Silver-Based Composite Photocatalytic Materials and Their Visible-Light-Driven Degradation of Tetracycline Hydrochloride in Aqueous Solutions

Abstract

This study fabricated a silver-based composite photocatalytic material (Ag/AgVO₃/g-C₃N₄) for antibiotic degradation driven by visible light. The composite was prepared using the water bath method, and its morphology, structure and optical properties were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and Ultraviolet-Visible (UV-vis) diffuse reflection spectroscopy. The photocatalytic activity of the composite was evaluated within 240 min under visible-light irradiation, and its degradation effect was quantified using tetracycline hydrochloride as the target pollutant, applying different conditions of composite proportions and dosage, initial concentration of the contaminant, and pH. The experimental results demonstrated that the composite doped with Ag⁺ had a higher degradation effect on tetracycline hydrochloride. The highest degradation rate of tetracycline hydrochloride (reaching 86.64%) was achieved at a composite ratio of Ag/AgVO₃/5g-C₃N₄, dosage of 0.8 g·L⁻¹, and initial concentration of tetracycline hydrochloride of 10mg·L⁻¹. This rate is approximately 1.4 times and 1.7 times those of the pure-phase g-C₃N₄ and AgVO₃, and the degradation rate remains stable at 81.52% after 4 cycles of experiments. The favorable stability of the photocatalyst was also verified through a stability test.

1. Introduction

Since the onset of the 21st century, antibiotics have been extensively utilized in healthcare, animal husbandry, aquaculture, and other industries [1,2]. Notably, tetracycline antibiotics represent one of the most widely used classes of antibiotics, in terms of both application scope and quantity. Driven by their high efficiency and cost-effectiveness, they are primarily utilized for disease prevention and treatment as well as promoting organism growth. They play a crucial role in the prevention and treatment of animal diseases, enhancement of production efficiency, and improvement of livestock and poultry product quality [2,3]. However, due to their overuse and incomplete metabolism, tetracycline residues have been frequently detected in wastewater, surface water, sediments and agricultural soils, raising concerns regarding ecological toxicity, antibiotic-resistance selection and human health risks [4,5].

Currently, methods for removing antibiotics from the environment encompass biological treatment, chemical precipitation, adsorption, membrane separation technology, advanced oxidation processes and other approaches [6,7,8]. Conventional wastewater treatment processes, such as biological treatment and chemical precipitation, can realize the migration or partial transformation of pollutants between different phases but cannot ensure complete removal of antibiotics from water. Adsorption and membrane separation technologies impose high requirements on materials and may generate concentrated waste streams that require further treatment [6,7]. Photocatalytic technology, as an emerging method for degrading antibiotics in recent years, has garnered significant attention owing to its low energy consumption, high efficiency, environmental friendliness, and lack of secondary pollution when properly designed [8,9,10].

In photocatalytic technology, photocatalytic materials are employed to drive chemical reactions under light irradiation. Photocatalytic technology can be applied to the treatment of contaminated water, air and soil, water-based hydrogen production, sterilization, and other relevant areas. Regarding pollutant control, this technology can be adopted to decompose pollutants into less toxic or more biodegradable substances [9,10,11]. Essentially, photocatalytic reaction lies in the fact that, under irradiation of a semiconductor photocatalyst by a solar light source, when the energy of the incident light is greater than or equal to the band gap width, electrons in the valence band of the semiconductor are excited and promoted to the conduction band, leaving corresponding holes in the valence band. As a result, electron (e⁻) and hole (h⁺) pairs form within the semiconductor [10,11]. Therefore, photocatalytic efficiency can be effectively improved only by inhibiting the recombination of photogenerated electrons and photogenerated holes.

Silver-based semiconductors show great potential in the field of photocatalysis because the unique photosensitivity and surface plasmon resonance effect of Ag can improve visible-light harvesting and interfacial charge transfer [12,13]. However, Ag-containing photocatalyst materials may suffer from photocorrosion or phase instability under light exposure. Therefore, silver-based photocatalyst materials are often combined with other semiconductor materials to improve photocatalytic activity, enhance stability and reduce use cost, thus promoting the practical application of silver-based materials in energy generation and environmental purification [13,14,15]. Among silver-based semiconductors, silver vanadate series compounds are commonly used owing to their favorable visible light response and stability [16,17]. Considering the photosensitive characteristics of silver-based semiconductors and the unique electron-rich structure and stability of g-C₃N₄, constructing silver-based/g-C₃N₄ composite materials is an effective strategy to improve photocatalytic performance [18,19,20]. Zhao et al. reported that Ag/AgVO₃/g-C₃N₄ ternary photocatalysts exhibited significantly higher degradation performance than single-component photocatalysts for dye degradation [21]. Nevertheless, their research mainly focused on material synthesis and dye removal, and the photocatalytic activity of Ag/AgVO₃/g-C₃N₄ toward tetracycline hydrochloride, a typical antibiotic pollutant, still requires further evaluation.

