Enhanced visible light-driven photodegradation of tetracycline by salicylic acid-modified graphitic carbon nitride and toxicity assessment

The tetracycline (TC) in water has led to serious concern. Graphitic carbon nitride (g-C3N4) photocatalysts were produced via copolymerization of mono-benzene ring-mediated precursors (urea, melamine, and dicyandiamide) involving salicylic acid (SA) for TC degradation. The SA-modified g-C3N4 samples showed improved visible light absorbance, transfer and separation of photogenerated electrons, and prospective photocatalytic application in TC degradation. As a result, the optimal SA-modified g-C3N4 (2 wt% of SA) using urea (CNU-SA-2) showed 2 times higher TC degradation than that of pristine g-C3N4. The process of TC degradation was evaluated by the reduction of antibacterial activity and extensively studied by varying the types of TC, initial pH values, co-existing anions, and natural organic materials. In addition, the catalyst could be reused for at least four cycles, indicating good reusability. The main active species were revealed to be h+ and ·O2− by scavenging experiments and electron spin resonance. The CNU-SA-2 photocatalyst and TC intermediates during degradation had no adverse impact on zebrafish embryos. This work could provide a design strategy and a perspective on the practical application of g-C3N4-based photocatalysts for the treatment of wastewater containing antibiotics.


Introduction
Antibiotics have been extensively used in human health, animal husbandry, and agriculture. The existence of antibiotics in aquatic environments leads to the development of antibiotic-resistant bacteria and genes, posing threats to organisms and humans . Tetracycline (TC), as a typical antibiotic, is frequently detected in water (Wei et al. 2010). Conventional wastewater treatment approaches, such as filtration, adsorption, and biodegradation, cannot completely degrade TC because of the naphthalene ring structure and hydrophilic property (Watkinson et al. 2007). Recently, solar light-driven nanomaterial-based semiconductor photocatalysis is a promising strategy due to its high efficiency, easy operation, high mineralization rate, and environmental compatibility (Li et al. 2022;Lin et al. 2021).
Among the semiconductor photocatalysts, metal-free graphitic carbon nitride (g-C 3 N 4 ) is a highly expected candidate for wastewater remediation. g-C 3 N 4 can be prepared using a simple thermal condensation with economic precursors (urea, melamine, and dicyandiamide). g-C 3 N 4 possesses high physicochemical stability as well as unique optical and electronic properties . However, the ineffective light absorption and fast photogenerated charge recombination of g-C 3 N 4 restrict its practical application (Liu et al. 2019a;Zhou et al. 2019b). To address the limitations, several efforts including nanostructure design (Liu et al. 2019b;Xia et al. 2017), heterojunctions with semiconductors, and copolymerization (Zhang et al. 2012) have been conducted. The copolymerization of g-C 3 N 4 precursors with non-metal aromatic structure is a promising technique since metal leaching into the aquatic environment, especially for toxic ones like Ag + , Co 2+ , and Zn 2+ , can be avoided. Additionally, the aromatic structure is similar to that of the triazine phase of g-C 3 N 4 . Anchoring aromatic rings in the g-C 3 N 4 network could regulate the intrinsic electronic property and manipulate the bandgap. The photocatalytic activity is consequently enhanced by increasing visible light harvesting and improving charge pair separation. The photocatalytic performance of g-C 3 N 4 could be improved by copolymerizing organic acid (anthroic acid, benzoic acid, and naphthoic acid) with urea due to the fact that the benzene ring in the g-C 3 N 4 framework could increase the visible light absorbance . Salicylic acid (SA) has a benzene-based structure and was proved to improve photogenerated charge carrier transmission of g-C 3 N 4 nanotubes after the copolymerization with melamine (Jia et al. 2019).
In previous studies, SA-modified g-C 3 N 4 showed excellent performance in the degradation of sulfamethazine and TC (Zhou et al. 2019a). However, it is not yet known if g-C 3 N 4 with benzene ring synthesized using different precursors have optimized degradation activities. At the same time, the mechanism responsible for TC removal needs to be clarified. On the other hand, in the process of degradation, the treatment of antibiotic-contaminated wastewater has recently raised new concerns because of the generation of high toxic intermediates (Osin et al. 2018;Zhao et al. 2020). For example, Han et al. (2020) reported that the toxicity of a tetracycline solution increased with the increased photocatalytic time and the intermediate (m/z 477.15) was deemed as the compound leading to the solution toxicity. Additionally, Zhao et al. (2020) found that many products with more toxicity were generated during the degradation of tetracycline. But, to the best of our knowledge, there are no reports to evaluate the possible toxicity of the transformation products during the degradation process (benzene ring-modified g-C 3 N 4 system). Meanwhile, the increasing applications of g-C 3 N 4 photocatalysts have also triggered safety concerns (He et al. 2020;Lin et al. 2018). Their potential risks are seldomly reported. So, the ecotoxicity of the benzene ring-modified g-C 3 N 4 photocatalysts, TC, and the intermediates needs assessment, which is a crucial factor of the potential application.
In this report, g-C 3 N 4 photocatalysts with benzene ring were constructed by thermal copolymerization of salicylic acid (SA) with different precursors (urea, melamine, and dicyandiamide). Salicylic acid was chosen as the typical acid with an aromatic ring due to its widely use in the pharmaceutical industry. The photocatalytic performance was investigated by TC degradation under visible LED light irradiation. The effects of the initial pH values, the co-existing ions, natural organic matter, and types of TCs were studied. Regeneration tests were investigated as well to assess the reusability. The mechanism of TC photodegradation was determined. The toxicity of the photocatalysts and TC degradation intermediates was evaluated using zebrafish embryo and/or bacteria inhibition tests. This study presents a reference for modifying g-C 3 N 4 with aromatic structure and facilitates the practical application of g-C 3 N 4 -based photocatalysts.

