Activation of sulfite autoxidation with CuFe2O4 prepared by MOF-templated method for abatement of organic contaminants

Environmental Pollution 260 (2020) 114038

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Activation of sulfite autoxidation with CuFe2O4 prepared by MOFtemplated method for abatement of organic contaminants* Xiaodan Zhao , Wenjing Wu , Guohua Jing , Zuoming Zhou * College of Chemical Engineering, Huaqiao University, Xiamen, 361021, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 September 2019 Received in revised form 30 December 2019 Accepted 21 January 2020 Available online 21 January 2020

Copper ferrite (denoted as CuFe2O4MOF), prepared via a complexation reaction to obtain bimetaleorganic frameworks (Cu/Fe bi-MOFs), followed by a combustion process to remove the MOF template, is employed as a heterogeneous activator to promote sulfite autoxidation for the removal of organic contaminants. At pH 8.0, more than 80% of the recalcitrant organic contaminant iohexol (10 mM) can be removed within 2 min by the activation of sulfite (500 mM) with CuFe2O4MOF (0.1 g L
1). CuFe2O4MOF exhibits more pronounced catalytic activity in accelerating sulfite autoxidation for iohexol abatement compared to that fabricated by hydrothermal and solegel combustion methods. Radical quenching studies suggest that the sulfate radical (SO
4 ) is the main reactive species responsible for iohexol abatement. The performance of CuFe2O4MOF/sulfite for iohexol abatement can be affected by several critical influencing factors, including the solution pH and the presence of humic acid, Cl
, and HCO
3 . The effect of the ionic strength and the results of the attenuated total reflectanceeFourier transform infrared (ATReFTIR) analysis indicate that sulfite autoxidation in the presence of CuFe2O4MOF involves an innersphere interaction with the surface Cu(II) sites of CuFe2O4MOF. X-ray photoelectron spectroscopy (XPS) characterization suggests that the surface Cu(II)eCu(I)eCu(II) redox cycle is responsible for efficient SO
4 production from sulfite. Overall, CuFe2O4MOF can be considered an alternative activator for sulfite autoxidation for potential application in the treatment of organic-contaminated water. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Metaleorganic frameworks Sulfite Sulfate radical Advanced oxidation process Iohexol

1. Introduction Efficient abatement of organic contaminants in aquatic environments is of great importance as they pose a potential threat to humans and ecosystems. Conventional oxidation processes such as chlorination, chloramination, and ozonation are not effective for the removal of recalcitrant organic contaminants (Ning and Graham, 2008; Wendel et al., 2014). As an alternative, the application of highly reactive radical species including hydroxyl radicals (HO∙) and sulfate radicals (SO
4 ) for the abatement of recalcitrant organic pollutants has attracted considerable research interest (Chan et al., 2010; Guo et al., 2013; Dong et al., 2017; Zhou et al., 2017). In particular, SO
4 , which has a high redox potential of 2.6e3.1 V as well as good selectivity and stability, can react rapidly with various organic contaminants in water with apparent second-

* This paper has been recommended for acceptance by Charles Wong. * Corresponding author. Department of Environmental Science & Engineering, Huaqiao University, Xiamen, Fujian, 361021, China. E-mail address: [email protected] (Z. Zhou).

https://doi.org/10.1016/j.envpol.2020.114038 0269-7491/© 2020 Elsevier Ltd. All rights reserved.

order rate constants of 108e1010 M
1 s
1 (Deng and Ezyske, 2011). Therefore, the development of advanced oxidation processes (AOPs) based on the in-situ generation of SO
4 has attracted the attention of many researchers. Conventional SO
4 precursors are mainly persulfate, including peroxymonosulfate (PMS) and peroxydisulfate (PDS) (Wang and Wang, 2018). However, an important issue associated with persulfate is that a large proportion of PMS/PDS remains undecomposed due to their durable stability (Johnson et al., 2008). In addition, intensive energy inputs and high oxidant costs can constrain the application of PMS/PDS. In recent years, SO
4 generated from sulfite due to its cost-effectiveness and safety has received increasing attention as an alternative to PMS/PDS (Zhang et al., 2018; Zhou et al., 2018; Zhao et al., 2019). Considering that sulfite oxidation in the presence of dissolved oxygen is very slow, with a low reactivity towards organic contaminants, appropriate activators are needed for its potential application. Transition metal ions (Cu(II), Mn(II), Fe(II), Co(II)) have been found to be effective sulfite autoxidation activators for organic contaminant removal or bacterial inactivation (Chen et al., 2017;

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X. Zhao et al. / Environmental Pollution 260 (2020) 114038

