Photocatalytic Degradation of Tetracycline Hydrochloride by Mn/g-C₃N₄/BiPO₄ and Ti/g-C₃N₄/BiPO₄ Composites: Reactivity and Mechanism

Abstract

The environmental pollution caused by antibiotics is becoming more serious. In this study, the Mn/BiPO₄/g-C₃N₄ composite (Mn-BPC) and the Ti/g-C₃N₄/BiPO₄ composite (Ti-BPC) were prepared by hydrothermal reaction method and solvent method, respectively, and applied to the degradation of tetracycline (TC) in an aqueous environment. The XRD and HRTEM results showed that these materials had the crystalline rod-like structure of BiPO₄ and abundant carbon, nitrogen and carbon–oxygen surface functional groups. The degradation of TC by Ti-BPC and Mn-BPC were nearly 92% and 79%, respectively. The degradation processes of TC were well consistent with the pseudo-second-order kinetics model and R² values were closer to 1. The trapping experiment showed that electron holes (h⁺) were the main reactive species for the degradation of tetracycline, OH· and O₂⁻ also have certain effects. Also, the possible photocatalytic degradation mechanism of Ti-BPC and Mn-BPC composites was thereby proposed. TC was firstly adsorbed on the surface of catalysts, and subsequently degraded by reactive species such as h⁺, OH· and O₂⁻ generated under visible light excitation. This study shows that the Ti-BPC and Mn-BPC photocatalysts have great potential in antibiotic degradation and can provide new ideas for antibiotic removal in aqueous environments.

1. Introduction

Tetracycline (TC) compounds have been increasingly promoted due to their broad spectrum, low cost and easy availability, accounting for approximately 26% of all antibiotic use. The first-generation TCs (tetracycline, oxytetracycline, and chlortetracycline) are the most used TCs [1]. China consumes approximately 6950 tons of TCs every year, and the total TC consumption in Europe has reached 2957 tons [2]. In recent years, TCs have been widely found in the environment, with the highest levels of TC pollution in water environments [3]. In the Pearl River basin of China, the concentration of antibiotics is as high as 3384 ng/L. The concentration of antibiotics in the medical wastewater discharged by some enterprises exceeds standards. The highest concentration of medical wastewater has been tested, and the concentration of tetracycline antibiotics is 53.688 μg/L [4,5]. At present, the techniques for removal of TCs from wastewater mainly include physical treatment, biological treatment, and chemical oxidation. However, traditional biological treatment methods have a low degradation rate and high cost; physical treatment may cause secondary pollution; and chemical oxidation can reduce the difficulty of subsequent treatment of pollutants by deactivating them. In contrast, the photocatalytic technique achieves thorough treatment through a simple, nontoxic, and environmentally friendly process; thus, this approach has attracted the attention of researchers [6,7,8,9].

In recent years, BiPO₄ has been used to degrade organic pollutants due to its high electron–hole separation rate, high chemical stability, simple preparation method, and photocatalytic ability under solar irradiation [10]. For example, the composite material g-C₃N₄/BiPO₄ can be obtained by hydrothermal reaction of BiPO₄ and graphitic carbon nitride (g-C₃N₄). BiPO₄ has a wide band gap and a high electron–hole rate, while g-C₃N₄ can improve the stability of photogenerated electrons and holes in BiPO₄, thus improving the antibiotic degradation efficiency of BiPO₄ without causing secondary pollution [11]. The degradation rate of antibiotics by a g-C₃N₄/BiPO₄ composite photocatalyst was 1.5 times higher than that by pure BiPO₄ under UV irradiation and 2.5 times than that by g-C₃N₄ under visible light irradiation [12,13]. However, g-C₃N₄/BiPO₄ materials still cannot absorb visible light with wavelengths exceeding 420 nm, and effective utilization methods need to be sought to maximize the excitation efficiency of g-C₃N₄/BiPO₄ materials [14,15]. Some researchers have prepared different g-C₃N₄/BiPO₄ based composite photocatalysts, such as loaded CDs. And the optical properties of the composite material, such as diffusion reflection UV characterization and Tauc plot analysis, were used to determine whether the composite catalyst successfully expanded the absorption range of the target pollutant. The results showed that the addition of CD reduced the energy band gap and led to a wider light absorption range. Qian et al.’s study showed that the degradation rate of tetracycline in water by loaded CDs composite catalysts is 75.5% [16]. Related studies have shown that metal ions can modify g-C₃N₄/BiPO₄ through complexation, effectively improving the catalytic activity and efficiency of g-C₃N₄/BiPO₄ in degrading antibiotics [17]. Darkwah [18] used TiO₂ and g-C₃N₄/BiPO₄ to prepare a ternary composite catalyst, and Li [19] used AgBr and g-C₃N₄/BiPO₄ to prepare ternary composite catalysts; these ternary composite catalysts are more effective than g-C₃N₄/BiPO₄ in degrading antibiotics.