Material synthesis
All the chemicals used in this study were analytical reagent grade, and detailed information is shown in Text S1.
The g-C 3 N 4 samples (CNU, CNM, and CND) were prepared from urea, melamine, and dicyandiamide by heating at 550 °C (5 °C/min) in a muffle furnace for 2 h. The SA-modified CNU, CNM, and CND were synthesized by mixing 10 g of precursors (urea, melamine, and dicyandiamide) and fixed amounts of SA (0.05, 0.2, and 0.3 g) in 15 mL of 33% (V/V) ethanol solution for 2 h. Then, the mixed solutions were treated with ultrasonication for 0.5 h and then dried at 80 °C overnight. Subsequently, the dried mixtures were annealed at 550 °C (5 °C/min) in a muffle furnace for 2 h. The obtained solids were ground into powder. The synthesized samples were denoted as CNU-SA-X , CNM-SA-x, and CND-SA-x, where x represents the mass percentage of SA.

Characterizations
X-ray diffraction (XRD) with Cu Kα as the radiation source was used to study the crystal phase of the synthesized photocatalysts (Rigaku Ultimate IV, Japan). Fourier transform infrared spectroscopy (FT-IR) was conducted on a Thermo Scientific Nicolet iS5 FT-IR spectrometer between wavenumbers of 4000 and 400 cm −1 . The detailed morphologies of the samples were observed using scanning electron microscopy (SEM) (Zeiss, Sigma 300, Germany). The specific surface areas and pore volume of the photocatalysts were determined by the Brunner-Emmet-Teller (BET) analysis (ASAP2460, USA). X-ray photoelectron spectroscopy (XPS) analysis was performed by an XPS analyzer (Thermo Fisher Scientific, K-alpha, USA). UV-Vis diffuse reflectance spectroscopy (DRS) was obtained in 200-800 nm region (UV-3600, Japan). The analysis of electron spin resonance (ESR) aimed to identify the active species using the trap reagent (DMPO) under the Xenon lamp (Bruker Corporation, Bruker A300, USA). The transient photocurrent responses and electrochemical impedance spectroscopy (EIS) were performed in a three-electrode cell with a CHI-760E workstation (USA).

Photocatalytic experiments
The photocatalytic activity was tested via the degradation of TC (20 mg·L −1 ) in visible LED light (5 W, λ > 420 nm) (PCX-50B Multichannel Photochemical Reaction System, Beijing). A total of 20 mg of samples was suspended in 40 mL TC solution. The suspension was magnetically stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. Afterwards, the suspension was irradiated in visible light for 180 min. A total of 1.0 mL suspension was extracted at given time intervals and filtered with a filter membrane (0.22 μm). Then, the residual TC was analyzed by a microplate reader (Thermo Fisher Scientific, Varioskan LUX, USA) at 357 nm . The removal efficiency of TC and the first-order kinetics could be calculated by the following equations (Eqs. (1) and (2)): where η represents the removal efficiency of TC; C 0 represents the initial concentration of TC; C t represents the concentration of TC at irradiation time (t); k is the first-order rate constant.