Zhang et al., 2018; Zhou et al., 2018; Zhao et al., 2019). The activation of sulfite with transition metal ions relies on the redox cycle of M(II)/M(III) (M ¼ Mn, Fe, Co) or M(I)/M(II) (M ¼ Cu) (Chen et al., 2017). However, a disadvantage related to these homogeneous AOPs is that the activators cannot be collected and recycled for consecutive reactions. In contrast, heterogeneous activators are more favorable due to their excellent reusability, reactivity, and stability. Sulfite activation using metal oxides as heterogeneous activators has been confirmed in previous reports (Ding et al., 2019; Luo et al., 2019; Wu et al., 2019). In particular, copper oxide can effectively activate sulfite to SO
4 for the abatement of organic contaminants. However, the application of copper oxide is found to be limited due to the relatively high rate of copper leaching (0.12 mg L
1 at a CuO dosage of 0.02 g L
1 and pH 8.0) (Wu et al., 2019). Regarding the high activity of the surface Cu(II), an alternative copper-based metal oxide to efficiently induce SO
4 production from sulfite autoxidation would be favorable. Spinel copper ferrite (CuFe2O4), as a heterogeneous catalyst, has been demonstrated to be effective for sulfite autoxidation to degrade organic compounds, attributed to its good magnetic property, stable structure, and excellent activation capability (Chen et al., 2018a). In particular, copper leaching from the solid particles can be greatly reduced due to the stable spinel structure of CuFe2O4. However, the efficiency of the sulfite autoxidation system activated by CuFe2O4 under mild pH conditions (pH 6.0e8.0) is unsatisfactory. To further enhance the catalytic activity of CuFe2O4, one solution is to increase the specific surface area and correspondingly increase the active sites of the copper ferrite. Conventional CuFe2O4 synthesis methods including coprecipitation, sol-gel, and thermal decomposition are not favorable for the preparation of CuFe2O4 with a larger specific surface area and more active sites (Selima et al., 2019). Recently, metaleorganic frameworks (MOFs) have emerged as ideal sacrificial templates for fabricating a variety of porous metal oxides or their composites (Zhou and Kitagawa, 2014; Xu et al., 2017; Li et al., 2018; Guo et al., 2019). Due to their relatively low thermal stability, the organic content can be burned up by the appropriate thermal treatment process and the morphology, porous structure, and large specific surface area of the parent MOFs can be retained (Yang et al., 2018; Huang et al., 2019). The MOF-templated strategy is to take advantage of the intrinsic properties of MOFs to fabricate metal oxides, which is more efficient than the existing methods. A previous study demonstrated that a Co/Fe bimetaleorganic framework (bi-MOF)-derived catalyst (CoFe2O4) showed greater activity in PMS activation (Yang et al., 2018). Therefore, it can be postulated that CuFe2O4 prepared by employing Cu/Fe bi-MOFs as the template can efficiently activate sulfite to SO
4 for the abatement of organic contaminants. This study aimed to (i) analyze the morphology, phase, and specific surface area of CuFe2O4 prepared by the MOF-templated method; (ii) investigate the activity and performance of CuFe2O4 in sulfite autoxidation for the abatement of organic pollutants and identify the reactive radicals by electron paramagnetic resonance (EPR) spectroscopy and the inhibition of radical scavengers on the abatement of the target compound; (iii) evaluate the influence of critical factors including the pH and CuFe2O4 dosage on the abatement efficiencies of the target compound; and (iv) explore the activation mechanisms of sulfite by CuFe2O4 via attenuated total reflectanceeFourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Iohexol, an iodinated X-ray contrast media that is commonly detected and recalcitrant in aquatic environments, was selected as the target compound.

2. Materials and methods 2.1. Materials Copper nitrate (Cu(NO3)2$3H2O, purity >99%), citric acid (C6H8O7$H2O, >99.5%), N,N-dimethylformamide (DMF, >99.5%), and ethanol (>99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Iron(III) acetylacetonate (Fe(acac)3, >98%), 5,5-dimethyl-l-pyrroline N-oxide (DMPO, >97%), tert-butanol (TBA, >99.5%), methanol (>99.9%), bisphenol A (BPA, >99%), benzoic acid (BA, >99.8%), p-acetamidophenol (APAP, >99.0%), and tetracycline (TA, >98%) were supplied by Sigma-Aldrich Co. (China). Iohexol (>99.5%), 5,50 -dithiobis-(2-nitrobenzoic acid) (DTNB, >99%), ethylenediaminetetraacetic acid (EDTA, >99.5%), and anhydrous sodium sulfite (>97.0%) were purchased from Xilong Scientific Co., Ltd. (China). All chemical reagents were directly used without further purification and all solutions were prepared with 18.2 MU cm
1 Milli-Q water. 2.2. Sample preparation A homogeneous Cu/Fe bi-MOF solution was prepared by mixing Fe(acac)3 (1.0 mmol), Cu(NO3)2$3H2O (0.5 mmol), and terephthalic acid (2.0 mmol) in a DMF/ethanol (40 mL, v/v ¼ 1:1) solvent under continuous stirring. Subsequently, this solution was transferred to a Teflon-lined stainless-steel autoclave and kept at a temperature of 393 K overnight. The obtained Cu/Fe bi-MOFs were washed with ethanol several times, dried at 338 K for 6 h, and then calcined at 673 K with a temperature gradient of 1 K min
1. The calcined product was denoted as CuFe2O4MOF. In contrast, CuFe2O4 was prepared according to two conventional fabrication methods, i.e., a one-step hydrothermal process and a citric-acid-assisted solegel combustion method, denoted as CuFe2O4therm and CuFe2O4gel, respectively (Zhang et al., 2013; Chen et al., 2018a). 2.3. Characterization techniques The surface morphology was obtained by field-emission scanning electron microscopy (FESEM; Hitachi SU8020). The BET surface area was measured by a Quantachrome NOVA 2000e analyzer. The structure and crystal phase of CuFe2O4MOF were analyzed by Xray diffraction (XRD; Bruker D8 Advance diffractometer). The magnetic properties were characterized by a vibrating-sample magnetometer (VSM; Quantum Design PPMS-9). Spectral changes of the sulfite in the presence or absence of CuFe2O4MOF were measured using a PerkinElmer ATR-FTIR spectrometer (Nicolet iS50, Thermo Fisher, USA). The copper valence states of CuFe2O4MOF were measured by XPS (Thermo Scientific Escalab 250Xi). 2.4. Experimental procedures 2.4.1. Batch reaction Abatement experiments were carried out at room temperature in a 200 mL glass vessel equipped with a stirrer. Sulfite and the solid activator were added to 100 mL of the solution containing the organic contaminant (10 mM) to initiate the reaction. At different time intervals, an appropriate amount of sample was obtained and filtered through a 0.22 mm polyethersulfone membrane to remove the solid activator in preparation for the organic contaminant measurements and sulfite measurements, respectively. The filtered sample was immediately quenched by methanol for the organic contaminant measurements. For the sulfite measurements, the sample was added to a colorimetric tube, containing a mixture of 1 mL of ethylenediaminetetraacetic acid (1 mM), 2 mL of DTNB

X. Zhao et al. / Environmental Pollution 260 (2020) 114038

(1 mM), and 5 mL of Na2HPO4/KH2PO4 buffer (pH ¼ 7.0) (Jiang et al., 2015). The initial solution pH was adjusted by diluting the NaOH and H2SO4. For the reusability experiment, the solid activator was recovered from the reaction mixture after each run by magnetic separation, washed with Milli-Q water several times, and dried at 338 K for 12 h.

