Catalytic Degradation of Tetracycline Hydrochloride by Coupled UV−Peroxydisulfate System: Efficiency, Stability and Mechanism

Abstract

Magnetic CuFe₂O₄ powder obtained by sol−gel method and coupled photocatalysis was used to activate peroxydisulfate for tetracycline (TC) removal. A scanning electron microscope, X−ray diffraction Raman spectroscopy and FT−TR were used to characterize the catalysts. The degradation efficiency and stability of TC were highest under neutral conditions. The TC degradation rate reached 91.1% within 90 min. The removal rate of total organic carbon reaches 39.6% under optimal conditions. The unique electron transfer property of CuFe₂O₄ was utilized to achieve the synergistic effect of photocatalysis and persulfate oxidation. The main oxidizing substances involved in the decomposition were sulfate radicals and hydroxyl radicals, and the removal rate of over 84% could be maintained after five cycles of experiments.

1. Introduction

As a kind of drug to prevent and treat bacterial diseases, antibiotics are used in a wide range of industries such as the medical industry, pharmaceutical industry and breeding industry [1]. Conventional wastewater treatment plants are unable to effectively treat antibiotics entering water bodies. Moreover, due to the long residence time of antibiotics and the low rate of microbial degradation in water, resulting in large amounts of antibiotic residues accumulating in natural water bodies [2]. This may increase the probability of the antibiotic resistance gene appearing, thus posing a threat to ecosystems. Tetracycline (TC) is widely used in veterinary medicine and aquaculture. The production and use of TC is the second largest antibiotic in global [3]. It plays a significant role in environmental protection and human health to explore an efficient and low−cost technology to remove TC from water bodies.

Advanced oxidation technology of sulfate radical (SO₄^•−) has attracted widespread interest [4]. Persulfate advanced oxidation method has been shown to be a very effective strategy for antibiotic degradation [5,6]. The persulfate itself is a strong oxidant (E₀ = 2.01 V) [7]. The activation can generate SO₄^•− with higher potential. The rapid decomposition and final mineralization of organic matter can be achieved through SO₄^•−. Compared with Fenton advanced oxidation technology which generates hydroxyl radical (•OH), the advanced oxidation technology based on persulfate has the following characteristics: the redox potential of SO₄^•− standard (E₀ = 2.5–3.1 V) is higher than that of •OH (E₀ = 1.9–2.7 V). The oxidation principle is also similar to that of •OH, but with a longer stabilization time. Additionally, it has a wider pH applicability range. Advanced oxidation of persulfate is more environmentally friendly than other degradation methods because it does not produce large amounts of waste material, nor does it consume a great deal of energy [8].

In recent years, CuFe₂O₄ magnetic materials have attracted great interest as an anon−homogeneous catalyst applied to advanced oxidation process systems (AOPs). Li et al. [9] prepared Cu/CuFe₂O₄ composites using the solvothermal method. It was observed that both SO₄^•− and •OH are the main activation products, indicating that CuFe₂O₄ could activate persulfate. Xu et al. [10] have found that the peroxymonosulfate (PMS) can produce free radicals to remove bisphenol A in aqueous solutions with CuFe₂O₄ magnetic nanoparticles, and the removal rate of BPA reached 92.3% within 60 min. Furthermore, CuFe₂O₄ can be used as a photocatalyst. Jing et al. [11] studied the separate porous tetragonal− CuFe₂O₄ nanotubes coupled with photocatalytic degradation of acid fuchsin, indicating that CuFe₂O₄ has good photocatalytic activity and durability. The photocatalytic and persulfate oxidation system also has an abundance of highly reactive oxide species. Persulfate can generate SO₄^•− and •OH by capturing photo−generated electrons. Thus, using UV activated persulfate can achieve highly efficient removal of pollutants [12].