Toxicity evaluation
The toxicity of TC and its intermediates was monitored using Gram-negative bacteria (Escherichia coli) and Grampositive bacteria (Bacillus subtilis) as the targets. A total of 0.5 g/L CNU-SA-2 was used for TC degradation. In the test, the parent TC and intermediates at specific time intervals were mixed into the culture medium with bacteria separately. The bacterial suspensions were incubated at 37 °C for 24 h at 160 rpm. The antimicrobial toxicity was estimated by measuring the absorbance of solutions at 600 nm, and calculated according to Eq. (3) (Coledam et al. 2016): where A 0 and A are the absorbance in the absence and presence of photocatalysts, respectively. The acute toxicity of catalysts, TC solution, and treated TC solutions were assessed using zebrafish embryos (He et al. 2020). In each well of 96-well plates, one zebrafish embryo and 200 μL of suspension were added. H-buffer solution was used as the negative control. These culture plates were incubated at 28 ± 0.5 °C. The survival rate and hatchability of the zebrafish embryos were observed at 72 h. Each treatment was carried out in three replicates of 12 embryos each.

Structure and morphology characterization
The XRD patterns were applied to analyze the crystal structure of the samples (Fig. 1a). All prepared samples had two typical diffraction peaks of g-C 3 N 4 . The prominent diffraction peaks at ~12.8° and ~27.5° were associated with (100) and (002) planes of g-C 3 N 4 , which belonged to the in-plane structural packing motif of tri-s-triazine units and interlayer stacking of aromatic segments, respectively (Chi et al. 2022;Dong et al. 2018). Once SA was introduced, the (002) peak intensities of CNU-SA-2, CNM-SA-0.5, and CND-SA-0.5 decreased due to the disturbance of graphitic structure. The changes in peak intensity confirmed that SA was introduced in the framework of g-C 3 N 4 . Additionally, FT-IR spectra (b) of synthesized g-C 3 N 4 photocatalysts the (002) peak intensity of g-C 3 N 4 prepared with urea (CNU and CNU-SA-2) was weaker than those of g-C 3 N 4 prepared with melamine and dicyandiamide (CNM and CND, CNM-SA-0.5 and CND-SA-0.5), demonstrating relatively loose structure (Luo et al. 2020). The functional groups of synthesized samples were analyzed by FT-IR spectroscopy (Fig. 1b). All samples maintained the same characteristic signals, indicating that the SA modification and different precursors did not change the functional groups of g-C 3 N 4 . The peaks from 1200 to 1700 cm −1 belonged to the typical stretching modes of C-N heterocycles ). The small peak at ~813 cm −1 represented the characteristic breathing mode of s-triazine (Yu et al. 2021). The broad absorption bands at 3000-3400 cm −1 were assigned to the vibration modes of the N-H and O-H stretches (Feng et al. 2016;Qin et al. 2021).
The sur face composition of CNU-SA-2 was characterized by XPS spectra (Fig. S1). Three peaks of C 1s, N 1s, and O 1s appeared in CNU-SA-2 (Fig. S1a). The C/N molar ratio in the CNU-SA-2 was 0.71, which was consistent with the theoretical value of g-C 3 N 4 (0.75). As shown in the high-resolution C 1s spectra (Fig. S1b), two characteristic peaks at ~288.0 eV and ~284.5 eV attributed to aromatic carbon atoms (N-C=N) and the sp 2 hybridized carbon (C-NH 2 ) respectively . Three peaks at ~400.7 eV, ~399.6 eV, and ~398.6 eV in the high-resolution N 1s spectra were assigned to C-NH, N-C 3 , and C-N=C groups, respectively (Fig. S1c). For O 1s spectrum (Fig. S1d), peaks at ~531.8 eV, ~531.9 eV, and ~532.5 eV belonging to -OH, C=O groups, and N-C-O bonds respectively were observed (Jia et al. 2019;Zhou et al. 2019a).
The SEM images of CNU, CNM, and CND exhibited similar stacked nanosheet morphologies with a porous surface (Fig. 2a, b, c). CNM and CND showed a tight aggregation of nanoplates. CNU had loose sheets with reduced thickness due to the gaseous CO 2 and H 2 O molecules generated during synthesis. The reduction in thickness of CNU might benefit the migration of charge carriers and introduce more active sites. After SA addition, the nanosheets of CNU-SA-2 tended to curve, and CNM-SA-0.5 and CND-SA-0.5 presented similar morphologies as CNM and CND (Fig. 2d, e, f).
The specific surface area and porous structure of synthesized photocatalysts were verified by N 2 adsorptiondesorption isotherm (Fig. S2). All samples presented type IV isotherms with H1 hysteresis loops, indicating the formation of mesoporous structure. Compared to CNM (26.89 m 2 /g) and CND (20.43 m 2 /g), CNU possessed a higher surface area (90.11 m 2 /g) (Table S1). After SA addition, the specific surface area of CNU-SA-2 decreased while the pore volume reduced from 0.56 to 0.19 cm 3 /g since the addition of SA destructed the intrinsic structural units and collapsed the pores. The specific surface area of CNM-SA-0.5 and CND-SA-0.5 had no obvious change, which illustrated that the introduction of SA did not cause lattice collapse in g-C 3 N 4 prepared with melamine and dicyandiamide.
To confirm efficient photogenerated charge separation and migration after the addition of SA in CNU, the photocurrent response was investigated. Figure 3c shows the transient photocurrent measurements of CNU and CNU-SA-2 via several on-off cycles under visible Xenon lamp irradiation (200 W). CNU-SA-2 displayed higher photocurrent response than CNU, implying reduced photo-induced charge carrier recombination due to the introduction of SA. The smaller impedance arc radius of CNU-SA-2 than that of CNU ( Fig. 3d, measured by EIS) suggested that the addition of SA promoted photogenerated charge transfer efficiency.

