined periodic network structure [23]. Compared with conventional Fenton-like catalysts, MOFs with the peculiar structure have many fascinating properties, for example, large specific surface area, abundant nanoscale cavities, tunable porosity, open pore channels, and good thermal stability [24]. In particular, the rich nanoscale cavities and open pore channels can provide particularly favourable pathways for entering and escaping molecules, which could help in overcoming the mass transport limitation during catalysis. In addition to their fascinating properties mentioned above, the well-dispersed metallic components in MOFs can also serve as active sites for catalytic reactions [25]. With these intriguing characteristics, MOFs have been recently anticipated as superior candidates for heterogeneous Fenton-like catalysts [25,26]. However, only a few MOF structures have shown attractive performances in heterogeneous Fenton reaction, because the metal nodes in the majority of MOFs are fully occupied by coordinated organic ligands, which often blocks the active metal sites and makes them unavailable for H2O2 activation [27]. In this regard, the incorporation of coordinatively unsaturated metal sites (CUS) in 5



MOFs seems to be a feasible strategy to provide more exposed active metal sites as catalytic centers for •OH generation. In principle, coordinatively unsaturated metal sites, also referred to as open metal sites, can be created by removing some non-bridging terminal ligands (e.g. H2O, halogen/hydroxide anionic ligands) from the metal nodes at elevated temperatures under inert atmosphere or vacuum condition. The formed CUS are accessible to guest molecules (adsorbates or substrates), which would bring a better affinity of incoming guest molecules with MOFs, and thus enhance the adsorption capacity of MOFs [27,28]. Besides, CUS, due to the Lewis acidity of metal cations, can behave as Lewis acid sites, while H2O2, as a Lewis base, tends to adsorb onto the Lewis acid sites [25,27,28]. During the heterogeneous Fenton process, the presence of CUS may induce regioselectivity towards H2O2 molecules, which can facilitate the interaction between CUS and H2O2. Furthermore, given that the electron-rich ligands would donate electrons to the center metal ions, the CUS obtained from the departure of those terminal ligands could further promote decomposition of H2O2 to •OH because of their reducibility [28,29]. Hereto, it is believed that constructing CUS on the structure of MOFs would create a high-efficiency heterogeneous catalyst for Fenton-like degradation of organic pollutants. As one of the most prominent MOF material, MIL-100(Fe) with chemical composition FeIII3O(H2O)2F·{C6H3(CO2)3}2·nH2O (n∼14.5) is extremely attractive in virtue of its relatively easy synthesis, excellent chemical stability and water stability 6


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[30,31]. Apart from this, MIL-100(Fe) has been successfully used as Fenton-like catalyst for the decolorization of azo-dye wastewater, yet the dissatisfactory performance of pure MIL-100(Fe), derived from the lack of coordinatively unsaturated metal sites, still needs to be improved [31,32]. Toward this end, it is of significance to choose MIL-100(Fe) as a substrate material to fabricate novel MOF-based Fenton catalyst by incorporating CUS and exploit its corresponding high catalytic activity. Therefore, in this study, MIL-100(Fe) with FeII/FeIII mixed-valence coordinatively unsaturated iron center (CUS-MIL-100(Fe)) was synthesized and employed as Fenton-like catalyst. Sulfamethazine (SMT), one of the commonly used sulfonamide antibiotics, was chosen as a target pollutant because of its ubiquitous occurrence in aquatic environment, adverse effects on living being, and non-biodegradability to conventional wastewater treatment processes [3,33-35]. The physicochemical properties of CUS-MIL-100(Fe) were characterized and the catalytic performances were evaluated in view of the effects of various key parameters, such as initial pH, H2O2 concentration, catalyst dosage, and initial SMT concentration. The reaction kinetics, catalyst stability, as well as degradation mechanism were also investigated. To the best of our knowledge, this is the first report that a well-defined CUS-MIL-100(Fe) was used as a highly efficient catalyst to activate H2O2 for the degradation of sulfamethazine.

2. Experimental Section 7



2.1. Chemicals and reagents All chemicals and reagents used in this study were of analytical grade and used as received without further purification. Purified sulfamethazine (SMT, >99%) were supplied by Alfa Aesar (Beijing, China). Reduced iron powder (Fe, ≥ 99%) and hydrofluoric acid (HF, ≥ 40%) were obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). 1,3,5-benzenetricarboxylic acid (H3BTC, 99%) was bought from Ourchem Chemical Co., Ltd. (Shanghai, China). Other commonly used chemicals such nitric acid (HNO3, 65%~68%), ethanol (C2H5OH, ≥ 99%), hydrogen peroxide (H2O2, 30 wt.%) and the reagents for sample analysis, were purchased from Beijing Chemical Reagent Co., Ltd. (Beijing, China). The deionized water was used throughout the experiments.

