Photocatalytic Removal of Antibiotics from Wastewater Using the CeO2/ZnO Heterojunction

CeO2/ZnO-based photocatalytic materials were synthesized by the sol-gel method in order to establish heterojunctions that increase the degradation efficiency of some types of antibiotics by preventing the recombination of electron–hole pairs. The synthesized materials were analysed by XRD, SEM, EDAX, FTIR, and UV-Vis. After several tests, the optimal concentration of the catalyst was determined to be 0.05 g‧L−1 and 0.025 g‧L−1 for chlortetracycline and 0.05 g‧L−1 for ceftriaxone. CeO2/ZnO assemblies showed much better degradation efficiency compared to ZnO or CeO2 tested individually. Sample S3 shows good photocatalytic properties for the elimination of ceftriaxone and tetracycline both from single solutions and from the binary solution. This work provides a different perspective to identify other powerful and inexpensive photocatalysts for wastewater treatment.


Introduction
In our constantly and rapidly evolving world, finding solutions to protect the environment has been a priority lately. Among the substances that improve our life, but also make it more difficult, antibiotics are indispensable products due to the positive health benefits, though they have negative effects on natural ecosystems. Antibiotics are necessary medicines; many of them are on the WHO list of essential medicines, and their consumption is continuously increasing [1,2]. Unfortunately, some of these substances are excreted from the body unmetabolized in percentages that vary depending on the compound (up to 80% for tetracycline [3]), and their accumulation in wastewater and soil causes complications [4][5][6][7][8]. The presence of antibiotics in wastewater and also in drinking water has been detected in small amounts, but this is quite alarming for environmental safety [2,9]. The use of antibiotics in excess also leads to the appearance of antibioticresistant microorganisms [5,6,10,11]. Antibiotic resistance is a current problem leading to a decrease in the effectiveness of these classes of drugs. Tetracycline and ceftriaxone are widely used to treat various bacterial infections and have been detected in wastewater at varying concentrations. They are stable over time in the complex wastewater mixtures and are difficult to remove using classic water treatment methods; they can adversely affect the wastewater treatment processes (for example the ammonia removal process or chemical oxygen demand removal). There are several ways to remove antibiotics and other contaminants from wastewater, such as adsorption, sonochemical processes, ozonation, membrane technology, aerobic or anaerobic treatment, phytoremediation, biogeochemical methods such as wetlands, chemical disinfection methods such as chlorination, electrochemical oxidation processes, osmosis and electro-osmosis, electro-flocculation, ionizing radiation, and hybrid technologies [8,10,[12][13][14][15][16][17][18][19][20][21][22][23][24]. However, it seems that the most effective methods of reducing the level of antibiotics in water are the methods that involve photocatalysis from Chemical Company, Romania) and PVA (average Mw 89,000-98,000, 99%, Aldrich) as anti-agglomeration agents. The precursors were mixed with ammonia solution in different molar ratios (S2-2ZnO:CeO 2 ; S3-ZnO:2CeO 2 ), the mixture was kept at 120 • C for 8 h in a Teflon autoclave, and then the precipitates obtained were separated by centrifugation and washed with bidistilled water until the total elimination of NH 4 OH, NH 4 NO 3 , and CH 3 COOH. The precipitates thus obtained (cerium and zinc hydroxide) were immersed in a 0.001 mol·L −1 PVA solution for 30 min, filtered, and then calcined at 400 • C (at a heating rate of 5 grd·min −1 ). Polyvinyl alcohol has the task of avoiding the agglomeration of particles and is eliminated by calcination. The samples obtained were named S1: ZnO, S2: 2ZnO-CeO 2 , S3: ZnO-2CeO 2 , and S4: CeO 2 .
The antibiotics tested were chlortetracycline and ceftriaxone (Sigma Aldrich), presented in Table 1.