To search for loading materials that can more efficiently stimulate the degradation effect of g-C₃N₄/BiPO₄ materials. We prepared two different composite photocatalysts based on the precursor g-C₃N₄/BiPO₄ and supported metal ions, namely, TiO₂/g-C₃N₄/BiPO₄ (hereinafter referred to as Ti-BPC) and Mn/g-C₃N₄/BiPO₄ (hereinafter referred to as Mn-BPC). This article compares the improvement effect of two material loads on the g-C₃N₄/BiPO₄ material. The physical and chemical characteristics of the two composite catalysts were analyzed to explore the effects of different influencing factors on the ability of the composite photocatalysts to degrade antibiotics, and the underlying mechanisms were analyzed based on radical trapping experiments; accordingly, the findings of this study provide a reference for the treatment of antibiotics in wastewater.

2. Results

2.1. Effect of the Mass Fractions of TiO₂ and MnCl₂ on the Properties of Composites

Based on the data collected from the experimental results, the degradation efficiency map was drawn using Origin software, and the time when the xenon lamp was turned on was used as the zero point in time (i.e., the start of the photoreaction).

The composite photocatalysts were prepared by adding different mass fractions of TiO₂ and MnCl₂, and their measured properties are shown in Figure 1. The figure shows that in the adsorption reaction stage, both Ti-BPC and Mn-BPC had limited adsorption efficiency, and their adsorption removal rates were both in the range of 8% to 20%. In the photocatalytic reaction stage (The determination of adsorption time can be found in Supplementary Figure S1), Ti-BPC samples prepared with different TiO₂ mass fractions all had a satisfactory degradation effect on TC. The best photocatalytic effect occurred at TiO₂ mass fractions of 12.5 wt% and 10 wt%, at which the degradation rate of TC reached 79.3% in 180 min. Considering the preparation cost, a TiO₂ mass fraction of 10 wt% was used in the subsequent preparation of Ti-BPC. When the mass fraction of MnCl₂ increased from 5 wt% to 25 wt%, the degradation rate of TC decreased. When the mass fraction of Mn was 30 wt%, the degradation rate of TC reached 84%; when the mass fraction of Mn was 5 wt%, the degradation rate was 85%. Based on comprehensive consideration, a MnCl₂ mass fraction of 5 wt% was used to prepare Mn-BPC.

2.2. Analysis of the Physicochemical Properties of the Photocatalytic Materials

The morphology and crystal structure of the two composites were determined by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), and the results are shown in Figure 2. Figure 2a shows that the constituent molecules of the composite photocatalyst Ti-BPC formed a compact cluster structure with tightly packed rod-shaped and spherical crystals [20]. The BiPO₄ nanorods shown in Figure 2b are approximately 60–75 nm in diameter with tiny spherical crystals around the edges. Figure 2c shows tiny spherical crystal structures uniformly attached to the crystal rods, which are presumed to be TiO₂ crystals [21]. The HRTEM image in Figure 2d shows that the crystal spacing is 0.2669 nm, corresponding to the (210) plane of elemental Bi. Figure 2e,f shows that the composite photocatalyst Mn-BPC is composed of agglomerates of flower-shaped or granular particles and overlapping layered nanosheet structures. The morphological characteristics of Mn-BPC are the same as those reported by Chen, and the layered graphite structure is g-C₃N₄. The lattice spacings measured in Figure 2g,h are 0.283 nm and 0.312 nm, corresponding to the (111) plane of BiPO₄ and the (012) plane of elemental Bi [22].

