Study On The Influence Mechanism Of Polystyrene Microplastics On The Adsorption Behavior Of Levofloxacin In Light Sierozem

In recent years,microplastics and antibiotics have received a lot of attention for causing serious environmental problems.Microplastics,with their small size and high migration and loading capacity,can act as good carriers of pollutants in the environment,affecting their migration and transformation.The content of microplastics in soil accounts for about 80%of terrestrial microplastic content.Microplastics entering the soil environment are aged to varying degrees and the adsorption of aged microplastics to environmental pollutants is the key to understanding the environmental behaviour of pollutants.Antibiotics entering the soil through soil amendment application,sewage irrigation and organic fertiliser use can lead to large residues of antibiotics in the soil,however,the effect of microplastics entering the soil environment on the migration of antibiotics in the soil and the sorption of antibiotics by the soil has been less studied.The proportion of microplastics in the soil and how microplastic ageing affects soil sorption of antibiotics are currently not studied systematically enough.Exogenous dissolved organic matter（DOM）plays an important role in soil sorption and is important for understanding the sorption behaviour of microplastics on soil sorbed typical antibiotics in the presence of DOM.In this study,polystyrene microplastic（PS）was used as a representative microplastic and the aged microplastic was subjected to ultraviolet（UV）ageing treatment and the aged microplastic was noted as UV-PS.levofloxacin（LEV）,an antibiotic with a high detection rate in the soil environment,was used as the test soil and fulvic acid（FA）,a dissolved organic matter（DOM）,was selected as the exogenous DOM in a light sierozem widely distributed in the northwest region.The main results of the study are as follows:（1）Ageing caused yellowing of the microplastic surface,increasing the degree of roughness and wrinkling of the PS surface.The adsorption of LEV by UV-PS was greater than that by PS.The adsorption kinetic data for both PS and UV-PS on LEV were well described by quasi-secondary kinetic equations.The R²obtained from the Freundlich model fit was the largest in the isothermal adsorption model,indicating that the adsorption of LEV by PS and UV-PS adsorption of LEV was mainly dominated by the inhomogeneous adsorption of multilayers.The effect of solution p H on the adsorption of LEV by PS and UV-PS was significant,with the maximum adsorption values of PS and UV-PS at p H=7.The adsorption of LEV by PS and UV-PS was not favoured by either too high or too low a p H.K⁺in solution promoted the adsorption of LEV by UV-PS at low concentrations and inhibited it at high concentrations.The adsorption of LEV by PS and UV-PS.The kinetic fitting data,the adsorption effects of PS and UV-PS on LEV at different p H values,combined with the Fourier infrared spectroscopy（FTIR）characterization results,led to the conclusion that the adsorption mechanisms of PS and UV-PS on LEV were mainly electrostatic attraction,hydrogen bonding andπ-πinteractions.（2）The addition of different masses of PS to the light sierozem before and after ageing had a facilitative effect on the adsorption of LEV by the light sierozem.The possible reasons for this are that the addition of PS to the light sierozem increases the pore structure of the light sierozem,and that the composition of the light sierozem contains a certain amount of mucilaginous material which makes the light sierozem colloid easy to agglomerate,and the addition of PS to the light sierozem makes the particles of the light sierozem disperse each other.The greater the proportion of PS in the microplastic-soil system,the stronger the dispersion effect.The aged PS has more folds and roughness,which enhances this dispersion effect,so the adsorption amount increases.The sorption of LEV by the microplastic-soil system is a spontaneous process with a decreasing degree of confusion.The maximum sorption value of LEV by microplastics on light sierozems is reached at p H=3,and the amount of sorption decreases as the p H increases.When NH₄⁺is present in the solution,it has a facilitating effect on the adsorption of LEV by the microplastic-soil system,while the three ions Na⁺,Mg²⁺and K⁺show an inhibiting effect on the adsorption of LEV by the microplastic-light sierozem system.When the ionic strength in the microplastic-soil system increases,the microplastic-soil system has an inhibiting effect on the adsorption of LEV by the microplastic-soil system because the ions and LEV compete for the adsorption sites on the surface of the system,thus making the adsorption of when the ionic strength of the microplastic-soil system increased,the microplastic-soil system showed a weakening of the sorption capacity of LEV due to competition between ions and LEV.The effect of p H and ionic strength on the adsorption,combined with kinetic fitting data and FTIR analysis,led to the conclusion that the mechanism of LEV adsorption by the microplastic-soil system mainly includes hydrogen bonding,electrostatic gravitational force andπ-πinteractions,and that the pore filling is also a factor affecting the adsorption of LEV by the microplastic-soil system due to the presence of a certain pore structure in the light sierozem.（3）When fulvic acid（FA）was added to the microplastic-soil system,the adsorption of LEV by the three systems of soil+FA,soil+1%PS+FA and soil+1%UV-PS+FA increased significantly compared to the adsorption of LEV by the microplastic-soil system without FA.The results of the infrared spectroscopy of FA showed that FA contains a large number of reactive functional groups and the electron-deficient sites in FA can undergo nucleophilic addition reactions with the electron-rich groups in LEV molecules,thus allowing the adsorption of LEV molecules.The isothermal adsorption behaviour of all three systems for LEV was well fitted by the Temkin model,indicating that the response of the heat of adsorption to changes in coverage during the adsorption process was sensitive.Acidic conditions were more favourable to the adsorption of LEV by the systems.Ions in solution can compete for adsorption,and the inhibition of adsorption by the same valence but different ions shows different effects,and the adsorption effect is influenced to some extent by the type and concentration of ions in solution.Combined with the experimental results and FTIR characterisation,the adsorption mechanisms of LEV by soil-FA and microplastic-soil-FA systems are mainly hydrogen bonding,π-πinteractions,hydrophobic partitioning,van der Waals forces and electrostatic gravitational effects.