This study aimed to systematically explore the performance and mechanism of tetracycline hydrochloride degradation by Ag/AgVO₃/g-C₃N₄ under visible light and determine the best degradation conditions by optimizing the proportion and dosage of Ag/AgVO₃/g-C₃N₄ composites, initial concentration of the pollutant, and pH of the solution. To this end, g-C₃N₄ was prepared using the thermal condensation method and subsequently modified by doping with AgVO₃ and reducing Ag nanoparticles to enhance photocatalytic efficiency and activity. The morphology, structure and optical properties of the prepared materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), ultraviolet–visible diffuse reflectance spectroscopy (UV-vis DRS), photoluminescence spectroscopy (PL), and Fourier transform infrared spectroscopy (FT-IR).

2. Materials and Methods

2.1. Chemicals and Materials

Silver nitrate (analytical reagent, AR) was supplied by the Shanghai Institute of Fine Chemical Materials (Shanghai, China); ammonium metavanadate (AR) and urea (AR) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China); tetracycline hydrochloride (guaranteed reagent, GR) was obtained from Dalian Meilun Biotechnology Co., Ltd. (Dalian, China); hydrochloric acid (GR) was supplied by Beijing Chemical Plant Co., Ltd. (Beijing, China); and sodium hydroxide (GR) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China).

2.2. Synthesis of Nanomaterials

2.2.1. Synthesis of g-C₃N₄

For the preparation of g-C₃N₄, 10 g of urea was weighed into a covered crucible, placed in a muffle furnace, and heated to 550 °C at a heating rate of 2 °C/min. A light-yellow powder was obtained after calcination for 3 h. Thereafter, the powder was ground and stored in a sealed bottle for further use.

2.2.2. Synthesis of AgVO₃/g-C₃N₄

For AgVO₃/g-C₃N₄, 0.0184 g of g-C₃N₄ powder was weighed and dissolved in 40 mL of deionized water. The solution was subjected to ultrasonic treatment for 15 min, then 0.0136 g of AgNO₃ was added, and the mixture was stirred for 30 min. Subsequently, 0.094 g of NH₄VO₃ was weighed and dissolved in 12 mL of deionized water. The resulting solution was added dropwise to the previous mixture under continuous stirring for 2 h. After the reaction was complete, the sample was washed, filtered, and dried under vacuum at 70 °C for 2 h, and the product was finally ground and stored in a sealed bottle for further use.

2.2.3. Synthesis of Ag/AgVO₃/g-C₃N₄

Various proportions of the binary catalytic material were weighed (0.02 g) into the catalytic tube, 30 mL of deionized water was added, and the mixture was subjected to ultrasonic treatment for 15 min. Subsequently, the mixture was placed in the photocatalytic instrument and irradiated under filtered visible light for 2 h, during which the solution changed in color from light yellow to light green. After filtration and washing, the sample was dried under vacuum at 70 °C for 2 h. Finally, Ag/AgVO₃/g-C₃N₄ was obtained in the form of a green powder, ground and stored in a sealed bottle for further use.

2.3. Batch Degradation Experiments

For the degradation experiments, 0.02 g of the photocatalyst powder was weighed and dispersed in 50 mL of a 10 mg·L⁻¹ tetracycline hydrochloride solution. The mixture was subjected to ultrasonic treatment for 15 min and transferred to a photocatalytic instrument. The solution was stirred under dark conditions for 30 min to achieve adsorption–desorption equilibrium and an initial sample was collected. Subsequently, the degradation experiment was conducted under light irradiation using a 500 W xenon lamp with a UV-cutoff filter (λ > 420 nm), collecting samples at fixed intervals. The samples were centrifuged at 10,000 rpm for 10 min, after which the supernatant was collected and filtered. The absorbance of the supernatant was measured at the maximum absorption wavelength of 358 nm using a UV–visible spectrophotometer (PERSEE, Beijing, China), and the degradation rate of TC-HCl was subsequently calculated.