Figure 3 shows the X-ray photoelectron spectroscopy (XPS) spectra of the two catalysts. The XPS spectrum of Ti-BPC shows that the composite catalyst contains C, N, O, P, Bi, and Ti with different molecular weights. For the Bi spectrum, Bi4f has the highest intensity, the smallest peak width (158–162 eV) and the best symmetry, and it is the main spectral line of Bi. The Mn-BPC spectral analysis results show that the peak at 133.08 eV is the P2p peak, the peak at 160.60 eV is the Bi4f peak, with the highest peak value, and the peak at 531.08 eV is the characteristic peak of O1s, with the second highest peak value. The peaks of C1s, N1s, and Mn2p appear at 285.05 eV, 339.08 eV, and 642.08 eV, respectively.

From the information in

Table 1. XPS peak information of Ti-BPC and Mn-BPC catalysts.

	TiO2-BPC	Mn-BPC
Name	Atomic %	Atomic %
C1s	23.89	32.41
N1s	5.44	12.74
O1s	43.12	36.31
P2p	7.17	8.96
Bi4f	4.25	6.69
Ti2p	16.12	/
Mn2p	/	2.89

, it can be seen that the molar ratio of each element of Ti-BPC is 23.89% for C1s, 5.44% for N1s, 43.12% for O1s, 7.17% for P2p, 4.25% for Bi4f, and 16.12% for Ti2p. The content of oxygen is the highest in the composite catalyst, which is consistent with the chemical formula of the catalyst [23]. The element with the lowest molar ratio is bismuth, with a content of only 4.25%, which is lower than the content of both carbon and titanium. The XPS peak information of the Mn-BPC catalyst shows that the molar ratio of each element is 32.41% for C1s, 12.74% for N1s, 36.31% for O1s, 2.89% for P2p, 8.96% for Bi4f, and 6.69% for Ti2p. The results showed that the content of oxygen was the highest, followed by carbon and nitrogen, and the content of manganese was the lowest. For the catalyst material, the manganese content is relatively low.

Figure 4 shows the XPS peak position, peak height, and peak width of each element in Ti-BPC. The peak areas of Bi and O are the largest; there are obvious peaks at the binding energies corresponding to P=O and P-O; obvious main peaks corresponding to P, Bi, and Ti binding to O are visible; and the main peaks of C and N correspond to the binding energies of C-C and C=N, respectively. The larger the peak area is, the higher the crystal phase content; the narrower the peak is, the larger the crystal grains. The XPS analysis of Mn in Figure 4g shows that the signal peaks at 641.28 eV and 653.58 eV correspond to Mn³⁺ and Mn²⁺, respectively, indicating the presence of both trivalent and divalent manganese ions in the catalyst. Figure 4h analyzes the P element. In the Mn BPC system, the P element has a characteristic peak at 132.88 eV, which is close to the binding energy of P2p. Figure 4j shows the analysis of Bi, with two peaks at 159.5 eV and 164.9 eV, corresponding to the binding energies of Bi4f_5/2 and Bi4f_7/2, indicating that the Bi element of BiPO₄ exists in the +3 valence state in the composite photocatalyst. Figure 4k shows the analytical spectrum of C, where the two peaks of 284.6 eV and 288.4 eV correspond to N-C=N and C=C bonds. The analysis spectra of O and N correspond to Figure 4i,l, respectively. The peak photoelectron analysis spectra of O are at 531.3 eV, indicating the presence of Bi-O bonds in the catalyst. The analysis spectra of N correspond to C=N-C, N-(C)₃, C-N-H at 398.4 eV, 399.5 eV, and 400.1 eV, respectively. The increase in peak area of C=N-C bonds indicates the chemical interaction between g-C₃N₄, BiPO₄, and manganese ions [24], This leads to an increase in the strength of carbon nitrogen conjugated π bonds, as shown in Figure 4.

