A High-Efficiency CuO/CeO2 Catalyst for Diclofenac Degradation in Fenton-Like System

Highlights

- CuO/CeO₂ was prepared with ultrasound to remove diclofenac in Fenton-like system.

- Ultrasound made CuO/CeO₂ had large surface area, high porosity and fine elements dispersion.

- More oxygen vacancies caused by Cu doping were in favor of the catalytic reaction.

- 86.62% diclofenac removal was achieved under the optimal conditions.

- ·OH and adsorbed oxygen species were responsible for diclofenac degradation.

Introduction

Advanced oxidation processes (AOPs) for wastewater treatment have attracted extensive attention due to the quick and efficient pollutant removal by strong oxidizing free radicals, like hydroxyl radical (Silveira et al., 2015; Zhu et al., 2019). The most widely used AOP is Fenton reaction (Fe²⁺ activate H₂O₂), which is a homogeneous reaction that requires low pH (2.0–4.0) and forms large amount of iron sludge (Blanco et al., 2016). To overcome these disadvantages, heterogeneous catalysts have been adopted as the alternative (Lei et al., 2015), forming heterogeneous Fenton-like systems (Nidheesh, 2015).

The frequently used catalysts in heterogeneous Fenton-like systems are transition metal-based catalysts (Anipsitakis and Dionysiou, 2004; Bokare and Choi, 2014), due to the good catalytic activity, low cost, easily available materials, and abundant reserves. Among them, catalysts containing Cu²⁺ are in the limelight (Gu et al., 2019), because of the similar redox properties of Cu²⁺/Cu⁺ to Fe³⁺/Fe²⁺, broad pH workable range, few sludge production and easily decomposition of Cu²⁺ complexes by ·OH (Bokare and Choi, 2014; Peng et al., 2019). As one of the most common and simplest copper compounds, CuO has successfully activated H₂O₂ to form ·OH to remove pollutants (Sehati and Entezari, 2017). However, CuO nanoparticles tend to agglomerate in water which is unfavorable for catalytic reaction. Besides, the leached Cu ions are poisonous. Loading CuO on support is an effective way to overcome the above problems.

Some metal oxides (such as CeO₂, Al₂O₃), clays, zeolites and carbon materials have been used as supports (Rao et al., 2018; Xu et al., 2018). The support increases the specific surface area of catalyst and reduces the leaching of metal ions (Du et al., 2016), which are beneficial to the adsorption property and stability of catalyst. Especially, CeO₂ can enhance the catalytic property of catalyst for its unique structure and redox property. CeO₂ has abundant oxygen vacancy defects and Ce⁴⁺/Ce³⁺ redox couple (Chong et al., 2016), so it has high oxygen storage capacity, which is beneficial for catalytic reaction.

Doping transition metal ion into CeO₂ can create more oxygen vacancies due to the different ionic radii between Ce ion and transition metal ion, and the number of oxygen vacancies depends on the ionic radius and electrons of transition metal ions (Raj et al., 2017). Thus, the catalytic activity is improved by enhancing the oxygen storage and redox capacity (Parvas et al., 2014). The catalytic performance improvement by Cu doped into CeO₂ has been reported before (Wang et al., 2006; Yang et al., 2009; Lin et al., 2011).

The traditional impregnation method takes a long doping time and metal species agglomerate easily, due to the small mass transfer force between CeO₂ and the doping ions (Wang et al., 2006; Yang et al., 2009; Lin et al., 2011). Ultrasound has been used in the impregnation stage to overcome these disadvantages. Ultrasound waves induce the cavitation effect in water, which is related to the formation, growth, and rapid collapse of cavitation bubbles (Mirtamizdoust et al., 2017; Zhang et al., 2017). The ultrasonic cavitation can significantly accelerate the mass transfer velocity and provide thermal effects (Zhu and Zhang, 2008; Li et al., 2018). Therefore, ultrasound impregnation increases active components (transition metal ions) doped onto the surface of support (like CeO₂). Meanwhile, some active components may be introduced into the structure of support under the power of ultrasound, thus the catalyst prepared by ultrasound impregnation would have better catalytic performance (Chong et al., 2016). Moreover, ultrasound can significantly shorten the preparation time of catalyst by providing faster mass transfer rate.

In this paper, CuO was doped into CeO₂ by ultrasonic impregnation to form CuO/CeO₂, which was then applied as a catalyst in Fenton-like process. Diclofenac, a typical pharmaceutical and emerging contaminant, was used as the target pollutant to check the activity of the catalyst. The catalyst was characterized by N₂ adsorption-desorption, scanning electron microscope (SEM), X-ray powder diffraction (XRD), X-ray photoelectron spectra (XPS), high resolution transmission electron microscope (HRTEM), and Raman analyses. Influences of experimental parameters including pH, catalyst dosage, and H₂O₂ dosage on diclofenac removal were investigated. The potential reaction mechanism was proposed. The objective was to obtain a highly active Fenton-like catalyst with facial synthesis.

Materials and Methods

Materials

All chemicals used were analytical grade. Cu(NO₃)₂·4H₂O and Ce(NO₃)₃·6H₂O were bought from Tianjin Guangfu Fine Chemical Co. (China). Thirty percent of H₂O₂ was obtained from Beijing Chemical (China). Diclofenac (98%) was bought from Tokyo Chemical (Japan). HPLC grade acetonitrile was purchased from Fisher Scientific (USA).

Synthesis of Catalysts

CuO was prepared by precipitation method (Ghasemi and Karimipour, 2018). One hundred and fifty milliliter NaOH solution (16.67 mol/L) was slowly injected into 150 mL of Cu(NO₃)₂ solution (0.67 mol/L). The mixture was stirred at ~1,000 rpm at a constant temperature of 80°C for 3 h. The precipitate was separated by centrifugation, in which the solution was centrifuged for 20 min at 3,500 rpm. The precursor of CuO was dried for 3 h at 105°C and then calcined for 3 h at 700°C. The CuO powder was obtained after grinding.

