Enhanced Photocatalytic Degradation of Tetracycline and Oxytetracycline Antibiotics by BiVO4 Photocatalyst under Visible Light and Solar Light Irradiation

The efficient degradation of a toxic antibiotic from an aqueous solution is essential for environmental protection. Our research aimed to fabricate a bismuth vanadate (BiVO4) catalyst via a facile hydrothermal method. The prepared catalyst exhibited a monoclinic phase with a band gap energy of 2.33 eV, indicating the excellent visible-light-active properties of a semiconductor. The photocatalytic performance of the synthesized BiVO4 catalyst was studied by determining the removal of tetracycline (TC) and oxytetracycline (OTC) antibiotics. After 240 min, under sunlight conditions, a high performance of 72% and 83% degradation of TC and OTC, respectively, was achieved. The photocatalytic degradation of the antibiotics correlates well with a first-order reaction, with a high rate constant of 0.0102 min−1. Photogenerated electrons and holes played an important role in the removal of the pollutant. After photocatalytic study, the structural stability of the prepared bismuth vanadate photocatalyst was confirmed. The photocatalyst provided a promising performance even after five successive runs. The result indicates the excellent cycling ability of the sample. The present work demonstrates a promising route for the preparation of a BiVO4 catalyst for the complete removal of toxic antibiotics in aqueous solutions.


Introduction
Water pollution is widely accepted as an increasingly serious environmental hazard. Therefore, there is a tremendous attempt to remove organic pollutants that contaminate natural water resources [1][2][3]. Oxytetracycline (OTC) and tetracycline (TC) are the typical antibiotics widely used in human and veterinary medicine. In addition, these drugs were also used in agriculture, aquaculture, and other fields [4][5][6][7][8][9]. However, these drugs cause a serious problem to the environment [4][5][6][7][8][9]. Therefore, the complete removal of these antibiotics from aqueous phases has become a major aim.
Various treatment methods have been used for the incomplete degradation of pollutants in wastewater. In some cases, however, the creation of secondary toxic organic pollutants was detected [10]. Alternatively, an advanced oxidation process based on semiconducting photocatalysis provides an alternative route for the complete degradation of toxic organic pollutants [11][12][13]. In general, it is known that the commercially available TiO 2 photocatalyst is active under UV light. This photocatalyst, however, shows low photoactivity under natural sunlight. Interestingly, visible-light-active catalysts have gained much attention due to the advantages of utilizing abundant solar light [3,[14][15][16][17].
Visible-light-responsive photocatalysts based on bismuth, such as bismuth molybdate, bismuth vanadate, and bismuth oxyhalides, have been fabricated [18][19][20][21][22][23][24]. Interestingly, Antibiotics 2022, 11, 761 2 of 18 the bismuth vanadate (BiVO 4 ) catalyst has gained much attention. This catalyst can be categorized into three crystal phases: tetragonal, monoclinic, and tetragonal zircon (z-t). It has been shown that BiVO 4 with a monoclinic crystal structure provides maximum photoactivity in comparison to tetragonal and tetragonal zircon crystal structures. Moreover, the BiVO 4 photocatalyst with an Aurivillius structure has the advantages of low toxicity, good structural stability, and high visible-light-active performance [17,18]. Furthermore, this catalyst is cheap and stable against photo-corrosion. Both physical and chemical methods have been reported for the fabrication of BiVO 4 [21,22,[25][26][27][28]. It is well accepted that the hydrothermal/solvothermal route provides the benefits of being less complicated, low cost, and providing an excellent yield, with a high potential for large-scale preparation [20].
In this work, the hydrothermal growth of BiVO 4 was demonstrated. The photocatalytic performance of the synthesized photocatalyst was examined by studying the degradation of tetracycline (TC) and oxytetracycline (OTC) antibiotics. The enhanced sunlight-active performance of about 72% and 93% degradation of TC and OTC antibiotics, respectively, was obtained. This promising performance indicates the excellent environmental remediation property of a synthesized BiVO 4 catalyst for the detoxification of harmful pollutants in aqueous solutions.

