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Article

Efficient Removal of Levofloxacin by Activated Persulfate with Magnetic CuFe2O4/MMT-k10 Nanocomposite: Characterization, Response Surface Methodology, and Degradation Mechanism

1
College of Natural Resources and Environment, South China Agricultural University, Guangzhou 510642, China
2
College of Environmental Science and Engineering, Yangzhou University, Yangzhou 225127, China
3
Guangdong Provincial Key Laboratory of Eco-Circular Agriculture, South China Agricultural University, Guangzhou 510642, China
*
Authors to whom correspondence should be addressed.
Water 2020, 12(12), 3583; https://doi.org/10.3390/w12123583
Received: 7 November 2020 / Revised: 5 December 2020 / Accepted: 15 December 2020 / Published: 21 December 2020
(This article belongs to the Special Issue Remediation of Contaminated or Degraded Soil and Water Resources)

Abstract

In this study, a magnetic copper ferrite/montmorillonite-k10 nanocomposite (CuFe2O4/MMT-k10) was successfully fabricated by a simple sol-gel combustion method and was characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), the Brunner–Emmett–Teller (BET) method, vibrating sample magnetometer (VSM), and X-ray photoelectron spectroscopy (XPS). For levofloxacin (LVF) degradation, CuFe2O4/MMT-k10 was utilized to activate persulfate (PS). Due to the relative high adsorption capacity of CuFe2O4/MMT-k10, the adsorption feature was considered an enhancement of LVF degradation. In addition, the response surface methodology (RSM) model was established with the parameters of pH, temperature, PS dosage, and CuFe2O4/MMT-k10 dosage as the independent variables to obtain the optimal response for LVF degradation. In cycle experiments, we identified the good stability and reusability of CuFe2O4/MMT-k10. We proposed a potential mechanism of CuFe2O4/MMT-k10 activating PS through free radical quenching tests and XPS analysis. These results reveal that CuFe2O4/MMT-k10 nanocomposite could activate the persulfate, which is an efficient technique for LVF degradation in water.
Keywords: magnetic; copper ferrite; montmorillonite-k10; levofloxacin; persulfate; response surface methodology magnetic; copper ferrite; montmorillonite-k10; levofloxacin; persulfate; response surface methodology

