Synthesis of Spinel Ferrite MFe2O4 (M = Co, Cu, Mn, and Zn) for Persulfate Activation to Remove Aqueous Organics: Effects of M-Site Metal and Synthetic Method

Metal species and synthetic method determine the characteristics of spinel ferrite MFe₂O₄. Herein, a series of MFe₂O₄ (M = Co, Cu, Mn, Zn) were synthesized to investigate the effect of M-site metal on persulfate activation for the removal of organics from aqueous solution. Results showed that M-site metal of MFe₂O₄ significantly influenced the catalytic persulfate oxidation of organics. The efficiency of the removal of organics using different MFe₂O₄ + persulfate systems followed the order of CuFe₂O₄ > CoFe₂O₄ > MnFe₂O₄ > ZnFe₂O₄. Temperature-programmed oxidation and cyclic voltammetry analyses indicated that M-site metal affected the catalyst reducibility, reversibility of M²⁺/M³⁺ redox couple, and electron transfer, and the strengths of these capacities were consistent with the catalytic performance. Besides, it was found that surface hydroxyl group was not the main factor affecting the reactivity of MFe₂O₄ in persulfate solution. Moreover, synthetic methods (sol–gel, solvothermal, and coprecipitation) for MFe₂O₄ were further compared. Characterization showed that sol–gel induced good purity, porous structure, large surface area, and favorable element chemical states for ferrite. Consequently, the as-synthesized CuFe₂O₄ showed better catalytic performance in the removal of organics (96.8% for acid orange 7 and 62.7% for diclofenac) along with good reusability compared with those obtained by solvothermal and coprecipitation routes. This work provides a deeper understanding of spinel ferrite MFe₂O₄ synthesis and persulfate activation.

GRAPHICAL ABSTRACT

Graphical Abstract. Effects of M-site metal and synthetic method of spinel ferrite on persulfate activation for organics removal.

Highlights

- Effects of M-site metal and synthetic method on ferrite were investigated.

- Suitable ferrite and its synthetic method for PS activation were screened out.

- Catalytic PS performance was CuFe₂O₄ > CoFe₂O₄ > MnFe₂O₄ > ZnFe₂O₄.

- M-site metal affected MFe₂O₄ reducibility, M²⁺/M³⁺ reversibility, and electron transfer.

- Sol–gel method was ideal to synthesize ferrite to activate PS for organics removal.

Introduction

Advanced oxidation processes (AOPs) that involve highly reactive radicals are powerful treatment techniques for the removal of organics in water, especially for removing highly toxic, persistent, and nonbiodegradable organics (Wang and Wang, 2018; Malvestiti et al., 2019). Among AOPs, activated persulfate (PS) process has received extensive attention. Compared with hydroxyl radical generated in conventional AOPs (Fenton or Fenton-like), sulfate radical ($\mathrm{\text{S}}{{\mathrm{\text{O}}}_{\mathrm{\text{4}}}}^{•-}$) generated from PS has comparable oxidizing power (E⁰ = 2.5-3.1 V), higher selectivity (for benzene ring and unsaturated bond), longer half-life (30-40 μs), and greater stability, and is less influenced by natural organic materials (Oh et al., 2016; Alexopoulou et al., 2019). Moreover, PS offers some advantages over other AOP oxidants [e.g., H₂O₂ and peroxymonosulfate (PMS)], such as the ease of storage, high stability, high redox potential, good solubility, and relatively low cost (Xu and Li, 2010; Wacławek et al., 2017). Therefore, activated PS process is expected to be promising for treating organics.

