Study on efficient degradation of tetracycline by titania nanoribbons modified with silver

TiO2 nanoribbons were prepared by hydrothermal method using P25 as titanium source, and Ag/TiO2 composites were prepared by REDOX method. Xenon lamp was used as the light source to study the photocatalytic degradation efficiency of tetracycline by Ag/TiO2 and tetracycline degradation efficiency under different conditions. When the initial concentration of tetracycline was 50mg/L, the pH value was 8, and the catalyst dosage was 0.05g. The optimal reaction conditions showed that the degradation efficiency of tetracycline by Ag/TiO2 composite could reach 88.9%, which was 12% higher than that of TiO2.


Introduction
Tetracyclic antibiotics are widely used in the treatment of diseases worldwide, but 90% of them are released into the environment because they cannot be fully used [1].Nowadays, tetracycline can be detected in a variety of environments, and because it can be enriched in the food chain and has chronic toxic effects, it poses a serious threat to the living environment and human health [2].Photocatalytic oxidation, as an emerging advanced oxidation technology, has been proven to be able to effectively degrade refractory antibiotics in the environment, among which the most representative catalyst is TiO 2 with good stability, strong photocatalytic activity, cheap and easy to obtain [3][4].
In the present study, TiO 2 nanoribbons were prepared by hydrothermal method using P25 as a titanium source [5].Ag/TiO 2 nanocomposites were prepared by the REDOX method.Under the optimal experimental conditions, the degradation efficiency of Ag/TiO 2 composite material for tetracycline can be increased by 12% compared to the degradation rate of TiO 2 nanoribbons.

Preparation of TiO 2 nanoribbon
An appropriate amount of P25 was weighed into a beaker, and 25 mL10M potassium hydroxide solution was added, mixed well, then moved to a polytetrafluoroethylene reactor, and heated at 220℃ for 24h.Cooling to room temperature naturally, the white product was washed with methanol and

Preparation of Ag/TiO 2 composites
An appropriate amount of TiO 2 powder was dissolved in anhydrous ethanol and dispersed by ultrasonic.We added 10 drops of sodium borohydride (2mol/L) solution to the dispersed solution and manually oscillate for 10 minutes so that the TiO 2 surface is reduced as much as possible.The upper layer of liquid was transferred to the centrifuge tube, and 3 drops of AgNO 3 (2mg/mL) solution were added.The ultrasonic was carried out for 8 minutes with a power of 50W.Through centrifugal separation and drying, Ag/TiO 2 composite materials can be obtained.

Photocatalytic degradation experiment
Tetracycline in water was treated by photocatalytic oxidation with Ag/TiO 2 as the catalyst and xenon lamp as the light source.The efficiency of Ag/TiO 2 photocatalytic degradation of tetracycline in water was investigated, and the effects of different factors on the degradation rate of tetracycline in water were studied.

The high-resolution images
As can be seen from Figure 1(a), the formed TiO 2 nanomaterial is banded with varying lengths, poor dispersion and obvious aggregation phenomenon.It can be seen from Figure 1(b) that Ag/ TiO 2 still has a banded structure, and Ag particles are evenly distributed on the surface of TiO 2 nanomaterials.As can be seen from Figure 3, when the initial concentration of tetracycline was 50mg/L, the degradation rate could reach 88.24% in 1h.As the initial concentration of tetracycline increased, the degradation rate decreased successively.According to the analysis results, when the catalyst dosage is constant, the electron-hole pair generated on the catalyst surface and the free base generated by the reaction remain unchanged, and too high or too low an initial concentration of tetracycline will affect the degradation rate.When the light source is constant, the irradiation amount of each molecule is almost constant.If the initial concentration is increased, the number of photons absorbed by each molecule will decrease, thus affecting the direct photodegradation of tetracycline.4, the degradation rate of tetracycline was similar under different catalyst dosages.When the dosage of the catalyst is 0.25g, the 1-hour degradation rate of tetracycline is only 64.93%.When the dosage of the catalyst was 0.05g, the degradation rate of tetracycline could reach 80.43%.The addition amount of catalyst is small, the free radical generated by photoexcitation is small, and the oxidation rate is slow.The degradation of tetracycline can be accelerated by the addition of a large amount of catalyst and the free base generated by light excitation.However, if the amount of catalyst is too large, the suspended matter in the solution will cause the solution turbidity and shade the light, which will reduce the lighting efficiency and hinder the generation of free radicals, resulting in the decrease of the catalytic speed.

Effect of pH value on degradation effect.
The degradation rate of tetracycline increases with the increase of pH value before pH 8, and the degradation rate of tetracycline is 85.82% when the pH value is 8, in Figure 5.If the pH value continues to increase, the degradation rate of tetracycline decreases.The main reason is that the surface charge of the catalyst is affected by the pH.In an acidic environment, the concentration of OH -is very low, which affects the generation of the dominant free radicals in the catalytic reaction and reduces the photocatalytic degradation efficiency.When the pH value is too high, it will affect the catalyst potential, reduce the oxidation of holes, and also affect the efficiency of photocatalytic degradation.

Conclusion
TiO 2 nanoribbons were synthesized by a simple hydrothermal method, and Ag/TiO 2 composites were prepared by the oxidation-reduction method.The morphology and structure of TiO 2 nanoribbons were not changed by Ag loading.Under the optimal experimental conditions, the degradation rate of tetracycline by TiO 2 nanomaterial 1h was 76.9%, and the degradation rate of modified Ag/TiO 2 composite was 88.91%, which increased by 12%.Experimental results show that Ag doping can modify TiO 2 well, promote electron-hole separation, and effectively increase the photocatalytic performance of TiO 2 .It is a photocatalytic composite material with practical value and provides a reference for the photocatalytic degradation of tetracycline antibiotics.

Figure 2 (
b) shows the XRD pattern of Ag/ TiO 2 .The main diffraction peaks of TiO 2 nanoribbons and Ag/TiO 2 are basically consistent, indicating that Ag doping does not change the crystal phase structure of TiO 2 .The diffraction peak at 44.7˚ can prove the existence of metal Ag.Due to the low content of Ag compared to the main material TiO 2 , the diffraction peak area at 44.7˚ is small.

Figure 3 .
Figure 3.The degradation rate of tetracycline at different initial concentrations.

Figure 4 .
Figure 4.The degradation efficiency of tetracycline with different catalyst dosages.

Figure 5 .
Figure 5.The degradation rate of tetracycline in different pH solutions.

3. 3 . 4
Comparison of tetracycline degradation effects of TiO 2 , Ag/TiO 2. As shown in Figure6, under the same optimal conditions, the titanium dioxide degradation rate of tetracycline is 76.9%, and the modified Ag/TiO 2 composite degradation rate of tetracycline is 88.9%.The main reason is that Ag broadens the optical absorption range of TiO 2 , increases the absorption rate of TiO 2 to light, reduces the electron-hole pair recombination rate of TiO 2 , and enhances the photocatalytic ability of TiO 2 .