CeO₂ was prepared by precipitation method (Zhao et al., 2014) and CuO/CeO₂ was prepared by ultrasonic impregnation method. As such, 2 g prepared CeO₂ powder was added to 20 mL 1 mol/L Cu(NO₃)₂ solution and impregnated for 30 min under 0.5 W/cm³ ultrasound, and then the filtered solid particulars were calcined for 2 h at 450°C in muffle furnace to obtain CuO/CeO₂.

Characterization of CuO/CeO₂

The porosity and specific surface area of CuO/CeO₂ were characterized by N₂ adsorption-desorption test using 3H-2000PS2, BeiShiDe Instrument-S&T Company. CuO/CeO₂ morphology was recorded by SEM with energy dispersive spectroscopy (EDS) (Hitachi S 4700 SEM analyzer). The XRD patterns were carried out by Rigaku D/maxrc diffractometer. The HRTEM measurement was investigated by Thermo Scientific Dionex UltiMate 3000 instrument. The oxidation state of CuO/CeO₂ was characterized by XPS analysis (EScalab250Xi spectrometer), and the binding energies were calibrated by C 1s peak at 284.8 eV.

Diclofenac Removal Experiments

The operation processes of reactions were as follows: 50 mL diclofenac solution (20 mg/L) was added to a 150 mL beaker and adjusted to the desired pH using 1 mol/L NaOH and H₂SO₄ solutions. After that, certain dosages of CuO/CeO₂ and H₂O₂ were poured into the solution under continuous magnetic stirring. Finally, a small sample of the mixture was taken out by syringe at certain times and filtered by a 0.45 μm membrane filter. The filtered solution was measured by HPLC. The experiments were done in triplicate.

The diclofenac concentration in the experiment was analyzed by Thermo Scientific Dionex UltiMate 3000 liquid chromatographt with a C18 column. The operation parameters were as follows: the flowing phases were 70% acetonitrile and 30% acetic acid solution (volume fraction was 0.2%), the flow rate was 1 mL/min, the injection volume was 10 μL, and the detection wavelength was 275 nm. The leached metal ions were detected by inductively coupled plasma optical emission spectrometer (ICP-OES) (IRIS Intrepid II XSP, ThermoFisher).

Results and Discussion

Catalyst Characterization

N₂ Adsorption-Desorption Analysis

Figure 1 shows the N₂ adsorption-desorption isotherm of CuO-CeO₂, which was typical type-IV isotherm with a H1-type hysteresis loop. This implied that mesoporous structure existed in the obtained CuO/CeO₂. The pore diameter distribution of CuO/CeO₂ was calculated from the adsorption branch and suggested the existence of mesopores at ~10 nm with the average diameter 3.9 nm (the embedded diagram of Figure 1). Table 1 summarized the basic structural parameters of CuO, CeO₂, and CuO/CeO₂. The specific surface area of CuO/CeO₂ reached 114.8 m²/g which was higher than that of CuO and CeO₂, and was also much higher than that of other CuO-containing catalysts such as CuO/rGO (56.1 m²/g) (Du et al., 2019) and CuO/Ti₆O₁₃ (6.93 m²/g) (Sehati and Entezari, 2017). The high specific surface area allowed the rapid electron transfer (Prathap et al., 2012) and would benefit the catalytic performance of CuO/CeO₂. These results indicated that ultrasonic preparation might facilitate catalytic reaction by changing the surface structure of the catalyst. For one thing, the mechanical effect of ultrasound could cut CeO₂ into smaller particles increasing specific surface area. Furthermore, the ultrasonic cavitation effect made CuO more uniformly distributed on the surface of CeO₂. The decreases of pore volume and pore diameter after ultrasonic impregnation of CeO₂ (Table 1) exactly indicated that CuO was indeed loaded on CeO₂.

FIGURE 1

Figure 1. N₂ adsorption-desorption isotherm and pore size distribution of CuO/CeO₂.

TABLE 1

Table 1. Basic structural parameters of CuO, CeO₂, and CuO/CeO₂.

SEM Analysis

SEM was used to characterize the morphology of CuO/CeO₂, and the results were shown in Figure 2. Figure 2A shows an overview of CuO/CeO₂ in which the size of particles was uniform. CuO was finely loaded on CeO₂ (Figure 2B). Element mapping results of Cu, Ce, and O (Figures 2D–F) indicated that the three elements were all well-dispersed in the catalyst. To investigate how ultrasound affected the structure of catalyst, Mahdiani et al. (2018) prepared PbFe₁₂O₁₉ with and without ultrasound, and found that ultrasound could decompress the large structures and form fine and homogeneous structures. Thus, the fine dispersion of elements and uniform size of CuO/CeO₂ benefitted from the cavitation effect of ultrasound which created an intense environment in the reaction mixture (Shende et al., 2018). Moreover, Hočevar et al. (2000) showed that the high dispersion of Cu on CeO₂ had positive effect on the activity and selectivity of copper cerium oxide. The Cu, Ce, and O atomic percentages confirmed by EDS were 10.56, 18.07, and 71.37%, respectively. The lower content of Cu showed that the dispersion of Cu was relatively sparse.

FIGURE 2

Figure 2. SEM analysis of CuO/CeO₂: (A) general view, (B) detailed image, (C) mapping region, (D) Cu mapping image, (E) Ce mapping image, and (F) O mapping image.