Heterogeneous catalysis is the most studied method for PS activation, not only because of the energy conservation and ease of operation (vs. thermolysis, photolysis, radiolysis, etc.) (Oh et al., 2016; Zhu et al., 2019) but also owing to the mild reaction conditions, retrievability, and little metal dissolution (vs. homogeneous catalysis) (Wang and Wang, 2018). Available and efficient catalytic material is the priority in heterogeneous catalysis. Over the past decades, iron oxides have been generally used as heterogeneous catalysts because of their low price, abundant reserves, and nontoxicity (Li et al., 2017; Silveira et al., 2018). However, their weak catalytic activity limits the efficiency of pollutant removal (Lei et al., 2015). Hitherto, multimetallic iron-based materials can relieve this problem and render catalytic processes more efficient toward long-term application (Deng et al., 2017; Wacławek et al., 2017). With ongoing explorations, a typical bimetallic iron-based oxide, spinel ferrite with the general formula of MFe₂O₄ (M is a divalent 3d transition metal such as Co, Cu, Mn, and Zn), has attracted much attention (Lassoued et al., 2017). The excellent activity and desirable magnetic recovery property render it useful in several applications (Garcia-Muñoz et al., 2020). For example, CoFe₂O₄ was effective for activating PMS to degrade atrazine (Li et al., 2018). CuFe₂O₄ and MnFe₂O₄ could be applied as catalysts of PS for acetaminophen and phenol removal (Stoia et al., 2017; Zhang et al., 2019). Further, in combination with PS, ZnFe₂O₄ exhibited good photocatalytic performance in the degradation of Orange II (Cai et al., 2016).

As mentioned, although certain ferrites have been applied to activate PS, the differences in the effectiveness of various ferrites in organics treatment have not been studied well. For example, metal species in a catalyst can critically impact the catalytic performance. Anipsitakis and Dionysiou (2004) reported that Fe²⁺ was the most efficient metal ion to activate H₂O₂, while Co²⁺ was the best for PMS activation and Ag⁺ showed the best results toward PS activation. The metal in M-site of MFe₂O₄ was known to be the main catalytic center for PS activation (Equation 1) (Li et al., 2017). However, the effect of different M-site metals in ferrite on PS activation to remove organics is not yet clear. Moreover, synthetic method is important for catalyst, which usually results in distinction on morphology, particle size, surface property, magnetism, etc., and thereby influences the catalytic performance (Kennaz et al., 2017; Zhang et al., 2018). Gupta and Garg (2017) found that compared with those prepared by coprecipitation and sol–gel methods, CuO/CeO₂ synthesized by solution combustion method led to the maximum oxidation of organics and showed the minimum metal leaching in catalytic H₂O₂ system. Priyanka et al. (2019) found that modified TiO₂ with lower band gap energy synthesized by sol–gel method had better mineralization of gray water in photocatalysis vs. those synthesized by ultrasonication and microwave methods. Ferrite can be also prepared by various methods including sol–gel, solvothermal, coprecipitation, and high-energy milling (Zhang et al., 2018). Hence, it is necessary to explore the effect of synthetic method of ferrite on organics removal in PS system.

$\begin{array}{ll}\text{M}(\text{II})+{\text{S}}_2{{\text{O}}_8}^{2-}\to \text{M}(\text{III})+{\text{S}{\text{O}}_4}^{•-}+{\text{S}{\text{O}}_4}^{2-}& (1)\end{array}$

In this work, the differences and causes in the catalytic performance of a series of MFe₂O₄ (M = Co, Cu, Mn, and Zn) were explored. Then, the characteristics and catalytic performances of ferrites synthesized by different methods, i.e., sol–gel, solvothermal, and coprecipitation, for activated PS process were investigated with CuFe₂O₄ as the representative. The efficacy of the catalyst was evaluated by applying it in the removal of two model refractory organics, a traditional dye pollutant [acid orange 7 (AO7)] and an emerging pharmaceutical pollutant (diclofenac). The main objectives were to (i) scrutinize the high-efficiency PS activator out and reveal the effect of M-site metal on the reactivity of ferrite and (ii) determine the effect of synthetic method on the performance of ferrite and find an ideal ferrite synthetic method for PS activation in organics treatment. The results can contribute to better understanding of the synthesis and application of ferrite and promote decontamination with activated PS process.

