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Article

Molten-Salt-Assisted Preparation of g-C3N4 for Photocatalytic Degradation of Tetracycline Hydrochloride: Degradation Mechanism, Pathway, and Toxicity Assessment

by
Yujie Jiao
1,
Yaqi Mao
1,
Qikai Liu
1,
Yongxia Ma
1,
Fei Fu
1,
Shenglong Jian
2,
Yang Liu
1,3,* and
Sujin Lu
1,*
1
College of Eco-Environmental Engineering, Qinghai University, Xining 810016, China
2
Qinghai Fisheries Technology Extension Centre, Xining 810016, China
3
Key Laboratory of Plateau Cold-Water Fish Culture and Eco-Environmental Conservation (Co-Construction by Ministry and Province), Ministry of Agriculture and Rural Affairs Qinghai University, Xining 810016, China
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(3), 1166; https://doi.org/10.3390/su17031166
Submission received: 11 December 2024 / Revised: 31 December 2024 / Accepted: 13 January 2025 / Published: 31 January 2025
(This article belongs to the Special Issue Pollution, Toxicology and Sustainable Solutions in Aquatic System)
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Abstract

:
The sustainability of aquaculture tailwater plays a key role in the aquaculture industry. Photocatalytic degradation of recalcitrant antibiotics in aquaculture tailwater has emerged as a significant research focus, with gCN-based photocatalysis offering a promising approach. To address the issue of inefficient degradation associated with gCN, melamine was modified using NaCl solution, resulting in the synthesis of NaMe-x with distinctive microstructure through molten salt assistance. The ability of NaMe-x to degrade tetracycline hydrochloride (TC-HCl) was examined, including an analysis of its degradation pathway, intermediate products, mechanism, and toxicity of the by-products. The results demonstrated that NaCl-based precursor modification markedly enhanced the degradation capacity of gCN for TC-HCl, achieving a maximum degradation rate of 0.02214 min−1, which is 2.1 times higher than that of unmodified gCN. LC-MS analysis revealed intermediates at various degradation stages, and two potential pathways for TC-HCl degradation in the presence of NaMe-1 were identified. In this process, ·O2− and ·OH are the reactive radicals that play a dominant role, and their degradation mechanism is thus proposed. It was confirmed by toxicity experiments that the products after the degradation of TC-HCl by NaMe-1 were not significantly toxic to Chlorella vulgaris (p ˃ 0.05). However, it had a significant effect on Vibrio fischeri (p < 0.01). These findings suggest that the synthesis of NaMe-x via melamine precursor modification substantially improves the degradation performance of gCN and enhances the sustainability of aquaculture tailwater.

1. Introduction

The rapid growth of the global population and the diminishing availability of water resources have led to water pollution becoming a critical global issue [1]. In aquaculture, antibiotics such as penicillin, tetracycline, and sulfonamides are extensively utilized to enhance fish immunity, suppress bacterial growth, reduce mortality, and increase productivity [2,3]. Worldwide, over 100,000 tons of antibiotics are consumed annually, with about 50% allocated to the cultivation industry [4]. These antibiotics possess complex and stable chemical structures that resist degradation and exhibit significant biotoxicity and ecological risks [5]. These antibiotics limit the sustainability of the aquaculture industry while substantial quantities are discharged into natural water systems in their unaltered forms. This discharge results in antibiotic-laden effluents that promote resistance in aquatic organisms, and prolonged exposure to low concentrations of residual antibiotics poses potential risks to human health [6,7]. Antibiotic wastewater is characterized by high organic content, intense chromaticity, challenging degradability, and significant biotoxicity, necessitating stringent treatment methods [8]. Among such pollutants, tetracyclines (TCs) are widely used broad-spectrum antibiotics in aquaculture and livestock farming [9,10]. Residual tetracycline antibiotics frequently persist in water and soil [11], imposing severe stress on aquatic ecosystems. Therefore, it is of great significance to realize the efficient degradation of antibiotics in aquaculture tailwater and reduce the ecological damage caused by their inflow into natural water bodies for the sustainable development of aquaculture tailwater.
Research indicates that traditional wastewater treatment methods are insufficient for completely removing antibiotics, resulting in their entry into the environment through treated effluents and posing risks to ecological systems and human health [12]. Thus, it is imperative to effectively degrade these contaminants into non-toxic or smaller molecular by-products, such as H2O and CO2. To overcome the limitations of conventional treatment methods, advanced technologies such as biological treatments [13], adsorption techniques [14,15], filtration methods [16], advanced oxidation processes [17,18], Fenton reactions [19], and photocatalysis [20,21] have been developed. Among these, photocatalytic oxidation is particularly promising due to its capacity to utilize solar energy efficiently, low energy requirements, and effectiveness in degrading organic pollutants. g-C3N4, a non-metallic green photocatalyst, has diverse applications, including hydrogen production through water photolysis [22], CO2 reduction [23], Cr(VI) reduction [24], and the degradation of organic pollutants [25]. However, the practical application of unmodified g-C3N4 is limited by factors such as rapid recombination of photogenerated electron–hole pairs, poor visible light utilization, low surface area [26,27], and inadequate hydrophilicity. To enhance the photocatalytic efficiency of g-C3N4, various modifications have been explored, including adjustments to its electronic structure [28,29], innovative nanostructure designs [30], and heterostructure development [31]. Utilizing molten salt methods to control polymerization has been shown to improve crystallinity and charge separation efficiency, significantly enhancing the photocatalytic performance of g-C3N4 [32,33,34]. The crystal surface structure directly affects the charge transfer and photocatalytic efficiency [35]. The molten salt method is one of the most efficient methods for the large-scale synthesis of crystalline carbon nitride, and the microstructure of g-C3N4 during thermal polymerization can be modulated by using one or more salts as a medium to tightly stack the layers and facilitate the migration of photogenerated charge carriers [36,37]. Dou Q et al. [38] synthesized Na-doped g-C3N4 in a facile manner, which improved the sample’s crystallinity and increased its specific surface area, which showed excellent catalytic performance not only in degrading pollutants, but also in H2 production and CO2 reduction. Previous studies have demonstrated that modifying precursors with saline solutions followed by molten salt calcination enhances both structural and catalytic properties. However, the precise mechanisms behind these improvements remain to be fully elucidated.
In this study, the melamine precursor was modified using an aqueous solution, and NaMe-x was synthesized through a molten-salt-assisted method. The influence of environmental factors on the degradation of TC-HCl by NaMe-x was examined, and the degradation mechanism was thoroughly analyzed. Additionally, the degradation pathways and the toxicity of the resultant degradation products were assessed by identifying the intermediate products formed during the process. The synthesis of NaMe-x through melamine precursor modification demonstrated a significant enhancement in photocatalytic efficiency. This approach offers an economically viable and effective method for the treatment of antibiotic pollutants in aquaculture tailwater while enhancing the sustainability of aquaculture tailwaters.