Reagents and Preparation
Photocatalytic materials were obtained by hydrothermal synthesis [42,43] starting from Zn(CH3COO)2·2H2O and Ce(NO₃)₃‧6H₂O (Sigma-Aldrich) as precursors and NH4OH (25% from Chemical Company, Romania) and PVA (average Mw 89,000-98,000, 99%, Aldrich) as anti-agglomeration agents. The precursors were mixed with ammonia solution in different molar ratios (S2-2ZnO:CeO2; S3-ZnO:2CeO2), the mixture was kept at 120 °C for 8 h in a Teflon autoclave, and then the precipitates obtained were separated by centrifugation and washed with bidistilled water until the total elimination of NH4OH, NH4NO3, and CH3COOH. The precipitates thus obtained (cerium and zinc hydroxide) were immersed in a 0.001 mol‧L −1 PVA solution for 30 min, filtered, and then calcined at 400 °C (at a heating rate of 5 grd‧min −1 ). Polyvinyl alcohol has the task of avoiding the agglomeration of particles and is eliminated by calcination. The samples obtained were named S1: ZnO, S2: 2ZnO-CeO2, S3: ZnO-2CeO2, and S4: CeO2.
The antibiotics tested were chlortetracycline and ceftriaxone (Sigma Aldrich), presented in Table 1. Table 1. Chemical structure and some characteristics of antibiotics.

Chemical Composition and Structure Properties
CT-Chlortetracycline hydrochloride C22H23ClN2O8‧HCl The first antibiotic discovered and used; 515. 34

Sample Characterization
The synthesized samples were characterized by X-ray diffraction (Rigaku, Tokyo, Japan) and CuKα radiation (2θ angle, range from 10 to 80; step 0.02°/s); the possible functional groups remaining on the surface of the photocatalyst were identified by FTIR (Perkin Elmer Spectrum 100, PerkinElmer Inc., Shelton, CT, USA), resolution 2 cm −1 using 32 scans in the range 4000-400 cm −1 ; all samples were prepared as KBr pellets (ratio 5/95 wt.%). The morphology of the prepared samples was observed with a Quanta 200 scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy analyzer (Bruker Optics Inc, Billerica, MA, USA). The UV-Vis absorption spectra of the solid samples were obtained with a Jasco V-550 device (Jasco International CO, Kyoto, Japan) equipped with an integrating sphere. Monitoring of the photocatalytic degradation of antibiotics was performed also with a Jasco V-550 device (Kyoto, Japan).

Reagents and Preparation
Photocatalytic materials were obtained by hydrothermal synthesis [42,43] starting from Zn(CH3COO)2·2H2O and Ce(NO₃)₃‧6H₂O (Sigma-Aldrich) as precursors and NH4OH (25% from Chemical Company, Romania) and PVA (average Mw 89,000-98,000, 99%, Aldrich) as anti-agglomeration agents. The precursors were mixed with ammonia solution in different molar ratios (S2-2ZnO:CeO2; S3-ZnO:2CeO2), the mixture was kept at 120 °C for 8 h in a Teflon autoclave, and then the precipitates obtained were separated by centrifugation and washed with bidistilled water until the total elimination of NH4OH, NH4NO3, and CH3COOH. The precipitates thus obtained (cerium and zinc hydroxide) were immersed in a 0.001 mol‧L −1 PVA solution for 30 min, filtered, and then calcined at 400 °C (at a heating rate of 5 grd‧min −1 ). Polyvinyl alcohol has the task of avoiding the agglomeration of particles and is eliminated by calcination. The samples obtained were named S1: ZnO, S2: 2ZnO-CeO2, S3: ZnO-2CeO2, and S4: CeO2.
The antibiotics tested were chlortetracycline and ceftriaxone (Sigma Aldrich), presented in Table 1. Table 1. Chemical structure and some characteristics of antibiotics.

Chemical Composition and Structure Properties
CT-Chlortetracycline hydrochloride C22H23ClN2O8‧HCl The first antibiotic discovered and used; 515. 34

Sample Characterization
The synthesized samples were characterized by X-ray diffraction (Rigaku, Tokyo, Japan) and CuKα radiation (2θ angle, range from 10 to 80; step 0.02°/s); the possible functional groups remaining on the surface of the photocatalyst were identified by FTIR (Perkin Elmer Spectrum 100, PerkinElmer Inc., Shelton, CT, USA), resolution 2 cm −1 using 32 scans in the range 4000-400 cm −1 ; all samples were prepared as KBr pellets (ratio 5/95 wt.%). The morphology of the prepared samples was observed with a Quanta 200 scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy analyzer (Bruker Optics Inc, Billerica, MA, USA). The UV-Vis absorption spectra of the solid samples were obtained with a Jasco V-550 device (Jasco International CO, Kyoto, Japan) equipped with an integrating sphere. Monitoring of the photocatalytic degradation of antibiotics was performed also with a Jasco V-550 device (Kyoto, Japan).