1. Introduction

In recent years, pharmaceuticals and personal care products (PPCPs), as emerging contaminants, have been detected frequently in the environment, which has caused widespread concern. Antibiotics, as one kind of PPCPs, are widely used in the treatment of pathogenic-organism diseases of humans or livestock [1,2]. However, most antibiotics are not completely transformed or metabolized after administration, which means excreta maintain the state of pharmacological activity and can be further excreted into the environment [3,4]. The long-term discharge of low-dose antibiotics into the environment will lead to the prompted drug resistance of sensitive bacteria. In addition, drug-resistant genes might develop and interfere with environment, posing a potential threat to the ecological environmental security and human health [5]. Unfortunately, the traditional water treatment process may not meet the demands of effective removal of antibiotic contamination. It is important to develop an efficient and eco-friendly technique to remove antibiotics in waste water.
Based on free radicals, the advanced oxidation process (AOP) has been widely used in water treatment. Compared with hydroxyl radicals (HOs; E0 = 1.9–2.7 eV) in the AOP [6,7], sulfate radical (SO4·−; E0 = 2.5–3.1 eV) has more advantages, such as higher redox potential, longer half-life, and wider pH range. Therefore, with regard to SO4·− the AOP has an advantage of pollutant removal in waste treatment. Generally, SO4·− is produced by peroxymonosulfate (PMS) and persulfate (PS). Compared with PMS, PS has a higher solubility, better stability, and lower cost, and is considered to be a better source of SO4·− [8]. Alkali [9], heat [10,11], and radiation [12,13] are good methods to activate PS and further generate SO4·−. However, high cost and harsh activation conditions confine these approaches, as they are not practical for large-scale application. Instead, the heterogeneous catalytic oxidation technique is a potential countermeasure for these problems. Spinel ferrites (MFe2O4; M=Co, Cu, Mn, Zn, Ni) are characterized by low-cost, efficient heterogenous catalysis and easy magnetic separation from aqueous solution [14]. CoFe2O4 is considered as the most efficient catalyst for activating PS to treat organic pollutants in water [15]. However, the International Agency for Research on Cancer (IARC) has identified cobalt as a potential human carcinogen [16]. Thus, the leaching toxicity of CoFe2O4 damages its application in practical water treatment. It has been demonstrated that CuFe2O4 is more effective at activating PMS during the process of di (n-butyl) phthalate degradation than other spinel ferrites, except for CoFe2O4 [17]. Although CuFe2O4 is not as effective as CoFe2O4, copper is not recognized as potential carcinogen, according to IARC [8]. In other words, considering the catalytic activity and the risk of metal leaching of catalysts, CuFe2O4 is the optimal PS activator to choose for removing LVF in this experiment. However, CuFe2O4 nanoparticles are characterized by magnetic properties, and are likely to agglomerate together to form larger clusters, and therefore decrease the catalytic efficiency [18]. Such problems might be mostly solvable with the introduction of supporting materials.
Montmorillonite (MMT) is an abundant, typical aluminosilicate mineral with a structure of 2:1 layers consisting of one Al3+ octahedron sheet placed between two Si4+ tetrahedral sheets [19]. Due to its special structure, MMT possesses a large surface area, high cation exchange capacity, and smooth expansion capacity [20]. With its favorable properties and low cost, MMT is a suitable support material for various nanoparticles purposes. In recent years, quaternary ammonium salt (QAC)-modified organic montmorillonite has caught researchers’ attention, due to its outstanding features listed above. Researchers have utilized cetyltrimethyl–ammonium bromide-modified organic montmorillonite as a supporting material for nanoscale, zero-valent iron to remove tetrabromobisphenol-A and bisphenol-A [21]. Researchers have also used didodecyldimethyl–ammonium bromide-modified organic montmorillonite as an absorbent material for thiophanate-methyl removal [22]. However, QACs have been released in the environment extensively in recent years, and this causes serious toxicity to protozoa [23] and shrimp [24], and even induces drug resistance in bacteria [25]. This has caused great concern from environmental scientists and bacteria researchers [26]. As a green and friendly material, montmorillonite k10 (MMT-k10) has been extensively applied in aspects of catalysis [27]. Relative to other materials, MMT-k10 offers several advantages, such as non-corrosive properties, non-toxicity, low cost, ease of handling, and mild reaction conditions [28]. Fe(III) immobilized on MMT-k10 as a catalyst have been applied in an H2O2 Fenton-like oxidation process to effectively degrade reactive black 5 and acid red 1 dye [29,30]. In the presence of H2O2, the montmorillonite K10-Cu(Ⅱ)ethylenediamine catalyst could efficiently catalyze the degradation of Acid blue29 and Chromotrope 2R [31,32].We suppose that CuFe2O4 supported via MMT-k10 provides effective PS activation and rapid degradation of LVF.
In this study, levofloxacin (LVF), as one of extensively utilized third-generation fluroquinolones, was chosen as target antibiotic. CuFe2O4/MMT-k10 was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), the Brunner–Emmett–Teller (BET) method, and a vibrating sample magnetometer (VSM). The adsorption capacity of CuFe2O4/MMT-k10 was examined in our study. The Box–Behnken design for the response surface methodology (RSM) model was applied to optimize LVF removal in aqueous solution. Additionally, the radicals involved in the CuFe2O4/MMT-k10/PS system were revealed by free radical quenching test, as well as a potential PS activation mechanism. The purpose of this study is to explore an effective, environmentally friendly, and economical method to remove antibiotics in waste water.

2. Materials and Methods

2.1. Materials

Montmorillonite-k10 (MMT-k10) was purchased from Macklin (Shanghai, China). Ferric nitrate (Fe(NO3)3·9H2O), cupric nitrate (Cu(NO3)2·3H2O), citric acid monohydrate (C6H8O7·H2O), and potassium persulfate (K2S2O8) were bought from Sinopharm Chemical Reagent Company (Shanghai, China). Potassium hydroxide (KOH) and sulfuric acid (H2SO4) were obtained from Guangzhou Chemical Reagent Factory (Guangzhou, China). Ethanol (EtOH) and t-butanol (TBA) from Aladdin Ltd. (Shanghai, China) were used for the quenching experiments. All chemicals were of analytical grade. All solutions were prepared with ultrapure water (resistivity > 18 MΩ).