XRD Analysis

XRD analysis was applied to identify the component and crystallography of the catalyst. The results were shown in Figure 3. The diffraction peaks at 28.46°, 32.9°, 47.44°, 56.24°, 76.260°, and 79.22° could be attributed to the cubic fluorite CeO₂ (JCPDS No. 48-1548), which are characteristics of the (111), (200), (220), (311), (400), and (331) crystal faces. There were also characteristic diffraction peaks owing to CuO crystal, and 35.46°, 38.66° were assigned to the (002) and (111) faces (JCPDS No. 04-0593). The characteristic diffraction peaks of CeO₂ in CuO/CeO₂ were slightly broadened compared with that in CeO₂. This might be associated with the incorporation of Cu²⁺ ions with a smaller ionic radius (0.73 Å) compared with Ce⁴⁺ (0.97 Å) (Cau et al., 2014). These phenomena indicated that lattice constriction occurred in CuO/CeO₂.

FIGURE 3

Figure 3. XRD pattern of CuO/CeO₂.

TEM and HRTEM Analyses

The TEM images of CuO/CeO₂ and CeO₂ were shown in Figures 4A,C. The particle size of CuO/CeO₂ was uniform, and was generally smaller than that of CeO₂, which was a favorable result of ultrasonic impregnation method and was good for catalytic reaction. The lattice fringes of CuO/CeO₂ were measured at 2.77 and 3.14 Å (Figure 4B), corresponding to (200) and (111) crystal faces. Compared with the two crystal faces of CeO₂ (Figure 4D), the lattice fringe spacing in CuO/CeO₂ had a slight increase. This phenomenon might be caused by some Cu atoms doped into CeO₂ structure_. CuO/CeO₂ had lattice constriction and generated a solid solution structure. The XRD analysis showed similar results.

FIGURE 4

Figure 4. (A) TEM and (B) HRTEM images of CuO/CeO₂, (C) TEM and (D) HRTEM images of CeO₂.

The crystal face (111) of CeO₂ was the most stable crystal face in CeO₂ for its minimal surface energy. Thus, the stability of the catalyst would be enhanced with the increase of exposed crystal face (111). The catalytic activity could be improved by the crystal face (200) for its high surface energy. Moreover, oxygen vacancies were more favored to form on unstable face (200) than on (111). Thus, exposed (200) and (111) faces were beneficial to the catalytic performance and stability of CuO/CeO₂.

Raman Analysis

To investigate the effect of CuO doping on the quantities of oxygen vacancies over CuO/CeO₂, CuO/CeO₂, and CeO₂ were both characterized by Raman spectra. As shown in Figure 5, a strong peak at 462 cm⁻¹ was observed, which was attributed to the vibration mode of the F2 g symmetry in cubic fluorite CeO₂ lattice. This peak of CuO/CeO₂ presented a red shift compared to pure CeO₂ because of the reduction in the lattice parameter as a result of shorter M-O bond length (Cau et al., 2014). The weak band at 1,172 cm⁻¹ was assigned to the second-order phonon mode of fluorite structure. The band at 593 cm⁻¹ was attributed to oxygen vacancies in CeO₂. With Cu doping into CeO₂, more oxygen vacancies were generated, which would promote the catalytic property of CuO/CeO₂.

FIGURE 5

Figure 5. Raman spectra of CuO/CeO₂ and CeO₂.

XPS Analysis

The valence states of Cu, Ce, and O in CuO/CeO₂ were investigated by XPS analysis. Figure 6A shows the XPS spectrum of Cu 2p of CuO/CeO₂. The peak at 935.09 eV was the main peak of Cu 2p_3/2, which was related to CuO (Jawad et al., 2018). CuO had a satellite peak accompanied by the main peak, which was approximately 9 eV higher binding energy than the main peak (Zeng et al., 2013). Thus, 944.27 eV was the satellite peak of CuO.

FIGURE 6

Figure 6. XPS spectra of (A) Cu 2p, (B) Ce 3d, and (C) O 1s in CuO/CeO₂.

The XPS spectrum of Ce 3d is shown in Figure 6B. The four main 3d_5/2 features appeared at 883.23, 885.2, 889.68, and 899.19 eV, while the 3d_3/2 features appeared at 901.68, 903.38, 908.39, and 917.64 eV. Among them, the peaks at 883.23 and 901.68 eV were assigned to characteristic of Ce³⁺, indicating that there were two valence states of Ce (+3 and +4) existed in CuO/CeO₂.

The O 1s XPS spectrum was shown in Figure 6C. O 1s existed in two groups, O²⁻ and OH⁻, which were fitted with peaks at 529.89 and 532.12 eV, respectively. O²⁻ groups represented the lattice oxygen (O_latt) in metal oxides, generating from CuO and CeO₂. The peak at 532.12 eV was the characteristic of adsorbed oxygen species or surface OH species. The chemical adsorbed oxygen (O_ads) on the surface of CuO/CeO₂ might be transformed from O_latt through oxygen vacancies, which was a reactive oxygen specie to attack organics (Chong et al., 2017). This kind of conversion demonstrated that O_latt species participated in the degradation process of diclofenac, which was consistent with the study by Sedmak et al. (2003). Furthermore, XPS results indicated that CuO doped in CeO₂, which could improve the formation of O_latt in CuO/CeO₂ (Zou et al., 2011).

Diclofenac Removal in CuO/CeO₂-H₂O₂ Fenton-Like System

The comparison of diclofenac removal efficiency by H₂O₂, CuO/CeO₂, CuO-H₂O₂, CeO₂-H₂O₂, and CuO/CeO₂-H₂O₂ was shown in Figure 7. Single H₂O₂ and single CuO/CeO₂ could hardly remove diclofenac, the same for CuO-H₂O₂ and CeO₂-H₂O₂ system. However, CuO/CeO₂ could effectively catalyze H₂O₂ to remove diclofenac. The highest removal efficiency was 81.05, 30.01, and 19.67% for CuO/CeO₂-H₂O₂, CuO-H₂O₂ and CeO₂-H₂O₂ processes, respectively. Compared with CuO-H₂O₂ and CeO₂-H₂O₂ systems, diclofenac removal efficiency after 60 min in CuO/CeO₂-H₂O₂ system was increased by 63.99 and 65.05%, respectively.