Sample Characterization
The synthesized samples were characterized by X-ray diffraction (Rigaku, Tokyo, Japan) and CuKα radiation (2θ angle, range from 10 to 80; step 0.02 • /s); the possible functional groups remaining on the surface of the photocatalyst were identified by FTIR (Perkin Elmer Spectrum 100, PerkinElmer Inc., Shelton, CT, USA), resolution 2 cm −1 using 32 scans in the range 4000-400 cm −1 ; all samples were prepared as KBr pellets (ratio 5/95 wt.%). The morphology of the prepared samples was observed with a Quanta 200 scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy analyzer (Bruker Optics Inc, Billerica, MA, USA). The UV-Vis absorption spectra of the solid samples were obtained with a Jasco V-550 device (Jasco International CO, Kyoto, Japan) equipped with an integrating sphere. Monitoring of the photocatalytic degradation of antibiotics was performed also with a Jasco V-550 device (Kyoto, Japan).

Photodegradation Experiments
The magnetically stirred aqueous suspensions were UV-irradiated in a flat cylinder reactor (total volume: 100 cm 3 ) exposed to air. The radiant flux entering the reactor was about 0.21 mW·cm −2 (Hamamatsu C9536-01 m with H9958 detector for 310-380 nm), calculated from the distance between the samples and the light source produced by a UV-B lamp with Hg (18 W) (OSRAM, Munich, Germany). The volume of the solution was 75 cm 3 , and the catalyst dose was 0.05 g·L −1 . Aqueous solutions were prepared using deionized bidistilled water (Milli-Q, Millipore, Darmstadt, Germany). The degradation operations were carried out at room temperature at natural pH. The aerated suspension was first stirred in the dark for 40 min, which was sufficient to achieve equilibrated adsorption. The initial concentration was 0.025 mg·mL −1 chlortetracycline and ceftriaxone 0.05 mg·mL −1 .
The tests were performed without changing the pH of the native antibiotic solution. Samples were taken at fixed timed intervals and centrifuged to remove the solid, then the absorbance of the supernatant was read three times, noting the mean value. The level of antibiotic degradation was quantified using the correlation Equation (1): where R(%) is the antibiotic degradation yield, A 0 and A i are the initial and t i time antibiotic absorbance values at the same time values.

Characterization of the Photocatalysts
The X-ray diffraction spectra of ZnO and CeO 2 and those of the as-synthesized composites are shown in Figure  The magnetically stirred aqueous suspensions were UV-irradiated in a flat cylinder reactor (total volume: 100 cm 3 ) exposed to air. The radiant flux entering the reactor was about 0.21 mW‧cm −2 (Hamamatsu C9536-01 m with H9958 detector for 310-380 nm), calculated from the distance between the samples and the light source produced by a UV-B lamp with Hg (18 W) (OSRAM, Munich, Germany). The volume of the solution was 75 cm 3 , and the catalyst dose was 0.05 g‧L −1 . Aqueous solutions were prepared using deionized bidistilled water (Milli-Q, Millipore, Darmstadt, Germany). The degradation operations were carried out at room temperature at natural pH. The aerated suspension was first stirred in the dark for 40 min, which was sufficient to achieve equilibrated adsorption. The initial concentration was 0.025 mg‧mL −1 chlortetracycline and ceftriaxone 0.05 mg‧mL −1 .
The tests were performed without changing the pH of the native antibiotic solution. Samples were taken at fixed timed intervals and centrifuged to remove the solid, then the absorbance of the supernatant was read three times, noting the mean value. The level of antibiotic degradation was quantified using the correlation Equation (1): where R(%) is the antibiotic degradation yield, A0 and Ai are the initial and ti time antibiotic absorbance values at the same time values.