2.2. Synthesis of CUS-MIL-100(Fe) The CUS-MIL-100(Fe) was synthesized by a previously reported hydrothermal method with certain post-synthetic modifications [28]. Briefly, 0.82 g of Fe powder and 2.06 g of H3BTC were firstly mixed in 80 mL of deionized water, followed by the addition of 1.14 mL of HNO3 and 0.6 mL of HF. The reactant mixture was vigorously stirred for 1 h and then transferred into a dried 100 mL Teflon-lined steel autoclave. The autoclave was progressively heated to 150 oC within 8 h and maintained for 24 h. After cooling naturally, the obtained precipitate was recovered by filtration, washed sequentially by stirring in 80 oC hot deionized water for 5 h and in 60 oC hot ethanol for 3 h to remove the residual unreacted reactants and the colored impurities, and then 8


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dried in a vacuum oven at 70 oC for 12 h. Subsequently, the prepared light-orange powder was activated under vacuum conditions at 230 oC for 12 h to generate the iron CUS with mixed valence FeII/FeIII. Finally, the resulting brown product was collected and stored in a covered glass container until needed. For comparison, the MIL-100(Fe) was synthesized as above hydrothermal procedure without vacuum activation treatment.

2.3. Characterization X-ray diffraction (XRD) patterns were collected on a D8-Advance X-ray diffractometer (Bruker, Germany) with a Cu-Kα radiation (λ = 1.5406 Å) over a 2θ range of 2.5–50°. The morphology of catalysts was characterized by a SU8010 field emission

scanning

electron

Brunauer-Emmett-Teller

microscopy

(BET)

surface

(FESEM, area

was

Hitachi,

Japan).

obtained

by

The N2

adsorption-desorption isotherms measured at 77 K on a NOVA4000 surface area analyzer (Quantachrome, USA), and pore structural parameters were determined by the Barrett-Joyner-Halenda (BJH) method. The chemical state of surface elements was analyzed by a ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS, Thermo Scientific, USA) with a monochromatic Al-Kα source (1486.6 eV). Fourier transform infrared spectra (FTIR) were recorded on a Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA). The identification of the formation of CUS was performed by in-situ IR spectroscopic analysis using NO as a probe gas in a nitrogen gas stream (1% NO in nitrogen). The reflection absorption IR spectra were recorded 9



on a Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA) equipped with an extended KBr beam splitting device and a mercury cadmium telluride (MCT) cryodetector. Before testing, samples were pretreated at 120 °C for 3 h under N2 flow and cooled at room temperature.

2.4. Catalytic degradation experiments All experiments were performed in a constant temperature shaking at 200 rpm and 30 oC in a dark environment. In a typical procedure, a required amount of CUS-MIL-100(Fe) were added into a 100 mL sealed serum bottles containing 80 mL of 20 mg L-1 SMT solution that was pH-adjusted to desired values by 0.1 M HCl or 0.1 M NaOH. Subsequently, a known concentration of H2O2 was added to the suspension to initiate the reaction. At regular intervals, samples were withdrawn using a 1 mL syringe, filtered through a 0.22 µm polytetrafluoroethylene (PTFE) filters, and immediately quenched with excess pure n-butanol. An Agilent 1200 Series high-performance liquid chromatograph (HPLC, Agilent, USA) equipped with a UV diode array detector and an Agilent XDB-C18 column (5µm, 4.6 mm × 150 mm) was applied to determine the concentration of SMT. The detection wavelength and the column temperature was set at 275.4 nm and 30 oC, respectively. The mobile phase consisted of water and acetonitrile with a volume ratio of 65:35 at a flow rate of 1.0 mL min−1. Total organic carbon (TOC) was measured by a Multi N/C 2100 TOC/TN analyzer (Analytik Jena AG Corporation, Germany). Dissolved iron concentrations were examined by ZA3000 atomic absorption spectroscopy (AAS, Hitachi, Japan). 10


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H2O2 concentration was measured by Lambda 25 UV-vis spectrophotometer (PerkinElmer, USA) using a potassium titanium(IV) oxalate method (λ = 400 nm). For the successive recycle tests, the catalyst was recovered by filtration, washed with deionized water, dried at 105 oC under vacuum and then reused for the next run under similar conditions.

3. Results and discussion

3.1. Synthesis and characterization of CUS-MIL-100(Fe) Since it is the iron CUS of CUS-MIL-100(Fe) that plays a significant role in the decomposition of H2O2 to generate •OH radicals, the formation of the iron CUS with mixed valence FeII/FeIII in CUS-MIL-100(Fe) is illustrated in Fig. S1 based on literature reported by Yoon et al. [28] In the case of the secondary building unit of MIL-100(Fe), each iron octahedron possesses one terminal group (i.e. H2O, OH- or F-) that is removable from the framework by vacuum thermal activation [28,36]. The coordinated H2O molecules could be easily removed when heating above 100 oC under vacuum, resulting in the exposure of FeIII CUS. Additionally, the FeII CUS could be created above 150 oC owing to the departure of anionic ligands (OH- or F-). Thus, in our experimental condition, FeIII CUS together with FeII CUS embedded in the framework of MIL-100(Fe) could be triumphantly achieved with the removal of H2O and anionic ligands. To further justify the formation of CUS, the in-situ IR spectroscopy of adsorbed NO probe molecules was performed at room temperature 11