2.2. Synthesis of CuFe2O4/MMT-k10 and CuFe2O4

CuFe2O4/MMT-k10 was synthesized by a citrate combustion method, according to previous research [33], with slight modification. At first, Cu(NO3)2·3H2O and Fe(NO3)3·9H2O were dissolved in 150 mL ultrapure water at a certain stoichiometric ratio and were stirred for 30 min. Then a certain amount of MMT-k10 was added into the homogeneous solution with continuous stirring for 2 h. After adding specific amount of citric acid monohydrate, the suspension was stirred vigorously at 90 °C for 2 h, and the final solid was calcined at 500 °C for 3 h. Finally, the obtained brown products were grinded to powder with mortar, washed with ultrapure water and ethanol, and dried in an oven at 55 °C for 24 h. The preparation of pure CuFe2O4 was the same method as the above-mentioned synthesis approach, but without the MMT-k10 addition.

2.3. Characterization of CuFe2O4/MMT-k10

The crystalline phase of CuFe2O4, MMT-k10, and CuFe2O4/MMT-k10 was analyzed by X-ray diffraction (XRD; Ultima IV, Rikagu, Tokyo, Japan) with Cu K-α radiation (λ = 0.15410 nm) and a 2θ range from 10° to 90°. The internal structure and external morphology of the prepared samples were observed by scanning electron microscopy (SEM; Zeiss Sigma 500) and transmission electron microscopy (TEM; FEI Tecnai G2 f20 s-twin 200kV). The Brunaure–Emmett–Teller (BET) method with pure CuFe2O4 and CuFe2O4/MMT-k10 was determined by Micromeritics ASAP 2020 from N2 adsorption–desorption isotherms. A vibrating sample magnetometer (VSM; Quantum DesignSQUID, San Diego, CA, USA) was explored for measuring the magnetic properties of CuFe2O4 and CuFe2O4/MMT-k10. The surface elements of CuFe2O4/MMT-k10 before and after the reaction were measured by X-ray photoelectron spectra (XPS; Thermo Fisher Scientific K-Alpha, Boston, MA, USA).

2.4. Batch Experiment

The batch experiments were carried out with the addition of 0.2 g CuFe2O4/MMT-k10 into 200 mL of LVF solution (1 g/L) in conical flasks. The conical flasks were placed in a constant temperature shaker at 160 rpm under 25 °C. At certain time intervals (30 min), 5 mL of the resulting reaction solution was taken from the conical flask, filtered through a 0.45 μm membrane, and immediately measured at 285 nm [34] in a UV–1800 spectrophotometer (MAPADA, Shanghai, China). In a batch experiment, the pH of the LVF solution (pH = 6) was not adjusted. For its reusability determination, CuFe2O4/MMT-k10 was separated after each run. After washing with ultrapure water, CuFe2O4/MMT-k10 was dried at 55 °C for the next cycle. The regenerated catalyst of heat treatment was also tested in cycle experiments. Moreover, the quench test of the scavenger was utilized for evaluation of the catalytic mechanism. In detail, 10 mL of TBA or 10 mL of EtOH was added into the reaction system before the shaker run to determine the removal rate of LVF.

2.5. Response Surface Methodology Optimization Design

The response surface methodology (RSM), a semi-empirical technology, is useful for designing the experiments, developing models by considering the interactions of parameters, and optimizing experimental procedures [35,36]. The Box–Behnken design has observable advantages, such as the estimation of quadratic model parameters, establishment of sequential design, investigation of the lack-of-fit of the model, and determination of block usage [37,38]. The Box–Behnken design is more efficient than the central composite design or three-level, full factorial design [39]. Hence, the Box–Behnken design was selected for this experiment. Parameters like temperature, pH value, CuFe2O4/MMT-k10 dosage, and PS dosage were considered as independent variables affecting the removal rate of LVF. As a solution pH regulator, KOH (2 mol/L) or H2SO4 (2 mol/L) were used in LVF solution. Before experiments, the pH value was recorded by a pH meter (PB-10, sartorius, Shanghai). All data was analyzed using statistical software Design Expert 8.0.