Referring to the relevant literature, the wavelength of the UV–visible spectrophotometer was set to 358 nm. A TC-HCl standard solution with a concentration of 100 mg·L⁻¹ was prepared. Subsequently, 2.5, 5.0, 10.0, 20.0, and 30.0 mL aliquots of the tetracycline hydrochloride standard solution were transferred into 50 mL colorimetric tubes and diluted to volume with deionized water. The solutions were then shaken and allowed to stand for 30 min. After the instrument was corrected using the blank solution, the absorbance of the solutions was measured in ascending order of concentration, and the standard curve for TC-HCl was subsequently constructed.

The removal rate ($\mathrm{\eta }$) for the degradation of TC-HCl by the Ag/AgVO₃/g-C₃N₄ system was calculated using Equation (1).

$$\mathrm{\eta }=\left(1-{\displaystyle \frac{\mathrm{c}}{{\mathrm{c}}_0}}\right)\times 100\%$$

(1)

where η is the degradation rate, c is the concentration of TC-HCl in the degraded solution (mg·L⁻¹), and c₀ is the concentration of TC-HCl in the solution before degradation (mg·L⁻¹).

2.4. Characterization

The instruments used in this study included an electric blast dryer (PV-213, Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China), multi-head magnetic stirrer (HJ-4A, Jiangsu Keyi Instrument Co., Ltd., Wuxi, China), high-speed refrigerated centrifuge (TGR-20M, Changsha Xiangzhi Centrifuge Instrument Co., Ltd., Changsha, China), electronic balance (FA-224, Shanghai Shunyu Science and Technology Instrument Co., Ltd., Shanghai, China), temperature-programmed muffle furnace (KSL-1200X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China), UV–visible spectrophotometer (T6, Beijing Puyang General Instrument Co., Ltd., Beijing, China), CNC ultrasonic cleaner (KQ5200DB, Kunlun Ultrasonic Instrument Co., Ltd., Kunlun, China), suction filter pump (AP-9950, Tianjin Aoxenes Instrument Co., Ltd., Tianjin, China), vacuum drying oven (DZF-6020, Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China), and pH meter (PHS-3C, Shanghai Yizheng Scientific Instrument Co., Ltd., Shanghai, China). The XRD patterns were obtained using an X’Pert 3 Powder diffractometer (PANalytical, Almelo, The Netherlands) under Cu Kα radiation (λ = 0.15406 nm). FTIR spectra were acquired on a NEXUS spectrometer (Madison, WI, USA) with KBr pellets over 500–4000 cm⁻¹. Morphological observations were performed via field emission SEM (ZEISS, Oberkochen, Germany). XPS data were recorded on an ESCALA 260Xi system (WI, USA)employing a monochromatic Al Kα X-ray source. Diffuse reflectance (light absorption) spectra were measured with a UV-1050 UV–vis–NIR spectrometer (Techcomp, Shanghai, China). PL spectra were collected on a Hitachi F-4600 fluorometer (Naka, Japan) using a Xe lamp-920 (Edinburgh Instruments, Livingston, Scotland, UK) at an excitation wavelength of 368 nm.

3. Results and Discussion

3.1. Structural Characterization of Materials

The elemental composition and occurrence state of the composite material were determined through XPS analysis (Figure 1). The C 1s peak of adventitious carbon at 284.8 eV was used as the internal binding-energy reference for all spectra. As shown in Figure 1a, the material contains five elements: C, Ag, N, O and V. In Figure 1b, the absorption peaks at the electron binding energies of 288.1 eV, 286.8 eV and 284.8 eV correspond to the N–C=N bond, C=O bond, and C–C bond, respectively [18,22]. Figure 1c shows the high-resolution distribution spectrum of Ag 3d. The doublet located at 368.5 eV and 374.5 eV is assigned to the Ag 3d₅/₂ and Ag 3d₃/₂ core levels of Ag⁰, respectively, with a spin–orbit splitting of 6.0 eV, which is characteristic of elemental silver. In addition, a second doublet component at 367.8 eV and its corresponding higher-binding-energy counterpart are attributed to Ag⁺ species associated with AgVO₃, in good agreement with previously reported values for silver vanadates [17,23]. The characteristic peaks at 401.2 eV, 396.8 eV and 395.3 eV in Figure 1d correspond to the sub-peaks of N–C3, amino N, and C=N–C in the graphitic carbon nitride structure, respectively [18,22]. In Figure 1e, the characteristic peak at 526.7 eV corresponds to the lattice oxygen of AgVO₃, while the peak at 528.6 eV may be attributed to surface-adsorbed hydroxyl groups or oxygen in water molecules absorbed from the air [24]. The characteristic peaks at 520.6 eV and 513.4 eV in Figure 1f correspond to V 2p_1/2 and V 2p_3/2, respectively [25].

