Antimicrobial properties of heterojunction BiSnSbO6-ZnO composites in wastewater treatment

BiSnSbO6-ZnO composite photocatalytic material with type II heterojunction structure was synthesized by a simple solid-phase sintering method, it was characterized by XRD, UV–vis, and PT methods. The photocatalytic antibacterial experiments were carried out under LED light irradiation. The experimental results showed that the photocatalytic antibacterial properties of BiSnSbO6-ZnO composites against bacteria and fungi were significantly stronger than those of single BiSnSbO6 and ZnO. Under light conditions, the antibacterial efficiencies of 500 mg/L BiSnSbO6-ZnO composites against E. coli, S. aureus, and P. aeruginosa reached 99.63%, 100%, and 100% for 6 h, 4 h, and 4 h, respectively. The best antibacterial concentration of BiSnSbO6-ZnO composite against the eukaryotic microorganism Candida albicans was 250 mg/L, and the antibacterial efficiency reached the highest 63.8% at 6 h. Antibacterial experiments were carried out on domestic livestock and poultry wastewater, which showed that the BiSnSbO6-ZnO composite photocatalytic material has broad-spectrum antibacterial activity against bacteria, and the antibacterial effect has species differences. Through the MTT experiment, it is proved that the prepared BiSnSbO6-ZnO composite photocatalytic material has no toxicity at the experimental concentration. According to the free radical scavenging experiment and SEM observation of the morphological changes of the bacteria after light treatment, the prepared BiSnSbO6-ZnO composite photocatalytic material can generate active species OH, h+, and e− through light irradiation to achieve the purpose of sterilization, where e− play a major role, indicating that the BiSnSbO6-ZnO composite photocatalytic material has broad application prospects in the actual antibacterial field.


