Migration Characteristics And Numerical Simulation Of Sulfadiazine And Cadmium In Iron-coated Quartz Sand

In recent years,the environmental problems have become increasingly prominent caused by widespread use of antibiotics.Antibiotics enter environmental media through surface runoff,infiltration and many other ways.They also occur some behaviors such as adsorption,desorption,degradation and transportation and so on in these media.At present,antibiotics have been detected in soil,water and sediment,which have potential harm to biodiversity,ecological environment balance and human health.Iron oxide minerals are ubiquitous in the soil environment,and have an important impact on the adsorption and transport behavior of antibiotics in the environment as result of their large specific surface area and surface activity.While,the influence of iron oxide minerals on the transport of sulfanilamide antibiotics is still lack of in-depth study.Therefore,sulfadiazine（SDZ）as model compound for sulfonamide antibiotics and using quartz sand（QS）for the preparation of iron oxides coated sand（IOCS）.A large number of indoor soil column experiment were used to explore the influence of various physical（particle size,velocity）and chemical（iron content,ionic strength,ionic type,pH,cadmium）fators on the transport of sulfadiazine in iron oxide coated sand.So,provide theoretical basis for the risk assessment and remediation of antibiotic pollution in soil and groundwater.The following works were mainly carried out in this paper.Firstly,the transport characteristics of sulfadiazine in quartz sand and iron oxide coated sand were explored under the factors of iron content,ionic strength,ionic type,pH value,particle size and pore water velocity and so on.Then,the co-transport experiments of sulfadiazine and cadmium（Cd）were carried out.Lastly,the transport processes of sulfadiazine and Cd under different conditions were simulated by Hydrous-1D software.Some characteristic parameters that related to adsorption were obtained by inversion and quantify the transport process.The main conclusions are as follows:（1）The characteristics of adsorption and transport of sulfadiazine in quartz sand and iron oxide coated sand affected by different physical and chemical factors.The surface of iron oxide coated sand was mainly goethite.The more the iron content,the higher the roughness,the large the specific surface area and the more the positive charge is carried through,which is beneficial to the adsorption and inhibit the transport of sulfadiazine.The increase of ionic strength promoted the transport of sulfadiazine in quartz sand and iron oxide coated sand.Ca²⁺competed with sulfadiazine and the competition increased significantly with the increase of Ca²⁺concentration.In the experiments of different ions type,in iron oxide coated sand,in the presence of Ca²⁺,the concentration of sulfadiazine was the highest,followed by Mg²⁺,K⁺and Na⁺.The competitive adsorption of divalent cations such as Ca²⁺and Mg²⁺were greater to promote the transport of sulfadiazine.Under different pH conditions,low pH value could inhibit the transport of sulfadiazine.It was mainly due to hydrophobic,electrostatic repulsion and hydrogen bond forces.Among the three kinds of quartz sands with different particle sizes,the silt was the most unfavorable to the transport of sulfadiazine.The higher velocity was easy to promote the transport process of sulfadiazine in the porous media under different pore water velocities.（2）The action mechanism of sulfadiazine in quartz sand and iron oxide coated sand were different in different concentrations of cadmium when antibiotics and heavy metals were transport together.The outflow concentration of sulfadiazine increased with the increase of Cd concentration in quartz sand.In iron oxides coated sand,Cd promoted the sulfadiazine outflow under the concentration of 1 mg/L and inhibited under the 5 mg/L,the main mechanisms of the two concentrations were competitive adsorption and the formation of ternary surface complexes respectively.The competitive adsorption between Cd and sulfadiazine was greater than the complexation when the concentration of Cd was 100 mg/L,which was conducive to promoting sulfadiazine transport in the quartz sand;While the coordination force between the two was greater when the concentration of Cd was 500 mg/L,which was conducive to promoting sulfadiazine adsorption.In the iron oxide coated sand,the increase of Cd concentration promoted the transport process of sulfadiazine.（3）The Hydrous-1D software developed by American salty soil laboratory was used to simulate the transport of sulfadiazine and the co-transport of cadmium and sulfadiazine in quartz sand and iron oxide coated sand respectively,and the related adsorption characteristic parameters were retrieved.The transport characteristics of sulfadiazine under different conditions in quartz sand and iron oxide coated sand simulated by One-site Sorption Model（OSM）.The fitted curves and the measured curves were almost coincide,R²>0.90,RMSE<0.051.The adsorption partition coefficient k _F that obtained by inversion varied with the experimental conditions.The retardation factor R_d also showed different trends.The adsorption characteristic parameterβwas about at 1.The first-order kinetic rate constantαwas smaller than 0.0020.Using OSM and Two-site Sorption Model（TSM）to simulate the transport process of Cd,the fittied effect had little difference,where R²>0.91,RMSE<0.060.The fraction f of the instantaneous exchange point was less than 0.1 in the adsorption equilibrium,the instantaneous adsorption of cadmium was small.The k _F value was larger in quartz sand.The comparison of fitted parameters of sulfadiazine and cadmium showed that sulfadiazine had a stronger adsorption in the iron oxide coated sand,while cadmium has a stronger adsorption in the quartz sand,and the iron oxide coated sand was more conducive to promote cadmium transport.