The XRD pattern of Ti-BPC in Figure 5 shows diffraction peaks at 2θ of 25.281°, 34.946°, 37.800°, 38.575°, and 48.049°, which are the characteristic peaks of TiO₂, corresponding to the (101), (103), (004), (112), and (200) planes of the JCPDS standard card PDF#21-1281, respectively. The diffraction peaks at 2θ of 29.033°, 31.316°, 37.850°, 41.866° and 52.001° are characteristic peaks of BiPO₄, corresponding to the (111), (102), (112), (211), and (203) planes of the JCPDS standard card PDF#15-0766, respectively [25,26]. The diffraction peaks at 2θ of 42.738° and 49.268° are characteristic peaks of g-C₃N₄, corresponding to the (111) and (102) planes of the JCPDS standard card PDF#0848, respectively. Mn-BPC has diffraction peaks at 2θ of 19.00°, 36.78°, 38.64°, and 44.96°, which are the characteristic peaks of MnO₂, corresponding to the (111), (311), (222), and (400) planes of MnO₂ in the JCPDS standard card PDF#44-0992 [27], respectively. Since BiPO₄ was prepared in this study by the hydrothermal method, which is the same hydrothermal method used by Jin [28] to prepare BiPO₄ (BiPO₄ was monoclinic in both studies), the seven diffraction peaks at 2θ of 21.72°. The characteristic peaks of BiPO₄ are 29.08°, 31.18°, 34.44°, 46.28° and 52.84°, which correspond to the (−111), (120), (012), (−202), (212), and (132) planes of the JCPDS standard card PDF#80-0209. The diffraction peaks of BiPO₄ exhibit an increased background and width, which is mainly caused by a reduction in particle size. A weak peak and a strong peak appear at 2θ of 18.58° and 27.12°, respectively. By comparison with the XRD results of Azam and Xu, it is concluded that the two peaks correspond to the (100) and (002) planes, respectively, where the (002) plane corresponds to the interlayer stacking of conjugated aromatics. The other characteristic peaks of g-C₃N₄ correspond to the (220) and (310) planes.

Figure 6 shows that the relatively broad peak of Ti-BPC between 3200 cm⁻¹ and 3000 cm⁻¹ is caused by C-H bending; there are several sharp peaks near 1600 cm⁻¹ and 1240 cm⁻¹, all of which are caused by the stretching vibration of C=N, corresponding to C-N and C=N bonds activated by infrared light [29]; the absorption peak at 1400 cm⁻¹ is caused by the bending vibration of C-O bonds; the peaks between 1100 cm⁻¹ and 1000 cm⁻¹ correspond to the strong stretching of O-P-O; the peak near 800 cm⁻¹ corresponds to the bending of O-P-O; and the peaks between 500 cm⁻¹ and 750 cm⁻¹ [30] correspond to the stretching and bending vibration of Ti-O bonds. These results confirm the XRD analysis results. The peaks of Mn-BPC in the range of 529 to 553 cm⁻¹ represent the bending vibration of BiPO₄, while the peak near 553 cm⁻¹ corresponds to O-P-O [31]. There is a weak peak near 1250 cm⁻¹, which corresponds to the stretching vibration of C-N, and there is a sharp peak near 1100 cm⁻¹ formed by the C-O stretching vibration, with a steep slope of approximately 6.9%. The absorption peak at 3100 cm⁻¹ corresponds to the N-H stretching vibration. The peak near 1500 cm⁻¹ is the characteristic absorption peak of Mn ions, and the characteristic peak of C=N stretching vibrations is attributed to the asymmetric stretching vibration of polypyrrole rings [32]. The peak near 1700 cm⁻¹ has a small width and a low peak height, so it is attributed to the stretching vibration of C=O in carboxyl groups on the surface of Mn-BPC. Mn-BPC has similar peaks to BiPO₄ and g-C₃N₄, which indicates that the catalyst was successfully prepared.

2.3. Degradation of Tetracycline Performance Analysis

The experimental data in Figure 7a,b show that Ti-BPC had the best TC degradation performance when the pH was weakly acidic (pH = 5), with the degradation rate reaching 58.71% in 120 min. The photocatalytic TC degradation efficiency of Mn-BPC gradually decreased with increasing pH. At pH = 3, the TC degradation rate reached 76.47%. In general, acidic conditions improve the TC degradation efficiency of composite catalysts. The photocatalytic degradation activity of composite catalysts is closely related to their surface charge. A change in pH changes the properties of the photocatalyst surface and solvent molecules, as well as the electrostatic interaction between the substance to be degraded and OH, thus affecting the photodegradation rate [33].

According to the data in Figure 7c,d, as the dosage of composite catalyst increases, the degradation rate also increases. When the dosages of Ti-BPC and Mn-BPC were 1 g/L, the corresponding degradation rates of TC were 91.54% and 79%, respectively. The main reason is that the greater the catalyst dosage is, the more active sites are available for contact between the catalyst and TC, and the better the catalytic degradation performance is. Therefore, the larger the amount of catalyst added, the higher the degradation rate, which is similar to the conclusions of previous research [34,35].