3. Results and Discussion

3.1. XRD Results

The phase structure of MMT-k10 and CuFe2O4/MMT-k10 composite was investigated by XRD pattern (Figure 1). As shown in the XRD pattern of MMT-k10, the diffraction peaks at 5.72°, 19.71°, 35.76°, 61.80°, and 73.22° were attributed to the typical crystal structure of bentonite (JCPDS No.03-0015) [40]. Moreover, the pure CuFe2O4 prepared by sol-gel method showed the diffraction peaks at 18.28°, 30.24°, 35.55°, 37.07°, 43.97°, 57.20°, and 62.43°, which was based on the analysis for the characteristic peaks’ positions at the (111), (220), (311), (222), (400), (511), and (440) planes of the copper ferrite (JCPDS No.25-0283) [41]. The CuFe2O4/MMT-k10 synthetic catalysts presented here had a two-phase composition: MMT-k10 and CuFe2O4. The XRD results indicated that the CuFe2O4 particles were successfully immobilized on MMT-k10.

3.2. SEM and TEM Results

The morphology and microstructure of MMT-k10, CuFe2O4, and CuFe2O4/MMT-k10 were examined using SEM/EDS and TEM. As shown in Figure 2a, the irregular lamellar structure of MMT-k10 was found. This structure was favorable for loading CuFe2O4 on the surface of MMT-k10. Generally, the sol-gel-synthesized CuFe2O4 was granular, but its magnetism makes it easy to form a large chain structure (Figure 2b). Agglomeration reduces the active site of CuFe2O4 for the SO4·− AOP procedure. Interestingly, CuFe2O4 particles were seen uniformly anchored on the surface of MMT-k10 (Figure 2c). Furthermore, the EDS spectrum clearly revealed that the Cu (Figure 2d), Fe (Figure 2e), and O (Figure 2f) were uniformly distributed throughout the CuFe2O4/MMT-k10 composite, and the molar ratio of Fe/Cu was estimated to be 1.89 (close to 2.00; Figure S1). Therefore, the information of the SEM/EDS results confirmed the immobilization of CuFe2O4 on the surface of MMT-k10, which was consistent with the XRD results. In addition, TEM was used to further reveal the morphology and structure of as-prepared samples, as depicted in Figure 3. Here, as observed in Figure 3a, MMT-k10 sheets with smooth and well-defined edges were stacked together. Clearly, the CuFe2O4 particles with a size of about 10–20 nm were uniformly distributed on the MMT-k10 (Figure 3b). These results were well-suited to those from the SEM and XRD analyses.

3.3. BET Results

Porosity and the specific surface area of CuFe2O4 and CuFe2O4/MMT-k10 were determined by the BET technique. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the N2 adsorption–desorption isotherms for both two samples (CuFe2O4 and CuFe2O4/MMT-k10) were categorized as type IV, with the apparent hysteresis loops, indicating mesoporous structures (Figure S2a). Figure S2b shows the pore size distribution of CuFe2O4 and CuFe2O4/MMT-k10 calculated by the BJH model. The pore size distributions of CuFe2O4 and CuFe2O4/MMT-k10 were in the range of 2–50 nm, with median values of 11.1 nm and 4.3 nm, respectively, which is compatible with the results of N2 adsorption–desorption isotherms. However, the BET surface area of CuFe2O4/MMT-k10 was 66.63 m2g−1 (Table S1), much larger than that of CuFe2O4 (25.15 m2g−1). The larger surface area and the smaller average pore size of CuFe2O4/MMT-k10 than CuFe2O4 particles may make it favorable for the absorption and catalytic performance of as-prepared composites.

3.4. VSM Results

The magnetic property of CuFe2O4 and CuFe2O4/MMT-k10 was measured by VSM (Quantum Design SQUID) at room temperature. As can be seen in Figure 4, CuFe2O4 and CuFe2O4/MMT-k10 showed symmetrical S-type magnetization curves, indicating the super para-magnetic behavior and nanosized dimension behavior of the samples [42]. The saturation magnetization of pure CuFe2O4 and CuFe2O4/MMT-k10 were 27.1 emu·g−1 and 13.0 emu·g−1, respectively. The saturation magnetization value of CuFe2O4 was larger than that of CuFe2O4/MMT-k10, and this was due to the introduction of non-magnetic MMT-k10 for a hybrid catalyst. From the inset of Figure 4, CuFe2O4/MMT-k10 might be completely separated from the water by an external magnet.