Compared with previously reported AgVO₃- or g-C₃N₄-based heterojunctions, the coexistence of Ag, AgVO₃ and g-C₃N₄ observed in this study indicates that the presence of the corresponding elements/chemical species in each component of the Ag/AgVO₃/g-C₃N₄ has been confirmed. Similar interfacial Ag-related species have been reported to promote visible-light utilization and charge separation in Ag₃PO₄/AgVO₃ and Ag/BiVO₄/rGO photocatalysts [17,24]. Therefore, the XPS results support the subsequent photocatalytic performance results, in which the ternary Ag/AgVO₃/5g-C₃N₄ sample exhibits higher TC-HCl removal than the single components.

To characterize the composites, the absorption capacity of g-C₃N₄, AgVO₃/5g-C₃N₄ and Ag/AgVO₃/5g-C₃N₄ to visible light was studied using ultraviolet–visible diffuse reflectance spectroscopy (UV-vis DRS), and the results are shown in Figure 2. All the samples were responsive to visible light (Figure 2a), with Ag/AgVO₃/5g-C₃N₄ exhibiting stronger absorption. Therefore, the doping of g-C₃N₄ with Ag and AgVO₃ can enhance its absorption capacity in the visible light region and improve its photocatalytic activity. The band gap E was calculated according to the Kubelka–Munk formula. As shown in Figure 2b, the band gap values of g-C₃N₄, AgVO₃/5g-C₃N₄ and Ag/AgVO₃/5g-C₃N₄ are 2.94 eV, 2.87 eV and 2.84 eV, respectively. Therefore, the addition of AgVO₃ can lead to a narrower band gap, which can excite more electron–hole pairs, reduce the energy required for the reaction, and improve the degradation performance of the catalyst [26,27].

The band-gap narrowing observed here is moderate, which is consistent with other g-C₃N₄-based visible-light photocatalysts where activity enhancement is usually attributed not only to a reduced band gap but also to improved interfacial charge migration and inhibited recombination [18,19,20]. For example, Ag/BiOI/g-C₃N₄ and WO₃/g-C₃N₄ systems have shown that heterojunction construction can extend the visible-light response while simultaneously strengthening charge separation [20,28]. Therefore, the improved light absorption of Ag/AgVO₃/5g-C₃N₄ provides an optical basis for its enhanced TC-HCl degradation performance, but it should be interpreted together with PL and active-species experiments rather than as the sole cause.

$${\left(\mathrm{\alpha }\mathrm{h}\mathrm{\nu }\right)}^{1/\mathrm{n}}=\mathrm{A}(\mathrm{h}\mathrm{\nu } - {\mathrm{E}}_{\mathrm{g}})$$

(2)

where α is the absorption coefficient, h is the Planck constant, ν is the frequency, A is the proportionality constant, Eg is the semiconductor bandgap width, and n is the exponent (n = 1/2).

SEM images of g-C₃N₄ are shown in Figure 3a,b. As shown in the figure, the pure-phase g-C₃N₄ exhibits a lamellar structure formed by the stacking of nano-sheets. Figure 3c illustrates g-C₃N₄ doped with AgVO₃, which appears as well-distributed rod-like structures on the surface of g-C₃N₄. The SEM image of composite Ag/AgVO₃/g-C₃N₄ is presented in Figure 3d. The morphology of this composite shows significant changes from rod-like structures to granular structures, presumably attributed to the influence of visible light irradiation during the preparation process.