Photocatalytic performance of synthesized samples
The photocatalytic performance was studied by degrading TC under visible LED light (Fig. 4). The adsorption-desorption equilibrium could be reached in 30 min in the dark. The adsorption efficiency of all samples was less 14.7%. The CNU and CNU-SA-x products had higher adsorption capacity than the CNM, CNM-SA-x, CND, and CND-SA-x, due to the relatively higher specific surface area (Table S1).
TC was hardly degraded without catalysts under visible light irradiation. All SA-modified products using urea (Fig. 4a), melamine (Fig. 4b), and dicyandiamide (Fig. 4c) precursors exhibited superior photocatalytic performance than the corresponding pristine g-C 3 N 4 . After the addition of SA, the highest reaction rate rates (k) were observed ( Fig. 4 (inserts)), suggesting that optimal doping amounts of SA could enhance the photocatalytic efficiency. It could be seen that CNU-SA-x had the highest photocatalytic efficiency among CNU-SA-x, CNM-SA-x, and CND-SA-x photocatalysts. Moreover, the CNU-SA-2 displayed the best photocatalytic activity of 82.3% within 180 min under the visible LED light among CNU-SA-x photocatalysts. The k value of CNU-SA-2 (0.0095 min −1 ) was about twice high as

Effect of the initial pH
The initial pH is an important parameter in the photocatalysis. The effect of the initial pH on photocatalytic activity was assessed in the pH range of 3-10 (Fig. 5a). The removal efficiency of TC increased from 74.0 to 82.3% with the increase of pH from 3.08 to 5.53, then decreased to 80.6% and 77.8% with further increase of pH to 8.12 and 10.04. The TC degradation efficiency peaked at 82.3% with an initial pH of 5.53. TC molecules exist as TCH 3 + cationic species (pH < 3.3), zwitterionic species (pH = 3.3-7.7), TCH − anionic species (7.7 < pH < 9.6), and TC 2− anionic species (pH > 9.6) ( Table S2) (Ye et al. 2019;Zhao et al. 2020). The pH pzc of CNU-SA-2 was ~5.73 (Text S2 and Fig. S3).  CNU-SA-2 is positively charged when solution pH is < 5.73, and is negatively charged as pH is > 5.73. The electrostatic interaction between CNU-SA-2 and TC was repulsive at pH 3.08, 8.12, and 10.04. To the best of our knowledge, there are no reports focusing on adsorption performance of g-C 3 N 4 toward antibiotics on an atomistic level. It is worth mentioning that the adsorption energy between g-C 3 N 4 and TC should be calculated by density functional theory (DFT) in the future work. The electrostatic repulsion between CNU-SA-2 and TC was lowest at pH 5.53, resulting in the highest adsorption capacity promoting further TC degradation (Li et al. 2022). On the other hand, photodegradation of TC was favored under the weakly alkaline condition (pH 8.12 and pH 10.04) since the deprotonated TC species are instable . Overall, the photocatalytic degradation performance of TC by CNU-SA-2 remained above 74.0% over a wide pH range, revealing potential practical application in real-life water bodies.