2. Experimental Section

2.1. Chemicals

The drugs used in this experiment are shown in Text S1.

2.2. Synthesis of Photocatalysts

2.2.1. Preparation of Precursors

A total of 6 g of melamine was completely dissolved in 200 mL of deionized water at 120 °C. Subsequently, 10 mL of a 1 mol·L−1 NaCl solution, prepared using deionized water, was added to the solution. The mixture was maintained at 100 °C for one hour. After crystal precipitation, the solution was allowed to stand for 8–10 h, following which the precipitate was washed multiple times with deionized water and dried at 80 °C. This precursor was designated as NaCl/Me-1. To investigate the effect of NaCl concentration, the procedure was repeated using NaCl solutions of 0.5 mol·L−1 and 2 mol·L−1, resulting in precursors labeled as NaCl/Me-0.5 and NaCl/Me-2, respectively. Unmodified melamine was processed similarly and designated as Me.

2.2.2. Synthesis of NaMe-x

The precursor materials prepared above were thoroughly ground and calcined in a crucible at 550 °C for 4 h. The resulting yellow powders, obtained after natural cooling, were ground again, washed with deionized water and alcohol, and then dried at 80 °C for 12 h (Figure 1). These photocatalysts were labeled as NaMe-0.5, NaMe-1, and NaMe-2 based on the precursor used. For comparison, unmodified melamine was calcined under the same conditions (550 °C for 4 h), naturally cooled to room temperature, ground thoroughly, and prepared as gCN.

2.3. Effect of Environmental Factors on the Degradation Efficiency of NaMe-x

In the photocatalysis experiments, the effects of various parameters on the degradation performance of precursor-synthesized photocatalysts (NaMe-x) were systematically evaluated. These parameters included the concentration of TC-HCl (10, 20, 40, 60, 80, and 100 ppm), photocatalyst dosage (0.08, 0.12, 0.16, 0.20, 0.24, and 0.28 g·L−1), pH levels (2, 3, 5, 7, 9, and 11), the presence of natural organic matter (humic acid and sodium acetate), and co-existing ions (CO32−, HCO3−, Cl−, NO3−, Ca2+, and Fe3+). Additionally, the stability of the photocatalysts under repeated use and their effectiveness in treating actual aquaculture tailwater were examined to assess their practical applicability. The light source employed in the photocatalysis experiments was a 300 W tritium light source, the details of which are provided in Text S3.

2.4. Degradation Pathway and Degradation Mechanism Study of TC-HCl by NaMe-x

To identify the intermediate products generated during the degradation of TC-HCl, samples collected at various intervals during the photocatalytic reaction were analyzed using LC-MS technology. Intermediate compounds were inferred by evaluating total ionic currents and mass-to-charge ratios (m/z) in the mass spectra, along with the relevant literature, to elucidate the degradation pathway of TC-HCl. Specific trapping agents EDTA-2Na (100 ppm) for h+, isopropanol (IPA) for ·OH, L-histidine (100 ppm) for 1O2, and p-benzoquinone (BQ, 100 ppm) for··O2 were utilized to identify reactive species involved. After two hours of photocatalysis, during which 5 mL of each free radical trapping agent was added, the concentration of TC-HCl was measured, enabling the determination of the primary reactive species contributing to its degradation mechanism.

2.5. Toxicity Studies on Chlorella and Luminescent Bacteria

In toxicity experiments, ECOSAR (Version 2.2) was employed to predict the biotoxicity indices of TC-HCl and its degradation intermediates on fish, Daphnia, and green algae. For experiments involving Chlorella vulgaris and luminescent bacteria, the toxicity of TC-HCl and its by-products was evaluated. In one group, the concentration of TC-HCl in the algal fluid was increased to 20 ppm, while in another, C. vulgaris and luminescent bacteria were cultured with the degradation products obtained from the NaMe-1 reaction after 2 h. A blank group was established as a control. Toxicity assessment was based on the growth of C. vulgaris (OD680), variations in chlorophyll content in C. vulgaris, and changes in the luminescence intensity of luminescent bacteria.

2.6. Sample Collection and Data Analysis

Antibiotics were detected using HPLC (Zhengzhou Jinshimai Technology Co., Ltd, Zhengzhou, China) to determine the concentration of TC-HCl in the degradation system. The procedure involved filtering the sample through a 0.22 μm aqueous membrane (Nanjing Kingsley Technology Co., Ltd., Nanjing, China) to collect the filtrate, which was subsequently analyzed via liquid chromatography under the conditions specified in Table S1. Verification was performed by measuring absorbance at 357 nm using a dual-beam UV–visible spectrophotometer (Shanghai Shuangxu Electronics Co., Ltd., Shanghai, China). To identify the by-products generated during the degradation of TC-HCl by NaMe-1, samples collected at specific intervals were centrifuged and filtered through 0.22 μm aqueous membranes before being analyzed using LC-MS (Shanghai Shanfu Electronic Technology Co., Shanghai, China).
Photocatalytic degradation properties of antibiotics were evaluated in terms of degradation efficiency (Re) and degradation rate constant (k). The degradation process was modeled using quasi-primary kinetic equations as detailed in Text S2. Statistical significance between data groups was assessed using the Student’s t-test, with * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001 indicating levels of significance.