3.5. LVF Removal

3.5.1. Adsorption Studies

Adsorption equilibrium isotherms provide the most paramount parameter for designing a desirable adsorption system [43]. The isotherm for LVF adsorption on CuFe2O4/MMT-k10 is shown in Figure 5. Langmuir and Freundlich isotherm models were used to fit the adsorption isotherms (Figure 5). In Table S2, the results showed that it was fitted to both Langmuir (R2 = 0.994) and Freundlich (R2 = 0.952) isotherm models with high determination coefficients. The R2 of Langmuir was higher than that of Freundlich. Hence, LVF adsorption by CuFe2O4/MMT-k10 tended to be monolayer adsorption. The maximum theoretical adsorption capacity of LVF was 35.8 mg/g (Table S2). Clearly, good adsorption capacity of the catalysts effectively improves the efficiency to activate PS to degrade organic pollutants [44]. The adsorption process could contribute to the removal efficiency of LVF.

3.5.2. LVF Removal in Different Systems

The LVF removal curves were displayed in different reaction systems in Figure 6. It can be seen that only 9.43% of the LVF was degraded within 120 min by PS alone; this indicates that single persulfate oxidant barely remove organic pollutant in aqueous solution [6]. Besides, LVF can hardly be effectively removed by CuFe2O4 and MMT-k10 (12.67% and 15.75%, respectively). Instead, 37.47% of the LVF was removed by CuFe2O4/MMT-k10, meaning that CuFe2O4/MMT-k10 had higher adsorption capacity compared to CuFe2O4 and MMT-k10. Figure 5 also shows that 45.64% of the LVF was degraded by adding CuFe2O4 and PS, indicating that PS under the activation of CuFe2O4 generates many radicals for degradation. Interestingly, 85.55% of LVF was degraded in the CuFe2O4/MMT-k10/PS system, meaning this system was more effective than other systems in Figure 6.

3.5.3. RSM Analysis of LVF Removal

In total, 29 experiments were conducted in a random order. The experimental data were analyzed by factorial regression, and the quadratic model was expressed by second-order polynomial equation.
Y = −20.186 + 0.185A + 1.798B + 62.587C + 61.287D + 0.021AB + 1.375AC + 2.075AD − 0.170BC − 0.370BD − 3.700CD − 0.592A2 − 0.015B2 − 22.627C2 − 18.827D2
where Y is the LVF removal rate (%), and A, B, C, and D are independent variables that represent pH value, temperature (°C), catalyst dosage (g/L), and PS dosage (g/L), respectively. To verify accuracy of the model, a normal plot of the residuals, and a residuals vs. predicted map were adopted. As shown in Figures S3 and S4, the normal distribution of residuals was in a straight line, and the map of residuals vs. predicted was scattered, implying that the model was suitable. According to the RSM analysis, the optimal experimental conditions were compatible, as the Cu2Fe2O4/MMT-k10 dosage was 1.5 g/L, PS dosage was 1.5 g/L, temperature was 45 °C, and pH value was 4.0. Under optimal conditions, the maximum degradation rate of LVF was 95.47%.
Analysis of variance (ANOVA) was used to test the significance and effectiveness of the second-order model, in order to predict LVF in the Cu2Fe2O4/MMT-k10/PS system. The results are shown in Table S3. The F-value of the model was 21.77, and the p-value was less than 0.0001, indicating that the fitting result of the model was significant. In addition, the correlation coefficient R2 was used to effectively evaluate the variability and applicability of the model. The correlation coefficient R2 in the Cu2Fe2O4/MMT-k10/PS system was 0.9561, representing 95.61% of the variation explained by this model. However, R2 is considered a misleading statistical value, because in some cases, a high R2 does not necessarily mean that the model is suitable [36]. To avoid such biases, the adjusted R2 was used. Adjusted R2 is a modified version of R2 that is adjusted for the different number of factors in the model [36]. The adjusted R2 was 0.9122 in this model, which was similar to the correlation coefficient R2. We concluded that the RSM is a suitable model to effectively determine the optimal conditions of the Cu2Fe2O4/MMT-k10/PS system.
The correlative relationships, such as Cu2Fe2O4/MMT-k10 dosage and PS dosage, Cu2Fe2O4/MMT-k10 dosage and temperature, Cu2Fe2O4/MMT-k10 dosage and pH value, and temperature and PS dosage, were evaluated. As shown in Figure 7a,b, with the increase of Cu2Fe2O4/MMT-k10 dosage and PS dosage, the removal efficiency of LVF improved. With the increase of Cu2Fe2O4/MMT-k10 dosage, the number of adsorption sites for LVF and the catalytic sites for PS increased. Thus, Cu2Fe2O4/MMT-k10 dosage could improve the removal rate of LVF. PS is the main source of SO4·−. PS dosage increase promotes the production of SO4·−, which also effectively improves the removal rate of LVF. As shown in Figure 7c,d, the pH value decreases, stimulates the removal rate of LVF. To approach the actual pH value of wastewater, the pH range of 4–8 was selected. In this range, the optimum pH of Cu2Fe2O4/MMT-k10/PS system was 4. The removal efficiency under acidic conditions is better than under alkaline conditions. This is attributed to several factors: (1) the lifetimes of HO· and SO4·− decrease in alkaline conditions, but this is insufficient for the radicals to diffuse into the bulk phase for further degradation [45]; (2) under acidic conditions, Cu2Fe2O4/MMT-k10 could release more Fe and Cu ions and activate PS to produce more SO4·−, in order to degrade LVF [46]. As shown in Figure 7e,f, a temperature increase promotes the removal rate of LVF. This leads to the increase of Brownian motion and the improvement of catalytic efficiency, due to thermally activated persulfate.

