Adsorption-desorption And Vertical Migration Of Sulfamethoxazole In A Soil Under The Impact Of Microplastics

As two environmental emerging contaminants,microplastics and antibiotics have received extensive attention in recent years.Microplastics refer to plastic pollutants with a particle size of less than 5 mm in the environment,including fragments,fibers,particles,foams,films,etc.The polymer types of common microplastics detected in the environmental samples include polyethylene（PE）and polystyrene（PS）,polypropylene（PP）,polyamide（PA）and polyvinyl chloride（PVC）,etc.Due to its complex surface structure and large specific surface area,microplastics can adsorb and accumulate environmental chemicals and act as a carrier for the chemicals in environment and from environment to biotas.This effect is also named of"vector effect"of the microplastics.Large amount application of organic fertilizers in farmlands may lead to residual of antibiotics in soils.The interaction between the antibiotics and microplastics in environments might have an effect on the change of environmental risks of both contaminants.A lot of surveys have been concudcted regarding the contamination of antibiotics and microplastics in agricultural environment.However,there is a knowledge gap on the adsorption-desorption and migration of antibiotics in soils under the impact of microplastics.Therefore,the objectives of this study are to understanding the adsorption-desorption and vertical migration of sulfamethoxazole in a soil under the impact of microplastics using the batch equilibrium adsorption and desorption tests,column migration experiment and simulated extraction of gastric and intestinal fluid for the soil and microplastics mixture.A typical paddy soil in south China developed from red soil was selected and five polymer types of microplastics including PE,PS,PP,PA and PVC were used in the study.It contained following three aspects:（1）exploring the influence of polymer type and spiked amount of microplastics,solution ions and their ionic strength,water soluble organic matter on the adsorption of sulfamethoxazole in the soil in the presence of microplastics using batch equilibrium adsorption test.（2）Studying the release of sulfamethoxazole from an aged soil in the presence of different types of microplastics and their spiked amounts,as well as the influence of ions and dissolved organic matter on the desorption using a series of desorption kinetic experiments.（3）Simulation of sulfamethoxazole migration in the soil and releasing of sulfamethoxazole from the soil inside the biotas in the presence of microplastics using soil column test and simulation gastrointestinal extraction.The major conclusions of this paper are shown as following:In a soil-microplastic mixture system,sulfamethoxazole in the solution was mainly adsorbed by the soil rather than the microplastics.However,presence of microplastics in the soil do affect the adsorption of sulfamethoxazole.The maximum equilibrium adsorption capacity of PP and PVC microplastics reached 154.89 and 126.62μg g^-1,and the Kd values reach 2.613 and 2.596 L kg^-1.PP and PVC microplastics was observed having the most significant effects among the five microplastic types.Different microplastic types had different dominant mechanisms for the adsorption of sulfamethoxazole.For example,the adsorption of sulfamethoxazole by PVC was mainly governed by its polarity of the structure,while the adsorption by PE was dominated by the rubber domain of PE structure,both of which increased the sulfamethoxazole adsorption by soil-microplastic mixture.With the increase of the ionic strength of the sodium chloride solution,the equilibrium adsorption capacity of sulfamethoxazole increased from 13.275～14.639μg g^-1to 18.633～20.12μg g^-1,while the presence of calcium ions reduced its adsorption.The adsorption of sulfamethoxazole was inhibited in the presence of fulvic acid and the adsorption capacity will decrease with the increase concentration of fulvic acid.The impact of polyethylene and polystyrene microplastics on the desorption of sulfamethoxazole in the aged soil could be observed in the study.Desorption rate of sulfamethoxazole declined significantly and a slow desorption phase was observed from 10 h to 48 h after the addition of polyethylene and polystyrene microplastics,and the pseudo-second-order kinetic desorption constants（k2）of the PE and PS microplastic groups were 1.54 mg L^-1h-¹and 1.531 mg L^-1h-¹.The equilibrium desorbed concentration of sulfamethoxazole declined significantly（p<0.05）in the presence of polyethylene and polyvinyl chloride microplastics.The influence of sodium and calcium ions on sulfamethoxazole desorption from the soil was not affected by the addition of microplastics.However,the influence of fulvic acid on sulfamethoxazole desorption was mitigated after the addition of microplastics in general.Importantly,with an elevated concentration of fulvic acid,the decline of sulfamethoxazole desorption was negligible.Vertical migration of sulfamethoxazole in soil column was significantly affected by the presence of microplastics.The effect mainly caused by the added amount of microplastics.In the case of the addition of 5%of the PP microplastic,it mainly changed soil density and porous volume of the soil column,which resulted to the increase of sulfamethoxazole dispersion.However,in the case of the addition of 1%of the PP microplastic,sulfamethoxazole dispersion decreased,the adsorption of sulfamethoxazole by the soil-microplastic mixture dominated the vertical migration.The release of adsorbed sulfamethoxazole in the soil-microplastic mixture could be predicted by simulated gastric and intestinal fluid extraction.The aged PP microplastic was assumed to significantly（p<0.05）decrease the release of sulfamethoxazole inside of biotas,while unaged PP microplastic was assumed to affect mainly the release of sulfamethoxazole in gastric environment.