3. Results and Discussion

3.1. Characterization of the Materials

3.1.1. Characterization of Precursor Structure

FT-IR mapping analysis (Figure 2a) revealed that the precursor modified with NaCl exhibited sharp, intense vibrations and a broad, strong peak in the 3300–3500 cm−1 region, corresponding to -NH2 and -OH functional groups [39]. Peaks observed at 1400–1700 cm−1 and 500–850 cm−1 were attributed to the bending vibrations of secondary amines, triazine derivatives, and para substitutions between triazine ring neighbors, respectively. XRD analysis (Figure 2b) indicated that the diffraction peak positions of the precursor remained unchanged after the introduction of NaCl, suggesting that the crystal structure of the modified melamine precursor was preserved. However, variations were observed in the intensity of the crystal faces, particularly at 21.88°, highlighting differences in crystallinity. This difference in crystallinity leads to a significant change in the surface of the resulting calcined NaMe-1, with an increase in disorder, an increase in specific surface area, an increase in the number of available active sites, etc., which improves its photocatalytic performance.

3.1.2. SEM, FI-RT, and XRD Analysis of NaMe-x (Questions 1–4)

SEM analysis revealed that the surface morphology of gCN exhibits an irregular layered and stacked structure characteristic of graphitic phase carbon nitride, with visible interstitial gaps between layers and good crystallinity (Figure 3a). In contrast, NaMe-x displays noticeable clumps of varying shapes, with these clumps overlapping and stacking on one another (Figure 3b–d). As the NaCl content in the precursor increased, the stacking of the blocks became progressively denser, with the surface looseness initially decreasing and then increasing. NaMe-1 exhibited the most loosely packed surface structure. However, an excessive NaCl concentration in the precursor led to overly tight packing on the surface of the synthesized graphitic phase carbon nitride, collapsing parts of the structure and slightly reducing surface looseness. FT-IR profiles of NaMe-x and gCN, synthesized from different precursors, were found to be similar, indicating that their chemical structures remained consistent (Figure 3h). A prominent peak at 808 cm−1 corresponded to the vibrations of tri-s-triazine structural units, while peaks in the 1200–1700 cm−1 range were associated with benzene ring vibrations, as well as C-N and C=N bond vibrations [40,41,42]. In the FT-IR spectral analysis, NaMe-x exhibited all characteristic absorption peaks typical of gCN, indicating that the fundamental framework of gCN remained unaltered following NaCl introduction during precursor preparation. However, a new absorption peak at 2170 cm−1 was observed, attributed to incomplete polymerization effects on tri-s-triazine structures caused by NaCl, leading to the formation of -C≡N groups integrated into the overall architecture [gCN] [43,44,45]. XRD analysis of NaMe-x compared with pure gCN (Figure 3i) revealed diffraction peaks at angles of 13.1° and 27.5°, corresponding to the (100) facets associated with tri-s-triazine structures and (002) interlayer aromatic stacking facets, respectively. The (002) crystalline surfaces showed significant enhancement in the NaMe-x samples, suggesting increased crystallinity due to the melamine modification using NaCl. However, excessive crystallinity, particularly in NaMe-2, adversely impacted interlayer arrangements, leading to structural collapse and reduced specific surface areas (Figure 3d). EDS elemental mapping confirmed the distribution of carbon, nitrogen, and sodium elements throughout the sample matrices, while SEM imaging further substantiated successful interactions between the incorporated components. These interactions altered π–π stacking dynamics within graphene-like lamellar frameworks, facilitating improved charge transport efficiencies, broadening spectral responses toward visible wavelengths, and effectively mitigating photogenerated electron–hole pair recombination issues, as illustrated in Figure 3e–g. Therefore, it can be reasonably surmised from the above conclusions that the basic skeleton of both the NaCl-modified melamine precursor and the synthesized NaMe-x remains unchanged. However, the change in crystal surface growth in the precursor leads to a change in the surface structure of the synthesized NaMe-x, with an increase in the specific surface area (Table 1), the formation of agglomerates with different morphologies, and an increase in the surface looseness.

3.1.3. BET and UV-Vis-DRS Analysis

The microstructure of synthesized materials plays a crucial role in determining their degradation ability [46,47]. A large specific surface area enables more binding sites for TC-HCl, enhancing catalytic activity under visible light. BET and pore distribution analyses of gCN and NaMe-x, based on N2 adsorption–desorption (Figure 4), revealed that both materials exhibit H3 hysteresis loops, characteristic of type IV isotherms, indicative of a well-defined mesoporous structure likely formed by slit-like interstitial accumulation [48,49]. The pore size distributions are shown in Figure S3, and the pore volumes of gCN, NaMe-0.5, NaMe-1, and NaMe-2 are 0.14, 0.39, 0.41, and 0.30 cc·g−1, respectively, indicating that the appropriate addition of NaCl in the precursor can effectively induce the formation of defects, resulting in an increase in pore volume and specific surface area [50,51]. This structural difference contributes to a significant increase in the specific surface area of NaMe-x compared to gCN (Table 1), resulting in more binding sites and enhanced interaction with TC-HCl [52]. Notably, NaMe-1 demonstrated a larger surface area, improving visible light absorption and enhanced degradation efficiency [53]. In contrast, NaMe-2 exhibited a markedly lower specific surface area (45.754 m2·g−1), likely due to excessive polymerization and interactions between surplus sodium ions and gCN. These interactions may cause partial structural collapse, reducing the specific surface area [54], a result consistent with SEM observations. The photocatalysts were analyzed by ultraviolet–visible diffuse reflectance spectroscopy (UV-Vis-DRS) to analyze their ability to absorb light and to determine their band gap (Figure 4b,c). The results showed that the Eg value of NaMe-1 was 2.66 eV, which was smaller than that of gCN (2.71 eV). The narrowing of the band gap undoubtedly indicates that the synthesis of NaMe-1 by the NaCl-modified melamine precursor can absorb more incident light energy than pure gCN, which can accelerate the charge transfer and is conducive to enhancing photocatalytic performance.