Introduction
With the enhancement of people's health awareness, the requirements for the living environment are also continuously improved, and we gradually realize that bacteria exist in all aspects of our daily life and are one of the microorganisms that humans are in close contact with. Bacteria are not only the main participants in the material cycle of nature but also play an important role in human body functions. However, the problem of microbial contamination caused by bacteria and fungi in global environmental pollution cannot be ignored. Due to the high adaptability of microorganisms, it has always been a challenging problem to efficiently remove harmful microorganisms in environments such as drinking water treatment, food processing, medical device manufacturing, or textile industry (Qu et al. 2013;Vikesland and Wigginton 2010;Hajipour et al. 2021). Even with the rapid development of science and technology, research on antibacterial agents that inhibit the growth and reproduction of pathogenic microorganisms is still a crucial task, so photocatalytic antibacterial materials have emerged as the times require. Photocatalytic antibacterial materials are a kind of antibacterial materials (Surapaneni et al. 2015). Inorganic antibacterial materials have many advantages such as long-lasting antibacterial and environmental protection Responsible Editor: Angeles Blanco Jing Chen and Mengde Shan contributed equally to this work and should be considered co-first authors. (Sun et al. 2021). The composite antibacterial material refers to the composite material of two or more kinds of antibacterial materials combined together to form a composite material with a more excellent antibacterial effect. At present, the field of composite photocatalytic materials has been well developed because of their better photocatalytic performance, higher light utilization, and better chemical stability compared with single catalytic materials (Liang et al. 2019;Xu et al. 2014;Ghafoor et al. 2018;Yang et al. 2018;Huang et al. 2019;Gratzel. 2018).
Among all photocatalytic antibacterial materials, n-type ZnO has been widely studied in photocatalytic applications due to its unique electronic structure and suitable properties and is a new type of antibacterial material (Cunha et al. 2020;Hu et al. 2019;Singh et al. 2019;Ozdemir and Soyer 2020). However, ZnO has certain defects, such as a wide bandgap around 3.2 eV, which makes the activity of ZnO limited to the ultraviolet region and not active in the visible region (V. Kumar et al. 2017;Aditya et al. 2018;Qi et al. 2017;Rahman et al. 2021). But this defect can be ameliorated by doping or forming composites with narrow-bandgap materials (Yan et al. 2018;Wang 2004;Podasca et al. 2016;Wang et al. 2018;Rokhsat and Akhavan 2016).
In the past 10 years, due to the advantages of narrow band gap, low price, and wide light absorption range, bismuthbased photocatalytic materials have entered people's field of vision as a visible light catalytic material with excellent performance (Ma et al. 2018;Wu et al. 2019;M. Kamli et al. 2021). There has been significant research progress in theoretical research and practical application of bismuth-based photocatalytic materials (Rather and Lo 2020). The research on binary and multi-component bismuth-based photocatalytic composite materials has gradually deepened, and its application in various fields has gradually become wider. However, there is still no systematic conclusion about its specific photocatalytic mechanism, and the research on the reaction mechanism and energy band structure of bismuthbased photocatalytic composites still needs to be strengthened. In 2019, researchers synthesized β-AgVO 3 /BiVO 4 composite visible light catalytic antibacterial material. After being excited by visible light irradiation, β-AgVO 3 / BiVO 4 composite material showed strong resistance to P. aeruginosa . The photocatalytic antibacterial activity, which can be attributed to the formation of a heterojunction structure in the β-AgVO 3 /BiVO 4 composite, greatly improves the separation degree and lifetime of photogenerated charges in the composite . In 2018, Jingfei Luan et al. synthesized BiSnSbO 6 , which has a pyrochlore structure and has photocatalytic degradation activity for BZT and RhB under visible light irradiation (Luan and Huang 2018). The surface of pyrochlore structural materials has more oxygen vacancies, which can improve the photon efficiency, and has shown great potential in the field of photocatalysis applications. However, the problem with Bi-based photocatalytic material BiSnSbO 6 is that it can only absorb and utilize visible light, which greatly reduces its application range and limits its application.
In this work, a simple solid-phase sintering method was used to synthesize BiSnSbO 6 materials, and then different ratios of BiSnSbO 6 -ZnO composite photocatalytic materials were prepared by changing the molar ratio. The crystal structure characteristics and photoelectric properties of the prepared BiSnSbO 6 -ZnO composite photocatalytic materials were investigated by XRD, EDS, SEM, EIS, and other characterization methods. E. coli, S. aureus, P. aeruginosa, and Candida albicans were used as model bacteria, to explore the antibacterial activity of BiSnSbO 6 -ZnO composite photocatalytic material under LED light illumination. Using L929 cells as model cells, in vitro cytotoxicity experiments were carried out. In order to understand the reasons for the enhanced antibacterial activity of BiSnSbO 6 -ZnO composite photocatalytic materials, the photocatalytic antibacterial mechanism was explored through SEM observation of bacterial surface morphology and free radical scavenging experiments, and the structure and reactive oxygen generation of BiSnSbO 6 -ZnO composite photocatalytic materials were analyzed relationship. Finally, the broad-spectrum antibacterial activity and practical applicability of the composite material were determined by the livestock and poultry wastewater collected in real life. E. coli (ATCC 25,922), S. aureus (ATCC 6538), P. aeruginosa (ATCC 9027), and Candida albicans (ATCC 10,231) used in this experiment were from the China Culture Collection Center. The LB medium was used as the nutrient source for bacteria, and the Martin medium was used as the nutrient source for fungi. Bi 2 O 3 , SnO 2 , Sb 2 O 5 , ZnO, and NaCl were purchased from Aladdin Biochemical Technology Company. Agar, glutaraldehyde, and isoamyl acetate were from Obersin Biotechnology.

Preparation of BiSnSbO 6
The solid phase sintering method is used. Mix Bi 2 O 3 : SnO 2 : Sb 2 O 5 in a molar ratio of 1:2:1 (Huang et al. 2019), put the materials in an agate mortar and mix and grind, and dry for 4 h. Put the ground powder into a corundum crucible and put it into a high-temperature muffle furnace for calcination. The calcination temperature changed as follows: 80 min from 20 to 350 °C, hold for 240 min, 80 min to 720 °C, hold for 240 min, and finally, 60 min from 720 to 900 °C, hold for 1500 min.

Preparation of BiSnSbO 6 -ZnO composite
The first step was to weigh and mix BiSnSbO 6 and ZnO in a certain molar ratio. The second step was to grind the mixed material. The third step was to put the powder into a corundum crucible and press it into a cake, paying attention to the thickness. Finally, it was put into a high-temperature muffle furnace for calcination. The BiSnSbO 6 -ZnO composite photocatalytic material was obtained by maintaining the heating rate of 4.4 ℃/min for 200 min, heating up to 900 ℃, holding at 900 ℃ for 1500 min, and cooling to room temperature after calcination. BiSnSbO 6 -ZnO composite photocatalytic materials were prepared by changing the molar ratio of 1:2, 1:3, 1:4, and 1:5, respectively.