Chemicals and Reagents
All chemicals were used without further purification. The ultrapure water (DI, 18.2 MΩ·cm −1 ) was used.

Hydrothermal Syntesis of BiVO 4
The BiVO 4 catalyst was hydrothermally synthesized by using an autoclave. Firstly, solution A, based on dissolving 3.3954 g of Bi(NO 3 ) 3 5H 2 O in 30 mL of 1.5 M HNO 3, was prepared. Secondly, solution B, based on dissolving 0.8189 g of NH 4 VO 3 in 30 mL of 1.5 M NaOH, was prepared. Thirdly, solution B was added to solution A. Afterwards a yellow color and precipitation were detected. Finally, the mixture was transferred into a 100 mL Teflon-lined autoclave. A temperature of 180 • C and a time of 15 h were selected. After that, the precipitate was collected, washed with water and ethanol, and then dried at 60 • C for 12 h.

Characterization
The sample was characterized using the same procedures which can be found elsewhere [3,[11][12][13]. The crystal structure of the sample was elucidated by a powder X-ray diffraction method using monochromatic Cu Kα radiation. The vibrational spectrum was monitored using a FT-IR spectrophotometer. Preparation of the sample was performed by using KBr pellets. The morphological structure and the elemental composition of the sample were elucidated by scanning electron microscopy and transmission electron microscopy. The oxidation states of all elements in the sample were studied by X-ray photoelectron spectroscopy (XPS) at BL 5.3, SLRI, Nakhon Ratchasima, Thailand. An ULVA-PHI 500 VersaProbe II with monochromatic Al Kα radiation was used as an excitation source. A C 1S peak at 284.6 eV was applied as a reference peak to calibrate the binding energy.

Photocatalytic Degradation of the Antibiotics
The photocatalytic performance of the photocatalyst was investigated by studying the degradation of tetracycline (TC) and oxytetracycline hydrochloride (OTC) antibiotics under the simulated visible light (a Panasonic daylight lamp, 15 W) and natural solar light irradiation. The details of the photocatalytic study were reported previously [3,[11][12][13].
The blank experiment was performed by illuminating the antibiotic solution without the addition of the BiVO 4 photocatalyst. This confirmed the low contribution in the photolysis of the antibiotic. In addition, the control experiment, based on the addition Antibiotics 2022, 11, 761 3 of 18 of the photocatalyst to the antibiotic solution without light, was also performed. This dark condition provided data based on the contribution of the adsorption process. The photodegradation study was performed in an aqueous solution of either the TC or OTC antibiotic (concentration of 10 ppm and total volume of 200 cm 3 ). We used 50 mg of BiVO 4 catalyst. A solution of 5 cm 3 was sampled after photo illumination. The exact content of TC and OTC antibiotics was examined by a spectrophotometric method. The λ max of 358 and 368 nm were used for TC and OTC, respectively.
The performance of the antibiotic removal was calculated by Equation (1): where C 0 and C represent the initial concentration and the concentration of the pollutant solution after different time periods of photo illumination, respectively. The photoactivity of the prepared BiVO 4 was also examined by determining the photocatalytic degradation rate as follows: where k 1 is the rate constant of the reaction.

Study of the Photocatalytic Degradation Mechanism
To study the major species that play an important role in the degradation of the antibiotics, additions of t-butanol, NaN 3 , EDTA-2Na, and K 2 Cr 2 O 7 to quench hydroxyl radicals, superoxide anion radicals, holes, and electrons, respectively, were performed. Additionally, KI was used as a quencher of surface hydroxyl radicals and holes. About 5 mM of each quencher was incorporated into the presence of the catalyst (50 mg).
To detect the hydroxyl radicals, the dispersion of the catalyst in a terephthalic acid solution (TA) was performed. The creation of these radicals was investigated by using a spectrofluorometric method (λ excitation of 315 nm).