Figure 7e,f shows that the photocatalytic degradation efficiency of Ti-BPC and Mn-BPC decreased with increasing TC concentration. One reason for this is that when the dosage of the photocatalyst is fixed, the number of active sites in the system is constant, so the composite photocatalyst has a limited ability to degrade TC in high-concentration TC solution. Another reason is that as the photocatalytic reaction progresses, many degradation intermediates generated in high-concentration TC solution will compete with TC molecules for active photocatalytic sites on the catalyst surface, hindering the formation of photocatalytically active species [36].

In summary, the effects of various influencing factors on the two composite catalysts are basically the same. Under the optimal conditions, the best TC degradation efficiencies of Ti-BPC and Mn-BPC were 91.54% and 79%, respectively. From this, it can be concluded that Ti-BPC may have better degradation effects.

3. Discussion

3.1. Photocatalytic Kinetic Analysis

Figure 8 and

Table 2. Kinetic fitting results of Ti-BPC photocatalytic degradation experiment.

Initial Concentration of Tetracycline Hydrochloride/(mg/L)	Pseudo-First Order Kinetic Equation	k	R2	Pseudo-Second Order Kinetic Equation	k	R2
4	y = 0.226x + 1.478	0.00463	0.98554	y = 0.00991x − 0.15833	0.00991	0.99878
6	y = 0.00397x + 0.084	0.00397	0.9925	y = 0.00407x − 0.10778	0.00407	0.97465
8	y = 0.0027x + 0.102	0.0027	0.98722	y = 0.00172x − 0.01973	0.00172	0.98407
10	y = 0.00217x + 0.064	0.00217	0.95762	y = 0.00102x − 0.01235	0.00102	0.97955
12	y = 0.00187 + 0.048	0.00187	0.98616	y = 0.000657x − 0.00995	0.000657	0.98529

and

Table 3. Kinetic fitting results of Mn-BPC photocatalytic degradation experiment.

Initial Concentration of Tetracycline Hydrochloride/(mg/L)	Pseudo-First Order Kinetic Equation	k	R2	Pseudo-Second Order Kinetic Equation	k	R2
3	y = 0.0080x + 0.6955	0.0080	0.9237	y = 0.0156x − 0.1503	0.0156	0.8248
4	y = 0.0087x + 0.5869	0.0087	0.9814	y = 0.0096x − 0.0591	0.0096	0.9779
5	y = 0.0070x + 0.4042	0.0070	0.9626	y = 0.0048x + 0.0137	0.0048	0.9904
7	y = 0.0041x + 0.4435	0.0041	0.9349	y = 0.0014x + 0.0636	0.0014	0.9616
9	y = 0.0032x + 0.2984	0.0032	0.9260	y = 0.0006x + 0.0335	0.0006	0.9549

show that the degradation of TC by Ti-BPC can be well simulated by first- and second-order kinetics. The minimum R² values of the pseudo-first-order and pseudo-second-order models were 0.9576 and 0.97465, respectively. The half-life of TC increased with increasing concentration. The half-life values of TC at 4 mg/L and 12 mg/L were 25.23 min and 126.84 min, respectively. The strong photocatalysis effect of Mn-BPC was better simulated by pseudo-second-order kinetics. At low concentrations, the half-life of TC degradation under photocatalysis by Mn-BPC was shorter. When the initial TC concentration was 12 mg/L, the half-life of TC was 138.89 mg/L. Overall, the two catalysts induced higher reaction rates and better TC removal effects at lower initial TC concentrations.

3.2. Analysis of the Capture Experiment of the Active Object Capture Agent

Figure 9a shows that the TC degradation efficiency of Ti-BPC in the control group reached 60% in 2 h. After different radical trapping agents were added, the catalytic effect in each test tube changed to different degrees, and the degradation rate of TC was only 12% 2 h after adding EDTA to trap holes (h⁺). The results show that the photocatalytic degradation effect of the catalyst was significantly reduced after the addition of EDTA, whereas the addition of tert-butanol or BQ as the radical trapping agent only slightly altered the photocatalytic degradation effect. Figure 9b shows that the photocatalytic TC degradation efficiency of Mn-BPC in the control group was 82.11%, while the addition of tert-butanol or BQ as a radical trapping agent had a certain effect on the degradation efficiency; these results demonstrate that hydroxyl radicals (OH·) and electrons (e⁻) have a certain effect on the degradation efficiency but are not the main effective substances. When EDTA-2Na was added as a hole (h⁺) trapping agent, the degradation effect was greatly reduced, with a degradation rate of only 20.33%. The results indicate that holes and hydroxyl radicals are the main active species for the degradation of TC by the two catalysts and that electrons also have certain effects on the degradation efficiency.