Characterization
The material crystal structure was tested using Cu ka radiation XRD (Rigaku Smart-Lab). SEM (Regulus 8100), TEM (H-8100EM), and EDS (Bruker XFlash 6l60) were used to understand the microscopic morphology and elemental composition of the samples for characterization.

Photoelectrochemical tests
The light absorption capacity of the photocatalytic materials was analyzed by UV-vis DRS (Varian Cary 500). EIS and PT tests were performed on an electrochemical workstation (CHI660E).

Photocatalytic antibacterial performance experiment
The antibacterial properties of the materials were determined by the gradient dilution method and plate colony counting method. Cultivation of bacteria in LB liquid medium, dilute the cultured bacterial liquid by 10 times, 10 2 times, 10 3 times, 10 4 times, 10 5 times, 10 6 times, 10 7 times, 10 8 times, and 10 9 times. One hundred μL of each was uniformly spread on the solid medium and cultured upside down for 18 h, and the number of colonies obtained was observed and recorded. According to the final result, we chose to design the experiment by diluting the bacterial solution 10 6 times.
The photocatalytic antibacterial activities of the composites against E. coli, S. aureus, P. aeruginosa, and Candida albicans were determined by the plate colony counting method. All glassware were sterilized at 121 °C for 20 min before the experiments were carried out. All experiments were performed in triplicate. Materials with different experimental concentrations are set for experiments. Too low or too high experimental concentrations are not conducive to the photocatalytic reaction. Therefore, it is particularly important to choose the most excellent concentration of antibacterial. In this experiment, four different concentrations were set from low to high, and the concentrations of BiSnSbO 6 -ZnO composite photocatalytic materials were 125 mg/L, 250 mg/L, 500 mg/L, and 1000 mg/L, respectively. Weigh samples of different masses and add them to beakers, add 0.9% NaCl solution to dilute, and ultrasonically disperse them evenly. Pipette the bacterial solution from the cultured bacterial solution and add it to the beaker containing the sample, so that the bacteria and the material are fully contacted. A 35-W LED light was used as the light source to irradiate vertically downward, and the stirring was maintained at a fixed speed throughout the experiment. Bacteria were cultured at 37 °C for 18 h, fungi were cultured at 28 °C for 48 h, and the colonies on the medium were counted to calculate the photocatalytic antibacterial rate.

In vitro cytotoxicity assay
The biosafety of BiSnSbO 6 -ZnO composite photocatalytic antibacterial materials was tested by in vitro cytotoxicity experiments, and normal mouse skin fibroblasts (L929) were selected for cytotoxicity detection of different materials. About 5000 cells were added to each well of a 96-well plate, cultured for 12 h, and 100 μL of material solutions of different concentrations were added to each well. After 24 h of culture, 10% MTT solution was added, and the culture was terminated after 4 h. After that, DMSO was added. The absorbance (OD) was measured at 490 nm in an enzymelinked immunosorbent assay to calculate the cell viability.

SEM study of bacterial surface morphology
SEM study was used to observe the differences in structure and morphology of bacteria and fungi treated with and without materials under light conditions, which more intuitively explained the influence of BiSnSbO 6 -ZnO composite photocatalytic materials on the production of pathogenic microorganisms. Bacterial samples were fixed with 2.5% glutaraldehyde solution, washed with PBS, dehydrated with different concentrations of ethanol gradient for 15 min/time, and finally replaced with isoamyl acetate twice, 20 min each time. After freeze-drying at − 80 ℃, the bacteria suspension was made with ethanol and added dropwise to the silicon wafer to observe the morphological difference of bacteria.

Photocatalytic antibacterial mechanism research
In order to further explore the photocatalytic antibacterial mechanism of BiSnSbO 6 -ZnO composite photocatalytic materials, radical scavenging experiments were carried out. S. aureus was used as a model bacterium to study the active substances that play a major role in the photocatalytic antibacterial process. During the experiment, different free radical scavengers were added to remove the generated ROS. 1 mM BQ, IPA, Na 2 C 2 O 4 , and K 2 Cr 2 O 7 were used as scavengers for ·O 2 − , ·OH, h + , and e − , respectively.