[37]. Fig. 1 shows the IR spectra of the NO adsorption stretching region on MIL-100(Fe) and CUS-MIL-100(Fe). NO adsorption gives rise to three ṽ(NO) bands at ṽ =1901, 1842 and 1828 cm-1. The band appeared at 1901 cm-1 belongs to nitrosyls on associated FeIII CUS, while the doublet at 1842 and 1828 cm-1 correspond to the species coordinated on FeII CUS. It could be found that only a weak ṽ(NO) band for FeIII CUS was detected in the IR spectra of MIL-100(Fe). The detectable FeIII CUS in MIL-100(Fe) can be attributed to the pretreatment at 120 oC under inert gas flow, which provokes the departure of water molecules and the formation of FeIII CUS. Compared with the IR spectra of MIL-100(Fe), the band intensity for FeIII CUS of CUS-MIL-100(Fe) was significantly increased and two additional ṽ(NO) bands for FeII CUS were also observed, testifying the successful formation of FeIII CUS and FeII CUS via the 200 oC vacuum thermal treatment. As a decrease in the atomic ratio of O/Fe and F/Fe occurred in the activation of MIL-100(Fe) (see Table 1 element semi-quantitative analysis below), these important features also prove that the creation of FeII CUS has direct correlation with the loss of anionic ligands (OH- or F-). Fig. 1.

The surface morphologies of MIL-100(Fe) and CUS-MIL-100(Fe) were observed with FESEM. From Fig. 2a, the MIL-100(Fe) displays a typical cubic octahedral structure with smooth surface and diameters of 250-500 nm. After incorporation of CUS by vacuum thermal activation, the CUS-MIL-100(Fe) retained 12


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the morphology and size of MIL-100(Fe) except that its surface became rough with the new emergence of uniformly distributed pores (Fig. 2b). XRD patterns in Fig. 2c was determined to identify the crystal structures of the as-synthesized samples. The sharp and strong diffraction peaks of MIL-100(Fe) indicate the well-crystallization of the sample, and the characteristic peaks at 3.4o, 3.9o, 5.3o, 6.3o, 10.2o, 11o, 18.2o, 20.1o, 24o, 27o are in good agreement with previous reports [38,39]. The XRD pattern of CUS-MIL-100(Fe) exhibits similar features to MIL-100(Fe), which signifies that the crystal structure of the host framework was well preserved after activation. In addition, it should be noted that the characteristic peaks of CUS-MIL-100(Fe) are broadened and weakened in comparison with those of MIL-100(Fe), indicating the formation of iron CUS along with the partial loss of long-range order [40,41]. Fig. 2d records the FTIR spectra of MIL-100(Fe) and CUS-MIL-100(Fe). It is obvious that CUS-MIL-100(Fe) shows highly similar FTIR patterns as that for MIL-100(Fe), which indicates that the incorporation of iron CUS has a negligible effect on the functional groups of MIL-100(Fe). Noteworthily, compared with MIL-100(Fe), the broad absorption band between 3000 and 3700 cm-1 attributable to the O-H stretching vibration of adsorbed water species and/or coordinated hydroxyl groups is narrowed and its relative intensity decrease significantly in the FTIR spectra of CUS-MIL-100(Fe) [42,43]. This suggests that partial water molecules and coordinated hydroxyl groups could be removed from the MIL-100(Fe) structure during the activation process, resulting in the generation of FeIII CUS and FeII CUS. 13



Fig. 2.

The specific surface area and the porous nature of the resulting samples were determined by N2 adsorption-desorption measurements and are depicted in Fig. 3. From Fig. 3a, the MIL-100(Fe) presents a perfect type I isotherm of IUPAC (International Union of Pure and Applied Chemistry) classification without a discernible hysteresis loop, indicating the existence of uniform micropores in the structure. Different from the curve of MIL-100(Fe), the isotherm of CUS-MIL-100(Fe) is of type IV with a distinct H3 hysteresis loop reflecting the presence of mesoporous structure in the catalyst. Fig. 3b points out the BJH pore size distribution of MIL-100(Fe) and CUS-MIL-100(Fe), from which we can see that both the samples possess the sharp peak below 2 nm, characteristic of micropores. Discarding that sharp peak, two major peaks of around 3.82 and 6.57 nm can be observed in the pore size distribution curve of CUS-MIL-100(Fe), which further manifests that the introduction of iron CUS could result in the creation of mesopores in CUS-MIL-100(Fe) in addition to intrinsic mircopores. The specific BET surface areas and pore structure parameters of each catalyst are summarized in Table 1. The specific BET surface area and total pore volume of MIL-100(Fe) were calculated to be 1271.31 m2 g-1 and 0.656 cm3 g-1, while those of CUS-MIL-100(Fe) were 568.91 m2 g-1 and 0.711 cm3 g-1, respectively. Despite the surface area of CUS-MIL-100(Fe) being diminished, its surface area value remains high. Thus, it is expected that the high 14


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porosity and surface area of CUS-MIL-100(Fe) would facilitate the adsorption and diffusion of catalytic substrates to the exposed active sites in catalysis. Fig. 3. Table 1.

