Adsorptin,Migration And Numercical Simulation Of Sulfadiazine In Soil Under Silica Colloids

The use of antibiotics is huge and it has increased year by year.However,lower utilization of antibiotics leads to a huge amount of antibiotics enter into the environment.At present,The distribution of antibiotics in east of China is generally higher than that in the west.Antibiotics in densely-populated areas was wide varieties and high pollution.Therefore,it is of utmost important to solve antibiotics pollution.Antibiotics in the environment can expand the range of polluted areas by migrating with water.It is of extremely important to research the pollution pathway and migration patterns of antibiotics in the environment.Soil system is an essential substrate of antibiotics migration.Soil colloids play a viral role in antibiotics migration.Compared with other sulfonamide antibiotics,sulfadiazine has a low degradation rate,simple structure and universal application.At present,there are few studies on the adsorption and migration characteristics of antibiotics in soil under colloidal action.In this work,the neutral silica sol was used as the exogenous colloid,SDZ as targeted pollutant.Indoor batch equilibrium adsorption experiment and column experiment was used to study the effect of soil colloids on adsorption and migration of SDZ.The results of adsorption and migration behaviors will guide the risk assessment and prevention of sulfa antibiotics.Researches in this work mainly include two parts:The first one was the static batch adsorption experiment of SDZ in soil under different ionic strength?with or without colloids?,different colloidal concentrations,different soil types?with or without colloids?.Two isothermal adsorption models were performed to study static adsorption characteristics.The second one was the influence of different ionic strength?with or without colloids?,colloidal concentrations,layered soils?with or without colloids?on SDZ migration in soil column.Hydrus-1D software simulated the migration of SDZ in soil columns under different ion mobility and layered soil,and the migration process of colloids in layered soil.Concluded as follow:?1?The results of batch equilibrium adsorption experiments were as follows.Colloids could inhibit adsorption of SDZ at the ionic strengths of 0.001 and 0.003 mol/L CaCl₂.However,when ionic strengths were 0.005 and 0.01 mol/L CaCl₂ and the colloids were added,the effect of the SDZ adsorption on soil was related to the initial concentration of SDZ.When the ionic strength was 0.01 mol/L CaCl₂,with different concentrations of colloids,adding colloids could promote the adsorption of SDZ in high organic loam?R1?.Ca²⁺can affect the adsorption capacity of SDZ by compressing colloidal double electric layer to promote colloidal flocculation and increase the adsorption site and compete with SDZ for the adsorption site.Adsorption of SDZ showed great difference in various soil type.The adsorption capacity in silt loam?F?was the strongest,followed by soil R1,the lowest was low organic loam?R2?.This is mainly the influence of soil physical and chemical properties,the influence of colloids on the adsorption capacity of SDZ in soil is affected by the soil organic matter,cation exchange capacity and clay content.?2?Freundlich and Linear model were performed to study static adsorption characteristics under different condition.The Freundlich model was appropriate to describe adsorption of SDZ in high organic loam under different ion intensities.R² was around 0.958⁰.997.When the concentration of colloids were 0?58 and 290 mol/L,the R²of Freundlich model was around 0.976⁰.997.While the concentration of colloids were116?174 and 232 mg/L,R² of Freundlich model was 0.964⁰.989.No model was appropriate to perform adsorption of SDZ in low organic loam.However,when colloids were added,this characteristics was more suitable for the Freundlich model.In addition,the adsorption of SDZ in both other soils was suitable for the Freundlich model with or without colloidals existence?3?Soil column migration experiments indicated that adding colloids could improve migration of SDZ and inhibit adsorption of SDZ at lower ion intensities.However,results were opposite at higher ion intensities.When the ionic strength was 0.01mol/L,the colloidal concentration was in the range of 0²90 mg/L,which reduced the outflow of SDZ.With the increase of colloidal concentration,the ability to inhibit the migration of SDZ decreased.This is due to the fact that the role of Ca²⁺and SDZ in the adsorption site is dominant when no colloids are added.Decreased velocity of pore water improved the diffusion effect when colloids and SDZ effluent.With colloids addition,higher velocity promote migration and lower velocity inhibited migration.This is because the decrease of the pore water flow rate enhances the dispersion of colloids and SDZ during outflow.Time which discharge and effluent peak arrived in low organic loam-silt loam was sooner than that in silt loam-high organic loam with or without colloids existence.Increased ion intensities could suppressed colloids effluent,and soil configuration could also influence colloids effluent.?4?Using One-site Soption Model?OSM?and Two-site Soprtion Model?TSM?to simulate the migration of SDZ in different ionic strength and soil layers,TSM?R²>0.959,RMSE<0.004?can better describe the migration of SDZ in soil.The R_d values obtained by fitting the two models were smaller than the R_d values obtained from the static adsorption experiments.This is due to the fact that the soil is more in contact with SDZ during static adsorption experiments.The coefficients?of the two models were all smaller than 0.3.The attachment-detachment Convection Dispersion Equation could better fit the migration process of Si O₂ colloids in silt loam-low organic loam.The fitted R²=0.996,RMSE=2.60E-6,and the release ability of silt loam to colloids was stronger than adsorption of colloids,while loam1 was opponent.