FIGURE 7

Figure 7. Removal efficiency of diclofenac in different systems. Reaction conditions: pH = 5, H₂O₂ = 200 mg/L, catalyst = 1 g/L.

Clearly, CuO/CeO₂-H₂O₂ system had higher diclofenac removal than the sum of CuO-H₂O₂ system and CeO₂-H₂O₂ system, showing a synergistic effect between CuO and CeO₂. The synergistic effect might be caused by two reasons. Firstly, more oxygen vacancies were formed for Cu doped in CeO₂, which was shown in the above characterization analyses. Lu et al. (2011) found that the formation energy of oxygen vacancy in Cu-doped ceria was lower than bare ceria, demonstrating that Cu dopant could serve as a seed for the formation of oxygen vacancies. Secondly, there were electronic interactions between metal and the support, i.e., the facilitation of redox interactions between Cu²⁺/Cu⁺ and Ce⁴⁺/Ce³⁺ redox couples, which would favor the CuO reduction (Konsolakis, 2016). Studies (Szabová et al., 2010, 2013) also reported that Cu doped on CeO₂ surface accompanied electron transfer process between Cu and neighboring Ce⁴⁺, generating more Ce³⁺.

Optimization of Operation Parameters for Diclofenac Removal

pH is an important parameter in AOPs. An advantage of Fenton-like process over Fenton reaction is avoiding of too acidic condition (pH < 3.5). As known, diclofenac was more likely to dissociate to ionic structure when the pH was higher than its pK_a (4.2), which was favorable to its adsorption and degradation. Thus, the effect of pH above 5 was investigated. As shown in Figure 8A, pH had a significant effect on diclofenac removal in CuO/CeO₂-H₂O₂ system, which decreased by 79.31% when the pH changed from 5 to 11. The result might attribute to two reasons: (1) the oxidation potential of ·OH decreased, and more H₂O₂ decomposed to O₂ and H₂O at high pH value; (2) deprotonation of the catalyst gradually increased, which was unfavorable to the electrostatic attraction between diclofenac and CuO/CeO₂ (Hassani et al., 2018).

FIGURE 8

Figure 8. Influencing factors of diclofenac removal in CuO/CeO₂-H₂O₂ system: (A) pH, (B) CuO/CeO₂ dosage, (C) H₂O₂ dosage. Reaction conditions: (A) H₂O₂ = 200 mg/L, catalyst = 1 g/L; (B) pH = 5, H₂O₂ = 200 mg/L; (C) pH = 5, catalyst = 1.2 g/L.

Different dosages of CuO/CeO₂ were added to the Fenton-like system. Figure 8B shows that the diclofenac removal efficiency was improved from 25.21 to 86.62% when the catalyst dosage increased from 0.1 to 1.2 g/L. The active sites were mostly on the surface of the catalyst, so more catalyst meant that more active sites exposed. But when the active sites were adequate in CuO/CeO₂-H₂O₂ system, further addition of catalyst would bring little improvement. Thus, the optimum CuO/CeO₂ dosage was 1.2 g/L.

The effect of H₂O₂ on diclofenac removal was studied within the dosage of 25–500 mg/L. Figure 8C shows that with the continuous increase of H₂O₂ dosage, the removal efficiency of diclofenac reached 86.62% at 200 mg/L H₂O₂, then the reaction tended to stabilize. Excessive H₂O₂ reacted with ·OH, thus go against the diclofenac removal (Hassani et al., 2018). Taking into account the removal efficiency and cost, 200 mg/L was considered as the optimum dosage of H₂O₂.

Lee et al. (2014) found only about 24% diclofenac could be removed in 60 min by Cu(II)/H₂O₂ system. Xu et al. (2016) used Cu-doped α-FeOOH as Fenton-like catalyst to degrade diclofenac, <50% diclofenac was removed in 60 min. Zhou et al. (2018) found 80% diclofenac was removed in a magnetic field enhanced Fe⁰/EDTA Fenton-like system. In comparsion, CuO/CeO₂ was a high-efficiency Fenton-like catalyst for diclofenac removal. Moreover, leaching test showed that there was almost no Ce leached (<0.05 mg/L) under the above optimal reaction conditions. The leached Cu concentration in the solution was 1.7 mg/L, which met the wastewater discharge standard of China (Cu < 2 mg L⁻¹) (Wang et al., 2016) and was only 9‰ of the catalyst dosage. Besides, the removal of diclofenac still reached 70.12% in the third run of CuO/CeO₂-H₂O₂ system. These results showed a good stability and reusability of the CuO/CeO₂ catalyst.

Potential Mechanism of Diclofenac Degradation in CuO/CeO₂-H₂O₂ System

Potential mechanism of diclofenac degradation in CuO/CeO₂-H₂O₂ system was proposed in Figure 9. CuO could decompose H₂O₂ to form highly active ·OH (Drijvers et al., 1999), and the reaction was represented by Equations (1) and (2) (Yamaguchi et al., 2018). ·OH radical could also be produced by Ce³⁺ with H₂O₂, which was achieved by the division of O-O bond of H₂O₂, as shown in Equation (3) (Chong et al., 2017). Moreover, the synergistic copper and ceria interaction (Equation 4) facilitated the redox cycles of Cu²⁺/Cu⁺ and further was conducive to ·OH generation (Konsolakis, 2016). Sehati and Entezari (2017) found that ·OH on the catalyst surface (·OH_ads) was the major reactive species, while ·OH in the solution had little effect on pollutant removal in Fenton-like system. During diclofenac degradation, diclofenac molecules were adsorbed on the surface of the catalyst, meanwhile, H₂O₂ was decomposed to ·OH by Cu and Ce species on the catalyst surface. Then, the adsorbed diclofenac molecules reacted with ·OH_ads and transformed into smaller molecules (intermediate products), H₂O or CO₂. The process continued until the complete decomposition of diclofenac was achieved (Ziylan et al., 2013).