3

2.4.2. Analysis A high-performance liquid chromatography system (Essentia LC-15C, Shimadzu, Japan) equipped with an Agilent HC-C18 column (5 mm, 250  4.6 mm) was used to quantify the organic contaminants. To determine the residual sulfite concentration, a UVeVis spectrophotometer (V-1600) was employed by measuring the absorbance at 412 nm after the sample was prepared by the above modified colorimetric procedure with DTNB. A Bruker EMX-10/12 spectrometer was used to conduct EPR measurements after the radicals were trapped by the DMPO. Copper dissolution from CuFe2O4MOF was measured by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7800). The intermediate products were analyzed by ultra-high performance liquid chromatography and quadrupole time-of-flight mass spectrometer (UHPLC-QTOFMS; 1290 UHPLC/6545 QTOF, Agilent, USA), based on the detailed descriptions in a previous study (Wang et al., 2019).

oxides. Fig. 1e presents a comparison of XRD patterns for CuFe2O4MOF, CuFe2O4therm and CuFe2O4gel. The major crystal phase of CuFe2O4MOF is identical to that of spinel CuFe2O4MOF (JCPDS: 25-0283). Several well-defined peaks at 2q ¼ 18.5 , 30.2 , 35.6 , 37.2 , 43.1, 57.1, and 62.8 demonstrate that CuFe2O4MOF exhibits a typical cuprospinel pattern. These peaks are corresponding to the Bragg planes of (111), (220), (311), (222), (400), (511) and (440), respectively. According to Scherrer’s equation (Yang et al., 2018), the average crystallite size is calculated, which are shown in Table S1. The average crystallite size for CuFe2O4MOF is in the range of 14.1e26.3 nm. Compared with the CuFe2O4therm and CuFe2O4gel, CuFe2O4MOF possesses a higher crystallinity, which indicates the high stability of CuFe2O4MOF spinel structure and the corresponding stable activity in the sulfite activation process (Zhang et al., 2013). Fig. 1f depicts the magnetic characteristics of CuFe2O4MOF via hysteresis loops at room temperature. As expected, the Cu/Fe biMOFs are nonmagnetic because ferric copper oxide is not formed prior to the thermal treatment. On the other hand, CuFe2O4MOF possesses ferromagnetic properties with the saturation magnetization determined as 26.5 emu g
1. It can be predicted that CuFe2O4MOF is easily separated from the aqueous solution if the heterogeneous solution is exposed to an external magnetic field.

3. Results and discussion

3.2. Activity and stability of CuFe2O4MOF for sulfite activation

3.1. Characterization of CuFe2O4MOF

The sulfite activation performance of CuFe2O4MOF for the abatement of various types of organic contaminants including iohexol, BPA, APAP, BA, and TC is assessed (Fig. 2a). The removal of the above organic compounds with efficiencies higher than 80% in the CuFe2O4MOF/sulfite process can be obtained. Organic abatement by single CuFe2O4MOF adsorption and single sulfite autoxidation is negligible (data not shown). Thus, the combination of CuFe2O4MOF and sulfite exhibits significant performance for the abatement of organic contaminants with different structures. Moreover, CuFe2O4therm and CuFe2O4gel were used as references to compare the performance of copper ferrite prepared by different methods. Fig. 2b shows the decrease in the iohexol concentration in a single sulfite solution and sulfite combined with CuFe2O4MOF, CuFe2O4therm, or CuFe2O4gel. It can be seen that iohexol abatement is more pronounced with CuFe2O4MOF than with CuFe2O4therm and CuFe2O4gel, which is probably due to the higher number of active sites induced by the larger specific surface area of CuFe2O4MOF. In the control experiments performed with the Cu-MOF- and Fe-MOFderived catalysts, a single component of copper ferrite or sulfite (Figs. S3 and S4), the abatement of iohexol can be neglected. The decrease in the sulfite concentration in the presence of CuFe2O4MOF, CuFe2O4therm, and CuFe2O4gel also suggests that the activity of the copper ferrite in the sulfite activation follows the order of CuFe2O4MOF > CuFe2O4therm z CuFe2O4gel (Fig. S5). In the homogeneous activation of sulfite autoxidation with Fe(III) and Cu(II) under circumneutral conditions (pH 8.0), it has been proven that Cu(II) exhibits considerable activity for accelerating the sulfite autoxidation whereas the activity of Fe(III) is negligible (Zhao et al., 2019). The abatement of organic compounds by heterogeneous sulfite activation with iron oxide (Fe2O3) and copper oxide (CuO) is consistent with that using homogeneous systems under the same experimental conditions (Wu et al., 2019). Therefore, it can be considered that the activity of Fe(III) is negligible and Cu(II) is the active metal site of CuFe2O4MOF. Apparently, CuFe2O4MOF with the surface-active Cu(II) can significantly activate sulfite to produce the reactive radicals for iohexol abatement, in which dissolved oxygen is critical for producing the reactive radicals (Fig. S6). It has been reported that a radical generation chain reaction in

Fig. 1a shows thermogravimetric variation of Cu/Fe bi-MOFs in the process of pyrolysis treatment. It can be seen that there are two distinct stages with a total weight loss of ~76%. The Cu/Fe bi-MOFs undergo a removal of the crystalline and coordinated water in the first stage and the burning of organic content in the second stage (Yang et al., 2018). After 400  C, the decline of the residual weight is negligible and thus determined as pyrolysis temperature. Fig. 1b and Fig. S1 illustrate the morphological information of CuFe2O4MOF and Cu/Fe bi-MOFs, respectively. SEM image clearly indicates that both Cu/Fe bi-MOFs and CuFe2O4MOF exhibit the rodlike regular structure. Combustion of Cu/Fe bi-MOFs leads to contraction of this rod-like regular structure, which is reasonable due to the pyrolysis of MOFs precursor. Thus, it can be assumed that CuFe2O4MOF is successfully prepared via the complexation reaction to obtain Cu/Fe bi-MOFs followed by the combustion process to remove the MOF template. Fig. 1c shows the BET surface area and pore volume of CuFe2O4MOF, CuFe2O4therm and CuFe2O4gel. The specific surface area for CuFe2O4MOF is 80 m2 g
1, which is significantly higher than that of CuFe2O4therm (32 m2 g
1) and CuFe2O4gel (28 m2 g
1). This result demonstrates the assumption that the CuFe2O4 prepared by employing Cu/Fe bi-MOFs as template exhibits great specific surface area. Higher surface area of CuFe2O4 with more active sites is favorable for abatement of organic contaminants via efficient sulfite activation process. The pore volume of CuFe2O4MOF is 0.54 cm3g
1, much higher than that of CuFe2O4therm and CuFe2O4gel. According to Fig. S2, the nitrogen adsorption isotherms belong to typical type IV isotherms with H3-shaped hysteresis loops based on the IUPAC classification, indicating that CuFe2O4MOF possesses a typical mesoporous structure. This can also be further confirmed by the pore size distribution data. Fig. 1d shows pore size distribution of CuFe2O4MOF, CuFe2O4therm and CuFe2O4gel. It can be illustrated that dominant pore sizes of CuFe2O4MOF are in the range of 20e80 mm with a peak at 35 mm. The MOF-templated preparation method can significantly increase the amount of mesopore of CuFe2O4. The pore volume of CuFe2O4MOF is primarily contributed by mesopore, which is favorable for the interaction between sulfite and the metal