Materials and Methods

Materials

All chemicals used were of analytical grade. Ni(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, Zn(NO₃)₂·6H₂O, NaOH, H₂SO₄ (95-98%), HCl (36-38%), C₂H₆O₂ (ethylene glycol), and C₂H₅OH (ethanol) were purchased from Beijing Chemical Works, China. Cu(NO₃)₂·3H₂O and Na₂S₂O₈ were obtained from Tianjin Fuchen Chemical Reagents Factory, China. Co(NO₃)₂·6H₂O and diclofenac sodium were purchased from Shanghai Macklin Biochemical Co. Ltd., China. Mn(NO₃)₂ (50% solution), C₆H₈O₇·H₂O (citric acid), C₂H₃NaO₂·3H₂O (NaAc), and Na₂SO₄ were obtained from Sinopharm Chemical Reagent Co. Ltd., China. AO7 was purchased from Tianjin Guangfu Fine Chemical Research Institute, China.

Synthesis of Ferrite

A series of spinel ferrite MFe₂O₄ (M = Co, Cu, Mn, and Zn) were prepared by sol–gel method (Li et al., 2017) to investigate the effect of M-site metal on PS activation. Then, the three most common methods, sol–gel, solvothermal (Ueda Yamaguchi et al., 2016), and coprecipitation (Jaafarzadeh et al., 2017), were used to study the effect of synthetic method on the properties of ferrite obtained for PS activation, with CuFe₂O₄ as a representative. CuFe₂O₄ synthesized by sol–gel, solvothermal, and coprecipitation methods were denoted as CuFe₂O₄-SG, CuFe₂O₄-ST, and CuFe₂O₄-CP, respectively. The detailed synthetic procedures were described in the Supplementary Information.

Characterization of Ferrite

X-ray powder diffraction (XRD) of ferrite was carried out on a Rigaku D/max-rc diffractometer using Cu K_α radiation. The morphology of ferrite was observed on a Hitachi S 4700 scanning electron microscope (SEM). N₂ adsorption–desorption analysis was performed on a QuadraSorb Station 4 instrument. X-ray photoelectron spectra (XPS) were measured on a Thermo Fisher Scientific EscaLab 250Xi system with a monochromatic Al K_α source.

Catalytic PS Oxidation Experiment

The typical experimental steps were as follows: known amounts of ferrite and PS solution were added simultaneously into a 20 mg/L organics solution (AO7 or diclofenac) under magnetic stirring. At known intervals, 3 ml solution was taken using a syringe and filtered through a 0.22 μm filter head. The concentration of the organics in the filtrate was analyzed to evaluate the efficacy of ferrite. The used ferrite was collected using a magnet, washed several times with ethanol and deionized water, and dried for the next run to investigate its reusability. The experiments were done in triplicate.

Analytical Methods

The concentration of AO7 was determined by TU-1900 UV-visible spectrophotometer (Beijing Persee General Instrument Co., Ltd.) at a maximum absorbance wavelength of 484 nm. The concentration of diclofenac was determined by Ultimate 3000 high performance liquid chromatography (Thermo Fisher Scientific Inc.). The UV detection wavelength was 275 nm, and the mobile phase consisted of acetonitrile and 0.2% acetic acid solution at a volume ratio of 7:3.

The redox property of ferrite was evaluated by oxygen temperature-programmed oxidation (O₂-TPO) and cyclic voltammetry (CV). O₂-TPO was performed from 200 to 500°C at a rate of 10°C/min on a ChemBET Pulsar TPR/TPD instrument (Quantachrome Instruments Inc.) equipped with a thermal conductivity detector (TCD) to measure the change of gas composition. A 5% O₂/He (vol.) gas mixture with a flow rate of 100 ml/min was used in the analysis. CV was conducted on a CHI 760E electrochemical workstation (Shanghai Chenhua instrument Co. Ltd.) with a foamed nickel working electrode, a platinum sheet counter electrode, and a saturated Ag/AgCl reference electrode. Before use, the foamed nickel electrode was dipped in the suspension of ferrite for 10 min to load the catalyst and then air-dried. A mixture of 0.1 mol/L Na₂SO₄ and 0.4 mmol/L PS was used as the electrolyte.