Application of composite photocatalytic materials in livestock and poultry wastewater
The above antibacterial experiments in this paper all used five typical pathogenic bacteria as the target substrate model bacteria. In real life, many kinds of bacteria often exist in water bodies at the same time. Therefore, in order to be more suitable for practical application, the livestock and poultry sewage from the farm was selected as the real water sample to test the removal effect of the BiSnSbO 6 -ZnO composite photocatalytic material on various bacteria in the actual wastewater. The collected livestock and poultry wastewater was first precipitated and then centrifuged, and the supernatant was taken. Livestock and poultry wastewater after centrifugation is diluted with physiological saline, dilute 5 times, 10 times, and 20 times respectively, take 100 μL and spread them evenly on LB solid medium, inverted culture for 24 h, observe, and record the number of colonies obtained. Find out the dilution ratio that conforms to the experimental design. When the number of colonies in the medium is about 150, it is considered to conform to the experimental design. Finally, the experiment was designed by diluting livestock and poultry wastewater by 20 times. Among them, the bacterial colony concentration in livestock wastewater was 294/ mL. The BiSnSbO 6 -ZnO composite photocatalytic material was added to the centrifuged and diluted livestock and poultry sewage of the farm, treated with LED light, and then 100 μL was taken out and spread on a solid LB plate for 24 h. The antibacterial effect was also evaluated by the colony counting method. BiSnSbO 6 diffraction crystal plane, which is consistent with the literature report (Luan and Huang 2018). Compared with the characteristic peaks at the molar ratios of 1:2 and 1:3, the characteristic peaks of the composites with the molar ratios of 1:4 and 1:5 are clearer and sharper, indicating that the composites with the molar ratios of 1:4 and 1:5 have higher crystallinity. In the spectrum of BiSnSbO 6 -ZnO composite photocatalytic material, both the characteristic peaks of BiSnSbO 6 and ZnO appeared, which indicated that the BiSnSbO 6 -ZnO composite photocatalytic material was successfully synthesized.

Structure and morphology characterizations
The morphology, structure, and elemental composition of BiSnSbO 6 -ZnO composite photocatalytic materials were analyzed by SEM, TEM, and EDS. Figure 2a is the SEM image of BiSnSbO6, the particles have random three-dimensional shapes, and the particle size is about 200-900 μm. Figures 2b and c are the SEM images of BiSnSbO 6 -ZnO composites with molar ratios of 1:2 and 1:3, respectively. It can be seen that the composite morphology of these two concentrations is aggregated. Figures 2d and e are the SEM and TEM images of the composite with a molar ratio of 1:4, respectively, while Figs. 2f and g are the SEM and TEM images of the composite with a molar ratio of 1:5. Figures 2d and f are the SEM pictures of BiSnSbO 6 -ZnO composite photocatalytic material. It can be seen from the figure that the irregularly shaped BiSnSbO 6 particles are attached to ZnO nanoparticles with smaller particle size, indicating that BiSnSbO 6 and ZnO are successfully compounded together to form a BiSnSbO 6 -ZnO composite photocatalytic material. From Figs. 2e, and g, BiSnSbO 6 -ZnO composite photocatalytic material TEM pictures can be seen from the larger BiSnSbO 6 particles and smaller diameter of ZnO. The results further proved that the BiSnSbO 6 -ZnO composite photocatalytic materials in 1:4 and 1:5 molar ratios were successfully composited. Among them, when scanning and observing the BiSnSbO 6 -ZnO composite photocatalytic materials with a molar ratio of 1:2 and 1:3, it was found that the composite photocatalytic materials of these two ratios appeared material agglomeration. This phenomenon may be because BiSnSbO 6 accounts for more. According to the literature, it can be known that BiSnSbO 6 is prone to agglomeration when sintered at high temperature for a long time, so the 1:2 and 1:3 molar ratio composite materials were not used in the subsequent antibacterial experiments.
The BiSnSbO 6 -ZnO composite photocatalytic material was scanned by EDS to further verify whether the effective components in the composite material were evenly distributed during the material composite process. Scanning the marked area of the SEM image, all elements (Bi, Sn, Sb, Zn, and O) are uniformly distributed in the BiSnSbO 6 -ZnO composite photocatalytic material as shown in Fig. 3. It shows that BiSnSbO 6 and ZnO are successfully coupled to form BiSnSbO 6 -ZnO composites.