3.1.4. XPS Analysis

The elemental composition and structural characteristics of NaMe-1 were analyzed using XPS. Both gCN and NaMe-1 exhibited the presence of C, N, and O elements in their full spectra (Figure 5a), with sodium detected in NaMe-1, while chlorine was absent, indicating that sodium exists in its ionic form [55]. The O 1s peak at 531.75 eV corresponded to O-H bonds (Figure 5b), suggesting the presence of hydroxyl groups associated with surface-bound water molecules [56]. The C 1s spectrum for NaMe-1 showed three peaks at 284.80 eV, 288.28 eV, and 285.81 eV (Figure 5c), corresponding to contributions from C-C bonds within the material, C-NHx structures, and sp2-hybridized carbon in benzene rings (N=C-N), respectively [57]. Similarly, the N 1s spectrum revealed three main peaks at 398.63 eV, 399.78 eV, and 401.12 eV (Figure 5d), attributed to sp2 nitrogen in triazine motifs (C=N-C), bridging nitrogen atoms among benzene rings (N-(C)3), and hybridized nitrogen species (N-H), respectively [58]. The fitted peak for Na+ observed at 1071.73 eV (Figure 5e) confirmed its presence in a +1-oxidation state, indicating its potential role as a charge compensator. This suggests that Na+ facilitates carrier transfer processes, thereby contributing to the enhanced photocatalytic performance of NaMe-1 [33,59].