The surface chemical information of MIL-100(Fe) and CUS-MIL-100(Fe) was further explored by XPS survey. The wide-scan XPS spectra represented in Fig. 4a reveals that both as-synthesized samples are only composed of C, O, F, and Fe elements. As summarized in Table 1, the O and F contents decreased noticeably while the contents of C and Fe were increased after the vacuum activation process. The significant decreased atomic ratios of O/Fe and F/Fe in CUS-MIL-100(Fe), compared with the MIL-100(Fe), demonstrated that the ligands containing O and F atoms were partially eliminated with iron CUS exposed. Fig. 4b illustrates the high-resolution XPS spectra of Fe 2p after Gaussian curve fitting. For MIL-100(Fe), the typical peaks of Fe 2p3/2 and Fe 2p1/2 were mainly centered at around 711.4 eV and 724.8 eV, respectively, which could be both deconvoluted into two components. Thereinto, the fitted peaks at 711.3, 713.9, 724.8, and 727.3 eV are normally assigned to the FeIII cation, and the two shake-up satellite peaks located at 717.8 eV and731.7 eV are the fingerprint of FeIII species, which indicate that the iron in MIL-100(Fe) is predominantly in the FeIII state [31,44]. Conversely, new multiplet peaks at 709.6 eV and 723.1 eV, which is attributable to the characteristics of FeII, appeared in the 15



deconvoluted curves of Fe2p3/2 and Fe2p1/2 of CUS-MIL-100(Fe) [45,46]. This infers that some FeIII center of MIL-100(Fe) undergo a valence variation to generate FeII CUS during the vacuum activation process. Fig. 4.

3.2. Catalytic properties of CUS-MIL-100(Fe) The catalytic performance of CUS-MIL-100(Fe) were assessed for SMT degradation under the assistance of H2O2. Fig. 5 shows the removal profiles of SMT under various reaction systems. It can be seen that negligible SMT removal (2.5%) was obtained when H2O2 alone was present, which indicates that H2O2 itself can scarcely induce the degradation of SMT in the absence of solid catalyst. Meanwhile, without H2O2, CUS-MIL-100(Fe) alone exhibited a limited performance of SMT removal (17.6%) due to the adsorption of CUS-MIL-100(Fe). Notably, the SMT was almost completely removed within 180 min in the coexistence of CUS-MIL-100(Fe) and H2O2, indicating that the SMT was mainly degraded by the Fenton-like reaction rather than the adsorption process. In order to clarify the role of the iron CUS in the CUS-MIL-100(Fe) on the catalytic reaction, control experiment using MIL-100(Fe) + H2O2 system were also evaluated. The MIL-100(Fe) showed a much lower removal efficiency (only 11.1% of SMT degradation within 180 min) than the CUS-MIL-100(Fe). The result demonstrates that the incorporation of iron CUS is important for the catalyst to get a satisfactory catalytic activity in Fenton-like 16


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degradation of SMT. Together with the physical-chemical characterization, the incorporation of iron CUS could induce the formation of mesopores with enlarged pore channels that allow reactants to diffuse easily into the interior structure and enable the interior and external active sites for the full contact with the reactive media. In addition, the iron CUS in the pore channels, owing to its Lewis acidic character, could induce region-selectivity towards H2O2, thereby improving the opportunities for H2O2 to contact with the active sites [25,27,28]. Meanwhile, the iron CUS with variable chemical valences (FeII CUS and FeIII CUS) would accelerate the decomposition of H2O2 to generate •OH radicals through the one-electron transfer process, and thus enhance the catalytic activity for the SMT degradation. Fig. 5.

Table 2 lists a comparison of removal efficiencies of sulfonamides in present work with some reported Fe-based Fenton-like catalysts [12,47-50]. As clearly displayed, our designed CUS-MIL-100(Fe) catalysts exhibit a relatively higher catalytic activity than those reported in the literatures even with a lower initial pH value. These results indicate that the CUS-MIL-100(Fe) scaﬀold with high porosity and large surface area seems to be an efficient Fenton-like catalyst for enhanced degradation of sulfonamide antibiotics in aquatic environment. Table 2

17



Since the reaction parameters (e.g. initial pH, H2O2 concentration, catalyst dosage, and initial SMT concentration) have remarkable effect on the performance of Fenton-like reactions, further investigations were carried out to evaluate the effect of these main reaction parameters on the degradation of SMT in CUS-MIL-100(Fe). The degradation of SMT was followed a two-stage pseudo first-order kinetics (i.e., an initially rapid decrease stage and a subsequently slow decay stage) over different reaction parameters (Text S1, Fig. 6 and S2). The rapid SMT degradation stage in the first 2 min can be explained by the presence of the abundant FeII CUS in the catalyst and the high H2O2 concentration in the solution. In this case, the FeII CUS could react quickly with the surrounding H2O2, producing a large amount of •OH radicals to attack the target compound, and thus contributing to the fast degradation of SMT. Due to the dramatic consumption of FeII CUS in the first stage, the reaction rate in the second stage would be determined by rate of regeneration of FeII CUS from FeIII CUS, which leads to the subsequently slower degradation of SMT. Similar phenomena were also obtained by other researches [51,52]. As the initial degradation stage occurred in a much short time could not accurately describe the overall reaction process, the kinetic rate (k) of second stage was chosen as the average reaction rate constant for the entire degradation period. Fig. 6a and S2a display the effect of initial pH on the degradation of SMT with CUS-MIL-100(Fe). It was observed that the SMT removal efficiency decreased markedly from 100% to 71.1% and the corresponding k value decreased from 0.0467 18