In this study, the combination of ultraviolet light (UV) and peroxodisulfate (PDS) was investigated for the removal of TC. The magnetically separable nano copper ferrite was prepared by sol−gel method and their structural features were characterized. The CuFe₂O₄/UV/PDS system was constructed to degrade TC and the total organic carbon (TOC) was investigated. The effect of pH on the degradation performance and the reusability of CuFe₂O₄ were investigated. Additionally, the active species in the system were obtained by quenching experiments.

2. Materials and Methods

2.1. Chemicals

The water used in the experiment was deionized water. The other reagents were analytical reagents.

2.2. Synthesis of CuFe₂O₄

Magnetic nanometer copper ferrite was prepared by the sol−gel method [13]. Firstly, adding Cu(NO₃)₂·3H₂O (Shanghai Maclean Biochemical Technology Co., Shanghai, China), Fe(NO₃)₃·9H₂O (Tianjin Komio Chemical Reagent Co., Tianjin, China) and citric acid (Tianjin Fengchuan Sliding Chemical Reagent Technology Co., Tianjin, China) in the molar ratio of 1:2:3 to deionized water and stir well, and stirred until dissolved. The pH of the mixed solution was then adjusted to 7 with ammonia (Yantai Shuangshuang Chemical Co., Yantai, Shandong, China). Secondly, the solution was transferred to a water bath at 70 °C with continuous stirring until the water evaporated to form a dark green gel. The gel was dried for 7 h at 130 °C. Finally, the obtained dry gel was calcined at 200 °C, 300 °C, 400 °C, 500 °C and 600 °C for 2 h to afford the catalyst.

2.3. Analysis Methods

The scanning electron microscope (SEM, JSM−6700F, JEOL, Tokyo, Japan) was used to observe the microscopic morphology and structure of catalytic materials. The X−ray diffraction (XRD) patterns of CuFe₂O₄ was carried out on Dmax 2500 V Advance diffractometer (Bruker Co, Billerica, MA, USA) using Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 5°/min with 2θ ranging between 10° and 80°. Raman spectrometer (HORIBA HR Evolution, HORIBA Co., Kyoto, Japan) with a laser wavelength of 532 nm was used in the range of 200–1200 cm⁻¹. The different functional groups and lattice vibrations of prepared systems were monitored by Fourier−transform infrared. (FT−IR, Is50FT−IR, American Thermal Power Company, Boston, MA, USA). The absorbance of TC solution was analyzed by UV−VIS spectrometer (N5000, Shanghai Youke Co., Shanghai, China) at a wavelength of 357 nm. To determine the removal rate of total organic carbon by TOC analyzer (Trace Elemental Instruments, XPERT, Delft, The Netherlands) was used. The pH_zpc of CuFe₂O₄ material was determined by intermittent equilibrium technique.

2.4. TC Degradation Experiments

The 0.02 g of CuFe₂O₄ was weighed into a beaker. Then, add 100 mL of TC solution with a concentration of 20 mg/L to the beaker. The solution was put into a dark box and stirred magnetically for 30 min. After that, 0.7 mM of PDS was added under continuous stirring and UV light (Royal Dutch Philips, λ = 254 nm) source was turned on. The light source is located above the beaker and the distance to the beaker is 15 cm. Samples were acquired when the reaction time was 0, 5, 10, 15, 30, 45 and 60 min, respectively. The TC solution concentration was determined by UV−VIS spectrometer after filtration with 0.45 μm filter membrane.

3. Results and Discussion

3.1. Characterization of CuFe₂O₄

The SEM analysis of CuFe₂O₄ calcined at 300 °C was carried out, and the results are shown in Figure 1. It is seen that the particles are on the nanometer scale. There are many particles on the surface of the material with uniform particle size dispersion, and these raised particles are CuFe₂O₄ crystals.