3.2. Photocatalytic Performance of TC-HCl Degradation by NaMe-x and the Effect of Environmental Factors

The photocatalytic properties of gCN and NaMe-x (x = 0.5, 1, 2), synthesized by calcining Me and NaCl/Me-x precursors, were evaluated for tetracycline hydrochloride (TC-HCl) degradation. To eliminate the influence of catalyst adsorption on photocatalysis, a 30 min dark reaction was conducted prior to light exposure. During the dark reaction, neither the gCN nor NaMe-x materials exhibited significant adsorption properties (Figure 6a). Therefore, no dark reaction adsorption experiments were carried out for the various experiments below. Under photoreaction conditions, the catalytic performance of gCN was found to be poor. The degradation efficiency of the synthesized NaMe-x varied with the NaCl concentration in the precursor. At a concentration of 1 mol·L−1 NaCl, NaMe-1 demonstrated the highest degradation efficiency for TC-HCl, increasing from 74.03% to 94.36%, with the corresponding degradation rate rising from 0.0109 min−1 to 0.0239 min−1. A first-order kinetic model was applied to the photocatalytic degradation of TC-HCl by NaMe-x (Figure 6b), and the degradation rate constant for NaMe-1 was determined to be 0.02214 min−1, which is 2.1 times higher than that of gCN. SEM images of NaMe-x revealed changes in surface morphology with varying NaCl content in the precursor, showing evident clump formation and stacking. These results suggest that the surface morphology and, consequently, the photocatalytic performance of NaMe-x can be effectively controlled by adjusting the NaCl concentration in the precursor. This suggests that NaMe-1 synthesized by the molten salt method has enhanced degradation ability, which is the same as the results of Quan Y [60] and Fernandes E et al. [61]. Meanwhile, Table S5 shows that NaMe-1 synthesized by the NaCl-modified melamine precursor degraded TC-HCl better than most photocatalysts of the same type [62,63,64,65,66,67]. In the above experiments, the concentration of TC-HCl in the reaction was controlled to be 20 ppm, and the dosage of various catalysts was 0.20 g·L−1.
In dosage experiments with NaMe-1 (Figure 7a), increasing the catalyst concentration from 0.08 g·L−1 to 0.20 g·L−1 enhanced the degradation rate of TC-HCl from 0.00718 min−1 to 0.0239 min−1 (Table S2). This increase is attributed to the production of free radicals such as ·OH and ·O2−, which accelerate reactions and improve overall reaction rates. However, excessive catalyst amounts can hinder light penetration and lead to surplus free radicals, potentially causing self-quenching effects that reduce photocatalytic efficiency [68,69]. The initial pH of aquaculture tailwater also significantly impacted TC-HCl degradation efficiency. Under visible light, degradation efficiency initially increased with rising pH, peaking at pH 5, but declined between pH 7 and 11 (Figure 7b,c). Specifically, efficiency rose from 85% to 95% when shifting from pH 2 to 5. NaMe-1 exhibited optimal catalytic performance in acidic conditions, particularly between pH 3 and 5, under visible illumination. This behavior is attributed to improved separation rates of photogenerated carriers in acidic environments, promoting radical generation and enhancing degradation efficiency [70]. However, excessive H+ concentrations in highly acidic environments can lead to protonation, causing electrostatic repulsion between the catalyst and TC-HCl, and scavenging ·OH radicals, thereby diminishing degradation efficiency [71]. This is in line with the findings of Z Ciğeroğlu et al. [72], where the region with the best catalyst effect is around 3.5. These findings suggest that while acidic or neutral conditions are generally suitable for TC-HCl degradation in most water bodies, reaction conditions should be controlled to remain within neutral or slightly acidic ranges for optimal performance.
Variations in the initial concentration of TC-HCl significantly influenced the degradation performance of NaMe-1. At lower concentrations, NaMe-1 effectively degraded TC-HCl without notable efficiency losses. However, at higher concentrations, its efficiency declined (Figure 7d; Table S2). The maximum degradation capacity of NaMe-1 reached 278 mg·g−1, over 1.5 times greater than that of pure gCN (Figure S1). At low TC-HCl concentrations, the photocatalyst generates sufficient free radicals for degradation. As concentrations increase, saturation occurs, blocking active sites on NaMe-1 and reducing light absorption, thereby diminishing photocatalytic efficiency [73]. Increased initial concentrations also create a shielding effect, where incident light is obstructed, reducing free radical generation and preventing the complete degradation of TC-HCl [74]. Furthermore, intermediate products generated during TC-HCl oxidation may compete with undegraded TC-HCl for active sites on the catalyst surface, further impairing photocatalytic performance [75]. The presence of natural organic matter, such as humic acid and sodium acetate, also affected the degradation ability of NaMe-1. With increasing humic acid concentration, the degradation efficiency of NaMe-1 for TC-HCl decreased from 95.39% to 57.54%, and the degradation rate constant fell from 0.02378 min−1 to 0.00758 min−1 (Figure 7e,f; Table S2). This reduction can be attributed to UV filtration, photon absorption, and free radical inhibition by natural organic matter, which interferes with the photocatalytic process [76]. These findings underscore the impact of both TC-HCl concentration and natural organic matter on the efficacy of NaMe-1 in photocatalytic degradation. The adsorption of humic acid onto the photocatalyst surface competes with TC-HCl for active binding sites, thereby reducing catalytic efficiency. Additionally, higher humic acid concentrations decrease light transmission, limit visible light availability, and subsequently lower catalytic activity. Conversely, increasing sodium acetate concentrations did not significantly affect the degradation efficiency or reaction rate of TC-HCl, indicating a negligible impact on the photocatalytic performance of NaMe-1. The degradation capacity of TC-HCl was also influenced by various co-existing ions commonly found in aquaculture tailwater (Figure 7g,h; Table S2). The concentration of various co-existing ions in 20 ppm was added up to 50 ppm before the photocatalytic experiments to explore their effects on the ability of NaMe-1 to degrade TC-HCl. Among these ions, Fe3+ notably enhanced the degradation efficiency of TC-HCl by NaMe-1, with the corresponding degradation rate increasing to 0.02962 min−1. In contrast, other ions exhibited inhibitory effects, with NO3− and HCO3− having the most significant impact on degradation efficiency, while Cl− exerted minimal influence. As shown in Equations (1) and (2), CO32− in water forms HCO3−, which is capable of quenching free radicals, thus impairing the photocatalytic process. These findings highlight the varying impacts of natural organic matter and co-existing ions on the efficiency of NaMe-1 in degrading TC-HCl, with implications for optimizing photocatalytic conditions in real-world applications. NO3− significantly reduces degradation efficiency by creating a light-shielding effect and competing with TC-HCl for photogenerated carriers. Additionally, Cl− quenches free radicals through the reactions described in Equations (3) and (4), further inhibiting the degradation process. Ca2+ reacts with ·OH in the reaction system to form Ca(OH)2, which possesses light-trapping or light-avoiding properties, thereby reducing the catalyst’s ability to absorb visible light. In contrast, Fe3+ enhances TC-HCl degradation efficiency through multiple mechanisms. It interacts with free water molecules to form complexes or gels capable of adsorbing TC-HCl and exhibits intrinsic photochemical properties, generating reactive species under light exposure to promote the degradation reaction. To confirm the role of Fe3+ in enhancing TC-HCl degradation, a validation experiment was conducted by adding Fe3+ to the system without the NaMe-1 photocatalyst. The results (Figure 7i) indicated that Fe3+ could independently achieve a certain level of TC-HCl degradation within two hours of light exposure. However, the degradation efficiency of Fe3+ alone was considerably lower than that observed with NaMe-1, demonstrating that its promoting effects are supplementary to the primary photocatalytic mechanism of NaMe-1.
CO32− + H+ → HCO3−,
HCO3− + ·OH → CO32− + H2O,
·OH + Cl− → Cl· + OH−,
Cl· + Cl− → Cl2−,
To assess the practical application of NaMe-1, 20 ppm of TC-HCl was added to actual aquaculture tailwater. Details of the actual culture tailwater are compiled in Text S4. The results indicated high degradation efficiency for NaMe-1 in both tap water and culture tailwater from the Qinghai Fisheries Technology Extension Centre. However, in culture tailwater from the Qinghai Lake Naked Carp Research Centre, the degradation rate decreased to 0.01093 min−1 (Figure 8a; Table S3). This reduction may be attributed to the presence of numerous co-existing ions in the tailwater, which likely caused light-trapping or light-avoiding effects, reducing the visible light absorbed by NaMe-1 and thereby lowering catalytic activity. Nonetheless, all samples demonstrated degradation efficiencies exceeding 80%. Meanwhile, Table S6 shows that the ability of NaMe-1 to degrade TC-HCl is much higher than that of gCN, regardless of the water body. These findings confirm the capability of NaMe-1 to effectively degrade antibiotic residues in aquaculture tailwater, significantly improving water quality, minimizing toxic effects on aquatic life, and aiding in ecological restoration of water bodies. To evaluate the recyclability of NaMe-1, photocatalyst samples were recovered post-photocatalysis via centrifugation, washed, dried, and reused under identical conditions. After five cycles, the degradation efficiency of NaMe-1 towards TC-HCl decreased by only about 10%, with the rate declining from 0.0234 min−1 to 0.01527 min−1 (Figure 8b and Figure S2; Table S3). XRD analysis before and after photocatalytic reactions (Figure 8c) revealed that the (002) peak position of NaMe-1 slightly decreased compared to gCN, indicating a minor loss of the ordered structure and changes in interlayer distances during reactions. However, the overall crystal structure remained intact, demonstrating that NaMe-1 is a stable photocatalyst suitable for practical applications.