$\begin{array}{ll}C{u}^{2+}+H_2{O}_2\to C{u}^{+}+·OOH+H^{+}& (1)\end{array}$

$\begin{array}{ll}C{u}^{+}+H_2{O}_2\to C{u}^{2+}+·OH+O{H}^{-}& (2)\end{array}$

$\begin{array}{ll}C{e}^{3+}+H_2{O}_2+H^{+}\to C{e}^{4+}+·OH+H_{2}O& (3)\end{array}$

$\begin{array}{ll}C{u}^{2+}+C{e}^{3+}\to C{u}^{+}+C{e}^{4+}& (4)\end{array}$

More oxygen vacancies were generated by Cu doped into CeO₂, which was proven by the Raman spectra (Figure 5). The oxygen storage capacity of a ceria-based catalyst benefited from oxygen vacancies (Soler et al., 2016). In the degradation process, the adsorbed O₂ on the surface of CuO/CeO₂ and bulk O_latt could replenish oxygen vacancies by diffusion from the surface and inner of the catalyst (Balcaen et al., 2010), then transformed to active oxygen (O_ads) accompanied by valence state transformation of CeO₂ and CuO (Ce³⁺/Ce⁴⁺ and Cu⁺/Cu²⁺) (Dong et al., 2019). O_ads could also participate in the oxidation of diclofenac (Xian et al., 2019). Besides, large specific surface area and high porosity of ultrasonically prepared CuO/CeO₂ catalyst allowed rapid electron transfer, which improved the catalytic property (Prathap et al., 2012).

FIGURE 9

Figure 9. Potential mechanisms of diclofenac degradation in CuO/CeO₂-H₂O₂ system.

Conclusions

An efficient heterogeneous Fenton-like catalyst CuO/CeO₂ was synthesized by ultrasonic impregnation method and used to remove diclofenac from water. The prepared CuO/CeO₂ had large specific surface area, high porosity and fine elements dispersion. CuO and CeO₂ crystals coexisted in CuO/CeO₂ with a lattice constriction. HRTEM analysis demonstrated that the main exposed crystal faces of CeO₂ contained (200) face which readily formed oxygen vacancies and improved the catalytic property of CuO/CeO₂. Oxygen vacancies in CeO₂ were increased by Cu doping. The optimal operating conditions of CuO/CeO₂-H₂O₂ system were pH = 5, CuO/CeO₂ dosage = 1.2 g/L, and H₂O₂ dosage = 200 mg/L, with 86.62% diclofenac removal. The synergistic effect between CuO and CeO₂ on diclofenac removal might be caused by more oxygen vacancies generation and electronic interactions between Cu and Ce species in CuO/CeO₂. The degradation of diclofenac was mainly by ·OH and adsorbed oxygen species which were enhanced by oxygen vacancies.

Data Availability Statement

All datasets generated for this study are included in the article/supplementary material.

Author Contributions

GZ, GX, and JZ designed the experiments. GX, NZ, and JL performed the experiments. GZ and GX wrote the paper. GZ, GX, NZ, and JZ 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 the financial support from the National Natural Science Foundation of China (51778374) and National Water Pollution Control and Treatment Science and Technology Major Project of China (2018ZX07110003).

References

Anipsitakis, G. P., and Dionysiou, D. D. (2004). Radical generation by the interaction of transition metals with common oxidants. Environ. Sci. Technol. 38, 3705–3712. doi: 10.1021/es035121o

PubMed Abstract | CrossRef Full Text | Google Scholar

Balcaen, V., Roelant, R., Poelman, H., Poelman, D., and Marin, G. B. (2010). TAP study on the active oxygen species in the total oxidation of propane over a CuO-CeO₂/γ-Al₂O₃ catalyst. Catal. Today 157, 49–54. doi: 10.1016/j.cattod.2010.02.048

CrossRef Full Text | Google Scholar

Blanco, L., Hermosilla, D., Merayo, N., and Blanco, Á. (2016). Assessing the use of zero-valent iron microspheres to catalyze Fenton treatment processes. J. Taiwan Inst. Chem. E 69, 54–60. doi: 10.1016/j.jtice.2016.08.014

CrossRef Full Text | Google Scholar

Bokare, A. D., and Choi, W. (2014). Review of iron-free Fenton-like systems for activating H₂O₂ in advanced oxidation processes. J. Hazard. Mater. 275, 121–135. doi: 10.1016/j.jhazmat.2014.04.054

PubMed Abstract | CrossRef Full Text | Google Scholar

Cau, C., Guari, Y., Chave, T., Larionova, J., and Nikitenko, S. I. (2014). Thermal and sonochemical synthesis of porous (Ce,Zr)O₂ mixed oxides from metal β-diketonate precursors and their catalytic activity in wet air oxidation process of formic acid. Ultrason. Sonochem. 21, 1366–1373. doi: 10.1016/j.ultsonch.2014.01.006

PubMed Abstract | CrossRef Full Text | Google Scholar

Chong, S., Zhang, G., Zhang, N., Liu, Y., Huang, T., and Chang, H. (2017). Diclofenac degradation in water by FeCeO_x catalyzed H₂O₂: Influencing factors, mechanism and pathways. J. Hazard. Mater. 334, 150–159. doi: 10.1016/j.jhazmat.2017.04.008