Figure 2 shows the XRD patterns of CuFe₂O₄ at different calcination temperatures. The diffraction peaks of CuFe₂O₄−300 at 2θ = 30.17°, 35.54°, 37.18°, 43.20°, 53.60°, 57.14° and 62.74° corresponding to the (220), (311), (222), (400), (422), (511) and (440) crystal faces of CuFe₂O₄ (#JCPDS77−0010), respectively. This indicates that the CuFe₂O₄ prepared by the sol−gel method has generated an obvious spinel structure, which is in agreement with the findings of Selima et al. [14]. According to Scherre formula, when the calcination temperature is 300 °C, the lattice size on the (311) crystal plane is 18.2 nm [15]. There is a phase transformation from cubic CuFe₂O₄ (c−CuFe₂O₄) to tetragonal CuFe₂O₄ (t−CuFe₂O₄) at temperatures above 300 °C. The arrangement of crystal particles is more complete and denser. The CuFe₂O₄−600 at 2θ = 18.32°, 34.72°, 35.86°, 41.82°, 53.92°, 62.16° and 63.63°, corresponding to the tetragonal CuFe₂O₄ at (101), (103), (211), (004), (312), (224) and (400) crystal faces, which are in agreement with the standard card (#JCPDS34−0425). When the calcination temperature is 400–600 °C, there is the appearance of an impurity phase. The characteristic peak of CuO (#JCPDS48−1548) appears at 53.49°, indicating that there is a small amount of CuO. The characteristic peaks of α−Fe₂O₃ (JCPDS 84−0306) appear at 57.27°, 62.92° and 64.01°, confirming the presence of this phase. The XRD results show that the crystalline formation and particle size of the catalysts are affected by the calcination temperature.

Raman spectroscopy technique is one of the powerful techniques for a direct probe of cation redistribution, structural transformation, and lattice distortion. Figure 3 shows Raman spectra of CuFe₂O₄ calcined at 200, 300, 400, 500, and 600 °C. The structural transformation from cubic (Fd−3m) to tetragonal (I4₁/amd) symmetry occurs due to strengthening of the Jahn–Teller distortion. In this study, five Raman characteristic peaks were observed for the CuFe₂O₄ calcined at 200 °C and 300 °C, which were present near 155 cm⁻¹ (T_2g(3)), 280 cm⁻¹ (Eg), 331 cm⁻¹ (T_2g(2)), 470 cm⁻¹ (T_2g(1)), 574 cm⁻¹ (A_1g(2)) and 676 cm⁻¹ (A_1g(1)). The Raman activity patterns present in the synthesized copper ferrate match well with the group theory results. These belong to cubic inverse−spinel copper ferrite. With the increase of calcination temperature, the copper ferrite samples gradually transformed into tetragonal phase structure, and new Raman spectral peaks appeared, such as 780 cm⁻¹, 840 cm⁻¹ and 930 cm⁻¹. In the prepared CuFe₂O₄, the presence of Raman modes the T_2g mode and the Eg mode in the lower frequency range demonstrates the stability of the spinel structure [16]. The peak at the T_2g(1) mode is due to the asymmetric stretching of the oxygen anion with tetrahedral and octahedral cations. It can be observed that the vibrational frequency of the Raman−activated mode shifts to a lower frequency (red−shift) due to the redistribution of the cations and the relative increase of the particle size. The width of the Raman active modes is also increased. Due to the redistribution of cations between the tetrahedral and octahedral positions, the intensity of the peak the A_1g(1) mode decreases with increasing calcination temperature, while the intensity of the remaining peaks increases [17].