3.3. Investigation of Degradation Mechanism and Degradation Pathway of TC-HCl by NaMe-1

In the free radical trapping experiments, the degradation rate of TC-HCl decreased to 0.01527 min−1 following the addition of BQ, which caused a marked reduction in photocatalytic degradation efficiency. This rate further dropped to 0.01006 min−1 when IPA was introduced (Figure 9a,b; Table S3). In contrast, the introduction of EDTA-2Na and L-histidine did not significantly affect the degradation efficiency or rate constant, indicating that ·OH radicals are the primary contributors to the photocatalytic degradation of TC-HCl, with ·O2− as secondary contributors, while h+ and 1O2 play negligible roles in the photocatalytic activity of NaMe-1. Based on these observations, the photocatalytic mechanism of NaMe-1 against TC-HCl can be inferred (Figure 9c). Under visible light irradiation, NaMe-1 absorbs photons, exciting electrons (e−) from its valence band (VB) to the conduction band (CB), leaving holes (h+) in the VB. The e− reacts with dissolved O2 molecules to form ·O2−, which can further interact with h+ and e− to generate H2O2. This H2O2 subsequently reacts with e− to produce ·OH radicals. Additionally, OH− ions react with water molecules, yielding more ·OH radicals. These ·OH and ·O2− radicals then interact with TC-HCl molecules, breaking them down into CO2 and H2O, completing the degradation process.
To elucidate the degradation pathway of TC-HCl by NaMe-1, intermediates formed during the degradation process were identified through LC-MS analysis of samples collected at different time intervals. The peak area corresponding to TC-HCl decreased progressively with increased degradation time (Figure S4). Seven intermediates were detected and identified (Table S4), which facilitated the determination of the photocatalytic degradation pathway (Figure 10). These intermediates formed due to two primary mechanisms: loss of functional groups and ring-opening reactions. In pathway I, TC-HCl undergoes dehydration. Ji Yuefei et al. [77] described the dehydration process, and our analysis confirmed the presence of intermediate P2 (m/z = 427.15), supporting this mechanism. Dehydration, particularly under acidic conditions, likely occurs at the C6 and C5a positions, as reported by B. Halling-Sørensen et al. [78]. Intermediate P2 is thus formed in this system. Additionally, ·OH radicals contribute to the formation of P5 (m/z = 431.14), identified as 4-demethyltetracycline [79]. Under the combined attack of ·OH and ·O2−, intermediates P3 (m/z = 410.12) and P6 (m/z = 364.14) are produced, ultimately leading to small molecular compounds such as H2O and CO2. In pathway II, the double bonds in the C 11a and C 12 positions are more susceptible to ·OH attack, undergoing 1,3-dipolar cycloaddition to form the primary intermediate P7 (m/z = 461.15). This intermediate is further oxidized to generate P4 (m/z = 459.14) and P1 (m/z = 495.16) [80,81]. These findings outline a detailed mechanism by which NaMe-1 facilitates TC-HCl degradation, providing insights into the roles of specific intermediates and reactive species.

3.4. Toxicity Experiments with TC-HCl and Its Degradation Products

The intermediates generated during the degradation of TC-HCl may exhibit higher toxicity than the parent compound itself [82,83]. To evaluate the toxicological impacts of TC-HCl and its degradation by-products formed via NaMe-1, both acute and chronic toxicity simulations were performed for fish, Daphnia, and green algae using ECOSAR software (Table 2). The analysis revealed that TC-HCl posed chronic toxicity risks to Daphnia while being non-toxic to fish and green algae. During the degradation process, the toxicity of intermediates P1, P2, P3, P5, and P6 was observed. Acute toxicity assessments indicated that P1 was harmful to both Daphnia and green algae, while P3 exhibited toxicity exclusively to Daphnia. Chronic toxicity evaluations showed that P2 negatively affected both Daphnia and green algae, while P1 and P3 were harmful to fish and exhibited severe impacts on Daphnia and green algae, with Daphnia being particularly affected. Additionally, P5 and P6 displayed chronic toxicity to Daphnia, whereas P4 and P7 were non-toxic. Although some intermediates demonstrated high toxicity, the overall concentration of TC-HCl and its intermediates could be significantly reduced after 2 h of reaction with NaMe-1, lowering the associated risks to acceptable levels. These results align with the findings of Yu et al. [82], indicating that while NaMe-1 can degrade TC-HCl into less or non-hazardous products, certain intermediates retain high toxicity, necessitating further consideration in practical applications.
To further assess the toxic effects of TC-HCl and its degradation products, C. vulgaris was exposed to 20 ppm of TC-HCl and its degradation by-products. Over 96 h of continuous culture, OD680 values of algal cells exhibited dynamic changes, gradually increasing in the blank group and in cultures treated with degradation products after 2 h of photocatalysis. In contrast, the OD680 values of cells exposed to untreated TC-HCl prodrugs increased more slowly, indicating significantly inhibited growth of C. vulgaris (p < 0.001) (Figure 11a,b). This suggests that untreated TC-HCl prodrugs exerted greater toxicity on Chlorella, while the degradation products exhibited weaker toxic effects. The chlorophyll concentration of algal cells corresponded to their OD680 values. In the blank group, chlorophyll concentration increased from 1.12 ppm to 3.07 ppm over 96 h, while under TC-HCl degradation product treatment, concentrations rose from 0.92 ppm to 2.65 ppm. In contrast, untreated TC-HCl prodrugs caused chlorophyll concentration to increase only marginally, from 1.12 ppm to 1.36 ppm over the same period (Figure 11c,d). This indicates that untreated TC-HCl prodrugs caused significant damage to chloroplasts, severely inhibiting chlorophyll production (p < 0.0001). Conversely, the degradation products of TC-HCl had a much weaker impact on the chlorophyll a level, suggesting reduced acute toxicity following degradation. These findings align with prior reports indicating that TC concentrations at ppm levels can strongly inhibit the growth of C. vulgaris, degrading algal cell density and damaging chloroplasts [82]. The present study similarly demonstrates that untreated TC-HCl prodrugs are highly toxic to Chlorella, whereas the degraded products exhibit significantly reduced toxicity.
The acute toxicity of NaMe-1 photodegradation products of TC-HCl to luminescent bacteria (Vibrio fischeri) was evaluated by measuring the luminescence intensity of V. fischeri exposed to undegraded TC-HCl and to products photodegraded for 2 h with NaMe-1, at intervals of 0 min and 15 min. The results showed that TC-HCl strongly suppressed the luminescence intensity of V. fischeri, indicating significant acute toxicity (p < 0.0001). The products of NaMe-1 photocatalytic degradation after 2 h also inhibited luminescence intensity, though to a lesser extent than the undegraded TC-HCl (Figure 11e). These findings suggest that while the degraded products retain some acute toxicity, their inhibitory effects on V. fischeri luminescence are markedly reduced compared to the original TC-HCl prodrug, reflecting a partial mitigation of toxicity through photocatalytic degradation.

