Study on photocatalytic degradation and reaction kinetics of tetracycline antibiotics in biogas slurry

Abstract: As a bactericidal drug, antibiotics are widely used in animal husbandry to prevent animal diseases and treat sick animals. With the rapid development of Chinese livestock and poultry breeding industry, the use of veterinary antibiotics continues to increase. In 2013, the annual use of antibiotics in China was about 162 000 t, of which 52% was used for livestock. The production and the actual usage of tetracycline antibiotics in livestock and poultry breeding industry of the world are the largest than other antibiotics. A large number of antibiotics are difficult to be absorbed and metabolized completely in animals, then they will be excreted and stay in biogas slurry during anaerobic digestion. High concentrations of antibiotics in biogas slurry may cause some microorganisms produce resistance genes and won't be degraded into the environment, and these resistance genes can transmit, it may make some pathogenic bacteria resist drugs, which in turn poses a great threat to ecosystems and human health. At present, the main methods of removing antibiotics include photocatalytic degradation, hydrolysis, oxidative degradation, sludge adsorption, and biodegradation. Among them, the photocatalytic degradation technology uses TiO2 as a photocatalyst, which has the advantages of high catalytic activity, high chemical stability, low cost, and low toxicity product, and has become a promising method for pollutant degradation. In this study, the photocatalytic degradation pathway was used to explore the optimal light source, pH value and the degradation effect of photocatalysis under different initial concentration of tetracyclines antibiotics in biogas slurry, and the degradation kinetics of antibiotics under different initial concentrations and pH value were studied. The results showed that the degradation effects of different light sources on tetracycline antibiotics were: High pressure mercury lamp > ultraviolet disinfection lamp > long arc xenon lamp>dark. After 2 h of high pressure mercury lamp catalysis, the degradation rates of tetracycline, oxytetracycline and chlortetracycline reached 91.68%, 85.58% and 81.18%, respectively. The lower the initial mass concentration of the tetracycline antibiotic, the better the photocatalytic effect. When the initial concentration of tetracycline, oxytetracycline and chlortetracycline is 5 mg/L, the degradation rate was up to 94.80%, 88.35% and 95.39%, the reaction rate constant was 1.505 6 h(1, and the half-life was 0.431 7 h. There was a significant difference in the degradation rate of tetracycline and chlortetracycline in the initial pH value of biogas slurry (P<0.05). When pH value was 6, the degradation rate of tetracycline was 96.16%, the reaction rate constant was 1.597 1 h(1, and the half-life was 0.355 3 h; when pH value was10, the degradation rate of chlortetracycline was 90.47%, the reaction rate constant was 1.084 4 h(1, half-life was 0.338 3 h. There was no significant difference in the degradation rate of oxytetracycline between the initial pH value of biogas slurry (P>0.05). When pH value was 10, the average degradation rate of three antibiotics was 89.88%. when the initial pH value of biogas slurry was 10, the concentration of tetracycline antibiotics was 5 mg/L, it is the best to catalytic degradation tetracycline antibiotics using the high-pressure mercury lamp.