| As bacteriostatic agents,antibiotics are widely used in the treatment of human and animal diseases and animal growth promoters.Among them,the antibiotics used in livestock and poultry breeding reached 46%of the total amount,and about 50%to 90%of the antibiotics used in livestock and poultry breeding were directly discharged into the environment in the form of mother or metabolites.Different concentrations of antibiotics have been detected in surface water,groundwater,human urine and even breast milk in China.These antibiotics can cause serious harm to the aquatic environment.Livestock and poultry wastewater is one of the main sources of antibiotic pollution,but compared with high concentration of COD,turbidity of them,there is often a lack of attention to trace pollutants of antibiotics.As a commonly used treatment process for livestock and poultry wastewater,coagulation and sedimentation also have a certain influence on antibiotics in wastewater,but there are few studies on the removal efficiency of tetracycline and whether there is structural change in this process.Therefore,this study took tetracycline,a typical antibiotic,as the research target,and studied its adsorption characteristics in the soil directly discharged from villages and towns and the coagulation adsorption characteristics in the coagulation process in the process of centralized treatment.Furthermore,the removal efficiency and mechanism of tetracycline by electroflocculation,the direct coupling technology of oxidation and adsorption,which is mainly used to remove heavy metals,were further studied.The study provided a scientific basis for the removal of tetracycline from livestock and poultry wastewater in the process from discharge to treatment.According to the water quality characteristics livestock and poultry breeding wastewater which is low temperature and low load in cold area Jilin Province,the adsorption characteristics of sand with different particle sizes were studied at low temperature.The efficiency of three commonly used coagulants in treating tetracycline and their influence on the material structure were explored,and the"additional" effect of the coagulation process in sewage treatment plants on the removal of tetracycline in livestock and poultry wastewater was analyzed.It makes up for the lack of research on tetracycline removal efficiency and mechanism of coagulation process in sewage treatment plant,and optimizes and in-depth research on the removal efficiency and mechanism of tetracycline by electric flocculation.Based on the above research,the following conclusions are drawn:The water quality characteristics of livestock and poultry breeding wastewater is low temperature and low load in cold area Jilin Province.(1)The adsorption characteristics of sand with different particle sizes were studied at low temperature,which provides the theory basis for water pollution risk as tetracycline in the livestock wastewater discharge,and(2)the removal efficiency and mechanism of tetracycline were studied using three kinds of commonly used coagulant under different conditions,to explore the presence of the structural change tetracycline,parsing removal "additional" role of tetracycline in the livestock wastewater in coagulation process;(3)In order to save economic cost and improve the removal efficiency as the guide,and then explore the "optimized" process-electroflocculation under the conditions of DC and pulse,to select the optimal program of tetracycline removal under the different influencing factors,intermediate products,removal path and mechanism analysis.It makes up for the lack of studies on the adsorption characteristics of tetracycline in soil and the removal efficiency and mechanism of tetracycline by coagulation and electric flocculation.Based on the above research,the following conclusions are drawn:(1)Adsorption of tetracycline from sandy soil conforms to the quasi-second-order kinetic equation and Freundlich adsorption isotherm curve,and the adsorption process can be divided into three stages:rapid adsorption,slow adsorption and adsorption equilibrium.The adsorption rates of tetracycline of sand were all about 95%.In the initial rapid adsorption stage,with the increase of sand particle size,the specific surface area and adsorption sites decreased,and the adsorption rate decreased.However,due to the collision in the adsorption process will lead to the occurrence of resolution,and the relatively small clay mineral content of fine sand,the adsorption rate of tetracycline fluctuates more than that of coarse sand and medium sand.As for the adsorption factors,the increase of pH,which affects the form of tetracycline,will reduce the electrostatic repulsion between sand and tetracycline anions,and reduce the adsorption rate of tetracycline.The increase of ionic strength and the presence of high-priced metal cations will decrease the electrostatic repulsion and increase the bridging effect.The sand has a strong adsorption capacity for tetracycline.When the sand is at a higher concentration of 20 mg/L and faster than the groundwater flow rate of 3 mL/min,the penetration rate of 8 PV is less than 0.1,indicating that the pollution of tetracycline to soil is more serious than the pollution to groundwater.