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X. Zhao et al. / Environmental Pollution 260 (2020) 114038

Fig. 1. Characterization of CuFe2O4MOF: (a) TG curve of Cu/Fe bi-MOFs; (b) SEM image; (c) BET specific surface area and pore volume of CuFe2O4MOF; (d) pore size distribution; (e) XRD pattern; (f) M-H curve.

Fig. 2. Abatement of various organic contaminants in CuFe2O4MOF/sulfite process (a) and comparison of the iohexol abatement efficacy by sulfite in the pesence of CuFe2O4MOF, CuFe2O4therm or CuFe2O4gel (b). Experimental conditions: [CuFe2O4] ¼ 0.1 g L
1, [Na2SO3]0 ¼ 500 mM, [organics]0 ¼ 10 mM, reaction time ¼ 30 min, pH 8.0.

X. Zhao et al. / Environmental Pollution 260 (2020) 114038

the homogeneous activation of sulfite with Cu(II) is initiated after the formation of Cu(II)eSO3 complexes: (i) one electron transfer in 
the Cu(II)eSO3 complexes leads to the formation of SO
3 , (ii) SO3 
reacts with dissolved oxygen to generate SO
, and (iii) SO reacts 5 5 with sulfite to form SO
4 (Chen et al., 2018a). It can be speculated that the CuFe2O4MOF/sulfite process undergoes similar radical production reactions. EPR measurements were used to qualitatively verify the major species in this process, as shown in Fig. 3a. No obvious signal is observed in the single CuFe2O4MOF system. For the CuFe2O4MOF/sulfite system, significant signals ascribed to N H DMPOeSO
3 adducts (a ¼ 14.7 G, ab ¼ 16.0 G) are identified (Ranguelova et al., 2012), and is consistent with the weaker signal observed in the single sulfite system. This can be expected because the initiation of sulfite autoxidation results in the formation of SO
3 
whereas SO
5 and SO4 are secondary radicals from the initially produced SO
3 . To further quantitatively differentiate the dominant radical species for iohexol abatement in the CuFe2O4MOF/sulfite system, radical quenching experiments were performed (Fig. 3b). Previous studies have reported that methanol can scavenge SO
4 and HO at a rate of 3.2  106 M
1 s
1 and 9.7  108 M
1 s
1, respectively. In contrast, TBA reacts with SO
at a rate of 4 8.0  105 M
1 s
1, three orders of magnitude lower than HO∙ (6  108 M
1 s
1) (Neta et al., 1988). Other radicals including SO
3 and SO
5 generated from sulfite autoxidation are fairly inert towards methanol and TBA (kapp  103 M
1 s
1). As shown in Fig. 3b, the significant inhibition of iohexol abatement by methanol and the negligible inhibition by TBA indicate that SO
4 is the dominant species contributing to iohexol abatement in the CuFe2O4MOF/sulfite system. To evaluate the stability and reusability of CuFe2O4MOF, the particles were recollected and reused in consecutive heterogeneous reactions. The iohexol abatement efficiency of CuFe2O4MOF recycled five times is shown in Fig. 4a. It can be seen that the removal of iohexol remains nearly constant. The crystallinity change of CuFe2O4MOF after five consecutive reactions was qualitatively measured by XRD (Fig. 4b). According to the XRD patterns of CuFe2O4MOF, no apparent variation is observed in the diffraction intensity compared to the original one. Considering that the diffraction intensity can reflect the crystallinity of the metal oxides (Faungnawakij et al., 2007), the crystallinity of CuFe2O4MOF is well maintained. The excellent stability of CuFe2O4MOF is also confirmed by its negligible rate of copper leaching described in the following section, which is significantly lower than that for CuO in our previous study (Wu et al., 2019). The above results demonstrate that CuFe2O4MOF can be considered an alternative catalyst for the activation of sulfite autoxidation in potential water treatment processes.

