The Study On The Influence Of Typical Components Of Environmental Water On The Aggregation-suspension Behavior Of Polymethyl Methacrylate And Polystyrene Micro (Nano) Plastics

In the aqueous environment,micro-nano plastics have received much attention due to their widespread distribution and various toxicities.The aggregation and sedimentation behavior of micro-nano plastics in environmental water is an important factor affecting their migration and diffusion.A comprehensive understanding of the aggregation and sedimentation characteristics of micro-nano plastics in environmental water will provide a better basis for understanding their migration and diffusion behavior.However,current research mainly focuses on the aggregation behavior of micro-nano particles,and the understanding of their sedimentation behavior is limited.In order to better understand the aggregation and sedimentation behavior of micro-nano plastics,this study investigated the effects of typical water components such as Na+ and natural organic matter(NOM)on the aggregation and sedimentation behavior of three plastic particles,including 180 nm polystyrene nanoparticles(PS NPs),100 nm poly(methyl methacrylate)nanoparticles(PMMANPs),and 1000 nm poly(methyl methacrylate)microplastics(PMMA MPs).The study also verified the specific differences in these behaviors in common environmental waters,such as seawater,river water,lake water,and wastewater,as well as other potential influencing factors.The relevant research results and conclusions are as follows:(1)Na+ plays an important role in regulating the aggregation and sedimentation behavior of PS NPs and PMMA N/MPs through electrostatic repulsion,but has different effects on plastic particles of different sizes and materials.When the Na+ concentration is low(less than 20 mM),the three types of plastic particles do not aggregate,but their sedimentation rates increase significantly with the increase of Na+concentration.When the Na+ concentration is in the range of20-80 mM,the aggregational behavior of the three types of plastic particles shows a significant difference.PMMA NPs aggregate rapidly due to the introduction of Na+,while PS NPs and PMMA MPs show no significant change in aggregational size compared to their initial states.However,due to the increase of water density caused by the increase of Na+ concentration,the sedimentation behavior of the three types of plastic particles is suppressed to varying degrees.When the Na+concentration reaches above 80 mM,the surface double electric layer(EDL)of the three types of plastic particles continues to be compressed.PS NPs and PMMA NPs quickly aggregate under the action of van der Waals attraction as the ion concentration increases,while PMMA MPs do not show significant aggregation phenomenon.Compared with the material itself,plastic particles with smaller initial sizes are more susceptible to the influence of Na+concentration on their zeta potential and aggregational behavior.Due to the continuous increase in water density,the sedimentation behavior of the three materials is basically eliminated.Compared with previous studies,the overall effect of Na+concentration on the two types of NPs shows a trend of "sink but not aggregate,or aggregate but not sink".(2)Natural organic matter(NOM)plays an important role in regulating the aggregation and settling behavior of PS NPs and PMMA N/MPs,with the concentration of Na+.When the Na+ concentration is 20 mM,increasing the NOM concentration slightly increases the particle size of both PMMA NPs,but has no significant effect on the particle size of PMMA MPs;when the Na+ concentration is 100 mM,as the NOM concentration increases,the PMMA and PS NPs aggregates show significant dissociation.The particle size of PMMA NPs decreases from about 650 nm to below 200 nm,and the particle size of PS NPs decreases from almost 300 nm to nearly 200 nm,but PMMA MPs are not significantly affected.Sedimentation kinetics experiments show that the influence of NOM on the sedimentation behavior of the three plastic particles varies greatly under different Na+ concentrations.When the Na+concentration is 20 mM,as the NOM concentration increases,the sedimentation rate of the three plastic particles first increases and then decreases.When the NOM concentration reaches 1,5 and 0.5 mg/L respectively,the sedimentation rate of PS NPs,PMMA NPs and PMMA MPs reaches the maximum value.When the Na+ concentration is 100 mM,NOM has a inhibitory effect on the sedimentation behavior of the three plastic particles throughout the process.Through density experiments,it was observed that NOM can continuously reduce the water density,and the adsorption amount of NOM on the surfaces of the three plastic particles increases with its concentration.Based on this,it is inferred that the reason why the sedimentation rate of the three plastic particles continuously decreases with increasing NOM concentration is because the potential adsorption of NOM on the particle surface reduces the average density of the aggregates.(3)Unlike PMMA MPs,PS NPs and PMMA NPs exhibit similar behaviors in environmental waters.In high salinity waters,PS NPs and PMMA NPs tend to aggregate and have weak sedimentation properties,making them easier to diffuse in the water.In low salinity waters,NOM increases the stability of PS NPs and PMMA NPs,reduces the density of the water,and causes them to aggregate in the sediment of the water.PMMA MPs,on the other hand,undergo sedimentation under the influence of Na+and NOM,eventually aggregating in the sediment of the water.This study systematically investigates the effects of Na+ concentration and NOM on the aggregation and settlement behavior of three types of plastic particles(PS NPs,PMMA NPs,and PMMA MPs).It has important research value in deepening our understanding of the migration and settlement behavior of plastic particles in aquatic environments,while also providing a theoretical basis for the environmental risk assessment and control of plastic particles.