(2)The mineral content,morphological characteristics and characteristic functional groups of sand with three different particle sizes were characterized,and combined with the adsorption efficiency of tetracycline,it was found that the sand minerals were quartz,potassium feldspar,plagioclase and clay minerals.The surface of sand before adsorption was relatively smooth and smooth,and the surface after adsorption was mostly rough fold.Due to the different content of clay minerals in sand with different particle sizes,the strength of-OH absorption peak caused by the absorption of water molecules at 3400 cm-1 wave number of the three kinds of sand is different.The absorption of tetracycline leads to the enhancement of-OH absorption peak,and the combination with Al-O and Si-O weakens the characteristic peak caused by the different content ratio of non-clay minerals,which leads to the different key position.It is proved that the decrease of tetracycline concentration is due to the adsorption of sand,and the influence of sand with different particle size on the adsorption of tetracycline is revealed.(3)Improve the turbidity of distribution water by adding diatomite,simulate the turbidity of actual livestock and poultry breeding wastewater and provide an environment for flocs generation.The optimal conditions for removing tetracycline with FeCl3 flocculant are:sodium bicarbonate 1mM,ferric chloride 0.2mm,pH=5.The optimal conditions for the removal of tetracycline by AlCl3 flocculant are:sodium bicarbonate 2 mM,aluminum chloride 0.2 mM(in Al),pH=5.The optimal removal conditions of tetracycline by PAC flocculant were as follows:sodium bicarbonate 2 mM,PAC 0.2 mM(in Al),pH=5.It is worth noting that the AlCl3 and PAC quantities are not the optimal quantities for the removal of tetracycline,but the optimal quantities under the condition of flocs generation,which meet the prerequisite conditions for the study of the "additional" effect of tetracycline on the treatment of livestock and poultry wastewater by coagulation.The optimal efficiency may be caused by the change of reaction environment caused by the ratio of sodium bicarbonate to coagulant,and the chemical reaction of tetracycline degradation.The adsorption kinetics and thermodynamics of the three coagulants accord with the quasi-second-order kinetic equation and Freundlich adsorption isotherm curve.The flocs formed by FeCl3 coagulant adsorbed tetracycline and formed dense granular floes,while AIC13 and PAC coagulant adsorbed flocs formed stacked network.Contrast analysis shows that FeCl3 flocculant has the best adsorption efficiency for tetracycline.And according to the liquid-mass full-sweep spectrum,the removal process of tetracycline by coagulation and precipitation is not only adsorbed,but also changes in the morphology and structure of tetracycline,which may be caused by the oxidation of high-value metal ions or the existence of hydroxylated metal ions and Cl-.(4)By investigating the optimal conditions of tetracycline removal by DC and pulse methods under the conditions of different pH,current intensity,conductivity and plate spacing,the optimal conditions of the two methods are the same:pH=4,1=0.2A,EC=2 mS/cm and D=1 cm.Through the analysis of the interaction of the four factors by response surface method,it can be seen that only the current intensity I has interaction with the other three factors.Under the monopulse duty cycle of 60%,the removal rate of tetracycline was about 96%.Using the optimal monopulse condition to treat the actual wastewater,it is found that the maximum removal rate is 60%,which indicates that the components of wastewater are complex and there is a competitive relationship among them.Evaluation of oxidation performance of the reaction under the optimal conditions of the two ways:oxidation rate:adsorption rate of about 80%:20%,oxidation plays a dominant role,which is due to the low concentration of tetracycline in the early stage of electric flocculation has not produced a large number of flocs when the electrochemical action is dominant.Compared with DC,pulse condition is more economical and energy saving,and provides more contact time for the adsorption reaction environment at the same time,which makes the efficiency better.According to the measurement of morphology and functional groups,the morphology of flocs is more compact after adsorption,the content of C increases,the appearance of N element,and the absorption peaks of N-OH and C=O increase.The morphology and structure of tetracycline were determined by mass spectrogram,and a possible reaction path was deduced by analyzing intermediate products.The antibiotic pollution in livestock and poultry wastewater is discharged directly by runoff,then removed indirectly by sewage treatment process,and finally"optimized" electric flocculation treatment for tetracycline is a step-by-step process from adsorption,adsorption-degradation to adsorption-strong degradation.This study not only provides a theoretical basis for the systematic treatment of tetracycline,but also provides a basis and optimal treatment conditions for the environmental pollution of soil and water and potential human health risks caused by tetracycline in livestock and poultry wastewater. |
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