The intimate contact between rod-like AgVO₃ species and lamellar g-C₃N₄ is beneficial for constructing interfacial charge-transfer channels. Similar morphology-dependent enhancement has been discussed in AgVO₃/g-C₃N₄ and Ag-containing bismuth-based photocatalysts, where well-dispersed Ag species or Ag-based semiconductors on a two-dimensional support can provide more accessible active sites and shorten the migration distance of photogenerated carriers [21,24,29]. In this study, the observed granular structure after photoreduction suggests the generation of Ag nanoparticles, which may further contribute to light harvesting through the surface plasmon resonance effect.

The crystal structure and composition of g-C₃N₄, AgVO₃/g-C₃N₄ and Ag/AgVO₃/g-C₃N₄ were characterized by X-ray diffraction (XRD). As shown in Figure 4, the diffraction peaks of AgVO₃/g-C₃N₄ and Ag/AgVO₃/g-C₃N₄ are similar, and the characteristic peaks of g-C₃N₄ and AgVO₃/g-C₃N₄ can be observed in the diffraction peak spectrum of Ag/AgVO₃/g-C₃N₄, indicating the coexistence of Ag, AgVO₃ and g-C₃N₄ in the composite. Specifically, we have now performed a complete phase identification by indexing all diffraction peaks against standard JCPDS/ICSD reference cards. The characteristic peaks of g-C₃N₄ at 2θ = 12.3° and 27.5° correspond to the (100) and (002) planes, respectively, which are consistent with the typical interlayer stacking of heptazine-based graphitic carbon nitride (JCPDS No. 87-1526 [30]). The diffraction peaks of AgVO₃ at 2θ = 17.2°, 19.6°, 25.1°, 30.5°, 32.1°, 34.3°, 38.6°, and 45.8° are in good agreement with the monoclinic phase of AgVO₃ (JCPDS No. 29-1154 [31]). Regarding metallic Ag⁰, we have now clearly identified its characteristic reflection at 2θ = 38.1° corresponding to the (111) plane (JCPDS No. 04-0783 [32]), which was previously overlooked. Additionally, the peaks at 44.3° and 64.5° correspond to the (200) and (220) planes of metallic Ag, respectively. The presence of these Ag⁰ peaks confirms the successful photoreduction of Ag⁺ to Ag⁰ during the visible-light irradiation step in the synthesis process, which is consistent with the composite nomenclature “Ag/AgVO₃/g-C₃N₄”.

Furthermore, the relatively broad diffraction peaks observed for g-C₃N₄ indicate its low degree of crystallinity, which is typical for polymeric carbon nitride materials prepared by thermal condensation. In contrast, the sharper and more intense peaks of AgVO₃/g-C₃N₄ suggest its well-developed crystalline structure. Notably, in the ternary composite, the diffraction peaks of both g-C₃N₄ and AgVO₃ are well preserved, with no additional impurity peaks detected, confirming that the composite formation does not alter the intrinsic crystal structures of the individual components. The slightly reduced intensity of the g-C₃N₄ (002) peak in the composite can be attributed to the surface coverage by AgVO₃ nanorods and Ag nanoparticles.

Fluorescence intensity can be used to analyze the separation of photogenerated electron–hole pairs. As shown in Figure 5a, the order of the peak signal intensities of the three materials is g-C₃N₄ > AgVO₃/5g-C₃N₄ > Ag/AgVO₃/5g-C₃N₄, with stronger fluorescence signal intensities indicating easier recombination of the photogenerated electron–hole pairs, which leads to lower photodegradation efficiency. It is concluded that the utilization rate of photogenerated carriers has been improved by doping Ag and AgVO₃, thus improving the photodegradation performance. Figure 5b shows the FT-IR spectra of three samples, with g-C₃N₄, AgVO₃/g-C₃N₄ and Ag/AgVO₃/g-C₃N₄ all exhibiting vibrations at 3100 cm⁻¹. The vibrations at 3100 cm⁻¹ are presumably attributable to surface N–H or O–H stretching and adsorbed water, whereas the vibrations in the range of 1000–1643 cm⁻¹ may be attributed to C–N stretching [18,22].