Cycling Ability of the Photocatalyst
To confirm the cycling ability of the prepared catalyst, after the first round of photocatalytic study, the catalyst was filtered. It was then washed (server times) with ethanol and water. The catalyst was dried and then used in the second run. This reuse was performed for five cycles.

Characterization of the BiVO 4 Catalyst
The XRD pattern of the sample in Figure 1a   14-0688 file [29]. Figure 1b shows the FT-IR spectrum of the sample. The vibrational bands at 725 cm −1 and 824 cm −1 indicated the ν 3 asymmetric and ν 1 symmetric stretching vibration of VO 4 3 , respectively. The Bi-O bond was confirmed by the existence of the vibrational peak at 580 cm −1 [29].  Figure 1b shows the FT-IR spectrum of the sample. The vibrational bands at 725 cm −1 and 824 cm −1 indicated the ν3 asymmetric and ν1 symmetric stretching vibration of VO4 3 , respectively. The BiO bond was confirmed by the existence of the vibrational peak at 580 cm −1 [29]. Figure 1c shows the UV-Vis diffuse reflectance spectrum of the BiVO4 catalyst with a band gap energy (Eg) of 2.33 eV, calculated by the Kubelka-Munk formula [24,[30][31][32]. Accordingly, a visible-light-active absorption edge of 532 nm was obtained. Furthermore, the electron-hole recombination rate of the sample was studied by monitoring the photoluminescence (PL) spectrum, as shown in Figure 1d. The spectrum showed a peak at about 537 nm, corresponding to near band edge (NBE) emission, which is normally found in a sample with high crystallinity [20]. Figure 2a,b show the SEM micrographs of BiVO4 with cubic microstructures of 5.6-9.0 μm. The prepared catalyst showed the particles to have a well-faceted structure and high surface quality [3]. The SEM images of the mapping area and elemental color mapping ( Figure 2c Figure 1c shows the UV-Vis diffuse reflectance spectrum of the BiVO4 catalyst with a band gap energy (E g ) of 2.33 eV, calculated by the Kubelka-Munk formula [24,[30][31][32]. Accordingly, a visible-light-active absorption edge of 532 nm was obtained. Furthermore, the electron-hole recombination rate of the sample was studied by monitoring the photoluminescence (PL) spectrum, as shown in Figure 1d. The spectrum showed a peak at about 537 nm, corresponding to near band edge (NBE) emission, which is normally found in a sample with high crystallinity [20]. Figure 2a,b show the SEM micrographs of BiVO 4 with cubic microstructures of 5.6-9.0 µm. The prepared catalyst showed the particles to have a well-faceted structure and high surface quality [3]. The SEM images of the mapping area and elemental color mapping In addition, TEM and SAED methods were carried out to examine the structural formation of the prepared BiVO4 catalyst ( Figure 3). The TEM images of the catalys Figure 3a,b display a size of about 5.28 μm. In addition, the crystalline structure was tained from the high-resolution TEM (HR-TEM) micrograph ( Figure 3c). On examinat of the lattice fringes, the interplanar lattice spacing (d) of 0.32 nm is due to the reflect from the (121) crystal plane of the monoclinic BiVO4. The selected area electron diffract (SAED) pattern in Figure 3d revealed the monocrystalline nature of the catalyst [3,33]. SAED pattern was mainly caused by the reflection from the (202), (220), and (251) cry planes. This is in good agreement with those found in the XRD diffractogram results (F ure 1a). In addition, TEM and SAED methods were carried out to examine the structural information of the prepared BiVO 4 catalyst ( Figure 3). The TEM images of the catalyst in Figure 3a,b display a size of about 5.28 µm. In addition, the crystalline structure was obtained from the high-resolution TEM (HR-TEM) micrograph ( Figure 3c). On examination of the lattice fringes, the interplanar lattice spacing (d) of 0.32 nm is due to the reflection from the (121) crystal plane of the monoclinic BiVO 4 . The selected area electron diffraction (SAED) pattern in Figure 3d revealed the monocrystalline nature of the catalyst [3,33]. The SAED pattern was mainly caused by the reflection from the (202), (220), and (251) crystal planes. This is in good agreement with those found in the XRD diffractogram results (Figure 1a).  