Electrocatalytic Degradation Of Tetracycline Antibiotics In Wastewater With Titanium Based PbO₂ Eelectrode

Tetracyclines were class of broad-spectrum antibiotics,including tetracycline,oxytetracycline and chlortetracycline,which were used massively among all antibiotics and treating clinical treatment of rickettsia disease,chlamydia and mycoplasmosis.They expressed bacteriostatic effect at low concentration and strong bactericidal effect at high concentration to prevent infectious diseases of human.And tetracyclines that were added into feedstuff can promote the absorption and digestion of animals and accelerate their growth.Previous studies showed that antibiotics entering human and animal bodies can not be completely metabolized,and most of them keeping the original activity were excreted through feces and urine.Tetracyclines can be detected in sewage treatment plant effluent,livestock farm,river water and sediment,soil and groundwater.The antibiotics residues in the environment not only harmed non-target organisms,contaminate food and drinking water sources,but also increased the resistance of pathogens and produce resistant genes,which seriously threatening the safety of the ecological environment and human health.Therefore,the development of rapid and simple treatment technology of tetracyclines in wastewater was greatly significant for the protection of water quality and human health.At present,the common methods to treat tetracyclines in water were traditional treatment,adsorption,biological and advanced oxidation techniques.Due to the antibiotics had bacteriostasis,traditional treatment and biological method can not completely remove antibiotics,and advanced treatment needed to increase time and cost investment.Although the adsorption method had good treatment effect,there was no real degradation of pollutants.Advanced oxidation technologies were important methods to treat antibiotics by using strong oxidizing active components to degrade pollutants.Electrochemical technology that had the advantages of simplicity and efficiency used?OH produced on the electrode surface to react with organic compounds to achieve the degradation of organic compounds.Lead dioxide?PbO₂?,as an inactive anode,had excellent electrocatalytic performance and stable structure,which was widely used in the treatment of organic pollution.Therefore,the titanium based PbO₂ electrode was prepared by electrodeposition method and the electrocatalytic degradation of tetracyclines in simulated wastewater were studied.The kinetic characteristics of electrocatalytic degradation of tetracyclines,possible degradation pathway and toxicity of degradation products were analyzed.The main results were as follows:1.The optimum preparation conditions of lead dioxide electrode was studied,and the deposition current of 10 mA?cm^-2 and the deposition time of 1 h were obtained.Under these conditions,the degradation of tetracyclines was greater.Scanning electron microscopy?SEM?,X-ray diffraction?XRD?and cyclic voltammetry analysis showed that PbO₂ was loaded on the substrate smoothly and formed a compact active layer,which was?-crystalline PbO₂,indicating that the titanium based PbO₂ electrode was successfully prepared.2.The electrocatalytic degradation conditions of tetracyclines were optimized.Compared with other electrolytes,Na₂SO₄ solution of 0.06 mol?L^-1 can obtain high degradation efficiencies without second pollution.The degradation rates of antibiotics increased with the increasing current density,but the current efficiencies decreased,and when the current density was 5 mA?cm^-2,high current efficiences and antibiotic degradation rates were maintained.Moreover,the degradation rates decreased with the increasing pH and concentrations of antibiotics,indicating that the acidic environment was conducive to enhance the electrochemical degradation of tetracyclines.Finally,The electrolyte of 0.06 mol?L^-1 Na₂SO₄,current density of 5mA?cm^-2,pH 4.6 of initial reaction solution was considered for electrocatalytic degradation.3.Increasing concentrations of antibiotics resulted on a strong competitive reaction between tetracyclines and intermediates that decreased the degradation rates,and tetracyclines of 100 ng?L^-1 were completely degraded after 3 min.Under the selected experimental conditions,the degradation efficiencies of tetracycline,oxytetracycline,and chlortetracycline of 50 mg?L^-1 reached 89.76%,91.58%and93.98%at 3 h,respectively.The degradation processes accorded with the first-order reaction kinetics equation,and the degradation rate constants were 0.7828,0.8390and 0.9667 h^-1,respectively.The degradation efficiency and degradation rate constant of chlortetracycline were higher than those of other two antibiotics,which was related to easy dechlorination of chlortetracycline at the cathode.4.The degradation products of tetracyclines were identified by liquid chromatography-tandem mass spectrometry?LC-MS/MS?and the possible degradation pathways were analysed.It was found that the three antibiotics had similar degradation pathway induluding the oxidation of enolyl and hydroxyl,deamination and dimethylamino,and ring-opening reaction.5.The concentration and reaction time of tetracyclines were setted to 10 mg?L^-1and 8 h.The acute toxicity for luminescent bacteria was analyzed after the degradation processes of tetracyclines.The luminescence inhibition ratios of tetracycline,oxytetracycline and chlortetracycline were 75.88%,7.76%and 76.82%without reaction,respectively.And after 2 hours of reaction,the luminescence inhibition rate remained unchanged which indicated the intermediate products remained biotoxic.The luminescence inhibition ratios began to decrease at 4 h,and reduced to 22.41%,34.75%and 37.86%at 8 h,indicating that tetracyclines were deeply degradated and their ecotoxicity was significantly decreased.