Sorption Of Organic Contaminants By Microplastic In Water And Soil: Behavior And Mechanisms

Plastic products are a great convenience and widely used in our daily life,because of their lightness,anticorrosion,strength and cheapness,but they have elicited serious environmental issues due to the poor level of recycling and management of plastic wastes.Microplastics are defined as plastic particles smaller than 5 mm in diameter,and have been ubiquitously detected in aquatic and soil environments.Owing to the surface hydrophobicity,microplastics tend to sorb various kinds of organic pollutants.The sorbed organic pollutants on microplastics might be transported to other environments or transferred to biota,thus causing the potential risk to environments.Therefore,this study investigated the sorption behavior of organic pollutants to microplastics in the aqueous solution and in the soil-water system,and evaluated the influence of various factors on the sorption process.The main results were as follows:?1?Characteristics of microplastic samples.Polyethylene?PE?,polypropylene?PP?,and polystyrene?PS?were selected in this study.Despite of the different sources of microplastics used in the sorption experiments of aqueous solution and soil-water system,microplastics in the same type had similar chemical structures and properties.Microplastics had a high purity and there were no oxygen-containing functional groups on the surface of microplastics.PE had the largest specific surface area and micropore volume among three microplastics.The chemical structures of three microplastics are different.PE and PP microplastics are rubbery polymers,with expanded and flexible structures,while PS microplastic is glassy polymer,with condensed and rigid structure.?2?Sorption of sulfamethoxazole?SMX?to microplastics in aqueous solution.Only the sorption of SMX to PE microplastics was observed among three types of microplastics.The sorption isotherms of SMX to PE microplastics can be well fitted by the linear model,with the partition coefficient of 591.7±24.1 L kg^–1.Non-specific van der Waals force was considered as the main sorption mechanism.Solution p H,salinity and the presence of fulvic acids?FA?had negligible effect on the sorption of SMX.?3?Sorption of tetracycline?TC?to microplastics in aqueous solution.The sorption isotherms of TC to PE,PP and PS microplastics can be well fitted by the Langmuir model.Hydrophobic interaction,pore-filling and van der Waals force were regarded as the main sorption mechanisms for TC sorption to microplastics.The sorption capacity of TC to three microplastics followed the order PS(167±3.63?g g^–1)>PP(113±4.45?g g^–1)>PE(109±7.74?g g^–1).The highest sorption capacity on PS microplastics was attributed to polar and?-?interactions between microplastics and TC.With the increasing p H,the sorption capacity of TC increased gradually,peaked at p H 6.0 and then decreased,because of the electrostatic repulsion between microplastics and TC under acidic or alkaline conditions,which inhibited the sorption of TC.With the increase of FA concentration,the sorption capacity of TC decreased sharply,indicating that the presence of dissolved organic matter?DOM?would hinder the sorption TC,and therefore microplastics might not be a carrier of TC in natural waters.?4?Sorption of phenanthrene?PHE?to microplastics in aqueous solution.The sorption kinetics of PHE to PE and PP microplastics can be fitted with the pseudo-second-order model,and that to PS microplastics can be fitted with the intraparticle diffusion model.The sorption equilibrium of PHE to PE,PP and PS microplasitcs was reached at 24,36 and 72 h,repectively.The sorption isotherms to three microplastics can be described by the linear model.The main sorption mechanism was hydrophobic interaction and the sorption process was dominated by partitioning.The partition coefficient of PHE to three microplastics followed the order PE(16821±410 L kg^–1)>PP(9048±175 L kg^–1)>PS(1892±79 L kg^–1).The expanded and flexible structures of PE and PP were benificial for the sorption of PHE,while rigid structure in PS microplastics was not conducive to PHE sorption.Solution p H showed negligible effects on PHE sorption to three microplastics.The presence of FA significantly inhibited PHE sorption to PS microplastics,but not to PE and PP microplastics.The increase of salinity facilitated the sorption of PHE to three microplastics to different extents due to the salting-out effect.?5?Microplastics significantly affected the sorption and distribution of PHE in soils.The sorption of PHE was increased by the addition of microplastics in soils.PE and PP microplastics at 1%and 10%?w/w?significantly enhanced the sorption of PHE in Oxisol and Inceptisol?p<0.01?,and PS only at 10%?w/w?could significantly increase that in two soils?p<0.01?.In the Oxisol system,the partition coefficients of PHE to microplastics and soil organic carbon followed the order Log K_PE(4.09±0.08 L kg^–1)>Log K_oc(3.74±0.15 L kg^–1)>Log K_PP(3.60±0.02 L kg^–1)>Log K_PS(3.10±0.03L kg^–1),with the same order in the Inceptisol system.Moreover,microplastics affected the distribution of PHE in soils.At the addition ratio of 0.1%?w/w?,the relative proportions of PHE to PE microplastics were 38%and 9%in Oxisol and Inceptisol,respectively.Soil treatment and the presence of DOM can affect the sorption of PHE to PS microplastics,while soil DOM hardly altered the sorpotin of PHE to PE and PP microplastics.In summary,microplastics can act as an effective carrier for SMX and PHE,but not for TC in natural waters,and microplastics can also be a“sink”for PHE in the soil environment,possbily with a more pronounced effect than soil organic matter.These results are significant for predicting and assessing the ecological risk of microplastics in the aquatic and soil environments.