4. Conclusions

In this study, NaMe-x composites with distinctive microstructures were synthesized by calcination of NaCl-modified melamine precursors to examine their degradation performance for tetracycline hydrochloride and their ecotoxicity before and after degradation. The experimental results demonstrated that the addition of synthetic NaCl improved the catalytic performance of the calcined NaMe-x material, expanding its ability to absorb visible light and effectively inhibiting the formation of photogenerated electron–hole pairs compared to unmodified gCN obtained by calcination. When 1 mol·L−1 of NaCl was incorporated into the precursor, the resulting NaMe-1 exhibited the highest degradation capacity for TC-HCl. At a dosage of 0.2 g·L−1 of NaMe-1, the degradation efficiency of TC-HCl with an initial concentration of 20 ppm reached 94.36% after 120 min of visible light exposure, and the degradation rate constant was 0.239 min−1, about 2.1 times greater than that of gCN. After five cycles of recycling, the degradation efficiency decreased by only about 10%, demonstrating the high chemical stability and reusability of NaMe-1. Free radical quenching experiments suggested that ·OH and ·O2− were the primary active radicals responsible for the photodegradation of TC-HCl, and a degradation mechanism was proposed. The identification of seven intermediates during the degradation of TC-HCl by the NaMe-1 catalyst using LC-MS analysis led to the proposal of two possible degradation pathways. Toxicity calculations of the intermediate products using ECOSAR software indicated the presence of highly toxic substances in the degradation products of TC-HCl. While the undegraded TC-HCl was acutely toxic to V. fischeri and C. vulgaris, after 2 h of degradation, the products exhibited reduced acute toxicity toward these organisms. In this study, NaMe-x was synthesized by the modified precursor method, which improved the ability of pure gCN to degrade TC-HCl and degrade the ecotoxicity of antibiotics in aquaculture tailwater and provided basic data and theoretical guidance for the sustainable development of aquaculture tailwater.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su17031166/s1: Text S1. Chemicals and reagents; Text S2. Formulation of degradation efficiency and quasi-primary kinetics; Text S3. Details of the light source used in the photocatalysis experiments as a tritium light source.; Text S4. Detailed information on the actual culture tailwater.; Table S1. Liquid chromatographic parameters for detecting antibiotics; Table S2. Kinetic parameters of photocatalytic degradation of tetracycline hydrochloride; Table S3. Kinetic parameters in actual culture tailwater, free radical quenching experiments and cycling curves; Table S4. Intermediates detected by LC-MS techniques; Table S5. Comparison of photocatalytic performance of g-C3N4 composite photocatalysts for degradation of similar pollutants; Table S6. Comparison of the ability of NaMe-1 and gCN to degrade actual culture tailwater; Figure S1. Effect of different initial TC-HCl concentrations on the degradation rate of gCN (Investigation of maximum degradation); Figure S2. Exploration of material cycling properties (degradation rate graph); Figure S3. BJH pore size distribution of gCN and NaMe-x; Figure S4. Variation of peak area of TC-HCl with reaction time by LC-MS technique.