3.2. Degradation of Ag/AgVO₃/g-C₃N₄ on TC-HCl

The ability of materials to remove pollutants is key to evaluating their catalytic activity. In this study, the effect of preparation conditions on the reducing activity of Ag/AgVO₃/g-C₃N₄ was investigated with TC-HCl as the contaminant. Because the experiment quantified residual TC-HCl concentration using UV-vis absorbance, the term “degradation rate” in this paper refers to the apparent removal of TC-HCl rather than complete mineralization [33,34]. The effects of degradation rate of as-prepared photocatalysts on the tetracycline hydrochloride were explored under the conditions (pH = 4.7, 0.02 g material dose, and 20 mg/L initial concentration of TC). As depicted in Figure S1, to ascertain the adsorption–desorption characteristics of the synthesized materials, a dark reaction was conducted for 240 min. The removal rate of Ag/AgVO₃/g-C₃N₄ heterojunction remained essentially unchanged at both 30 min and 240 min; hence, 30 min was chosen as the adsorption–desorption equilibrium point for this experiment. The control experiment indicated that TC-HCl hardly degraded under visible light irradiation without the addition of a catalyst, suggesting that TC-HCl was relatively stable. As shown in Figure 6, the degradation rate of pure-phase g-C₃N₄ and AgVO₃ reached 61.81% and 51.63%, respectively, after 240 min of irradiation. The degradation rate of AgVO₃ was the lowest due to the fact that AgVO₃ alone as a catalyst has low response to visible light, whereas pure-phase g-C₃N₄ can compensate for the shortcomings of Ag⁺ and enhance its response to visible light. Therefore, Ag/AgVO₃/5g-C₃N₄ exhibits the strongest degradation effect, with the degradation rate reaching 86.64%, which is 1.4 times that of ordinary g-C₃N₄. The reason is that after doping with Ag⁺, the recombination of photo-generated electron–hole pairs can be effectively reduced, the ability to respond to visible light can be improved, and the energy required for the reaction can be reduced, thus enhancing the degradation efficiency of TC-HCl. However, when excessive g-C₃N₄ participates in the reaction, the active sites of the composite will be reduced, the mobility of photogenerated carriers will decrease, and thus the degradation rate of TC-HCl will be reduced [20,21]. Compared with these systems, the present Ag/AgVO₃/5g-C₃N₄ sample reaches 83.98% TC-HCl removal under the tested conditions, indicating that the composite design is effective for tetracycline-type antibiotic removal. However, the reaction time and pollutant concentration differ among studies, so the comparison should be interpreted as qualitative rather than directly ranking catalyst superiority.

3.2.1. Effect of Material Dosage

The degradation effects of different doses (0.2 g·L⁻¹, 0.4 g·L⁻¹, 0.6 g·L⁻¹, and 0.8 g·L⁻¹) of Ag/AgVO₃/5g-C₃N₄ on tetracycline hydrochloride were explored at 4.7 initial pH and 10 mg/L initial concentration of tetracycline hydrochloride. The experimental results are shown in Figure 7. After 240 min of light catalysis, the degradation rates were 71.72%, 78.16%, 79.72%, and 82.22%, respectively. The degradation efficiency of tetracycline hydrochloride increased with increasing dosage. The reason is that at constant TC-HCl concentration, the increase in the dosage of composite materials increases the active sites available for the reaction and enhances the photocatalytic efficiency of the reaction, thereby increasing the degradation efficiency of TC-HCl. However, the gap in degradation rate between different dosages becomes narrow over time. Considering the cost issue in practical applications, the recommended maximum dosage is 0.8 g·L⁻¹ in this study.

A similar dosage-dependent trend has been widely reported in photocatalytic antibiotic degradation: increasing catalyst dosage can initially increase active sites and photon absorption, but excessive dosage may cause light scattering, particle aggregation and shielding effects [29,35]. In the present study, no decline was observed up to 0.8 g·L⁻¹, but the incremental improvement became smaller from 0.6 to 0.8 g·L⁻¹. This suggests that the reaction may gradually shift from active-site limitation to photon-transfer or mass-transfer limitation at higher catalyst loadings. Therefore, 0.8 g·L⁻¹ is reasonable for the present laboratory system, while economic and hydraulic factors should be further considered in scale-up applications.