PubMed Abstract | CrossRef Full Text | Google Scholar

Chong, S., Zhang, G., Zhang, N., Liu, Y., Zhu, J., Huang, T., et al. (2016). Preparation of FeCeO_x by ultrasonic impregnation method for heterogeneous Fenton degradation of diclofenac. Ultrason. Sonochem. 32, 231–240. doi: 10.1016/j.ultsonch.2016.03.019

PubMed Abstract | CrossRef Full Text | Google Scholar

Dong, C.-D., Huang, C. P., Nguyen, T.-B., Hsiung, C.-F., Wu, C.-H., Lin, Y.-L., et al. (2019). The degradation of phthalate esters in marine sediments by persulfate over iron-cerium oxide catalyst. Sci. Total Environ. 696:133973. doi: 10.1016/j.scitotenv.2019.133973

PubMed Abstract | CrossRef Full Text | Google Scholar

Drijvers, D., Langenhove, H. V., and Beckers, M. (1999). Decomposition of phenol and trichloroethylene by the ultrasound/H₂O₂/CuO process. Water Res. 33, 1187–1194. doi: 10.1016/S0043-1354(98)00308-X

CrossRef Full Text | Google Scholar

Du, X., Zhang, Y., Si, F., Yao, C., Du, M., Hussain, I., et al. (2019). Persulfate non-radical activation by nano-CuO for efficient removal of chlorinated organic compounds: reduced graphene oxide-assisted and CuO (001) facet-dependent. Chem. Eng. J. 356, 178–189. doi: 10.1016/j.cej.2018.08.216

CrossRef Full Text | Google Scholar

Du, Y., Ma, W., Liu, P., Zou, B., and Ma, J. (2016). Magnetic CoFe₂O₄ nanoparticles supported on titanate nanotubes (CoFe₂O₄/TNTs) as a novel heterogeneous catalyst for peroxymonosulfate activation and degradation of organic pollutants. J. Hazard. Mater. 308, 58–66. doi: 10.1016/j.jhazmat.2016.01.035

PubMed Abstract | CrossRef Full Text | Google Scholar

Ghasemi, S., and Karimipour, A. (2018). Experimental investigation of the effects of temperature and mass fraction on the dynamic viscosity of CuO-paraffin nanofluid. Appl. Therm. Eng. 128, 189–197. doi: 10.1016/j.applthermaleng.2017.09.021

CrossRef Full Text | Google Scholar

Gu, T., Dong, H., Lu, T., Han, L., and Zhan, Y. (2019). Fluoride ion accelerating degradation of organic pollutants by Cu(II)-catalyzed Fenton-like reaction at wide pH range. J. Hazard. Mater. 377, 365–370. doi: 10.1016/j.jhazmat.2019.05.073

PubMed Abstract | CrossRef Full Text | Google Scholar

Hassani, A., Celikdag, G., Eghbali, P., Sevim, M., Karaca, S., and Metin, O. (2018). Heterogeneous sono-Fenton-like process using magnetic cobalt ferrite-reduced graphene oxide (CoFe₂O₄-rGO) nanocomposite for the removal of organic dyes from aqueous solution. Ultrason. Sonochem. 40, 841–852. doi: 10.1016/j.ultsonch.2017.08.026

PubMed Abstract | CrossRef Full Text | Google Scholar

Hočevar, S., Krašovec, U. O., Orel, B., Aricó, A. S., and Kim, H. (2000). CWO of phenol on two differently prepared CuO-CeO₂ catalysts. Appl. Catal. B Environ. 28, 113–125. doi: 10.1016/S0926-3373(00)00167-3

CrossRef Full Text | Google Scholar

Jawad, A., Lang, J., Liao, Z., Khan, A., Ifthikar, J., Lv, Z., et al. (2018). Activation of persulfate by CuO_x@Co-LDH: A novel heterogeneous system for contaminant degradation with broad pH window and controlled leaching. Chem. Eng. J. 335, 548–559. doi: 10.1016/j.cej.2017.10.097

CrossRef Full Text | Google Scholar

Konsolakis, M. (2016). The role of Copper-Ceria interactions in catalysis science: Recent theoretical and experimental advances. Appl. Catal. B Environ. 198, 49–66. doi: 10.1016/j.apcatb.2016.05.037

CrossRef Full Text | Google Scholar

Lee, H.-J., Lee, H., and Lee, C. (2014). Degradation of diclofenac and carbamazepine by the copper(II)-catalyzed dark and photo-assisted Fenton-like systems. Chem. Eng. J. 245, 258–264. doi: 10.1016/j.cej.2014.02.037

CrossRef Full Text | Google Scholar

Lei, Y., Chen, C. S., Tu, Y. J., Huang, Y. H., and Zhang, H. (2015). Heterogeneous degradation of organic pollutants by persulfate activated by CuO-Fe₃O₄: mechanism, stability, and effects of pH and bicarbonate ions. Environ. Sci. Technol. 49, 6838–6845. doi: 10.1021/acs.est.5b00623

PubMed Abstract | CrossRef Full Text | Google Scholar

Li, J. X., Qin, Z. B., Li, Y. H., and Cui, G. H. (2018). Sonochemical synthesis and properties of two new nanostructured silver(I) coordination polymers. Ultrason. Sonochem. 48, 127–135. doi: 10.1016/j.ultsonch.2018.05.016

PubMed Abstract | CrossRef Full Text | Google Scholar

Lin, K.-S., Chowdhury, S., Yeh, H.-P., Hong, W.-T., and Yeh, C.-T. (2011). Preparation and characterization of CuO/ZnO–Al₂O₃ catalyst washcoats with CeO₂ sols for autothermal reforming of methanol in a microreactor. Catal. Today 164, 251–256. doi: 10.1016/j.cattod.2010.11.038