3.3. Analysis of Adsorption Mechanism

Figure 10 shows the possible photocatalytic degradation mechanism of TiO₂-BPC and Mn-BPC [37]. The semiconductor BiPO₄ is combined with g-C₃N₄ to form a heterogeneous structure with Ti and Mn attached to the surface and interface. The semiconductor material BiPO₄ can directly absorb photons with wavelengths less than 320 nm in sunlight, and g-C₃N₄ can directly absorb photons with wavelengths less than 420 nm [38]. These two materials can absorb visible-light photons with a wavelength greater than 500 nm through the Ti and Mn particles attached to their surfaces to improve their effectiveness. When the semiconductors BiPO₄ and g-C₃N₄ are in contact with each other, g-C₃N₄ tends to lose e⁻ and thus carries a net positive charge, and the semiconducting BiPO₄ tends to accept e⁻ and thus carries a net negative charge. Charge transfer continues until the Fermi energy levels are balanced and an internal electric field is formed at the interface, which accelerates the migration and separation of photogenerated e⁻-h⁺ [39]. After the two materials absorb solar energy, the electrons in the valence band (VB) are excited into the conduction band (CB), thereby generating holes in the VB. Holes can directly degrade TC into the intermediate products CO₂ and H₂O and can also react with OH- or H₂O to generate -OH. Hydroxyl radicals can directly act on TC degradation. The hopping electrons combine with O² to produce O²⁻. O²⁻ and superoxide radicals can catalyze TC degradation.

4. Materials and Methods

4.1. Materials

The TC used in this experiment was purchased from Hefei Ge’en Technology Co., Ltd. (Hefei, China) and stored at 4 °C; melamine was purchased from the Wuxi Yuanwang Chemical Reagent Co., Ltd. (Wuxi, China); Tert-Butyl alcohol, disodium Ethylenediaminetetraacetic acid, anhydrous ethanol (99.7% by mass), and other reagents were purchased from the Aladdin Reagent Company; all chemical substances used in the experiment are analytical pure.

4.2. Preparation of Composite Photocatalyst

The g-C₃N₄ was prepared by calcining 20 g of melamine with high temperature in a muffle furnace at 550 °C; (1) Weighing a fixed ratio of TiO₂, g-C₃N₄, Bi(NO₃)₃-5H₂O, NaH₂PO₄-2H₂O, where the ratio of g-C₃N₄ and BiPO₄ masses was a constant 1:5, changing only the mass fraction ratio of TiO₂ in the catalyst [40]. Deionized water was added for ultrasonic stirring, followed by washing the solution in small amounts into the Teflon bottle (the solution in the bottle did not exceed 80% of the total capacity of the Teflon bottle [41]). Subsequently it was placed in a hydrothermal reactor for 8 h at 160°; after extraction, drying, and grinding to obtain 5 wt%, 7.5 wt%, 10 wt%, 12.5 wt%, 15 wt%, and 17.5 wt% Ti-BPC. (2) Weigh an appropriate amount of g-C₃N₄ and add an appropriate amount of Bi(NO₃)₃-5H₂O with NaH₂PO₄-2H₂O with ultrasonic stirring to form suspension; subsequently placed in a hydrothermal reactor at 160° for 6 h [42]; the precursor complex g-C₃N₄/BiPO₄ was obtained as a white solid, an appropriate amount of MnCl₂ dissolved in deionized water was added, the mixed solution was poured into the autoclave, sealed and compacted in an oven at 160° for 8 h; 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt% of Mn-BPC, respectively.

4.3. Analysis and Testing Methods

To perform a more detailed analysis of the semiconductor composites Mn-BPC and Ti-BPC, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD; model: D8 Advance, Bruker, Germany), and high-resolution transmission electron microscopy (X-ray photoelectron spectroscopy) were used. The material was characterized by HRTEM and Fourier transform infrared spectroscopy (FTIR; model: Nicolet iS10, Thermo Fisher, USA), and the physical and chemical properties of the material were obtained from the data. XRD was used to characterize the crystal form and grain size of the composite photocatalyst, and the scanning range was 2θ = 10–90°. FTIR was used to explore the functional groups of the composite photocatalyst, and the measurement range was 4000–400 cm⁻¹. The morphological characteristics, particle size, and lattice width of the material could be observed and compared with the standard crystal plane spacing d of the crystal, and the crystal plane to which the material belongs could be determined.