3.6. Reusability and Stability of CuFe2O4/MMT-k10

The stability and reusability of catalyst plays an important role in actual water treatment. Therefore, cyclic experiments were conducted to evaluate the stability and reusability of CuFe2O4/MMT-k10. Figure S5a showed the removal rate of CuFe2O4/MMT-k10 for LVF, which was reused three times. The LVF removal rate of the second and third cycles reached 72.0% and 62.43%, respectively, and CuFe2O4/MMT-k10 still maintained high efficiency. The LVF removal rate decrease may be attributed to two aspects. On the one hand, inevitable metal element consumption in a catalyst leads to the catalytic activity decrease. On the other hand, the intermediate products in the degradation process may be adsorbed on the surface of the catalyst, resulting in the reduction of absorption and catalytic activity sites. According to previous literature, the adsorbed intermediate can be removed by heat treatment, which can regenerate the catalyst [34]. Therefore, after the first cycle and the second cycle, CuFe2O4/MMT-k10 was calcined at 400 °C for 1 h. As shown in Figure S5a, after heat treatment, the catalytic performance of CuFe2O4/MMT-k10 improved significantly. Therefore, heat treatment is a kind of regeneration technology for CuFe2O4/MMT-k10, conducive to the recycling use of CuFe2O4/MMT-k10 in practical water treatment.