Characterization of the Photocatalysts
The X-ray diffraction spectra of ZnO and CeO2 and those of the as-synthesized composites are shown in Figure 1. They present several peaks that could be indexed in accordance with the diffraction spectrum of ZnO (JCPDS card no. 36-1451) and CeO2 (JCPDS card no. 34-0394). For S1 (ZnO), the peaks were positioned at the 2θ angles: 31 (100), (002)     In order to get more information about the appearance of the surfaces of the prepared samples, the SEM (Scanning Electron Microscopy) images were examined. The SEM electron microscopy images showing the morphology of the surfaces at different magnifications and the EDAX (Energy Dispersive X-ray) profiles are shown in Figure 2. Samples S1-S4 present porous structures with different surface morphologies, from small and uneven aggregates for S1-S3 to aggregates with an isometric structure for S4. The boundaries between the particles are not well defined.
In order to get more information about the appearance of the surfaces of the prepared samples, the SEM (Scanning Electron Microscopy) images were examined. The SEM electron microscopy images showing the morphology of the surfaces at different magnifications and the EDAX (Energy Dispersive X-ray) profiles are shown in Figure 2. Samples S1-S4 present porous structures with different surface morphologies, from small and uneven aggregates for S1-S3 to aggregates with an isometric structure for S4. The boundaries between the particles are not well defined. For S4, crystallite size varies from below 0.1 µm × 0.1 µm × 0.1 µm to approximately 1 µm × 2 µm × 3 µm. The accumulation of micro-crystallites is a normal process; the crystallites try to reach the minimum energy state, minimizing the contact area with the external environment. The small size of the obtained crystallites explains the good photocatalytic activity. EDAX analysis confirms the existence of Zn, Ce, and O elements, so CeO2 For S4, crystallite size varies from below 0.1 µm × 0.1 µm × 0.1 µm to approximately 1 µm × 2 µm × 3 µm. The accumulation of micro-crystallites is a normal process; the crystallites try to reach the minimum energy state, minimizing the contact area with the external environment. The small size of the obtained crystallites explains the good photocatalytic activity. EDAX analysis confirms the existence of Zn, Ce, and O elements, so CeO 2 and ZnO oxides are present.
The FTIR spectra are presented in Figure 3. FTIR analysis confirms that the organic phase has been eliminated by calcination. The peaks in the 3400-3450 cm −1 range are due to adsorbed water molecules (the O-H bond stretching vibration), and those in the 550-400 cm −1 range are generated by the vibrations of the metal oxide bonds [49,51]. For S4, crystallite size varies from below 0.1 µm × 0.1 µm × 0.1 µm to approximately 1 µm × 2 µm × 3 µm. The accumulation of micro-crystallites is a normal process; the crystallites try to reach the minimum energy state, minimizing the contact area with the external environment. The small size of the obtained crystallites explains the good photocatalytic activity. EDAX analysis confirms the existence of Zn, Ce, and O elements, so CeO2 and ZnO oxides are present.
The FTIR spectra are presented in Figure 3. FTIR analysis confirms that the organic phase has been eliminated by calcination. The peaks in the 3400-3450 cm −1 range are due to adsorbed water molecules (the O-H bond stretching vibration), and those in the 550-400 cm −1 range are generated by the vibrations of the metal oxide bonds [49,51]. Next, solid-state UV-Vis spectroscopy was used to obtain information on the optical properties of the synthesized samples and to calculate the Eg values ( Figure 4). The optical band gap was determined using the Tauc formula, ahν 2 = A hν -E g , in which a is the absorption coefficient, A is a constant, h is Planck's constant, ν is the frequency of incident radiation, Eg is the optical band gap, and n = 1/2 (for ZnO and CeO2) [48,52]. The Eg values are obtained by extrapolating the straight lines to the point of intersection with the x-axis. Compared to pure ZnO and pure CeO2, the composite materials S2 and S3 have a different Eg value. Next, solid-state UV-Vis spectroscopy was used to obtain information on the optical properties of the synthesized samples and to calculate the E g values (Figure 4). The optical band gap was determined using the Tauc formula, (ahν) 2 = A hν − E g , in which a is the absorption coefficient, A is a constant, h is Planck's constant, ν is the frequency of incident radiation, E g is the optical band gap, and n = 1/2 (for ZnO and CeO 2 ) [48,52]. The E g values are obtained by extrapolating the straight lines to the point of intersection with the x-axis. Compared to pure ZnO and pure CeO 2 , the composite materials S2 and S3 have a different E g value.  The observed decrease in E g values can be explained by the occurrence of numerous surface defect states such as oxygen vacancies, the coexistence of Ce 4+ and Ce 3+ in the CeO 2 /ZnO heterostructure, and the interaction between ZnO and CeO 2 nanocrystals [53,54].