Author Contributions

Conceptualization, Y.J., Y.L. and S.L.; methodology, Y.J., Y.L. and S.L.; software, Y.J. and Y.M. (Yaqi Mao).; validation, Y.J. and Q.L.; formal analysis, Y.M. (Yongxia Ma); investigation, F.F. and Y.J.; resources, S.J.; writing—original draft preparation, Y.J. and Y.L.; writing—review and editing, Y.J., Y.L. and S.L.; visualization, Y.J. and Y.M.; supervision, Y.L. and S.L.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (42107272) and the Qinghai Provincial Science and Technology Department (2022-ZJ-923).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data supporting the reported results are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Preparation process of NaMe-x (x = 0.5, 1, 2).
Figure 1. Preparation process of NaMe-x (x = 0.5, 1, 2).
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Figure 2. (a) FT-IR pattern and (b) XRD pattern of Me and NaCl/Me-x precursors.
Figure 2. (a) FT-IR pattern and (b) XRD pattern of Me and NaCl/Me-x precursors.
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Figure 3. Characterization of NaMe-x samples. (a) Scanning electron micrographs of gCN; (b) scanning electron micrographs of NaMe-0.5; (c) scanning electron micrographs of NaMe-1; (d) scanning electron micrographs of NaMe-2; (e–i) EDS energy spectral scans of NaMe-1; (h) FT-IR spectra of gCN and NaMe-x; (i) XRD spectra of gCN and NaMe-x.
Figure 3. Characterization of NaMe-x samples. (a) Scanning electron micrographs of gCN; (b) scanning electron micrographs of NaMe-0.5; (c) scanning electron micrographs of NaMe-1; (d) scanning electron micrographs of NaMe-2; (e–i) EDS energy spectral scans of NaMe-1; (h) FT-IR spectra of gCN and NaMe-x; (i) XRD spectra of gCN and NaMe-x.
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Figure 4. (a) Nitrogen adsorption–desorption isotherms for gCN and NaMe-x; (b) UV-Vis-DRS absorption spectra; (c) Kubelka–Munk curves for gCN and NaMe-1.
Figure 4. (a) Nitrogen adsorption–desorption isotherms for gCN and NaMe-x; (b) UV-Vis-DRS absorption spectra; (c) Kubelka–Munk curves for gCN and NaMe-1.
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Figure 5. (a) XPS full spectra of gCN and NaMe-1; (b) NaMe-1 high-resolution XPS spectra of O 1s, (c) C 1s, (d) N 1s, and (e) Na 1s.
Figure 5. (a) XPS full spectra of gCN and NaMe-1; (b) NaMe-1 high-resolution XPS spectra of O 1s, (c) C 1s, (d) N 1s, and (e) Na 1s.
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Figure 6. (a) Catalytic performance of gCN and NaMe-x; (b) first-order kinetic simulation of gCN and NaMe-x.
Figure 6. (a) Catalytic performance of gCN and NaMe-x; (b) first-order kinetic simulation of gCN and NaMe-x.
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Figure 7. (a) Effect of different dosages; (b,c) effect of pH; (d) effect of different initial concentrations; (e) effect of humic acid; (f) effect of sodium acetate; (g,h) effect of co-existing ions; (i) validation test for Fe3+.
Figure 7. (a) Effect of different dosages; (b,c) effect of pH; (d) effect of different initial concentrations; (e) effect of humic acid; (f) effect of sodium acetate; (g,h) effect of co-existing ions; (i) validation test for Fe3+.
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Figure 8. (a) Application of actual aquaculture tailwater; (b) investigation of material recycling properties; (c) XRD patterns of NaMe-1 before and after five cycles of NaMe-1 compared to gCN.
Figure 8. (a) Application of actual aquaculture tailwater; (b) investigation of material recycling properties; (c) XRD patterns of NaMe-1 before and after five cycles of NaMe-1 compared to gCN.
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Figure 9. (a,b) Free radical trapping experiments; (c) photocatalytic mechanism of TC-HCl degradation by NaMe-1.
Figure 9. (a,b) Free radical trapping experiments; (c) photocatalytic mechanism of TC-HCl degradation by NaMe-1.
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Figure 10. Possible degradation pathways of TC-HCl over NaMe-1 catalyst (pathway I on the left; pathway II on the right).
Figure 10. Possible degradation pathways of TC-HCl over NaMe-1 catalyst (pathway I on the left; pathway II on the right).
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Figure 11. (a,b) Changes in OD680 of C. vulgaris; (c,d) changes in chlorophyll a concentration of C. vulgaris; (e) changes in toxicity of luminescent bacteria. * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.001.
Figure 11. (a,b) Changes in OD680 of C. vulgaris; (c,d) changes in chlorophyll a concentration of C. vulgaris; (e) changes in toxicity of luminescent bacteria. * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.001.
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Table 1. The physical properties of as-prepared samples.
Table 1. The physical properties of as-prepared samples.
SamplesSBET(m2·g−1)Vtotal (cc·g−1)DBJH (nm)
gCN19.4100.143.70
NaMe-0.555.1530.393.71
NaMe-155.2090.413.71
NaMe-245.7540.303.71
Table 2. Acute and chronic toxicity analyses of TC and its major degradation products during the degradation of tetracycline hydrochloride by NaMe-1 by the Ecological Structure Activity Relationship (ECOSAR) software.
Table 2. Acute and chronic toxicity analyses of TC and its major degradation products during the degradation of tetracycline hydrochloride by NaMe-1 by the Ecological Structure Activity Relationship (ECOSAR) software.
Acute Toxicity (ppm)Chronic Toxicity (ppm)
Intermediate ProductsFish (LC50)Daphnid (LC50)Green Algae (EC50)Fish (ChV)Daphnid (ChV)Green Algae (ChV)
TC-HCl13,10010601890249059.90474
P1 m/z = 495.1643446.1047.9036.103.3814.60
P2 m/z = 427.1512101181461337.9041.80
P3 m/z = 410.1292691.4011096.106.2431.80
P4 m/z = 459.14332,00021,00060,700133,00093412,800
P5 m/z = 431.1417,50013802570357076.10635
P6 m/z = 364.14511043869381226.10181
P7 m/z = 461.1528,1002150427063101151030
Note: Green: non-hazardous (LC50/EC50/ChV > 100 ppm); yellow: hazardous (100 ppm > LC50/EC50/ChV > 10 ppm); red: toxic (10 ppm > LC50/EC50/ChV > 1 ppm).
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Jiao, Y.; Mao, Y.; Liu, Q.; Ma, Y.; Fu, F.; Jian, S.; Liu, Y.; Lu, S. Molten-Salt-Assisted Preparation of g-C3N4 for Photocatalytic Degradation of Tetracycline Hydrochloride: Degradation Mechanism, Pathway, and Toxicity Assessment. Sustainability 2025, 17, 1166. https://doi.org/10.3390/su17031166

AMA Style

Jiao Y, Mao Y, Liu Q, Ma Y, Fu F, Jian S, Liu Y, Lu S. Molten-Salt-Assisted Preparation of g-C3N4 for Photocatalytic Degradation of Tetracycline Hydrochloride: Degradation Mechanism, Pathway, and Toxicity Assessment. Sustainability. 2025; 17(3):1166. https://doi.org/10.3390/su17031166

Chicago/Turabian Style

Jiao, Yujie, Yaqi Mao, Qikai Liu, Yongxia Ma, Fei Fu, Shenglong Jian, Yang Liu, and Sujin Lu. 2025. "Molten-Salt-Assisted Preparation of g-C3N4 for Photocatalytic Degradation of Tetracycline Hydrochloride: Degradation Mechanism, Pathway, and Toxicity Assessment" Sustainability 17, no. 3: 1166. https://doi.org/10.3390/su17031166

APA Style

Jiao, Y., Mao, Y., Liu, Q., Ma, Y., Fu, F., Jian, S., Liu, Y., & Lu, S. (2025). Molten-Salt-Assisted Preparation of g-C3N4 for Photocatalytic Degradation of Tetracycline Hydrochloride: Degradation Mechanism, Pathway, and Toxicity Assessment. Sustainability, 17(3), 1166. https://doi.org/10.3390/su17031166

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