The surface elemental composition and chemical oxidation state of the prepared catalyst was examined by an XPS technique. The correction of binding energies was carried out by using a peak of carbon (C) 1s at 284.6 eV as a reference. The XPS survey spectrum ( Figure 4a) confirmed the existence of Bi, V, and O elements, correlating with the results from the XRD and EDS. Figure 4b shows the XPS spectrum of Bi 4f with two XPS peaks at 163.9 and 158.5 eV, assigned to those of Bi 4f5/2 and Bi 4f7/2, respectively. The result confirmed the presence of Bi 3+ . In the case of the vanadium element (V 2p), the two peaks at 523.87 and 516.40 eV, as seen in Figure 4c, were assigned to V 2p1/2 and V 2p3/2, respectively. Moreover, in the case of the O 1s spectrum (Figure 4d), the deconvolution of the spectrum ends up with three XPS peaks at 532.94, 531.51, and 529.46 eV; this can be The surface elemental composition and chemical oxidation state of the prepared catalyst was examined by an XPS technique. The correction of binding energies was carried out by using a peak of carbon (C) 1s at 284.6 eV as a reference. The XPS survey spectrum (Figure 4a) confirmed the existence of Bi, V, and O elements, correlating with the results from the XRD and EDS. Figure 4b shows the XPS spectrum of Bi 4f with two XPS peaks at 163.9 and 158.5 eV, assigned to those of Bi 4f 5/2 and Bi 4f 7/2 , respectively. The result confirmed the presence of Bi 3+ . In the case of the vanadium element (V 2p), the two peaks at 523.87 and 516.40 eV, as seen in Figure 4c, were assigned to V 2p1/2 and V 2p3/2, respectively. Moreover, in the case of the O 1s spectrum (Figure 4d), the deconvolution of the spectrum ends up with three XPS peaks at 532.94, 531.51, and 529.46 eV; this can be Antibiotics 2022, 11, 761 7 of 18 attributed to the O 1s from the adsorbed water on the surface, the hydroxyl group, and the lattice oxygen in the catalyst, respectively [3,34]. attributed to the O 1s from the adsorbed water on the surface, the hydroxyl group, and the lattice oxygen in the catalyst, respectively [3,34]. The textural properties of the BiVO4 catalyst were examined from a nitrogen (N2) adsorption-desorption isotherm. According to the IUPAC classification, the BiVO4 catalyst showed type IV isotherm properties. In addition, a distinct H3 hysteresis loop was detected at a high relative pressure (Figure 5a) [3,35]. The catalyst showed a mesoporous structure with a confirmed specific surface area of 0.42 m 2 /g. In addition, a Barett-Joyner-Halenda (BJH) pore volume of about 0.020 cm 3 /g and an average pore diameter of about 27.31 nm were observed. The textural properties of the BiVO 4 catalyst were examined from a nitrogen (N 2 ) adsorption-desorption isotherm. According to the IUPAC classification, the BiVO 4 catalyst showed type IV isotherm properties. In addition, a distinct H 3 hysteresis loop was detected at a high relative pressure (Figure 5a) [3,35]. The catalyst showed a mesoporous structure with a confirmed specific surface area of 0.42 m 2 /g. In addition, a Barett-Joyner-Halenda (BJH) pore volume of about 0.020 cm 3 /g and an average pore diameter of about 27.31 nm were observed.
An electrochemical method was used to study the spatial transfer and separation of electron-hole pairs. Firstly, linear sweep voltammetry (LSV) was investigated. As seen in 6a, the photocurrent density of BiVO 4 under photo illumination was greater than that obtained under the dark condition. We conclude that, upon the light irradiation of the prepared catalyst, the enhancement of its photogenerated carrier ability is produced. This results in an improvement in the photoactivity of the synthesized photocatalyst [3]. An electrochemical method was used to study the spatial transfer and separation of electron-hole pairs. Firstly, linear sweep voltammetry (LSV) was investigated. As seen in 6a, the photocurrent density of BiVO4 under photo illumination was greater than that obtained under the dark condition. We conclude that, upon the light irradiation of the prepared catalyst, the enhancement of its photogenerated carrier ability is produced. This results in an improvement in the photoactivity of the synthesized photocatalyst [3].
Secondly, the electron-hole separation rate of the catalyst was determined by using electrochemical impedance spectroscopy (EIS) after photo illumination (the visible light from a Panasonic day light lamp, 15W). In principle, the radius of the arc provides information regarding the nature of the charge transfer process at the electrode/electrolyte interface being studied. In general, an increase in radius indicates an enhancement in charge-transfer resistance [36][37][38]. As shown in Figure 6b, the arc radius of the BiVO4 catalyst in the dark was larger than that obtained after visible light irradiation, indicating that there were fewer electrons present across the electrolyte interface under the