Antibacterial activity and in vitro cytotoxicity evaluation
Using a 35-W LED lamp as an excitation light source, the photocatalytic antibacterial properties of pure BiSnSbO 6 material, pure ZnO material, and BiSnSbO 6 -ZnO composite photocatalytic material were determined by colony counting method. Compared with pure BiSnSbO 6 and ZnO materials, the antibacterial properties of BiSnSbO 6 , ZnO, and BiSnSbO 6 -ZnO composite photocatalytic materials against E. coli, S. aureus, P. aeruginosa, and Candida albicans were compared. Firstly, the photocatalytic efficiency of BiSnSbO 6 -ZnO composite photocatalytic materials with different molar ratios of 1:4 and 1:5 was compared with S. aureus as model bacteria, so as to determine the optimal composite material ratio for subsequent experiments.
Using a control group without catalytic materials in a light environment, the effect of LED light on the inhibition or killing of pathogenic microorganisms was removed. Set up a dark group with catalytic materials in a dark environment, and test whether the materials themselves have the ability to inhibit or kill microorganisms. Provided 35-W LED lighting simultaneously under the same ambient conditions. Using S. aureus as model bacteria, the antibacterial ability of BiSnSbO 6 -ZnO composite photocatalytic materials at different ratios and concentrations against S. aureus was compared. It can be clearly seen from Fig. 4 that the BiSnSbO 6 -ZnO composite photocatalytic material with a molar ratio of 1:4 achieved 100% antibacterial efficiency against S. aureus at 4 h. The antibacterial efficiency against S. aureus is higher than that of the BiSnSbO 6 -ZnO composite photocatalytic material with a molar ratio of 1:5. Therefore, the BiSnSbO 6 -ZnO composite photocatalytic material with the best molar ratio of 1:4 was selected for the subsequent experiments.
From the antibacterial results, it can be found that the antibacterial effect of the composite with a molar ratio of 1:4 is the most significant. When the molar ratio reaches 1:5, the antibacterial effect of the BiSnSbO 6 -ZnO composite material is not better than the previous ratio. The reason may be that the excessive doping of ZnO may mask the active sites of BiSnSbO 6 material. Figure 5a shows the photocatalytic antibacterial efficiency of the BiSnSbO 6 -ZnO composite photocatalytic material with different concentrations of 1:4 molar proportions on E. coli. In the antibacterial process, the best antibacterial efficiencies of single BiSnSbO 6 and ZnO against E. coli within 6 h were 67.46% and 28.57%, respectively, indicating that both BiSnSbO 6 and ZnO have a certain inactivation effect on E. coli under LED light, but the effect is not ideal, and different concentrations of BiSnSbO 6 -ZnO composite photocatalytic materials have better antibacterial properties than single BiSnSbO 6 and ZnO. After 6 h of light irradiation, 500mg/L BiSnSbO 6 -ZnO composite photocatalytic material had the highest antibacterial efficiency against E. coli, reaching 99.63%. Figure 5c is a graph showing the antibacterial efficiency of BiSnSbO 6 -ZnO composite photocatalytic material against S. aureus. It can be seen that the highest antibacterial efficiencies of the monomers BiSnSbO 6 and ZnO are 21.60% and 84.79% respectively within 6 h, and the BiSnSbO 6 -ZnO composite photocatalytic material exhibits better antibacterial properties. The  antibacterial efficiency of the catalytic material reached 100% at 4 h. Nano-ZnO has been shown to be selectively toxic to S. aureus, and the experimental results are consistent with the reported literature (Zheng et al. 2012). Figure 5e is a graph showing the antibacterial efficiency of BiSnSbO 6 -ZnO composite photocatalytic material against P. aeruginosa. Compared with the monomers BiSnSbO 6 and ZnO, the BiSnSbO 6 -ZnO composite photocatalytic material exhibited stronger antibacterial properties against P. aeruginosa, It can be clearly seen that the antibacterial efficiency of the 500-mg/L BiSnSbO 6 -ZnO composite photocatalytic material against P. aeruginosa reached 100% when the light was irradiated for 4 h. Compared with the antibacterial efficiency of other different concentrations, the antibacterial efficiency of the BiSnSbO 6 -ZnO composite photocatalytic material at a concentration of 500 mg/L is better, indicating that the optimal antibacterial concentration of the BiSnSbO 6 -ZnO composite photocatalytic material for bacteria is 500 mg/L. Too much concentration or too little concentration will affect the photocatalytic activity of BiSnSbO 6 -ZnO composite photocatalytic material. Figures 5b, d, and f are the antibacterial plate images of monomer BiSnSbO 6 , ZnO, and BiSnSbO 6 -ZnO composite photocatalytic materials with different concentrations against E. coli, S. aureus, and P. aeruginosa.
In order to explore the antibacterial efficiency of materials with different concentrations, the antibacterial time of light exposure was extended to continue to observe. From  Fig. 6, it can be found that at 12 h, the BiSnSbO 6 -ZnO composite photocatalytic materials with four different antibacterial concentrations of 125 mg/L, 250 mg/L, 500 mg/L, and 1000 mg/L had better effects on E. coli, S. aureus, and P. aeruginosa three bacteria were 100% antibacterial efficiency. It shows that under the condition of maintaining light conditions, the BiSnSbO 6 -ZnO composite photocatalytic material with different concentrations can also continue to exert an antibacterial effect to kill bacteria.
According to the above research results, it can be found that the optimal antibacterial concentration of BiSnSbO 6 -ZnO composite photocatalytic material for bacteria is 500 mg/L, but there are species differences in the antibacterial effect on different bacteria.
In order to understand the antibacterial properties of BiSnSbO 6 -ZnO composite photocatalytic materials on fungi, this paper also used Candida albicans as model bacteria to conduct antibacterial experiments on fungi. Figure 7 shows the antibacterial efficiency diagram and antibacterial plate diagram of single BiSnSbO 6 , ZnO, and BiSnSbO 6 -ZnO composite photocatalytic materials with different concentrations against Candida albicans. The antibacterial efficiency against Candida albicans reached the highest at h, which was 63.80%.
The cytotoxicity of antibacterial materials is of great significance for further practical applications of the materials. In this study, normal mouse skin fibroblasts (L929) were selected to detect the cytotoxicity of BiSnSbO 6 -ZnO composite photocatalytic materials. According to the results of the previous antibacterial experiments, three different concentrations were set, which were 250 mg/L, 500 mg/L, and 1000 mg/L, respectively. Figure 8 shows the results of the MTT assay. There was no significant difference in the survival rate of L929 cells between the three different concentrations of BiSnSbO 6 -ZnO composite photocatalytic material and the control group (p > 0.05). This can explain the non-cytotoxicity of BiSnSbO 6 -ZnO composite photocatalytic material. However, the survival rate of L929 cells incubated with monomeric ZnO at a concentration of 500 mg/L was significantly reduced, which was significantly different from that of the blank control group (p < 0.05). Figure 9 shows the UV-vis DRS spectra of BiSnSbO 6 , ZnO, and BiSnSbO 6 -ZnO. It can be seen from Fig. 9a that ZnO absorbs light with a wavelength below 400 nm, mainly absorbing ultraviolet light, and the absorption band edges of BiSnSbO 6 and BiSnSbO 6 -ZnO are above 400 nm, indicating that they are responsive to visible light. With the addition of ZnO, the light absorption intensity of BiSnSbO 6 -ZnO was enhanced. The band gap energy (E g ) can be estimated according to the formula: ɑhv = A(hv − Eg) n/2 (Yosefi and Haghighi, 2018;Li et al. 2019). Since ZnO is a direct transition semiconductor material, n = 2 in the calculation process (Kumar et al. 2014). From Figs. 9b, c, and d, it can be concluded that the BiSnSbO 6 -ZnO composite photocatalytic material, the forbidden bandwidths of BiSnSbO 6 -ZnO, BiSnSbO 6 , and ZnO materials are 2.81 eV, 2.66 eV, and 3.2 eV, respectively. This is similar to the data reported in the previous literature (Luan and Huang 2018). The E VB and E CB edge potentials of BiSnSbO 6 and ZnO calculated according to the formula are shown in Table 1. According to the edge potentials of E VB and E CB , BiSnSbO 6 -ZnO composites belong to type II heterojunction structure composites.