Figure 4 shows the FT−IR spectra of CuFe₂O₄ calcined at 300 °C before and after the degradation reaction. Spinel compounds are characterized by two lattice vibrations that correspond, respectively, to the vibrations of tetrahedral and octahedral metal centers against the O²⁻⁻ cage. The first one appears in Figure 4 around 570 cm⁻¹ while the other is not complete and is expected to be centered slightly below 400 cm⁻¹. [18]. These two bands confirm the formation of spinel structure and there is no significant change before and after the reaction. The absorption broadband at 3400 cm⁻¹ indicates the stretching mode of the H₂O molecule −OH group. The formation of the bands at 2925 cm⁻¹ and 2960 cm⁻¹ is due to the stretching vibrations of CH₂ and CH₃. Moreover, bands due to C=O stretching vibrations of the citric acid COOH group appear at 1635 cm⁻¹, indicating the binding of citric acid radicals to the magnetite surface. This band may also be the bending vibration of water [19]. The characteristic absorption band located near 1036 cm⁻¹ is attributed to the C−O−C stretching vibration. The CuFe₂O₄ also exhibits weak characteristic bands at 1380 cm⁻¹ and 878 cm⁻¹, which represent the symmetric stretching vibration of the nitrate ion and the asymmetric stretching, etc. [20] From the FT−IR spectrum, it can be seen that citrate and nitrate impurities are very small. High−purity CuFe₂O₄ materials can be effectively prepared by the sol−gel method.

3.2. TC Degradation Behavior

3.2.1. A Comparative Study of TC Removal in the Different System

As displayed in Figure 5a, the degradation efficiencies of different reaction systems were investigated. The degradation rate of TC was 0.3% when only PDS was added. This indicates that PDS could not produce free radicals rapidly in the absence of external catalysis [21]. When only CuFe₂O₄ was used (i.e., without PDS), the TC removal efficiency reached 34.0%, obviously as a result of adsorption but not degradation In summary, the removal of TC by absorption or single PDS degradation is relatively low. In the absence of PDS, the CuFe₂O₄, UV and CuFe₂O₄/UV were 34.0%, 7.8% and 50.6%, respectively. The highest degradation rate of TC reached 91.1% after adding PDS, which verified that PDS has a crucial role to play in the system, as the addition of PDS could generate more free radicals [22]. The UV lamp was the activator in the photocatalytic reaction. The degradation rate of TC was 61.5% in the case of activating PDS by UV radiation, which confirmed that PDS can be directly excited by UV light to make its O−O bond break into SO₄^•− (Equation (1)). At the same time, UV radiation excites electrons from the valence band of CuFe₂O₄ into the conduction band. These electrons act as a strong reducing agent and result in the formation of SO₄^•− radicals as shown in Equation (2) [23]. The addition of CuFe₂O₄ was more effective compared to PDS activation by UV. The degradation of TC reached 91.1% in the CuFe₂O₄/UV/PDS system, indicating the high catalytic efficiency of CuFe₂O₄ in activating PDS. In addition, CuFe₂O₄ acts as a photocatalyst, where it can excite more different kinds of radicals in the system [24]. The above discussion shows that CuFe₂O₄ coupled with UV activated persulfate provides excellent degradation conditions for degrading TC. Firstly, both CuFe₂O₄ and UV can activate PDS. Secondly, the CuFe₂O₄ as a photocatalyst, it could excite more different kinds of radicals in the system.

Meanwhile, the mineralization efficiency of TC at various catalytic conditions was evaluated by TOC measurement in the solution after 90 min of reaction. As shown in Figure 5b, the CuFe₂O₄/UV/PDS system had the highest TOC removal rate which was 39.6%, followed by CuFe₂O₄/PDS. It indicates that the CuFe₂O₄/UV/PDS system can achieve a high level of pollutant degradation.

$${{\mathrm{S}}_2{\mathrm{O}}_8}^{2-}\stackrel{\mathrm{UV}}{\to }2{{\mathrm{SO}}_4}^{•-}$$

(1)

$${{\mathrm{S}}_2{\mathrm{O}}_8}^{2-}+{\mathrm{e}}^{-}\to {{\mathrm{SO}}_4}^{•-}+{ \mathrm{S}{\mathrm{O}}_4}^{2-}$$

(2)