The surface hydroxyl group of ferrite was quantified by the saturated deprotonation method (Ren et al., 2015). In this method, 20 ml of a 0.05 mol/L NaOH solution dispersed with 0.12 g of ferrite was shaken for more than 4 h at 25°C. After separating the solid by filtration, the solution was titrated with a diluted HCl solution.

Results and Discussion

Catalytic Performance of Ferrites With Different M-Site Metals

To investigate the effect of M-site metal on the catalytic performance of spinel ferrite, a series of MFe₂O₄ (M = Co, Cu, Mn, and Zn) were synthesized by sol–gel method. XRD patterns presented in Supplementary Figure 1 confirmed the successful synthesis of the ferrite samples.

Figure 1 clearly shows that the removal efficiencies of organics in different MFe₂O₄ + PS systems were different, but all were higher than that of PS oxidation and MFe₂O₄ adsorption (Supplementary Figure 2). For both AO7 and diclofenac removal (Figures 1A,C), the catalytic performance of ferrite ranked as follows: CuFe₂O₄ > CoFe₂O₄ > MnFe₂O₄ > ZnFe₂O₄. The organics removal processes involved two stages: rapid adsorption-dominated stage (0-1 min) and catalytic degradation stage (> 1 min), which could be fitted by pseudo-first order reaction. The degradation rate constant (Figures 1B,D) was also significantly affected by M-site metal, and the trend was basically consistent with the aforementioned catalytic performance order of MFe₂O₄. In detail, CuFe₂O₄ presented the best and fastest catalytic performance in organics removal. Almost 87.6% AO7 was removed in PS solution coupled with CuFe₂O₄. In comparison, Yue et al. (2016) found that only 53.5% AO7 removal was obtained in Fe₃O₄ + PS system. Moreover, the diclofenac degradation rate constant of CuFe₂O₄ + PS system was about 3.5 times of that of thermally activated PS system at 60°C (Chen et al., 2016). The high removal of AO7 and diclofenac (which have different molecular structures: AO7 is an azo dye and diclofenac is a secondary aromatic amine drug) indicated that CuFe₂O₄ + PS could effectively remove multiple organic pollutants.

FIGURE 1

Figure 1. Organics removal and degradation rate constants in PS systems activated by ferrites with different M-site metals: (A,B) AO7 removal; (C,D) diclofenac removal. Conditions: [organics] = 20 mg/L, (A,B) catalyst dosage = 0.2 g/L, [PS] = 0.8 g/L, unadjusted pH = 6.5; (C,D) catalyst dosage = 0.6 g/L, [PS] = 0.1 g/L, pH = 5.

These results demonstrated that M-site metal indeed affected the catalytic performance of ferrite. Among the ferrites, CuFe₂O₄ was found to be the best activator of PS for organics removal.

Redox Properties of Ferrites With Different M-Site Metals

According to above degradation experiments, it has been identified that ferrites with different M-site metals exhibit different catalytic performances. As is known, for PS activation by transition metal, the basic mechanism is chemical reduction of PS through electron transfer (Wacławek et al., 2017). Thus, the reducibility of catalyst is probably the vital factor affecting the effectiveness of PS activation system (Wang and Wang, 2018). Therefore, O₂-TPO and CV were carried out to investigate the redox properties of ferrites with different M-site metals.