Electrochemical characterization
Semiconductor photocatalytic materials can understand the interfacial mobility of photogenerated electron-hole pairs in semiconductor materials through an electrochemical impedance, where the smaller the radius of the semicircle arc, the higher the charge transport efficiency of semiconductor photocatalytic materials. As shown in Fig. 10, compared with pure BiSnSbO 6 and pure ZnO materials, BiSnSbO 6 -ZnO composite photocatalytic material has the smallest arc radius, indicating that BiSnSbO 6 -ZnO composite photocatalytic material has the smallest charge transfer resistance. Therefore, the BiSnSbO 6 -ZnO composite photocatalytic material has the strongest photocatalytic antibacterial performance. Figure 11 is the PT map of different photocatalytic materials under intermittent illumination conditions. The photocurrent of the BiSnSbO 6 -ZnO composite photocatalytic material is stronger than that of the two single materials. The higher the photocurrent intensity, the faster the separation rate of electron-hole pairs. It shows that the BiSnSbO 6 -ZnO composite photocatalytic material has better photocatalytic antibacterial efficiency than the single BiSnSbO 6 and ZnO materials.
PL analysis can study the process of photoinduced electron-hole pair migration, transfer, and recombination in   Fig. 12. After exciting the material with 469nm light, it can be found that the material shows a strong emission peak in the range of 800-900 nm, and the emission peak intensity of the BiSnSbO 6 -ZnO composite photocatalytic material is lower than that of the pure ZnO material, indicating that the recombination rate of BiSnSbO 6 and ZnO Fig. 9 UV-vis DRS spectra of the prepared samples a UV-vis DRS spectra of BiSnSbO 6 , ZnO, and BiSnSbO 6 -ZnO, b E g spectra of BiSnSbO 6 -ZnO, c E g spectra of BiSnSbO 6 , d E g spectra of ZnO inhibited the charge recombination rate. It is further demonstrated that the BiSnSbO 6 -ZnO composite photocatalytic material can reduce the charge recombination rate, thereby improving the photocatalytic antibacterial activity.