In this work, the catalytic performance of calcination at different temperatures (200~600 °C) was compared, and the results are shown in Figure 6a. The degradation rates of TC were 87.4%, 91.1%, 86.2%, 85.7% and 82.6% at different calcination temperatures (200–600 °C), respectively. It shows that TC has high degradation efficiency at low calcination temperature, and the degradation rate of TC decreases with higher calcination temperature. Proper calcination can improve the catalytic performance of CuFe₂O₄ catalyst. As the calcination temperature increases, the crystal structure, specific surface area and oxygen vacancies of the catalyst will change, thereby affecting its catalytic performance. At an appropriate calcination temperature, the specific surface area and pore structure of the catalyst are optimized, and the catalytic activity and selectivity are improved. Excessive temperature can lead to an increase in particle size and lattice damage, thereby reducing catalytic activity [25]. In addition, as the calcination temperature increases, CuFe₂O₄ decomposes into separate oxides α−Fe₂O₃ and CuO. To confirm that Fe₂O₃ and CuO affect the degradation rate of TC, a comparison test was also conducted in this section (Figure 6b), and under the same conditions, the removal rate by CuO as catalyst was 85.1% and that by Fe₂O₃ as a catalyst was 39.1%.

3.2.2. The Effect of Initial pH

Since the initial pH usually has an important influence on the generation process of free radicals, the physicochemical properties of the reactants and the oxidation capacity of the free radicals, the findings of the investigation of the initial solution pH are presented in Figure 7. Firstly, under alkaline conditions, the OH− reacts with SO₄^•− to form •OH, and the oxidation activity is weakened. The •OH reacts with •OH to form water and the free radicals are reduced, which leads to a decrease in the TC degradation rate (Equations (3)–(5)). The findings indicate that the TC removal is affected when the free radicals in the system are reduced [26]. Secondly, the TC degradation is also influenced by the charge of TC and the surface charge of CuFe₂O₄ [27,28]. The point of zero charge (pH_zpc) of CuFe₂O₄ was 5.95 (Figure 8a). The chemical structure of tetracycline was shown in Figure 8b. The hydroxyl groups are deprotonated at high pH, hence the negative charge, at low pH the amine groups are protonated and hence the positive charge. When pH < 3, the TC solution has a positive charge. When pH = 3–7.7, the TC in the solution exists as amphoteric ions. When pH > 7.7, it exists as monovalent anions or divalent anions. When pH < pH_zpc, the positive surface charge of the material will help to adsorb more SO₄^•− oxidized organic matter [29]. When pH > pH_zpc, the material surface is negatively charged and TC exists as monovalent anions, which generate an electrostatic repulsion between TC and the surface of CuFe₂O₄. At pH = 11, there is no adsorption in the 30 min−period before the reaction. At pH = 9 and pH = 3, the amount adsorbed is much less than at pH = 5 and pH = 7. Under strong acid or alkali conditions, the surface of copper ferrite will suffer serious structural damage and loss of active site, leading to a significant reduction in its adsorption capacity. Under neutral condition, copper ferrite shows better adsorption performance, because its surface structure and active site can maintain good stability at this time. In addition, some experimental studies have also shown that copper ferrite under neutral conditions may undergo hydrogen bonding, van der Waals forces and charge transfer interactions with the adsorbent, thereby enhancing its adsorption capacity [30]. It prevents the reaction of TC on the material surface. Showing the high efficiency of CuFe₂O₄/UV/PDS at neutral pH, the process is suitable for surface water and groundwater treatment.

$${{\mathrm{S}\mathrm{O}}_4}^{•-}+{\mathrm{O}\mathrm{H}}^{-}\to {{\mathrm{S}\mathrm{O}}_4}^{2-}+•\mathrm{O}\mathrm{H}$$

(3)

$${{\mathrm{S}\mathrm{O}}_4}^{•-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}{\mathrm{S}\mathrm{O}}_4}^{-}+•\mathrm{O}\mathrm{H}$$

(4)

$$•\mathrm{O}\mathrm{H}+•\mathrm{O}\mathrm{H}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(5)

3.2.3. Stability and Recyclability of CuFe₂O₄

The stability and reusability of CuFe₂O₄ photocatalysts were confirmed through five consecutive reuse experiments. The CuFe₂O₄ is easily removed from the solution through magnetic means such as simple magnets [31]. It was separated from the system using a magnet after each degradation reaction and rinsed three times with deionized water. Then, put into a 60 °C oven to dry. Subsequently, the degradation experiment was performed again. As displayed in Figure 9, even after four cycles, the TC degradation rate remained above 84%. From the changes in TC adsorption, it is inferred that the minor decrease in the efficiency of removal may be attributed to limited poisoning of the catalyst surface by reaction intermediates and loss of CuFe₂O₄ particles during washing [32].