Antibiotic Photocatalytic Degradation
To evaluate the photocatalytic activity, the synthesized samples were contacted with solutions of chlortetracycline, ceftriaxone, and a chlortetracycline-ceftriaxone mixture (Figures 5-7). Work was carried out without adjustments to the natural pH value of the solutions. The doses of photocatalytic material have been studied previously; only the results for the 0.05 g·L −1 concentration are presented here. Experiments performed with UV irradiation but without a photocatalyst showed that both antibiotics are relatively stable to UV exposure. The experimental results have similar profiles; only a small fraction of the antibiotics is degraded after exposure to ultraviolet light in the absence of photocatalysts. The substantial increase in the photocatalytic performance of samples S2 and S3 compared to S1 and S4 (pure oxides) is due to the process of delaying the recombination of electronhole pairs, owing to the formation of heterojunctions between the two oxides; this is advantageous for keeping the promoted electron in the conduction band of ZnO for a longer period of time. In this situation, adsorbed oxygen is more likely to form superoxide O 2 − radicals.   When CeO 2 /ZnO -based photocatalytic materials are irradiated with UV rays, pairs of electric charge holes in the valence band and electric charge electrons in the conduction band are formed. The holes immediately react with water molecules or hydroxyl ions and form hydroxyl radicals, which are very strong oxidizing agents of organic molecules, according to the following Equations (2)-(9) [29,43,49]: ZnO h + + OH − → · OH + ZnO (5)     When CeO2/ZnO -based photocatalytic materials are irradiated with UV rays, pairs of electric charge holes in the valence band and electric charge electrons in the conduction band are formed. The holes immediately react with water molecules or hydroxyl ions and form hydroxyl radicals, which are very strong oxidizing agents of organic molecules, according to the following Equations (2)-(9) [29,43,49]  Oxidation processes of antibiotics take place in hole species (h + ) and O 2 − . Photocatalytic activity is influenced by crystal structure, specific surface area, particle size distribution, porosity, surface hydroxyl group density, etc. All these properties affect the formation of electron-hole pairs, the adsorption-desorption surface area, and the redox process. The photocatalytic activity of the studied materials was enhanced by delaying the recombination of electron-hole pairs. The main method of slowing down is through the formation of heterojunctions in the CeO 2 /ZnO mixture (Figure 8), a process described in other studies [49,52,53]. The electrons in the 4f orbitals of Ce interfere with the 3d electrons of Zn and the 2p electrons of O, resulting in the formation of a new band between BV and BC that changes the characteristics of the oxide mixture. The highest level of degradation was achieved for CFTX at 71.23% in the presence of S3, followed by CT at 58.65%, also for S3 (Figure 9). The higher proportion of CeO2 in the synthesized materials has resulted in a higher degradation rate; Ce 4+ ions act as a trap to prevent the recombination of electron-hole pairs generated by irradiation with ultraviolet rays, with E > Eg; thereby, the photocatalytic process is accelerated. Compared to pure CeO2 and pure ZnO, the superior catalytic performance of samples S2 and S3 is attributed to the formation of heterojunctions, which are an effective method for modifying the properties of mixed oxides.
The information gathered in Table 2 reveals the large diversity of materials that can be used as photocatalytic materials for ceftriaxone and tetracycline degradation. The electrons in the 4f orbitals of Ce interfere with the 3d electrons of Zn and the 2p electrons of O, resulting in the formation of a new band between BV and BC that changes the characteristics of the oxide mixture. The highest level of degradation was achieved for CFTX at 71.23% in the presence of S3, followed by CT at 58.65%, also for S3 ( Figure 9). The higher proportion of CeO 2 in the synthesized materials has resulted in a higher degradation rate; Ce 4+ ions act as a trap to prevent the recombination of electron-hole pairs generated by irradiation with ultraviolet rays, with E > E g ; thereby, the photocatalytic process is accelerated. The electrons in the 4f orbitals of Ce interfere with the 3d electrons of Zn and the 2p electrons of O, resulting in the formation of a new band between BV and BC that changes the characteristics of the oxide mixture. The highest level of degradation was achieved for CFTX at 71.23% in the presence of S3, followed by CT at 58.65%, also for S3 (Figure 9). The higher proportion of CeO2 in the synthesized materials has resulted in a higher degradation rate; Ce 4+ ions act as a trap to prevent the recombination of electron-hole pairs generated by irradiation with ultraviolet rays, with E > Eg; thereby, the photocatalytic process is accelerated. Compared to pure CeO2 and pure ZnO, the superior catalytic performance of samples S2 and S3 is attributed to the formation of heterojunctions, which are an effective method for modifying the properties of mixed oxides.
The information gathered in Table 2 reveals the large diversity of materials that can be used as photocatalytic materials for ceftriaxone and tetracycline degradation. Compared to pure CeO 2 and pure ZnO, the superior catalytic performance of samples S2 and S3 is attributed to the formation of heterojunctions, which are an effective method for modifying the properties of mixed oxides.
The information gathered in Table 2 reveals the large diversity of materials that can be used as photocatalytic materials for ceftriaxone and tetracycline degradation.