dark condition. On the contrary, a decrease in charge transfer resistance was found after light irradiation. It can be concluded that the PL spectrum, the LSV plots, and the EIS plots confirm the enhanced photoactivity of the synthesized BiVO4 catalyst. Secondly, the electron-hole separation rate of the catalyst was determined by using electrochemical impedance spectroscopy (EIS) after photo illumination (the visible light from a Panasonic day light lamp, 15W). In principle, the radius of the arc provides information regarding the nature of the charge transfer process at the electrode/electrolyte interface being studied. In general, an increase in radius indicates an enhancement in charge-transfer resistance [36][37][38]. As shown in Figure 6b, the arc radius of the BiVO 4 catalyst in the dark was larger than that obtained after visible light irradiation, indicating that there were fewer electrons present across the electrolyte interface under the dark condition. On the contrary, a decrease in charge transfer resistance was found after light irradiation. It can be concluded that the PL spectrum, the LSV plots, and the EIS plots confirm the enhanced photoactivity of the synthesized BiVO 4 catalyst.
To determine the mechanism of the improved photocatalytic performance of the BiVO 4 catalyst, the band structure of the prepared catalyst was determined by examining the Mott-Schottky plot (Figure 6c). In principle, the positive slope of the plot can be obtained from the n-type semiconducting photocatalyst and vice versa [3,39]. Determination of the flat band potential (V FB ) can be carried out by an extrapolation to 1/C 2 = 0 [40]. Additionally, a calculation of the potential versus Ag/AgCl (known as E AgCl ) to that versus NHE (called E NHE ) was carried out using E NHE = E AgCl + 0.210 V. The value of V FB of the prepared BiVO 4 catalyst was 0.080 eV. It is well accepted that the conduction band potential (V CB ) of a sample can be approximated from the V FB of the semiconductor [40]. Thus, the sample showed a V CB of about 0.080 eV. By utilizing a band energy of 2.33 eV, the sample showed a valence band potential (V VB ) of 2.41 eV. An electrochemical method was used to study the spatial transfer and separation of electron-hole pairs. Firstly, linear sweep voltammetry (LSV) was investigated. As seen in 6a, the photocurrent density of BiVO4 under photo illumination was greater than that obtained under the dark condition. We conclude that, upon the light irradiation of the prepared catalyst, the enhancement of its photogenerated carrier ability is produced. This results in an improvement in the photoactivity of the synthesized photocatalyst [3].
Secondly, the electron-hole separation rate of the catalyst was determined by using electrochemical impedance spectroscopy (EIS) after photo illumination (the visible light from a Panasonic day light lamp, 15W). In principle, the radius of the arc provides information regarding the nature of the charge transfer process at the electrode/electrolyte interface being studied. In general, an increase in radius indicates an enhancement in charge-transfer resistance [36][37][38]. As shown in Figure 6b, the arc radius of the BiVO4 catalyst in the dark was larger than that obtained after visible light irradiation, indicating that there were fewer electrons present across the electrolyte interface under the dark condition. On the contrary, a decrease in charge transfer resistance was found after light irradiation. It can be concluded that the PL spectrum, the LSV plots, and the EIS plots confirm the enhanced photoactivity of the synthesized BiVO4 catalyst. To determine the mechanism of the improved photocatalytic performance of the BiVO4 catalyst, the band structure of the prepared catalyst was determined by examining the Mott-Schottky plot (Figure 6c). In principle, the positive slope of the plot can be obtained from the n-type semiconducting photocatalyst and vice versa [3,39]. Determination of the flat band potential (VFB) can be carried out by an extrapolation to 1/C 2 = 0 [40]. Additionally, a calculation of the potential versus Ag/AgCl (known as EAgCl) to that versus NHE (called ENHE) was carried out using ENHE = EAgCl + 0.210 V. The value of VFB of the prepared BiVO4 catalyst was 0.080 eV. It is well accepted that the conduction band potential (VCB) of a sample can be approximated from the VFB of the semiconductor [40]. Thus, the sample showed a VCB of about 0.080 eV. By utilizing a band energy of 2.33 eV, the sample showed a valence band potential (VVB) of 2.41 eV.