Figure 2 shows the O₂-TPO profiles of various MFe₂O₄. The temperature of O₂ consumption surge is an important parameter for evaluating the ease of oxidation-state change of M-site metal ion. As shown, the four ferrites exhibited distinct peaks with temperature increasing from 200 to 500°C, implying the oxidation reaction occurrence of Mⁿ⁺ to M⁽ⁿ⁺¹⁾⁺. The peak temperatures of CuFe₂O₄, CoFe₂O₄, MnFe₂O₄, and ZnFe₂O₄ gradually increased, at 330.8, 353.8, 384.6, and 401.9°C, respectively, which were similar to some other studies. For example, Wang et al. (2011) found that the initial oxidation of CoFe₂O₄ occurred at 350°C resulting from the oxidation of Co. Cihlar et al. (2017) reported that Mn²⁺ in binary oxide was mostly oxidized in the 350–500°C region. The lower oxidation temperature of MFe₂O₄, that was, the easier transition of the oxidation state of M-site metal ion accounted for its better performance in the activation of PS (Su et al., 2017).

FIGURE 2

Figure 2. O₂-TPO profiles of ferrites with different M-site metals.

To further reveal the redox properties of ferrites, CV curves of different MFe₂O₄ on electrodes were recorded (Figure 3). Except for the couple of redox peaks, the curves of all samples were identical in shape to the control curve, indicating that there was no interference of impossible peak in the solution. CuFe₂O₄ electrode exhibited a well-defined oxidation peak at 0.268 V, which was attributed to the Cu(II)/Cu(III) redox cycle. Likewise, the peaks at 0.350 and 0.401 V were assigned to the oxidation of Co(II) and Mn(II), respectively. ZnFe₂O₄ electrode gave an indistinct oxidation peak at 0.410 V. The lower potential of oxidation peak meant that it was easier for the catalyst to donate electrons, which was favorable for PS activation (Duan et al., 2018). Thus, the reducibility of the ferrites could be ranked as CuFe₂O₄ > CoFe₂O₄ > MnFe₂O₄ > ZnFe₂O₄. Moreover, there was no certain rule about the reduction peak positions of MFe₂O₄ electrodes. However, interestingly, for these ferrite electrodes, the trend of potential separation of redox peaks (ΔE_p) was similar to the trend of oxidation peak potential. CuFe₂O₄ electrode displayed the lowest ΔE_p of 0.179 V. Meanwhile, the ΔE_p values of CoFe₂O₄ and MnFe₂O₄ electrodes were higher at 0.269 and 0.276 V, respectively. In the case of ZnFe₂O₄, it might need much more negative potential than that in the CV curve to make it accept electrons to generate reduction peak. The lower ΔE_p of an electrode indicated the stronger reversibility and more electron transfer of M²⁺/M³⁺ redox reaction (Bard et al., 1980), which were good for the catalytic reaction.

FIGURE 3

Figure 3. Cyclic voltammograms obtained on electrodes of ferrites with different M-site metals after 4th cycle. Scan rate = 50 mV/s, Scan range = −0.5-1 V.

Therefore, from the above results, it can be concluded that M-site metal would affect the catalytic performance of MFe₂O₄ by affecting the reducibility, reversibility of M²⁺/M³⁺ redox couple, and electron transfer on the catalyst surface.

Surface Oxygen Functional Groups of Ferrites With Different M-Site Metals

The surface oxygen functional groups of a catalyst might participate in the activation of PS, and thereby affect the catalytic performance of the catalyst (Xiao et al., 2018). Therefore, ferrites with different M-site metals were characterized by FTIR spectra (Figure 4). The broad bands at about 3,416 cm⁻¹ of all samples indicated the obvious presence of hydroxyl group (-OH) (Zhang et al., 2019). The peak at 1,625 cm⁻¹ was due to the deformation vibration of water molecules in the interlayer (Parvas et al., 2014). The two adsorption bands at 1,560 and 1,410 cm⁻¹ (COO- stretching) implied the residual of some citrates in the pores of ferrite. The peak at around 570 cm⁻¹ was associated with the metal–oxygen bond (Zhao et al., 2018).

FIGURE 4

Figure 4. FTIR spectra of ferrites with different M-site metals.

