Adsorption/desorption Behavior Of Antibiotics With Different Polarity Veterinary Drugs On The Different Loess Soil

The adsorption behavior of veterinary antibiotics（VAs）on loess soil determines the main processes of migration,plant absorption and soil microbial availability in the environment.The adsorption behavior is related to the physicochemical properties of soil and types of target pollutants.However,the adsorption mechanism of VAs on different loess soil are not fully understand.Thus,the adsorption kinetics,thermodynamic process,p H and ionic strength of CTC and OTC with different polarity on five loess soils from Northwest China were studied by batch equilibrium experiments.A systematic study of the adsorption characteristics of different polar VAs on different loess soils between the adsorption coefficient（K_d）and the physical and chemical properties of different soils will help to predict their fate in the natural environment.In order to provide a theoretical basis for the control and remediation of VAs in soil,the following main conclusions are obtained:（1）The analysis the physical and chemical properties of loess shows that the chemical properties of loess show weak alkalinity,low organic matter and weak cation exchange capacity.Loess usually belongs to silt clay deposits.Generally based on classification of soil text（sand,silt,and clay）,the clay is relatively small more than 85%of the soil.The chemical composition of loess is dominant by Si O₂（56%）,followed Al₂O₃（11.8%）,Ca O（8%）,Fe₂O₃（4.5%）,K₂O（2.7%）,Mg O（2.1%）,and Na₂O（1.7%）.The content of Ca O varies greatly in different loess soils.The highest content of Ca O in sierozems is 10.5%and the lowest content of Ca O in cultivated loessial soil is 5.0%.The mineral composition of loess is mainly quartz,feldspar and mica,80%of the detrital minerals.Clay minerals are mainly montmorillonite.Based on XRD spectrum analysis,five kinds of loess are mainly quartz（55%～70%）,followed by albite ordered（10%～20%）,orthoclase（8%～15%）,clinochlore（5%～10%）and a small amount of calcite,and dolomite.The cultivated loessial soil also contains a very small amount of goethite.There are obvious differences in the mineral contents of five loess clastic minerals,but the mineral composition difference is small.The infrared spectrum shows that the 5 kinds of loess belongs to montmorillonite type,and kaolinite was not found in different loess soils.（2）The adsorption kinetics of CTC and OTC on different loess showed that the adsorption equilibrium of CTC and OTC on five loess was reached 1 and 2 h,respectively.The difference of adsorption equilibrium time is small among five different loess soils.However,the adsorption rate and the maximum adsorption capacity were varied greatly.The adsorption process was accords with both the elovich equation（R²≥0.95）and the pseudo-second-order kinetic model（R²≥0.94）,indicating that the adsorption was heterogeneous diffusion process with two diffusion stages,fast and slow adsorption.The results also show a bigger b values of the reaction rate constant.The greater the b of the reaction rate constant,the faster the adsorption rate,and the shorter the adsorption equilibrium time is.The analysis of particle diffusion shows that external diffusion is the main adsorption rate control mechanism.（3）The thermodynamic process of adsorption of CTC and OTC on different loess shows:Both Linear（R²≥0.817）and Freundlich（R²≥0.805）adsorption models can fitted well with the adsorption behavior of CTC and OTC on 5 kinds of loess,which indicates that hydrophobic distribution plays an important role in this process,and the target pollutants are adsorbed on the loess with heterogeneous surface multilayer.The affinity（K_d）of CTC onto five kinds of loess soils is stronger than that of OTC.The affinity of CTC onto loess was as follow:Gray-cinnamon soil>sierozems>Cultivated loessial soil>Dark loessial soil>Lou soil;OTC affinity to Lou loessial soil>Dark loessial soil>Sierozems>Cultivated loessial soil>Gray-cinnamon soil.The hydrophobic distribution of adsorption behavior on five kinds of loess for target pollutants plays an important role in adsorption process,and the target pollutant has multilayer adsorption on heterogeneous surface of loess soils.The desorption hysteresis of CTC and OTC on five kinds of loess was obvious,and the adsorption rate of CTC and OTC on five kinds of loess is larger than the desorption rate,which is positive hysteresis.With the increase of concentration CTC,the desorption rate of OTC on loess was also accelerating.However,the affinity of the hydrophobic CTC on the five loess was almost opposite to the content of soil organic matter（SOM）,but for the weak hydrophobicity of OTC,the affinity of five kinds of loess was basically consistent with the content of SOM.After the analysis of the structure-activity relationship,CTC adsorption K_dvalues on loess were positively correlated with Na₂O and dolomi,but negatively correlated with clay,K₂O and quartz.OTC adsorption K_dvalues on loess were positively correlated with silt,Fe₂O₃,Al₂O₃and K₂O,but it were negatively correlated with sand,Na₂O,dolomit and orthoclase in loess soils.Although the content of SOM does not show a strong correlation with K_d,it also proves that the adsorption affinity of CTC on loess is negatively correlated with the content of SOM.While the adsorption affinity od OTC on loess was positively correlated with SOM.The correlation analysis showed that the specific surface complexation and cationic bridging on inorganic minerals were contributed greatly to the adsorption affinity of CTC and OTC on different loess soils.The affinity prediction model of CTC on different loess was:K_d=719.419-283.472K₂O+0.041Albite+32.745Clinochlore-65.851TOC（R²=0.575）.The affinity predictionmodelofOTCondifferentloesswas:K_d=-424.889+192.275K₂O+0.647Albite-7.396Clinochlore+18.003TOC（R²=0.848）.（4）The effect of different p H on CTC and OTC adsorption showed that the loess particles were always negatively charged and electrostatic attraction plays a major role for sorption of ionizable CTC and OTC.When the p H value was<p K_a1,hydrophobic distribution,surface complexation,and electron donor-acceptor play a major role in adsortion process.When the p H values were between p K_a1and p K_a2,the negatively charged target pollutants and negatively charged soil particles have enhanced electrostatic repulsion,but the cationic adsorption bridging or anion exchange promote adsorption more obviously.Thus,the cation adsorption bridging or anion exchange were dominant.With the p H value of the reaction system increases gradually,and the K_dvalue of the CTC on the five loess is the largest in weak alkalinity of the reaction system,which indicates that the cationic adsorption bridging or anion exchange plays a major role in the adsorption of CTC on five loess soils,but there was no obvious change in the K_dvalue of the OTC.（5）The effect of different ionic strength on CTC and OTC adsorption showed that the increase of K⁺,Na⁺ionic will compete with the charged antibiotic ions for the negative potential of mineral surface,weakened the affinity of target antibiotics to inorganic minerals in loess soils,while,Ca²⁺and Mg²⁺can not only compete with antibiotic ions,and they may form ionic bond bridges between mineral surfaces and target antibiotics.If the concentrations of Na⁺,K⁺,Ca²⁺,and Mg²⁺in background solution were increased from 0.01 M to 0.05 M and0.1 M,respectively,The adsorption affinity of target antibiotics onto loess soils decreased with the increase of ion concentration.Only the differences were that when the Na ion concentration changes,and the special property of adsorption affinity CTC on Cultivated loessial soil,Gray-cinnamon soil and OTC on Dark loessial soil and Cultivated loessial soil decreases first and then increases,which may be related to its sliding double electron layer.Mg²⁺increase to 0.1 M at 0.01,always inhibit the occurrence of adsorption,but its specificity needs further study.