4.4. Degradation Experiment

4.4.1. Tube Experiment

The experiment explored the effect of four different influencing factors, namely preparation ratio, catalyst dosage, initial concentration of TC, and ph value of solution, on the degradation effect. One of the conditions was changed, respectively, and the other three influencing factors were maintained unchanged. Except for the TC initial concentration as an influencing factor on the experiment, all other three groups were set with an initial concentration of 5 mg/L for tetracycline wastewater. The experiment was carried out in a 10 mL quartz tube, and the photocatalytic reactor was opened to start agitation. The first 1 h was used to remove the adsorption. After 1 h, the absorbance of the solution in the test tube was measured by sampling, and the data were recorded. The photocatalytic reaction lasted for a total of 90 min. The Xenon lamp was turned on and adjusted to 1000 w (the maximum setting), stirring and rotating were started. Samples were taken every 20 min within the first 1 h of the reaction, and then every 30 min, the absorbance was measured and the data were recorded. The removal rate of tetracycline in wastewater (Formula (1)) was used to represent the effect, and the adsorption rates of the composite photocatalyst were calculated, respectively.

The removal efficiency of TC is expressed by the following formula:

$$\eta =\frac{C_t}{C_0}\times 100\%$$

(1)

In the formula: $C_0$—the concentration of TC at the beginning of catalysis, mg/L; $C_t$—concentration of TC-HCl at time t in the catalytic process, mg/L.

4.4.2. Active Substance Capture Experiment

By adding different trapping agents in the degradation process, the different degradation performance is obtained, and then the degradation rate determines the active substance. In this experiment, tert-butanol, EDTA-2Na and BQ. were added during the degradation process as trapping agents of ·OH, Vacancy (h⁺) and e⁻, respectively.

4.5. Study on Photocatalytic Kinetics

In order to further explore whether the photocatalytic degradation of TC solution by composite materials conforms to the kinetic reaction order, this experiment takes a TC solution with different initial concentrations as the condition to measure its degradation effect. According to the first order (Formula (2)) [43] and the second-order-reaction kinetic equation (Formula (3)) [44], the simulation study was carried out:

$$ln\frac{C_0}{C_t}=k_{1}t$$

(2)

$$\frac{1}{C_t}-\frac{1}{C_0}=k_{2}t$$

(3)

The half-life (t_1/2) of the quasi-primary kinetic and quasi-secondary kinetic models is calculated by the equation (Equations (4) and (5))

$$t_{1/2}=ln(2)\frac{1}{K_1}$$

(4)

$$t_{1/2}=\frac{1}{{C_{0}K}_2}$$

(5)

In the formula: C₀—the concentration of TC-HCl at the beginning of catalysis, mg/L; C_t—concentration of TC-HCl at time t in the catalytic process, mg/L. k₁, k₂-kinetic model rate constants; t-Reaction time.

5. Conclusions

In this study, the Mn/BiPO₄/g-C₃N₄ composite (Mn-BPC) and the Ti/g-C₃N₄/BiPO₄ composite (Ti- BPC) photocatalysts were successfully prepared and applied to the degradation of TC in water. The experimental results showed that the reactivity of Ti-BPC and Mn-BPC composite were higher than that of the original binary g-C₃N₄/BiPO₄. Meanwhile, the main factors affecting the photocatalytic process and degradation efficiency of TC were investigated, and the degradation efficiencies of Ti-BPC and Mn-BPC for TC were about 92% and 79% under the optimal conditions, respectively. The data fitting of reaction kinetics showed that TC degradation was well consistent with pseudo-second-order kinetics model. Several reactive species such as h⁺, OH· and O₂⁻ were generated in this photocatalytic system, and the possible reaction mechanism of TC degradation was proposed. In the experiment for exploring catalytic performance, the optimal conditions for the degradation of antibiotics by the catalyst were obtained. The materials used in the preparation process are common and will not cause secondary pollution to the water body during the application process. In subsequent applications, powdered biochar will be prepared into spherical carbon for easy recovery and utilization during the application process. However, in practical applications, the types of wastewater are complex and diverse, and the degradation mechanisms of different modified photocatalysts are also different. Currently, it is difficult to apply them on a large scale. Subsequent research can be conducted in different wastewater to develop highly selective catalysts, and to seek more energy-saving and low-cost stable preparation processes.