Photocatalytic Degradation of the Antibiotics
The photocatalytic degradation of the OTC and TC antibiotics was studied under simulated visible light (a Panasonic daylight lamp, 15 W) and natural solar light irradiation.

Photodegradation of OTC Antibiotic
As seen in Figure 7a, a decrease of absorbance with time was observed, implying the removal of OTC under photo illumination. By monitoring the OTC concentration over time (Figure 7b), the photolysis of this antibiotic was observed to be negligible. Additionally,

Photocatalytic Degradation of the Antibiotics
The photocatalytic degradation of the OTC and TC antibiotics was studied under simulated visible light (a Panasonic daylight lamp, 15 W) and natural solar light irradiation.

Photodegradation of OTC Antibiotic
As seen in Figure 7a, a decrease of absorbance with time was observed, implying the removal of OTC under photo illumination. By monitoring the OTC concentration over time (Figure 7b), the photolysis of this antibiotic was observed to be negligible. Additionally, about 30% of OTC was removed by the adsorption of the antibiotic upon the surface of BiVO 4 . Interestingly, nearly complete degradation of OTC was observed under photo illumination. A degradation performance of about 62% and 93% was observed after illumination of artificial visible light and natural sunlight, respectively (Figure 7c). The photodegradation reaction follows a first-order reaction (Figure 7d). Corresponding rate constants (k) of 0.0063 and 0.0093 min −1 were detected after visible light and sunlight illumination, respectively. Photocatalytic performance under sunlight was higher than under simulated visible light, supporting the practical use of the synthesized BiVO 4 catalyst via the utilization of abundant natural sunlight. The removal of toxic antibiotics can be carried out easily and economically using solar energy. nation of artificial visible light and natural sunlight, respectively (Figure 7c). The photodegradation reaction follows a first-order reaction (Figure 7d). Corresponding rate constants (k) of 0.0063 and 0.0093 min −1 were detected after visible light and sunlight illumination, respectively. Photocatalytic performance under sunlight was higher than under simulated visible light, supporting the practical use of the synthesized BiVO4 catalyst via the utilization of abundant natural sunlight. The removal of toxic antibiotics can be carried out easily and economically using solar energy.  Figure 8a shows a lowering of TC absorbance with time. This indicates the removal of TC under photoirradiation. As seen in Figure 8b, the removal of TC via photolysis can be neglected. Moreover, lower than 10% of the removal of TC via adsorption processes was observed. The enhanced photocatalytic degradation of TC was observed in the presence of both the photocatalyst and light. Photoactivities of about 59% and 72% were obtained after artificial visible light and natural solar light irradiation, respectively (Figure 8c). The photodegradation reaction correlates well with a first-order reaction (Figure 8d) [11][12][13]. Accordingly, rate constants of 0.0045 and 0.0102 min −1 were observed.  Figure 8a shows a lowering of TC absorbance with time. This indicates the removal of TC under photoirradiation. As seen in Figure 8b, the removal of TC via photolysis can be neglected. Moreover, lower than 10% of the removal of TC via adsorption processes was observed. The enhanced photocatalytic degradation of TC was observed in the presence of both the photocatalyst and light. Photoactivities of about 59% and 72% were obtained after artificial visible light and natural solar light irradiation, respectively (Figure 8c). The photodegradation reaction correlates well with a first-order reaction (Figure 8d) [11][12][13]. Accordingly, rate constants of 0.0045 and 0.0102 min −1 were observed. Antibiotics 2022, 11, x FOR PEER REVIEW 11 of 19

