Photocatalytic Degradation Of Tetracyclines By TiO₂/Molecular Sieves And Assessment Of Residual Antibacterial Activity

Tetracycline antibiotics are widely used in human medicine,animal husbandry,fishery,etc.After entering the human body or animal body,tetracycline antibiotics are discharged into the sewage system or directly into the water environment mainly as prototype without metabolism.The sewage of hospitals,pharmaceutical factories,pastures and fishing grounds often contains tetracycline antibiotics.In addition,the traditional biological treatment of sewage can not remove tetracycline antibiotics completely due to the stability and antibacterial properties.The presence of antibiotics in the environment is susceptible to microbial resistance gene,which is a serious threat to ecology and human health.In order to remove antibiotics in sewage,many advanced oxidation methods have been developed,such as composite photocatalysts containing nano-Ti02 and other materials.Molecular sieves are very good absorbents,which can load nano-TiO2 to gain excellent adsorption and photocatalytic performance.In this study,TiO2 was supported on microporous molecular sieve ZSM-5 and mesoporous molecular sieve MCM-41 for photocatalytic degradation of tetracycline antibiotics in aqueous solution.The surface features,skeleton structure,specific surface area,pore size,gap energy,functional group and zero potential of composite photocatalyst were analyzed.The effects of photocatalytic conditions such as pH value,temperature,TiO2 ratio,catalyst dose,pollutant concentration and ion insolution were analyzed by uniform design and single factor experiment.The reuse performance of composite photocatalyst was investigated.The residual antimicrobial effect of tetracycline antibiotics and its post-degradation solution on Escherichia coli DH5a was assessed using half effect concentration,maximum specific growth rate and maximum biomass.The main conclusions are listed as follows:(1)MCM-41 with ordered pores was successfully prepared at room temperature.The structure of TiO2,ZSM-5 and MCM-41 in TiO2/ZSM-5 and TiO2/MCM-41 composite photocatalyst prepared by solid dispersion method did not change obviously.TiO2 was mainly loaded on the surface of ZSM-5 and MCM-41,while there were chemical bonds between them.(2)The adsorption performance of ZSM-5 and TiO2 was poor,while MCM-41 was good.The adsorption performance of Ti02/ZSM-5 was slightly improved compared to ZSM-5.The adsorption performance of TiO2/MCM-41 was not decreased obviously after loading Ti02 compared to MCM-41.The adsorption rate of material was fast.The saturated adsorption capacity of the materials reached the largest value at pH 7 or 5,and was very small at pH 3 or 11,because the pH value determines the surface charge of the adsorbent and adsorbate.The adsorption process conformed with a quasi-second-order kinetic model and Langmuir isothermal adsorption model.(3)In the uniform design experiment,the most significant photocatalytic condition to TiO2/ZSM-5 and TiO2/MCM-41 was temperature and dosage respectively.The interactive effect of dosage and Ti02 ratio to both TiO2/ZSM-5 and TiO2/MCM-41 was important,indicating that the TiO2 content affected the photocatalyst degradation.The results of single factor experiments were basically consistent with the trend of uniform design formula.Under the same conditions,the photocatalytic performance of TiO2/MCM-41 was better than Ti02/ZSM-5.The photocatalytic degradation process conformed to quasi-first order kinetic model and Langmuir-Hinshelwood kinetic model.(4)In reuse of 5 times,the removal efficiency of TiO2/ZSM-5 composite photocatalyst was slightly decreased.while the removal efficiency of TiO2/MCM-41 was almost unchanged.Both of them were better than that of TiO2.(5)The removal rate of oxytetracycline in simulated waste water of chicken farm was lower than that of pure water or pure oxytetracycline solution.The main reason was that the ions in natural water had an effect on photocatalytic degradation reaction.It was found that K+ and NO3-had little effect,and Ca2+,Mg2+ and SO42-had slight inhibition on photocatalytic degradation reaction,and HCO3-reduced the degradation rate,although the effect of each ion was small.(6)During the degradation by TiO2/ZSM-5 and TiO2/MCM-41 composite photocatalysts,the decrease of two inhibition rates of E.coli,the maximum specific growth rate inhibition and the maximum biomass inhibition,had good linear relationships with the decrease of oxytetracycline concentration,while during the UV photodegradation process,there was a positive deviation.Compared with the maximum specific growth rate inhibition,the maximum biomass inhibition required less data and its error was relatively small.The intermediate products with extra toxicity was not detected during photodegradation or photocatalytic degradation of tetracycline,oxytetracycline.and chlorotetracycline.(7)The results of HPLC-MS/MS showed 10 major intermediate products in the degradation process of tetracycline,oxytetracycline,and chlorotetracycline.All the intermediate products had a tendency of increase first and then decrease,because they degraded with the degradation of their parent compound,which was one of the causes of no additional biotoxicity.(8)Ti02/MCM-41 composite photocatalyst has better adsorption and photocatalytic activity than Ti02/ZSM-5 composite photocatalyst,which has potential in the treatment of antibiotic wastewater.