3.2.2. Effect of Initial TC-HCl Concentration

The degradation rates of tetracycline hydrochloride with initial concentrations of 10 mg·L⁻¹, 20 mg·L⁻¹, 30 mg·L⁻¹, 40 mg·L⁻¹, and 50 mg·L⁻¹ with Ag/AgVO₃/5g-C₃N₄ were explored under an initial pH of 4.7 and Ag/AgVO₃/5g-C₃N₄ dose of 0.8 g·L⁻¹. The experimental results are shown in Figure 8. With increasing initial concentration of tetracycline hydrochloride, the degradation rates of tetracycline hydrochloride were 86.46%, 83.44%, 82.25%, 82.01%, and 81.19%, respectively. The degradation effect was the strongest at the initial concentration of 10 mg·L⁻¹. The reason may be that at a constant dose, on the one hand, the degradation ability of the catalyst itself is limited; on the other hand, the active sites provided by the catalyst are limited and cannot meet the degradation requirements of pollutants [28,33]. Therefore, the photocatalytic effect of the material is stronger at lower initial concentration of pollutants [34,35].

3.2.3. Effect of Initial pH

The degradation of tetracycline hydrochloride was explored at pH values of 4, 6, 7, 9, and 11 under the conditions of 0.8 g·L⁻¹ dosage of Ag/AgVO₃/5g-C₃N₄ and 10 mg·L⁻¹ initial concentration of tetracycline hydrochloride. At pH values of 4, 6, 7, 9, and 11, the degradation rates reached 85.99%, 75.03%, 68.61%, 69.04%, and 70.04%, respectively. The experimental results are shown in Figure 9. The best degradation effect of antibiotics at pH = 4 (approximately 4.7) presumably occurred because pH influences the existing form of pollutants in the solution and the adsorption characteristics of the photocatalyst surface. Under acidic conditions, the TC-HCl solution primarily exists in a positively charged or zwitterionic form, which is more likely to interact with the catalyst surface and reactive species, facilitating the degradation of pollutants. Under alkaline conditions, the pH of the solution may promote the formation of ·OH within a certain range, thereby affecting the reaction rate [36,37]. Under neutral conditions, pollutants exist mainly as neutral or zwitterionic species; therefore, the catalytic degradation effect is not significant. Under highly acidic or alkaline conditions, ions are easily adsorbed on the surface of the catalyst, which blocks the absorption of visible light by materials and increases the coverage of active sites, thus limiting the migration ability of photogenerated carriers and reducing photocatalytic efficiency. Moreover, the degradation rate is higher under alkaline conditions than under neutral conditions. As the initial pH of the solution was 4.7, which is close to the optimal pH, the pH of the solution was not specially adjusted in the remaining experiments.

The pH-dependent behavior is also consistent with the amphoteric nature of tetracycline molecules, which possess multiple acid dissociation constants and may occur as cationic, zwitterionic or anionic species depending on solution pH [38]. Previous studies have shown that the optimal pH for tetracycline photocatalytic degradation varies with catalyst surface charge, adsorption affinity and dominant active species [39,40]. The stronger performance under acidic conditions in this study suggests that adsorption between TC-HCl species and Ag/AgVO₃/5g-C₃N₄ is favorable near the natural pH of the solution. This is advantageous because additional pH adjustment may not be necessary for weakly acidic antibiotic wastewater, reducing chemical consumption.

3.3. Stability of Ag/AgVO₃/5g-C₃N₄

In order to explore the stability of Ag/AgVO₃/5g-C₃N₄, a stability test experiment was carried out. Under the conditions of 4.7 pH and 10 mg·L⁻¹ concentration of hydrochloric acid, 0.8 g·L⁻¹ of the composite material was weighed, fully dissolved, and placed in a photocatalytic instrument. The mixture was tested four times, and the degradation rates in each test were 89.34%, 86.95%, 85.52%, and 81.52%, respectively. As shown in Figure 10, the degradation rate of TC-HCl decreased from 89.34% to 81.52% after four cycles, proving that the photocatalyst had relatively stable performance.

The 7.82% decrease after four cycles may be associated with partial surface coverage by intermediates, unavoidable catalyst loss during recovery, and possible photocorrosion of Ag-containing species. Similar stability issues have been reported for silver-based photocatalysts, where coupling with g-C₃N₄ or other semiconductors can mitigate but not completely eliminate activity loss [13,15,41]. Compared with many Ag-based systems, the present material retains more than 90% of its initial removal capacity after four cycles, indicating acceptable reusability. Nevertheless, for water submission, it should be noted that XRD/XPS after cycling and Ag leaching analysis would further strengthen the claim of structural stability and environmental safety.

