Study On The Adsorption Characteristics And Leaching,degradation Behavior Of The Tetracycline-based Veterinary Drug In The Northwest Loess Soil

The environmental behaviour of tetracyclines（（Tetracyclines,TCs））antibiotics on different soils are related to the properties of loess,such as particle size,organic matter content,clay content,acidity and alkalinity of soil.It is of great significance to study the environmental behavior of tetracycline antibiotics on different types of loess in Northwest China in order to understand the fate of tetracycline antibiotics in Northwest loess area and to provide scientific basis for regional soil antibiotic pollution control,remediation and regional ecological security.Tetracycline is widely used in China and has a high detection rate in the environment.Therefore,tetracycline（Tetracycline,TC）was used as the target pollutant,and four loess types,such as sierozem,loessal soil,gray cinnamon soil and lou soil,were selected as experimental media in Northwest China in this study.The adsorption/desorption kinetics,adsorption thermodynamics and influencing factors（such as particle size,pH,ionic types and strength,and temperature et al.,）of the TC in the loess soils of Northwest China were studied.The soil samples were characteried by the method of particle size analysis,organic matter measurement and infrared spectroscopy.The migration ability of TC in the loess soils and its influence factors（such as different soil type,different ionic strength,different pH,different soil layers and different initial concentration et al.,）were also studied by the column leaching method.Bio-degradation of TC in different types of loess by control of test conditions（sterilization/non-sterilization）to reveal the adsorption characteristics and environmental behavior of tetracycline veterinary antibiotics in soil environment,so as to provide some theoretical guidance for soil restoration.The results of the study are as follows:（1）The physical and chemical properties shows that the pH values of sierozem,gray cinnamon soil,loessial soil and lou soil are slightly greater than 7,and indicating that the loess are weak alkaline.The level of organic matter in loess is gray cinnamon soil>lou soil>sierozem>loessal soil.The sand content of loessial soil is the most than that of other types of loess,while the silt clay content and the largest cationic exchange capacity of gray cinnamon soil are more.The trend of wave peak in the infrared spectrum of different loess soils is approximately the similar,and the functional groups contained in loess are also same.（2）The adsorption kinetics of TC on sierozem,loessial soil,gray cinnamon soil and lou soil show that the adsorption equilibrium time are1,1.5,2,0.8 h respectively.Under the same concentration,the order of adsorption maximum capacity for TC on different loess soil is:gray cinnamon soil>lou soil>sierozem>loessial soil.The dynamic data is fitted well with the pseudo-second-order model,The intraparticle diffusion model is also used to explore the adsorption mechanism.The results show that the fitting line is not through the origin of the coordinate,and the adsorption mechanism of TC on the different loess soils is involved the diffusion of liquid film and the adsorption of particle surface sites.The de-sorption amount of TC from the loess soils is in the order of sierozem.The desorption amount of TC on the gray cinnamon soil is the least.The Pseudo-second-order model could illustrate the de-sorption process.The HI of TC in loess is less than 0.7,which indicates the existence of adsorption hysteresis.The de-sorption rate of TC in soil is less than the adsorption rate.The result also showed that with the increase of initial TC concentration,HI also increased,that is to say,the higher the contamination concentration,the faster the desorption rate of TC in loess soils.The adsorption capacity of TC on sierozem,loessal soil,gray cinnamon soil and lou soil increases with the increase of temperature,which indicates that the adsorption belongs to endothermic reaction,that is,the increase of temperature is beneficial to the adsorption of TC on different loess soils.The adsorption thermodynamics of TC is more in accordance with Freundlich isotherm adsorption model.The n value is approximate 1,indicating that the adsorption process is easy to carry out,and the E value is less than 8kJ/mol,the adsorption is dominated by physical adsorption.The partition coefficient KF represents the adsorption capacity of soil to pollutants,and the higher the value is,the stronger the adsorption capacity is.The K_F of TC is between 0.122 and 0.804 with a sequence as gray cinnamon soil>lou soil>sierozem>loessal soil.The gibbs free energy changes（ΔG^θ）of TC adsorption by sierozem,loess soil,gray cinnamon soil and lou soil is less than0,the enthalpy change（?H^θ）and entropy change（?S^θ）are greater than 0,which indicates that the adsorption process of TC by sierozem,loess soil,gray cinnamon soil and lou soil are spontaneous,endothermic and chaotic.（3）The adsorption capacity of TC on four kinds of loess was gray cinnamon soil>lou soil>sierozem>loess soil,and the desorption amount of TC from four loess soils was gray cinnamon soil<loessal soil<lou soil<sierozem.The adsorption capacity of TC on sierozem,loessal soil,gray cinnamon soil and lou soil increased with the increase of temperature.That is,the increase of temperature is beneficial to the adsorption reaction.The adsorption capacity of TC decreases with the increase of Ca²⁺/Na⁺ion,and with the decrease of loess particle size,the adsorption capacity was increased.The pH has a significant effect on the adsorption of TC on lou soil,at pH<5,the adsorption capacity of TC decreased with the increase of pH,the adsorption capacity of TC increased with the increase of pH among 5 to 7,the adsorption capacity of TC decreased with the increase of pH between 7 and 9.（4）The leaching migration of TC in soil column is related to loess type,different soil layer,ionic strength,pH and the initial concentration of TC.Soil texture plays an important role in the migration of VAs in soil,TC has higher leachability in loessial soil.With the deepening of the soil layer,the migration ability of TC in the soil column is enhanced,indicating different soil layers will affect TC migration ability,but the effect is not significant.Ca²⁺and Na⁺inhibit the leaching migration of TC in loess,and the inhibition of Na⁺is more significant than that of loess.With the increase of ionic strength,the concentration of TC in leachate decreases obviously,and the adsorption amount of TC on soil column increases.The leaching migration of TC in loess increases with the pH value of leaching filtrate.Acidic leaching filtrate can promote the retention of TC in surface soil and reduce its downward leaching migration.With the increase of TC initial concentration,a large number of TC migrate to loess soil,and the pollution process higher the degree,the stronger TC longitudinal migration ability.The Koc values of TC on sierozem,loessal soil and lou soil ranged from 0.283 to 0.895 cm³/g<300 cm³/g,and the t_1/2/2 values ranged from 141 to 365 d>21 d,indicating that TC had high leaching capacity.The GUS value of TC in sierozem,loessal soil and lou soil was greater than 2.8,indicating that TC was leachable chemistry.（5）TC degradation in loess is divided into static degradation and dynamic degradation.The static degradation studies showed that the degradation rate of TC in loess decreased gradually with the prolongation of time.The degradation rate of TC was as follows:lou soil>sierozem>loessal soil;The results showed that the R² of TC degradation was more than 0.96,which indicated that the static degradation on lime-calcium soil,yellow cotton soil and coriander soil was more consistent with the First-order degradation kinetics,indicating that the degradation of TC on loess showed exponential decay relationship.The static degradation amount of TC was in an order as follows:lou soil>sierozem>loessal soil.The dynamic degradation curve of the TC in the sterilization group is similar to the dynamic degradation curve of the microorganism group（not sterilized）,which indicated that microorganisms have little significant effect on the dynamic degradation of TC on loess.The degradation studies show that the half-life period of TC in loess between 4.7 and 12 months,the degradation in sierozem,loessal soil and lou soil is difficult to degrade.