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min-1 to 0.0042 min-1 with increasing the initial pH from 3.0 to 6.0. The decreased SMT removal efficiency at higher pH value could be attributed to the lower oxidation potential of •OH radicals (E0 = +2.8 V at pH = 0; E0 = +2.0 V at pH = 14), and the auto-decomposition of H2O2 [11,53]. Moreover, CUS-MIL-100(Fe) demonstrated relative

good

catalytic

performance

at

near-neutral

pH,

suggesting

that

CUS-MIL-100(Fe) could be able to work in a broad initial pH range as compared to the conventional heterogeneous Fenton-like catalysts [11,48]. The effect of H2O2 concentration on the degradation of SMT was investigated as illustrated in Fig. 6b and S2b. With the increase of H2O2 concentration from 3 mM to 6 mM, both the SMT removal efficiency and the k value increased (91.0-98.4%, 0.0104-0.0158 min-1), which probably due to that the increased H2O2 concentration could generate more •OH radicals for the SMT degradation. Nevertheless, when H2O2 concentration was further increased to 12 mM, the SMT removal efficiency and the k value slightly decreased to 91.8% and 0.0100 min-1, respectively. This can be explained by the unwanted self-scavenging effect of •OH radicals by excessive H2O2 as described by Eqs. (1) and (2) [1,54]. The consumption of H2O2 at different initial H2O2 concentration investigated was within the range of 15.3–30.8%. H2O2 + •OH → HO2•+ H2O

(1)

HO2• + •OH → H2O + O2

(2)

Fig. 6c and S2c disclose the SMT degradation at different catalyst dosage. It was apparent that the increase of catalyst dosage surely accelerated the degradation of 19



SMT. As the catalyst dosage increased from 0.2 g L-1 to 1.5 g L-1, the k value increased gradually from 0.0054 min-1 to 0.0160 min-1, and the final SMT removal efficiency was increased from 70.3% to 98.8%. This originates from the fact that higher dosage of catalyst would provide more active sites available for the generation of •OH radicals, leading to the enhancement of catalytic efficiency. The effect of initial SMT concentration on the degradation of SMT was evaluated as depicted in Fig. 6d and S2d. Clearly, the SMT degradation rates tended to decline quickly when increasing the initial SMT concentration from 10 mg L-1 (0.0166 min-1) to 40 mg L-1 (0.0083 min-1). The likely reason for this phenomenon was that the rise of initial SMT concentration would result in the coverage of active sites on the catalyst, hampering the generation of •OH radicals and subsequent degradation of SMT [55]. In addition, more intermediates were produced at higher initial SMT concentration, which may cause an undesirable competitive consumption of hydroxyl radicals and therefore decrease the degradation rate [56]. Fig. 6.

During the oxidation process, the change of TOC value of the reaction solution was examined to reflect the mineralization degree of SMT. As observed in Fig. 7, the reduction of TOC proceeded more slowly than did the removal of SMT. After 180 min reaction, over 98% of SMT in the solution was eliminated, while the efficiency of TOC removal only reached 52.4%. Thus, it was speculated that some organic residues 20


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derived from SMT decomposition remained in the solution. The efficiency of H2O2 utilization in terms of mineralization, defined as the amount of TOC converted per gram of H2O2 decomposed, was also calculated to establish the efficiency of the Fenton-like catalyst [57]. Under these reaction conditions, the degradation of 180 min consumed 30.8% of the initial H2O2 concentration, which corresponded in turn to 86.3 mg TOC g-1 H2O2. This value obtained is in the range of the reported literature values, suggesting a high efficiency of H2O2 utilization and a good degradation of SMT [57-59]. Fig. 7.

3.3. Stability and reusability of CUS-MIL-100(Fe) The stability of the catalyst is of great significance for heterogeneous catalysis. To assess it, the concentrations of iron ions leaching from CUS-MIL-100(Fe) were also detected during the heterogeneous Fenton process over five cycles. As shown in Fig. 8a, the dissolved iron concentration gradually increased with the reaction time. The highest concentration of dissolved iron (0.41 mg L-1) was lower than the environmental standard (2 mg L-1) imposed by the European Union, avoiding the further treatment for iron ions. Additionally, the concentration of dissolved iron after each cycle were ranged from 0.36 mg L-1 to 0.41 mg L-1, which suggested the good structural stability of the catalyst. To estimate the possible contribution of the dissolved Fe ions to the reaction, the filtrate that collected by removing 0.5 g L-1 21