5

3.3. Effect of various parameters 3.3.1. Effect of CuFe2O4MOF and sulfite dosage Fig. 5a shows the effect of the CuFe2O4MOF dosage on iohexol abatement in the CuFe2O4MOF/sulfite process at pH 8.0. As illustrated in Fig. 5a, the abatement of iohexol exhibits a strong dependence on the CuFe2O4MOF dosage. The abatement efficiencies of iohexol within the reaction time of 20 min rise from 63% to 88% with an increase in the CuFe2O4MOF dosage from 0.01 to 0.2 g L
1. The abatement rate can be significantly accelerated. When the CuFe2O4MOF dosage is controlled at 0.1 g L
1, iohexol abatement can be completed within 2 min. Apparently, an increase in the CuFe2O4MOF dosage provides more active catalytic sites, which can significantly activate the sulfite autoxidation process and improve the concomitant iohexol abatement. A further increase in the CuFe2O4MOF dosage from 0.1 to 0.2 g L
1 will not lead to an obvious difference in the iohexol abatement, probably due to the selfquenching effect (Chen et al., 2018b). Thus, a CuFe2O4MOF dosage of 0.1 g L
1 was adopted in the following experiments. Fig. 5b shows the influence of the sulfite dosage on the abatement of iohexol in the CuFe2O4MOF/sulfite process. The iohexol abatement efficiency is considerably increased from 55% to 88% when the initial sulfite concentration rises from 100 to 500 mM. When the concentration is increased to 1000 mM, the removal of iohexol is reduced, which can be attributed to the fact that sulfite with a high concentration can compete with the reactive radicals with the organic contaminants (Xie et al., 2017). 3.3.2. Effect of pH Fig. 6a shows the effect of the solution pH (4.0e10.0) on iohexol abatement by the CuFe2O4MOF/sulfite process. A significant pH dependence of iohexol abatement is observed. Furthermore, iohexol adsorption by CuFe2O4MOF is negligible (Fig. S7). In the pH range of 4.0e10.0, the maximum iohexol abatement in the CuFe2O4MOF/sulfite process is achieved at pH 8.0 and 10.0. The abatement efficiency is reduced for relatively acidic solutions (pH 4.0e6.0). Sulfite consumption shows a similar trend (Fig. S8). Considering that the pKa value of bisulfite is 7.2, it can be assumed that a large fraction of HSO
3 is present in relatively acidic solutions while more SO2
exists in relatively alkaline solutions (pH 3 8.0e10.0). Therefore, a higher efficiency of the CuFe2O4MOF/sulfite process under higher pH conditions is probably because SO2
3 is mainly involved in the complexation reactions with CuFe2O4MOF and favors the replacement of the hydroxyl group of the surface Cu(II)eOH to form surface Cu(II)eSO2
complexes (Chen et al., 3 2018a). In the homogeneous activation of sulfite with Cu(II), a high pH value up to 10 is more favorable for contaminant removal

Fig. 3. EPR spectra for CuFe2O4MOF/sulfite process in the presence of DMPO (a) and effect of radical scavengers on iohexol abatement in CuFe2O4MOF/sulfite process (b). Experimental conditions: [CuFe2O4MOF] ¼ 0.1 g L
1, [sulfite]0 ¼ 500 mM, pH ¼ 8.0, (a) [DMPO] ¼ 100 mM; (b) [iohexol] ¼ 10 mM, [methanol] or [TBA] ¼ 20 mM.

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X. Zhao et al. / Environmental Pollution 260 (2020) 114038

Fig. 4. Iohexol abatement in consecutive reactions with CuFe2O4MOF (a) and XRD patterns of CuFe2O4MOF before and after five recycles (b). Experimental conditions: [CuFe2O4MOF] ¼ 0.1 g L
1, [sulfite]0 ¼ 500 mM, [iohexol]0 ¼ 10 mM, pH ¼ 8.0.

Fig. 5. Effect of CuFe2O4MOF dosage (a) and sulfite dosage (b) on iohexol abatement in CuFe2O4MOF/sulfite process. Experimental conditions: pH ¼ 8.0, [iohexol]0 ¼ 10 mM, (a) [sulfite]0 ¼ 500 mM, (b) [CuFe2O4MOF] ¼ 0.1 g L
1.

Fig. 6. Effect of pH on iohexol abatement (a) and copper leaching (b) in CuFe2O4MOF/sulfite process. Experimental conditions: [CuFe2O4MOF] ¼ 0.1 g L
1, [sulfite]0 ¼ 500 mM, [iohexol]0 ¼ 10 mM.

compared to pH 8.0 (Zhao et al., 2019). However, this discrepancy is not observed in the CuFe2O4MOF/sulfite process. Regarding the fact that the pHpzc of CuFe2O4MOF is approximately 8.0, the increase in the negatively charged surface is not favorable for the static interaction between the CuFe2O4MOF surface and the sulfite. Moreover, an increase in the solution pH leads to a significant reduction in the copper leaching (Fig. 6b). The copper dissolution is only 0.01 mg L
1 at a CuFe2O4MOF dosage of 0.1 g L
1 for pH 8.0, which confirms the stability of CuFe2O4MOF. This high stability is closely related to its spinel structure.

3.3.3. Effect of anions and humic acid 2
The presence of anions (Cl
, HCO
3 , HPO4 , etc.) is ubiquitous in aquatic environments and their reactions with SO
4 could inhibit the abatement of organic contaminants due to the formation of weaker radicals (Wang et al., 2011; Bennedsen et al., 2012; Hu et al., 2017). As shown in Fig. S9a, iohexol abatement is greatly prohibited in the presence of Cl
. It can be seen that the iohexol abatement efficiency is reduced by ~50% in the presence of 2 mM Cl
. This is reasonable because the oxidation of Cl
with SO
4 (kCl
þSO4
¼ 2.8  108 M
1 s
1) produces chlorine radicals (Cl
2 ), which have a lower oxidation potential (2.09 V) than that of SO
4

X. Zhao et al. / Environmental Pollution 260 (2020) 114038

(2.5e3.1 V) (Anipsitakis et al., 2006). As the concentration of Cl
rises from 2 to 8 mM, a slight decrease is found in the efficacy for iohexol abatement. This experimental result suggests that Cl
2 may mainly serve as the reactive radicals in the abatement of iohexol in the normal Cl
concentration range of water bodies. Fig. S9b shows the impact of HCO
3 on iohexol abatement CuFe2O4MOF/sulfite process. It can be seen that the presence of HCO
3 has an inhibitory effect on the iohexol abatement. Fewer reactive carbonate radicals (CO
3 ) with an oxidation potential of 
1.6 V can be produced by SO
4 /HO oxidation of HCO3 , leading to a decline in iohexol abatement (Wang et al., 2019). The presence of 2
NO
shows a negligible influence on the iohexol 3 and HPO4 abatement (Figs. S9c and d), which can be ascribed to the lower 2

reactivity of NO
3 and HPO4 towards SO4 as compared to iohexol. Natural organic matter in the natural aquatic system could compete with the highly reactive species with trace organic contaminants and thus inhibit their abatement efficiency by AOPs (Hu and Long, 2016). Fig. S9e shows the effect of humic acid (HA), typical natural organic matter, on iohexol abatement in the CuFe2O4MOF/sulfite process. The iohexol abatement efficiency is reduced when the concentration of humic acid is increased from 0 to 4 mg L
1. The inhibitory effect could be due to the following two aspects: (i) humic acid at a relatively high concentration can consume the reactive radical, and (ii) humic acid can inhibit the complexation reactions of the catalysts with sulfite via strong pep interactions (Duan et al., 2016). Iohexol abatement in a sample of urban wastewater effluent shows a decline compared with that in deionized water (Fig. S9f). This can be expected considering that the presence of natural organic matter and some anions can consume radicals and exert an inhibitory effect on the abatement of organic contaminants.