Study of Photocatalytic Degradation Mechanism
The mechanism for the photocatalytic degradation of OTC antibiotics was determined from the trapping experiment [11][12][13]. The influence of some quenchers on antibiotic removal was elucidated. A dramatic decrease in photocatalytic performance was detected in the presence of K2Cr2O7 and EDTA-2Na (Figure 9a and b), implying the important role of both photogenerated electrons and holes in the degradation of the antibiotic.
In addition, the determination of hydroxyl radical (•OH) after photo illumination was performed by using a terephthalic acid (TA) probe method [12]. The detection of the fluorescent product 2-hydroxyterephthalic acid (TA-OH) was confirmed from an additional PL peak intensity at 425 nm over time (Figure 9c). The result indicates that the radicals also have a role in the degradation of the antibiotic.

Study of Photocatalytic Degradation Mechanism
The mechanism for the photocatalytic degradation of OTC antibiotics was determined from the trapping experiment [11][12][13]. The influence of some quenchers on antibiotic removal was elucidated. A dramatic decrease in photocatalytic performance was detected in the presence of K 2 Cr 2 O 7 and EDTA-2Na (Figure 9a,b), implying the important role of both photogenerated electrons and holes in the degradation of the antibiotic.
In addition, the determination of hydroxyl radical (•OH) after photo illumination was performed by using a terephthalic acid (TA) probe method [12]. The detection of the fluorescent product 2-hydroxyterephthalic acid (TA-OH) was confirmed from an additional PL peak intensity at 425 nm over time (Figure 9c). The result indicates that the radicals also have a role in the degradation of the antibiotic. Firstly, upon photo illumination, the generation of the photogenerated electrons and holes can be found within the conduction band (CB) and the valence band (VB), respectively. Secondly, the generation of the reactive species had occurred. The level of the CB and VB potentials of the prepared BiVO4 catalyst can be determined by using Mulliken electronegativity theory [41] as shown: Firstly, upon photo illumination, the generation of the photogenerated electrons and holes can be found within the conduction band (CB) and the valence band (VB), respectively. Secondly, the generation of the reactive species had occurred. The level of the CB and VB potentials of the prepared BiVO 4 catalyst can be determined by using Mulliken electronegativity theory [41] as shown: where E VB , E CB and E C , and χ are the VB, the CB, and the standard hydrogen electrode potential (≈4.5 eV), respectively. χ (about 2.34 eV) is the absolute value of the electronegativity of the BiVO 4 catalyst. The calculated V VB and V CB of the BiVO 4 catalyst were found to be 2.71 and 0.37 eV, respectively. Practically, however, the CB edge of 0.080 eV was detected from the prepared BiVO4 catalyst (Figure 6c). The band energy of the BiVO 4 catalyst is 2.33 eV. Thus, the VB edge was found to be 2.41 eV.
In summary, the mechanism regarding the degradation of the antibiotic can be expressed as: h + + OH − → •OH (9) •OH + antibiotic → products (10) h + + antibiotic → products (11) The details of the photocatalytic degradation mechanism of the toxic antibiotic by the prepared BiVO 4 catalyst are summarized in Figure 10.
Antibiotics 2022, 11, x FOR PEER REVIEW 13 o where EVB, ECB and EC, and χ are the VB, the CB, and the standard hydrogen electro potential (≈4.5 eV), respectively. χ (about 2.34 eV) is the absolute value of the electron ativity of the BiVO4 catalyst. The calculated VVB and VCB of the BiVO4 catalyst were fou to be 2.71 and 0.37 eV, respectively. Practically, however, the CB edge of 0.080 eV w detected from the prepared BiVO4 catalyst (Figure 6c). The band energy of the BiVO4 c alyst is 2.33 eV. Thus, the VB edge was found to be 2.41 eV.
In summary, the mechanism regarding the degradation of the antibiotic can be pressed as: The details of the photocatalytic degradation mechanism of the toxic antibiotic by t prepared BiVO4 catalyst are summarized in Figure 10.