Among the observed functional groups, -OH should be of particular concern. It was reported that phenol removal was related to the surface hydroxyl concentration of TiO₂ during catalytic ozonation (Song et al., 2010). Hydroxyl group on CuFe₂O₄ surface was found to be critical for radical generation in PMS activation (Guan et al., 2013). Therefore, surface -OH quantities of ferrites with different M-site metals were measured. As shown in Table 1, different M-site metals led to different -OH quantities. Unexpectedly, ferrite with good catalytic performance (e.g., CuFe₂O₄) did not have many surface -OH. This phenomenon was different from the finding of Ren et al. (2015) that MFe₂O₄ containing more surface -OH showed better catalytic performance for PMS. Ren et al. proposed that surface -OH was the main binding site for PMS (surface -OH of ferrite formed hydrogen bond with side -O-OH of PMS), and then PMS accepted electron from metal ion and its O-OH bond was broken to generate ${\text{S}{\text{O}}_4}^{•-}$. The results of this study showed that surface -OH was not crucial for the catalytic performance of MFe₂O₄ in PS system. The reason might be that PS activation involved a different process; ${\text{S}{\text{O}}_4}^{•-}$ in MFe₂O₄ + PS system was mainly generated from the fission of middle O-O bond of PS (Wang and Wang, 2018).

TABLE 1

Table 1. Surface hydroxyl quantities of ferrites with different M-site metals.

Characterization of CuFe₂O₄ Synthesized by Different Methods

According to the above results, CuFe₂O₄ was selected as the representative ferrite to further explore the effect of synthetic method on the physicochemical property and catalytic performance of ferrite for PS activation.

XRD Analysis

The XRD patterns of CuFe₂O₄ synthesized by sol–gel, solvothermal, and coprecipitation methods (CuFe₂O₄-SG, CuFe₂O₄-ST, and CuFe₂O₄-CP) were shown in Figure 5. The major crystal phase of the samples was in agreement with typical spinel CuFe₂O₄ (JCPDS 25-0283), indicating that CuFe₂O₄ was indeed formed by all three methods. Moreover, no obvious impurity peak was found in the XRD pattern of CuFe₂O₄-SG, while two weak Cu (JCPDS 89-2838) diffraction peaks were observed in CuFe₂O₄-ST and CuFe₂O₄-CP. Meanwhile, the XRD peaks of CuFe₂O₄-SG were stronger and sharper. These results showed that ferrite prepared by sol–gel method was purer with better crystallinity than the samples prepared by the other two methods.

FIGURE 5

Figure 5. XRD patterns of CuFe₂O₄ synthesized by different methods.

SEM Analysis

The SEM images in Figure 6 show that the morphological structure of ferrite strongly depended on synthetic method. CuFe₂O₄-CP particles agglomerated into large and compact bulk forms, which might be caused by particle sintering that occurred during the calcination of coprecipitation precursor. This tended to reduce the contact area between the catalyst and other reactants, which was not conducive to pollutant removal (Xue et al., 2007). CuFe₂O₄-ST had a typical morphology of a solvothermal catalyst (Ueda Yamaguchi et al., 2016; Chen et al., 2017) with high dispersion, relatively uniform spherical-like shape, and minimum particle size, which could increase the external surface area. CuFe₂O₄-SG particles were found to be of moderate size and irregular shape. By the observation of enlarged SEM images of the three samples (Figures 6D–F), it is noteworthy that CuFe₂O₄-SG showed a spongy structure. Owing to the volatilization of citric acid, CuFe₂O₄-SG did not sinter as CuFe₂O₄-CP did, but had many discernible tiny pores. This porous structure was significantly advantageous in catalytic reaction because it could afford a large amount of reactive sites and enhance the reactant diffusion (Hou et al., 2018).

FIGURE 6

Figure 6. SEM images of CuFe₂O₄ synthesized by different methods: (A–C) 1250 times magnification; (D) 1730 times magnification; (E,F) 3840 times magnification.

