Study About The Environmental Aging Of Microplastics And Their Effects On Adsorption Behavior Of Heavy Metals

Due to the advantages of cheap,durability and strong adaptability,plastic products have been widely used in human daily life and industrial production,which has become the most important part of modern life.The global annual production of plastic still shows a trend of annual growth.However,plastic waste can be inevitably released into the natural environment,where it is gradually broken up into microplastics and even nanoplastics through the synergistic effects of long-term environmental aging and biodegradation.Aquatic environment is not only an important sink of microplastics,but also can accumulate heavy metal pollutants.Although the interaction between microplastics and heavy metals has been studied,the interfacial adsorption behavior of metal ions onto the biodegradable microplastics and tire wear particles is still poorly understood.Based on this,this study comprehensively evaluated the effects of environmental aging on the physicochemical properties of microplastics,and then explored the interfacial adsorption behavior of heavy metals lead(Pb(Ⅱ))and copper(Cu(Ⅱ))onto typical aging microplastics.This study provides a theoretical basis for clarifying the interaction and potential risks between microplastics and coexisting pollutants in aquatic environment.The main research work and results of this paper can be summarized as the following three sections:The environmental aging effect of microplastics was comprehensively evaluated in the first section.A series of characterization methods including visual measurement,scanning electron microscopy,specific surface area and pore column analysis,particle size distribution,contact angle,Zeta potential,Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used.The effects of ultraviolet radiation and mechanical abrasion on conventional polystyrene microplastics(PS),biodegradable polylactic acid microplastics(PLA)and tire wear particles(TWP)were evaluated.Meanwhile,the colonization characteristics and microbial community abundance of surface biofilms onto aging microplastics in the aquatic environment were also studied.The UV-aging can significantly modify the surface morphology,specific surface area and pore size of PS and PLA,potentially destroy the polymer structure to produce smaller microplastics,and increase the negatively charged range of Zeta potential.The increase of carbonyl index and oxygen/carbon ratio indicates the increase of oxygen-containing functional groups on the surface of aging microplastics.On the other hand,the mechanical abrasion mainly caused damage to the surface morphology of TWP.In addition,after 35 day exposure of actual sewage colonization experiment,a layer of biofilm was formed on the surface of the aging microplastics,of which Proteobacteria and Bacteroidetes were the most abundant microbial communities.Notably,there are some differences in bacterial community structure between different aging microplastics and wood chips.In the second section,the adsorption behavior of lead(Pb(Ⅱ))and copper(Cu(Ⅱ))onto typical aging-microplastics and pristine microplastics in the aquatic environment was studied.UV-aging polystyrene(UV-PS),UVaging polylactic acid(UV-PLA),and TWP obtained by mechanical abrasion were used as typical aging microplastics,and Pb(Ⅱ)and Cu(Ⅱ)were used as typical heavy metal pollutants.The adsorption behavior between three kinds of aged microplastics/two kinds of pristine microplastics(PS and PLA)and metal ions was investigated.The adsorption kinetics of microplastics were studied by the Pseudo-second-order kinetic model,the Pseudo-first-order kinetic model and the inparticle diffusion model.The adsorption isotherms of microplastics were studied by the Langmuir and Freundlich models.The Pseudo-second-order kinetics of the adsorption of Pb(Ⅱ)and Cu(Ⅱ)by five kinds of microplastics can be well fitted,indicating that the adsorption behavior is dominated by chemisorption.In terms of adsorption capacity,UV-PLA,TWP and PLA had higher adsorption capacity for Pb(Ⅱ)and Cu(Ⅱ).In addition,the adsorption capacity of aging microplastics to metal ions is generally stronger than that of pristine microplastics.Environmental aging can significantly alter the physicochemical properties of microplastics,which may affect the metal ion adsorption behavior of aging microplastics.According to the analysis,the surface morphology,specific surface area,pore volume and crystallinity are closely related to the adsorption capacity of microplastics.Furthermore,chemical properties such as difference of raw materials,electrostatic interaction and oxygen-containing functional groups are the main factors affecting the adsorption capacity and mechanism of microplastics.On the basis of second section,the effects of environmental factors(pH,humic acid,salinity,surfactant,Cu(Ⅱ)coexistence and suspended sediment)and desorption behavior(sediment and complex aquatic environment)on Pb(Ⅱ)adsorption by aging microplastics(UV-PLA,UV-PS and TWP)were further investigated in the third section.Results showed that pH value,salinity and surfactant significantly increased Pb(Ⅱ)adsorption capacity of aging microplastics,and different environmental factors showed differentiated promoting effects at the different concentrations.However,the addition of coexisting Cu(Ⅱ)generally inhibited the adsorption behavior of Pb(Ⅱ)onto aging-microplastics.In addition,humic acid showed a low concentration to promote adsorption and high concentration to inhibit adsorption.Notably,the influence mechanism of suspended sediment on Pb(Ⅱ)adsorption by aging-microplastics is complex.For the desorption behavior of aging microplastics,Pb-laden UV-PLA has the strongest desorption capacity.Futhermore,environmental factors significantly affected the desorption behavior of the three kinds of aging microplastics.Humic acid inhibits the Pb(Ⅱ)desorption behavior of aging microplastics in the sediment environment,while salinity can promote Pb(Ⅱ)desorption of aging microplastics in sediment environment.Moreover,both humic acid and salinity promoted the Pb(Ⅱ)desorption behavior of the three kinds of aging microplastics in complex aquatic environment,while the effects of humic acid was greater than that of salinity.In this paper,the environmental aging processes of microplastics and their effects on the adsorption of heavy metals were comprehensively studied.The interfacial adsorption behavior of metal ions Pb(Ⅱ)and Cu(Ⅱ)onto typical aging microplastics was elucidated,which provided some meaningful information for the assessment of the potential risk of microplastics and co-existing pollutants in the aquatic environment.