Effect of inorganic anions and natural organic matter
As an integral part of real-life water bodies, the inorganic anions (Cl − , CO 3 2− , H 2 PO 4 − , and SO 4 2− ) and natural organic matters (humic acid (HA)) may affect the degradation performance of photocatalysts (Du et al. 2021;Zhao et al. 2021). Therefore, it is necessary to explore their influences on the photocatalytic activity (Fig. 5b). The degradation efficiency of TC fell from 82.3 to 54.1% in the presence of HA (10 mg/L), which might be due to the light attenuation and radical scavenger of HA, and competition of adsorption active sites between HA and TC .
By contrast, the removal efficiency of TC by CNU-SA-2 in the presence of different anions declined at acceptable levels. The H 2 PO 4 − and SO 4 2− hindered the adsorption capacity of CNU-SA-2 due to competition between the anions and TC, while other anions had no significant effect on the adsorption process. The H 2 PO 4 − and SO 4 2− anions could react with h + and •OH, thus leading to the inhibition of TC degradation. The coexistence of Cl − had little negative effect on the degradation of TC due to generation of Cl• with weaker oxidative ability ). The CO 3 2− anion could consume h + and •OH to induce CO 3 − with less reactivity to affect TC degradation .

Degradation of four different TCs
To testify the universal effectiveness, the removals of various TCs (tetracycline (TC), chlortetracycline (CTC), oxytetracycline (OTC), and doxycycline (DXC)) were conducted (Fig. 5c). Physical and chemical information of TC, CTC, OTC, and DXC is shown in Table S2. CNU-SA-2 could absorb TC (14.7%) and DTC (7.6%) at an initial concentration of 20 mg/L, while no obvious adsorption toward CTC and OTC. The subsequent 180-min photocatalysis under visible light increased the removal efficiency of TC, DTC, CTC, and OTC to 82.3%, 55.3%, 72.1%, and 46.9%, respectively. These results demonstrated that the CNU-SA-2 photocatalyst could damage the tetraphenyl backbone of the TCs.

Recyclability
The reusability of photocatalysts also plays significant roles in the practical applications. The performance of four repeated cycle tests of CNU-SA-2 on TC degradation was studied. After each cycle, the used CNU-SA-2 was separated by centrifugation, washed thoroughly with water, and dried at 80 °C. As shown in Fig. 5d, CNU-SA-2 exhibited a good stability and maintained the photocatalytic efficiency of ~80% after four cycles. Hence, the results indicated CNU-SA-2 catalyst possessed excellent reusability in the photocatalytic application.

Photocatalytic mechanism
The above results suggested that the incorporation of benzene ring to g-C 3 N 4 could boost the photocatalytic efficiency. To further verify the conjugated effect of the numbers of benzene rings on the photocatalytic performance of CNU, CNU-naphthoic acid was synthesized by thermal polymerization of urea and naphthoic acid with two benzene rings. The amount of naphthoic acid was estimated based on the portion of benzene rings (56.5 wt%) in SA. As shown in Fig. 6a, CN-SA-2 showed the highest photocatalytic activity for TC degradation compared to CNU and CNUnaphthoic acid. The TC degradation efficiency in the presence of CNU-naphthoic acid was 67.4% in 180 min, i.e., a 9.6% enhancement relative to pristine CNU. Thus, the single/double benzene ring-modified CNU exhibited higher photocatalytic efficiency than that of CNU due to the accelerated mobility of electron-hole pairs in the modified g-C 3 N 4 photocatalysts. But the photocatalytic performance of CNU-naphthoic acid was inferior to that of CNU-SA-2. The double benzene rings decorated in the CNU network might result in weaker separation of photogenerated charge carriers . Therefore, a single benzene ring was crucial to improve the photocatalytic performance for TC degradation. More importantly, the results revealed that the conjugation of aromatic rings with g-C 3 N 4 was important to improve photocatalytic activity and brought a direction for designing polycyclic aromatic compound-modified g-C 3 N 4 as efficient photocatalysts in the future work.
To investigate the mechanism of CNU-SA-2 on the TC degradation under visible light, the generated active species were determined using the free radical trapping tests. Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, 1 mM), isopropanol (IPA, 1 mM), and benzoquinone (BQ, 1 mM) acted as the quenchers of holes (h + ), hydroxyl radical (•OH), and superoxide radical (•O 2 − ) respectively. As shown in Fig. 6a, the addition of IPA had a minor effect on the photocatalytic efficiency, while the addition of EDTA-2Na and BQ had significant effects. The results indicated that h + and •O 2 − played important roles for the degradation of TC. ESR technique was further employed to detect the reactive species (•OH and •O 2 − ) in the photocatalytic system (Fig. 6b, c). DMPO-OH and DMPO-O 2 − were not detected in the dark but became detectable under visible light irradiation for 10 min, confirming that CNU-SA-2 could form free radicals under light irradiation, same as the above trapping experiments. Based on the above analyses, the mechanism of TC removal by CNU-SA-2 is proposed in Scheme 1. Under visible light irradiation, CNU-SA-2 with a single benzene ring can be excited, where electrons (e − ) are produced in the conduction band and h + in the valence band. The e − in