3.4. Photocatalytic Mechanism

To investigate the photocatalytic reaction mechanism of Ag/AgVO₃/5g-C₃N₄, active species capture experiments were conducted. Isopropanol (IPA), disodium ethylenediaminetetraacetic acid (EDTA-2Na), and p-benzoquinone (BQ) were used to capture •OH, h⁺, and ·O₂⁻, respectively, with a dosage of 2 mmol for all three capture agents, as shown in Figure S2. After adding EDTA-2Na, the degradation rate of TC-HCl decreased from 86.64% to 9.59%, and the degradation was significantly inhibited. After adding BQ, the degradation rate decreased to 46.78%, and the degradation performance of the catalyst was greatly affected. However, after adding IPA, the degradation rate only decreased by 7.54%, and there was basically no effect. Therefore, ·O₂⁻ and h⁺ are the main active species in the degradation process.

Based on the collective evidence from scavenger tests, optical characterization, photocatalytic performance, and literature-reported band edge positions, a possible Ag-mediated Z-scheme-type charge-transfer pathway for the Ag/AgVO₃/5g-C₃N₄ heterojunction is proposed. As illustrated in Figure 11, under visible-light excitation, photogenerated electrons are promoted to the conduction band (CB), while holes remain in the valence band (VB). The electrons in the CB of AgVO₃ can transfer to the surface of Ag. Concurrently, holes in the VB of g-C₃N₄ can migrate to the surface of Ag, thereby facilitating efficient separation of photogenerated electron–hole pairs. Literature reports indicate that the VB potential of AgVO₃ is close to or slightly lower than the standard redox potential of H₂O/·OH (+2.72 eV vs. NHE), depending on the synthesis route and measurement method [21,25]. Therefore, these holes may mainly directly oxidize organic contaminants rather than efficiently oxidize water to generate ·OH radicals. On the other hand, the CB potential of g-C₃N₄ is generally more negative than the O₂/·O₂⁻ redox potential (−0.33 eV vs. NHE) [18,39]. Consequently, the photogenerated electrons can reduce O₂ to superoxide radicals (·O₂⁻), which can directly participate in photocatalytic oxidation or undergo further reactions to produce other oxidative species for pollutant degradation. Moreover, the surface plasmon resonance effect of Ag broadens the light absorption range and enhances visible-light utilization [42,43]. The synergistic interaction between the Ag/AgVO₃/5g-C₃N₄ heterojunction and Ag nanoparticles contributes to the improved photocatalytic activity.

4. Conclusions

In this study, Ag/AgVO₃/5g-C₃N₄ was prepared using the water bath method. The crystal phase of the prepared catalyst was analyzed via XRD, the microstructure and morphology of the catalyst were observed via SEM, the elements and molecular structure of the catalyst were analyzed via XPS and FT-IR, and the separation of the photogenerated electron–hole pairs in the prepared catalyst was studied via UV-vis DRS and PL. In addition, the photodegradation performance of the prepared photocatalyst was explored, and the optimum conditions for the degradation of tetracycline hydrochloride were explored by changing the ratio of samples, dosage, initial concentration, and pH. The following main conclusions can be drawn:

As a matrix, g-C₃N₄ was doped with Ag and AgVO₃, which enhanced its ability to degrade tetracycline hydrochloride.

The introduction of Ag and AgVO₃ broadens the visible light response, inhibits the recombination of photogenerated electron–hole pairs, and effectively improves the photocatalytic performance of g-C₃N₄.

Ag/AgVO₃/g-C₃N₄ exhibits higher photocatalytic performance than AgVO₃/g-C₃N₄ and g-C₃N₄. However, an excessive amount of g-C₃N₄ will negatively affect the degradation of TC-HCl, mainly because excessive g-C₃N₄ reduces the number of active sites available in the composite. In this study, the optimal ratio of Ag, AgVO₃ and g-C₃N₄ for antibiotic degradation was found to be 1:1:5.

pH can affect the adsorption performance of photocatalyst surfaces. In general, the degradation effect is stronger under acidic conditions than under alkaline and neutral conditions. The composite achieves the strongest degradation effect, reaching 86.64%, at room temperature of 25 °C, pH of 4.7, initial tetracycline hydrochloride concentration of 10 mg·L⁻¹, and photocatalyst dosage of 0.8 g·L⁻¹.