3.4. Mechanistic aspects A previous report indicated that the variation of ionic strength exerts a significant influence on the outer-sphere complexation between the ligands and the surface-active metal sites of the solid particles with regard to their equilibrium and kinetics. Conversely, inner-sphere interactions are not influenced by the increase in ionic strength. Therefore, the effect of ionic strength on the abatement of target contaminants can reflect the interactions between the ligands and the active metal sites of particles. In this study, NaClO4 was employed to control the solution ionic strength in order to study its influence on iohexol abatement in the CuFe2O4MOF/sulfite process. As shown in Fig. 7a, when the ionic strength is increased

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from 0 to 200 mM, the abatement of iohexol in the CuFe2O4MOF/ sulfite process is almost not affected. This result indicates the occurrence of a strong inner-sphere complexation between the sulfite and the active Cu(II) sites on the CuFe2O4MOF surface. With respect to the formation of inner-sphere complexation, in situ analysis of the interactions between the sulfite and the CuFe2O4MOF surface was performed with ATR-FTIR, as shown in Fig. 7b. According to Goodsel et al., the IR band at the wavenumber of 941 cm
1 can be assigned to the SeO stretching vibration of sulfite (Goodsel et al., 1972). A blueshift of 18 cm
1 for this band in the presence of CuFe2O4 MOF is observed compared to the sulfite solution alone. Based on the predictions of Cotton and Francis, there would be a shift of the SeO stretching vibration of sulfite to higher frequencies when monodentate sulfite surface complexes are formed via the sulfur atom (Cotton and Francis, 1960). In a recent study on the CoFe2O4/sulfite process, the surface Co(II) sites react directly with the sulfite with the concomitant formation of a surface Co(II)eSO2
3 complex (Liu et al., 2017). It can be assumed that there is a formation of surface Cu(II)eSO2
3 complexes (Eq. (1)), in which one electron transfer could occur, producing Cu(I) and SO
3 (Eq. (2)).
2
≡Cu(II)eOH þ SO2
3 / ≡Cu(II)eSO3 þ OH

(1)


≡Cu(II)eSO2
3 / ≡Cu(I) þ SO3

(2)

The metal valence states of CuFe2O4MOF before and after the reaction are characterized by XPS (ex situ and indirect analysis). As illustrated in Fig. 8, no variations are identified from the characteristic satellites of Fe 2p, demonstrating that there is no occurrence of a redox reaction for iron on the CuFe2O4MOF surface after the sulfite activation reaction. The binding energies appearing at 933.5 and 934.7 eV are attributed to the Cu 2p3/2 of Cu(I) and Cu(II). According to the peak area, the proportion of Cu(II) can be calculated. The decrease in the Cu(II) proportion from 59.1% to 42.4% and the corresponding increase in the Cu(I) proportion confirm that surface Cu(II)eSO2
complexes may undergo intramolecular electron 3 transfer with a concomitant formation of Cu(I). For the O 1s spectra, the peak at 531.3 eV represents the adsorbed oxygen or the surface hydroxyl groups occupying 22.2% while the peak at 529.8 eV represents the lattice oxygen occupying 77.8%. After the activation process, the proportion of adsorbed oxygen or surface hydroxyl groups is increased from 22.2% to 28.2%. This increase may be due to the formation of surface Cu(II)eOH complexes. In summary, the surface Cu(II)eCu(I)eCu(II) redox cycle is critical for the efficient SO
4 production from sulfite autoxidation.

Fig. 7. Effect of ionic strength on iohexol abatement in CuFe2O4MOF/sulfite process (a) and ATR-FTIR spectra of the sulfite solution alone, the oxide particles in water, and the oxide particles in sulfite solution (b). Experimental conditions: (a) [CuFe2O4MOF] ¼ 0.1 g L
1, [sulfite]0 ¼ 500 mM, [iohexol]0 ¼ 10 mM, pH ¼ 8.0; (b) [CuFe2O4MOF] ¼ 0.1 g L
1, [sulfite]0 ¼ 500 mM, pH ¼ 8.0.

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X. Zhao et al. / Environmental Pollution 260 (2020) 114038

Fig. 8. XPS spectra of CuFe2O4MOF before and after reaction.

TOC abatement efficiency was measured in order to assess the mineralization process of iohexol in the CuFe2O4MOF/sulfite process (Fig. S10). TOC exhibits a negligible decline (<5%) whereas the iohexol abatement efficiency can reach 80% within 30 min of the reaction. This negligible mineralization and substantial abatement

of iohexol indicate the occurrence of side chain reactions for iohexol. Therefore, oxidation intermediates (OI) produced from iohexol (10 mM) in the CuFe2O4MOF/sulfite process at pH 8.0 were detected by UPLCeQTOFeMS. Several major oxidation intermediates of iohexol are identified and summarized in Table S2.

Fig. 9. Proposed pathways for iohexol abatement in CuFe2O4MOF/sulfite process.