CUS-MIL-100(Fe) after vigorous agitation for 180 min was applied to catalyze the degradation of SMT. It can be seen from Fig. 8b that the degradation of SMT catalyzed by the dissolved Fe ions was only 13.2% after 180 min, which manifested that the degradation of SMT was mainly dominated by the heterogeneous Fenton reaction catalyzed by CUS-MIL-100(Fe). Moreover, five consecutive cycling tests of SMT degradation were performed to evaluate the reusability of CUS-MIL-100(Fe) using the recovered catalyst under the same reaction conditions (Figure 8c). Specifically, a significantly drop in the catalytic activity of CUS-MIL-100(Fe) was found after the first application. In order to check the possible change in the surface chemistry of the CUS-MIL-100(Fe) before and after use, XPS measurement were performed. The XPS results (Fig. 9) revealed that two new elements of N and S originating from the SMT were detected on the surface of the used CUS-MIL-100(Fe), and the amount of FeII CUS in the CUS-MIL-100(Fe) was decreased after reaction. Therefore, the decreased activity of the used CUS-MIL-100(Fe) may be attributable to the loss of exposed FeII CUS as well as the inhibition effect of the adsorbed organic species on catalyst surface. It should be remarked, however, that the catalytic activity of used CUS-MIL-100(Fe) was recovered to the original level as the fresh catalyst after reactivating under the same conditions described above. These results indicate that reactivation treatment under vacuum condition can recover the performance of the catalyst and enhance its reusability. Moreover, the physicochemical properties of the reactivated CUS-MIL-100(Fe) were also characterized by FESEM, XRD, FTIR 22


Page 22 of 47

Page 23 of 47


and XPS measurements (Fig. 9 and S3). The results showed no significant change in the morphology, crystal structures, functional group as well as the amount of FeII CUS in comparison to the fresh catalyst, which provided further evidence for the good structural stability of the catalyst. Fig. 8. Fig. 9.

3.4. Possible catalytic mechanism To identify the reactive oxidizing species for SMT degradation in the heterogeneous catalytic process, n-butanol, which has high reactivity with •OH radicals, was introduced as an effective radical scavenger for the reaction [8]. As shown in Fig. 10, the SMT removal efficiency declined significantly from 98.4% to 16.1% in the presence of excess n-butanol as compared to the reaction without scavenger, which indicated that the degradation of SMT is primarily attributed to the outstanding oxidation ability of •OH radicals in the Fenton-like system. Fig. 10.

On the basis of all the results obtained and previous studies, a possible mechanism for H2O2 activation and SMT degradation by CUS-MIL-100(Fe) was proposed and illustrate in Fig. 11. As mentioned above, CUS-MIL-100(Fe) with high porosity and open pore network (~6.57 nm pore diameter) could be valuable for the fast molecule diffusion of reactants towards the interior materials. Benefiting from the 23



Page 24 of 47

high accessibility of coordinatively unsaturated metal sites, the SMT molecules were initially adsorbed on the framework of catalyst through the π-π interaction between the aromatic rings of SMT and CUS-MIL-100(Fe), leading to the enrichment of SMT in the vicinity of iron CUS. Once H2O2 was added, complexes (≡FeII CUS-H2O2 and ≡FeIII CUS-H2O2) were immediately formed due to the strong affinity between the iron CUS and H2O2 molecules (Eqs. (3) and (4)). The ≡FeII CUS-H2O2 subsequently underwent intramolecular electron transfer to form surface-bound •OH radical (•OHads) and ≡FeIII CUS (Eq. (5)). Meanwhile, the ≡FeIII CUS-H2O2 could be reduced to ≡FeII CUS via reactions (Eqs. (6) and (7)). The ≡FeII CUS and ≡FeIII CUS was continuously to each other under this circumstance, generating a plenty of •OHads. The generated •OHads can directly attack the SMT in the vicinity of iron CUS (Eq. (8)). Furthermore, the high instantaneous concentration of SMT and •OHads in such a local microenvironment could provide a driving force to facilitate the degradation of SMT. Eventually, in the bulk solution, the dissolved iron resulted from the leaching of CUS-MIL-100(Fe) also initiates the decomposition of H2O2 through a chain reaction, producing free •OH radicals (•OHfree) to degrade SMT (Eqs. (9)-(12)). Fig. 11.

≡FeII CUS + H2O2 → ≡FeII CUS-H2O2

(3)

≡FeIII CUS + H2O2 → ≡FeIII CUS-H2O2

(4)

≡FeII CUS-H2O2 → ≡FeIII CUS + •OHads + OH-

(5)

24


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≡FeIII CUS-H2O2 → ≡FeII CUS + HO2• + H+

(6)

≡FeIII CUS + HO2• → ≡FeII CUS + O2 + H+

(7)

•OHads + SMT → degraded products

(8)

Fe2+ + H2O2 → Fe3+ + •OHfree + OH-

(9)

Fe3+ + H2O2 → Fe2+ + HO2• + H+

(10)

Fe3+ + HO2• → Fe2+ + O2 + H+

(11)

•OHfree + SMT → degraded products

(12)

The incorporation of iron CUS with mixed valence FeII/FeIII can significantly enhance the catalytic activity of MIL-100(Fe) for SMT degradation. This catalyst showed good reusability and stability with very low leaching of iron species. The findings of this work could offer a new insight into the application of MOFs materials in Fenton-like degradation of environmental contaminants.

Acknowledgements This research was supported by the National Natural Science Foundation of China (51338005) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13026).