3.7. Mechanism Discussion

HO· and SO4·− are the two main free radicals in the degradation of organic pollutants via PS activated by transition metal catalysts. To identify the major reactive radical species generated in the CuFe2O4/MMT-k10/PS system, free radical quenching tests were carried out. According to the previous studies, TBA reacts with HO· much more strongly than with SO4·−, while EtOH reacts with both HO· and SO4·− at a high rate [47]. As can been seen in Figure S5b, 85.48% of the LVF was degraded in the no-scavenger system. The LVF degradation efficiency decreased to 58.98% and 30.99% while adding TBA or EtOH into the system, respectively. The results clearly reveal that both HO· and SO4·− are involved in the CuFe2O4/MMT-k10/PS system, and participate in the LVF degradation process.
To better understand the surface composition and chemical states of CuFe2O4/MMT-k10 before and after use, the chemical compositions and the elemental state were characterized by XPS spectra (Figure 8). The XPS spectra of Cu 2p before and after use are shown in Figure 8a. For CuFe2O4/MMT-k10 before use, the XPS peaks at 933.8 eV and 953.8 eV were assigned to Cu 2p3/2 and Cu 2p1/2, respectively. The splitting width between Cu 2p3/2 and Cu 2p1/2 was about 20.0 eV, which agreed with the standard spectrum of Cu2+ in copper oxides, suggesting the existence of Cu2+ in CuFe2O4/MMT-k10 [8]. In addition, it reveals that there were two obvious satellite peaks at 942.0 eV and 962.1 eV in Figure 8a, which might correspond to the open 3d9 shell of Cu2+ ions [48]. Compared with the fresh CuFe2O4/MMT-k10, the new peak at 933.2 eV was formed in the hybrid catalyst after use, suggesting that the valence of Cu changed, and Cu2+ was partially transferred to Cu+ [49]. As seen from Figure 8b, the observed Fe 2p 1/2 (724.4 eV) and Fe 2p 3/2 (711.4eV) peaks implied that the presence of Fe3+ and Fe2+, respectively. At the beginning of use, Fe3+ and Fe2+ accounted for 63.18% and 36.82%, based on the Fe 2p signal deconvolution. After the degradation experiment, no obvious peak shifting was seen in the Fe 2p spectrum, but the intensity of each peak changed slightly. After the deconvolution of Fe 2p, Fe3+ and Fe2+ were detected as accounting for 70.07% and 29.93%, respectively. The results indicate that Fe3+/Fe2+ was involved in the CuFe2O4/MMT-k10/PS system. The O 1s spectrum was revealed in Figure 8c as well. The peak at about 530.0 eV, 531.4 eV, and 532.2 eV were ascribed to surface hydroxyls (M-(OH)2 or MO-OH), physic- and chemisorbed H2O on the surface, and the lattice oxygen (O2), respectively. For fresh CuFe2O4/MMT-k10, the proportions of -OH, H2O, and O2 were 51.65%, 12.4%, and 35.87%, respectively. After use, the proportions of -OH and H2O decreased to 53.67% and 10.46%, while the proportion of O2 was not significantly changed. The results of O 1s showed that the water adsorbed on the catalyst surface was partially converted to -OH, while the composition variation was not significant. According to the above results and previous researchers, the reaction mechanism was proposed for PS activation by CuFe2O4/MMT-k10 (reactions (1–11)) [50,51,52,53], and the concept map of the proposed mechanism is shown in Figure S6. Firstly, SO4·− was produced by PS activated with Cu2+ and Fe2+ (reactions (1–2)), while HO· was produced through the H2O and SO4·− reaction (reaction (3)). SO4·− and HO· probably destroy the absorbed LVF to the degraded products (reactions (4–5)). In addition, S2O82− possibly produces HSO5 by means of hydrolysis (reaction (6)), and Cu3+, Cu2+, and Fe3+ possibly react with HSO5 to produce SO5·− (reactions (7–9)). Meanwhile, Cu3+ likely is restored to Cu+ (reaction (10)), while Cu+ reacts with Fe3+ to realize the regeneration of Cu2+ and Fe2+ (reaction (11)).
Cu(II) + S2O82− → Cu(III) + SO4·− + SO42−
Fe(II) + S2O82− → Fe(III) + SO4·− + SO42−
H2O + SO4·− → H+ + HO· + SO42−
SO4·− + LVF → degraded products
HO· + LVF → degraded products
S2O82− +H2O → HSO5 +HSO4
Cu(III) + HSO5 → Cu(II) + SO5·− + H+
Cu(II) + HSO5 → Cu(I) + SO5·− + H+
Cu(II) + HSO5 → Cu(I) + SO5·− + H+
Cu(III) + H2O → Cu(II) + H+ + HO·
Fe(III) + Cu(I) → Fe(II) + Cu(II)

4. Conclusions

The CuFe2O4/MMT-k10 nanocomposite was successfully synthesized by a simple citric acid combustion method. CuFe2O4/MMT-k10 effectively activates PS to remove LVF in aqueous solution. In the removal process, the adsorption of LVF on CuFe2O4/MMT-k10 promotes this reaction. By establishing an RSM model, where the Cu2Fe2O4/MMT-k10 dosage was 1.5 g/L, PS dosage was 1.5 g/L, temperature was 45 °C, and pH value was 4.0, the CuFe2O4/MMT-k10/PS system achieves the best LVF removal efficiency. The cyclic experiments confirmed the good reusability and stability of CuFe2O4/MMT-k10. To disclose the underlying activation mechanism, XPS was used to analyze the change of CuFe2O4/MMT-k10 before and after use, and the active radicals were identified by free radical quenching tests. Overall, CuFe2O4/MMT-k10 reveals great application potential in advanced oxidation technology for antibiotic waste water treatment.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4441/12/12/3583/s1, Figure S1: EDS spectroscopy of CuFe2O4/MMT-k10, Table S1: Nitrogen adsorption data for CuFe2O4 and CuFe2O4/MMT-k10.