X. Zhao et al. / Environmental Pollution 260 (2020) 114038

According to the above previous report and the current study, the possible iohexol degradation pathways are illustrated in Fig. 9. For 
the SO
4 -based oxidation process, SO4 reacts with organic contaminants via H-abstraction, addition, and electron transfer (Yang et al., 2015; Wang et al., 2019). OI818 is generated from iohexol attacked by SO
4 via H-abstraction (reaction (1)). Then, OI708 is formed from OI818 via the substitution by hydroxyl groups at the lost iodine sites (reaction 4). OI744 can be generated from iohexol via H-abstraction (reaction (1)) and the transformation of aromatic amides to aromatic carbamoyl (reaction 2), which is further transformed into OI732 via amino oxidation (reaction 3) and amide hydrolysis (reaction 7). OI622 can then be produced from OI732 via the deiodination (reaction 6) and subsequent hydroxyl substitution. Alcohol oxidation (reaction 5) can generate OI837 and the subsequent amide hydrolysis (reaction 7) leads to the formation of O806. In summary, deiodination, hydroxyl addition, amide hydrolysis, and amine oxidation are the major reaction pathways for the transformation of iohexol in the CuFe2O4MOF/sulfite process, which is consistent with the SO
4 -based oxidation of iohexol in a previous study (Wang et al., 2019). 4. Conclusions In this study, CuFe2O4MOF is prepared via a MOF-templated method and characterized by a series of techniques. CuFe2O4MOF exhibits superior physicochemical properties with excellent crystallinity, considerable specific surface area, and magnetic separation ability. CuFe2O4MOF demonstrates excellent and stable catalytic performance for the activation of sulfite autoxidation to SO
4 for abatement of organic contaminants. Increasing the dosages of the catalyst and sulfite (up to 500 mM), and a higher solution pH are favorable for iohexol abatement. The presence of humic acid, Cl
, and HCO
3 shows an inhibitory influence on iohexol abatement due 
to their quenching effect on SO
4 . Therefore, this SO4 -based advanced oxidation process is suitable for the treatment of water containing a low concentration of natural organic matter and low salinity. The occurrence of inner-sphere complexation between the sulfite and the surface of CuFe2O4MOF is confirmed by ATR-FTIR analysis and the negligible ionic effect on the iohexol abatement. The results of this study suggest that activation of sulfite autoxidation with CuFe2O4MOF might be an alternative for the construction of a simple, nontoxic, cost-effective, and highly effective SO
4 based advanced oxidation process. Funding This work was funded by the National Natural Science Foundation of China (Grant No. 51808233). Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled Activation of sulfite autoxidation with CuFe2O4 prepared by MOF-templated method for abatement of organic contaminants. CRediT authorship contribution statement Xiaodan Zhao: Conceptualization, Methodology, Writing original draft, Project administration, Funding acquisition. Wenjing Wu: Formal analysis, Investigation, Validation. Guohua Jing:

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Writing - review & editing. Zuoming Zhou: Supervision, Writing review & editing. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2020.114038. References Anipsitakis, G.P., Dionysiou, D.D., Gonzalez, M.A., 2006. Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. Implications of chloride ions. Environ. Sci. Technol. 40, 1000e1007. Bennedsen, L.R., Muff, J., Sogaard, E.G., 2012. Influence of chloride and carbonates on the reactivity of activated persulfate. Chemosphere 86, 1092e1097. Chan, T.W., Graham, N.J.D., Chu, W., 2010. Degradation of iopromide by combined UV irradiation and peroxydisulfate. J. Hazard Mater. 181, 508e513. Chen, L., Tang, M., Chen, C., Chen, M., Luo, K., Xu, J., Zhou, D., Wu, F., 2017. Efficient bacterial inactivation by transition metal catalyzed auto-oxidation of sulfite. Environ. Sci. Technol. 51, 12663e12671. Chen, L., Luo, T., Yang, S., Xu, J., Liu, Z., Wu, F., 2018a. Efficient metoprolol degradation by heterogeneous copper ferrite/sulfite reaction. Environ. Chem. Lett. 16, 599e603. Chen, L., Ding, D., Liu, C., Cai, H., Qu, Y., Yang, S., Gao, Y., Cai, T., 2018b. Degradation of norfloxacin by CoFe2O4-GO composite coupled with peroxymonosulfate: a comparative study and mechanistic consideration. Chem. Eng. J. 334, 273e284. Cotton, F.A., Francis, R., 1960. Sulfoxides as ligands. I. A preliminary survey of methyl sulfoxide complexes. J. Am. Chem. Soc. 82, 2986e2991. Deng, Y., Ezyske, C.M., 2011. Sulfate radical-advanced oxidation process (SR-AOP) for simultaneous removal of refractory organic contaminants and ammonia in landfill leachate. Water Res. 45, 6189e6194. Ding, W., Huang, X., Zhang, W., Wu, F., Li, J., 2019. Sulfite activation by a lowleaching silica-supported copper catalyst for oxidation of As(III) in water at circumneutral pH. Chem. Eng. J. 359, 1518e1526. Dong, H., Qiang, Z., Hu, J., Sans, C., 2017. Accelerated degradation of iopamidol in iron activated persulfate systems: roles of complexing agents. Chem. Eng. J. 316, 288e295. Duan, X., Ao, Z., Zhou, L., Sun, H., Wang, G., Wang, S., 2016. Occurrence of radical and nonradical pathways from carbocatalysts for aqueous and nonaqueous catalytic oxidation. Appl. Catal. B Environ. 188, 98e105. Faungnawakij, K., Tanaka, Y., Shimoda, N., Fukunaga, T., Kikuchi, R., Eguchi, K., 2007. Hydrogen production from dimethyl ether steam reforming over composite catalysts of copper ferrite spinel and alumina. Appl. Catal. B Environ. 74, 144e151. Goodsel, A.J., Low, M.J.D., Takezawa, N., 1972. Reactions of gaseous pollutants with solids. II. Infrared study of sorption of SO2 on MgO. Environ. Sci. Technol. 6, 268e273. Guo, Y., Lou, X., Fang, C., Xiao, D., Wang, Z., Liu, J., 2013. Novel photo-sulfite system: toward simultaneous transformations of inorganic and organic pollutants. Environ. Sci. Technol. 47, 11174e11181. Guo, T., Wang, K., Zhang, G., Wu, X., 2019. A novel a-Fe2O3@g-C3N4 catalyst: synthesis derived from Fe-based MOF and its superior photo-Fenton performance. Appl. Surf. Sci. 469, 331e339. Hu, P., Long, M., 2016. Cobalt-catalyzed sulfate radical-based advanced oxidation: a review on heterogeneous catalysts and applications. Appl. Catal. B Environ. 181, 103e117. Hu, J., Dong, H., Qu, J., Qiang, Z., 2017. Enhanced degradation of iopamidol by peroxymonosulfate catalyzed by two pipe corrosion products (CuO and d-MnO2). Water Res. 112, 1e8. Huang, Y., Sun, X., Huo, S., Li, Y., Zhong, C., 2019. Core-shell dual-MOF heterostructures derived magnetic CoFe2O4/CuO (sub)microcages with superior catalytic performance. Appl. Surf. Sci. 466, 637e646. Jiang, B., Liu, Y., Zheng, J., Tan, M., Wang, Z., Wu, M., 2015. Synergetic transformations of multiple pollutants driven by Cr(VI)-sulfite reactions. Environ. Sci. Technol. 49, 12363e12371. Johnson, R.L., Tratnyek, P.G., Johnson, R.O., 2008. Persulfate persistence under thermal activation conditions. Environ. Sci. Technol. 42, 9350e9356. Li, H., Li, Q., He, Y., Zhang, N., Xu, Z., Wang, Y., 2018. Facile fabrication of magnetic metal-organic framework composites for the highly selective removal of cationic dyes. Materials 11, 744e756. Liu, Z., Yang, S., Yuan, Y., Xu, J., Zhu, Y., Li, J., Wu, F., 2017. A novel heterogeneous system for sulfate radical generation through sulfite activation on a CoFe2O4 nanocatalyst surface. J. Hazard Mater. 324, 583e592. Luo, T., Yuan, Y., Zhou, D., Luo, L., Li, J., Wu, F., 2019. The catalytic role of nascent Cu(OH)2 particles in the sulfite-induced oxidation of organic contaminants. Chem. Eng. J. 363, 329e336. Neta, P., Huie, R.E., Ross, A.B., 1988. Rate constants for reactions of inorganic radicals in aqueous solution. J. Phys. Chem. Ref. Data 17, 1027e1284. Ning, B.N., Graham, N.J.D., 2008. Ozone degradation of iodinated pharmaceutical compounds. J. Environ. Eng. 134, 944e953. Ranguelova, K., Rice, A.B., Khajo, A., Triquigneaux, M., Garantziotis, S., Magliozzo, R.S., Mason, R.P., 2012. Formation of reactive sulfite-derived free