N₂ Adsorption–Desorption Analysis

The N₂ adsorption–desorption isotherms of the three CuFe₂O₄ were presented in Figure 7. The isotherms all belonged to Type IV curve with H3 hysteresis loop, which pointed to the disordered, lamellar mesoporous structure of the catalysts. Obviously, the adsorption capacity of CuFe₂O₄ ranked as follows: CuFe₂O₄-SG > CuFe₂O₄-ST > CuFe₂O₄-CP. Table 2 summarizes the basic structural parameters of the various as-synthesized CuFe₂O₄ samples. CuFe₂O₄-SG had the largest surface area and pore volume, followed by CuFe₂O₄-ST, and the values of CuFe₂O₄-CP were much lower than those of the former two, which was consistent with the SEM observation. Interestingly, the sequences of surface area and pore volume were completely consistent with the adsorption capacity of the prepared sample, but the mesopore size and particle size (observed in SEM images) did not follow this rule. These results indicated that sol–gel method endowed ferrite with a large surface area and pore volume, which played a pivotal role in the material's adsorption capacity, while the pore size and particle size were not the key factors affecting the adsorption capacity. When the adsorption capacity of ferrite was stronger, the amounts of pollutant and PS gathered were greater, and when the surface area was larger, the amount of active component exposed was greater, which were conducive to promoting the catalytic degradation of organics.

FIGURE 7

Figure 7. N₂ adsorption-desorption isotherms of CuFe₂O₄ synthesized by different methods.

TABLE 2

Table 2. Basic structural parameters of CuFe₂O₄ synthesized by different methods.

XPS Analysis

Figure 8 presents the surface elements chemical state of CuFe₂O₄ synthesized by different methods. As shown in Figure 8A, all samples yielded Cu(II) 2p_3/2 peak at around 933.4 eV along with two satellite peaks at 941.3 and 943.8 eV (Lei et al., 2015). However, the surface of CuFe₂O₄-SG consisted uniquely of Cu(II); the surfaces of CuFe₂O₄-ST and CuFe₂O₄-CP also contained a small proportion of Cu(0) (the peak at 932.2 eV) (Li et al., 2019). The calculation of peak area showed that CuFe₂O₄-ST contained more surface Cu(0) than CuFe₂O₄-CP contained, which was in agreement with the diffraction peak intensities in the XRD patterns.

FIGURE 8

Figure 8. XPS spectra of CuFe₂O₄ synthesized by different methods: (A) Cu 2p, (B) Fe 2p, and (C) O 1s.

Figure 8B shows that Fe(III) (2p_3/2 712.2 eV, 2p_1/2 725.9 eV) and Fe(II) (2p_3/2 710.4 eV, 2p_1/2 723.8 eV) coexisted on the surface of the three CuFe₂O₄ samples (Li et al., 2019; Zhang et al., 2019). Obviously, the proportion of Fe(II) in CuFe₂O₄-SG and CuFe₂O₄-CP was much higher than that in CuFe₂O₄-ST. These results were favorable for organics removal because Fe was also an electron donor for PS activation (Zhang et al., 2019).

The O 1s spectra of CuFe₂O₄ synthesized by the three methods were shown in Figure 8C. All three samples showed peaks of lattice oxygen (O_latt, 529.9 eV) and OH adsorbed on the surface (531.3 eV) (Li et al., 2017). The highest proportion of O_latt in CuFe₂O₄-SG suggested the good crystal structure obtained by sol–gel method. The spectra of CuFe₂O₄-ST and CuFe₂O₄-CP contained another peak at 532.6 eV ascribed to adsorbed H₂O (Zeng et al., 2017). The high proportion of adsorbed OH and H₂O on the surface indicated the strong hydroxylation of CuFe₂O₄ during coprecipitation and solvothermal processes (Wang et al., 2019).

Catalytic Performance of CuFe₂O₄ Synthesized by Different Methods

Figure 9 displays AO7 and diclofenac removal in different CuFe₂O₄ + PS systems. As seen, the effectiveness and reusability of the catalyst were significantly influenced by synthetic method.