Supplementary data Supplementary data associated with this article can be found, in the online 25



version. The supporting information contains the kinetic analysis of SMT degradation by pseudo-first-order model and includes 3 figures. Fig. S1. Schematic illustration of the creation of FeII CUS and FeIII CUS by vacuum thermal activation; Fig. S2. The pseudo-first-order kinetic fitting for SMT degradation over CUS-MIL-100(Fe) with various reaction parameters: (a) initial pH, (b) H2O2 concentration, (c) catalyst dosage, and (d) initial SMT concentration; Fig. S3. FESEM images of (a) used CUS-MIL-100(Fe) and (b) reactivated CUS-MIL-100(Fe); (c) XRD and (d) FTIR spectra of used CUS-MIL-100(Fe) and reactivated CUS-MIL-100(Fe).

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Figure Captions Fig. 1. IR spectra of MIL-100(Fe) and CUS-MIL-100(Fe) under NO (1% NO in N2, 25 ccmin) flow at room temperature after pretreatment of the sample under N2 flow at 120 oC for 3 h. Fig. 2. FESEM images of (a) MIL-100(Fe) and (b) CUS-MIL-100(Fe); (c) XRD and (d) FTIR spectra of MIL-100(Fe) and CUS-MIL-100(Fe). Fig. 3. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distribution plots of MIL-100(Fe) and CUS-MIL-100(Fe). Fig. 4. (a) Wide-scan XPS spectra and (b) high-resolution scanning XPS spectra for Fe 2p regions of MIL-100(Fe) and CUS-MIL-100(Fe). Fig. 5. Degradation of SMT at various reaction conditions. Except for the investigated parameter, other parameters were set as follows: pH = 4.0, SMT concentration = 20 mg L-1, catalyst dosage = 0.5 g L-1, H2O2 concentration = 6 mM. Fig. 6. The effect of various parameters (a) initial pH; (b) H2O2 concentration; (c) catalyst dosage; (d) initial SMT concentration on the degradation of SMT in CUS-MIL-100(Fe) catalyzed Fenton-like system. Except for the investigated parameter, other parameters were set as follows: pH = 4.0, SMT concentration = 20 mg L-1, catalyst dosage = 0.5 g L-1, H2O2 concentration = 6 mM. Fig. 7. The efficiency of TOC removal during the degradation of SMT. Reaction conditions: pH = 4.0, SMT concentration = 20 mg L-1, [CUS-MIL-100(Fe)]0 = 0.5 g L-1, H2O2 concentration = 6 mM. Fig. 8. (a) Iron leakage during the degradation of SMT in repeated experiments. (b) The effect of homogeneous and heterogeneous catalysis, and (c) Reusability of CUS-MIL-100(Fe) on the degradation of SMT. Reaction conditions: pH = 4.0, SMT concentration = 20 mg L-1, catalyst dosage = 0.5 g L-1, H2O2 concentration = 6 mM. 33



Fig. 9. (a) Wide-scan XPS spectra and (b) high-resolution scanning XPS spectra for Fe 2p regions of used CUS-MIL-100(Fe) and reactivated CUS-MIL-100(Fe). Fig. 10. Effect of n-butanol scavenger on the degradation of SMT. Reaction conditions: pH = 4.0, SMT concentration = 20 mg L-1, [CUS-MIL-100(Fe)]0 = 0.5 g L-1, H2O2 concentration = 6 mM. Fig. 11. Schematic diagram of the reaction mechanism of the H2O2 activation by CUS-MIL-100(Fe).

Table 1 Textural properties and surface elemental contents of MIL-100(Fe) and CUS-MIL-100(Fe). Table 2 Comparison of the Fenton-like catalytic activity between CUS-MIL-100(Fe) and various Fe-based catalysts reported in literature.

34


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Absorbance

1828

1901

1842

Fig. 1.

CUS-MIL-100(Fe)

MIL-100(Fe)

2000

1960

1920

1880

1840

1800

-1

Wavenumber (cm )

35



Fig. 2.

36


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Fig. 3.

500

3

-1

Volume adsorbed (cm g )

(a) 600 400 300 200 100

MIL-100(Fe) CUS-MIL-100(Fe)

0 0.0

0.2

0.4

0.6

0.8

1.0

-1

Relative pressure (P P0 ) 6.57 nm

4

3

MIL-100(Fe) CUS-MIL-100(Fe)

3

-1

-1

dV/d(log D) (cm nm g )

(b)

2 3.82 nm

1

0 0

2

4

6

8

10

12

14

Pore diameter (nm)

37



Fig. 4.

F 1s

Intensity (a.u.)

Fe 2p

C 1s

O 1s

(a)

MIL-100(Fe)

CUS-MIL-100(Fe)

1200

1000

800

600

400

200

0

Binding energy (eV)

(b)

Fe 2p3/2

Fe 2p1/2

III

Intensity (a.u.)

Fe

MIL-100(Fe) Fe 2p3/2

Fe 2p1/2

III

Fe

II

Fe CUS-MIL-100(Fe) 740

735

730

725

720

715

710

705

700

Binding energy (eV)

38


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Fig. 5.

1.0

0.8 H2O2 alone

Ct/C0

0.6

CUS-MIL-100(Fe) alone CUS-MIL-100(Fe) + H2O2

0.4

MIL-100(Fe) + H2O2

0.2

0.0 0

30

60

90

120

150

180

Time (min)

39



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Fig. 6.