Toxicity assessment
Considering the real-life application of the synthesized CNU-SA-2 photocatalyst, it was necessary to assess the potential environmental impact of TC and its degradation products. Firstly, antimicrobial tests based on E. coli and B. subtilis were performed. As shown in Fig. 7a, the E. coli and B. subtilis inhibition ratio gradually fell to 32.2% and 25.6%, respectively after 180 min of TC degradation. The decrease in the bacterial inhibition ratio indicated that most intermediates had reduced antibacterial activity. Toxicity assessment is important in some cases, which is due to the toxicity of some degradation TC products that may be higher than that of the parent TC. The zebrafish embryo is an attractive biological model due to its fecundity, optical transparency, and genetic malleability (Sharma and Saneja 2022). Furthermore, the acute toxicity of TC and the degraded intermediates on zebrafish embryos were also examined. The hatchability and survival rate of zebrafish embryos in the CNU-SA-2 photocatalytic system was investigated (Fig. 7b). TC solution without any treatment showed 100% hatchability and survival rate of the embryos after an incubation of 72 h. There was no significant change in hatchability and survival rate of the zebrafish embryos with increasing duration of the CNU-SA-2 photocatalytic reaction (Fig. 7b). It was reasonable to conclude that the TC intermediates had no toxic impact on the zebrafish embryos.
The biosafety of CNU-SA-2 photocatalyst was also evaluated using zebrafish embryos. Zebrafish embryos were treated with CNU-SA-2 at different dosages (0.13, 0.25, 0.50, 0.75, and 1.0 g/L) for 72 h. The hatching and survival rates of zebrafish embryos were above 80% during the exposure period (Fig. 7c), indicating no significant toxicity of CNU-SA-2 on zebrafish embryos.

Conclusion
In summary, a series of SA-modified g-C 3 N 4 samples were synthesized via copolymerization using different precursors (urea, melamine, and dicyandiamide). The doping of SA with a benzene ring in g-C 3 N 4 framework showed extended visible light absorbance and decreased recombination of electron-hole pairs, leading to effective TC removal under visible light. The CNU-SA-2 achieved a photocatalytic rate of 0.0095 min −1 for TC degradation, which was 2 times higher than that of CNU. Various experimental conditions (initial pH values, co-existing anions, HA, and different TCs) were evaluated to demonstrate the potential of the photocatalyst for environmental remediation. Additionally, the photocatalytic activity of CNU-SA-2 remained stable after four cycle tests. The trapping tests and ESR analyses revealed that h + and ·O 2 − were the main active species in the photocatalytic process of TC degradation. The bacterial inhibition tests and hatching/survival tests of zebrafish embryos revealed the reduced toxicity of the intermediates and no acute toxicity of CNU-SA-2. This study offers a scheme for improving photocatalytic activity of g-C 3 N 4 for remediation of real-life antibiotic-contaminated water.
Author contribution Mengmeng Chen: conceptualization, data curation, investigation, methodology, writing-review and editing. Mengxue Li: writing-review and editing. Peng Li: writing-review Fig. 7 Antibacterial activity of effluent during the photocatalytic degradation of TC by CNU-SA-2 (a); hatching rate and survival rate of zebrafish embryos exposed to TC and its intermediates in CNU-SA-2 photocatalytic system (b), and at different dosages of CNU-SA-2 (c)