Kinetic Analysis
The photocatalytic process was assumed to follow a pseudo-first-order model according to Equation (10).

Conclusions
In this work, we have reported a CeO2/ZnO mixed oxide powder with good photocatalytic activity for the degradation of some antibiotics (ceftriaxone and chlortetracycline) that are relatively stable to UV radiation. Four samples were synthesized by the hydrothermal method, two of which were pure oxides (ZnO and CeO2) and two of which were mixed oxides with different molar ratios (CeO2/ZnO). These were characterized by XRD, FTIR, SEM + EDAX, and UV-Vis on the solid. All four samples showed photocatalytic activity to reduce the level of antibiotics in wastewater (CFTX, CT, and the mixture of the two); the experimental results showed that mixed oxides behave better than pure oxides (Ce:Zn ratio = 2.1) and have better photocatalytic activity in all three tested situations (CFTX, CT, CFTX + CT). Since the radiation dose used is very low and the synthesized materials are chemically inert, we consider that they are useful for reducing the level of antibiotics in wastewater. The heterogeneous photocatalytic processes have a net advantage in the reduction in some contaminants from the aqueous medium (antibiotics), including the non-selective breakdown of pollutants to very low concentrations, normal pressure and temperature, use of oxygen as the primary oxidant, and the possibility of simultaneously inducing both oxidation reactions and reduction reactions.  Fits of the pseudo-first-order kinetic model for the photocatalytic chlortetracycline + ceftriaxone mixture degradation using the S1-S4 photocatalyst (0.05 g·L −1 , chlortetracycline concentration 0.025 g·L −1 , ceftriaxone concentration 0.05 g·L −1 ).

Conclusions
In this work, we have reported a CeO 2 /ZnO mixed oxide powder with good photocatalytic activity for the degradation of some antibiotics (ceftriaxone and chlortetracycline) that are relatively stable to UV radiation. Four samples were synthesized by the hydrothermal method, two of which were pure oxides (ZnO and CeO 2 ) and two of which were mixed oxides with different molar ratios (CeO 2 /ZnO). These were characterized by XRD, FTIR, SEM + EDAX, and UV-Vis on the solid. All four samples showed photocatalytic activity to reduce the level of antibiotics in wastewater (CFTX, CT, and the mixture of the two); the experimental results showed that mixed oxides behave better than pure oxides (Ce:Zn ratio = 2.1) and have better photocatalytic activity in all three tested situations (CFTX, CT, CFTX + CT). Since the radiation dose used is very low and the synthesized materials are chemically inert, we consider that they are useful for reducing the level of antibiotics in wastewater. The heterogeneous photocatalytic processes have a net advantage in the reduction in some contaminants from the aqueous medium (antibiotics), including the non-selective breakdown of pollutants to very low concentrations, normal pressure and temperature, use of oxygen as the primary oxidant, and the possibility of simultaneously inducing both oxidation reactions and reduction reactions.