3.3. Degradation Reaction Mechanism

3.3.1. Identification of Active Species

To investigate the activation mechanism of the CuFe₂O₄/UV/PDS system, quenching experiments were performed. Based on the summary of previous studies [33,34,35], it was concluded that SO₄^•− and •OH may be produced in the CuFe₂O₄/UV/PDS system. Because of the different reaction rates of anhydrous ethanol (EtOH) and tert−butanol (TBA) with SO₄^•− and •OH, the EtOH capture group and the TBA capture group were selected. The reaction rates of EtOH with SO₄^•− and •OH are (1.6–7.7) × 10⁷ M⁻¹s⁻¹ and (1.2–2.8) × 10⁹ M⁻¹s⁻¹ [36], respective1y. As presented in Figure 10a, EtOH concentration and the degradation rate of TC were inversely related. When the EtOH concentration increased from 0 mM to 100 mM, the degradation rate of TC decreased by 32.2%. In addition, the reaction rate of TBA with •OH was higher than that of SO₄^•− (k_•OH = (3.8–7.6) × 10⁸ M⁻¹s⁻¹, k_SO4^•− = (4.0–9.1) × 10⁵ M⁻¹s⁻¹) [37]. The removal rate of TC was 83.4% when the concentration of TBA was 100 mM (Figure 10b). Therefore, according to the difference between the degradation rate of TC under the participation of EtOH and TBA, it can be inferred that SO₄^•− plays a dominant role compared with •OH. It is worth noting that the addition of ethanol and tert butanol does not completely stop the reaction, indicating that there may be other free radical or non− free radical pathways in the reaction system [38,39].

3.3.2. Mechanism of Degradation

According to the mentioned results and discussions, the possible mechanism for the TC degradation by CuFe₂O₄/UV/PDS is proposed as shown in Figure 11. In the first step, the energy of some incident photons can destroy the chemical bond of TC molecule, TC molecules will directly self−decompose. In addition, UV radiation can also directly activate PDS, generating sulfate radicals. Furthermore, the presence of photogenerated electrons on the surface of CuFe₂O₄ can activate PDS to generate SO₄^•−. Secondly, the effective Fe³⁺/Fe²⁺ (Cu²⁺/Cu³⁺) conversion enhances the PDS activation and generates SO₄^•− and •OH for pollutant removal [40,41]. These produce free radicals react with TC, breaking it down into harmless H₂O, CO₂ and some intermediates.

4. Conclusions

To summarize, the CuFe₂O₄ exhibits excellent performance in the degradation of tetracycline. In this process, several methods (XRD, Raman spectra spectroscopy, SEM and FT−IR) were carried out to characterize CuFe₂O₄, demonstrating that magnetic separable CuFe₂O₄ nanoparticles can be prepared by the sol−gel method. Through the coupling of CuFe₂O₄/UV/PDS process, the degradation rate of antibiotics was improved. It was confirmed that the coupling of the CuFe₂O₄/UV has a synergistic effect on the catalytic PDS to produce more active species. Under optimal conditions, the TC degradation rate reached 91.1% within 90 min. Magnetic CuFe₂O₄ powder can be effectively recycled, and reusability and stability experiments confirmed the excellent stability and cyclicity of the CuFe₂O₄ catalyst. In addition, through quenching experiments, it was concluded that there is SO₄^•− and •OH generation in the system. A possible mechanism of degradation was proposed. It confirmed its potential application in degrading antibiotics.