(a) 1.0 0.8

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

30

60

90

120

150

3 mM 6 mM 9 mM 12 mM

0.8

Ct/C0

0.6 Ct/C0

(b)1.0

pH = 3.0 pH = 4.0 pH = 5.0 pH = 6.0

180

0

30

60

Time (min)

90

120

180

Time (min)

(c) 1.0

(d)1.0 -1

-1

0.2 g L -1 0.5 g L -1 1.0 g L -1 1.5 g L

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

30

60

90

120

150

10 mg L -1 20 mg L -1 30 mg L -1 40 mg L

0.8

Ct/C0

0.8

Ct/C0

150

180

0

30

Time (min)

60

90

120

Time (min)

40


150

180

Page 41 of 47


Fig. 7.

1.0 SMT Ct/C0

Removal efficiency (%)

0.8

TOCt/TOC0

0.6

0.4

0.2

0.0 0

30

60

90

120

150

180

Time (min)

41



Fig. 8.

-1

Dissolved iron (mg L )

(a)

0.4

0.3

0.2

First cycle Second cycle Third cycle Fourth cycle Fifth cycle

0.1

0.0 0

30

60

90

120

150

180

Time (min)

(b)

1.0 0.8

Ct/C0

0.6

CUS-MIL-100(Fe) Dissolved Fe ions

0.4 0.2 0.0 0

30

60

90

120

150

180

Time (min)

Removal efficiency (%)

(c) 100 80

After reactivation

60 40 20 0 1

2

3

4

5

Cycle number

42


Page 42 of 47

Page 43 of 47


Fig. 9.

(b)

O 1s

(a)

Fe 2p3/2

Fe 2p1/2

III

Intensity (a.u.)

Used CUS-MIL-100(Fe)

S 2p

N 1s

C 1s

Fe 2p

Intensity (a.u.)

Fe

Fe

II

Used CUS-MIL-100(Fe)

Fe 2p3/2

Fe 2p1/2

Fe

III

II

Fe

Reactivated CUS-MIL-100(Fe) Reactivated CUS-MIL-100(Fe)

1200

1000

800

600

400

200

0

740

Binding energy (eV)

735

730

725

720

715

710

Binding energy (eV)

43


705

700


Fig. 10.

1.0 0.8

Ct/C0

0.6

None n-Butanol

0.4 0.2 0.0 0

30

60

90

120

150

180

Time (min)

44


Page 44 of 47

Page 45 of 47


Fig. 11.

45



Page 46 of 47

Table 1 Textural properties and surface elemental contents of MIL-100(Fe) and CUS-MIL-100(Fe) Sample

SBET

Total pore volume

Average pore diameter

Surface atomic content

(m2 g-1)

(cm3 g-1)

(nm)

C (%)

O (%)

F (%)

Fe (%)

O/Fe

F/Fe

MIL-100(Fe)

1271.31

0.656

1.702

55.21

36.59

1.71

6.49

5.36

0.26

CUS-MIL-100(Fe)

568.91

0.711

6.572

57.26

32.80

0.64

9.30

3.53

0.07

46


Atomic ratio

Page 47 of 47


Table 2 Comparison of the Fenton-like catalytic activity between CUS-MIL-100(Fe) and various Fe-based catalysts reported in literature. Target pollutants

Catalysts

SBET

(m2 Reaction conditions

Catalytic performance

References

100% removal in 60 min

This work

g-1) Sulfamethazine (SMT) Sulfamethazine (SMT) Sulfamethazine (SMT) Sulfamethazine (SMT) Sulfamethoxazole (SMX) Sulfamethoxazole (SMX) Sulfanilamide (SA)

CUS-MIL-100(Fe)

568.91

CUS-MIL-100(Fe)

568.91

Fe3O4/Mn3O4

124.1

Fe3O4/MWCNTs

178

Fe3O4

7.5

CX/CoFe

530

Fe/Cu/Al-pillared clays

124

pH: 3.0; Catalyst: 0.5 g L-1; [H2O2]: 6.0 mM; [SMT]0: 20 mg L-1;T: 25 oC pH: 4.0; Catalyst: 0.5 g L-1; [H2O2]: 6.0 mM; [SMT]0: 20 mg L-1;T: 25 oC pH: 3.0; Catalyst: 0.5 g L-1; [H2O2]: 6.0 mM; [SMT]0: 20 mg L-1;T: 35 oC pH: 3.0; Catalyst: 0.5 g L-1; [H2O2]: 6.0 mM; [SMT]0: 20 mg L-1;T: 30 oC pH: 5.0; Catalyst: 1.0 g L-1; [H2O2]: 25 mg L-1; [SMX]0: 5 mg L-1;T: 25 oC pH: 3.0; Catalyst: 80 mg L-1; [H2O2]: 500 mg L-1; [SMX]0: 500 µg L-1;T: 25 oC pH: 3.5; Catalyst: 2.0 g L-1; [H2O2]: 5.23 mmol L-1; [SMX]0: 0.29 mmol L-1;T: 50 oC

47


98.4% removal in 180 This work min 95% removal in 240 min [47] 98.3% removal in 180 [48] min 100% removal in 210 min [12] 96.8% removal in 360 [49] min 98% removal in 240 min [50]

Metal Organic Framework with Coordinatively Unsaturated Sites as

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