Understanding of the mechanism for antimicrobial activity
In order to understand the role of different active substances in the antibacterial process of BiSnSbO 6 -ZnO composite photocatalytic materials, the photocatalytic antibacterial mechanism of BiSnSbO 6 -ZnO composite photocatalytic materials was explored through radical scavenging experiments. BQ (P-benzoquinone), IPA (Isopropanol), Na 2 C 2 O 4 (sodium oxalate), and K 2 Cr 2 O 7 (potassium dichromate) were used as scavengers of O 2 − ,·OH, h + , and e − , respectively. It can be seen from Fig. 13 that after adding BQ, the antibacterial efficiency against the model bacteria S. aureus was enhanced, reaching 100% in 1 h. This should be because the conduction band position of ZnO is − 0.31 eV, which is more correct than E θ O 2 /O 2 − (− 0.33 eV), so the e − on the conduction band of ZnO cannot combine with O 2 to form O 2 − (Abbas and Bidin 2017). After adding BQ, BQ cannot combine with O 2 − , and because BQ is a broad-spectrum antibacterial agent, it can interact with DNA, protein, mitochondria, etc. in cells, resulting in bacterial death (Liu et al. 2009;Wong et al. 2017). This may be the reason why the antibacterial efficiency against S. aureus was improved after adding BQ, indicating that O 2 − was not generated during the photocatalytic antibacterial process. The valence band position of BiSnSbO 6 is 2.81 eV, which is more correct than E θ ·OH/H 2 O (2.38 eV) (Jiang et al. 2017), so the h + at the valence band of BiSnSbO 6 can react with H 2 O to generate OH, and then react with bacteria. After adding IPA, the antibacterial efficiency decreased slightly, indicating that the role of ·OH in the photocatalytic antibacterial process was almost negligible. When the Na 2 C 2 O 4 solution was added, the antibacterial efficiency decreased, indicating that the effect of h + was weak in the photocatalytic antibacterial process. After adding the K 2 Cr 2 O 7 solution, the antibacterial efficiency decreased to a large extent, which indicated that the active substance e − played a major role. The free radical clearing experiment shows that the BiSnSbO 6 -ZnO composite photocatalytic material can generate ROS under light conditions, and OH, h + , and e − all have certain antibacterial effects, of which e − plays the main antibacterial effect.
The photocatalyst can be excited to generate e − and h + under sunlight irradiation. e − and h + can directly react with bacterial cell membranes and their intracellular substances, causing the inactivation of functional units, resulting in bacterial death.
In order to more clearly understand the effect of BiSnSbO 6 -ZnO composite photocatalytic material on the morphological changes of bacteria and fungi, SEM was used to observe the morphology of experimental bacteria treated with added material light and the control bacteria only treated with light. From Fig. 14, it can be found that compared with the control group, the structures and shapes of bacteria and fungi treated with BiSnSbO 6 -ZnO composite photocatalytic materials are significantly different. E. coli, S. aureus, P. aeruginosa, and Candida albicans all exhibited wrinkled atrophy, rupture, and irregular morphology to a certain extent, unlike bacteria that had not been treated with the material whose surface was intact. The results showed that the BiSnSbO 6 -ZnO composite photocatalytic material contacted the surface of bacteria and fungi, causing the surface of bacteria and fungi to atrophy or even rupture,  In summary, it is found that the BiSnSbO 6 -ZnO composite photocatalytic material produces active substances OH, h + , and e − in the photocatalytic antibacterial process, OH, h + , and e − all have a certain antibacterial effect, and the role of e − occupies the leading position. After the active substance is in contact with the bacteria, the structural integrity of the bacteria is destroyed, and the internal substances of the bacteria leak, thereby achieving an antibacterial effect. According to published studies, BiSnSbO6 and ZnO are p-type and n-type semiconductors, respectively (Huang et al. 2019;Jaffari et al. 2019). At the same time, combining the UV-vis valence and conduction band positions of the two, it is concluded that the BiSnSbO6-ZnO composite photocatalytic material is a type II heterojunction structure, as shown in Fig. 15.