FIGURE 9

Figure 9. Organics removal in different CuFe₂O₄ activated PS systems and reusability of CuFe₂O₄ synthesized by different methods: (A,B) AO7 removal; (C,D) diclofenac removal. Conditions: [organics] = 20 mg/L, [catalyst] = 0.6 g/L, (A,B) [PS] = 0.8 g/L, unadjusted pH = 6.5; (C,D) [PS] = 0.1 g/L, pH = 5.

Figure 9A shows that CuFe₂O₄-SG had the best PS catalytic performance. AO7 removal efficiency in CuFe₂O₄-SG + PS, CuFe₂O₄-ST + PS, and CuFe₂O₄-CP + PS systems were 96.8, 90.1, and 45.3%, respectively. In contrast, only 46.5, 53.4, and 36.3% of AO7 were adsorbed by the corresponding catalyst alone. In the early stage of reaction, the relatively large external surface area of the catalyst enabled CuFe₂O₄-ST to activate PS to remove more AO7. However, as the reaction proceeded, PS and AO7 gradually diffused into the pores of CuFe₂O₄-SG and reacted on the abundant active sites, ultimately leading to a better removal of AO7. Meanwhile, the corrosion and dissolution of the metallic Cu impurity observed on CuFe₂O₄-ST reduced the content of active metal and led to a gradual loss of activation ability of CuFe₂O₄-ST (Li et al., 2019). As for CuFe₂O₄-CP, its adsorption and catalytic capacity were greatly hindered owing to the serious sintering. Moreover, the reusability of CuFe₂O₄ synthesized by different methods was also found to be different (Figure 9B). After three runs, the decrease of AO7 removal in CuFe₂O₄-SG + PS system (19.9%) was lower than that in CuFe₂O₄-ST + PS system (22.0%). AO7 removal efficiency in CuFe₂O₄-SG + PS, CuFe₂O₄-ST + PS and CuFe₂O₄-CP + PS systems became 76.9, 68.1, and 35.8%, respectively. The lower decreasing trend of activity and higher pollutant removal efficiency after repeated uses suggested that CuFe₂O₄-SG had a good reusability. The removal of diclofenac in different CuFe₂O₄ + PS systems (Figures 9C,D) was similar to that of AO7. Once again, CuFe₂O₄-SG showed the best performance, followed by CuFe₂O₄-ST and then CuFe₂O₄-CP.

These results demonstrated that synthetic method would influence the catalytic performance of ferrite from morphological structure, surface area, and element chemical state. Sol–gel method was the ideal one to synthesize ferrite applicable in activated PS process.

Conclusions

M-site metal and synthetic method significantly influenced the catalytic performance and physicochemical property of spinel ferrite. The sequence of the effectiveness of ferrite-activated PS system for organics removal was CuFe₂O₄ > CoFe₂O₄ > MnFe₂O₄ > ZnFe₂O₄. The high catalytic performance of MFe₂O₄ resulted from its good reducibility, strong reversibility of M²⁺/M³⁺ redox couple, and active electron transfer on the surface, which were affected by M-site metal. Surface -OH was not crucial for the catalytic performance of MFe₂O₄ in PS system. Moreover, sol–gel method was found to be the ideal one to synthesize ferrite to effectively activate PS for organics removal. The as-prepared ferrite had good purity, porous structure, large surface area, and favorable element chemical states, leading to superior catalytic performance and reusability compared with those prepared by solvothermal and coprecipitation methods. The results served as a reference for screening ferrite and promoting PS activation.

Data Availability Statement

All datasets generated for this study are included in the article/Supplementary Material.

Author Contributions

GX, SK, GZ, and NZ designed the experiments. GX, SK, QL, and HD performed the experiments. GX, SK, and GZ wrote the paper. GX, SK, QL, GZ, NZ, HD, and LN discussed the results and analyzed the data.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors thank for financial support by the National Water Pollution Control and Treatment Science and Technology Major Project of China (2018ZX07110003).

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2020.00177/full#supplementary-material

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