10

X. Zhao et al. / Environmental Pollution 260 (2020) 114038

radicals by the activation of human neutrophils: an ESR study. Free Radic. Biol. Med. 52, 1264e1271. Selima, S.S., Khairy, M., Mousa, M.A., 2019. Comparative studies on the impact of synthesis methods on structural, optical, magnetic and catalytic properties of CuFe2O4. Ceram. Int. 45, 6535e6540. Wang, Z., Yuan, R., Guo, Y., Xu, L., Liu, J., 2011. Effects of chloride ions on bleaching of azo dyes by Co2þ/oxone reagent: kinetic analysis. J. Hazard Mater. 190, 1083e1087. Wang, J., Wang, S., 2018. Activation of persulfate (PS) and peroxymonosulfate (PMS) and application for the degradation of emerging contaminants. Chem. Eng. J. 334, 1502e1517. Wang, X., Wang, Z., Tang, Y., Xiao, D., Zhang, D., Huang, Y., Guo, Y., Liu, J., 2019. Oxidative degradation of iodinated X-ray contrast media (iomeprol and iohexol) with sulfate radical: an experimental and theoretical study. Chem. Eng. J. 368, 999e1012. Wendel, F.M., Eversloh, C.L., Machek, E.J., Duirk, S.E., Plewa, M.J., Richardson, S.D., Ternes, T.A., 2014. Transformation of iopamidol during chlorination. Environ. Sci. Technol. 48, 12689e12697. Wu, W., Zhao, X., Jing, G., Zhou, Z., 2019. Efficient activation of sulfite autoxidation process with copper oxides for iohexol degradation under mild conditions. Sci. Total Environ. 695, 133836e133846. Xie, P., Guo, Y., Chen, Y., Wang, Z., Shang, R., Wang, S., Ding, J., Wan, Y., Jiang, W., Ma, J., 2017. Application of a novel advanced oxidation process using sulfite and zero-valent iron in treatment of organic pollutants. Chem. Eng. J. 314, 240e248. Xu, J., Gao, J., Liu, Y., Li, Q., Wang, L., 2017. Fabrication of In2O3/Co3O4-palygorskite composites by the pyrolysis of In/Co-MOFs for efficient degradation of

methylene blue and tetracycline. Mater. Res. Bull. 91, 1e8. Yang, Y., Jiang, J., Lu, X., Ma, J., Liu, Y., 2015. Production of sulfate radical and hydroxyl radical by reaction of ozone with peroxymonosulfate: a novel advanced oxidation process. Environ. Sci. Technol. 49, 7330e7339. Yang, S., Qiu, X., Jin, P., Dzakpasu, M., Wang, X.C., Zhang, Q., Zhang, L., Yang, L., Ding, D., Wang, W., Wu, K., 2018. MOF-templated synthesis of CoFe2O4 nanocrystals and its coupling with peroxymonosulfate for degradation of bisphenol A. Chem. Eng. J. 353, 329e339. , J.P., 2013. Production of sulfate radical from peroxZhang, T., Zhu, H., Croue ymonosulfate induced by a magnetically separable CuFe2O4 spinel in water: efficiency, stability, and mechanism. Environ. Sci. Technol. 47, 2784e2791. Zhang, J., Ma, J., Song, H., Sun, S., Zhang, Z., Yang, T., 2018. Organic contaminants degradation from the S(IV) autoxidation process catalyzed by ferrousmanganous ions: a noticeable Mn(III) oxidation process. Water Res. 133, 227e235. Zhao, X., Wu, W., Yan, Y., 2019. Efficient abatement of an iodinated X-ray contrast media iohexol by Co(II) or Cu(II) activated sulfite autoxidation process. Environ. Sci. Pollut. Res. 26, 24707e24719. Zhou, H.C.‘‘Joe’’, Kitagawa, S., 2014. Metaleorganic frameworks (MOFs). Chem. Soc. Rev. 43, 5415e5418. Zhou, L., Ferronato, C., Chovelon, J.M., Sleiman, M., Richard, C., 2017. Investigations of diatrizoate degradation by photo-activated persulfate. Chem. Eng. J. 311, 28e36. Zhou, D., Chen, L., Li, J., Wu, F., 2018. Transition metal catalyzed sulfite autooxidation systems for oxidative decontamination in waters: a state-of-the-art minireview. Chem. Eng. J. 346, 726e738.