CrossRef Full Text | Google Scholar

Lu, Z., Yang, Z., He, B., Castleton, C., and Hermansson, K. (2011). Cu-doped ceria: oxygen vacancy formation made easy. Chem. Phys. Lett. 510, 60–66. doi: 10.1016/j.cplett.2011.03.091

CrossRef Full Text | Google Scholar

Mahdiani, M., Soofivand, F., and Salavati-Niasari, M. (2018). Investigation of experimental and instrumental parameters on properties of PbFe₁₂O₁₉ nanostructures prepared by sonochemical method. Ultrason. Sonochem. 40, 271–281. doi: 10.1016/j.ultsonch.2017.06.023

PubMed Abstract | CrossRef Full Text | Google Scholar

Mirtamizdoust, B., Travnicek, Z., Hanifehpour, Y., Talemi, P., Hammud, H., and Joo, S. W. (2017). Synthesis and characterization of nano-peanuts of lead(II) coordination polymer [Pb(qcnh)(NO₃)₂]_n with ultrasonic assistance: a new precursor for the preparation of pure-phase nano-sized PbO. Ultrason. Sonochem. 34, 255–261. doi: 10.1016/j.ultsonch.2016.05.041

PubMed Abstract | CrossRef Full Text | Google Scholar

Nidheesh, P. V. (2015). Heterogeneous Fenton catalysts for the abatement of organic pollutants from aqueous solution: a review. RSC Adv. 5, 40552–40577. doi: 10.1039/C5RA02023A

CrossRef Full Text | Google Scholar

Parvas, M., Haghighi, M., and Allahyari, S. (2014). Degradation of phenol via wet-air oxidation over CuO/CeO₂-ZrO₂ nanocatalyst synthesized employing ultrasound energy: physicochemical characterization and catalytic performance. Environ. Technol. 35, 1140–1149. doi: 10.1080/09593330.2013.863952

PubMed Abstract | CrossRef Full Text | Google Scholar

Peng, J., Zhang, C., Zhang, Y., Miao, D., Zhang, Y., Liu, H., et al. (2019). Enhanced Cu(II)-mediated fenton-like oxidation of antimicrobials in bicarbonate aqueous solution: kinetics, mechanism and toxicity evaluation. Environ. Pollut. 252, 1933–1941. doi: 10.1016/j.envpol.2019.05.148

PubMed Abstract | CrossRef Full Text | Google Scholar

Prathap, M. U., Kaur, B., and Srivastava, R. (2012). Hydrothermal synthesis of CuO micro-/nanostructures and their applications in the oxidative degradation of methylene blue and non-enzymatic sensing of glucose/H₂O₂. J. Colloid Interf. Sci. 370, 144–154. doi: 10.1016/j.jcis.2011.12.074

PubMed Abstract | CrossRef Full Text | Google Scholar

Raj, A. K. V., Prabhakar Rao, P., Sreena, T. S., and Aju Thara, T. R. (2017). Influence of local structure on photoluminescence properties of Eu(³⁺) doped CeO₂ red phosphors through induced oxygen vacancies by contrasting rare earth substitutions. Phys. Chem. Chem. Phys. 19, 20110–20120. doi: 10.1039/C7CP02741A

CrossRef Full Text | Google Scholar

Rao, W., Lv, G., Wang, D., and Liao, L. (2018). Enhanced degradation of Rh 6G by zero valent iron loaded on two typical clay minerals with different structures under microwave irradiation. Front. Chem. 6:463. doi: 10.3389/fchem.2018.00463

PubMed Abstract | CrossRef Full Text | Google Scholar

Sedmak, G., Hočevar, S., and Levec, J. (2003). Kinetics of selective CO oxidation in excess of H₂ over the nanostructured Cu_0.1Ce_0.9O₂-y catalyst. J. Catal. 213, 135–150. doi: 10.1016/S0021-9517(02)00019-2

CrossRef Full Text | Google Scholar

Sehati, S., and Entezari, M. H. (2017). Sono-incorporation of CuO nanoparticles on the surface and into the mesoporous hexatitanate layers: enhanced Fenton-like activity in degradation of orange-G at its neutral pH. Appl. Surf. Sci. 399, 732–741. doi: 10.1016/j.apsusc.2016.12.079

CrossRef Full Text | Google Scholar

Shende, T. P., Bhanvase, B. A., Rathod, A. P., Pinjari, D. V., and Sonawane, S. H. (2018). Sonochemical synthesis of Graphene-Ce-TiO₂ and Graphene-Fe-TiO₂ ternary hybrid photocatalyst nanocomposite and its application in degradation of crystal violet dye. Ultrason. Sonochem. 41, 582–589. doi: 10.1016/j.ultsonch.2017.10.024

PubMed Abstract | CrossRef Full Text | Google Scholar

Silveira, J. E., Zazo, J. A., Pliego, G., Bidóia, E. D., and Moraes, P. B. (2015). Electrochemical oxidation of landfill leachate in a flow reactor: optimization using response surface methodology. Environ. Sci. Pollut. Res. 22, 5831–5841. doi: 10.1007/s11356-014-3738-2

PubMed Abstract | CrossRef Full Text | Google Scholar

Soler, L., Casanovas, A., Escudero, C., Perez-Dieste, V., Aneggi, E., Trovarelli, A., et al. (2016). Ambient pressure photoemission spectroscopy reveals the mechanism of carbon soot oxidation in ceria-based catalysts. ChemCatChem 8, 2748–2751. doi: 10.1002/cctc.201600615

CrossRef Full Text | Google Scholar

Szabová, L., Camellone, M. F., Huang, M., Matolin, V., and Fabris, S. (2010). Thermodynamic, electronic and structural properties of Cu/CeO₂ surfaces and interfaces from first-principles DFT+U calculations. J. Chem. Phys. 133:234705. doi: 10.1063/1.3515424