Cycling Ability of the Catalyst
Recycling ability is an important factor affecting the practical application of the p pared photocatalyst [3,[11][12][13][14]. Therefore, the reuse of the BiVO4 catalyst after the remo of OTC and TC was studied. The synthesized catalyst still showed a promising perf mance, even after the fifth run ( Figure 11). The chemical structure of the catalyst af antibiotic degradation was investigated. The stability of the structure was confirmed (F ure 12a). Identical FT-IR ( Figure 12b) and PL spectra (Figure 12c) also evidence the stru tural stability of the prepared photocatalyst. Furthermore, the morphological structure

Cycling Ability of the Catalyst
Recycling ability is an important factor affecting the practical application of the prepared photocatalyst [3,[11][12][13][14]. Therefore, the reuse of the BiVO 4 catalyst after the removal of OTC and TC was studied. The synthesized catalyst still showed a promising performance, even after the fifth run ( Figure 11). The chemical structure of the catalyst after antibiotic degradation was investigated. The stability of the structure was confirmed (Figure 12a). Identical FT-IR ( Figure 12b) and PL spectra (Figure 12c) also evidence the structural stability of the prepared photocatalyst. Furthermore, the morphological structure of the used catalyst is the same as the unused catalyst (Figure 12d) indicating the morphological stability of the sample. The photocatalytic efficiency of photocatalysts toward the removal of TC and OTC antibiotics has been previously studied [2][3][4][6][7][8][9][42][43][44][45][46][47][48][49][50][51][52]. In this work, the BiVO4 catalyst  the used catalyst is the same as the unused catalyst (Figure 12d) indicating the morphological stability of the sample.
(a) (b) Figure 11. Reusability of the BiVO4 catalyst toward degradation of OTC (a) and TC (b).

Conclusions
A BiVO 4 photocatalyst was fabricated via a hydrothermal method. The monoclinic photocatalyst showed a band energy of 2.34 eV. Under sunlight conditions, a performance of 72% and 93% degradation of TC and OTC, respectively, was achieved. The photodegradation of the antibiotic followed a first-order reaction. Photogenerated electrons and holes are two major active species concerning the removal of antibiotics. The synthesized BiVO 4 catalyst still showed a high performance after five times of use, indicating its excellent cycling ability. The presented findings offer a promising route for the creation of a BiVO 4 catalyst for environmental protection. For future work, the preparation of a floating, or polymer-film, photocatalyst for the degradation of pollutants in wastewater would be worth investigating. This would provide real-scale application of the catalyst in terms of practical work.