Author Contributions

J.Y.: Conceptualization, Data curation, Data curation, Investigation, Methodology, Project administration, Software, Visualization, Writing—original draft, Writing—review & editing. M.H.: Supervision, Validation. S.W.: Writing—review & editing. X.M.: Conceptualization, Funding acquisition, Project administration, Resources, Writing—review & editing. Y.H.: Formal analysis. X.C.: Resources, Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [the Science and Technology Project of Guangdong Province] grant number [2017A050501029 and 2019B030301007] and [the National Natural Science Foundation of China] grant number [41071162, U1901601 and 42007084].

Conflicts of Interest

The authors declare that there are no conflict of interest regarding the publication of this article.

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Figure 1. X-ray diffraction (XRD) patterns of montmorillonite k10 (MMT-k10), CuFe2O4 and CuFe2O4/MMT-k10.
Figure 1. X-ray diffraction (XRD) patterns of montmorillonite k10 (MMT-k10), CuFe2O4 and CuFe2O4/MMT-k10.
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Figure 2. Scanning electron microscopy (SEM) images of (a) MMT-k10, (b) CuFe2O4, (c) CuFe2O4/MMT-k10, and EDS mapping images of Cu (d), Fe (e), and O (f) for CuFe2O4/MMT-k10.
Figure 2. Scanning electron microscopy (SEM) images of (a) MMT-k10, (b) CuFe2O4, (c) CuFe2O4/MMT-k10, and EDS mapping images of Cu (d), Fe (e), and O (f) for CuFe2O4/MMT-k10.
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Figure 3. Transmission electron microscopy (TEM) images of (a1, 100 nm; a2, 50 nm; a3, 20 nm) MMT-k10 and (b1, 100 nm; b2, 50 nm; b3, 20 nm) CuFe2O4/MMT-k10.
Figure 3. Transmission electron microscopy (TEM) images of (a1, 100 nm; a2, 50 nm; a3, 20 nm) MMT-k10 and (b1, 100 nm; b2, 50 nm; b3, 20 nm) CuFe2O4/MMT-k10.
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Figure 4. Magnetic hysteresis loops of CuFe2O4 and CuFe2O4/MMT-k10, as well as a magnetic recovery diagram (inset).
Figure 4. Magnetic hysteresis loops of CuFe2O4 and CuFe2O4/MMT-k10, as well as a magnetic recovery diagram (inset).
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Figure 5. The isotherm for levofloxacin (LVF) adsorption on CuFe2O4/MMT-k10, as well as Langmuir and Freundlich isotherm model fittings.
Figure 5. The isotherm for levofloxacin (LVF) adsorption on CuFe2O4/MMT-k10, as well as Langmuir and Freundlich isotherm model fittings.
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Figure 6. Effects of different reaction systems on the removal of LVF. Reaction conditions: C0 [LVF] = 10 mg/L, C0 [persulfate (PS)] = 1.0 g/L, C0 [catalyst] = 1 g/L, T = 25 °C and initial pH = 6 (unadjusted).
Figure 6. Effects of different reaction systems on the removal of LVF. Reaction conditions: C0 [LVF] = 10 mg/L, C0 [persulfate (PS)] = 1.0 g/L, C0 [catalyst] = 1 g/L, T = 25 °C and initial pH = 6 (unadjusted).
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Figure 7. With regard to the LVF removal efficiency parameters, three-dimensional (3D) response surfaces and two-dimensional (2D) contours of Cu2Fe2O4/MMT-k10 dosage and PS dosage (a,b), Cu2Fe2O4/MMT-k10 dosage and temperature (c,d), Cu2Fe2O4/MMT-k10 dosage and pH value (e,f), and temperature and PS dosage (g,h) are provided.
Figure 7. With regard to the LVF removal efficiency parameters, three-dimensional (3D) response surfaces and two-dimensional (2D) contours of Cu2Fe2O4/MMT-k10 dosage and PS dosage (a,b), Cu2Fe2O4/MMT-k10 dosage and temperature (c,d), Cu2Fe2O4/MMT-k10 dosage and pH value (e,f), and temperature and PS dosage (g,h) are provided.
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Figure 8. X-ray photoelectron spectroscopy (XPS) of (a1,a2) Cu 2p, (b1,b2) Fe 2p, and (c1,c2) O 1s enveloping Cu2Fe2O4/MMT-k10 (1) before and (2) after use.
Figure 8. X-ray photoelectron spectroscopy (XPS) of (a1,a2) Cu 2p, (b1,b2) Fe 2p, and (c1,c2) O 1s enveloping Cu2Fe2O4/MMT-k10 (1) before and (2) after use.
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