Application of composite photocatalytic materials in livestock and poultry wastewater
A variety of bacteria have been found in the collected livestock and poultry wastewater, and these bacteria are extremely harmful to humans and the environment. This serious problem needs to be solved urgently to protect the environment. The practical application of antibacterial properties of BiSnSbO 6 -ZnO composite photocatalytic materials was tested with livestock and poultry wastewater.
As shown in Fig. 16, after the livestock and poultry wastewater was treated by light with the BiSnSbO 6 -ZnO composite photocatalytic material with the optimal antibacterial concentration of 500 mg/L, with the increase of time, the colonies in the livestock and poultry wastewater diluted 20 times were obviously reduced or even disappeared. After 3 h of LED light illumination, the antibacterial rate of BiSnSbO 6 -ZnO composite photocatalytic material reached 99.41%. When the time was extended to 6 h, it was found that the bacteria had been completely killed. It is proved that the BiSnSbO 6 -ZnO composite photocatalytic material has a good antibacterial effect on a variety of coexisting bacteria, which also shows that the BiSnSbO 6 -ZnO composite photocatalytic material is a broad-spectrum antibacterial material, which can be applied in practical applications.

Conclusion
BiSnSbO 6 -ZnO composite photocatalytic materials were synthesized by a simple solid-phase sintering method. XRD, EDS, EIS, and other characterizations further proved that the BiSnSbO 6 -ZnO composite photocatalytic material was composed of BiSnSbO 6 and ZnO, and the composite was successful and the structure did not change. Comparing the photocatalytic and antibacterial effects of composites with different molar ratios on pathogenic microorganisms under 35-W LED light. The results showed that the antibacterial effect was the best when the molar ratio of BiSnSbO 6 -ZnO composite was 1:4 and the concentration was 500 mg/L. The antibacterial efficiency against E. coli at 6 h can reach 99.63%, and the antibacterial efficiency of 500-mg/L BiSnSbO 6 -ZnO composite photocatalytic material against S. aureus and P. aeruginosa can reach 100% under illumination for 4 h. With the prolongation of the illumination time, the antibacterial efficiency of the composite materials with different concentrations against the four bacteria can reach 100% in 12 h. It shows that under the condition of maintaining light conditions, the BiSnSbO 6 -ZnO composite photocatalytic materials with different concentrations can continue to exert antibacterial effect to achieve the good antibacterial effect. The antibacterial efficiency of 250-mg/L BiSnSbO 6 -ZnO composite photocatalytic material against Candida albicans can reach 63.8% after 6 h of illumination. It can be seen that the composite photocatalytic material has a better antibacterial effect on bacteria. In order to further elucidate the antibacterial properties of BiSnSbO 6 -ZnO composite photocatalytic materials in practical applications, antibacterial experiments were conducted on livestock and poultry wastewater collected in daily life. The results show that the antibacterial rate of BiSnSbO 6 -ZnO composite photocatalytic material to bacteria reaches 99.41% after 3 h of LED light illumination, indicating that BiSnSbO 6 -ZnO composite photocatalytic material has broad-spectrum antibacterial activity against bacteria and can be widely used in real life. According to the free radical scavenging experiment, SEM observation of the morphological changes of the bacteria after light treatment and analysis of the energy band structure, the prepared BiSnSbO 6 -ZnO composite photocatalytic material can generate active species·OH, h + , and e − through light irradiation to achieve the purpose of sterilization, where e − play the main role. Cytotoxicity experiments proved that the prepared BiSnSbO 6 -ZnO composite photocatalytic material was non-toxic at the experimental concentration, indicating that the BiSnSbO 6 -ZnO composite photocatalytic material was a green, safe, and efficient photocatalytic material with broad-spectrum antibacterial properties and species differences.