PubMed Abstract | CrossRef Full Text | Google Scholar

Szabová, L., Skála, T., Matolínová, I., Fabris, S., Farnesi Camellone, M., and Matolín, V. (2013). Copper-ceria interaction: a combined photoemission and DFT study. Appl. Surf. Sci. 267, 12–16. doi: 10.1016/j.apsusc.2012.04.098

CrossRef Full Text | Google Scholar

Wang, C., Wan, J., Ma, Y., and Wang, Y. (2016). Insights into the synergy of zero-valent iron and copper oxide in persulfate oxidation of Orange G solutions. Res. Chem. Intermed. 42, 481–497. doi: 10.1007/s11164-015-2035-0

CrossRef Full Text | Google Scholar

Wang, X., Rodriguez, J. A., Hanson, J. C., Gamarra, D., Martínez-Arias, A., and Fernández-García, M. (2006). In situ studies of the active sites for the water gas shift reaction over Cu-CeO₂ catalysts: complex interaction between metallic copper and oxygen vacancies of ceria. J. Phys. Chem. B 110, 428–434. doi: 10.1021/jp055467g

PubMed Abstract | CrossRef Full Text | Google Scholar

Xian, G., Zhang, G., Chang, H., Zhang, Y., Zou, Z., and Li, X. (2019). Heterogeneous activation of persulfate by Co₃O₄-CeO₂ catalyst for diclofenac removal. J. Environ. Manage. 234, 265–272. doi: 10.1016/j.jenvman.2019.01.013

PubMed Abstract | CrossRef Full Text | Google Scholar

Xu, D., Lv, H., and Liu, B. (2018). Encapsulation of metal nanoparticle catalysts within mesoporous zeolites and their enhanced catalytic performances: a review. Front. Chem. 6:550. doi: 10.3389/fchem.2018.00550

PubMed Abstract | CrossRef Full Text | Google Scholar

Xu, J., Li, Y., Yuan, B., Shen, C., Fu, M., Cui, H., et al. (2016). Large scale preparation of Cu-doped α-FeOOH nanoflowers and their photo-Fenton-like catalytic degradation of diclofenac sodium. Chem. Eng. J. 291, 174–183. doi: 10.1016/j.cej.2016.01.059

CrossRef Full Text | Google Scholar

Yamaguchi, R., Kurosu, S., Suzuki, M., and Kawase, Y. (2018). Hydroxyl radical generation by zero-valent iron/Cu (ZVI/Cu) bimetallic catalyst in wastewater treatment: heterogeneous Fenton/Fenton-like reactions by Fenton reagents formed in-situ under oxic conditions. Chem. Eng. J. 334, 1537–1549. doi: 10.1016/j.cej.2017.10.154

CrossRef Full Text | Google Scholar

Yang, W., Li, D., Xu, D., and Wang, X. (2009). Effect of CeO₂ preparation method and Cu loading on CuO/CeO₂ catalysts for methane combustion. J. Nat. Gas Chem. 18, 458–466. doi: 10.1016/S1003-9953(08)60141-3

CrossRef Full Text | Google Scholar

Zeng, S., Zhang, W., Śliwa, M., and Su, H. (2013). Comparative study of CeO₂/CuO and CuO/CeO₂ catalysts on catalytic performance for preferential CO oxidation. Int. J. Hydrogen Energ. 38, 3597–3605. doi: 10.1016/j.ijhydene.2013.01.030

CrossRef Full Text | Google Scholar

Zhang, N., Zhao, H., Zhang, G., Chong, S., Liu, Y., Sun, L., et al. (2017). Preparation of a magnetic N-Fe/AC catalyst for aqueous pharmaceutical treatment in heterogeneous sonication system. J. Environ. Manage. 187, 201–211. doi: 10.1016/j.jenvman.2016.11.043

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhao, H., Zhang, G., and Zhang, Q. (2014). MnO₂/CeO₂ for catalytic ultrasonic degradation of methyl orange. Ultrason. Sonochem. 21, 991–996. doi: 10.1016/j.ultsonch.2013.12.002

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhou, T., Feng, K., Xiang, W., Lv, Y., Wu, X., Mao, J., et al. (2018). Rapid decomposition of diclofenac in a magnetic field enhanced zero-valent iron/EDTA Fenton-like system. Chemosphere 193, 968–977. doi: 10.1016/j.chemosphere.2017.11.090

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhu, L., and Zhang, H. (2008). A novel method for the modification of zinc powder by ultrasonic impregnation in cerium nitrate solution. Ultrason. Sonochem. 15, 393–401. doi: 10.1016/j.ultsonch.2007.09.016

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhu, Y., Zhu, R., Xi, Y., Zhu, J., Zhu, G., and He, H. (2019). Strategies for enhancing the heterogeneous Fenton catalytic reactivity: a review. Appl. Catal. B Environ. 255, 117739. doi: 10.1016/j.apcatb.2019.05.041

CrossRef Full Text | Google Scholar

Ziylan, A., Koltypin, Y., Gedanken, A., and Ince, N. H. (2013). More on sonolytic and sonocatalytic decomposition of Diclofenac using zero-valent iron. Ultrason. Sonochem. 20, 580–586. doi: 10.1016/j.ultsonch.2012.05.005

PubMed Abstract | CrossRef Full Text | Google Scholar

Zou, H., Chen, S., Liu, Z., and Lin, W. (2011). Selective CO oxidation over CuO-CeO₂ catalysts doped with transition metal oxides. Powder Technol. 207, 238–244. doi: 10.1016/j.powtec.2010.11.005

CrossRef Full Text | Google Scholar